Internet Draft                                                Fred Baker
Expiration: May 1999                                               Cisco
File: draft-ietf-rsvp-md5-07.txt                             Bob Lindell
                                                            Mohit Talwar

                   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-Drafts.

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   and may be updated, replaced, or obsoleted by other documents at any
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   Comments on this draft should be made on the list


   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 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
   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

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   cryptographic authentication, with other proposals optional (see
   section 7 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",
   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 [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.

   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 that a flow traverses for each flow
   making reservations.

   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.

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   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                 |
         |                                                       |
         +                                                       +
         |                                                       |

   (1)  Flags

           An 8-bit field with the following format


                            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 should be set to 0.

           + 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.

   (2)  Key Identifier

           An unsigned 48-bit number that MUST be unique for a given
           sender.  Locally unique Key Identifiers can be generated

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           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).

   (3)  Sequence Number

           An unsigned 64-bit monotonically increasing, unique sequence

           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.

   (4)  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:

   + Authentication algorithm and algorithm mode being used.

   + Key used with the authentication algorithm.

   + Lifetime of the key.

   + Associated sending interface [REQUIRED at Sending System].

   + Source Address of the sending system [REQUIRED at Receiving

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

   + 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.

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   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 [10].

   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.

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
   to be larger than any possible value of the counter at the time of
   the failure.  This can be computed, knowing the interval at which the
   counter was saved to stable storage and incrementing the stored value
   by that amount.

3.2.  Sequence Numbers Based on a Real Time Clock

   Most devices will probably not have the capability to save sequence
   number counters to stable storage for each key.  A more universal
   solution is to base sequence numbers on the stable storage of a real
   time clock.  Many computing devices have a real time clock module
   that includes stable storage of the clock.  These modules generally

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   include some form of nonvolatile memory to retain clock information
   in the event of a power failure.

   In this approach, we could use an NTP based timestamp value as the
   sequence number.  The roll-over period of an NTP timestamp is about
   136 years, much longer than any reasonable lifetime of a key.  In
   addition, the granularity of the NTP timestamp is fine enough to
   allow the generation of an RSVP message every 200 picoseconds for a
   given key.  Many real time clock modules do not have the resolution
   of an NTP timestamp.  In these cases, the least significant bits of
   the timestamp can be generated using a message counter, which is
   reset every clock tick.  For example, when the real time clock
   provides a resolution of 1 second, the 32 least significant bits of
   the sequence number can be generated using a message counter.  The
   remaining 32 bits are filled with the 32 least significant bits of
   the timestamp.  Assuming that the recovery time after failure takes
   longer than one tick of the real time clock, the message counter for
   the low order bits can be safely reset to zero after a restart.

3.3.  Sequence Numbers Based on a Network Recovered Clock

   If the device does not contain any stable storage of sequence number
   counters or of a real time clock, it could recover the real time
   clock from the network using NTP.  Once the clock has been recovered
   following a restart, the sequence number generation procedure would
   be identical to the procedure described above.

4.  Message Processing

   Implementations SHOULD allow specification of interfaces that are to
   be secured, for either sending messages, or receiving them, or both.
   The sender must ensure that all RSVP messages sent on secured sending
   interfaces include an INTEGRITY object, generated using the
   appropriate Key.  Receivers verify whether RSVP messages, except of
   the type "Integrity Challenge" (section 4.3), arriving on a secured
   receiving interface contain the INTEGRITY object.  If the INTEGRITY
   object is absent, the receiver discards the message.

   Authentication Keys are simplex - the key that a sending system uses
   to sign its messages may be different from the key that its receivers
   use to sign theirs.  Hence, each key is associated with a unique
   sending system and (possibly) multiple receiving systems.

   Each sender has at least one key configured per secured sending
   interface (or LIH).  While administrators may configure all the
   routers and hosts on a subnet (or for that matter, in their network)
   with the same key, implementations MUST assume that each sender may
   send with a different key on each secured interface.  At the sender,

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   Authentication Key selection is based on the interface through which
   the message is sent.  This selection MAY include additional criteria,
   such as the destination address (when sending the message unicast,
   over a broadcast LAN with a large number of hosts) or user identity
   [9].  Finally, all intended message recipients should be configured
   with this Authentication Key.  Route flaps in a non RSVP cloud might
   cause messages for the same receiver to be sent on different
   interfaces at different times.  In such cases, the receiver should be
   configured with keys associated with all possible interfaces through
   which the message might be sent.

