[Docs] [txt|pdf] [Tracker] [Email] [Diff1] [Diff2] [Nits]

Versions: 00 01 02

Network Working Group                                        W A Simpson
Internet Draft                                                Daydreamer
expires in six months                                      November 1994


                 IPv6 Neighbor Discovery -- Processing
                draft-simpson-ipv6-discov-process-01.txt



Status of this Memo

   This document is a submission to the IPng Working Group of the
   Internet Engineering Task Force (IETF).  Comments should be submitted
   to the ipng@sunroof.eng.sun.com mailing list.

   Distribution of this memo is unlimited.

   This document is an Internet-Draft.  Internet Drafts are working
   documents of the Internet Engineering Task Force (IETF), its Areas,
   and its Working Groups.  Note that other groups may also distribute
   working documents as Internet Drafts.

   Internet Drafts are draft documents valid for a maximum of six
   months, and may be updated, replaced, or obsoleted by other documents
   at any time.  It is not appropriate to use Internet Drafts as
   reference material, or to cite them other than as a ``working draft''
   or ``work in progress.''

   To learn the current status of any Internet-Draft, please check the
   ``1id-abstracts.txt'' listing contained in the internet-drafts Shadow
   Directories on ds.internic.net (US East Coast), nic.nordu.net
   (Europe), ftp.isi.edu (US West Coast), or munnari.oz.au (Pacific
   Rim).


Abstract

   This document discusses the implementation techniques for
   identification of and forwarding to adjacent IPv6 nodes, including
   Next Hop Determination and Router Discovery.










Simpson                  expires in six months                  [Page i]


DRAFT                   IPv6 Neighbor Discovery            November 1994


1.  Introduction

   This document describes how to

   -  determine the availability of other IPv6 nodes as demand for
      communication occurs;

   -  detect the presence of available IPv6 routers;

   -  learn the appropriate media address for sending to its neighbors;

   -  and redirect traffic where appropriate.

   The design requirements are more completely described in [D-Sign].
   The ICMP packet formats are described in [D-Form].

   This document contains only that information which is particularly
   relevant to IPv6 Neighbor Discovery, or that differs from IPv4
   techniques.  Other IPv6 documents should be consulted for further
   details.



2.  Link-Layers

   This document anticipates that link-layer material will be covered in
   a separate Link Layer Requirements document.  Specific link-layer
   protocol implementation details are beyond the scope of this
   document.



2.1.  Address Resolution Protocol (ARP)

   ARP [RFC-826] is no longer used for IPv6.



2.2.  Trailers

   Because ARP is no longer used for IPv6, trailer encapsulation [RFC-
   893] MUST NOT be used.



2.3.  Maximum Transmission Unit (MTU)

   The MTU is a internetwork-layer indication of the maximum datagram



Simpson                  expires in six months                  [Page 1]


DRAFT                   IPv6 Neighbor Discovery            November 1994


   size which can be sent on an interface.  This does not include link-
   layer encapsulation and framing, but includes padding which can be
   added for small frames.

   Many link-layer protocols define a maximum frame size that may be
   sent.  In such cases, a node MUST NOT allow a datagram to be sent
   which would require frames larger than those allowed by the link-
   layer protocol.

   The MTU of each logical interface MUST be configurable.  However, a
   node MUST be able to receive a packet as large as the maximum frame
   size, even if that is larger than the currently configured MTU.



2.4.  Maximum Receive Unit (MRU)

   The MRU is a internetwork-layer indication of maximum datagram size
   that can be received by a peer.  This does not include link-layer
   encapsulation and framing, but includes padding which can be added
   for small frames.

   Some link-layer protocols [RFC-1661] define a mechanism for adjusting
   the maximum frame size.  In addition, this protocol provides the
   advertisement of an MRU independently of the link-layer.

   When the advertised MRU of a peer node is less than the configured
   link MTU, that MRU MUST be maintained on a per peer basis.



2.5.  Incoming Interface

   On receipt of a link-layer unicast, broadcast or multicast packet,
   the node SHOULD check against a list of valid link addresses.

   For each received packet, the link-layer MUST pass the following
   information to the internetwork-layer:

   (1)   the datagram itself.

   (2)   the length of the data portion of the link-layer frame (not
         including encapsulation and framing).

   (3)   the identity of the physical interface from which the datagram
         was received.

   (4)   the classification of the received destination link-layer



Simpson                  expires in six months                  [Page 2]


DRAFT                   IPv6 Neighbor Discovery            November 1994


         address as unicast, broadcast, or multicast.

   In addition, the link-layer SHOULD provide:

   (5)   the source link-layer address, if any.

   RATIONALE:
      This section is included because other parts of this document
      require specific information to be passed across this layer
      boundary.

      Although every different medium typically has a different address
      format, the broadcast and multicast addresses are an important
      special case.

      Some link adapters check only a coarse grained hash or suffix of
      the link destination.  It is the responsiblity of the interface
      implementation to prevent leaking.  See also "Processing
      Datagrams".

      The source link address might be required in some implementations
      to map an essentially transient link-layer address (such as, a
      Frame-Relay DLCI) to the more stable family media address (that
      is, E.164).



2.6.  Outgoing Interface

   For each transmitted packet, the internetwork-layer MUST pass the
   following information to the link-layer:

   (1)   the datagram itself.

   (2)   the length of the datagram.

   (3)   the destination physical interface.

   (4)   the destination link-layer address, if any.

   In addition, the internetwork-layer SHOULD provide:

   (5)   the link-layer priority value.

   The link-layer MUST notify the internetwork-layer if the packet to be
   transmitted causes a link-layer precedence-related error.





Simpson                  expires in six months                  [Page 3]


DRAFT                   IPv6 Neighbor Discovery            November 1994


2.7.  Unreachable

   The link-layer MUST NOT report a Destination Unreachable error to the
   internetwork-layer solely because there is no Hop Cache entry for a
   particular Destination.














































Simpson                  expires in six months                  [Page 4]


DRAFT                   IPv6 Neighbor Discovery            November 1994


3.  Sending Datagrams

   For outgoing datagrams, the internetwork-layer:

   (1)   sets any fields not set by the transport-layer.

   (2)   chooses the interface and next hop.

   (3)   fragments the datagram if necessary, when intentional
         fragmentation is configured.

   (4)   passes the packet(s) to the appropriate link-layer interface.



3.1.  Choosing a Source Address

   When a node sends a datagram, the IPv6 Source MUST be one of its own
   IPv6 Unicast Addresses (not an IPv6 Cluster or Multicast Address).

   If the datagram is sent in response to a received datagram, the
   Destination from that datagram SHOULD be used as the Source for the
   response (unless it was an IPv6 Cluster or Multicast Address).

   An application MUST be able to explicitly specify the Source for
   initiating a connection or a request.

   If the Source to be used is unknown, a Source MUST be selected by the
   internetwork-layer.

   (a)   When no Router Advertisements have been heard, the Destination
         is assumed to be on an attached link.

         The Source is chosen from an IPv6 Unicast Address which is
         bound to any interface.

   (b)   When one or more Router Advertisements have been heard, the
         Router List is examined.

         If the Destination exactly matches the Primary Identifier of a
         router, a Routing-Information entension, or an Other-Identifier
         extension, then the Source is chosen from the interface on
         which the advertisement was received.

   (c)   The Destination is compared against the Cluster-prefixes
         configured for each interface, or learned from Routing-
         Information and Other-Identifier extensions.




Simpson                  expires in six months                  [Page 5]


DRAFT                   IPv6 Neighbor Discovery            November 1994


         If the Destination exactly matches one of the Cluster-prefixes,
         then the Source is chosen from the indicated interface.

   (d)   If the Destination does not match any Cluster-prefixes, then
         the Source is chosen from the interface based on the preferred
         router, as described in the "Router Selection" section which
         follows.

   When more than one IPv6 Unicast Address meets the criteria, the
   Source chosen SHOULD have the longest prefix match with the
   Destination.

   Other selection preferences are implementation dependent.

