AU8443498A - Flow control in a telecommunications network - Google Patents

Description

WO 99/04536 PCT/F198/00591 1 Flow control in a telecommunications network Field of the invention This invention relates generally to flow control in a telecommunica 5 tions network. More particularly, the invention relates to congestion control in a packet switched telecommunications network, especially in a network where Transmission Control Protocol (TCP) is used as a transport layer protocol. Background of the invention 10 As commonly known, TCP is the most popular transport layer pro tocol for data transfer. It provides a connection-oriented reliable transfer of data between two communicating hosts. (Host refers to a network-connected computer, or to any system that can be connected to a network for offering services to another host connected to the same network.) TCP uses several 15 techniques to maximize the performance of the connection by monitoring dif ferent variables relating to the connection. For example, TCP includes an internal algorithm for avoiding congestion. ATM (Asynchronous Transfer Mode), in turn, is a (newer) connec tion-oriented packet-switching technique which the international telecommuni 20 cation standardization organization ITU-T has chosen as the target solution of a broadband integrated services digital network (B-ISDN). The problems of conventional packet networks have been eliminated in the ATM network by using short packets of a standard length (53 bytes), known as cells. ATM net works are quickly being adopted as backbones for the various parts of TCP/IP 25 networks (such as Internet). Although ATM has been designed to provide an end-to-end trans port level service, it is very likely that also in the future networks will be imple mented in such a way that (a) TCP/IP remains as the de-facto standard of the networks and (b) only part of the end-to-end path of a connection is imple 30 mented using ATM. Thus, even though ATM will continue to be utilized, TCP will still be needed to provide the end-to-end transport functions. The introduction of ATM also means that implementations must be able to support the huge legacy of existing data applications, in which TCP is widely used as transport layer protocol. To migrate the existing upper layer 35 protocols to ATM networks, several approaches to congestion control in ATM networks have been considered in the past.

