problems with complex import/export attributes

Frank Bohnsack (Frank.Bohnsack at de.uu.net) on February 1:
> Hi,
> 
> Yes, I know this is not a RPSL forum, but i think someone
> of you could help me ;-)
> 
> I found a problem for RPSL parsers conform with RFC2622. In the
> RFC2622 (section 6.6) is an example of an import attribute:
> 
>  > import: from AS1 action pref = 1; accept as-foo;
>  >         except {
>  >            from AS2 action pref = 2; accept AS226;
>  >            except {
>  >               from AS3 action pref = 3; accept {128.9.0.0/16};
>  >            }
>  >         }
> 
> Appendix B "Grammar Rules" describes the syntax like below:
> 
>  > ...
>  > import_attribute: ATTR_IMPORT
>  > | ATTR_IMPORT opt_protocol_from opt_protocol_into import_factor
>  >

This is actually my carelessness. The first rule is cut short by a
script I wrote that generated the appendix from the lex rules in
rpsl.y. The long line that is cut is:

import_attribute: ATTR_IMPORT opt_protocol_from opt_protocol_into import_expression
| ATTR_IMPORT opt_protocol_from opt_protocol_into import_factor

We need both lines. Because the first line always requires a ; at the
end. 

>  > import_expression: import_term
>  > | import_term KEYW_REFINE import_expression
>  > | import_term KEYW_EXCEPT import_expression
>  >
>  > import_term: import_factor ';'
>  > | '{' import_factor_list '}'
>  >
>  > import_factor_list: import_factor ';'
>  > | import_factor_list import_factor ';'
>  >
>  > import_factor: import_peering_action_list KEYW_ACCEPT filter
>  >
>  > import_peering_action_list: KEYW_FROM peering opt_action
>  > | import_peering_action_list KEYW_FROM peering opt_action
>  >
>  > ...
> 
> I feel in import_attribute lakes a line like:
> 
>  | ATTR_IMPORT opt_protocol_from opt_protocol_into import_expression
> 
> and a look into ftp://ftp.isi.edu/ra/RAToolSet/RAToolSet-4.6.3.tar.gz
> file src/rpsl/rpsl/rpsl.y agree with me:
> 
>  > ...
>  > import_attribute: ATTR_IMPORT
>  >                   opt_protocol_from opt_protocol_into
>  >                   import_expr TKN_EOA {
>  >    $$ = changeCurrentAttr(new AttrImport($2, $3, $4));
>  > }
>  > | ATTR_IMPORT opt_protocol_from opt_protocol_into import_factor TKN_EOA {
>  >    PolicyTerm *term = new PolicyTerm;
>  >    term->append($4);
>  >
>  >    $$ = changeCurrentAttr(new AttrImport($2, $3, term));
>  > }
>  > | ATTR_IMPORT error TKN_EOA {
>  >    $$ = $1;
>  >    handle_error("Error: from <peering> expected.\n");
>  >    yyerrok;
>  > }
>  > ;
>  > ..
> 
> OK as so far, but rpslcheck as well as my perl parser module breaks
> while parsing the example above ;-(
> 
>  > fb at sun5 1315> rpslcheck < example 
>  > ...
>  > import: from AS1 action pref = 1;
>  >         accept as-foo;
>  >         except {
>  >           from AS2 action pref = 2; accept AS226;
>  >            except {
>  >            ^^^^^^
>  > ***Error: from <peering> expected.
>  >             from AS3 action pref = 3; accept {128.9.0.0/16};
>  >           }
>  >        }
>  > ...
> 

The example is also wrong. If careful examination of the grammar
suggests that one can only sequence the excepts, but not nest them
inside {}'s. The example should have been:
     
   import: from AS1 action pref = 1;
	   accept as-foo;
	   except {
	     from AS2 action pref = 2; accept AS226;
	   }  
	      except {
	       from AS3 action pref = 3; accept {128.9.0.0/16};
	     }

This is because the moment you use {, you can only have an import-term
in it, which can not have an except, because that is a feature of an
import-expression. 

They are evaluated right to left, which is similar to nesting.

> For me looks the grammar correct, but my knowledge about automata theory,
> languages and computation is worser than my english ;-)
> 
> Has anybody a solution for this problem ?
> 
> 
> cheers
> frank

Thanks for noticing. I will fix these when we get a permission to
correct this kind of mistakes.

Cengiz

-- 
Cengiz Alaettinoglu           Packet Design Inc.