K. Hedayat
Internet Draft Brix Networks
Expires: December 27,2005 P. Jones
Cisco Systems, Inc.
A. Roychowdhury
Flextronics Software Systems
C. SivaChelvan
Cisco Systems, Inc.
N. Stratton
BroadVoice
June 27, 2005
An Extension to the Session Description Protocol (SDP) for Media
Loopback
draft-ietf-mmusic-media-loopback-01.txt
Status of this Memo
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Copyright Notice
Copyright (C) The Internet Society (2005).
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Abstract
The wide deployment of VoIP and Video over IP services has
introduced new challenges in managing and maintaining voice/video
quality, reliability, and overall performance. In particular,
media delivery is an area that needs attention. One method of
meeting these challenges is monitoring the media delivery
performance by looping media back to the transmitter. This is
typically referred to as "active monitoring" of services. Media
loopback is especially popular in ensuring the quality of transport
to the edge of a given VoIP or Video over IP service. Today in
networks that deliver real-time media, short of running 'ping' and
'traceroute' to the edge, service providers are left without the
necessary tools to actively monitor, manage, and diagnose quality
issues with their service. The extension defined herein adds new
SDP media attributes which enables establishment of media sessions
where the media is looped back to the transmitter. Such media
sessions will serve as monitoring and troubleshooting tools by
providing the means for measurement of more advanced VoIP and Video
Over IP performance metrics.
Table of Contents
1. Introduction..................................................3
2. Terminology...................................................3
3. Offering Entity Behavior......................................4
4. Answering Entity Behavior.....................................4
5. SDP Constructs Syntax.........................................4
5.1 Loopback Type Attribute...................................4
5.2 Loopback Mode Attribute...................................6
5.3 Generating the Offer for Loopback Session.................6
5.4 Generating the Answer for Loopback Session................7
5.5 Offerer Processing of the Answer..........................8
5.6 Modifying the Session.....................................8
6. RTP Requirements..............................................8
7. RTCP Requirements.............................................9
8. Examples......................................................9
8.1 Offer for specific media loopback type....................9
8.2 Offer for choice of media loopback type..................10
8.3 Offer for choice of media loopback type with
rtp-start-loopback...........................................11
8.4 Response to INVITE request rejecting loopback media......12
8.5 Response to INVITE request rejecting loopback media with
rtp-start-loopback...........................................13
9. Security Considerations......................................13
10. IANA Considerations.........................................14
11. Acknowledgements............................................14
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12. References..................................................14
12.1 Normative References....................................14
1. Introduction
The overall quality, reliability, and performance of VoIP and Video
over IP services relies on the performance and quality of the media
path. In order to assure the quality of the delivered media there
is a need to monitor the performance of the media transport. One
method of monitoring and managing the overall quality of VoIP and
Video over IP Services is through monitoring the quality of the
media in an active session. This type of "active monitoring" of
services is a method of pro-actively managing the performance and
quality of VoIP based services.
The goal of active monitoring is to measure the media quality of a
VoIP or Video over IP session. A way to achieve this goal is to
request an endpoint to loop media back to the other endpoint and to
provide media statistics (e.g., RTCP and RTCP XR information).
Another method involves deployment of special endpoints that always
loop incoming media back for sessions. Although the latter method
has been used and is functional, it does not scale to support large
networks and introduces new network management challenges.
Further, it does not offer the granularity of testing a specific
endpoint that may be exhibiting problems.
The extension defined in this memo introduces new SDP media
attributes that enable establishment of media sessions where the
media is looped back to the transmitter. The offer/answer model
per RFC 3264 [RFC3264] is used to establish a loopback connection.
Furthermore, this extension provides guidelines on handling RTP
(RFC 3550) [RFC3550], as well as usage of RTCP (RFC 3550) [RFC3550]
and RTCP XR (RFC 3611) [RFC3611] for reporting media related
measurements.
2. Terminology
In this document, the key words "MUST", "MUST NOT", "REQUIRED",
"SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY",
and "OPTIONAL" are to be interpreted as described in RFC 2119
[RFC3264] and indicate requirement levels for compliant
implementations.
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3. Offering Entity Behavior
An offering entity compliant to this memo and attempting to
establish a media session with media loopback MUST include
"loopback" media attributes for each individual media description
in the offer message. The offering entity MUST look for the
"loopback" media attributes in the media description(s) of the
response from the answering entity for confirmation that the
request is accepted.
