draft-ietf-issll-atm-mapping-04.txt   draft-ietf-issll-atm-mapping-05.txt 
INTERNET-DRAFT Mark W. Garrett, INTERNET-DRAFT Mark W. Garrett,
ISSLL WG Bellcore ISSLL WG Bellcore
Expires: 12 September 1998
Marty Borden, Marty Borden,
New Oak Communications Bay Networks
21 November 1997
Interoperation of Controlled-Load Service and Guaranteed Service with ATM Interoperation of Controlled-Load Service and Guaranteed Service with ATM
<draft-ietf-issll-atm-mapping-04.txt> <draft-ietf-issll-atm-mapping-05.txt>
Status of this Memo Status of this Memo
This document is an Internet-Draft. Internet-Drafts are working This document is an Internet-Draft. Internet-Drafts are working
documents of the Internet Engineering Task Force (IETF), its areas, documents of the Internet Engineering Task Force (IETF), its areas,
and its working groups. Note that other groups may also distribute and its working groups. Note that other groups may also distribute
working documents as Internet-Drafts. working documents as Internet-Drafts.
Internet-Drafts are draft documents valid for a maximum of six months Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
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technologies. The service mappings are useful for providing technologies. The service mappings are useful for providing
effective interoperation and end-to-end Quality of Service for IP effective interoperation and end-to-end Quality of Service for IP
Integrated Services networks containing ATM subnetworks. Integrated Services networks containing ATM subnetworks.
The discussion and specifications given here support the IP The discussion and specifications given here support the IP
integrated services protocols for Guaranteed Service (GS), integrated services protocols for Guaranteed Service (GS),
Controlled-Load Service (CLS) and the ATM Forum UNI specification, Controlled-Load Service (CLS) and the ATM Forum UNI specification,
versions 3.0, 3.1 and 4.0. Some discussion of IP best effort service versions 3.0, 3.1 and 4.0. Some discussion of IP best effort service
over ATM is also included. over ATM is also included.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119. (Note, in
many cases the use of "MUST" reflects our interpretation of the
requirements of a related standard, e.g., ATM Forum UNI 4.0, rsvp,
etc.)
Table of Contents Table of Contents
1.0 Introduction ....................................................... 3 1.0 Introduction ....................................................... 3
1.1 General System Architecture .................................... 4 1.1 General System Architecture .................................... 4
1.2 Related Documents .............................................. 7 1.2 Related Documents .............................................. 7
2.0 Major Protocol Features for Traffic Management and QoS ............. 8
2.0 Major Protocol Features for Traffic Management and QoS ............. 7
2.1 Service Category and Bearer Capability ......................... 8 2.1 Service Category and Bearer Capability ......................... 8
2.1.1 Service Categories for Guaranteed Service ................ 9 2.1.1 Service Categories for Guaranteed Service ................ 9
2.1.2 Service Categories for Controlled Load ................... 10 2.1.2 Service Categories for Controlled Load ................... 10
2.1.3 Service Categories for Best Effort ....................... 11 2.1.3 Service Categories for Best Effort ....................... 11
2.2 Cell Loss Priority Bit, Tagging and Conformance Definitions .... 11 2.2 Cell Loss Priority Bit, Tagging and Conformance Definitions .... 12
2.3 ATM Adaptation Layer ........................................... 13 2.3 ATM Adaptation Layer ........................................... 13
2.4 Broadband Low Layer Information ................................ 13 2.4 Broadband Low Layer Information ................................ 14
2.5 Traffic Descriptors ............................................ 14 2.5 Traffic Descriptors ............................................ 14
2.5.1 Translating Traffic Descriptors for Guaranteed Service ... 15 2.5.1 Translating Traffic Descriptors for Guaranteed Service ... 15
2.5.2 Translating Traffic Descriptors for Controlled Load Service 18 2.5.2 Translating Traffic Descriptors for Controlled Load Service 19
2.5.3 Translating Traffic Descriptors for Best Effort Service ....19 2.5.3 Translating Traffic Descriptors for Best Effort Service ....20
2.6 QoS Classes and Parameters ..................................... 19 2.6 QoS Classes and Parameters ..................................... 20
2.7 Additional Parameters -- Frame Discard Mode .................... 22 2.7 Additional Parameters -- Frame Discard Mode .................... 22
3.0 Additional IP-Integrated Services Protocol Features ................ 23
3.0 Additional IP-Integrated Services Protocol Features ................ 22 3.1 Path Characterization Parameters for IP Integrated Services .... 23
3.1 Path Characterization Parameters for IP Integrated Services .... 22
3.2 Handling of Excess Traffic ..................................... 24 3.2 Handling of Excess Traffic ..................................... 24
3.3 Use of Guaranteed Service Adspec Parameters and Slack Term ..... 25 3.3 Use of Guaranteed Service Adspec Parameters and Slack Term ..... 26
4.0 Miscellaneous Items ................................................ 27
4.0 Miscellaneous Items ................................................ 26 4.1 Units Conversion ............................................... 27
4.1 Units Conversion ............................................... 26
5.0 Summary of ATM VC Setup Parameters for Guaranteed Service .......... 28 5.0 Summary of ATM VC Setup Parameters for Guaranteed Service .......... 28
5.1 Encoding GS Using Real-Time VBR ................................ 28 5.1 Encoding GS Using Real-Time VBR ................................ 28
5.2 Encoding GS Using CBR .......................................... 30 5.2 Encoding GS Using CBR .......................................... 30
5.3 Encoding GS Using Non-Real-Time VBR ............................ 31 5.3 Encoding GS Using Non-Real-Time VBR ............................ 31
5.4 Encoding GS Using ABR .......................................... 31 5.4 Encoding GS Using ABR .......................................... 31
5.5 Encoding GS Using UBR .......................................... 31 5.5 Encoding GS Using UBR .......................................... 31
5.6 Encoding GS Using UNI 3.0 and UNI 3.1. ......................... 32 5.6 Encoding GS Using UNI 3.0 and UNI 3.1. ......................... 31
6.0 Summary of ATM VC Setup Parameters for Controlled Load Service ..... 33 6.0 Summary of ATM VC Setup Parameters for Controlled Load Service ..... 33
6.1 Encoding CLS Using ABR ......................................... 34 6.1 Encoding CLS Using Non-Real-Time VBR ........................... 33
6.2 Encoding CLS Using Non-Real-Time VBR ........................... 35 6.2 Encoding CLS Using ABR ......................................... 34
6.3 Encoding CLS Using Real-Time VBR ............................... 36 6.3 Encoding CLS Using CBR ......................................... 36
6.4 Encoding CLS Using CBR ......................................... 37 6.4 Encoding CLS Using Real-Time VBR ............................... 36
6.5 Encoding CLS Using UBR ......................................... 37 6.5 Encoding CLS Using UBR ......................................... 36
6.6 Encoding CLS Using UNI 3.0 and UNI 3.1. ........................ 37 6.6 Encoding CLS Using UNI 3.0 and UNI 3.1. ........................ 36
7.0 Summary of ATM VC Setup Parameters for Best Effort Service ......... 38 7.0 Summary of ATM VC Setup Parameters for Best Effort Service ......... 38
7.1 Encoding Best Effort Service Using UBR ......................... 39 7.1 Encoding Best Effort Service Using UBR ......................... 38
8.0 Security ........................................................... 39 8.0 Security Considerations ............................................ 39
9.0 Acknowledgements ................................................... 39 9.0 Acknowledgements ................................................... 40
Appendix 1 Abbreviations .............................................. 40 Appendix 1 Abbreviations .............................................. 40
References ............................................................. 41 References ............................................................. 41
Authors' Addresses ..................................................... 43 Authors' Addresses ..................................................... 43
1.0 Introduction 1.0 Introduction
We consider the problem of providing IP Integrated Services [1] with We consider the problem of providing IP Integrated Services [1] with
an ATM subnetwork. This document is intended to be consistent with an ATM subnetwork. This document is intended to be consistent with
the rsvp protocol [2] for IP-level resource reservation, although it the rsvp protocol [2] for IP-level resource reservation, although it
applies also in the general case where GS and CLS services are applies also in the general case where GS and CLS services are
supported through other mechanisms. In the ATM network, we consider supported through other mechanisms. In the ATM network, we consider
ATM Forum UNI Signaling, versions 3.0, 3.1 and 4.0 [3, 4, 5]. The ATM Forum UNI Signaling, versions 3.0, 3.1 and 4.0 [3, 4, 5]. The
latter uses the more complete service model of the ATM Forum's TM 4.0 latter uses the more complete service model of the ATM Forum's TM 4.0
specification [6, 7]. specification [6, 7].
This is a complex problem. In this document, we focus on the service This is a complex problem with many facets. In this document, we
types, parameters and signalling elements needed for service focus on the service types, parameters and signalling elements needed
interoperation. The resulting service mappings can be used to for service interoperation. The resulting service mappings can be
provide effective end-to-end Quality of Service (QoS) for IP traffic used to provide effective end-to-end Quality of Service (QoS) for IP
that traverses ATM networks. traffic that traverses ATM networks.
The IP services considered are Guaranteed Service (GS) [8] and The IP services considered are Guaranteed Service (GS) [8] and
Controlled Load Service (CLS) [9]. We also discuss the default Best Controlled Load Service (CLS) [9]. We also discuss the default Best
Effort Service (BE) in parallel with these. Our recommendations for Effort Service (BE) in parallel with these. Our recommendations for
BE are intended to be consistent with RFC 1755 [10], and its revision BE are intended to be consistent with RFC 1755 [10], and [11], which
[11], which define how ATM VCs can be used in support of normal BE IP define how ATM VCs can be used in support of normal BE IP service.
service. The ATM services we consider are: The ATM services we consider are:
CBR Constant Bit Rate CBR Constant Bit Rate
rtVBR Real-time Variable Bit Rate rtVBR Real-time Variable Bit Rate
nrtVBR Non-real-time Variable Bit Rate nrtVBR Non-real-time Variable Bit Rate
UBR Unspecified Bit Rate UBR Unspecified Bit Rate
ABR Available Bit Rate ABR Available Bit Rate
In the case of UNI 3.x signalling, where these service are not all In the case of UNI 3.x signalling, where these service are not all
clearly distinguishable, we identify the appropriate available clearly distinguishable, we identify the appropriate available
services. services.
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consists of one or more ATM switches. It uses SVCs to provide both consists of one or more ATM switches. It uses SVCs to provide both
resources and QoS within the ATM cloud. These connections are set resources and QoS within the ATM cloud. These connections are set
up, added to (in the case of multipoint trees), torn down, and up, added to (in the case of multipoint trees), torn down, and
controlled by the edge devices, which act as both IP routers and ATM controlled by the edge devices, which act as both IP routers and ATM
interfaces, capable of initiating and managing VCs across the ATM interfaces, capable of initiating and managing VCs across the ATM
user-to-network (UNI) interface. The edge devices are assumed to be user-to-network (UNI) interface. The edge devices are assumed to be
fully functional in both the IP int-serv/RSVP protocols and the ATM fully functional in both the IP int-serv/RSVP protocols and the ATM
UNI protocols, as well as translating between them. UNI protocols, as well as translating between them.
ATM Cloud ATM Cloud
------------------ -----------------
H ----\ ( ) /------- H H ----\ ( ) /------- H
H ---- R -- R -- E --( ATM Sw -- ATM Sw ) -- E -- R -- R -- H H ---- R -- R -- E-( X -- X -- X )-E -- R -- R -- H
H ----/ | ( ) \ H ----/ | ( ) \
| ------------------ \------ H | ----------------- \------ H
H ----------R H ----------R
Figure 1: Network Architecture with Hosts (H), Figure 1: Network Architecture with Hosts (H),
Routers (R) and Edge Devices (E). Routers (R), Edge Devices (E) and ATM
Switches (X).
When considering the edge devices with respect to traffic flowing When considering the edge devices with respect to traffic flowing
from source to destination, the upstream edge device is called the from source to destination, the upstream edge device is called the
"ingress" point and the downstream device the "egress" point. The "ingress" point and the downstream device the "egress" point. The
edge devices may be considered part of the IP internet or part of the edge devices may be considered part of the IP internet or part of the
ATM cloud, or both. They process RSVP messages, reserve resources, ATM cloud, or both. They process RSVP messages, reserve resources,
and maintain soft state (in the control path), and classify and and maintain soft state (in the control path), and classify and
schedule packets (in the data path). They also initiate ATM schedule packets (in the data path). They also initiate ATM
connections by signalling, and accept or refuse connections signalled connections by signalling, and accept or refuse connections signalled
to them. They police and schedule cells going into the ATM cloud. to them. They police and schedule cells going into the ATM cloud.
The service mapping function occurs when an IP-level reservation The service mapping function occurs when an IP-level reservation
(RESV message) triggers the edge device to translate the RSVP service (RESV message) triggers the edge device to translate the RSVP service
requirements into ATM VC (UNI) semantics. requirements into ATM VC (UNI) semantics.
A range of VC management policies are possible, which determine A range of VC management policies are possible, which determine
whether a flow should initiate a new VC or join an existing one. VCs whether a flow initiates a new VC or joins an existing one. VCs are
are managed according to a combination of standards and local policy managed according to a combination of standards and local policy
rules, which are specific to either the implementation (equipment) or rules, which are specific to either the implementation (equipment) or
the operator (network service provider). Point-to-multipoint the operator (network service provider). Point-to-multipoint
connections within the ATM cloud can be used to support general IP connections within the ATM cloud can be used to support general IP
multicast flows. In ATM, a point to multipoint connection can be multicast flows. In ATM, a point to multipoint connection can be
controlled by the source (or root) node, or a leaf initiated join controlled by the source (or root) node, or a leaf initiated join
(LIJ) feature in ATM may be used. The topic of VC management is (LIJ) feature in ATM may be used. The topic of VC management is
considered at length in other ISSLL working group drafts [12,13,14]. considered at length in other ISSLL documents [12,13,14].
Figure 2 shows the functions of an edge device, summarizing the work Figure 2 shows the functions of an edge device, summarizing the work
not part of IP or ATM abstractly as an InterWorking Function (IWF), not part of IP or ATM abstractly as an InterWorking Function (IWF),
and segregating the control and data planes. and segregating the control and data planes.
IP ATM IP ATM
____________________ ____________________
| IWF | | IWF |
| | | |
admission and <--> | service mapping | <--> ATM admission and <--> | service mapping | <--> ATM
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| | | |
classification, |ATM Adaptation Layer| cell classification, |ATM Adaptation Layer| cell
policing & <--> | Segmentation and | <--> scheduling/ policing & <--> | Segmentation and | <--> scheduling/
scheduling | Reassembly | shaping scheduling | Reassembly | shaping
| Buffering | | Buffering |
____________________ ____________________
Figure 2: Edge Device Functions showing the IWF Figure 2: Edge Device Functions showing the IWF
In the logical view of Figure 2, some functions, such as scheduling, In the logical view of Figure 2, some functions, such as scheduling,
are shown separately, since these functions are required of both the are shown separately, since these functions are present on both the
IP and ATM sides. However it may be possible in an integrated IP and ATM sides. However it may be possible in an integrated
implementation to combine such functions. implementation to combine such functions.
The service mapping and VC management functions can be highly The service mapping and VC management functions can be highly
interdependent. For example: (i) Multiple integrated-services flows interdependent. For example: (i) Multiple integrated-services flows
may be aggregated to use one point-to-multipoint VC. In this case, may be aggregated to use one point-to-multipoint VC. In this case,
we assume the IP flows are of the same service type and their we assume the IP flows are of the same service type and their
parameters have been merged appropriately. (ii) The VC management parameters have been merged appropriately. (ii) The VC management
function may choose to allocate extra resources in anticipation of function may choose to allocate extra resources in anticipation of
further reservations or based on an empiric of changing TSpecs. further reservations or based on an empiric of changing TSpecs.
(iii) There must exist a path for best effort flows and for sending (iii) There MUST exist a path for best effort flows and for sending
the rsvp control messages. How this interacts with the establishment the rsvp control messages. How this interacts with the establishment
of VCs for QoS traffic may alter the characteristics required of of VCs for QoS traffic may alter the desired characteristics of those
those VCs. See [12,13,14] for further details on VC management. VCs. See [12,13,14] for further details on VC management.
Therefore, in discussing the service mapping problem, we will assume Therefore, in discussing the service mapping problem, we will assume
that the VC management function of the IWF can always express its that the VC management function of the IWF can always express its
result in terms of an IP-level service with some QoS and TSpec. The result in terms of an IP-level service with some QoS and TSpec. The
service mapping algorithm can then identify the appropriate VC service mapping algorithm can then identify the appropriate VC
parameters as if a new VC were to be created for this service. The parameters as if a new VC were to be created for this service. The
VC management function can then use this information consistent with VC management function can then use this information consistent with
its own policy, which determines whether the resulting action uses its own policy, which determines whether the resulting action uses
new or existing VCs. new or existing VCs.
1.2 Related Documents 1.2 Related Documents
Earlier ATM Forum documents combined signaling, traffic management Earlier ATM Forum documents combined signalling, traffic management
and other areas into a single document, e.g., UNI 3.0 [3] and UNI 3.1 and other areas into a single document, e.g., UNI 3.0 [3] and UNI 3.1
[4]. The 3.1 release was used to correct errors and fix alignment [4]. The 3.1 release was used to correct errors and fix alignment
with the ITU. While UNI 3.0 and 3.1 are incompatible in terms of with the ITU. While UNI 3.0 and 3.1 are incompatible in terms of
actual codepoints, the semantics are generally the same. Therefore, actual codepoints, the semantics are generally the same. Therefore,
we will often refer to UNI 3.x to mean either version of the ATM we will often refer to UNI 3.x to mean either version of the ATM
protocol. protocol.
After 3.1, the ATM Forum released documents separately for each After 3.1, the ATM Forum released documents separately for each
technical working group. The UNI Signalling 4.0 [5] and Traffic technical working group. The UNI Signalling 4.0 [5] and Traffic
Management 4.0 [6] documents represent a consistent overall ATM Management 4.0 [6] documents represent a consistent overall ATM
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ATM. These do not appear explicitly as set-up parameters in the ATM. These do not appear explicitly as set-up parameters in the
above list, since they are implied by the policing method used. above list, since they are implied by the policing method used.
