Re: [Lsr] Clarification on inconsistency between RFC7794 and draft-ietf-lsr-isis-srv6-extensions

Peter Psenak <ppsenak@cisco.com> Wed, 24 February 2021 09:39 UTC

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To: "Van De Velde, Gunter (Nokia - BE/Antwerp)" <gunter.van_de_velde@nokia.com>, "draft-ietf-lsr-isis-srv6-extensions@ietf.org" <draft-ietf-lsr-isis-srv6-extensions@ietf.org>
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From: Peter Psenak <ppsenak@cisco.com>
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Date: Wed, 24 Feb 2021 10:39:07 +0100
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Subject: Re: [Lsr] Clarification on inconsistency between RFC7794 and draft-ietf-lsr-isis-srv6-extensions
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Hi Gunter,

On 24/02/2021 07:24, Van De Velde, Gunter (Nokia - BE/Antwerp) wrote:
> Hi Peter, All,
> 
> I’m am trying to clarify a potential inconsistency between RFC7794 and 
> draft-ietf-lsr-isis-srv6-extensions.
> 
> draft-ietf-lsr-isis-srv6-extensions says that we should advertise 
> identical prefix-attribute tlv for the ipv6 reachability tlv and for the 
> locator tlv.

yes, for algo 0 only.

> 
> RFC7794 document says that we should not set the X flag in case of ipv6 
> routes because the ipv6 reachability tlv already has an external indication.
> 
> Can you advise.
> 
>  1. draft-ietf-lsr-isis-srv6-extensions
> 
> The Prefix Attribute Flags Sub-TLV can be carried in the SRv6 Locator
> 
>     TLV as well as the Prefix Reachability TLVs.  When a router
> 
>     originates both the Prefix Reachability TLV and the SRv6 Locator TLV
> 
>     for a given prefix, and the router is originating the Prefix
> 
>     Attribute Flags Sub-TLV in one of the TLVs, the router SHOULD
> 
>     advertise identical versions of the Prefix Attribute Flags Sub-TLV in

For locator TLV, the is X-flag obtained from Prefix Attribute Flags 
Sub-TLV, unlike the TLVs 236 and 237. I will add the text to clarify 
that difference.

thanks,
Peter

> 
> both TLVs.
> 
>  2. RFC7794
> 
> Prefix Attribute Flags
> 
>       Type:   4
> 
>       Length: Number of octets of the Value field.
> 
>       Value:
> 
>            (Length * 8) bits.
> 
>         0 1 2 3 4 5 6 7...
> 
>        +-+-+-+-+-+-+-+-+...
> 
>        |X|R|N|          ...
> 
>        +-+-+-+-+-+-+-+-+...
> 
>     Bits are defined/sent starting with Bit 0 defined below.  Additional
> 
>     bit definitions that may be defined in the future SHOULD be assigned
> 
>     in ascending bit order so as to minimize the number of bits that will
> 
>     need to be transmitted.
> 
>     Undefined bits MUST be transmitted as 0 and MUST be ignored on
> 
>     receipt.
> 
>     Bits that are NOT transmitted MUST be treated as if they are set to 0
> 
>     on receipt.
> 
>     X-Flag:  External Prefix Flag (Bit 0)
> 
>        Set if the prefix has been redistributed from another protocol.
> 
>        This includes the case where multiple virtual routers are
> 
>        supported and the source of the redistributed prefix is another
> 
>        IS-IS instance.
> 
>        The flag MUST be preserved when leaked between levels.
> 
>    In TLVs 236 and 237, this flag SHOULD always be sent as 0and MUST
> 
>        be ignored on receipt.  This is because there is an existing X
> 
>        flag defined in the fixed format of these TLVs as specified in
> 
> [RFC5308 <https://tools.ietf.org/html/rfc5308>] and [RFC5120 
> <https://tools.ietf.org/html/rfc5120>].
> 
> G/
>