Abstract:
Embodiments of the invention are directed to providing a method for selecting a link for transmitting a data packet, from links of a Multi-Link Point-to-Point Protocol (MLPPP) bundle, by compiling a list of links having a minimum queue depth and selecting the link in a round robin manner from the list. Some embodiments of the invention further provide for a flag to indicate if the selected link has been assigned to a transmitter so that an appropriate link will be selected even if link queue depth status is not current.

Description:
FIELD OF THE INVENTION 
       [0001]    The invention is directed to packet switching communication networks, particularly to distributing data packet fragments among links of a bundle of Multi-Link Point to Point Protocol (MLPPP) interfaces in a packet switching network. 
       BACKGROUND OF THE INVENTION 
       [0002]    Multi-Link Point to Point Protocol (MLPPP) is used to aggregate the bandwidth of several lower speed links to effectively provide a single higher speed link, along with low delay, low jitter and maintain the order of packets. IP packets of a traffic flow to be carried over the MLPPP (higher speed) link are distributed across the lower speed links at the transmitting end of the MLPPP link by fragmenting whole PPP frames and sending the fragments over different physical links also called MLPPP links. MLPPP is formally known as PPP Multilink Protocol, but is also known as ML PPP, PPP-MP, PPP-MLP, MPPP and as defined in IETF RFC1990 which specifies that fragments of a packet are to be distributed to the link transmit queues on the basis of the least occupied queue. That is, the occupancy (fill level in bytes) of each queue is determined and the current fragment of a packet to be transmitted is inserted into the queue with the lowest occupancy. 
         [0003]    Generally, the aforementioned approach works well. However, in practice, many IP data switches use data memory with linked list pointers to implement data queues. In these scenarios, the data queues can be implemented in blocks of X bytes (e.g. 64 bytes) in which case the link transmit queue occupancy is often measured in terms of blocks. Link transmit queue occupancy of several queues could be equal in terms of blocks when in fact their actual occupancy measured in bytes can be different. Additionally, the block management process updates the queue occupancy in a finite amount of time, which introduces latency in available queue occupancy information. When packets are processed back-to-back, the packet fragments in a MLPPP bundle can be assigned to links faster than the true occupancy of each link can be determined, which can prevent the system from utilizing the appropriate link. 
         [0004]    Therefore, in such cases, an improved means for determining which link should be selected for the current packet fragment for transmission is highly desirable. 
       SUMMARY OF THE INVENTION 
       [0005]    In general terms, embodiments of the present invention determine the occupancy of the MLPPP link transmit queues in terms of blocks (X bytes), as dictated by hardware limitations of the system. Where multiple queues have the same level occupancy and it is the equivalent lowest level, packets are distributed to those queues at the lowest level in a round-robin manner. These embodiments can minimize packet transfer delay, differential delay, and packet delay variation across the links that comprise the MLPPP link. 
         [0006]    An embodiment of the present invention is directed to providing a method of selecting a link (selected link) for transmitting a data packet, from a plurality of links of a Multi-Link Point-to-Point Protocol (MLPPP) bundle. The method comprises steps of: compiling a first list of active links from the plurality of links in the MLPPP bundle; compiling from the first list, a second list of links having a minimum queue depth; and if the second list has more than one member, the selected link is selected in a round robin manner. 
         [0007]    In some embodiments of the present invention, the round robin manner comprises addressing the links in a circular sequence and wherein the selected link comprises the link following a previously-selected link in the sequence. 
         [0008]    Some embodiments of the present invention further comprise steps of: setting a flag associated with the selected link when assigning the data packet for transmission on the selected link; and reading the flag when selecting the selected link, wherein, if the flag is not set, the selected link comprises the previously-selected link in the sequence. 
         [0009]    Some embodiments of the present invention further comprise a step of determining if at least one link in the first list has a queue depth below its respective link queue threshold, and if so, restricting the first list to links having a queue depth below their respective link queue threshold. 
         [0010]    In some embodiments of the present invention, the packet comprises a fragment of a larger packet. 
         [0011]    Another aspect of the present invention provides a program storage device readable by a machine, tangibly embodying a program of instructions executable by the machine to perform the method steps described above. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]    Some embodiments of apparatus and/or methods in accordance with embodiments of the present invention are now described, by way of example only, and with reference to the accompanying drawings in which: 
           [0013]      FIG. 1  illustrates an arrangement of transmit data queues in a MLPPP bundle according to an embodiment of the present invention; 
           [0014]      FIG. 2  illustrates the relationship between data tables and the process of embodiments of the present invention; and 
           [0015]      FIG. 3  illustrates a flowchart of a method for distributing data packet fragments in a MLPPP bundle according to an embodiment of the invention. 
       
