Patent Publication Number: US-2005129039-A1

Title: RDMA network interface controller with cut-through implementation for aligned DDP segments

Description:
BACKGROUND OF THE INVENTION  
      1. Technical Field  
      The present invention relates generally to data transfer, and more particularly, to an RDMA enabled network interface controller (RNIC) with a cut-through implementation for aligned DDP segments.  
      1. Related Art  
      1. Overview  
      Referring to  FIG. 1A , a block diagram of a conventional data transfer environment  1  is shown. Data transfer environment  1  includes a data source  2  (i.e., a peer) that transmits a data transfer  3 A via one or more remote memory data access (RDMA) enabled network interface controller(s) (RNIC)  4  to a data sink  5  (i.e., a peer) that receives data transfer  3 B. RNIC  4  includes, inter alia (explained further below), reassembly buffers  6 . Networking communication speeds have significantly increased recently from 10 mega bits per second (Mbps) through 100 Mbps to 1 giga bits per second (Gbps), and are now approaching speeds in the range of 10 Gbps. The communications bandwidth increase, however, is now beginning to outpace the rate at which central processing units (CPUs) can process data efficiently, resulting in a bottleneck at server processors, e.g., RNIC  4 . For example, a common 1 Gbps network connection, if fully utilized, can be a large burden to a 2 GHz CPU. In particular, a CPU such as this can extend approximately half of its processing power just handling low-level transmission control protocol (TCP) processing from data coming from a network card.  
      One approach to solving this problem has been to implement the transmission control and Internet protocol (TCP/IP) stack in hardware finite state machines (FSM) rather than as software to be processed by a CPU. This approach allows for very fast packet processing resulting in wire speed processing of back-to-back short packets. In addition, this approach presents a very compact and powerful solution with low cost. Unfortunately, since the TCP/IP stack was defined and developed for implementation in software, generating a TCP/IP stack in hardware has resulted in a wide range of new problems. For example, problems that arise include: how to implement a software-based protocol in hardware FSMs and achieve improved performance, how to design an advantageous and efficient interface to upper layer protocols (ULPs) (e.g., application protocols) to provide a faster implementation of the ULP, and how to avoid new bottle-necks in a scaled-up implementation.  
      In order to address these new problems, new communication layers have been developed to lay between the traditional ULP and the TCP/IP stack. Unfortunately, protocols placed over a TCP/IP stack typically require many copy operations because the ULP must supply buffers for indirect data placement, which adds latency and consumes significant CPU and memory resources. In order to reduce the amount of copy operations, a suite of new protocols, referred to as iWARP, have been developed.  
      2. The Protocols  
      Referring to  FIG. 1B , a brief overview of various protocols, including the iWARP protocols, and data transfer format structure will now be described. As can be seen, each data transfer may include information related to a number of different protocols, each for providing different functionality relative to the data transfer. For example, as shown in  FIG. 1B , an Ethernet protocol  100  provides local area network (LAN) access as defined by IEEE standard 802.3; an Internet protocol (IP)  102  adds necessary network routing information; a transfer control protocol (TCP)  104  schedules outbound TCP segments  106  and satisfies delivery guarantees; and a marker with protocol data unit (PDU) alignment (MPA) protocol  108  provides an MPA frame  109  that includes a backward MPA marker(s)  110  at a fixed interval (i.e., every 512 bytes) across DDP segments  112  (only one shown, but may be stream) and also adds a length field  114  and cyclic redundancy checking (CRC) field  116  to each MPA frame  109 . In addition, a direct data placement (DDP) protocol  120  segments outbound messages into one or more DDP segments  112 , and reassembles one or more DDP segments into a DDP message  113 ; and a remote data memory access (RDMA) protocol  122  converts RDMA Write, Read, Sends into/out of DDP messages. Although only one DDP segment  112  has been shown for clarity, it should be recognized that numerous DDP segments  112  can be provided in each TCP segment  106 .  
      With special regard to RDMA protocol  122 , this protocol, developed by the RDMA Consortium, enables removal of data copy operations and reduction in latencies by allowing one computer to directly place information in another computer&#39;s memory with minimal demands on memory bus bandwidth and central processing unit (CPU) processing overhead, while preserving memory protection semantics. RDMA over TCP/IP promises more efficient and scalable computing and data transport within a data center by reducing the overhead burden on processors and memory, which makes processor resources available for other work, such as user applications, and improves infrastructure utilization. In this case, as networks become more efficient, applications are better able to scale by sharing tasks across the network as opposed to centralizing work in larger, more expensive systems. With RDMA functionality, a transmitter can use framing to put headers on Ethernet byte streams so that those byte streams can be more easily decoded and executed in an out-of-order mode at the receiver, which will boost performance—especially for Internet Small Computer System Interface (iSCSI) and other storage traffic types. Another advantage presented by RDMA is the ability to converge functions in the data center over fewer types of interconnects. By converging functions over fewer interconnects, the resulting infrastructure is less complex, easier to manage and provides the opportunity for architectural redundancy, which improves system resiliency.  
      With special regard to the DDP protocol, this protocol introduces a mechanism by which data may be placed directly into an upper layer protocol&#39;s (ULP) receive buffer without intermediate buffers. DDP reduces, and in some cases eliminates, additional copying (to and from reassembly buffers) performed by an RDMA enabled network interface controller (RNIC) when processing inbound TCP segments.  
      3. Challenges  
      One challenge facing efficient implementation of TCP/IP with RDMA and DDP in a hardware setting is that standard TCP/IP off-load engine (TOE) implementations include reassembly buffers in receive logic to arrange out-of-order received TCP streams, which increases copying operations. In addition, in order for direct data placement to the receiver&#39;s data buffers to be completed, the RNIC must be able to locate the destination buffer for each arriving TCP segment payload  127 . As a result, all TCP segments are saved to the reassembly buffers to ensure that they are in-order and the destination buffers can be located. In order to address this problem, iWARP specifications strongly recommend to the transmitting RNIC to perform segmentation of RDMA messages in such way that the created DDP segments would be “aligned” to TCP segments. Nonetheless, non-aligned DDP segments are oftentimes unavoidable, especially where the data transfer passes through many interchanges.  
      Referring to  FIG. 1B , “alignment” means that a TCP header  126  is immediately followed by a DDP segment  112  (i.e., MPA header follows TCP header, then DDP header), and the DDP segment  112  is fully contained in the one TCP segment  106 . More specifically, each TCP segment  106  includes a TCP header  126  and a TCP payload/TCP data  127 . A “TCP hole”  130  is a missing TCP segment(s) in the TCP data stream. MPA markers  110  provide data for when an out-of-order TCP segment  106  is received, and a receiver wants to know whether MPA frame  109  inside TCP segment  106  is aligned or not with TCP segment  106 . Each marker  110  is placed at equal intervals (512 bytes) in a TCP stream, starting with an Initial Sequence Number of a particular connection, and points to a DDP/RDMA header  124  of an MPA frame  109  that it travels in. A first sequential identification number is assigned to a first TCP segment  106 , and each Initial Sequence Number in subsequent TCP segments  106  includes an incremented sequence number.  
      In  FIG. 1B , solid lines illustrate an example of an aligned data transfer in which TCP header  126  is immediately followed by MPA length field  114  and DDP/RDMA header  124 , and DDP segment  112  is fully contained in TCP segment  106 . A dashed line in DDP protocol  120  layer indicates a non-aligned DDP segment  112 NA in which TCP header  126  is not immediately followed by MPA length field  114  and DDP/RDMA header  124 . A non-aligned DDP segment may result, for example, from re-segmentation by a middle-box that may stand in-between sending and receiving RNICs, or a reduction of maximum segment size (MSS) on-the-fly. Since a transmitter RNIC cannot change DDP segmentation (change location of DDP headers in TCP stream), a retransmit operation may require a new, decreased MSS despite the original DDP segments creation with a larger MSS. In any case, the increase in copying operations reduces speed and efficiency. Accordingly, there is a need in the art for a way to handle aligned DDP segment placement and delivery in a different fashion than non-aligned DDP segment placement and delivery.  
      Another challenge relative to non-aligned DDP segment  112 NA handling is created by the fact that it is oftentimes difficult to determine what is causing the non-aligmnent. For example, the single non-aligned DDP segment  112 NA can be split between two or more TCP segments  106  and one of them may arrive and another may not arrive. In another case, some DDP segments  112 NA may fall between MPA markers  110 , a header may be missing, or a segment tail may be missing (in the latter case, you can partially place the segment and need to keep some information to understand where to place the remaining part, when it arrives), etc. Relative to this latter case,  FIG. 1C  shows a block diagram of possible situations relative to MPA marker references for one or more non-aligned DDP segments  112 NA. Case A illustrates a situation in which a DDP segment header  160  of a newly received DDP segment  162  is referenced by an MPA length field  164  of a previously processed DDP segment  166 . Case B illustrates a situation in which newly received DDP segment  162  header  160  is referenced by a marker  168  located inside newly received DDP segment  162 . That is, marker  168  is referring to the beginning of newly received DDP segment  162 . Case C illustrates a situation in which marker  168  is located in newly received DDP segment  162 , but points outside of the segment. Case D illustrates a situation in which marker  168  is located in newly received DDP segment  162 , and points inside the segment. Case E illustrates a situation in which no marker is located in newly received DDP segment  162 . In any case, where the cause of DDP segment non-alignment cannot be determined, an RNIC cannot conduct direct data placement because there are too many cases to adequately address, and too much information/partial segments to hold in the intermediate storage. Accordingly, any solution that provides different handling of aligned and non-aligned DDP segments should address the various situations that may cause the non-alignment.  
      4. DDP/RDMA Operational Flow  
      Referring to  FIGS. 1D-1H , a brief overview of DDP/RDMA operational flow will now be described for purposes of later description. With special regard to DDP protocol  120  ( FIG. 1B ), DDP provides two types of messages referred to as tagged and untagged messages. Referring to  FIG. 1D , in a “tagged message,” each DDP segment  112  ( FIG. 1B ) carries a steering tag (“STag”) in DDP/RDMA header  124  that identifies a memory region/window in a destination buffer (e.g., a memory region  232  in  FIG. 1G ) on a receiver to which data can be placed directly, a target offset (TO) in this region/window and a segment payload (not shown). In this case, availability of the destination buffer is “advertised” via the STag. Referring to  FIG. 1E , an “untagged message” is one in which a remote transmitter does not know buffers at a receiver, and sends a message with a queue ID (QN), a message sequence number (MSN) and a message offset (MO), which may be used by the receiver to determine appropriate buffers.  
      Referring to  FIGS. 1F-1H , the RDMA protocol defines four types of messages: a Send  200 , a Write  202 , a Read  204 , and a Read Response  206 . Returning to  FIG. 1A , a verb interface  7  presents RNIC  4  to a consumer, and includes methods to allocate and de-allocate RNIC  4  resources, and to post work requests (WR)  208  to RNIC  4 . Verb interface  7  usually is implemented by a verb library  8  having two parts: user space library  9 A that serves user space consumers and kernel module  9 B that serves kernel space consumers. Verb interface  7  is RNIC-specific software that works with RNIC  4  hardware and firmware. There is no strict definition of what should be implemented in verb interface  7  (verb library  8 ), hardware and firmware. Verb interface  7  can be viewed as a single package that provides RNIC  4  services to a consumer, so the consumer can perform mainly two types of operations: management of RNIC  4  resources (allocation and de-allocation), and posting of work request(s) (WR) to RNIC  4 . Examples of RNIC  4  resource management are: a queue pair allocation and de-allocation, a completion queue (hereinafter “CQ”) allocation and de-allocation or memory region allocation and de-allocation. These management tasks will be described in more detail below.  
