[RPRWG] RPRWG: Buffer insertion rings operate per definition as cut-thro

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INSERTION-BUFFER RINGS OPERATE PER DEFINITION AS CUT-THROUGH

Unfortunately, all emails on the issue cut-through are discussing scheduling. These discussions have nothing to do with the difference between cut-through and store-and-forward! Buffer-insertion rings are always cut-through. Also Cisco uses cut-through, even when they call it wrongly store-and-forward. This is the origin of this confusion. If they really would apply store-and-forward, the performance would be disastrous. Thus, in the May meeting the voting decision on this issue can only be cut-through! How to schedule between buffers is an orthogonal issue.

Buffer-Insertion rings were first used in the late 70’s. The only purpose to use an insertion buffer in the transmission path is to hold up the data stream that already is on the ring, when the node itself pulses out it own packet from its transmit buffer. Otherwise, both transmissions would overlap and both packets would be destroyed as in CSMA/CD. Assuming an empty insertion-buffer at the beginning of the node’s transmission, an upstream packet on the ring is only buffered during the collision time. Thus, the filling of the insertion buffer is not necessary a complete packet. Assuming that the node has no other packets to transmit, then the possibly partly buffered packet on the ring is immediately pulsed out on the transmission link. The additional insertion-buffer delay given by the amount of data that had to be hold up is then experienced by all passing packets until the insertion buffer can be emptied during the absence of data on that part of the ring. This is cut-through! In case of store-and-forward, first the complete packet on the ring must be buffered in the insertion buffer and then the decision is made to relay it to the next node. This would mean that in case that none of the k nodes between source and destination has a packet to transmit, one has a principal and unnecessary delay corresponding to k times the size of the packet. This would correspond to the degradation of the RPR to a ring of store-and-forward Ethernet switches.

Scheduling between insertion buffers and transmit buffers comes only into play when the insertion buffer (or one of the insertion buffers in case of priorities) has buffered at least one single data unit of a ring packet. The scheduling sequence could then for instance be: first insertion buffer of priority 1, then transmit buffer of priority 1, then insertion buffer of priority 2, then transmit buffer of priority 2. Note however that in case of selecting the insertion buffer, it might contain only a part of an upstream packet and the rest is still coming in as the packet is pulsed out. This is clearly, cut-through. Insertion buffers are used to avoid loss during the interfering time of packet transmissions on the ring and the node itself, not to buffer complete packets before forwarding as basic operation.

Another thing that should be noted is that for a lossless operation on the ring, a node can only transmit its own packet when there is enough buffer space left in the insertion buffer to hold up an upstream packet of the same size.

With respect to a node structure, the transmission path in a node consists of a part for removing a packet destined to that node, a part for classifying packets into priority streams, a part containing the insertion buffers, and a part for scheduling the next packet from the insertion and transmit buffers.

I hope this resolves the confusion.

Best regards

Harmen R. van As

[RPRWG] RPRWG: Buffer insertion rings operate per definition as cut-through