IP Packet Header

The Internet Protocol (IP) uses a Datagram service to transfer packets of data between end systems using routers.

The IP packet header consists of 20 bytes of data. An option exists within the header which allows further optional bytes to be added, but this is not normally used. The full header is shown below:

The header fields are discussed below:

• Version (always set to the value 4, which is the current version of IP)
• IP Header Length (number of 32 -bit words forming the header, usually five)
• Type of Service, now known as Differentiated Services Code Point (DSCP) (usually set to 0, but may indicate particular Quality of Service needs from the network, the DSCP defines one of a set of class of service)
• Size of Datagram (in bytes, this is the combined length of the header and the data)
• Identification ( 16-bit number which together with the source address uniquely identifies this packet - used during reassembly of fragmented datagrams)
• Flags (a sequence of three flags (one of the 4 bits is unused) used to control whether routers are allowed to fragment a packet (i.e. the Don't Fragment, DF, flag), and to indicate the parts of a packet to the receiver)
• Fragmentation Offset (a byte count from the start of the original sent packet, set by any router which performs IP router fragmentation)
• Time To Live (Number of hops /links which the packet may be routed over, decremented by most routers - used to prevent accidental routing loops)
• Protocol (Service Access Point (SAP) which indicates the type of transport packet being carried (e.g. 1 = ICMP; 2= IGMP; 6 = TCP; 17= UDP).
• Header Checksum (A 2's complement checksum inserted by the sender and updated whenever the packet header is modified by a router - Used to detect processing errors introduced into the packet inside a router or bridge where the packet is not protected by a link layer cyclic redundancy check. Packets with an invalid checksum are discarded by all nodes in an IP network)
• Source Address (the IP address of the original sender of the packet)
• Destination Address (the IP address of the final destination of the packet)
• Options (not normally used, but when used the IP header length will be > 5 32-bit words to indicate the size of the options field)

http://www.erg.abdn.ac.uk/users/gorry Date: 10/11/1995 Revised: 7/10/2003