Error detection/correction and fault detection/recovery – Pulse or data error handling – Digital data error correction
Reexamination Certificate
1998-12-14
2001-05-01
Tu, Christine T. (Department: 2784)
Error detection/correction and fault detection/recovery
Pulse or data error handling
Digital data error correction
C714S776000
Reexamination Certificate
active
06226771
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to computer networks, and more specifically, to a method and apparatus for generating error detection information for insertion into encapsulated frames.
BACKGROUND OF THE INVENTION
A computer network typically comprises a plurality of interconnected entities that transmit (i.e., “source”) or receive (i.e., “sink”) messages, such as frames or packets. A common type of computer network is a local area network (“LAN”) which typically refers to a privately owned network within a single building or campus. LANs employ 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), such as the Open Systems Interconnection (OSI) Reference Model from the International Standards Organization.
To form a wide area network (“WAN”), metropolitan area network (“MAN”) or intranet, one or more intermediate devices are often used to couple multiple LANs together. For example, a bridge may be used to provide a “bridging” function between two or more LANs. Alternatively, 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 a source port and transferring them to at least one destination port.
Virtual Local Area Networks
A computer network may also be segregated into a series of network groups. For example, U.S. Pat. No. 5,394,402, to Ross (the “'402 Patent”) discloses an arrangement that is capable of associating any port of a switch with any particular segregated network group. Specifically, according to the '402 Patent, any number of physical ports of a particular switch may be associated with any number of groups within the switch by using a virtual local area network (VLAN) arrangement that virtually associates the port with a particular VLAN designation. More specifically, Ross discloses a system that associates VLAN designations with at least one internal switch port and further associates those VLAN designations with messages transmitted from any of the ports to which the VLAN designation has been assigned. Thus, those entities having the same VLAN designation function as if they are all part of the same LAN.
The VLAN designation for each internal port is stored in a memory portion of the switch such that every time a message is received by the switch on an internal port the VLAN designation of that port is associated with the message. Association is accomplished by a flow processing element which looks up the VLAN designation in a memory based on the internal port where the message originated. Message exchanges between parts of the network having different VLAN designations are specifically prevented in order to preserve the boundaries of each VLAN segment. In addition to the '402 patent, the Institute of Electrical and Electronics Engineers (IEEE) is preparing a standard for Virtual Bridged Local Area Networks. See IEEE Standard 802.1q (draft).
Error Detection
To deal with errors caused during transmission of frames, most LAN standards include some form of error detection. Error detection allows the receiver to deduce that an error exists in a received frame. In response, the receiver may request retransmission of the frame. The IEEE Token Ring (802.5) and Ethernet (802.3) standards, for example, specify a 32 bit error detection algorithm known as the cyclic redundancy check (CRC) algorithm, which provides up to 10
10
bits of error detection. In particular, a k-bit data frame is processed to generate an n-bit Frame Check Sequence (FCS) that is appended to the frame such that the resulting frame, consisting of k+n bits, is exactly divisible by some number. A receiving entity divides the frame by the agreed-upon number and determines whether a remainder exists. If so, the receiving entity concludes that an error exists and requests retransmission of the frame.
When an Ethernet or Token Ring frame is received at a switch, the FCS value is typically stripped off and used to check whether an error exists with the frame. If an error is found, the frame is discarded. Since most source entities retransmit frames that are not acknowledged, the discarded frame will be retransmitted by the source entity. If no error is detected, the fame is switched onto the corresponding destination port for forwarding to the intended recipient. Before the frame is forwarded, however, the switch generates a new FCS value which is appended to the frame. By checking received frames, switches avoid transporting defective frames, thereby conserving network resources.
Fast Ethernet
The original Ethernet standard, referred to as 10 Base-T, is capable of transmitting data at 10 Mbs. In 1995, the IEEE approved a Fast Ethernet standard, referred to as 100 Base-T, which is capable of operating at 100 Mbs. Fast Ethernet is typically used on links or tis between two or more switches to improve network performance. Frames received at a first switch and destined to entities coupled to a neighboring switch are transported across such high speed links. Such messages, however, may be associated with more than one VLAN designation. To preserve VLAN associations of messages transported across trunks or links, both Ross and the 802.1 q standard disclose appending a VLAN identifier field to the corresponding frames.
In addition, U.S. Pat. No. 5,742,604 to Edsall et al. (the “'604 patent”), is which is commonly owned with the present application, discloses an Interswitch Link (ISL) encapsulation mechanism for efficiently transporting packets or frames, including VLAN-modified frames, between switches while maintaining the VLAN association of the frames. In particular, an ISL link, which may utilize the Fast Ethernet standard, connects ISL interface circuitry disposed at each switch. The transmitting ISL circuitry encapsulates the frame being transported within an ISL header and ISL error detection information, while the ISL receiving circuitry strips off this information and recovers the original frame.
FIG. 1
is a block diagram of an ISL encapsulated frame
100
. The encapsulated frame
100
includes an original frame, such as Ethernet frame
102
, that is encapsulated within an ISL header
104
and an ISL FCS field
106
. The Ethernet frame
102
includes a plurality of well-known fields, such as a Medium Access Control (MAC) destination address (DA)
108
, a MAC source address (SA)
110
, a data field
112
and an Ethernet FCS field
114
, among others. The ISL header
104
also includes a plurality of fields. In particular, the ISL header
104
includes an ISL DA field
116
, a frame type field
118
, an ISL SA field
120
, a length (LEN) field
122
and a VLAN identifier (ID) field
124
.
The ISL DA field
116
identifies the ISL interface circuitry of the receiving switch and may be a multicast or unicast address. The frame type field
118
indicates the type of original frame that is encapsulated. For example, although the ISL encapsulated frame
100
is shown encapsulating an Ethernet frame
102
, other frame types, such as Token Ring, Fiber Distributed Data Interface (FDDI) or Asynchronous Transfer Mode (ATM), may be encapsulated within ISL header
104
and ISL FCS
106
. The ISL SA field
120
may identify the ISL interface circuitry forwarding the frame
100
and the length field
112
indicates the length of frame
100
. The VLAN ID field
124
carries the VLAN designation associated with the original frame
102
being encapsulated. To the extent the original frame
102
includes its own VLAN ID field, such as a Tag Protocol Identifier Field (TPID) field as specified in the 802.1 q draft stanard, this designation may be repeated in field
124
.
As shown, the ISL encapsulated frame
100
includes two FCS fields: an Ethernet FCS fi
Edwards James M.
Hilla Stephen C.
Jennings William E.
Masterson Timothy F.
Cesari and McKenna LLP
Cisco Technology Inc.
Tu Christine T.
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