Appendix A - IP Header

IP Header [STEVENS94]

• Version
  Always set to the value 4, which is the current version of 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.

• Fragment 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) indicates the type of transport packet being carried (e.g. 1=ICMP; 2=IGMP; 4=IpinIP; 6=TCP; 17=UDP; 94=IPinIP).

• 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 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 IP Address
  The IP address of the original sender of the packet.

• Destination IP 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.