Abstract:
A method and apparatus are provided for implementing automatic congestion avoidance for differentiated service flows in a communications network. A bandwidth control signal representing congestion information is identified. Responsive to the identified bandwidth control signal, dual transmit probabilities are calculated for each flow. The dual transmit probabilities include a first transmit probability for a first color and a first transmit probability for a second color. A marked color for a packet is identified. The marked packet color and the calculated dual transmit probabilities are selectively utilized for making a packet discard determination.

Description:
FIELD OF THE INVENTION 
   The present invention relates generally to the data processing field, and more particularly, relates to a method and apparatus for implementing automatic congestion avoidance for differentiated service flows over a communications network. 
   DESCRIPTION OF THE RELATED ART 
   With Internet Protocol (IP), communication between network devices is accomplished with packets. Each packet can contain various messages with varying widths. Because not all packets have the same priority, the Internet standards committee has proposed a method to differentiate packets by using 6 bits in the header of an Internet packet. This is known as Differential Services or DiffServ. In order to make a differentiated service work, a routing device must read these header bits and provide a method of dealing with high priority traffic in a different manner than low priority traffic. 
   Also, the Internet task force has defined a standard for monitoring and policing incoming traffic. This is accomplished by using two methods of policing, Single Rate Two Color Marking and Two Rate Two Color Marking, RFC2698/RFC2697. In either method, an incoming packet is checked against the history of packets on the same flow. If an incoming flow exceeds certain defined bounds, the packet is marked yellow. If a packet exceeds an upper limit, the packet is marked red. If no bounds are exceeded, the color is green. The colors green, yellow or red can be sent to the next hop along with the packet to aid in the next hops decision to pass the packet along or to discard it. 
   If there is sufficient bandwidth in a router, there is no reason to discard yellow or red packets. However, if the router experiences congestion, then the packets of color red and yellow should be the first ones to be discarded. The Request for Comment (RFC 2698/RFC 2697) specifies that the drop rate for red should be greater than or equal to the drop rate for yellow which should be greater than or equal to the drop rate for green. 
   Differentiated Services attempts to classify incoming Internet packets into flows and when a router experiences congestion, different probabilities for discarding red and yellow packets from each flow are needed. How to calculate the drop probabilities and keep them in synchronization with the system has been a manual trial and error process. 
   Various methods have been proposed over the years to randomly drop packets according to some drop probabilities. One method still in use today is known as Random Early Discard (RED). By setting system parameters that must be set before congestion occurs, a drop probability can be created. Another commonly used and improved method is known as Weighted Random Early Discard. The Weighted Random Early Discard method attempts to provide some improvements over RED by allowing some lines or paths higher priority without dropping packets from multiple sources. Again, a manual approach is used in setting up the system to handle the drop probabilities. Both methods employ threshold monitoring and then a drop rate supplied by the system administrators are applied. However, neither method takes in account the Differentiated Service information. Neither method is dynamic, that is, these methods do not provide discard rates based on the system utilization. 
   International Business Machines Corporation of Armonk, N.Y. has developed a more recent method called Bandwidth Allocation Technology. The Bandwidth Allocation Technology (BAT) method provides an automatic system to deal with allocating the bandwidth resources of a chip or system based upon the system utilization. The BAT method applies control theory to create an automatic system that reacts to congestion. 
   In the BAT method, the transmit ratio Ti for a given flow can be determined by the following equations:
 
if  fi &lt;=mini then  Ti ( t+Dt )=min(1, Ti ( t )+0.125)
 
else if( fi &gt;maxi) then  Ti ( t+Dt )=0.857 *Ti ( t )
 
else
 
if  B= 1 Ti ( t+Dt )=min(1, Ti ( t )+ Ci*E ( t ))
 
else  Ti ( t+Dt )=max(0, Ti ( t )− Di*fi ( t ))
 
The value fi represents the amount of bytes transmitted for any given line or channel i input to a network router. Ti is the transmit probability for any given input i. Ti(t+Dt) represents the transmit rate for a next period that starts after the computation is performed. Ti(t) represents the previous transmission rate, for what was just being transmitted. The transmit probability Ti is a value between 0 and 1. Mini represents a lower limit or guaranteed level where the transmit probability Ti will not be reduced. The bytes transmitted over a sample time value fi is compared with the mini value to determine whether the flow is below the lower limit or guaranteed level.
 
