AVT Working Group V. Singh Internet-Draft T. Karkkainen Intended status: Experimental J. Ott Expires: August 29, 2011 S. Ahsan Aalto University L. Eggert Nokia February 25, 2011 Multipath RTP (MPRTP) draft-singh-avtcore-mprtp-00 Abstract The Real-time Transport Protocol (RTP) is used to deliver real-time content and, along with the RTP Control Protocol (RTCP), forms the control channel between the sender and receiver. However, RTP and RTCP assume a single delivery path between the sender and receiver and make decisions based on the measured characteristics of this single path. Increasingly, endpoints are becoming multi-homed, which means that they are connected via multiple Internet paths. Network utilization can be improved when endpoints use multiple parallel paths for communication. The resulting increase in reliability and throughput can also enhance the user experience. This document extends the Real-time Transport Protocol (RTP) so that a single session can take advantage of the availability of multiple paths between two endpoints. Status of this Memo This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at http://datatracker.ietf.org/drafts/current/. 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." This Internet-Draft will expire on August 29, 2011. Copyright Notice Singh, et al. Expires August 29, 2011 [Page 1]
Internet-Draft Multipath RTP February 2011 Copyright (c) 2011 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 Simplified BSD License. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4 1.1. Requirements Language . . . . . . . . . . . . . . . . . . 4 1.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 4 1.3. Use-cases . . . . . . . . . . . . . . . . . . . . . . . . 4 2. Goals . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 2.1. Functional goals . . . . . . . . . . . . . . . . . . . . . 5 2.2. Compatibility goals . . . . . . . . . . . . . . . . . . . 6 3. RTP Topologies . . . . . . . . . . . . . . . . . . . . . . . . 6 4. MPRTP Architecture . . . . . . . . . . . . . . . . . . . . . . 6 4.1. Relationship of MPRTP with Session Signaling . . . . . . . 8 5. Example Media Flow diagrams . . . . . . . . . . . . . . . . . 8 5.1. Streaming use-case . . . . . . . . . . . . . . . . . . . . 8 5.2. Conversational use-case . . . . . . . . . . . . . . . . . 9 5.3. Challenges with Multipath Interface Discovery . . . . . . 10 6. MPRTP Functional blocks . . . . . . . . . . . . . . . . . . . 10 7. Available mechanisms within the functional blocks . . . . . . 11 7.1. Session Setup . . . . . . . . . . . . . . . . . . . . . . 11 7.2. Expanding RTP . . . . . . . . . . . . . . . . . . . . . . 11 7.3. Adding New Interfaces . . . . . . . . . . . . . . . . . . 11 7.4. Expanding RTCP . . . . . . . . . . . . . . . . . . . . . . 12 7.5. Checking and Failure Handling . . . . . . . . . . . . . . 12 8. MPRTP Protocol . . . . . . . . . . . . . . . . . . . . . . . . 12 8.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . . 13 8.1.1. Subflow or Interface Advertisement . . . . . . . . . . 14 8.1.2. Path selection . . . . . . . . . . . . . . . . . . . . 14 8.1.3. Opening subflows . . . . . . . . . . . . . . . . . . . 15 8.2. RTP Transmission . . . . . . . . . . . . . . . . . . . . . 15 8.3. Playout Considerations at the Receiver . . . . . . . . . . 15 8.4. Flow specific RTCP Statistics and RTCP Aggregation . . . . 15 8.5. RTCP Transmission . . . . . . . . . . . . . . . . . . . . 16 9. Packet Formats . . . . . . . . . . . . . . . . . . . . . . . . 16 9.1. MPRTP RTP Header Extension . . . . . . . . . . . . . . . . 16 Singh, et al. Expires August 29, 2011 [Page 2]
Internet-Draft Multipath RTP February 2011 9.1.1. MPRTP RTP header extension for a subflow . . . . . . . 16 9.2. MPRTP RTCP Header Extension . . . . . . . . . . . . . . . 18 9.2.1. MPRTP RTCP header extension for flow specific SR/RR . 18 9.2.2. MPRTP RTCP header extension for Interface advertisement . . . . . . . . . . . . . . . . . . . . 18 9.2.3. Interface Address Advertisement block . . . . . . . . 19 10. SDP Considerations . . . . . . . . . . . . . . . . . . . . . . 21 10.1. Increased Throughput . . . . . . . . . . . . . . . . . . . 21 10.2. MPRTP using preloaded ICE . . . . . . . . . . . . . . . . 21 11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 21 12. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 22 13. Security Considerations . . . . . . . . . . . . . . . . . . . 22 14. References . . . . . . . . . . . . . . . . . . . . . . . . . . 22 14.1. Normative References . . . . . . . . . . . . . . . . . . . 22 14.2. Informative References . . . . . . . . . . . . . . . . . . 23 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 23 Singh, et al. Expires August 29, 2011 [Page 3]
