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

Dave Dolson <ddolson@sandvine.com> Tue, 02 May 2017 13:24 UTC

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From: Dave Dolson <ddolson@sandvine.com>
To: "mohamed.boucadair@orange.com" <mohamed.boucadair@orange.com>, James N Guichard <james.n.guichard@huawei.com>, Greg Mirsky <gregimirsky@gmail.com>
CC: "sfc@ietf.org" <sfc@ietf.org>, Ron Parker <Ron_Parker@affirmednetworks.com>
Thread-Topic: [sfc] TTL field within the NSH base header
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Date: Tue, 02 May 2017 13:19:45 +0000
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Subject: Re: [sfc] TTL field within the NSH base header
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I still think the conceptual difficulty goes away if we say that the 6 bits 000000 represents the value 64.



From: mohamed.boucadair@orange.com [mailto:mohamed.boucadair@orange.com]
Sent: Tuesday, May 2, 2017 8:53 AM
To: Dave Dolson; James N Guichard; Greg Mirsky
Cc: sfc@ietf.org; Ron Parker
Subject: RE: [sfc] TTL field within the NSH base header

Re-,

You have a point. Acked.

My concern was how to avoid misbehaving SFFs that would forward packets with SHL==0 to a next-hop SFF even if that SFF parses the SHL field.

What about having a configuration knob to indicate the behavior to follow when receiving SHL=0? A domain that involves classifiers/SFFs that understand SHL must not forward or accept receiving packets with SHL=0.

Cheers,
Med

De : Dave Dolson [mailto:ddolson@sandvine.com]
Envoyé : mardi 2 mai 2017 14:41
À : BOUCADAIR Mohamed IMT/OLN; James N Guichard; Greg Mirsky
Cc : sfc@ietf.org<mailto:sfc@ietf.org>; Ron Parker
Objet : RE: [sfc] TTL field within the NSH base header

Med,
I agree.
Consider the case when a “new” SFF is added to an existing system:
For all NSH packets, the Hop Limit/TTL field is zero, not just those from the classifier.

I object to this statement of yours:
> add a sentence saying that “an incoming value of 0” is only acceptable from a classifier.

As I see it, an incoming value of 0 is acceptable from any source.

-Dave

From: mohamed.boucadair@orange.com<mailto:mohamed.boucadair@orange.com> [mailto:mohamed.boucadair@orange.com]
Sent: Tuesday, May 2, 2017 7:58 AM
To: Dave Dolson; James N Guichard; Greg Mirsky
Cc: sfc@ietf.org<mailto:sfc@ietf.org>; Ron Parker
Subject: RE: [sfc] TTL field within the NSH base header

Hi Dave,

SFFs that do not support SHL will preserve these bits as received. In other words, these SFFs won’t reset these bits to zeros.

Cheers,
Med

De : Dave Dolson [mailto:ddolson@sandvine.com]
Envoyé : mardi 2 mai 2017 12:19
À : BOUCADAIR Mohamed IMT/OLN; James N Guichard; Greg Mirsky
Cc : sfc@ietf.org<mailto:sfc@ietf.org>; Ron Parker
Objet : Re: [sfc] TTL field within the NSH base header

Med,
For the backwards compatibility case, we also support ‎SFFs that do not decrement TTL/Hop Limit. Saying a value of zero can only come from classifiers defeats that, as well as adding unnecessary complexity.

David Dolson
‎
From: mohamed.boucadair@orange.com<mailto:mohamed.boucadair@orange.com>
Sent: Tuesday, May 2, 2017 2:17 AM
To: James N Guichard; Greg Mirsky; Dave Dolson
Cc: sfc@ietf.org<mailto:sfc@ietf.org>; Ron Parker
Subject: RE: [sfc] TTL field within the NSH base header


Hi Jim,

Thank you for this updated proposal. This is much more better.

Giving the clarification discussed with Joel (i.e., do not reset reserved bits by intermediate devices), I wonder whether it makes sense to add a sentence saying that “an incoming value of 0” is only acceptable from a classifier.

Cheers,
Med

De : James N Guichard [mailto:james.n.guichard@huawei.com]
Envoyé : lundi 1 mai 2017 19:39
À : Greg Mirsky; Dave Dolson
Cc : sfc@ietf.org<mailto:sfc@ietf.org>; BOUCADAIR Mohamed IMT/OLN; Ron Parker
Objet : RE: [sfc] TTL field within the NSH base header

Okay lots of conversation but let me offer a revised version of Med’s text that hopefully captures most of the discussion (changes highlighted):

SHL (SFF Hop Limit): Indicates the maximum SFF hops for an SFP. The initial SHL value SHOULD be configurable via the control plane; the configured initial value can be specific to one or more SFPs. 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 the SHL value by 1 prior to NSH forwarding lookup. Decrementing by 1 from an incoming value of 0 shall result in a SHL value of 63. The packet MUST NOT be forwarded if SHL is, after decrement, 0 e.g. decrement SHALL occur before testing SHL for 0.

Jim


From: Greg Mirsky [mailto:gregimirsky@gmail.com]
Sent: Monday, May 01, 2017 1:02 PM
To: Dave Dolson <ddolson@sandvine.com<mailto:ddolson@sandvine.com>>
Cc: sfc@ietf.org<mailto:sfc@ietf.org>; mohamed.boucadair@orange.com<mailto:mohamed.boucadair@orange.com>; James N Guichard <james.n.guichard@huawei.com<mailto:james.n.guichard@huawei.com>>; Ron Parker <Ron_Parker@affirmednetworks.com<mailto:Ron_Parker@affirmednetworks.com>>
Subject: Re: [sfc] TTL field within the NSH base header

"Be liberal what you accept. Be conservative what you send."
I think this sums what we have discussed - accept TTL 0, don't forward TTL 0.

Regards, Greg

On May 1, 2017 8:58 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<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 ☺.

   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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