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

[Roger Cover]

Greetings Simon,

I have implemented this by adding a flag bit for the struct pbuf field "flags". 
There were two unused bits, so I named one PBUF_FLAG_DF. I then altered ip.c to 
use this flag to decide whether to set the DF flag in the IP packet header upon 
transmission. This method only added 2 lines to pbuf.h and added 3 lines and 
altered one in ip.c.

Here is the diff for pbuf.h (I am using lwIP 1.4.1):
diff -r 
C:\Users\RCover\Documents\C_Source\9870\lwIP_1_4_1_lib\src\include\lwip\pbuf.h 
M:\DRAWINGS\9870-_ECO3389\9870-.zip\lwIP_1_4_1_lib\src\include\lwip\pbuf.h
78,79d77
< /** indicates the IP header for this pbuf should have the DF flag set on 
transmission */
< #define PBUF_FLAG_DF          0x40U

Here is the diff for ip.c:
diff -r 
C:\Users\RCover\Documents\C_Source\9870\lwIP_1_4_1_lib\src\core\ipv4\ip.c 
M:\DRAWINGS\9870-_ECO3389\9870-.zip\lwIP_1_4_1_lib\src\core\ipv4\ip.c
738,741c738
<     IPH_OFFSET_SET(iphdr, (p->flags & PBUF_FLAG_DF) ? PP_HTONS(IP_DF) : 0);
< #if CHECKSUM_GEN_IP_INLINE
<     chk_sum += iphdr->_offset;
< #endif /* CHECKSUM_GEN_IP_INLINE */
---
>     IPH_OFFSET_SET(iphdr, 0);

I have tested this, and it works well. It has minimal impact on the lwIP 
source, so hopefully it will be accepted.

Regards,
Roger W. Cover
________________________________________
From: lwip-users <lwip-users-bounces+rcover=address@hidden> on behalf of 
address@hidden <address@hidden>
Sent: Tuesday, March 10, 2020 1:26 PM
To: Mailing list for lwIP users
Subject: Re: [lwip-users] Do not fragment (DF) in a UDP transmission.

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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