rfc1476.txt

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RFC 1476                          RAP                          June 1993


   Type 0 indicates a null (no-operation) option.  It should be class
   zero, but an implementation that "understands" the null option may
   decline to propagate it.

   Note that since an implementation may delete an option of class 1 by
   simply setting its type to 0 and forwarding the route description,
   class 1 does not provide any confidentiality of the content of an
   option.

3.1.3  Format

   The format field specifies the format of the data included after the
   option header.  Formats:

   0        none, no data present.
   1        one or more 32-bit signed integers
   2        a character string, null terminated
   3        one or more real numbers
   4        an octet string
   5        one real, followed by a character string

   Format is also orthogonal to type, but a particular type is usually
   only reasonably represented by one format.  This allows decoding of
   all option values for logging and other troubleshooting, even when
   the option type is unknown.  (A new unknown format will still present
   a problem.)

   Format 4, octet string, is to be represented in dotted-decimal byte
   form when printed; it is normally an internet address.

   Format 5 is intended for dimensioned parameters with the character
   string giving the dimension or scale.

3.2  Metrics and Options

   As much as possible, metrics are kept in the base units of bytes and
   seconds, by analogy to the physics systems of MKS (meter-kilogram-
   second) and CGS (centimeter-gram-second) of base units.

   Bytes aren't the real primitive, the bit is.  We are thus using a
   multiple of 8 that isn't part of what one would come to expect from a
   decimal metric system that uses the other prefixes.  However, since K
   (kilo) is often taken to be 1024, and M (mega) to be 1,048,576 (or
   even 1,024,000) we allow this liberty.

   Distance is measured in units also unique to the field.  It is the
   integer number of times that a datagram must be forwarded to reach
   the destination.  (Hop count.)



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RFC 1476                          RAP                          June 1993


3.2.1  Distance

   The Distance metric counts the number of hops on a route; this is
   included in the RAP route command header.

   The initial distance at insertion into the RAP domain by the origin
   of the route MUST be less than or equal to 2z, where z is the number
   of zero bits in the route mask.

   If the origin derives the route from RIP or OSPF, and the distance
   exceeds 2z, the route must not be used.

   When a router originates a route designed to permit aggregation, the
   distance is usefully set to more than 0; this allows simple subset
   aggregation without propagating small distance changes repeatedly as
   the internal diameter of the described network changes.

   For example, for routers designated to announce a default route for
   an AD, with a 24/48 mask, the maximum initial distance is 96.

3.2.2  Delay

   The Delay metric (Type = 2) measures the one-way path delay.  It is
   usually the sum of delays configured for the gateways and interfaces,
   but might also include path segments that are actually measured.

   Format is real (3), with one value.  The units are seconds.

3.2.3  MTU

   The MTU metric (Type = 3) measures the minimum value over the route
   of the Maximum Transmission Unit, i.e., the largest IP datagram that
   can be routed without resulting in fragmentation.

   Format is one integer, measuring the MTU in bytes.

3.2.4  Bandwidth

   The Bandwidth metric (Type = 4) measures the minimum bandwidth of the
   path segments that make up the route.

   Format is one real, representing bandwidth in bytes/second.

3.2.5  Origin

   The origin attribute (type = 5) identifies the router that originally
   inserted the route into the RAP domain.  It is one of the IP
   addresses of the router, format is 4.



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RFC 1476                          RAP                          June 1993


3.2.6  Target

   The target attribute (type = 6) identifies a host or network toward
   which the route should be propagated, regardless of proximity
   filtering that would otherwise occur.  This aids in the establishment
   of tunnels for hosts or subnets "away from home." It can be used to
   force the route to propagate all the way to the home network, or to
   try to propagate a better route to a host that the origin has
   established a connection (e.g., TCP) with.  Note that a router can
   distinguish these two cases during proximity filtering by comparing
   the route described with the host or network identified by the target
   option.

   Format is 4.

3.2.7  Packet Cost

   The packet cost metric (type = 7) measures the actual cost (to
   someone) of sending data over the route.  It is probably either class
   3 or 0.  Format is 5.

   The real number is the cost in currency units/byte.  Tariffs set in
   packets or "segments" should be converted using the nominal (or
   actual path) size.  For example, Sprintnet charges for DAF
   connections within its network are US$1.40/Ksegment thus for segments
   of 64 bytes, the cost is 0.000021875 USD.

