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Using packet drop precedence for graceful degradation

Using packet drop precedence for graceful degradation. Authors:. Date: 2009-01-18. Abstract. As one of its objectives, the 802.11aa PAR has:

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Using packet drop precedence for graceful degradation

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  1. Using packet drop precedencefor graceful degradation Authors: Date: 2009-01-18 Alex Ashley, NDS Ltd

  2. Abstract As one of its objectives, the 802.11aa PAR has: • Graceful degradation of audio video streams when there is insufficient channel capacity, by enabling packet discarding without any requirement for deep packet inspection There are many issues to consider to meet this goal. The aim of this presentation is to highlight some of these issues and to propose a simple end-to-end solution. Alex Ashley, NDS Ltd

  3. Assumption • The assumption in this presentation is that packet discarding for graceful audio video degradation only makes sense for UDP (and UDP-Lite) based streaming • TCP has its own congestion control algorithms. Dropping frames will not provide graceful degradation, just cause retransmissions and probably trigger congestion avoidance algorithms Alex Ashley, NDS Ltd

  4. MAC header IP header UDP header (RTP header) Video Frame UDP (RTP) Packets • An MPEG-2 transport packet is 188 bytes long and contains data for one component (e.g. video, audio, data) • When sending audio video data that is inside MPEG-2 transport packets, typically 7 TS packets are placed inside one UDP packet • Most UDP packets (typically >95%) contain a mix of components Alex Ashley, NDS Ltd

  5. Components of a Transport Stream Alex Ashley, NDS Ltd

  6. Components of a Transport Stream (2) • The tuning information (1.01%) and data components (4.3%) are continuously repeated in the transport stream • Loss of these packets reduces the speed of channel change or use of interactive applications, but has no impact on audio video experience • Therefore, these transport packets make good candidates for discarding when there is congestion • But they don’t normally occur on UDP packet boundaries • Application layer needs to be aware of UDP packing to place these components in their own UDP packets. Alex Ashley, NDS Ltd

  7. Elementary Stream RTP • Another method of audio video streaming is to place individual components in their own RTP stream • The MPEG-2 Transport Stream and PES headers are removed • Each component is sent to a different port • Timestamp field in RTP header is used to synchronize components • Selective packet dropping in RTP ES is much easier, because only one component per UDP packet • But at the moment not widely deployed due to increased receiver complexity Alex Ashley, NDS Ltd

  8. Forward Error Correction (FEC) • To cope with UDP packet loss, some systems add FEC packets • E.g. when streaming IPTV over xDSL • E.g. Adding 16 UDP packets of FEC to every 204 UDP packets of audio video data. Any 16 UDP packets can be lost without information loss • Example FEC schemes: • RFC 2733 RTP Payload Format for Generic FEC • Reed-Solomon • RFC 5053 Raptor FEC • RFC 5170 LDPC Staircase and Triangle FEC • SMPTE2022-1 Pro-MPEG Code of Practice #3 r2 • ETSI TS 102 034 DVB-AL-FEC • Packet discarding very easy as any packet can be a discard candidate • Just don’t throw away too many in one protection period! Alex Ashley, NDS Ltd

  9. Scalable Video Coding • The video encoding is partitioned in a manner that has a baseline and one or more enhancement layers • E.g. an SD baseline plus enhancement to make HD • However, SVC is not widely deployed • Encoder complexity • Decoder complexity • Not necessarily bandwidth efficient • It’s often more efficient to send two separately encoded streams! Alex Ashley, NDS Ltd

  10. So how does this relate to 802.11 ? • There are many ways in which audio video data is streamed over IP networks • Some methods lend themselves to selective packet discarding, some do not • Probably the best pragmatic approach is some simple signalling per UDP packet and let the application decide when it can provide discard hints Alex Ashley, NDS Ltd

  11. DSCP • At layer 3, per hop behaviour defined by DSCP (TOS) field • Typical PHBs: • AF - Assured Forwarding based upon service level agreement • BE - Best Effort • EF - Expedited Forwarding for low-loss, low-latency traffic • CS – Class selector preserves backward compatibility with IP-precedence scheme • AF defines 3 levels of drop precedence • But does EF make more sense for audio video? • Some standards bodies & industry forums have already pre-defined DSCP tags for AV in the home • DVB-IPI uses AF (video=100010 or 100100) • DLNA uses CS (video=101000) • In conclusion, layer 3 tag might not help us • A layer 2 based tagging solution is needed Alex Ashley, NDS Ltd

  12. Priority & DE Drop Eligible Indicator 802.1ad VLAN Tags Alex Ashley, NDS Ltd

  13. PCP field in C-Tag Format Management decision on which row to choose. All Ports on a LAN should select the same row. Alex Ashley, NDS Ltd

  14. Stream Classification Service • dot11ShortRetryLimit is expanded by 32 entries • Allows lower latency for AV streams without impact on legacy traffic • Allows reduced retry counts for packets marked discard eligible • Request to AP to signal a stream that wants to make use of packet drop precedence • Uses the frame classifier TCLAS Element to specify frame marking • Provides suggested drop and non-drop retry limits • Allows flexibility in packet signalling • No need for new 802.11 frame signalling • Using 802.1ad C-TAGs allows end-to-end use • Marked packets are dropped when AP’s buffers are exhausted or the retry limit is reached Alex Ashley, NDS Ltd

  15. AP Example Request SCS AV stream Source device tags UDP packets with priority and drop eligibility (DEI). IEEE 802.3 used to connect source to the AP Destination makes an SCS request to the AP to inform it that the stream has DEI and the method of tagging When active, the AP can use the DEI tagging to choose packets to discard if conditions deteriorate. Alex Ashley, NDS Ltd

  16. Deep Packet Inspection • “Deep Packet Inspection is a term used to describe the capabilities of a firewall or an Intrusion Detection System (IDS) to look within the application payload of a packet or traffic stream and make decisions on the significance of that data based on the content of that data.” • “Deep Packet Inspection (DPI) (also called complete packet inspection and Information eXtraction - IX -) is a form of computer network packet filtering that examines the data and/or header part of a packet as it passes an inspection point, searching for protocol non-compliance, viruses, spam, intrusions or predefined criteria to decide if the packet can pass or if it needs to be routed to a different destination, or for the purpose of collecting statistical information. This is in contrast to shallow packet inspection (usually called Stateful Packet Inspection) which just checks the header portion of a packet.” Alex Ashley, NDS Ltd

  17. Conclusions • Even without scalable video coding, it is possible to arrange for some UDP packets to be more readily sacrificed than others • Not all audio video streams will be amenable to graceful packet discarding • A simple addition to 802.11 allows suitable streams to make use of graceful degradation Alex Ashley, NDS Ltd

  18. References • Slides 4 to 12 in document IEEE 802.11-08/0717r1 provide a good background on MPEG video encoding • Deep packet inspection • http://en.wikipedia.org/wiki/Deep_packet_inspection • http://www.securityfocus.com/infocus/1716 Alex Ashley, NDS Ltd

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