draft-ietf-ngtrans-isatap-12.txt   draft-ietf-ngtrans-isatap-13.txt 
Network Working Group F. Templin Network Working Group F. Templin
Internet-Draft Nokia Internet-Draft Nokia
Expires: July 25, 2003 T. Gleeson Expires: September 25, 2003 T. Gleeson
Cisco Systems K.K. Cisco Systems K.K.
M. Talwar M. Talwar
D. Thaler D. Thaler
Microsoft Corporation Microsoft Corporation
January 24, 2003 March 27, 2003
Intra-Site Automatic Tunnel Addressing Protocol (ISATAP) Intra-Site Automatic Tunnel Addressing Protocol (ISATAP)
draft-ietf-ngtrans-isatap-12.txt draft-ietf-ngtrans-isatap-13.txt
Status of this Memo Status of this Memo
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Copyright Notice Copyright Notice
Copyright (C) The Internet Society (2003). All Rights Reserved. Copyright (C) The Internet Society (2003). All Rights Reserved.
Abstract Abstract
This document specifies an Intra-Site Automatic Tunnel Addressing This document specifies an Intra-Site Automatic Tunnel Addressing
Protocol (ISATAP) that connects IPv6 hosts and routers within IPv4 Protocol (ISATAP) that connects IPv6 hosts and routers within IPv4
sites. ISATAP treats the site's IPv4 infrastructure as a link layer sites. ISATAP treats the site's IPv4 infrastructure as a link layer
skipping to change at page 2, line 13 skipping to change at page 2, line 13
or private IPv4 addresses are used. or private IPv4 addresses are used.
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Applicability Statement . . . . . . . . . . . . . . . . . . . 3 2. Applicability Statement . . . . . . . . . . . . . . . . . . . 3
3. Requirements . . . . . . . . . . . . . . . . . . . . . . . . . 3 3. Requirements . . . . . . . . . . . . . . . . . . . . . . . . . 3
4. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4 4. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
5. Basic IPv6 Operation . . . . . . . . . . . . . . . . . . . . . 4 5. Basic IPv6 Operation . . . . . . . . . . . . . . . . . . . . . 4
6. Automatic Tunneling . . . . . . . . . . . . . . . . . . . . . 5 6. Automatic Tunneling . . . . . . . . . . . . . . . . . . . . . 5
7. Neighbor Discovery . . . . . . . . . . . . . . . . . . . . . . 7 7. Neighbor Discovery . . . . . . . . . . . . . . . . . . . . . . 6
8. Deployment Considerations . . . . . . . . . . . . . . . . . . 10 8. Deployment Considerations . . . . . . . . . . . . . . . . . . 9
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11 9. Site Administration Considerations . . . . . . . . . . . . . . 9
10. Security considerations . . . . . . . . . . . . . . . . . . . 11 10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 12 11. Security considerations . . . . . . . . . . . . . . . . . . . 10
Normative References . . . . . . . . . . . . . . . . . . . . . 12 12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 10
Informative References . . . . . . . . . . . . . . . . . . . . 13 Normative References . . . . . . . . . . . . . . . . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 15 Informative References . . . . . . . . . . . . . . . . . . . . 11
A. Major Changes . . . . . . . . . . . . . . . . . . . . . . . . 16 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 12
B. Rationale for Interface Identifier Construction . . . . . . . 17 A. Major Changes . . . . . . . . . . . . . . . . . . . . . . . . 13
C. ISATAP Interface MTU Considerations . . . . . . . . . . . . . 18 B. Rationale for Interface Identifier Construction . . . . . . . 15
Intellectual Property and Copyright Statements . . . . . . . . 23 Intellectual Property and Copyright Statements . . . . . . . . 17
1. Introduction 1. Introduction
This document presents a simple approach called the Intra-Site This document presents a simple approach called the Intra-Site
Automatic Tunnel Addressing Protocol (ISATAP) that enables Automatic Tunnel Addressing Protocol (ISATAP) that enables
incremental deployment of IPv6 [1] within IPv4 [2] sites. ISATAP incremental deployment of IPv6 [RFC2460] within IPv4 [RFC0791] sites.
allows dual-stack nodes that do not share a physical link with an ISATAP allows dual-stack nodes that do not share a physical link with
IPv6 router to automatically tunnel packets to the IPv6 next-hop an IPv6 router to automatically tunnel packets to the IPv6 next-hop
address through IPv4, i.e., the site's IPv4 infrastructure is treated address through IPv4, i.e., the site's IPv4 infrastructure is treated
as a link layer for IPv6. as a link layer for IPv6.
Specific details for the operation of IPv6 and automatic tunneling Specific details for the operation of IPv6 and automatic tunneling
over ISATAP links are given, including an interface identifier format using ISATAP are given, including an interface identifier format that
that embeds an IPv4 address. This format supports IPv6 address embeds an IPv4 address. This format supports IPv6 address
configuration and simple link-layer address mapping. Also specified configuration and simple link-layer address mapping. Also specified
is the operation of IPv6 Neighbor Discovery and deployment/security is the operation of IPv6 Neighbor Discovery and deployment/security
considerations. considerations.
2. Applicability Statement 2. Applicability Statement
ISATAP provides the following features: ISATAP provides the following features:
o treats site's IPv4 infrastructure as a link layer for IPv6 using o treats site's IPv4 infrastructure as a link layer for IPv6 using
automatic IPv6-in-IPv4 tunneling automatic IPv6-in-IPv4 tunneling
o enables incremental deployment of IPv6 hosts within IPv4 sites o enables incremental deployment of IPv6 hosts within IPv4 sites
with no aggregation scaling issues at border gateways with no aggregation scaling issues at border gateways
o requires no special IPv4 services within the site (e.g., o requires no special IPv4 services within the site (e.g.,
multicast) multicast)
o supports both stateless address autoconfiguration and manual o supports both stateless and stateful autoconfiguration as well as
configuration manual configuration
o supports networks that use non-globally unique IPv4 addresses o supports networks that use non-globally unique IPv4 addresses
(e.g., when private address allocations [10] are used) (e.g., when private address allocations [RFC1918] are used)
o compatible with other NGTRANS mechanisms (e.g., 6to4 [11]) o compatible with other NGTRANS mechanisms (e.g., 6to4 [RFC3056])
3. Requirements 3. Requirements
The keywords MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD, The keywords MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD,
SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL, when they appear in this SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL, when they appear in this
document, are to be interpreted as described in [3]. document, are to be interpreted as described in [RFC2119].
This document also makes use of internal conceptual variables to This document also makes use of internal conceptual variables to
describe protocol behavior and external variables that an describe protocol behavior and external variables that an
implementation must allow system administrators to change. The implementation must allow system administrators to change. The
specific variable names, how their values change, and how their specific variable names, how their values change, and how their
settings influence protocol behavior are provided to demonstrate settings influence protocol behavior are provided to demonstrate
protocol behavior. An implementation is not required to have them in protocol behavior. An implementation is not required to have them in
the exact form described here, so long as its external behavior is the exact form described here, so long as its external behavior is
consistent with that described in this document. consistent with that described in this document.
