HI Priscilla, Thanks for your response. This problem is driving me nuts.
Looking at sh int, sh frame pvc etc I can't see any significant errors or problems. We have been using auth for some time. No significant changes, other than a power down on Friday, problem notice don Monday. I have gone over the router configs and nothing looks wrong. No routes flapping, all times on routes are consistent. There are no firewalls or access lists in this equation. >From the client side, this is the packet flow: 3 way handshake ok Final ack from the client (WINXP) declares a 62420 byte window size. SMTP starts, ehlo, auth, mail from, etc all ok. After "send data..." from the server, client sends an ack, with Windows size 63795 (I guess the client is thinking at this point that it has a good fast connection, and the server is emptying it's buffers quickly?). I then see 5 packets of SMTP data (from client to server) at 1514 bytes a piece. I then see one packet with smtp data from client to server at 946 bytes. Then I see 3 acks from the server to the client, ack'ing the 3rd, 5th, and get this the NEXT SMTP data packet. It looks like the packet and ack were recieved at the same time. THe acks all have a window size of 8760. This tells me that either the recieve buffers are full or set too small on the server? (I have since increased them). The above 5 packets and 3 acks happens another 4 times (the email is about 400 Kbytes), then the re-transmissions start. I get 6 retransmissions of the 4th to last SMTP data packet from the client to the server, then the whole sequence starts all over again from 3 way handshake. No rst or fin or anything. On the mail server I can see it logging that it is waiting for the message body, and times out waiting. Perhaps the buffer fills up on the server and it can't empty it anymore? In this case I would expect to see some sort of notification from the server to stop sending (a reduced window size perhpas?) I'm going to train it home (with my nose in TCP/IP illustrated) then study the captures some more. If anyone is interested, I can send you the captures, they are from Sniffer Pro. Cheers, Symon ________________________________ From: Priscilla Oppenheimer [mailto:[EMAIL PROTECTED] Sent: Thu 06/03/2003 6:32 PM To: [EMAIL PROTECTED] Subject: RE: SMTP Time outs and re-transmissions, multiple TCP [7:64617] Symon Thurlow wrote: > > Hi all, > > I am trying to problem solve and issue where I have multiple > frame sites > (1750's) all connecting to a site (3640) to send and receive > email (POP3 > SMTP). > > They are having problems sending SMTP. Looking at a packet > trace from a > site trying to send SMTP, I see lots of re-transmissions. > > I have checked with the Frame provider and they assure me that > no > packets are being dropped, even though there are a lot getting > marked > DE. > > I have pretty much excluded the server from being an issue. > > IN the packet sniff I also see multiple acks with the same > sequence > number, one after the other. This does not look right to me. I > am about > to delve in to TCP/IP illustrated to find out about it, but > does this > behaviour trigger anything in your minds? Multiple ACKs with the same sequence number are probably just TCP keepalives. The original TCP RFC didn't mention a keepalive process but the Host Requirements RFC does. That's how it works. After some time of silence a host just sends the last ACK over and over again for a couple hours (typical timeout). Is the client side sending these multiple ACKs? If the server were sending them, and you're seeing them on the client side, then you would have some proof that the server can at least get through, but I bet it's the client side sending them? If you're sniffing on the client side and you see it send retransmissions and multiple ACKs, but you don't see much from the server, I would suspect that the server's packets are getting lost. But what would cause SMTP to get lost but not POP3? A firewall misconfiguration? An MTU issue? What is the last packet exchange you see for SMTP? What error do the users see? For SMTP, did you recently start using authentication or any other new features? We recently started requiring our users to use authentication and had lots of strange problems due to the mix of operating systems. Some of them didn't behave correctly at all, although their misbehavior wasn't exactly what you're describing. Can the users still receive mail? I think you implied that they can. POP3 is based on TCP too. Does it still work? If it still works, but SMTP doesn't, that's a huge clue that TCP and the internetwork are fine. If POP has problems too, start bugging the Frame provider more and do some checking yourself with "show int serial?" Are there lots of errors? Actually, before you blame the Frame provider, though, if possible do some sniffing on the server side and make sure the server is really still sending packets and they are making it through local routers and firewalls. Make sure it is sending packets with data, not just ACKs. It might be hard to sniff on the outside side of the firewall, but if you can that would a good spot. Speaking of firewalls, are there any in the picture that could be messing with TCP and causing problems or timing out? It scares me how much you tell a firewall to mess with TCP. What else has changed recently, if anything? Feel free to send us more info. Thanks, Priscilla > > Cheers, > > Symon ============================================= This email has been content filtered and subject to spam filtering. If you consider this email is unsolicited please forward the email to [EMAIL PROTECTED] and request that the sender's domain be blocked from sending any further emails. ============================================= Message Posted at: http://www.groupstudy.com/form/read.php?f=7&i=64697&t=64617 -------------------------------------------------- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
