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Network Working Group R. Ullmann Request for Comments: 1476 Process Software Corporation

                                                            June 1993
                RAP: Internet Route Access Protocol

Status of this Memo

 This memo defines an Experimental Protocol for the Internet
 community.  It does not specify an Internet standard.  Discussion and
 suggestions for improvement are requested.  Please refer to the
 current edition of the "IAB Official Protocol Standards" for the
 standardization state and status of this protocol.  Distribution of
 this memo is unlimited.


 This RFC describes an open distance vector routing protocol for use
 at all levels of the internet, from isolated LANs to the major
 routers of an international commercial network provider.

Table of Contents

 1.       Introduction  . . . . . . . . . . . . . . . . . . . 2
 1.1       Link-State and Distance-Vector . . . . . . . . . . 3
 1.2       Terminology  . . . . . . . . . . . . . . . . . . . 3
 1.3       Philosophy . . . . . . . . . . . . . . . . . . . . 3
 2.       RAP Protocol  . . . . . . . . . . . . . . . . . . . 4
 2.1       Command Header Format  . . . . . . . . . . . . . . 4
 2.1.1     Length field . . . . . . . . . . . . . . . . . . . 4
 2.1.2     RAP version  . . . . . . . . . . . . . . . . . . . 5
 2.2       RAP Commands . . . . . . . . . . . . . . . . . . . 5
 2.2.1     No operation . . . . . . . . . . . . . . . . . . . 5
 2.2.2     Poll . . . . . . . . . . . . . . . . . . . . . . . 6
 2.2.3     Error  . . . . . . . . . . . . . . . . . . . . . . 7
 2.2.4     Add Route  . . . . . . . . . . . . . . . . . . . . 8
 2.2.5     Purge Route  . . . . . . . . . . . . . . . . . . . 9
 3.       Attributes of Routes  . . . . . . . . . . . . . . . 9
 3.1       Metric and Option Format . . . . . . . . . . . . .10
 3.1.1     Option Class . . . . . . . . . . . . . . . . . .  10
 3.1.2     Type . . . . . . . . . . . . . . . . . . . . . .  10
 3.1.3     Format . . . . . . . . . . . . . . . . . . . . .  11
 3.2       Metrics and Options  . . . . . . . . . . . . . .  11
 3.2.1     Distance . . . . . . . . . . . . . . . . . . . .  12
 3.2.2     Delay  . . . . . . . . . . . . . . . . . . . . .  12
 3.2.3     MTU  . . . . . . . . . . . . . . . . . . . . . .  12
 3.2.4     Bandwidth  . . . . . . . . . . . . . . . . . . .  12

Ullmann [Page 1] RFC 1476 RAP June 1993

 3.2.5     Origin . . . . . . . . . . . . . . . . . . . . .  12
 3.2.6     Target . . . . . . . . . . . . . . . . . . . . .  13
 3.2.7     Packet Cost  . . . . . . . . . . . . . . . . . .  13
 3.2.8     Time Cost  . . . . . . . . . . . . . . . . . . .  13
 3.2.9     Source Restriction . . . . . . . . . . . . . . .  14
 3.2.10    Destination Restriction  . . . . . . . . . . . .  14
 3.2.11    Trace  . . . . . . . . . . . . . . . . . . . . .  14
 3.2.12    AUP  . . . . . . . . . . . . . . . . . . . . . .  15
 3.2.13    Public . . . . . . . . . . . . . . . . . . . . .  15
 4.       Procedure   . . . . . . . . . . . . . . . . . . .  15
 4.1       Receiver filtering . . . . . . . . . . . . . . .  16
 4.2       Update of metrics and options  . . . . . . . . .  16
 4.3       Aggregation  . . . . . . . . . . . . . . . . . .  17
 4.4       Active route selection . . . . . . . . . . . . .  17
 4.5       Transmitter filtering  . . . . . . . . . . . . .  17
 4.6       Last resort loop prevention  . . . . . . . . . .  18
 5.       Conclusion  . . . . . . . . . . . . . . . . . . .  18
 6.       Appendix: Real Number Representation  . . . . . .  19
 7.       References  . . . . . . . . . . . . . . . . . . .  20
 8.       Security Considerations . . . . . . . . . . . . .  20
 9.       Author's Address  . . . . . . . . . . . . . . . .  20

