Fibre channel over ethernet

Description of equivalent WO 2007121101 A2

PATENT APPLICATION FIBRE CHANNEL OVER ETHERNET

0002Inventors: Claudio DeSanti

0003444 Coventry Rd Kensington, CA 94707

0004A Citizen of Italy

0005Silvano Gai 15840 Alta Vista Way

0006San Jose, Ca 95127

0007A Citizen of Italy

0008Andrea Baldini

00094153 23^rd Street

0010San Francisco, CA 94114 A Citizen of Italy

0011Assignee: Cisco Technology, Inc.

0012170 West Tasman Drive San Jose, California 95134-1706 A corporation of California

FIBRE CHANNEL OVER ETHERNET

CROSS-REFERENCE TO RELATED APPLICATIONS

0015This application is a continuation-in-part of United States Patent Application No. 11/078,992 (Attorney Docket No. CISCP409), entitled "Fibre Channel Over Ethernet" and filed on March 10, 2005, which claims priority to United States Provisional Application No. 60/621,396 (Attorney Docket No. CISCP404P), entitled "FC Over Ethernet" and filed on October 22, 2004, both of which are hereby incorporated by reference.

0016This application is related to United States Patent Application No. 11/084,587

0017(Attorney Docket No. CISCP404), entitled "Ethernet Extension for the Data Center" and filed on March 18, 2005, which is also hereby incorporated by reference.

BACKGROUND OF THE INVENTION

0019Fig. 1 depicts a simplified version of a data center of the general type that an enterprise that requires high availability and network storage (e.g., a financial institution) might use. Data center 100 includes redundant Ethernet switches with redundant connections for high availability. Data center 100 is connected to clients via network 105 via a firewall 115. Network 105 may be, e.g., an enterprise Intranet, a DMZ and/or the Internet. Ethernet is well suited for TCP/IP traffic between clients (e.g., remote clients 180 and 185) and a data center.

0020Within data center 105, there are many network devices. For example, many servers are typically disposed on racks having a standard form factor (e.g., one "rack unit" would be 19" wide and about 1.25" thick). A "Rack Unit" or "U" is an Electronic Industries Alliance (or more commonly "EIA") standard measuring unit for rack mount type equipment. This term has become more prevalent in recent times due to the proliferation of rack mount products showing up in a wide range of commercial, industrial and military markets. A "Rack Unit" is equal to 1.75" in height. To calculate the internal useable space of a rack enclosure you would simply multiply the total amount of Rack Units by 1.75". For example, a 44U rack enclosure would have 77" of internal usable space (44 x 1.75). Racks within a data center may have, e.g., about 40 servers each. A data center may have thousands of servers, or even more. Recently, some vendors have announced "blade servers," which allow even higher-density packing of servers (on the order of 60 to 80 servers per rack).

0021However, with the increasing numbers of network devices within a data center, connectivity has become increasingly complex and expensive. At a minimum, the servers, switches, etc., of data center 105 will typically be connected via an

0022Ethernet. For high availability, there will be at least 2 Ethernet connections, as shown in Fig. 1.

0023Moreover, it is not desirable for servers to include a significant storage capability. For this reason and other reasons, it has become increasingly common for enterprise networks to include connectivity with storage devices such as storage array 150. Historically, storage traffic has been implemented over SCSI (Small Computer System Interface) and/or FC (Fibre Channel).

0024In the mid-1990's SCSI traffic was only able to go short distances. A topic of key interest at the time was how to make SCSI go "outside the box." Greater speed, as always, was desired. At the time, Ethernet was moving from 10 Mb/s to 100Mb/s. Some envisioned a future speed of up to lGb/s, but this was considered by many to be nearing a physical limit. With 10 Mb/s Ethernet, there were the issues of half duplex and of collisions. Ethernet was considered to be somewhat unreliable, in part because packets could be lost and because there could be collisions. (Although the terms "packet" and "frame" have somewhat different meanings as normally used by those of skill in the art, the terms will be used interchangeably herein.)

0025FC was considered to be an attractive and reliable option for storage applications, because under the FC protocol packets are not intentionally dropped and because FC could already be run at 1 Gb/s. However, during 2004, both Ethernet and FC reached speeds of 10 Gb/s. Moreover, Ethernet had evolved to the point that it was full duplex and did not have collisions. Accordingly, FC no longer had a speed advantage over Ethernet. However congestion in a switch may cause Ethernet packets to be dropped and this is an undesirable feature for storage traffic.

0026During the first few years of the 21^st century, a significant amount of work went into developing iSCSI, in order to implement SCSI over a TCP/IP network. Although these efforts met with some success, iSCSI has not become very popular: iSCSI has about l%-2% of the storage network market, as compared to approximately 98%-99% for FC.

0027One reason is that the iSCSI stack is somewhat complex as compared to the FC stack. Referring to Fig. 7A, it may be seen that iSCSI stack 700 requires 5 layers: Ethernet layer 705, IP layer 710, TCP layer 715, iSCSI layer 720 and SCSI layer 725. TCP layer 715 is a necessary part of the stack because Ethernet layer 705 may lose packets, but yet SCSI layer 725 does not tolerate packets being lost. TCP layer 715 provides SCSI layer 725 with reliable packet transmission. However, TCP layer 715 is a difficult protocol to implement at speeds of 1 to 10 Gb/s. In contrast, because FC does not lose frames, there is no need to compensate for lost frames by a TCP layer or the like. Therefore, as shown in Fig. 7B, FC stack 750 is simpler, requiring only FC layer 755, FCP layer 760 and SCSI layer 765.

0028Accordingly, the FC protocol is normally used for communication between servers on a network and storage devices such as storage array 150 in Fig. 1. Therefore, data center 105 includes FC switches 140 and 145, provided by Cisco Systems, Inc. in this example, for communication between servers 110 and storage array 150.

0029Blade Servers are very popular because they are relatively inexpensive, powerful, standardized and can run any of the most popular operating systems. It is well known that in recent years the cost of a typical server has decreased and its performance level has increased. Because of the relatively low cost of servers and the potential problems that can arise from having more than one type of software application run on one server, each server is typically dedicated to a particular application. The large number of applications that is run on a typical enterprise network continues to increase the number of servers in the network.

0030However, because of the complexities of maintaining various types of connectivity (e.g., Ethernet and FC connectivity) with each server, each type of connectivity preferably being redundant for high availability, the cost of connectivity for a server is becoming higher than the cost of the server itself. For example, a single FC interface for a server may cost as much as the server itself. A server's connection with an Ethernet is typically made via a network interface card ("NIC") and its connection with an FC network is made with a host bus adaptor ("HBA").

0031The roles of devices in an FC network and a Ethernet network are somewhat different with regard to network traffic, mainly because packets are routinely dropped in response to congestion in a TCP/IP network, whereas frames are not intentionally dropped in an FC network. Accordingly, FC will sometimes be referred to herein as one example of a "no-drop" network, whereas Ethernet will be referred to as one manifestation of a "drop" network. When packets are dropped on a TCP/IP network, the system will recover quickly, e.g., in a few hundred microseconds. However, the protocols for an FC network are generally based upon the assumption that frames will not be dropped. Therefore, when frames are dropped on an FC network, the system does not recover quickly and SCSI may take minutes to recover.

0032Currently, a port of an Ethernet switch may buffer a packet for up to about 100 milliseconds before dropping it. As 10 Gb/s Ethernet is implemented, each port of an Ethernet switch would need more than 100 MB of RAM in order to buffer a packet for 100 milliseconds. This would be prohibitively expensive.

0033For some enterprises, it is desirable to "cluster" more than one server, as indicated by the dashed line around servers S2 and S3 in Fig. 1. Clustering causes an even number of servers to be seen as a single server. For clustering, it is desirable to perform remote direct memory access ("RDMA"), wherein the contents of one virtual memory space (which may be scattered among many physical memory spaces) can be copied to another virtual memory space without CPU intervention. The RDMA should be performed with very low latency. In some enterprise networks, there is a third type of network that is dedicated to clustering servers, as indicated by switch 175. This may be, for example, a "Myrinet," a "Quadrix" or an "Infiniband" network.

0034Therefore, clustering of servers can add yet more complexity to data center networks. However, unlike Quadrix and Myrinet, Infiniband allows for clustering and provides the possibility of simplifying a data center network. Infiniband network devices are relatively inexpensive, mainly because they use small buffer spaces, copper media and simple forwarding schemes. However, Infiniband has a number of drawbacks. For example, there is currently only one source of components for Infiniband switches. Moreover, Infiniband has not been proven to work properly in the context of, e.g., a large enterprise's data center. For example, there are no known implementations of Infiniband routers to interconnect Infiniband subnets. While gateways are possible between Infiniband and Fibre Channel and Infiniband to Ethernet, it is very improbable that Ethernet will be removed from the datacenter. This also means that the hosts would need not only an Infiniband connection, but also an Ethernet connection.

0035Accordingly, even if a large enterprise wished to ignore the foregoing shortcomings and change to an Infiniband-based system, the enterprise would need to have a legacy data center network (e.g., as shown in Fig. 1) installed and functioning while the enterprise tested an Infiniband-based system. Therefore, the cost of an Infiniband-based system would not be an alternative cost, but an additional cost.

0036It would be very desirable to simplify data center networks in a manner that would allow an evolutionary change from existing data center networks. An ideal system would provide an evolutionary system for consolidating server I/O and providing low latency and high speed at a low cost.

SUMMARY OF THE INVENTION

0038The present invention provides methods and devices for implementing a Low Latency Ethernet ("LLE") solution, also referred to herein as a Data Center Ethernet ("DCE") solution, which simplifies the connectivity of data centers and provides a high bandwidth, low latency network for carrying Ethernet, storage and other traffic. Some aspects of the invention involve transforming FC frames into a format suitable for transport on an Ethernet. Some preferred implementations of the invention implement multiple virtual lanes ("VLs") (also referred to as virtual links) in a single physical connection of a data center or similar network. Some VLs are "drop" VLs, with Ethernet-like behavior, and others are "no-drop" lanes with FC-like behavior.

0039A VL may be implemented, in part, by tagging a frame. Each VL should be treated independently from other VLs with respect to scheduling, flow control and resource assignment (e.g., queues, buffers); each VL may have its own credits. To allow a more complex topology and to allow better management of a frame inside a switch, TTL information may be added to a frame as well as a frame length field. There may also be encoded information regarding congestion, so that a source may receive an explicit message to slow down.

0040Some preferred implementations of the invention provide guaranteed bandwidth based on scheduling, credits and VL. Different VLs may be assigned different guaranteed bandwidths that can change over time. Preferably, a VL will remain a drop or no drop lane, but the bandwidth of the VL may be dynamically changed depending on the time of day, tasks to be completed, etc.

0041Active buffer management allows for both high reliability and low latency while using small frame buffers, even with 10 GB/s Ethernet. Preferably, the rules for active buffer management are applied differently for drop and no drop VLs. Some embodiments of the invention are implemented with copper media instead of fiber optics. Given all these attributes, I/O consolidation may be achieved in a competitive, relatively inexpensive fashion. According to some aspects of the invention, an FC frame, including FC addressing information, is encapsulated in an Ethernet frame for transmission on a DCE network. The Ethernet address fields may indicate that the frame includes an FC frame, e.g., by the use of a predetermined Organization Unique Identifier ("OUI") code in the D MAC field, but also include Ethernet addressing information. Such DCE frames can be forwarded properly by switches in the DCE network whether or not these switches are configured for operation according to the FC protocol. Accordingly, only a subset of switches in the DCE network needs to be FC-enabled. Only switches so configured will require an FC Domain_ID.

