Retransmission-based stream repair and stream join

Abstract

The Real-time Transport Protocol (RTP) and its related standards define a retransmission packet format and a way to give feedback via Negative ACKnowledge (NACK) packets for data that has been lost. In one embodiment, a unicast RTP repair session is associated with a main Source Specific Multicast (SSM) multicast session. Real-time Transport Control Protocol (RTCP) NACK packets are then used for feedback to a SSM feedback target address. This dynamically instantiates unicast RTP repair for multicast sessions. The repair scheme can be used for repairing multicast channels or joining new multicast channels. In another embodiment, a media transmission device shares an IP address with one or more other media transmission devices. The shared IP address can also be used to route multiple identical multicast media streams to different media stream receivers.

Description

TECHNICAL FIELD

The present disclosure relates generally to a scheme for retransmission-based repair and fast stream join for Internet Protocol (IP) based media streams.

BACKGROUND

The Real-time Transport Protocol (RTP) and its related standards define a retransmission packet format and a way to give feedback via Negative ACKnowledgment (NACK) packets that data has been lost. The following standards RTP (RFC3550), RTP Retransmission (RFC4588), RTCP Feedback (RFC4585), and RTCP with SSM Sessions (draft-ietf-avt-rtcpssm-11.txt) are all incorporated by reference and describe unicast feedback and retransmission for unicast sessions, and unicast feedback with multicast retransmission for multicast sessions.

However, the RTP protocol suite has limitations when used with certain types of Internet Protocol (IP) media transmissions, such as transmitting streaming video to different endpoints. For example, neither RTP, nor any other common media transmission protocol, can perform unicast repair of multicast media streams or quickly and efficiently switch among different multicast media streams without loss of data.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a repair scheme used in a lookaside mode for multicast media streams.

FIG. 2 shows the repair scheme in FIG. 1 using IP anycast addresses.

FIG. 3 is a diagram showing how different media streams can be associated with different IP anycast addresses.

FIG. 4 is a diagram showing how the repair scheme is used in a source mode.

FIG. 5 is a diagram showing another embodiment of a retransmission scheme used for channel joining.

FIG. 6 is a flow diagram explaining in more detail the retransmission scheme shown in FIG. 5.

DESCRIPTION OF EXAMPLE EMBODIMENTS

Overview

A Real-time Transport Protocol (RTP)-based unicast repair scheme is used for repairing errors in RTP multicast streams. By modeling the joining of a new media stream as a repair operation, the repair scheme is extended to also rapidly join media channels. It should be noted that the terms channel and stream are used interchangeably below.

Referring to FIG. 1, a media stream source 12 may be a server, computer, or any other type of network processing device that can source IP media, such as video, audio, voice, data, etc., over an Internet Protocol (IP) network 10. In this example, the media stream source 12 transmits a multicast media stream 14 over the IP network 10 which is received by different media stream receivers 24A-24N and also by a retransmission system 18.

The media stream receivers 24 can be any device that receives and stores or renders the multicast media stream 14. For example, the media stream receivers 24 could be Set Top Boxes (STB), Digital Video Recorders (DVR), computer terminals, Personal Computers (PCs), televisions with IP interfaces, Voice over IP (VoIP) phones, cell phones, Personal Digital Assistants (PDA), etc. The retransmission system 18 can be any type of media server that caches and retransmits portions of the multicast media stream 14. The media stream source 12 and retransmission system 18 are shown as separate devices but could reside in the same physical location and be connected together via a Local Area Network (LAN) or other backplane connectivity.

In one embodiment, a Source Specific Multicast (SSM) multicast session is established for transmitting the multicast media stream 14 between the media stream source 12 and one or more of the media stream receivers 24. The media stream source 12 knows the IP addresses and ports to use for transmitting a particular media stream. The retransmission system 18 knows what IP addresses and ports to use for receiving the media stream and what address and port to use for sending retransmissions. The media stream receivers 24 know what IP address and port to listen for the media stream and the retransmissions, and where to send retransmission requests. All of this address and port information can be described using Session Description Protocol (SDP), but other media description schemes could be used.

The feedback target IP address 16 is used by the media stream receivers 24 as a destination address for requesting retransmissions for portions of the multicast media stream 14. For example, packets from the media stream 14 may not successfully arrive at particular media stream receivers, or may be received in a corrupted state.

