Internet Protocol IP Internet Layer and IP Protocol

14.4.2 Internet Layer and IP Protocol

The Internet layer is a layer above the network access layer in the protocol hierarchy. The best−known protocols in this layer are: Internet protocol IP and Internet Control Message Protocol ICMP.

14.4.2.1 Internet Protocol IP

The most important protocol in this layer is the Internet protocol, better known as the IP. IP is the core of TCPIP, and it provides the basic packet delivery service on which TCPIP networks are built. All protocols in the layers above and below the Internet layer are dependent in some way on the IP to deliver data. All incoming and outgoing TCPIP data flows deal with IP, regardless of their real destinations. IP functions include: Defining the datagram, which is the basic unit of transmission in the TCPIP network • Defining the Internet addressing scheme • Moving data between the layer below, the network access layer, and the layer above, the host−to−host transport layer • Routing datagrams to remote hosts • Performing fragmentation and reassembly of datagrams • IP is a connectionless protocol. This means that IP does not exchange control information, known as handshaking, to establish an end−to−end connection before transmitting data. Rather, the opposite is true: connection−oriented protocols perform handshaking with the remote system to verify that a connection is established before data transmission starts see more details later about TCP and in Figure 14.11. IP relies on protocols in other layers to establish the connection if they require connection−oriented service. IP is also an unreliable protocol because it contains no error detection and recovery code. Of course, this does not mean that reliable data delivery cannot be based on IP, it only means that IP does not check to ensure the data was correctly received at the remote system. TCPIP protocols transmit data over the network in packets. A packet contains a block of data to be transferred, as well as the full information that identifies the destination of a packet itself. Each packet travels over the network independently of any other packet. Long data structures are divided into packets for transfer over the network and reassembled at the receiving end. The datagram is the packet format defined by IP, and it is presented in Figure 14.8. The first five or six 32−bit words the sixth word is optional form the header. The header length is specified in the field IHL Internet Header Length. The header contains all the information necessary to identify and deliver the datagram. The source and destination addresses are crucial for delivery; they are the IP addresses of the source and destination hosts in the network. Two hosts in the communication mostly do not reside in the same subnet local area network, so datagrams may travel through many network devices until they reach their destinations. 332 Figure 14.8: IP datagram format. The Total Length field determines the length of a data part of the datagram and should be decreased by IHL. Sometimes, when traveling through different networks, datagrams must be further divided into smaller packets because of the network type they are passing through. This procedure is called fragmentation, and corresponding identification information is also included in the datagram header.

14.4.2.2 Internet Control Message Protocol ICMP