Network Working Group                                        W A Simpson
Internet Draft                                                Daydreamer
                          SIP System Discovery

Status of this Memo

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Abstract

   This document specifies ICMP messages for the identification and
   location of adjacent SIP systems.  This is intended to replace ARP,
   ICMP Router Advertisement, ICMP Redirect, ICMP Information, ICMP
   Mask, and OSPF Hello in the SIP environment.  There are also elements
   of the OSI ES-IS and IS-IS Hello.

1.  Terminology

   The following terms have a precise meaning when used in this
   document:
   system          a device that implements the Internet Protocol, IP [9].

   intermediate-system
                   a system that forwards datagrams, as specified in [2].
                   Often called a router or gateway.  This does not include
                   systems that, though capable of forwarding, have that
                   capability turned off.  Nor does it include systems that
                   do forwarding only as required to obey Source Route
                   options.

   end-system      any system that is not acting as an intermediate-system.
                   Often called a host.

   dumb            the minimal implementation.  This is not meant in a
                   perjorative sense.  It is intended that every mechanism
                   be defined in such a way that it is implementable on a
                   minimal system.

   smart           an improved implementation, possibly requiring more
                   internal resources, while using less external resources.

   multicast       unless otherwise qualified, means the use of either IP
                   multicast [4] or IP broadcast [6] service.

   link            a communication facility or medium over which systems
                   can communicate at the link layer; that is, the protocol
                   layer immediately below IP.  The term "physical network"
                   has sometimes been used (imprecisely) for this.
                   Examples of links are LANs (possibly bridged to other
                   LANs), wide-area store-and-forward networks, satellite
                   channels, and point-to-point links. circuits.

   multicast link  a link over which IP multicast or IP broadcast service
                   is supported.  This includes broadcast media such as
                   LANs and satellite channels, single point-to-point
                   links,
                   circuits, and some store-and-forward networks such as
                   SMDS networks [8].

   interface       a system's attachment point to a link.  It is possible
                   (though unusual) for a system to have more than one
                   interface to the same link.  Interfaces are uniquely
                   identified by an identifier; a single interface may have
                   more than one such identifier.

   multicast interface
                   an interface to a multicast link; that is, an interface
                   to a link over which IP multicast or IP broadcast
                   service is supported.

   identifier      uniquely identifies each interface; a single interface
                   may have more than one such identifier.

   primary identifier
                   uniquely identifies each system; only one such
                   identifier is used, to simplify discovery of neighbors.

   subnet          either a single link of a subnetted IP network [7] or
                   a single non-subnetted link.

   prefix          the part of an identifier which may be used for routing
                   to a particular subnet, defined by logically ANDing with
                   its assigned subnet mask.  More than one subnet prefix
                   may identify the same link.

   zone            the part of a special identifier which indicates a
                   unique subnet within an administrative domain.

   neighbor        having an IP address identifier belonging to the same subnet.

2.  Criteria

   Historically, the methods for discovery of the next hop next-hop evolved
   separately from those for location of neighbors and auto-
   configuration of systems.  With the advent of SIP, the old techniques
   must be re-implemented, usually due to larger field sizes.
   Unfortunately, older implementations frequently did not take proper
   care in differentiating existing variable field lengths, version
   numbers, and new types of messages.  Therefore, the techniques used
   for SIP are required to be distinguishable from previous versions.

   None of the current protocols are readily extensible.  While some
   have the ability to change in simple terms, such as larger addresses,
   none were designed to add new kinds of information to be carried in
   the same packet.

   This can be viewed is an opportunity to design a uniform and coherent
   method for accomplishing these goals, rather than a liability.

   Through prior experience, the following criteria were established, in
   order of relative importance.  It is understood that many of these
   criteria may conflict, and require numerous tradeoffs.

   Multicast support

      All SIP systems are required to support multicast.

      This is the primary technique for distinguishing the new messages.
      Older systems will ignore multicast messages at the link layer.

      There are numerous other advantages to using multicast for the new
      messages.  In particular, when compared to broadcast, reduced
      overhead for processing messages which are not ultimately intended
      for the local system.

      Not all media supports multicast.  Since multicast is directly
      supported by the SIP header, this technique will work even when
      using link-layer broadcast, or link-layer unicast to each
      recipient.

   Reduced net traffic

      Currently, there are separate packets sent for media address
      resolution, router discovery, and the Hello protocols for the
      various routing algorithms.  Since much of the same information is
      contained in each of these packets, it would be helpful to combine
      the functions in a single packet where possible.

      Also, the most common next hop next-hop resolution protocol, the Address
      Resolution Protocol (ARP), requires an additional two packets at
      the beginning of each connection.  The Request is sent, a Reply is
      received, and then the first datagram can be sent to the next hop. next-hop.
      This causes a significant amount of traffic, and considerable
      latency in establishing a connection.

      Several alternative methods were proposed:

      1)    The ISO solution (ES-IS) eliminates some of these problems.
            Each end-system and intermediate-system sends Hellos on a
            periodic basis.  Each  Every system must remember all of the media
            addresses for the other systems on the local network.  This
            does eliminate the latency of ARP, at the expense of many
            additional packets sent on a regular basis, and a large
            amount of storage overhead in each system.

      Two alternative solutions were proposed:

      1)    Sending the

      2)    The first packet destined for an unknown system may be sent
            to the all-systems multicast, or to a media multicast based
            on a hash function of the destination.  The appropriate
            system accepts the packet, and sends a redirect indicating
            the appropriate media address to be used for future packets.
            This reduces the traffic from 3 to 2 packets at the
            beginning of a connection, and eliminates the latency, as
            the discovery packet sent is also the data packet.  However,
            the  The
            destination identifer in the network header will be unicast,
            while the media address will be multicast, possibly
            resulting in some confusion.  Intermediate-systems multicast.  Intermediate-
            systems would require extra intelligence to recognize those
            packets destined beyond the local link, while multi-homed end-
            systems
            end-systems require that capability already.  Also, this
            method is not extensible to include other information useful
            in mobile environments.

      2)

      3)    Using advertisements for the (fewer) intermediate systems,
            and an ARP-like protocol for those end-system connections
            that are on the local media.  For those packets which are
            clearly destined off the local media, the packet can be sent
            directly to the appropriate intermediate system.  When most
            of the traffic is between systems that are not on the same
            local media, this is very efficient.  When most of the
            traffic is between end-systems on the local media (client-
            server), the extra discovery packets will be rare.

      The
            intelligence solution that is split between detailed here is a combination of the intermediate best
      features of the preceding techniques.

      Intermediate-systems advertise their locations.  When an
      intermediate-system needs the location of an end-system, it
      requests the location of the end-system, and end
            systems.

      The latter the end-system
      replies.  Knowledge about end-systems is concentrated in the solution
      intermediate-systems, but only for the systems that are actually
      communicating.

      End-systems send all datagrams directly to the intermediate-
      systems.  If there is detailed here.  However, a more direct path to the
      former end-system,
      because it is not mutually exclusive, and could directly accessible on the local link or another
      intermediate-system would be used in parallel. more appropriate, the intermediate-
      system issues a redirect.

      Also, by carrying media addresses within the advertisements and
      redirect packets, a further ARP-like query/response can be avoided
      entirely.

   Low storage overhead

      It is desirable that systems require as little storage overhead as
      possible.  In particular, mobile systems often have significantly
      reduced processing power and memory.

      An end-system should need only retain information for those end-systems
      with which it is directly communicating.  To improve efficiency
      and reduce net traffic, this

      This design requires the sufficient storage of in an end-system for
      information about at least one intermediate-system.  In addition,
      storage is required for at least one location of each intermediate-system service
      (such as a domain name server) which is heard. used.

      An intermediate-system may require more processing power and
      memory, since participation
      memory.  Participation in routing protocols requires the knowledge
      of every neighboring intermediate-system.  It

      When subnet prefix-routing is not
      expected that in the general case use, it is not necessary for an
      intermediate-system to determine the location of every an end-system
      will be maintained.

   Autoconfiguration

      This design handles initial self-identification and propagation of
      changes in identification.  Other aspects of configuration are
      specified elsewhere, such as loading the operating system and
      environment, and additional facilities and servers
      until traffic for
      registration.

   Mobility support

      This the end-system arrives.  If prefix-routing is sometimes considered a subset of
      not used, particularly in radio and mobile environments, the above, as related to
      dynamically changing addresses while moving.  Other systems
      location of each reachable end-system must be notified of the changes.

      In addition, continuously
      retained.

   Auto-configuration

      It would be highly desirable that the "hidden transmitter" problem is considered (you
      can hear another system, it can't hear you, but there is connection procedures for a path to
      configuring a third new system which it can hear, completing are reduced to the circuit).  This
      is not well supported in any minimal set of "plug
      it in, turn on the past protocols.

      Although basic support for mobility is provided, descriptions of
      additional facilities power, and servers are specified elsewhere.

   Black hole detection

      In determining whether run".

      -  Each system, or more precisely each interface, should be
         assigned an identifier, within the next hop number space assigned to the
         local subnet.

      -  Each system is still available,
      there is should be assigned a basic tradeoff between frequent queries and resources
      used.  This design trades fewer queries against more information name within each query and response.

      Explicit holding times are used to limit the exposure to black
      holes. local domain.
         The times may name, and the associated identifiers, should be dynamically shortened by registered
         in the responsible local domain name server.

      -  The system when a resource is critical, or when should discover the external routes provided by the
         intermediate-systems attached to the local subnet, so that it
         can exchange packets with remote systems.

      -  The system is actively
      moving.

   Media independence

      There are many instances where system discovery is accomplished
      differently over different media, such as point-to-point versus
      broadcast versus Large Public Data Networks.  This design places
      the system discovery above the network layer, where it enjoys
      greater independence.  It also encompasses media level redirects
      between logical subnets on the same physical media.

      There are difficulties with carrying media addresses within
      packets, especially in should discover the presence location of multi-media bridges.
      Rather than allowing translation by bridges in the path, this
      design exercizes control at the destination system, servers that it
         needs for configuration, loading, dumping, printing, and requires
      all such media addresses to be in canonical form,

   Optimal route determination

      This is essentially a superset of next hop discovery, combined other
         services.

      In evaluating previous experience with resource reservation and possible policy considerations, and autoconfiguration
      procedures, the ability to redirect traffic under changing conditions.  This following constraints were determined:

      1)    It is not well supported in any of possible to embed an IEEE-802 component within
            every SIP identifier, since the current protocols.

      To balance remaining prefix would be
            too small for global routing.  Using the 48-bit IEEE-802
            number to identify one system overhead against within a local network traffic, this design
      attempts to adapt that is
            not designed to accomodate more than a continuum of system capabilities.  Dumb
      end-systems few hundred systems
            is considerable overkill.  It may simply send packets to a default intermediate-
      system, and be redirected worthwhile to use the correct next hop by more capable
      intermediate-systems.  Smarter end-systems learn sufficient
      information
            address during initial configuration.

      2)    Random identifier assignments are to make informed choices.

   Simplicity

      All be avoided.  They do
            not scale well to large networks, are difficult to track and
            manage, and lead to administrative confusion.  Relying on
            broadcast collision resolution procedures for avoiding
            duplicate assignments results in conflicts when systems
            occupy partitioned subnets, or are frequently powered down
            or taken off-line.

      3)    Reassignment of identifiers should be transparent to the above desires,
            human users.  In particular, renumbering, and they want to keep it simple, too.
      This design reduces the number assignment of packet types which must be
      supported in
            alias identifiers as a pure SIP system, and reduces mobile system moves should be
            automatic.

      4)    End-system users should not be concerned with routing
            prefixes, or the number of systems routing methods extant on the local
            network.  When used, such prefixes should be automatically
            determined, and dynamic changes should propagate
            automatically.

      5)    It is important to allow users to choose a system name which recognize
            is memorable and respond comfortable to each type. them.  The extensions are
      designed with 32 and 64 bit boundaries for efficient processing.

3.  Design name should be
            automatically registered, and Use

   This proposal describes two packets, not much different from those
   already deployed.  These familiar forms are re-packaged changes to join
   common functions into the same packet to reduce traffic, and are
   designed to associated
            identifers should be more extensible maintained automatically.

      This design handles initial self-identification and propagation of
      changes in the future.

   In order to foster media independence, the packets are part identification.  Other aspects of ICMP,
   which allows configuration, such
      as loading the protocols to be used over broadcast, multicast,
   partial-mesh, operating system and point-to-point media. environment, and additional
      facilities and servers for registration, are specified elsewhere.

   Mobility support

      This is similar to sometimes considered a subset of the
   positioning above, as related to
      dynamically changing addresses while moving.  Other systems must
      be notified of ES-IS.

   The Where-Are-You solicitation the changes.

      In addition, the "hidden transmitter" problem is used considered (you
      can hear another system, it can't hear you, but there is a path to find other systems, and
   associated resources and services.  General solicitations are sent
   when
      a third system interface which it can hear, completing the circuit).  This
      is initialized.  Specific solicitations are
   sent when one system is ready to communicate with another particular
   system.

   The I-Am-Here advertisement is the answer to the Where-Are-You
   solicitation.  Advertisements are also sent on a periodic basis to
   indicate special resources and services.  Periodic advertisements
   from a few commonly requested systems result not well supported in less traffic than
   repeated solicitations from many systems.

