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2. BASICS .R
The basic building block for communication is the socket. A socket is an endpoint of communication to which a name may be bound. Each socket in use has a type and one or more associated processes. Sockets exist within communication domains. A communication domain is an abstraction introduced to bundle common properties of processes communicating through sockets. One such property is the scheme used to name sockets. For example, in the UNIX communication domain sockets are named with UNIX path names; e.g. a socket may be named \*(lq/dev/foo\*(rq. Sockets normally exchange data only with sockets in the same domain (it may be possible to cross domain boundaries, but only if some translation process is performed). The 4.4BSD IPC facilities support four separate communication domains: the UNIX domain, for on-system communication; the Internet domain, which is used by processes which communicate using the Internet standard communication protocols; the NS domain, which is used by processes which communicate using the Xerox standard communication protocols*; .FS * See Internet Transport Protocols, Xerox System Integration Standard (XSIS)028112 for more information. This document is almost a necessity for one trying to write NS applications. .FE and the ISO OSI protocols, which are not documented in this tutorial. The underlying communication facilities provided by these domains have a significant influence on the internal system implementation as well as the interface to socket facilities available to a user. An example of the latter is that a socket \*(lqoperating\*(rq in the UNIX domain sees a subset of the error conditions which are possible when operating in the Internet (or NS) domain. Socket types
Sockets are typed according to the communication properties visible to a user. Processes are presumed to communicate only between sockets of the same type, although there is nothing that prevents communication between sockets of different types should the underlying communication protocols support this.
Four types of sockets currently are available to a user. A stream socket provides for the bidirectional, reliable, sequenced, and unduplicated flow of data without record boundaries. Aside from the bidirectionality of data flow, a pair of connected stream sockets provides an interface nearly identical to that of pipes\(dg. .FS \(dg In the UNIX domain, in fact, the semantics are identical and, as one might expect, pipes have been implemented internally as simply a pair of connected stream sockets. .FE
A datagram socket supports bidirectional flow of data which is not promised to be sequenced, reliable, or unduplicated. That is, a process receiving messages on a datagram socket may find messages duplicated, and, possibly, in an order different from the order in which it was sent. An important characteristic of a datagram socket is that record boundaries in data are preserved. Datagram sockets closely model the facilities found in many contemporary packet switched networks such as the Ethernet.
A raw socket provides users access to the underlying communication protocols which support socket abstractions. These sockets are normally datagram oriented, though their exact characteristics are dependent on the interface provided by the protocol. Raw sockets are not intended for the general user; they have been provided mainly for those interested in developing new communication protocols, or for gaining access to some of the more esoteric facilities of an existing protocol. The use of raw sockets is considered in section 5.
A sequenced packet socket is similar to a stream socket, with the exception that record boundaries are preserved. This interface is provided only as part of the NS socket abstraction, and is very important in most serious NS applications. Sequenced-packet sockets allow the user to manipulate the SPP or IDP headers on a packet or a group of packets either by writing a prototype header along with whatever data is to be sent, or by specifying a default header to be used with all outgoing data, and allows the user to receive the headers on incoming packets. The use of these options is considered in section 5.
Another potential socket type which has interesting properties is the reliably delivered message socket. The reliably delivered message socket has similar properties to a datagram socket, but with reliable delivery. There is currently no support for this type of socket, but a reliably delivered message protocol similar to Xerox's Packet Exchange Protocol (PEX) may be simulated at the user level. More information on this topic can be found in section 5. Socket creation
To create a socket the socket system call is used: s = socket(domain, type, protocol); This call requests that the system create a socket in the specified domain and of the specified type. A particular protocol may also be requested. If the protocol is left unspecified (a value of 0), the system will select an appropriate protocol from those protocols which comprise the communication domain and which may be used to support the requested socket type. The user is returned a descriptor (a small integer number) which may be used in later system calls which operate on sockets. The domain is specified as one of the manifest constants defined in the file <sys/socket.h>. For the UNIX domain the constant is AF_UNIX*; for the Internet .FS * The manifest constants are named AF_whatever as they indicate the ``address format'' to use in interpreting names. .FE domain AF_INET; and for the NS domain, AF_NS. The socket types are also defined in this file and one of SOCK_STREAM, SOCK_DGRAM, SOCK_RAW, or SOCK_SEQPACKET must be specified. To create a stream socket in the Internet domain the following call might be used: s = socket(AF_INET, SOCK_STREAM, 0); This call would result in a stream socket being created with the TCP protocol providing the underlying communication support. To create a datagram socket for on-machine use the call might be: s = socket(AF_UNIX, SOCK_DGRAM, 0);
The default protocol (used when the protocol argument to the socket call is 0) should be correct for most every situation. However, it is possible to specify a protocol other than the default; this will be covered in section 5.
