JP2009089197A - Repeating device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To improve communication performance of a communication system in which a packet is transferred using a repeating device. <P>SOLUTION: A repeating device 4 repeats a data packet transmitted from a communication apparatus 1 to a communication apparatus 2. When the data packet is discarded, the repeating device 4 produces a retransmission request packet and transmits it to the communication apparatus 1. In response to the retransmission request packet sent from the repeating device 4, the communication apparatus 1 retransmits the data packet. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a relay device that relays packets in a communication system.

  In general, in a network that performs packet communication such as the Internet, reachability of packets is not guaranteed in a network layer protocol (IP (Internet Protocol) in the Internet) that defines end-to-end communication. For this reason, reliable communication is realized by a transport layer protocol (TCP (Transport Control Protocol) in the Internet) which is an upper layer of the network layer.

  In IP, usually, end-to-end communication is realized by installing a plurality of relay devices (relay devices are also a type of communication device) between communication devices. However, the relay apparatus is not aware of information on protocols higher than IP. For this reason, reliability assurance and congestion control in the upper layer TCP are basically performed only between communication devices. That is, for example, when the data packet is discarded in the relay device, in TCP, the transmission side communication device retransmits the data packet. When the receiving communication device receives the data packet, an acknowledgment (ACK) packet is returned to the transmitting communication device as shown in FIG. Then, the communication device on the transmitting side confirms that the data packet has been normally received by using this ACK packet.

  However, the Internet is a very large network that is not centrally managed, and link latency and quality vary greatly. In addition, although IP enables network construction at a low cost by not guaranteeing packet reachability, TCP has a problem that link performance cannot be utilized to the maximum for communication control between communication devices. For example, it is known that the communication performance of TCP is greatly reduced in a network having a high link delay and a high packet discard rate.

  In TCP, communication control between communication apparatuses is mainly performed using a sequence number, an ACK number, a reception window size, a maximum segment size, a packet round trip time (RTT (Round Trip Time)), and the like. The sequence number represents the byte position of the transmission data, and the ACK number represents the sequence number expected to be received next.

  In TCP, when a packet discard occurs in the middle of a network path, the transmission side device retransmits the packet. For example, when the ACK packet does not arrive from the receiving side within a certain time, the transmitting side device retransmits the corresponding packet (retransmission by the timer). Alternatively, the transmission side apparatus retransmits the corresponding packet when receiving a predetermined number (for example, 3) or more of duplicate ACK packets from the reception side (high-speed retransmission). That is, the transmission side device determines whether to perform retransmission according to information obtained from the reception side.

  In TCP, since communication control is performed between communication devices (that is, between ends), if RTT increases, the time required from the occurrence of packet discard to the start of retransmission increases. Further, when retransmission occurs, the transmission side reduces the number of packets to be transmitted without waiting for an ACK packet. For this reason, when packet discard occurs on a link with a large RTT, the performance of TCP communication may be greatly degraded.

For example, Patent Documents 1 and 2 propose techniques related to retransmission when a packet is discarded, in addition to the functions provided by the TCP described above.
In the method described in Patent Document 1, when an overflow of the reception buffer is detected in the relay node, the payload of the received packet is discarded, and a loss notification packet including the header information of the packet and necessary information is generated and transmitted to the receiving terminal. It is done. Then, the retransmission procedure is started with this loss notification packet as a trigger.

In the method described in Patent Document 2, a function for realizing packet retransmission between a pair of relay apparatuses on a route is provided. The packet retransmission procedure performed between the relay apparatuses is basically the same as TCP.
JP 2003-198612 A JP 2004-241872 A

  In general TCP, as described above, since communication control is performed between ends, the time required from the occurrence of packet discard to the start of retransmission increases. Also, once the retransmission procedure occurs, the transmission side apparatus decreases the transmission rate without waiting for the ACK packet, so the communication performance deteriorates.

  In the method described in Patent Document 1, the amount of data between the relay node and the receiving terminal can be reduced when the network is congested. However, since the retransmission procedure itself is controlled between ends, it is the same as general TCP. Problems can arise. In the method described in Patent Document 2, it is necessary to provide two or more relay apparatuses having a retransmission function on the route. For this reason, in order to realize this method, the cost of the entire network becomes high.

