On the Scalability of Router Forwarding Tables: Nexthop-Selectable FIB Aggregation Qing Li * , Dan Wang † , Mingwei Xu * and Jiahai Yang ‡ * Dept. of Comp. Sci. & Tech., Tsinghua Univ., Tsinghua National Laboratory for Information Science and Technology † Dept. of Computing, The Hong Kong Polytechnic Univ. ‡ Network Research Center, Tsinghua Univ. Abstract—In recent years, the core-net routing table, e.g., Forwarding Information Base (FIB), is growing at an alarming speed and this has become a major concern for Internet Service Providers. One effective solution for this routing scalability problem, which requires only upgrades on individual routers, is FIB aggregation. Intrinsically, IP prefixes with numerical prefix matching and the same next hop can be aggregated. Very commonly, all previous studies assume that each IP prefix has one corresponding next hop, i.e., towards one optimal path. In this paper, we argue that a packet can be delivered to its destination through a path other than the one optimal path. Based on this observation, we for the first time propose Nexthop- Selectable FIB Aggregation that is fundamentally different from all previous aggregation schemes. IP prefixes are aggregated if they have numerical prefix matching and share one common next hop. Consequently, IP prefixes that cannot be aggregated, due to lack of the same next hop, are aggregated; and we achieve a substantially higher aggregation ratio. In this paper, we provide a systematic study on this Nexthop- Selectable FIB Aggregation problem. We present several practical choices to build the sets of selectable next hops for the IP prefixes. To maximize the aggregation, we formulate the problem as an optimization problem. We show that the problem can be solved by dynamic programming. While the straightforward application of dynamic programming has exponential complexity, we propose a novel algorithm that is O(N ). We then develop an optimal online algorithm with constant running time. We evaluate our algorithms through a comprehensive set of simulations with BRITE with RIBs collected from RouteViews. Our evaluation shows that we can reduce more than an order of the FIB size. I. I NTRODUCTION The global Internet has experienced tremendous growth over the last decade. The sheer growth of user population, as well as such factors as multi-homing, traffic engineering, policy routing, have driven the growth of Default Free Zone (DFZ) routing table size at an alarming rate [1–3] (see the routing table size from 1994 to 2010 in Fig. 1). The Internet Service Providers (ISPs) are forced to upgrade routers in an unanticipated pace. Instead of upgrading their routers, a few ISPs have resorted to filtering out some small prefixes (mostly /24s) which implies that parts of the Internet may not have reachability to each other. This suggests that ISPs are undergoing some pain to avoid the cost of router upgrades. To handle this severe Internet routing scalability problem, many solutions are proposed. One set of proposals is to get rid of the IP-oriented infrastructure and design a fully scalable The works of Q. Li, M. Xu and J. Yang are supported by grants of NSFC 61073166 (only for Q. Li and M. Xu), 973 Pro. 2009CB320502, 863 Pro. 2009AA01Z251, National Sci. & Tech. Pillar Pro. 2008BAH37B05 (only for J. Yang). D. Wang’s work is supported by grants Hong Kong PolyU/G-YG78, A-PJ19, 1-ZV5W, and RGC/GRF PolyU 5305/08E. 1994 96 98 00 02 04 06 08 10 0 0.5 1 1.5 2 2.5 3 3.5 x 10 5 Time from 1994 to 2010 Active BGP Entries (FIB) Fig. 1. The historical FIB size. Data obtained from bgp.potaroo.net [1]. addressing system. Example protocols and frameworks include [4–6]. Another set of proposals is to protect the core-net router tables by address encapsulation or translation. These proposals [7–9] require a significant transition time, as what we have learned from the deployment of IPv6. A more immediate solution is Forwarding Information Base (FIB) aggregation. FIB aggregation can reduce FIB size with only local router upgrade and requires no protocol change. In FIB aggregation, multiple prefixes can be aggregated into one if two conditions hold: 1) the prefixes are numerically aggregatable; 2) their next hops are the same. Some vendors have already implemented simple FIB aggregation schemes and many techniques [10, 11] are proposed in academia. In [10], systematic analysis of FIB aggregation is presented. All these previous studies focus on the first condition of FIB aggregation, that is, how to find or construct the numerical aggregatable prefixes. Very commonly, these algo- rithms consider that each IP prefix has one corresponding next hop. As a sharp contrast, in this paper, we make a key observation that it is practically possible that there are multiple selectable next hops for each IP prefix; and through any one of these next hops, the packets can be delivered to the destination. In practice, such schemes as equal-cost multi-path routing (ECMP), loop-free alternative (LFA) [12] naturally exist. There are also many multi-path routing (path protection) schemes proposed in literature, making a selection of multiple next hops possible. Consequently, we propose a fundamentally different FIB aggregation approach, where multiple IP prefixes can be aggregated into one if 1) they are numerically aggregatable, and 2) one of their selectable next hops is the same. We illustrate our idea with a simple example: An Example: Consider two prefix entries, < 158/8,a >, < 158.128/9,b>. They cannot be aggregated in any previous FIB aggregation schemes, as they have different next hops. Assume that both next hops a and b can deliver the packets