Patent Publication Number: US-2022217085-A1

Title: Server fabric adapter for i/o scaling of heterogeneous and accelerated compute systems

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of priority to U.S. Provisional Patent Application No. 63/134,586, filed Jan. 6, 2021, the entire contents of which are incorporated by reference herein. 
    
    
     TECHNICAL FIELD 
     This disclosure relates to a communication system that can improve communication speeds within a processing system having several processors and/or the speed of communication between such a system and a network. 
     BACKGROUND 
     Servers are processing larger and larger quantities of data for various applications in the cloud, such as business intelligence and analytics, information technology automation and productivity, content distribution, social networking, gaming and entertainment, etc. In recent years, the slowing down of Moore&#39;s Law and Dennard Scaling in industry-standard semiconductor processors, coupled with an increase in specialized workloads that require high data processing performance such as machine learning and database acceleration has given rise to server acceleration. As a result, a standard central processing unit (CPU), often in a multiprocessor architecture, is augmented by other peripheral component interconnect express (PCIe)-attached domain-specific processors such as graphics processing units (GPUs) or field-programmable gate arrays (FPGAs) to form a heterogeneous compute server. However, the existing designs of the heterogeneous compute server using network interface controllers (NICs), private network fabric, etc., have a lot of shortcomings. These shortcomings include, but are not limited to, bandwidth bottleneck, complex packet processing, scaling limitations, insufficient load balancing, lack of visibility and control, etc. 
     SUMMARY 
     To address the aforementioned shortcomings, a server fabric adapter (SFA) communication system is provided. In some embodiments, the SFA communication system comprises an SFA communicatively coupled to a plurality of controlling hosts, a plurality of endpoints, and a plurality of network ports. The SFA receives a network packet from a network port of the plurality of network ports. The SFA separates the network packet into different portions, each portion including a header or a payload. The SFA then maps each portion of the network packet to: (i) a controlling host of the plurality controlling hosts, the controlling host being designated as a destination controlling host, or (ii) an endpoint of the plurality of endpoints, the endpoint being designated as a destination endpoint. The SFA further forwards a respective portion of the network packet to the destination controlling host or the destination endpoint. 
     The above and other preferred features, including various novel details of implementation and combination of elements, will now be more particularly described with reference to the accompanying drawings and pointed out in the claims. It will be understood that the particular methods and apparatuses are shown by way of illustration only and not as limitations. As will be understood by those skilled in the art, the principles and features explained herein may be employed in various and numerous embodiments. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The disclosed embodiments have advantages and features which will be more readily apparent from the detailed description, the appended claims, and the accompanying figures (or drawings). A brief introduction of the figures is below. 
         FIG. 1A  illustrates an example prior art accelerated server architecture using a network interface controller (NIC). 
         FIG. 1B  illustrates an alternative example prior art accelerated server architecture using a private network fabric. 
         FIG. 2  illustrates an exemplary server fabric adapter architecture for accelerated and/or heterogeneous computing systems, according to some embodiments. 
         FIG. 3A  illustrates components of a server fabric adapter architecture, according to some embodiments. 
         FIG. 3B  illustrates components of ingress and egress pipelines, according to some embodiments. 
         FIG. 4  illustrates an exemplary data flow diagram from a network to a host or endpoint, according to some embodiments. 
         FIG. 5  illustrates an exemplary data flow diagram from a host/endpoint to a network, according to some embodiments. 
         FIG. 6  illustrates an exemplary peer-to-peer data packet flow diagram, according to some embodiments. 
         FIG. 7  illustrates a host view of packet flow to a network, according to some embodiments. 
         FIG. 8  illustrates an exemplary process of transmitting data from a network to a host/endpoint in a server fabric adapter (SFA) communication system, according to some embodiments. 
         FIG. 9  illustrates an exemplary process of transmitting data from a host/endpoint to a network in an SFA communication system, according to some embodiments. 
         FIG. 10  illustrates an exemplary process of peer-to-peer data transfer in an SFA communication system, according to some embodiments. 
         FIG. 11  illustrates an exemplary process of a host view of data transfer in an SFA communication system, according to some embodiments. 
     
    
    
     DETAILED DESCRIPTION 
     The Figures (FIGS.) and the following description relate to preferred embodiments by way of illustration only. It should be noted that from the following discussion, alternative embodiments of the structures and methods disclosed herein will be readily recognized as viable alternatives that may be employed without departing from the principles of what is claimed. 
     Reference will now be made in detail to several embodiments, examples of which are illustrated in the accompanying figures. It is noted that wherever practicable similar or like reference numbers may be used in the figures and may indicate similar or like functionality. The figures depict embodiments of the disclosed system (or method) for purposes of illustration only. One skilled in the art will readily recognize from the following description that alternative embodiments of the structures and methods illustrated herein may be employed without departing from the principles described herein. 
       FIG. 1A  illustrates an example prior art accelerated server architecture  100  using a network interface controller (NIC). The server architecture  100  is usually used in a data center for applications such as distributed neural network training, ray-tracing graphics processing, or scientific computing, etc. A PCIe switch tree is a collection of PCIe switches that connect to each other. As depicted, a PCIe switch tree  102  connects graphics processing units (GPUs), field programmable gate arrays (FPGAs), or other accelerators  104 . PCIe switch tree  102  may also connect storage elements (e.g., flash-based solid state drives (SSDs)) or storage-class memories to central processing units (CPUs)  108  and GPUs  104 . In order to communicate with other server systems in the data center, architecture  100  also includes a network interface controller (NIC). A NIC may forward packets between processors/storage (e.g.,  108 / 106 ) and top-of-rack Ethernet switches  112 . A recent variant of such NIC devices is a NIC  110 . NIC  110  typically includes an advanced processor complex based on a multi-core instruction set, and the processor complex is similar to the type of processing engine used in the CPUs of a server. 
     Prior accelerated computing systems, such as system  100  shown in  FIG. 1A , suffer from numerous problems. These problems include, but are not limited to, low radix or scalability, PCIe bandwidth constraints, the complexity in implementing coherent compute express link (CXL) .mem and .cache operations, poor resiliency, weakened security, etc. For example, a single GPU may saturate an 800G input/output (I/O) link, which may exceed the I/O bandwidth of a PCIe/CXL interface between a CPU and the GPU, as well as the I/O bandwidth of the NIC or NIC  110 . 
