rfc2747.txt

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Network Working Group                                           F. Baker
Request for Comments: 2747                                         Cisco
Category: Standards Track                                     B. Lindell
                                                                 USC/ISI
                                                               M. Talwar
                                                               Microsoft
                                                            January 2000


                   RSVP Cryptographic Authentication


Status of this Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2000).  All Rights Reserved.

Abstract

   This document describes the format and use of RSVP's INTEGRITY object
   to provide hop-by-hop integrity and authentication of RSVP messages.

1.  Introduction

   The Resource ReSerVation Protocol RSVP [1] is a protocol for setting
   up distributed state in routers and hosts, and in particular for
   reserving resources to implement integrated service.  RSVP allows
   particular users to obtain preferential access to network resources,
   under the control of an admission control mechanism.  Permission to
   make a reservation will depend both upon the availability of the
   requested resources along the path of the data, and upon satisfaction
   of policy rules.

   To ensure the integrity of this admission control mechanism, RSVP
   requires the ability to protect its messages against corruption and
   spoofing.  This document defines a mechanism to protect RSVP message
   integrity hop-by-hop.  The proposed scheme transmits an
   authenticating digest of the message, computed using a secret
   Authentication Key and a keyed-hash algorithm.  This scheme provides
   protection against forgery or message modification.  The INTEGRITY
   object of each RSVP message is tagged with a one-time-use sequence



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   number.  This allows the message receiver to identify playbacks and
   hence to thwart replay attacks.  The proposed mechanism does not
   afford confidentiality, since messages stay in the clear; however,
   the mechanism is also exportable from most countries, which would be
   impossible were a privacy algorithm to be used.  Note: this document
   uses the terms "sender" and "receiver" differently from [1].  They
   are used here to refer to systems that face each other across an RSVP
   hop, the "sender" being the system generating RSVP messages.

   The message replay prevention algorithm is quite simple.  The sender
   generates packets with monotonically increasing sequence numbers.  In
   turn, the receiver only accepts packets that have a larger sequence
   number than the previous packet.  To start this process, a receiver
   handshakes with the sender to get an initial sequence number.  This
   memo discusses ways to relax the strictness of the in-order delivery
   of messages as well as techniques to generate monotonically
   increasing sequence numbers that are robust across sender failures
   and restarts.

   The proposed mechanism is independent of a specific cryptographic
   algorithm, but the document describes the use of Keyed-Hashing for
   Message Authentication using HMAC-MD5 [7].  As noted in [7], there
   exist stronger hashes, such as HMAC-SHA1; where warranted,
   implementations will do well to make them available.  However, in the
   general case, [7] suggests that HMAC-MD5 is adequate to the purpose
   at hand and has preferable performance characteristics.  [7] also
   offers source code and test vectors for this algorithm, a boon to
   those who would test for interoperability.  HMAC-MD5 is required as a
   baseline to be universally included in RSVP implementations providing
   cryptographic authentication, with other proposals optional (see
   Section 6 on Conformance Requirements).

   The RSVP checksum MAY be disabled (set to zero) when the INTEGRITY
   object is included in the message, as the message digest is a much
   stronger integrity check.

1.1.  Conventions used in this document

   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 [8].

1.2.  Why not use the Standard IPSEC Authentication Header?

   One obvious question is why, since there exists a standard
   authentication mechanism, IPSEC [3,5], we would choose not to use it.
   This was discussed at length in the working group, and the use of
   IPSEC was rejected for the following reasons.



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   The security associations in IPSEC are based on destination address.
   It is not clear that RSVP messages are well defined for either source
   or destination based security associations, as a router must forward
   PATH and PATH TEAR messages using the same source address as the
   sender listed in the SENDER TEMPLATE.  RSVP traffic may otherwise not
   follow exactly the same path as data traffic.  Using either source or
   destination based associations would require opening a new security
   association among the routers for which a reservation traverses.

   In addition, it was noted that neighbor relationships between RSVP
   systems are not limited to those that face one another across a
   communication channel.  RSVP relationships across non-RSVP clouds,
   such as those described in Section 2.9 of [1], are not necessarily
   visible to the sending system.  These arguments suggest the use of a
   key management strategy based on RSVP router to RSVP router
   associations instead of IPSEC.

2.  Data Structures

2.1.  INTEGRITY Object Format

   An RSVP message consists of a sequence of "objects," which are type-
   length-value encoded fields having specific purposes.  The
   information required for hop-by-hop integrity checking is carried in
   an INTEGRITY object.  The same INTEGRITY object type is used for both
   IPv4 and IPv6.

