Re: [sfc] TTL field within the NSH base header

["Joel M. Halpern" <[e-mail]>]

There are really two separate issues.
First, and most important, what behavior does the working group want?
     The discussion started with a proposal along the lines Ron 
suggests, and moved to what is in the summary Jim provided.  We are 
trying as chairs to reflect the WG agreement, and to bring this to a 
close.  Yes, we are incurring technical debt (of different forms and 
degrees) in any solution that allows backwards compatibility.  So, yes, 
we can still change this if the WG wants, but this was the rough 
consensus Jim and I saw.

Second, there is the question of whether the text is clear.  It is 
always a balancing act between explaining the motivation for everything 
at great length 9and losing the reader), vs just giving the behavior 
with no explanation (and tending to get incorrect implementations.)  We 
need to be somewhere in between.  We may not be in the right place.

Yours,
Joel

On 4/27/17 5:55 PM, Joe Clarke wrote:
> On 4/27/17 16:37, Joel M. Halpern wrote:
>> That alternative was discussed.  It also works.
>> However, with that alternative, if the Initial classifier is an old
>> system (not RFC compliant), we will not get any protection from the TTL.
>> If we take this (admittedly slightly odd) approach to decrementing the
>> TTL, then even though we do not recommend it, things still work with an
>> initial classifier that generates a TTL of 0.
>>
>> It is thus more robust, so the WG selected it in discussion atht
>> einterim adn on the list.
>
> Thanks, Joel.  I know it was discussed, but reading the final text, I
> wonder if this is just building technical debt into the protocol from
> the beginning.
>
> While this makes sense today, will it be as obvious (or as needed) in a
> year?  In two?
>
> Ron's text does seem clearer at the expense of the backward compat
> explicitness.
>
> Joe
>
>>
>> Yours,
>> Joel
>>
>> On 4/27/17 4:05 PM, Ron Parker wrote:
>>> Thanks, Dave.    Do you think it is possible to combine that backward
>>> compatibility with the simpler (to me) concept of testing for a packet
>>> that arrives with TTL=1 and conditioning behavior on whether the SFF is
>>> the terminating SFF of the path?
>>>
>>>
>>>
>>>    Ron
>>>
>>>
>>>
>>>
>>>
>>> *From:* Dave Dolson [mailto:[e-mail]]
>>> *Sent:* Thursday, April 27, 2017 4:03 PM
>>> *To:* Ron Parker <[e-mail]>; James N Guichard
>>> <[e-mail]>; [e-mail]
>>> *Subject:* RE: TTL field within the NSH base header
>>>
>>>
>>>
>>> Ron,
>>>
>>> Decrementing from zero to reach 63, and checking for zero after
>>> decrementing provides a degree of backwards compatibility with the prior
>>> header format.
>>>
>>> This permits packets on the wire to have 0 in the bits that were
>>> previously “reserved”.
>>>
>>>
>>>
>>> -Dave
>>>
>>>
>>>
>>>
>>>
>>> *From:*sfc [mailto:[e-mail]] *On Behalf Of *Ron Parker
>>> *Sent:* Thursday, April 27, 2017 3:21 PM
>>> *To:* James N Guichard; [e-mail] <mailto:[e-mail]>
>>> *Subject:* Re: [sfc] TTL field within the NSH base header
>>>
>>>
>>>
>>> I had to really think about the language around decrementing by 1 from 0
>>> and reaching 63.   This would only occur in an error scenario where a
>>> preceding SFF misbehaved, wouldn’t it?   Why are we insistent that the
>>> test for 0 happen after decrement, thereby creating this problem.     I
>>> suspect that this has been already discussed and I apologize if I missed
>>> it, but if the language were simplified to say an SFF that receives an
>>> NSH with TTL=1 shall not forward it to another SFF (e.g., allowing it to
