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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 26, 2012
An Extension to the Session Description Protocol (SDP)
and Real-time Transport Protocol (RTP) for Media Loopback
draft-ietf-mmusic-media-loopback-18
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
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This Internet-Draft will expire on August 12, 2012.
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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/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,
administrators 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
enable 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...................................................6
3. Overview of Operation.........................................6
3.1 SDP Offerer Behavior......................................6
3.2 SDP Answerer Behavior.....................................6
4. New SDP Attributes............................................7
4.1 Loopback Type Attribute...................................7
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4.2 Loopback Role Attributes: loopback-source and loopback-
mirror........................................................8
5. Rules for Generating the SDP offer/answer.....................9
5.1 Generating the SDP Offer for Loopback Session.............9
5.2 Generating the SDP Answer for Loopback Session...........10
5.3 Offerer Processing of the SDP Answer.....................11
5.4 Modifying the Session....................................12
5.5 Establishing Sessions Between Entities Behind NAT........12
6. RTP Requirements.............................................12
7. Payload formats for Packet loopback..........................13
7.1 Encapsulated Payload format..............................13
7.2 Direct loopback RTP payload format.......................16
8. RTCP Requirements............................................17
9. Congestion Control...........................................17
10. Examples....................................................18
10.1 Offer for specific media loopback type..................18
10.2 Offer for choice of media loopback type.................18
10.3 Answerer rejecting loopback media.......................19
11. Security Considerations.....................................20
12. Implementation Considerations...............................21
13. IANA Considerations.........................................21
13.1 SDP Attributes..........................................21
13.2 MIME Types..............................................22
14. Acknowledgements............................................31
15. Normative References........................................31
16. Informative References......................................32
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
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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 document introduces new SDP media
attributes that enable establishment of media sessions where the
media is looped back to the transmitter. The SDP 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, using the SDP offer/answer exchange.
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 RTP packet that
is specific to this use case and specified in 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 encapsulating
outer RTP header and the encapsulated RTP packet payload. The
loopback source can also play back the returned media content for
evaluation.
Because the encapsulating RTP packet header 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
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encapsulating payload format 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.
1.1.2 Direct Loopback
In the direct loopback case, the loopback mirror copies the payload
of the incoming RTP packet into a new RTP packet, using a payload
format specific to this use case and specified in 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 re-encoded according to an outgoing format determined by SDP
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.
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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.
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 signaling protocol for
carrying the SDP.
The following terms are borrowed from [RFC3264] definitions: offer,
offerer, answer, answerer, and agent.
3. Overview of Operation
This document defines two loopback 'types', two 'roles', and two
encoding formats for loopback. For any given SDP offerer or
answerer pair, one side is the source of RTP packets, while the
other is the mirror looping packets/media back. Those define the
two loopback roles. As the mirror, two 'types' of loopback can be
performed: packet-level or media-level. When media-level is used,
there is no further choice of encoding format - there is only one
format: whatever is indicated for normal media, since the "looping"
is performed at the codec level. When packet-level looping is
performed, however, the mirror can either send back RTP in an
encapsulated format or direct-loopback format. The rest of this
document describes these loopback types, roles, and encoding
formats, and the SDP offer/answer rules for indicating them.
3.1 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.
3.2 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
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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 this section.
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 (i.e., 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 attributes 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.
4. New SDP Attributes
Three new SDP media-level attributes are defined: one indicates the
type of loopback, and the other two define the role of the agent.
4.1 Loopback Type Attribute
This specification defines a new 'loopback' attribute, which
indicates that the agent wishes to perform loopback, and 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:" SP loopback-type
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
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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. This type of
loopback applies to the encapsulated and direct loopback use-cases
described in Section 1.1.
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. This type of loopback applies to the media loopback
use-case described in Section 1.1.3.
4.2 Loopback Role Attributes: loopback-source and loopback-mirror
The loopback role defines two value media attributes that are used
to indicate the role of the agent generating the SDP offer or
answer. The syntax of the two loopback role media attributes are as
follows:
a=loopback-source
and
a=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.
The "m=" line in the SDP MUST include all the payload types that
will be used during the loopback session. The complete payload
space for the session 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.
