An Extension to the Session Description Protocol (SDP) for Media Loopback
draft-ietf-mmusic-media-loopback-17
The information below is for an old version of the document.
| Document | Type |
This is an older version of an Internet-Draft that was ultimately published as RFC 6849.
|
|
|---|---|---|---|
| Authors | Hadriel Kaplan , Kaynam Hedayat , Nagarjuna Venna , Paul Jones , Arjun Roychowdhury , C Sivachelvan , Nathan Stratton | ||
| Last updated | 2012-03-12 | ||
| RFC stream | Internet Engineering Task Force (IETF) | ||
| Formats | |||
| Additional resources | Mailing list discussion | ||
| Stream | WG state | WG Document | |
| Document shepherd | Miguel Angel García | ||
| IESG | IESG state | Became RFC 6849 (Proposed Standard) | |
| Consensus boilerplate | Unknown | ||
| Telechat date | (None) | ||
| Responsible AD | (None) | ||
| Send notices to | (None) |
draft-ietf-mmusic-media-loopback-17
Internet Draft H. Kaplan (ed.)
Expires: August 12, 2012 Acme Packet
K. Hedayat
EXFO
N. Venna
Saperix
P. Jones
Cisco Systems, Inc.
A. Roychowdhury
Hughes Systique Corp.
C. SivaChelvan
Cisco Systems, Inc.
N. Stratton
BlinkMind, Inc.
March 10, 2012
An Extension to the Session Description Protocol (SDP) for Media
Loopback
draft-ietf-mmusic-media-loopback-17
Status of this Memo
This Internet-Draft is submitted to IETF in full conformance with
the provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet-
Drafts. The list of current Internet-Drafts is at
http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six
months and may be updated, replaced, or obsoleted by other
documents at any time. It is inappropriate to use Internet-Drafts
as reference material or to cite them other than as "work in
progress."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt.
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
This Internet-Draft will expire on August 12, 2012.
Kaplan, et al. Expires August 2012 [Page 1]
Internet-Draft SDP Media Loopback March 2012
Copyright Notice
Copyright (c) 2012 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with
respect to this document. Code Components extracted from this
document must include Simplified BSD License text as described in
Section 4.e of the Trust Legal Provisions and are provided without
warranty as described in the Simplified BSD License.
Abstract
The wide deployment of Voice over IP (VoIP), Text and Video over IP
services has introduced new challenges in managing and maintaining
real-time 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
1.1 Use Cases Supported.......................................4
2. Terminology...................................................5
3. Offering Entity Behavior......................................6
4. Answering Entity Behavior.....................................6
5. SDP Constructs Syntax..............Error! Bookmark not defined.
5.1 Loopback Type Attribute...................................7
5.2 Loopback Mode Attribute...................................7
Kaplan, et al. Expires August 2012 [Page 2]
Internet-Draft SDP Media Loopback March 2012
5.3 Generating the Offer for Loopback Session.................8
5.4 Generating the Answer for Loopback Session................9
5.5 Offerer Processing of the Answer.........................11
5.6 Modifying the Session....................................11
5.7 Establishing Sessions Between Entities Behind NAT........12
6. RTP Requirements.............................................12
7. Payload formats for Packet loopback..........................12
7.1 Encapsulated Payload format..............................13
7.2 Direct loopback RTP payload format.......................15
8. RTCP Requirements............................................16
9. Congestion Control...........................................17
10. Examples....................................................17
10.1 Offer for specific media loopback type..................17
10.2 Offer for choice of media loopback type.................18
10.3 Response to INVITE request rejecting loopback media.....19
11. Security Considerations.....................................19
12. Implementation Considerations...............................20
13. IANA Considerations.........................................20
13.1 SDP Attributes..........................................20
13.2 MIME Types..............................................21
14. Normative References........................................30
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 real-time VoIP, 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 proactively managing the performance and quality of VoIP based
services.
The goal of active monitoring is to measure the media quality of a
VoIP, 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.
Kaplan, et al. Expires August 2012 [Page 3]
Internet-Draft SDP Media Loopback March 2012
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.
1.1 Use Cases Supported
As a matter of terminology in this document, packets flow from one
peer acting as a "loopback source", to the other peer acting as a
"loopback mirror", which in turn returns packets to the loopback
source. In advance of the session, the peers negotiate to determine
which one acts in which role. The negotiation also includes details
such as the type of loopback to be used.
This specification supports three use cases: "encapsulated packet
loopback", "direct loopback", and "media loopback". These are
distinguished by the treatment of incoming RTP packets at the
loopback mirror.
1.1.1 Encapsulated Packet Loopback
In the encapsulated packet loopback case, the entire incoming RTP
packet is encapsulated as payload within an outer payload type that
is specific to this use case and specified below (Section 7.1).
The encapsulated packet is returned to the loopback source. The
loopback source can generate statistics for one-way path
performance up to the RTP level for each direction of travel by
examining sequence numbers and timestamps in the outer header and
the encapsulated RTP packet payload. The loopback source can also
play back the returned media content for evaluation.
Because the encapsulating payload extends the packet size, it could
encounter difficulties in an environment where the original RTP
packet size is close to the path MTU size. The encapsulating
payload type therefore offers the possibility of RTP-level
fragmentation of the returned packets. The use of this facility
could affect statistics derived for the return path. In addition,
the increased bit rate required in the return direction may affect
these statistics more directly in a restricted-bandwidth situation.
