Internet Engineering Task Force                                   MMUSIC WG
INTERNET-DRAFT                                                 Mark Handley
draft-ietf-mmusic-sap-00.txt                                            ISI
                                                              19th Nov 1996

                   SAP: Session Announcement Protocol

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     This document  describes  the  SAP  -  the  session  directory
     announcement  protocol, and the related issues affecting secu-
     rity and scalability that should be taken into account by  the
     implementors  of  session  directory tools.  It is a companion
     document to draft-ietf-mmusic-sdp.

This document is a product of the Multiparty Multimedia Session  Control
(MMUSIC) working group of the Internet Engineering Task Force.  Comments
are solicited and should be addressed to  the  working  group's  mailing
list at and/or the author.

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1.  Introduction

An mbone session directory is used to advertise multimedia  conferences,
and  to communicate the session addresses (whether multicast or unicast)
and conference-tool-specific information  necessary  for  participation.
Such sessions are described using the Session Description Protocol (SDP)
which is described in a companion draft.  This  document  describes  the
issues  involved  in  the  multicast announcement of session description
packets and defines a packet format to  be  used  by  session  directory
clients.   SAP  v0  is currently implemented in Sdr and other compatible
tools.  This document describes SAP v1, which contains some enhancements
to the basic announcement model.  The differences between SAP v1 and SAP
v0 are described in Appendix A.  Much of this document is concerned with
security  considerations  -  these  security considerations have not yet
been subject to suitable peer-review, and this document  should  not  be
considered authoritative in this area.

2.  Background

IP Multicast is an extension of internet routing that permits  efficient
many-to-many  communication.  It is used extensively for multimedia con-
ferencing.  Such multimedia sessions  usually  have  the  property  that
tight  coordination  of session membership is not necessary; in order to
receive a session, a user at a multicast-capable site only has  to  know
the  correct  multicast  group address for the session and the transport
ports the conferencing applications will use to receive  the  conference
data streams.

In order to assist the advertisement of multicast sessions and  to  com-
municate  the relevant session setup information to prospective partici-
pants, a distributed session directory is used.  An instance of  such  a
session  directory periodically multicasts packets containing a descrip-
tion of a multimedia session, and these advertisements are  received  by
potential  participants who can use the session description to start the
tools required to participate  in  the  session.   The  companion  draft
``SDP:  Session  Description Protocol'' describes a payload format suit-
able for such session descriptions.  This draft describes the  distribu-
tion mechanism and packet format.

3.  The SAP Protocol

SAP is an announcement protocol for multicast conference  sessions.   An
SAP  client  that announces a conference session periodically multicasts
an announcement packet to a well known multicast address and port.   The
announcement  is  multicast  with  the  same  scope (as defined by group
address range or TTL) as the session it  is  announcing.   This  ensures
that the recipients of the announcement can also be potential recipients

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of the session the announcement describes (bandwidth and other such con-
straints permitting).  This is also important for the scalability of the
protocol, as it keeps local session announcements local.

The time period between one announcement and its repetition is dependent
on two factors - the scope (TTL) of the session, and the number of other
sessions currently being announced by other session  directory  clients.
The  goal  is  to keep the total bandwidth being used below a predefined
level for each scope.

Session Announcement

A session to be announced is simply multicast to the  appropriate  well-
known  multicast  address  and port. The announcement contains a session
description and, optionally,  an  authentication  header.   The  session
description may be encrypted.

Session Deletion

Sessions may be deleted in one of several ways:

Explicit Timeout
    The session description contains timestamp information which  speci-
    fies  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 current time, it is ignored.

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 mes-
    sage has not been received for ten times the announcement period, or
    half  an hour, whichever is the greater, then the session is deleted
    from the receiver's session cache.  The  half  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. If the cached session contains an authentica-
    tion header, the session deletion packet must  contain  a  signature
    signed  by  the same key.  If the cached session does not contain an
    authentication header, but the deletion  packet  has  the  same  IP-
    source  address (not the SAP-stated source address in the packet) as
    that from which the session announcement was  originally  announced,
    then  the  session  is deleted.    If neither of these conditions is

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    not the case, then the session deletion  packet  is  ignored.   Note
    that IP source addresses can be spoofed, and although the RPF filter
    in most multicast routing algorithms will result in the  packet  not
    being  delivered  in  some cases, this is insufficient protection in
    many cases, and an authentication header should  be  used  for  such

Session Modification

A pre-announced session can be modified by simply announcing  the  modi-
fied  session  description.   In  this case, the version hash in the SAP
header should be changed to indicate to receivers that the  packet  con-
tents  should  be  parsed  (or decrypted and parsed if it is encrypted).
The session itself is uniquely identified by the SDP origin field in the
payload, and not by the version hash in the SAP header.

