One document matched: draft-ietf-avtext-sdes-hdr-ext-01.xml
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<?rfc tocindent="yes"?>
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<rfc category="std" docName="draft-ietf-avtext-sdes-hdr-ext-01"
ipr="trust200902">
<front>
<title abbrev="RTP HE for RTCP SDES">RTP Header Extension for RTCP Source
Description Items</title>
<author fullname="Magnus Westerlund" initials="M." surname="Westerlund">
<organization>Ericsson</organization>
<address>
<postal>
<street>Farogatan 6</street>
<city>SE-164 80 Stockholm</city>
<country>Sweden</country>
</postal>
<phone>+46 10 714 82 87</phone>
<email>magnus.westerlund@ericsson.com</email>
</address>
</author>
<author fullname="Bo Burman" initials="B." surname="Burman">
<organization>Ericsson</organization>
<address>
<postal>
<street>Kistavagen 25</street>
<city>Stockholm</city>
<code>16480</code>
<country>Sweden</country>
</postal>
<email>bo.burman@ericsson.com</email>
</address>
</author>
<author fullname="Roni Even" initials="R." surname="Even">
<organization>Huawei Technologies</organization>
<address>
<postal>
<street></street>
<city>Tel Aviv</city>
<region></region>
<code></code>
<country>Israel</country>
</postal>
<phone></phone>
<facsimile></facsimile>
<email>roni.even@mail01.huawei.com</email>
<uri></uri>
</address>
</author>
<author fullname="Mo Zanaty" initials="M." surname="Zanaty">
<organization>Cisco Systems</organization>
<address>
<postal>
<street>7100 Kit Creek</street>
<city>RTP</city>
<region>NC</region>
<code>27709</code>
<country>USA</country>
</postal>
<phone></phone>
<facsimile></facsimile>
<email>mzanaty@cisco.com</email>
<uri></uri>
</address>
</author>
<date day="11" month="May" year="2015" />
<abstract>
<t>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.</t>
</abstract>
</front>
<middle>
<section title="Introduction">
<t>This specification defines an <xref target="RFC3550">RTP header
extension</xref><xref target="RFC5285"></xref> 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.</t>
<t>It is important to note that not all SDES items are appropriate to
transmit using RTP header extensions. Some SDES items performs 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.</t>
<t>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.</t>
</section>
<section title="Definitions">
<t></t>
<section title="Requirements Language">
<t>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 <xref
target="RFC2119">RFC 2119</xref>.</t>
</section>
<section title="Terminology">
<t>This document uses terminology defined in <xref
target="I-D.ietf-avtext-rtp-grouping-taxonomy">"A Taxonomy of Grouping
Semantics and Mechanisms for Real-Time Transport Protocol (RTP)
Sources"</xref>. In particular the following definitions:<list
style="empty">
<t>Media Source</t>
<t>RTP Stream</t>
<t>Media Encoder</t>
<t>Encoded Stream</t>
<t>Participant</t>
</list></t>
<t></t>
</section>
</section>
<section title="Motivation">
<t>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.</t>
<t>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.</t>
<t>For the case of rapid media synchronization, one may use the RTP
header extension for <xref target="RFC6051">Rapid Synchronization of RTP
Flows</xref>. 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.</t>
<t>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 <xref target="RFC6051"></xref>). 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.</t>
<t>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.</t>
<t>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:<list style="hanging">
<t hangText="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 <xref
target="I-D.ietf-mmusic-sdp-bundle-negotiation"></xref>.</t>
</list></t>
<t></t>
</section>
<section title="Specification">
<t>This section first specifies the SDES item RTP header extension
format, followed by some usage considerations.</t>
<section title="SDES Item Header Extension">
<t>The RTP header extension scheme that allows for multiple extensions
to be included is defined in <xref target="RFC5285">"A General
Mechanism for RTP Header Extensions"</xref>. 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.</t>
<t>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 <xref target="IANA">IANA
consideration</xref> for the known SDES items appropriate to use.</t>
<section title="One-Byte Format">
<t>The one-byte header format for an SDES item extension element
consists of the One-Byte header (defined in Section 4.2 of <xref
target="RFC5285"></xref>), 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.</t>
<figure anchor="fig-short-header">
<artwork><![CDATA[
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 ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
]]></artwork>
</figure>
</section>
<section title="Two-Byte Format">
<t>The two-byte header format for an SDES item extension element
consists of the two-byte header (defined in Section 4.3 of <xref
target="RFC5285"></xref>), 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.</t>
<figure anchor="fig-long-header">
<artwork><![CDATA[
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 ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
]]></artwork>
</figure>
<t></t>
</section>
</section>
<section title="Usage of the SDES Item Header Extension">
<t>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.</t>
<section title="One or Two Byte Headers">
<t>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.</t>