   Receivers select keys based on the Key Identifier and the sending
   system's IP address.  The Key Identifier is included in the INTEGRITY
   object.  The sending system's address can be obtained either from the
   RSVP_HOP object, or if that's not present (as is the case with
   PathErr and ResvConf messages) from the IP source address.  Since the
   Key Identifier is unique for a sender, this method uniquely
   identifies the key.

   The integrity mechanism slightly modifies the processing rules for
   RSVP messages, both when including the INTEGRITY object in a message
   sent over a secured sending interface and when accepting a message
   received on a secured receiving interface.  These modifications are
   detailed below.

4.1.  Message Generation

   For an RSVP message sent over a secured sending interface, the
   message is created as described in [1], with these exceptions:

   (1)  The RSVP checksum field is set to zero.  If required, an RSVP
        checksum can be calculated after step (8), when the processing
        of the INTEGRITY object is complete.

   (2)  The INTEGRITY object is inserted in the appropriate place, and
        its location in the message is remembered for later use.

   (3)  The sending interface and other appropriate criteria (as
        mentioned above) are used to determine the Authentication Key
        and the hash algorithm to be used.

   (4)  The unused flags and the reserved field in the INTEGRITY object
        should be set to 0.  The Handshake Flag (HF) should be set
        according to rules specified in section 2.1.

   (5)  The sending sequence number must be updated to ensure a unique,
        monotonically increasing number.  It is then placed in the
        Sequence Number field of the INTEGRITY object.

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   (6)  The Keyed Message Digest field is set to zero.

   (7)  The Key Identifier is placed into the INTEGRITY object.

   (8)  An authenticating digest of the message is computed using the
        Authentication Key in conjunction with the keyed-hash algorithm.
        When the HMAC-MD5 algorithm is used, the hash calculation is
        described in [7].

   (9)  The digest is written into the Cryptographic Digest field of the
        INTEGRITY object.

4.2.  Message Reception

   When the message is received on a secured receiving interface, and is
   not of the type "Integrity Challenge", it is processed in the
   following manner:

   (1)  The RSVP checksum field is set to zero.

   (2)  The Cryptographic Digest field of the INTEGRITY object is set

   (3)  The Key Identifier field and the sending system address are used
        to uniquely determine the Authentication Key and the hash
        algorithm to be used.  Processing of this packet might be
        delayed when the Key Management System (section 6) is queried
        for this information.

   (4)  For messages of type "Integrity Response", this step is ignored
        and processing continues at step 5.  Otherwise the sequence
        number is validated to prevent replay attacks, and messages with
        invalid sequence numbers are ignored by the receiver.

        When a message is accepted, the sequence number of that message
        could update a stored value corresponding to the largest
        sequence number received to date.  Each subsequent message must
        then have a larger (modulo 2^64) sequence number to be accepted.
        This simple processing rule prevents message replay attacks, but
        it must be modified to tolerate limited out-of-order message
        delivery.  For example, if several messages were sent in a burst
        (in a periodic refresh generated by a router, or as a result of
        a tear down function), they might get reordered and then the
        sequence numbers would not be received in an increasing order.

        An implementation SHOULD allow administrative configuration that
        sets the receiver's tolerance to out-of-order message delivery.

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        A simple approach would allow administrators to specify a
        message window corresponding to the worst case reordering
        behavior.  For example, one might specify that packets reordered
        within a 32 message window would be accepted.  If no reordering
        can occur, the window is set to one.

        The receiver must store a list of all sequence numbers seen
        within the reordering window.  A received sequence number is
        valid if (a) it is greater than the maximum sequence number
        received or (b) it is a past sequence number lying within the
        reordering window and not recorded in the list.  Acceptance of a
        sequence number implies adding it to the list and removing a
        number from the lower end of the list.  Messages received with
        sequence numbers lying below the lower end of the list or marked
        seen in the list are silently discarded.

