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

[address@hidden]

Am 09.03.2020 um 18:08 schrieb 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.

That will be for TCP only? Since we don't suppor MTU path discovery
(yet?), that's of no use to us.
>
> 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."

That could be an option for lwIP. The real problem is that there doesn't
seem to be a standard way for this, which is why we don't support it
yet. But I guess adding this for IPv4 UDP and RAW would be a valid option.

Regards,
Simon

>
> 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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