





Network Working Group                                         M. Handley
Request for Comments: 2974                                         ACIRI
Category: Experimental                                        C. Perkins
                                                                 USC/ISI
                                                               E. Whelan
                                                                     UCL
                                                            October 2000


                     Session Announcement Protocol

Status of this Memo

   This memo defines an Experimental Protocol for the Internet
   community.  It does not specify an Internet standard of any kind.
   Discussion and suggestions for improvement are requested.
   Distribution of this memo is unlimited.

Copyright Notice

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

Abstract

   This document describes version 2 of the multicast session directory
   announcement protocol, Session Announcement Protocol (SAP), and the
   related issues affecting security and scalability that should be
   taken into account by implementors.

1  Introduction

   In order to assist the advertisement of multicast multimedia
   conferences and other multicast sessions, and to communicate the
   relevant session setup information to prospective participants, a
   distributed session directory may be used.  An instance of such a
   session directory periodically multicasts packets containing a
   description of the session, and these advertisements are received by
   other session directories such that potential remote participants can
   use the session description to start the tools required to
   participate in the session.

   This memo describes the issues involved in the multicast announcement
   of session description information and defines an announcement
   protocol to be used.  Sessions are described using the session
   description protocol which is described in a companion memo [4].






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2  Terminology

   A SAP announcer periodically multicasts an announcement packet to a
   well known multicast address and port.  The announcement is multicast
   with the same scope as the session it is announcing, ensuring that
   the recipients of the announcement are within the scope of the
   session the announcement describes (bandwidth and other such
   constraints permitting).  This is also important for the scalability
   of the protocol, as it keeps local session announcements local.

   A SAP listener learns of the multicast scopes it is within (for
   example, using the Multicast-Scope Zone Announcement Protocol [5])
   and listens on the well known SAP address and port for those scopes.
   In this manner, it will eventually learn of all the sessions being
   announced, allowing those sessions to be joined.

   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 [1].

3  Session Announcement

   As noted previously, a SAP announcer periodically sends an
   announcement packet to a well known multicast address and port.
   There is no rendezvous mechanism - the SAP announcer is not aware of
   the presence or absence of any SAP listeners - and no additional
   reliability is provided over the standard best-effort UDP/IP
   semantics.

   That announcement contains a session description and SHOULD contain
   an authentication header.  The session description MAY be encrypted
   although this is NOT RECOMMENDED (see section 7).

   A SAP announcement is multicast with the same scope as the session it
   is announcing, ensuring that the recipients of the announcement are
   within the scope of the session the announcement describes. There are
   a number of possibilities:

   IPv4 global scope sessions use multicast addresses in the range
      224.2.128.0 - 224.2.255.255 with SAP announcements being sent to
      224.2.127.254 (note that 224.2.127.255 is used by the obsolete
      SAPv0 and MUST NOT be used).









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   IPv4 administrative scope sessions using administratively scoped IP
      multicast as defined in [7].  The multicast address to be used for
      announcements is the highest multicast address in the relevant
      administrative scope zone.  For example, if the scope range is
      239.16.32.0 - 239.16.33.255, then 239.16.33.255 is used for SAP
      announcements.

   IPv6 sessions are announced on the address FF0X:0:0:0:0:0:2:7FFE
      where X is the 4-bit scope value.  For example, an announcement
      for a link-local session assigned the address
      FF02:0:0:0:0:0:1234:5678, should be advertised on SAP address
      FF02:0:0:0:0:0:2:7FFE.

   Ensuring that a description is not used by a potential participant
   outside the session scope is not addressed in this memo.

   SAP announcements MUST be sent on port 9875 and SHOULD be sent with
   an IP time-to-live of 255 (the use of TTL scoping for multicast is
   discouraged [7]).

   If a session uses addresses in multiple administrative scope ranges,
   it is necessary for the announcer to send identical copies of the
   announcement to each administrative scope range.  It is up to the
   listeners to parse such multiple announcements as the same session
   (as identified by the SDP origin field, for example).  The
   announcement rate for each administrative scope range MUST be
   calculated separately, as if the multiple announcements were
   separate.

