Abstract:
A method and apparatus for providing notification to a client that an access server is on-line during a communication session is disclosed. The client and the access server execute an embedded protocol, such as telnet, to define the communication session. The protocol defines a negotiation-style communication session in which a client issues a request and a server must issue a mandatory acknowledgment that identifies how the request is handled. A command option is sent from the client to the access server, wherein the option is an invalid telnet option. The access server responds, indicating that the option is invalid. The access server&#39;s response provides a positive indication to the client that the access server is on-line. The process can be repeated frequently without requiring any significant system resources. If the access server does not immediately respond, the communication session between the client and access server can thereafter be recovered or terminated, thereby maximizing system resources.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to telecommunications. The invention relates more specifically to apparatus and methods for allowing either a client or host, which are communicating by way of an access server during a remote session, to know when the other has gone offline. 
     BACKGROUND OF THE INVENTION 
     Many telecommunication functions require computer users to connect to remote services to retrieve or transmit information. Increasingly, these remote services arc accessed using an asynchronous dial up connection. This class of functions may include, but is not limited to, dial up connections to the Internet, connections to bulletin boards, connections to internal and external databases and sending and receiving faxes. These functions are carried out during interactive, character-oriented communication sessions via a modem. This is typically known as outbound modem dialing. 
     To support remote login sessions, various computer manufacturers have developed facilities that allow their users to log in remotely from one computer to another. Within the Internet, the most commonly used method is a facility called “telnet,” which is the name of the protocol used to support remote login sessions and also the name of the Transmission Control Protocol/Internet Protocol (“TCP/IP”) remote login program. TCP/IP refers to the suite of protocols that define the Internet. Originally designed for the UNIX™ operating system, TCP/IP software is now available for every major kind of computer operating system. To be on the Internet, a computer must have TCP/IP software. The telnet protocol defines how local and remote computers talk to each other to support a remote login session. A more complete discussion of remote login using telnet is described in D. Dem, “The Internet Guide For New Users,” pp. 247-67 (McGraw Hill 1994). 
     Many computer users are connected to the Internet by access servers on local area networks or enterprise networks. An example of an access server is model number (AS5200), commercially available from Cisco Systems, Inc. To help defer the cost of installing and maintaining additional phone lines, which may be used very little per user, many equipment manufacturers have added the ability to establish remote sessions on the outbound ports of access servers and routers. These remote sessions are supported by an embedded telnet protocol operating in conjunction with other communication software, such as a communication port director. 
     To support remote sessions via an access server, as opposed to a direct personal computer/remote service connection, the telnet protocol has undergone revision. The name and the result of the process for disseminating information about a proposed standard on the Internet is known as Request for Comments (“RFC”). The standards are currently proposed and published on-line at http:http://info.intcrnet.isi.edu/innotes/rfc/files/rfc2217.txt. 
     In RFC 2217, entitled “Telnet Com Port Control Option” and published in Oct. of 1997, the present inventor identified three new areas of functionality within the telnet protocol which needed updating to successfully support the needs of outbound modem dialing. A first new function is the ability for a client to send corn port configuration information to an access server which is connected to the outbound modem. This ensures that data transmitted and received by the modem is formatted correctly at the byte level. A second new function is the ability for an access server to inform a client of any modem line or signal changes, such as carrier detect (RLSD) changes. This information is vital, since many client software packages use this information to determine if a session with a remote service has been established. The third new function provides the ability to manage flow control between the client and the access server which does not interfere with the flow control mechanisms used by the session between the client and the remote service. 
     The first new function is carried out by causing a client and server to negotiate a corn port configuration by exchanging messages. The negotiation of the corn port control option protocol uses the standard telnet negotiation protocol mechanism. This mechanism involves the exchange of messages that request an action (a “DO” or “DON&#39;T” messages) and messages that respond to such requests (“WILL” or “WON&#39;T” messages). RFC 2217 identifies the following negotiation messages having the following meanings: 
     
       
         IAC WILL COM-PORT-OPTION 
       
     
     The sender of this command is willing to send corn port control option commands. 
     
