Patent Publication Number: US-10320952-B2

Title: System-wide synchronized switch-over of multicast flows

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Patent Application 62/336,763, filed May 16, 2016, whose disclosure is incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to communication networks, and particularly to methods and systems for synchronized switch-over of multicast flows. 
     BACKGROUND OF THE INVENTION 
     Various communication systems, applications and protocols transmit data to multiple recipients using multicast packet flows. Multicast is used extensively, for example, in media production and distribution applications, and in IP television (IPTV). 
     Several protocols have been specified for managing multicast groups. One such protocol is the Internet Group Management Protocol (IGMP), which is specified by the Internet Engineering Task Force (IETF), for example in “Internet Group Management Protocol, Version 3,” Request For Comments (RFC) 3376, October, 2002, which is incorporated herein by reference. Another example protocol is Multiple MAC Registration Protocol (MMRP). MMRP is specified in IEEE Standard 802.1Q-2005, entitled “IEEE Standard for Local and Metropolitan Area Networks—Virtual Bridged Local Area Networks,” May 19, 2006, which is incorporated herein by reference. 
     One popular protocol for sending media content such as audio and video over Internet Protocol (IP) networks is the Real-time Transport Protocol (RTP). RTP is specified by the IETF in “RTP: A Transport Protocol for Real-Time Applications,” RFC 3550, July, 2003, which is incorporated herein by reference. RTP payload format structure and format, and in particular the RTP timestamp, are specified in “RTP Payload Format for Uncompressed Video,” RFC 4175, September, 2005, which is incorporated herein by reference. 
     SUMMARY OF THE INVENTION 
     An embodiment of the present invention that is described herein provides a network device including multiple ports, for communicating over a communication network, and packet processing circuitry. The packet processing circuitry is configured to receive via the ports packets belonging to a plurality of multicast flows, to receive, for each of the multicast flows, (i) a first configuration that specifies clients that are to receive the multicast flow prior to a specified switch-over time, and (ii) a second configuration that specifies the clients that are to receive the multicast flow after the specified switch-over time, to forward the multicast flows via the ports in accordance with the first configuration, to extract from a field in at least one of the packets a value that is indicative of the switch-over time, and, based on the extracted value, to switch-over forwarding of the multicast flows from the first configuration to the second configuration simultaneously at the switch-over time. 
     In some embodiments, the multicast flows include Real-time Transport Protocol (RTP) media flows, and the extracted value includes an RTP timestamp. In an embodiment, the multicast flows convey video frames, and the packet processing circuitry is configured to determine a video-frame boundary based on the extracted value, and to switch-over to the second configuration at the video-frame boundary. 
     In a disclosed embodiment, the packet processing circuitry is configured to extract the value from a single selected multicast flow, and to switch-over all the multicast flows to the second configuration at the switch-over time indicated by the single selected multicast flow. In an example embodiment, the packet processing circuitry is configured to determine the switch-over time, and to switch-over to the second configuration, irrespective of time synchronization with any other network device. 
     In example embodiments, the packet processing circuitry includes a table, which holds a rule that triggers a switch-over to the second configuration based on the extracted value. In an embodiment, the packet processing circuitry is configured to delete the rule from the table after the switch-over to the second configuration has been triggered. 
     In some embodiments, the packet processing circuitry is configured to forward the multicast flows by looking-up a table, which points to the first configuration prior to the switch-over time and to the second configuration after the switch-over time. In an embodiment, in response to a decision to switch-over to the second configuration, the packet processing circuitry is configured to update the table to point to the second configuration. 
     There is additionally provided, in accordance with an embodiment of the present invention, a method including receiving in a network device packets belonging to a plurality of multicast flows. First and second configurations are received in the network device for each of the multicast flows. The first configuration specifies clients that are to receive the multicast flow prior to a specified switch-over time, and the second configuration specifies the clients that are to receive the multicast flow after the specified switch-over time. The multicast flows are forwarded in accordance with the first configuration. A value, which is indicative of the switch-over time, is extracted from a field in at least one of the packets. Based on the extracted value, forwarding of the multicast flows is switched-over from the first configuration to the second configuration simultaneously at the switch-over time. 
