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Re: [lwip-users] Sending a large amount of data using UDP

From: Jon Bean
Subject: Re: [lwip-users] Sending a large amount of data using UDP
Date: Sun, 19 Jul 2020 15:27:49 +0100
User-agent: Mozilla/5.0 (Windows NT 10.0; WOW64; rv:68.0) Gecko/20100101 Thunderbird/68.10.0

Hi Patrick

Yes I did try with fragmentation enabled but that didn't seem to work. I can see in wireshark that there is a packet with IP4 type that mentions fragmentation , but I don't get any UDP packets.

I also did a test where I just send two packets of 1400, one after another, but only one get sent. I am wondering if I need to check some send buffer like in TCP?

If I just allocate a pbuf of 2800 and then send it I dont actually get any errors, it just doesnt send anything.

The problem is that I cant really do anything with the data in terms of adding sequence numbers, as I am developing the code for the TIVA for a pc application that is already defined. I kind of thought that UDP could send up to 64k. I am a bit surprised I cant just ask LWIP to send a large amount of data and then it send multiple packets.



On 19/07/2020 15:18, Patrick Klos wrote:
On 7/19/2020 6:44 AM, Jon Bean wrote:

I have an application were I need to send around 4k of data using UDP. What I would like to know is how I go about sending this using LWIP. I can send small packets no problem. I first allocate a pbuf. Then copy my data to the pbuf. Then send the data. But sending 4k of data doesnt work. I realise that the Ethernet has a max payload of 1500 bytes. Is there a different way to send large amounts of data? Do I need to configure something and then do it the same way I have previously? I am using a embedded TIVA micro controller.



Have you enabled IP fragmentation? (IP_FRAG)

Or you could just break up the data yourself, add a little custom header, and essentially fragment the data yourself.  Depending on what's in this data, you could add sequence numbers and a way for the peer to report they missed a chunk?  How often are you sending these blocks of data?


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