1. Introduction

 RAP is a general protocol for distributing routing information at all
 levels of the Internet, from private LANs to the widest-flung
 international carrier networks.  It does not distinguish between
 "interior" and "exterior" routing (except as restricted by specific
 policy), and therefore is not as restricted nor complex as those
 protocols that have strict level and area definitions in their
 The protocol encourages the widest possible dissemination of topology
 information, aggregating it only when limits of thrust, bandwidth, or
 administrative policy require it.  Thus RAP permits aggressive use of
 resources to optimize routes where desired, without the restrictions
 inherent in the simplifications of other models.
 While RAP uses IPv7 [RFC1475] addressing internally, it is run over
 both IPv4 and IPv7 networks, and shares routing information between
 them.  A IPv4 router will only be able to activate and propagate
 routes that are defined within the local Administrative Domain (AD),
 loading the version 4 subset of the address into the local IP
 forwarding database.

Ullmann [Page 2] RFC 1476 RAP June 1993

1.1 Link-State and Distance-Vector

 Of the two major classes of routing algorithm, link-state and
 distance vector, only distance vector seems to scale from the local
 network (where RIP is existence-proof of its validity) to large scale
 inter-domain policy routing, where the number of links and policies
 exceeds the ability of each router to map the entire network.
 In between, we have OSPF, an open link state (specifically, using
 shortest-path-first analysis of the graph, hence the acronym)
 protocol, with extensive development in intra-area routing.
 Since distance vector has proven useful at both ends of the range, it
 seems reasonable to apply it to the entire range of scales, creating
 a protocol that works automatically on small groups of LANs, but can
 apply fairly arbitrary policy in the largest networks.
 This helps model the real world, where networks are not clearly
 divided into hierarchical domains with identifiable "border" routers,
 but have many links across organizational structure and over back

1.2 Terminology

 The RAP protocol propagates routes in the opposite direction to the
 travel of datagrams using the routes.  To avoid confusion explaining
 the routing protocol, several terms are distinguished:
 source          where datagrams come from, the source of the
 destination     where datagrams go to, the destination of the
 origin          where routing information originates, the router
                 initially inserting route information into the
                 RAP domain
 target          where routing information goes, the target uses the
                 information to send datagrams

1.3 Philosophy

 Protocols should become simpler as they evolve.

Ullmann [Page 3] RFC 1476 RAP June 1993

2. RAP Protocol

 The RAP protocol operates on TCP port 38, with peers opening a
 symmetric TCP connection between the RAP ports on each system.  Thus
 only one RAP connection exists between any pair of peers.
 RAP is also used on UDP port 38, as a peer discovery method.  Hosts
 (i.e., non-routing systems) may listen to RAP datagrams on this port
 to discover local gateways.  This is in addition to, or in lieu of,
 an Internet Standard gateway discovery protocol, which does not exist
 at this writing.
 The peers then use RAP commands to send each other all routes
 available though the sending peer.  This occurs as a full-duplex
 (i.e., simultaneous) exchange of information, with no acknowledgement
 of individual commands.
 Once the initial exchange has been completed, the peers send only
 updates to routes, new routes, and purge commands to delete routes
 previously offered.
 When the connection is broken, each system purges all routes that had
 been offered by the peer.

2.1 Command Header Format

 Each RAP command starts with a header.  The header contains a length
 field to identify the start of the next packet in the TCP stream, a
 version number, and the code for the command.  On UDP, the length
 field does not appear:  each UDP datagram must contain exactly one
 RAP command and not contain data or padding after the end of the
   0                   1                   2                   3
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
  |        length                                                 |
  |        RAP version            |       command code            |

2.1.1 Length field

 The length is a 32 bit unsigned number specifying the offset in bytes
 from the first byte of the length field of this command packet to the
 start of the length field of the next.  The minimum value is 8.
 There is no specific limit to the length of a command packet;
 implementations MUST be able to at least count and skip over a packet

Ullmann [Page 4] RFC 1476 RAP June 1993

 that is too large and then MAY send an error indication.
 Each version of the protocol will profile what size should be
 considered the limit for senders, and what (larger) size should be
 considered by receivers to mean that the connection is insane:
 either unsynchronized or worse.
 For version 1 of the protocol, senders MUST NOT send command packets
 greater than 16384 bytes.  Receivers SHOULD consider packets that
 appear to be greater than 162144 bytes in length to be an indication
 of an unrecoverable error.
 Note that these limits probably will not be approached in normal
 operation of version 1 of the protocol; receivers may reasonably
 decline to use routes described by 16K bytes of metrics and policy.
 But even the most memory-restricted implementation MUST be able to
 skip such a command packet.

