Re: [tcpm] Q&C regarding tcpsecure-09 recommendations

[Joe Touch <[e-mail]>]

Lars Eggert wrote:
> Hi,
> 
> On 2008-6-2, at 20:41, ext Joe Touch wrote:
>> This is, IMO, a signal to review the recommendations in Section 1.1. I 
>> was always concerned that these mitigations would be misinterpreted as 
>> applying to hosts in general, which they do not.
> 
> do you think that a short paragraph in Section 1.1 that discusses the 
> downsides of implementing the checks where not needed would be helpful?

Yes.

> Currently the document says "The mitigations suggested in this draft 
> SHOULD be implemented in devices where the TCP connections are most 
> vulnerable to the attacks described in this document. (...) These 
> mitigations MAY be implemented in other cases." I think you're saying 
> that the last sentence may not be detailed enough to allow implementors 
> to judge the tradeoffs?

Yes, that's partly it. The key is that there are two issues that don't 
come out just because they're ref'd off to the document (i.e., 
"described in this document" isn't sufficient; it's important to 
summarize these):

	1. these mitigations help only a very small class of situations
	that may be typical for unprotected routers, but are currently
	very rare otherwise.

	2. of particular recent note, these mitigations are not useful
	to on-path attacks, such as ISP-issued RSTs

	2. adding these mitigations complicates the TCP implementation,
	and makes it less robust to legitimate RSTs, esp. in the
	presence of reordering; it can increase the time needed to
	reset a connection by one or more RTTs

Joe

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