AVT B. VerSteeg Internet-Draft A. Begen Intended status: Standards Track Cisco Expires: May 20, 2010 T. VanCaenegem Alcatel-Lucent Z. Vax Microsoft Corporation November 16, 2009 Unicast-Based Rapid Acquisition of Multicast RTP Sessions draft-ietf-avt-rapid-acquisition-for-rtp-05 Abstract When an RTP receiver joins a primary multicast session, it may need to acquire and parse certain Reference Information before it can process any data sent in the multicast session. Depending on the join time, length of the Reference Information repetition interval, size of the Reference Information as well as the application and transport properties, the time lag before an RTP receiver can usefully consume the multicast data, which we refer to as the Acquisition Delay, varies and may be large. This is an undesirable phenomenon for receivers that frequently switch among different multicast sessions, such as video broadcasts. In this document, we describe a method using the existing RTP and RTCP protocol machinery that reduces the acquisition delay. In this method, an auxiliary unicast RTP session carrying the Reference Information to the receiver precedes/accompanies the primary multicast stream. This unicast RTP flow may be transmitted at a faster than natural rate to further accelerate the acquisition. The motivating use case for this capability is multicast applications that carry real-time compressed audio and video. However, the proposed method can also be used in other types of multicast applications where the acquisition delay is long enough to be a problem. Status of this Memo This Internet-Draft is submitted to IETF in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. VerSteeg, et al. Expires May 20, 2010 [Page 1] Internet-Draft Rapid Acquisition of RTP Sessions November 2009 Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. This Internet-Draft will expire on May 20, 2010. Copyright Notice Copyright (c) 2009 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 (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 BSD License. This document may contain material from IETF Documents or IETF Contributions published or made publicly available before November 10, 2008. The person(s) controlling the copyright in some of this material may not have granted the IETF Trust the right to allow modifications of such material outside the IETF Standards Process. Without obtaining an adequate license from the person(s) controlling the copyright in such materials, this document may not be modified outside the IETF Standards Process, and derivative works of it may not be created outside the IETF Standards Process, except to format it for publication as an RFC or to translate it into languages other than English. VerSteeg, et al. Expires May 20, 2010 [Page 2] Internet-Draft Rapid Acquisition of RTP Sessions November 2009 Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 5 2. Requirements Notation . . . . . . . . . . . . . . . . . . . . 7 3. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . 8 4. Elements of Delay in Multicast Applications . . . . . . . . . 8 5. Protocol Design Considerations and Their Effect on Resource Management for Rapid Acquisition . . . . . . . . . . 10 6. Rapid Acquisition of Multicast RTP Sessions . . . . . . . . . 12 6.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . . 12 6.2. Message Flows . . . . . . . . . . . . . . . . . . . . . . 13 6.3. Shaping the Unicast Burst . . . . . . . . . . . . . . . . 21 6.4. Failure Cases . . . . . . . . . . . . . . . . . . . . . . 23 7. Encoding of the Signaling Protocol in RTCP . . . . . . . . . . 24 7.1. Extensions . . . . . . . . . . . . . . . . . . . . . . . . 25 7.1.1. Vendor-Neutral Extensions . . . . . . . . . . . . . . 25 7.1.2. Private Extensions . . . . . . . . . . . . . . . . . . 26 7.2. RAMS Request . . . . . . . . . . . . . . . . . . . . . . . 26 7.3. RAMS Information . . . . . . . . . . . . . . . . . . . . . 28 7.4. RAMS Termination . . . . . . . . . . . . . . . . . . . . . 30 8. SDP Definitions and Examples . . . . . . . . . . . . . . . . . 31 8.1. Definitions . . . . . . . . . . . . . . . . . . . . . . . 31 8.2. Examples . . . . . . . . . . . . . . . . . . . . . . . . . 31 9. NAT Considerations . . . . . . . . . . . . . . . . . . . . . . 35 10. Known Implementations . . . . . . . . . . . . . . . . . . . . 35 10.1. Open Source RTP Receiver Implementation by Cisco . . . . . 35 10.2. IPTV Commercial Implementation by Microsoft . . . . . . . 36 11. Security Considerations . . . . . . . . . . . . . . . . . . . 36 12. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 37 12.1. Registration of SDP Attributes . . . . . . . . . . . . . . 38 12.2. Registration of SDP Attribute Values . . . . . . . . . . . 38 12.3. Registration of FMT Values . . . . . . . . . . . . . . . . 38 12.4. SFMT Values for RAMS Messages Registry . . . . . . . . . . 39 12.5. RAMS TLV Space Registry . . . . . . . . . . . . . . . . . 39 12.6. RAMS Response Code Space Registry . . . . . . . . . . . . 40 13. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 41 14. Change Log . . . . . . . . . . . . . . . . . . . . . . . . . . 42 14.1. draft-ietf-avt-rapid-acquisition-for-rtp-05 . . . . . . . 42 14.2. draft-ietf-avt-rapid-acquisition-for-rtp-04 . . . . . . . 42 14.3. draft-ietf-avt-rapid-acquisition-for-rtp-03 . . . . . . . 42 14.4. draft-ietf-avt-rapid-acquisition-for-rtp-02 . . . . . . . 42 14.5. draft-ietf-avt-rapid-acquisition-for-rtp-01 . . . . . . . 43 14.6. draft-ietf-avt-rapid-acquisition-for-rtp-00 . . . . . . . 43 14.7. draft-versteeg-avt-rapid-synchronization-for-rtp-03 . . . 43 14.8. draft-versteeg-avt-rapid-synchronization-for-rtp-02 . . . 43 14.9. draft-versteeg-avt-rapid-synchronization-for-rtp-01 . . . 44 15. References . . . . . . . . . . . . . . . . . . . . . . . . . . 44 15.1. Normative References . . . . . . . . . . . . . . . . . . . 44 VerSteeg, et al. Expires May 20, 2010 [Page 3] Internet-Draft Rapid Acquisition of RTP Sessions November 2009 15.2. Informative References . . . . . . . . . . . . . . . . . . 45 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 47 VerSteeg, et al. Expires May 20, 2010 [Page 4] Internet-Draft Rapid Acquisition of RTP Sessions November 2009 1. Introduction Most multicast flows carry a stream of inter-related data. Certain information must first be acquired by the receivers to start processing any data sent in the multicast session. This document refers to this information as Reference Information. The Reference Information is conventionally sent periodically in the multicast session and usually consists of items such as a description of the schema for the rest of the data, references to which data to process, encryption information including keys, as well as any other information required to process the data in the primary multicast stream. Real-time multicast applications require the receivers to buffer data. The receiver may have to buffer data to smooth out the network jitter, to allow loss-repair methods such as Forward Error Correction and retransmission to recover the missing packets, and to satisfy the data processing requirements of the application layer. When a receiver joins a multicast session, it has no control over what point in the flow is currently being transmitted. Sometimes the receiver may join the session right before the Reference Information is sent in the session. In this case, the required waiting time is usually minimal. Other times, the receiver may join the session right after the Reference Information has been transmitted. In this case, the receiver has to wait for the Reference Information to appear again in the flow before it can start processing any multicast data. In some other cases, the Reference Information is not contiguous in the flow but dispersed over a large period, which forces the receiver to wait for all of the Reference Information to arrive before starting to process the rest of the data. The net effect of waiting for the Reference Information and waiting for various buffers to fill up is that the receivers may experience significantly large delays in data processing. In this document, we refer to the difference between the time an RTP receiver joins the multicast session and the time the RTP receiver acquires all the necessary Reference Information as the Acquisition Delay. The acquisition delay may not be the same for different receivers; it usually varies depending on the join time, length of the Reference Information repetition interval, size of the Reference Information as well as the application and transport properties. The varying nature of the acquisition delay adversely affects the receivers that frequently switch among multicast sessions. In this specification, we address this problem for RTP-based multicast applications and describe a method that uses the fundamental tools offered by the existing RTP and RTCP protocols [RFC3550]. In this VerSteeg, et al. Expires May 20, 2010 [Page 5] Internet-Draft Rapid Acquisition of RTP Sessions November 2009 method, either the multicast source (or the distribution source in a single-source multicast (SSM) session) retains the Reference Information for a period after its transmission, or an intermediary network element (that we refer to as Retransmission Server) joins the multicast session and continuously caches the Reference Information as it is sent in the session and acts as a feedback target (See [I-D.ietf-avt-rtcpssm]) for the session. When an RTP receiver wishes to join the same multicast session, instead of simply issuing a Source Filtering Group Management Protocol (SFGMP) Join message, it sends a request to the feedback target for the session asking for the Reference Information. The Retransmission Server starts a unicast retransmission RTP session and sends the Reference Information to the RTP receiver over that session. If there is spare bandwidth, the Retransmission Server may also burst the Reference Information at a faster than its natural rate. As soon as the receiver acquires the Reference Information, it can join the multicast session and start processing the multicast data. This method potentially reduces the acquisition delay. We refer to this method as Unicast-based Rapid Acquisition of Multicast RTP Sessions. A simplified network diagram showing this method through an intermediary network element is depicted in Figure 1. +-----------------------+ +--->| Intermediary | | ...| Network Element | | : |(Retransmission Server)| | : +-----------------------+ | : | v +-----------+ +----------+ +----------+ | Multicast | | Router |---------->| Joining | | Source |------->| |..........>| RTP | +-----------+ +----------+ | Receiver | | +----------+ | | +----------+ +---------------->| Existing | | RTP | | Receiver | +----------+ ...