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RTP Multiplexing using RTP Multiplexing using Tunnels (TCRTP)Tunnels (TCRTP)

Bruce ThompsonTmima Koren

Cisco Systems Inc

March 2000 IETF 2

StatusStatus

• RTP Multiplexing using TunnelsRTP Multiplexing using Tunnels– Original draft submitted in OsloOriginal draft submitted in Oslo

• draft-wing-avt-tcrtp-00.txtdraft-wing-avt-tcrtp-00.txt

– Broken into distinct parts:Broken into distinct parts:• IP Tunneling, PPP Multiplexing, CRTP IP Tunneling, PPP Multiplexing, CRTP

enhancementsenhancements

• Builds on existing standardsBuilds on existing standards

• No modifications to existing RTP applicationsNo modifications to existing RTP applications

– New draft reflects these changesNew draft reflects these changes• draft-ietf-avt-tcrtp-00.txtdraft-ietf-avt-tcrtp-00.txt

March 2000 IETF 3

Tunneled CRTPTunneled CRTP•Encapsulation for end to end multiplexing•Consists of:

Compression - RFC 2508Multiplexing - PPP layer multiplexingIP tunneling for PPP - L2TPCRTP negotiation - RFC 2509

•Application runs with RTP encapsulationCRTP, multiplexing in lower layersTunnel adds destination IP header

March 2000 IETF 4

CompressionCompression• RFC 2508 ExtensionsRFC 2508 Extensions

– draft-koren-avt-crtp-enhance-01.txtdraft-koren-avt-crtp-enhance-01.txt

– Compressed UDP CU* packet formatCompressed 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 deliveryEnhanced state update delivery• N mode or ACK modeN mode or ACK mode

– CRTP Headers checksumCRTP Headers checksum• Used if UDP checksum not presentUsed if UDP checksum not present

March 2000 IETF 5

CompressionCompression

• CRTP OperationCRTP Operation– State resynchronizationState resynchronization

• Packet loss must result in resynchronizationPacket loss must result in resynchronization

• Twice algorithm should be usedTwice algorithm should be used– UDP checksumsUDP checksums– CRTP Headers checksum if no UDP checksumCRTP Headers checksum if no UDP checksum

• Compression state independent of Compression state independent of multiplexmultiplex

• Compression contexts can be distributedCompression contexts can be distributed

• Other compression algorithms could be Other compression algorithms could be specifiedspecified

– draft-jonsson-robust-hc-03.txtdraft-jonsson-robust-hc-03.txt

March 2000 IETF 6

MultiplexingMultiplexing

• PPP MultiplexingPPP Multiplexing– draft-ietf-pppext-pppmux-00.txtdraft-ietf-pppext-pppmux-00.txt

– Allow amortization of tunnel headerAllow amortization of tunnel header

– New payload type for multiplexed payloadsNew payload type for multiplexed payloads

– 1 to 3 bytes overhead / payload1 to 3 bytes overhead / payload

PayloadLength

1 Byte

PayloadType

0-2 Byte

PayloadLength

1 Byte

PayloadType

0-2 Byte

Payload TypeMUXED_PPP_FRAME

2 Bytes

March 2000 IETF 7

TunnelingTunneling

• 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 IETF 8

TunnelingTunneling• Efficient tunnel header neededEfficient tunnel header needed

• L2TP with Header Compression (L2TPHC)L2TP with Header Compression (L2TPHC)

– draft-ietf-l2tpext-l2tphc-03.txtdraft-ietf-l2tpext-l2tphc-03.txt

– Removes session ID, tunnel ID from L2TPRemoves session ID, tunnel ID from L2TP

– Removes UDP headerRemoves UDP header• Negotiated IP protocol IDNegotiated IP protocol ID

