Internetworking: Addressing Sept. 18, 2003

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Internetworking AddressingSept. 18, 2003
15-441Computer Networking

• Topics
• Hierarchical Addressing
• Evolution of Internet Addressing
• Algorithmic Issues

class08.ppt
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What is an Internetwork?

• Multiple incompatible LANs can be physically
  connected by specialized computers called
  routers.
• The connected networks are called an
  internetwork.
• The Internet is one (very big successful)
  example of an internetwork

...
...
host
host
host
host
host
host
LAN 1
LAN 2
router
router
router
WAN
WAN
LAN 1 and LAN 2 might be completely different,
totally incompatible LANs (e.g., Ethernet and ATM)
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Issues in Designing an Internetwork

• How do I designate a distant host?
• Addressing / naming
• How do I send information to a distant host?
• Underlying service model
• What gets sent?
• How fast will it go?
• What happens if it doesnt get there?
• Routing
• Challenges
• Heterogeneity
• Assembly from variety of different networks
• Scalability
• Ensure ability to grow to worldwide scale

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Logical Structure of Internet
host
router
router
router
host
router
router
router

• Ad hoc interconnection of networks
• No particular topology
• Vastly different router link capacities
• Send packets from source to destination by
  hopping through networks
• Router forms bridge from one network to another
• Different packets may take different routes

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Routing Through Single Network
host/ router
router/ host

• Path Consists of Series of Hops
• Source -- Router
• Router -- Router (typically high-speed,
  point-to-point link)
• Router -- Destination
• Each Hop Uses Link-Layer Protocol
• Determine hop destination
• Based on destination destination
• Send over local network
• Put on header giving MAC address of intermediate
  router (or final destination)

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Router Operation

• Destination-Based Routing
• Move packet through network via series of hops
• When Packet Arrives at Router
• Examine header to determine intended destination
• Look up in table to determine next hop in path
• Send packet out appropriate port
• Terminology
• Each router forwards packet to next router
• Overall goal is to route packet from source to
  destination

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Possible Addressing Schemes

• Flat
• e.g., every host identified by its 48-bit MAC
  address
• Router would need entry for every host in the
  world
• Too big (although technology can help this)
• Too hard to maintain as hosts come go
• Hierarchy
• Address broken into segments of increasing
  specificity
• 412 (Pittsburgh area) 268 (Oakland exchange) 8821
  (Bryants office)
• Pennsylvania / Pittsburgh / Oakland / CMU /
  Bryant
• Route to general region and then work toward
  specific destination
• As people and organizations shift, only update
  affected routing tables

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Hierarchical Addressing Schemes

• Uniform Hierarchy
• Segment sizes same for everyone
• 412 (Pittsburgh area) 268 (Oakland exchange) 8821
  (Bryants office)
• System is more homogeneous and easier to control
• Requires more centralized planning
• Nonuniform Hierarchy
• Number sizes of segments vary according to
  destination
• Pennsylvania / Pittsburgh / Oakland / CMU /
  Bryant
• Delaware / Smallville / Bob Jones
• System is more heterogenous decentralized
• Allows more local autonomy

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IP Addressing

• IPv4 32-bit addresses
• Typically, write in dotted decimal format
• E.g., 128.2.198.135
• Each number is decimal representation of byte
• Big-Endian Order
• Translation from Network Names
• Performed by Domain Name Server (DNS)
• unix host bryant.vlsi.cs.cmu.edu
• bryant.vlsi.cs.cmu.edu has address 128.2.198.135

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IP Addressing and Forwarding

• Routing Table Requirement
• For every possible destination IP address, give
  next hop
• Nearly 232 (4.3 x 109) possibilities!
• Hierarchical Addressing Scheme
• Address split into network ID and host ID
• E.g., CMU has one network ID shared by all hosts
  within CMU
• All packets to given network follow same route
• Until they reach destination network
• Fields
• pfx Prefix to specify split between network
  host IDs
• network 2x possibilities
• host 2y possibilities

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IP Address Classes

• Class A
• mit.edu 18.7.22.69
• Class B
• cmu.edu 128.2.11.43
• Class C
• bryant.dsl.telerama.com 205.201.9.200
• Classes D, E, F
• Not commonly used

First digit 1126
First digit 128191
First digit 192223
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IP Address Classes

• Partitioning too Coarse
• Not enough big (class A) addresses
• No organization needs 16.7 million hosts
• Large organization likely to be geographically
  distributed
• Many organizations must make do with multiple
  class Cs
• Too many different Network IDs
• Routing tables must still have 2.1 million entries

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Subnetting

• Add Another Layer to Hierarchy
• From the outside, appears as one monolithic
  network
• Single entry in routing table
• Within network, manage as multiple subnetworks
• Internal routers must route according to subnet
  ID
• Subnet Mask
• Way to specify break between subnet ID and host
  ID
• Similar masks used in many contexts

pfx
network
host
subnet
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Host Routing Table Example
cygwinroute PRINT
Inter
face List 0x1 ........................... MS TCP
Loopback interface 0x1000003 ...00 03 47 b8 e5 f3
...... Intel(R) PRO/100 SP Mobile Combo
Adapter


Active Routes Network Destination
Netmask Gateway Interface
Metric 0.0.0.0 0.0.0.0
128.2.254.36 128.2.222.198 1
127.0.0.0 255.0.0.0 127.0.0.1
127.0.0.1 1 128.2.0.0 255.255.0.0
128.2.222.198 128.2.222.198 1
128.2.222.198 255.255.255.255 127.0.0.1
127.0.0.1 1 128.2.255.255
255.255.255.255 128.2.222.198 128.2.222.198
1 224.0.0.0 224.0.0.0
128.2.222.198 128.2.222.198 1
255.255.255.255 255.255.255.255 128.2.222.198
128.2.222.198 1 Default Gateway
128.2.254.36