     The use of NIC  110  may cause additional problems for system  100 . Since all paths in and out of the accelerated complex are through a single path network interface (e.g., NIC  110 ), this tends to create a bandwidth bottleneck (e.g., 200 Gigabits) to and from the network fabric. The complex packet processing offload also places an operational burden on the infrastructure operator and deployment engineers. Further, given a small target form factor (typically a mezzanine card attached to the CPU motherboard), the cost and power scaling to provide the required bandwidth with features would be intolerable. There is also an operating system (OS) instance explosion problem outside the host CPU. That is, while the NIC effectively doubles the number of OS instances in the entire data center, half of these OS instances are running on a different, customized instruction-set architecture (ISA) as compared to the industry-standard ISA of the CPU (e.g., x86 or ARM). Moreover, software stack investment and portability are challenging. Using NIC  110  in  FIG. 1A , the entire communication stack for every single application networking operation or remote procedure call in the cloud must be ported, qualified, and secured on the processing architecture of the new NIC  110 . This creates a high non-recoverable investment for both the NIC vendors as well as the data center operator. 
       FIG. 1B  illustrates an alternative example prior art accelerated compute system  150 , which employs a private network fabric between accelerators (e.g., GPUs), separate from the main data center network. As depicted, the private network fabric typically uses a dedicated NIC  152  per GPU (or per two GPUs) connected via isolated PCIe interfaces. NIC  152  uses a dedicated communication scheme implemented in hardware such as GPU-direct over Infiniband (IB) or remote direct memory access (RDMA) over Converged Ethernet (RoCE), to enable the GPU to transmit data to and from the NIC&#39;s network port(s) without copying the data to the server&#39;s CPU. This mechanism can be generally described as zero-copy I/O. 
     This private fabric NIC  152 , however, still has a number of drawbacks that significantly impact the performance of system  150 . Since NIC  152  and its adjoint accelerator need to be on the same level of a PCIe tree, significant stranding of computational resources in the accelerator and the bandwidth provisioned occurs. With this hard assignment (i.e., absence of elasticity in the design), each path has to be maximally provisioned. Also, using private fabric NIC  152 , load balancing is only feasible within the accelerator domain attached to the NIC but not across multiple accelerators. In addition, the transport protocol is codified in the NIC hardware instead of being disaggregated, which creates scaling limitations as to how many accelerators can be networked in a stable manner. Again, the transport capacity of each NIC must be the maximum, as no other NIC can provide capacity if the load exceeds the capacity of the NIC. Furthermore, since the embedded transport resides in part inside the private fabric NIC, this creates a lack of visibility and control for operations of the data center network. 
     Additional infrastructure problems exist with the deployment using PCIe switch trees and NICs. First, provisioning and management systems have to constantly reconcile the PCIe domain and the network domain. For example, security posture is expressed in memory terms for PCIe but in network addresses for the network. Also, isolation in the network is very different from isolation and reachability in the PCIe fabrics. The reconciliation may also reflect in job placement and performance management. In addition to the continuous reconciliation problem, there is also the problem of poor resource utilization. Because each link in the PCIe trees that connects to a NIC has to be maximally configured, the link consumes half of the total bandwidth of the PCIe switch even before any real data starts to transmit. Moreover, using this infrastructure in  FIG. 1B , transport is locked to hardware ISA, operations are costly and non-agile, etc. 
     Generally, the NIC, and PCI switches in prior systems (e.g., as shown in  FIGS. 1A and 1B ) map their I/O communication packets into memory-mapped PCIe address space of a single device. This can be inefficient and/or ineffective, if that single device is not designated to process the entire packet routed thereto. This is often the case when specialized processors are used in conjunction with a general purpose processor, where a portion of the packet, typically a header, is analyzed by the general purpose processor, and another portion of the same packet is analyzed/processed by a specialized processor. In this case, routing the entire packet to a general purpose processor or to a specialized processor can create bottlenecks at these devices since the remainder of the packet may need to be forwarded internally, adding to the communication burden. A new infrastructure, as shown below in  FIGS. 2-7  will be described in this disclosure to improve the performance of the prior accelerated compute systems. 
     Server Fabric Adapter (SFA) System Overview 
       FIG. 2  illustrates an exemplary server fabric adapter architecture  200  for accelerated and/or heterogeneous computing systems in a data center network. In some embodiments, a server fabric adapter (SFA)  202  may connect to one or more controlling host CPUs  204 , one or more endpoints  206 , and one or more Ethernet ports  208 . An endpoint  206  may be a GPU, accelerator, FPGA, etc. Endpoint  206  may also be a storage or memory element  212  (e.g., SSD), etc. SFA  202  may communicate with the other portions of the data center network via the one or more Ethernet ports  208 . 
     In some embodiments, the interfaces between SFA  202  and controlling host CPUs  204  and endpoints  206  are shown as over PCIe/CXL  214   a  or similar memory-mapped I/O interfaces. In addition to PCIe/CXL, SFA  202  may also communicate with a GPU/FPGA/accelerator  210  using wide and parallel inter-die interfaces (IDI) such as Just a Bunch of Wires (JBOW). The interfaces between SFA  202  and GPU/FPGA/accelerator  210  are therefore shown as over PCIe/CXL/IDI  214   b.    
     SFA  202  is a scalable and disaggregated I/O hub, which may deliver multiple terabits-per-second of high-speed server I/O and network throughput across a composable and accelerated compute system. In some embodiments, SFA  202  may enable uniform, performant, and elastic scale-up and scale-out of heterogeneous resources. SFA  202  may also provide an open, high-performance, and standard-based interconnect (e.g., 800/400 GbE, PCIe Gen 5/6, CXL). SFA  202  may further allow I/O transport and upper layer processing under the full control of an externally controlled transport processor. In many scenarios, SFA  202  may use the native networking stack of a transport host and enable ganging/grouping of the transport processors (e.g., of x86 architecture). 
     As depicted in  FIG. 2 , SFA  202  connects to one or more controlling host CPUs  204 , endpoints  206 , and Ethernet ports  208 . A controlling host CPU or controlling host  204  may provide transport and upper layer protocol processing, act as a user application “Master,” and provide infrastructure layer services. An endpoint  206  (e.g., GPU/FPGA/accelerator  210 , storage  212 ) may be producers and consumers of streaming data payloads that are contained in communication packets. An Ethernet port  208  is a switched, routed, and/or load balanced interface that connects SFA  202  to the next tier of network switching and/or routing nodes in the data center infrastructure. 
     In some embodiments, SFA  202  is responsible for transmitting data at high throughput and low predictable latency between:
         Network and Host;   Network and Accelerator;   Accelerator and Host;   Accelerator and Accelerator; and/or   Network and Network.       

