Summary of Module 11

Summary
This page summarizes the topics discussed in this module.


The primary duties of the transport layer, Layer 4 of the OSI model, are to transport and regulate the flow of information from the source to the destination reliably and accurately.

The transport layer multiplexes data from upper layer applications into a stream of data packets. It uses port (socket) numbers to identify different conversations and delivers the data to the correct application.

The Transmission Control Protocol (TCP) is a connection-oriented transport protocol that provides flow control as well as reliability. TCP uses a three-way handshake to establish a synchronized circuit between end-user applications. Each datagram is numbered before transmission. At the receiving station, TCP reassembles the segments into a complete message. If a sequence number is missing in the series, that segment is retransmitted.

Flow control ensures that a transmitting node does not overwhelm a receiving node with data. The simplest method of flow control used by TCP involves a “not ready” signal that notifies the transmitting device that the buffers on the receiving device are full. When the receiver can handle additional data, the receiver sends a “ready” transport indicator.

Positive acknowledgment with retransmission is another TCP protocol technique that guarantees reliable delivery of data. Because having to wait for an acknowledgment after sending each packet would negatively impact throughput, windowing is used to allow multiple packets to be transmitted before an acknowledgment is received. TCP window sizes are variable during the lifetime of a connection.

Positive acknowledgment with retransmission is another TCP protocol technique that guarantees reliable delivery of data. Because having to wait for an acknowledgment after sending each packet would negatively impact throughput, windowing is used to allow multiple packets to be transmitted before an acknowledgment is received. TCP window sizes are variable during the lifetime of a connection.

If an application does not require flow control or an acknowledgment, as in the case of a broadcast transmission, User Datagram Protocol (UDP) can be used instead of TCP. UDP is a connectionless transport protocol in the TCP/IP protocol stack that allows multiple conversations to occur simultaneously but does not provide acknowledgments or guaranteed delivery. A UDP header is much smaller than a TCP header because of the lack of control information it must contain.

Some of the protocols and applications that function at the application level are well known to Internet users:

• Domain Name System (DNS) - Used in IP networks to translate names of network nodes into IP addresses

• File Transfer Protocol (FTP) - Used for transferring files between networks

• Hypertext Transfer Protocol (HTTP) - Used to deliver hypertext markup language (HTML) documents to a client application, such as a WWW browser

• Simple Mail Transfer Protocol (SMTP) - Used to provide electronic mail services

• Simple Network Management Protocol (SNMP) - Used to monitor and control network devices and to manage configurations, statistics collection, performance and security

• Telnet - Used to login to a remote host that is running a Telnet server application and then to execute commands from the command line