4. Terminology 4. Terminology
The terminology of RFC 2460 [1] applies to this document. The The terminology of [RFC2460] applies to this document. The following
following additional terms are defined: additional terms are defined:
link, on-link, off-link: link, on-link, off-link:
same definitions as ([4], section 2.1). same definitions as ([RFC2461], section 2.1).
underlying link: underlying link:
a link layer that supports IPv4 (for ISATAP), and MAY also support a link layer that supports IPv4 (for ISATAP), and MAY also support
IPv6 natively. IPv6 natively.
ISATAP link:
one or more underlying links used for tunneling. The IPv4 network
layer addresses of the underlying links are used as link-layer
addresses on the ISATAP link.
ISATAP interface: ISATAP interface:
a node's attachment to an ISATAP link. an interface configured over one or more underling links.
advertising ISATAP interface: advertising ISATAP interface:
same meaning as "advertising interface" in ([4], section 6.2.2). same meaning as "advertising interface" in ([RFC2461], section
6.2.2).
ISATAP address: ISATAP address:
an on-link address on an ISATAP interface and with an interface an on-link address on an ISATAP interface and with an interface
identifier constructed as specified in Section 5.1 identifier constructed as specified in Section 5.1
5. Basic IPv6 Operation 5. Basic IPv6 Operation
ISATAP links transmit IPv6 packets via automatic tunnels using the ISATAP interfaces automatically tunnel IPv6 packets using the site's
site's IPv4 infrastructure as a link layer for IPv6, i.e., IPv6 IPv4 infrastructure as a link layer for IPv6, i.e., IPv6 treats the
treats the site's IPv4 infrastructure as a Non-Broadcast, Multiple site's IPv4 infrastructure as a Non-Broadcast, Multiple Access (NBMA)
Access (NBMA) link layer. The following considerations for IPv6 on link layer. The mechanisms in [RFC2491] are used, with the following
ISATAP links are noted: noted exceptions for ISATAP:
5.1 Interface Identifiers and Unicast Addresses 5.1 Interface Identifiers and Unicast Addresses
ISATAP interface identifiers use "modified EUI-64" format ([5], ISATAP interface identifiers use "modified EUI-64" format ([ARCH],
section 2.5.1) and are formed by appending an IPv4 address on the section 2.5.1) and are formed by appending an IPv4 address assigned
ISATAP link to the 32-bit string '00-00-5E-FE'. Appendix B includes to an underlying link to the 32-bit string '00-00-5E-FE'. Appendix B
non-normative rationale for this construction rule. includes non-normative rationale for this construction rule.
With reference to ([5], sections 2.5.4, 2.5.6), global and local-use IPv6 global and local-use ([ARCH], sections 2.5.4, 2.5.6) ISATAP
ISATAP addresses are constructed as follows: addresses are constructed as follows:
| 64 bits | 32 bits | 32 bits | | 64 bits | 32 bits | 32 bits |
+------------------------------+---------------+----------------+ +------------------------------+---------------+----------------+
| global or local-use unicast | 0000:5EFE | IPv4 Address | | global/local unicast prefix | 0000:5EFE | IPv4 Address |
| prefix | | of ISATAP link |
+------------------------------+---------------+----------------+ +------------------------------+---------------+----------------+
5.2 ISATAP Link/Interface Configuration 5.2 ISATAP Interface Configuration
ISATAP links consist of one or more underlying links that support
IPv4 for tunneling within a site.
ISATAP interfaces are configured over ISATAP links; each IPv4 address ISATAP interfaces are configured over one or more underlying links
that support IPv4 for tunneling within a site; each IPv4 address
assigned to an underlying link is seen as a link-layer address for assigned to an underlying link is seen as a link-layer address for
ISATAP. ISATAP.
Neighbor discovery on ISATAP links (see: Section 7) provides the
functional equivalent of unicast virtual circuits (VCs) required for
other NBMA media types ([6], section 4.6). Neighbor state
information MAY be kept in the Conceptual Neighbor Cache ([4],
section 5.1).
5.3 Link Layer Address Options 5.3 Link Layer Address Options
With reference to ([6], section 5.2), when the [NTL] and [STL] fields With reference to ([RFC2491], section 5.2), when the [NTL] and [STL]
in an ISATAP link layer address option encode 0, the [NBMA Number] fields in an ISATAP link layer address option encode 0, the [NBMA
field encodes a 4-octet IPv4 address. Number] field encodes a 4-octet IPv4 address.
5.4 Multicast and Anycast 5.4 Multicast and Anycast
ISATAP interfaces recognize a node's required addresses as specified ISATAP interfaces recognize an IPv6 node's required addresses
in ([5], section 2.8). ([ARCH], section 2.8), including certain multicast/anycast addresses.
Mechanisms for multicast/anycast emulation on ISATAP links (e.g., Mechanisms for multicast/anycast emulation on ISATAP interfaces
adaptations of MLD [12], PIM-SM [13], MARS [14], etc.) are subject (e.g., adaptations of MLD [RFC2710], PIM-SM [RFC2362], MARS
for future companion document(s). [RFC2022], etc.) are subject for future companion document(s).
6. Automatic Tunneling 6. Automatic Tunneling
The common tunneling mechanisms specified in ([7], sections 2 and 3) The common tunneling mechanisms specified in ([MECH], sections 2 and
are used, with the following noted considerations for ISATAP: 3) are used, with the following noted considerations for ISATAP:
6.1 Dual IP Layer Operation
ISATAP uses the same specification found in ([7], section 2). That
is, ISATAP nodes provide complete IPv4 and IPv6 implementations and
are able to send and receive both IPv4 and IPv6 packets.
Address configuration and DNS considerations are the same as ([7],
sections 2.1 through 2.3).
6.2 Encapsulation/Decapsulation
The specifications in ([7], sections 3.1 and 3.6) are used.
Additionally, the IPv6 next-hop address for packets encapsulated on
an ISATAP link MUST be an ISATAP address; other packets are discarded
and an ICMPv6 destination unreachable indication with code 3 (Address
Unreachable) ([8], section 3.1) is returned to the source.
6.3 Tunnel MTU and Fragmentation 6.1 Tunnel MTU and Fragmentation
ISATAP automatic tunnel interfaces may be configured over multiple ISATAP automatic tunnel interfaces may be configured over multiple
underlying links with diverse maximum transmission units (MTUs). The underlying links with diverse maximum transmission units (MTUs). The
minimum MTU for IPv6 interfaces is 1280 bytes ([1], Section 5), but minimum MTU for IPv6 interfaces is 1280 bytes ([RFC2460], Section 5),
the following considerations apply for ISATAP interfaces: but the following considerations apply for ISATAP interfaces:
o Nearly all IPv4 nodes connect to physical links with MTUs of 1500 o Nearly all IPv4 nodes connect to physical links with MTUs of 1500
bytes or larger (e.g., Ethernet) bytes or larger (e.g., Ethernet)
o Sub-IPv4 layer encapsulations (e.g., VPN) may occur on some paths o Sub-IPv4 layer encapsulations (e.g., VPN) may occur on some paths
o Commonly-deployed VPN interfaces use an MTU of 1400 bytes o Commonly-deployed VPN interfaces use an MTU of 1400 bytes
To maximize efficiency and minimize IPv4 fragmentation for the To maximize efficiency and minimize IPv4 fragmentation for the
predominant deployment case, the ISATAP interface MTU, or "LinkMTU" predominant deployment case, the ISATAP interface MTU, or "LinkMTU"
(see: [4], Section 6.3.2 ), SHOULD be set to no more than 1380 bytes (see: [RFC2461], Section 6.3.2), SHOULD be set to no more than 1380
(1400 minus 20 bytes for IPv4 encapsulation). LinkMTU MAY be set to bytes (1400 minus 20 bytes for IPv4 encapsulation). LinkMTU MAY be
larger values when a dynamic link layer MTU discovery mechanism is set to larger values when a dynamic link layer MTU discovery
used or when a static MTU assignment is used and additional mechanism is used or when a static MTU assignment is used and
fragmentation in the site's IPv4 network is deemed acceptable. See additional fragmentation in the site's IPv4 network is deemed
Appendix C for non-normative ISATAP interface MTU considerations. acceptable.