     The details of data transmission between various entities (e.g., network, host, accelerator) will be described below with reference to  FIGS. 4-7 . However, in general, when transmitting data/packets between the entities, SFA  202  may separate/parse arbitrary portions of a network packet and map each portion of the packet to a separate device PCIe address space. In some embodiments, an arbitrary portion of the network packet may be a transport header, an upper layer protocol (ULP) header, or a payload. SFA  202  is able to transmit each portion of the network packet over an arbitrary number of disjoint physical interfaces toward separate memory subsystems or even separate compute (e.g., CPU/GPU) subsystems. 
     By identifying, separating, and transmitting arbitrary portions of a network packet to separate memory/compute subsystems, SFA  202  promotes the aggregate packet data movement capacity of a network interface into heterogeneous systems consisting of CPUs, GPUs/FPGAs/accelerators, and storage/memory. SFA  202  may also factor, in the various physical interfaces, capacity attributes (e.g., bandwidth) of each such heterogeneous systems/computing components. 
     In some embodiments, SFA  202  may interact with or act as a memory manager. SFA  202  provides virtual memory management for every device that connects to SFA  202 . This allows SFA  202  to use processors and memories attached to it to create arbitrary data processing pipelines, load balanced data flows, and channel transactions towards multiple redundant computers or accelerators that connect to SFA  202 . 
     SFA System Components 
       FIG. 3A  illustrates components of a server fabric adapter architecture  300 , according to some embodiments. SFA system  300  is used in a data center network for accommodating applications such as distributed neural network training, ray-tracing graphics processing, or scientific computing, etc. As shown in  FIG. 3A , SFA  202  also connects with controlling hosts  204  and endpoints  206  and communicates with the other portions of the data center network through Ethernet ports  208 . Endpoints  206  may include GPU/FPGA/accelerator  210  and/or storage/memory element  212 . In some embodiments, SFA system  300  may implement one or more of the following functionalities:
         splitting a network packet into partial packets, e.g., a transport header, a ULP header, or a payload.   mapping a full packet or partial packets (e.g., payload) to and from a set of P endpoints  206 , where P is an integer, and the P endpoints  206  are capable of arbitrary packet processing and/or packet storage.   mapping a full packet or partial packets (e.g. a transport header and a ULP header) to and from a set of N controlling hosts  204 , where N is an integer, and the N controlling hosts  204  are capable of arbitrary packet processing or packet header processing.   maintaining dynamic associations between active sessions on any of the N controlling hosts  204  to the I/O buffers in any of the P endpoints  206 .   performing arbitrary routing and capacity allocation from the network towards GPUs/FPGAs/accelerators  210 , where the arbitrary routing and capacity allocation may include sharding and aggregation.   building arbitrary data flow pipelines, where data can reside on any device attached to SFA  202 , and other similarly connected devices can access SFA  202  in a safe and isolated manner.   performing arbitrary homing of accelerators to compute complexes on a given SFA, with a low latency path for quick message passing and a high bandwidth path for streaming data amongst devices connected to the SFA.       

     In some embodiments, SFA  202  identifies the partial packet parts of a network packet that may constitute a header. SFA  202  also identifies a payload of the network packet at arbitrary protocol stack layers. The arbitrary protocol stack layers may include message-based protocols layered on top of byte stream protocols. SFA  202  makes flexible yet precise demarcations as to the identified header and payload. Responsive to identifying the header and payload, SFA  202  selects which parts or combinations of the header and payload should be sent to which set of destinations. 
     Unlike a NIC (e.g., NIC  110 ), SFA  202  enables a unified application and communication software stack on the same host complex. To accomplish this, SFA  202  transmits the transport headers and ULP headers exclusively to controlling hosts  204  although the controlling hosts may be different CPUs or different cores within the same CPU. As such, SFA  202  enables parallelized and decoupled processing of protocol layers in the host CPU, and further confines that layer of processing to dedicated CPUs or cores. 
     In some embodiments, SFA  202  provides protocol headers (e.g., transport headers) in a first queue, ULP headers in a second queue, and data/payload in a dedicated third queue, where the first, second, and third queues may be different queues. In this way, SFA  202  may allow the stack to make forward progress in parallel, and further allow a native mechanism with little contention where multiple CPUs or CPU cores can be involved in handling the packet if it is desired. 
     SFA  202  enables per-flow packet sequencing and coalesced steering per CPU core. Therefore, SFA system  300  allows a solution where a standard CPU complex with a familiar stack can be made a data processing unit (DPU) processor and achieve significantly higher performance. In some embodiments, the present SFA architecture  300  may also eliminate operational dependency on hidden NIC firmware from operators of the data center network. 
     In some embodiments, SFA  202  includes one or more per-port Ethernet MACs &amp; port schedulers  302 , one or more network ingress and egress processing pipelines  304 , a switching core  306 , one or more host/endpoint egress and ingress pipelines  308 , one or more memory transactors (e.g., direct memory access (DMA) engines), and an embedded management processor  312 . Surrounding the host/endpoint egress and ingress pipelines  308  is a shared memory complex, which allows the SFA to directly buffer the packets to the corresponding flows instead of overprovisioning and stranding, or underprovisioning and dropping. 
       FIG. 3B  illustrates components of ingress and egress pipelines. In some embodiments, network ingress and egress processing pipeline  304  includes a network ingress pipeline  304   a  and a network egress pipeline  304   b . As shown in  FIG. 3B , each network ingress pipeline  304   a  includes a packet parser  352 , a packet header classification &amp; lookup engine  354 , and a steering engine  356 . Each network egress pipeline  304   b  includes a virtual queuing engine  358  and a packet editor  360 . In some embodiments, host/endpoint ingress and egress processing pipeline  308  includes a host/endpoint ingress pipeline  308   a  and a host/endpoint egress pipeline  308   b . Each host/endpoint ingress pipeline  308   a  includes a packet parser  362 , a packet header classification &amp; lookup engine  364 , an egress protocol handler  366 , and a load balancing engine  368 . Each host/endpoint egress pipeline  308   b  includes a virtual queuing engine  372 , an ingress protocol handler  374 , a flow classification and lookup engine  376 , and a flow steering/queuing engine  378 . 
     Example Packet Receiving Procedure 
     As described above, SFA  202  may be used to transmit data between network and host, between network and accelerator, between accelerator and host, between accelerator and accelerator, and between network and network. Instead of describing every data transmission process between different entities, the present disclosure illustrates an example procedure for packet receiving from the network to the host/endpoint herein. The data transmission flows between different entities are also described below in  FIGS. 4-7 . 