WO 99/04536 PCT/F198/00591 2 Congestion control relates to the general problem of traffic man agement for packet switched networks. Congestion means a situation in which the number of transmission requests at a specific time exceeds the transmis sion capacity at a certain network point (called a bottle-neck resource). Con 5 gestion usually results in overload conditions. As a result, the buffers overflow, for instance, so that packets are retransmitted either by the network or by the subscriber. In general, congestion arises when the incoming traffic to a specific link is more than the outgoing link capacity. The primary function of congestion control is to ensure good throughput and delay performance while maintaining 10 a fair allocation of network resources to users. For the TCP traffic, whose traffic patterns are often highly bursty, congestion control poses a challenging problem. It is known that packet losses result in a significant degradation in TCP throughput. Thus, for the best possible throughput, a minimum number of packet losses should occur. 15 The present invention relates to congestion control in packet switched networks. For the above-mentioned reasons, most of such networks are, and will in the foreseeable future be, TCP networks or TCP over ATM networks (i.e. networks in which TCP provides the end-to-end transport func tions and the ATM network provides the underlying "bit pipes"). In the follow 20 ing, the congestion control mechanisms of these networks are described briefly. ATM Forum has specified five different service categories which relate traffic characteristics and the quality of service (QoS) requirements to network behavior. These service classes are: constant bit rate (CBR), real-time 25 variable bit rate (rt-VBR), non-real time variable bit rate (nrt-VBR), available bit rate (ABR), and unspecified bit rate (UBR). These service classes divide the traffic between guaranteed traffic and so-called "best effort traffic", the latter being the traffic which fills in the left-over bandwidth after the guaranteed traffic has been served. 30 One possible solution for the best effort traffic is to use ABR (Available Bit Rate) flow control. The basic idea behind the ABR flow control is to use special cells, so-called RM (Resource Management) cells, to adjust source rates. ABR sources periodically probe the network state (factors such as bandwidth availability, the state of congestion and impending congestion) 35 by sending RM cells intermixed with data cells. The RM cells are turned around at the destination and sent back to the source. Along the way, ATM WO 99/04536 PCT/FI98/00591 3 switches can write congestion information on these RM cells. Upon receiving returned RM cells, the source can then increase, decrease or maintain its rate according to the information carried by the cells. In TCP over ATM networks, the source and the destination are 5 interconnected through an IP/ATM/IP sub-network. Figure 1 illustrates a con nection between a TCP source A and a TCP destination B in a network, where the connection path goes through an ATM network using ABR flow control. When congestion is detected in the ATM network, ABR rate control becomes effective and forces the edge router R1 to reduce its transmission rate to the 10 ATM network. Thus, the purpose of the ABR control loop is to command the ATM sources of the network to reduce their transmission rate. If congestion persists, the buffer in the router will reach its maximum capacity. As a conse quence, the router starts to discard packets, resulting in the reduction of the TCP congestion window (the congestion window concept will be explained in 15 more detail later). From the point of view of congestion control, the network of Figure 1 comprises two independent control loops: an ABR control loop and a TCP control loop. However, this kind of congestion control, which relies on dual congestion control schemes on different protocol layers, may have an unex 20 pected and undesirable influence on the performance of the network. To put it more accurately, the inner control loop (ABR loop) may cause unexpected delays in the outer control loop (TCP loop). An alternative approach to support the best effort traffic is to use UBR service with sufficiently large buffers and let the higher layer protocols, 25 such as TCP, handle overload or congestion situations. Figure 2 illustrates this kind of network, i.e. a TCP over UBR network. The nodes of this kind of net work comprise packet discard mechanisms which discard packets or cells when congestion occurs. When a packet is discarded somewhere in the net work, the corresponding TCP source does not receive an acknowledgment. As 30 a result, the TCP source reduces its transmission rate. The UBR service employs no flow control and provides no numeri cal guarantees on the quality of service; it is therefore also the least expensive service to provide. However, because of its simplicity, plain UBR without ade quate buffer sizes gives poor performance in a congested network. 35 To eliminate this drawback, more sophisticated congestion control mechanisms have been proposed. One is the so-called early packet discard WO 99/04536 PCT/F198/00591 4 (EPD) scheme. According to the early packet discard scheme, an ATM switch drops entire packets prior to buffer overflow. In this way the throughput of TCP over ATM can be much improved, as the ATM switches need not transmit cells of a packet with corrupted cells, i.e. cells belonging to packets in which at least 5 one cell is discarded (these packets would be discarded during the re assembly of packets in any case). Another advantage of the EPD scheme is that it is relatively inexpensive to implement in an ATM switch. For those inter ested in the subject, a detailed description of the EPD method can be found, for example, in an article by A. Romanow and S. Floyd, Dynamics of TCP 10 Traffic over ATM Networks, Proc. ACM SIGCOMM '94, pp. 79-88, August 1994. However, the EPD method still deals unfairly with the users. This is due to the fact that the EPD scheme discards complete packets from all con nections, without taking into account their current rates or their relative shares 15 in the buffer, i.e. without taking into account their relative contribution to an overload situation. To remedy this drawback, several variations for selective drop policies have been proposed. One of these is described in an article by Rohit Goyal, Performance of TCP/IP over UBR+, ATMForum/96-1269. This method uses a FIFO buffer at the switch, and performs some per-VC ac 20 counting to keep track of the buffer occupancy of each virtual circuit. In this way only cells from overloading connections can be dropped, whereas the underloading connections can increase their throughput. Despite all the improvements mentioned above, the prior art con gestion control methods still have the major drawback that there is no means 25 of giving early warning to the traffic source when excessive load is detected in the network. In other words, the traffic source is not informed quickly of over load so that it could reduce its output rate. Summary of the invention 30 The purpose of the invention is to eliminate the above-mentioned drawback and to create a method by means of which it is possible, using a simple implementation, to inform the traffic source at a very early stage that the network is becoming overloaded or congested and to ask the source to slow down its transmission rate. The purpose is also that the method allows 35 the co-operation of TCP and ATM flow control mechanisms in an efficient way.

