Communication device and communication method

1. A communication device, comprising:

a memory configured to hold a packet-length table storing, in association with an identifier of a network, a length of a packet to be transmitted to a device belonging to the network;

a transmitter configured to obtain a first length from the packet-length table, and to transmit a certain packet to a destination device when a second length is equal to or less than the first length, the first length being a length associated with a network to which the destination device belongs, and the second length being a length of the certain packet;

a receiver configured to receive a notification message indicating an address of the destination device and a third length, when the second length is greater than the third length, the third length being a maximum packet length corresponding to a maximum length of a packet that is able to be transmitted to a network included in a route leading to the destination device; and

a processor configured to update the first length in the packet-length table to the third length,

wherein

the packet-length table stores a routing prefix of an address of the destination device and a length of a packet that is able to be transmitted to a network designated by the routing prefix in association with each other,

and

the processor changes a value of packet length associated with the routing prefix of the address of the destination device to the maximum packet length.

2. A relay apparatus that relays a packet transmitted from a transmission source device, the relay apparatus comprising:

a receiver configured to receive a relay packet;

a memory configured to hold a packet-length table storing, in association with an identifier of a network including a destination device which is a destination of the packet, a length of a packet that is able to be transmitted to a device belonging to the network;

a processor configured to decide a transfer destination network to which the relay packet is to be transferred, and to determine that the relay packet is to be transferred when a first length is less than a second length, the first length being a length of the relay packet, and the second length being a length stored in association with an identifier that identifies the network including the destination device; and

a transmitter configured to transmit the packet determined to be transferred to the transfer destination network, wherein

the receiver receives a notification message indicating an address of the destination device and a third length when the first length is greater than the third length, the third length being a maximum length corresponding to a maximum length of a packet that is able to be transmitted to a network included in a route leading to the destination device, and

the second length in the packet-length table is updated to the third length,

wherein

the packet-length table stores a routing prefix of an address of the destination device and a length of a packet that is able to be transmitted to a network designated by the routing prefix in association with each other,

and

the processor changes a value of packet length associated with the routing prefix of the address of the destination device to the maximum packet length.

3. The relay apparatus according to claim 2 , wherein

the processor generates a notification message indicating that a length of the relay packet is greater than the third length and indicating a value of the third length when the length of the relay packet is greater than the maximum packet length, and

the transmitter transmits the notification message to the transmission source device.

4. The relay apparatus according to claim 3 , wherein

the relay packet is generated according to a first protocol, and when a second protocol different from the first protocol is used in the transfer destination network, the processor adds a header for performing a process with the second protocol to the relay packet, and

the packet-length table stores a value calculated by subtracting a length of the header from the maximum packet length in association with the identifier that identifies the network including the destination device.

5. A communication method in which a process is performed by a relay apparatus that relays a packet transmitted from a transmission source device, the process comprising:

receiving the packet;

deciding a transfer destination network to which the packet is to be transferred;

determining that the packet is to be transferred when a first length is less than a second length, the first length being a length of the packet, and the second length being a length stored in association with an identifier that identifies a network including a destination device which is a destination of the packet;

transmitting the packet determined to be transferred to the transfer destination network;

receiving a notification message indicating an address of the destination device and a third length when the first length is greater than the third length, the third length being a maximum packet length corresponding to a maximum length of a packet that is able to be transmitted to a network included in a route leading to the destination device; and

updating the second length to the third length,

wherein

the packet-length table stores a routing prefix of an address of the destination device and a length of a packet that is able to be transmitted to a network designated by the routing prefix in association with each other,

and

the processor changes a value of packet length associated with the routing prefix of the address of the destination device to the maximum packet length.