GB2238934A

GB2238934A - A copy fabric for a multicast fast packet switch

Info

Publication number: GB2238934A
Application number: GB9026316A
Authority: GB
Inventors: Peter Newman
Original assignee: ADAPTIVE CORP
Current assignee: ADAPTIVE CORP
Priority date: 1989-12-06
Filing date: 1990-12-04
Publication date: 1991-06-12
Anticipated expiration: 2010-12-04
Also published as: GB8927625D0; GB2238934B; GB9026316D0

Abstract

A copy fabric 11 based upon a slotted ring 15 replicates a number of copies of input packets and each copy is then routed to the required destination in a conventional point-to-point fast packet switch. The copy fabric is embodied in a fast packet switch, which can support many diverse forms of telecommunications services across a single integrated network, when communications traffic from a single source is concurrently to be transmitted to and received by many destinations. <IMAGE>

Description

TITLE A COPY FABRIC FOR A MULTICAST FAST PACKET SWITCH BACKGROUND OF THE INVENTION 1. Introduction Fast packet switching, also known as asynchronous transfer mode (ATM), has been proposed as an integrated switching mechanism both in the public broadband ISDN and within broadband private networks, Other areas of application include metropolitan area networks MANs and high speed local area networks LANs. Many fast packet switch designs only support unicast operation, i.e. point-to-point operation, where one incoming packet produces one outgoing packet routed to a single specific destination. There are a number of applications that require the switches of the network to be capable of multicast operation where a single incoming packet produces multiple outgoing packets. Each outgoing copy of the multicast packet contains the same information but is routed to a different destination.Such applications include: video conferencing, audio conferencing, entertainment video distribution, the interconnection of local area networks (LANs) and some aspects of distributed data processing.

The simplest means to achieve multicast operation is for the source to send multiple copies of each multicast packet to each destination, one after the other. This simple technique has several major drawbacks. The delay between the generation of the first copy of each multicast packet and the last will be large. The source must waste valuable time individually transmitting many copies of the same packet. Also, network resources such as bandwidth would be more efficiently utilized if copies of multicast packets were not generated until later nodes in the network wherever possible. A slightly more efficient approach uses a dedicated server at each switching node to make the required number of copies of every incident multicast packet and to route each copy individually to its required destination.The copying and routing is still performed in series, however, thus a large delay may still exist between the generation of the first copy and the last. This approach is therefore not suitable for the multicast operation of real-time traffic such as voice or video and may not be fast enough for some distributed computing applications. It may even prove unsuitable for the interconnection of high speed LANs via bridges that use broadcast messages to locate unknown destinations.

For such applications the operations of copying and routing must be performed in parallel, i.e. all of the copies of each incident multicast packet must be copied and routed concurrently.

Also, many different multicast packets from separate switch ports should be capable of being handled at the same time. For fast packet switches that use a single path switch fabric the operations of copying and routing the multicast packets may be implemented in the same switch fabric that handles the switching of the unicast traffic, but such switches may not be constructed with a total switch capacity above a few Gbits/sec. Fast packet switch designs that usa a multi-stage switch fabric with output buffered switching elements may alsc implement multicast operation within the same switch fabric that performs the routing function but this makes the design of the switching element very complex. Also, non-buffered multi-stage switch fabrics exist that can handle both unicast and multicast traffic within a single witch fabric, e.g. the Richards network [4] and also [2].

In general, however, these tend to be more suited to synchronous transfer mode (STM) with centralized control than for the distributed control that is essential for a high capacity fast packet (ATM) switch.

A number of multicast packet systems and interconnection networks have been described in the prior art including the following.

[1.] R.G. Bubernik and J.S. Turner. Performance of a broadcast packet switch. IEEE Trans. Commun., 37 (1), 60-69, Jan. 1989.

[2.] C.T. Lea. A new broadcast switching network.

IEEE Trans. Commun., 36 (10), 1128-1137, Oct. 1988.

[3.] T.T. Lee. Nonblocking copy networks for multicast packet switching. IEEE J. Select Areas in Commun., 6 (9), 14551467, Dec. 1988.

