Multiplex communications – Pathfinding or routing – Switching a message which includes an address header
Reexamination Certificate
2002-09-30
2003-08-12
Lim, Krisna (Department: 2153)
Multiplex communications
Pathfinding or routing
Switching a message which includes an address header
C370S389000, C370S354000, C370S352000, C709S238000
Reexamination Certificate
active
06606318
ABSTRACT:
BACKGROUND ART
The present invention generally relates to a hop-count management method and to a node apparatus using this method.
In a node apparatus performing packet communications by using, for example, Internet Protocol (IP), control information for assigning specific labels to specific packet streams can be exchanged between nodes. Each node then stores input labels (and input interfaces) and output labels (and output interfaces) assigned to the individual streams, and performs a packet transfer operation (switching operation) in accordance with the relationship between the stored input and output labels. This scheme is referred to as a “label switching technique”.
Generally, the lengths of the labels are fixed, and faster processing is achieved and more flexible path control is enabled by employing the above-described label-switching packet transfer operation than by utilizing a conventional packet transfer operation performed by analyzing packet header information (destination IP address prefix, etc.).
The path through which packet streams are label-switched is referred to as a “label switched path (LSP)”. A node, which is located at the ingress of the LSP for transferring a packet stream, is referred to as an “ingress node”, and performs packet transfer after analyzing an IP packet which belongs to the packet stream and attaching a label to the packet. A node, which is located at the egress of the LSP for transferring the packet stream, is referred to as an “egress node”, and performs packet transfer after removing the label from the received packet. A node, which is placed between the ingress node and the egress node in the LSP so as to connect these nodes, is referred to as an “intermediate node”, and performs packet transfer in accordance with the relationship between the input and output labels.
A stream to be transferred in a certain LSP may be various packets, such as a packet provided with a specific destination IP address, a packet provided with a specific destination IP address prefix (network address), and a packet passing through a specific egress node belonging to a certain domain.
LSPs may include at least point-to-point LSPs for unicast streams (one ingress node and one egress node are provided), and point-to-multipoint LSPs (one ingress node and a plurality of egress nodes are provided, and the LSP is branched off at an intermediate node into at least two portions) for multicast streams. Additionally, multipoint-to-point LSPs (a plurality of ingress nodes and a single egress node are provided) may be defined if a stream merge function of relating a plurality of input labels to a single output label is provided in an intermediate node.
To specifically implement the above-described label switching, a header defined in the known datalink switching technique, such as an asynchronous transfer mode (ATM) and a frame relay, may be used. Alternatively, a label header newly defined specifically for label switching may be used.
In the second technique, a label header field (hereinafter referred to as a “generic label header”) is inserted between a point-to-point protocol link (PPP link) frame header and an IP header or between a logical link control (LLC) header of a local area network (LAN) and an IP header, and a label value filled in the generic label header field is used as switching information.
In the generic label header, not only a label field used as the switching information, but also a Time-To-Live (TTL) field for checking the number of nodes through which a packet passes (hereinafter sometimes referred to as a “hop-count”) and a Class of Service (CoS) field for indicating the priority of performing transfer operation are defined. The TTL field and the CoS field are also defined in a conventional IP packet header. If each node (not only the ingress node and the egress node, but also an intermediate node) decrements the TTL value in the generic label header one-by-one upon transferring a packet by label switching, it is possible to discard the packet which have passed through a number of nodes greater than a predetermined number of nodes due to a reason, such as circling in the transfer path (routing loop).
In contrast, if the aforementioned first technique, i.e., a known datalink switching technique, is used for implementing label switching, a Virtual Path Identifier/Virtual Channel Identifier (VPI/VCI) field of an ATM cell header or a Data Link Circuit Identifier (DLCI) field of a frame relay header may be used as a label field.
In this case, a standardized ATM cell header format or frame relay header format is used, but the TTL field and the CoS field discussed previously are not defined for these header formats. Accordingly, once a packet (cell) is input into an LSP formed by the ATM virtual connection (VC), it may not be discarded and circles indefinitely in the LSP due to, for example, routing loop.
As one of the measures to overcome the above-described drawback, the following method has been proposed. By using a Label Distribution Protocol (LDP) for establishing, maintaining, and releasing an LSP, the number of nodes through which a packet passes (the hop-count) in each LSP from an ingress node to an egress node is stored in the ingress node or in the egress node. Then, when a packet is actually transferred, the TTL value of the IP packet header is decremented by the hop-count stored in the ingress node or the egress node.
In this method, the ingress node transmits a “label request message” for requesting an assignment of a label to a specific packet stream defined by, for example, a destination network prefix. Then the downstream node receives the label request message and further transmits it to its neighboring node toward the egress node. In this manner, the label request message is forwarded to the egress node, and the egress node sends back a “label mapping message” for assigning the label to the specific packet stream. Alternatively, the egress node may voluntarily transmit a label mapping message to the upstream node, even if it does not receive any label request message. Then the upstream node receives the label mapping message and further transmits it to its neighboring node toward the ingress node.
The label mapping message transmitted from the egress node includes not only the label value to be informed to the upstream nodes, but also the hop-count representing a value of one. When a node receives the label mapping message from its neighboring downstream node, the node transmits the label mapping message to its neighboring upstream node after incrementing the hop-count value by one. Ultimately, the ingress node that has transmitted the label request message receives the label mapping message and stores the hop-count value indicated in the label mapping message, which represents the number of nodes between the ingress node and the egress node.
Then, when the ingress node receives a packet belonging to a packet stream corresponding to the above-described label, it decrements the TTL value of the packet header by the stored hop-count value and checks whether the decremented TTL value has reached zero before transmitting the packet. If the TTL value has reached zero, the packet may be discarded at the ingress node.
The aforementioned method, where the number of hops in the LSP from the ingress node to the egress node is informed to the ingress node by the label mapping messages and the ingress node decrements the TTL value of a packet to be transmitted through the LSP by the informed hop-count value, is effective only when all the intermediate nodes located on an LSP are unable to check and update the TTL value in a packet transferred. That is, it is effective only in a system where all the nodes forming the LSP use the same known datalink switching technique, such as ATM or frame relay.
Considering an actual label switching network, both ATM and frame relay may be employed, or both ATM and a Synchronous Optical Network (SONET)-based point-to-point link may be used, in a single network. However, the control mechanism previously discussed
Katsube Yasuhiro
Tanaka Hisako
Kabushiki Kaisha Toshiba
Lim Krisna
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