Routing
Jennifer Rexford
Advanced Computer Networks
http://www.cs.princeton.edu/courses/archive/fall08/cos561/
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
2
3
2
1
1
1
3
5
4
3
Interdomain Routing: Between Networks
• AS-level topology
– Nodes are Autonomous Systems (ASes)
– Destinations are prefixes (e.g., 12.0.0.0/8)
– Edges are links and business relationships
4
3
5
2
1
Client
7
6
Web server
AS Numbers (ASNs)
ASNs are 16 bit values.
64512 through 65535 are “private”
Currently around 30,000 in use.
•
•
•
•
•
•
•
•
•
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
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?
“12.34.158.0/24: path (2,1)”
3
“12.34.158.0/24: path (1)”
1
2
data traffic
data traffic
12.34.158.5
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
TTL=1
source
Time
exceeded
destination
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
12.1.2.0/24
0.0.0.0/0
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 routing
• Other causes
– Asymmetric link weights
in intradomain routing
– Cold-potato routing,
where AS requests traffic
enter at particular place
Customer B
Provider B
multiple
peering
points
• Consequences
Early-exit
routing
Provider A
Customer A
– Lots of asymmetry
– One-way delay is not
necessarily half of the
round-trip time
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