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OSI Transport Layer

Network Fundamentals – Chapter 4

Dr. C. BouSaba

OSI transport layer

 OSI model layer 4

Application

Presentation

Session

Transport

Network

Data link

Physical

Application

Transport

Internet

Network Access

TCP, UDP

Ethernet,

WAN

technologies

HTTP, FTP,

TFTP, SMTP

etc

Segment

Packet

Frame

Bits

Data

stream

 TCP/IP model Transport layer

Purpose of transport layer

 Responsible for the overall end-to-end transfer of application data.

 Enables multiple applications on the same device to send data over the network at the same time

 Provides “reliability” and error handling if required. (Checks if data has arrived and re-sends if it has not.)

Transport Layer TCP and UDP

 Preparing application data for transport over the network and processing network data for use by applications.

 Tracking the individual communication between (one or more) applications on the source and destination hosts

 Segmenting data and managing each piece

 Reassembling the segments into application data

 Identifying the different applications using an identifier (port number)

Transport Layer:

Controlling the Conversations

Supporting Reliable Communication

Transport Layer TCP and UDP

Why two transport protocols?

 Some applications need their data to be complete with no errors or gaps and they can accept a slight delay to ensure this. They use TCP.

 Some applications can accept occasional errors or gaps in the data but they cannot accept any delay. They use UDP.

Reliable

Fast

TCP and UDP headers

TCP UDP

 Sets up a connection with the receiving host before sending data.

 Checks if segments have arrived and resends if they were lost. (Reliability)

 Sorts segments in right order before reassembling the data.

 Sends at a speed to suit the receiving host. (Flow control)

 But – this takes time and resources (High overhead).

 Robust

 Connectionless. Does not contact receiving host before sending data.

 Does not check if data arrived and does not re-send. (Unreliable)

 Does not sort into the right order.

 “Best effort”.

 Low overhead.

 Used for VoIP, streaming video, DNS, TFTP (data sensitive to delays)

Port Numbers and Sockets

 Used by TCP and UDP as a form of addressing.

 Identifies the application and the conversation.

 Common application protocols have default port numbers e.g. 80 for HTTP, 110 for POP3 mail, 20/21 for FTP, 23 for Telnet

Client PC uses port 80.

Identifies HTTP as application.

Requesting a web page.

Client PC uses port 49152.

Chosen at random.

Remembers this to identify

application and conversation.

Port + IP address = socket. E.g. 192.168.2.12:80

Transport Layer Role and Services

Port Addressing: Identifying the Conversations

Port numbers

 The Internet Assigned Numbers Authority (IANA) assigns port numbers.

 Well Known Ports (0 to 1023) - Reserved for common services and applications such as HTTP, FTP, Telnet, POP3, SMTP.

 Registered Ports (1024 to 49151) - Assigned to user processes or applications. Can be dynamically selected by a client as its source port.

 Dynamic or Private or Ephemeral Ports (49152 to 65535) – Can be assigned dynamically to client applications when initiating a connection.

The IANA assigns port numbers

Port Addressing: netstat command  Sometimes it is necessary to know which active TCP connections are open

and running on a networked host.

Unexpected connections may mean there is a security problem.

 Netstat is an important network utility that can be used to verify those connections. Netstat lists:

protocol in use, local address and port number, foreign address and port number, state of the connection.

Segment and sequence

 Both TCP and UDP split application data into suitably sized pieces for transport and re-assemble them on arrival.

 TCP has sequence numbers in the segment headers. It re-assembles segments in the right order.

 UDP has no sequence numbers. It assembles datagrams in the order they arrive.

Connection oriented

 TCP sets up a connection between end hosts before sending data

 The two hosts go through a synchronization process to ensure that both hosts are ready and know the initial sequence numbers.

 This process is the Three-way handshake

 When data transfer is finished, the hosts send signals to end the session.

Three way handshake

Send SYN

seq = x

Receive SYN

seq = x

Receive SYN

ack = y

seq = x+1

Receive ACK

ack = y+1

Send ACK

ack = y+1

Send SYN

ack = y

seq = x+1

Terminating connection

TCP-Making Conversations Reliable  The key distinction between TCP and UDP is reliability.

 The reliability of TCP communication is performed using connection-oriented sessions. (3 way handshake)

Before a host using TCP sends data to another host, the Transport layer initiates a process to create a connection with the destination.

This process ensures that each host is aware of and prepared for communication.

 After a session has been established (3 way handshake), the destination sends acknowledgements to the source for the segments that it receives.