      As shown in  FIG. 1F-1H , a consumer allocates a queue pair to which work requests  208  are posted. A “queue pair” (hereinafter “QP”) is associated with a TCP connection and includes a pair of work queues (e.g., send and receive)  210 ,  212  as well as a posting mechanism (not shown) for each queue. Each work queue  210 ,  212  is a list of Work Queue Elements (WQE)  216  where each WQE holds some control information describing one work request (WR)  208  and refers (or points) to the consumer buffers. A consumer posts a work request (WR)  208  to work queues  210 ,  212  in order to get verb interface  7  ( FIG. 1A ) and RNIC  4  ( FIG. 1A ) to execute posted work requests (WR)  208 . In addition, there are resources that may make up the QP with which the consumer does not directly interact such as a read queue  214  ( FIG. 1H ) and work queue elements (WQEs)  216 .  
      The typical information that can be held by a WQE  216  is a consumer work request (WR) type (i.e., for a send WR  208 S it can be RDMA Send, RDMA Write, RDMA Read, etc., for a receive WR  208 R it can be RDMA Receive only), and a description of consumer buffers that either carry data to transmit or represent a location for received data. A WQE  216  always describes/corresponds to a single RDMA message. For example, when a consumer posts a send work request (WR)  208 S of the RDMA Write type, verb library  8  ( FIG. 1A ) builds a WQE  216 S describing the consumer buffers from which the data needs to be taken, and sent to the responder, using an RDMA Write message. In another example, a receive work request (WR)  208 R ( FIG. 1F ) is present. In this case, verb library  8  ( FIG. 1A ) adds a WQE  216 R to receive queue (RQ)  212  that holds a consumer buffer that is to be used to place the payload of the received Send message  200 .  
      When verb library  8  ( FIG. 1A ) adds a new WQE  216  to send queue (SQ)  210  or receive queue (RQ)  212 , it notifies (referred to herein as “rings doorbell”) of RNIC  4  ( FIG. 1A ) that a new WQE  216  has been added to send queue (SQ)/receive queue (RQ), respectively. This “doorbell ring” operation is usually a write to the RNIC memory space, which is detected and decoded by RNIC hardware. Accordingly, a doorbell ring notifies the RNIC that there is new work that needs to the done for the specified SQ/RQ, respectively.  
      RNIC  4  ( FIG. 1A ) holds a list of send queues (SQs)  210  that have pending (posted) WQEs  216 . In addition, the RNIC arbitrates between those send queues (SQs)  210 , and serves them one after another. When RNIC  4  picks a send queue (SQ)  210  to serve, it reads the next WQE  216  to serve (WQEs are processed by the RNIC in the order they have been posted by a consumer), and generates one or more DDP segments  220  belonging to the requested RDMA message.  
      Handling of the particular types of RDMA messages will now be described with reference to  FIGS. 1F-1H . As shown in  FIG. 1F , RNIC (Requester) selects to serve particular send queue (SQ)  210 S. It reads WQE  216 S from send queue (SQ)  210 S. If this WQE  216 S corresponds to an RDMA Send request, RNIC generates a Send message, and sends this message to the peer RNIC (Responder). The generated message may include, for example, three DDP segments  220 . When RNIC (Responder) receives the Send message, it reads WQE  216 R from receive queue (RQ)  212 , and places the payload of received DDP segments  220  to the consumer buffers (i.e. responder Rx buff)  230  referred by that WQE  216 R. If Send Message  200  is received in-order, then the RNIC picks the first unused WQE  216 R from receive queue (RQ)  212 . WQEs  216 R are chained in request queue (RQ)  212  in the order they have been posted by a consumer. In terms of an untagged DDP message, Send message  200  carries a Message Sequence Number (MSN) ( FIG. 1E ), which is initialized to one and monotonically increased by the transmitter with each sent DDP message  220  belonging to the same DDP Queue. (Tagged messages will be described relative to RDMA Write message  202  below). A DDP Queue is identified by Queue Number (QN) ( FIG. 1E ) in the DDP header. The RDMA protocol defines three DDP Queues: QN # 0  for inbound RDMA Sends, QN # 1  for inbound RDMA Read Requests, and QN # 2  for inbound Terminates. Accordingly, when Send message  200  arrives out-of-order, RNIC  4  may use the MSN of that message to find the WQE  216 R that corresponds to that Send message  200 . One received Send message  200  consumes one WQE  216 R from receive queue (RQ)  212 . Lack of a posted WQE, or message data length exceeding the length of the WQE buffers, is considered as a critical error and leads to connection termination.  
      Referring to  FIGS. 1G and 1H , an RDMA Write message  202 , using tagged operations, and part of RDMA Read message  204  will now be described. To use tagged operations, a consumer needs to register a memory region  232 . Memory region  232  is a virtually contiguous chunk of pinned memory on the receiver, i.e., responder in  FIG. 1G . A memory region  232  is described by its starting virtual address (VA), length, access permissions, and a list of physical pages associated with that memory region  232 . As a result of memory region  232  registration, a consumer receives back a steering tag (STag), which can be used to access that registered memory region  232 . Access of memory region  232  by a remote consumer (e.g., requester in  FIG. 1G ) is performed by RNIC  4  without any interaction with the local consumer (e.g., responder in  FIG. 1G ). When the consumer wants to access remote memory  232 , it posts a send work request (WR)  208 W or  208 R ( FIG. 1H ) of the RDMA Write or RDMA Read type, respectively. Verb library  8  ( FIG. 1A ) adds corresponding WQEs  216 W ( FIG. 1G ) or  216 R ( FIG. 1H ) to send queue (SQ)  210 W or  210 R, respectively, and notifies RNIC  4 . When connection wins arbitration, RNIC  16  reads WQEs  216 W or  216 R, and generates RDMA Write message  202  or RDMA Read message  204 , respectively.  
      With special regard to RDMA Write message  202 , as shown in  FIG. 1G , when an RDMA Write message  202  is received by RNIC  4 , the RNIC uses the STag and TO ( FIG. 1D ) and length in the header of DDP segments (belonging to that message) to find the registered memory region  232 , and places the payload of RDMA Write message  202  to memory  232 . The receiver software or CPU (i.e., responder as shown) is not involved in the data placement operation, and is not aware that this operation took place.  
      With special regard to an RDMA Read message  204 , as shown in  FIG. 1H , when the message is received by RNIC  4  ( FIG. 1A ), the RNIC generates a RDMA Read Response message  206 , and sends it back to the remote host, i.e., requester as shown. In this case, the receive queue is referred to as a read queue  214 . Generation of RDMA Read Response  206  is also performed without involvement of the local consumer (i.e., responder), which is not aware that this operation took place. When the RDMA Read Response  206  is received, RNIC  4  ( FIG. 1A ) handles this message similarly to handling an RDMA Write message  204 . That is, it writes to memory region  232  on the requester side.  
      In addition to handling consumer work requests, RNIC  4  ( FIG. 1A ) also notifies a consumer about completion of those requests, as shown in  FIGS. 1F-1H . Completion notification is made by using completion queues  240 , another RNIC resource, which is allocated by a consumer (via a dedicated function provided by verb library  8 ). A completion queue  240  includes completion queue elements (CQE)  242 . CQEs  242  are placed to a completion queue (CQ)  240  by RNIC  4  ( FIG. 1A ) when it reports completion of a consumer work request (WR)  208 S,  208 W,  208 RR. Each work queue (i.e., send queue (SQ)  210 , receive queue (RQ)  212 ) has an associated completion queue (CQ)  240 . (Note: read queue  214  is an internal queue maintained by hardware, and is invisible to software. Therefore, no CQ  240  is associated with this queue, and the consumer does not allocate this queue nor know about its existence). It should be noted, however, that the same completion queue (CQ)  240  can be associated with more than one send queue (SQ)  210  and receive queue (RQ)  212 . Association is performed at queue pair (QP) allocation time. In operation, when a consumer posts a work request WR  208  to a send queue (SQ)  210 , it can specify whether it wants to get a notification when this request is completed. If the consumer requested a completion notification, RNIC  4  places a completion queue element (CQE)  242  to an associated completion queue (CQ)  240  associated with send queue (SQ)  210  upon completion of the work request (WR). The RDMA protocol defines very simple completion ordering for work requests (WR)  208  posted to a send queue (SQ)  210 . In particular, RDMA send work requests (WR)  208 S and RDMA write work requests (WR)  208 W are completed when they have been reliably transmitted. An RDMA read work request (WR)  208 R is completed when the corresponding RDMA Read Response message  206  has been received, and placed to memory region  232 . Consumer work requests (WR) are completed in the order they are posted to send queue (SQ)  210 . Referring to  FIG. 1F , each work request (WR) posted to a receive queue (RQ)  212  also requires completion notification. Therefore, when RNIC  4  ( FIG. 1A ) finishes placement of a received Send message  200 , it places a completion queue element (CQE)  242  to completion queue (CQ)  240  associated with that receive queue (RQ)  212 .  
      In view of the foregoing, there is a need in the art for a way to handle aligned DDP segment placement and delivery differently than non-aligned DDP segment placement and delivery.  
     SUMMARY OF THE INVENTION  
      The invention includes an RNIC implementation that performs direct data placement to memory where all received DDP segments of a particular connection are aligned, or moves data through reassembly buffers where some DDP segments of a particular connection are non-aligned. The type of connection that cuts-through without accessing the reassembly buffers is referred to as a “Fast” connection, while the other type is referred to as a “Slow” connection. When a consumer establishes a connection, it specifies a connection type. For example, a connection that would go through the Internet to another continent has a low probability to arrive at a destination with aligned segments, and therefore should be specified by a consumer as a “Slow” connection type. On the other hand, a connection that connects two servers in a storage area network (SAN) has a very high probability to have all DDP segments aligned, and therefore would be specified by the consumer as a “Fast” connection type. The connection type can change from Fast to Slow and back. The invention reduces memory bandwidth, latency, error recovery using TCP retransmit and provides for a “graceful recovery” from an empty receive queue, i.e., a case when the receive queue does not have a posted work queue element (WQE) for an inbound untagged DDP segment. A conventional implementation would end with connection termination. In contrast, a Fast connection according to the invention would drop such a segment, and use a TCP retransmit process to recover from this situation and avoid connection termination. The implementation also may conduct cyclical redundancy checking (CRC) validation for a majority of inbound DDP segments in the Fast connection before sending a TCP acknowledgement (Ack) confirming segment reception. This allows efficient recovery using TCP reliable services from data corruption detected by a CRC check.  