4. Answering Entity Behavior
An answering entity compliant to this specification and receiving
an offer containing media descriptions with the "loopback" media
attributes, MUST acknowledge the request by including the received
"loopback" media attributes for each media description in its
response. The server MAY reject the "loopback" request for
specific media types as defined in section 5.4.1 of this
specification.
An answering entity that is not compliant to this specification
and which receives an offer with the "loopback" media attributes
MAY safely ignore the attribute and treat the incoming offer as a
normal request.
5. SDP Constructs Syntax
Two new media attributes are defined: one indicates the type of
loopback and one indicates the mode of the loopback.
5.1 Loopback Type Attribute
The loopback type is a property media attribute with the following
syntax:
a=loopback:<loopback-type>
Following is the Augmented BNF (RFC 2234) [RFC2234] for
loopback-type:
loopback-type = loopback-type-choice [ space loopback-type-choice ]
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loopback-type-choice = "rtp-pkt-loopback" | "rtp-media-loopback |
rtp-start-loopback"
The loopback type is used to indicate the type of loopback. The
loopback-type values are rtp-pkt-loopback, rtp-media-loopback, and
rtp-start-loopback.
rtp-pkt-loopback: In this mode, the RTP packets are looped back to
the sender at a point before the encoder/decoder function in the
receive direction to a point after the encoder/decoder function in
the send direction. This effectively re-encapsulates the RTP
payload with the RTP/UDP/IP overheads appropriate for sending it in
the reverse direction. Any type of encoding related functions,
such as packet loss concealment, MUST NOT be part of this type of
loopback path.
rtp-media-loopback: This loopback is activated as close as possible
to the analog interface and after the decoder so that the RTP
packets are subsequently re-encoded prior to transmission back to
the sender.
rtp-start-loopback: In certain scenarios it is possible that the
media transmitted by the offering entity is blocked by a network
element until the answering entity starts transmitting packets.
One example of this scenario is the presence of an RTP relay in the
path of the media. RTP relays exist in VoIP networks for purpose
of NAT and Firewall traversal. If an RTP relay is present the
offering entityÆs packets are dropped by the RTP relay until the
answering entity has started transmitting media and the media state
within the RTP relay is established. This loopback attribute is
used to specify the media type for transmitting media packets by
the answering entity prior to the loopback process for the purpose
of setting media state within the network. In the presence of this
loopback attribute the answering entity will transmit media,
according to the description that contains this attribute, until it
receives media from the offering entity. After the first media
packet is received from the offering entity, the answering entity
MUST terminate the transmission of rtp-start-loopback media and
MUST start looping back media as defined by the other loopback
attributes present in the offer. If an offer includes the
rtp-start-loopback attribute it MUST also include at least one
other attribute as defined in this section. The offering entity is
able to filter rtp-start-loopback packets from other types of
loopback with the payload type of the packet. The media port number
for rtp-start-loopback MUST be the same as the corresponding
loopback attribute that will take over after the reception of first
media packet from the offering entity.
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It is recommended that an offering entity specifying media with
either rtp-pkt-loopback or rtp-media-loopback attribute also
specify the rtp-start-loopback attribute unless the offering entity
is certain that its media will not be blocked by a network entity
as explained above.
5.2 Loopback Mode Attribute
The loopback mode is a value media attribute that is used to
indicate the mode of the loopback. These attributes can be viewed
as additional mode attributes similar to sendonly, recvonly, etc.
The syntax of the loopback mode media attribute is:
a=<loopback-mode>
The loopback-mode values are loopback-source and loopback-mirror.
loopback-source: This attribute specifies that the sender is the
media source and expects the receiver to act as a loopback-mirror.
loopback-mirror: This attribute specifies that the receiver will
mirror (echo) all received media back to the sender of the RTP
stream. No media is generated locally by the reciver for
transmission in the mirrored stream.
The loopback mode attribute does not apply to rtp-start-loopback
attribute and MAY be omitted. If the loopback mode attribute is
present with the rtp-start-loopback attribute it MUST be ignored by
the answering entity.
