EP1131922A1

EP1131922A1 - Method for reducing congestion in a network

Info

Publication number: EP1131922A1
Application number: EP00960822A
Authority: EP
Inventors: Nadine Fabiano; Bernard Maingueno; René Martin
Original assignee: Bull SA
Current assignee: Bull SA
Priority date: 1999-09-16
Filing date: 2000-09-07
Publication date: 2001-09-12
Also published as: WO2001020857A1; FR2798798B1; FR2798798A1; JP2003509962A

Abstract

The invention concerns a machine (15) acting as router implementing the inventive method to reduce congestion in its network layer (16) when it accumulates in a waiting list datagrams (12) to be transmitted on a network (18). The method comprises a first step (29) which consists in measuring the filling up level of said waiting list (20) to prepare a signal (NIV) based on said filling level; a second step (30) which consists in detecting any datagram received from said network (18), whereof a field (28) of a transport layer (6) contains a received window value (VFE); a third step (31) which works out a transmitted window value (VFE) based on said signal (NIV) to process the detected datagram by including therein said value (VFE) in said field; a fourth step (32) which consists in routing the datagram processed on the network (17) addressed to a transport layer (4) limiting its transmission rate on the basis of the transmitted window value (VFE).

Description

METHOD FOR REDUCING CONGESTION IN A NETWORK

The field of the invention is that of communication networks. To communicate, data terminal equipment DTE (Data Terminal Equipment DTE in English) use different protocols.

There are usually several communication layers, for example an application layer, a transport layer and a network layer. The application layer is not directly concerned with the locations of function execution. By way of nonlimiting example, we can cite different protocols usable in an application layer such as TELNET to couple a local terminal to a remote machine, FTP to transfer files between machines, HTTP to access WEB pages. Typically, a client application issues requests to a server application from which it receives responses regardless of whether the server application can run in a physical and software environment different from that of the client application. The transport layer has the function of communicating two applications taking into account the physical and software environment of each of them. There are connected type transport layer protocols such as TCP and unconnected type protocols such as UDP. The advantage of a connected type transport protocol is that it ensures the reliability of exchanges while an unconnected type protocol allows greater speed. The function of the network layer is to route messages between two DTEs, taking into account the networks on which the two DTEs are connected. For example, network protocols such as IP or CLNP provide a datagram-type connectionless service. This means that for a message made up of datagrams, the network protocol retransmits each datagram from machine to machine according to the availability of the paths offered without ensuring that each transmitted datagram actually arrives at its destination, for example in the event of congestion of a network or an intermediate machine. Such an intermediate machine, responsible for propagating datagrams between two distinct networks, is generally called a router. According to recommendation 1.113 ITU-T "Vocabulary of Terms for Broadband Aspects of ISDN", Helsinki, March 1993, a connectionless service is defined as a service allowing the transfer of information between two users of the service without the need for procedures of end-to-end call establishment. When the number of datagrams to propagate between two networks exceeds the sending capacity of the router, the datagrams are put in queues inside the router for further processing. When the number of datagrams waiting to be processed in a queue exceeds a threshold, the router discards the new datagrams which arrive and which should have taken place in this queue because the memory capacity of the router is limited.

These delay and loss phenomena mean that in the event of congestion, the DTE transport layers re-emit, multiplying the number of datagrams to be propagated by the router and thus aggravating the congestion.

Known solutions exist at the transport layer level such as for example slowing down to avoid congestion (Slow Start with Congestion Avoidance) of the TCP protocol. The transport layer of the transmitting IΕTTD detects network congestion when it finds that it must retransmit data. It relieves the routers crossed by its traffic by temporarily and spontaneously reducing its transmission capacity for a connection concerned. This is done for example by increasing the time interval between any two retransmissions, by sending a smaller amount of information than that authorized by the receiving DTE. A disadvantage of this solution is that congestion has already occurred when it is implemented, retransmission is inevitable and the need to increase the delay considerably decreases communication performance.

