[ AETN1102 ] (10) Transport Layer

Transport Layer

• Responsible for logical communications between applications running on different hosts.

• The link between the application layer and the lower layers that are responsible for network transmission

Multiplexing

This is to interleave multiple conversations on the same network.

Port Numbers

This is used to identify the applications.

Sequence Numbers

This is added as a header information to reassemble the segment in the correct order.

Segmenting

This is the process of breaking down a whole into smaller, distinct parts

Transport Layer Protocols

• Specify how to transfer messages between hosts.

• Are responsible for managing reliability requirements of a conversation.

• Include the TCP and UDP transport layer protocols.

Transmission Control Protocol

This provides reliability and flow control using these basic operations:

• Must first establish a connection between the sender and the receiver order to maintain the state of a conversation and track the information

• Number and track data segments transmitted to a specific host from a specific application

• Sequence data that might arrive in wrong order

• Acknowledge received data

• Retransmit any unacknowledged data after a certain amount of time

• Send data at an efficient rate

Connection-Oriented Protocol

TCP must first establish a connection between the sender and the receiver order to maintain the state of a conversation and track the information.

User Datagram Protocol

A lightweight transport protocol – offers the same data segmentation and reassembly as TCP, but without TCP reliability and flow control

Best Effort Protocol

• Does not provide reliability and flow control.

• Delivers datagrams between the applications, with very little overhead and no data checking

Connectionless Protocol

UDP does not require an established connection.

Stateless Protocol

UDP does not track information sent or received.

TCP

This is used for applications that require that all data arrives in its proper sequence

UDP

This is used for:

• Request-and-reply applications where the data is minimal, and retransmission can be done quickly by the application layer (e.g., DNS).

• Applications such as Voice over IP (VoIP) that can tolerate some data loss during transmission, but delays in transmission are unacceptable

Established a Session

TCP negotiates and establishes a permanent connection (or session) between source and destination devices prior to forwarding any traffic

Stateful Protocol

Keeps track of the state of the communication session

Ensures Reliable Delivery

TCP ensures that each segment that is sent by the source arrives at the destination, by keeping track of the information sent and acknowledged.

Provides Same-Order Delivery

Because networks may provide multiple routes that can have different transmission rates, data can arrive in the wrong order

Supports Flow Control

TCP can request that the sending application reduce the rate of data flow when the resources (i.e., memory and processing power) in the receiving host are overwhelmed.

20 Bits

How many bits can be found in a TCP header?

Source Port

A 16-bit field used to identify the source application by port number.

Destination Port

A 16-bit field used to identify the destination application by port number.

Sequence Number

A 32-bit field used for data reassembly purposes.

Acknowledgment Number

A 32-bit field used to indicate that data has been received and the next byte expected from the source.

Header Length

A 4-bit field known as data offset that indicates the length of the TCP segment header

Reserved

A 6-bit field that is reserved for future use.

Control Bits

A 6-bit field used that includes bit codes, or flags, which indicate the purpose and function of the TCP segment

Window Size

A 16-bit field used to indicate the number of bytes that can be accepted at one time

Checksum

A 16-bit field used for error checking of the segment header and data

Urgent

A 16-bit field used to indicate if the contained data is urgent

8 Bits

How many bits does a UDP header have?

Length

A 16-bit field that indicates the length of the UDP datagram header.

Checksum

A 16-bit field used for error checking of the datagram header and data

Socket Pair

Source socket + Destination socket

Sockets

These enable multiple processes, running on a client, to distinguish themselves from each other, and multiple connections to a server process to be distinguished from each other.

Well-Known Ports

0 to 1023;

• These port numbers are reserved for common or popular services and applications such as web browsers, email clients, and remote access clients.

• Defined _______ for common server applications enables clients to easily identify the associated service required.

Registered Ports

1024 to 49151;

• These port numbers are assigned by IANA to a requesting entity to use with specific processes or applications.

• These processes are primarily individual applications that a user has chosen to install, rather than common applications that would receive a well-known port number.

  • For example, Cisco has registered port 1812 for its RADIUS server authentication process

Private or Dynamic Ports

49152 to 65535;

• These ports are also known as ephemeral ports.

• The client’s OS usually assign port numbers dynamically when a connection to a service is initiated.

  • The dynamic port is then used to identify the client application during communication.

Flow Control

• Adjusts the rate of data flow between source and destination for a given session

• Uses a segment header field called window size.

• Window size represents the number of bytes: that:

  • the destination device can accept and process at one time.

  • can be sent before an acknowledgment.

• Window size is changed by the destination at any time depending on the buffer availability

Sliding Window

Continual adjustment of send window by number of bytes already received by the source: 10,000 + 4,380 = 14,380