       
         IAC WONT COM-PORT-OPTION 
       
     
     The sender of this command refuses to send corn port control option commands. 
     
       
         IAC DO COM-PORT-OPTION 
       
     
     The sender of this command is willing to accept corn port control option commands. 
     
       
         IAC DONT COM-PORT-OPTION 
       
     
     The sender of this command refuses to accept corn port control option commands. 
     The client can send these commands at any time and at multiple times throughout a telnet session. Each command transmitted from the client to the access server must be acknowledged once the command has been processed by the access server. This confirmation informs the client of the value set at the access server after the processing of the command. This acknowledgment is not used to acknowledge the receipt of the command, which is handled at the TCP protocol layer. Its purpose is to inform the client of the value in use, which may be different than the value requested in the client&#39;s command. For example, the client may request a baud rate higher than the access server can provide. If the client does not receive an acknowledgment within a reasonable time, the client may wish to re-send the command or terminate the session. 
     Once DO and WILL commands have been negotiated, the client may send any of the following Com-Port Control Option commands: 
     
       
         
               
               
               
             
               
               
               
             
           
               
                   
               
               
                   
                 Client 
                 Access 
               
               
                 Command Name 
                 to Access Server 
                 Server to Client 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 SIGNATURE 
                 Text 
                 Text 
               
               
                 SET-BAUDRATE 
                 1 
                 101 
               
               
                 SET-DATASIZE 
                 2 
                 102 
               
               
                 SET-PARITY 
                 3 
                 103 
               
               
                 SET-STOPSIZE 
                 4 
                 104 
               
               
                 SET-CONTROL 
                 5 
                 105 
               
               
                 NOTIFY-LINESTATE 
                 6 
                 106 
               
               
                 NOTIFY-MODEMSTATE 
                 7 
                 107 
               
               
                 FLOWCONTROL-SUSPEND 
                 8 
                 108 
               
               
                 FLOWCONTROL-RESUME 
                 9 
                 109 
               
               
                 SET-LINESTATE-MASK 
                 10 
                 110 
               
               
                 SET-MODEMSTATE-MASK 
                 11 
                 111 
               
               
                 PURGE-DATA 
                 12 
                 112 
               
               
                   
               
             
          
         
       
     
