draft-ietf-ippm-bw-capacity-04.txt   draft-ietf-ippm-bw-capacity-05.txt 
IP Performance Metrics Working P. Chimento IP Performance Metrics Working P. Chimento
Group JHU Applied Physics Lab Group JHU Applied Physics Lab
Internet-Draft J. Ishac Internet-Draft J. Ishac
Expires: June 2, 2007 NASA Glenn Research Center Intended status: Informational NASA Glenn Research Center
November 29, 2006 Expires: December 1, 2007 May 30, 2007
Defining Network Capacity Defining Network Capacity
draft-ietf-ippm-bw-capacity-04 draft-ietf-ippm-bw-capacity-05
Status of this Memo Status of this Memo
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Copyright Notice Copyright Notice
Copyright (C) The Internet Society (2006). Copyright (C) The IETF Trust (2007).
Abstract Abstract
Measuring capacity is a task that sounds simple, but in reality can Measuring capacity is a task that sounds simple, but in reality can
be quite complex. In addition, the lack of a unified nomenclature on be quite complex. In addition, the lack of a unified nomenclature on
this subject makes it increasingly difficult to properly build, test, this subject makes it increasingly difficult to properly build, test,
and use techniques and tools built around these constructs. This and use techniques and tools built around these constructs. This
document provides definitions for the terms 'Capacity' and 'Available document provides definitions for the terms 'Capacity' and 'Available
Capacity' related to IP traffic traveling between a source and Capacity' related to IP traffic traveling between a source and
destination in an IP network. By doing so, we hope to provide a destination in an IP network. By doing so, we hope to provide a
common framework for the discussion and analysis of a diverse set of common framework for the discussion and analysis of a diverse set of
current and future estimation techniques. current and future estimation techniques.
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.1 Links and Paths . . . . . . . . . . . . . . . . . . . . . 5
2.2 Definition: Nominal Physical Link Capacity . . . . . . . . 5
2.3 Capacity at the IP Layer . . . . . . . . . . . . . . . . . 5
2.3.1 Definition: IP Layer Bits . . . . . . . . . . . . . . 6
2.3.1.1 Standard or Correctly Formed Packets . . . . . . . 6
2.3.2 Definition: IP Layer Link Capacity . . . . . . . . . . 7
2.3.3 Definition: IP Layer Path Capacity . . . . . . . . . . 7
2.3.4 Definition: IP Layer Link Usage . . . . . . . . . . . 7
2.3.5 Definition: Average IP Layer Link Utilization . . . . 8
2.3.6 Definition: IP Layer Available Link Capacity . . . . . 8
2.3.7 Definition: IP Layer Available Path Capacity . . . . . 8
3. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . 9 2. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . 6
3.1 Time and Sampling . . . . . . . . . . . . . . . . . . . . 9 2.1. Links and Paths . . . . . . . . . . . . . . . . . . . . . 6
3.2 Hardware Duplicates . . . . . . . . . . . . . . . . . . . 9 2.2. Definition: Nominal Physical Link Capacity . . . . . . . . 6
3.3 Other Potential Factors . . . . . . . . . . . . . . . . . 9 2.3. Capacity at the IP Layer . . . . . . . . . . . . . . . . . 6
3.4 Common Literature Terminology . . . . . . . . . . . . . . 10 2.3.1. Definition: IP Layer Bits . . . . . . . . . . . . . . 7
3.5 Comparison to Bulk Transfer Capacity (BTC) . . . . . . . . 10 2.3.1.1. Standard or Correctly Formed Packets . . . . . . . 7
3.6 Type P Packets . . . . . . . . . . . . . . . . . . . . . . 11 2.3.1.2. Type P Packets . . . . . . . . . . . . . . . . . . 8
2.3.2. Definition: IP-type-P Link Capacity . . . . . . . . . 8
2.3.3. Definition: IP-type-P Path Capacity . . . . . . . . . 9
2.3.4. Definition: IP-type-P Link Usage . . . . . . . . . . . 9
2.3.5. Definition: IP-type-P Link Utilization . . . . . . . . 9
2.3.6. Definition: IP-type-P Available Link Capacity . . . . 9
2.3.7. Definition: IP-type-P Available Path Capacity . . . . 10
4. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . 12 3. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . 11
3.1. Time and Sampling . . . . . . . . . . . . . . . . . . . . 11
3.2. Hardware Duplicates . . . . . . . . . . . . . . . . . . . 11
3.3. Other Potential Factors . . . . . . . . . . . . . . . . . 11
3.4. Common Literature Terminology . . . . . . . . . . . . . . 12
3.5. Comparison to Bulk Transfer Capacity (BTC) . . . . . . . . 12
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 13 4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 14
6. Security Considerations . . . . . . . . . . . . . . . . . . . 14 5. Security Considerations . . . . . . . . . . . . . . . . . . . 15
7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 15 6. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . 16
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 16 7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 17
8.1 Normative References . . . . . . . . . . . . . . . . . . . 16
8.2 Informative References . . . . . . . . . . . . . . . . . . 16
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 16 8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 18
8.1. Normative References . . . . . . . . . . . . . . . . . . . 18
8.2. Informative References . . . . . . . . . . . . . . . . . . 18
Intellectual Property and Copyright Statements . . . . . . . . 18 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 19
Intellectual Property and Copyright Statements . . . . . . . . . . 20
1. Introduction 1. Introduction
Measuring the capacity of a link or network path is a task that Measuring the capacity of a link or network path is a task that
sounds simple, but in reality can be quite complex. Any physical sounds simple, but in reality can be quite complex. Any physical
medium requires that information be encoded and, depending on the medium requires that information be encoded and, depending on the
medium, there are various schemes to convert information into a medium, there are various schemes to convert information into a
sequence of signals that are transmitted physically from one location sequence of signals that are transmitted physically from one location
to another. to another.
