Datagram Congestion Control Protocol (DCCP)

1
Datagram Congestion Control Protocol (DCCP)

• CISC 856 - TCP/IP and Upper Layer Protocols
• Presented by
• Ke Li (kli_at_cis.udel.edu)
• 2007/12/4
• Thanks to Prof Amer, Kireeti Valicherla, and
  Xiaofeng Han

2
Overview

• Motivation
• Connections
• Unreliable datagram transfer
• Modular congestion control
• Miscellaneous issues

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DCCP Which Layer?
SCTP
DCCP
Adapted from Figure 2-11 TCP/IP Protocol Suite,
Behrouz A. Forouzan
4
Streaming Media

• What streaming media needs?
• Timeliness of data
• What streaming media doesnt need?
• Retransmissions of lost/expired packets
• Annoying rebuffering HOL blocking

Source http//streaming.wisconsin.edu/Accessible_
Tutorials/Tutorial1/p1-3.htm
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Streaming Media Over TCP
Server
Client
D12
A12
D13
D14 - D16
A12
Data is not useful now
Retransmit D13
D data TCP-PDU A ack TCP-PDU
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Streaming Media Over UDP
Server
Client

• No congestion control in UDP flows
• Harmful to Internet health

7
Streaming Media with SCTP

• Multi-streams over a single association
• Uses TCP-like congestion control
• Retransmission
• Partial Reliability require at least 1 RTT

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Other target applications

• Internet Telephony
• Constant-packet-rate sources
• Change data rate by adjusting packet size
• Extremely sensitive to delay
• Demands a slower congestion response
• Interactive games
• Can quickly make use of available bandwidth
• Prefers TCP-like sawtooth congestion response

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Solution DCCP

• provides unreliable flow of datagrams
• provides congestion control using
• Acknowledgment
• Sequence number
• Connection oriented
• does not provide
• Full reliability no-loss no-error in-order
  no-duplicate
• flow control
• streaming
• DCCP UDP congestion control
• or
• TCP bytestream semantics
  full reliability

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DCCP connections
DCCP A
DCCP B

• Full-duplex bi-directional connection
• Two logical half connections
• A-to-B half connection
• Application data sent from A to B
• Corresponding acks from B to A
• In practice overlapped DataAck
• Each half connection can have independent
  features negotiated during connection initiation,
  e.g., different congestion control mechanism

Data
Ack
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DCCP Connection Initiation
Client
Server
CLOSED
CLOSED
LISTEN
DCCP Request
REQUEST
RESPOND
DCCP Response
DCCP Ack
PARTOPEN
OPEN
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DCCP Data Transfer Phase
Client
Server
DCCP A
DCCP B
OPEN
PARTOPEN
DCCP Data
OPEN
DCCP Ack
DCCP Data
DCCP DataAck

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DCCP Connection Termination
Client
Server
Client
Server
OPEN
OPEN
OPEN
OPEN
DCCP Close
DCCP CloseReq
CLOSING
CLOSEREQ
DCCP Reset
CLOSED
CLOSING
DCCP Close
TIMEWAIT
DCCP Reset
CLOSED
Wait 2 MSL
CLOSED
TIMEWAIT
Wait 2 MSL
CLOSED
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DCCP Data Transfer Example 1. without loss
DCCP A
DCCP B

• Seq on DCCP-PDU, not byte
• Each PDU carries a Seq
• Seq increases per PDU
• detect loss congestion control
• network duplicate ignored
• Pure acks also consume Seq
• possible to detect ack loss
• No cum ack ack is the Greatest Seq Received
  (GSR) normally

Data(seq 1)
Ack(seq 10, ack 1)
Data(seq 2)
Ack(seq 11, ack 2)
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DCCP Data Transfer Example 2. non-large burst
of loss
DCCP A
DCCP B

• Maybe loss no data retransmissions
• Separate options indicate PDU loss or ECN info
  Ack Vector (SACK-like)
• Ack of Ack clear receivers state
• A PDU is ackable its header has been
  successfully processed (e.g., valid header
  checksum and seq )
• Acked PDUs may be dropped -- no guarantee of data
  delivery
• Due to receiver buffer overflow or corruption --
  endpoint loss
• Data dropped option

Data(seq 1)
Ack(seq 10, ack 1)
Data(seq 2)
Data(seq 3)
Ack(seq 11, ack 3)
Data(seq 4)
Ack(seq 12, ack 4)
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Sequence Validity Check

• Both endpoints keep expected seq/ack -- in sync
• To detect seq attacks, significant reordering,
  or one endpoint crash
• Out of sync after large burst of loss
• No cum ack, use separate DCCP-Sync/SyncAck PDU to
  recover
• Sequence number variables
• Maintained at each endpoint for each connection
• GSR Greatest Seq Received
• GSS Greatest Seq Sent
• Sequence validity windows
• Window width W Seq Win feature
• Expected seq SWL, SWH, SWHGSR3W/4
• Expected ack AWL, AWH, AWHGSS
• Seq/ack out of range seq invalid PDU, ignore
  and send Sync PDU

exp seq window
exp ack window
W/4
3W/4
W
GSR
GSS
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DCCP Data Transfer Example 3. large burst of
loss
DCCP A
DCCP B
Exp ack
Exp seq
Data(seq 20)
13, 20
GSS20
19, 26
GSR20
Ack(seq 60, ack 20)
Data(seq 21)
14, 21
GSS21

