draft-ietf-mmusic-sap-v2-02.txt   draft-ietf-mmusic-sap-v2-03.txt 
Mark Handley Mark Handley
ACIRI ACIRI
Colin Perkins Colin Perkins
UCL UCL
Edmund Whelan Edmund Whelan
UCL UCL
Session Announcement Protocol Session Announcement Protocol
draft-ietf-mmusic-sap-v2-02.txt draft-ietf-mmusic-sap-v2-03.txt
Status of this memo Status of this memo
This document is an Internet-Draft and is in full conformance with all This document is an Internet-Draft and is in full conformance with
provisions of Section 10 of RFC2026. all provisions of Section 10 of RFC2026.
Internet-Drafts are working documents of the Internet Engineering Task Internet-Drafts are working documents of the Internet Engineering
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This document is a product of the Multiparty Multimedia Session Control This document is a product of the Multiparty Multimedia Session Control
working group of the Internet Engineering Task Force. Comments are working group of the Internet Engineering Task Force. Comments are
solicited and should be addressed to the working group's mailing list at solicited and should be addressed to the working group's mailing
confctrl@isi.edu and/or the authors. list at confctrl@isi.edu and/or the authors.
Abstract Abstract
This document describes version 2 of the multicast session directory This document describes version 2 of the multicast session
announcement protocol, SAP, and the related issues affecting security directory announcement protocol, SAP, and the related issues
and scalability that should be taken into account by the implementors affecting security and scalability that should be taken
of multicast session directory tools. into account by the implementors of multicast session directory
tools.
1 Introduction 1 Introduction
In order to assist the advertisement of multicast multimedia conferences In order to assist the advertisement of multicast multimedia conferences
and other multicast sessions, and to communicate the relevant session and other multicast sessions, and to communicate the relevant session
setup information to prospective participants, a distributed session setup information to prospective participants, a distributed session
directory may be used. An instance of such a session directory periodically directory may be used. An instance of such a session directory periodically
multicasts packets containing a description of the session, and these multicasts packets containing a description of the session, and these
advertisements are received by potential participants who can use advertisements are received by potential participants who can use the
the session description to start the tools required to participate session description to start the tools required to participate in the
in the session. session.
This memo describes the issues involved in the multicast announcement This memo describes the issues involved in the multicast announcement of
of session description information and defines an announcement protocol session description information and defines an announcement protocol to be
to be used by session directory clients. Sessions are described used by session directory clients. Sessions are described using the
using the session description protocol which is described in a companion session description protocol which is described in a companion memo [4].
memo [4].
2 Terminology 2 Terminology
A SAP announcer periodically multicasts an announcement packet to A SAP announcer periodically multicasts an announcement packet to a well
a well known multicast address and port. The announcement is multicast known multicast address and port. The announcement is multicast with the
with the same scope as the session it is announcing, ensuring that same scope as the session it is announcing, ensuring that the recipients of
the recipients of the announcement can also be potential recipients the announcement can also be potential recipients of the session the
of the session the announcement describes (bandwidth and other such announcement describes (bandwidth and other such constraints permitting).
constraints permitting). This is also important for the scalability This is also important for the scalability of the protocol, as it keeps
of the protocol, as it keeps local session announcements local. local session announcements local.
A SAP listener learns of the multicast scopes it is within (for example, A SAP listener learns of the multicast scopes it is within (for example,
using the Multicast-Scope Zone Announcement Protocol [5]) and listens using the Multicast-Scope Zone Announcement Protocol [5]) and listens on
on the well known SAP address and port for those scopes. In this the well known SAP address and port for those scopes. In this manner, it
manner, it will eventually learn of all the sessions being announced, will eventually learn of all the sessions being announced, allowing those
allowing those sessions to be joined. sessions to be joined.
The key words `MUST', `MUST NOT', `REQUIRED', `SHALL', `SHALL NOT', The key words `MUST', `MUST NOT', `REQUIRED', `SHALL', `SHALL NOT',
`SHOULD', `SHOULD NOT', `RECOMMENDED', `MAY', and `OPTIONAL' in this `SHOULD', `SHOULD NOT', `RECOMMENDED', `MAY', and `OPTIONAL' in this
document are to be interpreted as described in [1]. document are to be interpreted as described in [1].
3 Session Announcement 3 Session Announcement
As noted previously, a SAP announcer periodically sends an announcement As noted previously, a SAP announcer periodically sends an announcement
packet to a well known multicast address and port. There is no rendezvous 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 mechanism - the SAP announcer is not aware of the presence or absence of
of any SAP listeners - and no additional reliability is provided any SAP listeners - and no additional reliability is provided over the
over the standard best-effort UDP/IP semantics. standard best-effort UDP/IP semantics.
