Transport layer TCP and UDP.ppt

PROCESS-TO-PROCESS DELIVERY
The transport layer is responsible for process-to-
process delivery—the delivery of a packet, part of a
message, from one process to another. Two processes
communicate in a client/server relationship, as we will
see later.

The transport layer is responsible for
process-to-process delivery.
Note

Figure 23.1 Types of data deliveries

 Client, server
 1. Local host
 2. Local process
 3. Remote host
 4. Remote process

 lANA Ranges
 The lANA (Internet Assigned Number Authority) has divided the port numbers into
 three ranges: well known, registered, and dynamic (or private)
 o Well-known ports.
 The ports ranging from 0 to 1023 are assigned and controlled
 by lANA. These are the well-known ports.
 Registered ports. The ports ranging from 1024 to 49,151 are not assigned or
controlled
 by lANA.
 Dynamic ports. The ports ranging from 49,152 to 65,535 are neither controlled
 nor registered. They can be used by any process. These are the ephemeral ports

Figure 23.3 IP addresses versus port numbers

Figure 23.6 Multiplexing and demultiplexing

Connectionless Versus
Connection-Oriented Service
Connectionless Service
 In a connectionless service, the packets are sent from one party to
another with no need for connection establishment or connection
release. The packets are not numbered; they may be delayed or lost or
may arrive out of sequence. There is no acknowledgment either. We
will see shortly that one of the transport layer protocols in the Internet
model, UDP, is connectionless.
Connection-Oriented Service
 In a connection-oriented service, a connection is first established
between the sender and the receiver. Data are transferred. At the end,
the connection is released. TCP and SCTP are connection-oriented
protocols.

Reliable Versus Unreliable
 The transport layer service can be reliable or unreliable. If the
application layer program needs reliability, we use a reliable transport
layer protocol by implementing flow and error control at the transport
layer. This means a slower and more complex service.
 On the other hand, if the application program does not need reliability
because it uses its own flow and error control mechanism or it needs
fast service or the nature of the service does not demand flow and error
control (real-time applications), then an unreliable protocol can be
used.
 UDP is connectionless and unreliable; TCP and SCTP (Stream Control
Transmission Protocol) are connection oriented and reliable.
 These three can respond to the demands of the application layer
programs.

Figure 23.8 Position of UDP, TCP, and SCTP in TCP/IP suite

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.

Figure 23.9 User datagram format

UDP length
= IP length – IP header’s length
Note

Figure 23.10 Pseudoheader for checksum calculation

Figure 23.11 shows the checksum calculation for a very
small user datagram with only 7 bytes of data. Because
the number of bytes of data is odd, padding is added for
checksum calculation. The pseudoheader as well as the
padding will be dropped when the user datagram is
delivered to IP.
Example 23.2

Figure 23.11 Checksum calculation of a simple UDP user datagram

UDP Operation
 Connectionless service- no relation
between datagram, not numbered
 No Flow and error control- no flow control
so no window mechanics.
 No error control except checksum (silently
discard packet)
 Encapsulation and decapsulation – IP
Datagrams

Queuing
 Incoming and outgoing queue
 It will obtain only one port number
 Port unreachable icmp message (if queue
is not created)

Uses of UDP
 Suitable for process that require simple request response
communication with little concern for flow and error control.
 Suitable for multicasting
 Used for management process such as SNMP
 Used for routing updating protocol : RIP

23-3 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 the transport level.
TCP Services
TCP Features
Segment
A TCP Connection
Flow Control
Error Control
Topics discussed in this section:

Figure 23.14 Sending and receiving buffers

TCP
 Connection oriented phase-
 Reliable
 Features
 Numbering system
 use byte number – sequence number, ack number
 Flow control
 Error control
 Congestion control

The bytes of data being transferred in
each connection are numbered by TCP.
The numbering starts with a randomly
generated number.
Note

The following shows the sequence number for each
segment:
Example 23.3

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

The value of the acknowledgment field
in a segment defines
the number of the next byte a party
expects to receive.
The acknowledgment number is
cumulative.
Note

Figure 23.16 TCP segment format

Table 23.3 Description of flags in the control field

3-Way Handshake Connection Establishment Process
It will support communication between a web browser on the client and
server sides whenever a user navigates the Internet.

Synchronization Sequence Number (SYN) − The client sends
the SYN to the server
• When the client wants to connect to the server, then it sends
the message to the server by setting the SYN flag as 1.
• The message carries some additional information like the
sequence number (32-bit random number).
• The ACK is set to 0. The maximum segment size and the window
size are also set. For example, if the window size is 1000 bits and
the maximum segment size is 100 bits, then a maximum of 10
data segments can be transmitted in the connection by dividing
(1000/100=10).

Synchronization and Acknowledgement (SYN-ACK) to the client
• The server acknowledges the client request by setting the ACK flag to 1.
• The ACK indicates the response of the segment it received and SYN
indicates with what sequence number it will start the segments.
• For example, if the client has sent the SYN with sequence number = 500,
then the server will send the ACK using acknowledgment number = 5001.
• The server will set the SYN flag to '1' and send it to the client if the server
also wants to establish the connection.
• The sequence number used for SYN will be different from the client's
SYN.
• The server also advertises its window size and maximum segment size to
the client. And, the connection is established from the client-side to the
server-side.

Acknowledgment (ACK) to the server
• The client sends the acknowledgment (ACK) to the server
after receiving the synchronization (SYN) from the server.
• After getting the (ACK) from the client, the connection is
established between the client and the server.
• Now the data can be transmitted between the client and
server sides.

3 -Way Handshake Closing Connection Process

To close a 3-way handshake connection:
• First, the client requests the server to terminate the
established connection by sending FIN.
• After receiving the client request, the server sends back the
FIN and ACK request to the client.
• After receiving the FIN + ACK from the server, the client
confirms by sending an ACK to the server.

Figure 23.18 Connection establishment using three-way handshaking

A SYN segment cannot carry data, but it
consumes one sequence number.
Note

A SYN + ACK segment cannot
carry data, but does consume one
sequence number.
Note

An ACK segment, if carrying no data,
consumes no sequence number.
Note

 Syn flooding attack
 Simultaneous open

Figure 23.20 Connection termination using three-way handshaking

The FIN segment consumes one
sequence number if it does
not carry data.
Note

The FIN + ACK segment consumes
one sequence number if it
does not carry data.
Note

Transport layer TCP and UDP.ppt

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