There are several reasons a socket call may fail. Aside from the rare occurrence of lack of memory (ENOBUFS), a socket request may fail due to a request for an unknown protocol (EPROTONOSUPPORT), or a request for a type of socket for which there is no supporting protocol (EPROTOTYPE). Binding local names
A socket is created without a name. Until a name is bound to a socket, processes have no way to reference it and, consequently, no messages may be received on it. Communicating processes are bound by an association. In the Internet and NS domains, an association is composed of local and foreign addresses, and local and foreign ports, while in the UNIX domain, an association is composed of local and foreign path names (the phrase ``foreign pathname'' means a pathname created by a foreign process, not a pathname on a foreign system). In most domains, associations must be unique. In the Internet domain there may never be duplicate <protocol, local address, local port, foreign address, foreign port> tuples. UNIX domain sockets need not always be bound to a name, but when bound there may never be duplicate <protocol, local pathname, foreign pathname> tuples. The pathnames may not refer to files already existing on the system in 4.3; the situation may change in future releases.
The bind system call allows a process to specify half of an association, <local address, local port> (or <local pathname>), while the connect and accept primitives are used to complete a socket's association.
In the Internet domain, binding names to sockets can be fairly complex. Fortunately, it is usually not necessary to specifically bind an address and port number to a socket, because the connect and send calls will automatically bind an appropriate address if they are used with an unbound socket. The process of binding names to NS sockets is similar in most ways to that of binding names to Internet sockets.
The bind system call is used as follows: bind(s, name, namelen); The bound name is a variable length byte string which is interpreted by the supporting protocol(s). Its interpretation may vary from communication domain to communication domain (this is one of the properties which comprise the \*(lqdomain\*(rq). As mentioned, in the Internet domain names contain an Internet address and port number. NS domain names contain an NS address and port number. In the UNIX domain, names contain a path name and a family, which is always AF_UNIX. If one wanted to bind the name \*(lq/tmp/foo\*(rq to a UNIX domain socket, the following code would be used*: .FS * Note that, although the tendency here is to call the \*(lqaddr\*(rq structure \*(lqsun\*(rq, doing so would cause problems if the code were ever ported to a Sun workstation. .FE #include <sys/un.h> ... struct sockaddr_un addr; ... strcpy(addr.sun_path, "/tmp/foo"); addr.sun_family = AF_UNIX; bind(s, (struct sockaddr *) &addr, strlen(addr.sun_path) + sizeof (addr.sun_len) + sizeof (addr.sun_family)); Note that in determining the size of a UNIX domain address null bytes are not counted, which is why strlen is used. In the current implementation of UNIX domain IPC, the file name referred to in addr.sun_path is created as a socket in the system file space. The caller must, therefore, have write permission in the directory where addr.sun_path is to reside, and this file should be deleted by the caller when it is no longer needed. Future versions of 4BSD may not create this file.
In binding an Internet address things become more complicated. The actual call is similar, #include <sys/types.h> #include <netinet/in.h> ... struct sockaddr_in sin; ... bind(s, (struct sockaddr *) &sin, sizeof (sin)); but the selection of what to place in the address sin requires some discussion. We will come back to the problem of formulating Internet addresses in section 3 when the library routines used in name resolution are discussed.
Binding an NS address to a socket is even more difficult, especially since the Internet library routines do not work with NS hostnames. The actual call is again similar: #include <sys/types.h> #include <netns/ns.h> ... struct sockaddr_ns sns; ... bind(s, (struct sockaddr *) &sns, sizeof (sns)); Again, discussion of what to place in a \*(lqstruct sockaddr_ns\*(rq will be deferred to section 3. Connection establishment
Connection establishment is usually asymmetric, with one process a \*(lqclient\*(rq and the other a \*(lqserver\*(rq. The server, when willing to offer its advertised services, binds a socket to a well-known address associated with the service and then passively \*(lqlistens\*(rq on its socket. It is then possible for an unrelated process to rendezvous with the server. The client requests services from the server by initiating a \*(lqconnection\*(rq to the server's socket. On the client side the connect call is used to initiate a connection. Using the UNIX domain, this might appear as, struct sockaddr_un server; ... connect(s, (struct sockaddr *)&server, strlen(server.sun_path) + sizeof (server.sun_family)); while in the Internet domain, struct sockaddr_in server; ... connect(s, (struct sockaddr *)&server, sizeof (server)); and in the NS domain, struct sockaddr_ns server; ... connect(s, (struct sockaddr *)&server, sizeof (server)); where server in the example above would contain either the UNIX pathname, Internet address and port number, or NS address and port number of the server to which the client process wishes to speak. If the client process's socket is unbound at the time of the connect call, the system will automatically select and bind a name to the socket if necessary; c.f. section 5.4. This is the usual way that local addresses are bound to a socket.
