Re: [mpls] Request for comments on draft-malis-mpls-sfc-encapsulation-00.txt

[Eric C Rosen <[e-mail]>]

It's always a bit tricky to specify the precise rules for creating a 
label stack.  I think the rules for creating the label stack atop an NSH 
packet are probably something like the following.  Suppose one has an 
NSH packet, and one has chosen a target node on the network, where that 
target node is (or contains) the SFF that next needs to receive the NSH 
packet (the target SFF).  Then create the label stack as follows:

- Push on zero or more labels that are to be interpreted by the target 
SFF.  (I guess the GAL label could be an example here.)

- Push on the SFF label.  (A label that, at the target node, is 
associated with the target SFF.)

- Push on zero or more additional labels such that (a) the resulting 
label stack will cause the packet to be transported to the target node, 
and (b) when the packet arrives at the target node, either:

   * the SFF label will be at the top of the label stack, or

   * the SFF label will rise to the top of the label stack before the 
packet is forwarded to another node and before the packet is dispatched 
to a higher layer.

It's probably best to also state that PHP MUST NOT be applied to the SFF 
label.  (I realize that the control plane is out of scope here, but 
whatever the control plane is, it needs to set up the PHP appropriately.)

It would be good to say what the SFF is supposed to do if it gets an 
MPLS packet rather than an NSH packet (i.e., if the SFF label is not 
BoS).  The SFF might or might not understand the label following the SFF 
label.  Also, if the following label is v4-explicit-null or 
v6-explicit-null, does that convey any semantics?  Frankly, I wonder if 
it's a bit dangerous to allow labels below the SFF label, as this will 
cause problems if the SFF label is programmed to cause a dispatch of the 
remaining packet to a procedure that does not understand MPLS at all and 
is expecting to see an NSH packet.

It would be good to say where something like the entropy label would 
appear in the stack.  Before the SFF label?  Or after?  Or either?