Can a TCP listener receive multiple connections from the same IP/port combination?

user_197229 0
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I know that multiple TCP clients can connect to the same remote endpoint (e.g. my server runs on 127.0.0.1:8080).

I know that multiple TCP clients can connect from the same IP address. But when I test this (in my case using .Net's TcpClient class, they seem to be automatically given unique ports, but is that enforced?

Can my TCP listener/server have multiple concurrent connections from the same IP/port combination? If not, how can I uniquely distinguish my connections?

One edge case I considered is if two clients have the same IP4 address... or on any given network is IP4 uniqueness also enforced?

tcp
2 Answers

When speaking about TCP, a connection between two entities is identified by the quadruple

<clientIP, clientPort, serverIP, serverPort>

This is why the same service (running on serverIP, serverPort) can accept

  • multiple connections from different clients (entities with different IPs)
  • multiple connections from the same client (same clientIP BUT different clientPort)

I'm not sure about .NET's implementation, altough I think it would work anyway, but when estabilishing a connection on the client side, you can also specify the port to which bind your client side socket; this way, on the server sidem you could identify incoming connections connections from the same host by their clientPort value.

user_93288310

Given the TCP/IP stack definition, no, you cannot enforce which port a client listens on from the server side. When a client hits a server it sends a port number that it expects the server to respond to (the client will listen on this port) called a Source Port. The server responds to that unique port for that particular client.

https://packetlife.net/blog/2010/jun/7/understanding-tcp-sequence-acknowledgment-numbers/

Looking at this capture you can see the client sent a packet with the request to (Destination) port 80 on the server with a (source) port 54841. This 54841 should be unique to that client from the servers perspective especially when combined with the Client IP address.

In more detail, the client sends a packet like this: IP: Src: 192.168.1.23 (client), Dst: 192.168.1.11 (server) TCP: Src Port: 54841, Dst Port: 80

The server responds with a packet that looks like this: IP: Src: 192.168.1.11 (server), Dst: 192.168.1.23 (client) TCP: Src Port: 80, Dst Port: 54841

Hopefully you can see that the server is able to uniquely track each request by the client IP/Port combination. A new request from the same client would have the same IP but a different Port.

IPv4 address conflicts are a problem for most networks. Most networks will not tolerate an IP address conflict. It is usually enforce by a competent network administrator or a DHCP server handling the IP addresses. Someone can manually force an IP address on a client and this will cause problems with the other host that has the same IP address. But this seems to be a different question from your original.

user_114383360
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