Internet-Draft Multipath RTP February 2011 1. Introduction Multi-homed endpoints are becoming common in today's Internet, e.g., devices that support multiple wireless access technologies such as 3G and Wireless LAN. This means that often there is more than one network path available between two endpoints. Transport protocols, such as RTP, have not been designed to take advantage of the availability of multiple concurrent paths and therefore cannot benefit from the increased capacity and reliability that can be achieved by pooling their respective capacities. Multipath RTP (MPRTP) is an OPTIONAL extension to RTP [1] that allows splitting a single RTP stream into multiple subflows that transmit over different paths. In effect, this pools the resource capacity of multiple paths. Multipath RTCP (MPRTCP) is an extension to RTCP, it is used along with MPRTP to report per-path sender and receiver characteristics. Other IETF transport protocols that are capable of using multiple paths include SCTP [9], MPTCP MPTCP [10] and SHIM6 [11]. However, these protocols are not suitable for realtime communications. 1.1. Requirements Language The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [2]. 1.2. Terminology o Endpoint: host either initiating or terminating an RTP connection. o Interface: A logical or physical component that is capable of acquiring a unique IP address. o Path: sequence of links between a sender and a receiver. Typically, defined by a set of source and destination addresses. o Subflow: A flow of RTP packets along a specific path, i.e., a subset of the packets belonging to an RTP stream. The combination of all RTP subflows forms the complete RTP stream. 1.3. Use-cases The primary use-case for MPRTP is transporting high bit-rate streaming multimedia content between endpoints, where at least one is multi-homed. Such endpoints could be residential IPTV devices that connect to the Internet through two different Internet service Singh, et al. Expires August 29, 2011 [Page 4]
Internet-Draft Multipath RTP February 2011 providers (ISPs), or mobile devices that connect to the Internet through 3G and WLAN interfaces. By allowing RTP to use multiple paths for transmission, the following gains can be achieved: o Higher quality: Pooling the resource capacity of multiple Internet paths allows higher bit-rate and higher quality codecs to be used. From the application perspective, the available bandwidth between the two endpoints increases. o Load balancing: Transmitting one RTP stream over multiple paths can reduce the bandwidth usage, compared to transmitting the same stream along a single path. This reduces the impact on other traffic. o Fault tolerance: When multiple paths are used in conjunction with redundancy mechanisms (FEC, re-transmissions, etc.), outages on one path have less impact on the overall perceived quality of the stream. A secondary use-case for MPRTP is transporting Voice over IP (VoIP) calls to a device with multiple interfaces. Again, such an endpoint could be a mobile device with multiple wireless interfaces. In this case, little is to be gained from resource pooling, i.e., higher capacity or load balancing, because a single path should be easily capable of handling the required load. However, using multiple concurrent subflows can improve fault tolerance, because traffic can shift between the subflows when path outages occur. This results in very fast transport-layer handovers that do not require support from signaling. 2. Goals This section outlines the basic goals that multipath RTP aims to meet. These are broadly classified as Functional goals and Compatibility goals. 2.1. Functional goals Allow unicast RTP session to be split into multiple subflows in order to be carried over multiple paths. This may prove beneficial in case of video streaming. o Increased Throughput: Cumulative capacity of the two paths may meet the requirements of the multimedia session. Therefore, MPRTP MUST support concurrent use of the multiple paths. Singh, et al. Expires August 29, 2011 [Page 5]
Internet-Draft Multipath RTP February 2011 o Improved Reliability: MPRTP SHOULD be able to send redundant or re-transmit packets along any available path to increase reliability. The protocol SHOULD be able to open new subflows for an existing session when new paths appear and MUST be able to close subflows when paths disappear. 2.2. Compatibility goals MPRTP MUST be backwards compatible; an MPRTP stream needs to fall back to be compatible with legacy RTP stacks if MPRTP support is not successfully negotiated. o Application Compatibility: MPRTP service model MUST be backwards compatible with existing RTP applications, i.e., an MPRTP stack MUST be able to work with legacy RTP applications and not require changes to them. Therefore, the basic RTP APIs MUST remain unchanged, but an MPRTP stack MAY provide extended APIs so that the application can configure any additional features provided by the MPRTP stack. o Network Compatibility: individual RTP subflows MUST themselves be well-formed RTP flows, so that they are able to traverse NATs and firewalls. This MUST be the case even when interfaces appear after session initiation. Interactive Connectivity Establishment (ICE) [3] MAY be used for discovering new interfaces or performing connectivity checks. 3. RTP Topologies RFC 5117 [12] describes a number of scenarios using mixers and translators in single-party (point-to-point), and multi-party (point- to-multipoint) scenarios. RFC 3550 [1] (Section 2.3 and 7.x) discuss in detail the impact of mixers and translators on RTP and RTCP packets. MPRTP assumes that if a mixer or translator exists in the network, then either all of the multiple paths or none of the multiple paths go via this component. 4. MPRTP Architecture In a typical scenario, an RTP session uses a single path. In an MPRTP scenario, an RTP session uses multiple subflows that each use a different path. Figure 1 shows the difference. Singh, et al. Expires August 29, 2011 [Page 6]
