| 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
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.
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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 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.
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].
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:
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:
This section first specifies the SDES item RTP header extension format, followed by some usage considerations.
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 [IANA] for the known SDES items appropriate to use.
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
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
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.
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.
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.
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.
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 [sec-different-usages] have different recommendations. First some general considerations for getting the header extensions delivered to the receiver:
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.
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.
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.
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 [sec-new-ssrc]. 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.
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.
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.
This section makes the following requests to IANA:
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.
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.
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].
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 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.
The authors likes to thanks the following individuals for feedback and suggestions; Colin Perkins.
| [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. |