2.1 Service Category and Bearer Capability 2.1 Service Category and Bearer Capability
The highest level of abstraction distinguishing features of ATM VCs The highest level of abstraction distinguishing features of ATM VCs
is in the service category or bearer capability. Service categories is in the service category or bearer capability. Service categories
were introduced in TM/UNI 4.0; previously the bearer capability was were introduced in TM/UNI 4.0; previously the bearer capability was
used to discriminate at this level. used to discriminate at this level.
These elements indicate the general properties required of a VC: These elements indicate the general properties of a VC: whether there
whether there is a real-time delay constraint, whether the traffic is is a real-time delay constraint, whether the traffic is constant or
constant or variable rate, the applicable traffic and QoS description variable rate, the applicable traffic and QoS description parameters
parameters and (implicitly) the complexity of some supporting switch and (implicitly) the complexity of some supporting switch mechanisms
mechanisms (e.g., ABR). (e.g., ABR).
For UNI 3.0 and UNI 3.1, there are only two distinct options for For UNI 3.0 and UNI 3.1, there are only two distinct options for
bearer capabilities (in our context): bearer capabilities (in our context):
BCOB-A: constant rate, timing required, unicast/multipoint; BCOB-A: constant rate, timing required, unicast/multipoint;
BCOB-C: variable rate, timing not required, unicast/multipoint. BCOB-C: variable rate, timing not required, unicast/multipoint.
A third capability, BCOB-X, can be used as a substitute for the above A third capability, BCOB-X, can be used as a substitute for the above
two capabilities, with its dependent parameters (traffic type and two capabilities, with its dependent parameters (traffic type and
timing requirement) set appropriately. The distinction between the timing requirement) set appropriately. The distinction between the
BCOB-X formulation and the "equivalent" (for our purposes) BCOB-A and BCOB-X formulation and the "equivalent" (for our purposes) BCOB-A and
BCOB-C constructs is whether the ATM network is to provide pure cell BCOB-C constructs is whether the ATM network is to provide pure cell
relay service or interwork with other technologies (with relay service or interwork with other technologies (with
interoperable signalling), such as narrowband ISDN. Where this interoperable signalling), such as narrowband ISDN. Where this
distinction is applicable, the appropriate code should be used (see distinction is applicable, the appropriate code SHOULD be used (see
[5] and related ITU specs, e.g., I.371). [5] and related ITU specs, e.g., I.371).
In TM/UNI 4.0 the service categories are: In TM/UNI 4.0 the service categories are:
Constant Bit Rate (CBR) Constant Bit Rate (CBR)
Real-time Variable Bit Rate (rtVBR) Real-time Variable Bit Rate (rtVBR)
Non-real-time Variable Bit Rate (nrtVBR) Non-real-time Variable Bit Rate (nrtVBR)
Unspecified Bit Rate (UBR) Unspecified Bit Rate (UBR)
Available Bit Rate (ABR) Available Bit Rate (ABR)
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significantly more efficient to use the other ATM services where significantly more efficient to use the other ATM services where
applicable and available [17]. applicable and available [17].
2.1.1 Service Categories for Guaranteed Service 2.1.1 Service Categories for Guaranteed Service
There are two possible mappings for GS: There are two possible mappings for GS:
CBR (BCOB-A) CBR (BCOB-A)
rtVBR rtVBR
GS requires real-time support. Thus in UNI 3.x, the bearer class Real-time support is REQUIRED for GS. Thus in UNI 3.x, the Bearer
BCOB-A (or an equivalent BCOB-X formulation) must be used. In TM/UNI Class BCOB-A (or an equivalent BCOB-X formulation) MUST be used. In
4.0 either CBR or rtVBR is appropriate. The use of rtVBR may TM/UNI 4.0 either CBR or rtVBR is appropriate. The use of rtVBR may
encourage recovery of allocated bandwidth left unused by a source. encourage recovery of allocated bandwidth left unused by a source.
It also accommodates more bursty sources with a larger token bucket It also accommodates more bursty sources with a larger token bucket
burst parameter, and permits the use of tagging for excess traffic burst parameter, and permits the use of tagging for excess traffic
(see Section 2.2). (see Section 2.2).
Neither the BCOB-C bearer class, nor nrtVBR, UBR, ABR are good Neither the BCOB-C Bearer Class, nor nrtVBR, UBR, ABR are good
matches for the GS service. These provide no delay estimates and matches for the GS service. These provide no delay estimates and
cannot guarantee consistently low delay for every packet. cannot guarantee consistently low delay for every packet.
Specification of BCOB-A or CBR requires specification of a peak cell Specification of BCOB-A or CBR REQUIRES specification of a peak cell
rate (PCR). In these cases, PCR is the nominal clearing rate with a rate (PCR). In these cases, PCR is the nominal clearing rate with a
nominal jitter toleration (bucket size), CDVT. nominal jitter toleration (bucket size), CDVT.
Specification of rtVBR requires two rates, PCR and SCR. This models Specification of rtVBR REQUIRES two rates, PCR and SCR. This models
bursty traffic with specified peak and sustainable rates. The bursty traffic with specified peak and sustainable rates. The
corresponding ATM token bucket depth values are CDVT, and CDVT+BT, corresponding ATM token bucket depth values are CDVT, and CDVT+BT,
respectively. respectively.
2.1.2 Service Categories for Controlled Load 2.1.2 Service Categories for Controlled Load
There are three possible good mappings for CLS: There are three possible good mappings for CLS:
CBR (BCOB-A) CBR (BCOB-A)
ABR
nrtVBR (BCOB-C) nrtVBR (BCOB-C)
ABR
Note that under UNI 3.x, there are equivalent services to CBR and Note that under UNI 3.x, there are equivalent services to CBR and
nrtVBR, but not ABR. The first, with a CBR/BCOB-A connection, nrtVBR, but not ABR. The first, with a CBR/BCOB-A connection,
provides a higher level of QoS than is necessary, but it may be provides a higher level of QoS than is necessary, but it may be
convenient to simply allocate a fixed-rate "pipe", which we expect to convenient to simply allocate a fixed-rate "pipe", which we expect to
be ubiquitously supported in ATM networks. However unless this is be ubiquitously supported in ATM networks. However unless this is
the only choice available, it would probably be wasteful of network the only choice available, it would probably be wasteful of network
resources. resources.
The nrtVBR/BCOB-C category is perhaps the best match, since it The nrtVBR/BCOB-C category is perhaps the best match, since it
provides for allocation of bandwidth and buffers with an additional provides for allocation of bandwidth and buffers with an additional
peak rate indication, similar to the CLS TSpec. peak rate indication, similar to the CLS TSpec. Excess traffic can
be handled by CLP bit tagging with VBR.
The ABR category with a positive MCR aligns with the CLS idea of The ABR category with a positive MCR aligns with the CLS idea of
"best effort with a floor." The ATM network agrees to forward cells "best effort with a floor." The ATM network agrees to forward cells
with a rate of at least MCR, which should be directly converted from with a rate of at least MCR, which MUST be directly converted from
the token bucket rate of the receiver TSpec. The bucket size the token bucket rate of the receiver TSpec. The bucket size
parameter measures approximately the amount of buffer required at the parameter measures approximately the amount of buffer necessary at
IWF. This buffer serves to absorb the bursts allowed by the token the IWF. This buffer serves to absorb the bursts allowed by the
bucket, since they cannot be passed directly into an ABR VC. token bucket, since they cannot be passed directly into an ABR VC.
The rtVBR category can be used, although the edge device must The rtVBR category can be used, although the edge device MUST then
determine values for CTD and CDV. Since there are no corresponding determine values for CTD and CDV. Since there are no corresponding
IP-level parameters, their values are set as a matter of local IP-level parameters, their values are set as a matter of local
policy. policy.
The UBR category does not provide enough capability for Controlled The UBR category does not provide enough capability for Controlled
Load. The point of CLS is to allow an allocation of resources. This Load. The point of CLS is to allow an allocation of resources. This
is facilitated by the token bucket traffic descriptor, which is is facilitated by the token bucket traffic descriptor, which is
unavailable with UBR. unavailable with UBR.
2.1.3 Service Categories for Best Effort 2.1.3 Service Categories for Best Effort
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a nrtVBR connection could also be used. In these cases the rate a nrtVBR connection could also be used. In these cases the rate
parameter used reflects a bandwidth allocation in support of the parameter used reflects a bandwidth allocation in support of the
ingress edge device's best effort connectivity to the egress edge ingress edge device's best effort connectivity to the egress edge
router. It would be normal for traffic from many source/destination router. It would be normal for traffic from many source/destination
pairs to be aggregated on this connection; indeed, since Best Effort pairs to be aggregated on this connection; indeed, since Best Effort
is the default IP behavior, the individual flows are not normally is the default IP behavior, the individual flows are not normally
identified or accounted for. CBR may be a preferred solution in the identified or accounted for. CBR may be a preferred solution in the
case where best effort traffic is sufficiently highly aggregated that case where best effort traffic is sufficiently highly aggregated that
a simple fixed-rate pipe is efficient. Both CBR and nrt-VBR provide a simple fixed-rate pipe is efficient. Both CBR and nrt-VBR provide
explicit bandwidth allocation which may be useful for billing explicit bandwidth allocation which may be useful for billing
purposes. purposes. In the case of UBR, the network operator SHOULD allocate
bandwidth for the overall service through the admission control
function, although such allocation is not done explicitly per VC.
An ABR connection could similarly be used to support Best Effort An ABR connection could similarly be used to support Best Effort
traffic. Indeed, the support of data communications protocols such traffic. Indeed, the support of data communications protocols such
as TCP/IP is the explicit purpose for which ABR was designed. It is as TCP/IP is the explicit purpose for which ABR was designed. It is
conceivable that a separate ABR connection would be made for each IP conceivable that a separate ABR connection would be made for each IP
flow, although the normal case would probably have all IP Best Effort flow, although the normal case would probably have all IP Best Effort
traffic with a common egress router sharing a single ABR connection. traffic with a common egress router sharing a single ABR connection.
The rt-VBR service category may be considered less suitable, simply The rt-VBR service category may be considered less suitable, simply
because both the real-time delay constraint and the use of SCR/BT add because both the real-time delay constraint and the use of SCR/BT add
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with CLP=1 are said to be "tagged" or "marked" and have lower with CLP=1 are said to be "tagged" or "marked" and have lower
priority. This tagging may be done by the source, to indicate priority. This tagging may be done by the source, to indicate
relative priority within the VC, or by a switch, to indicate traffic relative priority within the VC, or by a switch, to indicate traffic
in violation of policing parameters. Options involving the use of in violation of policing parameters. Options involving the use of
tagging are decided at call setup time. tagging are decided at call setup time.
A Conformance Definition is a rule that determines whether a cell is A Conformance Definition is a rule that determines whether a cell is
conforming to the traffic descriptor of the VC. The conformance conforming to the traffic descriptor of the VC. The conformance
definition is given in terms of a Generic Cell Rate Algorithm (GCRA), definition is given in terms of a Generic Cell Rate Algorithm (GCRA),
also known as a "leaky bucket" algorithm, for CBR and VBR services. also known as a "leaky bucket" algorithm, for CBR and VBR services.
(UBR and ABR have network implementation-specific conformance The conformance definition also specifies rules for tagging traffic
definitions. Note, the term "compliance" in ATM is used to describe in excess of the {SCR, MBS} GCRA traffic descriptor. (Note, the term
the behavior of a connection, as opposed to "conformance", which "compliance" in ATM is used to describe the behavior of a connection,
applies to a single cell.) as opposed to "conformance", which applies to a single cell.)
The network may tag cells that are non-conforming, rather than The network may tag cells that are non-conforming, rather than
dropping them if the VC set-up requests tagging and the network dropping them if the VC set-up requests tagging and the network
supports the tagging option. When congestion occurs, a switch must supports the tagging option. When tagging is used and congestion
attempt to discard tagged cells in preference to discarding CLP=0 occurs, a switch MUST attempt to discard tagged cells in preference
cells. However, the mechanism for doing this is completely to discarding CLP=0 cells. However, the mechanism for doing this is
implementation specific. The behavior that best meets the completely implementation specific. The behavior that best meets the
requirements of IP Integrated Services is where tagged cells are requirements of IP Integrated Services is where tagged cells are
treated as "best effort" in the sense that they are transported when treated as "best effort" in the sense that they are transported when
bandwidth is available, queued when buffers are available, and bandwidth is available, queued when buffers are available, and
dropped when resources are overcommitted. ATM standards, however, do dropped when resources are overcommitted. ATM standards, however, do
not explicitly specify treatment of tagged traffic. Providers of GS not explicitly specify treatment of tagged traffic. Providers of GS
and CLS service with ATM subnetworks should ascertain the actual and CLS service with ATM subnetworks SHOULD ascertain the actual
behavior of ATM implementation with respect to tagged cells. behavior of ATM implementation with respect to tagged cells.
Since GS and CLS services require excess traffic to be treated as Since GS and CLS services REQUIRE excess traffic to be treated as
best effort, the tagging option should always be chosen (if best effort, the tagging option SHOULD always be chosen (if
supported) in the VC setup as a means of "downgrading" the cells supported) in the VC setup as a means of "downgrading" the cells
comprising non-conformant packets. The term "best effort" can be comprising non-conformant packets. The term "best effort" can be
interpreted in two ways. The first is as a service class that, for interpreted in two ways. The first is as a service class that, for
example, may be implemented as a separate queue. The other sense is example, may be implemented as a separate queue. The other sense is
more generic, meaning that the network makes a best effort to more generic, meaning that the network makes a best effort to
transport the traffic. A reasonable interpretation of this is that a transport the traffic. A reasonable interpretation of this is that a
network with no contending traffic would transport the packet, while network with no contending traffic would transport the packet, while
a very congested network would drop the packet. A mechanism that a very congested network would drop the packet. A mechanism that
tags best effort packets with lower loss priority (such as with the tags best effort packets with lower loss priority (such as with the
ATM CLP bit) would drop some of these packets, but would not reorder ATM CLP bit) would drop some of these packets, but would not reorder
the remaining ones with respect to the conforming portion of the the remaining ones with respect to the conforming portion of the
flow. The "best effort" mechanism for excess traffic does not flow. The "best effort" mechanism for excess traffic does not
necessarily have to be the same as that for best effort "service", as necessarily have to be the same as that for best effort "service", as
long as it fits this generic sense of best effort. long as it fits this generic sense of best effort.
There are three conformance definitions of VBR service (for both There are three conformance definitions of VBR service (for both
rtVBR and nrtVBR) to consider. In VBR, only the conformance rtVBR and nrtVBR) to consider. In VBR, only the conformance
definition VBR.3 supports tagging and applies the GCRA with rate PCR definition VBR.3 supports tagging and applies the GCRA with rate PCR
to the aggregate CLP=0+1 cells, and another GCRA with rate SCR to the to the aggregate CLP=0+1 cells, and another GCRA with rate SCR to the
CLP=0 cells. This conformance definition should always be used with CLP=0 cells. This conformance definition SHOULD always be used with
a VBR service supporting IP integrated services. For UBR service, a VBR service supporting IP integrated services. For UBR service,
conformance definition UBR.2 supports the use of tagging, but a CLP=1 conformance definition UBR.2 supports the use of tagging, but a CLP=1
cell does not imply non-conformance; rather, it may be used to cell does not imply non-conformance; rather, it may be used by the
indicate network congestion. network to indicate congestion.
In TM/UNI 4.0 tagging does not apply to the CBR or ABR services. In TM/UNI 4.0 tagging is not a feature of the conformance definitions
More precisely, the conformance definitions listed in TM 4.0 for CBR for the CBR or ABR service categories. (Since conformance
and ABR do not use tagging. Since conformance definitions are definitions are generally network specific, some implementations CBR
network specific, it may be possible that implementations of CBR or or ABR may, in fact, use tagging in some way.) Wherever an ATM
ABR with tagging can exist. Wherever an ATM network does support network does support tagging, in the sense of transporting CLP=1
tagging, in the sense of transporting CLP=1 cells on a "best effort" cells on a "best effort" basis, it is a useful and preferable
basis, it is a useful and preferable mechanism for handling excess mechanism for handling excess traffic.
traffic.
It is always better for the IWF to tag cells when it can anticipate It is always better for the IWF to tag cells when it can anticipate
that the ATM network would do so. This is because the IWF knows the that the ATM network would do so. This is because the IWF knows the
IP packet boundaries and can tag all of the cells corresponding to a IP packet boundaries and can tag all of the cells corresponding to a
packet. If left to the ATM layer UPC, the network would inevitably packet. If left to the ATM layer UPC, the network would inevitably
drop some of the cells of a packet while carrying others, which would drop some of the cells of a packet while carrying others, which would
then be dropped by the receiver. Therefore, the IWF, knowing the VC then be dropped by the receiver. Therefore, the IWF, knowing the VC
GCRA parameters, should always anticipate the cells which will be GCRA parameters, SHOULD always anticipate the cells which will be
tagged by the ATM UPC and tag all of the cells uniformly across each tagged by the ATM UPC and tag all of the cells uniformly across each
affected packet. affected packet. See Section 3.2 for further discussion of excess
traffic.
2.3 ATM Adaptation Layer 2.3 ATM Adaptation Layer
The AAL type 5 encoding must be used, as specified in RFC 1483 and The AAL type 5 encoding SHOULD be used, as specified in RFC 1483 and
RFC 1755. AAL5 requires specification of the maximum SDU size in both RFC 1755. AAL-5 REQUIRES specification of the maximum SDU size in
the forward and reverse directions. Both GS and CLS specify a maximum both the forward and reverse directions. Both GS and CLS specify a
packet size as part of the TSpec and this value shall be used as the maximum packet size, M, as part of the TSpec and this value SHOULD be
maximum SDU in each direction for unicast connections, and for used (corrected for AAL headers) as the maximum SDU in each direction
unidirectional point-to-multipoint connections. When multiple flows for unicast connections, and for unidirectional point-to-multipoint
are aggregated into a single VC, the M parameters of the receiver connections. When multiple flows are aggregated into a single VC,
TSpecs are merged according to rules given in the GS and CLS specs. the M parameters of the receiver TSpecs are merged according to rules
given in the GS and CLS specs.
