About the SIDs assignment, I think it is better to have a
scheduled assignment than a random assignment as Les mentioned.
[1]
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Thanks,
Weiqiang Cheng
----邮件原文----
*发件人：*Louis Chan <[email protected]>
*收件
人：*"Les Ginsberg (ginsberg)" <[email protected]>,Acee Lindem
<[email protected]>
*抄 送:
*lsr <[email protected]>,Krzysztof Szarkowicz
<[email protected]>,Weiqiang Cheng <[email protected]>
*发送时间：*2023-04-12 10:45:56
*主题：*Re: [Lsr] IETF-
116 LSR - IGP extensions for Advertising Offset
forFlex-Algorithm
Hi Les,
Thanks for the prompt reply. Please see inline for clarification
[lc2].
/Louis
*From:* Les Ginsberg (ginsberg) <[email protected]>
*Sent:* Tuesday, April 11, 2023 11:03 PM
*To:* Louis Chan <[email protected]>; Acee Lindem
<[email protected]>
*Cc:* lsr <[email protected]>; Krzysztof Szarkowicz
<[email protected]>; Weiqiang Cheng
<[email protected]>
*Subject:* RE: [Lsr] IETF-116 LSR - IGP extensions for Advertising
Offset for Flex-Algorithm
*[External Email. Be cautious of content]*
Louis -
Please see inline.
> -----Original Message-----
> From: Louis Chan <[email protected]
<mailto:[email protected]>>
> Sent: Monday, April 10, 2023 11:01 PM
> To: Les Ginsberg (ginsberg) <[email protected]
<mailto:[email protected]>>; Acee Lindem
> <[email protected] <mailto:[email protected]>>
> Cc: lsr <[email protected] <mailto:[email protected]>>; Krzysztof
Szarkowicz <[email protected]
<mailto:[email protected]>>;
> Weiqiang Cheng <[email protected]
<mailto:[email protected]>>
> Subject: RE: [Lsr] IETF-116 LSR - IGP extensions for Advertising
Offset for
> Flex-Algorithm
>
> Hi Les,
>
> Thanks for your questions. Please see inline [lc] below.
>
> /Louis
>
> -----Original Message-----
> From: Les Ginsberg (ginsberg) <[email protected]
<mailto:[email protected]>>
> Sent: Saturday, April 8, 2023 7:34 AM
> To: Acee Lindem <[email protected]
<mailto:[email protected]>>; Louis Chan <[email protected]
<mailto:[email protected]>>
> Cc: lsr <[email protected] <mailto:[email protected]>>
> Subject: RE: [Lsr] IETF-116 LSR - IGP extensions for Advertising
Offset for
> Flex-Algorithm
>
> [External Email. Be cautious of content]
>
>
> OK - since Acee opened the door - here are some
comments from me -
> starting with the most important.
>
> (BTW - I still have limited enthusiasm for this
draft.)
>
> 1)The proposal places some restrictions on how operators
provision their
> network in terms of assigning SIDs and reserving
space for future
> assignments.
> If operators do not use compatible assignment schemes, then this
will never
> get deployed. It is therefore not enough to come with a nice idea
- you must
> have some enthusiasm from the operator community.
>
>
> [lc] If the operator only wants to deploy flex-algo, there is no
change in their
> Node-sid numbering scheme. For the Adj-sid, these are local
labels with local
> significant only, and there is no need for any special planning
for Adj-sid,
> unless you are suggesting they want to make fixed assignment of
Adj-sid
> label for each link. Even with fixed, the proposed draft has
benefit on that. I
> will explain later.
>
[LES:] Let's discuss this in the context of prefix-sids - the same
applies to adj-sids.
Today (i.e., in the absence of your proposal) an operator is free to
assign any label within the SRGB for a given prefix/algo pair so
long as it is not assigned to some other prefix/algo context.
Your proposal places some new restrictions. Now, for a given
flex-algo, whenever an operator assigns a given label for a prefix
in Algo 0 (call it Label-A0), they must guarantee that
"Label-A0+offset" for an advertised flex-algo specific offset is
available to be assigned for the prefix/flex-algo pair - and this
must be true for all prefixes advertised in algo 0.
This is certainly possible to do, but is not guaranteed to be the
case in current deployments.