   (5)  The Cryptographic Digest field of the INTEGRITY object is set to

   (6)  A new keyed-digest is calculated using the indicated algorithm
        and the Authentication Key.

   (7)  If the calculated digest does not match the received digest, the
        message is discarded without further processing.

4.3.  Integrity Handshake at Restart or Initialization of the Receiver

   To obtain the starting sequence number for a live Authentication Key,
   the receiver MAY initiate an integrity handshake with the sender.
   This handshake consists of a receiver's Challenge and the sender's
   Response, and may be either initiated during restart or postponed
   until a message signed with that key arrives.

   Once the receiver has decided which Authentication Key to initiate an
   integrity handshake for, it identifies the sender using the sending
   system's address configured in the corresponding security
   association.  The receiver then sends an RSVP Integrity Challenge
   message to the sender.  This message contains the Key Identifier to
   identify the sender's key and a unique sequence number generated by
   the standard methods outlined earlier.

   An RSVP Integrity Challenge message will carry a message type of 11.
   The message format is as follows:

   <Integrity Challenge message> ::= <Common Header> <CHALLENGE>

   The CHALLENGE object has the following format:

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                 CHALLENGE Object: Class = 16, C-Type = 1

         |        0 (Reserved)       |                           |
         +-------------+-------------+                           +
         |                    Key Identifier                     |
         |                    Sequence Number                    |
         |                                                       |

   The sender accepts the "Integrity Challenge" without doing an
   integrity check.  It returns an RSVP "Integrity Response" message
   that contains the original CHALLENGE object.  It also includes an
   INTEGRITY object, signed with the key specified by the Key Identifier
   included in the "Integrity Challenge".

   An RSVP Integrity Response message will carry a message type of 12.
   The message format is as follows:

   <Integrity Response message> ::= <Common Header> <INTEGRITY>


   The "Integrity Response" message is accepted by the receiver
   (challenger) only if the returned CHALLENGE object matches the one
   sent in the "Integrity Challenge" message.  This prevents replay of
   old "Integrity Response" messages.  If the match is successful, the
   receiver saves the Sequence Number from the INTEGRITY object as the
   latest sequence number received with the key identifier included in

   If a response is not received within a given period of time, the
   challenge is repeated.  When the integrity handshake successfully
   completes, the receiver begins accepting normal RSVP signaling
   messages from that sender and ignores any other "Integrity Response"

   An integrity handshake may not be necessary in all environments.  A
   common use of RSVP integrity will be between peering domain routers,
   which are likely to be processing a steady stream of RSVP messages
   due to aggregation effects.  When a router restarts after a crash,
   valid RSVP messages from peering senders will probably arrive within
   a short time.  Assuming that replay messages are injected into the
   stream of valid RSVP messages, there may be only a small window of
   opportunity for a replay attack before a valid message is processed.

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   This valid message will set the largest sequence number seen to a
   value greater than any number that had been stored prior to the
   crash, preventing any further replays.

   The Handshake Flag (HF) is used to allow implementations the
   flexibility of not including the integrity handshake mechanism.  By
   setting this flag to 1, message senders that implement the integrity
   handshake distinguish themselves from those that do not.  Receivers
   SHOULD NOT attempt to handshake with senders whose INTEGRITY object
   has HF = 0.

   On the other hand, not using an integrity handshake could allow
   exposure to replay attacks if there is a long period of silence from
   a given sender following a restart of a receiver.  Hence, it SHOULD
   be an administrative decision whether or not the receiver performs an
   integrity handshake with senders that are willing to respond to
   "Integrity Challenge" messages, and whether it accepts any messages
   from senders that refuse to do so.  These decisions will be based on
   assumptions related to a particular network environment.

5.  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
   Identifier 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

5.1.  Key Management Procedures

   Each key has a lifetime associated with it that is recorded in all
   systems (sender and receivers) configured with that key.  The concept
   of a "key lifetime" merely requires that the earliest (KeyStartValid)
   and latest (KeyEndValid) times that the key is valid be programmable
   in a way the system understands.  In general, no key is ever used
   outside its lifetime (but see section 5.3).  Possible mechanisms for
   managing key lifetime include the Network Time Protocol and hardware
   time-of-day clocks.