> Unicast RTP Flow ---> Multicast RTP Flow Figure 1: Rapid acquisition through an intermediary network element A primary design goal in this solution is to use the existing tools VerSteeg, et al. Expires May 20, 2010 [Page 6] Internet-Draft Rapid Acquisition of RTP Sessions November 2009 in the RTP/RTCP protocol family. This improves the versatility of the existing implementations, and promotes faster deployment and better interoperability. To this effect, we use the unicast retransmission support of RTP [RFC4588] and the capabilities of RTCP to handle the signaling needed to accomplish the acquisition. The packet(s) carrying the Reference Information are sent by the Retransmission Server in the auxiliary unicast RTP session for rapid acquisition. These are constructed as retransmission packets that would have been sent in a unicast RTP session to recover the missing packets at an RTP receiver that has never received any packet. In fact, a single RTP session SHOULD be used for both rapid acquisition and retransmission-based loss repair. This session can be used to simultaneously provide the unicast burst for the rapid acquisition and the repair packets requested by the RTP receivers when they detect lost burst packets or lost RTP packets in the primary multicast stream. The conventional RTCP feedback (NACK) message that requests the retransmission of the missing packets [RFC4585] indicates their sequence numbers. However, upon joining a new session the RTP receiver has never received a packet, and thus, does not know the sequence numbers. Instead, the RTP receiver sends a newly defined RTCP feedback message to request the Reference Information needed to rapidly get on the track with the primary multicast session. It is also worth noting that in order to issue the initial RTCP message to the feedback target, the SSRC of the session to be joined must be known prior to any packet reception, and hence, needs to be signaled out-of-band (or otherwise communicated to the RTP receiver in advance of the initiation of the rapid acquisition operation). In a Session Description Protocol (SDP) description, the SSRC MUST be signaled through the 'ssrc' attribute [I-D.ietf-avt-rtcpssm]. In the rest of this specification, we have the following outline: In Section 4, we describe the delay components in generic multicast applications. Section 5 presents an overview of the protocol design considerations for rapid acquisition. We provide the protocol details of the rapid acquisition method in Section 6 and Section 7. Section 8 and Section 9 discuss the SDP signaling issues with examples and NAT-related issues, respectively. Note that Section 3 provides a list of the definitions frequently used in this document. 2. Requirements Notation 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 [RFC2119]. VerSteeg, et al. Expires May 20, 2010 [Page 7] Internet-Draft Rapid Acquisition of RTP Sessions November 2009 3. Definitions This document uses the following acronyms and definitions frequently: Primary Multicast Session: The multicast RTP session to which RTP receivers can join at a random point in time. Primary Multicast Stream: The RTP stream carried in the primary multicast session. Source Filtering Group Management Protocol (SFGMP): Following the definition in [RFC4604], SFGMP refers to the Internet Group Management Protocol (IGMP) version 3 [RFC3376] and the Multicast Listener Discovery Protocol (MLD) version 2 [RFC3810] in the IPv4 and IPv6 networks, respectively. However, the rapid acquisition method introduced in this document does not depend on a specific version of either of these group management protocols. In the remainder of this document, SFGMP will refer to any group management protocol that has Join and Leave functionalities. Feedback Target: (Unicast RTCP) Feedback target as defined in [I-D.ietf-avt-rtcpssm]. Retransmission Packet: An RTP packet that is formatted as defined in [RFC4588]. Reference Information: The set of certain media content and metadata information that is sufficient for an RTP receiver to start usefully consuming a media stream. The meaning, format and size of this information are specific to the application and are out of scope of this document. (Unicast) Burst (Stream): A unicast stream of RTP retransmission packets that enable an RTP receiver to rapidly acquire the Reference Information. The burst stream is typically transmitted at an accelerated rate. Retransmission Server (RS): The RTP/RTCP endpoint that can generate the retransmission packets and the burst stream. RS may also generate other non-retransmission packets to aid the RAMS process. 4. Elements of Delay in Multicast Applications In an any-source (ASM) or a single-source (SSM) multicast delivery system, there are three major elements that contribute to the overall acquisition delay when an RTP receiver switches from one multicast session to another one. These are: VerSteeg, et al. Expires May 20, 2010 [Page 8] Internet-Draft Rapid Acquisition of RTP Sessions November 2009 o Multicast switching delay o Reference Information latency o Buffering delays Multicast switching delay is the delay that is experienced to leave the current multicast session (if any) and join the new multicast session. In typical systems, the multicast join and leave operations are handled by a group management protocol. For example, the receivers and routers participating in a multicast session may use the Internet Group Management Protocol (IGMP) version 3 [RFC3376] or the Multicast Listener Discovery Protocol (MLD) version 2 [RFC3810]. In either of these protocols, when a receiver wants to join a multicast session, it sends a message to its upstream router and the routing infrastructure sets up the multicast forwarding state to deliver the packets of the multicast session to the new receiver. Depending on the proximity of the upstream router, the current state of the multicast tree, the load on the system and the protocol implementation, the join times vary. Current systems provide join latencies usually less than 200 milliseconds (ms). If the receiver had been participating in another multicast session before joining the new session, it needs to send a Leave message to its upstream router to leave the session. In common multicast routing protocols, the leave times are usually smaller than the join times, however, it is possible that the Leave and Join messages may get lost, in which case the multicast switching delay inevitably increases. Reference Information latency is the time it takes the receiver to acquire the Reference Information. It is highly dependent on the proximity of the actual time the receiver joined the session to the next time the Reference Information will be sent to the receivers in the session, whether the Reference Information is sent contiguously or not, and the size of the Reference Information. For some multicast flows, there is a little or no interdependency in the data, in which case the Reference Information latency will be nil or negligible. For other multicast flows, there is a high degree of interdependency. One example of interest is the multicast flows that carry compressed audio/video. For these flows, the Reference Information latency may become quite large and be a major contributor to the overall delay. Refer to [I-D.begen-avt-rtp-mpeg2ts-preamble] for details. The buffering component of the overall acquisition delay is driven by the way the application layer processes the payload. In many multicast applications, an unreliable transport protocol such as UDP [RFC0768] is often used to transmit the data packets, and the reliability, if needed, is usually addressed through other means such VerSteeg, et al. Expires May 20, 2010 [Page 9] Internet-Draft Rapid Acquisition of RTP Sessions November 2009 as Forward Error Correction and retransmission [I-D.ietf-rmt-pi-norm-revised]. These loss-repair methods require buffering at the receiver side to function properly. In many applications, it is also often necessary to de-jitter the incoming data packets before feeding them to the application. The de- jittering process also increases the buffering delays. Besides these network-related buffering delays, there are also specific buffering needs that are required by the individual applications. For example, standard video decoders typically require an amount, sometimes a significant amount, of coded video data to be available in the pre- decoding buffers prior to starting to decode the video bitstream. 5. Protocol Design Considerations and Their Effect on Resource Management for Rapid Acquisition Rapid acquisition is an optimization of a system that must continue to work correctly whether or not the optimization is effective, or even fails due to lost control messages, congestion, or other problems. This is fundamental to the overall design requirements surrounding the protocol definition and to the resource management schemes to be employed together with the protocol (e.g., QoS machinery, server load management, etc). In particular, the system needs to operate within a number of constraints: o First, a rapid acquisition operation must fail gracefully. The user experience must, except perhaps in pathological circumstances, be not significantly worse for trying and failing to complete rapid acquisition compared to simply joining the multicast session. o Second, providing the rapid acquisition optimizations must not cause collateral damage to either the multicast session being joined, or other multicast sessions sharing resources with the rapid acquisition operation. In particular, the rapid acquisition operation must avoid self-interference with the multicast session that may be simultaneously being received by other hosts. In addition, it must also avoid interference with other multicast sessions sharing the same network resources. These properties are possible, but are usually difficult to achieve. One challenge is the existence of multiple bandwidth bottlenecks between the receiver and the server(s) in the network providing the rapid acquisition service. In commercial IPTV deployments, for example, bottlenecks are often present in the aggregation network connecting the IPTV servers to the network edge, the access links (e.g., DSL, DOCSIS) and in the home network of the subscribers. Some of these links may serve only a single subscriber, limiting VerSteeg, et al. Expires May 20, 2010 [Page 10] Internet-Draft Rapid Acquisition of RTP Sessions November 2009 congestion impact to the traffic of only that subscriber, but others can be shared links carrying multicast sessions of many subscribers. Also note that the state of these links may be varying over time. The receiver may have knowledge of a portion of this network, or may have partial knowledge of the entire network. The methods employed by the devices to acquire this network state information is out of scope for this document. The receiver should be able to signal the server with the bandwidth that it believes it can handle. The server also needs to be able to rate limit the flow in order to stay within the performance envelope that it knows about. Both the server and receiver need to be able to inform the other of changes in the requested and delivered rates. However, the protocol must be robust in the presence of packet loss, so this signaling must include the appropriate default behaviors. A second challenge is that for some uses (e.g., high-bitrate video) the unicast burst bitrate is high while the flow duration of the unicast burst is short. This is because the purpose of the unicast burst is to allow the RTP receiver to join the multicast quickly and thereby limit the overall resources consumed by the burst. Such high-bitrate, short-duration flows are not amenable to conventional admission control techniques. For example, end-to-end per-flow signaled admission control techniques such as RSVP have too much latency and control channel overhead to be a good fit for rapid acquisition. Similarly, using a TCP (or TCP-like) approach with a 3-way handshake and slow-start to avoid inducing congestion would defeat the purpose of attempting rapid acquisition in the first place by introducing many RTTs of delay. These observations lead to certain unavoidable requirements and goals for a rapid acquisition protocol. These are: o The protocol must be designed to allow a deterministic upper bound on the extra bandwidth used (compared