Re: [bess] Signaling Control Word in EVPN

[Alexander Vainshtein <[e-mail]>]

Andy, James and all,
One answer for Muthu’s question could be that the EVPN CP (including the EVPN Layer 2 Attributes Extended Community) cannot provide support for negotiating the CW usage in the case of MP2MP EVPN services.

I agree that this can hardly be called a good answer, but I suspect that there is no other one.

Regards,
Sasha

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From: Andrew G. Malis [mailto:[e-mail]]
Sent: Tuesday, October 9, 2018 5:16 PM
To: [e-mail]
Cc: Alexander Vainshtein <[e-mail]>; [e-mail]; Shell Nakash <[e-mail]>; Yechiel Rosengarten <[e-mail]>; Michael Gorokhovsky <[e-mail]>; Dmitry Valdman <[e-mail]>; Ron Sdayoor <[e-mail]>; [e-mail]; Rotem Cohen <[e-mail]>
Subject: Re: [bess] Signaling Control Word in EVPN

James,

It's much harder to mandate use of EL than the CW for several reasons:
- CW implementation is much more common than EL implementation
- PWs and/or EVPN are rarely the only traffic in an MPLS traffic tunnel, rather, they will be multiplexed with other MPLS-based applications that are using the traffic tunnel to reach a common destination. Thus, by using the CW, you can disable ECMP only for those MPLS packets that cannot tolerate reordering.

That said, I'm also concerned that because of the existing text in 7432, implementations may not be using the CW even for P2P EVPN.

And we still don't have a good answer for Muthu's original question. :-)

Cheers,
Andy


On Tue, Oct 9, 2018 at 8:49 AM James Bensley <[e-mail]<mailto:[e-mail]>> wrote:
On Tue, 9 Oct 2018 at 11:29, Alexander Vainshtein
<[e-mail]<mailto:[e-mail]>> wrote:
> I fully agree that reordering due to lack of the CW happens, and that usage of the CW is the right way to eliminate that.

Hi Sasha,

If I may rudely interject here. You have to be careful with your
wording. The PWMCW doesn't eliminate reordering.

https://tools.ietf.org/html/draft-ietf-pals-ethernet-cw-07#section-4:

4.  Recommendation

   The ambiguity between an MPLS payload that is an Ethernet PW and one
   that is an IP packet is resolved when the Ethernet PW control word is
   used.  This document updates [RFC4448] to state that where both the
   ingress PE and the egress PE support the Ethernet pseudowire control
   word, then the CW MUST be used.

The CW doesn't fix the problem in every case, but it does fix it in most cases.

The only way to truly fix this problem is to use entropy labels. The
problem with the CW draft is that it only recommends entropy labels or
FAT labels when ECMP is required, not when it isn't require *but*
exists anyway within the packet switched core:

   Where the application of ECMP to an Ethernet PW traffic is required,
   and where both the ingress and the egress PEs support [RFC6790]
   (Entropy Label Indicator/Entropy Label (ELI/EL)) or both the ingress
   and the egress PEs support [RFC6391] (FAT PW), then either method may
   be used.

For this reason my personal opinion is that only EL should be
recommended by the EVPN draft. I don't think a solution that doesn't
completely fix the problem (PWMCW) should be recommend when we have
one that does fully fix the problem (El/ELI or ELC/RLD in SR).

> Or should we agree that consistent usage of the CW in the EVPN encapsulation has to be delegated to some network-wide management mechanism/LSO? For the reference, the latest (expired) version of the EVPN YANG data model does not include any attributes that indicate usage or non-usage of the CW in the encapsulation...

This is perhaps the more difficult question ^ what to recommend in the
case that EL isn't supported? You could recommend that all devices
should be configured to *not* hash on the VPN payload and only the
label stack, then there can be no mistaking what the VPN contents are
(Ethernet/IPv4/IPv6/other) but, some devices might not support that.
Alternatively, you could recommend that the CW is used only when EL
isn't available but, some devices might not support that. I'll leave
this up to you guys as you're the experts here, I just wanted to point
out the wording issue above regarding CW and EL and "fixing"
reordering.

Cheers,
James.

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