Note that these problems do not occur only in TCP but may also occur in other communication protocols (for example, network layer protocols such as IP).
The subject of this invention is improving the communication performance of the communication system which transfers a packet using a relay apparatus.

  The relay device of the present invention relays a packet in a communication system. When the relay device discards the received packet, a determination unit that determines whether or not the received packet is discarded in the relay device, A generating unit that generates a retransmission request for requesting retransmission of the discarded packet; and a requesting unit that transmits the retransmission request to a transmission source of the received packet.

  When a packet is discarded in the relay device, a retransmission request is sent from the relay device to the transmission terminal before the packet loss is detected in the reception terminal. Therefore, the time from when the packet is discarded until the retransmission procedure is started is shortened.

  A relay apparatus according to another aspect of the present invention relays a packet in a communication system, and includes a cache unit that holds a packet to be transferred to a transmission destination, and a transmission unit that transmits the packet to be transferred to the transmission destination. And a retransmission means for taking out and retransmitting the corresponding packet from the cache means when a retransmission request is received.

  When a retransmission request is generated due to packet loss or the like in the communication system, the relay device performs packet retransmission on behalf of the transmitting terminal. Therefore, this configuration also shortens the time until the retransmission procedure is started.

  ADVANTAGE OF THE INVENTION According to this invention, the communication performance of the communication system which transfers a packet using a relay apparatus improves.

  Embodiments of the present invention will be described with reference to the drawings. The relay apparatus according to the embodiment is used in a communication system that transfers packets. In the following, the communication system according to the embodiment transfers a packet by TCP / IP between ends as an example. However, the present invention is not limited to this, and the network does not guarantee the reachability of the packet. Applicable.

  FIG. 1 is a diagram illustrating a basic configuration of a relay apparatus that relays packets. In FIG. 1, a network interface 11 transmits and receives packets to and from other relay devices or terminal devices. Packets received via the network interface 11 are temporarily held in the reception packet buffer 12. The packet transfer processing unit 13 refers to the routing table 14 using the header information of the received packet, and writes the packet in the transmission packet buffer 15 connected to the network interface 11 corresponding to the transmission destination. The route table 14 is a routing table, for example. Then, the packet written in the transmission packet buffer 15 is read and transmitted by the network interface 11.

  Here, in a known general relay device, packet control is performed in the network layer (IP in this embodiment). For this reason, a known general relay apparatus does not directly relate to control for retransmitting a packet when the packet is discarded on the route. On the other hand, the relay apparatus according to the embodiment provides a function for retransmitting a packet when the packet is discarded on the route.

  FIG. 2 is a diagram illustrating a basic configuration of the relay apparatus according to the embodiment. The relay apparatus according to the embodiment includes a retransmission processing unit 16 in addition to the network interface 11, the reception packet buffer 12, the packet transfer processing unit 13, the route table 14, and the transmission packet buffer 15.

  The retransmission processing unit 16 determines whether packet retransmission is necessary. When it is determined that it is necessary to retransmit a certain packet, a retransmission request is transmitted to the transmission source of the packet. In addition, when receiving a retransmission request for a packet, the retransmission processing unit 16 performs the retransmission process itself.

<First Embodiment>
FIG. 3 is a diagram for explaining the outline of the retransmission procedure according to the first embodiment. Here, a communication system is assumed in which a plurality of relay devices are provided between a communication device 1 that is a data packet transmission source terminal and a communication device 2 that is a data packet transmission destination terminal.

  The relay device 3 is a relay device that processes a packet in the network layer, and does not directly relate to packet retransmission control. On the other hand, the relay device 4 is a relay device including the retransmission processing unit 16 illustrated in FIG. 2 and provides a function related to packet retransmission. The communication system according to the first embodiment includes one or a plurality of relay devices 4 on the path between the communication devices 1 and 2.