to just joining the multicast session). A reasonable size bound is e*B, where B is the "nominal" bandwidth of the primary multicast stream, and e is an "excess-bandwidth" coefficient The total duration of the unicast burst must have a reasonable bound; long unicast bursts devolve to the bandwidth profile of multi-unicast for the whole system. o The scheme should minimize (or better eliminate) the overlap of the unicast burst and the primary multicast stream. This minimizes the window during which congestion could be induced on a bottleneck link compared to just carrying the multicast or unicast packets alone. VerSteeg, et al. Expires May 20, 2010 [Page 11] Internet-Draft Rapid Acquisition of RTP Sessions November 2009 o The scheme must minimize (or better eliminate) any gap between the unicast burst and the primary multicast stream, which has to be repaired later, or in the absence of repair, will result in loss being experienced by the application. In addition to the above, there are some other protocol design issues to be considered. First, there is at least one RTT of "slop" in the control loop. In starting a rapid acquisition burst, this manifests as the time between the client requesting the unicast burst and the burst description and/or the first unicast burst packets arriving at the receiver. For managing and terminating the unicast burst, there are two possible approaches for the control loop: The receiver can adapt to the unicast burst as received, converge based on observation and explicitly terminate the unicast burst with a second control loop exchange (which takes a minimum of one RTT, just as starting the unicast burst does). Alternatively, the server generating the unicast burst can pre-compute the burst parameters based on the information in the initial request and tell the receiver the burst duration. The protocol described in the next section allows either method of controlling the rapid acquisition unicast burst. 6. Rapid Acquisition of Multicast RTP Sessions We start this section with an overview of the rapid acquisition of multicast sessions (RAMS) method. 6.1. Overview [I-D.ietf-avt-rtcpssm] specifies an extension to the RTP Control Protocol (RTCP) to use unicast feedback in an SSM session. It defines an architecture that introduces the concept of Distribution Source, which - in an SSM context - distributes the RTP data and redistributes RTCP information to all RTP receivers. This RTCP information is retrieved from the Feedback Target, to which RTCP unicast feedback traffic is sent. The Feedback Target MAY be implemented in one or more entities different from the Distribution Source, and different RTP receivers MAY use different Feedback Targets. This document builds further on these concepts to reduce the acquisition time when an RTP receiver joins a multicast session at a random point in time by introducing the concept of the Burst Source and new RTCP feedback messages. The Burst Source has a cache where the most recent packets from the primary multicast session are continuously stored. When an RTP receiver wants to receive the VerSteeg, et al. Expires May 20, 2010 [Page 12] Internet-Draft Rapid Acquisition of RTP Sessions November 2009 primary multicast stream, prior to joining the SSM session, it may first request a unicast burst from the Burst Source. In this burst, the packets are formatted as RTP retransmission packets [RFC4588] and carry the Reference Information. This information allows the RTP receiver to start usefully consuming the RTP packets sent in the primary multicast session. Using an accelerated rate (as compared to the rate of the primary multicast stream) for the unicast burst implies that at a certain point in time, the payload transmitted in the unicast burst is going to be the same as the payload multicast in the SSM session, i.e., the unicast burst will catch up with the primary multicast stream. At this point, the RTP receiver no longer needs to receive the unicast burst and can join the primary multicast session. This method is referred to as the Rapid Acquisition of Multicast Sessions (RAMS). This document proposes extensions to [RFC4585] for an RTP receiver to request a unicast burst as well as for additional control messaging that can be leveraged during the acquisition process. 6.2. Message Flows Figure 2 shows the main entities involved in rapid acquisition: o Multicast Source o Feedback Target (FT) o Burst/Retransmission Source o RTP Receiver (RTP_Rx) VerSteeg, et al. Expires May 20, 2010 [Page 13] Internet-Draft Rapid Acquisition of RTP Sessions November 2009 +----------------------------------------------+ | Retransmission Server | | (RS) | | +----------+ +---------------------------+ | | | Feedback | | Burst/Retransmission | | | | Target | | Source | | | +----------+ +---------------------------+ | +----------------------------------------------+ ^ ^ : | ' : | ' : | ' v +-----------+ +----------+ +----------+ | | | |'''''''''''>| | | Multicast |------->| Router |...........>| RTP | | Source | | |<~~~~~~~~~~~| Receiver | | | | |----------->| (RTP_Rx) | +-----------+ +----------+ +----------+ '''> Unicast RTCP Messages ~~~> SFGMP Messages ...> Unicast RTP Flow ---> Multicast RTP Flow Figure 2: Flow diagram for unicast-based rapid acquisition The feedback target (FT) is the entity as defined in [I-D.ietf-avt-rtcpssm], to which RTP_Rx sends its RTCP feedback messages indicating packet loss in the primary multicast stream by means of an RTCP NACK message or indicating RTP_Rx's desire to rapidly acquire the primary multicast stream by means of an RTCP feedback message defined in this document. While the Burst/ Retransmission Source is responsible for responding to these messages and for further RTCP interaction with RTP_Rx in the case of a rapid acquisition process, it is assumed in the remainder of the document that these two logical entities (FT and Burst/Retransmission Source) are combined in a single physical entity and they share state. In the remainder of the text, the term Retransmission Server will be used whenever appropriate, to refer to the combined functionality of the FT and Burst/Retransmission Source. However, it must be noted that only FT is involved in the primary multicast session, whereas the Burst/Retransmission Source transmits the unicast burst and retransmission packets both formatted as RTP retransmission packets [RFC4588] in a single separate unicast RTP retransmission session to each RTP_Rx. In the retransmission session, as in any other RTP session, RS and RTP_Rx regularly send VerSteeg, et al. Expires May 20, 2010 [Page 14] Internet-Draft Rapid Acquisition of RTP Sessions November 2009 the periodic sender and receiver reports, respectively. Note also that the same method (with the identical message flows) would also apply in a scenario where rapid acquisition is performed by a feedback target co-located with the media source. The unicast burst is triggered by an RTCP feedback message that is defined in this document, whereas an RTP retransmission is triggered by an RTCP NACK message defined in [RFC4585]. Based on its design, in an RAMS implementation, there may be a gap between the end of the burst and the reception of the primary multicast stream because of the imperfections in the switch-over. If needed, RTP_Rx may make use of the RTCP NACK message to request a retransmission for the missing packets in the gap. Figure 3 depicts an example of messaging flow for rapid acquisition. The RTCP feedback messages are explained below. Note that the messages indicated in parentheses may or may not be present during rapid acquisition. VerSteeg, et al. Expires May 20, 2010 [Page 15] Internet-Draft Rapid Acquisition of RTP Sessions November 2009 +-----------+ +----------------+ +----------+ +------------+ | Multicast | | Retransmission | | | | RTP | | Source | | Server | | Router | | Receiver | | | | (RS) | | | | (RTP_Rx) | +-----------+ +----------------+ +----------+ +------------+ | | | | |-- RTP Multicast ------------------->| | | | | | |-- RTP Multicast ->| | | | | | | | |<'''''''''''''''''' RTCP RAMS-R ''| | | | | | | | | | |'' (RTCP RAMS-I) ''''''''''''''''>| | | | | | | | | | |.. Unicast RTP Burst ............>| | | | | | |<''''''''''''''''''(RTCP RAMS-R)''| | | | | | | | | | |'' (RTCP RAMS-I) ''''''''''''''''>| | | | | | | | | | | |<~ SFGMP Join ~~| | | | | | | | | |-- RTP Multicast ------------------------------------>| | | | | | | | | | |<'''''''''''''''''' RTCP RAMS-T ''| | | | | | | | | | |<''''''''''''''''''' (RTCP NACK)''| | | | | | | | | | |.. (Unicast Retransmissions) ....>| | | | | | | | | | |<''''''''''''''''''' (RTCP BYE) ''| | | | | | | | | '''> Unicast RTCP Messages ~~~> SFGMP Messages ...> Unicast RTP Flow ---> Multicast RTP Flow VerSteeg, et al. Expires May 20, 2010 [Page 16] Internet-Draft Rapid Acquisition of RTP Sessions November 2009 Figure 3: Message flows for unicast-based rapid acquisition This document defines the expected behaviors of RS and RTP_Rx. It is instructive to have independently operating implementations on RS and RTP_Rx that request the burst, describe the burst, start the burst, join the multicast session and stop the burst. These implementations send messages to each other, but there MUST be provisions for the cases where the control messages get lost, or re-ordered, or are not being delivered to their destinations. The following steps describe rapid acquisition in detail: 1. Request: RTP_Rx sends a rapid acquisition request for the new multicast RTP session to the feedback target address of that session. The request contains the SSRC of RTP_Rx and the SSRC of the media source. This RTCP feedback message is defined as the RAMS-Request (RAMS-R) message and MAY contain parameters, which may constrain the burst, such as the bandwidth limit. Other parameters may be related to the amount of buffering capacity available at RTP_Rx, which may be used by RS to prepare a burst that conforms with RTP_Rx's requirements. Before joining the primary multicast session, a new joining RTP_Rx learns the addresses associated with the new multicast session (addresses for the multicast source, group and retransmission server) by out-of-band means. Also note that since no RTP packets have been received yet for this session, the SSRC must be obtained out-of-band. See Section 8 for details. 