Method and apparatus for mapping asynchronous ports to HDLC addresses

The present invention provides a method and apparatus for transmitting packets. One embodiment of the present invention includes an access server receiving a packet from a first unit through a first interface of a plurality of interfaces. The access server maps the packet into a first frame to transmit the packet over a predetermined point-to-point connection. An interface number representing the first interface is stored in an address field of the first frame. The first frame is then forwarded via the point-to-point connection to a second unit. The second unit maps a response to the packet into a second frame and stores the interface number representing the first interface in an address field of the second frame. The second unit then forwards the second frame to the access server. Upon receipt, the access server reads the address field of the frame to determine through which interface the response is to be forwarded.

FIELD OF THE INVENTION
 The invention relates generally to computer networking systems, and, in
 particular, to addressing frames of data.
 BACKGROUND OF THE INVENTION
 Typically, when a remote node 152 uses a service provider to establish a
 remote connection with a separate network, the remote node 152 will dial
 into the service provider's local Network Access Server (NAS) 154. As
 shown in FIG. 1, the NAS 154 usually includes several port interfaces.
 Each remote node dials into the NAS 154 through a separate port interface.
 Data received through interfaces may in turn be multiplexed over one
 synchronized line 156 providing a connection to a Wide Area Network (WAN)
 access device 260.
 The data is usually transmitted between the remote nodes and the NAS 154
 over an asynchronous line (e.g. telephone line). Therefore, the NAS
 typically needs to frame the packets of data received from the remote node
 into data-link control protocol frames (e.g. High-level Data Link Control
 (HDLC)) in order to transmit the packets over the synchronous line 156.
 (References herein to the data-link layer and the network layer are
 references to the Open Systems Interconnection (OSI) model developed by
 the International Standardization Organization).
 As shown in FIG. 1, the synchronous line 156 between the NAS 154 and the
 WAN access device 160 is usually a point-to-point connection. That is, a
 connection with out any intervening nodes or switches.
 As a result of the point-to-point nature of the connection, the address
 field of the data-link frame is not used because the frame has only one
 possible destination once it is transmitted. Usually, the address field of
 a data-link frame is used to indicate the physical destination of the
 frame so that any switches between a frame's source and destination will
 be able to read the frame's address field and know where to forward the
 frame. However, when it is known that there will be no intervening
 switches between the frame's source and destination (e.g. point-to-point
 connection) no address is necessary. In fact, the RFC 1662 states that for
 point-to-point connections, the address field of the HDLC frame should
 contain 0.times.FF.
 When the NAS 154 receives responses from the WAN access device 160, the NAS
 154 usually performs a routing function to determine where the response is
 to be forwarded. That is, the NAS 154 will strip the response packet from
 its data-link frame and read the packet's address information, which is
 usually provided at the network layer (e.g. the Internet Protocol (IP)
 address). The routing procedures, however, slow down the transmission of
 the responses and usually only work if the node to which a response is
 being delivered, has been given a network address.
 In the case of a user dialing into a NAS 154 from a unit that does not have
 a network address (e.g. a terminal), the NAS will usually assign a network
 address to each packet received from that unit. The assigned network
 addresses will in turn will be used by the NAS to forward responses using
 the routing procedures described above. Assigning network addresses,
 however, has the disadvantage of preallocating several network addresses,
 when in fact, some of these addresses may not be used.
 Therefore, what is needed is a way for an access server to forward
 data-link frames received from a point-to-point connection without having
 to perform routing operations.
 SUMMARY OF THE INVENTION
 The present invention provides a method and apparatus for transmitting
 packets. One embodiment of the present invention includes an access server
 receiving a packet from a first unit through a first interface of a
 plurality of interfaces. The access server maps the packet into a first
 frame to transmit the packet over a predetermined point-to-point
 connection. An interface number representing the first interface is stored
 in an address field of the first frame. The first frame is then forwarded
 via the point-to-point connection to a second unit.
 The second unit maps a response to the packet into a second frame and
 stores the interface number representing the first interface in an address
 field of the second frame. The second unit then forwards the second frame
 to the access server.
 Upon receipt, the access server reads the address field of the second frame
 to determine through which interface the response is to be forwarded.

DETAILED DESCRIPTION
 A method and apparatus is described for having a NAS forward data-link
 layer frames received from a point-to-point connection without performing
 routing operations.
