Re: [bess] CW in EVPN: Was Signaling Control Word in EVPN

James Bensley <> Mon, 29 October 2018 13:34 UTC

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From: James Bensley <>
Date: Mon, 29 Oct 2018 13:33:52 +0000
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Subject: Re: [bess] CW in EVPN: Was Signaling Control Word in EVPN
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On Tue, 23 Oct 2018 at 09:28, Alexander Vainshtein
<> wrote:
> James,
> I am adding PALS WG to the list of addressees because, AFAIK, the PW CW is defined in this WG.
> I think that the really observed problem with incorrect ECMP behavior exists, but it is different from your description in your earlier email:
> Any LSR on the path between ingress and egress LER is free to look beyond the MPLS label stack and misinterpret the 0x00 0x00 at the start of a control-word as a valid MAC that starts 00:00:XX:XX:XX:XX and try to hash on Ethernet headers starting directly after the MPLS label stack.
> I have not seen (or heard about) such behavior in any deployed networks.
> However, I am aware of some modern forwarding chipsets that (correctly) treat the ‘0000’ in the first nibble of the payload of a labeled packet (i.e., immediately following the bottom of the label stack) as the indication of a 32-bit PW control word but (incorrectly), consider this as a CW of an Ethernet PW (as if no other PWs exist!) and try to hash on the presumed MAC addresses, Ethertype etc.  Such behavior is really deadly for, say TDM PWs that, AFAIK, are still widely deployed in many places.

Hi Sasha,

Well in either case we have both provided different examples of when
the PWMCW doesn't prevent reordering when ECMP is present in the PSN.
Perhaps I need to actually step back a bit here and ask a different
question, to further understand the non-technical problem first.

What is the scope/remit of the WG in this scenario? To be clear on
what I mean by this, the PWMCW is a flawed method for preventing
reordering when ECMP is present (see our two examples above). It also
doesn't add any entropy to improve ECMP when ECMP is required. Entropy
labels help to prevent reordering and help to add entropy, despite
these technical benefits but they may have other non-technical
disadvantages. So what is the WGs remit with recommending one
technology over another?

In this specific case, is it:

Option 1. The WG's remit is to phase out or discourage flawed
technologies if a superior one exists, so it should look to deprecate
CWs because ELs are superior from a purely technical view?
Option 2. The WG's remit is to support as many technologies as
possible, so it shouldn't look to deprecate CWs because ELs may have
other non-technical draw backs?
Option 3. The WG's remit is to remain neutral on the subject of CWs vs
other methods, and simply ensure that all drafts follow the correct
due diligence process regardless of whether one technology is
technically "better" than another?
Option 4. The WG's remit is something else?