MMUSIC Working Group F. Andreasen
Internet-Draft Cisco Systems, Inc.
Intended status: Standards Track G. Camarillo
Expires: July 26, 2008 Ericsson
D. Oran
D. Wing
Cisco Systems, Inc.
January 23, 2008
Connectivity Preconditions for Session Description Protocol Media
Streams
draft-ietf-mmusic-connectivity-precon-04.txt
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Copyright (C) The IETF Trust (2008).
Abstract
This document defines a new connectivity precondition for the Session
Description Protocol (SDP) precondition framework. A connectivity
precondition can be used to delay session establishment or
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modification until media stream connectivity has been successfully
verified. The method of verification may vary depending on the type
of transport used for the media. For unreliable datagram transports
such as UDP, verification involves probing the stream with data or
control packets. For reliable connection-oriented transports such as
TCP, verification can be achieved simply by successful connection
establishment or by probing the connection with data or control
packets, depending on the situation.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Connectivity Precondition Definition . . . . . . . . . . . . . 3
3.1. Syntax . . . . . . . . . . . . . . . . . . . . . . . . . . 3
3.2. Operational Semantics . . . . . . . . . . . . . . . . . . 4
3.3. Status Type . . . . . . . . . . . . . . . . . . . . . . . 4
3.4. Direction Tag . . . . . . . . . . . . . . . . . . . . . . 5
3.5. Precondition Strength . . . . . . . . . . . . . . . . . . 5
4. Verifying Connectivity . . . . . . . . . . . . . . . . . . . . 6
4.1. Media Stream to Dialog Correlation . . . . . . . . . . . . 6
4.2. Explicit Connectivity Verification Mechanisms . . . . . . 7
4.3. Verifying Connectivity for Connection-Oriented
Transports . . . . . . . . . . . . . . . . . . . . . . . . 9
5. Connectivity and Other Precondition Types . . . . . . . . . . 9
6. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
7. Security Considerations . . . . . . . . . . . . . . . . . . . 14
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 15
9. Change Log . . . . . . . . . . . . . . . . . . . . . . . . . . 15
9.1. Changes since -03 . . . . . . . . . . . . . . . . . . . . 15
9.2. Changes since -02 . . . . . . . . . . . . . . . . . . . . 15
9.3. Changes since -01 . . . . . . . . . . . . . . . . . . . . 15
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 15
10.1. Normative References . . . . . . . . . . . . . . . . . . . 15
10.2. Informative References . . . . . . . . . . . . . . . . . . 16
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 17
Intellectual Property and Copyright Statements . . . . . . . . . . 18
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1. Introduction
The concept of a Session Description Protocol (SDP) [RFC4566]
precondition in the Session Initiation Protocol (SIP) [RFC3261] is
defined in [RFC3312] (updated by [RFC4032]). A precondition is a
condition that has to be satisfied for a given media stream in order
for session establishment or modification to proceed. When the
precondition is not met, session progress is delayed until the
precondition is satisfied or the session establishment fails. For
example, [RFC3312] defines the Quality of Service precondition, which
is used to ensure availability of network resources prior to
establishing a session (i.e., prior to starting alerting the callee).
SIP sessions are typically established in order to setup one or more
media streams. Even though a media stream may be negotiated
successfully through an SDP offer-answer exchange, the actual media
stream itself may fail. For example, when there is one or more
Network Address Translators (NATs) or firewalls in the media path,
the media stream may not be received by the far end. In cases where
the media is carried over a connection-oriented transport such as TCP
[RFC0793], the connection-establishment procedures may fail. The
connectivity precondition defined in this document ensures that
session progress is delayed until media stream connectivity has been
verified.
The connectivity precondition type defined in this document follows
the guidelines provided in [RFC4032] to extend the SIP preconditions
framework.
2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
3. Connectivity Precondition Definition
3.1. Syntax
The connectivity precondition type is defined by the string "conn"
and hence we modify the grammar found in [RFC3312] as follows:
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precondition-type = "conn" | "qos" | token
This precondition tag is registered with the IANA in Section 8.
3.2. Operational Semantics
According to [RFC4032], documents defining new precondition types
need to describe the behavior of UAs (User Agents) from the moment
session establishment is suspended due to a set of preconditions,
until it is resumed when these preconditions are met. An entity that
wishes to delay session establishment or modification until media
stream connectivity has been established uses this precondition-type
in an offer. When a mandatory connectivity precondition is received
in an offer, session establishment or modification is delayed until
the connectivity precondition has been met (i.e., until media stream
connectivity has been established in the desired direction or
directions). The delay of session establishment defined here implies
that alerting of the called party does not occur until the
precondition has been satisfied.