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various techniques and tools. various techniques and tools.
We are interested in information-carrying capacity, but even this is We are interested in information-carrying capacity, but even this is
not straightforward. Each of the layers, depending on the medium, not straightforward. Each of the layers, depending on the medium,
adds overhead to the task of carrying information. The wired adds overhead to the task of carrying information. The wired
Ethernet uses Manchester coding or 4/5 coding which cuts down Ethernet uses Manchester coding or 4/5 coding which cuts down
considerably on the "theoretical" capacity. Similarly RF (radio considerably on the "theoretical" capacity. Similarly RF (radio
frequency) communications will often add redundancy to the coding frequency) communications will often add redundancy to the coding
scheme to implement forward error correction because the physical scheme to implement forward error correction because the physical
medium (air) is lossy. This can further decrease the information medium (air) is lossy. This can further decrease the information
efficiency. capacity.
In addition to coding schemes, usually the physical layer and the In addition to coding schemes, usually the physical layer and the
link layer add framing bits for multiplexing and control purposes. link layer add framing bits for multiplexing and control purposes.
For example, on SONET there is physical layer framing and typically For example, on SONET there is physical layer framing and typically
also some layer 2 framing such as HDLC, PPP or ATM. also some layer 2 framing such as HDLC, PPP or ATM.
Aside from questions of coding efficiency, there are issues of how Aside from questions of coding efficiency, there are issues of how
access to the channel is controlled which also may affect the access to the channel is controlled which also may affect the
capacity. For example, a multiple-access medium with collision capacity. For example, a multiple-access medium with collision
detection, avoidance and recovery mechanisms has a varying capacity detection, avoidance and recovery mechanisms has a varying capacity
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whatever layer may in fact provide a skewed picture (either whatever layer may in fact provide a skewed picture (either
optimistic or pessimistic) of what is actually available. optimistic or pessimistic) of what is actually available.
2. Definitions 2. Definitions
In this section, we specify definitions for capacity. We begin by In this section, we specify definitions for capacity. We begin by
first defining "link" and "path" clearly and then we define a first defining "link" and "path" clearly and then we define a
baseline capacity that is simply tied to the physical properties of baseline capacity that is simply tied to the physical properties of
the link. the link.
2.1 Links and Paths 2.1. Links and Paths
To define capacity, we need to broaden the notions of link and path To define capacity, we need to broaden the notions of link and path
found in the IPPM framework document [RFC2330] to include network found in the IPPM framework document [RFC2330] to include network
devices that can impact IP capacity without being IP aware. In devices that can impact IP capacity without being IP aware. For
example, consider an Ethernet switch that can operate ports at example, consider an Ethernet switch that can operate ports at
different speeds. different speeds.
We define nodes as hosts, routers, Ethernet switches, or any other We define nodes as hosts, routers, Ethernet switches, or any other
device where the input and output links have different device where the input and output links can have different
characteristics. A link is a connection between two of these network characteristics. A link is a connection between two of these network
devices or nodes. We then define a path P of length n as a series of devices or nodes. We then define a path P of length n as a series of
links (L1, L2, ..., Ln) connecting a sequence of nodes (N1, N2, ..., links (L1, L2, ..., Ln) connecting a sequence of nodes (N1, N2, ...,
Nn+1). A source, S, and destination, D, reside at N1 and Nn+1 Nn+1). A source, S, and destination, D, reside at N1 and Nn+1
respectively. Furthermore, we define a link L as a special case respectively. Furthermore, we define a link L as a special case
where the path size is one. where the path length is one.
