Abstract:
A relay device for relaying a packet communication between communication devices; comprising: communication quality parameter conversion means for converting a communication quality parameter described in a received quality information notification packet into a communication quality parameter measured by the relay device from a received data packet.

Description:
INCORPORATION BY REFERENCE 
       [0001]    This application is based upon and claims the benefit of priority from Japanese patent application No. 2007-264524, filed on Oct. 10, 2007, the disclosure of which is incorporated herein in its entirety by reference. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to a relay device including a quality information notification function capable of detecting or measuring a communication quality and capable of notifying a transmitting device of a detected or measured communication quality parameter in a communication system in which one terminal device transmits a packet to another terminal device and the relay device is disposed on a communication section between the terminal devices, and a communication system including the relay device, and a relay method for the relay device. 
         [0004]    2. Description of the Related Art 
         [0005]    In real-time media communication, it can be said that ideal communication is such that each data packet continuously transmitted from a transmitting device is transferred to a receiving device without exceeding preset certain transmission time and without packet loss. Further, a “communication quality parameter” such as a transmission delay, a jitter or a discard rate for data packets on a communication section generated to follow transmission of real-time data has a great influence on reproducing of data in a receiving device. 
         [0006]    The real-time media communication uses RTP (Real-time Transport Protocol)/RTCP (RTP Control Protocol) as a communication protocol to manage the communication quality parameters. Research and development of a method of measuring communication quality parameters, a communication method control method and the like are underway in various institutions. 
         [0007]    As related techniques, a technique for causing a transmitting device to adjust a bit rate of transmission data and adjust an error correcting level based on feedback information including a jitter or a loss rate for reception packets and measured by a receiving device is disclosed in Japanese Patent Application Laid-Open No. 2002-204278. With this technique, the receiving device notifies the transmitting device of a communication quality parameter measured from a data packet reception state, whereby the transmitting device can optimize a data packet communication method. As a result, the receiving device reproduces real-time media with high quality. 
         [0008]    However, if mobile terminals are included in transmitting devices or receiving devices transmitting or receiving real-time media, wired sections and wireless sections such as access points and wireless base stations are normally used simultaneously on a communication path. In this respect, degradation in transmission quality particularly due to transmission error has greater influence particularly on the wireless sections than the wired sections. Due to this, it is desired to ensure high communication quality specialized in the wireless sections. 
         [0009]    However, with the technique disclosed in the Japanese Patent Application Laid-Open No. 2002-204278, the communication parameter such as packet discard rate, the jitter or the packet delay time measured by the receiving device has a value integrally estimating the entire communication path without discrimination between the two different types of sections, i.e., the wired sections and the wireless sections on the communication path from the transmitting device to the receiving device. Due to this, a communication method specialized in the wireless sections is not optimized only by causing the transmitting device to control the communication method based on the communication quality parameters. 
         [0010]    In view of these problems, a pamphlet of International Patent Application Publication No. 2005/027394 discloses a transmission technique for causing each wireless base station disposed at a boundary point between one wired section and one wireless section to calculate a communication quality parameter in the wireless section and to use a communication method appropriate for the wireless section based on the calculated communication quality parameters. 
         [0011]    A content of the communication quality parameter disclosed in the pamphlet of the International Patent Application Publication No. 2005/027394 includes a packet discard rate, a jitter, packet delay time or the like. Examples of the communication method include a data packet retransmission count control, an error correction level control, and a transmission priority control. 
         [0012]    With the technique disclosed in the pamphlet of the International Patent Application Publication No. 2005/027394, the wireless base station relays an RTP packet from a transmitting device, thereby measuring a communication parameter in the wired section from the transmitting device to the wireless base station. Furthermore, the wireless base station refers to contents of an RTCP packet fed back from a receiving-side mobile terminal (terminal device) to the transmitting device, thereby grasping integral communication quality parameter in the wired section and the wireless section from the transmitting device to the receiving-side mobile terminal. The wireless base station calculates a difference between the integral communication quality parameter in the wired section and the wireless section and those in the wired section, thereby making it possible to calculate a communication quality parameter in the wireless section. As a result, the wireless base station can use an appropriate communication method for the wireless section according to the calculated current communication quality in the wireless section. Namely, by using the technique disclosed in the pamphlet of the International Patent Application Publication No. 2005/027394, a receiving device can receive a data packet by an appropriate communication method specialized in the wired section if the receiving device receives the data packet from the wireless base station. 