Sync.ack out-of-sync seq recvd
Data(seq 30)
23, 30
GSS30
Data(seq 31)
24, 31
GSS31
seq 31gt26, out of range, send Sync.ack31
Sync(seq 61, ack 31)
SyncAck(seq 32, ack 61)
25, 32
GSS32
31, 38
GSR32
If Sync is ackable, SyncAck.ack Sync.seq
If valid ack update GSR, back to sync

• GSR Greatest Sequence number Received
• GSS Greatest Sequence number Sent
• Window Size 8

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DCCP Data Transfer Example 4. slight reordering
DCCP A
DCCP B
Exp ack
Exp seq
Data (seq 20)
13, 20
GSS20
19, 26
GSR20
Ack (seq 60, ack 20)
Data (seq 21)
14, 21
GSS21
Data (seq 22)
15, 22
GSS22
GSR22
21, 28
Ack (seq 61, ack 22)
seq 21 within range, Ack.ack GSR
Data may be delivered out-of-order
Ack (seq 62, ack 22)
Data (seq 23)
16, 23
GSS23
Ack (seq 63, ack 23)
22, 29
GSR23

• GSR Greatest Sequence number Received
• GSS Greatest Sequence number Sent
• Window Size 8

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DCCP Data Transfer Example 5. medium reordering
DCCP A
DCCP B
Exp ack
Exp seq
Data (seq 18)

Data (seq 22)
15, 22
GSS22
GSR22
21, 28
Ack (seq 61, ack 22)
Sync (seq 62, ack 18)
seq18lt21, out of range, send Sync.ack18
SyncAck (seq 23, ack 62)
16, 23
GSS23
22, 29
GSR23
If Sync is ackable, SyncAck.ack Sync.seq
If valid ack update GSR, back to sync

• GSR Greatest Sequence number Received
• GSS Greatest Sequence number Sent
• Window Size 8

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DCCP Data Transfer Example 6. significant
reordering (or blind attack)
DCCP A
DCCP B
Exp ack
Exp seq
Data (seq 10)

Data (seq 22)
15, 22
GSS22
GSR22
21, 28
Ack (seq 61, ack 22)
Sync (seq 62, ack 10)
seq10lt21, out of range, send Sync.ack10
ack10lt15, out of range, nonackable Sync, ignored
Data (seq 23)
16, 23
GSS23
Ack (seq 63, ack 23)
22, 29
GSR23

• GSR Greatest Sequence number Received
• GSS Greatest Sequence number Sent
• Window Size 8

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DCCP PDU Types
DCCP-Request
DCCP-Response
DCCP-Ack
DCCP-Data
DCCP-DataAck
DCCP-CloseReq
DCCP-Close
DCCP-Reset
DCCP-Sync, DCCP-SyncAck
Connection Initialization
Data Transfer
Connection Termination
Resynchronization
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DCCP PDU Formats
Generic Header
Additional Fields (depending on type)
Options (optional )
Application Data Area

• Generic header 16 bytes (using 48bits seq) or
  12 bytes (using short 24 bits seq)
• Additional fields fixed length field, e.g. ack
  (48b, 24b)
• Options variable length field up to1008 bytes,
  e.g. Init Cookie, Ack Vector, Change, Confirm,
  Slow Receiver, Data Dropped

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DCCP Generic Header
0
8
16
24
Source Port Source Port Source Port Source Port Source Port Destination Port
Data Offset Data Offset Data Offset CCVal CsCov (4bit) Checksum
Res Type (4bit) 1 Reserved Reserved Sequence Number (high bits)
Sequence Number (low bits) Sequence Number (low bits) Sequence Number (low bits) Sequence Number (low bits) Sequence Number (low bits) Sequence Number (low bits)
X 1
24
DCCP Generic Header short
0
8
16
24
Source Port Source Port Source Port Source Port Source Port Destination Port
Data Offset Data Offset Data Offset CCVal CsCov Checksum
Res Type 0 Sequence Number Sequence Number Sequence Number
X 0
25
Acknowledgement Sub-Header
0
8
16
24
Reserved Acknowledgement Number (high bits)
Acknowledgement Number (low bits) Acknowledgement Number (low bits)
X 1
X 0
Reserved Acknowledgement Number(low bits)
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DCCP Checksum
0
4
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CsCov Checksum

• Header Checksum Coverage (CsCov) 4 bits
• CsCov 0 covers the DCCP header (generic,
  additional), DCCP options, network-layer
  pseudoheader, and all application data in the
  packet (possibly some padding)
• CsCov 1-15 covers the DCCP header, DCCP
  options, network-layer pseudoheader, and the
  initial (CsCov-1)4 bytes of the packet's
  application data.
• Applications that can tolerate corruption can
  request header checksum only covers part or no
  app data at all
• Corrupted data could be delivered, impact on
  congestion control
• Improve delivery rate and perceived performance
• Data checksum option provides strong CRC checksum
  for all application data