  In the communication system, it is assumed that the data packet transmitted from the communication device 1 is discarded in the relay device 4. In this case, the relay device 4 generates a retransmission request packet and returns it to the communication device 1. Then, the communication device 1 retransmits the previously transmitted data packet according to the retransmission request packet. As a result, this data packet is received by the communication device 2.

FIG. 4 is a sequence diagram illustrating a retransmission procedure according to the first embodiment. Here, the relay device 3 is omitted.
The communication device 1 transmits a TCP data packet using the communication device 2 as a transmission destination address. The relay device 4 normally transfers the TCP data packet to the communication device 2. However, for example, when the network is congested, the received TCP data packet is discarded without being transferred to the communication device 2. Then, when discarding the TCP data packet, the relay device 4 generates a retransmission request packet and sends it to the communication device 1. When receiving the retransmission request packet, the communication device 1 executes a retransmission process. As a result, the previously transmitted TCP data packet is retransmitted. The relay device 4 transfers the TCP data packet to the communication device 2 if the network is not congested.

  When receiving the TCP data packet, the communication device 2 returns a corresponding ACK packet. This ACK packet is relayed by the relay device 4 and received by the communication device 1. As a result, the communication device 1 recognizes that the transmitted TCP data packet has been received by the communication device 2.

  As described above, in the communication system according to the first embodiment, a retransmission request is generated by the relay device 4 provided between the communication devices 1 and 2, and the retransmission procedure is executed. That is, before the packet discard is detected at the transmission destination terminal, the packet discard is detected at the relay device, and the retransmission procedure is started. For this reason, the time required from the occurrence of packet discard to the start of retransmission is shorter than in the conventional configuration in which retransmission is controlled between ends by TCP. Further, the retransmission function according to the first embodiment can be realized by providing only one relay device 4 including the retransmission processing unit 16 on the path. That is, the retransmission function according to the first embodiment is realized relatively easily and at low cost. This effect is particularly remarkable when a part of the conventional relay device 3 is replaced with the relay device 4 of the embodiment on the existing network.

  FIG. 5 is a diagram illustrating a configuration of the relay device 4 according to the first embodiment. In FIG. 5, packets (including data packets and ACK packets) received via the network interface unit 11 are written in the received packet buffer 12. Further, the network interface unit 11 reads out the packet stored in the transmission packet buffer 15 and sends it out to the network. In FIG. 5, the route table 14 is omitted.

  The retransmission processing unit 16 includes a session information recording unit 21, a session information table 22, a packet discard detection unit 23, and a retransmission request packet generation unit 24. The session information recording unit 21 extracts session information from the header of each packet held in the reception packet buffer 12 and writes it in the session information table 22. At this time, the session information recording unit 21 refers to the IP header and the TCP header. The format of the IP header is shown in FIG. The format of the TCP header is shown in FIG. In this embodiment, “source IP address”, “destination IP address”, “protocol type information”, “source port number”, “destination port number”, “sequence number”, and “ACK number” are extracted as session information. The

  FIG. 8 is an example of the session information table 22. The session information table 22 manages “latest sequence number”, “latest ACK number”, and “number of duplicate ACKs received” for each session. Here, each session is identified by a combination of “transmission source IP address”, “transmission destination IP address”, “protocol type information”, “transmission source port number”, and “transmission destination port number”. A session identified by the combination of “source IP address”, “destination IP address”, “protocol type information”, “source port number”, and “destination port number” of the newly received packet is displayed in the session information table 22. If not registered, a new record is added.

  “Sequence number” is information indicating the position of transmission data in units of bytes, and is basically given by the transmission source terminal of the TCP data packet. The “ACK number” is information indicating the sequence number expected to be received next, and is basically given by the destination terminal of the TCP data packet. In the session information table 22, “sequence number” and “ACK number” are updated to values written in the TCP header of the latest received packet.

  In this embodiment, it is assumed that the duplicate ACK function is adopted. That is, when a receiving device detects a loss or an error for a certain packet, the receiving device returns a response packet with the same ACK number assigned to the subsequent packet until the packet is normally received. In this case, the transmission side apparatus starts a retransmission procedure when the same ACK number is detected a predetermined number of times (for example, 3). If the ACK number of the received packet is duplicated, the session information recording unit 21 counts the number of times and records it in the session information table 22.

