Patent Publication Number: US-7720994-B2

Title: Method for suppression of multicast join/prune messages from extranet receivers

Description:
FIELD OF THE INVENTION 
   The present invention generally relates to suppressing the number of messages sent from a receiver node to a source node in a network multicast group. The invention relates more specifically to a method for suppressing PIM messages sent by extranet receivers in an extranet VPN multicast computer network. 
   BACKGROUND OF THE INVENTION 
   The approaches described in this section could be pursued, but are not necessarily approaches that have been previously conceived or pursued. Therefore, unless otherwise indicated herein, the approaches described in this section are not prior art to the claims in this application and are not admitted to be prior art by inclusion in this section. 
   Multicasting technology is a bandwidth-conserving network technology that reduces network traffic by simultaneously delivering a single stream of information to thousands of recipients. Applications that can use multicast include videoconferencing, corporate communications, distance learning, and distribution of software, stock quotes, and news. Internet Protocol (IP) Multicast delivers source traffic to multiple receivers without adding any additional burden on the source or the receivers, and while using the least network bandwidth of any competing technology. 
   Multicasting is based on the concept of a group. An arbitrary group of receivers expresses an interest in receiving a particular data stream. Receivers may include network infrastructure elements, such as routers and switches, or end station devices, such as workstations or personal computers. A multicast group does not have any physical or geographical boundaries; receivers (also termed “hosts”) can be located anywhere in the Internet. Hosts that are interested in receiving data flowing to a particular group must join the group. In a typical approach, Internet Group Management Protocol (IGMP) is used to dynamically register individual hosts in a multicast group on a particular LAN. Hosts identify group memberships by sending IGMP messages to a local multicast-enabled router. Under IGMP, routers listen to IGMP messages and periodically send out queries to discover which groups are active or inactive on a particular subnet. 
   To receive a data stream associated with a group, a receiver must be a member of the group. Multicast traffic flows from the source to the multicast group using a distribution tree that connects all of the sources to all of the receivers in the group. Distribution trees define the path that multicast traffic takes through the network to arrive at group members. Distribution trees comprise either source trees or shared trees. A distribution tree may be shared by all sources (a shared tree), or a separate distribution tree can be built for each source (a source tree). The shared tree may be one-way or bi-directional. 
   For a source to build a distribution tree, sources such as routers implement Protocol Independent Multicast (PIM) in software. PIM software forwards IP Multicast traffic using the standard Unicast routing table that is maintained in the router. PIM uses the Unicast routing table to decide if the source of an IP multicast packet has arrived on an optimal path from the source. Typically, the router sends PIM messages upstream to the source to refresh the forwarding state once every sixty (60) seconds. Thus, receivers notify a source that they want to receive the data stream by sending the PIM messages to the source. Using PIM in this manner results in the most efficient delivery of data to multiple receivers possible. 
   Because of increasing usage of IP Multicast in the networks of business enterprises, network service providers who offer Layer 3 Virtual Private Network (VPN) services have moved to offer IP Multicast-enabled VPNs. A VPN provides secure, private network connectivity across a non-secure or shared infrastructure, such as the internetworks owned and operated by Internet Service Providers (ISPs). VPN multicasting offers the same multicasting technology as conventional IP multicast, but traverses shared infrastructure elements between the source and the receivers. Multicast Virtual Private Network service aims to provide the same policies and performance as a private network, without requiring enterprises to purchase dedicated lines or service. 
   In Multicast Virtual Private Network (MVPN) service, a service provider establishes a secure logical connection or “tunnel” between provider edge (PE) routers in the ISP network for communication of multicast information. The PE routers transmit IP multicast traffic for default Multicast Distribution Trees (MDTs) and Data MDTs across the MDT Tunnel. The MDT Tunnel provides protection and integrity for the private data involved in the multicast VPN, since that data travels over public connections of the ISP. Information identifying the PE routers involved in a VPN, and the receivers that are authorized to receive VPN traffic, are stored in a VPN routing/forwarding (“VRF”) table. The number of multicast sources in a default MDT group is equal to the number of PE routers connected to the VRF table using the default MDT group. 
