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IP Addressing

EE 122: Intro to Communication Networks

Fall 2010 (MW 4-5:30 in 101 Barker)

Scott Shenker

TAs: Sameer Agarwal, Sara Alspaugh, Igor Ganichev, Prayag Narula

http://inst.eecs.berkeley.edu/~ee122/

Materials with thanks to Jennifer Rexford, Ion Stoica, Vern Paxsonand other colleagues at Princeton and UC Berkeley

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Goals of Today’s Lecture

• IP addressing

• Address allocation

• Brief security analysis of IP’s header design– Leftover from last lecture, will cover if have time

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IP Addressing

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Designing IP’s Addresses

• Question #1: what should an address be associated with?– E.g., a telephone number is associated not with a person

but with a handset

• Question #2: what structure should addresses have? What are the implications of different types of structure?

• Question #3: who determines the particular addresses used in the global Internet? What are the implications of how this is done?

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IP Addresses (IPv4)

• A unique 32-bit number

• Identifies an interface (on a host, on a router, …)

• Represented in dotted-quad notation. E.g, 12.34.158.5:

00001100 00100010 10011110 00000101

12 34 158 5

Examples

• What address is this?

• How would you represent 68.115.183.7

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01010000 00010011 11110000 00110011

1000100 1110011 10110111 00000111

80.19.240.51

What Are Addresses Used For?

• Network uses addresses to figure out where to forward packets

• Routers are the network devices that forward packets based on IP addresses

• What do “switches” do?– Route on Layer-2 addresses (e.g., MAC addresses)

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Routers

• Router consists of– Set of input interfaces where packets arrive– Set of output interfaces from which packets depart– Some form of interconnect connecting inputs to outputs

• Router implements– Forward packet to corresponding output interface– Manage bandwidth and buffer space resources

host host host

LAN 1

... host host host

LAN 2

...

router router routerWAN WAN

Router

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Forwarding Table

• Store mapping between IP addresses and output interfaces– Forward incoming packets based on destination address

……

31.2.3.6 11.2.3.5

1

21.2.3.5

1.2.3.4

1.2.3.4 2

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Scalability Challenge

• Suppose hosts had arbitrary addresses– Then every router would need a lot of information– …to know how to direct packets toward the host

host host host

LAN 1

... host host host

LAN 2

...

router router routerWAN WAN

1.2.3.4 5.6.7.8 2.4.6.8 1.2.3.5 5.6.7.9 2.4.6.9

1.2.3.4

1.2.3.5

forwarding table

Two Universal Tricks in CS

• When you need more flexibility, you add…– A layer of indirection

• When you need more scalability, you impose…– A hierarchical structure

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Hierarchical Addressing in U.S. Mail

• Addressing in the U.S. mail– Zip code: 94704– Street: Center Street– Building on street: 1947– Location in building: Suite 600– Name of occupant: Scott Shenker

• Forwarding the U.S. mail– Deliver letter to the post office in the zip code– Assign letter to mailman covering the street– Drop letter into mailbox for the building/room– Give letter to the appropriate person

???

Who Knows What?

• Does anyone in the US Mail system know where every house is?

• Separate routing tables at each level of hierarchy– Each of manageable scale

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Hierarchical Structure

• The Internet is an “inter-network”– Used to connect networks together, not hosts– Natural two-level hierarchy:

o WAN delivers to right LANo LAN delivers to right host

host host host

LAN 1

... host host host

LAN 2

...

router router routerWAN WAN

LAN = Local Area NetworkWAN = Wide Area Network

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Hierarchical Addressing

• Prefix is network address: suffix is host address

• 12.34.158.0/23 is a 23-bit prefix with 29 addresses– Terminology: “Slash 23”

00001100 00100010 10011110 00000101

Network (23 bits) Host (9 bits)

12 34 158 5

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IP Address and a 23-bit Subnet Mask

00001100 00100010 10011110 00000101

12 34 158 5

11111111 11111111 11111110 00000000

255 255 254 0

Address

Mask

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Scalability Improved

• Number related hosts with same prefix– 1.2.3.0/24 on the left LAN– 5.6.7.0/24 on the right LAN

host host host

LAN 1

... host host host

LAN 2

...

router router routerWAN WAN

1.2.3.4 1.2.3.7 1.2.3.156 5.6.7.8 5.6.7.9 5.6.7.212

1.2.3.0/24

5.6.7.0/24

forwarding table

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Easy to Add New Hosts

• No need to update the routers– E.g., adding a new host 5.6.7.213 on the right– Doesn’t require adding a new forwarding entry

host host host

LAN 1

... host host host

LAN 2

...

router router routerWAN WAN

1.2.3.4 1.2.3.7 1.2.3.156 5.6.7.8 5.6.7.9 5.6.7.212

1.2.3.0/24

5.6.7.0/24

forwarding table

host

5.6.7.213

Original Internet Addresses

• First eight bits: network address (/8)

• Last 24 bits: host address

Assumed 256 networks were more than enough!

