10. THE TRANSPORT LAYER

Description of the transport layer

The transport layer:

1. defines the protocols for structuring messages,
2. checks the validity of transmissions, and
3. translates transport layer addresses into transport layer logical names.

To provide error and flow control services, transport layer protocols utilize connection services. The two types of connection services are:

1. Connection-oriented: In this type of connection the two computers use acknowledgements and response signals to maintain a constant connection. Because of this, error and flow control are easily controlled. If a packet is lost, not only is it detected, it is, also, corrected by the sending station retransmitting the dropped packet.
2. Connectionless Services: Connectionless services don't have error and flow control, but they make up for this by being significantly faster then connection-oriented services. In this type of service since error and flow control checks aren't made, any packets that get lost lost or dropped are not retransmitted.

The transport layer also handles logical address to logical name resolution. Some protocol stacks use name resolution services in place of network addresses to allow humans to work with them more easily.

Transport Layer Implementations

IPX/SPX Protocol (Internet Packet eXchange/Sequenced Packet eXchange): Here there are two transport protocols; IPX, which is connectionless and, thus, enjoys the increased speed this type of connection technology facilitates, and SPX, which uses connection-oriented services. IPX/SPX has no name resolution by default. Name resolution services can be implemented on a Netware server when it is running NDS (Novell Directory Services) and the NDS directory requester, which runs at higher levels of the OSI model can make requests of an NDS database.

TCP/IP Protocol (Transmission Control Protocol/Internet Protocol): This protocol uses two transport protocols. TCP uses a transmission control protocol that is connection oriented and UDP, which uses a datagram protocol and is non-connection oriented. Different upper layer protocols rely on each one to perform their functions. FTP (File Transfer Protocol) and HTTP (Hyper Text Transfer Protocol) require reliable connection oriented services and, therefore, use TCP. TFTP (Trivial File Transfer Protocol) and NFS (Network File System) require increased speed and use UDP. For network address to name resolution, TCP/IP uses Domain Name Service, also known as DNS. Many operating systems use DNS for name resolution, but UNIX uses DNS almost exclusively. DNS is probably the most cross-platform name resolution method available.

NetBEUI: This method is based on the NetBIOS protocol. NetBEUI uses datagrams for connectionless transport. It doesn't have any support for connection-oriented services. NetBIOS does allow hosts to have logical names, but the naming service operates at the upper layers of the protocol stack.

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