2.4 Broadband Low Layer Information 2.4 Broadband Low Layer Information
The B-LLI Information Element is transferred transparently by the ATM The B-LLI Information Element is transferred transparently by the ATM
network between the edge devices and is used to specify the network between the edge devices and is used to specify the
encapsulation method. Multiple B-LLI IEs may be sent as part of encapsulation method. Multiple B-LLI IEs may be sent as part of
negotiation. The default encapsulation LLC/SNAP [18] must be negotiation. The LLC/SNAP encapsulation [18] SHOULD be supported as
supported as specified in RFC 1577 [19] and RFC 1755 [10]. See RFC the default, but "null" or "VC encapsulation" MAY also be allowed.
1755 for information on additional encapsulations. Implementations SHOULD follow RFC 1577 [19] and RFC 1755 [10] for
BLLI usage.
2.5 Traffic Descriptors 2.5 Traffic Descriptors
The ATM traffic descriptor always contains a peak cell rate (PCR) The ATM traffic descriptor always contains a peak cell rate (PCR)
(for each direction). For variable rate services it also contains a (for each direction). For VBR services it also contains a
sustainable cell rate (SCR) and maximum burst size (MBS). The SCR sustainable cell rate (SCR) and maximum burst size (MBS). The SCR
and MBS form a leaky bucket pair (rate, depth), while the bucket and MBS form a leaky bucket pair (rate, depth), while the bucket
depth parameter for PCR is CDVT. Note that CDVT is not signalled depth parameter for PCR is CDVT. Note that CDVT is not signalled
explicitly, but is determined by the network operator, and serves as explicitly, but is determined by the network operator, and can be
a measure of the jitter imposed by the network. viewed as a measure of the jitter imposed by the network.
Since CDVT is generally presumed to be small (equivalent to a few Since CDVT is generally presumed to be small (equivalent to a few
cells of token bucket depth), and cannot be set independently for cells of token bucket depth), and cannot be set independently for
each connection, it cannot be used to account for the burstiness each connection, it cannot be used to account for the burstiness
permitted by b of the IP-layer TSpec. Additional buffering is needed permitted by b of the IP-layer TSpec. Additional buffering may be
at the IWF to account for the depth of the token bucket. needed at the IWF to account for the depth of the token bucket.
The ATM Burst Tolerance (BT) is equivalent to MBS (see TM 4.0 [6] for The ATM Burst Tolerance (BT) is equivalent to MBS (see TM 4.0 [6] for
the exact equation). They are both expressions of the bucket depth the exact equation). They are both expressions of the bucket depth
parameter that goes with SCR. The units of BT is time while the parameter associated with SCR. The units of BT are time while the
units of MBS is cells. Since both SCR and MBS are signalled, they units of MBS are cells. Since both SCR and MBS are signalled, they
can be computed directly from the IP layer traffic description. The can be computed directly from the IP layer traffic description. The
specific manner in which resources are allocated from the traffic specific manner in which resources are allocated from the traffic
description is implementation specific. Note that when translating description is implementation specific. Note that when translating
the traffic parameters, the segmentation overhead and minimum policed the traffic parameters, the segmentation overhead and minimum policed
unit need to be taken into account (see Section 4.1 below). unit need to be taken into account (see Section 4.1 below).
In ATM UNI Signalling 4.0 there are the notions of Alternative In ATM UNI Signalling 4.0 there are the notions of Alternative
Traffic Descriptors and Minimal Traffic Descriptors. Alternative Traffic Descriptors and Minimal Traffic Descriptors. Alternative
Traffic Descriptors enumerate other acceptable choices for traffic Traffic Descriptors enumerate other acceptable choices for traffic
descriptors and are not considered here. Minimal Traffic Descriptors descriptors and are not considered here. Minimal Traffic Descriptors
are used in "negotiation," which refers to the specific way in which are used in "negotiation," which refers to the specific way in which
an ATM connection is set up. To illustrate, roughly, taking PCR as an ATM connection is set up. To illustrate, roughly, taking PCR as
an example: A minimal PCR and a requested PCR are signalled, the an example: A minimal PCR and a requested PCR are signalled, the
requested PCR being the usual item signalled, and the minimal PCR requested PCR being the usual item signalled, and the minimal PCR
being the absolute minimum that the source edge device will accept. being the absolute minimum that the source edge device will accept.
When sensing the existence of both minimal and requested parameters, When both minimal and requested parameters are present, the
the intermediate switches along the path may reduce the requested PCR intermediate switches along the path may reduce the requested PCR to
to a "comfortable" level. This choice is part of admission control, a "comfortable" level. This choice is part of admission control, and
and is therefore implementation dependent. If at any point the is therefore implementation specific. If at any point the requested
requested PCR falls below the minimal PCR then the call is cleared. PCR falls below the minimal PCR then the call is cleared. Minimal
Minimal Traffic Descriptors can be used to present an acceptable Traffic Descriptors can be used to present an acceptable range for
range for parameters and ensure a higher likelihood of call parameters and ensure a higher likelihood of call admission. In
admission. In general, our discussion of connection parameters general, our discussion of connection parameters assumes the values
assumes the values resulting from successful connection setup. resulting from successful connection setup.
The Best Effort indicator (used only with UBR) and Tagging indicators The Best Effort indicator (used only with UBR) and Tagging indicators
are also part of the signalled information element (IE) containing (see Section 2.2) are also part of the signalled information element
the traffic descriptor. In the UNI 4.0 traffic descriptor IE there (IE) containing the traffic descriptor. In the UNI 4.0 traffic
is an additional parameter, the Frame Discard indicator, which is descriptor IE there is an additional parameter, the Frame Discard
discussed below in Section 2.7. indicator, which is discussed below in Section 2.7.
2.5.1 Translating Traffic Descriptors for Guaranteed Service 2.5.1 Translating Traffic Descriptors for Guaranteed Service
For Guaranteed Service the source TSpec contains peak rate, rate and For Guaranteed Service the source TSpec contains peak rate, rate and
and bucket depth parameters, p_s, r_s, b_s. The receiver TSpec and bucket depth parameters, p_s, r_s, b_s. The receiver TSpec
contains corresponding parameters p_r, r_r, b_r. The (receiver) contains corresponding parameters p_r, r_r, b_r. The (receiver)
RSpec also has a rate, R. The two different TSpec rates are intended RSpec also has a rate, R. The two different TSpec rates are intended
to support receiver heterogeneity, in the sense that receivers can to support receiver heterogeneity, in the sense that receivers can
accept different rates representing different subsets of the sender's accept different rates representing different subsets of the sender's
traffic. Whenever rates from different receivers differ, the values traffic. Whenever rates from different receivers differ, the values
will always be merged appropriately before being mapping into ATM MUST always be merged appropriately before being mapping into ATM
parameters. parameters.
Note that when the sender and receiver TSpec rates r_s, r_r differ, Note that when the sender and receiver TSpec rates r_s, r_r differ,
there is no mechanism specified (in either rsvp or the int-serv there is no mechanism specified (in either rsvp or the int-serv
specs) for indicating which subset of the traffic is to be specs) for indicating which subset of the traffic is to be
transported. Implementation of this feature is therefore completely transported. Implementation of this feature is therefore completely
network specific. Hence the ambiguity in how policing and scheduling network specific. The policing and scheduling mechanisms may simply
use the two rates is an inherent and currently unresolved issue in be parameterized with the (lower) receiver rate, resulting in the
IP-IS technology. random loss of traffic sufficient to make up the difference in rates.
The receiver TSpec rate describes the traffic for which resources are The receiver TSpec rate describes the traffic for which resources are
to be reserved, and may be used for policing, while the RSpec rate to be reserved, and may be used for policing, while the RSpec rate
(which cannot be smaller) is the allocated service bandwidth (or (which cannot be smaller) is used (perhaps in an implementation
strictly speaking, a lower bound on this). A receiver increases R specific way) to modify the allocated service bandwidth in order to
over r_r to reduce the delay. reduce the delay.
When mapping Guaranteed Service onto a rtVBR VC, the ATM traffic When mapping Guaranteed Service onto a rtVBR VC, the ATM traffic
descriptor parameters (PCR, SCR, MBS) can often be set cannonically descriptor parameters (PCR, SCR, MBS) can be set cannonically as:
as:
PCR = p_r PCR = p_r
SCR = R SCR = R
MBS = b_r. MBS = b_r.
There are a number of conditions that may lead to different choices. There are a number of conditions that may lead to different choices.
The following discussion is not intended so much to set hard The following discussion is not intended to set hard requirements,
requirements, but to provide some interpretation and guidance on the but to provide some interpretation and guidance on the bounds of
bounds of possible parameter mappings. The ingress edge device possible parameter mappings. The ingress edge device generally
generally includes a buffer preceding the ATM network interface. includes a buffer preceding the ATM network interface. This buffer
This buffer can be used to absorb bursts that fall within the IP- can be used to absorb bursts that fall within the IP-level TSpec, but
level TSpec, but not within the ATM traffic descriptor. The minimal not within the ATM traffic descriptor. The minimal REQUIREMENT for
requirement for guaranteed service is that the delay in this buffer guaranteed service is that the delay in this buffer MUST NOT exceed
may not exceed b/R, and the delays within the ATM network must be b/R, and the delays within the ATM network MUST be accurately
accurately accounted for in the values of Adspec parameters C and D accounted for in the values of Adspec parameters C and D advertised
advertised by the ingress router (see Section 3.3 below). by the ingress router (see Section 3.3 below).
In general, if either an edge device buffer of size b_r exists or the If either an edge device buffer of size b_r exists or the ATM maximum
ATM maximum burst size (MBS) parameter is at least b_r, then the burst size (MBS) parameter is at least b_r, then the various rate
various rate parameters will generally exhibit the following parameters will generally exhibit the following relationship:
relationship:
r_r <= SCR <= R <= PCR <= APB <= line rate r_r <= SCR <= R <= PCR <= APB <= line rate
r_r <= p_r <= APB r_r <= p_r <= APB
APB refers to the General Characterization Parameter, APB refers to the General Characterization Parameter,
AVAILABLE_PATH_BANDWIDTH, which is negotiated in the Adspec portion AVAILABLE_PATH_BANDWIDTH, which is negotiated in the Adspec portion
of the PATH message. APB reflects the narrowest bottleneck rate of the PATH message. APB reflects the narrowest bottleneck rate
along the path, and so is always bounded by the local line rate. The along the path, and so is always no larger than the local line rate.
receiver must choose a peak rate no greater than APB for the The receiver SHOULD choose a peak rate no greater than APB for the
reservation to be accepted, although the source peak rate, p_s, could reservation to be accepted, although the source peak rate, p_s, could
be higher, as the source does not know the value of APB. There is no be higher, as the source does not know the value of APB. There is no
advantage to allocating any rate above APB of course, so it is an advantage to allocating any rate above APB of course, so it is an
upper bound for all the other parameters. upper bound for all the other parameters.
We might normally expect to find R <= p_r, as would be necessary for We might normally expect to find R <= p_r, as would be necessary for
the simple mapping of PCR = p_r, SCR = R given above. However, a the simple mapping of PCR = p_r, SCR = R given above. However, a
receiver is free to choose R > p_r to lower the GS delay [8]. In receiver is free to choose R > p_r to lower the GS delay [8]. In
this case, PCR cannot be set below R, because a burst of size b this case, PCR cannot be set below R, because a burst of size b
arriving into the buffer must be cleared at rate R to keep the first arriving into the buffer MUST be cleared at rate R to keep the first
component of GS delay down to b/R. So here we will have PCR = R. component of GS delay down to b/R. So here we will have PCR = R. It
may seem that PCR = p_r would be sufficient to avoid buffer overflow,
since data is generated at the source at a rate bounded by p_r.
However, setting PCR < R, can result in the delay bound advertised by
C and D not being met. Also, traffic is always subject to jitter in
the network, and the arrival rate at a network element can exceed p_r
for short periods of time.
In the case R <= p_r, we may still choose R <= PCR < p_r. The edge In the case R <= p_r, we may still choose PCR such that R <= PCR <
device buffer is then necessary (and sufficient) to absorb the bursts p_r. The edge device buffer is then necessary (and sufficient) to
(limited to size b_r) which arrive faster than they depart. For absorb the bursts (limited to size b_r + C_sum + R D_sum) which
example, it may be the case that the cost of the ATM VC depends on arrive faster than they depart. For example, it may be the case that
PCR, while the cost of the Internet service reservation is not the cost of the ATM VC depends on PCR, while the cost of the Internet
strongly dependent on the IP-level peak rate. The user may the have service reservation is not strongly dependent on the IP-level peak
an incentive to set p_r to APB, while the operator of the IP/ATM edge rate. The user may then have an incentive to set p_r to APB, while
router has an incentive to reduce PCR as much as possible. This may the operator of the IP/ATM edge router has an incentive to reduce PCR
be a realistic concern, since the charging models of IP and ATM are as much as possible. This may be a realistic concern, since the
historically different as far as usage sensitivity, and the value of charging models of IP and ATM are historically different as far as
p_r, if set close to APB, could be many times the nominal GS usage sensitivity, and the value of p_r, if set close to APB, could
allocated rate of R. Thus, we can set PCR to R, with a buffer of be many times the nominal GS allocated rate of R. Thus, we can set
size b, with no loss of traffic, and no violation of the GS delay PCR to R, with a buffer of size b_r + C_sum + R D_sum, with no loss
bound. of traffic, and no violation of the GS delay bound.
A more subtle, and perhaps controversial case is where we set SCR to A more subtle, and perhaps controversial case is where we set SCR to
a value below R. The major feature of the GS service is to allow a a value below R. The major feature of the GS service is to allow a
receiver to specify the allocated rate R to be larger than the rate receiver to specify the allocated rate R to be larger than the rate
r_r sufficient to transport the traffic, in order to lower the r_r sufficient to transport the traffic, in order to lower the
queueing delay (roughly) from b/r + C_TOT/r + D_TOT to b/R + C_TOT/R queueing delay (roughly) from b/r + C_TOT/r + D_TOT to b/R + C_TOT/R
+ D_TOT. To effectively allocate bandwidth R to the flow, we set SCR + D_TOT. To effectively allocate bandwidth R to the flow, we set SCR
to match R. (Note it is unnecessary in any case to set SCR above R, to match R. (Note it is unnecessary in any case to set SCR above R,
so the relation, SCR <= R, is still true.) It is possible to set SCR so the relation, SCR <= R, is still true.) It is possible to set SCR
to a value as low as r_r, without violating the delay bounds or to a value as low as r_r, without violating the delay bounds or
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for allocating SCR = r_r rather than R is that the delay in the ATM for allocating SCR = r_r rather than R is that the delay in the ATM
network will have a component of MBS/SCR, which will be b/r rather network will have a component of MBS/SCR, which will be b/r rather
than b/R, contained in the D term advertised for the ATM sub-network than b/R, contained in the D term advertised for the ATM sub-network
(see further discussion in Section 3.3 below). It is also true that (see further discussion in Section 3.3 below). It is also true that
allocating r instead of R in a portion of the path is rather against allocating r instead of R in a portion of the path is rather against
the spirit of GS. As mentioned above, this mapping may however be the spirit of GS. As mentioned above, this mapping may however be
useful in practice in the case where pricing in the ATM network leads useful in practice in the case where pricing in the ATM network leads
to different incentives in the tradeoff between delay and bandwidth to different incentives in the tradeoff between delay and bandwidth
than those of the user who buys IP integrated services. than those of the user who buys IP integrated services.
Another point of view on parameter mapping suggests that SCR should Another point of view on parameter mapping suggests that SCR may
merely reflect the traffic description, hence SCR = r_r, while the merely reflect the traffic description, hence SCR = r_r, while the
service requirement is expressed in the QoS parameter as CDV = b/R. service requirement is expressed in the QoS parameter as CDV = b/R.
Thus the ATM network may internally allocate bandwidth R, but it is Thus the ATM network may internally allocate bandwidth R, but it is
free to use other methods as well to achieve the delay constraint. free to use other methods as well to achieve the delay constraint.
Mechanisms such as statistical flow/connection aggregation may be Mechanisms such as statistical flow/connection aggregation may be
implemented in the ATM network and hidden from the user (or parameter implemented in the ATM network and hidden from the user (or parameter
mapping module in the edge router) which sees only the interface mapping module in the edge router) which sees only the interface
implemented in the signalled parameters. implemented in the signalled parameters.
Note that this discussion considers an edge device buffer size of Note that this discussion considers an edge device buffer size of
b_r. In practice, it may be necessary for the AAL/segmentation b_r. In practice, it may be necessary for the AAL/segmentation
module to buffer M bytes in converting packets to cells. Also an module to buffer M bytes in converting packets to cells. Also an
additional amount of buffer equal to C_sum + R D_sum is generally additional amount of buffer equal to C_sum + R D_sum is generally
necessary to absorb jitter imposed by the upstream network [8]. necessary to absorb jitter imposed by the upstream network [8].
With ATM, it is possible to have little or no buffer in the edge With ATM, it is possible to have little or no buffer in the edge
router, because the ATM VC can be set to accept bursts at peak rate. router, because the ATM VC can be set to accept bursts at peak rate.
This may be unusual, since the edge router normally has enough buffer This may be unusual, since the edge router normally has enough buffer
to absorb bursts according to the TSpec token bucket parameters. We to absorb bursts according to the TSpec token bucket parameters. We
consider two cases. First, if PCR >= p_r, then MBS can be set to b_r consider two cases. First, if PCR >= p_r, then MBS can be set to b_r
and no buffering is necessary to absorb normal bursts. The extra and no buffering is necessary to absorb non-excessive bursts. The
buffering needed to absorb jitter can also be transferred to MBS. extra buffering needed to absorb jitter can also be transferred to
MBS. This effectively moves the buffering across the UNI into the
This effectively moves the buffering across the UNI into the ATM ATM network.
network.
For completeness, we consider an edge router with no burst-absorbing For completeness, we consider an edge router with no burst-absorbing
buffers and an MBS parameter of approximately zero. In this case it buffers and an MBS parameter of approximately zero. In this case it
is sufficient to set the rate parameters to PCR = SCR = max (R, p_r). is sufficient to set the rate parameters to PCR = SCR = max (R, p_r).