WO 99/04536 PCT/F198/00591 5 This goal can be attained by using the solution defined in the inde pendent patent claims. The basic idea of the invention is to delay the acknowledgments being transferred from the destination towards the sender. This can be done at 5 the same network point where congestion has been detected, or, alternatively, a network point detecting overload or congestion can direct another network point to delay the acknowledgments. Thus, with this invention congestion control is performed on the return path of the connection, whereas prior art systems control traffic on the forward path. Instead of discarding packets or 10 cells on the forward path, the network according to the present invention de lays acknowledgments on the return path and thus causes the TCP source to reduce its output rate. The invention offers an inexpensive solution for giving the TCP source an early warning of impending overload or congestion in the network. It 15 is also important to note that the transport protocol TCP itself does not have to be amended in any way. To put the invention into use, a congestion control algorithm must be introduced into the network, but for this purpose many ex isting control algorithms in the TCP over UBR can be utilized with only slight modifications. 20 Moreover, by means of the present invention the variations in the output rate of the TCP source can be smoothed, which in turn results in better bandwidth utilization. Further, because the amount of variation is lessened, the buffer capacity requirements are also reduced. According to one preferred embodiment of the invention, load level 25 information from an ATM network point is transported in RM cells to a node providing access to the ATM network, and the acknowledgments are delayed in said access node on the basis of the information contained in the RM cells. In this way the TCP and the ATM flow control mechanisms can be made de pendent on each other so that they function efficiently together. 30 By means of the invention the performance of connections can be significantly improved, especially in large latency networks. Brief description of the drawings In the following, the invention and its preferred embodiments are 35 described in closer detail with reference to examples shown in the appended drawings, wherein WO 99/04536 PCT/F198/00591 6 Figure 1 illustrates a TCP connection path through an ABR-based ATM sub network, Figure 2 illustrates a TCP connection path through a UBR-based ATM sub network, 5 Figure 3 illustrates the flow control loop according to the present invention in a TCP over ATM network, Figure 4a illustrates one possible implementation of the new method in an IP switch, Figure 4b is a time diagram showing the significant moments of the imple 10 mentation of Figure 4a, Figure 5 is a flow chart illustrating a method for determining delay values, Figure 6a illustrates another possible implementation of the delaying of ac knowledgments in a switch, Figure 6b illustrates an alternative way of using acknowledgment buffers, 15 Figure 7a illustrates one way of applying the method to an IP network, Figure 7b illustrates another way of applying the method to an IP network, Figure 8a illustrates one way of applying the method to an ATM network, Figure 8b illustrates another way of applying the method to an ATM network, Figure 9 illustrates the interworking of the TCP and ATM flow control loops 20 according to a preferred embodiment of the invention, Figure 10 illustrates an example of packet transfer between the traffic source and the traffic destination in a known TCP network, Figure 11 illustrates an example of packet transfer between the traffic source and the traffic destination in a TCP network utilizing the method ac 25 cording to the invention, Figure 12 is a flow diagram illustrating a further embodiment of the method, and Figure 13 illustrates one possible implementation of the method according to Figure 12 in an IP switch. 30 Detailed description of the invention Figure 3 illustrates the basic principle of the invention by showing a connection between two user terminals (A and B) in a TCP over ATM network, i.e. the user terminals using TCP as a transport layer protocol. In addition to 35 the access nodes (AN1 and AN2) of the user terminals, only one intermediate WO 99/04536 PCT/FI98/00591 7 node (Ni) and the transmission lines (TL1, TL2) connecting the nodes are shown. The TCP connection between the hosts A and B starts out, the same as any other TCP connection, with a negotiation between the hosts to 5 open the connection. This initial negotiation is called a three-way handshake, as three opening segments are transmitted during this handshake phase. The term "segment" refers to the unit of information passed by TCP to IP (Internet Protocol). IP headers are attached to these TCP segments to form IP data grams, i.e. TCP segments are transferred to the receiver within IP datagrams 10 which is the information unit used by IP. During the initial handshaking proc ess, the hosts inform each other for example of the maximum segment size they will accept. This is done to avoid fragmentation of the TCP segments, as fragmentation would slow down the performance of the TCP connection con siderably. 15 After the initial handshake has been completed, the hosts begin to send data by means of the TCP segments. Each uncorrupted TCP segment, including the handshaking segments, is acknowledged. To illustrate the basic idea of the invention, let us assume that host A sends one TCP segment to host B. At the network layer, host A adds an IP header to this TCP segment to 20 form an IP datagram. This datagram is converted into standard ATM cells in an access node AN1 located at the edge of the ATM network ANW. The cells of the datagram are then routed through the ATM network to the access node AN2 of host B. This access node reconstructs the original IP datagram from the arriving cells and sends the IP datagram to host B. Host B removes the IP 25 header to reveal the TCP segment. If the segment is received correctly, host B sends an acknowledging TCP segment ACK1 back to host A. Up till now the network has operated in a known manner. The load of the network is monitored in the access node AN1, for example, by monitoring the occupancy of one or more of the buffers buffering 30 the traffic to the ATM network. If overload is detected, i.e. if buffer occupancy exceeds a predefined level, a congestion notification CM is sent inside the node to delay the acknowledgments traveling at that moment through the switch towards the traffic sources. Thus, also our exemplary acknowledgment (ACK1) is delayed when passing through access node AN1, provided that 35 node AN1 experiences overload during that particular time period.

