wdiff rfc7941v1.txt rfc7941.txt

Internet Engineering Task Force (IETF) M. Westerlund
Request for Comments: 7941 B. Burman
Category: Standards Track Ericsson
ISSN: 2070-1721 R. Even
Huawei Technologies
M. Zanaty
Cisco Systems
July
August 2016

RTP Header Extension for the RTP Control Protocol (RTCP)
Source Description Items

Abstract

Source Description (SDES) items are normally transported in the RTP
Control Protocol (RTCP). In some cases, it can be beneficial to
speed up the delivery of these items. Mainly, The main case is when a new
sychronization source (SSRC) joins an RTP session and the receivers
need this source's identity, relation to other sources, or its
synchronization context, all of which may be fully or partially
identified using SDES items. To enable this optimization, this
document specifies a new RTP header extension that can carry SDES
items.

Status of This Memo

This is an Internet Standards Track document.

This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.

Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc7941.

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
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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.

Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.1. Requirements Language . . . . . . . . . . . . . . . . . . 3
2.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
3. Motivation . . . . . . . . . . . . . . . . . . . . . . . . . 4
4. Specification . . . . . . . . . . . . . . . . . . . . . . . . 5
4.1. SDES Item Header Extension . . . . . . . . . . . . . . . 5
4.1.1. One-Byte Format . . . . . . . . . . . . . . . . . . . 5
4.1.2. Two-Byte Format . . . . . . . . . . . . . . . . . . . 6
4.2. Usage of the SDES Item Header Extension . . . . . . . . . 6
4.2.1. One-Byte or Two-Byte Headers . . . . . . . . . . . . 6
4.2.2. MTU and Packet Expansion . . . . . . . . . . . . . . 7
4.2.3. Transmission Considerations . . . . . . . . . . . . . 7
4.2.4. Different Usages . . . . . . . . . . . . . . . . . . 9
4.2.5. SDES Items in RTCP . . . . . . . . . . . . . . . . . 9
4.2.6. Update Flaps . . . . . . . . . . . . . . . . . . . . 10
4.2.7. RTP Header Compression . . . . . . . . . . . . . . . 10
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11
5.1. Registration of an SDES Base URN . . . . . . . . . . . . 11
5.2. Creation of the "RTP SDES Compact Header Extensions" Sub-
Registry . . . . . . . . . . . . . . . . . . . . . . . . 11
5.3. Registration of SDES Item . . . . . . . . . . . . . . . . 12
6. Security Considerations . . . . . . . . . . . . . . . . . . . 12
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 13
7.1. Normative References . . . . . . . . . . . . . . . . . . 13
7.2. Informative References . . . . . . . . . . . . . . . . . 14
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 15
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 16

1. Introduction

This specification defines an RTP header extension [RFC3550][RFC5285]
that can carry RTCP source description Source Description (SDES) items. Normally, the
SDES items are carried in their own RTCP packet type [RFC3550]. By
including selected SDES items in a header extension, the
determination of relationship and synchronization context for new RTP
streams (SSRCs) in an RTP session can be optimized. Which
relationship and what information depends on the SDES items carried.
This becomes a complement to using only RTCP for SDES item delivery.

It is important to note that not all SDES items are appropriate to
transmit using RTP header extensions. Some SDES items perform
binding or identify synchronization contexts with strict timeliness
requirements, while many other SDES items do not have such
requirements. In addition, security and privacy concerns for the
SDES item information need to be considered. For example, the Name
and Location SDES items are highly sensitive from a privacy
perspective and should not be transported over the network without
strong security. No use case has identified where this that such information is
required at the same time as when the first RTP packets arrive. A delay of a few seconds
before such information is available to the receiver and appears
acceptable. Therefore, only appropriate SDES items, such as CNAME,
will be registered for use with this header extension.

Requirements language and terminology are defined in Section 2. The
next section motivates
Section 3 describes why this header extension is sometimes required
or at least provides a significant improvement compared to waiting
for regular RTCP packet transmissions of the information.
This is followed by Section 4
provides a specification of the header extension and usage
recommendations. Next, Section 5 defines a subspace of the header
extension URN is
defined to be used for existing and future SDES items, items and then
registers the appropriate existing SDES items are registered. items.