<t>For example using CNAMEs that are generated according to <xref
target="RFC7022">"Guidelines for Choosing RTP Control Protocol
(RTCP) Canonical Names (CNAMEs)"</xref>, 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.</t>
</section>
<section title="MTU and Packet Expansion">
<t>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 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.</t>
<t>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.</t>
<t>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.</t>
</section>
<section title="Transmission Considerations">
<t>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
<xref target="sec-different-usages">different usages</xref> have
different recommendations. First some general considerations for
getting the header extensions delivered to the receiver:<list
style="numbers">
<t>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.</t>
<t>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.</t>
<t>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.</t>
<t>The use of RTP level robustness mechanisms, such as <xref
target="RFC4588">RTP retransmission</xref>, or Forward Error
Correction, e.g., <xref target="RFC5109"> </xref> 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.</t>
</list></t>
<t>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
<xref target="RFC3611">RTCP XR Loss RLE report block</xref>, which
will indicate succesful delivery of particular packets. If the
RTP/AVPF <xref target="RFC4585">Transport Layer Feedback Messages
for generic NACK</xref> 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.</t>
</section>
<section anchor="sec-different-usages" title="Different Usages">
<t></t>
<section anchor="sec-new-ssrc" title="New SSRC">
<t>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.</t>
</section>
<section title="Late Joiner">
<t>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 <xref
target="RFC6051"></xref>. 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.</t>
</section>
<section title="Information Change">
<t>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 <xref target="sec-new-ssrc">new SSRCs</xref>.
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.</t>
</section>
</section>
<section title="SDES Items in RTCP">
<t>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.</t>
<t>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.</t>
</section>
<section title="Update Flaps">
<t>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.</t>
<t>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.</t>
</section>
</section>
</section>
<section anchor="IANA" title="IANA Considerations">
<t>This section makes the following requests to IANA:<list
style="symbols">
<t>Register and reserve for SDES items the URN sub-space
"urn:ietf:params:rtp-hdrext:sdes:" in the RTP Compact Header
Extensions registry.</t>
<t>Register the SDES items appropriate for use with the RTP header
extension defined in this document.</t>
</list></t>
<t></t>
<section title="Reservation of the SDES URN sub-space">
<t>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 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.</t>
<t>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.</t>
<t>IANA is requested to register the below in the RTP Compact Header
Extensions:</t>
<figure>
<artwork><![CDATA[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]
]]></artwork>
</figure>
<t>RFC-editor note: Please replace all occurances of RFCXXXX with the
RFC number this specification receives when published.</t>
</section>
<section title="Registration of SDES Items">
<t>It is requested that the following SDES item is registered in the
RTP Compact Header Extensions registry:</t>
<figure>
<artwork><![CDATA[Extension URI: urn:ietf:params:rtp-hdrext:sdes:cname
Description: Source Description: Canonical End-Point Identifier
(SDES CNAME)
Contact: Authors of [RFCXXXX]
Reference: [RFCXXXX]
]]></artwork>
</figure>
<t>We also note that the MID SDES item is already registered in the
registry by <xref
target="I-D.ietf-mmusic-sdp-bundle-negotiation"></xref>.</t>
</section>
</section>
<section anchor="Security" title="Security Considerations">
<t>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
<xref target="RFC7022">short term CNAME identifier generated using a
random number generator</xref> 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.</t>
<t>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 <xref target="RFC6904"></xref>. 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.</t>
<t>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.</t>
</section>
<section anchor="Acknowledgements" title="Acknowledgements">
<t>The authors likes to thanks the following individuals for feedback
and suggestions; Colin Perkins.</t>
</section>
</middle>
<back>
<references title="Normative References">
<?rfc include="reference.RFC.2119"?>
<?rfc include='reference.RFC.3550'?>
<?rfc include='reference.RFC.5285'?>
<?rfc include='reference.RFC.6904'?>
</references>
<references title="Informative References">
<?rfc include='reference.RFC.3611'?>
<?rfc include='reference.RFC.4585'?>
<?rfc include='reference.RFC.4588'?>
<?rfc include='reference.RFC.5109'?>
<?rfc include='reference.RFC.6051'?>
<?rfc include='reference.RFC.7022'?>
<?rfc include='reference.I-D.ietf-mmusic-sdp-bundle-negotiation'?>
<?rfc include='reference.I-D.ietf-avtext-rtp-grouping-taxonomy'?>
</references>
</back>
</rfc>
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