Network multicasting

In general, in one aspect, the disclosure describes a technique of determining forwarding information for at least a sub-set of members of a multi cast group, and sending, toward a downstream entity, at most a single copy of data to be multicasted to the sub-set of members and the determined forwarding information.

BACKGROUND

Networks enable computers and other devices to communicate. For example, networks can carry data representing video, audio, e-mail, and so forth. Typically, data sent across a network is divided into smaller messages. The structure and contents of the messages depend on the networking technology being used. For example, Internet Protocol (IP) datagrams include a destination address that is much like an address written on the outside of an envelope. Devices, known as routers, receive datagrams and can determine how to forward the datagram further toward its destination based on the destination address. Another network technology is known as Asynchronous Transfer Mode (ATM). In ATM, data messages, known as cells, include identification of a “circuit” that leads from the sender to the receiver. That is, rather than identifying a destination, an ATM cell identifies a path connecting the sender and receiver.

To complicate matters conceptually, IP and ATM can both be used to support the other technology. For example, an IP datagram can be divided across different ATM cells. A receiver can reassemble the IP datagram after receiving the ATM cells.

IP datagrams and ATM cells are examples of protocol data units (PDUs). A PDU includes a payload and header. The data in the header is often used by network protocols in handling the PDU (e.g., determining where to forward the PDU, whether transmission errors occurred, and so forth). Other examples of PDUs include frames (e.g., Ethernet and Synchronous Optical Network (SONET) frames) and segments (e.g., Transmission Control Protocol (TCP) segments).

A sender can send data to a single receiver. This is known as “unicasting”. Alternately, a sender (or multiple senders) can send the same data to multiple members of a group. For example, a sender can send streaming video data to many different group members located at different points in the network. This is known as “multicasting”.

Different protocols support multicasting in different ways. For example, in the Internet Protocol, instead of specifying a single destination, an IP datagram can specify a group address. ATM networks may also support multicasting in a variety of ways. For example, multicasting may be provided by a “tree” of circuits where a given circuit may feed diverging circuits downstream. Again, the above is merely a sampling of a wide variety of approaches to multicasting.

DETAILED DESCRIPTION

Multicasting can increase traffic handled by network devices. For example, where paths to different multi cast group members diverge, a device may need to construct multiple protocol data units (PDUs) (e.g., IP datagrams or ATM cells) having copies of the same multicast data, but destined for different group members.FIG. 1illustrates a technique that can potentially reduce traffic associated with multicasting. In particular, the technique shown can delay replication of multi cast data to reduce traffic between a device100and a downstream device106. As shown, instead of carrying multiple PDUs storing copies of the same multi cast data104between devices100and106,FIG. 1depicts a scheme where device100transmits one copy of the multi cast data104with control data102. The downstream device106, in turn, generates PDUs carrying the multi cast data and transmits the generated PDUs via the appropriate egress interfaces (e.g., links to remote network devices). The technique illustrated inFIG. 1may not only reduce traffic between devices100and106, but may also offload duties from device100. For example, construction of the out-bound PDUs by the downstream device106can conserve resources of device100.

In greater detail, the sample system ofFIG. 1includes a device100, such as a network processor or an Application-Specific Integrated Circuit (ASIC), that processes PDUs received over a network. The processing can include a determination of how to forward a PDU. For example, a network processor may be programmed to determine which egress interface(s) (labeled “port 1”-“port n”) of a downstream device106to send out received multi cast data. The downstream device106shown inFIG. 1may be local to device100. For example, the device106may be a medium access controller (MAC) (e.g., an Ethernet MAC), SONET framer, or other link layer device. Alternately, the downstream device106may be a remote device (i.e., one separated from device100by one or more network connections).

As shown inFIG. 1, the device100transmits multi cast data104(e.g., the payload(s) of one or more received multi cast PDUs) to the downstream device106. The device100also transmits control information102that instructs the downstream device106how to handle multi cast data104. For example, as shown, the control information102includes forwarding information such as identification of the downstream device106egress interfaces that should be used to transmit the multi cast data. In the example shown, the control information102also includes PDU header data. The downstream device106can generate multiple out-bound PDUs108by adding (e.g., pre-pending) the headers to copies of the multi cast data104. The downstream device106can then transmit the generated PDUs108via the identified egress interfaces.

The network device100can store the control information102for a multi cast group to speed PDU processing. For example, the device100may store a table of headers and interfaces to be used for members of different multi cast groups (or sub-groups). Thus, determining the control information102becomes a matter of a fast table lookup. The network device100may be statically provisioned with the control information102or may update the control information102as members join and leave a multi cast group. For example, the network device100may receive group membership data transmitted via IGMP (Internet Group Management Protocol) messages from group members and multi cast routers or switches. Additionally, the device100may also dynamically modify the control information102based on changing network characteristics (e.g., as network connections become congested or go off-line).