[4.] G.W. Richards and F.K. Hwang. A two-stage rearrangeable broadcast packet switching network. IEEE Trans.

Commun., 33 (10), 1025-1035, Oct. 1985.

[5.] J.S. Turner. Design of a broadcast packet switching network. IEEE Trans. Commun., 36 (6), 734-743, June 1988.

The most general solution (which is well known in the art) is shown in FIG.1. A unicast (point-to-point) fast packet switch 10 is preceded by a copy fabric 11 and a set of multicast controllers 12-1, ..., 12-N. The multicast controllers 12 add a copy tag to each of the incoming packets. The copy tag defines the number of copies of the packet that are required. The copy fabric 11 then generates the required number of copies of each packet in parallel and each copy exits from the copy fabric on a separate one of the output ports, 13-1, ..., 13-N, at approximately the same time. All packets that exit from the copy fabric 11, whether originally unicast or multicast, may now be handled in the same manner by the unicast switch 10 and routed to their respective destinations.

2. Existing Copy Fabric Designs Two designs of copy fabric have been proposed in the literature both based upon a banyan network constructed from 2x2 broadcast switching elements that have the ability to produce either a single copy or two copies of an incident packet.

Turner's switch [5,1] uses buffered nodes in his copy fabric.

A fanout field, appended to the front of each packet, specifies the required number of copies and each node in the network uses this field to determine whether to produce a single copy of each incident packet or two copies. The copy fabric is blocking in that incident packets may contend for the same resources (nodes and links) within the copy fabric therefore each node must be buffered and a busyback signal is used between nodes in each stage of the network. Due to the buffering, each copy of the packet is not guaranteed to exit the copy fabric at the same time and there is some randomness built into the network as to the outputs of the network from which each copy will emerge.

Lee's copy fabric [3] is also based upon a banyan network but is non-blocking and also non-buffered. The fanout field at the head of each incident multicast packet is presented to a running adder network which computes a range of contiguous output ports from which the required number of copies will emerge. This address range is prefixed to the packet as a minimum and maximum output address. All packets are applied to the copy fabric in alignment and from the minimum and maximum address fields each node may decide on which output to route the packet or whether to produce two copies. In this copy fabric, all copies will exit the fabric at exactly the same time but the ports upon which they will exit is dependent upon the other traffic within the network.

Furthermore, for Lee's copy fabric to work, active ports may not be interspersed with inactive ports thus a concentration fabric is required prior to the copy fabric.

The above examples of a copy fabric do not offer simple implementation in current gate array technology. They are also difficult to partition into a single component that may be replicated in order to implement copy fabrics of any size. There are many applications that do not require the extremes of performance that the above examples of copy fabric may be able to offer. Also, many applications cannot justify the investment in custom silicon required to implement the above copy fabrics.

Although proposals are known in the art for the use of slotted rings to construct switching fabrics, slotted rings have not been used to form copy fabrics. In a slotted ring used as a switching fabric each packet must exit on a specific output port of the ring depending upon its required destination. For traffic with a random distribution of destinations each packet will on average have to travel half way round the ring to reach the destination it requires. Thus, on average, for a single slot ring no more than two packets maya be serviced for each complete rotation of the slot.

In accord with the above background, there is a need for an improved copy fabric.

SUMMARY The present invention is a copy fabric in which a slotted ring replicates a number of copies of input packets. Typically, each copy from the copy fabric is routed to the required output destination by a conventional point-to-point fast packet switch which supports many diverse forms of telecommunications services across a single integrated network. The copy fabric is used with a fast packet switch when communications traffic from a single source is concurrently to be transmitted to and received by multiple destinations.

The copy fabric of the present invention forms a number of copies of incoming packet signals on input ports. The incoming packet signals have copy tags which specify the number of copies to be formed on the output ports. The copy fabric forms the copies using a slotted ring. The ring includes a plurality of signal path nodes, each node connected between an upstream node and a downstream node where the nodes are adapted to circulate the packet signals, together with copy tags, around the ring.