     The following format may be used to send the commands: IAC SB COM-PORT-OPTION COMMAND NAME&lt;value&gt;IAC SE or IAC SB COM-PORT-OPTION COMMAND NAME&lt;text&gt;IAC SE. “IAC” means Interpret As Command, and is defined for use in Telnet in RFC 854, character decimal code 255; “SB” means Subnegotiation Begin, and is defined for character code 250; “SE” means Subnegotiation End, and is defined for character code 240. The Command Name can be any one of those listed above with the appropriate value or text inserted. 
     If a client sends a command and there is no acknowledgment by the access server within a reasonable time, the client may wish to re-send the command or terminate the session to save system resources. Generally, a reasonable time period to re-send or terminate would be twice the delay acknowledgement (“delay ack”) timer in TCP/IP If the delay ack timer is ten seconds, then the client would wait approximately 20 or 30 seconds before re-sending or terminating. This 20-30 second time period ensures that commands will not be re-sent or that the receiver will not be terminated unnecessarily in the event that the receiver is only slowing down or interrupted temporarily. 
     Unfortunately, when telnet operates as an embedded protocol to support remote TCP/IP sessions, there is no efficient way for either a client or an access server to know when a client or server at the opposite end of the connection has gone offline. In a past approach, a user would execute telnet using a terminal interface, so that the user could type commands to the telnet program and view responses by the remote system. In this past approach, there are visual cues to indicate when either the client or access server had gone off-line. If a user of the client, a PC user for example, depressed a key at the client machine and received no response, the user could surmise that the host had gone offline, and could thereafter terminate the session. Alternatively, the user could test whether the PC had stopped operating or crashed. In this context, a device is “off-line” when it is disconnected, crashed, or otherwise logically or physically unavailable. 
     When telnet is operated as an embedded protocol within an access server or router, however, there are no visual cues. The telnet program is executed by the access server or router within a network, but such devices do not provide a visual display to the end user when they are in operation. Currently, the only way to know whether the client or access server has gone off-line or become unavailable is to use a timeout mechanism. In some telnet systems, timeout code will disconnect the access server and client after a pre-defined period of inactivity. This approach is a waste of system resources, however, because the access servers and/or outbound modems are occupied and unavailable during this period. Maintaining a TCP/IP connection for an Internet activity using the telnet protocol, for example, ties up buffer space and control sources in operating systems on both ends. Most systems have limited resources, and it is undesirable to leave open connections where there is no communication. Leaving an open telnet connection can also be a security breach. 
     Accordingly, it is desirable to have a method or system that enables a client or server to know immediately when the other has gone off-line. With such a method or system, the connection could be terminated or a recovery option could be implemented immediately. 
     One solution is to create a new protocol to handle this situation. However, this approach is undesirable because of the burden associated with creating and obtaining approval for a new standard. It is desirable to have a method or system that does not require development of a new system protocol and that could use the available telnet protocol options. Further, it is desirable to have a method that would allow a client or access server to let the other know it is “alive” on a frequent basis without requiring an inordinate amount of system resources. 
     SUMMARY OF THE INVENTION 
     The foregoing needs, and other needs and objectives that will become apparent from the description herein, are achieved by the present invention, which comprises, in one aspect, a method for a first computer process to determine that a second computer process is active during a communication session using a protocol that defines one or more request messages and one or more corresponding mandatory acknowledgment messages, comprising sending a command from the first computer process to the second computer process, in which the command comprises at least one parameter value that is invalid under the protocol; sending a response from the second computer process to the first computer process, in which the response indicates that the parameter value is invalid; and determining, at the first computer process, that the second computer process is active. 
     One feature of this aspect involves testing for immediate receipt of the response from the second computer process; and at the first computer process, determining that the second computer process is off-line when the response is not immediately received. A related feature is terminating the communication session when the second computer process does not immediately respond to the command. Another related feature is implementing a recovery procedure when the second computer process does not immediately respond to the command. 
     According to another feature, the first computer process is a client and the second computer process is an access server that executes the protocol in embedded fashion. In still another feature, the second computer process includes a corn port interface coupled to a modem interface, in which the modem interface is coupled to a remote service that provides information to the client. 