This amounts to peak-rate allocation of bandwidth, which will not This amounts to peak-rate allocation of bandwidth, which will not
usually be very cost effective. One reason for mentioning this case usually be very cost effective. This case may be relevant where the
might be that IP routers and ATM switches differ so substantially in IP routers and ATM switches differ substantially in their buffering
their buffering designs that IP-level users typically specify much designs. IP-level users may typically specify much larger burst
larger burst parameters than can be handled in the ATM subnet. parameters than can be handled in the ATM subnet. Peak-rate
Peak-rate bandwidth allocation provides a means to work around this bandwidth allocation provides a means to work around this problem.
problem. It is also true that intermediate tradeoffs can be It is also true that intermediate tradeoffs can be formulated, where
formulated, where the burst-absorbing buffer is less than b bytes, the burst-absorbing buffer is less than b bytes, and SCR is set above
and SCR is set above R and below p_r. Note that jitter-absorbing R and below p_r. Note that jitter-absorbing buffers (C_sum + R
buffers (C_sum + R D_sum) can not be avoided, generally, by D_sum) can not be avoided, generally, by increasing ATM rates, unless
increasing ATM rates, unless SCR is set to exceed the physical line SCR is set to exceed the physical line rate(s) into the edge device
rate(s) into the edge device for the flow. for the flow.
For GS over CBR, the value of PCR may be mapped to the RSpec rate R, For GS over CBR, the value of PCR may be mapped to the RSpec rate R,
if the edge device has a buffer of size b_r. With little or no burst if the edge device has a buffer of size b_r + C_sum + R D_sum. With
buffering, the requirements resemble the zero-buffer case above, and little or no burst buffering, the requirements resemble the zero-
we have PCR = max (R, p_r). Additional buffers sufficient to absorb buffer case above, and we have PCR = max (R, p_r). Additional
network jitter, given by C_sum, D_sum, must always be provided in the buffers sufficient to absorb network jitter, given by C_sum, D_sum,
edge router, or in the ATM network via MBS. MUST always be provided in the edge router, or in the ATM network via
MBS.
2.5.2 Translating Traffic Descriptors for Controlled Load Service 2.5.2 Translating Traffic Descriptors for Controlled Load Service
The Controlled Load service TSpec has a peak rate, p, a "token The Controlled Load service TSpec has a peak rate, p, a "token
bucket" rate, r, and a corresponding token bucket depth parameter, b. bucket" rate, r, and a corresponding token bucket depth parameter, b.
The receiver TSpec values are used to determine resource allocation, The receiver TSpec values are used to determine resource allocation,
and a simple mapping for the nrtVBR service category is given by, and a simple mapping for the nrtVBR service category is given by,
PCR = p_r PCR = p_r
SCR = r_r SCR = r_r
MBS = b_r. MBS = b_r.
The discussions in the preceding section on using edge device buffers The discussions in the preceding section on using edge device buffers
to reduce PCR, increasing buffers to reduce PCR and trading off to reduce PCR and/or MBS apply generally to the CLS over nrtVBR case
between such buffers and MBS, apply generally to the CLS over nrtVBR as well. Extra buffers to accommodate jitter accumulated (beyond the
case as well. Extra buffers to accommodate jitter accumulated b_r burst size allowed at the source) MUST be provided. For CLS,
(beyond the b_r burst size allowed at the source) must be provided. there are no Adspec parameters C and D, so the dimensioning of such
For CLS, there are no Adspec parameters C and D, so the estimation of buffers is an implementation design issue.
such buffers is an implementation design issue.
For ABR VCs, the TSpec rate r_r is used to set the minimum cell rate For ABR VCs, the TSpec rate r_r is used to set the minimum cell rate
(MCR) parameter. Since there is no corresponding signalled bucket (MCR) parameter. Since there is no corresponding signalled bucket
depth parameter, the edge device must have a buffer of at least b_r depth parameter, the edge device SHOULD have a buffer of at least b_r
bytes. Since the actual transfer rate can vary substantially with bytes, plus additional buffers to absorb jitter. With ABR, the ATM
ABR, the buffering should not be made so large that the, in an network can quickly throttle the actual transfer rate down to MCR, so
attempt to avoid loss, that delays exceed higher-layer timeouts, a source transmitting above that rate can experience high loss at the
e.g., TCP retransmission. ingress edge device when the ATM network becomes congested.
For CBR, the TSpec rate r_r sets a lower bound on PCR, and again, the For CBR, the TSpec rate r_r sets a lower bound on PCR, and again, the
available buffering in the edge device must be adequate to available buffering in the edge device SHOULD be adequate to
accommodate possible bursts of b_r. accommodate possible bursts of b_r.
The requirement for CLS that network delays approximate "best-effort The REQUIREMENT for CLS that network delays approximate "best-effort
service under unloaded conditions", is interpreted here to mean that service under unloaded conditions", is interpreted here to mean that
an allocation of (at least) r_r, resulting in the last byte of a it would be sufficient to allocate bandwidth resources so that the
burst of size b_r having delay approximately b_r/r_r, is sufficient. last byte of a burst of size b_r sees a delay approximately b_r/r_r.
A network element e.g., with no cross-traffic, work conserving For example, a network element with no cross-traffic, a work
scheduling and output link rate of r_L might provide delays in the conserving scheduler and an output link rate of r_L, might provide
range from M/r_L to b_r/r_L, which may be much better. delays in the range from M/r_L to b_r/r_L, that are much lower than
b_r/r_r. While the access to the full link bandwidth (r_L), as
reflected in this example, is a more literal interpretation of delay
"under unloaded conditions" for a shared link, an ATM VC may only
have access to bandwidth equal to its nominal allocation (some
implementation specific function of SCR and PCR).
2.5.3 Translating Traffic Descriptors for Best Effort Service 2.5.3 Translating Traffic Descriptors for Best Effort Service
For Best Effort service, there is no traffic description. The UBR For Best Effort service, there is no traffic description. The UBR
service category allows negotiation of PCR, simply to allow the service category allows negotiation of PCR simply to allow the source
source to discover the smallest physical bottleneck along the path. to discover the smallest physical bottleneck along the path. The
(The ingress edge router should set PCR to the ATM line rate, and may ingress edge router may set PCR to the ATM line rate, and then when
wish to make use of the returned value when the VC is set up.) Often the VC setup is complete, the returned value indicates an upper bound
a service provider will want to statically configure large VCs with a on throughput. For UBR service, resources may be allocated for the
overall service (i.e., not per-VC) using the (implementation
specific) admission control features of the ATM switches.
Often a service provider will statically configure large VCs with a
certain bandwidth allocation to handle all best effort traffic certain bandwidth allocation to handle all best effort traffic
between two edge routers. ABR, CBR or nrtVBR VCs are appropriate for between two edge routers. ABR, CBR or nrtVBR VCs are appropriate for
this with traffic parameters set to comfortably accommodate the this design, with traffic parameters set to comfortably accommodate
expected traffic load. See [10,11]. the expected traffic load. See IETF ION specifications for IP over
ATM signalling [10,11].
2.6 QoS Classes and Parameters 2.6 QoS Classes and Parameters
In UNI 3.x the quality of service is indicated by a single parameter In UNI 3.x the quality of service is indicated by a single parameter
called "QoS Class," which is essentially an index to a network called "QoS Class," which is essentially an index to a network
specific table of values for the actual QoS parameters. In TM/UNI specific table of values for the actual QoS parameters. In TM/UNI
4.0 three QoS parameters may be individually signalled, and the 4.0 three QoS parameters may be individually signalled, and the
signalled values override those implied by the QoS Class, which is signalled values override those implied by the QoS Class, which is
still present. These parameters are the Cell Loss Ratio (CLR), Cell still present. These parameters are the Cell Loss Ratio (CLR), Cell
Transfer Delay (CTD), and Cell Delay Variation (CDV) [6]. Transfer Delay (CTD), and Cell Delay Variation (CDV) [6].
A network provider may choose to associate other parameters, such as A network provider may choose to associate other parameters, such as
Severely Errored Cell Block Ratio, with a QoS Class definition, but Severely Errored Cell Block Ratio, with a QoS Class definition, but
these cannot be signalled individually. The ATM Forum UNI 3.0, 3.1 these cannot be signalled individually. The ATM Forum UNI 3.0, 3.1
and TM 4.0 specs, following prior ITU specs, give vague qualitative and TM 4.0 specs, following prior ITU specs, give vague qualitative
definitions for QoS Classes 1 to 4. (QoS Class 0 is well-defined as definitions for QoS Classes 1 to 4. (QoS Class 0 is well-defined as
"no QoS parameters defined".) Since our mapping is based on these "no QoS parameters defined".) Since our mapping is based on these
specifications, we generally follow this guidance by setting QoS specifications, we generally follow this guidance by setting the QoS
Class value to 0 for UBR and ABR (as required), 1 for CBR and rtVBR Class value to 0 for UBR and ABR (as REQUIRED), 1 for CBR and rtVBR
and 3 for nrtVBR. Note that the QoS Class follows the ATM service and 3 for nrtVBR. Note that the QoS Class follows the ATM service
category, and not the IP service, to avoid combination that are category, and not the IP service, to avoid combination that are
unlikely to be supported. For example, if only nrtVBR is available unlikely to be supported. For example, if only nrtVBR is available
for GS, then choosing QoS Class = 1 would probably result in for GS, then choosing QoS Class = 1 would probably result in
connection failure, rather than a way to add real-time behavior to an connection failure. The QoS Class MUST NOT be interpreted as a way
inherently non-real-time service. to add real-time behavior to an inherently non-real-time service.
The ITU has recently included a standard set of parameter values for The ITU has included a standard set of parameter values for a (small)
a (small) number of QoS Classes in the latest version of number of QoS Classes in the latest version of Recommendation I.356
Recommendation I.356, October 1996. Network providers may choose to [20]. Network providers may choose to define further network-
define further network-specific QoS Classes in addition to these. specific QoS Classes in addition to these. Note that the QoS class
Note that the QoS class definitions in the new I.356 version may not definitions in the new I.356 version might not align with the model
align with the model we follow from the UNI specs. Apart from these we follow from the ATM Forum UNI specs. Apart from these
definitions, the problem of agreement between network providers as to definitions, there is no consistent agreement on QoS Class
the definition of QoS Classes has not, to our knowledge, been definitions among providers in practice.
addressed.
The ATM QoS parameters have no explicitly signalled IP layer The ATM QoS parameters have no explicitly signalled IP layer
counterparts. The values that should be signalled in the ATM network counterparts. The values that are signalled in the ATM network are
are determined by knowledge of certain network characteristics and determined by the IP service definitions and knowledge of the
the IP service definitions. underlying ATM network characteristics, as explained below.
The ingress edge router must keep a table of QoS information for the The ingress edge router SHOULD keep a table of QoS information for
set of egress routers that it may establish VCs with. This table may the set of egress routers that it may establish VCs with. This table
be simplified by using default values, but it will probably be good may be simplified by using default values, but it will probably be
network practice to maintain a table of current data for the most good practice to maintain a table of current data for the most
popular egress points. An ATM network generally needs to have some popular egress points. An edge device that initiates VC setup
way to propose initial value for CDV and CTD, even if changed by generally needs to have some way to propose initial value for CDV and
negotiation; so by positing such a table, we are not creating any new CTD, even if they are changed by negotiation; so by positing such a
design burden. Cached information can be updated when VCs are table, we are not creating any new design burden. Cached information
successfully established, and to the extent that IP-layer can be updated when VCs are successfully established, and to the
reservations can wait for VCs to complete, the values can be refined extent that IP-layer reservations can wait for VCs to complete, the
through iterated negotiation. In general the construction of this values can be refined through iterated negotiation.
table is implementation specific.
Both GS and CLS require that losses of packets due to congestion Both GS and CLS REQUIRE that losses of packets due to congestion be
should be minimized, so that the loss rate is approximately the same minimized, so that the loss rate is approximately the same as for an
as for an unloaded network. The characteristic loss behavior of the unloaded network. The characteristic loss behavior of the physical
link-layer medium not due to congestion (e.g., bit errors or fading medium not due to congestion (e.g., bit errors or fading on wireless
on wireless channels) determines the order of the permitted packet channels) determines the order of the permitted packet loss rate.
loss rate. The ingress edge device will choose a value of CLR that The ingress edge device MUST choose a value of CLR that provides the
provides the appropriate IP-level packet loss rate. The CLR value appropriate IP-level packet loss rate. The CLR value may be uniform
may be uniform over all egress points in the ATM network, or may over all egress points in the ATM network, or may differ, e.g., when
differ, e.g., when wireless or satellite ATM links in the path. The wireless or satellite ATM links are in some paths. The determination
determination of CLR should account for the effects of packet size of CLR MUST account for the effects of packet size distribution and
distribution and ATM Frame Discard mode (which can change the ATM Frame Discard mode (which can change the effective packet loss
effective packet loss rate by orders of magnitude, given the same rate by orders of magnitude [21]).
underlying cell loss rate [20]).
The ingress router will also tabulate values for the Minimum Path The ingress router will also tabulate values for the Minimum Path
Latency (MPL) and estimated queueing delays (D_ATM) for each egress Latency (MPL) and estimated queueing delays (D_ATM) for each egress
point. The latter will be used as part of the Adspec "D" parameter point. The latter will be used as part of the Adspec "D" parameter
for GS, but its use here applies to CLS as well. MPL represents all for GS, but its use here applies to CLS as well (when the VC setup
non-congestion related delays, including propagation delay. D_ATM includes delay parameters). MPL represents all constant (non-
congestion related) delays, including propagation delay. D_ATM
accounts for the variable component of delays in the ATM network. accounts for the variable component of delays in the ATM network.
(It may depend on parameters such as CDVT, etc.) Hence, when a VC is (It may depend on non-signalled parameters such as CDVT.) Given
set up, the delay-related QoS parameters are given by these quantities, a new VC can be set up with delay-related QoS
parameters given by
CDV = D_ATM CDV = D_ATM
CTD = D_ATM + MPL. CTD = D_ATM + MPL.
(CDV and CTD may be increased by the slack term in GS, see Section (CDV and CTD may be adjusted (increased) by the slack term in GS, see
3.3 below.) For rtVBR, the value of CDV will generally have a Section 3.3 below.)
component of MBS/SCR analogous to the b/R term in the delay of GS
service. It may have other components that depend on the ATM switch
implementation. In cases where the ATM network uses statistical
resource allocation methods, it may be possible to establish VCs with
CDV less than MBS/SCR. This capability should be reflected in the
D_ATM values advertised in GS and used to determine CDV in for VCs
supporting both GS and CLS.
It is interesting (and perhaps unfortunate) to note that in a typical It is interesting (and perhaps unfortunate) to note that in a typical
GS/rtVBR service, the delay bound advertised can contain two GS/rtVBR service, the delay bound advertised can contain two
components of b/R instead of one. Consider the case where SCR = R components of b/R instead of one. Consider the simple case where SCR
and MBS = b. Parekh's theory, which is the basis of the GS delay = R is the rate allocated to the flow in both IP routers and ATM
formula [8] states that the b/R delay term occurs only once, because switches along the path, and the buffer allocation is MBS = b.
once a burst of size b has been served by a congested node at rate R, Parekh's theory [22], which is the basis of the GS delay formula [8]
the packets will not arrive at a subsequent node as a single burst. states that the b/R delay term occurs only once, because once a burst
However, we can't tell if this bottleneck will occur in the ATM of size b has been served by a congested node at rate R, the packets
network or elsewhere in the IP network, so the declaration of CDV will not arrive at a subsequent node as a single burst. However, we
must account for it. Once CDV is set, the ATM network can impose can't tell a priori if this bottleneck will occur in the ATM network
that delay. Since the delay b/R can also occur elsewhere, it cannot or elsewhere in the IP network, so the declaration of CDV SHOULD
be removed from the first term of the GS delay formula. The ATM b/R account for it (i.e., CDV >= b/R). Once CDV is set, the ATM network
delay component appears in the third term, D_tot. See Section 3.3 can impose this delay, whether or not the traffic arrives in a burst.
below for more on GS Adspec parameters. This effect may be Since the delay b/R can also occur elsewhere, it cannot be removed
unapparent when the ATM network employs more efficient statistical from the first term of the GS delay formula. The ATM b/R delay
resource allocation schemes. component appears in the third term of the GS delay formula, D_tot.
See Section 3.3 below for more on GS Adspec parameters. This effect
may be mitigated when the ATM network employs more efficient
statistical resource allocation schemes.
2.7 Additional Parameters -- Frame Discard Mode 2.7 Additional Parameters -- Frame Discard Mode
TM/UNI 4.0 allows the user to choose a mode where the ATM network is TM/UNI 4.0 allows the user to choose a mode where the ATM network is
aware, for the purpose of congestion management, of PDUs larger than aware, for the purpose of congestion management, of PDUs larger than
an ATM cell (i.e., AAL PDUs that correspond in our context to IP an ATM cell (i.e., AAL PDUs that correspond in our context to IP
packets). This facilitates implementation of algorithms such as packets). This facilitates implementation of algorithms such as
partial packet discard, where a dropped cell causes subsequent cells partial packet discard, where a dropped cell causes subsequent cells
in the AAL5 PDU to be dropped as well. Several other applicable in the same AAL-5 PDU to be dropped as well. Several other
buffer management schemes have been proposed [20, 21]. applicable buffer management schemes have been proposed [21, 23].
Frame discard can improve efficiency and the performance of end-to- Frame discard can improve the efficiency and performance of end-to-
end protocols such as TCP, since the remaining cells of a damaged PDU end protocols such as TCP, since the remaining cells of a damaged PDU
are generally useless to the receiver. For IP over ATM, Frame are generally useless to the receiver. For IP over ATM, Frame
Discard should always be indicated, if available. Discard MUST always be indicated, if available.
3.0 Additional IP-Integrated Services Protocol Features 3.0 Additional IP-Integrated Services Protocol Features
3.1 Path Characterization Parameters for IP Integrated Services with ATM 3.1 Path Characterization Parameters for IP Integrated Services with ATM
This section discusses the setting of General Characterization This section discusses the setting of General Characterization
Parameters (GCPs) at an ATM egress edge router. GCPs are signalled Parameters (GCPs) at an ATM egress edge router. GCPs are signalled
from source to destination, and modified by intermediate nodes using from IP source to IP destination, and modified by intermediate nodes
the Adspec portion of PATH messages in rsvp. The GS-specific Adspec using the Adspec portion of PATH messages in rsvp. The GS-specific
parameters are discussed below in Section 3.3. These parameters are Adspec parameters are discussed below in Section 3.3. These
denoted as <x,y> where x is the service and y is the parameter parameters are denoted as <x,y> where x is the service and y is the
number. Service number 1 indicates default or general parameter parameter number. Service number 1 indicates default or general
values. Please refer to [22] for definitions and details. parameter values. Please refer to [24] for definitions and details.
