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ndp(7P)								       ndp(7P)

       ndp - Neighbor Discovery Protocol, NDP

       Neighbor	 Discovery  Protocol  (NDP)  is	 a  protocol used by hosts and
       routers to:

	      1.     Find the link-layer address of the neighbors known to  be
		     attached to the same link.

	      2.     Find  the neighboring routers that are willing to forward
		     packets on their behalf.

	      3.     Actively keep track of which neighbors are reachable  and
		     which are not.

	      4.     Search  for  alternate  routers when the path to a router

       To accomplish the above mentioned tasks, NDP defines the following pro‐

       1.     Router and Prefix Discovery

	      Router  discovery	 is  a	process through which hosts locate the
	      neighboring routers and learn prefix plus other parameters  nec‐
	      essary for address autoconfiguration.

	      Prefix discovery is used by the hosts to learn the range of IPv6
	      addresses that reside on-link and can be reached	without	 going
	      through a router.

	      Routers  send  Router  Advertisements  which will make the hosts
	      treat them as the default routers.   The	Router	Advertisements
	      will  also contain prefix information options that will identify
	      the range of IPv6 addresses that are on-link (Subnet prefix).

       2.     Router and Host Requirements

	      Router requirements in NDP specify a set of rules	 for  host  to
	      act as a router.	These rules include:

	      ·	 Router configuration variables.

		 These	configuration variables include intervals between suc‐
		 cessive unsolicited router advertisements, etc.

	      ·	 How to make an interface an advertising interface.

		 When an interface is made an advertising interface, it	 means
		 that the node is going to send periodic router advertisements
		 and is willing to forward packets on behalf of hosts on  that

	      ·	 Message content for router advertisements.

		 A  router  will  send	periodic  as  well as solicited Router
		 Advertisements on an advertising  interface.	NDP  specifies
		 the format of these messages.

	      ·	 Sending unsolicited router advertisements.

		 Apart	 from	sending	 solicited  router  advertisements  in
		 response to router  solicitations,  routers  can  send	 unso‐
		 licited  router  advertisements.   For	 example,  unsolicited
		 router advertisements can be sent to expire a	prefix	or  to
		 advertise a new prefix, etc.

	      ·	 Stopping router advertisements on an interface.

		 A router can stop advertising prefixes on an interface.  This
		 can happen due to system management decisions when  a	router
		 may be stopped from being one.	 NDP specifies what the router
		 should be doing under these circumstances.

	      ·	 Processing router solicitation messages.

		 Hosts as part of the stateless autoconfiguration process will
		 send  Router  Solicitations.	Routers should respond to such
		 solicitations with a router advertisement.

	      ·	 Steps to be taken when the link-local address for the	router

		 Normally  the	link-local  address  of	 a  Router  should not
		 change.  However, NDP still defines the steps should be taken
		 by  the router when its link-local address changes for any of
		 its interfaces.

	      Host requirements are a set of rules that apply for a IPv6 host.
	      They are:

	      ·	 IPv6 variables that have to be maintained.

		 These	variables  include the time between retransmissions of
		 neighbor solicitations, link MTU for each interface, etc.

	      ·	 Processing router advertisements.

		 This rule discusses what actions should be taken  on  receipt
		 of router advertisements.

	      ·	 Timing out prefixes and default routers.

		 Whenever routers send router advertisements, they include the
		 lifetime of the router as well	 as  the  prefixes  that  they
		 advertise.   NDP  specifies what actions the host should take
		 when these lifetimes expire.

	      ·	 Selecting a default router.

		 When there is more than one router in the link,  the  default
		 router selection algorithm comes into effect.	This algorithm
		 helps select the default router based on factors like reacha‐
		 bility, etc.

	      ·	 Sending a router solicitation.

		 When  an  interface  is enabled, a host need not wait for the
		 unsolicited router advertisement.  Instead,  it  can  send  a
		 router	 solicitation  and  get	 a  router  advertisement as a
		 response.  This will help in receiving the default router and
		 prefix information as soon as the interface is enabled.

       3.     Algorithm for Sending a Packet

	      Any  IPv6 host is required to maintain some data structures that
	      will be used by the algorithm for sending a packet.  These  data
	      structures are:

	      Neighbor Cache
		     A set of entries that will maintain IPv6 Address to link-
		     layer address mappings for neighbors to  which  a	packet
		     has been sent recently.  In addition to that it maintains
		     information needed for neighbor unreachability  detection
		     like the reachability state, etc.

	      Destination Cache
		     A set of entries for hosts to whom packets have been sent
		     recently.	This includes hosts which are both on-link and
		     off-link.	 It  contains  a  level	 of indirection to the
		     neighbor cache.

	      Prefix List
		     This is a list of prefixes which define the set  of  IPv6
		     address that are on-link.	This information is maintained
		     on a per interface basis.	Typically this list  is	 built
		     from Router Advertisements received from the router.

	      Default Router List
		     A list of routers which will forward packets on behalf of
		     this host.	 This list will again  have  a	pointer	 to  a
		     neighbor cache entry for the respective router.