     Other aspects and features will become apparent from the following description and appended claims. In particular, the invention encompasses an apparatus and a computer-readable medium in the form of a carrier wave that are configured, in certain embodiments, to carry out the foregoing steps. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention is illustrated by way of example, and not by way of limitation, in the figures of the accompanying drawings and in which like reference numerals refer to similar elements and in which: 
     FIG. 1 is a block diagram of a communication system; 
     FIG. 2 is a flow chart showing a method of notifying a client or server that a client or server has gone off-line, according to a preferred embodiment; and 
     FIG. 3 is a block diagram of a computer system that may be used in the implementation of the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     A method of notifying a client or server that a client or server has gone off-line is described. In the following description, for purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of the present invention. It will be apparent, however, to one skilled in the art that the present invention may be practiced without these specific details. In other instances, well-known structures and devices are shown in block diagram form in order to avoid unnecessarily obscuring the present invention. 
     OVERVIEW OF OPERATIONAL CONTEXT COMMUNICATION SYSTEM 
     FIG. 1 is a block diagram of a communication system  112  illustrating a client  100  that is coupled to a remote service  110  through access server  104 . In the preferred embodiment, the client  100  resides on a local area or enterprise network  102  supported by an access server  104 . The client  100  may be any network device that can carry out a telnet session with the access server  104 . Client  100  may specifically refer to a personal computer or any device having a communication port wherefrom data may be transferred. Client  100  may also refer to any software program on the computer or device that is used to contact and obtain data from a server software program, where the client and server are either located on the same premise or are in different locations separated by a significant distance. Any client  100  programs may be designed to work with one or more specific kinds of server programs, and each server may require a specific kind of client. 
     The communication port traffic from the client  100  is converted to a telnet session and sent to the access server  104 . In the preferred embodiment, the access server  104  is any network device which accepts telnet sessions and passes the data received to a corn port, and passes data received from the communication port to the client  100  via the telnet session. The access server  104  may be a computer, or a software package, which provides a specific kind of service to client software running on other computers. Access server  104  may therefore refer to a particular piece of software, or to the machine on which the software is running. A single server machine could have several different server software packages running on it, thus providing many different servers to clients on the network. 
     In the preferred embodiment, the access server  104  will have a protocol interface, such as the telnet interface  104   a  shown in FIG.  1 . Access server  104  also has at least one communication port interface  104   b  (“comport interface”), and an interface to a local area or enterprise network  102 . The access server  104  may provide point-to-point protocol connections, meaning that the access server  104  uses a regular telephone line and a modem to make network connections. Alternatively, the access server  104  may use any protocol which allows for connection to an external network such as the global, packet-switched network known as the Internet. 
     The modem  106  interfaced with the corn port interface  104   b  can be any modem device that connects the server to a public switched network by network connection  108 , allowing the client to communicate with a remote service  110  through the phone system. Modem  106  may comprise a single modem device or a plurality (“bank”) of modems. The remote service  110  can be any service that accepts dial-up connections, such as an Internet Service Provider, a bulletin board or even a fax machine. 
     In this configuration, access server  104  may provide selective access of client  100  to the remote service  110  or to network connection  108 . For example, the access server  104  may be configured with appropriate software to authenticate client  100  when the client seeks to access the network connection. The access server  104  also may be a router or packet data switch. 
     Client  100  and access server  104  are examples of network nodes or computer processes with which embodiments of the invention may be used. The network nodes may be any element of a network. A client/server relationship among the nodes is not required. Client  100  and access server  104  may be implemented in hardware, software, or a combination of both hardware and software. 
     EMBEDDED PROTOCOL 
     Preferably, access server  104  executes an embedded protocol that defines a communication session between client  100  and access server  104 . Within the Internet, telnet is the most commonly used facility for supporting remote login sessions. The telnet protocol defines how local and remote computers talk to one another to support a communications session whereby a local computer remotely contacts and logs into the remote computer (“remote login session”). Telnet currently has approximately forty-five (45) on/off parameters that are set during a negotiation phase. In the negotiation phase, the client and access server determine the parameters which will later guide and facilitate the transfer and communication of data. The parameters include, for example, terminal speed, window size, display location, terminal location number, end of record, echo and other aspects of the communication session. The parameters may be set by issuing a command having the syntax “DO [value]”, where “value” is a two-octet (2 bytes) value that can represent any value from 0-255. 