We can also cite the Source Quench of the ICMP control protocol. When the router finds that the number of messages in a queue reaches an intermediate threshold when putting a new datagram in the queue, it sends a specific message to IΕTTD which has issued the datagram to indicate that the risk of congestion increases. The transmitting DTE then reduces its transmission capacity. One drawback of this solution is that the transmitting DTE is poorly informed of the extent to which it can increase its throughput. On the other hand, this solution requires to send additional messages on the networks. To control the speed of the sending DTE, so as to reduce it before congestion occurs, without generating additional traffic, the invention uses datagrams which pass through the router in the direction from the receiving DTE to the sending DTE. and which contain window information. This window information is that generated at the transport protocol level by the receiving DTE to indicate to the sending DTE, the amount of information that the latter is authorized to transmit on a connection before receipt of an acknowledgment indicating that the previously transmitted information has been received correctly. The subject of the invention is a method for reducing congestion in a network layer of a router machine when it accumulates datagrams to be sent on a network in a queue, characterized in that it comprises:

- a first step which measures a filling level of said queue to develop a signal as a function of said filling level;

a second step which detects any datagram received from said network, a field of a transport layer of which contains a first value of window received;

a third step which elaborates a second window value transmitted as a function of said signal to process the detected datagram by putting there said second value in said field;

a fourth step which routes the datagram processed on a network to a transport layer limiting its transmission rate as a function of the window value transmitted.

Thus, in an environment with a network protocol making a connectionless service of the datagram type and a transport protocol making a reliable connection service using a window system for end-to-end flow control, an intermediate machine which sees passing all datagrams exchanged by two terminal data processing equipment can control the flow of datagrams which crosses it, by acting from its network layer on the transport layer of the transmitting terminal data processing equipment. This has the advantage of reducing congestion in the intermediate machine without requiring any special procedure in the terminal data processing equipment.

A preferred example of implementation of the invention is explained in the description which follows with reference to the figures in which: - Figure 1 shows a router of the state of the art;

- Figure 2 shows a transport layer segment;

- Figure 3 shows a network layer datagram;

- Figure 4 shows a router which implements the invention; - Figure 5 shows a datagram with window information;

- Figure 6 shows process steps according to the invention.

With reference to FIG. 1, a sending DTE (terminal data processing equipment) 1 communicates messages to a receiving DTE 2 by means of a transport protocol 3. To do this, a transport layer 4 of IΕTTD 1 generates segments 5 intended for a transport layer 6 of the DTE 2. More generally, the segments are transport protocol data units TPDU, acronym for Transport Protocol Data Unit in English.

With reference to FIG. 2, each segment 5 of the transport layer comprises at least one transport field 7. When the message relates to an application, the transport layer 4 receives information from an application layer, not shown from the machine 1, by means of an interface 8. The transport layer 4 then incorporates this information into a field 9 of segment 5.

With reference to FIG. 1, the transport layer 4 transmits the segment 5 to a network layer 10 of the DTE 1 by means of an interface 11.

With reference to FIG. 3, the network layer 10 juxtaposes a field 13 with the segment 5 to constitute a datagram 12 intended for a network layer 14 of the receiving DTE 2. The field 13 contains data for implementing a network protocol between DTEs 1 and 2. In the case where, for example, the network is of the IP type, field 13 contains an IP address which identifies the receiving IDTTD 2 and an IP address which identifies the sending DTE 1.

Depending on the topoiogical network configuration, the datagram 12 is routed directly from DTE 1 to DTE 2 or indirectly through one or more router devices 15. With reference to FIG. 1, the network layer 10 routes the datagram 12 to a network layer 16 of router equipment 15, by means of a physical network 17. The network layer 16 then routes the datagram 12 to layer 14 by means of a physical network 18. When the physical network 18 is not directly connected to the DTE 2, the datagram 12 passes through as many routing equipment as necessary to reach a physical network to which is directly connected to IΕTTD 2.

When the network layer 16 of the router equipment 15 receives a datagram 19 which it cannot immediately retransmit on the physical network 18 due to congestion, it accumulates the datagram 19 in a first queue 20 which it empties as the physical network becomes available 18.

When the network layer 14 of IΕTTD 2 receives the datagram 12, it removes the field 13 to transfer it in the form of segment 5 to the transport layer 6 by means of an interface 21. When the segment 5 comprises a field 9 intended for an application, the transport layer 6 transfers the field 9 to an application layer not shown of IΕTTD 2 by means of an interface 22. On the other hand, the transport layer 6 sends to the transport layer 4, an acknowledgment segment for tell it that it has received the segment 5. To do this, the transport layer 6 transmits to the network layer 14, the acknowledgment segment which generally includes only the field 7. The network layer 14 then juxtaposes a field 13 at the field 7 so as to obtain a route an acknowledgment datagram which is routed to the network layer 10. The network layer 10 then transmits field 7 of the acknowledgment datagram to the transport layer 4 by means of the interface 11 .

With reference to FIG. 1, the network layer 14 routes the datagrams on the physical network 18 to the network layer 16 of the router equipment 15 which redirects them over the physical network 17 to the network layer 10. When the network layer 16 of the router equipment 15 receives a datagram 24 which it cannot immediately retransmit on the physical network 17 due to congestion, it accumulates the datagram 24 in a second queue 25 which it empties as and when measuring the availability of the physical network 17.

