Draft         RSVP Cryptographic Authentication      June 1996
                        RSVP Cryptographic Authentication                 |
                               Status of this Memo
          This document is an Internet Draft.  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
          Internet Drafts are 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 a "work in progress".
          This document describes the format and use of RSVP's INTEGRITY
          object to provide hop-by-hop integrity and authentication of
          RSVP messages.
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          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
          To protect the integrity of this admission control mechanism,
          RSVP requires the ability to protect its messages against
          corruption and spoofing.  This document proposes a mechanism
          to protect RSVP message integrity hop-by-hop.  The proposed
          scheme transmits the result of applying a cryptographic
          algorithm to a one-way function or ``digest'' of the message
          together with a secret Authentication Key.  This scheme
          affords protection against forgery or message modification,
          but not replays.  It is possible to replay a message until the
          sequence number changes, but the sequence number makes replays
          less of an issue.  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.
          The proposed mechanism is independent of a specific
          cryptographic algorithm, but the document describes the use of
          Keyed MD5 [2] for this purpose.
          The cost of computing a Keyed MD5 message digest far exceeds
          the cost of computing an RSVP checksum; therefore the RSVP      |
          checksum should be disabled (set to zero) if MD5                |
          authentication is used, as the MD5 digest is a much stronger
          integrity check.
          Two uses are envisioned: authentication of RSVP messages or     |
          message fragments (should a fragmentation procedure be defined  |
          in the future), and authentication of sessions. The INTEGRITY   |
          object used in both is the same, and is defined in this         |
          document. The use of the INTEGRITY object for those purposes    |
          is defined in other more appropriate documents [1] [9].
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          1.1.  Why not use the Standard IPSEC Authentication Header?     |
          One obvious question is why, since there exists a standard      |
          mechanism for authentication, we would choose to not use it.
          This was discussed at length in the working group, and was
          rejected due to the operational impact of manually opening a
          new security association among the routers that a flow
          traverses for each flow making reservations.                    |
          It is also not clear that RSVP messages are well defined for
          the 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, as in RSVP tunnels traffic may
          not follow exactly the same IP path otherwise.
          These matters are simplified if a secure key management         |
          protocol exists which can be used to open and key the security  |
          associations; should such a protocol come into existence, it    |
          may be worthwhile reviewing this decision.  However, the        |
          addressing considerations conspire against using the same       |
          solution as one which would work for IPSEC.  Therefore, this    |
          consideration cannot be understood as a promise that this       |
          procedure will go away.
          2.  Data Structures
          2.1.  INTEGRITY Object Format
          The 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 contents of INTEGRITY object are defined as a "Keyed
          Message Digest" structure, with one of the following formats:   |
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               IP4 Keyed Message Digest INTEGRITY Object: Class = 4, C-Type = 1|
               |                    Key Identifier                     |
               |                    Sequence Number                    |  |
               |               Sending System IP4 Address              |  |
               +-------------+-------------+-------------+-------------+  |
               |                                                       |  |
               +                                                       +  |
               |                                                       |  |
               +              Keyed Message Digest                     |  |
               |                                                       |  |
               +                                                       +  |
               |                                                       |  |
               +-------------+-------------+-------------+-------------+  |
               IP6 Keyed Message Digest INTEGRITY Object: Class = 4, C-Type = 2|
               +-------------+-------------+-------------+-------------+  |
               |                    Key Identifier                     |  |
               +-------------+-------------+-------------+-------------+  |
               |                    Sequence Number                    |
               |                                                       |
               +                                                       +
               |                                                       |
               +              Sending System IP6 Address               +  |
               |                                                       |
               +                                                       +
               |                                                       |
               |                                                       |  |
               +                                                       +  |
               |                                                       |  |
               +              Keyed Message Digest                     +  |
               |                                                       |  |
               +                                                       +  |
               |                                                       |  |
               +-------------+-------------+-------------+-------------+  |
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          (1)  Key Indentifier
                  An unsigned 32-bit number that acts as a key selector.  |
                  With the key, the system stores an algorithm for its    |
          (2)  Sending System Address
                  This is the same address as would be carried in the
                  Next Hop or Previous Hop object, the address of the     |
                  interface of the RSVP system that sent this message.
          (3)  Sequence Number
                  An unsigned 32-bit non-decreasing sequence number.
                  Any non-decreasing sequence of numbers may be used as
                  Sequence Number values.  For example, a timestamp on
                  the message's creation or a simple message counter
                  might be used.
                  This sequence number is reset to zero upon any key
          (4)  Keyed Message Digest                                       |
                  The digest must be a multiple of 4 octets long.  For
                  MD5, it will be 16 bytes long.