     Telnet has a standard negotiation mechanism. The negotiation mechanism involves communicating request and response messages between the client and server or between a local computer and a remote computer. The negotiation protocol uses four (4) commands in the form of“WILL,” WON&#39;T”, “DO” and “DON&#39;T” commands: 
     IAC WILL [value]. The sender of this command is willing to send commands represented by “value”. 
     IAC WONT [value]. The sender of this command refuses to send commands represented by “value”. 
     IAC DO [value]. The sender of this command is willing to accept commands represented by “value”. 
     IAC DONT [value]. The sender of this command refuses to accept commands represented by “value”. 
     In the preferred embodiment, either the client  100  or access server  104  could generate the initial DO or WILL command. In response, either the client  100  or access server  104  responds with the WONT or DONT command. 
     Telnet is merely one example of a protocol that can be used with embodiments of the invention. No particular protocol is required. What is important is that the protocol defines a request-response mode of operation and defines a mandatory response when an invalid request or command is received. For example, a first network node issues a request to a second network node, and the protocol dictates that the second network node shall respond with a message that acknowledges how the second network node is handling the request. The responsive message may be in the nature of an acknowledgment, a refusal, a success report, an error message, a confirmation, or the like. In one embodiment, the protocol may define a particular mandatory responsive message. In another embodiment, the protocol may define that the responsive message must be returned to the client promptly or immediately. These characteristics, however, are not required. 
     DETERMINING WHETHER CLIENT OR SERVER IS OFF-LINE 
     There is no efficient way for either the client  100  or access server  104  to know whether the other is off-line. Prior to the use of telnet as an embedded protocol, the telnet prompt commands and responses could be viewed on a computer display as a user typed requests and received responses from a remote system. If a command was sent and there was no response, a client user could recognize this immediately and either re-send the command or terminate the session. When telnet is operated as an embedded protocol layered with other software, in an access server, router, or other device that does not have a visual user interface, there is no visual way for the client  100  or access server  104  to know whether the other is off-line, whether to recover or terminate the session, thereby saving system resources. 
     USE OF INVALID OPTION TO NOTIFY CLIENT OR SERVER THAT A DEVICE IS OFF-LINE 
     Telnet&#39;s four primary negotiating commands (DO, DONT, WILL, WONT) are used to negotiate the values of parameters prior to the transfer of data between a client and an access server. There are approximately 45 parameters currently defined in the telnet standards from among 255 available parameters. Approximately 210 available parameters are not currently defined by any RFC or other Internet standard, and are available for future use. 
     For the 45 parameters that are defined, if a DO command is executed on one end, from the client  100  for example, the access server  104  will respond with an appropriate WILL or WONT. For example, if the client  100  sends a “DO” command with the value “44”, possibly followed by other parameter values, the access server  104  will respond with either a “WILL 44” message or a “WONT 44”, depending on whether the access server can logically or physically implement the things required by a “44” parameter. The “DO 44” command represents the client  100  telling the access server  104  to set one or more parameters associated with the value “44” to those values that are included with the “DO 44” command. A “WILL 44” response from the access server  104  indicates that the access server  104  is willing to set the parameters and has set them. A WONT response, of course, indicates that the access server  104  cannot or is not willing to set the parameters. In the preferred embodiment, the client  100  can send these commands at any time and at multiple times throughout a telnet session. Each command transmitted from the client  100  to the access server  104  must be acknowledged once the command has been processed by the access server  104 . Thus, an important aspect of the telnet protocol is that whenever a server receives a DO or DONT command, the server must respond with either a WILL message or a WONT message. Moreover, telnet requires that the server response must be immediate. This confirmation informs the client  100  of the value set at the access server  104  after the processing of the command. 
     This acknowledgment is not used to acknowledge the receipt of the command, which is handled at the TCP protocol layer. Instead, its purpose is to inform the client  100  of the value in use, which may be different than the value requested in the client&#39;s command. If an acknowledgment is not received by the client  100  within a reasonable time, the client  100  may wish to re-send the command or terminate the session. 
     To determine whether the client  100  or access server  104  is on-line, the present approach utilizes the standard telnet negotiation protocol and leverages the fact that certain telnet command options, particularly 45-253, have not been allocated. In the preferred embodiment, the client  100  sends a DO [value] command to the access server  104 , where “value” is any undefined telnet parameter value. For example, in one embodiment, “value” is a numeric value between 45 and 253. 
