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To better represent LANs and reduce the number of OSPF packets that are exchanged, OSPF handles LAN differently. When OSPF routers boot on a LAN, they elect [#felection]_ one of them as the `Designated Router (DR)` :rfc:`2328`. The `DR` router `represents` the local area network, and advertises the LAN's subnet. Furthermore, LAN routers only exchange HELLO packets with the `DR`. Thanks to the utilization of a `DR`, the topology of the LAN appears as a set of point-to-point links connected to the `DR` router.
How to quickly detect a link failure ?
Network operators expect an OSPF network to be able to quickly recover from link or router failures [VPD2004]_. In an OSPF network, the recovery after a failure is performed in three steps [FFEB2005]_ :
the routers that are adjacent to the failure detect it quickly. The default solution is to rely on the regular exchange of HELLO packets. However, the interval between successive HELLOs is often set to 10 seconds... Setting the HELLO timer down to a few milliseconds is difficult as HELLO packets are created and processed by the main CPU of the routers and these routers cannot easily generate and process a HELLO packet every millisecond on each of their interfaces. A better solution is to use a dedicated failure detection protocol such as the Bidirectional Forwarding Detection (BFD) protocol defined in [KW2009]_ that can be implemented directly on the router interfaces. Another solution to be able to detect the failure is to instrument the physical and the datalink layer so that they can interrupt the router when a link fails. Unfortunately, such a solution cannot be used on all types of physical and datalink layers.
the routers that have detected the failure flood their updated link state packets in the network
all routers update their routing table
A last, but operationally important, point needs to be discussed about intradomain routing protocols such as OSPF and IS-IS. Intradomain routing protocols always select the shortest path for each destination. In practice, there are often several equal paths towards the same destination. When a router computes several equal cost paths towards one destination, it can use these paths in different ways.
A first approach is to select one of the equal cost paths (e.g. the first or the last path found by the SPF computation) and install it in the forwarding table. In this case, only one path is used to reach each destination.
A second approach is to install all equal cost paths [#fmaxpaths]_ in the forwarding table and load-balance the packets on the different paths. Consider the case where a router has `N` different outgoing interfaces to reach destination `d`. A first possibility to load-balance the traffic among these interfaces is to use `round-robin`. `Round-robin` allows equally balancing the packets among the `N` outgoing interfaces. This equal load-balancing is important in practice because it allows better spreading the load throughout the network. However, few networks use this `round-robin` strategy to load-balance traffic on routers. The main drawback of `round-robin` is that packets that belong to the same flow (e.g. TCP connection) may be forwarded over different paths. If packets belonging to the same TCP connection are sent over different paths, they will probably experience different delays and arrive out-of-sequence at their destination. When a TCP receiver detects out-of-order segments, it sends duplicate acknowledgments that may cause the sender to initiate a fast retransmission and enter congestion avoidance. Thus, out-of-order segments may lead to lower TCP performance. This is annoying for a load-balancing technique whose objective is to improve the network performance by spreading the load.
To efficiently spread the load over different paths, routers need to implement `per-flow` load-balancing. This implies that they must forward all the packets that belong to the same flow on the same path. Since a TCP connection is always identified by the four-tuple (source and destination addresses, source and destination ports), one possibility would be to select an outgoing interface upon arrival of the first packet of the flow and store this decision in the router's memory. Unfortunately, such a solution does not scale since the required memory grows with the number of TCP connections that pass through the router.
Fortunately, it is possible to perform `per-flow` load balancing without maintaining any state on the router. Most routers today use hash functions for this purpose :rfc:`2991`. When a packet arrives, the router extracts the Next Header information and the four-tuple from the packet and computes :
:math:`hash(NextHeader,IP_{src},IP_{dst},Port_{src},Port_{dst}) \pmod{N}`
In this formula, `N` is the number of outgoing interfaces on the equal cost paths towards the packet's destination. Various hash functions are possible, including CRC, checksum or MD5 :rfc:`2991`. Since the hash function is computed over the four-tuple, the same hash value will be computed for all packets belonging to the same flow. This prevents reordering due to load balancing inside the network. Most routers support this kind of load-balancing today [ACO+2006]_.
Footnotes
See http://bgp.potaroo.net/index-as.html for reports on the evolution of the number of Autonomous Systems over time.
OSPF can support `virtual links` to connect routers together that belong to the same area but are not directly connected. However, this goes beyond this introduction to OSPF.
The OSPF Designated Router election procedure is defined in :rfc:`2328`. Each router can be configured with a router priority that influences the election process since the router with the highest priority is preferred when an election is run.
In some networks, there are several dozens of paths towards a given destination. Some routers, due to hardware limitations, cannot install more than 8 or 16 paths in their forwarding table. In this case, a subset of the computed paths is installed in the forwarding table.