Internet Routing (COS 598A) Today: Addressing and Routing

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Internet Routing (COS 598A)Today Addressing and
Routing

• Jennifer Rexford
• http//www.cs.princeton.edu/jrex/teaching/spring2
  005
• Tuesdays/Thursdays 1100am-1220pm

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Outline

• IP addressing
• Address allocation blocks
• Packet forwarding
• Routing protocols
• Autonomous Systems
• Interdomain routing
• Intradomain routing
• Practical realities
• Asymmetric routing
• IP address ! host
• Autonomous System ! institution
• Discussion of Saltzer84 and Clark88

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IP Addressing

• 32-bit number in dotted-quad notation
  (12.34.158.5)
• Divided into network host portions (left and
  right)
• 12.34.158.0/24 is a 24-bit prefix with 28
  addresses

12
34
158
5
Network (24 bits)
Host (8 bits)
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Some History Why Dotted-Quad Notation?

• In the olden days
• Class A 0
• Very large /8 blocks (e.g., MIT has 18.0.0.0/8)
• Class B 10
• Large /16 blocks (e.g,. Princeton has
  128.112.0.0/16)
• Class C 110
• Small /24 blocks (e.g., ATT Labs has
  192.20.225.0/24)
• Class D 1110
• Multicast groups
• Class E 11110
• Reserved for future use (sounds a bit scary)
• And then, address space became scarce

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Classless Inter-Domain Routing (CIDR)
Use two 32-bit numbers to represent a network.
Network number IP address Mask
IP Address 12.4.0.0 IP Mask 255.254.0.0
Usually written as 12.4.0.0/15
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CIDR Hierarchy in Address Allocation

• Prefixes are key to Internet scalability
• Address allocation by ARIN/RIPE/APNIC and by ISPs
• Routing protocols and packet forwarding based on
  prefixes
• Today, routing tables contain 150,000-200,000
  prefixes

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Figuring Out Who Owns an Address

• Address registries
• Public record of address allocations
• ISPs should update when giving addresses to
  customers
• However, records are notoriously out-of-date
• Ways to query
• UNIX whois h whois.arin.net 128.112.136.35
• http//www.arin.net/whois/
• http//www.geektools.com/whois.php

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Example Output for 128.112.136.35

• OrgName Princeton University
• OrgID PRNU
• Address Office of Information Technology
• Address 87 Prospect Avenue
• City Princeton
• StateProv NJ
• PostalCode 08544-2007
• Country US
• NetRange 128.112.0.0 - 128.112.255.255
• CIDR 128.112.0.0/16
• NetName PRINCETON
• NetHandle NET-128-112-0-0-1
• Parent NET-128-0-0-0-0
• NetType Direct Allocation
• RegDate 1986-02-24

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Longest Prefix Match Forwarding

• Forwarding tables in IP routers
• Maps each IP prefix to next-hop link(s)
• Destination-based forwarding
• Packet has a destination address
• Router identifies longest-matching prefix
• Cute algorithmic problem very fast lookups

forwarding table
4.0.0.0/8 4.83.128.0/17 12.0.0.0/8 12.34.158.0/24
126.255.103.0/24
destination
12.34.158.5
outgoing link
Serial0/0.1
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Where do Forwarding Tables Come From?

• Routers have forwarding tables
• Map prefix to outgoing link(s)
• Entries can be statically configured
• E.g., map 12.34.158.0/24 to Serial0/0.1
• But, this doesnt adapt
• To failures
• To new equipment
• To the need to balance load
• That is where routing protocols come in

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Routing Protocols
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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
• Led to the use of different protocols

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Connections Between Networks
interdomain
protocols
dial-in access
ISP 2
private peering
intradomain
destination
protocols
IXP
ISP 1
gateway router
access router
ISP 3
destination
commercial
customer
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Internet Routing Architecture

• Divided into Autonomous Systems
• Distinct regions of administrative control
• Routers/links managed by a single institution
• Service provider, company, university,
• Hierarchy of Autonomous Systems
• Large, tier-1 provider with a nationwide backbone
• Medium-sized regional provider with smaller
  backbone
• Small network run by a single company or
  university
• Interaction between Autonomous Systems
• Internal topology is not shared between ASes
• but, neighboring ASes interact to coordinate
  routing

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AS Numbers (ASNs)
ASNs are 16 bit values.
64512 through 65535 are private
Currently around 20,000 in use.

• Level 3 1
• MIT 3
• Harvard 11
• Yale 29
• Princeton 88
• ATT 7018, 6341, 5074,
• UUNET 701, 702, 284, 12199,
• Sprint 1239, 1240, 6211, 6242,

ASNs represent units of routing policy
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Interdomain Routing (Between ASes)
Path 6, 5, 4, 3, 2, 1
4
3
5
2
6
7
1
Web server
Client
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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 ASs operator
• Path selection which of the paths to use?
• Path export which neighbors to tell?