   Each advertisement includes a lifetime field, specifying the maximum
   length any of time that the advertisements are to be considered valid in the absence past protocols.

      Although basic support for mobility is provided, descriptions of further advertisements.  This ensures that other
   systems eventually forget about systems that become unreachable,
      additional facilities and servers are specified elsewhere.

   Black hole detection

      In determining whether that is because the next-hop system has failed, or it no longer
   provides the advertised service.

   Each message contains "optional" extensions, designed to allow
   flexibility and extensibility.

   One of the common extensions is the media address.  Each message
   contains enough information to return a reply directly to the sender,
   without additional location traffic.

   Another common extension still available,
      there is a list of the intermediate-systems which
   have been heard.  This allows intermediate-systems to build a map of
   the paths between intermediate-systems, and basic tradeoff between intermediate-
   systems frequent queries and end-systems. resources
      used.  This is designed design trades fewer queries against more information
      within each query and response.

      Explicit holding times are used to limit the exposure to black
      holes.  The times may be used dynamically shortened by most
   commonly deployed routing protocols.  This also solves the "hidden
   transmitter" problem, responsible
      system when a resource is critical, or when used together with the well-known link-
   state class of routing protocols.

   Several methods of routing system is actively
      moving.

   Media independence

      There are supported.

3.1.  System Identification

   Zone numbers may be combined many instances where system discovery is accomplished
      differently over different media, such as point-to-point versus
      broadcast versus Large Public Data Networks.  This design places
      the system discovery above the network layer, where it enjoys
      greater independence.  It also encompasses media level redirects
      between multiple logical subnets on the same physical media.

      There are difficulties with carrying media addresses within
      packets, especially in the last hop presence of multi-media bridges.
      Rather than allowing translation by bridges in the path, this
      design exercizes control at the destination system, and requires
      all such media address addresses to make
   a locally significant identifier.  This is useful for initial
   configuration and local communication within an administrative
   domain.  These identifiers may be routed in canonical form,

   Optimal route determination

      This is essentially a similar manner superset of next-hop discovery, combined
      with resource reservation and possible policy considerations, and
      the ability to
   prefix-routed subnets.

   Prefix-routed subnet identifiers are redirect traffic under changing conditions.  This
      is not well supported for addressing
   globally connected networks in any of the metro/provider addressing model.
   The prefix part past protocols.

      To balance system overhead against network traffic, this design
      attempts to adapt to a continuum of each identifier system capabilities.  Dumb
      end-systems may simply send packets to a default intermediate-
      system, and be used redirected to locate the subnet
   link for correct next-hop by more capable
      intermediate-systems.  Smarter end-systems learn sufficient
      information to make informed choices.

   Simplicity

      All of the final hop. above desires, and they want to keep it simple, too.
      This is design reduces the routing technique with number of packet types which we
   have greatest familiarity.

   End-Point identifiers, or any other globally unique identifier, may
   be used with future routing techniques.  A mobile system may must be
   treated as having an end-point identifier when it appears
      supported in a
   prefix-routed subnet, since it will not have pure SIP system, and reduces the same prefix as other number of systems in the subnet.

   Facilities
      which recognize and respond to each type.  The extensions are provided for exchange of redirects
      designed with 32 and translation
   between the various 64 bit boundaries for efficient processing.

3.  Design Overview

   This proposal describes two packets, not much different from those
   already deployed.  These familiar forms of identifiers.

3.2.  Intermediate System Advertisements

   Each intermediate-system peridodically sends the I-Am-Here message are re-packaged to
   advertise its forwarding capability.  End-systems discover join
   common functions into the
   location of their neighboring intermediate-systems simply by
   listening for same packet to reduce traffic, and are
   designed to be more extensible in the advertisements.  This eliminates future.

   In order to foster media independence, the need for
   manual configuration packets are part of intermediate-system addresses ICMP,
   which allows the protocols to be used over broadcast, multicast,
   partial-mesh, and point-to-point media.  This is
   independent similar to the
   positioning of any specific routing protocol. ES-IS.

   The advertisements include such important information as the media
   address Where-Are-You solicitation is used to access the system, find other subnets directly accessible
   through the intermediate-system, systems, and neighboring intermediate-systems
   heard.

   Identifiers

      Each intermediate-system advertisement includes one or more
      identifier fields.  These indicate the identifiers which
   associated resources and services.  General solicitations are
      presently in use for each sent
   when a system interface of the intermediate-system.

   Prefix Size

      Each advertised identifier includes a prefix size field. is initialized.  Specific solicitations are
   sent when one system is ready to communicate with another particular
   system.

   The
      value ranges from 0 I-Am-Here advertisement is the answer to 62, the Where-Are-You
   solicitation.  Advertisements are also sent on a periodic basis to
   indicate special resources and indicates services.  Periodic advertisements
   from a few commonly requested systems result in less traffic than
   repeated solicitations from many systems.

   Each advertisement includes a lifetime field, specifying the number maximum
   length of bits time that the advertisements are to be considered valid in
   the
      Identifier which define absence of further advertisements.  This ensures that other
   systems eventually forget about systems that become unreachable,
   whether that is because the prefix mask for system has failed, or it no longer
   provides the link.  A value advertised service.

   Each message contains "optional" extensions, designed to allow
   flexibility and extensibility.

   One of
      zero indicates an end-point identifier.  When the value common extensions is not
      zero, the identifier may be used to discern zone or prefix-routed
      subnet mapping.

   Preference media address.  Each advertised identifier includes a preference field.  This is
      used message
   contains enough information to choose return a default intermediate-system for reply directly to the first-hop
      when no other information sender,
   without additional location traffic.

   Another common extension is available (for a particular
      destination, list of the end-system has not yet intermediate-systems which
   have been redirected or
      configured heard.  This allows intermediate-systems to use build a specific intermediate-system).  The end-system map of
   the paths between intermediate-systems, and between intermediate-
   systems and end-systems.  This is expected designed to choose from those intermediate-systems that have be used by most
   commonly deployed routing protocols.  This also solves the highest preference level for the best prefix-routing match.
      When there is no match, or prefix-routing is not in use, the
      preference value is "hidden
   transmitter" problem, when used alone. together with the well-known link-
   state class of routing protocols.

   Several methods of routing are supported.

3.1.  System Identification

   Zone

      A network administrator can configure intermediate-system
      preference levels Zone is defined to encourage or discourage the use be a collection of particular
      intermediate-systems links which may be
      accessed as the default first hop.  The use of
      separate preferences per prefix allows the choice same next-hop.  A Zone is usually a single link,
      or a collection of different
      intermediate-systems for each prefix, when there are bridged links.  When a single intermediate-
      system is connected to multiple
      prefixes in use for the same link.  This point-to-point links, these links
      may be useful where
      multiple organizations share resources.

      [I am not sure this works when there are multiple identifiers per
      end-system interface.] collected into a single zone.

      The Zone number is a fixed size.  The preference value 0 is not the same as the "metric" only used in many
      routing protocols.  It to
      indicate the local zone.  The values 1 through 255 indicate each
      zone within an administrative domain.

      Zone numbers may be combined with an interface media address to
      make a locally significant identifier.  This is used only by end-systems in determining useful for initial
      configuration and local communication within the default first-hop, rather than by intermediate-systems administrative
      domain.  These identifiers may be routed in
      choosing a link for transit traffic. similar manner to
      prefix-routed subnets.

      The values are not additive.
      Therefore, generation of these local identifiers depends upon the range
      availability of values is smaller, and a higher value
      indicates a higher preference.

3.2.1.  Constants

      MAX_INITIAL_ADVERTISEMENTS        3 transmissions

      MAX_INITIAL_ADVERT_INTERVAL      16 seconds

      MAX_RESPONSE_DELAY                2 seconds

3.2.2.  Configuration

   An intermediate-system MUST allow the following variables to be
   configured by system management.  Default values are specified which
   make it unnecessary to re-configure these variables in most cases.

   For each interface:

   MaxAdvertisementInterval

      The maximum time (in seconds) allowed between sending
      intermediate-system advertisements from the interface.  Must be no
      less than 4 seconds and no greater than 1800 seconds.

      Default: 600 seconds

   MinAdvertisementInterval

      The minimum time (in seconds) allowed between sending unsolicited
      intermediate-system advertisements from the interface.  Must be no
      less than 1 second and registered unique number, such as an IEEE-802
      number.  When there is no greater than MaxAdvertisementInterval.

      Default: 0.75 * MaxAdvertisementInterval

   AdvertisementLifetime

      The value (in seconds) IEEE-802 number to be placed found anywhere in
      the Lifetime field of
      intermediate-system advertisements sent from machine, such as when the interface.  Must machine is connected exclusively to
      point-to-point links, an external link-level mechanism MUST be no less than MaxAdvertisementInterval and no greater than 9000
      seconds.

      Default: 3 * MaxAdvertisementInterval

   For each of
      used to negotiate a unique identifier.  Such a mechanism is beyond
      the identifiers scope of each interface:

   Advertise this document.

   Prefix

      A flag indicating whether or not the identifier Prefix is similar to be
      advertised.

      Default: TRUE

   PreferenceLevel

      The preferability of the interface as a default intermediate-
      system choice, relative to other intermediate-system interfaces
      serving the same prefix on the same link.

      Values are Zone, in the range 0 to 255.  Higher values mean more
      preferable.  The minimum value 0 is used to indicate that the
      identifier, even though it identifies a collection
      of links which may be advertised, is not to be used by
      neighboring end-systems accessed as the same next-hop.  The Prefix
      may indicate a default intermediate-system address.

      Default: 1

   It is useful to configure single zone, a collection of zones, an identifier with entire
      administrative domain, or a preference level collection of 0
   (rather than simply setting its Advertise flag administrative domains.

      The Prefix is variable in size.  The Prefix Size ranges from 1 to FALSE) when
   advertisements
      62.  The value of 63 cannot be used, since at least 2 bits of the
      SIP 64-bit identifier are being used for "black hole" detection.  In
   particular, an intermediate-system that is reserved to identify a particular
      system.

      Prefix-routed subnet identifiers are supported for addressing
      globally connected networks in the metropolitan and/or provider
      addressing models.

   End-Point Identifiers

      End-Point identifiers, or any other globally unique identifier,
      may be used to reach only
   specific destinations could advertise with future routing techniques.  An End-Point
      Identifier is indicated as having a preference level Prefix Size of 0 (so
   that neighboring end-systems 0.  A mobile
      system may be treated as having an End-Point Identifier when it
      appears in a prefix-routed subnet, since it will not use it have the same
      prefix as a default
   intermediate-system other systems in the subnet.

   Facilities are provided for reaching arbitrary destinations) exchange of redirects and a non-
   zero lifetime (so that neighboring end-systems that have been
   redirected or configured to use it can detect its failure by timing
   out translation
   between the reception various forms of its advertisements).

   It has been suggested that, when identifiers.

3.2.  Multicast Support

   Every SIP system MUST join the preference level of an
   identifier has not been explicitly configured, an intermediate-system
   could set it according to all-systems multicast group on all
   interfaces on which the metric of system supports multicast.

   Every SIP intermediate-system MUST also join the intermediate-system's
   "default route" (if it has one), rather than defaulting as suggested
   above.  Thus, an intermediate-system with a better metric for its
   default route would advertise all-routers
   multicast group.

   Every SIP end-system which offers a higher preference level particular service MUST also join
   the multicast group for its
   identifier.  (Note that routing metrics that are encoded such that
   "lower is better" would have to be inverted before being used service.  Intermediate-systems do not
   join the service multicast group, as
   preference levels in intermediate-system advertisement messages.)
   Such their services are discovered
   under a strategy might reduce separate process.

4.  Intermediate-System Discovery

   Before an end-system can send datagrams beyond its directly attached
   link, it must discover the amount location of redirect traffic at least one operational
   intermediate-system on some
   links by making it more likely that an end-system's first choice for
   reaching an arbitrary destination link.  This is accomplished through
   intermediate-system advertisement messages.

   The intermediate-system advertisements also the best choice.

   On the other hand, redirect traffic is rarely a significant load on a serve to indicate zone
   and subnet prefixes for each link, and there are some cases where such a strategy would result in
   more redirect traffic (on links from which the most frequently chosen
   destinations are best reached via intermediate-systems other than the
   one to establish neighbor
   relationships with other intermediate-systems.

   Each intermediate-system periodically sends the best default route).  Also, since I-Am-Here message to
   advertise its forwarding capability.  End-systems and intermediate-
   systems discover the routing algorithms
   learn location of their neighboring intermediate-systems from the advertisements,
   and the default routes are learned from the routing algorithms, intermediate-
   systems simply by listening for the
   calculated preference may be unstable from time to time. advertisements.  This
   document makes no recommendation concerning this issue, and
   implementors are free to try such a strategy, as long as they also
   support static eliminates
   the need for manual configuration of preference levels as specified above.

3.2.3.  Implementation

   Every SIP intermediate-system MUST implement Intermediate-System
   Advertisements. addresses
   and is independent of any specific routing protocol.

   The intermediate-system joins advertisement messages do not constitute a routing protocol,
   although they might be used by a routing protocol to build a map.
   They enable systems to discover the all-routers multicast group on all
   interfaces on existence of neighboring
   intermediate-systems, but not necessarily which the intermediate-system supports multicast.