Packets may be both sent and received on the media streams in
question. However, such packets SHOULD be limited to packets that
are necessary to verify connectivity between the two endpoints
involved on the media stream. That is, the underlying media stream
SHOULD NOT be cut through. For example, STUN packets
[I-D.ietf-behave-rfc3489bis], RTP [RFC3550] No-Op
[I-D.ietf-avt-rtp-no-op] packets and their corresponding RTCP
reports, as well as TCP SYN and ACK packets can be exchanged on media
streams that support them as a way of verifying connectivity.
Some media streams are described by a single 'm' line but,
nevertheless, involve multiple addresses. For example, [RFC5109]
specifies how to send FEC (Forward Error Correction) information as a
separate stream (the address for the FEC stream is provided in an
'a=fmtp' line). When a media stream consists of multiple destination
addresses, connectivity to all of them MUST be verified in order for
the precondition to be met. In the case of RTP-based media streams,
RTCP connectivity MAY be verified, but it is not a requirement.
3.3. Status Type
[RFC3312] defines support for two kinds of status types, namely
segmented and end-to-end. The connectivity precondition-type defined
here MUST be used with the end-to-end status type; use of the
segmented status type is undefined.
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3.4. Direction Tag
The direction attributes defined in [RFC3312] are interpreted as
follows:
o send: the party that generated the session description is sending
packets on the media stream to the other party, and the other
party has received at least one of those packets. That is, there
is connectivity in the forward (sending) direction.
o recv: the other party is sending packets on the media stream to
the party that generated the session description, and this party
has received at least one of those packets. That is, there is
connectivity in the backwards (receiving) direction.
o sendrecv: both the send and recv conditions hold.
Note that a "send" connectivity precondition from the offerer's point
of view corresponds to a "recv" connectivity precondition from the
answerer's point of view, and vice versa. If media stream
connectivity in both directions is required before session
establishment or modification continues, the desired status needs to
be set to "sendrecv".
3.5. Precondition Strength
Connectivity preconditions may have a strength-tag of either
"mandatory" or "optional".
When a mandatory connectivity precondition is offered and the
answerer cannot satisfy the connectivity precondition (e.g., because
the offer does not include parameters that enable connectivity to be
verified without media cut through) the offer MUST be rejected as
described in [RFC3312].
When an optional connectivity precondition is offered, the answerer
MUST generate its answer SDP as soon as possible. Since session
progress is not delayed in this case, it is not known whether the
associated media streams will have connectivity. If the answerer
wants to delay session progress until connectivity has been verified,
the answerer MUST increase the strength of the connectivity
precondition by using a strength-tag of "mandatory" in the answer.
Note that use of a "mandatory" precondition requires the presence of
a SIP "Require" header with the option tag "precondition". Any SIP
UA that does not support a mandatory precondition will reject such
requests. To get around this issue, an optional connectivity
precondition and the SIP "Supported" header with the option tag
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"precondition" can be used instead.
Offers with connectivity preconditions in re-INVITEs or UPDATEs
follow the rules given in Section 6 of [RFC3312]. That is:
"Both user agents SHOULD continue using the old session parameters
until all the mandatory preconditions are met. At that moment,
the user agents can begin using the new session parameters."
4. Verifying Connectivity
Media stream connectivity is ascertained by use of a connectivity
verification mechanism between the media endpoints. A connectivity
verification mechanism may be an explicit mechanism, such as ICE
[I-D.ietf-mmusic-ice] or ICE TCP [I-D.ietf-mmusic-ice-tcp], or it may
be an implicit mechanism, such as TCP. Explicit mechanisms provide
specifications for when connectivity between two endpoints using an
offer/answer exchange is ascertained, whereas implicit mechanisms do
not. The verification mechanism is negotiated as part of the normal
offer/answer exchange, however it is not identified explicitly. More
than one mechanism may be negotiated, but the offerer and answerer
need not use the same. The following rules guide which connectivity
verification mechanism to use:
1. if an explicit connectivity verification mechanism (e.g., ICE) is
negotiated, the precondition is met when the mechanism verifies
connectivity successfully, otherwise
2. if a connection-oriented transport (e.g., TCP) is negotiated, the
precondition is met when the connection is established.
3. in other cases, an implicit verification mechanism may be
provided by the transport itself or the media stream data using
the transport (e.g., RTP No-Op)
4. if none of the above apply, connectivity cannot be verified
reliably and the connectivity precondition will never be
satisfied if requested.