  Returning to FIG. The packet discard detection unit 23 detects packet discard in the relay device. That is, in this relay apparatus, when the usage rate of the transmission packet buffer 15 exceeds a preset threshold, it is considered that a congestion state has occurred (or there is a possibility that congestion will occur), and the transmission packet Is discarded. When the packet discard detection unit 23 detects packet discard, the packet discard detection unit 23 notifies the retransmission request packet generation unit 24 to that effect.

  Upon receiving the notification from the packet discard detection unit 23, the retransmission request packet generator 24 generates a retransmission request packet corresponding to the discarded packet. The transmission destination of this retransmission request packet is the transmission source terminal of the discarded packet. Here, the retransmission request packet is specified based on information such as an IP header address field, a protocol field, a TCP header port number field, a sequence number field, and an ACK number field. The retransmission request packet is realized by, for example, a TCP duplicate ACK, a TCP selection ACK, and a TCP NAK option. A selective ACK (SACK) is a message for notifying which packet is being delivered in detail, is defined in RFC2018, and is realized by using an option field of a TCP header. The retransmission request packet is not limited to these forms, and may be realized by notification by ICPM, for example.

The operation of the relay apparatus according to the first embodiment having the above-described configuration is as follows. That is, the relay device 4 executes the following process every time it receives a data packet from the communication device 1 on the transmission side. (1) Extract and record session information necessary for generating a retransmission request packet from the received data packet. This process is performed by the session information recording unit 21. The session information is written in the session information table 22.
(2) It is detected whether or not a data packet is discarded due to a buffer overflow. This process is performed by the packet discard detection unit 23. When the data packet is discarded, the retransmission request packet generator 24 generates a retransmission request packet for requesting retransmission of the discarded data packet. The generated retransmission request packet is transmitted to the communication device 1 on the transmission side by the packet transfer processing unit 13. On the other hand, when the data packet is not discarded, the packet transfer processing unit 13 transmits the data packet to the communication device 2 on the receiving side.

The relay device 4 executes the following process every time it receives an ACK packet from the communication device 2 on the receiving side.
(1) Session information is extracted from the received ACK packet and recorded. This process is performed by the session information recording unit 21 in the same manner as when a data packet is received.
(2) The received ACK packet is transmitted to the communication device 1 on the transmission side.

  By performing the above processing, retransmission is immediately urged from the relay apparatus 4 that has detected the packet discard to the communication apparatus 1 on the transmission side, and the retransmission procedure is quickly started. Therefore, particularly in a network with a large RTT, the performance of TCP communication is greatly improved. Further, since the relay device 4 operates transparently with respect to the existing TCP protocol, it is effective for the communication device 1 on the transmission side and the communication device 2 on the reception side without requiring a change of the TCP protocol. .

  In the above-described embodiment, the packet discard is detected based on the overflow of the buffer provided in the relay device 4, but the present invention is not limited to this. That is, for example, the relay device 4 may detect a packet loss on the route based on the sequence number and transmit the retransmission request packet.

<Second Embodiment>
FIG. 9 is a diagram illustrating an outline of a retransmission procedure according to the second embodiment. In FIG. 9, the relay device 5 is a relay device according to the second embodiment of the present invention. The communication system according to the second embodiment includes one or a plurality of relay devices 5 on the path between the communication devices 1 and 2.

  In the above communication system, the relay device 5 copies the data packet and holds it in the packet cache 6 when relaying the data packet transmitted from the communication device 1. Thereafter, when the data packet is discarded on the route, the communication device 2 transmits a retransmission request packet toward the communication device 1. However, the retransmission request packet is captured by the relay device 5. Then, the relay device 5 extracts the corresponding data packet from the packet cache 6 in response to the retransmission request packet, and transmits the data packet to the communication device 2. As a result, this data packet is received by the communication device 2.