          2.2.  Keyed MD5 Message Trailer
          The Keyed MD5 algorithm requires appending the following
          message trailer to the message to be sent, before the hash is
          computed.  However, this trailer is not transmitted, since the
          receiver can reconstruct it knowing the message length and
          hash algorithm.
          The trailer consists of bytes to pad the length appropriately
          followed by the a 64-bit unsigned integer equal to the length
          of the RSVP message without the trailer.
          | zero or more pad bytes (defined by [2] or[8] when MD5 is used)||
          |                        64 bit message length MSW              |
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          |                        64 bit message length LSW              |
          3.  Message Processing Rules
          3.1.  Message Generation
          An RSVP message is created as usual, with these exceptions:
          (1)  The RSVP checksum is not calculated, but it set to zero.
          (2)  The INTEGRITY object is inserted in the appropriate
               place, and its location in the message is remembered for
               later use.
          (3)  The current sequence number is placed in the Sequence
               Number field of the INTEGRITY object.
               If several messages are being created simultaneously (for
               example, in a periodic refresh generated by a router),
               the messages should all use the same sequence number.
               This is to assure that message reordering between RSVP     |
               peers (in non-FIFO queues or in an RSVP tunnel) does not   |
               cause authentication to fail for some of them.
          (4)  The appropriate Authentication Key is selected and placed
               in the Keyed Message Digest field of the INTEGRITY         |
          (5)  The Key Identifier is placed into the INTEGRITY object.    |
          (6)  The Keyed MD5 message trailer is appended to the end of
               the message in memory.
          (7)  A Keyed Message Digest of the augmented message is         |
               calculated using the appropriate hash algorithm.  When
               the Keyed MD5 algorithm is used, the hash calculation is   |
               described in [2] and [8].
          (8)  The digest is written into the Cryptographic Digest field
               of the INTEGRITY object, overlaying the Authentication
          In the sender, Authentication Key selection is based on the     |
          interface through which the message is sent, there being a key  |
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          configured per interface.  While administrations may configure  |
          all the routers and hosts on a subnet (or for that matter, in   |
          their network) with the same key, implementations should        |
          assume that each sender may send with a different key on each   |
          numbered interface, and that they keys are simplex - the key    |
          that a system uses to sign its messages need he same key that   |
          its recievers use to sign theirs.  Implementations SHOULD       |
          maintain a separate key per interface that they sign with.
          This restriction to numbered interfaces is intentional; if an   |
          RSVP system peers with another through a set of non-RSVP        |
          routers, and it might be able to reach systems through that     |
          domain from either a numbered interface or an unnumbered        |
          interface using the same address as a router id, the choice of  |
          key would otherwise be ambiguous.  Therefore, on unnumbered     |
          interfaces, an RSVP router must use the same key as it uses on  |
          the related numbered interface.  User interfaces SHOULD         |
          provide convenient ways to configure these keys.                |
          3.2.  Message Reception
          When the message is received, the process is reversed:
          (1)  The RSVP checksum is not calculated.
          (2)  The Cryptographic Digest field of the INTEGRITY object is
               set aside.
          (3)  The Key Identifer field and Sending System Address are
               used to determine the Authentication Key and the hash
               algorithm to be used.  Implementations SHOULD maintain a   |
               key per neighboring RSVP system address or CIDR prefix,    |
               as the keys used by neighbors to sign their messages need  |
               not be the same key that the recieving system uses.
          (4)  If the received sequence number is less than the last      |
               sequence number received from the sending system with      |
               that key identifier, the message is discarded              |
               unprocessed.                                               |
          (5)  The Cryptographic Digest field of the INTEGRITY object is
               overlaid with the Authentication Key.
          (6)  The Keyed MD5 message trailer is reconstructed at the end
               of the message.
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          (7)  A new digest calculated using the indicated algorithm.
          (8)  If the calculated digest does not match the received
               digest, the message is discarded unprocessed.              |
          If a system detects the loss of a neighbor or interface, or     |
          the RSVP process is restarted on a system, the system should    |
          start with a new key if possible.  In this way, the sequence    |
          number may be reset without exposure to a replay attack.  In    |
          the event that no other key is available, the sequence number   |
          should be stored in non-volatile memory around failures, so     |
          that it may continue without decreasing.
          4.  Key Management
          It is likely that the IETF will define a standard key
          management protocol.  It is strongly desirable to use that key
          management protocol to distribute RSVP Authentication Keys
          among communicating RSVP implementations.  Such a protocol
          would provide scalability and significantly reduce the human
          administrative burden.  The Key ID can be used as a hook
          between RSVP and such a future protocol.  Key management
          protocols have a long history of subtle flaws that are often
          discovered long after the protocol was first described in
          public.  To avoid having to change all RSVP implementations
          should such a flaw be discovered, integrated key management
          protocol techniques were deliberately omitted from this
          4.1.  Key Management Procedures
          Each key has a lifetime associated with it.  No key is ever
          used outside its lifetime.  If more than one key is currently
          alive, then the youngest key (the key whose lifetime most
          recently started) should be used.
