Internet Engineering Task Force A. Bansal, T. Przygienda, A. Patel
INTERNET DRAFT Fore, Bell Labs/Lucent
20 Jan 1998
IS-IS over IPv4
<draft-ietf-isis-wg-over-ip-00.txt>
Status of This Memo
This document is an Internet Draft, and can be found as
draft-ietf-isis-wg-over-ip-02.txt in any standard internet drafts
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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].
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.
By defining an encapsulation of IS-IS in IP, we save on special OSI
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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 have the size of [InterfaceMTU - IP headersize] (1)
and have the DF bit set. IIH's TTL MUST be set to 1 to prevent
them from traveling multiple hops.
- SNPs on IP encapsulated interfaces MUST NOT be larger than
the minimum of [InterfaceMTU - IP headersize] and respective
originatingLSPBufferSize.
- LSP fragments MAY BE built with a size up to the value of
corresponding originatingLSPBufferSize.
- LSP fragments MUST NOT be larger than the respective
originatingLSPBufferSize.
___________________________________________
1. not of maximum of Buffer and DataLinkSize used normally
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- 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.
The other important features of IS-IS in IP's IP encapsulation are:
- 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.? 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.
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.? 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.
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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.? 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.
- IS-IS in IP is IP protocol number ??. This number has been
requested with the Network Information Center. IP protocol
number assignments are documented in [RP94].
Note: For development purposes, IP Protocol number 9 (Private
IGP protocol number) [RP94] will be used until an official number
is granted.
- 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 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 NLPID as indicated in figure 1.
+------------+-----------+----------------+
| PPP Header | IP Header | NLPID|ISIS PDU |
+------------+-----------+----------------+
Figure 1: Encapsulation of ISIS frames over PPP
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For P2P ATM links using VC muxing, the payload format must not
include the PPP header as indicated in figure 2.
+-------------+-----------+-----------------+
| AAL5 Header | IP Header | NLPID|ISIS PDU |
+-------------+-----------+-----------------+
Figure 2: Encapsulation of ISIS frames over ATM
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
when receiving frames. On the other hand, introducing an artificial
`bridging' encapsulation header preceding the Intradomain Routeing
Protocol Discriminator within the routed frame was perceived as
a very bulky solution. As yet another approach, based on the
observation that ethernets always have at least one IP address,
MAC address necessary for adjacency maintenance could be built
algorithmically using the source address of the IP packet received.
However, this would necessitate e.g. the allocation of a 2-byte
prefix within the 802.2/3 MAC space. Given the difficulties
surrounding each of these solutions, no special encapsulation is
defined for the ethernet case and the protocol implementation is
either required to received the complete frame, including layer 2
encapsulation or obtain the appropriate MAC address from the source
IP address using an interface to the lower layers of the IP stack.
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 invariable lead to the
problem of partitioning of broadcast media, such a solution is not
defined.
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5. Acknowledgments
The encapsulation description part has been "borrowed" from a
well-known RFC [Moy97] with the author's consent. Tony Li, Mike
Shand, Henk Smit, Rajesh Varadarajan, Jeff Swinton, Stacy Smith gave
many 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.
[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.
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[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
Bell Labs, Lucent Technologies
101 Crawfords Corner Road
Holmdel, NJ 07733-3030
prz@dnrc.bell-labs.com
Ajay Patel
Bell Labs, Lucent Technologies
101 Crawfords Corner Road
Holmdel, NJ 07733-3030
ajayp@dnrc.bell-labs.com
Atul Bansal
FORE Systems,
1595 Spring Hill Rd
Vienna, VA 22181
bansal@fore.com
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