This document does not mandate any particular connectivity
verification mechanism; however, in the following, we provide
additional considerations for verification mechanisms.
4.1. Media Stream to Dialog Correlation
SIP and SDP do not provide any inherent capabilities for associating
an incoming media stream packet with a particular dialog. Thus, when
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an offerer is trying to ascertain connectivity, and an incoming media
stream packet is received, the offerer may not know which dialog had
its "recv" connectivity verified. Explicit connectivity verification
mechanisms therefore typically provide a means to correlate the media
stream, whose connectivity is being verified, with a particular SIP
dialog. However, some connectivity verification mechanisms may not
provide such a correlation. In the absence of a dialog-to-media-
stream correlation mechanism (e.g., ICE), a UAS (User Agent Server)
MUST NOT require the offerer to confirm a connectivity precondition.
4.2. Explicit Connectivity Verification Mechanisms
Explicit connectivity verification mechanisms typically use probe
traffic with some sort of feedback to inform the sender whether
reception was successful. Below we provide two examples of such
mechanisms, and how they are used with connectivity preconditions:
Interactive Connectivity Establishment (ICE) [I-D.ietf-mmusic-ice]
provides one or more candidate addresses in signaling between the
offerer and the answerer and then uses STUN Binding Requests to
determine which pairs of candidate addresses have connectivity. Each
STUN Binding Request contains a password which is communicated in the
SDP as well; this enables correlation between STUN Binding Requests
and candidate addresses for a particular media stream. It also
provides correlation with a particular SIP dialog.
ICE implementations may be either Full or Lite (see
[I-D.ietf-mmusic-ice]). Full implementations generate and respond to
STUN Binding Requests, whereas Lite implementations only respond to
them. With ICE, one side is a controlling agent, and the other side
is a controlled agent. A Full implementation can take on either
role, whereas a Lite implementation can only be a controlled agent.
The controlling agent decides which valid candidate to use and
informs the controlled agent of it by identifying the pair as the
nominated pair. This leads to the following connectivity
precondition rules:
o A Full implementation ascertains both "send" and "recv"
connectivity when it operates as a STUN client and has sent a STUN
Binding Request that resulted in a successful check for all the
components of the media stream (as defined further in ICE).
o A Full or a Lite implementation ascertains "recv" connectivity
when it operates as a STUN server and has received a STUN Binding
Request that resulted in a successful response for all the
components of the media stream (as defined further in ICE).
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o A Lite implementation ascertains "send" and "recv" connectivity
when the controlling agent has informed it of the nominated pair
for all the components of the media stream.
A simpler and slightly more delay-prone alternative to the above
rules is for all ICE implementations to ascertain "send" and "recv"
connectivity for a media stream when the ICE state for that media
stream has moved to Completed.
Note that there is never a need for the answerer to request
confirmation of the connectivity precondition when using ICE: the
answerer can determine the status locally. Also note, that when ICE
is used to verify connectivity preconditions, the precondition is not
satisfied until connectivity has been verified for all the component
transport addresses used by the media stream. For example, with an
RTP-based media stream where RTCP is not suppressed, connectivity
MUST be ascertained for both RTP and RTCP; this is a tightening of
the general operational semantics provided in Section 3.2, which is
imposed by ICE. Finally, it should be noted, that although
connectivity has been ascertained, a new offer/answer exchange may be
required before media can flow (per ICE).
RTP No-Op [I-D.ietf-avt-rtp-no-op] enables the sender of an RTP No-Op
payload to verify send connectivity by examining the RTCP report(s)
being returned. In particular, the source SSRC in the RTCP report
block is used for correlation. The RTCP report block also contains
the SSRC of the sender of the report and the SSRC of incoming RTP
No-Op packets identifies the sender of the RTP packet. Thus, once
send connectivity has been ascertained, receipt of an RTP No-Op
packet from the same SSRC provides the necessary correlation to
determine receive connectivity. Alternatively, the duality of send
and receive preconditions can be exploited, with one side confirming
when his send precondition is satisfied, which in turn implies the
other sides recv precondition is satisfied.
The above are merely examples of explicit connectivity verification
mechanisms. Other techniques can be used as well. It is however
RECOMMENDED that ICE be supported by entities that support
connectivity preconditions. Use of ICE has the benefit of working
for all media streams (not just RTP) as well as facilitate NAT and
firewall traversal, which may otherwise interfere with connectivity.