     SFA  202 , in the packet receiving direction from the network to the host/endpoint, delivers streaming payloads to GPUs/FPGAs/accelerators  210  and storage  212  using zero-copy I/O without requiring controlling CPU/host  204  to first complete the receipt of the headers. In some embodiments, a data-receiving processing flow is as follows: 
     At step  1 , a packet is received from an outside network via Ethernet port  208 . The packet is delineated in Ethernet MAC into one or more cells via Ethernet MACs &amp; port schedulers  302 . A cell is a portion of the packet, e.g., a payload cell. Ethernet MACs &amp; port schedulers  302 , along with its arbitration engine (not shown), then schedules and passes the one or more cells of the packet into network ingress pipeline  304   a.    
     At step  2 , packet parser  352  of network ingress pipeline  304   a  parses the packet and obtains the packet header(s). Packet header classification &amp; lookup engine  354  of network ingress pipeline  304   a  then classifies the packet header(s) as a flow or flow aggregate in the network ingress pipeline based on one or more table lookups. 
     At step  3 , packet header classification &amp; lookup engine  354  determines whether to split the packet based on the result of the table lookups performed at step  2  in network ingress. In some embodiments, if it is determined that the packet should not be split, packet header classification &amp; lookup engine  354  may record the forwarding result to be a single destination among the N controlling hosts  204 . That is, the entire packet will be sent atomically and in-order to the single destination of the N controlling hosts  204 . However, if it is determined that the packet should be split, packet header classification &amp; lookup engine  354  may record a first forwarding result and a second forwarding result. Packet header classification &amp; lookup engine  354  may record the first forwarding result to indicate that one or more headers of the packet (e.g. transport header, ULP header) should be forwarded to a destination among the N controlling hosts  204 . Packet header classification &amp; lookup engine  354  may also record the second forwarding result to indicate that the payload of the packet should be forwarded to a different destination among the P endpoints. 
     At step  4 , packet header classification &amp; lookup engine  354  sends the packet, the metadata recording the parsing, and the forwarding and classification results to steering engine  356  of network ingress pipeline  304   a . Steering engine  356  performs the requested action (could be direct mapped or load balanced via a consistent hash) on the parsed packet header to determine which host/endpoint egress pipeline  308   b  the packet should be switched to. 
     At step  5 , steering engine  356  of network ingress pipeline  304   a  forwards the packet/cells to switch core  306  such that switch core  306  may write the packet header, metadata, and payload cells into a switch core buffer. This shared switch core buffer allows SFA  202  to make a steering decision without having to move the payload around different entities. 
     At step  6 , upon a specific host/endpoint egress pipeline  308   b  being determined, virtual queuing engine  372  of this host/endpoint egress pipeline  308   b  stores multiple linked lists of the packets or cells written into the switch core buffer as the packets/cells arrive. In this way, each host/endpoint egress pipeline  308   b  may maintain multiple pointer queues with at least per ingress port granularity, per class granularity, or per flow granularity. 
     At step  7 , virtual queue engine  372  enqueues a packet header descriptor in an appropriate virtual queue based on at least one of the network ingress classification result or the steering result received from packet header classification &amp; lookup engine  354  and steering result  356 . The packet metadata cell is the only component that is operated on, and it represents all the information in the header while also carrying references to the real packet header and payload. 
     At step  8 , when a packet can be dequeued from the appropriate queue, virtual queuing engine  372  reads a packet metadata cell corresponding to the packet from the switch core buffer and sends the packet metadata cell to flow classification &amp; lookup engine  376  of host/endpoint egress pipeline  308   b . In the meanwhile, virtual queuing engine  372  reads the corresponding first cell of the packet payload from the switch core buffer and sends the first cell of the packet payload to ingress protocol handler  374  of host/endpoint egress pipeline  308   b.    
     At step  9 , flow classification &amp; lookup engine  376  of host/endpoint egress pipeline  308   b  classifies the packet metadata corresponding to the packet descriptor and searches/looks up the packet metadata in a flow table. 
     At step  10 , based on the result of the lookups in the flow table (e.g., flow lookups), flow steering engine  378  of host/endpoint egress pipeline  308   b  writes the packet header descriptor to an appropriate per-flow header queue destined for a given host interface. Flow steering engine  378  also writes a packet data descriptor to an appropriate data queue destined for an endpoint interface. The packet data descriptor is a compact structure that describes the packet payload. 
     It should be noted that the metadata is placed into virtual queues of the switch core buffer such that SFA  202  can classify the network packets at an early stage of data switching. In addition, these virtual queues or flow queues can keep the flows consistent and treat the flows coherently throughout the SFA system. 
     At step  11 , when the specific host posts an internal I/O buffer indicating that the host is ready to receive a packet from SFA  202 , flow steering engine  378  retrieves the corresponding headers and payload data from corresponding queues. For example, flow steering engine  378  dequeues the packet header descriptor at the head of the packet from the per-flow header queue. Flow steering engine  378  also reads the corresponding packet header and payload data cells from the switch core buffer into the memory transactor and writes the packet header and payload data cells to a DMA engine at the host and/or endpoint interface. 
     At step  12 , when the payload transfer over DMA to the endpoint host is completed for the packet, host egress pipeline  308   b  of SFA  202 , e.g., via flow steering engine  378 , may signal the host to write a completion queue entry corresponding to the header submission queue entry of the packet. 
     Because the payload is buffered in the switch core, manipulation of the packet for the purpose of adjusting quality of service (QoS) or coalescing to improve the effective packet rate is a function of purely manipulating the metadata cells. This allows the switch core buffering to operate independently from the packet processing pipeline while allowing a large number of temporary contexts for packet processing. At step  13 , generic receive offload (GRO) may be performed. For example, a large number of packets in a transmission control protocol (TCP) stream may be collapsed into a single packet without having to copy or move the packet data around. 
     Switch Core 
     Switch core  306  of SFA  202  uses a shared memory output queued scheme. In some embodiments, switch core  306  manages a central pool of memory. As a result, any ingress port of SFA  202  can write any memory location, and any egress port of SFA  202  can read from any memory location. In addition, switch core  306  allows packet buffer memory to be managed in units of cells, where the cell size is globally defined to trade-off memory fragmentation. The memory fragmentation may be caused by the partial occupancy in relation to mapping data structure cost. Switch core  306  is also the central point of allocation of packet pointers from internal memory cells in memory banks. Further, using the shared memory output queued scheme of switch core  306 , network ingress and host ingress pipelines can read and write into arbitrary offsets of a packet. For example, the network ingress and host ingress pipelines may specify the cell address, the operation requested, and the data to be written (when there is a “write” operation). The “write” operations update the entire cell, and thus there are no partial writes. Moreover, switch core  306  allows any packet cell to be partially filled. 