That announcement contains a session description and SHOULD contain That announcement contains a session description and SHOULD contain an
an authentication header. The session description MAY be encrypted authentication header. The session description MAY be encrypted although
although this is NOT RECOMMENDED (see section 8). this is NOT RECOMMENDED (see section 7).
A SAP announcement is multicast with the same scope as the session A SAP announcement is multicast with the same scope as the session
it is announcing, ensuring that the recipients of the announcement it is announcing, ensuring that the recipients of the announcement
can also be potential recipients of the session being advertised. can also be potential recipients of the session being advertised.
There are four possiblities: There a number of possiblities:
IPv4 global scope sessions use multicast addresses in the range IPv4 global scope sessions use multicast addresses in the range
224.2.128.0 - 224.2.255.255 with SAP announcements being sent 224.2.128.0 - 224.2.255.255 with SAP announcements being sent
to 224.2.127.254 (note that 224.2.127.255 is used by the obsolete to 224.2.127.254 (note that 224.2.127.255 is used by the obsolete
SAPv0 and MUST NOT be used). SAPv0 and MUST NOT be used).
IPv4 administrative scope sessions using administratively scoped IPv4 administrative scope sessions using administratively scoped
IP multicast as defined in [7]. The multicast address to be IP multicast as defined in [7]. The multicast address to be
used for announcements is the highest multicast address in the used for announcements is the highest multicast address in the
relevant administrative scope zone. For example, if the scope relevant administrative scope zone. For example, if the scope
range is 239.16.32.0 - 239.16.33.255, then 239.16.33.255 is used range is 239.16.32.0 - 239.16.33.255, then 239.16.33.255 is used
for SAP announcements. for SAP announcements.
IPv6 sessions are announced on the address FF0X:0:0:0:0:0:2:7FFE 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 where X is the 4-bit scope value. For example, an announcement for
for a link-local session assigned the address FF02:0:0:0:0:0:1234:5678, 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. 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
sessions.
SAP announcements MUST be sent on port 9875 and SHOULD be sent with SAP announcements MUST be sent on port 9875 and SHOULD be sent with
an IP time-to-live of 255. an IP time-to-live of 255.
If a session uses addresses in multiple administrative scope ranges, it is If a session uses addresses in multiple administrative scope ranges, it is
necessary for the announcer to send identical copies of the announcement to 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 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 multiple announcements as the same session (as identified by the SDP origin
field, for example). The announcement rate for each administrative scope field, for example). The announcement rate for each administrative scope
range MUST be calculated separately, as if the multiple announcements were range MUST be calculated separately, as if the multiple announcements were
separate. separate.
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If multiple announcements are being made for a session, then each If multiple announcements are being made for a session, then each
announcement MUST carry an authentication header signed by the same announcement MUST carry an authentication header signed by the same
key, or be treated as a completely separate announcement by listeners. key, or be treated as a completely separate announcement by listeners.
An IPv4 SAP listener SHOULD listen on the IPv4 global scope SAP address An IPv4 SAP listener SHOULD listen on the IPv4 global scope SAP address
and on the SAP addresses for each IPv4 administrative scope zone and on the SAP addresses for each IPv4 administrative scope zone
it is within. The discovery of administrative scope zones is outside it is within. The discovery of administrative scope zones is outside
the scope of this memo, but it is assumed that each SAP listener 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 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. listener which supports IPv6 SHOULD also listen to the IPv6 SAP addresses.
Support for directory sessions is OPTIONAL.
3.1 Announcement Interval 3.1 Announcement Interval
The time period between repetitions of an announcement is chosen The time period between repetitions of an announcement is chosen
such that the total bandwidth used by all announcements on a single such that the total bandwidth used by all announcements on a single
SAP group remains below a preconfigured limit. If not otherwise SAP group remains below a preconfigured limit. If not otherwise
specified, the bandwidth limit SHOULD be assumed to be 4000 bits specified, the bandwidth limit SHOULD be assumed to be 4000 bits
per second. per second.
