draft-ietf-mmusic-sap-v2-00.txt   draft-ietf-mmusic-sap-v2-01.txt 
Mark Handley
Colin Perkins
Edmund Whelan
Internet Engineering Task Force MMUSIC WG Session Announcement Protocol
Internet Draft Maryann P. Maher draft-ietf-mmusic-sap-v2-01.txt
draft-ietf-mmusic-sap-v2-00.txt ISI
16 November 1998 Colin Perkins
Expires: May 30, 1998 UCL
Session Announcement Protocol: Version 2 Status of this memo
Status of this Memo This document is an Internet-Draft and is in full conformance with all
provisions of Section 10 of RFC2026.
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This document is a product of the Multiparty Multimedia Session Control
working group of the Internet Engineering Task Force. Comments are
solicited and should be addressed to the working group's mailing list at
confctrl@isi.edu and/or the authors.
Abstract Abstract
This document describes modifications and enhancements to SAP, the This document describes version 2 of the multicast session directory
session directory announcement protocol, for support of IPv6 announcement protocol, SAP, and the related issues affecting security
conferencing environments. Along with support for IPv6, a couple new and scalability that should be taken into account by the implementors
features are also introduced. This document compliments [SAP], which of multicast session directory tools.
fully describes SAP in the context of IPv4. Readers are assumed to be
familiar with [SAP].
Note: At this time, this document presents an initial set of ideas 1 Introduction
aimed solely at starting discussion within the working group. We
await the RFC publication of [SAP] before finalizing any protocol
This document is a product of the Multiparty Multimedia Session In order to assist the advertisement of multicast multimedia conferences
Control (MMUSIC) working group of the Internet Engineering Task and other multicast sessions, and to communicate the relevant session
Force. Comments are solicited and should be addressed to the working
group's mailing list at confctrl@isi.edu and/or the author.
1. Overview 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 potential participants who can use
the session description to start the tools required to participate
in the session.
This document describes a new version of SAP, the session directory This memo describes the issues involved in the multicast announcement
announcement protocol, for support of IPv6 conferencing environments. of session description information and defines an announcement protocol
SAP is a protocol used for handling multicast and unicast session to be used by session directory clients. Sessions are described
description packets and defines an encapsulating packet format to be using the session description protocol which is described in a companion
used by session directory clients. As currently defined, only IPv4 memo [4].
session announcements are supported mainly due to the fact that an
IPv4 address field is contained in the SAP packet header. This docu-
ment describes the modifications to SAP for supporting IPv6 session
announcements and introduces some additional new features. The two
new features described in this document are:
1) a MIME Content-Type specifier, and 2 Terminology
2) a URL scheme for referencing SAP announcements. 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 can also be potential recipients
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.
Note: At this time, this document presents an initial set of ideas A SAP listener learns of the multicast scopes it is within (for example,
aimed solely at starting discussion within the working group. We using the Multicast-Scope Zone Announcement Protocol [5]) and listens
await the RFC publication of [SAP] before finalizing any protocol on the well known SAP address and port for those scopes. In this
specifications. manner, it will eventually learn of all the sessions being announced,
allowing those sessions to be joined.
2. Modifications to the SAP Packet Format The key words `MUST', `MUST NOT', `REQUIRED', `SHALL', `SHALL NOT',
document are to be interpreted as described in [1].
Current SAPv1 data packets are of the following format: 3 Session Announcement
0 1 2 3 As noted previously, a SAP announcer periodically sends an announcement
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 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
| V=1 | MT |E|C| auth len | msg id hash | any SAP listeners - and no additional reliability is provided over the
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ standard best-effort UDP/IP semantics.
| originating source |
| optional authentication header |
| .... |
| optional timeout |
| optional random field |
| text payload |
| .... |
In order to support session directory clients in IPv6 environments,
the SAP packet format must be modified to contain a 128-bit IPv6
source address instead of the 32-bit IPv4 address.