The IS break bit <1,2> should, of course, be left alone by The IS break bit <1,2> MUST, of course, be left alone by
implementations following these guidelines (as they are presumably implementations following these guidelines (as they are presumably
IS-aware). Similarly, the router should always increment IS_HOPS IS-aware). Similarly, the router MUST always increment IS_HOPS
<1,4>. The GS and CLS service-specific break bits, <2,2> and <5,2> <1,4>. The GS and CLS service-specific break bits, <2,2> and <5,2>
respectively, should be set if the support of the service is respectively, MUST be set if the support of the service is
inadequate. In general GS is adequately supported by CBR (BCOB-A) inadequate. In general GS is adequately supported by CBR (BCOB-A)
and rtVBR service categories, and not adequately supported by UBR, and rtVBR service categories, and not adequately supported by UBR,
ABR and nrtVBR because delays are not controlled. CLS may be ABR and nrtVBR because delays are not controlled. CLS may be
adequately supported by all service categories except UBR (or Best adequately supported by all service categories except UBR (or Best
Effort in UNI 3.x). See Sections 5, 6 for further discussion. Effort in UNI 3.x). See Sections 5, 6 for further discussion.
For GS, the ATM network must meet the delay performance advertised For GS, the ATM network MUST meet the delay performance advertised
through the Adspec parameters, MPL, C, and D. If it cannot through the Adspec parameters, MPL, C, and D. If it cannot
predictably meet these requirements, the GS break bit should be set. predictably meet these requirements, the GS break bit MUST be set.
Similarly both break bits should be set if reservations are honored, Similarly both break bits MUST be set if reservations are honored,
but sufficient resources to avoid congestion loss are not allocated but sufficient resources to avoid congestion loss are not allocated
in practice. If the service break bits are not set, then the in practice. If the service break bits are not set, then the
corresponding service hop counters, <2,4>, <5,4>, should be corresponding service hop counters, <2,4>, <5,4>, MUST be
incremented. incremented.
The Available Path Bandwidth (APB) parameters <x,6> indicate the The Available Path Bandwidth (APB) parameters <x,6> indicate the
minimum physical bottleneck rate along the path. This may be minimum physical bottleneck rate along the path. This may be
discoverable in an ATM network as the negotiated PCR value for a UBR discoverable in an ATM network as the negotiated PCR value for any
VC along the path. This value should be corrected for AAL, ATM and UBR VC along the same path. This value MUST be corrected for AAL,
physical-layer headers, as necessary, to reflect the effective IP ATM and physical-layer headers, as necessary, to reflect the
datagram bandwidth. With ATM, it is possible that there is some effective IP datagram bandwidth. With ATM, it is possible that there
policy limitation on the value of PCR, below the physical link is some policy limitation on the value of PCR, below the physical
bottleneck. In this case, the advertised value of APB (in general link bottleneck. In this case, the advertised value of APB (in
and for each service if different) should reflect this limit, since general, and for each service if the values of APB signalled are
excess traffic beyond this rate will be dropped. (Note that there is service specific) MUST reflect this limit, since excess traffic
no tagging of traffic in excess of PCR for TM/UNI 4.0.) These values beyond this rate will be dropped. (Note that there is no tagging of
should generally be cached by the ingress router for the set of traffic in excess of PCR for TM/UNI 4.0.) These values SHOULD
egress routers that it typically needs to establish VCs to. The generally be cached by the ingress router for the set of egress
Adspec parameters <x,6> are only adjusted down, to reflect the routers with which it typically needs to establish VCs. The APB
minimum as the composed value. parameters are only adjusted down, to reflect the minimum as the
composed value.
In the case of a multipoint VC, several parameters can be different In the case of a multipoint VC, several parameters can be different
for each egress point. In this case, the IWF at the egress routers for each egress point, e.g., because the characteristics of the
must correct these values in PATH messages as they exit the ATM physical links of the VC branches differ. When this occurs, the IWF
network. This is the only case where the egress router needs to at the egress routers MUST correct these values in PATH messages as
operate on rsvp control messages. (A similar correction must be they exit the ATM network. (We use the word "correct" because the
implemented for any non-rsvp set-up mechanism). The parameters that ingress router SHOULD set the parameters to a value that is
require such correction are specifically the Available Path Bandwidth appropriate for the largest number of branches, or a value that would
(APB), the Minimum Path Latency (MPL), the Path MTU (although for do the least harm if the egress routers failed to correct such
ATM/AAL5 this may typically be constant), and the ATM-specific parameters for each branch.) This is the only case where the egress
components of the GS Adspec parameters C_ATM and D_ATM. router needs to operate on rsvp control messages. (A similar
correction MUST be implemented for any non-rsvp set-up mechanism).
The parameters for which such correction is REQUIRED are the
Available Path Bandwidth (APB), the Minimum Path Latency (MPL), the
Path MTU (although for ATM/AAL-5 this may typically be constant), and
the ATM-specific components of the GS Adspec parameters C_ATM and
D_ATM.
The ingress router table must store values for the ATM-network MPL The ingress router table SHOULD store values for the ATM-network MPL
<x,7> for the various egress points. The composed values <x,8> are <x,7> for the various egress points. The composed values <x,8> are
formed by addition and forwarded along the path. In the cases where formed by addition and forwarded along the path. In the cases where
ATM routing chooses different paths for VCs to a given egress point, ATM routing chooses different paths, depending on the service
depending on the service category, the table will generally reflect category, for VCs to a given egress point, the table will generally
different values for each service. If the ATM network is very large reflect different values for each service. If the ATM network is
and complex, it may become difficult to predict the routes that VCs very large and complex, it may become difficult to predict the routes
will take once they are set up. This could be a significant source that VCs will take once they are set up. This could be a significant
of misconfiguration, resulting in discrepancies between GS delay source of misconfiguration, resulting in discrepancies between GS
advertisements and actual results. The RSpec Slack term may be delay advertisements and actual results. The RSpec Slack term may be
useful in mitigating this problem. useful in mitigating this problem.
AAL 5 will support any message size up to 65,535 bytes, so setting AAL-5 will support any message size up to 65,535 bytes, so setting
the AAL SDU to the receiver TSpec M parameter value should generally the AAL SDU to the receiver TSpec M parameter value (plus 8 bytes for
not be a issue. In the PATH Adspec, however, the PATH_MTU parameter the LLC/SNAP header) will generally not be an issue. In the PATH
<x,10> for each service should be set to 9180 bytes, which is the Adspec, however, the PATH_MTU parameter <x,10> for each service
default MTU for AAL 5. SHOULD be set to 9188 bytes, which is the default MTU for AAL-5 [19].
3.2 Handling of Excess Traffic 3.2 Handling of Excess Traffic
CLS requires and GS recommends that network elements transport CLS REQUIRES and GS RECOMMENDS that network elements transport
traffic in excess of the TSpec parameters whenever physical resources traffic in excess of the TSpec parameters whenever physical resources
(bandwidth, buffers and processing) are available. While excess (bandwidth, buffers and processing) are available. While excess
traffic should be supported on a best effort basis, it should not traffic SHOULD be supported on a best effort basis, it MUST NOT
interfere with the QoS (delay and loss) of conforming CLS and GS interfere with the QoS (delay and loss) of conforming CLS and GS
traffic, nor with normal service of non-reserved best effort traffic. traffic, nor with normal service of non-reserved best effort traffic.
There are several solutions with ATM: the most attractive is to use a There are several solutions with ATM: the most attractive is to use a
VBR service category (with an appropriate conformance definition) and VBR service category (with an appropriate conformance definition) and
tag excess traffic as low priority using the CLP bit. This avoids tag excess traffic as low priority using the CLP bit. This avoids
reordering of the flow, but requires care in the design of the egress reordering of the flow, but necessitates careful design of the egress
router scheduler. To avoid reordering, the excess traffic would be router scheduler. To avoid reordering, the excess traffic can be
queued with conforming traffic. A threshold must be used to ensure queued with conforming traffic. A threshold SHOULD be used to ensure
that conforming traffic is not unnecessarily delayed by the excess. that conforming traffic is not unnecessarily delayed by the excess.
Also, for GS, the extra delay that would be incurred due to excess Also, for GS, the extra delay that would be incurred due to excess
traffic below the threshold would have to be accurately reflected in traffic in the queue ahead of conforming packets would have to be
the delay advertisement. Note that the egress router should accurately reflected in the delay advertisement. Note that the
uniformly tag all the cells of each non-conforming packet, rather ingress router SHOULD tag all cells of each non-conforming packet,
than letting the ATM network apply tagging due to ATM-level non- rather than letting the ATM network apply tagging due to ATM-level
conformance. non-conformance.
There is no requirement in ATM standards that tagged cells, marked There is no requirement in ATM standards that tagged cells, marked
either by the user or by policing, must be transported if possible. either by the user or by policing, be transported if possible.
Therefore, the operator of an edge router supporting IP-IS should Therefore, the operator of an edge router supporting IP-IS SHOULD
ascertain the actual behavior of the ATM equipment in the path, which ascertain the actual behavior of the ATM equipment in the path, which
may span multiple administrative domains in the ATM network. If may span multiple administrative domains in the ATM network. If
tagged cells are simply dropped at some point, regardless of load, tagged cells are simply dropped at some point, regardless of load,
then the operator may consider setting the break bit, at least for then the operator may consider setting the break bit, at least for
CLS service. CLS service.
The other solutions generally involve a separate VC to carry the The other solutions generally involve a separate VC to carry the
excess. A distinct VC can be set up for each VC supporting a GS or excess. A distinct VC can be set up for each VC supporting a GS or
CLS flow, or, if many flows are aggregated into a single QoS VC, then CLS flow, or, if many flows are aggregated into a single QoS VC, then
another VC can handle the excess traffic for that set of flows. A VC another VC can handle the excess traffic for that set of flows. A VC
can be set up to handle all excess traffic from the ingress router to can be set up to handle all excess traffic from the ingress router to
the egress point. Since the QoS of the excess traffic is not the egress point. Since the QoS of the excess traffic is not
particularly constrained, the design is quite flexible. The service particularly constrained, the design is quite flexible. However,
category for the excess-traffic VC may typically be UBR or ABR, using a separate VC may cause misordering of packets within a flow.
although one could use CBR or nrtVBR if the excess traffic were The service category for the excess-traffic VC may typically be UBR
predictable enough to know what rate to allocate. (This wouldn't or ABR, although one could use CBR or nrtVBR if the excess traffic
normally be expected for excess traffic, though.) were predictable enough to know what rate to allocate. (This
wouldn't normally be expected for excess traffic, though.)
Whether a separate VC is used may be influenced by the design of the Whether a separate VC is used may be influenced by the design of the
router scheduler. The CLS spec suggests two possible router scheduler. The CLS spec suggests two possible
implementations: one where excess traffic shares the Best Effort implementations: one where excess traffic shares the Best Effort
class scheduler allocation, but at lower priority than other best class scheduler allocation, but at lower priority than other best
effort traffic. The other where a separate allocation is made. The effort traffic. The other, where a separate allocation is made. The
first would allow excess traffic to use the same VC as normal best first would allow excess traffic to use the same VC as normal best
effort traffic, and the second would suggest a separate VC. effort traffic, and the second would suggest a separate VC.
TM/UNI 4.0. does not support tagging of traffic in excess of PCR. TM/UNI 4.0. does not support tagging of traffic in excess of PCR.
Although UNI 3.x does have a separate PCR parameter for CLP=0 cells Although UNI 3.x does have a separate PCR parameter for CLP=0 cells
only, we do not recommend using this feature for reasons of only, we do not recommend using this feature for reasons of
interoperability. This restricts CBR VCs to use solutions other than interoperability with TM/UNI 4.0 equipment. This restricts CBR VCs
tagging. The value of PCR can be set higher than necessary for to use solutions other than tagging. The value of PCR can be set
conformant traffic, in an effort to support excess traffic on the higher than necessary for conformant traffic, in an effort to support
same VC. In some cases this may be a viable solution, such as when excess traffic on the same VC. In some cases this may be a viable
there is little additional cost imposed for a high PCR. If PCR can solution, such as when there is little additional cost imposed for a
be set as high as APB, then the excess traffic is fully accommodated. high PCR. If PCR can be set as high as APB, then the excess traffic
is fully accommodated.
3.3 Use of Guaranteed Service Adspec Parameters and Slack Term 3.3 Use of Guaranteed Service Adspec Parameters and Slack Term
The Adspec is used by the Guaranteed Service to allow a receiver to The Adspec is used by the Guaranteed Service to allow a receiver to
calculate the worst-case delay associated with a GS flow. Three calculate the worst-case delay associated with a GS flow. Three
quantities, C, D, and MPL, are accumulated (by simple addition of quantities, C, D, and MPL, are accumulated (by simple addition of
components corresponding to each network element) in the PATH message components corresponding to each network element) in the PATH message
from source to receiver. The resulting delay values can be different from source to receiver. The resulting delay values can be different
for each unique receiver. The maximum delay is then computed as for each unique receiver. The maximum delay is computed as
delay <= b_r/R + C_TOT/R + D_TOT + MPL delay <= b_r/R + C_TOT/R + D_TOT + MPL
The Minimum Path Latency (MPL) includes propagation delay, while The Minimum Path Latency (MPL) includes propagation delay, while
b_r/R accounts for bursts and C and D include other queueing, b_r/R accounts for bursts due to the source and C and D include other
scheduling and serialization delays. (We neglect the effect of queueing, scheduling and serialization delays. (We neglect the
maximum packet size and peak rate here; see the GS specification [8] effect of maximum packet size and peak rate here; see the GS
for a more detailed equation.) The service rate requested by the specification [8] for a more detailed equation.) The service rate
receiver, R, can be greater than the TSpec rate, r_r, resulting in requested by the receiver, R, can be greater than the TSpec rate,
lower delay. The burst size, b_r, is the leaky bucket parameter from r_r, resulting in lower delay. The burst size, b_r, is the leaky
the receiver TSpec. bucket parameter from the receiver TSpec.
The values of C and D that a router advertises depend on both the The values of C and D that a router advertises depend on both the
router packet scheduler, and the characteristics of the subnet router packet scheduler and the characteristics of the subnet
attached to the router. Each router (or the source host) takes attached to the router. Each router (or the source host) takes
responsibility for its downstream subnet in its advertisement. For responsibility for its downstream subnet in its advertisement. For
example, if the subnet is a simple point-to-point link, the subnet- example, if the subnet is a simple point-to-point link, the subnet-
specific parts of C and D need to account for the link transmission specific parts of C and D need to account for the link transmission
rate and MTU. An ATM subnet is generally more complex. rate and MTU. An ATM subnet is generally more complex.
For this discussion, we consider only the ATM subnet-specific For this discussion, we consider only the ATM subnet-specific
components, denoted C_ATM and D_ATM. The ATM network can be components, denoted C_ATM and D_ATM. The ATM network can be
represented as a "pure delay" element, where the variable queueing represented as a "pure delay" element, where the variable queueing
delay, given by CVD is captured in D_ATM, and C_ATM = 0. It is delay, given by CVD is captured in D_ATM, and C_ATM is set to zero.
possible to use C_ATM only when the ATM service rate equals R. This It is possible to use C_ATM only when the ATM service rate equals R.
may be the case, for example with a CBR VC with PCR = R. Usually it This may be the case, for example with a CBR VC with PCR = R.
will be simpler to just advertise the total delay variation (CDV) in Usually it will be simpler to just advertise the total delay
D_ATM. variation (CDV) in D_ATM.
As discussed in Section 2.6, the edge router keeps a table with As discussed in Section 2.6, the edge router keeps a table with
values of MPL and D_ATM for each egress router it needs to share VCs values of MPL and D_ATM for each egress router it needs to share VCs
with. The values of D_ATM contribute to the D parameter advertised with. The value of D_ATM contributes to the D parameter advertised
by the edge router, and should accurately reflect the CDV that the by the edge router, and SHOULD accurately reflect the CDV that the
router will get in a VC when it is set up. Factors that affect CDV, router will get in a VC when it is set up. Factors that affect CDV,
such as statistical multiplexing in the ATM network, should be taken such as statistical multiplexing in the ATM network, SHOULD be taken
into account when compiling data for the router's table. In case of into account when compiling data for the router's table. In case of
uncertainty, D_ATM can be set to an upper bound. uncertainty, D_ATM can be set to an upper bound. When an RESV
message arrives, causing a VC to be set up, the requested values for
When a RESV message arrives, causing a VC to be set up, the requested CTD and CDV can be relaxed using the slack term in the receiver
values for CTD and CDV can be relaxed using the slack term in the RSpec:
receiver RSpec:
CTD = D_ATM + MPL + S_ATM CTD = D_ATM + MPL + S_ATM
CDV = D_ATM + S_ATM. CDV = D_ATM + S_ATM.
The term S_ATM is the portion of the slack term applied to the ATM The term S_ATM is the portion of the slack term applied to the ATM
portion of the path. Recall that the slack term [8] is positive when portion of the path. Recall that the slack term [8] is positive when
the receiver can afford more delay than that computed from the the receiver can afford more delay than that computed from the
Adspec. The ATM edge device may take part (or all) of the slack term Adspec. The ATM edge device may take part (or all) of the slack
S. The distribution of delay slack among the nodes and subnets is term, S. The distribution of delay slack among the nodes and subnets
network specific. is network specific.
Note that with multipoint VCs the egress edge router may need to Note that with multipoint VCs the egress edge router may need to
correct advertised values of C and D. See discussion in Section 3.1. correct advertised values of C and D. See discussion in Section 3.1.