    
    
       [0016]    In the figures like features are denoted by like reference characters. 
       DETAILED DESCRIPTION 
       [0017]      FIG. 1  conceptually depicts a telecommunication system  100  having an MLPPP transmit interface  102 , and a corresponding MLPPP receiving interface  104 , typically on a different network node. The MLPPP transmit interface is configured as a bundle of links  106 ,  108 ,  110 ,  112  each with corresponding transmit queues  114 ,  116 ,  118 ,  120 , respectively. Each transmit queue  114 ,  116 ,  118 ,  120  has a configurable queue threshold illustrated by dashed lines  124 ,  126 ,  128 ,  130  respectively. The queue thresholds can be set higher for higher speed links for example, or set lower for lower latency such as for the case where an individual link is selected for transmitting control plane data in addition to being configured as part of an MLPPP bundle. 
         [0018]    MLPPP can optionally support fragmentation of packets to better utilize bandwidth of the links in the MLPPP bundle. Thus large packets can be fragmented into packet fragments, each of which can be transmitted over individual links. Embodiments of the present invention support both packets and packet fragments. Packets (or packet fragments)  1 ,  2 ,  3 ,  4 ,  5 ,  6 ,  7  are transmitted on links having the lowest transmit queue depth so as to distribute the packet traffic across all the configure d links to maximize throughput and minimize latency. 
         [0019]    Thus, packet fragment  1  is en-queued on transmit queue  114  for transmission on link  106 .  FIG. 1  illustrates that packet fragment  1  is the first packet fragment for transmission on link  106 , with no other packet fragments pending for transmission on that link. Similarly, packet fragment  2  is en-queued on transmit queue  116  for transmission on link  108 , packet fragment  3  is en-queued on transmit queue  118  for transmission on link  110  and packet fragment  4  is en-queued on transmit queue  120  for transmission on link  112 . The round robin aspect is illustrated by the circular arrow  122 . 
         [0020]    The round robin process is intended to break ties for load-balanced links. The function is two-fold: In a situation where a bundle has very low traffic, there may be no packets queued for any of the links, in which case the round robin algorithm will ensure that each link will have a turn at sending a packet so that all links will be used. In a fragmentation application, a packet is split up into smaller packets. The queue depth update mechanism cannot always update the whole chain of memories to the current values in real time before the next fragment is requesting assignment to a bundle member. The round robin process will bump the start point for link selection if a link has been polled since the last round robin update and will not if it has not been polled, thus preventing “phantom jumps” in the assigned link. 
         [0021]    Table 1 illustrates a Bundle Status Memory (BSM) table. The BSM holds the state information for the links of each MLPPP bundle, including whether a link is valid (configured as part of the bundle), the depth of the link transmit queue buffer, and whether the link is in back pressure. The BSM is indexed by the Bundle ID field which is configured to address up to N (e.g., 512) MLPPP bundles, identified by their Bundle ID. For ease of description, in the described embodiment, the BSM is configured to handle up to 16 links per bundle, thus the Link Valid state can be represented by a 16-bit field with each bit representing a corresponding link and each valid link represented by a “1”. Note that other embodiments of the present invention contemplates other numbers of links to be supported. For ease of description, in the described embodiment, the queue depth of each link is measured in blocks of 40 bytes and the queue depth of 63 blocks can be represented by 6 bits, thus the Link Depth field can be represented by 16×6 bits. The Back Pressure field indicates whether a link is in back pressure which is defined as having a queue depth exceeding a predefined threshold for the link. Typically, when the queue for a particular link exceeds the queue threshold, it is an indication that packets are arriving faster than they can be transmitted and a “backpressure” flag is set. 
         [0000]    
       
         
               
             
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Bundle Status Memory (BSM) 
               
             
          
           
               
                   
                   
                 Link Valid 
                 Link Depths 
                 Back Pressure 
               
               
                   
                   
                 (16 columns 
                 (16 columns 
                 (16 columns 
               
               
                   
                 Bundle ID 
                 of 1 bit) 
                 of 6 bits) 
                 of 1 bit) 
               
               
                   
                   
               
             
          
           
               
                   
                  1 
                   
                   
                   
               
               
                   
                  2 
                   
                   
                   
               
               
                   
                 . . . 
                   