2.2 Definition: Nominal Physical Link Capacity 2.2. Definition: Nominal Physical Link Capacity
Nominal Physical Link Capacity, NomCap(L), is the theoretical maximum Nominal Physical Link Capacity, NomCap(L), is the theoretical maximum
amount of data that the link L can support. For example, an OC-3 amount of data that the link L can support. For example, an OC-3
link would be capable of 155.520 Mbps. We stress that this is a link would be capable of 155.520 Mbps. We stress that this is a
measurement at the physical layer and not the network IP layer, which measurement at the physical layer and not the network IP layer, which
we will define separately. While NomCap(L) is typically constant we will define separately. While NomCap(L) is typically constant
over time, there are links whose characteristics may allow otherwise, over time, there are links whose characteristics may allow otherwise,
such as the dynamic activation of additional transponders for a such as the dynamic activation of additional transponders for a
satellite link. satellite link.
The nominal physical link capacity is provided as a means to help The nominal physical link capacity is provided as a means to help
distinguish between the commonly used link layer capacities and the distinguish between the commonly used link layer capacities and the
remaining definitions for IP layer capacity. As a result, the value remaining definitions for IP layer capacity. As a result, the value
of NomCap(L) does not influence the other definitions presented in of NomCap(L) does not influence the other definitions presented in
this document. this document. Instead, it provides an upper bound on those values.
2.3 Capacity at the IP Layer 2.3. Capacity at the IP Layer
There are many factors that can reduce the IP information carrying There are many factors that can reduce the IP information carrying
capacity of the link, some of which have already been discussed in capacity of the link, some of which have already been discussed in
the introduction. However, the goal of this document is not to the introduction. However, the goal of this document is not to
become an exhaustive list of such factors. Rather, we outline some become an exhaustive list of such factors. Rather, we outline some
of the major examples in the following section, thus providing food of the major examples in the following section, thus providing food
for thought to those implementing the algorithms or tools that for thought to those implementing the algorithms or tools that
attempt to measure capacity accurately. attempt to measure capacity accurately.
The remaining definitions are all given in terms of "IP layer bits" The remaining definitions are all given in terms of "IP layer bits"
in order to distinguish these definitions from the nominal physical in order to distinguish these definitions from the nominal physical
capacity of the link. capacity of the link.
2.3.1 Definition: IP Layer Bits 2.3.1. Definition: IP Layer Bits
IP layer bits are defined as eight (8) times the number of octets in IP layer bits are defined as eight (8) times the number of octets in
all IP packets received, from the first octet of the IP header to the all IP packets received, from the first octet of the IP header to the
last octet of the IP packet payload, inclusive. last octet of the IP packet payload, inclusive.
IP layer bits are recorded at the destination, D, beginning at time T IP layer bits are recorded at the destination, D, beginning at time T
and ending at a time T+I. Since the definitions are based on and ending at a time T+I. Since the definitions are based on
averages, the two time parameters, T and I, must accompany any report averages, the two time parameters, T and I, must accompany any report
or estimate of the following values in order for them to remain or estimate of the following values in order for them to remain
meaningful. It is not required that the interval boundary points meaningful. It is not required that the interval boundary points
fall between packet arrivals at D. However, boundaries that fall fall between packet arrivals at D. However, boundaries that fall
within a packet will invalidate the packets on which they fall. within a packet will invalidate the packets on which they fall.
Specifically, the data from the partial packet that is contained Specifically, the data from the partial packet that is contained
within the interval will not be counted. This may artificially bias within the interval will not be counted. This may artificially bias
some of the values, depending on the length of the interval and the some of the values, depending on the length of the interval and the
amount of data received during that interval. We elaborate on what amount of data received during that interval. We elaborate on what
constitutes correctly received data in the next section. constitutes correctly received data in the next section.
2.3.1.1 Standard or Correctly Formed Packets 2.3.1.1. Standard or Correctly Formed Packets
The definitions in this document specify that IP packets must be The definitions in this document specify that IP packets must be
received correctly. The IPPM framework recommends a set of criteria received correctly. The IPPM framework recommends a set of criteria
for such standard-formed packet in section 15 of [RFC2330]. However, for such standard-formed packet in section 15 of [RFC2330]. However,
it is inadequate for use with this document. Thus, we outline our it is inadequate for use with this document. Thus, we outline our
own criteria below while pointing out any variations or similarities own criteria below while pointing out any variations or similarities
to [RFC2330]. to [RFC2330].
