Data Communications Networking Devices - 4th Ed.pdf

214 __________________________________________________________ WIDE AREA NETWORKSSource and destination port ®eldsThe source and destination port ®elds are each 16 bits in length and as previouslydescribed for TCP identify the port number of the sending and receiving process,respectively. Here each port number process identi®es as application running atthe corresponding IP address in the IP header pre®xed for the UDP header. Theuse of a port number provides a mechanism for identifying network services asthey denote communications points where particular services can be accessed. Forexample, a value of 161 in a port ®eld is used in UDP to identify SNMP.Length ®eldThe length ®eld indicates the length of the UDP packets in octets to include theheader and user data. The checksum, which is a one's complement arithmetic sum,is computed over a pseudo-header and the entire UDP packet. The pseudo-headeris created by the conceptual pre®x of 12 octets to the header previously illustratedin Figure 2.27. The ®rst 8 octets are used by source and destination IP addressesobtained from the IP packet. This is followed by a zero-®lled octet and an octetwhich identi®es the protocol. The last two octets in the pseudo-header denote thelength of the UDP packet. By computing the UDP checksum over the pseudoheaderand user data a degree of additional data integrity is obtained.The IP headerAs previously mentioned, IP provides a datagram-oriented gateway service fortransmission between subnetworks. This provides a mechanism for hosts to accessother hosts on a best-effort basis but does not enhance reliability as it relies onupper layer protocols for error detection and correction. As a Layer 3 protocol IPis responsible for the routing and delivery of datagrams. To accomplish this taskIP performs number of communications functions to include addressing, statusinformation, management and the fragmentation and reassembly of datagramswhen necessary.Figure 2.28 illustrates the IP header format while Table 2.4 provides a briefdescription of the ®elds in the IP header.Version ®eldThe four-bit version ®eld identi®es the version of the IP protocol used to create thedatagram. The current version of the IP protocol is 4 and is encoded as 0100 inbinary. The next generation IP protocol is version 6, which is encoded as 0110 inbinary. In our discussion of IP we will primarily focus on IPv4 in this section;however, we will also examine the key features of IPv6 which represent the nextgeneration of Internet addressing.