Multiplex communications – Data flow congestion prevention or control
Reexamination Certificate
1997-12-31
2002-04-09
Olms, Douglas (Department: 2661)
Multiplex communications
Data flow congestion prevention or control
C370S230000, C370S395430
Reexamination Certificate
active
06370114
ABSTRACT:
BACKGROUND OF THE INVENTION
A. Field of the Invention
The present invention relates generally to digital networks, and in particular to apparatus and methods for increasing throughput in a connection carried at least partially over an asynchronous transfer mode (ATM) network.
B. Description of the Prior Art
Data transported over a connection frequently crosses several networks, each having a different protocol. The source and destination nodes may employ one protocol, such as Transport Control Protocol (TCP), and a network linking the nodes travels may employ another, such as ATM.
As networks become congested, net throughput decreases. Constant efforts have been made to increase connection throughput and correspondingly decrease network congestion. Moderate increases in throughput are attributable to new congestion avoidance algorithms serving as part of a protocol used to implement a connection. For example, TCP uses a congestion window algorithm to alter source to destination data transfer, and ATM has a variety of services, such as Available Bit Rate Service (ABR), to optimize throughput over a connection. When TCP is carried over ABR, this is referred to as “TCP over ABR.”
Using TCP over ABR increases throughput dramatically. The increased throughput, however, is only realized over ATM networks. Therefore, the advantages are only realized when both source and destination are connected over a pure ATM connection. When a TCP connection is partially carried through an ATM network and partially over a non-ATM network, such as an Ethernet LAN, the non-ATM network can become a bottleneck to the overall connection throughput. This is because the TCP congestion window algorithm does not optimize throughput as much as possible. Therefore, even though TCP over ABR raises throughput on the ATM portion of the connection, increased throughput is not realized over the non-ATM portion of the connection.
FIG. 1
is a block diagram showing a prior art network architecture. Ethernet local area network (LAN)
110
is connected to switch
114
and user
116
, and ATM network
112
is connected to switch
114
and user
118
. The architecture supports communications between user
116
and user
118
.
In a typical connection, several protocols may be used simultaneously for a connection between user
116
and user
118
. One common combination of protocols is the combined use of TCP and ATM. TCP is a protocol for transporting a byte stream between pairs of hosts, such as user
116
and user
118
. ATM is a high bandwidth transmission technology over which TCP connections can be transferred. A TCP byte stream is packetized according to the ATM protocol, transported over the ATM network, depacketized, and reassembled into the original TCP byte stream at the other end.
FIG. 2
is a block diagram showing a conventional switch which may be used as switch
114
of FIG.
1
. Switch
114
receives TCP data from Ethernet LAN
110
, packetizes the data into ATM data, and depacketizes ATM data from an ATM network into TCP data. Switch
114
is connected to LAN
110
via Ethernet physical interfaces
210
and
218
. Data coming from LAN
110
into switch
114
is transferred from Ethernet physical interface
210
to demux
211
. Demux
211
associates TCP connections with ATM connections by demultiplexing the TCP data stream into connection buffers
212
. Data is then transferred from connection buffers
212
into segmentation and reassembly processor (SAR)
214
. Finally, the data is transferred to ATM network
112
by ATM physical interface
216
.
In the opposite direction, data is received from ATM network
112
by ATM physical interface
226
. ATM physical interface
226
transfers the data through Resource Management (RM) cell demultiplexer
224
to SAR
222
, which in turn transfers the data to buffer system
220
. Finally, the data is transferred from buffer system
220
to Ethernet physical interface
218
, which transfers the data to Ethernet LAN
110
.
RM cell demultiplexer
224
extracts RM cells from the incoming ATM data stream and transfers them to SAR
214
. RM cells are used in ATM to transfer information regarding resources on the ATM network. For example, RM cells may be used to transfer congestion information. RM cells also include ATM information that identifies the particular connection the resource information is associated with. In response to congestion information in the RM cells, SAR
214
adjusts its output in accordance with the ABR service used on the ATM connection.
ATM supports four service categories that have a variety of performance levels: Constant Bit Rate (CBR), Variable Bit Rate (VBR), Available Bit Rate (ABR), and Unspecified Bit Rate (UBR).
ABR service in ATM networks is primarily used for the transport of best effort data services. ABR sessions share the network bandwidth left over after serving CBR and VBR traffic. This available bandwidth varies with the requirements of the ongoing CBR/VBR sessions by providing rate based feedback in RM cells to the switches carrying ABR sessions. Because TCP is currently the predominant data transport protocol that applications run on over the Internet, TCP traffic is a likely candidate to be carried over the ABR service category.
Data is transferred over ATM network
112
using virtual connections. A virtual connection receives data to be transferred, packetizes the data, transmits the packets over any available path toward the destination, reassembles the packets into the order in which they were transferred, and finally transmits the original data that is transferred. The ATM protocol manages the virtual connection to ensure that data being transferred is reassembled in the same order it was sent. From the user's standpoint, the virtual connection appears like a physical connection because data is received in the same order it was sent.
Each virtual connection is routed through the network and includes a forward path (from source to destination) and a backward path (from destination to source). For both bidirectional point-to-point and point-to-multipoint connections, the forward and backward components of a virtual connection use the same connection identifiers, and pass through identical transmission facilities.
A source for the ABR service can submit cells into the network at a variable but controlled or shaped rate. SAR
214
performs this function. The ABR source and destination forms an ABR control loop: the ABR source transmits cells for conveying feedback information towards the destination and the destination returns them towards the source.
The ABR service uses RM cells to provide network congestion information to SAR
214
. In particular, SAR
214
reduces or increases cell transmission rates depending on the availability of bandwidth in the network as indicated by the RM cells.
Congestion feedback information is used by switch
114
to respond to changes in the available bandwidth by appropriately modifying submission rates of data being transmitted onto ATM
112
. This controls or avoids congestion, and the available bandwidth is used.
For ABR connections, the source creates a connection with a call setup request. During this call setup, the values for a set of ABR-specific parameters are identified. Some values are requested by the source and may be modified by the network (e.g., the lower and upper bounds on the source rate), while other values are directly chosen by the network (e.g., the parameters characterizing the process for dynamically updating rates).
Once the source has received permission, it begins cell transmission. The rate at which an ABR source is allowed to schedule cells for transmission is denoted by the Allowed Cell Rate (ACR). In ABR, ACR is initially set to the Initial Cell Rate (ICR), and is always bounded between the Minimum Cell Rate (MCR) and the Peak Cell Rate (PCR). Transmission of data cells is preceded by the sending of an ABR RM cell. The source rate is controlled by the return- of these RM cells, which are looped back by the destination or by a virtual destination.
An RM cell
Bernstein Greg M.
Chhabra Gurpreet S.
Gullicksen Jeffrey T.
Finnegan Henderson Farabow Garrett & Dunner L.L.P.
Olms Douglas
Pizarro Ricardo M.
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