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

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

+   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.

If an announcement is received containing a  authentication  header  and
the  cached announcement did not contain an authentication header, or it
contained  an  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.

4.  Packet Format

Unencrypted SAP data packets are of the following format:

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 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 | MT  |E|C|   auth len    |        msg id hash            |
|                         orig source                           |
|                optional authentication header                 |
|                             ....                              |
|                         text payload                          |
|                             ....                              |

Encrypted SAP data packets contain additional fields

 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 | MT  |E|C|   auth len    |        msg id hash            |
|                         orig source                           |
|                optional authentication header                 |
|                             ....                              |
|                             key id                            |
|                            timeout                            |
|P|                        random field                         |
|                         text payload                          |
|                             ....                              |

Only fields from * onwards are encrypted.

V: Version Number

SAP version number = 1

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MT: Message Type

One of the following:

0   Session description announcement packet.  The text payload is an SDP
    session description, as described in draft-ietf-mmusic-sdp.

1   Session description deletion packet.  The text payload is  a  single
    SDP  line  consisting  of the origin field of the announcement to be

E - Encryption Bit

If the encryption bit is set, the text payload  of  the  SAP  packet  is
encrypted,  and  additional  fields  are  added  to  the packet: Key-ID,
Timeout, P (padding) and Random.  The Key-ID and Timeout fields are  not
encrypted, but the P and Random fields are encrypted along with the text
payload.  Note the encryption algorithm is not specified in the packet -
this  is  communicated to permitted receivers out-of-band along with the
corresponding decryption key.

C - Compressed bit

This bit indicates that  the  payload  was  compressed  using  the  gzip
compression algorithm [3].

Authentication Length:

A 8 bit unsigned quantity giving the number of 32  bit  words  following
the  main SAP header that contain authentication data (and padding bytes
if present).  If it is zero, no authentication header is present.

Authentication Header

This contains a digital signature (encrypted cryptographic hash) of  the
text  payload  (including key-id, expiry timestamp, and encrypted random
field and text payload if the payload is encrypted) from the end of  the
authentication  header  onwards.   It  also contains the public key with
which the authentication header can be checked, and information to iden-
tify  the  encryption  algorithm  and mode used.  It can be used for two

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

+   Authentication of the identity of the session creator.

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In some circumstances only Verification is possible because  a  certifi-
cate  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 ses-
sions claiming to be from the same person.  This may or may not be  suf-
ficient depending on the purpose of the session and the people involved.

Clearly the key given in the authentication header 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  announce-

SAP is not tied to any single authentication mechanism.   Authentication
Headers must be self-describing, but their precise format depends on the
authentication mechanism (signature and encryption scheme) in  use,  and
so is not defined here.

Message Identifier Hash

A 16 bit quantity that, used in combination with the originating source,
provides  a  globally  unique id identifying the precise version of this
announcement.  The message id hash should be changed if any field of the
session  description  is changed.  A value of zero means the hash should
be ignored and the message should always be parsed.

Originating Source

This gives the IP address of the original source of the message.  It  is
permissible  to  be  zero  if the message has not passed through a proxy
relay and if the message id hash is also zero, though this  is  intended
only for backwards compatibility with SAPv0 clients.

Key ID

The key identifier is a 32 bit network byte-order integer which is  used
as a hint to identify which encryption key was used to encrypt a packet.
Key id's should be randomly generated  when  a  new  encryption  key  is
chosen  for  a group of users, and so they are not guaranteed to be glo-
bally unique.  If a receiver has multiple keys with the same key-id,  to
perform  decryption each key in turn must be used until one of them suc-
cessfully decrypts the data.

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When the session payload is encrypted, and the  session  description  is
being  relayed  or  announced via a proxy, the detailed timing fields in
the SDP description are not available to the proxy as they are encrypted
and  the  proxy is not trusted with the decryption key.  Under such cir-
cumstances, SAP includes an additional 32-bit  timestamp  field  stating
when  the session should be timed out.  This field is included after the
authentication header, and the digital signature in  the  authentication
header  encompasses  the timeout so that a session cannot be maliciously
deleted by modifying its timeout in an announcing proxy.

The value is an unsigned quantity giving the NTP time [2] in seconds  at
which time the session is timed out.  It is in network byte order.

P: Encryption Padding

This bit indicates that the payload was padded prior to encryption.  The
last byte of the decrypted payload indicates how many padding bytes were


This field is only  present  when  the  payload  is  encrypted.   It  is
encrypted  along with the payload, and is used to perform the randomiza-
tion task normally performed by an initialization vector  in  algorithms
such  as  cipher-block  chained DES.  This 31 bit field should contain a
genuinely random number.  After decryption, this field is discarded.

