Re: [Ospf-wireless-design] Advantages of aligning adjacencies with relays
Richard Ogier <rich.ogier@earthlink.net> Tue, 08 November 2005 00:39 UTC
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Date: Mon, 07 Nov 2005 16:39:16 -0800
From: Richard Ogier <rich.ogier@earthlink.net>
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Subject: Re: [Ospf-wireless-design] Advantages of aligning adjacencies with relays
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I would like to go into more depth regarding the following advantage of having a set of relays (MDRs) that are responsible for forwarding all LSAs (independent of the originator) and for maintaining synchronization with neighbors. > Extendability to non-ackable LSAs (i.e., periodic flooding). > By having a fixed set of nodes responsible for flooding LSAs > (independently of the originator) 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. One might argue that MPRs can be used for periodic flooding of LSAs, as in OLSR. But let's take a closer look at this in the context of OSPF. To accomplish periodic flooding using MPRs, the originator (advertising router) must periodically flood its router-LSA (e.g., every 5 seconds) whether or not the LSA changes. Let's first suppose that the LSA sequence number is incremented only when the LSA changes. Then an MPR that receives a periodically flooded LSA from an MPR selector, which is not a new LSA (has the same sequence number as the database copy), must still flood the LSA for this to work. But this is contrary to OSPF, in which a received LSA is forwarded only if it is a new instance, and it is not clear what the forwarding rule should be if the same MPR receives the same LSA from another MPR selector 1 second later, or 5 seconds later, etc. Now suppose that the LSA sequence number is incremented (by the originator) with each periodic flood, whether or not the LSA has changed. This is also contrary to OSPF, and will cause unnecessary flooding of the LSA over non-MANET interfaces that are in the same OSPF area. Now consider the periodic flooding method (using non-ackable LSAs) proposed in the MDR draft. As in OSPF, the originator increments the LSA sequence number only when the LSA actually changes. As in OSPF, a received LSA is forwarded only if it is a new instance. The only difference regards local retransmissions: each MDR periodically multicasts each LSA until a new instance of the LSA is received. (If topology changes are frequent, the new instance will often be received before the old instance is multicast a second time.) This method does not work with MPRs because each MPR is responsible for forwarding only LSAs received from its MPR selectors, and it is not clear what to do when the set of MPR selectors changes without receiving a new instance of the LSA. This is just one example of how the use of source-independent MDRs makes the design more extensible (and more compatible with OSPF). I will mention here another advantage of the fact that OSPF-MDR treats a MANET as a generalized OSPF broadcast network. This allows OSPF-MDR to treat an OSPF broadcast network as a special case of a MANET, at least for the purpose of flooding LSAs between a broadcast interface and a MANET interface. This is described in Section 8.1.2 of the MDR draft. 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
- Re: [Ospf-wireless-design] Advantages of aligning… Richard Ogier