A transport protocol window system indicates to the transport layer 4, the quantity of information that it can send to the transport layer 6 before receiving the acknowledgment segment. To do this, the transport layer 6 regularly sends segments containing an indicator of the amount of information it can process without saturation. A simple way to do this is for example to put this indicator in field 7 of the acknowledgment segments.

There are several reasons why transport layer 4 does not receive acknowledgment to segments generated destined for transport layer 6. For example, the segments generated destined for the transport layer or the acknowledgment segments may have been lost in network layers. The transport layer 4 can then retransmit unacknowledged segments until it receives acknowledgment.

Referring to Figure 4, the router machine 15 comprises a device for reducing congestion in its network layer 16 when it accumulates in the queue 20 datagrams 12 to be transmitted on the network 18. The device comprises means 33 to detect any datagram received from the network 18, including a field 28 of the transport layer 6 containing a received window value VFR and putting therein a transmitted window value VFE, which is a function of a filling level 26 of the queue 20 before routing the datagram detected on the network 17 to the transport layer 4. When the filling level of the first queue 20 exceeds a prevention threshold 26, the network layer 16 picks up the acknowledgment datagrams 27 in from the network 18 and performs the processing of the content of their field 28 before retransmitting them on the network 17. The processing of the field 28 is done so that the window value slows the bit rate of datagrams entering router 15 to network 18. Network layer 10 then receives datagram 27 with a window value in field 28 which not only takes into account the processing capacity of transport layer 6 but which takes into account also the processing capacity of the network layer 16. The network layer 10 subtracts or moves the field 13 of the datagram 27 to obtain a segment to be transmitted to the transport layer 4. By respecting the window value of the field 28, the transport layer 4 then generates a number of datagrams destined for the transport layer 6, less than or equal to the number of datagrams admissible by the transport layer 6 of the DTE 2.

Referring to Figure 6, a method for reducing congestion in a machine includes four steps. A first step 29 measures the level of filling of the first queue 20 of the datagrams to be transmitted on the network 18, to develop a VIN signal. A second step 30 detects the datagrams coming from the network 18 containing the field 28 with a received window value VFR. A third step 31 processes the value VFR to develop a transmitted window value VFE and replace the value VFR by the value VFE in the detected datagram, as a function of the signal VIN. A fourth step 32 retransmits the datagram detected on the destination network 17.

The method, although implemented at the network layer 16 of the router equipment 15, by replacing the window value VFR with the value VFE, modifies a field of the transport layer. The constraints related to the transport protocol must be taken into account. Step 30 therefore begins by identifying in field 7 of the received datagram, the transport protocol.

For example, in the case of the known transport protocol TCP, the segments are transmitted in sequences of bytes, each numbered from the first to the last byte of the sequence. On receipt of the last octet of a sequence, the transport layer 6 of the receiving DTE 2 sends an acknowledgment if it is the first sequence or if it has already sent an acknowledgment for the immediately preceding sequence. This acknowledgment generally indicates the number of the first byte of the following sequence awaiting reception. In the same segment as that containing the acknowledgment, IΕTTD receiver 2, transmits a VFR window value representative of the number of bytes that the transmitting DTE 1 can transmit in the sequences to come. The VFR value takes into account a possible value already transmitted with a previous acknowledgment, the number of bytes already received and the number of bytes admissible on reception.

For each connection established between the transport layers 4 and 6, the datagrams of the network layers 10 and 14 pass through the network layer 16 of the router equipment 15, the router equipment 15 detects the type of upstream transport protocol. In the case where the detected transport protocol is of the TCP type, the router equipment 15 calculates, in parallel with steps 29 to 32, a remaining window value VFER representative of the number of bytes that the transmitting DTE can still transmit 1 when it is calculated. So that the remaining window value VFER is representative of reality, it is advisable to force the passage of all datagrams of the same connection by the router equipment that constitutes the machine 15.

The remaining window value VFER is calculated as follows. Each time the means 33 receive a datagram containing an acknowledgment, the value which it indicates is stored in a variable called ACK. An ACKp variable, initialized to zero, contains the value indicated by the previous acknowledgment. A Diff value is calculated by the formula:

Diff = ACK - ACKp. The Diff value then represents a number of bytes transmitted within a VFEp window previously transmitted to the DTE 1. The VFER value is then given by the formula:

VFER = VFEp - Diff.

The value VFE obtained in step 31 is then equal to the greater of two window values VFER and VFI where VFI is an intermediate window value calculated according to different possible embodiments as explained in the following description, left to the choice of a network administrator.