2. Response: RS receives the RAMS-R message and decides whether to accept it or not. RS MUST send an (at least one) RAMS- Information (RAMS-I) message to RTP_Rx. The first RAMS-I message MAY precede the unicast burst or it MAY be sent during the burst. Additional RAMS-I messages MAY be sent during the burst and these RAMS-I messages may or may not be a direct response to an RAMS-R message. The RAMS-I message is sent by the Burst/Retransmission Source logical entity that is part of RS. Note that RS learns the IP address information for RTP_Rx from the RAMS-R message it received. (This description glosses over the NAT details. Refer to Section 9 for a discussion of NAT- related issues.) If RS cannot provide a rapid acquisition service, RS rejects the request and informs RTP_Rx immediately via an RAMS-I message. If RTP_Rx receives a message indicating that its rapid acquisition request has been denied, it abandons the rapid acquisition attempt and MAY immediately join the multicast session by sending VerSteeg, et al. Expires May 20, 2010 [Page 17] Internet-Draft Rapid Acquisition of RTP Sessions November 2009 an SFGMP Join message towards its upstream multicast router for the new primary multicast session. If RS accepts the request, it sends an RAMS-I message to RTP_Rx (before commencing the unicast burst or during the unicast burst) that comprises fields that can be used to describe the unicast burst (e.g., the maximum bitrate and the duration of the unicast burst). A particularly important field in the RAMS-I message carries the RTP sequence number of the first packet transmitted in the retransmission session to allow RTP_Rx to detect any missing initial packet(s). Note that the first RTP packet transmitted in the retransmission session is not necessarily a burst packet. It could be a payload-specific RTP packet (See [I-D.begen-avt-rtp-mpeg2ts-preamble] for an example). When RS accepts the request, this field MUST be populated in the RAMS-I message and it is RECOMMENDED that the RAMS-I message is sent early enough in the session to be useful. If RS rejects the request, this field SHALL NOT exist in the RAMS-I message. It is RECOMMENDED to include a sender report with the RAMS-I message in the same compound RTCP packet. This allows rapid synchronization among multiple RTP flows [I-D.ietf-avt-rapid-rtp-sync]. The unicast burst duration MAY be calculated by RS, and its value MAY be updated by messages received from RTP_Rx. If RS accepts the request, the join time information (for the new multicast session) MUST be populated in at least one of the RAMS-I messages. If RS rejects the request, including the join time information in a RAMS-I message is OPTIONAL. RS MAY send the RAMS-I message after a significant delay, so RTP_Rx SHOULD NOT make protocol dependencies on quickly receiving an RAMS-I message. 3. Unicast Burst: If the request is accepted, RS starts sending the unicast burst that comprises one or more RTP retransmission packets (The burst packet(s) are sent by the Burst/Retransmission Source logical entity). In addition, there MAY be optional payload-specific information that RS chooses to send to RTP_Rx. Such an example is discussed in [I-D.begen-avt-rtp-mpeg2ts-preamble] for transmitting the payload-specific information for MPEG2 Transport Streams. 4. Updated Request: RTP_Rx MAY send a new RAMS-R message (to the FT entity of RS) with a different value for one or more fields of an earlier RAMS-R message. Upon receiving an updated request, RS MAY use the updated values for sending/shaping the burst, or VerSteeg, et al. Expires May 20, 2010 [Page 18] Internet-Draft Rapid Acquisition of RTP Sessions November 2009 refine the values and use the refined values for sending/shaping the burst. RS MAY send a new RAMS-I message to indicate the changes it made. However, note that RS does not have to send a new RAMS-I, or the new RAMS-I message may get lost. It is also possible that the updated RAMS-R message could have been lost. Thus, RTP_Rx SHOULD NOT make protocol dependencies on quickly (or ever) receiving a new RAMS-I message, or assume that RS will honor the requested changes. RTP_Rx may be in an environment where the available resources are time-varying, which may or may not deserve sending a new updated request. Determining the circumstances where RTP_Rx should or should not send an updated request and the methods that RTP_Rx can use to detect and evaluate the time-varying available resources are not specified in this document. 5. Updated Response: RS may send more than one RAMS-I messages, e.g., to update the value of one or more fields in an earlier RAMS-I message and/or to signal RTP_Rx in real time to join the primary multicast session. RTP_Rx usually depends on RS to learn the join time, which can be conveyed by the first RAMS-I message, or can be sent/revised in a later RAMS-I message. If RS is not capable of determining the join time in the first RAMS-I message, it MUST send another RAMS-I message (with the join time information) later. 6. Multicast Join Signaling: In principal, RTP_Rx can join the primary multicast session any time during or after the end of the unicast burst via an SFGMP Join message. However, there may be missing packets if RTP_Rx joins the primary multicast session too early or too late. For example, if RTP_Rx starts receiving the primary multicast stream while it is still receiving the unicast burst at a high excess bitrate, this may result in an increased risk of packet loss. Or, if RTP_Rx joins the primary multicast session some time after the unicast burst is finished, there may be a gap between the burst and multicast data (a number of RTP packets may be missing). In both cases, RTP_Rx MAY issue retransmissions requests (via RTCP NACK messages) [RFC4585] to fill the gap. Yet, there are cases where the remaining available bandwidth may limit the number of retransmissions that can be provided within a certain time period, causing the retransmission data to arrive too late at RTP_Rx (from an application-layer point of view). To cope with such cases, the RAMS-I message allows RS to signal explicitly when RTP_Rx should send the SFGMP Join message. VerSteeg, et al. Expires May 20, 2010 [Page 19] Internet-Draft Rapid Acquisition of RTP Sessions November 2009 Alternatively, RS may pre-compute the burst duration and the time RTP_Rx should send the SFGMP Join message. This information may be conveyed in the RAMS-I message and can be updated in a subsequent RAMS-I message. While RTP_Rx MAY use a locally calculated join time, it SHOULD use the information from the most recent RAMS-I message. 7. Multicast Receive: After the join, RTP_Rx starts receiving the primary multicast stream. 8. Terminate: RS may know when it needs to stop the unicast burst based on the burst parameters, or RTP_Rx MAY explicitly let RS know the sequence number of the first RTP packet it received from the multicast session, or RTP_Rx MAY request RS to terminate the burst immediately. Regardless of whether or not RS knows when it needs to stop the burst, RTP_Rx SHALL use the RAMS-Termination (RAMS-T) message at an appropriate time. RTP_Rx can choose to send the RAMS-T message before or after it starts receiving the multicast data. In the latter case, RTP_Rx SHALL include the sequence number of the first RTP packet received in the primary multicast session in the RAMS-T message, and RS SHOULD terminate the burst after it sends the unicast burst packet whose Original Sequence Number (OSN) field in the RTP retransmission payload header matches this number minus one. If RTP_Rx wants to stop the burst prior to receiving the multicast data, it sends an RAMS-T message without an RTP sequence number. RTP_Rx MUST send at least one RAMS-T message (if an RTCP BYE message has not been issued yet as described in Step 9), and the RAMS-T message MUST be addressed to the RTCP port of the retransmission session. Against the possibility of a message loss, RTP_Rx MAY repeat the RAMS-T message multiple times as long as it follows the RTCP timer rules defined in [RFC4585]. 9. Terminate with RTCP BYE: When RTP_Rx is receiving the burst, if RTP_Rx becomes no longer interested in the primary multicast stream, RTP_Rx SHALL issue an RTCP BYE message for the RTP retransmission session and another RTCP BYE message for the primary multicast session. Upon receiving an RTCP BYE message, RS MUST terminate the rapid acquisition operation, and cease transmitting any further regular retransmission packets as well as retransmission packets associated with the unicast burst. If support for [RFC5506] has VerSteeg, et al. Expires May 20, 2010 [Page 20] Internet-Draft Rapid Acquisition of RTP Sessions November 2009 been signaled, the RTCP BYE message MAY be sent in a reduced-size RTCP packet. Otherwise, Section 6.1 of [RFC3550] mandates the RTCP BYE message always to be sent with a sender or receiver report in a compound RTCP packet (If no data has been received, an empty receiver report MUST be included). With the information contained in the receiver report, RS can also figure out how many duplicate RTP packets have been delivered to RTP_Rx (Note that this will be an upper-bound estimate as one or more packets might have been lost during the burst transmission). The impact of duplicate packets and measures that can be taken to minimize the impact of receiving duplicate packets will be addressed in Section 6.3. Note that an RTCP BYE message issued for the RTP retransmission session terminates the whole session and ceases transmitting any further packets in that RTP session. Thus, in this case there is no need for sending an (explicit) RAMS-T message, which would only terminate the burst. Note that for the purpose of gathering detailed information about RTP_Rx's rapid acquisition experience, [I-D.begen-avt-rapid-sync-rtcp-xr] defines an RTCP Extended Report (XR) Block. This report is designed to be payload-independent, thus, it can be used by any multicast application that supports rapid acquisition. Support for this XR report is, however, optional. 6.3. Shaping the Unicast Burst This section provides informative guidelines about how RS can shape the transmission of the unicast burst. A higher bitrate for the unicast burst naturally conveys the Reference Information and media content to RTP_Rx faster. This way, RTP_Rx can start consuming the data sooner, which results in a faster acquisition. A higher rate also represents a better utilization of RS resources. As the burst may continue until it catches up with the primary multicast stream, the higher the bursting rate, the less data RS needs to transmit. However, a higher rate for the burst also increases the chances for congestion-caused packet loss. Thus, as discussed in Section 5, there must be an upper bound on the extra bandwidth used by the burst. When RS transmits the burst, it SHOULD take into account all available information to prevent any packet loss that may take place during the bursting as a result of buffer overflow on the path between RS and RTP_Rx and at RTP_Rx itself. The bursting rate may be VerSteeg, et al. Expires May 20, 2010 [Page 21] Internet-Draft Rapid Acquisition of RTP Sessions November 2009 determined by taking into account the following data, when available: a. Information obtained via the RAMS-R message, such as Max RAMS Buffer Fill Requirement and/or Max Receive Bitrate (See Section 7.2). b. Information obtained via RTCP receiver reports provided by RTP_Rx in the retransmission session, allowing in-session rate adaptations for the burst. When these receiver reports indicate packet loss, this may indicate a certain congestion state in the path from RS to RTP_Rx. Heuristics or algorithms that deduce such congestion state and how subsequently the RS should act, are outside the scope of this document. c. Information obtained via RTCP NACKs provided by RTP_Rx in the primary multicast session, allowing in-session rate adaptations for the burst. Such RTCP NACKs are transmitted by RTP_Rx in response to packet loss detection by RTP_Rx in the burst. NACKs may indicate a certain congestion state on the path from RS to RTP_Rx. Heuristics or algorithms that deduce such congestion state and how subsequently the RS should act, are outside the scope of this document. d. There may be other feedback received from RTP_Rx, e.g., in the form of ECN-CE RTCP feedback messages [I-D.westerlund-avt-ecn-for-rtp] that may influence in-session rate adaptations. e. Information obtained via updated RAMS-R messages, allowing in- session rate adaptations, if supported by RS. f. Pre-configured settings for each RTP_Rx or a set of RTP_Rxs that indicate the upper-bound bursting rates for which no packet loss will occur as a result of congestion along the path of RS to RTP_Rx. For example, in managed IPTV networks, where the bottleneck bandwidth along the end-to-end path is known (which is generally