Re: [Lsr] New Version Notification for draft-ietf-lsr-isis-area-proxy-01.txt

[Les Ginsberg (ginsberg)]

Resuming this thread…the authors were kind enough to meet with me and educate 
me on how Area Proxy works. The resulting conclusions are:

1)Area Proxy TLV is meant to  be sent ONLY in L1L2 LSPs (NOT Proxy LSPs).

2)Advertisement of Area SID in the Area Proxy TLV is needed so that Inside 
Nodes can install a receive entry for that SID

3)Advertisement of Area SID in Binding TLV is needed in Proxy LSPs so that 
outside nodes can install a forwarding entry towards the Inside Node(s).
This SID MUST be identical to the SID advertised in the Area Proxy TLV in L2 
LSPs.
Use of the Binding TLVs to do this is the current choice.

4)Other uses of an Area SID not related to Area Proxy are outside the scope of 
the Area Proxy draft. However, as the concept seems potentially useful in other 
scenarios, being able to advertise an Area SID in the Binding TLV allows for 
these other uses.
But, any advertisement of Area SID in Binding TLV which appears in L1L2 LSPs is 
unrelated to Area Proxy. Whether that SID is the same value as that used for 
Area Proxy is something future use cases for Area SID will have to decide.

I therefore think it makes sense to allow advertisement of the Area SID in both 
the Area Proxy TLV and the Binding TLV, subject to the constraints above. This 
will NOT result in “duplicate advertisements”.

Further details regarding the encoding will need to be provided in future 
revisions of the draft.

   Les (both as WG member and Designated Expert for IS-IS TLV Codepoints 
Registry)


From: Lsr  On Behalf Of Les Ginsberg (ginsberg)
Sent: Wednesday, July 08, 2020 11:19 AM
To: tony...@tony.li
Cc: lsr@ietf.org
Subject: Re: [Lsr] New Version Notification for 
draft-ietf-lsr-isis-area-proxy-01.txt

Tony –

Inline.

From: Tony Li mailto:tony1ath...@gmail.com>> On Behalf 
Of tony...@tony.li<mailto:tony...@tony.li>
Sent: Wednesday, July 08, 2020 11:12 AM
To: Les Ginsberg (ginsberg) mailto:ginsb...@cisco.com>>
Cc: lsr@ietf.org<mailto:lsr@ietf.org>
Subject: Re: [Lsr] New Version Notification for 
draft-ietf-lsr-isis-area-proxy-01.txt


Hi Les,


Again, the subTLVs of the area proxy TLV are for the coordination of the Inside 
Area. The Area Proxy TLV appears in the Inside Node’s normal LSP.

The Proxy LSP is for informing the Outside Area.

[Les:] Understood – but I do not see why this requires you to advertise the SID 
in two different TLVs. As you allow the Binding SID TLVs to be advertised in 
both standard LSPs and Proxy LSPs, there seems to be no need for two different 
TLVs to include the advertisement.
??


The semantics are completely different. Advertising it in the Binding TLV is 
permission to use the SID for packet forwarding.

The advertisement in the Area Proxy LSP is to instruct the Inside Nodes to use 
that SID to establish the forwarding state.
[Les:] Sorry, I do not see why this requires you to advertise the SID in two 
different places.
The SID gets advertised in one place (which TLV is TBD for the moment). The 
instruction to enable use of it for Area Proxy comes in the Area Proxy TLV. 
That only requires (at most) a bit – and perhaps nothing other than the 
presence of the Area Proxy TLV. It certainly does not require the SID itself to 
be readvertised.

Let’s see if we can agree that the SID only needs to be advertised in one place 
before proceeding further.

Les



[Les:] Yes – of course – this is pathological. (Probably not hilarious to the 
customer. 😊 )


No doubt, but I’ve found that if one doesn’t regard bugs with some humor, our 
profession would be morose, in the extreme.


I am just saying by having two sources for the advertisement you introduce the 
possibility of inconsistency – and the spec would have to speak to this – even 
if it should not happen.


Ummm…. Not sure what I can say other than “don’t write bugs”.  But ok, I’ll 
happily do so.


We are abusing the definition of the Binding TLV.  2.4 says: "The SID/Label 
Binding TLV may be used to advertise prefixes to SID/Label mappings.”  That’s 
not what we’re doing, so it’s not too surprising that there’s some conflicts.

[Les:] Yes – the Binding TLV has some issues being generalized. There is 
history here as to why the format is the way it is and why it isn’t more easily 
extensible – and that is open for discussion AFAIAC, but we cannot break 
backwards compatibility for SR.