2. Definitions

2.1. Requirements Language

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 [RFC2119].

2.2. Terminology

This document uses terminology defined in "A Taxonomy of Semantics
and Mechanisms for Real-Time Transport Protocol (RTP) Sources"
[RFC7656]. In particular, the following terms are used:

Media Source

RTP Stream

Media Encoder

Participant

3. Motivation

SDES items are associated with a particular SSRC and thus with a
particular RTP stream. The source description Source Description items provide various
metadata associated with the SSRC. How important it is to have this
data no later than when the first RTP packets is received depends on
the item itself. The CNAME item is one item that is commonly needed
either at reception of the first RTP packet for this SSRC or at least
by the time the first media can be played out. If it is not
available, the synchronization context cannot be determined; thus,
any related streams cannot be correctly synchronized. Therefore,
this is a valuable example for having this information early when a
new RTP stream is received.

The main reason for new SSRCs in an RTP session is when media sources
are added. This can be because either an endpoint is adding a new
actual media source or additional participants in a multi-party
session are added to the session. Another reason for a new SSRC can
be an SSRC collision that forces both colliding parties to select new
SSRCs.

For the case of rapid media synchronization, one may use the RTP
header extension for rapid synchronization of RTP flows [RFC6051].
This header extension carries the clock information present in the
RTCP sender report (SR) packets. However, it assumes that the CNAME
binding is known, which can be provided via signaling [RFC5576] in
some cases, but not all. Thus, an RTP header extension for carrying
SDES items like CNAME is a powerful combination to enable rapid
synchronization in all cases.

The "Rapid Synchronisation of RTP Flows" specification [RFC6051] does
provide an analysis of the initial synchronization delay for
different sessions depending on the number of receivers as well as on
session bandwidth (Section 2.1 of [RFC6051]). These results are also
applicable for other SDES items that have a similar time dependency
until the information can be sent using RTCP. These figures can be
used to determine the benefit of reducing the initial delay before
information is available for some use cases.

[RFC6051] also discusses the case of late joiners and defines an RTCP
Feedback format to request synchronization information, which is
another potential use case for SDES items in the RTP header
extension. It would, for example, be natural to include a CNAME SDES
item with the header extension containing the NTP-formatted reference
clock to ensure synchronization.

The ongoing work on bundling Session Description Protocol (SDP) media
descriptions [SDP-BUNDLE] has identified a new SDES item that can
benefit from timely delivery. A corresponding RTP SDES compact
header extension is therefore also defined and registered in that
document:

MID: This is a media description identifier that matches the value
of the SDP [RFC4566] a=mid attribute [RFC5888], to associate RTP
streams multiplexed on the same transport with their respective
SDP media description.

4. Specification

This section first specifies the SDES item RTP header extension
format, followed by some usage considerations.

4.1. SDES Item Header Extension

An RTP header extension scheme allowing for multiple extensions is
defined in "A General Mechanism for RTP Header Extensions" [RFC5285].
That specification defines both short and long item headers. The
short headers (one byte) are restricted to 1 to 16 bytes of data,
while the long format (two bytes) supports a data length of 0 to 255
bytes. Thus, the RTP header extension formats are capable of
supporting any SDES item from a data length perspective.

The ID field, independent of a short or long format, identifies both
the type of RTP header extension and, in the case of the SDES item
header extension, the type of SDES item. The mapping is done in
signaling by identifying the header extension and SDES item type
using a URN, which is defined in Section 5 ("IANA Considerations")
for the known SDES items appropriate to use.

4.1.1. One-Byte Format

The one-byte header format for an SDES item extension element
consists of the one-byte header (defined in Section 4.2 of
[RFC5285]), which consists of a 4-bit ID followed by a 4-bit length
field (len) that identifies the number of data bytes (len value +1)
following the header. The data part consists of len+1 bytes of UTF-8
[RFC3629] text. The type of text and its mapping to the SDES item
type are determined by the ID field value.

