Re: Comments on draft-herbert-ipv6-update-dest-ops

[Tom Herbert <[e-mail]>]

On Mon, Aug 20, 2018 at 12:07 PM, Ole Troan <[e-mail]> wrote:
> Tom,
>
>> Consider the folling scenario:
>>
>> Someone is developing a new Destination Option that might be of
>> interest for use with tunnels and might even become one of those
>> options "that makes sense to support" . Maybe it's an option for the
>> aforementioned insitu-OAM, or maybe a general packet CRC, etc. The
>> developer dutifully uses experimental option type 0x1e for the type
>> value of their option to test it. In particular, the action taken if
>> the option is unknown to a receiver is to skip over it. They made that
>> choice because that allows incremental deployment of the support for
>> the option in any combination supporting sending side or receivers. In
>> other words, they don't want a flag day for the option that requires
>> all nodes to support the new option.
>>
>> Now they go to test backwards comapatbility of the option by sending
>> it to a node that hasn't been updated to receive it. If the receiving
>> node is a Linux system, then the option is ignored and the tunneled
>> packet is processed as before-- expected behavior per the spec.
>> However, if it's sent to a VPP node then the packet is dropped-- not
>> expected. Even if this doesn't lead to incorrectness, it does breaks
>> interoperability and violates "be liberal in what you receive". This
>> net effect is that this makes development, test, and pilot deployment
>> of new options really hard. Just implementing the TLV loop, even if
>> the implementation doesn't process any of them, would resolve this.
>
> Absolutely. The set of _optional_ destination options would benefit from this approach.
> Of which we have none. I’d much rather just implement new options as they come available (if they ever will)

Ole,

I'm not sure what "comes available" mean here. Does that mean VPP
would only support TLVs parsing once one becomes standardized and the
VPP maintainers think is worth supporting? As shown in the scenario
above, with that approach its difficult to develop optional TLVs if
implementations don't properly skip over them.

> If I get time, I’ll try to do some performance measurements on what the actual cost is for a software based system.
>
Our testing in Linux showed that the performance hit is heavily
dependent on the number of TLVs to be processed. Processing a small
number of TLVs was fairly inconsequential relative the rest of the
header processing. A a packet filled with tiny TLVs so that a single
MTU sized packet has hundreds of them will kill performance on any
software or hardware system and really has no other practical purpose
other than to be a DOS attack. This why we needed to add limits in
RFC6434bis. In Linux, the default  limit of number of options a stack
will process is eight.

Tom

> Best regards,
> Ole