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Design Considerations for a Wireless OSPF Interface draft-spagnolo-manet-ospf-design Tom Henderson, Phil Spagnolo, Gary Pei {[email protected]} IETF-60 MANET WG meeting August 2004 Problem statement (draft-baker-manet-ospf-problem-statement-00) • OSPF does not have suitable interface type for MANET (wireless, multi-access subnet) operation – Leads to scalability problems with respect to overhead (primarily flooding overhead) • OSPF seems extensible to cover this case – proposals have centered on a new interface type – could be for IPv4 or v6, or both Purpose Design Considerations for a Wireless OSPF Interface draft-spagnolo-manet-ospf-design • examine fundamental performance problems of OSPF in this environment • study the performance trends of different OSPF MANET proposals OSPF analysis (Sec. 3) • Multicast-capable Point-to-Multipoint interface type is the benchmark • Finding: LSU flooding and acknowledgment is by far the dominant contributor to overhead – backed up by simulations as well Methodology • Simulation-based study using QualNet 3.7 – 802.11-based and Rockwell Collins USAP TDMA – Ricean fading model, no power control – OSPFv2 implementation (validated against Moy ospfd implementation) – random waypoint mobility on square grid • Performance metrics – OSPFv2 overhead measured at IP layer – User data delivery ratio Scenario-independent parameters • Number of nodes • Number of neighbors per node – averaged over all nodes Network size Network density • Number of neighbor state changes per unit Network churn time – averaged over all nodes • (Number of external LSAs) – not included in this study OSPFv2 benchmark simulations Mobility | Low Medium High -----------------------------------Hello | 2.20 2.00 1.71 LSU-flood| 43.55 66.33 67.59 LSU-rxmt | 35.62 72.04 87.28 LSAck | 3.70 7.28 9.16 LSR | 0.04 0.10 0.20 DDESC | 2.67 4.91 6.80 Total | 87.80 152.70 172.70 Figure 8: Summary of overhead (kbps) at the three mobility levels. Dominant overhead factor (reliable) Flooding optimizations • Lin’s SI-CDS reduced overhead by 23% against benchmark • Lin’s SI-CDS plus …. – Multicast ACKs reduced additional 32% – Ogier’s receiver-based ACK suppression reduced overhead by 8% (created more overhead) – Originator-based LSA suppression reduced overhead by 28% – Retransmit-timer backoff reduced overhead by 24% Unreliable flooding advantage (draft-spagnolo-manet-ospf-wireless-interface) | best SI-CDS MPR w/out flag | (reliable) (unreliable) MPR w/ flag (unreliable) -----------------------------------------------------Total | 110.0 17.70 28.40 Hello | 1.65 1.79 1.79 LSA Flood | 34.94 15.97 26.63 LSA Rxmt | 57.13 - - LSAck | 8.07 - - LSR | 0.39 - - DDESC | 7.81 - - Deliv ratio | 0.78 0.78 0.78 Figure 17: Summary of overhead (kbps) for comparison of reliable and unreliable flooding. Summary • LSU flooding is by far the dominant contributor to overhead – can reliable flooding optimizations do better than 50% reduction? • unreliable flooding can provide up to 10x reduction without sacrificing performance – large numbers of external LSAs are a concern • Database exchange optimization also may be important in a frequently partitioning network Next steps Fundamental design choices • Broadcast-based interface Network LSA (desig. rtr.) – provides abstraction – may be most scalable for large networks • Point-to-multipoint-based interface – provides visibility into structure of MANET – important for picking good entry points into network, over bandwidth-constrained links Layer-2 triggers • Should we specify how implementations might make use of layer-2 information? – neighbor discovery suppression – link quality issues • How does this affect interoperability? • Examples: – A Triggered Interface: draft-corson-triggered00.txt (expired) – PPPoE interface for link metrics: draft-bberrypppoe-credit-01.txt