Internet-Draft Multipath RTP February 2011 +--------------+ Signaling +--------------+ | |------------------------------------>| | | Client |<------------------------------------| Server | | | Single RTP flow | | +--------------+ +--------------+ +--------------+ Signaling +--------------+ | |------------------------------------>| | | Client |<------------------------------------| Server | | |<------------------------------------| | +--------------+ MPRTP sub-flows +--------------+ Figure 1: Comparison between traditional RTP streaming and MPRTP +-----------------------+ +-------------------------------+ | Application | | Application | +-----------------------+ +-------------------------------+ | | | MPRTP | + RTP + +- - - - - - - -+- - - - - - - -+ | | | RTP subflow | RTP subflow | +-----------------------+ +---------------+---------------+ | UDP | | UDP | UDP | +-----------------------+ +---------------+---------------+ | IP | | IP | IP | +-----------------------+ +---------------+---------------+ Figure 2: MPRTP Architecture Figure 2 illustrates the differences between the standard RTP stack and the MPRTP stack. MPRTP receives a normal RTP session from the application and splits it into multiple RTP subflows. Each subflow is then sent along a different path to the receiver. To the network, each subflow appears as an independent, well-formed RTP flow. At the receiver, the subflows are combined to recreate the original RTP session. The MPRTP layer performs the following functions: o Path Management: The layer is aware of alternate paths to the peer, which may, for example, be the peer's multiple interfaces to send differently marked packets along separate paths. MPRTP also selects interfaces to send and receive data. Furthermore, it manages the port and IP address pair bindings for each subflow. o Packet Scheduling: the layer splits a single RTP flow into multiple subflows and sends them across multiple interfaces (paths). The splitting MAY BE done using different path characteristics. Singh, et al. Expires August 29, 2011 [Page 7]
Internet-Draft Multipath RTP February 2011 o Subflow recombination: the layer creates the original stream by recombining the independent subflows. Therefore, the multipath subflows appear as a single RTP stream to applications. 4.1. Relationship of MPRTP with Session Signaling Session signaling (e.g., SIP [13], RTSP [14]) SHOULD be done over a failover-capable or multipath-capable transport for e.g., SCTP [9] or MPTCP [10] instead of TCP or UDP. 5. Example Media Flow diagrams There may be many complex technical scenarios for MPRTP, however, this memo only considers the following two scenarios: 1) an unidirectional media flow that represents the streaming use-case, and 2) a bidirectional media flow that represents a conversational use- case. 5.1. Streaming use-case In the unidirectional scenario, the receiver (client) initiates a multimedia session with the sender (server). The receiver or the sender may have multiple interfaces and both endpoints are MPRTP- capable. Figure 3 shows this scenario. In this case, host A has multiple interfaces. Host B performs connectivity checks on host A's multiple interfaces. If the interfaces are reachable, then host B streams multimedia data along multiple paths to host A. Moreover, host B also sends RTCP Sender Reports (SR) for each subflow and host A responds with a standard RTCP Receiver Report (RR) for the overall session and receiver statistics for each subflow. Host B distributes the packets into the subflows based on the individually measured path characteristics. Alternatively, to reduce media startup time, host B may start streaming multimedia data to host A's initiating interface and then perform connectivity checks for the other interfaces. This method of updating a single path session to a multipath session is called "multipath session upgrade". Singh, et al. Expires August 29, 2011 [Page 8]
Internet-Draft Multipath RTP February 2011 Host A Host B ----------------------- ---------- Address A1 Address A2 Address B1 ----------------------- ---------- | Session Setup | |------------------------------------->| connection for the |<-------------------------------------| peers may be "preloaded" | | | (e.g., with ICE) | | | | (RTP data B1->A1, B1->A2) | |<=====================================| | |<========================| | | | Note: there maybe more scenarios. Figure 3: Unidirectional media flow 5.2. Conversational use-case In the bidirectional scenario, multimedia data flows in both directions. The two hosts exchange their lists of interfaces with each other and perform connectivity checks. Communication begins after each host finds suitable address, port pairs. All interfaces that receive data send back RTCP