Abstract:
In a technique for performing flow control in a wide-area network, a user-specified flow control message such as a Pause message is provisioned at a first node, and a user-specified flow control message filter is provisioned at a second node coupled to the first node. At the first node, the availability of a buffer for data received from the second node is monitored, and in response to a predetermined availability condition, the provisioned flow control message is sent to the second node. In a Pause/Resume protocol, the buffer may be monitored for an Almost Full condition, and the flow control message may be a Pause message indicating that the first node should suspend transmission. The second node compares messages from the first node against the flow control message filter. Upon detecting the flow control message from the first node, the second node takes a predetermined flow control action with respect to the transmission of data to the first node. In the case of a Pause/Resume protocol, the flow control action may be to stop transmission for a specified period.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
         [0001]    —None— 
         STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT  
         [0002]    —Not Applicable— 
         BACKGROUND OF THE INVENTION  
         [0003]    The present invention relates to the field of flow control in wide-area networks.  
           [0004]    Data communications networks often employ a technique referred to as “flow control” to manage the transmission of data among network nodes. Generally, “flow control” refers to techniques for making the transmission of data from one node to another conditional upon some real-time criteria, such as whether there is sufficient buffer space at a receiver to receive a transmission, or whether the transmitter is complying with a specified transmission rate. There are different types of flow control mechanisms or protocols. On network links employing the Gigabit Ethernet data communications protocol, for example, a mechanism using Pause and Resume messages is utilized. When a receiver senses that receive buffers are close to full, it sends a Pause message to the transmitter indicating that the transmitter should stop transmission for a specified time. Later, when the buffers have become emptier, the receiver sends a Resume message indicating that transmission can be resumed. This Pause/Resume protocol keeps the risk of buffer overrun and loss of data at an acceptably low level.  
           [0005]    Data communications networks often have a hybrid nature, such that an end-to-end data communications path traverses links or subnetworks that employ different transmission protocols. At the interfaces between subnetworks of different types, devices that perform encapsulation, protocol mapping, and related functions are used to enable the successful transfer of data from one subnetwork to another. The different subnetworks may use different flow control protocols, or in some cases a subnetwork using a flow control protocol may be connected to another subnetwork that does not. One configuration is the interconnection of two or more relatively local links, such as Gigabit Ethernet (GbE) links, via a more wide-area network, such as a Synchronous Optical Network (SONET) long-haul network. In such a configuration, the wide-area network (WAN) serves as a transport medium for the traffic of the local links, i.e., it functions much as a wire that simply moves data from one GbE link to another without participating in the GbE protocols. SONET, for example, has no provision for flow control, either internally or in relation to external network segments such as GbE links.  
           [0006]    It may be desirable in networks having local-area segments interconnected by a WAN to employ flow control within the WAN in addition to whatever flow control is used on the local segments. However, for some WANs, such as SONET-based WANs, there may be no specification of a flow-control protocol, nor any anticipation of a need for flow control. Additionally, it may be desirable that protocol-mapping network devices used at the edges of a WAN be operable with different types of attached local networks, so that it may be necessary to support different flow control protocols or different methods for enabling subnetworks using different flow control protocols to interoperate with each other. Therefore, there is a need for mechanisms that can be flexibly utilized for flow control in a wide-area network and protocol-mapping devices attached thereto.  
         BRIEF SUMMARY OF THE INVENTION  
         [0007]    In accordance with the present invention, an apparatus and method are disclosed for performing flow control in a flexible manner in a wide-area network.  
           [0008]    A user-specified flow control message is provisioned at a first node, and a user-specified flow control message filter is provisioned at a second node coupled to the first node. In one embodiment, the flow control message may be provisioned as a Pause message, and the flow control message filter provisioned to recognize the Pause message when received at the second node.  
           [0009]    At the first node, the availability of a buffer temporarily storing user data received from the second node is monitored, and in response to a predetermined availability condition of the buffer, the provisioned flow control message is sent to the second node. In a Pause/Resume protocol, for example, the buffer may be monitored for an Almost Full condition, and upon such condition being detected, a Pause message is sent indicating that the first node should suspend transmission for a specified period of time.  
           [0010]    The second node continually compares messages received from the first node against the provisioned flow control message filter. Upon detecting the receipt of the flow control message sent from the first node, the second node takes a predetermined flow control action with respect to the transmission of user data to the first node. In the case of a Pause/Resume protocol, the flow control action may be to stop transmission for the specified period.  
           [0011]    The user provisioning of the flow control message and flow control message filter provide flexibility in the type of flow control protocols that can be supported and the manner in which they are implemented in a given system. A user can provision the flow control message to have a particular structure that is advantageous in a given system, and then provision the filter in a corresponding fashion.  
           [0012]    Other aspects, features, and advantages of the present invention will be apparent from the detailed description that follows. 
