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Spring 91 - MACTCP COOKBOOK: CONSTRUCTING NETWORK-AWARE APPLICATIONS

MACTCP COOKBOOK: CONSTRUCTING NETWORK-AWARE APPLICATIONS

STEVE FALKENBURG

[IMAGE p46-69_Falkenburg_text_1.GIF]

The Macintosh is now a full-fledged player in the world of TCP/IP networking. MacTCP, an implementation of TCP/IP for the Macintosh, lets applications take advantage of a protocol suite that is used extensively by many makes of computers. This article attempts to demystify the process of MacTCP programming and provides a library of calls that can be used easily by anyone familiar with Macintosh programming.

TCP/IP, which stands for Transmission Control Protocol/Internet Protocol, was developed by the U.S. Department of Defense Advanced Research Products Agency (DARPA) and used initially on the ARPANET, a national research network created by DARPA in the late 1960s. Although the ARPANET no longer exists, the TCP/IP protocols are used on many large-scale networks. Many of these networks are interconnected and are known collectively as the Internet.

The TCP/IP protocol stack, shown in Figure 1, is composed of several layers. At the lowest layer, the Internet Protocol (IP) handles transmitting packets of information from one host to another. Above this network level, TCP/IP provides two transport layer protocols: Transmission Control Protocol (TCP) and User Datagram Protocol (UDP). TCP provides reliable connection-based service, while UDP is not connection based. The MacTCP ® driver gives the programmer interfaces to TCP and UDP, but not to the lower-level IP. This article deals only with TCP programming. For information on MacTCP UDP programming, consult the MacTCP Programmer's Guide.

Several application-level protocols use TCP to provide user-level service. The Simple Mail Transfer Protocol (SMTP) is used to send electronic mail, the Network News Transfer Protocol (NNTP) is used to transfer and post news, the File Transfer Protocol (FTP) is used to transfer files between machines, and the Finger protocol is used to retrieve user information. MacTCP does not include programming interfaces or implementations for any of these application-level protocols.

With connection-based protocols, such as TCP, a connection is defined as a bidirectional open line of communication between two hosts. Data is guaranteed to be received in the same order as it was sent, and in TCP, data reliability is ensured. To open a connection between two computers, the initiating program sends an open command containing the network address of the remote computer to MacTCP. If the remote computer is listening for a connection, it acknowledges the connection, and data can then be transferred on the connection stream. If the remote computer is not listening for a connection, the open command fails. Once all transactions have been completed, the connection may be closed by either computer.

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Figure 1 TCP/IP Protocol Stack

Network addressing is essential to this process. Each device connected to a TCP/IP network is assigned a unique 4-byte address, also known as the IP number, as shown in Table 1. A unique name can also be assigned to each network entity. MacTCP provides a name service mechanism, called the Domain Name Resolver (DNR), which translates network names to addresses and vice versa. In addition, there's a 2-byte port number associated with TCP connections. Each port number is usually mapped to the type of application-level service the sender is requesting. For example, the NNTP protocol always operates on TCP port 119. This mapping is shown in Table 2. MacTCP does not provide a service for translating between service name and port number.


Table 1 Network Name to Address Mapping

Network NameIP Number
sol.engin.umich.edu141.212.4.65
mondo.engin.umich.edu141.212.68.14
freebie.engin.umich.edu141.212.68.23
daneel.engin.umich.edu141.212.68.30
maize.engin.umich.edu141.212.10.56

Table 2 Service to Port Number Mapping

Service
ftp21
telnet23
smtp25
finger79
nntp119

MACTCP PROGRAMMING BASICS

This section focuses on the basics of MacTCP programming, while the remainder of the article discusses high-level, easy-to-use routines. MacTCP is a driver for the Macintosh that provides access to TCP/IP through the Device Manager. By making standard Device Manager PBControl calls, programs can access TCP/IP transport protocols to control and maintain connections. The standard TCP parameter block, defined in TCPPB.h, is shown here:
typedef struct TCPiopb {
    char        fill12[12];
    TCPIOCompletionProc ioCompletion;
                                  /* address of completion routine */
    short       ioResult;   /* result of call (>0 = incomplete) */
    char        *ioNamePtr;
    short       ioVRefNum;
    short       ioCRefNum;  /* MacTCP driver reference number */
    short       csCode;     /* command code */
    StreamPtr   tcpStream;  /* pointer to stream buffer */
    union {
        struct TCPCreatePB create; /* var for TCPCreate,TCPRelease */
        struct TCPOpenPB    open;   
                           /* var for TCPActiveOpen,TCPPassiveOpen */
        struct TCPSendPB    send;   /* var for TCPSend */
        struct TCPReceivePB receive;    
                    /* var for TCPNoCopyRcv,TCPRcvBfrReturn,TCPRcv */
        struct TCPClosePB   close;  /* var for TCPClose */
        struct TCPAbortPB   abort;  /* var for TCPAbort */
        struct TCPStatusPB  status; /* var for TCPStatus */
        struct TCPGlobalInfoPB globalInfo;  
                                          /* var for TCPGlobalInfo */
        } csParam;
} TCPiopb;

