TSIN02 - Internetworking

© 2004 Image Coding Group, Linköpings Universitet

Lecture 4: Transport Layer

Literature:� Forouzan: ch 11-12

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� Transport layer responsibilities� UDP� TCP

TSIN02 - Internetworking

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Transport layer in OSI model

Figure from Forouzan 4

� The physical layer� transmits signals representing individual bits on a

channel� The link layer

� provides reliable link communication throughframing, physical addressing, flow- error- and access control

� The network layer� provides end to end delivery of individual packets

through logical addressing and routing support

5Figure from Forouzan 6

The transport layer is responsible for end to end delivery of messages. This is achieved through� Service point addressing� Segmentation and reassembly � Connection control� Flow control� Error control

7Figure from Forouzan 8

Port numbers are used as a process identifier.

They are needed since computers can run multiple programs at the same time.� IP addresses identify computers� Port numbers identify processes

An (IP address, port number) pair is called a socket.

Messages must define source and destination sockets

TSIN02 - Internetworking

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Addresses and Ports

Figure from Forouzan

10Figure from Forouzan

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The IANA has divided the port numbers into three ranges:� Well known ports (0-1023)

Assigned and controlled by IANA

� Registered ports (1024-49151)Ports in this range can be registered with IANA to prevent duplication.

� Dynamic ports (49152-65535)The ephemeral ports can be used by any process.

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Port number Application20 FTP23 Telnet25 SNMP80 HTTP

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There are two transport layer protocols in the TCP/IP stack:� UDP - User Datagram Protocol

� Connectionless unreliable service

� TCP - Transmission Control Protocol� Connection-oriented reliable stream service

IPICMPIGMP

ARP RARP

TCP UDP

Underlying networks 14

UDP is a connectionless, unreliable transport protocol

UDP provides � process to process communication� limited error checking

UDP does not provide� acknowledgement for recieved packets� segmentation and reassembly� flow control

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Why use UDP?

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UDP is suitable for� processes that provide internal flow and error

control mechanisms, eg TFTP� multicasting and broadcasting� management processes, eg SNMP� some route updating protocols, eg RIP� processes that only send short messages

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UDP packets (datagrams):� 8 byte header� max 65507 bytes of data

(=IP length – IP header – UDP header)

Figure from Forouzan 18

The UDP checksum includes� pseudoheader� UDP header� data from the

application layer� pad byte (if needed)

8 bits 8 bits

.

.

.

8 bits 8 bits

All 0s Protocol (17) UDP total length

Source IP address

Destination IP address

Source port address

Destination port address

UDP total length UDP Checksum

Data

19Figure from Forouzan 20

� Messages are encapsulated in UDP datagrams� Usually each port is associated with one or two

queues� UDP handles multiplexing and demultiplexing

of messages.

21Figure from Forouzan 22Figure from Forouzan

23Figure from Forouzan 24Figure from Forouzan

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TCP is a connection-oriented transport protocol

TCP provides� full duplex connections� reliable stream service

� flow control� error control

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Sending and receiving buffers are used for storage. This makes it possible to handle differences in speed between the sending and receiving processes.

Buffers are also used in flow and error control mechanisms.

29Figure from Forouzan 30Figure from Forouzan

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TCP provides reliable service by� breaking application data into suitably sized blocks

called segments� using an acknowledgement mechanism � retransmission when errors occur� providing flow control� checksum control of header and data

32Figure from Forouzan

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� All bytes transmitted in a connection are numbered, starting from a random position

� 32 bits are used for numbering� The value in the header is the number of the

first byte of data contained in the segment.

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� This number is used to announce the sequence number of the next expected segment and thus acknowledging that all prior segments (with lower sequence numbers) have been received.

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6 flags can be used to determine the purpose of the segment (more than one can be set)� URG - urgent pointer valid (set when sender wants the receiver to

read a piece of data urgently and possibly out of order)

� ACK - acknowledgment number valid

� PSH - push data, receiver should immediately pass the data to the application (buffers should be emptied!)

� RST - reset the connection

� SYN - synchronize sequence numbers to initiate connection

� FIN - terminate the connection

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A three-way handshake is used to establish a connection

�Negotiation on initial sequence numbers

�Initial segment number (ISN) chosen so that each active connection has its own ISN

�Normally initiation is made by the client

Figure from Forouzan

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4 segments needed to close a connection

Figure from Forouzan 38

� MSS is the largest block of data TCP will send to the other side.

� MSS can be announced in the options field during connection establishment.

� Default MSS is 536 � The larger the better (until fragmentation

occurs)

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�Up to 40 bytes of optional informationcan be included in the TCP header

�Used to convey additional information or to align other options

Figure from Forouzan 40

� How much can a source send without receiving an ACK?

� The sliding window protocol is used in TCP� offered window� usable window

A sliding window is used to make transmission A sliding window is used to make transmission more efficient as well as to control the flow of data more efficient as well as to control the flow of data so that the destination does not become so that the destination does not become overwhelmed with data. TCP’s sliding windows are overwhelmed with data. TCP’s sliding windows are byte oriented.byte oriented.

41Figure from Forouzan 42

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� Sender sends 1-byte segments because the application produces data slowly.� Solution - Nagle's algorithm

� After sending one segment, wait until either an ack is received or a full window can be transmitted.

� 1-byte windows are announced by the receiver because the application consumes data slowly.� Solution - Clark's

� Send acks as soon as possible, but announce zero window size until a maximum size segment fits the buffer

� Solution - delayed ack� Wait with acknowledgments until there is room in the

buffer 44

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� The source does not have to send a full window's worth of data.

� The size of the window can be increased or decreased by the destination

� The destination can send an acknowledgement at any time.

46Figure from Forouzan

47Figure from Forouzan 48Figure from Forouzan

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� TCP assumes that the cause of a lost segment is due to congestion in the network.

� If the cause of the lost segment is congestion, retransmission of the segment will not help, it will make things worse.

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� Handles bottlenecks in the network� Without congestion control the network will

collaps!� Solution:

� Congestion window (CWND)� slow start� Additive increase of CWND� Multiplicative decrease of CWND� Fast retransmit and Fast recovery

51Figure from Forouzan 52

� Retransmission timer� how long should sender wait for an ACK ?

� Persistence timer� how long should sender wait if window size = 0?

� Keepalive timer� Prevents connections to live forever

� Time-Waited timer� Each connection held in limbo before actually

closed

53Figure from Forouzan

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TCP packet

Figure from Forouzan

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� Transport layer basics� UDP - a fairly simple connectionless protocol� TCP - a very complex protocol

� Reliability� Connection management� Flow control� Congestion avoidance� Timers