Transcript 投影片 1

DCCP: Transport Protocol with
Congestion Control and Unreliability
Source：Yuan-Cheng Lai IEEE Internet
Computing ,2008 October
Reporter : Chang, Yi Jui
Student ID : M9756003
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Outline
Introduction
 Packet format
 Connection Phases

◦ Connection establishment
◦ Data transfer
◦ Connection termination

Congestion Control
◦ CCID 2
◦ CCID 3

Comparisons
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TCP Version
Tahoe
 Reno
 New Reno
 SACK

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TCP behavior
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Introduction-Why not use TCP?

IP phones, video conferencing, video on
demand (VoD), and online games have
consequently become extremely popular.

Real-time operation, delay sensitivity, and
low reliability.
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Introduction-Why not use TCP?

TCP’s reliability mechanism retransmits lost
packets, but retransmitting expired packets is
redundant.

TCP reduces the transmission rate on
detecting packet loss, reducing quality of
service (QoS) for the user.

TCP executes a congestion-control
mechanism, it uses less than its fair share of
bandwidth when compared to UDP.
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Introduction

Multimedia applications have increased,
UDP traffic has become widespread
throughout the Internet, causing network
congestion and threatening TCP traffic.

DCCP has two basic functions:
◦ Establish, maintain, and tear down(or close) unreliable
connections.
◦ Utilize the congestion-control mechanism for
unreliable connections.
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Introduction

DCCP’s main characteristics：
◦
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Unreliable data transfer
Reliable connection establishment
Adequate packet options
Dynamic choice of congestion control
Dynamic adjustment of acknowledgment rate
Prevention of SYN flooding attack
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Packet format
The diagram shows
(a) DCCP’s packet format.
(b) a DCCP generic header.
(c) the acknowledgment number sub-header.
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Packet format
CCval: indicates the congestion control
mechanism used (CCID 2 or CCID 3).
 CsCov: (checksum coverage) determines
what range the checksum covers.


These two particular fields, CCval and
CsCov, didn’t appear in previous
protocols.
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Packet format

CCval lets the application dynamically
adopt a suitable congestion-control
mechanism according to its requirement
and network.

CsCov enhances performance on errorprone links for applications.

DCCP has defined 10 packet types.
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Connection Phases

DCCP is a connection-oriented protocol
and provides a bidirectional connection.

Connection establishment, data transfer,
and connection termination.

DCCP has defined nine states that signify
these phases’ processes: CLOSED, LISTEN,
REQUEST, RESPOND, PARTOPEN, OPEN,
CLOSEREQ, CLOSING, and TIMEWAIT.
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Connection establishment
DCCP uses a three-way handshake
to establish a connection.
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Connection establishment

Why not go to OPEN state？ Prevent a
deadlock.

If a DCCPAck/DCCP-DataAck packet is
lost, the server doesn’t enter the OPEN
state and thus can’t send data.

Maximum segment lifetime(MSL) expires
4 times.
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Data transfer

The data packet exchange between the
client and server is unreliable but
restricted to congestion control.

During the transfer process, DCCP
endpoints might use DCCP-Sync and
DCCP-SyncAck packets for
synchronization.
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Data transfer
DCCP can use
DCCP-Sync and
DCCP-SyncAck
packets to
synchronize the
sequence numbers
of both sides.
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Connection termination
DCCP uses two- or three-way handshakes
to terminate a connection.
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Congestion Control
CCID2: additive increase/ multiplicative
decrease (AIMD) congestion-control
mechanism whereby the sender adopts a
congestion window to control the
transmission rate.
 Congestion window-based congestion
control.

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Congestion Control

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CCID3: TFRC(TCP-Friendly Rate Control)
congestion control. The sender uses the loss
event ratio the receiver measures, calculates
the transmission rate T via the following
formula, and accordingly adjusts its sending
rate.
Rate-based congestion control.
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T is transmission rate
S is packet size
RTT is round-trip time
tRTO is retransmission timeout
p is loss event ratio
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Comparisons
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