From the start of the parameter block up to the tcpStream parameter, this structure is a standard Device Manager control block. Three fields must be filled in:

  • csCode, which specifies the driver command to be executed.
  • ioCRefNum, which contains the MacTCP driver reference number.
  • tcpStream, which contains a pointer to the pertinent TCP stream. This stream pointer, which is described in more detail later, is the unique identifier for a connection.

If the call is made asynchronously, as all MacTCP calls may be, a pointer to a completion routine can be specified in ioCompletion. Depending on the type of call made, there are various ways to fill in the union at the end of this parameter block.

TCP DRIVER CALLS
A description of some of the common commands and their parameter blocks follows. Unless otherwise specified, a value of zero for any parameter indicates the default value.

TCPCreate   (csCode = 30)
TCPRelease  (csCode = 42)
typedef struct TCPCreatePB {
    Ptr             rcvBuff;    /* pointer to area allocated
                                   for stream buffer */
    unsigned long   rcvBuffLen; /* length of stream buffer */
    TCPNotifyProc   notifyProc; /* address of asynchronous
                                   notification routine */
    Ptr userDataPtr;
}TCPCreatePB;

TCPCreate allocates a MacTCP stream to be used for opening or listening for a connection. The rcvBuff parameter should be a pointer to a block of memory previously allocated as a stream buffer; set rcvBuffLen to the length of that buffer. If you want to be interrupted when the connection state changes, set notifyProc to the address of an asynchronous notification routine (ASR). The code in this article doesn't make use of the ASR feature, so in this case notifyProc should be set to nil. A pointer to the created stream is returned in tcpStream.

TCPRelease removes the stream pointed to by tcpStream from all MacTCP-internal stream lists. It returns a pointer to the stream buffer in rcvBuff. When TCPRelease completes successfully, this buffer should be disposed of by the calling program.

TCPPassiveOpen  (csCode = 31)
TCPActiveOpen   (csCode = 32)
typedef struct TCPOpenPB {
    byte        ulpTimeoutValue;     /* upper-layer protocol
                                        timeout */
    byte        ulpTimeoutAction;    /* upper-layer protocol
                                        timeout action */
    byte        validityFlags;       /* validity flags for options */
    byte        commandTimeoutValue; /* timeout value for command */
    ip_addr     remoteHost;          /* IP address of the remote
                                        host */
    tcp_port    remotePort;          /* TCP port to connect to on the
                                        remote host */
    ip_addr     localHost;           /* local IP number */
    tcp_port    localPort;           /* local port from which
                                        connection originates */
    byte        tosFlags;            /* type of service requested */
    byte        precedence;          /* priority of connection */
    Boolean     dontFrag;            /* if true, don't fragment
                                        packets */
    byte        timeToLive;          /* maximum number of hops for
                                        packets */
    byte        security;            /* security option byte */
    byte        optionCnt;           /* number of IP options */
    byte        options[40];         /* other IP options (def in
                                        RFC 894) */
    Ptr         userDataPtr;
}TCPOpenPB;

TCPPassiveOpen listens for an incoming connection from a specific host and port. The command completes when a remote host connects to the port monitored by this command. Store the remote host address in remoteHost and specify the remote TCP port number in remotePort. Connections from any host and port are possible if these values are set to zero. Set the localPort parameter to the local TCP port number or to zero to assign an unused port. ULP ("ulp" in the parameter names) stands for upper-layer protocol. The ulpTimeoutValue parameter is the maximum amount of time MacTCP allows for a connection to complete after the connection process has started. If this timeout is reached, and the value of ulpTimeoutAction is zero, the ASR, if present, is called and the ULP timer is reset. When the timeout is reached, if ulpTimeoutAction is nonzero, the command returns an error. The validityFlags parameter indicates which of the other command parameters are specified explicitly. Bit 6 is set if the ULP timeout action is valid; bit 7 is set if the ULP timeout value is valid. Descriptions for the rest of the entries in this structure can be found in the MacTCP Programmer's Guide. In most cases, they can all be set to zero, indicating default values should be used.

TCPActiveOpen attempts to initiate a connection with a remote host and completes when this connection is established. The parameters are identical to TCPPassiveOpen, with the following exceptions: There's no command timeout parameter, although the ULP timeout is available. You must fully specify the remote host address and remote port in remoteHost and remotePort, respectively, since it's impossible to initiate a connection to an arbitrary host without direction.

