Lecture 13: Introduction to Networking

Networking is simply communicating between two computers connected on a network. You can actually set up a network connection on a single computer, as well.
A network requires one computer to act as the server, waiting patiently for an incoming connection from another computer, the client.
Server-side applications set up a socket that listens to a particular port. The server socket is an integer identifier associated with a local IP address, and a the port number is a 16-bit integer with up to 65535 allowable ports.
- You can think of a port number as a virtual process ID the host associates with the true pid of the server application.
- You can see some of the ports your machine is monitoring using netstat:

myth66:/usr/class/cs110/lecture-examples/networking$ netstat -plnt
(Not all processes could be identified, non-owned process info
 will not be shown, you would have to be root to see it all.)
Active Internet connections (only servers)
Proto Recv-Q Send-Q Local Address           Foreign Address         State       PID/Program name
tcp        0      0 127.0.0.1:25            0.0.0.0:*               LISTEN      -               
tcp        0      0 0.0.0.0:10050           0.0.0.0:*               LISTEN      -               
tcp        0      0 127.0.0.1:587           0.0.0.0:*               LISTEN      -               
tcp        0      0 127.0.1.1:53            0.0.0.0:*               LISTEN      -               
tcp        0      0 0.0.0.0:22              0.0.0.0:*               LISTEN      -               
tcp        0      0 127.0.0.1:631           0.0.0.0:*               LISTEN      -               
tcp6       0      0 :::10050                :::*                    LISTEN      -               
tcp6       0      0 :::22                   :::*                    LISTEN      -             
myth66:/usr/class/cs110/lecture-examples/networking$

Chris Gregg contributed to these slides.

Lecture 13: Introduction to Networking

Some common ports are listed above. You can see a full list here and here.
- Ports 25 and 587 are the SMTP (Simple Mail Transfer Protocol), for sending and receiving email.
- Port 53 is the DNS (Domain Name Service) port, for associating names with IP addresses.
- Port 22 is the port for SSH (Secure Shell)
- Port 631 is for IPP (Internet Printing Protocol)
For your own programs, generally try to stay away from port numbers listed in the links above, but otherwise, ports are up for grabs to any program that wants one.

myth66:/usr/class/cs110/lecture-examples/networking$ netstat -plnt
(Not all processes could be identified, non-owned process info
 will not be shown, you would have to be root to see it all.)
Active Internet connections (only servers)
Proto Recv-Q Send-Q Local Address           Foreign Address         State       PID/Program name
tcp        0      0 127.0.0.1:25            0.0.0.0:*               LISTEN      -               
tcp        0      0 0.0.0.0:10050           0.0.0.0:*               LISTEN      -               
tcp        0      0 127.0.0.1:587           0.0.0.0:*               LISTEN      -               
tcp        0      0 127.0.1.1:53            0.0.0.0:*               LISTEN      -               
tcp        0      0 0.0.0.0:22              0.0.0.0:*               LISTEN      -               
tcp        0      0 127.0.0.1:631           0.0.0.0:*               LISTEN      -               
tcp6       0      0 :::10050                :::*                    LISTEN      -               
tcp6       0      0 :::22                   :::*                    LISTEN      -               
tcp6       0      0 ::1:631                 :::*                    LISTEN      -               
myth66:/usr/class/cs110/lecture-examples/networking$

Lecture 13: Introduction to Networking

Let's create our first server (entire program here):

int main(int argc, char *argv[]) {
  int server = createServerSocket(12345);
  while (true) {
    int client = accept(server, NULL, NULL); // the two NULLs could instead be used to
                                             // surface the IP address of the client
    publishTime(client);
  }
  return 0;
}

accept (found in sys/socket.h) returns a descriptor that can be written to and read from. Whatever's written is sent to the client, and whatever the client sends back is readable here.
- This descriptor is one end of a bidirectional pipe bridging two processes—on different machines!

