K. Hedayat
Internet Draft Brix Networks
Expires: December 2006 P. Jones
Cisco Systems, Inc.
A. Roychowdhury
Hughes
C. SivaChelvan
Cisco Systems, Inc.
N. Stratton
June 2006
An Extension to the Session Description Protocol (SDP) for Media
Loopback
draft-ietf-mmusic-media-loopback-03
Status of this Memo
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Copyright Notice
Copyright (C) The Internet Society (2006).
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Abstract
The wide deployment of Voice over IP (VoIP), Real-time Text and
Video over IP services has introduced new challenges in managing
and maintaining voice/real-time Text/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,
Real-time Text 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, Real-time Text 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......................................14
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10. IANA Considerations.........................................14
11. Acknowledgements............................................14
12. References..................................................14
1. Introduction
The overall quality, reliability, and performance of VoIP,
Real-time Text and Video over IP services rely 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, Real-time Text 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, Real-time Text 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
[RFC3264] is used to establish a loopback connection. Furthermore,
this extension provides guidelines on handling RTP [RFC3550], as
well as usage of RTCP [RFC3550] and RTCP XR [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
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 [RFC2234] for loopback-type:
loopback-type = 1*2(loopback-type-choice) [ space "rtp-start-
loopback" ]
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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 entitys 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. The answering entity MAY
include this attribute in the answer if it is not present in the
offer. This may be necessary if the answering entity is aware of
NATs, firewalls, or RTP relays on the path of the call. In this
case the offering entity MUST accept media according to
rtp-start-loopback attribute. 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
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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.
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 receiver for
transmission in the mirrored stream unless rtp-start-loopback is
requested
The loopback mode attribute does not apply to rtp-start-loopback
attribute and MUST be ignored if received 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.
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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
that are 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 indicate 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.
Note: NAT devices may change the actual port number that is used
for transmission and the expected receive port.
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
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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
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 in the answer as per RFC 3264 [RFC3264].
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 entity that is compliant to this specification and
accepting a media with rtp-pkt-loopback loopback-type MUST loopback
the incoming RTP packets while re-generating only the SSRC field of
the RTP header. Note that during the rtp-pkt-loopback mode the
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answering entity does not have control over the encoding of the
media and cannot perform certain functions including congestion
control on the looped back media. However, since the purpose of the
loopback is to characterize the round-trip path at the RTP level,
this limitation is acceptable.
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 answering entity MAY choose to encode the
loopback media according to any of the media descriptions supported
by the offering entity. Furthermore, in cases where the same media
type is looped back, the answering entity 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
Loss RLE report block, Duplicate RLE report block, Statistics
Summary report block, and VoIP Metric Reports Block per sections
4.1, 4.2, 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
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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 192.168.0.12/127
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 192.168.0.12/127
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 192.168.0.12/127
t=0 0
m=audio 49170 RTP/AVP 0
a=loopback:rtp-media-loopback rtp-pkt-loopback
a=loopback-source
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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:
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 192.168.0.12/127
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 192.168.0.12/127
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
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i=An example session
e=user@example.com
c=IN IP4 192.168.0.12/127
t=0 0
m=audio 49170 RTP/AVP 0
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 192.168.0.12/127
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 192.168.0.12/127
t=0 0
m=audio 0 RTP/AVP 0
a=loopback:rtp-media-loopback
a=loopback-mirror
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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.
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 192.168.0.12/127
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 192.168.0.12/127
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.
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9. Security Considerations
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
is limited by the server.
10. IANA Considerations
There are no IANA considerations associated with this
specification.
11. Acknowledgements
The authors wish to thank Nagarjuna Venna, Flemming Andreasen, Jeff
Bernstein, Paul Kyzivat, and Dave Oran for their comments and
suggestions.
12. 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.
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and M. Westerlund, "RTP Control Protocol Extended
Reports (RTCP XR)", RFC 3611, STD 1, November 2003.
[RFC2234] Crocker, P. Overell, "Augmented ABNF for Syntax
Specification: ABNF, 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
Hughes Systique Corp.
15245 Shady Grove Rd, Ste 330
Rockville MD 20850
US
Phone: +1 301 527 1629
EMail: arjun@hsc.com
URI: http://www. hsc.com/
Chelliah SivaChelvan
Cisco Systems, Inc.
2200 East President George Bush Turnpike
Richardson, TX 75082
US
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Phone: +1 972 813 5224
EMail: chelliah@cisco.com
URI: http://www.cisco.com/
Nathan Stratton
663 Salem St.
Lynnfield, MA 01940
Phone: +1 410 908 7587
EMail: nathan@robotics.net
URI: http://www.robotics.net/
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This document and the information contained herein are provided on
an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE
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REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND
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This document is subject to the rights, licenses and restrictions
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Acknowledgment
Funding for the RFC Editor function is currently provided by the
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