Bliss, Aaron wrote:
I know this is a bit of a networking question more than a redhat
question, however I'm sure you guys will be able to explain this to me;
I'm trying to understand exactly how this works, and have not been able
to find any solid authority on the subject; I understand the concept of
ports, well know ports, and part of the initial 3 way handshake of a new
tcp session; My question is, does the initial receiving host of a
session (usually a server) use the port that it's listening for the tcp
session, or does it negotiate a port number that is not in use like the
client does? For example, host A is a client, host B is a webserver
listening on port 80; host A wants some data from host B's port 80; lets
say host A wants to use port 6785; will the tcp session then use port
6785 for host A and port 80 for host B, or will they also pick an
available port from host B as well; if port 80 is used, how are multiple
sessions possible; i.e. how can the host B serve up more web traffic if
port 80 is busy? Thanks for your help.
Aaron
Here is what happens:
1) TCP Server listens on port X at address IP1
2) TCP Client connects ("handshake")
to port X at address IP1
from port Z at address IP2
3) TCP Server code will spawn a thread or process to handle this
connection. The
abstraction used to represent the connection is the 'socket', which is
a source and sink
of bytes, but basically it's a I/O buffer.
The TCP session identifier is: (IP1,Z,IP2,X), all TCP packets carry that
information.
The OS at IP1 will correctly route the packet to its destination I/O
buffer (to be picked up
by the handling thread) based on that identifier. Thus the packets can
STILL go to the
same port X on the Server.
Note that for FTP, there is first a connection from the client to server
port 21, then
a negotiation over the TCP connection whereupon the server opens another
server
port (high-numbered) on the the server machine and the client
additionally connects to
that to transfer data. But this means *two* TCP connections.
Best regards,
-- David
"four fifty -- press return"
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