Re: [lisp] [spring] IPv6-compressed-routing-header-crh

[Greg Mirsky <[e-mail]>]

Hi Tom,
thank you for your feedback and the suggestion. In proposing the use of 20
bits-long SID we've followed the existing IGP-SR extensions. Both
draft-ietf-ospf-segment-routing-extensions
and draft-ietf-isis-segment-routing-extensions advertisement of 4
octets-long SIDs as well as 20 bits-long (the latter as 20 rightmost bits
in 3 octets-long field). Though the proposal to using 20 bits-long SIDs in
IPv6 SR EH may be considered as duplication of
the draft-ietf-mpls-sr-over-ip, SR EH has very useful property, for example
in OAM, of preserving the SR path.
I think that we can expand the length of the field in SR EH to support 16
and 64 bits-long SIDs in addition to ones being proposed in the draft.
Much appreciate your comments.

Regards,
Greg

On Sat, Apr 20, 2019 at 12:50 PM Tom Herbert <[e-mail]> wrote:

>
>
> On Fri, Apr 19, 2019, 5:54 PM Greg Mirsky <[e-mail]> wrote:
>
>> Hi Tom,
>> in draft-mirsky-6man-unified-id-sr
>> <https://tools.ietf.org/html/draft-mirsky-6man-unified-id-sr-02> we've
>> proposed the use of 20 and 32 bits-long SIDs in SR EH. Two bits-long field
>> also defined in the Flags to identify the length of SID element in the SR
>> EH:
>>       0b00 - 128-bits SID;
>>       0b01 - 20-bits SID;
>>       0b10 - 32-bits SID
>>       0b11 - reserved for future use.
>>
>
> Hi Greg,
>
> 20 bit fields in a list seems a little odd; how is this packed in a
> packet? It's more typical to have byte alignment at least and if the fields
> hold numerical values they would usually be bytes, words, double words,
> etc. with natural alignment maintained. In a two bit representation of
> length, I think best possibilities are 16, 32, 64, and 128 bits.
>
> Tom
>
>
>> Much appreciate your comments on that draft, suggestions.
>>
>> Regards,
>> Greg
>>
>>
>> On Fri, Apr 12, 2019 at 3:09 PM Tom Herbert <[e-mail]> wrote:
>>
>>> On Fri, Apr 12, 2019 at 1:48 PM Mark Smith <[e-mail]>
>>> wrote:
>>> >
>>> > Hi Tom,
>>> >
>>> > On Sat, 13 Apr 2019 at 00:26, Tom Herbert <[e-mail]> wrote:
>>> > >
>>> > > On Sun, Mar 31, 2019 at 7:40 AM Robert Raszuk <[e-mail]>
>>> wrote:
>>> > > >
>>> > > > Hi Mark,
>>> > > >
>>> > > > > As MPLS SR SIDs are 20 bits, then rounding up to an octet
>>> boundary and a 32 bit alignment,
>>> > > > > I'd think 32 bit SIDs would be adequate to perform SR in an IPv6
>>> network.
>>> > > > >
>>> > > > > As 32 bit SIDs are also the same size as IPv4 addresses, that
>>> may also create some opportunities to
>>> > > > > leverage IPv4 support in existing protocols to suite carrying
>>> and processing 32 bit SIDs with some, possibly
>>> > > > > slight, modification. For example, perhaps IPv4 Address Family
>>> support in OSPFv3 (RFC 5838) could be
>>> > > > > somehow leveraged to suit SR.
>>> > > >
>>> > > >
>>> > > > Thank you for describing your understanding of fundamentals of SR.
>>> > > >
>>> > > > I think SR while indeed started with the story of "less control
>>> plane is good for you" now clearly has evolved into not only reduction of
>>> control plane but what can be even more important to some users ability to
>>> request specific behavior via programmed functions of network elements on a
>>> per flow basis without actually per flow or per path signalling or state.
>>> > > >
>>> > > > Yes for some it may be very useful feature and I am sure some will
>>> call it overload of data plane or . There is no one size fits all.
>>> > > >
>>> > > > With that let's observe that till today SR did not require any new
>>> mapping plane to be distributed in control plane and to be inserted into
>>> data plane. This is clearly a precedent.
>>> > > >
>>> > > > Furthermore as we see in companion documents all additional
>>> network functionality is being taken away from SRH and is being shifted to
>>> Destination Options .
>>> > > >
>>> > > > As far as mapping plane I already pointed out in my Vector Routing
>>> proposal that we have one already it is called BGP. One needs to also
>>> observe that we as industry worked number of years of protocol suite called
>>> LISP allowing not only very good mapping plane, but also data plane
>>> integration. CC-ing lisp authors for their comments. Note also work for
