Hi Les,
My responses are inline below with [HC].
Best Regards,
Huaimo
Folks –
The term “backwards compatibility” is getting abused here.
What does “backwards compatibility” mean in the context of a routing protocol
like IS-IS?
It means that protocol extensions can be advertised and safely used in the
presence of legacy nodes (which do not understand the new advertisements).
Neither MP nor Big-TLV are backwards compatible.
[HC]: Big-TLV is backwards compatible according to your definition of
“backwards compatibility”.
The authors of MP draft do not claim it is backwards compatible.
The authors of Big-TLV claim it is “backwards compatible” – but this is a false
statement. Any attempt to use Big-TLV advertisements in the presence of legacy
nodes will result in inconsistent and potentially dangerous behavior.
[HC]: Using Big-TLV advertisements in the presence of legacy nodes will not
result in any inconsistent or potentially dangerous behavior. I have explained
this in detail in the LSR mailing list.
Big-TLV authors like to say “you can send Big-TLV but not use it until all
nodes support it” – but this does meet the criteria for backwards
compatibility. If Big-TLV were “backwards compatible” there would be no need
for a capability advertisement to determine when it is safe to use the
advertisements.
[HC]: Big-TLV capability advertisement is a part of the Big-TLV
solution/extension.
Capability advertisements seem used for backwards compatibility by other
protocols. Is there any restriction in IS-IS that does not allow any IS-IS
extension to use a capability advertisement for backwards compatibility?
Using a capability for Big-TLV backwards compatibility is suggested by Chris in
the LSR mailing list.
https://mailarchive.ietf.org/arch/msg/lsr/cXyGnSJSdEQR9BVxykXhiuUWtgE/
Les
From: Les Ginsberg (ginsberg) <[email protected]>
Sent: Tuesday, December 5, 2023 2:11 AM
To: [email protected]; Huaimo Chen <[email protected]>; Tony Li
<[email protected]>; Linda Dunbar <[email protected]>
Cc: Yingzhen Qu <[email protected]>;
[email protected]; lsr <[email protected]>
Subject: RE: Re: [Lsr] WG Adoption Call - draft-pkaneria-lsr-multi-tlv
(11/17/2023 - 12/09/2023)
Folks –
The term “backwards compatibility” is getting abused here.
What does “backwards compatibility” mean in the context of a routing protocol
like IS-IS?
It means that protocol extensions can be advertised and safely used in the
presence of legacy nodes (which do not understand the new advertisements).
Neither MP nor Big-TLV are backwards compatible.
The authors of MP draft do not claim it is backwards compatible.
The authors of Big-TLV claim it is “backwards compatible” – but this is a false
statement. Any attempt to use Big-TLV advertisements in the presence of legacy
nodes will result in inconsistent and potentially dangerous behavior.
Big-TLV authors like to say “you can send Big-TLV but not use it until all
nodes support it” – but this does meet the criteria for backwards
compatibility. If Big-TLV were “backwards compatible” there would be no need
for a capability advertisement to determine when it is safe to use the
advertisements.
Les
From: [email protected]<mailto:[email protected]>
<[email protected]<mailto:[email protected]>>
Sent: Monday, December 4, 2023 10:35 PM
To: Huaimo Chen <[email protected]<mailto:[email protected]>>;
Les Ginsberg (ginsberg) <[email protected]<mailto:[email protected]>>; Tony
Li <[email protected]<mailto:[email protected]>>; Linda Dunbar
<[email protected]<mailto:[email protected]>>
Cc: Yingzhen Qu <[email protected]<mailto:[email protected]>>;
[email protected]<mailto:[email protected]>;
lsr <[email protected]<mailto:[email protected]>>
Subject: Re: Re: [Lsr] WG Adoption Call - draft-pkaneria-lsr-multi-tlv
(11/17/2023 - 12/09/2023)
Hello All,
It seems Big-TLV is backward compatible. Backward compatible is an important
point that should be considered when we introduce new features in a protocol,
especially the widely used protocols like ISIS, BGP etc.
________________________________
Best Regards,
Zhenqiang Li
China Mobile
[email protected]<mailto:[email protected]>
From: Huaimo Chen<mailto:[email protected]>
Date: 2023-12-04 21:57
To: Les Ginsberg (ginsberg)<mailto:[email protected]>; Tony
Li<mailto:[email protected]>; Linda Dunbar<mailto:[email protected]>
CC: Yingzhen Qu<mailto:[email protected]>;
[email protected]<mailto:[email protected]>;
lsr<mailto:[email protected]>
Subject: Re: [Lsr] WG Adoption Call - draft-pkaneria-lsr-multi-tlv (11/17/2023
- 12/09/2023)
Hi Les,
My responses are inline below with [HC].
Best Regards,
Huaimo
Linda –
When we have polarized positions (for whatever reasons), coming to consensus is
often difficult. Each side tends to dismiss the arguments of the other –
sometimes regardless of merit.
So, maybe the following won’t help – but I am going to give it a try.
Point #1: There are existing TLVs for which MP behavior was explicitly stated
in existing RFCs and there are already many deployments that conform to those
RFCs (see Introduction of MP draft for a list).