the access network link) and where the network between RS and this link is provisioned and dimensioned to carry the burst streams, the bursting rate does not exceed the provisioned value. These settings may also be dynamically adapted using application-aware knowledge. The initial bursting rate of the unicast burst to RTP_Rx is determined by parameters directly obtained from RTP_Rx (a) or by pre- configured settings (f). If such information is not available, RS may choose an appropriate initial bursting rate, and could increase or decrease the rate based on the feedback information (b, c, d or e). However, this may not be an easy task as by the time packet loss VerSteeg, et al. Expires May 20, 2010 [Page 22] Internet-Draft Rapid Acquisition of RTP Sessions November 2009 is reported back to RS triggering a rate reduction, packet loss may have occurred. A specific situation occurs near the end of the unicast burst, when RS has almost no more additional data to sustain the relatively higher bursting rate, thus, the upper-bound bursting rate automatically gets limited by the nominal rate of the primary multicast stream. During this time frame, RTP_Rx will join the primary multicast session because it was instructed to do so via an RAMS-I message or based on some heuristics. This means that both the burst packets and the primary multicast stream packets will be simultaneously received by RTP_Rx for a period of time. In this case, when the unicast burst is close to catch up with the primary multicast stream, RS may, for example, keep on sending burst packets but should reduce the rate accordingly by taking the nominal rate of the primary multicast stream into account. Alternatively, RS may immediately cease transmitting the burst packets, when being close to catch-up. Any gap resulting from an imperfect switch by RTP_Rx in receiving first the burst packets and then only primary multicast stream packets, can be later repaired by requesting retransmissions of the missing packets from RS. The retransmissions may also be shaped by RS to make sure that they do not cause collateral loss in the primary multicast and retransmission sessions. 6.4. Failure Cases All RAMS messages MAY be sent several times against the possibility of message loss as long as RS/RTP_Rx follows the RTCP timer rules defined in [RFC4585]. In the following, we examine the implications of losing the RAMS-R, RAMS-I or RAMS-T messages. When RTP_Rx sends an RAMS-R message to initiate a rapid acquisition but the message gets lost and RS does not receive it, RTP_Rx will not get an RAMS-I message, nor a unicast burst. In this case, RTP_Rx MAY resend the request when it is eligible to do so. Or, after a reasonable amount of time, RTP_Rx MAY time out (based on the previous observed response times) and immediately join the primary multicast session. In this case, RTP_Rx MUST still send an RAMS-T message. In the case RTP_Rx starts receiving a unicast burst but it does not receive a corresponding RAMS-I message within a reasonable amount of time, RTP_Rx MAY either discard the burst data and stop the burst by sending an RAMS-T message to RS, or decide not to interrupt the unicast burst and be prepared to join the primary multicast session at an appropriate time it determines or indicated in a subsequent RAMS-I message (if available). In either case, RTP_Rx SHALL send an RAMS-T message to RS at an appropriate time. VerSteeg, et al. Expires May 20, 2010 [Page 23] Internet-Draft Rapid Acquisition of RTP Sessions November 2009 In the case the RAMS-T message sent by RTP_Rx does not reach its destination, RS may continue sending the unicast burst even though RTP_Rx no longer needs it. In some cases, RS has not pre-computed the burst duration and does not know when to stop the burst. To cover that case, RTP_Rx MAY repeat the RAMS-T message multiple times as long as it follows the RTCP timer rules defined in [RFC4585]. RS MUST be provisioned to deterministically terminate the burst at some point, even if it never receives an RAMS-T message for an ongoing burst. If RTP_Rx becomes no longer interested in receiving the primary multicast stream and the associated unicast burst, RTP_Rx SHALL issue an RTCP BYE message to RS to terminate the RTP retransmission session. Only after that, RTP_Rx MAY send a new rapid acquisition request for another primary multicast session. 7. Encoding of the Signaling Protocol in RTCP This section defines the formats of the RTCP transport-layer feedback messages that are exchanged between the Retransmission Server (RS) and RTP Receiver (RTP_Rx) during rapid acquisition. These messages are referred to as the RAMS Messages. They are payload-independent and MUST be used by all RTP-based multicast applications that support rapid acquisition regardless of the payload they carry. Payload-specific feedback messages are not defined in this document, but an extension mechanism is provided where further optional payload-independent and payload-specific information can be included in the exchange. The common packet format for the RTCP feedback messages is defined in Section 6.1 of [RFC4585]. Each feedback message has a fixed-length field for version, padding, feedback message type (FMT), payload type (PT), length, SSRC of packet sender, SSRC of media source as well as a variable-length field for feedback control information (FCI). In the RAMS messages, the PT field is set to RTPFB (205) and the FMT field is set to RAMS (6). Individual RAMS messages are identified by a sub-field called Sub Feedback Message Type (SFMT). Depending on the specific scenario and timeliness/importance of a RAMS message, it may be desirable to send it in a reduced-size RTCP packet [RFC5506]. However, unless support for [RFC5506] has been signaled, compound RTCP packets MUST be used by following [RFC3550] rules. VerSteeg, et al. Expires May 20, 2010 [Page 24] Internet-Draft Rapid Acquisition of RTP Sessions November 2009 7.1. Extensions To improve the functionality of the RAMS method in certain applications, it may be desirable to define new fields in the RAMS Request, Information and Termination messages. Such fields MUST be encoded as TLV elements as described below and sketched in Figure 4: o Type: A single-octet identifier that defines the type of the parameter represented in this TLV element. o Length: A two-octet field that indicates the length of the TLV element excluding the Type and Length fields in octets. Note that this length does not include any padding that is required for alignment. o Value: Variable-size set of octets that contains the specific value for the parameter. If a TLV element does not fall on a 32-bit boundary, the last word must be padded to the boundary using further bits set to 0. In an RAMS message any vendor-neutral or private extension MUST be placed after the mandatory fields (if any). The support for extensions is OPTIONAL. 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type | Length | Value | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Value contd. / +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 4: Structure of a TLV element 7.1.1. Vendor-Neutral Extensions If the goal in defining new TLV elements is to extend the functionality in a vendor-neutral manner, they MUST be registered with IANA through the guidelines provided in Section 12.5. The current document defines several vendor-neutral extensions in the following sections. VerSteeg, et al. Expires May 20, 2010 [Page 25] Internet-Draft Rapid Acquisition of RTP Sessions November 2009 7.1.2. Private Extensions It is desirable to allow vendors to use private extensions in TLV format. For interoperability, such extensions MUST NOT collide with each other. A certain range of TLV Types is reserved for private extensions (Refer to Section 12.5). IANA management for these extensions is unnecessary and they are the responsibility of individual vendors. The structure that MUST be used for the private extensions is depicted in Figure 5. Here, the enterprise numbers are used from http://www.iana.org/assignments/enterprise-numbers. This will ensure the uniqueness of the private extensions and avoid any collision. 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type | Length | Ent. Number | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Ent. Number contd. | Value | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Value contd. / +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 5: Structure of a private extension 7.2. RAMS Request The RAMS Request message is identified by SFMT=1. The FCI field MUST contain only one RAMS Request. The RAMS Request is used by RTP_Rx to request rapid acquisition for a new multicast RTP session. The FCI field has the structure depicted in Figure 6. 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | SFMT=1 | Optional TLV-encoded Fields | +-+-+-+-+-+-+-+-+ | : Optional TLV-encoded Fields (and Padding, if needed) : +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ VerSteeg, et al. Expires May 20, 2010 [Page 26] Internet-Draft Rapid Acquisition of RTP Sessions November 2009 Figure 6: FCI field syntax for the RAMS Request message o Min RAMS Buffer Fill Requirement (32 bits): Optional TLV element that denotes the minimum milliseconds of data that RTP_Rx desires to have in its buffer before allowing the data to be consumed by the application. RTP_Rx may have knowledge of its buffering requirements. These requirements may be application and/or device specific. For instance, RTP_Rx may need to have a certain amount of data in its application buffer to handle transmission jitter and/or to be able to support error-control methods. If RS is told the minimum buffering requirement of the receiver, it may tailor the burst more precisely, e.g., by choosing an appropriate starting point. The methods used by RTP_Rx to determine this value are application specific, and thus, out of the scope of this document. If specified, the amount of backfill that will be provided by the unicast burst SHOULD NOT be smaller than the specified value since it will not be able to build up the desired level of buffer at RTP_Rx and may cause buffer underruns. Type: 1 o Max RAMS Buffer Fill Requirement (32 bits): Optional TLV element that denotes the maximum milliseconds of data that RTP_Rx can buffer without losing the burst data due to buffer overflow. RTP_Rx may have knowledge of its buffering requirements. These requirements may be application or device specific. For instance, one particular RTP_Rx may have more physical memory than another RTP_Rx, and thus, can buffer more data. If RS knows the buffering ability of the receiver, it may tailor the burst more precisely. The methods used by the receiver to determine this value are application specific, and thus, out of scope. If specified, the amount of backfill that will be provided by the unicast burst SHOULD NOT be larger than this value since it may cause buffer overflows at RTP_Rx. Type: 2 o Max Receive Bitrate (32 bits): Optional TLV element that denotes the maximum bitrate (in bits per second) that the RTP receiver can process the unicast burst. This rate should include whatever knowledge the receiver has that would provide an upper bound on the unicast burst bitrate. The limits may include local receiver limits as well as network limits that are known to the receiver. VerSteeg, et al. Expires May 20, 2010 [Page 27] Internet-Draft Rapid Acquisition of RTP Sessions November 2009 If specified, the unicast burst bitrate SHOULD NOT be larger than this value since it may cause congestion and packet loss. Type: 3 The semantics of the RAMS-R feedback message is independent of the payload type. 7.3. RAMS Information The RAMS Information message is identified by SFMT=2. The FCI field MUST contain only one RAMS Information. The RAMS Information is used to describe the unicast burst that will be sent for rapid acquisition. It also includes other useful information for RTP_Rx as described below. Optional payload-specific information MAY follow RAMS Information. The FCI field has the structure depicted in Figure 7. 