TCP: Transmission Control Protocol, Slides of Computer Science

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Transmission Control Protocol (TCP),

The Need for TCP

 Network Layer (IP) provides a best-effort service

 Need to build a reliable layer on top of IP

 Possible problems sending messages from one host

to another:

 May be lost  May be reordered  May be delivered multiple times  May be delivered after a long delay  May be broken into smaller messages  May be corrupted

Can this be done hop-by-hop

instead of end-to-end?

 Think of this like the postal service

 One major problem: Nodes would not know what

happened two or more hops ahead of them  Routing tables incorrect, packets dropped, machine crashed, link failed…  Only ends can be sure of receipt/non-receipt of a message

 For others: read “End-to-end arguments for

System Design”… Classic, must-read paper, could be on exam

TCP Usage Model

 Connection setup

 􀁻􀁻 3-way handshake

 Data transport

 Sender writes data

 TCP

 Breaks data into segments

• Sends each segment over IP
• Retransmits, reorders and removes duplicates as necessary
• Receiver reads some data

 Teardown

 4 step exchange

TCP is connection-oriented

Connection Setup

3-way handshake

(Active)

Client

(Passive)

Server

Syn (ISNA)

Syn (ISNB) + Ack (ISNA+1)

Ack (ISNB+1)

Connection Close/Teardown

2 x 2-way handshake

(Active)

Client

(Passive)

Server

Fin (SNA)

(Data +) Ack (SN (^) A+1)

Fin (SNB)

Ack (SNB+1)

TCP supports a “stream of

bytes” service

Host A

Host B

The TCP Segment Format

IP Hdr

IP Data TCP Data TCP Hdr

Src port Dst port

Sequence #

Ack Sequence # HLEN 4 RSVD 6 URG ACK PSH RST SYN FIN

Flags Window Size

Checksum Urg Pointer

(TCP Options)

0 15 31

TCP Data

TCP Segment Header

 16-bit source and destination ports

 32-bit send and ACK sequence numbers

 4-bit header length in 4-byte words

 􀁻􀁻 Minimum value of 5  􀁻􀁻 Offset to first data byte

 6 1-bit flags

 􀁻􀁻 URG: Segment contains urgent data  􀁻􀁻 ACK: ACK sequence number is valid  􀁻􀁻 PSH: Do not delay delivery of data  􀁻􀁻 RST: Reset connection (reject or abn. termination)  􀁻􀁻 SYN: Synchronize segment for setup  􀁻􀁻 FIN: Final segment for teardown

TCP Options

 Negotiate maximum segment size (MSS)

 Each host suggests a value  Minimum of two values is chosen  Prevents IP fragmentation over first and last hops

 Packet timestamp

 Allows RTT calculation for retransmitted packets  Extends sequence number space for identification of stray packets

 Negotiate advertised window granularity

 Allows windows to be scaled to larger sizes  Good for routes with large bandwidth-delay products

Sequence Numbers

Host A

Host B

TCP Data

TCP Data

TCP HDR

TCP HDR

ISN (initial sequence number)

Sequence number = 1st byte Ack sequence number = next expected byte

TCP State Descriptions

CLOSED Disconnected

LISTEN Waiting for incoming connection

SYN_RCVD Connection request received

SYN_SENT Connection request sent

ESTABLISHED Connection ready for data transport

CLOSE_WAIT Connection closed by peer

LAST_ACK Connection closed by peer, closed locally, await ACK

FIN_WAIT_1 Connection closed locally

FIN_WAIT_2 Connection closed locally and ACK’d

CLOSING Connection closed by both sides simultaneously

TIME_WAIT Wait for network to discard related packets

TCP State Transition Diagram

TCP Sliding Window

 How much data can a TCP sender have

outstanding in the network?

 How much data should TCP retransmit

when an error occurs? Just selectively

repeat the missing data?

 How does the TCP sender avoid over-

running the receiver’s buffers?

TCP Sliding Window

Window Size

Outstanding Un-ack’d data

Data OK to send

Data not OK to send yet

Data ACK’d

 Window is meaningful to the sender.

 Current window size is “advertised” by receiver

(usually 4k – 8k Bytes when connection set-up).

 TCP’s Retransmission policy is “Go Back N”.