Therefore, SAP v2 data packets are of the following format: 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:
IPv4 global scope sessions use multicast addresses in the range - with SAP announcements being sent
to (note that is used by the obsolete
SAPv0 and MUST NOT be used).
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 -, then 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.
Directory sessions are announced on an address which is itself announced
by a SAP announcement. See section 6 for details for directory
SAP announcements MUST be sent on port 9875 and SHOULD be sent with
an IP time-to-live of 255.
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
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.
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.
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
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
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 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.
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
current time, it is ignored. If the payload is encrypted, and
the receiver does not have the correct decryption key, the timeout
field in the header should be used as an explicit timeout.
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 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.
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.
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.
o=cperkins 2890844526 2890842807 IN IP4
s=Sample directory session
m=directory 9875 SAP application/sdp
c=IN IP4
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
announcer may announce sessions in an announced SAP group, there
is no explicit access control.
If the announcement of a directory is deleted, announcers of sessions
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
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 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| MT|E|C| auth len | msg id hash | | V=1 |A|R|T|E|C| auth len | msg id hash |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | | |
| originating source | : originating source (32 or 128 bits) :
| .... | : :
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| optional authentication header | | optional authentication header |
| .... | : .... :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| optional timeout | | optional timeout |
| optional payload type |
+ +-+- - - - - - - - - -+
| |0| |
+ - - - - - - - - - - - - - - - - - - - - +-+ |
| |
: payload :
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| optional random field |
Figure 2: Packet format
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 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.
Authentication Header 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
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.
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.
Timeout. When the session payload is encrypted the detailed timing
fields in the payload are not available to listeners which are
not trusted with the decryption key. Under such circumstances,
the header includes an additional 32-bit timestamp field stating
when the session should be timed out.
Note that the timeout field in the header is intended for use
only by those listeners which do not have the correct key to
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 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
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.
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
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
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 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| text payload | | V=1 |P| Auth | |
| .... | +-+-+-+-+-+-+-+-+ |
| Format specific authentication subheader |
: .................. :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Note: the version number in the packet header has NOT changed. Figure 3: Format of the authentication header
A Address type: 0 = IPv4, 1 = IPv6 9 Authenticated Announcements
If the A bit is 0, the orig source contains a 32-bit IPv4 address. If The authentication header can be used for two purposes:
the A bit is 1, the orig source contains a 128-bit IPv6 address.
3. Scoping SAP Announcements o Verification that changes to a session description or deletion
of a session are permitted.
A SAP client that announces a conference session periodically multi- o Authentication of the identity of the session creator.
casts an announcement packet to a well known multicast address and
port. The well known IPv6 SAP address is FF0X:0:0:0:0:0:2:7FFE,
where X is the 4-bit scope value. The following scope values are
currently defined in IPv6:
Value Scope Recommended Bandwidth Limit In some circumstances only verification is possible because a certificate
------ ----------- ----------------------------- signed by a mutually trusted person or authority is not available.
0x1 Node-local n/a However, under such circumstances, the session originator may still
0x2 Link-local 20 Kbps be authenticated to be the same as the session originator of previous
0x5 Site-local 10 Kbps sessions claiming to be from the same person. This may or may not
0x8 Organization-local 1 Kbps be sufficient depending on the purpose of the session and the people
0xE Global 200 bps involved.
The announcement is multicast with the same scope as the session it Clearly the key used in the authentication header should not be trusted
is announcing, ensuring that the recipients of the announcement can to belong to the session originator unless it has been separately
also be potential recipients of the session being advertised. Having authenticated by some other means, such as being certified by a trusted
a scope field within the address itself removes the need to carve out third party. Such certificates are not normally included in an SAP
scope ranges within the multicast address space or scope addresses header because they take more space than can normally be afforded
according to a corresponding TTL. Therefore, unlike in IPv4, TTL- in an SAP packet, and such verification must therefore take place
scoped announcements do not exist in IPv6 environments. The scope by some other mechanism. However, as certified public keys are normally
value to be used in the well-know SAP address is simply the same used locally cached, authentication of a particular key only has to take
in the multicast address assigned for the session. place once, rather than every time the session directory retransmits
the announcement.
