Re: [lwip-users] Do not fragment (DF) in a UDP transmission.

[Roger Cover]

Greetings Simon,

The Linux man pages mention the IP_PMTUDISC_DO flag that "forces the 
don't-fragment flag to be set on all outgoing packets" in the setsockopt() 
description.

The documentation for setsockopt() for BSD 4.3 lists the similar flag 
"IP_DONTFRAG", with the qualifier "This option is supported for sockets with an 
address family of AF_INET and type of SOCK_DGRAM or SOCK_RAW only."

The Microsoft documentation at: 
https://docs.microsoft.com/en-us/windows/win32/winsock/ipproto-ip-socket-options
 lists an option called IP_DONTFRAGMENT, about which they state "Microsoft 
TCP/IP providers respect this option for UDP and ICMP."

There is also a reference for the .NET framework at 
https://docs.microsoft.com/en-us/dotnet/api/system.net.sockets.socket.dontfragment?view=netframework-4.8
 if you have any interest in that.

There is also a comment I do not completely understand in the lwIP ip.c file 
(line 790 in the ip_output_if() function, version 1.4.1) that states "don't 
fragment if interface has mtu set to 0."

I am not very familiar with sockets programming. I did some 20 years ago, but I 
am sure it has changed since then. I have been using the lwIP callback API in 
all my products for the last 14 years. Since I am using the callback API, I 
will need something that works through that API. I hope this information is 
sufficient. I am very willing to patch my system and do testing for you. I just 
need some guidance about where to make the changes. I have been using lwIP for 
some time, but I have not paid that much attention to its interior. It has 
always just worked for me, straight out of the box.

Regards,
Roger W. Cover
________________________________________
From: lwip-users <lwip-users-bounces+rcover=address@hidden> on behalf of 
address@hidden <address@hidden>
Sent: Saturday, March 7, 2020 2:02 AM
To: Mailing list for lwIP users
Subject: Re: [lwip-users] Do not fragment (DF) in a UDP transmission.

Am 06.03.2020 um 21:37 schrieb Roger Cover:
> Greetings,
>
> I am writing a video transmitter. The protocol I am using requires that a 
> "test packet" has its do not fragment bit set. This is used to determine the 
> maximum usable MTU of the intervening network nodes.
>
> I would like to know the recommended method for setting this bit in my UDP 
> transmission. I am currently using the ip_output_if() function, and it does 
> not have any arguments to allow this.

That's currently not supported (hasn't been requested so far). If you
tell me a standard way to do that (e.g. how is it done on sockets), I
guess adding this should not be too hard.

Regards,
Simon

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