          Possible mechanisms for managing key lifetime include:  the
          use of the Network Time Protocol, hardware time-of-day clocks,
          or waiting some time before emitting the first message to
          determine what key other systems are signing with.  The matter
          is left for the implementor.  Note that the concept of a "key
          lifetime" does not require a hardware time-of-day clock or the
          use of NTP, although one or the other is advised; it merely
          requires that the earliest and latest times that the key is
          valid must be programmable in a way the system understands.
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          To maintain security, it is necessary to change the RSVP
          Authentication Key on a regular basis.  It must be possible to
          switch the RSVP Authentication Key without loss of RSVP state
          or denial of reservation service, and without requiring people
          to change all the keys at once.  This requires the RSVP
          implementation to support the storage and use of more than one
          RSVP Authentication Key on a given interface at the same time.
          For each key there will be a locally-stored Key Identifier.
          The combination of the Key Identifier and the interface
          associated with the message uniquely identifies the
          cryptographic algorithm and Authentication Key in use by RSVP.
          As noted above, the party creating the RSVP message will
          select a valid key from the set of valid keys for that
          interface.  The receiver will use the Key Identifier and
          interface to determine which key to use for authentication of
          the received message.  More than one key may be associated
          with an interface at the same time.
          To ensure a smooth switch-over, each communicating RSVP system
          must be updated with the new key several minutes before the
          current key will expire and several minutes before the new key
          lifetime begins.  The new key should have a lifetime that
          starts several minutes before the old key expires.  This gives
          time for each system to learn of the new RSVP Authentication
          Key before that key will be used.  It also ensures that the
          new key will begin being used and the current key will go out
          of use before the current key's lifetime expires.  For the
          duration of the overlap in key lifetimes, a system may receive
          messages using either key and authenticate the message.
          There are four important times for each key:
            + KeyStartReceive: the time the system starts accepting
               received packets signed with the key.
            + KeyStartSign: the time the system starts signing packets
               with the key.
            + KeyStopSign: the time the system stops signing packets
               with the key, which implies that it starts signing with
               the next key, if any.
            + KeyStopReceive: the time the system stops accepting
               received packets signed with the key.
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          The times in the order listed SHOULD form a non-decreasing
          sequence.  There needs to be some distance between start times
          and stop times, to achieve a seamless transition.  Each system
          sends using the key with the most recent "start" time and
          makes its first attempt at validation of incoming traffic with
          this same key.  If this validation fails and another (older)
          key is also active, the system should attempt to validate with
          any other active keys it may possess.
          4.2.  Key Management Requirements
          Requirements on an implementation are as follows.
          (1)  It is strongly desirable that a hypothetical security
               breach in one Internet protocol not automatically
               compromise other Internet protocols.  The Authentication
               Key of this specification SHOULD NOT be stored using
               protocols or algorithms that have known flaws.
          (2)  An implementation MUST support the storage of more than
               one key at the same time, although normally only one key
               will be active on an interface.
          (3)  An implementation MUST associate a specific lifetime
               (i.e., KeyStartSign and KeyStopSign) with each key and
               corresponding Key Identifier.
          (4)  An implementation MUST support manual key distribution
               (e.g., the privileged user manually typing in the key,
               key lifetime, and key identifier on the console).  The
               lifetime may be infinite.
          (5)  If more than one algorithm is supported, then the
               implementation MUST require that the algorithm be
               specified for each key at the time the other key
               information is entered.
          (6)  Keys that are out of date MAY be deleted at will by the
               implementation without requiring human intervention.
          (7)  Manual deletion of active keys SHOULD also be supported.
          (8)  Key storage SHOULD persist across a system restart, warm
               or cold, to avoid operational issues, and the sequence
               number in use SHOULD be stored with it. Implementations
               should note, however, that systems with no non-volatile
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               storage may reset the sequence number to zero when
          4.3.  Pathological Cases
          An implementation of this document must handle two
          pathological cases.  Both of these should be exceedingly rare.
          (1)  During key switch-over, devices may exist which have not
               yet been successfully configured with the new key.
               Therefore, systems MAY implement (and would be well
               advised to implement) an algorithm that detects the set
               of keys being used by its neighbors, and transmits its
               messages using both the new and old keys until all the
               neighbors are using the new key or the lifetime of the
               old key expires.  Under normal circumstances, this
               elevated transmission rate will exist for a single
               refresh interval.
          (2)  It is possible that the last key associated with an
               interface may expire.