FIG. 10 is a sequence diagram illustrating a retransmission procedure according to the second embodiment. Here, the relay device 3 is omitted.
The communication device 1 transmits a TCP data packet using the communication device 2 as a transmission destination address. The relay device 5 transfers the TCP data packet to the communication device 2, copies the TCP data packet, and holds it in the packet cache 6. Thereafter, it is assumed that the TCP data packet is discarded on the path between the relay device 5 and the communication device 2. Then, the communication device 2 detects that the TCP data packet has been discarded, generates a retransmission request packet, and transmits it to the communication device 1. The retransmission request packet is captured by the relay device 5. When the relay apparatus 5 receives the retransmission request packet, the relay apparatus 5 executes a retransmission process. That is, the TCP data packet corresponding to the retransmission request packet is taken out from the packet cache 6 and the TCP data packet is transmitted to the communication device 2.

  When receiving the TCP data packet, the communication device 2 returns a corresponding ACK packet. This ACK packet is relayed by the relay device 5 and received by the communication device 1. As a result, the communication device 1 recognizes that the transmitted TCP data packet has been received by the communication device 2.

As described above, in the communication system according to the second embodiment, when a data packet is discarded between the relay device 5 and the communication device 2 that is the reception-side terminal, the communication device 1 that is the transmission-side terminal is replaced. At the same time, the relay device 5 executes the retransmission process. For this reason, as compared with the conventional configuration in which retransmission is controlled between ends by TCP, the time required from the occurrence of packet discard to the start of retransmission is shortened. Further, since the transmission side terminal does not receive the retransmission request, the transmission rate does not decrease and the communication performance does not decrease. Furthermore, the retransmission function according to the second embodiment can be realized by providing only one relay device 5 including the retransmission processing unit 16 on the path, as in the first embodiment.

  FIG. 11 is a diagram illustrating a configuration of the relay device 5 according to the second embodiment. The basic configuration of the relay device 5 of the second embodiment is the same as that of the relay device 4 of the first embodiment described above. However, in the first and second embodiments, the configuration of the retransmission processing unit 16 is different from each other.

  The retransmission processing unit 16 of the relay device 5 according to the second embodiment includes a session information recording unit 21, a session information table 22, a packet cache processing unit 25, and a retransmission request packet detection unit 26. Here, since the session information recording unit 21 and the session information table 22 are the same as those described in the first embodiment, description thereof is omitted. The packet cache processing unit 25 corresponds to the packet cache 6 shown in FIG.

  The packet cache processing unit 25 includes a memory and a circuit that controls writing / reading of the memory, and copies a packet relayed by the relay device 5 and stores it for a certain period. At this time, the packet is stored in association with the session information so as to be searchable. Note that the packet cache processing unit 25 may store all the packets, store only the data packets, or store only some of the packets.

  The retransmission request packet detector 26 detects a retransmission request packet from the received packets. When the retransmission request packet is detected, the corresponding packet is extracted from the memory of the packet cache processing unit 25 using the session information. Then, the extracted packet is transmitted.

The operation of the relay device of the second embodiment having the above configuration is as follows. That is, the relay device 5 executes the following process every time it receives a data packet from the communication device 1 on the transmission side. (1) Extract and record session information necessary for generating a retransmission request packet from the received data packet. This process is performed by the session information recording unit 21. The session information is written in the session information table 22.
(2) If the packet cache buffer is empty, the data packet is copied and stored. (3) The data packet is transmitted to the communication device 2 on the receiving side.

The relay device 5 executes the following process every time it receives an ACK packet from the communication device 2 on the receiving side.
(1) Session information is extracted from the received ACK packet and recorded. This process is performed by the session information recording unit 21 in the same manner as when a data packet is received. However, when duplicate ACK is adopted, the number of duplicate ACKs is monitored. When the number of duplicate ACKs reaches a predetermined threshold, the same transmission process as when a retransmission request packet is received is performed.
(2) The received ACK packet is transmitted to the communication device 1 on the transmission side.