When a dynamic MTU discovery mechanism is not used, the ISATAP link
layer encapsulates IPv6 packets with the Don't Fragment (DF) bit not
set in the encapsualting IPv4 header.
6.4 Handling IPv4 ICMP Errors When a dynamic IPv4 MTU discovery mechanism is not used, the ISATAP
interface encapsulates IPv6 packets with the Don't Fragment (DF) bit
not set in the encapsualting IPv4 header.
IPv4 ICMP errors and ARP failures are processed as link error 6.2 Handling IPv4 ICMP Errors
notifications.
6.5 Local-Use IPv6 Unicast Addresses ARP failures and persistent ICMPv4 errors SHOULD be processed as
The specification in ([7], section 3.7) is not used. Instead, local link-specific information indicating that a path to a neighbor has
use IPv6 unicast addresses are formed as specified in Section 5.1. failed ([RFC2461], section 7.3.3).
6.6 Ingress Filtering 6.3 Local-Use IPv6 Unicast Addresses
The specification in ([7], section 3.9) is used. In particular, The specification in ([MECH], section 3.7) is not used; the
ISATAP nodes that forward decapsulated packets MUST verify the tunnel specification in Section 5.1 is used instead.
source address is acceptable.
7. Neighbor Discovery 7. Neighbor Discovery
The specification in ([7], section 3.8) applies only to configured The specification in ([MECH], section 3.8) applies only to configured
tunnels. RFC 2461 [4] provides the following guidelines for tunnels. [RFC2461] provides the following guidelines for
non-broadcast multiple access (NBMA) link support: non-broadcast multiple access (NBMA) link support:
"Redirect, Neighbor Unreachability Detection and next-hop "Redirect, Neighbor Unreachability Detection and next-hop
determination should be implemented as described in this document. determination should be implemented as described in this document.
Address resolution and the mechanism for delivering Router Address resolution and the mechanism for delivering Router
Solicitations and Advertisements on NBMA links is not specified in Solicitations and Advertisements on NBMA links is not specified in
this document." this document."
ISATAP links SHOULD implement Redirect, Neighbor Unreachability ISATAP interfaces SHOULD implement Redirect, Neighbor Unreachability
Detection, and next-hop determination exactly as specified in [4]. Detection, and next-hop determination exactly as specified in
Address resolution and the mechanisms for delivering Router [RFC2461]. Address resolution and the mechanisms for delivering
Solicitations and Advertisements for ISATAP links are not specified Router Solicitations and Advertisements are not specified by
by [4]; instead, they are specified in the following sections of this [RFC2461]; instead, they are specified in the following sections of
document. this document.
7.1 Address Resolution and Neighbor Unreachability Detection 7.1 Address Resolution and Neighbor Unreachability Detection
ISATAP addresses are resolved to link-layer addresses (IPv4) by a ISATAP addresses are resolved to link-layer (IPv4) addresses by a
static computation, i.e., the last four octets are treated as an IPv4 static computation, i.e., the last four octets are treated as an IPv4
address. address.
Following static address resolution, hosts SHOULD perform an initial Hosts SHOULD perform an initial reachability confirmation by sending
reachability confirmation by sending Neighbor Solicitation (NS) Neighbor Solicitation (NS) message(s) and receiving a Neighbor
message(s) and receiving a Neighbor Advertisement (NA) message using Advertisement (NA) message as specified in ([RFC2461], section 7.2).
the mechanisms specified in ([4], section 7.2.). When the ISATAP Unless otherwise specified in a future document, solicitations are
interface provides a multicast emulation mechanism (see: Section 5.4)
solicitations are sent to the solicited-node multicast address
corresponding to the target address. Otherwise, the solicitation is
sent to the target's unicast address. sent to the target's unicast address.
Hosts SHOULD additionally perform Neighbor Unreachability Detection Hosts SHOULD additionally perform Neighbor Unreachability Detection
(NUD) as specified in ([4], section 7.3). Routers MAY perform the (NUD) as specified in ([RFC2461], section 7.3). Routers MAY perform
above-specified reachability detection and NUD procedures, but this these reachability confirmation and NUD procedures, but this might
might not scale in all environments. not scale in all environments.
All ISATAP nodes MUST send solicited neighbor advertisements ([4], All ISATAP nodes MUST send solicited neighbor advertisements
section 7.2.4). ([RFC2461], section 7.2.4).
7.2 Duplicate Address Detection 7.2 Duplicate Address Detection
Duplicate Address Detection ([9], section 5.4) is not required for Duplicate Address Detection ([RFC2462], section 5.4) is not required
ISATAP addresses, since duplicate address detection is assumed for ISATAP addresses, since duplicate address detection is assumed
already performed for the IPv4 addresses from which they derive. already performed for the IPv4 addresses from which they derive.
7.3 Router and Prefix Discovery 7.3 Router and Prefix Discovery
The following sections describe mechanisms to support the router and The following sections describe mechanisms to support the router and
prefix discovery process ([4], section 6) on ISATAP links: prefix discovery process ([RFC2461], section 6):
7.3.1 Conceptual Data Structures 7.3.1 Conceptual Data Structures
ISATAP nodes use the conceptual data structures Prefix List and ISATAP nodes use the conceptual data structures Prefix List and
Default Router List exactly as in ([4], section 5.1). ISATAP links Default Router List exactly as in ([RFC2461], section 5.1). ISATAP
add a new conceptual data structure "Potential Router List" (PRL) and adds a new conceptual data structure "Potential Router List" (PRL)
the following new configuration variable: and the following new configuration variable:
PrlRefreshInterval PrlRefreshInterval
Time in seconds between successive refreshments of the PRL after Time in seconds between successive refreshments of the PRL after
initialization. SHOULD be no less than 3,600 seconds. initialization. SHOULD be no less than 3,600 seconds.
Default: 3,600 seconds Default: 3,600 seconds
A PRL is associated with every ISATAP link. Each entry in the PRL A PRL is associated with every ISATAP interface. Each entry in the
("PRL(i)") has an IPv4 address ("V4ADDR(i)") that represents an PRL ("PRL(i)") has an IPv4 address ("V4ADDR(i)") that represents an
advertising ISATAP interface and an associated timer ("TIMER(i)"). advertising ISATAP interface and an associated timer ("TIMER(i)").
