Date: Sat, 7 Mar 1998 18:21:49 +0100 (MET) From: Luigi Rizzo <luigi@labinfo.iet.unipi.it> To: hackers@FreeBSD.ORG Subject: weird problem (lost packets) in iijppp Message-ID: <199803071721.SAA03615@labinfo.iet.unipi.it>
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Since i wanted to play with ppp (to implement better compression for
UDP/RTP traffic, and implement "preemption" of long, non real-time
packets), i decided to do some tests using ppp over tcp.
Setting up everything with two ppp instances on the same machine
(details at the end of this email) i tried some pings and, surprise
surprise:
# ping 10.0.0.1
PING 10.0.0.1 (10.0.0.1): 56 data bytes
...
64 bytes from 10.0.0.1: icmp_seq=21 ttl=255 time=1.839 ms
64 bytes from 10.0.0.1: icmp_seq=22 ttl=255 time=1.748 ms
64 bytes from 10.0.0.1: icmp_seq=24 ttl=255 time=1.786 ms
... NOTE THE MISSING PKT 23 ...
64 bytes from 10.0.0.1: icmp_seq=30 ttl=255 time=1.716 ms
64 bytes from 10.0.0.1: icmp_seq=31 ttl=255 time=1.750 ms
^C
--- 10.0.0.1 ping statistics ---
32 packets transmitted, 31 packets received, 3% packet loss
round-trip min/avg/max = 1.716/1.826/3.646 ms
so the packet has been lost (and not because i interrupted
the ping, it is in the middle of the sequence), but presumably outside
the low level (tcp) link since the ppp stats on the two sides seem
to suggest that the packet has been correctly transferred over the
link:
Protocol in out
client: COMPD : 31, 32
server: COMPD : 32, 31
Apart from some weird interaction with TCP delayed acks on the TCP
circuit which carries ppp packets (since packets around 108 bytes
and above 1500 will have a RTT of 300-400ms), there are some
occasional packet losses (got up to 10-15 lost in some tests) and
i have no idea where they occur. At first sight the losses seem to
be more frequent when there is a short (<100ms) RTT.
Disabling "pred1" compression seems not to make much difference --
just the effect of the predictor changes the sizes for which i get the
delayed acks, but the losses remain.
For what matters, i got the same loss pattern using ppp on a real
modem line (frequently the losses occur at multiple of 8 pkts if
that matters). I thought they were due to the modem (i.e. congestion
somewhere) but that's not possible since ping is sending 1 pkt/s
and the queues are 10pkts at least (i think) -- perhaps even 20 or
50 pkts.
Any idea on this problem ? I am seeing it on 2.2.1 -- maybe newer
relases of ppp do not have the problem. In any case something to check.
For those willing to duplicate the problem, i have patched modem.c so
that a negative port number will instruct ppp to open the socket in
server mode. On the two sides i run
[server] % ppp tcps
[client] % ppp tcpc
then go into "term" mode at the ppp prompt, and on one of the parties
press ~p to enter packet mode. The relevant lines in ppp.conf are:
#server for tcp connection
tcps:
set device 127.0.0.1:-2400
set ifaddr 10.0.0.1 10.0.0.2
tcpc:
set device 127.0.0.1:2400
set ifaddr 10.0.0.2 10.0.0.1
while the patches for modem.c are as following
cheers
luigi
--- ../ppp.22R/modem.c Sun Jan 12 22:52:49 1997
+++ modem.c Sat Mar 7 15:25:04 1998
@@ -334,6 +334,7 @@
struct hostent *hp;
struct servent *sp;
+bzero(&dest, sizeof(dest) );
dest.sin_family = AF_INET;
dest.sin_addr.s_addr = inet_addr(host);
if (dest.sin_addr.s_addr == INADDR_NONE) {
@@ -357,6 +358,27 @@
}
LogPrintf(LOG_PHASE_BIT, "Connected to %s:%s\n", host, port);
+ if (ntohs(dest.sin_port) & 0x8000) {
+ struct sockaddr_in src;
+int r , l;
+
+ bzero(&src, sizeof(src) );
+ dest.sin_port = htons(- (short)(ntohs(dest.sin_port)) ) ;
+ dest.sin_addr.s_addr = INADDR_ANY ;
+ printf("using server socket at %d\n", ntohs(dest.sin_port));
+ sock = socket(PF_INET, SOCK_STREAM, 0);
+ if (sock < 0) {
+ return(sock);
+ }
+ r = bind(sock, &dest, sizeof(dest) );
+ printf("bind returns %d\n", r);
+ listen(sock, 10);
+ printf("listen returns %d\n", r);
+ l = sizeof(src) ;
+ sock = accept(sock, &src, &l );
+ printf("accept returns %d\n", sock);
+
+ } else {
sock = socket(PF_INET, SOCK_STREAM, 0);
if (sock < 0) {
return(sock);
@@ -364,6 +386,7 @@
if (connect(sock, (struct sockaddr *)&dest, sizeof(dest)) < 0) {
printf("connection failed.\n");
return(-1);
+ }
}
return(sock);
}
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