Re: [Idr] Bug in draft-ietf-idr-rfc5575bis, worth fixing?

[John Scudder <[e-mail]>]

Hi Bruno,

Thanks for your comments. Some replies in line.

On Sep 25, 2020, at 12:56 PM, [e-mail]<mailto:[e-mail]> wrote:

...
Just as a side note, “error handling according to Section 10” points us to RFCs 7606 and 4760, which end up telling us to reset the session if the NLRI is malformed.

[Bruno] Thanks for providing the full picture and hence the importance of the point.
_IMHO_, RFC 7606 calls for session reset as a ‘last’  resort error handling behaviour.
8.  Guidance for Authors of BGP Specifications
[…]
The "treat-as-withdraw" approach is generally preferred and the "session reset" approach is discouraged.
https://tools.ietf.org/html/rfc7606#section-8<https://urldefense.com/v3/__https://tools.ietf.org/html/rfc7606*section-8__;Iw!!NEt6yMaO-gk!VbvkPXEkoG_mG12V_w5W4g3uJjg0nhKD6gBw8TalRXnfznNSdAmgpOnjLHVOBw$>

Treat-as-withdraw cannot be performed when the NLRI can’t be found in the update, or the NLRI can’t be effectively withdrawn typically because the peer won’t accept it in the withdraw (e.g. and IPv4 address of 5 octets). Or (my reading) we feel that the received NLRI seem so wrong that what we read may not be what was intended to be send. IOW, there is an error in the syntax.

Exactly right.

IHMO, RFC 7606 specifies “session reset” when the MP_REACH_NLRI attribute is malformed or the NLRI is syntactically incorrect.

I’m not familiar with FS, but so far, I don’t feel that an unknown component type in FS:
- prevents the “treat-as-withdraw” approach.
- matches the RFC 7606 description of a syntactically incorrect NLRI field https://tools.ietf.org/html/rfc7606#section-5.3<https://urldefense.com/v3/__https://tools.ietf.org/html/rfc7606*section-5.3__;Iw!!NEt6yMaO-gk!VbvkPXEkoG_mG12V_w5W4g3uJjg0nhKD6gBw8TalRXnfznNSdAmgpOmjo43yJQ$>

My reading was that since the FS document carefully says “is considered malformed” and then references Section 10, which in turn references 7606 and 4760, and since 7606 and 4760 both say “reset the session if it’s malformed” that a session reset was called for. I think that is the right thing, too: FS uses T,V and not T,L,V for its component types, the lengths are implicit. So, if my parser encounters an unknown type code it is impossible for me to know how to skip over that type code. At that point, parsing breaks down.

Also, I don’t see capability negotiation so how is the sender supposed to know the list of component types which is supported by the receiver?

It’s not, this is a deficiency in FS that I think the WG needs to address in the proposed FSv2 work.

Regards,

—John