Re: [spring] Typo correction Re: Question from SPRING regarding draft-filsfilscheng-spring-srv6-srh-compression

[Brian E Carpenter <[e-mail]>]

On 20-Oct-21 05:02, Tom Herbert wrote:
> 
> 
> On Tue, Oct 19, 2021, 12:10 AM Mark Smith <[e-mail] <mailto:[e-mail]>> wrote:
> 
>     Hi Brian,
> 
>     On Tue, 19 Oct 2021 at 15:51, Brian E Carpenter
>     <[e-mail] <mailto:[e-mail]>> wrote:
>     >
>     > Hi,
>     >
>     > After reading a lot of messages, I'm going to offer my considered
>     > opinion as a direct response to Joel's OP.
>     >
>     > Firstly, I don't believe that in the end this draft raises any
>     > concerns that are *significantly* different than those raised
>     > when RFC 8986 was in draft. As Ted Hardie mentioned, section 5
>     > of RFC 8754 explains that SIDs of any shape or size are only
>     > meaningful within an SR domain. That applies to srh-compression
>     > too.
>     >
>     > Secondly, I was concerned about how these strange looking
>     > "addresses" would potentially interfere with normal IPv6
>     > addresses and their handling by normal IPv6 nodes. Well, I
>     > now believe that they won't. The reason is that in the SR model
>     > these "addresses" are *never used for final delivery of IPv6
>     > packets to a host.* All SRv6 participants are routers. The
>     > last hop for a packet whose DA is set to (say) 2001:db8:a:1900::
>     > is *not* the last hop on a LAN, mediated by neighbor discovery
>     > for 2001:db8:a:1900::. It's just a hop from one router to another,
>     > using the entry for 2001:db8:a:1900::/64 in the FIB of the last
>     > router that actually forwards the packet. 2001:db8:a:1900:: is
>     > not assigned to a physical interface so RFC 4861 is never invoked.
>     >
> 
>     So I'm guessing you're probably thinking of a router as a physical
>     device (as most of us will by default), rather than as routing as a
>     function.
> 
>     RFC8200 definitions:
> 
>     "router       a node that forwards IPv6 packets 
not explicitly
>                     addressed to itself.  (See Note below.)
> 
>        host         any node that is 
not a router.  (See Note below.)"
>     (the note is about hosts having multiple interfaces)
> 
>     (A node doesn't have to be a device, and a device doesn't have to be
>     physical, so the above are really function descriptions.)
> 
>     If a router as a physical device has IPv6 addresses that are sent to
>     the device itself, then the router as a physical device is also
>     performing host functions. For packets with those "router addresses",
>     the router as a physical device is not "forward[ing] IPv6 packets not
>     explicitly addressed to itself. "
> 
>     In a router as a physical device, the forwarding plane is performing
>     the router function above. In a router as a physical device, the
>     "control plane" (really just a fancy name for a host) is performing
>     host functions on packets that are directly to and from it (BGP, OSPF,
>     SSH, SNMP etc. packets).
> 
>     The consequence is that all IPv6 addresses are host addresses, and all
>     processing of packets at the device that holds a packet's DA is host
>     processing of the packet.
> 
>     It is the same in IPv4  - from RFC791, "The internet protocol provides
>     for transmitting blocks of data called datagrams from sources to
>     destinations, where sources and destinations are hosts identified by
>     fixed length addresses."
> 
>     So these packets with CSID rotating IPv6 DAs are being host processed
>     by the router (as a device), after the forwarding plane in the router
>     (as a device) has forwarded the packet to the colocated host function.
> 
>     (A more practical example to support the above observation. If a
>     router as a device is bought from a router vendor, and then is run as
>     a BGP route reflector, meaning it is never in a packet forwarding
>     path, and therefore never "forwards IPv6 packets not explicitly
>     addressed to itself" is the router device still a router? Functionally
>     no, it is a host, even though it looks like a router (as a device).)
> 
> 
> Mark,
> 
> On the other hand, when a node is processing a routing it header it does forward packets addressed to it self (but not being the final destination). Processing a router header is intuitively router functionality and not host functionality. The RFC8200 definition of host and router don't seem to consider routing headers, perhaps this should be amended to say that a host is the final destination of the packet.