Each of the nodes has a packet input for receiving incoming packet signals from one of the input ports, has a packet output for providing a copy of the incoming packet signals to one of the output ports, has a ring input for receiving packet signals from an upstream node in the ring, and has a ring output for providing packet signals to a downstream node in the ring.

Each of the nodes in the ring operates by examining copy tags to control the input of the packet signals from the packet input or from the ring input and to control the output of the packet signals to the ring output and/or to the packet output.

Each node examines a copy tag from the ring input and, if the copy tag from the ring input is equal to zero and a packet signal is present at the packet input, copies the packet signal from the packet input to the packet output and decrements the copy tag from the packet input toward zero and transmits the copy tag to the ring output.

Each node examines a copy tag from the ring input and, if the copy tag from the ring input is greater than zero, copies the packet signal from the ring input to the packet output and decrements the copy tag from the ring input toward zero and transmits the copy tag to the ring output.

Each node examines a copy tag from the ring input and, if the copy tag from the ring input is equal to zero and a packet signal is not present at the packet input, transmits the copy tag with a zero value to the ring output.

Each of said nodes decrements the copy tag toward zero each time a packet signal is copied to the packet output.

The copy fabric of the present invention is prefixed to a unicast fast packet (ATM) switch to support multicast operation without necessarily requiring any modification to the unicast switch.

The copy fabric of the present invention has a simple design that can be easily implemented in gate array technology while still offering an acceptable multicast capacity.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 depicts a multicast controller connected to each input port of a copy fabric with the output ports of the copy fabric feeding a unicast switch.

FIG. 2 depicts a slotted ring copy fabric in accord with the present invention.

FIG. 3 depicts a slot structure including a copy tag followed by the packet.

Fig. 4 depicts a single slot ring copy fabric under saturation for one complete rotation of the slot.

FIG. 5 depicts an embodiment in which unicast traffic is routed directly to the unicast switch without passing through the copy fabric.

DETAILED DESCRIPTION In FIG. 2, a copy fabric 11 is based upon a slotted ring 15 having nodes 16-1, ..., 16-N. Each node on the ring 15 supports a single input port 17 of ports 17-1, ..., 17-N and a single output port 13 of ports 13-1, ..., 13-N of the copy fabric 11.

Sufficient storage (typically eight bits or less) is located in each node 16 of the ring 15 for an integer number of slots to circulate on the ring. Each slot is long enough to contain a complete packet and packets must be of fixed length. The packet length will typically be in the region of 32 to 64 bytes which is common for ATM switches.

The slot structure is illustrated in FIG. 3 and consists of the copy tag 18 followed by the packet 19. The copy tag 18 specifies the required number of copies of the packet. The slot structure may be maintained by a single timing source and distributed to each node 16 of the ring 15 as the copy fabric 11 is likely to be implemented within a single cabinet.

Fast packet switching requires that all packets have a header that contains a label. The label defines to which connection each packet belongs. Each multicast controller 12 performs a table look-up on the label of every incident packet and extracts from the table the copy tag 18 associated with the connection to which that packet belongs. The controller 12 prefixes the copy tag 18 to the front of each incoming packet and stores the packets in a first in first out (FIFO) queue.

When a node 16 on the copy fabric 11 has a packet waiting in the FIFO queue of its associated multicast controller 12, it waits for the beginning of the next slot to arrive on the ring.

If that slot is free, the packet is written into that slot or, if the slot is full, the node 16 waits for the next free slot.

A slot is considered free if the copy tag 18 at the head of the slot is zero. As a full slot passes each node on the ring, a copy of the packet is transmitted to the output port 13 of that node 16 and the copy tag 18 at the head of the slot is decremented by one. Thus the required number of copies of the packet 19 will exit from the copy fabric 11, the first copy from the output port 13 of the node 16 at which the packet entered and further copies from consecutive downstream ports. As soon as the required number of copies of the packet have been produced, the copy tag 18 becomes zero hence the slot is free for use by the next node on the ring that has a packet waiting. The copy tag should be removed from each packet on exit from the copy fabric 11 to restore the packet to the format expected by the unicast switch 10.

