Patent Application
20050135248
The present invention relates to network communication and, more specifically, to methods and applications for avoiding slow-start restart in Transmission Control Protocol (TCP) network communication.
In mobile or wireless networks, especially those that implement interactive services, such as web services, web browsing and the like, it is important to maintain low user-perceived latency (i.e., delay). Latency in the mobile network environment will typically negatively impact application performance and overall user satisfaction.
The mobile network is more prone to latency problems than a standard wireline network because wireless networks rely on a radio link that is characterized by low radio bandwidth (e.g., around 30 Kilobits per second (Kbps) usable bandwidth in a General Packet Radio Service (GPRS) connection). In addition, a significant latency is also introduced by the radio link layer that combats high bit error rates. The algorithms and implementation of underlying transport protocols further intensify the latency problem in mobile networks.
Mobile Web Services (MWS) use HTTP/TCP/IP (HyperText Transfer Protocol/Transmission Control Protocol/Internet Protocol) for communication between the web service client and the service provider, which includes the radio link. In an attempt to control network congestion, TCP implements congestion avoidance and control algorithms. While these congestion avoidance algorithms are advantageous in the wireline communication network, studies have shown that these algorithms can be overly conservative, resulting in performance degradation and adding unwarranted latency into the data transmission process.
At the onset of communication between the network proxy and the client, TCP implements a congestion avoidance algorithm as a means of trying to eliminate or lessen congestion in the network. This algorithm is referred to as the “slow-start” algorithm and it attempts to efficiently allocate network resources by identifying underutilized network connections. The “slow-start” algorithm functions in the following manner. Upon receipt of the first communication from a terminal, the network proxy sets a congestion window. The congestion window indicates the maximum amount of data (i.e., the number of bytes) that can be sent out on a connection without being acknowledged (i.e, without an acknowledgement message being sent from the client). TCP detects congestion when it fails to receive an acknowledgement for a packet within the estimated retransmit timeout (RTO) period. Thus, when the network proxy fails to receive an acknowledgement from the client within the estimated timeout, the network proxy assumes that the data packet that it sent out was lost (i.e., dropped) and that the network is, therefore, congested. In this instance, the network proxy decreases the congestion window to one maximum segment size (MSS) and retransmits the data packet. Under other conditions, in which the network proxy receives an acknowledgement, it increases the congestion window by one MSS.
At stage 210, the network proxy responds to the SYN request and, at stage 220, the client sends an acknowledgement to the network proxy. Since the network proxy received the acknowledgement within the estimated timeout period (referred to as the retransmit timeout period), the network proxy increases the congestion window by one MSS. In this example, the congestion window expands to two segments or 1912 bytes. Once the congestion window is expanded, absent receipt of an acknowledgement from the client, the network proxy is now allowed to send the client up to 1912 bytes (i.e., two segments).
At stage 230, the client sends the network proxy an HTTP request, which requests access to a specified web page and, at stage 240, the network proxy sends an acknowledgment to the client that the request has been received. In turn the network proxy, contacts the origin server to retrieve the web page for the client.
Once the network proxy server communicates with the origin server, at stage 250, the network proxy responds to the client with the requested web page or some other response from the origin server. It is noted that at stage 250, what is referred to in the art as the “slow-start restart” has occurred. The “slow-start restart” entails contraction of the congestion window if the TCP connection between the client and the network proxy remains idle for a period of time in excess of a timeout period (referred to herein as the second timeout period). In the example of
The “slow-start restart” concept was introduced in TCP communication to avoid sudden bursts of data into a network whose conditions are unknown because the connection has been idle for ‘too long’. However, the “slow-start restart” can unnecessarily throttle the performance of an already slow OTA (over-the-air) connection, even when there is no congestion on the network. Additionally, it is noted that in typical Mobile Web Services (MWS) communication the transmissions are typically short (only a few packets) and, as such, the TCP “slow-start restart” can severely limit the MWS performance.
There are numerous factors that can cause the TCP connection to remain idle for a long time. In one example, it may take the server a long time to reply to the client's request due to various reasons. One reason could be that the task the server needs to perform takes a lot of time (for example, it may need to request information from other servers and assemble the information to form the requisite reply). In another example, typically a network proxy first performs a Domain Name Server (DNS) query to map the domain name to servers on the Internet before contacting the origin server. Once the DNS query has been performed, the network proxy establishes a new TCP connection with the origin server, forwards the client's HTTP request and waits for the response before it can start sending any data to the client.
