1 RTP Multiplexing using Tunnels (TCRTP) Bruce Thompson Tmima Koren Cisco Systems Inc

March 2000 IETF2 Status RTP Multiplexing using TunnelsRTP Multiplexing using Tunnels –Original draft submitted in Oslo draft-wing-avt-tcrtp-00.txtdraft-wing-avt-tcrtp-00.txt –Broken into distinct parts: IP Tunneling, PPP Multiplexing, CRTP enhancementsIP Tunneling, PPP Multiplexing, CRTP enhancements Builds on existing standardsBuilds on existing standards No modifications to existing RTP applicationsNo modifications to existing RTP applications –New draft reflects these changes draft-ietf-avt-tcrtp-00.txtdraft-ietf-avt-tcrtp-00.txt

March 2000 IETF3 Tunneled CRTP Encapsulation for end to end multiplexing Consists of: Compression - RFC 2508 Multiplexing - PPP layer multiplexing IP tunneling for PPP - L2TP CRTP negotiation - RFC 2509 Application runs with RTP encapsulation CRTP, multiplexing in lower layers Tunnel adds destination IP header

March 2000 IETF4 Compression RFC 2508 ExtensionsRFC 2508 Extensions –draft-koren-avt-crtp-enhance-01.txt –Compressed UDP CU* packet format Sender/Receiver stay in sync in event of packet lossSender/Receiver stay in sync in event of packet loss Less use of CONTEXT_STATELess use of CONTEXT_STATE –Enhanced state update delivery N mode or ACK modeN mode or ACK mode –CRTP Headers checksum Used if UDP checksum not presentUsed if UDP checksum not present

March 2000 IETF5 Compression CRTP OperationCRTP Operation –State resynchronization Packet loss must result in resynchronizationPacket loss must result in resynchronization Twice algorithm should be usedTwice algorithm should be used –UDP checksums –CRTP Headers checksum if no UDP checksum Compression state independent of multiplexCompression state independent of multiplex Compression contexts can be distributedCompression contexts can be distributed Other compression algorithms could be specifiedOther compression algorithms could be specified –draft-jonsson-robust-hc-03.txt

March 2000 IETF6 Multiplexing PPP MultiplexingPPP Multiplexing –draft-ietf-pppext-pppmux-00.txt –Allow amortization of tunnel header –New payload type for multiplexed payloads –1 to 3 bytes overhead / payload PayloadLength 1 Byte Payload Type 0-2 Byte PayloadLength 1 Byte Payload Type 0-2 Byte Payload Type MUXED_PPP_FRAME 2 Bytes

March 2000 IETF7 Tunneling Tunneled PPP session from end to endTunneled PPP session from end to end L2TP for PPP session tunnelingL2TP for PPP session tunneling RFC 2509 for CRTP session negotiationRFC 2509 for CRTP session negotiation Tunnels may be established statically or dynamicallyTunnels may be established statically or dynamically

March 2000 IETF8 Tunneling Efficient tunnel header neededEfficient tunnel header needed L2TP with Header Compression (L2TPHC)L2TP with Header Compression (L2TPHC) –draft-ietf-l2tpext-l2tphc-03.txt –Removes session ID, tunnel ID from L2TP –Removes UDP header Negotiated IP protocol IDNegotiated IP protocol ID L2TPHC encapsulation: |T=0|L=0|x|x|S=0|I=0|O=0|P| PPP packet

March 2000 IETF9 Tunneled CRTP Encapsulation PPP 1 Byte VOIP G.729A, AAL-5, IP, Tunneled CRTP, 1 Audio Sample / 1 Cell Payload Cell Header 5 Bytes AAL-5 Trailer 8 Bytes PAD AAL-5 Voice Payload 10 bytes CRTP 2 Bytes L2TP 1 Byte IP (L2TP payload type) 20 Bytes PPP Mux 2 Bytes Length 1 Byte 4 Bytes IP HDLC 20 Bytes VOIP G.729A, HDLC, IP, Tunneled CRTP, 3 Audio Samples / Packet RTP G.729a, 10 bytes 4 Bytes Voice Payload G.729a, 10 bytes PPP/ CRTP 3 Bytes Voice Payload PPP/ CRTP Voice Payload PPP/ CRTP 4 BytesG.729a, 10 bytes CRC 4 Bytes L2TP/ PPP Mux 2 Bytes

March 2000 IETF10 Example Implementation

March 2000 IETF11 Bandwidth Efficiency Factors Multiplexing gainMultiplexing gain –Depends on traffic in tunnel –Large pipes more efficient –Only affects tunnel overhead Tunnel loss rateTunnel loss rate –N mode good enough for audio with loss rates <5% –N=2 good enough for audio with loss rates <1% –Only used when IP/RTP fields change

March 2000 IETF12 Bandwidth Efficiency Factors IP/RTP field changesIP/RTP field changes –RTP Time Stamp Voice applications with VADVoice applications with VAD Not a big factorNot a big factor –IPID IPID may change randomlyIPID may change randomly Larger factor in bandwidth efficiencyLarger factor in bandwidth efficiency Layer 2 encapsulationLayer 2 encapsulation –Multiplexing gains large for IP/AAL-5 ATM cell taxATM cell tax –Multiplexing gains less for variable length packet encaps HDLC, Frame RelayHDLC, Frame Relay

March 2000 IETF13 TCRTP vs. RTP / CRTP Bandwidth 10 msec packetization, Constant IPID delta