For example - and this is only an example...today an operator might
utilize a provisioning tool to assign prefix-sids for all supported
algorithms on all nodes in the network.
To do this, the tool might maintain a database of assigned labels.
When provisioning a new node/prefix/algorithm, the logic in the tool
might simply take the next available label in the database.
The result of this would not be consistent with the requirements of
your draft.
Which is why I say in order to deploy the extension you propose,
such an operator would have to modify its provisioning tool.
[lc2] There might be some misunderstanding of our proposal. Let me
give some examples.
Case 1: Flex-Algo only
Prefix offset advertisement: “no”
Adj-sid offset advertisement: yes
In slide 8’s example, FA129 is using label “402001”, and the
advertisement of this label is using existing methods.
e.g. SRGB = 400000-460000
FA129: index 2001 (preferred value), or one can choose
111,
222
FA130 (new): index 3001 (preferred value), or 333,
4444
This does not change how the operator to assign label for prefix-sid
with their current method. Any index/label could be used for FA
prefix within SRGB.
The only change is the Adj-sid label allocation, but this “mostly”
is only “local” to one node. There is no effect on global label
allocation.
This draft will be compatible to what operators are doing today.
Case 2: VFA only
Prefix offset advertisement: yes
Adj-sid offset advertisement: yes
I agree, with VFA, there would be impact to global allocation to
node-sid/prefix-sid. But VFA is a totally new concept. No one has
deployed that yet.
There is no impact to operators which stick to deploy only Flex-algo.
Other case: Flex-Algo w/o Adj-sid offset
Continue the example of Case#1 above
Another FA131 is added, but no Adj-sid offset is
advertised
The question would be
*
Either allow this configuration, and FA131 will fallback using
Algo 0’s adj-sid
*
Or, disallow this configuration
I tend to “allow” such configuration with mix of FA129, FA130 (with
adj-sid offset) and FA131 (w/o adj-sid offset)
[/lc2]
Could this be done? Sure.
Do operators want to do this? I do not know.
But since this would be necessary in order to use your proposed
extension, it is necessary to gauge operator enthusiasm for making
such changes in order to know whether there is any point in
proceeding with your proposal.
> In slide 8 of the below presentation
>
https://urldefense.com/v3/__https://datatracker.ietf.org/meeting
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>
> FA129 is a prefix-sid (400201) allocated by operator, and it can
be any label.
> There is no connection to how Adj-sid is derived.
> Per Flex-Algo adj-sid assignment is not affecting
network wide label
> assignment from operation perspective. Each node could have
different local
> block for such adj-sid assignment. One might need to estimate
total possible
> number of link in one chassis for such allocation, or it could be
estimated by
> OS software itself. I also mentioned in the session, if there is
100 x FA with
> 1000 links (high end), it is only 100k labels. Is it difficult to
allocate such.
[LES:] Your proposal does not reduce the number of labels which need
to be "allocated" and installed in forwarding, it only reduces the
number of bytes used to advertise this information in LSPs/LSAs.
[lc2] You are correct.
I think, at the same time, it helps reducing time for global
convergence since the advertisement size is smaller, especially in
network with long diameter with multi-hops.
Also, in
https://urldefense.com/v3/__https://datatracker.ietf.org/doc/dra
ft
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ietf-lsr-isis-fast-flooding/__;!!NEt6yMaO-gk!EX4RtSVg7I0jO4M9ZMr
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af
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tc j G l2sqxcDIFHFZRnk4pocYHUxv9_pAb8cfeVdGYX_3gOEDi1kl_1f1RjQ$
(which you participated in)
>>>
As IS-IS is deployed in greater scale both in the
number of nodes in
an area and in the number of neighbors per node,
the impact of the
historic flooding rates becomes more significant.
Consider the
bringup or failure of a node with 1000 neighbors.
This will result
in a minimum of 1000 LSP updates. At typical LSP flooding rates
used
today (33 LSPs/second), it would take 30+ seconds
simply to send the
updated LSPs to a given neighbor. Depending on
the diameter of the
network, achieving a consistent LSDB on all nodes
in the network
could easily take a minute or more.
<<<
This proposed draft will certainly help.
[/lc2]
>
> So, this is why I do not understand your question in full.
