Pierre Francois

Achieving sub-50 milliseconds recovery upon BGP peering link failures

Recent measurements show that BGP peering links can fail as frequently as intradomain links and usually for short periods of time. We propose a new fast-reroute technique where routers are prepared to react quickly to interdomain link failures. For each of its interdomain links, a router precomputes a protection tunnel, i.e., an IP tunnel to an alternate nexthop which can reach the same destinations as via the protected link.We propose a BGP-based auto-discovery technique that allows each router to learn the candidate protection tunnels for its links. Each router selects the best protection tunnels for its links and when it detects an interdomain link failure, it immediately encapsulates the packets to send them through the protection tunnel. Our solution is applicable for the links between large transit ISPs and also for the links between multi-homed stub networks and their providers. Furthermore, we show that transient forwarding loops (and thus the corresponding packet losses) can be avoided during the routing convergence that follows the deactivation of a protection tunnel in BGP/MPLS VPNs and in IP networks using encapsulation.

An efficient algorithm to enable path diversity in link state routing networks

Multipath routing allows for load balancing and fast re-routing in order to improve the reliability and the efficiency of the network. Current IP routers only support Equal Cost MultiPath (ECMP) which guarantees that the forwarding paths do not contain loops. However, ECMP provides limited path diversity. In this paper, we present an efficient algorithm that allows routers to enable more path diversity: our algorithm let all routers compute at least the two best first hop distinct paths towards each destination and achieves a good tradeoff between path diversity and overhead. In addition, we propose a multipath routing scheme whose goal is to combine fast re-routing and load balancing loop-free routes. The low overhead of our scheme (no additional signaling messages and low complexity) and the nature of its loop-free rules allow to incrementally deploy it on current IP routers. Using actual, inferred, and generated topologies, we compare our algorithm to existing solutions.

Disruption Free Topology Reconfiguration in OSPF Networks

A few modifications to software and/or hardware of routers have been proposed recently to avoid the transient micro loops that can occur during the convergence of link-state interior gateway protocols like IS-IS and OSPF. We1 propose in this paper a technique that does not require modifications to ISIS and OSPF, and that can be applied now by ISPs. Roughly, in the case of a manual modification of the state of a link, we progressively change the metric associated with this link to reach the required modification by ensuring that each step of the progression will be loop-free. The number of changes that are applied to a link to reach the targeted state by ensuring the transient consistency of the forwarding inside the network is minimized. Analysis performed on real regional and tier-1 ISP topologies show that the number of required transient changes is small. The solution can be applied in the case of link metric updates, manual set up, and shut down of links.

Avoiding transient loops during IGP convergence in IP networks

When the topology of an IP network changes due to a link failure or a link metric modification, the routing tables of all the routers must be updated. Each of those updates may cause transient loops. In this paper, we prove that by ordering the updates of the routing tables on the routers, it is possible to avoid all transient loops during the convergence of ISIS or OSPF after a planned link failure, an unplanned failure of a protected link and after a link metric modification. We then propose a protocol that allows the routers to order the update of their routing tables to avoid transient loops without requiring any complex computation.

An evaluation of IP-based fast reroute techniques

Today, IP-based networks are used to carry all types of traffic, from the traditional best-effort Internet access to traffic with much more stringent requirements such as realtime voice or video services and Virtual Private Networks. Some of those services have strong requirements in terms of restoration time in case of failure. When a link or a router fails in an IP network, the routers adjacent to the failing ressource must react by distributing new routing information to allow each router of the network to update its routing table. A realistic estimate of the convergence time of a tuned intradomain routing protocol in a large network is a few hundred of milliseconds [1]. For some mission critical services like voice or video over IP, achieving a restoration time in the order of a few tens of milliseconds after a failure is important [2]. In this paper, we first present several techniques that can be used to achieve such a short restoration time. While most of the work on fast restoration has focussed on MPLS-based solutions [2], recent work indicate that fast restoration techniques can be developed also for pure IP networks. Recently, the RTGWG working group of the IETF started to work actively on this problem and several fast reroute techniques are being discussed. However, as of today, no detailed evaluation of the various proposed IP-based fast reroute techniques has been published. The goal of this short paper is to firstly provide a brief overview of fast restoration techniques suitable for pure IP networks, in section 2. Then, in section 3, we evaluate by simulation how many links can be protected by each technique in large ISP networks based on their actual topology. This coverage is an important issue as some techniques cannot protect all links from failures.