0042Some embodiments of the invention provide a network that includes a first plurality of switches configured for operation according to the Fibre Channel ("FC") protocol and a second plurality of switches configured for operation according to the DCE protocol but not configured for operation according to the FC protocol. Each of the first plurality of switches and each of the second plurality of switches are configured to carry traffic according to multiple protocols on virtual lanes of the same physical link. Each virtual lane may carry traffic according to a predetermined protocol. The multiple protocols may include the Ethernet protocol, the FC protocol and/or a clustering protocol.

0043At least some of the traffic on the network may comprise FC frames that are encapsulated as Ethernet frames. Each of the Ethernet frames may have an Ethertype field that indicates the Ethernet frame encapsulates an FC frame.

0044In some implementations of the network, each of the first plurality of switches is assigned an FC Domain ID and each of the second plurality of switches is not assigned an FC Domain_ID. Each of the second plurality of switches may be configured to forward any encapsulated FC frame to a predetermined switch of the first plurality of switches.

0045Each device in the network that is configured to encapsulate an FC frame in an Ethernet frame may also be configured to determine whether the Ethernet frame has a minimum size required by an Ethernet protocol and to pad the Ethernet frame when it is determined that the Ethernet frame does not have the minimum size. The determining step may be based the Ethertype value and/or the information in the destination media access control address ("D MAC") field.

0046The network may include a third plurality of host devices. An FC frame may be formed and encapsulated in an Ethernet frame by one of the host devices or one of the first plurality of switches.

0047An Ethernet frame that encapsulates an FC frame may include: (a) a D MAC field having a first portion with an Organization Unique Identifier ("OUI") value indicating that the DMAC address is mapping an FC D ID and a second portion that is the mapped FC destination identification ("D_ID") information corresponding to the D_ID of the encapsulated FC frame; and (b) a source MAC (S MAC) field that indicates the MAC address of the device that generated the FC frame.

0048Predetermined switches of the first plurality of switches may be configured to perform the following steps: receive the Ethernet frame; determine that the Ethernet frame contains an FC frame; decapsulate the FC frame; either forward or locally process the decapsulated FC frame. In some instances, the decapsulated FC frame is locally processed and the processing implies a response. Then, the predetermined switch may be further configured to perform the following steps: form an appropriate response; encapsulate the response in an additional Ethernet frame having a D MAC field indicating the device that sent the received frame; and transmit the additional Ethernet frame to the device that sent the received frame.

0049The predetermined switches may be further configured to determine whether the additional Ethernet frame has a minimum size required by an Ethernet protocol. When it is determined that the additional Ethernet frame does not have the minimum size, the predetermined switches may pad the additional Ethernet frame.

0050Some implementations of the invention provide a data networking method that includes the steps of forming a first FC frame and of encapsulating the first FC frame to form a first Ethernet frame. The encapsulation step may involve the following: including FC addressing information with the encapsulated FC frame; inserting a MAC address of a first device performing the forming and encapsulating steps into an S MAC field of the first Ethernet frame; and inserting a code in an Ethertype field of the first Ethernet frame. The code may indicate that the first Ethernet frame includes the first FC frame.

0051The method may include the step of determining whether the first Ethernet frame has a minimum size required by an Ethernet protocol. When it is determined that the first Ethernet frame does not have the minimum size, the method may in the step of padding the first Ethernet frame.

0052The first FC frame may be an FLOGI request frame. If so, the method may include these steps: receiving, by a switch that is operating according to the FC protocol, the first Ethernet frame; determining that the first Ethernet frame comprises an FLOGI request; decapsulating the first Ethernet frame; determining an address responsive to the FLOGI request; forming an FLOGI ACCEPT frame that includes the address; encapsulating the FLOGI ACCEPT frame as a second Ethernet frame; and transmitting the second Ethernet frame to the first device.

0053The encapsulation step may involve the following: inserting an OUI code into a first portion of a D MAC field of the first Ethernet frame, the OUI code indicating that the first Ethernet frame contains FC information; and inserting FC destination identification ("D ID") information from the first FC frame in a second portion of the D MAC field of the first Ethernet frame. When a first switch that is not operating according to the FC protocol receives the first Ethernet frame, the first switch may forward the first Ethernet frame to a predetermined second switch that is operating according to the FC protocol.

0054The step of encapsulating the FLOGI ACCEPT frame may involve inserting the MAC address of the first device into a D_MAC field of the second Ethernet frame. Accordingly, the method may also include these steps: receiving, by the first device, the second Ethernet frame; decapsulating the second Ethernet frame; reading the address in the FLOGI ACCEPT frame; and using the address as an S ID for subsequent transmissions that originate from the first device.

0055Some embodiments of the invention provide a gateway configured to enable communication between a storage network and a Data Center Ethernet ("DCE") network. The DCE network includes switches configured to carry traffic according to multiple protocols on virtual lanes of the same physical link. According to some such embodiments, the gateway includes a plurality of FC ports configured for communication with an FC network and a plurality of Ethernet ports configured for communication with the DCE network.

0056The gateway also includes at least one logic device configured to perform the following steps: receive a first FC frame from one of the plurality of FC ports; encapsulate the first FC frame as a first Ethernet frame; and forward the Ethernet frame to one of the plurality of Ethernet ports for transmission on a virtual lane of the DCE network. The encapsulation step may involve the following: inserting a MAC address of a first device performing the forming and encapsulating steps into an S_MAC field of the first Ethernet frame; inserting a code in an Ethertype field of the first Ethernet frame, the code indicating that the first Ethernet frame includes the first FC frame; and including FC addressing information with other encapsulated fields of the FC frame.

0057At least one logic device of the gateway may be configured to perform the following steps: receive a second Ethernet frame from one of the plurality of Ethernet FC ports, the second Ethernet frame encapsulating a second FC frame; decapsulate the second Ethernet frame; and forward the second FC frame to one of the plurality of FC ports for transmission on the storage network.

0058The methods described herein may be implemented and/or manifested in various ways, including as hardware, software, firmware or the like.

BRIEF DESCRIPTION OF THE DRAWINGS

0060The invention may best be understood by reference to the following description taken in conjunction with the accompanying drawings, which are illustrative of specific implementations of the present invention. Fig. 1 is a simplified network diagram that depicts a data center.

0061Fig. 2 is a simplified network diagram that depicts a data center according to one embodiment of the invention.

0062Fig. 3 is a block diagram that depicts multiple VLs implemented across a single physical link.

0063Fig. 4 illustrates one format of an Ethernet frame that carries additional fields for implementing DCE according to some implementations of the invention.

0064Fig. 5 illustrates one format of a link management frame according to some implementations of the invention.

0065Fig. 6A is a network diagram that illustrates a simplified credit-based method of the present invention.

0066Fig. 6B is a table that depicts a crediting method of the present invention.

0067Fig. 6C is a flow chart that outlines one exemplary method for initializing a link according to the present invention.

0068Fig. 7A depicts an iSCSI stack.

0069Fig. 7B depicts a stack for implementing SCSI over FC.

0070Fig. 8 depicts a stack for implementing SCSI over DCE according to some aspects of the invention.

0071Figs. 9A and 9B depict a method for implementing FC over Ethernet according to some aspects of the invention.

0072Fig. 10 is a simplified network diagram for implementing FC over Ethernet according to some aspects of the invention. Fig. 11 is a simplified network diagram for aggregating DCE switches according to some aspects of the invention.

0073Fig. 12 depicts the architecture of a DCE switch according to some embodiments of the invention.

0074Fig. 13 is a block diagram that illustrates buffer management per VL according to some implementations of the invention.

0075Fig. 14 is a network diagram that illustrates some types of explicit congestion notification according to the present invention.

0076Fig. 15 is a block diagram that illustrates buffer management per VL according to some implementations of the invention.

0077Fig. 16 is a graph that illustrates probabilistic drop functions according to some aspects of the invention.

0078Fig. 17 is a graph that illustrates an exemplary occupancy of a VL buffer over time.

0079Fig. 18 is a graph that illustrates probabilistic drop functions according to alternative aspects of the invention.

0080Fig. 19 illustrates a network device that may be configured to perform some methods of the present invention.

0081Fig. 20 is a simplified network diagram that illustrates some features of the invention.

0082Fig. 21 is a flow chart that outlines an exemplary method of the invention.

0083Fig. 22 depicts a DCE frame that includes an encapsulated FC frame according to some implementations of the invention.

0084Fig. 23 depicts another DCE frame that includes an encapsulated FC frame according to some implementations of the invention.

0085Fig. 24 indicates a discovery process between FCoE switches. Figs. 25 and 26 illustrate an ELP exchange.

0086Fig. 27 illustrates an example of a routing table for an FCoE switch.

0087Fig. 28 illustrates an example of a routing table for an FCoE switch.

0088Fig. 29 indicates a host device sending an FLOGI request.

0089Fig. 30 shows a responsive FLOGI accept returned to the host device.

0090Figs. 31 and 32 illustrate communication of 2 host devices across a DCE network.

0091Figs. 33 and 34 illustrate communication between a host devices in a DCE network and another device in an FC fabric.

0092Fig. 35 illustrates one use of a virtual FCoE switch.

0093Fig. 36 illustrates an FCoE multicast.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

0095Reference will now be made in detail to some specific embodiments of the invention including the best modes contemplated by the inventors for carrying out the invention. Examples of these specific embodiments are illustrated in the accompanying drawings. While the invention is described in conjunction with these specific embodiments, it will be understood that it is not intended to limit the invention to the described embodiments. On the contrary, it is intended to cover alternatives, modifications, and equivalents as may be included within the spirit and scope of the invention as defined by the appended claims. Moreover, numerous specific details are set forth below in order to provide a thorough understanding of the present invention. The present invention may be practiced without some or all of these specific details. In other instances, well known process operations have not been described in detail in order not to obscure the present invention.

0096The present invention provides methods and devices for simplifying the connectivity of data centers and providing a high bandwidth, low latency network for carrying Ethernet and storage traffic. Some preferred implementations of the invention implement multiple VLs in a single physical connection of a data center or similar network. A flow control mechanism is used, preferably per VL, either maintaining buffer-to-buffer credits or PAUSE frames. Some VLs are "drop" VLs, with Ethernet-like behavior, and others are "no-drop" lanes with FC-like behavior.

0097Some implementations provide intermediate behaviors between "drop" and "no-drop." Some such implementations are "delayed drop," wherein frames are not immediately dropped when a buffer is full, but instead there is an upstream "push back" for a limited time (e.g., on the order of milliseconds) before dropping a frame. Delayed drop implementations are useful for managing transient congestion.

0098Preferably, a congestion control scheme is implemented at layer 2. Some preferred implementations of the invention provide guaranteed bandwidth based on credits, scheduling and VL. An alternative to the use of credits is the use of the standard IEEE 802.3 (PAUSE frame) or an extension of such standard per VL to implement the "no drop" or "delayed drop" VLs. The IEEE 802.3 standard is hereby incorporated by reference for all purposes. For example, Annex 3 IB of the 802.3ae- 2002 standard, entitled "MAC Control PAUSE Operation," is specifically incorporated by reference. It is also understood that this invention will work in the absence of VLs but in that case the overall link will assume either a "drop" or "delayed drop" or "no drop" behavior.

0099Preferred implementations support a negotiation mechanism, for example one such as is specified by IEEE 802. Ix, which is hereby incorporated by reference. The negotiation mechanism can, e.g., determine whether a host device supports DCE and, if so, allow the host to receive VL information, e.g., which is the behavior of each individual VL.