5.3 Generating the Offer for Loopback Session
If an offerer wishes to make a loopback request, it MUST include
both the loopback-type and loopback-mode attribute in a valid SDP
offer:
Example: a=loopback:rtp-media-loopback
a=loopback-source
Note: A loopback offer in a given media description MUST NOT
contain the standard mode attributes sendonly, recvonly, sendrecv
or inactive.
The offerer may offer more than one loopback-type in the SDP offer.
In this case the answer MUST include only one of the loopback types
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that is accepted by the answerer. The answerer SHOULD give
preference to the first loopback-type in the SDP offer.
For loopback-source media (e.g. audio) streams, the port number and
the address in the offer indicates where the offerer would like to
receive the media stream. The payload type numbers indicate the
value of the payload the offerer expects to receive, and would
prefer to send. However, the answer might indicate a different
payload type number for the same codec. In that case, the offerer
MUST send the payload type received in the answer.
5.4 Generating the Answer for Loopback Session
If an answerer wishes to accept the loopback request it MUST
include both the loopback mode and loopback type attribute in the
answer. If a stream is offered with loopback-source or
loopback-mirror attributes, the corresponding stream MUST be
loopback-mirror or loopback-source respectively, provided that
answerer is capable of supporting the requested loopback-type.
For example, if the offer contains:
a=loopback:rtp-media-loopback
a=loopback-source
The answer that is capable of supporting the offer MUST contain:
a=loopback:rtp-media-loopback
a=loopback-mirror
As previously stated if a stream is offered with multiple loopback
type attributes, the corresponding stream MUST contain only one
loopback type attribute selected by the answerer.
For example, if the offer contains:
a=loopback:rtp-media-loopback rtp-pkt-loopback
a=loopback-source
The answer that is capable of supporting the offer and chooses to
loopback the media using the rtp-media-loopback type MUST contain:
a=loopback:rtp-media-loopback
a=loopback-mirror
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5.4.1 Rejecting the Loopback Offer
An offered stream with loopback-source MAY be rejected, if the
loopback-type is not specified, the specified loopback-type is not
supported, or the endpoint cannot honor the offer for any other
reason. The Loopback request may be rejected by setting the media
port number to zero, according to RFC 3264 [RFC3264], in the
answer.
5.5 Offerer Processing of the Answer
The answer to a loopback-source MUST be loopback-mirror. The
answer to a loopback-mirror MUST be loopback-source. In addition,
the "m=" line MUST contain at least one codec that the answerer is
willing to both send and receive.
If the answer does not contain a=loopback-mirror or
a=loopback-source or contains any other standard mode attributes,
it is assumed that the loopback extensions are not supported by the
target UA.
5.6 Modifying the Session
At any point during the loopback session, either participant may
issue a new offer to modify the characteristics of the previous
session. In case of SIP this is defined in section 8 of RFC 3264
[RFC3264]. This also includes transitioning from a normal media
processing mode to loopback mode, and vice a versa.
6. RTP Requirements
An answering entitity that is compliant to this specification and
accepting a media with rtp-pkt-loopback loopback-type MUST
re-generate all of the RTP header fields as it does when
transmitting other media. However, the answering entity MUST
maintain the timing information of the received RTP packets when
generating the RTP timestamp for the transmit packets. Maintaining
the timing information of the RTP packets enables the offerer to
re-construct the incoming media and take account for impairments
from gaps in the media due to packet loss. Note that RTP Sequence
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numbers are re-generated by the UAS and will not provide packet
loss information to the receiver of the loopback media.
An answering entity that is compliant to this specification and
accepting a media with rtp-media-loopback loopback-type MUST
transmit all received media back to the sender . The incoming media
MUST be treated as if it were to be played (e.g. the media stream
MAY receive treatment from PLC algorithms). The answering entity
MUST re-generate all the RTP header fields as it would when
transmitting media. The UAS MAY choose to encode the loopback media
according to any of the media descriptions supported by the UAC.
Furthermore, in cases where the same media type is looped back, the
UAS MAY choose to preserve number of frames/packet and bitrate of
the encoded media according to the received media.
7. RTCP Requirements
The use of the loopback attribute is intended for monitoring of
media quality of the session. Consequently the media performance
information should be exchanged between the offering and the
answering entities. An offering or answering entity that is
compliant to this specification SHOULD support RTCP per [RFC3550]
and RTCP-XR per RFC 3611 [RFC3611]. Furthermore, if the client or
the server choose to support RTCP-XR, they SHOULD support RTCP-XR
Statistics Summary Report Block and VoIP Metric Reports Block per
sections 4.6 and 4.7 of RFC 3611 [RFC3611]. The client and the
server MAY support other RTCP-XR reporting blocks as defined by RFC
3611 [RFC3611].