Lecture 13: Introduction to Networking

The publishTime function is straightforward:

The first five lines here produce the full time string that should be published.
- Let these five lines represent more generally the server-side computation needed for the service to produce output.
- Here,the payload is the current time, but it could have been a static HTML page, a Google search result, an image, or a movie on Netflix.
The remaining lines publish the time string to the client socket using the raw, low-level I/O we've seen before.

static void publishTime(int client) {
  time_t rawtime;
  time(&rawtime);
  struct tm *ptm = gmtime(&rawtime);
  char timestr[128]; // more than big enough
  /* size_t len = */ strftime(timestr, sizeof(timestr), "%c\n", ptm);
  size_t numBytesWritten = 0, numBytesToWrite = strlen(timestr);
  while (numBytesWritten < numBytesToWrite) {
    numBytesWritten += write(client,
                             timestr + numBytesWritten,
                             numBytesToWrite - numBytesWritten);
  }
  close(client);
}

Lecture 13: Introduction to Networking

Note that the while loop for writing bytes is a bit more important now that we are networking: we are more likely to need to write multiple times.
- A socket descriptor is attached to a network driver with a finite amount of space
- That means write's return value could very well be less than what was supplied by the third argument. For example, you may try to publish the contents of a 62GB movie and be told that only 4MB went through.
Ideally, we'd rely on either C streams (e.g. the FILE *) or C++ streams (e.g. the iostream class hierarchy) to layer over data buffers and manage the while loop around exposed write calls for us.
Fortunately, there's a stable, easy-to-use third-party library—one called socket++—that provides precisely this.
- socket++ provides iostream subclasses that respond to operator<<, operator>>, getline, endl, and so forth, just like cin, cout, and file streams do.
- We are going to operate as if this third-party library was just part of standard C++.
The next slide shows a prettier version of publishTime.

Lecture 13: Introduction to Networking

Here's the new implementation of publishTime:

static void publishTime(int client) {
  time_t rawtime;
  time(&rawtime);
  struct tm *ptm = gmtime(&rawtime);
  char timestr[128]; // more than big enough
  /* size_t len = */ strftime(timestr, sizeof(timestr), "%c", ptm);
  sockbuf sb(client);
  iosockstream ss(&sb);
  ss << timestr << endl;
} // sockbuf destructor closes client

We rely on the same C library functions to generate the time string.
This time, however, we insert that string into an iosockstream that itself layers over the client socket.
Note that the intermediary sockbuf class takes ownership of the socket and closes it when its destructor is called.

Lecture 13: Introduction to Networking

You've already seen two examples—the myth-buster and Assignment 5's aggregate—where multithreading can signiﬁcantly improve the performance of networked applications.
Our time server can beneﬁt from multithreading as well. The work a server needs to do in order to meet the client's request might be time consuming—so time consuming, in fact, that the server is slow to iterate and accept new client connections.
As soon as accept returns a socket descriptor, spawn a child thread—or reuse an existing one within a ThreadPool—to get any intense, time consuming computation oﬀ of the main thread. The child thread can make use of a second processor or a second core, and the main thread can quickly move on to its next accept call.
Here's a new version of our time server, which uses a ThreadPool (you'll be implementing one for Assignment 5) to get the computation oﬀ the main thread.

int main(int argc, char *argv[]) {
    int server = createServerSocket(12345);
    ThreadPool pool(4);
    while (true) {
        int client = accept(server, NULL, NULL); // the two NULLs could instead be used
                                                 // to surface the IP address of the client
        pool.schedule([client] { publishTime(client); });
    }
    return 0;
}

Lecture 13: Introduction to Networking

The implementation of publishTime needs to change just a little if it's to be thread safe.
The change is simple but important: we need to call a diﬀerent version of gmtime.
gmtime returns a pointer to a single, statically allocated global that's used by all calls.
If two threads make competing calls to it, then both threads race to pull time information from the shared, statically allocated record.
Of course, one solution would be to use a mutex to ensure that a thread can call gmtime without competition and subsequently extract the data from the global into local copy.
Another solution—one that doesn't require locking and one I think is better—makes use of a second version of the same function called gmtime_r. This second, reentrant version just requires that space for a dedicated return value be passed in.

Lecture 13: Introduction to Networking

A function is reentrant if a call to it can be interrupted in the middle of its execution and called a second time before the ﬁrst call has completed.
While not all reentrant functions are thread-safe, gmtime_r itself is, since it doesn't depend on any shared resources.
A thread-safe version of publishTime is presented below.

static void publishTime(int client) {
    time_t rawtime;
    time(&rawtime);
    struct tm tm;
    gmtime_r(&rawtime, &tm);
    char timestr[128]; // more than big enough
    /* size_t len = */ strftime(timestr, sizeof(timestr), "%c", &tm);
    sockbuf sb(client); // destructor closes socket
    iosockstream ss(&sb);
    ss << timestr << endl;
}