TCPSend (csCode = 34)
typedef struct TCPSendPB {
    byte           ulpTimeoutValue;   /* upper-layer protocol
                                         timeout */
    byte           ulpTimeoutAction;  /* upper-layer protocol
                                         timeout action */
    byte           validityFlags;     /* validity flags for
                                         options */
    Boolean        pushFlag;          /* true if data should be sent
                                         immediately */
    Boolean        urgentFlag;        /* identifies the data as
                                         important */
    Ptr            wdsPtr;            /* pointer to write data
                                         structure */
    unsigned long  sendFree;
    unsigned short sendLength;
    Ptr            userDataPtr;
}TCPSendPB;

TCPSend sends data to a remote host along an open connection stream. The definitions of ulpTimeoutValue, ulpTimeoutAction, and validityFlags are the same as in TCPPassiveOpen. The pushFlag parameter is set if the data should be sent immediately and the urgentFlag option can be set to indicate that the data is important. The data to be sent is stored in a write data structure (WDS). The format of this structure is simply an array of wdsEntry structures, which are length/pointer pairs. You terminate the WDS by setting the last entry's length to zero.

typedef struct wdsEntry {
    unsigned short length;  /* length of buffer */
    char * ptr;     /* pointer to buffer */
} wdsEntry;

TCPRcv     (csCode = 37)
typedef struct TCPReceivePB {   /* for receive and return rcv buff
                                   calls */
    byte           commandTimeoutValue;   /* timeout value for 
                                             command */
    byte           filler;
    Boolean        markFlag;   /* true if start of read data
                                  structure is urgent data */
    Boolean        urgentFlag; /* true if read data structure
                                  ends in urgent data */
    Ptr            rcvBuff;    /* pointer to data that has been
                                  received */
    unsigned short rcvBuffLen; /* amount of data in bytes that has
                                  been received */
    Ptr            rdsPtr;     /* pointer to read data structure */
    unsigned short rdsLength;   
    unsigned short secondTimeStamp;
    Ptr            userDataPtr;
}TCPReceivePB;

TCPRcv receives incoming data on an already established connection. Allocate a buffer for the incoming data and store a pointer to this location in rcvBuff. Store the maximum length to be received in rcvBuffLen. This value is changed to the number of bytes received when the command completes. Store the timeout value for this command in commandTimeoutValue. Finally, use markFlag and urgentFlag to delimit the start and end of urgent data blocks.

TCPClose    (csCode = 38)
typedef struct TCPClosePB {
    byte ulpTimeoutValue;   /* upper-layer protocol timeout */
    byte ulpTimeoutAction;  /* upper-layer protocol timeout action */
    byte validityFlags;     /* validity flags for options */
    Ptr  userDataPtr;
}TCPClosePB;

TCPClose indicates to the remote host that the caller has no more data to send on the connection. It's assumed that the remote host will then issue a close command, which permits the connection to close. However, the connection will not close until both hosts issue this command. The parameters to this call are described in other calls.

TCPAbort    (csCode = 39)
typedef struct TCPAbortPB {
    Ptr userDataPtr;
}TCPAbortPB;

TCPAbort closes a connection without asking the remote host for permission. This command does not ensure that all data transfers have completed. The parameters to this call are described earlier.

MEDIUM-LEVEL TCP CALLS
Since calling the Device Manager is a painful experience for some programmers, a small library of intermediate routines can speed up development time. This article includes such a library. There's one medium-level call provided for each associated TCP driver command, so these calls simply isolate programmers from filling out parameter blocks. This seems to be a big plus with most programmers. The source code for the calls is on the Developer Essentials disc.

Several of the medium-level calls have hooks that allow them to be called asynchronously. Any procedure containing an async flag can be called in this manner. If this is done, the parameter block used to make the call is returned in returnBlock. The calling program must then poll returnBlock- >ioResult to determine when the command has completed. As with any other Device Manager call, the ioResult field remains positive while the command is executed and then changes to zero or a negative value upon completion, indicating the call's result code. Any number of calls may be awaiting completion, since medium-level routines dynamically allocate space for parameter blocks. Completion routine hooks are not provided by these routines, but could easily be added.

If a medium-level routine is called with the async flag false, or if the routine doesn't have an async flag, the underlying PBControl call is still called asynchronously. While awaiting completion of the command, the medium-level routines call a routine defined by

Boolean GiveTime(unsigned long sleepTime);

This callback permits the application to carry out other small tasks. In the examples in this article, GiveTime spins the cursor and calls WaitNextEvent from a secondary main event loop to handle a subset of normal program operation and to give other applications time to execute.

At this point, you may be thinking that figuring out proper values for these routines is as much of a pain as filling out parameter blocks. For this reason, another set of high-level routines is provided for sending and receiving data.