Various different solutions have been proposed to overcome the latency problems related to the “slow-start restart” prevalent in TCP communications. While some of these solutions have been successful in addressing a portion of the latency problems attributable to “slow-start restart”, none of the known solutions address the problem related to idle time at the TCP sender. The following provides examples of some of the known solutions for overcoming latency problems related to “slow-start restart”.
Recently, it is becoming more common to implement a Performance Enhancing Proxy (PEP) within the network architecture to improve the performance of Internet protocols on networks where native performance is poor due to link characteristics. These PEPs can provide performance enhancements at various protocol layers. A transport-layer PEP has been suggested for mobile networks, which addresses the “slow-start restart” problem in situations where the mobile terminal experiences temporary disconnections (such as during handoffs). Such a transport-layer PEP is proposed in the article entitled “M-TCP: TCP for Mobile Cellular Networks”, K. Brown et al., ACM Computer Communications Review, Volume 27(5), October 1997. Although, the transport-layer PEP described in the article focuses on avoiding the “slow-start restart”, it does so in the context of data being available for delivery while the receiving device is not ready to receive it. As such, the transport-layer PEP does not address the problem of avoiding the “slow-start restart” when the receiving device is awaiting delivery of information but no data is available for transmission causing the idle time to exceed the timeout period.
Additionally, various methods have been implemented to that use probes to detect whether a connection is alive. For example, the TCP Keepalive option provides for probes to be sent to a client if the connection has been idle for a sustained period of two hours (this interval value is system-wide and any changes to this interval will affect all applications using TCP). The probe is usually an empty data packet (or a 1-byte packet) with a sequence number that is smaller than the receiver's expected sequence number. This forces the client to respond with an ACK, thus indicating that the connection is still alive. However, the TCP Keepalive option is used only to detect whether the other endpoint is still active and it performs such at a prolonged interval of two hours. In addition, the TCP Keepalive option does not detect network conditions or otherwise improve TCP performance.
A TCP Fast Start algorithm has been suggested, which provides a method for resuming data communication after long periods of idleness in the TCP connection. See “Addressing the Challenges of Web Data Transport”, V. N. Padmanabhan, PhD Thesis, University of California at Berkeley, September 1988. The proposed algorithm works in the following manner. After a long idle period, instead of performing ‘slow-start restart’, the sender uses the same value of congestion window as before the idle period and initiates the ‘fast start’ phase by sending out a burst of packets. Packets sent during the fast start phase are marked with a low priority so that if the network had become congested while the connection remained idle, the intermediate routers can simply drop the fast start packets. When a congested condition is detected, the algorithm falls back to the standard slow-start. Although this algorithm is capable of avoiding unnecessary “slow-start restarts”, it violates the congestion avoidance principles of TCP by sending a burst of packets into a network whose conditions are unknown.
Additionally, several solutions have focused on a proposal to have a ‘faster’ slow-start. An example is increasing the initial congestion window to two or four times the MSS. However, as it should be evident, these solutions are mere stop gate solutions, in that, the “slow-start restart” is still implemented in case the connection idle time exceeds the second timeout period.
The need exists to develop a method and application for avoiding the “slow-start restart” in TCP communications when network conditions dictate such. The methods and applications will characteristically eliminate latency in the network and, especially in a mobile or wireless network, improve web services performance. The desired solution shall address the problem of avoiding the “slow-start restart” when the receiving device is awaiting delivery of information but no data is available for transmission, resulting in the connection remaining idle for a period longer than the timeout. In addition, the desired solution shall be implemented while cognizant of current network capacity conditions. In this regard, the desired solution shall avoid the “slow-start restart” only if there is no current congestion within the network.
The present invention provides for methods, applications and devices for avoiding the “slow-start restart” in TCP communications. As previously discussed, the TCP “slow-start restart”, which was employed to combat network congestion, has a general tendency to add latency to communications in a mobile or wireless network environment. This invention serves to overcome the latency related to the “slow-start restart” by assessing network conditions and avoiding the “slow-start restart” when network conditions justify the avoidance.