First, data that is in error at layers below IP and cannot be First, data that is in error at layers below IP and cannot be
properly passed to the IP layer should not be counted. For example, properly passed to the IP layer must not be counted. For example,
wireless media often has a considerably larger error rate than wired wireless media often has a considerably larger error rate than wired
media, resulting in a reduction in IP Link Capacity. In accordance media, resulting in a reduction in IP Link Capacity. In accordance
with the framework, packets that fail validation of the IP header with the IPPM framework, packets that fail validation of the IP
should be discarded. Specifically, the requirements in [RFC1812] header must be discarded. Specifically, the requirements in
section 5.2.2 on IP header validation should be checked, which [RFC1812] section 5.2.2 on IP header validation must be checked,
includes a valid length, checksum, and version field. which includes a valid length, checksum, and version field.
The framework specifies further restrictions, requiring that any The IPPM framework specifies further restrictions, requiring that any
transport header be checked for correctness and that any packets with transport header be checked for correctness and that any packets with
IP options be ignored. However, the definitions in this document are IP options be ignored. However, the definitions in this document are
concerned with the traversal of IP layer bits. As a result, data concerned with the traversal of IP layer bits. As a result, data
from the higher layers is not required to be valid or understood as from the higher layers is not required to be valid or understood as
they are simply regarded as part of the IP packet. The same holds they are simply regarded as part of the IP packet. The same holds
true for IP options. Valid IP fragments should also be counted as true for IP options. Valid IP fragments must also be counted as they
they expend the resources of a link even though assembly of the full expend the resources of a link even though assembly of the full
packet may not be possible. The framework differs in this area, packet may not be possible. The IPPM framework differs in this area,
discarding IP fragments. discarding IP fragments.
In summary, any IP packet that can be properly processed should be For a discussion of duplicates, please see Section 3.2.
In summary, any IP packet that can be properly processed must be
included in these calculations. included in these calculations.
2.3.2 Definition: IP Layer Link Capacity 2.3.1.2. Type P Packets
The definitions in this document refer to "Type P" packets to
designate a particular type of flow or sets of flows. As defined in
RFC 2330, Section 13, "Type P" is a placeholder for what may be an
explicit specification of the packet flows referenced by the metric,
or it may be a very loose specification encompassing aggregates. We
use the "Type P" designation in these definitions in order to
emphasize two things: First, that the value of the capacity
measurement depends on the types of flows referenced in the
definition. This is because networks may treat packets differently
(in terms of queuing and scheduling) based on their markings and
classification. Networks may also arbitrarily decide to flow balance
based on the packet type or flow type and thereby affect capacity
measurements. Second, the measurement of capacity depends not only
on the type of the reference packets, but also on the types of the
packets in the "population" with which the flows of interest share
the links in the path.
All of this indicates two different approaches to measuring: One is
to measure capacity using a broad spectrum of packet types,
suggesting that "Type P" should be set as generic as possible. The
second is to focus narrowly on the types of flows of particular
interest, which suggests that "Type P" should be very specific and
narrowly defined. The first approach is likely to be of interest to
providers, the second to application users.
2.3.2. Definition: IP-type-P Link Capacity
We define the IP layer link capacity, C(L,T,I), to be the maximum We define the IP layer link capacity, C(L,T,I), to be the maximum
number of IP layer bits that can be transmitted from the source S and number of IP layer bits that can be transmitted from the source S and
correctly received by the destination D over the link L during the correctly received by the destination D over the link L during the
interval [T, T+I], divided by I. interval [T, T+I], divided by I.
Using this, we can then extend this notion to an entire path, such 2.3.3. Definition: IP-type-P Path Capacity
that the IP layer path capacity simply becomes that of the link with
the smallest capacity along that path.
2.3.3 Definition: IP Layer Path Capacity Using our definition for link capacity, we can then extend this
notion to an entire path, such that the IP layer path capacity simply
becomes that of the link with the smallest capacity along that path.
C(P,T,I) = min {1..n} {C(Ln,T,I)} C(P,T,I) = min {1..n} {C(Ln,T,I)}
The previous definitions specify a link's capacity, namely the IP The previous definitions specify the number of IP layer bits that can
layer bits that can be transmitted across a link or path should the be transmitted across a link or path should the resource be free of
resource be free of any congestion. Determining how much capacity is any congestion. It represents the full capacity available for
available for use on a congested link is potentially much more traffic between the source and destination. Determining how much
useful. However, in order to define the available capacity we must capacity is available for use on a congested link is potentially much
first specify how much is being used. more useful. However, in order to define the available capacity we
must first specify how much is being used.
