JP3859521B2 - Mobile communication system - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to retransmission control when data is transferred using a transport control protocol (TCP) in a mobile communication system.
[0002]
[Prior art]
Conventionally, a transport control protocol (hereinafter referred to as “TCP”) is known as a network protocol used for communication in a network. This TCP is located in the fourth layer (transport layer) in the OSI (Open Systems Interconnection) reference model, and is a protocol that performs flow control, retransmission control, and the like. TCP is used as a standard network protocol for communication on the Internet. The TCP is also used when communicating from a mobile terminal such as a mobile phone by connecting to the Internet via a mobile communication network including a wireless communication line.
[0003]
FIG. 4 shows an example of a connection establishment process and a data transmission sequence when a client in the network requests and establishes a TCP connection to the server.
In the TCP connection establishment process, as shown in FIG. 4, first, the client 101 requesting the TCP connection transmits the segment 1 with the SYN flag turned on to the server 100. Receiving segment 1, the server 100 turns on the SYN flag and transmits segment 2 of ACK for SYN to the client. The client that has received the segment 2 transmits the ACK segment 3 for SYN to the server 100. As a result, the TCP connection between the client 101 and the server 100 is established. Such connection establishment is called three-way handshake. When the connection is established, for example, as illustrated in FIG. 4, when the client 101 transmits the segment 4 for requesting data to the server 100, the server 100 returns the segment 5 of the ACK for this segment 4 to the client 101. Response data (not shown) to the request is transmitted. In this way, the client 101 can obtain predetermined data from the server 100.
[0004]
In addition, when the ACK corresponding to the transmitted segment does not return for a certain period of time, the client 101 that is the transmission source retransmits the segment. This fixed time is called retransmission timeout time (RTO), and RTO is based on a round trip time (RTT: Round Trip Time) from when the client 101 transmits a segment to the server 100 until it receives an ACK segment from the server 100. It is calculated according to the line quality at that time. For example, as shown in FIG. 5, when the client transmits the segment 4 of data 1 (DATA1) to the server 100, it is assumed that the segment 4 disappears in the middle and the server cannot receive it. In this case, since the server 100 cannot receive the segment 4, it does not return an ACK. When the client 101 transmits the segment 4, the client 101 starts a retransmission timer with the RTO value set. When the retransmission timer times out, the client 101 retransmits the ACK segment 10 as shown in FIG. Assuming that the server 100 receives the retransmitted segment 4, the server 100 returns the segment 11 of the ACK for the segment 10 to the client 101, and the client 101 that has received this ACK has received the data of the segment 10 normally. Know. By performing such retransmission control, the reliability of communication using TCP can be improved. In retransmission control, retransmission is performed over several orders until ACK is returned.
[0005]
[Problems to be solved by the invention]
By the way, in a mobile communication system, an intermediate node for absorbing differences between communication lines is generally provided between the wireless communication line side and the wired communication line side. FIG. 6 is a sequence diagram showing an example of retransmission control when data is transferred from the server 200 to the mobile device 201 using TCP in this case. In FIG. 6, between the server 200 and the intermediate node 202, retransmission control is performed by TCP located in the fourth layer (transport layer). That is, the RTO value calculated by the server 200 based on the RTT is set in the retransmission timer, and retransmission is performed when the retransmission timer times out. In contrast, retransmission control is performed between the intermediate node 202 and the mobile device 201 in the second layer (data link layer) of the OSI reference model. The retransmission control performed in the second layer is generally performed in a shorter time than the TCP retransmission control performed in the fourth layer.
[0006]
In particular, wireless communication lines have poor line quality compared to wired communication lines, and their quality is not constant, so in wireless communication lines, the round-trip time RTT from transmission of a segment to reception of an ACK segment becomes longer. There is a tendency to end up. For this reason, in a communication network including a wireless communication line, there is a possibility that the segment may be retransmitted based on the elapse of the retransmission timeout time RTO even though no packet loss has occurred on the communication line. In addition to being retransmitted from the server 200 by TCP retransmission control, this retransmission is also retransmitted from the intermediate node 202 by second-layer retransmission control. In this way, when the TCP retransmission control and the second layer retransmission control compete, or when the retransmission segment is retransmitted erroneously, useless packets exist on the communication line, and the communication line utilization efficiency is increased. It will fall. That is, there is a problem that the data band of the wireless line is used by the retransmission segment, and the band of the wireless line is compressed. Since the bandwidth is narrow in the case of a wireless line, it is a big problem that the data bandwidth is wasted due to retransmission segments.