The methods, applications and devices of the present invention implement communication of a probe packet that is generated and transmitted from a TCP sender to a TCP receiver during a TCP network connection. Receipt of the probe packet by the TCP receiver will trigger the transmission of an acknowledgment message by the TCP receiver. Upon receipt of the acknowledgement message the TCP sender will restart measurement of the idleness period and, as such the “slow-start restart” is avoided. The communication of the probe packets, referred to as a probing process, and the receipt of the probe packet acknowledgement allows for the TCP sender to be aware of current network congestion conditions so that the TCP sender can avoid the “slow-start restart” when network conditions dictate such. Therefore, the present invention reduces the total data transfer time over an air interface by avoiding unnecessary “slow-start restarts” under non-congested network condition.
One embodiment of the invention is defined by a method for avoiding “slow-start restart” in Transmission Control Protocol (TCP) network communications. The method includes the step of establishing a TCP connection between a TCP sender and a TCP receiver and setting an idle counter at the TCP sender that measures the time interval between the current time and the last communication with the client, which could be the receipt of an acknowledgement from the client or the transmission of a packet to the client. The TCP “slow-start restart” is triggered when the TCP sender resumes sending data to the receiver after the connection has remained idle for longer than the maximum allowed period of idleness. The method further includes the steps of transmitting, from the TCP sender to the TCP receiver, a probe packet and the corresponding transmission of an acknowledgement by the TCP receiver, upon receipt of the probe packet. The probe packet will typically be a data packet having an empty payload and indicating a sequence number that is one less than the sequence number anticipated by the TCP receiver. The method concludes by resetting the idle counter upon receipt of the probe packet's acknowledgement to avoid occurrence of the slow-start restart in the TCP connection.
The period for transmission of the probe packets may be the median value of the round-trip time (RTT) and the transmit timeout (RTO); one-half the retransmit timeout (RTO) or any other designated period may be defined.
The probe packet will typically be transmitted from the TCP sender periodically to estimate the congestion of the network and insure that the idleness time does not exceed the allowable limit for triggering the “slow-start restart” process. The period for transmission of the probe packets may be derived from the values of the round-trip time (RTT) or the retransmit timeout (RTO). For example, it may be the median value of the round-trip time (RTT) and the retransmit timeout (RTO); one-half the retransmit timeout (RTO/2) or any other designated period may be defined.
The method may further include the step of expanding the congestion window upon receipt of the acknowledgement for the probe packet in order to allow for a greater volume of data to be transmitted when the TCP sender becomes ready to send data to the TCP receiver. This provides additional performance enhancement by not only avoiding “slow-start restart”, but also resuming communication at a higher flow rate.
The invention is further embodied in an application for avoiding Transmission Control Protocol (TCP) “slow-start restart” at a TCP sender. The application includes a computer readable storage medium having computer-readable program instructions embodied in the medium. The computer-readable program instructions including first instructions for establishing a TCP connection between the TCP sender and a networked TCP receiver and second instructions for generating a probe packet at a TCP sender that is repeatedly transmitted to a TCP receiver during the TCP connection to avoid occurrence of the “slow-start restart” in the TCP connection upon receipt, by the TCP sender, of a probe packet acknowledgement. The probe packet will typically include an empty payload and a sequence number that is one less than a TCP receiver anticipated sequence number. Receipt of the acknowledgement by the TCP sender will reset the idleness counter and avoid the occurrence of the “slow-start restart” occurring at the TCP sender.
Additionally, the invention is defined by a network node device that includes a controller that establishes a TCP connection with a client, generates a probe packet and transmits the probe packet. The network node also includes a counter that counts an idle time that corresponds to an interval between the current time and the last packet sent or received, which could be the receipt of an acknowledgement from the client or the transmission of a packet to the client. The controller resets the idle counter, upon receipt of a probe packet acknowledgement from the client, to avoid occurrence of the “slow-start restart” in the TCP connection. The controller of the network node additionally provides for establishing a congestion window at the node upon establishment of the connection. The congestion window defines the maximum amount of data that can be transmitted by the node. The controller may additionally provide for expanding the congestion window upon receipt of the probe packet acknowledgement. Expanding the congestion window provides for a greater amount of data to be transmitted from the node to the client.
Thus, the present invention serves to overcome the latency related to the “slow-start restart” by assessing network conditions and avoiding the “slow-start restart” when network conditions justify the avoidance. The implementation of the invention is especially relevant in wireless or mobile networks, in which latency adversely affects the performance of web services and diminishes user satisfaction.