2.1.2 RAP version

 The version field is a 16 bit unsigned number.  It identifies the
 version of RAP used for that command.  Note that commands with
 different versions may be mixed on the same connection, although the
 usual procedure will be to do the serious protocol (exchanging route
 updates) only at the highest version common to both ends of the
 Each side starts the connection by sending a poll command, using the
 highest version supported and continues by using the highest version
 received in any command from the remote.  The response to the poll
 will either be a no-operation packet at that version or an error
 packet at the highest version supported by the remote.
 This document describes version 1 of the RAP protocol.

2.2 RAP Commands

 There five simple RAP commands, described in the following sections.

2.2.1 No operation

 The no operation command serves to reset the poll timer (see next
 section) of the receiver, or (as a side effect) to tell the receiver
 that a particular version is supported.  It never contains option
 specific data and its length is always 8.
 The no operation command is also used in a UDP broadcast to inform
 other systems that the sender is running RAP actively on the network

Ullmann [Page 5] RFC 1476 RAP June 1993

 and is both a possible gateway and a candidate peer.  If this command
 is being sent in response to a broadcast poll, it should be sent only
 to the poller.
 A RAP process may send such broadcasts in a startup sequence, or it
 may persist indefinitely to inform other systems coming on line.  If
 it persists, it must not send them more than once every 10 minutes
 (after the initial startup sequence).  If the RAP process sends polls
 as part of its startup, it must not persist in sending them after the
 startup sequence.
 The command code for no-operation is always 0, regardless of RAP

2.2.2 Poll

 A poll command packet requests that the other side transmit either a
 no-operation packet, or some other packet if sent without delay.
 (i.e., receivers MUST NOT delay a response to a poll by waiting for
 some other packet expected to be queued soon.)
 The poll command code is always 1, regardless of version, and the
 length is always 8.
 Each RAP implementation runs a timer for each connection, to ensure
 that if the other system becomes unreachable, the connection will be
 closed or reset.  The timers run at each end of the connection are
 independent; each system is responsible for sending polls in time to
 reset its own timer.
 The timer MUST be reset (restarted) on the receipt of any RAP packet,
 regardless of whether the version or command code is known.
 In normal operation, if route updates are being sent in both
 directions, polls may not be necessary for long periods of time as
 the timers are continually reset.  When the connection is quiescent,
 both timers will typically get reset as a result of the side with the
 shorter timer doing a poll, and then getting a no-operation in
 response.  RAP implementations MUST NOT be dependent in any way on
 the size or existence of the remote timer.
 An implementation that has access to information from the TCP layer,
 such as the results of TCP layer keepalives, MAY use this instead of
 or in addition to a timer.  However, the use of TCP keepalives is
 discouraged, and this procedure does not ensure that the remote RAP
 process is alive, only that its TCP is accepting data.  Thus a
 failure mode exists that would not exist for active RAP layer polls.

Ullmann [Page 6] RFC 1476 RAP June 1993

 The timer MUST be implemented, SHOULD be configurable in at least the
 range 1 to 10 minutes on a per-peer basis, and MAY be infinite
 (disabled) by explicit configuration.
 On UDP, a system (router or non-routing host) may send RAP polls to
 attempt to locate candidate peers or possible gateways.  Such a
 system must not persist in sending polls after its startup sequence,
 except that a system which actually has offered traffic for non-local
 destinations, and has no available gateways, may continue to send
 periodic polls to attempt to acquire a gateway.