         [0013]    By using the above-stated conventional technique, the wireless base station can grasp the communication quality parameter in the wireless section from the wireless base station to the receiving-side terminal device. Due to this, the wireless base station can transmit a packet to the receiving-side terminal device using the appropriate communication method for the wireless section. However, no considerations are given to a method for causing the receiving-side terminal device to transmit a packet using an appropriate communication method for a wireless section opposite in a direction to the wireless section from the wireless base station to the receiving-side terminal device. 
         [0014]    Due to this, with the conventional technique, a transmitting-side terminal device cannot transmit a packet using an appropriate communication method for a wireless section from the transmitting-side terminal device to a relay device such as the wireless base station in a communication system in which a packet is transmitted from the transmitting-side terminal device to a receiving-side terminal device and the relay device is disposed on a communication section between the transmitting-side terminal device and the receiving-side terminal device. This follows that a packet cannot be transmitted using an appropriate communication method for a communication section from a transmitting-side terminal device to a relay device whether the communication section is wired or wireless. This problem is not limited to an instance in which a transmitting-side communication device is a terminal device. A packet cannot be transmitted using an appropriate communication method for a communication section from a certain relay device to each of all other relay devices if the certain relay device transmits (or transfers) the packet to each of all the other relay devices. 
         [0015]    Examples of the relay device include a wireless base station, a public wireless LAN, a router, and a gateway. Examples of the terminal device include a portable telephone, a PDA (Personal Digital Assistance), and a PC (Personal Computer). 
         [0016]    Therefore, the problem to be solved by the present invention is that an appropriate communication method for a communication section between two communication devices out of all communication devices except for a receiving-side terminal device if a packet is transmitted (transferred) between the two communication devices. 
       SUMMARY OF THE INVENTION 
       [0017]    It is, therefore, an object of the present invention to provide a communication device including a quality information notification function capable of acquiring a communication quality parameter on a communication section between two communication devices out of all communication devices except for a receiving-side terminal device, that is, a communication section from a communication device transmitting (or transferring) a packet to a communication device receiving the packet if the packet is transmitted (or transferred) between the two communication devices in a communication system in which each terminal device transmits a packet to another terminal device and a relay device is disposed on a communication section between the terminal devices, and the communication system including the communication device, and a relay method for the communication device. 
         [0018]    According to the present invention, there is provided a relay device for relaying a packet communication between communication devices; comprising: communication quality parameter conversion means for converting a communication quality parameter described in a received quality information notification packet into a communication quality parameter measured by the relay device from a received data packet. 
         [0019]    In addition, according to the present invention, there is provided a communication device transmitting and receiving packets, comprising a function as the relay device according to claim  1 , and further comprising:transmission means for transmitting a data packet using a transmission method according to a communication interval based on the communication quality parameter. 
         [0020]    Further, according to the present invention, there is provided a communication system in a network, comprising a transmission device and a receiving device communicating a data packet and a communication quality notification packet therebetween, further comprising: the relay device according to claim  1 , wherein the relay device relays the data packet and the communication quality notification packet. 
         [0021]    Still further, there is provided a relay method for relaying a packet communication between communication methods; comprising: a communication quality parameter conversion step of converting a communication quality parameter described in a received quality information notification packet into a communication quality parameter measured by the relay device from a received data packet. 