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | SFMT=2 | MSN | Response | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ : Optional TLV-encoded Fields (and Padding, if needed) : +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 7: FCI field syntax for the RAMS Information message o Message Sequence Number (8 bits) : Mandatory field that denotes the sequence number of this RAMS-I message. During rapid acquisition, multiple RAMS-I messages MAY be sent and/or the same RAMS-I message MAY be repeated. The first RAMS-I message SHALL have an MSN value of 0. This value SHALL NOT be changed if the same RAMS-I message is sent to the same RTP_Rx multiple times for redundancy purposes. If a new information is conveyed in a new RAMS-I message, the MSN value SHALL be incremented by one. o Response (16 bits): Mandatory field that denotes the RS response code for this RAMS-I message. This document defines several initial response codes and registers them with IANA. If a new vendor-neutral response code will be defined, it MUST be registered with IANA through the guidelines specified in Section 12.6. If the new response code is intended to be used privately by a vendor, there is no need for IANA management. VerSteeg, et al. Expires May 20, 2010 [Page 28] Internet-Draft Rapid Acquisition of RTP Sessions November 2009 Instead, the vendor MUST use the private extension mechanism (Section 7.1.2) to convey its message and MUST indicate this by putting zero in the Response field. o RTP Seqnum of the First Packet (16 bits): TLV element that specifies the RTP sequence number of the first packet that will be sent in the retransmission session. This allows RTP_Rx to know whether one or more packets sent by RS have been dropped at the beginning of the retransmission session. If RS accepts the RAMS request, this element MUST exist. If RS rejects the RAMS request, this element SHALL NOT exist. Type: 31 o Earliest Multicast Join Time (32 bits): Optional TLV element that specifies the time difference (i.e., delta time) between the arrival of this RAMS-I message and the earliest time instant when RTP_Rx could join the primary multicast session in RTP ticks. A zero value in this field means that RTP_Rx can join the primary multicast session right away. Note that if the RAMS request has been accepted, RS MUST send this field at least once, so that RTP_Rx knows when to join the primary multicast session. If the burst request has been rejected as indicated in the Response field, this field MAY be omitted or set to 0. In that case, it is up to RTP_Rx when or whether to join the primary multicast session. Type: 32 o Burst Duration (32 bits): Optional TLV element that denotes the duration of the burst that RS is planning to send (in RTP ticks). In the absence of additional stimulus, RS will send a burst of this duration. However, the burst duration may be modified by subsequent events, including changes in the primary multicast stream and reception of RAMS-T messages. Note that RS MUST terminate the flow in a deterministic timeframe, even if it does not get an RAMS-T or a BYE from RTP_Rx. It is optional to send this field in an RAMS-I message when the burst request is accepted. If the burst request has been rejected as indicated in the Response field, this field MAY be omitted or set to 0. Type: 33 o Max Transmit Bitrate (32 bits): Optional TLV element that denotes the maximum bitrate (in bits per second) that will be used by RS. VerSteeg, et al. Expires May 20, 2010 [Page 29] Internet-Draft Rapid Acquisition of RTP Sessions November 2009 Type: 34 The semantics of the RAMS-I feedback message is independent of the payload type. The RAMS-I message MAY be sent multiple times at the start of, prior to, or during the unicast burst. The subsequent RAMS-I messages MAY signal changes in any of the fields. 7.4. RAMS Termination The RAMS Termination message is identified by SFMT=3. The FCI field MUST contain only one RAMS Termination. The RAMS Termination may be used to assist RS in determining when to stop the burst. If prior to sending the RAMS-T message RTP_Rx has already joined the primary multicast session and received at least one RTP packet from the multicast session, RTP_Rx includes the sequence number of the first RTP packet in the RAMS-T message. With this information, RS can decide when to terminate the unicast burst. If RTP_Rx issues the RAMS-T message before it has joined and/or begun receiving RTP packets from the primary multicast session, RTP_Rx does not specify any sequence number in the RAMS-T message, which indicates RS to stop the burst immediately. However, the RAMS-T message may get lost and RS may not receive this message. The FCI field has the structure depicted in Figure 8. 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | SFMT=3 | Optional TLV-encoded Fields | +-+-+-+-+-+-+-+-+ | : Optional TLV-encoded Fields (and Padding, if needed) : +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 8: FCI field syntax for the RAMS Termination message o Extended RTP Seqnum of First Multicast Packet (32 bits): Optional TLV element that specifies the extended RTP sequence number of the of the first multicast packet received by RTP_Rx. If no RTP packet has been received from the primary multicast session, this field does not exist and tells RS to stop the burst immediately. VerSteeg, et al. Expires May 20, 2010 [Page 30] Internet-Draft Rapid Acquisition of RTP Sessions November 2009 Type: 61 The semantics of the RAMS-T feedback message is independent of the payload type. 8. SDP Definitions and Examples 8.1. Definitions The syntax of the 'rtcp-fb' attribute has been defined in [RFC4585]. Here we add the following syntax to the 'rtcp-fb' attribute (the feedback type and optional parameters are all case sensitive): (In the following ABNF [RFC5234], fmt, SP and CRLF are used as defined in [RFC4566].) rtcp-fb-syntax = "a=rtcp-fb:" rtcp-fb-pt SP rtcp-fb-val CRLF rtcp-fb-pt = "*" ; wildcard: applies to all formats / fmt ; as defined in SDP spec rtcp-fb-val = "nack" SP "ssli" The following parameter is defined in this document for use with 'nack': o 'ssli' stands for Stream Synchronization Loss Indication and indicates the use of RAMS messages as defined in Section 7. This document also defines a new SDP attribute ('rams-updates') that indicates whether RS supports updated request messages or not. This attribute is used in a declarative manner. If RS supports updated request messages and this attribute is included in the SDP description, RTP_Rx MAY send updated requests. RS may or may not be able to accept value changes in every field in an RAMS-R message. However, if the 'rams-updates' attribute is not included in the SDP description, RTP_Rx SHALL NOT send updated requests (RTP_Rx MAY repeat its initial request without changes, though). 8.2. Examples This section provides a declarative SDP [RFC4566] example for enabling rapid acquisition of multicast RTP sessions. The following example uses the SDP grouping semantics [I-D.ietf-mmusic-rfc3388bis], the RTP/AVPF profile [RFC4585], the RTP retransmissions [RFC4588], the RTCP extensions for SSM sessions with unicast feedback [I-D.ietf-avt-rtcpssm] and the source-specific media attributes VerSteeg, et al. Expires May 20, 2010 [Page 31] Internet-Draft Rapid Acquisition of RTP Sessions November 2009 [RFC5576]. In the example shown Figure 9, we have a primary multicast stream and a unicast retransmission stream. The source stream is multicast from a distribution source (with a source IP address of 192.0.2.2) to the multicast destination address of 233.252.0.2 and port 41000. A Retransmission Server including feedback target functionality (with an address of 192.0.2.1 and port of 41001) is specified with the 'rtcp' attribute. The RTP receiver(s) can report missing packets on the source stream to the feedback target and request retransmissions. In the RAMS context, the parameter 'rtx-time' specifies the time in milliseconds that the Retransmission Server keeps an RTP packet in its buffer available for retransmission (measured from the time the packet was received by the Retransmission Server). The RTP retransmissions are sent on a unicast session with a destination port of 41002. The RTCP port for the unicast session (41003) is specified with the 'rtcp' attribute. In this example, both the conventional retransmission and rapid acquisition support are enabled. This is achieved by the additional "a=rtcp-fb:98 nack ssli" line. Note that this SDP includes the "a=sendonly" line for the media description of the retransmission stream and is for the Retransmission Server (RS). Its counterpart for the RTP Receiver (RTP_Rx) includes the "a=recvonly" line as shown in Figure 10. When an RTP receiver requires rapid acquisition for a new multicast session it wants to join, it sends an RAMS-R message to the feedback target of that primary multicast session. This feedback message has to have the SSRC of the primary multicast stream for which rapid acquisition is requested for. However, since this RTP receiver has not received any RTP packets from the primary multicast session yet, the RTP receiver MUST learn the SSRC value from the 'ssrc' attribute of the media description [I-D.ietf-avt-rtcpssm]. In addition to the SSRC value, the 'cname' source attribute MUST also be present in the SDP description [RFC5576]. Note that listing the SSRC values for the primary multicast sessions in the SDP file does not create a problem in SSM sessions when an SSRC collision occurs. This is because in SSM sessions, an RTP receiver that observed an SSRC collision with a media source MUST change its own SSRC [I-D.ietf-avt-rtcpssm] by following the rules defined in [RFC3550]. A feedback target that receives an RAMS-R feedback message becomes aware that the prediction chain at the RTP receiver side has been broken or does not exist any more. If the necessary conditions are satisfied (as outlined in Section 7 of [RFC4585]) and available resources exist, RS MAY react to the RAMS-R message by sending any VerSteeg, et al. Expires May 20, 2010 [Page 32] Internet-Draft Rapid Acquisition of RTP Sessions November 2009 transport-layer and payload-specific feedback message(s) and starting the unicast burst. v=0 o=ali 1122334455 1122334466 IN IP4 rams.example.com s=Rapid Acquisition Example t=0 0 a=group:FID 3 4 a=rtcp-unicast:rsi m=video 41000 RTP/AVPF 98 i=Primary Multicast Stream #2 c=IN IP4 233.252.0.2/255 a=source-filter: incl IN IP4 233.252.0.2 192.0.2.2 a=recvonly a=rtpmap:98 MP2T/90000 a=rtcp:41001 IN IP4 192.0.2.1 a=rtcp-fb:98 nack a=rtcp-fb:98 nack ssli a=ssrc:123321 cname:iptv-ch32@rams.example.com a=rams-updates a=mid:3 m=video 41002 RTP/AVPF 99 i=Unicast Retransmission Stream #2 (Ret. and Rapid Acq. Support) c=IN IP4 192.0.2.1 a=sendonly a=rtpmap:99 rtx/90000 a=rtcp:41003 a=fmtp:99 apt=98; rtx-time=5000 a=mid:4 Figure 9: Example SDP for RS when RAMS support is enabled VerSteeg, et al. Expires May 20, 2010 [Page 33] Internet-Draft Rapid Acquisition of RTP Sessions November 2009 v=0 o=ali 1122334455 1122334466 IN IP4 rams.example.com s=Rapid Acquisition Example t=0 0 a=group:FID 3 4 