HIGH-LEVEL TCP CALLS
High-level TCP calls further simplify and generalize the process of calling MacTCP. Write data structures are not required to send data, only a single timeout value is allowed, and other TCP specifics (mark, push, and so on) have been removed. Instead, the high-level calls are a core set of routines that are both understandable and easy to use. In most cases, it's to your advantage to use these routines, since they abstract the MacTCP programming interface to resemble a generic connection-based protocol scheme. This opens the possibility of porting these high-level routines to another protocol stack, such as AppleTalk ®. In fact, if the high-level routines were modified to use AppleTalk Datastream Session Protocol (ADSP), any code written using the high-level calls for MacTCP could be compiled for use on an AppleTalk network. The source code for these routines is on the Developer Essentials disc.

The operation of high-level MacTCP calls is fairly straightforward. For each asynchronous routine, there's a corresponding routine to call when the asynchronous command completes. The moment of completion can be determined by polling returnBlock->ioResult. This returnBlock parameter is the same as the one returned by the medium-level routines and contains the MacTCP parameter block used in the pending asynchronous call.

NAME RESOLVER CALLS
Before you can build a useful network application, you must consider name-to-address resolution. Name-to-address resolution provides a means of converting from domain names (unique string identifiers for computers on a TCP/IP network) to IP numbers (4-byte addresses) and vice versa. In general, people can remember the name of a computer (for example, goofy.apple.com) more easily than they can remember a network address (for example, 90.1.0.10). Translation tables between the names and numbers can be stored on the local machine (the MacTCP Hosts file, for example) or on a remote server. Remote access to network numbers is provided through the Domain Name Server protocol. The MacTCP Domain Name Resolver allows name/address translations using both the static table and remote server methods.

The MacTCP Developer's Kit (APDA #M0230LL/D) ships with the file dnr.c, a set of C routines providing a programming interface to the name resolver. Several important calls from this code module are described here.

OSErr OpenResolver(char *fileName);

OpenResolver initializes the name resolver. As a parameter to the call, you can supply a local file containing important hosts. Passing nil for this filename defaults to the Hosts file in the current System Folder.

OSErr StrToAddr(char *hostName, struct hostInfo *hostInfoPtr,
        ResultProcPtr ResultProc, char *userDataPtr);

StrToAddr converts a string of the form "W.X.Y.Z" or "goofy.apple.com" to its 4-byte IP address. The hostInfo struct that you pass in looks like this:

typedef struct hostInfo {
    long            rtnCode;
    char            cname [255];
    unsigned long   addr [NUM_ALT_ADDRS];
};

OSErr AddrToName(ip_addr addr, struct hostInfo *hostInfoPtr,
    ResultProcPtr ResultProc, char *userDataPtr);

AddrToName performs a reverse lookup to retrieve a host name, given a 4-byte IP address.

OSErr CloseResolver();

CloseResolver closes the resolver and deallocates the resources and the address cache that has accumulated.

Calls to these dnr.c routines can be combined to construct a fairly simple routine to convert host names to IP addresses:

/* CvtAddr.c
Converts host names to IP numbers
written by Steve Falkenburg
*/
#include <Types.h>
#include <MacTCPCommonTypes.h>
#include <AddressXLation.h>
#include "CvtAddr.h"

pascal void DNRResultProc(struct hostInfo *hInfoPtr,char *userDataPtr);

/*  ConvertStringToAddr is a simple call to get a host's IP number, given the name
    of the host.
*/

OSErr ConvertStringToAddr(char *name,unsigned long *netNum)
{
    struct   hostInfo hInfo;
    OSErr    result;
    char     done = 0x00;

    if ((result = OpenResolver(nil)) == noErr) {
        result = StrToAddr(name,&hInfo,DNRResultProc,&done);
        if (result == cacheFault)
            while (!done)
                ; /* wait for cache fault resolver to be called by
                     interrupt */
        CloseResolver();
        if ((hInfo.rtnCode == noErr) ||
            (hInfo.rtnCode == cacheFault)) {
            *netNum = hInfo.addr[0];
            strcpy(name,hInfo.cname);
            name[strlen(name)-1] = '\0';
            return noErr;
        }
    }
    *netNum = 0;

    return result;
}

/*  This is the completion routine used for name resolver calls.
    It sets the userDataPtr flag to indicate the call has completed.
*/
pascal void DNRResultProc(struct hostInfo *hInfoPtr,
        char *userDataPtr)
{
#pragma unused (hInfoPtr)

    *userDataPtr = 0xff;    /* Setting the user data to nonzero means
                               we're done. */
}

PROGRAMMING A SIMPLE MACTCP APPLICATION

The core set of routines provided in the earlier sections makes it fairly easy to write a small but useful TCP application. These core routines can be stacked together to form a framework for a basic MacTCP application, as shown in Figure 2. Note that no module accesses a module that's farther than one level away. This provides the programmer the flexibility needed to improve the networking library or switch protocol stacks without losing functionality.

