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The TCP/IP Guide  9  TCP/IP Lower-Layer (Interface, Internet and Transport) Protocols (OSI Layers 2, 3 and 4)       9  TCP/IP Internet Layer (OSI Network Layer) Protocols            9  Internet Protocol (IP/IPv4, IPng/IPv6) and IP-Related Protocols (IP NAT, IPSec, Mobile IP)                 9  Internet Protocol Version 4 (IP, IPv4)                      9  IP Datagram Encapsulation and Formatting

IP Datagram General Format 1 2 3 IP Datagram Size, Maximum Transmission Unit (MTU), Fragmentation and Reassembly

IP Options and Source Routing

Normally, IP datagrams are routed without any specific instructions from devices regarding the path a datagram should take from the source to the destination. It's the job of routers, using routing protocols, to figure out those details. In some cases, however, it may be advantageous to have the source of a datagram specify the route a datagram takes through the network. This is called source routing.

There are two IP options that support source routing. In each, the option includes a list of IP addresses specifying the routers that must be used, to reach the destination. When strict source routing is used, this means that the path specified in the option must be used exactly, in sequence, with no other routers permitted to handle the datagram at all. In contrast, loose source routing specifies a list of IP addresses that must be followed in sequence, but having intervening hops in between the devices on the list is allowed.

For full details on the exact structure used by each option type, please refer to RFC 791.

IP Datagram General Format 1 2 3 IP Datagram Size, Maximum Transmission Unit (MTU), Fragmentation and Reassembly