Having thus described the invention in general terms, reference will now be made to the accompanying drawings, which are not necessarily drawn to scale, and wherein:
The present inventions now will be described more fully hereinafter with reference to the accompanying drawings, in which some, but not all embodiments of the invention are shown. Indeed, these inventions may be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will satisfy applicable legal requirements. Like numbers refer to like elements throughout.
The present invention provides for methods and applications for avoiding the “slow-start restart” in TCP communications. The methods and applications implement the communication of a probe packet transmitted from a TCP sender, such as a network node, to a TCP receiver, such as a client, during a network connection. Receipt of the probe packet by the TCP receiver will trigger the transmission of an acknowledgment message by the TCP receiver. Upon receipt of the acknowledgement message the TCP sender will restart measurement of the idleness period and, as such the “slow-start restart” is avoided. The communication of the probe packets, referred to as a probing process, and the receipt of an acknowledgement by the TCP sender provides the TCP sender with awareness that current network conditions do not require the occurrence of the “slow-start restart”. In other words, if the TCP sender sends the probe packet and does not receive an acknowledgement then the TCP sender assumes that the packet was lost or dropped due to network congestion, in which case, the “slow-start restart” will occur. As such, the present invention reduces the total data transfer time over an air interface by avoiding unnecessary “slow-start restarts” under non-congested network condition.
The provision that provides for probe packets to be communicated from a network TCP sender can be implemented at the Operating System (OS) kernel level. The exact implementation will vary depending on the type and function of the TCP sender that generates and communicates the probe packets.
Probe packets can be sent repeatedly in various fashions, but in one embodiment, probe packets are sent periodically. In this embodiment, the periodic time interval to trigger the sending of probe packets may be derived from the current values of RTT and RTO. In one embodiment of the invention the periodic time interval is defined as the median value of RTT and RTO. In an alternate embodiment the periodic time interval is defined as RTO/2. Other measures of the period may also be employed.
At stage 310, the TCP sender responds to the SYN request and, at stage 320, the TCP receiver sends an acknowledgement to the TCP sender. Upon receipt of the acknowledgement, the TCP sender increases the congestion window by one MSS (Maximum Segment Size). The congestion window is expanded because the TCP receiver has been responsive to the TCP sender's response (i.e., an acknowledgement has been sent), which indicates that the network is not currently experiencing congestion and, as such, the TCP sender can send more information. In this example, the congestion window expands to two segments or 1912 bytes. Once the congestion window is expanded, the TCP sender is now allowed to send the TCP receiver up to 1912 bytes (i.e., two segments) prior to receiving an acknowledgement from the TCP receiver.
At stage 330, the TCP receiver sends the TCP sender an HTTP request, which requests access to a specified web page and, at stage 340, the TCP sender sends an acknowledgment to the TCP receiver that the request has been received. In turn the TCP sender, contacts the origin server to retrieve the web page for the TCP receiver.
While the TCP sender is communicating with the origin server or conducting other related network communications, such as a DNS query or the like, idle time in the communication between the TCP sender and the TCP receiver ensues. In order for the idle time to not accrue to a level that would invoke the “slow-start restart”, at stage 350, the TCP sender transmits a probe packet to the TCP receiver. The probe packet will be sent after a certain predefined period of idleness and if there is no other communication expected from the TCP receiver, e.g. the TCP sender has not sent the TCP receiver any recent communications for which the TCP receiver is expected to acknowledge. The probe packet will typically be an empty data packet with a sequence number that is one less than the TCP receiver's expected sequence number. At stage 360, the TCP receiver responds to the probe packet with an acknowledgement. Receipt of the acknowledgement by the TCP sender will trigger the restart of the idleness counter, i.e., the measurement of the period of time for which the connection has been idle. Additionally, upon receipt of the acknowledgement by the TCP sender, the congestion window may be expanded by the TCP sender. In this example, the congestion window is expanded to three segments or 2868 bytes. Once the congestion window is expanded the TCP sender is now allowed to send the TCP receiver up to 2868 bytes of data (i.e., three segments) prior to receiving an acknowledgement from the TCP receiver.