2.3.4 Definition: IP Layer Link Usage 2.3.4. Definition: IP-type-P Link Usage
The average usage of a link L, Used(L,T,I), is the actual number of The average usage of a link L, Used(L,T,I), is the actual number of
IP layer bits from any source, correctly received over link L during IP layer bits from any source, correctly received over link L during
the interval [T, T+I], divided by I. the interval [T, T+I], divided by I.
An important distinction between usage and capacity is that An important distinction between usage and capacity is that
Used(L,T,I) is not the maximum number, but rather, the actual number Used(L,T,I) is not the maximum number, but rather, the actual number
of IP bits sent that are correctly received. The information of IP bits sent that are correctly received. The information
transmitted across the link can be generated by any source, including transmitted across the link can be generated by any source, including
those who may not be directly attached to either side of the link. those who may not be directly attached to either side of the link.
In addition, each information flow from these sources may share any In addition, each information flow from these sources may share any
number (from one to n) of links in the overall path between S and D. number (from one to n) of links in the overall path between S and D.
Next, we express usage as a fraction of the overall IP layer link
capacity.
2.3.5 Definition: Average IP Layer Link Utilization 2.3.5. Definition: IP-type-P Link Utilization
We express usage as a fraction of the overall IP layer link capacity.
Util(L,T,I) = ( Used(L,T,I) / C(L,T,I) ) Util(L,T,I) = ( Used(L,T,I) / C(L,T,I) )
Thus, the utilization now represents the fraction of the capacity Thus, the utilization now represents the fraction of the capacity
that is being used and is a value between zero, meaning nothing is that is being used and is a value between zero, meaning nothing is
used, and one, meaning the link is fully saturated. Multiplying the used, and one, meaning the link is fully saturated. Multiplying the
utilization by 100 yields the percent utilization of the link. By utilization by 100 yields the percent utilization of the link. By
using the above, we can now define the capacity available over the using the above, we can now define the capacity available over the
link as well as the path between S and D. Note that this is link as well as the path between S and D. Note that this is
essentially the definition in [PDM]. essentially the definition in [PDM].
2.3.6 Definition: IP Layer Available Link Capacity 2.3.6. Definition: IP-type-P Available Link Capacity
We can now determine the amount of available capacity on a congested
link by multiplying the IP layer link capacity with the complement of
the IP layer link utilization. Thus, the IP layer available link
capacity becomes:
AvailCap(L,T,I) = C(L,T,I) * ( 1 - Util(L,T,I) ) AvailCap(L,T,I) = C(L,T,I) * ( 1 - Util(L,T,I) )
2.3.7 Definition: IP Layer Available Path Capacity 2.3.7. Definition: IP-type-P Available Path Capacity
Using our definition for IP layer available link capacity, we can
then extend this notion to an entire path, such that the IP layer
available path capacity simply becomes that of the link with the
smallest available capacity along that path.
AvailCap(P,T,I) = min {1..n} {AvailCap(Ln,T,I)} AvailCap(P,T,I) = min {1..n} {AvailCap(Ln,T,I)}
Since measurements of available capacity are more volatile than that Since measurements of available capacity are more volatile than that
of capacity, it is important that both the time and interval be of capacity, we stress the importance that both the time and interval
specified as their values have a great deal of influence on the be specified as their values have a great deal of influence on the
results. In addition, a sequence of measurements may be beneficial results. In addition, a sequence of measurements may be beneficial
in offsetting the volatility when attempting to characterize in offsetting the volatility when attempting to characterize
available capacity. available capacity.
3. Discussion 3. Discussion
3.1 Time and Sampling 3.1. Time and Sampling
We must emphasize the importance of time in the basic definitions of We must emphasize the importance of time in the basic definitions of
these quantities. We know that traffic on the Internet is highly these quantities. We know that traffic on the Internet is highly
variable across all time scales. This argues that the time and variable across all time scales. This argues that the time and
length of measurements are critical variables in reporting available length of measurements are critical variables in reporting available
capacity measurements and must be reported when using these capacity measurements and must be reported when using these
definitions. definitions.
The closer to "instantaneous" a metric is, the more important it is The closer to "instantaneous" a metric is, the more important it is
to have a plan for sampling the metric over a time period that is to have a plan for sampling the metric over a time period that is
sufficiently large. By doing so, we allow valid statistical sufficiently large. By doing so, we allow valid statistical
inferences to be made from the measurements. An obvious pitfall here inferences to be made from the measurements. An obvious pitfall here
is sampling in a way that causes bias. For example, a situation is sampling in a way that causes bias. For example, a situation
where the sampling frequency is a multiple of the frequency of an where the sampling frequency is a multiple of the frequency of an
underlying condition. underlying condition.