   The string is the 3 capital letter ISO code [ISO4217] for the
   currency used.  Funds codes and codes XAU, XBA, XBB, XBC, XBD, and
   XXX are not used.

   If a route already has a packet cost in a different currency
   associated with it, another instance of this option should be added.
   RAP implementations MUST NOT attempt to convert the currency units
   except when actually making a route selection decision.  That is, the
   effects of a currency conversion should never be propagated, except
   for the proper effect of such a selection decision.

3.2.8  Time Cost

   The time cost metric (type = 8) measures the actual cost of holding
   one or more paths in the route open to send data.  It is probably
   either class 3 or 0.  Format is 5.

   The real number is the cost in currency units/second.  For example,
   Sprintnet charges for international connections (to typical
   destinations) are US$10/hour so the cost is 0.002777778 USD.




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RFC 1476                          RAP                          June 1993


   The other notes re codes used and conversions in the previous section
   also apply.

3.2.9  Source Restriction

   A source restriction option (type 9, format 4, class 2 or 3)
   indicates that a route may only be used by datagrams from a
   particular source or set of sources.  The data consists of a network
   or host number, and a mask to qualify it.  If multiple source
   restriction options are included, the restriction is the logical
   union of the sources specified; i.e., any are permitted.

   Source restrictions must be added to routes when the RAP system has
   security filters set in the IP forwarding layer.  This is necessary
   to prevent datagrams from taking "better" routes that end in the
   datagram being silently discarded at the filter.  Note that this
   propagates confidential information about the security configuration,
   but only toward the net authorized to use the route if the RAP
   implementation is careful about where it is propagated.

3.2.10  Destination Restriction

   A destination restriction option (type 10, format 4, class 3) serves
   only to provide a non-contiguous mask, the destination already having
   been specified in the command header.  Data is the destination
   network and mask.

3.2.11  Trace

   Trace (type 11, format 4, class 0) provides an indication that the
   route has propagated through a particular system.  This can be used
   for loop detection, as well as various methods of troubleshooting.
   The data is one internet address, one of the addresses of the system.
   If an arriving route already carries a trace identifying this system,
   and is not an update, it is discarded.  If it is an update, the route
   is purged.

   Trace SHOULD NOT be simply added to every route traversing a system.
   Rather, it should be added (if being used for loop detection) when
   there is a suspicion that a loop has formed.

   When the distance to a destination has increased twice in a row in a
   fairly short period of time, and the number of trace options present
   in the route did not increase as a result of the last update, the RAP
   process should add a trace option identifying itself to the route.
   Effectively, when a loop forms, one router will select itself to be a
   tracer, adding itself and breaking the loop after one more turn.  If
   that fails for some reason, another router will add its trace.  Each



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RFC 1476                          RAP                          June 1993


   router thus depends in the end only on its own trace and will break
   the loop, even if the other routers are using other methods, or
   simply counting-out the route.

3.2.12  AUP

   The AUP (Acceptable Use Policy) option (type 12, format 2, class
   any), tags a route as being useable only according to the policy of a
   network.  This may be used to avoid traversal of the net by (for
   example) commercial traffic, or to prevent un-intentional use of an
   organization's internal net.  (It does not provide a security barrier
   in the sense of forwarding filters, but does provide cooperative
   exchange of information on the useability of a net.)

   The data is a domain name, probably the name of the network, although
   it may be the name of another organization.  E.g., the routers that
   are subject to the NSF AUP might add NSF.NET as the descriptor of
   that policy.

3.2.13  Public

   Public (type 13, format 0, class 2 or 3) marks the route as
   consisting in part of a public broadcast medium.  Examples of a
   public medium are direct radio broadcast or a multi-drop cable in
   which other receivers, not associated with the destination may read
   the traffic.  I.e., a TV cable is a public medium, a LAN within an
   organization is not, even if it can be easily wiretapped.

   This is intended for use by cable TV providers to identify routes
   that should not be used for private communications, in spite of the
   attractively high bandwidth being offered.

4.  Procedure

   Routing information arrives in the RAP process from other peers, from
   (local) static route and interface configuration, and from other
   protocols (e.g., RIP).  The RAP process filters out routes that are
   of no interest (too detailed or too "far away" in the topology) and
   builds an internal database of available routes.

   From this database, it selects routes that are to be active and loads
   them into the IP forwarding database.

   It then advertises those routes to its peers, at a greater distance.







Ullmann                                                        [Page 15]