Lecture 4: Routing - University of California, San...

CSE 222A: Computer Communication Networks Alex C. Snoeren

Lecture 4:Routing

Thanks: Nick Feamster & Mike Freedman

Lecture 4 Overview

●  Routing review ◆  Alternative designs ◆  BGP

●  Paxson discussion

CSE 222A – Lecture 4: Routing 2

●  Building a forwarding table ◆  Computing paths between network elements ◆  … and figuring out where the end-hosts are ◆  … to map a destination address to an outgoing link

●  How to find the hosts? ◆  Learning/flooding ◆  Injecting into routing protocol ◆  Dissemination via different protocol ◆  Directory service

Finding the Hosts

3 CSE 222A – Lecture 4: Routing

Learning and Flooding●  When a frame arrives

◆  Inspect the source address

◆  Associate address with the incoming interface

●  When the frame has an unfamiliar destination ◆  Forward out all interfaces ◆  … except for the one where

the frame arrived

A

B

C

D

Switch learns how to reach A.

A

B

C

D

When in doubt, shout!

Used in Ethernet LANs


Inject into Routing Protocol●  Treat the end host (or subnet) as a node

◆  And disseminate in the routing protocol ◆  E.g., flood information about where addresses attach

3 2

2

1

1 4

1

4

5

3

u

s6Used in OSPF and IS-IS, especially in enterprise networks

. . .5 CSE 222A – Lecture 4: Routing

Disseminate w/Other Protocol●  Distribute using another protocol

◆  One router learns the route ◆  … and shares the information with other routers

learn a route to d(e.g., via BGP)

disseminate route to other routers

Internal BGP (iBGP) used in backbone networks


Directory Service●  Contact a service to learn the location

◆  Lookup the end-host or subnet address ◆  … and learn the label to put on the packet ◆  … to get the traffic to the right egress point

s

d

i

e

directory

“Host d is at egress e”

Encapsulate packet to send to egress e.

Used in some data centers


To Each His Own…●  Ethernet LAN: spanning tree, MAC learning, flooding

●  Enterprise: link-state routing, injecting subnet addresses

●  Backbone: link-state routing inside, path-vector routing with neighboring domains, and iBGP dissemination

●  Data centers: many different solutions, still in flux »  E.g., link-state routing or multiple spanning trees »  E.g., directory service or injection of subnets into routing protocol


Internet Routing: BGP

Route Advertisement

Autonomous Systems (ASes)

Session

Traffic Destination Next-hop AS Path

130.207.0.0/16

130.207.0.0/16

192.5.89.89

66.250.252.44

10578..2637

174… 2637


Two Flavors of BGP

●  External BGP (eBGP): exchanging routes between ASes

●  Internal BGP (iBGP): disseminating routes to external destinations among the routers within an AS

eBGP iBGP


Example BGP Routing Table> show ip bgp Network Next Hop Metric LocPrf Weight Path *>i3.0.0.0 4.79.2.1 0 110 0 3356 701 703 80 i *>i4.0.0.0 4.79.2.1 0 110 0 3356 i *>i4.21.254.0/23 208.30.223.5 49 110 0 1239 1299 10355 10355 i * i4.23.84.0/22 208.30.223.5 112 110 0 1239 6461 20171 i

The full routing table

> show ip bgp 130.207.7.237 BGP routing table entry for 130.207.0.0/16 Paths: (1 available, best #1, table Default-IP-Routing-Table) Not advertised to any peer 10578 11537 10490 2637 192.5.89.89 from 18.168.0.27 (66.250.252.45) Origin IGP, metric 0, localpref 150, valid, internal, best Community: 10578:700 11537:950 Last update: Sat Jan 14 04:45:09 2006

Specific entry. Can do longest prefix lookup: Prefix

AS path Next-hop


●  Local preference: numerical value assigned by routing policy. Higher values are more preferred.

●  AS path length: number of AS-level hops in the path ●  Multiple exit discriminator (“MED”): allows one AS to specify that

one exit point is more preferred than another. Lower values are more preferred.