Kaplan, et al. Expires August 2012 [Page 4]
Internet-Draft SDP Media Loopback March 2012
1.1.2 Direct Loopback
In the direct loopback case, the loopback mirror copies the payload
of the incoming RTP packet into a new packet, the payload type of
which is again specific to this use case and specified below
(Section 7.2). The loopback mirror returns the new packet to the
packet source. There is no provision in this case for RTP-level
fragmentation.
This use case has the advantage of keeping the packet size the same
in both directions. The packet source can compute only two-way
path statistics from the direct loopback packet header, but can
play back the returned media content.
It has been suggested that the loopback source, knowing that the
incoming packet will never be passed to a decoder, can store a
timestamp and sequence number inside the payload of the packet it
sends to the mirror, then extract that information from the
returned direct loopback packet and compute one-way path statistics
as in the previous case. Obviously, playout of returned content is
no longer possible if this is done.
1.1.3 Media Loopback
In the media loopback case, the loopback mirror submits the
incoming packet to a decoder appropriate to the incoming payload
type. The packet is taken as close as possible to the analog level,
then reencoded according to an outgoing format determined by
negotiation. The reencoded content is returned to the loopback
source as an RTP packet with payload type corresponding to the
reencoding format.
This usage allows trouble-shooting at the codec level. The
capability for path statistics is limited to what is available from
RTCP reports.
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 RFC 2119.
Kaplan, et al. Expires August 2012 [Page 5]
Internet-Draft SDP Media Loopback March 2012
SDP: Session Description Protocol, as defined in [RFC4566]. This
document assumes the SDP offer/answer model is followed, per
[RFC3264], but does not assume any specific protocol for carrying
the SDP.
The following terms are borrowed from [RFC3264] definitions: offer,
offerer, answer, answerer, and agent.
3. SDP Offerer Behavior
An SDP offerer 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 offerer MUST look for the "loopback" media attributes
in the media description(s) of the response from the answer for
confirmation that the request is accepted.
4. SDP Answerer Behavior
An SDP answerer 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
asnwer if it agrees to do the loopback. If the answerer does not
want to do loopback or wants to reject the "loopback" request for
specific media types, it MAY do so as defined in section Error!
Reference source not found. of this specification.
An answerer MAY reject an offered stream (either with loopback-
source or loopback-mirror) 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
MUST be rejected by setting the stream's media port number to zero
in the answer as defined in RFC 3264 [RFC3264], or by rejecting the
entire offer (e.g., by rejecting the session request entirely).
Note that an answerer that is not compliant to this specification
and which receives an offer with the "loopback" media attributes
would ignore the attribute and treat the incoming offer as a normal
request. If the offerer does not wish to establish a "normal" RTP
session, it would need to terminate the session upon receiving such
an answer.
Kaplan, et al. Expires August 2012 [Page 6]
Internet-Draft SDP Media Loopback March 2012
5. New SDP Attributes
Three new SDP media-level attributes are defined: one indicates the
type of loopback, and the other two define the mode of the
loopback.
5.1 Loopback Type Attribute
This specification defines a new 'loopback' attribute, which
indicates the type of loopack that the agent is able to do. The
loopback type is a property media attribute with the following
syntax:
a=loopback:<loopback-type>
Following is the Augmented BNF [RFC5234] for loopback-type:
Loopback-attr = "a=loopback:"
loopback-type = loopback-choice [1*SP loopback-choice]
loopback-choice = loopback-type-pkt / loopback-type-media
loopback-type-pkt = "rtp-pkt-loopback"
loopback-type-media = "rtp-media-loopback"
The loopback type is used to indicate the type of loopback. The
loopback-type values are rtp-pkt-loopback, and rtp-media-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 headers 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. In this mode the RTP packets are looped back with a
new payload type and format. Section 7 describes the payload
formats that MUST be used for this type of loopback.
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.
5.2 Loopback Mode Attribute
The loopback mode defines two value media attributes that are used
to indicate the mode of the loopback. These attributes are
Kaplan, et al. Expires August 2012 [Page 7]
Internet-Draft SDP Media Loopback March 2012
additional mode attributes like sendonly, recvonly, etc. The
syntax of the loopback mode media attributes are based on the
following:
a=<loopback-mode>:<fmt>...
The loopback-mode values are 'loopback-source' and 'loopback-
mirror'.
loopback-source: This attribute specifies that the entity that
generated the SDP is the media source and expects the receiver of
the SDP message to act as a loopback-mirror.
loopback-mirror: This attribute specifies that the entity that
generated the SDP will mirror (echo) all received media back to the
sender of the RTP stream. No media is generated locally by the
looping back entity for transmission in the mirrored stream.
<fmt> is a media format description. The format description has the
semantics as defined in section 5.14 of RFC 4566[RFC4566]. When
loopback-mode is specified as loopback-source, the media format
corresponds to the RTP payload types the entity that generated the
SDP is willing to send. When loopback-mode is specified as
loopback-mirror, the media format corresponds to the RTP payload
types the mirror is willing to receive. The "m=" line in the SDP
MUST include all the payload types that will be used during the
loopback session including those specified in the loopback-mode
attribute line. The complete payload space for the call is
specified in the "m=" line and the rtpmap attribute is used to map
from the payload type number to an encoding name denoting the
payload format to be used.