a=rtcp-unicast:rsi m=video 41000 RTP/AVPF 98 i=Primary Multicast Stream #2 c=IN IP4 233.252.0.2/255 a=source-filter: incl IN IP4 233.252.0.2 192.0.2.2 a=recvonly a=rtpmap:98 MP2T/90000 a=rtcp:41001 IN IP4 192.0.2.1 a=rtcp-fb:98 nack a=rtcp-fb:98 nack ssli a=ssrc:123321 cname:iptv-ch32@rams.example.com a=rams-updates a=mid:3 m=video 41002 RTP/AVPF 99 i=Unicast Retransmission Stream #2 (Ret. and Rapid Acq. Support) c=IN IP4 192.0.2.1 a=recvonly a=rtpmap:99 rtx/90000 a=rtcp:41003 a=fmtp:99 apt=98; rtx-time=5000 a=mid:4 Figure 10: Example SDP for RTP_Rx when RAMS support is enabled In the above SDP descriptions, the ports for the unicast session are also declaratively specified and the RTP receivers are required to use them. In certain scenarios, however, RTP receivers may want to use other desired ports to receive the burst and retransmission packets. A current work considers this feature in more general terms and provides the recommended mechanisms [I-D.begen-avt-ports-for-ucast-mcast-rtp]. In this section, we considered the simplest scenario where the primary multicast session carried only one multicast stream and the RTP receiver wanted to rapidly acquire this stream only. Discussions on scenarios where the primary multicast session carries two or more multicast streams or the RTP receiver wants to acquire one or more multicast streams from multiple multicast RTP sessions at the same time are presented in [I-D.begen-avt-rams-scenarios]. VerSteeg, et al. Expires May 20, 2010 [Page 34] Internet-Draft Rapid Acquisition of RTP Sessions November 2009 9. NAT Considerations For a variety of reasons, one or more NAPT devices (hereafter simply called NAT) are expected to exist between RTP_Rx and RS. NATs have a variety of operating characteristics for UDP traffic [RFC4787]. For a NAT to permit traffic from RS to arrive at RTP_Rx, the NAT(s) must first either: a. See UDP traffic sent from RTP_Rx (which is on the 'inside' of the NAT) to RS (which is on the 'outside' of the NAT). This traffic is sent to the same transport address as the subsequent response traffic, OR; b. Be configured to forward certain ports (e.g., using HTML configuration, UPnP IGD [UPnP-IGD], DLNA [DLNA]). Details of this are out of scope of this document. For both (a) and (b), RTP_Rx is responsible for maintaining the NAT's state if it wants to receive traffic from the RS on that port. For (a), RTP_Rx MUST send UDP traffic to keep the NAT binding alive, at least every 30 seconds [RFC4787]. Note that while (a) is more like an automatic/dynamic configuration, (b) is more like a manual/static configuration. 10. Known Implementations 10.1. Open Source RTP Receiver Implementation by Cisco An open source RTP Receiver code that implements the functionalities introduced in this document is available. For documentation, visit the following URL: http://www.cisco.com/en/US/docs/video/cds/cda/vqe/3_4/user/guide/ vqe_guide3_4.html The code is also available at: ftp://ftpeng.cisco.com/ftp/vqec/ Note that this code is under development and may be based on an earlier version of this document. As we make progress in the draft, the source code will also be updated to reflect the changes. Some preliminary results based on this code are available in [CCNC09] and [IC2009]. VerSteeg, et al. Expires May 20, 2010 [Page 35] Internet-Draft Rapid Acquisition of RTP Sessions November 2009 10.2. IPTV Commercial Implementation by Microsoft Rapid Acquisition of Multicast RTP Sessions is supported as part of the Microsoft Mediaroom Internet Protocol Television (IPTV) and multimedia software platform. This system is in wide commercial deployment. More information can be found at: http://www.microsoft.com/mediaroom http://informitv.com/articles/2008/10/13/channelchangetimes/ 11. Security Considerations Applications that are using RAMS make heavy use of the unicast feedback mechanism described in [I-D.ietf-avt-rtcpssm] and the payload format defined in [RFC4588]. Thus, these applications are subject to the general security considerations discussed in [I-D.ietf-avt-rtcpssm] and [RFC4588]. In this section, we give an overview of the guidelines and suggestions described in these specifications from a RAMS perspective. We also discuss the security considerations that explicitly apply to RAMS applications. First of all, much of the session description information is available in the SDP descriptions that are distributed to the media sources, Retransmission Servers and RTP Receivers. Adequate security measures are RECOMMENDED to ensure the integrity and authenticity of the SDP descriptions so that transport addresses of the media sources, Feedback Targets as well as other session-specific information can be authenticated. Compared to an RTCP NACK message that triggers one or more retransmissions, an RAMS Request (RAMS-R) message may trigger a new burst stream to be sent by the Retransmission Server. Depending on the application-specific requirements and conditions existing at the time of the RAMS-R reception by the Retransmission Server, the resulting burst stream may contain potentially a large number of retransmission packets. Since these packets are sent at a faster than the nominal rate of the multicast session, RAMS consumes more resources on the Retransmission Server, the RTP Receiver and the network. This particularly makes denial-of-service attacks more intense, and hence, more harmful than attacks that target ordinary retransmission sessions. Following the suggestions given in [RFC4588], counter-measures SHOULD be taken to prevent tampered or spoofed RTCP packets. Tampered RAMS-R messages may trigger inappropriate burst streams or alter the existing burst streams in an inappropriate way. For example, if the VerSteeg, et al. Expires May 20, 2010 [Page 36] Internet-Draft Rapid Acquisition of RTP Sessions November 2009 Max Receive Bitrate field is altered by a tampered RAMS-R message, the updated burst may overflow the buffer on the receiver side, or oppositely, may slow down the burst to the point that it is useless. Tampered RAMS Termination (RAMS-T) messages may terminate valid burst streams pre-maturely resulting in gaps in the received RTP packets. RAMS Information (RAMS-I) messages contain fields that are critical for the success of the RAMS operation. Any tampered information in the RAMS-I message may easily cause the RTP Receiver to make wrong decisions. Consequently, the RAMS operation may fail. While most of the denial-of-service attacks can be prevented by the integrity and authenticity checks enabled by SRTP, an attack can still be started by legitimate endpoints that send several valid RAMS-R messages to a particular Feedback Target in a synchronized fashion and very short amount of time. Since a RAMS operation may temporarily consume a large amount of resources, a series of the RAMS-R messages may temporarily overload the Retransmission Server. In these circumstances, the Retransmission Server may, for example, reject incoming RAMS requests until its resources become available again. One means to ameliorate this threat is to apply a per- endpoint policing mechanism on the incoming RAMS requests. A reasonable policing mechanism should consider application-specific requirements and minimize false negatives. In addition to the denial-of-service attacks, man-in-the-middle and replay attacks can also be harmful. However, RAMS itself does not bring any new risks or threats other than the ones discussed in [I-D.ietf-avt-rtcpssm]. [RFC4588] RECOMMENDS that the cryptography mechanisms are used for the retransmission payload format to provide protection against known plaintext attacks. As discussed in [RFC4588], the retransmission payload format sets the timestamp field in the RTP header to the media timestamp of the original packet and this does not compromise the confidentiality. Furthermore, if cryptography is used to provide security services on the original stream, then the same services, with equivalent cryptographic strength, MUST be provided on the retransmission stream per [RFC4588]. 12. IANA Considerations The following contact information shall be used for all registrations in this document: VerSteeg, et al. Expires May 20, 2010 [Page 37] Internet-Draft Rapid Acquisition of RTP Sessions November 2009 Ali Begen abegen@cisco.com 170 West Tasman Drive San Jose, CA 95134 USA Note to the RFC Editor: In the following, please replace "XXXX" with the number of this document prior to publication as an RFC. 12.1. Registration of SDP Attributes This document registers a new attribute name in SDP. SDP Attribute ("att-field"): Attribute name: rams-updates Long form: Support for Updated RAMS Request Messages Type of name: att-field Type of attribute: Media level Subject to charset: No Purpose: See this document Reference: [RFCXXXX] Values: None 12.2. Registration of SDP Attribute Values This document registers a new value for the 'nack' attribute to be used with the 'rtcp-fb' attribute in SDP. For more information about 'rtcp-fb', refer to [RFC4585]. Value name: ssli Long name: Stream Synchronization Loss Indication Usable with: nack Reference: [RFCXXXX] 12.3. Registration of FMT Values Within the RTPFB range, the following format (FMT) value is registered: VerSteeg, et al. Expires May 20, 2010 [Page 38] Internet-Draft Rapid Acquisition of RTP Sessions November 2009 Name: RAMS Long name: Rapid Acquisition of Multicast Sessions Value: 6 Reference: [RFCXXXX] 12.4. SFMT Values for RAMS Messages Registry This document creates a new sub-registry for the sub-feedback message type (SFMT) values to be used with the FMT value registered for RAMS messages. The registry is called the SFMT Values for RAMS Messages Registry. This registry is to be managed by the IANA according to the Specification Required policy of [RFC5226]. The length of the SFMT field in the RAMS messages is a single octet, allowing 256 values. The registry is initialized with the following entries: Value Name Reference ----- -------------------------------------------------- ------------- 1 RAMS Request [RFCXXXX] 2 RAMS Information [RFCXXXX] 3 RAMS Termination [RFCXXXX] The SFMT values 0 and 255 are reserved for future use. Any registration for an unassigned SFMT value MUST contain the following information: o Contact information of the one doing the registration, including at least name, address, and email. o A detailed description of what the new SFMT represents and how it shall be interpreted. Note that new RAMS functionality should be introduced by using the extension mechanism within the existing RAMS message types not by introducing new message types unless it is absolutely necessary. 12.5. RAMS TLV Space Registry This document creates a new IANA TLV space registry for the RAMS extensions. The registry is called the RAMS TLV Space Registry. This registry is to be managed by the IANA according to the Specification Required policy of [RFC5226]. The length of the Type field in the TLV elements is a single octet, VerSteeg, et al. Expires May 20, 2010 [Page 39] Internet-Draft Rapid Acquisition of RTP Sessions November 2009 allowing 256 values. The Type values 0 and 255 are reserved for future use. The Type values between (and including) 128 and 254 are reserved for private extensions. The registry is initialized with the following entries: Type Description Reference ---- -------------------------------------------------- ------------- 1 Min RAMS Buffer Fill Requirement [RFCXXXX] 2 Max RAMS Buffer Fill Requirement [RFCXXXX] 3 Max Receive Bitrate [RFCXXXX] 31 RTP Seqnum of the First Packet [RFCXXXX] 32 Earliest Multicast Join Time [RFCXXXX] 33 Burst Duration [RFCXXXX] 34 Max Transmit Bitrate [RFCXXXX] 61 Extended RTP Seqnum of First Multicast Packet [RFCXXXX] Any registration for an unassigned Type value MUST contain the following information: o Contact information of the one doing the registration, including at least name, address, and email. o A detailed description of what the new TLV element represents and how it shall be interpreted. 