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- 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.
-
- Abstract
-
- 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.
-
- 1.0 Introduction
-
- 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).
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- Katz Standards Track [Page 1]
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- RFC 2113 Router Alert Option February 1997
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-
- 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 [1] 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
- decided advantage of allowing automatic tunneling through routers
- that don't understand RSVP, since the packets will naturally flow
- toward the ultimate destination. However, the performance cost of
- making this Small Change may be unacceptable, since the mainline
- forwarding path of routers tends to be highly tuned--even the
- addition of a single instruction may incur penalties of hundreds of
- packets per second in performance.
-
- 2.0 Router Alert Option
-
- The goal, then, is to provide a mechanism whereby routers can
- intercept packets not addressed to them directly, without incurring
- any significant performance penalty. This document defines a new IP
- option type, Router Alert, for this purpose.
-
- The Router Alert option has the semantic "routers should examine this
- packet more closely". By including the Router Alert option in the IP
- header of its protocol message, RSVP can cause the message to be
- intercepted while causing little or no performance penalty on the
- forwarding of normal data packets.
-
- Routers that support option processing in the fast path already
- demultiplex processing based on the option type field. If all option
- types are supported in the fast path, then the addition of another
- option type to process is unlikely to impact performance. If some
- option types are not supported in the fast path, this new option type
- will be unrecognized and cause packets carrying it to be kicked out
- into the slow path, so no change to the fast path is necessary, and
- no performance penalty will be incurred for regular data packets.
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- Katz Standards Track [Page 2]
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- RFC 2113 Router Alert Option February 1997
-
-
- Routers that do not support option processing in the fast path will
- cause packets carrying this new option to be forwarded through the
- slow path, so no change to the fast path is necessary and no
- performance penalty will be incurred for regular data packets.
-
- 2.1 Syntax
-
- The Router Alert option has the following format:
-
- +--------+--------+--------+--------+
- |10010100|00000100| 2 octet value |
- +--------+--------+--------+--------+
-
- Type:
- Copied flag: 1 (all fragments must carry the option)
- Option class: 0 (control)
- Option number: 20 (decimal)
-
- Length: 4
-
- Value: A two octet code with the following values:
- 0 - Router shall examine packet
- 1-65535 - Reserved
-
- 2.2 Semantics
-
- Hosts shall ignore this option. Routers that do not recognize this
- option shall ignore it. Routers that recognize this option shall
- examine packets carrying it more closely (check the IP Protocol
- field, for example) to determine whether or not further processing is
- necessary. Unrecognized value fields shall be silently ignored.
-
- The semantics of other values in the Value field are for further
- study.
-
- 3.0 Impact on Other Protocols
-
- For this option to be effective, its use must be mandated in
- protocols that expect routers to perform significant processing on
- packets not directly addressed to them. Currently such protocols
- include RSVP [1] and IGMP [2].
-
- 4.0 Security Considerations
-
- If the Router Alert option is not set and should be set, the behavior
- of the protocol using the Router Alert, e.g., RSVP or IGMPv2, will be
- adversely affected since the protocol relies on the use of the Router
- Alert option.
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- Katz Standards Track [Page 3]
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- RFC 2113 Router Alert Option February 1997
-
-
- If the Router Alert option is set when it should not be set, it is
- likely that the flow will experience a performance penalty, as a
- packet whose Router Alert option is set will not go through the
- router's fastpath and will be processed in the router more slowly
- than if the option were not set.
-
- 5.0 References
-
- [1] Braden, B. (ed.), L. Zhang, D. Estrin, S. Herzog, S. Jamin,
- "Resource ReSerVation Protocol (RSVP)," work in progress, March
- 1996.
-
- [2] Fenner, W., "Internet Group Management Protocol, Version 2
- (IGMPv2)," work in progress, October 1996.
-
- Author's Address
-
- Dave Katz
- cisco Systems
- 170 W. Tasman Dr.
- San Jose, CA 95134-1706 USA
-
- Phone: +1 408 526 8284
- Email: dkatz@cisco.com
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- Katz Standards Track [Page 4]
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