Abstract:
A virtual link bandwidth control scheme is described that is a software-based mechanism for achieving distributed and coordinated bandwidth control. The scheme can be seamlessly integrated with the data transfer protocols utilizing queuing delay as part of rate control mechanism for data transfer and allows multiple processes to self-limit their transfer speeds, so that the aggregate rate stabilizes at a pre-configured level below the physical bottleneck capacity.

Description:
BACKGROUND 
       [0001]    There are many situations in which it may be desired to limit the amount of bandwidth utilized by a specific group of processes transferring data over a network. For example, it is often of interest to limit the bandwidth usage of certain user processes traversing one or more segments inside a network in order to reserve network resources for other user processes sharing the same set of network segments. It may also be desired to bound the aggregate bandwidth of a group of processes between a specific set of servers and clients, so that the residue bandwidth resources in front of the servers may be used by other user processes connected with the same set of servers. In what follows, a virtual link control system is presented to realize the functionality of the aforementioned aggregate bandwidth control. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0002]      FIG. 1  is a diagram showing multiple processes utilizing a common network pathway in which a virtual link has been inserted. 
           [0003]      FIG. 2  illustrates an exemplary implementation of a virtual link controller and rate control mechanism. 
           [0004]      FIG. 3  illustrates an exemplary server configuration for utilizing the virtual bandwidth control scheme. 
       
    
    
     DETAILED DESCRIPTION 
       [0005]    Described herein is a method and system by which a group of independently executing processes residing in the same or different computers may limit the aggregate amount of bandwidth that they use in transferring data over a network, wherein these processes may consist of a set of data transfers traversing either one or more common segments inside the network, or all segments along a particular network pathway. Such bandwidth limiting may be used, for example, to ensure some residual capacity along the shared network segments for use by other processes that also traverse through those segments. In order to provide aggregate bandwidth control, a virtual link is implemented by a virtual link controller integrated into each process. The virtual link controllers communicate with one another either over a network or within a computer. The virtual link is a software construct that emulates a physical link with some throughput capacity. The throughput capacity may thus be specified as some value c l  representing the desired maximum bandwidth to be utilized by the group of processes.  FIG. 1  illustrates an exemplary configuration of N processes P 1  through P N  that transfer data over a network  100  (the dashed line box), and traverse common network segments  100 - 1 ,  100 - 2  and  100 - 3 . A virtual link  101  is shown as being interposed within the network  100  that emulates a physical link with limited capacity. The shared segments  100 - 1 ,  100 - 2  and  100 - 3  may be, for example, shared by the processes P 1  through P N  as well as other processes, and it may be desired to reserve some residual bandwidth along these segments for use by those other processes. 
         [0006]    In order to implement the virtual link, the virtual link controller of each process communicates with each other to determine the total amount of data traffic flowing to or from the group of processes over the virtual link. The virtual link controller may determine the traffic passing through the virtual link from intermittent broadcasts by each process that signal to each other process the amount of data transferred by it. Such broadcasts may, for example, contain the amount of data transferred over some period of time or may signify that some specified amount of data has been transferred. In order to determine the amount of traffic passing by quickly and accurately, virtual link controllers may be deployed in locations close to each other, so that a broadcast message sent by a particular virtual link controller may be heard by all controllers soon after its disposal. In the simplistic case, for example, virtual link controllers, as well as their hosting processes, may be deployed on a single computer and jointly realize a virtual link. In another configuration, these processes may be deployed on distinct nodes across a connected network as, for example, a group of computer nodes on a common local area network, or a set of server nodes integrated together through a common network backplane. 
         [0007]    The virtual link controller uses the collected traffic information to update a virtual queue that simulates the queueing maintained by the buffer in a physical link with limited bandwidth. No data is actually queued, however, as the virtual queue is merely a number representing the amount of data that would be queued in a physical link. Whenever the traffic over the virtual link exceeds the link capacity c l , the virtual queue is filled with data at a rate equal to the difference, similar to the way an actual queue in the buffer of a physical link would be filled. Conversely, when the traffic is below the link capacity, the virtual queue is emptied at a rate equal to the difference between the link capacity and the overall traffic flow rate until no more data is left in the virtual queue. When the system stabilizes, the amount of data inside the virtual queue shall stay at some fixed level below the maximal virtual queue size Q max , indicating that the overall data traffic rate equals the virtual link capacity c l . The amount of data in the virtual queue is thus an indication of the extent to which the traffic flow over the virtual link over-utilizes or under-utilizes the desired bandwidth limit c l  and may be used as a feedback signal by a rate control mechanism incorporated into each process to regulate its data transfer rate. The rate control mechanism may control the rate at which the process transfers data based upon one or more virtual queue parameters, such as the absolute amount of data in the virtual queue Q(t), the corresponding virtual queuing delay q(t) (where (q(t)=Q(t)/c l ), and/or the relative occupancy of the virtual queue Q(t)/Q max , which indicates how close the amount of virtual queueing is to the maximal virtual queue size Q max . 
