Re: [Idr] Questions to RFC5512: Encapsulation sub-TLV and Opaque extended community to indicate the Encapsulation protocol?

[Eric C Rosen <[e-mail]>]

I think it would be better if you looked at draft-ietf-idr-tunnel-encaps 
instead of RFC 5512.  The draft has passed WG LC, and if it ever gets 
sent to the IESG, it will obsolete RFC 5512.

On 6/26/2018 1:56 PM, Linda Dunbar wrote:
>
> BGP experts:
>
> If the RFC5512 has “distinguished SAFI value” and “the Encapsulation 
> SAFI”. Do those two terms have the same meaning?
>
> The Section 3 of RFC5512 has SAFI value of 7 to represent 
> Encapsulation SAFI.
>
> Does it mean that the “distinguished SAFI value” is just “7” for 
> speaker to advertise its supported Tunnel information?
>

Yes, per RFC 5512 the Tunnel Encapsulation attribute can only be carried 
on UPDATEs of SAFI 7.

Draft-ietf-idr-tunnel-encaps deprecates this SAFI, and explicitly allows 
the Tunnel Encapsulation attribute to be carried on UPDATEs whose 
AFI/SAFI is 1/1, 1/4, 2/4, 1/128, 2/128, and 25/70.  Use of the 
attribute on other AFI/SAFIs is outside the scope of the draft.

> The Section 4 goes on defining the Tunnel Encapsulation Type (such as 
> L2TPv3 with Type =1; etc), and a list of sub-TLVs, one of the SubTLV 
> is Protocol Type (section 4.2) which can be used to represent the 
> Encapsulation Protocol (i.e. protocol type of data frames carried by 
> the tunnel)
>

Just to be clear, the Protocol Type sub-TLV identifies the protocol type 
of the payload.  If a Protocol Type sub-TLV identifying "IPv4" appears 
inside a Tunnel Type of "GRE", the meaning is that IPv4 packets may be 
carried inside a GRE tunnel.

> Why need the Opaque extended community to indicate the Encapsulation 
> protocol?
>

Well, the argument was "I just want to indicate that packets directed to 
a given prefix need to be encapsulated in GRE.  I don't want to have to 
use a new SAFI to say that, all it takes is putting a community on the 
UPDATE that mentions that prefix in its NLRI". That's fine as long as 
you don't want to convey any additional information about how to 
construct the encapsulation.  In practice, people have invented 
additional extended communities to carry information that should have 
been carried in the Tunnel Encapsulation attribute, in order to avoid 
the use of SAFI 7.

This extended community is not really necessary when 
draft-ietf-idr-tunnel-encaps is used, but it is preserved for backwards 
compatibility purposes; see section 4.5 of that draft.