   The approach described above for VPN multicasting was designed for applications on an Intranet, in which all sources and receivers are in the same domain and have the same administrative scope. However, there is a present need to expand VPN multicasting for use in an Extranet, in which sources and receivers are not in the same domain or same administrative scope. To extend VPN multicasting to an Extranet, a VRF can be established on an Extranet source, so that the Extranet source can communicate with each VPN associated with a PE router that wishes to receive data from the Extranet source. However, this approach to VPN Multicasting on an Extranet causes complications because of the VPN boundary line and the different administrative scopes. 
   In particular, in the current approach PIM messaging applied to VPN Multicasting on an Extranet causes certain participating network elements to send far too many messages. For example, by conforming to conventional PIM protocol operations, the Multicast VPN Extranet routers send PIM messages for each VRF in a PE router to a Reverse Path Forwarding (RPF) neighbor. In the case of multicast VPN, the RPF interface is the MDT Tunnel. Therefore, for every VRF receiver, a PIM message is sent through the MDT Tunnel, even though sending only one PIM message is needed. 
   The resulting number of PIM messages causes an unnecessary amount of network traffic. For example, if a multicast group has 100 VPNs established in a PE router, then 100 PIM messages are sent through the MDT Tunnel. Processing the PIM messages consumes resources on each router. Additionally, the MDT tunnel becomes flooded with unnecessary PIM messages, thereby consuming unnecessary bandwidth. 
   Based on the foregoing, there is a clear need for an approach to reduce the number of PIM messages that are sent from an Extranet receiver in a VPN multicasting network. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention is depicted 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 that shows a configuration of a multicast Virtual Private Network (VPN) as applied to an Extranet. 
       FIG. 2  is a block diagram illustrating a configuration of multiple VPNs on one Provider Edge (PE) router. 
       FIG. 3A  is a flow diagram showing steps in one embodiment of a method of suppressing PIM Join messages sent by Extranet receivers. 
       FIG. 3B  is a flow diagram showing steps in one embodiment of a method of suppressing triggered PIM Join messages sent by Extranet receivers. 
       FIG. 3C  is a flow diagram showing steps in one embodiment of a method of suppressing PIM Prune messages sent by Extranet receivers. 
       FIG. 4  is a block diagram of a computer system on which embodiments of the invention may be implemented. 
   

   DETAILED DESCRIPTION 
   A. Structural Overview 
   A method and apparatus for suppressing the number of PIM messages from Extranet receivers in a VPN multicast network are described. In the following description, for the 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. 
     FIG. 1  is a block diagram that shows a configuration of a multicast Virtual Private Network (VPN) as applied to an Extranet. A multicast information source element, such as a workstation, router or switch, is located in Intranet  118 , which also includes a customer edge (CE) router  106 A. The CE router  106 A is communicatively coupled to provider edge (PE) router  110 A, which is an element of a service provider or ISP network  112 . The ISP network  112  further includes a second PE router  110 B that is communicatively coupled to a CE router  106 B that can route information to one or more receivers  104 . 
   Another multicast information source element is located in Extranet  120 , and for convenience this element is also termed Extranet source  120  in this description. Extranet source  120  may comprise a workstation, router, switch, etc. 
   For purposes of illustrating a simple example,  FIG. 1  shows a limited number of routers, networks, links and other elements. However, in other embodiments, the techniques described herein can be practiced with other configurations that have any number of elements. The use of thousands of virtual private networks and receivers is specifically contemplated. 
   The configuration of  FIG. 1  supports one or more VPNs. As an example, receivers  104  participate in a Blue VPN  102  that is supported using a first Blue VRF  109  at PE router  110 A and a second Blue VRF  108  at PE router  110 B. Extranet source  120  participates in a Red VPN that is supported using a Red VRF  116  at an extranet network element, a Red VRF  121  at PE router  11 A, and a source Red VRF  122  at PE router  110 B. 
   CE Router  106 A is coupled to Blue VRF  109 . A Multicast Distribution Tree (MDT) Tunnel  114  connects Red VRFs  121 ,  122 . Blue VRF  108  is coupled to CE Router  106 B. 