Each announcer is expected to listen to other announcements in order Each announcer is expected to listen to other announcements in order to
to determine the total number of sessions being announced on a particular determine the total number of sessions being announced on a particular
group. Sessions are uniquely identified by the combination of the group. Sessions are uniquely identified by the combination of the message
message identifier hash and originating source fields of the SAP identifier hash and originating source fields of the SAP header (note that
header (note that SAP v0 clients always set the message identifier SAP v0 clients always set the message identifier hash to zero, and if such
hash to zero, and if such an announcement is received the entire an announcement is received the entire message MUST be compared to
message MUST be compared to determine uniqueness). determine uniqueness).
Announcements are made by periodic multicast to the group. The base Announcements are made by periodic multicast to the group. The base
interval between announcements is derived from the number of announcements interval between announcements is derived from the number of announcements
being made in that group, the size of the announcement and the configured being made in that group, the size of the announcement and the configured
bandwidth limit. The actual transmission time is derived from this bandwidth limit. The actual transmission time is derived from this base
base interval as follows: interval as follows:
1.The announcer initialises the variable tp to be the last time 1.The announcer initialises the variable tp to be the last time
a particular announcement was transmitted (or the current time a particular announcement was transmitted (or the current time
if this is the first time this announcement is to be made). if this is the first time this announcement is to be made).
2.Given a configured bandwidth limit in bits/second and an announcement 2.Given a configured bandwidth limit in bits/second and an announcement
of ad_size bytes, the base announcement interval in seconds is of ad_size bytes, the base announcement interval in seconds is
interval = max(300; (8*no_of_ads*ad_size)/limit) interval =max(300; (8*no_of_ads*ad_size)=limit)
3.An offset is calculated based on the base announcement interval 3.An offset is calculated based on the base announcement interval
offset = rand(interval* 2/3)-(interval/3) offset= rand(interval* 2=3)-(interval=3)
4.The next transmission time for an announcement derived as 4.The next transmission time for an announcement derived as
tn = tp + interval + offset tn = tp + interval + offset
The announcer then sets a timer to expire at tn and waits. When The announcer then sets a timer to expire at tn and waits. When this timer
this timer expires, the announcement is transmitted. expires, the announcement is transmitted.
If, at some time tc <tn, the no_of_ads changes (eg: if a new announcer 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 starts up, or an existing announcement is deleted), the announcer SHOULD
SHOULD recalculate the next transmission time. If the new value recalculate the next transmission time. If the new value of tn is less
of tn is less than tc the announcement is sent immediately. Otherwise than tc the announcement is sent immediately. Otherwise the transmission
the transmission is rescheduled for the new tn. is rescheduled for the new tn.
4 Session Deletion 4 Session Deletion
Sessions may be deleted in one of several ways: Sessions may be deleted in one of several ways:
Explicit Timeout The session description payload contains timestamp Explicit Timeout The session description payload contains timestamp
information which specifies a start and end time for the session. information which specifies a start and end time for the session.
If the current time is later than the 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. then the session is deleted from the receiver's session cache.
If an announcement packet arrives with an end-time before the If an announcement packet arrives with an end-time before the
skipping to change at line 228 skipping to change at line 222
one hour minimum is to allow for transient network partitionings. one hour minimum is to allow for transient network partitionings.
Explicit Deletion A session deletion packet is received specifying Explicit Deletion A session deletion packet is received specifying
the version of the session to be deleted. The deletion packets the version of the session to be deleted. The deletion packets
should be ignored, unless they contain an authentication header should be ignored, unless they contain an authentication header
which authenticates correctly and matches that used to authenticate which authenticates correctly and matches that used to authenticate
the announcement which is being deleted. the announcement which is being deleted.
5 Session Modification 5 Session Modification
A pre-announced session can be modified by simply announcing the A pre-announced session can be modified by simply announcing the modified
modified session description. In this case, the version hash in session description. In this case, the version hash in the SAP header MUST
the SAP header MUST be changed to indicate to receivers that the be changed to indicate to receivers that the packet contents should be
packet contents should be parsed (or decrypted and parsed if it is parsed (or decrypted and parsed if it is encrypted). The session itself,
encrypted). The session itself, as distinct from the session announcement, as distinct from the session announcement, is uniquely identified by the
is uniquely identified by the payload and not by the message identifier payload and not by the message identifier hash in the header.
hash in the header.
The same rules apply for session modification as for session deletion: The same rules apply for session modification as for session deletion:
o Either the modified announcement must contain an authentication o Either the modified announcement must contain an authentication
header signed by the same key as the cached session announcement header signed by the same key as the cached session announcement
it is modifying, or: it is modifying, or:
o The cached session announcement must not contain an authentication o The cached session announcement must not contain an authentication
header, and the session modification announcement must originate header, and the session modification announcement must originate
from the same host as the session it is modifying. 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.