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 attributes in a valid SDP
offer:
Example: m=audio 41352 RTP/AVP 0 8 100
a=loopback:rtp-media-loopback
a=loopback-source:0 8 100
a=rtpmap:0 pcmu/8000
a=rtpmap:8 pcma/8000
a=rtpmap:100 G7221/16000/1
A loopback offer in a given media description MUST NOT contain the
standard mode attributes sendonly, recvonly, sendrecv, or inactive.
Kaplan, et al. Expires August 2012 [Page 8]
Internet-Draft SDP Media Loopback March 2012
The loopback-mode attributes (loopback-source and loopback-mirror)
replace the standard attributes.
The offerer may offer more than one loopback-type in the SDP offer.
The port number and the address in the offer (m/c= lines) indicate
where the offerer would like to send and receive the media stream.
The payload type numbers indicate the value of the payload the
offerer expects to send and receive. If the offerer is the
loopback-source, the subset of payload types indicated in the
a=loopback-source line are the payload types for the codecs the
offerer is willing to send. However, the answer might indicate a
different payload type number for the same codec in the loopback-
mirror line. In that case, the offerer MUST send the payload type
received in the answer. If the offerer is the loopback-mirror, the
subset of payload types indicated in the a=loopback-mirror line are
the payload types for the codecs the offerer is willing to receive.
If loopback-type is rtp-pkt-loopback, the loopback-mirror MUST send
and the loopback-source MUST receive the looped back packets
encoded in one of the two payload formats (encapsulated RTP or
direct loopback) as defined in section 7.
Example: m=audio 41352 RTP/AVP 0 8 112
a=loopback:rtp-pkt-loopback
a=loopback-source:0 8
a=rtpmap:112 encaprtp/8000
Example: m=audio 41352 RTP/AVP 0 8 112
a=loopback:rtp-pkt-loopback
a=loopback-source:0 8
a=rtpmap:112 rtploopback/8000
5.4 Generating the Answer for Loopback Session
As with the offer, an SDP answer for loopback MUST NOT contain the
standard mode attributes sendonly, recvonly, sendrecv, or inactive.
The port number and the address in the answer (m/c= lines) indicate
where the answerer would like to receive the media stream. The
payload type numbers indicate the value of the payload types the
answerer expects to send and receive. The loopback-mode attributes
(a=loopback-source or a=loopback-miror) MUST contain at least one
codec the answerer is willing to send or receive depending on
whether it is the loopback-source or the loopback-mirror. In
addition, the "m=" line MUST contain at least one codec that the
answerer is willing to send or receive depending on whether it is
the loopback-mirror or the loopback-source.
Kaplan, et al. Expires August 2012 [Page 9]
Internet-Draft SDP Media Loopback March 2012
If the offerer is the loopback-source, the answerer MUST be a
loopback-mirror and the subset of payload types indicated in the
a=loopback-mirror line are the payload types for the codecs the
answerer is willing to receive. Similarly, if the offerer is the
loopback-mirror, the answerer MUST be aloopback-source and the
subset of payload types indicated in the a=loopback-source line are
the payload types for the codecs the answerer is willing to send.
If an answerer wishes to accept the loopback request it MUST
include both the loopback mode and loopback type attributes in the
answer. When a stream is offered with the loopback-source
attribute, the corresponding stream in the response MUST be
loopback-mirror and vice versa, provided that answerer is capable
of supporting the requested loopback-type.
For example, if the offer contains the loopback-source attribute:
m=audio 41352 RTP/AVP 0 8
a=loopback:rtp-media-loopback
a=loopback-source:0 8
The answer that is capable of supporting the offer MUST contain the
loopback-mirror attribute:
m=audio 41352 RTP/AVP 0 8
a=loopback:rtp-media-loopback
a=loopback-mirror:0 8
If a stream is offered with multiple loopback type attributes, 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 example, if the offer contains:
m=audio 41352 RTP/AVP 0 8 112
a=loopback:rtp-media-loopback rtp-pkt-loopback
a=loopback-source:0 8
a=rtpmap:112 encaprtp/8000
The answer that is capable of supporting the offer and chooses to
loopback the media using the rtp-media-loopback type MUST contain:
m=audio 41352 RTP/AVP 0 8
a=loopback:rtp-media-loopback
a=loopback-mirror:0 8
Kaplan, et al. Expires August 2012 [Page 10]
Internet-Draft SDP Media Loopback March 2012
As specified in section 7, if the loopback-type is
rtp-pkt-loopback, either the encapsulated RTP payload format or
direct loopback RTP payload format MUST be used for looped back
packets.
For example, if the offer contains:
m=audio 41352 RTP/AVP 0 8 112 113
a=loopback:rtp-pkt-loopback
a=loopback-source:0 8
a=rtpmap:112 encaprtp/8000
a=rtpmap:113 rtploopback/8000
The answer that is capable of supporting the offer must contain one
of the following:
m=audio 41352 RTP/AVP 0 8 112
a=loopback:rtp-pkt-loopback
a=loopback-mirror:0 8
a=rtpmap:112 encaprtp/8000
m=audio 41352 RTP/AVP 0 8 113
a=loopback:rtp-pkt-loopback
a=loopback-mirror:0 8
a=rtpmap:113 rtploopback/8000
The previous examples used the 'encaprtp' and 'rtploopback'
encoding names, which will be defined in sections 7.1.3 and 7.2.3.
