Multiplex communications – Data flow congestion prevention or control – Flow control of data transmission through a network
Reexamination Certificate
1998-03-18
2001-10-30
Cangialosi, Salvatore (Department: 2661)
Multiplex communications
Data flow congestion prevention or control
Flow control of data transmission through a network
C370S410000, C370S403000
Reexamination Certificate
active
06310858
ABSTRACT:
BACKGROUND OF THE INVENTION
(1) Field of the Invention
The present invention generally relates to a frame relay system connecting networks to each other, particularly to a frame relay system which relays frames each of which has a destination address and a frame time-to-live (TTL) as control information. The frame TTL indicates the number of times which a frame can be relayed.
(2) Description of the Related Art
In a frame relay system connecting network to each other, a frame having a format as shown in
FIG. 1
is used. That is, the frame has a destination address
1
, a frame time-to-live (TTL)
2
and data
3
.
A conventional communication is preformed in accordance with a procedure in which a transmission terminal transmits frames, each frame having the frame TTL set at a value, and the frame relay system decrements the value of the frame TTL when relaying each of the frames. The frame relay system has a unit for deleting a frame when the value of the frame TTL reaches zero “0”. That is, each of the frames is provided with a term of life so that frames are not infinitely circulating through a network.
A decrement process of the frame TTL in the frame relay system is applied to all the frames which should be relayed. As a result, the delay time of the relay process is increased, so that the relay process is inhibited from being performed at a high speed.
FIG. 2
is a block diagram illustrating a conventional frame relay system. Referring to
FIG. 2
, the frame relay system has a frame receiving unit
4
and a frame transmitting unit
5
. The frame receiving unit
4
receives frames from a network and converts a format of each of the received frames into a format which can be processed in the system. The frame transmitting unit
5
converts a format of each of frame processed in the system into a format which can be output to the network and outputs the processed frame to the network. A set of the frame receiving unit
4
and the frame transmitting unit
5
is provided for each of the networks to which the system is connected. In this example shown in
FIG. 2
, a set of the frame receiving unit
4
and the frame transmitting unit
5
is provided for each of the networks
1
and
2
.
The frame relay system further has a frame relay control unit
10
, a path control unit
20
and a destination address extracting unit
6
. The frame relay control unit
10
is connected to the frame receiving unit
4
and the frame transmitting unit
5
and carries out a frame relay control process. The frame relay control unit
10
has a frame forwarding block
11
for forwarding frames to the network, a TTL decrement block
12
for decrementing the value of the frame TTL and a header-check-sum calculation block
13
.
The path control unit
20
is connected to the frame relay control unit
10
and controls paths. The path control unit
20
has a routing table
21
, a table retrieving block
22
and a table forming/updating block
23
. The routing table
21
indicates a destination address of each frame and control information corresponding to the destination address. The table retrieving block
22
retrieves that routing table
21
. The table forming/updating block
23
forms and updates the routing table
21
.
The destination address extracting unit
6
extracts a destination address from each frame received by the frame receiving unit
4
. The destination address extracted by the destination address extracting unit
6
is supplied to the path control unit
20
.
The conventional frame relay system performs processes in accordance with procedures as shown in
FIG. 3. A
description will now be given, with reference to
FIG. 3
, of the frame relay operation.
The frame receiving unit
4
receives a frame from the network and converts the format of the frame into a format which can be processed in the system (a frame receiving process S
1
).
The destination address extracting unit
6
extracts a destination address from the frame received by the frame receiving unit
4
(S
2
). The table retrieving block
22
receives the destination address from the destination address extracting unit
6
and retrieves the routing table
21
based on the received destination address (S
3
). It is checked whether the received destination address corresponds to the present frame relay system (the present station) (S
4
). The routing table
21
indicates a relationship between destination addresses and output ports as shown in
FIG. 4
so that an output port can be decided based on a destination address.
The routing table
21
is formed by deciding a path, namely an output port, corresponding to a destination address in a received path information frame.
When the destination address does not correspond to the present frame relay system, the TTL decrement block
12
decrements a value of the frame TTL to avoid infinitely circulating through the network. If the value of the frame TTL reaches zero “0”, the frame is deleted.
The frame forwarding block
11
forwards the frame to the frame transmitting unit
5
corresponding to the output port decided by the table retrieving block
22
.
The frame transmitting unit
5
corresponding to the output port converts the frame format used in the system into a frame format which can be output to the network and then outputs the frame to the network.
When it is determined, in step S
4
, that the destination address corresponds to the present frame relay system, the system determines whether the received frame is a path information frame (S
5
). If the received frame is the path information frame, the system forms or updates the routing table
21
(S
6
).
When the frame is relayed with the decrement of the value of the frame TTL, the process time is increased. Thus, to achieve the fast relay process, a method for relaying the frame without the decrement of the value of the frame TTL may be used.
Even if frames are relayed without the decrement of the value of the frame TTL, due to forwarding of frames in accordance with the routing table, the frames can be relayed without errors in a case where the network is formed in a tree-structure. However, if the network structure is complex so that a circular path exits in the network, frames may circulates through the network so as to not reach any stations.
A description will now be given of states where the frame circulation occurs.
(1) Change of Routing Table
Each frame relay system receives path information frames from an adjacent frame relay system and decides paths to which frames should be relayed. The routing table indicating the relationship between destination addresses and output ports is then formed (see FIG.
4
). In a state where a sufficient time elapses from a start of services in the network and the network is in a stationary state, an output port corresponding to each of the destination address is uniquely decided in each frame relay system. In the stationary state, frames are relayed to paths in accordance with the routing table, so that the frames can reach the destinations without circulating through the network.
In cases where network equipment trouble and disconnection of a path in the network occur, relaying paths are changed based on information relating to the trouble included in the path information frame so that restoration of the relaying paths is attempted. In this case, if all the frame relay systems in the network simultaneously change the relaying paths, that is, if the routing tables are simultaneously updated in all the frame relay systems, the relay paths are restored without problems.
However, the routing tables are updated separately in the respective frame relay systems, and it takes a long time until the routing tables are updated in all the frame relay systems so that the network becomes in a stationary state. In a transition period until the network becomes in the stationary state, the circulating paths of the frames may occur, so that a frame transmitted in the transition period may be circulated through the network. A description will be given of an example of the circulation of the frame.
FIGS. 5
Chugo Akira
Kano Shinya
Cangialosi Salvatore
Fujitsu Limited
Rosenman & Colin LLP
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