Furthermore, the ICE recommendation provides a baseline to ensure
that all entities that require probe traffic to support the
connectivity preconditions have a common way of ascertaining
connectivity.
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4.3. Verifying Connectivity for Connection-Oriented Transports
Connection-oriented transport protocols generally provide an implicit
connectivity verification mechanism. Connection establishment
involves sending traffic in both directions thereby verifying
connectivity at the transport protocol level. When a three-way (or
more) handshake for connection establishment succeeds, bi-directional
communication is confirmed and both the "send" and "recv"
preconditions are satisfied whether requested or not. In the case of
TCP for example, once the TCP three-way handshake has completed (SYN,
SYN-ACK, ACK), the TCP connection is established and data can be sent
and received by either party (i.e., both a send and a receive
connectivity precondition has been satisfied). SCTP [RFC4960]
connections have similar semantics as TCP and SHOULD be treated the
same.
When a connection-oriented transport is part of an offer, it may be
passive, active, or active/passive [RFC4145]. When it is passive,
the offerer expects the answerer to initiate the connection
establishment, and when it is active, the offerer wants to initiate
the connection establishment. When it is active/passive, the
answerer decides. As noted earlier, lack of a media-stream-to-dialog
correlation mechanism can make it difficult to guarantee with whom
connectivity has been ascertained. When the offerer takes on the
passive role, the offerer will not necessarily know which SIP dialog
originated an incoming connection request. If the offerer instead is
active, this problem is avoided.
5. Connectivity and Other Precondition Types
The role of a connectivity precondition is to ascertain media stream
connectivity before establishing or modifying a session. The
underlying intent is for the two parties to be able to exchange media
packets successfully. Connectivity by itself however may not fully
satisfy this. Quality of Service for example may be required for the
media stream; this can be addressed by use of the "qos" precondition
defined in [RFC3312]. Similarly, succesful security parameter
negotiation may be another prequisite; this can be addressed by use
of the "sec" precondition defined in [RFC5027].
6. Examples
The first example uses the connectivity precondition with TCP in the
context of a session involving a wireless access medium. Both UAs
use a radio access network that does not allow them to send any data
(not even a TCP SYN) until a radio bearer has been setup for the
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connection. Figure 1 shows the message flow of this example (the
required PRACK transaction has been omitted for clarity):
A B
| INVITE |
| a=curr:conn e2e none |
| a=des:conn mandatory e2e sendrecv |
| a=setup:holdconn |
|----------------------------------->|
| |
| 183 Session Progress |
| a=curr:conn e2e none |
| a=des:conn mandatory e2e sendrecv |
| a=setup:holdconn |
|<-----------------------------------|
| |
| UPDATE |
| a=curr:conn e2e none |
| a=des:conn mandatory e2e sendrecv |
A's radio | a=setup:actpass |
bearer is +----------------------------------->|
up | |
| 200 OK |
| a=curr:conn e2e none |
| a=des:conn mandatory e2e sendrecv |
| a=setup:active |
|<-----------------------------------|
| |
| |
| |
| | B's radio
|<---TCP Connection Establishment--->+ bearer is up
| | B sends TCP SYN
| |
| |
| 180 Ringing | TCP connection
|<-----------------------------------+ is up
| | B alerts the user
| |
Figure 1: Message flow with two types of preconditions
A sends an INVITE requesting connection-establishment preconditions.
The setup attribute in the offer is set to holdconn [RFC4145] because
A cannot send or receive any data before setting up a radio bearer
for the connection.
B agrees to use the connectivity precondition by sending a 183
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(Session Progress) response. The setup attribute in the answer is
also set to holdconn because B, like A, cannot send or receive any
data before setting up a radio bearer for the connection.
When A's radio bearer is ready, A sends an UPDATE to B with a setup
attribute with a value of actpass. This attribute indicates that A
can perform an active or a passive TCP open. A is letting B choose
which endpoint will initiate the connection.
Since B's radio bearer is not ready yet, B chooses to be the one
initiating the connection and indicates so with a setup attribute
with a value of active. At a later point, when B's radio bearer is
ready, B initiates the TCP connection towards A.
Once the TCP connection is established successfully, B knows the
"sendrecv" precondition is satisfied, and B proceeds with the session
(i.e., alerts the Callee), and sends a 180 (Ringing) response.
The second example shows a basic SIP session establishment using SDP
connectivity preconditions and RTP No-Op (the required PRACK
transaction and some SDP details have been omitted for clarity). The
message flow for this scenario is shown in Figure 2 below.