   The term "advertising interface" refers
   is best to any functioning and
   enabled interface reach a particular destination.  If a system chooses a
   poor intermediate-system for a particular destination, it should
   receive a redirect from that has at least one identifier whose configured
   Advertise flag is TRUE.

   From each advertising interface, intermediate-system.

   However, the system transmits periodic advertisements containing the following values:

   -  In the Destination Address field often contain sufficient information to
   make a good choice of the first-hop.  This may be important for choosing
   among intermediate-systems which are participating in a security
   group or policy-based routing.

4.1.  Solicitations

   Every SIP header: the all-
      systems multicast, with end-system MUST implement Intermediate-System Solicitation.

   When any end-system starts up, it MUST send the scope set Where-Are-You
   solicitation to local.

   -  In the Source Address field of the SIP header: prompt the primary
      identifier advertisement of the system.  The same identifier intermediate-systems.
   This is also used for all
      interfaces.

   -  In by the Code field intermediate-systems to construct a map of
   accessible end-systems, to discover partitions in the ICMP header: 2 for Intermediate-System
      Advertisement.

   -  In the Lifetime field: the interface's configured
      AdvertisementLifetime.

   -  For each of that interface's identifiers whose Advertise flags local subnet,
   and to support mobile systems.

   If (and only if) no advertisements from neighboring intermediate-
   systems are
      TRUE, forthcoming, the Routing-Information extension.

   -  For each of that system's other interface's identifiers which have
      not already been included through prefix subsumption, end-system MAY retransmit the Other-
      Identifier extension.

   -  For each service advertisement Where-
   Are-You a small number of times, but then MUST desist from sending
   more solicitations.

   Any intermediate-systems that is offered, subsequently start up, or has been
      learned from another advertisement, the Service-Information
      extension.

   -  For each intermediate-system advertisement that has been heard,
      the System-Heard extension.

   -  For interfaces which are were not point-to-point links, the Media-
      Access extension.

   In the unlikely event
   discovered because of packet loss or temporary link partitioning, are
   eventually discovered by reception of their periodic (unsolicited)
   advertisements.  Links that not all extensions fit in a single
   advertisement, as constrained suffer high packet loss rates or frequent
   partitioning are accommodated by increasing the MTU rate of
   intermediate-system advertisements, rather than increasing the link, multiple
   advertisements number
   of solicitations that end-systems are sent, permitted to send.

4.1.1.  Constants

      MAX_SOLICITATIONS                 3 transmissions

      MAX_SOLICITATION_DELAY            1 second

      SOLICITATION_INTERVAL             3 seconds

4.1.2.  Implementation

   The intermediate-system solicitation is sent to the all-routers
   multicast, with each except the last containing as many
   extensions as can fit.

      CONTROVERSIAL:

      When an intermediate-system discovers that it scope set to local.

   An end-system is receiving required to transmit up to MAX_SOLICITATIONS Where-
   Are-You messages from any of its own
      advertisements, that interfaces after any of the
   following events:

   -  The interface is an indication that it has more than one
      interface on same link.  The initialized at system MUST choose only one
      advertising startup time.

   -  The interface for each link.  Identifiers associated with
      the remaining interfaces on the same link are indicated with the
      Other-Identifier extension.  Redirect is used to move specific
      traffic to the alternate interfaces.

      An intermediate-system MAY proxy for the identifers of other
      systems, using the Other-Identifier extension.  This SHOULD only
      be used when the intermediate-system is translating to another
      network-layer protocol format.

   The advertisements are not strictly periodic. reinitialized after a temporary interface failure
      or after being temporarily disabled by system management.

   -  The interval between
   subsequent transmissions is randomized system has its forwarding capability turned off by system
      management.

   If a system chooses to reduce the probability send a solicitation after one of
   synchronization with the advertisements from other intermediate-
   systems on the same link.  This is done by maintaining a separate above
   events, it should delay transmission interval timer for each advertising interface.  Each
   time an advertisement is sent from an interface, that interface's
   timer is reset to a uniformly-distributed random value amount of time
   between the
   configured MinAdvertisementInterval 0 and MaxAdvertisementInterval.
   Expiration of the timer causes the next advertisement MAX_SOLICITATION_DELAY.  This serves to be sent, and alleviate
   congestion when many systems start up on a new random value to be chosen. link at the same time,
   such as might happen after recovery from a power failure.

   It is recommended that intermediate-systems systems include some unique
   value, such value (such as one
   of their interface or link-layer addresses, identifiers) in the seed used to
   initialize their pseudo-random number generators.  Although the
   randomization range is configured specified in units of seconds, the actual
   randomly-chosen values value should not be in units of whole seconds, but
   rather in units of the highest available timer resolution.

   For the first few advertisements sent from an interface (up to
   MAX_INITIAL_ADVERTISEMENTS),

   The small number of retransmissions of a solicitation, which are
   permitted if the randomly chosen interval no advertisement is
   greater than MAX_INITIAL_ADVERT_INTERVAL, the timer received, should be set sent at
   intervals of SOLICITATION_INTERVAL seconds, without further
   randomization.

   Upon receiving a valid advertisement from any intermediate-system
   subsequent to
   MAX_INITIAL_ADVERT_INTERVAL instead.  Using this smaller interval for one of the initial advertisements increases above events, the likelihood of an
   intermediate-system being discovered quickly when it first becomes
   available, in system MUST NOT send any
   solicitation on that interface (even if none have been sent yet)
   until the presence next time one of possible packet loss.

   In addition to the periodic unsolicited advertisements, an
   intermediate-system sends advertisements in response to valid
   solicitations above events occurs.

4.1.3.  Receipt

   An end-system MUST silently discard any received on Intermediate-System
   Solicitation messages.

   An intermediate-system MUST silently discard any of its advertising interfaces.  If the
   solicitation does received
   Intermediate-System Solicitation messages that do not contain any Intermediate-Systems-Heard
   extension, and satisfy the time since
   following validity checks:

   -  ICMP Checksum is correct.

   -  ICMP length (derived from the previous advertisement payload length) is greater
   than MAX_INITIAL_ADVERT_INTERVAL, 16 or more
      octets.

   -  Source Address is either 0 or the identifier of a neighbor (an
      identifier that matches one of the intermediate-system's own
      identifiers on the arrival interface under the prefix mask
      associated with that identifer, or the zone associated with that
      interface).

4.2.  Advertisements

   Every SIP intermediate-system MUST
   multicast an advertisement from that interface. implement Intermediate-System
   Advertisements.

   The interface's
   interval timer is reset to a new random value, intermediate-system advertisements include such important
   information as with unsolicited
   advertisements.  The response MUST be delayed for a small random
   interval not greater than MAX_RESPONSE_DELAY, in order the media address to prevent
   synchronization with access the system, other responding intermediate-systems, subnets
   directly accessible through the system, services available through
   the system, and to
   allow multiple closely-spaced solicitations to be answered with a
   single advertisement.

   An interface may become an advertising interface at times other than
   system startup, as a result of recovery from an interface failure or
   through actions of system management such as:

   -  enabling the interface, if it had been administratively disabled
      and it has neighboring intermediate-systems heard.

   Identifiers

      Each intermediate-system advertisement includes one or more
      identifier fields.  These indicate the identifiers whose Advertise flag is TRUE,

   -  enabling SIP forwarding capability (changing which are
      presently in use for each interface of the system intermediate-system.

   Zone

      Each advertised identifier includes a zone field.  The value
      ranges from an
      end-system 0 to 255, and indicates a subnet number which is
      unique to an intermediate-system), when the interface administrative domain.  A value of zero indicates
      that no zone number has one
      or more identifiers whose Advertise flag been assigned.  It may be combined with an
      interface media address to make a locally significant identifier.

      If all advertised zone values are zero, then zone routing is TRUE,

   -  setting not
      available beyond that link.  This does not prevent the Advertise flag of one or more use of the interface's
      locally significant identifiers to TRUE (or adding a new identifier for communication with a TRUE
      Advertise flag), when previously other
      systems on the interface had no local link.

   Prefix Size

      Each advertised identifier
      whose Advertise flag was TRUE.

   In such cases, includes a prefix size field.  The
      value ranges from 0 to 62, and indicates the intermediate-system must commence transmission number of
   periodic advertisements on bits in the new advertising interface, limiting
      Identifier which define the first few advertisements to intervals no greater than
   MAX_INITIAL_ADVERT_INTERVAL.  In prefix mask for the case link.  A value of
      zero indicates an end-system becoming
   an intermediate-system, the system must also join the all-routers
   multicast group on all interfaces on which end-point identifier.  When the intermediate-system
   supports multicast (whether or value is not they are advertising interfaces).

   An interface
      zero, the identifier may also cease to be an advertising interface, through
   actions of system management such as:

   -  shutting down the system,

   -  administratively disabling the interface,

   -  disabling SIP forwarding capability (changing the system from an
      intermediate-system used to an end-system),

   -  setting the Advertise flags of discern prefix-routed subnet
      mapping.

      If all of the interface's identifiers advertised prefix values are zero, then subnet prefix-
      routing is not in use on that link.

   Preference

      Each advertised identifier includes a preference field.  This is
      used to FALSE.

   In such cases, the intermediate-system SHOULD transmit choose a final
   multicast advertisement on default intermediate-system for the interface, identical first-hop
      when the end-system has not yet been redirected or configured to its previous
   transmission, but with
      use a Lifetime field of zero.  In the case of an specific intermediate-system becoming an end-system, the system must also
   depart for a particular destination.
      The end-system is expected to choose from those intermediate-
      systems that have the all-routers multicast group on all interfaces on
   which highest preference level for the intermediate-system supports multicast (whether best
      prefix-routing match.  When there is no match, or prefix-routing
      is not they
   had been advertising interfaces).

   When in use, the Advertise flag of one preference value is used alone.

      A network administrator can configure intermediate-system
      preference levels to encourage or more discourage the use of an interface's identifiers
   are set to FALSE by system management, but there remain other
   identifiers on that interface whose Advertise flags are TRUE, particular
      intermediate-systems as the
   intermediate-system SHOULD send a single multicast advertisement
   containing only those identifiers whose Advertise flags were set to
   FALSE, with a Lifetime field default first-hop.  The use of zero.

3.3.  Intermediate System Discovery

   Before an end-system can send datagrams beyond its directly attached
   link, it must discover
      separate preferences per prefix allows the location choice of at least one operational
   intermediate-system on that different
      intermediate-systems for each prefix, when there are multiple
      prefixes in use for the same link.  This is accomplished through the
   intermediate-system advertisement messages described above. may be useful where
      multiple organizations share resources.

      [I am not sure how this works when there are multiple identifiers
      per end-system interface.]

      The advertisement messages do preference value is not constitute a routing protocol.
   They enable systems to discover the existence of neighboring
   intermediate-systems, but not necessarily which intermediate-system same as the "metric" used in many
      routing protocols.  It is best to reach a particular destination.  If a system chooses a
   poor intermediate-system for a particular destination, it should
   receive a redirect from that intermediate-system.

   However, used only by end-systems in determining
      the advertisements often contain sufficient information to
   make default first-hop, rather than by intermediate-systems in
      choosing a good choice of first hop.  This may be important link for choosing
   among intermediate-systems which transit traffic.  The values are participating in not additive.
      Therefore, the range of values is smaller, and a security
   group or policy-based routing. higher value
      indicates a higher preference.

      It should be understood that preference levels learned from
      intermediate-system advertisements do not affect any system's
      cached route entries.  For example, if a system has been
      redirected to use a particular intermediate-system to reach a
      specific destination, it continues to use that intermediate-system
      for that destination, even if it discovers another intermediate-system intermediate-
      system identifier with a higher preference level.  Preference
      levels influence the choice of intermediate-system only for a
      destination for which there is no cached or configured route, or
      whose cached route points to an intermediate-system that is
      subsequently determined to be unreachable.

3.3.1.  Configuration

   The Host Requirements -- Communication Layers [1], Section 3.3.1.6,
   specifies that each end-system must support a configurable list of
   default intermediate-system identifiers.  The purpose of the

4.2.1.  Constants

      MAX_INITIAL_ADVERTISEMENTS        3 transmissions

      MAX_INITIAL_ADVERT_INTERVAL      16 seconds

      MAX_RESPONSE_DELAY                2 seconds

4.2.2.  Configuration

   An intermediate-system discovery messages is to eliminate MUST allow the need following variables to
   configure that list.  On links for be
   configured by system management.  Default values are specified which intermediate-system
   discovery is administratively disabled,
   make it MAY continue unnecessary to re-configure these variables in most cases.

   For each interface:

   MaxAdvertisementInterval

      The maximum time (in seconds) allowed between sending
      intermediate-system advertisements from the interface.  Must be
   necessary no
      less than 4 seconds and no greater than 1800 seconds.

      Default: 600 seconds

   MinAdvertisementInterval

      The minimum time (in seconds) allowed between sending unsolicited
      intermediate-system advertisements from the interface.  Must be no
      less than 1 second and no greater than MaxAdvertisementInterval.