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ID | len | SDES item text value ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 1

4.1.2. Two-Byte Format

The two-byte header format for an SDES item extension element
consists of the two-byte header (defined in Section 4.3 of
[RFC5285]), which consists of an 8-bit ID followed by an 8-bit length
field (len) that identifies the number of data bytes following the
header. The data part consists of len bytes of UTF-8 text. The type
of text and its mapping to the SDES item type is are determined by the
ID field value.

Figure 2

4.2. Usage of the SDES Item Header Extension

This section discusses various usage considerations: which form of
the header extension to use, the packet expansion, and when to send
SDES items in the header extension.

4.2.1. One-Byte or Two-Byte Headers

The RTP header extensions for SDES items MAY use either the one-byte
or two-byte header formats, depending on the text value size for the
used SDES items and the requirement from any other header extensions
used. The one-byte header SHOULD be used when all non-SDES item
header extensions support the one-byte format and all SDES item text
values contain at most 16 bytes. Note that the RTP header extension
specification [RFC5285] does not allow mixing one-byte and two-byte
headers for the same RTP stream (SSRC), so if the value size of any of the SDES
items value item requires
the two-byte header, then all other header extensions MUST also use
the two-byte header format.

For example, if using CNAMEs that are generated according to
"Guidelines for Choosing RTP Control Protocol (RTCP) Canonical Names
(CNAMEs)" [RFC7022], if using short-term persistent values, and if
96-bit random values prior to base64 encoding are sufficient, then
they will fit into the one-byte header format.

An RTP middlebox needs to take care choosing between one-byte headers
and two-byte headers when creating the first packets for an outgoing
stream (SSRC) with header extensions. First of all, it needs to
consider all the header extensions that may potentially be used.
Second, it needs to know the size of the SDES items that are going to
be included and use two-byte headers if any are longer than 16 bytes.
An RTP middlebox that forwards a stream, i.e., not mixing it or
combining it with other streams, may be able to base its choice on
the header size in incoming streams. This is assuming that the
middlebox does not modify the stream or add additional header
extensions to the stream it sends, in which case it needs to make its
own decision.

4.2.2. MTU and Packet Expansion

The RTP packet size will clearly increase when a header extension is
included. How much depends on the type of header extensions and
their data content. The SDES items can vary in size. There are also
some use cases that require transmitting multiple SDES items in the
same packet to ensure that all relevant data reaches the receiver.
An example of that is when CNAME, a MID, and the rapid time
synchronization extension from RFC 6051 are all needed. Such a
combination is quite likely to result in at least 16+3+8 bytes of
data plus the headers, which will be another 7 bytes for one-byte
headers, plus two bytes of header padding to make the complete header
extension 32-bit word aligned, thus 36 bytes in total.

If the packet expansion cannot be taken into account when producing
the RTP payload, it can cause an issue. An RTP payload that is
created to meet a particular IP-level Maximum Transmission Unit
(MTU), taking the addition of IP/UDP/RTP headers but not RTP header
extensions into account, could exceed the MTU when the header
extensions are present, thus resulting in IP fragmentation. IP
fragmentation is known to negatively impact the loss rate due to
middleboxes unwilling or not capable of dealing with IP fragments, as
well as increasing the target surface for other types of packet
losses.

As this is a real issue, the media encoder and payload packetizer
should be flexible and be capable of handling dynamically varying
payload size restrictions to counter the packet expansion caused by
header extensions. If that is not possible, some reasonable worst-
case packet expansion should be calculated and used to reduce the RTP
payload size of all RTP packets the sender transmits.

4.2.3. Transmission Considerations

The general recommendation is to only send header extensions when
needed. This is especially true for SDES items that can be sent in
periodic repetitions of RTCP throughout the whole session. Thus, the
different usages (Section 4.2.4) have different recommendations. The
following are some general considerations for getting the header
extensions delivered to the receiver:

1. The probability for packet loss and burst loss determine how many
repetitions of the header extensions will be required to reach a
targeted delivery probability and, if burst loss is likely, what
distribution would be needed to avoid getting all repetitions of
the header extensions lost in a single burst.

2. If a set of packets are all needed to enable decoding, there is
commonly no reason for including the header extension in all of
these packets, as they share fate. Instead, at most one instance
of the header extension per independently decodable set of media
data would be a more efficient use of the bandwidth.