   Multiple announcers may announce a single session, as an aid to
   robustness in the face of packet loss and failure of one or more
   announcers.  The rate at which each announcer repeats its
   announcement MUST be scaled back such that the total announcement
   rate is equal to that which a single server would choose.
   Announcements made in this manner MUST be identical.

   If multiple announcements are being made for a session, then each
   announcement MUST carry an authentication header signed by the same
   key, or be treated as a completely separate announcement by
   listeners.

   An IPv4 SAP listener SHOULD listen on the IPv4 global scope SAP
   address and on the SAP addresses for each IPv4 administrative scope
   zone it is within.  The discovery of administrative scope zones is
   outside the scope of this memo, but it is assumed that each SAP
   listener within a particular scope zone is aware of that scope zone.
   A SAP listener which supports IPv6 SHOULD also listen to the IPv6 SAP
   addresses.



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3.1 Announcement Interval

   The time period between repetitions of an announcement is chosen such
   that the total bandwidth used by all announcements on a single SAP
   group remains below a preconfigured limit.  If not otherwise
   specified, the bandwidth limit SHOULD be assumed to be 4000 bits per
   second.

   Each announcer is expected to listen to other announcements in order
   to determine the total number of sessions being announced on a
   particular group.  Sessions are uniquely identified by the
   combination of the message identifier hash and originating source
   fields of the SAP header (note that SAP v0 announcers always set the
   message identifier hash to zero, and if such an announcement is
   received the entire message MUST be compared to determine
   uniqueness).

   Announcements are made by periodic multicast to the group.  The base
   interval between announcements is derived from the number of
   announcements being made in that group, the size of the announcement
   and the configured bandwidth limit.  The actual transmission time is
   derived from this base interval as follows:

      1. The announcer initializes the variable tp to be the last time a
         particular announcement was transmitted (or the current time if
         this is the first time this announcement is to be made).

      2. Given a configured bandwidth limit in bits/second and an
         announcement of ad_size bytes, the base announcement interval
         in seconds is

                interval =max(300; (8*no_of_ads*ad_size)/limit)

      3. An offset is calculated based on the base announcement interval

                offset= rand(interval* 2/3)-(interval/3)

      4. The next transmission time for an announcement derived as

                tn =tp+ interval+ offset

   The announcer then sets a timer to expire at tn and waits.  At time
   tn the announcer SHOULD recalculate the next transmission time.  If
   the new value of tn is before the current time, the announcement is
   sent immediately.  Otherwise the transmission is rescheduled for the
   new tn.  This reconsideration prevents transient packet bursts on
   startup and when a network partition heals.




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4  Session Deletion

   Sessions may be deleted in one of several ways:

   Explicit Timeout The session description payload may contain
      timestamp information specifying the start- and end-times of the
      session.  If the current time is later than the end-time of the
      session, then the session SHOULD be deleted from the receiver's
      session cache.

   Implicit Timeout A session announcement message should be received
      periodically for each session description in a receiver's session
      cache.  The announcement period can be predicted by the receiver
      from the set of sessions currently being announced.  If a session
      announcement message has not been received for ten times the
      announcement period, or one hour, whichever is the greater, then
      the session is deleted from the receiver's session cache.  The one
      hour minimum is to allow for transient network partitionings.

   Explicit Deletion A session deletion packet is received specifying
      the session to be deleted.  Session deletion packets SHOULD have a
      valid authentication header, matching that used to authenticate
      previous announcement packets.  If this authentication is missing,
      the deletion message SHOULD be ignored.

5  Session Modification

   A pre-announced session can be modified by simply announcing the
   modified session description.  In this case, the version hash in the
   SAP header MUST be changed to indicate to receivers that the packet
   contents should be parsed (or decrypted and parsed if it is
   encrypted).  The session itself, as distinct from the session
   announcement, is uniquely identified by the payload and not by the
   message identifier hash in the header.