      Default: 0.75 * MaxAdvertisementInterval

   AdvertisementLifetime

      The value (in seconds) to configure be placed in the default Lifetime field of
      intermediate-system list in advertisements sent from the interface.  Must
      be no less than MaxAdvertisementInterval and no greater than 9000
      seconds.

      Default: 3 * MaxAdvertisementInterval

   For each
   end-system.

   Each entry in of the list contains (at least) identifiers of each interface:

   Advertise

      A flag indicating whether or not the following configurable
   variables:

   RouterAddress

      An identifier of a default intermediate-system. is to be
      advertised.

      Default: (none) TRUE

   PreferenceLevel

      The preferability of the RouterAddress interface as a default intermediate-
      system choice, relative to other intermediate-system interfaces
      serving the same prefix on the same link.

      Values are in the range 0 to 255.  Higher values mean more
      preferable.  The Host Requirements
      RFC does not specify how this minimum value 0 is used to indicate that the
      identifier, even though it may be advertised, is not to be encoded.  The values used here are defined above. by
      neighboring end-systems as a default intermediate-system address.

      Default: 255

3.3.2.  Implementation

   To process an Intermediate-System Advertisement, an end-system scans
   the list of Routing-Information extensions contained in it.  For each
   identifier, the end-system does the following:

   -  If the prefix size 1

   It is not zero, the useful to configure an identifier and prefix size are
      compared against any identifiers associated with the interface on
      which the message was received.  If there is a match, the
      interface prefix size is set preference level of 0
   (rather than simply setting its Advertise flag to the advertised prefix size.

   -  If the identifier is not already present in the end-system's FALSE) when
   advertisements are being used for "black hole" detection.  In
   particular, an intermediate-system list, a new entry that is added to the list,
      containing the identifier along with its accompanying be used to reach only
   specific destinations could advertise a preference
      level, level of 0 (so
   that neighboring end-systems will not use it as a default
   intermediate-system for reaching arbitrary destinations) and a timer initialized non-
   zero lifetime (so that neighboring end-systems that have been
   redirected or configured to use it can detect its failure by timing
   out the Lifetime value from the
      advertisement.

   -  If the identifier is already present in the end-system's
      intermediate-system list as a result reception of a previously-received
      advertisement, its advertisements).

   It has been suggested that, when the preference level is updated and its timer is
      reset of an
   identifier has not been explicitly configured, an intermediate-system
   could set it according to the value in the newly-received advertisement.

   -  If the identifier is already present in metric of the end-system's
      intermediate-system list intermediate-system's
   "default route" (if it has one), rather than defaulting as a result of system configuration, no
      change is made to its preference level.  There is no timer
      associated with a configured identifier.

   -  If a Media-Access extension is present, the suggested
   above.  Thus, an intermediate-system
      list is updated with the location information.

   Whenever the timer expires in any entry that was created as a result
   of better metric for its
   default route would advertise a received advertisement, higher preference level for its
   identifier.  (Note that entry is discarded.

      Note routing metrics that any intermediate-system identifiers acquired from the
      "Gateway" subfield of the vendor extensions field of a BOOTP
      packet [11] are considered encoded such that
   "lower is better" would have to be configured identifiers; they are
      assigned the default inverted before being used as
   preference level of 255, and they do not have
      an associated timer.

      Note further that any identifier found levels in intermediate-system advertisement messages.)
   Such a strategy might reduce the "giaddr" field amount of a
      BOOTP packet [3] identifies a BOOTP forwarder which redirect traffic on some
   links by making it more likely that an end-system's first choice for
   reaching an arbitrary destination is not
      necessarily also the best choice.

   On the other hand, redirect traffic is rarely a SIP intermediate-system; significant load on a
   link, and there are some cases where such an identifier should
      not be installed a strategy would result in
   more redirect traffic (on links from which the end-system's default intermediate-system
      list.

   To limit most frequently chosen
   destinations are best reached via intermediate-systems other than the storage needed for
   one with the best default intermediate-system list,
   an end-system MAY choose not to store all of route).  Also, since the intermediate-system
   identifiers discovered via advertisements.  The end-system SHOULD
   discard those identifiers with lower preference levels in favor routing algorithms
   learn of
   those with higher levels.  It is desirable to retain more than one
   default intermediate-system identifier in neighboring intermediate-systems from the list; if advertisements,
   and the current
   choice of default intermediate-system is discovered to be down, routes are learned from the
   end-system routing algorithms, the
   calculated preference may immediately choose another default intermediate-system
   without having be unstable from time to wait for the next advertisement time.  This
   document makes no recommendation concerning this issue, and
   implementors are free to arrive.

   Any intermediate-system identifier advertised with try such a preference level strategy, as long as they also
   support static configuration of zero preference levels as specified above.

4.2.3.  Implementation

   The intermediate-system advertisement is not sent to be used by the end-system as default intermediate-
   system identifier.  Such an identifier may be omitted from all-systems
   multicast, with the
   default intermediate-system list, unless its timer is being use as a
   "black-hole" detection mechanism.

3.4.  Initial Intermediate-System Solicitations

   When scope set to local.

   The term "advertising interface" refers to any end-system starts up, which functioning and
   enabled interface that has at least one identifier whose configured
   Advertise flag is not otherwise sending TRUE.

   From each advertising interface, the system MUST transmit periodic
   I-Am-Here messages.

      CONTROVERSIAL:

      When an intermediate-system discovers that it is receiving its own
      advertisements, that is an indication that it has more than one
      interface on same link.  The system MUST instead send the Where-
   Are-You solicitation to begin discovery of intermediate-systems.

   If (and choose only if) no advertisements from neighboring intermediate-
   systems are forthcoming, the end-system may retransmit one
      advertising interface for each link.  Identifiers associated with
      the Where-
   Are-You a small number of times, but then must desist from sending
   more solicitations.

   Any systems that subsequently start up, or that were not discovered
   because of packet loss or temporary remaining interfaces on the same link partitioning, are eventually
   discovered by reception of their periodic (unsolicited)
   advertisements.  Links that suffer high packet loss rates or frequent
   partitioning are accommodated by increasing the rate of
   advertisements, rather than increasing indicated with the number of solicitations
   that systems are permitted
      Other-Identifier extension.  Redirect is used to move specific
      traffic to send.

3.4.1.  Constants

      MAX_SOLICITATIONS                 3 transmissions

      MAX_SOLICITATION_DELAY            1 second

      SOLICITATION_INTERVAL             3 seconds

3.4.2.  Implementation

   Every SIP system MUST implement the System Solicitation.

   Every SIP system joins alternate interfaces.

      An intermediate-system MAY proxy for the all-systems multicast group on all
   interfaces on which identifers of other
      systems, using the system supports multicast.

   An end-system Other-Identifier extension.  This SHOULD only
      be used when the intermediate-system is permitted (but not required) translating to transmit up another
      network-layer protocol format.

   The advertisements are not strictly periodic.  The interval between
   subsequent transmissions is randomized to
   MAX_SOLICITATIONS solicitation messages from any of its interfaces
   after any reduce the probability of
   synchronization with the following events:

   -  The interface advertisements from other intermediate-
   systems on the same link.  This is initialized at system startup time.

   -  The interface done by maintaining a separate
   transmission interval timer for each advertising interface.  Each
   time an advertisement is reinitialized after sent from an interface, that interface's
   timer is reset to a temporary interface failure
      or after being temporarily disabled by system management.

   -  The system has its SIP forwarding capability turned off by system
      management.

   -  The system has its SIP service capability turned off by system
      management.

   From each such interface, the system transmits solicitations
   containing the following values:

   -  In the Destination Address field of the SIP header: the all-
      routers multicast, with the scope set to local.

   -  In the Source Address field of the SIP header: the primary
      identifier associated with that interface.  It MAY contain zero if
      the system has not yet determined an identifier for the interface.

   -  In the Code field of uniformly-distributed random value between the ICMP header: 2 for Intermediate-System
      Solicitation.

   -  For each
   configured MinAdvertisementInterval and MaxAdvertisementInterval.
   Expiration of that system's interface identifiers other than the
      primary identifier, the Other-Identifier extension, with timer causes the
      prefix size set to zero.

   -  For each intermediate-system next advertisement that has been heard,
      the System-Heard extension.

   -  For interfaces which are not point-to-point links, the Media-
      Access extension.

   If a system chooses to send a solicitation after one of the above
   events, it should delay transmission for be sent, and
   a new random amount of time
   between 0 and MAX_SOLICITATION_DELAY.  This serves value to alleviate
   congestion when many systems start up on a link at the same time,
   such as might happen after recovery from a power failure. be chosen.

   It is recommended that systems intermediate-systems include some unique value, such value
   (such as one of their interface or link-layer identifiers, addresses) in the seed
   used to initialize their pseudo-random number generators.  Although
   the randomization range is specified configured in units of seconds, the actual
   randomly-chosen value values should not be in units of whole seconds, but
   rather in units of the highest available timer resolution.

   The small number of retransmissions of a solicitation, which are
   permitted

   For the first few advertisements sent from an interface (up to
   MAX_INITIAL_ADVERTISEMENTS), if no advertisement the randomly chosen interval is received,
   greater than MAX_INITIAL_ADVERT_INTERVAL, the timer should be sent at
   intervals set to
   MAX_INITIAL_ADVERT_INTERVAL instead.  Using this smaller interval for
   the initial advertisements increases the likelihood of SOLICITATION_INTERVAL seconds, without further
   randomization.

   Upon receiving an
   intermediate-system being discovered quickly when it first becomes
   available, in the presence of possible packet loss.

   An interface may become an advertising interface at times other than
   system startup, as a valid advertisement result of recovery from any intermediate-system
   subsequent to one an interface failure or
   through actions of system management such as:

   -  enabling the above events, interface, if it had been administratively disabled
      and it has one or more identifiers whose Advertise flag is TRUE,

   -  enabling SIP forwarding capability (changing the system MUST NOT send any
   solicitation on that from an
      end-system to an intermediate-system), when the interface (even if none have been sent yet)
   until has one
      or more identifiers whose Advertise flag is TRUE,

   -  setting the next time Advertise flag of one or more of the above events occurs.  The
   intermediate-system advertisements (described above) contain interface's
      identifiers to TRUE (or adding a
   summary of all of new identifier with a TRUE
      Advertise flag), when previously the available information for interface had no identifier
      whose Advertise flag was TRUE.

   In such cases, the link.

3.5.  Service Advertisments

   Each system offering one intermediate-system must commence transmission of
   periodic advertisements on the special configuration services
   detailed below, whether an end-system or intermediate-system,
   periodically sends new advertising interface, limiting
   the I-Am-Here message first few advertisements to advertise its service
   availablility.  All systems discover intervals no greater than
   MAX_INITIAL_ADVERT_INTERVAL.  In the location case of these services
   simply by listening for an end-system becoming
   an intermediate-system, the advertisements.  This eliminates system must also join the need
   for manual configuration, periodic probes, and special handling of
   certain packet types by intermediate-systems.

   The learned service information is included in any neighboring
   intermediate-system advertisements.  In this fashion, the
   intermediate-system advertisements provide a summary of all-routers
   multicast group on all available
   network services, and pass information beyond the link where the
   advertisement originated.  This results in a reduction of network
   traffic when compared to interfaces on which the broadcast or multicast of service
   discovery requests/replies over a wide area.

   The service advertisements use similar configuration constants and
   variables as intermediate-system advertisements.

3.5.1.  Implementation

   Services offered by intermediate-systems
   supports multicast (whether or not they are included in the
   intermediate-system advertisements described above.

   From each advertising interface, interfaces).

   An interface MAY also cease to be an end-system transmits periodic
   advertisements containing the following values: advertising interface, through
   actions of system management such as:

   -  In  shutting down the Destination Address field of system,

   -  administratively disabling the interface,

   -  disabling SIP header: the all-
      systems multicast, with forwarding capability (changing the scope set system from an
      intermediate-system to local. an end-system),

   -  In  setting the Source Address field Advertise flags of all of the SIP header: the primary
      identifier associated with that interface.

   - interface's identifiers
      to FALSE.

   In such cases, the Code intermediate-system SHOULD transmit a final
   multicast advertisement on the interface, identical to its previous
   transmission, but with a Lifetime field of the ICMP header: 1 for End-System
      Advertisement.

   - zero.  In the Lifetime field: the interface's configured
      AdvertisementLifetime.

   -  For each case of that system's interface identifiers other than the
      primary identifier, an
   intermediate-system becoming an end-system, the Other-Identifier extension, with system must also
   depart from the
      prefix size set to zero.

   -  For each service advertisement that is offered, the Service-
      Information extension.

   -  For each intermediate-system advertisement that has been heard,
      the System-Heard extension.

   -  For all-routers multicast group on all interfaces on
   which are not point-to-point links, the Media-
      Access extension.

   In the unlikely event that intermediate-system supports multicast (whether or not all extensions fit in a single
   advertisement, as constrained by they
   had been advertising interfaces).

   When the MTU Advertise flag of the link, multiple
   advertisements are sent, with each except the last containing as many
   extensions as can fit.

   End-system service advertisements one or more of an interface's identifiers
   are sent using the same periodicity
   as intermediate-system advertisements.

   Advertising interfaces set to FALSE by system management, but there remain other
   identifiers on that interface whose Advertise flags are established and terminated in TRUE, the same
   manner as
   intermediate-system advertisements.