3. How early the SDES item information is needed, from the first
received RTP data or only after some set of packets are received,
can guide if the header extension(s) should be in all of the
first N packets or be included only once per set of packets, for
example, once per video frame.

4. The use of RTP-level robustness mechanisms, such as RTP
retransmission [RFC4588] or forward error correction [RFC5109],
may treat packets differently from a robustness perspective, and
SDES header extensions should be added to packets that get a
treatment corresponding to the relative importance of receiving
the information.

As a summary, the number of header extension transmissions should be
tailored to a desired probability of delivery, taking the receiver
population size into account. For the very basic case, N repetitions
of the header extensions should be sufficient but may not be optimal.
N is selected so that the header extension target delivery
probability reaches 1-P^N, where P is the probability of packet loss.
For point-to-point or small receiver populations, it might also be
possible to use feedback, such as RTCP, to determine when the
information in the header extensions has reached all receivers and to
stop further repetitions. Feedback that can be used includes the
RTCP Extended Report (XR) Loss RLE Report Block [RFC3611], which
indicates successful delivery of particular packets. If the RTP/AVPF
transport-layer feedback message for generic NACK [RFC4585] is used,
it can indicate the failure to deliver an RTP packet with the header
extension, thus indicating the need for further repetitions. The
normal RTCP report blocks can also provide an indicator of successful
delivery, if no losses are indicated for a reporting interval
covering the RTP packets with the header extension. Note that loss
of an RTCP packet reporting on an interval where RTP header extension
packets were sent does not necessarily mean that the RTP header
extension packets themselves were lost.

4.2.4. Different Usages

4.2.4.1. New SSRC

A new SSRC joins an RTP session. As this SSRC is completely new for
everyone, the goal is to ensure, with high probability, that all RTP
session participants receive the information in the header extension.
Thus, header extension transmission strategies that allow some
margins in the delivery probability should be considered.

4.2.4.2. Late Joiner

In a multi-party RTP session where one or a small number of receivers
join a session where the majority of receivers already have all
necessary information, the use of header extensions to deliver
relevant information should be tailored to reach the new receivers.
The trigger to send header extensions can, for example, be either
RTCP from a new receiver(s) or an explicit request like the Rapid
Resynchronization Request defined in [RFC6051]. In centralized
topologies where an RTP middlebox is present, it can be responsible
for transmitting the known information, possibly stored, to the new
session participant only and not repeat it to all the session
participants.

4.2.4.3. Information Change

If the SDES information is tightly coupled with the RTP data, and the
SDES information needs to be updated, then the use of the RTP header
extension is superior to RTCP. Using the RTP header extension
ensures that the information is updated on reception of the related
RTP media, ensuring synchronization between the two. Continued use
of the old SDES information can lead to undesired effects in the
application. Thus, header extension transmission strategies with
high probability of delivery should be chosen.

4.2.5. SDES Items in RTCP

The RTP header extension information, i.e., SDES items, can and will
be sent also in RTCP. Therefore, it is worth making some reflections
on this interaction. As an alternative to the header extension, it
is possible to schedule a non-regular RTCP packet transmission
containing important SDES items, if one uses an RTP-/AVPF-based RTP
profile. Depending on which the mode in which one's RTCP feedback
transmitter is
working on, working, extra RTCP packets may be sent as immediate
or early packets, enabling more timely SDES information delivery.

There are, however, two aspects that differ between using RTP header
extensions and any non-regular transmission of RTCP packets. First,
as the RTCP packet is a separate packet, there is no direct relation
and also no fate sharing between the relevant media data and the SDES
information. The order of arrival for the packets will matter. With
a header extension, the SDES items can be ensured to arrive if the
media data to play out arrives. Second, it is difficult to determine
if an RTCP packet is actually delivered. This, delivered, as the RTCP packets lack
both a sequence number and a mechanism providing feedback on the RTCP
packets themselves.

