Re: [spring] Spring protection - determining applicability

"Ketan Talaulikar (ketant)" <ketant@cisco.com> Fri, 14 August 2020 12:00 UTC

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From: "Ketan Talaulikar (ketant)" <ketant@cisco.com>
To: "Joel M. Halpern" <jmh@joelhalpern.com>, Alexander Vainshtein <Alexander.Vainshtein@rbbn.com>, Shraddha Hegde <shraddha=40juniper.net@dmarc.ietf.org>, "EXT-Andrew.Alston@liquidtelecom.com" <Andrew.Alston@liquidtelecom.com>, Robert Raszuk <robert@raszuk.net>
CC: "spring@ietf.org" <spring@ietf.org>
Thread-Topic: [spring] Spring protection - determining applicability
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Subject: Re: [spring] Spring protection - determining applicability
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Hi All,

I would like to share a different perspective on this.

First, thanks to Joel for bringing up the discussion. Clearly we need a well-defined applicability statement for determining applicability of protection for segment used in an SR Policy. Some of this is captured in [1].

This is about local repair at a PLR. By it's very nature, the PLR does not have a notion of how "strict or not" is the SLA that is being provided by the SR Policy. Awareness of that notion exists at the SR Policy headend and/or computation-node.

We have protected and un-protected variants of adjacency SIDs to enable the computation to pick or the other based on the "strictness" of the SLA requirement for picking that link. We do not have such a notion for Prefix SIDs. One can say that we could introduce signalling (e.g. a B flag) to indicate whether a Prefix SID can be bypassed or not. This provides the opportunity for the computation to use one or the other flavor depending on the nature of the SLA for the SR Policy.

I have a problem and a concern in the assumption that PLRs can assume that the currently defined variant of Prefix SIDs in RFC8402 (and IGP specs) are "bypass-able".

As Joel and others have brought out, the Prefix SID could be just a topological instruction or may also be used to steer the flow to a node which is applying a service function to it. In order to support a mix of SR Policies of different SLAs (strict and not-strict), we need to enable the choice of SIDs that indicates to the PLR whether they are "bypass-able" or not.

For the cases, where the SR Policy has a specific SLA, it is required for nodes to drop the packets meant for the "active segment" than to bypass it. When this mechanism is used along side SRTE path monitoring mechanisms, it enables the headend to detect the failure and fallback to an alternate path using the path protection approach. This is something that is described and in use in deployments today [1].

Thanks,
Ketan

[1] https://tools.ietf.org/html/draft-ietf-spring-segment-routing-policy-08#section-9
[2] https://tools.ietf.org/html/draft-ietf-spring-segment-routing-policy-08#section-9.3

-----Original Message-----
From: spring <spring-bounces@ietf.org> On Behalf Of Joel M. Halpern
Sent: 04 August 2020 20:25
To: Alexander Vainshtein <Alexander.Vainshtein@rbbn.com>om>; Shraddha Hegde <shraddha=40juniper.net@dmarc.ietf.org>rg>; EXT-Andrew.Alston@liquidtelecom.com <Andrew.Alston@liquidtelecom.com>om>; Robert Raszuk <robert@raszuk.net>
Cc: spring@ietf.org; Joel M. Halpern <jmh@joelhalpern.com>
Subject: Re: [spring] Spring protection - determining applicability

There are, as far as I can tell, a number of ways to address this family of related questions.
What struck me, and prompted the starting question, was that none of them were spelled out.  I see lots of interesting ideas / proposals.
Some of them are compatible with others.   Some are not.
It would be good if we could reach agreement on how we thought it should be handled.