5.  Encrypted Announcements

Announcements may be encrypted using any encryption algorithm  or  mode.
However,  the  use  of DES in cipher-block chaining (CBC) mode is recom-
mended as the default case.  The choice of encryption algorithm and mode
is conveyed to potential recipients along with the encryption key itself
and a 32 bit key identifier which should be randomly chosen and is  used
as a non-globally-unique identifier for the key.

In normal usage, a {decryption-key,keyid,algorithm,mode} tuple  will  be
conveyed  in  advance  to  the  intended group recipients.  This process
takes place out-of-band and is not described in this draft.  However, if
keys  are  to be communicated as plain text, the use of MD5 as described
in [4] is recommended to manipulate the key prior to use.

Session announcements may  then  be  made  to  the  appropriate  session
announcement  address,  encrypted so that they can be decrypted with the
group key.  The key-id is  carried  in  the  announcement  packets,  and

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serves as an index into a sparse key-ring at each receiver.  As key-id's
are allocated randomly, in some cases more than one  decryption-key  may
be  identified by the same key id - this is expected to be a rare event,
but may happen.  When more that one key is identified by a key-id,  each
of the decryption keys in turn must be tried.

5.1.  Encrypting Announcements

If the payload is to be  compressed,  this  is  performed  first  before
encryption or padding.

When an announcement is to be encrypted, a 32-bit word is  prepended  to
the  session  description  payload.   The  most  significant bit of this
number (in network byte order) is set to zero if the session description
does  not  require  padding for encryption, and set to one if padding is
required for encryption, in which case the last byte of the padded  ses-
sion  description contains the number of padding bytes added.  The least
significant 31 bits of this 32 bit quantity should contain random data.

The padded and 32-bit pre-pended session description is  then  encrypted
using  the  relevant  encryption algorithm, key and mode.  The encrypted
payload is then sent with an extended SAP header which has the E bit set
and contains the key id and timeout fields as described above.

5.2.  Decrypting Announcements

Upon receiving a  new  announcement  with  the  encryption  bit  set,  a
receiver should attempt to decrypt the announcement with each of its set
of session  decryption  keys  that  has  the  key-id  appearing  in  the
announcement.   If  it  succeeds,  then  the session is displayed to the
user.  If it has no key that matches the announcement  key-id,  or  does
not  succeed  with any key that does match, then the session is ignored.
If one of more keys did match the key-id, but decryption failed with all
of  these matching keys, then the version hash, original source and key-
id are cached to avoid having to attempt to  decrypt  this  announcement
every  time  it is received in future.  To avoid possible denial of ser-
vice  attacks,  such  incoming  announcements  should  occasionally   be
attempted  to  be  decrypted  on  a random basis as available processing
power allows.  This cache can be  safely  timed  out  when  the  timeout
specified in encrypted packets expires.

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  announce-
ment  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 modifica-
tion has occurred.  This  does  not  guarantee  that  all  proxies  have

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deleted  the  session,  and so receivers of encrypted sessions should be
prepared to discard old versions of session announcements that they  may
receive  (as  identified  by the SDP version field).  In most cases how-
ever, 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.

6.  SDP announcement by periodic multicast.

SAP announces  multicast  sessions  by  periodic  multicast  of  session
descriptions  to  an  appropriate well known multicast address and port.
The appropriate address is determined by the scope mechanisms  in  force
at the sites of the intended participants.  IP multicast sessions can be
either TTL-scoped or administratively scoped.   One  well-known  address
and port is used for all TTL-scoped announcements, and additionally, one
well-known address (within the corresponding scope  zone)  and  port  is
used  for each administrative scope zone that an instance of the session
directory is within. Thus an instance of the  session  directory  should
listen  on multiple multicast addresses, but should normally only send a
particular announcement to the single multicast address corresponding to
the  scope of the session being described.  The discovery of administra-
tive scope zones and the appropriate announcement address for each  zone
are  outside  the  scope  of  this  draft,  but  it is assumed that each
instance of the session directory within  a  particular  scope  zone  is
aware of that scope zone, and of the corresponding announcement address,
port, TTL, and session address allocation range.

TTL Scoped Announcement

The well-known address is and the UDP port is  9875.   The
session  announcements  should be multicast with the same TTL with which
the conference session will be multicast. If the different media  in  an
announcement  are  given different TTLs, then multiple announcements are
necessary to ensure that anyone  joining  the  conference  can  in  fact
receive  data  for  each  media  started.   For  example,  if we have an
announcement to make containing audio at TTL 127 and video  at  TTL  63,
then  we  make  an  announcement  at TTL 63 containing both media, and a
separate announcement at TTL 127 containing only the audio.  It is up to
the  receiving session directory to parse both announcements as the same
announcement (as identified by the SDP origin field) if it is within the
appropriate  scope to get both announcements.  If multiple announcements
are being made for the same session in this way, then each  announcement
must  carry  an  authentication  header  signed  by  the same key, or be
treated as a completely separate announcement.