   When any system ceases to offer an advertised service, the system SHOULD transmit send a final single multicast advertisement on the interface,
   identical
   containing only those identifiers whose Advertise flags were set to its previous transmission, but
   FALSE, with a Lifetime field of zero.

3.6.  Service Discovery

   Because

   In addition to the service periodic unsolicited advertisements, an
   intermediate-system MUST send advertisements learned in response to valid
   advertisements or solicitations received on any of its advertising
   interfaces.  If the advertisement or solicitation does not contain
   any System-Heard extension, and the time since the previous
   advertisement is greater than MAX_INITIAL_ADVERT_INTERVAL, the
   intermediate-system MUST multicast an advertisement from a link that
   interface.

   Whenever these response advertisements are
   promulgated by sent, the intermediate-system advertisements, no further
   effort is required to solicit service advertisements.

   [what advertisement
   MUST be delayed for a small random interval not greater than
   MAX_RESPONSE_DELAY, in order to do when there are no IS Adverts]

   The services advertised are limited primarily to configuration.  The
   locations of prevent synchronization with other facilities may
   responding intermediate-systems, and to allow multiple closely-spaced
   solicitations to be learned from these basic
   servers.

3.6.1.  Domain Name Service

   Before answered with a system can communicate single advertisement.  The
   interface's interval timer is reset to a new random value, as with another system, it must learn
   unsolicited advertisements.

4.2.4.  Receipt

   All systems MUST silently discard any received Intermediate-System
   Advertisement messages that system's identifiers and location.  The Domain Name System (DNS) do not satisfy the following validity
   checks:

   -  ICMP Checksum is usually used for this purpose.

   Therefore, correct.

   -  ICMP length (derived from the location of at payload length) is 16 or more
      octets.

   -  At least one DNS server must be learned.
   This is accomplished through Routing-Information extension.

   -  For interfaces which are not point-to-point links, the service advertisement messages
   described above.

   In Media-
      Access extension.

4.3.  Processing Advertisements

   Every intermediate-system saves the past, this was accomplished information contained in the
   advertisements, in order to respond to future requests.  Any other
   action on receipt of such messages by reading a list an intermediate-system (for
   example, as part of servers from a (possibly remote) configuration file at startup time.  Some systems
   discovered servers by sending periodic probes "peer discovery" process) is beyond the scope
   of this document.

   An end-system saves the information contained in the advertisements,
   in order to determine the first-hop when sending datagrams.  First-
   hop determination is elaborated in a broadcast or
   multicast address.  Both of these methods have serious drawbacks. subsequent section.

4.3.1.  Configuration files

   The Host Requirements -- Communication Layers [1], Section 3.3.1.6,
   specifies that each end-system must be maintained manually (a significant
   administrative burden when ther are large numbers support a configurable list of systems), and
   are unable to track dynamic changes in DNS availability.  Periodic
   probes are restricted from using recursion (see Host Requirements --
   Application and Support [2], Section 6.1.3.2), and are thus limited
   to information about the local domain.

   In practice, only systems which are users or stub resolvers
   default intermediate-system identifiers.  The purpose of the
   DNS will use
   intermediate-system discovery messages is to eliminate the DNS server advertisements.  Full-Service resolvers
   MUST need to
   configure that list.  On links for which intermediate-system
   discovery is administratively disabled, it MAY continue to be manually configured
   necessary to ensure a heirarchy of
   believability within the network.

3.6.2.  Self-Configuration Service

   Before a system can communicate with another system, it must learn
   its own identity.  The Bootstrap Protocol (BOOTP) is frequently used
   for this purpose.

   Therefore, configure the location of at least one BOOTP server must be learned.
   This is accomplished through default intermediate-system list in each
   end-system.

   Each entry in the service advertisement messages
   described above.

   In list contains (at least) the past, this was accomplished by ad hoc passing following configurable
   variables:

   RouterAddress

      An identifier of BOOTP
   requests by routers.  This method has several serious drawbacks.
   Presence a default intermediate-system.

      Default: (none)

   PreferenceLevel

      The preferability of the feature cannot be relied upon.  It is not of much use
   for mobile, roving or portable systems.

3.7.  Prefix Discovery

   which define RouterAddress as a default intermediate-
      system choice, relative to other intermediate-system interfaces
      serving the same prefix mask for on the same link.  The Host Requirements
      RFC does not specify how this value ranges from 0 is to 62.

   The value of 63 cannot be used, since at least 2 bits encoded.  The values
      used here are reserved
   for a valid host portion defined above.

      Default: 255

4.3.2.  Implementation

   To process an Intermediate-System Advertisement, an end-system scans
   the list of Routing-Information extensions contained in it.  For each
   identifier, the identifier.

   Each IS, when configured, or address assigned.  The prefixes are
   assigned by hand.  Can ask DNS or BOOTP.

   Unlike previous practice, an end-system does the following:

   -  If the prefix size MUST NOT be
   preconfigured.  Instead, is not zero, the identifier and prefix size is dynamically learned from
   matching are
      compared against any identifiers associated with the intermediate-system advertisements, as described
   in Intermediate-System Discovery above.

   more than one prefix interface on same link.

3.8.  Zone Discovery

   A Zone is defined to be a collection of systems
      which may be
   aggregated as the same next hop.  A Zone may be as small as message was received.  If there is a single
   link segment, or as large as an entire administrative domain.

3.8.1.  Abstraction Algorithm

   The zones are learned from match, the intermediate-system advertisements,
   which contain
      interface prefix size is set to the necessary advertised prefix information.

3.9.  Next Hop Determination

   Within size.

   -  If the identifier is not already present in the end-system's
      intermediate-system list, a directly attached link, each system must be able new entry is added to locate
   other systems the list,
      containing the identifier along with which it desires its accompanying preference
      level, and a timer initialized to communicate.  This is
   accomplished using the Where-Are-You and I-Am-Here messages described
   below.  This is independent of any specific media.

   When Lifetime value from the system has heard one or more intermediate-system
   advertisements, it first uses these advertisements to determine if
      advertisement.

   -  If the desired system identifier is directly accessible on already present in the local link.

   When an end-system has not heard any end-system's
      intermediate-system
   advertisements, it is assumed that all end-systems are only
   accessible on the local link.

3.9.1.  Configuration

3.9.2.  Implementation

3.9.3.  Examples list as a result of Use

       Simple case -- J to K on the same fully-connected link.

           J sends the Where-Are-You (which contains its own media address)
           to all-systems.  K sends the I-Am-Here (which contains a previously-received
      advertisement, its own
           media address) directly to J.  At this point, they both know
           that they can talk directly to each other, without regard to
           subnet.

       Routed case -- J to K not on the same fully-connected link.

           If no resource reservation or policy routing preference level is desired, J
           simply sends updated and its packets directly timer is
      reset to the "preferred" router that
           it has learned from value in the Advertisements. newly-received advertisement.

   -  If there is a better
           router for the first hop, that router sends the I-Am-Here
           redirect to J, but never-the-less forwards the packet.

           In identifier is already present in the presence end-system's
      intermediate-system list as a result of RR or PR, J sends the Where-Are-You system configuration, no
      change is made to its preference level.  There is no timer
      associated with a configured identifier.

   -  If a Media-Access extension is present, the
           "preferred" router that it has learned from the Advertisements.
           That router always returns the I-Am-Here (even if the correct
           hop intermediate-system
      list is itself), which contains updated with the requested RR or PR status location information.  J then sends its packets to

   Whenever the first hop router timer expires in any entry that was created as determined a result
   of a received advertisement, that entry is discarded.

      Note that any intermediate-system identifiers acquired from the I-Am-Here.
      "Gateway" subfield of the vendor extensions field of a BOOTP
      packet [11] are considered to be configured identifiers; they are
      assigned the default preference level of 255, and they do not have
      an associated timer.

      Note further that any identifier found in the "giaddr" field of a
      BOOTP packet [3] identifies a BOOTP forwarder which is not
      necessarily a SIP intermediate-system; such an identifier should
      not be installed in the end-system's default intermediate-system
      list.

   To limit the storage needed for the default intermediate-system list,
   an end-system MAY choose not to store all of the intermediate-system
   identifiers discovered via advertisements.  The end-system SHOULD
   discard those identifiers with lower preference levels in favor of
   those with higher levels.  It is desirable to retain more than one
   default intermediate-system identifier in the list; if the current
   choice of default intermediate-system is discovered to be down, the
   end-system may immediately choose another default intermediate-system
   without having to wait for the next advertisement to arrive.

   Any intermediate-system identifier advertised with a preference level
   of zero is not to be used by the end-system as default intermediate-
   system identifier.  Such an identifier may be omitted from the
   default intermediate-system list, unless its timer is being use as a
   "black-hole" detection mechanism.

5.  End-System Discovery

   Within a directly attached link, each system must be able to locate
   end-systems with which it desires to communicate.  This is
   accomplished using the Where-Are-You and I-Am-Here messages described
   below.  This is independent of any specific media.

   When an intermediate-system needs the location of an end-system, it
   sends the Where-Are-You solicitation.  The target end-system responds
   with the I-Am-Here advertisement.

   When no intermediate-system advertisements have been heard, an end-
   system sends the Where-Are-You solicitation itself.  The target end-
   system responds with the I-Am-Here advertisement as usual.

   When an end-system has heard one or more intermediate-system
   advertisements, the default behavior is to send all datagrams to the
   preferred intermediate-system.  If the target end-system is
   accessible on the local link, the intermediate-system sends a
   redirect back indicating the appropriate media address.

   When an end-system has heard one or more intermediate-system
   advertisements, and no zone or prefix-routing is being used, or no
   prefix matches any current interface identifier, the end-system can
   assume that it is operating as a mobile end-system.  The mobile end-
   system advertises on a periodic basis, just as an intermediate-
   system.

5.1.  Solicitations

   Every SIP system MUST implement End-System Solicitation for discovery
   of local end-systems.

   When a system is ready to send a datagram to another system, it
   examines its cache of system locations.  If no intermediate-system
   advertisements have been received, the system MUST send the Where-
   Are-You solicitation to prompt the advertisement of the target
   system.

   If (and only if) no advertisements from the target system are
   forthcoming, the system MAY retransmit the Where-Are-You a small
   number of times, but then MUST desist from sending more
   solicitations.

5.1.1.  Implementation

   The end-system solicitation is sent to the all-systems multicast.

   The end-system solicitations use the same configuration constants as
   intermediate-system solicitations.

   Unlike intermediate-system solicitations, end-system solicitations
   are sent only when a particular end-system location is needed, rather
   than on startup.

   End-system solicitations are sent using the same periodicity
   calculations as intermediate-system solicitations.

   Upon receiving a valid advertisement from any intermediate-system, an
   end-system MUST NOT send any end-system solicitations.

5.1.2.  Receipt

   An intermediate-system MUST silently discard any received End-System
   Solicitation messages.

   An end-system MUST silently discard any received End-System
   Solicitation messages that do not satisfy the following validity
   checks:

   -  ICMP Checksum is correct.

   -  ICMP length (derived from the payload length) is 16 or more
      octets.

   -  Source Address is either 0 or the identifier of a neighbor (an
      identifier that matches one of the end-system's own identifiers on
      the arrival interface under the prefix mask associated with that
      identifer, or the zone associated with that interface).

5.2.  Advertisements

   Every SIP end-system MUST implement End-System Advertisements.

   Usually, end-system advertisements are sent in response to end-system
   solicitations.  In addition, mobile end-system advertisements and
   service end-system advertisements (described below) are sent on a
   periodic basis.

   The end-system advertisements include such important information as
   the media address to access the system, and neighboring
   intermediate-systems heard.

5.2.1.  Implementation

   The periodic mobile end-system advertisement is sent to the all-
   routers multicast.

   The single end-system advertisement in respnse to a solicitation is
   sent to the all-systems multicast.

   In either case, the scope is set to local.

      CONTROVERIAL: The all-systems multicast is used for end-system
      advertisements, rather than responding directly to the soliciting
      system.  This is under the assumption that all intermediate-
      systems need to update the list of active end-systems, when the
      query is sent by a router.  Logically, the response could be sent
      to all-routers.

      However, when the query is sent by an end-system, there are no
      routers present.  The response could be sent directly to the
      requesting end-system.

      There is no easy way to determine that the sender was an
      intermediate-system rather than an end-system.  The only multicast
      which covers both cases is all-systems.

   Mobile advertisements use similar configuration constants and
   variables as intermediate-system advertisements.

   Mobile advertisements are sent using the same periodicity
   calculations as intermediate-system advertisements.

   Advertising interfaces are established and terminated in the same
   manner as intermediate-system advertisements.

6.  Service Discovery

   Each system offering one of the special configuration services
   detailed below, whether an end-system or intermediate-system,
   includes that service availability in every advertisement that it
   sends.  All systems discover the location of these services simply by
   listening for the advertisements.  This eliminates the need for
   manual configuration, periodic probes, and special handling of
   certain packet types by intermediate-systems.

   The learned service information is included in any neighboring
   intermediate-system advertisements.  In this fashion, the
   intermediate-system advertisements provide a summary of all available
   network services, and pass information beyond the link where the
   advertisement originated.  This results in a reduction of network
   traffic when compared to the broadcast or multicast of service
   discovery requests/replies over a wide area.