The use of a slotted ring as a copy fabric, as distinguished from its use as a switching fabric, places no restrictions upon which output ports of the copy fabric the packets must exit.

The only constraint is that each copy of a packet exits the copy fabric on a separate output port. Each copy of a packet may therefore exit the ring on the nearest available output port.

This occurs when each copy of a packet exits the ring on consecutive downstream output ports. Thus under saturation, during a single rotation of a single slot ring, every output port of the ring may produce a copy of a packet. By way of comparison, a slotted ring used as a switching fabric has on average only two ports producing output packets. This operation of the copy fabric will remain whatever the distribution of the incident traffic or the distribution of copy requests in the copy tags. Thus, a slotted ring used as a copy fabric in the above manner has a far greater capacity than a slotted ring used as a switching fabric.

At saturation, a single slot N node ring used as a copy fabric may generate Npacket copies, one on each output port, for every rotation of the slot regardless of the distribution of copy requests or the number of multicast packets served. Due to the fixed slot size, the copy fabric is only designed to handle fixed length packets. Furthermore, the packet length should not be too large else the amount of storage required in each node will increase which will increase the delay between each copy of a multicast packet on exit from the copy fabric. A packet length of 32 or 64 bytes or so is considered reasonable.

Fig. 4 illustrates a single slot ring copy fabric 11 under saturation for one complete rotation of the slot. Although every input 17 is assumed to have a multicast packet waiting to access the ring, only five have gained access to the ring during a single rotation of the slot. Every output 13, however, is busy producing one copy of a packet for every rotation of the slot.

Therefore, the copy fabric is operating at saturation. (The illustration of FIG. 4 suggests that all packets exit the copy fabric with their packet headers aligned, whereas in reality there will be a small delay between the header of each packet due to the storage in each node of the ring.) Unfortunately, at high loads the copy fabric may become unfair in that some busy input ports may 'hog' the ring preventing downstream ports from gaining a fair share of access to the ring. If the bandwidth of the ring can be made sufficiently high, this effect may not become important, otherwise a mechanism that ensures fairness may need to be implemented. One possible mechanism is to use a counterrotating ring containing a single bit per ring node which is used to indicate which downstream nodes have packets to send.

Upstream nodes may therefore take into account the traffic of downstream nodes before inserting their own packets onto the ring. This mechanism might also be used to include priority information into the ring access algorithm to ensure that high priority traffic is serviced before traffic of lower priority.

This mechanism is likely to be important for delay sensitive multicast traffic such as conference voice and video in the presence of delay insensitive traffic. In the unicast switch, the connection to which a packet belongs is identified by the label in the packet header. In general. each input port of a unicast switch allocates its own labels. A multicast packet will be replicated on the copy fabric and identical copies of the packet with identical labels will appear at a number of input ports to the unicast switch. Care must therefore be take to ensure that there is no conflict between the labels of unicast and multicast traffic. A simple method of doing so is to allocate unicast labels from the bottom end of the address space and multicast labels from the top of the address space.A more efficient method is to translate the labels of multicast packets as they pass through the multicast controllers. Thus each port may select both its unicast and multicast labels independently of all other ports and the translation of multicast labels in the multicast controllers permits any conflict between labels to be avoided.

In this simple slotted ring copy fabric, the output port of the copy fabric at which each copy of a multicast packet will exit is fixed and known at the time the connection is established. This operation removes the requirement for label translation at the output of the copy fabric and permits the copy fabric to be used with any design of unicast switch without requiring that the unicast switch be modified. In more complex schemes, e.g. Lee [3], any copy of a multicast packet may emerge at any output port of the copy fabric depending on the current traffic load within the copy fabric. In this case, every output port of the copy fabric requires a translation table that contains a label translation for every copy of each of the multicast labels. This requirement rapidly leads to a very large translation table on every output. of the copy fabric even for moderate sizes of switches.