WO 99/04536 PCT/F198/00591 8 TCP is one of the few transport protocols that natively has a con gestion control mechanism. The solution of the invention relies on this known TCP control mechanism, i.e. no other control mechanisms are needed in the source or in the destination. Therefore, this mechanism is described briefly in 5 the following. TCP congestion control is based on two variables: the receiver's advertised window (Wrcvr) and the congestion window (CNWD). The re ceiver's advertised window is maintained at the receiver as a measure of the buffering capacity of the receiver, and the congestion window is maintained at 10 the sender as a measure of the capacity of the network. The TCP source can never send more segments than the minimum of the receiver's advertised window and the congestion window. The TCP congestion control method comprises two phases: slow start and congestion avoidance. A variable called SSTHRES (slow start 15 threshold) is maintained at the source to distinguish between the two phases. The source starts to transmit in the slow start phase by sending one TCP segment, i.e. the value of CWND is set to one in the beginning. When the source receives an acknowledgment, it increments CWND by one, and, as a consequence, sends two more segments. In this way the value of CWND 20 doubles every round trip time during the slow start phase, as each segment is acknowledged by the destination terminal. The slow start phase ends and the congestion avoidance phase begins when CWND reaches the value of SSTHRES. If a packet is lost in a TCP connection, the source does not receive 25 acknowledgment and times out. The source sets SSTHRES to half the CWND value when the packet was lost. More precisely, SSTHRES is set to max{2, min{CWND/2, Wrcvr}}, and CWND is set to one. As a result, the source enters the congestion avoidance phase. During the congestion avoidance phase, the source increments its CWND by 1/CWND every time a segment is acknowl 30 edged. As the invention does not in any way change the above-described known TCP congestion control mechanism, the latter is not described in more detail here. Anyone interested in the matter can find more detailed information from several books describing the field. (For example, see W. 35 Richard Stevens, TCP/IP Illustrated Volume 1, The protocols, Addison Wesley, 1994, ISBN 0-201-63346-9) WO 99/04536 PCT/F198/00591 9 According to the invention, when overload or congestion is detected at a network point, one or more acknowledgments traveling towards the source on the return path are delayed. In this way the TCP source, which operates in the manner described above, automatically starts to slow down its 5 transmission rate, or at least it does not increase its transmission rate as quickly as it otherwise would. This is because the delay slows down the rate at which the source increases the size of its congestion window. Figure 4a illustrates this principle by showing an example in which the acknowledgments are delayed at the output port OP of an IP switch. A 10 load measurement unit LMU measures the load level of the switch by meas uring the fill rates (occupancies) of the buffers buffering the traffic passing through the switch in the forward direction. It is to be noted that the load level can be determined in any known manner. The IP datagrams passing through the switch in the backward di 15 rection are first routed to their correct output port. At this port the received datagrams are stored in a FIFO-type output buffer OB. A traffic splitter TS reads the stored packets out from the output buffer, one packet at a time from the first memory location ML1 of the buffer. The traffic splitter operates in the following ways. 20 If the congestion signal CS from the load measurement unit indi cates that the load of the switch is below a predefined level, the traffic splitter forwards all the datagrams (packets) directly to the outgoing link OL, irrespec tive of whether they include acknowledgments or not. On the other hand, if the congestion signal CS indicates that the 25 load level has reached a predefined level, the traffic splitter starts to read the acknowledgment bit of each TCP header inside each IP datagram. If this bit is valid, i.e. if the datagram includes an acknowledgment, the traffic splitter for wards the packet to an acknowledgment buffer AB. If the bit is not valid, the traffic splitter forwards the packet directly to the outgoing link OL. Thus, only 30 packets including an acknowledgment are delayed. In the acknowledgment buffer, each IP datagram is delayed for a certain period. The length of the period is preferably directly proportional to the current load level measured by the unit LMU. After the delay period for each outgoing acknowledgment packet has elapsed, the packet is sent to the out 35 going link.

WO 99/04536 PCT/IFI98/00591 10 If ACKTi denotes the moment in time when a packet with an ac knowledgment is output from the traffic splitter to the acknowledgment buffer and ACKTo denotes the moment in time when a packet is output from the acknowledgment buffer to the link, ACKTo can be defined as follows: 5 ACKTo(j) = ACKTi(j) + dj, j=1,2,... where j is the packet sequence number, and d, is the value of the delay asso ciated with a packet with sequence number j. Figure 4b illustrates the moments when the packets leave the traffic splitter and the acknowledgment buffer, respectively. It is assumed that exces 10 sive load is detected after ACKTo(7) (until then the acknowledgments have not been delayed). If the congestion signal received by the delay control unit DCU indicates that the level of the load has exceeded a predefined value, the delay control unit executes an algorithm defining how long the next packet to be transferred to the link should be delayed. The calculated value may depend on 15 one or more parameters, such as the current traffic rate, the current buffer occupancy, or the previous delay value (dp). As can be seen from Figure 4b, the value of the delay may vary from one packet to another. Figure 5 is a flow chart illustrating an example of the algorithm which is executed by the delay control unit for each packet to be read out from 20 the acknowledgment buffer AB. If congestion is detected, the delay value d, for the current packet to be read out from the acknowledgment buffer (i.e. the length of the time that the current packet is stored in the buffer) is calculated by the following formula: di = ady 1 + (1-a)dM (1) 25 where dj 1 is the delay value of the previous packet, dM is a measured delay value, and a is a smoothing factor (preferably a<0,5<1). The measured delay is the actual delay as measured between the moment when a packet is re ceived by the acknowledgment buffer and the moment when that packet is read out from the acknowledgment buffer. This delay can be measured as a 30 mean value over a certain period of time or over a certain number of packets. The delay control unit can perform this measurement. If congestion is detected, and if d 1 1 =0 and dM=O, i.e. if the previous packet was not delayed and there have been no packets in the acknowledg ment buffer AB for a certain predefined preceding period, the delay value di of 35 the current packet gets the value of a predefined delay parameter di, i. e. d = dinitiai.

