WO1996000468A1

WO1996000468A1 - Method for using point-to-point protocol over an imperfect mesh network

Info

Publication number: WO1996000468A1
Application number: PCT/US1995/007442
Authority: WO
Inventors: Brett Galloway
Original assignee: Metricom, Inc.
Priority date: 1994-06-24
Filing date: 1995-06-08
Publication date: 1996-01-04
Also published as: AU2902095A

Abstract

A lightweight transport protocol (i.e. a transport protocol that does not provide reliable ordered delivery of packets and thus does not require the exchange of acknowledgement packets) is employed to tunnel point-to-point protocol (ppp) packets between one or more source nodes (14) and a gateway node (18) through an imperfect mesh network (IMN)(12). The gateway node (18) may be configured either as a single device operative to internally map between physical IMN ports and logical ports, or as a combination of a traditional communications server and one or more special network interfaces for coupling Internet Protocol network (20) to the IMN.

Description

METHOD FOR USING POINT-TO-POINT PROTOCOL OVER AN IMPERFECT MESH NETWORK

COPYRIGHT NOTICE A portion of the disclosure of this patent document contains material which is subject to copyright protection. The copyright owner has no objection to the facsimile reproduction by anyone of the patent document or the patent disclosure as it appears in the Patent and Trademark Office patent file or records, but otherwise reserves all copyright rights whatsoever.

BACKGROUND OF THE INVENTION This invention relates to the emulation of point-to- point data communication links over an imperfect mesh communication network as hereinafter defined. A point-to-point data communication link serves to connect two nodes or stations directly over a single two-way link. In a typical implementation of a point-to-point link, the two nodes are each connected together by a common transmission medium such as a twisted pair transmission line, fiber optic channel, or a radio communications channel. The two nodes cooperate to transmit data aetween themselves using a common data communication protocol. An example of a point- to-point data communication protocol is^' the Point-to-Point Protocol (PPP) , a standard adopted by the Internet Engineering Task Force for use in point-to-point data links that connect to the Internet. PPP is described in an IETF protocol document cited as "The Point-to-Point Protocol (PPP) ; RFC 1548" Internet Request for Comments, no. 1548, December 1993, the contents of which are herein incorporated by reference.

In accordance with PPP, data to be communicated over a point-to-point link is included in an information field of a special PPP frame or packet which further includes other fields used for link operation. The data included in the information field may be a packet formatted in accordance with some other protocol such as Internet Protocol (IP) .

An imperfect mesh network (IMN) is a network with an interconnected set of nodes in which the nodes typically do not have knowledge of all other nodes in the network and which do not share a common transmission medium. A source node with data to transmit to an arbitrary target node does not normally know of the intermediate path to the target node, nor is there a direct link between the source node and its ultimate target node. Data transmitted from the source node to the target node is instead routed through a path comprising a series of intermediate nodes belonging to the IMN with each node in the path having a direct link to a successor in its route to the ultimate target node, but generally not being able to communicate directly or to even be aware of the source node and/or the target node.

If a sequence of packets is transmitted from the source node to target node, they may follow disparate paths and thus may arrive out of order. Furthermore, a fault at an intermediate node in any one of the paths followed may terminate transport of a packet and thus disrupt the sequence.

To address these problems, applications requiring reliable ordered delivery of packets of data through an IMN employ so-called heavyweight transport protocols. In a heavyweight transport protocol, for each set of one or more data packets successfully transported from a source node to a target node, an acknowledgement packet is returned from the target node to the source node. If reliable ordered delivery is not required, a so-called lightweight transport protocol may be employed. A lightweight transport protocol does not provide reliable ordered delivery but offers increased network efficiency by operating without the use of acknowledgement packets. Because of the widespread availability of software that implements PPP at a node such as a personal computer or workstation, it is desirable to emulate a PPP data communication link over an IMN. In such an emulation, the node which operates PPP forwards a PPP packet to a source node connected in the IMN. The PPP packet is encapsulated at the source node within an IMN forwarding packet for transport across the IMN to a target node connected in the IMN. The encapsulation process appends an address of the target node and any other header information required for transport across the IMN. Upon receipt at the target node, the PPP packet is extracted from the IMN forwarding packet by stripping away information particular to IMN operation. The extracted PPP packet may then be forwarded to another node equipped with PPP software.

The process of encapsulating, transporting, and then extracting the PPP packet is known as tunneling. Prior art methods of tunneling PPP packets across IMNs have all employed heavyweight transport protocols.

