Principles of Computer Systems

Winter 2021

Stanford University

Computer Science Department

Instructors: Chris Gregg and

Nick Troccoli

PDF of this presentation

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 at an IP address (the network address that identifies a computer on the network). 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 that the host associates with the true pid of the server application.
- You can see some of the ports your computer is listening to with the netstat command:

cgregg@myth59: $ 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      0 127.0.0.1:587           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 :::22                   :::*                    LISTEN      -
tcp6       0      0 ::1:631                 :::*                    LISTEN      -

Lecture 13: Introduction to Networking

cgregg@myth59: $ 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      0 127.0.0.1:587           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 :::22                   :::*                    LISTEN      -
tcp6       0      0 ::1:631                 :::*                    LISTEN      -

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.

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 RSS document, 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 on a network.
- The socket descriptor is bound to a network driver that may have a limited amount of space
- That means write's return value could very well be less than what was supplied by the third argument.
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 exactly 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

In assignment 5, you've already seen an example (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 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.
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.
The thread-safe version of publishTime is presented on the next slide.

Lecture 13: Introduction to Networking

Here's the updated version of publishTime:

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;
}

Lecture 13: Introduction to Networking

Implementing your ﬁrst client! (code here)
- The protocol—that's the set of rules both client and server must follow if they're to speak with one another—is very simple.
  - The client connects to a speciﬁc server and port number. The server responds to the connection by publishing the current time into its own end of the connection and then hanging up. The client ingests the single line of text and then itself hangs up.
  - We'll soon discuss the implementation of createClientSocket. For now, view it as a built-in that sets up a bidirectional pipe between a client and a server running on the speciﬁed host (e.g. myth64) and bound to the speciﬁed port number (e.g. 12345).

int main(int argc, char *argv[]) {
    int clientSocket = createClientSocket("myth64.stanford.edu", 12345);
    assert(clientSocket >= 0);
    sockbuf sb(clientSocket);
    iosockstream ss(&sb);
    string timeline;
    getline(ss, timeline);
    cout << timeline << endl;
    return 0;
}

Lecture 13: Introduction to Networking

Emulation of wget
- wget is a command line utility that, given its URL, downloads a single document (HTML document, image, video, etc.) and saves a copy of it to the current working directory.
- Without being concerned so much about error checking and robustness, we can write a simple program to emulate the wget's most basic functionality.
- To get us started, here are the main and parseUrl functions.
parseUrl dissects the supplied URL to surface the host and pathname components.

static const string kProtocolPrefix = "http://";
static const string kDefaultPath = "/";
static pair<string, string> parseURL(string url) {
    if (startsWith(url, kProtocolPrefix))
        url = url.substr(kProtocolPrefix.size());
    size_t found = url.find('/');
    if (found == string::npos)
        return make_pair(url, kDefaultPath);
    string host = url.substr(0, found);
    string path = url.substr(found);
    return make_pair(host, path);
}

int main(int argc, char *argv[]) {
    pullContent(parseURL(argv[1]));
    return 0;
}

Lecture 13: Introduction to Networking

Emulation of wget (continued)

pullContent, of course, needs to manage everything, including the networking.

static const unsigned short kDefaultHTTPPort = 80;
static void pullContent(const pair<string, string>& components) {
    int client = createClientSocket(components.first, kDefaultHTTPPort);
    if (client == kClientSocketError) {
        cerr << "Could not connect to host named \"" << components.first << "\"." << endl;
        return;
    }
    sockbuf sb(client);
    iosockstream ss(&sb);
    issueRequest(ss, components.first, components.second);
    skipHeader(ss);
    savePayload(ss, getFileName(components.second));
}

We've already used this createClientSocket function for our time-client. This time, we're connecting to real but arbitrary web servers that speak HTTP.
The implementations of issueRequest, skipHeader, and savePayload subdivide the client-server conversation into manageable chunks.

The implementations of these three functions have little to do with network connections, but they have much to do with the protocol that guides any and all HTTP conversations.

Lecture 13: Introduction to Networking

Emulation of wget (continued)

issueRequest

static void issueRequest(iosockstream& ss, const string& host, const string& path) {
    ss << "GET " << path << " HTTP/1.0\r\n";
    ss << "Host: " << host << "\r\n";
    ss << "\r\n";
    ss.flush();
}

It's standard HTTP-protocol practice that each line, including the blank line that marks the end of the request, end in CRLF (short for carriage-return-line-feed), which is '\r' following by '\n'.
The flush is necessary to ensure all character data is pressed over the wire and consumable at the other end.
After the flush, the client transitions from supply to ingest mode. Remember, the iosockstream is read/write, because the socket descriptor backing it is bidirectional.

Lecture 13: Introduction to Networking

skipHeader reads through and discards all of the HTTP response header lines until it encounters either a blank line or one that contains nothing other than a '\r'. The blank line is, indeed, supposed to be "\r\n", but some servers—often hand-rolled ones—are sloppy, so we treat the '\r' as optional. Recall that getline chews up the '\n', but it won't chew up the '\r'.

static void skipHeader(iosockstream& ss) {
    string line;
    do {
        getline(ss, line);
    } while (!line.empty() && line != "\r");
}

In practice, a true HTTP client—in particular, something as HTTP-compliant as the wget we're imitating—would ingest all of the lines of the response header into a data structure and allow it to inﬂuence how it treats payload.

For instance, the payload might be compressed and should be expanded before saved to disk.
I'll assume that doesn't happen, since our request didn't ask for compressed data.

Lecture 13: Introduction to Networking

Everything beyond the response header and that blank line is considered payload—that's the timestamp, the JSON, the HTML, the image, or the cat video.

Every single byte that comes through should be saved to a local copy.

static string getFileName(const string& path) {
    if (path.empty() || path[path.size() - 1] == '/') return "index.html";
    size_t found = path.rfind('/');
    return path.substr(found + 1);
}

static void savePayload(iosockstream& ss, const string& filename) {
    ofstream output(filename, ios::binary); // don't assume it's text
    size_t totalBytes = 0;
    while (!ss.fail()) {
        char buffer[2014] = {'\0'};
        ss.read(buffer, sizeof(buffer));
        totalBytes += ss.gcount();
        output.write(buffer, ss.gcount());
    }
    cout << "Total number of bytes fetched: " << totalBytes << endl;
}

HTTP dictates that everything beyond that blank line is payload, and that once the server publishes that payload, it closes its end of the connection. That server-side close is the client-side's EOF, and we write everything we read.

Lecture 13: Introduction to Networking