draft-ietf-mmusic-sap-v2-04.txt   draft-ietf-mmusic-sap-v2-05.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-v2-04.txt draft-ietf-mmusic-sap-v2-05.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.
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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 list at
confctrl@isi.edu and/or the authors. 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 implementors.
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 the advertisements are received by potential participants who can use the
session description to start the tools required to participate in the session description to start the tools required to participate in the
session. session.
This memo describes the issues involved in the multicast announcement of This memo describes the issues involved in the multicast announcement of
session description information and defines an announcement protocol to be session description information and defines an announcement protocol to be
used by session directory clients. Sessions are described using the used. Sessions are described using the session description protocol which
session description protocol which is described in a companion memo [4]. is described in a companion 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 on using the Multicast-Scope Zone Announcement Protocol [5]) and listens on
the well known SAP address and port for those scopes. In this manner, it the well known SAP address and port for those scopes. In this manner, it
will eventually learn of all the sessions being announced, allowing those will eventually learn of all the sessions being announced, 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].
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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 for where X is the 4-bit scope value. For example, an announcement
a link-local session assigned the address FF02:0:0:0:0:0:1234:5678, 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. should be advertised on SAP address FF02:0:0:0:0:0:2:7FFE.
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 (the use of TTL scoping for multicast is
discouraged [7]).
If a session uses addresses in multiple administrative scope ranges, If a session uses addresses in multiple administrative scope ranges,
it is necessary for the announcer to send identical copies of the it is necessary for the announcer to send identical copies of the
announcement to each administrative scope range. It is up to the announcement to each administrative scope range. It is up to the
listeners to parse such multiple announcements as the same session listeners to parse such multiple announcements as the same session
(as identified by the SDP origin field, for example). The announcement (as identified by the SDP origin field, for example). The announcement
rate for each administrative scope range MUST be calculated separately, rate for each administrative scope range MUST be calculated separately,
as if the multiple announcements were separate. as if the multiple announcements were separate.
Multiple announcers may announce a single session, as an aid to robustness 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 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 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 that the total announcement rate is equal to that which a single server
would choose. Announcements made in this manner MUST be identical. would choose. Announcements made in this manner MUST be identical.
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
and on the SAP addresses for each IPv4 administrative scope zone on the SAP addresses for each IPv4 administrative scope zone it is within.
it is within. The discovery of administrative scope zones is outside The discovery of administrative scope zones is outside the scope of this
the scope of this memo, but it is assumed that each SAP listener memo, but it is assumed that each SAP listener within a particular scope
within a particular scope zone is aware of that scope zone. A SAP zone is aware of that scope zone. A SAP listener which supports IPv6
listener which supports IPv6 SHOULD also listen to the IPv6 SAP addresses. SHOULD also listen to the IPv6 SAP addresses.
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 to Each announcer is expected to listen to other announcements in order 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 message group. Sessions are uniquely identified by the combination of the message
identifier hash and originating source fields of the SAP header (note that identifier hash and originating source fields of the SAP header (note that
SAP v0 clients always set the message identifier hash to zero, and if such SAP v0 announcers always set the message identifier hash to zero, and if
an announcement is received the entire message MUST be compared to such an announcement is received the entire 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 base bandwidth limit. The actual transmission time is derived from this 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
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tn the announcer SHOULD recalculate the next transmission time. If tn the announcer SHOULD recalculate the next transmission time. If
the new value of tn is before the current time, the announcement the new value of tn is before the current time, the announcement
is sent immediately. Otherwise the transmission is rescheduled for is sent immediately. Otherwise the transmission is rescheduled for
the new tn. This reconsideration prevents transient packet bursts the new tn. This reconsideration prevents transient packet bursts
on startup and when a network partition heals. on startup and when a network partition heals.
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 may contain timestamp
information which specifies a start and end time for the session. information specifying the start- and end-times of 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 of the session,
then the session is deleted from the receiver's session cache. then the session SHOULD be deleted from the receiver's session
If an announcement packet arrives with an end-time before the cache.