       
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING  
       [0013]    The invention will be more fully understood by reference to the following Detailed Description of the Invention in conjunction with the Drawing, of which:  
         [0014]    [0014]FIG. 1 is a block diagram of a network in accordance with the present invention;  
         [0015]    [0015]FIG. 2 is a block diagram of a multiplexer/transport node in the network of FIG. 1;  
         [0016]    [0016]FIG. 3 is a block diagram of processing logic for a client link in the multiplexer/transport node of FIG. 2;  
         [0017]    [0017]FIG. 4 is a flow diagram depicting the operation of a pair of multiplexer/transport nodes in the network of FIG. 1; and  
         [0018]    [0018]FIG. 5 is a diagram showing the structure of flow-control messages used in the network of FIG. 1. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0019]    In FIG. 1, multiplexer/transport (mux/transport) nodes  10  and  12  are connected between a wide-area network (WAN)  14  and respective sets of client links  16  and  18 . The WAN  14  is a long-haul, circuit-oriented network such as a Synchronous Optical Network (SONET) network or the emerging Optical Transport Network (OTN). The client links  16  and  18  are high-speed connections to client data network devices such as switches or routers (not shown), such as Gigabit Ethernet (GbE) or Fibre Channel (FC) links. As shown, there are several client links  16  and  18  connected to the respective mux/transport nodes  10  and  12 . The mux/transport nodes  10  and  12  provide for multiplexing and demultiplexing client data streams from the client links  16  and  18  to and from higher-rate data streams carried in the WAN  14 . The mux/transport nodes  10  and  12  also perform other functions pertaining to the transport of the client data streams through the WAN  14 , including for example data buffering, channel allocation, performance monitoring, flow control, and other functions.  
         [0020]    Generally, the client links  16  are distinguished from the WAN  14  by the relative distances over which they are specified to operate. GbE links, for example, are typically specified to operate over a distance of 5-10 km. for single-mode fiber, and considerably shorter distances for other physical transmission media. A SONET link, however, is specified to operate over distances as long as hundreds of kilometers. These differences give rise to different operational characteristics. In particular, it is common to employ a flow control mechanism on shorter-distance data links such as the client links  16 , whereas flow control has generally not been utilized on long-haul links such as used in the WAN  14 . The term “flow control” refers to the ability of a data receiver to influence the pattern of data transmission from a corresponding data transmitter, so as to provide for high data throughput while minimizing the risk of data loss due to buffer overflow at the receiver. In long-haul networks, it has generally been the responsibility of equipment at the transmitting end to conform the transmission rate to the transmission capacity available on the long-haul link, without relying on any flow-control mechanism on the link itself.  
         [0021]    Client data frames received from the client links  16  and  18  by a mux/transport node  10  or  12  are re-framed or encapsulated for transmission through the WAN  14 . One known framing protocol is referred to as High Level Data Link Control (HDLC), and an emerging encapsulation technique is referred to as Generic Framing Procedure (GFP). In addition to providing adaptation between the frame format of the client links  16 ,  18  and that of the WAN  14 , re-framing or encapsulation logic can also provide protocol support for ancillary operational features that may be desirable from a system perspective. For example, GFP allows for the definition of “client management” frames that can be exchanged between devices for various purposes, such as exchanging performance monitoring information or reporting events such as Client Signal Fail (CSF). As described below, such an ancillary function of the WAN  14  is utilized to carry flow-control messages between the mux/transport nodes  10  and  12  as part of a flow control scheme in the WAN  14 . Such ancillary traffic may be carried over the same physical path(s) through the WAN  14  as the related user data, but occupies logical channels that are separate from the user data streams.  
         [0022]    [0022]FIG. 2 shows the general structure of a mux/transport node  10  or  12 . Traffic from the WAN  14  is received by a WAN receive (RX) section  20 . The components of the received traffic that are associated with the different client links  16  or  18  are routed through a demultiplexer  22  to a corresponding one of a plurality of client link egress processing sections  24 , wherein each section  24  provides the transmit function for a single associated client link  16  or  18 . The demultiplexer  22  separates the traffic of the different client links  16  or  18 , and may be implemented in a variety of ways. In a SONET WAN, for example, different “virtual channels” (VCs) or groups of base STS-1 signals can carry the traffic associated with the different links  16  or  18 , and the demultiplexer  22  may take the form of a buffer employing an addressing scheme based on numeric identifiers of the different VCs. The outputs of the demultiplexer  22  are the data streams carried in the various VCs.  
         [0023]    In the other direction, traffic from the client links  16  or  18  is received by a group of client link ingress sections  26 , and then provided through a multiplexer  28  to a WAN transmit (TX) section  30 , from which it is transmitted on the WAN  14 . As with the demultiplexer  22 , the multiplexer  28  may be implemented in a variety of ways. In a SONET WAN, the output of the multiplexer  28  is a single high-rate, VC-organized data stream carrying the traffic from the various client links  16  or  18 . This data stream is further processed by the WAN TX section  30  to create a complete multiplexed STS signal, such as an STS-48 signal, for transmission on the WAN  14 .  