>
> If the operator plans to use VFA, that would be a different
discussion. VFA,
> today, does not exist in deployment.
>
[LES:] My comments here have nothing to do with VFA.
> [/lc]
>
> Have you discussed this idea with any operators?
>
> [lc] I include Wei-qiang from China mobile in this thread. He has
shown his
> need on this kind of solution. Maybe, he could give his
perspective here. [/lc]
>
> If so, what has been their response?
> If they are open to the idea, how might they migrate from their
existing
> assignment schemes to an assignment scheme
compatible with the
> proposal?
>
> These are questions that need to be answered before considering
this idea.
>
> [lc] In slide 8, if you see these label numbers
>
> Prefix-sid: 400001, 402001, 406001, 407001
> Adj-sid: 32, 2032, 6032, 7032
>
> From operator perspective or troubleshooting perspective, value
xxx001
> represent the same node, and value x032 represent
the same link. This
> makes things more organized and easier to understand.
>
> If all are random labels, I do not see any benefit at all.
> [/lc]
[LES:] I am not commenting on whether the label assignment scheme
you propose is better or worse than any other.
I am only pointing out that you are imposing new restrictions on how
labels are allocated.
As you are not in charge of how operators provision their networks
(nor am I), it is presumptuous of you to think that simply because
you think this is a better way to do things that operators will be
happy to modify their existing networks to conform to your proposed
restrictions.
This isn’t academia - you need to vet this with the operator
community.
[lc2] Please refer to the examples at the top. The picture should be
clear by now. There is no restriction to what is
deployed today. [/lc2]
>
> 2)Section 5 Compatibilty
>
> There is no "compatibility" with legacy nodes - because all nodes
in the
> network have to have a consistent understanding of what SID is
assigned to a
> given context (for prefixes and adjacencies) since they might
need to install
> forwarding entries for that context.
> I do not see any point in deploying this until all nodes support
it. If you did do
> so, you would need to advertise old and new forms -
which does the
> opposite of what you are trying to achieve. Instead of reducing
LSP space
> used you would increase it.
>
> [lc] If the operator just plans to use only Flex-Algo, and no
VFA, it should be
> compatible with legacy implementation. If legacy nodes do not
understand
> adj-sid offset notation, these nodes could just ignore it. The
forwarding
> plane should work with co-existence of old and new nodes. Per
flex-algo adj-
> sid is only local significant to one node. New nodes should
detect whether
> legacy nodes exist in the network via such new extension
advertisement.
> And new nodes should use only algo 0 adj-sid from legacy nodes
for any TI-
> LFA.
[LES:] Consider a network of 100 nodes.
Let's say the "left-hand-side" of the network consist of legacy
nodes who do not understand your new advertisements.
The "right-hand-side" of the network consists of upgraded nodes who
support the new advertisements.
Consider nodes PE-LEFT and PE-RIGHT.
PE-RIGHT advertises a prefix-SID of 1000 for 2.2.2.2/32, and an
offset of 1000 for flex-algo 128.
PE-LEFT supports flex-algo 128 and wants to install a forwarding
entry for 2.2.2.2 for flex-algo 128.
It looks in the LSPs originated by PE-RIGHT. It does not see any
assigned SID for 2.2.2.2/32 flex-algo 128.
It cannot create a forwarding entry. And neither can any other node
in the left hand side of the network.
When PE-RIGHT stops advertising the explicit prefix SID for
2.2.2.2/32 Algo 128, all legacy nodes are unable to create
forwarding entries for the prefix/algo tuple.
This isn’t backwards compatible.
In general, you cannot advertise information legacy nodes require in
a new container that legacy nodes do not understand and claim that
you are backwards compatible.
[lc2] Please refers to the examples for clarification.
1.
For existing Flex-Algo deployment, it would be compatible. There
is no change in the container format on how prefix-sid
2.2.2.2/32 in FA128 is advertised.
1.
For new VFA, it would not be compatible. But….it does not mean
that we could not have VFA running in the same network.
There could be procedures to enhance such. With your example, one
workaround could be:
For VFA 600, PE-RIGHT detects that PE-LEFT does not participate in
VFA600 (due to no offset advertisement seen),
*
Either, it spawns new CSPF for VFA600 instead of sharing FA129’s
result. Bypass PE-LEFT as a result.
