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Network Working Group                                         M. HandleyRequest for Comments: 2974                                         ACIRICategory: Experimental                                        C. Perkins                                                                 USC/ISI                                                               E. Whelan                                                                     UCL                                                            October 2000Session Announcement ProtocolStatus 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].Handley, et al.               Experimental                      [Page 1]

RFC 2974             Session Announcement Protocol          October 20002  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 (seesection 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).Handley, et al.               Experimental                      [Page 2]

RFC 2974             Session Announcement Protocol          October 2000   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.Handley, et al.               Experimental                      [Page 3]

RFC 2974             Session Announcement Protocol          October 20003.1 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.Handley, et al.               Experimental                      [Page 4]

RFC 2974             Session Announcement Protocol          October 20004  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.Handley, et al.               Experimental                      [Page 5]

RFC 2974             Session Announcement Protocol          October 2000   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 format6  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).Handley, et al.               Experimental                      [Page 6]

RFC 2974             Session Announcement Protocol          October 2000   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.  Seesection 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.  Seesection 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.Handley, et al.               Experimental                      [Page 7]

RFC 2974             Session Announcement Protocol          October 2000   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 isHandley, et al.               Experimental                      [Page 8]

RFC 2974             Session Announcement Protocol          October 2000   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 beHandley, et al.               Experimental                      [Page 9]

RFC 2974             Session Announcement Protocol          October 2000   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 header8  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.Handley, et al.               Experimental                     [Page 10]

RFC 2974             Session Announcement Protocol          October 2000   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 sections8.1 and8.2   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 aHandley, et al.               Experimental                     [Page 11]

RFC 2974             Session Announcement Protocol          October 2000   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 ofHandley, et al.               Experimental                     [Page 12]

RFC 2974             Session Announcement Protocol          October 2000   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 (sections7 and8).   As stated insection 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.Handley, et al.               Experimental                     [Page 13]

RFC 2974             Session Announcement Protocol          October 2000A. 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.Handley, et al.               Experimental                     [Page 14]

RFC 2974             Session Announcement Protocol          October 2000    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.Handley, et al.               Experimental                     [Page 15]

RFC 2974             Session Announcement Protocol          October 2000D. 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.ukHandley, et al.               Experimental                     [Page 16]

RFC 2974             Session Announcement Protocol          October 2000References   [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.Handley, et al.               Experimental                     [Page 17]

RFC 2974             Session Announcement Protocol          October 2000Full Copyright Statement   Copyright (C) The Internet Society (2000).  All Rights Reserved.   This document and translations of it may be copied and furnished to   others, and derivative works that comment on or otherwise explain it   or assist in its implementation may be prepared, copied, published   and distributed, in whole or in part, without restriction of any   kind, provided that the above copyright notice and this paragraph are   included on all such copies and derivative works.  However, this   document itself may not be modified in any way, such as by removing   the copyright notice or references to the Internet Society or other   Internet organizations, except as needed for the purpose of   developing Internet standards in which case the procedures for   copyrights defined in the Internet Standards process must be   followed, or as required to translate it into languages other than   English.   The limited permissions granted above are perpetual and will not be   revoked by the Internet Society or its successors or assigns.   This document and the information contained herein is provided on an   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING   TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING   BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF   MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.Acknowledgement   Funding for the RFC Editor function is currently provided by the   Internet Society.Handley, et al.               Experimental                     [Page 18]