LC‐RTP (Loss Collection RTP)
Michael Zink · 2005
In this appendix a protocol is presented which allows the lossless transport of audio and video data into caches while, in parallel, this data is streamed to one or several clients.In contrast to existing approaches on reliable multicast (see section 3.6) the protocol presented here was especially designed for use in content distribution infrastructures for multimedia data.An important characteristic of AfV content is the fact that it should be transmitted in real-time.This implies that a client cannot wait too long (assuming a limited buffer) for any resent packets instead of displaying the current data, so the normal data flow must persist and any retransmission must happen aside from the normal data flow.The situation is different for caches where the arrival of data is not time-critical (at least not as critical as at the client).Therefore, additional functionality in server and caches in combination with LC-RTP allows a lossless transport into caches while data is streamed to clients as usual.LC-RTP is designed as an RFC-compliant extension to RTP for reliable file transfer that requires no infrastructure modifications except on servers and caches.LC-RTP provides lossless transfer of real-time data by using loss collection (Le).The sender sends RTP packets via multicast to all receivers (clients and cache servers) in the multicast group.If a cache server detects a packet loss during the transmission it will be memorized in a list.At the end of the session, caches which are storing the video from this multicast transmission request the missing parts from the sender.The sender retransmits all missing blocks and waits until no more packets are requested.LC-RTP works also in simple unicast mode where data is streamed from the server through the cache to the client.