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Routing Metrics. The ARPANET Experience. History. The original ARPAnet was actually a terminal concentrator network so lots of dumb terminals could use a few big, expensive machines
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Routing Metrics The ARPANET Experience
History • The original ARPAnet was actually a terminal concentrator network so lots of dumb terminals could use a few big, expensive machines • In the early Internet, the ARPAnet became an access network for little IP/TCP clients to use a few big, expensive IP/TCP servers • In the adolescent Internet, the ARPAnet became a transit network for widely distributed IP/TCP local area networks • In the mature Internet, the ARPAnet faded to the museums, but MILnet and clones remain for IP/TCP and ITU-T legacy stuff
Importance of cost metric • Choice of link cost defines traffic load • low cost = high probability link belongs to SPT and will attract traffic, which increases cost. • Main problem: convergence • avoid oscillations • achieve good network utilization
Metric choices • Static metrics (e.g., hop count) • good only if links are homogeneous • definitely not the case in the Internet • Static metrics do not take into account: • link delay • link capacity • link load (hard to measure)
Original routing algorithm • Circa 1969 • Distance vector algorithm • Routing tables exchanged every 2/3 seconds
Original ARPANET metric • Cost proportional to queue size • instantaneous queue length as delay estimator • Problems: • did not take into account link speed • poor indicator of expected delay due to rapid fluctuations • delay may be longer even if queue size is small due to contention for other resources
New algorithm • Link state algorithm • D-SPF (delay shortest path tree) • Only link cost disseminated into the network (standard LS approach), not routes
New metric • Delay = (depart time - arrival time) + transmission time + link propagation delay • (depart time - arrival time) captures queuing • transmission time captures link capacity • link propagation delay captures the physical length of the link • Measurements averaged over 10 seconds • Update sent if difference > threshold, or every 50 seconds
Performance of new metric • Works well for light to moderate load • static values dominate • Oscillates under heavy load • queuing dominates • Reason: there is no correlation between original and new values of delay after re-routing!
Specific problems • Range is too wide • 9.6 Kbps highly loaded link can appear 127 times costlier than 56 Kbps lightly loaded link • can make a 127-hop path look better than 1-hop • No limit in reported delay variation • All nodes calculate routes simultaneously • triggered by link update
Example A Net X Net Y B
..example After everyone re-calculates routes: A Net X Net Y B .. Oscillations!
Consequences • Low network utilization (50% in example) • Congestion can spread elsewhere • Routes could oscillate between short and long paths • Large swings lead to frequent route updates • more messages • frequent SPT re-calculation
Revised link metric Better metric: packet delay = f(queueing, transmission, propagation). When lightly loaded, transmission and propagation are good predictors When heavily loaded queuing delay is dominant and so transmission and propagation are bad predictors
Revised Metric Avg utilization measurements limit range of change….5*sample + .5*last average Normalize according to link type (e.g., satellite should look good when queuing on other links increases) Max change allowed is link type specific change per update cannot be more than 1/2 of that hops delay value (e.g. if max is 90 and min is 30, worst case is only 2 hops worse than best)
Routing metric v.s. link utilization 225 New metric (routing units) 9.6 satellite 140 90 9.6 terrestrial 75 56 satellite 60 56 terrestrial 30 0 25% 50% 75% 100% Utilization
Observations • Cost of highly loaded link never more than 3*cost when idle • Most expensive link is 7 * least expensive link • High-speed satellite link is more attractive than low-speed terrestrial link
..observations • Cost = f(link utilization) only at moderate to high loads • Allows routes to be gradually shed from link • also takes into account link characteristics
Routing Characteristics • Vern Paxson used traceroute to study 40,000 routes • Probability of encountering serious route failure 1/30 with problem lasting 30 seconds • 2/3 of routes persist for days or weeks • 1/3 of route use different path in each direction. • Routes becoming less predictable
Architecture Entities • Mobile Node • Home Agent: • where the mobile node is registered permanently • Foreign Agent: • where the mobile node visits currently
Protocol Overview • Agent Discovery: • Home agents and foreign agents may advertise their availability. • On the contrary, a newly arrived mobile node can send a solicitation to learn if any prospective agents are present. • Registration: • When the mobile node is away from home, it registers its care-of address with its home agent.
Sending host Home agent Foreign agent (10.0.0.3) (12.0.0.6) Internetwork Home network (network 10) (10.0.0.9) Routing for Mobile Hosts
Mobile Routers • A mobile node can also be a foreign agent (similar to a router) of other mobile nodes. • Ex: servers in airplane, train, car, etc. home agent of x Airplane home agent of y Airport x y Internet foreign agent of y foreignagent of x a sender to y Ground Station
Proxy ARP • Proxy ARP: (RFC 925) • an ARP Reply sent by one node on behalf of another node which is either unable or unwilling to answer its own ARP Requests; • The ARP Proxy supplies ARP Reply with: • its own link-layer address • IP of the proxyee (the mobile node). • The receiver of the Reply then associates the proxy’s link-layer address with the proxyee’s IP address, causing future datagrams being directed to the proxy.