3.2 Hardware Duplicates 3.2. Hardware Duplicates
The base definitions make no mention of hardware duplication of We briefly consider the impacts of paths where hardware duplication
packets. While hardware duplication has no impact on the nominal of packets may occur. In such an environment, a node in the network
capacity, it can impact the IP link layer capacity. For example, path may duplicate packets and the destination may receive multiple,
consider a link which can normally carry a capacity of 2X on average. identical copies of these packets. Both the original packet and the
However, the link has developed a syndrome where it duplicates every duplicates can be properly received and appear to be originating from
incoming packet. The link would still technically carry a capacity the sender. Thus, in the most generic form, duplicate IP packets are
of 2X, however the link has a effective capacity of X or lower, counted in these definitions. However, hardware duplication can
depending on framing overhead to send the duplicates, etc. Since the impact these definitions depending on the use of "Type P" to add
definitions specify bits sent and correctly received, duplicates are additional restrictions on packet reception. For instance, a
not counted in the usage and capacity definitions. Thus, a value for restriction to only count uniquely sent packets may be more useful to
C(L,T,I) and AvailCap(L,T,I) will reflect the duplication with the users concerned with capacity for meaningful data. In contrast, the
lower value. more general, unrestricted metric may be suitable for a user who is
concerned with raw capacity. Thus, it is up to the user to properly
scope and interpret results in situations where hardware duplicates
may be prevalent.
3.3 Other Potential Factors 3.3. Other Potential Factors
IP encapsulation does not impact the definitions as all IP header and IP encapsulation does not impact the definitions as all IP header and
payload bits should be counted regardless of content. However, payload bits must be counted regardless of content. However,
different sized IP packets can lead to a variation in the amount of different sized IP packets can lead to a variation in the amount of
overhead needed at the lower layers to transmit the data, thus overhead needed at the lower layers to transmit the data, thus
altering the overall IP link layer capacity. altering the overall IP link layer capacity.
Should the link happen to employ a compression scheme such as ROHC Should the link happen to employ a compression scheme such as ROHC
[RFC3095] or V.44 [V44], some of the original bits are not [RFC3095] or V.44 [V44], some of the original bits are not
transmitted across the link. However, the inflated (not compressed) transmitted across the link. However, the inflated (not compressed)
number of IP-layer bits should be counted. number of IP-layer bits should be counted.
3.4 Common Literature Terminology 3.4. Common Literature Terminology
Certain terms are often used to characterize specific aspects of the Certain terms are often used to characterize specific aspects of the
presented definitions. The link with the smallest capacity is presented definitions. The link with the smallest capacity is
commonly referred to as the "narrow link" of a path. Also, the value commonly referred to as the "narrow link" of a path. Also, the link
of n that satisfies AvailCap(P,T,I), is often referred to as the with the smallest available capacity is often referred to as the
"tight link" within a path. So, while Ln may have a very large "tight link" within a path. So, while Ln may have a very large
capacity, the overall congestion level on the link makes it the capacity, the overall congestion level on the link makes it the
likely bottleneck of a connection. Conversely, a link that has the likely bottleneck of a connection. Conversely, a link that has the
smallest capacity may not be a bottleneck should it be lightly loaded smallest capacity may not be a bottleneck should it be lightly loaded
in relation to the rest of the path. in relation to the rest of the path.
Also, common literature often overloads the term "bandwidth" to refer Also, common literature often overloads the term "bandwidth" to refer
to what we have described as capacity in this document. For example, to what we have described as capacity in this document. For example,
when inquiring about the bandwidth of a 802.11b link, a network when inquiring about the bandwidth of a 802.11b link, a network
engineer will likely answer with 11 Mbps. However, an electrical engineer will likely answer with 11 Mbps. However, an electrical
engineer may answer with 25 MHz, and an end user may tell you that engineer may answer with 25 MHz, and an end user may tell you that
his observed bandwidth is 8 Mbps. In contrast, the term capacity is his observed bandwidth is 8 Mbps. In contrast, the term capacity is
not quite as overloaded and is an appropriate term that better not quite as overloaded and is an appropriate term that better
reflects what is actually being measured. reflects what is actually being measured.