SUMMARY OF THE INVENTION In accordance with the invention, a lightweight transport protocol (i.e. a transport protocol that does not provide reliable ordered delivery of packets and thus does not require the exchange of acknowledgement packets) is employed to tunnel Point-to-Point Protocol (PPP) packets between one or more source nodes and a gateway node through an imperfect mesh network (IMN) . The gateway node may be configured either as a single device operative to internally map between physical IMN ports and logical ports, or as a combination of a conventional communications server and one or more special network interfaces for coupling to the IMN. The invention stems from recognition of a problem posed by the use of a heavyweight transport protocol to tunnel PPP packets through an IMN from a source node to a gateway node connected in a network employing Internet Protocol (IP) , e.g. the Internet, namely that use of a heavyweight transport protocol often results in the excessive generation of acknowledgement packets which degrade performance.

While it is known that protocols overlying IP generate and exchange acknowledgement packets to assure reliable ordered delivery of packets, i.e., for each set of one or more data transport packets there is an acknowledgement packet, if a heavyweight transport protocol were used to tunnel PPP packets through the IMN, the heavyweight protocol 5 would generate and exchange its own acknowledgement packets for the PPP packets carried across the IMN, including PPP packets that themselves encapsulate the acknowledgement packets generated by a higher level protocol. Thus, a flood of performance-degrading acknowledgment packets would result

10 from the interaction between the heavyweight transport protocol used across the IMN and the protocol(s) overlying IP.

By recognizing that the protocol(s) overlying IP already provide reliable ordered delivery of packets, acknowledgment packets specific to IMN operation become

15 unnecessary. Substituting a lightweight transport protocol which does not generate acknowledgement packets across the IMN provides a significant performance advantage in that a superfluous layer of acknowledgement packet exchange is avoided.

20 In accordance with a first aspect of the invention, a source node network interface is provided that includes means for encapsulating PPP packets received from a source node within IMN forwarding packets and means for forwarding the IMN forwarding packets to a gateway node via the IMN by

25. employing a lightweight transport protocol. The source node thus can communicate over the IMN by operating generally- available PPP software to generate and receive PPP packets. In accordance with a second aspect of the invention, logical ports at a gateway node corresponding to potential active PPP sessions are decoupled from physical ports for interfacing to the IMN. Decoupling logical ports from physical ports provides a significant advantage in that a packet to be forwarded or received during the course of an active PPP session may make use of any currently available physical port. Thus, the number of active PPP sessions that the gateway node can support is not limited by the number of physical ports. The ability to make efficient use of available physical ports at the gateway node is particularly beneficial when the IMN is a wireless network and each physical port includes an expensive radio transceiver.

In a first configuration, the gateway node includes a single device that couples directly to the IMN and internally maps between physical ports and logical ports. In a second configuration, the gateway node includes a conventional communications server with serial ports directly coupled to logical ports and a special interface which includes physical ports to the IMN and performs the necessary mapping between the physical ports and communications server serial ports. In a third configuration of the gateway node, devices substantially similar to the source node network interface of the first aspect of the invention couple individual serial ports of the communications server to the IMN. In the third configuration, there is no decoupling of logical and physical ports. The invention will be better understood by reference to the following detailed description in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 depicts a data communication system in accordance with the invention, wherein PPP packets are transported across an imperfect mesh network (IMN) encapsulated within IMN forwarding packets. Fig. 2 is a flowchart describing the steps of establishing and employing an emulated PPP link over an IMN in accordance with the invention.

Fig. 3 depicts a representation of a gateway node of the invention, wherein physical ports coupled to an IMN are decoupled from logical ports employed to operate individual virtual PPP sessions.

Fig. 4 depicts a gateway node of the invention configured as a combination of a communications server having multiple serial ports directly linked to internal logical ports and a special network interface for coupling to an IMN.

Fig. 5 depicts a gateway node of the invention configured as a combination of a communications server having multiple serial ports directly linked to internal logical ports and a plurality of special network : rfaces, each one for coupling a given serial port to an IM DESCRIPTION OF SPECIFIC EMBODIMENTS Fig. 1 depicts a data communication system 10 in accordance with the invention, wherein PPP packets are transported across an imperfect mesh network (IMN) 12 encapsulated within IMN forwarding packets. Data communication system 10 includes a representative source node 14, an associated source node network interface 16, IMN 12, and a gateway node 18. Gateway node 18 acts as an interface between IMN 12 and an Internet Protocol network 20. IMN 12 consists of repeater nodes labelled R₀ through R₉ interconnected by paths representing allowable communication links between nodes. Data transmitted between the source node and gateway node is routed through a path comprising a series of the repeater nodes belonging to the IMN with each node in the path having a direct link to a successor in its route to the ultimate target node (gateway or source) but generally no direct link to the ultimate target node itself.