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 Implicit Timeout A session announcement message should be received
periodically for each session description in a receiver's session periodically for each session description in a receiver's session
cache. The announcement period can be predicted by the receiver cache. The announcement period can be predicted by the receiver
from the set of sessions currently being announced. If a session from the set of sessions currently being announced. If a session
announcement message has not been received for ten times the announcement message has not been received for ten times the
announcement period, or one hour, whichever is the greater, then announcement period, or one hour, whichever is the greater, then
the session is deleted from the receiver's session cache. The the session is deleted from the receiver's session cache. The
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 session to be deleted. Session deletion packets SHOULD have
should be ignored, unless they contain an authentication header a valid authentication header, matching that used to authenticate
which authenticates correctly and matches that used to authenticate previous announcement packets. If this authentication is missing,
the announcement which is being deleted. the deletion message SHOULD be ignored.
5 Session Modification 5 Session Modification
A pre-announced session can be modified by simply announcing the modified 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 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 be changed to indicate to receivers that the packet contents should be
parsed (or decrypted and parsed if it is encrypted). The session itself, parsed (or decrypted and parsed if it is encrypted). The session itself,
as distinct from the session announcement, is uniquely identified by the as distinct from the session announcement, is uniquely identified by the
payload and not by the message identifier hash in the header. payload and not by the message identifier hash in the header.
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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.
If an announcement is received containing an authentication header If an announcement is received containing an authentication header
and the cached announcement did not contain an authentication header, and the cached announcement did not contain an authentication header,
or it contained a different authentication header, then the modified or it contained a different authentication header, then the modified
announcement MUST be treated as a new and different announcement, 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
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 |
: .... : : .... :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| optional timeout |
*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-* *-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*
| optional payload type | | optional payload type |
+ +-+- - - - - - - - - -+ + +-+- - - - - - - - - -+
| |0| | | |0| |
+ - - - - - - - - - - - - - - - - - - - - +-+ | + - - - - - - - - - - - - - - - - - - - - +-+ |
| | | |
: payload : : payload :
| | | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: Packet format Figure 1: Packet format
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 circumstances, being able to authenticate the message originator is
the only way to discover which session is the correct session. the only way to discover which session is the correct session.
6 Packet Format 6 Packet Format
SAP data packets have the format described in figure 1. 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,
skipping to change at line 296 skipping to change at line 290
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. If this bit is 0 the packet
added to the packet header. If this bit is 0 the packet is is not encrypted. See section 7 for details of the encryption
not encrypted and the timeout MUST NOT be present. See section 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 of 32 bit words following the main SAP header that contain
authentication data. If it is zero, no authentication header is authentication data. If it is zero, no authentication header is
present. present.
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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 zero. SAP announcers SHOULD NOT set the message identifier hash to
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 zero, else it is an IPv6 address. The address is stored to zero, else it is an IPv6 address. The address is stored
in network byte order. in network byte order.
SAPv0 permitted the originating source to be zero if the message SAPv0 permitted the originating source to be zero if the message
skipping to change at line 343 skipping to change at line 337
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 zero, else it is an IPv6 address. The address is stored to zero, else it is an IPv6 address. The address is stored
in network byte order. in network 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
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 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 payload data itself. If the E or C bits are set in the header both
the payload type and payload are encrypted and/or compressed. the payload type and payload are encrypted and/or compressed.
The payload type field is a MIME content type specifier, describing
the format of the payload. This is a variable length ASCII text The payload type field is a MIME content type specifier, describing the
string, followed by a single zero byte (ASCII NUL). The payload type format of the payload. This is a variable length ASCII text string,
SHOULD be included in all packets. If the payload type is `application/sdp' 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, 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' [4]. All implementations MUST support payloads of type `application/sdp' [4].
Other formats MAY be supported although since there is no negotiation in Other formats MAY be supported although since there is no negotiation in
SAP an announcer which chooses to use a session description format other SAP an announcer which chooses to use a session description format other
than SDP cannot know that the listeners are able to understand the than SDP cannot know that the listeners are able to understand the
announcement. A proliferation of payload types in announcements has the announcement. A proliferation of payload types in announcements has the
potential to lead to severe interoperability problems, and for this reason, 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 the use of non-SDP payloads is NOT RECOMMENDED.
announcement packet, the payload contains a session description.
If the packet is an announcement packet, the payload contains a session
description.
