📄 rfc2892.txt
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Tsiang & Suwala Informational [Page 6] RFC 2892 The Cisco SRP MAC Layer Protocol August 2000 FIGURE 2. Global and Local Re-Use . . . . . . . . . . . . . . . . . . . ----- . ---------------->| N |----------------- . | ---------------| 1 |<-------------- | . | | ----- | | . | | | | . ----- ----- . . .>| N | | N |. .. . . | 6 | | 2 | . . . ----- ----- . . . ^ | ^ | . . . o | | i | | . . . u | | n | | . . . t | | n | | . . . e | | e | | . . . r | | r | | . . . | v | v . . . ----- ----- . . . . | N | | N |<. . . | 5 | | 3 | . ----- ----- . | | | | . | | ----- | | . | -------------->| N |--------------- | . -----------------| 4 |<---------------- . ----- . ^ . . . . . . . .<. . . . . . . . . . . .2.4. Transit Buffer To be able to detect when to transmit and receive packets from the ring, SRP makes use of a transit (sometimes referred as insertion) buffer as shown in Figure 3 below. High priority packets and low priority packets can be placed into separate fifo queues.Tsiang & Suwala Informational [Page 7] RFC 2892 The Cisco SRP MAC Layer Protocol August 2000 FIGURE 3. Transit buffer ^^ || || vv |----| |----| | | | | |----|Rx |----|Tx | |Buffer | |Buffer |----| |----| | | | | |----| |----| | | | | |----| |----| | | | | |----| |----| ^^ Transit || || Buffer || || |------| vv | H | ===========>|------|==========> | L | |------|3. SRP Overview3.1. Receive Operation Overview Receive Packets entering a node are copied to the receive buffer if a Destination Address (DA) match is made. If a DA matched packet is also a unicast, then the packet will be stripped. If a packet does not DA match or is a multicast and the packet does not Source Address (SA) match, then the packet is placed into the Transit Buffer (TB) for forwarding to the next node if the packet passes Time To Live and Cyclic Redundancy Check (CRC) tests.3.2. Transmit Operation Overview Data sent from the node is either forwarded data from the TB or transmit data originating from the node via the Tx Buffer. High priority forwarded data always gets sent first. High priority transmit data may be sent as long as the Low Priority Transit Buffer (LPTB) is not full. A set of usage counters monitor the rate at which low priority transmit data and forwarded data are sent. Low priority data may be sent as long as the usage counter does not exceed an allowed usage governed by the SRP-fa rules and the LPTB has not exceeded the low priority threshold.Tsiang & Suwala Informational [Page 8] RFC 2892 The Cisco SRP MAC Layer Protocol August 20003.3. SRP Fairness Algorithm (SRP-fa) Overview If a node experiences congestion, then it will advertise to upstream nodes via the opposite ring the value of its transmit usage counter. The usage counter is run through a low pass filter function to stabilize the feedback. Upstream nodes will adjust their transmit rates so as not to exceed the advertised values. Nodes also propagate the advertised value received to their immediate upstream neighbor. Nodes receiving advertised values who are also congested propagate the minimum of their transmit usage and the advertised usage. Congestion is detected when the depth of the low priority transit buffer reaches a congestion threshold. Usage messages are generated periodically and also act as keepalives informing the upstream station that a valid data link exists.3.4. Intelligent Protection Switching (IPS) Protocol Overview An SRP Ring is composed of two counter-rotating, single fiber rings. If an equipment or fiber facility failure is detected, traffic going towards and from the failure direction is wrapped (looped) back to go in the opposite direction on the other ring (subject to the protection hierarchy). The wrap around takes place on the nodes adjacent to the failure, under control of the IPS protocol. The wrap re-routes the traffic away from the failed span. An example of the data paths taken before and after a wrap are shown in Figures 4 and 5. Before the fiber cut, N4 sends to N1 via the path N4->N5->N6->N1. If there is a fiber cut between N5 and