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COMPUTER NETWORKS

MODULE 4

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Transport Layer

Transport layer provides a process-to-process communication between two application layers, one at local host and other at the remote host

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Only the two end systems(Alice and Bob) use the services of the transport layer;
All intermediate routers use only the first three layers. �

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Transport Layer Services

Process to process communication
Addressing: Port Numbers
ICANN Ranges(Internet corporation for assigned numbers and names)
Encapsulation and Decapsulation
Multiplexing and Demultiplexing
Flow Control
Error Control
Congestion Control

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Process to Process Communication

A process is an application-layer entity (running program) that uses the services of the transport layer.

A transport-layer protocol is responsible for delivery of the message to the appropriate process. � �

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Addressing: Port Numbers

A process on the local host, called a client, needs services from a process usually on the remote host, called a server.
To define the processes, we need second identifiers, called port numbers. In the TCP/IP protocol suite, the port numbers are integers between 0 and 65,535 (16 bits)
The client program defines itself with a port number, called the ephemeral port number. �

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If the computer at the server site runs a server process and assigns a random number as the port number, the process at the client site that wants to access that server and use its services will not know the port number.

Of course, one solution would be to send a special packet and request the port number of a specific server, but this creates more overhead. TCP/IP has decided to use universal port numbers for servers; these are called well-known port numbers. ��

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Port Numbers

IP addresses versus port numbers

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ICANN Ranges

Well-known ports. The ports ranging from 0 to 1023 are assigned and controlled by ICANN.

Registered Ports. The ports ranging from 1024 to 49,151 are not assigned or controlled by ICANN.

Dynamic Ports. The ports ranging from 49,152 to 65,535 are neither controlled nor registered. ��

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Socket Addresses

The combination of an IP address and a port number is called a socket address. �

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Encapsulation and Decapsulation

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Encapsulation happens at the sender site. When a process has a message to send, it passes the message to the transport layer along with a pair of socket addresses and some other pieces of information, which depend on the transport-layer protocol. �

Decapsulation happens at the receiver site. When the message arrives at the destination transport layer, the header is dropped and the transport layer delivers the message to the process running at the application layer. �

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Whenever an entity accepts items from more than one source, this is referred to as multiplexing (many to one); whenever an entity delivers items to more than one source, this is referred to as demultiplexing (one to many). �

Multiplexing and Demultiplexing

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Flow Control

Whenever an entity produces items and another entity consumes them, there should be a balance between production and consumption rates. If the items are produced faster than they can be consumed, the consumer can be overwhelmed and may need to discard some items. �

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Pushing or Pulling

If the sender delivers items whenever they are produced without a prior request from the consumer the delivery is referred to as pushing. If the producer delivers the items after the consumer has requested them, the delivery is referred to as pulling. �

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Flow control at transport layer

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Buffers

Flow control can be implemented by maintaining 2 buffers.

One at sending transport layer and the other at the receiving transport layer.

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Error Control

Error control at the transport layer is responsible for

1. Detecting and discarding corrupted packets.

2. Keeping track of lost and discarded packets and resending them.

3. Recognizing duplicate packets and discarding them.

4. Buffering out-of-order packets until the missing packets arrive. �

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Sliding Window

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Congestion Control

Congestion in a network may occur if the load on the network—the number of packets sent to the network—is greater than the capacity of the network—the number of packets a network can handle. Congestion control refers to the mechanisms and techniques that control the congestion and keep the load below the capacity. �

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Connectionless protocols

Connectionless and Connection-Oriented Protocols

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Connection Oriented Protocols

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FSM of Connectionless and Connection Oriented Service

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Transport Layer Protocols

Simple Protocol
Stop and Wait Protocol
Go-Back N Protocol
Selective Repeat Protocol

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Go-Back N Protocol

To improve the efficiency of transmission (to fill the pipe), multiple packets must be in transition while the sender is waiting for acknowledgment.
The key to Go-back-N is that we can send several packets before receiving acknowledgments, but the receiver can only buffer one packet. ��

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Sequence Numbers

The sequence numbers are modulo 2^m, where m is the size of the sequence number field in bits.

