Network Working Group M. Westerlund
Internet-Draft B. Burman
Intended status: Standards Track Ericsson
Expires: November 12, 2015 R. Even
Huawei Technologies
M. Zanaty
Cisco Systems
May 11, 2015
RTP Header Extension for RTCP Source Description Items
draft-ietf-avtext-sdes-hdr-ext-01
Abstract
Source Description (SDES) items are normally transported in RTP
control protocol (RTCP). In some cases it can be beneficial to speed
up the delivery of these items. Mainly when a new source (SSRC)
joins an RTP session and the receivers needs 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
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This Internet-Draft will expire on November 12, 2015.
Copyright Notice
Copyright (c) 2015 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
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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 or Two Byte Headers . . . . . . . . . . . . . . . 6
4.2.2. MTU and Packet Expansion . . . . . . . . . . . . . . 6
4.2.3. Transmission Considerations . . . . . . . . . . . . . 7
4.2.4. Different Usages . . . . . . . . . . . . . . . . . . 8
4.2.5. SDES Items in RTCP . . . . . . . . . . . . . . . . . 9
4.2.6. Update Flaps . . . . . . . . . . . . . . . . . . . . 10
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
5.1. Reservation of the SDES URN sub-space . . . . . . . . . . 10
5.2. Registration of SDES Items . . . . . . . . . . . . . . . 11
6. Security Considerations . . . . . . . . . . . . . . . . . . . 11
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 12
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 12
8.1. Normative References . . . . . . . . . . . . . . . . . . 12
8.2. Informative References . . . . . . . . . . . . . . . . . 13
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 13
1. Introduction
This specification defines an RTP header extension [RFC3550][RFC5285]
that can carry RTCP source description (SDES) items. By including
selected SDES items in an header extension the determination of
relationship and synchronization context for new RTP streams (SSRCs)
in an RTP session can be speeded up. 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 performs
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binding or identifies synchronization context 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 needs 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 information
is required at the same time as the first RTP packets arrive. A few
seconds delay before such information is available to the receiver
appears acceptable. Therefore only appropriate SDES items will be
registered for use with this header extension, such as CNAME.
First, some requirements language and terminology is defined. The
following section motivates 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 a specification of the header extension and usage
recommendations. Next, a sub-space of the header-extension URN is
defined to be used for existing and future SDES items, and then the
appropriate SDES items are registered.
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 Grouping
Semantics and Mechanisms for Real-Time Transport Protocol (RTP)
Sources" [I-D.ietf-avtext-rtp-grouping-taxonomy]. In particular the
following definitions:
Media Source
RTP Stream
Media Encoder
Encoded Stream
Participant
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3. Motivation
Source Description (SDES) items are associated with a particular SSRC
and thus RTP stream. The source description items provide various
meta data associated with the SSRC. How important it is to have this
data no later than when receiving the first RTP packets depends on
the item itself. The CNAME item is one item that is commonly needed
if not at reception of the first RTP packet for this SSRC, then at
least by the time the first media can be played out. If not, the
synchronization context cannot be determined and thus any related
streams cannot be correctly synchronized. Thus, 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 either because an end-point 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. It however assumes that the CNAME
binding is known, which can be provided via signaling 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 Synchronization of RTP Flows specification does provide an
analysis of the initial synchronization delay for different sessions
depending on number of receivers as well as on session bandwidth
(Section 2.1 of [RFC6051]). These results are applicable also for
other SDES items that have a similar time dependency until the
information can be sent using RTCP. Thus the benefit of reducing the
initial delay before information is available can be determined for
some use cases from these figures.
That document 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 RTP header extension.
It would for example be natural to include CNAME SDES item with the
header extension containing the NTP formatted reference clock to
ensure synchronization.
There is an additional, newly defined SDES item that can benefit from
timely delivery, and an RTP header extension SDES item is therefore
defined for it:
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MID: This is a media description identifier that matches the value
of the SDP a=mid attribute, to associate RTP streams multiplexed
on the same transport with their respective SDP media description
as described in [I-D.ietf-mmusic-sdp-bundle-negotiation].
4. Specification
This section first specifies the SDES item RTP header extension
format, followed by some usage considerations.
