IP Router Alert Option
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Extracted elements (15)
Routers already demultiplex options by type in the fast path; adding a new option type adds negligible overhead. Routers that do not support option processing in the fast path already forward option-bearing packets through the slow path, so normal data packet performance is unaffected in all cases.
RSVP places the ultimate destination in the IP Destination Address field rather than the next RSVP-capable hop, enabling automatic tunneling through non-RSVP routers. The Router Alert option complements this by asking RSVP-aware routers to intercept and process such packets without requiring changes to the destination address.
The copied flag is set to 1 so that all IP fragments carry the Router Alert option; this ensures that every fragment is intercepted by routers implementing the option, not just the first fragment.
The motivation for loosely coupling new routing protocols to unicast routing is to allow incremental deployment without reinventing basic routing functions, but this limits new protocols to least-common-denominator forwarding information (next hop only).
RSVP and IGMPv2 are the two protocols identified in this document that require the Router Alert option to function correctly when routers along the path are not directly addressed by the packet.
Hosts shall ignore the Router Alert option.
Protocols that expect routers to perform significant processing on packets not directly addressed to them must mandate use of the Router Alert option. RSVP and IGMPv2 are cited as current examples.
Routers that do not recognize the Router Alert option shall ignore it, ensuring backward compatibility with non-implementing routers.
Routers that recognize the Router Alert option shall examine packets carrying it more closely — for example, checking the IP Protocol field — to determine whether further processing is necessary.
Unrecognized value fields in the Router Alert option shall be silently ignored by routers.
The 2-octet value field in the Router Alert option encodes the action: value 0 means 'Router shall examine packet'; values 1-65535 are reserved for future use.
The Router Alert option type byte (0x94) has copied flag set to 1, meaning all IP fragments must carry the option. Option class is 0 (control) and option number is 20 decimal.
If the Router Alert option is absent when it should be set, the protocol relying on it (e.g., RSVP or IGMPv2) will be adversely affected because those protocols depend on the option to trigger router interception.
If the Router Alert option is set when it should not be, packets will be ejected from the router's fast path into the slow path, causing a performance penalty relative to normal data packets.
The Router Alert option is 4 octets: type byte 0x94 (10010100 binary), length byte 0x04 (00000100 binary), and a 2-octet value field. The type byte encodes copied flag=1, option class=0 (control), and option number=20 decimal.