0100Active buffer management allows for both high reliability and low latency while using small frame buffers. Preferably, the rules for active buffer management are applied differently for drop and no drop VLs.

0101Some implementations of the invention support an efficient RDMA protocol that is particularly useful for clustering implementations. In some implementations of the invention, network interface cards ("NICs") implement RDMA for clustering applications and also implement a reliable transport for RDMA. Some aspects of the invention are implemented via user APIs from the User Direct Access Programming Library ("uDAPL"). The uDAPL defines a set of user APIs for all RDMA-capable transports and is hereby incorporated by reference.

0102Fig. 2 is a simplified network diagram that illustrates one example of an LLE solution for simplifying the connectivity of data center 200. Data center 200 includes LLE switch 240, having router 260 for connectivity with TCP/IP network 205 and host devices 280 and 285 via firewall 215. The architecture of exemplary LLE switches is set forth in detail herein. Preferably, the LLE switches of the present invention can run 10 Gb/s Ethernet and have relatively small frame buffers. Some preferred LLE switches support only layer 2 functionality.

0103Although LLE switches of the present invention can be implemented using fiber optics and optical transceivers, some preferred LLE switches are implemented using copper connectivity to reduce costs. Some such implementations are implemented according to the proposed IEEE 802.3ak standard called 10Base-CX4, which is hereby incorporated by reference for all purposes. The inventors expect that other implementations will use the emerging standard IEEE P802.3an (10GBASE-T), which is also incorporated by reference for all purposes.

0104Servers 210 are also connected with LLE switch 245, which includes FC gateway 270 for communication with disk arrays 250. FC gateway 270 implements FC over Ethernet, which will be described in detail herein, thereby eliminating the need for separate FC and Ethernet networks within data center 200. Gateway 270 could be a device such as Cisco Systems' MDS 9000 IP Storage Service Module that has been configured with software for performing some methods of the present invention. Ethernet traffic is carried within data center 200 as native format. This is possible because LLE is an extension to Ethernet that can carry FC over Ethernet and RDMA in addition to native Ethernet.

0105Fig. 3 illustrates two switches 305 and 310 connected by a physical link 315. The behavior of switches 305 and 310 is generally governed by IEEE 802.1 and the behavior of physical link 315 is generally governed by IEEE 802.3. In general, the present invention provides for two general behaviors of LLE switches, plus a range of intermediate behaviors. The first general behavior is "drop" behavior, which is similar to that of an Ethernet. A second general behavior is "no drop" behavior, which is similar to that of FC. Intermediate behaviors between "drop" and "no drop" behaviors, including but not limited to the "delayed drop" behavior described elsewhere herein, are also provided by the present invention.

0106In order to implement both behaviors on the same physical link 315, the present invention provides methods and devices for implementing VLs. VLs are a way to carve out a physical link into multiple logical entities such that traffic in one of the VLs is unaffected by the traffic on other VLs. This is done by maintaining separate buffers (or separate portions of a physical buffer) for each VL. For example, it is possible to use one VL to transmit control plane traffic and some other high priority traffic without being blocked because of low priority bulk traffic on another VL. VLANs may be grouped into different VLs such that traffic in one set of VLANs can proceed unimpeded by traffic on other VLANs.

0107In the example illustrated by Fig. 3, switches 305 and 310 are effectively providing 4 VLs across physical link 315. Here, VLs 320 and 325 are drop VLs and VLs 330 and 335 are no drop VLs. In order to simultaneously implement both "drop" behavior and "no drop" behavior, there must be at least one VL assigned for each type of behavior, for a total of 2. (It is theoretically possible to have only one VL that is temporarily assigned to each type of behavior, but such an implementation is not desirable.) To support legacy devices and/or other devices lacking LLE functionality, preferred implementations of the invention support a link with no VL and map all the traffic of that link into a single VL at the first LLE port. From a network management perspective, it is preferable to have between 2 and 16 VLs, though more could be implemented.

0108It is preferable to dynamically partition the link into VLs, because static partitioning is less flexible. In some preferred implementations of the invention, dynamic partitioning is accomplished on a packet-by-packet basis (or a frame-by- frame basis), e.g., by adding an extension header or using specific fields of the frame header. For example, some implementations are based on the IEEE 802. IQ format. The present invention encompasses a wide variety of formats for such a header. In some implementations of the invention, there are two types of frames sent on a DCE link: these types are data frames and link management frames.

0109Although Figs. 4 and 5 illustrate formats for an Ethernet data frame and a link management frame, respectively, for implementing some aspects of the invention, alternative implementations of the invention provide frames with more or fewer fields, in a different sequence and other variations. Fields 405 and 410 of Fig. 4 are standard Ethernet fields for the frame's destination address and source address, respectively. Similarly, protocol type field 430, payload 435 and CRC field 440 may be those of a standard Ethernet frame.

0110However, protocol type field 420 indicates that the following fields are those of DCE header 425. If present, the DCE header will preferably be as close as possible to the beginning of the frame, as it enables easy parsing in hardware. The DCE header may be carried in Ethernet data frames, as shown in Fig. 4, as well as in link management frames (see Fig. 5 and the corresponding discussion). This header is preferably stripped by the MAC and does not need to be stored in a frame buffer. In some implementations of the invention, a continuous flow of link management frames is generated when there is no data traffic present or if regular frames cannot be sent due to lack of credits.

0111Most information carried in the DCE header is related to the Ethernet frame in which the DCE header is contained. However, some fields are buffer credit fields that are used to replenish credit for the traffic in the opposite direction. In this example, buffer credit fields are only carried by frames having a long DCE header. The credit fields may not be required if the solution uses the Pause frames instead of credits.

0112TTL field 445 indicates a time to live, which is a number decremented each time frame 400 is forwarded. Normally, a Layer 2 network does not require a TTL field. Ethernet uses a spanning tree topology, which is very conservative. A spanning tree puts constraints on the active topology and allows only one path for a packet from one switch to another.

0113In preferred implementations of the invention, this limitation on the active topology is not followed. Instead, it is preferred that multiple paths are active at the same time, e.g. via a link state protocol such as OSPF (Open Shortest Path First) or IS-IS (Intermediate System to Intermediate System). However, link state protocols are known to cause transient loops during topology reconfiguration. Using a TTL or similar feature ensures that transient loops do not become a major problem. Therefore, in preferred implementations of the invention, a TTL is encoded in the frame in order to effectively implement a link state protocol at layer 2. Instead of using a link state protocol, some implementations of the invention use multiple spanning trees rooted in the different LLE switches and obtain a similar behavior.

0114Field 450 identifies the VL of frame 400. Identification of the VL according to field 450 allows devices to assign a frame to the proper VL and to apply different rules for different VLs. As described in detail elsewhere herein, the rules will differ according to various criteria, e.g., whether a VL is a drop or a no drop VL, whether the VL has a guaranteed bandwidth, whether there is currently congestion on the VL and other factors.

0115ECN (explicit congestion notification) field 455 is used to indicate that a buffer (or a portion of a buffer allocated to this VL) is being filled and that the source should slow down its transmission rate for the indicated VL. In preferred implementations of the invention, at least some host devices of the network can understand the ECN information and will apply a shaper, a/k/a a rate limiter, for the VL indicated. Explicit congestion notification can occur in at least two general ways. In one method, a packet is sent for the express purpose of sending an ECN. In another method, the notification is "piggy-backed" on a packet that would have otherwise been transmitted.

0116As noted elsewhere, the ECN could be sent to the source or to an edge device. The ECN may originate in various devices of the DCE network, including end devices and core devices. As discussed in more detail in the switch architecture section below, congestion notification and responses thereto are important parts of controlling congestion while maintaining small buffer sizes.

0117Some implementations of the invention allow the ECN to be sent upstream from the originating device and/or allow the ECN to be sent downstream, then back upstream. For example, the ECN field 455 may include a forward ECN portion ("FECN") and a backward ECN portion ("BECN"). When a switch port experiences congestion, it can set a bit in the FECN portion and forward the frame normally. Upon receiving a frame with the FECN bit set, an end station sets the BECN bit and the frame is sent back to the source. The source receives the frame, detects that the BECN bit has been set and decreases the traffic being injected into the network, at least for the VL indicated.

0118Frame credit field 465 is used to indicate the number of credits that should be allocated for frame 400. There are many possible ways to implement such a system within the scope of the present invention. The simplest solution is to credit for an individual packet or frame. This may not be the best solution from a buffer management perspective: if a buffer is reserved for a single credit and a credit applies to each packet, an entire buffer is reserved for a single packet. Even if the buffer is only the size of an expected full-sized frame, this crediting scheme will often result in a low utilization of each buffer, because many frames will be smaller than the maximum size. For example, if a full-sized frame is 9KB and all buffers are 9KB, but the average frame size is 1500 bytes, only about 1/6 of each buffer is normally in use. A better solution is to credit according to a frame size. Although one could make a credit for, e.g., a single byte, in practice it is preferable to use larger units, such as 64B, 128B, 256B, 512B, 1024B, etc. For example, if a credit is for a unit of 512B, the aforementioned average 1500-byte frame would require 3 credits. If such a frame were transmitted according to one such implementation of the present invention, frame credit field 465 would indicate that the frame requires 3 credits.

0119Crediting according to frame size allows for a more efficient use of buffer space. Knowing the size of a packet not only indicates how much buffer space will be needed, but also indicates when a packet may be moved from the buffer. This may be particularly important, for example, if the internal transmission speed of a switch differs from the rate at which data are arriving at a switch port.

0120This example provides a longer version and a shorter version of the DCE header. Long header field 460 indicates whether or not the DCE header is a long or a short version. In this implementation, all data frames contain at least a short header that includes TTL, VL, ECN, and Frame Credit information in fields 445, 450, 455 and 465, respectively. A data frame may contain the long header if it needs to carry the credit information associated with each VL along with the information present in the short header. In this example, there are 8 VLs and 8 corresponding fields for indicating buffer credits for each VL. The use of both short and long DCE headers reduces the overhead of carrying credit information in all frames.

0121When there is no data frame to be sent, some embodiments of the invention cause a link management frame ("LMF") to be sent to announce credit information. An LMF may also be used to carry buffer credit from a receiver or to carry transmitted frame credit from a Sender. An LMF should be sent uncredited (Frame Credit = 0) because it is preferably consumed by the port and not forwarded. An

0122LMF may be sent on a periodic basis and/or in response to predetermined conditions, for example, after every 10MB of payload has been transmitted by data frames.

0123Fig. 5 illustrates an example of an LMF format according to some implementations of the invention. LMF 500 begins with standard 6B Ethernet fields 510 and 520 for the frame's destination address and source address, respectively. Protocol type header 530 indicates that DCE header 540 follows, which is a short DCE header in this example (e.g., Long Header field = 0). The VL, TTL, ECN and frame credit fields of DCE header 540 are set to zero by the sender and ignored by the receiver. Accordingly, an LMF may be identified by the following characteristics: Protocol_Type = DCE_Header and Long_Header = 0 and Frame_Credit = 0.

0124Field 550 indicates receiver buffer credits for active VLs. In this example, there are 8 active VLs, so buffer credits are indicated for each active VL by fields 551 through 558. Similarly, field 560 indicates buffer credits for the sending device, so frame credits are indicated for each active VL by fields 561 through 568.

0125LMF 500 does not contain any pay load. If necessary, as in this example, LMF 500 is padded by pad field 570 to 64 Bytes in order to create a legal minimum-sized Ethernet frame. LMF 500 terminates with a standard Ethernet CRC field 580.