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data traffic
data traffic
12.34.158.5
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Zooming in to AS 3

• Border router
• Learns BGP route from neighbor AS
• Creates forwarding-table entry for prefix
• But, how do the other routers get there?

Border router
12.34.158.0/24
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How do Other Routers Learn the BGP Route?

• Internal BGP
• iBGP sessions between the routers
• Allows other routers to get the big picture
• Simplest case full mesh of iBGP sessions

12.34.158.0/24 through red router
12.34.158.0/24
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How To Get to the Egress Router?

• Interior Gateway Protocol (OSPF/IS-IS)
• Routers flood information to learn topology
• Routers determine next hop to other routers
• Compute shortest paths based on the link weights
• Link weights configured by the operator

2
1
3
1
3
2
1
5
Use Serial0/0.1 to get to the red router
4
3
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Constructing the Forwarding Table

• Three protocols
• External BGP learn the external route
• Internal BGP propagate inside the AS
• IGP learn outgoing link on path to other router
• Router joins the data
• Prefix 12.34.158.0/24 reached through red router
• Red router reached via link Serial0/0.1
• Forwarding entry 12.34.158.0/24 ? Serial0/0.1
• Router forwards packets
• Lookup destination 12.34.158.5 in table
• Forward packet out link Serial0/0.1

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What if There are Multiple Choices?
Hot-potato routing
192.44.78.0/24
egress 2
egress 1
IGP distances
56
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This router has two BGP routes to 192.44.78.0/24.
Hot potato get traffic off of your network as
soon as possible. Go for egress 1!
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Two Kinds of Routing Protocols
Link State
Vectoring

• Topology information is flooded within the
  routing domain
• Best end-to-end paths are computed locally at
  each router.
• Best end-to-end paths determine next-hops.
• Based on minimizing some notion of distance
• Works only if policy is shared and uniform
• Examples OSPF, IS-IS

• Each router knows little about network topology
• Only best next-hops are chosen by each router for
  each destination.
• Best end-to-end paths result from composition of
  all next-hop choices
• Does not require any notion of distance
• Does not require uniform policies at all routers
• Examples RIP, BGP

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Practical Realities
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IP Address ! Host Machine

• Dynamic IP address assignment (DHCP)
• Single client may have multiple addresses over
  time
• Address may correspond to multiple clients over
  time
• Shared machines
• Multiple users on a shared compute server
• Transfers traveling through proxies and firewalls
• Multiple Web sites hosted on a single machine
• Replicated sites
• Multiple machines hosting a single (popular) Web
  site
• Addresses do not correspond to geographic
  location
• Similar prefix does not necessarily imply nearby
  hosts
• Single prefix may span hosts in large geographic
  region
• Source IP address may be spoofed (e.g., DoS
  attack)

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AS ! Institution

• Not equivalent to an AS
• Many institutions span multiple autonomous
  systems
• Some institutions do not have their own AS number
• Ownership of an AS may be hard to pinpoint
  (whois)
• Not equivalent to a block of IP addresses
  (prefix)
• Many institutions have multiple (non-contiguous)
  prefixes
• Some institutions are a small part of a larger
  address block
• Ownership of a prefix may be hard to pinpoint
  (whois)
• Not equivalent to a domain name (att.com)
• Some sites may be hosted by other institutions
• Some institutions have multiple domain names
  (att.net)

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Routing is Not Symmetric
Web request and TCP ACKs
client
server
Web response
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Discussion of Saltzer84

• End-to-end argument
• Better to implement functions close to
  application
• except when performance requires otherwise
• Why?
• What should be the end for routing?
• Router?
• End host?
• Enterprise edge?
• Autonomous System?

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Discussion of Clark88

• Basic story of Clark88
• Enumerate (and prioritize) system goals
• and see what decisions that leads you to make
• Clark88 doesnt say much about routing, but
• Some of the most significant problems with the
  Internet today relate to lack of sufficient tools
  for distributed management, especially in the
  area of routing.
• What should be goals priorities for routing?

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For Next Tuesdays Class

• Topology Inside an Autonomous System
• Measuring ISP topologies with RocketFuel
  (SIGCOMM02)
• A first-principles approach to understanding the
  Internets router-level topology (SIGCOMM04)
• Written review, one page each (hard-copy)
• Brief summary of the paper
• Reasons to accept the paper
• Reasons to reject the paper
• Three suggestions for future research directions
• Other materials
• Short paper on path diversity (Teixeira03)
• Internet topology pictures (Maps)