   The terms “Blue” and “Red” are arbitrary labels that are used to uniquely identify particular VPNs; in a practical embodiment, any other label may be used. 
   B. Functional Overview 
   1. Suppressing Unnecessary Join Messages 
   Referring again to  FIG. 1 , in operation, PE router  110 A transmits IP multicast traffic for default Multicast Distribution Trees and Data MDTs across the MDT Tunnel  114 . The MDT tunnel provides protection and integrity of the data traveling over elements of the ISP network  112 . When one of the receivers  104  in the Blue VPN  102  wants to join or leave the Multicast group, the receiver sends a PIM message to the CE router  106 B, which passes the PIM message to PE router  110 B. The Blue VRF  108  for the Blue VPN located in the PE router  110 B then sends the PIM message to the Red VRF  116  located at the Extranet source  120 . The Extranet source  120  can then build its distribution tree and send multicast information to the group, if needed. 
   In operation with multiple VPNs, each VRF for each VPN acts independently and periodically sends, for the purpose of preventing timeout of forwarding state information, a PIM message to its RPF neighbor. In the example of  FIG. 1 , MDT Tunnel  114  is considered an RPF neighbor for both the Blue VRF  108  and the source Red VRF  122 . Accordingly, in conventional practice, both periodically send a PIM message to the MDT Tunnel  114  in an effort to update forwarding state for any upstream routers and prevent timeout. Each such PIM message is carried by core network elements of ISP network  112  along the MDT Tunnel  114 . However, elements in the ISP network  112  only need to receive one PIM message to accomplish the goal of preventing timeout. 
   According to one embodiment, when a VPN multicast Extranet receiver—such as a VRF acting as a multicast VPN Extranet receiver—needs to send a periodic PIM Join message, the receiver first searches to determine whether any multicast VPN Extranet receivers other than itself currently exist. If there are no other receivers, then the receiver is free to send a PIM Join message. Otherwise, the receiver does not send the periodic PIM Join message. Using this approach, unnecessary PIM Join messages are suppressed. 
     FIG. 3A  is a flow diagram showing steps in one embodiment of a method of suppressing PIM Join messages sent by Extranet receivers. At step  302 , a VPN Multicast Extranet Receiver determines that it needs to send a periodic PIM Join message. For example, a specified timer, such as a 60-second timer as typically used in PIM implementations, expires and a VRF at a PE router determines that it needs to send a PIM Join message to prevent timeout of values relating to a MDT Tunnel. 
   In step  304 , the receiver searches for any other receivers present in the same PE router. Step  304  may be implemented in software that causes a PIM process to search an internally maintained list of receivers. If other receivers are present, as tested at step  305 , then sending a PIM Join message is suppressed, as shown by step  308 . If no receivers are present, then a PIM Join message is sent, at step  306 . Step  306  may involve enqueuing a triggered PIM Join message to a neighbor queue of a PE router that is performing the steps of  FIG. 3A . 
   As a first example, referring again to  FIG. 1 , assume that source Red VRF  122  determines that it needs to send a periodic PIM Join message. VRF  122  searches data stored in PE router  110 B to identify any other VPN multicast receivers. VRF  122  determines that another VPN receiver, Blue VRF  108 , is present. Therefore, VRF  122  does not send a periodic PIM Join message to MDT Tunnel  114 , and always suppresses the sending of such messages. Instead, VRF  122  allows Blue VRF  108  to send such messages. 
   Conversely, assume that in the configuration of  FIG. 1 , the Blue VPN is not established, but receivers  104  are in a multicast group that receives from the Extranet source  120 . In that case, VRF  122  would not find any other VPN multicast receivers at PE router  110 B. Therefore, in that case, VRF  122  would send a normal periodic PIM Join message to MDT Tunnel  114 . 
