PROBLEM: small socket send/receive buffers on TCP stream result in data not being transferred

From: Nicholas Daley (
Date: Thu Feb 01 2001 - 04:12:18 EST

I'm pretty sure this is a kernel problem

1. small socket send/receive buffers result in data not being transferred

2. I was playing with socket buffers using setsockopt() and SO_SNDBUF
and SO_RCVBUF (see small attached programs), I found that given small
enough buffers, data would stop being sent, although more was available.
I suspect what is happening is that one of the buffers fills, stops
accepting data, but doesn't reenable it when there's room again.
When I use netstat I find the buffer on the send side full, and the
buffer on the receive end empty.
This behaviour may depend on the speed of the system, on mine I can
usually get it to happen with buffer sizes around 100, especially if the
side sending data has a larger buffer.
I've tested this on a couple of machines running a 2.2 kernel, and one
running a 2.4 kernel.

3. Keywords: Networking, TCP, kernel

4. Kernel version (output of /proc/version):
Linux version 2.4.0 (root@viola) (gcc version egcs-2.91.66
19990314/Linux (egcs-1.1.2 release)) #2 Thu Jan 25 19:56:28 NZDT 2001

6. Attached at the end are two C++ programs (though they could as easily
have been C),
fakefeed - run first, acts as a server that just keeps sending data on a
fakesim - run second, connects to fake feed, and outputs what's received.
They both take as parameters a port number, and a buffer size.
e.g. fakefeed 8001 500
fakesim 8001 100

7.1. output of ver_linux script:
-- Versions installed: (if some fields are empty or look
-- unusual then possibly you have very old versions)
Linux viola 2.4.0 #2 Thu Jan 25 19:56:28 NZDT 2001 i586 unknown
Kernel modules 2.4.0
Gnu C egcs-2.91.66
Gnu Make 3.77
Linux C Library 2.1.2
Dynamic linker ldd: version 1.9.9
Procps 2.0.2
Mount 2.9v
Net-tools 1.52
Kbd 0.99
Sh-utils 1.16
Modules Loaded ppp_deflate ipt_state ipt_MASQUERADE
iptable_filter ip_nat_ftp iptable_nat ip_conntrack ip_tables de4x5
v_midi opl3 mpu401 sb sb_lib uart401 sound bsd_comp ppp_async

7.2 output of /proc/cpuinfo
: 0
: GenuineIntel
cpu family : 5
: 4
model name : Pentium MMX
: 4
cpu MHz : 167.049
: no
: no
: yes
: no
: yes
: yes
cpuid level : 1
: yes
: fpu vme de pse tsc msr mce cx8 mmx
: 333.41

7.3 output of /proc/modules
ppp_deflate 40912 0 (autoclean)
ipt_state 800 2 (autoclean)
ipt_MASQUERADE 1312 1 (autoclean)
iptable_filter 1856 0 (autoclean) (unused)
ip_nat_ftp 3440 0 (unused)
iptable_nat 12608 1 [ipt_MASQUERADE ip_nat_ftp]
ip_conntrack 12992 2 [ipt_state ipt_MASQUERADE ip_nat_ftp
ip_tables 10656 6 [ipt_state ipt_MASQUERADE
iptable_filter iptable_nat]
de4x5 42496 1
v_midi 5264 0 (unused)
opl3 11792 0 (unused)
mpu401 18960 0 (unused)
sb 6656 0 (unused)
sb_lib 34352 0 [sb]
uart401 6416 0 [sb_lib]
sound 57280 0 [v_midi opl3 mpu401 sb_lib uart401]
bsd_comp 4160 0
ppp_async 6464 1


bits and peace

Nicholas Daley <> insert :-)=s as appropriate

#include <stdlib.h> #include <errno.h> #include <netdb.h> #include <sys/types.h> #include <sys/socket.h> #include <netinet/in.h> #include <netinet/tcp.h> #include <unistd.h> #include <string.h> #include <stdio.h> #include <iostream.h>

int openList(int port) { int fd=-1; sockaddr_in sa; fd=socket(PF_INET, SOCK_STREAM, 0); if(fd<0) return -1;

sa.sin_family=AF_INET; sa.sin_port=htons(port); sa.sin_addr.s_addr=htonl(INADDR_ANY); int s=1; setsockopt(fd,SOL_SOCKET,SO_REUSEADDR,&s,sizeof(s)); if(bind(fd,(struct sockaddr*)&sa,sizeof(sa))<0) { shutdown(fd,2); fd=-1; return fd; }; if(listen(fd,1)<0) { shutdown(fd,2); fd=-1; return fd; }; return fd; };

int main(int argc, char** argv) { int listener=openList(argc>1?atoi(argv[1]):8001); int stream=accept(listener,NULL,NULL); size_t s; s=argc>2?atoi(argv[2]):3000; setsockopt(stream,SOL_SOCKET,SO_RCVBUF,&s,sizeof(s)); setsockopt(stream,SOL_SOCKET,SO_SNDBUF,&s,sizeof(s));

close(listener); char buff[200]; long long sent=0; for(int i=1;i;i++) { sprintf(buff,"%d\n",i); int writeres=write(stream,buff,strlen(buff)); if(writeres<0) { cerr<<"Write error "<<writeres<<','<<errno<<"\a\n";break;} if(writeres==0) { cerr<<"Write EOF\a\n";break;} if(writeres!=strlen(buff)) {cerr <<"Didn't finish writing\a\n";break;} cerr<<buff<<" Total bytes sent="<<(sent+=writeres)<<endl; } };

#include <stdlib.h> #include <errno.h> #include <netdb.h> #include <sys/types.h> #include <sys/socket.h> #include <netinet/in.h> #include <netinet/tcp.h> #include <unistd.h> #include <string.h> #include <stdio.h> #include <iostream.h> typedef double sk_time;

int openCon(const char* hostname, int port) { int fd=-1; sockaddr_in sa; hostent *he; fd=socket(PF_INET, SOCK_STREAM, 0); if(fd<0) return -1;

sa.sin_family=AF_INET; sa.sin_port=htons(port); he=gethostbyname(hostname); if(he==NULL) return fd=-1; sa.sin_addr=*(struct in_addr*) he->h_addr; if(connect(fd,(struct sockaddr*) &sa, sizeof(sa))<0) return fd=-1;

return fd; } int main(int argc, char** argv) { int tracestream=openCon("localhost",argc>1?atoi(argv[1]):8001); size_t s; s=argc>2?atoi(argv[2]):3000; setsockopt(tracestream,SOL_SOCKET,SO_RCVBUF,&s,sizeof(s)); setsockopt(tracestream,SOL_SOCKET,SO_SNDBUF,&s,sizeof(s));

while(1) { char c; if(read(tracestream,&c,1)<=0) break; cerr<<c; } close(tracestream); };

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This archive was generated by hypermail 2b29 : Wed Feb 07 2001 - 21:00:11 EST