   NOTE:
      This process is essentially the same as choosing the next hop (see
      "Next Hop Decision").  Implementors might be able to combine the
      two functions.



3.2.  Hop Cache

   To efficiently send a series of datagrams to the same Destination,
   each node MUST keep a cache of prior decisions, indexed by
   Destination.  The cache entry might point directly to the
   Destination, or to a router which handles the Destination.

   The cache entry MUST include the media address (if any) to be used to
   send the datagram.  It MAY contain other information which records
   previous experience related to the Destination.

   If the cache contains no information for a particular Destination, a
   determination is made where to send the datagram.  This is described
   in the "Next Hop Decision" section which follows.

   While scanning or making changes to the Hop Cache entries, whenever
   the LifeTime expires in any entry that was created as a result of a
   received advertisement (of any kind), that entry is discarded.



3.3.  Next Hop Decision

   The next hop decision is based on the IPv6 Destination.  If the
   Destination can be readily determined to be on an attached link, the
   datagram is sent directly to the Destination.  Otherwise, it is sent
   to a router.



Simpson                  expires in six months                  [Page 6]


DRAFT                   IPv6 Neighbor Discovery            November 1994


   To determine the next hop, the following algorithm MUST be used:

   (a)   If the Destination is the IPv6 Loopback Address, or an IPv6
         Multicast Address with scope intra-node, process as an incoming
         datagram.

   (b)   If the Destination is another scope of IPv6 Multicast Address,
         simply pass the datagram to the link-layer for any indicated
         interface(s).

   (c)   When no Router Advertisements have been heard, the Destination
         is assumed to be on an attached link.

         For every interface, the Destination is located as described in
         "Sending General Solicitations".

   (d)   When one or more Router Advertisements have been heard, the
         Router List is examined.

         If the Destination exactly matches the Primary Identifier of a
         router, a Routing-Information entension, or an Other-Identifier
         extension, then the datagram is sent directly to the indicated
         router (using the Media-Access extension provided, if any).

   (e)   The Destination is compared against the Cluster-prefixes
         configured for each interface, or learned from Routing-
         Information and Other-Identifier extensions.  The Cluster-bit
         indicates whether the Cluster-prefix is confined to a single
         link.

         If the Destination exactly matches one of the Cluster-prefixes,
         and the Cluster-bit is set, then the Destination is assumed to
         be on that specific link.  For that interface, the Destination
         is located as described in "Sending General Solicitations".

   (f)   If the Destination exactly matches one of the Cluster-prefixes,
         but the Cluster-bit is not set, then the datagram is sent
         directly to the indicated router (using the Media-Access
         extension provided, if any).

   (g)   If the Destination does not match any Cluster-prefixes, then
         the datagram is sent to a single preferred router, as described
         in the "Router Selection" section which follows.

   For a node with multiple interfaces, when one or more Router
   Advertisements have been heard on some interfaces, but no Router
   Advertisements have been heard on other interfaces, the datagram is
   duplicated, and sent to both the preferred router and all those



Simpson                  expires in six months                  [Page 7]


DRAFT                   IPv6 Neighbor Discovery            November 1994


   interfaces without routers for which a peer entity is unknown.  This
   allows a node to continue operation in the presence of private or
   partitioned links.  The correct interface will be learned through the
   future advertisements of the target node.

   Every host MUST operate correctly in a minimal environment.  For
   example, if the host insists on finding at least one router to
   initialize, the host will be unable to operate on an isolated link.



3.4.  Router Selection

   The router selection is based on the IPv6 Source.  To decide which
   router to send a datagram, the following procedure is used:

   (a)   Cluster-prefixes are learned from the Routing-Information
         extension of Router Advertisements.  The Prefix_Size is the
         number of valid bits in the Cluster-prefix.

   (b)   The Source is compared to the list of Cluster-prefixes in the
         Router List.

   (c)   If a Cluster-prefix exactly matches the Source prefix extracted
         by the same Prefix_Size, then that router is one of the
         preferred routers for that Source.  The node selects the
         highest preference value among those matching routers.

   (d)   If there are no matching Cluster-prefixes, or the Source is the
         Unknown Address, then there is no preferred router for the
         Source.  The node selects the highest preference value among
         all routers found on all interfaces.

   (e)   If that router is not the best next-hop to the Destination,
         that router will forward the datagram to the best next-hop, and
         return a Local Redirect message to the sending node.  See
         "Sending Local Redirects".

   (f)   When the sending node receives a Local Redirect, it updates the
         next-hop in the appropriate Hop Cache entry, so that later
         datagrams to the same Destination will go directly to the best
         next-hop.  See "Processing Local Redirects".

   When the Destination is determined to be accessible through a router,
   a separate entry is created in the Hop Cache for that Destination,
   and the cache entry for the router is used to send the datagram.  The
   Router List entry might be duplicated in the Hop Cache, or a system
   of pointers could be used.  In any case, the Hop Cache entry for the



Simpson                  expires in six months                  [Page 8]


DRAFT                   IPv6 Neighbor Discovery            November 1994


   Destination MUST have the same LifeTime as the cache entry for the
   router.



3.5.  Static Routes

   A static route is typically a particular preset mapping for a
   Destination IPv6 Unicast or Cluster Address.  Static routes would be
   setup by administrators to override the normal automatic routing
   mechanism, and to handle exceptional situations.

   However, any static routing information is a potential source of
   failure as configurations change or equipment fails.

   Each such static route MUST be overridden by Local Redirects for the
   LifeTime indicated.  Otherwise, every datagram sent will result in a
   repeated Redirect message.



3.6.  Dead Node Detection

   The internetwork-layer MUST be able to detect the failure of a target
   that is listed in its Hop Cache.

   Each cache entry has a LifeTime, which is obtained through the
   advertisement messages.  When an entry expires, the routing
   availability of the Destination MUST be redetermined as if no prior
   entry had existed.

   Negative "advice" from other layers, such as excessive
   retransmissions by a transport-layer protocol, or a down indication
   from a link-layer interface, SHOULD be used to invalidate a cache
   entry.

   Positive "advice" from other layers, such as returning
   acknowledgements from a transport-layer protocol, MUST NOT extend the
   LifeTime of a cache entry.

   Promiscuous observation of link-layer or internetwork-layer Source
   fields MUST NOT extend the LifeTime of a cache entry.

   ICMP Echo "pings" MUST NOT be used to actively check a cache entry.

   RATIONALE:
      The exact constraints on the timeliness of "black hole" detection
      may vary somewhat depending on the nature of the node's mission,



Simpson                  expires in six months                  [Page 9]


DRAFT                   IPv6 Neighbor Discovery            November 1994


      but a node generally needs to detect a failed first-hop router
      quickly enough that transport-layer connections will not break
      before an alternate router can be selected.

      Passing advice from other layers of the protocol stack complicates
      the interfaces between the layers, but it is the preferred
      approach.

      The detection mechanism must not cause unacceptable load on the
      node, on congested links, or on first-hop router(s).

      Packets arriving from a particular link-layer address are evidence
      that the system at this address is alive.  However, turning this
      information into advice requires mapping the link-layer address
      into an IPv6 address, and then checking that IPv6 address against
      the entries in the Hop Cache.  This is probably prohibitively
      inefficient.

      Positive advice that is given for every datagram received could
      cause unacceptable overhead in the implementation.

      Ping scales poorly.





























Simpson                  expires in six months                 [Page 10]


DRAFT                   IPv6 Neighbor Discovery            November 1994


4.  Processing Datagrams

   For incoming datagrams, the internetwork-layer:

   (1)   verifies that the datagram is correctly formatted for the IP
         version.

   (2)   verifies that it is destined to the local host.

   (3)   processes subsequent headers and options.

   (4)   reassembles the datagram as necessary.

   (5)   passes the final payload to the appropriate transport-layer
         protocol module.



4.1.  Address List

   Each interface requires at least one IPv6 Address.

   Each IPv6 Address is bound to at most one interface.

   Each interface contains (at least) the following configurable
   variables:

   Address

      The IPv6 Unicast Address which is presently in use for the
      interface.

