Re: Comments on draft-shaikh-rtgwg-policy-model

[Jeffrey Haas <[e-mail]>]

Stephane,

> On Jul 20, 2015, at 2:21 PM, <[e-mail]> <[e-mail]> wrote:
> 
> [SLI] Do we really need to differentiate from a policy point of view ? from an import policy perspective, matching a tag, means learning the tag value available in the protocol (if available) and when the route ins inserted into RIB the tag value is copied from the protocol value if not overrided by import policy action; from an export policy perspective (talking about export from rib to protocol), matching a tag means matching the tag value in the RIB (which may come from protocol or not),  setting a tag means fill the protocol field if available. From a RIB point of view, the tag associated with the route is protocol agnostic, even if the protocol does not support tags in encoding you may associate a local tag for policy processing.
>  
> Having two types of tags is also possible : protocol-tag and local-tag but I see more complexity and do not see more flexibility : but maybe there is some use case that I do not see.

The messy detail with this attribute is that while it's useful as a generic policy element, in specific protocol context it needs to have differing constraints.  OSPF has one set of constraints, RIP a slightly different one (zero is reserved), and ISIS has different sizes with some option for one or two tags plus the 64-bit tag previously discussed.

This set of context specific constraints probably removes some level of the flexibility that you'd want for it to be a generic marker - unless you can live within the least common denominator.

-- Jeff