     FIG. 2  illustrates a case in which multiple receivers exist.  FIG. 2  is a block diagram illustrating a configuration of multiple VPNs on one Provider Edge (PE) router. As a second operational example, in the configuration of  FIG. 2 , each VRF receiver  108 ,  124 ,  126 ,  128  that needs to send a periodic PIM Join message first searches for any other receiver other than itself. In this case, any of the VRF receivers  108 ,  124 ,  126 ,  128  will identify at least one of the others at the time that it needs to send a periodic PIM Join message. Therefore, each such VRF receiver will cease attempting to send the PIM Join message, with one exception. 
   One and only one PIM Join message needs to be sent to MDT Tunnel  114  to prevent timeout of data that establishes the tunnel. Therefore, according to an embodiment, one VRF Extranet receiver is designated as a principal, and is responsible for sending the PIM Join message. For example, in  FIG. 2 , Red Source VRF  122  is designated as principal as part of a configuration step. The Red Source VRF  122  detects that there are multiple VPN receivers participating in a multicast that originates at Extranet source  120 . Therefore, Red Source VRF  122  sends periodic PIM Join messages to the MDT Tunnel  114 . 
   In one embodiment, the search operation specified above is implemented using the PIM software that executes as part of an operating system of PE router  110 B and other PE routers in a network. In one specific embodiment, in which a PE router is a Cisco router from Cisco Systems, Inc., San Jose, Calif., the search operation involves searching a receiver “olist” in the source multicast VRF of the PE router to find information identifying other receivers. If a local receiver or any other extranet receiver is present, then no PIM Join message is queued to the RPF neighbor queue. 
   Additionally, a counter may be established in an Extranet VRF context, such as the Red Source VRF  122 . The counter maintains a number of PIM Join messages that the Red Source VRF  122  has sent to the Extranet source  120  within a specified time period. Every time the Red Source VRF  122  needs to send a PIM Join message, the Red Source VRF examines the counter to determine if a PIM Join message was already sent within the specified time period. If so, then the Red Source VRF  122  does not send, or suppresses sending, another PIM Join message. Thus, The VPN Extranet receiver only sends one PIM message across the MDT Tunnel  114  once every specified time period. At the end of the time period, the counter is reset. 
   Using these approaches suppresses the number of PIM Join messages that are sent upstream by an Extranet VPN receiver. Therefore, fewer router resources are consumed, and less bandwidth is consumed in the MDT tunnel. 
   2. Suppressing Unnecessary Triggered Join Messages 
   Conventional use of the PIM protocol as typically implemented in a router may result in sending too many triggered PIM Join messages to upstream neighbor devices. Under conventional use of PIM, each time that a different Extranet VPN receiver joins a particular multicast group, each source multicast VPN VRF that previously joined sends a Triggered PIM Join message to upstream RPF neighbors on a reverse path towards the multicast source. Thus, the conventional approach results in sending duplicate PIM Join messages. Referring again to  FIG. 1  and  FIG. 2 , assume that both Source Red VRF  122  and another Extranet VPN receiver, not shown, both join a multicast group that includes receivers  104  at approximately the same time. Such a join operation triggers both the Extranet VPN receivers to send PIM Join messages across the MDT Tunnel  114 . However, sending multiple messages is unnecessary, because the same destination multicast group is involved in both join operations. Therefore, a second and subsequent triggered PIM Join messages, all sent within a specified timeout period, do not serve any purpose. 
   In one embodiment, when a receiver VRF sends a Triggered PIM Join packet after joining a multicast group, each Extranet source VRF on the same PE router checks a list of receivers associated with the source VPN. Excluding the receiver that triggered the join operation, if any local VPN receiver or any Extranet VPN receiver is present, then the Extranet source VRF does not send a PIM Join message. If no other such receiver is present in the list, then a PIM Join message is sent. In one embodiment, the PIM Join message is queued to the neighbor queue, and is sent across the MDT Tunnel  114 . 
     FIG. 3B  is a flow diagram showing steps in one embodiment of a method of suppressing triggered PIM Join messages sent by Extranet receivers. At step  310 , a VPN Multicast Extranet Receiver sends a triggered PIM Join message in response to a join operation initiated by another receiver. For example, another receiver joins a multicast group of which the VPN Multicast Extranet Receiver performing  FIG. 3B  is a part. 