[0007]
Therefore, in order to solve this problem, when establishing communication using TCP between the mobile terminal and the server, the server is given the information of the retransmission timeout time held in the mobile terminal. It has been proposed to perform retransmission control according to the information on the retransmission timeout period. According to such re-transmission control, it is possible to prevent the use of the bandwidth of the radio line due to the re-transmission control, and to suppress a reduction in the utilization efficiency of the communication line.
However, there are cases where the intermediate node supports circuit-switched bearers and packet-switched bearers, and the circuit-switched bearer (packet-switched bearer) used so far when the mobile terminal is handed over is the packet-switched bearer (circuit-switched bearer). ) May occur. In such a case, it is preferable to set a retransmission timeout time (RTO value) suitable for the changed bearer.
[0008]
Therefore, an object of the present invention is to provide a mobile communication system capable of changing the retransmission timeout period even during a TCP session.
[0009]
[Means for Solving the Problems]
In order to achieve the above object, a mobile communication system of the present invention is a mobile communication system that enables data transfer between a mobile terminal in a radio access network and a server in a core network using a transport control protocol (TCP). In a communication system, during a session using the transport control protocol between the mobile terminal and the server, when the mobile terminal is handed over and a bearer located under the transport control protocol is changed The mobile terminal sends a segment to which the option for changing to a retransmission timeout time suitable for the changed bearer is sent to the server, so that the server performs control for changing the retransmission timeout time based on the option. I have to.
[0011]
According to the present invention as described above, during the session using the transport control protocol, the server adds control for changing the retransmission timeout time to the TCP session by sending a segment with an option to change the retransmission timeout time to the server. You will be able to do it inside. For this reason, since the server can perform control to change the retransmission timeout period during communication with the mobile device and can set the optimum retransmission timeout period, it is possible to prevent the communication line utilization efficiency from being lowered. it can. That is, it is possible to prevent the bandwidth of the wireless line from being used wastefully. In particular, even when the protocol located under the TCP is changed when the mobile device is handed over, it is possible to set the optimum retransmission timeout time for the changed protocol.
Furthermore, since retransmission control by TCP is performed between the mobile terminal and the server, an intermediate node for absorbing the difference between the retransmission control of TCP and the retransmission control of the second layer can be omitted.
[0012]
DETAILED DESCRIPTION OF THE INVENTION
FIG. 1 shows a schematic configuration of a mobile communication system according to an embodiment of the present invention.
In FIG. 1, a mobile device 1 is a mobile terminal comprising a mobile phone as an information terminal device, and can perform data communication with a web server 4 on the Internet via a mobile communication network. . The mobile communication network is composed of a radio access network (RAN) including a radio communication line between the mobile device 1 and the base station 2, and a core network including a wired communication line provided with various nodes. When the mobile device 1 accesses the web server 4, an HTML file or an image file is transferred from the web server 4 by starting up a browser and specifying and transmitting a URL (Uniform Resource Locators) of the web server 4. 1 is displayed back on the browser screen of the mobile device 1. Transmission and reception of HTML files and image files are performed using HTTP (Hypertext Transfer Protocol) located at the fifth layer (session layer) or higher of the OSI reference model.
[0013]
In this case, End-to-End between the mobile device 1 and the web server 4 is connected by a transport control protocol (TCP) located in the fourth layer (transport layer). Also, a circuit-switched bearer that supports circuit switching located in the second layer (data link layer) or a packet bearer that supports packet switching is between the mobile station 1 and the base station 2 and between the base station 2 and the switch 3. Connected by Further, the exchange 3 and the web server 4 are connected by a wired line such as Ethernet located in the second layer (data link layer).