If the packet is a session deletion packet, the payload contains a session If the packet is a session deletion packet, the payload contains a session
deletion message. If the payload format is `application/sdp' the deletion deletion message. If the payload format is `application/sdp' the deletion
message is a single SDP line consisting of the origin field of the message is a single SDP line consisting of the origin field of the
announcement to be deleted. 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
multiplier effect, which is known to significantly affect the reliability
has a loss multiplier effect, which is known to significantly affect the of announcements. It is RECOMMENDED that SAP packets are smaller than
reliability of announcements. It is RECOMMENDED that SAP packets are 1kByte in length, although if it is known that announcements will use a
smaller than 1kByte in length, although if it is known that announcements network with a smaller MTU than this, then that SHOULD be used as the
will use a network with a smaller MTU than this, then that SHOULD be used maximum recommended packet size.
as the maximum recommended packet size.
7 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
skipping to change at line 425 skipping to change at line 405
details using another mechanism. There are, however, certain scenarios details using another mechanism. There are, however, certain scenarios
where encrypted announcements may be useful. For this reason, the where encrypted announcements may be useful. For this reason, the
encryption bit is included in the SAP header to allow experimentation encryption bit is included in the SAP header to allow experimentation
with encrypted announcements. with encrypted announcements.
This memo does not specify details of the encryption algorithm to This memo does not specify details of the encryption algorithm to
be used or the means by which keys are generated and distributed. be used or the means by which keys are generated and distributed.
An additional specification should define these, if it is desired An additional specification should define these, if it is desired
to use encrypted SAP. to use encrypted SAP.
Note that if an encrypted announcement is being announced via a proxy, Note that if an encrypted announcement is being announced via a proxy, then
then there may be no way for the proxy to discover that the announcement there may be no way for the proxy to discover that the announcement has
has been superseded, and so it may continue to relay the old announcement been superseded, and so it may continue to relay the old announcement in
in addition to the new announcement. SAP provides no mechanism to addition to the new announcement. SAP provides no mechanism to chain
chain modified encrypted announcements, so it is advisable to announce modified encrypted announcements, so it is advisable to announce the
the unmodified session as deleted for a short time after the modification unmodified session as deleted for a short time after the modification has
has occurred. This does not guarantee that all proxies have deleted occurred. This does not guarantee that all proxies have deleted the
the session, and so receivers of encrypted sessions should be prepared session, and so receivers of encrypted sessions should be prepared to
to discard old versions of session announcements that they may receive. discard old versions of session announcements that they may receive. In
In most cases however, the only stateful proxy will be local to (and most cases however, the only stateful proxy will be local to (and known to)
known to) the sender, and an additional (local-area) protocol involving the sender, and an additional (local-area) protocol involving a handshake
a handshake for such session modifications can be used to avoid this for such session modifications can be used to avoid this 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
skipping to change at line 458 skipping to change at line 436
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 asymmetric 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.
8 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.
1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| V=1 |P| Auth | |
+-+-+-+-+-+-+-+-+ |
| Format specific authentication subheader |
: .................. :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2: Format of the authentication data in the SAP header
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.
However, under such circumstances, the session originator may still However, under such circumstances, the session originator may still be
be authenticated to be the same as the session originator of previous authenticated to be the same as the session originator of previous sessions
sessions claiming to be from the same person. This may or may not claiming to be from the same person. This may or may not be sufficient
be sufficient depending on the purpose of the session and the people 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 locally cached, authentication of a particular key only has to take
place once, rather than every time the session directory retransmits place once, rather than every time the session directory retransmits
the announcement. the announcement.
SAP is not tied to any single authentication mechanism. Authentication SAP is not tied to any single authentication mechanism. Authentication
data in the header is self-describing, but the precise format depends data in the header is self-describing, but the precise format depends
on the authentication mechanism in use. The generic format of the on the authentication mechanism in use. The generic format of the
1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| V=1 |P| Auth | |
+-+-+-+-+-+-+-+-+ |
| Format specific authentication subheader |
: .................. :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2: Format of the authentication data in the SAP header
authentication data is given in figure 2. The structure of the format authentication data is given in figure 2. The structure of the format
specific authentication subheader, using both the PGP and the CMS specific authentication subheader, using both the PGP and the CMS
formats, is discussed in sections 8.1 and 8.2 respectively. 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
skipping to change at line 531 skipping to change at line 508
of the information. This defines the format of the authentication of the information. This defines the format of the authentication
subheader and can take the values: 0 = PGP format, 1 = CMS subheader and can take the values: 0 = PGP format, 1 = CMS
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 digital signature.
