Re: [Spud] Putting Network-Layer Information in the Network Layer

[Joe Touch <[e-mail]>]

On 7/20/2015 9:46 AM, Toerless Eckert wrote:
> On Mon, Jul 20, 2015 at 09:22:38AM -0700, Joe Touch wrote:
>> Yes and no. UDP has a length field, which means it's actually possible
>> to use space in an IP payload that isn't part of the UDP frame without
>> compliant UDP implementations breaking.
> 
> It's a nice possible trick, but it would break the ability to build an
> app purely on top of unmodified UDP APIs.

An unmodified UDP API, by definition, would not be able to do anything
except use vanilla UDP.

> Those APIs do not provide you
> with the ability to create a UDP packet with UDP payload + "metadata"
> payload after the UDP length. So you'd need to get support for this API extensions
> again in all OSs and programming language SDKs. 8 years minimum. 

It didn't take that long for MPTCP.

> Worse yet,
> there are likely  a good set of firewalls that would drop these packets.

Firewalls typically either look at UDP ports or part of the UDP content.
It might complicate firewalls that try to parse the entire IP payload as
if it were UDP, i.e., a device that didn't process the UDP length field
correctly. But that's a bug - we really can't build protocols to operate
over devices that do such things.

Joe