The MPLS WG Archive

[Leonardo Balliache <lbb@opalsoft.net>]

Hi, all,

Hoping Loa can help me with these questions.

I'm trying to implement LDP in the ns-2 simulator. It has already an 
implementation but I want something closer to the specification. I tell 
this to take for granted that I'm not working in any commercial implementation.

My questions refer to the specification, and are:

** Q1.- Here Propagating is set in independent control but not in ordered 
control; but it is used as parameter in ordered control below.

A.1.1. Receive Label Request

     ...........

       LRq.9   Perform LSR Label Distribution procedure:

             For Downstream Unsolicited Independent Control OR
             For Downstream On Demand Independent Control

                1. Has LSR previously received and retained a label map-
                   ping for FEC from Next Hop?.
                   Is so, set Propagating to IsPropagating.
                   If not, set Propagating to NotPropagating.

                2. Execute procedure Prepare_Label_Map-
                   ping_Attributes(MsgSource, FEC, RAttributes, SAt-
                   tributes, Propagating, StoredHopCount).

                3. Execute procedure Send_Label (MsgSource, FEC, SAt-
                   tributes).

                4. Is LSR egress for FEC? OR
                   Has LSR previously received and retained a label map-
                   ping for FEC from Next Hop?
                   If so, goto LRq.11.
                   If not, goto LRq.10.

             For Downstream Unsolicited Ordered Control OR
             For Downstream On Demand Ordered Control

                1. Is LSR egress for FEC? OR
                   Has LSR previously received and retained a label map-
                   ping for FEC from Next Hop?  (See Note 3.)
                   If not, goto LRq.10.

                2. Execute procedure Prepare_Label_Map-
                   ping_Attributes(MsgSource, FEC, RAttributes, SAt-
                   tributes, IsPropagating, StoredHopCount)

                3. Execute procedure Send_Label (MsgSource, FEC, SAt-
                   tributes).
                   Goto LRq.11.

** Q2.- Here, in PMpA.7, Hop Count in RAttribute is incremented but it is 
used then in PMpA.14. In this (14), Hop Count to be used is the original 
Hop Count in RAttribute or the incremented value in PMpA.7?

A.2.8. Prepare_Label_Mapping_Attributes

     .........

       PMpA.7  Increment RAttributes Hop Count and copy the resulting
               Hop Count to SAttributes.  See Note 2.  Goto PMpA.9.

       PMpA.8  Include Hop Count of unknown (0) in SAttributes.

       PMpA.9  Is Loop Detection configured on LSR?
               If not, goto PMpA.21.

       PMpA.10 Do RAttributes have a Path Vector?
               If so, goto PMpA.19.

       PMpA.11 Is LSR propagating a received Label Mapping?
               If not, goto PMpA.20.

       PMpA.12 Does LSR support merging?
               If not, goto PMpA.14.

       PMpA.13 Has LSR previously sent a Label Mapping for FEC to Peer?
               If not, goto PMpA.20.

       PMpA.14 Do RAttributes include a Hop Count?
               If not, goto PMpA.21.

       PMpA.15 Is Hop Count in Rattributes unknown(0)?
               If so, goto PMpA.20.

** Q3.- I've got some problem interpreting the sentences "Install label" as 
below:

     .......

       LRq.12  Perform LSR Label Use procedure.

             For Use Immediate OR
             For Use If Loop Not Detected

                1. Install label sent to MsgSource and label from Next
                   Hop (if LSR is not egress) for forwarding/switching
                   use.

     .......

       LMp.15  Install Label for forwarding/switching use.

     .......

               2. Install label received and label sent to Peer for
                  forwarding/switching use.

       SL.3   Install Label for forwarding/switching use.

     .......

As I understand MPLS we should have three tables: NHLFE, FTN and ILM.

FTN contains fecs and points to NHLFE. Then, it should have the information 
about the incoming interface to have the tuple <fec, incoming_itfc).

ILM contains labels and points to NHLFE too. Then it should have the 
information about the incoming interface to have the tuple <label, 
incoming_itfc).

NHLFE should have the information about the outgoing label (if it is 
required or the null (0) label to indicate that the last label must be 
popped to expose the L3-header), the outgoing interface and the operation 
to be done (push, swap, pop) to have the tuple <label, outgoing_itfc, 
operation).

Then, I'm not very sure how to interpret the sentences above ordering to 
"install label". Could you be a little bit more explicit about this 
information?

Best regards,

Leonardo Balliache


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