--- 1/draft-ietf-mmusic-sap-v2-01.txt	2006-02-05 00:27:39.000000000 +0100
+++ 2/draft-ietf-mmusic-sap-v2-02.txt	2006-02-05 00:27:39.000000000 +0100
@@ -1,19 +1,19 @@
                                                       Mark Handley
                                                              ACIRI
                                                      Colin Perkins
                                                                UCL
                                                      Edmund Whelan
                                                                UCL
 
                        Session Announcement Protocol
-                      draft-ietf-mmusic-sap-v2-01.txt
+               draft-ietf-mmusic-sap-v2-02.txt
 
                             Status of this memo
 
 This document is an Internet-Draft and is in full conformance with all
 provisions of Section 10 of RFC2026.
 
 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.
 
@@ -75,23 +75,23 @@
 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.
+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 8).
 
 A SAP announcement is multicast with the same scope as the session
 it is announcing, ensuring that the recipients of the announcement
 
 can also be potential recipients of the session being advertised.
 There are four possiblities:
@@ -140,47 +140,67 @@
 
 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.
 Support for directory sessions is OPTIONAL.
 
-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.
+3.1 Announcement Interval
 
-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.  If these
-fields are non-zero they form a unique identifier for the announcement, if
-they are zero then the entire message MUST be compared to determine
-uniqueness.
+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.
 
-Thus you should calculate the available bandwidth for your session's scope
-by dividing the bandwidth limit by the number of other sessions being
-announced in your scope.  This gives you your bandwidth allocation, which,
-given the size of your data packets, can be used to derive the base
-interval for announcements.  That is, given a limit in bits/second (as
-above) and a ad_size in bytes, the base announcement interval in seconds is
+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 clients always set the message identifier
+hash to zero, and if such an announcement is received the entire
+message MUST be compared to determine uniqueness).
 
-         interval = MAX(300; (8* no_of_ads* ad_size)/limit)
+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:
 
-For every interval between announcement packets (i.e, every time you send a
-packet), you must add a random value (+/- 1/3 of the base interval) to the
-value used for the inter-announcement period to prevent announcement
-synchronisation.  It is also important to keep monitoring other
-announcements and adjust the base interval accordingly.
+  1.The announcer initialises 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.  When
+this timer expires, the announcement is transmitted.
+
+If, at some time tc <tn, the no_of_ads changes (eg: if  a new announcer
+starts up, or an existing announcement is deleted), the announcer
+SHOULD recalculate the next transmission time.  If the new value
+of tn is less than tc the announcement is sent immediately.  Otherwise
+the transmission is rescheduled for the new tn.
 
 4  Session Deletion
 
 Sessions may be deleted in one of several ways:
 
 Explicit Timeout The session description payload contains timestamp
     information which specifies a start and end time for the session.
     If the current time is later than the end-time for the session,
     then the session is deleted from the receiver's session cache.
     If an announcement packet arrives with an end-time before the
@@ -198,61 +218,62 @@
     one hour minimum is to allow for transient network partitionings.
 
 Explicit Deletion A session deletion packet is received specifying
     the version of the session to be deleted.  The deletion packets
     should be ignored, unless they contain an authentication header
     which authenticates correctly and matches that used to authenticate
     the announcement which is being deleted.
 
 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.
+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.
 
+v=0
+o=cperkins 2890844526 2890842807 IN IP4 126.16.64.4
+s=Sample directory session
+m=directory 9875 SAP application/sdp
+c=IN IP4 224.2.127.12/255
+t=2873397496 2873404696
+
+        Figure 1:  Example SDP for a directory session
+
 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.
 
-v=0
-o=cperkins 2890844526 2890842807 IN IP4 126.16.64.4
-s=Sample directory session
-m=directory 9875 SAP application/sdp
-c=IN IP4 224.2.127.12/255
-t=2873397496 2873404696
-
-        Figure 1:  Example SDP for a directory session
-
 6  Directory Sessions
 
 It is possible to announce sessions which describe other directories.
 Such directory sessions should each announce a single directory.  Receivers
 MUST ignore announcements which describe multiple directories.  Receivers
 SHOULD ignore non-authenticated directory sessions.
 
 Directory sessions may be described in SDP using the media type `directory'
 with transport `SAP' (see figure 1 for example).  Directory sessions
 SHOULD be announced with TTL 255 and SHOULD use port 9875.  Any SAP
@@ -263,106 +284,106 @@
 within that directory MUST stop announcing those sessions provided
 the deletion packet authenticates with the same key as the original
 announcement of the directory session.  If the deletion packet is
 not authenticated, or is authenticated with a different key, then
 it SHOULD be ignored.
 