In one embodiment, source node 14 is a computer equipped with a conventional modem port and standard PPP software for transmitting and receiving PPP packets. The modem port is coupled to source node network interface 16 by a serial link. If IMN 12 is a wireless network, source node network interface 16 incorporates a radio transceiver. In operation, PPP packets are exchanged between source node 14 and source node network interface 16 through the modem port and serial link. Source node network interface 16 encapsulates PPP packets and modem commands received from source node 14 within IMN forwarding packets, packets specially formatted for transport across IMN 12. Source node network interface 16 also extracts PPP packets and modem commands from IMN forwarding packets received via IMN 12. Fig. 2 is a flowchart describing the steps of establishing and employing an emulated PPP link over IMN 12 in accordance with the invention. At step 100, source node 14 requests the establishment of an emulated PPP data link. To establish the link, source node 14 issues a conventional connection request to source node network interface 16. Source node network interface 16 then encapsulates the connection request within an IMN forwarding packet. The IMN forwarding packet includes an IMN-specific address of gateway node 18 and any other information required for transport across IMN 12. This IMN forwarding packet is forwarded across IMN 12 to gateway node 18 which responds to the encapsulated connection request by registering an active PPP connection to source node 14 and returning a connection verification indication encapsulated within another IMN forwarding packet which includes an IMN-specific address of source node network interface 16. If gateway node 18 cannot in fact support a new PPP connection, a connection busy indication will be sent instead. Source node network interface 16 extracts this information from the IMN forwarding packet and forwards it to source node 14. Source node 14 thus establishes the link by employing a conventional connection request which may be an "AT" modem command and without awareness of the operation of IMN 12. Alternatively, gateway node 18 may request the establishment of an emulated PPP link to source node 14 at step 110. Such a callout request may come about due to the operation of Internet Protocol network 20 in which gateway node 18 is connected or for some other reason. Gateway node 18 sends the connection request encapsulated within an IMN forwarding packet addressed to source node network interface 16 via IMN 12. Upon receipt of this IMN forwarding packet, source node network interface 16 extracts the connection request and forwards it to source node 14. Source node 14 responds by registering an active PPP connection to gateway node 18 and forwarding a connection verification indication to source node network interface 16 which encapsulates it within another IMN forwarding packet addressed for transport to gateway node 18. Again, the connection request and connection verification indication may be conventional "AT" modem commands and source node 14 need not be aware of IMN 12.

An emulated PPP data link established as described in connection with steps 100 and 110 may be employed to forward a PPP packet from source node 14 to gateway node 18 at step 120 or to forward a PPP packet from gateway node 18 to source node 14 at step 130. If gateway node 18 is connected in Internet Protocol network 20 and source node 14 operates software which implements Internet Protocol, the information fields of PPP packets exchanged via IMN 12 may include

Internet Protocol packets. Source node 14 may then exchange Internet Protocol packets with any node accessible via Internet Protocol network 20, enabling a broad range of applications imp emented by protocols overlying Internet Pro .%col includir; electronic mail and remote data access.

At step 120, if a PPP packet is to be forwarded from source node 14 to gateway node 18, the PPP packet is sent to source node network interface 16 for encapsulation within an IMN forwarding packet for transport to gateway node 18 across IMN 12. Gateway node 18 responds by extract:;r.g the PPP packet and processing it appropriately. If an Internet Protocol packet (IP packet) generated by source node 14 is encapsulated within an information field of the PPP packet, the IP packet is extracted and forwarded to its destination in Internet Protocol network 20 in accordance with an address field of the IP packet. Again, the operation of IMN 12 is transparent to source node 14 which need only generate the PPP packets. At step 130, if a PPP packet is to be forwarded from gateway node 18 to source node 14, it is similarly encapsulated within an IMN forwarding packet for transport across IMN 12 to source node network interface 16. Source node network interface 16 then extracts the PPP packet and sends it to source node 14. The motivation for generating the PPP packet at gateway node 18 may be receipt from Internet Protocol network 20 of an IP packet addressed to source node 14. Again, the operation of IMN 12 is transparent to source node 14. Protocols overlying Internet Protocol typically employ acknowledgement packets to assure reliable ordered delivery of data transport packets. For each set of data transport packets forwarded, an acknowledgement packet is returned. If a heavyweight protocol were employed to transport IMN forwarding packets across IMN 12, the heavyweight protocol would similarly generate and exchange acknowledgement packets, including IMN-specific acknowledgement packets which themselves encapsulate the acknowledgement packets generated by overlying protocols. However, since the protocols overlying Internet Protocol assure reliable ordered delivery between source node 14 and the ultimate destination in Internet Protocol network 20, there is no need to provide reliable ordered delivery across IMN 12. Thus, in the preferred embodiment, a lightweight transport protocol that does not itself generate acknowledgement packets is employed to transport the IMN forwarding packets which encapsulate the PPP packets. An emulated PPP link established in accordance with the invention is discontinued at the request of source node 16, at step 140, or at the request of gateway node 18, at step 150. Discontinuation of the link is effected by procedures similar to those employed to establish the link except disconnection requests are employed instead of connection requests. Gateway node 18 may request disconnection of a link to source node 14 if a predetermined interval passes with no link activity. To avoid a timed disconnection, source node 14 may periodically encapsulate and forward so-called "keep- alive" messages to gateway node 18.