WO 99/04536 PCT/FI98/00591 11 When the switch recovers from congestion or an overload state, the delay control unit calculates the delay value di with the formula: d = ade 1 - (1-a)dm (2). The purpose of the second term in formula (1) is to smoothly in 5 crease the delay when congestion is detected, and in formula (2) to smoothly decrease the delay when the network is recovering from congestion. Figure 6a shows the solution according to Figure 4a in a shared buffer switch architecture. In the embodiment of Figure 6a, all the packets are buffered in a shared buffer SB prior to routing each packet to the correct out 10 put port OP, of the switch. In other respects, the embodiment of Figure 6a correspond to the embodiment shown in Figure 4a. The traffic splitters TS, (i=1..n) can also form a single unit which reads one packet at a time from the shared buffer and delivers the packet to the correct port. The delay control unit DCU (not shown in Figure 6a) can also be implemented as a common unit for 15 all the output ports. In the embodiments of Figures 4a and 6a, the acknowledgment buffer contains packets from several connections, and all the packets are delayed according to the same delay algorithm. Alternatively, the packets may be stored on a per-connection basis at each output port, i.e. the data packets 20 of each IP connection (or each TCP connection) can be stored in a separate buffer. In these cases each buffer can be a FIFO-type buffer, as the packets of a single queue do not have to be re-sequenced even though different connec tions were delayed in different ways. Also the relative share of each connec tion in the forward buffer can be determined through measurement of the load 25 level, and the connections can be delayed on the basis of the measured val ues. In this way the acknowledgments of connections loading the network more heavily can be delayed longer. Figure 6b illustrates this alternative em bodiment in which the output port has a buffer unit BFU, including separate queues for at least some of the connections. 30 If connection-specific buffers are not used, and if different connec tions are delayed in different ways, the buffers can, for example, be shift register type memories, allowing the re-sequencing of packets so that packets of underloading connections can pass packets of overloading connections. As mentioned earlier, the congestion control method in accordance 35 with the invention can be utilized in packet networks. This means that the network comprises user terminals, network access points providing access to WO 99/04536 PCT/F198/00591 12 the network, and switches. The user terminals act as traffic sources and desti nations, i.e. as points transmitting and receiving data. The switches can be packet switches or ATM switches. An access point can be, for example, a router, or an access point can carry out packet assembling/reassembling, 5 routing or switching. The delaying of acknowledging packets is preferably carried out at the access points, but it can also be carried out in the switches within the network, as described later. Figures 7a and 7b show two different ways of implementing the invention in an IP network. In the embodiment of Figure 7a, the congestion 10 detection as well as the delaying of acknowledgments are carried out within the access switch IPS1, which provides access to the IP network. In the em bodiment of Figure 7b, congestion detection is carried out in the access node, whereas the delaying of acknowledgments is carried out in the TCP/IP proto col stack of the user terminal UT. Congestion notifications CS are transmitted 15 to the user terminal, where the packets with acknowledgments are delayed in the above-described manner prior to their being sent to the TCP source. Figures 8a and 8b show two different ways of implementing the invention in association with an ATM network. In the embodiment of Figure 8a, the congestion detection and the delaying of acknowledgments are carried out 20 in the access node AN. The access node can be divided into an interface card unit ICU and an ATM switch ASW. The interface card unit includes the ATM Adaptation Layer (AAL) functions for the segmentation and reassembly of the IP datagrams. Congestion is monitored in the ATM switch part of the node by monitoring, for example, the fill rates (occupancies) of the buffers buffering the 25 subscriber traffic towards the network. Congestion notifications are transferred to the interface card unit, where the reassembled IP packets are delayed in the above-described manner. In the embodiment of Figure 8b, congestion is monitored in switch ASW, whereas the acknowledging packets are delayed in the TCP/IP protocol stack of the user terminal UT. 30 The embodiments of Figures 7a and 8a are the more advantageous ones because it is much more economical to implement the delaying of ac knowledgments in a single access node than in several terminals located in user premises. Furthermore, it is naturally preferable that the user terminals need not be amended in any way to put the invention into use. 35 As mentioned earlier, one network element in the connection path can command another network element of the same path to perform the de- WO 99/04536 PCT/FI98/00591 13 laying. Figure 9 illustrates this principle in a TCP over ATM network by show ing a connection between two user terminals (A and B), using TCP as a trans port layer protocol. In addition to the access nodes (ANS and AND) of the user terminals, only one intermediate ATM node (Ni) and the transmission lines 5 connecting the nodes are shown. It is assumed that the network nodes have channels in two directions; a forward channel and a backward channel. In order to simplify the description, we assume that the data packets are sent from terminal A to terminal B via access node ANS, one or more ATM switches, and access node AND (forward direction), while the acknowledg 10 ments are returned from terminal B to terminal A via access node AND, one or more ATM switches, and access node ANS (backward direction). As indicated above, the access nodes can be divided into an interface card unit ICU and an ATM switch ASW. The interface card unit includes the ATM Adaptation Layer (AAL) functions for the segmentation and reassembly of the IP datagrams. As 15 in the example of Figure 8a, the delaying of acknowledgments is performed in the interface card unit. However, in this case congestion is not monitored in the ATM switch part of the access node, but in an ATM switch located further within the ATM network. In Figure 9, said ATM switch, which commands the access node to delay the acknowledgments, is switch N1. 