Agreed, trying not to. :-)


But I am also responding (in part) to your desire to make the Area Segment SID 
a more general tool – usable outside of Area Proxy – which seems like a good 
goal.


And you’ve requested that we put the Area Segment SID in the Binding TLV.  I’ve 
tried to do so and you don’t like how I’ve done it.  Please tell me what I can 
do that will satisfy you.  From what you’ve said so far, there is no legal way 
to use the Binding TLV.

Catch-22.  Yossarian!

Regards,
Tony

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Re: [Lsr] New Version Notification for draft-ietf-lsr-isis-area-proxy-01.txt

[Les Ginsberg (ginsberg)]

Tony –

Inline.

From: Tony Li  On Behalf Of tony...@tony.li
Sent: Wednesday, July 08, 2020 11:12 AM
To: Les Ginsberg (ginsberg) 
Cc: lsr@ietf.org
Subject: Re: [Lsr] New Version Notification for 
draft-ietf-lsr-isis-area-proxy-01.txt


Hi Les,


Again, the subTLVs of the area proxy TLV are for the coordination of the Inside 
Area. The Area Proxy TLV appears in the Inside Node’s normal LSP.

The Proxy LSP is for informing the Outside Area.

[Les:] Understood – but I do not see why this requires you to advertise the SID 
in two different TLVs. As you allow the Binding SID TLVs to be advertised in 
both standard LSPs and Proxy LSPs, there seems to be no need for two different 
TLVs to include the advertisement.
??


The semantics are completely different. Advertising it in the Binding TLV is 
permission to use the SID for packet forwarding.

The advertisement in the Area Proxy LSP is to instruct the Inside Nodes to use 
that SID to establish the forwarding state.
[Les:] Sorry, I do not see why this requires you to advertise the SID in two 
different places.
The SID gets advertised in one place (which TLV is TBD for the moment). The 
instruction to enable use of it for Area Proxy comes in the Area Proxy TLV. 
That only requires (at most) a bit – and perhaps nothing other than the 
presence of the Area Proxy TLV. It certainly does not require the SID itself to 
be readvertised.

Let’s see if we can agree that the SID only needs to be advertised in one place 
before proceeding further.

Les




[Les:] Yes – of course – this is pathological. (Probably not hilarious to the 
customer. 😊 )


No doubt, but I’ve found that if one doesn’t regard bugs with some humor, our 
profession would be morose, in the extreme.



I am just saying by having two sources for the advertisement you introduce the 
possibility of inconsistency – and the spec would have to speak to this – even 
if it should not happen.


Ummm…. Not sure what I can say other than “don’t write bugs”.  But ok, I’ll 
happily do so.


We are abusing the definition of the Binding TLV.  2.4 says: "The SID/Label 
Binding TLV may be used to advertise prefixes to SID/Label mappings.”  That’s 
not what we’re doing, so it’s not too surprising that there’s some conflicts.

[Les:] Yes – the Binding TLV has some issues being generalized. There is 
history here as to why the format is the way it is and why it isn’t more easily 
extensible – and that is open for discussion AFAIAC, but we cannot break 
backwards compatibility for SR.


Agreed, trying not to. :-)



But I am also responding (in part) to your desire to make the Area Segment SID 
a more general tool – usable outside of Area Proxy – which seems like a good 
goal.


And you’ve requested that we put the Area Segment SID in the Binding TLV.  I’ve 
tried to do so and you don’t like how I’ve done it.  Please tell me what I can 
do that will satisfy you.  From what you’ve said so far, there is no legal way 
to use the Binding TLV.

Catch-22.  Yossarian!

Regards,
Tony

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Re: [Lsr] New Version Notification for draft-ietf-lsr-isis-area-proxy-01.txt

[tony . li]

Hi Les,


> Again, the subTLVs of the area proxy TLV are for the coordination of the 
> Inside Area. The Area Proxy TLV appears in the Inside Node’s normal LSP.
>  
> The Proxy LSP is for informing the Outside Area.
>  
> [Les:] Understood – but I do not see why this requires you to advertise the 
> SID in two different TLVs. As you allow the Binding SID TLVs to be advertised 
> in both standard LSPs and Proxy LSPs, there seems to be no need for two 
> different TLVs to include the advertisement.
> ??


The semantics are completely different. Advertising it in the Binding TLV is 
permission to use the SID for packet forwarding.

The advertisement in the Area Proxy LSP is to instruct the Inside Nodes to use 
that SID to establish the forwarding state.  