L2TPHC encapsulation:0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8+---+---+-+-+---+---+---+-+-+-+-+-+-+-+-+-+-+-+|T=0|L=0|x|x|S=0|I=0|O=0|P| PPP packet... +---+---+-+-+---+---+---+-+-+-+-+-+-+-+-+-+-+-+

March 2000 IETF 9

Tunneled CRTP EncapsulationTunneled CRTP Encapsulation

PPP

1 Byte

VOIP G.729A, AAL-5, IP, Tunneled CRTP, 1 Audio Sample / 1 Cell

PayloadCell Header

5 Bytes

AAL-5Trailer

8 Bytes

PADAAL-5

Voice Payload

10 bytes

CRTP

2 Bytes

L2TP

1 Byte

IP(L2TP payload type)

20 Bytes

PPPMux

2 Bytes

Length

1 Byte

4 Bytes

IPHDLC

20 Bytes

VOIP G.729A, HDLC, IP, Tunneled CRTP, 3 Audio Samples / Packet

RTP

G.729a, 10 bytes4 Bytes

Voice Payload

G.729a, 10 bytes

PPP/CRTP

3 Bytes

Voice PayloadPPP/CRTP

Voice PayloadPPP/CRTP

4 Bytes G.729a, 10 bytes

CRC

4 Bytes

L2TP/PPP Mux

2 Bytes

March 2000 IETF 10

Example ImplementationExample Implementation +---+---+---+---+---+---+---+---+ + | Application | | +---+---+---+---+---+---+---+---+ | | RTP | | +---+---+---+---+---+---+---+---+ Application | UDP | | +---+---+---+---+---+---+---+---+ | | IP | | +---+---+---+---+---+---+---+---+ + | | IP forwarding | + +---+---+---+---+---+---+---+---+ + | CRTP | | +---+---+---+---+---+---+---+---+ | | PPPMUX | | +---+---+---+---+---+---+---+---+ Tunnel | PPP | Interface +---+---+---+---+---+---+---+---+ | | L2TP | | +---+---+---+---+---+---+---+---+ | | IP | | +---+---+---+---+---+---+---+---+ + | | IP forwarding | + +---+---+---+---+---+---+---+---+ + | Layer 2 | | +---+---+---+---+---+---+---+---+ Physical | Phys | Interface +---+---+---+---+---+---+---+---+ +

March 2000 IETF 11

Bandwidth Efficiency Bandwidth Efficiency FactorsFactors

• Multiplexing gainMultiplexing gain– Depends on traffic in tunnelDepends on traffic in tunnel

– Large pipes more efficientLarge pipes more efficient

– Only affects tunnel overheadOnly affects tunnel overhead

• Tunnel loss rateTunnel loss rate– N mode good enough for audio with loss rates <5%N mode good enough for audio with loss rates <5%

– N=2 good enough for audio with loss rates <1%N=2 good enough for audio with loss rates <1%

– Only used when IP/RTP fields changeOnly used when IP/RTP fields change

March 2000 IETF 12

Bandwidth Efficiency Bandwidth Efficiency FactorsFactors

• IP/RTP field changesIP/RTP field changes– RTP Time StampRTP Time Stamp

• Voice applications with VADVoice applications with VAD

• Not a big factorNot a big factor

– IPIDIPID• 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-5Multiplexing gains large for IP/AAL-5• ATM cell taxATM cell tax

– Multiplexing gains less for variable length packet encapsMultiplexing gains less for variable length packet encaps• HDLC, Frame RelayHDLC, Frame Relay

March 2000 IETF 13

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1 2 3 4 5 6 7 8 9 10

Calls

Ba

nd

wid

th G.729 TCRTP HDLC 10 msec

G.729 RTP HDLC 10 msec

G.729 CRTP HDLC 10 msec

G.729 TCRTP AAL-5 10 msec

G.729 RTP AAL-5 10 msec

G.729 CRTP AAL-5 10 msec

TCRTP vs. RTP / CRTP Bandwidth10 msec packetization, Constant IPID delta