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Chapter 12

Transmission Control Protocol(TCP) – Part Two

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CONTENTS

Part Two

7. Retransmission Timer8. Congestion Control 9. Segment10. Options11. Checksum12. Connection13. State Transition Diagram14. TCP Oeration15. TCP Package

Part One

1. Process-to-process Communication

2. TCP Services3. Numbering Bytes4. Flow Control5. Silly Window Syndrome6. Error Control7. TCP Timers

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12.7 TCP Timers

Missing ACK 0-Sized

WindowDetect Idle

clients Connection Termination

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12.7.1 Retransmission Timer

How long to wait for an ACK of a previously sent segment before retransmission.Depends on distance and network traffic density.

Retransmission time should be dynamic.Retransmission time = 2 × RTT

Dynamic Calculation of RTT:Use a timestamp TCP option (discussed later), or

a) Actually measure RTT of first two segments of a connectionb) RTT new estimate = α × RTT old estimate + ( 1 – α ) × RTTactual

c) Typically, α = 0.90d) Do NOT consider retransmitted segments into the above

calculation of RTT

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12.8 Congestion Control

Congestion occurs when some routers along the path run out of buffers and drop packets.Sending TCP assumes that a missing ACK is due to congestion.

Retransmission will only aggravate the congestionA Congestion Window (CongWin) is used to control number of segments transmitted simultaneously (i.e. before waiting for an ACK)

Sender Window = Min ( RecvWin , CongWin)

During connection establishment, each party specifies its maximum Receiving Window (RecvWinMax), and the Maximum Segment Size (MSS) it can receive.

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The (Slow Start, Additive Increase, Multiplicative Decrease) Cycle

Received ACKs

Initially,CongWin = Receiver’s MSS = NThreshold = ½ RecvWinMax

Slow: CongWin = 2 × CongWinAdditive: CongWin = 1 + CongWinIf time-out,

Threshold = ½ CongWinCongWin = N

≤ RecvWin

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12.9 TCP Segment20 to 60 Bytes

Starting from ISN

+ 2

Sliding Window

sizeIn Bytes

6 Control Bits

#of 4-Byte words Offset of

last byte of urgent data

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Control Fields

Enable flow control, connection establishment & termination, andmode of data transfer.One or more bits may be set at the same timeDiscussed in detail later

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12.10 Options

Up to 40 bytes of optional information to the destination

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No-operation Option

A Filler option used for alignment purposes only.

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End-of-option Option

Used as a padding to terminate options. Only ONE may be present.The remainder of this 32-bit word is garbage

Payload data starts at the next 32-bit word.

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Maximum segment size (MSS) Option

The maximum number of data bytes I can receive in one segmentEach party determines its MSS during connection establishment

Cannot be changed later.Defaults to 536 bytes

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Window scale factor Option

The 16-bit Window Size in the header is too small for high-speed links (e.g. OC-24 @ 1.2Gbps)Actual Sender Window size = WinSize in header × 2 WinScaleFactor

WinScaleFactor ≤ 16This option may only appear during connection establishment.

Header.WinSize may change, but WinScaleFactor is constant throughout the entire connection

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Timestamp Option

Set by the sender just before the segment leaves (to IP layer)Echoed by the receiver just before it ACKs the segmentThe sender measure RTT as:

the arrival time of the ACK - the timestamp echo reply.

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12.11 Checksum

Same as in UDP, but mandatory

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12.12 Connection

Connections must be established to facilitate acknowledgements, and retransmissions.Connections are terminated at the end.A connection may be reset at any time in one of these 3 cases:1. A destination rests a connection requested for a nonexistent port2. One party encounters abnormal situation, so it resets the

connection3. One party detects that the other party has been idle for too long.

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Connection Establishment: A 3-way Handshaking

Client Server

ISNPossibly MSS and/or

WinScaleFactor

WinSize, ISNPossibly MSS and/or

WinScaleFactor

WinSize,Possibly real data

LISTENingState

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Connection Termination: A 4-way handshakingUsually, Client Usually, Server

Active Close

Passive Close

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12.13 TCP FSM: State Transition Diagram

Client states Server states

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Complete State Diagram

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12.14 TCP Operation

Encapsulation and decapsulationBufferingMultiplexing and demultiplexingPushing Data

Send NOW, deliver to server application ASAPUrgent Data

Delivered to other application out-of-order

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Encapsulation and decapsulation

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Multiplexing and demultiplexing

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12.15 TCP Package

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Transmission Control Blocks

of the Connection