It is possible that in many applications, most multicast packets will only require a few copies to be made. In this case, if the multicast traffic load is fairly low, a ring much smaller than the number of input ports may be used as a copy fabric.

Each node of the copy fabric would handle the traffic from several input ports and any multicast packets that require more copies than the size of the copy fabric could circulate around the ring more than once. The information in the copy tag of the packet would have to be used to distinguish between multiple copies of the same packet arriving at the same output port of the copy fabric. This would require a simple translation table on each output of the copy fabric.

In the above discussion, it has been assumed that unicast packets (which do not require multiple copies) are handled in the same manner as multicast packets. The copy tag for a unicast packet is set to one and it passes through the copy fabric in exactly the same manner as multicast traffic. It emerges on the output port of the copy fabric at which it entered without further copies being made. It therefore suffers the same delay through the copy fabric as multicast traffic. If the delay performance for unicast traffic is to be optimized it is better if unicast traffic is routed directly to the unicast switch without passing through the copy fabric.

FIG. 5 illustrates one method of achieving this routing.

A larger unicast switch 20 is used and the outputs of the smaller copy fabric 21 feed into separate inputs 27 on the unicast switch 20. The copy fabric 21 does not need to be the same size as the number, N, of input ports. A better distribution of traffic across the unicast switch 20 is also offered by this design. If it is undesirable to use a larger unicast switch 20 than the number of external input and output ports, the unicast and multicast traffic may be recombined after the copying operation if the appropriate label translation is provided on the output of the copy fabric 21.

It will be appreciated that the above-described and illustrated exemplary embodiments may be modified in various ways within the scope of the invention hereinbefore defined.

Claims

What is claimed is:

1. A copy fabric for forming a number of copies of incoming packet signals comprising: a plurality of input ports for receiving said incoming packet signals with copy tags specifying the number of copies to be formed, a plurality of output ports1 a ring including a plurality of signal path nodes, each node connected between an upstream node and a downstream node for circulating packet signals including copy tags, each of said nodes having, a packet input for receiving incoming packet signals from one of said input ports, a packet output for providing a copy of said incoming packet signals to one of said output ports, a ring input for receiving packet signals from an upstream node, a ring output for providing packet signals to a downstream node, control means for examining copy tags to control input of packet signals from the packet input or from the ring input and to control output of the packet signals to the ring output and/or to the packet output.

2. The copy fabric of Claim 1 wherein said control means in each node examines a copy tag from the ring input and, if the copy tag from the ring input is equal to zero and a packet signal is present at the packet input, copies the packet signal from the packet input to the packet output and decrements the copy tag from the packet input toward zero and transmits the copy tag to the ring output.

3. The copy fabric of Claim 1 wherein said control means in each node examines a copy tag from the ring input and, if the copy tag from the ring input is greater than zero, copies the packet signal from the ring input to the packet output and decrements the copy tag from the ring input toward zero and transmits the copy tag to the ring output

4. The copy fabric of Claim 1 wherein said control means in each node examines a copy tag from the ring input and, if the copy tag from the ring input is equal to zero and a packet signal is not present at the packet input, transmits the copy tag with a zero value to the ring output.

5. The copy fabric of Claim 1 wherein in each of said nodes, said control means decrements the copy tag toward zero each time a packet signal is copied to the packet output.

6. The copy fabric of Claim 1 wherein said ring is a slotted ring having a head of slot and said control means for each node operates on the head of slot to control said input and output of packet signals.

7. The copy fabric of Claim 6 wherein said copy fabric includes N nodes and said control means controls input of up to N packets into said ring.

8. The copy fabric of Claim 6 wherein said ring includes a plurality of slots for circulating packets and each slot includes a head of slot.

9. The copy fabric of Claim 6 wherein said ring includes N nodes and said ring circulates up to N packets.

10. The copy fabric of Claim 1 wherein said ring includes N nodes and up to N copies of an incoming packet are formed by said N nodes, each copy appearing at a different one of said output ports.