5.5 Offerer Processing of the Answer
If the received answer does not contain a=loopback-mirror or
a=loopback-source, it is assumed that the loopback extensions are
not supported by the remote agent. This is not a protocol failure,
and instead merely completes the SDP offer/answer exchange with
whatever normal rules apply; the offerer MAY decide to end the
established RTP session (if any) through normal means of the upper-
layer signaling protocol (e.g., by sending a SIP BYE).
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, as 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.
Kaplan, et al. Expires August 2012 [Page 11]
Internet-Draft SDP Media Loopback March 2012
5.7 Establishing Sessions Between Entities Behind NAT
ICE/STUN/TURN provide a general solution to establishing media
sessions between entities that are behind NATs. Loopback sessions
that involve one or more end points behind NATs SHOULD use these
general solutions wherever possible.
6. RTP Requirements
A loopback-mirror that is compliant to this specification and
accepting a media with rtp-pkt-loopback loopback-type MUST loopback
the incoming RTP packets using either the encapsulated RTP payload
format or the direct loopback RTP payload format as defined in
section 7 of this specification.
An answering entity that is compliant to this specification and
accepting a media with the loopback type rtp-media-loopback MUST
transmit all received media back to the sender, unless congestion
feedback or other lower-layer constraints prevent it from doing so.
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. Payload formats for Packet loopback
The payload formats described in this section MUST be used by a
loopback-mirror when rtp-pkt-loopback is the specified
loopback-type. Two different formats are specified here - an
encapsulated RTP payload format and a direct loopback RTP payload
format. The encapsulated RTP payload format should be used when
the incoming RTP header information needs to be preserved during
the loopback operation. This is useful in cases where loopback
source needs to measure performance metrics in both directions.
However, this comes at the expense of increased packet size as
described in section 7.1. The direct loopback RTP payload format
should be used when bandwidth requirement prevents the use of
encapsulated RTP payload format.
Kaplan, et al. Expires August 2012 [Page 12]
Internet-Draft SDP Media Loopback March 2012
To keep the implementation of loopback-mirrors simple it is
mandated that no payload format other than encapsulated or direct
loopback formats can be used in the packets generated by a
loopback-mirror. As described in RFC 3550 [RFC3550], sequence
numbers and timestamps in the RTP header are generated with initial
random values for security reasons. If this were not mandated and
the source payload is sequence number aware, the loopback-mirror
will be required to understand that payload format to generate
looped back packets that do not violate RFC 3550 [RFC3550].
Requiring looped back packets to be in one of the two formats means
loopback-mirror does not have to look into the actual payload
received before generating the loopback packets.
7.1 Encapsulated Payload format
A received RTP packet is encapsulated in the payload section of the
RTP packet generated by a loopback-mirror. Each received packet
MUST be encapsulated in a different packet, the encapsulated packet
MUST be fragmented only if required (for example: due to MTU
limitations).
7.1.1 Usage of RTP Header fields
Payload Type (PT): The assignment of an RTP payload type for this
packet format is outside the scope of this document; it is either
specified by the RTP profile under which this payload format is
used or more likely signaled dynamically out-of-band (e.g., using
SDP; section 7.1.3 defines the name binding).
Marker (M) bit: If the received RTP packet is looped back in
multiple RTP packets, the M bit is set to 1 in the last packet,
otherwise it is set to 0.
Extension (X) bit: Defined by the RTP Profile used.
Sequence Number: The RTP sequence number SHOULD be generated by the
loopback-mirror in the usual manner with a constant random offset
as described in RFC 3550 [RFC3550].
Timestamp: The RTP timestamp denotes the sampling instant for when
the loopback-mirror is transmitting this packet to the loopback-
source. The RTP timestamp MUST use the same clock rate used by the
loopback-source. The initial value of the timestamp SHOULD be
random for security reasons (see Section 5.1 of RFC 3550
[RFC3550]).
SSRC: set as described in RFC 3550 [RFC3550].
Kaplan, et al. Expires August 2012 [Page 13]
Internet-Draft SDP Media Loopback March 2012
CC and CSRC fields are used as described in RFC 3550 [RFC3550].
7.1.2 RTP Payload Structure
The RTP header in the encapsulated packet MUST be followed by the
payload header defined in this section. If the received RTP packet
has to be looped back in multiple packets due to fragmentation, the
RTP header in each packet MUST be followed by the payload header
defined in this section. The header is devised so that the
loopback-source can decode looped back packets in the presence of
moderate packet loss [RFC3550].
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| receive timestamp |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| F | R | CC |M| PT | sequence number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| transmit timestamp |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| synchronization source (SSRC) identifier |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| contributing source (CSRC) identifiers |
| .... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The 12 octets after the receive timestamp are identical to the RTP
header in the received packet except for the first 4 bits of the
first octet.
Receive Timestamp: 32 bits
The Receive timestamp denotes the sampling instant for when the
last octet of the received media packet that is being encapsulated
by the loopback-mirror is received from the loopback-source. The
Receive timestamp MUST be based on the same clock used by the
loopback-source. The initial value of the timestamp SHOULD be
random for security reasons (see Section 5.1 of RFC 3550
[RFC3550]).
