Design Considerations for a Wireless OSPF Interface draft-spagnolo-manet-ospf-design Tom Henderson, Phil Spagnolo, Gary Pei 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 examine fundamental performance problems of OSPF in this environment study the performance trends of different OSPF MANET proposals Design Considerations for a Wireless OSPF Interface draft-spagnolo-manet-ospf-design

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 – 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 Number of neighbor state changes per unit time –averaged over all nodes (Number of external LSAs) –not included in this study Network sizeNetwork densityNetwork churn

OSPFv2 benchmark simulations Mobility | Low Medium High Hello | LSU-flood| LSU-rxmt | LSAck | LSR | DDESC | Total | 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 MPR w/ flag | (reliable) (unreliable) (unreliable) Total | Hello | LSA Flood | LSA Rxmt | LSAck | LSR | DDESC | Deliv ratio | 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 –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 Network LSA (desig. rtr.)

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-triggered- 00.txt (expired) –PPPoE interface for link metrics: draft-bberry- pppoe-credit-01.txt