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

No, I like decrement at only SFF.   But, it does introduce the question of whether behavior should exploit knowledge of position within the chain or not, including the added complexity of carrying this technical debt of the "old" classifier (e.g., should all SFF's blindly accept 0 as legal?).

   Ron

-----Original Message-----
From: Joel M. Halpern [mailto:jmh@joelhalpern.com] 
Sent: Monday, May 1, 2017 12:55 PM
To: Ron Parker <Ron_Parker@affirmednetworks.com>; Dave Dolson <ddolson@sandvine.com>
Cc: mohamed.boucadair@orange.com; sfc@ietf.org; James N Guichard <james.n.guichard@huawei.com>
Subject: Re: [sfc] TTL field within the NSH base header

The summary I sent to the list, based on what little I could tell of rough consensus, was to only decrement TTL at SFF.  If you want to argue for decrementing at all NSH-aware devices, state your case.

Yours,
Joel

On 5/1/17 12:50 PM, Ron Parker wrote:
> Is there a decrement for each SF, too.?   In the past we've talked about
> decrement only between SFFs.
>
> On May 1, 2017, at 11:56 AM, Dave Dolson <ddolson@sandvine.com 
> <mailto:ddolson@sandvine.com>> wrote:
>
>> Ron,
>>
>> I too would like simple, but your table looks pretty complicated. And 
>> I think it adds configuration for an SFF to know whether there are 
>> others before it.
>>
>> My version of simple:
>>
>>   TTL field of 0 means 64.
>>
>>   Every SFF decrements TTL each time it forwards the NSH packet. If 
>> decrementing from 1 would result in under-flow to 64, discard.
>>
>>
>>
>>
>>
>>
>>
>>
>>
>> *From:*Ron Parker [mailto:Ron_Parker@affirmednetworks.com]
>> *Sent:* Monday, May 1, 2017 11:41 AM
>> *To:* Dave Dolson; James N Guichard; mohamed.boucadair@orange.com 
>> <mailto:mohamed.boucadair@orange.com>; sfc@ietf.org 
>> <mailto:sfc@ietf.org>
>> *Subject:* RE: TTL field within the NSH base header
>>
>>
>>
>> Agree that I'd opt for precision over simplicity.   Which is what I
>> tried to convey a ways back on this thread regarding differentiated 
>> behavior at the last SFF in the SFP vs. the non-last SFF in the
>> SFP.    Subsequent comments didn't seem to embrace that way of
>> describing things.    Also, if there were consensus to supporting that
>> approach, backward compatibility for classifiers that emit TTL=0 
>> could be addressed by also describing behavior at the first SFF vs. non-first.
>>
>>
>>
>>
>>
>> 	
>>
>> TTL=1
>>
>> 	
>>
>> TTL=0
>>
>> Only SFF (First and last)
>>
>> 	
>>
>> Valid - service local SF's
>>
>> 	
>>
>> Valid for backward compatibility - service local SF's
>>
>> First SFF and not Last
>>
>> 	
>>
>> Invalid - drop (since propagation to subsequent SFF with TTL=0 is 
>> illegal)
>>
>> 	
>>
>> Valid for backward compatibility - service local SF's and propagate 
>> to subsequent SFF with TTL=63
>>
>> Last SFF when >1 SFF's for SFP
>>
>> 	
>>
>> Valid - service local SF's
>>
>> 	
>>
>> Invalid -drop (due to bad behavior of previous SFF)
>>
>>
>>
>> *From:* Dave Dolson [mailto:ddolson@sandvine.com]
>> *Sent:* Monday, May 1, 2017 11:25 AM
>> *To:* Ron Parker <Ron_Parker@affirmednetworks.com 
>> <mailto:Ron_Parker@affirmednetworks.com>>; James N Guichard 
>> <james.n.guichard@huawei.com <mailto:james.n.guichard@huawei.com>>;
>> mohamed.boucadair@orange.com <mailto:mohamed.boucadair@orange.com>;
>> sfc@ietf.org <mailto:sfc@ietf.org>
>> *Subject:* RE: TTL field within the NSH base header
>>
>>
>>
>> I think precision in the definition is important for when expects 
>> certain behavior for small values of TTL. E.g., by using trace-route 
>> type of tools.
>>
>> We can't just wave our hands and say drop packets with TTL near 0 or 
>> 1 or 2.
>>
>>
>>
>> If one has a chain of 3 items, a TTL of 6 should work. (SFFs receive 
>> packets 6 times in the system diagram that is usually drawn).
>>
>>
>>
>> (Simplified: I realize in some systems more SFF interactions are
>> required.)
>>
>>
>>
>>
>>
>>
>>
>>
>>
>> *From:*Ron Parker [mailto:Ron_Parker@affirmednetworks.com]
>> *Sent:* Monday, May 1, 2017 11:14 AM
>> *To:* Dave Dolson; James N Guichard; mohamed.boucadair@orange.com 