0126In general, the buffer-to-buffer crediting scheme of the present invention is implemented according to the following two rules: (1) a Sender transmits a frame when it has a number of credits from the Receiver greater or equal to the number of credits required for the frame to be sent; and (2) a Receiver sends credits to the Sender when it can accept additional frames. As noted above, credits can be replenished using either data frames or LMFs. A port is allowed to transmit a frame for a specific VL only if there are at least as many credits as the frame length (excluding the length of the DCE header).

0127Similar rules apply if a Pause Frame is used instead of credits. A Sender transmits a frame when it has not been paused by the Receiver. A Receiver sends a PAUSE frame to the Sender when it cannot accept additional frames.

0128Following is a simplified example of data transfer and credit replenishment. Fig. 6A illustrates data frame 605, having a short DCE header, which is sent from switch B to switch A. After packet 605 arrives at switch A, it will be kept in memory space 608 of buffer 610. Because some amount of the memory of buffer 610 is consumed, there will be a corresponding decrease in the available credits for switch B. Similarly, when data frame 615 (also having a short DCE header) is sent from switch A to switch B, data frame 615 will consume memory space 618 of buffer 620 and there will be a corresponding reduction in the credits available to switch A. However, after frames 605 and 615 have been forwarded, corresponding memory spaces will be available in the buffers of the sending switches. At some point, e.g., periodically or on demand, the fact that this buffer space is once again available should be communicated to the device at the other end of the link. Data frames having a long DCE header and LMFs are used to replenish credits. If no credits are being replenished, the short DCE header may be used. Although some implementations use the longer DCE header for all transmissions, such implementations are less efficient because, e.g., extra bandwidth is being consumed for packets that contain no information regarding the replenishment of credits.

0129Fig. 6B illustrates one example of a credit signaling method of the present invention. Conventional credit signaling scheme 650 advertises the new credits that the receiver wants to return. For example, at time t4 the receiver wants to return 5 credits and therefore the value 5 is carried in the frame. At time t5 the receiver has no credit to return and therefore the value 0 is carried in the frame. If the frame at time t4 is lost, five credits are lost.

0130DCE scheme 660 advertises the cumulative credit value. In other words, each advertisement sums the new credit to be returned to the total number of credits previously returned modulo m (with 8 bits, m is 256). For example, at time t3 the total number of credits returned since link initialization is 3; at time t4, since 5 credits need to be returned, 5 is summed to 3 and 8 is sent in the frame. At time t5 no credits need to be returned and 8 is sent again. If the frame at time t4 is lost, no credits are lost, because the frame at time t5 contains the same information.

0131According to one exemplary implementation of the invention, a receiving DCE switch port maintains the following information (wherein VL indicates that the information is maintained per virtual lane):

0132• BufCrd[VL] - a modulus counter which is incremented by the number of credits which could be sent;

0133• BytesFromLastLongDCE - the number of bytes sent since the last Long DCE header;

0134• BytesFromLastLMF - the number of bytes sent since the last LMF; • MaxIntBetLongDCE - the maximum interval between sending Long DCE header;

0135• MaxIntBetLMF - the maximum interval between sending LMF; and

0136• FrameRx - a modulus counter which is incremented by the FrameCredit field of the received frame.

0137A sending DCE switch port maintains the following information:

0138• LastBufCrdfVL] - The last estimated value of the BufCrd[VL] variable of the receiver; and

0139• FrameCrd[VL] - a modulus counter which is incremented by the number of credits used to transmit a frame.

0140When links come up, the network devices on each end of a link will negotiate the presence of a DCE header. If the header is not present, the network devices will, for example, simply enable the link for standard Ethernet. If the header is present, the network devices will enable features of a DCE link according to some aspect of the invention.

0141Fig. GC is a flow chart that indicates how a DCE link is initialized according to some implementations of the invention. One of skill in the art will appreciate that the steps of method 680 (like other methods described herein) need not be, and in some cases are not, performed in the order indicated. Moreover, some implementations of these methods include more or fewer steps than are indicated.

0142In step 661, the physical link comes up between two switch ports and in step 663 a first packet is received. In step 665, it is determined (by the receiving port) whether the packet has a DCE header. If not, the link is enabled for standard Ethernet traffic. If the packet has a DCE header, the ports perform steps to configure the link as a DCE link. In step 671, the receiver and sender zero out all arrays relating to traffic on the link. In step 673, the value of MaxIntBetLongDCE is initialized to a configured value and in step 675, MaxIntBetLMF is initialized to a configured value. In step 677, the two DCE ports exchange available credit information for each VL, preferably by sending an LMF. If a VL is not used, its available credit is announced as 0. In step 679, the link is enabled for DCE and normal DCE traffic, including data frames, may be sent on the link according to the methods described herein.

0143To work properly in the presence of a single frame loss, the DCE self- recovering mechanism of preferred implementations requires that the maximum number of credits advertised in a frame be less than 1/2 of the maximum advertisable value. In some implementations of the short DCE header each credit field is 8 bits, i.e. a value of 256. Thus, up to 127 additional credits can be advertised in a single frame. The maximum value of 127 credits is reasonable, since the worst situation is represented by a long sequence of minimum size frames in one direction and a single jumbo frame in the opposite direction. During the transmission of a 9KB jumbo frame, the maximum number of minimum size frames is approximately 9220B/84B = 110 credits (assuming a 9200-byte maximum transmission unit and 20 bytes of IPG and Preamble).

0144If multiple consecutive frames are lost, an LMF recovery method can "heal" the link. One such LMF recovery method works on the idea that, in some implementations, internal counters maintained by the ports of DCE switches are 16 bits, but to conserve bandwidth, only the lower 8 bits are transmitted in the long DCE header. This works well if there are no consecutive frame losses, as explained before. When the link experiences multiple consecutive errors, the long DCE header may no longer be able to synchronize the counters, but this is achieved through LMFs that contain the full 16 bits of all the counters. The 8 additional bits allow the recovery of 256 times more errors for a total of 512 consecutive errors. Preferably, before this situation is encountered the link is declared inoperative and reset.

0145In order to implement a low latency Ethernet system, at least 3 general types of traffic must be considered. These types are IP network traffic, storage traffic and cluster traffic. As described in detail above, LLE provides "no drop" VLs with FC- like characteristics that are suitable for, e.g., storage traffic. The "no drop" VL will not lose packets/frames and may be provided according to a simple stack, e.g., as shown in Fig. 8. Only a small "shim" of FC over LLE 810 is between LLE layer 805 and FC Layer 2 (815). Layers 815, 820 and 825 are the same as those of FC stack 750. Therefore, storage applications that were previously running over FC can be run over LLE.

0146The mapping of FC frames to FC over Ethernet frames according to one exemplary implementation of FC over LLE layer 810 will now be described with reference to Figs. 9A, 9B and 10. A preferred implementation is discussed below with reference to Figs. 22 and 23.

0147Fig. 9A is a simplified version of an FC frame. FC frame 900 includes SOF 905 and EOF 910, which are ordered sets of symbols used not only to delimit the boundaries of frame 900, but also to convey information such as the class of the frame, whether the frame is the start or the end of a sequence (a group of FC frames), whether the frame is normal or abnormal, etc. At least some of these symbols are illegal "code violation" symbols. FC frame 900 also includes 24-bit source FC ID field 915, 24-bit destination FC ID field 920 and payload 925.

0148One goal of the present invention is to convey storage information contained in an FC frames, such as FC frame 900, across an Ethernet. Fig. 10 illustrates one implementation of the invention for an LLE that can convey such storage traffic. Network 1000 includes LLE cloud 1005, to which devices 1010, 1015 and 1020 are attached. LLE cloud 1005 includes a plurality of LLE switches 1030, exemplary architecture for which is discussed elsewhere herein. Devices 1010, 1015 and 1020 may be host devices, servers, switches, etc. Storage gateway 1050 connects LLE cloud 1005 with storage devices 1075. For the purposes of moving storage traffic, network 1000 may be configured to function as an FC network. Accordingly, the ports of devices 1010, 1015 and 1020 each have their own FC ID and ports of storage devices 1075 have FC IDs.

0149In order to move efficiently the storage traffic, including frame 900, between devices 1010, 1015 and 1020 and storage devices 1075, some preferred implementations of the invention map information from fields of FC frame 900 to corresponding fields of LLE packet 950. LLE packet 950 includes SOF 955, organization ID field 965 and device ID field 970 of destination MAC field, organization ID field 975 and device ID field 980 of source MAC field, protocol type field 985, field 990 and payload 995.

0150Preferably, fields 965, 970, 975 and 980 are all 24-bit fields, in conformance with normal Ethernet protocol. Accordingly, in some implementations of the invention, the contents of destination FC ID field 915 of FC frame 900 are mapped to one of fields 965 or 970, preferably to field 970. Similarly, the contents of source FC ID field 920 of FC frame 900 are mapped to one of fields 975 or 980, preferably to field 980. It is preferable to map the contents of destination FC ID field 915 and source FC ID field 920 of FC frame 900 to fields 970 and 980, respectively, of LLE packet 950 because, by convention, many device codes are assigned by the IEEE for a single organization code. This mapping function may be performed, for example, by storage gateway 1050.

0151Therefore, the mapping of FC frames to LLE packets may be accomplished in part by purchasing, from the IEEE, an Organization Unique Identifier ("OUI") codes that correspond to a group of device codes. In one such example, the current assignee, Cisco Systems, pays the registration fee for an OUI, assigns the OUI to "FC over Ethernet." A storage gateway configured according to this aspect of the present invention (e.g., storage gateway 1050) puts the OUI in fields 965 and 975, copies the 24-bit contents of destination FC ID field 915 to 24-bit field 970 and copies the 24-bit contents of source FC ID field 920 to 24-bit field 980. The storage gateway inserts a code in protocol type field 985 that indicates FC over Ethernet and copies the contents of payload 925 to payload field 995.

0152Because of the aforementioned mapping, no MAC addresses need to be explicitly assigned on the storage network. Nonetheless, as a result of the mapping, an algorithmically derived version of the destination and source FC IDs are encoded in corresponding portions of the LLE frame that would be assigned, in a normal Ethernet packet, to destination and source MAC addresses. Storage traffic may be routed on the LLE network by using the contents of these fields as if they were MAC address fields.

0153The SOF field 905 and EOF field 910 contain ordered sets of symbols, some of which (e.g., those used to indicate the start and end of an FC frame) are reserved symbols that are sometimes referred to as "illegal" or "code violation" symbols. If one of these symbols were copied to a field within LLE packet 950 (for example, to field 990), the symbol would cause an error, e.g., by indicating that LLE packet 950 should terminate at that symbol. However, the information that is conveyed by these symbols must be retained, because it indicates the class of the FC frame, whether the frame is the start or the end of a sequence and other important information.

0154Accordingly, some implementations of the invention provide another mapping function that converts illegal symbols to legal symbols. These legal symbols may then be inserted in an interior portion of LLE packet 950. In one such implementation, the converted symbols are placed in field 990. Field 990 does not need to be very large; in some implementations, it is only 1 or 2 bytes in length.

0155To allow the implementation of cut-through switching field 990 may be split into two separate fields. For example, one field may be at the beginning of the frame and one may be at the other end of the frame.

0156The foregoing method is but one example of various techniques for encapsulating an FC frame inside an extended Ethernet frame. Alternative methods include any convenient mapping that involves, for example, the derivation of the tuple {VLAN, DST MAC Addr, Src MAC Addr} from the tuple {VSAN, D_ID, S_ID}. One preferred encapsulation method is discussed below with reference to Figs. 21- 23.