      Default: None

   Prefix_Size

      Each Address entry bound to a link interface has an associated
      Prefix_Size.  The value ranges from 0 to 127, and indicates the
      number of bits in the Address which define the Cluster-prefix for
      the link.

      When the value is not zero, the Address may be used to discern
      Cluster-prefix mapping.

      If all associated Prefix_Size values are zero, then prefix routing
      is not in use on that link.

      Default: 0



Simpson                  expires in six months                 [Page 11]


DRAFT                   IPv6 Neighbor Discovery            November 1994


   LifeTime

      The value for the time that the Address is associated with an
      interface.

      Default: infinity

   The Cluster-prefix(es) for a host interface SHOULD NOT be configured
   manually.

   The Cluster-prefix(es) for a router interface SHOULD be configured
   manually, until such time in the future that an automatic algorithm
   is developed.



4.2.  Details

   An incoming datagram is destined for the node when the IPv6
   Destination is:

   (1)   (one of) the IPv6 Unicast Address(es) on any interface of the
         node.

         A host MUST NOT discard an incoming datagram whose Destination
         does not correspond to the logical interface through which it
         is received.

   (2)   an IPv6 Cluster Address which corresponds to the incoming
         interface.

   (3)   an IPv6 Multicast group of which the host is a member on the
         incoming interface.

   A host MUST silently discard any IPv6 datagram which is not destined
   for itself.

   A router MUST silently discard any IPv6 unicast datagram which is not
   destined for itself, and that has arrived with a link-layer broadcast
   or multicast indication.  Other unicast, cluster and multicast
   routing details are beyond the scope of this document.

   All nodes MUST silently discard any IPv6 datagram containing an
   invalid IPv6 Source, such as an IPv6 Cluster or Multicast Address.
   This validation could be done in either the internetwork-layer, or by
   each protocol in the transport-layer.

   RATIONALE:



Simpson                  expires in six months                 [Page 12]


DRAFT                   IPv6 Neighbor Discovery            November 1994


      A mis-addressed datagram might be caused by a link-layer broadcast
      of a unicast datagram, or by any node that is confused or mis-
      configured.

      All nodes are required to check for a link-layer broadcast or
      multicast, as well as an internetwork-layer address.  This is
      necessary to prevent propagation of mis-addressed datagrams, which
      can result in broadcast storms.

      An architectural goal for hosts is to allow addresses to be
      featureless numbers, avoiding algorithms that required a knowledge
      of the format.  Otherwise, any future change in the format or
      interpretation of addresses will require host software changes.

      However, validation of IPv6 Cluster and Multicast Addresses
      violates this goal.  This is mitigated by the need to explicitly
      learn or join these groups.


































Simpson                  expires in six months                 [Page 13]


DRAFT                   IPv6 Neighbor Discovery            November 1994


5.  Sending General Solicitations

   Every IPv6 node MUST implement General Solicitations.

   The General Solicitation is used by any node to determine both the
   reachability and the link-layer destination of a neighboring node.



5.1.  Constants

      MAX_SOLICITATION_DELAY               1 second



5.2.  Configuration

   A node SHOULD 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:

      General_Solicitation_Interval

         The value to be placed in the Lifetime field of the Hop Cache
         entry for General Solicitations sent from the interface.  MUST
         NOT be less than 2 * MAX_SOLICITATION_DELAY, and SHOULD NOT be
         greater than 10 seconds.

         Default: 3 seconds



5.3.  Details

   A node is required to transmit a single General Solicitation, at the
   times specified in "Next Hop Decision", only when there is no entry
   in the Hop Cache.

   Whenever a solicitation is sent, a new Hop Cache entry is added with
   a LifeTime of General_Solicitation_Interval, and an unknown media
   address indication.  No further solicitations are sent until the Hop
   Cache entry expires.

   RATIONALE:
      This mechanism prevents flooding (repeating a solicitation at a
      high rate).



Simpson                  expires in six months                 [Page 14]


DRAFT                   IPv6 Neighbor Discovery            November 1994


      The General_Solicitation_Interval is chosen to allow sufficient
      round trip time for low bandwidth or congested links, and response
      time for heavily loaded nodes.

      A LifeTime this short could create noticeable overhead traffic on
      a link with large number of nodes.  Therefore, it may be necessary
      to configure busy routers or active servers with a longer
      General_Solicitation_Interval.

   The following method is used to send the solicitation:

   (a)   If the interface has no broadcast capability (a point-to-point
         link), and the peer entity is unknown (no advertisements
         received), the General Solicitation is sent on that interface.
         No media address is needed.

   (b)   If a virtual interface has no broadcast capability (a Frame-
         Relay or X.25 link), the General Solicitation is duplicated on
         each virtual circuit for which there is no known peer entity,
         as if they were each a separate point-to-point interface on a
         node with multiple physical interfaces.  The media address used
         is determined by the virtual circuit setup.

   (c)   If an interface has no multicast capability, the General
         Solicitation is sent as a link-layer broadcast.  The IPv6
         Destination is unchanged.

   (d)   For an interface with multicast capability, the General
         Solicitation is sent as a link-layer multicast.  The IPv6
         Destination is used to determine the appropriate multicast.

   If a node chooses to send a solicitation, it MUST delay transmission
   for a random amount of time between 0 and MAX_SOLICITATION_DELAY.

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

   Upon receiving a valid advertisement (of any kind) from the target
   Destination, the node MUST NOT send any solicitation on that
   interface (even if no solicitation has been sent yet) until the
   advertisement LifeTime expires.

   RATIONALE:



Simpson                  expires in six months                 [Page 15]


DRAFT                   IPv6 Neighbor Discovery            November 1994


      This serves to alleviate congestion when many nodes start up on a
      link at the same time, such as might happen after recovery from a
      power failure, or the periodic Hop Cache refresh of a large number
      of clients sharing a server.

The original datagram SHOULD be held (rather than discarded) until a
valid advertisement is received.

When additional datagrams for the same Destination are received, while
waiting for an advertisement, the most recent are saved, and earlier
datagrams MAY be discarded.

When the Hop Cache entry expires, any held datagrams MUST be discarded.

RATIONALE:
   Failure to follow this recommendation causes the first packet of
   every exchange to be lost.  Although transport-layer protocols can
   generally cope with packet loss by retransmission, packet loss does
   impact performance.

   For example, loss of a TCP open request causes the initial round-trip
   time estimate to be inflated.  UDP applications, such as the Domain
   Name System, are more seriously affected.




























Simpson                  expires in six months                 [Page 16]


DRAFT                   IPv6 Neighbor Discovery            November 1994


6.  Processing General Solicitations

   Every IPv6 node MUST process General Solicitations.

   All IPv6 nodes MUST accept the calculated Solicited-Nodes IPv6
   Multicast Address for every address bound to every interface.

   This is calculated by starting with the exclusive-or of each byte of
   the target IPv6 Unicast Address, then adding the result to the base
   Solicited-Nodes multicast (FFxx::70).

   For example, to calculate the destination value for target A::BC, the
   exclusive-or is D.  The calculated destination would be FFxx::7D.

   On receipt of a valid General Solicitation, the target node sends a
   General Advertisement, using the extension information provided.



6.1.  Validity

   All nodes MUST silently discard any received General Solicitation
   messages that do not satisfy the following validity checks:

   -  IPv6 Version is correct.

   -  IPv6 Source is a Unicast Address, is not the Unspecified Address,
      and is not a Cluster or Multicast Address.

   -  ICMP Checksum is correct.

   -  ICMP length (derived from the payload length) is 8 or more octets.

   -  The Other-Identifier extension indicates one of the IPv6 Unicast
      Addresses which is bound to any interface of the node.

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



6.2.  Details

   The solicitation has no LifeTime.  The extension information is used
   only for returning the advertisement, and then discarded.

   No new Hop Cache entry is added, and any existing entry is not
   updated.



