Internet Engineering Task Force A. Bansal, T. Przygienda, A. Patel
INTERNET DRAFT Fore,
Siara
Oct
1999
IS-IS over IPv4
<draft-ietf-isis-wg-over-ip-02.txt>
Status of This Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026. Internet-Drafts are working
documents of the Internet Engineering Task Force (IETF), its areas,
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Abstract
This draft describes an optional implementation technique within
IS-IS [ISO90 , Cal90a , Cal90b ] used today by several ISPs for routing
within their clouds. IS-IS is an interior gateway routing protocol
developed originally by OSI and used with IP extensions as IGP. This
draft describes how to encapsulate IS-IS packets in IPv4 [Pos81 ]
format. Such an encapsulation has many advantages, one of those
being the possibility to run integrated IS-IS on anything that
understands IPv4, including avian carriers [Wai90 ]. Encapsulation of
IS-IS PDUs in IPv6 is outside the scope of this document.
1. Introduction
Encapsulation of IS-IS as defined in ISO 10589 [ISO90 , Cal90a ,
Cal90b] uses directly over Link Layer protocols as opposed to IP
routing protocols [Moy97 ] that are encapsulated over IP directly.
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By defining an encapsulation of IS-IS in IP, we save on special OSI
encapsulation on several media types. Such an encapsulation would
solve fragmentation problems of large LSPs and remove the necessity
for OSI PPP extensions [Kat92 ] when IS-IS is run over negotiated
PPP links. Additionally, on certain media, such as P2P ATM links,
no LLC/SNAP encapsulation is necessary to provide multi-protocol
routing, allowing for gains in efficiency.
2. Encapsulation of IS-IS and ISO 9542 packets over IP
IS-IS is encapsulated directly over the Internet Protocol's network
layer. IS-IS packets are therefore encapsulated by IP and local
data-link headers. Within this encapsulation, IS-IS packets are
propagated as usual, however without the appropriate link-layer
fields but starting at NLPI.
IS-IS does not normally provide a way to transmit packets larger
than MTU size. This proposal allows to use IP fragmentation when
transmitting such packets. If necessary, the length of IS-IS packets
over IP can be up to 65,535 bytes (including the IP header). The
IS-IS packet types that are likely to be large (LSPs, CSNPs, PSNPs)
can usually be split into several separate protocol packets, without
loss of functionality. This is recommended; IP fragmentation SHOULD
be avoided whenever possible since it can lead to different problems,
such as loss of fragments causing the retransmission of complete IP
packets. Following rules apply:
- IIHs MUST not exceed the size of [InterfaceMT U - IP headersize]
(1) and have the DF bit set. MTU size padding rules should be
followed as described in ISO 10589.
- SNPs MUST NOT be larger than the respective
originatingLSPBufferSize.
- SNPs MUST be sent allowing IP fragmentation (DF bit not set).
- SNPs SHOULD NOT be larger than the minimum of [InterfaceMT U -
IP headersize] and respective originatingLSPBufferSize.
- LSP fragments MAY BE built with a size up to the value of
corresponding originatingLSPBufferSize.
___________________________________________
1. not of maximum of Buffer and DataLinkSize used normally
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- LSP fragments MUST NOT be larger than the respective
originatingLSPBufferSize.
- LSPs MUST be sent allowing IP fragmentation (DF bit not set).
- LSP fragments SHOULD NOT exceed the size of the minimum of
dataLinkBlockSize and respective originatingLSPBufferSize.
This set of rules allows to configure a network with respective
originatingLSPBufferSize larger than some interfaces' MTUs.
In a mixed environment, care must be taken that respective
originatingLSPBufferSize does net exceed the MTU size of interfaces
without IP encapsulation. A default originatingLSPBufferSize of 1472
is recommended.
The other important features of IS-IS in IP's IP encapsulation are:
- IS-IS in IP has been assigned IP protocol number 124. IP
protocol number assignments are documented in [RP94 ].
Historical note: For development purposes, IP Protocol number
9 (Private IGP protocol number) [RP94 ] has been used until an
official number was granted.
- Use of IP multicast. Some IS-IS in IP messages are multicast,
when sent over broadcast networks. Three distinct IP multicast
addresses are used. Packets sent to these multicast addresses
should never be forwarded; they are meant to travel a single hop
only. To ensure that these packets will not travel multiple
hops, their IP TTL must be set to 1.
IPAllL1ISs
OSI multicast value of this address was O1-80-C2-00-00-14.
This multicast address has been assigned the IP address
value 224.0.0.19 for IP encapsulated IS-IS. All routers
running L1 IS-IS in IP should be prepared to receive
packets sent to this address. Hello packets are always
sent to this destination. Also, certain IS-IS in IP
protocol packets are sent to this address during the
flooding procedure.
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IPAllL2ISs
OSI multicast value of this address was O1-80-C2-00-00-15.
This multicast address has been assigned the IP address
value 224.0.0.20 for IP encapsulated IS-IS. All routers
running L2 IS-IS in IP should be prepared to receive
packets sent to this address. Hello packets are always
sent to this destination. Also, certain IS-IS in IP
protocol packets are sent to this address during the
flooding procedure.
IPAllIntermediateSystems
OSI multicast value of this address was 09-00-2B-0O-00-05.
This multicast address has been assigned the IP address
value 224.0.0.21 for IP encapsulated IS-IS. All routers
running IS-IS in IP should be prepared to receive packets
sent to this address. ISO 9542 is using this address.
- All IS-IS in IP routing protocol packets are sent using the
normal service TOS value of binary 0000 defined in [Alm92 ].