0157The aforementioned mapping and symbol conversion processes produce an

0158LLE packet, such as LLE packet 950, that allows storage traffic to and from FC-based storage devices 1075 to be forwarded across LLE cloud 1005 to end node devices 1010, 1015 and 1020. The mapping and symbol conversion processes can be run, e.g., by storage gateway 1050, on a frame-by-frame basis. For implementations using a method such as that discussed below with reference to Figs. 21- 23, an encapsulation or decapsulation process is performed instead of a mapping/symbol conversion process.

0159Some preferred implementations of the invention provide DCE networks that include devices that do not exploit all of the capabilities of the DCE protocols. For example, only a subset of the switches of a DCE network may implement or use the Fibre Channel over Ethernet feature. One such DCE network is illustrated in Fig. 20. Here, DCE network 2005 includes switches 2010, 2015, 2030 and 2035. Switches 2010 and 2035 are configured to operate according to the FC over Ethernet protocol, whereas switches 2015 and 2030 are not. Host device 2020 is connected to switch 2015 via host bus adapter ("HBA") 2022 and host device 2025 is connected to switch 2015 via HBA 2027.

0160As understood by those of skill in the art, when an interface of a device is connected to a port of an FC switch, the HBA initiates a fabric log-in ("FLOGI") process. The standard "Fibre Channel Framing and Signaling Interface (FC-FS)" (ANSI INCITS 373-2003) describes relevant information, including details of FLOGI requests and responses (see, for example, sections 12.3.2.41 and 12.3.2.7), and is hereby incorporated by reference for all purposes.

0161By sending an FLOGI request, the HBA is saying, in effect "I am here and I need to be provisioned." The FC switch assigns an address to the new interface and provides other provisioning information. The address will be used as an S ID for data sent from the HBA and will be used as a D ID for data sent to the HBA.

0162This process is more involved when implemented in a DCE network that includes switches that are not configured to operate according to the FC protocol. Referring again to Fig. 20, for example, suppose that HBA 2022 of host device 2020 forms an FLOGI request and converts the FLOGI request to a DCE frame. This process is preferably done according to the method described below with reference to Figs. 22 and 23. The D_MAC field of the DCE frame 2024 would indicate FF.FF.FE, the characteristic D ID of an FLOGI request. The S MAC field would indicate 00.00.00, the characteristic S ID of an FLOGI request from a host device.

0163DCE switch 2015 is not configured to operate according to the FC protocol and therefore cannot provide a response to FLOGI request 2024. However, DCE switch 2015 could have the address of a predetermined FC-enabled fabric switch in its forwarding table. Therefore, switch 2015 could forward the mapped FLOGI request 2024 to switch 2035, for example.

0164However, there is nothing in the addressing structure of an FLOGI request that uniquely identifies a device. IfFLOGI request 2029 were also formed according to the previously-described mapping process, it would be identical to FLOGI request 2024. Therefore, switch 2035 would not be able to determine what interface should be assigned an address or to what interface an FLOGI ACCEPT should be transmitted in response to FLOGI request 2024 or FLOGI request 2029.

0165Moreover, being configured to support the FC protocols, switch 2010 and

01662035 exchanges between themselves FC control protocol messages (e.g., ELP messages), as defined in the standard "Fibre Channel Switch Fabric - 3 (FC-SW-3)" (ANSI INCITS 384-2004), hereby incorporated by reference for all purposes. Most of these messages are encoded in FC frames having the same source and destination FC ID (i.e., FF.FF.FD). If switch 2010 mapped these messages in DCE frames according to the previously-described mapping process, the resulting DCE frames would violate the Ethernet protocol because the D MAC and S MAC fields of the resulting DCE frame would have the same value. This would cause the DCE frame to be dropped by switch 2015 or switch 2030.

0167Therefore, some preferred implementations of the present invention provide alternative methods of forming DCE frames. One such method 2100 will now be described with reference to Fig. 21. The method may be used to form various types of DCE frames. Accordingly, the following example in the FLOGI context is merely illustrative. In step 2110, an FC frame (which is an FLOGI request in this example) is created. The FLOGI request may be created, for example, by a host device or a switch.

0168In step 2115, the FC frame is encapsulated by an Ethernet frame for transmission on a DCE network. In this example, step 2115 involves forming a frame similar to the frame depicted in Fig. 22. Frame 2200 is a frame that has been formed by HBA 2022 of host 2020. Only a simplified version of frame 2200 is depicted in Fig. 22: it will be appreciated by those of skill in the art that frame 2200 may include a number of fields not shown or described herein.

0169Frame 2200 includes Ethernet portion 2205 and encapsulated FC portion 2255. Here, the D_MAC field 2270 (also known as a DA field according to IEEE 802.3) of frame 2200 includes the FC over Ethernet OUI (i.e., FC-OUI) 2210 and the encapsulated D ID 2215. As described elsewhere herein, FC-OUI 2210 may indicate that the frame 2200 includes FC information. In this implementation, most or all of the FC information is in FC portion 2255. As described elsewhere herein, 802.1Q field (not shown, but which would follow field 2220) associates frame 2200 with the appropriate priority that enables no-drop behavior. Ethertype field 2225 also indicates that frame 2200 encapsulates an FC frame.

0170However, unlike that of a DCE frame formed according to the previously- described mapping procedures, S MAC field 2220 (also known as an SA field according to IEEE 802.3) does not include FC addressing information. Instead, S_MAC field 2220 carries the MAC address of the device in host 2020 (here, HBA 2022) that formed the FLOGI request.

0171SOF field 2230 delineates the first part of FC portion 2255. DJD field 2235 would normally indicate a specific destination ID. In this example, D_ID field indicates FF.FF.FE, because the encapsulated FC frame comprises an FLOGI request. Similarly, S ID field 2240 would normally indicate a specific source ID. However, in this example S_ID field 2240 indicates 00.00.00, because the FLOGI request originated from a host device.

0172The additional fields of the FLOGI request, as described in the FC-FS standard, are carried in portion 2245 of DCE frame 2200. EOF field 2250 indicates the end of the encapsulated FC frame 2255.

0173Preferred implementations of the invention take into account the variable length of FC frames and the differing minimum sizes of FC and Ethernet frames. A zero length Data Field is legal for an FC frame and therefore the minimum size for a valid FC frame, not including SOF and EOF, is 24 bytes for the Frame Header plus 4 bytes for the CRC. On the other hand, Ethernet mandates a minimum frame size of 64 bytes when the 802.1 Q tag is used.

0174Therefore, encapsulation step 2115 preferably includes a determination of whether the resulting DCE frame is at least the minimum size required by the Ethernet protocol. If not, a padding field 2260 is added to increase the DCE frame to the minimum required length. Frame 2200 ends with frame check sequence ("FCS") 2265 as specified by Ethernet protocol. An FLOGI frame does not require padding. Referring once again to Fig. 21 and Fig.20, after the FLOGI request has been formed, and encapsulated, it is transmitted (step 2120). Here, HBA 2022 transmits frame 2200 to switch 2015.

0175In step 2125, it is determined whether the switch receiving frame 2200 is configured to operate according to the FC protocol. Being not configured to operate according to the FC protocol (and therefore being unable to provide a response to the FLOGI request), switch 2015 includes an entry in its forwarding table indicating that such frames should be forwarded to FC-enabled switch 2035. (Step 2130.)

0176After receiving frame 2200, switch 2045 determines from Ethertype field 2225 and/or FC-OUI field 2210 that frame 2200 includes an encapsulated FC frame. Based on the information in D MAC field 2270, switch 2035 can determine if frame 2200 has to be forwarded toward the FC cloud or if it has to be locally processed. If it has to be locally processed, as in the case of FLOGI, switch 2035 decapsulates frame 2200. (Step 2135.)

0177In response to the FLOGI request, switch 2035 assigns an address (in this example, 1.2.3) and determines other configuration information for HBA 2022. (Step 2140.) Switch 2035 updates its forwarding table and notifies other switches in the network of the new address, then forms a response (here, an FLOGI ACCEPT) that includes the address and provisioning information. (Step 2145.)

0178Switch 2035 then encapsulates the FLOGI ACCEPT (step 2150). In this example, switch 2035 forms an encapsulated FLOGI ACCEPT like the simplified frame 2300, shown in Fig. 23. As part of the encapsulation process, switch 2035 inserts the address from S_MAC field of frame 2200 into D_MAC field 2315 of frame 2300. Switch 2035 inserts a MAC address associated with itself (e.g., of a line card or the like that is performing the operation) into S_MAC field 2320. These are normal Ethernet addresses, not FC addresses, and can be forwarded by any device in DCE network 2005.

0179The encapsulated FC frame includes the newly-assigned address, 1.2.3, in DJD field 2335 and the S_ID of a typical FLOGI ACCEPT (FF.FF.FE) in S_ID field 2340. As before, the encapsulation process includes a determination of whether frame 2300 would be at least as large as a minimum-sized frame required by the Ethernet protocol. In this example, frame 2300 does not require padding in field 2360 to attain this minimum size.

0180The encapsulated FLOGI ACCEPT is then returned to HBA 2022. (Step 2155.) Frame 2300 is decapsulated (step 2160) and HBA 2022 uses the address and provisioning information of the FLOGI ACCEPT. (Step 2165.) Subsequent DCE frames from HBA 2022 will indicate an S_ID of 1.2.3, which will be recognized by the FC switches of the network as being associated with HBA 2022. The process ends in step 2170.

0181When an ELP message is sent from one switch of a DCE network to another switch, both implementing the FC protocols, a similar process occurs. For example, switch 2010 of DCE network 2005 could send an ELP request encapsulated in an Ethernet frame having a D_MAC field that contains an FC-OUI and the D_ID of the Fabric Controller (FF.FF.FD). The S_MAC field of the Ethernet portion would contain a MAC address of switch 2010 that formed the frame. The frame would also include an encapsulated FC portion with a D ID of FF.FF.FD and an S_ID of FF.FF.FD.

0182The frame could be sent to either switch 2015 or switch 2030. When either of these switches receives such a frame, it will forward the frame to a predetermined switch that supports the FC protocol (e.g., the nearest switch that supports the FC protocol), according to a corresponding entry in its forwarding table. In this example, the predetermined switch is switch 2035.

0183Switch 2035 will determine that the frame contains an ELP request from another FC switch. Accordingly, switch 2035 will return an ELP response to switch 2010. The D_MAC field of the returned frame will include the address that was in the S_MAC field of the frame from switch 2035, which is a normal Ethernet address. Either switch 2015 or switch 2030 will forward the frame to switch 2010, as indicated in the D MAC field, according to the Ethernet protocol.

0184The present invention provides increased flexibility regarding the deployment of devices in DCE networks. In particular, it is not necessary for each switch in the DCE network to run FC protocols. In fact, it is envisioned that in many networks only a minority of the switches will be running FC. For example, an Ethernet cloud may appear (e.g., to storage network 2040 of Fig. 20) as a single FC switch. The control protocols may be run in a single place and the traffic may be forwarded according to Spanning Tree (or link state routing protocol). In order to permit the deployment of the Fibre Channel control protocols in only a subset of the DCE network, DCE switches use the MAC addresses to send a DCE frame. According to some implementations of the invention, the physical topology of a Fibre Channel Fabric is replaced with a logical topology implemented over the DCE network by encapsulating FC frames into DCE frames and using MAC addresses. This allows a set of DCE switches to appear as a single switch in the FC realm.