         [0022]    Further more, there is provided a relay program for relaying a packet communication between communication devices causing a computer to execute: a communication quality parameter conversion function of converting a communication quality parameter described in a received quality information notification packet into a communication quality parameter measured by the relay device from a received data packet. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0023]      FIG. 1  is a configuration diagram of a communication system according to a first embodiment of the present invention; 
           [0024]      FIG. 2  is an internal block diagram of a relay device including a communication quality notification function according to the first embodiment of the present invention; 
           [0025]      FIG. 3  is a sequence diagram showing a sequence for a data packet transmission operation from a transmitting device to a receiving device according to the first embodiment of the present invention; 
           [0026]      FIG. 4  is a sequence diagram showing a sequence for a feedback information packet transmission operation from the receiving device to the transmitting device according to the first embodiment of the present invention; 
           [0027]      FIG. 5  is a block diagram of a communication system performing real-time media streaming according to a second embodiment of the present invention; 
           [0028]      FIG. 6  is a sequence diagram showing a sequence for a data packet transmission operation from a transmitting device to a receiving device according to the second embodiment of the present invention; 
           [0029]      FIG. 7  is a sequence diagram showing a sequence for a feedback information packet transmission operation from the receiving device to the transmitting device according to the second embodiment of the present invention; and 
           [0030]      FIG. 8  is a sequence diagram of an operation sequence for transmitting an information packet according to the second embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0031]    Embodiments of the present invention will be described hereinafter with reference to the accompanying drawings. 
       First Embodiment  
       [0032]      FIG. 1  is a configuration diagram showing a configuration of a communication system according to a first embodiment of the present invention. 
         [0033]    As shown in  FIG. 1 , the communication system according to the first embodiment of the present invention includes a network  100 , a network  101 , a transmitting device  102 , a receiving device  103 , and a relay device  104 . The transmitting device  102  and the receiving device  103  are communication devices according to the first embodiment, respectively. 
         [0034]    Each of the networks  100  and  101  is a communication network for transmitting a packet by making correct routing with the packet addressed to a destination address. The transmitting device  102  is connected to the relay device  104  via the network  101 . Likewise, the relay device  104  is connected to the receiving device  103  via the network  100 . 
         [0035]    The relay device  104  according to the first embodiment is advantageous particularly if the networks  101  and  102  greatly differ in network communication characteristics, for example, one of the networks  100  and  101  is a wired section and the other is a wireless section. The reason is as follows. Because of the great difference in network communication characteristics between the networks  100  and  101  in the communication system, the networks  100  and  101  also differ in an appropriate communication method. Due to this, the relay device  104  according to the first embodiment capable of adopting appropriate communication methods for the networks  100  and  101  is desirably disposed at a boundary point between communication sections different in communication characteristics. 
         [0036]    Specific examples of the great difference in communication characteristics between the networks  100  and  101  include an example in which one of the networks  100  and  101  is a wireless link and the other is a wired network such as Ethernet (registered trademark) network or ADSL. Examples of the relay device  104  for the example in which one of the networks  100  and  101  is a wireless link and the other is a wired network include a wireless base station, an access point of a public wireless LAN, a router, and a gateway. 
         [0037]    However, the communication device according to the first embodiment is applicable to any types of the communication sections, that is, the networks  100  and  101 . Namely, even if both the networks  100  and  101  are wired sections or wireless sections, the communication device according to the first embodiment is applicable to the communication sections, that is, the networks  100  and  101 . Even if the networks  100  and  101  are a wired section and a wireless section, respectively, and the communication device according to the first embodiment is applicable to the communication sections, that is, the networks  100  and  101 . 
         [0038]    The transmitting device  102  is the communication device transmitting real-time media to the receiving device  103 . The relay device  104  transfers a data packet transmitted from the transmitting device  102  to the receiving device  103  and also transfers a feedback information packet received from the receiving device  103  to the transmitting device  102 . 
         [0039]    The receiving device  103  is the communication device decoding the received data packet to an image/a voice. The receiving device  103  also notifies the transmitting device  102  via the relay device  104  of a communication quality parameter acquired by measuring a reception state of the received data packet as the feedback information packet. 
         [0040]    The transmitting device  102  can further include a function as the receiving device  103  and the receiving device  103  can further include a function as the transmitting device  102 . In this case, the relay device  104  functions as the relay device  104  according to the first embodiment on both an up transmission path and a down transmission path (that is, irrespectively of whether one of the communication devices  102  and  103  functions as a transmitting device and the other functions as a receiving device). 
         [0041]      FIG. 2  is a block diagram for describing an internal configuration of the relay device  104  including a quality notification function according to the present invention in detail. 