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A B
| |
|-------------(1) INVITE SDP1--------------->|
| |
|<------(2) 183 Session Progress SDP2--------|
| |
|<~~~~~ Connectivity check to A ~~~~~~~~~~~~~|
|~~~~~ Connectivity to A OK ~~~~~~~~~~~~~~~~>|
| |
|~~~~~ Connectivity check to B ~~~~~~~~~~~~~>|
|<~~~~ Connectivity to B OK ~~~~~~~~~~~~~~~~~|
| |
|-------------(3) UPDATE SDP3--------------->|
| |
|<--------(4) 200 OK (UPDATE) SDP4-----------|
| |
|<-------------(5) 180 Ringing---------------|
| |
| |
Figure 2: Connectivity precondition with RTP No-Op
SDP1: A includes a mandatory end-to-end connectivity precondition
with a desired status of "sendrecv"; this will ensure media stream
connectivity in both directions before continuing with the session
setup. Since media stream connectivity in either direction is
unknown at this point, the current status is set to "none". A's
local status table (see [RFC3312]) for the connectivity precondition
is as follows:
Direction | Current | Desired Strength | Confirm
-----------+----------+------------------+----------
send | no | mandatory | no
recv | no | mandatory | no
and the resulting offer SDP is:
m=audio 20000 RTP/AVP 0 96
c=IN IP4 192.0.2.1
a=rtpmap:96 no-op/8000
a=curr:conn e2e none
a=des:conn mandatory e2e sendrecv
SDP2: When B receives the offer, B sees the mandatory sendrecv
connectivity precondition. B can ascertain connectivity to A ("send"
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from B's point of view) by use of the RTP No-Op, however B wants A to
inform it about connectivity in the other direction ("recv" from B's
point of view). B's local status table therefore looks as follows:
Direction | Current | Desired Strength | Confirm
-----------+----------+------------------+----------
send | no | mandatory | no
recv | no | mandatory | no
Since B wants to ask A for confirmation about the "recv" (from B's
point of view) connectivity precondition, the resulting answer SDP
becomes:
m=audio 30000 RTP/AVP 0 96
a=rtpmap:96 no-op/8000
c=IN IP4 192.0.2.4
a=curr:conn e2e none
a=des:conn mandatory e2e sendrecv
a=conf:conn e2e recv
Meanwhile, B performs a successful send connectivity check to A by
sending an RTP No-Op packet to A and receiving a corresponding RTCP
report. B's local status table is updated as follows:
Direction | Current | Desired Strength | Confirm
-----------+----------+------------------+----------
send | yes | mandatory | no
recv | no | mandatory | no
Since the "recv" connectivity precondition (from B's point of view)
is still not satisfied, session establishment remains suspended.
SDP3: When A receives the answer SDP, A notes that confirmation was
requested for B's "recv" connectivity precondition, which is the
"send" precondition from A's point of view. A performs a successful
send connectivity check to B by sending an RTP No-Op packet to B and
receiving a corresponding RTCP report, and A's local status table
becomes:
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Direction | Current | Desired Strength | Confirm
-----------+----------+------------------+----------
send | yes | mandatory | yes
recv | no | mandatory | no
Since B asked for confirmation about the "send" connectivity (from
A's point of view), A now sends an UPDATE (5) to B to confirm the
connectivity from A to B:
m=audio 20000 RTP/AVP 0 96
a=rtpmap:96 no-op/8000
c=IN IP4 192.0.2.1
a=curr:conn e2e send
a=des:conn mandatory e2e sendrecv
B has both send and recv connectivity confirmed at this point and the
session can continue.
7. Security Considerations
In addition to the general security considerations for preconditions
provided in [RFC3312], the following security issues, which are
specific to connectivity preconditions, should be considered.
Connectivity preconditions rely on mechanisms beyond SDP such as
TCP[RFC0793] connection establishment, RTP No-Op
[I-D.ietf-avt-rtp-no-op], or STUN [I-D.ietf-behave-rfc3489bis] to
establish and verify connectivity between an offerer and an answerer.
An attacker that prevents those mechanism from succeeding can prevent
media sessions from being established and hence it is RECOMMENDED
that such mechanisms are adequately secured by message authentication
and integrity protection. Also, the mechanisms SHOULD consider how
to prevent denial of service attacks. Similarly, an attacker that
can forge packets for these mechanisms can enable sessions to be
established when there in fact is no media connectivity, which may
lead to a poor user experience. Authentication and integrity
protection of such mechanisms can prevent this type of attacks and
hence use of it is RECOMMENDED.