   The initial services listed here are primarily concerned with
   configuration.  The locations of other facilities may be learned from
   these basic servers.

   Domain Name Service

      Before a system can communicate with another system, it must learn
      that system's identifiers and location.  The Domain Name System
      (DNS) is usually used for this purpose.

      In the past, this was accomplished by reading a list of servers
      from a (possibly remote) configuration file at startup time.  Some
      systems discovered servers by sending periodic probes to a
      broadcast or multicast address.  Both of these methods have
      serious drawbacks.  Configuration files must be maintained
      manually (a significant administrative burden when ther are large
      numbers of systems), and are unable to track dynamic changes in
      DNS availability.  Periodic probes are restricted from using
      recursion (see Host Requirements -- Application and Support [2],
      Section 6.1.3.2), and are thus limited to information about the
      local domain.

      In practice, only systems which are users or stub resolvers of the
      DNS can use the DNS server advertisements.  Full-Service resolvers
      MUST continue to be manually configured to ensure a heirarchy of
      believability within the network.

   Self-Configuration Service

      Before a system can communicate with another system, it must learn
      its own identity.  The Bootstrap Protocol (BOOTP) is frequently
      used for this purpose.

      In the past, this was accomplished by ad hoc passing of BOOTP
      requests by routers.  This method has several serious drawbacks.
      Presence of the feature cannot be relied upon.  It is not of much
      use for mobile, roving or portable systems.

6.1.  Solicitations

   Every SIP end-system SHOULD implement End-System Solicitation for
   discovery of local services.

   When a system is ready to use a particular service, it examines its
   cache of such services.  If no intermediate-system or other service
   advertisements have been received, the system MAY send the Where-
   Are-You solicitation to prompt the advertisement of the service.

   If (and only if) no advertisements from desired services are
   forthcoming, the system MAY retransmit the Where-Are-You a small
   number of times, but then MUST desist from sending more
   solicitations.

6.1.1.  Implementation

   The service solicitation is sent to the special multicast for each
   particular service, with the scope set to local.

   The service solicitations use the same configuration constants as
   intermediate-system and end-system solicitations.

   Unlike intermediate-system solicitations, service solicitations are
   sent only when a particular service is utilized, rather than on
   startup.

   Service solicitations are sent using the same periodicity
   calculations as intermediate-system and end-system solicitations.

   Upon receiving a valid advertisement from any intermediate-system,
   the system MUST NOT send any service solicitation.

   Service solicitations require the same validity checks as end-system
   solicitations.

6.2.  Advertisements

   Like intermediate-system and mobile end-system advertisements,
   service end-systems advertisements are sent on a periodic basis.

   Services offered by intermediate-systems are included in the
   intermediate-system advertisements described above.

6.2.1.  Implementation

   The service advertisement is sent to the all-systems multicast, with
   the scope set to local.

      CONTROVERIAL: The all-systems multicast is used for service
      advertisements, rather than different multicasts for each service.
      This is under the assumption that all systems need to learn of
      services.

      This corresponds to the design for intermediate-system
      advertisements.  Thus, intermediate-system advertisements can be
      viewed as a special case of service advertisements.

      This ensures that the design will operate when there are no
      routers, and when the routing protocols are still initializing.

   The service advertisements use similar configuration constants and
   variables as intermediate-system advertisements.

   Service advertisements are sent using the same periodicity
   calculations as intermediate-system advertisements.

   Advertising interfaces are established and terminated in the same
   manner as intermediate-system advertisements.

   When any system ceases to offer an advertised service, the system
   SHOULD transmit a final multicast advertisement on the interface,
   identical to its previous transmission, but with a Lifetime field of
   zero.

7.  Self Discovery

7.1.  End-Systems

   At startup, each SIP end-system solicits the advertisements of
   intermediate-systems, as described in Intermediate-System Discovery
   above.  Until an intermediate-system is discovered, an end-system is
   limited to accessing systems and services for the links to which it
   is directly attached.

   In the absence of an intermediate-system, each SIP end-system
   solicits the advertisements of services as described in Service
   Discovery above.  Until self-configuration services are discovered,
   an end-system is limited to accessing systems and services according
   to prior configuration.

7.1.1.  Zone Determination

   Until an intermediate-system is discovered, an end-system assumes a
   zone number of zero.  When combined with any IEEE-802 number found in
   the machine, or other identifier negotiated at the link level, this
   yields a local identifier which is unique to the system.

   When an intermediate-system is discovered, the advertisements are
   examined for zone information, as described in Intermediate-System
   Discovery above.  If all advertised zone values are zero, then zone
   routing is not available beyond that link.  If more than one zone
   number is discovered for the same interface, only the highest zone
   number is used.

   When there is more than one interface on a multi-homed end-system,
   each interface MUST answer to all of the local identifers generated.

   When more than one IEEE-802 number is available, the primary system
   identifier is composed of the highest zone discovered, combined with
   the highest IEEE-802 number found.

7.1.2.  Initialization

   Once a system has becomed locally addressable, it can engage in
   exchanges with local servers.  Some of these local servers could be a
   bootstrap service, for loading and configuring the system.  Another
   server could be a registration service, in charge of managing the
   local name and identifier space.

   When the registration service is unable to find a match for the
   system, the system SHOULD request the operator to provide a name for
   the system.  The registration service would be responsible for
   ensuring uniqueness, and assigning appropriate identifiers for the
   name.

   Further specification of such services is beyond the scope of this
   document.

7.1.3.  Identifier Determination

   Once the Domain Name has been determined for a system, the Domain
   Name Service SHOULD be consulted to determine the globally advertised
   identifiers for the system.  In this fashion, system is coordinated
   with the most current information actually propagated within the
   internet.

   Each DNS identifier has a Time-To-Live associated with it.  When any
   identifier expires, another request SHOULD be made to the DNS for a
   list of identifiers.

   When there is more than one interface on a multi-homed end-system,
   each interface MUST answer to all of the identifers learned.

   When more than one identifier is returned for a system, the primary
   system identifier is the identifier with the highest TTL, or the
   first listed identifier of those with the highest TTL.

7.1.4.  Prefix Determination

   The prefix size is dynamically learned from matching interface
   identifiers against the intermediate-system advertisements, as
   described in Intermediate-System Discovery above.

   Unlike previous practice, an end-system prefix sizes SHOULD NOT be
   preconfigured.  Any preconfigured value MUST be superceded by new
   values and changes propagated in intermediate-system advertisements.

7.1.5.  Changing Identifiers

7.2.  Intermediate-Systems

   The zones and prefixes are assigned by hand.

8.  Next-Hop Determination

   When an end-system has not heard any intermediate-system
   advertisements, it is assumed that all end-systems are only
   accessible on the local link.

   multi-homed

   preferred router

   smart selection

   local redirect

   remote redirect

8.1.  Examples of Use

       Simple case -- J to K on the same fully-connected link.

           J sends the Where-Are-You (which contains its own media address)
           to all-systems.  K sends the I-Am-Here (which contains its own
           media address) directly to all-systems.  At this point, they
           both know that they can talk directly to each other, without
           regard to subnet.

       Routed case -- J to K not on the same fully-connected link.

           If no resource reservation or policy routing is desired, J
           simply sends its packets directly to the "preferred" router that
           it has learned from the Advertisements.  If there is a better
           router for the first-hop, that router sends the I-Am-Here
           redirect to J, but never-the-less forwards the packet.

           In the presence of RR or PR, J sends a Where-Are-You to the
           "preferred" router that it has learned from the Advertisements.
           That router always returns an I-Am-Here redirect (even if the
           correct hop is itself), which contains the requested RR or PR
           status information.  J then sends its packets to the first-hop
           router as determined from the I-Am-Here.

       General case -- J to K over disconnected partial mesh (radio/framerelay).

           J sends K over disconnected partial mesh (radio/framerelay).

           J periodically sends the I-Am-Here (which contains its own media
           address, and the addresses of its "heard" routers) to the
           all-routers multicast.  The routers use such messages to
           construct a map of the current state of the topology.  The
           routers now know who J hears, and who hears J.

           If the routing map doesn't contain a current whereabouts of K,
           the Destination Unreachable message is returned by the "best"
           router on J's "heard" list.

           If the routing map contains the current whereabouts of K, the
           "best" router on K's "heard" list sends a Where-Are-You to K,
           with a list of routers which can hear K.  The list is ordered by
           the intersection of those routers which can also hear J,
           minimizing the number of hops.

           When K hears the Where-Are-You, it sends the I-Am-Here to the
           all-systems address.  The "best" router on J's "heard" list
           sends an I-Am-Here redirect to J, with a substitute list of
           routers which can hear J.  The list is ordered by the
           intersection of those routers which can also hear K.

           Of course, J may have heard K's I-Am-Here directly.

           At this point, the routing fabric knows which routers are heard
           by J and K, and which routers can hear J and K.  J and K know
           whether they can hear each other directly.  If not, they know
           the "best" next-hop router (which may not be the same in both
           directions).

           Unlike the fully-connected scenarios, this scheme requires that
           the I-Am-Here is sent from time to time to keep the map updated.
           However, only routers need store the information.

9.  Additional ICMP Packets

   The Packet format and basic facilities are already defined for ICMP
   [3], as modified for SIP [1].

   Up-to-date values of the ICMP Type field are specified in the most
   recent "Assigned Numbers" RFC [2].  This document concerns the
   following values:

      <TBD>   Where-Are-You
      <TBD>   I-Am-Here

9.1.  Where-Are-You

   A summary of the Where-Are-You message format is shown below.  The
   fields are transmitted from left to right.

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |     Code      |          Checksum             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                           Reserved                            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +                       System Identifier                       +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   Extensions ...
   +-+-+-+-+-+-+-+-+-+-+-+-+

   Type

      <TBD>

   Code

      The Code field is one octet.  Up-to-date values of the I-Am-Here
      Code field are specified in the most recent "Assigned Numbers" RFC
      [2].  Current values are assigned as follows:

           0     RESERVED
           1     End-System Solicitation
           2     Intermediate-System Solicitation

   Checksum

      The ICMP Checksum.

   System Identifier

      The System Identifier field is eight octets in length, and
      contains an identifier of the system which is sought.  When the
      identifer of the system is unknown, the field is zero filled.

   Extensions

      The Extensions field is variable in length and contains zero or
      more Extensions.  These Extensions are described in a later
      section.

The contents of the Reserved field are ignored.  Future backward-
compatible changes to the Where-Are-You (which contains its own media address,
           and protocol may specify the addresses contents of its "heard" routers) to the all-systems
           address.  The routers use such messages to construct a map
Reserved field or of additional octets at the current state end of the topology. message.

9.1.1.  End-System Solicitation

   The routers now know who J
           hears, and who hears J.

           If End-System Solicitation contains the routing map doesn't contain a current whereabouts of K, following values:

   -  In the Destination Unreachable message is returned by Address field of the "best"
           router on J's "heard" list.

           If SIP header: For service
      solicitations, the routing map contains special multicast group associated with the current whereabouts of K,
      service.  For other solicitations, the
           "best" router on K's "heard" list sends a copy of all-systems multicast.  In
      either case, the
           Where-Are-You scope is set to K, local.

   -  In the Source Address field of the SIP header: any identifier
      associated with a substitute list the sending interface.  It MAY contain zero if the
      system has not yet determined an identifier for the interface.

   -  In the Code field of routers the ICMP header: 1 for End-System
      Solicitation.

   -  For each intermediate-system advertisement that has been heard,
      the System-Heard extension.

   -  For interfaces which can
           hear K.  The list is ordered are not point-to-point links, the Media-
      Access extension.

   In the unlikely event that not all extensions fit in a single
   solicitaion, as constrained by the intersection MTU of those
           routers which can also hear J, minimizing the number of hops.

           Of course, K may have heard J's Where-Are-You directly, in which
           case it adds its own address to link, the remaining
   extensions are removed.  Only a single solicitation is sent.

9.1.2.  Intermediate-System Solicitation

   The Intermediate-System Solicitation contains the front of following values:

   -  In the list Destination Address field of routers.

           When K hears the J Where-Are-You, it sends SIP header: the I-Am-Here all-
      routers multicast, with the scope set to local.

   -  In the
           all-systems address.  The "best" router on J's "heard" list
           sends a copy Source Address field of the I-Am-Here to J, SIP header: any identifier
      associated with a substitute list of
           routers which can hear J.  The list is ordered by the
           intersection sending interface.  It MAY contain zero if the
      system has not yet determined an identifier for the interface.

   -  In the Code field of those routers which can also hear K.

           At this point, the routing fabric knows which routers are heard
           by J and K, and which routers can hear J and K.  J and K know
           whether they can hear ICMP header: 2 for Intermediate-System
      Solicitation.

   -  For each of that system's interface identifiers other directly.  If not, they know than the "best" next hop router (which may not be
      primary identifier, the same in both
           directions).

           Unlike Other-Identifier extension, with the fully-connected scenarios, this scheme requires
      prefix size set to zero.

   -  For each intermediate-system advertisement that has been heard,
      the I-Am-Here is sent from time to time to keep System-Heard extension.

   -  For interfaces which are not point-to-point links, the map updated.
           However, only routers need store Media-
      Access extension.

   In the information.