 In the following description, numerous specific details are set forth in
 order to provide a thorough understanding of the present invention. It
 will be apparent, however, to one of ordinary skill in the art that the
 present invention may be practiced without these specific details. In
 other instances, well-known standards, structures, and techniques have not
 been shown in order not to unnecessarily obscure the present invention.
 Referring to FIG. 2, a network configuration is shown, which is capable of
 implementing the present invention to one embodiment. As shown, a remote
 node 252 dials into the NAS 254, when attempting to establish a connection
 through the WAN 270. The remote node sends a packet of information to the
 NAS 254 across an asynchronous line. The information is received by the
 NAS through one of the NAS's several port interfaces 258.
 The packet of information received from a remote node 252 is framed by the
 NAS into a data-link frame to be sent to the WAN access device 260. In the
 present invention, the number of the port interface through which the
 packet was received is mapped into the address field of the data-link
 frame.
 As shown in FIG. 2, the line connecting the NAS 254 and the WAN access
 device 260 is a point-to-point connection. In one embodiment, the line is
 also synchronous connection.
 Once the WAN access device 260 receives the data-link frame from the NAS
 254, the WAN access device 260 establishes a logical channel 264 between
 the WAN access device 260 and a WAN 270. Afterwards, the WAN access device
 stores the number of the NAS port interface, through which the NAS
 received the packet, into a table entry corresponding to channel
 established between the WAN access device 260 and the WAN 270. The table
 entry is included in a mapping table 262 stored at the WAN access device.
 When the WAN access device 260 receives a response over one of its channels
 264, the WAN access device checks its mapping table 262 to see if a NAS
 port interface number has been stored in a table entry corresponding to
 the channel over which the response was received. If a NAS port interface
 number is present in the table entry, the port interface number is mapped
 into the address field of the data-link frame when the response is framed.
 The data-link frame is then sent to the NAS 254 over the point-to-point
 connection.
 When the NAS 254 receives the response, rather than performing a standard
 routing procedure of the response, the NAS 254 reads the address field of
 the data-link frame. The NAS then forwards the response through the NAS
 port interface corresponding with the port interface number represented in
 the address field of the data-link frame. As a result, the NAS 254 is able
 to forward data-link frames received from the point-to-point connection
 faster than had it performed standard routing operations.
 Referring to FIG. 3, a flow diagram is shown describing the steps of one
 embodiment of the invention in more detail. At step 302, a user dials into
 a NAS through a remote node and transmits a packet of information to the
 NAS. In one embodiment, the connection between the remote node and the NAS
 is an asynchronous line.
 In step 304, the packet is received by the NAS through one of several port
 interfaces, each of which is assigned a port interface number. The NAS, in
 step 306, then strips the asynchronous data-link frame from the packet.
 In step 308, the NAS determines whether the packet is to be multiplexed
 over a point-to-point connection with a WAN access device (e.g. a Frame
 Relay Access Device). For example, in one embodiment, the NAS
 predetermines that all packets of data received on a particular set of
 port interfaces are to be multiplexed over a particular synchronized
 point-to-point connection line. In addition, the point-to-point connection
 may provided a connection between the NAS and a device other than a WAN
 access device without departing from the scope of the invention.
 If the NAS determines that the packet is not to be multiplexed (i.e. was
 not received via one of the specified interfaces), in step 310 standard
 routing operations are performed to determine the next hop for the packet
 and the interface through which the packet should be forwarded.
 If, on the other hand, it is determined in step 308 that the packet is to
 be multiplexed over the synchronized point-to-point connection, then in
 step 312 the NAS frames the packet into a data-link control protocol (e.g.
 HDLC). In particular, the NAS maps the number of the NAS port interface
 through which the packet was received, into the destination address field
 of the data-link frame. Recall that RFC 1662 states that ordinarily in the
 case of a point-to-point connection the destination field in a data-link
 control protocol should be set to all ones because there are no
 alternative destinations on the point-to-point connection and no address
 is necessary.
 In step 314, the WAN access device attached to the other end of the
 point-to-point connection receives the frame transmitted from the NAS. In
 step 316, the WAN access device reads the destination address field of the
 frame to determine whether the address field contains 0.times.ff or some
 other address.
