IPv6 over Non-Broadcast Multiple Access
There are some usages for Non-broadcast Multiple Access (NMBA) :
We will see the architecture of IPv6 over NMBA networks (ATM , Frame-Relay) .
Large part will be dedicated to the managing of point-to-point and point to multipoint calls between interfaces on the NBMA network .
The mechanism of neighbor discovery is very different from the one that appears at the IPv4 mechanism :
In the IPv4 mechanism there was a usage of the link layer (Ethernet ARP) for neighbor discovery . On the other hand , the IPv6 developers
have chosen not to use in the same approach : each IP interface has the ability of native multicasting . But most of the NBMA networks
required convergence protocols to emulate the desired service .
A shortcut is an NBMA level call (VC) that directly connected 2 IP endpoints which are logically separated (by one or more routers) .
IPv6 packets traversing this VC are said to “shortcut” the routers that are in the logical IPv6 pah between the VC’s endpoints.
The NBMA shortcuts are a mechanism that makes shortcuts , minimizing the consumption of resources within an IP over NBMA
cloud (for example : routers hops). The NBMA shortcut for IPv6 over NBMA is achieved through mixture of neighbor-discovery
an NHRP (Next Hop Routing Protocol) .
Key components of the IPv6 over NBMA architecture :
NBMA networks providing PVC (Permanent Virtual Circuit) support
When we use NBMA network with PVC mode , each PVC connects exactly 2 nodes and the use of neighbor-discovery
(and more IPv6 features) is limited .
On each link the IPv6 over NBMA interfaces has only one neighbor on each link .
Dynamically discovered shortcuts are not supported also .
Therefore, in the following issues we will discuss NBMA networks which use only SVC (Switched Virtual Circuit) .
NBMA networks providing SVC (Switched Virtual Circuit) support
When we use NBMA network in SVC mode the key components are the following :
1. The IPv6 neighbor model – where neighbors are discovered through the use of messages multicast to members of an
IPv6 interface’s local IPv6 link
2. The MARS (Multicast Address Resolution Server) model – allowing emulation of general multicast calls provided by the
underlying NBMA network .
3. The NHRP service for seeking out the NBMA identities of IP interfaces who are logically distant in an IP topological sense .
4. The modeling of IP traffic as “flows” , and optionally using the existence of a flow as the basis for attempting to set up
a shortcut link level connection .
The IPv6 Link is a logical link in NBMA environments . IPv6 over NBMA interfaces uses MARS for general intra-logical link
multicasting .For destinations (which aren’t neighbors) a host sends packets to its default routers .
The egress (exit) router from a logical link is responsible for detecting the existence of an IP packet flow using a shortcut connection .
The last router/NHS before the target of the NHRP query verifies that the target interface's preferred NBMA address.
The originally querying router then issues a redirect to the IP source, identifying the flow's destination as a temporarily neighbor.
Host-initiated triggering of shortcut discovery, regardless of the existence of a packet flow, is also supported through specific Neighbor
Solicitations sent to a source host's default router.
Logical Links and Transient Neighbors
Neighbors – neighbors are those nodes that are considered on-link and whose link-layer addresses
may therefore be located using Neighbor-Discovery .
On Link – an address that is assigned to a neighbor’s interface on a shared link . A host considers an
address to be on-link if :
it is covered by one of the link's prefixes, or a neighboring router specifies the address as the target of
a Redirect message, or a Neighbor Advertisement message is received for the target address,
or a Neighbor-Discovery message is received from the address.
Off Link
- the opposite of "on-link": an address that is not assigned to any
interfaces attached to a
shared link . Off-link nodes are considered to only be accessible through one of the routers
directly attached to the link .
An LL (Logical Layer) consists of nodes administratively configured to be 'on link' with respect to
each other.
The members of an LL are an IPv6 interface's initial set of neighbors, and each interface's Link Local address
only needs to be unique amongst this set. It should be noted that whilst members of an LL are IPv6 Neighbors,
it is possible for neighbors to exist that are not, administratively, members of the same LL.
Neighbor- Discovery events can result in the expansion of an IPv6 interface's set of neighbors. However,
this does not change the set of interfaces that make up its LL. This leads to three possible relationships
between any two IPv6 interfaces:
On LL, Neighbor.
Off LL, Neighbor.
Off LL, not Neighbor.
Off LL Neighbors represent the 'shortcut' connections, where it has been ascertained that direct connectivity
at the NBMA level is possible to a target that is not a member of the source's
LL.