	      A host will use the above data structures while sending a packet
	      to a host.  Following is the conceptual algorithm for sending  a
	      packet to a unicast destination.

	      a.     Before  a	packet	is  sent  out,	the next hop should be
		     determined.  Normally, next hop determination is not done
		     on	 all packets.  The results of a next hop determination
		     are stored in the destination  cache.   The  host	should
		     first  check  the	destination  cache  for any entry that
		     matches with the  current	destination  address.	If  it
		     finds a match, then it proceeds to step c, below.

	      b.     If	 there is no entry for the destination in the destina‐
		     tion cache, a longest prefix match is made with all  pre‐
		     fixes  in the prefix list.	 If there is a match, the des‐
		     tination is determined to be on-link and the  destination
		     address  will  be considered as the next hop.  Otherwise,
		     the next hop is determined from the routing table.

	      c.     Once the next hop is determined, the  address  resolution
		     process  and  neighbor  unreachability detection are done
		     for the next hop.	This process is explained in the  next

	      d.     Once the neighbor is known to be reachable, the packet is
		     sent to that destination.

       4.     Address Resolution and Neighbor Unreachability Detection

	      Address resolution is a process used to determine the link-layer
	      address  of  a neighbor.	The IPv6 Address to link-layer address
	      mapping found through this process is  cached  in	 the  Neighbor
	      Cache.  Following are the steps involved in Address Resolution.

	      a.     First,  the  neighbor cache is checked for an entry which
		     matches the current destination address.  If the entry is
		     not  present, the host sends a Neighbor Solicitation Mes‐
		     sage to the solicited-node multicast group.  This	multi‐
		     cast  address  is	derived	 based on the destination IPv6
		     address and all nodes with the  particular	 IPv6  address
		     are required to join that group.

	      b.     If	 a  host with the specified IPv6 address is present in
		     the network, it  will  reply  this	 solicitation  with  a
		     Neighbor Advertisement Message.

	      c.     On	 receiving  the	 Neighbor Advertisement, the node will
		     search for	 an  entry  in	the  neighbor  cache  for  the
		     sender's  IPv6  address.	A  new entry is created in the
		     neighbor cache and the reachability flag is set to REACH‐

	      Once  the Address resolution is completed, neighbor unreachabil‐
	      ity detection will be performed.	This process  depends  on  the
	      reachability  field  of  the  neighbor  cache.   An entry in the
	      neighbor cache can have any of the following states:

	      INCOMPLETE     The address resolution is	in  progress  and  the
			     link-layer	 address  of the destination is yet to
			     be determined.

	      REACHABLE	     The destination is reachable until recently.

	      STALE	     The destination is no longer known to  be	reach‐
			     able, but reachability detection need not be made
			     until a packet has to be sent  to	that  destina‐

	      DELAY	     This  state  is  an optimization that gives addi‐
			     tional time for the upper layer protocols to pro‐
			     vide the reachability confirmation.

	      PROBE	     A reachability confirmation is actively requested
			     by repeatedly sending Neighbor Solicitations.

	      During neighbor unreachability detection, the  node  checks  for
	      the  state in the neighbor cache.	 If the state for the destina‐
	      tion is REACHABLE, the packet is sent.  Otherwise, the following
	      steps are taken:

	      a.     When  an  address resolution is made on a destination, an
		     entry is created in the neighbor cache for that  destina‐
		     tion  and	the  reachability  state will be set to INCOM‐
		     PLETE.  If the address resolution	fails,	the  entry  is

	      b.     When  the	address	 resolution  passes, the entry will be
		     filled with the destination's link-layer address and  the
		     state will be set to REACHABLE.

	      c.     There is a timer maintained called the Reachability timer
		     which will expire the state of an entry in	 the  neighbor
		     cache.   Once  this timer expires, the reachability state
		     changes from REACHABLE to STALE.

	      d.     When a packet is being sent to a destination whose	 state
		     is	 STALE	in the neighbor cache, the node sets the state
		     to DELAY and starts a timer associated with  that	state.
		     By the time the timer expires if the node received reach‐
		     ability confirmation, the	state  is  set	to  REACHABLE.
		     Otherwise, it is set to PROBE.

	      e.     Once  the	entry's state is in PROBE, the node sends uni‐
		     cast neighbor solicitations  to  the  link-layer  address
		     specified in the entry.  If it receives a neighbor adver‐
		     tisement in response the state is set to REACHABLE.  This
		     solicitation  will be sent repeatedly; the maximum number
		     of times is configurable.	If the reachability  confirma‐
		     tion  is  not  received  after maximum solicitations, the
		     entry is deleted from the neighbor cache and the  address
		     resolution is done again.

	      Note:  Entries  in  the  neighbor cache can also be created as a
	      result of node receiving	unsolicited  Neighbor  Advertisements,
	      Router  Advertisements  and Router Solicitations, etc.  However,
	      for the entry created under these circumstances the reachability
	      state will always be set to STALE.

       5.     Redirect Function

	      A	 router will send a host a redirect message when it finds that
	      there is a better next-hop router on the same link.  This	 is  a
	      requirement for a router.

	      On receiving a router redirect message, a host should update its
	      destination cache with the new next hop address.

       NDP was developed by the IPng Working Group of the Internet Engineering
       Task Force.

       ifconfig(1M), ndp(1M), ip6(7P), lan(7).

       Neighbor	 Discovery  for	 IPv6, RFC2461, T. Narten et al.  NDP Neighbor
       Discovery Protocol


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