Routing Jennifer Rexford Advanced Computer Networks Tuesdays/Thursdays 1:30pm-2:50pm

Goals of Today’s Lecture Routing –Routing vs. forwarding –Properties of routing protocols Internet routing architecture –Separation of intradomain and interdomain –Intra: metric-based and router level –Inter: policy-based at Autonomous System level Measuring the routing system –Challenges of measuring a decentralized, heterogeneous system –Discussion of Paxson97 (Labovitz98 next time)

Routing

What is a Route? A famous quotation from RFC 791 “A name indicates what we seek. An address indicates where it is. A route indicates how we get there.” -- Jon Postel

Forwarding vs. Routing Forwarding: data plane –Directing a data packet to an outgoing link –Individual router using a forwarding table Routing: control plane –Computing paths the packets will follow –Routers talking amongst themselves –Individual router creating a forwarding table

Why Does Routing Matter? End-to-end performance –Quality of the path affects user performance –Propagation delay, throughput, and packet loss Use of network resources –Balance of the traffic over the routers and links –Avoiding congestion by balancing load Transient disruptions during changes –Failures, maintenance, and load balancing –Limiting packet loss and delay during changes Realizing business objectives –Maximizing revenue or minimizing cost –Avoiding paths going through untrusted parties

Choosing a Routing Protocol Who is in charge of selecting the path? –The network or the end host? How complex are the path-selection goals? –Shortest-path vs. policy-based routing Are participants willing to cooperate? –Willing to share information? –Have a common goal in selecting paths? Is large-scale behavior a concern? –Stability of the network topology –State and message overhead –Disruptions during routing convergence

Many Kinds of Routing Protocols Link-state routing (Dijkstra) –Routers flood topology information –And compute (shortest) paths Distance-vector routing (Bellman-Ford) –Routers learn path costs from their neighbors –And select the neighbor along shortest path Policy-based path-vector routing –Routers learn full path from their neighbors –And select the most desirable path

Many Kinds of Routing Protocols (Continued) Source routing –End host or edge router learn the topology –And select the end-to-end path Route servers –Set of servers learn topology and compute routes –And tell all the routers how to forward packets Ad hoc routing –Routers keep track of a small neighborhood –And forward packets in (hopefully) right direction

Internet Routing Architecture

Two-Tiered Internet Routing Architecture Goal: distributed management of resources –Internetworking of multiple networks –Networks under separate administrative control Solution: two-tiered routing architecture –Intradomain: inside a region of control Okay for routers to share topology information Routers configured to achieve a common goal –Interdomain: between regions of control Not okay to share complete information Networks may have different/conflicting goals

Intradomain Routing: E.g., Shortest Path Routers belong to the same institution –Share a common, network-wide goal Metric-based routing protocols –Typically shortest-path routing –With configurable link weights

Interdomain Routing: Between Networks AS-level topology –Nodes are Autonomous Systems (ASes) –Destinations are prefixes (e.g., /8) –Edges are links and business relationships ClientWeb server

AS Numbers (ASNs) ASNs are 16 bit values through are “private” Level 3: 1 MIT: 3 Harvard: 11 Yale: 29 Princeton: 88 AT&T: 7018, 6341, 5074, … UUNET: 701, 702, 284, 12199, … Sprint: 1239, 1240, 6211, 6242, … … ASNs represent units of routing policy Currently around 30,000 in use.

Interdomain Routing: Border Gateway Protocol ASes exchange info about who they can reach –IP prefix: block of destination IP addresses –AS path: sequence of ASes along the path Policies configured by the AS’s operator –Path selection: which of the paths to use? –Path export: which neighbors to tell? “ /24: path (2,1)” “ /24: path (1)” data traffic

Measuring Internet Routing

Motivations for Measuring the Routing System Characterizing the Internet –Internet path properties –Demands on Internet routers –Routing convergence Improving Internet health –Protocol design problems –Protocol implementation problems –Configuration errors or attacks Operating a network –Detecting and diagnosing routing problems –Traffic shifts, routing attacks, flaky equipment, …

Techniques for Measuring Internet Routing Active probing –Inject probes along path through the data plane –E.g., using traceroute Passive route monitoring –Capture control-plane messages between routers –E.g., using tcpdump or a software router –E.g., dumping the routing table on a router Injecting network events –Cause failure/recovery at planned time and place –E.g., BGP route beacon, or planned maintenance

Internet Routing is Hard to Measure Nobody knows the Internet topology –No central registry of the AS-level graph –Little public information about intra-AS topologies Deploying monitoring infrastructure is hard –Forwarding: active probes of end-to-end paths –Routing: passive monitoring of routing messages Many measurement challenges –Network conditions vary by location –Network conditions change over time –One-way measurements are hard to collect –Controlled experiments are hard to do

Two Papers for Today Both early measurement studies of routing –Initially appeared at SIGCOMM’96 and ’97 –Both won the “best student paper” award And recently won the SIGCOMM “test of time” award! –Early glimpses into the health of Internet routing –Early wave of papers on Internet measurement Differences in emphasis –Paxson96: end-to-end active probing to measure the characteristics of the data plane –Labovitz97: passive monitoring of BGP update messages from several ISPs to characterize (in)stability of the interdomain routing system

Active Measurement: Traceroute Time-To-Live field in IP packet header –Source sends a packet with a TTL of n –Each router along the path decrements the TTL –“TTL exceeded” sent when TTL reaches 0 Traceroute tool exploits this TTL behavior source destination TTL=1 Time exceeded TTL=2 Send packets with TTL=1, 2, 3, … and record source of “time exceeded” message

Paxson Study: Forwarding Loops Forwarding loop –Packet returns to same router multiple times May cause traceroute to show a loop –If loop lasted long enough –So many packets traverse the loopy path Traceroute may reveal false loops –Path change that leads to a longer path –Causing later probe packets to hit same nodes Heuristic solution –Require traceroute to return same path 3 times

Paxson Study: Causes of Loops Transient vs. persistent –Transient: routing-protocol convergence –Persistent: likely configuration problem Challenges –Appropriate time boundary between the two? –What about flaky equipment going up and down? –Determining the cause of persistent loops? Causes of persistent loops –E.g., misconfiguration / /24

Paxson Study: Path Fluttering Rapid changes between paths –Multiple paths between a pair of hosts –Load balancing policies inside the network Packet-based load balancing –Round-robin or random –Multiple paths for packets in a single flow Flow-based load balancing –Hash of some fields in the packet header –E.g., IP addresses, port numbers, etc. –To keep packets in a flow on one path

Paxson Study: Routing Stability Route prevalence –Likelihood of observing a particular route –Relatively easy to measure with sound sampling –Poisson arrivals see time averages (PASTA) –Most host pairs have a dominant route Route persistence –How long a route endures before a change –Much harder to measure through active probes –Look for cases of multiple observations –Typical host pair has path persistence of a week

Paxson Study: Route Asymmetry Hot-potato routingOther causes –Asymmetric link weights in intradomain routing –Cold-potato routing, where AS requests traffic enter at particular place Consequences –Lots of asymmetry –One-way delay is not necessarily half of the round-trip time Customer A Customer B multiple peering points Provider A Provider B Early-exit routing

Conclusions Internet routing architecture –Two-tiered system –Intradomain is metric-based, with common goal –Interdomain is policy-based, reconciling different goals across ASes Behavior of complete systems is mysterious –Challenging to measure –Challenging to characterize, and diagnose Next time –Border Gateway Protocol (BGP) –Discussion of the Labovitz97 paper