20 In the network of Figure 9, ABR flow control occurs between a sending end-system (ANS) and a receiving end-system (AND). As regards the RM cell flow in this bidirectional ABR connection, each termination point is both the sending and the receiving end-system. As shown in Figure 9, for the forward information flow from access node ANS to access node AND, there is 25 a control loop consisting of two RM cell flows, one in the forward direction and the other in the backward direction. Access node ANS generates forward RM cells, which are turned around by access node AND and sent back to access node ANS as backward RM cells. These backward RM cells carry feedback information provided by the network nodes and/or the access node AND. A 30 network node within the ATM network, such as node N1, can: - insert feedback control information directly into RM cells when they pass the node in the forward or backward direction, - indirectly inform the source about congestion by setting the EFCI bit (Explicit Forward Congestion Indication) in the headers of data cells (i.e. 35 user cells) traveling in the forward direction. In this case, the access node AND updates the backward RM cells according to this congestion information, WO 99/04536 PCT/FI98/00591 14 - generate backward RM cells. Thus, there are at least three different ways of controlling the de laying of the acknowledgments in the access node from within the network. In RM cells, the congestion information can be inserted, for exam 5 ple, in the 45 octet long "Function Specific Fields", or in the subsequent "Reserved" part having a length of 6 bits. The traffic parameters forwarded to the user of ABR capability via RM cells are described in item 5.5.6.3 of the ITU-T specification 1.371, and the structure of an RM cell in item 7.1 of said specification, where an interested reader can find a more detailed description 10 of RM cells. The EFCI bit, in turn, is the middlemost bit in the 3 bit wide PTI (Payload Type Indicator) field in the ATM cell header. According to this preferred embodiment of the invention, when overload or congestion is detected at an ATM network node, the correspond 15 ing access node receives backward RM cells containing the congestion infor mation. On the basis of this information, the ATM switch part of the access node adjusts its output rate towards the ATM network, and the flow control mechanism delays the acknowledgments traveling towards the traffic source on the backward channel. In this way the TCP source automatically starts to 20 slow down its transmission rate, or at least it does not increase its transmission rate as quickly as it otherwise would. As mentioned earlier, this is because the delay slows down the rate at which the source increases the size of its con gestion window. In the above-described way the end-to-end ABR flow control can be 25 performed without changing the interworking TCP protocol. In other words, the interworking of the ATM and TCP flow control loops can be implemented in an inexpensive way. Figures 10 and 11 are time lines illustrating the exchange of seg ments between a TCP source and a TCP destination. The source is shown on 30 the left side and the destination on the right side. The transmission and recep tion events have been marked with numbers starting from 3. Figure 10 gives an example of how the source and the destination behave in a conventional network, i.e. in a network without the implementation of the inventive method on the return path of the connection. At first, the 35 source is in the slow start phase. Let us assume that the load of the network increases gradually, and, as a result, packet P10 transmitted at number 21 is WO 99/04536 PCT/FI98/00591 15 lost in the overloaded network point. After this, the source still sends packets, as the acknowledgments it receives are in sequence. At number 37 the source finally notices that the acknowledgment number received was out of sequence and stops transmitting. 5 At 41 the timer of the source goes off and the source retransmits packet P10. Simultaneously the source moves over to the congestion avoid ance phase. Figure 11 gives an example of the data exchange when the network utilizes the present invention. Here, overload is detected after the destination 10 has transmitted the seventh acknowledgment (ACK7). As a result, this and the subsequent acknowledgments (ACK8.. .ACK1 1) are delayed in the network. As can be seen from the figure, already at number 24 the source begins to slow down its output rate, continuing in the slow start phase. As shown, the conventional network behaves in a more uneven way, i.e. first the 15 source sends a lot of packets, and when congestion is detected, no packets are sent. On the contrary, a network implementing the present invention be haves in a much smoother way. This is because delaying the acknowledg ments prevents the source from incrementing its congestion window as quickly as in the known network. Because of this, the buffering capacity of the access 20 network can be diminished. The above-described