receiver statistics for each path. The peers balance their multimedia stream over multiple links based on the reception statistics from its peer and its own volume of traffic. Figure 4 describes an example of a bidirectional flow. Host A Host B ----------------------- ----------------------- Address A1 Address A2 Address B1 Address B2 ----------------------- ----------------------- | | | | | Session Setup | | connection for |----------------------------------->| | the peers may |<-----------------------------------| | be "preloaded" | | | | (e.g., with ICE) | | | | | (RTP data B1<->A1, B2<->A2) | | |<===================================| | | |<===================================| |===================================>| | | |===================================>| | | | | Note: the address pairs may have other permutations, and they maybe symmetric or asymmetric combinations. Figure 4: Bidirectional flow Singh, et al. Expires August 29, 2011 [Page 9]
Internet-Draft Multipath RTP February 2011 5.3. Challenges with Multipath Interface Discovery For some applications, where the user expects immediate playback, e.g., High Definition Media Streaming or IPTV, it may not be possible to perform connectivity checks within the given time bound. In these cases, connectivity checks MAY need to be done ahead of time. [Open Issue: ICE or any other system would have to be aware of the endpoint's interfaces ahead of time]. 6. MPRTP Functional blocks This section describes some of the functional blocks needed for MPRTP. We then investigate each block and consider available mechanisms in the next section. 1. Session Setup: Multipath session setup is an upgrade or add-on to a typical RTP session. Interfaces may appear or disappear at anytime during the session. To preserve backward compatibility with legacy applications, a multipath session MUST look like a bundle of individual RTP sessions. 2. Expanding RTP: For a multipath session, each subflow MUST look like an independent RTP flow, so that individual RTCP messages can be generated per subflow. Furthermore, MPRTP splits the single multimedia stream into multiple subflows based on path characteristics (e.g. RTT, loss-rate, receiver rate, bandwidth- delay product etc.) and dynamically adjusts the load on each link. 3. Adding Interfaces: Interfaces on the host need to be regularly discovered and signaled. This can be done at session setup and/or during the session. When discovering and receiving new interfaces, the MPRTP layer needs to select address and port pairs. 4. Expanding RTCP: MPRTP MUST recombine RTCP reports from each path to re-create a single RTCP message to maintain backward compatibility with legacy applications. 5. Maintenance and Failure Handling: In a multi-homed endpoint interfaces may appear and disappear. If this happens at the sender, it has to re-adjust the load on the available links. On the other hand, if this occurs on the receiver, then the multimedia data transmitted by the sender to those interfaces is lost. This data may be re-transmitted along a different path i.e., to a different interface on the receiver. Furthermore, the Singh, et al. Expires August 29, 2011 [Page 10]
Internet-Draft Multipath RTP February 2011 receiver has to explicitly signal the disappearance of an interface, or the sender has to detect it. What happens if the interface that setup the session disappears? does the control channel also failover? re-start the session? 6. Teardown: The MPRTP layer releases the occupied ports on the interfaces. 7. Available mechanisms within the functional blocks This section discusses some of the possible alternatives for each functional block mentioned in the previous section. 7.1. Session Setup MPRTP session can be set up in many possible ways e.g., during handshake, or upgraded mid-session. The capability exchange may be done using out-of-band signaling (e.g., SDP [15] in SIP [13], RTSP [14]) or in-band signaling (e.g., RTP/RTCP header extension). Furthermore, ICE [3] may be used for discovering and performing connectivity checks during session setup. 7.2. Expanding RTP RTCP [1] is generated per media session. However, with MPRTP, the media sender spreads the RTP load across several interfaces. The media sender SHOULD make the path selection, load balancing and fault tolerance decisions based on the characteristics of each path. Therefore, apart from normal RTP sequence numbers defined in [1], the MPRTP sender MUST add subflow-specific sequence numbers to help calculate fractional losses, jitter, RTT, playout time, etc., for each path and a flow identifier to associate the characteristics to a path. The RTP header extension for MPRTP is shown in Section 9). 7.3. Adding New Interfaces When interfaces appear and disappear mid-session, ICE [3] may be used for discovering interfaces and performing connectivity checks. However, MPRTP may require a capability re-negotiation (using SDP) to include all these new interfaces. This method is referred to as out- of-band multipath advertisement. Alternatively, when new interfaces appear the interface advertisements may be done in-band using RTP/RTCP extensions. The peers perform connectivity checks (see Figure 5 for more details). If the connectivity packets are received by the peers, then multimedia data can flow between the new address, port pairs. Singh, et al. Expires August 29, 2011 [Page 11]
