Switched token ring over ISL (TR-ISL) network

Details Switched token ring over ISL (TR-ISL) network Switched token ring over ISL (TR-ISL) network

1999-03-10

2004-03-09

Pham, Chi (Department: 2667)

Multiplex communications

Pathfinding or routing

Switching a message which includes an address header

Reexamination Certificate

active

06704318

ABSTRACT:

BACKGROUND OF THE INVENTION
Communication in a computer internetwork involves the exchange of data between two or more entities interconnected by communication media configured as local area networks (LANs) and wide area networks (WANs). The entities are typically software programs executing on hardware computer platforms, such as end stations and intermediate stations. In particular, communication software executing on the end stations correlate and manage data communication with other end stations. The stations typically communicate by exchanging discrete packets or frames of data according to predefined protocols. A protocol, in this context, consists of a set of rules defining how the stations interact with each other. For example, a LAN employs a data communication protocol (LAN standard), such as Token Ring, Ethernet or Token Bus, that defines the functions performed by the data link and physical layers of a communications architecture (i.e., a protocol stack).
To form a WAN, one or more intermediate devices are often used to interconnect multiple LANs. A bridge is an example of an intermediate station that may be used to provide a “bridging” function between two or more LANs to form a relatively small domain of stations, such as a subnetwork. Subnetworks or subnets provide an organizational overlay to an internetwork that facilitates transmission of data between the end stations. A switch may be utilized to provide a “switching” function for transferring information, such as data frames, between LANs. Typically, the switch is a computer having a plurality of ports that couple the switch to several LANs and to other switches. The switching function includes receiving data frames at an inbound port and transferring them to at least one outbound port of the switch. A router is an intermediate station that interconnects subnets and executes network routing software to allow expansion of communication to end stations of other subnets. Collectively, these hardware and software components comprise a communications internetwork.
FIG. 1
is a schematic block diagram of a conventional Token Ring (TR) internetwork
100
comprising a plurality of TR LANs interconnected by conventional bridges and a router (R). Each token ring is assigned a ring number (RN), such as RN
001
, RN
222
and RN
123
, and each bridge is assigned a bridge number (BN), such as BN
1
-
3
. The RNs assigned to the token rings must be unique within each bridged TR subnetwork that extends to the router. That is, RNs assigned to the token rings within each subnetwork must be different, although BNs assigned to the bridges within each subnetwork may be similar. An exception to this latter rule involves the use of redundant bridges coupling common TR LANs; here, the redundant bridges must have unique BNs in order to distinguish one another.
In the TR internetwork, there may be multiple paths between a source end station and a destination end station. To send a TR frame from a source (such as Station A) to a destination (such as Station B) along a particular path of the internetwork, the source may insert information within a routing information field (RIF) of the frame that specifies the particular path to the destination.
FIG. 2
is a schematic diagram of a portion of a conventional TR frame
200
comprising destination address (DA) and source address (SA) medium access control (MAC) fields
202
-
204
and a RIF header
210
. The RIF header
210
, in turn, comprises a type (TYPE) field
212
, a RIF length indicator (LENGTH) field
214
, a direction bit (DIRECTION) field
216
and a ROUTE field
220
that may include a plurality of RN/BN pairs needed to describe the path. Each RN/BN pair comprises 2 bytes, wherein the RN is 12 bits and the BN is 4 bits. The RIF header
210
terminates with a 4-bit padding (PAD) field
228
of zeros.
The source typically acquires the information for insertion into the RIF through the issuance of a special TR frame called an All Routes Explorer (ARE) frame that is broadcasted throughout the TR subnetwork. An ARE frame is typically used to find all paths to a particular destination; an example of a frame used to strictly find the destination is a Spanning Tree Explorer (STE) frame. The STE frame only propagates over network segments that are along a defined spanning tree path to the destination; consequently, the destination only receives one copy of the frame. Execution of a spanning tree algorithm within the bridges results in blocking of certain ports to obviate propagation of frames around loops.
Source Route Bridging (SRB) describes a bridging technique that forwards TR frames based on the RIF information stored in the frame; an example of a frame that has a RIF is called a Specifically Routed Frame (SRF). In contrast, Transparent Bridging (TB) is a bridging technique that forwards TR frames based on their MAC addresses using a forwarding table. Source Route Transparent (SRT) bridging is a merging of the SRB and TB techniques; that is, if there is a RIF in the frame transported over an SRT bridge network, forwarding decisions are based on that RIF, whereas if there is no RIF in the frame, forwarding decisions are made based on the MAC address of the frame using the forwarding table. A TR frame that does not have a RIF is called a Non-Source Route (NSR) frame.
When issuing an ARE frame, the source (Stn A) initially sets the RIF length
214
to “2” (the length of the header
210
) signifying that there is no information contained in the route field
220
of the RIF, and loads the type field
212
of the header with information specifying the type of frame, e.g., an ARE frame. Stn A then transmits the ARE frame over token ring RN
001
where it is received by each station, including each bridge, connected to the token ring. Upon receiving the frame, each bridge inserts information into the RIF prior to forwarding a copy of the ARE frame onto its connected token ring.
In general, each bridge inserts into the RIF (i) its bridge number and (ii) the ring number of the token ring to which it is forwarding the frame; however, when a bridge receives an ARE frame having a RIF length of “2”, the bridge also inserts into the RIF the ring number of the token ring from which the frame is received. For example, a first BN
1
inserts into the RIF the following information: the RN of the token ring from which the frame is received, its BN and the RN of the ring to which it is forwarding the frame <001.1.123>. The contents of the RIF thus describe the path followed by the ARE frame to reach token ring RN
123
.
The RIF contents for other copies of the ARE frame broadcasted throughout the TR subnetwork include (i) RIF=<001.1.222>and (ii) RIF=<001.2.222>. These copies of the ARE frame are forwarded over RN
222
and the bridges connected to the ring update the RIF of the ARE frames prior to forwarding them to their connected LANs. For example when bridge BN
3
forwards the ARE frame to RN
123
, it updates the RIF header
210
, including the length field
214
, as a result of inserting its bridge number and connected ring number into the RIF. Thus, the contents of the RIF of an ARE frame propagating over RN
123
are <001.1.222.3.123>. Destination (Stn B) receives three ARE frames, one of which has a RIF with contents <001.1.123>, another having RIF contents <001.1.222.3.123>and a third having RIF contents of <001.2.222.3.123>.
Stn B chooses one of the ARE frames (and its RIF contents) as the route over which it returns a response frame; typically, the destination chooses the frame it received first, which may be the frame having the shortest RIF to the source. Stn B thus returns a SRF frame to the source over a path <001.1.123>specified in the RIF. The frame type is indicated as a SRF frame and the direction bit is altered to enable interpretation of the contents of the RIF. In the case of a response frame, the direction bit is inverted to denote that the RIF contents are interpreted in a reverse direction to describe the path to the source.

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