The process for initializing and refreshing the PRL is described
below:
When a node enables an ISATAP link, it initializes the PRL with IPv4 When a node enables an ISATAP interface, it initializes the PRL with
addresses. The addresses MAY be discovered via a DHCPv4 [15] option IPv4 addresses. The addresses MAY be discovered via a DHCPv4
for ISATAP (option code TBD), manual configuration, or an unspecified [RFC2131] option for ISATAP, manual configuration, or an unspecified
alternate method (e.g., DHCPv4 vendor-specific option). alternate method (e.g., DHCPv4 vendor-specific option).
When no other mechanisms are available, a DNS fully-qualified domain When no other mechanisms are available, a DNS fully-qualified domain
name (FQDN) [16] established by an out-of-band method (e.g., DHCPv4, name (FQDN) [RFC1035] established by an out-of-band method (e.g.,
manual configuration, etc.) MAY be used. The FQDN is resolved into DHCPv4, manual configuration, etc.) MAY be used. The FQDN is resolved
IPv4 addresses for the PRL through a static host file, a into IPv4 addresses for the PRL through a static host file, a
site-specific name service, querying a DNS server within the site, or site-specific name service, querying a DNS server within the site, or
an unspecified alternate method. There are no mandatory rules for an unspecified alternate method. There are no mandatory rules for the
the selection of a FQDN, but manual configuration MUST be supported. selection of a FQDN, but manual configuration MUST be supported. When
When DNS is used, client resolvers use the IPv4 transport. DNS is used, client resolvers use the IPv4 transport.
After initialization, nodes periodically refresh the PRL (i.e., using After initialization, nodes periodically refresh the PRL (i.e., using
one or more of the methods described above) after PrlRefreshInterval. one or more of the methods described above) after PrlRefreshInterval.
7.3.2 Validation of Router Advertisements Messages 7.3.2 Validation of Router Advertisements Messages
The specification in ([4], section 6.1.2) is used. The specification in ([RFC2461], section 6.1.2) is used.
Additionally, received RA messages that contain Prefix Information Additionally, received RA messages that contain Prefix Information
options and/or encode non-zero values in the Cur Hop Limit, Router options and/or encode non-zero values in the Cur Hop Limit, Router
Lifetime, Reachable Time, or Retrans Timer fields (see: [4], section Lifetime, Reachable Time, or Retrans Timer fields (see: [RFC2461],
4.2) MUST satisfy the following validity check for ISATAP: section 4.2) MUST satisfy the following validity check for ISATAP:
o the network-layer (IPv6) source address is an ISATAP address and o the network-layer (IPv6) source address is an ISATAP address and
embeds V4ADDR(i) for some PRL(i) embeds V4ADDR(i) for some PRL(i)
7.3.3 Router Specification 7.3.3 Router Specification
Routers with advertising ISATAP interfaces behave the same as Routers with advertising ISATAP interfaces behave the same as
described in ([4], section 6.2). As permitted by ([4], section described in ([RFC2461], section 6.2). As permitted by ([RFC2461],
6.2.6), advertising ISATAP interfaces SHOULD send unicast RA messages section 6.2.6), advertising ISATAP interfaces SHOULD send unicast RA
to a soliciting host's address when the solicitation's source address messages to a soliciting host's unicast address when the
is not the unspecified address. solicitation's source address is not the unspecified address.
7.3.4 Host Specification 7.3.4 Host Specification
When no unsolicited RA messages containing prefix information options Hosts behave the same as described in ([RFC2461], section 6.3) and
and/or non-zero router lifetime values are received, hosts MAY send ([RFC2462], section 5.5) with the following additional considerations
Router Solicitation (RS) messages using the specification in Section for ISATAP:
7.3.4.1. RA messages (whether solicited or unsolicited) are
processed using the specification in Section 7.3.4.2.
7.3.4.1 Sending Router Solicitations 7.3.4.1 Soliciting Router Advertisements
All PRL(i)'s are assumed to represent active advertising ISATAP Hosts solicit Router Advertisements (RAs) by sending Router
interfaces within the site, i.e., the PRL provides trust basis only; Solicitations (RSs) to advertising ISATAP interfaces in the PRL. The
not reachability detection. Hosts periodically solicit information manner of selecting PRL(i)'s for solicitation is up to the
from one or more PRL(i) by sending Router Solicitation (RS) messages. implementation. Hosts add the following variable to support the
The manner of selecting a PRL(i) for solicitation and/or deprecating
a previously-selected PRL(i) is outside the scope of this
specification. Hosts add the following variable to support the
solicitation process: solicitation process:
MinRouterSolicitInterval MinRouterSolicitInterval
Minimum time in seconds between successive solicitations of the Minimum time in seconds between successive solicitations of the
same advertising ISATAP interface. SHOULD be no less than 900 same advertising ISATAP interface. SHOULD be no less than 900
seconds. seconds.
Default: 900 seconds Default: 900 seconds
Solicitation consists of sending RS messages using the interface's
link-local unicast addresses as the source address. When the ISATAP
interface provides a multicast emulation mechanism (see: Section
5.4), RS messages are sent to the All-Routers multicast address.
Otherwise, they are sent to the link-local ISATAP address constructed
from V4ADDR(i) for some PRL(i) selected for solicitation. The RS
messages are otherwise sent exactly as in ([4], section 6.3.7).
7.3.4.2 Processing Router Advertisements RS messages use a link-local unicast address from the ISATAP
interface as the IPv6 source address. Unless otherwise specified in a
Hosts process received RA messages exactly as in ([4], section 6.3.4) future document, RS messages use the link-local ISATAP address
and ([9], section 5.5.3). (But, see Appendix C for non-normative constructed from V4ADDR(i) for the PRL(i) being solicited as the IPv6
considerations for RA messages containing MTU options.) destination address.
When the source address of the RA message is an ISATAP address that
embeds V4ADDR(i) for some PRL(i) selected for solicitation, hosts
additionally reset TIMER(i). Let "MIN_LIFETIME" be the minimum value
in the router lifetime or the lifetime(s) encoded in options included
in the RA message. Then, TIMER(i) is reset to:
MAX((0.5 * MIN_LIFETIME), MinRouterSolicitInterval) 7.3.4.2 Router Advertisement Processing
8. Deployment Considerations When the source address of an RA message is an ISATAP address that
embeds V4ADDR(i) for some PRL(i), hosts reset TIMER(i) to schedule
the next solicitation event (see: Section 7.3.4.1). Let
"MIN_LIFETIME" be the minimum value in the router lifetime or the
lifetime(s) encoded in options included in the RA message. Then,
TIMER(i) is reset as follows:
8.1 Host And Router Deployment Considerations TIMER(i) = MAX((0.5 * MIN_LIFETIME), MinRouterSolicitInterval)
For hosts, if an underlying link supports both IPv4 (over which 7.3.4.3 Stateful Autoconfiguration
ISATAP is implemented) and also supports IPv6 natively, then ISATAP
MAY be enabled if the native IPv6 layer does not receive Router
Advertisements (i.e., does not have connection with an IPv6 router).