4.  Additional ICMP Packets

   The Packet format and basic facilities are already defined for ICMP
   [3], unlikely event that not all extensions fit in a single
   solicitaion, as modified for SIP [1].

   Up-to-date values constrained by the MTU of the ICMP Type field link, multiple
   solicitations are specified in the most
   recent "Assigned Numbers" RFC [2].  This document concerns sent, with each except the
   following values:

      <TBD>   Where-Are-You
      <TBD> last containing as many
   extensions as can fit.

9.2.  I-Am-Here

4.1.  Where-Are-You

   A summary of the Where-Are-You I-Am-Here message format is shown below.  The fields
   are transmitted from left to right.

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |     Code      |          Checksum             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                           Reserved        Sequence Number        |          LifeTime             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +                       System Identifier                       +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   Extensions ...
   +-+-+-+-+-+-+-+-+-+-+-+-+

   Type

      <TBD>

   Code

      The Code field is one octet.  Up-to-date values of the I-Am-Here
      Code field are specified in the most recent "Assigned Numbers" RFC
      [2].  Current values are assigned as follows:

           0     RESERVED
           1     End-System Solicitation Advertisement
           2     Intermediate-System Solicitation Advertisement
           3     Local Redirect
           4     Remote Redirect

   Checksum

      The ICMP Checksum.

   System Identifier

   Sequence Number

      The System Identifier Sequence Number field is eight two octets in length, and contains
      the identifier number of I-Am-Here messsages sent since the system which was
      initialized.  This number MUST include this advertisement.

   LifeTime

      The LifeTime field is sought.  For an
      Intermediate-System Solicitation, two octets in length, and indicates the
      seconds remaining before the entry is considered invalid.

   System Identifier

      The System Identifier field is unused eight octets in length, and remains
      zero filled.
      contains the primary identifier for this system.  Other
      identifiers are indicated with the Other-Identifier extension.

   Extensions

      The Extensions field is variable in length and contains zero or
      more Extensions.  These Extensions are described in a later
      section.

4.1.1.  Description

9.2.1.  End-System Advertisement

   The Where-Are-You message is used to determine End-System Advertisement contains the presence and
   availability of following values:

   -  In the next hop.  This message is also used for resource
   reservation and policy route determination.

4.2.  I-Am-Here

   A summary Destination Address field of the I-Am-Here message format is shown below.  The fields
   are transmitted from left to right.

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |     Code      |          Checksum             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |        Sequence Number        |          LifeTime             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +                       System Identifier                       +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   Extensions ...
   +-+-+-+-+-+-+-+-+-+-+-+-+

   Type

      <TBD>

   Code

      The Code field SIP header: For periodic
      mobile end-system advertisements, the all-routers multicast.  For
      other end-system advertisements, the all-systems multicast.  In
      either case, the scope is one octet.  Up-to-date values set to local.

   -  In the Source Address field of the I-Am-Here SIP header: For service
      advertisements, the primary identifier associated with that
      system.  For responses to solicitations, the identifier specified
      in the solicitation.

   -  In the Code field of the ICMP header: 1 for End-System
      Advertisement.

   -  In the Lifetime field: the interface's configured
      AdvertisementLifetime.

   -  For each of that system's interface identifiers other than the
      primary identifier, the Other-Identifier extension, with the
      prefix size set to zero.

   -  For each service advertisement that is offered, the Service-
      Information extension.

   -  For each intermediate-system advertisement that has been heard,
      the System-Heard extension.

   -  For interfaces which are specified not point-to-point links, the Media-
      Access extension.

   In the unlikely event that not all extensions fit in a single
   advertisement, as constrained by the most recent "Assigned Numbers" RFC
      [2].  Current values MTU of the link, multiple
   advertisements are assigned sent, with each except the last containing as follows:

           0     RESERVED
           1     End-System Advertisement
           2 many
   extensions as can fit.

9.2.2.  Intermediate-System Advertisement
           3     Local Redirect
           4     Remote Redirect

   Checksum

      The ICMP Checksum.

   Sequence Number

   The Sequence Number field is two octets in length, and Intermediate-System Advertisement contains the number of I-Am-Heres sent.  This number MUST include this
      advertisement.

   LifeTime

      The LifeTime following values:

   -  In the Destination Address field is two octets in length, and indicates of the
      seconds remaining before SIP header: the entry is considered invalid.

   System Identifier

      The System Identifier all-
      systems multicast, with the scope set to local.

   -  In the Source Address field is eight octets in length, and
      contains of the SIP header: the primary
      identifier for this system.  Other
      identifiers are indicated with of the Other-Identifier extension.

   Extensions

      The Extensions field is variable in length and contains zero or
      more Extensions.  These Extensions are described in a later
      section.

4.2.1.  Description system.  The I-Am-Here message same identifier is used to announce for all
      interfaces.

   -  In the presence Code field of an
   intermediate or end system, to indicate changes in the topology, and
   to support system mobility.

   It contains all ICMP header: 2 for Intermediate-System
      Advertisement.

   -  In the Lifetime field: the interface's configured
      AdvertisementLifetime.

   -  For each of that interface's identifiers whose Advertise flags are
      TRUE, the information now in Routing-Information extension.

   -  For each of that interface's recently changed identifiers, the old Router
   Advertisement, ES Hello, IS Hello, OSPF Hello and RSPF Hello.

   Intermediate Systems

      The message is sent by
      Change-Identifier extension.

   -  For each intermediate-system periodically to of that system's other interface's identifiers which have
      not already been included through prefix subsumption, the all-systems multicast.  The information is stored by all
      systems.

      The message is also sent in response to a Where-Are-You.

   End-Systems

      The message is sent in response to a Where-Are-You.  The
      information Other-
      Identifier extension.

   -  For each service that is stored only by offered, or has been learned from another
      advertisement, the affected systems.

   Local Redirect

      The message is sent in response to changes in Service-Information extension.

   -  For each intermediate-system advertisement that has been heard,
      the routing.  The
      information is stored only by System-Heard extension.

   -  For interfaces which are not point-to-point links, the affected systems.

   Remote Redirect

      The message is sent to indicate movement of Media-
      Access extension.

   In the unlikely event that not all extensions fit in a system beyond the
      local zone.  The information is stored only single
   advertisement, as constrained by the affected
      systems.

5. MTU of the link, multiple
   advertisements are sent, with each except the last containing as many
   extensions as can fit.

10.  Extensions

   Extensions allow variable amounts of information to be carried within
   each Advertisement or Solicitation Advertisement packet.  Some extensions are
   common to both packet types.

   The end of the list of Extensions is indicated by the Payload Length
   of the SIP packet.

   A summary of the Extensions format is shown below.  The fields are
   transmitted from left to right.

    0                   1
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |    Length     |    Data ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Type

      The Type field is one octet and indicates the type of Extension.
      Up-to-date values of the Extension Type field are specified in the
      most recent "Assigned Numbers" RFC [2].  Current values are
      assigned as follows:

           1     Media-Access
           2     Other-Identifier     Change-Identifier
           3     Routing-Information     Other-Identifier
           4     System-Heard
           5     Security-Information     Routing-Information
           6     Service-Information
           7     Transit-Information
           8     Authentication
           9     Security-Information
          10     Redirected-Header

   Length

      The Length field is one octet and indicates the length of the Data
      field which has been used.

      Each Extension ends on an octet boundary which is an integral
      multiple of four octets.  Any unused portion of the Data field is
      padded with zeros.

      length          actual
      0 through 2        4
      3 through 6        8
      7 through 10      12

   Data

      The Data field is zero or more octets and contains the value or
      other information for this Extension.  The format and length of
      the Data field is determined by the Type and Length fields.

5.1.

10.1.  Media-Access

A summary of the Media-Access extension format is shown below.  The
fields are transmitted from left to right.

 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|     Type      |    Length     |           Media Type          |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|     MAC Address ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Type

     1

     1

Length

     >= 3

Media Type

   The Media Type field is two octets in length.  The value of this
   field is the same as the Hardware Type used in ARP.  Up-to-date
   values of the Hardware Type field are specified in the most recent
   "Assigned Numbers" RFC [2].

      [Should we use the ifType from MIB-II instead?]

MAC Address

   The MAC Address field is variable in length, and contains the media
   address which is used to access this system.

   The MAC Address is always specified in Canonical order.

The Media-Access extension MUST be included in those messages sent from
an interface on a multi-access media.

It MUST NOT be included in a message sent from a point-to-point
interface, or in messages such as the Remote Redirect which pass through
intermediate systems.

10.2.  Change-Identifier

A summary of the Change-Identifier extension format is shown below.  The
fields are transmitted from left to right.

 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|     Type      |    Length     |                   |Prefix Size|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                                                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                                                               |
+                         Old Identifier                        +
|                                                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                                                               |
+                         New Identifier                        +
|                                                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Type

     2

Length

     >= 3

Media Type

    22

Prefix Size

   The Media Type Prefix Size field is two octets six bits in length.  The value of this
   field is the same as length, and indicates the Hardware Type used in ARP.  Up-to-date
   values number
   of the Hardware Type field are specified bits in both Identifiers which define the most recent
   "Assigned Numbers" RFC [2].

      [Should we use prefix mask for the ifType
   link.  The value ranges from MIB-II instead?]

MAC Address 0 to 62.

   End-Systems MUST have a Prefix Size of zero.

Old Identifier

   The MAC Address Old Identifier field is variable eight octets in length, and contains the media
   address which is used to access
   old identifier for this system. interface.

New Identifier

   The MAC Address New Identifier field is always specified eight octets in Canonical order.

The Media-Access extension MUST length, and contains one
   of the identifiers for this interface.  This may be included in those messages sent from
an another
   identifier for the same interface on a multi-access media.

It MUST NOT be included in a message that sent from a point-to-point
interface, the message, or in messages such as may
   identify another interface on the Remote Redirect same system which pass through
intermediate systems.

5.2. sent the message.

10.3.  Other-Identifier

A summary of the Other-Identifier extension format is shown below.  The
fields are transmitted from left to right.

 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|     Type      |    Length     |                   |Prefix Size|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                            Metric                             |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                                                               |
+                      Interface Identifier                     +
|                                                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Type

     2

     3

Length

    14

Prefix Size

   The Prefix Size field is six bits in length, and indicates the number
   of bits in the Interface Identifier which define the prefix mask for
   the link.  The value ranges from 0 to 62.

   If the Interface Identifier does not indicate a valid prefix, the
   value is zero.

   End-Systems MUST have a Prefix Size of zero.

Metric

   The Metric field is four octets in length, and indicates the
   preference level for use of this system to forward packets to the
   Interface Identifier.  Lower values indicate greater preference.

   End-Systems MUST set this field to zero.

Interface Identifier

   The Interface Identifier field is eight octets in length, and
   contains an identifier one of the identifiers for this system.  This may be another
   identifier for the same interface that sent the message, or may
   identify another interface on the same system which sent the message.

Every identifier for every interface is listed in each I-Am-Here
message.

This supports multiple identifiers per interface, as well as multi-homed
systems.

When a number of interfaces, such as point-to-point interfaces, may be
aggregated with the same prefix, only one extension need be included.

This enables end-systems to determine the best next hop next-hop without sending
a Where-Are-You solicitation when the next hop next-hop is on another interface
attached to the same advertising system.

5.3.  Routing-Information

10.4.  System-Heard

A summary of the Routing-Information System-Heard extension format is shown below.  The
fields are transmitted from left to right.

 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|     Type      |    Length     |  Preference     Speed     |D|B|Prefix Size|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|              MRU              |     Zone      |   Priority                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                                                               |
+                      Interface                       System Identifier                       +
|                                                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Type

     3

Length

    14

Preference

   The Preference field is one octet in length, and indicates the
   preference level for use of this system to forward packets to the
   Interface Identifier.  Higher values indicate greater preference.
|        Sequence Number        |      Remaining LifeTime       |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                            Quality                            |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                      Advertisement Count                      |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                          Error Count                          |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Type

     4

Length

    30

Designated Bit

   The Designated Bit indicates that the System Identifier is the
   Designated Router.

Backup Bit

   The Backup Bit indicates that the System Identifier is the Backup
   Designated Router.

Prefix Size

   The Prefix Size field is six bits six bits in length, and indicates the number
   of bits in the System Identifier which define the prefix mask for the
   link.  The value ranges from 0 to 62.

   If the System Identifier does not indicate a valid prefix, the value
   is zero.

   End-Systems MUST have a Prefix Size of zero.

MRU

   The Maximum Receive Unit field is two octets in length, and indicates
   the maximum size packet that the system will receive over the link.

Speed

   The Speed field is one octet in length, and indicates the speed of
   the link over which the advertisement or solicitation was heard.
   Higher values indicate greater speed.  The speed value is related to
   int( 10 * ln( speed / 100 ) ) in bits per second.

      After considerable trial and error, this formula was used because
      it gave the best distribution for distinguishing medium speed
      links, and fit reasonably well in the realm of currently
      envisioned speeds.  It has an upper limit of 11.87 Terabits per
      second.  (It also has a convenient button on the calculator.)