4.1. SDES Item Header Extension
The RTP header extension scheme that allows for multiple extensions
to be included 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-byte) 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 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 the IANA consideration (Section 5)
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 how many bytes (len value +1) of data
following the header. The data part consists of len+1 bytes of UTF-8
text. The type of text is determined by the ID field value and its
mapping to the type of SDES item.
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
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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 how many bytes of data that follows the
header. The data part consists of len bytes of UTF-8 text. The type
of text is determined by the ID field value and its mapping to the
type of SDES item.
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 2
4.2. Usage of the SDES Item Header Extension
This section discusses various usage considerations; which form of
header extension to use, the packet expansion, and when to send SDES
items in header extension.
4.2.1. One 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 supports the one-byte format and all SDES item text
values contain at most 16 bytes. Note that the RTP header extension
specification 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 requires the two-byte header, the all other header
extensions MUST also use the two-byte header format.
For example using CNAMEs that are generated according to "Guidelines
for Choosing RTP Control Protocol (RTCP) Canonical Names (CNAMEs)"
[RFC7022], 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.
4.2.2. MTU and Packet Expansion
The RTP packet size will clearly increase when it includes the header
extension. How much depends on which header extensions and their
data parts. The SDES items can vary in size. There are also some
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use-cases which require transmitting multiple SDES items in the same
packet to ensure that all relevant data reaches the receiver. An
example of that is when you need both the CNAME, a MID, and the rapid
time synchronization extension from RFC 6051. 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 padding headers to make the complete header extension word
aligned, thus in total 36 bytes.
The packet expansion can cause an issue when it cannot be taken into
account when producing the RTP payload. 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.
First 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
dispersion would be needed to avoid getting multiple 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.
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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, e.g.,
[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.
In 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
stop further repetitions. Feedback that can be used includes the
RTCP XR Loss RLE report block [RFC3611], which will indicate
succesful delivery of particular packets. If the RTP/AVPF Transport
Layer Feedback Messages 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 succesful
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
receivers receives the information in the header extension. Thus,
header extension transmission strategies that allow some margins in
the delivery probability should be considered.
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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 either be RTCP
from 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
In cases when the SDES item text value is changed and the new SDES
information is tightly coupled to and thus needs to be synchronized
with a related change in the RTP stream, use of a header extension is
far superior to RTCP SDES. In this case it is equal or even more
important with timely SDES information than in the case of new SSRCs
(Section 4.2.4.1). 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 extensions information, i.e. SDES Items, can and will
be sent also in RTCP. Therefore, it is worth some reflections on
this interaction. An alternative to the header extension is the
possibility to schedule a non-regular RTCP packet transmission
containing important SDES items, if one uses a RTP/AVPF based RTP
profile. Depending on which mode one's RTCP feedback transmitter is
working on, extra RTCP packets may be sent as immediate or early
packets, enabling more timely delivery of SDES information.
There is 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. Secondly, it is difficult to
determine if an RTCP packet is actually delivered. This, as the RTCP
packets lack both sequence number or a mechanism providing feedback
on the RTCP packets themselves.
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4.2.6. Update Flaps
The SDES item may arrive both in RTCP and in RTP header extensions,
this can cause 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
senders SDES item parameter, can take different time to propagate.
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 performing some
checks before updating the receiver's stored value. To handle flaps
caused by reordering, only SDES items received in RTP packets with a
higher extended sequence number than the last change shall 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
Sender Report (SR) packet (assuming an active RTP sender), the
receiver can compare the RTCP Sender Report's Timestamp field, to
determine at what approximate time it was transmitted. If the
timestamp is earlier than the last received RTP packet 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
only lously couple with the actual transmission time.
5. IANA Considerations
This section makes the following requests to IANA:
o Register and reserve for SDES items the URN sub-space
"urn:ietf:params:rtp-hdrext:sdes:" in the RTP Compact Header
Extensions registry.
o Register the SDES items appropriate for use with the RTP header
extension defined in this document.
5.1. Reservation of the SDES URN sub-space
The reason to require registering a URN within an SDES sub-space is
that the name represents an RTCP Source Description item, where a
specification is strongly recommended. The formal policy is
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maintained from the main space, i.e. Expert Review. However, some
additional considerations are provided here that needs to be
considered when applying for a registration within this sub-space of
the RTP Compact Header Extensions registry.
