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Versions: 02 RFC 2521

Network Working Group                                             P Karn
Internet Draft                                                  Qualcomm
                                                             W A Simpson
expires in six months                                         April 1996

                    ICMP Security Failures Messages

Status of this Memo

   Distribution of this memo is unlimited.

   This document is an Internet-Draft.  Internet Drafts are working doc-
   uments of the Internet Engineering Task Force (IETF), its Areas, and
   its Working Groups.  Note that other groups may also distribute work-
   ing documents as Internet Drafts.

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   This document specifies ICMP messages for indicating failures when
   using IP Security Protocols (AH and ESP).

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1.  Introduction

   This mechanism is intended for use with the Internet Security Proto-
   cols [RFC-1825] for authentication and privacy.  For statically con-
   figured Security Associations, these messages indicate that the oper-
   ator needs to manually reconfigure, or is attempting an unauthorized
   operation.  These messages may also be used to trigger automated ses-
   sion-key management.

   The datagram format and basic facilities are already defined for ICMP

   Up-to-date values of the ICMP Type field are specified in the most
   recent "Assigned Numbers" [RFC-1700].  This document concerns the
   following values:

       40  Security Failures

2.  Message Formats

   |     Type      |     Code      |          Checksum             |
   |           Reserved            |          Pointer              |
   |                                                               |
   ~     Original Internet Headers + 64 bits of Payload            ~
   |                                                               |

   Type              40

   Code             Indicates the kind of failure:

                      0 = Bad SPI
                      1 = Authentication Failed
                      2 = Decompression Failed
                      3 = Decryption Failed
                      4 = Need Authentication
                      5 = Need Authorization

   Checksum         Two octets.  The ICMP Checksum.

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   Reserved         Two octets.  For future use; MUST be set to zero
                    when transmitted, and MUST be ignored when received.

   Pointer          Two octets.  An offset into the Original Internet
                    Headers that locates the most significant octet of
                    the offending SPI.  Will be zero when no SPI is pre-

   Original Internet Headers ...
                    The original Internet Protocol header, any interven-
                    ing headers up to and including the offending SPI
                    (if any), plus the first 64 bits (8 octets) of the
                    remaining payload data.

                    This data is used by the host to match the message
                    to the appropriate process.  If a payload protocol
                    uses port numbers, they are assumed to be in the
                    first 64-bits of the original datagram's payload.

   Usage of this message is elaborated in the following sections.

2.1.  Bad SPI

   Indicates that a received datagram includes a Security Parameters
   Index (SPI) that is invalid or has expired.

2.2.  Authentication Failed

   Indicates that a received datagram failed the authenticity or
   integrity check for a given SPI.

   Note that the SPI may indicate an outer Encapsulating Security Proto-
   col when a separate Authentication Header SPI is hidden inside.

2.3.  Decompression Failed

   Indicates that a received datagram failed a decompression check for a
   given SPI.

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2.4.  Decryption Failed

   Indicates that a received datagram failed a decryption check for a
   given SPI.

2.5.  Need Authentication

   Indicates that a received datagram will not be accepted without addi-
   tional authentication.

   In this case, either no SPI is present, or an unsuitable SPI is pre-
   sent.  For example, an encryption SPI without integrity arrives from
   a secure operating system with mutually hostile users.

2.6.  Need Authorization

   Indicates that a received datagram will not be accepted because it
   has insufficient authorization.

   In this case, an authentication SPI is present that is inappropriate
   for the target transport or application.  The principle party denoted
   by the SPI does not have proper authorization for the facilities used
   by the datagram.  For example, the party is authorized for Telnet
   access, but not for FTP access.

3.  Error Procedures

   As is usual with ICMP messages, upon receipt of one of these error
   messages that is uninterpretable or otherwise contains an error, no
   ICMP error message is sent in response.  Instead, the message is
   silently discarded.  However, for diagnosis of problems, a node
   SHOULD provide the capability of logging the error, including the
   contents of the silently discarded datagram, and SHOULD record the
   event in a statistics counter.

   On receipt, special care MUST be taken that the ICMP message actually
   includes information that matches a previously sent IP datagram.
   Otherwise, this might provide an opportunity for a denial of service

   The sending implementation MUST be able to limit the rate at which
   these messages are generated.  The rate limit parameters SHOULD be
   configurable.  How the limits are applied (such as, by destination or
   per interface) is left to the implementor's discretion.

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Security Considerations

   When a prior Security Association between the parties has not
   expired, these messages SHOULD be sent with authenticatation.

   However, the node MUST NOT dynamically establish a new Security Asso-
   ciation for the sole purpose of authenticating these messages.  Auto-
   mated key management is computationally intensive.  This could be
   used for a very serious denial of service attack.  It would be very
   easy to swamp a target with bogus SPIs from random IP Sources, and
   have it start up numerous useless key management sessions to authen-
   tically inform the putative sender.

   In the event of loss of state (such as a system crash), the node will
   need to send failure messages to all parties that attempt subsequent
   communication.  In this case, the node may have lost the key manage-
   ment technique that was used to establish the Security Association.

   Much better to simply let the peers know that there was a failure,
   and let them request key management as needed (at their staggered
   timeouts).  They'll remember the previous key management technique,
   and restart gracefully.  This distributes the restart burden among
   systems, and helps allow the recently failed node to manage its com-
   putational resources.

   In addition, these messages inform the recipient when the ICMP sender
   is under attack.  Unlike other ICMP error messages, the messages pro-
   vide sufficient data to determine that these messages are in response
   to previously sent messages.

   Therefore, it is imperative that the recipient accept both authenti-
   cated and unathenticated failure messages.  The recipient's log
   SHOULD indicate when the ICMP messages are not validated, and when
   the ICMP messages are not in response to a valid previous message.

   There is some concern that sending these messages may result in the
   leak of security information.  For example, an attacker might use
   these messages to test or verify potential forged keys.  However,
   this information is already available through the simple expedient of
   using Echo facilities, or waiting for a TCP 3-way handshake.

   The rate limiting mechanism also limits this form of leak, as many
   messages will not result in an error indication.  At the very least,
   this will lengthen the time factor for verifying such information.

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   Some of the text of this specification was derived from "Requirements
   for Internet Hosts -- Communication Layers" [RFC-1122] and "Require-
   ments for IP Version 4 Routers" [RFC-1812].

   Naganand Doraswamy and Hilarie Orman provided useful critiques of
   earlier versions of this document.

   Stimulating comments were also received from Jeffrey Schiller.


            Postel, J., "Internet Control Message Protocol", STD 5,
            September 1981.

            Braden, R., Editor, "Requirements for Internet Hosts -- Com-
            munication Layers", USC/Information Sciences Institute,
            October 1989.

            Reynolds, J., and Postel, J., "Assigned Numbers", STD 2,
            USC/Information Sciences Institute, October 1994.

            Baker, F., Editor, "Requirements for IP Version 4 Routers",
            Cisco Systems, June 1995.

            Atkinson, R., "Security Architecture for the Internet Proto-
            col", Naval Research Laboratory, July 1995.

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   Comments about this document should be discussed on the ipsec-
   dev@terisa.com mailing list.

   Questions about this document can also be directed to:

      Phil Karn
      Qualcomm, Inc.
      6455 Lusk Blvd.
      San Diego, California  92121-2779

          karn@unix.ka9q.ampr.org (preferred)

      William Allen Simpson
      Computer Systems Consulting Services
      1384 Fontaine
      Madison Heights, Michigan  48071

          wsimpson@GreenDragon.com (preferred)

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