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
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|R|T|E|C| auth len | msg id hash | | V=1 |A|R|T|E|C| auth len | msg id hash |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | | |
: originating source (32 or 128 bits) : : originating source (32 or 128 bits) :
: : : :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| optional authentication data | | optional authentication data |
skipping to change at line 317 skipping to change at line 262
*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-* *-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*
| optional payload type | | optional payload type |
+ +-+- - - - - - - - - -+ + +-+- - - - - - - - - -+
| |0| | | |0| |
+ - - - - - - - - - - - - - - - - - - - - +-+ | + - - - - - - - - - - - - - - - - - - - - +-+ |
| | | |
: payload : : payload :
| | | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2: Packet format Figure 1: Packet format
address announcers of sessions within that directory MUST move their If an announcement is received containing an authentication header
announcement to the new directory, provided the modification packet and the cached announcement did not contain an authentication header,
authenticates with the same key as the original announcement of the or it contained a different authentication header, then the modified
directory session. If the modification packet is not authenticated, announcement MUST be treated as a new and different announcement,
or is authenticated with a different key, then it SHOULD be ignored. 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.
7 Packet Format 6 Packet Format
SAP data packets have the format described in figure 2. SAP data packets have the format described in figure 1.
V: Version Number. The version number field MUST be set to 1 (SAPv2 V: Version Number. The version number field MUST be set to 1 (SAPv2
announcements which use only SAPv1 features are backwards compatible, announcements which use only SAPv1 features are backwards compatible,
those which use new features can be detected by other means, those which use new features can be detected by other means,
so the SAP version number doesn't need to change). so the SAP version number doesn't need to change).
A: Address type. If the A bit is 0, the originating source field 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 contains a 32-bit IPv4 address. If the A bit is 1, the originating
source contains a 128-bit IPv6 address. source contains a 128-bit IPv6 address.
R: Reserved. SAP announcers MUST set this to 0, SAP listeners MUST R: Reserved. SAP announcers MUST set this to 0, SAP listeners MUST
ignore the contents of this field. ignore the contents of this field.
T: Message Type. If the T field is set to 0 this is a session announcement 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. packet, if 1 this is a session deletion packet.
E: Encryption Bit. If the encryption bit is set to 1, the payload 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 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 added to the packet header. If this bit is 0 the packet is
not encrypted and the timeout MUST NOT be present. See section not encrypted and the timeout MUST NOT be present. See section
8 for details of the encryption process. 7 for details of the encryption process.
C: Compressed bit. If the compressed bit is set to 1, the payload C: Compressed bit. If the compressed bit is set to 1, the payload
is compressed using the zlib compression algorithm [3]. If the is compressed using the zlib compression algorithm [3]. If the
payload is to be compressed and encrypted, the compression MUST payload is to be compressed and encrypted, the compression MUST
be performed first. be performed first.
Authentication Length. An 8 bit unsigned quantity giving the number Authentication Length. An 8 bit unsigned quantity giving the number
of 32 bit words following the main SAP header that contain authentication of 32 bit words following the main SAP header that contain
data. If it is zero, no authentication header is present. authentication data. If it is zero, no authentication header
is present.
Authentication data 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 with length as specified by the authentication length header
field. See section 9 for details of the authentication process. field. See section 8 for details of the authentication process.
Message Identifier Hash. A 16 bit quantity that, used in combination Message Identifier Hash. A 16 bit quantity that, used in combination
with the originating source, provides a globally unique identifier with the originating source, provides a globally unique identifier
indicating the precise version of this announcement. The choice indicating the precise version of this announcement. The choice
of value for this field is not specified here, except that it of value for this field is not specified here, except that it
MUST be unique for each session announced by a particular SAP MUST be unique for each session announced by a particular SAP
announcer and it MUST be changed if the session description is announcer and it MUST be changed if the session description is
modified. modified.
Earlier versions of SAP used a value of zero to mean that the Earlier versions of SAP used a value of zero to mean that the
hash should be ignored and the payload should always be parsed. hash should be ignored and the payload should always be parsed.
This had the unfortunate side-effect that SAP announcers had This had the unfortunate side-effect that SAP announcers had
to study the payload data to determine how many unique sessions to study the payload data to determine how many unique sessions
were being advertised, making the calculation of the announcement were being advertised, making the calculation of the announcement
interval more complex that necessary. In order to decouple the interval more complex that necessary. In order to decouple the
session announcement process from the contents of those announcements, session announcement process from the contents of those announcements,
SAP announcers SHOULD NOT set the message identifier hash to SAP announcers SHOULD NOT set the message identifier hash to zero.
zero.