That is the case even if the host is also a router and if it recognises the DA of a packet delivered to it in its role as a router as a DA that it 
handles in its role as a host. (In Linux jargon, that means "if the DA is 
assigned to its loopback interface", but that is an implementation detail.)

The point here is that the *previous* router doesn't know that the DA is assigned that way; it just forwards the packet to whoever announced the longest-matching prefix. It *does not* send the packet to DA.

   Brian

> 
> Tom
> 
> 
>     Regards,
>     Mark.
> 
>     > Another way to say it is RFC 7608 is the relevant architectural
>     > standard. CIDR rules, even within an SR domain.
>     >
>     > For that reason, the fact that the bottom 64 bits in the
>     > "address" look funny or change is simply irrelevant. They are
>     > invisible to routing (which is done based on the prefix)
>     > and invisible to neighbor discovery (because it never happens).
>     >
>     > I apologise if this is all obvious to everybody, but I needed
>     > to spell it out for my own understanding.
>     >
>     > Now back to Joel's questions:
>     >
>     >
>     > On 13-Oct-21 20:37, Joel M. Halpern wrote:
>     > > There is a typo in the below which if not understood as a typo would be
>     >
>     > > quite confusing.   I wrote that I raised the issue with
>     > > "with the Internet ADs and SPRING chairs".
>     > > That should have read "with the Internet ADs and 6man chairs".
>     > > The SPRING co-chairs are recused, and the charter requirement leads to
>     > > the 6man chairs.  Which is who I talked to.
>     > >
>     > > Also, I am sending a courtesy copy to the routing ADs, which I should
>     > > have done originally.
>     > >
>     > > Thank you and enjoy.
>     > > Yours,
>     > > Joel
>     > >
>     > > On 10/12/2021 11:52 PM, Joel M. Halpern wrote:
>     > >> The SPRING working group is in the midst of an adoption call on
>     > >> https://datatracker.ietf.org/doc/draft-filsfilscheng-spring-srv6-srh-compression/ <https://datatracker.ietf.org/doc/draft-filsfilscheng-spring-srv6-srh-compression/>.
>     > >>
>     > >>
>     > >> The SPRING charter has text that is explicit that modifications to data
>     > >> planes and architectures standardized by other working groups may not be
>     > >> modified in SPRING unless the chairs and ADs responsible for that data
>     >
>     > >> plane and / or architecture agree.
>     > >>
>     > >> To complete the context, as my SPRING co-chairs are co-authors 
on the
>     > >> document in question, they have recused themselves from decisional
>     > >> activities regarding the document.  Therefore, this message is coming
>     > >> just from my as the responsible SPRING co-chair managing this adoption
>     >
>     > >> call.
>     > >>
>     > >> As you have seen, multiple questions have been raised about the
>     > >> relationship of the document to the IPv6 defined data plane and
>     > >> architecture (particularly RFC 4291 and 8200). In particular the
>     > >> questions seem to revolve around what the document describes as the
>     > >> NEXT-C-SID flavor of compressed SID, and its relationship to the IPv6
>     > >> standards.  (For those seeking more context without reading the full
>     > >> document, a paraphrase and simplification of the NEXT-C_SID flavor is
>     > >> provided as a postscript.)
>     > >>
>     > >> I raised the question of concurrence as required by the SPRING 
charter
>     >
>     > >> with the Internet ADs and SPRING chairs.  They quite reasonably asked me
>     > >> to write a note to 6man explaining the concerns as clearly as a can, so
>     > >> that they can then determine how to proceed.
>     > >>
>     > >> The questions that prompted my inquiry are:
>     > >>
>     > >> 1) Does the placement of a list of sids in the IPv6 DA field change the
>     > >> IPv6 architectural description of that field.
>     >
>     > I think it should be noted explicitly somewhere that since the contents
>     > of the DA field are *never* used for last-hop neighbor discovery,
>     > the IID aspect of RFC 4291 is irrelevant, and RFC 4861 + RFC 5942
>     > are irrelevant. Another citation is RFC 7608: for routing, all that
>     > counts is the prefix, and it can be anything up to 128.
>     >
>     > Perhaps this should have been in section 5 of RFC 8754, but I leave
>     > that to the wordsmiths.
>     >
>     > >> 2) Does the operation of shifting information around in the IPv6
>     > >> destination address field represent a modification or extension of the
>     >
>     > >> IPv6 data plane.