         [0008]    An example of a virtual link controller and rate control mechanism as implemented by the execution of sequential steps within a process is illustrated in  FIG. 2 . At step S 1 , the process transfers data at a prescribed rate x(t). At step S 2 , the process broadcasts to other processes of the group an indication of the amount of data that has been transferred by it. At step S 3 , similar broadcasts from other processes are received and tabulated so that the overall traffic rate y(t) over the virtual link can be calculated at step S 4 . Based upon the overall traffic rate, the amount of data in the virtual queue is updated at step S 5 . At step S 6 , the prescribed data transfer rate x(t) is adjusted in accordance with a parameter reflective of the amount of virtual queuing, such as the virtual queuing delay itself, the amount of data in the virtual queue, and/or a parameter reflective of the relative occupancy of the virtual queue, such as the ratio of the amount of data in the virtual queue to the maximal virtual queue size. The process then returns to step S 1  to continue transferring data at the rate x(t). 
         [0009]    The virtual link bandwidth control scheme as described above may be implemented in processes utilizing any data transfer protocol that permits adjustment of its overall data transfer rate based on some form of queueing delay information. An example of a protocol that is particularly suitable for implementation of the virtual link control scheme is the FASP protocol as described in U.S. patent application Ser. No. 11/317,663, filed on Dec. 23, 2005 and hereby incorporated by reference. The FASP protocol derives an actual queuing delay from measured round trip times for data packets. This derived delay represents the total amount of queueing delay generated at all physical links along the network path through which FASP sends data packets, and is used as an input parameter by the rate control mechanism in FASP for adjusting the data transfer rate. The protocol increases the data transfer rate with decreasing queuing delay and vice-versa. By basing adjustment of the data transfer rate on queuing delay rather than packet timeouts, the FASP protocol is, for example, able to more efficiently utilize networks where loss of data may frequently occur. In order to implement the virtual link bandwidth control scheme, the rate control mechanism of the FASP protocol may be modified to use the virtual queuing delay provided by the virtual link controller or a combination of an actual measured queuing delay and the virtual queuing delay. 
         [0010]    Virtual link control and FASP may jointly form an end-to-end, application-layer bandwidth control solution with superior configurability. The bandwidth caps enforced by virtual link control are bi-directional, and can be adjusted on-the-fly, either through configuration files or through commands from other software modules. Each FASP session maintains its own virtual link controller and computes its rate independently. By eliminating centralized point of control, the virtual link design avoids single point of failure, and embeds distributed control into application protocols. This distinguishes the virtual link bandwidth control scheme from other forms of active queue management (AQM) schemes, which are often implemented at centralized strategic points, e.g., inside network routers.  FIG. 3  shows an example of a group of servers  301  executing processes that communicate with a group of clients  303  over a common network pathway  302 . Each server process maintains its own FASP session as well as virtual link controller. Each virtual link controller constantly manages its internal virtual queue, which provides necessary delay information for the corresponding FASP session to perform dynamic rate updates. In spite of independent rate update activities, FASP sessions sharing a common virtual link are able to jointly enforce a pre-defined bandwidth cap via exchanging incoming/outgoing traffic information. 
         [0011]    In addition to FASP, any rate control mechanism that adjusts the rate of its process based on some form of queueing delay information may be combined with the virtual link control scheme to realize aggregate bandwidth capping. Instead of solely relying on the queueing delay information that it is already aware of, a rate control mechanism may also use a combination of that queueing delay and the virtual queueing delay to adjust the transfer rate of its hosting process. In one embodiment, a rate control mechanism of an individual process adjusts its data transfer rate based on a combination of the virtual queueing delay and the queueing delay derived from the round trip delay measurements collected by its process. For instance, the data transfer rate x(t) may be updated at the end of a period T as: 
         [0000]        x ( t+T )= x ( t )+ K[α−x ( t )( q ( t )+ q   f ( t ))] 
         [0000]    where q(t) is the amount of virtual queueing delay informed by the virtual link controller, q f (t) is the amount of queueing delay measured by the rate control mechanism, K is some constant, and α is the setpoint for x(t)(q(t)+q f (t)). In this case, x(t)q(t) captures the amount of queueing present in the virtual queue, and x(t)q f (t) represents the amount of queueing in the queues maintained by physical links along the network path the process travels through. Their sum x(t)(q(t)+q f (t)), is the total amount of data accumulated at all the queues (both physical and virtual) that are accessed by the process. By comparing this amount of data with the setpoint α, the rate control mechanism adjusts the transfer rate in the next period x(t+T) up and down. In particular, if x(t) exceeds the capacity of a particular link, be it the virtual link or one of the physical links, the queueing delay at that link will grow and drive up the total amount of queueing delay (q(t)+q f (t)). Once (q(t)+q f (t)) becomes so large that its product with x(t) surpasses the setpoint α, the rate control mechanism will force the process to reduce its transfer rate as a measure to avoid further congesting the link. 
