Electrical computers and digital processing systems: multicomput – Computer-to-computer protocol implementing – Computer-to-computer data transfer regulating
Reexamination Certificate
1999-05-27
2002-10-08
Meky, Moustafa M. (Department: 2153)
Electrical computers and digital processing systems: multicomput
Computer-to-computer protocol implementing
Computer-to-computer data transfer regulating
C370S233000
Reexamination Certificate
active
06463476
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention consists in a method of controlling congestion in a node of an asynchronous transmission system.
The invention concerns asynchronous transfer mode (ATM) transmission. In this transmission mode, cells of fixed size are transmitted in a connection-oriented mode. A connection is set up by marking a path, i.e. a chain of identifiers. Locally, only a user's local identifier known as the VPVC (virtual path virtual channel) is used in an ATM node; identifiers are translated locally in each node to transmit the packets. For more details on the structure and operation of the ATM see the publications of the ITU, the ATM Forum or the ETSI.
ATM is currently the support for layer
2
of the ISO model for the Internet protocol (IP). An IP-ATM adaptation layer known as the ATM adaptation layer (AAL-5) is used for this purpose. In AAL-5, an IP packet is transported by segmenting it into ATM cells; an ITU recommendation proposes to introduce into the header of ATM cells a bit representative of the end of the IP packet, i.e. the last ATM cell of an IP packet. This AUU (ATM user to user indication) bit is used to mark the end of an IP packet; it is also used to mark the beginning of an IP packet in that the ATM cell following a cell with an AUU bit at
1
corresponds to the start of an IP packet. These AAL-5 functions are documented in ITU-T 1361, BisDN ATM Layer Specification, Helsinki, March 1993.
One problem encountered with ATM is controlling congestion at the nodes. The advantage of ATM is that the cell rate is flexible because transmission is asynchronous; locally, in a node having finite resources, this flexibility causes congestion control problems.
Various solutions to the problem of controlling congestion have been proposed. A first solution consists in not controlling congestion in the nodes, i.e. in multiplexing incoming cells blind. This solution has a high cost in terms of resources if it is to achieve adequate performance. New service models seek to provide more suitable solutions, in particular so-called “best effort” solutions, which take account of the fact that the same grade of service is not necessarily required by all types of application. “Packet discard” and “frame discard” solutions have been suggested, in particular at the suggestion of the ATM Forum.
One solution proposed in the document ATM Forum Traffic Management Specification 4.0 is known as tail packet discard (TPD). This entails rejecting or discarding all subsequent cells up to the end of the packet if a cell is rejected or discarded because of a buffer overflow. This solution uses the AUU bit to mark the end of a packet; after a cell is discarded, all subsequent cells having the same VPVC are discarded, until a cell is discarded whose AUU bit is set, which is the end of a packet. This solution recovers space in the buffer, given that a packet from which a cell has been eliminated is unusable by the application.
Another solution proposed in the document ATM Forum Traffic Management Group Living List, December 1997, pages 18-20, is known as early packet discard (EPD). The idea of this solution is to decide, as soon as reception of a packet begins, whether the cells constituting the packet must be accepted or not, depending on the space available in the buffer when the first cell is received. To this end an EPD threshold is defined in the buffer. When the first cell of a packet reaches the buffer it and all the subsequent cells of the packet are rejected if the buffer has already been filled beyond the EPD threshold. Conversely, the cell is accepted if the EPD threshold in the buffer has not been reached when the first cell arrives; this does not prejudge accepting subsequent cells of the packet.
The above solutions are not totally satisfactory. TPD is not optimal in that it is possible to accept most cells of a packet and still to have to retransmit the whole packet if the last cells are rejected; EPD is relatively inefficient in a small queue or if the transmission node is heavily loaded. It is also difficult to set the EPD threshold because it is sensitive to the size of the packets and to their mode of arrival. Statistical information on cell size and mode of arrival is generally used to set the EPD threshold.
A service model of the ATM layer known as ATM block transfer with immediate transmission (ABT/IT) has also been proposed and provides the facility to mark blocks in the ATM layer. The blocks are not necessarily tied to a higher level protocol data unit (PDU)—a packet in the case of the IP. The marking is done by inserting resource management (RM) cells which define the ends of the blocks and carry information on the cell rate of the block transmitted between two successive RM cells and optional block size information. This service model is described, for example, in the document ITU-T Recommendation 1.371, Toronto, September 1997, page 19. It is not used in current systems because of the complexity entailed in implementing this network solution. Also, inserting RM cells leads to an increase in traffic (overhead).
SUMMARY OF THE INVENTION
The invention proposes an ATM node congestion control solution enabling more efficient control than the prior art EPD and TPD solutions and which is simpler to implement than the ABT/IT solutions.
To be more precise, the invention proposes a method of controlling congestion in a node of an asynchronous transmission system in which cells are transmitted, characterized in that it consists in:
evaluating the cell rate of the connection from the peak cell rate negotiated for the connection,
evaluating the remaining transmission capacity that the node can accept at the time concerned, and
comparing the peak cell rate and the remaining transmission capacity and accepting or rejecting cells according to the result of the comparison.
In one particular embodiment, the method is applied to cells which form packets and is characterized in that it consists in performing these three operations when the first cell of each packet arrives.
The remaining transmission capacity at the node is preferably taken as equal to the difference between the maximum total cell rate that the node can accept and the sum of the cell rates of the packets in process of transmission via the node.
The packets in process of transmission via the node are advantageously packets whose last cell has not been transmitted.
In another embodiment, the packets in process of transmission via the node comprise packets whose last cell has not been transmitted and packets whose last cell has been transmitted in a time period less than the average time period between cells of the packet.
The method preferably comprises inserting spacers into at least one connection and the evaluation of the peak cell rate of said connection is corrected to allow for the introduction of said spacers.
It is also possible to reject all the cells of a packet whose first cell has been rejected.
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Alcatel
Meky Moustafa M.
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