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Computer Science
Computer Catlog
Comm Network Catlog

Network Components
Network Types
The OSI Model
Protocol Notations
Physical Layer
Transmission Media
Digitization and Synchronization
Physical Layer Standards
DataLink Layer
Error Checking
Retrans - Flow Control
Sliding Window Protocol
Data Link Layer Standards
Network Layer
Switching Methods
Congestion Control
Network Sub layers
Transport Layer
Transport Protocol
Transport Layer Standards
Session Layer
Session Layer Role
Session Protocol
Presentation Layer
Abstract Syntax Notation
Application Layer
Common Application
Specific Application
Message Handling
IEEE 802 Standards
ANSI FDDI Standard
Frame Relay
Broadband ISDN & ATM

Transport Layer Standards

    Transport services are defined by the ISO 8072 and CCITT X.214 standards. ISO 8073 and CCITT X.224 define the transport protocol, and were the basis of the descriptions in Section 5.2. Below we will look at TCP- a de facto transport layer standard that has gained considerable acceptance in the industry.


    The Transmission Control Protocol (TCP) originated in ARPANET, and was designed to cope with Type C networks. Consequently, the CCITT Class 4 protocol has borrowed many of its concepts. TCP sits directly on top of IP and is well suited to internetworking. The combination is commonly referred to as TCP/IP.

    TCP accepts arbitrary size messages from a service user and, if necessary, segments them into smaller blocks. The block is constructed in two stages. First, a TCP header is added to user data; this header is essentially a transport-layer-style header. Next, a datagram header is added to the result to prepare it for a datagram network service.

    Given the unreliable network service expectations of TCP (Type C), blocks may be corrupted, lost, duplicated, or out of sequence. TCP uses sequence numbers and resequences blocks when necessary. Unlike other protocols, sequencing is performed on an octet rather than block basis. The sequence numbers for the octets in a block are determined by the Sequence Number and the Total Length fields. Flow control is also octet-oriented. It is based on a credit-style sliding window, where the window size determines how many octets may be transmitted.

    Corrupted and lost blocks are retransmitted (the T1 timer is used for detecting lost blocks). Delayed duplicate control blocks for establishing connections are dealt with using the three-way handshake method. In this method, to establish a connection between A and B, when A requests a connection and receives a confirmation from B, it must acknowledge (and therefore finalize) the connection as intended. Should B receive a delayed duplicate connection request and confirm it, A will know that this is due to a duplicate message and reject the connection on that basis.