CODEWITHSUNDEEP
socketslanguage-agnostic
William Wisdom
William Wisdom

Reputation: 131

Why do outgoing sockets need port numbers?

I understand why a server would need sockets for incoming data, but I do not understand why it is necessary that a socket connecting to another computer needs a source port.

Upvotes: 3

Views: 423

Answers (4)

slebetman
slebetman

Reputation: 113866

While others have mentioned the exact reason why, let me illustrate the point by giving you an example:

  1. Say you want to ssh to your server. OK, you ssh in and do some stuff. Then you tail a log file. So now you don't have access to the console anymore. No problem you think, I'll ssh again...

    With one port number, if you ssh again that second connection will be a mirror of the first since the server won't know that there are two connections (no source port number to tell the difference) so you're out of luck.

    With two port numbers you can ssh a second time to get a second console.

  2. Say you browse a website, say Stackoverflow. You're reading a question but you think you've seen it before. You open a new tab in your browser to stackoverflow to do a search.

    With only one port number the server have no way of knowing which packet belongs to which socket on the client so opening a second page will not be possible (or worse, both pages receive mixed data from each other).

    With two port numbers the server will see two different connections from the client and send the correct data to the correct tab.

So you need two port numbers for client to tell what data is coming from what server and for the server to tell what data is coming from which socket from the client.

Upvotes: 3

user207421
user207421

Reputation: 310884

A TCP connection is defined in terms of the source and destination IP addresses and port numbers.

Otherwise for example you could never distinguish between two connections to the same server from the same client host.

Upvotes: 2

Aasmund Eldhuset
Aasmund Eldhuset

Reputation: 37950

TCP communication is two-way. A segment being sent from the server, even if it is in response to a segment from the client, is an incoming segment as seen from the client. If a client opens multiple connections to the same port on the server (such as when you load multiple StackOverflow pages at once), both the server and the client need to be able to tell the TCP segments from the different connections apart; this is done by looking at the combination of source port and destination port.

Upvotes: 0

JCutting8
JCutting8

Reputation: 774

Check out this link: http://compnetworking.about.com/od/basiccomputerarchitecture/g/computer-ports.htm

Ultimately, they allow different applications and services to share the same networking resources. For example, your browser probably uses port 80, but your email application may use port 25.

Upvotes: 0

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