Intra-LL and Inter-LL Discovery
The document makes a distinction between the discovery of neighbors within a Logical Link (intra-LL) and neighbors
beyond the LL .
ND assumes that a link layer interface will do something meaningful with an ICMPv6 packet sent to a multicast IP destination address. Assumes multicast support will be provided using the RFC 2022 (MARS) . An IPv6 LL maps directly onto an IPv6 MARS Cluster same way an IPv4 LIS maps directly onto an IPv4 MARS Cluster.
Intra-LL operation : IPv6 layer must be able to simply pass multicast ICMPv6 packets down to the IPv6/NBMA driver without any special, NBMA specific processing .
IPv6/NBMA interfaces shall register as MARS Cluster members ,and SHALL send certain classes of outgoing IPv6 packets directly to their local MARS .
The MARS itself shall then re-transmit these packets, by:
When receives an IPv6 packet, find the NBMA addreses by scan membership database , then checks for pt-pt control VC open to the MARS.
Then send a copy of the data packet to each group over the pt-pt VCs.
If a group does not have an open pt-pt VC to the MARS, the packet is sent out ClusterControlVC instead. Without copies of the packet.
Shortcuts are created between Transient Neighbors. The key to creating transient neighbors is the Redirect message. IPv6 allows by Router Redirect message to inform of a better 'first hop'.
A transmitting host only listens to Router Redirects. If a Redirect arrives that indicates a better first hop, with a link layer (NBMA) address, the associated Neighbor Cache entry in the source host is updated. This involves building a new VC to the new NBMA address. If this is successful, the old VC is torn down only if it no longer required.
The modification of forwarding paths based on the dynamic detection of IP packet flows.
Such a router:
shall track flows originate from a directly attached host .shall not use IP packets
arriving from another router to trigger the Redirect .It only consider IPv6 packets with FlowID of zero .It utilize NHRP to ascertain a better first-hop.
A source host MAY also trigger a redirection to a transient neighbor. To perform by a source host shall :
Create a Neighbor Solicitation message .Use the standard ND hop limit of 255 to ensure the NS won't discarded by the router .The value should be equal to the hop limit of the data flow .This ensures that the router is able to restrict the .
Forward the NS packet to the router that would be the next hop for Traffic sent
towards the off-LL target.
Actual shortcut discovery is optional for IPv6 routers. If shortcut discovery is to be supported :
NHRP Request is constructed and sent .The original NS message should be discarded. When NHRP Reply is received, the router shall construct a Redirect
message , and the NBMA link layer address returned .
The resulting Redirect message shall transmitted back to the source host. When
message received, the source host shall update its Neighbor caches.
The next packet sent to the Transient Neighbor will result in the creation of the direct, shortcut VC.
If a NHRP NAK or error indication is received, the requesting router shall construct a Redirect message identifying the router itself as the best 'shortcut'.
Once flow detection has occurred, or a host trigger has been detected, routers shall use NHRP in an NHS to NHS mode to establish the IPv6 to link level address mapping of a better first hop.
The destination of the flow that caused the trigger is used as the target for resolution in a NHRP Request. The router then forwards this NHRP Request to the next closest NHS. The process continues until the Request reaches an NHS that believes the IP target is within link-local scope of one of its interfaces.
The last hop router shall resolve the NHRP Request from mapping information contained in its neighbor cache for the interface on which the specified target is reachable. If there is no appropriate entry in the Neighbor cache, or the destination is currently considered unreachable, the last hop router shall perform Neighbor Discovery on the local interface, and build the NHRP Reply from the resulting answer. The NHRP Reply is propagated back to the source of the NHRP Request, using a hop-by-hop path as it would for normal NHRP.
Upon receipt of a NHRP NAK reply or error indication for a flow- triggered shortcut attempt, no indication is sent to the source of the flow.
The following translation rules are meant to augment the packet format of the NHRP. NHRP messages are constructed and sent according to the rules. The value of the NBMA technology specific fields are: ar$afn, ar$pro.type, ar$pro.snap and link layer address. Source, destination or client protocol addresses are always IPv6 addresses of length 16.
The ar$hopcnt field MUST be smaller than the shortcut limit value. This ensures that hosts have control over the reach of their shortcut request. The Flags field in the common header of the NHRP resolution request should have the Q , U and S bits set.
NBMA and protocol source addresses are those of the router constructing the request.
The target address is used as the NHRP destination protocol address.
When constructing a NHRP resolution request as a result of flow detection, the choice of
values is configuration dependent.
A successful NHRP resolution reply for a host-triggered shortcut attempt is translated into an IPv6 Redirect message .
All NHRP extensions have no effect on NHRP/ND translation and MAY be used in NHRP messages for IPv6.
Purges are generated by NHRP when changes are detected that invalidate a previously issued NHRP Reply. Any IPv6 shortcut previously established on the basis of newly purged information should be torn down.
Routers shall keep track of NHRP cache entries for which they have issued Neighbor Advertisements or Router Redirects. If a NHRP Purge is received that invalidates information previously issued to local host, the router shall issue a Router Redirect specifying the router itself as the new best next-hop for the affected IPv6 target.
Routers shall keep track of Neighbor cache entries that have previously been used to generate an NHRP Reply. The expiry of any such Neighbor cache entry shall result in a NHRP Purge being sent towards the router that originally requested the NHRP Reply.
Duplicate Address Detection is only required within the link-local scope,
which in this case is the LL-local scope. Transient Neighbors are outside
the scope of the LL. No particular interaction is required between the
mechanism for establishing shortcuts an