         [0042]    The relay device  104  includes a first receiving unit  401 , a communication quality measurement unit  402 , a second transmitting unit  403 , a second receiving unit  404 , a packet rewrite unit  405 , a first transmitting unit  406 , and a communication quality parameter storage unit  407 . 
         [0043]    The first receiving unit  401  is a data packet receiving unit receiving a data packet on the down transmission path from the transmitting device  102  to the receiving device  103 . 
         [0044]    The communication quality measurement unit  402  decides a communication quality parameter based on a sequence number and time information such as a timestamp of the received data packet. The communication quality measurement unit  402  stores the communication quality parameter in the communication quality parameter storage unit  407  while linking the communication quality parameter to a transmission source (hereinafter, “source”) address of the data packet. 
         [0045]    The second transmitting unit  403  is a data packet transmitting unit on the down transmission path from the transmitting device  102  to the receiving device  103 . The second receiving unit  404  is a feedback information packet receiving unit on the up transmission path from the receiving device  103  to the transmitting device  102 . 
         [0046]    The packet rewrite unit  405  receives the feedback information packet and reads a transmission destination (hereinafter, “destination”) address. The packet rewrite unit  405  requests the communication quality parameter storage unit  407  to notify the linked communication quality parameter corresponding to the read destination address. Further, the packet rewrite unit  405  rewrites contents of the received feedback information packet to the communication quality parameter received from the communication quality parameter storage unit  407 . 
         [0047]    The first transmitting unit  406  is a feedback information packet transmitting unit on the up transmission path from the receiving device  103  to the transmitting device  102 . 
         [0048]    The communication quality parameter storage unit  407  is a unit holding information identifying each communication device (“identification information”) and a communication quality parameter while linking the identification information to the communication quality parameters. Examples of the identification information include an IP address. 
         [0049]    Furthermore, if receiving a communication parameter notification request with the source address designated, the communication quality parameter storage unit  407  notifies a requester of the communication quality parameter linked to the designated source address. At this time, examples of the communication quality parameter include a packet discard rate, a packet delay time, and a jitter. 
         [0050]    Referring to  FIGS. 3 and 4 , overall operation of the communication system according to the first embodiment will next be described in detail. For convenience of description, a path from the transmitting device  102  to the relay device  104  is assumed as a section A. 
         [0051]      FIG. 3  shows a data packet transmission operation from the transmitting device  102  to the receiving device  103 . The first receiving unit  401  of the relay device  104  receives a data packet transmitted from the transmitting device  102  to the receiving device  103  (step S 100 ). The first receiving unit  401  also transmits the received data packet to the communication quality measurement unit  402  (step S 101 ). 
         [0052]    The communication quality measurement unit  402  reads a sequence number and time information such as a timestamp from the received data packet, thereby measuring a communication quality parameter (step S 102 ). 
         [0053]    Further, the communication quality measurement unit  402  stores the source address of the data packet and the measured communication quality parameter in the communication quality parameter storage unit  407  while linking the source address to the communication quality parameter (step S 103 ). The communication quality parameter measured at this time will be referred to as “communication quality parameter A” on the section A. 
         [0054]    Next, the communication quality measurement unit  402  transmits the data packet to the second transmitting unit  403  (step S 104 ). The second transmitting unit  403  transmits the received data packet to the receiving device  103  (step S 105 ). 
         [0055]      FIG. 4  shows transmission operation if the receiving device  103  transmits the feedback information packet to the transmitting device  102  according to necessity. The feedback information packet is an information packet in which the communication quality parameter such as a packet loss rate, a transmission delay or a jitter is described. 
         [0056]    It is to be noted that the receiving device  103  is configured to measure the communication quality parameter by reading the sequence number and the time information such as the timestamp. 
         [0057]    The receiving device  103  transmits a feedback information packet to the second receiving unit  404  (step S 106 ). The second receiving unit  404  transfers the received feedback information packet to the packet rewrite unit  405  (step S 107 ). 