Figure 2 Finger Code Modularization

One of the simplest and most widely used network utilities is finger. Implemented on most UNIX workstations, finger is used to retrieve personal information (such as phone number and address) for a particular user. The finger utility accesses information through the Finger User Information Protocol, discussed in RFC 1194 on the Developer Essentials disc. The protocol operates on a client/server model. Figure 3 is a diagram of a sample transaction.

Client Server1. Client sends "sfalken" to retrieve information on user sfalken.

[IMAGE p46-69_Falkenburg_text_4.GIF]

  1. Client receives "Phone: 555-1234"
  2. Server retrieves information from from server and closes connection. database: "Phone: 555-1234"

Figure 3Finger Protocol Transaction


The code for a simple MPW tool to implement the Finger protocol is shown here, with accompanying explanation:


/* MacTCP finger client            */
/* written by Steven Falkenburg     */
/*                      */

#include <Types.h>
#include <Memory.h>
#include <Packages.h>
#include <CursorCtl.h>
#include <String.h>

#include "CvtAddr.h"
#include "MacTCPCommonTypes.h"
#include "TCPPB.h"
#include "TCPHi.h"

/* constants */

#define kFingerPort 79  /* TCP port assigned for finger protocol */
#define kBufSize 16384  /* Size in bytes for TCP stream buffer and
                           receive buffer */
#define kTimeOut 20     /* Timeout for TCP commands (20 sec. pretty
                           much arbitrary) */

/* function prototypes */

void main(int argc,char *argv[]);
OSErr Finger(char *userid,char *hostName,Handle *fingerData);
OSErr GetFingerData(unsigned long stream,Handle *fingerData);
void FixCRLF(char *data);
Boolean GiveTime(short sleepTime);

/* globals */

Boolean gCancel = false;  /* This is set to true if the user
                             cancels an operation. */

/*  main entry point for finger                 */
/*                                              */
/*  usage: finger <user>@<host>                 */
/*                                              */
/*  This function parses the args from the command line,        */
/*  calls Finger() to get info, and prints the returned info.   */

void main(int argc,char *argv[])
{
    OSErr err;
    Handle theFinger;
    char userid[256],host[256];
    
    if (argc != 2) {
        printf("Wrong number of parameters to finger call\n");
        return;
    }
    
    sscanf(argv[1],"%[^@]@%s",userid,host);
    
    strcat(userid,"\n\r");
    
    err = Finger(userid,host,&theFinger);
    
    if (err == noErr) {
        HLock(theFinger);
        FixCRLF(*theFinger);
        printf("\n%s\n",*theFinger);
        DisposHandle(theFinger);
    }
    else
        printf("An error has occurred: %hd\n",err);
}

/*  Finger()    */
/*  This function converts the host string to an IP number, */
/*  opens a connection to the remote host on TCP port 79, sends */
/*  the id to the remote host, and waits for the information on */
/*  the receiving stream. After this information is sent, the   */
/*  connection is closed down.      */

OSErr Finger(char *userid,char *hostName,Handle *fingerData)
{
    OSErr err;
    unsigned long ipAddress;
    unsigned long stream;
    
    /* open the network driver */
    
    err = InitNetwork();
    if (err != noErr)
        return err;
    
    /* get remote machine's network number */
    
    err = ConvertStringToAddr(hostName,&ipAddress);
    if (err != noErr)
        return err;
    
    /* open a TCP stream */
    
    err = CreateStream(&stream,kBufSize);
    if (err != noErr)
        return err;
    
    err = OpenConnection(stream,ipAddress,kFingerPort,kTimeOut);
    if (err == noErr) {
        err = SendData(stream,userid,
            (unsigned short)strlen(userid),false);
        if (err == noErr)
            err = GetFingerData(stream,fingerData);
        CloseConnection(stream);
    }
    
    ReleaseStream(stream);
    return err;
}

OSErr GetFingerData(unsigned long stream,Handle *fingerData)
{
    OSErr err;
    long bufOffset = 0;
    unsigned short dataLength;
    Ptr data;
    
    *fingerData = NewHandle(kBufSize);
    err = MemError();
    if (err != noErr)
        return err;
        
    HLock(*fingerData);
    data = **fingerData;
    dataLength = kBufSize;
    
    do {
        err = RecvData(stream,data,&dataLength,false);
        if (err == noErr) {
            bufOffset += dataLength;
            dataLength = kBufSize;
            HUnlock(*fingerData);
            SetHandleSize(*fingerData,bufOffset+kBufSize);
            err = MemError();
            HLock(*fingerData);
            data = **fingerData + bufOffset;
        }
    } while (err == noErr);
    
    data[0] = '\0';
    