   The same rules apply for session modification as for session
   deletion:

    o Either the modified announcement must contain an authentication
      header signed by the same key as the cached session announcement
      it is modifying, or:

    o The cached session announcement must not contain an authentication
      header, and the session modification announcement must originate
      from the same host as the session it is modifying.






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   If an announcement is received containing an authentication header
   and the cached announcement did not contain an authentication header,
   or it contained a different authentication header, then the modified
   announcement MUST be treated as a new and different announcement, and
   displayed in addition to the un-authenticated announcement.  The same
   should happen if a modified packet without an authentication header
   is received from a different source than the original announcement.

   These rules prevent an announcement having an authentication header
   added by a malicious user and then being deleted using that header,
   and it also prevents a denial-of-service attack by someone putting
   out a spoof announcement which, due to packet loss, reaches some
   participants before the original announcement.  Note that under such
   circumstances, being able to authenticate the message originator is
   the only way to discover which session is the correct session.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | V=1 |A|R|T|E|C|   auth len    |         msg id hash           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   :                originating source (32 or 128 bits)            :
   :                                                               :
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    optional authentication data               |
   :                              ....                             :
   *-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*
   |                      optional payload type                    |
   +                                         +-+- - - - - - - - - -+
   |                                         |0|                   |
   + - - - - - - - - - - - - - - - - - - - - +-+                   |
   |                                                               |
   :                            payload                            :
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                     Figure 1: Packet format

6  Packet Format

   SAP data packets have the format described in figure 1.

   V: Version Number. The version number field MUST be set to 1 (SAPv2
      announcements which use only SAPv1 features are backwards
      compatible, those which use new features can be detected by other
      means, so the SAP version number doesn't need to change).




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   A: Address type. If the A bit is 0, the originating source field
      contains a 32-bit IPv4 address.  If the A bit is 1, the
      originating source contains a 128-bit IPv6 address.

   R: Reserved. SAP announcers MUST set this to 0, SAP listeners MUST
      ignore the contents of this field.

   T: Message Type. If the T field is set to 0 this is a session
      announcement packet, if 1 this is a session deletion packet.

   E: Encryption Bit. If the encryption bit is set to 1, the payload of
      the SAP packet is encrypted.  If this bit is 0 the packet is not
      encrypted.  See section 7 for details of the encryption process.

   C: Compressed bit. If the compressed bit is set to 1, the payload is
      compressed using the zlib compression algorithm [3].  If the
      payload is to be compressed and encrypted, the compression MUST be
      performed first.

   Authentication Length. An 8 bit unsigned quantity giving the number
      of 32 bit words following the main SAP header that contain
      authentication data.  If it is zero, no authentication header is
      present.

   Authentication data containing a digital signature of the packet,
      with length as specified by the authentication length header
      field.  See section 8 for details of the authentication process.

   Message Identifier Hash. A 16 bit quantity that, used in combination
      with the originating source, provides a globally unique identifier
      indicating the precise version of this announcement.  The choice
      of value for this field is not specified here, except that it MUST
      be unique for each session announced by a particular SAP announcer
      and it MUST be changed if the session description is modified (and
      a session deletion message SHOULD be sent for the old version of
      the session).

      Earlier versions of SAP used a value of zero to mean that the hash
      should be ignored and the payload should always be parsed.  This
      had the unfortunate side-effect that SAP announcers had to study
      the payload data to determine how many unique sessions were being
      advertised, making the calculation of the announcement interval
      more complex that necessary.  In order to decouple the session
      announcement process from the contents of those announcements, SAP
      announcers SHOULD NOT set the message identifier hash to zero.

      SAP listeners MAY silently discard messages if the message
      identifier hash is set to zero.



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   Originating Source. This gives the IP address of the original source
      of the message.  This is an IPv4 address if the A field is set to
      zero, else it is an IPv6 address.  The address is stored in
      network byte order.

