rfc3215.txt

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      d) Otherwise, ignore the message.

2.3. ATM Switch LSR with VC-merge

2.3.1 VC-merge

   A VC-merge capable LSR can map multiple incoming labels (VPI/VCI)
   into one outgoing label.  It is possible that this LSR can only merge
   a limited number of incoming labels into a single outgoing label.  As
   described in [2], suppose, for example, that due to some hardware
   limitation a node is capable of merging four incoming labels into a
   single outgoing label.  Suppose however, that this particular node
   has six incoming labels arriving at it for a particular FEC.  In this
   case, this node may merge these into two outgoing labels.

   When an upstream LSR has a limited merging capability, it is
   difficult for a downstream LSR to know how many labels should be
   assigned to each FEC.  In this case, downstream-on-demand is
   recommended.

2.3.2 Control Block

   There are 3 types of control blocks involved: Upstream LSP Control
   Block, Downstream LSP Control Block, and Next Hop Trigger Control
   Block.




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   There is one Upstream LSP Control Block for each LDP-REQUEST
   received.

   There is one Downstream LSP Control Block for each unique LDP-REQUEST
   sent to a downstream LSR.  There can be multiple Downstream LSP
   Control Blocks per FEC in an LSR.  This can be the result of an
   upstream LSR asking for a label for an FEC.  This LSR must assign a
   unique upstream label and it can not merge this upstream label into
   an existing downstream label for this FEC.

   There is one Next Hop Trigger Control Block for each FEC for which a
   better next hop has been detected and the LSR has decided to switch
   to this better next hop.  It could be the result of "local repair" or
   "global repair" if the LSR is the ingress LSR of the LSP.

   A Downstream LSP Control Block contains a list of pointers to
   Upstream LSP Control Blocks or the Next Hop Trigger Control Block.
   This means that this LSR has decided to map the multiple labels
   listed in the Upstream LSP Control Blocks and the Next Hop Trigger
   Control Block into a single label listed in the Downstream LSP
   Control Block.

   An Upstream LSP Control Block may contain the following information:

      -  Upstream LDP Session Identifier

      -  State

      -  Upstream Label (assigned by this LSR)

      -  Downstream LSP Control Block pointer

      -  Upstream LDP Request ID (assigned by the upstream LSR in
         downstream-on-demand mode)

      -  Next_Hop_Trigger_Block pointer

   Upstream Label and Upstream LDP Session Identifier can be used to
   locate a unique Upstream LSP Control Block.

   If an LSR is using downstream-on-demand mode, it can use the Upstream
   LDP Request ID and the Upstream LDP Session Identifier to locate a
   unique Upstream LSP Control Block.








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   An Next_Hop_Trigger LSP Control Block may contain the following
   information:

      -  Upstream LSP Control Block pointer, that points to the one that
         is needed to switch over to the better next hop

      -  State

      -  Downstream LSP Control Block pointer

   A Downstream LSP Control Block may contain the following information:

      -  FEC

      -  State

      -  Downstream LDP Session Identifier

      -  list of pointers to the Upstream LSP Control Blocks or the
         Trigger_Control_Blocks that are merged at this LSR for this FEC

      -  Downstream Label (assigned by the downstream LSR)

      -  Downstream Label Request ID (assigned by the LSR itself if it
         is using downstream-on-demand mode)

   Downstream Label, Downstream LDP Session Identifier can be used to
   locate a unique Downstream LSP Control Block.

   If an LSR is using downstream-on-demand mode, it can also use the
   Downstream Label Request ID and the Downstream LDP Session Identifier
   to locate a unique Downstream LSP Control Block.

   The following diagram details the relationship between these 2 types
   of control blocks:

   For example, the LSR has decided to merge 3 LDP-REQUESTs of a FEC
   from upstream LSR1, LSR2, LSR3 into one LDP-REQUEST and sent it to a
   downstream LSR4.












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   +---------------------+
   |                     |
   | Upstream_LSP_Control|
   | _Block  for Upstream|------+
   | LSR1                |      |
   +---------------------+      |
                                |
   +---------------------+      |
   |                     |      |
   | Upstream_LSP_Control|      | (merged into)
   | _Block for Upstream |------+
   |  LSR2               |      |
   +---------------------+      |    +------------------------------+
                                |    |                              |
   +---------------------+      +--->| Downstream LSP Control Block |
   | Next_Hop_Trigger_   |      |    |   for Downstream LSR4        |
   | LSP Control Block   |------+    |                              |
   |                     |           +------------------------------+
   +---------------------+

2.3.3   State Machines for Downstream-on-demand Mode

   The following sections describe the state machines used in
   downstream-on-demand mode.

2.3.3.1 State of the Upstream LSP Control Block's State Machine for
        Downstream-on-demand Mode

   -- IDLE

   This is the initial LSP state.

   -- RESPONSE_AWAITED

   This state means that the LSR has received and processed an LDP-
   REQUEST from an upstream LSR, and has sent a new LDP-REQUEST towards
   a downstream LSR.  The LSR is waiting for the LDP-MAPPING from the
   downstream LSR.

   -- ESTABLISHED

   This state means that the LSR has received the LDP-MAPPING from the
   downstream LSR and the LSP is up and operational.

   -- RELEASE_AWAITED

   This state means that the LSR has sent a LDP-WITHDRAW upstream and is
   waiting for the LDP-RELEASE before freeing up the label resource.