      A first aspect of the invention is directed to a method of handling a data transfer in a network interface controller (NIC), the method comprising the steps of: a) receiving the data transfer wherein the data transfer is denoted as one of a first type and a second type; b) calculating a cyclical redundancy check (CRC) for the data transfer, wherein the CRC is one of valid and invalid; and c) conducting one of: 1) dropping the data transfer and not confirming reception; 2) placing the data transfer to a reassembly buffer of the NIC; and 3) placing the data transfer to an internal buffer of the NIC for direct data placement to a destination buffer.  
      A second aspect of the invention is directed to a network interface controller (NIC) for handling a data transfer, the NIC comprising: first storage means for storing the data transfer for reassembly; second storage means for storing the data transfer for direct data placement to a destination buffer; means for receiving the data transfer wherein the data transfer is denoted as one of a first type and a second type; means for calculating a cyclical redundancy check (CRC) for the data transfer, wherein the CRC is one of valid and invalid; and means for conducting one of: 1) dropping the data transfer and not confirming reception; 2) placing the data transfer to a reassembly buffer of the NIC; and 3) placing the data transfer to an internal buffer of the NIC for direct data placement to a destination buffer.  
      A third aspect of the invention is directed to a computer program product comprising a computer useable medium having computer readable program code embodied therein for handling a data transfer in a network interface controller (NIC), the program product comprising the steps of: program code configured to receive the data transfer wherein the data transfer is denoted as one of a first type and a second type; program code configured to calculate a cyclical redundancy check (CRC) for the data transfer, wherein the CRC is one of valid and invalid; and program code configured to conduct one of: 1) dropping the data transfer and not confirming reception; 2) placing the data transfer to a reassembly buffer of the NIC; and 3) placing the data transfer to an internal buffer of the NIC for direct data placement to a destination buffer.  
      The foregoing and other features of the invention will be apparent from the following more particular description of embodiments of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The embodiments of this invention will be described in detail, with reference to the following figures, wherein like designations denote like elements, and wherein:  
       FIG. 1A  shows a block diagram of a conventional data transfer environment and RNIC.  
       FIG. 1B  shows a block diagram of conventional MPA/RDMA/DDP over TCP/IP data transfer structure.  
       FIG. 1C  shows a block diagram of possible MPA marker references for one or more DDP segments.  
       FIG. 1D  shows a block diagram of a conventional tagged DDP header.  
       FIG. 1E  shows a block diagram of a conventional untagged DDP header.  
      FIGS.  1 F-LH show block diagrams of various conventional RDMA message data transfers.  
       FIG. 2A  shows a block diagram of a data transfer environment and RNIC according to the invention.  
       FIG. 2B  shows a block diagram of a connection context of the RNIC of  FIG. 2A .  
       FIG. 2C  shows a block diagram of a validation unit of the RNIC of  FIG. 2A .  
       FIG. 3  shows a flow diagram of RNIC input logic (i.e., InLogic) functions.  
       FIGS. 4A-4B  show flow diagrams for a limited retransmission attempt mode embodiment for the InLogic of  FIG. 3 .  
       FIG. 5  shows a block diagram illustrating handling of TCP segments after connection downgrading according to an alternative embodiment.  
       FIG. 6  shows a flow diagram for a connection upgrade embodiment for the InLogic of  FIG. 3 .  
       FIG. 7  shows an MPA request/reply frame for use with an initial sequence number negotiation implementation for cyclical redundancy checking (CRC) calculation and validation.  
       FIG. 8  shows a flow diagram for an alternative modified MPA length implementation for CRC calculation and validation.  
       FIG. 9  shows a flow diagram for a first alternative embodiment of InLogic using a no-markers cut-through implementation for CRC calculation and validation.  
       FIG. 10  shows a flow diagram for a second alternative embodiment of InLogic using the no-markers cut-through implementation for CRC calculation and validation.  
       FIG. 11  shows a block diagram of RDMA Read and Read Response message data transfers including a Read Queue according to the invention.  
       FIG. 12  shows a block diagram of work queue elements (WQEs) and TCP holes for messages processed by RNIC output logic (i.e., OutLogic).  
       FIG. 13  shows a block diagram of RDMA Send message data transfers including a completion queue element (CQE) according to the invention.  
       FIG. 14  shows a block diagram of the CQE of  FIG. 13 .  
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      The following outline is provided for organizational purposes only: I. Overview, II. InLogic, III. OutLogic, and IV. Conclusion.  
      I. Overview  
      A. Environment  
      With reference to the accompanying drawings,  FIG. 2A  is a block diagram of data transfer environment  10  according to one embodiment of the invention. Data transfer environment  10  includes a data source  12  (i.e., a peer) that transmits a data transfer  14 A via one or more remote memory data access (RDMA) enabled network interface controller(s) (RNIC)  16  to a data sink  18  (i.e., a peer) that receives data transfer  14 B. For purposes of description, an entity that initiates a data transfer will be referred to herein as a “requester” and one that responds to the data transfer will be referred to herein as a “responder.” Similarly, an entity that transmits data shall be referred to herein as a “transmitter,” and one that receives a data transfer will be referred to herein as a “receiver.” It should be recognized that each one of data source  12  and sink  18  may, at different times, be a transmitter or a receiver of data or a requester or a responder, and that the labels “source” and “sink” are provided only for purposes of initially denoting that entity which holds the data to be transferred. The following description may also refer to one of the above entities as a “consumer” (for its consuming of RNIC  16  resources), where a more specific label is not necessary. “Destination buffers” shall refer to the data storage that ultimately receives the data at a receiver, i.e., data buffers  50  of data source  12  or data sink  18 . Data source  12  and data sink  18  each include data buffers  50  for storage of data.  
      In terms of hardware, RNIC  16  is any network interface controller such as a network I/O adapter or embedded controller with iWARP and verbs functionality. RNIC  16  also includes a verb interface  20 , an access control  30 , RNIC input logic (hereinafter “InLogic”)  32 , reassembly buffers  34 , an internal data buffer  38 , RNIC output logic (hereinafter “OutLogic”)  40 , a connection context  42 , a validation unit  44  and other components  46 . Verb interface  20  is the presentation of RNIC  16  to a consumer as implemented through the combination of RNIC  16  hardware and an RNIC driver (not shown) to perform operations. Verb interface  20  includes a verb library  22  having two parts: a user space library  24  and a kernel module  26 . Access control  30  may include any now known or later developed logic for controlling access to InLogic  32 . Reassembly buffers  34  may include any mechanism for temporary storage of data relative to a data transfer  14 A,  14 B. In particular, reassembly buffers  34  are commonly used for temporary storage of out-of-order TCP streams, as will be described in greater detail below. Other components  46  may include any other logic, hardware, software, etc., necessary for operation of RNIC  16 , but not otherwise described herein.  
      Referring to  FIG. 2B , connection context  42  includes a number of fields for storing connection-specific data. Other context data  60  provides connection-specific data not otherwise explained herein but recognizable to one having ordinary skill in the art. In accordance with the invention, two connection types are defined: a Fast (hereinafter “FAST”) connection and a Slow (hereinafter “SLOW”) connection. The terms “Fast” and “Slow” refer to the connection&#39;s likelihood of delivering aligned DDP segments. The connection type is identified in a connection context field called ConnectionType  62 . The SLOW connection may be used for RDMA connections which either were created as SLOW connections, or were downgraded by RNIC  16  during processing of inbound data, as will be described in greater detail below. Other fields shown in  FIG. 2B  will be described relative to their associated processing elsewhere in this disclosure. Referring to  FIG. 2C , validation unit  44  includes cyclic redundancy checking (CRC) logic  64 , TCP checksum logic  66  and store-and-forward buffers  68  as may be necessary for validation processing.  
      B. RNIC General Operation  
      Returning to  FIG. 2A , in operation, RNIC  16  receives data transfer  14 A via an access control  30  that controls access to InLogic  32 . Information for sustaining the connection is retained in other context data  60  ( FIG. 2B ) of connection context  42 , as is conventional. InLogic  32  processes inbound TCP segments in data transfer  14 A, performs validation of received TCP segments via TCP checksum logic  66  ( FIG. 2C ), calculates MPA CRC via CRC logic  64  ( FIG. 2C ), and separates FAST connection data streams from SLOW connection data streams. With regard to the latter function, InLogic  32 , as will be described more fully below, directs all data received by RNIC  16  on a SLOW connection to reassembly buffers  34 , and handles a FAST connection in a number of different ways. With regard to the FAST connections, if InLogic  32  detects an alignment violation (i.e., a TCP header is not immediately followed by a DDP Header, and the DDP segment is not fully contained in the one TCP segment), the connection is downgraded to a SLOW connection and data is directed to reassembly buffers  34 . In contrast, if an alignment violation is not present, InLogic  32  directs the aligned inbound DDP stream to an internal data buffer  38  and then to OutLogic  40  for direct placement to a destination data buffer  50 . Alternatively, a TCP segment  106  may be dropped, and no acknowledgement (Ack) sent, thus necessitating a re-transmission of the segment.  
      OutLogic  40  arbitrates between FAST and SLOW connections, and performs data placement of both connection type streams to data sink  18  data buffers  50 . The situation in which aligned DDP segments on a FAST connection are directed to internal data buffer  38  for direct placement to a destination buffer is referred to as the “cut-through mode” since FAST connections having aligned DDP segments are placed directly by OutLogic  40 , bypassing reassembly buffer  34 . For both connection types, however, only an in-order received data stream is delivered to data sink  18  via OutLogic  40 .  
      II. InLogic  
      With reference to  FIG. 3 , a flow diagram of InLogic  32  ( FIG. 2A ) according to the invention and its processing of a data transfer  14 A will be described in further detail. As noted above, InLogic  32  processes inbound TCP segments, performs TCP validation of received segments, calculates MPA CRC, and separates FAST connection data streams from SLOW connection data streams. Unless otherwise noted, reference numerals not followed by an “S” refer to structure shown in  FIGS. 2A-2C .  
      In a first step S 1 , InLogic  32  filters TCP segments  106  of a data transfer  14 A belonging to RNIC  16  connections, and obtains packets with calculated CRC validation (via validation unit  44 ) results for the received segments. (Note that CRC validation should be done before InLogic  32  decision processing. CRC validation can also be done simultaneously with TCP checksum calculation, before TCP segment  106  is identified as one belonging to a FAST connection—step S 2 .)  
      In step S 2 , InLogic  32  determines whether TCP segment  106  belongs to a SLOW connection. In this case, InLogic  32  determines how the transmitter labeled the connection. If YES, TCP segment  106  is directed to reassembly buffers  34 , and TCP logic considers this segment as successfully received, at step S 3 .  
      If NO, InLogic  32  proceeds, at step S 4 , to determine whether TCP segment  106  length is greater than a stated MPA segment length. That is, whether TCP segment  106  length, which is stated in TCP header  126 , is longer than an MPA length stated in MPA length field  114 . If YES, this indicates that TCP segment  106  includes multiple DDP segments  112 , the processing of which will be described below. If NO, this indicates that TCP segment  106  includes a single DDP segment  112  or  112 NA.  