The media stream receivers 24 send retransmission requests 28 to the retransmission system 18 associated with the feedback target IP address 16 that includes information 30 identifying the lost media packets. In one example, retransmission requests 28 are unicast Real-time Transport Control Protocol (RTCP) Negative ACKnowledge (NACK) packets that use the feedback target address 16 as a destination address 29. Sending the unicast RTCP NACK packet 28 to the retransmission system 18 dynamically instantiates a unicast RTP repair session from the retransmission system to the requesting receiver, if one does not already exist.

The retransmission system 18 includes a media cache 22 that caches the recent history of packets from the multicast media stream 14. During the unicast RTP repair session, lost packets for media stream 14 are identified in lost packet information 30 of the retransmission request 28. The retransmission system 18 identifies the packets in media cache 22 corresponding to the lost or corrupted packet information 30 in retransmission request 28.

The identified media packets in media cache 22 are sent as unicast media repair packets 34 back to the requesting media stream receiver 28. The media stream receiver 24 then inserts the received unicast media repair packets 34 into corresponding lost packet locations in the media stream 14. Thus, in one embodiment, a repair session uses unicast NACK packets 28 and unicast media repair packets 34 for repairing a multicast media session.

The media configuration shown in FIG. 1 is referred to as a “lookaside mode” because the retransmission system 18 (repair element) is not required to relay the original media stream 14 to the media stream receivers 24. Instead, the media stream source 12 multicasts the media stream 14 directly to the receivers 24. The lookaside mode may produce higher availability since the media stream 14 is not necessarily disrupted if the retransmission system 18 crashes or becomes unavailable.

In this example, only the unicast repair function ceases when the retransmission system 18 fails. Higher performance may also be provided since the retransmission system 18 only receives and caches the multicast media stream 14 and does not also have to retransmit the media stream 14 to the media stream receivers 24. Thus, by only providing media stream repair, the constant work factor for the retransmission system 18 is halved.

In the lookaside mode, the retransmission system 18 is distinguished from the media stream source 12 by using a feedback target address 16 for the retransmission system 18 that is different from the source IP address associated with the SSM media stream source 12.

IP Anycast Addressing

Referring to FIG. 2, in order to provide both high availability and load sharing, one embodiment of the repair scheme uses an IP anycast address 46 as the feedback target address for the retransmission system 18. This allows repair operations to be adaptively directed to the optimal currently available retransmission system 18A or 18B. The anycast addressing scheme exploits the metrics of IP unicast routing that automatically route packets to network processing devices associated with the cheapest IP routing cost.

To explain in more detail, message 40 identifies a single IP anycast address 46 for multiple different retransmission systems 18A and 18B available for repairing the multicast media stream 14. This SDP is provided to both the retransmission systems 18A and 18B, as well as to the receivers so that they will direct their retransmission requests to that anycast feedback address. Both retransmission system 18A and 18B cache the media stream 14 and are available for retransmitting lost packets to any of the media stream receivers 24A-24D.

Both of the retransmission systems 18A and 18B share the same IP anycast source address 46 identified in message 40. An IP anycast address refers to a unicast IP address that is used as a destination address simultaneously by multiple network processing devices. In this example, the network processing devices sharing IP address 46 include retransmission systems 18A and 18B.

The retransmission systems 18A and 18B and the media stream receivers 24 operate in substantially the same manner as described above in FIG. 1. In this example, all of the media stream receivers 24A-24D each receive the same multicast media stream 14 from media stream source 12. Alternatively, the media stream receivers 24 could indirectly receive the multicast media stream 14 via a separate multicast stream sourced from one of the retransmission systems 18A or 18B. This embodiment is referred to as “source mode” and is described in more detail below in FIG. 4.

Any of the media stream receivers 24A-24D may detect lost or corrupted packets or frames from the multicast media stream 14. In response, the receiver 24 sends out a unicast NACK retransmission request message 44. For explanation purposes, each of the media stream receivers 24A-24D in FIG. 2 is currently receiving the media stream 14 and one or more of them has identified lost packets from the media stream 14. Respectively, the receivers 24A-24D experiencing loss or corruption send out retransmission request messages 44A-44D.

Each of the retransmission request messages 44A-44D includes the same IP anycast destination address 46 along with the associated media stream receiver source address 48A-48D, respectively. The retransmission request messages 44A-44D also include lost packet information identifying which of the packets were lost from the multicast media stream 14.