>     >
>     > No. As my text above indicates, the SRv6 DA field is only ever used
>     > by routing, where RFC 7608 rules. And of course it vanishes as soon
>     > as the packet is decapsulated.
>     >
>     > Regards
>     >     Brian
>     >
>     > >>
>     > >> On a related note, the document in question also defines two other
>     > >> flavors, REPLACE-C-SID, and NEXT-and-REPLACE-C-SID.  The
>     > >> NEXT-and-REPLACE-C_SID flavor is defined to include the NEXT-C_SID
>     > >> flavor operation, so seems to be affected by the same question.
>     > >>
>     > >>  From my own reading, it appears that the REPLACE-C-SID flavor does not
>     > >> raise issues requiring 6man leadership concurrence.
>     > >>
>     > >> Yours,
>     > >> Joel M. Halpern for the SPRING working group
>     > >>
>     > >>
>     > >> PS:
>     > >> Clearly, understanding the question requires some understanding of what
>     > >> the NEXT-C_SID flavor does.   This explanation is a simplification for
>     > >> length and context.  Really, the best place to understand 
it is the
>     > >> draft.  However, to give you enough information to let you decide
>     >
>     > >> whether you care, I will try to provide a fair summary.  My apologies in
>     > >> advance to the authors for necessary liberties for length.  
Also,
>     >
>     > >> discussion of the draft contents (as distinct from the interaction with
>     > >> the IPv6 data plane and architecture) belongs on the SPRING list, and
>     > >> should not clutter up 6man.
>     > >>
>     > >> SIDs are the identifiers used in segment routing.
>     > >> In SRv6, as document in the current RFCs, these are 128 bits.   As
>     > >> defined in the relevant RFCs, SIDs which identify endpoints to 
which
>     > >> packets are directed are identified by endpoint SIDs.  These can have
>     > >> behaviors (decapsulate and forward is one example).  They 
can have
>     > >> flavors such as where the SRH is removed.
>     > >>
>     > >> The topic under discussion is means to compress these SIDs in the
>     > >> packets on the wire.  The document under discussion provides three
>     > >> flavors of compression.
>     > >>
>     > >> The fundamental mechanism of the draft is to use a single SRH entry as
>     > a
>     > >> container for multiple SIDs.  In the NEXT-C_SID mechanism, when it is
>     > >> first encountered the entire container is copied into the desination
>     > >> address of the IPv6 packet.  The container has a common routing prefix
>     > >> used for all the NEXT-C-SID SIDs.  It is followed by a sequence of
>     > >> compressed SIDs of a configured length.  One could configure 16, 24, or
>     > >> 32 bits.  Or whatever length.  The routing advertisements are arranged
>     > >> so that the IPv6 packet is directed to the node represented by 
the first
>     > >> compressed SID on the basis of longest prefix match matching the
>     > >> combination of the common routing prefix and that compressed SID.
>     > >>
>     > >> When the packet arrives at that node, it looks up the configured
>     > >> portion, the compressed SID, and determines the behavior and flavor.  In
>     > >> the case of the NEXT-C-SID flavor, the resulting operation is to shift
>     >
>     > >> the entire remaining contents of the IPv6 address (the bits past the
>     > >> first compressed sid) so as to over-write the first compressed 
SID.  0
>     > >> bits are shifted into the low order positions.  If the result is a
>     > >> non-zero new first compressed SID, then the packets is forwarded and the
>     > >> process repeats.  When all that is left are 0s, if there is an SRH, it
>     > >> is consulted to find the next SRH entry, which is, per normal SRv6
>     > >> processing, put into the IPv6 DA.
>     > >> Note that in the common case where the SIDS needed all fit in to a
>     > >> single container, the analysis also assumes the use of the reduced
>     > >> encapsulation options which omits the SRH that is not needed as it would
>     > >> have no entries.  This the packet contains a normal IPv6 header, with a
>     > >> sequence of compressed SIDs (what one might or might not call a source
>     >
>     > >> route) in the IPv6 destination address field.
>     > >>
>     > >> PPS: If the authors of the NEXT-C-SID flavor feel I have mis-represented
>     > >> the work, please, send clarifications or corrections.   Again, the best
>     > >> source of information is the draft itself.  I was asked to provide extra
>     > >> context in this email.
>     > >
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