Re: [spring] PSP and a logical application of RFC8200

"Pablo Camarillo (pcamaril)" <pcamaril@cisco.com> Mon, 02 March 2020 20:03 UTC

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From: "Pablo Camarillo (pcamaril)" <pcamaril@cisco.com>
To: Brian E Carpenter <brian.e.carpenter@gmail.com>, "Darren Dukes (ddukes)" <ddukes=40cisco.com@dmarc.ietf.org>, 6man WG <ipv6@ietf.org>, SPRING WG List <spring@ietf.org>
Thread-Topic: PSP and a logical application of RFC8200
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Date: Mon, 2 Mar 2020 20:02:54 +0000
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Subject: Re: [spring] PSP and a logical application of RFC8200
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Brian,

The PSP pseudocode is presented as a modification to the End pseudocode starting at line S14 of such.
Please go through the PSP pseudocode in conjunction with the End pseudocode (Section 4.1). 
You will see that the ingress state of the packet is (Segments Left == 1 and Destination Address == the PSP node's address).

Many thanks,
Pablo.

-----Original Message-----
From: ipv6 <ipv6-bounces@ietf.org> on behalf of Brian E Carpenter <brian.e.carpenter@gmail.com>
Date: Monday, 2 March 2020 at 20:34
To: "Darren Dukes (ddukes)" <ddukes=40cisco.com@dmarc.ietf.org>rg>, 6man WG <ipv6@ietf.org>rg>, SPRING WG List <spring@ietf.org>
Subject: Re: PSP and a logical application of RFC8200

    Darren,
    
    Regardless of whether you accept Fernando's comment about the intention of RFC 8200, there is also the fact that the description of the PSP flavor cheats by considering the packet to have
     (Segments Left == 0 and Destination Address == the PSP node's address).
    In fact that is *never* the state of the packet on the wire, which is either
     (Segments Left == 1 and Destination Address == the PSP node's address)
    or
     (Segments Left == 0 and Destination Address == the final node's address)
    
    OK, maybe it's not cheating, maybe it's only a side effect of the pseudocode, but the fact is that the test "S14.1.   If (Segments Left == 0) {" in section 4.16.1 is very confusing because it's applied to a packet that is half way through processing of the routing header (Segments Left has been updated, but Destination Address has not been updated). This makes it very unclear how the spec is claiming to interpret RFC 8200.
    
    Regards
       Brian Carpenter
    
    On 03-Mar-20 03:52, Darren Dukes (ddukes) wrote:
    > What follows has been made clear on the list for a while, 
    > I am re-stating it.
    > 
    > The draft-ietf-spring-srv6-network-programming PSP behavior 
    > strictly follows the letter of RFC 8200.
    >  
    >      RFC8200 section 4 says:
    >  
    >      Extension headers (except for the Hop-by-Hop Options header) are not
    >      *processed, inserted, or deleted* by any node along a packet's delivery
    >      path, until the packet reaches *the node* (or each of the set of nodes,
    >      in the case of multicast) *identified in the Destination Address field*
    >     * of the IPv6 header.*
    >   
    > 
    > The processing, insertion and deletion restrictions only apply 
    > “until the packet reaches the node identified in the Destination
    > Address field of the IPv6 header”.
    >  
    > At the penuptimate segment of the segment list, the endpoint IS
    > “the node identified in the Destination Address field of the IPv6
    > header” and hence the PSP operation programmed by the source SR 
    > node strictly follows the letter of RFC 8200.
    > 
    > 
    > Thanks,
    >   Darren
    > 
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