Routers 42 and 44 in the IP network 10 use internal routing metrics to select which of the retransmission systems 18A or 18B has the cheapest IP routing cost for routing the messages 44A-44D. Since the IP anycast address 46 is shared by two different network processing devices 18A and 18B, the conventional routing metrics in the routers 42 and 43 will automatically select one of the two devices 18A or 18B for forwarding any messages 44A-44D. In this example, router 42 determines that retransmission system 18A has the shortest routing path for the retransmission request messages 44A and 44B received from receivers 24A and 24B, respectively. Alternatively, router 43 determines that retransmission system 18B has the shortest routing path for the retransmission request messages 44C and 44D received from receivers 24C and 24D, respectively.

Retransmission system 18A receives unicast NACK retransmission request messages 44A and 44B and retransmission system 18B receives unicast NACK retransmission request messages 44C and 44D. The retransmission system 18A responds back by sending unicast media repair packets from its cache of multicast media stream 14 back to receivers 24A and 24B. For example, retransmission system 18A sends unicast media repair packets 34 back to the media stream receiver 24B identified in message 44B. Retransmission system 18B responds to retransmission request messages 44C and 44D by sending unicast media repair packets from its cache of multicast media stream 14 back to receivers 24C and 24D, respectively.

This distributed routing of retransmission requests 44A-44D increases both availability and load sharing capacity while using substantially the same repair scheme described above in FIG. 1. For example, if either of the retransmission systems 18A or 18B is disabled, the routers 42 and 44 will automatically drop the disabled retransmission system from internal routing tables. Accordingly, repair requests previously routed to the disabled retransmission system 18 are automatically rerouted to a different one of the operational retransmission systems 18.

FIG. 3 shows different IP anycast addresses used as feedback addresses for different media streams 54 and 56. This further increases the scalability of the media repair scheme by allowing one or more different retransmission systems 50 and 52 to be associated with different multicast media streams.

For example, one or more retransmission systems 50 can be specifically designated to provide media packet repair for a particular multicast media stream 54. The feedback target IP anycast address identified in the Source Specific Multicast (SSM) multicast session for multicast media stream 54 is used as the source address for each of the retransmission systems 50. Similarly, one or more retransmission systems 52 can be designated to provide media packet repair support for a different multicast media stream 56. The feedback target IP anycast source address identified in the SSM multicast session for the multicast media stream 56 is used as the source address for each of retransmission systems 52.

In this example, media stream receiver 24B is receiving packets for multicast media stream 54. If any of the multicast packets are lost, media stream receiver 24B sends a unicast NACK retransmission request message 58 to the IP anycast destination address X associated with multicast media stream 54. The routers in the IP infrastructure (not shown) automatically route the retransmission request message 58 to one of the retransmission systems 50 which then sends back unicast media repair packets containing the identified lost packets.

Thus, different retransmission systems can be associated with different media streams to further increase the repair scheme scalability. However, in other embodiments, a retransmission system 50 may provide repair support for more than one media stream.

FIG. 4 shows an alternative “source mode” for the repair scheme. In the source mode, retransmission sources 70 provide media stream repair and also operate as SSM distribution sources for the multicast stream 14 originally generated by media stream source 12. The media stream source 12 could be a local encoder, a local splicer, or a separate media stream server operating remotely in the IP network 10 from the retransmission sources 70A and 70B.

The retransmission source 70A caches the original media stream 14 in media cache 22A and if necessary “re-sources” the received media stream 14, operating as a legal RTP mixer/translator according to the RTP specifications. Similarly, retransmission source 70B retransmits the original media stream 14 cached in media cache 22B as multicast media stream 64.

In the source mode, the retransmission sources 70A and 70B still receive unicast NACK retransmission request messages 44 from the media stream receivers 24 when multicast media stream packets are lost. The retransmission sources 70 accordingly send back unicast media repair packets 34 containing the requested lost media.

In one embodiment, the two retransmission sources 70A and 70B also still share the same IP source address 46. This common IP source address 46 for the feedback target can again be identified for the multicast media session using out-of-band or in-band messaging. The messages 60 exchanged between the media stream source 12, retransmission sources 70, and the media stream receivers 24, associate media stream 14 with feedback target IP anycast address 46.