Acknowledgment Numbers

An acknowledgment number in this protocol is cumulative and defines the sequence number of the next packet expected.
In the Go-Back-N protocol, the acknowledgment number is cumulative and defines the sequence number of the next packet expected to arrive.�

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Send Window

The send window is an imaginary box covering the sequence numbers of the data packets that can be in transit or can be sent.

In each window position, some of these sequence numbers define the packets that have been sent; others define those that can be sent. The maximum size of the window is 2^m − 1�

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Sliding the send window

The figure shows how a send window can slide one or more slots to the right when an acknowledgment arrives from the other end. In the figure, an acknowledgment with ackNo = 6 has arrived. This means that the receiver is waiting for packets with sequence number 6. �

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Receive Window

In Go-Back-N, the size of the receive window is always 1. The receiver is always looking for the arrival of a specific packet. Any packet arriving out of order is discarded and needs to be resent. �

The receive window is an abstract concept defining an imaginary box of size 1 with a single variable Rn. The window slides when a correct packet has arrived; sliding occurs one slot at a time.

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Timers

There can be a timer for each packet that is sent, in our protocol we use only

One. The reason is that the timer for the first outstanding packet always expires first. Resend all outstanding packets when this timer expires.

Resending packets

When the timer expires, the sender resends all outstanding packets.

For example, suppose the sender has already sent packet 6 (Sn = 7), but the only timer expires. If Sf = 3, this means that packets 3, 4, 5, and 6 have not been acknowledged; the sender goes back and resends packets 3, 4, 5, and 6. That is why the protocol is called Go-Back-N. On a time-out, the machine goes back N locations and resends all packets.

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FSM of Go-Back N

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Send Window Size

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Flow Diagram of Go-Back-N

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Selective Repeat Protocol

Each time a single packet is lost or corrupted, the sender resends all outstanding packets, even though some of these packets may have been received safe and sound but out of order.

If the network layer is losing many packets because of congestion in the network, the resending of all of these outstanding packets makes the congestion worse, and eventually more packets are lost.

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The Selective-Repeat protocol allows as many packets as the size of

the receive window to arrive out of order and be kept until there is a set of consecutive packets to be delivered to the application layer.

Because the sizes of the send window and receive window are the same, all the packets in the send packet can arrive out of order and be stored until they can be delivered.

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Selective Repeat Protocol

Selective Repeat Protocol resend only selective packets

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Send window for Selective-Repeat Protocol

The send window maximum size can be 2m−1. For example, if m = 4, the sequence numbers go from 0 to 15, but the maximum size of the window is just 8.

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Receive Window for Selective Repeat Protocol

The size of the receive window is the same as the size of the send window (maximum 2m−1). The Selective-Repeat protocol allows as many packets as the size of the receive window to arrive out of order and be kept until there is a set of consecutive packets to be delivered to the application layer.

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Timer

Selective-Repeat uses one timer for each outstanding packet. When a timer expires, only the corresponding packet is resent.

However, most transport-layer protocols that implement SR use only a single timer. For this reason, we use only one timer.�

Acknowledgments

In SR, an ackNo defines the sequence number of a single packet that is received safe and sound; there is no feedback for any other. �

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FSM of Selective Repeat

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Flow Diagram of SR

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Piggybacking

piggybacking is used to improve the efficiency of the bidirectional protocols. When a packet is carrying data from A to B, it can also carry acknowledgment feedback about arrived packets from B; when a packet is carrying data from B to A, it can also carry acknowledgment feedback about the arrived packets from A. �

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Introduction to Transport Layer Protocols

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Services of Transport Layer Protocols

UDP

UDP is an unreliable connectionless transport-layer protocol used for its simplicity and efficiency in applications where error control can be provided by the application-layer process.

TCP

TCP is a reliable connection-oriented protocol that can be used in any application where reliability is important.

SCTP

SCTP is a new transport-layer protocol that combines the features of UDP and TCP �

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Port Numbers

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User Datagram Protocol

The User Datagram Protocol (UDP) is a connectionless, unreliable transport protocol. It does not add anything to the services of IP except for providing process-to-process communication instead of host-to-host communication. �

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User Datagram

UDP packets, called user datagrams, have a fixed-size header of 8 bytes made of four fields, each of 2 bytes (16 bits). �

UDP Packet Format

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UDP Services

Process – process Communication
Connectionless Service
Flow Control
Error Control
Checksum
Congestion control
Encapsulation and Decapsulation
Queuing
Multiplexing and Demultiplexing

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Process- to Process Communication

UDP provides process-to-process communication using socket addresses, a combination of IP addresses and port numbers. �

Connectionless Services

User datagram sent by UDP is an independent datagram.