     Network, Host and Accelerator Interfaces 
     Network interfaces are used to connect an SFA system to one or more networks. The network interfaces are address less bi-directional streams following well-defined formats at each of the layers. These formats at each layer may include: serializer/deserializer (SERDES) and physical coding sublayer (PCS) at layer 1, Ethernet with or without virtual local area network (VLAN) headers at layer 2, internet protocol IPv4 and IPv6 at layer 3, two levels of inner and outer headers for network overlays, configurable transport headers with native support for transmission control protocol (TCP) and user datagram protocol (UDP) at layer 4, and also up to two transport layer headers (e.g., RDMA over UDP). 
     As depicted in  FIG. 3A , there are two types of network interfaces:  310   a  and  310   b . A DMA host interface  310   a  is used to connect SFA  202  to one or more controlling hosts  204 . This interface  310   a  is essentially PCIe- or CXL-based. That is, interface  310   a  may be used for address-based load, or store plus memory posted writes and read split transactions. PCIe/CXL naturally defines the SERDES and PCS at layer 1, the PCIe/CXL at layer 2 including flow control, and the PCIe/CXL transport layer, e.g., transaction layer packet/flow control unit (TLP/FLIT), at layer 4. 
     A DMA or memory mapped accelerator interface  310   b  is used to connect SFA  202  to an endpoint  206 . The endpoint includes a GPU, accelerator ASIC, or even storage media. At the transaction level, interface  310   b  consists of memory type transactions. These transactions are transported over the SERDES and PCS at layer 1 of interface  310   b . Depending on the protocol and accelerator type, interface  310   b  may have different layer 2 and optional layer 4. However, in all cases, interface  310   b  may use an adaptation layer above layers 2 and 4 to expose memory/read semantics into the individual memory space of each accelerator. 
     Protocol Handlers 
     In some embodiments, a protocol handler is a functional block inside host ingress/egress pipeline, for example, egress protocol handler  366  inside host ingress pipeline  308   a  and ingress protocol handler  374  inside host egress pipeline  308   b . The protocol handler is capable of processing packets at a very high individual rate while allowing its functionalities to be programmable. 
     Protocol handlers inside host ingress and host egress pipelines operate at the stream level (e.g., above TCP layer) or at the packet level. A protocol handler, e.g., an ingress protocol handler, may have some unique characters and perform specific functionalities. 
     In some embodiments, an ingress protocol handler is asynchronously triggered by packet arrivals or by timers. The ingress protocol handler completes its operation upon the packet disposition. 
     In some embodiments, the ingress protocol handler may extract ULP headers of L5 protocols above TCP by requesting specific byte offsets or ranges to be delivered to the protocol handler. The host egress pipeline skips intermediate bytes and directly moves the intermediate bytes (the L5 payload) towards the corresponding memory transactors based on the previous dispositions of the protocol handler determined for that flow. The offsets or ranges are within a host egress TCP reassembled contiguous byte stream. 
     In some embodiments, the ingress protocol handler determines packet dispositions through a set of actions. The set of actions may include: (1) writing metadata results into a packet metadata cell; (2) queuing the dispositions, e.g., discard, recirculation (e.g., through a different queue), steering, etc.; (3) Trimming the payload to extract the bytes per the embedded protocol; (4) triggering a pre-configured packet towards a programmable destination. 
     In some embodiments, the ingress protocol handler may have read and/or write access to any offset of a packet when the packet is queued in memory. In some embodiments, the ingress protocol handler may also have local memory and thus may create cross packet-state in the local memory for custom use of the ingress protocol handler. 
     Flow Diagram of Packet Transfer Using SFA 
     SFA  202  can be used to transmit data at high throughput and low predictable latency between network and host, between network and accelerator, between accelerator and host, between accelerator and accelerator, and between network and network. 
       FIG. 4  illustrates an exemplary data flow diagram  400  from a network (e.g., network  402 ) to a host or endpoint (e.g., host/endpoint  206 ). In some embodiments, upon receiving a network packet  404 , SFA  202  breaks the packet apart into portions, e.g., using host/endpoint egress processing engine/pipeline  308   b . These portions include a transport header  406 , an upper layer protocol (ULP) header  408 , and a data payload  410 . SFA  202  then combines payload  410  with a PCIe header/DMA descriptor  412  and sends the combined data to the endpoint/host  206 , as shown in arrow  414 . SFA  202  also sends transport header  406  and ULP header  408  to the host memory of controlling host  204  as shown in arrows  416  and  418  respectively. 
     As shown in  FIG. 4 , an atomic message or packet  404  can be split into a number of header fields and payload. The header fields are sent to a set of controlling host CPUs, while the payload is sent to the endpoints. Therefore, instead of switching on the level of packets or messages, the SFA system described herein maintains an arbitrary association between the host CPUs and endpoints, and performs switching or steering from inside a packet. The SFA system herein separates the packet into portions and sends each portion to different PCIe addresses. A key advantage of the novel SFA system described herein is that existing NIC systems map an entire packet into a single PCIe address space or Root Complex, while SFA  202  allows each portion of a packet to be separated and sent to different PCIe address spaces or Root Complexes. 
     While network packet  404  in  FIG. 4  is separated into three different PCIe addresses, this is only an exemplary case. SFA  202  allows a packet to be separated into an arbitrary number of headers and sent into different places, where the headers will be sent to one or more controlling hosts while the payload is sent to an endpoint. Since SFA  202  disaggregates a packet into an arbitrary number of portions and sends the portions across PCIe address spaces, the SFA system described herein achieves scalable association and uniqueness (i.e., switching portions of packet). 
       FIG. 5  illustrates an exemplary data flow diagram  500  for packet transmission from a host/endpoint (e.g., host  204 , endpoint  206 ) to a network (e.g., network  502 ). In some embodiments, host/endpoint ingress processing engine/pipeline  308   a  of SFA  202  receives transport header  504  and upper layer protocol (ULP) header  506  from controlling host  204 , and receives payload  508  from endpoint  206  (e.g., SSD  212 ). Although headers and payload are from different sources, a header is not disassociated with a payload. SFA  202  maintains a relationship between the header and the payload. For example, a header indicates where the payload should be routed to. Based on such a relationship, SFA  202  combines the received headers  504 ,  506  and payload  508  into a network packet  510 . SFA  202  then sends network packet  510  out to network  502 . 
