Datagram Congestion Control Protocol (DCCP)

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Datagram Congestion Control Protocol (DCCP)

• CISC 856 - TCP/IP and Upper Layer Protocols
• Presentation by
• Xiaofeng Han
• xiaofeng_at_udel.edu
• Thanks for Kireeti Valicherlas slides

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Overview

• Motivation
• Connection process
• Unreliable packets flow with acknowledgement
• Features negotiation
• Choice of congestion control
• Miscellaneous features

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

• What streaming media needs?
• Timeliness of data
• What streaming media doesnt need?
• Retransmissions of lost and expired packets

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
D13
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Streaming Media Over UDP
Server
Client

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

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Streaming Media with SCTP

• Multi-streams over a single association
• Uses TCP-like congestion control
• Retransmission

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Solution

• DCCP
• Reliable connection establishment and termination
• Unreliable packet flow
• On-demand congestion control
• Optional features
• Security concerns

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DCCP Connection Setup
Client
Server
DCCP A
DCCP B
Similar to TCP connection setup
DCCP Request
DCCP Response
DCCP Ack
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DCCP Data Transfer Phase
Client
Server
DCCP DATA
DCCP ACK
DCCP DATA
DCCP DATA ACK
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DCCP Connection Termination
Client
Server
Client
Server
DCCP CloseReq
DCCP Close
DCCP Reset
DCCP Close
DCCP Reset
DCCP Reset
Wait 2 MSL
Wait 2 MSL
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DCCP Data Transfer Example - 1

• Client

Server

• Pure Seq , not bytes
• Each packet carries a Seq
• Seq increase per packet
• Pure Acks also consume Seq

DCCP-DATA (Seq 1)
DCCP-ACK (Seq 10, Ack 1)
DCCP-DATA(seq 2)
DCCP-ACK(seq 11, ACK 2)
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DCCP Data Transfer Example - 2

• Client

Server

• No Retransmissions
• Acks the largest Seq received

DCCP-DATA (Seq 1)
DCCP-ACK (Seq 10, Ack 1)
DCCP-DATA(seq 2)
DCCP-DATA(seq 3)
DCCP-ACK(seq 11, ACK 3)
DCCP-DATA(seq 4)
DCCP-ACK(seq 12, ACK 4)
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DCCP Data Transfer Example - 3

• Client

Server
GSR 1 (W/4)
DATA (Seq 20)
ACK (Seq 60, Ack 20)
DATA(seq 21)
DATA(seq 22)
DATA(seq 23)
DATA (Seq 24)
ACK(seq 61, ACK 24)
GSR 3 (W/4)

• GSR Greatest Sequence Number Received
• GSS Greatest Ack Number Received
• Window Size 8

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DCCP Data Transfer Example - 4

• Client

Server
GSR 1 (W/4)
DATA (Seq 20)
ACK (Seq 60, Ack 20)
DATA(Seq 21) DATA(Seq 30)
Seq out of range
DATA (Seq 31)
Sync(Seq 61, ACK 31)
SyncAck (Seq 32, Ack 61)
GSR 3 (W/4)

• GSR Greatest Sequence Number Received
• GSS Greatest Ack Number Received
• Window Size 8

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DCCP Packet Types
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DCCP Packet Formats

• DCCP header can be from 12 to 1020 bytes
• Generic header 12 bytes
• Additional fields fixed length field
• Options variable length field

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DCCP Generic Header
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DCCP Generic Header
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Acknowledgement Sub-Header
X 1
X 0
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DCCP Checksum

• Checksum Coverage (CsCov) 4 bits
• CsCov 0 covers the DCCP header, 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.
• Option provides CRC checksum for all application
  data

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Congestion Control in DCCP

• Each congestion control mechanism supported by
  DCCP is assigned a 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

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CCID 2 TCP-like Congestion Control