12.6. RAMS Response Code Space Registry This document creates a new IANA TLV space registry for the RAMS response codes. The registry is called the RAMS Response Code Space Registry. This registry is to be managed by the IANA according to the Specification Required policy of [RFC5226]. The length of the Response field is two octets, allowing 65536 codes. However, the response codes have been classified and registered following an HTTP-style code numbering in this document. New response codes SHALL follow the guidelines below: Code Level Purpose ---------- --------------- 1xx Informational 2xx Success 3xx Redirection 4xx RTP Receiver Error 5xx Retransmission Server Error VerSteeg, et al. Expires May 20, 2010 [Page 40] Internet-Draft Rapid Acquisition of RTP Sessions November 2009 The Response code 65536 is reserved for future use. The registry is initialized with the following entries: Code Description Reference ----- -------------------------------------------------- ------------- 0 A private response code is included in the message [RFCXXXX] 100 Parameter update for RAMS session [RFCXXXX] 101 RAMS session was terminated by RAMS-T [RFCXXXX] 102 RAMS session was terminated by RS [RFCXXXX] 200 RAMS request has been accepted [RFCXXXX] 201 Unicast burst has been completed [RFCXXXX] 400 Invalid RAMS-R message syntax 401 Invalid min buffer requirement in RAMS-R message [RFCXXXX] 402 Invalid max buffer requirement in RAMS-R message [RFCXXXX] 403 Invalid max bw requirement in RAMS-R message [RFCXXXX] 500 An unspecified RS internal error has occurred [RFCXXXX] 501 RS has no bandwidth to start RAMS session [RFCXXXX] 502 RS has no CPU available to start RAMS session [RFCXXXX] 503 RAMS functionality is not available on RS [RFCXXXX] 504 RAMS functionality is not available for RTP_Rx [RFCXXXX] 505 RAMS functionality is not available for the requested multicast stream [RFCXXXX] 506 RS has no a valid starting point available for the requested multicast stream [RFCXXXX] 507 RS has no reference information available for the requested multicast stream [RFCXXXX] Any registration for an unassigned Response code MUST contain the following information: o Contact information of the one doing the registration, including at least name, address, and email. o A detailed description of what the new Response code describes and how it shall be interpreted. 13. Acknowledgments The authors would like to specially thank Peilin Yang for his contributions to this document and detailed reviews. VerSteeg, et al. Expires May 20, 2010 [Page 41] Internet-Draft Rapid Acquisition of RTP Sessions November 2009 The authors also thank the following individuals for their contributions, comments and suggestions to this document: Dave Oran, Dan Wing, Tony Faustini, Jeff Goldberg, Muriel Deschanel, Orit Levin, Roni Even, Guy Hirson, Tom Taylor, Xavier Marjou, Ye-Kui Wang, Zixuan Zou, Ingemar Johansson, Haibin Song, Ning Zong, Jonathan Lennox and Sean Sheedy. 14. Change Log 14.1. draft-ietf-avt-rapid-acquisition-for-rtp-05 The following are the major changes compared to version 04: o Editorial changes throughout the document. 14.2. draft-ietf-avt-rapid-acquisition-for-rtp-04 The following are the major changes compared to version 03: o Clarifications for the definition of RS. o Response codes have been defined. 14.3. draft-ietf-avt-rapid-acquisition-for-rtp-03 The following are the major changes compared to version 02: o Clarifications for the RAMS-I message. o Type values have been assigned. 14.4. draft-ietf-avt-rapid-acquisition-for-rtp-02 The following are the major changes compared to version 01: o Port mapping discussion has been removed since it will be discussed in a separate draft. o Security considerations section has been added. o Burst shaping section has been completed. o Most of the outstanding open issues have been addressed. VerSteeg, et al. Expires May 20, 2010 [Page 42] Internet-Draft Rapid Acquisition of RTP Sessions November 2009 14.5. draft-ietf-avt-rapid-acquisition-for-rtp-01 The following are the major changes compared to version 00: o Formal definitions of vendor-neutral and private extensions and their IANA registries have been added. o SDP examples were explained in more detail. o The sub-FMT field has been introduced in the RAMS messages for message type identification. o Some terminology has been fixed. o NAT considerations section has been added. 14.6. draft-ietf-avt-rapid-acquisition-for-rtp-00 This is a resubmission of version 03 as a WG item. 14.7. draft-versteeg-avt-rapid-synchronization-for-rtp-03 The following are the major changes compared to version 02: o The title and message names have been changed. o RTCP message semantics have been added. RAMS protocol has been revised to handle updated requests and responses. o Definitions have been revised. o RTP/RTCP muxing reference has been added. 14.8. draft-versteeg-avt-rapid-synchronization-for-rtp-02 The following are the major changes compared to version 01: o The discussion around MPEG2-TS has been moved to another document. o The RAMS-R, RAMS-I and RAMS-T messages have been extensively modified and they have been made mandatory. o IANA Considerations section has been updated. o The discussion of RTCP XR report has been moved to another document. VerSteeg, et al. Expires May 20, 2010 [Page 43] Internet-Draft Rapid Acquisition of RTP Sessions November 2009 o A new section on protocol design considerations has been added. 14.9. draft-versteeg-avt-rapid-synchronization-for-rtp-01 The following are the major changes compared to version 00: o The core of the rapid synchronization method is now payload- independent. But, the draft still defines payload-specific messages that are required for enabling rapid synch for the RTP flows carrying MPEG2-TS. o RTCP APP packets have been removed, new RTCP transport-layer and payload-specific feedback messages have been defined. o The step for leaving the current multicast session has been removed from Section 6.2. o A new RTCP XR (Multicast Join) report has been defined. o IANA Considerations section have been updated. o Editorial changes to clarify several points. 15. References 15.1. Normative References [RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson, "RTP: A Transport Protocol for Real-Time Applications", STD 64, RFC 3550, July 2003. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC3376] Cain, B., Deering, S., Kouvelas, I., Fenner, B., and A. Thyagarajan, "Internet Group Management Protocol, Version 3", RFC 3376, October 2002. [RFC3810] Vida, R. and L. Costa, "Multicast Listener Discovery Version 2 (MLDv2) for IPv6", RFC 3810, June 2004. [RFC4604] Holbrook, H., Cain, B., and B. Haberman, "Using Internet Group Management Protocol Version 3 (IGMPv3) and Multicast Listener Discovery Protocol Version 2 (MLDv2) for Source- Specific Multicast", RFC 4604, August 2006. [RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session VerSteeg, et al. Expires May 20, 2010 [Page 44] Internet-Draft Rapid Acquisition of RTP Sessions November 2009 Description Protocol", RFC 4566, July 2006. [I-D.ietf-mmusic-rfc3388bis] Camarillo, G. and H. Schulzrinne, "The SDP (Session Description Protocol) Grouping Framework", draft-ietf-mmusic-rfc3388bis-04 (work in progress), November 2009. [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. [I-D.ietf-avt-rtcpssm] Ott, J. and J. Chesterfield, "RTCP Extensions for Single- Source Multicast Sessions with Unicast Feedback", draft-ietf-avt-rtcpssm-19 (work in progress), November 2009. [RFC5576] Lennox, J., Ott, J., and T. Schierl, "Source-Specific Media Attributes in the Session Description Protocol (SDP)", RFC 5576, June 2009. [RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax Specifications: ABNF", STD 68, RFC 5234, January 2008. [RFC5506] Johansson, I. and M. Westerlund, "Support for Reduced-Size Real-Time Transport Control Protocol (RTCP): Opportunities and Consequences", RFC 5506, April 2009. [RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA Considerations Section in RFCs", BCP 26, RFC 5226, May 2008. 15.2. Informative References [RFC0768] Postel, J., "User Datagram Protocol", STD 6, RFC 768, August 1980. [I-D.ietf-rmt-pi-norm-revised] Adamson, B., Bormann, C., Handley, M., and J. Macker, "NACK-Oriented Reliable Multicast Transport Protocol", draft-ietf-rmt-pi-norm-revised-14 (work in progress), September 2009. VerSteeg, et al. Expires May 20, 2010 [Page 45] Internet-Draft Rapid Acquisition of RTP Sessions November 2009 [I-D.begen-avt-rams-scenarios] Begen, A., "Considerations for RAMS Scenarios", draft-begen-avt-rams-scenarios-00 (work in progress), October 2009. [I-D.begen-avt-rtp-mpeg2ts-preamble] Begen, A. and E. Friedrich, "RTP Payload Format for MPEG2-TS Preamble", draft-begen-avt-rtp-mpeg2ts-preamble-03 (work in progress), October 2009. [I-D.begen-avt-rapid-sync-rtcp-xr] Begen, A. and E. Friedrich, "Multicast Acquisition Report Block Type for RTCP XR", draft-begen-avt-rapid-sync-rtcp-xr-03 (work in progress), October 2009. [I-D.ietf-avt-rapid-rtp-sync] Perkins, C. and T. Schierl, "Rapid Synchronisation of RTP Flows", draft-ietf-avt-rapid-rtp-sync-07 (work in progress), November 2009. [I-D.westerlund-avt-ecn-for-rtp] Westerlund, M., Johansson, I., Perkins, C., and K. Carlberg, "Explicit Congestion Notification (ECN) for RTP over UDP", draft-westerlund-avt-ecn-for-rtp-02 (work in progress), October 2009. [I-D.ietf-avt-rtp-and-rtcp-mux] Perkins, C. and M. Westerlund, "Multiplexing RTP Data and Control Packets on a Single Port", draft-ietf-avt-rtp-and-rtcp-mux-07 (work in progress), August 2007. [RFC4787] Audet, F. and C. Jennings, "Network Address Translation (NAT) Behavioral Requirements for Unicast UDP", BCP 127, RFC 4787, January 2007. [RFC5389] Rosenberg, J., Mahy, R., Matthews, P., and D. Wing, "Session Traversal Utilities for NAT (STUN)", RFC 5389, October 2008. [I-D.begen-avt-ports-for-ucast-mcast-rtp] Begen, A. and B. Steeg, "Port Mapping Between Unicast and Multicast RTP Sessions", draft-begen-avt-ports-for-ucast-mcast-rtp-01 (work in progress), October 2009. VerSteeg, et al. Expires May 20, 2010 [Page 46] Internet-Draft Rapid Acquisition of RTP Sessions November 2009 [UPnP-IGD] Forum, UPnP., "Universal Plug and Play (UPnP) Internet Gateway Device (IGD)", November 2001. [DLNA] , DLNA., "http://www.dlna.org/home". [CCNC09] Begen, A., Glazebrook, N., and W. VerSteeg, "A Unified Approach for Repairing Packet Loss and Accelerating Channel Changes in Multicast IPTV (IEEE CCNC)", January 2009. [IC2009] Begen, A., Glazebrook, N., and W. VerSteeg, "Reducing Channel Change Times in IPTV with Real-Time Transport Protocol (IEEE Internet Computing)", May 2009. Authors' Addresses Bill VerSteeg Cisco 5030 Sugarloaf Parkway Lawrenceville, GA 30044 USA Email: billvs@cisco.com Ali Begen Cisco 170 West Tasman Drive San Jose, CA 95134 USA Email: abegen@cisco.com Tom VanCaenegem Alcatel-Lucent Copernicuslaan 50 Antwerpen, 2018 Belgium Email: Tom.Van_Caenegem@alcatel-lucent.be VerSteeg, et al. Expires May 20, 2010 [Page 47] Internet-Draft Rapid Acquisition of RTP Sessions November 2009 Zeev Vax Microsoft Corporation 1065 La Avenida Mountain View, CA 94043 USA Email: zeevvax@microsoft.com VerSteeg, et al. Expires May 20, 2010 [Page 48]