> [Les:] Yes – of course – this is pathological. (Probably not hilarious to the 
> customer. 😊 )


No doubt, but I’ve found that if one doesn’t regard bugs with some humor, our 
profession would be morose, in the extreme.


> I am just saying by having two sources for the advertisement you introduce 
> the possibility of inconsistency – and the spec would have to speak to this – 
> even if it should not happen.


Ummm…. Not sure what I can say other than “don’t write bugs”.  But ok, I’ll 
happily do so.


> We are abusing the definition of the Binding TLV.  2.4 says: "The SID/Label 
> Binding TLV may be used to advertise prefixes to SID/Label mappings.”  That’s 
> not what we’re doing, so it’s not too surprising that there’s some conflicts.
>  
> [Les:] Yes – the Binding TLV has some issues being generalized. There is 
> history here as to why the format is the way it is and why it isn’t more 
> easily extensible – and that is open for discussion AFAIAC, but we cannot 
> break backwards compatibility for SR.


Agreed, trying not to. :-)


> But I am also responding (in part) to your desire to make the Area Segment 
> SID a more general tool – usable outside of Area Proxy – which seems like a 
> good goal.


And you’ve requested that we put the Area Segment SID in the Binding TLV.  I’ve 
tried to do so and you don’t like how I’ve done it.  Please tell me what I can 
do that will satisfy you.  From what you’ve said so far, there is no legal way 
to use the Binding TLV.

Catch-22.  Yossarian!

Regards,
Tony

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Re: [Lsr] New Version Notification for draft-ietf-lsr-isis-area-proxy-01.txt

[Les Ginsberg (ginsberg)]

Tony –

Inline.

From: Tony Li  On Behalf Of tony...@tony.li
Sent: Wednesday, July 08, 2020 8:29 AM
To: Les Ginsberg (ginsberg) 
Cc: lsr@ietf.org
Subject: Re: [Lsr] New Version Notification for 
draft-ietf-lsr-isis-area-proxy-01.txt


Hi Les,



The new definitions in the latest version in the draft are very close to what 
we discussed in the earlier thread – so this looks pretty good to me.


Excellent, thanks.



I still have some concerns regarding the Area Segment SID.
You propose to advertise this in two places:

1)As a sub-TLV of the new Area Proxy TLV
2)As a new sub-TLV of the existing Binding TLVs (149, 150)

I am not sure why you need this in the Area Proxy TLV as you allow Binding TLVs 
to be advertised in the Proxy LSPs (Section 4.4.10).
???


Again, the subTLVs of the area proxy TLV are for the coordination of the Inside 
Area. The Area Proxy TLV appears in the Inside Node’s normal LSP.

The Proxy LSP is for informing the Outside Area.

It’s fairly important that we install the forwarding state for the Area Segment 
SID and distribute the Proxy System ID before we advertise the Proxy LSP.

[Les:] Understood – but I do not see why this requires you to advertise the SID 
in two different TLVs. As you allow the Binding SID TLVs to be advertised in 
both standard LSPs and Proxy LSPs, there seems to be no need for two different 
TLVs to include the advertisement.
??


If this is what is intended, it raises a number of concerns:

If both are present and inconsistent how are they used?


If both are present and inconsistent, then we have a major malfunction of the 
Area Leader, since it is the single system that is intentionally advertising 
both.

The Inside Nodes would follow the contents of the Area Proxy TLV and employ one 
SID value.

The Outsiide Nodes would follow the contents of the Proxy LSP and employ a 
different SID value.

Hilarity ensues.

Note that this is somewhat analogous to a system that wished to advertise a 
loopback interface and advertised one prefix into L1 and another prefix into L2.

[Les:] Yes – of course – this is pathological. (Probably not hilarious to the 
customer. 😊 )
I am just saying by having two sources for the advertisement you introduce the 
possibility of inconsistency – and the spec would have to speak to this – even 
if it should not happen.


As Area Proxy TLV does not support MT (not suggesting that it should) it isn’t 
clear how this relates to MT context – which exists for TLVs 149/150.

Encoding wise, if we are to support Area Segment SID in the Binding TLVs, I 
think more detail needs to be provided as to flag settings when the new sub-TLV 
is present.
The following flags are currently defined for the Binding TLVs:

0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
|F|M|S|D|A| |
+-+-+-+-+-+-+-+-+

F,S,D flags seem applicable w/o change.
However, M flag would need to be clear when Area Segment SID is present.
The A flag seems not applicable to Area Segment SID
And your encoding violates the current definition of Binding TLVs.
Specifically, https://www.rfc-editor.org/rfc/rfc8667.html#section-2.4.4 states:

“The Prefix-SID sub-TLV is defined in Section 
2.1<https://www.rfc-editor.org/rfc/rfc8667.html#PREFIXSIDSUBTLV> and contains 
the SID/Index/Label value associated with the prefix and range.
 The Prefix-SID sub-TLV MUST be present in the SID/Label Binding TLV when the 
M-Flag is clear.
 The Prefix-SID sub-TLV MUST NOT be present when the M-Flag is set.”