      SAPv0 permitted the originating source to be zero if the message
      identifier hash was also zero.  This practise is no longer legal,
      and SAP announcers SHOULD NOT set the originating source to zero.
      SAP listeners MAY silently discard packets with the originating
      source set to zero.

   The header is followed by an optional payload type field and the
   payload data itself.  If the E or C bits are set in the header both
   the payload type and payload are encrypted and/or compressed.

   The payload type field is a MIME content type specifier, describing
   the format of the payload.  This is a variable length ASCII text
   string, followed by a single zero byte (ASCII NUL).  The payload type
   SHOULD be included in all packets.  If the payload type is
   `application/sdp' both the payload type and its terminating zero byte
   MAY be omitted, although this is intended for backwards compatibility
   with SAP v1 listeners only.

   The absence of a payload type field may be noted since the payload
   section of such a packet will start with an SDP `v=0' field, which is
   not a legal MIME content type specifier.

   All implementations MUST support payloads of type `application/sdp'
   [4].  Other formats MAY be supported although since there is no
   negotiation in SAP an announcer which chooses to use a session
   description format other than SDP cannot know that the listeners are
   able to understand the announcement.  A proliferation of payload
   types in announcements has the potential to lead to severe
   interoperability problems, and for this reason, the use of non-SDP
   payloads is NOT RECOMMENDED.

   If the packet is an announcement packet, the payload contains a
   session description.

   If the packet is a session deletion packet, the payload contains a
   session deletion message.  If the payload format is `application/sdp'
   the deletion message is a single SDP line consisting of the origin
   field of the announcement to be deleted.

   It is desirable for the payload to be sufficiently small that SAP
   packets do not get fragmented by the underlying network.
   Fragmentation has a loss multiplier effect, which is known to
   significantly affect the reliability of announcements.  It is



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   RECOMMENDED that SAP packets are smaller than 1kByte in length,
   although if it is known that announcements will use a network with a
   smaller MTU than this, then that SHOULD be used as the maximum
   recommended packet size.

7  Encrypted Announcements

   An announcement is received by all listeners in the scope to which it
   is sent.  If an announcement is encrypted, and many of the receivers
   do not have the encryption key, there is a considerable waste of
   bandwidth since those receivers cannot use the announcement they have
   received.  For this reason, the use of encrypted SAP announcements is
   NOT RECOMMENDED on the global scope SAP group or on administrative
   scope groups which may have many receivers which cannot decrypt those
   announcements.

   The opinion of the authors is that encrypted SAP is useful in special
   cases only, and that the vast majority of scenarios where encrypted
   SAP has been proposed may be better served by distributing session
   details using another mechanism.  There are, however, certain
   scenarios where encrypted announcements may be useful.  For this
   reason, the encryption bit is included in the SAP header to allow
   experimentation with encrypted announcements.

   This memo does not specify details of the encryption algorithm to be
   used or the means by which keys are generated and distributed.  An
   additional specification should define these, if it is desired to use
   encrypted SAP.

   Note that if an encrypted announcement is being announced via a
   proxy, then there may be no way for the proxy to discover that the
   announcement has been superseded, and so it may continue to relay the
   old announcement in addition to the new announcement.  SAP provides
   no mechanism to chain modified encrypted announcements, so it is
   advisable to announce the unmodified session as deleted for a short
   time after the modification has occurred.  This does not guarantee
   that all proxies have deleted the session, and so receivers of
   encrypted sessions should be prepared to discard old versions of
   session announcements that they may receive.  In most cases however,
   the only stateful proxy will be local to (and known to) the sender,
   and an additional (local-area) protocol involving a handshake for
   such session modifications can be used to avoid this problem.