Thank you,
Joel

On 8/4/2020 3:54 AM, Alexander Vainshtein wrote:
> Hi all,
> 
> I am still not sure that the problem of bypass going thru undesirable 
> links/nodes exists in the case of topological SIDs.
> 
> AFAIK, Facility Protection in RSVP-TE FRR (RFC 4090
> <https://tools.ietf.org/html/rfc4090>) has been successfully deployed 
> for many years before SR-MPLS has been introduced. What’s more, 
> signaling of bypass tunnels he PLR usually did not include any of the 
> constraints used for computing of any specific LSP that the bypass LSP 
> would protect – because in the Facility Protection mode the same 
> bypass LSP would be used to protect multiple LSPs passing thru the 
> failed link/node.
> 
>  From my POV the only difference between this behavior and that 
> introduced by the “bypassing” drafts in SR is that, in the case of 
> RSVP-TE, the operator would explicitly indicate, as part of LSP 
> signaling, whether it would or would not use FRR; LSPs that would not 
> use FRR would then drop traffic rather than delivering it the wrong way.
> 
> Such an option indeed does not exist in SR-TE today, but would be easy 
> to provide if so desired IMHO.
> 
> Did I miss something substantial?
> 
> Regards, and lots of thanks in advance,
> 
> Sasha
> 
> Office: +972-39266302
> 
> Cell:      +972-549266302
> 
> Email:   Alexander.Vainshtein@ecitele.com
> 
> *From:* spring <spring-bounces@ietf.org> *On Behalf Of *Shraddha Hegde
> *Sent:* Tuesday, August 4, 2020 9:41 AM
> *To:* EXT-Andrew.Alston@liquidtelecom.com
> <Andrew.Alston@liquidtelecom.com>om>; Robert Raszuk <robert@raszuk.net>
> *Cc:* spring@ietf.org; Joel M. Halpern <jmh@joelhalpern.com>
> *Subject:* Re: [spring] Spring protection - determining applicability
> 
> All,
> 
> This is a very interesting discussion and thanks to Joel for starting 
> this discussion. IMO, when there are strict requirements of avoiding 
> certain nodes/links it can be realized  either by defining a flex-algo 
> avoiding those
> 
> Nodes and links or by using a stack of unprotected adj-sids that avoid 
> restricted nodes and links. When a stack of adj-sids is used to 
> realize the path, the head-end based (sBFD) protection mechanisms can be applied.
> 
> If Node-sids/prefix-sid/anycast-sids are used to build the stack, the 
> failure events may cause traffic to go through restricted nodes and 
> links. This would happen regardless of whether any kind of protection 
> is in use or not.
> 
> Rgds
> 
> Shraddha
> 
> Juniper Business Use Only
> 
> *From:* spring <spring-bounces@ietf.org 
> <mailto:spring-bounces@ietf.org>> *On Behalf Of *Andrew Alston
> *Sent:* Tuesday, August 4, 2020 5:41 AM
> *To:* Robert Raszuk <robert@raszuk.net <mailto:robert@raszuk.net>>
> *Cc:* spring@ietf.org <mailto:spring@ietf.org>; Joel M. Halpern 
> <jmh@joelhalpern.com <mailto:jmh@joelhalpern.com>>
> *Subject:* Re: [spring] Spring protection - determining applicability
> 
> *[External Email. Be cautious of content]*
> 
> Robert this is actually far more difficult when – it can be an entire
> (long) series of nodes that need to be avoided.
> 
> It could potentially be made to work but I’d worry that to do this – 
> you’d have to stack 10 – 20 – 30 negative labels – and that wouldn’t 
> be viable.
> 
> It’s easier to use algorithms and adjacency sids and other such things 
> to calculate paths – the biggest trick is about the stack depth.  When 
> you have this need for node avoidance – the need for 10+ label depth 
> is critical – unless you wanna be applying one hell of a lot of 
> binding labels along the way which is a nightmare.
> 
> But to answer your question, is this a common use case – it’s a use 
> case that most of the people I discuss this with certain have – I cant 
> comment on a global scale, or for anyone else, but every indication I 
> have is that yes – its something people need, and want
> 
> Andrew
> 
> *From:* Robert Raszuk <robert@raszuk.net <mailto:robert@raszuk.net>>
> *Sent:* Tuesday, 4 August 2020 01:27
> *To:* Andrew Alston <Andrew.Alston@liquidtelecom.com 
> <mailto:Andrew.Alston@liquidtelecom.com>>
> *Cc:* Joel M. Halpern <jmh@joelhalpern.com 
> <mailto:jmh@joelhalpern.com>>; spring@ietf.org 
> <mailto:spring@ietf.org>
> *Subject:* Re: [spring] Spring protection - determining applicability
> 
> Is this a common use case ie.  "but rather – which nodes / network 
> segments it can never touch or flow through."
> 
> If so perhaps its time to define notion of *negative-SID* ie. list in 
> the packet resources which given packet MUST not ever traverse.