     It should be noted that since SFA  202  is scheduling and tracking the state of the network ports and all the input queues (e.g., transport, ULP and data sources), SFA  202  may make precise decisions about enforcing quality of service (QoS), isolation and fairness between consumers of the network ports. From tracking, SFA  202 , specifically switch core  306 , may determine if progress is being made for each traffic class that the classified packets belong to and each of the output ports corresponding to the traffic classes. Progress happens when the egress virtual queues are draining, and the data of the queues are not being blocked or backing up. The traffic class can be port-based or packet-based classification. In either way, the destination scheduling state is known to the SFA for that class as described herein. From scheduling, SFA  202 , e.g., a QoS scheduler (not shown), maintains statistics for each of the inputs. An input may be an input port, a transmit queue, or a network port. The QoS scheduler of SFA  202  may stop fetching a packet from the input port if, upon a forwarding decision, the packet was destined to a port that was blocked. As compared to the Host to Network path in a traditional system, where a packet transfer follows the route like Host→NIC→Switch→Switch, each layer can only make local decisions. Therefore, even if the path to a destination is blocked, the NIC may still send the packets to the switch, and the switch then drops or discards the packets. In contrast, since SFA  202  tracks and knows the state of switch ports, SFA  202  will keep the packets on the host (without even fetching them) when the path to a destination is blocked. SFA  202  may start packet transfer again when SFA  202  determines that the output port is going to make progress. It is critical that SFA  202  has visibility into the state of the host (e.g., a NIC interface) and the network (e.g., a switch interface), which allows SFA  202  to make the arbitration decision precisely. This is significantly advantageous as compared to statistical decisions based on sampling of the data on ports used in standard top-of-rack switch systems. 
     In addition, the SFA system described herein allows much higher overall packet bandwidth to be streamed or scaled than existing systems because, on the host/endpoint side, there is typically much higher data rate required for payload transfers than for header transfers. Because the SFA can dedicate more PCIe/CXL I/O bandwidth to/from endpoints (e.g. GPU/FPGA/accelerators or storage/memory) versus to/from controlling hosts, the SFA based system can transmit or receive complete packets and make progress in either direction at a higher throughput than a system where header and payload transfers share the same fixed PCIe/CXL I/O bandwidth. Further, the SFA system described herein allows a much higher packet processing rate to be scaled because controlling host CPUs can be scaled and aggregated as opposed to some existing systems that limit one, or a few, controlling host CPUs to only a single NIC. 
       FIG. 6  illustrates an exemplary data packet flow diagram  600  for peer-to-peer transmission. Peer-to-peer data transmission can be between network and network as shown in dash-lined circle  602 , or between endpoint and endpoint as shown in dash-lined circle  604 . For network to network packet flow shown in  602 , an Ethernet switch/router may be responsible for forwarding, classification, and load balancing of network packets when the packets move through SFA  202  via the data path of network ingress, switch core, and then network egress. For endpoint to endpoint communications shown in  604 , the host/endpoint ingress processing pipeline (e.g.,  308   a ) of SFA  202  receives the payload of a packet flows from a source endpoint (e.g. GPU  210 ). This payload is passed to switch core (e.g.  306 ). The switch core then routes the payload to the host/endpoint egress processing pipe (e.g.,  308   b ) of SFA  202  of a destination endpoint (e.g. storage  212 ). In some embodiments, the payload transfer from a source endpoint may be triggered by either (1) a scatter gather list (SGL) write initiated by the source endpoint to the destination endpoint memory address via standard queue interfaces of SFA  202 , or (2) a host-initiated cache line operation through the memory mapped windows of SFA  202 , as shown in  606 . 
       FIG. 7  illustrates a host view  700  of packet flow to a network. SFA  202  selects p ports that connect SFA  202  to GPUs  210  and storage (e.g., SSDs  212 ). SFA  202  selects n ports that connect SFA  202  to controlling host  204 . In some embodiments, each p port is a PCIe endpoint to a unique n port. Any SFA queue can be mapped into any p or n port. n ports have header queues. p ports have queues to submit scatter-gather lists. Memory Mapped windows allow any-to-any cache line exchange. As explained above, n and p port packets combine (e.g., data from header queues with SGL&#39;s) to generate network packets. n and p ports may also combine to generate peer-to-peer traffic, such that the header queues can indicate which local ports to send the SGL&#39;s to. For example, the payload is combined with the header on the source side (e.g., in a TransmitHeaderQueue). The combined data is passed to a Host Egress virtual queue, where the payload is stripped and directly placed into the receive SGL&#39;s on the destination side (e.g., in a ReceiveHeaderQueue). For peer to peer signaling, memory mapped windows may provide extremely low latency inter-process communication (IPC) or doorbell primitives, which can operate in interrupt or polling with memory waiters. 
       FIG. 8  illustrates an exemplary process  800  of transmitting data from a network to a host/endpoint in an SFA communication system, according to some embodiments. In some embodiments, the SFA communication system includes an SFA (e.g., SFA  202 ) communicatively coupled to a plurality of controlling hosts, a plurality of endpoints, and a plurality of network ports. Process  800  corresponds to the data flow in  FIG. 4 . 
     At step  805 , the SFA receives a network packet from a network port of the plurality of network ports. At step  810 , the SFA separates the network packet into different portions, each portion including a header or a payload. In some embodiments, prior to separating the network packet, the SFA also determines whether to separate the packet. If it is determined not to split the network packet, the SFA may transmit the entire other network packet to a controlling host of the plurality controlling hosts. 
     At step  815 , the SFA maps each portion of the network packet to a destination controlling host or a destination endpoint. In some embodiments, the SFA may identify a controlling host of the plurality controlling hosts and designate the controlling host as the destination controlling host. The SFA may also identify an endpoint of the plurality of endpoints and designate the endpoint as a destination endpoint. In some embodiments, the SFA is configured to perform a consistent hash function on the parsed headers to determine the destination controlling host or the destination endpoint. 
     At step  820 , the SFA forwards a respective portion of the network packet to the destination controlling host or the destination endpoint. In some embodiments, the SFA moves each portion of the network packet over one or more disjoint physical interfaces toward separate memory subsystems or separate compute subsystems. 
       FIG. 9  illustrates an exemplary process  900  of transmitting data from a host/endpoint to a network in an SFA communication system, according to some embodiments. In some embodiments, the SFA communication system includes an SFA (e.g., SFA  202 ) communicatively coupled to a plurality of controlling hosts, a plurality of endpoints, and a plurality of network ports. Process  900  corresponds to the data flow in  FIG. 5 . 
     At step  905 , the SFA receives from one or more of the plurality controlling hosts a plurality of headers, each header having a respective descriptor. At step  910 , the SFA receives a plurality of payloads from one or more of the plurality of endpoints. At step  915 , the SFA identifies, based on respective descriptors of the plurality of headers, a header and a corresponding payload. In some embodiments, a descriptor from the respective descriptors includes a data descriptor or a header descriptor. At step  920 , the SFA combines the identified header and payload into a network packet. At step  925 , the SFA transmits the network packet to a network port of the plurality of network ports. 
