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Computer Communication &Networks

Lecture 23 & 24

Transport Layer: UDP and TCP

http://web.uettaxila.edu.pk/CMS/coeCCNbsSp09/index.asp

Waleed Ejaz

waleed.ejaz@uettaxila.edu.pk

http://web.uettaxila.edu.pk/CMS/coeCCNbsSp09/index.aspmailto:waleed.ejaz@uettaxila.edu.pkmailto:waleed.ejaz@uettaxila.edu.pkhttp://web.uettaxila.edu.pk/CMS/coeCCNbsSp09/index.asp

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Transport Layer

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Transport Layer Topics to Cover

UDP, TCP

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Process-to-Process Delivery

The transport layer is responsible for

process-to-process deliverythe delivery of

a packet, part of a message, from one

process to another. Two processescommunicate in a client/server relationship,

as we will see later.

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Types of Data Deliveries

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Port Numbers

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IP addresses versus port numbers

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Internet Assigned Number Authority (IANA) Ranges

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Socket address

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Multiplexing and demultiplexing

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Error control

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Position of UDP, TCP, and SCTP in TCP/IP suite

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User Datagram Protocol (UDP)

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User Datagram Protocol (UDP)

The User Datagram Protocol (UDP) is called

a connectionless, unreliable transport

protocol. It does not add anything to the

services of IP except to provide process-to-process communication instead of host-to-

host communication.

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Well-known ports used with UDP

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User Datagram Format

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UDP Operation

Connectionless Services

Flow and Error Control

No flow control

No Error Control except Checksum

Encapsulation & Decapsulation

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Queues in UDP

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TCP

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TCP

TCP is a connection-oriented protocol; it

creates a virtual connection between two

TCPs to send data. In addition, TCP uses

flow and error control mechanisms at thetransport level.

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Well-known ports used by TCP

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Stream delivery

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Sending and receiving buffers

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TCP segments

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Example The bytes of data being transferred in each connection are

numbered by TCP. The numbering starts with a randomlygenerated number.

The value in the sequence number field of a segment defines

the number of the first data byte contained in that segment. The value of the acknowledgment field in a segment defines

the number of the next byte a party expects to receive. Theacknowledgment number is cumulative.

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TCP segment format20 to 60 Byte header

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Connection establishment using three-way

handshaking A SYN segment

cannot carry data,but it consumes onesequence number.

A SYN + ACK

segment cannotcarry data, but doesconsume onesequence number.

An ACK segment, if

carrying no data,consumes nosequence number.

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Data transfer

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Connection termination using three-way

handshaking

The FIN segmentconsumes onesequencenumber if it does

not carry data. The FIN + ACK

segmentconsumes

one sequencenumber if itdoes not carrydata.

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Half Close

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Sliding Window Protocol A sliding window is used to make

transmission more efficient as well as tocontrol the flow of data so that the destination

does not become overwhelmed with data.

TCP sliding windows are byte-oriented.

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F igure on next sl ide shows an unrealistic example of a

sl iding window. The sender has sent bytes up to 202. We

assume that cwnd is 20 (in reali ty this value is thousands

of bytes). The receiver has sent an acknowledgment

number of 200 with an rwnd of 9 bytes (in reali ty thisvalue is thousands of bytes). The size of the sender

window is the minimum of rwnd and cwnd, or 9 bytes.

Bytes 200 to 202 are sent, but not acknowledged. Bytes

203 to 208 can be sent without worrying aboutacknowledgment. Bytes 209 and above cannot be sent.

Example

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Example

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ACK segments do not consumesequence numbers and are not

acknowledged.

Note

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In modern implementations, aretransmission occurs if the

retransmission timer expires or three

duplicate ACK segments have arrived.

Note

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No retransmission timer is set for anACK segment.

Note

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Data may arrive out of order and betemporarily stored by the receiving TCP,

but TCP guarantees that no out-of-order

segment is delivered to the process.

Note

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Normal operation

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Lost Segment

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The receiver TCP delivers only ordered

data to the process.

Note

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Fast Retransmission