    HUnlock(*fingerData);
    if (err == connectionClosing)
        err = noErr;
}

/* FixCRLF() removes the linefeeds from a text buffer. This is  */
/* necessary, since all text on the network is embedded with    */
/* carriage return linefeed pairs.      */

void FixCRLF(char *data)
{
    register char *source,*dest;
    long length;
    
    length = strlen(data);
    
    if (*data) {
        source = dest = data;
        while ((source - data) < (length-1)) {
            if (*source == '\r')
                source++;
            *dest++ = *source++;
        }
        if (*source != '\r' && (source - data) < length)
            *dest++ = *source++;
        length = dest - data;
    }
    
    *dest = '\0';
}

/* This routine would normally be a callback for giving time to */
/* background apps.                             */
    
Boolean GiveTime(short sleepTime)
{
    SpinCursor(1);
    return true;
}

The main points in the execution of this program can be traced as follows:

  1. Get userid, host
  2. Initialize MacTCP InitNetwork();
  3. Get address of host ConvertStringToAddr(hostName,&ipAddress);
  4. Make a TCP streamCreateStream(&stream,kBufSize);
  5. Connect to finger hostOpenConnection(stream,ipAddress,kFingerPort,kTimeOut);
  6. Send userid across stream SendData(stream,userid,(unsigned short)strlen(userid),false);
  7. Receive finger information RecvData(stream,data,&dataLength,false); from stream
  8. Close connection CloseConnection(stream);
  9. Release stream ReleaseStream(stream);
  10. Quit program

Once the host name and user ID are received, MacTCP is initialized by a call to InitNetwork. The IP number of the host is then retrieved by a call to ConvertStringToAddr. If this is successful, CreateStream creates a TCP stream, and OpenConnection opens a connection on that stream to the finger port on the remote host. Next, SendData sends the user ID along this connection. The fingerinformation is received through repeated calls to RecvData. Once all data has been sent, CloseConnection closes the connection, and the stream is removed with

ReleaseStream. Finally, the program terminates, leaving the MacTCP driver open. Here's a sample run of the finger tool in action:

finger sfalken mondo.engin.umich.edu <enter> 
Login name: sfalken In real life: Steven
Falkenburg
Phone: 555-1234
Directory: /u/sfalken Shell: /bin/csh
On since Feb 26 07:08:28 on ttyp2 from rezcop.engin.umi
No Plan.

The code for the finger tool can be compiled and linked with the high- and medium-level routines into an MPW tool. This example shows how a standard network protocol can be easily encapsulated into a simple program. The high-level networking calls act somewhat like a Macintosh Toolbox Manager, since they encapsulate the complexity of TCP programming. These routines can be used by any programmer with a user-level knowledge of networking, making network programming less of a mysterious art.

NEWSWATCHER--A COMPLEX TCP APPLICATION

The finger tool is limited in scope, but the same techniques can be used to construct more complex programs. NewsWatcher, a Macintosh-based network news reader, provides an example.

One of the most popular services available on the Internet is network news, an international forum for discussion of almost any topic you can imagine. Anyone can post and read messages. Since thousands of messages are posted every day, the messages are divided into more than a thousand newsgroups. Examples of these diverse groups include

comp.sys.mac.programmerMacintosh programming discussions
comp.sys.mac.miscMiscellaneous Macintosh ramblings
alt.flame.spellingPeople insulting others' spelling skills
alt.alien.visitorsDiscussions of alien life--intelligent or otherwise

The volume of network news bombarding readers necessitates a usable interface. I wrote NewsWatcher to provide that interface.

NAVIGATING THE NET WITH NEWSWATCHER
Before plunging into a deep technical discussion of the NewsWatcher code structure, a brief description of the interface is in order. A typical NewsWatcher screen is shown in Figure 4.

The program is based on a multiwindow browser interface, similar to the Finder (without the icons). A window containing all active newsgroups is always available, and users can get a full article subject list for a group by double-clicking that group's name. This opens up another browsing window, containing the subjects. These subjects, in turn, can be opened to display the full text of individual articles in separate windows. A user who wants to keep track of a few specific newsgroups and see only new articles in those groups can "subscribe" to the specific groups. This is done by choosing New Group Window from the File menu, which creates an empty group window. The desired groups can then be dragged from the main Newsgroups window to this custom group window. The new window can be browsed like any other group window. When the user is ready to quit, the custom group list can either be saved to disk or uploaded to a UNIX ® news file (using FTP). This gives users access to their group list from multiple computers.