   In step  312 , the receiver searches for any other receivers present in the same PE router. Step  312  may be implemented in software that causes a PIM process to search an internally maintained list of receivers. If other receivers are present, as tested at step  314 , then sending a triggered PIM Join message to upstream devices is suppressed, as shown by step  318 . If no receivers are present, then a PIM Join message is sent, at step  316 . Step  316  may involve enqueuing a triggered PIM Join message to a neighbor queue of a PE router that is performing the steps of  FIG. 3B . 
   3. Suppressing Unnecessary Prune Messages 
   Conventional use of the PIM protocol as typically implemented in a router may result in sending too many PIM Prune messages to upstream neighbor devices. Under conventional use of PIM, when an Extranet VPN receiver leaves a multicast group, a PE router sends a PIM prune message corresponding to each member in the group that the receiver left. As a result, in the context of  FIG. 1 ,  FIG. 2 , duplicate PIM Prune messages are sent across MDT Tunnel  114  to the same Extranet source  120 . This causes logical gaps or “black holes” in the information relating to the group upon each member&#39;s departure. Further, the Extranet source  120  could interpret the PIM Prune messages as indicating that the entire VPN has left the group, therefore the Extranet source could close the MDT Tunnel  114 . 
   In this context, as an example, the term “black hole” refers to a state in which a multicast Extranet receiver has information indicating that it is on the multicast distribution tree and expecting to receive the traffic from Extranet Source  120 , but in fact if a PIM Prune message is sent by the Black VRF in PE Router  10 B, because all Black VRF receivers have left, MDT Tunnel  114  will be relinquished, and all other VRFs, such as the Blue, Green and Yellow VRF of  FIG. 2  will stop receiving traffic for up to 3 minutes. Relinquishment of MDT Tunnel  114  is done by Red VRF  201  in PE Router  110 A. Extranet Source  120  is not aware of this problem, and will keep sending traffic. 
   In one embodiment, when a receiver VRF sends a PIM Prune packet after leaving a multicast group, each Extranet source VRF on the same PE router checks a list of receivers associated with the source VPN. Excluding the receiver that triggered the leave operation, if any local VPN receiver or any Extranet VPN receiver is present, then the Extranet source VRF does not send a PIM Prune message. If no other such receiver is present in the list, then a PIM Prune message is queued to the neighbor queue, and is sent across the MDT Tunnel  114 . 
     FIG. 3C  is a flow diagram showing steps in one embodiment of a method of suppressing PIM Prune messages sent by Extranet receivers. At step  320 , a VPN Multicast Extranet Receiver sends a PIM Prune message in response to a group leave operation performed by another receiver. For example, step  320  may involve determining that another receiver is leaving a multicast group of which the VPN Multicast Extranet Receiver performing  FIG. 3C  is a part. 
   In step  322 , the receiver searches for any other receivers present in the same PE router. Step  322  may be implemented in software that causes a PIM process to search an internally maintained list of receivers. If other receivers are present, as tested at step  324 , then sending a PIM Prune message to upstream devices is suppressed, as shown by step  328 . If no receivers are present, then a PIM Prune message is sent, at step  326 . Step  326  may involve enqueuing a PIM Prune message to a neighbor queue of a PE router that is performing the steps of  FIG. 3C . 
   4. Results of Suppression Processes 
   In certain embodiments, each of the processes of  FIG. 3A ,  FIG. 3B ,  FIG. 3C , applied in the context of a network of the type shown in  FIG. 1 ,  FIG. 2 , provides a useful, concrete, and tangible result. For example, embodiments of the processes that are performed in a computer apparatus, when performing at least steps  305 ,  306 , and  308  of  FIG. 3A , or steps  314 ,  316 , and  318  of  FIG. 3B , or steps  324 ,  326 , or  328  of  FIG. 3C , involve manipulation and modification of data that is stored in tangible computer-readable media, such as electronic memory. 
   Further, performing the processes results in changing data structures that are created by software elements that implement the processes, which in turns results in changing a state of data recorded in the memory, which in turn results in changing a state of energization of particular semiconductor transistors, MOSFETs and other structures that comprise the computer memory. Performing the processes results in an apparatus either sending or not sending specific kinds of electronic messages that are communicated using real signals in the form of electric voltages on tangible media such as copper wire. 