2.2.3 Error

 The error packet is used to report an error, whether fatal, serious
 or informational.  It includes a null terminated text description in
 ISO-10646-UTF-1 of the condition, which may be useful to a human
 administrator, and SHOULD be written to a log file.  (The machine is
 not expected to understand the text.)
 Errors are actual failures (in the interpretation of the receiver) to
 use the correct syntax and semantics of the RAP protocol itself, or
 "failure" of the receiver to implement a version of the protocol.
 Other conditions that may require action on the part of the peer
 (such as purging a route) are given their own command codes.
   0                   1                   2                   3
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
  |        length                                                 |
  |        RAP version (1)        |       command code (2)        |
  |        error code (0)  [reserved]                             |
  |        description                                            |
  +                                                               +
  |                       ...                                     |
 The RAP system receiving an Error packet MUST NOT regard it as fatal,
 and close the connection or discard routes.  If the sending system
 desires the condition to be fatal (unrecoverable), its proper action
 is to close the connection.  This requirement is to prevent the kind
 of failure mode demonstrated by hosts that killed off TCP connections
 on the receipt of ICMP Host-Unreachable notifications, even when the
 condition is transient.  We do not want to discourage the reporting
 of errors, in the way that some implementations avoided sending ICMP
 datagrams to deal with overly sensitive hosts.

Ullmann [Page 7] RFC 1476 RAP June 1993

 An error packet MUST NOT be sent in response to something that is (or
 might be) an error packet itself.  Subsequent versions of RAP should
 keep the command code point (2) of the error packet.

2.2.4 Add Route

 The add route command offers a route to the receiving peer.  As noted
 later, it MUST be a route actually loaded into the forwarding
 database of the offering peer at the time the add route is sent.
   0                   1                   2                   3
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
  |        length                                                 |
  |        RAP version (1)        |       command code (3)        |
  |        distance               |     (MBZ)     |     mask      |
  |        destination network                                    |
  +                                                               +
  |                    ...                                        |
  |        route identifier                                       |
  +                                                               +
  |                    ...                                        |
  |        metrics and options    ....                            |
 The add route command describes a single offered route, with the
 metrics and other options (such as policies) associated with the
 Distance is a simple count of the hops to the RAP process (or other
 routing process) that originated the route, incremented every time
 the route is forwarded.  Its initial value may be greater than 1,
 particularily for a route that is administratively configured to
 aggregate routes for a large network or AD.  It may also enter the
 RAP routing domain for the first time with a non-zero distance
 because the route originated in RIP, OSPF, or BGP; if so, the
 distance carried in that protocol is copied into the RAP route.
 The mask is a count of the number of bits of prefix ones in the
 binary representation of the network mask.  Non-contiguous masks are
 not supported directly.  (The destination restriction option may be
 used to give another, non-contiguous, mask; the header mask would
 then describes the number of contiguous ones.)

Ullmann [Page 8] RFC 1476 RAP June 1993

 The route identifier is a 64 bit value that the IP forwarding module
 on the sending host can use to rapidly identify the route and the
 next hop for each incoming datagram.  The host receiving the route
 places this identifier into the forward route ID field of the
 datagrams being sent to this host.
 The route ID is also used to uniquely identify the route in the purge
 route operation.

2.2.5 Purge Route

 The purge route command requires that the receiving peer delete a
 route from its database if in use, and requires that it revoke that
 route from any of its peers to whom it has offered the route.  This
 command should preferably be sent before the route is deleted from
 the sending peer's forwarding database, but this is not (cannot be)
 required; it should be sent without delay when the route is removed.
 The command code is 4.  The format is the same as add route without
 any added metrics or options.
 If the route identifier in a purge route command is zero, the command
 requires the deletion of all routes to the destination previously
 offered by this peer.

3. Attributes of Routes

 There are a rather large number of possible attributes.
 Possibilities include both metrics, and other options describing for
 example policy restrictions and alterations of proximity.  Any
 particular route will usefully carry only a few attributes or none at
 all, particularily on an infrastructure backbone.  A reasonable
 policy for the routers that make up a backbone might be to strip all
 attributes before propagating routes (discarding routes that carry
 attributes with class indications prohibiting this), and then adding
 (for example) an AUP attribute to all routes propagated off of the
 backbone.  A less drastic method would be to simply prefer routes
 with no restrictions, but still propagate a route with restrictions
 if no other is available.
 Most options can occur more than once in a route if there is any
 sensible reason to do so.

