RE: Why we consider the method of "label sharing for fast PE protection"

[<[e-mail]>]

Hi Mingui,

Please find my comments inline [Bruno2]

>From: Mingui Zhang [mailto:[e-mail]] >Sent: Monday, November 18, 2013 10:32 AM
>
>Hi Bruno,
>
>Please find my comments inline below [Mingui].
>
[...]
>
>>[Bruno] First, consider that in a "POP" there may be multiple PE, i.e. more
>than
>>the 2 required for the dual homing. Let's say 5 PE.
>>The number of RG is flexible, but this also comes with drawbacks.
>>One option is to have a single RG. In this case, you indeed need a single
>vNH but
>>you require global label allocation for the perimeter of those 5 PE. Which
>roughly
>>divide the available label space by 5.
>
>[Mingui] That is a misinterpretation. Actually, all 5 PEs share the same
>label range (e.g., 1~1100) rather than dividing the label space. There is no
>scaling issue.

[Bruno2] Let's assume:
- 5 PE in the group, hence sharing the same range of labels (e.g., 1~1100).
- 5 VPNs in connect to this group of PE, 2 of which being dual-homed (VPN1 & VPN2).

With label sharing:
Label:1	2	3	4	5

PE1	VPN1	x	x	x	x	
PE2	VPN1	VPN2	x	x	x
PE3	x	VPN2	x	x	x	
PE4	x	x	VPN3	x	x
PE5	x	x	x	VPN4	VPN5

All labels marked as "x" are burned/lost because of the label sharing.

Without label sharing:
Label:1	2	3	4	5

PE1	VPN1					
PE2	VPN1	VPN2			
PE3	VPN2	VPN2				
PE4	VPN3	 			 
PE5	VPN4	VPN5	 	 	 


Here no label is lost.


So, in that specific example, 7 labels*PE are used out of 25 while 18 are lost.
Hence you need/use a label space which is 3.5 times bigger. (25/7)

>
>>Then if this represents a scaling issue, I
>>guess you can call for bigger labels, e.g. by stacking 2 labels. But this
>requires
>>more lookup work in the forwarding plane.
>
>[Mingui] As explained above, it does not need multiple lookup. Remember, we
>do not want to change the forwarding plane.


[Bruno2] I was referring to a different draft: draft-renwei-l3vpn-big-label-00

>>Roughly equivalent to the label
>>context space that you did not want in the first time...
>>Another option is to have one RG per couple of PE. In this case you need 10
>vNH
>>(IINM). That's roughly the number of PE but that's still *3 the number of
>states
>>in the IGP so I think the document should talk about this.
>
>[Mingui] On one hand, the above extreme case happens only in theory. The
>reason we scope the document to talk about 2 PEs is that duel homing is the
>BCP. Think about the real deployment, I believe most POPs will use two PEs
>for the redundant connection. On the other hand, even if you have 5 PEs in
>one POP, just put them into one RG. Therefore, the increase of the number of
>states in the IGP is not so big.
>
>[Mingui] But I agree with your advice that we can spend some words to
>address this scaling issue in the document.

[Bruno2] ok, thanks.

><snip>
>>>[Mingui] All in all, if customers really want the resilience, those
>>>routes via the primary PE SHOULD NOT be installed in the FIBs of backup
>PEs.
>>
>>[Bruno] Good point. So you probably should add this to the document.
>
>[Mingui] Sure.
>
>>>[Mingui] By the way, the scenario you designed may suffer from
>>>transient loop issue. Imagine that the more specific routes fails on
>>>the primary PE, the primary PE will resort to the backup PE. At that
>>>time, it's probably that the backup PE is unaware of the failure. It
>>>will send packet back to the primary PE. Oops, loop occurs. Following
>>>the above "SHOULD NOT be installed" requirement, such kind of loops can be
>>avoided.
>>
>>[Bruno] You are right. I suspect that RFC 4364 implies that a PE should not
>>reflect back to the core, a VPN packet received from the core. But I
>haven't
>>checked.
>
>[Mingui] OK. How about the local-repair and label-retention technique? With
>that technique, the primary PE may cache the route advertised by the backup
>PE and use it when the route it learns from the CE fails. I guess this issue
>is kind of off the track. We can talk more about it between us two.
>
>>[Bruno] Sure. But the solution relies on global VPN labels which is not
>what is
>>deployed today. So it all depends on whether you will be able to deploy
>such
>>change before. Which will requires all implementations to do it while draft
>minto
>>has no such dependency.
>
>[Mingui] The solution does not rely on *global* VPN labels. They are still
>locally assigned in the scope of the RG.
>
>>> If customers want this feature, I promise you will see this feature
>>>supported by products in several months.
>>
>>[Bruno] Ok. But I guess you mean a subset of the products currently
>deployed.
>>There may be multiple vendors/implementations in a given network. And some
>>implementations are sometimes even considered as legacy (by the vendor).
>
>[Mingui] For those legacy products, operators can update their software.

[Bruno2] not always. There is public/ietf example for this: draft-l3vpn-legacy-rtc-00

Bruno


> If
>bare legacy PEs and label-sharing PEs are interconnected, ICCP connection
>will not be established. It allows PEs falls back to the non-redundant
>connection. For those label-sharing PEs from different vendors, if they
>implement the same RFC, they are supposed to be interoperable.
>
>>>[Mingui] c.	The delegation of protector is topology dependent. I guess
>the
>>>number of possible alternative paths held by the protector is a much
>>>smaller set of all possible alternative paths.
>>
>>#Bruno: Indeed, currently the location of the protector is topology
>independent.
>>Simply because this relies on the existing FRR mechanism used in the core
>and
>>this FRR may have coverage limitations (e.g. LFA). There are plans to
>remove
>>this limitation.
>>I'm not sure to understand your second point (second sentence).
>
>[Mingui] The central protector is delegated as the PLR, right? This further
>reduces the coverage of LFA. As we know, a free LFA can have any router as
>the PLR.
>
>Thanks,
>Mingui

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