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Internet Layer

The Internet Protocol

The Internet Protocol is provided within the TCP-IP stack program like STinG or MiNTnet.
The picture below shows the position of the Internbet Layer in the TCP-IP model and also a comparison with the ISO/OSI model.

The purpose of the IP is to provide three services :

  • Enforce the basic unit of data transferused throughout the TCP-IP protocalsuite
  • erform the routing function by chossing a path to send data.
  • Include a set of rules that embodies the idea of unreliable packet delivery. These rules determine when and how the IP generates errir messages and discards packets.

The Internet Control Message Protocol

The Internet Control Message Protocol (ICMP) allows routers to send control or error messages to other routers and hosts. It provides the communication mechanism between the IP on one system and the IP on another system.

When an error occurs in the datagram, ICMP reports the error to the system which issued the datagram (the source). This communication by ICMP can include a control message (such as a redirect) or simply an errir message (such as Network is Unreachable).

Datagrams

Datagrams are the basic units of information passed across a TCP-IP internet. The datagram is the data area of an Ethernet frame. Within the datagram, is a datagram header that contains information, such as the source IP address and the destination IP address. The header contaions the information on what protocol IP is to pass the data to (such as UDP, TCP, or ICMP) and a TTL field that determines how many gateways or hosts can process a datagram before it expires.

Fragmentation

Fragments are units of data that have been broken into yet smaller units of data. Since data must be able to fit into the data protion of an Ethernet frame it may be necessary to fragment the application data so that it can be encapsulated into an Ethernet frame.

The fragment size is determined by the MTU of the network interface and the hardware layers. IPv4 specifies that fragmentation occur at each router based on the MTU of the interface through which the IP datagrams must pass. IPv6, however, requires that the initial sending router determine and use the smallest fragment size required by an intermediate router. This increases performance at the expence of flecibilty in that some paths, which use a smaller MTU, will not be available.