TCP/IP versus OSI

Comparison between TCP/IP and OSI

This chapter gives a brief comparison between OSI and TCP/IP protocols with a special focus on the similarities and on how the protocols from both worlds map to each other. The adoption of TCP/IP does not conflict with the OSI standards because the two protocol stacks were developed concurrently. In some ways, TCP/IP contributed to OSI, and vice-versa. Several important differences do exist, though, which arise from the basic requirements of TCP/IP which are:

The main differences between the OSI architecture and that of TCP/IP relate to the layers above the transport layer (layer 4) and those at the network layer (layer 3). OSI has both, the session layer and the presentation layer, whereas TCP/IP combines both into an application layer. The requirement for a connectionless protocol also required TCP/IP to combine OSI’s physical layer and data link layer into a network level.

Physical Layer

The physical layer may be either ethernet, SDH-DCC, or some timeslot of a PDH signal. Either OSI protocols and TCP/IP protocols build on the same physical layer standards, thus there is no difference between OSI and TCP/IP in this aspect.

Data Link Layer

The purpose of the data link layer is to provide error free data transmission even on noisy links. This is achieved by framing of data and retransmission of every frame until it is acknowledged from the far end, using flow control mechanisms. Error detection is done by means of error detection codes.

The data link layer in the OSI world makes use of the Q.921 LapD protocol which must support an information field length of at least 512 octets according to G.784. LapD is based on HDLC framing.

In the internet world there is no real data link layer protocol, but the subnet protocol which has quite many similarities. The subnet protocol consists of the IMP-IMP protocol which aims to provide a reliable connection between neighbored IMPs.

For ethernet based networks e.g. LANs (Local Area Network), the data link protocol LLC (Logical Link Control) is equally used in OSI and TCP/IP networks.

Network Layer

The network layer provides routing capabilities between source and destination system.

OSI uses the CLNS (Connection Less Network Service) protocols ES-IS for communication of an end system to an intermediate system and IS-IS for communication between intermediate systems.

TCP divides messages in datagrams of up to 64k length. Each datagram consists of a header and a text part. Besides some other information, the header contains the source and the destination address of the datagram. IP routes these datagrams through the network using e.g. the protocol OSPF (Open Shortest Path First) or RIP (Route Information Protocol) for path calculation purposes. The service provided by IP is not reliable. Datagrams may be received in the wrong order or they may even get lost in the network.

Transport Layer

The transport layer provides a reliable end-to-end connection between source and destination system on top of the network layer. It builds an integral part of the whole OSI layering principle and of the internet protocol.

The OSI transport layer protocol (TP4) and the internet tranport protocol (TCP) have many similarities but also some remarkable differences. Both protocols are built to provide a reliable connection oriented end-to-end transport service on top of an unreliable network service. The network service may loose packets, store them, deliver them in the wrong order or even duplicate packets. Both protocols have to be able to deal with the most severe problems e.g. a subnetwork stores valid packets and sends them at a later date. TP4 and TCP have a connect, transfer and a disconnect phase. The principles of doing this are also quite similar.

One difference between TP4 and TCP to be mentioned is that TP4 uses nine different TPDU (Transport Protocol Data Unit) types whereas TCP knows only one. This makes TCP simpler but every TCP header has to have all possible fields and therefore the TCP header is at least 20 bytes long whereas the TP4 header takes at least 5 bytes.

Another difference is the way both protocols react in case of a call collision. TP4 opens two bidirectional connections between the TSAPs whereas TCP opens just one connection.

TP4 uses a different flow control mechanism for its messages, it also provides means for quality of service measurement.