Datagram Congestion Control Protocol (DCCP)

CISC 856 - TCP/IP and Upper Layer Protocols

Presentation byXiaofeng Han

xiaofeng@udel.edu

Thanks for Kireeti Valicherla’s 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

Figure 2-11 TCP/IP Protocol Suite, Behrouz A. Forouzan

DCCP: Which Layer?

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Streaming Media

Source: http://streaming.wisconsin.edu/Accessible_Tutorials/Tutorial1/p1-3.htm•What streaming media needs?

•Timeliness of data

•What streaming media doesn’t need?

•Retransmissions of lost and expired packets

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D12

D13

A12

A12

D14 - D16

D13Data is not

useful

Server Client

Streaming Media Over TCP

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Streaming Media Over UDP

No congestion control in UDP

flows

Harmful to Internet health

Server Client

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Streaming Media with SCTP

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

IP network

IP A2

IP B2 IP B1

IP B3IP A1

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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 Response

DCCP Request

DCCP Ack

Client Server

DCCP Connection Setup

DCCP A DCCP B

Similar to TCP

connection setup

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DCCP Data Transfer Phase

Client Server

DCCP DATA

DCCP ACK

DCCP DATA ACK

DCCP DATA

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DCCP Connection Termination

DCCP Close

DCCP Reset

Client Server

DCCP CloseReq

Wait 2 MSL

DCCP Close

DCCP Reset

Client Server

Wait 2 MSL

DCCP Reset

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DCCP Data Transfer Example - 1

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

DCCP-DATA (Seq #1)

DCCP-DATA(seq # 2)

DCCP-ACK(seq # 11, ACK # 2)

ClientServer

DCCP-ACK (Seq # 10, Ack #1)

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No Retransmissions Acks the largest Seq # received

DCCP-DATA (Seq #1)

DCCP-DATA(seq # 2)

DCCP-ACK(seq # 11, ACK # 3)

ClientServer

DCCP-ACK (Seq # 10, Ack #1)

DCCP Data Transfer Example - 2

DCCP-DATA(seq # 3)

DCCP-DATA(seq # 4)

DCCP-ACK(seq # 12, ACK # 4)

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GSR – Greatest Sequence Number Received GSS – Greatest Ack Number Received Window Size = 8

DCCP Data Transfer Example - 3

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GSR + 3 * (W/4)

GSR + 1 – (W/4)

DATA (Seq #20)

DATA(seq # 21)

ACK(seq # 61, ACK # 24)

ClientServer

DATA(seq # 22)

ACK (Seq #60, Ack #20)

DATA(seq # 23)

DATA (Seq #24)

24

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GSR – Greatest Sequence Number Received GSS – Greatest Ack Number Received Window Size = 8

DCCP Data Transfer Example - 4

GSR + 3 * (W/4)

GSR + 1 – (W/4)

DATA (Seq #20)

DATA(Seq # 21)

:

DATA(Seq # 30)

Sync(Seq # 61, ACK # 31)

ClientServer

ACK (Seq #60, Ack #20)

DATA (Seq #31)

Seq # out of range

SyncAck (Seq #32, Ack 61)

2032

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DCCP Packet Types

DCCP-Request

DCCP-Response

DCCP-Ack

DCCP-Data

DCCP-DataAck

DCCP-CloseReq

DCCP-Close

DCCP-Reset

DCCP-Sync, DCCP-SyncAck

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DCCP Packet Formats

Generic Header

Additional Fields (depending on type)

Options (optional )

Application Data Area

• 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

Source Port Destination Port

Data Offset CCVal CsCov Checksum

Res Type X

=

0

Sequence Number

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DCCP Generic Header

Source Port Destination Port

Data Offset CCVal CsCov Checksum

Res Type X

=

1

Reserved Sequence Number (high bits)

Sequence Number (low bits)

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Acknowledgement Sub-Header

Reserved Acknowledgement Number (high bits)

Acknowledgement Number (low bits)

Acknowledgement Number(low bits)

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

CsCov Checksum

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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 -> Halve congestion window Abrupt rate changes

time

cwnd

Loss, e.g. timeout

(initial) ssthresh

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

Streaming Media doesn’t need responsiveness but prefer steadier and less burst traffic as provided by TFRC

CCID 3: TCP Friendly Rate Control

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CCID 3: TCP Friendly Rate Control

s

X = -------------------------------------------------------------------------------------

{R*sqrt(2*b*p/3) + (t_RTO * (3*sqrt(3*b*p/8) * p * (1+32*p^2)))}

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 sender’s 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

ECT set

ECN enabled sender ECN enabled receiver

CWR set

ECE set ACK

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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

A B

Feature location for all F/A

Feature location for all F/B

Feature Remote for all F/B

Feature Remote for all F/A

Change L (Local)

Change R (Remote)

Confirm L (Local)

Confirm R (Remote)

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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)

Type Length Feature # Value(s)

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Feature Negotiation Example

Change R (CCID, 2, 3)

Confirm L (CCID, 2)

Change L (CCID, 3, 4)

Confirm R (CCID, 4, 4 2)

CCID/Server agreed as 2

CCID/Server agreed as 4

Client Server

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Feature Negotiation Example

Change R(CCID, 2)

CCID/Server agreed as 2

Change R(CCID, 2)

Change R(CCID, 2)

Change R(CCID, 2)

Confirm L(CCID, 2)

Client Server

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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 Floyd[May 2005] http://www.icir.org/kohler/dcp/draft-ietf-dccp-spec-09.txt

DCCP OverviewEddie Kohler and Sally Floyd http://www.icir.org/kohler/dcp/summary.pdf

DCCP Eddie Kohler, Mark Handley, Sally Floyd, Jitendra Padhye

http://www.icir.org/kohler/dcp/

TCP Friendly Rate Control (TFRC) RFC 3448

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Questions and Comments ?