Internet-Draft Multipath RTP February 2011 7.4. Expanding RTCP To provide accurate per path information an MPRTP host MUST send (SR/RR) report for each unique subflow along with the overall overall session RTCP report. Therefore, the additional sub flow reporting affects the RTCP bandwidth and the RTCP reporting interval for each subflow. RTCP report scheduling for each subflow may cause a problem for RTCP recombination and reconstruction in cases when 1) RTCP for a subflow is lost, and 2) RTCP for a subflow arrives later than the other subflows. (There maybe other cases as well.) The sender distributes the media across different paths using the per path RTCP reports. However, this document doesn't cover algorithms for congestion control or load balancing. 7.5. Checking and Failure Handling [Note: If the original interface that setup the session disappears then does the session signaling failover to another interface? Can we recommend that SIP/RTSP be run over MPTCP, SCTP]. 8. MPRTP Protocol To enable a quick start of a multimedia session, a multipath session MUST be upgraded from a single path session. Therefore, no explicit changes are needed in multimedia session setup and the session can be setup as before. Singh, et al. Expires August 29, 2011 [Page 12]
Internet-Draft Multipath RTP February 2011 Host A Host B ----------------------- ----------------------- Address A1 Address A2 Address B1 Address B2 ----------------------- ----------------------- | | | | | | (1) | | |--------------------------------------->| | |<---------------------------------------| | | | (2) | | |<=======================================| | |<=======================================| (3) | | | (4) | | |<=======================================| | |<=======================================| | |<=======================================| | | | (5) | | |- - - - - - - - - - - - - - - - - - - ->| | |<=======================================| (6) | |<=======================================| | | |<========================================| |<=======================================| | | |<========================================| Key: | Interface ---> Signaling Protocol <=== RTP Packets - -> RTCP Packet Figure 5: MPRTP New Interface 8.1. Overview The bullet points explain the different steps shown in Figure 5 for upgrading a standard single path multimedia session to multipath session. (1) The first two interactions between the hosts describes the standard session setup. This may be SIP or RTSP. (2) Following the setup, like in a conventional RTP scenario, host B using interface B1 starts to stream data to host A at interface A1. (3) Host B is an MPRTP-capable media sender and becomes aware of another interface B2. Singh, et al. Expires August 29, 2011 [Page 13]
Internet-Draft Multipath RTP February 2011 (4) Host B advertises the multiple interface addresses using an RTP header extensions. (5) Host A is an MPRTP-capable media receiver and becomes aware of another interface A2. It advertises the multiple interface addresses using an RTCP RR extension. Side note, if an MPRTP-capable host has only one interface even then it SHOULD advertise its single interface. (6) Each host receives information about the additional interfaces and performs the connectivity tests (not shown in figure). If the paths are reachable then the host starts to stream the multimedia content using the additional paths. 8.1.1. Subflow or Interface Advertisement To advertise the multiple interfaces, an MPRTP-capable endpoint MUST add the MPRTP Interface Advertisement defined in Figure 8 with the RTCP Sender Report (SR). Each unique address is encapsulated in an Interface Advertisement block and contains the IP address, RTP and RTCP port addresses. The Interface Advertisement blocks are ordered based on a decreasing priority level. On receiving the MPRTP Interface Advertisement, an MPRTP-capable receiver MUST respond with its own set of interfaces. If the sender and receiver have only one interface, then the endpoints MUST respond with the default IP, RTP port and RTCP port addresses. If an endpoint receives an RTCP report without the MPRTP Interface Advertisement, then the endpoint MUST assume that the other endpoint is not MPRTP capable. 8.1.2. Path selection After MPRTP support has been discovered and interface advertisements have been exchanged, the sender MUST initiate connectivity checks to determine which interface pairs offer valid paths between the sender and the receiver. Each combination of IP addresses and port pairs (5-tuple) is a unique subflow. An endpoint MUST associate a Flow ID to each unique subflow. To initiate a connectivity check, the endpoints send an RTP packet using the appropriate MPRTP extension header (See Figure 7), associated Flow ID and no RTP payload. The receiving endpoint replies to each connectivity check with an RTCP packet with the appropriate packet type (See Figure 9) and Flow ID. After the endpoint receives the reply, the path is considered a valid candidate for sending data. An endpoint MAY choose to do any number of connectivity checks for any interface pairs at any point in a session. Singh, et al. Expires August 29, 2011 [Page 14]
