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Internet Engineering Task Force (IETF)                     M. WesterlundRequest for Comments: 7941                                     B. BurmanCategory: Standards Track                                       EricssonISSN: 2070-1721                                                  R. Even                                                     Huawei Technologies                                                               M. Zanaty                                                           Cisco Systems                                                             August 2016RTP Header Extension forthe RTP Control Protocol (RTCP) Source Description ItemsAbstract   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.  The main case is when a new   synchronization 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 inSection 2 of RFC 7841.   Information about the current status of this document, any errata,   and how to provide feedback on it may be obtained athttp://www.rfc-editor.org/info/rfc7941.Westerlund, et al.           Standards Track                    [Page 1]

RFC 7941                  RTP HE for RTCP SDES               August 2016Copyright Notice   Copyright (c) 2016 IETF Trust and the persons identified as the   document authors.  All rights reserved.   This document is subject toBCP 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.Table of Contents1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .32.  Definitions . . . . . . . . . . . . . . . . . . . . . . . . .32.1.  Requirements Language . . . . . . . . . . . . . . . . . .32.2.  Terminology . . . . . . . . . . . . . . . . . . . . . . .43.  Motivation  . . . . . . . . . . . . . . . . . . . . . . . . .44.  Specification . . . . . . . . . . . . . . . . . . . . . . . .54.1.  SDES Item Header Extension  . . . . . . . . . . . . . . .54.1.1.  One-Byte Format . . . . . . . . . . . . . . . . . . .64.1.2.  Two-Byte Format . . . . . . . . . . . . . . . . . . .64.2.  Usage of the SDES Item Header Extension . . . . . . . . .64.2.1.  One-Byte or Two-Byte Headers  . . . . . . . . . . . .64.2.2.  MTU and Packet Expansion  . . . . . . . . . . . . . .74.2.3.  Transmission Considerations . . . . . . . . . . . . .84.2.4.  Different Usages  . . . . . . . . . . . . . . . . . .94.2.5.  SDES Items in RTCP  . . . . . . . . . . . . . . . . .104.2.6.  Update Flaps  . . . . . . . . . . . . . . . . . . . .104.2.7.  RTP Header Compression  . . . . . . . . . . . . . . .115.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .115.1.  Registration of an SDES Base URN  . . . . . . . . . . . .11     5.2.  Creation of the "RTP SDES Compact Header Extensions"           Sub-Registry  . . . . . . . . . . . . . . . . . . . . . .125.3.  Registration of SDES Item . . . . . . . . . . . . . . . .126.  Security Considerations . . . . . . . . . . . . . . . . . . .137.  References  . . . . . . . . . . . . . . . . . . . . . . . . .147.1.  Normative References  . . . . . . . . . . . . . . . . . .147.2.  Informative References  . . . . . . . . . . . . . . . . .14   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .17   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .17Westerlund, et al.           Standards Track                    [Page 2]

RFC 7941                  RTP HE for RTCP SDES               August 20161.  Introduction   This specification defines an RTP header extension [RFC3550][RFC5285]   that can carry RTCP 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 that such information is   required when the first RTP packets arrive.  A delay of a few seconds   before such information is available to the receiver 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 inSection 2.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.Section 4   provides a specification of the header extension and usage   recommendations.Section 5 defines a subspace of the header   extension URN to be used for existing and future SDES items and   registers the appropriate existing SDES items.2.  Definitions2.1.  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 inRFC 2119 [RFC2119].Westerlund, et al.           Standards Track                    [Page 3]

RFC 7941                  RTP HE for RTCP SDES               August 20162.2.  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      Participant3.  Motivation   SDES items are associated with a particular SSRC and thus with a   particular RTP stream.  The 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 alsoWesterlund, et al.           Standards Track                    [Page 4]

RFC 7941                  RTP HE for RTCP SDES               August 2016   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 inSection 5 ("IANA Considerations")   for the known SDES items appropriate to use.Westerlund, et al.           Standards Track                    [Page 5]

RFC 7941                  RTP HE for RTCP SDES               August 20164.1.1.  One-Byte Format   The one-byte header format for an SDES item extension element   consists of the one-byte header (defined inSection 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 14.1.2.  Two-Byte Format   The two-byte header format for an SDES item extension element   consists of the two-byte header (defined inSection 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 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 24.2.  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 itemWesterlund, et al.           Standards Track                    [Page 6]

RFC 7941                  RTP HE for RTCP SDES               August 2016   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 any SDES 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 fromRFC 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, asWesterlund, et al.           Standards Track                    [Page 7]

RFC 7941                  RTP HE for RTCP SDES               August 2016   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.Westerlund, et al.           Standards Track                    [Page 8]

RFC 7941                  RTP HE for RTCP SDES               August 2016   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 Usages4.2.4.1.  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 relatedWesterlund, et al.           Standards Track                    [Page 9]

RFC 7941                  RTP HE for RTCP SDES               August 2016   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 the mode in which one's RTCP feedback   transmitter is 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, 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 theWesterlund, et al.           Standards Track                   [Page 10]

RFC 7941                  RTP HE for RTCP SDES               August 2016   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 ofRFC 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].Westerlund, et al.           Standards Track                   [Page 11]

RFC 7941                  RTP HE for RTCP SDES               August 20165.2.  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   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: SeeSection 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 ofRFC 7941   Reference:RFC 7941Westerlund, et al.           Standards Track                   [Page 12]

RFC 7941                  RTP HE for RTCP SDES               August 20166.  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 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.Westerlund, et al.           Standards Track                   [Page 13]

RFC 7941                  RTP HE for RTCP SDES               August 2016   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.  References7.1.  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>.Westerlund, et al.           Standards Track                   [Page 14]

RFC 7941                  RTP HE for RTCP SDES               August 2016   [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>.Westerlund, et al.           Standards Track                   [Page 15]

RFC 7941                  RTP HE for RTCP SDES               August 2016   [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-32, August 2016.Westerlund, et al.           Standards Track                   [Page 16]

RFC 7941                  RTP HE for RTCP SDES               August 2016Acknowledgments   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.comWesterlund, et al.           Standards Track                   [Page 17]