The MPLS WG Archive

[Mark Duffy <mduffy@quarrytech.com>]

Hi, 

In draft-ietf-mpls-ldp-mtu-extensions-00 I believe there are several
problems with the example in section 3.

At the very end of the example it states:

   At t[2]:

   1) Rx receives a Mapping for F from Ry with an MTU of 9216.  Rx
   compares 9212 (9216 - 4) to 4466, and sends a Mapping message for F
   with an MTU of 4466 to Ri.

   2) Ri has not received Mappings for F.

   At t[3]:

   1) Ri receives a Mapping for F from Rx with an MTU of 4462.  Ri
   compares 4466 to 9216, and sets the MTU for P1 to 4462 (4466 minus
   the overhead of 1 label pushed onto the stack).


The last step looks to me like it has several errors.
At t[2] item 1 Rx sends a mapping with mtu of 4466 to Ri.  Therefore at
t[3] step 1 Ri should receive a mapping from Rx with an mtu of 4466.  Also
in this same step Ri should subtract 4 bytes from both the MTU it learned
from Rx, as well as its local interface MTU, in making the calculation (to
account for the label pushed at Ri).

So it looks to me like the correct text for t[3] is:

   1) Ri receives a Mapping for F from Rx with an MTU of *4466*.  Ri
   compares *4462* to *9212*, and sets the MTU for P1 to 4462 (4466 minus
   the overhead of 1 label pushed onto the stack).

---------------------

Also, the example doesn't seem to correctly account for the encapsulation
of LSP P1 inside LSP P2 from Rx to Re.  It discusses mappings for FEC F to
LSP P1 distributed to Ry, however Ry may be unaware of FEC F, since LSP P1
is encapsulated in P2 at this point.  

I think the way to fix this would be to:
  Let FEC G represent traffic mapped to LSP P2.
  Change t[0] and t[1] items #1 to discuss G rather than F.
  Change t[0] and t[1] items #2 to include G.
  Chnage t[2] item #1 to receive mapping for G from Ry, send mapping for F
to Ri.
  Adding another column to the table at the right of the figure would help
as well, giving the "MTU with P1 and P2 overhead".


Apologies if I am misunderstanding this, or if this has already been
pointed out.

--Mark