After a non-link-local address has been configured and a default
router acquired on the native link, the host SHOULD discontinue the
router solicitation process described in the Host Specification
(Section 7.3.4) and allow existing ISATAP address configurations to
expire as specified in ([4], section 5.3) and ([9], section 5.5.4).
Any ISATAP addresses added to the DNS for this host should also be
removed. In this way, ISATAP use will gradually diminish as IPv6
routers are widely deployed throughout the site.
Routers MAY configure both a native IPv6 and ISATAP interface over If stateful autoconfiguration is invoked ([RFC2462], sections 5.5.2,
the same physical link. Routing will operate as usual between these 5.5.3), the "ALL_DHCP_Relay_Agents_and_Servers" multicast address
two domains. Note that the prefixes used on the ISATAP and native ([DHCPV6], section 5.1) is resolved to the link-local ISATAP address
IPv6 interfaces will be distinct. The IPv4 address(es) configured on constructed from V4ADDR(i) for some PRL(i).
a router's advertising ISATAP interface(s) SHOULD be added (either
automatically or manually) to the site's address records for
advertising ISATAP interfaces.
8.2 Site Administration Considerations 8. Deployment Considerations
The following considerations are noted for sites that deploy ISATAP: Hosts may enable ISATAP, e.g., when native IPv6 service is
unavailable. When native IPv6 service is acquired, hosts SHOULD
discontinue the ISATAP router solicitation process (Section 7.3.4)
and/or allow associated state to expire (see: [RFC2461], section 5.3
and [RFC2462], section 5.5.4). Any associated addresses added to the
DNS should also be removed.
o ISATAP links are administratively defined by a set of advertising Routers MAY configure both native IPv6 and ISATAP interfaces over the
ISATAP interfaces and set of nodes which discover those interface same physical link. The prefixes used on each interface will be
addresses. Thus, ISATAP links are defined by administrative (not distinct, and normal IPv6 routing between the interfaces may occur.
physical) boundaries.
o Hosts and routers that use ISATAP can be deployed in an ad-hoc 9. Site Administration Considerations
fashion. In particular, hosts can be deployed with little/no
advanced knowledge of existing routers, and routers can be
deployed with no reconfiguration requirements for hosts.
o Site administrators maintain a list of IPv4 addresses representing ISATAP sites are administratively defined by a set of advertising
advertising ISATAP interfaces and make them available via one or interfaces and set of nodes that solicit those interfaces. Thus,
more of the mechanisms described in Section 7.3.1. ISATAP nodes ISATAP sites are defined by administrative (not physical) boundaries.
use this list to initialize and periodically refresh the PRL.
Responsible site administration can reduce the control traffic.
At a minimum, administrators SHOULD ensure that dynamically
advertised information for the site's PRL is well maintained.
9. IANA Considerations Site administrators maintain a list of IPv4 addresses representing
advertising ISATAP interfaces and make them available via one or more
of the mechanisms described in Section 7.3.1. Responsible
administration can reduce control traffic overhead.
A DHCPv4 option code for ISATAP (TBD) [17] may be requested in the 10. IANA Considerations
event that this document or a derivative thereof is moved to
standards track.
Modifications to the IANA "ethernet-numbers" registry (e.g., based on Modifications to the IANA "ethernet-numbers" registry (e.g., based on
text in Appendix B) may be requested in the event that this document text in Appendix B) are requested.
or a derivative thereof is moved to standards track.
10. Security considerations
ISATAP site border routers and firewalls MUST implement IPv6 ingress 11. Security considerations
filtering and MUST NOT forward packets with site-local source and/or
destination addresses outside of the site [18].
In addition to possible attacks against IPv6, security attacks ISATAP site border routers and firewalls MUST implement IPv6 and IPv4
against IPv4 must also be considered. In particular, border routers ingress filtering, including ip-protocol-41 filtering. Packets with
and firewalls MUST implement IPv4 ingress filtering and local-use source and/or destination addresses MUST NOT be forwarded
ip-protocol-41 filtering. outside of the site.
Even with IPv4 and IPv6 ingress filtering, reflection attacks can Even with IPv4 and IPv6 ingress filtering, reflection attacks can
originate from compromised nodes within an ISATAP site that spoof originate from compromised nodes within an ISATAP site that spoof
IPv6 source addresses. Security mechanisms for reflection attack IPv6 source addresses. Security mechanisms for reflection attack
mitigation (e.g., [19], [20], etc.) SHOULD be used in routers with mitigation SHOULD be used in routers with advertising ISATAP
advertising ISATAP interfaces. At a minimum, ISATAP site border interfaces. At a minimum, border gateways SHOULD log potential source
gateways MUST log potential source address spoofing cases. address spoofing cases.
IPv6 Neighbor Discovery trust models and threats [21] apply also to
ISATAP. However, ([21], section 4.4.) shows that most of these
threats are mitigated in corporate networks that implement site
security mechanisms, i.e., the applicability space for ISATAP.
ISATAP addresses do not support privacy extensions for stateless ISATAP addresses do not support privacy extensions for stateless
address autoconfiguration [22]. However, since the ISATAP interface address autoconfiguration [RFC3041]. However, since the ISATAP
identifier is derived from the node's IPv4 address, ISATAP addresses interface identifier is derived from the node's IPv4 address, ISATAP
do not have the same level of privacy concerns as IPv6 addresses that addresses do not have the same level of privacy concerns as IPv6
use an interface identifier derived from the MAC address. (This is addresses that use an interface identifier derived from the MAC
especially true when private address allocations [10] are used.) address. (This is especially true when private address allocations
[RFC1918] are used.)
11. Acknowledgements 12. Acknowledgements
Some of the ideas presented in this draft were derived from work at Some of the ideas presented in this draft were derived from work at
SRI with internal funds and contractual support. Government sponsors SRI with internal funds and contractual support. Government sponsors
who supported the work include Monica Farah-Stapleton and Russell who supported the work include Monica Farah-Stapleton and Russell
Langan from U.S. Army CECOM ASEO, and Dr. Allen Moshfegh from U.S. Langan from U.S. Army CECOM ASEO, and Dr. Allen Moshfegh from U.S.
Office of Naval Research. Within SRI, Dr. Mike Frankel, J. Peter Office of Naval Research. Within SRI, Dr. Mike Frankel, J. Peter
Marcotullio, Lou Rodriguez, and Dr. Ambatipudi Sastry supported the Marcotullio, Lou Rodriguez, and Dr. Ambatipudi Sastry supported the
work and helped foster early interest. work and helped foster early interest.
The following peer reviewers are acknowledged for taking the time to The following peer reviewers are acknowledged for taking the time to
review a pre-release of this document and provide input: Jim Bound, review a pre-release of this document and provide input: Jim Bound,
Rich Draves, Cyndi Jung, Ambatipudi Sastry, Aaron Schrader, Ole Rich Draves, Cyndi Jung, Ambatipudi Sastry, Aaron Schrader, Ole
Troan, Vlad Yasevich. Troan, Vlad Yasevich.
