Home/ Questions/Q 8576573
In Process

The Archive Base Latest Questions

Editorial Team
  • 0
Asked: 2026-06-11T19:57:16+00:00

http header : Connection: Keep-Alive After reading alot about this , I still can’t

  • 0

http header : Connection: Keep-Alive

After reading alot about this , I still can’t understand how its working.

wiki :

A keepalive signal can
also be used to indicate to Internet infrastructure that the
connection should be preserved. Without a keepalive signal
intermediate NAT-enabled routers can drop the connection after
timeout.

I dont udnerstand :

a Server can have 1,000,000 cuncorrent connections .

John sends a request to the server.

Paul’s compter is on the same lan near paul. paul also sends a request to the same server.

John’s and paul organization is behind router.

enter image description here

How the hell the server knows how to keep connection alive for both paul and john ?

Also , when john sends request the second time , it “doesnt open a new conneciton” , so how does keep-alive is applied here ?

Share

Leave an answer

You must login to add an answer.

Need An Account,

1 Answer

  1. Editorial Team

    First of all, TCP/IP connection is not some thin wire that is temporarily connecting two computers. At the end of the day both TCP/IP and UDP are just a series of packets. It’s the operating system that pretends you have a connection by putting the IP packets back together in correct order.

    Now back to your question. Note that the problem is not really HTTP-specific, all of this works on TCP/IP layer. Say Paul has 192.168.0.100 and John has 192.168.0.101 internal IP addresses while NAT has public 1.2.3.4 address. When Paul connects to some server, his OS uses 192.168.0.100:54321 address (port is chosen randomly by OS). This request hits NAT which remembers that address and forwards request to external server. The external server sees 1.2.3.4:4321 (notice the different port) as the user is behind the NAT so internal IP is not visible.

    When the external server (let it be web server) sends a reply, it sends it to 1.2.3.4:4321. NAT, on the other hand, remembers that 4321 port should be forwarded to 192.168.0.100:54321` – and so it is.

    Now imagine John sends request to the same server. This TCP/IP connection is routed through NAT which remember that request from 192.168.0.101:32123 was made. This request is then forwarded using public 1.2.3.4:4322 (notice different port). When response arrives, NAT checks the port if it is 4322, it routes to 192.168.0.101:32123 (John). Otherwise (on port 4321) Paul will get his reply.

    Note: do not confuse client ephemeral port with server port (80 in HTTP by default).

    • 0