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Network Working Group                                        P. Riikonen
Internet-Draft
draft-riikonen-silc-multimedia-session-00.txt            15 January 2007
Expires: 15 July 2007

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.ce 2
Multimedia Sessions in SILC protocol
<draft-riikonen-silc-multimedia-session-00.txt>

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Status of this Draft

By submitting this Internet-Draft, each author represents that any
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Abstract

This document defines the use of multimedia protocols and the set up
of multimedia sessions in the Secure Internet Live Conferencing (SILC)
protocol [SILC1].


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Table of Contents

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1 Introduction ..................................................  2
  1.1 Requirements Terminology ..................................  2
2 Recommended Protocol ..........................................  2
3 Session Description Protocol (SDP) ............................  2
  3.1 SDP field usage in SILC ...................................  3
  3.2 SDP Examples ..............................................  5
4 Session Initiation Protocol (SIP) .............................  6
5 Other Protocols ...............................................  6
6 Security Considerations .......................................  7
7 References ....................................................  7
8 Author's Address ..............................................  7
9 Full Copyright Statement ......................................  7


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1 Introduction

This document defines the use of multimedia protocols and the set up
of multimedia sessions in the Secure Internet Live Conferencing (SILC)
protocol [SILC1].  The SILC protocol supports multimedia messages
with the Message Payload [SILC2] and SILC_MESSAGE_FLAG_DATA which
has the ability to define what type of content is delievered within
the payload.  The Message Payload is used to encapsulate the multimedia
session set up procedure and the actual multimedia session data.  We
define the recommended multimedia session protocol for SILC and also
consider some other protocols in the scope of SILC.


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1.1 Requirements Terminology

The keywords MUST, MUST NOT, REQUIRED, SHOULD, SHOULD NOT, RECOMMENDED,
MAY, and OPTIONAL, when they appear in this document, are to be
interpreted as described in [RFC2119].


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2 Recommended Protocol

Since SILC protocol can encapsulate practically any protocol for setting
up a multimedia session we have selected the Session Description Protocol
(SDP) as RECOMMENDED protocol.  It was chosen for its maturity, simplicity
and versatility.  If multimedia features are implemented in SILC
application it is recommended that at least support for SDP is added.


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3 Session Description Protocol (SDP)

The SDP [SDP] protocol defines a general purpose multimedia session
description protocol.  SDP is one of the simplest protocols to negotiate
multimedia sessions and is suited perfectly for SILC protocol.  Since SDP
does not itself define how it is used to set up the session, we define it
here for SILC.  The definition is based on the [RFC3264] and [RFC4145].

In SILC the SDP messages are sent as data messages (MIME message).  They
can be destined directly to a client for direct conferencing, or to a
channel for group conferencing.  It is also possible to send the message
directly to client to invite them to group conferencing before they have
joined the channel.  The MIME type used is application/sdp.

To set up a multimedia session a client sends SILC message with
SILC_MESSAGE_FLAG_DATA and SILC_MESSAGE_FLAG_REQUEST flags set and with
MIME SDP message in the message payload.  If the receiver wants to
participate in the multimedia session it sends MIME SDP message back with
SILC_MESSAGE_FLAG_DATA and SILC_MESSAGE_FLAG_REPLY flags set to the
sender.  If reply is not received after an application defined period of
time the message may be retransmitted or the session set up may be
terminated.

After reply has been received the multimedia session is started according
to the SDP and all multimedia data is sent using SILC data messages.  When
performing peer-to-peer connection the SDP defines which party initiates
the connection.  After initiation the SILC Key Exchange protocol MUST be
performed.  The resulted key material will be used to protect the multimedia
session.  Multimedia data transmission may start after the key exchange
has been performed.  When performing group conferencing all parties
independently connect to the SILC server specified in the SDP.  In other
cases when performing the multimedia session inside the SILC network, any
party may start transmitting the multimedia data after the SDPs have been
exchanged.