While some changes to this definition are likely required to support Area 
Segment SID no matter what, it is hard for me to see a good way to do this w/o 
adding a new flag.


I’m open to suggestions here.

We are abusing the definition of the Binding TLV.  2.4 says: "The SID/Label 
Binding TLV may be used to advertise prefixes to SID/Label mappings.”  That’s 
not what we’re doing, so it’s not too surprising that there’s some conflicts.

[Les:] Yes – the Binding TLV has some issues being generalized. There is 
history here as to why the format is the way it is and why it isn’t more easily 
extensible – and that is open for discussion AFAIAC, but we cannot break 
backwards compatibility for SR.
But I am also responding (in part) to your desire to make the Area Segment SID 
a more general tool – usable outside of Area Proxy – which seems like a good 
goal.

   Les

Tony


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Re: [Lsr] New Version Notification for draft-ietf-lsr-isis-area-proxy-01.txt

[tony . li]

Hi Les,


> The new definitions in the latest version in the draft are very close to what 
> we discussed in the earlier thread – so this looks pretty good to me.


Excellent, thanks.


> I still have some concerns regarding the Area Segment SID.
> You propose to advertise this in two places:
>  
> 1)As a sub-TLV of the new Area Proxy TLV
> 2)As a new sub-TLV of the existing Binding TLVs (149, 150)
>  
> I am not sure why you need this in the Area Proxy TLV as you allow Binding 
> TLVs to be advertised in the Proxy LSPs (Section 4.4.10).
> ???


Again, the subTLVs of the area proxy TLV are for the coordination of the Inside 
Area. The Area Proxy TLV appears in the Inside Node’s normal LSP.

The Proxy LSP is for informing the Outside Area.

It’s fairly important that we install the forwarding state for the Area Segment 
SID and distribute the Proxy System ID before we advertise the Proxy LSP.


> If this is what is intended, it raises a number of concerns:
>  
> If both are present and inconsistent how are they used?


If both are present and inconsistent, then we have a major malfunction of the 
Area Leader, since it is the single system that is intentionally advertising 
both.

The Inside Nodes would follow the contents of the Area Proxy TLV and employ one 
SID value.

The Outsiide Nodes would follow the contents of the Proxy LSP and employ a 
different SID value.

Hilarity ensues.

Note that this is somewhat analogous to a system that wished to advertise a 
loopback interface and advertised one prefix into L1 and another prefix into L2.


> As Area Proxy TLV does not support MT (not suggesting that it should) it 
> isn’t clear how this relates to MT context – which exists for TLVs 149/150.
>  
> Encoding wise, if we are to support Area Segment SID in the Binding TLVs, I 
> think more detail needs to be provided as to flag settings when the new 
> sub-TLV is present.
> The following flags are currently defined for the Binding TLVs:
>  
> 0 1 2 3 4 5 6 7
> +-+-+-+-+-+-+-+-+
> |F|M|S|D|A| |
> +-+-+-+-+-+-+-+-+
>  
> F,S,D flags seem applicable w/o change.
> However, M flag would need to be clear when Area Segment SID is present.
> The A flag seems not applicable to Area Segment SID
> And your encoding violates the current definition of Binding TLVs.
> Specifically, https://www.rfc-editor.org/rfc/rfc8667.html#section-2.4.4 
>  states:
>  
> “The Prefix-SID sub-TLV is defined in Section 2.1 
>  and contains 
> the SID/Index/Label value associated with the prefix and range.
>  The Prefix-SID sub-TLV MUST be present in the SID/Label Binding TLV when the 
> M-Flag is clear.
>  The Prefix-SID sub-TLV MUST NOT be present when the M-Flag is set.”
>  
> While some changes to this definition are likely required to support Area 
> Segment SID no matter what, it is hard for me to see a good way to do this 
> w/o adding a new flag.


I’m open to suggestions here.

We are abusing the definition of the Binding TLV.  2.4 says: "The SID/Label 
Binding TLV may be used to advertise prefixes to SID/Label mappings.”  That’s 
not what we’re doing, so it’s not too surprising that there’s some conflicts.

Tony


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