Multiplex communications – Diagnostic testing – Fault detection
Reexamination Certificate
2000-05-10
2003-11-04
Nguyen, Chau (Department: 2663)
Multiplex communications
Diagnostic testing
Fault detection
C370S248000
Reexamination Certificate
active
06643265
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to data communications networks and more particularly relates to a method of releasing stuck virtual circuits in an asynchronous transfer mode network.
BACKGROUND OF THE INVENTION
Asynchronous Transfer Mode
Currently, there is a growing trend to make Asynchronous Transfer Mode (ATM) networking technology the base of future global communications. ATM has already been adopted as a standard for broadband communications by the International Telecommunications Union (ITU) and by the ATM Forum, a networking industry consortium.
ATM originated as a telecommunication concept defined by the Comite Consulatif International Telegraphique et Telephonique (CCITT), now known as the ITU, and the American National Standards Institute (ANSI) for carrying user traffic on any User to Network Interface (UNI) and to facilitate multimedia networking between high speed devices at multi-megabit data rates. ATM is a method for transferring network traffic, including voice, video and data, at high speed. Using this connection oriented switched networking technology centered around a switch, a great number of virtual connections can be supported by multiple applications through the same physical connection. The switching technology enables bandwidth to be dedicated for each application, overcoming the problems that exist in a shared media networking technology, like Ethernet, Token Ring and Fiber Distributed Data Interface (FDDI). ATM allows different types of physical layer technology to share the same higher layer—the ATM layer.
ATM is a connection oriented transport service. To access the ATM network, a station requests a virtual circuit between itself and other end stations, using the signaling protocol to the ATM switch. ATM provides the User Network Interface (UNI) that is typically used to interconnect an ATM user with an ATM switch that is managed as part of the same network.
The current standard solution for routing in a private ATM network is described in Private Network Node Interface (PNNI) Phase 0 and Phase 1 specifications published by the ATM Forum. The previous Phase 0 draft specification is referred to as Interim Inter-Switch Signaling Protocol (IISP). The goal of the PNNI specifications is to provide customers of ATM network equipment some level of multi-vendor interoperability.
ATM uses very short, fixed length packets called cells. The first five bytes, called the header, of each cell contain the information necessary to deliver the cell to its destination. The cell header also provides the network with the ability to implement congestion control and traffic management mechanisms. The fixed length cells offer smaller and more predictable switching delays as cell switching is less complex than variable length packet switching and can be accomplished in hardware for many cells in parallel. The cell format also allows for multi-protocol transmissions. Since ATM is protocol transparent, the various protocols can be transported at the same time. With ATM, phone, fax, video, data and other information can be transported simultaneously.
A diagram illustrating the format of an ATM cell including the cell header format across the UNI is shown in FIG.
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. The header comprises an 8-bit Virtual Path Indicator (VPI) and a 16-bit Virtual Circuit Indicator (VCI). A four-bit Generic Flow Control (GFC) field supports multiplexing functions. The GFC mechanism is intended to support simple flow control in ATM connections. The Cell Loss Priority (CLP) one-bit field corresponds to the loss priority of a cell. In operation, lower priority cells (i.e. CLP=1) can be discarded under congestion situations. The Header Error Check (HEC) is an 8-bit field that is used for header error detection and correction. The importance of the information in the header requires use of the HEC. The payload follows the HEC field in the ATM cell header and comprises 48 bytes.
The Payload Identifier Type (PTI) is a 3-bit cell header field for encoding information regarding ATM Adaptation Layer (AAL) and Explicit Forward Congestion Indication (EFCI). The AAL is a collection of standardized protocols that adapts user traffic to a cell format. AAL
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is the protocol standard used to support the transport of Variable Bit Rate (VBR) traffic and signaling messages. EFCI is a one-bit field containing information whether congestion at an intermediate node has been experienced. The EFCI bit is set when a threshold has been exceeded.
ATM Adaptation Layer
In the AAL
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protocol, every AAL frame to be transmitted over the ATM network is segmented into cells. The last cell of the frame is indicated in the ATM cell header in the PTI field of the cell header. In particular, the last cell is indicated by the third bit in the PTI field being cleared, the second bit a don't care and the first bit being set, i.e. PTI=1 or 3. An ATM network device that receives AAL
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frames must reassemble all the cells received over each Virtual Circuit (VC), i.e. VPI/VCI pair, until the last cell of the frame is received. After the last cell of the cell is received, the entire frame is available and can be sent to its corresponding destination port, i.e. Ethernet port, host, etc.
A problem occurs, however, in the event the last cell of the frame on a specific VC gets lost and no subsequent ‘last cell’ is received (even from the next frame) on the VC. In the event the last cell is not received, the previously received cells of the frame remain stored in the network device and consume memory resources that are never released since a last cell indication will never be received on the specific VC. The problem is compounded in an ATM network device that establishes a large number of VCs (i.e. several thousand) wherein the last cell is lost for a large percentage of the VCs. In this case, a majority of the memory resources may be consumed and not available for other VCs.
For example, a network device such as an ATM switch or edge device that is connected to the ATM network may be accidentally disconnected. All virtual circuit connections that were already established and that were in the midst of receiving frames, will never be completed and closed. The frames waiting to complete consume memory resources and depending on the number of incomplete frames, the amount of memory may be large.
Consider an ATM edge device adapted to transport 10 Mbps Ethernet over the ATM network. Assume each Ethernet frame comprises 1518 bytes, thus occupying 32 ATM cells to send the corresponding AAL
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frame. Assuming one thousand open connections at the time the device is disconnected from the network, this translates to approximately 1.5 Mbytes of memory. Depending on the size of the memory in the device, this may amount to a sizeable portion of the memory. Note that even a relatively small portion of memory, e.g., 5 or 10%, creates a problem since the memory consumption is cumulative if it is not released.
In some cases, the hardware in the device will timeout and cause the software to clear the call, but the memory consumed by the incomplete frame remains. In other cases, both the virtual circuit connection remains and the memory is not cleared.
It is therefore desirable to have a mechanism adapted to locate virtual circuits that are stuck due to lost ‘last cells’ and to release these ‘stuck’ virtual circuits thus freeing up memory resources that could be used for new virtual circuits.
SUMMARY OF THE INVENTION
The present invention solves the problems associated with the prior art by providing an apparatus for and a method of locating and releasing stuck virtual circuits in an ATM network device. The invention functions to free memory resources that would otherwise be lost due to the last cell of a frame not being received on a particular virtual circuit. The memory savings made possible by the present invention increase as the number of virtual circuits increase wherein the last cell of the frame was not received.
Throughout this document the term stuck virtual circuit is meant to denote a virtual ci
3Com Corporation
Ng Christine
Nguyen Chau
Zaretsky Howard
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