This won’t be a tutorial, however, the purpose of this post is to highlight some common TCP/IP issues that degrade network performance.
Network Congestion – If your firewall/router graphs your network usage, use that or obtain your current throughput (up/down) to your ISP. Use the current usage and then run a few speed tests in order to determine if your hardware is capable of hitting your maximum download/upload speeds you currently pay for. So if you have a 100 meg fiber circuit and your firewall is showing you are currently using about 60 megs at any given time, you should be able to get somewhere close to 40 megs on a download test from speedtest.net. If not, either your provider is not giving you the bandwidth you are paying for or you have a hardware issue. If you have SNMP capable devices, a really simple tool is STG
Maximum Segment Size (MSS) – I’ve run into this issue a number of time, mostly when dealing with GRE/DMVPN tunnels. It’s usually fixed on Cisco routers by issuing ip tcp mss 1400. You can read more here: TCP MSS Adjustment
High TCP Retransmissions – This one you’ll probably have to take out Wireshark and start a packet capture from the device that is having issues in your network or use a network tap or even a span port on a switch. If TCP retransmissions are high, you have a problem somewhere in your network. Most commonly on wireless.
TCP Window Scaling – I’ve seen this most commonly on modern firewalls that use Deep Packet Inspection (DPI). Please read more here.
Slow DNS Responses – in most enterprise environments, PC’s will have a local DNS server to query from which would cut down on this problem, however if the server were having CPU/Memory/Disk issues, it can still happen. Likewise, if you do not have a local DNS resolver, and use a remote DNS server, this can also be a problem. I’ve used a tool from Google in the past to help troubleshoot and to point me to the best DNS server based on my network/location called Namebench.
Path MTU Discovery (PMTUD) – This is where the “secure by default” methodology of most firewall’s today may kick you in the butt. By disabling ICMP, you also disable Path MTU Discovery. It’s a simple fix but Cisco has the best explanation I’ve been able to find on it. You can read that here. Then someone felt very strongly about the subject and created a website dedicated to it here.
Routing Issues – In this case it would be poorly configured asymmetrical routing. Traffic goes out on a nice 100 meg connection but comes back to you on your backup 20 meg connection, likely due to a BGP configuration problem.
TCP Offload – Depending on how your servers are configured, this could be a blessing or a curse. I’ve most commonly had a problem with this on virtualized servers where the host/virtual NIC weren’t on the same page. Disabling it helped.
SMB 3.0 Multichannel – Read here for details on it, but simply put, add more network interfaces to your file server and you’ll have better performance.
I have re-created the wheel here somewhat but if you want a single source for some good information on most of these issues, please visit this site.
Also, learn Wireshark. It will become your best friend.