udp sending long messages broadcast

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udp sending long messages broadcast

samyuktar
Hello,

I am using lwip raw api functions to perform tcp, udp message sending. I am
trying to implement dhcp as well using the raw api.
I posted earlier in the forum about getting malformed DHCP request packets
from my device. I was foxed since the only function I had called was
dhcp_start(&mynetif). I was able to successfully send a broadcast udp
message before this, however when the length of the message exceeded about
210 bytes the message seen in wireshark started appearing corrupted. No
issues when sending a message less than 210 bytes. I believe this may be the
origin of the DHCP malformed packet, since DHCP uses UDP send.

Please advise me what may be occurring to make UDP send fail in longer
message lengths.



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Re: udp sending long messages broadcast

samyuktar
This post was updated on .
I have ascertained that just before calling ip_output function, i.e.
netif->output(netif, p, dest), the length of the payload was 336 bytes but
what I see on wireshark is only 96 bytes, which is why the DHCP request is
coming out malformed - it is truncated. In fact, any UDP packet with a total length of more than 255 bytes is corrupted or truncated.

I have also successfully been able to send a TCP message to my PC and see the payload for a short message, but when I construct a longer message of about 250 bytes TCP retransmits for a while and then gives up. There seems to be a pattern here.

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Re: udp sending long messages broadcast

samyuktar
Issue resolved, issue was with the low-level ethernet driver, the length of
the packet was saved as a uint8_t instead of a larger integer so the largest
length that it could handle was 256 bytes. So far I had never encountered a
packet larger than that - hence it also fixed the malformed DHCPDiscover
packet issue.



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