Method for consolidating backward resource management cells...

Details Method for consolidating backward resource management cells... Method for consolidating backward resource management cells...

1998-09-23

2002-06-04

Hsu, Alpus H. (Department: 2665)

Multiplex communications

Data flow congestion prevention or control

Flow control of data transmission through a network

C370S407000, C370S408000, C370S395200

Reexamination Certificate

active

06400688

ABSTRACT:

FIELD OF THE INVENTION
The present invention is directed to a method for consolidating backward resource management cells for Available Bit Rate service in an Asynchronous Transfer Mode network.
In an Asynchronous Transfer Mode (ATM) network a plurality of service categories are available, as described in “The ATM Forum Technical Committee Traffic Management Specification”, Version 4.0, af-tm-0056.000, April 1996, herein incorporated by reference. One such service is Available Bit Rate (ABR) in which the source rate, referred to as the allowed cell rate (ACR), at which the ABR source transmits data is adjusted in accordance with network congestion information feedback.
FIG. 1
shows an ABR flow control path between point-to-point end-systems (unicast) for controlling through feedback the source rate in response to changing ATM layer transfer characteristics. The point-to-point configuration includes a single sending end-system (source)
10
, a single receiving end-system (destination)
20
and a network cloud
30
disposed therebetween. Network cloud
30
is an interconnection of transmission links and switching devices which provides a path for ATM traffic between the source and destination end-systems. Congestion points may exist in the network cloud and/or at the destination end-system. ABR flow control schemes are employed to control congestion at the congestion points in the network cloud and/or destination end-system which may increase queue-length, delay and/or cell loss.
In order to control the source rate, information such as bandwidth availability, state of congestion, and impending congestion of the network is conveyed to the source using Resource Management (RM) cells. In particular, RM cells include an Explicit Rate (ER) field that carries a 16-bit floating-point number representing the explicit rate, a Congestion Indication (CI) bit, which is used to signal a source to increase/decrease its rate by some predefined amount, and a No-Increase (NI) bit to signal a source to maintain its current rate.
Initially, source
10
generates Forward Resource Management (FRM) cells which are transmitted along with data cells via the network cloud
30
towards the destination
20
. Backward Resource Management (BRM) cells are generated via looping back an FRM cell at the destination end-system and/or are directly created by various congestion points, e.g., ATM switches, in the network cloud. The BRM cells carry feedback information provided by the destination end-system as well as by other congestion points in the network cloud back to the source
10
which, in turn, adjusts its source rate based on the congestion information in the BRM cells. RM cells are marked with switch congestion information such as the ER field and CI and NI bits as they are transmitted in a forward direction (from the source toward the destination) or in a reverse direction (from the destination back toward the source).
End-systems may also be arranged in a more complex point-to-multipoint tree (multicast tree) including a source end-system (root) connected by branches to a plurality of destination end-systems (leaves) and at least one branch point disposed between the root and the leaves. Specifically, a branch is defined as any point-to-point segment of the point-to-multi-point tree and a branch point is an intersection of two or more branches. By way of example, a multicast tree having a root
40
, a branch point
60
, two leaves
80
,
100
and network clouds
50
,
70
,
90
is shown in FIG.
2
. Each network cloud
50
,
70
,
90
comprises an interconnection of transmission links and switching devices which provide the path between the root (source end-system) or branch point and another branch point or a leaf (destination end-system). Congestion points may exist in the network cloud and/or at the leaf (destination end-system). The multicast tree shown in
FIG. 2
is for illustrative purposes only and is not intended to limit the number of branches, branch points, leaves or arrangement thereof. FRM cells and data cells received by branch point
60
from the root
40
are replicated (multicast) along each branch downstream in the forward flow direction toward the leaves
80
,
100
. BRM cells are generated via looping back an FRM cell at the leaf (destination end-system) and/or are directly created by various congestion points, e.g., ATM switches, in the network clouds. BRM cells returning from the leaves and/or the congestion points to the root are consolidated at branch point
60
before being passed back upstream to the root
40
. Consolidation of the BRM cells at the branch point prevents BRM cell implosion on the root and maintains the same number of BRM cells returning to the root irrespective of the number of leaves in the tree. In short, branch point
70
replicates the RM cells traveling in a forward direction and consolidates the RM cells traveling in a reverse direction to ensure that cell flows on each branch of the point-to-multipoint tree maintain the same conformance definitions as point-to-point cell flows.
As each copy of the FRM cell travels from the root to the plural leaves in a point-to-multipoint tree, its respective ER field, CI bit and NI bit is marked to convey information regarding the degree of congestion/bandwidth availability of traversed congested points. The congestion information in the ER field, Cl bit and NI bit of the RM cell are continuously updated as the cell travels in the forward/reverse flow directions.
As discussed above, in a point-to-multipoint tree the number of BRM cells which reach a particular branch point must be consolidated in order to limit the bandwidth consumed by BRM cells of each multicast branch and ensure that the volume of RM cells for an ABR end-point does not increase with the number of leaves of the multicast tree. On average over an extended period for every FRM cell that reaches a branch point at most one BRM cell is returned or passed back through the branch point towards the root (source end-system).
One conventional solution for performing BRM cell consolidation will be referred to as the “Branch point generation of BRM”. In this approach congestion information memory devices are associated with each forward ABR multicast VC at a branch point for storing the ER, CI, NI congestion information. When a BRM cell returns from a branch, the congestion/bandwidth availability information, e.g. ER field, CI bit, and NI bit, of the BRM cell of the current cell is consolidated with the previously stored values in the respective local congestion information memory devices, the updated information is stored in the local congestion information memory devices and the BRM cell is discarded. In a preferred embodiment the ER, CI, NI congestion information memory devices store the result of the min(previous ER, incoming ER), the CI bit of the incoming BRM cell ORed with the previously stored CI bit, and the NI bit of the incoming BRM cell ORed with the previously stored NI bit, respectively. When an FRM cell arrives at a branch point it is multicast downstream along each of the branches of the branch point. In addition, arrival of the FRM cell at the branch point triggers generation of a BRM cell which is sent to the source. Before the generated BRM cell is sent to the source it “picks up” the congestion information stored in the congestion information memory devices. The BRM cell “picks-up” the consolidated congestion/bandwidth availability information by storing in the BRM cell the result of the min(ER stored in congestion information memory devices, ER of the BRM cell), the CI bit stored in congestion information memory devices ORed with the CI bit of the BRM cell, the NI bit stored in the congestion information memory devices ORed with the NI bit of the BRM cell. This approach is disadvantageous in that it requires the switch to generate a large number of BRM cells.
Another well-known conventional scheme for performing BRM cell consolidation is referred to as the “ready-bit” approach. In this approach, for each forward multicast ABR Virtual Chan

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