   Maxi represents an upper limit or a maximum number of bytes that the channel can transmit. The bytes transmitted over a sample time value fi is compared with the maxi value to determine whether the flow is above the upper limit or maximum level. If a channel exceeds this number, then the transmit rate for the channel is decreased. 
   B represents a control signal for excess bandwidth. If the routing device is not seeing congestion, then value B is set to true or one. Packets are allowed to flow in and out without any packets being dropped. However, when congestion is detected, then the value B is set to zero. 
   Ci represents a constant of increase and is used when an input flow is between minimum and maximum and there is no congestion. Ci is used to allow additional packets into the system. The value E represents a history of excess bandwidth and is a value between 0 and 1. If the router is congested for only a short period of time, E will be close to one. However, if congestion has occurred over a long period of time, E will be near or at zero. 
   Di represents a discard coefficient or constant of decrease and is used to decrease the amount of packets entering the system during congestion. The value Dt is a complex number taking bytes into a percentage of bandwidth used for a given flow. The discard coefficient Di has a wide range of values containing a scaling value and an exponential value. The multiply of Di*Fi(t) requires a large number of bits and the complexity of an exponent requiring floating point and exponent capabilities. 
   The equations have two functions, the first two lines monitor the rate in which frames come in and classify a packet or flow as below minimum, above minimum or between the two. The lines act as policers. 
   In the equations, the third line “else” is related to congestion avoidance. The final else clause begins with the line “if B=1” is defined as excess bandwidth, or in other words, the router is not experiencing congestion. The transmit probability if B=1 is min(1,Ti(t)+Ci*E(t)). This means a product of the linear increase constant Ci times a value of excess bandwidth over time E(t) is added to the current transmit probability Ti(t). 
   The final line begins when B=0, the transmit rate is max(0,Ti(t)−Di*fi(t)), meaning the transmit rate is the current rate minus the constant Di*fi where fi is the amount of bytes transmitted. 
   As shown in  FIG. 3 , the prior art BAT method has the following characteristics. When congestion occurs, the flow will see a reduction in its transmit probability Ti. When congestion is over, the transmit rate rises back to 100% when no further congestion is encountered. While the BAT method provides improvements over other arrangements, the BAT method has not been used for implementing automatic congestion avoidance for differentiated service flows. 
   A need exists for an improved mechanism for implementing automatic congestion avoidance for differentiated service flows in a communications network. 
   SUMMARY OF THE INVENTION 
   A principal object of the present invention is to provide a method and apparatus for implementing automatic congestion avoidance for differentiated service flows in a communications network. Other important objects of the present invention are to provide such method and apparatus for implementing automatic congestion avoidance for differentiated service flows substantially without negative effect and that overcome many of the disadvantages of prior art arrangements. 
   In brief, a method and apparatus are provided for implementing automatic congestion avoidance for differentiated service flows in a communications network. A bandwidth control signal representing congestion information is identified. Responsive to the identified bandwidth control signal, dual transmit probabilities are calculated for a flow. The dual transmit probabilities include a first transmit probability for a first color and a first transmit probability for a second color. A marked color for a packet is identified. The marked packet color and the calculated dual transmit probabilities are selectively utilized for making a packet discard determination. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention together with the above and other objects and advantages may best be understood from the following detailed description of the preferred embodiments of the invention illustrated in the drawings, wherein: 
       FIG. 1  is a schematic and block diagram illustrating apparatus for implementing automatic congestion avoidance for differentiated service flows in accordance with the preferred embodiment; 