0185Some preferred implementations also help to gain flexibility regarding the FC limitation on Domain IDs. Currently, there is a maximum of 239 Domain IDs in an FC fabric. Suppose the DCE cloud 2005 of Fig. 20 had 100 switches instead of 4. The prior implementation required each of these 100 switches to have its own Domain ID. Each one would count toward the maximum of 239, leaving only 139 for FC cloud 2040. However, in the new implementation, only those switches in the DCE cloud that are capable of running FC would need to count toward the 239-switch limit. Because it is envisioned that a minority of the switches in a DCE cloud would be running FC, the present implementation may significantly reduce the number of Domain IDs that need to be assigned to switches in a DCE network.

0186Accordingly, the present invention provides exemplary methods for encapsulating an FC frame inside an extended Ethernet frame at the ingress edge of an FC-Ethernet cloud. Analogous method of the invention provide for an inverse process that is performed at the egress edge of the Ethernet-FC cloud. An FC frame may be decapsulated from an extended Ethernet frame and then transmitted on an FC network.

0187For example, switch 2035 of DCE network includes a device, such as a line card, that acts as a gateway between FC network 2040 and DCE network 2005. The line card 2045 can encapsulate FC frames received from the FC network into DCE frames that are usable on the DCE network, as described above. The line card can also receive DCE frames from other devices on the DCE network and decapsulate the DCE frames into FC frames for transmission to the FC network. The gateway does not need to keep track of frame state information.

0188An alternative embodiment of how the present invention may be deployed in a DCE network is here described. In this alternative embodiment Fibre Channel services are provided at the logical edge of the DCE network, while the core of the DCE network is unaware of Fibre Channel. In this embodiment FCoE hosts (i.e., hosts implementing the Fibre Channel over DCE functionality and protocols) may be topologically directly connected to FCoE switches (i.e., DCE switches implementing the Fibre Channel over DCE functionality and protocols). FCoE switches are also logically directly connected via MAC-in-MAC tunnels implemented over the DCE network, forming a fully meshed logical topology at the logical edge of the DCE network. During operation, each FCoE switch acquires a Fibre Channel Domain ID, in addition to the DCE Switch ID, in order to provide FC services.

0189In this embodiment all switch to switch communications use the MAC-in- MAC encapsulation (i.e., the FCoE frames are further encapsulated in an Ethernet frame via the addition of another DCE header) and the following encapsulation rules are followed:

01901. A reserved DCE PortJD (FC-PID) identifies the FC end-point internal to an FCoE switch. This DCE PortJD is used for MAC-in-MAC encapsulation in both source and destination MAC addresses.

01912. A normal MAC address (FC-MAC) identifies the FC end-point internal to an FCoE switch. This FC-MAC address is used as source MAC for the FCoE encapsulation when the FCoE switch originates FCoE frames.

01923. The destination MAC address in the FCoE encapsulation is algorithmically derived from the D_ID of the encapsulated FC frame, by concatenating the D_ID of the FC frame to the Fibre Channel OUI (i.e., Destination MAC = FC-OUI || DJD). The FLOGI accept requires a special treatment, and therefore constitutes an exception to this rule.

01934. The source MAC address in the FCoE encapsulation is the MAC address of the generating station. To begin operations, an FCoE switch has to discover the other FCoE switches present on the DCE network (i.e., the DCE SwitchJD of the other FCoE switches). This may be done via manual configuration, or automatically via some information provided by the underlining DCE routing protocol (e.g., an extended version of the IS-IS routing protocol). As an example, FCoE enabled switches may advertise their presence with a flag in the IS-IS advertisements. Knowing the DCE Switch ID of a remote FCoE switch provides enough information to perform the MAC-in-MAC encapsulation to reach the remote FCoE Switch: using the encapsulation rules described above the Destination MAC address to use is then Switch ID || FC-PID.

0194Referring to network 2400 of Fig. 24, the FCoE switches A, B and C are interconnected via DCE core switches 2405. FCoE switch C provides a gateway between DCE core 2405 and FC fabric switches 2425. Accordingly, FCoE switch C is configured to encapsulate FC frames within Ethernet frames to form FCoE frames, as described elsewhere. Likewise, FCoE switch C is configured to decapsulate FCoE frames and extract FC frames. However, FCoE switches A and B are not necessarily configured to perform such encapsulation and decapsulation.

0195FCoE switches A, B and C discover each other, e.g., via a flag in an IS-IS TLV. This provides enough information (i.e., the Switch ID) to create a logical (i.e., MAC-in-MAC tunneled) full meshed topology among FCoE switches A, B and C, as represented by arrows 2410, 2415 and 2420.

0196Once discovery is performed, FCoE switches A, B and C begin the Fibre Channel operations. From a Fibre Channel perspective, each MAC-in-MAC tunnel is seen by an FCoE Switch as a Virtual E Port (VE_Port), over which the FC protocol packets are exchanged. The first step is an ELP exchange, represented in Figs. 25 and 26 with the associated frame formats. Fig. 25 indicates FCoE switch A sending ELP request 2505. Fig. 26 indicates switch B responding with ELP accept 2605.

0197The following step is the acquisition of a Domain_ID, which happens via the exchange of point-to-point messages, encapsulated in the same manner as the ELP messages represented in Figs. 25 and 26. The subsequent step is the FSPF operation of computing forwarding path over the tunneled topology. FSPF messages are encapsulated in the same manner as ELP messages. The FSPF costs associated with the tunnels need to be properly computed from the underlining DCE topology, in order to ensure optimal routing. Optimal routing is achieved when FSPF selects the direct tunnel to reach a destination FCoE switch. Considering the tunneled topology represented in Fig. 24 as an example, FCoE switch A in order to reach FCoE C shall select the VE_Port identifying the direct tunnel to C, and not the VE Port to B (i.e., the Destination MAC address in the MAC-in-MAC encapsulation shall be SwitchJD(C) || FC-PID, not Switch ΙD(B) || FC-PID). Otherwise, frames will have a destination MAC address of Switch ΙD(B) || FC-PID and will be delivered to FCoE switch B, that will have to decapsulate them from the MAC-in-MAC encapsulation and re-encapsulate them in MAC-in-MAC with the proper destination MAC address SwitchJD(C) || FC-PID.

0198A way to achieve optimal routing is computing the FSPF costs as the IS-IS path costs of he underlining DCE topology plus a constant (e.g., 1). In this way, whenever an FSPF path over the DCE cloud comprises more than one hop, that constant will be added more than one time in computing the FSPF path cost, while for a direct path will be counted only one time, providing a lower cost for the direct path that will be then selected by the routing protocol. By assigning FSPF costs in this way, the resulting FSPF routing table 2700 for FCoE switch A is represented in Fig. 27. A corresponding exemplary forwarding table 2800 for FCoE switch A is represented in Fig. 28, showing the MAC addresses to be used in the MAC-in-MAC encapsulation.

0199Once the FCoE Fabric is operational, the FLOGI process is handled as shown in Figs. 29 and 30, in which the FCoE encapsulation described above is used. Fig. 29 indicates host H2 sending FLOGI request 2905 to FCoE switch A. Fig. 30 illustrates FCoE switch A sending FLOGI Accept 3005 to host H2.

0200The example explains why the FLOGI accept needs to be handled in a different way than the general encapsulation rules expressed above: host H2 has to program the address FC-OUI || assigned-S_ID as one of its MAC Addresses, but it has not received yet the S ID from the switch (in fact, it is being assigned via the FLOGI accept message). Then the destination MAC address in the FCoE encapsulation of the FLOGI accept message has to be the source MAC address of host H2. Instead in all subsequent FCoE frames it will be the FC-OUI || D_ID(H2), where the D_ID(H2) is the Fibre Channel address identifier assigned to host H2.

0201After FLOGI is performed, FC host to host communication may take place. Figs. 31 and 32 show how the FCoE encapsulation object of the present invention is used between two FCoE hosts connected to the DCE network. Fig. 31 illustrates a communication from host H2 to host H3. First, host H2 sends frame 3105 to FCoE switch A. FCoE switch A forms MAC-in-MAC frame 3110 for transmission to FCoE switch C over DCE core 2405, the latter of which may include non-FCoE switches. FCoE switch C decapsulates frame 3110 by removing MAC-in-MAC portion 3111, then forwards decapsulated frame 3115 to host H3.

0202Fig. 32 illustrates a communication from host H3 to host H2. First, host H3 sends frame 3205 to FCoE switch C. FCoE switch C forms MAC-in-MAC frame 3210 (by adding MAC-in-MAC portion 3211) for transmission to FCoE switch A over DCE core 2405. FCoE switch A decapsulates frame 3210, then forwards decapsulated frame 3215 to host H2.

0203Analogously, Figs. 33 and 34 show how the FCoE encapsulation is used for communication between a host on the DCE network and a host on the FC Fabric. Fig. 33 illustrates a communication from host H2 to a device in FC fabric 2425. First, host H2 sends frame 3305 to FCoE switch A. FCoE switch A forms MAC-in-MAC frame 3310 for transmission to FCoE switch C over DCE core 2405. FCoE switch C decapsulates frame 3310 by removing MAC-in-MAC portion 3311, then forwards decapsulated frame 3315 to the indicated device in FC fabric 2425. Fig. 34 illustrates a communication from a device in FC fabric 2425 to host H2. First, the device sends FC frame 3405 to FCoE switch C. FCoE switch C forms MAC-in-MAC frame 3410 (by adding MAC-in-MAC portion 3411) for transmission to FCoE switch A over DCE core 2405. FCoE switch A decapsulates frame 3410, then forwards decapsulated frame 3415 to host H2.

0204The case of multiple connection points is illustrated by Fig. 35. Here, DCE network 3500 and FC fabric 3510 are in communication via links 3515 and 3520. Such situations may be properly handled via virtual FCoE switch V. Each of the "real" FCoE switches B and C may "impersonate" the virtual switch V (i.e., considering the MAC address SwitchJD(V) || FC-PID as one of their MAC addresses).

0205Fibre Channel broadcast frames (i.e., with an FC D_ID FF.FF.FF) could be handled over the FCoE topology by replicating each FC broadcast frame over the MAC-in-MAC tunnels. However this replication process is not desirable. A better way to handle FC broadcast frames is to define an Ethernet multicast address (i.e., FCoE-Multicast) for this purpose. Every FCoE switch joins this multicast group at the beginning of their operations. The FCoE-Multicast MAC address is then used as destination MAC address in the MAC-in-MAC encapsulation, as shown in Fig. 36. Transmissions to FCoE switch A and the other host devices are in the form of frame 3605. Frame 3610 indicates MAC-in-MAC encapsulated frames for transmission across the DCE core. Frame 3615 is a decapsulated FC frame for transmission to the FC network.

0206Some alternative methods include these steps: receiving an Ethernet frame (encapsulated, for example, as described herein); mapping destination contents of a first portion of a destination MAC field of the Ethernet frame to a destination FC ID field of an FC frame; mapping source contents of a second portion of a source MAC field of the Ethernet frame of a source FC ID field of the FC frame; converting legal symbols (encoded version of SOF and EOF) of the Ethernet frame to corresponding FC ordered sets (such symbols being illegal in Ethernet thus requiring the encoding in the first place, as previously described); inserting those ordered sets into selected fields of the FC frame; mapping payload contents of a payload field of the Ethernet frame to an FC frame payload field; and transmitting the FC frame on the FC network.

