A common question when you setup a new SaaS or just a fancy website is "Why do my remote users have hard time with my application?" while when I'm in a physical location that is close to my hosting provider I get a relatively good user experience.
It's the TCP/IP Doctor Watson
Window Size is a significant parameter in the TCP/IP stack. This parameter controls how much data can be sent by either party before receiving an ACK from the other. If your Window Size is set to 8KB and the round trip (RTT) between you and the server is 20ms, you could get 400KB/s (1sec/20ms*8KB) or ~4Mbps.
What If You Were in the Other Side of the Globe?
Well, if your RTT is more like 250ms, you could hardly get 32KB/sec (1sec/250ms*8KB), meaning that you could download a 1MB image in 30sec... Not something to proudly report back home.
CDN. The Question is: How Can I Solve This?
CDN (Content Delivery Network) helps bringing web content closer to the end user. This way it maximizes the throughput controlled by TCP/IP Window size.
How Does CDN Work?
- The CDN provider places many edge servers around the world (as many as dozens of thousands of locations in some cases).
- The CDN provider hosts your DNS record in a DNS that supports Anycast. Anycast enables the same DNS record to point to different locations around the world based on the network topology.
- The end user is directed to the closest edge server based on the Anycast DNS.
- The edge server tries to serve the file if it is cached on it (images, JavaScript files, static HTML, Streaming Video or just a static copy of your favorite news site are good examples).
- If the requested file is dynamic or not cached, the edge server requests the file from the origin server using a large TCP window size. This way the 230ms RTT (250ms-20ms) is done in a much larger Window Size (say 64KB) and the throughput will be much larger.
CDN can help you better your end user experience, use it when needed.
If you need assistance, don't hesitate to communicate,
Keep Performing,
Moshe Kaplan