      In this latter case, at step S 5 , InLogic  32  determines whether the MPA length is greater than TCP segment  106  length. If YES, this indicates one of three situations: 1) the single DDP segment  112 NA is not aligned to TCP segment  106 , and the field that was assumed to be an MPA length field is not a length field; 2) the beginning of the single DDP segment  112  is aligned to TCP segment  106 , but the length of the single DDP segment exceeds TCP segment  106  payload size; or 3) the received single DDP segment  112  is aligned to TCP segment  106 , but has a corrupted MPA length field  114 . The first two cases (1 and 2) indicate that the non-aligned single DDP segment  112 NA has been received on a FAST connection, and thus the connection should be downgraded to a SLOW connection, at step S 3 . The third case (3) does not require connection downgrade. However, since the reason for MPA frame  109  length exceeding TCP segment  106  length cannot be identified and confirmed, the drop (i.e., cancellation and non-transfer) of such TCP segment  106  is not advisable because it can lead to a deadlock (case 2, above). That is, if such TCP segment indeed carried a non-aligned DDP segment, the transmitter will retransmit the same non-aligned DDP segment, which following the same flow, would be repeatedly dropped by the receiver leading to a deadlock. Accordingly, InLogic  32 , at step S 3 , directs data transfer of TCP segment  106  to reassembly buffers  34 , schedules an Ack to confirm that TCP segment  106  was successfully received, and downgrades the connection to a SLOW connection (i.e., ConnectionType field  62  in  FIG. 2B  is switched from Fast to Slow). As will be described below, if MPA length field  114  is corrupted (case 3 above), this is detected by OutLogic  40 , and the connection would be closed due to a CRC error as detected by validation unit  44 . Therefore, the connection downgrade, at step S 3 , would not cause the FAST connection to permanently become a SLOW connection due to data corruption in an aligned DDP segment  112 .  
      Returning to step S 5 , if MPA length is not greater than TCP length, i.e., NO, this indicates that MPA frame  109  length matches (equals) TCP segment  106  length. InLogic  32  proceeds, at step S 6 , to determine whether the CRC validation results are valid for this TCP segment  106 . That is, whether CRC logic  64  returned a “valid” indication. If YES, this indicates that single DDP segment  112  exactly fits TCP segment  106  boundaries (i.e., lengths are equal to one another), and no data corruption has been detected for this segment. As a result, at step S 7 , single DDP segment  112  is processed in a “fast path mode” by placing the received TCP segment  106  to internal data buffer  38  of RNIC  16  for processing by OutLogic  40 , which places the received TCP segment  106  directly to the destination data buffers  50  of a receiver, e.g., of data sink  18 . In addition, an Ack is scheduled to confirm successful reception of this TCP segment  106 .  
      If CRC logic  64  returns an “invalid” indication, i.e, NO at step S 6 , this indicates one of five possible cases exist that can be determined according to the invention.  FIG. 1C  illustrates the five possible cases and steps S 8 -S 10  illustrate how InLogic  32  handles each case. In any case, the object of processing is to: 1) avoid termination of non-aligned connections, even if those were declared by a transmitter to be a FAST connection; 2) reduce probability of connection termination due to data corruption in aligned DDP segments belonging to a FAST connection; and 3) maintain Inlogic  32  as simple as possible while reducing the number of cases to be treated separately to a minimum.  
      At step S 8 , InLogic  32  determines, as shown as Case A in  FIG. 1C , whether a DDP segment header  160  of a newly received DDP segment  162  is referenced by an MPA length field  164  of a previously processed DDP segment  166 . In this case, the MPA length of previously processed DDP segment  166  was checked during validation of MPA CRC of newly received DDP segment  162 , and thus refers to the correct location of DDP header  160  in the next segment. CRC invalidation for Case A, at step S 6 , means that the single DDP segment  162  data or header  160  has been corrupted. TCP retransmit of newly received segment  162  resolves this problem. Accordingly, at step S 9 , TCP segment  106  is dropped, and segment reception is considered not confirmed.  
      If newly received DDP segment  162  header  160  is not referenced by MPA length field  164  of previously processed DDP segment  166  (i.e., NO at step S 8 ), InLogic  32  proceeds, at step S 10 , to determine, as shown as Case B in  FIG. 1C , whether newly received DDP segment  162  header  160  is referenced by a marker  168  located inside newly received DDP segment  162 . That is, marker  168  is referring to the beginning of newly received DDP segment  162 . In this case, CRC invalidation, at step S 6 , indicates that either: 1) marker  168  carries a correct value, and newly received DDP segment  162  has a corrupted DDP header  160  or data, or 2) marker  168  inside newly received DDP segment  162  has been corrupted. In both cases retransmit of newly received DDP segment  162  resolves the problem. Accordingly, at step S 9 , the TCP segment is dropped, and segment reception is not confirmed.  
      If newly received DDP segment  162  header  160  is not referenced by a marker  168  located inside newly received DDP segment  162 , i.e., NO at step S 10 , then one of three cases exist. First, as shown as Case C in  FIG. 1C , marker  168  is located in newly received DDP segment  162 , but points outside of the segment. Second, as shown as Case D in  FIG. 1C , marker  168  is located in newly received DDP segment  162 , but points inside the segment. Third, as shown as Case E in  FIG. 1C , no marker is located in newly received DDP segment  162 .  
      In Cases C, D and E, the reason for CRC logic  64  returning an invalid indication is uncertain and can be the result of data corruption and/or reception of a non-aligned DDP segment  112 NA ( FIG. 1B ). Unlimited retransmit of such a segment can lead to deadlock in the case of a non-aligned DDP segment  112 NA. To avoid potential deadlock, InLogic  32  handles Cases C, D and E by, as shown at step S 3 , directing newly received DDP segment  162  to reassembly buffers  34 , scheduling an Ack to confirm successful reception of the segment, and downgrading the connection to a SLOW connection. If CRC logic  64  returning an invalid indication was due to data corruption in an aligned DDP segment  112 , this error would be detected by OutLogic  40 , as will be described below, when processing the data of the SLOW connection and the connection would be terminated. Otherwise, the connection will remain a SLOW connection forever. However, a Limited Retransmission Attempt Mode, as will be described below, may prevent this problem.  
      Returning to step S 4  of  FIG. 3 , if InLogic  32  determines that TCP segment  106  length is greater than MPA frame  109  length this indicates that TCP segment  106  includes multiple DDP segments  112 . In this case, at step S 11 , a sequential checking of CRC logic  64  validation results is conducted from a first to a last DDP segment  112 . If all DDP segments  112  have a valid CRC, i.e., YES, all DDP segments  112  are fully contained in TCP segment  106 , and all are valid, properly aligned DDP segments  112 . In this case, InLogic  32  processes DDP segments  112 , at step S 7 , on the fast path mode by placing the received TCP segment  106  to internal data buffer  38  of RNIC  16  for processing by OutLogic  40 , which places the received TCP segment  106  to the destination data buffers, e.g., data buffers  50  of data sink  18 . In addition, an Ack is scheduled to confirm successful reception of this TCP segment  106 . InLogic  32  stops checking CRC validation results when a first failure has been detected, the management of which is explained relative to steps S 12 -S 13 .  
      In step S 12 , InLogic  32  determines whether a first DDP segment  112  has an invalid CRC as determined by CRC logic  64 . If YES, InLogic  32  processes the first DDP segment  112  similarly to an invalid CRC case for a single DDP segment (step S 8 ). That is, InLogic  32  treats the first DDP segment  112  with an invalid CRC as a single DDP segment  112  and proceeds to determine what caused the CRC invalidation, i.e., which of Cases A-E of  FIG. 1C  applies, and how to appropriately handle the case.  
      If step S 12  results in NO, i.e., the first DDP segment  112  has a valid CRC, then InLogic  32  proceeds to determine whether CRC invalidity has been detected when checking an intermediate or last DDP segment  112  at step S 13 . If YES, InLogic  32  ( FIG. 1 ) proceeds to step S 9 , since this error indicates that the data or header of DDP segment  112  that caused the CRC invalidation has been corrupted (i.e., length of previous DDP segment with valid CRC). That is, the CRC error was detected on the intermediate or last DDP segment  112  in the same TCP segment  106 , which means the preceding DDP segment has a valid CRC, and thus the length of the preceding DDP segment points to the header of the segment with the invalid CRC. This matches the description of Case A ( FIG. 1C ). Therefore, as described in Case A, the location of the header is known, and therefore, the CRC error is known to have been caused either by data or header corruption. Accordingly, a retransmit of the entire TCP segment should resolve this problem, without any risk of the deadlock scenario. At step S 9 , the TCP segment is dropped, and segment reception is not confirmed.  
      If step S 13  results in NO, i.e., an intermediate or last DDP segment  112  has not caused the CRC invalidation, then this indicates that MPA length field  114  of the last DDP segment  112  exceeds TCP segment  106  boundaries, i.e., the last DDP segment is outside of TCP segment  106  boundaries or is too long. In this case, InLogic  32  treats the situation identical to the single DDP segment  112  that is too long. In particular, InMogic  32  proceeds to, at step S 3 , direct data transfer  14 A of TCP segment  106  to reassembly buffers  34 , schedules an Ack to confirm that TCP segment  106  was successfully received, and downgrades the connection to a SLOW connection. In this way, deadlock is avoided. If RNIC  16  decides to drop one of the multiple DDP segments  112  contained in a TCP segment  106 , the entire TCP segment  106  is dropped, which simplifies implementation and reduces the number of cases that need to be handled.  
      Although not discussed explicitly above, it should be recognized that other data transfer processing may also be carried in conjunction with the above described operation of InLogic  32 . For example, filtering of TCP segments belonging to RNIC  16  connections and TCP/IP validations of received segments may also be performed including checksum validation via TCP checksum logic  66  ( FIG. 2C ). Processing of inbound TCP segment  106  may also include calculation of MPA CRC, and validation of this CRC via CRC logic  64  ( FIG. 2C ). One particular embodiment for CRC calculation and validation will be further described below.  
      A. Limited Retransmission Attempt Mode  
      As an alternative embodiment relative to the uncertainty of the cause of a detected error (e.g., NO at step S 10  of  FIG. 3  being one illustrative determination that may result in such a situation), a “limited retransmission attempt mode” may be implemented to limit the number of retransmit attempts to avoid deadlock and reduce the number of FAST connections that are needlessly reduced to SLOW connections. In particular, as noted above, Cases C, D and E represent several cases in which, due to uncertainty of the cause of a detected error, the connection may be downgraded to a SLOW connection (step S 3 ) with potential connection termination (by OutLogic  40 ) when the error was caused by data corruption and not loss of DDP segment  112  alignment.  
      In order to limit the number of retransmit attempts, the present invention provides additional fields to connection context  42  ( FIG. 2B ) to allow for a certain number of retransmissions before downgrading the connection. In particular, as shown in  FIG. 2B , connection context  42  includes a set of fields  290  including: a number of recovery attempts field (RecoveryAttemptsNum)  292 , a last recovery sequence number field (LastRecoverySN)  294  and a maximum recovery attempts number field (MaxRecoveryAttemptsNum)  296 . RecoveryAttemptsNum field  292  maintains the number of recovery attempts that were done for the connection since the last update; LastRecoverySN field  294  maintains a sequence number (SN) of the last initiated recovery operation; and MaxRecoveryAttemptsNum field  296  defines the maximum number of recovery attempts that should be performed by InLogic  32  before downgrading the connection.  