   Session announcements that are encrypted with a symmetric algorithm
   may allow a degree of privacy in the announcement of a session, but
   it should be recognized that a user in possession of such a key can
   pass it on to other users who should not be in possession of such a
   key.  Thus announcements to such a group of key holders cannot be



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   assumed to have come from an authorized key holder unless there is an
   appropriate authentication header signed by an authorized key holder.
   In addition the recipients of such encrypted announcements cannot be
   assumed to only be authorized key holders.  Such encrypted
   announcements do not provide any real security unless all of the
   authorized key holders are trusted to maintain security of such
   session directory keys.  This property is shared by the multicast
   session tools themselves, where it is possible for an un-trustworthy
   member of the session to pass on encryption keys to un-authorized
   users.  However it is likely that keys used for the session tools
   will be more short lived than those used for session directories.

   Similar considerations should apply when session announcements are
   encrypted with an asymmetric algorithm, but then it is possible to
   restrict the possessor(s) of the private key, so that announcements
   to a key-holder group can not be made, even if one of the untrusted
   members of the group proves to be un-trustworthy.

                        1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | V=1 |P| Auth  |                                               |
   +-+-+-+-+-+-+-+-+                                               |
   |              Format  specific authentication subheader        |
   :                        ..................                     :
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

    Figure 2:  Format of the authentication data in the SAP header

8  Authenticated Announcements

   The authentication header can be used for two purposes:

    o Verification that changes to a session description or deletion of
      a session are permitted.

    o Authentication of the identity of the session creator.

   In some circumstances only verification is possible because a
   certificate signed by a mutually trusted person or authority is not
   available.  However, under such circumstances, the session originator
   may still be authenticated to be the same as the session originator
   of previous sessions claiming to be from the same person.  This may
   or may not be sufficient depending on the purpose of the session and
   the people involved.






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   Clearly the key used for the authentication should not be trusted to
   belong to the session originator unless it has been separately
   authenticated by some other means, such as being certified by a
   trusted third party.  Such certificates are not normally included in
   an SAP header because they take more space than can normally be
   afforded in an SAP packet, and such verification must therefore take
   place by some other mechanism.  However, as certified public keys are
   normally locally cached, authentication of a particular key only has
   to take place once, rather than every time the session directory
   retransmits the announcement.

   SAP is not tied to any single authentication mechanism.
   Authentication data in the header is self-describing, but the precise
   format depends on the authentication mechanism in use.  The generic
   format of the authentication data is given in figure 2.  The
   structure of the format specific authentication subheader, using both
   the PGP and the CMS formats, is discussed in sections 8.1 and 8.2
   respectively.  Additional formats may be added in future.

   Version Number, V:  The version number of the authentication format
      specified by this memo is 1.

   Padding Bit, P:  If necessary the authentication data is padded to be
      a multiple of 32 bits and the padding bit is set.  In this case
      the last byte of the authentication data contains the number of
      padding bytes (including the last byte) that must be discarded.

   Authentication Type, Auth: The authentication type is a  4 bit
      encoded field that denotes the authentication infrastructure the
      sender expects the recipients to use to check the authenticity and
      integrity of the information.  This defines the format of the
      authentication subheader and can take the values:  0 = PGP format,
      1 = CMS format.  All other values are undefined and SHOULD be
      ignored.

   If a SAP packet is to be compressed or encrypted, this MUST be done
   before the authentication is added.

   The digital signature in the authentication data MUST be calculated
   over the entire packet, including the header.  The authentication
   length MUST be set to zero and the authentication data excluded when
   calculating the digital signature.

   It is to be expected that sessions may be announced by a number of
   different mechanisms, not only SAP.  For example, a session
   description may placed on a web page, sent by email or conveyed in a





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   session initiation protocol.  To ease interoperability with these
   other mechanisms, application level security is employed, rather than
   using IPsec authentication headers.

8.1 PGP Authentication

   A full description of the PGP protocol can be found in [2].  When
   using PGP for SAP authentication the basic format specific
   authentication subheader comprises a digital signature packet as
   described in [2].  The signature type MUST be 0x01 which means the
   signature is that of a canonical text document.