                   
                   
               
               
                   
                 512 
               
               
                   
                   
               
             
          
         
       
     
         [0022]    Table 2 illustrates a Minimum Depth Memory (MDM) table, which stores the link ID of the link of each MLPPP bundle which is selected as the best link (“Selected Link ID”) for transmitting the next packet or packet fragment for the MLPPP bundle. This is the link with the minimum queue depth at the time of the event, or in the case of more than one link having the same minimum queue depth, the link selected by a round robin process as the best link for transmitting the next packet or packet fragment for the MLPPP bundle. The Polled Bit indicates whether the MDM has been accessed and the selected link has been assigned to transmit the next packet or packet fragment. 
         [0000]    
       
         
               
             
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 Minimum Depth Memory (MDM) 
               
             
          
           
               
                 Bundle ID 
                 Polled Bit 
                 Selected Link ID 
               
               
                   
               
               
                  1 
                   
                   
               
               
                  2 
                   
                   
               
               
                 . . . 
                   
                   
               
               
                 512 
               
               
                   
               
             
          
         
       
     
         [0023]      FIG. 2 , illustrates the relationship between BSM  201  and MDM  203  and the processes which read or update them, thus a dequeue event  207  which remove a packet fragment from a link transmit queue buffer for transmitting it, or an enqueue event  209  which would add an incoming packet or packet fragment to be transmitted, would trigger an update to BSM  201 . The round robin link selection process  205  reads the queue depths from BSM  201  and reads the polled bit from MDM  203  and updates the Selected Link ID of MDM  203 , as described below with reference to  FIG. 3 . When the system has a packet or packet fragment to transmit ( 211 ) for a MLPPP bundle, a link is assigned ( 213 ) by reading the Selected Link for the bundle from MDM  203  and then setting the Polled it for the bundle. The Polled bit thus indicates that the Selected Link has been used (assigned) and if the round robin process accesses the MDM  203  for a subsequent packet or fragment before the BSM  201  can be updated, then the round robin process will consider selecting a different link at step  326  described below. Thus the step  213  will trigger the load balance process  300  described below. (“round robin” process which will update selected links in MDM  203 ). After the Selected link is assigned, the system then queues the packet or packet fragment in the appropriate queue for transmission ( 215 ). Thus event  215  is an enqueue event, which triggers ( 209 ) an update of BSM  201 . A single packet or fragment can generate several (e.g., 5-20 or more) enqueues and dequeues, as well as a MDM read event and each event can cause a recalculation of the selected link in the BSM  201 . This means a full packet need not be transmitted before the link it is being transmitted on could become the selected link. A packet or fragment stays in the queue until it reaches the head of the transmit queue (which is typically a fifo buffer), then bytes from the packet or fragment are transmitted on the link. 
         [0024]    Referring to  FIG. 3 , the load balance process  300  starts at step  301  and is triggered by an event that updates the queue depth information in BSM  201  such as an enqueue event  209  or dequeue event  207  for any link in the bundle, or by assigning the selected link ( 213 ) which updates the polled status bit in MDM  203 . Note that the same link could be reselected at this step. These trigger events mean that the determination of the selected link and the polled status bit in MDM  203  needs to be reassessed. At step  302 , the system builds a list of valid bundle links from BSM  201 . At step  304 , the system reads the link queue depths of the links configured for the MLPPP bundle from the BSM  201 . At step  306 , the process determines if there are any links in the MLPPP bundle that have a queue depth below its respective threshold. This can be determined from the backpressure bit (BP) for each link in the BSM. If at least one link has a queue depth below its respective threshold, then the process proceeds to step  308 , where the list of valid links is restricted to those links with a queue depth below their respective thresholds. This is useful when links have different thresholds, such as if one or more links have a low threshold so that they can be more responsive for use as a link for handling control signals for example. This helps keep these links from becoming congested unnecessarily while still being available for MLPPP bundles. If at step  306 , it is determined that no links have a queue depth below their respective threshold, then the process continues to step  310  with the list of links intact. That is, all the links from step  302  are still in consideration. 
         [0025]    At step  310 , the system identifies which links have the lowest queue depth. At step  312  the system determines if more than one link has the lowest queue depth. If only one link has the lowest queue depth then at step  314  it is selected for transmitting the pending packet fragment and at step  316  the system writes the selected link ID to MDM  203  and clears the corresponding polled status bit on MDM  203 . The process ends at step  320 . 