>> <mailto:mohamed.boucadair@orange.com>; sfc@ietf.org 
>> <mailto:sfc@ietf.org>
>> *Subject:* RE: TTL field within the NSH base header
>>
>>
>>
>> Agree that differentiating "forwarding" to attached SF instances vs 
>> subsequent SFF's is logical and intuitive (to me) and I prefer to
>> acknowledge that there is a difference.   But, I was addressing a
>> suggestion to simplify.     In practice, 61 or 62 or 63 decrements of
>> TTL all probably mean the same thing J.
>>
>>
>>
>>    Ron
>>
>>
>>
>>
>>
>> *From:* Dave Dolson [mailto:ddolson@sandvine.com]
>> *Sent:* Monday, May 1, 2017 11:11 AM
>> *To:* Ron Parker <Ron_Parker@affirmednetworks.com 
>> <mailto:Ron_Parker@affirmednetworks.com>>; James N Guichard 
>> <james.n.guichard@huawei.com <mailto:james.n.guichard@huawei.com>>;
>> mohamed.boucadair@orange.com <mailto:mohamed.boucadair@orange.com>;
>> sfc@ietf.org <mailto:sfc@ietf.org>
>> *Subject:* RE: TTL field within the NSH base header
>>
>>
>>
>> There is a difference between dropping the packet at receive vs. transmit.
>>
>> A **terminating** SFF may accept a packet with TTL=1.  I.e., where 
>> the NSH header is removed.
>>
>> What matters is that it is not decremented to zero and forwarded as NSH.
>>
>>
>>
>> Otherwise we'd say, "huh, no point in sending a packet if TTL=1. And 
>> huh, if received with TTL=2 then we might as well drop it." Recurse 
>> ad absurdum.
>>
>>
>>
>> Hence, the correct language needs to convey that a device 
>> decrementing TTL (in order to forward NSH) must check if it would result in zero.
>>
>>
>>
>> This conveys that, I think:
>>
>> The packet MUST NOT be forwarded to a next hop if SHL is decremented 
>> to zero.
>>
>>
>>
>>
>>
>> -Dave
>>
>>
>>
>>
>>
>> *From:*Ron Parker [mailto:Ron_Parker@affirmednetworks.com]
>> *Sent:* Monday, May 1, 2017 10:50 AM
>> *To:* James N Guichard; mohamed.boucadair@orange.com 
>> <mailto:mohamed.boucadair@orange.com>; Dave Dolson; sfc@ietf.org 
>> <mailto:sfc@ietf.org>
>> *Subject:* RE: TTL field within the NSH base header
>>
>>
>>
>> We could simplify this by saying that an SFF that receives an NSH
>> packet with TTL=1 shall drop the packet.   That doesn't require any
>> distinction of the type of forwarding.    You could argue that perhaps
>> 1 means take care of local SF's, only, but then we are back to having
>> to make that distinction.    Given that we are starting at 63 by
>> default, finessing what happens when receiving 1 doesn't seem 
>> worthwhile to me.
>>
>>
>>
>> *From:* James N Guichard [mailto:james.n.guichard@huawei.com]
>> *Sent:* Monday, May 1, 2017 10:45 AM
>> *To:* Ron Parker <Ron_Parker@affirmednetworks.com 
>> <mailto:Ron_Parker@affirmednetworks.com>>;
>> mohamed.boucadair@orange.com <mailto:mohamed.boucadair@orange.com>;
>> Dave Dolson <ddolson@sandvine.com <mailto:ddolson@sandvine.com>>; 
>> sfc@ietf.org <mailto:sfc@ietf.org>
>> *Subject:* RE: TTL field within the NSH base header
>>
>>
>>
>> Yes but my point was if the SFF is **terminating** the service chain 
>> there are no service functions left to be forwarded to so the 
>> sentence does not make any sense. If however the SFF is terminating 
>> the service chain and at termination the SHL reaches 0 then it should 
>> still forward the packet (after removal of NSH).
>>
>>
>>
>> Jim
>>
>>
>>
>> *From:* Ron Parker [mailto:Ron_Parker@affirmednetworks.com]
>> *Sent:* Monday, May 01, 2017 10:28 AM
>> *To:* James N Guichard <james.n.guichard@huawei.com 
>> <mailto:james.n.guichard@huawei.com>>; mohamed.boucadair@orange.com 
>> <mailto:mohamed.boucadair@orange.com>; Dave Dolson 
>> <ddolson@sandvine.com <mailto:ddolson@sandvine.com>>; sfc@ietf.org 
>> <mailto:sfc@ietf.org>
>> *Subject:* RE: TTL field within the NSH base header
>>
>>
>>
>> I'd like to disambiguate the word "forwarding".   From SFF
>> perspective, there is forwarding to attached SF instances and there 
>> is forwarding to other SFFs.
>>
>>
>>
>> "SFFs that terminate a service chain MUST forward the packet to 
>> attached service functions, even if SHL is decremented to 0"
>>
>>
>>
>>    Ron
>>
>>
>>
>>
>>
>>
>>
>> *From:* sfc [mailto:sfc-bounces@ietf.org] *On Behalf Of *James N 
>> Guichard