   To maintain security, it is advisable to change the RSVP
   Authentication Key on a regular basis.  It should be possible to

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   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 an RSVP implementation to
   support the storage and use of more than one active RSVP
   Authentication Key at the same time.  Hence a sender might have
   multiple active keys for a given interface and a receiver might have
   multiple active keys for a particular sending system.

   Since keys are shared between a sender and (possibly) multiple
   receivers, there is a region of uncertainty around the time of key
   switch-over during which some systems may still be using the old key
   and others might have switched to the new key.  The size of this
   uncertainty region is related to clock synchrony of the systems.
   Administrators should configure the overlap between the expiration
   time of the old key (KeyStopValid) and the validity of the new key
   (KeyStartValid) to be at least twice the size of this uncertainty
   interval.  This will allow the sender to make the key switch-over at
   the midpoint of this interval and be confident that all receivers are
   now accepting the new key.  For the duration of the overlap in key
   lifetimes, a receiver must be prepared to authenticate messages using
   either key.

5.2.  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 and use of more than
        one key at the same time, for both sending and receiving

   (3)  An implementation MUST associate a specific lifetime (i.e.,
        KeyStartValid and KeyStopValid) with each key and the
        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

   (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.

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   (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.

5.3.  Pathological Case

   It is possible that the last key associated with an interface

   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 configured.

6.  Key Management Interface

   This section describes a generic interface to Key Management.  At the
   start of execution, RSVP would use this interface to obtain the
   current set of relevant keys for sending and receiving messages.
   During execution, RSVP can query for specific keys given a Key
   Identifier and Source Address, discover newly created keys, and be
   informed of those keys that have been deleted.  The interface
   provides both a polling and asynchronous upcall style for wider

6.1.  Data Structures

   Information about keys is returned using the following KeyInfo data

   KeyInfo {
           Key Type (Send or Receive)
           Authentication Algorithm Type & Mode
           Status (Active or Deleted)
           Outgoing Interface (for Send only)
           Other Outgoing Selection Criteria (for Send only, optional)
           Sending System Address (for Receive Only)

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6.2.  Default Key Table

   This function returns a list of KeyInfo data structures corresponding
   to all of the keys that are configured for sending and receiving RSVP
   messages and have an Active Status.  This function is usually called
   at the start of execution but there is no limit on the number of
   times that it may be called.

   KM_DefaultKeyTable() -> KeyInfoList

6.3.  Querying for Unknown Receive Keys

   When a message arrives with an unknown Key Identifier & Sending
   System Address pair, RSVP can use this function to query the Key
   Management System for the appropriate key if it exists.  The status
   of the element returned, if any, must be Active.

   KM_GetRecvKey( INTEGRITY Object, SrcAddress ) -> KeyInfo

6.4.  Polling for Updates

   This function returns a list of KeyInfo data structures corresponding
   to any incremental changes that have been made to the default key
   table or requested keys since the last call to either
   KM_KeyTablePoll, KM_DefaultKeyTable, or KM_GetRecvKey.  The status of
   some elements in the returned list may be set to Deleted.

   KM_KeyTablePoll() -> KeyInfoList

6.5.  Asynchronous Upcall Interface

   Rather than repeatedly calling the KM_KeyTablePoll(), an
   implementation may choose to use an asynchronous event model.  This
   function registers interest to change in key for a given Key
   Identifier or for all keys if no Key Identifier is specified.  The
   upcall function is called each time a change is made to a key.

   KM_KeyUpdate ( Function [, KeyIdentifier ] )

   where the upcall function is parameterized as follows:

   Function ( KeyInfo )

7.  Conformance Requirements

   To conform to this specification, an implementation MUST support all
   of its aspects.  The HMAC-MD5 authentication algorithm defined in [7]
   MUST be implemented by all conforming implementations.  A conforming

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   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 roll over described
   under "Key Management Procedures."

   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 IETF.