Simpson                  expires in six months                 [Page 17]


DRAFT                   IPv6 Neighbor Discovery            November 1994


   RATIONALE:
      At the time of solicitation, the extension information might not
      be complete.  For example, the initial solicitation will not
      contain the Node-Heard for the target, and the target will not be
      assured that the path to the sender is complete.

      This also helps stagger solicitations.

   To process a General Solicitation, the node scans the list of
   extensions in it.



6.2.1.  Media-Access

   If a Media-Access extension is present, the information MAY be used
   to return the General Advertisement directly to the solicitor.  The
   Media-Access extension MAY appear anywhere in the list of extensions,
   but is most likely at the beginning or end.



6.2.2.  Node-Heard

   The absence of the Node-Heard extension serves as an indication that
   the solicitor has not yet heard any Router Advertisement.  The
   General Advertisement MUST be sent directly to the solicitor.

   If a Node-Heard extension is present which indicates that the
   solicitor has previously heard the node, it is confirmation of
   contact in those cases where the IPv6 Cluster is not entirely
   confined to the link.  The General Advertisement MAY be sent directly
   to the solicitor.


















Simpson                  expires in six months                 [Page 18]


DRAFT                   IPv6 Neighbor Discovery            November 1994


7.  Sending General Advertisements

   Every IPv6 node MUST implement General Advertisements.

   A General Advertisement is only sent in response to a General
   Solicitation.



7.1.  Constants

      GENERAL_ADVERTISEMENT_COUNT          4 entries



7.2.  Configuration

   A node SHOULD 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:

      Advertisement_LifeTime

         The value to be placed in the Lifetime field of General
         Advertisements sent from the interface.  MUST NOT be less than
         2 * MAX_SOLICITATION_DELAY, and SHOULD NOT be greater than 7200
         seconds (2 hours).

         Default: 600 seconds



7.3.  Details

   The IPv6 Source specified in the solicitation is used as the IPv6
   Destination in the advertisement, except under the following
   conditions:

   (a)   When the number of current Hop Cache entries (exclusive of
         static routes and router list entries) is
         GENERAL_ADVERTISEMENT_COUNT or more.

   (b)   When one or more Router Advertisements have been heard, and the
         Source does not match any Cluster-prefixes configured for an
         interface, or learned from Routing-Information extensions.




Simpson                  expires in six months                 [Page 19]


DRAFT                   IPv6 Neighbor Discovery            November 1994


   (c)   When the Source exactly matches one of the Cluster-prefixes,
         but the Cluster-bit is not set.

   In these cases, the advertisement is sent to the All-Nodes IPv6
   Multicast Address (FF02::1).  The scope is intra-link.

   RATIONALE:
      The decision to multicast an advertisement to all nodes, instead
      of repeating a unicast to each successive soliciting node, is a
      balance between disturbing a large number of nodes at the
      internetwork-layer against a greater amount of traffic.

      Assuming that each node communicates with every neighbor, the
      discovery traffic increases at the rate of 2n*n nodes.  When most
      nodes communicate only with a server, a multicast advertisement
      reduces the traffic to 2n.

      The multicast advertisements could be particularly disruptive when
      they interrupt the sleep mode of a battery powered device.
      However, the device might already have been disrupted by the
      solicitation when the link has broadcast and not multicast
      capability.  Also, it is precisely those devices which are most
      likely to be deployed on bandwidth-limited links, where a
      reduction of traffic is most important.

      In addition, roaming nodes which experience multipath and half-
      link conditions use the multicast advertisement to learn whether a
      direct contact is possible.























Simpson                  expires in six months                 [Page 20]


DRAFT                   IPv6 Neighbor Discovery            November 1994


8.  Processing General Advertisements

   Every IPv6 node MUST process General Advertisements.

   All IPv6 nodes MUST accept the All-Nodes IPv6 Multicast Address
   (FFxx::1) on every interface.

   On receipt of a valid General Advertisement, all nodes which have a
   Hop Cache entry for the Source update the cache entry with the
   current LifeTime and Media Address, and any other pertinent field
   values implemented.



8.1.  Validity

   All nodes MUST silently discard any received General Advertisement
   messages that do not satisfy the following validity checks:

   -  IPv6 Version is correct.

   -  IPv6 Source is a Unicast Address, is not the Unspecified Address,
      and is not a Cluster or Multicast Address.

   -  ICMP Checksum is correct.

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

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



8.2.  Details

   To process a General Advertisement, the node scans the list of
   extensions in it.



8.2.1.  Media-Access

   If a Media-Access extension is present, the Hop Cache is updated with
   the information.  The Media-Access extension MAY appear anywhere in
   the list of extensions, but is most likely at the beginning or end.





Simpson                  expires in six months                 [Page 21]


DRAFT                   IPv6 Neighbor Discovery            November 1994


8.2.2.  Other-Identifier

   The Other-Identifier extension is used to indicate IPv6 Addresses
   bound to other interfaces of the node, or other IPv6 addresses on the
   same interface which are not subsumed by the same Cluster-prefix.

   Each Other-Identifier MAY be used to add or update another Hop Cache
   entry.



8.2.3.  Node-Heard

   If a Node-Heard extension is present which indicates that the
   advertiser has previously heard the node, it is confirmation of
   contact in those cases where the IPv6 Cluster is not entirely
   confined to the link.

   If the Quality specified is not zero, but is less than the Quality
   for some other router Node-Heard extension, the Hop Cache entry MAY
   be updated to point to that router instead.  A Routing Header can be
   used to direct datagrams along the more reliable path.





























Simpson                  expires in six months                 [Page 22]


DRAFT                   IPv6 Neighbor Discovery            November 1994


9.  Sending Router Solicitations

   Every IPv6 node MUST implement Router Solicitations.

   When any node initializes, it MUST send the Router Solicitation to
   prompt the advertisement of neighboring routers.

   If (and only if) no advertisements from neighboring routers are
   forthcoming, the node MAY retransmit the Router Solicitation a small
   number of times, but then MUST desist from sending more
   solicitations.

   Any routers that subsequently start up, or that were not discovered
   because of packet loss or temporary 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 Router
   Advertisements, rather than increasing the number of solicitations
   that nodes are permitted to send.



9.1.  Constants

      MAX_SOLICITATIONS                    3 transmissions



9.2.  Configuration

   A node SHOULD 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:

      Router_Solicitation_Interval

         The value to be used for repeated Router Solicitations sent
         from the interface.  MUST NOT be less than 2 *
         MAX_SOLICITATION_DELAY, and SHOULD NOT be greater than 10
         seconds.

         Default: 3 seconds







Simpson                  expires in six months                 [Page 23]


DRAFT                   IPv6 Neighbor Discovery            November 1994


9.3.  Details

   A node is required to transmit up to MAX_SOLICITATIONS messages from
   any of its interfaces after any of the following events:

   -  The interface is initialized at system startup time.

   -  The interface is reinitialized after a temporary interface failure
      or after being temporarily disabled by system management.

   -  A router has its forwarding capability for that interface turned
      off by system management.

   If a node chooses to send a solicitation after one of the above
   events, it should delay transmission in the same manner as described
   in the "Sending General Solicitations" chapter "Details" section.

   The small number of retransmissions of a solicitation, which are
   permitted if no advertisement is received, should be sent at
   intervals of Router_Solicitation_Interval without further
   randomization.

   Upon receiving a valid Router Advertisement subsequent to one of the
   above events, the node MUST NOT send any solicitation on that
   interface (even if no solicitation has been sent yet) until the next
   time one of the above events occurs.

























Simpson                  expires in six months                 [Page 24]


DRAFT                   IPv6 Neighbor Discovery            November 1994


10.  Processing Router Solicitations

   Every IPv6 router MUST process Router Solicitations.

   All IPv6 routers MUST accept the All-Routers IPv6 Multicast Address
   (FFxx::2) on every interface for which forwarding is enabled.

   On receipt of a valid Router Solicitation, the target router sends a
   Router Advertisement.

   If the IPv6 Source does not match one of the router's own IPv6
   Cluster Addresses on the arrival interface, by matching the
   associated Cluster-prefix, the sender is considered a Mobile Node.
   The location of every reachable Mobile Node is maintained separately
   within the router.



