TCP

Learning objectives

• Reliable Transport in TCP

• TCP flow and Congestion Control

TCP

- TCP passed block of data to IP, consisting of the TCP header and application layer data, called segment- Stream of octets passed between sender/ receiver Deliver in same octate sequence- Connection-oriented service - Full duplex - Reliable service

byte stream

Send buffer

segments

Receive buffer

byte stream

Application Application

ACKs

Transmitter Receiver

TCP: Overview

TCP Segment

Source Port Destination Port

Sequence Number

Acknowledgement Number

Checksum Urgent Pointer

Options Padding

0 4 10 16 24 31

URG

ACK

PSH

RST

SYN

FIN

HeaderLength Reserved Window Size

Data

• Source, Destination port: 16 bits - identify applications at ends of the connection

• Client Program allocates a port (usually above 1023)

• Servers are known by ports number - FTP 20, TELNET 23, SMTP 25, HTTP 80

• Port numbers are generally allocated by -- 0 -- not used -- 1- 255 -- Reserved ports for well- known services -- 256- 1023 -- Other reserved ports -- 1024- 65535 -- user- defined server ports

• Unix store general used ports in /etc/ services

TCP header fields

Socket

• Connection identification by 5 tuple --- 2 IP address, 2 port, protocol number (TCP=6) -- Socket

• IP address is unique to a node, the port is unique on a node

• A connection is identified by the socket address at its to ends: - client socket: 158. 108.33. 3, 3000; 158.108.2.71,21 - server socket: 158.108.2.71,21; 158.108.33.3,3000;

Socket: Multiple connection

• Server’s unique socket address can be accessed simultaneously by clients

TCP: Reliable Transport

• Send and wait for acknowledgment with sequence number ---sender reset timer when receives ACK

• No ACK within a certain time, retransmit the packet

---Error Recovery

TCP: Reliable Transport

Set the appropriate size of sliding window size adaptively

• Use for flow control : - Prevent sender from overloading receiver with data, e. g. high- performance server to slow PC - Congestion inside network, e. g. router performance, slow link speed

• How to provide flow control?

• Receiver “ advertises” it’s windows size in acknowledgments - Window size specifies how many bytes the receiver is willing to accept - Limited by congestion window

• Sender will adjusts buffer pointer as receiver’s advertisement

Sliding window/ Transmission window

• A larger window size allows more data to be transmitted with pending acknowledgment

• Need not to wait for acknowledgment every segment

Sliding/ Send window buffer

Connection Concept

• Before data could be transferred, a connection must be opened ---servers do passive open (listen) ---clients do active open (connect)

• When it finished, the connection is closed

• TCP has general 3 phases -- connection setup phase -- data phase -- connection close phase

• Socket is stored in „TCP control block“ TCB - with sequence number and timer values

3-way Handshake for connection establishment

Host A Host B

SYN, Seq_no = x

SYN, Seq_no = y, ACK, Ack_no = x+1

Seq_no = x+1, ACK, Ack_no = y+1

Active open

Closed

Listen

Established

Established

Seq_no = 2000, Ack_no = 1, Win = 1024, Data = 2000-3023

Seq_no = 1, Ack_no = 4048, Win = 512, Data = 1-128

Seq_no = 3024, Ack_no = 1, Win = 1024, Data = 3024-4047

Seq_no = 4048, Ack_no = 129, Win = 1024, Data = 4048-4559

t1

t2

t3

t4

Seq_no = 1, Ack_no = 2000, Win = 2048, No Data t0

TCP Window control

FIN, seq = 5086

ACK = 5087

Data, seq. = 303, ACK = 5087Deliver 150 bytes

FIN, seq. =453, ACK = 5087

ACK = 454

Host B

ACK = 453

Connection Termination

Host A

Retransmission Timer

Retransmission Timer

• With RTT, Timeout increase

RTT Measurement and Timer Backoff

TCP Flow and Congestion Control

Principle

Zero Window Probing

Silly Window Syndrome

Solutions: Silly Window Syndrome

Delayed Acknowledgment

Delayed Transmission

TCP Congestion Control Algorithm

Multiplicative Decrease

Additive Increase

Congestion Avoidance

Congestion Control Algorithms

Congestion Control Algorithms

TCP Congestion Control figure

TCP Performance Measure

Thank You