Re: [v6ops] PMTUD issue discussion

[Ray Hunter <[e-mail]>]

Fred Baker (fred) wrote:
> On Aug 26, 2014, at 2:50 AM, Andrew 👽 Yourtchenko<[e-mail]>  wrote:
>
>> However, the value of 1 causes a large value of MSS to be used,
>> resulting in about 1.5 second hiccup. So, yeah it "works" but for
>> barely acceptable values of "works"
>>
>> Setting the value to 2 causes the initial MSS to be 512, which
>> "mitigates" the problem and avoided the hiccups on a 100kb file I was
>> testing with - at the expense of almost 3x more packets of course.
>>
>> I think it would be useful to see the discussion of this measure and
>> its applicability/tradeoffs in the draft.
>
> It seems like it might have value to also test the interaction with various initial window settings, and look at its inclusion in other relevant OS’s including FreeBSD, Windows, and MacOSX. My reading of 4821 says it’s grand theory, but I’m not precisely sure how I would write the code. As an initial setting, for example, I could imagine sending an initial burst containing a 9K byte packet, a 1500 byte packet, and a 1280 byte packet in that order, and set the MSS to the the size of the first acknowledgment I receive. How does that relate to IW=10 (RFC 6928)?

Whilst reading your discussion, I had a "what if" brainwave.

Paths are learned via routing. What if Path MTU was also learned via 
routing, rather than being probed for by individual end hosts?

Then I checked: EIGRP discovers the set of 6 metrics (bandwidth, load, 
delay, reliability, minimum MTU, hop count)

MTU is never used in the routing decision AFAICS, but couldn't the local 
router be programmed to generate ICMP PTB messages (as a proxy for 
downstream routers) with a link-local source address to each host if a 
path MTU is going to be exceeded, plus a hint of the actual PMTU 
discovered by routing?

Thus avoiding ICMP PTB message black holes?And avoiding the PLMTUD 
discovery process and extensive probing/ ramp up for every session?
And also avoiding trust issues (as hosts presumably trust their local 
on-link default router)?

Obviously for this to work internet wide, BGP would also need to carry 
an AS-PMTU attribute per AS path.

-- 
Regards,
RayH