An error is returned if the connection was unsuccessful (any name automatically bound by the system, however, remains). Otherwise, the socket is associated with the server and data transfer may begin. Some of the more common errors returned when a connection attempt fails are:
After failing to establish a connection for a period of time, the system decided there was no point in retrying the connection attempt any more. This usually occurs because the destination host is down, or because problems in the network resulted in transmissions being lost.
The host refused service for some reason. This is usually due to a server process not being present at the requested name.
These operational errors are returned based on status information delivered to the client host by the underlying communication services.
These operational errors can occur either because the network or host is unknown (no route to the network or host is present), or because of status information returned by intermediate gateways or switching nodes. Many times the status returned is not sufficient to distinguish a network being down from a host being down, in which case the system indicates the entire network is unreachable.
For the server to receive a client's connection it must perform two steps after binding its socket. The first is to indicate a willingness to listen for incoming connection requests: listen(s, 5); The second parameter to the listen call specifies the maximum number of outstanding connections which may be queued awaiting acceptance by the server process; this number may be limited by the system. Should a connection be requested while the queue is full, the connection will not be refused, but rather the individual messages which comprise the request will be ignored. This gives a harried server time to make room in its pending connection queue while the client retries the connection request. Had the connection been returned with the ECONNREFUSED error, the client would be unable to tell if the server was up or not. As it is now it is still possible to get the ETIMEDOUT error back, though this is unlikely. The backlog figure supplied with the listen call is currently limited by the system to a maximum of 5 pending connections on any one queue. This avoids the problem of processes hogging system resources by setting an infinite backlog, then ignoring all connection requests.
With a socket marked as listening, a server may accept a connection: struct sockaddr_in from; ... fromlen = sizeof (from); newsock = accept(s, (struct sockaddr *)&from, &fromlen); (For the UNIX domain, from would be declared as a struct sockaddr_un, and for the NS domain, from would be declared as a struct sockaddr_ns, but nothing different would need to be done as far as fromlen is concerned. In the examples which follow, only Internet routines will be discussed.) A new descriptor is returned on receipt of a connection (along with a new socket). If the server wishes to find out who its client is, it may supply a buffer for the client socket's name. The value-result parameter fromlen is initialized by the server to indicate how much space is associated with from, then modified on return to reflect the true size of the name. If the client's name is not of interest, the second parameter may be a null pointer.
Accept normally blocks. That is, accept will not return until a connection is available or the system call is interrupted by a signal to the process. Further, there is no way for a process to indicate it will accept connections from only a specific individual, or individuals. It is up to the user process to consider who the connection is from and close down the connection if it does not wish to speak to the process. If the server process wants to accept connections on more than one socket, or wants to avoid blocking on the accept call, there are alternatives; they will be considered in section 5. Data transfer
With a connection established, data may begin to flow. To send and receive data there are a number of possible calls. With the peer entity at each end of a connection anchored, a user can send or receive a message without specifying the peer. As one might expect, in this case, then the normal read and write system calls are usable, write(s, buf, sizeof (buf)); read(s, buf, sizeof (buf)); In addition to read and write, the new calls send and recv may be used: send(s, buf, sizeof (buf), flags); recv(s, buf, sizeof (buf), flags); While send and recv are virtually identical to read and write, the extra flags argument is important. The flags, defined in <sys/socket.h>, may be specified as a non-zero value if one or more of the following is required:
| MSG_OOB send/receive out of band data |
| MSG_PEEK look at data without reading |
| MSG_DONTROUTE send data without routing packets |
Once a socket is no longer of interest, it may be discarded by applying a close to the descriptor, close(s); If data is associated with a socket which promises reliable delivery (e.g. a stream socket) when a close takes place, the system will continue to attempt to transfer the data. However, after a fairly long period of time, if the data is still undelivered, it will be discarded. Should a user have no use for any pending data, it may perform a shutdown on the socket prior to closing it. This call is of the form: shutdown(s, how); where how is 0 if the user is no longer interested in reading data, 1 if no more data will be sent, or 2 if no data is to be sent or received. Connectionless sockets
To this point we have been concerned mostly with sockets which follow a connection oriented model. However, there is also support for connectionless interactions typical of the datagram facilities found in contemporary packet switched networks. A datagram socket provides a symmetric interface to data exchange. While processes are still likely to be client and server, there is no requirement for connection establishment. Instead, each message includes the destination address.
Datagram sockets are created as before. If a particular local address is needed, the bind operation must precede the first data transmission. Otherwise, the system will set the local address and/or port when data is first sent. To send data, the sendto primitive is used, sendto(s, buf, buflen, flags, (struct sockaddr *)&to, tolen); The s, buf, buflen, and flags parameters are used as before. The to and tolen values are used to indicate the address of the intended recipient of the message. When using an unreliable datagram interface, it is unlikely that any errors will be reported to the sender. When information is present locally to recognize a message that can not be delivered (for instance when a network is unreachable), the call will return -1 and the global value errno will contain an error number.
