Network Working Group                                      T. Przygienda
Request for Comments: 3358                                         Xebeo
Category: Informational                                      August 2002


                        Optional Checksums in
          Intermediate System to Intermediate System (ISIS)

Status of this Memo

  This memo provides information for the Internet community.  It does
  not specify an Internet standard of any kind.  Distribution of this
  memo is unlimited.

Copyright Notice

  Copyright (C) The Internet Society (2002).  All Rights Reserved.

Abstract

  This document describes an optional extension to the Intermediate
  System to Intermediate System (ISIS) protocol, used today by several
  Internet Service Proviers (ISPs) for routing within their clouds.
  ISIS is an interior gateway routing protocol developed originally by
  OSI and used with IP extensions as Interior Gateway Protocol (IGP).
  ISIS originally does not provide Complete Sequence Numbers Protocol
  Data (CSNP) and Partial Sequence Numbers Protocol Data Unit (PSNP)
  checksums, relying on the underlying layers to verify the integrity
  of information provided.  Experience with the protocol shows that
  this precondition does not always hold and scenarios can be imagined
  that impact protocol functionality.  This document introduces a new
  optional Type, Length and Value (TLV) providing checksums.

1.  Introduction

  ISIS [ISO90, Cal90a, Cal90b] CSNPs and PSNPs and IIHs can be
  corrupted in case of faulty implementations of L2 hardware or lack of
  checksuming on a specific network technology.  As a particularly ugly
  case, corruption of length and/or TLV length fields may lead to the
  generation of extensive numbers of "empty" LSPs in the receiving
  node.  Since we cannot rely on authentication as a checksum
  mechanism, this document proposes an optional TLV to add checksums to
  the elements.

  The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
  "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
  document are to be interpreted as described in [Bra97].




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2.  TLV Description

  This optional TLV MAY BE included in all CSNP, PSNP and IIH packets
  and an implementation that implements optional checksums MUST accept
  PDUs if they do NOT contain the optional checksum.  Implementations
  that receive an optional checksum TLV and support it MUST discard the
  PDU if the checksum is incorrect.  An implementation that does NOT
  implement optional checksums MUST accept a PDU that contains the
  checksum TLV.  An implementation that supports optional checksums and
  receives it within any other PDU than CSNP, PSNP or IIH MUST discard
  the PDU.  Such an implementation MUST discard the PDU as well if more
  than one optional checksum TLVs are included within it.
  Additionally, any implementation supporting optional checksums MUST
  accept PDUs with an optional checksum with the value 0 and consider
  such a checksum as correct.

3.  Checksum Computation

  The checksum is a fletcher checksum computed according to [ISO98],
  Annex C over the complete PDU.  To compute the correct checksum, an
  implementation MUST add the optional checksum TLV to the PDU with the
  initial checksum value of 0 and compute the checksum over such a PDU.

4.  Interaction with TLVs using PDU Data to Compute Signatures

  The implementation MUST either omit the optional checksum on an
  interface or send a 0 checksum value if it includes in the PDU
  signatures that provide equivalent or stronger functionality, such as
  HMAC or MD5.  Otherwise an implementation that handles such
  signatures but does not handle the optional checksums, may fail to
  compute the MD5 signature on the packet.  Such a failure would be
  caused by the fact that MD5 is computed with the checksum value set
  to 0 and only as a final step is the checksum value being filled in.

5.  TLV Format

  [Prz01] lists the according value of the TLV type and discusses
  issues surrounding the assignment of new TLV codepoints.

   0                   1                   2                   3
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  | TLV Type =12  | TLV Length =2 |       Checksum (16 bits)      |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+







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6.  Acknowledgments

  Tony Li mentioned the original problem.  Mike Shand provided
  comments.  Somehow related problems with purging on LSP checksum
  errors have been observed by others before.  Nischal Sheth spelled
  out the issues of interaction between MD5 and the optional checksums.

7.  Security Considerations

  ISIS security applies to the work presented.  No specific security
  issues as to the new element are known.

References

  [Bra97]  Bradner, S., "Key Words for Use in RFCs to Indicate
           Requirement Levels", BCP 14, RFC 2119, March 1997.

  [Cal90a] Callon, R., "OSI ISIS Intradomain Routing Protocol", RFC
           1142, February 1990.

  [Cal90b] Callon, R., "Use of OSI ISIS for Routing in TCP/IP and Dual
           Environments", RFC 1195, December 1990.

  [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.

  [ISO98]  ISO.  Information Technology - Protocol for Providing the
           Connectionless-Mode Network Service:  Protocol
           Specification.  ISO, 1998.

  [Prz01]  Przygienda, T., "Reserved Type, Length and Value (TLV)
           Codepoints in Intermediate System to Intermediate System",
           RFC 3359, August 2002.

Author's Address

  Tony Przygienda
  Xebeo
  One Cragwood Road
  South Plainfield, NJ 07080

  Phone: (908) 222 4225
  Email: [email protected]





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Full Copyright Statement

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Acknowledgement

  Funding for the RFC Editor function is currently provided by the
  Internet Society.



















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