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2.3.3.2 Events of the Upstream LSP Control Block's State Machine for
        Downstream-on-demand Mode

   -- LDP Request

   The LSR receives an LDP-REQUEST from an upstream LSR.

   -- Internal Downstream Mapping

   This event is sent by one Downstream LSP Control Block's state
   machine.  This Downstream LSP Control Block is the merged Downstream
   LSP Control Block of this Upstream LSP Control Block.  The event is
   the result of receiving an LDP-MAPPING by the Downstream LSP Control
   Block's state machine.

   -- LDP Release

   The LSR receives an LDP-RELEASE from an upstream LSR.

   -- Internal Downstream Withdraw

   This event is sent by one Downstream LSP Control Block's state
   machine.  This Downstream LSP Control Block is the merged Downstream
   LSP Control Block of this Upstream LSP Control Block.  The event is
   the result of receiving an LDP-WITHDRAW by the Downstream LSP Control
   Block's state machine.

   -- LDP Upstream Abort

   The LSR receives an LDP-ABORT from an upstream LSR.

   -- Internal Downstream NAK

   This event is sent by one Downstream LSP Control Block's state
   machine.  This Downstream LSP Control Block is the merged Downstream
   LSP Control Block of this Upstream LSP Control Block.  The event is
   the result of receiving an LDP-NAK by the Downstream LSP Control
   Block's state machine, or it detects an error.

   -- Upstream Lost

   The LSR loses the LDP session with its upstream LDP peer.

   -- Internal New NH

   The LSR detects there is better next hop and decides to establish the
   lsp through this better next hop.




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   -- Internal Re-Cross-Connect

   This event is used to trigger splicing into a different downstream
   LSP.  This can happens when it is switched over to a better LSP
   through the new next hop.

2.3.3.3 State Transitions of the Upstream LSP Control Block's State
        Machine for Downstream-on-demand Mode

                      +-------------------+
                      |                   |
            +-------->|  IDLE             |<-------------------+
            |         |                   |                    |
            |         +-------------------+                    |
            |(LDP Abort)       |                               |
            |(Internal         |(LDP Request)                  |
            | Downstream NAK)  |                               |
            |(Upstream Lost)   |               (Upstream Lost) |
            |                  v                 (LDP Release) |
            |         +-------------------+                    |
            |         |                   |                    |
            +---------|  RESPONSE_AWAITED |                    |
                      |                   |                    |
                      +-------------------+                    |
                               |                               |
                               |(Internal Downstream           |
                               |  mapping)                     |
                               |                               |
                               v                               |
                      +-------------------+                    |
                      |                   |                    |
                      |  ESTABLISHED      |------->------------+
                      |                   |                    |
                      +-------------------+                    |
                               |                               |
                               |                               |
                               |(Internal Downstream Withdraw) |
                               |(Internal Downstream NAK)      |
                               v                               |
                      +-------------------+    (LDP Upstream   |
                      |                   |     Abort)         |
                      |RELEASE_AWAITED    |------->------------+
                      |                   |
                      +-------------------+







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2.3.3.4 Upstream LSP Control Block's State Machine for Downstream-on-
        demand Mode

2.3.3.4.1 State -- "IDLE"

   State:          IDLE

   Event:          LDP Request

   New State:      Depends upon the action routine.

   Actions:

      If this LSR is the LSP Egress or Proxy Egress [2],

      Then:
         choose an upstream label, allocate the resources, connect this
         upstream label to the local IP forwarding module, send an LDP-
         MAPPING upstream with the upstream label and go to the state
         `ESTABLISHED'.

      else
         Obtain a next hop (or interface).  Find a Downstream LSP
         Control Block that has the same FEC and the same next hop and
         also is able to merge more input labels.  If not found, create
         a new Downstream LSP Control Block with the state `IDLE'.

         If the state of the Downstream LSP Control Block is
         `ESTABLISHED', choose an upstream label, connect the upstream
         label with the downstream label and send an LDP-MAPPING
         upstream with the upstream label, and go to the state
         `ESTABLISHED'.

         If the state of Downstream LSP Control Block is not
         `ESTABLISHED', set the state of the Upstream LSP Control Block
         to `RESPONSE_AWAITED'.  If the LSR use the independent control
         mode [2], choose an upstream label, and send an LDP-MAPPING
         upstream.

         Pass the event `Internal AddUpstream' to the Downstream LSP
         Control Block's state machine.

      If unable to process the request for any reason, issue an LDP-NAK
      to the sender with the appropriate error code, go to IDLE and
      delete the control block.

   State:          IDLE




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   Event:          Internal Downstream Mapping

   New State:      IDLE

   Actions:

      Ignore the event.  It is an internal implementation error.

   State:          IDLE

   Event:          LDP Release

   New State:      IDLE

   Actions:

      Ignore the event.  It is an internal implementation error.

   State:          IDLE

   Event:          Internal Downstream Withdraw

   New State:      IDLE

   Actions:

      Ignore the event.  It is an internal implementation error.

   State:          IDLE

   Event:          LDP Upstream Abort

   New State:      IDLE

   Actions:

      Ignore the event.  It is an internal implementation error.

   State:          IDLE

   Event:          Internal Downstream NAK

   New State:      IDLE

   Actions:

      Ignore the event.  It is an internal implementation error.




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