      Referring to  FIG. 4A , in operation, when InLogic  32  detects that a new in-order received data transfer includes an error (shown generically as step S 101  in  FIG. 4A ), rather than immediately downgrade the connection to a SLOW connection (at step S 3  in  FIG. 3 ), InLogic  32  provides for a certain number of retransmits to be conducted for that error-including data transfer. It should be recognized that step S 101  is generic for a number of error determinations (step S 101  may apply, e.g., for a YES at step S 5  of  FIG. 3  or a NO at step S 10  of  FIG. 3 ) that are caused either by a non-aligned DDP segment  112 NA or a data corruption. At step S  102 , InLogic proceeds to record this transmission attempt for this error-including data transfer, step S 102 , by increasing RecoveryAttemptsNum by one (1). In addition, InLogic updates LastRecoverySN to store the largest sequence number between the previously stored sequence number therein and that of the newly received (but dropped) data transfer. That is, InLogic updates LastRecoverySN to store the largest sequence number among at least one previously received error-including data transfer and the newly received error-including (but dropped) data transfer. The newly received error-including data transfer is determined to have a sequence number greater than the largest sequence number by comparing the sequence number of the newly received error-including data transfer to the stored largest sequence number. The significance of LastRecoverySN recordation will become apparent below.  
      Next, at step S 103 , InLogic  32  determines whether the RecoveryAttemptsNum (field  292 ) exceeds the MaxRecoveryAttemptsNum (field  296 ). If NO, at step S 104 , InLogic  32  drops TCP segment  106  and does not confirm successful receipt, which causes a retransmission of the TCP segment. Processing then returns to step SI ( FIG. 3 ). If TCP segment  106  was corrupted, then the retransmission should remedy the corruption such that data transfer  14 A is placed directly to memory as a FAST connection (at step S 7  of  FIG. 3 ). Alternatively, if processing continues to return other error detections (e.g., step S 10  of  FIG. 3 ), RecoveryAttemptsNum (field  292 ) will eventually exceed MaxRecoveryAttemptsNum (field  296 ) and result in a YES at step S 106 . In this case, InLogic  32  proceeds to step S 105  at which InLogic  32  downgrades the connection to a SLOW connection, places error-including data transfer  14 A to reassembly buffer  34  and schedules an Ack confirming successful reception of this TCP segment. The above process occurs for each error-including data transfer.  
       FIG. 4B  represents another component of the Limited Retransmission Attempt Mode that addresses the fact that data corruption usually does not occur in multiple consecutive TCP segments, but non-aligned segments may affect several subsequent TCP segments. For example, a FAST connection may be sustained for a long period of time, e.g., five hours, and from time-to-time, e.g., once an hour, may have data corruption such that CRC validation will fail. As this occurs, the RecoveryAttemptsNum (field  292 ) may be increased each time the error-including data transfer (i.e., corrupted segment) is dropped. This process addresses the situation where different segments are dropped due to data corruption at different periods of time, and after several (probably one) retransmit operation these segments are successfully received, and placed to the memory. Accordingly, the recovery operation for these segments was successfully completed, and the data corruption cases that are recovered from are not counted, i.e., when entering a new recovery mode due to reception of new errant segment.  
      In order to exit from the limited retransmission attempt mode, a determination as to whether a TCP segment Sequence Number (SN) of a newly received in-order data transfer (i.e., InOrderTCPSegmentSN) is greater than a LastRecovery Sequence Number (SN) (field  294  in  FIG. 2B ) is made at step S 105 . That is, a sequence number of each newly received in-order TCP segment belonging to a FAST connection is compared to a stored largest sequence number selected from the one or more previously received error-including data transfers. (Note that reception of an out-of-order segment with larger SN does not mean that error recovery was completed.) However, one indicator that recovery is complete is that a TCP segment is received that was transmitted after the segment(s) that caused entry to the recovery mode. This situation can be determined by comparing the InOrderTCPSegmentSN with LastRecoverySN. This determination can be made at practically any stage of processing of the TCP segment received for this connection. For example, after step S 9  in  FIG. 3 , or prior to step S 102  in  FIG. 4A . When the in-order segment SN is greater than the LastRecoverySN, i.e., a new TCP segment is received, and YES is determined at step S 105 , at step S 106 , RecoveryAttemptsNum (field  292  in  FIG. 2B ) is reset, i.e., set to zero. Relative to the above example, step S 105  prevents unnecessary downgrading of a FAST connection to a SLOW connection after the long period of time, e.g., five hours (i.e., because RecoveryAttemptsNum exceeds MaxRecoveryAttemptsNum), where the dropped segments were dropped due to data corruption and then, after the transmitter retransmitted the segment, were successfully received and processed as an aligned segment. If NO at step S 105  or after step S 106 , segment processing proceeds as usual, e.g., step S 1  of  FIG. 3 .  
      Using the above processing, the number of retransmits allowed can be user defined by setting MaxRecoveryAttemptsNum field  296 . It should be recognized that while the limited retransmission attempt mode has been described above relative to  FIGS. 4A-4B  and an error detection relative to step S 10  of  FIG. 3 , the limited retransmission attempt mode is applicable beyond just the error detection of step S 10 , as will be described further below. Note, that the limited retransmission attempt mode also finds advantageous use with part D, Speeding Up TCP Retransmit Process, described below, which sends an immediate Duplicate Ack when a segment was dropped due to ULP considerations.  
      B. Connection Downgrading  
      Referring to  FIG. 5 , discussion of handling of a unique situation in which a connection is downgraded (step S 3  in  FIG. 3 ) after one or more out-of-order received DDP segments  112  are placed to destination data buffers  50  in the fast path mode will now be described. As shown in  FIG. 5 , four TCP segments labeled packet (Pkt) are received out-of-order, i.e., in the order  3 ,  4 ,  1  and  2 . When a connection is downgraded to a SLOW connection, all data received from the moment of downgrading is placed to reassembly buffers  34  and is reassembled to be in-order, i.e., as Pkts  1 ,  2 ,  3  and  4 . In this case, according to the TCP protocol, InLogic  32  maintains records that those segments were received.  
      Although rare, a situation may arise where a segment(s), e.g., Pkt # 3  (shaded), is/are directly placed to destination data buffers  50 . This situation leads to the location in reassembly buffers  34  that would normally hold packet  3  (Pkt#  3 ) being filled with ‘garbage’ data, i.e., gaps or holes, even though InLogic  32  assumes that all data was received. If processing is allowed to continue uncorrected, when OutLogic  40  transfers reassembly buffers  34  to destination data buffers  50 , packet  3  (Pkt # 3 ) that was earlier transferred on the fast path mode will be overwritten with the ‘garbage’ data, which will corrupt the data.  
      To resolve this problem without adding hardware complexity, in an alternative embodiment, InLogic  32  directs TCP logic to forget about the segments that were out-of-order received when the connection was a FAST connection (i.e., Pkt#  3  in  FIG. 5 ). In particular, InLogic  32  is configured to clear a TCP hole for an out-of-order placed data transfer when downgrading the connection to a SLOW connection at step S 3  ( FIG. 3 ), and stops receipt reporting to the transmitter that these packets have been received (SACK option). As a result, a transmitter retransmits all not acknowledged data, including those segment(s) that were out-of-order directly placed to destination data buffers  50 , i.e., Pkt#  3 . When the retransmitted data is received, it is written to reassembly buffers  34 , and any out-of-order directly placed segments are overwritten at destination data buffers  50  when OutLogic  40  transfers the data from reassembly buffers  34 . This functionality effectively means that RNIC  16  ‘drops’ segments that were out-of-order placed to destination data buffers  50  in this connection. Such approach eliminates the case of ‘gapped’ in-order streams in reassembly buffers  34 , and does not cause visible performance degradation because of the rare conditions that would lead to such behavior.  
      C. Connection Upgrade  
      As another alternative embodiment, the present invention may include a connection upgrade procedure as illustrated in  FIG. 6 . The purpose of the fast path mode approach described above is to allow bypassing of reassembly buffers  34  for a connection carrying aligned DDP segments  112 . However, even in FAST connections, a data source  12  or intermediate network device can generate intermittent non-aligned DDP segments  112 NA, which causes FAST connections to be downgraded to SLOW connections according to the above-described techniques. The intermittent behavior can be caused, for example, by maximum segment size (MSS) changes during TCP retransmit, or other sporadic scenarios.  
      As shown in  FIG. 6 , to recover from this situation, the present invention may also provide a connection upgrade from a SLOW connection to a FAST connection after an earlier downgrade, e.g., at step S 3  ( FIG. 3 ). In order to accommodate the upgrade, a number of situations must be present. In a first step S 31  of the alternative embodiment, InLogic  32  determines whether reassembly buffers  34  are empty. If NO, then no upgrade occurs—step S 32 . If YES is determined at step S 31 , then at step S 33 , InLogic  32  determines whether aligned DDP segments  112  are being received. If NO, then no upgrade occurs—step S 32 . If YES is determined at step S 33 , then at step S 34 , InLogic  32  determines whether the connection was originated as a FAST connection by a transmitter, e.g., data source  12 . If NO is determined at step S 24 , then no upgrade occurs—step S 32 . If YES is determined at step S 34 , the connection is upgraded to a FAST connection at step S 35 .  
      D. Speeding Up TCP Retransmit Process  
      Another alternative embodiment addresses the situation in which a TCP segment  106  is received, but is dropped because of RDMA or ULP considerations, e.g., corruption, invalid CRC of DDP segments, etc. According to the above-described procedures, there are a number of times where a TCP segment  106  is received and has passed TCP checksum, but is dropped by InLogic  32  without sending a TCP Ack covering the segment (i.e., step S 9  of  FIG. 3 ). Conventional procedures would then cause a retransmission attempt of those packets. In particular, in the basic scheme (the so-called “Reno protocol”), a TCP transmitter starts the ‘Fast Retransmit’ mode when it gets three duplicated Acks (i.e., Acks that do not advance the sequence number of in-order received data). For example, assume two TCP segments A and B, and that segment B follows segment A in TCP order. If segment A is dropped, then the receiver would send a duplicate Ack only when it receives segment B. This duplicate Ack would indicate “I&#39;m waiting for segment A, but received another segment,” i.e., segment B. In the ‘Fast Retransmit’ mode under the Reno protocol, the transmitter sends one segment, then it waits for another three duplicate Acks to retransmit another packet. More advanced schemes (like the so-called “New-Reno protocol”) allow retransmitting of a segment for each received duplicate in its ‘Fast Recovery’ mode. The logic behind this process being that if one segment left the network, then the transmitter may put another packet to the network.  
      In order to facilitate re-transmission, according to an alternative embodiment of the invention, InLogic  32  generates a first duplicate TCP acknowledgement (Ack) covering a received TCP segment that is determined to be valid by TCP and was dropped by TCP based on an upper layer protocol (ULP) decision (e.g., at step S 9  of  FIG. 3 ); and transmits the duplicate TCP Ack. The ULP, as noted above, may include one or more of: an MPA protocol, a DDP protocol, and a RDMA protocol. The first duplicate TCP Ack is generated for a TCP segment regardless of whether the TCP segment is in-order or out-of-order, and even where a next in-order TCP segment has not been received. Inlogic  32  may also generate a second duplicate TCP acknowledgement (Ack) covering a next out-of-order received TCP segment, and transmit the second duplicate TCP Ack.  