To terminate the session, or to reject incoming request, an MD5 digest
MUST be computed from the original SDP data, and the digest is sent back
with the SILC_MESSAGE_FLAG_DATA and SILC_MESSAGE_FLAG_STOP flags set.
The receiver of such message should verify the MD5 digest and terminate
the session if it matches any active session.  The session may also be
terminated by closing network connection.  In group sessions simply by
leaving the channel terminates the session.  The original sender of the
SDP message may send the terminating message to notify all clients on the
channel to terminate the session.  If the original sender on channel
receives the terminating message it takes no action on it.

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3.1 SDP field usage in SILC

The Encryption Keys (k=) field describes encryption key to protect the
multimedia session.  As SILC protocol transport and the multimedia session
is secured by default this field SHOULD NOT be used.


The Origin (o=) field describes from where the session originates.  The
<username> sub-field is the sender's SILC nickname.  Examples:

	o=foobar 2890844521 2890842804 IN IP4 10.2.1.7


The Connection Data (c=) field describes the connection information for
the multimedia session.  When performing peer-to-peer multimedia session
the <network type> is 'IN', indicating Internet connection.  When
performing multimedia session inside SILC network it is 'SILC'.  When
the 'SILC' network type is used the <address type> and <connection address>
sub-fields are omitted.  Examples:

	c=SILC
	c=IN IP4 10.2.1.7


The Media Announcements (m=) field describes the media information for the
multimedia session.  If the network type in c= field is 'SILC' the <port>
sub-field MUST be set to 9 (discard).  The <transport> for RTP over UDP
is 'RTP/AVP', for RTP over TCP it is 'TCP/RTP/AVP', and for non-RTP protocol
over UDP it is 'udp' and over TCP it is 'tcp'.  The <fmt> sub-field
includes the RTP media payload number when using RTP.  When using non-RTP
protocol it includes MIME subtype.  Examples:

	c=SILC
	m=audio 9 TCP/RTP/AVP 3
	a=rtpmap:3 GSM/8000

	c=SILC
	m=audio 9 tcp mpeg


The Attributes (a=) field can be used to set various session and media
specific attributes.  For SILC we define attribute "silc".

	a=silc:<session type> <parameters>

The <session type> is either "direct" or "group".  When it is "direct"
and the c= field defines a connection point the connection will be
peer-to-peer connection to the remote client.  If it is "group" and the
the c= field defines a connection point the connection will be to a remote
SILC server for group conferencing.  If c= field includes "SILC" network
type, then "direct" is for direct session with a client in SILC network
and "group" is for group conferencing in SILC network.  If the "silc"
attribute is omitted the session type is expected to be "direct".  The
following parameters are defined for attribute "silc".

	channel		The name of the channel for group conferencing.
                        Can be used only with "group" session type.
                        More than one channel parameters may be defined.


The [RFC4145] specifies a "setup" attribute that defines which party of the
session will initiate the connection when performing peer-to-peer session.
Its use in SILC is as specified in [RFC4145] and MUST be present in SDP
when the c= field includes an actual connection point and when the "silc"
attribute session type is "direct", or if the attribute is not present at
all.  When performing group conferencing each party always need to create
the connection to the server and the "setup" attribute need not be present
in SDP.

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3.2 SDP Examples

	v=0
	o=foobar 2890844521 2890842804 IN IP4 10.2.1.100
	s=peer-to-peer example
	t=0 0
	m=audio 5000 TCP/RTP/AVP 3
	c=IN IP4 10.2.1.100
	a=rtpmap:3 GSM/8000
	a=silc:direct
	a=setup:active

This example sets up a peer-to-peer session to remote client at
10.2.1.100 at port 5000.

	v=0
	o=foobar 2890844521 2890842804 IN IP4 10.2.1.32
	s=Group conferencing example
	c=IN IP4 10.2.1.7
	t=0 0
	a=silc:group channel=foobar
	m=audio 706 TCP/RTP/AVP 3
	a=rtpmap:3 GSM/8000