         [0012]    The capacity of the virtual link c l  may be chosen to be a value that is smaller than the link capacities of the shared network segments, in order to reserve bandwidth on those segments for others users. In this scenario, the aggregate transfer rate will be bounded by the virtual link capacity, and therefore no queueing will accumulate at the physical links within those shared network segments. A rate control mechanism may thus only respond to the virtual queueing delay in this context. In one embodiment, a rate control mechanism of an individual process adjusts its data transfer rate in a manner that attempts to maintain the amount of data injected into the virtual queue at a constant level. For example, the data transfer rate x(t) may be updated at a period T as: 
         [0000]        x ( t+T ) =x ( t )+ K[α−x ( t ) q ( t )] 
         [0000]    where q(t) is the virtual queuing delay so that x(t)q(t) is the amount of data injected into the virtual queue by the process, K is some constant, and α is the setpoint for x(t)q(t). In this embodiment, the rate at which each process transfers data increases or decreases as the virtual queuing delay decreases or increases, respectively. The setpoint α and maximum size Q max  of the virtual queue are selected such that, when all of the processes in the group are transferring data at their maximal rates, the virtual queue is occupied between 0 and Q max , with each process injecting an equal amount of data into it. As the data transfer rate of certain processes decreases, the virtual queuing delay decreases to allow other processes to increase their data transfer rates in order to maintain their individual queue injection amounts constant. 
         [0013]    In another embodiment, the queue injection setpoint α is dynamically adjusted in accordance with the amount of data present in the virtual queue so that the setpoint is increased as the virtual queue is emptied and vice-versa. As explained below in greater detail, such dynamic tuning of the queue injection setpoint may also result in improved equilibrium and stability properties for the virtual link. The queue injection setpoint α may be adjusted on the basis of the current amount of virtual queueing Q(t) and the maximal virtual queue size Q max , via a setpoint adjustment function ƒ(Q(t), Q max ). In one embodiment, the setpoint adjustment function is simply a function of the relative occupancy Q(t)/Q max  itself. In particular, it may be defined as a non-increasing function of Q(t)/Q max , ranging from 1 to 0 as Q(t)/Q max  increases from 0 till it passing over 1. An example of such a function is ƒ(Q(t), Q max =[(1−Q(t)/Q max ) + ] k , where k is some positive number and “ + ” maps any negative number to zero. The rate control mechanism then adjusts the data transfer rate x(t) of each process at a period T as: 
         [0000]        x ( t+T ) =x ( t )+ K ( f ( Q ( t ),  Q   max   α−x ( t ) q ( t )) 
         [0014]    By appropriate specification of the queue injection setpoint α and the setpoint adjustment function ƒ(Q(t), Q max ), a single process will utilize nearly all of the bandwidth of the virtual link if no other processes are transferring data and will aggressively reduce its bandwidth utilization as the number of other processes transferring data increases. Particularly, at the equilibrium moment when the system stabilizes, the amount of data an individual process injects into the queue x(t)q(t) exactly matches the target queue set point f(Q(t), Q max )α, so that the rate control mechanism always computes a new rate x(t+T) that is equal to the old rate x(t). This can be reflected by the following equation which governs the relationship between the transfer rate of a process x*, the virtual queueing delay it experiences q*, and the queue target set point f(Q*, Q max )α, in an equilibrium state: 
         [0000]        x*q*=f ( Q*, Q   max )α 
         [0000]    Since each process in the group will experience the same amount of virtual queueing delay q* at equilibrium, by assigning the same a to all processes in the group, each individual process will have the same target queue set point and consequently attain the same transfer rate in a stabilized system. As the number of processes in the group N increases, each process will acquire an equal (or fair) share of the total virtual link capacity c l , which is c l /N. 
         [0015]    From another perspective, by properly selecting the setpoint adjustment function ƒ(Q(t), Q max ) as a non-increasing function of Q(t), the amount of virtual queueing Q* in a stabilized virtual queue increases as the total number of processes increases, so that a decreasing target queue set point f(Q*, Q max )α is provided to each process in a way that matches the decreasing pace of its transfer rate x*. Despite its increasing trend, the amount of total virtual queueing (delay) may still be bounded below a finite value related to the maximal virtual queue size Q max , no matter how many processes are present in the group. For example, when f(Q(t), Q max =[(1−Q(t)/Q max ) + ] k , the amount of data in the virtual queue in equilibrium Q* increases with N towards Q max , in order to match the decreasing transfer rate of an individual process, that is, c l /N. However, Q* will only gradually approach to Q max  but will not pass Q max  since the transfer rate is always greater than zero, no matter how large N may become. Bounding the total amount of virtual queueing below Q max  reduces the likely oscillation range of the virtual queueing delay in a system where the number of processes N varies over time. As mentioned before, the transfer rate of a process changes up and down according to the amount of virtual queueing delay, by virtue of the rate control mechanism. Hence, reducing the oscillation range of the virtual queueing delay also helps to smooth out the transfer rates of individual processes in dynamic settings. 
         [0016]    The invention has been described in conjunction with the foregoing specific embodiments. It should be appreciated that those embodiments may also be combined in any manner considered to be advantageous. Also, many alternatives, variations, and modifications will be apparent to those of ordinary skill in the art. Other such alternatives, variations, and modifications are intended to fall within the scope of the following appended claims.