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The Internet is the set of several autonomous systems (ASes) that are operated by different companies or institutions. The world-wide performance of routing system is essential to the entities operating the ASes. The routing between autonomous systems specifies the division of Internet routing as inter-domain routing.
Inter-domain routing is always apprehensive for the scalability and convergence of the networks. Border Gateway Protocol (BGP) is a large-scale inter-domain routing protocol that uses path vector routing. BGP router detects the change in the topology, select a new route and propagate this information to entire inter-network after the change takes place in the topology (Rekhter & Hares, 2005). Convergence occurs when any change takes place in the topology. The time interval between the first update and last update of any event determines convergence time. Convergence time and the number of exchanged updates messages has always been a matter of concern in BGP. To reduce the convergence time and exchanged updates the authors of (Rekhter & Hares, 2005; Jakma, 2008; Pei et al., 2005) suggested Minimum Route Advertisement Interval (MRAI). The effect of MRAI timers is significant in BGP.
MRAI is rate-limiting for advertised updates. Each autonomous system selects one BGP speaker to distribute path information (Rekhter & Hares, 2005) A BGP speaker may hold updates for a certain time interval when there is a frequent exchange of announcement of routes between BGP speakers. This time interval is known as MRAI. The selected MRAI value affects convergence time (Gill et al., 2012). BGP speaker advertises updates when MRAI timer expires.
MRAI timer can be implemented in two ways. The first approach is per-destination MRAI timer and the second approach is per-peer MRAI timer (Rekhter & Hares, 2005). Per-destination MRAI timer approach assigns a distinct timer with an MRAI value to each destination. It limits the rate of advertisements independently for each destination. This approach has limited scalability.
According to CAIDA (2017), AS ranking (updated on 14 Feb 2017), there are approximately 51,112 autonomous systems. Therefore, per-destination MRAI timer is not recommended for current internet size. Per-peer MRAI timer approach assigns a timer for each peer and may hold advertisements until the MRAI timer expires. Per-peer MRAI timer is scalable than per-destination MRAI timer, but it affects advertisements to connected BGP peers. It delays updates for all destinations reachable through a BGP peer.
Reusable MRAI timer (Gill et al., 2012) uses features of per-destination and per-peer approaches. Reusable MRAI timer associates all updates advertised within a specific time interval to one MRAI timer. However, the reusable MRAI timers have a complex implementation but worth for decrease in convergence time.
In large networks, frequent change in traffic load impacts more to convergence time of BGP. Additionally, there are numerous factors that affect the convergence time of BGP such as MRAI timer, the no. of update exchanged between BGP routers; time taken for convergence of BGP events, and MRAI value.