The time period between one announcement and its repetition is dependent

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on two factors - the scope (TTL) of the session, and the number of other
sessions currently being announced by other session directory instances.

The recommended bandwidth limits for each TTL are:

                TTL     bandwidth
                1-15     2Kbps
                16-63    1Kbps
                64-127   1Kbps
                128-255 200bps

Session announcers in the same scope band can normally  be  expected  to
hear  your announcements, and reduce their data rates accordingly.  Thus
you should calculate the available bandwidth for  your  session's  scope
band  by  dividing  the  appropriate  limit above by the number of other
announcers in your scope band.  This gives you  your  bandwidth  alloca-
tion,  which, given the size of your data packets, can be used to derive
the base interval for announcements.

I.e., given a limit in bits/second (as above) and a  ad_size  in  bytes,
the base announcement interval in seconds is:

         interval =MAX(300, (8*no_of_ads*ad_size)/limit)

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 inter-
val) 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.

There is possibility to adjust the scope band limits depending  on  pro-
perties of the sessions being announced, but this is left for future SAP
drafts to specify.

Administrative Scoped Announcements

For each administrative scope  zone  in  force  at  a  particular  site,
instances of the session directory running at that site need to know the
following information:

+   The multicast address to be used for announcement.  The is  normally
    the  highest  multicast address in the relevant administrative scope
    zone.  For  example,  if  the   scope   range   is   -, then the convention is that is used for
    session announcements.

+   The UDP port to which announcements should be sent.

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+   The TTL announcements should be made with.   This  should  be  large
    enough  to reach all sites in the admin scope zone, and will also be
    the TTL used for sessions announced to be using this scope zone.

+   The address range to be used for sessions in this scope zone.   This
    should  be  a  contiguous range, and currently should lie within the
    range to (but this is defined by IANA, not
    by this draft).

+   The total bandwidth to be used by the session directory for  session
    announcements in this admin scope zone.  A recommended default value
    for this is 500bps, but this may be inappropriate for some uses.

7.  Security Considerations

SAP contains mechanisms for ensuring integrity of session announcements,
for authenticating the origin of an announcement and for encrypting such
announcements.  These mechanisms have not yet been subject  to  suitable
peer-review, and this document should not be considered authoritative in
this area at this time.

SAP contains mechanisms that are designed to prevent an announcement  by
one  user  from  being  modified or deleted by another user, and also to
provide limited privacy by use of encryption.

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 recognised 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 assumed  to  have
come  from  an  authorised  key  holder  unless  there is an appropriate
authentication header signed by an authorised key holder.   In  addition
the recipients of such encrypted announcements cannot be assumed to only
be authorised key holders.  Such encrypted announcements do not  provide
any  real  security unless all of the authorised 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-authorised  users.   However  it is likely that keys used for the
session tools will be more short  lived  that  those  used  for  session

Similar considerations  should  apply  when  session  announcements  are
encrypted  with an assymetric algorithm, but then it is possible to res-
trict 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.

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As stated above, 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  detect-
able, and we do not provide any mechanism here to prevent it.

Appendix 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  Sdr  session  description tool.  SAPv1 is the proposed protocol
described in the document.  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:

+   Version Number: 0

+   Message Type: 0 (Announcement)

+   Authentication Type: 0 (No Authentication)

+   Encryption Bit: 0 (No Encryption)

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+   Compression Bit: 0 (No compression)

+   Message Id Hash: 0 (No Hash Specified)

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

Appendix B: Author's Address

Mark Handley
Information Sciences Institute,
University of Southern California,
c/o MIT Laboratory for Computer Science,
545 Technology Square,
Cambridge, MA 02139,
United States
electronic mail:


SAP and SDP were originally based on the protocol used by the sd session
directory  from  Van  Jacobson at LBNL.  The design of SAP was funded by
the European Commission under the Esprit 7602 "MICE"  project,  and  the
Telematics 1007 "MERCI" project.


[1] M.Handley,  V.  Jacobson,  ``SDP:  Session  Description  Protocol'',

[2] D. Mills, ``Network Time Protocol version 2 specification and imple-
mentation", RFC1119, 1st Sept 1989.

[3] P. Deutsch, ``GZIP file  format  specification  version  4.3'',  RFC
1952, May 1996.

[4] H. Schulzrinne, ``RTP Profile for Audio and Video  Conferences  with
Minimal Control'', RFC 1890, January 1996

Handley                                                        [Page 14]