4.0 Miscellaneous Items 4.0 Miscellaneous Items
4.1 Units Conversion 4.1 Units Conversion
All rates and token bucket depth parameters that are mapped from IP- All rates and token bucket depth parameters that are mapped from IP-
level parameters to ATM parameters must be corrected for the effects level parameters to ATM parameters MUST be corrected for the effects
of cell headers, AAL headers and segmentation of packets into cells. of added headers and the segmentation of packets into cells. At the
At the IP layer, bucket depths and rates are measured in bytes and IP layer, token bucket depths and rates are measured in bytes and
bytes/sec, respectively, whereas for ATM, they are measured in cells bytes/sec, respectively, whereas for ATM, they are measured in cells
and cells/sec. and cells/sec.
Packets are segmented into 53 byte cells of which the first 5 bytes Each IP Packet is wrapped into an AAL-5 PDU, having a number of
are header information. For additional header bytes (8 for LLC/SNAP and perhaps others, e.g. 12
for MPOA, etc.), and an 8-byte AAL-5 trailer. The AAL-5 PDU is then
segmented into multiple ATM cells, each having a 5-byte cell header
followed by a 48-byte cell payload. The number of cells used to
carry an IP packet with
B = number of Bytes, B = number of IP-packet Bytes,
H = number of AAL-5 header bytes (LLC/SNAP etc.)
C = number of cells, C = number of cells,
a rough approximation between the token bucket parameters (rate and is roughly
bucket depth) is
C = B/48.
This is actually a lower bound on C and does not take into account
the extra padding at the end of a partially filled cell, or the 8
byte trailer in the last cell of an AAL5 encoding. The actual
relationship between the number of cells and bytes of one packet is
C = 1 + int(B/48) + x,
where x = 1 if B mod 48 > 41
0 otherwise.
where int() is the rounding down operation. The third term is 0 or
1 and is 1 only when the remainder of B/48 is 41 or more. (An
additional cell is needed because the 41 bytes plus 8 byte trailer
will not fit in a cell.)
The above formula is not particularly amenable to engineering
considerations. By equating the number of bytes before and after
segmentation we have
48 C = B + 8 + A,
where A is the additional padding used in the last 2 cells and has
the range 0 <= A <= 47. From this we obtain a number of useful
observations.
For example, if one believes that the packet lengths are uniformly
distributed mod 48, then on average, 48 C = B + 8 + 47/2, or C = B/48
+ .65625.
We can also make use of the upper bound on A to state that 48 C <= B
+ 55. This is true for any one packet. Considering the number of
bytes in a stream of P packets, we have
48 C <= B + 55 P. C = B/48,
and more precisely
The number of packets P may not be a readily available quantity. C = floor[(H + B + 8 + 47)/48]
However, in terms of the minimum policed unit m, we know that P * m
<= B. Hence P <= B/m and 48 C <= B ( 1 + 55/m). That is,
C <= B/48 * (1 + 55/m). where floor[] is rounds down to the nearest integer. The '8'
accounts for the AAL-5 trailer and the '47' accounts for the last
cell which may be only partially filled.
5.0 Summary of ATM VC Setup Parameters for Guaranteed Service 5.0 Summary of ATM VC Setup Parameters for Guaranteed Service
This section describes how to create ATM VCs appropriately matched This section describes how to create ATM VCs appropriately matched
for Guaranteed Service. The key points differentiating among ATM for Guaranteed Service. The key points are that real-time timing is
choices are that real-time timing is required, that the data flow may REQUIRED, that the data flow may have a variable rate, and that
have a variable rate, and that demotion of non-conforming traffic to demotion of non-conforming traffic to best effort is REQUIRED to be
best effort is required to be in agreement with the definition of GS. in agreement with the definition of GS. For this reason, we prefer
For this reason, we prefer an rtVBR service in which tagging is an rtVBR service in which tagging is supported. Another good match
supported. Another good match is to use CBR with special handling of is to use CBR with special handling of any non-conforming traffic,
any non-conforming traffic, e.g., through another VC, since a CBR VC e.g., through another VC, since a CBR VC will not accommodate traffic
will not accommodate traffic in excess of PCR. in excess of PCR.
Note, these encodings assume point to multipoint connections, where Note, these encodings assume point to multipoint connections, where
the backward channel is not used. If the IP session is unicast only, the backward channel is not used. If the IP session is unicast only,
then a point-to-point VC may be used and the IWF may make use of the then a point-to-point VC may be used and the IWF may make use of the
backward channel, provided that the QoS parameters are mapped backward channel, with QoS parameters set appropriately for the
consistently for the service provided. service provided.
We provide a mapping for all combinations of IP service and ATM
service category, and comments indicating whether or not each
combination meets the requirements of the IP-IS service.
5.1 Encoding GS Using Real-Time VBR (ATM Forum TM/UNI 4.0) 5.1 Encoding GS Using Real-Time VBR (ATM Forum TM/UNI 4.0)
RtVBR with conformance definition VBR.3 [6] MEETS the requirements of
GS.
AAL AAL
Type 5 Type 5
Forward CPCS-SDU Size parameter M of receiver TSpec Forward CPCS-SDU Size parameter M of rcvr TSpec + 8 Bytes
Backward CPCS-SDU Size 0 Backward CPCS-SDU Size parameter M of rcvr TSpec + 8 Bytes
SSCS Type 0 (Null SSCS) SSCS Type 0 (Null SSCS)
Traffic Descriptor Traffic Descriptor
Forward PCR CLP=0+1 Note 1 Forward PCR CLP=0+1 Note 1
Backward PCR CLP=0+1 0 Backward PCR CLP=0+1 0
Forward SCR CLP=0 Note 1 Forward SCR CLP=0 Note 1
Backward SCR CLP=0 0 Backward SCR CLP=0 0
Forward MBS (CLP=0) Note 1 Forward MBS (CLP=0) Note 1
Backward MBS (CLP=0) 0 Backward MBS (CLP=0) 0
BE indicator NOT included BE indicator NOT included
skipping to change at page 29, line 4 skipping to change at page 29, line 13
Backward PCR CLP=0+1 0 Backward PCR CLP=0+1 0
Forward SCR CLP=0 Note 1 Forward SCR CLP=0 Note 1
Backward SCR CLP=0 0 Backward SCR CLP=0 0
Forward MBS (CLP=0) Note 1 Forward MBS (CLP=0) Note 1
Backward MBS (CLP=0) 0 Backward MBS (CLP=0) 0
BE indicator NOT included BE indicator NOT included
Forward Frame Discard bit 1 Forward Frame Discard bit 1
Backward Frame Discard bit 1 Backward Frame Discard bit 1
Tagging Forward bit 1 (Tagging requested) Tagging Forward bit 1 (Tagging requested)
Tagging Backward bit 1 (Tagging requested) Tagging Backward bit 1 (Tagging requested)
Broadband Bearer Capability Broadband Bearer Capability
Bearer Class 16 (BCOB-X) Note 2 Bearer Class 16 (BCOB-X) Note 2
ATM Transfer Capability 9 (Real time VBR) Note 3 ATM Transfer Capability 9 (Real time VBR) Note 3
Susceptible to Clipping 00 (bit encoding for Not Susceptible to Clipping 00 (Not Susceptible)
susceptible) User Plane Configuration 01 (Point-to-Multipoint)
User Plane Configuration 01 (bit encoding for pt-to-mpt)
Broadband Low Layer Information Broadband Low Layer Information
User Information Layer 2 User Information Layer 2
Protocol 12 (ISO 8802/2) Protocol 12 (ISO 8802/2)
User Information Layer 3 User Information Layer 3
Protocol 11 (ISO/IEC TR 9577) Note 4 Protocol 11 (ISO/IEC TR 9577) Note 4
ISO/IEC TR 9577 IPI 204 ISO/IEC TR 9577 IPI 204
QoS Class QoS Class
QoS Class Forward 1 Note 5 QoS Class Forward 1 Note 5
QoS Class Backward 1 Note 5 QoS Class Backward 1 Note 5
QoS Parameters Note 6 Extended QoS Parameters Note 6
Acceptable Forward CDV Acceptable Forward CDV
Acceptable Forward CLR Acceptable Forward CLR
Forward Max CTD Forward Max CTD
Note 1: See discussion Section 2.5.1. Note 1: See discussion in Section 2.5.1.
Note 2: Value 3 (BCOB-C) can also be used. Note 2: Value 3 (BCOB-C) can also be used.
Note 3: The ATC value 19 is not used. The value 19 implies CLR If Bearer Class C is chosen the ATC field MUST be absent.
objective applies to the aggregate CLP=0+1 stream and Note 3: The ATC value 19 is not used. The value 19 implies that the
that does not give desirable treatment of excess CLR objective applies to the aggregate CLP=0+1 stream and
traffic in the case of IP. that does not give desirable treatment of excess traffic.
Note 4: For QoS VCs supporting GS or CLS, the layer 3 protocol should Note 4: For QoS VCs supporting GS or CLS, the layer 3 protocol SHOULD
be specified. For BE VCs, it can be left unspecified, allowing be specified. For BE VCs, it can be left unspecified, allowing
the VC to be shared by multiple protocols, following RFC 1755. the VC to be shared by multiple protocols, following RFC 1755.
Note 5: Cf ITU I.365 (Oct 1996) for new QoS Class definitions. Note 5: Cf ITU Rec. I.356 [20] for new QoS Class definitions.
Note 6: See Section 2.6 for the values to be used. Note 6: See discussion in Section 2.6.
5.2 Encoding GS Using CBR (ATM Forum TM/UNI 4.0) 5.2 Encoding GS Using CBR (ATM Forum TM/UNI 4.0)
It is also possible to support GS using a CBR "pipe." The advantage A CBR VC MEETS the requirements of GS. The main advantage of this is
of this is that CBR is probably supported; the disadvantage is that that CBR is widely supported; the disadvantage is that data flows
data flows may not fill the pipe (utilization loss) and there is no might not fill the pipe (utilization loss) and there is no tagging
tagging option available. option available. Excess traffic MUST be handled using a separate
VC.
AAL AAL
Type 5 Type 5
Forward CPCS-SDU Size parameter M of receiver TSpec Forward CPCS-SDU Size parameter M of rcvr TSpec + 8 Bytes
Backward CPCS-SDU Size parameter M of receiver TSpec Backward CPCS-SDU Size parameter M of rcvr TSpec + 8 Bytes
SSCS Type 0 (Null SSCS) SSCS Type 0 (Null SSCS)
Traffic Descriptor Traffic Descriptor
Forward PCR CLP=0+1 Note 1 Forward PCR CLP=0+1 Note 1
Backward PCR CLP=0+1 0 Backward PCR CLP=0+1 0
BE indicator NOT included BE indicator NOT included
Forward Frame Discard bit 1 Forward Frame Discard bit 1
Backward Frame Discard bit 1 Backward Frame Discard bit 1
Tagging Forward bit 0 (Tagging not requested) Tagging Forward bit 0 (Tagging not requested)
Tagging Backward bit 0 (Tagging not requested) Tagging Backward bit 0 (Tagging not requested)
Broadband Bearer Capability Broadband Bearer Capability
Bearer Class 16 (BCOB-X) Note 2 Bearer Class 16 (BCOB-X) Note 2
ATM Transfer Capability 5 (CBR) Note 3, 4 ATM Transfer Capability 5 (CBR) Note 3
Susceptible to Clipping 00 (bit encoding for Not Susceptible to Clipping 00 (Not Susceptible)
susceptible) User Plane Configuration 01 (Point-to-Multipoint)
User Plane Configuration 01 (bit encoding for pt-to-mpt)
Broadband Low Layer Information Broadband Low Layer Information
User Information Layer 2 User Information Layer 2
Protocol 12 (ISO 8802/2) Protocol 12 (ISO 8802/2)
User Information Layer 3 User Information Layer 3
Protocol 11 (ISO/IEC TR 9577) Note 5 Protocol 11 (ISO/IEC TR 9577) Note 4
ISO/IEC TR 9577 IPI 204 ISO/IEC TR 9577 IPI 204
QoS Class QoS Class
QoS Class Forward 1 Note 6 QoS Class Forward 1 Note 5
QoS Class Backward 1 Note 6 QoS Class Backward 1 Note 5
QoS Parameters Note 7 Extended QoS Parameters Note 6
Acceptable Forward CDV Acceptable Forward CDV
Acceptable Forward CLR Acceptable Forward CLR
Forward Max CTD Forward Max CTD
Note 1: See discussion Section 2.5.1. Note 1: See discussion in Section 2.5.1.
Note 2: Value 1 (BCOB-A) can also be used. Note 2: Value 1 (BCOB-A) can also be used.
Note 3: If bearer class A is chosen the ATC field must be absent. If Bearer Class A is chosen the ATC field MUST be absent.
Note 4: The ATC value 7 is not used. The value 7 implies CLR Note 3: The ATC value 7 is not used. The value 7 implies CLR
objective applies to the aggregate CLP=0+1 stream and objective applies to the aggregate CLP=0+1 stream and
that does not give desirable treatment of excess that does not give desirable treatment of excess traffic.
traffic in the case of IP. Note 4: For QoS VCs supporting GS or CLS, the layer 3 protocol SHOULD
Note 5: For QoS VCs supporting GS or CLS, the layer 3 protocol should
be specified. For BE VCs, it can be left unspecified, allowing be specified. For BE VCs, it can be left unspecified, allowing
the VC to be shared by multiple protocols, following RFC 1755. the VC to be shared by multiple protocols, following RFC 1755.
Note 6: Cf ITU I.365 (Oct 1996) for new QoS Class definitions. Note 5: Cf ITU Rec. I.356 [20] for new QoS Class definitions.
Note 6: See discussion in Section 2.6.
Note 7: See Section 2.6 for the values to be used.
5.3 Encoding GS Using Non-Real-Time VBR (ATM Forum TM/UNI 4.0) 5.3 Encoding GS Using Non-Real-Time VBR (ATM Forum TM/UNI 4.0)
The remaining ATM service categories, including nrtVBR, do not NrtVBR does not provide delay guarantees and is NOT RECOMMENDED for
provide delay guarantees and cannot be recommended as the best fits. GS. If GS/nrtVBR is used and network utilization is low, the delay
However in some circumstances, the best fits may not be available. may be `reasonable', but will not be controlled. The encoding of GS
with nrtVBR is the same as that for CLS using nrtVBR. See Section
If nrtVBR is used, no hard delay can be given. However by using a 6.1 below.
variable rate service with low utilization, delay may be
`reasonable', but not controlled. The encoding of GS as nrtVBR is
the same as that for CLS using nrtVBR, except that the Forward PCR
would be derived from the TSpec peak rate. See Section 6.2 below.
5.4 Encoding GS Using ABR (ATM Forum TM/UNI 4.0) 5.4 Encoding GS Using ABR (ATM Forum TM/UNI 4.0)
GS using ABR is a very unlikely combination. The objective of the GS using ABR is a very unlikely combination, and DOES NOT meet the
ABR service is to provide "low" loss rates. The delay objectives for service requirements of GS. The objective of the ABR service is to
ABR should be expected to be very loose. If ABR were used for GS, provide "low" loss rates. The delay objectives for ABR SHOULD be
the VC parameters would follow as for CLS over ABR. See Section 6.1. expected to be very loose. If ABR were used for GS, the VC
parameters would follow as for CLS over ABR. See Section 6.2.
5.5 Encoding GS Using UBR (ATM Forum TM/UNI 4.0) 5.5 Encoding GS Using UBR (ATM Forum TM/UNI 4.0)
The UBR service is the default lowest common denominator of the The UBR service is the lowest common denominator of the services. It
services. It cannot provide delay or loss guarantees. However if it cannot provide delay or loss guarantees, and therefore DOES NOT meet
is used for GS, it will be encoded in the same way as Best Effort the requirements of GS. However if it is used for GS, it will be
over UBR, with the exception that the PCR would be determined from encoded in the same way as Best Effort over UBR, with the exception
the peak rate of the receiver TSpec. See Section 7.1. that the Forward PCR would be determined from the peak rate of the
receiver TSpec. See Section 7.1.
5.6 Encoding GS Using ATM Forum UNI 3.0/3.1 Specifications 5.6 Encoding GS Using ATM Forum UNI 3.0/3.1 Specifications
It is not recommended to support GS using UNI 3.x VBR mode for the It is not recommended to support GS using UNI 3.x VBR mode because
following reasons. The Class C bearer class does not represent the BCOB-C Bearer Class does not represent real-time behavior. Also,
real-time behavior. Appendix F of UNI 3.1 specification precludes Appendix F of the UNI 3.1 specification precludes the specification
the specification of traffic type "VBR" with the timing requirement of traffic type "VBR" with the timing requirement "End to End timing
"End to End timing Required" in conjunction with bearer class X. Required" in conjunction with Bearer Class X.
It is possible to support GS using a CBR "pipe." The following A CBR VC MEETS the requirements of GS. The following table specifies
table specifies the support of GS using CBR. the support of GS using CBR.
AAL AAL
Type 5 Type 5
Forward CPCS-SDU Size parameter M of receiver TSpec Forward CPCS-SDU Size parameter M of rcvr TSpec + 8 Bytes
Backward CPCS-SDU Size parameter M of receiver TSpec Backward CPCS-SDU Size parameter M of rcvr TSpec + 8 Bytes
Mode 1 (Message mode) Note 1 Mode 1 (Message mode) Note 1
SSCS Type 0 (Null SSCS) SSCS Type 0 (Null SSCS)
Traffic Descriptor Traffic Descriptor
Forward PCR CLP=0 Note 2 Forward PCR CLP=0 Note 2
Backward PCR CLP=0 0 Backward PCR CLP=0 0
Forward PCR CLP=0+1 Note 2 Forward PCR CLP=0+1 Note 2
Backward PCR CLP=0+1 0 Backward PCR CLP=0+1 0
BE indicator NOT included BE indicator NOT included
Tagging Forward bit 1 (Tagging requested) Tagging Forward bit 1 (Tagging requested)
Tagging Backward bit 1 (Tagging requested) Tagging Backward bit 1 (Tagging requested)
Broadband Bearer Capability Broadband Bearer Capability
Bearer Class 16 (BCOB-X) Note 3 Bearer Class 16 (BCOB-X) Note 3
Traffic Type 001 (bit encoding for Constant Bit Traffic Type 001 (Constant Bit Rate)
Rate) Timing Requirements 01 (Timing Required)
Timing Requirements 01 (bit encoding for Timing Susceptible to Clipping 00 (Not Susceptible)
Required) User Plane Configuration 01 (Point-to-Multipoint)
Susceptible to Clipping 00 (bit encoding for Not
susceptible)
User Plane Configuration 01 (bit encoding for pt-to-mpt)
Broadband Low Layer Information Broadband Low Layer Information
User Information Layer 2 User Information Layer 2
Protocol 12 (ISO 8802/2) Protocol 12 (ISO 8802/2)
User Information Layer 3 User Information Layer 3
Protocol 11 (ISO/IEC TR 9577) Note 4 Protocol 11 (ISO/IEC TR 9577) Note 4
ISO/IEC TR 9577 IPI 204 ISO/IEC TR 9577 IPI 204
QoS Class QoS Class Note 5
QoS Class Forward 1 QoS Class Forward 1
QoS Class Backward 1 QoS Class Backward 1
QoS Parameters
Parameters are implied by the QOS Class
Note 1: Only included for UNI 3.0. Note 1: Only included for UNI 3.0.