         [0024]    [0024]FIG. 3 shows the combined structure of a client link egress section  24  and a client link ingress section  26  for a given client link  16  or  18 . The egress path includes frame extraction logic  32 , an egress FIFO buffer  34 , and an egress MAC/PHY section  36  that performs media access control (MAC) and physical layer (PHY) functions. The frame extraction logic  32  provides egress frames to frame filter and capture logic  38 , whose function is described below. The ingress path includes an ingress MAC/PHY section  40 , rate shaping logic  42 , and frame encapsulation logic  44 . As shown, the ingress MAC/PHY section  40  exchanges client link flow control information with the egress MAC/PHY section  36 . Frame insertion logic  46  is coupled to the rate shaping logic  42 . The frame insertion logic  46  receives input signals from the egress FIFO buffer  34 . Also shown is a microprocessor  48  responsible for provisioning and other control functions.  
         [0025]    The frame insertion logic  46  includes two frame buffers whose contents are provisionable by the microprocessor  48 , and which are conditionally inserted into the stream of ingress frames in response to a hardware trigger event as described in more detail below. Each buffer is used to store a single frame, and has a storage area that dictates the maximum-sized frame that can be stored. For purposes of flow control signaling, a storage area for each buffer of 127 bytes may be more than adequate. The exact length of a provisioned frame in each buffer is programmed by the microprocessor  48 , so that the correct number of bytes for the frame are played out when the frame is inserted into the data stream.  
         [0026]    The triggers for insertion of each buffer of the frame insertion logic  46  into the ingress data stream are also programmed by the microprocessor  48 . These are summarized below:  
                                   Code   Trigger                   0   No triggers active       1   Insert on egress buffer almost full       2   Insert on egress buffer almost empty                  
 
         [0027]    It will be appreciated that trigger #1, “insert on almost full”, is useful for generating flow-control frames that reduce or stop transmission from the far end, and that trigger #2, “insert on almost empty”, is useful for generating flow-control frames that increase or resume transmission from the far end. The use of these triggers for specific flow-control protocols is described below.  
         [0028]    The frame filter and capture logic  38  includes two frame filters whose contents are provisionable by the microprocessor  48 , and which can be used to detect flow control frames in the egress direction and initiate corresponding flow control actions. The filters specify bit patterns that must be found in a predetermined location in received frames to declare a match. In connection with each filter is a corresponding provisionable mask that specifies the bits of a frame to be compared with corresponding filter bits, all other bits being deemed “don&#39;t cares”. In one embodiment, each filter and mask is 8 bytes in length, and the filter is compared with the 8 bytes immediately following the “core header” of each received egress frame. Exemplary frame structures and a description of the “core header” appear below.  
         [0029]    The actions to be taken upon the occurrence of a match between each filter and a received frame are programmed by the microprocessor  48 , and are summarized below:  
                                   Code   Action                   0   No action       1   Signal the rate shaping logic 42 to pause or resume           frame transmission, depending on which filter is           matched; discard received frame.                  
 
         [0030]    In the above, the function of pausing or resuming frame transmission is hard-associated with the identity of the filter. That is, if a given frame matches filter #1, for example, then the frame is interpreted as a Pause frame, and if the frame matches filter #2, then the frame is interpreted as a Resume frame. In alternative embodiments, it may be useful for this association to itself be provisionable, thus permitting a user to assign either function (pausing or resuming) to a given filter. Also, in different flow-control protocols it may be desirable to employ more or less than two filters.  
         [0031]    [0031]FIG. 4 illustrates the flow-control related operation of the mux/transport nodes  10  and  12  of FIG. 1. At steps  50  and  52 , the microprocessor  48  in each node  10  and  12  provisions a Pause frame, a Resume frame, a Pause filter, and a Resume filter. It will be appreciated that the provisioned frames must be consistent with the provisioned filters, i.e., that the Pause filter as provisioned will detect the Pause frame as provisioned, and likewise with the Resume filter and frame. Beyond this requirement and a few others, such as the 8-byte limit on filter size and any requirements of the encapsulation protocol being utilized, the frames and filters may be defined in any convenient way. It may be convenient to define the Resume frame as a special type of Pause frame, e.g., as a Pause frame with a Pause time of  0  (zero). However, other formats may be used.  
         [0032]    At steps  54  and  56 , the nodes  10  and  12  exchange user data frames via the WAN  14 .  