If we choose to use a different encoding to support > 255 bytes for other
codepoints, this both complicates implementations and confuses the definition
of new protocol extensions. When defining a new codepoint should I choose MP or
should I choose a different encoding? And what criteria can be used to make
this choice a sensible one?
And since MP is already REQUIRED for those TLVs where it was explicitly
defined, we will always have to support that – at least for some codepoints.
[HC]: When Big-TLV is used for a TLV > 255, it does not affect the existing
MP-TLV that has been used for another TLV > 255. When defining a new codepoint
for a new TLV, it seems better to choose Big-TLV if backward compatibility is
needed for the new TLV since Big-TLV is backward compatible.
I have explained it (i.e. ,“Big-TLV is backward compatible”) in detail. In
addition, some other people indicate it in the LSR mailing list.
Point #2: MP for IS-Neighbor/Prefix Reachability TLVs has already been
implemented by multiple vendors, tested in both partial deployment and full
deployment scenarios. We know that it works and we know what the problems are
in partial deployment.
This cannot be said for new alternatives.
With due respect to Huaimo, he tends to characterize the implementation
problems to be solved for Big-TLV as “easy to resolve” but given the absence of
implementation I think this is an overly optimistic and somewhat naïve POV. (No
offense intended)
[HC]: It seems that the implementation of an idea/solution in a draft is not
required by LSR WG for the draft to be adopted, or even for the draft to become
RFC.
Point #3: As documented in the IANA section of the MP draft, the problem
extends to sub-TLVs of top level TLVs as well. It is then not as simple as
reserving one encap TLV to handle the top-level TLV case. We also have to have
a solution when a sub-TLV requires more than 255 bytes. MP solves this without
additional changes. Big-TLV has yet to discuss this.
[HC]: It seems that MP-TLV has to discuss this.
Assume: a TLV (e.g., TLV 1) > 255 and its sub-TLV > 255, there are MP-TLVs
(e.g., MP-TLV 1 and MP-TLV 2) for the TLV and MP-TLVs (e.g., MP-TLV 3 and
MP-TLV 4) for the sub-TLV. All these MP-TLVs are TLVs. If there is another TLV
(e.g., TLV 2) > 255 and its sub-TLV > 255 and all these sub-TLVs (i.e., the
sub-TLV of TLV 1 and the sub-TLV of TLV 2) have the same type and there are
MP-TLVs (e.g., MP-TLV 5, MP-TLV 6) for TLV 2 and MP-TLVs (e.g., MP-TLV 7,
MP-TLV 8) for the sub-TLV (of TLV 2) > 255, how do you handle this case? How
do you map MP-TLVs (e.g., MP-TLV 3, 4, 7, 8) for the same type of the sub-TLVs
to their TLVs (e.g., TLV 1, 2)?
If Big-TLV actually solved the partial deployment case, we would have a
motivation to look at it more seriously. But it does not. It has the same issue
with partial deployment that MP does. So for me, there is no value add to
Big-TLV – and it does require additional implementation work – not all of which
has even been defined yet.
[HC]: Big-TLV actually solved the partial deployment case. I have explained
this in detail. In addition, some other people indicate that Big-TLV is
backward compatible in the LSR mailing list.
It isn’t better – it is just different – and comes with additional
implementation costs.
[HC]: Big-TLV is better as I have explained in detail.
Les
From: Tony Li <[email protected]<mailto:[email protected]>> On Behalf
Of Tony Li
Sent: Friday, December 1, 2023 2:44 PM
To: Linda Dunbar <[email protected]<mailto:[email protected]>>
Cc: Yingzhen Qu <[email protected]<mailto:[email protected]>>;
[email protected]<mailto:[email protected]>;
lsr <[email protected]<mailto:[email protected]>>
Subject: Re: [Lsr] WG Adoption Call - draft-pkaneria-lsr-multi-tlv (11/17/2023
- 12/09/2023)
Hi Linda,
* Suppose the information to be carried by the Extended IS Reachability
(type 22) (in Example 4.1) is larger than 255. Does it mean the recipient will
receive 2 TLVs (both with the Type 22) in one LSA? For legacy routers, the
second TLV (Type =22) might overwrite the first TLV.
Yes, a legacy implementation may well have bugs. The proposal is to fix that:
expect MP-TLVs.
[Linda] Are you saying only the legacy implementation with bugs will be
confused with two TLVs with the same Type in in one LSA?
No. All implementations have bugs. This is reality.
Implementations that do not understand MP-TLV may be confused. Correct
implementations of MP-TLV support will not be confused.
* Isn’t it more straightforward to have a new TYPE value for carrying the
extra information beyond the 255 bytes? So, the legacy routers can ignore the
TLVs with the unrecognized types.
You could do that, but code points are not free. We certainly cannot afford
another code point for each existing code point. Using just one code point is
less than helpful: it forces us to aggregate information that has no business
being aggregated. Ignoring information is a non-starter because it makes
partial deployments fatal: some of the domain operates with some information
and some of the domain operates with different information.
[Linda] Why not consider having just one additional TYPE code with sub-types to
indicate which original TLVs the value should be appended to?
We have considered it. See all of Les’ emails for why it’s a bad idea.
If it helps simplify this debate: we know that you work for Futurewei/Huawei
and that the discussion has polarized into your Big-TLV faction vs. everyone
else. Repetition of previously made points add zero value to the discussion.
Tony
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