      This above processing effectively means generation of a duplicate Ack (e.g., for segment A in example above) even though the next in-order segment (e.g., segment B in example above) may not have been received yet, and thus should speed up a process of re-entering the transmitter to the fast path mode under the above-described retransmission rules. More specifically, even if segment B has not been received, the transmitter would know that segment A, a valid TCP segment, was received and dropped due to ULP considerations. As a result, the additional duplicate Ack forces the transmitter to begin the retransmit procedure earlier where a number of duplicate Acks must be received before retransmission begins. This approach does not violate TCP principles, since TCP segment  106  has been successfully delivered to the ULP, and dropped due to ULP considerations (invalid CRC). Therefore the packet was not dropped or reordered by the IP protocol. This approach is particularly valuable when RNIC  16  implements the limited retransmission attempt mode as outlined relative to  FIG. 4A , i.e., an Ack is sent at step S 103 .  
      E. CRC Calculation and Validation  
      Conventional processing of incoming Ethernet frames starts with a filtering process. The purpose of filtering is to separate valid Ethernet frames from invalid ones. “Invalid frames” are not corrupted frames, but frames that should not be received by RNIC  16 , e.g., MAC filtering—frame selection based on MAC addresses, virtual local area network (VLAN) filtering—frame selection based on VLAD Tags, etc. The valid frames, that were allowed to get into RNIC  16 , are also separated into different types. One of these types is a TCP segment. The filtering process is done on the fly, without any need to perform store-and-forward processing of the entire Ethernet frame.  
      The next step of TCP segment processing is TCP checksum calculation and validation. Checksum calculation determines whether data was transmitted without error by calculating a value at transmission, normally using the binary values in a block of data, using some algorithm and storing the results with the data for comparison with the value calculated in the same manner upon receipt. Checksum calculation and validation requires store-and-forward processing of an entire TCP segment because it covers an entire TCP segment payload. Conventionally, calculation and validation of cyclical redundancy checking (CRC) normally follows TCP checksum validation, i.e., after a connection is recognized as an RDMA connection and after the boundaries of a DDP segment have been detected either using a length of a previous DDP segment or MPA markers. CRC calculation and validation determines whether data has been transmitted accurately by dividing the messages into predetermined lengths which, used as dividends, are divided by a fixed divisor. The remainder of the calculation is appended to the message for comparison with an identical calculation conducted by the receiver. CRC calculation and validation also requires store-and-forward of an entire DDP segment, which increases latency and requires large data buffers for storage. One requirement of CRC calculation is to know DDP segment boundaries, which are determined either using the length of the preceding DDP segment or using MPA markers  110  ( FIG. 1B ). The marker-based determination is very complicated due to the many exceptions and comer cases. CRC calculation of a partially received DDP segment is also a complicated process.  
      In order to address the above problems, as shown in  FIG. 2C , the present invention performs CRC calculation and validation via CRC logic  64  in parallel with TCP checksum calculation and validation via TCP checksum logic  66  using the same store-and-forward buffer  68 . In addition, the present invention does not immediately locate DDP segment boundaries, and then calculate and validate DDP segment CRC. Rather, the present invention switches the order of operations by calculating CRC and later determining DDP boundaries. In order to make this switch, CRC logic  64  assumes that each TCP segment (before it is known that the segment belongs to an RDMA connection) starts with an aligned DDP segment. In addition, the present invention assumes that the first two bytes of a TCP payload  127  ( FIG. 1B ) is an MPA length field  114  ( FIG. 1B ) of an MPA frame. This length is then used to identify the DDP segment boundaries and calculate CRC for that segment. After validation unit  44  identifies a boundary of the first possible DDP segment  112  in TCP segment  106 , it calculates and validates CRC for that DDP segment simultaneously with the checksum calculation for that portion of TCP segment payload  127 , and then proceeds to the next potential DDP segment  112  (if any) contained in the same TCP segment  106 . For each “potential” DDP segment discovered in TCP segment  106 , CRC validation results may be valid, invalid or too long. Results of CRC validation are stored for use as described above relative to  FIG. 3 .  
      In order to actually calculate CRC as described above, when the payload of a TCP segment  106  is processed, InLogic  32  needs to know where MPA markers  110  are in a TCP segment  106 . As discussed above relative to  FIG. 1B , MPA markers  110  are placed every 512 bytes apart in a TCP segment  106 , and the first MPA marker is 512 bytes from an Initial Sequence Number in TCP header  126  ( FIG. 1B ), which is stored as StartNum field  248  ( FIG. 2B ) of connection context  42 . Unfortunately, an evaluation of each MPA marker  110  does not reveal its position relative to StartNum  248  ( FIG. 2B ). In addition, MPA markers  110  are covered by CRC data  116 , but are not included in an MPA length field  114 , which includes only the payload of an MPA frame. Accordingly, to identify MPA markers  110 , RNIC  16  needs to know StartNum  248  ( FIG. 2B ), which must be fetched from connection context  42 . Unfortunately, reading connection context  42  is very inconvenient to conduct during TCP processing as it occurs very early in processing and breaks up or holds up packet processing.  
      In order to reduce or eliminate connection context  42  fetching, the present invention presents four alternatives allowing correct calculation of DDP segment  112  length, which is required to calculate and validate MPA CRC of that segment. These options are discussed in the following sections.  
      1. Connection Context Prefetch Method  
      A first alternative embodiment for correctly calculating DDP segment  112  length includes implementing a connection context  42  prefetch of an Initial Sequence Number stored as StartNum field  248  ( FIG. 2B ). No change to the MPA specification is proposed here. The current MPA specification requires knowledge of an Initial Sequence Number (StartNum) to identify the location of an MPA marker  110  in a TCP segment  106 . The Initial Sequence Number is a TCP connection attribute, which varies from connection to connection and is negotiated at connection establishment time. Therefore, a StartNum  248  ( FIG. 2B ) is maintained on a per connection basis. To identify the location of MPA marker  110 , CRC logic  64  ( FIG. 2C ) checks that the remainder of a particular segment&#39;s sequence number (SeqNum) and StartNum (SeqNum−StartNum) mod 512 is zero. That is, because each TCP segment  106  header carries the sequence number of the first byte of its payload, CRC logic  64  can determine where to look for a marker by taking a difference between the particular segment&#39;s sequence number and StartNum 248 , and then starting from this position, locate a marker every 512 bytes. The MPA specification defines the above-described marker detection method. In this way, a Hash lookup (based on TCP tuple) and a connection context  42  prefetch can be performed before the TCP checksum validation is performed. This is a normal connection context  42  fetch flow. If RNIC  16  wants to get connection context  42 , it first needs to understand where this context is located, or get the Connection ID. TCP segment  106  header carries TCP tuple (IP addresses (source and destination) and TCP ports (source and destination)). Tuple is an input to Hash function. The output of Hash function is a Connection ID. Of course, the same Connection ID for different tuples may result, which is called “collision.” To handle collisions, RNIC  16  reads connection context  42 , checks the tuple in connection context  42  with the tuple in the packet, and if it does not match, then RNIC  16  gets the pointer to the next connection context  42 . RNIC  16  keeps checking tuples until it either finds the match, or the segment is recognized as one that does not belong to any known connection. This process allows locating MPA markers  110  in TCP stream. As a result, CRC calculation and validation can be performed simultaneously with TCP checksum validation.  
      2. Initial Sequence Number Negotiation Method  
      In a second alternative embodiment, correctly calculating DDP segment length is possible without connection context fetching by making a number of changes to the MPA specification. First, the definition of MPA marker  110  placement in the MPA specification is changed. One disadvantage of the above-described Connection Context Prefetch Method is the need to perform a Hash lookup and connection context  42  prefetch to identify boundaries of the MPA frame  109  in a TCP segment  106 . In order to prevent this, the present invention places MPA markers  110  every 512 bytes rather than every 512 bytes starting with the Initial Sequence Number (SN)(saved as StartNum  248 ) (which necessitates the above-described SN-StartNum mod  512  processing). In this fashion, MPA markers  110  location may be determined by a sequence number mod  512  process to locate MPA markers  110 , and no connection context  42  fetch is required.  
      A second change to the MPA specification according to this embodiment acts to avoid the situation where one marker is split between two DDP segments  112 , i.e., where an Initial Sequence Number is not word-aligned. As a result, a sequence number mod  512  process may not work in all circumstances because the standard TCP implementation allows the Initial SN to have a randomly generated byte-aligned value. That is, whether an Initial Sequence Number is word-aligned is not controllable by RNIC  16 . As a result, a TCP stream for the given connection may not necessarily start with an MPA marker  110 . Accordingly, if CRC logic  64  picks the location of a marker  110  just by using the sequence number mod  512  process, it could get markers placed to the byte aligned location, which is unacceptable. To avoid this situation, the present invention adds padding to MPA frames exchanged during an MPA negotiation stage, i.e., the so called “MPA request/reply frame,” to make the Initial SN of an RDMA connection when it moves to RDMA mode, word-aligned. That is, as shown in  FIG. 7 , a correction factor  150  is inserted into an MPA request/reply frame  152  of a TCP segment  106  that includes the number of bytes needed to make the Initial SN word-aligned. It should be recognized that the exact location of correction factor  150  does not have to be as shown. In this way, CRC logic  64  may implement the sequence number mod  512  process to obtain the exact location of the MPA markers  110  in TCP stream without a connection context fetch. Using the above-described modifications of the MPA specification, the invention can locate MPA markers  110  and properly calculate the length of MPA segment without prefetching connection context  42 .  
      3. MPA Length Field Modification Method  
      In a third alternative embodiment for correctly calculating DDP segment  112  length without connection context fetching, a definition of MPA length field  114  is changed in the MPA specification. Conventionally, MPA length field  114  is defined to carry the length of the ULP payload of a respective MPA frame  109 , excluding markers  110 , padding  121  ( FIG. 1B ) and CRC data  116  added by the MPA layer. Unfortunately, this information does not allow locating of MPA frame boundaries using information provided by TCP segment  106 . In order to address this, according to this alternative embodiment, the definition of MPA length in the MPA specification is changed to specify a length of the entire MPA frame  109  including: 14 most-significant bits (MSBs) of MPA length field  114 , ULP payload  118  length, MPA markers  110 , CRC data  116 , 2 least-significant bits (LSBs) of MPA length field  114 , and valid bits in padding  121 .  
      This revised definition allows detection of MPA frame  109  boundaries using MPA length field  114  without locating all MPA Markers  110  embedded in that MPA frame. MPA layer protocol is responsible for stripping markers  110 , CRC data  116  and padding  121  and provide the ULP (DDP Layer) with ULP payload length.  