Fragmentation (F): 2 bits
First Fragment (00) /Last Fragment (01) /No Fragmentation(10)/
Intermediate Fragment (11). This field identifies how much of the
received packet is encapsulated in this packet by the loopback-
mirror. If the received packet is not fragmented, this field is
Kaplan, et al. Expires August 2012 [Page 14]
Internet-Draft SDP Media Loopback March 2012
set to 10; otherwise the packet that contains the first fragments
sets this field to 00, the packet that contains the last fragment
sets this field to 01, all other packets set this field to 11.
Reserved: 2 bits
This field is reserved for future definition. In the absence of
such a definition, the bits in this field MUST be set to zero and
MUST be ignored by the receiver.
Any padding octets in the original packet MUST NOT be included in
the loopback packet generated by a loopback-mirror. The
loopback-mirror MAY add padding octets if required.
7.1.3 Usage of SDP
The payload type number for the encapsulated stream can be
negotiated using a mechanism like SDP. There is no static payload
type assignment for the encapsulated stream, so dynamic payload
type numbers MUST be used. The binding to the name is indicated by
an rtpmap attribute. The name used in this binding is "encaprtp".
The following is an example SDP fragment for encapsulated RTP.
m=audio 41352 RTP/AVP 112
a=rtpmap:112 encaprtp/8000
7.2 Direct loopback RTP payload format
The direct loopback RTP payload format can be used in scenarios
where the 16 byte overhead of the encapsulated payload format is
significant. This payload format MUST NOT be used in cases where
the MTU on the loopback path will cause fragmentation of looped
back RTP packets. When using this payload format, the receiver
MUST loop back each received packet in a separate RTP packet.
7.2.1 Usage of RTP Header fields
Payload Type (PT): The assignment of an RTP payload type for this
packet format is outside the scope of this document; it is either
specified by the RTP profile under which this payload format is
used or more likely signaled dynamically out-of-band (e.g., using
SDP; section 7.2.3 defines the name binding).
Marker (M) bit: Set to the value in the received packet.
Kaplan, et al. Expires August 2012 [Page 15]
Internet-Draft SDP Media Loopback March 2012
Extension (X) bit: Defined by the RTP Profile used.
Sequence Number: The RTP sequence number SHOULD be generated by the
loopback-mirror in the usual manner with a constant random offset.
Timestamp: The RTP timestamp denotes the sampling instant for when
the loopback-mirror is transmitting this packet to the
loopback-source. The RTP timestamp MUST be based on the same clock
used by the loopback-source. The initial value of the timestamp
SHOULD be random for security reasons (see Section 5.1 of RFC 3550
[RFC3550]).
SSRC: set as described in RFC 3550 [RFC3550].
CC and CSRC fields are used as described in RFC 3550 [RFC3550].
7.2.2 RTP Payload Structure
This payload format does not define any payload specific headers.
The loopback-mirror simply copies the payload data from the payload
portion of the packet received from the loopback-source.
7.2.3 Usage of SDP
The payload type number for the payload loopback stream can be
negotiated using a mechanism like SDP. There is no static payload
type assignment for the stream, so dynamic payload type numbers
MUST be used. The binding to the name is indicated by an rtpmap
attribute. The name used in this binding is "rtploopback".
The following is an example SDP fragment for direct loopback RTP
format.
m=audio 41352 RTP/AVP 112
a=rtpmap:112 rtploopback/8000
8. 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
Kaplan, et al. Expires August 2012 [Page 16]
Internet-Draft SDP Media Loopback March 2012
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].
9. Congestion Control
All the participants in a loopback session SHOULD implement
congestion control mechanisms as defined by the RTP profile under
which the loopback mechanism is implemented. For audio video
profiles, implementations SHOULD conform to the mechanism defined
in Section 2 of RFC 3551.
10. 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.
10.1 Offer for specific media loopback type
An agent sends an SDP offer which looks like:
v=0
o=alice 2890844526 2890842807 IN IP4 host.atlanta.example.com
s=Example
i=An example session
e=alice@example.com
c=IN IP4 host.atlanta.example.com
t=0 0
m=audio 49170 RTP/AVP 0
a=loopback:rtp-media-loopback
a=loopback-source:0
a=rtpmap:0 pcmu/8000
The agent is offering to source the media and expects the answering
agent to mirror the RTP stream per rtp-media-loopback loopback
type.
An answering agent sends an SDP answer which looks like:
v=0
o=bob 1234567890 1122334455 IN IP4 host.biloxi.example.com
Kaplan, et al. Expires August 2012 [Page 17]
Internet-Draft SDP Media Loopback March 2012
s=Example
i=An example session
e=bob@example.com
c=IN IP4 host.biloxi.example.com
t=0 0
m=audio 49270 RTP/AVP 0
a=loopback:rtp-media-loopback
a=loopback-mirror:0
a=rtpmap:0 pcmu/8000
The answerer is accepting to mirror the media from the offerer at
the media level.
10.2 Offer for choice of media loopback type
An agent sends an SDP offer which looks like:
v=0
o=alice 2890844526 2890842807 IN IP4 host.atlanta.example.com
s=Example
i=An example session
e=alice@example.com
c=IN IP4 host.atlanta.example.com
t=0 0
m=audio 49170 RTP/AVP 0 112 113
a=loopback:rtp-media-loopback rtp-pkt-loopback
a=loopback-source:0
a=rtpmap:0 pcmu/8000
a=rtpmap:112 encaprtp/8000
a=rtpmap:113 rtploopback/8000
The offerer is offering to source the media and expects the
answerer to mirror the RTP stream at either the media or rtp level.
