9.1 Transportation of Data Flashcards
TRANSPORT LAYER
Application layer programs generate data that must be exchanged between 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 is the link between the application layer and the lower layers that are responsible for network transmission.
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:
Transmission Control Protocol (TCP)
User Datagram Protocol (UDP)
Transport Layer Responsibilities
1.Tracking Individual Conversations
2. Segmenting Data and Reassembling Segments
3. Add Header Information
4. Identifying the Applications
5. Conversation Multiplexing
- Tracking Individual Conversations
At the transport layer, each set of data flowing between a source application and a destination application is known as a conversation and is tracked separately. It is the responsibility of the transport layer to maintain and track these multiple conversations.
A host may have multiple applications that are communicating across the network simultaneously.
Most networks have a limitation on the amount of data that can be included in a single packet. Therefore, data must be divided into manageable pieces.
The PC simultaneously runs multiple network applications including an email client, instant messaging client, web browser web pages, streaming video, and a video conference client.
- Segmenting Data and Reassembling Segments
It is the transport layer responsibility to divide the application data into appropriately sized blocks. Depending on the transport layer protocol used, the transport layer blocks are called either segments or datagrams.
The transport layer divides the data into smaller blocks (i.e., segments or datagrams) that are easier to manage and transport.
- Add Header Information
The transport layer protocol also adds header information containing binary data organized into several fields to each block of data. It is the values in these fields that enable various transport layer protocols to perform different functions in managing data communication.
For instance, the header information is used by the receiving host to reassemble the blocks of data into a complete data stream for the receiving application layer program.
The transport layer ensures that even with multiple application running on a device, all applications receive the correct data.
The segments sent across the network have their own transport layer headers with control information that help the segments reach the correct destination applications and hosts.
- Identifying the Applications
The transport layer must be able to separate and manage multiple communications with different transport requirement needs.
To pass data streams to the proper applications, the transport layer identifies the target application using an identifier called a port number. As illustrated in the figure, each software process that needs to access the network is assigned a port number unique to that host.
- Conversation Multiplexing
Sending some types of data (e.g., a streaming video) across a network, as one complete communication stream, can consume all the available bandwidth. This would prevent other communication conversations from occurring at the same time. It would also make error recovery and retransmission of damaged data difficult.
The transport layer uses segmentation and multiplexing to enable different communication conversations to be interleaved on the same network.
Error checking can be performed on the data in the segment, to determine if the segment was altered during transmission.
SEGMENTATION
Segmentation facilitates data transport by the lower network layers.
MULTIPLEXING
Multiple applications can use the network at the same time.
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 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.
TCP
IP is concerned only with the structure, addressing, and routing of packets, from original sender to 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 which 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 provides reliability and flow 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 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.
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.
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 a successful delivery.
