CODEWITHSUNDEEP
linuxsockets
FunnyName
FunnyName

Reputation: 11

socket UDP recvfrom not working for large packets even if buffer given is enough on LINUX

I am calling a recvfrom api of a valid address , where i am trying to read data of size 9600 bytes , the buffer i have provided i of size 12KB , I am not even getting select read events. Even tough recommended MTU size is 1.5 KB, I am able to send and receive packets of 4 KB.

I am using android NDK , (Linux) for development.

Please help . Is there a socket Option i have to set to read large buffers ?

Upvotes: 1

Views: 1729

Answers (1)

bdonlan
bdonlan

Reputation: 231383

If you send a packet larger than the MTU, it will be fragmented. That is, it'll be broken up into smaller pieces, each which fits within the MTU. The problem with this is that if even one of those pieces is lost (quite likely on a cellular connection...), the entire packet will effectively disappear.

To determine whether this is the case you'll need to use a packet sniffer on one (or both) ends of the connection. Wireshark is a good choice on a PC end, or tcpdump on the android side (you'll need root). Keep in mind that home routers may reassemble fragmented packets - this means that if you're sniffing packets from inside a home router/firewall, you might not see any fragments arrive until all of them arrive at the router (and obviously if some are getting lost this won't happen).

A better option would be to simply ensure that you're always sending packets smaller than the MTU, of course. Fragmentation is almost never the right thing to be doing. Keep in mind that the MTU may vary at various hops along the path between server and client - you can either use the common choice of a bit less than 1500 (1400 ought to be safe), or try to probe for it by setting the MTU discovery flag on your UDP packets (via IP_MTU_DISCOVER) and always sending less than the value returned by getsockopt's IP_MTU option (including on retransmits!)

Upvotes: 1

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