[IMAGE p46-69_Falkenburg_text_5.GIF]

Figure 4 A Typical NewsWatcher Screen

Not surprisingly, the user interface code for NewsWatcher, though complex, has virtually nothing to do with MacTCP programming and is beyond the scope of this article. This code, however, is included on the Developer Essentials disc for those interested in a multiwindow browsing system.

NEWSWATCHER NNTP ROUTINES
The Network News Transfer Protocol (NNTP) is easily implemented from the medium-level TCP calls described in the first part of this article. (For information on NNTP, see "Introduction to NNTP.") The NNTP calls in NewsWatcher are encapsulated into a module named NNTPLow.c. The external functions in this module, along with a description of their purpose, are described here:

OSErr StartNNTP(void);

StartNNTP initializes the network resources needed to establish an NNTP connection with a remote server. Once all proper drivers have been initialized, this routine opens a connection to the local NNTP server, whose name is stored in the program's configuration file. This connection is maintained throughout the life span of the program.

OSErr CloseNewsConnection(void);

CloseNewsConnection gracefully terminates a connection with a remote NNTP server. This command is executed only when NewsWatcher is quitting.

OSErr ResetConnection(void);

ResetConnection is called when a communication error occurs between the client and server. This routine terminates and attempts to reestablish connection to the NNTP server.

OSErr GetGroupList(short *numGroups);

GetGroupList puts the list of newsgroups in the global variable gGroupList. To get the list of groups from the server, the command LIST is sent to the server, which responds with a list of allknown newsgroups.

OSErr GetMessages(char *newsGroup,long first,long last,
TSubject *subjects,long *numSubjects,char *hdrName);
GetMessages returns a set of subject headers specified by group name and article number range. This routine operates by sending the command XHDR groupname first-last to the NNTP server and parsing the response.

OSErr GetArticle(char *newsGroup,char *article,char **text,
        long *length, long maxLength);

GetArticle retrieves the full text of the article in group newsGroup named article . This procedure sends the command ARTICLE article to the NNTP server.

These procedures are called in response to user requests for articles and subject lists. As you can see, there's absolutely no network dependence at this point, even on the NNTP protocol. It would be trivial to write a set of routines with identical function prototypes that treated a file hierarchy on a hard disk as a set of newsgroups and articles. This layered isolation approach is of critical importance in network programming, since network-level protocols may change while applications remain the same.

OTHER PROTOCOLS IN NEWSWATCHER
In addition to NNTP, several other network protocols are implemented for NewsWatcher. These protocols include the Simple Mail Transfer Protocol (SMTP) and the File Transfer Protocol (FTP).

Simple Mail Transfer Protocol (SMTP). SMTP gives users the ability to respond to article postings through electronic mail. This protocol, like NNTP, operates on a request-response stream. SMTP servers listen on TCP port 25, and the protocol is described in detail in RFC 821 on the Developer Essentials disc. NewsWatcher contains a single procedure, SendSMTP, that takes care of setting up a connection to the server, sending the message, and disconnecting. The function prototype for this function is as follows:

Boolean SendSMTP(char *text,unsigned short tLength);

The SendSMTP code is contained within the SMTPLow.c code module. This separation allows the code to be used in other programs easily. The routine calls functions in TCPLow.c (the medium- level routines) and is called from netstuff.c.

File Transfer Protocol (FTP). NewsWatcher includes FTP-based routines that allow users to send lists of newsgroups to, and receive them from, a file on a remote machine. This protocol uses a control stream, running on TCP port 21, to set up file transfers. When a transfer is initiated, a secondary data stream is opened on a negotiated TCP port. The file transfer is completed by means of this secondary stream, and the data stream is then closed down. For a detailed description of the protocol and command set used, see RFC 959 on the Developer Essentials disc. To shield programmers from the complications of FTP, I wrote several high-level routines to implement it:

OSErr FTPInit(ProcPtr statusCallback);

FTPInit initializes network resources required for FTP transfers. The single parameter provided is a pointer to a callback procedure called when a status message is received from a remote host.

OSErr FTPFinish(void);

Call this routine when file transfers are finished to release network resources used to support the FTP session.

OSErr FTPConnect(unsigned long *connID,char *address,char *userID,
char *password);
Call FTPConnect with the address of the remote machine, along with the user ID and password of the account needed to access that machine.

OSErr FTPDisconnect(unsigned long connID);

After all transfers to a particular host have been completed, call this routine to disconnect from the remote host.

OSErr FTPViewFile(unsigned long connID,Ptr *file,char *fileName);

FTPViewFile retrieves a file from the remote machine and stores its data in a pointer allocated within the function.

OSErr FTPPutFile(unsigned long connID,char *fileName,char *data,
    long size);

FTPPutFile is used to send a file to a remote machine. The remote filename and the file data and length must be given.

These routines are contained in FTPLow.c, using the same modular layered approach as for other protocols.