>> *Sent:* Monday, May 1, 2017 10:22 AM
>> *To:* mohamed.boucadair@orange.com
>> <mailto:mohamed.boucadair@orange.com>; Dave Dolson 
>> <ddolson@sandvine.com <mailto:ddolson@sandvine.com>>; sfc@ietf.org 
>> <mailto:sfc@ietf.org>
>> *Subject:* Re: [sfc] TTL field within the NSH base header
>>
>>
>>
>> I don't understand this sentence as if an SFF is terminating a 
>> service chain then it **wont** be forwarding the packets to attached 
>> SFs; Shouldn't this sentence read "SFFs that terminate a service 
>> chain MUST forward the packet even if SHL is decremented to 0" ?
>>
>>
>>
>> Jim
>>
>>
>>
>> *From:* mohamed.boucadair@orange.com
>> <mailto:mohamed.boucadair@orange.com>
>> [mailto:mohamed.boucadair@orange.com]
>> *Sent:* Friday, April 28, 2017 10:38 AM
>> *To:* Dave Dolson <ddolson@sandvine.com 
>> <mailto:ddolson@sandvine.com>>; James N Guichard 
>> <james.n.guichard@huawei.com <mailto:james.n.guichard@huawei.com>>;
>> sfc@ietf.org <mailto:sfc@ietf.org>
>> *Subject:* RE: TTL field within the NSH base header
>>
>>
>>
>> Re-,
>>
>>
>>
>> I see SHL as a means to prevent SFF loops. There is no value in 
>> deleting a packet after an SFF decrement the SHL to 0, but that 
>> packet is to be passed to SFs that are attached to this SFF. The 
>> packet will be forwarded after stripping the NSH header; no risk for 
>> SFF loops out there. No?
>>
>>
>>
>> Cheers,
>>
>> Med
>>
>>
>>
>> *De :*Dave Dolson [mailto:ddolson@sandvine.com] *Envoyé :* vendredi 
>> 28 avril 2017 16:29 *À :* BOUCADAIR Mohamed IMT/OLN; James N 
>> Guichard; sfc@ietf.org <mailto:sfc@ietf.org> *Objet :* RE: TTL field 
>> within the NSH base header
>>
>>
>>
>> Med,
>>
>>
>>
>> "SFFs that terminate a service chain MUST forward the packet to 
>> attached SFs even if SHL is decremented to 0."
>>
>> I don't think this is right.
>>
>> If TTL/SHL is decremented to 0, this must not be forwarded as NSH.
>>
>> I see that **receiving** a packet with SHL=1 can result in chain 
>> termination, i.e., NSH decapsulation. But not forwarding as NSH with 
>> SHL=0.
>>
>>
>>
>> -Dave
>>
>>
>>
>>
>>
>> *From:*sfc [mailto:sfc-bounces@ietf.org] *On Behalf Of 
>> *mohamed.boucadair@orange.com <mailto:mohamed.boucadair@orange.com>
>> *Sent:* Friday, April 28, 2017 9:36 AM
>> *To:* James N Guichard; sfc@ietf.org <mailto:sfc@ietf.org>
>> *Subject:* Re: [sfc] TTL field within the NSH base header
>>
>>
>>
>> Hi Jim, all,
>>
>>
>>
>> I have the following comments :
>>
>> ·        Change "TTL" to "SFF Hop Limit (SHL)" because this field is
>> not about a time to live but about a limit of SFF hops to be crossed.
>>
>> ·        I don't understand what is meant by "testing".
>>
>> ·        I suggest to make this change to cover the following points:
>>
>> o   SHL should be configurable by the control plane.
>>
>> o   Packets can be forwarded to SFs even if SHL is decremented to 0
>> for the terminating SFF.
>>
>> o   I don't think it is a good idea to include "Decrementing by a
>> value of 1 from 0 shall result in a TTL value of 63" because this 
>> will lead to a broken mechanism.
>>
>>
>>
>> OLD:
>>
>> 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.
>>
>>
>>
>> NEW:
>>
>> SHL (SFF Hop Limit): Indicates the maximum SFF hops for a service 
>> chain. The initial SHL value SHOULD be configurable via the control 
>> plane; the configured initial value can be specific to a chain or all 
>> chains. If no initial value is explicitly provided, the default 
>> initial SHL value 63 MUST be used. Each SFF involved in forwarding an 
>> NSH packet MUST decrement SHL value by 1. The packet MUST NOT be 
>> forwarded to a next hop SFF if SHL is decremented to zero. SFFs that 
>> terminate a service chain MUST forward the packet to attached SFs 
>> even if SHL is decremented to 0.
>>
>>
>>
>> Thank you.
>>
>>
>>
>> Cheers,
>>
>> Med
>>
>>
>>
>> *De :*sfc [mailto:sfc-bounces@ietf.org] *De la part de* James N 
>> Guichard *Envoyé :* jeudi 27 avril 2017 20:54 *À :* sfc@ietf.org 
>> <mailto:sfc@ietf.org> *Objet :* [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
>>
>>
>>
>>
>>
>>
>>
>>
>>
>>
>>
>
>
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