[Ospf-wireless-design] Advantages of aligning adjacencies with relays
Richard Ogier <rich.ogier@earthlink.net> Mon, 07 November 2005 05:37 UTC
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Date: Sun, 06 Nov 2005 21:37:29 -0800
From: Richard Ogier <rich.ogier@earthlink.net>
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Subject: [Ospf-wireless-design] Advantages of aligning adjacencies with relays
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All, As has been discussed, using Smart Peering results in adjacencies that are not aligned with relays (MPRs or MDRs). Of course, in a OSPF broadcast network, adjacencies are aligned with the (Backup) DR, and perhaps there is a good reason for this. So I have been thinking about possible advantages of keeping this property of OSPF so that at least one endpoint of each adjacency is a relay. Here are some possible advantages. 1. Alignment helps take advantage of implicit ACKs. If a flooding relay (MDR) has several adjacent neighbors, then these neighbors can treat an LSA flooded by the relay as an implicit ACK. If Smart Peering is used with MPRs, then in general an MPR has only a few adjacent neighbors (same as any router), so only a few neighbors can treat a flooded LSA as an implicit ACK. 2. Alignment helps take advantage of explicit multicast ACKs. Every router must ACK every LSA explicitly or implicitly, so this advantage refers to sending an explicit multicast ACK in response to a duplicate LSA received as a unicast. By requiring adjacencies to be aligned with MDRs, a single multicast ACK sent by an MDR will inform several (adjacent) neighbors that the LSA has been received, thus suppressing a larger number of possible retransmissions than if adjacencies were not aligned with MDRs. 3. Extendability to non-ackable LSAs (i.e., periodic flooding). By having a fixed set of nodes responsible for flooding LSAs (independently of the advertising router) and maintaining synchronization with neighbors, it is easier to implement options such as non-ackable LSAs (described in Appendix D of the MDR draft), by having each MDR flood each LSA periodically until a new instance has been received. There are probably other advantages to having a source-independent set of relays. 4. Here is an example that shows it can take longer for an LSA to reach all routers if adjacencies are not alighed with relays. 1 | | 2 --- 3 | | | | 4 --- 5 In this example, nodes 2 and 3 are relays (MDRs or MPRs) and all neighbor pairs are adjacent except for (2,4). In particular, (4,5) is an adjacent pair even though neither 4 nor 5 is a relay. (I am making a general point so am not assuming that Backup MDRs/MPRs are used.) Suppose that node 1 floods an LSA and relay 2 forwards it. Suppose nodes 3 and 4 do not hear 2's flood. Then node 2 will retransmit the LSA to node 3 after 5 seconds, but will NOT retransmit the LSA to node 4 (not adjacent). Suppose node 3 receives the retransmitted LSA and relays it. If node 5 does not receive it the first time, node 3 will retransmit it after 5 seconds. Now, since node 5 is not a relay it does not immediately flood the LSA, but will retransmit it to node 4 (adjacent) after 5 seconds. This example shows that it can take much longer for the LSA to reach node 4 than if adjacencies were aligned with relays, so that the pair (2,4) is adjacent. Richard _______________________________________________ Ospf-wireless-design mailing list Ospf-wireless-design@lists.ietf.org https://www1.ietf.org/mailman/listinfo/ospf-wireless-design
- [Ospf-wireless-design] Advantages of aligning adj… Richard Ogier
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