       FIG. 2  is a flow diagram illustrating exemplary steps performed by a probability policy maker of  FIG. 1  for implementing automatic congestion avoidance methods for differentiated service flows for bandwidth allocation technology in accordance with the preferred embodiment; 
       FIG. 3  is a chart illustrating system response for the prior art Bandwidth Allocation Technology (BAT) method for implementing congestion control; 
       FIGS. 4 ,  5  and  6  are charts illustrating system response for the apparatus of  FIG. 2  for implementing automatic congestion avoidance for differentiated service flows in accordance with the preferred embodiment; and 
       FIG. 7  is a block diagram illustrating a computer program product in accordance with the preferred embodiment. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Having reference now to the drawings, in  FIG. 1 , there is shown apparatus for implementing automatic congestion avoidance for differentiated service flows in accordance with the preferred embodiment generally designated by the reference character  100 . Automatic congestion avoidance apparatus includes a classification unit  102 , a marker  104 , a bandwidth accounting unit  106 , a discard determination unit  108 , a probability policy-maker  110 , and congestion information  112 . Each received frame or packet is classified as belonging to a stream or flow based on a subset of bits that are carried in its header by the classification mechanism  102 . The marker  104  checks the incoming packet against the history of packets on the same flow. The marker  104  colors or marks the packet according to the method described in RFC 2698/RFC 2597. If an incoming flow exceeds certain defined bounds, the packet is marked yellow. If a packet exceeds an upper limit, the packet is marked red. If no bounds are exceeded, the color is green. The marker  104  applies a packet color of green, yellow or red and the flow number to the discard determination mechanism  108 . The bandwidth accounting mechanism  106  only measures the input bandwidth for a flow color on a flow-by-flow basis and has only one value used to calculate fi for later calculations. 
   In accordance with features of the preferred embodiment, the probability policy-maker  110  applies the BAT equations twice to each flow enabling automatic congestion avoidance for differentiated service flows. BAT equations are applied once for red and then for yellow to each flow. Separate red and yellow constants, Ci and Di are set such that yellow packets decrease transmission at a rate slower than red and such that yellow packets transmission rate increases faster than red. The values of minimum and maximum are not used because in determining the color of a packet, marker  104  has determined if the packet is green, yellow or red and has marked the packet accordingly. 
   In accordance with features of the preferred embodiment, the probability policy maker  110  contains two sets of the constants: Ci and Di and calculates Ti for the two different colors, yellow and red for each flow. Congestion information  112  is represented by the value B in the BAT equations. Classification  102  determines the flow number and the previous color for discard determination. Discard determination unit  108  is enhanced in the preferred embodiment. Discard determination unit  108  receives two discard probabilities from the probability policy maker  120 , one discard probability for yellow and one discard probability for red and uses the two discard probabilities to decide if the frame or packet is sent onto the rest of the system. 
   Referring now to  FIG. 2 , there are shown exemplary steps performed by a probability policy maker  110  for implementing automatic congestion avoidance methods for differentiated service flows for bandwidth allocation technology in accordance with the preferred embodiment. The dual probability maker  110  starts with a flow Ti, for example, for i=0 to 511, applied by bandwidth accounting  106  as indicated in a block  200 . The dual probability maker  110  checks whether the control signal for excess bandwidth B applied by congestion information  112  is equal to one as indicated in a decision block  202 . If the routing apparatus  100  is not seeing congestion, then value B is set to true or one. Then as indicated in a block  204  the dual probability maker  110  calculates the transmit probabilities for red and yellow, Ti(red) and Ti(yellow) with B=1 represented by the following equations, where Ci(red) represents:
 