     The present invention will be more fully understood from the following detailed description of the embodiments thereof, taken together with the drawings in which: 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram that schematically illustrates a media production system, in accordance with an embodiment of the present invention; 
         FIG. 2  is a block diagram that schematically illustrates a network device, in accordance with an embodiment of the present invention; 
         FIG. 3  is a flow chart that schematically illustrates a method for synchronized switch-over of multicast flows, in accordance with an embodiment of the present invention; and 
         FIGS. 4A-4C  are diagrams that schematically illustrate an implementation of multicast flow switch-over in a network device, in accordance with an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     Overview 
     Embodiments of the present invention that are described herein provide improved methods and systems for synchronized switch-over of multicast (MC) flows in communication networks. The embodiments described herein refer mainly to switch-over of MC flows that carry media content, such as in media production applications. The disclosed techniques, however, are applicable to other applications and use-cases, as well. 
     In some embodiments, a media production system comprises a first plurality of media sources that produce MC flows, and a second plurality of media clients that consume the MC flows. The MC flows carry video content, i.e., sequences of video frames. The sources and clients communicate over a packet network that comprises one or more network devices, such as switches or routers. At a given point in time, the system forwards each MC flow to a respective group of clients designated to receive that flow. For this purpose, each network device is provided in advance with a “MC configuration,” which specifies the respective group of clients designated to receive each MC flow. 
     In some practical scenarios it is desirable to switch-over the entire system, synchronously, from the current MC configuration to a different MC configuration. Switching-over to a different MC configuration involves modifying the designated client group for at least one MC flow. Synchronizing the switch-over is advantageous, for example, for minimizing visible glitches and other undesired artifacts in the video displayed at the clients. 
     In some embodiments of the present invention, the system performs a switch-over process that is synchronized in two respects. At the individual MC flow level, the network devices switch-over between MC configurations at a boundary between video frames, not during a video frame. At the system level, the network devices synchronize the switch-over between MC configurations to the same video-frame boundary in all the MC flows. System-wide synchronization is advantageous, for example, in multi-screen clients that simultaneously display multiple video streams originating from multiple media sources. 
     In some embodiments, each network device determines the switch-over time by examining the packets in at least one of the MC flows, and extracting from the packets a value of a field that is indicative of the switch-over time. For example, when the media content in each MC flow is sent using Real-time Transport Protocol (RTP), the network device may examine the RTP timestamp field, which is time-dependent and is incremented at the video-frame boundary. 
     In a typical embodiment, a central controller sends the network devices a switch-over command that is indicative of the desired switch-over time, and possibly of the new MC configuration. The switch-over command may be sent, for example, several tens of milliseconds before the desired switch-over time, in order to allow sufficient time for the network devices to pre-configure the new MC configuration in preparation for the switch-over. 
     In these embodiments, the media sources are assumed to be time-synchronized with one another, e.g., using the Precision Time Protocol (PTP). Under this assumption, if each network device switches-over the MC configuration when the RTP timestamp is first incremented following reception of the switch-over command, the switch-over is guaranteed to be synchronized system-wide. Note that in this solution, it is sufficient for each network device to examine the RTP timestamps only in one of the multiple MC flows it handles. 
     The disclosed techniques enable an entire system, comprising multiple media sources, multiple media clients and multiple network devices, to perform system-wide synchronized switch-over between MC configurations. This synchronization is achieved without any requirement to time-synchronize the network devices with one another. Moreover, the disclosed switch-over process does not involve any overlap period during which a client has to receive MC flows belonging to both the old and the new MC configurations. The media clients are therefore not required to reserve extra bandwidth or other resources to support such an overlap. 
     System Description 
       FIG. 1  is a block diagram that schematically illustrates a media production system  20 , in accordance with an embodiment of the present invention. System  20  comprises multiple media sources  24  and multiple media clients  28 , which communicate over a communication network  32 . 
     Sources  24  may comprise, for example, video cameras, video players or any other suitable type of equipment that generates media. Clients  28  may comprise, for example, video displays, multi-screen displays, computers that process video, video recorders, mixers, or any other suitable type of equipment that consumes media. 