   The INTEGRITY object has the following format:

      Keyed Message Digest INTEGRITY Object: Class = 4, C-Type = 1

       +-------------+-------------+-------------+-------------+
       |    Flags    | 0 (Reserved)|                           |
       +-------------+-------------+                           +
       |                    Key Identifier                     |
       +-------------+-------------+-------------+-------------+
       |                    Sequence Number                    |
       |                                                       |
       +-------------+-------------+-------------+-------------+
       |                                                       |
       +                                                       +
       |                                                       |
       +                  Keyed Message Digest                 |
       |                                                       |
       +                                                       +
       |                                                       |
       +-------------+-------------+-------------+-------------+




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     o    Flags: An 8-bit field with the following format:

                                      Flags

                          0   1   2   3   4   5   6   7
                        +---+---+---+---+---+---+---+---+
                        | H |                           |
                        | F |             0             |
                        +---+---+---+---+---+---+---+---+

          Currently only one flag (HF) is defined.  The remaining flags
          are reserved for future use and MUST be set to 0.

          o    Bit 0: Handshake Flag (HF) concerns the integrity
               handshake mechanism (Section 4.3).  Message senders
               willing to respond to integrity handshake messages SHOULD
               set this flag to 1 whereas those that will reject
               integrity handshake messages SHOULD set this to 0.

     o    Key Identifier: An unsigned 48-bit number that MUST be unique
          for a given sender.  Locally unique Key Identifiers can be
          generated using some combination of the address (IP or MAC or
          LIH) of the sending interface and the key number.  The
          combination of the Key Identifier and the sending system's IP
          address uniquely identifies the security association (Section
          2.2).

     o    Sequence Number: An unsigned 64-bit monotonically increasing,
          unique sequence number.

          Sequence Number values may be any monotonically increasing
          sequence that provides the INTEGRITY object [of each RSVP
          message] with a tag that is unique for the associated key's
          lifetime.  Details on sequence number generation are presented
          in Section 3.

     o    Keyed Message Digest: The digest MUST be a multiple of 4
          octets long.  For HMAC-MD5, it will be 16 bytes long.

2.2.  Security Association

   The sending and receiving systems maintain a security association for
   each authentication key that they share.  This security association
   includes the following parameters:







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     o    Authentication algorithm and algorithm mode being used.

     o    Key used with the authentication algorithm.

     o    Lifetime of the key.

     o    Associated sending interface and other security association
          selection criteria [REQUIRED at Sending System].

     o    Source Address of the sending system [REQUIRED at Receiving
          System].

     o    Latest sending sequence number used with this key identifier
          [REQUIRED at Sending System].

     o    List of last N sequence numbers received with this key
          identifier [REQUIRED at Receiving System].

3.  Generating Sequence Numbers

   In this section we describe methods that could be chosen to generate
   the sequence numbers used in the INTEGRITY object of an RSVP message.
   As previous stated, there are two important properties that MUST be
   satisfied by the generation procedure.  The first property is that
   the sequence numbers are unique, or one-time, for the lifetime of the
   integrity key that is in current use.  A receiver can use this
   property to unambiguously distinguish between a new or a replayed
   message.  The second property is that the sequence numbers are
   generated in monotonically increasing order, modulo 2^64.  This is
   required to greatly reduce the amount of saved state, since a
   receiver only needs to save the value of the highest sequence number
   seen to avoid a replay attack.  Since the starting sequence number
   might be arbitrarily large, the modulo operation is required to
   accommodate sequence number roll-over within some key's lifetime.
   This solution draws from TCP's approach [9].

   The sequence number field is chosen to be a 64-bit unsigned quantity.
   This is large enough to avoid exhaustion over the key lifetime.  For
   example, if a key lifetime was conservatively defined as one year,
   there would be enough sequence number values to send RSVP messages at
   an average rate of about 585 gigaMessages per second.  A 32-bit
   sequence number would limit this average rate to about 136 messages
   per second.

   The ability to generate unique monotonically increasing sequence
   numbers across a failure and restart implies some form of stable
   storage, either local to the device or remotely over the network.
   Three sequence number generation procedures are described below.



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3.1.  Simple Sequence Numbers

   The most straightforward approach is to generate a unique sequence
   number using a message counter.  Each time a message is transmitted
   for a given key, the sequence number counter is incremented.  The
   current value of this counter is continually or periodically saved to
   stable storage.  After a restart, the counter is recovered using this
   stable storage.  If the counter was saved periodically to stable
   storage, the count should be recovered by increasing the saved value