      Referring to  FIG. 8 , using this definition of MPA length, CRC logic  64  locates the boundaries of MPA frame  109  by the following process: In step S 100 , CRC logic  64  determines whether the first word of an MPA frame  109  equals zero. If YES, then InLogic  32  ( FIG. 2A ) reads MPA length field  114  from the next word at step S 102 . This is the case when a marker  110  falls between two MPA frames  109 . In this situation, MPA length field  114  is located in the next word as indicated at step S 104 . If NO is the determination at step S 100 , then this word holds MPA length field  114 . In step S 106 , the MPA length is used to find the location of the CRC data  116  covering this MPA frame  109 . The above process then repeats to locate other MPA frames  109  embedded in TCP segment  106 . This embodiment allows locating of MPA frame  109  boundaries without any additional information from connection context  42 .  
      4. No-Markers Cut-Through Implementation  
      In a fourth alternative embodiment, a no-marker cut-through implementation is used relative to CRC calculation and validation, as will be described below. A disadvantage of the above-described three alternative embodiments for correctly calculating DDP segment length is that each requires modification of the MPA specification or connection context  42  prefetching. This embodiment implements a cut-through processing of inbound segments without prefetching connection context  42  to calculate CRC of arriving MPA frames and without any additional changes to the MPA specification. In addition, this embodiment allows out-of-order direct data placement without use of MPA Markers. This embodiment is based, in part, on the ability of a receiver to negotiate a ‘no-markers’ option for a given connection according to a recent updated version of the MPA specification. In particular, the updated MPA specification allows an MPA receiver to decide whether to use markers or not for a given connection, and the sender must respect the receiver&#39;s decision. This embodiment changes validation unit  44  logic to allow CRC calculation on the fly concurrently with TCP checksum calculation and without prefetching connection context  42 .  
      The CRC calculation is done exactly as described for the case with markers. That is, the present invention assumes that the TCP segment starts with aligned DDP segment, and uses the MPA length field to find the location of CRC, and then calculates and validates CRC. The difference with this embodiment, however, is that there is no need to consider markers when calculating DDP segment length, given MPA length field of the MPA header.  
      Referring to  FIG. 9 , a flow diagram illustrating InLogic  32  functionality relative to a first alternative of this embodiment is shown. It should be recognized that much of InLogic  32  functionality is substantially similar to that described above relative to  FIG. 3 . For clarity purposes, where InLogic  32  functionality is substantially similar to that described above relative to  FIG. 3 , the steps have been repeated and delineated with a dashed box.  
      Under the updated MPA specification, a receiver negotiates a ‘no-marker’ option for a particular connection at connection initialization time. As shown in  FIG. 9 , in this embodiment, at step S 201 , InLogic  32  determines whether inbound TCP segment  106  includes markers  110 . If YES, InLogic  32  proceeds with processing as in  FIG. 3 , and some other method of CRC calculation and validation would be used, as described above. If NO, at step S 202 , inbound MPA frames  109  have their CRC calculated and validated on the fly using the same store-and-forward buffers  68  as TCP checksum logic  66 , but without fetching connection context  42 . A determination of whether the connection is a SLOW connection, steps S 2  and S 3  as in  FIG. 3 , may also be completed. Results of CRC validation can be one of the following: 1) the length of MPA frame  109  matches the length of TCP segment  106 , and MPA frame  109  has a valid MPA CRC; 2) the length of the MPA frame  109  matches the length of TCP segment  106 , but MPA frame  109  has an invalid CRC; 3) the length of MPA frame  109  exceeds the length of the TCP segment; and 4) the length of MPA frame  109  is smaller than the length of TCP segment  106 .  
      In case 1), InLogic  32  functions substantially similar to steps S 4 -S 7  of  FIG. 3 . That is, where MPA frame  109  has a same length as a TCP segment  106  (steps S 4  and S 5  of  FIG. 3 ), and carries a valid MPA CRC (step S 6 ), the frame is considered to be a valid MPA frame, and is passed to OutLogic  40  for further processing via internal data buffers  38  and to destination data buffers  50  on the fast path mode.  
      In case 2), where MPA frame  109  has a same length as a TCP segment  106  (steps S 4  and S 5  of  FIG. 3 ), but has an invalid CRC (step S 6  of  FIG. 3 ), InLogic  32  functions differently than described relative to  FIG. 3 . In particular, since received MPA frame  109  does not contain MPA markers  110 , the marker related information cannot be used for recovery (as in step S 10  of  FIG. 3 ). This leaves only two cases that need to be addressed: Case A: when MPA frame  109  is referred by the length of the previously received segment (and validated) MPA frame  109  (as determined at step S 8  of  FIG. 3 ); and Case B: all other cases. In Case A the MPA frame  109  is corrupted, and in Case B, MPA frame  109  can be either corrupted or not aligned. In both cases the received TCP segment  106  is dropped (step S 9  of  FIG. 3 ), and receipt is not confirmed. In this case, the limited retransmission attempt mode described relative to  FIG. 4  may be implemented to recover from the drop of that TCP segment  106 , which allows the sender to retransmit the dropped TCP segment  106  and resolve any potential data corruption. If MPA frame  109  was not aligned to TCP segment  106 , then the limited retransmission attempt mode will end with downgrading of the connection to a SLOW connection, as described above.  
      In case 3), where the length of MPA frame  109  exceeds a length of TCP segment  106  (step S 5  of  FIG. 3 ), either MPA frame  109  is not aligned to TCP segment  106 , or the length is corrupted. In this case, the received TCP segment  106  is dropped (step S 9  of  FIG. 3 ), and TCP does not confirm receipt. In this case, again, the limited retransmission attempt mode described relative to  FIG. 4  may be implemented to recover from the drop of that TCP segment  106 , which allows the sender to retransmit the dropped TCP segment and resolve any potential data corruption. Again, if MPA frame  109  is not aligned to TCP segment  106 , then the limited retransmission attempt mode will end with downgrading of the connection to a SLOW connection, as described above.  
      In case 4), where the length of MPA frame  109  is smaller than the length of TCP segment  106  (step S 4  of  FIG. 3 ), or TCP segment  106  potentially carries multiple MPA frames  109  (sender exercises a packing option), InLogic  32  sequentially checks the CRCs of all DDP segments  112  embedded in the received TCP segment  106  (steps S 11 -S 13  of  FIG. 3 ). If all DDP segments  112  have a valid CRC, InLogic  32  approves reception of that TCP segment  106 , and all MPA frames are forwarded for the further processing on the fast path mode (step S 7  of  FIG. 3 ). If one of DDP segments  112  has an invalid CRC, or the last segment is not fully contained in the TCP segment (steps S 12 -S 13  of  FIG. 3 ), the entire TCP segment is dropped (step S 9  of  FIG. 3 ), and InLogic  32  does not confirm reception of that TCP segment. As above, the limited retransmission attempt mode described relative to  FIG. 4  may be implemented to recover from the drop of that TCP segment  106 , which allows the sender to retransmit the dropped TCP segment and resolve any potential data corruption. If MPA frame  109  was not aligned to TCP segment  106 , then the limited retransmission attempt mode will end with downgrading of the connection to a SLOW connection, as described above.  
      Turning to  FIG. 10 , another alternative flow diagram illustrating InLogic  32  functionality relative to this embodiment, and including aspects of the Limited Retransmission Attempt Mode and TCP Retransmit Speed-Up is shown. In contrast to  FIG. 9 , InLogic  32  functionality is greatly simplified compared to  FIG. 3 . For clarity purposes, where InLogic  32  functionality is substantially similar to that described above relative to  FIG. 3 , the steps have been repeated and delineated with a dashed box.  
      In  FIG. 10 , steps S 151 -S 153  are substantially identical to step S 1 -S 3  of  FIG. 3 . At step S 154 , InLogic  32  determines whether CRC validation passed. This evaluation is different than step S 4  in  FIG. 3  in that instead of providing an indication per DDP segment, CRC logic  54  provides a CRCValidationPassed bit that indicates success or failure of CRC validation of all DDP segments in a received TCP segment. This bit is set if the CRC validation passed for all DDP segments contained in received TCP segment, and is cleared if either the CRC validation failed for one of the segments, or the last (only) segment was too long. If NO, InLogic  32  proceeds to step S 155 , where a determination as to whether RecoveryAttemptsNum (field  292  of  FIG. 2B ) is greater than MaxRecoveryAttemptsNum (field  296  of  FIG. 2B ). If YES, then InLogic proceeds to step S 153  where the DDP segment is placed to reassembly buffers  34 , an Ack is sent, and the connection is downgraded to a SLOW connection (if it was a FAST connection). If NO at step S 155 , then at step S 156 , the TCP segment  106  is dropped and no confirmation is scheduled. In addition, RecoveryAttemptNum (field  292  of  FIG. 2B ) is increased by one, and the LastRecoverySN (field  294  of  FIG. 2B ) is updated.  
      Returning to step S 154 , if the determination results in a YES, InLogic  32  proceeds, at step S 157 , to determine whether a newly received in-order data transfer&#39;s sequence number (In-order SN) is greater than LastRecoverySN (field  294  of  FIG. 1B ). If YES, then at step S 158 , InLogic  32  clears RecoveryAttemptsNum (field  292  in  FIG. 1B ), i.e., sets it to zero. If NO at step S 157  or subsequent to step S 158 , at step S 159 , the segment is processed on the “fast path mode” by placing the segment to destination data buffers  50 . Step S 159  may also include implementation of the duplicate Ack, as discussed above relative to the TCP Retransmit Speed-Up option.  
      The above-described  FIG. 10  embodiment implements the cut-through mode of the invention plus the limited retransmission attempt mode and TCP retransmit speed-up option without use of MPA markers.  
      III. OutLogic  
      OutLogic  40  ( FIG. 2A ) performs in-order delivery of RDMA messages without keeping information per RDMA message. There are two situations that are addressed: 1) for all RDMA Messages excepting a Send message, and 2) an RDMA Send message.  
      Returning to  FIGS. 1F-1H , operation of OutLogic  40  ( FIG. 2A ) will now be described. OutLogic processes aligned DDP segments  220  from internal data buffers  38  ( FIG. 2A ) that were placed there on the fast path mode, as described above, and conducts data placement and delivery of the aligned DDP segments to a receiver&#39;s data buffers. As used herein, “placement” refers to the process of actually putting data in a buffer, and “delivery” refers to the process of confirming completion of a data transfer. “Placement” may be applied to both segments and messages, while “delivery” applies to messages only. Under the RDMA protocol, aligned DDP segments may be placed in an out-of-order fashion, but delivery does not occur until all of the aligned DDP segments are placed in-order. For example, for three aligned DDP segments  1 ,  2  and  3 , where segments  2  and  3  are first placed without segment  1 , delivery does not occur until segment  1  is placed.  
      A. Placement  
      With regard to placement, OutLogic  40  provides conventional placement of RDMA messages except relative to RDMA Read messages, as will be described below.  
      With regard to tagged DDP segments, for example, returning to  FIG. 1D , according to the RDMA protocol, a header  124  of a tagged DDP segment carries an address of the receiver&#39;s previously registered memory region (e.g, memory region  232  in  FIG. 1G ). As indicated above, this address includes starting tag (STag) indicating a destination buffer that lies in memory region/window (e.g., memory region  232  in  FIG. 1G  for an RDMA Write message), a target offset (TO) in this region/window and a transaction length (segment payload). In this case, data placement is conducted by OutLogic  40  in a conventional manner, without retrieving any additional information from connection context  42  ( FIG. 2A ). Conventional Address Translation and Protection (ATP) processes, in which the STag and TO are translated to a list of physical buffers of a memory region describing the destination data buffer, precedes the data placement by OutLogic  40 .  