VFE = max (VFER, VFI) This ensures that the VFE value will never be lower than the VFER window value for which ETTD 1 continues to transmit bytes before receiving the new VFE window value.

According to a first possible embodiment, in step 29, the VIN signal is set to a binary alarm state when the filling level exceeds a first threshold.

In step 31, when the signal VIN is at an initial state, the value VFI is equal to the value VFR, it is the transport layer 6 which imposes the window value on the transport layer 4 to regulate the transmission thereof datagrams. Steps 30 and 31 can be short-circuited, that is to say that window datagrams can be directly retransmitted from network 18 to network 17. When the VIN signal is in the binary alarm state, the value VFI is obtained by taking the lowest value from the VFR value and a predetermined VFT stub value as a function of the capacity of the network 18 to empty the queue 20. This has the effect of temporarily reducing exchanges on transport layers 4, 6 at high communication speed without necessarily reducing them on transport layers 4, 6 at lower speed whose window values are already below the VFT stub value. A variant consists in obtaining the VFI value by multiplying the VFR value by a coefficient less than one. This has the effect of temporarily reducing the exchanges on all the transport layers 4, 6 in a proportionally identical manner, both for those at low speed and for those at high communication speed. When the VIN signal is reset, datagrams with the VFR window value are retransmitted as is. The VIN signal is reset in step 29 when the filling level falls below the first threshold or when the filling level falls below a second threshold below the first threshold. The hysteresis thus caused on the limitation of the size of the windows has the effect of avoiding instabilities. The second threshold can be very low, to the point of corresponding to an empty state of the queue 20.

According to a second possible embodiment, in step 29, the VIN signal is the complement to one of a RATE number obtained by dividing the filling level measured by the total capacity of the queue 20. Thus, when the queue waiting 20 is empty, the VIN signal is equal to one, when the queue 20 is full, the VIN signal is equal to zero. In step 31, the value VFI is obtained by multiplying the value VFR by the signal VIN. Thus, when the queue 20 is empty, the value VFI is equal to the value VFR and the datagrams remain unchanged. When the queue 20 is full, the value VFI is zero, that is to say that the transport layer 4 can only transmit a datagram to the network layer 10 after having received an acknowledgment for a datagram previously transmitted. Between these two extremes, the size of the windows is gradually reduced with a PFD value between VFR and zero. In the event of a temporary overload of the network 18, the filling level of the queue 20 tends to stabilize around an intermediate value which makes it possible to anticipate a subsequent reduction in load. It is possible to act on this intermediate value by involving the number TAUX in polynomial form in the calculation of the VIN signal.

Claims

CLAIMS:

1. Method for reducing congestion in a network layer (16) of a machine (15) when it accumulates in a queue (20) datagrams (12) to be transmitted over a network (18), characterized in what he understands:

- a first step (29) which measures a filling level of said queue (20) to generate a signal (VIN) as a function of said filling level;

- a second step (30) which detects any datagram received from said network (18), a field (28) of a transport layer (6) of which contains a received window value (VFR); - a third step (31) which generates a transmitted window value (VFE) as a function of said signal (VIN) to process the detected datagram by putting said value (VFE) in said field (28), the value of the transmitted window (VFE) being at least equal to a remaining window value (VFER) representative for each connection established, of the number of bytes transmissible at the time of preparation; - A fourth step (32) which routes the datagram processed on a network (17) to a transport layer (4) limiting its transmission rate as a function of the value of window transmitted (VFE).

2. Method according to claim 1, characterized in that the signal (VIN) is produced by means of a binary function which gives an alarm state when the filling level of the queue (20) exceeds a first threshold.

3. Method according to claim 1, characterized in that the signal (NTV) is produced by means of a polynomial function proportional to the filling level and inversely proportional to the capacity of the queue (20).

4. Method according to claim 2, characterized in that the transmitted window value (VFE) is developed by limiting the received window value (VFR) when the signal (NTV) is in the alarm state.

5. Device for reducing congestion in a network layer (16) of a machine (15) when it accumulates in a queue (20) in a memory of said machine (15), datagrams (12) to transmit on a network (18), characterized in that it comprises means (33) in said memory, for detecting any datagram received from said network (18), a field (28) of a transport layer (6) of which contains a received window value (VFR) and putting therein a transmitted window value ( VFE) as a function of a level (26) of filling said queue (20) before routing the datagram detected on a network (17) to a transport layer (4) limiting its transmission rate by function of the transmitted window value (VFE), the value of the transmitted window (VFE) being at least equal to a remaining window value (VFER) representative for each connection established, of the number of bytes transmissible at the time of development.