10.1.  Validity

   A non-router MUST silently discard any received Router Solicitation
   messages.

   A router MUST silently discard any received Router Solicitation
   messages that do not satisfy the following validity checks:

   -  IPv6 Version is correct.

   -  IPv6 Source is the Unspecified Address, or an IPv6 Unicast
      Address, but is not a Cluster or Multicast Address.

   -  ICMP Checksum is correct.

   -  ICMP length (derived from the payload length) is 8 or more octets.

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



10.2.  Details

   To process a Router Solicitation, the node scans the list of
   extensions in it.







Simpson                  expires in six months                 [Page 25]


DRAFT                   IPv6 Neighbor Discovery            November 1994


10.2.1.  Media-Access

   If a Media-Access extension is present, the information MAY be used
   to return the Router Advertisement directly to the solicitor.  The
   Media-Access extension MAY appear anywhere in the list of extensions,
   but is most likely at the beginning or end.



10.2.2.  Node-Heard

   The absence of the Node-Heard extension serves as an indication that
   the solicitor has not yet heard any Router Advertisement.  The Router
   Advertisement MUST be sent promptly, at the accelerated initial rate.

   If a Node-Heard extension is present which indicates that the
   solicitor has previously heard the node, it is confirmation of
   contact in those cases where the IPv6 Cluster is not entirely
   confined to the link.
































Simpson                  expires in six months                 [Page 26]


DRAFT                   IPv6 Neighbor Discovery            November 1994


11.  Sending Router Advertisements

   Every IPv6 router MUST implement Router Advertisements.

   A Router Advertisement is sent periodically, and also in response to
   a Router Solicitation.



11.1.  Constants

      MAX_INITIAL_ADVERTISEMENTS           3 transmissions

      MAX_INITIAL_ADVERT_INTERVAL         10 seconds

      MAX_RESPONSE_DELAY                   2 seconds



11.2.  Configuration

   A router 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:

      Minimum_Advertisement_Interval

         The minimum time allowed between sending unsolicited Router
         Advertisements from the interface.  MUST NOT be less than 2 *
         MAX_SOLICITATION_DELAY.

         Default: 120 seconds

      Maximum_Advertisement_Interval

         The maximum time allowed between sending Router Advertisements
         from the interface.  MUST NOT be less than
         Minimum_Advertisement_Interval, and SHOULD NOT be greater than
         1800 seconds (half hour).

         Default: 1.50 * Minimum_Advertisement_Interval

      Advertisement_LifeTime

         The value to be placed in the Lifetime field of Router
         Advertisements sent from the interface.  MUST NOT be less than



Simpson                  expires in six months                 [Page 27]


DRAFT                   IPv6 Neighbor Discovery            November 1994


         Maximum_Advertisement_Interval, and SHOULD NOT be greater than
         7200 seconds (2 hours).

         Default: 3 * Maximum_Advertisement_Interval

   For each of the IPv6 Unicast or Cluster Addresses of each interface:

      Advertise

         A flag indicating whether or not the IPv6 Address is to be
         advertised.

         Default: TRUE

      Preference

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

         Values are in the range 0 to 255.  Higher values mean more
         preferable.  The minimum value zero is reserved to indicate
         that the IPv6 Address, even though it may be advertised, is not
         to be used by neighboring hosts as a default Router Address.
         The maximum value 255 is reserved to indicate that the
         preference was locally configured, and not learned through
         advertisments.

         Default: 128

      It is useful to configure an IPv6 Address with a preference level
      of zero (rather than simply setting its Advertise flag to FALSE)
      when advertisements are being used for "black hole" detection.  In
      particular, a router that is to be used to reach only specific
      destinations could advertise a preference level of zero (so that
      neighboring hosts will not use it as a default router for reaching
      arbitrary destinations) and a non-zero lifetime (so that
      neighboring hosts that have been redirected or configured to use
      it can detect its failure by timing out the reception of its
      advertisements).

      DISCUSSION:

         It has been suggested that, when the preference level of an
         IPv6 Address has not been explicitly configured, a router could
         set it according to the metric of the router's "default route"
         (if it has one), rather than defaulting as suggested above.
         Thus, a router with a better metric for its default route would



Simpson                  expires in six months                 [Page 28]


DRAFT                   IPv6 Neighbor Discovery            November 1994


         advertise a higher preference level for its IPv6 Address.
         (Note that routing metrics that are encoded such that "lower is
         better" would have to be inverted before being used as
         preference levels in Router Advertisement messages.) Such a
         strategy might reduce the amount of redirect traffic on some
         links by making it more likely that the host's first choice for
         reaching an arbitrary destination is also the best choice.

         On the other hand, redirect traffic is rarely a significant
         load on a 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
         routers other than the one with the best default route).  Also,
         since the routing algorithms learn of neighboring routers from
         the advertisements, and the default routes are learned from the
         routing algorithms, the calculated preference may be unstable
         from time to time.  This document makes no recommendation
         concerning this issue, and implementors are free to try such a
         strategy, as long as they also support static configuration of
         preference levels as specified above.



11.3.  Details

   The term "advertising interface" refers to any functioning and
   enabled interface that has at least one IPv6 Address whose configured
   Advertise flag is TRUE.

   From each advertising interface, the router MUST transmit Router
   Advertisements.

   The advertisements are not strictly periodic.  The interval between
   subsequent transmissions is randomized to reduce the probability of
   synchronization with the advertisements from other routers on the
   same link.  This is done by maintaining a separate 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 between the configured
   Minimum_Advertisement_Interval and Maximum_Advertisement_Interval.
   Expiration of the timer causes the next advertisement to be sent, and
   a new random value to be chosen.

   For the first few advertisements sent from an interface (up to
   MAX_INITIAL_ADVERTISEMENTS), if the randomly chosen interval is
   greater than MAX_INITIAL_ADVERT_INTERVAL, the timer should be set to
   MAX_INITIAL_ADVERT_INTERVAL instead.  Using this smaller interval for
   the initial advertisements increases the likelihood of a router being



Simpson                  expires in six months                 [Page 29]


DRAFT                   IPv6 Neighbor Discovery            November 1994


   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 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 one or more IPv6 Addresses whose Advertise flag is
      TRUE,

   -  enabling IPv6 forwarding capability (changing the node from a host
      to a router), when the interface has one or more IPv6 Addresses
      whose Advertise flag is TRUE,

   -  setting the Advertise flag of one or more of the interface's IPv6
      Addresses to TRUE (or adding a new IPv6 Address with a TRUE
      Advertise flag), when previously the interface had no IPv6 Address
      whose Advertise flag was TRUE.

   In such cases, the router MUST commence transmission of periodic
   advertisements on the new advertising interface, limiting the first
   few advertisements to intervals no greater than
   MAX_INITIAL_ADVERT_INTERVAL.  In the case of a host becoming a
   router, the node MUST also accept the All-Routers IPv6 Multicast
   Address on all interfaces on which the router supports multicast
   (whether or not they are advertising interfaces).

   An interface MAY also cease to be an advertising interface, through
   actions of system management such as:

   -  shutting down the node,

   -  administratively disabling the interface,

   -  disabling IPv6 forwarding capability (changing the node from a
      router to a host),

   -  setting the Advertise flags of all of the interface's IPv6
      Addresses to FALSE.

   In such cases, the router MUST transmit a final multicast
   advertisement on the interface, identical to its previous
   transmission, but with a Lifetime of zero.  In the case of a router
   becoming a host, the node MUST also drop the All-Routers IPv6
   Multicast Address on all interfaces on which the router supports
   multicast (whether or not they had been advertising interfaces).




Simpson                  expires in six months                 [Page 30]


DRAFT                   IPv6 Neighbor Discovery            November 1994


   When the Advertise flag of one or more of an interface's IPv6
   Addresses are set to FALSE by system management, but there remain
   other IPv6 Addresses on that interface whose Advertise flags are
   TRUE, the router SHOULD send a single multicast advertisement
   containing only those IPv6 Addresses whose Advertise flags were set
   to FALSE, with a Lifetime of zero.

