The MPLS WG Archive

[Guangzhi Li <gli@research.att.com>]

Dear GMPLS authors and all experts:

During GMPLS last call, I posted the same question on the mailing list
without response. Please somebody spend a little time and check with the
following example? Something seems not clear when the current GMPLS
contention resution schemes are applied on a framework with both
bi-directional and uni-directional LSPs. Your clarification is very much
appreciated.

Sincerely,

Guangzhi

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The issue arises because contention is resolved between bi-directional
LSPs by the node with the higher node index while for uni-directional
LSPs, contention is resolved by the downstream node. Consider the
following example of two nodes with paired, bi-directional interfaces
(i.e., a transmitter/receiver pair of ports).  Node 1 with ID=100 and
node 2 with ID = 50.  Node 1 uses label 1 for the transmitter port and
label 2 for the receiver port; node 2 uses label 4 for the transmitter
port and label 3 for the receiver port.   We assume that a
bi-directional LSP requires a single I/O interface.

We consider two LSPs - a uni-directional LSP (LSP A) and a
bi-directional LSP (LSP B). Both LSPs are going from node 1 to node 2,
with the uni-directional LSP setup request arriving marginally before
the bi-directional LSP.  LSP A does not use a suggested label, and thus
is assigned a label (port) by node 2.  Label 3 is assigned,
corresponding to label 1 at node 1.  At the same time, for LSP B (the
bi-directional LSP) node 1 assigns label 2, with suggested label 1.
Because node 1 has the higher node ID, node 2 will assume (due to the
contention resolution rule for bi-directional LSPs) that LSP B wins the
contention and thus label 3 is assigned to LSP B.  Thus label 1 at node
1 (label 3 at node 2) has been assigned to two different LSPs.  Both
LSPs have “won” the contention.