       FIG. 10  illustrates an exemplary process  1000  of peer-to-peer data transfer in an SFA communication system, according to some embodiments. In some embodiments, the SFA communication system includes an SFA (e.g., SFA  202 ) communicatively coupled to a plurality of controlling hosts, a plurality of endpoints, and a plurality of network ports. Process  1000  corresponds to the data flow in  FIG. 6 . 
     At step  1005 , the SFA receives a trigger event. In some embodiments, the trigger is based on the receipt of associated descriptors that include at least one of a data descriptor or a header descriptor. At step  1010 , the SFA retrieves payload data of a network packet from a source endpoint of the plurality of endpoints. 
     At step  1015 , the SFA routes, based on associated descriptors, the payload data to a destination endpoint of the plurality of endpoints. In some embodiments, the trigger event is a scatter gather list (SGL) write initiated by the source endpoint to the destination endpoint memory address, and the data descriptor includes data pointing to the SGL. In other embodiments, the trigger event is a host-initiated cache line operation. 
       FIG. 11  illustrates an exemplary process  1100  of a host view of data transfer in an SFA communication system, according to some embodiments. In some embodiments, the SFA communication system includes an SFA (e.g., SFA  202 ) communicatively coupled to a plurality of controlling hosts, a plurality of endpoints, and a plurality of network ports. Process  1100  corresponds to the data flow in  FIG. 7 . 
     At step  1105 , the SFA identifies a first set of endpoint ports that connect the SFA to the plurality of endpoints. Each respective endpoint port of the first set of endpoint ports has a respective queue to submit one or more scatter-gather lists (SGLs). At step  1110 , the SFA identifies a second set of host ports that connect the SFA to a controlling host of the plurality of controlling hosts. Each respective host port of the second set of host ports includes one or more respective header queues. At step  1115 , the SFA combines data from the header queues with the SGLs to generate network packets. 
     In some embodiments, the SFA is also configured to receive a network packet and divide the network packet into one or more headers and a payload. The SFA is then configured to identify, from the first set of endpoint ports, an endpoint port, and route the payload to an SGL of a queue associated with the identified endpoint port. The SFA is further configured to identify, from the second set of host ports, one or more host ports, and route the one or more headers to one or more header queues associated with the identified host ports. In some embodiments, the payload is transmitted to an arbitrary memory geometry based on the SGL of memory locations. 
     Additional Considerations 
     In some implementations, at least a portion of the approaches described above may be realized by instructions that upon execution cause one or more processing devices to carry out the processes and functions described above. Such instructions may include, for example, interpreted instructions such as script instructions, or executable code, or other instructions stored in a non-transitory computer readable medium. The storage device  830  may be implemented in a distributed way over a network, for example as a server farm or a set of widely distributed servers, or may be implemented in a single computing device. 
     Although an example processing system has been described, embodiments of the subject matter, functional operations and processes described in this specification can be implemented in other types of digital electronic circuitry, in tangibly-embodied computer software or firmware, in computer hardware, including the structures disclosed in this specification and their structural equivalents, or in combinations of one or more of them. Embodiments of the subject matter described in this specification can be implemented as one or more computer programs, i.e., one or more modules of computer program instructions encoded on a tangible nonvolatile program carrier for execution by, or to control the operation of, data processing apparatus. Alternatively or in addition, the program instructions can be encoded on an artificially generated propagated signal, e.g., a machine-generated electrical, optical, or electromagnetic signal that is generated to encode information for transmission to suitable receiver apparatus for execution by a data processing apparatus. The computer storage medium can be a machine-readable storage device, a machine-readable storage substrate, a random or serial access memory device, or a combination of one or more of them. 
     The term “system” may encompass all kinds of apparatus, devices, and machines for processing data, including by way of example a programmable processor, a computer, or multiple processors or computers. A processing system may include special purpose logic circuitry, e.g., an FPGA (field programmable gate array) or an ASIC (application specific integrated circuit). A processing system may include, in addition to hardware, code that creates an execution environment for the computer program in question, e.g., code that constitutes processor firmware, a protocol stack, a database management system, an operating system, or a combination of one or more of them. 
     A computer program (which may also be referred to or described as a program, software, a software application, a module, a software module, a script, or code) can be written in any form of programming language, including compiled or interpreted languages, or declarative or procedural languages, and it can be deployed in any form, including as a standalone program or as a module, component, subroutine, or other unit suitable for use in a computing environment. A computer program may, but need not, correspond to a file in a file system. A program can be stored in a portion of a file that holds other programs or data (e.g., one or more scripts stored in a markup language document), in a single file dedicated to the program in question, or in multiple coordinated files (e.g., files that store one or more modules, sub programs, or portions of code). A computer program can be deployed to be executed on one computer or on multiple computers that are located at one site or distributed across multiple sites and interconnected by a communication network. 
     The processes and logic flows described in this specification can be performed by one or more programmable computers executing one or more computer programs to perform functions by operating on input data and generating output. The processes and logic flows can also be performed by, and apparatus can also be implemented as, special purpose logic circuitry, e.g., an FPGA (field programmable gate array) or an ASIC (application specific integrated circuit). 
     Computers suitable for the execution of a computer program can include, by way of example, general or special purpose microprocessors or both, or any other kind of central processing unit. Generally, a central processing unit will receive instructions and data from a read-only memory or a random access memory or both. A computer generally includes a central processing unit for performing or executing instructions and one or more memory devices for storing instructions and data. Generally, a computer will also include, or be operatively coupled to receive data from or transfer data to, or both, one or more mass storage devices for storing data, e.g., magnetic, magneto optical disks, or optical disks. However, a computer need not have such devices. Moreover, a computer can be embedded in another device, e.g., a mobile telephone, a personal digital assistant (PDA), a mobile audio or video player, a game console, a Global Positioning System (GPS) receiver, or a portable storage device (e.g., a universal serial bus (USB) flash drive), to name just a few. 
     Computer readable media suitable for storing computer program instructions and data include all forms of nonvolatile memory, media and memory devices, including by way of example semiconductor memory devices, e.g., EPROM, EEPROM, and flash memory devices; magnetic disks, e.g., internal hard disks or removable disks; magneto optical disks; and CD-ROM and DVD-ROM disks. The processor and the memory can be supplemented by, or incorporated in, special purpose logic circuitry. 