method can also be used together with other flow control mechanisms. As the above-described method needs a long ac knowledgment buffer, if the congestion situation lasts for a long time, it may in some applications be advantageous to combine it with another mechanism 25 which takes care of the more severe congestion situations. According to a further embodiment of the invention, the delaying of acknowledgments is used together with a method which is otherwise similar to the above method but which generates duplicate acknowledgments, instead of delaying acknowl edgments. By duplicating the acknowledgments the TCP source can be made 30 to slow down its output rate, i.e. duplication has a same kind of effect on the TCP source as delaying. This is based on the fast retransmission and fast recovery algorithms which the source automatically performs after receiving a certain number (typically three) of duplicate acknowledgments. These algo rithms are nowadays widely implemented in different TCP versions. According 35 to the algorithms the source performs, after receiving a certain number of duplicate acknowledgments, a retransmission of what appears to be the miss- WO 99/04536 PCT/FI98/00591 16 ing segment, without waiting for a retransmission timer to expire (the fast re transmission algorithm). After this the source performs congestion avoidance, instead of slow start, in order not to reduce the data flow abruptly (the fast recovery algorithm). 5 Figure 12 is a flow chart illustrating the combinatory method. If con gestion is not detected on the forward path, the acknowledgments are for warded without delay and with the incoming acknowledgment number. If the load measurement detects that the load level on the forward path exceeds a predetermined value (phase 111), it is tested (phase 112) whether the fill rate 10 of the acknowledgment buffer has exceeded a predetermined value. If this is the case, duplicate acknowledgments are generated. Otherwise acknowledg ments are only delayed. Thus, if congestion occurs only slightly and for a short period, delaying of acknowledgments is performed. However, should there be a more severe congestion situation, the system always moves over to gener 15 ate duplicate acknowledgments. This means that a network node sends to wards the source M successive acknowledgments in which the acknowledg ment number, which indicates the next sequence number that the destination expects to receive, are equal to each other. Figure 13 illustrates how this preferred embodiment is implemented 20 in the node of Figure 4a. As mentioned above in connection with Figure 4a, the IP datagrams passing through the switch in the backward direction are first routed to their correct output port. At this port the received datagrams are stored in a FIFO type output buffer OB. 25 The traffic splitter reads the stored packets out from the output buffer, one packet at a time from the first memory location ML1 of the buffer. If the congestion signal CS1 from the load measurement unit LMU indicates that the load of the switch on the forward path is below a predefined level, the traffic splitter forwards all the datagrams (packets) directly to the 30 outgoing link OL, irrespective of whether they include acknowledgments or not. On the other hand, if the congestion signal CS1 indicates that the load level has reached a predefined level, the traffic splitter starts to read the acknowledgment bit of each TCP header inside each IP datagram. If this bit is valid, i.e. if the datagram includes an acknowledgment, the traffic splitter for 35 wards the packet to an acknowledgment buffer AB. If the bit is not valid, the traffic splitter forwards the packet directly to the outgoing link OL. Thus, only WO 99/04536 PCT/F198/00591 17 packets including an acknowledgment are delayed. In the acknowledgment buffer, each IP datagram is delayed for a certain period. The length of the period is preferably directly proportional to the current load level measured by the unit LMU. After the delay period for each 5 outgoing acknowledgment packet has elapsed, the packet is sent to the out going link. The load measurement unit LMU also measures the fill rate of the acknowledgment buffer AB. If this fill rate exceeds a predetermined value, the load measurement unit sends the control unit CU a second congestion signal 10 CS2 indicating that the control unit should now begin to produce duplicate acknowledgments. The duplication can be done for example by modifying the acknowledgment number of the acknowledgments in the packet buffer OB. The traffic splitter is also informed to direct all the traffic directly to the output link. The command can be given either by the load measurement unit or by the 15 control unit. Although the invention has been described here in connection with the examples shown in the attached figures, it is clear that the invention is not limited to these examples, as it can be varied in several ways within the limits set by the attached patent claims. The following describes briefly some possi 20 ble variations. As indicated above, a prerequisite for a user terminal is that it ac knowledges correctly received (i.e. uncorrupted) data units. Therefore, the idea can in principle be applied to any other protocol which sends acknowledg ments and slows down its output rate if the acknowledgments are delayed. 25 The formula used for calculating the absolute delay value can also vary in many ways. The measurement unit can inform about the load level in many ways; as an ON/OFF type information, or more than one bit can be used to indicate the value of the measured load. The signal (CS) informing about the load level can also include information on the particular connections that 30 should be subject to delaying of acknowledgments. User terminals can also have wireless access to the network.