         [0026]    If more than one link has the lowest queue depth, then the process proceeds to step  322  where the system reads the previous selected link and polled status bit link from the MDM memory. At step  324 , the system determines if the previous selected link is still in the list of links having the lowest queue depths (which is a subset of the list of valid links) and if not, the process proceeds to step  326  where the system selects the next link in the list after the previous selected link in round robin fashion, and the process then continues to step  316  previously described above. If at step  324 , the system determines that previous selected link is in the list of links having the lowest queue depths, then the process proceeds to step  328  which determines if the polled status bit is set, and if it is, the process proceeds to step  326  previously described above. If at step  328  it is determined that the polled status bit is not set, then the MDM  203  remains unchanged and the selected link in MDM  203  will be the previous selected link (indicated by box  330 ) and the process stops at step  320 . Thus the process  300  updates the MDM  203  so that when the system needs to assign a pending packet of step  213  to a bundle member or link, the system reads the selected link from the MDM  203 . 
         [0027]    The round robin arbitration process is intended to break ties for load balanced links. The method can have advantages in cases of low traffic and high traffic. In the situation where a bundle has very low traffic, there may be no packets in memory for any of the links of a MLPP bundle. The round robin arbitration process will ensure that each link will have a turn at sending a packet fragment so that traffic load is balanced over all the links. Ensuring a more even distribution of sparse traffic can also accelerate detection of links that are down and reduce the transmission time of a packet when a packet is fragment. 
         [0028]    In high traffic scenarios, packets can be fragmented, i.e. split up into smaller packets or fragments. This fragmentation is supported by MLPPP RFC 1990. In high traffic scenarios with very small time intervals between packets or fragments, the queue depth update mechanism can not always update the whole chain of memories to reflect the actual current queue depths for all the links and it is possible that process  300  could be executed multiple times before the queue depth information for each link can be updated. 
         [0029]    There is value in selecting the least occupied link to avoid introducing unnecessary delays and jitter and possibly even packet loss at the MLPPP on the receiving link in cases where the receiver wouldn&#39;t have enough buffers to process the packets or packet fragments and perform packet re-ordering, depending on the MLPPP receiver on the other end. 
         [0030]    A person of skill in the art would readily recognize that steps of various above-described methods can be performed by programmed computers. Herein, some embodiments are also intended to cover program storage devices, e.g., digital data storage media, which are machine or computer-readable and encode machine-executable or computer-executable programs of instructions, wherein said instructions perform some or all of the steps of said above-described methods. The program storage devices may be, e.g., digital memories, magnetic storage media such as a magnetic disks and magnetic tapes, hard drives, or optically readable digital data storage media. The embodiments are also intended to cover computers programmed to perform said steps of the above-described methods. 
         [0031]    The description and drawings merely illustrate the principles of the invention. It will thus be appreciated that those skilled in the art will be able to devise various arrangements that, although not explicitly described or shown herein, embody the principles of the invention and are included within its spirit and scope. Furthermore, all examples recited herein are principally intended expressly to be only for pedagogical purposes to aid the reader in understanding the principles of the invention and the concepts contributed by the inventor(s) to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions. Moreover, all statements herein reciting principles, aspects, and embodiments of the invention, as well as specific examples thereof, are intended to encompass equivalents thereof. 
         [0032]    The functions of the various elements shown in the Figures, including any functional blocks labeled as “processors”, may be provided through the use of dedicated hardware as well as hardware capable of executing software in association with appropriate software. When provided by a processor, the functions may be provided by a single dedicated processor, by a single shared processor, or by a plurality of individual processors, some of which may be shared. Moreover, explicit use of the term “processor” or “controller” should not be construed to refer exclusively to hardware capable of executing software, and may implicitly include, without limitation, digital signal processor (DSP) hardware, network processor, application specific integrated circuit (ASIC), field programmable gate array (FPGA), read only memory (ROM) for storing software, random access memory (RAM), and non volatile storage. Other hardware, conventional and/or custom, may also be included. Similarly, any switches shown in the FIGS. are conceptual only. Their function may be carried out through the operation of program logic, through dedicated logic, through the interaction of program control and dedicated logic, or even manually, the particular technique being selectable by the implementer as more specifically understood from the context. 
         [0033]    It should be appreciated by those skilled in the art that any block diagrams herein represent conceptual views of illustrative circuitry embodying the principles of the invention. Similarly, it will be appreciated that any flow charts, flow diagrams, state transition diagrams, pseudo code, and the like represent various processes which may be substantially represented in computer readable medium and so executed by a computer or processor, whether or not such computer or processor is explicitly shown. 
         [0034]    Numerous modifications, variations and adaptations may be made to the embodiment of the invention described above without departing from the scope of the invention, which is defined in the claims.