3.5 Comparison to Bulk Transfer Capacity (BTC) 3.5. Comparison to Bulk Transfer Capacity (BTC)
Bulk Transfer Capacity (BTC) [RFC3184] provides a distinct Bulk Transfer Capacity (BTC) [RFC3184] provides a distinct
perspective on path capacity that differs from the definitions in perspective on path capacity that differs from the definitions in
this document in several fundamental ways. First, BTC operates at this document in several fundamental ways. First, BTC operates at
the transport layer, gauging the amount of capacity available to an the transport layer, gauging the amount of capacity available to an
application that wishes to send data. Only unique data is measured, application that wishes to send data. Only unique data is measured,
meaning header and retransmitted data are not included in the meaning header and retransmitted data are not included in the
calculation. In contrast, IP layer link capacity includes the IP calculation. In contrast, IP layer link capacity includes the IP
header and is indifferent to the uniqueness of the data contained header and is indifferent to the uniqueness of the data contained
within the packet payload (Hardware duplication of packets is an within the packet payload (Hardware duplication of packets is an
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consider a single event where a link suffers a large duration of bit consider a single event where a link suffers a large duration of bit
errors. The event could cause IP layer packets to be discarded, and errors. The event could cause IP layer packets to be discarded, and
the lost packets would reduce the IP layer link capacity. However, the lost packets would reduce the IP layer link capacity. However,
the same event and subsequent losses would trigger loss recovery for the same event and subsequent losses would trigger loss recovery for
a BTC measurement resulting in the retransmission of data and a a BTC measurement resulting in the retransmission of data and a
potentially reduced sending rate. Thus, a measurement of BTC does potentially reduced sending rate. Thus, a measurement of BTC does
not correspond to any of the definitions in this document. Both not correspond to any of the definitions in this document. Both
techniques are useful in exploring the characteristics of a network techniques are useful in exploring the characteristics of a network
path, but from different perspectives. path, but from different perspectives.
3.6 Type P Packets 4. IANA Considerations
Note that these definitions do not make mention of "Type P" packets,
while other IPPM definitions do. We could add the packet type as an
extra parameter. This would have the effect of defining a large
number of quantities, relative to the QoS policies that a given
network or concatenation of networks may have in effect in the path.
It would produce metrics such as "estimated EF IP Link/Path Capacity"
or "estimated EF IP Link Utilization".
Such metrics may indeed be useful. For example, this would yield
something like the sum of the capacities of all the QoS classes
defined along the path as the link or path capacity. The breakdown
then gives the user an analysis of how the link or path capacity (or
at least the "tight link" capacity) is allocated among classes.
These QoS-based capacities become difficult to measure on a path if
there are different capacities defined per QoS class on different
links in the path. Possibly the best way to approach this would be
to measure each link in a path individually, and then combine the
information from individual links.
4. Conclusion
In this document, we have defined a set of quantities related to the
capacity of links in an IP network. In these definitions, we have
tried to be as clear as possible and take into account various
characteristics that links can have. The goal of these definitions
is to enable researchers who propose capacity metrics to relate those
metrics to these definitions and to evaluate those metrics with
respect to how well they approximate these quantities.
In addition, we have pointed out some key auxiliary parameters and
opened a discussion of issues related to valid inferences from
available capacity metrics.
5. IANA Considerations
This document makes no request of IANA. This document makes no request of IANA.
Note to RFC Editor: this section may be removed on publication as an Note to RFC Editor: this section may be removed on publication as an
RFC. RFC.
6. Security Considerations 5. Security Considerations
This document specifies definitions regarding IP traffic traveling This document specifies definitions regarding IP traffic traveling
between a source and destination in an IP network. These definitions between a source and destination in an IP network. These definitions
do not raise any security issues and do not have a direct impact on do not raise any security issues and do not have a direct impact on
the networking protocol suite. the networking protocol suite.
Tools that attempt to implement these definitions may introduce Tools that attempt to implement these definitions may introduce
security issues specific to each implementation. Both active and security issues specific to each implementation. Both active and
passive measurement techniques can be abused, impacting the security, passive measurement techniques can be abused, impacting the security,
privacy, and performance of the network. Any measurement techniques privacy, and performance of the network. Any measurement techniques
based upon these definitions must include a discussion of the based upon these definitions must include a discussion of the
techniques needed to protect the network on which the measurements techniques needed to protect the network on which the measurements
are being performed. are being performed.
6. Conclusion
In this document, we have defined a set of quantities related to the
capacity of links and paths in an IP network. In these definitions,
we have tried to be as clear as possible and take into account
various characteristics that links and paths can have. The goal of
these definitions is to enable researchers who propose capacity
metrics to relate those metrics to these definitions and to evaluate
those metrics with respect to how well they approximate these
quantities.
In addition, we have pointed out some key auxiliary parameters and
opened a discussion of issues related to valid inferences from
available capacity metrics.