8.2 CMS Authentication

   A full description of the Cryptographic Message Syntax can be found
   in [6].  The format specific authentication subheader will, in the
   CMS case, have an ASN.1 ContentInfo type with the ContentType being
   signedData.

   Use is made of the option available in PKCS#7 to leave the content
   itself blank as the content which is signed is already present in the
   packet.  Inclusion of it within the SignedData type would duplicate
   this data and increase the packet length unnecessarily.  In addition
   this allows recipients with either no interest in the authentication,
   or with no mechanism for checking it, to more easily skip the
   authentication information.

   There SHOULD be only one signerInfo and related fields corresponding
   to the originator of the SAP announcement.  The signingTime SHOULD be
   present as a signedAttribute.  However, due to the strict size
   limitations on the size of SAP packets, certificates and CRLs SHOULD
   NOT be included in the signedData structure.  It is expected that
   users of the protocol will have other methods for certificate and CRL
   distribution.

9  Scalability and caching

   SAP is intended to announce the existence of long-lived wide-area
   multicast sessions.  It is not an especially timely protocol:
   sessions are announced by periodic multicast with a repeat rate on
   the order of tens of minutes, and no enhanced reliability over UDP.
   This leads to a long startup delay before a complete set of
   announcements is heard by a listener.  This delay is clearly
   undesirable for interactive browsing of announced sessions.

   In order to reduce the delays inherent in SAP, it is recommended that
   proxy caches are deployed.  A SAP proxy cache is expected to listen
   to all SAP groups in its scope, and to maintain an up-to-date list of



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   all announced sessions along with the time each announcement was last
   received.  When a new SAP listeners starts, it should contact its
   local proxy to download this information, which is then sufficient
   for it to process future announcements directly, as if it has been
   continually listening.

   The protocol by which a SAP listener contacts its local proxy cache
   is not specified here.

10 Security Considerations

   SAP contains mechanisms for ensuring integrity of session
   announcements, for authenticating the origin of an announcement and
   for encrypting such announcements (sections 7 and 8).

   As stated in section 5, if a session modification announcement is
   received that contains a valid authentication header, but which is
   not signed by the original creator of the session, then the session
   must be treated as a new session in addition to the original session
   with the same SDP origin information unless the originator of one of
   the session descriptions can be authenticated using a certificate
   signed by a trusted third party.  If this were not done, there would
   be a possible denial of service attack whereby a party listens for
   new announcements, strips off the original authentication header,
   modifies the session description, adds a new authentication header
   and re-announces the session.  If a rule was imposed that such spoof
   announcements were ignored, then if packet loss or late starting of a
   session directory instance caused the original announcement to fail
   to arrive at a site, but the spoof announcement did so, this would
   then prevent the original announcement from being accepted at that
   site.

   A similar denial-of-service attack is possible if a session
   announcement receiver relies completely on the originating source and
   hash fields to indicate change, and fails to parse the remainder of
   announcements for which it has seen the origin/hash combination
   before.

   A denial of service attack is possible from a malicious site close to
   a legitimate site which is making a session announcement.  This can
   happen if the malicious site floods the legitimate site with huge
   numbers of (illegal) low TTL announcements describing high TTL
   sessions.  This may reduce the session announcement rate of the
   legitimate announcement to below a tenth of the rate expected at
   remote sites and therefore cause the session to time out.  Such an
   attack is likely to be easily detectable, and we do not provide any
   mechanism here to prevent it.