> 
> Put in the packet set of nodes or links which the packet should never 
> traverse.
> 
> That goes in line of recent wave of negative routing implementations
> (RIFT) or discussions (LSR)
> 
> Best,
> R.
> 
> On Mon, Aug 3, 2020 at 11:46 PM Andrew Alston 
> <Andrew.Alston@liquidtelecom.com 
> <mailto:Andrew.Alston@liquidtelecom.com>> wrote:
> 
>     So –
> 
>     One of the use cases, in fact, some very major use cases in any
>     spring technology for us revolve around the following
> 
>     a.The explicit avoidance of certain nodes
> 
>     b.The explicit avoidance of certain sections of the network
> 
>     Anything that could result in that explicit avoidance being violated
>     – would create, shall we say significant problems.
> 
>     Much of the use case is not a case of which nodes the packets flow
>     through – but rather – which nodes / network segments it can never
>     touch or flow through.  Effectively, to be used as a technology to
>     avoid certain things for specific reasons.
> 
>     This is also one of the reasons for needing such deep label stacks –
>     this kind of detailed path programming tends to deepen the stack
>     because you sometimes have to be pretty explicit.
> 
>     It is absolutely critical to us that this functionality is there –
>     and that we can avoid situations which could cause traffic to
>     accidently hit things explicitly avoided.
> 
>     I wish I could be more specific than this, but it is what it is.
> 
>     Thanks
> 
>     Andrew
> 
>     *From:* spring <spring-bounces@ietf.org
>     <mailto:spring-bounces@ietf.org>> *On Behalf Of *Joel M. Halpern
>     *Sent:* Monday, 3 August 2020 21:36
>     *To:* Robert Raszuk <robert@raszuk.net <mailto:robert@raszuk.net>>
>     *Cc:* spring@ietf..org <mailto:spring@ietf.org>
>     *Subject:* Re: [spring] Spring protection - determining 
> applicability
> 
>     (Since the thread has gotten long enough, reiterating that this is as a
>     participant, not a WG chair.)
> 
>     Yes, we are talking IP networks. And yes, I have seen IP networks that
>     choose to drop packets. For all sorts of reasons.
>     I think there are likely other reasons why one may not want a random
>     path rather than a chosen TE path. I think it is important we be clear
>     about what constraints may be / are violated when we tell people they
>     have this tool (protective rerouting) that is intended to preserve QoS.
> 
>     Let's be clear. I am not arguing that this is not a good idea. It is a
>     good idea. And useful. I am trying to figure otu what combination of
>     additional mechanisms and clear descriptions will lead to everyone
>     getting the behavior they expect (which may not be the behavior they
>     desire, but sometimes is the best we can do.)
> 
>     Yours,
>     Joel
> 
>     On 8/3/2020 2:30 PM, Robert Raszuk wrote:
>      > Joel,
>      >
>      > Are we still talking about IP networks here ? Or perhaps some hard
>      > slicing with real resource reservations or detnets ?
>      >
>      > Because if we are talking about IP networking I have two
>     observations:
>      >
>      > A) If you need to traverse via a specific node (ie. firewall) you
>     better
>      > apply IP encapsulation to that node. I don't think IP
>     encapsulation can
>      > be hijacked today such that destination address of the packet is
>     ignored.
>      >
>      > B) Have you seen any IP network where upon topology change (link
>     or node
>      > failure) you suddenly start dropping flows in spite of SPT offering
>      > perhaps few ms longer path with 10 ms more jitter ?
>      >
>      > Or are some SR marketing slides promise to turn IP networks in
>      > something new ? Worse ... do they mention path quality guarantees,
>      > resource reservations ? I hope not.
>      >
>      > Thx,
>      > R.
>      >
>      >
>      >
>      >
>      >
>      >
>      >
>      >
>      >
>      > On Mon, Aug 3, 2020 at 8:10 PM Joel M. Halpern <jmh@joelhalpern.com
>     <mailto:jmh@joelhalpern.com%20%0b>> <mailto:jmh@joelhalpern.com>> wrote:
>      >
>      > Well less serious for TE SIDs, I am not sure the problem is
>     restricted
>      > to just service SIDs.
>      >
>      > Suppose that the PCE has specified the path to meet some complex te
>      > objective.  The bypass node has no way of knowing what those
>      > constraints
>      > were.  And for some kinds of traffic, it is better to drop the packet
>      > than to deliver it outside the envelop.  I suspect that the right