Furthermore, every time the relay apparatus 5 receives a retransmission request packet from the receiving-side communication apparatus 2, the relay apparatus 5 executes the following processing.
(1) Using the session information of the retransmission request packet, it is searched whether or not the data packet corresponding to the request is stored in the packet cache buffer.
(2) When a corresponding data packet is found, the data packet read from the packet cache buffer is transmitted to the communication device 2 on the receiving side. In this case, the retransmission request packet is not relayed to the communication device 1 on the transmission side.
(3) When the corresponding data packet is not found, the received retransmission request packet is transmitted to the communication device 1 on the transmission side.

  By performing the above processing, the data packet can be retransmitted in the relay device 5 that has detected the retransmission request packet. However, since unlimited retransmissions may promote congestion, the data packet transmission rate may be adjusted as necessary. Further, the relay device 5 operates transparently with respect to the existing TCP protocol, and does not require a change of the TCP protocol. The packet stored in the packet cache buffer may be periodically deleted based on the buffer free capacity, the maximum lifetime (TTL (Time To Live)), session information, and the like.

<Third Embodiment>
FIG. 12 is a sequence diagram illustrating a retransmission procedure according to the third embodiment. Here, the relay device 7 (7A, 7B) is provided on the path between the communication devices 1 and 2. The relay device 7 has the retransmission function of the relay device 4 of the first embodiment and the retransmission function of the relay device 5 of the second embodiment. In FIG. 12, other relay devices on the route are omitted.

  The communication device 1 transmits a TCP data packet using the communication device 2 as a transmission destination address. The relay device 7A transfers the TCP data packet to the communication device 2, and copies the TCP data packet and holds it in the packet cache. Thereafter, it is assumed that the TCP data packet is discarded in the relay device 7B. Then, the relay device 7B generates a retransmission request packet and transmits it to the communication device 1. This retransmission request packet is captured by the relay device 7A. When the relay device 7A receives the retransmission request packet, the relay device 7A executes a retransmission process. That is, the TCP data packet corresponding to the retransmission request packet is taken out from the packet cache, and the TCP data packet is transmitted to the communication device 2. The relay device 7B caches the TCP data packet and transfers it to the communication device 2.

  When receiving the TCP data packet, the communication device 2 returns a corresponding ACK packet. The ACK packet is relayed by the relay devices 7B and 7A and received by the communication device 1. As a result, the communication device 1 recognizes that the transmitted TCP data packet has been received by the communication device 2.

  In the example shown in FIG. 12, two relay devices 7 (7A, 7B) are provided on one route, and the retransmission processing is completed by these two relay devices 7. However, only one relay device 7 may be provided on one route. However, in this case, the retransmission request packet generated by the relay device 7 is received by the transmission terminal, and the transmission terminal retransmits the packet. The retransmission request packet received by the relay device 7 is generated by the receiving terminal.

  FIG. 13 is a diagram illustrating a configuration of the relay device 7 according to the third embodiment. As described above, the relay device 7 of the third embodiment has the function of the relay device 4 of the first embodiment and the function of the relay device 5 of the second embodiment. That is, the retransmission processing unit 16 of the relay device 7 includes a session information recording unit 21, a session information table 22, a packet discard detection unit 23, a retransmission request packet generation unit 24, a packet cache processing unit 25, and a retransmission request packet detection unit 26. .

The operation of the relay device of the third embodiment having the above-described configuration is as follows. That is, the relay device 7 executes the following process every time it receives a data packet from the communication device 1 on the transmission side. (1) Extract and record session information necessary for generating a retransmission request packet from the received data packet.
(2) It is detected whether or not the data packet is discarded due to the buffer overflow. When discarding the data packet, a retransmission request packet is generated and transmitted to the communication device 1 on the transmission side.
(3) If the data packet is not discarded, the data packet is transmitted to the communication device 2 on the receiving side. At this time, if there is an empty packet cache buffer, the data packet is copied and stored.

The relay device 7 executes the following process every time an ACK packet is received from the receiving-side communication device 2.
(1) Session information is extracted from the received ACK packet and recorded. However, when the number of duplicate ACKs reaches a predetermined threshold, the same transmission process as when a retransmission request packet is received is performed.
(2) The received ACK packet is transmitted to the communication device 1 on the transmission side.