4.2.6. Update Flaps

The SDES item may arrive both in RTCP and in RTP header extensions,
potentially causing the value to flap back and forth at the time of
updating. There are at least two reasons for these flaps. The first
one is packet reordering, where a pre-update RTP or RTCP packet with
an SDES item is delivered to the receiver after the first RTP/RTCP
packet with the updated value. The second reason is the different
code paths for RTP and RTCP in implementations. An update to the
sender's SDES item parameter can take a different time to propagate
to the receiver than the corresponding media data. For example, an
RTCP packet with the SDES item included that may have been generated
prior to the update can still reside in a buffer and be sent
unmodified. The update of the item's value can, at the same time,
cause RTP packets to be sent including the header extension, prior to
the RTCP packet being sent.

However, most of these issues can be avoided by the receiver
performing some checks before updating the receiver's stored value.
To handle flaps caused by reordering, SDES items received in RTP
packets with the same or a lower extended sequence number than the
last change MUST NOT be applied, i.e., discard items that can be
determined to be older than the current one. For compound RTCP
packets, which will contain an SR packet (assuming an active RTP
sender), the receiver can use the RTCP SR timestamp field to
determine at what approximate time it was transmitted. If the
timestamp is earlier than the last received RTP packet with a header
extension carrying an SDES item, and especially if carrying a
previously used value, the SDES item in the RTCP SDES packet can be
ignored. Note that media processing and transmission pacing can
easily cause the RTP header timestamp field as well as the RTCP SR
timestamp field to not match with the actual transmission time.

4.2.7. RTP Header Compression

When Robust Header Compression (ROHC) [RFC5225] is used with RTP, the
RTP header extension [RFC5285] data itself is not part of what is
being compressed and thus does not impact header compression
performance. The extension indicator (X) bit in the RTP header is,
however, compressed. It is classified as rarely changing, which may
no longer be true for all RTP header extension usage, in turn leading
to lower header compression efficiency.

5. IANA Considerations

This section details the following updates made by IANA:

o Creation of a new sub-registry reserved for RTCP SDES items with
the URN subspace "urn:ietf:params:rtp-hdrext:sdes:" in the "RTP
Compact Header Extensions" registry.

o Registration of the SDES items appropriate for use with the RTP
header extension defined in this document.

5.1. Registration of an SDES Base URN

IANA has registered the following entry in the "RTP Compact Header
Extensions" registry:

Extension URI: urn:ietf:params:rtp-hdrext:sdes
Description: Reserved as base URN for RTCP SDES items that are also
defined as RTP compact header extensions.
Contact: Authors of RFC 7941
Reference: RFC 7941

The reason to register a base URN for an SDES subspace is that the
name represents an RTCP Source Description item, for which a
specification is strongly recommended [RFC3550].

5.2. Creation of the "RTP SDES Compact Header Extensions" Sub-Registry

IANA has created a sub-registry to the "RTP Compact Header
Extensions" registry, with the same basic requirements, structure,
and layout as the "RTP Compact Header Extensions" registry.

o Registry name: RTP SDES Compact Header Extensions

o Specification: RFC 7941 and RFCs updating RFC 7941

o Information required: Same as for the "RTP Compact Header
Extensions" registry [RFC5285]

o Review process: Same as for the "RTP Compact Header Extensions"
registry [RFC5285], with the following requirements added to the
Expert Review [RFC5226]:

1. Any registration using an extension URI that starts with
"urn:ietf:params:rtp-hdrext:sdes:" (Section 5.1) MUST also
have a registered Source Description item in the "RTP SDES
item types" registry.

2. Security and privacy considerations for the SDES item MUST be
provided with the registration.

3. Information MUST be provided on why this SDES item requires
timely delivery, motivating it to be transported in a header
extension rather than as RTCP only.

o Size and format of entries: Same as for the "RTP Compact Header
Extensions" registry [RFC5285].

o Initial assignments: See Section 5.3 of this document.