- Routing protocol packets are sent with IP precedence set to
Internetwork Control. IS-IS in IP protocol packets should be
given precedence over regular IP data traffic, in both sending
and receiving. Setting the IP precedence field in the IP header
to Internetwork Control [Pos81 ] may help to implement this
objective.
3. Internal Encapsulation
On point-to-point links no MAC addresses are used by IS-IS. Therefore
the Intradomain Routeing Protocol Discriminator or ISO 9542 Network
Layer Protocol Identifier starts directly after the IP header.
For P2P link running PPP, the Payload format will consist of PPP
header, followed by the IP header and the usual NLPID as indicated in
figure 1.
For P2P ATM links using VC muxing, the payload format MUST NOT
include the PPP header as indicated in figure 2.
In the case of broadcast media, MAC addresses are used for adjacency
identification. It is rather painful from the implementation
perspective to assume that an IS-IS must have access to MAC headers
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+------------+-----------+-----------+
| PPP Header | IP Header | ISIS PDU |
+------------+-----------+-----------+
Figure 1: Encapsulation of ISIS frames over PPP
+-------------+-----------+----------+
| AAL5 Header | IP Header | ISIS PDU |
+-------------+-----------+----------+
Figure 2: Encapsulation of ISIS frames over ATM
when receiving frames. However, based on the observation that
ethernets have at least one IP address assigned, MAC address
necessary for adjacency maintenance can be built algorithmically
using the source address of the IP packet received. To prevent
collisions with universally administered addresses, the addresses
are designated by having bit one in byte zero of the MAC set to 1 to
indicate locally administered address as specified by the according
IEEE standard. When receiving an IP encapsulated ISIS PDU, the
artificial MAC address is assumed to consist (in network order) of
4 bytes of source IP address aligned as least significant bytes,
``locally administered'' bit being set and all remaining bits being
zero. Figure 3 visualizes such an address for a source IP address
128.127.128.1.
0 2 4 6 0 2 4 6 0 2 4 6 0 2 4 6 0 2 4 6 0 2 4 6
+--------+--------+--------+--------+--------+--------+
|01000000|00000000|10000000|01111111|10000000|00000001|
+--------+--------+--------+--------+--------+--------+
Figure 3: Artificial MAC for IP address 128.127.128.1
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When operating on an interface encapsulating IS-IS within IP,
encapsulated PDUs MUST be sent with a constant IP source address.
Any change of the IP address on the interface MUST be considered
equivalent to change of MAC address in ISO mode. In all places in
which MAC addresses are being used in ISO mode, source IP address
MUST be used to compute artificial MACs when sending and parsing
received PDUs.
Any PDU received on IP encapsulated broadcast interface and
containing MACs with either the ``locally administered'' bit not
being set or any of the remaining bits in the first two bytes being
set SHOULD be discarded.
4. Interoperability with Devices without IP Encapsulation
An interoperability solution for devices using IP encapsulation and
OSI encapsulation of ISIS frames would be only useful if it could
significantly ease the migration path in the existing networks.
Given the fact that graceful migration is not a paramount issue for
existing networks and any solution would invariably lead to the
problem of partitioning of broadcast media, such a solution is not
defined.
5. Acknowledgments
The encapsulation description part has been "borrowed" from a
well-known RFC [Moy97 ] with the author's consent. Tony Li, Dave
Katz, Christian Hopps, Mike Shand, Henk Smit, Rajesh Varadarajan,
Jeff Swinton, Stacy Smith and others provided constructive comments.
6. Security Consideration
ISIS security applies to the work presented. No specific security
issues with the proposed solutions are known. Things like IPSec may
influence the work in strange and unknown ways ;-)
References
[Alm92] P. Almquist. Type of Service in the Internet Protocol
Suite. INTERNET-RFC, Internet Engineering Task Force, July
1992.
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[Cal90a] R. Callon. OSI ISIS Intradomain Routing Protocol.
INTERNET-RFC, Internet Engineering Task Force, February
1990.
[Cal90b] R. Callon. Use of OSI ISIS for Routing in TCP/IP and Dual
Environments. INTERNET-RFC, Internet Engineering Task
Force, December 1990.
[Hei93] J. Heinanen. Rfc 1483, Multiprotocol Encapsulation over ATM
Adaptation Layer 5. Internet Engineering Task Force, July
1993.
[ISO90] ISO. Information Technology - Telecommunications and
Information Exchange between Systems - Intermediate System
to Intermediate System Routing Exchange Protocol for
Use in Conjunction with the Protocol for Providing the
Connectionless-Mode Network Service. ISO, 1990.
[Kat92] D. Katz. Rfc 1377, The PPP OSI Network Layer Control
Protocol (OSINLCP). Internet Engineering Task Force,
November 1992.
[MD90] J. Mogul and S. Deering. Rfc 1191, Path MTU Discovery.
Internet Engineering Task Force, November 1990.
[Moy97] J. Moy. OSPFv2, RFC 2178. Internet Engineering Task Force,
July 1997.
[Pos81] J. Postel. Rfc 791, rfc Internet Protocol. Internet
Engineering Task Force, September 1981.
[RP94] J. Reynolds and J. Postel. Rfc 1700, Assigned Numbers.
Internet Engineering Task Force, October 1994.
[Wai90] D. Waitzman. Rfc 1149, standard for the Transmission of
IP Datagrams on Avian Carriers. Internet Engineering Task
Force, April 1990.
Authors' Addresses
Tony Przygienda
Siara Systems
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300 Ferguson Drive
Mountain View, CA 94043
prz@siara.com
Ajay Patel
Siara Systems
300 Ferguson Drive
Mountain View, CA 94043
ajayp@siara.com
Atul Bansal
FORE Systems,
1595 Spring Hill Rd
Vienna, VA 22181
bansal@fore.com
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