real-world data about fragmentation

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1

Hi all,

It's common wisdom that a datagram that needs to be fragmented between endpoints (because it is bigger than the path MTU) will demonstrate less reliable delivery and reassembly than a datagram that doesn't need to be fragmented, because math, firewall, other, take your pick.

Is anybody aware of any wide-scale studies that examine the probability of fragmentation of datagrams of different sizes?

For example, I could reasonable expect an IPv4 packet of 576 bytes not to be fragmented very often (to choose a size not at random). The probability of a 10,000 octet IPv4 packet getting fragmented seems likely to be 100%, if we're talking about arbitrary paths across the Internet.

What does the curve look like between 576 bytes and 10,000 bytes?

I might expect exciting curve action around 1500 bytes (because ethernet), 1492 (PPPoE), 1480 (GRE), etc. But I'm interested in actual data.

Anybody have any pointers? IPv4 and IPv6 are both interesting.

Joe

2

I can send you a copy of an invited presentation at AINTEC from 2009.

/bill

3

This isn't a direct answer to the question, but I find this paper pretty useful (even though it is dated now):

  Beyond Folklore: Observations on Fragmented Traffic
  by Colleen Shannon, David Moore, and k claffy
  IEEE/ACM Transactions on Networking, December 2002
  http://www.caida.org/publications/papers/2002/Frag/frag.pdf

(Bill, I'd be curious to see your AINTEC slides, too.)

-- Jen

4

Hi, Joe,

Is anybody aware of any wide-scale studies that examine the
probability of fragmentation of datagrams of different sizes?

We're in the process of measuring some (kind of related stuff). If
you're interested in this data, we might be able to provide something
along these lines in 1 month or so...

It seems to be mostly about measuring the MTU to as many destinations as
possible, so to speak...

For example, I could reasonable expect an IPv4 packet of 576 bytes
not to be fragmented very often (to choose a size not at random).

Note: there shouldn't be any special magic around this number (usualy
mistakenly interpreted as the minimum IPv6 MTU, but rather being the
minimum IPv4 reassembly buffer size).

The
probability of a 10,000 octet IPv4 packet getting fragmented seems
likely to be 100%, if we're talking about arbitrary paths across the
Internet.

What does the curve look like between 576 bytes and 10,000 bytes?

I might expect exciting curve action around 1500 bytes (because
ethernet), 1492 (PPPoE), 1480 (GRE), etc. But I'm interested in
actual data.

Anybody have any pointers? IPv4 and IPv6 are both interesting.

Probably off-topic, but since you mentioned reliability of IPv6
fragmentation:

5

Seems a good thing for RIPE Atlas probes to measure. But they are probably not generally connected to representative networks (read: poor networks).

-d