Telecommunications - IP Internet Protocol

TCP/IP is a set of protocols developed by a community of researchers in USA including ARPAnet and other military, university and research institutions.

The IP related protocols can be mapped onto the OSI 7 layer model

Layer Protocol
6 Presentation
5 Session
4 Transport TCP UDP  
3 Network IP
2 Data Link Ethernet, Token Ring, etc.
1 Physical


In addition to LANs these protocols can be carried over wide area networks.

TCP/IP is a family of protocols, include:

These protocols are designed to allow a large number of small independent networks to be connected together by gateways.

Higher Level Protocols

Internet addresses

see internet addressing and routing.

As far as the user is concerned, all they need to know in order to access another system is an "Internet address". This is an address that looks like It is actually a 32-bit number. However it is normally written as 4 decimal numbers, each representing 8 bits of the address.

Generally the structure of the address gives you some information about how to get to the system. For example, 128.6 is a network number assigned by a central authority to Rutgers University. Rutgers uses the next octet to indicate which of the campus Ethernets is involved. 128.6.4 happens to be an Eithernet used by the Computer Science Department. The last octet allows for up to 254 systems on each Ethernet. (0 and 255 are reserved).

information Transfer

TCP/IP is a 'connectionless'technology, information is transferred as a sequence of 'datagrams'. A datagram is a collection of data that is sent as a single message. Each of these datagrams is sent through the network individually. There are provisions to open connections (i.e. to start a conversation that will continue for some time). However at some level, information from those connections is broken up into datagrams, and those datagrams are treated by the network as completely separate. For example, suppose you want to transfer a 15000 octet file. Most networks can't handle a 15000 octet datagram. So the protocols will break this up into something like 30 500-octet datagrams. Each of these datagrams will be sent to the other end. At that point, they will be put back together into the 15000-octet file. However while those datagrams are in transit, the network doesn't know that there is any connection between them. It is perfectly possible that datagram 14 will actually arrive before datagram 13. It is also possible that somewhere in the network, in error will occur, and some datagram won't get through at all. In that case, that datagram has to be sent again.

The format of a datagram is:

Ethernet Header

Ethernet destination address (first 32 bits)
Ethernet dest (last 16 bits) Ethernet source (first 16 bits)
Ethernet source address (last 32 bits)
Type code

IP header - IP handles routing.

Version IHL Type of Service Total length
Identification Flags Fragment Offset
Time to live Protocol Header Checksum
Source Address
Destination Address

TCP header - TCP handles, breaking up the message into datagrams, reassembling them at the other end, resending anything that gets lost, and putting things back in the right order.

Source Port Destination Port
Sequence Number
Acknowledgement Number
Data Offset Reserved URG ACK PSH RST SYN FIN window
Checksum Urgent Pointer
Options Padding
User data
Ethernet Checksum

<---------------------32 bits------------------------------->

DLCI coding

The DLCI has 'local significance' on the link concerned only.

The DLCI is the equivalent of an X.25 'logical channel'. In the case of the LAN Interconnect service, the equivalent of a PVC has to be configured between all the LANs which the a user is linked to form a fully connected mesh.

For phase 1. Each frame relay trunk or access link is limited to a maximum of 126 DLCIs. The routing through the network of each of these DLCIs is manually calculated and administered.

To ease the administrative burden and to prevent accidental duplication or 'cross-connection' of DLCIs belonging to different customer communities, the DLCIs are assumed to be globally unique throughout the BT Tymnet frame relay network. Every customer router connecting to the network is allocated a unique DLCI from the pool of 126 DLCIs. Whenever a particular DLCI is used as a data sink by any other user regardless of the location of the source connection into the network, that data is routed by the network to the router associated with that unique DLCI.

To prevent unauthorised access, DLCIs are configured and allocated by the network provider. The customer has visibility of and can only communicate with DLCIs of other customers who are part of his community. Only the network management systems have access to all the customer routers on the network.

Lans in OSI framework

In considering how LANs fit into the framework of OSI, we need to view the 7 layers as an assembly of functional blocks relating to Interconnection Standards (Layers1-4) and Interworking Standards (Layers5-7).

OSI Layer
FTAM X.400 VT 6 Interworking
Transport 4
2 Standards

In the OSI framework, the 7layers are viewed as an assembly of functional block.

Interconnection Standards can be based on LANs, WANs or ISDN. Whatever the underlying network structure happens to be is of no concern to the Interworking Standards, which can be grouped according to particular applications such as FTAM, X.400, VT and so on.

The Transport Layer plays an important part in segregating the 2halves of the communications stack. The Transport Protocol creates a transport service which provides the upper layers with a reliable and error free end-to-end data path. The protocol must comply with a service quality requested by the upper layers and be able to implement this independently of the quality of the underlying network service. The transport service is divided into 5classes of supported functions and the Transport Protocol is structured accordingly:-

(Multiplexing is the ability to establish more than one transport connection over a single network connection.) Class4 provides a full implementation of service functions and will be needed for use over a LAN.

Let us now consider the way in which LANs are able to support OSI through the implementation of standards. At the lowest level, Layers1 and2, protocols are concerned with the network hardware.

They determine how station access to the network is controlled, ie contention or token passing, and ensure that physical transmission errors are detected and rectified.

Protocol Analyser

To view packets being sent or received by your computer you can use Protocol Analyser Software such as Ethereal.


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