To receive messages on an unconnected datagram socket, the recvfrom primitive is provided: recvfrom(s, buf, buflen, flags, (struct sockaddr *)&from, &fromlen); Once again, the fromlen parameter is handled in a value-result fashion, initially containing the size of the from buffer, and modified on return to indicate the actual size of the address from which the datagram was received.
In addition to the two calls mentioned above, datagram sockets may also use the connect call to associate a socket with a specific destination address. In this case, any data sent on the socket will automatically be addressed to the connected peer, and only data received from that peer will be delivered to the user. Only one connected address is permitted for each socket at one time; a second connect will change the destination address, and a connect to a null address (family AF_UNSPEC) will disconnect. Connect requests on datagram sockets return immediately, as this simply results in the system recording the peer's address (as compared to a stream socket, where a connect request initiates establishment of an end to end connection). Accept and listen are not used with datagram sockets.
While a datagram socket socket is connected, errors from recent send calls may be returned asynchronously. These errors may be reported on subsequent operations on the socket, or a special socket option used with getsockopt, SO_ERROR, may be used to interrogate the error status. A select for reading or writing will return true when an error indication has been received. The next operation will return the error, and the error status is cleared. Other of the less important details of datagram sockets are described in section 5. Input/Output multiplexing
One last facility often used in developing applications is the ability to multiplex i/o requests among multiple sockets and/or files. This is done using the select call: #include <sys/time.h> #include <sys/types.h> ... fd_set readmask, writemask, exceptmask; struct timeval timeout; ... select(nfds, &readmask, &writemask, &exceptmask, &timeout); Select takes as arguments pointers to three sets, one for the set of file descriptors for which the caller wishes to be able to read data on, one for those descriptors to which data is to be written, and one for which exceptional conditions are pending; out-of-band data is the only exceptional condition currently implemented by the socket If the user is not interested in certain conditions (i.e., read, write, or exceptions), the corresponding argument to the select should be a null pointer.
Each set is actually a structure containing an array of long integer bit masks; the size of the array is set by the definition FD_SETSIZE. The array is be long enough to hold one bit for each of FD_SETSIZE file descriptors.
The macros FD_SET(fd, &mask) and FD_CLR(fd, &mask) have been provided for adding and removing file descriptor fd in the set mask. The set should be zeroed before use, and the macro FD_ZERO(&mask) has been provided to clear the set mask. The parameter nfds in the select call specifies the range of file descriptors (i.e. one plus the value of the largest descriptor) to be examined in a set.
A timeout value may be specified if the selection is not to last more than a predetermined period of time. If the fields in timeout are set to 0, the selection takes the form of a poll, returning immediately. If the last parameter is a null pointer, the selection will block indefinitely*. .FS * To be more specific, a return takes place only when a descriptor is selectable, or when a signal is received by the caller, interrupting the system call. .FE Select normally returns the number of file descriptors selected; if the select call returns due to the timeout expiring, then the value 0 is returned. If the select terminates because of an error or interruption, a -1 is returned with the error number in errno, and with the file descriptor masks unchanged.
Assuming a successful return, the three sets will indicate which file descriptors are ready to be read from, written to, or have exceptional conditions pending. The status of a file descriptor in a select mask may be tested with the FD_ISSET(fd, &mask) macro, which returns a non-zero value if fd is a member of the set mask, and 0 if it is not.
To determine if there are connections waiting on a socket to be used with an accept call, select can be used, followed by a FD_ISSET(fd, &mask) macro to check for read readiness on the appropriate socket. If FD_ISSET returns a non-zero value, indicating permission to read, then a connection is pending on the socket.
As an example, to read data from two sockets, s1 and s2 as it is available from each and with a one-second timeout, the following code might be used: #include <sys/time.h> #include <sys/types.h> ... fd_set read_template; struct timeval wait; ... for (;;) { wait.tv_sec = 1; /* one second */ wait.tv_usec = 0; FD_ZERO(&read_template); FD_SET(s1, &read_template); FD_SET(s2, &read_template); nb = select(FD_SETSIZE, &read_template, (fd_set *) 0, (fd_set *) 0, &wait); if (nb <= 0) { An error occurred during the select, or the select timed out. } if (FD_ISSET(s1, &read_template)) { Socket #1 is ready to be read from. } if (FD_ISSET(s2, &read_template)) { Socket #2 is ready to be read from. } }
In 4.2, the arguments to select were pointers to integers instead of pointers to fd_sets. This type of call will still work as long as the number of file descriptors being examined is less than the number of bits in an integer; however, the methods illustrated above should be used in all current programs.
Select provides a synchronous multiplexing scheme. Asynchronous notification of output completion, input availability, and exceptional conditions is possible through use of the SIGIO and SIGURG signals described in section 5.