Re: [Idr] locator length : draft-li-idr-flowspec-srv6

Huaimo Chen <huaimo.chen@futurewei.com> Fri, 12 March 2021 16:44 UTC

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From: Huaimo Chen <huaimo.chen@futurewei.com>
To: Joel Halpern Direct <jmh.direct@joelhalpern.com>, "Joel M. Halpern" <jmh@joelhalpern.com>
CC: "idr@ietf.org" <idr@ietf.org>, Lizhenbin <lizhenbin@huawei.com>
Thread-Topic: locator length : draft-li-idr-flowspec-srv6
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Date: Fri, 12 Mar 2021 16:44:54 +0000
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Archived-At: <https://mailarchive.ietf.org/arch/msg/idr/tOLPnAh4WhtUEDNPa8eJkLsphSk>
Subject: Re: [Idr] locator length : draft-li-idr-flowspec-srv6
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Hi Joel,

    Thanks much for your further comment.

    Just checking the FUNC bits should be limited. We will add some
text to state that in order to check the FUNC bits, the LOC needs to
be examined and matched first.

Best Regards,
Huaimo

________________________________
From: Joel Halpern Direct <jmh.direct@joelhalpern.com>
Sent: Friday, March 12, 2021 9:58 AM
To: Huaimo Chen <huaimo.chen@futurewei.com>om>; Joel M. Halpern <jmh@joelhalpern.com>
Cc: idr@ietf.org <idr@ietf.org>rg>; Lizhenbin <lizhenbin@huawei.com>
Subject: Re: locator length : draft-li-idr-flowspec-srv6

An operator can assign B::/48 and C::?46  for Locators.  Sure, it would
usually be a single prefix with a single length.  But that is not required.

When one is examining the LOC, sure, you can use the value length to
handle it.
But the way the mechanism is described, one could try to check just the
FUNC bits, without matching the LOC.
First, that has the problem of needing exogenous information about the
LOC length.

And it is actually worse than that.  Testing the FUNC bits of the
destination field of an IP packet without checking the LOC bits is
actually meaningless.  You don't even know if the DA is an SRv6 SID.

An yet further, there is no requirement that the encoding of the FUNC in
different SIDs uses the same value representation.  The standardized
values are for advertising in routing protocols, not for the packets.

Net: I don't think having the field identification works.

Yours,
Joel

On 3/12/2021 9:51 AM, Huaimo Chen wrote:
> Hi Joel,
>
>      Thank you very much for your comment during the IETF 110.
>
>      Regarding to the lengths of locator(LOC)s and function(FUNCT)s in
> SIDs,
> RFC8986 says that the locator length, is flexible, and an operator is free
> to use the locator length of their choice. This seems indicating that the
> operator can select the length for the locator. After their selection, the
> the locator length is determined/fixed. This is illustrated by examples
> in RFC8986.
>
>      One example in the beginning of section 3.2 is as follows:
>         For example, a network operator may:
>            Assign block B::/48 to the SR domain
>            Assign a unique B:N::/64 block to each SRv6-enabled node in
> the domain.
> After this assignment, the length of the locators of the SIDs in the domain
> is 64 bits.
>
>      In the end of section 3.2, the text shows the Function fields of SIDs.
> The length of function(FUNCT)s is 16 bits.
>
>      When a SID is used in the domain, its locator length and function
> length
> should have been determined.
>
>      When an operator configures a SRv6 flow specification, involving
> a SID or a group of SIDs, s/he should have known the locator length and
> function length in the SID(s).
>
> Best Regards,
> Huaimo