 Ti (red)&lt;= Ti (red)+ Ci (red)* E 
 
 Ti (yellow)&lt;= Ti (yellow)+ Ci (yellow)* E 
 
Ci(red) and Ci(yellow) respectively represent a constant of increase for red and yellow packets used when there is no congestion. Ci(red) and Ci(yellow) allows additional packets into the system. Ci(yellow) is a larger value than Ci(red) so that yellow packets return to 100% capacity faster than the red packets following a period of congestion. The value E represents a history of excess bandwidth and is a value between 0 and 1; with congestion for only a short period of time, E is close to one and when congestion has occurred over a long period of time, E is near or at zero.
 
   Otherwise, when congestion is detected the value B is zero. Then as indicated in a block  206  the dual probability maker  110  calculates the transmit probabilities for red and yellow, Ti(red) and Ti(yellow) with B=0 represented by the following equations:
 
 Ti (red)&lt;= Ti (red)− Di (red)* fi 
 
 Ti (yellow)&lt;= Ti (yellow)− Di (yellow)* fi 
 
Di(red) and Di(yellow) respectively represent a discard coefficient or constant of decrease for red and yellow packets used to decrease the amount of packets entering the system during congestion. Di(red) is a larger value than Di(yellow) so that red packets are discarded faster than the yellow packets during a period of congestion. The value fi represents the amount of bytes transmitted for the flow. This completes the dual transmit probabilities calculations for the flow Ti of the probability policy-maker  110  as indicated in a block  208 .
 
   The Discard Determination unit  108  takes the flow number and the packet color after it has been marked by marker  104 , and uses the probabilities Ti(red) or Ti(yellow) supplied by probability policy-maker  110  and decides if the packet should be discarded or not. 
   Referring to  FIGS. 4 ,  5 , and  6 , there are shown exemplary system responses for the apparatus  100  for implementing automatic congestion avoidance for differentiated service flows in accordance with the preferred embodiment. As shown in  FIGS. 4 and 5 , packets are discarded at different rates depending on the color of the packet. This results from using the dual drop probability as described. 
     FIG. 4  shows the system response provided by apparatus  100  if congestion is encountered once over the period of time. When congestion is encountered, the red packets are discarded faster than the yellow packets. When the period of congestion is over, the yellow packets will return to 100% faster than the red. 
     FIG. 5  shows the system response provided by apparatus  100  if there is sustained congestion. As shown with the sustained congestion what will happen is the discards will occur followed by a small period of time when frames allowed to enter the system rise slightly until the congestion reoccurs. After a point the yellow frames reach an equilibrium as shown in  FIG. 5 . If the system is over subscribed, red frames will be dropped until the yellow frames reach equilibrium. If the system is significantly over subscribed, the yellow frames eventually reach 0% too. 
     FIG. 6  illustrates that packets are discarded at different rates depending on the priority of the flow. Each flow can have different probabilities. As shown, a high priority flow has a slower decrease than a low priority flow during a region of congestion. Then the high priority flow has a faster increase than a low priority flow following the region of congestion. 
   Referring now to  FIG. 7 , an article of manufacture or a computer program product  700  of the invention is illustrated. The computer program product  700  includes a recording medium  702 , such as, a floppy disk, a high capacity read only memory in the form of an optically read compact disk or CD-ROM, a tape, a transmission type media such as a digital or analog communications link, or a similar computer program product. Recording medium  702  stores program means  704 ,  706 ,  708 ,  710  on the medium  702  for carrying out the methods for implementing automatic congestion avoidance for differentiated service flows of the preferred embodiment with apparatus  100  of  FIG. 1 . 
   A sequence of program instructions or a logical assembly of one or more interrelated modules defined by the recorded program means  704 ,  706 ,  708 ,  710 , direct the apparatus  100  for implementing automatic congestion avoidance for differentiated service flows of the preferred embodiment. 
   While the present invention has been described with reference to the details of the embodiments of the invention shown in the drawing, these details are not intended to limit the scope of the invention as claimed in the appended claims.