Note 2: See discussion, Section 2.5.1. PCR CLP=0 should be set identical Note 2: See discussion in Section 2.5.1. PCR CLP=0 SHOULD be set identical
to PCR CLP=0+1. Although this could potentially allow a CBR VC to PCR CLP=0+1. Although this could potentially allow a CBR VC
to carry excess traffic as tagged cells, it is not recommended to carry excess traffic as tagged cells, it is not recommended
since it is not supported in UNI 4.0 since it is not supported in UNI 4.0
Note 3: Value 1 (BCOB-A) can also be used. If BCOB-A is used Traffic Note 3: Value 1 (BCOB-A) can also be used. If BCOB-A is used Traffic
Type and Timing Requirements fields are not included. Type and Timing Requirements fields are not included.
Note 4: For QoS VCs supporting GS or CLS, the layer 3 protocol should
Note 4: For QoS VCs supporting GS or CLS, the layer 3 protocol SHOULD
be specified. For BE VCs, it can be left unspecified, allowing be specified. For BE VCs, it can be left unspecified, allowing
the VC to be shared by multiple protocols, following RFC 1755. the VC to be shared by multiple protocols, following RFC 1755.
Note 5: QoS Parameters are implied by the QoS Class.
6.0 Summary of ATM VC Setup Parameters for Controlled Load Service 6.0 Summary of ATM VC Setup Parameters for Controlled Load Service
This section describes how to create ATM VCs appropriately matched This section describes how to create ATM VCs appropriately matched
for Controlled Load. CLS traffic is partly delay tolerant and of for Controlled Load Service. CLS traffic is partly delay tolerant
variable rate. NrtVBR and ABR (for TM/UNI 4.0 only) are the best and has variable rate. NrtVBR and ABR (TM/UNI 4.0 only) are the best
choices in supporting CLS. choices for supporting CLS.
Note, these encodings assume point to multipoint connections, where Note, these encodings assume point to multipoint connections where
the backward channel is not used. If the IP session is unicast only, the backward channel is not used. If the IP session is unicast only,
then a point-to-point VC may be used and the IWF may make use of the then a point-to-point VC may be used and the IWF may make use of the
backward channel, provided that the QoS parameters are mapped backward channel, with QoS parameters set appropriately for the
consistently for the service provided. service provided.
6.1 Encoding CLS Using ABR (ATM Forum TM/UNI 4.0) We provide a mapping for all combinations of IP service and ATM
service category, and comments indicating whether or not each
combination meets the requirements of the IP-IS service.
6.1 Encoding CLS Using Non-Real-Time VBR (ATM Forum TM/UNI 4.0)
NrtVBR MEETS the requirements for CLS.
AAL AAL
Type 5 Type 5
Forward CPCS-SDU Size parameter M of receiver TSpec Forward CPCS-SDU Size parameter M of rcvr TSpec + 8 Bytes
Backward CPCS-SDU Size parameter M of receiver TSpec Backward CPCS-SDU Size parameter M of rcvr TSpec + 8 Bytes
SSCS Type 0 (Null SSCS) SSCS Type 0 (Null SSCS)
Traffic Descriptor Traffic Descriptor
Forward PCR CLP=0+1 Note 1 Forward PCR CLP=0+1 Note 1
Backward PCR CLP=0+1 0 Backward PCR CLP=0+1 0
Forward MCR CLP=0+1 Note 1 Forward SCR CLP=0 Note 1
Backward MCR CLP=0+1 0 Backward SCR CLP=0 0
Forward MBS (CLP=0) Note 1
Backward MBS (CLP=0) 0
BE indicator NOT included BE indicator NOT included
Forward Frame Discard bit 1 Forward Frame Discard bit 1
Backward Frame Discard bit 1 Backward Frame Discard bit 1
Tagging Forward bit 0 (Tagging not requested) Tagging Forward bit 1 (Tagging requested)
Tagging Backward bit 0 (Tagging not requested) Tagging Backward bit 1 (Tagging requested)
Broadband Bearer Capability Broadband Bearer Capability
Bearer Class 16 (BCOB-X) Note 2 Bearer Class 16 (BCOB-X) Note 2
ATM Transfer Capability 12 (ABR) ATM Transfer Capability 10 (Non-real time VBR) Note 3
Traffic Type 010 (Variable Bit Rate) Susceptible to Clipping 00 (Not Susceptible)
Timing Requirements 10 (Timing Not Required) User Plane Configuration 01 (Point-to-Multipoint)
Susceptible to Clipping 00 (Not susceptible)
User Plane Configuration 00 (For pt-to-pt)
Broadband Low Layer Information Broadband Low Layer Information
User Information Layer 2 User Information Layer 2
Protocol 12 (ISO 8802/2) Protocol 12 (ISO 8802/2)
User Information Layer 3 User Information Layer 3
Protocol 11 (ISO/IEC TR 9577) Note 3 Protocol 11 (ISO/IEC TR 9577) Note 4
ISO/IEC TR 9577 IPI 204 ISO/IEC TR 9577 IPI 204
QoS Class QoS Class
QoS Class Forward 0 Note 4 QoS Class Forward 3 Note 5
QoS Class Backward 0 Note 4 QoS Class Backward 3 Note 5
QoS Parameters Note 5 Extended QoS Parameters Note 6
Acceptable Forward CDV Acceptable Forward CDV
Acceptable Forward CLR Acceptable Forward CLR
Forward Max CTD Forward Max CTD
ABR Setup Parameters Note 6 Note 1: See discussion in Section 2.5.2.
ABR Additional Parameters Note 6
Note 1: See discussion, Section 2.5.2.
Note 2: Value 3 (BCOB-C) can also be used. Note 2: Value 3 (BCOB-C) can also be used.
Note 3: For QoS VCs supporting GS or CLS, the layer 3 protocol should If Bearer Class C is used, the ATC field MUST be absent.
Note 3: The ATC value 11 is not used. The value 11 implies CLR
objective applies to the aggregate CLP=0+1 stream and
that does not give desirable treatment of excess traffic.
Note 4: For QoS VCs supporting GS or CLS, the layer 3 protocol SHOULD
be specified. For BE VCs, it can be left unspecified, allowing be specified. For BE VCs, it can be left unspecified, allowing
the VC to be shared by multiple protocols, following RFC 1755. the VC to be shared by multiple protocols, following RFC 1755.
Note 4: Cf ITU I.365 (Oct 1996) for new QoS Class definitions. Note 5: Cf ITU Rec. I.356 [20] for new QoS Class definitions.
Note 5: See Section 2.6 for the values to be used. Note 6: See discussion in Section 2.6.
Note 6: The ABR-specific parameters are beyond the scope of this document.
These generally depend on local implementation and not on values 6.2 Encoding CLS Using ABR (ATM Forum TM/UNI 4.0)
mapped from IP level service parameters (with the exception of MCR).
See [6, 11] for further information. ABR MEETS the requirements for CLS when MCR is set to the CLS TSpec
rate.
6.2 Encoding CLS Using Non-Real-Time VBR (ATM Forum TM/UNI 4.0)
AAL AAL
Type 5 Type 5
Forward CPCS-SDU Size parameter M of receiver TSpec Forward CPCS-SDU Size parameter M of rcvr TSpec + 8 Bytes
Backward CPCS-SDU Size parameter M of receiver TSpec Backward CPCS-SDU Size parameter M of rcvr TSpec + 8 Bytes
SSCS Type 0 (Null SSCS) SSCS Type 0 (Null SSCS)
Traffic Descriptor Traffic Descriptor
Forward PCR CLP=0+1 Note 1 Forward PCR CLP=0+1 Note 1
Backward PCR CLP=0+1 0 Backward PCR CLP=0+1 0
Forward SCR CLP=0 Note 1 Forward MCR CLP=0+1 Note 1
Backward SCR CLP=0 0 Backward MCR CLP=0+1 0
Forward MBS (CLP=0) Note 1
Backward MBS (CLP=0) 0
BE indicator NOT included BE indicator NOT included
Forward Frame Discard bit 1 Forward Frame Discard bit 1
Backward Frame Discard bit 1 Backward Frame Discard bit 1
Tagging Forward bit 1 (Tagging requested) Tagging Forward bit 0 (Tagging not requested)
Tagging Backward bit 1 (Tagging requested) Tagging Backward bit 0 (Tagging not requested)
Broadband Bearer Capability Broadband Bearer Capability
Bearer Class 16 (BCOB-X) Note 2 Bearer Class 16 (BCOB-X) Note 2
ATM Transfer Capability 10 (Non-real time VBR) Note 3, 4 ATM Transfer Capability 12 (ABR)
Susceptible to Clipping 00 (bit encoding Not susceptible) Susceptible to Clipping 00 (Not Susceptible)
User Plane Configuration 01 (bit encoding pt-to-mpt) User Plane Configuration 00 (Point-to-Point)
Broadband Low Layer Information Broadband Low Layer Information
User Information Layer 2 User Information Layer 2
Protocol 12 (ISO 8802/2) Protocol 12 (ISO 8802/2)
User Information Layer 3 User Information Layer 3
Protocol 11 (ISO/IEC TR 9577) Note 5 Protocol 11 (ISO/IEC TR 9577) Note 3
ISO/IEC TR 9577 IPI 204 ISO/IEC TR 9577 IPI 204
QoS Class QoS Class
QoS Class Forward 3 Note 6 QoS Class Forward 0 Note 4
QoS Class Backward 3 Note 6 QoS Class Backward 0 Note 4
QoS Parameters Note 7 Extended QoS Parameters Note 5
Acceptable Forward CDV Acceptable Forward CDV
Acceptable Forward CLR Acceptable Forward CLR
Forward Max CTD Forward Max CTD
Note 1: See discussion, Section 2.5.2. ABR Setup Parameters Note 6
ABR Additional Parameters Note 6
Note 1: See discussion in Section 2.5.2.
Note 2: Value 3 (BCOB-C) can also be used. Note 2: Value 3 (BCOB-C) can also be used.
Note 3: If bearer class C is used, the ATC field must be absent If Bearer Class C is chosen the ATC field MUST be absent.
Note 4: The ATC value 11 is not used. The value 11 implies CLR Note 3: For QoS VCs supporting GS or CLS, the layer 3 protocol SHOULD
objective applies to the aggregate CLP=0+1 stream and
that does not give desirable treatment of excess
traffic in the case of IP.
Note 5: For QoS VCs supporting GS or CLS, the layer 3 protocol should
be specified. For BE VCs, it can be left unspecified, allowing be specified. For BE VCs, it can be left unspecified, allowing
the VC to be shared by multiple protocols, following RFC 1755. the VC to be shared by multiple protocols, following RFC 1755.
Note 6: Cf ITU I.365 (Oct 1996) for new QoS Class definitions. Note 4: Cf ITU Rec. I.356 [20] for new QoS Class definitions.
Note 7: See Section 2.6 for the values to be used. Note 5: See discussion in Section 2.6.
Note 6: The ABR-specific parameters are beyond the scope of this document.
6.3 Encoding CLS Using Real-Time VBR (ATM Forum TM/UNI 4.0) These generally depend on local implementation and not on values
mapped from IP level service parameters (except for MCR).
The encoding of CLS using rtVBR imposes a hard limit on the delay, See [6, 11] for further information.
which is specified as an end-to-end delay in the ATM network. This
is more stringent than the CLS service requires.
If rtVBR is used to encode CLS, then the encoding is essentially the
same as that for GS. See Section 5.1 and discussion in Section
2.5.2.
6.4 Encoding CLS Using CBR (ATM Forum TM/UNI 4.0) 6.3 Encoding CLS Using CBR (ATM Forum TM/UNI 4.0)
Although CBR does not explicitly take into account the variable rate Although CBR does not explicitly take into account the variable rate
of source data, it may be convenient to use ATM connectivity between of source data, it may be convenient to use ATM connectivity between
edge routers to provide a simple "pipe" service, as a leased line edge routers to provide a simple "pipe" service, as a leased line
replacement. replacement. Since no tagging option is available with CBR, excess
traffic MUST be handled using a separate VC. Under this condition,
CBR MEETS the requirements of CLS.
To use CBR for CLS, the same encoding for GS over CBR (Section 5.2 To use CBR for CLS, the same encoding for GS over CBR (Section 5.2)
would be used. See Section 2.5.2. would be used. See discussion in Section 2.5.2.
6.4 Encoding CLS Using Real-Time VBR (ATM Forum TM/UNI 4.0)
The encoding of CLS using rtVBR implies a hard limit on the end-to-
end delay in the ATM network. This creates more complexity in the VC
setup than the CLS service requires, and is therefore not a preferred
combination, although it DOES MEET the requirements of CLS.
If rtVBR is used to encode CLS, then the encoding is essentially the
same as that for GS. See discussions in Section 5.1 and Section
2.5.2.
6.5 Encoding CLS Using UBR (ATM Forum TM/UNI 4.0) 6.5 Encoding CLS Using UBR (ATM Forum TM/UNI 4.0)
This encoding gives no QoS guarantees. If used, it is coded in the This encoding gives no QoS guarantees and DOES NOT MEET the
same way as for BE over UBR, except that the PCR would be determined requirements of CLS. If used, it is coded in the same way as for BE
from the peak rate of the receiver TSpec. See Section 7.1. over UBR (Section 7.1), except that the PCR would be determined from
the peak rate of the receiver TSpec.
6.6 Encoding CLS Using ATM Forum UNI 3.0/3.1 Specifications 6.6 Encoding CLS Using ATM Forum UNI 3.0/3.1 Specifications
This encoding is equivalent to the nrtVBR service category.
This encoding is equivalent to the nrtVBR service category. It MEETS
the requirements of CLS.
AAL AAL
Type 5 Type 5
Forward CPCS-SDU Size parameter M of receiver TSpec Forward CPCS-SDU Size parameter M of rcvr TSpec + 8 Bytes
Backward CPCS-SDU Size 0 Backward CPCS-SDU Size parameter M of rcvr TSpec + 8 Bytes
Mode 1 (Message mode) Note 1 Mode 1 (Message mode) Note 1
SSCS Type 0 (Null SSCS) SSCS Type 0 (Null SSCS)
Traffic Descriptor Traffic Descriptor
Forward PCR CLP=0+1 Note 2 Forward PCR CLP=0+1 Note 2
Backward PCR CLP=0+1 0 Backward PCR CLP=0+1 0
Forward SCR CLP=0 Note 2 Forward SCR CLP=0 Note 2
Backward SCR CLP=0 0 Backward SCR CLP=0 0
Forward MBS (CLP=0) Note 2 Forward MBS (CLP=0) Note 2
Backward MBS (CLP=0) 0 Backward MBS (CLP=0) 0
BE indicator NOT included BE indicator NOT included
Tagging Forward bit 1 (Tagging requested) Tagging Forward bit 1 (Tagging requested)
Tagging Backward bit 1 (Tagging requested) Tagging Backward bit 1 (Tagging requested)
Broadband Bearer Capability Broadband Bearer Capability
Bearer Class 16 (BCOB-X) Note 3 Bearer Class 16 (BCOB-X) Note 3
Traffic Type 010 (bit encoding for Variable Bit Traffic Type 010 (Variable Bit Rate)
Rate) Timing Requirements 00 (No Indication)
Timing Requirements 00 (bit encoding for No Indication) Susceptible to Clipping 00 (Not Susceptible)
Susceptible to Clipping 00 (bit encoding for Not User Plane Configuration 01 (Point-to-Multipoint)
susceptible)
User Plane Configuration 01 (bit encoding for For pt-to-mpt)
Broadband Low Layer Information Broadband Low Layer Information
User Information Layer 2 User Information Layer 2
Protocol 12 (ISO 8802/2) Protocol 12 (ISO 8802/2)
User Information Layer 3 User Information Layer 3
Protocol 11 (ISO/IEC TR 9577) Note 4 Protocol 11 (ISO/IEC TR 9577) Note 4
ISO/IEC TR 9577 IPI 204 ISO/IEC TR 9577 IPI 204
QoS Class QoS Class
QoS Class Forward 3 QoS Class Forward 3 Note 5
QoS Class Backward 3 QoS Class Backward 3 Note 5
QoS Parameters
Parameters are implied by the QOS Class
Note 1: Only included for UNI 3.0. Note 1: Only included for UNI 3.0.
Note 2: See discussion, Section 2.5.2. Note 2: See discussion in Section 2.5.2.
Note 3: Value 3 (BCOB-C) can also be used. If BCOB-C is used Traffic Note 3: Value 3 (BCOB-C) can also be used. If BCOB-C is used Traffic
Type and Timing Requirements fields are not included. Type and Timing Requirements fields are not included.
Note 4: For QoS VCs supporting GS or CLS, the layer 3 protocol should Note 4: For QoS VCs supporting GS or CLS, the layer 3 protocol SHOULD
be specified. For BE VCs, it can be left unspecified, allowing be specified. For BE VCs, it can be left unspecified, allowing
the VC to be shared by multiple protocols, following RFC 1755. the VC to be shared by multiple protocols, following RFC 1755.
Note 5: Cf ITU Rec. I.356 [20] for new QoS Class definitions.
QoS Parameters are implied by the QoS Class.
