Internet DRAFT - draft-hsmit-isis-ip-aal5mux

draft-hsmit-isis-ip-aal5mux



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Network Working Group                                      Henk Smit
Internet Draft                                         cisco Systems
Expiration Date: December 1999

      IS-IS and IP over the same ATM VC with aal5mux encapsulation

                   draft-hsmit-isis-ip-aal5mux-00.txt

1. Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026 except that the right to
   produce derivative works is not granted.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
   other groups may also distribute working documents as Internet-
   Drafts.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as ``work in progress.''

   The list of current Internet-Drafts can be accessed at
   http://www.ietf.org/ietf/1id-abstracts.txt

   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html.

2. Abstract

   This document describes a way to send and receive both IP and IS-IS
   packets over one and the same ATM Virtual Circuit without using an
   extra layer of encapsulation.

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3. Introduction

   IS-IS is a routing protocol used for IP and CLNS routing.  A number
   of large ISP networks use IS-IS as their IP IGP.  The IS-IS protocol
   is specified in [1], with extensions for supporting IPv4 specified in
   [2].  IS-IS packets are encapsulated directly in the datalink layer,
   as part of the OSI protocol family.  IS-IS packets are not
   encapsulated in an IP header, nor in a CLNS header.

   ATM is a technology used to transmit data, voice and video.  When
   data packets are sent over ATM, they are encapsulated in an AAL5
   frame header, and the AAL5 frame is divided in a number of fixed size
   cells. Cell headers are 5 bytes. The AAL5 headers are 8 bytes. Cells
   have a payload of 48 bytes.

3.1. ATM encapsulation methods

   When data packets are sent over an ATM VC, the receiver must have a
   way to recognize each packet. Currently there are 3 ways to do this:

      - Reserve a separate VC for each layer-3 protocol.
        This is called 'Null Encapsulation', 'VC multiplexing',
        or 'aal5mux'. This encapsulation is described in [3].

      - Add a 8-byte LLC/SNAP header in front of the layer-3 header.
        This is called 'LLC/SNAP Encapsulation' or aal5snap.
        This encapsulation is also described in [3].

      - Add two bytes with the NLPID in front of the data packet.
        This is called aal5nlpid encapsulation.

   It is common practice to send both routing protocol packets and user
   traffic over the same physical or logical path. Doing so prevents
   certain failure modes. Thus IS-IS packets and IP packets must be sent
   over the same ATM VC. Because IS-IS packets are not encapsulated in
   IP packets, users of IS-IS for IP routing must use aal5snap or
   aal5nlpid encapsulation.

3.2. Motivation

   The current estimate is that 40% of the packet on the Internet are
   small packets, of 40 bytes or less. Most of these are TCP
   acknowledgements. They consist of a 20 byte IP header, and a 20 byte
   TPC header, which makes them exactly 40 bytes long.

   The ATM cell payload is 48 bytes. The 48 bytes of AAL5, IP and TCP

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Internet Draft     draft-hsmit-isis-ip-aal5mux-00.txt          June 1999

   headers will exactly fill one ATM cell. If one uses aal5nlpid or
   aal5snap, we need two cells to store all 50 or 56 bytes of AAL5,
   SNAP/NLPID, IP and TCP headers. Because cells are fixed in length,
   the biggest part of the second cell will be unused.

   Bandwidth used by ATM, which is not used to transport user traffic,
   is called cell-tax. A quick calculation will show that using aal5snap
   increases the cell-tax by 5% to 7% of the total bandwidth.

4. Sending IS-IS and IP over the same aal5mux Virtual Circuit

   Unfortunately aal5mux supports only one layer-3 protocol per VC.  The
   reason is that the receiver must be able to de-multiplex packets to
   different layer-3 stacks. Without extra information in the AAL5
   frame, indicating the layer-3 protocol, layer-2 at the receiver can
   not distinguish packets from different layer-3 protocols.

   But if there is no extra information in the AAL5 frame, can the
   receiver not make the distinction between IP and IS-IS by just
   looking at the bare layer-3 packet ?  Fortunately the answer is yes.
   By looking at the first byte of each packet, the receiver can
   determine if a packet is IP or IS-IS.

   The first two fields in the IP header are the 4-bit version number
   and the 4-bit header length. The value of the first byte is normally
   0x45. If there are IP header options attached to the IP header, the
   first byte can be between 0x46 and 0x4F.

   The first byte in an IS-IS packet is always 0x83.

   Thus by looking at the first byte of an incoming packet, the receiver
   can separate IP and IS-IS packets. Because of this feature we do not
   depend on the ATM layer anymore to help us with the demultiplexing.
   Routers can now send and receive both IS-IS and IP packets in a non-
   ambiguous way over the same aal5mux VC.

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Internet Draft     draft-hsmit-isis-ip-aal5mux-00.txt          June 1999

5. Remaining issues

   If IS-IS is used in Dual-mode, to build both IP and CLNS routing
   tables, it is necessary that CLNS packets are forwarded over the same
   path as IP packets.  Implementations supporting IP and CLNS routing
   might want to consider extending the check for the first byte in the
   packet. CLNS packets always start with 0x81. ES-IS packets start with
   0x82.

   This proposal depends on the fact that all IP packets in the global
   Internet are version 4. The method of looking at the first byte can
   be extended to distinquish between IS-IS and any version of the IP
   protocol, except version 8.

6. Security Considerations

   The extension described in this document might have the surprising
   effect to some network administrators that IS-IS or IP packets can
   now be sent over VCs which did not carry those type of packets in the
   past.

7. Acknowledgements

   The author would like to thank Jonathan Gardner for the discussion
   that initiated this proposal.

8. References

   [1] ISO 10589, "Intermediate System to Intermediate System
       Intra-Domain Routeing Exchange Protocol for use in Conjunction
       with the Protocol for Providing the Connectionless-mode Network
       Service (ISO 8473)" [Also republished as RFC 1142]

   [2] RFC 1195, "Use of OSI IS-IS for routing in TCP/IP and dual
       environments", R.W. Callon, Dec. 1990

   [3] RFC 1483, "Multiprotocol Encapsulation over ATM Adaptation
       Layer 5", Juha Heinanen, July 1993

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Internet Draft     draft-hsmit-isis-ip-aal5mux-00.txt          June 1999

9. Author's Address

   Henk Smit
   Cisco Systems, Inc.
   170 West Tasman Drive
   San Jose, CA 95134
   e-mail: hsmit@cisco.com