         [0058]    The packet rewrite unit  405  reads a destination address of the received feedback information packet. The packet rewrite unit  405  designates the source address (which refers to the address stored as the source address in the step S 103 ) corresponding to the read destination address, and request the communication parameter storage unit  407  to notify the communication quality parameter stored in the communication parameter storage unit  407  (step S 108 ). 
         [0059]    The communication quality parameter storage unit  407  notifies the packet rewrite unit  405  of the communication quality parameter A compatible with the source address designated by the packet rewrite unit  405  among the communication quality parameters measured in the step S 102  (step S 109 ). 
         [0060]    The packet rewrite unit  405  rewrites a value of the communication quality parameter described in the feedback information packet received from the second receiving unit  404  to the communication quality parameter A (step S 110 ). Further, the packet rewrite unit  405  transfers the feedback information packet in which the communication quality parameter is rewritten to the communication quality parameter A to the first transmitting unit  406  (step S 111 ). The first transmitting unit  406  transfers the received feedback information packet to the transmitting device  102  (step S 112 ). 
         [0061]    As a result, the transmitting device  102  can grasp the communication quality parameter A on the section A. The transmitting device  102  optimizes a data packet communication method on the section A based on the communication quality parameter A on the section A (step S 113 ). 
         [0062]    By applying the communication device according to the first embodiment to an IP relay device, the transmitting device  102  can use an appropriate communication method for the section A if the transmitting device  102  includes a transmission control function using the grasped communication quality parameter on the section A. It is to be noted that the transmitting device  102  is configured to be able to use an appropriate packet communication method based on communication quality parameter. 
         [0063]    Moreover, the relay device  104  can grasp a communication quality parameter on a communication section between the relay device  104  and the receiving device  103  by calculating a difference between the communication quality parameter read from the feedback information packet and the communication quality parameter A on the section A. Accordingly, the relay device  104  may be configured to optimize a data packet communication method on the communication section between the relay device  104  and the receiving device  103  based on the grasped communication quality parameters. 
       Second Embodiment  
       [0064]    A second embodiment of the present invention will be described in detail with reference to the drawings. By applying a relay device including a quality information notification function according to the present invention to each of a plurality of relay devices on communication sections, the relay device can be used to improve packet transmission quality. 
         [0065]      FIG. 5  shows a communication system according to the second embodiment. The communication system includes a transmitting device  102 , a receiving device  103 , and relay devices  105  to  109 . Since the transmitting device  102  and the receiving device  103  are similar in configuration to those according to the first embodiment, they will not be repeatedly described herein. Further, since the relay devices  105  to  109  are similar in configuration to the relay device  104  according to the first embodiment, they will not be repeatedly described herein. Similarly to the first embodiment, the transmitting device  102  can further include a function as the receiving device  103  and the receiving device  103  can further include a function as the transmitting device  102 . 
         [0066]    Each of the relay devices  105  to  109  is a relay device transferring a data packet transmitted from the transmitting device  102  to the receiving device  103  by making correct routing and transferring a feedback information packet received from the receiving device  103  to the transmitting device  102  by making correct routing. Examples of the relay devices  105  to  109  include routers, gateways, wireless base stations, and access points. 
         [0067]    The second embodiment shows one of network configuration examples in which a plurality of relay devices according to the present invention is disposed on communication sections between a transmitting device and a receiving device. The network configuration is not necessarily similar to that of the second embodiment so as to realize the present invention. 
         [0068]    Referring to  FIGS. 6 and 7 , overall transmission operation of the communication system according to the second embodiment will be described. For convenience of description, a path from the transmitting device  102  to the relay device  106  is assumed as a section A, a path from the transmitting device  102  to the relay device  107  is assumed as a section B, and a path from the transmitting device  102  to the relay device  108  is assumed as a section C. Further, a path from the transmitting device  102  to the receiving device  103  is assumed as a section D and a path from the relay device  107  to the relay device  108  is assumed as a section E. 
         [0069]      FIG. 6  shows a detailed transmission signal operation sequence if the transmitting device  102  transmits a data packet to the receiving device  103 . The relay devices  105  to  109  transfer the data packet transmitted from the transmitting device  102  to the receiving device  103  by performing similar operation sequences to that of the steps S 100  to S 105  shown in  FIG. 3 . 
