Re: Linux UDP Implementation

["Gary Chan" <hpchan5@ie.cuhk.edu.hk>]

Dear Rick,

Thank you for your reply.

I am sorry that I don't quite understand your point. As far as I know, the 
function call udp_flush_pending_frames() in net/ipv4/udp.c is invoked 
regardless of whether the socket is set to either a blocking mode or a 
non-blocking mode. Do you mean that the implementation in the function 
sendto() handles the packet drop at the interface queue by caching data at 
the socket level ? If so, could you please advise me the Linux Kernel source 
file which contains the exact implementation of sendto() API function call 
in Linux ? I tried to locate this souce file using a bottom-up approach 
starting at the function call udp_sendmsg(), -> inet_sendmsg()  in 
net/ipv4/af_inet.c -> sock_sendmsg() in net/socket.c-> sys_sendto() in 
net/socket.c ... but I finally got lost in sys_sendto().

Thank you for your help.

Regards,

Gary

----- Original Message ----- 
From: "Rick Jones" <rick.jones2@hp.com>
To: "Gary Chan" <hpchan5@ie.cuhk.edu.hk>
Sent: Friday, September 01, 2006 1:00 AM
Subject: Re: Linux UDP Implementation


> Gary Chan wrote:
>> According to the function call udp_sendmsg() in the source file 
>> net/ipv4/udp.c (Linux Kernel 2.6.17.11), when an error value is returned 
>> from the function ip_append_data() due to local device congestion, say 
>> interface queue overflow,  pending packets in the queue 
>> sk->sk_write_queue are simply flushed (udp_flush_pending_frames() is 
>> invoked) without caching for future retransmission.
>>
>> I called a network API function sendto() to transmit UDP packets in a 
>> blocking I/O mode at a rate of 100Mbps over the 802.11b wireless ad hoc 
>> network, the network was overloaded as the maximum transfer rate for 
>> 802.11b was just 11Mbps. Therefore, the outgoing interface queue must be 
>> full and UDP packets will be dropped eventually. However, I checked that 
>> there was no packet loss at the receiver side, i.e. the number of packets 
>> sent from the sender is equal to that received.
>>
>> It seems that the implementation (at code level) does not match with the 
>> actual behaviour. I would like to seek expertise on clarifying my 
>> understanding in UDP implementation so that this phenomenon can be 
>> explained.
>
> Perhaps you are seeing a difference in the behaviour of blocking vs 
> non-blocking sockets?
>
> rick jones 


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