●  eBGP over iBGP ●  Shortest IGP path cost to next hop: implements “hot potato”

routing ●  Router ID tiebreak: arbitrary tiebreak, since only a single “best”

route can be selected

BGP routes have the following attributes, on which the route selection process is based:


BGP Route Selection

Other BGP Attributes

●  Next-hop: IP address to send packets en route to destination. (Question: How to ensure that the next-hop IP address is reachable?)

●  Community value: Semantically meaningless. Used for passing around “signals” and labelling routes. More in a bit.

Next-hop: 4.79.2.1 iBGP

4.79.2.1 4.79.2.2

Next-hop: 192.5.89.89


Local Preference

●  Control over outbound traffic ●  Not transitive across ASes ●  Coarse hammer to implement route preference ●  Useful for preferring routes from one AS over another

(e.g., primary-backup semantics)

Primary

Backup

Higher local pref

Lower local pref

Destination


Communities and Local Pref

●  Customer expresses provider that a link is a backup ●  Affords some control over inbound traffic

Primary

Backup

“Backup” Community

Destination


AS Path Length

●  Among routes with highest local preference, select route with shortest AS path length

●  Shortest AS path != shortest path, for any interpretation of “shortest path”

Destination

Traffic


Hot-Potato Routing●  Prefer route with shorter IGP path cost to next-hop ●  Idea: traffic leaves AS as quickly as possible

I

New York Atlanta

Washington, DC

5 10

Dest.

Common practice: Set IGP weights in accordance with propagation delay (e.g., miles, etc.)

Traffic


Hot-Potato Dynamics●  Small changes in IGP weights can have significant

impact

18

I

San Fran New York

LA

5 10

Dest.

Traffic

11

CSE 222A – Lecture 4: Routing

Internet Business Model (Simplified)

●  Customer/Provider: One AS pays another for reachability to some set of destinations

●  “Settlement-free” Peering: Bartering. Two ASes exchange routes with one another.

Provider

Peer

Customer

Preferences implemented with local preference manipulation

Destination

Pay to use

Get paid to use

Free to use


End-to-End Routing Behavior●  Importance of paper

◆  Revitalized field of network measurement ◆  Use of statistical techniques to capture new types of

measurements ◆  Empirical findings of routing behavior

(motivation for future work)

●  Various routing pathologies ◆  Routing loops ◆  Erroneous ◆  Connectivity altered mid-stream ◆  Fluttering…


Measurement With Traceroute●  Traceroute tool to measure the forwarding path

◆  Send packets with TTL=1, 2, 3… ◆  Record the source of the “time exceeded” message

●  Useful, but introduces many challenges ◆  Path changes ◆  Non-participating nodes ◆  Inaccurate, two-way measurements

source destination

TTL=1Time exceeded

TTL=2


Questions●  Why can’t we measure the Internet more directly?

◆  What can we do about it?

●  Right division of labor between host and network? ◆  For path selection ◆  For network monitoring

●  How do we fix these routing problems? ◆  In a decentralized, federated network ◆  How to incentivize better network management


For Next Class…

●  Read and review Wetherall ’98

●  Keep thinking about term project ideas/groups ◆  Initial ideas due on Tuesday ◆  Email to Bhanu


Pathology type

Prevalence in 1995

Prevalence in 1996

Long-lived Routing loops

Short-lived Routing loops

Outage>30s

Total

0.065% ~

0.14% ~ same

same

0.96% 2.2%

3.4% 1.5%

End-to-End Routing Behavior


Routing Prevalence and Persistence●  Prevalence: How often is the route present in the routing

tables? ◆  Internet paths are strongly dominated by a single route

●  Persistence: How long do routes endure before changing? ◆  Routing changes occur over a variety of time scales


Routing Loops●  Persistent Routing Loops

◆  10 persistent routing loops in D1 ◆  50 persistent routing loops in D2

●  Temporary Routing Loops ◆  2 loops in D1 ◆  21 in D2

●  Location of Routing Loops: All in one AS


Erroneous and Transient Routing●  Transatlantic route to London via Israel!

●  Connectivity altered mid-stream ◆  10 cases in D1 ◆  155 cases in D2

●  Fluttering: Packets to the same flow changing mid-stream


Backup Slides on Paxson Paper

28

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?

●  Anecdote on recent study of persistent loops ◆  Provider has static route for customer prefix ◆  Customer has default route to the provider

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

●  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

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