I know you can't have two different process using the same port, but what happens if one is using tcp and the other one udp? Can you have two different process each one binding a socket to the same port but different protocol?
The 5-tuple (protocol, source ip, source port, dest ip, dest port) must be unique. That means that not only can you have TCP and UDP using the same port number, but even outgoing connections with the same protocol and local port number, but different destinations.
When listening however, sockets usually must be unique in their protocol, i.e. you can/should not open another TCP socket with the same port number.
TCP ports and UDP ports are not related to each other at all.
Yes. Two sockets can bind same port number but different protocol.
It's not the same port, just happens to have the same number.
Related
After doing some search for this, I got to know a few points
We cannot multiplex a port for TCP.
If two connections use the same protocol and have the same destination ports, they must have the same connection.
I am quite confused about how some sites say that TCP can only have one application listening on the same port at one time while others say multiple listening TCP sockets, all bound to the same port, can co-exist, provided they are all bound to different local IP addresses.
Reading the above stuff has left me more confused than ever. Can a destination port be associated with more than one TCP connection?
We cannot multiplex a port for TCP.
This is wrong. You can run multiple TCP connections on the same port, as long as they are unique connections. And it is not very difficult to write code that multiplexes I/O on multiple TCP sockets within the same process.
If two connections use the same protocol and have the same destination ports, they must have the same connection.
This is wrong. A TCP connection is uniquely identified by a combination of protocol + local IP/port + and remote IP/port.
Two connections that use the same protocol and same destination IP/port are still unique if they use different source IP/port to connect from. For instance, multiple clients running on the same machine can connect to the same server if they use a different local port to connect from. Which is typically the case, as most clients use a random available local port, selected by the OS, for the outbound connection.
Likewise, two connections that use the same protocol and the same source IP/port are still unique if they connect to different destination IP/port. For instance, multiple clients running on the same machine can use the same local IP/port to connect from if they connect to different servers.
some sites say that TCP can only have one application listening on the same port at one time
This is correct, but only if all of the listeners are trying to use the same local IP/port at the same time. Only 1 listener is allowed on it.
others say multiple listening TCP sockets, all bound to the same port, can co-exist, provided they are all bound to different local IP addresses.
This is correct.
Can a destination port be associated with more than one TCP connection?
Yes. Even if there is only 1 listener on that port, every connection it accepts will be using that same local port on the server side, but a different source IP/port from the client side. This allows multiple clients from different remote machines to connect to the same server at the same time.
I have read contradictory definitions of what a socket comprise of (mainly in this question).
The first definition is that a socket comprise of the following:
{Source IP Address, Source Port Number}
The second definition is that a socket comprise of the following:
{Source IP Address, Source Port Number, Destination IP Address,
Destination Port Number}
Is there an official document or something that states what the correct definition is?
Also, is the Transport protocol included in the socket?
If you look at the RFCs, e.g. RFC 193, TRANSMISSION CONTROL PROTOCOL, you will see the definition:
Multiplexing:
To allow for many processes within a single Host to use TCP
communication facilities simultaneously, the TCP provides a set of
addresses or ports within each host. Concatenated with the network
and host addresses from the internet communication layer, this forms a
socket. A pair of sockets uniquely identifies each connection. That
is, a socket may be simultaneously used in multiple connections.
The first definition applies to an unconnected TCP or UDP socket.
The second definition applies to a connected TCP or UDP socket.
I have the following statement.
"In TCP, the receiver host uses all of source IP, source port, destination IP and destination port to direct datagram to appropriate socket. While in UDP, the receiver only checks destination port number to direct the datagram. "
Is the above statement true?
If yes, does it mean that in TCP the same port can be used for multiple socket in one process, while in UDP only one socket can use on a port in one process? What about sockets in different processes? Can multiple processes use the same port in TCP/UDP? (in programming language: C/C++/Java)
If not, why?
"In TCP, the receiver host uses all of source IP, source port, destination IP and destination port to direct datagram to appropriate socket. While in UDP, the receiver only checks destination port number to direct the datagram. "
Is the above statement true?
Yes.
If yes, does it mean that in TCP the same port can be used for multiple socket in one process,
Yes, under some circumstances.
while in UDP only one socket can use on a port in one process?
No, see below.
What about sockets in different processes? Can multiple processes use the same port in TCP/UDP? (in programming language: C/C++/Java)
Under some circumstances, yes. A UDP port has to be designated as reusable by all processes that want to share it. A TCP port can only be reused by sockets bound to different interfaces: there is no sharing.
What that means is, in TCP, a unique communication "channel" can be described as the four-tuple: (src-ip, src-port, dst-ip, dst-port).
In UDP, all packets destined to a certain port are delivered to the only UDP socket listening on that port, regardless of the source address and port of said packet. I like to think of it as a funnel.
I have a weird query. I have learned that a socket is a combination of IP and Port. So what is socket descriptor? Is it just an integer? What does it do?
Can I have to different socket descriptors on the same port? If yes, then can those be of different types (TCP/UDP)?
I know these are silly questions; I have been blindly using SD for quite a time now :P
TCP and UDP are independent, so you can have TCP and UDP sockets on the same port.
A socket descriptor is to a socket as a file descriptor is to a file.
A TCP connection is actually defined by the tuple: local IP, local port, remote IP, remote port. You can have multiple connections with the same local IP and port, as long as they have different remote IP and/or port.
For instance, a web server uses its local port 80 for all the connections. But each client connection will either come from a different machine (and hence a different remote IP) or different sockets on the same machine (so they'll have the same remote IP but different remote ports).
A socket descriptor is a unique integer returned by the system when you ask it to create a socket with the socket call. Each socket is identifiable by its socket descriptor.
As regards the second part of your question, You will get a different socket descriptor for the same IP+PORT+PROTOCOL, so yes, you can have tcp and udp sockets on the same port, but you will get two different socket descriptors
You should read network programming tutorials like these first: Beej's Network Programming Tutorial
First of all, is there any problem with using both UDP and TCP on the same server?
Secondly, can I use the same port number?
Yes, you can use the same port number for both TCP and UDP. Many protocols already do this, for example DNS works on udp/53 and tcp/53.
Technically the port pools for each protocol are completely independent, but for higher level protocols that can use either TCP or UDP it's convention that they default to the same port number.
When writing your server, bear in mind that the sequence of events for a TCP socket is much harder than for a UDP socket, since as well as the normal socket and bind calls you also have to listen and accept.
Furthermore that accept call will return a new socket and it's that socket that you'll then have to also poll for receive events. Your server should be prepared to continue accepting connections on the original socket whilst simultaneously servicing multiple clients each of which will be triggering receive events on their own sockets.
Firstly,there is no problem using both tcp and udp on the server.
Secondly,we can have both UDP and TCP requests on same port ,because each request is identified by a quintuple contained by source IP ,Destination IP, Source Port, Destination Port, PROTOCOL(as protocol can be TCP or UDP).