An answering agent sends an SDP answer which looks like:
v=0
o=box 1234567890 1122334455 IN IP4 host.biloxi.example.com
s=Example
i=An example session
e=bob@example.com
c=IN IP4 host.biloxi.example.com
t=0 0
m=audio 49270 RTP/AVP 0 112
a=loopback:rtp-pkt-loopback
a=loopback-mirror:0
a=rtpmap:0 pcmu/8000
Kaplan, et al. Expires August 2012 [Page 18]
Internet-Draft SDP Media Loopback March 2012
a=rtpmap:112 encaprtp/8000
The answerer is accepting to mirror the media from the offerer at
the packet level using the encapsulated RTP payload format.
10.3 Answerer rejecting loopback media
An agent sends an SDP offer which looks like:
v=0
o=alice 2890844526 2890842807 IN IP4 host.atlanta.example.com
s=Example
i=An example session
e=user@example.com
c=IN IP4 host.atlanta.example.com
t=0 0
m=audio 49170 RTP/AVP 0
a=loopback:rtp-media-loopback
a=loopback-source:0
a=rtpmap:0 pcmu/8000
The offerer is offering to source the media and expects the
answerer to mirror the RTP stream at the media level.
An answering agent sends an SDP answer which looks like:
v=0
o=bob 1234567890 1122334455 IN IP4 host.biloxi.example.com
s=Example
i=An example session
e=user@example.com
c=IN IP4 host.biloxi.example.com
t=0 0
m=audio 0 RTP/AVP 0
a=loopback:rtp-media-loopback
a=loopback-mirror:0
a=rtpmap:0 pcmu/8000
11. Security Considerations
The security considerations of [RFC3264] 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.
Kaplan, et al. Expires August 2012 [Page 19]
Internet-Draft SDP Media Loopback March 2012
12. Implementation Considerations
The media loopback approach described in this document is a
complete solution that would work under all scenarios. However, it
is believed that the solution may not be light-weight enough for
the common case. In light of this concern, this section clarifies
which features of the loopback proposal MUST be implemented for all
implementations and which features MAY be deferred if the complete
solution is not desired.
All implementations MUST support the rtp-pkt-loopback option for
loopback-type attribute. In addition, for the loopback-mode
attribute, all implementations of an offerer MUST at a minimum be
able to act as a loopback-source. All implementation MUST also at a
minimum support the direct media loopback payload type. The rtp-
media-loopback attribute MAY be implemented in complete
implementations of this draft.
13. IANA Considerations
13.1 SDP Attributes
This document defines three new media-level SDP attributes. IANA
has registered the following attributes:
Contact name: Kaynam Hedayat
<kaynam.hedayat@exfo.com>.
Attribute name: "loopback".
Type of attribute: Media level.
Subject to charset: No.
Purpose of attribute: The 'loopback' attribute is used to
indicate the type of media loopback.
Allowed attribute values: The parameters to 'loopback' may be
one or more of "rtp-pkt-loopback" and
"rtp-media-loopback". See section 5
of this document for syntax.
Contact name: Kaynam Hedayat
<kaynam.hedayat@exfo.com>.
Attribute name: "loopback-source".
Type of attribute: Media level.
Subject to charset: No.
Purpose of attribute: The 'loopback-source' attribute
specifies that the sender is the media
Kaplan, et al. Expires August 2012 [Page 20]
Internet-Draft SDP Media Loopback March 2012
source and expects the receiver to act
as a loopback-mirror.
Allowed attribute values: The parameter to 'loopback-source' is
a media format ("<fmt>") description
as defined in RFC 4566 Section 5.14.
Contact name: Kaynam Hedayat
<kaynam.hedayat@exfo.com>.
Attribute name: "loopback-mirror".
Type of attribute: Media level.
Subject to charset: No.
Purpose of attribute: The 'loopback-mirror' attribute
specifies that the receiver will
mirror (echo) all received media back
to the sender of the RTP stream.
Allowed attribute values: The parameter to 'loopback-mirror' is
a media format ("<fmt>") description
as defined in RFC 4566 Section 5.14.
13.2 MIME Types
The IANA has registered the following MIME types:
13.2.1 audio/encaprtp
To: ietf-types@iana.org
Subject: Registration of media type audio/encaprtp
Type name: audio
Subtype name: encaprtp
Required parameters:
rate:RTP timestamp clock rate, which is equal to the
sampling rate. The typical rate is 8000; other rates
may be specified.
Optional parameters: none
Encoding considerations: This media type is framed
binary data.
Security considerations: See Section 12 of this document.
Kaplan, et al. Expires August 2012 [Page 21]
Internet-Draft SDP Media Loopback March 2012
Interoperability considerations: none
Published specification: This MIME type is described fully
within this document.
Applications which use this media type: Applications wishing
to monitor and ensure the quality of transport to the
edge of a given VoIP, Real-Time Text or Video Over IP
Service.
Additional information: none
Person & email address to contact for further information:
Kaynam Hedayat
EMail: kaynam.hedayat@exfo.com
Intended usage: COMMON
Restrictions on usage: This media type depends on RTP
framing, and hence is only defined for transfer via
RTP. Transfer within other framing protocols is not
defined at this time.