For example, an announcement for a link-local session assigned the SAP is not tied to any single authentication mechanism. Authentication
multicast address of FF02:0:0:0:0:0:1234:5678, should be advertised headers are self-describing, but their precise format depends on the
on SAP address FF02:0:0:0:0:0:2:7FFE. (Note: Issues related to IPv6 authentication mechanism in use. The generic format of the Authentication
multicast address allocation are being addressed in the MALLOC work- Header is given in figure 3. The structure of the Format Specific
ing group.) Authentication Subheader, using both the PGP and the CMS formats,
is discussed in sections 9.1 and 9.2 respectively.
In the table above, recommended bandwidth limits are given for ses- Version Number, V: The version number of the authentication header
sions within the defined scopes. The total bandwidth to be used for specified by this memo is 1.
SAP announcements should be one-fourth of the session bandwidth,
though this may be inappropriate for some uses.
4. SAP Payload 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.
In previous versions of SAP, the payload was required to be a session Authentication Type, Auth: The authentication type is a 4 bit encoded
description in the SDP format [RFC2327]. With the introduction of new field that denotes the authentication infrastructure the sender
session description formats, such as SMIL [SMIL], it is believed that expects the recipients to use to check the authenticity and integrity
that restriction is no longer appropriate. SAP v2 therefore allows of the information. This defines the format of the authentication
for other content to be carried. That content should begin with a subheader and can take the values: 0 = PGP format, 1 = CMS
MIME Content-Type specifier. format. All other values are undefined and SHOULD be ignored.
Content-Type: application/sdp If a SAP packet is to be compressed or encrypted, this MUST be done
before the authentication is added.
v=0 The digital signature in the authentication header MUST be calculated
o=... 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.
If the content type is "application/sdp" the MIME header may be omit- 9.1 PGP Authentication
ted, for backwards compatibility with SAP v1 applications. If any
other content is carried the MIME header MUST be present.
5. SAP URL scheme 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.
In certain cases, it is desirable to reference a SAP announcement. 9.2 CMS Authentication
For example, if it is desired that a new participant join an existing
session yet it is not known if that participant is within the scope
of the announcement then an explicit reference to the announcement
will enable them to determine if it can be received. Providing the
session description contained within that announcement merely allows
them to join the session, when they then notice that the media
streams are not visible. Moreover, the addresses contained in a ses-
sion description for one scope may not be valid outside that scope
We therefore define a URL scheme for SAP announcements. The combina- Support for this format is OPTIONAL.
tion of msg-id-hash and originating-source fields in the SAP header
is sufficient to identify a particular announcement.
sap:msg-id@orig-source 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
TBD: What is the effect of 10.x.x.x assignments on this? 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
6. Compatibility There SHOULD be only one signerInfo and related fields corresponding
to the originator of the SAP announcement. The signingTime SHOUD
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.
SAPv2 announcement are backwards compatible with SAPv1 provided that 10 Scalability and caching
IPv4 sessions are announced, and that the payload is SDP with the
content-type omitted.
If IPv6 announcements are made, they will be discarded by SAPv1 tools SAP is intended to announce the existence of long-lived wide-area
since they represent illegal MT values in that protocol. 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.
If the Content-Type header is present in the payload, the announce- In order to reduce the delays inherent in SAP, it is recommended
ment is invalid in SAPv1. Those tools require that SDP is used, hence that proxy caches are deployed. A SAP proxy cache is expected to
the payload for a SAPv1 announcement MUST start with "v=". listen to all SAP groups in its scope, and to maintain an up-to-date
list of 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.
7. Security Considerations The protocol by which a SAP listener contacts its local proxy cache
is not specified here.