*
Or, it uses legacy node FA129 prefix-sid and adj-sid as
replacement (note: this method needs more comment)
In either ways, VFA600 could work without issue even with legacy
nodes co-existence.
After PE-LEFT upgraded, VFA600 would be using FA129 CSPF result
instead, and save CPU resources in each node.
Another question: do we need FAD for VFA600? Currently, no. Not
mandatory.
But it could be considered if “good to have” parameters are passed
along with FAD.
[/lc2]
>
> I do not see a major problem. Please give me an example to
illustrate your
> concern if possible.
>
> Of course, we need to do double check on the claim
and possibly lab
> verification to see if the backward compatibility could be
achieved. It could
> be vendor specific.
>
> [/lc]
>
>
> What does deserve discussion is a "hitless migration strategy".
When full
> support is available, if you were to switch to the new scheme,
you would
> want to do so without changing any existing SIDs as
this would avoid
> forwarding disruption. Which means operators would have to modify
their
> SID assignment scheme in advance of deploying the new scheme.
>
> [lc] For VFA, there would be issue for legacy nodes. I agree. In
this case,
> solution could be
> - either have a fallback plan for newer nodes if
detection of legacy nodes
> exist in the network. E.g. spawn new CSPF
> - or, totally not to deploy VFA unless all nodes are
upgraded.
>
> Section 5 is not updated with VFA inclusion.
[LES:] My comments have nothing to do with VFA.
Please reconsider them after you have understood the backwards
compatibility issues.
>
> [/lc]
>
> 3)Virtual Flex Algorithm
>
> You have introduced a new concept with very little explanation of
what it is
> nor how it can be supported.
> For example, how would we determine which nodes support a given
VFA?
> Since the algorithm value must be greater than 255, it cannot be
advertised in
> the existing SR Algorithm sub-TLV.
>
> If you are serious about this idea, please provide
a more complete
> discussion.
>
> [lc] We could illustrate application examples in next
presentation. For
> ethernet, we have port, and then we have VLAN and stacked vlan.
History
> has some hints on this.
>
[LES:] You are writing a normative specification. Hoping that all
readers/implementors have the same "intuition" isn’t sufficient.
Les
> [/lc]
>
> 4)Section 4.3
>
> "R" and "N" flags are now defined in prefix attributes sub-TLV
(RFC7794)
> They were originally defined in the SR sub-TLV because RFC 7794
did not exist
> at the time.
> The only reason they continue to exist in RFC 8667
is for backwards
> compatibility with early implementation of SR-MPLS based on early
drafts of
> what became RFC 8667.
> Please do not introduce them in new sub-TLVs - there is no need.
>
> [lc] noted with thanks [/lc]
>
> 5)ADJ-SIDs are NOT allocated from the SRGB as they are local in
scope.
> They MAY be allocated from the SRLB - or outside either GB range.
> Please correct the document in this regard.
>
> [lc] noted [/lc]
>
> Les
>
> > -----Original Message-----
> > From: Lsr <[email protected] <mailto:[email protected]>>
On Behalf Of Acee Lindem
> > Sent: Friday, April 7, 2023 11:43 AM
> > To: Louis Chan <[email protected]
<mailto:[email protected]>>
> > Cc: lsr <[email protected] <mailto:[email protected]>>
> > Subject: Re: [Lsr] IETF-116 LSR - IGP extensions
for Advertising
> > Offset for Flex-Algorithm
> >
> > Hi Louis,
> >
> > In the interest of initiating discussion, I would like to
propose the
> > term "Flex Algorithm Traffic Class (FATC)" for
the sub-division of
> > flex-algorithm traffic referred to in the draft as “Virtual
Flex Algorithm”.
> >
> > Also, in your terminology, you refer referred to TLVs and
fields with
> > simply “Algorithm” when RFC 9350 uses “Flex Algorithm”.
> >
> > Thanks,
> > Acee
> >
> >
> >
> > _______________________________________________
> > Lsr mailing list
> > [email protected] <mailto:[email protected]>
> >
https://urldefense.com/v3/__https://www.ietf.org/mailman/listinf
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> > EoixpkxsfMnbqwbR0bpHgoS9E$
>
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