Subject: [Dtls-iot] Fwd: [Int-area] Quick MTU measurements

From: Hannes Tschofenig <[email-masked]>
Date: Thu, 18 Dec 2014 16:03:15 +0100

Just got this mail; very timely.

-------- Forwarded Message --------
Subject: [Int-area] Quick MTU measurements
Date: Thu, 18 Dec 2014 15:58:13 +0100
From: Iljitsch van Beijnum <[email-masked]>
To: [email-masked]

Hi all,

We had some discussions on MTU issues on the v6ops list, which prompted
me to do some quick measurements, please see:

Maximum packet sizes on the internet
http://www.bgpexpert.com/article.php?article=151

Internet packet sizes part 2: IPv4 path MTU discovery is dead
http://www.bgpexpert.com/article.php?article=152

Quick summary: all IPv6 hosts and 99.7% of IPv4 hosts advertise they can
handle at least 1280 bytes in their TCP MSS and about 65% can handle 1500.

In almost a week, I didn't get a single IPv4 too big, so it seems that
in practice, IPv4 PMTUD is no longer used. (Please confirm with a larger
dataset before taking any actions based on this!) However, depending on
PMTUD doesn't lead to obvious problems with IPv6.
_______________________________________________
Int-area mailing list
[email-masked]
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From: Rene Struik <[email-masked]>
Date: Thu, 18 Dec 2014 11:08:48 -0500

Hi Hannes:

I was intrigued by the following snippet in your second pointer:

    "With IPv6, if you want to send packets larger than 1280 bytes,
    PMTUD is mandatory, as routers aren't allowed to fragment IPv6 packets."

Does this have implications if the MAC layer only supports small frame 
sizes (such as 127 bytes with classic 802.15.4)?

Best regards, Rene

On 12/18/2014 10:03 AM, Hannes Tschofenig wrote:
> dtls-iot mailing list
> [email-masked]
> https://www.ietf.org/mailman/listinfo/dtls-iot

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From: Hannes Tschofenig <[email-masked]>
Date: Mon, 22 Dec 2014 09:33:11 +0100

Hi Rene,

First, I believe the analysis by Iljitsch does not show an any IoT
deployments that use IEEE 802.15.4 radios (or IPv6 over similar low
power radios). Hence, he came to the conclusion that we pretty much have
an MTU size of at least 1280 bytes.

In many deployments the path MTU might well be the MTU of the link the
IoT device is attached to the network.

Why there aren't any low power radios showing up in this analysis is an
interesting question. There are probably a number of reasons, including
networks that are separated from the rest of the Internet (using
firewalls), few IEEE 802.15.4 network deployments, IPv6 over Bluetooth
Smart just getting started, many IoT deployments using IPv4 or radios
that are not low power, etc.

In context of our work there are a few questions:

* Is the recommendation to do a path MTU discovery before the DTLS
exchange meaningful? (Consider that the text below  preliminarily
concludes that 'IPv4 PMTUD is no longer used'.)

* Is there really a problem with overlapping fragments since you will
have to be prepared to buffer at minimum a DTLS message anyway and at
maximum an entire flight.

Ciao
Hannes