Further, each time the relay device 7 receives a retransmission request packet from the receiving-side communication device 2 or another relay device 7, the relay device 7 executes the following processing.
(1) It is searched whether or not the data packet corresponding to the retransmission request packet is stored in the packet cache buffer.
(2) When a corresponding data packet is found, the data packet read from the packet cache buffer is transmitted to the communication device 2 on the receiving side. In this case, the retransmission request packet is not relayed to the communication device 1 on the transmission side.
(3) When the corresponding data packet is not found, the received retransmission request packet is transmitted to the communication device 1 on the transmission side.

<Flowchart>
FIG. 14 is a flowchart illustrating an operation when the relay apparatus according to the embodiment receives a data packet. The process of this flowchart is executed by the retransmission processing unit 16 every time a data packet is received.

  In step S1, it is checked whether or not the target data packet is discarded. This check is determined, for example, based on the usage rate of the transmission packet buffer 15. If this data packet is discarded, a corresponding retransmission request packet is generated in step S2. In step S3, the retransmission request packet is transmitted to the transmission side of the data packet.

  On the other hand, if the target data packet is not discarded, the session information is collected and recorded in the session information table 22 in step S4. In step S5, the target data packet is copied and stored in the packet cache. In step S6, the data packet is transmitted to the receiving side.

Note that steps S1 to S4 and S6 are executed in the first embodiment, steps S4 to S6 are executed in the second embodiment, and steps S1 to S6 are executed in the third embodiment.
FIG. 15 is a flowchart illustrating an operation when the relay apparatus according to the embodiment receives a response packet. The process of this flowchart is executed by the retransmission processing unit 16 every time a response packet is received. The response packet includes a normal ACK packet and a retransmission request packet.

  In step S11, it is checked whether or not the response packet is a retransmission request packet. If the response packet is a retransmission request packet, it is checked in step S12 whether the requested data packet is stored in the packet cache. If the requested data packet is stored in the packet cache, the data packet is extracted from the packet cache and transmitted to the receiving communication terminal in step S13.

On the other hand, if the requested data packet is not stored in the packet cache, the session information of the response packet is collected and recorded in the session table 22 in step S14. In step S15, the response packet is transmitted to the communication device on the transmission side.

In the first embodiment, steps S14 to S15 are executed, and in the second and third embodiments, steps S11 to S15 are executed.
<Other embodiments>
In the first to third embodiments described above, the description has been made on the assumption of TCP, but the present invention is not limited to this. That is, the present invention is applicable to, for example, an IP network that does not support TCP. However, in an IP network, the retransmission request function to the communication device on the transmission side cannot be applied. Further, the retransmission request packet takes a notification (enhancement according to the present invention) method using ICMP. Further, the retransmission packet is specified based on information such as an IP header address field, a protocol field, an IP identifier field, and a transport header port number field.

(Appendix 1)
A relay device that relays packets in a communication system,
Determining means for determining whether or not to discard the received packet in the relay device;
Generating means for generating a retransmission request for requesting retransmission of the discarded packet when the received packet is discarded in the relay device;
Request means for transmitting the retransmission request to a transmission source of the received packet;
A relay device.

(Appendix 2)
The relay device according to attachment 1, wherein
It further comprises acquisition means for acquiring session information for each received packet,
The relay device according to claim 1, wherein the generation unit generates a retransmission request based on the session information.

(Appendix 3)
The relay device according to attachment 1, wherein
Provided with a transmission buffer that temporarily holds received packets when they are forwarded to the destination.
The determination unit determines whether or not to discard a received packet in the relay device based on a usage rate of the transmission buffer.

(Appendix 4)
A relay device that relays packets in a communication system,
Cache means for holding packets to be transferred to the destination;
Transmitting means for transmitting the packet to be transferred to a transmission destination;
A retransmission unit that, when receiving a retransmission request, retrieves and retransmits the corresponding packet from the cache unit;
A relay device.

(Appendix 5)
The relay device according to attachment 4, wherein
It further comprises acquisition means for acquiring session information for each received packet,
The relay apparatus, wherein the retransmission means extracts a packet to be retransmitted from the cache means based on the session information.