5.3. Registration of SDES Item

IANA has registered the following SDES item in the newly formed "RTP
SDES Compact Header Extensions" registry:

Extension URI: urn:ietf:params:rtp-hdrext:sdes:cname
Description: Source Description: Canonical End-Point Identifier
(SDES CNAME)
Contact: Authors of RFC 7941
Reference: RFC 7941

6. Security Considerations

Source Description items may contain data that are sensitive from a
security perspective. There are SDES items that are or may be
sensitive from a user privacy perspective, like CNAME, NAME, EMAIL,
PHONE, LOC, and H323-CADDR. Some may contain sensitive information,
like NOTE and PRIV, while others may be sensitive from profiling
implementations for vulnerability or other reasons, like TOOL. The
CNAME sensitivity can vary depending on how it is generated and what
persistence it has. A short-term CNAME identifier generated using a
random number generator [RFC7022] may have minimal security
implications, while a CNAME of the form user@host has privacy
concerns, and a CNAME generated from a Media Access Control (MAC)
address has long-term tracking potentials.

In RTP sessions where any type of confidentiality protection is
enabled for RTCP, the SDES item header extensions MUST also be
protected. This implies that to provide confidentiality, users of
the Secure Real-time Transport Protocol (SRTP) need to implement and
use encrypted header extensions per [RFC6904]. SDES items carried as
RTP header extensions MUST then use commensurate strength algorithms
and SHOULD use the same cryptographic primitives (algorithms, modes)
as applied to RTCP packets carrying corresponding SDES items. If the
security level is chosen to be different for an SDES item in RTCP and
an RTP header extension, it is important to motivate justify the exception and
to consider the security properties as the worst in each aspect for
the different configurations. It is worth noting that the current
SRTP [RFC3711] only provides protection for the next trusted RTP/RTCP
hop, which is not necessarily end to end.

The general RTP header extension mechanism [RFC5285] does not itself
contain any functionality that is a significant risk for a
denial-of-service attack, neither from processing nor from storage
requirements. The extension for SDES items defined in this document
can potentially be a risk. The risk depends on the received SDES
item and its content. If the SDES item causes the receiver to
perform a large amount of processing, create significant storage
structures, or emit network traffic, such a risk does exist. The
CNAME SDES item in the RTP header extension is only a minor risk, as
reception of a CNAME item will create an association between the
stream carrying the SDES item and other RTP streams with the same
SDES item. This usually results in time synchronizing the media
streams; thus, some additional processing is performed. However, the
application's media quality is likely more affected by an erroneous
or changing association and media synchronization than the
application quality impact caused by the additional processing.

As the SDES items are used by the RTP-based application to establish
relationships between RTP streams or between an RTP stream and
information about the originating participant, there SHOULD be strong
integrity protection and source authentication of the header
extensions. If not, an attacker can modify the SDES item value to
create erroneous relationship bindings in the receiving application.
For information regarding options for securing RTP, see [RFC7201].

7. References

7.1. Normative References

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.

[RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V.
Jacobson, "RTP: A Transport Protocol for Real-Time
Applications", STD 64, RFC 3550, DOI 10.17487/RFC3550,
July 2003, <http://www.rfc-editor.org/info/rfc3550>.

[RFC5285] Singer, D. and H. Desineni, "A General Mechanism for RTP
Header Extensions", RFC 5285, DOI 10.17487/RFC5285, July
2008, <http://www.rfc-editor.org/info/rfc5285>.

[RFC6904] Lennox, J., "Encryption of Header Extensions in the Secure
Real-time Transport Protocol (SRTP)", RFC 6904,
DOI 10.17487/RFC6904, April 2013,
<http://www.rfc-editor.org/info/rfc6904>.

7.2. Informative References

[RFC3611] Friedman, T., Ed., Caceres, R., Ed., and A. Clark, Ed.,
"RTP Control Protocol Extended Reports (RTCP XR)",
RFC 3611, DOI 10.17487/RFC3611, November 2003,
<http://www.rfc-editor.org/info/rfc3611>.

[RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO
10646", STD 63, RFC 3629, DOI 10.17487/RFC3629, November
2003, <http://www.rfc-editor.org/info/rfc3629>.

[RFC3711] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
Norrman, "The Secure Real-time Transport Protocol (SRTP)",
RFC 3711, DOI 10.17487/RFC3711, March 2004,
<http://www.rfc-editor.org/info/rfc3711>.

[RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
Description Protocol", RFC 4566, DOI 10.17487/RFC4566,
July 2006, <http://www.rfc-editor.org/info/rfc4566>.