11. The copy fabric of Claim 1 wherein said ring includes N nodes and any number of copies of an incoming packet are formed by said N nodes, said copies appearing at one or more of said output ports a multiple number of times.

12. The copy fabric of Claim 10 wherein each said different one of said output ports is a downstream port from the port of an incoming packet.

13. The copy fabric of Claim 10 wherein an input packet from any one of said input ports may be copied to any number of downstream output ports.

14. The copy fabric of Claim 13 wherein each of said output ports are consecutive downstream ports from the port of an incoming packet.

15. The copy fabric of Claim 1 wherein said ring is a slotted ring having a head of slot and wherein, in each of said nodes, said control means operates to decrement the copy tag toward zero each time a packet signal is copied to the packet output and operates at the head of slot to control said input of packet signals from the packet input when said copy tag is zero.

16. A copy fabric for forming a number of copies of incoming packet signals comprising: a plurality of input ports for receiving said incoming packet signals with copy tags specifying the number of copies to be formed, a plurality of output ports, two or more rings, each ring including a plurality of signal path nodes, each node in each ring connected between an upstream node and a downstream node for circulating packet signals including copy tags, each of said nodes having, a packet input for receiving incoming packet signals from one of said input ports, a packet output for providing a copy of said incoming packet signals to one of said output ports, a ring input for receiving packet signals from an upstream node, a ring output for providing packet signals to a downstream node, control means for examining copy tags to control input of packet signals from the packet input or from the ring input and to control output of the packet signals to the packet output and/or to the ring output.

17. The copy fabric of Claim 16 wherein said control means in each node examines a copy tag from the ring input and, if the copy tag from the ring input is equal to zero and a packet signal is present at the packet input, copies the packet signal from the packet input to the packet output and decrements the copy tag from the packet input toward zero and transmits the copy tag to the ring output.

18. The copy fabric of Claim 16 wherein said control means in each node examines a copy tag from the ring input and, if the copy tag from the ring input is greater than zero, copies the packet signal from the ring input to the packet output and decrements the copy tag from the ring input toward zero and transmits the copy tag to the ring output.

19. The copy fabric of Claim 16 wherein said control means in each node examines a copy tag from the ring input and, if the copy tag from the ring input is equal to zero and a packet signal is not present at the packet input, transmits the copy tag with a zero value to the ring output.

20. The copy fabric of Claim 16 wherein in each of said nodes, said control means decrements the copy tag toward zero each time a packet signal is copied to the packet output.

21. The copy fabric of Claim 16 wherein said rings include a first ring circulating packets in a first direction and include a second ring circulating packets in a second direction opposite to said first direction, and wherein for each node, said packet input transmits incoming packet signals to either said first ring or said second ring.

22. A fabric for forming a number of copies of incoming packet signals from fabric inputs and switching the copies to fabric outputs comprising: a copy fabric including, a plurality of copy input ports for receiving said incoming packet signals from fabric inputs with copy tags specifying the number of copies to be formed, a plurality of copy output ports, two or more rings, each ring including a plurality of signal path nodes, each node in each ring connected between an upstream node and a downstream node for circulating packet signals including copy tags, each of said nodes having, a packet input for receiving incoming packet signals from one of said copy input ports, a packet output for providing a copy of said incoming packet signals to one of said copy output ports, a ring input for receiving packet signals from an upstream node, a ring output for providing packet signals to a downstream node, control means for examining copy tags to control input of packet signals from the packet input or from the ring input and to control output of the packet signals to the packet output and/or to the ring output, a switch fabric having switch fabric inputs for receiving the copy packet outputs from said copy fabric.

23. The fabric of Claim 22 wherein fabric inputs connect through multicast controllers to a first group of switch fabric inputs for said switch fabric and to said copy fabric inputs, and wherein said copy fabric outputs connect to a second group of switch fabric inputs whereby copies of packets on said fabric inputs are input to said switch fabric.

24. A copy fabric constructed and arranged substantially as herein particularly described with reference to the accompanying drawings.