Application layer programs generate data that must be exchanged between the source and destination hosts. The transport layer is responsible for logical communications between applications running on different hosts. This may include services such as establishing a temporary session between two hosts and the reliable transmission of information for an application.
The transport layer has no knowledge of the destination host type, the type of media over which the data must travel, the path taken by the data, the congestion on a link, or the size of the network.
The transport layer includes two protocols:
The transport layer includes two protocols:
- Transmission Control Protocol (TCP)
- User Datagram Protocol (UDP)
Transport Layer Responsibilities
The transport layer has many responsibilities.
Transport Layer Protocols
IP is concerned only with the structure, addressing, and routing of packets. IP does not specify how the delivery or transportation of the packets takes place.
Transport layer protocols specify how to transfer messages between hosts, and are responsible for managing the reliability requirements of a conversation. The transport layer includes the TCP and UDP protocols.
Different applications have different transport reliability requirements. Therefore, TCP/IP provides two transport layer protocols.
shows how application layer protocols like FTP, HTTP, SMTP use TCP at the transport layer and DNS and TFTP use UDP. How they all use IP at the internet layer regardless of whether they connect to a LAN or a WAN at the network access layer.
Transmission Control Protocol (TCP)
IP is concerned only with the structure, addressing, and routing of packets, from the original sender to the final destination. IP is not responsible for guaranteeing delivery or determining whether a connection between the sender and receiver needs to be established.
TCP is considered a reliable, full-featured transport layer protocol, which ensures that all of the data arrives at the destination. TCP includes fields that ensure the delivery of the application data. These fields require additional processing by the sending and receiving hosts.
Note: TCP divides data into segments.
TCP transport is analogous to sending packages that are tracked from source to destination. If a shipping order is broken up into several packages, a customer can check online to see the order of the delivery.
TCP provides reliability and flows control using these basic operations:
- Number and track data segments transmitted to a specific host from a specific application
- Acknowledge received data
- Retransmit any unacknowledged data after a certain amount of time
- Sequence data that might arrive in the wrong order
- Send data at an efficient rate that is acceptable by the receiver
In order to maintain the state of a conversation and track the information, TCP must first establish a connection between the sender and the receiver. This is why TCP is known as a connection-oriented protocol.
Click Play in the figure to see how TCP segments and acknowledgments are transmitted between sender and receiver.
Click Play in the figure to see how TCP segments and acknowledgments are transmitted between sender and receiver.
The animation shows a connection to an FTP server initiated with a TCP 3-way handshake and the data segments being accounted for by using sequence numbers and acknowledgments. A file is sent to a server using the File Transfer Protocol (FTP) application. TCP tracks the conversation and divides the data to be sent into 6 segments.
TCP Header
TCP is a stateful protocol which means it keeps track of the state of the communication session. To track the state of a session, TCP records which information it has sent and which information has been acknowledged. The stateful session begins with the session establishment and ends with the session termination.
A TCP segment adds 20 bytes (i.e., 160 bits) of overhead when encapsulating the application layer data. The figure shows the fields in a TCP header.
shows the fields in the TCP header.
TCP Header Fields
The table identifies and describes the ten fields in a TCP header.
| TCP Header Field | Description |
|---|---|
| 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 ʺdata offsetʺ 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 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. |
User Datagram Protocol (UDP)
UDP is a simpler transport layer protocol than TCP. It does not provide reliability and flow control, which means it requires fewer header fields.
Because the sender and the receiver UDP processes do not have to manage reliability and flow control, this means UDP datagrams can be processed faster than TCP segments. UDP provides the basic functions for delivering datagrams between the appropriate applications, with very little overhead and data checking.
Note: UDP divides data into datagrams that are also referred to as segments.
UDP is a connectionless protocol. Because UDP does not provide reliability or flow control, it does not require an established connection. Because UDP does not track information sent or received between the client and server, UDP is also known as a stateless protocol.
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UDP is also known as a best-effort delivery protocol because there is no acknowledgment that the data is received at the destination. With UDP, there are no transport layer processes that inform the sender of successful delivery.
UDP is like placing a regular, nonregistered, letter in the mail. The sender of the letter is not aware of the availability of the receiver to receive the letter. Nor is the post office responsible for tracking the letter or informing the sender if the letter does not arrive at the final destination.
UDP Header
UDP is a stateless protocol, meaning neither the client nor the server, tracks the state of the communication session. If reliability is required when using UDP as the transport protocol, it must be handled by the application.
One of the most important requirements for delivering live video and voice over the network is that the data continues to flow quickly. Live video and voice applications can tolerate some data loss with minimal or no noticeable effect and are perfectly suited to UDP.
The blocks of communication in UDP are called datagrams, or segments. These datagrams are sent as a best effort by the transport layer protocol.
The UDP header is far simpler than the TCP header because it only has four fields and requires 8 bytes (i.e., 64 bits). The figure shows the fields in a UDP header.
UDP datagram diagram shows 4 header fields: source port, destination port, length, and checksum as well as the non-header application layer data
UDP Header Fields
The table identifies and describes the four fields in a UDP header.
| UDP Header Field | Description |
|---|---|
| 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. |
| 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 Pairs
The source and destination ports are placed within the segment. The segments are then encapsulated within an IP packet.
The IP packet contains the IP address of the source and destination. The combination of the source IP address and source port number, or the destination IP address and destination port number is known as a socket.
In the example in the figure, the PC is simultaneously requesting FTP and web services from the destination server.
The figure depicts a PC making both an FTP connection and a web connection to a server. The requests have source and destination port numbers that identify the host PC and the requested application service respectively.
The socket is used to identify the server and service being requested by the client. A client socket might look like this, with 1099 representing the source port number: 192.168.1.5:1099
The socket on a web server might be 192.168.1.7:80
Together, these two sockets combine to form a socket pair: 192.168.1.5:1099, 192.168.1.7:80
Together, these two sockets combine to form a socket pair: 192.168.1.5:1099, 192.168.1.7:80
Sockets 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.
The source port number acts as a return address for the requesting application. The transport layer keeps track of this port and the application that initiated the request so that when a response is returned, it can be forwarded to the correct application.
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