SAP listeners MAY silently discard messages if the message identifier SAP listeners MAY silently discard messages if the message identifier
hash is set to zero. hash is set to zero.
Originating Source. This gives the IP address of the original source 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 of the message. This is an IPv4 address if the A field is set to
to zero, else it is an IPv6 address. The address is stored zero, else it is an IPv6 address. The address is stored in network
in network byte order. byte order.
SAPv0 permitted the originating source to be zero if the message SAPv0 permitted the originating source to be zero if the message
identifier hash was also zero. This practise is no longer legal, identifier hash was also zero. This practise is no longer legal,
and SAP announcers SHOULD NOT set the originating source to zero. and SAP announcers SHOULD NOT set the originating source to zero.
SAP listeners MAY silently discard packets with the originating SAP listeners MAY silently discard packets with the originating
source set to zero. source set to zero.
Timeout. When the session payload is encrypted the detailed timing Timeout. When the session payload is encrypted the detailed timing
fields in the payload are not available to listeners which are fields in the payload are not available to listeners which are
not trusted with the decryption key. Under such circumstances, not trusted with the decryption key. Under such circumstances,
skipping to change at line 426 skipping to change at line 381
string, followed by a single zero byte (ASCII NUL). The payload type 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' 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, both the payload type and its terminating zero byte MAY be omitted,
although this is intended for backwards compatibility with SAP v1 although this is intended for backwards compatibility with SAP v1
listeners only. listeners only.
The absence of a payload type field may be noted since the payload 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 section of such a packet will start with an SDP `v=0' field, which
is not a legal MIME content type specifier. is not a legal MIME content type specifier.
All implementations MUST support payloads of type `application/sdp' All implementations MUST support payloads of type `application/sdp' [4].
[4]. Other formats MAY be supported although since there is no negotiation Other formats MAY be supported although since there is no negotiation in
in SAP an announcer which chooses to use a session description format SAP an announcer which chooses to use a session description format other
other than SDP cannot know that the listeners are able to understand than SDP cannot know that the listeners are able to understand the
the announcement. A proliferation of payload types in announcements announcement. A proliferation of payload types in announcements has the
has the potential to lead to severe interoperability problems, and potential to lead to severe interoperability problems, and for this reason,
for this reason, the use of non-SDP payloads is NOT RECOMMENDED. 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 an announcement packet, the payload contains a session
description. description.
If the packet is a session deletion packet, the payload contains If the packet is a session deletion packet, the payload contains
a session deletion message. If the payload format is `application/sdp' a session deletion message. If the payload format is `application/sdp'
the deletion message is a single SDP line consisting of the origin the deletion message is a single SDP line consisting of the origin
field of the announcement to be deleted. field of the announcement to be deleted.
It is desirable for the payload to be sufficiently small that SAP It is desirable for the payload to be sufficiently small that SAP packets
packets do not get fragmented by the underlying network. Fragmentation do not get fragmented by the underlying network. Fragmentation has a loss
has a loss multiplier effect, which is known to significantly affect multiplier effect, which is known to significantly affect the reliability
the reliability of announcements. It is RECOMMENDED that SAP packets of announcements. It is RECOMMENDED that SAP packets are smaller than
are smaller than 1kByte in length, although if it is known that 1kByte in length, although if it is known that announcements will use a
announcements network with a smaller MTU than this, then that SHOULD be used as the
maximum recommended packet size.
will use a network with a smaller MTU than this, then that SHOULD
be used as the maximum recommended packet size.
8 Encrypted Announcements 7 Encrypted Announcements
An announcement is received by all listeners in the scope to which 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 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 do not have the encryption key, there is a considerable waste of
bandwidth since those receivers cannot use the announcement they have bandwidth since those receivers cannot use the announcement they have
received. For this reason, the use of encrypted SAP announcements received. For this reason, the use of encrypted SAP announcements
is NOT RECOMMENDED on the global scope SAP group or on administrative is NOT RECOMMENDED on the global scope SAP group or on administrative
scope groups which may have many receivers which cannot decrypt those scope groups which may have many receivers which cannot decrypt those
announcements. announcements.
skipping to change at line 498 skipping to change at line 452
problem. problem.