   E. Hardware Overview 
     FIG. 4  is a block diagram that illustrates a computer system  400  upon which an embodiment of the invention may be implemented. Computer system  400  includes a bus  402  or other communication mechanism for communicating information, and a processor  404  coupled with bus  402  for processing information. Computer system  400  also includes a main memory  406 , such as a random access memory (RAM) or other dynamic storage device, coupled to bus  402  for storing information and instructions to be executed by processor  404 . Main memory  406  also may be used for storing temporary variables or other intermediate information during execution of instructions to be executed by processor  404 . Computer system  400  further includes a read only memory (ROM)  408  or other static storage device coupled to bus  402  for storing static information and instructions for processor  404 . A storage device  410 , such as a magnetic disk or optical disk, is provided and coupled to bus  402  for storing information and instructions. 
   Computer system  400  may be coupled via bus  402  to a display  412 , such as a cathode ray tube (CRT), for displaying information to a computer user. An input device  414 , including alphanumeric and other keys, is coupled to bus  402  for communicating information and command selections to processor  404 . Another type of user input device is cursor control  416 , such as a mouse, a trackball, or cursor direction keys for communicating direction information and command selections to processor  404  and for controlling cursor movement on display  412 . This input device typically has two degrees of freedom in two axes, a first axis (e.g., x) and a second axis (e.g., y), that allows the device to specify positions in a plane. 
   The invention is related to the use of computer system  400  for suppressing PIM messages from extranet receivers. According to one embodiment of the invention, suppressing PIM messages from extranet receivers is provided by computer system  400  in response to processor  404  executing one or more sequences of one or more instructions contained in main memory  406 . Such instructions may be read into main memory  406  from another computer-readable medium, such as storage device  410 . Execution of the sequences of instructions contained in main memory  406  causes processor  404  to perform the process steps described herein. One or more processors in a multi-processing arrangement may also be employed to execute the sequences of instructions contained in main memory  406 . 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  404  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  410 . Volatile media includes dynamic memory, such as main memory  406 . Transmission media includes coaxial cables, copper wire and fiber optics, including the wires that comprise bus  402 . 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  404  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  400  can receive the data on the telephone line and use an infrared transmitter to convert the data to an infrared signal. An infrared detector coupled to bus  402  can receive the data carried in the infrared signal and place the data on bus  402 . Bus  402  carries the data to main memory  406 , from which processor  404  retrieves and executes the instructions. The instructions received by main memory  406  may optionally be stored on storage device  410  either before or after execution by processor  404 . 
   Computer system  400  also includes a communication interface  418  coupled to bus  402 . Communication interface  418  provides a two-way data communication coupling to a network link  420  that is connected to a local network  422 . For example, communication interface  418  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  418  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  418  sends and receives electrical, electromagnetic or optical signals that carry digital data streams representing various types of information. 
   Network link  420  typically provides data communication through one or more networks to other data devices. For example, network link  420  may provide a connection through local network  422  to a host computer  424  or to data equipment operated by an Internet Service Provider (ISP)  426 . ISP  426  in turn provides data communication services through the worldwide packet data communication network now commonly referred to as the “Internet”  428 . Local network  422  and Internet  428  both use electrical, electromagnetic or optical signals that carry digital data streams. The signals through the various networks and the signals on network link  420  and through communication interface  418 , which carry the digital data to and from computer system  400 , are exemplary forms of carrier waves transporting the information. 
   Computer system  400  can send messages and receive data, including program code, through the network(s), network link  420  and communication interface  418 . In the Internet example, a server  430  might transmit a requested code for an application program through Internet  428 , ISP  426 , local network  422  and communication interface  418 . In accordance with the invention, one such downloaded application provides for suppressing PIM messages from extranet receivers as described herein. 
   The received code may be executed by processor  404  as it is received, and/or stored in storage device  410 , or other non-volatile storage for later execution. In this manner, computer system  400  may obtain application code in the form of a carrier wave. 
   F. Extensions and Alternatives 
   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.