Sharing the same IP destination address 46 provides high availability and load sharing for the repair scheme similar in a similar manner as described above in FIG. 2. However, both retransmission sources 70A and 70B in FIG. 4 also operate as retransmission sources for the media stream 14. Therefore, sharing the same SSM source address 46 also provides higher availability and load sharing with respect to the multicasting of the original media stream 14. Use of an anycast IP address as the SSM source address for the media source provides this capability as well.

For example, retransmission source 70A re-originates media stream 14 as multicast media stream 62 and retransmission source 70B re-originates media stream 14 as multicast media stream 64. The multicast packets 72 for multicast media stream 62 and the multicast packets 74 from multicast media stream 64 each use the same source IP address 46.

Any routers 42 in the IP network 10 receiving both media streams 62 and 64 with the same source address automatically drop packets for one of the two streams according to a basic characteristic of multicast routing known as “reverse path forwarding”. For packets being forwarded on a multicast tree, only those received from the upstream branch leading to the source IP address at the root of the tree are accepted for forwarding. In this example, the router 42 drops the multicast packets 74 for media stream 64 and only routes the multicast packets 72 for media stream 62 to the media stream receiver 24.

If retransmission source 70A is ever disabled, the router 42 will re-compute internal routing metrics and then automatically start routing the multicast packets 74 for media stream 64 to media stream receiver 24. Accordingly, using the shared IP anycast address 46 and the SSM source address also provides redundancy and load sharing for the multicast media stream source.

The shared IP source address 46 still increases retransmission repair redundancy and load balancing as described above in FIG. 3 by allowing either of the retransmission sources 70A or 70B to receive and respond to the retransmission requests 44 sent by any of the media stream receivers 24 as described above in FIG. 3. It is also possible that one of the retransmission sources 70 may end up re-originating the multicast media stream to the receiver 24 while the other retransmission source 70 provides media stream repair support by receiving unicast retransmission requests 44 and responding back with media repair packets 34.

Extension of Repair Scheme for Fast Stream Join

Several observations can be made with respect to a stream join as compared with a media stream repair operation. A media stream receiver joining a new RTP session may have no idea what packets are needed to render the current stream. Also since multicast video sessions may be involved, the media stream receiver is in all likelihood “behind” the multicast stream in time and may need to “catch up”. This is important because the media stream receiver may not be able to render the media stream until it receives an intra-coded frame that may have passed by shortly before the media stream receiver attempts to join the multicast session.

Referring to FIG. 5, a fast stream join scheme (alternatively referred to as a fast channel join) uses a variant of the repair schemes described above. Multiple different multicast media streams 82A-82N are generated by different media stream sources 80A-80N, respectively. Synchronization SouRCes (SSRCs) used by the media streams are communicated out of band to the media stream receivers 24 in the SDP messages 84. The SDP messages 84 also include the feedback target IP addresses for the one or more retransmission systems 86 associated with repairing the multicast media streams as described above.

A media stream receiver 24 detects a request to join a new multicast media stream and sends a unicast request 96 to the retransmission system 86 specifying the new channel which the receiver wishes to receive. A channel join request may detected, for example, by a set top box that detects a user using user interface 94 to select a new or different multicast media channel.

The unicast channel join request 96 in one embodiment is substantially the same RTCP NACK retransmission request message described above in FIGS. 1-4. However, in this embodiment, the message 96 is a NACK packet containing a Picture Loss Indication (PLI) 98. The PLI indication 98 notifies the retransmission system 86 to send all of the information needed to join a new identified media stream 102. This is different from the retransmission request messages 28 in FIG. 1 that instead requests specific lost packets from an already connected media stream.

The channel join request 96 is sent by the media stream receiver 24 to the feedback target address 100 for the retransmission system 86 associated with the identified new media stream 102. It is worth noting that initiation of the fast channel/stream join scheme is similar to the repair scheme described above since it exploits the same NACK and retransmission machinery.

Referring to both FIG. 5 and FIG. 6, the retransmission system 86 performs the following operations when the channel join request 96 is received. In operation 110, the retransmission system 86 determines which media stream channel the receiver 24 is joining using the SSRC 102 in the unicast NACK channel join request 96 and the destination transport address and port of the NACK packet. As described above, the SSRC 102 was previously communicated to the receiver 24 in the channel description of SDP message 84.