8. Examples
This section provides examples for media descriptions using SDP for
different scenarios. The examples are given for SIP based
transactions and are abbreviated and do not show the complete
signaling for convenience.
8.1 Offer for specific media loopback type
A client sends an INVITE request with SDP which looks like:
v=0
o=user1 2890844526 2890842807 IN IP4 126.16.64.4
s=Example
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i=An example session
e=user@example.com
c=IN IP4 224.2.17.12/127
t=0 0
m=audio 49170 RTP/AVP 0
a=loopback:rtp-media-loopback
a=loopback-source
The client is offering to source the media and expects the server
to mirror the RTP stream per rtp-media-loopback loopback type.
A server sends a response with SDP which looks like:
v=0
o=user1 2890844526 2890842807 IN IP4 126.16.64.4
s=Example
i=An example session
e=user@example.com
c=IN IP4 224.2.17.12/127
t=0 0
m=audio 49170 RTP/AVP 0
a=loopback:rtp-media-loopback
a=loopback-mirror
The server is accepting to mirror the media from the client at the
media level.
8.2 Offer for choice of media loopback type
A client sends an INVITE request with SDP which looks like:
v=0
o=user1 2890844526 2890842807 IN IP4 126.16.64.4
s=Example
i=An example session
e=user@example.com
c=IN IP4 224.2.17.12/127
t=0 0
m=audio 49170 RTP/AVP 0
a=loopback:rtp-media-loopback rtp-pkt-loopback
a=loopback-source
The client is offering to source the media and expects the server
to mirror the RTP stream at either the media or rtp level.
A server sends a response with SDP which looks like:
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v=0
o=user1 2890844526 2890842807 IN IP4 126.16.64.4
s=Example
i=An example session
e=user@example.com
c=IN IP4 224.2.17.12/127
t=0 0
m=audio 49170 RTP/AVP 0
a=loopback:rtp-pkt-loopback
a=loopback-mirror
The server is accepting to mirror the media from the client at the
packet level.
8.3 Offer for choice of media loopback type with rtp-start-loopback
A client sends an INVITE request with SDP which looks like:
v=0
o=user1 2890844526 2890842807 IN IP4 126.16.64.4
s=Example
i=An example session
e=user@example.com
c=IN IP4 224.2.17.12/127
t=0 0
m=audio 49170 RTP/AVP 0
a=loopback:rtp-media-loopback rtp-pkt-loopback
a=loopback-source
m=audio 49170 RTP/AVP 100
a=loopback:rtp-start-loopback
The client is offering to source the media and expects the server
to mirror the RTP stream at either the media or rtp level. The
client also expects the server to source media until it receives
packets from the server per media described with the
rtp-start-loopback attribute.
A server sends a response with SDP which looks like:
v=0
o=user1 2890844526 2890842807 IN IP4 126.16.64.4
s=Example
i=An example session
e=user@example.com
c=IN IP4 224.2.17.12/127
t=0 0
m=audio 49170 RTP/AVP 0
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a=loopback:rtp-pkt-loopback
a=loopback-mirror
m=audio 49170 RTP/AVP 100
a=rtpmap:100 pcmu/8000
a=loopback:rtp-start-loopback
The server is accepting to mirror the media from the client at the
packet level. The server is also accepting to source media until
it receives media packets from the client.
8.4 Response to INVITE request rejecting loopback media
A client sends an INVITE request with SDP which looks like:
v=0
o=user1 2890844526 2890842807 IN IP4 126.16.64.4
s=Example
i=An example session
e=user@example.com
c=IN IP4 224.2.17.12/127
t=0 0
m=audio 49170 RTP/AVP 0
a=loopback:rtp-media-loopback
a=loopback-source
The client is offering to source the media and expects the server
to mirror the RTP stream at the media level.
A server sends a response with SDP which looks like:
v=0
o=user1 2890844526 2890842807 IN IP4 126.16.64.4
s=Example
i=An example session
e=user@example.com
c=IN IP4 224.2.17.12/127
t=0 0
m=audio 0 RTP/AVP 0
a=loopback:rtp-media-loopback
a=loopback-mirror
NOTE: Loopback request may be rejected by either not including the
loopback mode attribute (for backward compatibility) or setting the
media port number to zero, or both, in the response.