There is no relationship between the different user datagrams even if they are coming from the same source process and going to the same destination program. The user datagrams are not numbered. �

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Flow Control

UDP is a very simple protocol. There is no flow control, and hence no window mechanism. �

Error Control

There is no error control mechanism in UDP except for the checksum. This means that the sender does not know if a message has been lost or duplicated. �

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Checksum

UDP checksum calculation includes three sections:

a pseudoheader,

the UDP header,

and the data coming from the application layer.

The pseudoheader is the part of the header of the IP packet in which the user datagram is to be encapsulated with some fields filled with 0s �

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Congestion Control

Since UDP is a connectionless protocol, it does not provide congestion control. UDP assumes that the packets sent are small and sporadic and cannot create congestion in the network.�

Encapsulation and Decapsulation

To send a message from one process to another, the UDP protocol encapsulates and decapsulates messages. �

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Queuing

In UDP, queues are associated with ports �

Multiplexing and Demultiplexing

In a host running a TCP/IP protocol suite, there is only one UDP but possibly several processes that may want to use the services of UDP. To handle this situation, UDP multiplexes and demultiplexes. �

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UDP Applications

UDP Features

Connectionless Service

UDP is a connectionless protocol.

Each UDP packet is independent from other packets sent by the same application program.

It is an advantage if, for example, a client application needs to send a short request to a server and to receive a short response.�

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Lack of Error Control

UDP does not provide error control; it provides an unreliable service. Most applications expect reliable service from a transport-layer protocol �

Lack of Congestion Control

UDP does not provide congestion control. However, UDP does not create additional traffic in an error-prone network. �

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Typical Applications

❑ UDP is suitable for a process that requires simple request-response communication

with little concern for flow and error control.

❑ UDP is suitable for a process with internal flow- and error-control mechanisms.

For example, the Trivial File Transfer Protocol (TFTP) process includes flow and

error control. It can easily use UDP.

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❑ UDP is a suitable transport protocol for multicasting. Multicasting capability is

embedded in the UDP software but not in the TCP software.

❑ UDP is used for management processes such as SNMP

❑ UDP is used for some route updating protocols such as Routing Information Protocol (RIP)

❑ UDP is normally used for interactive real-time applications that cannot tolerate

uneven delay between sections of a received message�

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Transmission Control Protocol (TCP)

Transmission Control Protocol (TCP) is a connection-oriented, reliable protocol. TCP explicitly defines connection establishment, data transfer, and connection teardown phases to provide a connection-oriented service. �

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TCP Services

Process – to process communication
Stream Delivery Service
Full-Duplex Communication
Multiplexing and Demultiplexing
Connection Oriented Service
Reliable Service

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Process-to-Process Communication

TCP provides process-to-process communication using port numbers. �

Stream Delivery Service

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TCP, unlike UDP, is a stream-oriented protocol.
TCP, on the other hand, allows the sending process to deliver data as a stream of bytes and allows the receiving process to obtain data as a stream of bytes.
TCP creates an environment in which the two processes seem to be connected by an imaginary “tube” that carries their bytes across the Internet. � �

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Sending & Receiving Buffers

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TCP needs buffers for storage. There are two buffers, the sending buffer and the receiving buffer, one for each direction.
One way to implement a buffer is to use a circular array of 1-byte locations
Two buffers of 20 bytes each; normally the buffers are hundreds or thousands of bytes, depending on the implementation
The figure shows the movement of the data in one direction.
At the sender, the buffer has three types of chambers.
The white section contains empty chambers that can be filled by the sending process (producer).
The colored area holds bytes that have been sent but not yet acknowledged.
The TCP sender keeps these bytes in the buffer until it receives an acknowledgment. ��

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The operation of the buffer at the receiver is simpler.
The circular buffer is divided into two areas (shown as white and colored).
The white area contains empty chambers to be filled by bytes received from the network.
The colored sections contain received bytes that can be read by the receiving process.
When a byte is read by the receiving process, the chamber is recycled and added to the pool of empty chambers. ��

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Segments

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The network layer, as a service provider for TCP, needs to send data in packets, not as a stream of bytes.
At the transport layer, TCP groups a number of bytes together into a packet called a segment.
TCP adds a header to each segment (for control purposes) and delivers the segment to the network layer for transmission.
The segments are encapsulated in an IP datagram and transmitted. This entire operation is transparent to the receiving process. �

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Full-Duplex Communication

TCP offers full-duplex service, where data can flow in both directions at the same time.
Each TCP endpoint then has its own sending and receiving buffer, and segments move in both directions.

Multiplexing and Demultiplexing

Like UDP, TCP performs multiplexing at the sender and demultiplexing at the receiver.

However, since TCP is a connection-oriented protocol, a connection needs to be established for each pair of processes �

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Connection-Oriented Service

TCP, unlike UDP, is a connection-oriented protocol. When a process at site A wants to send to and receive data from another process at site B, the following three phases occur:

1. The two TCP’s establish a logical connection between them.

2. Data are exchanged in both directions.

3. The connection is terminated. �

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Reliable Service

TCP is a reliable transport protocol. It uses an acknowledgment mechanism to check the safe and sound arrival of data. �

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TCP Features

Numbering System

- Byte Number

Sequence Number
Acknowledgement Number

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Numbering System

Although the TCP software keeps track of the segments being transmitted or received, there is no field for a segment number value in the segment header. Instead, there are two fields, called the sequence number and the acknowledgment number. �

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Byte Number

TCP numbers all data bytes (octets) that are transmitted in a connection.
Numbering is independent in each direction.
When TCP receives bytes of data from a process, TCP stores them in the sending buffer and numbers them.
The numbering does not necessarily start from 0.
Instead, TCP chooses an arbitrary number between 0 and 232 − 1 for the number of the first byte �

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Sequence Number

After the bytes have been numbered, TCP assigns a sequence number to each segment that is being sent. The sequence number, in each direction, is defined as follows:

1. The sequence number of the first segment is the ISN (initial sequence number),

which is a random number.

2. The sequence number of any other segment is the sequence number of the previous

segment plus the number of bytes (real or imaginary) carried by the previous segment. �

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Acknowledgment Number

Communication in TCP is full duplex; when a connection is established, both parties can send and receive data at the same time.
Each party numbers the bytes, usually with a different starting byte number.
The sequence number in each direction shows the number of the first byte carried by the segment.
Each party also uses an acknowledgment number to confirm the bytes it has received.
The term cumulative here means that if a party uses 5643 as an acknowledgment number, it has received all bytes from the beginning up to 5642. � �

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Segment- TCP Segment Format

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Source port address. This is a 16-bit field that defines the port number of the application program in the host that is sending the segment.

❑ Destination port address. This is a 16-bit field that defines the port number of the application program in the host that is receiving the segment.

Sequence number. This 32-bit field defines the number assigned to the first byte of data contained in this segment.

Acknowledgment number. This 32-bit field defines the byte number that the receiver of the segment is expecting to receive from the other party. If the receiver of the segment has successfully received byte number x from the other party, it returns x + 1 as the acknowledgment number. ��

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Header length. This 4-bit field indicates the number of 4-byte words in the TCP header. The length of the header can be between 20 and 60 bytes.

Control. This field defines 6 different control bits or flags, ��

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Window size. This field defines the window size of the sending TCP in bytes. The length of this field is 16 bits, which means that the maximum size of the window is 65,535 bytes �Checksum. This 16-bit field contains the checksum. The calculation of the checksum for TCP follows the same procedure as the one described for UDP. �

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Urgent pointer. This 16-bit field, which is valid only if the urgent flag is set, is used when the segment contains urgent data. It defines a value that must be added to the sequence number to obtain the number of the last urgent byte in the data section of the segment.

Options. There can be up to 40 bytes of optional information in the TCP header. � �Encapsulation

A TCP segment encapsulates the data received from the application layer. The TCP segment is encapsulated in an IP datagram, which in turn is encapsulated in a frame at the data-link layer. �

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