In operation 112, the retransmission system 86 uses the cached video information for the selected media stream to extract from the cache all the elements the receiver 24 may need to “prime” its decoder 97. This may include a Moving Pictures Experts Group (MPEG) Program Association Table (PAT) and Program Map Table (PMT) elements, Encryption Control Messages (ECMs) and possibly other non-video data needed by the decoder 97. The retransmission system 86 in operation 114 constructs a Real-time Transport Control Protocol (RTCP) APPlication-specific (APP) decoder packet 88 and in operation 116 sends the decoder priming packet 88 to the media stream receiver 26 that requested the channel join.

In operation 118, the retransmission system 86 references back into media cache 87 for the RTP data containing the most recent cached intra-coded frame (I-frame). In operation 120, the retransmission system 86 sends the cached RTP packets containing the I-frame and all subsequent frames 90 up to a current cache time to the media stream receiver 24. The identified frames 90 are burst to the receiver 24 at a much faster speed than what the media in the packets is actually rendered by receiver 24 (i.e., faster than real-time). This speeds up the channel join process by allowing the receiver to render video going back to the previous I-frame.

The portions of the stream sent back are not necessary to simply join the new stream. The receiver can join the stream just by knowing the SDP for the media stream and performing a conventional IGMP join operation without asking for channel join. The burst of the cached data back from the prior I-frame allows the receiver to start rendering before the join completes and the next I-frame arrives. If the data were not burst faster than real-time, the receiver would not be able to “catch up” with the multicast stream, which is ahead of the receiver in time. Thus, the burst frames are essentially “back filling” to the previous I-frame so the receiver is able to render media prior to when the next I-frame arrives on the multicast stream.

The decoder 97 in the receiver 24 is primed with the information from the decoder packet 88 and media frames 90. After being primed and receiving the burst, the receiver 24 can join the multicast group for the new stream and can start rendering any subsequent media from the joined multicast media stream 82N.

As with the basic packet repair operations, the fast channel join scheme can exploit any of the anycast-based availability and load sharing features provided either by the look aside mode scheme in FIGS. 2 and 3 or the source mode scheme shown in FIG. 4. Various embodiments are anticipated whereby the repair or channel join schemes are used by service providers offering IPTV service over a variety of access networks, including, but not limited to, Digital Subscriber Loop (DSL), cable, WiMax, etc.

It is also worth noting that the media repair and channel join schemes are essentially stateless. They maintain no state about individual receivers except during a repair or channel join operation. This allows the system to scale much better than a system in which permanent or semi-permanent state is kept in the retransmission system about each receiver potentially wishing to request repair or channel join operations. The normal routing states in the routers in the IP network provide any knowledge required for ensuring retransmission requests or channel join requests are directed to operational retransmission systems 18. The routing metrics in the IP network routers also, as a by-product, provide a degree of load balancing.

Thus, the media source, retransmission systems, and media stream receivers associated with a media stream are not required to keep track of which media stream sources or which retransmission systems are operational, or which media stream receivers are currently connected to which media streams.

The RTP unicast repair scheme also does not require the retransmission systems to remember what repair or channel join packets have been previously received or sent to media stream receivers. Each retransmission or channel join request can be a single unitary request that is answered with a single response or group of responses by the retransmission system with no required state knowledge of what other repair operations have been previously performed for the requesting receiver.

Thus, the use of RTP protocol machinery with the above described extensions allows the creation of a unified technique for both retransmission-based media stream repair and fast channel joining (i.e., stream joining). Operation in either the lookaside mode or source mode can use anycast-based feedback addressing to improve scalability, robustness, and performance. Separate operations are unified with common mechanisms and low protocol, state, and computational overhead.

The system described above can use dedicated processor systems, micro controllers, programmable logic devices, or microprocessors that perform some or all of the operations. Some of the operations described above may be implemented in software and other operations may be implemented in hardware.

For the sake of convenience, the operations are described as various interconnected functional blocks or distinct software modules. This is not necessary, however, and there may be cases where these functional blocks or modules are equivalently aggregated into a single logic device, program or operation with unclear boundaries. In any event, the functional blocks and software modules or features of the flexible interface can be implemented by themselves, or in combination with other operations in either hardware or software.

Having described and illustrated the principles of the invention in a preferred embodiment thereof, it should be apparent that the invention may be modified in arrangement and detail without departing from such principles. I/We claim all modifications and variation coming within the spirit and scope of the following claims.

