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Network Working Group                                            F. Gont
Internet-Draft                                                   UTN/FRH
Expires: January 31, 2005                                 August 2, 2004


             Increasing the payload of ICMP error messages
                     draft-gont-icmp-payload-00.txt

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Copyright Notice

   Copyright (C) The Internet Society (2004).  All Rights Reserved.

Abstract

   The original ICMP specification states that when a packet elicits an
   ICMP error message, the IP header plus the next 64 bits of the
   original datagram must be returned in the payload of the ICMP error
   message.  This imposes a constraint on the design of transport-layer
   protocols, which are forced to include all the relevant information
   needed to identify an instance of communication in the first 64 bits



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   of their protocol header.  It also limits the amount of data from the
   original packet available to the transport-layer when acting on the
   ICMP error message.  Including only the first 64 bits of the original
   datagram's payload may also not be enough to demultiplex ICMP error
   messages if IP is being used to tunnel some other network-layer
   protocol.  This document proposes to increase the amount of data of
   the original datagram to be included in the payload of ICMP error
   messages.

1.  Introduction

   The Internet Control Message Protocol (ICMP) [1] is used in the
   Internet Architecture to perform the fault isolation function, that
   is, the group of actions that hosts and routers take to determine
   that there is some network failure [4].

   The original ICMP specification [1] states that, whenever a packet
   elicits an ICMP error message, the internet header plus the first 64
   bits of the original datagram's data must be included in the payload
   of the ICMP error message.  These data are used by the receiving host
   to match the error message to the instance of communication that
   elicited it.

   This limit on the amount information returned in the payload of ICMP
   error messages has two drawbacks:

   o  It imposes a constraint on the design of transport-layer
      protocols, which are forced to include all the relevant
      information needed to identify a communication instance in the
      first 64 bits of their protocol header.

   o  It limits the amount of data the transport-protocol has available
      to perform, for example, security checks on the returned datagram.

   o  If IP [5] is being used for tunneling purposes, including just the
      first 8 bytes of the payload of the original datagram may not be
      enough information to demultiplex the ICMP error message.

   As discussed in [1] and [6], in order to allow ICMP error messages to
   be demultiplexed, transport protocols are forced to include in the
   first 64 bits of their headers all the information needed to identify
   a communication instance.  Thus, this limit somehow constrains the
   design of transport protocols.

   There are a number of scenarios in which a larger amount of data from
   the original datagram may be needed, or, at least, desirable.  For
   example, additional data from the original datagram could be used to
   perform security checks on the received ICMP error message [7].



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   Also, in case IP is being used to tunnel some other protocol, the
   first 64 bits of the original datagrams's payload may not provide
   enough information to the demultiplex the ICMP error message.

   Even when the Host Requirements RFC [2] states that more than 8
   octects of the original datagram's payload MAY be included in the
   payload of an ICMP error message, it does not require any specific
   amount of data, and thus does not remove the constraints discussed
   above.

   This document proposes a modification to the original ICMP
   specification to increase the amount of data of the original packet
   to be included in the payload of ICMP error messages.

2.  Conventions

   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 RFC 2119 [3].

3.  Specification

   When a host or router sends an ICMP error message, it MUST include in
   the payload of the ICMP error message as many bytes of the original
   datagram as possible.  However, the resulting IP datagram MUST NOT be
   greater than 576 bytes.

   It must be noted that 576 is the minimum reassembly buffer size [2].

4.  Security Considerations

   This document proposes a minor modification to the original ICMP
   specification [1], to increase the amount of data of the original
   packet to be included in the payload of ICMP error messages.  This
   modification does not raise any new security implications.

5.  Acknowledgements

   The author would like to thank Guillermo Gont and Michael Kerrisk for
   providing many valuable comments.

6.  References

6.1  Normative References

   [1]  Postel, J., "Internet Control Message Protocol", STD 5, RFC 792,
        September 1981.




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   [2]  Braden, R., "Requirements for Internet Hosts - Communication
        Layers", STD 3, RFC 1122, October 1989.

   [3]  Bradner, S., "Key words for use in RFCs to Indicate Requirement
        Levels", BCP 14, RFC 2119, March 1997.

6.2  Informative References

   [4]  Clark, D., "Fault isolation and recovery", RFC 816, July 1982.

   [5]  Postel, J., "Internet Protocol", STD 5, RFC 791, September 1981.

   [6]  Clark, D., "Name, addresses, ports, and routes", RFC 814, July
        1982.

   [7]  Gont, F., "ICMP attacks against TCP", (work in progress)
        draft-gont-tcpm-icmp-attacks-00.txt, 2004.


Author's Address

   Fernando Gont
   Universidad Tecnologica Nacional
   Evaristo Carriego 2644
   Haedo, Provincia de Buenos Aires  1706
   Argentina

   Phone: +54 11 4650 8472
   EMail: fernando@gont.com.ar






















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