7.0 Summary of ATM VC Setup Parameters for Best Effort Service 7.0 Summary of ATM VC Setup Parameters for Best Effort Service
This section should be considered informational only. RFC 1755 [10] and This section is provided for completeness only. The IETF ION working
the IETF ION working group draft on ATM signalling support for IP over group documents on ATM signalling support for IP over ATM [10, 11]
ATM using UNI 4.0 [11] provide more definitive specification of Best provide definitive specifications for Best Effort IP service over
Effort IP service over ATM. ATM.
The best-matched ATM service category to IP Best Effort is UBR. We The best-matched ATM service category to IP Best Effort is UBR. We
provide the setup details for this case below. The BE service does not provide the setup details for this case below. The BE service does
require reservation of resources. not involve reservation of resources. ABR and nrtVBR are also well
suited to BE service. See discussion in Section 2.1.3.
Note, VCs supporting best effort service are usually point to point, Note, VCs supporting best effort service are usually point to point,
rather than point to multipoint, and the backward channels of VCs are rather than point to multipoint, and the backward channels of VCs are
used. In specific cases where VCs are set up to support best effort used. In cases where VCs are set up to support best effort multicast
multicast sessions, multipoint VCs can be used and the backward channels sessions, multipoint VCs can be used and the backward channels would
would be not have resources reserved. Related situations include be not have resources reserved. Related situations include transport
transport of excess traffic on IP-multicast QoS sessions, or to support of excess traffic on IP-multicast QoS sessions, or to support the
the subset of multicast end systems that have not made rsvp subset of multicast end systems that have not made rsvp reservations.
reservations. See the discussion on VC management in [12].
7.1 Encoding Best Effort Service Using UBR (ATM Forum TM/UNI 4.0) 7.1 Encoding Best Effort Service Using UBR (ATM Forum TM/UNI 4.0)
AAL AAL
Type 5 Type 5
Forward CPCS-SDU Size 9180 (default MTU for AAL5) Forward CPCS-SDU Size 9188 Bytes (default MTU for AAL-5)
Backward CPCS-SDU Size 9180 (default MTU for AAL5) Backward CPCS-SDU Size 9188 Bytes (default MTU for AAL-5)
SSCS Type 0 (Null SSCS) SSCS Type 0 (Null SSCS)
Traffic Descriptor Traffic Descriptor
Forward PCR CLP=0+1 Note 1 Forward PCR CLP=0+1 Note 1
Backward PCR CLP=0+1 0 Backward PCR CLP=0+1 0
BE indicator included BE indicator included
Forward Frame Discard bit 1 Forward Frame Discard bit 1
Backward Frame Discard bit 1 Backward Frame Discard bit 1
Tagging Forward bit 1 (Tagging requested) Tagging Forward bit 1 (Tagging requested)
Tagging Backward bit 1 (Tagging requested) Tagging Backward bit 1 (Tagging requested)
Broadband Bearer Capability Broadband Bearer Capability
Bearer Class 16 (BCOB-X) Note 2 Bearer Class 16 (BCOB-X) Note 2
ATM Transfer Capability 10 (Non-real time VBR) Note 3 ATM Transfer Capability 10 (Non-real time VBR)
Susceptible to Clipping 00 (bit encoding for Not susceptible) Susceptible to Clipping 00 (Not Susceptible)
User Plane Configuration 01 (bit encoding for pt-to-mpt) User Plane Configuration 01 (Point-to-Multipoint)
Broadband Low Layer Information Broadband Low Layer Information
User Information Layer 2 User Information Layer 2
Protocol 12 (ISO 8802/2) Note 4 Protocol 12 (ISO 8802/2) Note 3
QoS Class QoS Class
QoS Class Forward 0 QoS Class Forward 0
QoS Class Backward 0 QoS Class Backward 0
Note 1: See discussion, Section 2.5.3. Note 1: See discussion in Section 2.5.3.
Note 2: Value 3 (BCOB-C) can also be used. Note 2: Value 3 (BCOB-C) can also be used.
Note 3: If bearer class C is used, the ATC field must be absent If Bearer Class C is used, the ATC field MUST be absent
Note 4: For QoS VCs supporting GS or CLS, the layer 3 protocol should Note 3: For QoS VCs supporting GS or CLS, the layer 3 protocol SHOULD
be specified. For BE VCs, it can be left unspecified, allowing be specified. For BE VCs, it can be left unspecified, allowing
the VC to be shared by multiple protocols, following RFC 1755. the VC to be shared by multiple protocols, following RFC 1755.
8.0 Security 8.0 Security Considerations
Some security issues are raised in the rsvp specification [2], which IP Integrated Services (including rsvp) and ATM are both complex
would apply here as well. There are no additional security resource reservation protocols, and SHOULD be expected to have
considerations raised in this document. complex feature interactions.
Differences in IP and ATM billing styles could cause unforeseen
problems since RESV messages can set up VCs. For example, an end-
user paying a flat rate for (non-rsvp aware) internet service may
send an rsvp RESV message that encounters a (perhaps distant) ATM
network with a usage-sensitive billing model. Insufficient
authentication could result in services being accidentally billed to
an innocent third party, intentional theft of service, or malicious
denial of service attacks where high volumes of reservations consume
transport or processing resources at the edge devices.
The difference in styles of handling excess traffic could result in
denial of service attacks where the ATM network uses transport
resources (bandwidth, buffers) or connection processing resources
(switch processor cycles) in an attempt to accommodate excess traffic
that was admitted by the internet service.
Problems associated with translation of resource reservations at edge
devices are probably more complex and susceptible to abuse when the
IP-ATM edge is also an administrative boundary between service
providers. Note also that administrative boundaries can exist within
the ATM cloud, i.e., the ingress and egress edge devices are operated
by different service providers.
Note, the ATM Forum Security Working Group is currently defining
ATM-level security features such as data encryption and signalling
authentication. See also the security issues raised in the rsvp
specification [2].
9.0 Acknowledgements 9.0 Acknowledgements
The authors would like to thank the members of the ISSLL working The authors received much useful input from the members of the ISSLL
group for their input. In particular, thanks to Drew Perkins and Jon working group. In particular, thanks to Drew Perkins and Jon Bennett
Bennett of Fore Systems, Roch Guerin of IBM, Susan Thomson and Sudha of Fore Systems, Roch Guerin of IBM, Susan Thomson and Sudha Ramesh
Ramesh of Bellcore. of Bellcore.
Appendix 1 Abbreviations Appendix 1 Abbreviations
AAL ATM Adaptation Layer AAL ATM Adaptation Layer
ABR Available Bit Rate ABR Available Bit Rate
APB Available Path Bandwidth (int-serv GCP) APB Available Path Bandwidth (int-serv GCP)
ATC ATM Transfer Capability
ATM Asynchronous Transfer Mode ATM Asynchronous Transfer Mode
B-LLI Broadband Low Layer Information B-LLI Broadband Low Layer Information
BCOB Broadband Connection-Oriented Bearer Capability BCOB Broadband Connection-Oriented Bearer Capability
BCOB-{A,C,X} Bearer Class A, C, or X BCOB-{A,C,X} Bearer Class A, C, or X
BE Best Effort BE Best Effort
BT Burst Tolerance BT Burst Tolerance
CBR Constant Bit Rate CBR Constant Bit Rate
CDV Cell Delay Variation CDV Cell Delay Variation
CDVT Cell Delay Variation Tolerance CDVT Cell Delay Variation Tolerance
CLP Cell Loss Priority (bit) CLP Cell Loss Priority (bit)
CLR Cell Loss Ratio CLR Cell Loss Ratio
CLS Controlled Load Service CLS Controlled Load Service
CPCS Common Part Convergence Sublayer CPCS Common Part Convergence Sublayer
CTD Cell Transfer Delay CTD Cell Transfer Delay
EOM End of Message EOM End of Message
FFS For Further Study
GCP General Characterization Parameter GCP General Characterization Parameter
GCRA Generic Cell Rate Algorithm GCRA Generic Cell Rate Algorithm
GS Guaranteed Service GS Guaranteed Service
IE Information Element IE Information Element
IETF Internet Engineering Task Force IETF Internet Engineering Task Force
ION IP Over Non-broadcast multiple access networks
IP Internet Protocol IP Internet Protocol
IPI Initial Protocol Identifier IPI Initial Protocol Identifier
IS Integrated Services IS Integrated Services
ISSLL Integrated Services over Specific Link Layers ISSLL Integrated Services over Specific Link Layers
ITU International Telecommunication Union ITU International Telecommunication Union
IWF Interworking Function IWF Interworking Function
LIJ Leaf Initiated Join LIJ Leaf Initiated Join
LLC Logical Link Control LLC Logical Link Control
MBS Maximum Burst Size MBS Maximum Burst Size
MCR Minimum Cell Rate MCR Minimum Cell Rate
MPL Minimum Path Latency MPL Minimum Path Latency
MTU Maximum Transfer Unit MTU Maximum Transfer Unit
nrtVBR Non-real-time VBR nrtVBR Non-real-time VBR
PCR Peak Cell Rate PCR Peak Cell Rate
PDU Protocol Data Unit PDU Protocol Data Unit
PVC Permanent Virtual Connection PVC Permanent Virtual Connection
QoS Quality of Service QoS Quality of Service
RESV Reservation Message (of rsvp protocol) RESV Reservation Message (of rsvp protocol)
RFC Request for Comment RFC Request for Comments
RSVP Resource Reservation Protocol RSVP Resource Reservation Protocol
RSpec Reservation Specification RSpec Reservation Specification
rtVBR Real-time VBR rtVBR Real-time VBR
SCR Sustained Cell Rate SCR Sustainable Cell Rate
SDU Service Data Unit SDU Service Data Unit
SNAP Subnetwork Attachment Point SNAP Subnetwork Attachment Point
SSCS Service-Specific Convergence Sub-layer SSCS Service-Specific Convergence Sub-layer
SVC Switched Virtual Connection SVC Switched Virtual Connection
Sw Switch
TCP Transport Control Protocol TCP Transport Control Protocol
TM Traffic Management TM Traffic Management
TSpec Traffic Specification TSpec Traffic Specification
UBR Unspecified Bit Rate UBR Unspecified Bit Rate
UNI User-Network Interface UNI User-Network Interface
UPC Usage Parameter Control (ATM traffic policing function) UPC Usage Parameter Control (ATM traffic policing function)
VBR Variable Bit Rate VBR Variable Bit Rate
VC (ATM) Virtual Connection VC (ATM) Virtual Connection
REFERENCES REFERENCES
skipping to change at page 41, line 36 skipping to change at page 41, line 43
"Resource ReSerVation Protocol (RSVP) - Version 1 Functional "Resource ReSerVation Protocol (RSVP) - Version 1 Functional
Specification", RFC 2205, September 1997. Specification", RFC 2205, September 1997.
[3] The ATM Forum, "ATM User-Network Interface Specification, Ver- [3] The ATM Forum, "ATM User-Network Interface Specification, Ver-
sion 3.0", Prentice Hall, Englewood Cliffs NJ, 1993. sion 3.0", Prentice Hall, Englewood Cliffs NJ, 1993.
[4] The ATM Forum, "ATM User-Network Interface Specification, Ver- [4] The ATM Forum, "ATM User-Network Interface Specification, Ver-
sion 3.1", Prentice Hall, Upper Saddle River NJ, 1995. sion 3.1", Prentice Hall, Upper Saddle River NJ, 1995.
[5] The ATM Forum, "ATM User-Network Interface (UNI) Signalling [5] The ATM Forum, "ATM User-Network Interface (UNI) Signalling
Specification, Version 4.0", July 1996. Publication by Prentice Specification, Version 4.0", July 1996. Available at
Hall pending; available at ftp://ftp.atmforum.com/pub/approved- ftp://ftp.atmforum.com/pub/approved-specs/af-sig-0061.000.ps.
specs/af-sig-0061.000.ps.
[6] The ATM Forum, "ATM Traffic Management Specification, Version [6] The ATM Forum, "ATM Traffic Management Specification, Version
4.0", April 1996. Publication by Prentice Hall pending, avail- 4.0", April 1996. Available at
able at ftp://ftp.atmforum.com/pub/approved-specs/af-tm- ftp://ftp.atmforum.com/pub/approved-specs/af-tm-0056.000.ps.
0056.000.ps.
[7] M. W. Garrett, "A Service Architecture for ATM: From Applica- [7] M. W. Garrett, "A Service Architecture for ATM: From
tions to Scheduling", IEEE Network Mag., Vol. 10, No. 3, pp. 6- Applications to Scheduling", IEEE Network Mag., Vol. 10, No. 3,
14, May 1996. pp. 6-14, May 1996.
[8] S. Shenker, C. Partridge and R. Guerin, "Specification of [8] S. Shenker, C. Partridge and R. Guerin, "Specification of
Guaranteed Quality of Service", RFC 2212, September 1997. Guaranteed Quality of Service", RFC 2212, September 1997.
[9] J. Wroclawski, "Specification of the Controlled-Load Network [9] J. Wroclawski, "Specification of the Controlled-Load Network
Element Service", RFC 2211, September 1997. Element Service", RFC 2211, September 1997.
[10] M. Perez, F. Liaw, A. Mankin, E. Hoffman, D. Grossman and A. [10] M. Perez, F. Liaw, A. Mankin, E. Hoffman, D. Grossman and A.
Malis, "ATM Signaling Support for IP over ATM", RFC 1755, Febru- Malis, "ATM Signaling Support for IP over ATM", RFC 1755, Febru-
ary 1995. ary 1995.
skipping to change at page 42, line 22 skipping to change at page 42, line 29
[11] M. Perez and A. Mankin, "ATM Signaling Support for IP over ATM [11] M. Perez and A. Mankin, "ATM Signaling Support for IP over ATM
UNI 4.0 Update", Internet Draft, October 1997. <draft-ietf- UNI 4.0 Update", Internet Draft, October 1997. <draft-ietf-
ion-sig-uni4.0-05.txt> ion-sig-uni4.0-05.txt>
[12] E. Crawley, L. Berger, S. Berson, F. Baker, M. Borden, J. [12] E. Crawley, L. Berger, S. Berson, F. Baker, M. Borden, J.
Krawczyk, "A Framework for Integrated Services and RSVP over Krawczyk, "A Framework for Integrated Services and RSVP over
ATM", Internet Draft, July 1997. <draft-ietf-issll-atm- ATM", Internet Draft, July 1997. <draft-ietf-issll-atm-
framework-00.txt> framework-00.txt>
[13] L. Berger, "RSVP over ATM Implementation Requirements", Internet [13] L. Berger, "RSVP over ATM Implementation Requirements", Internet
Draft, July 1997. <draft-ietf-issll-atm-imp-req-00.txt> Draft, January 1998. <draft-ietf-issll-atm-imp-req-02.txt>
[14] L. Berger, "RSVP over ATM Implementation Guidelines", Internet [14] L. Berger, "RSVP over ATM Implementation Guidelines", Internet
Draft, July 1997. <draft-ietf-issll-imp-guide-01.txt> Draft, January 1998. <draft-ietf-issll-imp-guide-03.txt>
[15] S. Shenker and J. Wroclawski, "Network Element Service Specifi- [15] S. Shenker and J. Wroclawski, "Network Element Service Specifi-
cation Template", RFC 2216, September 1997. cation Template", RFC 2216, September 1997.
[16] J. Wroclawski, "The Use of RSVP with IETF Integrated Services", [16] J. Wroclawski, "The Use of RSVP with IETF Integrated Services",
RFC 2210, September 1997. RFC 2210, September 1997.
[17] M. Borden, E. Crawley, B. Davie and S. Batsell, "Integration of [17] M. Borden, E. Crawley, B. Davie and S. Batsell, "Integration of
Real-time Services in an IP-ATM Network Architecture", RFC 1821, Real-time Services in an IP-ATM Network Architecture", RFC 1821,
August 1995. August 1995.
[18] J. Heinanen, "Multiprotocol Encapsulation over ATM Adaptation [18] J. Heinanen, "Multiprotocol Encapsulation over ATM Adaptation
Layer 5", RFC 1483, July 1993. Layer 5", RFC 1483, July 1993.
[19] M. Laubach, "Classical IP and ARP over ATM", RFC 1577, January [19] M. Laubach, "Classical IP and ARP over ATM", RFC 1577, January
1994. 1994.
[20] A. Romanow, S. Floyd, "Dynamics of TCP Traffic over ATM Net- [20] ITU Recommendation I.356, "B-ISDN ATM layer cell transfer per-
formance", International Telecommunication Union, Geneva,
October 1996.
[21] A. Romanow, S. Floyd, "Dynamics of TCP Traffic over ATM Net-
works", IEEE J. Sel. Areas in Commun., Vol. 13, No. 4, pp. 633- works", IEEE J. Sel. Areas in Commun., Vol. 13, No. 4, pp. 633-
-41, May 1995, 41, May 1995.
[21] S. Floyd, V. Jacobson, "Link-sharing and Resource Management [22] A. K. Parekh, R. G. Gallager, "A Generalized Processor Sharing
Approach to Flow Control in Integrated Services Networks: The
Multiple Node Case", IEEE/ACM Trans. Networking, Vol. 2, No. 2,
pp. 137-150, April 1994.
[23] S. Floyd, V. Jacobson, "Link-sharing and Resource Management
Models for Packet Networks", IEEE/ACM Trans. Networking, Vol. 3, Models for Packet Networks", IEEE/ACM Trans. Networking, Vol. 3,
No. 4, August 1995. No. 4, August 1995.
[22] S. Shenker and J. Wroclawski, "General Characterization Parame- [24] S. Shenker and J. Wroclawski, "General Characterization Parame-
ters for Integrated Service Network Elements", RFC 2215, Sep- ters for Integrated Service Network Elements", RFC 2215, Sep-
tember 1997. tember 1997.
Authors' Addresses Authors' Addresses
Mark W. Garrett Marty Borden Mark W. Garrett Marty Borden
Bellcore New Oak Communications, Inc. Bellcore Bay Networks
445 South Street 42 Nagog Park 445 South Street 42 Nagog Park
Morristown, NJ 07960 Acton MA, 01720 Morristown, NJ 07960 Acton MA, 01720
USA USA USA USA
phone: +1 201 829-4439 phone: +1 508 266-1011 phone: +1 201 829-4439 phone: +1 508 266-1011
email: mwg@bellcore.com email: mborden@newoak.com email: mwg@bellcore.com email: mborden@baynetworks.com
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