N6, N5 and N6 will wrap the inner ring to the outer ring. After the wraps have been set up, traffic from N4 to N1 initially goes through the non-optimal path N4->N5->N4->N3->N2->N1->N6->N1. Subsequently a new ring topology is discovered and a new optimal path is used N4->N3->N2-N1 as shown in Figure 6. Note that the topology discovery and the subsequent optimal path selection are not part of the IPS protocol.Tsiang & Suwala Informational [Page 9] RFC 2892 The Cisco SRP MAC Layer Protocol August 2000 FIGURE 4. Data path before wrap, N4 -> N1 ----- ################>| N |----------------- # ---------------| 1 |<-------------- | # | ----- | | # | | | ----- ----- | N | | N | | 6 | | 2 | ----- ----- ^ | ^ | # | | | # | | | # | | | # | | | # | | | # v | v ----- ----- | N | | N | | 5 | | 3 | ----- ----- # | | | # | ----- | | # -------------->| N |--------------- | #################| 4 |<---------------- ----- The ring wrap is controlled through SONET BLSR [3][4] style IPS signaling. It is an objective to perform the wrapping as fast as in the SONET equipment or faster. The IPS protocol processes the following request types (in the order of priority, from highest to lowest): 1. Forced Switch (FS): operator originated, performs a protection switch on a requested span (wraps at both ends of the span) 2. Signal Fail (SF): automatic, caused by a media Signal Failure or SRP keep-alive failure - performs a protection switch on a requested spanTsiang & Suwala Informational [Page 10] RFC 2892 The Cisco SRP MAC Layer Protocol August 2000 FIGURE 5. Data path after the wrap, N4 -> N1 ----- ################>| N |----------------- # ###############| 1 |<############## | # # ----- # | # v # | ----- ----- | N | | N | | 6 | | 2 | ----- ----- ^ # wrap ^ | ### # | _________ # | fiber cut # | --------- # | ### # | # v wrap # v ----- ----- | N | | N | | 5 | | 3 | ----- ----- # # # | # # ----- # | # ##############>| N |############### | #################| 4 |<---------------- 3. Signal Degrade (SD): automatic, caused by a media Signal Degrade (e.g. excessive Bit Error Rate) - performs a protection switch on a requested span 4. Manual Switch (MS): operator originated, like Forced Switched but of a lower priority 5. Wait to Restore (WTR): automatic, entered after the working channel meets the restoration criteria after SF or SD condition disappears. IPS waits WTR period before restoring traffic in order to prevent protection switch oscillations If a protection (either automatic or operator originated) is requested for a given span, the node on which the protection has been requested issues a protection request to the node on the other end of the span using both the short path (over the failed span, as the failure may be unidirectional) and the long path (around the ring).Tsiang & Suwala Informational [Page 11] RFC 2892 The Cisco SRP MAC Layer Protocol August 2000 FIGURE 6. Data path after the new topology is discovered ----- -----------------| N |----------------- | ---------------| 1 |<############## | | | ----- # | | v # | ----- ----- | N | | N | | 6 | | 2 | ----- ----- ^ | wrap ^ | -- # | _________ # | fiber cut # | --------- # | -- # | | v wrap # v ----- ----- | N | | N | | 5 | | 3 | ----- ----- | | # | | | ----- # | | -------------->| N |############### | -----------------| 4 |<---------------- ----- As the protection requests travel around the ring, the protection hierarchy is applied. If the requested protection switch is of the highest priority e.g. Signal Fail request is of higher priority than the Signal Degrade than this protection switch takes place and the lower priority switches elsewhere in the ring are taken down, as appropriate. If a lower priority request is requested, it is not allowed if a higher priority request is present in the ring. The only exception is multiple SF and FS switches, which can coexist in the ring. All protection switches are performed bidirectionally (wraps at both ends of a span for both transmit and receive directions, even if a failure is only unidirectional).⌨️ 快捷键说明
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