SAP contains mechanisms for ensuring integrity of session announce- 11 Security Considerations
ments, for authenticating the origin of an announcement and for
encrypting such announcements. The strengths and limitations of these
mechanisms are described in the 'Security Considerations' section of
[SAP]. Those considerations apply to this document as well.
8. Acknowledgements SAP contains mechanisms for ensuring integrity of session announcements,
for authenticating the origin of an announcement and for encrypting
such announcements (sections 8 and 9). These mechanisms have not
yet been subject to suitable peer-review, and this memo should not
be considered authoritative in this area at this time.
REFERENCES As stated in section 5, if a session modification announcement is
received that contains a valid authentication header, but which is
not signed by the original creator of the session, then the session
must be treated as a new session in addition to the original session
with the same SDP origin information unless the originator of one
of the session descriptions can be authenticated using a certificate
signed by a trusted third party. If this were not done, there would
be a possible denial of service attack whereby a party listens for
new announcements, strips off the original authentication header,
modifies the session description, adds a new authentication header
[SAP] Handley, M.J., Session Announcement Protocol, draft-ietf- and re-announces the session. If a rule was imposed that such spoof
mmusic-sap-01.txt, work in progress. 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.
[ADDARCH] Hinden, R., and S. Deering, "IP Version 6 Addressing Archi- A similar denial-of-service attack is possible if a session announcement
tecture", RFC 2373, July 1998. 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.
[IPV6] Deering, S., and R. Hinden, Editors, "Internet Protocol, Ver- A Summary of differences between SAPv0 and SAPv1
sion 6 (IPv6) Specification", RFC 1883, December 1995.
[MASGN] Hinden, R., and S. Deering, "IPv6 Multicast Address Assign- For this purpose SAPv0 is defined as the protocol in use by version
ments", RFC 2375, July 1998. 2.2 of the session directory tool, sdr. SAPv1 is the protocol described
in the 19 November 1996 version of this memo (draft-ietf-mmusic-sap-00.txt).
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:
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate o Version Number: 0
Requirement Levels", BCP 14, RFC 2119, March 1997.
[SMIL] o Message Type: 0 (Announcement)
[RFC2327] Handley, M., and Jacobson, V., "SDP: Session Description o Authentication Type: 0 (No Authentication)
Protocol", RFC2327, April 1998.
Author's Address o Encryption Bit: 0 (No Encryption)
Maryann P. Maher <maher@isi.edu> o Compression Bit: 0 (No compression)
4350 N. Fairfax Drive, Suite 620 o Message Id Hash: 0 (No Hash Specified)
Arlington VA 22203
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
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 clients (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.
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 SAPv2 makes the concept of directory session explicit.
C Acknowledgments
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. The idea of using SAP to announce other SAP sessions
is due to Ross Finlayson.
D Authors' Addresses
Mark Handley <mjh@aciri.org>
AT&T Center for Internet Research at ICSI,
International Computer Science Institute,
1947 Center Street, Suite 600,
Berkeley, CA 94704, USA
Colin Perkins <c.perkins@cs.ucl.ac.uk> Colin Perkins <c.perkins@cs.ucl.ac.uk>
Department of Computer Science Department of Computer Science,
University College London University College London,
Gower Street Gower Street,
London WC1E 6BT London, WC1E 6BT, UK
Edmund Whelan <e.whelan@cs.ucl.ac.uk>
Department of Computer Science,
University College London,
Gower Street,
London, WC1E 6BT, UK
[1] S. Bradner. Key words for use in RFCs to indicate requirement levels,
March 1997. RFC2119.
[2] J. Callas, L. Donnerhacke, H. Finney, and R. Thayer. OpenPGP message
format, November 1998. RFC2440.
[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.
[7] D. Mayer. Administratively scoped IP multicast, July 1998. RFC2365.
[8] D. Mills. Network time protocol version 3, March 1992. RFC1305.
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