IP Fragmentation - Not reliable over the Internet?

Owen DeLong owen at delong.com
Thu Aug 29 02:22:28 UTC 2013


Has the path MTU been measured for all vantage point pairs?

Is it known to be 1500 or just the end-point MTUs?

That could affect your results very differently.

Owen

On Aug 28, 2013, at 02:26 , Emile Aben <emile.aben at ripe.net> wrote:

> On 28/08/2013 08:05, Tore Anderson wrote:
>> * Owen DeLong
>> 
>>> On Aug 27, 2013, at 07:33 , Valdis.Kletnieks at vt.edu wrote:
>>> 
>>>> Saku Ytti and Emile Aben have numbers that say otherwise.  And there must
>>>> be a significantly bigger percentage of failures than "pretty close to 0",
>>>> or Path MTU Discovery wouldn't have a reputation of being next to useless.
>>> 
>>> No, their numbers describe what happens to single packets of differing sizes.
>>> 
>>> Nothing they did describes results of actually fragmented packets.
>> 
>> Yes, it did.
>> 
>> Hint: 1473 + 8 + 20
> 
> For Saku: yes. For me: that was my intention, but later I discovered the
> Atlas ping does include the ICMP header in it's 'size' parameter so what
> I did in effect was 1473 + 20 = 1493 (and not the 1501 I intended).
> 
> Redid the tests to a "known good" destination where I knew interface MTU
> (1500) and could tcpdump which confirmed that I was looking at
> fragmentation. I also took an offline recommendation to do different
> packet sizes to try to distinguish fragmentation issues from general
> corruption-based packet loss.
> 
> Results:
> size = ICMP packet size, add 20 for IPv4 packet size
> fail% = % of vantage points where 5 packets where sent, 0 where received.
> #size	fail%   vantage points
> 100	0.88	2963
> 300	0.77	3614
> 500	0.88	1133
> 700	1.07	3258
> 900	1.13	3614
> 1000	1.04	770
> 1100	2.04	3525
> 1200	1.91	3303
> 1300	1.76	681
> 1400	2.06	3014
> 1450	2.53	3597
> 1470	3.01	2192
> 1470	3.12	3592
> 1473	4.96	3566
> 1475	4.96	3387
> 1480	6.04	679
> 1480	4.93	3492 [*]
> 1481	9.86	3489
> 1482	9.81	3567
> 1483	9.94	3118
> 
> There is a ~5% difference going up from 1480 to 1481.
> 
> As to interpreting this: Leo Bicknell's observations (this is to a
> "known good" host, and the RIPE Atlas vantage points may very well have
> a clueful-operator bias) stand, so interpret with care. Also: roughly
> 2/3 of these vantage points are behind NATs that may also have some
> firewall(ish) behaviour.
> 
> Hope this data point helps interpreting the magnitude of IPv4
> fragmentation problems.
> 
> Emile Aben
> RIPE NCC
> 
> [*] redid the 'size 1480' experiment because the first time around it
> had significantly less vantage points.




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