Session announcements that are encrypted with a symmetric algorithm Session announcements that are encrypted with a symmetric algorithm
may allow a degree of privacy in the announcement of a session, but 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 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 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 a key. Thus announcements to such a group of key holders cannot
be assumed to have come from an authorised key holder unless there be assumed to have come from an authorised key holder unless there
is an appropriate authentication header signed by an authorised key is an appropriate authentication header signed by an authorised key
holder. In addition the recipients of such encrypted announcements holder. In addition the recipients of such encrypted announcements
cannot be assumed to only be authorised key holders. Such encrypted cannot be assumed to only be authorised key holders. Such encrypted
announcements do not provide any real security unless all of the announcements do not provide any real security unless all of the
authorised key holders are trusted to maintain security of such session authorised key holders are trusted to maintain security of such session
directory keys. This property is shared by the multicast session directory keys. This property is shared by the multicast session
tools themselves, where it is possible for an un-trustworthy member tools themselves, where it is possible for an un-trustworthy member
of the session to pass on encryption keys to un-authorised users. 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 However it is likely that keys used for the session tools will be
more short lived than those used for session directories. more short lived than those used for session directories.
Similar considerations should apply when session announcements are Similar considerations should apply when session announcements are
encrypted with an assymetric algorithm, but then it is possible to encrypted with an assymetric algorithm, but then it is possible to
restrict the possessor(s) of the private key, so that announcements 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 to a key-holder group can not be made, even if one of the untrusted
members of the group proves to be un-trustworthy. members of the group proves to be un-trustworthy.
9 Authenticated Announcements 8 Authenticated Announcements
The authentication header can be used for two purposes: The authentication header can be used for two purposes:
o Verification that changes to a session description or deletion o Verification that changes to a session description or deletion
of a session are permitted. of a session are permitted.
o Authentication of the identity of the session creator. o Authentication of the identity of the session creator.
In some circumstances only verification is possible because a certificate In some circumstances only verification is possible because a certificate
signed by a mutually trusted person or authority is not available. signed by a mutually trusted person or authority is not available.
skipping to change at line 538 skipping to change at line 490
be sufficient depending on the purpose of the session and the people be sufficient depending on the purpose of the session and the people
involved. involved.
Clearly the key used for the authentication 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 to belong to the session originator unless it has been separately
authenticated by some other means, such as being certified by a trusted authenticated by some other means, such as being certified by a trusted
third party. Such certificates are not normally included in an SAP third party. Such certificates are not normally included in an SAP
header because they take more space than can normally be afforded header because they take more space than can normally be afforded
in an SAP packet, and such verification must therefore take place in an SAP packet, and such verification must therefore take place
by some other mechanism. However, as certified public keys are normally by some other mechanism. However, as certified public keys are normally
locally cached, authentication of a particular key only has to take
place once, rather than every time the session directory retransmits
the announcement.
SAP is not tied to any single authentication mechanism. Authentication
data in the header is self-describing, but the precise format depends
on the authentication mechanism in use. The generic format of the
authentication data is given in figure 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.
1 2 3 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 |P| Auth | | | V=1 |P| Auth | |
+-+-+-+-+-+-+-+-+ | +-+-+-+-+-+-+-+-+ |
| Format specific authentication subheader | | Format specific authentication subheader |
: .................. : : .................. :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 3: Format of the authentication data in the SAP header Figure 2: Format of the authentication data in the SAP header
locally cached, authentication of a particular key only has to take
place once, rather than every time the session directory retransmits
the announcement.
SAP is not tied to any single authentication mechanism. Authentication
data in the header is self-describing, but the precise format depends
on the authentication mechanism in use. The generic format of the
authentication data is given in figure 2. The structure of the format
specific authentication subheader, using both the PGP and the CMS
formats, is discussed in sections 8.1 and 8.2 respectively.
Version Number, V: The version number of the authentication format Version Number, V: The version number of the authentication format
specified by this memo is 1. specified by this memo is 1.
Padding Bit, P: If necessary the authentication data is padded Padding Bit, P: If necessary the authentication data is padded
to be a multiple of 32 bits and the padding bit is set. In 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 this case the last byte of the authentication data contains the
number of padding bytes (including the last byte) that must be number of padding bytes (including the last byte) that must be
discarded. discarded.
skipping to change at line 584 skipping to change at line 536
format. All other values are undefined and SHOULD be ignored. format. All other values are undefined and SHOULD be ignored.
If a SAP packet is to be compressed or encrypted, this MUST be done If a SAP packet is to be compressed or encrypted, this MUST be done
before the authentication is added. before the authentication is added.