Ullmann [Page 9] RFC 1476 RAP June 1993

3.1 Metric and Option Format

 Each metric or option for a route begins with a 32 bit header:
   0                   1                   2                   3
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
  |   length      | C |  format   |           type                |
  |        option data                 ...        |   padding     |
                 RAP Option/Metric Header Format

A description of each field:

 length       length of the option or metric
 C            option class, see below
 format       data format
 type         option type identifier
 data         variable length

3.1.1 Option Class

 This field tells implementations what to do with routes containing
 options or metrics they do not understand.  No implementation is
 required to implement (i.e., understand) any given option or metric
 by the RAP specification itself, except for the distance metric in
 the RAP header.
 0        use, propagate, and include this option unmodified
 1        use, propagate, but do not include this option
 2        use this route, but do not propagate it
 3        discard this route
 Note that class 0 is an imperative:  if the route is propagated, the
 option must be included.
 Class and type are entirely orthogonal, different implementations
 might use different classes for the same option or metric.

3.1.2 Type

 The type code identifies the specific option or metric.  The codes
 are part of the option descriptions following.

Ullmann [Page 10] 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

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.)

Ullmann [Page 11] 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.

Ullmann [Page 12] 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
 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.

Ullmann [Page 13] 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

Ullmann [Page 14] 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] RFC 1476 RAP June 1993

  1. ——————————————————————
         [incoming routes]
         [proximity filtering/aggregation]       [static routes]
                 |                                  |
                 v                                  v
         [route database]  --->  [selected active routes]
                 ^                       |
                 |                       v
         [RIP, etc. routes]      [output filtering]
                                 [routes advertised]
  1. ——————————————————————

4.1 Receiver filtering

 The first step is to filter out offered routes that are too "far
 away" or too specific.  The filter consists of a maximum distance at
 which a route is considered usable for each possible (contiguous)
 Routers that need universal connectivity must either pass through the
 filter all routes regardless of distance (short of "infinity"), and
 use aggregation to reduce them, or have a default route to a router
 that does this.
 The filter may be adjusted dynamically to fit limited resources, but
 if the filter is opened, i.e., made less restrictive, there may be
 routes that have already been offered and discarded that will never
 become available.

4.2 Update of metrics and options

 The process then updates any metrics present on the route to reflect
 the path to the RAP peer.  MTU and bandwidth are minimized, delay and
 cost are added in.  Distance is incremented.  Any unknown options
 cause class-dependent processing:  discarding the option (class 2) or
 route (3), or marking the route as non-propagatable (1).
 Policy options that are known may cause the route to be discarded at
 this stage.

Ullmann [Page 16] RFC 1476 RAP June 1993

4.3 Aggregation

 The next step is to aggregate routes that are subsetted by other
 routes through the same peer.  This should not be done automatically
 in every possible case.  The more information that is propagated, the
 more effective the use of forward route identifiers is likely to be,
 particularily in the case of aggregating into a default route.
 In general, a route can be included in an aggregate, and not
 propagated further, if it is through the same peer (next hop) and has
 a smaller distance metric than the containing route.  (Thus datagrams
 will always travel "downhill" as they take more specific routes.)
 The usual case of aggregation is that routes derived from interface
 configurations on the routers from which they originated are subsumed
 into routes offered by routers explicitly configured to route for an
 entire network, area, or AD.  If the larger area becomes partitioned,
 unaggregatable routes will appear (as routes outside the area become
 the shortest distance routes) and traffic will flow around the
 Attributes of routes, particularily policy options, may prevent
 aggregation and may result in routes simply being discarded.
 Some information about aggregation also needs to be represented in
 the forwarding database, if the route is made active:  the router
 will need to make a decision as to which forward route identifier to
 use for each datagram arriving on the active route.

4.4 Active route selection

 The router selects those routes to be entered into the IP forwarding
 database and actively used to forward datagrams from the set of
 routes after aggregation, combined with routes derived from other
 protocols such as RIP.  This selection may be made on any combination
 of attributes and options desired by local policy.

4.5 Transmitter filtering

 Finally, the RAP process must decide which routes to offer to its
 peers.  These must be a subset of the active routes, and may in turn
 be a selected subset for each peer.  Arbitrary local policies may be
 used in deciding whether or not to offer any particular route to a
 given peer.
 However, the transmitter must ensure that any datagram filters are
 represented in the offered route, so that the peer (and its peers)
 will not route into a black hole.