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8.5 Response to INVITE request rejecting loopback media with
rtp-start-loopback
A client sends an INVITE request with SDP which looks like:
v=0
o=user1 2890844526 2890842807 IN IP4 126.16.64.4
s=Example
i=An example session
e=user@example.com
c=IN IP4 224.2.17.12/127
t=0 0
m=audio 49170 RTP/AVP 0
a=loopback:rtp-media-loopback
a=loopback-source
m=audio 49170 RTP/AVP 100
a=loopback:rtp-start-loopback
The client is offering to source the media and expects the server
to mirror the RTP stream at the media level. The client also
expects the server to source media until it receives packets from
the server per media described with the rtp-start-loopback
attribute.
A server sends a response with SDP which looks like:
v=0
o=user1 2890844526 2890842807 IN IP4 126.16.64.4
s=Example
i=An example session
e=user@example.com
c=IN IP4 224.2.17.12/127
t=0 0
m=audio 0 RTP/AVP 0
a=loopback:rtp-media-loopback
a=loopback-mirror
m=audio 0 RTP/AVP 0
a=loopback:rtp-start-loopback
NOTE: Loopback request may be rejected by either not including the
loopback mode attribute(for backward compatibility) or setting the
media port number to zero, or both, in the response.
9. Security Considerations
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The security considerations of [RFC3261] apply. Furthermore, given
that media loopback may be automated without the end user's
knowledge, the server of the media loopback should be aware of
denial of service attacks. It is recommended that sessions with
media loopback are authenticated and the frequency of such sessions
are limited by the server.
10. IANA Considerations
There are no IANA considerations associated with this
specification.
11. Acknowledgements
The authors wish to thank Flemming Andreasen, Jeff Bernstein, Paul
Kyzivat, and Dave Oran for their comments and suggestions.
12. References
12.1 Normative References
[RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G.,
Johnston, A., Peterson, J., Sparks, R., Handley, M.
and E. Schooler, "SIP: Session Initiation Protocol",
RFC 3261, STD 1, June 2002.
[RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer
Model with the Session Description Protocol (SDP)",
RFC 3264, STD 1, June 2002.
[RFC3550] Schulzrinne, H., Casner, S., Frederick, R. and V.
Jacobson, "RTP: A Transport Protocol for Real-Time
Applications", RFC 3550, STD 1, July 2003.
[RFC3611] Almeroth, K., Caceres, R., Clark, A., Cole, R.,
Duffield, N., Friedman, T., Hedayat, K., Sarac, K.
and M. Westerlund, "RTP Control Protocol Extended
Reports (RTCP XR)", RFC 3611, STD 1, November 2003.
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[RFC2234] Crocker, P. Overell, "Augmented ABNF for Syntax
Specification: ABNFö, RFC 3611, STD 1, November 1997.
Authors' Addresses
Kaynam Hedayat
Brix Networks
285 Mill Road
Chelmsford, MA 01824
US
Phone: +1 978 367 5611
EMail: khedayat@brixnet.com
URI: http://www.brixnet.com/
Paul E. Jones
Cisco Systems, Inc.
7025 Kit Creek Rd.
Research Triangle Park, NC 27709
US
Phone: +1 919 392 6948
EMail: paulej@packetizer.com
URI: http://www.cisco.com/
Arjun Roychowdhury
Flextronics Software Systems
11717 Exploration Lane
Germantown, MD 20876
US
Phone: +1 301 212 7860
EMail: arjun.roy@flextronicssoftware.com
URI: http://www.flextronicssoftware.com/
Chelliah SivaChelvan
Cisco Systems, Inc.
2200 East President George Bush Turnpike
Richardson, TX 75082
US
Phone: +1 972 813 5224
EMail: chelliah@cisco.com
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URI: http://www.cisco.com/
Nathan Stratton
BroadVoice
900 Chelmsford Street
Tower Three
Lowell, MA 01851
US
Phone: +1 978 418 7320
EMail: nstratton@broadvoice.com
URI: http://www.broadvoice.com/
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EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT
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Copyright Notice
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This document is subject to the rights, licenses and restrictions
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