Re: [spring] USD/USP question in draft-ietf-spring-srv6-network-programming-06.txt

[Gyan Mishra <[e-mail]>]

Hi Wang

I have a question regarding the PSP, USP and USD sections I pasted below.

I just sent an email to Spring WG related to PSP and technically why that
is necessary as that is a legacy concept that has parity to MPLS but is not
used today due to QOS issues.  Please see that email related to that topic.


In the PSP section can If we have to keep PSP can we add verbiage that
states that PSP removal of the SRH header occurs on the Penultimate egress
P node.

In the USP section can we also add that all remaining SRH present in the
packet are popped on the egress PE ultimate node.

In looking at these 3 SID functions the PSP and USP pop the EH and the USP
removes the 6in6 encapsulation so that the other end.x dt4 dt6 etc can pop
the services L3vpn headers.

Why can’t the USD 6in6 encapsulation removal be done on with the USP SID?

Why does the USP and USD SID have to be separate?

4.16.1 <https://tools.ietf.org/html/draft-ietf-spring-srv6-network-programming-05#section-4.16.1>.
PSP: Penultimate Segment Pop of the SRH

   The SRH processing of the End, End.X and End.T behaviors are
   modified: after the instruction "S14.  Update IPv6 DA with Segment
   List[Segments Left]" is executed, the following instructions must be
   executed as well:

   S14.1.   If (updated SL == 0) {
   S14.2.      Pop the SRH
   S14.3.   }
4.16.2 <https://tools.ietf.org/html/draft-ietf-spring-srv6-network-programming-05#section-4.16.2>.
USP: Ultimate Segment Pop of the SRH

   The SRH processing of the End, End.X and End.T behaviors are
   modified: the instructions S02-S04 are substituted by the following
   ones:

   S02.   If (Segments Left == 0) {
   S03.       Pop the SRH
   S04.   }

4.16.3 <https://tools.ietf.org/html/draft-ietf-spring-srv6-network-programming-05#section-4.16.3>.
USD: Ultimate Segment Decapsulation


   The SRH processing of the End, End.X and End.T behaviors are
   modified: the instructions S02-S04 are substituted by the following
   ones:

   S02.   If (Segments Left == 0) {
   S03.      Skip the SRH processing and proceed to the next header
   S04.   }


   Further on, the Upper-layer header processing of the End, End.X and
   End.T behaviors are modified as follows:



Kind regards,


Gyan


On Sat, Dec 14, 2019 at 9:08 PM Wang, Weibin (NSB - CN/Shanghai) <
[e-mail]> wrote:

> Hi Pablo:
>
>
>
> After the 2 context assumption in previous version of this draft,  “we
> assume that there is no other extension header than the SRH.”  and  “We
> assume
>
>    that the SRH may be present multiple times inside each packet”, are
> removed in this last draft, I feel a bit confusion on USD SID, as well as
> combination of USD & USP.
>
>
>
> First, within the content of this last draft, the word “Further on” marked
> red in the following pseudocode in section “4.16.3” is hard to understand
> if the packet being processed has other EH embed between SRH and
> Upper-layer header, such as AH or other EH, then the processing control of
> this packet will be passed to normal IPv6 module from current SRH
> processing module in SR-Node, so my question is : Can its control after
> completing AH processing (for example)  be back to SRH module (or call it
> pseudocode module) to proceed the next header like “upper-lay header type
> ==41 or 4”.
>
> Or, if not, Did you created a new EH processing protocol stack instance in
> parallel to normal IPv6 module within the scope of SRH processing in
> SR-node.
>
>
>
> 4.16.3.  USD: Ultimate Segment Decapsulation
>
>
>
> S02.   If (Segments Left == 0) {
>
>    S03.      Skip the SRH processing and proceed to the next header
>
>    S04.   }
>
>
>
> Further on, the Upper-layer header processing of the End, End.X and
>
>    End.T behaviors are modified as follows:
>
>
>
>    End:
>
>    S01. If (Upper-layer Header type == 41 || 4) {
>
>    S02.    Remove the outer IPv6 Header with all its extension headers
>
>    S03.    Submit the packet to the egress IP FIB lookup and
>
>               transmission to the new destination
>
>    S04. } Else {
>
>    S05.    Send an ICMP Parameter Problem message to the Source Address
>
>               Code 4 (SR Upper-layer Header Error),
>
>               Pointer set to the offset of the upper-layer header.
>
>               Interrupt packet processing and discard the packet.
>
>
>
>    S06. }
>
>
>
> From my understanding, the all processing action about specific SID must
> be completed successively. That is to say, upon USD, the upper-layer header
> (type 41 or 4) must be followed the SRH header being processed currently,
> or second SRH following the same rule (of course, the draft not considering
> 2 or more successive SRHs).
>
>
>
> Second, the mixed SIDs function with combination of USD and USP (even
> PSP&USD&USP), I think, it is easy to understand when the two assumption
> above exist, but now I think it isn’t clear if you only provide the
> following sentence in this draft, i.e.  “if … else…” statement:
>
> “An implementation that supports the USD flavor in conjunction with
>
>    the USP flavor MAY optimize the packet processing by first looking
>
>    whether the conditions for the USD flavor are met, in which case it
>
>    can proceed with USD processing else do USP processing.”
>
> This confusion is also described in my another mail. Of course, if the
> first question is addressed then this confusion does not exist.
>
>
>
> By the way, is it really no different in text description before and after
> the two context assumption above removed?
>
>
>
>
>
> *Cheers !*
>
>
>
> *WANG Weibin*
> _______________________________________________
> spring mailing list
> [e-mail]
> https://www.ietf.org/mailman/listinfo/spring
>
-- 

Gyan S. Mishra

IT Network Engineering & Technology

Verizon Communications Inc. (VZ)

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