Multiplex communications – Pathfinding or routing – Switching a message which includes an address header
Reexamination Certificate
1998-09-02
2004-02-24
Olms, Douglas (Department: 2732)
Multiplex communications
Pathfinding or routing
Switching a message which includes an address header
C370S401000, C370S475000, C709S220000
Reexamination Certificate
active
06697360
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to computer networks, and more specifically, to a method and apparatus for auto-configuring layer
3
devices with network configuration parameters.
BACKGROUND OF THE INVENTION
A computer network typically comprises a plurality of interconnected entities. An entity may consist of any device, such as a host or a node, that “sources” (i.e., transmits) or “sinks” (i.e., receives) data frames. A common type of network is a local area network (“LAN”) which typically refers to a privately owned network within a single building or campus. LANs typically employ a data communication protocol (LAN standard), such as Ethernet, FDDI or token ring, that defines the functions performed by the data link and physical layers of a communications architecture (i.e., a protocol stack), such as the Open Systems Interconnection (OSI) or the Transport Control Protocol/Internet Protocol (TCP/IP) Reference Models. In many instances, LANs of one or more companies, each referred to as a subnetwork, may be interconnected by point-to-point links, microwave transceivers, satellite hook-ups, etc. to form a wide area network (“WAN”), an internet or a larger network that may span an entire campus, country or continent.
One or more intermediate devices are often used to couple LANs together and allow the corresponding entities to exchange information. For example, a switch may be utilized to provide a “switching” function for transferring information, such as data frames, among entities of a computer network. Typically, the switch is a computer and includes a plurality of ports that couple the switch to several LANs and to other switches. The switching function includes receiving data at a source port from an entity and transferring that data to at least one destination port for receipt by another entity.
Switches may operate at various levels of the communication protocol stack. For example, a switch may operate at layer
2
which, in the OSI Reference Model, is called the data link layer. Data frames at the data link layer typically include a header containing the Media Access Control (MAC) address of the entity transmitting the message (source address) and the MAC address of the entity to whom the message is being sent (destination address). To perform the switching function, layer
2
switches examine the MAC destination address of each data frame received on a source port. The frame is then switched onto the destination port or ports associated with that MAC destination address. Layer
2
switches typically do not perform any modification to the data frames being switched and, therefore, are only used to interconnect subnetworks operating the same data link layer standard (e.g., Ethernet).
A MAC address is 48 bits long and is uniquely associated with the network interface card that resides within the entity and provides the connectivity to the network. In particular, each manufacturer of network interface cards is provided with a block of available MAC addresses from a central authority. The manufacturer then assigns to each network interface card a particular MAC address from its authorized block. The MAC address of a particular network interface card thus does not change over time, even though the corresponding entity (or possibly just the network interface card) may be moved from one subnetwork to another. Moreover, upon initialization, an entity may poll its network interface card and learn its MAC address.
Other devices, including switches, may operate at higher communication layers, such as layer three of the OSI Reference Model which is called the network layer. In TCP/IP Model, the network layer corresponds to the Internet Protocol (IP). Data frames at the network or IP layer also include a header. For TCP/IP, the network header contains the IP source address of the entity transmitting the data frame and the IP destination address of the entity to whom the message is being sent. Layer
3
switches typically strip away the data link headers from received data frames to reveal the IP or network header. Layer
3
switches may re-assemble or convert received data frames from one data link format (e.g., Ethernet) to another (e.g. token ring). Thus, layer
3
switches are often used to interconnect dissimilar subnetworks.
Each host or node implementing the TCP/IP protocol stack typically has only one network connection and is therefore assigned a single IP address. A layer
3
switch, however, typically has multiple ports each connected to the network. Associated with each port or physical connection, moreover, may be one or more logical connections or interfaces that provide connectivity between the IP software layer and the data link software layer. These interfaces may each be assigned a different IP address. Thus, a single layer
3
switch typically has many different IP addresses.
An IP version
4
address is 32 bits long and consists of a network number followed by a host number. The network number corresponds to the particular network on which the host resides and is used for routing purposes. The host number is used to address an individual entity located on the corresponding network. Network numbers are assigned from a central authority and each network number uniquely identifies a specific network. Host numbers are assigned by the local network administrator using any desired method. The combination of network number and host number results in a unique IP address across all networks. Nonetheless, unlike MAC addresses, there is nothing inherent in the configuration of an entity (like a particular network interface card) which determines its IP address.
A given network, moreover, may be divided into several parts called subnets for internal routing purposes. With subnets, the original host number is split into a subnet number and a new host number. The resulting IP address now includes a network number (which has not changed), a subnet number and a host number. Each entity on the same subnet has the same subnet number. To entities outside the network subnetting is not visible, since the network number of all entities on all subnets has not changed. Thus, subnetting allows an organization to segregate its various departments (e.g., marketing, engineering, etc.) without having to obtain new network numbers or change any external databases.
To determine which portion of an IP address corresponds to a subnet, a subnet mask is provided. The subnet mask is a 32 bit combination. By ANDing the subnet mask with the IP address, a device, such as a layer
3
switch, may learn the subnet number of the corresponding IP address. The number of available host numbers for a given subnet, moreover, depends on the number of bits selected to represent the subnet number. As additional hosts are added to a subnet, a network administrator may run out of available host numbers, requiring the assignment of a new subnet number and host number to all of the hosts. Also, if a large subnet is assigned to only a few hosts, valuable host numbers will be wasted. Planning for and implementation of subnets is thus an important task which demands substantial time and energy of network administrators.
When an entity wishes to send a message to another entity, upper layers of the communication software build a message packet and hand the packet along with the IP address of the recipient to the IP software layer for transmission. The IP address of the recipient may be learned through the well-known Domain Name System. Before passing the message packet down to the data link layer, the IP layer needs to determine the corresponding MAC address of the recipient. Typically, the IP layer utilizes the Address Resolution Protocol (ARP) to identify a MAC address based on a given IP address. With ARP, a device broadcasts a message asking which entity owns a given IP address. The broadcast will arrive at every entity on the corresponding subnetwork and each entity will check its IP address. The entity having the requested IP address will respond with its MAC address. The IP layer
Gai Silvano
McCloghrie Keith
Rekhter Yakov
Cesari and McKenna LLP
Cisco Technology Inc.
Olms Douglas
Phunkulh Bob A.
Reinemann Michael R.
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