Re: [spring] WG adoption call - draft-hu-spring-segment-routing-proxy-forwarding

[[e-mail]]

Hi,

 

Please find more inline.

 

From: Huzhibo <[e-mail]> 
Sent: mercredi 26 janvier 2022 09:31
To: [e-mail]; [e-mail]; 'SPRING WG'
<[e-mail]>
Subject: RE: [spring] WG adoption call -
draft-hu-spring-segment-routing-proxy-forwarding

 

Hi slitkows :

 

Thanks for your comments, Please see inline.

 

Thanks

 

Zhibo Hu

From: spring [mailto:[e-mail]] On Behalf Of
[e-mail] <mailto:[e-mail]> 
Sent: Wednesday, January 26, 2022 1:13 AM
To: [e-mail] <mailto:[e-mail]> ; 'SPRING
WG' <[e-mail] <mailto:[e-mail]> >
Subject: Re: [spring] WG adoption call -
draft-hu-spring-segment-routing-proxy-forwarding

 

Hi 

 

I'm NOT supporting this draft for the following reasons:

 

1.	The WG already have a WG document which is dealing with this
problem, I don't think that WG should come with multiple documents/solutions
for the same solution space as it may just confuse the industry and create
deployment issues as different vendors may pick different solutions.

-----> [I-D.ietf-spring-segment-protection-sr-te-paths] defines local
behaviors to implement SR-TE node protection.
draft-hu-spring-segment-routing-proxy-forwarding enhances SR-TE node
protection. 

 

It optimized the number of entries in the Context Table. This solution
solves the connectivity problem after IGP convergence, and protects binding
segments.

 

[SLI] While I think your arguments are not completely valid (see discussion
below), this has nothing to do with the one draft vs two drafts discussion.
As there is already a WG doc, I don't see any reason for creating another
one except creating artificial work for the IETF and confusing readers.

 

 

2.	Adding protocols extensions adds complexity in the solution without
adding a strong value.

 

The document claims that "[I-D.ietf-spring-segment-protection-sr-te-paths] .
may not work for some cases such as some of nodes in the network not
supporting this solution.". While this is true, the proposed solution in
draft-hu-spring-segment-routing-proxy-forwarding has exactly the same caveat
and requires all nodes in the network to support the solution.

 

Considering the following straight line network: A -B -C -D - E - F - G -H
and an SR policy from A to H using SID_G, routers A to F have to support the
extension to make the solution working, if one of the router doesn't support
the extension, traffic will be dropped. 

 

Then, there is no value compared to the timer-based solution of
[I-D.ietf-spring-segment-protection-sr-te-paths]

 

Authors of draft-hu-spring-segment-routing-proxy-forwarding argued that G
may have multiple upstream neighbors let's say F and F' and the solution
allows for F' to support the extension while F may not support, so the
solution will send the traffic to F'. Well yes, but this still requires all
routers upstream to F' to support this extension and maybe F is on the path
to F'. So, I don't think the argument is valid as it may possibly work
tactically depending on the network topology when we look at a small portion
of the network, but when we look at the whole network, operator will have to
upgrade all their nodes to support the extension to ensure the benefit is
there. 

 

In addition, in term of traffic, forwarding traffic to a neighbor of the
failed node which wasn't initially on the path, could lead to traffic
congestion or high traffic peaks on links that were not sized to carry this
traffic. We could easily expect some traffic tromboning, where traffic goes
to this non-natural neighbor of the failed node and then goes back over some
part of the same path before reaching the destination.

 

So these protocol extensions are bringing complexity for no value here.

---------> Protocols extensions can accurately direct traffic to a node that
can perform proxy forwarding and solve the problem that traffic cannot be
forwarded to a proxy forwarding node after IGP convergence. This protocol
extension is necessary.

This solution does not require that all network nodes support this
extension, take the example you have mentioned :

but it still requires that all routers upstream to F' support this extension
---> This description is inaccurate, assuming that the previous segment is
node B, when node G fails. When the node B converges, the node B finds the
PF

node F' adjacent to G, and can push the node Sid of the node F',Even if C
and D do not support this protocol extension, this is not affected.

