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

Joel Halpern Direct <jmh.direct@joelhalpern.com> Fri, 12 March 2021 14:58 UTC

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To: Huaimo Chen <huaimo.chen@futurewei.com>, "Joel M. Halpern" <jmh@joelhalpern.com>
Cc: "idr@ietf.org" <idr@ietf.org>, Lizhenbin <lizhenbin@huawei.com>
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From: Joel Halpern Direct <jmh.direct@joelhalpern.com>
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Date: Fri, 12 Mar 2021 09:58:00 -0500
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Archived-At: <https://mailarchive.ietf.org/arch/msg/idr/s4PlAu6735l7m7zcFqMAlr34sxE>
Subject: Re: [Idr] locator length : draft-li-idr-flowspec-srv6
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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