         [0070]    In the second embodiment, the overall operation sequence of the relay devices  105  to  109  will be described while paying attention particularly to the relay devices  106 ,  107 , and  108  in the detailed transmission signal operation sequence. 
         [0071]    Each of the relay devices on a communication section from the relay device  105  to the relay device  106  performs a transmission operation in the steps S 100  to S 105  shown in  FIG. 3 , thereby transferring the data packet transmitted from the transmitting device  102  to the relay device  106  (step S 200 ). 
         [0072]    The relay device  106  measures and records a communication quality parameter A on the section A from the received data packet by performing the transmission operation in the steps S 100  to S 103  shown in  FIG. 3  (step S 201 ). The relay device  106  transfers the data packet to the relay device  107  by performing the transmission operation in the steps S 104  to S 106  (step S 202 ). 
         [0073]    The relay device  107  measures and records a communication quality parameter B on the section B by performing the transmission operation in the steps S 100  to S 105  shown in  FIG. 3  (step S 203 ), and transfers the data packet to the relay device  108  (step S 204 ). 
         [0074]    Likewise, the relay device  108  measures and records a communication quality parameter C on the section C by performing the transmission operation in the steps S 100  to S 105  shown in  FIG. 3  (step S 205 ), and transfers the data packet to a next relay device. Thereafter, each of the relay devices disposed on a communication path from the relay device  108  to the receiving device  103  repeatedly performs a similar transmission operation and transfers the data packet to the receiving device  103  (step S 206 ). 
         [0075]    At this time, each of the relay devices  105  to  109  disposed on communication sections from the transmitting device  102  to the receiving device  103  holds one communication quality parameter on the communication section from the transmitting device  102  to each relay device. 
         [0076]      FIG. 7  shows a detailed transmission signal operation sequence if the receiving device  103  transmits a communication quality parameter judged from a data packet reception state to the transmitting device  102  as the feedback information packet. 
         [0077]    At this time, the communication quality parameter described in the feedback information packet transmitted from the receiving device  13  is a communication quality parameter D on the section D. If the receiving device  103  transmits the feedback information packet to the transmitting deice  102 , the relay devices  109  to  105  perform similar operation sequences to that of the steps S 107  to S 112  shown in  FIG. 4 , thereby transferring the feedback information packet transmitted from the receiving device  103  to the transmitting device  102 .  5  In this respect, the overall operation sequence of the relay devices  109  to  105  will be described while paying attention to the relay devices  106 ,  107 , and  108 . 
         [0078]    Each of the relay devices disposed on a communication path from the receiving device  103  to the relay device  108  performs a transmission operation similar to that of the steps S 107  to S 112  shown in  FIG. 4 , thereby transferring the feedback information packet transmitted from the receiving device  103  to the relay device  108  (step S 207 ). 
         [0079]    The relay device  108  performs a transmission operation similar to that of the steps S 107  to S 110 , thereby rewriting a content of the received feedback information packet to the communication quality parameter C decided in the step S 205  (Step S 208 ). Further, the relay device  108  performs a transmission operation similar to that of the steps S 111  to S 112 , thereby transferring the feedback information packet to the relay device  107  (step S 209 ). 
         [0080]    Similarly to the relay device  108 , the relay device  107  performs the transmission operation similar to that of the steps S 107  to S 110 , thereby rewriting the content of the received feedback information packet to the communication quality parameter B decided in the step S 203  (Step S 210 ). Further, the relay device  107  performs the transmission operation similar to that of the steps S 111  to S 112 , thereby transferring the feedback information packet to the relay device  106  (step S 211 ). 
         [0081]    Similarly to the relay device  108 , the relay device  106  performs the transmission operation similar to that of the steps S 107  to S 110 , thereby rewriting the content of the received feedback information packet to the communication quality parameter A decided in the step S 201  (step S 212 ). Further, the relay device  106  performs the transmission operation similar to that of the steps S 111  to S 112 , thereby transferring the feedback information packet to the adjacent relay device closer to the transmitting device  102 . Thereafter, each of the relay devices disposed on a communication path from the relay device  106  to the transmitting device  102  repeatedly performs similar transmission operations, thereby transferring the feedback information packet to the transmitting device  102  (step S 213 ). 