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A. Summary of differences between SAPv0 and SAPv1

   For this purpose SAPv0 is defined as the protocol in use by version
   2.2 of the session directory tool, sdr.  SAPv1 is the protocol
   described in the 19 November 1996 version of this memo.  The packet
   headers of SAP messages are the same in V0 and V1 in that a V1 tool
   can parse a V0 announcement header but not vice-versa.  In SAPv0, the
   fields have the following values:

     o Version Number:  0

     o Message Type:  0 (Announcement)

     o Authentication Type:  0 (No Authentication)

     o Encryption Bit:  0 (No Encryption)

     o Compression Bit:  0 (No compression)

     o Message Id Hash:  0 (No Hash Specified)

     o Originating Source:  0 (No source specified, announcement has
       not been relayed)

B. Summary of differences between SAPv1 and SAPv2

   The packet headers of SAP messages are the same in V1 and V2 in that
   a V2 tool can parse a V1 announcement header but not necessarily
   vice-versa.

    o The A bit has been added to the SAP header, replacing one of the
      bits of the SAPv1 message type field.  If set to zero the
      announcement is of an IPv4 session, and the packet is backwards
      compatible with SAPv1.  If set to one the announcement is of an
      IPv6 session, and SAPv1 listeners (which do not support IPv6) will
      see this as an illegal message type (MT) field.

    o The second bit of the message type field in SAPv1 has been
      replaced by a reserved, must-be-zero, bit.  This bit was unused in
      SAPv1, so this change just codifies existing usage.

    o SAPv1 specified encryption of the payload.  SAPv2 includes the E
      bit in the SAP header to indicate that the payload is encrypted,
      but does not specify any details of the encryption.

    o SAPv1 allowed the message identifier hash and originating source
      fields to be set to zero, for backwards compatibility.  This is no
      longer legal.



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    o SAPv1 specified gzip compression.  SAPv2 uses zlib (the only known
      implementation of SAP compression used zlib, and gzip compression
      was a mistake).

    o SAPv2 provides a more complete specification for authentication.

    o SAPv2 allows for non-SDP payloads to be transported.  SAPv1
      required that the payload was SDP.

    o SAPv1 included a timeout field for encrypted announcement, SAPv2
      does not (and relies of explicit deletion messages or implicit
      timeouts).

C. Acknowledgements

   SAP and SDP were originally based on the protocol used by the sd
   session directory from Van Jacobson at LBNL.  Version 1 of SAP was
   designed by Mark Handley as part of the European Commission MICE
   (Esprit 7602) and MERCI (Telematics 1007) projects.  Version 2
   includes authentication features developed by Edmund Whelan, Goli
   Montasser-Kohsari and Peter Kirstein as part of the European
   Commission ICE-TEL project (Telematics 1005), and support for IPv6
   developed by Maryann P. Maher and Colin Perkins.




























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D. Authors' Addresses

   Mark Handley
   AT&T Center for Internet Research at ICSI,
   International Computer Science Institute,
   1947 Center Street, Suite 600,
   Berkeley, CA 94704, USA

   EMail: mjh@aciri.org


   Colin Perkins
   USC Information Sciences Institute
   4350 N. Fairfax Drive, Suite 620
   Arlington, VA 22203, USA

   EMail: csp@isi.edu


   Edmund Whelan
   Department of Computer Science,
   University College London,
   Gower Street,
   London, WC1E 6BT, UK

   EMail: e.whelan@cs.ucl.ac.uk

























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References

   [1] Bradner, S., "Key words for use in RFCs to indicate requirement
       levels", BCP 14, RFC 2119, March 1997.

   [2] Callas, J., Donnerhacke, L., Finney, H. and R. Thayer. "OpenPGP
       message format", RFC 2440, November 1998.

   [3] Deutsch, P. and J.-L. Gailly, "Zlib compressed data format
       specification version 3.3", RFC 1950, May 1996.

   [4] Handley, M. and V. Jacobson, "SDP: Session Description Protocol",
       RFC 2327, April 1998.

   [5] Handley, M., Thaler, D. and R. Kermode, "Multicast-scope zone
       announcement protocol (MZAP)", RFC 2776, February 2000.

   [6] Housley, R., "Cryptographic message syntax", RFC 2630, June 1999.

   [7] Mayer, D., "Administratively scoped IP multicast", RFC 2365, July
       1998.






























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Full Copyright Statement

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Acknowledgement

   Funding for the RFC Editor function is currently provided by the
   Internet Society.



















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