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Next Design: Classful Addressing

– Class A: if first byte in [0..127], assume /8 (top bit = 0)

o Very large blocks (e.g., MIT has 18.0.0.0/8)

– Class B: first byte in [128..191] assume /16 (top bits = 10)

o Large blocks (e.g,. UCB has* 128.32.0.0/16)

– Class C: [192..223] assume /24 (top bits = 110)

o Small blocks (e.g., ICIR has 192.150.187.0/24)o (My house has a /25)

0******* ******** ******** ********

10****** ******** ******** ********

110***** ******** ******** ********

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Classful Addressing (cont’d)

– Class D: [224..239] (top bits 1110)

o Multicast groups

– Class E: [240..255] (top bits 11110)

o Reserved for future use

• What problems can classful addressing lead to?– Only comes in 3 sizes– Routers can end up knowing about many class C’s

1110**** ******** ******** ********

11110*** ******** ******** ********

Today’s Addressing: CIDR

• CIDR = Classless Interdomain Routing

• Flexible boundary between network and host addresses

• Must specify both address and mask, to clarify where the network address ends and the host address begins– Classful addressing communicate this with first few bits– CIDR requires explicit mask

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CIDR Addressing

IP Address : 12.4.0.0 IP Mask: 255.254.0.0

00001100 00000100 00000000 00000000

11111111 11111110 00000000 00000000

Address

Mask

for hosts Network Prefix

Use two 32-bit numbers to represent a network. Network number = IP address + Mask

Written as 12.4.0.0/15 or 12.4/15

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CIDR: Hierarchal Address Allocation

12.0.0.0/8

12.0.0.0/15

12.253.0.0/16

12.2.0.0/1612.3.0.0/16

::

12.3.0.0/2212.3.4.0/24

::

12.3.254.0/23

12.253.0.0/1912.253.32.0/1912.253.64.0/1912.253.64.108/3012.253.96.0/1812.253.128.0/17

:::

• Prefixes are key to Internet scalability– Addresses allocated in contiguous chunks (prefixes)– Routing protocols and packet forwarding based on prefixes

:

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Scalability: Address Aggregation

Provider is given 201.10.0.0/21 (201.10.0.x .. 201.10.7.x)

201.10.0.0/22 201.10.4.0/24 201.10.5.0/24 201.10.6.0/23

Provider

Routers in the rest of the Internet just need to know how to reach 201.10.0.0/21. The provider can direct the

IP packets to the appropriate customer.

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Aggregation Not Always Possible

201.10.0.0/21

201.10.0.0/22 201.10.4.0/24 201.10.5.0/24 201.10.6.0/23

Provider 1 Provider 2

Multi-homed customer with 201.10.6.0/23 has two providers. Other parts of the Internet need to know how

to reach these destinations through both providers. /23 route must be globally visible

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Growth in Routed Prefixes (1989-2005)

Initial growth super-linear; no aggregation

Advent of CIDR allows aggregation: linear growth

Internet boom: multihoming drives superlinear growth

Dot-com implosion; Internet bubble bursts

Back in business

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Special-Purpose Address Blocks• Private addresses

– By agreement, not routed in the public Internet– For networks not meant for general Internet connectivity– Blocks: 10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16

• Link-local– By agreement, not forwarded by any router– Used for single-link communication only– Intent: autoconfiguration (especially when DHCP fails)– Block: 169.254.0.0/16

• Loopback– Address blocks that refer to the local machine– Block: 127.0.0.0/8– Usually only 127.0.0.1/32 is used

• Limited broadcast– Sent to every host attached to the local network– Block: 255.255.255.255/32

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Summary of IP Addressing

Scalability Through Non-Uniform Hierarchy

• Hierarchical addressing – Critical for scalable system– Don’t require everyone to know everyone else– Reduces amount of updating when something changes

• Non-uniform hierarchy – Useful for heterogeneous networks of different sizes– Class-based addressing was far too coarse– Classless InterDomain Routing (CIDR) more flexible

Editorial

• Internet started with simple addressing design:– Two-layer hierarchy: network and host– Addresses in each hierarchy not tied to network topology

• Address exhaustion led to a less clean design– CIDR is based on flexible aggregation– Aggregation relies on topological numbering

• In “our” design, names were– Semantic-free– Verifiable

• Internet addresses violate both of these conditions– Spoofing, problems with mobility, etc.

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5 Minute Break

Questions Before We Proceed?