   In addition to the periodic unsolicited advertisements, a router MUST
   send advertisements in response to valid Router Advertisements or
   Router Solicitations received on any of its advertising interfaces.
   If the received advertisement or solicitation does not contain any
   Node-Heard extension, and the time since the previous advertisement
   is greater than MAX_INITIAL_ADVERT_INTERVAL, the router MUST
   multicast an advertisement from that interface.

   Whenever these response advertisements are sent, the node MUST delay
   transmission for a random amount of time between 0 and
   MAX_RESPONSE_DELAY, in order to prevent synchronization with other
   responding routers, and to allow multiple closely-spaced
   solicitations to be answered with a single advertisement.  The
   interface's interval timer is reset to a new random value, as with
   unsolicited advertisements.

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






















Simpson                  expires in six months                 [Page 31]


DRAFT                   IPv6 Neighbor Discovery            November 1994


12.  Processing Router Advertisements

   Every IPv6 node MUST process Router Advertisements.

   All IPv6 nodes MUST accept the All-Nodes IPv6 Multicast Address
   (FFxx::1) on every interface.

   Each router saves the information contained in the advertisements, in
   order to respond to future requests.  Any other action on receipt of
   such messages by a router (for example, as part of a "peer discovery"
   process) is beyond the scope of this document.

   Each host saves the information contained in the advertisements, in
   order to determine the next-hop when sending datagrams.  Hop
   determination is elaborated in "Sending Datagrams".



12.1.  Validity

   All nodes MUST silently discard any received Router Advertisement
   messages that do not satisfy the following validity checks:

   -  IPv6 Version is correct.

   -  IPv6 Source is a Unicast Address, is not the Unspecified Address,
      and is not a Cluster or Multicast Address.

   -  ICMP Checksum is correct.

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

   -  At least one Routing-Information extension is present.

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



12.2.  Router List

   Host Requirements -- Communication Layers [1], Section 3.3.1.6,
   specifies that each host (must) support a configurable list of
   default routers.  The purpose of the Router Advertisement messages is
   to eliminate the need to configure that list.

   Each entry in the list contains (at least) the following configurable



Simpson                  expires in six months                 [Page 32]


DRAFT                   IPv6 Neighbor Discovery            November 1994


   variables:

   Router_Identifier

      An IPv6 Unicast Address of a default router.

      Default: (none)

   Prefix_Size

      Each router entry has an associated Prefix_Size.  The value ranges
      from 0 to 127, and indicates the number of bits in the IPv6
      Address which define the Cluster-prefix for the link.  A value of
      zero indicates an end-point IPv6 Address.  When the value is not
      zero, the IPv6 Address may be used to discern Cluster-prefix
      mapping.

      If all associated Prefix_Size values are zero, then prefix routing
      is not in use on that link.

      Default: 0

   Preference

      The preferability of the Router_Identifier as a default router
      choice, relative to other router interfaces serving the same
      Cluster-prefix on the same link.  Host Requirements does not
      specify how this value is to be encoded.

      The values used here are defined as in Router Advertisements.
      Values are in the range 0 to 255.  Higher values mean more
      preferable.  The minimum value zero is reserved to indicate that
      the IPv6 Address, even though it may be advertised, is not to be
      used by neighboring hosts as a default Router Address.  The
      maximum value 255 is reserved to indicate that the preference was
      locally configured, and not learned through advertisments.

      Default: 255

   Default routers and preference levels SHOULD NOT be configured
   manually.  On links for which router discovery is administratively
   disabled, it MAY continue to be necessary to configure the default
   Router List in each host.

      NOTES: Any router IPv4 Address acquired from the "Gateway"
      subfield of the vendor extensions field of a BOOTP packet [RFC-
      BOOTP?11] are considered to be configured; they are assigned the
      default preference level of 255, and they do not have an



Simpson                  expires in six months                 [Page 33]


DRAFT                   IPv6 Neighbor Discovery            November 1994


      associated LifeTime.

      Any IPv4 Address found in the "giaddr" field of a BOOTP packet
      [RFC-BOOTP?3] identifies a BOOTP forwarder which is not
      necessarily a router; an entry SHOULD NOT be installed in the
      default Router List.



12.3.  Details

   To process a Router Advertisement, the node scans the list of
   extensions in it.



12.3.1.  Media-Access

   If a Media-Access extension is present, the Router List is updated
   with the information.  The Media-Access extension MAY appear anywhere
   in the list of extensions, but is most likely at the beginning or
   end.



12.3.2.  Change-Identifier

   This extension gives advance indication that an address or prefix
   will no longer be routable.  Applications SHOULD cease to accept new
   connections with the old value.  Existing connections SHOULD issue a
   Remote Redirect.

   Change-Identifier extensions MUST preceed Routing-Information
   extensions.

   -  If the Prefix_Size is zero, the IPv6 Address indicates the change
      of a single node, without affecting other nodes on that link.

   -  If the Prefix_Size is not zero, the IPv6 Address indicates the
      change of Cluster-prefix for all nodes on that link.

   -  The IPv6 Address and Prefix_Size are compared against any IPv6
      Addresses defined for the node.  If there is a match, a Remote
      Redirect MAY be sent to correspondents to inform them of the
      change.

   The node MUST continue to accept datagrams destined for the old IPv6
   Address(es), until such time as all stimulus for maintaining the



Simpson                  expires in six months                 [Page 34]


DRAFT                   IPv6 Neighbor Discovery            November 1994


   entry has expired.  This implies that the node will maintain a
   LifeTime for most sources of IPv6 Addresses, such as DNS records and
   dynamic configuration.



12.3.3.  Routing-Information

   Routing-Information extensions MUST preceed Other-Identifier
   extensions.

   -  If the Prefix_Size is not zero, the IPv6 Address and Prefix_Size
      are compared against any IPv6 Addresses defined for the node.  If
      there is a match, the IPv6 Address is associated with the
      interface on which the message was received, and the Prefix_Size
      is set to the advertised Prefix_Size.

   -  If the IPv6 Address is not already present in the Router List, a
      new entry is added to the list, containing the IPv6 Address along
      with its accompanying preference level, and the Lifetime value
      from the advertisement.

   -  If the IPv6 Address is already present in the Router List as a
      result of a previously-received advertisement, its preference
      level is updated and its LifeTime is reset to the value in the
      newly-received advertisement.

   -  If the IPv6 Address is already present in the Router List as a
      result of static configuration, no change is made to its
      preference level.  There is no LifeTime associated with a
      configured IPv6 Address.  To limit the storage needed for the
      default Router List, the host MAY choose not to store all of the
      router IPv6 Addresses discovered via advertisements.  The host
      SHOULD discard those IPv6 Addresses with lower preference levels
      in favor of those with higher levels.

   It is desirable to retain more than one default router in the list;
   if the current choice of default router is discovered to be down, the
   host may immediately choose another default router without having to
   wait for the next advertisement to arrive.

   Any router IPv6 Address advertised with a preference level of zero is
   not to be used by the host as default router IPv6 Address.  Such an
   IPv6 Address may be omitted from the default Router List, unless its
   LifeTime is being used as a "black-hole" detection mechanism.






Simpson                  expires in six months                 [Page 35]


DRAFT                   IPv6 Neighbor Discovery            November 1994


12.3.4.  Other-Identifier

   The Other-Identifier extension is used to indicate IPv6 Addresses
   bound to other interfaces of the router, or other IPv6 Addresses on
   the same interface which are not subsumed by the same Cluster-prefix.

   -  If the IPv6 Address is not already present in the Router List, a
      new entry MAY be added, containing the IPv6 Address, the
      preference level set to zero, and the Lifetime value from the
      advertisement.

   -  If the IPv6 Address is already present in the Router List as a
      result of a previously-received advertisement, and its preference
      level is zero, its LifeTime is reset to the value in the newly-
      received advertisement.