The authors acknowledge members of the NGTRANS community who have The authors acknowledge members of the NGTRANS community who have
made significant contributions to this effort, including Rich Draves, made significant contributions to this effort, including Rich Draves,
Alain Durand, Nathan Lutchansky, Karen Nielsen, Art Shelest, Margaret Alain Durand, Nathan Lutchansky, Karen Nielsen, Art Shelest, Margaret
Wasserman, and Brian Zill. Wasserman, and Brian Zill.
The authors also wish to acknowledge the work of Quang Nguyen [23] The authors also wish to acknowledge the work of Quang Nguyen [VET]
under the guidance of Dr. Lixia Zhang that proposed very similar under the guidance of Dr. Lixia Zhang that proposed very similar
ideas to those that appear in this document. This work was first ideas to those that appear in this document. This work was first
brought to the authors' attention on September 20, 2002. brought to the authors' attention on September 20, 2002.
Normative References Normative References
[1] Deering, S. and R. Hinden, "Internet Protocol, Version 6 (IPv6) [ARCH] Hinden, R. and S. Deering, "IP Version 6 Addressing
Specification", RFC 2460, December 1998.
[2] Postel, J., "Internet Protocol", STD 5, RFC 791, September 1981.
[3] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[4] Narten, T., Nordmark, E. and W. Simpson, "Neighbor Discovery for
IP Version 6 (IPv6)", RFC 2461, December 1998.
[5] Hinden, R. and S. Deering, "IP Version 6 Addressing
Architecture", draft-ietf-ipngwg-addr-arch-v3-11 (work in Architecture", draft-ietf-ipngwg-addr-arch-v3-11 (work in
progress), October 2002. progress), October 2002.
[6] Armitage, G., Schulter, P., Jork, M. and G. Harter, "IPv6 over [MECH] Gilligan, R. and E. Nordmark, "Basic Transition Mechanisms
Non-Broadcast Multiple Access (NBMA) networks", RFC 2491, for IPv6 Hosts and Routers", draft-ietf-v6ops-mech-v2-00
January 1999. (work in progress), February 2003.
[7] Gilligan, R. and E. Nordmark, "Basic Transition Mechanisms for
IPv6 Hosts and Routers", draft-ietf-ngtrans-mech-v2-01 (work in
progress), November 2002.
[8] Conta, A. and S. Deering, "Internet Control Message Protocol
(ICMPv6) for the Internet Protocol Version 6 (IPv6)
Specification", RFC 2463, December 1998.
[9] Thomson, S. and T. Narten, "IPv6 Stateless Address [RFC0791] Postel, J., "Internet Protocol", STD 5, RFC 791, September
Autoconfiguration", RFC 2462, December 1998. 1981.
Informative References [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[10] Rekhter, Y., Moskowitz, R., Karrenberg, D., Groot, G. and E. [RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6
Lear, "Address Allocation for Private Internets", BCP 5, RFC (IPv6) Specification", RFC 2460, December 1998.
1918, February 1996.
[11] Carpenter, B. and K. Moore, "Connection of IPv6 Domains via [RFC2461] Narten, T., Nordmark, E. and W. Simpson, "Neighbor
IPv4 Clouds", RFC 3056, February 2001. Discovery for IP Version 6 (IPv6)", RFC 2461, December
1998.
[12] Deering, S., Fenner, W. and B. Haberman, "Multicast Listener [RFC2462] Thomson, S. and T. Narten, "IPv6 Stateless Address
Discovery (MLD) for IPv6", RFC 2710, October 1999. Autoconfiguration", RFC 2462, December 1998.
[13] Estrin, D., Farinacci, D., Helmy, A., Thaler, D., Deering, S., [RFC2463] Conta, A. and S. Deering, "Internet Control Message
Handley, M. and V. Jacobson, "Protocol Independent Protocol (ICMPv6) for the Internet Protocol Version 6
Multicast-Sparse Mode (PIM-SM): Protocol Specification", RFC (IPv6) Specification", RFC 2463, December 1998.
2362, June 1998.
[14] Armitage, G., "Support for Multicast over UNI 3.0/3.1 based ATM [RFC2491] Armitage, G., Schulter, P., Jork, M. and G. Harter, "IPv6
Networks", RFC 2022, November 1996. over Non-Broadcast Multiple Access (NBMA) networks", RFC
2491, January 1999.
[15] Droms, R., "Dynamic Host Configuration Protocol", RFC 2131, Informative References
March 1997. [DHCPV6] Droms, R., "Dynamic Host Configuration Protocol for IPv6
(DHCPv6)", draft-ietf-dhc-dhcpv6-28 (work in progress),
November 2002.
[16] Mockapetris, P., "Domain names - implementation and [RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, November 1987. specification", STD 13, RFC 1035, November 1987.
[17] Droms, R., "Procedures and IANA Guidelines for Definition of [RFC1918] Rekhter, Y., Moskowitz, R., Karrenberg, D., Groot, G. and
New DHCP Options and Message Types", BCP 43, RFC 2939, E. Lear, "Address Allocation for Private Internets", BCP
September 2000. 5, RFC 1918, February 1996.
[18] Hinden, R., "IPv6 Globally Unique Site-Local Addresses",
draft-hinden-ipv6-global-site-local-00 (work in progress),
December 2002.
[19] Savola, P., "Security Considerations for 6to4",
draft-savola-ngtrans-6to4-security-01 (work in progress), March
2002.
[20] Bellovin, S., Leech, M. and T. Taylor, "ICMP Traceback
Messages", draft-ietf-itrace-03 (work in progress), January
2003.
[21] Nikander, P., "IPv6 Neighbor Discovery trust models and
threats", draft-ietf-send-psreq-01 (work in progress), January
2003.
[22] Narten, T. and R. Draves, "Privacy Extensions for Stateless
Address Autoconfiguration in IPv6", RFC 3041, January 2001.
[23] Nguyen, Q., "http://irl.cs.ucla.edu/vet/report.ps", spring
1998.
[24] Braden, R., "Requirements for Internet Hosts - Communication
Layers", STD 3, RFC 1122, October 1989.
[25] Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191, [RFC2022] Armitage, G., "Support for Multicast over UNI 3.0/3.1
November 1990. based ATM Networks", RFC 2022, November 1996.
[26] Postel, J., "Internet Control Message Protocol", STD 5, RFC [RFC2131] Droms, R., "Dynamic Host Configuration Protocol", RFC
792, September 1981. 2131, March 1997.
[27] Baker, F., "Requirements for IP Version 4 Routers", RFC 1812, [RFC2362] Estrin, D., Farinacci, D., Helmy, A., Thaler, D., Deering,
June 1995. S., Handley, M. and V. Jacobson, "Protocol Independent
Multicast-Sparse Mode (PIM-SM): Protocol Specification",
RFC 2362, June 1998.
[28] Lahey, K., "TCP Problems with Path MTU Discovery", RFC 2923, [RFC2710] Deering, S., Fenner, W. and B. Haberman, "Multicast
September 2000. Listener Discovery (MLD) for IPv6", RFC 2710, October
1999.
[29] McCann, J., Deering, S. and J. Mogul, "Path MTU Discovery for [RFC3041] Narten, T. and R. Draves, "Privacy Extensions for
IP version 6", RFC 1981, August 1996. Stateless Address Autoconfiguration in IPv6", RFC 3041,
January 2001.
