Enhancements to CRTP

1
Enhancements to CRTP
draft-koren-avt-crtp-enhance-01.txt T. Koren,
S. Casner, P. Ruddy, B. Thompson, A. Tweedly, D.
Wing Cisco Systems John Geevarghese Motorola India
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Why Enhance CRTP

• CRTP does not perform well on links with packet
  loss and long round trip delays
• Packet loss causes context invalidation
• CRTP error recovery mechanism using CONTEXT_STATE
  messages is not efficient due to the long round
  trip delay.
• The suggested enhancements make CRTP more robust
  and resilient to packet loss, which in turn will
  reduce context invalidation.

3
Packet Loss andContext Invalidation

• When will a lost packet cause context
  invalidation?
• Packet includes changes to context state fields
• New delta value
• Significant packet its loss causes context
  invalidation

4
Example Audio Stream

• Sample sent every 10 ms
• The first talk spurt is 1 second long.
• Then there are 2 seconds silence
• Then another talk spurt.

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Shortcuts

• FH - FULL_HEADER
• CR - COMPRESSED_RTP
• CU - COMPRESSED_UDP
• CS - CONTEXT_STATE
• T, dT - RTP timestamp (absolute and delta)
• S, dS - RTP seq (absolute and delta)
• I, dI - IP ID (absolute and
  delta)
• pt - RTP payload type
• CC - Number of CSRC identifiers

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Classic CRTP Sequence

• seq Time pkt type
• 1 10 FH
  x
• 2 20 CR dI dT10
  x
• 3 30 CR
• 4 40 CR
• ...
• 100 1000 CR
• 101 3010 CR dT2010
  x
• 102 3020 CR dT10
  x
• 103 3030 CR
• 104 3040 CR
• ...

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Remove Dependency

• seq Time pkt type
• 100 1000 CR
• 101 3010 CR dT2010
  x
• 102 3020 CR dT10
  x
• 103 3030 CR
• Replace 101, 102
• 100 1000 CR
• 101 3010 ?? T3010 dT2010
• 102 3020 ?? T3020 dT10
  x
• 103 3030 CR

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Express change in one packet

• seq Time pkt type
• 100 1000 CR
• 101 3010 CR dT2010
  x
• 102 3020 CR dT10
  x
• 103 3030 CR
• Replace 101, 102
• 100 1000 CR
• 101 3010 ?? T3010
  x
• 102 3020 CR

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Enhanced CU CU
CU
CU
msb of context ID
msb of context ID
lsb of context ID
lsb of context ID
CRTP seq
dI
0
0
0
CRTP seq
dI
0
dT
I
UDP Checksum
UDP Checksum
RANDOM fields
RANDOM fields
Delta IPv4 ID
Delta IPv4 ID
UDP data (uncompressed RTP header)
Delta RTP timestamp
IP ID
UDP data (uncompressed RTP header)
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Enhanced CU CU (CR style)
msb of context ID
RTP Timestamp
lsb of context ID
CRTP seq
dI
1
dT
I
CC
pt
M
S
T
UDP Checksum
RTP Payload Type
CSRC List
RANDOM fields
Delta IPv4 ID
RTP Header Extension
Delta RTP timestamp
RTP Data
IP ID
RTP Seq
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Using CU with T and dT

• seq Time pkt type
• 100 1000 CR
• 101 3010 CR dT2010
  x
• 102 3020 CR dT10
  x
• 103 3030 CR
• Replace 101, 102
• 100 1000 CR
• 101 3010 CU T3010 dT2010
• 102 3020 CU T3020 dT10
  x
• 103 3030 CR

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Using CU with flag T

• seq Time pkt type
• 100 1000 CR
• 101 3010 CR dT2010
  x
• 102 3020 CR dT10
  x
• 103 3030 CR
• Replace 101, 102
• 100 1000 CR
• 101 3010 CU T3010
  x
• 102 3020 CR

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The N SchemeRepeat

• When to use
• At most N adjacent packets may get lost
• How to use
• Repeat change N1 times

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Using the N scheme (T, dT)

• seq Time pkt type
• 100 1000 CR
• 101 3010 CU T3010 dT2010
• 102 3020 CU T3020 dT10
  x
• 103 3030 CR
• N1
• 100 1000 CR
• 101 3010 CU T3010 dT2010
• 102 3020 CU T3020 dT10
  b
• 103 3030 CU T3030 dT10
  b
• 104 3040 CR

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Using the N scheme (T)

• seq Time pkt type
• 100 1000 CR
• 101 3010 CU T3010
  x
• 102 3020 CR
• N1
• 100 1000 CR
• 101 3010 CU T3010
  b
• 102 3020 CU T3020
  b
• 103 3030 CR

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N Can Be Learned

• Increase / decrease N according to the number of
  invalidated contexts

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The ACK SchemeRepeat until ACKd

• When to use
• Avoid context invalidation
• How to use
• Repeat change until ACKd

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ACK Packet Format
Type code4 ACK, 8-bit CID
Context Count
Session Context ID
RTP Sequence
...
Session Context ID
RTP Sequence
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ACK Packet Format
Type code5 ACK, 16-bit CID
Context Count
Session Context ID
RTP Sequence
...
Session Context ID
RTP Sequence
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Using the ACK Scheme (T,dT)