 

 

[SLI] Your statement is purely theoretical and life in real networks is not
theoretical. You cannot predict which router will converge first (routers
may have different CPUs, may have different tasks to execute.). B may
converge first maybe, but maybe it will be C or D. no one knows and it's
unpredictable. So at the end, if you want to guarantee the mechanism to
work, all routers have to support the mechanism.

 

In addition, the Hold timers solution mentioned in
[I-D.ietf-spring-segment-protection-sr-te-paths] does not extend protocols,
but is also complex. In addition, slow deletion is required for node faults.
In addition, loop prevention is implemented to prevent loops.Moreover, it
cannot accurately direct traffic to a node that can perform proxy
forwarding.

[SLI] Directing traffic to few nodes that could do proxy forwarding can have
serious traffic impact and at the end cause damages to traffic that has
nothing to do with the failure. It's the solution, but it has major
drawbacks from an operational point of view.

 

 

3.	Regarding BSID, I'm not fan of advertising BSIDs in IGP as there may
be hundreds or thousands of BSID on a node which again will create a lot of
burden in IGP. The proposed way will have to be discussed in LSR, not in
SPRING (see next comment).

 

Note that [I-D.ietf-spring-segment-protection-sr-te-paths] could also work
with BSIDs as long as BSID information of failed node is available in the
control-plane of PLRs by whatever mechanism. I think this BSID handling is
orthogonal to the proxy-forwarding controlplane behavior. The forwarding
operations for BSID will have to be discussed more in details, we could not
expect all HW to be able to do 3 or 4 lookups without any perf degradation.

-------> Binding segments need to be exchanged only between neighbors and do
not need to be flooded to the entire IGP domain. Therefore, binding segments
do not exert pressure on IGP performance.The control-plane processing and
forwarding-plane processing of the BSID are not strongly coupled.

 

[SLI] Control plane aspects of IGPs have to be discussed in LSR, not in
SPRING. So please take the discussion to LSR for the control plane and
forwarding aspects could be further described in
[I-D.ietf-spring-segment-protection-sr-te-paths] if WGs agrees that BSID is
interesting to solve.

 

 

SR-TE protection    

takes effect only from the time during a fault occurs to the TE path
converges. Therefore, SR-TE protection does not take effect during normal
forwarding,Compared with impaired connectivity, performance degradation is
acceptable.

 

4.	The document is currently a bit borderline between SPRING and LSR as
it talks in good details about IGP protocol extensions. If it's a SPRING
doc, it should detail reqs for protocols but nothing beyond.

                ------->As you said, this document defines the detail
requests for IGP protocols

[SLI] No it goes beyond requirements and already talks about encoding: 

"For supporting binding SID proxy forwarding, a new IS-IS TLV, called

   Binding Segment TLV, is defined.  It contains a binding SID and a

   list of segments (SIDs).  This TLV may be advertised in IS-IS Hello

   (IIH) PDUs, LSPs, or in Circuit Scoped Link State PDUs (CS-LSP)

   [RFC7356].

 

This is not a requirement; this is an IS-IS solution description that has to
be discussed in LSR not in SPRING.

 

 

 

 

 

Brgds,

 

Stephane

 

 

From: spring <[e-mail] <mailto:[e-mail]> > On
Behalf Of [e-mail] <mailto:[e-mail]> 
Sent: jeudi 13 janvier 2022 11:19
To: SPRING WG <[e-mail] <mailto:[e-mail]> >
Subject: [spring] WG adoption call -
draft-hu-spring-segment-routing-proxy-forwarding

 

Dear WG,

 

This message starts a 2 week WG adoption call, ending 27/01/2022, for
draft-hu-spring-segment-routing-proxy-forwarding

https://datatracker.ietf.org/doc/draft-hu-spring-segment-routing-proxy-forwa
rding/

 

After review of the document please indicate support (or not) for WG
adoption of the document to the mailing list.

 

Please also provide comments/reasons for your support (or lack thereof) as
this is a stronger way to indicate your (non) support as this is not a vote.


 

If you are willing to work on or review the document, please state this
explicitly. This gives the chairs an indication of the energy level of
people in the working group willing to work on the document.

 

Thanks!

Bruno, Jim, Joel

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