     Network  32  may comprise any suitable type of packet network, for example a Local-Area Network (LAN) that operates in accordance with a suitable protocol such as Ethernet or Infiniband. Typically, network  32  comprises an Internet Protocol (IP) network. Network  32  comprises one or more network devices  36 , e.g., switches or routers. The network devices are connected to one another, and to the media sources and media clients, using physical network links  40 . Links  40  may comprise, for example, copper links or optical fibers. 
     Each media source  24  generates one or more media flows. Each media flow comprises a stream of multicast (MC) packets, which carries a sequence of video frames. The video-frame rate is considerably slower than the packet rate, i.e., each video frame spans multiple packets. The media flows are also referred to herein as MC flows, and the two terms are used interchangeably. 
     System  20  further comprises a central controller  44 , typically implemented using a workstation or other suitable computer. Among other tasks, central controller  44  specifies the distribution of media flows from sources  24  to clients  28 . A given MC flow may be distributed to a group of one or more clients  28 . A given client  28  may receive one or more flows, originating from one or more sources  24 . The system-wide configuration, which specifies the respective group of clients designated to receive each MC flow, is referred to herein as a “MC configuration.” 
     In some embodiments, central controller  44  defines the MC configuration to be applied in system  20  at any given time. In addition, central controller  44  sends instructions to network devices  36  to switch-over from one MC configuration to another. Methods for synchronized switch-over between MC configurations, which are carried out by network devices  36 , are described in detail below. 
       FIG. 2  is a block diagram that schematically illustrates a simplified internal structure of a network device  36 , in accordance with an embodiment of the present invention. In the present example the network device comprises a packet switch. The switch comprises multiple ports  48  for connecting to respective links  40 , and a configurable switch fabric  52  that forwards packets between ports  48 . The switch further comprises a switch controller that configures fabric  52  and generally manages the operation of the switch. In some embodiments, switch controller  56  receives MC configurations from central controller  44 , and configures fabric  52  to forward the MC flows according to the received MC configurations. In particular, switch controller  56  performs the MC configuration switch-over processes described below. 
     The configurations of system  20  and network device  36  shown in  FIGS. 1 and 2  are example configurations that are depicted purely for the sake of conceptual clarity. In alternative embodiments, any other suitable network and/or network device configuration can be used. System and network-device elements that are not mandatory for understanding of the disclosed techniques have been omitted from the figures for the sake of clarity. 
     The different elements of network devices  36  may be implemented using any suitable hardware, such as in an Application-Specific Integrated Circuit (ASIC) or Field-Programmable Gate Array (FPGA). In some embodiments, some elements of network devices  36  can be implemented using software, or using a combination of hardware and software elements. 
     In the context of the present patent application and in the claims, fabric  52  and controller  56  are referred to as packet processing circuitry that carries out the disclosed techniques. In alternative embodiments, the packet processing circuitry can be implemented in any other suitable manner. 
     In some embodiments, central controller  44 , and switch controllers  56  of network devices  36 , comprise general-purpose processors, which are programmed in software to carry out the functions described herein. The software may be downloaded to the processors in electronic form, over a network, for example, or it may, alternatively or additionally, be provided and/or stored on non-transitory tangible media, such as magnetic, optical, or electronic memory. 
     System-Wide Synchronized Switch-Over of Multicast Flows 
     In some embodiments, network devices  36  switch-over to a different MC configuration at a specified switch-over time, which is synchronized to a specific video-frame boundary across the entire system  20 . An underlying assumption is that media sources  24  are time-synchronized with one another with regard to the video-frame timing. In other words, it is assumed that the boundary between the video frames occurs at the same time in all media sources. 
     In a real-life implementation, the synchronization of the video-frame boundary between different MC flows may vary slightly, e.g., due to residual timing offsets between media sources and processing-time differences in network  32 . Typically, although not necessarily, such synchronization offsets are on the order of several tens of microseconds. Thus, when switching from one MC configuration to another in a network device  36 , a particular MC flow may experience a transient of this size. Media clients  28  are typically configured to handle such transients with little or no noticeable degradation in video quality. 