Ullmann [Page 17] RFC 1476 RAP June 1993

4.6 Last resort loop prevention

 RAP is designed to support many different kinds of routing selection
 algorithms, and allow them to interact to varying extents.  Routes
 can be shared among administrations, and between systems managed with
 more or less sophistication.
 This leaves one absolute requirement:  routing loops must be self-
 healing, regardless of the algorithm used on each host.  There are
 two caveats:
   1.  A loop will not fix itself in the presence of an error that
       continually recurs (thus re-generating the loop)
   2.  The last resort algorithm does not provide rapid breaking of
       loops, only eventual breaking of them even in the absence of
       any intervention by (human) intelligence.
 The algorithm relies on the distance in the RAP route header.  This
 count must be updated (i.e., incremented by one) at each router
 forwarding the route.
 Routers must also impose some limit on the number of hops permitted
 in incoming routes, discarding any routes that exceed the limit.
 This limit is "infinity" in the classic algorithm.  In RIP, infinity
 is 15, much too low for general inter-domain routing.
 In RAP, infinity is defined as 2z + i, where z is the number of zero
 bits in the mask (as described previously) and i is a small number
 which MUST be configurable.
 Note that RAP depends on the last resort algorithm, "counting to
 infinity," much less than predecessors such as RIP.  Routes in the
 RAP domain will usually be purged from the net as the purge route
 command is flooded without the delays typical of periodic broadcast
 algorithms.  Only in some cases will loops form, and they will be
 counted out as fast as the routing processes can exchange the

5. Conclusion

 Unlike prior routing protocols, RAP is designed to solve the entire
 problem, from hands-off autoconfiguration of LAN networks, to
 implementing the most complex policies of international carriers.  It
 provides a scaleable solution to carry the Internet forward to a
 future in which essentially all users of data transmission use IP as
 the fabric of their networks.

Ullmann [Page 18] RFC 1476 RAP June 1993

6. Appendix: Real Number Representation

 Real numbers are represented by a one byte exponent, e, in excess-128
 notation, and a fraction, f, in excess-8388608 notation, with the
 radix point at the right.  (I.e., the "fraction" is actually an
 e is thus in the range 0 to 255, representing exponents (powers of 2)
 in the range 2^-128 to 2^127.
 f is in the range 0 to 16777215, representing numbers from -8388608
 to 8388607
 The value is (f-8338608) x 2^(e-128)
 The real number is not necessarily normalized, but a normalized
 representation will, of course, provide more accuracy for numbers not
 exactly representable.
 Example code, in C:
 #include <math.h>
 typedef struct {
         unsigned e : 8;
         unsigned f : 24;
         } real;
 double a;          /* input value */
 real r;
 double b;          /* output value */
 double d;
 int e32;
 /* convert to real: */
 d = frexp(a, &e32);
 r.e = e32+105;
 r.f = (int)(d*8388608.0) + 8388608;
 /* convert back: */
 b = ldexp((double)r.f - 8388608.0, (int)r.e - 128);

Ullmann [Page 19] RFC 1476 RAP June 1993

7. References

 [ISO3166]   International Organization for Standardization.  Codes
             for the Representation of Names of Countries.  ISO
             3166, ISO, 1988.
 [ISO4217]   International Organization for Standardization.  Codes
             for the representation of currencies and funds.  ISO
             4217, ISO, 1981.
 [RFC791]    Postel, J., "Internet Protocol - DARPA Internet Program
             Protocol Specification", STD 5, RFC 791, DARPA,
             September 1981.
 [RFC1058]   Hedrick, C., "Routing Information Protocol", STD 34,
             RFC 1058, Rutgers University, June 1988.
 [RFC1247]   Moy, J., "OSPF Version 2", RFC 1247, Proteon, Inc.,
             July 1991.
 [RFC1287]   Clark, D., Chapin, L., Cerf, V., Braden, R., and
             R. Hobby, "Towards the Future Internet Architecture",
             RFC 1287, MIT, BBN, CNRI, ISI, UCDavis, December 1991.
 [RFC1338]   Fuller, V., Li, T., Yu, J., and K. Varadhan,
             "Supernetting: an Address Assignment and Aggregation
             Strategy", RFC 1338, BARRNet, cicso, Merit, OARnet,
             June 1992.
 [RFC1475]   Ullmann, R., "TP/IX: The Next Internet", RFC 1475,
             Process Software Corporation, June 1993.

8. Security Considerations

 Security issues are discussed in sections 3.2.9 and 3.2.12.

9. Author's Address

 Robert Ullmann
 Process Software Corporation
 959 Concord Street
 Framingham, MA 01701
 Phone: +1 508 879 6994 x226
 Email: Ariel@Process.COM

Ullmann [Page 20]

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