>>> integrating SRv6 with LISP which is already is published.
>>> > > >
>>> > > > Since you correctly observed that now SID can be 32 bit and that
>>> is similar to the size of IPv4 my fundamental question is why not use
>>> something which already exists instead of defining some sort of new  from
>>> scratch ?
>>> > > >
>>> > > Robert,
>>> > >
>>> > > I don't see in the SRH draft where 32 bit SIDs are defined. Can you
>>> > > please provide a reference?
>>> > >
>>> >
>>> > To clarify, I've been thinking about the idea of a smaller SID size
>>> > for IPv6 for a while now (since inserting EHs came up), and thought
>>> > about what would be a generic single size that might suit SR that
>>> > wasn't the same size as an IPv6 address. 32 bits seemed suitable to
>>> > me, although if people wanted bigger, I'd be suggesting 64 bits (not
>>> > entirely coincidentally the common IID size.)
>>> >
>>> > Ron and others have written this draft, which supports SIDS of various
>>> > sizes - 8, 16 or 32 bits - that triggered this discussion.
>>> >
>>> Mark,
>>>
>>> Why not just put a SID length field in the header (like RFC6554 but
>>> more generic). That would allow lengths of 1-16 bytes. Additional
>>> flags could be used to indicate the semantics of the entries. For
>>> instance, they might be actual addresses (128 bits for IPv6, 32 bits
>>> for IPv4), parts of addresses (prefixes of suffixes like in RFC6554)
>>> where the rest of the address can be inferred, indices into a table,
>>> labels, etc.
>>>
>>> Tom
>>>
>>> > "The IPv6 Compressed Routing Header (CRH)"
>>> > https://tools.ietf.org/html/draft-bonica-6man-comp-rtg-hdr-03
>>> >
>>> > Regards,
>>> > Mark.
>>> >
>>> >
>>> > > As for trying to use something that already exists, why does SR used
>>> a
>>> > > fixed size format for SIDs instead of a variable length format like
>>> > > that described in RFC6554? Similarly, why does SR define it's own TLV
>>> > > format instead of using Hop-by-Hop and Destination Options defined in
>>> > > RFC8200?
>>> > >
>>> > > Tom
>>> > >
>>> > > > It will be perfectly fine to have full proper SRv6 with SRH and
>>> LISP or Vector Routing as an alternative options. I really do not see a
>>> room or need for yet one more mapping plane. What problem does it solve
>>> which would not be already solved elsewhere ?
>>> > > >
>>> > > > Kind regards,
>>> > > > Robert
>>> > > >
>>> > > >
>>> > > >>> 2) Is there an agreement that solutions which require additional
>>> per SR path state in both control plane and now in data plane are really
>>> something we should be endorsing here ?
>>> > > >>
>>> > > >>
>>> > > >> I think so.
>>> > > >>
>>> > > >> My understanding of what SR is fundamentally about is to reduce
>>> control plane state and processing. The trade-off for reduced control plane
>>> state and processing is to instead carry and encode most or all of that
>>> information or its semantics as per-packet overhead.
>>> > > >>
>>> > > >> If the per-packet overhead becomes too large and expensive, then
>>> pushing some of that information and processing back into the control plane
>>> should be ok, as long as there is still a beneficial overall reduction in
>>> control plane state and processing.
>>> > > >>
>>> > > >> As MPLS SR SIDs are 20 bits, then rounding up to an octet
>>> boundary and a 32 bit alignment, I'd think 32 bit SIDs would be adequate to
>>> perform SR in an IPv6 network.
>>> > > >>
>>> > > >> As 32 bit SIDs are also the same size as IPv4 addresses, that may
>>> also create some opportunities to leverage IPv4 support in existing
>>> protocols to suite carrying and processing 32 bit SIDs with some, possibly
>>> slight, modification. For example, perhaps IPv4 Address Family support in
>>> OSPFv3 (RFC 5838) could be somehow leveraged to suit SR.
>>> > > >>
>>> > > >> Regards,
>>> > > >> Mark.
>>> > > >
>>> > > >
>>> --------------------------------------------------------------------
>>> > > > IETF IPv6 working group mailing list
>>> > > > [e-mail]
>>> > > > Administrative Requests:
>>> https://www.ietf.org/mailman/listinfo/ipv6
>>> > > >
>>> --------------------------------------------------------------------
>>>
>>> _______________________________________________
>>> spring mailing list
>>> [e-mail]
>>> https://www.ietf.org/mailman/listinfo/spring
>>>
>>