         [0033]    At step  58 , it is detected at node  10  that the egress FIFO  34  for client “n” is Almost Full. The definition of Almost Full varies depending on several considerations in a given system, such as the size of the FIFO  34 , the round-trip delay in the WAN  14 , etc. It may be advantageous to make the Almost Full threshold user-provisionable for greater flexibility. As a result of the detection of Almost Full, a Pause frame is inserted in the stream of ingress frames for this client. The Pause frame specifies a provisioned non-zero time for which transmission on behalf of this client should be suspended.  
         [0034]    At step  60 , frame filter and capture logic  38  at the node  12  captures the Pause frame from the stream of egress frames, and signals the rate shaping logic  42  to cease transmission of ingress frames for this client. It will be appreciated that if transmission is interrupted long enough, the local client link flow control may be activated to prevent buffers (not shown) within the ingress MAC/PHY section  40  from overflowing.  
         [0035]    At step  62 , it is detected at node  10  that the egress FIFO  34  for client “n” is Almost Empty. The definition of Almost Empty varies depending on several considerations in a given system, as described above for Almost Full. As a result of the detection of Almost Empty, a Resume frame is inserted in the stream of ingress frames for this client. At step  64 , the Resume frame is captured by the node  12 , which triggers the re-starting of the transmission of ingress frames for this client.  
         [0036]    It will be appreciated that the process depicted in FIG. 4 is generally carried out in the reverse direction as well, between mux/transport node  12  and mux/transport node  10 .  
         [0037]    In the above description, it is assumed that the flow of frames in the same direction as the Pause and Resume messages is not itself paused. However, this is not a necessary condition, and in fact in operation the opposite may often be true, i.e., a Pause or Resume message must be sent when the flow of frames to the remote node is paused. Because of the critical importance of sending and receiving Pause and Resume messages under a variety of operational circumstances, these message must have greater priority for transmission and reception than more routine messages, such as user data frames. This explains why the frame filter and capture logic  38  resides ahead of the egress FIFO  34  in the egress data path. Also, provision must be made in the rate shaping logic  42  to enable the insertion of Pause and Resume frames whether or not the transmission of lower-priority frames is paused.  
         [0038]    [0038]FIG. 5 shows the structure of the GFP Pause and Resume frames. The overall frame structure is shown at  66 , and includes a core header  68  and payload area  70 . The payload area includes a payload header  74  and payload  76 , which in this case is an 8-byte Client Management Frame (CMF) or control payload. The payload header  74  includes a payload type indicator (PTI)  78 , payload FCS indicator (PFI)  80 , extension header identifier (EXI)  82 , user payload identifier (UPI)  84 , and type header error control (tHEC) bytes  86 . For the Pause and Resume frames, the value of PTI  78  indicates “Client Management”. Both PFI  80  and EXI  82  indicate null values (i.e., that no FCS or extension header is employed), and UPI  84  indicates that the user control payload is a Pause/Resume frame.  
         [0039]    The payload  76  contains a type identifier  88  indicating a Pause message and two bytes  90  and  92  constituting a Pause time (MSB and LSB respectively). As previously indicated, a Resume message is a Pause message with a zero pause time  90 ,  92 . The remaining bytes of the payload  76  constitute spare bytes  94 .  
         [0040]    While in the above description, completely user-specified Pause and Resume frames can be provisioned in the frame insertion logic  46  for use in flow-control operations, in alternative embodiments it may be convenient to hard-wire portions of these frames and allow more limited user provisioning. In particular, it may be convenient to hard-wire much of the contents of a GFP CMF Pause/Resume frame as described above with reference to FIG. 5, and allow the user to provision only the Pause time. In such a case, both the frame as defined in the frame insertion logic  46  and the filter as defined in the frame filter and capture logic  38  consist of a predefined pattern of logic Os and Is, which can be implemented very compactly, and only enough writeable storage to hold the variable pause time.  
         [0041]    With respect to the use of hard-wired frames and filters versus the more fully provisionable frames and filters as described above, it will be appreciated that there is a trade-off between greater logic efficiency (i.e. compactness) and greater flexibility and versatility. With the more generic provisioning approach described above, it is easier to operate the mux/transport node  12  with protocols other than GFP, for example, or to change the structure of the payload  76  as system needs may dictate. One particular use of the generic capability may be in FibreChannel systems, which employ a credit-based flow control scheme in contrast to the pause/resume scheme described above. Using the generic provisioning of flow control frames as described above, it may be easier to create flow-control frames that function similar to Pause and Resume frames but are understood by credit-based flow-control logic. Other applications of the generic provisioning capability are also possible.  
         [0042]    Additionally, in alternative embodiments it may be convenient to use other than two provisionable flow control frames, depending on the type of flow control protocol, operating conditions, and other circumstances.  
         [0043]    It will be apparent to those skilled in the art that other modifications to and variations of the disclosed methods and apparatus are possible without departing from the inventive concepts disclosed herein, and therefore the invention should not be viewed as limited except to the full scope and spirit of the appended claims.