IP Router Alert Option (RFC2113)
Original Publication Date: 1997-Feb-01
Included in the Prior Art Database: 2019-Feb-15
Internet Society Requests For Comment (RFCs)
This memo describes a new IP Option type that alerts transit routers to more closely examine the contents of an IP packet. [STANDARDS-TRACK]
Network Working Group D. Katz Request for Comments: 2113 cisco Systems Category: Standards Track February 1997
IP Router Alert Option
Status of this Memo
This document specifies an Internet standards track protocol for the Internet community, and requests discussion and suggestions for improvements. Please refer to the current edition of the "Internet Official Protocol Standards" (STD 1) for the standardization state and status of this protocol. Distribution of this memo is unlimited.
This memo describes a new IP Option type that alerts transit routers to more closely examine the contents of an IP packet. This is useful for, but not limited to, new protocols that are addressed to a destination but require relatively complex processing in routers along the path.
A recent trend in routing protocols is to loosely couple new routing functionality to existing unicast routing. The motivation for this is simple and elegant -- it allows deployment of new routing functionality without having to reinvent all of the basic routing protocol functions, greatly reducing specification and implementation complexity.
The downside of this is that the new functionality can only depend on the least common denominator in unicast routing, the next hop toward the destination. No assumptions can be made about the existence of more richly detailed information (such as a link state database).
It is also desirable to be able to gradually deploy the new technology, specifically to avoid having to upgrade all routers in the path between source and destination. This goal is somewhat at odds with the least common denominator information available, since a router that is not immediately adjacent to another router supporting the new protocol has no way of determining the location or identity of other such routers (unless something like a flooding algorithm is implemented over unicast forwarding, which conflicts with the simplicity goal).
Katz Standards Track [Page 1]
RFC 2113 Router Alert Option February 1997
One obvious approach to leveraging unicast routing is to do hop-by- hop forwarding of the new protocol packets along the path toward the ultimate destination. Each system that implements the new protocol would be responsible for addressing the packet to the next system in the path that understood it. As noted above, however, it is difficult to know the next system implementing the protocol. The simple, degenerate case is to assume that every system along the path implements the protocol. This is a barrier to phased deployment of the new protocol, however.
RSVP  finesses the problem by instead putting the address of the ultimate destination in the IP Destination Address field, and then asking that every RSVP router make a "small change in its ... forwarding path" to look for the specific RSVP packet type and pull such packets out of the mainline forwarding path, performing local processing on the packets before forwarding them on. This has the...