        0      link is down
        1 - 9  reserved
       10      300 or less
       24    1,200                  96      1,544,000 T1
       31    2,400                  99      2,048,000 E1
       38    4,800                 106      4,000,000 Token Ring
       42    7,200                 110      6,312,000 T2
       45    9,600                 115     10,000,000 Ethernet
       49   14,400                 119     16,000,000 Token Ring
       52   19,200
       56   28,800                 130     44,736,000 T3
       59   38,400                 142    155,520,000 STS-3,STM-1
       63   57,600                 202    622,080,000 STS-12,STM-4
       64   64,000                 216  2,488,320,000 STS-48,STM-16
       71  128,000
       73  153,600
       78  256,000

System Identifier

   The System Identifier field is eight octets in length, and indicates the number
   of bits in the Interface Identifier which define contains
   the prefix mask primary identifier for the link.  The value ranges system, taken from 0 to 62.

   If the Interface Identifier does not indicate a valid prefix, Source Address
   field of the
   value is zero.

MRU advertisement heard.

Sequence Number

   The Maximum Receive Unit Sequence Number field is two octets in length, and indicates
   the maximum size packet that contains the system will receive over
   last heard sequence number from the link.

Zone system.

Remaining LifeTime

   The Zone Remaining LifeTime field is one octet two octets in length, and indicates
   the zone for seconds remaining before the
   link.  A value entry is considered invalid.

Quality

   The Quality field is four octets in length, and contains an
   indication of zero indicates that no zone has been assigned.

Priority the signal quality received from this system.  Higher
   values indicate greater quality.

Advertisement Count

   The Priority Advertisement Count field is one octet four octets in length, and indicates
   the priority
   for election to Designated Backup.  A value number of zero indicates advertisements that have been heard from the system is not eligible.

Interface Identifier identified
   system.

Error Count

   The Interface Identifier Error Count field is eight four octets in length, and
   contains an identifier for this interface. indicates the
   number of errors which have been detected on the link with the
   identified system.

This extension is sent only by Intermediate-Systems.

5.4.  System-Heard included in every I-Am-Here.

10.5.  Routing-Information

A summary of the System-Heard Routing-Information extension format is shown below.
The fields are transmitted from left to right.

 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|     Type      |    Length     |     Speed  Preference   |D|B|Prefix Size|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|              MRU              |     Zone      |   Priority    |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                                                               |
+                       System                      Interface Identifier                     +
|                                                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|        Sequence Number        |      Remaining LifeTime       |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                            Quality                            |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                      Advertisement Count                      |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                          Error Count                          |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Type

     4

     5

Length

    30

    14

Preference

   The Preference field is one octet in length, and indicates the
   preference level for use of this system to forward packets to the
   Interface Identifier.  Higher values indicate greater preference.

Designated Bit

   The Designated Bit indicates that the System Identifier system is the Designated
   Router.

Backup Bit

   The Backup Bit indicates that the System Identifier system is the Backup Designated
   Router.

Prefix Size

   The Prefix Size field is six bits in length, and indicates the number
   of bits in the System Interface Identifier which define the prefix mask for
   the link.  The value ranges from 0 to 62.

   If the System Interface Identifier does not indicate a valid prefix, the
   value is zero.

   End-Systems MUST have a Prefix Size of zero.

MRU

   The Maximum Receive Unit field is two octets in length, and indicates
   the maximum size packet that the system will receive over the link.

Speed

Zone

   The Speed Zone field is one octet in length, and indicates the speed of
   the link over which zone for the advertisement or solicitation was heard.
   Higher values indicate greater speed.  The speed
   link.  A value of zero indicates that no zone has been assigned.

Priority

   The Priority field is related to
   int( 10 * ln( speed / 100 ) ) one octet in bits per second.

      After considerable trial length, and error, this formula was used because
      it gave indicates the best distribution priority
   for distinguishing medium speed
      links, and fit reasonably well in the realm of currently
      envisioned speeds.  It has an upper limit election to Designated Backup.  A value of 11.87 Terabits per
      second.  (It also has a convenient button on zero indicates that
   the calculator.)

        0      link system is down
        1 - 9  reserved
       10      300 or less
       24    1,200                  96      1,544,000 T1
       31    2,400                  99      2,048,000 E1
       38    4,800                 106      4,000,000 Token Ring
       42    7,200                 110      6,312,000 T2
       45    9,600                 115     10,000,000 Ethernet
       49   14,400                 119     16,000,000 Token Ring
       52   19,200                 130     44,736,000 T3
       56   28,800
       59   38,400                 142    155,520,000 STS-3/STM-1
       63   57,600                 202    622,080,000 STS-12/STM-4
       64   64,000                 216  2,488,320,000 STS-48/STM-16
       71  128,000
       73  153,600
       78  256,000

System not eligible.

Interface Identifier

   The Interface Identifier field is eight octets in length, and
   contains one of the identifiers for this interface.

This extension is sent only by Intermediate-Systems.

When more than one of these extensions is present, the Designated and
Backup bits, MRU, Zone and Priority fields MUST be the same in each
copy.

10.6.  Service-Information

A summary of the Service-Information extension format is shown below.
The fields are transmitted from left to right.

 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|     Type      |    Length     |           Service             |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|        Sequence Number        |      Remaining LifeTime       |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                                                               |
+                       System Identifier                       +
|                                                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|     Data ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Type

     6

Length

    >= 14

Service

   The Service field is eight two octets in length, and contains length.  The value of this field
   is usually the primary identifier for same as the system. well-known port number.  Up-to-date values
   of the Service field are specified in the most recent "Assigned
   Numbers" RFC [2].

Sequence Number

   The Sequence Number field is two octets in length, and contains the
   last heard sequence number from the system.

Remaining LifeTime

   The Remaining LifeTime field is two octets in length, and indicates
   the seconds remaining before the entry is considered invalid.

Quality

System Identifier

   The Quality System Identifier field is four eight octets in length, and contains an
   indication of
   the signal quality received from primary identifier for this system.  Higher
   values indicate greater quality.

Advertisement Count

Data

   The Data field is variable in length, and contains information
   specific to the service.  For example, it could contain a string with
   the description of the service.

   The format of the Data field is entirely service dependent, and is
   always treated as a binary value.

10.7.  Transit-Information

A summary of the Transit-Information extension format is shown below.
The fields are transmitted from left to right.

 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|     Type      |    Length     |               |      QoS      |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                             Metric                            |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Type

     7

Length

     6

QoS

   The Advertisement Count Quality of Service field is four octets one octet in length, and indicates
   the number of advertisements that have been heard from the identified
   system.

Error Count a
   service for which transit will be accepted.

Metric

   The Error Count Metric field is four octets in length, and indicates the
   number
   preference level for use of errors which have been detected on the this network link with to forward packets of
   the
   identified system.

The System Heard indicated Quality of Service.  Lower values indicate greater
   preference.

This extension MUST be is included in every I-Am-Here.

5.5.  Security-Information the Intermediate-System I-Am-Here to
indicate that it will accept transit traffic.  If this extension is not
included, other intermediate-systems will treat the link as a stub
network.

10.8.  Authentication

A summary of the Security-Information Authentication extension format is shown below.  The
fields are transmitted from left to right.

 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|     Type      |    Length     |                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                                                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                                                               |
|                                                               |
|                                                               |
|                         Compartments                          |
|                                                               |
|                                                               |
|                                                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+     Data ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Type

     5

     8

Length

    22

Compartments

Data

   The Compartments Data field is sixteen octets variable in length. length, and contains information
   specific to the authentication method,

This extension is included in the Intermediate-System I-Am-Here to
indicate that it will accept transit traffic for the designated security
compartments.

5.6.  Service-Information any I-Am-Here.

10.9.  Security-Information

A summary of the Service-Information Security-Information extension format is shown below.
The fields are transmitted from left to right.

 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|     Type      |    Length     |                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                                                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                                                               |
+                       System Identifier                       +
|                                                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+     Compartments ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Type

     6

     9

Length

    14

System Identifier

    22

Compartments

   The System Identifier Compartments field is eight sixteen octets in length, and contains
   an identifier for this system. length.

This may be another identifier for extension is included in the same interface Intermediate-System I-Am-Here to
indicate that sent the message, or may identify another
   interface on the same system which sent it will accept transit traffic for the message.

5.7.  Transit-Information designated security
compartments.

10.10.  Redirected-Header

A summary of the Transit-Information Redirected-Header extension format is shown below.  The
fields are transmitted from left to right.

 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|     Type      |    Length     |                               |      QoS      |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                             Metric                                                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|     SIP Header ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Type

     7

    10

Length

     6

QoS

   The Quality of Service field is one octet in length, and indicates a
   service for which transit will be accepted.

Metric

    22

SIP Header

   The Metric SIP Header field is four 48 octets in length, and indicates the
   preference level for use of this network link to forward packets of
   the indicated service.  Lower values indicate greater preference. length.

This extension is included in the Intermediate-System I-Am-Here Local or Remote Redirect to
indicate verifiy
the traffic that it will accept transit traffic.  If this extension is not
included, the system will treat the link as a stub network. being redirected.

Security Considerations

References

   [1]

   [2]

Acknowledgments

Chair's Address

   The working group can be contacted via the current chairs:

Author's Address

   Questions about this memo can also be directed to:

      William Allen Simpson
      Daydreamer
      Computer Systems Consulting Services
      P O Box 6205
      East Lansing, MI  48826-6205

      EMail: Bill.Simpson@um.cc.umich.edu
                           Table of Contents

     1.     Terminology ...........................................    1

     2.     Criteria ..............................................    2

     3.     Design and Use ........................................    7 Overview .......................................    8
        3.1       System Identification ...........................    8
        3.2       Intermediate System Advertisements ..............    9
           3.2.1  Constants .......................................
        3.2       Multicast Support ...............................   10
           3.2.2  Configuration

     4.     Intermediate-System Discovery .........................   11
        4.1       Solicitations ...................................   10
           3.2.3   11
           4.1.1  Constants .......................................   12
           4.1.2  Implementation ..................................   12
        3.3       Intermediate System Discovery ...................   16
           3.3.1
           4.1.3  Receipt .........................................   13
        4.2       Advertisements ..................................   13
           4.2.1  Constants .......................................   15
           4.2.2  Configuration ...................................   16
           3.3.2   15
           4.2.3  Implementation ..................................   17
        3.4       Initial Intermediate-System Solicitations .......   19
           3.4.1  Constants .......................................   19
           3.4.2
           4.2.4  Receipt .........................................   20
        4.3       Processing Advertisements .......................   20
           4.3.1  Configuration ...................................   20
           4.3.2  Implementation ..................................   19
        3.5       Service Advertisments ...........................   22
           3.5.1   21

     5.     End-System Discovery ..................................   23
        5.1       Solicitations ...................................   23
           5.1.1  Implementation ..................................   22
        3.6       Service Discovery ...............................   24
           3.6.1  Domain Name Service .............................
           5.1.2  Receipt .........................................   24
           3.6.2  Self-Configuration Service ......................
        5.2       Advertisements ..................................   24
        3.7       Prefix Discovery ................................   26
        3.8       Zone Discovery
           5.2.1  Implementation ..................................   25

     6.     Service Discovery .....................................   26
           3.8.1  Abstraction Algorithm ...........................   26
        3.9       Next Hop Determination ..........................   27
           3.9.1  Configuration
        6.1       Solicitations ...................................   27
           3.9.2
           6.1.1  Implementation ..................................   27
           3.9.3
        6.2       Advertisements ..................................   28
           6.2.1  Implementation ..................................   28

     7.     Self Discovery ........................................   29
        7.1       End-Systems .....................................   29
           7.1.1  Zone Determination ..............................   29
           7.1.2  Initialization ..................................   29
           7.1.3  Identifier Determination ........................   30
           7.1.4  Prefix Determination ............................   30
           7.1.5  Changing Identifiers ............................   30
        7.2       Intermediate-Systems ............................   30

     8.     Next-Hop Determination ................................   31
        8.1       Examples of Use .................................   28

     4.   32

     9.     Additional ICMP Packets ...............................   30
        4.1   34
        9.1       Where-Are-You ...................................   31
           4.1.1  Description .....................................   32
        4.2   35
           9.1.1  End-System Solicitation .........................   37
           9.1.2  Intermediate-System Solicitation ................   38
        9.2       I-Am-Here .......................................   33
           4.2.1  Description .....................................   34

     5.   39
           9.2.1  End-System Advertisement ........................   41
           9.2.2  Intermediate-System Advertisement ...............   42

     10.    Extensions ............................................   36
        5.1   43
        10.1      Media-Access ....................................   38
        5.2   45
        10.2      Change-Identifier ...............................   46
        10.3      Other-Identifier ................................   39
        5.3       Routing-Information .............................   41
        5.4   48
        10.4      System-Heard ....................................   43
        5.5       Security-Information ............................   46
        5.6   50
        10.5      Routing-Information .............................   53
        10.6      Service-Information .............................   47
        5.7   55
        10.7      Transit-Information .............................   48   57
        10.8      Authentication ..................................   58
        10.9      Security-Information ............................   59
        10.10     Redirected-Header ...............................   60

     SECURITY CONSIDERATIONS ......................................   49   61

     REFERENCES ...................................................   49   61

     ACKNOWLEDGEMENTS .............................................   49   61

     CHAIR'S ADDRESS ..............................................   49   61

     AUTHOR'S ADDRESS .............................................   49   61