Internet Routing Instability.ppt
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1、Internet Routing Instability,Craig Labovitz G. Robert Malan Farnam Jahanian,Presenters:Supranamaya RanjanMohammed Ahamed,Appeared: SIGCOMM 97,Internet Structure,Many small ISPs at lowest level,Small number of big ISPs at core,The Core of the Internet,Verio,UUNet,Sprint,rice.edu,Routing done using BG
2、P at core,Inter-domain routing could be RIP/OSPF etc,BGP Overview,Verio,UUNet,Sprint,128.42.x.x 196.29.x.x,128.42.x.x,100.100.x.x,196.29.x.x,92.92.x.x,100.100.x.x 196.29.x.x,92.92.x.x,BGP Overview (contd.),Similar to Distance Vector routing,Path Vector protocol,Loop detection done using AS_PATH fiel
3、d,Peering session (TCP),R1,R2,Exchange full routing table at start,Updates sent incrementally,Key Point,The volume of BGP messages exchanged is abnormally high,Most messages are redundant / unnecessary and do notcorrespond to and topology or policy changes,Consequence: Instability,Normal data packet
4、s handled by dedicated hardware,BGP packet processing consumes CPU time,Severe CPU processing overhead takes the router offline,Route Flap Storm:,C,B,A,Router A temporarily fails,When A becomes alive B & Csend full routing tables,B & C failcascading effect,How do we avoid /lessen the impact of these
5、 problems?,Route Dampening,Router does not accept frequent route updates to a destination,Might signal that network has erratic connectivity,Increment counter for destination when route changes,Counter exceeds threshold stop accepting updates,Problem:,Future legitimate announcements are accepted onl
6、yafter a delay,Decrement counter with time,Prefix Aggregation/Super-netting,Core router advertises a less specific network prefix,Reduces size of routing tables exchanged,Problems:,- Internet addresses largely non-hierarchically assigned,Prefix aggregation is not effective because:,- 25% of prefixes
7、 multi-homed,- Multi-homed prefixes should be exposed at the core,- Domain renumbering not done when changing ISPs,Route Servers,O(N) peering sessions per Router,1 peering session per router,Route Server,In-spite of all these measures the BGP message overhead is unexpectedly high,Evaluation Methodol
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