>      > answer
>      > to this is "too bad".  If so, as with the distinction regarding
>     service
>      > nodes, we should say so, shouldn't we?
>      >
>      > Yours,
>      > Joel
>      >
>      > On 8/3/2020 2:36 AM, Alexander Vainshtein wrote:
>      > > Mach, Joel and all,
>      > >
>      > > I think that in most cases:
>      > >
>      > > 1.There is clear differentiation between "topological" and
>     "service"
>      > > instructions in SID advertisements. E.g.:
>      > >
>      > > oIGP Prefix Node SIDs IGP Adj-SIDs (identified as such in the
>      > > corresponding IGP advertisements) represent topological
>     instructions
>      > >
>      > > oService SIDs for SRv6 (see SRv6 BGP-Based Overlay Services
>      > >
>      >
>     <https://datatracker.ietf.org/doc/html/draft-ietf-bess-srv6-services-04
>     <https://clicktime.symantec.com/3GC5af2z3JzphZDkPncHAQi6H2?u=https%3A%2F%2Furldefense.com%2Fv3%2F__https%3A%2Fdatatracker.ietf.org%2Fdoc%2Fhtml%2Fdraft-ietf-bess-srv6-services-04__%3B%21%21NEt6yMaO-gk%21S0Yusx9FYNE8E_R1oiQAtQxgm0x0wxqguoZHgwp6vpRHOGt8AkTRDuiDo4T-L0nl%24>>
>      >
>      > > draft) unsurprisingly represent “service” instructions
>      > >
>      > > 2.Segments that represent topological instructions can be bypassed,
>      > > while segments that represent service instructions require
>      > alternative
>      > > protection mechanisms.
>      > >
>      > > This view seems to be aligned with RFC 8402
>      > > <https://tools.ietf.org/html/rfc8402
>     <https://clicktime.symantec.com/37PzUKAD82cjSvmVcGvpkhF6H2?u=https%3A%2F%2Furldefense.com%2Fv3%2F__https%3A%2Ftools.ietf.org%2Fhtml%2Frfc8402__%3B%21%21NEt6yMaO-gk%21S0Yusx9FYNE8E_R1oiQAtQxgm0x0wxqguoZHgwp6vpRHOGt8AkTRDuiDo0I4Ybtm%24>>
>     that says in Section 1:
>      > >
>      > >     In the context of an IGP-based distributed control plane, two
>      > >
>      > > topological segments are defined: the IGP-Adjacency segment and the
>      > >
>      > >     IGP-Prefix segment.
>      > >
>      > >     In the context of a BGP-based distributed control plane, two
>      > >
>      > > topological segments are defined: the BGP peering segment and the
>      > >
>      > >     BGP-Prefix segment.
>      > >
>      > > In the case of SR-MPLS this differentiation is assumed in Section
>      > 3.4 of
>      > > the Node Protection for SR-TE Path
>      > >
>      >
>     <https://datatracker.ietf.org/doc/html/draft-hegde-spring-node-protection-for-sr-te-paths-07#section-3.4
>     <https://clicktime.symantec.com/3CrUgARW8somAbw6TisjgJ16H2?u=https%3A%2F%2Furldefense.com%2Fv3%2F__https%3A%2Fdatatracker.ietf.org%2Fdoc%2Fhtml%2Fdraft-hegde-spring-node-protection-for-sr-te-paths-07%2Asection-3.4__%3BIw%21%21NEt6yMaO-gk%21S0Yusx9FYNE8E_R1oiQAtQxgm0x0wxqguoZHgwp6vpRHOGt8AkTRDuiDo9wO-Ssn%24>>
>      >
>      > > draft that says:
>      > >
>      > >     The node protection mechanism described in the previous
>     sections
>      > >
>      > >     depends on the assumption that the label immediately below
>      > the top
>      > >
>      > > label in the label stack is understood in the IGP domain.  When the
>      > >
>      > >     provider edge routers exchange service labels via BGP or some
>      > other
>      > >
>      > >     non-IGP mechanism the bottom label is not understood in the IGP
>      > >
>      > >     domain.
>      > >
>      > >     The egress node protection mechanisms described in the draft
>      > >
>      > >     [RFC8679 <https://datatracker.ietf.org/doc/html/rfc8679
>     <https://clicktime.symantec.com/36dMAjuYTQovo8jHwmm3eJw6H2?u=https%3A%2F%2Furldefense.com%2Fv3%2F__https%3A%2Fdatatracker.ietf.org%2Fdoc%2Fhtml%2Frfc8679__%3B%21%21NEt6yMaO-gk%21S0Yusx9FYNE8E_R1oiQAtQxgm0x0wxqguoZHgwp6vpRHOGt8AkTRDuiDo8MGipXc%24>>]
>     is
>      > > applicable to this use case and no additional changes
>      > >
>      > >     will be required for SR based networks
>      > >
>      > > The scenarios in which  differentiation between “topological” and
>      > > “service” instructions is broken are indeed problematic. E.g.,
>      > consider
>      > > the use case in which a Node SID in the ERO of a SR-TE path
>      > identifies a
>      > > node that acts as a firewall for all packets it receives, i.e.,
>      > provides
>      > > the firewall service without any dedicated service SID
>      > identifying it.
>      > > One could say that the Node SID of such a node would combine
>      > topological