         [0082]    In this way, the relay devices  105  to  109  sequentially rewrite the content of the feedback information packet transmitted from the receiving device  103 . Each relay device can, therefore, notify the adjacent relay device closer to the transmitting device  102  (the adjacent relay device corresponds to the transmitting device  102  if the previous relay device is the relay device  105 ) of the measured communication quality parameters. 
         [0083]    In this respect, if each of the relay devices  105  to  109  includes a packet content read unit reading a communication quality parameter from the feedback information packet, a communication quality calculation unit calculating a difference between the read communication quality parameter and the communication quality parameter held by the communication quality measurement unit  402 , and a transmission control unit and the transmitting device  102  includes a transmission control unit, real-time media can be transmitted with high quality according to the second embodiment. 
         [0084]    The reason for this will be described while taking the transmission operations performed by the relay devices  106 ,  107 , and  108  as an example with reference to  FIG. 8 . 
         [0085]    In a step S 209 , the relay device  107  is notified of the communication quality parameter C from the relay device  108  and holds the communication quality parameter C in the packet content read unit. 
         [0086]    The communication quality parameters held in the relay device  107  at this time are the communication quality parameter C and the communication quality parameter B on the section B measured in the step S 203 . By causing the communication quality calculation unit  402  to calculate the difference between the two communication quality parameters B and C, a communication quality parameter E on the section E can be calculated (step S 301 ). 
         [0087]    A method of calculating the communication quality parameter E on the section E if the communication quality parameter E is the packet discard rate or the jitter will be described specifically. It is to be noted that the packet discard rate or the jitter is only an example and is not intended to limit the content of the communication parameter according to the second embodiment. As long as information enables measuring a communication quality on a communication path, even the information other than the packet discard rate or the jitter can be used as the communication quality parameter. 
         [0088]    If the communication quality parameter measured by each relay device according to the second embodiment is the packet discard rate, the communication quality parameter E is calculated by the following calculation equation. 
         [0000]      Packet discard rate  E  [%](100×( C  [%]− B  [%]))/(100− B  [%]) 
         [0089]    Moreover, if the communication quality parameter measured by each relay device according to the second embodiment is the jitter, the communication quality parameter E is calculated by the following calculation equation. 
         [0000]      Jitter  E  [ms]= C  [ms]− B  [ms] 
         [0090]    If the relay device  107  transfers the data packet received from the relay device  106  to the relay device  108 , the relay device  107  can exercise a transmission control using the communication quality parameter E calculated in the transmission operation stated above and can thereby use an appropriate communication method for the section E (step S 302 ). In addition, the relay device  107  performs the transmission operation similar to that of the steps S 210  and S 211 . 
         [0091]    As a result, according to the second embodiment, the communication quality parameters between the communication sections of the communication system including the transmitting device  102 , the receiving device  103 , and the relay device  105  to  109  can be acquired. Appropriate communication methods for the communication sections can be used based on the acquired communication quality parameters, respectively. 
         [0092]    In the first and second embodiments, an IP address is used as the information identifying each communication device. Alternatively, a network address that is an address representing a network on which a communication device is present can be used as the information identifying the communication device. 
         [0093]    The relay device according to the present invention can be constructed by hardware. Alternatively, the relay device according to the present invention can be constructed by causing a computer to read and execute a program for allowing the computer to function as the relay device according to the present invention. 
         [0094]    The present invention can be applied to an IP relay device disposed on a communication path between transmitting or receiving devices in a communication system and can be used to improve packet transmission quality. Examples of the IP relay device include a wireless base station of a portable terminal, an access point of a public wireless LAN, a router, and a gateway. The present invention can be further applied to a relay device in a dynamically generated network such as an ad hoc network besides a statically generated network. 
         [0095]    Although the exemplary embodiments of the present invention have been described in detail, it should be understood that various changes, substitutions and alternatives can be made therein without departing from the sprit and scope of the invention as defined by the appended claims. Further, it is the inventor&#39;s intent to retain all equivalents of the claimed invention even if the claims are amended during prosecution.