[30] Jacobson, V., Braden, B. and D. Borman, "TCP Extensions for [RFC3056] Carpenter, B. and K. Moore, "Connection of IPv6 Domains
High Performance", RFC 1323, May 1992. via IPv4 Clouds", RFC 3056, February 2001.
[31] Templin, F., "Neighbor Affiliation Protocol for [VET] Nguyen, Q., "http://irl.cs.ucla.edu/vet/report.ps", spring
IPv6-over-(foo)-over-IPv4", draft-templin-v6v4-ndisc-01 (work 1998.
in progress), November 2002.
Authors' Addresses Authors' Addresses
Fred L. Templin Fred L. Templin
Nokia Nokia
313 Fairchild Drive 313 Fairchild Drive
Mountain View, CA 94110 Mountain View, CA 94110
US US
Phone: +1 650 625 2331 Phone: +1 650 625 2331
skipping to change at page 16, line 7 skipping to change at page 13, line 33
Microsoft Corporation Microsoft Corporation
One Microsoft Way One Microsoft Way
Redmond, WA 98052-6399 Redmond, WA 98052-6399
US US
Phone: +1 425 703 8835 Phone: +1 425 703 8835
EMail: dthaler@microsoft.com EMail: dthaler@microsoft.com
Appendix A. Major Changes Appendix A. Major Changes
changes from version 11 to version 12: changes from version 12 to version 13:
o Added comments from co-authors o Added comments from co-authors
o Revised PRL initialization o Text cleanup; removed extraneous text
o Updated MTU section o Revised ISATAP interface/link terminology
changes from version 10 to version 11: o Returned to using symbolic reference names
o Added multicast/anycast subsection o Revised MTU section; moved non-normative MTU text to seperate
document
changes from earlier versions to version 12:
o Added multicast/anycast subsection
o Revised PRL initialization o Revised PRL initialization
o Updated neighbor discovery, security consideration sections o Updated neighbor discovery, security consideration sections
o Updated MTU section
changes from version 09 to version 10:
o Rearranged/revised sections 5, 6, 7 o Rearranged/revised sections 5, 6, 7
o updated MTU section
changes from version 08 to version 09:
o Added stateful autoconfiguration mechanism o Added stateful autoconfiguration mechanism
o Normative references to RFC 2491, RFC 2462 o Normative references to RFC 2491, RFC 2462
o Moved non-normative MTU text to appendix C o Moved non-normative MTU text to appendix C
changes from version 07 to version 08:
o updated MTU section
changes from version 06 to version 07:
o clarified address resolution, Neighbor Unreachability Detection o clarified address resolution, Neighbor Unreachability Detection
o specified MTU/MRU requirements o specified MTU/MRU requirements
changes from earlier versions to version 06:
o Addressed operational issues identified in 05 based on discussion o Addressed operational issues identified in 05 based on discussion
between co-authors between co-authors
o Clarified ambiguous text per comments from Hannu Flinck; Jason o Clarified ambiguous text per comments from Hannu Flinck; Jason
Goldschmidt Goldschmidt
o Moved historical text in section 4.1 to Appendix B in response to o Moved historical text in section 4.1 to Appendix B in response to
comments from Pekka Savola comments from Pekka Savola
o Identified operational issues for anticipated deployment scenarios o Identified operational issues for anticipated deployment scenarios
o Included reference to Quang Nguyen work o Included reference to Quang Nguyen work
skipping to change at page 18, line 19 skipping to change at page 17, line 5
including support for encapsulating legacy EUI-48 interface including support for encapsulating legacy EUI-48 interface
identifiers (e.g., an IANA EUI-48 format multicast address such as: identifiers (e.g., an IANA EUI-48 format multicast address such as:
'01-00-5E-01-02-03' is encapsulated as: '01-00-5E-FF-FE-01-02-03'). '01-00-5E-01-02-03' is encapsulated as: '01-00-5E-FF-FE-01-02-03').
But, the specification also provides a special TYPE (0xFE) to But, the specification also provides a special TYPE (0xFE) to
indicate an IPv4 address is embedded. Thus, when the first four indicate an IPv4 address is embedded. Thus, when the first four
octets of an IPv6 interface identifier are: '00-00-5E-FE' (note: the octets of an IPv6 interface identifier are: '00-00-5E-FE' (note: the
'u/l' bit MUST be 0) the interface identifier is said to be in 'u/l' bit MUST be 0) the interface identifier is said to be in
"ISATAP format" and the next four octets embed an IPv4 address "ISATAP format" and the next four octets embed an IPv4 address
encoded in network byte order. encoded in network byte order.
Appendix C. ISATAP Interface MTU Considerations
ISATAP encapsulators and decapsulators are IPv6 neighbors that may be
separated by multiple link layer (IPv4) forwarding hops. Thus, the
path MTU of the underlying IPv4 network may determine the uni-
directional IPv6 per-neighbor MTU from the encapsulator to the
decapsulator. (Note that this constitutes the MTU of only one hop in
what may be a multiple-hop IPv6 path.) When the encapsulator's ISATAP
interface configures a large LinkMTU value (see: Section 6.3),
special considerations apply as described in the following
non-normative sections:
C.1 Stateless (Static) MTU Assignment
Nodes that connect to the Internet should be able to reassemble and/
or discard IPv4 packets up to 64KB in length when the DF bit is not
set in the encapsulating IPv4 header. Nodes that cannot reassemble/
discard maximum-length IPv4 packets are vulnerable to buffer overrun
attacks. This issue may be obviated for nodes that are accessed only
within a site (i.e., do not connect directly to the Internet) since
site border gateways, etc. can filter and discard fragments of large
packets before they reach constrained node(s).
When the ISATAP encapsulator does not implement a dynamic link layer
mechanism to determine per-neighbor MTUs, all IPv6 packets are
encapsulated with the DF bit not set in the IPv4 header.
Additionally, LinkMTU may be set to a value that is no more than the
smallest Effective MTU to Receive (EMTU_R) (see: RFC 1122 [24],
section 3.3.2) for all potential decapsulators in the site. The
value chosen for LinkMTU must be at least 1280 bytes (the minimum
IPv6 MTU) and such that the potential worst-case level of
fragmentation in the underlying IPv4 network is deemed "acceptable"
by the site's standards.
For example, when all decapsulators in the site are known to have an
EMTU_R of 10KB and the site's IPv4 routers are optimized for IPv4
fragmentation, encapsulators may be able to use LinkMTU values as
large as 10KB (minus 20 bytes for IPv4 encapsulation). Conversely,
when IPv4 fragmentation causes performance degradation along some
paths, LinkMTU should be set to a smaller value.
Nodes that use a static MTU assignment SHOULD copy the value in an
MTU option received in any Router Advertisement message into LinkMTU
for the ISATAP interface as specified in ([4], section 6.3.4).
C.2 Stateful (Dynamic) MTU Determination
When the encapsulator implements a dynamic MTU determination
mechanism it keeps a link layer cache of per-neighbor MTU values
(e.g., as ancillary data in the IPv6 neighbor cache, in the IPv4 path
MTU discovery cache, etc.). IPv4 path MTU discovery [25] uses ICMPv4
"fragmentation needed" messages, but these generally do not provide
enough information for stateless translation to ICMPv6 "packet too
big" messages (see: RFC 792 [26] and RFC 1812 [27], section 4.3.2.3).
