Intradomain routing in IP networks follows leastcost paths according to administrative link costs. Routing optimization modifies these values to minimize an objective function for a network with given link capacities and traffic matrix. An example for an objective function is the maximum utilization of all links under failure-free conditions or also after rerouting in case of network failures. Many papers have provided heuristic algorithms for routing optimization using different objective functions, but the investigation and comparison of various objective functions has not attracted much attention so far. In this work we present several objective functions for resilient IP routing. We also propose a new combined optimization approach which can simultaneously optimize different objective functions with almost no additional computation effort and describe new techniques to minimize overall computation time. The different objective functions and combinations thereof are then analyzed and compared experimentally.

Abstract—Pre-congestion notification (PCN) defines admissible rates (AR) and supportable rates (SR) per link and marks the PCN traffic rate above these thresholds as AR- or SR-overload. The IETF standardizes simple mechanisms for admission control (AC) and flow termination (FT) based on this PCN-feedback for high-priority DiffServ traffic. While admission control (AC) has been extensively discussed in the literature, flow termination (FT) is a new control function. In this paper we propose an algorithm that converts marked AR-overload into marked SRoverload by unmarking appropriate packets. Classic marked flow termination (MFT) is based on marked SR-overload and works well even with a small number of PCN flows per ingress-egress aggregate and in case of multipath routing. Thanks to the new marking converter MFT also works with marked AR-overload so that a single marking scheme suffices to support AC and FT. We investigate whether MFT with marking conversion based on AR-overload retains the benefits classic MFT. I.

Abstract—Pre-congestion notification (PCN) is a new packet marking scheme based on which simple measurement-based admission control (AC) and flow termination (FT) are implemented. FT is useful for traffic management in unexpected events, e.g., when admitted flows lead to overload on a link after rerouting which may be due to a link or node failure. While AC is a classic flow control function, FT is new and only little understood so far. The limited literature on FT focuses mainly on a single overloaded link. However, when a link or node fails, redirected traffic is likely to cause overload on multiple backup links (bottlenecks) at the same time. As the packet marking probability for flows traversing multiple bottlenecks is larger than for flows traversing only the most severe bottleneck, more traffic is possibly terminated than needed, i.e. overtermination occurs. This paper quantifies potential overtermination in case of multiple bottlenecks for different FT mechanisms which are currently discussed by the IETF. I.

Intradomain routing in IP networks follows shortest paths according to administrative link costs. When several equalcost shortest paths exist, routers that use equal-cost multipath (ECMP) distribute the traffic over all of them. To produce singleshortest path (SSP) routing, a selection mechanism (tie-breaker) chooses just one of the equal-cost paths. Tie-breakers are poorly standardized and use information that may change over time, which makes SSP routing unpredictable. Therefore, link costs producing unique shortest paths (USP) are preferred. In this paper, we show that optimized SSP routing can lead to significantly higher link utilization than expected in case of non-deterministic tie-breakers. We investigate the impact of the allowed link cost range on the general availability of USP routing. We use a heuristic algorithm to generate link costs for USP routing and to minimize the maximum link utilization in networks with and without failures. Fast reroute (FRR) mechanisms can repair failures faster than conventional IP rerouting by pre-computing shortest backup paths around failed network elements. However, when multiple equal-cost paths exist, the backup path layout is unpredictable. We adapt our heuristic to optimize USP routing for IP-FRR using not-via addresses and MPLS-FRR with facility and oneto-one backup. Finally, we compare the performance of USP with various other routing schemes using realistic Rocketfuel topologies.

Abstract—Pre-congestion notification (PCN) uses packet metering and marking within a PCN domain to notify PCN egress nodes about high load regimes in the network. One question is how to encode the PCN markings in packet headers. The problem is that the IPv4 packet header is short of available codepoints and that tunnelling mechanisms constrain solutions. This paper proposes packet-specific dual marking (PSDM) as a new encoding scheme that avoids these problems and also explains how to apply it to achieve PCN-based admission control and flow termination. Therefore, our proposal may improve the deployability of PCN in spite of the limited extensibility of the current Internet architecture. I.

Abstract—Pre-congestion notification (PCN) provides feedback about load conditions in a network to its boundary nodes. The PCN working group of the IETF discusses the use of PCN to implement admission control (AC) and flow termination (FT) for prioritized realtime traffic in a DiffServ domain. Admission control (AC) is a well-known flow control function that blocks admission requests of new flows when they need to be carried over a link whose admitted PCN rate already exceeds an admissible rate. Flow termination (FT) is a new flow control function that terminates some already admitted flows when they are carried over a link whose admitted PCN rate exceeds a supportable rate. The latter condition can occur in spite of AC, e.g., when traffic is rerouted due to network failures. This survey gives an introduction to PCN and is a primer for this new technology. It presents and discusses the multitude of architectural design options in an early stage of the standardization process in a comprehensive and streamlined way before only a subset of them is standardized by the IETF. It brings PCN from the IETF to the research community and serves as historical record. I.

Abstract—Pre-congestion notification (PCN) marks packets when the PCN traffic rate exceeds an admissible link rate and this marking information is used as feedback from the network to take admission decisions for new flows. This idea is currently under standardization in the IETF. Different marking algorithms are discussed and various admission control (AC) algorithms are proposed that decide based on the packet markings whether further flows should be accepted or blocked. In this paper, we investigate the applicability of PCN-based AC under challenging conditions and show the limitations of different marking and AC algorithms. Index Terms—Admission control, QoS, feedback systems, packet marking. I.

Overload in a packet-based network can be prevented by admitting or blocking new flows depending on its load conditions. However, overload can occur in spite of admission control due to unforseen events, e.g., when admitted traffic is rerouted in the network after a failure. To restore quality of service for the majority of admitted flows in such cases, flow termination has been proposed as a novel control function. We present several flow termination algorithms that measure so-called pre-congestion notification (PCN) feedback. We analyze their advantages and shortcomings in particular under challenging conditions. The results improve the understanding of PCN technology which is currently being standardized by