Claims

1. A method comprising: receiving a multicast media stream at a media retransmission device;caching the multicast media stream in a local media cache at the media retransmission device, wherein the media retransmission device shares an IP address with at least one other media retransmission device, and wherein the shared IP address is for addressing retransmission requests from at least one media stream receiver;receiving a unicast Internet Protocol (IP) request from the at least one media stream receiver for retransmission of the cached multicast media stream, wherein the retransmission request is directed to the media retransmission device according to an IP topology relationship between the media retransmission device and the at least one media stream receiver, and wherein the retransmission requests are Real-Time Transport Control Protocol (RTCP) Negative Acknowledgement (NACK) packets that include the shared IP address as a destination address;sending a unicast IP message to the requesting media stream receiver to dynamically instantiate a unicast Real Time Transport (RTP) repair session; andunicasting a cached requested portion of the multicast media stream to the media stream receiver.

2. The method according to claim 1 wherein the common IP address triggers IP network routers to distribute retransmission requests to the one or more media retransmission devices having a common IP address according to a least expensive IP routing cost.

3. The method according to claim 1, further comprising sending a cached portion of the multicast media stream identified as lost packets in the request by inspecting the request for retransmission.

4. The method according to claim 1, further comprising acting as a media stream source by sending the multicast media stream to one or more receivers prior to the caching the multicast media stream.

5. The method according to claim 1, wherein the common IP address is different from a source IP address associated with a media stream source.

6. The method according to claim 1, wherein a different common IP address is associated with a different media stream.

7. The method according to claim 1, wherein the common IP address is associated with a different media stream.

8. Software encoded on a non-transitory computer-readable media and when executed operable to: cache an Internet Protocol (IP) transmitted media stream at a media retransmission device, wherein the media retransmission device shares an IP address with at least one other media retransmission device and wherein the shared IP address is for addressing retransmission requests from at least one media stream receiver;identify the shared IP address to the at least one receiver receiving the media stream, wherein the shared IP address is associated with the media stream;receive a retransmission request addressed to the shared IP address, wherein the retransmission request is directed to the media retransmission device according to an IP topology relationship between the media retransmission device and the at least one media stream receiver, and wherein the retransmission requests are Real-Time Transport Control Protocol (RTCP) Negative Acknowledgement (NACK) packets that include the shared IP address a destination address;identify the retransmission request as a media stream repair request; andresponsive to identifying the media stream repair request, send a portion of the cached media stream indicated as lost or corrupted in the repair request;wherein the repair request dynamically instantiates a unicast RTP repair session with a requesting receiver.

9. The software encoded on the non-transitory computer-readable media of claim 8, wherein the retransmission request is art the Real-time Transport Control Protocol (RTCP) Negative Acknowledgment (HACK NACK) packet further includes lost packet information.

10. The software encoded on the non-transitory computer-readable media of claim 8, when executed is further operable to re-originate the IP transmitted media stream to send the media stream to the at least one media stream receiver.

11. The software encoded on the non-transitory computer-readable media of claim 8, wherein the shared IP address is an anycast address.

12. The software encoded on the non-transitory computer-readable media of claim 8, wherein a different shared IP address is associated with a different media stream.

13. The software encoded on the non-transitory computer-readable media of claim 8, wherein the shared IP address is associated with a different media stream.

14. Logic encoded on a non-transitory tangible media for execution and when executed operable to: cache an Internet Protocol (IP) transmitted media stream at a media retransmission device, wherein the media retransmission device shares an IP address with at least one other media retransmission device and wherein the shared IP address is for addressing retransmission requests from at least one media stream receiver;identify the shared IP address to the at least one media stream receiver receiving the media stream;receive a retransmission request addressed to the shared IP address, wherein the retransmission request is directed to the media retransmission device according to an IP topology relationship between the media retransmission device and the at least one media stream receiver, and wherein the retransmission requests are Real-Time Transport Control Protocol (RTCP) Negative Acknowledgement (NACK) packets that include the shared IP address a destination address;identify the retransmission request as a channel join request; andresponsive to identifying the channel join request, send a portion of the cached media stream to the receiver, wherein the join request dynamically instantiates a unicast RTP repair session with a requesting receiver for sending decoder information and a group of cached media stream frames required for decoding a new media stream.