     In some embodiments, each MC flow in system  20  comprises an RTP flow as specified, for example, in RFC 3550, cited above. In particular, the packets of each MC flow comprise an RTP timestamp as specified, for example, in RFC 4175, cited above. The RTP timestamp serves as a sequence number for the video frames, and is incremented at the video-frame boundary. In these embodiments, controllers  56  of network devices  36  identify the boundary between successive video frames by detecting that the RTP timestamp is incremented. The RTP timestamp is located at a known fixed offset from the beginning of the packet, specified in RFC 4175, and controllers  56  use this offset to access the appropriate location in the packets. 
     In these embodiments, central controller  44  decides that a switch-over to a new MC configuration is to be performed system-wide at a certain time. Having decided on the desired switch-over time, central controller  44  sends a switch-over command to network devices  36 , instructing them to perform the switch-over. The switch-over command typically specifies the time remaining until the switch-over time, or any other indication that enables the network devices to choose the correct video-frame boundary at which to perform the switch-over. In an example embodiment, the central controller sends the switch-over command a predefined time period prior to the desired switch-over time, e.g., 60 mS ahead. The network devices are aware of this time period, and can therefore select the correct video-frame boundary at which to perform the switch-over to the new MC configuration. 
     In some embodiments, the switch-over command may also specify the new MC configuration to be applied. Alternatively, the new MC configuration may be provided to the network devices at any other suitable time and in any other suitable way. 
       FIG. 3  is a flow chart that schematically illustrates a method for switch-over of MC flows, in accordance with an embodiment of the present invention. This method is typically carried out by each network device  36  in system  20 , independently of the other network devices and regardless of any time synchronization with any other network device. The assumption is that, when appropriate, central controller  44  issues a switch-over command that notifies network devices  36  of the desired switch-over time (desired video-frame boundary) and the new MC configuration. 
     The method begins with network device  36  processing multiple MC flows, at a processing step  60 . Typically, network device  36  receives multiple MC flows via ports  48 . Switch fabric  52  forwards each MC flow to its designated group of clients  28  via ports  40 , in accordance with the currently-applicable MC configuration. Fabric  52  is configured with the currently-applicable MC configuration by controller  56 . 
     At a timestamp extraction step  64 , controller  56  in network device  36  extracts the RTP timestamps from one or more packets in one or more of the MC flows. In an example embodiment, controller  56  configures fabric  52  to temporarily mirror the packets of one of the MC flows to controller  56  for examination. Controller  56  then extracts the RTP timestamps from this flow. The RTP timestamp extraction is performed in parallel to the normal forwarding of the MC flow by fabric  52 . 
     Since the video-frame boundaries are assumed to be synchronized (up to some finite offset as explained above) among media sources  24  (and therefore among the various MC flows), it is sufficient for controller  56  to examine the RTP timestamps of only a single MC flow. The actual value of the RTP timestamp may vary slightly from one MC flow to another. 
     At a checking step  68 , fabric  52  checks whether the RTP timestamp(s) extracted at step  64  above indicate that a switch-over of MC configuration is to be performed. If no switch-over is to be performed, the method loops back to step  60 . Otherwise, i.e., if switch-over is to be performed, fabric  52  begins to process the MC flows in accordance with the new MC configuration, as instructed by central controller  44 . The method then loops back to step  60 . 
     The method of  FIG. 3  is an example method that is depicted purely for the sake of conceptual clarity. In alternative embodiments, network devices  36  may use any other suitable method for carrying out synchronized switch-over of the MC configuration. For example, alternatively to examining RTP timestamps, controllers  56  may examine any other suitable packet field value that is indicative of video-frame boundary. 