• Congestion control as in TCP/IP
• Slow start
• Timeouts
• Congestion event -gt Halve congestion window
• Abrupt rate changes

(initial) ssthresh
cwnd
Loss, e.g. timeout
time
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CCID 2 TCP-like Congestion Control

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

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

• TFRC, RFC 3448
• Equation-based congestion control
• Minimizes abrupt changes in sending rate
• Maintains longer-term fairness with TCP

Streaming Media doesnt need responsiveness but
prefer steadier and less burst traffic as
provided by TFRC
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CCID 3 TCP Friendly Rate Control

• s
• X ---------------------------------------------
  ----------------------------------------
• Rsqrt(2bp/3) (t_RTO (3sqrt(3bp/8)
  p (132p2)))
• Where
• X is the transmit rate in bytes/second.
• s is the packet size in bytes.
• R is the round trip time in seconds.
• p is the loss event rate, between 0 and 1.0, of
  the number of loss events as a fraction of the
  number of packets transmitted.
• t_RTO is the TCP retransmission timeout value in
  seconds.
• b is the number of packets acknowledged by a
  single TCP acknowledgement.

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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
• Data dropped option
• Option indicates that some packets reported as
  received actually had their data dropped before
  it reached the application.

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Explicit Congestion Notification
ECE set ACK
ECT set
CWR set
ECN enabled sender
ECN enabled receiver
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Features Negotiation

• Connection attribute on what value two endpoints
  agree
• Examples
• Congestion control identifier (CCID)
• ECN capable / incapable
• Data checksum
• Sequence Window
• DCCP features are identified by a feature number
  and an endpoint
• Notation F/X is used

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F/X Notation
Feature location for all F/B
Feature location for all F/A
B
A
Feature Remote for all F/B
Feature Remote for all F/A
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Feature Negotiation

• Anytime during the connection process
• Carried in a reliable way
• Multiple values, priority order
• Endpoints keep sending packets containing change
  options, until agreement is reached ( and
  signalled by Confirm Option)

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Feature Negotiation Example
Client
Server
CCID/Server agreed as 2
Change R (CCID, 2, 3)
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
Client
Server
Change R(CCID, 2)
Change R(CCID, 2)
Change R(CCID, 2)
Change R(CCID, 2)
Confirm L(CCID, 2)
CCID/Server agreed as 2
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DCCP Miscellaneous Features

• Security Concerns
• Prevents DDoS attacks init cookie
• Prevents Sequence Number Attack
• Large Sequence Number
• Sequence and Acknowledgement Number Windows
• Data Corruption
• CRC data Checksum Option

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DCCP Miscellaneous Features

• Path MTU discovery
• DCCP should NOT fragment data
• DCCP must maintain maximum packet size (MPS)
• Applications can usually get better error
  tolerance by producing packets smaller than the
  PMTU
• Methods
• On IPv4 connections whose applications have
  requested fragmentation, the sender SHOULD send
  packets with the DF bit not set
• On IPv6 connections whose applications have
  requested fragmentation, the sender SHOULD use
  fragmentation extension headers to fragment
  packets larger than PMTU into suitably-sized
  chunks (Those chunks are, of course,
  unfragmentable.)

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DCCP and RTP
DCCP
Figure 28-8 TCP/IP Protocol Suite, Behrouz A.
Forouzan
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DCCP Summary

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

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References

• Datagram Congestion Control Protocol (DCCP)Eddie
  Kohler, Mark Handley, and Sally FloydMay 2005
  http//www.icir.org/kohler/dcp/draft-ietf-dccp-spe
  c-09.txt
• DCCP OverviewEddie Kohler and Sally Floyd
  http//www.icir.org/kohler/dcp/summary.pdf
• DCCP
• Eddie Kohler, Mark Handley, Sally Floyd,
  Jitendra Padhyehttp//www.icir.org/kohler/dcp/
• TCP Friendly Rate Control (TFRC) RFC 3448

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Questions and Comments ?