If at this stage, the TCP sender still does not have data available to be sent to the TCP receiver, then the additional communication idle time between the TCP sender and the TCP receiver prompts, at stage 370, the TCP sender to transmit an additional probe packet to the TCP receiver once the idleness counter maintained by the network server reaches the predefined period of idleness. This repeated probing of the TCP receiver provides the TCP sender awareness as to current network congestion conditions and obviates the need to perform a “slow-start restart”. At stage 380, the TCP receiver responds to the probe packet with an acknowledgement. Receipt of the acknowledgement by the TCP sender will, once again, trigger the restart of the idleness counter. Upon receipt of the acknowledgement by the TCP sender, the congestion window may once again be expanded. In this example, the congestion window is expanded to four segments or 3824 bytes. Once the congestion window is expanded the TCP sender is now allowed to send the TCP receiver up to 3824 bytes of data (i.e., four segments) prior to receiving an acknowledgement from the TCP receiver.
Once the TCP sender communicates with the origin server, at stage 390, the TCP sender responds to the TCP receiver with the requested web page or some other response from the origin server. It is noted that a “slow-start restart” has not occurred (i.e., the congestion window has not been contracted) because the TCP sender has received acknowledgements from the TCP receiver while the TCP sender was communicating with the origin server. Additionally, the congestion window has opened such that TCP sender can communicate data to the TCP receiver in fewer round trips than would be required if the congestion window contracted or remained constant.
In the embodiment of
At optional step 520, a congestion window is established at the onset of the establishment of the connection. The congestion window will define the maximum allowable volume of data that can be transmitted from the TCP sender to the TCP receiver at any time. In a typical “slow-start restart” decreasing the congestion window will aid in providing less congestion to the overall network environment.
At step 530, at a predetermined time, the TCP sender will transmit a probe packet to the TCP receiver. The probe packet will typically include an empty payload and a sequence number that is one less than the sequence number anticipated by the TCP sender. The predetermined time will be less than the maximum allowable idle time, typically the median of the RTT and RTO, one-half the RTO or any other suitable interval. In most embodiments, the TCP sender will transmit the probe packets at the predetermined time interval, if the connection is in an idle state. Periodic transmission of the probe packets allows for the TCP sender to assess current network congestion levels and to avoid a “slow-start restart” if network conditions dictate such. At step 540, the TCP receiver will respond to the receipt of the probe packet by transmitting, to the TCP sender, an acknowledgement.
Upon receipt by the TCP sender of the acknowledgement, at step 550, the TCP sender will reset the idle counter to avoid occurrence of the slow-start restart in the TCP connection upon receipt of the acknowledgement. Receipt of the acknowledgement by TCP sender results in the TCP sender assuming that the network is not congested because the probe packet reached its specified destination without being lost or dropped. In this instance, a “slow-start restart” can be avoided.
Additionally, at optional step 560, upon receipt of the acknowledgement by the TCP sender, the congestion window may be expanded to allow for a greater volume of data to be transmitted by the TCP sender to the TCP receiver during the predefined interval.
The invention is also defined by a network node, such as the network proxy, that includes a controller, such as a processor or other computing device that operates under control of an application comprising a computing readable medium, such as memory, for storing computer readable instructions for performing the various functions of the network node including those in
In this regard,
Accordingly, blocks or steps of the flowcharts support combinations of means for performing the specified functions, combinations of steps for performing the specified functions and program instruction means for performing the specified functions. It will also be understood that each block or step of the flowchart, and combinations of blocks or steps in the flowchart, can be implemented by special purpose hardware-based computer systems that perform the specified functions or steps, or combinations of special purpose hardware and computer instructions.
Thus, the present invention serves to overcome the latency related to the “slow-start restart” by assessing network conditions and avoiding the “slow-start restart” when network conditions justify the avoidance. The implementation of the invention is especially relevant in wireless or mobile networks, in which latency adversely affects the performance of web services and diminishes user satisfaction.
Many modifications and other embodiments of the inventions set forth herein will come to mind to one skilled in the art to which these inventions pertain having the benefit of the teachings presented in the foregoing descriptions and the associated drawings. Therefore, it is to be understood that the inventions are not to be limited to the specific embodiments disclosed and that modifications and other embodiments are intended to be included within the scope of the appended claims. Although specific terms are employed herein, they are used in a generic and descriptive sense only and not for purposes of limitation.