Any registration using an Extension URI that starts with
"urn:ietf:params:rtp-hdrext:sdes:" MUST also have a registered Source
Description item in the "RTP SDES item types" registry. Secondly, a
security and privacy consideration for the SDES item must be provided
with the registration, preferably in a publicly available reference.
Thirdly, information must be provided on why this SDES item requires
timely delivery, motivating it to be transported in an header
extension rather than as RTCP only.
IANA is requested to register the below in the RTP Compact Header
Extensions:
Extension URI: urn:ietf:params:rtp-hdrext:sdes
Description: Reserved as base URN for SDES items that are also
defined as RTP Compact header extensions.
Contact: Authors of [RFCXXXX]
Reference: [RFCXXXX]
RFC-editor note: Please replace all occurances of RFCXXXX with the
RFC number this specification receives when published.
5.2. Registration of SDES Items
It is requested that the following SDES item is registered in the RTP
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 [RFCXXXX]
Reference: [RFCXXXX]
We also note that the MID SDES item is already registered in the
registry by [I-D.ietf-mmusic-sdp-bundle-negotiation].
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
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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 MAC address has long term
tracking potentials.
The above security concerns may have to be put in relation to third
party monitoring needs. In RTP sessions where any type of
confidentiality protection is enabled, the SDES item header
extensions SHOULD also be protected per default. This implies that
to provide confidentiality, users of SRTP need to implement encrypted
header extensions per [RFC6904]. Commonly, it is expected that the
same security level is applied to RTCP packets carrying SDES items,
and to an RTP header extension containing SDES items. If the
security level is different, it is important to consider the security
properties as the worst in each aspect for the different
configurations.
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
requirements on integrity 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.
7. Acknowledgements
The authors likes to thanks the following individuals for feedback
and suggestions; Colin Perkins.
8. References
8.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V.
Jacobson, "RTP: A Transport Protocol for Real-Time
Applications", STD 64, RFC 3550, July 2003.
[RFC5285] Singer, D. and H. Desineni, "A General Mechanism for RTP
Header Extensions", RFC 5285, July 2008.
[RFC6904] Lennox, J., "Encryption of Header Extensions in the Secure
Real-time Transport Protocol (SRTP)", RFC 6904, April
2013.
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8.2. Informative References
[I-D.ietf-avtext-rtp-grouping-taxonomy]
Lennox, J., Gross, K., Nandakumar, S., Salgueiro, G., and
B. Burman, "A Taxonomy of Grouping Semantics and
Mechanisms for Real-Time Transport Protocol (RTP)
Sources", draft-ietf-avtext-rtp-grouping-taxonomy-06 (work
in progress), March 2015.
[I-D.ietf-mmusic-sdp-bundle-negotiation]
Holmberg, C., Alvestrand, H., and C. Jennings,
"Negotiating Media Multiplexing Using the Session
Description Protocol (SDP)", draft-ietf-mmusic-sdp-bundle-
negotiation-19 (work in progress), March 2015.
[RFC3611] Friedman, T., Caceres, R., and A. Clark, "RTP Control
Protocol Extended Reports (RTCP XR)", RFC 3611, November
2003.
[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, July
2006.
[RFC4588] Rey, J., Leon, D., Miyazaki, A., Varsa, V., and R.
Hakenberg, "RTP Retransmission Payload Format", RFC 4588,
July 2006.
[RFC5109] Li, A., "RTP Payload Format for Generic Forward Error
Correction", RFC 5109, December 2007.
[RFC6051] Perkins, C. and T. Schierl, "Rapid Synchronisation of RTP
Flows", RFC 6051, November 2010.
[RFC7022] Begen, A., Perkins, C., Wing, D., and E. Rescorla,
"Guidelines for Choosing RTP Control Protocol (RTCP)
Canonical Names (CNAMEs)", RFC 7022, September 2013.
Authors' Addresses
Magnus Westerlund
Ericsson
Farogatan 6
SE-164 80 Stockholm
Sweden
Phone: +46 10 714 82 87
Email: magnus.westerlund@ericsson.com
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Bo Burman
Ericsson
Kistavagen 25
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
USA
Email: mzanaty@cisco.com
Westerlund, et al. Expires November 12, 2015 [Page 14]