[0014]
Next, characteristic points in the mobile communication system according to the embodiment of the present invention will be described with reference to the sequence diagram shown in FIG.
The sequence diagram shown in FIG. 2 is a sequence during a session after the mobile device 1 in the mobile communication system establishes a TCP connection with the web server 4. The mobile device 1 is connected by a circuit-switched bearer or a packet-switched bearer located in the lower layer of TCP, and the current bearer is indicated as bearer A. That is, the current TCP session is performed using bearer A. Here, it is assumed that the mobile device 1 moves and is handed over. At this time, the base station where the mobile device 1 is located is changed from the base station 2 to another base station. For this reason, the bearer used as shown in the figure is changed from the bearer A to the other bearer B. Further, the optimum value of the retransmission timeout time RTO differs depending on the type of bearer, that is, whether it is a circuit switched bearer or a packet switched bearer.
[0015]
Therefore, when the mobile device 1 detects that the bearer has been changed, the mobile device 1 adds a request for changing the value of the retransmission timeout time RTO using the option area to the segment and transmits it to the web server 4. In the example shown in FIG. 2, this segment is the ACK segment of the segment in the TCP session (bearer A). However, the segment is not limited to this, and any segment may be used as long as the mobile station 1 transmits during the session. . When the web server 4 receives the ACK segment 1 to which the RTO value change request option is added, the RTO value is changed so that the RTO value is optimum for the bearer changed according to the RTO change request. Next, the web server 4 transmits an ACK segment 2 indicating that the RTO value change request has been received to the mobile device 1. Thereby, in the subsequent TCP session (bearer B), retransmission control based on the RTO value optimum for the changed bearer B is performed.
[0016]
Here, the data structure of the TCP segment is shown in FIG. In the TCP segment shown in FIG. 3, up to options are used as TCP headers. The TCP header includes a transmission source port number (16 bits), a destination port number (16 bits), a sequence number (32 bits), an acknowledgment number (ACK number: 32 bits 9 and a header length (HLen). : 4 bits) and reserve (6 bits) fields, and 6 flag bits consisting of urgent flag (URG), ACK flag, PSH flag, RST flag, SYN flag, and FIN flag Here, when the ACK flag is “1”, ACK is valid, and when the SYN flag is “1”, it indicates that the sequence number field has an initial value. Further, when the FIN flag is set to “1”, it indicates that there is no more data to be sent, and a window field (16 for indicating the buffer amount to the other party). Bit), a checksum field (16 bits) used to detect errors in the header and data, an urgent pointer field (16 bits), and an optional field for setting an RTO value during the session in the present invention. The field following the TCP header is a data field.
[0017]
By the way, when the transmission source is the web server 4 and the destination is the mobile device 1, if the ACK corresponding to the segment transmitted from the web server 4 is not returned from the mobile device 1, the retransmission timer of the web server 4 When the RTO value set in Timeout expires, the segment is retransmitted. This RTO value is set according to the RTO value change request transmitted from the mobile device 1 to the web server 4 during the TCP session described above. Since the set RTO value is the optimum RTO value for the mobile communication system and the bearer to be used, it can be set to the optimum retransmission timeout time, so that the utilization efficiency of the communication line is reduced. Can be prevented. The mobile communication system may be a mobile communication system such as PDC (Personal Digital Cellular), WCDMA (Wideband Code Division Multiple Access), GSM (global system for mobile communication), or PHS (Personal Handy-phone System). .
[0018]
Further, when the transmission source is the mobile device 1 and the destination is the web server 4, if an ACK corresponding to the segment transmitted from the mobile device 1 is not returned from the web server 4, the retransmission timer of the mobile device 1 When the RTO value set in Timeout expires, the segment is retransmitted. This RTO value is a fixed RTO value held in the mobile device 1 and is an optimum RTO value according to the mobile communication system and the bearer to be used. As much as possible, it is possible to prevent the use efficiency of the communication line from being lowered.
As described above, even if TCP retransmission control is performed in the end-to-end between the mobile device 1 and the web server 4, it is possible to prevent the use efficiency of the communication line from being lowered. An intermediate node for absorbing the difference from the retransmission control of the second layer (data link layer) can be omitted.