(Appendix 6)
The relay device according to attachment 4, wherein
Analyzing means for receiving and analyzing a response packet corresponding to the packet transmitted by the transmitting means;
If the response packet is a retransmission request, the retransmission means performs packet retransmission, and if the response packet is not a retransmission request, the response packet is transferred according to header information of the response packet.

(Appendix 7)
A relay device that relays packets in a communication system,
Determining means for determining whether or not to discard the received packet in the relay device;
Generating means for generating a retransmission request for requesting retransmission of the discarded packet when the received packet is discarded in the relay device;
Request means for transmitting the retransmission request to a transmission source of the received packet;
Transmitting means for transmitting to the destination when not discarding the received packet;
Cache means for holding a packet to be transmitted by the transmission means to a transmission destination;
A retransmission unit that, when receiving a retransmission request, retrieves and retransmits the corresponding packet from the cache unit;
A relay device.

It is a figure which shows the basic composition of a relay apparatus. It is a figure which shows the basic composition of the relay apparatus of embodiment. It is a figure explaining the outline | summary of the resending procedure of 1st Embodiment. It is a sequence diagram explaining the resending procedure of 1st Embodiment. It is a figure which shows the structure of the relay apparatus of 1st Embodiment. It is a figure which shows the format of an IP header. It is a figure which shows the format of a TCP header. It is an Example of a session information table. It is a figure explaining the outline | summary of the resending procedure of 2nd Embodiment. It is a sequence diagram explaining the resending procedure of 2nd Embodiment. It is a figure which shows the structure of the relay apparatus of 2nd Embodiment. It is a sequence diagram explaining the resending procedure of 3rd Embodiment. It is a figure which shows the structure of the relay apparatus of 3rd Embodiment. It is a flowchart which shows operation | movement when a data packet is received. It is a flowchart which shows operation | movement when a response packet is received. It is a figure explaining TCP communication.

Explanation of symbols

1 Communication device (data packet sender)
2 Communication device (data packet destination)
3 relay device 4 relay device (first embodiment)
5. Relay device (second embodiment)
6 Packet cache 7 Relay device (third embodiment)
DESCRIPTION OF SYMBOLS 11 Network interface 12 Reception packet buffer 13 Packet transfer process part 14 Path | route table 15 Transmission packet buffer 16 Retransmission process part 21 Session information recording part 22 Session information table 23 Packet discard detection part 24 Retransmission request packet generation part 25 Retransmission request packet detection part 26 Packet cache processor

Claims (5)

A relay device that relays packets in a communication system,
Determining means for determining whether or not to discard the received packet in the relay device;
Generating means for generating a retransmission request for requesting retransmission of the discarded packet when the received packet is discarded in the relay device;
Request means for transmitting the retransmission request to a transmission source of the received packet;
A relay device. The relay device according to claim 1,
It further comprises acquisition means for acquiring session information for each received packet,
The relay device according to claim 1, wherein the generation unit generates a retransmission request based on the session information. The relay device according to claim 1,
Provided with a transmission buffer that temporarily holds received packets when they are forwarded to the destination.
The determination unit determines whether or not to discard a received packet in the relay device based on a usage rate of the transmission buffer. A relay device that relays packets in a communication system,
Cache means for holding packets to be transferred to the destination;
Transmitting means for transmitting the packet to be transferred to a transmission destination;
A retransmission unit that, when receiving a retransmission request, retrieves and retransmits the corresponding packet from the cache unit;
A relay device. A relay device that relays packets in a communication system,
Determining means for determining whether or not to discard the received packet in the relay device;
Generating means for generating a retransmission request for requesting retransmission of the discarded packet when the received packet is discarded in the relay device;
Request means for transmitting the retransmission request to a transmission source of the received packet;
Transmitting means for transmitting to the destination when not discarding the received packet;
Cache means for holding a packet to be transmitted by the transmission means to a transmission destination;
A retransmission unit that, when receiving a retransmission request, retrieves and retransmits the corresponding packet from the cache unit;
A relay device.

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