      Relative to untagged DDP segments such as an RDMA Read message, referring to  FIG. 1H , the RDMA protocol defines the maximal number of pending inbound Read Requests  222 , which is exchanged at negotiation time. Each RDMA Read message  204  consumes a single DDP segment  222 . When RNIC  16  receives RDMA Read message  204 , it posts an RDMA Read Response WQE  216 RR to a Read Queue  214 . In another example, referring to  FIG. 1F , each Send message  200  is placed to receive queue (RQ)  212  of a responder, e.g., data sink  18  ( FIG. 2A ). As noted above, each receive queue (RQ)  212  is a buffer to which control instructions are placed, and includes a WQE  216 R to which a payload is placed. Receive queue (RQ)  212  includes WQEs  216 R. Each WQE  216 R holds control information describing a receive WR  208 R posted by a consumer. Each WQE  216 R also points on consumer buffer(s) posted in that WR  208 R. Those buffers are used to place the payload. Accordingly, each message  200  consumes a WQE  216 R.  
      Referring to  FIG. 11 , a representation of an RDMA Read message  204  and RDMA Read Response  206  similar to  FIG. 1H  is shown. In accordance with the invention, however, a Read Queue  414  is provided as a special work queue (WQ) implemented as a cyclic buffer, and each entry of this cyclic buffer is a WQE  216 RR describing the RDMA Read Response that needs to be generated by transmit logic. This allows easy and efficient placement of out-of-order RDMA Read Requests  222  since for each inbound RDMA Read Request there is a well known location in the Read Queue  414 , i.e., WQE  216 RR. For example, when RDMA Read message # 3  is received and RDMA Read message # 2  is lost, RDMA Read message # 3  is placed. This placement is done upon reception of RDMA Read Request message  222 , i.e., message sent due to posting of Read WR  208 R on requester. Location of WQE  216 RR in Read Queue  414  is identified by the MSN in RDMA Read message header  124  ( FIG. 1D ).  
      B. Delivery  
      The RDMA protocol allows out-of-order data placement but requires in-order delivery. Accordingly, conventional implementations require maintaining information about each message that was placed (fully or partially) to the memory, but not delivered yet. Loss of a single TCP segment, however, can lead to the reception of many out-of-order RDMA messages, which would be placed to the destination buffers, and not completed until the missing segment would be retransmitted, and successfully placed to the memory. Under conventional circumstances, limited resources are available to store an out-of-order stream such that only a certain number of subsequent messages can be stored after an out-of-order stream is received.  
      According to the invention, however, instead of holding some information for each not delivered RDMA message and therefore limiting the number of supported out-of-order received messages, an unlimited number of not delivered RDMA messages are supported by storing information on a per TCP hole basis. A “TCP hole” is a term that describes a vacancy created in the TCP stream as a result of reception of an out-of-order TCP segment.  
      Referring to  FIG. 12 , white blocks indicate missing TCP segments  400  that form TCP holes  130 A- 130 C, and shaded/gray blocks  402  indicate a continuously received TCP stream. Per TCP hole  130 A- 130 C information is stored in connection context  42  ( FIG. 2B ). A limited number of supported TCP holes  130 A- 130 C is a characteristic inherited from the TCP protocol implementation. In particular, the TCP protocol usually limits the number of supported TCP holes  130 A- 130 C to, for example, one, two or three holes. Typically, support of limited number of TCP holes  130 A- 130 C effectively means that when an out-of-order TCP segment arrives, opening a new TCP hole, this segment is dropped by TCP logic.  FIG. 12  illustrates a three-TCP hole implementation. In this case, if a new segment arrives after the bottom TCP hole  130 C, i.e., after the two bottom missing segments  400 , this segment will “open” a fourth hole that is not supported. As a result, that segment would be dropped.  
      In order to address this situation, the present invention implements tracking of TCP holes  130  ( FIG. 12 ) via connection context  42  ( FIGS. 2A and 2B ) rather than tracking of out-of-order messages/segments. In particular, as shown in  FIG. 2B , the invention stores a PendingReadResponseNum field  300  to count completed RDMA Read Requests, a CompletedSendsNum field  302  to count completed Send messages and a CompletedReadResponseNum field  306  to count completed RDMA Read Responses. As those skilled in the art should recognize, other fields may be required for each hole, the description of which will not be made for brevity sake. This approach allows an unlimited number of out-of-order received RDMA messages waiting for completion and in-order delivery. This approach does not limit ability to share a completion queue  240  ( FIGS. 1F-1H ) both by receive  212  and send  210  queues without any limitation. The details of handling of particular types of messages will now be described.  
      First, it should be recognized that delivery of RDMA Write messages  202  ( FIG. 1G ) does not lead to any report to a responder, or any notification to other hardware logic because of the nature of the operation. Accordingly, no delivery concerns exist relative to this type RDMA message.  
      Second, returning to  FIG. 11 , with regard to an RDMA Read Response message  206 , this operation represents the completion of a pending RDMA Read message  204 . In this case, storing a CompletedReadResponseNum field  306  ( FIG. 2B ) in connection context  42  that includes a number of completed RDMA Read Response messages  206  per TCP hole  130  is sufficient to provide completion handling logic of the requester with enough information to complete pending RDMA Read work requests  208 R. When the TCP hole closes, the number of completed RDMA Read Responses associated with this hole is reported to completion handling logic of the requester to indicate completion of pending RDMA Read work requests  208 R.  
      With regard to RDMA Read Requests, operation of WQE  216 RR post includes two steps: placement of WQE  216 RR to Read Queue  414 , and a notification, i.e., doorbell ring, to notify RNIC  16  that this WQE can be processed. Placement of WQE  216 RR can be done out-of-order. However, as noted above, the start of the WQE processing (and thus doorbell ring) must be compliant to RDMA ordering rules. That is, the RDMA protocol requires delay of processing of inbound RDMA Read messages  204  until all previously transmitted RDMA messages of any kind are completed. Thus, the doorbell ring, i.e., notification, should be delayed until all in-order preceding RDMA Read messages  204  are completed. A single doorbell ring, i.e., notification, can indicate posting of several WQEs  216 RR.  
      To resolve the above problem, RNIC  16  according to the invention stores in connection context  42  (PendingReadResponseNum field  300  ( FIG. 2B )) the number of posted RDMA read response WQEs  216 RR waiting for the doorbell ring (notification) for each TCP hole  130  ( FIG. 1B ). When a TCP hole  130  is closed, RNIC  16  rings the doorbell (notifies) to confirm posting of PendingReadResponseNum WQEs  216 RR to Read Queue  214 . This indicates that all preceding read messages  204  have been completed, and RNIC  16  can start processing of the posted read response WQEs  216 RR.  
      Referring to  FIG. 13 , an RDMA Send message  500  represents a unique situation. In particular, delivery of a completed Send message includes placing of a CQE  542  to CQ  540 . CQE  542  carries information describing the completed message (e.g., length, Invalidate STag, etc.). This information is message specific information, and therefore should be kept for each pending Send message  500 . RNIC  16  cannot place a CQE  542  before a Send message  500  has been completed (similarly to the placement of RDMA Read Response WQE  508 RR in received Read work requests  508 R), because a CQ  540  can be shared by several send  510  and receive  512  queues, as indicated above.  
      To resolve this issue without consuming additional RNIC resources, and providing scalable implementation, OutLogic  40  according to the present invention places all information that needs to be included in CQE  542  to the WQE  516 R consumed by that Send message  500 . This information is then retrieved from WQE  516 R by verb interface  20  ( FIG. 2A ) upon a Poll-For-Completion request. RNIC  16  needs to keep the number of completed send messages  500  (in CompletedSendsNum field  302 ) per TCP hole  130  in connection context  42 , which is used to post CQEs  542  to CQ  540 , when corresponding TCP hole closes. When the TCP hole  130  closes, RNIC  16  places CQEs  542  to CQ  540 . The number of CQEs  542  to be placed equals the number of completed Send messages  500  counted for this hole. This approach involves  2 N write operations, when N is a number of completed Send messages  500 .  
      One disadvantage of the approach presented above relative to delivery of an RDMA Send message  500  is that it doubles the number of write operations performed by RNIC  16 . That is, there is one write to WQE  516 R and one write of CQE  542  for each completed Send message  500 . In order to address this issue, as shown in  FIG. 14 , according to an alternative embodiment of the present invention, the content of a CQE  542  is changed to carry a reference counter  544  of WQEs  516 R that the particular CQE  542  completes. Reference counter  544  is initialized by RNIC  16  to the number of Send messages  500  completed for the given TCP hole  130 . Verb interface  20 , for each Poll-For-Completion operation, reduces reference counter  544 , and removes CQE  542  from CQ  540  only if the counter becomes zero. In addition, RNIC  16  updates a WQE  516 S only if it is holds greater than a threshold (M) outstanding Send messages  500  waiting for completion. M is a configurable parameter, indicating an amount of internal resources allocated to keep information for pending inbound Send messages  500 . If M equals zero, then any out-of-order received Send message  500  involves update of WQE  516 R (no updated is needed for in-order received Send messages  500 ).  
      This embodiment also includes defining two kinds of CQEs  542  and providing an indicator  546  with a CQE  542  to indicate whether the CQE is one carrying all completion data in the CQE&#39;s body, or one that carries part of completion data-with the remainder of the completion information stored in WQE  516 R associated with one or more RDMA Send messages. This alternative embodiment reduces the number of write operations to N+1, where N is a number of completed Send messages  500 , that were pending before TCP hole  130  was closed.  
      IV. Conclusion  
      In the previous discussion, it will be understood that the method steps are preferably performed by a specific use computer, i.e., finite state machine, containing specialized hardware for carrying out one or more of the functional tasks of the invention. However, the method steps may also be performed by a processor, such as a CPU, executing instructions of a program product stored in memory. It is understood that the various devices, modules, mechanisms and systems described herein may be realized in hardware, software, or a combination of hardware and software, and may be compartmentalized other than as shown. They may be implemented by any type of computer system or other apparatus adapted for carrying out the methods described herein. A typical combination of hardware and software could be a general-purpose computer system with a computer program that, when loaded and executed, controls the computer system such that it carries out the methods described herein. The present invention can also be embedded in a computer program product, which comprises all the features enabling the implementation of the methods and functions described herein, and which—when loaded in a computer system—is able to carry out these methods and functions. Computer program, software program, program, program product, or software, in the present context mean any expression, in any language, code or notation, of a set of instructions intended to cause a system having an information processing capability to perform a particular function either directly or after the following: (a) conversion to another language, code or notation; and/or (b) reproduction in a different material form.  
      While this invention has been described in conjunction with the specific embodiments outlined above, it is evident that many alternatives, modifications and variations will be apparent to those skilled in the art. Accordingly, the embodiments of the invention as set forth above are intended to be illustrative, not limiting. Various changes may be made without departing from the spirit and scope of the invention as defined in the following claims. In particular, the described order of steps may be changed in certain circumstances or the functions provided by a different set of steps, and not depart from the scope of the invention.