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5. Rules for Generating the SDP offer/answer
5.1 Generating the SDP Offer for Loopback Session
If an offerer wishes to make a loopback request, it MUST include
both the loopback-type and loopback-role attributes in a valid SDP
offer:
Example: m=audio 41352 RTP/AVP 0 8 100
a=loopback:rtp-media-loopback
a=loopback-source
a=rtpmap:0 pcmu/8000
a=rtpmap:8 pcma/8000
a=rtpmap:100 G7221/16000/1
Since media loopback requires bidirectional RTP, its normal
direction mode is "sendrecv"; the "sendrecv" direction attribute
MAY be encoded in SDP or not, as per section 5.1 of [RFC3264],
since it is implied by default. If either the loopback source or
mirror wish to disable loopback use during a session, the direction
mode attribute "inactive" MUST be used as per [RFC3264]. The
direction mode attributes "recvonly" and "sendonly" are
incompatible with the loopback mechanism and MUST NOT be indicated
when generating an SDP Offer or Answer. When receiving an SDP
Offer or Answer, if "recvonly" or "sendonly" is indicated for
loopback, the SDP-receiving agent SHOULD treat it as a protocol
failure of the loopback negotiation and terminate the session
through its normal means (e.g., by sending a SIP BYE if SIP is
used).
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 receive the media stream(s). The
payload type numbers indicate the value of the payload the offerer
expects to receive. However, the answer might indicate a subset of
payload type numbers from those given in the offer. In that case,
the offerer MUST only send the payload types received in the
answer, per normal SDP offer/answer rules.
If the offer indicates rtp-pkt-loopback support, the offer MUST
also contain either an encapsulated or direct loopback encoding
format encoding names, or both, as defined in later sections of
this document. If the offer only indicates rtp-media-loopback
support, then neither encapsulated nor direct loopback encoding
formats apply and they MUST NOT be in the offer.
If loopback-type is rtp-pkt-loopback, the loopback-mirror MUST send
and the loopback-source MUST receive the looped back packets
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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
a=rtpmap:112 encaprtp/8000
Example: m=audio 41352 RTP/AVP 0 8 112
a=loopback:rtp-pkt-loopback
a=loopback-source
a=rtpmap:112 rtploopback/8000
5.2 Generating the SDP Answer for Loopback Session
As with the offer, an SDP answer for loopback MUST follow SDP
offer/answer rules for the direction attribute, but directions of
"sendonly" or "recvonly" do not apply for loopback operation. \
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.
If an answerer wishes to accept the loopback request it MUST
include both the loopback role 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
The answer that is capable of supporting the offer MUST contain the
loopback-mirror attribute:
m=audio 12345 RTP/AVP 0 8
a=loopback:rtp-media-loopback
a=loopback-mirror
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.
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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
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 12345 RTP/AVP 0 8
a=loopback:rtp-media-loopback
a=loopback-mirror
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
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 12345 RTP/AVP 0 8 112
a=loopback:rtp-pkt-loopback
a=loopback-mirror
a=rtpmap:112 encaprtp/8000
m=audio 12345 RTP/AVP 0 8 113
a=loopback:rtp-pkt-loopback
a=loopback-mirror
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.
5.3 Offerer Processing of the SDP Answer
If the received SDP answer does not contain an a=loopback-mirror or
a=loopback-source attribute, it is assumed that the loopback
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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).
5.4 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.
5.5 Establishing Sessions Between Entities Behind NAT
ICE/STUN/TURN provide a general solution to establishing media
sessions between entities that are behind NATs, as defined in
[RFC5245]. Loopback sessions that involve one or more end points
behind NATs SHOULD use these general solutions wherever possible.
Furthermore, if the mirroring entity is behind a NAT, it MUST send
some packets to the identified address/port(s) of the peer, in
order to open the NAT pinhole. Using ICE this would be
accomplished with the STUN connectivity check process, or through a
TURN server connection. If ICE is not supported, either [RFC6263]
or Section 10 of ICE [RFC5245] SHOULD be followed to open the
pinhole and keep the NAT binding alive/refreshed.
Note that for any form of NAT traversal to function, symmetric
RTP/RTCP MUST be used. In other words both agents MUST send
packets from the same source address and port they receive packets
on.
