RTP Multiplexing using Tunnels (TCRTP)

1
RTP Multiplexing using Tunnels (TCRTP)
Bruce Thompson Tmima Koren Cisco Systems Inc
2
Status

• RTP Multiplexing using Tunnels
• Original draft submitted in Oslo
• draft-wing-avt-tcrtp-00.txt
• Broken into distinct parts
• IP Tunneling, PPP Multiplexing, CRTP enhancements
• Builds on existing standards
• No modifications to existing RTP applications
• New draft reflects these changes
• draft-ietf-avt-tcrtp-00.txt

3
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

4
Compression

• RFC 2508 Extensions
• draft-koren-avt-crtp-enhance-01.txt
• Compressed UDP CU packet format
• Sender/Receiver stay in sync in event of packet
  loss
• Less use of CONTEXT_STATE
• Enhanced state update delivery
• N mode or ACK mode
• CRTP Headers checksum
• Used if UDP checksum not present

5
Compression

• CRTP Operation
• State resynchronization
• Packet loss must result in resynchronization
• Twice algorithm should be used
• UDP checksums
• CRTP Headers checksum if no UDP checksum
• Compression state independent of multiplex
• Compression contexts can be distributed
• Other compression algorithms could be specified
• draft-jonsson-robust-hc-03.txt

6
Multiplexing

• PPP Multiplexing
• draft-ietf-pppext-pppmux-00.txt
• Allow amortization of tunnel header
• New payload type for multiplexed payloads
• 1 to 3 bytes overhead / payload

7
Tunneling

• Tunneled PPP session from end to end
• L2TP for PPP session tunneling
• RFC 2509 for CRTP session negotiation
• Tunnels may be established statically or
  dynamically

8
Tunneling

• Efficient tunnel header needed
• L2TP with Header Compression (L2TPHC)
• draft-ietf-l2tpext-l2tphc-03.txt
• Removes session ID, tunnel ID from L2TP
• Removes UDP header
• Negotiated IP protocol ID

L2TPHC encapsulation 0 1 2 3 4 5 6 7 8
9 0 1 2 3 4 5 6 7 8 --------------------
-------- T0L0xxS0I0O0P PPP
packet... ------------------------
----
9
Tunneled CRTP Encapsulation


10
Example Implementation
11
Bandwidth Efficiency Factors

• Multiplexing gain
• Depends on traffic in tunnel
• Large pipes more efficient
• Only affects tunnel overhead
• Tunnel loss rate
• N mode good enough for audio with loss rates lt5
• N2 good enough for audio with loss rates lt1
• Only used when IP/RTP fields change

12
Bandwidth Efficiency Factors

• IP/RTP field changes
• RTP Time Stamp
• Voice applications with VAD
• Not a big factor
• IPID
• IPID may change randomly
• Larger factor in bandwidth efficiency
• Layer 2 encapsulation
• Multiplexing gains large for IP/AAL-5
• ATM cell tax
• Multiplexing gains less for variable length
  packet encaps
• HDLC, Frame Relay

13
TCRTP vs. RTP / CRTP Bandwidth 10 msec
packetization, Constant IPID delta