Internet-Draft Multipath RTP February 2011 [Open Issue: How should the endpoint adjust the RTCP Reporting interval/schedule the RTCP packet on receiving a connectivity check containing a new FlowID? Editor: One option is send immediately as defined in [4].] 8.1.3. Opening subflows The sender MAY open any number of subflows from the set of candidate subflows after performing connectivity checks. To use the subflow, the sender simply starts sending the RTP packets with an MPRTP extension shown in Figure 6. The MPRTP extension carries a mapping of a subflow packet to the aggregate flow. Namely, sequence numbers and timestamps associated with the subflow. [Open Issue: How to differentiate between Passive and Active connections?] [Open Issue: How to keep a passive connection alive, if not actively used?] 8.2. RTP Transmission The MPRTP layer SHOULD associate an RTP packet to a subflow based on a scheduling strategy. The scheduling strategy may either choose to augment the paths to create higher throughput or use the alternate paths for enhancing resilience or error-repair. Due to the changes in path characteristics, an MPRTP sender can change its scheduling strategy during an ongoing session. The MPRTP sender MUST also populate the flow specific fields described in the MPRTP extension header (see Section 9.1). 8.3. Playout Considerations at the Receiver A media receiver, irrespective of MPRTP support or not, should be able to playback the media stream because the received RTP packets are compliant to [1], i.e., a non-MPRTP receiver will ignore the MPRTP header and still be able to playback the RTP packets. However, the variation of jitter and loss per path may affect proper playout. By calculating optimum skew across all paths, the receiver can compensate for the jitter by modifying the playout delay (adaptive playout) of the received RTP packets. 8.4. Flow specific RTCP Statistics and RTCP Aggregation Aggregate RTCP provides the overall media statistics and follows the standard RTCP defined in RFC3550 [1]. However, flow specific RTCP provides the per path media statistics because the aggregate RTCP report may not provide sufficient per path information to an MPRTP Singh, et al. Expires August 29, 2011 [Page 15]
Internet-Draft Multipath RTP February 2011 scheduler. Specifically, the scheduler should be aware of each path's RTT and loss-rate, which an aggregate RTCP cannot provide. The sender/receiver MUST use non-compound RTCP reports defined in RFC5506 [5] to transmit the aggregate and flow-specific RTCP reports. Also, each subflow and the aggregate RTCP report MUST follow the timing rules defined in [4]. The RTCP reporting interval is locally implemented and the scheduling of the RTCP reports may depend on the the behavior of each path. For instance, the RTCP interval may be different for a passive path than an active path to keep port bindings alive. Additionally, a peer may decide to share the RTCP reporting bit rate equally across all its paths or schedule based on the receiver rate on each path. 8.5. RTCP Transmission The sender sends an RTCP SR on each active path. For each SR the receiver gets, it echoes one back to the same IP address-port pair that sent the SR. The receiver tries to choose the symmetric path and if the routing is symmetric then the per-path RTT calculations will work out correctly. However, even if the paths are not symmetric, the sender would at maximum, under-estimate the RTT of the path by a factor of half of the actual path RTT. 9. Packet Formats In this sub-section we define the protocol structures described in the previous sections. 9.1. MPRTP RTP Header Extension The MPRTP header extension is used to 1) distribute a single RTP stream over multiple subflows, 2) advertise the endpoint's multiple interface addresses, and 3) perform connectivity checks on the advertised interfaces. 9.1.1. MPRTP RTP header extension for a subflow The RTP header for each subflow is defined below: Singh, et al. Expires August 29, 2011 [Page 16]
Internet-Draft Multipath RTP February 2011 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |V=2|P|X| CC |M| PT | sequence number | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | timestamp | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | synchronization source (SSRC) identifier | +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+ | 0x10 | 0x00 | length=2 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | RTP H-Ext ID | length | MPR_Type=0x00 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Flow ID | Flow specific Sequence Number | +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+ | RTP payload | | .... | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 6: MPRTP header for subflow RTP H-Ext ID and length: 8-bits each It conforms to the 2-byte RTP header extension defined in [6]. RTP H-Ext=TBD The 8-bit length field is the length of extension data in bytes not including the RTP H-Ext ID and length fields. MPR_Type: 16-bits The MPR_Type field corresponds to the type of RTP packet. Namely: +---------------+---------------------------------------------------+ | MPR_Type | Use | | Value | | +---------------+---------------------------------------------------+ | 0x00 | Subflow RTP Header. For this case the Length is | | | set to 7 | | 0x01 | Connectivity Check. For this case the length is | | | set to 0 | | TBD | Keep Alive Packet. | +---------------+---------------------------------------------------+ Figure 7: RTP header extension values for MPRTP (MPR_Type) Singh, et al. Expires August 29, 2011 [Page 17]