Author:
Kaynam Hedayat.
Change controller: IETF Audio/Video Transport working
group delegated from the IESG.
13.2.2 video/encaprtp
To: ietf-types@iana.org
Subject: Registration of media type video/encaprtp
Type name: video
Subtype name: encaprtp
Required parameters:
rate:RTP timestamp clock rate, which is equal to the
sampling rate. The typical rate is 8000; other rates
may be specified.
Optional parameters: none
Kaplan, et al. Expires August 2012 [Page 22]
Internet-Draft SDP Media Loopback March 2012
Encoding considerations: This media type is framed
binary data.
Security considerations: See Section 12 of this document.
Interoperability considerations: none
Published specification: This MIME type is described fully
within this document.
Applications which use this media type: Applications wishing
to monitor and ensure the quality of transport to the
edge of a given VoIP, Real-Time Text or Video Over IP
Service.
Additional information: none
Person & email address to contact for further information:
Kaynam Hedayat
EMail: kaynam.hedayat@exfo.com
Intended usage: COMMON
Restrictions on usage: This media type depends on RTP
framing, and hence is only defined for transfer via
RTP. Transfer within other framing protocols is not
defined at this time.
Author:
Kaynam Hedayat.
Change controller: IETF Audio/Video Transport working
group delegated from the IESG.
13.2.3 text/encaprtp
To: ietf-types@iana.org
Subject: Registration of media type text/encaprtp
Type name: text
Subtype name: encaprtp
Required parameters:
Kaplan, et al. Expires August 2012 [Page 23]
Internet-Draft SDP Media Loopback March 2012
rate:RTP timestamp clock rate, which is equal to the
sampling rate. The typical rate is 8000; other rates
may be specified.
Optional parameters: none
Encoding considerations: This media type is framed
binary data.
Security considerations: See Section 12 of this document.
Interoperability considerations: none
Published specification: This MIME type is described fully
within this document.
Applications which use this media type: Applications wishing
to monitor and ensure the quality of transport to the
edge of a given VoIP, Real-Time Text or Video Over IP
Service.
Additional information: none
Person & email address to contact for further information:
Kaynam Hedayat
EMail: kaynam.hedayat@exfo.com
Intended usage: COMMON
Restrictions on usage: This media type depends on RTP
framing, and hence is only defined for transfer via
RTP. Transfer within other framing protocols is not
defined at this time.
Author:
Kaynam Hedayat.
Change controller: IETF Audio/Video Transport working
group delegated from the IESG.
13.2.4 application/encaprtp
To: ietf-types@iana.org
Subject: Registration of media type
application/encaprtp
Kaplan, et al. Expires August 2012 [Page 24]
Internet-Draft SDP Media Loopback March 2012
Type name: application
Subtype name: encaprtp
Required parameters:
rate:RTP timestamp clock rate, which is equal to the
sampling rate. The typical rate is 8000; other rates
may be specified.
Optional parameters: none
Encoding considerations: This media type is framed
binary data.
Security considerations: See Section 12 of this document.
Interoperability considerations: none
Published specification: This MIME type is described fully
within this document.
Applications which use this media type: Applications wishing
to monitor and ensure the quality of transport to the
edge of a given VoIP, Real-Time Text or Video Over IP
Service.
Additional information: none
Person & email address to contact for further information:
Kaynam Hedayat
EMail: kaynam.hedayat@exfo.com
Intended usage: COMMON
Restrictions on usage: This media type depends on RTP
framing, and hence is only defined for transfer via
RTP. Transfer within other framing protocols is not
defined at this time.
Author:
Kaynam Hedayat.
Change controller: IETF Audio/Video Transport working
group delegated from the IESG.
13.2.5 audio/rtploopback
Kaplan, et al. Expires August 2012 [Page 25]
Internet-Draft SDP Media Loopback March 2012
To: ietf-types@iana.org
Subject: Registration of media type audio/rtploopback
Type name: audio
Subtype name: rtploopback
Required parameters:
rate:RTP timestamp clock rate, which is equal to the
sampling rate. The typical rate is 8000; other rates
may be specified.
Optional parameters: none
Encoding considerations: This media type is framed
binary data.
Security considerations: See Section 12 of this document.
Interoperability considerations: none
Published specification: This MIME type is described fully
within this document.
Applications which use this media type: Applications wishing
to monitor and ensure the quality of transport to the
edge of a given VoIP, Real-Time Text or Video Over IP
Service.
Additional information: none
Person & email address to contact for further information:
Kaynam Hedayat
EMail: kaynam.hedayat@exfo.com
Intended usage: COMMON
Restrictions on usage: This media type depends on RTP
framing, and hence is only defined for transfer via
RTP. Transfer within other framing protocols is not
defined at this time.
Author:
Kaynam Hedayat.
Change controller: IETF Audio/Video Transport working
Kaplan, et al. Expires August 2012 [Page 26]
Internet-Draft SDP Media Loopback March 2012
group delegated from the IESG.
13.2.6 video/rtploopback
To: ietf-types@iana.org
Subject: Registration of media type video/rtploopback
Type name: video
Subtype name: rtploopback
Required parameters:
rate:RTP timestamp clock rate, which is equal to the
sampling rate. The typical rate is 8000; other rates
may be specified.
Optional parameters: none
Encoding considerations: This media type is framed
binary data.