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Address Allocation

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Obtaining a Block of Addresses

• Allocation is also hierarchical– Prefix: assigned to an institution– Addresses: assigned by the institution to their nodes

• Who assigns prefixes?– Internet Corporation for Assigned Names and Numbers

o Allocates large address blocks to Regional Internet Registrieso ICANN is politically charged

– Regional Internet Registries (RIRs)o E.g., ARIN (American Registry for Internet Numbers)o Allocates address blocks within their regionso Allocated to Internet Service Providers and large institutions ($$)

– Internet Service Providers (ISPs)o Allocate address blocks to their customers (could be recursive)

• Often w/o charge

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Figuring Out Who Owns an Address

• Address registries–Public record of address allocations–Internet Service Providers (ISPs) should update

when giving addresses to customers–However, records are notoriously out-of-date

• Ways to query–UNIX: “whois –h whois.arin.net 169.229.60.27”–http://www.arin.net/whois/–http://www.geektools.com/whois.php–…

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Are 32-bit Addresses Enough?

• Not all that many unique addresses– 232 = 4,294,967,296 (just over four billion)– Plus, some (many) reserved for special purposes– And, addresses are allocated in larger blocks

• And, many devices need IP addresses– Computers, PDAs, routers, tanks, toasters, …

• Long-term solution (perhaps): larger address space– IPv6 has 128-bit addresses (2128 = 3.403 × 1038)

• Short-term solutions: limping along with IPv4– Private addresses– Dynamically-assigned addresses (DHCP)– Network address translation (NAT)

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Network Address Translation (NAT)

Before NAT…– Every machine connected to Internet had unique IP address

1.2.3.4

1.2.3.5

5.6.7.8

LAN

Clients

Server

Internet1.2.3.45.6.7.880 1001

dest addr src addrdst port

src port

5.6.7.8 1.2.3.4 80 1001

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NAT (cont’d)

• Independently assign addresses to machines behind same NAT– Usually in address block 192.168.0.0/16

• Use bogus port numbers to multiplex/demultiplex internal addresses

192.2.3.4

192.2.3.5

5.6.7.8

Clients

Server

Internet

NAT

1.2.3.4

5.6.7.8 192.2.3.4 80 1001

192.2.3.4:1001 1.2.3.4:2000

5.6.7.8 1.2.3.4 80 2000

1.2.3.45.6.7.880 2000

5.6.7.8 192.2.3.480 1001

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NAT (cont’d)

• Independently assign addresses to machines behind same NAT– Usually in address block 192.168.0.0/16

• Use bogus port numbers to multiplex demultiplex internal addresses

192.2.3.4

192.2.3.5

5.6.7.8

Clients

Server

Internet

NAT

1.2.3.4

192.2.3.4:1001 1.2.3.4:2000

5.6.7.8 1.2.3.4 80 2001

1.2.3.45.6.7.880 2001

5.6.7.8 192.2.3.580 1001

192.2.3.5:1001 1.2.3.4:2001

5.6.7.8 192.2.3.5 80 1001

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Hard Policy Questions

• How much address space per geographic region?– Equal amount per country?– Proportional to the population?– What about addresses already allocated?

• Address space portability?– Keep your address block when you change providers?– Pro: avoid having to renumber your equipment– Con: reduces the effectiveness of address aggregation

• Keeping the address registries up to date?– What about mergers and acquisitions?– Delegation of address blocks to customers?– As a result, the registries are often out of date

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Summary of IP Addressing

• 32-bit numbers identify interfaces

• Allocated in prefixes

• Non-uniform hierarchy for scalability and flexibility– Routing is based on CIDR

• A number of special-purpose blocks reserved

• Address allocation:– ICANN RIR ISP customer network host

• Issues to be covered later– How hosts get their addresses (DHCP)– How to map from an IP address to a link address (ARP)

Quick Security Analysis

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Focus on Sender Attacks

• Ignore (for now) attacks by others:– Traffic analysis– Snooping payload– Denial of service

• Here we look at vulnerabilities sender can exploit

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IP Packet Structure

4-bitVersion

4-bitHeaderLength

8-bitType of Service

(TOS)16-bit Total Length (Bytes)

16-bit Identification3-bitFlags 13-bit Fragment Offset

8-bit Time to Live (TTL) 8-bit Protocol 16-bit Header Checksum

32-bit Source IP Address

32-bit Destination IP Address

Options (if any)

Payload

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IP Address Integrity

• Source address should be the sending host– But, who’s checking, anyway?– You could send packets with any source you want– Why is checking hard?

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IP Address Integrity, con’t

• Why would someone use a bogus source address?