Internet-Draft Multipath RTP February 2011 Flow ID: Identifier of the subflow. Every RTP packet belonging to the same subflow carries the same unique flow identifier. Flow specific Sequence Number: Sequence of the packet in the subflow. Each subflow has its own strictly monotonically increasing sequence number space. 9.2. MPRTP RTCP Header Extension The MPRTP RTCP header extension is used 1) to provide RTCP feedback per subflow to gauge the characteristics of each path, 2) to advertise the multiple interface addresses for a media receiver, and 3) perform connectivity check on the new interfaces. 9.2.1. MPRTP RTCP header extension for flow specific SR/RR TBD 9.2.2. MPRTP RTCP header extension for Interface advertisement This sub-section defines the RTCP header extension for in-band interface advertisement by the receiver, instead of relying on ICE or in situations when the interface appears after SDP session establishment. 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |V=2|P| RC | PT=MP_IA=2xx | length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | SSRC of packet sender | +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+ | SSRC_1 (SSRC of first source) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | MPRR_Type=0x00 | length | RESV | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Interface #1 Advertisement block | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Interface #2 Advertisement block | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Interface #... Advertisement block | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Interface #m Advertisement block | +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+ Figure 8: MPRTP Interface Advertisement. (RTCP SR/RR header extension) Singh, et al. Expires August 29, 2011 [Page 18]
Internet-Draft Multipath RTP February 2011 MP_IA: 8 bits Indicates that it is a RTCP extension for interface advertisement. MPRR_Type: 16-bits The MPRR_Type field corresponds to the type of MPRTP RTCP packet. Namely: +---------------+---------------------------------------------------+ | MPRR_Type | Use | | Value | | +---------------+---------------------------------------------------+ | 0x00 | Interface Advertisement | | | | | 0x01 | Connectivity Check. For this case the length is | | | set to 0 | | TBD | Keep Alive Packet. | +---------------+---------------------------------------------------+ Figure 9: RTP header extension values for MPRTP (MPR_Type) length: 8-bits The 8-bit length field is the length of extension data in bytes not including the MPRR_Type and length fields. The value zero indicates there is no data following. Interface Advertisement block: variable size Defined later in the section. 9.2.3. Interface Address Advertisement block This block describes a method to represent IPv4, IPv6 and generic DNS-type addresses in a block format. It is based on the sub- reporting block in RFC 5760 [7]. Singh, et al. Expires August 29, 2011 [Page 19]
Internet-Draft Multipath RTP February 2011 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={0,1,2} | Length | RTP Port | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | RTCP Port | Address | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + : : | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 10: Interface Address Advertisement block during path discovery Type: 8 bits The Type corresponds to the type of address. Namely: 0: IPv4 address 1: IPv6 address 2: DNS name Length: 8 bits The length of the Interface Advertisement block in bytes. For an IPv4 address, this should be 9 (i.e., 5 octets for the header and 4 octets for IPv4 address). For an IPv6 address, this should be 21. For a DNS name, the length field indicates the number of octets making up the string plus the 5 byte header. RTP Port: 2 octets The port number to which the sender sends RTP data. A port number of 0 is invalid and MUST NOT be used. RTCP Port: 2 octets The port number to which receivers send feedback reports. A port number of 0 is invalid and MUST NOT be used. Singh, et al. Expires August 29, 2011 [Page 20]