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Modular Congestion Control

• Each congestion control mechanism supported by
  DCCP is assigned a 1-byte congestion control
  identifier, or CCID a number from 0 to 255.
• CCID 0 and CCID 1 are reserved
• TCP-like congestion control CCID 2
• TCP friendly rate control (TFRC) CCID 3
• CCID 4-255 are reserved
• CCID is a feature to be negotiated and agreed on
  both endpoints

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CCID 2 TCP-like Congestion Control

• Congestion control like TCP RFC4341
• Ack contains Seq of all received packets within
  some window
• Congestion event packet loss, or ECN -gt Halve
  congestion window
• Abrupt rate changes
• Reverse-path (Ack) congestion control
• Ack Ratio R, integer, 2, cwnd/2
• For each congestion window of data where at least
  one of the corresponding acks was lost or
  ECN-marked, R is doubled
• For each cwnd/(R2-R) consecutive congestion
  window of data whose acks were not lost or
  ECN-marked, R is decreased by 1
• The above formula comes from wanting to increase
  the number of acks per congestion window, namely
  cwnd/R, by 1 for every congestion-free window
  that passes.

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CCID 2 TCP-like Congestion Control RFC4341

• Applications using this
• Respond quickly to changes in available
  bandwidth
• Must tolerate abrupt rate changes -- sawtooth

Online interactive games prefer this kind of
congestion control
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CCID 3 TCP Friendly Rate Control RFC 4342

• Receiver-based feedback mechanism
• Equation-based congestion control
• Minimizes abrupt changes in sending rate
• Maintains longer-term fairness with TCP

Streaming Media prefer steadier and less bursty
traffic as provided by TFRC
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CCID 3 TCP Friendly Rate Control

• The receiver measures the loss event rate and
  feeds this information back to the sender
• The sender uses these feedback messages to
  measure the round-trip time (RTT)
• The loss event rate and RTT are then fed into
  TFRC's throughput equation, giving the acceptable
  transmit rate
• The sender then adjusts its transmit rate to
  match the calculated rate

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Congestion related options

• Slow receiver option
• Receiver sends this option to its sender to
  indicate it is having trouble keeping up with the
  senders data
• Sender shouldn't increase sending rate for about
  1 RTT time
• Data dropped option
• indicates that a packet was dropped due to
  corruption, receiver buffer overflow, application
  requirement, or other non congestion reasons.

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Feature Negotiation Options

• DCCP features on what value two endpoints agree
• Examples
• Congestion control identifier (CCID)
• ECN capable / incapable
• Ack Ratio
• Allow Short Seq
• DCCP features are identified by a feature number
  and an endpoint
• Notation F/X is used
• Use Change and Confirm options to negotiate

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F/X Notation
Feature location for all F/B
Feature location for all F/A
B
A
Change L (Local)
Change R (Remote)
Confirm L (Local)
Confirm R (Remote)
Feature Remote for all F/B
Feature Remote for all F/A
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Feature Negotiation

• General-purpose reliable negotiating
• Almost always during the connection initiation
  handshake, but it can begin at any time
• Each happens in a single option exchange
• Multiple values, preference order

Type Length Feature Value(s)
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Feature Negotiation Example 1
Client
Server
CCID/Server agreed as 2
Change R (CCID, 2)
Confirm L (CCID, 2)
Change L (CCID, 3 4)
CCID/Server agreed as 4
Confirm R (CCID, 4, 4 2)
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Feature Negotiation Example 2
Client
Server
Change L(CCID, 3 2)
Change L(CCID, 3 2)
Change L(CCID, 3 2)
Change L(CCID, 3 2)
Confirm R(CCID, 3, 3 2)
CCID/Client agreed as 3
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DCCP Miscellaneous issues

• Maximum Packet Size (MPS)
• Maintained for each DCCP session
• Minimum of congestion control MPS (CCMPS) and
  path MTU
• Generally, DCCP should NOT fragment data reduce
  robustness
• Applications can usually get better error
  tolerance by producing packets smaller than the
  PMTU
• Security Concerns
• Prevents SYN-flooding-like DDoS attacks init
  cookie
• Prevents Sequence Number Attack
• Large Sequence Number
• Sequence and Acknowledgement Number Windows

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DCCP Summary

• Transport layer protocol
• Unreliable datagrams
• Modular congestion control
• Negotiable features

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Implementation

• Linux Kernel Version 2.6.14
• Preliminary FreeBSD implementation
• tcpdump 3.9.4 and later includes DCCP support

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References

• Designing DCCP Congestion Control Without
  Reliability, by Eddie Kohler, Mark Handley, and
  Sally Floyd. Proc. ACM SIGCOMM 2006, September
  2006.
• RFC 4340 - Datagram Congestion Control Protocol
  (DCCP), Eddie Kohler, Mark Handley, and Sally
  Floyd, March 2006
• DCCP Overview, Eddie Kohler and Sally Floyd, July
  2003
• DCCP link from one of the authors
• http//www.read.cs.ucla.edu/dccp/

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Questions and Comments ?
Thank you! Have a nice holiday!