     Assume, for example, that a “DO 235” command is sent. The client  100  is telling the access server  104  to carry out the telnet command represented by value “235”. Since “235” is an undefined parameter value, it is invalid, and the access server  104  can only respond with the message “WONT 235”, meaning that the access server cannot implement parameter “235”. The “WONT” response by the access server provides a positive signal to the client  100  that the access server  104  is on-line and operating. If the client  100  does not immediately receive a WONT message from the access server  104 , the client knows immediately that the access server  104  is off-line. Accordingly, recovery or termination procedures may be implemented immediately. 
     FIG. 2 is a block diagram showing a method of notifying a client or server that a client or server has gone off-line, according to a preferred embodiment. In particular, FIG. 2 depicts messages exchanged between the dial out modules of the client  100  and access server  104 . The process of FIG. 2 may be implemented as one or more computer programs, functions, subroutines, processes, methods and the like, either in stand-alone form or callable from another program or process. 
     In block  200 , the client sends a command in a particular protocol, containing an invalid parameter, to the server. For example, the client sends a DO command using the telnet protocol, with an invalid telnet parameter value, such as a numeric value between “45” and “253”. 
     In block  201 , the server receives the protocol command and extracts parameters from it. In block  202 , the server tests whether the protocol command is valid based on the parameters. It should be noted that the steps of block  201  and block  202  form a part of conventional telnet protocol processing at the server. Thus, the present process is advantageous in that it does not require any modification of server-side telnet processing. 
     If the test of block  202  is TRUE, then control is passed to block  208 , in which the server processes or responds to the command, as appropriate depending on its contents. In the present process, from the perspective of the client, an error condition exists if control reaches block  208 . 
     If the test of block  202  is FALSE, and the server is on-line, then the server immediately responds to the invalid command by sending a responsive message that refuses to process the command, as shown in block  204 . For example, access server  104  responds to client  100  with a telnet WONT command. 
     In block  206 , the client tests whether a response is received promptly from the server. The response by the server is a positive indication to the client that the server is on-line. If a response is received, then the test of block  206  is TRUE, and the process then returns control to the calling process or program, as shown by block  210 . 
     In the preferred embodiment, client  100  executes the telnet DO command with an invalid option approximately every thirty (30) seconds. This ensures that the access server  104  will not be undetectably off-line for any significant period. 
     If the test of block  206  is FALSE, then the server is off-line, down, or unavailable. Accordingly, as shown by block  212 , the client determines how to recover from the failure of the server. A wide range of responsive measures may be undertaken, and the specific responsive measure taken is a decision to be made by the client. For example, the client may carry out a recovery procedure, or set a timer and wait for the server to come on-line, or notify the user through an alarm or appropriate message. Alternatively, the client may terminate the communication session between the client and server. 
     While the preferred embodiment focuses on outbound modem dialing as a primary use, alternate embodiments are used for any serial device attached to an access server. Such devices could be serial printers, plotters, monitoring devices such as pipe line monitors or medical monitors and general office equipment such as photocopiers and cash registers. 
     HARDWARE OVERVIEW 
     FIG. 3 is a block diagram that illustrates a computer system  300  upon which an embodiment of the invention may be implemented. Computer system  300  includes a bus  302  or other communication mechanism for communicating information, and a processor  303  coupled with bus  302  for processing information. Computer system  300  also includes a main memory  306 , such as a random access memory (RAM) or other dynamic storage device, coupled to bus  302  for storing information and instructions to be executed by processor  303 . Main memory  306  also may be used for storing temporary variables or other intermediate information during execution of instructions to be executed by processor  303 . Computer system  300  further includes a read only memory (ROM)  308  or other static storage device coupled to bus  302  for storing static information and instructions for processor  303 . A storage device  310 , such as a magnetic disk or optical disk, is provided and coupled to bus  302  for storing information and instructions. 
     Normally computer system  300  is configured without a display, keyboard, mouse or like devices because such input/output devices are not necessary for embedded operation in a network. Computer system  300  may be coupled via bus  302  to a communications interface  312  that can be coupled to an external terminal or host system, for displaying information to a computer user or for receiving configuration input. For example, communications interface  312  may be an RS-232 port that is coupled to a terminal or workstation. Computer system  300  executes an internetworking operating system, such as the IOS operating system available from Cisco Systems, Inc. The operating system provides a command-line interface to the terminal or workstation over communications interface  312 . 
