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

Peter Psenak <ppsenak@cisco.com> Wed, 24 February 2021 14:33 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 15:33:18 +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 15:28, Van De Velde, Gunter (Nokia - BE/Antwerp) wrote:
> Thanks Peter.
> 
> With algo-0 SRv6, then after the draft is updated, it will be allowed that the
> attribute flags are none-identical between locator-tlv (27) and TLV236/237?
> Is that understanding correct?

X-flag:

- for locator it is set in Prefix Attribute Flags Sub-TLV
- for  TLVs 236 and 237 it is set in the internal flags field

Nothing really changes, above was the only way to set X-flag and it will 
continue to be so.

We will just not mandate the "identical" Prefix Attribute Flags Sub-TLV 
in the two.

thanks,
Peter


> 
> G/
> 
> 
> -----Original Message-----
> From: Peter Psenak <ppsenak@cisco.com>
> Sent: Wednesday, February 24, 2021 16:39
> To: Van De Velde, Gunter (Nokia - BE/Antwerp) <gunter.van_de_velde@nokia.com>om>; draft-ietf-lsr-isis-srv6-extensions@ietf.org
> Cc: lsr@ietf.org
> Subject: Re: Clarification on inconsistency between RFC7794 and draft-ietf-lsr-isis-srv6-extensions
> 
> 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/
>>
> 
> 
>