       FIGS. 4A-4C  are diagrams that schematically illustrate an example implementation of MC flow switch-over in network device  36 , in accordance with an embodiment of the present invention. In the present example, a MC flow denoted “Flow A” enters the network device at an egress port denoted “Port 1 ”. In accordance with the initial MC configuration, shown in  FIG. 4A , Flow A is forwarded to a single egress port denoted “Port 2 ”. At a time denoted “T”, the network device switches-over to a new MC configuration, shown in  FIG. 4C , in which Flow A is forwarded to two egress ports denoted “Port 3 ” and “Port 4 ”.  FIG. 4B  illustrates an intermediate stage during the switch-over process, which is described below. Transitioning between the three stages is decided and performed by controller  56  of the network device. 
     In the example implementation of  FIGS. 4A-4C , the packet processing circuitry of the network device comprises three tables denoted “Match_table_ 1 ”, “Match_table_ 2 ” and “Flood_table”. The Flood_table holds the definitions of the two MC configurations (the initial MC configuration applicable up to time T, and the new MC configuration applicable from time T). Match_table_ 1  holds a rule that checks whether the current time t is equal to or greater than the switch-over time T (“t≥T?”). Match_table_ 2  holds the parameters (e.g., 5-tuple) of Flow A, for identifying the packets belonging to Flow A and forwarding them in accordance with the currently-applicable MC configuration. 
     The three tables, and the logic that accesses the tables and compares table entries with packet attributes, may be implemented, for example, as part of fabric  52 . The tables are populated and otherwise managed by controller  56 . It should be noted, however, that the implementation shown in  FIGS. 4A-4C  is depicted purely by way of example and that the functionality carried out by the tree tables can be implemented in various other ways. 
     Before time T, as seen in  FIG. 4A , the entry that matches Flow A in Match_table_ 2  points to the initial MC configuration in the Flood_table. Therefore, at this stage, fabric  52  forwards packets belonging to Flow A to Port  3  only. Forwarding in accordance with the initial MC configuration continues as long as the time comparison in Match_table_ 1  concludes that t&lt;T. 
     When the time comparison in Match_table_ 1  first detects that t≥T, controller  56  transitions to the intermediate stage seen in  FIG. 4B . (In an embodiment, controller  56  of network device  36  is notified by a mirrored packet that fabric  52  has started switching in accordance with Match_table_ 1 .) The intermediate stage of  FIG. 4B  typically spans the time interval T&lt;t≤T+Δ. During the intermediate stage, Match_table_ 1  has precedence over Match_table_ 2 . At this stage controller  56  updates the entry of Flow A in Match_table_ 2  to point to the new MC configuration in the Flood_table. During the intermediate stage, fabric  52  already forwards packets belonging to Flow A in accordance with the new MC configuration, i.e., to both Port  3  and Port  4 . 
     Once the update of Match_table_ 2  is completed, controller  56  proceeds to the stage shown in  FIG. 4C . The entry that matches Flow A in Matching table  2  points to the new MC configuration in the Flood_table. Fabric  52  thus forwards the packets of Flow A in accordance with the new MC configuration. 
     As can also be seen in  FIG. 4C , at this stage controller  56  has deleted the rule “t≥T?” from Match_table_ 1 . Since time T has passed, this rule is obsolete and there is no need to evaluate it any longer. Typically, Match_table_ 1  is expensive both in terms of cost and size and in terms of lookup complexity, since its rules involve examination of the packet payload. As such, it is highly desirable to remove obsolete rules as soon as possible, in order to keep the table as small as possible and minimize the number of rule evaluation operations. 
     Although the embodiments described herein mainly address MC flows that carry media content, such as in media production and IPTV applications, the methods and systems described herein can also be used in other applications that can benefit from synchronized switch-over of MC flows, such as financial/algorithmic trading systems. 
     It will thus be appreciated that the embodiments described above are cited by way of example, and that the present invention is not limited to what has been particularly shown and described hereinabove. Rather, the scope of the present invention includes both combinations and sub-combinations of the various features described hereinabove, as well as variations and modifications thereof which would occur to persons skilled in the art upon reading the foregoing description and which are not disclosed in the prior art. Documents incorporated by reference in the present patent application are to be considered an integral part of the application except that to the extent any terms are defined in these incorporated documents in a manner that conflicts with the definitions made explicitly or implicitly in the present specification, only the definitions in the present specification should be considered.