6. RTP Requirements
A looback source MUST NOT send multiple source streams on the same
5-tuple, since there is no means for the mirror to indicate which
is which in its mirrored RTP packets.
A loopback mirror that is compliant to this specification and
accepts 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.
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A device that is compliant to this specification and performing the
mirroring using 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
mirroring entity MUST re-generate all the RTP header fields as it
would when transmitting media. The mirroring 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 mirroring
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.
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 separate encapsulating RTP packet; the
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encapsulated packet MUST be fragmented only if required (for
example: due to MTU limitations).
7.1.1Usage 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 encapsulating RTP packets, the M bit is set to 1 in every
fragment except 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 as that of
the encapsulated packet. 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].
7.1.2RTP Payload Structure
The outer RTP header of the encapsulating packet MUST be followed
by the payload header defined in this section. If the received RTP
packet has to be looped back in multiple encapsulating packets due
to fragmentation, the encapsulating 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].
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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
encapsulated RTP header of the received packet except for the first
2 bits of the first octet. In effect, the received RTP packet is
encapsulated by creating a new outer RTP header followed by 4 new
bytes of a receive timestamp, followed by the original received RTP
header and payload, except that the first two bits of the received
RTP header are overwritten as defined here.
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
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.
7.1.3Usage of SDP
The payload type number for the encapsulated stream can be
negotiated using SDP. There is no static payload type assignment
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for the encapsulating 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 of
concern, or simply due to local policy. When using this payload
format, the receiver MUST loop back each received RTP packet
payload (not header) in a separate RTP packet.
Because a direct loopback format does not retain the original RTP
headers, there will be no indication of the original payload-type
sent to the mirror, in looped returning packets. Therefore, the
loopback source SHOULD only send one payload type per loopback RTP
session, if direct mode is used.
7.2.1Usage of RTP Header fields
Payload Type (PT): The assignment of an RTP payload type for the
encapsulating 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.
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
as that of the received RTP packet. 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].
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7.2.2RTP Payload Structure
This payload format does not define any payload specific headers.
The loopback-mirror simply copies the RTP payload data from the
payload portion of the RTP packet received from the loopback-
source.
7.2.3Usage 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 agent that is
compliant to this specification SHOULD support RTCP per [RFC3550]
and RTCP-XR per RFC 3611 [RFC3611]. Furthermore, if the offerer or
answerer 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 offerer and the
answerer 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.
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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=-
c=IN IP4 host.atlanta.example.com
t=0 0
m=audio 49170 RTP/AVP 0
a=loopback:rtp-media-loopback
a=loopback-source
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
s=-
c=IN IP4 host.biloxi.example.com
t=0 0
m=audio 49270 RTP/AVP 0
a=loopback:rtp-media-loopback
a=loopback-mirror
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
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o=alice 2890844526 2890842807 IN IP4 host.atlanta.example.com
s=-
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
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=-
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
a=rtpmap:0 pcmu/8000
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=-
c=IN IP4 host.atlanta.example.com
t=0 0
m=audio 49170 RTP/AVP 0
a=loopback:rtp-media-loopback
a=loopback-source
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.
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An answering agent sends an SDP answer which looks like:
v=0
o=bob 1234567890 1122334455 IN IP4 host.biloxi.example.com
s=-
c=IN IP4 host.biloxi.example.com
t=0 0
m=audio 0 RTP/AVP 0
a=rtpmap:0 pcmu/8000
Note in this case the answerer did not indicate loopback support,
although it could have and still used a port number of 0 to
indicate it does not wish to accept that media session.
Alternatively, the answering agent could have simply rejected the
entire SDP offer through some higher-layer signaling protocol means
(e.g., by rejecting the SIP INVITE request if the SDP offer was in
the INVITE).
11. Security Considerations
The security considerations of [RFC3264] and [RFC3550] apply.
Given that media loopback may be automated without the end user's
knowledge, the answerer of the media loopback should be aware of
denial of service attacks. It is recommended that session requests
for media loopback be authenticated and the frequency of such
sessions limited by the answerer.
If the higher-layer signaling protocol were not authenticated, a
malicious attacker could create a session between two parties the
attacker wishes to target, with each party acting as the loopback-
mirror to the other, of rtp-pkt-loopback type. A few RTP packets
sent to either party would then infinitely loop among the two, as
fast as they could process them, consuming their resources and
network bandwidth.