Claims (16)

1. A method for controlling overload in a packet switched network comprising traffic sources (A), traffic destinations (B) and network nodes (AN, Ni), the method comprising the steps of 5 - sending data units from a traffic source to a traffic destination, - sending an acknowledgment from the destination to the source, if a data unit is received correctly at the destination, and - measuring load level in at least one network node, characterized by 10 delaying the acknowledgments traveling towards the source when the measured load level exceeds a predetermined value.

2. A method according to claim 1, c h a r a c t e r i z e d in that the acknowledgments are delayed in the same network node where the load level is measured. 15

3. A method according to claim 1, c h a r a c t e r i z e d in that the acknowledgments are delayed in a different network node than where the load level is measured.

4. A method according to claim 3, c h a r a c t e r i z e d in that the acknowledgments are delayed in an access node (AN, ANS, AND) providing 20 the traffic sources and destinations access to the network, and the load level is measured in at least one network node (NI) located within the network.

5. A method according to claim 4, wherein the network between the access nodes is an ATM network, c h a r a c t e r i z e d by - transporting load level information in RM cells to the access node, 25 and - delaying the acknowledgments on the basis of the information contained in the RM cells.

6. A method according to claim 1, c h a r a c t e r i z e d in that the acknowledgments are delayed in at least one network node by 30 - storing at least part of the data packets traveling in a first direction through the node in a first buffer, - reading data packets out from the first buffer in such a way that (a) packets including an acknowledgment are transferred into a second buffer, and (b) packets failing to include an acknowledgment are transferred directly to 35 an outgoing link (OL), WO 99/04536 PCT/F198/00591 19 - determining a delay value for each packet in the second buffer, and - reading out a packet from the second buffer to the outgoing link when the delay value determined for said packet has elapsed. 5

7. A method according to claim 6, c h a r a c t e r i z e d in that the first and second buffers are used at each output port of the first direction.

8. A method according to claim 6, c h a r a c t e r i z e d in that the delay value is determined using the same determination rule for all packets in the second buffer. 10

9. A method according to claim 8, c h a r a c t e r i z e d in that the delay value for a packet is determined on the basis of the delay value of the preceding packet and of the delay value measured over a certain preceding period.

10. A method according to claim 1, c h a r a c t e r i z e d in that 15 only acknowledgments belonging to selected connections are delayed if the measured load level exceeds a predetermined value.

11. A method according to claim 1, wherein said data units travel along a forward path from the traffic source to the traffic destination and said acknowledgments travel along a backward path from the destination to the 20 source, ch a ra cte rized bythesteps of - measuring load level both on the forward path and on the back ward path, - delaying acknowledgments when the load level on the forward path is higher than a first predetermined value and the measured load level on 25 the backward path is lower than a second predetermined value, and - transmitting duplicate acknowledgments when the load level on the forward path is higher than the first predetermined value and the measured load level on the backward path is higher than the second predetermined value. 30

12. Packet switched telecommunications network comprising - nodes interconnected by transmission lines (TL1, TL2), - user terminals (UT) connected to the nodes, said user terminals acting as traffic sources which send data packets and as traffic destinations which receive data packets, and 35 - measuring means (LMU) for measuring current load level in a node, WO 99/04536 PCT/F198/00591 20 c h a r a c t e r i z e d in that the network further comprises - delaying means (AB, DCU), operably connected to the measuring means (LCU) for delaying data packets carrying acknowledgments from a destination towards a source. 5

13. A network according to claim 12, c h a r a c t e r i z e d in that at least one node comprises both the measuring means and the delaying means.

14. A network according to claim 13, c h a r a c t e r i z e d in that said at least one network node is an access node connecting at least one user terminal to the network. 10

15. An IP network according to claim 13, wherein the network nodes switch IP packets, c h a r a c t e r i z e d in that said at least one net work node can be any one or more of the network nodes.

16. A TCP over ATM network according to claim 12, c h a r a c t e r i z e d in that the delaying means are connected to the meas 15 uring means by an RM cell flow, said RM cells carrying information on the load level.