7. Acknowledgments 7. Acknowledgments
The authors would like to acknowledge Mark Allman, Patrik Arlos, Matt The authors would like to acknowledge Mark Allman, Patrik Arlos, Matt
Mathis, Al Morton, Stanislav Shalunov, and Matt Zekauskas for their Mathis, Al Morton, Stanislav Shalunov, and Matt Zekauskas for their
suggestions, comments, and reviews. We also thank members of the suggestions, comments, and reviews. We also thank members of the
IETF IPPM Mailing List for their discussions and feedback on this IETF IPPM Mailing List for their discussions and feedback on this
document. document.
8. References 8. References
8.1 Normative References 8.1. Normative References
8.2 Informative References
[PDM] Dovrolis, C., Ramanathan, P., and D. Moore, "Packet
Dispersion Techniques and a Capacity Estimation
Methodology", IEEE/ACM Transactions on Networking 12(6):
963-977, December 2004.
[RFC1812] Baker, F., "Requirements for IP Version 4 Routers", [RFC1812] Baker, F., "Requirements for IP Version 4 Routers",
RFC 1812, June 1995. RFC 1812, June 1995.
[RFC2330] Paxson, V., Almes, G., Mahdavi, J., and M. Mathis, [RFC2330] Paxson, V., Almes, G., Mahdavi, J., and M. Mathis,
"Framework for IP Performance Metrics", RFC 2330, "Framework for IP Performance Metrics", RFC 2330,
May 1998. May 1998.
8.2. Informative References
[PDM] Dovrolis, C., Ramanathan, P., and D. Moore, "Packet
Dispersion Techniques and a Capacity Estimation
Methodology", IEEE/ACM Transactions on Networking 12(6):
963-977, December 2004.
[RFC3095] Bormann, C., Burmeister, C., Degermark, M., Fukushima, H., [RFC3095] Bormann, C., Burmeister, C., Degermark, M., Fukushima, H.,
Hannu, H., Jonsson, L-E., Hakenberg, R., Koren, T., Le, Hannu, H., Jonsson, L-E., Hakenberg, R., Koren, T., Le,
K., Liu, Z., Martensson, A., Miyazaki, A., Svanbro, K., K., Liu, Z., Martensson, A., Miyazaki, A., Svanbro, K.,
Wiebke, T., Yoshimura, T., and H. Zheng, "RObust Header Wiebke, T., Yoshimura, T., and H. Zheng, "RObust Header
Compression (ROHC): Framework and four profiles: RTP, UDP, Compression (ROHC): Framework and four profiles: RTP, UDP,
ESP, and uncompressed", RFC 3095, July 2001. ESP, and uncompressed", RFC 3095, July 2001.
[RFC3184] Harris, S., "IETF Guidelines for Conduct", BCP 54, [RFC3184] Harris, S., "IETF Guidelines for Conduct", BCP 54,
RFC 3184, October 2001. RFC 3184, October 2001.
skipping to change at page 18, line 5 skipping to change at page 20, line 5
Joseph Ishac Joseph Ishac
NASA Glenn Research Center NASA Glenn Research Center
21000 Brookpark Road 21000 Brookpark Road
Cleveland, Ohio 44135 Cleveland, Ohio 44135
USA USA
Phone: +1-216-433-6587 Phone: +1-216-433-6587
Fax: +1-216-433-8705 Fax: +1-216-433-8705
Email: jishac@grc.nasa.gov Email: jishac@grc.nasa.gov
Intellectual Property Statement Full Copyright Statement
Copyright (C) The IETF Trust (2007).
This document is subject to the rights, licenses and restrictions
contained in BCP 78, and except as set forth therein, the authors
retain all their rights.
This document and the information contained herein are provided on an
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND
THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS
OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF
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WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Intellectual Property
The IETF takes no position regarding the validity or scope of any The IETF takes no position regarding the validity or scope of any
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skipping to change at page 18, line 29 skipping to change at page 20, line 45
such proprietary rights by implementers or users of this such proprietary rights by implementers or users of this
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The IETF invites any interested party to bring to its attention any The IETF invites any interested party to bring to its attention any
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Disclaimer of Validity
This document and the information contained herein are provided on an
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Copyright Statement
Copyright (C) The Internet Society (2006). This document is subject
to the rights, licenses and restrictions contained in BCP 78, and
except as set forth therein, the authors retain all their rights.
Acknowledgment Acknowledgment
Funding for the RFC Editor function is currently provided by the Funding for the RFC Editor function is provided by the IETF
Internet Society. Administrative Support Activity (IASA).
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