Re: [spring] Spring protection - determining applicability

"Joel M. Halpern" <jmh@joelhalpern.com> Mon, 03 August 2020 18:35 UTC

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To: Robert Raszuk <robert@raszuk.net>
Cc: "spring@ietf.org" <spring@ietf.org>
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From: "Joel M. Halpern" <jmh@joelhalpern.com>
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Subject: Re: [spring] Spring protection - determining applicability
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(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>> 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>
> 
>      > 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> 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>
> 
>      > 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>] 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>
>      >
>      > -----Original Message-----
>      > From: spring <spring-bounces@ietf.org
>     <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>>; 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>] 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>>
>      >
>      >  > 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
>      >
>      >  >
>      >
>      >  > _______________________________________________
>      >
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