Security considerations: See Section 12 of this document.
Interoperability considerations: none
Published specification: This MIME type is described fully
within this document.
Applications which use this media type: Applications wishing
to monitor and ensure the quality of transport to the
edge of a given VoIP, Real-Time Text or Video Over IP
Service.
Additional information: none
Person & email address to contact for further information:
Kaynam Hedayat
EMail: kaynam.hedayat@exfo.com
Intended usage: COMMON
Restrictions on usage: This media type depends on RTP
framing, and hence is only defined for transfer via
RTP. Transfer within other framing protocols is not
defined at this time.
Kaplan, et al. Expires August 2012 [Page 27]
Internet-Draft SDP Media Loopback March 2012
Author:
Kaynam Hedayat.
Change controller: IETF Audio/Video Transport working
group delegated from the IESG.
13.2.7 text/rtploopback
To: ietf-types@iana.org
Subject: Registration of media type text/rtploopback
Type name: text
Subtype name: rtploopback
Required parameters:
rate:RTP timestamp clock rate, which is equal to the
sampling rate. The typical rate is 8000; other rates
may be specified.
Optional parameters: none
Encoding considerations: This media type is framed
binary data.
Security considerations: See Section 12 of this document.
Interoperability considerations: none
Published specification: This MIME type is described fully
within this document.
Applications which use this media type: Applications wishing
to monitor and ensure the quality of transport to the
edge of a given VoIP, Real-Time Text or Video Over IP
Service.
Additional information: none
Person & email address to contact for further information:
Kaynam Hedayat
EMail: kaynam.hedayat@exfo.com
Intended usage: COMMON
Kaplan, et al. Expires August 2012 [Page 28]
Internet-Draft SDP Media Loopback March 2012
Restrictions on usage: This media type depends on RTP
framing, and hence is only defined for transfer via
RTP. Transfer within other framing protocols is not
defined at this time.
Author:
Kaynam Hedayat.
Change controller: IETF Audio/Video Transport working
group delegated from the IESG.
13.2.8 application/rtploopback
To: ietf-types@iana.org
Subject: Registration of media type
application/rtploopback
Type name: application
Subtype name: rtploopback
Required parameters:
rate:RTP timestamp clock rate, which is equal to the
sampling rate. The typical rate is 8000; other rates
may be specified.
Optional parameters: none
Encoding considerations: This media type is framed
binary data.
Security considerations: See Section 12 of this document.
Interoperability considerations: none
Published specification: This MIME type is described fully
within this document.
Applications which use this media type: Applications wishing
to monitor and ensure the quality of transport to the
edge of a given VoIP, Real-Time Text or Video Over IP
Service.
Additional information: none
Person & email address to contact for further information:
Kaplan, et al. Expires August 2012 [Page 29]
Internet-Draft SDP Media Loopback March 2012
Kaynam Hedayat
EMail: kaynam.hedayat@exfo.com
Intended usage: COMMON
Restrictions on usage: This media type depends on RTP
framing, and hence is only defined for transfer via
RTP. Transfer within other framing protocols is not
defined at this time.
Author:
Kaynam Hedayat.
Change controller: IETF Audio/Video Transport working
group delegated from the IESG.
14.
Normative References
[RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer
Model with the Session Description Protocol (SDP)",
RFC 3264, June 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.
[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, November 2003.
[RFC5234] Crocker, P. Overell, "Augmented ABNF for Syntax
Specification: ABNF", RFC 5234, October 2005.
[RFC2119] Bradner, S.,"Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2736] Handley, M., Perkins, C., "Guidelines for Writers of
RTP Payload Format Specifications", RFC 2736, BCP
0036, December 1999.
Kaplan, et al. Expires August 2012 [Page 30]
Internet-Draft SDP Media Loopback March 2012
[RFC3551] Schulzrinne, H., Casner, S., "RTP Profile for Audio
and Video Conferences with Minimial Control", STD 65,
RFC 3551, July 2003.
[RFC4566] Handley, M., Jacobson, V., Perkins, C., "SDP: Session
Description Protocol", RFC 4566, July 2006.
[RFC4855] Casner, S., "Media Type Registration of RTP Payload
Formats", RFC 4855, February 2007.
Authors' Addresses
Hadriel Kaplan
Acme Packet
100 Crosby Drive
Bedford, MA 01730
USA
EMail: hkaplan@acmepacket.com
URI: http://www.acmepacket.com
Kaynam Hedayat
EXFO
285 Mill Road
Chelmsford, MA 01824
US
Phone: +1 978 367 5611
EMail: kaynam.hedayat@exfo.com
URI: http://www.exfo.com/
Nagarjuna Venna
Saperix
738 Main Street, #398
Waltham, MA 02451
US
Phone: +1 978 367 5703
EMail: vnagarjuna@saperix.com
URI: http://www.saperix.com/
Paul E. Jones
Kaplan, et al. Expires August 2012 [Page 31]
Internet-Draft SDP Media Loopback March 2012
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
Phone: +1 972 813 5224
EMail: chelliah@cisco.com
URI: http://www.cisco.com/
Nathan Stratton
BlinkMind, Inc.
2027 Briarchester Dr.
Katy, TX 77450
Phone: +1 832 330 3810
EMail: nathan@robotics.net
URI: http://www.robotics.net/
Kaplan, et al. Expires August 2012 [Page 32]