0207No state information about the frames needs to be retained. Accordingly, the frames can be processed quickly, for example at a rate of 40 Gb/s. The end nodes can run storage applications based on SCSI, because the storage applications see the SCSI layer 825 of LLE stack 800, depicted in Fig. 8. Instead of forwarding storage traffic across switches dedicated to FC traffic, such as FC switches 140 and 145 shown in Fig. 1, such FC switches can be replaced by LLE switches 1030. Moreover, the functionality of LLE switches allows for an unprecedented level of management flexibility. Referring to Fig. 11, in one management scheme, each of the LLE switches 1130 of LLE cloud 1105 may be treated as separate FC switches. Alternatively, some or all of the LLE switches 1130 may be aggregated and treated, for management purposes, as FC switches. For example, virtual FC switch 1140 has been formed, for network management purposes, by treating all LLE switches in LLE cloud 1105 as a single FC switch. All of the ports of the individual LLE switches 1130, for example, would be treated as ports of virtual FC switch 1140. Alternatively, smaller numbers of LLE switches 1130 could be aggregated. For example, 3 LLE switches have been aggregated to form virtual FC switch 1160, and 4 LLE switches have been aggregated to form virtual FC switch 1165. A network manager may decide how many switches to aggregate by considering, inter alia, how many ports the individual LLE switches have. The control plane functions of FC, such as zoning, DNS, FSPF and other functions may be implemented by treating each LLE switch as an FC switch or by aggregating multiple LLE switches as one virtual FC switch.

0208Also, the same LLE cloud 1105 may support numerous virtual networks. Virtual local area networks ("VLANs") are known in the art for providing virtual Ethernet-based networks. United States Patent No. 5,742,604, entitled "Interswitch Link Mechanism for Connecting High-Performance Network Switches" describes relevant systems and is hereby incorporated by reference. Various patent applications of the present assignee, including United States Patent Application No. 10/034,160, entitled "Methods And Apparatus For Encapsulating A Frame For Transmission In A Storage Area Network" and filed on December 26, 2001, provide methods and devices for implementing virtual storage area networks ("VSANs") for FC-based networks. This application is hereby incorporated by reference in its entirety. Because LLE networks can support both Ethernet traffic and FC traffic, some implementations of the invention provide for the formation of virtual networks on the same physical LLE cloud for both FC and Ethernet traffic.

0209Fig. 12 is a schematic diagram that illustrates a simplified architecture of DCE switch 1200 according to one embodiment of the invention. DCE switch 1200 includes N line cards, each of which characterized by and ingress side (or input) 1205 and an egress side (or output) 1225. Line card ingress sides 1205 are connected via switching fabric 1250, which includes a crossbar in this example, to line card egress sides 1225.

0210In this implementation, buffering is performed on both the input and output sides. Other architectures are possible, e.g., those having input buffers, output buffers and shared memory. Accordingly, each of input line cards 1205 includes at least one buffer 1210 and each of output line cards 1225 includes at least one buffer 1230, which may be any convenient type of buffer known in the art, e.g., an external DRAM-based buffer or an on-chip SRAM-based buffer. The buffers 1210 are used for input buffering, e.g., to temporarily retain packets while awaiting sufficient buffer to become available at the output linecard to store the packets to be sent across switching fabric 1250. Buffers 1230 are used for output buffering, e.g., to temporarily retain packets received from one or more of the input line cards 1205 while awaiting sufficient credits for the packets to be transmitted to another DCE switch.

0211It is worthwhile noting that while credits may be used internally to a switch and also externally, there is not necessarily a one-to-one mapping between internal and external credits. Moreover, it is possible to use PAUSE frame either internally or externally. For example, any of the four possible combinations PAUSE-PAUSE, PAUSE-CREDITS, CREDITs-PAUSE and CREDIT-CREDIT may produce viable solutions.

0212DCE switch 1200 includes some form of credit mechanism for exerting flow control. This flow control mechanism can exert back pressure on buffers 1210 when an output queue of one of buffers 1230 has reached its maximum capacity. For example, prior to sending a frame, one of the input line cards 1205 may request a credit from arbiter 1240 (which may be, e.g., a separate chip located at a central location or a set of chips distributed across the output linecards) prior to sending a frame from input queue 1215 to output queue 1235. Preferably, the request indicates the size of the frame, e.g., according to the frame credit field of the DCE header. Arbiter 1240 will determine whether output queue 1235 can accept the frame (i.e., output buffer 1230 has enough space to accommodate the frame). If so, the credit request will be granted and arbiter 1240 will send a credit grant to input queue 1215. However, if output queue 1235 is too full, the request will be denied and no credits will be sent to input queue 1215.

0213DCE switch 1200 needs to be able to support both the "drop" and "no drop" behavior required for virtual lanes, as discussed elsewhere herein. The "no drop" functionality is enabled, in part, by applying internally to the DCE switch some type of credit mechanism like the one described above. Externally, the "no drop" functionality can be implemented in accordance with the buffer-to-buffer credit mechanism described earlier or PAUSE frames. For example, if one of input line cards 1205 is experiencing back pressure from one or more output line cards 1225 through the internal credit mechanism, the line card can propagate that back pressure externally in an upstream direction via a buffer-to-buffer credit system like that of FC.

0214Preferably, the same chip (e.g., the same ASIC) that is providing "no drop" functionality will also provide "drop" functionality like that of a classical Ethernet switch. Although these tasks could be apportioned between different chips, providing both drop and no drop functionality on the same chip allows DCE switches to be provided at a substantially lower price.

0215Each DCE packet will contain information, e.g., in the DCE header as described elsewhere herein, indicating the virtual lane to which the DCE packet belongs. DCE switch 1200 will handle each DCE packet according to whether the VL to which the DCE packet is assigned is a drop or a no drop VL.

0216Fig. 13 illustrates an example of partitioning a buffer for VLs. In this example, 4 VLs are assigned. VL 1305 and VL 1310 are drop VLs. VL 1315 and VL 1320 are no drop VLs. In this example, input buffer 1300 has specific areas assigned for each VL: VL 1305 is assigned to buffer space 1325, VL 1310 is assigned to buffer space 1330, VL 1315 is assigned to buffer space 1335 and VL 1320 is assigned to buffer space 1340. Traffic on VL 1305 and VL 1310 is managed much like normal Ethernet traffic, in part according to the operations of buffer spaces 1325 and 1330. Similarly, the no drop feature of VLs 1315 and 1320 is implemented, in part, according to a buffer-to-buffer credit flow control scheme that is enabled only for buffer spaces 1335 and 1340. In some implementations, the amount of buffer space assigned to a VL can be dynamically assigned according to criteria such as, e.g., buffer occupancy, time of day, traffic loads/congestion, guaranteed minimum bandwidth allocation, known tasks requiring greater bandwidth, maximum bandwidth allocation, etc. Preferably, principles of fairness will apply to prevent one VL from obtaining an inordinate amount of buffer space.

0217Within each buffer space there is an organization of data in data structures which are logical queues (virtual output queues or VOQs") associated with destinations. ("A Practical Scheduling Algorithm to Achieve 100% Throughput in Input-Queued Switches," by Adisak Mekkittikul and Nick McKeown, Computer Systems Laboratory, Stanford University (InfoCom 1998) and the references cited therein describe relevant methods for implementing VOQs and are hereby incorporated by reference.) The destinations are preferably destination port/virtual lane pairs. Using a VOQ scheme avoids head of line blocking at the input linecard caused when an output port is blocked and/or when another virtual lane of the destination output port is blocked.

0218In some implementations, VOQs are not shared between VLs. In other implementations, a VOQ can be shared between drop VLs or no-drop VLs. However, a VOQ should not be shared between no drop VLs and drop VLS.

0219The buffers of DCE switches can implement various types of active queue management. Some preferred embodiments of DCE switch buffers provide at least 4 basic types of active queue management: flow control; dropping for drop VLs or marking for no-drop VLs for congestion avoidance purposes; dropping to avoid deadlocks in no drop VLs; and dropping for latency control.

0220Preferably, flow control for a DCE network has at least two basic manifestations. One flow control manifestation is a buffer-to-buffer, credit-based flow control that is used primarily to implement the "no drop" VLs. Another flow control manifestation of some preferred implementations involves an explicit upstream congestion notification. This explicit upstream congestion notification may be implemented, for example, by the explicit congestion notification ("ECN") field of the DCE header, as described elsewhere herein. Fig. 14 illustrates DCE network 1405, including edge DCE switches 1410, 1415, 1425 and 1430 and core DCE switch 1420. In this instance, buffer 1450 of core DCE switch 1420 is implementing 3 types of flow control. One is buffer-to-buffer flow control indication 1451, which is communicated by the granting (or not) of buffer-to-buffer credits between buffer 1450 and buffer 1460 of edge DCE switch 1410.

0221Buffer 1450 is also transmitting 2 ECNs 1451 and 1452, both of which are accomplished via the ECN field of the DCE headers of DCE packets. ECN 1451 would be considered a core-to-edge notification, because it is sent by core device 1420 and received by buffer 1460 of edge DCE switch 1410. ECN 1452 would be considered a core-to-end notification, because it is sent by core device 1420 and received by NIC card 1465 of end-node 1440.

0222In some implementations of the invention, ECNs are generated by sampling a packet that is stored into a buffer subject to congestion. The ECN is sent to the source of that packet by setting its destination address equal to the source address of the sampled packet. The edge device will know whether the source supports DCE ECN, as end-node 1440 does, or it doesn't, as in the case of end-node 1435. In the latter case, edge device 1410 will terminate the ECN and implement the appropriate action.

0223Active queue management (AQM) will be performed in response to various criteria, including but not limited to buffer occupancy (e.g., per VL), queue length per VOQ and the age of a packet in a VOQ. For the sake of simplicity, in this discussion of AQM it will generally be assumed that a VOQ is not shared between VLs.

0224Some examples of AQM according to the present invention will now be described with reference to Fig. 15. Fig. 15 depicts buffer usage at a particular time. At that time, portion 1505 of physical buffer 1500 has been allocated to a drop VL and portion 1510 has been allocated to a no drop VL. As noted elsewhere herein, the amount of buffer 1500 that is allocated to drop VLs or no drop VLs can change over time. Of the portion 1505 allocated to a drop VL, part 1520 is currently in use and part 1515 is not currently in use.

0225Within portions 1505 and 1510, there numerous VOQs, including VOQs 1525,

02261530 and 1535. In this example, a threshold VOQ length L has been established. VOQs 1525 and 1535 have a length greater than L and , VOQ 1530 has a length less than L. A long VOQ indicates downstream congestion. Active queue management preferably prevents any VOQ from becoming too large, because otherwise downstream congestion affecting one VOQ will adversely affect traffic for other destinations.

0227The age of a packet in a VOQ is another criterion used for AQM. In preferred implementations, a packet is time stamped when it comes into a buffer and queued into the proper VOQ. Accordingly, packet 1540 receives time stamp 1545 upon its arrival in buffer 1500 and is placed in a VOQ according to its destination and VL designation. As noted elsewhere, the VL designation will indicate whether to apply drop or no drop behavior. In this example, the header of packet 1540 indicates that packet 1540 is being transmitted on a drop VL and has a destination corresponding to that of VOQ 1525, so packet 1540 is placed in VOQ 1525.

0228By comparing the time of time stamp 1545 with a current time, the age of packet 1540 may be determined at subsequent times. In this context, "age" refers only to the time that the packet has spent in the switch, not the time in some other part of the network. Nonetheless, conditions of other parts of the network may be inferred by the age of a packet. For example, if the age of a packet becomes relatively large, this condition indicates that the path towards the destination of the packet is subject to congestion.