Internet-Draft Multipath RTP February 2011 Address: 4 octets (IPv4), 16 octets (IPv6), or n octets (DNS name) The address to which receivers send feedback reports. For IPv4 and IPv6, fixed-length address fields are used. A DNS name is an arbitrary-length string. The string MAY contain Internationalizing Domain Names in Applications (IDNA) domain names and MUST be UTF-8 encoded [8]. 10. SDP Considerations The packet formats specified in this document define extensions for RTP and RTCP. The use of MPRTP is left to the discretion of the sender and receiver. A participant of a media session MAY use SDP to signal that it supports MPRTP. Not providing this information may/will make the sender or receiver ignore the header extensions. However, MPRTP MAY be used by either sender or receiver without prior signaling. mprtp-attrib = "a=" "mprtp" [":" mprtp-optional-parameter] CRLF ; flag to enable MPRTP The literal 'mprtp' MUST be used to indicate support for MPRTP. Generally, senders and receivers SHOULD indicate this capability if they support MPRTP and would like to use it in the specific media session being signaled. However, it is possible for an MPRTP sender to stream data using multiple paths to a non-MPRTP client. Currently, there are no extensions defined for the literal 'mprtp' but we provide the opportunity to extend it using the mprtp-optional- parameter. 10.1. Increased Throughput The MPRTP layer MAY choose to augment paths to increase throughput. If the desired media rate exceeds the current media rate, the peers MUST renegotiate the application specific ("b=AS:") [16] bandwidth. 10.2. MPRTP using preloaded ICE TBD 11. Acknowledgements Varun Singh, Saba Ahsan, and Teemu Karkkainen are supported by Singh, et al. Expires August 29, 2011 [Page 21]
Internet-Draft Multipath RTP February 2011 Trilogy (http://www.trilogy-project.org), a research project (ICT- 216372) partially funded by the European Community under its Seventh Framework Program. The views expressed here are those of the author(s) only. The European Commission is not liable for any use that may be made of the information in this document. 12. IANA Considerations This document defines a new SDP attribute, "mprtp", within the existing IANA registry of SDP Parameters. TBD. 13. Security Considerations All drafts are required to have a security considerations section. See RFC 3552 [17] for a guide. 14. References 14.1. Normative References [1] Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson, "RTP: A Transport Protocol for Real-Time Applications", STD 64, RFC 3550, July 2003. [2] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [3] Rosenberg, J., "Interactive Connectivity Establishment (ICE): A Protocol for Network Address Translator (NAT) Traversal for Offer/Answer Protocols", RFC 5245, April 2010. [4] 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. [5] Johansson, I. and M. Westerlund, "Support for Reduced-Size Real-Time Transport Control Protocol (RTCP): Opportunities and Consequences", RFC 5506, April 2009. [6] Singer, D. and H. Desineni, "A General Mechanism for RTP Header Extensions", RFC 5285, July 2008. [7] Ott, J., Chesterfield, J., and E. Schooler, "RTP Control Singh, et al. Expires August 29, 2011 [Page 22]
Internet-Draft Multipath RTP February 2011 Protocol (RTCP) Extensions for Single-Source Multicast Sessions with Unicast Feedback", RFC 5760, February 2010. [8] Yergeau, F., "UTF-8, a transformation format of ISO 10646", STD 63, RFC 3629, November 2003. 14.2. Informative References [9] Stewart, R., "Stream Control Transmission Protocol", RFC 4960, September 2007. [10] Ford, A., Raiciu, C., Handley, M., Barre, S., and J. Iyengar, "Architectural Guidelines for Multipath TCP Development", draft-ietf-mptcp-architecture-05 (work in progress), January 2011. [11] Nordmark, E. and M. Bagnulo, "Shim6: Level 3 Multihoming Shim Protocol for IPv6", RFC 5533, June 2009. [12] Westerlund, M. and S. Wenger, "RTP Topologies", RFC 5117, January 2008. [13] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A., Peterson, J., Sparks, R., Handley, M., and E. Schooler, "SIP: Session Initiation Protocol", RFC 3261, June 2002. [14] Schulzrinne, H., Rao, A., Lanphier, R., Westerlund, M., and M. Stiemerling, "Real Time Streaming Protocol 2.0 (RTSP)", draft-ietf-mmusic-rfc2326bis-26 (work in progress), November 2010. [15] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with Session Description Protocol (SDP)", RFC 3264, June 2002. [16] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session Description Protocol", RFC 4566, July 2006. [17] Rescorla, E. and B. Korver, "Guidelines for Writing RFC Text on Security Considerations", BCP 72, RFC 3552, July 2003. Singh, et al. Expires August 29, 2011 [Page 23]
Internet-Draft Multipath RTP February 2011 Authors' Addresses Varun Singh Aalto University School of Science and Technology Otakaari 5 A Espoo, FIN 02150 Finland Email: varun@comnet.tkk.fi Teemu Karkkainen Aalto University School of Science and Technology Otakaari 5 A Espoo, FIN 02150 Finland Email: teemuk@comnet.tkk.fi Joerg Ott Aalto University School of Science and Technology Otakaari 5 A Espoo, FIN 02150 Finland Email: jo@comnet.tkk.fi Saba Ahsan Aalto University School of Science and Technology Otakaari 5 A Espoo, FIN 02150 Finland Email: sahsan@cc.hut.fi Singh, et al. Expires August 29, 2011 [Page 24]
Internet-Draft Multipath RTP February 2011 Lars Eggert Nokia Research Center P.O. Box 407 Nokia Group 00045 Finland Phone: +358 50 48 24461 Email: lars.eggert@nokia.com URI: http://research.nokia.com/people/lars_eggert Singh, et al. Expires August 29, 2011 [Page 25]