phil@shl.com (Phil Trubey) (03/12/91)
I have a problem. We have just connected an HP 3000 computer to a remote LAN
connected to a campus ethernet through a 56 kbps line and bridge:
PCs, Vax, etc ===========B---------B====== HP 3000
Campus ethernet 56 kbps Remote ethernet
The problem is with ordinary telnet sessions from PCs on the campus ethernet
to the remote ethernet to the HP 3000. You can start a telnet session fine
from a PC to the HP. Intermittantly, when you display a large amount of
text on the screen, the telnet session slows to a crawl. You can see the
individual packets coming through, one every few seconds. The problem
fixes itself after about 30 seconds of this, and then activity is normal
again, until the next time it happens. This occurs with a single telnet
session active through the bridge.
We are running Wollongong's TCP/IP package on the HP, and used both FTP's
PC/TCP and Wollongong's Win/TCP on the PCs. The problem occurs more
frequently with FTP's software.
Interestingly enough, if we telnet from a terminal attached to the VAX, this
problem does not occur.
The campus ethernet has lots of hosts and PCs on it, the remote ethernet
has just the bridge and HP 3000 connected to it.
Any ideas?
--
Phil Trubey
SHL Systemhouse Inc.
(Internet: phil@shl.com UUCP: ...!uunet!shl!phil)phil@shl.com (Phil Trubey) (03/13/91)
In an earlier posting to comp.dcom.lans, I mentioned a problem we were having telneting to an HP 3000 through a bridge using PC/TCP. Well, it turns out that the problem is not related to the bridge, and is actually composed of three separate problems. The original problem was that telneting from PC/TCP on a PC to the HP 3000 would occationally slow way down when the HP was outputing lots of text to the PC's screen. In the middle of transmission, it would start to display one line at a time every four seconds. With a protocol analyzer we were able to determine what was going on. The problem occurs when the PC drops a packet. The HP doesn't realize that this has occured, of course, and merrily blasts the TCP window size number of bytes to the PC. The PC meanwhile keeps sending an ACK that acks everything up to the dropped packet. So far so good. The HP realizes that the PC has dropped a packet when its timeout timer kicks in (4 seconds later). It then proceeds to retransmit the dropped packet and just the dropped packet. The PC, not having kept the packets after the dropped packet, calmly acks the retransmitted packet from the HP and waits for the HP to send the rest of the packets. The HP waits for another retransmission interval and then retransmits the next packet that it had sent. This continues until the HP has sent all outstanding packets - at a rate of one every four seconds. Questions: 1. Is it normal for PC TCP implementations to drop packets fairly frequently? Ie about one packet every 100 or so? 2. The original TCP spec seems to be silent here, but surely this can't be the 'standard' way retransmission works? I would have though that the retransmitter would send all outstanding packets all at once after an ack timeout, rather than one packet every timeout interval, the way the HP 3000 does. 3. Is it normal for TCP implementations to discard all out of sequence packets? Thanks for any info! -- Phil Trubey | Internet: phil@shl.com SHL Systemhouse Inc. | UUCP: ...!uunet!shl!phil 50 O'Connor St., Suite 501 | Phone: 613-236-6604 x667 Ottawa, Ontario, Canada | Fax: 613-236-2043