               When this happens, it is unacceptable to revert to an
               unauthenticated condition, and not advisable to disrupt
               current reservations.  Therefore, the system should send
               a "last authentication key expiration" notification to
               the network manager and treat the key as having an
               infinite lifetime until the lifetime is extended, the key
               is deleted by network management, or a new key is
          5.  Conformance Requirements
          To conform to this specification, an implementation MUST
          support all of its aspects.  The MD5 authentication algorithm   |
          defined in [2] and [8] MUST be implemented by all conforming
          implementations.  A conforming implementation MAY also support
          other authentication algorithms such as NIST's Secure Hash
          Algorithm (SHA).  Manual key distribution as described above
          MUST be supported by all conforming implementations.  All
          implementations MUST support the smooth key rollover described
          under "Key Change Procedures."
          The user documentation provided with the implementation MUST
          contain clear instructions on how to ensure that smooth key
          rollover occurs.
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          Implementations SHOULD support a standard key management
          protocol for secure distribution of RSVP Authentication Keys
          once such a key management protocol is standardized by the
          6.  Acknowledgment
          This document is derived directly from similar work done for
          OSPF and RIP Version II, jointly by Ran Atkinson and Fred
          Baker, with modifications by Dino Farinacci for IDMR.
          7.  References
          [1]  Braden, R., Ed., Zhang, L., Estrin, D., Herzog, S., and
               S. Jamin, "Resource ReSerVation Protocol (RSVP) --
               Version 1 Functional Specificationq.  Internet Draft       |
               draft-ietf-rsvp-spec-12.ps, May 1996.
          [2]  Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321,
               April 1992.
          [3]  S.  Bellovin, "Security Problems in the TCP/IP Protocol
               Suite", ACM Computer Communications Review, Volume 19,
               Number 2, pp.32-48, April 1989.
          [4]  N.  Haller, R.  Atkinson, "Internet Authentication
               Guidelines", RFC-1704, October 1994.
          [5]  R. Braden, D. Clark, S. Crocker, & C. Huitema, "Report of
               IAB Workshop on Security in the Internet Architecture",
               RFC-1636, June 1994.
          [6]  R. Atkinson, "IP Authentication Header", RFC-1826, August
          [7]  R. Atkinson, "IP Encapsulating Security Payload", RFC-
               1827, August 1995.
          [8]  P. Metzger, W. Simpson, "IP Authentication using Keyed
               MD5", RFC-1828, August 1995.                               |
          [9]  S. Herzog, "Building Blocks for Accounting and Access      |
               Control", draft-ietf-rsvp-lpm-arch-00.ps, March 1996.
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          8.  Security Considerations
          This entire memo describes and specifies an authentication
          mechanism for RSVP that is believed to be secure against
          active and passive attacks.  Passive attacks are clearly
          widespread in the Internet at present.  Protection against
          active attacks is also needed even though such attacks are not
          currently widespread.
          Users need to understand that the quality of the security
          provided by this mechanism depends completely on the strength
          of the implemented authentication algorithms, the strength of
          the key being used, and the correct implementation of the
          security mechanism in all communicating RSVP implementations.
          This mechanism also depends on the RSVP Authentication Keys
          being kept confidential by all parties.  If any of these are    |
          incorrect or insufficiently secure, then no real security will  |
          be provided to the users of this mechanism.
          Confidentiality is not provided by this mechanism.  Work is
          underway within the IETF to specify a standard mechanism for
          IP-layer encryption.  That mechanism might be used to provide
          confidentiality for RSVP in the future.  Protection against
          traffic analysis is also not provided.  Mechanisms such as
          bulk link encryption might be used when protection against
          traffic analysis is required.
          9.  Author's Address
               Fred Baker
               Cisco Systems
               519 Lado Drive
               Santa Barbara, California 93111
               Phone: (408) 526-4257
               Email: fred@cisco.com
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          Table of Contents
          1 Introduction ..........................................    2
          1.1 Why not  use  the  Standard  IPSEC  Authentication
               Header?  ...........................................    3
          2 Data Structures .......................................    3
          2.1 INTEGRITY Object Format .............................    3
          2.2 Keyed MD5 Message Trailer ...........................    5
          3 Message Processing Rules ..............................    6
          3.1 Message Generation ..................................    6
          3.2 Message Reception ...................................    7
          4 Key Management ........................................    8
          4.1 Key Management Procedures ...........................    8
          4.2 Key Management Requirements .........................   10
          4.3 Pathological Cases ..................................   11
          5 Conformance Requirements ..............................   11
          6 Acknowledgment ........................................   12
          7 References ............................................   12
          8 Security Considerations ...............................   13
          9 Author's Address ......................................   13
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