   -  If the IPv6 Address is already present in the Router List as a
      result of static configuration, no change is made to its
      preference level.  There is no LifeTime associated with a
      configured IPv6 Address.

      To limit the storage needed for the default Router List, the host
      MAY choose not to store all of the other IPv6 Addresses discovered
      via advertisements.  The most preferred router is used for unknown
      Destinations, and it will send a redirect when appropriate.



12.3.5.  Node-Heard

   As in a General or Router Solicitation, the absence of the Node-Heard
   extension serves as an indication that the router has not yet heard
   any other Router Advertisement.

   If the Quality specified is not zero, but is less than the Quality
   for some other router Node-Heard extension, the Hop Cache entry MAY
   be updated to point to that router instead.  A Routing Header can be
   used to direct datagrams along the more reliable path.













Simpson                  expires in six months                 [Page 36]


DRAFT                   IPv6 Neighbor Discovery            November 1994


13.  Sending Local Redirects

   Every IPv6 router MUST implement Local Redirect.

   A router sends a Local Redirect when it receives datagrams for which
   that router is not the best next-hop to the Destination.  The router
   will forward the datagram to the best next-hop, and return a Local
   Redirect message to the sending node.

   A host SHOULD NOT send a Local Redirect.



14.  Processing Local Redirects

   Every IPv6 node MUST process Local Redirects.

   On receipt of a valid Local Redirect, a host MUST update its Hop
   Cache.



14.1.  Validity

   All nodes MUST silently discard any received Local Redirect messages
   that do not satisfy the following validity checks:

   -  IPv6 Version is correct.

   -  IPv6 Source is a Unicast Address, is not the Unspecified Address,
      is not a Cluster or Multicast Address, and is the current next hop
      router for the target specified in the Other-Identifier
      extension(s).

   -  ICMP Checksum is correct.

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

   -  The Other-Identifier extension indicates one of the IPv6 Unicast
      or Cluster Addresses in the Hop Cache.

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







Simpson                  expires in six months                 [Page 37]


DRAFT                   IPv6 Neighbor Discovery            November 1994


14.2.  Details

   Since the Cluster-prefixes bound to an interface are not required to
   be relevant for all Destinations, particularly in the presence of
   Mobile Nodes, the next hop specified is always presumed to be
   accessible via the same interface through which the Redirect arrived,
   and the Redirect MUST NOT be discarded.

   RATIONALE:
      A Mobile Node will likely not have a prefix which matches any
      router advertised prefixes.  When a local host (such as a printer
      or DNS) responds to a message from the Mobile Node, it will
      initially send to its preferred router.  That router will send a
      Redirect to the Mobile Node.

   Every host MUST be prepared to accept both Host and Network
   Redirects???  Since the Cluster-prefix appropriate to the Destination
   is generally not known, a Network Redirect message SHOULD be treated
   identically to a Host Redirect message.  That is, the cache entry for
   the Destination (only) would be updated (or created when an entry for
   that Destination did not exist), with a Prefix_Size of zero.

   RATIONALE:
      This recommendation is to protect against routers that erroneously
      send Network Redirects for a prefix routed link, in violation of
      the router requirements.  (Do we still have the Network
      Redirect???)
























Simpson                  expires in six months                 [Page 38]


DRAFT                   IPv6 Neighbor Discovery            November 1994


15.  Sending Remote Redirects

   Every IPv6 node MUST implement Remote Redirects.

   A node sends a Remote Redirect when it receives a Router
   Advertisement containing Change-Identifier extensions.  The Hop Cache
   is examined for Destinations accessed through that router.  Those
   remote nodes are sent the Remote Redirect with an indication of a
   Care-Of-Address to use in order to reach the expiring identification
   of the node.

   A Mobile Node MAY also send a Remote Redirect when it receives a
   datagram which does not have a Routing Header containing its current
   Care-Of-Address(es).

   The Remote Redirect is only sent to those remote nodes with which the
   node maintains a Security Association.



16.  Processing Remote Redirects

   Every IPv6 node MUST process Remote Redirects.

   On receipt of a valid Remote Redirect, the node uses a Routing Header
   to reach the sender.



16.1.  Validity

   All nodes MUST silently discard any received Remote Redirect messages
   that do not satisfy the following validity checks:

   -  IPv6 Version is correct.

   -  IPv6 Source is a Unicast Address, is not the Unspecified Address,
      is not a Cluster or Multicast Address, and indicates one of the
      IPv6 Unicast Addresses in the Hop Cache.

   -  Authentication Header is valid.

   -  ICMP Checksum is correct.

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





Simpson                  expires in six months                 [Page 39]


DRAFT                   IPv6 Neighbor Discovery            November 1994


16.2.  Details

   To process a Remote Redirect, the node scans the list of extensions
   in it.



16.2.1.  Other-Identifier

   The Other-Identifier extension is used to indicate the IPv6 Unicast
   or Cluster Address which is used as a Care-Of-Address to reach the
   Source.







































Simpson                  expires in six months                 [Page 40]


DRAFT                   IPv6 Neighbor Discovery            November 1994


A.  Configuration Summary

A.1.  Router Configuration

   A router requires at least one IPv6 Address to be configured.

   For each interface, a Prefix_Size is assigned to each IPv6 Address,
   unless automatic prefix discovery is in place.

      Note that this procedure minimizes the number of items to be
      configured, and possible configuration errors.

   Optionally, other values MAY be altered from their defaults, such as
   preference and advertisement lifetime.

   Optionally, routing protocols MAY require additional values to be
   configured, such as metric and priority.  Such functions are beyond
   the scope of this document.



A.2.  Host Configuration

   Most hosts need no prior configuration.

   A node attached to a multi-access media creates a local-use unicast
   address from the media address.

   A node attached to a point-to-point media (using the Point-to-Point
   Protocol [RFC-1661]) can be dynamically assigned either a global or
   local unicast address.

   Other nodes require configuration of an IPv6 Address, as described in
   "Address List".

















Simpson                  expires in six months                 [Page 41]


DRAFT                   IPv6 Neighbor Discovery            November 1994


B.  Hop Cache Implementation

   Each Hop Cache entry needs to include the following items:

   (1)  LifeTime
   (2)  Next-hop interface (when a node is multi-homed)
   (3)  Next-hop media address
   (4)  Destination IPv6 Address
   (5)  Destination Prefix_Size
   (6)  Source IPv6 Address
   (7)  Flow Label
   (8)  Path Maximum Transmission Unit
   (9)  Path Round Trip Time

   Field (4) MAY be the full IPv6 Address of the Destination, or the
   Cluster which includes the Destination.  This is determined by the
   Cluster-prefix size in (5).

   Field (7) SHOULD be included, as it is related to the Source in (6).

   DISCUSSION:
      Each Hop Cache entry defines the end-points of an internetwork
      path.  Although the connecting path may change dynamically in an
      arbitrary way, the transmission characteristics of the path tend
      to remain approximately constant over a time period longer than a
      single typical host-host transport connection.  Therefore, a Hop
      Cache entry is a natural place to cache data on the properties of
      the path.

      Examples of such properties might be the maximum unfragmented
      datagram size, or the average round-trip delay measured by a
      transport protocol.  This data will generally be both gathered and
      used by a higher layer protocol (that is, by TCP or by an
      application using UDP).  Experiments are currently in progress on
      caching path properties in this manner.

      There is no consensus on whether the Hop Cache should be keyed on
      destinations alone, or allow both Unicast and Cluster addresses.
      Those who favor the use of only node identifiers argue that:

      (1)   Redirect messages will generally result in entries keyed on
            nodes.  The simplest and most general scheme would be to
            only use node identifiers.

      (2)   The internetwork layer may not always know the Prefix_Size
            for a remote Cluster.

      (3)   The use of only node identifiers may allow the Internet



Simpson                  expires in six months                 [Page 42]


DRAFT                   IPv6 Neighbor Discovery            November 1994


            architecture to be more easily extended in the future
            without any change to the hosts.

      The opposing view is that allowing a mixture of destination nodes
      and clusters in the Hop Cache:

      (1)   Saves memory space.