This example sets up a session to a remote SILC server 10.2.1.7 at port
706.  Once connected the channel "foobar" will be joined for group
conferencing.

	v=0
	o=foobar 2890844521 2890842804 IN IP4 10.2.1.32
	s=SILC network chat example
	c=SILC
	t=0 0
	m=audio 9 TCP/RTP/AVP 3
	a=rtpmap:3 GSM/8000

This example sets up a session inside SILC network with the remote user
"foobar".

	v=0
	o=foobar 2890844521 2890842804 IN IP4 10.2.1.32
	s=SILC network group conferencing example
	t=0 0
	m=audio 9 TCP/RTP/AVP 3
	c=SILC
	a=rtpmap:3 GSM/8000
	a=silc:group channel=group-chat

This example sets up a group conferencing session inside SILC network on
channel "group-chat".


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4 Session Initiation Protocol (SIP)

The SIP [SIP] protocol is a general purpose protocol for setting up,
modifying and terminating different kinds of sessions, including
multimedia sessions.  The SIP protocol use the SDP to describe the
multimedia session.

In SILC the SIP messages are sent as data messages (MIME message).  They
can be destined directly to a client for direct conferencing, or to a
channel for group conferencing.  It is also possible to send the message
directly to client to invite them to group conferencing before they have
joined the channel.  The MIME type used is application/sip.  The
SILC_MESSAGE_FLAG_DATA flag must be set in each message and the message
payload includes a MIME SIP message.  The actual SIP session set up and
termination is described in the SIP protocol specification, and SILC
protocol merely provides a secure transport for the session.  After the
session is set up all multimedia data is sent using SILC data messages.
The MIME type for the multimedia data messages is defined during the SIP
session set up.

The rules for SDP fields described in previous section also applies for
SDP with SIP in the context of SILC.

Proxy and redirection servers usually would not be used in the context of
SILC, unless the sessions are redirected to outside SILC network.  This
may compromise the security of the session.

The S/MIME need not be used when using SIP in SILC protocol.  The SILC
protocol transport and the created multimedia session is secured by
default.


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5 Other Protocols

There are other open and proprietary protocols for setting up multimedia
sessions.  One important is H.323 using the H.225 to set up the session.
This document should later define the use of H.323 with SILC.
Practically any protocol to set up multimedia sessions may be used with
SILC by using SILC as a secure transport to set up the session, and to use
SILC data messages (MIME messages) to secure and deliver the actual
multimedia data once the session has been established.


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6 Security Considerations

Security is central to the design of this protocol, and these security
considerations permeate the specification.  Common security considerations
such as keeping private keys truly private and using adequate lengths for
symmetric and asymmetric keys must be followed in order to maintain the
security of this protocol.


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7 References

[SILC1]      Riikonen, P., "Secure Internet Live Conferencing (SILC),
             Protocol Specification", Internet Draft, June 2003.

[SILC2]      Riikonen, P., "SILC Packet Protocol", Internet Draft,
             June 2003.

[RFC3264]    Rosenberg, J., et. al., "An Offer/Answer Model with the
             Session Description Protocol (SDP)", RFC 3264, June 2002.

[RFC4145]    Yon, D., et. al., "TCP-Based Media Transport in the
             Session Description Protocol (SDP)", RFC 4145, September
             2005.

[SIP]        Rosenberg, J., et. al., "SIP: Session Initiation Protocol",
             RFC 3261, June 2002.



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8 Author's Address

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Pekka Riikonen
Helsinki
Finland

EMail: priikone@iki.fi


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9 Full Copyright Statement

Copyright (C) The Internet Society (2007).

This document is subject to the rights, licenses and restrictions
contained in BCP 78, and except as set forth therein, the authors
retain all their rights.

This document and the information contained herein are provided on an
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