• bryant-tp2.vlsi.cs.cmu.edu when plugged into CS
  ethernet
• Internet address 128.2.222.198
• Main CS router gigrouter.net.cs.cmu.edu
• Internet address 128.2.254.36

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Deciphering Table
Network Destination Netmask
Gateway Interface Metric 0.0.0.0
0.0.0.0 128.2.254.36
128.2.222.198 1 127.0.0.0
255.0.0.0 127.0.0.1 127.0.0.1 1
128.2.0.0 255.255.0.0 128.2.222.198
128.2.222.198 1 128.2.222.198
255.255.255.255 127.0.0.1 127.0.0.1
1 128.2.255.255 255.255.255.255
128.2.222.198 128.2.222.198 1
224.0.0.0 224.0.0.0 128.2.222.198
128.2.222.198 1 255.255.255.255
255.255.255.255 128.2.222.198 128.2.222.198
1 Default Gateway 128.2.254.36
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Resolving Table Ambiguities

• Address 128.2.222.198 matches 3 entries
• Longest Prefix Match
• Select entry with longest sequence of 1s in mask
• Most specific case

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Improving the Hierarchy

• Basic Idea of Hierarchy is Good
• Organizations of different sizes can be assigned
  different numbers of IP addresses
• Shortcomings of Class-Based Addressing
• Class A too coarse Class C too fine not enough
  Class Bs
• When fully deployed would have too many entries
  in routing table (2.1 million)
• Solution
• Hierarchy with finer gradation of network/host ID
  split

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Classless Interdomain Routing

• CIDR, pronounced cider
• Arbitrary Split Between Network Host IDs
• Specify either by mask or prefix length
• E.g., CMU can be specified as
• 128.2.0.0 with netmask 255.255.0.0
• 128.2.0.0/16

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Aggregation with CIDR

• Original Use Aggregate Class C Addresses
• One organization assigned contiguous range of
  class Cs
• e.g., Microsoft given all addresses 207.46.192.X
  -- 207.46.255.X
• Specify as CIDR address 207.46.192.0/18
• Represents 26 64 class C networks
• Use single entry in routing table
• Just as if were single network address

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Routing Table Entry Examples

• Snapshot From MAE-West Routing Table
• Probably out of date
• Note hole in table Nothing covers bytes 96 127

microsoft.com 207.46.245.214 207.46.245.222
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Splitting with CIDR

• Expose subnetting structure to external routers
• Example
• Class A address 12.X.X.X has 413 entries in
  MAE-WEST table
• Prefix lengths 8--24
• attbi.com
• Backbone services of ATT
• Geographically distributed
• Dont want all packets to concentrate to single
  region

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Size of Complete Routing Table

• Source www.cidr-report.org
• Shows that CIDR has kept table entries in check
• Currently require 124,894 entries for a complete
  table
• Only required by backbone routers

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IPv6 Addressing

• Main motivation for switch from IPv4
• Getting hard to manage 32-bit address allocation
• 128-Bit Addresses
• Standard unicast addresses 125 bits long (3-bit
  prefix)
• 4.2 x 1037 nodes
• Earth radius is 6371 km
• Metric 4.2 x 1037 / 4 ? (6.371 x 108)2 8 X
  1018 nodes / cm2
• Aggregation Levels (RFC 2374)
• 13 bits top-level (major providers)
• 24 bits next-level (intermediate level)
• 16 bits site-level (like current subnetting)
• 64 bits interface (like current MAC addresses)

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Implementing a Fast Router

• When Packet Arrives
• Must find matching table entries
• Choose match with longest prefix
• Send out on appropriate outbound port
• Performance Example
• Enterprise Router Entrasys XSR-4100
• 56 ports
• 500K packets/second
• 16K

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Implementing Matching
O(w) accesses w address length IPv4 w
32 IPv6 w 128

• Patricia Tree
• Binary Trie
• Follow tree branches according to address bits
• Look for longest match
• Performance
• OK for small tables and/or low-speed software
• Large tables yield poor cache performance

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01
111
000
001
1101
Extracted from slides by Marcel Waldvogel
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Hash Tables

• Generally good way to manage large data set
• Problem
• Finding maximum matching key
• Possible Solution
• Separate hash table for each prefix length
• Scan through them linearly

O(w) accesses
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Reducing Table Accesses

• Perform Binary Search on Prefix Lengths
• O(log w) lookups
• Difficulty
• Address can match for noncontiguous prefix
  lengths
• Must fill in some holes in table
• Reference
• Waldvogel, Varghese, Turner, Plattner Scalable
  High-Speed Prefix Matching SIGCOMM 1997

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Hardware Solutions

• Content-Addressable Memory
• Special memory that uses keys rather than
  addresses to lookup entries
• Can be extended to implement prefix matching
• Ternary CAM
• At each bit position, can specify either match
  1, match 0, or dont care
• Hard to make big fast enough
• But, its happening
• IDT (www.idt.com) Each CAM chip can hold 64K
  72-bit keys

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Important Concepts

• Hierarchical addressing critical for scalable
  system
• Dont require everyone to know everyone else
• Reduces amount of updating when something changes
• Nonuniform hiearchy useful for heterogenous
  networks
• Class-based addressing too coarse
• CIDR helps
• Move to IPv6 due to limited number of 32-bit
  addresses
• Implementation Challenge
• Longest prefix matching much more difficult than
  when no ambiguity