• Launch a denial-of-service attack– Send excessive packets to the destination– … to overload the node, or the links leading to the node– But: victim can identify/filter you by the source address

• Evade detection by “spoofing”– Put someone else’s source address in the packets

o Or: use a lot of different ones so can’t be filtered

• Or: as a way to bother the spoofed host– Spoofed host is wrongly blamed– Spoofed host may receive return traffic from the receiver

Security Implications of IP’s Design

4-bitVersion

4-bitHeaderLength

8-bitType of Service

(TOS)16-bit Total Length (Bytes)

16-bit Identification3-bitFlags 13-bit Fragment Offset

8-bit Time to Live (TTL) 8-bit Protocol 16-bit Header Checksum

32-bit Source IP Address

32-bit Destination IP Address

Options (if any)

Payload

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Security Implications, con’t

• Version field (4 bits) …. ?– Issue: fledgling IPv6 deployment means sometimes

connectivity exceeds security enforcemento E.g., firewall rules only set up for IPv4

• Header length (4 bits) …. ?– Controls presence of IP options

o E.g., Source Route lets sender control path taken through network - say, sidestep security monitoring

– Non-obvious difficulty: IP options often processed in router’s slow patho Allows attacker to stress router for denial-of-service

– Often, today’s firewalls configured to drop packets with options.

IP Packet Structure

4-bitVersion

4-bitHeaderLength

8-bitType of Service

(TOS)16-bit Total Length (Bytes)

16-bit Identification3-bitFlags 13-bit Fragment Offset

8-bit Time to Live (TTL) 8-bit Protocol 16-bit Header Checksum

32-bit Source IP Address

32-bit Destination IP Address

Options (if any)

Payload

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Security Implications of TOS? (8 bits)

• What if attacker sets TOS for their flooding traffic for prioritized delivery?– If regular traffic does not set TOS, then network prefers

the attack traffic, greatly compounding damage

• What if network charges for TOS traffic …– … and attacker spoofs the victim’s source address?

(denial-of-money)

• In general, in today’s network TOS does not work– Due to very hard problems with billing– TOS has now been redefined for Differential Service

o Discussed later in course

IP Packet Structure

4-bitVersion

4-bitHeaderLength

8-bitType of Service

(TOS)16-bit Total Length (Bytes)

16-bit Identification3-bitFlags 13-bit Fragment Offset

8-bit Time to Live (TTL) 8-bit Protocol 16-bit Header Checksum

32-bit Source IP Address

32-bit Destination IP Address

Options (if any)

Payload

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Security Implications of Fragmentation?

• Allows evasion of network monitoring/enforcement

• E.g., split an attack across multiple fragments– Packet inspection won’t match a “signature”

• E.g., split TCP header across multiple fragments– Firewall can’t tell anything about connection associated

with traffic

• Both of these can be addressed by monitor remembering previous fragments– But that costs state

Nasty-atOffset=0

tack-bytesOffset=8

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Fragmentation Attacks, con’t

• What if 2 overlapping fragments are inconsistent?

• How does network monitor know whether receiver sees USERNAME NICE or USERNAME EVIL?

USERNAMEOffset=0

NICE

Offset=8

EVILOffset=8

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Fragmentation Attacks, con’t

• What if fragments exceed IP datagram limit?

– Maximum size of 13-bit field: 0x1FFF = 8191Byte offset into final datagram = 8191*8 = 65528Length of final datagram = 65528 + 9 = 65537

• Result: kernel crash– Denial-of-service using just a few packets– Fixed in modern OS’s

NineBytes

Offset=65528

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Fragmentation Attacks, con’t

• What happens if attacker doesn’t send all of the fragments in a datagram?

• Receiver (or firewall) winds up holding the ones they receive for a long time– State-holding attack

IP Packet Structure

4-bitVersion

4-bitHeaderLength

8-bitType of Service

(TOS)16-bit Total Length (Bytes)

16-bit Identification3-bitFlags 13-bit Fragment Offset

8-bit Time to Live (TTL) 8-bit Protocol 16-bit Header Checksum

32-bit Source IP Address

32-bit Destination IP Address

Options (if any)

Payload

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Security Implications of TTL? (8 bits)

• Allows discovery of topology (a la traceroute)

• Can provide a hint that a packet is spoofed– It arrives at a router w/ a TTL different than packets from

that address usually haveo Because path from attacker to router has different # hops

– Though this is brittle in the presence of routing changes

• Initial value that’s picked is somewhat distinctive to sender’s operating system. This plus other such initializations allow OS fingerprinting …– Which in turn can allow attacker to infer its likely

vulnerabilities

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Security Implications of Remainder?

• No apparent problems with protocol field (8 bits)– It’s just a demux’ing handle– If value set incorrectly, next higher layer will find packet

ill-formed

• Similarly, bad IP checksum field (16 bits) will very quickly cause packet to be discarded by the network

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Next Lecture

• IP Forwarding; Transport protocols

• Read K&R: 3-3.4