Server Fault is a question and answer site for system and network administrators. Join them; it only takes a minute:

Sign up
Here's how it works:
  1. Anybody can ask a question
  2. Anybody can answer
  3. The best answers are voted up and rise to the top

I currently have a data center connected to two remote locations via MPLS - 2M and 6M respectively. The 2M connection (to Asia from the US) has about 300ms of latency and when transferring between clients (XP, 2003 Server) and server (2003 Server) a single flow won't exceed .65 Mbps.

I can get a very nice connection via FTP that fills up pretty much the entire 2 Mbps with a single threaded FTP connection but CIFS or whatever else TCP based seems to be gimped horribly.

It's been suggested that this is a TCP window size issue but when referring to this to change the window size to 64K (or larger, I've tried a range of window sizes) it doesn't change my transfer speed at all. I've peeked at Network Monitor to see what window sizes I'm negotiating and it's reporting that if I set it to 64K it's really negotiating 64K and so on.

I don't really have much experience with any of this insanity so was hoping someone might have some insight into what I'm missing or some things to consider.

share|improve this question
What kind of vendor QOS is implemented on your MPLS links? – Izzy Sep 1 '09 at 21:34
No specific QoS is implemented (like we have no voice / video / etc on the link). Our provider says that from on-site router to on-site router there's absolutely nothing to cause this on their end. – Dustin Sep 1 '09 at 23:41
up vote 0 down vote accepted

We have the same problem as you do, the only way to solve the problem of accessing CIFS via the MPLS link is to speed it up with CIFS accelerator like Riverbed devices or F5 Networks devices. It will be worst if you are using windows file sharing, and without a local domain controller due to the chatty protocol from MS. With exception of Vista and above due to SMBv2.

We get around 250ms to US DC from APAC and 220ms to our European DC.

share|improve this answer

IMHO, low level hacks like changing the window size aren't the first place to start. 300ms latency is horrible. Have you looked at packet loss? FTP has less overhead than Windows file copying so it's generally going to run faster.

share|improve this answer
300ms latency from Asia to US may be reasonable. – Igal Serban Sep 1 '09 at 21:08
@Igal, especially if it's to somewhere like Thailand – Mark Henderson Sep 1 '09 at 21:30
300ms latency from Asia may be resonable to expect but it's horrible from a performance standpoint. – joeqwerty Sep 1 '09 at 21:41
I'm with you on the horror of 300ms, but it's within the SLA for US to our location in Asia and is significantly better than like a site-to-site VPN over the internet on average. I've done Iperf tests from site to site and there isn't any notable packet loss at all. – Dustin Sep 1 '09 at 23:43

Your Answer


By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.