>>> engage its local SF instances, still).   Otherwise, it must decrement by
>>> 1 before forwarding to another SFF.
>>>
>>>
>>>
>>> An exact wording along these lines might be something like:
>>>
>>>
>>>
>>> TTL: Service plane time-to-live. An SFF MUST test the TTL before
>>> forwarding to another SFF for a given Service Function Chain.  If the
>>> received TTL value is 1, the SFF MUST drop packets that would otherwise
>>> have been forwarded to another SFF, but SHALL send such packets to
>>> attached service functions if the SFF terminates the Service Function
>>> Chain.   If the TTL value is greater than 1, the SFF must decrement the
>>> TTL by 1 before forwarding to another SFF.   The default for originating
>>> an NSH packet is a TTL value of 63.
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>> *From:* sfc [mailto:[e-mail]] *On Behalf Of *James N Guichard
>>> *Sent:* Thursday, April 27, 2017 2:54 PM
>>> *To:* [e-mail] <mailto:[e-mail]>
>>> *Subject:* [sfc] TTL field within the NSH base header
>>>
>>>
>>>
>>> Dear WG:
>>>
>>>
>>>
>>> Having reviewed all of the email discussion on the mailing list it
>>> appears to the chairs that we have consensus to add a TTL field to the
>>> NSH base header. We would like to propose the following changes:
>>>
>>>
>>>
>>> Section 3.2:
>>>
>>> Update figure 2 as follows:
>>>
>>>
>>>
>>>      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
>>>
>>>     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
>>>
>>>     |Ver|O|R|    TTL    |   Length  |R|R|R|R|MD Type| Next Protocol |
>>>
>>>     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
>>>
>>>
>>>
>>> Add the following text after figure 2:
>>>
>>>
>>>
>>> TTL: Service plane time-to-live. An SFF MUST decrement the TTL by a
>>> value of 1 for all NSH packets it receives. Decrementing by a value of 1
>>> from 0 shall result in a TTL value of 63. The default for originating an
>>> NSH packet is a TTL value of 63. The decrement SHALL occur before
>>> testing for 0. After decrement, if the TTL is 0, the NSH packet MUST NOT
>>> be forwarded.
>>>
>>>
>>>
>>> Section 3.4:
>>>
>>> Update figure 4 to reflect the new base header format as per section 3.2
>>> base header.
>>>
>>>
>>>
>>> Section 3.5:
>>>
>>> Update figure 5 to reflect the new base header format as per section 3.2
>>> base header.
>>>
>>>
>>>
>>> Section 12.2.1:
>>>
>>> Current text is as follows:
>>>
>>>
>>>
>>>    There are ten bits at the beginning of the NSH Base Header.  New bits
>>>
>>>    are assigned via Standards Action [RFC5226].
>>>
>>>
>>>
>>>    Bits 0-1 - Version
>>>
>>>    Bit 2 - OAM (O bit)
>>>
>>>    Bit 3 - Critical TLV (C bit)
>>>
>>>    Bits 4-9 - Reserved
>>>
>>>
>>>
>>> Replace entire text as follows:
>>>
>>>
>>>
>>>    There are eight reserved bits in the NSH Base Header. New bits
>>>
>>>    are assigned via Standards Action [RFC5226].
>>>
>>>
>>>
>>>    Bits 0-1 - Version
>>>
>>>    Bit 2 - OAM (O bit)
>>>
>>>    Bit 3 - Reserved
>>>
>>>    Bits 16-19 - Reserved
>>>
>>>
>>>
>>> Section 12.2.3:
>>>
>>> Current text has the MD-type as 8-bit values.
>>>
>>>
>>>
>>> Update text for this section and table 1 to reflect 4-bit values *not*
>>> 8-bit values.
>>>
>>>
>>>
>>> *Please review carefully and indicate support for these changes (or any
>>> changes to the suggested text).*
>>>
>>> * *
>>>
>>> Thanks,
>>>
>>>
>>>
>>> Jim & Joel
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>> _______________________________________________
>>> sfc mailing list
>>> [e-mail]
>>> https://www.ietf.org/mailman/listinfo/sfc
>>>
>>
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>