The digital signature in the authentication data MUST be calculated The digital signature in the authentication data MUST be calculated
over the entire packet, including the header. The authentication over the entire packet, including the header. The authentication
length MUST be set to zero and the authentication data excluded when length MUST be set to zero and the authentication data excluded when
calculating the digitial signature. calculating the digitial signature.
9.1 PGP Authentication 8.1 PGP Authentication
Implementations which support authentication MUST support this format. Implementations which support authentication MUST support this format.
A full description of the PGP protocol can be found in [2]. When A full description of the PGP protocol can be found in [2]. When
using PGP for SAP authentication the basic format specific authentication using PGP for SAP authentication the basic format specific authentication
subheader comprises a digital signature packet as described in [2]. subheader comprises a digital signature packet as described in [2].
The signature type MUST be 0x01 which means the signature is that The signature type MUST be 0x01 which means the signature is that
of a canonical text document. of a canonical text document.
9.2 CMS Authentication 8.2 CMS Authentication
Support for this format is OPTIONAL. Support for this format is OPTIONAL.
A full description of the Cryptographic Message Syntax can be found A full description of the Cryptographic Message Syntax can be found
in [6]. The format specific authentication subheader will, in the in [6]. The format specific authentication subheader will, in the
CMS case, have an ASN.1 ContentInfo type with the ContentType being CMS case, have an ASN.1 ContentInfo type with the ContentType being
signedData. signedData.
Use is made of the option available in PKCS#7 to leave the content 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 itself blank as the content which is signed is already present in
the packet. Inclusion of it within the SignedData type would duplicate the packet. Inclusion of it within the SignedData type would duplicate
this data and increase the packet length unnecessarily. In addition this data and increase the packet length unnecessarily. In addition
this allows recipients with either no interest in the authentication, this allows recipients with either no interest in the authentication,
or with no mechanism for checking it, to more easily skip the authentication or with no mechanism for checking it, to more easily skip the
information. authentication information.
There SHOULD be only one signerInfo and related fields corresponding There SHOULD be only one signerInfo and related fields corresponding
to the originator of the SAP announcement. The signingTime SHOUD to the originator of the SAP announcement. The signingTime SHOUD
be present as a signedAttribute. However, due to the strict size be present as a signedAttribute. However, due to the strict size
limitations on the size of SAP packets, certificates and CRLs SHOULD limitations on the size of SAP packets, certificates and CRLs SHOULD
NOT be included in the signedData structure. It is expected that NOT be included in the signedData structure. It is expected that
users of the protocol will have other methods for certificate and users of the protocol will have other methods for certificate and
CRL distribution. CRL distribution.
10 Scalability and caching 9 Scalability and caching
SAP is intended to announce the existence of long-lived wide-area SAP is intended to announce the existence of long-lived wide-area
multicast sessions. It is not an especially timely protocol: sessions multicast sessions. It is not an especially timely protocol: sessions
are announced by periodic multicast with a repeat rate on the order are announced by periodic multicast with a repeat rate on the order
of tens of minutes, and no enhanced reliability over UDP. This leads of tens of minutes, and no enhanced reliability over UDP. This leads
to a long startup delay before a complete set of announcements is to a long startup delay before a complete set of announcements is
heard by a listener. This delay is clearly undesirable for interactive heard by a listener. This delay is clearly undesirable for interactive
browsing of announced sessions. browsing of announced sessions.
In order to reduce the delays inherent in SAP, it is recommended In order to reduce the delays inherent in SAP, it is recommended
skipping to change at line 641 skipping to change at line 593
listen to all SAP groups in its scope, and to maintain an up-to-date 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 list of all announced sessions along with the time each announcement
was last received. When a new SAP listeners starts, it should contact was last received. When a new SAP listeners starts, it should contact
its local proxy to download this information, which is then sufficient its local proxy to download this information, which is then sufficient
for it to process future announcements directly, as if it has been for it to process future announcements directly, as if it has been
continually listening. continually listening.
The protocol by which a SAP listener contacts its local proxy cache The protocol by which a SAP listener contacts its local proxy cache
is not specified here. is not specified here.
11 Security Considerations 10 Security Considerations
SAP contains mechanisms for ensuring integrity of session announcements, SAP contains mechanisms for ensuring integrity of session announcements,
for authenticating the origin of an announcement and for encrypting for authenticating the origin of an announcement and for encrypting
such announcements (sections 8 and 9). These mechanisms have not such announcements (sections 7 and 8). These mechanisms have not
yet been subject to suitable peer-review, and this memo should not yet been subject to suitable peer-review, and this memo should not
be considered authoritative in this area at this time. be considered authoritative in this area at this time.