Fig. 3 depicts a representation of one configuration of gateway node 18 in accordance with the invention, wherein physical ports to IMN 12 are decoupled from logical ports employed to operate virtual PPP sessions. As depicted in Fig. 3, gateway node 18 incorporates physical ports PI through PN for coupling to IMN 12. If IMN 12 is a wireless network, physical ports P through P_N each incorporate a radio transceiver. Gateway node 14 may operate a plurality of emulated PPP links over IMN 12. Each link is operated by one of logical ports L_χ through 1^ which generates PPP packets for transmission and processes received PPP packets. A free logical port is selected to operate a given PPP link when the link is established.

In accordance with the invention, physical ports P_χ through P_N and logical ports L_λ through L_M are not directly coupled. Instead, PPP packets associated with a given link may be received (in encapsulated form) by any available physical port. A lightweight transport/switching system 200 extracts PPP packets from IMN forwarding packets. Lightweight transport/switching system 200 directs the extracted PPP packet to the logical port which operates the given link. Similarly, a PPP packet generated by a logical port and intended for transport across IMN 12 may employ any available physical port. Lightweight transport/switching system 200 encapsulates the PPP packet within an IMN forwarding packet and directs the IMN forwarding packet to the physical port with the shortest queue of packets awaiting transmission. Typical PPP sessions are not continuously active.

There are periods during which no PPP packets are exchanged. By decoupling logical ports from physical ports, the gateway node configuration depicted in Fig. 3 takes advantage of these idle periods to support a number of logical ports greater than the number of available physical ports. This decoupling is particularly advantageous when IMN 12 is a wireless network and each physical port necessarily incorporates an expensive radio transceiver.

An IP router 202 interfaces the logical ports to Internet Protocol network 20. If a PPP packet received by a logical port encapsulates an IP packet, the logical port forwards the IP packet to IP router 202. The IP router then forwards the IP packet to its ultimate destination in Internet Protocol network 20. Alternatively, if IP router 202 receives an IP packet addressed to a node in IMN 12, the IP packet is forwarded to a logical port in communication with that node. If no PPP link with the node is currently active, the call-out procedure described in reference to Fig. 2 is followed to create a link.

Fig. 4 depicts gateway node 18 configured as a combination of a communications server 300 having multiple serial ports directly linked to logical ports and a special network interface 302 for coupling to IMN 12. Communications server 300 is a conventional communications server, incorporating serial ports S through S_M which are individually assigned to logical ports L through 1^ for operating PPP sessions. Network interface 302 incorporates physical ports P through P_N for connecting to IMN 12 and is also connected to serial ports S_x through S_M. In accordance with the invention, network interface 302 incorporates lightweight transport/switching system 200. Thus the configuration of Fig. 4 provides the decoupling advantages provided by the configuration of Fig. 3 while employing a readily-available conventional communications server.

Fig. 5 depicts gateway node 18 configured as a combination of conventional communications server 300 and a plurality of special network interfaces 400 for coupling to IMN 12. Each network interface 400 directly couples a serial port of communications server 300 to a physical port of IMN 12. In operation, each network interface extracts PPP packets from IMN forwarding packets received over IMN 12 and encapsulates PPP packets received from communications server 300 within IMN forwarding packets for transport across IMN 12. The network interfaces 400 are each functionally equivalent to source node network interface 16 of Fig. 1, only they are coupled to gateway node 18 rather than source node 14. The gateway node configuration of Fig. 5 does not provide the decoupling of physical ports and logical ports provided by the configurations of Fig. 3 and Fig. 4.

