Linux's Internet domain socket, transport protocols (TCP/UDP)'s socket and port

Question

I have been bothered for a long while by some confusions among

• internet domain socket provided by Linux,
• transport protocols (TCP/UDP)'s socket and
• transport protocols (TCP/UDP)'s port.

Replies on some related posts on SO have lots of ambiguities and inconsistencies and make my confusions even more.

1. Both Linux and transport protocols (TCP/UDP) have concepts "socket". How do the two concepts differ? Is internet domain socket (represented as a file?) provided by Linux a (faithful) implementation of socket in transport protocols (TCP/UDP)? (I guess yes, and if that is true, we can interchangeably use the two terms.)

2. Conceptually, is it correct to think of a port in a transport protocol (TCP/UDP) as a tuple (IP address, transport protocol, port number) or just port number? (I guess a port is a tuple (IP address, transport protocol, port number), because I have been educated several times that the same port number with a different IP address or a different transport protocol represents a different port. In that sense, port and socket (in transport protocols) seem to be an identical concept.) It seems the established name "port" means "port number" only, and I will explicitly use "port number" in the following to avoid unnecessary confusions.

3. What are the relations between socket (in transport protocols) and tuple (IP address, transport protocol, port number)? Is there a bijective mapping between the set of sockets and the set of tuples (IP address, transport protocol, port number)? Must there be one or more sockets for each tuple (IP address, transport protocol, port number), and must there be one or more tuples (IP address, transport protocol, port number) for each socket? Can two sockets share the same tuple (IP address, transport protocol, port number)? Can two tuples (IP address, transport protocol, port number) share the same socket?

4. I heard that two processes can share the same socket (which I understand it in the way that two processes can share a file, assuming Linux's internet domain socket and transport protocols (TCP/UDP)'s socket can be used interchangeably). Can two processes share the same tuple (IP address, transport protocol, port number)?

5. I heard that two connections can't share the same socket (assuming Linux's internet domain socket and transport protocols (TCP/UDP)'s socket can be used interchangeably). Can two connections share the same tuple (IP address, transport protocol, port number)?

Thanks.

Answer 1

1. Sockets are an operating system API. This API lets applications on same or different systems communicate over the TCP and UDP (and other) protocols. UNIX domain sockets (not internet domain sockets as you write) provide similar functionality for communicating with applications on the same system only. The concepts for both are similar: the API provides ways to create a socket, bind, listen+accept and connect a socket, to read and write on it and to shut it down. Regarding read and write they match other file descriptors which relate to regular files, named pipes, anonymous pipes etc but the creating of the file descriptor is different and there are some more operations on the file descriptor compared to for example regular files.
2. A port number in TCP and UDP is an integer between 1 and 65535. The word "port" is used as short for "port number". The tuple of IP address and port number and protocol describes the endpoint address. Calling it port instead will cause confusion when reading other literature.
3. An unconnected (but already bound) socket represents only a single endpoint (ip,port,protocol). A connected socket represents a local endpoint and another (local or remote) endpoint, i.e. a connection. One cannot have multiple in-kernel sockets for the same connection but one can have multiple file descriptors for the same in-kernel socket. One can have the same endpoint in multiple connected sockets but not for the same connection, i.e. the other endpoint of the connection must be different. One can actually have multiple unconnected sockets representing the same endpoint but this is very unusual.
4. Sockets can be shared between processes since sockets are file descriptors and file descriptors can be shared. Sharing is typically done by forking, i.e. the parent opens some file or socket and the child inherits it. But there are also ways to send a file descriptor/socket from one process to another. Sharing means that both can write and read but no data will be duplicated, i.e. if the parent reads some data these data are taken from the socket and cannot be also read by the child. But it is not possible that one process creates a new socket (instead of sharing an existing one) which represents exactly the same connection as an existing socket on the same system.
5. Two sockets/connections can share the same port on one endpoint but they cannot share both endpoints, i.e. at least one of source IP, source port, destination IP, destination port or protocol needs to be different.

Answer 2

"port" is like a house, identified by city, street name and house number, and "port number" is like the house number of a house. I hope this will help you understand what I mean in my post.

I see. I agree, this is a useful concept to think about.

When we need to talk about some exact technical detail in the existing systems, the "port number" concept is easier to define. We can refer to the value of the port field in a TCP packet, or the sin_port field of struct sockaddr_in used with the UNIX socket API. In this type of discussion, we can let the reader look up the full story of how TCP packets are used (perhaps in the original RFC :-). Or how the socket API functions are used in a program, perhaps by looking at the man pages.

It is natural for this concept to become abbreviated. We can naturally say "port 80" instead of "port number 80".

The original TCP standard, RFC 793, talks about the concepts of "port" and "port number" as distinct things. (The introduction also uses "port identifier" to mean the same as "port number").

The man pages on current Linux, for example, are not so careful to make this distinction. man 7 ip frequently uses "port" as an abbreviation for "port number".

The Linux man pages are a very prominent document that programmers refer to, so the term "port" becomes ambiguous. If you are worried that you might be mis-interpreted, talking about the "port number" + IP address is a perfectly good idea.

5. I heard that two connections can't share the same socket (assuming Linux's internet domain socket and transport protocols (TCP/UDP)'s socket can be used interchangeably). Can two connections share the same tuple (IP address, transport protocol, port number)?

A single listening port can receive connections from multiple different source ports.

In Linux TCP programming, the listener gets one socket for each such connection (and is listening on a socket which is not connected).

The socket API doesn't give you any way to do the opposite, i.e. make several outgoing connections using the same source port. The transport protocol standard might technically allow it; I am not sure.

Notice that if you could do this, you would not be able to make multiple connections to the same target port. There would be no way for the target system to distinguish the two connections. I guess they felt this specific limitation was too weird to deal with. Then the broader limitation gets enshrined in the original APIs, including UNIX sockets. And later on, if anyone tries to do it, they will risk finding corner cases somewhere which have never actually been tested :-).

The UDP transport protocol doesn't have any concept of connections, so the question does not apply to it. You can call connect() on a UDP socket if you like, but it is only for convenience.

---

If you write "socket" in a modern document, people will interpret it as referring to the socket concept in the UNIX socket API. The definition of "socket" in RFC 793 is different. That definition, below, is not used any more.

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.