As stated in section 5, if a session modification announcement is As stated in section 5, if a session modification announcement is
received that contains a valid authentication header, but which is received that contains a valid authentication header, but which is
not signed by the original creator of the session, then the session 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 must be treated as a new session in addition to the original session
with the same SDP origin information unless the originator of one with the same SDP origin information unless the originator of one
of the session descriptions can be authenticated using a certificate of the session descriptions can be authenticated using a certificate
signed by a trusted third party. If this were not done, there would signed by a trusted third party. If this were not done, there would
skipping to change at line 740 skipping to change at line 692
o SAPv1 specified gzip compression. SAPv2 uses zlib (the only o SAPv1 specified gzip compression. SAPv2 uses zlib (the only
known implementation of SAP compression used zlib, and gzip compression known implementation of SAP compression used zlib, and gzip compression
was a mistake). was a mistake).
o SAPv2 provides a more complete specification for authentication. o SAPv2 provides a more complete specification for authentication.
o SAPv2 allows for non-SDP payloads to be transported. SAPv1 required o SAPv2 allows for non-SDP payloads to be transported. SAPv1 required
that the payload was SDP. that the payload was SDP.
o SAPv2 makes the concept of directory session explicit.
C Acknowledgments C Acknowledgments
SAP and SDP were originally based on the protocol used by the sd 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 session directory from Van Jacobson at LBNL. Version 1 of SAP was
designed by Mark Handley as part of the European Commission MICE designed by Mark Handley as part of the European Commission MICE
(Esprit 7602) and MERCI (Telematics 1007) projects. Version 2 includes (Esprit 7602) and MERCI (Telematics 1007) projects. Version 2 includes
authentication features developed by Edmund Whelan, Goli Montasser-Kohsari authentication features developed by Edmund Whelan, Goli Montasser-Kohsari
and Peter Kirstein as part of the European Commission ICE-TEL project and Peter Kirstein as part of the European Commission ICE-TEL project
(Telematics 1005), and support for IPv6 developed by Maryann P. Maher (Telematics 1005), and support for IPv6 developed by Maryann P. Maher
and Colin Perkins. The idea of using SAP to announce other SAP sessions and Colin Perkins.
is due to Ross Finlayson.
D Authors' Addresses D Authors' Addresses
Mark Handley <mjh@aciri.org> Mark Handley <mjh@aciri.org>
AT&T Center for Internet Research at ICSI, AT&T Center for Internet Research at ICSI,
International Computer Science Institute, International Computer Science Institute,
1947 Center Street, Suite 600, 1947 Center Street, Suite 600,
Berkeley, CA 94704, USA Berkeley, CA 94704, USA
Colin Perkins <c.perkins@cs.ucl.ac.uk> Colin Perkins <c.perkins@cs.ucl.ac.uk>
skipping to change at line 789 skipping to change at line 737
[2] J. Callas, L. Donnerhacke, H. Finney, and R. Thayer. OpenPGP message [2] J. Callas, L. Donnerhacke, H. Finney, and R. Thayer. OpenPGP message
format, November 1998. RFC2440. format, November 1998. RFC2440.
[3] P. Deutsch and J.-L. Gailly. Zlib compressed data format specification [3] P. Deutsch and J.-L. Gailly. Zlib compressed data format specification
version 3.3, May 1996. RFC1950. version 3.3, May 1996. RFC1950.
[4] M. Handley and V. Jacobson. SDP: Session Description Protocol, April [4] M. Handley and V. Jacobson. SDP: Session Description Protocol, April
1998. RFC2327. 1998. RFC2327.
[5] M. Handley, D. Thaler, and R. Kermode. Multicast-scope zone [5] M. Handley, D. Thaler, and R. Kermode. Multicast-scope zone announcement
announcement protocol (MZAP)(, February 1999. Work in progress. protocol (MZAP), February 1999. Work in progress.
[6] R. Housley. Cryptographic message syntax. Work in progress, [6] R. Housley. Cryptographic message syntax. Work in progress, April
April 1999. draft-ietf-smime-cms-13.txt. 1999.
draft-ietf-smime-cms-13.txt.
[7] D. Mayer. Administratively scoped IP multicast, July 1998. RFC2365. [7] D. Mayer. Administratively scoped IP multicast, July 1998. RFC2365.
[8] D. Mills. Network time protocol version 3, March 1992. RFC1305. [8] D. Mills. Network time protocol version 3, March 1992. RFC1305.
 End of changes. 

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