Furthermore, media-loopback provides a means of attack indirection,
whereby a malicious attacker creates a loopback session as the
loopback-source, and uses the mirror to reflect the attacker's
packets against a target - perhaps a target the attacker could not
reach directly, such as one behind a firewall for example. Or the
attacker could initiate the session as the loopback-mirror, in the
hopes of making the peer generate media against another target.
If end-user devices such as mobile phones answer loopback requests
without authentication and without notifying the end-user, then an
attacker could cause the battery to drain, and possibly deny the
end-user normal phone service or cause network data usage fees.
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This could even occur naturally if a legitimate loopback session
does not terminate properly and the end device does not have a
timeout mechanism for such.
For the reasons noted above, end user devices SHOULD provide a
means of indicating to the human user that the device is in a
loopback session, even if it is an authenticated session. Devices
which answer or generate loopback sessions SHOULD either perform
keepalive/refresh tests of the session state through some means, or
time out the session automatically.
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 at least support the rtp-pkt-loopback mode
for loopback-type, with direct media loopback payload encoding. In
addition, for the loopback role, all implementations of an SDP
offerer MUST at least be able to act as a loopback-source.
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.
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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
source and expects the receiver to act
as a loopback-mirror.
Allowed attribute values: None.
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: None.
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.
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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.
13.2.2 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.
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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.
13.2.3 text/encaprtp
To: ietf-types@iana.org
Subject: Registration of media type text/encaprtp
Type name: text
Subtype name: encaprtp
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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.
13.2.4 application/encaprtp
To: ietf-types@iana.org
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Subject: Registration of media type
application/encaprtp
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.
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13.2.5 audio/rtploopback
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:
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Kaynam Hedayat.
Change controller: IETF Audio/Video Transport working
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
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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.
13.2.7 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
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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.
13.2.8 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.
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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.
14. Acknowledgements
This document's editor would like to thank the original authors of
the document: Kaynam Hedayat, et al. The editor has made fairly
insignificant changes in the end. Also, we'd like to thank Magnus
Westerlund, Miguel Garcia, Flemming Andreason, Gunnar Hellstrom,
Emil Ivov and Dan Wing for their feedback, comments and
suggestions.
15. 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.
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[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.
[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.
16. Informative References
[RFC5245] Rosenberg, J., "Interactive Connectivity
Establishment (ICE): A Protocol for Network Address
Translator (NAT) Traversal for Offer/Answer
Protocols", RFC 5245, April 2010.
[RFC6263] Marjou, X., Sollaud, A., "Application Mechanism for
Keeping Alive the NAT Mappings Associated with RTP /
RTP Control Protocol (RTCP) Flows", RFC 6263, June
2011.
Authors' Addresses
Hadriel Kaplan
Acme Packet
100 Crosby Drive
Bedford, MA 01730
USA
EMail: hkaplan@acmepacket.com
URI: http://www.acmepacket.com
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Kaynam Hedayat
EXFO
285 Mill Road
Chelmsford, MA 01824
US
EMail: kaynam.hedayat@exfo.com
URI: http://www.exfo.com/
Nagarjuna Venna
Saperix
738 Main Street, #398
Waltham, MA 02451
US
EMail: vnagarjuna@saperix.com
URI: http://www.saperix.com/
Paul E. Jones
Cisco Systems, Inc.
7025 Kit Creek Rd.
Research Triangle Park, NC 27709
US
EMail: paulej@packetizer.com
URI: http://www.cisco.com/
Arjun Roychowdhury
Hughes Systique Corp.
15245 Shady Grove Rd, Ste 330
Rockville MD 20850
US
EMail: arjun@hsc.com
URI: http://www. hsc.com/
Chelliah SivaChelvan
Cisco Systems, Inc.
2200 East President George Bush Turnpike
Richardson, TX 75082
US
EMail: chelliah@cisco.com
URI: http://www.cisco.com/
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Nathan Stratton
BlinkMind, Inc.
2027 Briarchester Dr.
Katy, TX 77450
EMail: nathan@robotics.net
URI: http://www.robotics.net/
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