Network device100may perform other operations beyond determining the control information102for given multi cast data. For example, the device100can facilitate multicasting of an IP datagram over an ATM network by segmenting the datagram data across different ATM cells. Techniques for associating ATM circuits with IP multi cast groups is described in greater detail in Request For Comments (RFC) 2022 (G. Armitage, Support for Multi cast over UNI 3.0/3.1 based ATM Networks, November 1996).

In the sample scheme depicted inFIG. 1, the limited responsibilities of the downstream device100enable the device to be implemented relatively inexpensively (e.g., as a Field Programmable Gate Array (FPGA) or other circuitry). The division of duties can also conserve resources of the network device100. That is, instead of expending resources replicating PDUs, the device100can devote greater resources to other PDU processing tasks. Again, the technique illustrated above can also reduce traffic between device100and106as the multi cast data104may be transmitted, at most, once for a given multi cast group or sub-group.

FIG. 2illustrates an example of control120and multi cast data122messages sent to the downstream device106. As-shown, both messages120,122include a “type” identifier to distinguish data messages122from control messages120. Both messages also include a multi cast group identifier that enables the downstream device106to pair the messages120,122together.

The data message122includes the data being multi cast. This data can include the payload of a multi cast PDU received by device100and may or may not include portions of the received PDU header. The control message120can include data for the different paths to be taken by copies of the multi cast message. For example, as shown, the message120includes pairs of headers/egress interfaces. For example, the pairs may include Virtual Path Identifiers (VPI) and a Virtual Channel Identifiers (VCI) of an ATM header and port or virtual port identifiers. After receiving the messages120,122the downstream device106can construct the out-bound PDUs by copying the multi cast data and adding the header data. The device120can then output the constructed PDUs via the identified egress interfaces.

The messaging scheme shown inFIG. 2is merely illustrative and many variations of the above can be implemented. For instance, instead of sending two different messages120,122, a single message can be used that includes both control and multi cast data.

Potentially, members of a multi cast group may have different associated data rates. For example, some multi cast group members may have a data rate associated with Digital Subscriber Loop (DSL) connections while others have data rates associated with T1 connections. As shown inFIG. 3, the device100can categorize different multi cast group112members according to their different data rates114. In the example illustrated, multi cast group112xincludes data rate sub-groups114a,114n. As shown, the control data stored for members of a sub-group114a,114ncan include identification (e.g., port numbers or Ethernet addresses) of the downstream egress interfaces used to transmit a multi cast message to the sub-group114members and/or header data used to prepare the out-bound PDUs at the downstream device106. For instance, for sub-group114a, multi cast data110should be output via downstream ports “2”, “1”, and “3”.

As shown, device100can transmit control data114ato the downstream device106for individual data rate sub-groups. Transmitting control information114aand multi cast110data, at most, once per sub-group enables the downstream device106to efficiently handle transmission of the multi cast data. Additionally, when multi cast data is being sent to multiple sub-groups, the device100can flag the multi cast data for storage by the downstream entity106. This can eliminate retransmission of the multi cast data to the downstream entity106for each sub-group. As described below, grouping members by data rate can also potentially ease transmission scheduling and can reduce system complexity.

FIG. 4illustrates a scheme that device100can implement to coordinate efficient handling of multi cast data via the downstream device106. As shown, when data arrives at the device, receive logic130determines whether the data is a unicast or multi cast transmission and enters a request to transmit the data in the appropriate queue132,134. For multi cast data, an entry can be queued in a multi cast queue134for each multi cast sub-group.

A scheduler138determines when the set of egress interfaces used to multi cast to a sub-group are available. At the time scheduled, transmit logic136sends the multi cast and control data to the downstream device. The downstream device106can then generate the specified PDUs and output them via the egress interfaces.

FIG. 5illustrates an example of a scheduling scheme that synchronizes availability of the egress interfaces used by a particular multi cast group or sub-group. The scheme includes a interface vector for each multi cast group or data rate sub-group receiving multi cast data. A bit (labeled “intf #”) within the vector identifies the availability of an egress interface. Initially, the vector may be setup so that bits corresponding to the egress interfaces to be used to transmit multi cast data are set to “0” while the remaining vector bits are set to “1”. In the example shown, the multi cast data will be transmitted via interfaces “1” and “3”.

The scheme also includes an interface “wheel” that identifies when egress interfaces will become available for a transmission. As shown, in a given time-period (e.g., a set of downstream device cycles), illustrated as a pie-slice of the wheel, one or more interfaces may be scheduled for availability. For instance, in time-period “2”144begress interface “1” will become available, for example, after completing transmission of a previously scheduled PDU and awaiting some minimum gap time between transmissions. Thus, for time-period “2”, the bit for interface “1” is set to “1” and the use of interface “1” is reserved (e.g., the interface cannot be scheduled for other unicast or multi cast use until the multi cast data is transmitted). Finally, in time-period “3”144c, egress interface “3” will become available and the bit for interface “3” is set to “1”. Thus, at time-period “3”, all bits in the vector are set. Thus, the needed interfaces have been reserved for the multi cast sub-group. A schedule entry (e.g., the multi cast ID) can be made for the sub-group at the specified time. Other entries identify other scheduled multi cast groups/sub-groups and unicast transmissions. When time-period “3”, arrives, the schedule entry causes the device100to transmit the control and multi cast data to the downstream device.