The attached appendix contains a source-code listing containing one operational embodiment of selected elements of the claimed invention. In particular, the source-code instantiates elements of network interface 400 depicted in Fig. 5 which is equivalent to network interface 16 depicted in Fig. 1. The machine-readable form of the listing can be compiled using a C language compiler.

The invention has now been explained with reference to specific embodiments. Other embodiments will be apparent to those of ordinary skill in the art in view of the foregoing description. It is therefore not intended that this invention be limited except as indicated by the appended claims.

Claims

WHAT IS CLAIMED IS:

1. A source node network interface, coupled to a source node and connected in an imperfect mesh network (IMN) , comprising: means for encapsulating PPP packets received from the source node within IMN forwarding packets; and means for forwarding the IMN forwarding packets to a gateway node via the IMN by employing a lightweight transport protocol.

2. The source node network interface of claim 1 wherein the IMN is a wireless network and said forwarding means comprise a radio transceiver.

3. A gateway node for connecting in an imperfect mesh network (IMN) , said gateway node comprising: a plurality of physical ports for interfacing to the IMN; and means, coupled to said plurality of physical ports, for operating a plurality of PPP sessions over the IMN, wherein PPP packets exchanged during the course of any one of said plurality of PPP sessions may travel via any one of said plurality of physical ports.

4. The gateway node of claim 3 wherein said IMN is a wireless network and said plurality of physical ports includes a plurality of incorporated radio transceivers.

5. A network interface for coupling a serial port of a communications server to an imperfect mesh network (IMN) , said network interface comprising: means for encapsulating PPP packets received from the communications server serial port within IMN forwarding packets; and means for forwarding the IMN forwarding packets to a source node via the IMN by employing a lightweight transport protocol.

6. A network interface for coupling a communications server to an imperfect mesh network (IMN) , said network interface comprising: a plurality of serial ports for connection to serial ports of the communications server; a physical port connected to the IMN; means for extracting PPP packets from IMN forwarding packets received via said physical port; and means for operating a plurality of PPP sessions over the IMN, wherein PPP packets extracted from IMN forwarding packets during the course of any one of said plurality of PPP sessions are forwarded to a selected one of said serial ports associated with said PPP session.

7. A method for establishing an emulated PPP data communication link over an imperfect mesh network (IMN) comprising the steps of: using a source node to send a modem connection request command to a source node network interface; thereafter sending a connection request via the IMN to a gateway node; using the gateway node to select an idle logical PPP port included within the gateway node for connection to the source node; using the gateway node to send a connection establishment confirmation to the source node network interface via the IMN; using the source node network interface to send a connection establishment modem command to the source node.

8. A method for establishing an emulated PPP data communication link over an imperfect mesh network (IMN) comprising the steps of: using a gateway node to generate an IMN forwarding packet including a connection request; thereafter sending the IMN forwarding packet to a source node network interface over the IMN; using the source node network interface to send a connection request modem command to a source node; using the source node to send a connection establishment modem command to the source node network interface; and thereafter using the source node network interface to send a connection establishment confirmation to the gateway node via the IMN.

9. In a data communication system, a method for forwarding a PPP packet from a gateway node to a source node over an imperfect mesh network (IMN) comprising the steps of: using the gateway node to encapsulate the PPP packet within an IMN forwarding packet; thereafter using the gateway node to forward the IMN forwarding packet to a source node network interface over the IMN in accordance with a lightweight transport protocol; thereafter using the source node network interface to extract the PPP packet from the IMN forwarding packet; and thereafter using the source node network interface to forward the PPP packet to the source node.

10. The method of claim 9 wherein an information field of the PPP packet holds an Internet Protocol (IP) packet received by the gateway node from an Internet Protocol network in which the gateway node is connected.

11. In a data communication system, a method for forwarding a PPP packet from a gateway node to a source node over an imperfect mesh network (IMN) comprising the steps of: using the source node to forward the PPP packet to a source node network interface; thereafter using the source node network interface to encapsulate the PPP packet within an IMN forwarding packet; using the source node network interface to forward the IMN forwarding packet over the IMN to the gateway node in accordance with a lightweight transport protocol; and thereafter using the gateway node to extract the PPP packet from the IMN forwarding packet.

12. The method of claim 11 wherein an information field of the PPP packet holds an Internet Protocol (IP) packet addressed to a destination node accessible via an Internet Protocol network in which the gateway node is connected.