Subject: Proposed Terminology section for IPFRR framework document

From: mike shand <[email-masked]>
Date: Wed, 20 Oct 2004 14:09:02 +0100

Here is the proposed terminology section to be included in the framework 
document. Alia and I have had some discussion about this, and come up with 
the following. Note that we have changed from P to E for the primary next 
hop partly because there was some confusion with Provider routers and 
partly because we were both using "P" for different things. The equitable 
thing seemed be for neither of us to use P.

Alia's latest version of the basic draft uses this terminology, and we will 
be re-spinning our draft to use this terminology where appropriate.

If anyone has any comments please let me have them by the end of the week.

	Mike

TERMINOLOGY

    This section defines words and acronyms used in this draft.

     S                    Used to denote a router that is the source of
                          a repair that is computed in anticipation of
                          the failure of a neighboring router denoted as
                          E, or of the link between S and E. It is the
                          viewpoint from which IP Fast-Reroute is
                          described.

     D                    Used to denote the destination router under
                          discussion.

     E                    Used to denote the router which is the primary
                          next-hop neighbor to get from S to the
                          destination D. Where there is an ECMP set for
                          the shortest path from S to D, these are
                          referred to as E_1, E_2, etc.

     N_i                  The ith neighbor of S.

     R_i_j                The jth neighbor of N_i.

     Distance_opt(A,B)    The distance of the shortest path from A to B.

     Link(A->B)           A link connecting router A to router B.

     Loop-Free            This is a neighbor N, that is not a primary
     Alternate            next-hop neighbor E, whose shortest path to
                          the destination D does not go back through the
                          router S.  The neighbor N must meet the
                          following condition:-

                          Distance_opt(N, D) < Distance_opt(N, S) +
                          Distance_opt(S, D)

     Downstream Path      This is a subset of the loop-free alternates
                          where the neighbor N meet the following
                          condition:-

                          Distance_opt(N, D) < Distance_opt(S,D)

     Upstream             This is a forwarding loop which involves a set
     Forwarding Loop      of routers, none of which are directly
                          connected to the link which has caused the
                          topology change that triggered a new SPF in
                          any of the routers.

     Primary Neighbor     One or more of the primary next-hops for S to
                          reach the destination D goes directly to this
                          neighbor.

     Loop-Free            A Neighbor N_i which is not the primary
     Neighbor             neighbor and whose shortest path to D does not
                          go through S.

     Loop-Free            This is a path via a Loop-Free Neighbor N_i
     Link-Protecting      which does not go through the particular link
     Alternate            of S which is being protected to reach the
                          destination D.

     Loop-Free            This is a path via a Loop-Free Neighbor N_i
     Node-Protecting      which does not go through the particular
     Alternate            primary neighbor of S which is being protected
                          to reach the destination D.

     FIB                  Forwarding Information Base. The database used
                          by the packet forwarder to determine what
                          actions to perform on a packet.

     IPFRR                IP fast re-route

     Routing              The process whereby routers converge on a new
     transition           topology. In conventional networks this
                          process frequently causes some disruption to
                          packet delivery.

     Micro-loop           A temporary forwarding loop which exists
                          during a routing transition as a result of
                          temporary inconsistencies between FIBs.

     RPF                  Reverse Path Forwarding. I.e. checking that a
                          packet is received over the interface which
                          would be used to send packets addressed to the
                          source address of the packet.

     SPF                  Shortest Path First, e.g. Dijkstra's
                          algorithm.

     SPT                  Shortest path tree

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