The interface wheel is continually updated to reflect the future availability of interfaces. Thus, entries for interfaces “1” and “3” can be re-inserted into the wheel after time-period “3” based on the size and data rate of the transmission. Since the sub-groups may be based on data rate, the technique described above can keep the interfaces synchronized after the initial synchronization. That is, since the interfaces are operated at the same rate and may transmit the same amount of data, the interfaces may be scheduled at the same future time-periods. This can ease scheduling operations.

The scenarios illustrated above assumed that a given set of interfaces could be reserved within a given period of time. However, such scheduling may not always be possible. In such cases, the multi cast transmissions to the different interfaces may be enqueued in unicast queues.

Provisioning bandwidth for the multi cast traffic may be performed. Since the constituent ports in the multi cast group may be of different rates, apportioning bandwidth for the multi cast group can ensure the minimum bandwidth requirement at each port to process the multi cast stream. The definition of minimum bandwidth for multi cast traffic may be defined in cases where such traffic contains real-time data (video). For multi cast traffic that is processed at best-effort priority, the scheduler performs a different set of operations compared to multi cast groups that have bandwidth associated with it. The scheduler distinguishes between unicast and multi cast traffic. For multi cast traffic, if a minimum bandwidth is assigned to a multi cast group, a unique ID associated with the multi cast group will be used to populate the scheduler wheel. A schedule entry for unicast traffic will contain an index to the unicast queue and for multi cast traffic, a schedule entry will index into a table of bit-vectors. The significance of the bit-vector is to be able to identify the physical ports that are eligible for transmission. To determine the eligibility of each physical port in the multi cast group, the scheduler will use the physical port's inter-packet/cell gap along with other schedule information of other packets/cells on that port. The scheduler then reconciles the time when the multi cast packet/cell needs to be sent to meet the minimum bandwidth (of the multi cast group's bandwidth) criteria with the physical port bandwidth. For ports that are eligible, the bit-vector associated with the multi cast group will be updated. For ports that cannot be reconciled, the multi cast packet will be en-queued into the unicast queue since such ports will not be able to transmit that multi cast packet along with other ports in the multi cast group.

If multi cast packets/cells are sent with a best-effort priority, the scheduler will determine when ports in the multi cast group become eligible and, based on this information, it will group a subset of multi cast group physical ports for transmission. If such an alignment of transmission time among some subset of the physical ports in the multi cast group is not possible, the multi cast packet is replicated and copied into the respective unicast queues.

Packet/cell replication may be avoided by using a virtual queue concept, where a packet is not removed from a queue until the packet is transmitted among all the members of the multi cast group.

FIG. 6depicts a flow-chart illustrating operation of the sample system described above. After receiving152data to multi cast to a multi cast group, forwarding information for the message is determined154(e.g., egress interfaces of the downstream device and PDU headers). After receiving160the transmitted multi cast156and control data158, the downstream device can construct PDUs and forward162the PDUs via the appropriate egress interfaces based on the control information.

The techniques described above may be used in a wide variety of systems. For example,FIG. 7depicts a programmable network processor200that features multiple packet processors204. The network processor200shown is an Intel® Internet eXchange network Processor (IXP). Other network processors feature different designs.

As shown, the network processor200features an interface202(e.g., an Internet eXchange bus interface) that can carries PDUs between the processor200and network components. For example, the bus may carry PDUs received via physical layer (PHY) components (e.g., wireless, optic, or copper PHYs) and link layer component(s)222(e.g., MACs and framers). The processor200also includes an interface208for communicating, for example, with a host. Such an interface may be a Peripheral Component Interconnect (PCI) type interface such as a PCI-X bus interface. The processor200also includes other components such as memory controllers206,212, a hash engine, and scratch pad memory.

The network processor200shown features a collection of packet processors204. The packet processors204may be Reduced Instruction Set Computing (RISC) processors tailored for network PDU processing. For example, the packet processors may not include floating point instructions or instructions for integer multiplication or division commonly provided by general purpose central processing units (CPUs).

An individual packet processor204may offer multiple threads. The multi-threading capability of the packet processors204is supported by context hardware that reserves different general purpose registers for different threads and can quickly swap instruction and status data for the different threads between context registers and context storage.

The processor200also includes a core processor210(e.g., a StrongARM® XScale®) that is often programmed to perform “control plane” tasks involved in network operations. The core processor210, however, may also handle “data plane” tasks and may provide additional datagram processing threads.

The network processor200may implement the techniques described above in a variety of ways. For example, the control data may be stored as an array in DRAM while different packet processor204and core210threads process received PDUs and implement scheduling logic.

The techniques may be implemented in hardware, software, or a combination of the two. For example, the techniques may be implemented by programming a network processor or other processing system. The programs may be disposed on computer readable mediums and include instructions for causing processor(s) to execute instructions implementing the techniques described above.