>      > > and service instructions thus breaking the differentiation
>      > between the two.
>      > >
>      > > I am not sure if usage of such “combined” SIDs could be prevented
>      > or at
>      > > least discouraged.
>      > >
>      > > If not, providing an ability to identify such SIDs in the
>      > advertisement
>      > > mechanisms would be useful IMHO.
>      > >
>      > > My 2c,
>      > >
>      > > Sasha
>      > >
>      > > Office: +972-39266302
>      > >
>      > > Cell:      +972-549266302
>      > >
>      > > Email: Alexander.Vainshtein@ecitele.com
>     <mailto:Alexander.Vainshtein@ecitele.com>
>      > <mailto:Alexander.Vainshtein@ecitele.com>
>      > >
>      > > -----Original Message-----
>      > > From: spring <spring-bounces@ietf.org
>     <mailto:spring-bounces@ietf.org%0b>>
>     <mailto:spring-bounces@ietf.org>> On Behalf Of Mach Chen
>      > > Sent: Monday, August 3, 2020 6:30 AM
>      > > To: Joel M. Halpern <jmh@joelhalpern.com
>     <mailto:jmh@joelhalpern.com%0b>> <mailto:jmh@joelhalpern.com>>;
>     spring@ietf.org <mailto:spring@ietf.org> <mailto:spring@ietf.org>
>      > > Subject: Re: [spring] Spring protection - determining applicability
>      > >
>      > > Hi Joel,
>      > >
>      > > I think this is a good point that may not be discussed in the
>      > past. And
>      > > I also don't think there is a "can be bypassed" indication in the
>      > > routing advertisement for now.
>      > >
>      > > IMHO, the information advertised by routing is neutral, such
>      > information
>      > > (can or cannot be bypassed) is more path specific, thus
>     normally the
>      > > controller should be responsible for deciding whether/which SID
>      > can be
>      > > bypassed.
>      > >
>      > > Best regards,
>      > >
>      > > Mach
>      > >
>      > >  > -----Original Message-----
>      > >
>      > >  > From: spring [mailto:spring-bounces@ietf.org
>      > <mailto:spring-bounces@ietf.org>
>     <mailto:spring-bounces@ietf.org%0b%3e%20%3cmailto:spring-bounces@ietf.org%3e>]
>     On Behalf Of Joel M.
>      > >
>      > >  > Halpern
>      > >
>      > >  > Sent: Monday, August 3, 2020 7:51 AM
>      > >
>      > >  > To: spring@ietf.org <mailto:spring@ietf.org>
>     <mailto:spring@ietf.org>
>      > <mailto:spring@ietf.org <mailto:spring@ietf.org
>     <mailto:spring@ietf.org%20%3cmailto:spring@ietf.org>>>
>      > >
>      > >  > Subject: [spring] Spring protection - determining applicability
>      > >
>      > >  >
>      > >
>      > >  > (WG Chair hat Off, this is merely a note from a slightly
>      > confused WG
>      > >
>      > >  > participant.)
>      > >
>      > >  >
>      > >
>      > >  > I have been reading the various repair drafts, and the various
>      > >
>      > >  > networks programming and service programming draft, and I am
>      > trying to
>      > >
>      > >  > figure out one aspect of the combination.
>      > >
>      > >  >
>      > >
>      > >  > How does a node that is doing some form of bypass (suppose, for
>      > >
>      > >  > simplicity, it is Node N2 deciding to bypass the next SID for
>      > a failed
>      > >
>      > >  > node N3) know that it is safe to do so?
>      > >
>      > >  >
>      > >
>      > >  > If the path was just for TE, then it is "safe" if the new path
>      > meets
>      > >
>      > >  > the TE criteria.  or maybe it is safe if it is even close, as
>      > long as
>      > >
>      > >  > it is not used for too long.
>      > >
>      > >  >
>      > >
>      > >  > But what if the node were a Firewall, included to meet legal
>      > > requirements?
>      > >
>      > >  > Or was some other necessary programmatic transform (wince we are
>      > >
>      > >  > deliberately vague about what nodes can do when asked suitably.)
>      > >
>      > >  >
>      > >
>      > >  > Is there some "can be bypassed" indication in the routing
>      > >
>      > >  > advertisements that I missed?
>      > >
>      > >  >
>      > >
>      > >  > Thank you,
>      > >
>      > >  > Yours,
>      > >
>      > >  > Joel
>      > >
>      > >  >
>      > >
>      > >  > _______________________________________________
>      > >
>      > >  > spring mailing list
>      > >
>      > >  > spring@ietf.org <mailto:spring@ietf.org>
>     <mailto:spring@ietf.org>
>      > <mailto:spring@ietf.org <mailto:spring@ietf.org
>     <mailto:spring@ietf.org%20%3cmailto:spring@ietf.org>>>
>      > >
>      > >  >
>      > >
>      >
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