Re: [Lsr] WG Last Call draft-ietf-lsr-isis-srv6-extensions

[Peter Psenak <[e-mail]>]

Hi Chris,

please see inline (##PP)

On 29/01/2020 17:25, Chris Bowers wrote:
> I would like to proposed the following text to make section 6 more clear.
> 
> Thanks,
> Chris
> 
> ====================
> 
>   (existing text)
> 
> 
> 6.  Advertising Anycast Property
> 
>     Both prefixes and SRv6 Locators may be configured as anycast and as
> 
>     such the same value can be advertised by multiple routers.  It is
> 
>     useful for other routers to know that the advertisement is for an
> 
>     anycast identifier.
> 
>     A new flag in "Bit Values for Prefix Attribute Flags Sub-TLV"
> 
>     registry [RFC7794] is defined to advertise the anycast property:
> 
>         Bit #: 4 (Suggested - to be assigned by IANA)
> 
>         Name: Anycast Flag (A-flag)
> 
>         When the prefix/SRv6 locator is configured as anycast, the A-flag
> 
>         SHOULD be set. Otherwise, this flag MUST be clear.
> 
>     The A-flag MUST be preserved when leaked between levels.
> 
>     The A-flag and the N-flag MUST NOT both be set.
> 
> ==== start insert new text =======
> 
> 
> Certain use cases require prefixes/locators that uniquely belong to a node.
> 
> Since prefixes/locators which are not /128 should not have the N bit set,
> 
> this node local uniqueness is decided based on A bit for non-/128 prefixes.

##PP
above does not seem correct. Above seems to imply that for non-/128 
prefix, A-bit is replacement of N-bit.

A-bit applies to both /128 and non-/128 prefixes equally.

Current draft clearly states what to do when both N a A bits are set.



> 
>     When a prefix/locator iscategorized as anycast, it does not uniquely 
> belong
> 
>     to a node and cannot be used for such use cases.  The rules below 
> specify
> 
>     how to determine whether or not a prefix/locator should be treated 
> as anycast
> 
>     in various situations.
> 
> 
>     [RFC7794] contains the following restriction on the interpretation of the N-flag.
> 
>     "If the flag is set and the prefix length is NOT a host prefix
> 
>      (/32 for IPV4, /128 forIPv6), then the flag MUST be ignored."
> 
>     The current document does NOT modify this restriction on the interpretation of
> 
>     the N-flag imposed by [RFC7794].

##PP
I don't think above text is needed. And I don't think above is 
completely correct, as we define a new case in which the N-bit should be 
ignored (when A-bit is set).


> 
>     For a prefix/SRv6 Locator advertisement with a /128 prefix/locator,
> 
>     if both N-flag and A-flag are set, the receiving router MUST treat the
> 
>     prefix advertisement as anycast.

##PP
we have following text in the draft already:

"If both N-flag and A-flag are set in the prefix/SRv6 Locator
    advertisement, the receiving routers MUST ignore the N-flag."



> 
>     For a prefix/SRv6 Locator advertisement with a /128 prefix/locator,
> 
>     if the N-flag and A-flag are NOT set, the receiving routers
> 
>     MUST treat the prefix advertisement as anycast.  

##PP
I don't think above statement is correct. Why a node cannot advertise a 
/128 prefix which is not an anycast one and does not have a N-bit set?



> This rule ensures the
> 
>     correct interpretation of a prefix advertisement originated by
> 
>     a router that is not SRv6 capable and originates a legacy
> 
>     Prefix Attribute Flags Sub-TLV based on [RFC7794] alone.
> 
>     For a prefix/SRv6 Locator advertisement with a prefix/locator that
> 
>     is NOT /128, the N-flag must be ignored, so the setting of the
> 
>     A-flag determines the anycast treatment of the prefix advertisement.

##PP
A-flag does that regardless of the length of the prefix.


> 
>     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 both TLVs.

##PP
Above seems a good suggestion. Will add it.

> 
>   
> 
>     If a router receives one Prefix Attribute Flags Sub-TLV in the
> 
>     Prefix Reachability TLV and another in the SRv6 Locator TLV, the router should
> 
>     use the prefix attribute flags received in the Prefix Reachability TLV.

##PP
above contradicts what you suggest in the previous paragraph, where you 
suggest we need to advertise with both prefix and locator, and here you 
suggest we ignore what we received in the locator.

Are you talking about the case where the content of the Prefix Attribute 
Flags Sub-TLV is different in prefix vs locator?
> 
>   
> 
>     If a router receives a Prefix Attribute Flags Sub-TLV in the
> 
>     Prefix Reachability TLV but not in the SRv6 Locator TLV, the router should
> 
>     use the prefix attribute flags received in the Prefix Reachability TLV.

##PP
do we really need this? If the originator does the right thing, then we 
don't have the problem. Cross referencing data between different TLVs 
complicates the implementations.


> 
>   
> 
>     If a router receives a Prefix Attribute Flags Sub-TLV in the
> 
>     SRv6 Locator TLV but not in the Prefix Reachability TLV,
> 
>     the router should use the prefix attribute flags received in the SRv6 Locator TLV.

##PP
same as above.

thanks,
Peter

> 
> ==== end insert new text =========
> 
>     The same prefix/SRv6 Locator can be advertised by multiple routers.
> 
>     If at least one of them sets the A-Flag in its advertisement, the
> 
>     prefix/SRv6 Locator SHOULD be considered as anycast.
> 
> 
> 
> ===================
> 
> 
> On Tue, Jan 21, 2020 at 6:15 PM Christian Hopps <[e-mail] 
> <mailto:[e-mail]>> wrote:
> 
>     This begins a 2 week WG Last Call, ending after Feb 4, 2020, for
>     draft-ietf-lsr-isis-srv6-extensions
> 
>     https://datatracker.ietf..org/doc/draft-ietf-lsr-isis-srv6-extensions/
>     <https://datatracker.ietf.org/doc/draft-ietf-lsr-isis-srv6-extensions/>
> 
>     Authors please indicate if you aware of any other IPR beyond what is
>     posted:
> 
>     https://datatracker.ietf.org/ipr/3796/
> 
>     Thanks,
>     Chris & Acee.
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