NEWSWATCHER CODE MODULARITY
As in the finger example, the NewsWatcher source code is structured for maximum flexibility in case of a protocol switch and to allow for ease in code sharing. The logical code blocks are shown in Figure 5. The FTPLow.c module, for example, could easily be extended and used as a generic file transfer module in many applications. It simply requires the medium- and high-level TCP calls described earlier.

[IMAGE p46-69_Falkenburg_text_6.GIF]

Figure 5NewsWatcher Code Modularization

SUMMARY

Although programming with MacTCP can be fairly complicated from a low-level point of view, using high-level routines makes it much simpler to create an easy-to-use TCP networking module. Once difficult issues, such as asynchronous behavior, are handled at a low level, higher-level modules can successively add protocol support for mail, news, and file transfer. These high-level routines can be used by any programmer, regardless of network-level knowledge.

INTRODUCTION TO NNTP

The Network News Transfer Protocol (NNTP) is a popular protocol for transmitting and accessing network news on the Internet. This protocol, which runs out of TCP port 119 on a news host computer, is described in detail in RFC 977 on the Developer Essentials disc. NNTP, like finger, is based on the client-server model. An NNTP server, usually one per site, stores a copy of each newsgroup and new article. Clients contact this server to request news or post new articles.

The protocol is based on a request-response model. Clients contact the NNTP server, make a request, and receive a response. A sample session with an NNTP server is shown below. Commands sent from the client (in this case a Macintosh) are shown in italics.

200 srvr1.engin.umich.edu NNTP server version 1.5.10 + serve.c GNUS
patch (3 October 1990) ready at Tue Oct 9 23:46:12 1990 (posting ok).

HELP

100 This server accepts the following commands:
ARTICLE     BODY        GROUP
HEAD        LAST        LIST
NEXT        POST        QUIT
STAT        NEWGROUPS   HELP
IHAVE       NEWNEWS     SLAVE

Additionally, the following extension is supported:

XHDR Retrieve a single header line from a range of articles.

Bugs to Stan Barber (Internet: nntp@tmc.edu; UUCP: ...!bcm!nntp)
.
GROUP comp.sys.mac.misc
211 281 3035 3744 comp.sys.mac.misc
ARTICLE 3035
220 3035 <1990Sep14.145124.25214@midway.uchicago.edu> 
        Article retrieved; head and body follow.

...article text here...
.
QUIT
205 srvr1.engin.umich.edu closing connection.  Goodbye.

For information on the TCP/IP protocol stack and networking in general:

  • Douglas E. Comer: Internetworking with TCP/IP , Prentice Hall, 1991.
  • Andrew S. Tanenbaum: Computer Networks , Prentice Hall, 1988.

For an in-depth discussion of some of the protocols:

  • J. B. Postel: Simple Mail Transfer Protocol, RFC #821, August 1982.
  • J. B. Postel and J. K. Reynolds: File Transfer Protocol, RFC #959, October 1985.
  • Kantor and P. Lapsley: Network News Transfer Protocol, RFC #977, February 1986.
  • D. P. Zimmerman: Finger User Information Protocol, RFC #1194, November 1990.

These RFCs are on the Developer Essentials disc. The library of RFC (Request For Comment) memos is available by anonymous FTP from NIS.NSF.NET in the directory RFC. Use FTP to connect to this host and useanonymous for username and guest for password.

A useful on-line reference for TCP/IP:

  • Computer Science Facilities Group: Introduction to the Internet Protocols , Rutgers University, 1988.

This on-line reference is on the Developer Essentials disc.

STEVE FALKENBURG just started his new life as an Apple Developer Technical Support engineer, but when writing this he was still a computer engineering student at the University of Michigan at Ann Arbor. Last summer he worked in Apple's Advanced Technology Group as an intern, and he claims to have emerged from the experience totally normal (a summer in ATG may not be long enough, but a full-time job in DTS is another story altogether). He says there is absolutely nothing weird about him (which he thinks is a shame), but we're convinced we'll either unearth something or inspire it. In addition to attending classes, he's been working for the university's Computer-Aided Engineering Network, supporting and programming Macintoshes. When not working or studying, he's been seen in the stands at U of M football games (so don't misinterpret him when he yells "Go Big Blue"), attending loud rock concerts, going to movies, and reading Cyberpunk (his favorite is William Gibson). *

For a more complete reference to the TCP driver calls, please see the MacTCP Programmer's Guide. *

For interested programmers, full source code to both the finger tool and NewsWatcher are available on the Developer Essentials disc. *

Thanks to Our Technical Reviewers Pete (Luke) Alexander, Brian Bechtel, Harry Chesley, Larry Rosenstein, Andy Shebanow, Gordon Sheridan, John Veizades. *

 
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