Seamless network-wide IGP migrations

Network-wide migrations of a running network, such as the replacement of a routing protocol or the modification of its configuration, can improve the performance, scalability, manageability, and security of the entire network. However, such migrations are an important source of concerns for network operators as the reconfiguration campaign can lead to long and service-affecting outages. In this paper, we propose a methodology which addresses the problem of seamlessly modifying the configuration of commonly used link-state Interior Gateway Protocols (IGP). We illustrate the benefits of our methodology by considering several migration scenarios, including the addition or the removal of routing hierarchy in an existing IGP and the replacement of one IGP with another. We prove that a strict operational ordering can guarantee that the migration will not create IP transit service outages. Although finding a safe ordering is NP complete, we describe techniques which efficiently find such an ordering and evaluate them using both real-world and inferred ISP topologies. Finally, we describe the implementation of a provisioning system which automatically performs the migration by pushing the configurations on the routers in the appropriate order, while monitoring the entire migration process.

Evolution of Internet Address Space Deaggregation: Myths and Reality

Internet routing table size growth and BGP update churn are two prominent Internet scaling issues. There is widespread belief in a high and fast growing number of ASs that deaggregate prefixes, e.g., due to multi-homing and for the purpose of traffic engineering. Moreover, researchers often blame specific classes of ASs for generating a disproportionate amount of BGP updates. Our primary objective is to challenge such widespread assumptions (“myths”) and not solely to confirm previous findings. Surprisingly, we find severe discrepancies between existing myths and reality. According to our results, there is no trend towards more aggressive prefix deaggregation or traffic engineering over time. With respect to update dynamics, we observe that deaggregated prefixes generally do not generate a disproportionate number of BGP updates, with respect to their share of the BGP routing table. On the other side, we observe much more widespread traffic engineering in the form of AS path prepending and scoped advertisements compared to previous studies. Overall, our work gives a far more positive picture compared to the alarming discourses typically heard: The impact of “bad guys” on routing table size growth and BGP churn has not changed for the worse in recent years. Rather, it increases at the same pace as the Internet itself.

A Declarative and Expressive Approach to Control Forwarding Paths in Carrier-Grade Networks

SDN simplifies network management by relying on declarativity (high-level interface) and expressiveness (network flexibility). We propose a solution to support those features while preserving high robustness and scalability as needed in carrier-grade networks. Our solution is based on (i) a two-layer architecture separating connectivity and optimization tasks; and (ii) a centralized optimizer called framework, which translates high-level goals expressed almost in natural language into compliant network configurations. Our evaluation on real and synthetic topologies shows that framework improves the state of the art by (i) achieving better trade-offs for classic goals covered by previous works, (ii) supporting a larger set of goals (refined traffic engineering and service chaining), and (iii) optimizing large ISP networks in few seconds. We also quantify the gains of our implementation, running Segment Routing on top of IS-IS, over possible alternatives (RSVP-TE and OpenFlow).

The Segment Routing Architecture

Network operators anticipate the offering of an increasing variety of cloud-based services with stringent Service Level Agreements. Technologies currently supporting IP networks however lack the flexibility and scalability properties to realize such evolution. In this article, we present Segment Routing (SR), a new network architecture aimed at filling this gap, driven by use-cases defined by network operators. SR implements the source routing and tunneling paradigms, letting nodes steer packets over paths using a sequence of instructions (segments) placed in the packet header. As such, SR allows the implementation of routing policies without per-flow entries at intermediate routers. This paper introduces the SR architecture, describes its related ongoing standardization efforts, and reviews the main use-cases envisioned by network operators.

Avoiding disruptions during maintenance operations on BGP sessions

This paper presents a solution aimed at avoiding losses of connectivity when an eBGP peering link is shut down by an operator for a maintenance. Currently, shutting down an eBGP session can lead to transient losses of connectivity even though alternate path are available at the borders of the network. This is very unfortunate as ISPs face more and more stringent service level agreements, and maintenance operations are predictable operations, so that there is time to adapt to the change and preserve the respect of the service level agreement.