     The invention is related to the use of computer system  300  for notifying a client or server that another, remotely connected client or server is offline. According to one embodiment of the invention, a method of notifying a client or server that another, remotely connected client or server is offline is provided by computer system  300  in response to processor  303  executing one or more sequences of one or more instructions contained in main memory  306 . Such instructions may be read into main memory  306  from another computer-readable medium, such as storage device  310 . Execution of the sequences of instructions contained in main memory  306  causes processor  303  to perform the process steps described herein. In alternative embodiments, hard-wired circuitry may be used in place of or in combination with software instructions to implement the invention. Thus, embodiments of the invention are not limited to any specific combination of hardware circuitry and software. 
     The term “computer-readable medium” as used herein refers to any medium that participates in providing instructions to processor  303  for execution. Such a medium may take many forms, including but not limited to, non-volatile media, volatile media, and transmission media. Non-volatile media includes, for example, optical or magnetic disks, such as storage device  310 . Volatile media includes dynamic memory, such as main memory  306 . Transmission media includes coaxial cables, copper wire and fiber optics, including the wires that comprise bus  302 . Transmission media can also take the form of acoustic or light waves, such as those generated during radio-wave and infrared data communications. 
     Common forms of computer-readable media include, for example, a floppy disk, a flexible disk, hard disk, magnetic tape, or any other magnetic medium, a CD-ROM, any other optical medium, punch cards, paper tape, any other physical medium with patterns of holes, a RAM, a PROM, and EPROM, a FLASH-EPROM, any other memory chip or cartridge, a carrier wave as described hereinafter, or any other medium from which a computer can read. 
     Various forms of computer readable media may be involved in carrying one or more sequences of one or more instructions to processor  303  for execution. For example, the instructions may initially be carried on a magnetic disk of a remote computer. The remote computer can load the instructions into its dynamic memory and send the instructions over a telephone line using a modem. A modem local to computer system  300  can receive the data on the telephone line and use an infrared transmitter to convert the data to an infrared signal. An infrared detector can receive the data carried in the infrared signal and appropriate circuitry can place the data on bus  302 . Bus  302  carries the data to main memory  306 , from which processor  303  retrieves and executes the instructions. The instructions received by main memory  306  may optionally be stored on storage device  310  either before or after execution by processor  303 . 
     Computer system  300  also includes a communication interface  318  coupled to bus  302 . Communication interface  318  provides a two-way data communication coupling to a network link  320  that is connected to a local network  322 . For example, communication interface  318  may be an integrated services digital network (ISDN) card or a modem to provide a data communication connection to a corresponding type of telephone line. As another example, communication interface  318  may be a local area network (LAN) card to provide a data communication connection to a compatible LAN. Wireless links may also be implemented. In any such implementation, communication interface  318  sends and receives electrical, electromagnetic or optical signals that carry digital data streams representing various types of information. 
     Network link  320  typically provides data communication through one or more networks to other data devices. For example, network link  320  may provide a connection through local network  322  to a host computer  323  or to data equipment operated by an Internet Service Provider (ISP)  326 . ISP  326  in turn provides data communication services through the worldwide packet data network now commonly referred to as the “Internet”  328 . Local network  322  and Internet  328  both use electrical, electromagnetic or optical signals that carry digital data streams. The signals through the various networks and the signals on network link  320  and through communication interface  318 , which carry the digital data to and from computer system  300 , are exemplary forms of carrier waves transporting the information. 
     Computer system  300  can send messages and receive data, including program code, through the network(s), network link  320  and communication interface  318 . In the Internet example, a server  330  might transmit a requested code for an application program through Internet  328 , ISP  326 , local network  322  and communication interface  318 . In accordance with the invention, one such downloaded application provides for notifying a client or server that another, remotely connected client or server is offline as described herein. 
     The received code may be executed by processor  303  as it is received, and/or stored in storage device  310 , or other non-volatile storage for later execution. In this manner, computer system  300  may obtain application code in the form of a carrier wave. 
     VARIATIONS, EXTENSIONS 
     In the foregoing specification, the invention has been described with reference to specific embodiments thereof. It will, however, be evident that various modifications and changes may be made thereto without departing from the broader spirit and scope of the invention. The specification and drawings are, accordingly, to be regarded in an illustrative rather than a restrictive sense.