[0019]
In the above description, when the bearer is changed, the segment to which the RTO value change request is added is sent during the TCP session. However, when the RTO value is to be changed, the segment to which the RTO value change request is added is sent. It may be. The RTO value change request may be information indicating the RTO value to be set or the RTO value to be set. For example, since the retransmission control mechanism is different for each bearer type, the optimum RTO value varies depending on the bearer type. The optimum value can be set in advance in the table or the like as the RTO value for each bearer type. Therefore, information indicating a specific RTO value on the table may be sent instead of the RTO value.
[0020]
In addition, while performing retransmission control of TCP in the end-to-end between the mobile device 1 and the web server 4, the retransmission control is performed in the circuit switched bearer or the packet bearer located in the second layer (data link layer). Also good.
If the ACK number for the retransmitted segment is not returned, retransmission is performed again. In this case, the retransmission timeout time is set to a fixed RTO value set in the retransmission timer. In other words, retransmission is performed repeatedly, for example, every 10 seconds until the preset maximum time is reached. However, when the first RTO value is 10 sec in consideration of the failure, an exponential back-off that sets the second RTO value to 20 sec and the third RTO value to 40 sec may be incorporated. Further, the RTO value may be increased by adding a predetermined value to the RTO value every time it is timed out.
[0021]
In the above description, the retransmission timeout time RTO to be held in the mobile device 1 is set to 10 sec. However, the present invention is not limited to this. This retransmission timeout period RTO is preferably set to an optimum value based on transmission characteristics such as round trip time (RTT) in the radio access network for each bearer to be used.
In the above description, TCP is used as the protocol of the fourth layer (transport layer) between the mobile device 1 and the web server 4. However, instead of TCP, transaction type T / TCP is used. Also good.
Furthermore, the mobile terminal is not limited to a mobile phone, and may be a mobile information terminal or the like. The server is not limited to a web server on the Internet, and may be a server on a network.
[0022]
【The invention's effect】
As described above, according to the present invention, during the session using the transport control protocol, by sending a segment with an option for changing the retransmission timeout time to the server, the server controls the change of the retransmission timeout time in the TCP session. You will be able to do it inside. For this reason, since the server can perform control to change the retransmission timeout period during communication with the mobile device and can set the optimum retransmission timeout period, it is possible to prevent the communication line utilization efficiency from being lowered. it can. That is, it is possible to prevent the bandwidth of the wireless line from being used wastefully. In particular, even when the protocol located under the TCP is changed when the mobile device is handed over, it is possible to set the optimum retransmission timeout time for the changed protocol.
Furthermore, since retransmission control by TCP is performed between the mobile terminal and the server, an intermediate node for absorbing the difference between the retransmission control of TCP and the retransmission control of the second layer can be omitted.
[Brief description of the drawings]
FIG. 1 is a diagram showing a schematic configuration of a mobile communication system according to an embodiment of the present invention.
FIG. 2 is a sequence diagram showing feature points in the mobile communication system according to the embodiment of the present invention.
FIG. 3 is a diagram showing a data structure of a TCP segment in the mobile communication system according to the embodiment of the present invention.
FIG. 4 is a sequence diagram showing connection establishment processing and data transmission processing using conventional TCP.
FIG. 5 is a sequence diagram showing retransmission control processing using conventional TCP.
FIG. 6 is a sequence diagram showing retransmission control in a conventional mobile communication system.
[Explanation of symbols]
1 mobile device, 2 base station, 3 exchange, 4 web server, 100 server, 101 client, 200 server, 201 mobile device, 202 intermediate node

Claims (1)

A mobile communication system capable of transferring data using a transport control protocol (TCP) between a mobile terminal in a radio access network and a server in a core network,
During a session using the transport control protocol between the mobile terminal and the server, when the mobile terminal is handed over and a bearer located under the transport control protocol is changed, the mobile terminal The server is configured to control the change of the retransmission timeout time based on the option by sending a segment with an option to change the retransmission timeout time suitable for the changed bearer to the server. Mobile communication system.

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