[RFC4585] Ott, J., Wenger, S., Sato, N., Burmeister, C., and J. Rey,
"Extended RTP Profile for Real-time Transport Control
Protocol (RTCP)-Based Feedback (RTP/AVPF)", RFC 4585,
DOI 10.17487/RFC4585, July 2006,
<http://www.rfc-editor.org/info/rfc4585>.

[RFC4588] Rey, J., Leon, D., Miyazaki, A., Varsa, V., and R.
Hakenberg, "RTP Retransmission Payload Format", RFC 4588,
DOI 10.17487/RFC4588, July 2006,
<http://www.rfc-editor.org/info/rfc4588>.

[RFC5109] Li, A., Ed., "RTP Payload Format for Generic Forward Error
Correction", RFC 5109, DOI 10.17487/RFC5109, December
2007, <http://www.rfc-editor.org/info/rfc5109>.

[RFC5225] Pelletier, G. and K. Sandlund, "RObust Header Compression
Version 2 (ROHCv2): Profiles for RTP, UDP, IP, ESP and
UDP-Lite", RFC 5225, DOI 10.17487/RFC5225, April 2008,
<http://www.rfc-editor.org/info/rfc5225>.

[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226,
DOI 10.17487/RFC5226, May 2008,
<http://www.rfc-editor.org/info/rfc5226>.

[RFC5576] Lennox, J., Ott, J., and T. Schierl, "Source-Specific
Media Attributes in the Session Description Protocol
(SDP)", RFC 5576, DOI 10.17487/RFC5576, June 2009,
<http://www.rfc-editor.org/info/rfc5576>.

[RFC5888] Camarillo, G. and H. Schulzrinne, "The Session Description
Protocol (SDP) Grouping Framework", RFC 5888,
DOI 10.17487/RFC5888, June 2010,
<http://www.rfc-editor.org/info/rfc5888>.

[RFC6051] Perkins, C. and T. Schierl, "Rapid Synchronisation of RTP
Flows", RFC 6051, DOI 10.17487/RFC6051, November 2010,
<http://www.rfc-editor.org/info/rfc6051>.

[RFC7022] Begen, A., Perkins, C., Wing, D., and E. Rescorla,
"Guidelines for Choosing RTP Control Protocol (RTCP)
Canonical Names (CNAMEs)", RFC 7022, DOI 10.17487/RFC7022,
September 2013, <http://www.rfc-editor.org/info/rfc7022>.

[RFC7201] Westerlund, M. and C. Perkins, "Options for Securing RTP
Sessions", RFC 7201, DOI 10.17487/RFC7201, April 2014,
<http://www.rfc-editor.org/info/rfc7201>.

[RFC7656] Lennox, J., Gross, K., Nandakumar, S., Salgueiro, G., and
B. Burman, Ed., "A Taxonomy of Semantics and Mechanisms
for Real-Time Transport Protocol (RTP) Sources", RFC 7656,
DOI 10.17487/RFC7656, November 2015,
<http://www.rfc-editor.org/info/rfc7656>.

[SDP-BUNDLE]
Holmberg, C., Alvestrand, H., and C. Jennings,
"Negotiating Media Multiplexing Using the Session
Description Protocol (SDP)", Work in Progress, draft-ietf-
mmusic-sdp-bundle-negotiation-31, June
mmusic-sdp-bundle-negotiation-32, August 2016.

Acknowledgments

The authors would like to thank the following individuals for
feedback and suggestions: Colin Perkins, Ben Campbell, and Samuel
Weiler.

Authors' Addresses

Magnus Westerlund
Ericsson
Farogatan 6
SE-164 80 Stockholm
Sweden

Phone: +46 10 714 82 87
Email: magnus.westerlund@ericsson.com

Bo Burman
Ericsson
Gronlandsgatan 31
Stockholm 16480
Sweden

Email: bo.burman@ericsson.com

Roni Even
Huawei Technologies
Tel Aviv
Israel

Email: roni.even@mail01.huawei.com

Mo Zanaty
Cisco Systems
7100 Kit Creek
RTP, NC 27709
United States of America

Email: mzanaty@cisco.com