• seq Time pkt type
• 100 1000 CR
• 101 3010 CU T3010 dT2010
• 102 3020 CU T3020 dT10
  x
• 103 3030 CR
• ACK Scheme
• 100 1000 CR
• 101 3010 CU T3010 dT2010
• 102 3020 CU T3020 dT10
• Receive ACK 101
• 103 3030 CU T3030 dT10
• Receive ACK 102
• 104 3040 CR

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Using the ACK Scheme (T)

• seq Time pkt type
• 100 1000 CR
• 101 3010 CU T3010
  x
• 102 3020 CR
• ACK Scheme
• 100 1000 CR
• 101 3010 CU T3010
• 102 3020 CU T3020
• Receive ACK 101
• 103 3030 CR

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Bandwidth Overhead when using the N scheme

• Classic 3
• 101 3010 CR dT2010
  x
• 102 3020 CR dT10
  x
• Enhanced with (T, dT) 25 N6 overhead
• 101 3010 CU T3010 dT2010
• 102 3020 CU T3020 dT10
  b
• 103 3030 CU T3030 dT10
  b
• Enhanced with (T) 5 N5 overhead
• 101 3010 CU T3010
  b
• 102 3020 CU T3020
  b

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Bandwidth Overhead when using the N scheme

• Enhanced CRTP with N scheme
• (T, dT) transition 10 N6 bytes
• (T) transition 5 N5 bytes
• Average talk spurt length 1 second

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Bandwidth Overhead when using the ACK scheme

• Calculate same as in N scheme, selectN round
  trip delay (ms) / sample length (ms)
• ACK packet size 6 bytes

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Refresher Packet(CU Usage)

• seq Time pkt type
• N1
• 100 1000 CR
• 101 3010 CU T3010 dT2010
• 102 3020 CU T3020 dT10
  b
• 103 3030 CU T3030 dT10
  b
• 104 3040 CR
• 108 3080 CU I dI dT10 lt-- refresh
  all context state parameters

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The Random IP ID(CU Usage)

• dI is not constant
• Either dI or I must be included in each packet
• If dI is included in each packet, each lost
  packet invalidates the context
• Conclusion its better to include I
• Use CU with I flag instead of CR

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Sequence with changing dI

• seq Time pkt type
• 1 10 FH
  x
• 2 20 CR dI dT10
  x
• 3 30 CR dI
  x
• 4 40 CR dI
  x
• ...
• Enhanced sequence
• 1 10 FH
  x
• 2 20 CU I dT10
  x
• 3 30 CU I
• 4 40 CU I

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Including IP ID in the UDP Checksum

• UDP checksum is used to verify validity of
  reconstructed packets
• Reconstructed IP ID cannot be verified
• Include IP ID in UDP checksum

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Including IP ID in the UDP Checksum (cont.)

• When UDP checksum is present (nonzero)
• The compressor will 1's complement subtract the
  IP ID value from the UDP checksum before
  compression
• The decompressor will 1's complement add the IP
  ID value to the UDP checksum after any validation
  operations and before delivering the packet
  further downstream.

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Headers Checksum

• UDP Checksum used to validate reconstructed
  packets, especially when twice was used
• When UDP checksum is not present, CRTP may add
  Headers Checksum (HDRCKSUM)

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HDRCKSUM

• HDRCKSUM includes
• Pseudo-IP Header
• UDP Header
• Fixed part of RTP Header (first 12 bytes)
• IP ID
• Does not include
• The extended part of the RTP header
• RTP data

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HDRCKSUM (cont.)

• Calculated in the same way as a UDP checksum
• Placed in the COMPRESSED_UDP or COMPRESSED_RTP
  packets where a UDP checksum would have been
• The decompressor MUST zero out the UDP checksum
  field in the reconstructed packets

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NON-RTP stream flag

• Notifies the decompressor that this stream is not
  an RTP stream
• Decompressor can enter flow in negative cache
  without RTP compression attempts
• Less CID thrashing
• Useful for application nodes where compressor has
  hints from application layer.

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NON-RTP stream flag in the FULL_HEADER packet
New FULL_HEADER length fields format
For 8-bit context ID
CID
Generation
0
1
0
U
Seq
For 16-bit context ID
Seq
Generation
1
1
U
0
CID
Set U 1 to indicate a non-RTP stream
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Rejecting a new compressed stream

• Decompressor implementations may share resources
  across multiple links
• Decompressor may over commit decompression
  resources in RFC 2509 negotiation
• Decompressor may REJECT a compressed steam when
  out of resources

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Reject packet (Using CONTEXT_STATE opcode)
1CS, Reject 8-bit CID
2CS, Reject 16-bit CID
context count
context count
session context ID
session context ID
CRTP seq
0
1
1
0
generation
0
0
CRTP seq
0
1
1
0
generation
0
0
CID, CRTP sequence and generation are taken
from the FULL_HEADER
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Negotiating usage of enhanced-CRTP andACK scheme

• RFC 2509 specifies how the use of CRTP is
  negotiated on PPP links using the IP Compression
  Protocol option of IPCP
• IPCP option 2 IP compression protocol
• protocol 0x61 indicates RFC 2507 header
  compression
• sub-option 1 enables use of COMPRESSED_RTP,
  COMPRESSED_UDP and CONTEXT_STATE as specified in
  RFC 2508

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Negotiating usage of enhanced-CRTP andACK scheme
(cont.)

• For the enhancements defined in this document,
  two new sub-options are added
• sub-option 2 (length2) enables use of all
  CRTP enhancements except for the ACK scheme
• sub-option 3 (length2) enables use of all
  CRTP enhancements including the ACK scheme

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END