8.  Acknowledgments

   This document is derived directly from similar work done for OSPF and
   RIP Version II, jointly by Ran Atkinson and Fred Baker.  Significant
   editing was done by Bob Braden, resulting in increased clarity.
   Significant comments were submitted by Steve Bellovin, who actually
   understands this stuff.  Matt Crawford and Dan Harkins helped revise
   the document.

9.  References

   [1]  Braden, R., Ed., Zhang, L., Berson, S., Herzog, S., and S.
        Jamin, "Resource ReSerVation Protocol (RSVP) -- Version 1
        Functional Specification".  RFC 2205, September 1997.

   [2]  S.  Bellovin, "Security Problems in the TCP/IP Protocol Suite",
        ACM Computer Communications Review, Volume 19, Number 2, pp.32-
        48, April 1989.

   [3]  N.  Haller, R.  Atkinson, "Internet Authentication Guidelines",
        Request for Comments 1704, October 1994.

   [4]  R.  Braden, D.  Clark, S.  Crocker, & C.  Huitema, "Report of
        IAB Workshop on Security in the Internet Architecture", Request
        for Comments 1636, June 1994.

   [5]  R.  Atkinson, "IP Authentication Header", Request for Comments
        1826, August 1995.

   [6]  R.  Atkinson, "IP Encapsulating Security Payload", Request for
        Comments 1827, August 1995.

   [7]  Krawczyk, Bellare, and Canetti, "HMAC: Keyed-Hashing for Message
        Authentication", Request for Comments 2104, March 1996.

   [8]  Bradner, S., "Key words for use in RFCs to Indicate Requirement
        Levels", Request for Comments 2119, Harvard University, March

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   [9]  S.  Herzog, "RSVP Extensions for Policy Control", draft-ietf-
        rap-rsvp-ext-00.txt April 1998.

   [10] Postel, Jon, "Transmission Control Protocol", Request for
        Comments 793, September 1981.

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10.  Security Considerations

   This entire memo describes and specifies an authentication mechanism
   for RSVP that is believed to be secure against active and passive

   The quality of the security provided by this mechanism depends 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 assumptions are incorrect or
   procedures are insufficiently secure, then no real security will be
   provided to the users of this mechanism.

   While the handshake "Integrity Response" message is integrity-
   checked, the handshake "Integrity Challenge" message is not.  This
   was done intentionally to avoid the case when both peering routers do
   not have a starting sequence number for each other's key.
   Consequently, they will each keep sending handshake "Integrity
   Challenge" messages that will be dropped by the other end.  Moreover,
   requiring only the response to be integrity-checked eliminates a
   dependency on an integrity key in the opposite direction.

   This, however, lets an intruder generate fake handshaking challenges
   with a certain sequence number.  It could then save the response and
   attempt to play it against a receiver that is in recovery.  If it was
   lucky enough to have guessed the sequence number used by the receiver
   at recovery time it could use the saved response.  This response
   would be accepted, since it is properly signed, and would have a
   smaller sequence number for the sender because it was an old message.
   This opens the receiver up to replays. Still, it seems very difficult
   to exploit.  It requires not only guessing the challenge sequence
   number in advance, but also being able to masquerade as the receiver
   to generate a handshake "Integrity Challenge" with the proper IP
   address and not being caught.

   Confidentiality is not provided by this mechanism. If confidentiality
   is required, IPSEC ESP [6] may be the best approach, although it is
   subject to the same criticisms as IPSEC Authentication, and therefore
   would be applicable only in specific environments.  Protection
   against traffic analysis is also not provided.  Mechanisms such as
   bulk link encryption might be used when protection against traffic
   analysis is required.

11.  Authors' Addresses

        Fred Baker

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        Cisco Systems
        519 Lado Drive
        Santa Barbara,
        California 93111
        Phone: (408) 526-4257

        Bob Lindell
        USC Information Sciences Institute
        4676 Admiralty Way
        Marina del Rey, CA 90292
        Phone: (310) 822-1511
        Email: lindell@ISI.EDU

        Mohit Talwar
        USC Information Sciences Institute
        4676 Admiralty Way
        Marina del Rey, CA 90292
        Phone: (310) 822-1511
        Email: mtalwar@ISI.EDU

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