      (2)   Leads to a simpler data structure, easily combining the
            cache with the tables of default and static routes.

      (3)   Provides a more useful place to cache path properties.

   The cache needs to be large enough to include entries for the maximum
   number of destinations that may be in use at one time.

   Advertisement updates could indefinitely continue to refresh
   otherwise unused entries.  A Hop Cache entry SHOULD also include
   control information used to choose an entry for replacement.  For
   example, this might take the form of a "recently used" bit, a use
   count, or a last-used timestamp.  It is also recommended that it
   include the time of last modification of the entry, for diagnostic
   purposes.

   An implementation may wish to reduce the overhead of scanning the Hop
   Cache for every datagram to be transmitted.  This may be accomplished
   with a hash table to speed the lookup, or by giving a connection-
   oriented transport protocol a "hint", or temporary handle on the
   appropriate cache entry, to be passed to the internetwork-layer with
   each subsequent datagram.

   Although the Hop Cache, the Router List, and a table of static routes
   are described as conceptually distinct, in practice they may be
   combined into a single "routing table" data structure.

















Simpson                  expires in six months                 [Page 43]


DRAFT                   IPv6 Neighbor Discovery            November 1994


C.  Proxy Advertisements

   A router MAY proxy for the identifiers of other nodes, using the
   Other-Identifier extension.

   This SHOULD only be used when the router is translating to another
   internetwork protocol format.












































Simpson                  expires in six months                 [Page 44]


DRAFT                   IPv6 Neighbor Discovery            November 1994


D.  Examples

D.1.  Simple Solicitation

   Assume host A has address 1 and host B has addresses 2 and 3.  There
   is only one interface on A, and only one interface on B.

   A is trying to talk to B, so it sends a General Solicitation to B,
   fills in portions of a Hop Cache entry, and waits for the General
   Advertisement from B.

   The Other-Identifier extension in the solicitation is 2.

   B sends a General Advertisement with the Media-Access for its
   interface.  It also puts 3 in an Other-Identifier extension.

   A MUST update its cache entry for 2.

   A SHOULD also add another cache entry for 3, using the same media
   address.  This is not required in a minimal memory implementation.



D.2.  Complex Solicitation

   Assume that host B had a different interface for 3 on the same link.

   If host A already had a Hop Cache entry for 3 (using the original
   media address), but the advertisement (above) contains a different
   media address for 3, A MUST add another cache entry for 3, pointing
   to 2.

   This is required in order that the purpose of redundant interfaces on
   the same link be fulfilled, and 3 is accessible through 2 (and vice
   versa) when its interface fails.

   The pairing of IPv6 address and media address can be considered a
   tuple consisting of {IPv6 address, interface, media address}.

   This allows annealing of partitioned links with no effort by hosts.











Simpson                  expires in six months                 [Page 45]


DRAFT                   IPv6 Neighbor Discovery            November 1994


Security Considerations

   There are a lot of Security issues which are not discussed in this
   memo.



Acknowledgements

   The document was initially composed of quotations from the RFC-1122
   "Requirements for Internet Hosts -- Communication Layers" (Robert
   Braden, Editor), and RFC-1256 "ICMP Router Discovery Messages" (Steve
   Deering, Editor), and "Requirements for IP Routers" (Almquist and
   Kastenholz, Editors).

   Thanks also for suggestions and contributions from the Simple-IP
   Working Group.

   Special thanks for implementation review by Ran Atkinson (Naval
   Research Laboratory), Alex Conta (DEC), Dan McDonald (Naval Research
   Laboratory), Fred Rabouw (Network Systems Netherlands), and Brad
   Stone (Hewlett-Packard).



Author's Address

   Questions about this memo can also be directed to:

      William Allen Simpson
      Daydreamer
      Computer Systems Consulting Services
      1384 Fontaine
      Madison Heights, Michigan  48071

      Bill.Simpson@um.cc.umich.edu
          bsimpson@MorningStar.com














Simpson                  expires in six months                 [Page 46]


DRAFT                   IPv6 Neighbor Discovery            November 1994


                           Table of Contents


     1.     Introduction ..........................................    1

     2.     Link-Layers ...........................................    1
        2.1       Address Resolution Protocol (ARP) ...............    1
        2.2       Trailers ........................................    1
        2.3       Maximum Transmission Unit (MTU) .................    1
        2.4       Maximum Receive Unit (MRU) ......................    2
        2.5       Incoming Interface ..............................    2
        2.6       Outgoing Interface ..............................    3
        2.7       Unreachable .....................................    4

     3.     Sending Datagrams .....................................    5
        3.1       Choosing a Source Address .......................    5
        3.2       Hop Cache .......................................    6
        3.3       Next Hop Decision ...............................    6
        3.4       Router Selection ................................    8
        3.5       Static Routes ...................................    9
        3.6       Dead Node Detection .............................    9

     4.     Processing Datagrams ..................................   11
        4.1       Address List ....................................   11
        4.2       Details .........................................   12

     5.     Sending General Solicitations .........................   14
        5.1       Constants .......................................   14
        5.2       Configuration ...................................   14
        5.3       Details .........................................   14

     6.     Processing General Solicitations ......................   17
        6.1       Validity ........................................   17
        6.2       Details .........................................   17
           6.2.1  Media-Access ....................................   18
           6.2.2  Node-Heard ......................................   18

     7.     Sending General Advertisements ........................   19
        7.1       Constants .......................................   19
        7.2       Configuration ...................................   19
        7.3       Details .........................................   19

     8.     Processing General Advertisements .....................   21
        8.1       Validity ........................................   21
        8.2       Details .........................................   21
           8.2.1  Media-Access ....................................   21
           8.2.2  Other-Identifier ................................   22
           8.2.3  Node-Heard ......................................   22



Simpson                  expires in six months                 [Page ii]


DRAFT                   IPv6 Neighbor Discovery            November 1994


     9.     Sending Router Solicitations ..........................   23
        9.1       Constants .......................................   23
        9.2       Configuration ...................................   23
        9.3       Details .........................................   24

     10.    Processing Router Solicitations .......................   25
        10.1      Validity ........................................   25
        10.2      Details .........................................   25
           10.2.1 Media-Access ....................................   26
           10.2.2 Node-Heard ......................................   26

     11.    Sending Router Advertisements .........................   27
        11.1      Constants .......................................   27
        11.2      Configuration ...................................   27
        11.3      Details .........................................   29

     12.    Processing Router Advertisements ......................   32
        12.1      Validity ........................................   32
        12.2      Router List .....................................   32
        12.3      Details .........................................   34
           12.3.1 Media-Access ....................................   34
           12.3.2 Change-Identifier ...............................   34
           12.3.3 Routing-Information .............................   35
           12.3.4 Other-Identifier ................................   36
           12.3.5 Node-Heard ......................................   36

     13.    Sending Local Redirects ...............................   37

     14.    Processing Local Redirects ............................   37
        14.1      Validity ........................................   37
        14.2      Details .........................................   38

     15.    Sending Remote Redirects ..............................   39

     16.    Processing Remote Redirects ...........................   39
        16.1      Validity ........................................   39
        16.2      Details .........................................   40
           16.2.1 Other-Identifier ................................   40

     APPENDICES ...................................................   41

     A.     Configuration Summary .................................   41
        A.1       Router Configuration ............................   41
        A.2       Host Configuration ..............................   41

     B.     Hop Cache Implementation ..............................   42

     C.     Proxy Advertisements ..................................   44



Simpson                  expires in six months                [Page iii]

DRAFT                   IPv6 Neighbor Discovery            November 1994


     D.     Examples ..............................................   45
        D.1       Simple Solicitation .............................   45
        D.2       Complex Solicitation ............................   45

     SECURITY CONSIDERATIONS ......................................   46

     ACKNOWLEDGEMENTS .............................................   46

     AUTHOR'S ADDRESS .............................................   46


Html markup produced by rfcmarkup 1.129c, available from https://tools.ietf.org/tools/rfcmarkup/