It is also strongly RECOMMENDED that integrity protection be applied
to the SDP session descriptions. S/MIME [RFC3853] provides such end-
to-end integrity protection, as described in [RFC3261].
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8. IANA Considerations
IANA is hereby requested to register a new precondition type under
the Precondition Types used with SIP subregistry, which is located
under the Session Initiation Protocol (SIP) Parameters registry.
Precondition-Type Description Reference
----------------- ----------------------------------- ---------
conn Connectivity precondition [RFCxxxx]
[Note to the RFC Editor: replace RFCxxxx with the number assigned to
this RFC.]
9. Change Log
9.1. Changes since -03
Minor fixes here and there.
9.2. Changes since -02
Connectivity preconditions are now mechanism agnostic. Clarified
when and how to use ICE, RTP No-Op, and connection establishment
procedures to check connectivity. Clarified relation with other
precondition types.
9.3. Changes since -01
There are no changes since the previous version of the document.
10. References
10.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC5109] Li, A., "RTP Payload Format for Generic Forward Error
Correction", RFC 5109, December 2007.
[RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
A., Peterson, J., Sparks, R., Handley, M., and E.
Schooler, "SIP: Session Initiation Protocol", RFC 3261,
June 2002.
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[RFC3312] Camarillo, G., Marshall, W., and J. Rosenberg,
"Integration of Resource Management and Session Initiation
Protocol (SIP)", RFC 3312, October 2002.
[RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V.
Jacobson, "RTP: A Transport Protocol for Real-Time
Applications", STD 64, RFC 3550, July 2003.
[RFC3853] Peterson, J., "S/MIME Advanced Encryption Standard (AES)
Requirement for the Session Initiation Protocol (SIP)",
RFC 3853, July 2004.
[RFC4032] Camarillo, G. and P. Kyzivat, "Update to the Session
Initiation Protocol (SIP) Preconditions Framework",
RFC 4032, March 2005.
[RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
Description Protocol", RFC 4566, July 2006.
10.2. Informative References
[RFC0793] Postel, J., "Transmission Control Protocol", STD 7,
RFC 793, September 1981.
[RFC4145] Yon, D. and G. Camarillo, "TCP-Based Media Transport in
the Session Description Protocol (SDP)", RFC 4145,
September 2005.
[RFC4960] Stewart, R., "Stream Control Transmission Protocol",
RFC 4960, September 2007.
[RFC5027] Andreasen, F. and D. Wing, "Security Preconditions for
Session Description Protocol (SDP) Media Streams",
RFC 5027, October 2007.
[I-D.ietf-avt-rtp-no-op]
Andreasen, F., "A No-Op Payload Format for RTP",
draft-ietf-avt-rtp-no-op-04 (work in progress), May 2007.
[I-D.ietf-mmusic-ice]
Rosenberg, J., "Interactive Connectivity Establishment
(ICE): A Protocol for Network Address Translator (NAT)
Traversal for Offer/Answer Protocols",
draft-ietf-mmusic-ice-16 (work in progress), June 2007.
[I-D.ietf-mmusic-ice-tcp]
Rosenberg, J., "TCP Candidates with Interactive
Connectivity Establishment (ICE",
Andreasen, et al. Expires July 26, 2008 [Page 16]
Internet-Draft Connectivity Precondition January 2008
draft-ietf-mmusic-ice-tcp-03 (work in progress),
March 2007.
[I-D.ietf-behave-rfc3489bis]
Rosenberg, J., "Session Traversal Utilities for (NAT)
(STUN)", draft-ietf-behave-rfc3489bis-06 (work in
progress), March 2007.
Authors' Addresses
Flemming Andreasen
Cisco Systems, Inc.
499 Thornall Street, 8th Floor
Edison, NJ 08837
USA
Email: fandreas@cisco.com
Gonzalo Camarillo
Ericsson
Hirsalantie 11
Jorvas 02420
Finland
Email: Gonzalo.Camarillo@ericsson.com
David Oran
Cisco Systems, Inc.
7 Ladyslipper Lane
Acton, MA 01720
USA
Email: oran@cisco.com
Dan Wing
Cisco Systems, Inc.
170 West Tasman Drive
San Jose, CA 94301
USA
Email: dwing@cisco.com
Andreasen, et al. Expires July 26, 2008 [Page 17]
Internet-Draft Connectivity Precondition January 2008
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Andreasen, et al. Expires July 26, 2008 [Page 18]