     To provide for interaction with a user, embodiments of the subject matter described in this specification can be implemented on a computer having a display device, e.g., a CRT (cathode ray tube) or LCD (liquid crystal display) monitor, for displaying information to the user and a keyboard and a pointing device, e.g., a mouse or a trackball, by which the user can provide input to the computer. Other kinds of devices can be used to provide for interaction with a user as well; for example, feedback provided to the user can be any form of sensory feedback, e.g., visual feedback, auditory feedback, or tactile feedback; and input from the user can be received in any form, including acoustic, speech, or tactile input. In addition, a computer can interact with a user by sending documents to and receiving documents from a device that is used by the user; for example, by sending web pages to a web browser on a user&#39;s user device in response to requests received from the web browser. 
     Embodiments of the subject matter described in this specification can be implemented in a computing system that includes a back-end component, e.g., as a data server, or that includes a middleware component, e.g., an application server, or that includes a front-end component, e.g., a client computer having a graphical user interface or a Web browser through which a user can interact with an implementation of the subject matter described in this specification, or any combination of one or more such back-end, middleware, or front-end components. The components of the system can be interconnected by any form or medium of digital data communication, e.g., a communication network. Examples of communication networks include a local area network (“LAN”) and a wide area network (“WAN”), e.g., the Internet. 
     The computing system can include clients and servers. A client and server are generally remote from each other and typically interact through a communication network. The relationship of client and server arises by virtue of computer programs running on the respective computers and having a client-server relationship to each other. 
     While this specification contains many specific implementation details, these should not be construed as limitations on the scope of what may be claimed, but rather as descriptions of features that may be specific to particular embodiments. Certain features that are described in this specification in the context of separate embodiments can also be implemented in combination in a single embodiment. Conversely, various features that are described in the context of a single embodiment can also be implemented in multiple embodiments separately or in any suitable subcombination. Moreover, although features may be described above as acting in certain combinations and even initially claimed as such, one or more features from a claimed combination can in some cases be excised from the combination, and the claimed combination may be directed to a subcombination or variation of a subcombination. 
     Similarly, while operations are depicted in the drawings in a particular order, this should not be understood as requiring that such operations be performed in the particular order shown or in sequential order, or that all illustrated operations be performed, to achieve desirable results. In certain circumstances, multitasking and parallel processing may be advantageous. Moreover, the separation of various system components in the embodiments described above should not be understood as requiring such separation in all embodiments, and it should be understood that the described program components and systems can generally be integrated together in a single software product or packaged into multiple software products. 
     Particular embodiments of the subject matter have been described. Other embodiments are within the scope of the following claims. For example, the actions recited in the claims can be performed in a different order and still achieve desirable results. As one example, the processes depicted in the accompanying figures do not necessarily require the particular order shown, or sequential order, to achieve desirable results. In certain implementations, multitasking and parallel processing may be advantageous. Other steps or stages may be provided, or steps or stages may be eliminated, from the described processes. Accordingly, other implementations are within the scope of the following claims. 
     The phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. 
     The term “approximately”, the phrase “approximately equal to”, and other similar phrases, as used in the specification and the claims (e.g., “X has a value of approximately Y” or “X is approximately equal to Y”), should be understood to mean that one value (X) is within a predetermined range of another value (Y). The predetermined range may be plus or minus 20%, 10%, 5%, 3%, 1%, 0.1%, or less than 0.1%, unless otherwise indicated. 
     The indefinite articles “a” and “an,” as used in the specification and in the claims, unless clearly indicated to the contrary, should be understood to mean “at least one.” The phrase “and/or,” as used in the specification and in the claims, should be understood to mean “either or both” of the elements so conjoined, i.e., elements that are conjunctively present in some cases and disjunctively present in other cases. Multiple elements listed with “and/or” should be construed in the same fashion, i.e., “one or more” of the elements so conjoined. Other elements may optionally be present other than the elements specifically identified by the “and/or” clause, whether related or unrelated to those elements specifically identified. Thus, as a non-limiting example, a reference to “A and/or B”, when used in conjunction with open-ended language such as “comprising” can refer, in one embodiment, to A only (optionally including elements other than B); in another embodiment, to B only (optionally including elements other than A); in yet another embodiment, to both A and B (optionally including other elements); etc. 
     As used in the specification and in the claims, “or” should be understood to have the same meaning as “and/or” as defined above. For example, when separating items in a list, “or” or “and/or” shall be interpreted as being inclusive, i.e., the inclusion of at least one, but also including more than one, of a number or list of elements, and, optionally, additional unlisted items. Only terms clearly indicated to the contrary, such as “only one of or “exactly one of,” or, when used in the claims, “consisting of,” will refer to the inclusion of exactly one element of a number or list of elements. In general, the term “or” as used shall only be interpreted as indicating exclusive alternatives (i.e. “one or the other but not both”) when preceded by terms of exclusivity, such as “either,” “one of,” “only one of,” or “exactly one of.” “Consisting essentially of,” when used in the claims, shall have its ordinary meaning as used in the field of patent law. 
     As used in the specification and in the claims, the phrase “at least one,” in reference to a list of one or more elements, should be understood to mean at least one element selected from any one or more of the elements in the list of elements, but not necessarily including at least one of each and every element specifically listed within the list of elements and not excluding any combinations of elements in the list of elements. This definition also allows that elements may optionally be present other than the elements specifically identified within the list of elements to which the phrase “at least one” refers, whether related or unrelated to those elements specifically identified. Thus, as a non-limiting example, “at least one of A and B” (or, equivalently, “at least one of A or B,” or, equivalently “at least one of A and/or B”) can refer, in one embodiment, to at least one, optionally including more than one, A, with no B present (and optionally including elements other than B); in another embodiment, to at least one, optionally including more than one, B, with no A present (and optionally including elements other than A); in yet another embodiment, to at least one, optionally including more than one, A, and at least one, optionally including more than one, B (and optionally including other elements); etc. 
     The use of “including,” “comprising,” “having,” “containing,” “involving,” and variations thereof, is meant to encompass the items listed thereafter and additional items. 
     Use of ordinal terms such as “first,” “second,” “third,” etc., in the claims to modify a claim element does not by itself connote any priority, precedence, or order of one claim element over another or the temporal order in which acts of a method are performed. Ordinal terms are used merely as labels to distinguish one claim element having a certain name from another element having a same name (but for use of the ordinal term), to distinguish the claim elements. 
     Having thus described several aspects of at least one embodiment of this invention, it is to be appreciated that various alterations, modifications, and improvements will readily occur to those skilled in the art. Such alterations, modifications, and improvements are intended to be part of this disclosure, and are intended to be within the spirit and scope of the invention. Accordingly, the foregoing description and drawings are by way of example only.