 If the WAN access device determines that the destination address field
 contains 0.times.ff, in step 318, the WAN access device forwards the
 packet by performing standard routing procedures. This typically includes
 stripping off the data-link frame and reading the destination address
 information provided in the network layer header of the packet.
 If, on the other hand, the WAN access device determines that a number other
 than 0.times.ff is provided in the address field of the frame, in step 320
 the WAN access device establishes a direct logical channel (e.g. a
 permanent virtual circuit) between the WAN access device and the ultimate
 destination of the packet. In step 322, the WAN access device then stores
 the NAS port interface number provided in the address field of the
 data-link frame. In one embodiment, the interface number is stored in a
 mapping-table, and in particular, it is stored in a table-entry
 corresponding to the logical channel through which the packet has been
 forwarded from the WAN access device.
 When the WAN access device receives a response packet from the network 270
 to which it is providing access, in step 324 the WAN access device
 determines if the channel over which the response was received has a
 corresponding entry in the mapping-table. If the WAN access device
 determines no entry exist for the logical channel over which the response
 was received, in step 326 the WAN access device performs standard routing
 procedures to determine the next hop for the packet and the interface over
 which the packet is to be forwarded. In particular, if the packet is to be
 forwarded over the point-to-point connection 256 to the NAS 254, the
 packet is framed into a data-link frame and the address field of the frame
 is set to 0.times.ff. The packet is then forwarded over the synchronous
 line.
 If an entry in a table storing interface numbers is provided for the
 logical channel over which the packet was received, in step 328, the WAN
 access device frames the packets in a data-link frame. In particular, the
 WAN access device stores the NAS port interface number, which was stored
 in the table entry corresponding to the logical channel over which the
 packet was received, into the destination address field of the data-link
 frame. The packet is then forwarded over the NAS 254 over the
 point-to-point connection 256.
 When the NAS receives a data-link frame over the point-to-point line 256,
 in step 330, the NAS reads the destination address field of the data-link
 frame to determine if the frame has a destination address of 0.times.ff
 (i.e. it was routed) or has another address. If the packet has a data-link
 frame address of 0.times.ff, in step 332, the NAS performs standard
 routing procedures and forwards the packet.
 On the other hand, if the address field of the data-link frame is a port
 interface number (i.e. any number other than 0.times.ff), in step 334, the
 NAS bypasses the routing procedure and forwards the packet over the port
 interface specified in the destination address of the data-link frame.
 As a result, the present invention provides the increased performance
 advantage of forwarding packets received from point-to-point connections
 without having to perform routing operations on the packet.
 Moreover, it should be understood that the asynchronous line connecting the
 remote node and the NAS could be an Integrated Service Digital Network
 (ISDN) without requiring any changes to the implementation of the present
 invention because each ISDN line is understood to be uniquely identifiable
 with a simple number logically indistinguishable from the number of a port
 interface.
 In alternative embodiments, the present invention may be applicable to
 implementations of the invention in integrated circuits or chip sets,
 wireless implementations, switching systems products and transmission
 systems products. For purposes of this application, the terms switching
 systems products shall be taken to mean private branch exchange (PBXs),
 central office switching systems that interconnect subscribers,
 toll/tandem switching systems for interconnecting trunks between switching
 centers, and broadband core switches found at the center of a service
 provider's network that may be fed by broadband edge switches or access
 muxes, and associated signaling, and support systems and services.
 The term transmission systems products shall be taken to mean products used
 by service providers to provide interconnection between their subscribers
 and their networks such as loop systems, and which provide multiplexing,
 aggregation and transport between a service provider's switching systems
 across the wide area, and associated signaling and support systems and
 services.
 The present invention may be implemented on a storage medium having stored
 thereon instructions which can be used to program a computer to perform
 the present invention. The storage medium can include, but is not limited
 to, any type of disk including floppy disks, optical disks, CD-ROMs, and
 magneto-optical disks, ROMs, RAMs, EPROMs, EEPROMs, magnet or optical
 cards, or any type of media suitable for storing electronic instructions.
 Alternatively, the present invention could be implemented in discrete
 hardware components such as large-scale integrated circuits (LSI's),
 application-specific integrated circuits (ASIC's) or in firmware.
 Moreover, in the foregoing specification the invention has been described
 with reference to specific exemplary 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 restrictive sense.