 If the announcement of a directory times out, announcers of sessions
 within that directory SHOULD stop announcing those sessions.
 
 If the announcement of the directory is modified to use a different
-address announcers of sessions within that directory MUST move their
-announcement to the new directory, provided the modification packet
-authenticates with the same key as the original announcement of the
-directory session.  If the modification packet is not authenticated,
-or is authenticated with a different key, then it SHOULD be ignored.
-
-7  Packet Format
-
-SAP data packets have the format described in figure 2.
-
-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).
-
  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 header              |
+|                    optional authentication data               |
 :                              ....                             :
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                        optional timeout                       |
 *-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*
 |                      optional payload type                    |
 +                                         +-+- - - - - - - - - -+
 |                                         |0|                   |
 + - - - - - - - - - - - - - - - - - - - - +-+                   |
 |                                                               |
 :                            payload                            :
 |                                                               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 
                   Figure 2: Packet format
 
+address announcers of sessions within that directory MUST move their
+announcement to the new directory, provided the modification packet
+authenticates with the same key as the original announcement of the
+directory session.  If the modification packet is not authenticated,
+or is authenticated with a different key, then it SHOULD be ignored.
+
+7  Packet Format
+
+SAP data packets have the format described in figure 2.
+
+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).
+
 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.
+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 and the timeout field MUST be
     added to the packet header.  If this bit is 0 the packet is
     not encrypted and the timeout MUST NOT be present.  See section
     8 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.
+   of 32 bit words following the main SAP header that contain authentication
+   data.  If it is zero, no authentication header is present.
 
-Authentication Header containing a digital signature of the packet,
+Authentication data containing a digital signature of the packet,
     with length as specified by the authentication length header
     field.  See section 9 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.
 
     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 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.
 
 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
@@ -382,22 +403,21 @@
     decrypt the announcement.  A SAP listener which is capable of
     decrypting the announcement MUST determine when to timeout the
     session based on the payload information.
 
     The value is an unsigned quantity giving the NTP time [8] in
     seconds at which time the session is timed out.  It is in network
     byte order.
 
 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 optionally encrypted and/or compressed.
-
+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
@@ -412,27 +432,29 @@
 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 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.
+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 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.
 
 8  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
@@ -482,93 +504,94 @@
 of the session to pass on encryption keys to un-authorised 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 assymetric 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 3:  Format of the authentication header
-
 9  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.
 
-Clearly the key used in the authentication header should not be trusted
+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
-headers are self-describing, but their precise format depends on the
-authentication mechanism in use.  The generic format of the Authentication
-Header is given in figure 3.  The structure of the Format Specific
-Authentication Subheader, using both the PGP and the CMS formats,
-is discussed in sections 9.1 and 9.2 respectively.
+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 3.  The structure of the format
+specific authentication subheader, using both the PGP and the CMS
+formats, is discussed in sections 9.1 and 9.2 respectively.
 
-Version Number, V:  The version number of the authentication header
+                     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 3:  Format of the authentication data in the SAP header
+
+Version Number, V:  The version number of the authentication format
     specified by this memo is 1.
 
-Padding Bit, P:  If necessary the data in the authentication header
-    is padded to be a multiple of 32 bits and the padding bit  is
-    set.  In this case the last byte of the authentication header
-    contains the number of padding bytes (including the last byte)
-    that must be discarded.
+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 header MUST be calculated
+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 header field excluded
-when calculating the digitial signature.
+length MUST be set to zero and the authentication data excluded when
+calculating the digitial signature.
 
 9.1 PGP Authentication
 
 Implementations which support authentication MUST support this format.
+
 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.
 
 9.2 CMS Authentication
 
 Support for this format is OPTIONAL.
 
@@ -757,18 +782,16 @@
 
 [3] P. Deutsch and J.-L. Gailly. Zlib compressed data format specification
     version  3.3, May 1996. RFC1950.
 
 [4] M. Handley and V. Jacobson. SDP: Session Description Protocol, April
     1998. RFC2327.
 
 [5] M. Handley,  D. Thaler, and R. Kermode. Multicast-scope zone
     announcement  protocol (MZAP)(, February 1999. Work in progress.
 
-[6] R. Housley.  Cryptographic message syntax.  Work in progress, April
-    1999.  draft-ietf-smime-cms-13.txt.
+[6] R. Housley.  Cryptographic message syntax.  Work in progress,
+    April 1999.  draft-ietf-smime-cms-13.txt.
 
 [7] D. Mayer. Administratively scoped IP multicast, July 1998. RFC2365.
 
 [8] D. Mills. Network time protocol version 3, March 1992.  RFC1305.
-
-===============================================================================