IPv4 Options

IPv4 allows up to 40 bytes of options to follow the fixed 20-byte header. Although 10 different options are defined, the most commonly used is the source route option. Access

to these options is through the IP_OPTIONS socket option and we will demonstrate this with an example that uses source routing.

options following the 20-byte IPv4 header, the 4-bit header length field limits the total size of the IPv4 header to 15 32-bit words (60 bytes), so the size of the IP options is limited to 40 bytes.

1. NOP: no-operation A one-byte option typically used for padding to make a later option fall on a four-byte boundary.
2. EOL: end-of-list A one-byte option that terminates option processing. Since the total size of the IP options must be a multiple of four bytes, EOL bytes follow the final option.
3. LSRR: loose source and record route
4. SSRR: strict source and record route
5. Timestamp
6. Record route
7. Basic security (obsolete).
8. Extended security (obsolete).
9. Stream identifier (obsolete).
10. Router alert This option is included in IP datagrams that should be examined by all routers that forward the datagram.

The getsockopt and setsockopt functions (with a level of IPPROTO_IP and an optname of IP_OPTIONS) fetch and set the IP options.

When the IP options are set using setsockopt, the specified options will then be sent on all IP datagrams on that socket. This works for TCP, UDP, and raw IP sockets. To clear these options, call setsockopt and specify either a null pointer as the fourth argument or a value of 0 as the fifth argument (the length).

Setting the IP options for a raw IP socket does not work on all implementations if the IP_HDRINCL socket option is also set.

IPv4 Source Route Options

A source route is a list of IP addresses specified by the sender of the IP datagram. If the source route is strict, then the datagram must pass through each listed node and only the
listed nodes. That is, all the nodes listed in the source route must be neighbors. But if the source route is loose, the datagram must pass through each listed node, but can also pass through other nodes that do not appear in the source route.

We place an NOP before the source route option, which causes all the IP addresses to be aligned on a four-byte boundary. This is not required, but takes no additional space (the IP options are always padded to be a multiple of four bytes) and aligns the addresses.

In this figure, we show up to 10 IP addresses in the route, but the first listed address is removed from the source route option and becomes the destination address of the IP
datagram when it leaves the source host. Although there is room for only 9 IP addresses in the 40-byte IP option space (do not forget the 3-byte option header that we are about to describe), there are actually 10 IP addresses in an IPv4 header when the destination address is included.

The code is either 0x83 for an LSRR option or 0x89 for an SSRR option. The len that we specify is the size of the option in bytes, including the three-byte header, and including the extra destination address at the end. It will be 11 for a route consisting of one IP address, 15 for a route consisting of two IP addresses, and so on, up to a maximum of 43. The NOP is not part of the option and is not included in the len field, but is included in the size of the buffer that we specify to setsockopt. When the first address in the list is removed from the source route option and placed into the destination address field of the IP header, this len value is decremented by four. ptr is a pointer which contains the offset of the next IP address to be processed in the route, and we initialize it to 4, which points to the first IP address. The value of this field increases by four as the datagram is processed by each listed node.

When a received source route is returned to the application by getsockopt, the format is different

First, the order of the addresses has been reversed by the kernel from the ordering in the received source route. What we mean by "reversed" is that if the received source route contains the four addresses A, B, C, and D, in that order, the reverse of this route is D, C, B, and then A. The first 4 bytes contain the first IP address in the list, followed by a 1-byte NOP (for alignment), followed by the 3-byte source route option header, followed by the remaining IP addresses. Up to 9 IP addresses can follow the 3-byte header, and the len field in the returned header will have a maximum value of 39. Since the NOP is always present, the length returned by getsockopt will always be a multiple of 4 bytes.