0229In preferred implementations, a packet having an age that exceeds a predetermined age will be dropped. Multiple drops are possible, if at the time of age determination it is found that a number of packets in a VOQ exceed a predetermined age threshold.

0230In some preferred implementations, there are separate age limits for latency control (T_L) and for avoiding deadlocks (T_D). The actions to be taken when a packet reaches T_L preferably depend on whether the packet is being transmitted on a drop or a no drop VL. For traffic on a no drop lane, data integrity is more important than latency. Therefore, in some implementations of the invention, when the age of a packet in a no drop VL exceeds T_L, the packet is not dropped but another action may be taken. For example, in some such implementations, the packet may be marked and/or an upstream congestion notification may be triggered. For packets in a drop VL, latency control is relatively more important and therefore more aggressive action is appropriate when the age of a packet exceeds TL- For example, a probabilistic drop function may be applied to the packet.

0231Graph 1600 of Fig. 16 provides some examples of probabilistic drop functions. According to drop functions 1605, 1610 and 1615, when the age of a packet exceeds Tco, i.e., the latency cut-off threshold, the probability that the packet will intentionally be dropped increases from 0 % to 100% as its age increases up to T_L, depending on the function. Drop function 1620 is a step function, having a 0% probability of intentional dropping until TL is reached. All of drop functions 1605, 1610, 1615 and 1620 reach a 100% chance of intentional drop when the age of the packet reaches TL. Although Tco, T_L, and TD may be any convenient times, in some implementations of the invention Tco is in the order of tens of microseconds, T_L is on the order of ones to tens of milliseconds and T_D is on the order of hundreds of milliseconds, e.g., 500 milliseconds.

0232If the age of the packet in a drop or a no drop VL exceeds TD, the packet will be dropped. In preferred implementations, T_D is larger for no drop VLs than for drop VLs. In some implementations, TL and/or T_D may also depend, in part, on the bandwidth of the VL on which the packet is being transmitted and on the number of VOQs simultaneously transmitting packets to that VL.

0233For no drop VL, a probability function similar to those shown in Figure 16 may be used to trigger an upstream congestion notification or to set the Congestion Experienced bit (CE) in the header of TCP packets belonging to connections capable to support TCP ECN.

0234In some implementations, whether a packet is dropped, an upstream congestion notification is sent, or the CE bit of a TCP packet is marked depends not only on the age of a packet but also on the length of the VOQ in which the packet is placed. If such length is above a threshold L_max, the AQM action is taken; otherwise it will be performed on first packet dequeued from a VOQ whose length exceeds the L_max threshold. Use of Buffer Occupancy Per VL

0235As shown in Fig. 15, a buffer is apportioned to VLs. For parts of the buffer apportioned to drop VLs (such as portion 1505 of buffer 1500), a packet will be dropped if the occupancy of a VL, at any given time, is greater than a predetermined maximum value. In some implementations, an average occupancy of a VL is computed and maintained. An AQM action may be taken based on such average occupancy. For example, being portion 1505 associated with a no-drop VL, DCE ECNs will be triggered instead of packet drops as in the case of portion 1510, which is associated with a drop VL.

0236Fig. 17 depicts graph 1700 of VL occupancy B(VL) (the vertical axis) over time (the horizontal axis). Here, Bx is a threshold value of B(VL). In some implementations of the invention, some packets in a VL will be dropped at times during which it is determined that B(VL) has reached. The actual value of B(VL) over time is shown by curve 1750, but B(VL) is only determined at times t_\ through t_N. In this example, packets would be dropped at points 1705, 1710 and 1715, which correspond to times t₂, t₃ and tβ. The packets may be dropped according to their age (e.g., oldest first), their size, the QoS for the virtual network of the packets, randomly, according to a drop function, or otherwise.

0237In addition (or alternatively), an active queue management action may be taken when an average value of B(VL), a weighted average value, etc., reaches or exceeds Bx. Such averages may be computed according to various methods, e.g., by summing the determined values of B(VL) and dividing by the number of determinations. Some implementations apply a weighting function, e.g., by according more weight to more recent samples. Any type of weighting function known in the art may be applied.

0238The active queue management action taken may be, for example, sending an ECN and/or applying a probabilistic drop function, e.g., similar to one of those illustrated in Fig. 18. In this example, the horizontal axis of graph 1880 is the average value of B(VL). When the average value is below a first value 1805, there is a 0% chance of intentionally dropping the packet. When the average value reaches or exceeds a second value 1810, there is a 100% chance of intentionally dropping the packet. Any convenient function may be applied to the intervening values, whether a function similar to 1815, 1820 or 1825 or another function.

0239Returning to Fig. 15, it is apparent that the length of VOQs 1525 and 1535 exceed a predetermined length L. In some implementations of the invention, this condition triggers an active queue management response, e.g., the sending of one or more ECNs. Preferably, packets contained in buffer 1500 will indicate whether the source is capable of responding to an ECN. If the sender of a packet cannot respond to an ECN, this condition may trigger a probabilistic drop function or simply a drop. VOQ 1535 is not only longer than predetermined length Li, it is also longer than predetermined length L₂. According to some implementations of the invention, this condition triggers the dropping of a packet. Some implementations of the invention use average VOQ lengths as criteria for triggering active queue management responses, but this is not preferred due to the large amount of computation required.

0240It is desirable to have multiple criteria for triggering AQM actions. For example, while it is very useful to provide responses to VOQ length, such measures would not be sufficient for DCE switches having approximately 1 to 2 MB of buffer space per port. For a given buffer, there may be thousands of active VOQs. However, there may only be enough storage space for on the order of 10³ packets, possibly fewer. Therefore, it may be the case that no individual VOQ has enough packets to trigger any AQM response, but that a VL is running out of space.

0241Queue Management for No Drop VLs

0242In preferred implementations of the invention, the main difference between active queue management of drop and no drop VLs is that the same criterion (or criteria) that would trigger a packet drop for a drop VL will result in an DCE ECN being transmitted or a TCP CE bit being marked for no drop VL. For example, a condition that would trigger a probabilistic packet drop for a drop VL would generally result in a probabilistic ECN to an upstream edge device or an end (host) device. Credit-based schemes are not based on where a packet is going, but instead are based on where packets are coming from. Therefore, upstream congestion notifications help to provide fairness of buffer use and to avoid blocking that might otherwise arise if the sole method of flow control for no drop VLs were a credit-based flow control. For example, with regard to the use of buffer occupancy per VL as a criterion, packets are preferably not dropped merely because the buffer occupancy per VL has reached or exceeded a threshold value. Instead, for example, a packet would be marked or an ECN would be sent. Similarly, one might still compute some type of average buffer occupancy per VL and apply a probabilistic function, but the underlying action to be taken would be marking and/or sending an ECN. The packet would not be dropped.

0243However, even for a no drop VL, packets will still be dropped in response to blocking conditions_? e.g., as indicated by the age of a packet exceeding a threshold as described elsewhere herein. Some implementations of the invention also allow for packets of a no drop VL to be dropped in response to latency conditions. This would depend on the degree of importance placed on latency for that particular no drop VL. Some such implementations apply a probabilistic dropping algorithm. For example, some cluster applications may place a higher value on latency considerations as compared to a storage application. Data integrity is still important to cluster applications, but it may be advantageous to reduce latency by foregoing some degree of data integrity. In some implementations, larger values T_L (i.e., the latency control threshold) may be used for no drop lanes than the corresponding values used for drop lanes.

0244Fig. 19 illustrates an example of a network device that may be configured to implement some methods of the present invention. Network device 1960 includes a master central processing unit (CPU) 1962, interfaces 1968, and a bus 1967 (e.g., a PCI bus). Generally, interfaces 1968 include ports 1969 appropriate for communication with the appropriate media. In some embodiments, one or more of interfaces 1968 includes at least one independent processor 1974 and, in some instances, volatile RAM. Independent processors 1974 may be, for example ASICs or any other appropriate processors. According to some such embodiments, these independent processors 1974 perform at least some of the functions of the logic described herein. In some embodiments, one or more of interfaces 1968 control such communications-intensive tasks as media control and management. By providing separate processors for the communications-intensive tasks, interfaces 1968 allow the master microprocessor 1962 efficiently to perform other functions such as routing computations, network diagnostics, security functions, etc.

0245The interfaces 1968 are typically provided as interface cards (sometimes referred to as "line cards"). Generally, interfaces 1968 control the sending and receiving of data packets over the network and sometimes support other peripherals used with the network device 1960. Among the interfaces that may be provided are Fibre Channel ("FC") interfaces, Ethernet interfaces, frame relay interfaces, cable interfaces, DSL interfaces, token ring interfaces, and the like. In addition, various very high-speed interfaces may be provided, such as fast Ethernet interfaces, Gigabit Ethernet interfaces, ATM interfaces, HSSI interfaces, POS interfaces, FDDI interfaces, ASI interfaces, DHEI interfaces and the like.

0246When acting under the control of appropriate software or firmware, in some implementations of the invention CPU 1962 may be responsible for implementing specific functions associated with the functions of a desired network device. According to some embodiments, CPU 1962 accomplishes all these functions under the control of software including an operating system (e.g. Linux, Vx Works, etc.), and any appropriate applications software.

0247CPU 1962 may include one or more processors 1963 such as a processor from the Motorola family of microprocessors or the MIPS family of microprocessors. In an alternative embodiment, processor 1963 is specially designed hardware for controlling the operations of network device 1960. In a specific embodiment, a memory 1961 (such as non-volatile RAM and/or ROM) also forms part of CPU 1962. However, there are many different ways in which memory could be coupled to the system. Memory block 1961 may be used for a variety of purposes such as, for example, caching and/or storing data, programming instructions, etc.

0248Regardless of network device's configuration, it may employ one or more memories or memory modules (such as, for example, memory block 1965) configured to store data, program instructions for the general-purpose network operations and/or other information relating to the functionality of the techniques described herein. The program instructions may control the operation of an operating system and/or one or more applications, for example. Because such information and program instructions may be employed to implement the systems/methods described herein, the present invention relates to machine-readable media that include program instructions, state information, etc. for performing various operations described herein. Examples of machine-readable media include, but are not limited to, magnetic media such as hard disks, floppy disks, and magnetic tape; optical media such as CD-ROM disks; magneto-optical media; and hardware devices that are specially configured to store and perform program instructions, such as read-only memory devices (ROM) and random access memory (RAM). The invention may also be embodied in a carrier wave traveling over an appropriate medium such as airwaves, optical lines, electric lines, etc. Examples of program instructions include both machine code, such as produced by a compiler, and files containing higher level code that may be executed by the computer using an interpreter.

0249Although the system shown in Fig. 19 illustrates one specific network device of the present invention, it is by no means the only network device architecture on which the present invention can be implemented. For example, an architecture having a single processor that handles communications as well as routing computations, etc. is often used. Further, other types of interfaces and media could also be used with the network device. The communication path between interfaces/line cards may be bus based (as shown in Fig. 19) or switch fabric based (such as a cross-bar).

0250While the invention has been particularly shown and described with reference to specific embodiments thereof, it will be understood by those skilled in the art that changes in the form and details of the disclosed embodiments may be made without departing from the spirit or scope of the invention. For example, some implementations of the invention allow a VL to change from being a drop VL to a no drop VL. Thus, the examples described herein are not intended to be limiting of the present invention. It is therefore intended that the appended claims will be interpreted to include all variations, equivalents, changes and modifications that fall within the true spirit and scope of the present invention.