As the source receives an acknowledgement, it knows that the data has been successfully delivered and can quit tracking that data.

 If the source does not receive an acknowledgement within a predetermined amount of time, it retransmits that data to the destination.

There is also additional overhead on the individual hosts created by the necessity to keep track of which segments are awaiting acknowledgement and by the retransmission process.

Expectational acknowledgement

 TCP checks that data has been received.

 The receiving host sends an acknowledgement giving the sequence number of the byte that it expects next.

Window size

 Controls how many bytes are sent before an acknowledgement is expected.

Lost segments

 Send bytes 1 to 2999

 Receive 1 to 2999, send ACK 3000

 Send bytes 3000 to 4999

 Receive 3000 to 3999, send ACK 4000 (bytes 4000 to 4999 were lost)

 Send bytes 4000 to 5999

 Lost segments are re-sent.

 If no ACK – send them all again

Flow control

 The initial window size is agreed during the three-way handshake.

 If this is too much for the receiver and it loses data (e.g. buffer overflow) then it can decrease the window size.

 If all is well then the receiver will increase the window size.

Application and Operation of TCP Mechanisms

TCP Connection Establishment and Termination

TCP Connection Establishment: Three-way Handshake

 The three steps in TCP connection establishment are:

1. The initiating client sends a segment containing an initial sequence value, which serves as a request to the server to begin a communications session.

2. The server responds with a segment containing an acknowledgement value equal to the received sequence value plus 1, plus its own synchronizing sequence value.

The value is one greater than the sequence number because the ACK is always the next expected Byte or Octet.

This acknowledgement value enables the client to tie the response back to the original segment that it sent to the server.

3. Initiating client responds with an acknowledgement value equal to the sequence value it received plus one. This completes the process of establishing the connection.

TCP Termination  Look at the various values that the two hosts exchange.

 Within the TCP segment header, there are six 1-bit fields that contain control information used to manage the TCP processes. Those fields are:

URG - Urgent pointer field significant

ACK - Acknowledgement field significant

PSH - Push function

RST - Reset the connection

SYN - Synchronize sequence numbers

FIN - No more data from sender

 These fields are referred to as flags, because the value of one of these fields is only 1 bit and, therefore, has only two values: 1 or 0. When a bit value is set to 1, it indicates what control information is contained in the segment.

 Using a four-step process, flags are exchanged to terminate a TCP connection.

TCP Segment Reassembly

TCP Acknowledgement with Windowing

UDP: Low Overhead vs. Reliability

UDP – Low Overhead vs. Reliability  UDP is a simple protocol that provides the basic Transport layer functions.

It is not connection-oriented

It does not provide retransmission, sequencing, and flow control.

 This does not mean that applications that use UDP are always unreliable.

It simply means that these functions are not provided by the Transport layer protocol and must be implemented elsewhere if required.

 key Application layer protocols that use UDP include:

Domain Name System (DNS)

Simple Network Management Protocol (SNMP)

Dynamic Host Configuration Protocol (DHCP)

Routing Information Protocol (RIP)

Trivial File Transfer Protocol (TFTP)

Online games

 If these applications used TCP, they may experience large delays while TCP detects data loss and retransmits data.

These delays would be more detrimental to the application than small data losses.

UDP Protocol

UDP Datagram Reassembly

UDP Client Processes

Comparison of TCP and UDP

 Both TCP and UDP use port numbers

 Both split up application data if necessary

 TCP sets up a connection

 TCP uses acknowledgements and re-sends

 TCP uses flow control

 TCP can re-assemble segments in the right order if they arrive out of sequence

 UDP has less overhead so is faster

Comparison of TCP and UDP

TCP and UDP  The 2 most common Transport layer protocols:

User Datagram Protocol (UDP)

UDP is a connectionless protocol, described in RFC 768.

It provides low overhead data delivery.

The pieces of communication in UDP are called datagrams.

each UDP segment only has 8 bytes of overhead.

These datagrams are sent as "best effort".

Applications that use UDP include: Domain Name System (DNS), Video Streaming, Voice over IP (VoIP)

Transmission Control Protocol (TCP)

TCP is a connection-oriented protocol, described in RFC 793.

TCP incurs additional overhead to gain functions.

Additional functions specified by TCP are: the same order delivery, reliable delivery, and flow control.

Each TCP segment has 20 bytes of overhead in the header encapsulating the Application layer data,

Applications that use TCP are: Web Browsers, E-mail, File Transfers

Summary