Additionally, ICMPv4 "fragmentation needed" messages can be spoofed,
filtered, or not sent at all by some forwarding nodes. Thus, IPv4
Path MTU discovery used alone may be inadequate and can result in
black holes that are difficult to diagnose [28].
Alternate methods for determining per-neighbor MTUs should be used
when RFC 1191 path MTU discovery is deemed inadequate. In these
methods, the encapsulator uses periodic and/or on-demand probing of
the IPv4 path to the decapsulator to initialize and update cache
entries. The following three probing methods (among others) are
possible:
1. Encapsulator-driven - the encapsulator periodically sends probe
packets with the DF bit set in the IPv4 header and waits for a
positive acknowledgement from the decapsulator that the probe was
received
2. Decapsulator-driven - the encapsulator sends all packets with the
DF bit NOT set in the IPv4 header unless and until the
decapsulator sends a "Fragmentation Experienced" indication(s)
3. Hybrid - the encapsulator and decapsulator engage in a dialogue
and use "intelligent" probing to monitor the path MTU
These methods are discussed in detail in the following subsections:
C.2.1 Encapsulator-driven Method
In this method, the encapsulator sets the DF bit in the IPv4 header
of probe packets. Probe packets may be sent either when the
encapsulator's link layer forwards a large data packet to the
decapsulator (i.e., on-demand) or when the path MTU for the
decapsulator has not been verified for some time (i.e., periodic).
IPv6 Neighbor Solicitation (NS) or ICMPv6 ECHO_REQUEST packets with
padding bytes added could be used for this purpose, since successful
delivery results in a positive acknowledgement that the probe
succeeded vis-a-vis a response from the decapsulator.
While probing, the encapsulator maintains a queue of packets that
have the decapsulator as the IPv6 next-hop address. If the probe
succeeds, packets in the queue that are no larger than the probe size
are sent to the decapsulator. If the probe fails, packets that are
larger than the last known successful probe are dropped and an ICMPv6
"packet too big" message returned to the sender [29]. The queue
should be large enough to buffer the (delay*bandwidth) product for
the round-trip time to the decapsulator. When smaller queues are
used, loss of packets that are too big for the yet-to-be-determined
path MTU may occur with no ICMPv6 "packet too big" message returned.
Such loss may occur only in rare instances, but may result in
unpredictable behavior in senders that base their adaptation solely
on ICMPv6 "packet too big" messages.
This method has the advantage that the decapsulator need not
implement any special mechanisms, since standard IPv6 request/
response mechanisms are used. Additionally, the encapsulator is
assured that any packets that are too large for the decapsulator to
receive will be dropped by the network. Disadvantages for this
method include the fact that probe packets do not carry data and thus
consume network resources. Additionally, queues may become large on
Long, Fat Networks (LFNs) (see: RFC 1323 [30]).
C.2.2 Decapsulator-driven Method
In this method, the encapsulator sends all packets with the DF bit
NOT set in the IPv4 header with the expectation that the decapsulator
will send a "Fragmentation Experienced" indication if the IPv4
network fragments packets. In other words, the decapsulator simply
sends all packets that are no larger than LinkMTU unless and until it
receives "Fragmentation Experienced" messages from the decapsulator.
The decapsulator can use IPv6 Router Advertisement (RA) messages with
an MTU option as the means for both reporting fragmentation and
informing the encapsulator of a new MTU value to use.
This method has the advantage that the data packets themselves are
used as probes and no queuing on the encapsulator is necessary.
(When large data packets for probing are not available, smaller data
packets can be null-padded to the desired probe size by artificially
inflating the length field in the IPv4 header; leaving the IPv6
length unchanged.) An additional advantage is that fewer packets will
be lost since the decapsulator will quite often be able to reassemble
packets fragmented by the network. The primary disadvantage for this
method is that, using the current specifications, the encapsulator
has no way of knowing whether a particular decapsulator implements
the "fragmentation experienced" signaling capability. However, the
"fragmentation experienced" indication can be trivially implemented
in an application on the decapsulator that uses the Berkeley Packet
Filter (aka, libpcap) to listen for fragmented packets from
encapsulators.
When fragmented packets arrive, the decapsulator sends IPv6 RA
messages with an MTU option to inform the encapsulator that
fragmentation has been experienced and a new value for the neighbor's
MTU should be used. The decapsulator additionally sends ICMPv6
"packet too big" messages to the original source when a fragmented
packet is not correctly reassembled. This function need not be built
into the decapsulator's operating system and can be added as an
after-market feature. Finally, simply adding an extra bit in a
neighbor discovery message header would provide a means for the
decapsulator to inform the encapsulator that dynamic MTU discovery is
supported.
C.2.3 Hybrid Method
In this method, the encapsulator and decapsulator engage in a
"neighbor affiliation" protocol to negotiate link-layer parameters
such as MTU. (See: [31] for an example of such an approach.) This
approach has the advantage that bi-directional links are used and
both ends of the link have unambiguous knowledge that the other end
implements the protocol. However, the signaling protocol between the
endpoints is complicated and additional state is required in both the
encapsulator and decapsultor. The hybrid method seems best suited to
implementation in a reliable transport-layer protocol rather than at
the network/link layer.
C.2.4 Additional Notes
o In all dynamic methods, some packet loss due to link/buffer
restrictions may occur with no ICMPv6 "packet too big" message
returned to the sender. Unenlightened senders will interpret such
loss as loss due to congestion, which may result in longer
convergence to the actual path MTU. Enlightened senders will
interpret the loss as due to link/buffer restrictions and
immediately reduce their MTU estimate.
o In all dynamic methods, when a Router Advertisement (RA) message
includes an MTU option hosts SHOULD NOT copy the option's value
into LinkMTU for the ISATAP interface. Instead, when the ISATAP
interface uses a per-neighbor path MTU cache, hosts SHOULD copy
the MTU option's value into the cache entry for the neighbor that
sent the RA message. This leaves an ambiguous interpretation for
processing received RA messages which could be eliminated if [4]
were modified to allow Neighbor Advertisement (NA) messages to
carry MTU options.
o In all methods, a "minimum MTU" must be supported by all nodes for
multicast (i.e., even when multicast is emulated on the NBMA IPv4
network.) The mechanisms described above speak only to the unicast
case for MTU determination.
o To avoid denial-of-service attacks that would cause superfluous
probing based on counting down/up by small increments, plateau
tables (e.g., [25], section 7) should be used when the actual MTU
value is indeterminant.
o ICMPv4 "fragmentation needed" messages may result when a link
restriction is encountered but may also come from denial of
service attacks. Implementations should treat ICMPv4
"fragmentation needed" messages as "tentative" negative
acknowledgments and apply heuristics to determine when to suspect
an actual link restriction and when to ignore the messages. IPv6
packets lost due actual link restrictions are perceived as lost
due to congestion by the original source, but robust
implementations minimize instances of such packet loss without
ICMPv6 "packet too big" messages returned to the sender.
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