8.1 PGP Authentication It is to be expected that sessions may be announced by a number of
different mechanisms, not only SAP. For example, a session description
may placed on a web page, sent by email or conveyed in a session
initiation protocol. To ease interoperability with these other mechanisms,
application level security is employed, rather than using IPsec authentication
headers.
Implementations which support authentication MUST support this format. 8.1 PGP Authentication
A full description of the PGP protocol can be found in [2]. When Implementations which support authentication MUST support this format. A
using PGP for SAP authentication the basic format specific authentication full description of the PGP protocol can be found in [2]. When using PGP
subheader comprises a digital signature packet as described in [2]. for SAP authentication the basic format specific authentication subheader
The signature type MUST be 0x01 which means the signature is that comprises a digital signature packet as described in [2]. The signature
of a canonical text document. type MUST be 0x01 which means the signature is that of a canonical text
document.
8.2 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
itself blank as the content which is signed is already present in blank as the content which is signed is already present in the packet.
the packet. Inclusion of it within the SignedData type would duplicate Inclusion of it within the SignedData type would duplicate this data and
this data and increase the packet length unnecessarily. In addition increase the packet length unnecessarily. In addition this allows
this allows recipients with either no interest in the authentication, recipients with either no interest in the authentication, or with no
or with no mechanism for checking it, to more easily skip the mechanism for checking it, to more easily skip the authentication
authentication information. 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 SHOULD
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.
9 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
skipping to change at line 594 skipping to change at line 578
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.
10 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 7 and 8). These mechanisms have not such announcements (sections 7 and 8).
yet been subject to suitable peer-review, and this memo should not
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
be a possible denial of service attack whereby a party listens for be a possible denial of service attack whereby a party listens for
new announcements, strips off the original authentication header, new announcements, strips off the original authentication header,
skipping to change at line 663 skipping to change at line 647
B Summary of differences between SAPv1 and SAPv2 B Summary of differences between SAPv1 and SAPv2
The packet headers of SAP messages are the same in V1 and V2 in 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 that a V2 tool can parse a V1 announcement header but not necessarily
vice-versa. vice-versa.
o The A bit has been added to the SAP header, replacing one of 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 the bits of the SAPv1 message type field. If set to zero the
announcement is of an IPv4 session, and the packet is backwards announcement is of an IPv4 session, and the packet is backwards
compatible with SAPv1. If set to one the announcement is of compatible with SAPv1. If set to one the announcement is of
an IPv6 session, and SAPv1 clients (which do not support IPv6) an IPv6 session, and SAPv1 listeners (which do not support IPv6)
will see this as an illegal message type (MT) field. will see this as an illegal message type (MT) field.
o The second bit of the message type field in SAPv1 has been replaced 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, by a reserved, must-be-zero, bit. This bit was unused in SAPv1,
so this change just codifies existing usage. so this change just codifies existing usage.
o SAPv1 specified encryption of the payload. SAPv2 includes the o SAPv1 specified encryption of the payload. SAPv2 includes the
E bit in the SAP header to indicate that the payload is encrypted, E bit in the SAP header to indicate that the payload is encrypted,
but does not specify any details of the encryption. but does not specify any details of the encryption.
o SAPv1 allowed the message identifier hash and originating source o SAPv1 allowed the message identifier hash and originating source
fields to be set to zero, for backwards compatibility. This fields to be set to zero, for backwards compatibility. This
is no longer legal. is no longer legal.
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 known implementation of SAP compression used zlib, and gzip compression
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 SAPv1 included a timeout field for encrypted announcement, SAPv2
does not (and relies of explicit deletion messages or implicit
timeouts).
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. and Colin Perkins.
skipping to change at line 733 skipping to change at line 721
[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 protocol (MZAP), February 1999. Work in progress. announcement protocol (MZAP), February 2000, RFC2776.
[6] R. Housley. Cryptographic message syntax. Work in progress, [6] R. Housley. Cryptographic message syntax, June 1999. RFC2630.
April 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.
 End of changes. 

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