Re: [Pals] draft-ietf-pals-ethernet-cw and enhanced heuristics

["Andrew G. Malis" <[e-mail]>]

Stewart,

I agree that this should be addressed, as the behavior as described in the
online documentation would prevent guaranteed in-order delivery, which is
what we are trying to address.

I also agree that we should include the quote from 4385 and your suggestion
in your second-to-last paragraph.

I also think that we should include the following, as you suggest in your
last paragraph: “RFC 6391 is RECOMMENDED as the safe and proper way to
achieve ECMP for Ethernet PWs if that indeed is desired." I don’t think we
need to also reference 6790, but that could be just me. :-)

Cheers,
Andy


On Wed, Oct 25, 2017 at 7:07 AM, Stewart Bryant <[e-mail]>
wrote:

>
> The following has just been drawn to my attention:
>
> https://www.juniper.net/documentation/en_US/junos/topics/
> concept/mpls-encapsulated-payload-load-balancing-overview.html
>
> The writer contacted the authors, but not the list, and stated that this
> heuristic results in similar behaviour to that which we are trying to
> address in this draft, but that the draft as written does not address this
> problem.
>
> I have asked the writer to post to the PALS list or let me do it for them.
>
> In such implementations, even requiring the CW does not address the issue
> that is being reported to us.
>
> Looking at RFC4385 it says:
>
>   If a PW is sensitive to packet misordering and is being carried over
>    an MPLS PSN that uses the contents of the MPLS payload to select the
>    ECMP path, it MUST employ a mechanism that prevents packet
>    misordering.  A suitable mechanism is the PWMCW described in Section
>    3 for data, and the PWACH described in Section 5 for channel-
>    associated traffic.
>
> PWMCW is a reference to the use of the CW  starting with 0000.
>
> I think that the implication in RFC4385 is that implementations should not
> proceed past the CW. That was certainly my intention when I wrote the text
> in 2005. I must say that whilst I knew of implementations that tried to
> perform ECMP on IP payloads, I was not aware of implementations that would
> attempt to glean that the PW was Ethernet and that its payload was IP and
> then perform five-tuple ECMP on the Ethernet payload.
>
> I am wondering if we need to address this, and if so how to proceed.
>
> One way would be to add text quoting RFC4385 and noting the use of the
> control word only prevents incorrect ECMP in implementations that cease
> further packet analysis on encountering a CW. Implementations that do
> packet analysis beyond the CW should expect unpredictable results, and
> therefore as noted in RFC4385 MUST employ an alternate mechanism to prevent
> packet misordering.
>
> We could leave it there or we could suggest that using RFC6391 or RFC6790
> with no further heuristics is a safe way to achieve ECMP.
>
> - Stewart
>
>
>
>