IP Addressing

1. 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

2. 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

3. IP Addressing

4. 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?

5. 00001100 00100010 10011110 00000101 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: 12 34 158 5

6. 01010000 1000100 00010011 1110011 11110000 10110111 00110011 00000111 Examples • What address is this? • How would you represent 68.115.183.7 80.19.240.51

7. 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)

8. 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 host host host LAN 2 LAN 1 router router router WAN WAN Router

9. 1.2.3.4 1.2.3.5 Forwarding Table • Store mapping between IP addresses and output interfaces • Forward incoming packets based on destination address 1.2.3.5 1 1.2.3.6 3 1.2.3.4 2 … … 1 2

10. forwarding table 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 1.2.3.4 5.6.7.8 2.4.6.8 1.2.3.5 5.6.7.9 2.4.6.9 ... ... host host host host host host LAN 2 LAN 1 router router router WAN WAN 1.2.3.4 1.2.3.5

11. 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

12. 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 ???

13. 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

14. Hierarchical Structure • The Internet is an “inter-network” • Used to connect networks together, not hosts • Natural two-level hierarchy: • WAN delivers to right LAN • LAN delivers to right host ... ... host host host host host host LAN 2 LAN 1 router router router WAN WAN LAN = Local Area Network WAN = Wide Area Network

15. 00001100 00100010 10011110 00000101 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” 12 34 158 5 Network (23 bits) Host (9 bits)

16. 11111111 00001100 00100010 11111111 10011110 11111110 00000101 00000000 IP Address and a 23-bit Subnet Mask Address 12 34 158 5 255 255 254 0 Mask

17. 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 1.2.3.4 1.2.3.7 1.2.3.156 5.6.7.8 5.6.7.9 5.6.7.212 ... ... host host host host host host LAN 2 LAN 1 router router router WAN WAN 1.2.3.0/24 5.6.7.0/24 forwarding table

18. 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 1.2.3.4 1.2.3.7 1.2.3.156 5.6.7.8 5.6.7.9 5.6.7.212 ... ... host host host host host host LAN 2 LAN 1 router router router host WAN WAN 5.6.7.213 1.2.3.0/24 5.6.7.0/24 forwarding table

19. Original Internet Addresses • First eight bits: network address (/8) • Last 24 bits: host address Assumed 256 networks were more than enough!

20. 0******* 10****** 110***** ******** ******** ******** ******** ******** ******** ******** ******** ******** Next Design: Classful Addressing • Class A: if first byte in [0..127], assume /8 (top bit = 0) • Very large blocks (e.g., MIT has 18.0.0.0/8) • Class B: first byte in [128..191]  assume /16 (top bits = 10) • Large blocks (e.g,. UCB has* 128.32.0.0/16) • Class C: [192..223]  assume /24 (top bits = 110) • Small blocks (e.g., ICIR has 192.150.187.0/24) • (My house has a /25)

21. 1110**** 11110*** ******** ******** ******** ******** ******** ******** Classful Addressing (cont’d) • Class D: [224..239] (top bits 1110) • Multicast groups • Class E: [240..255] (top bits 11110) • 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

22. 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

23. 00001100 00000100 00000000 00000000 11111111 11111110 00000000 00000000 CIDR Addressing 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 Address Mask Network Prefix for hosts Written as 12.4.0.0/15 or 12.4/15

24. CIDR: Hierarchal Address Allocation • Prefixes are key to Internet scalability • Addresses allocated in contiguous chunks (prefixes) • Routing protocols and packet forwarding based on prefixes 12.0.0.0/15 : : : 12.2.0.0/16 12.3.0.0/22 12.3.0.0/16 12.3.4.0/24 : : : : 12.0.0.0/8 12.3.254.0/23 12.253.0.0/19 12.253.32.0/19 12.253.64.0/19 12.253.0.0/16 12.253.64.108/30 : 12.253.96.0/18 12.253.128.0/17

25. Scalability: Address Aggregation Provider is given 201.10.0.0/21 (201.10.0.x .. 201.10.7.x) Provider 201.10.0.0/22 201.10.4.0/24 201.10.5.0/24 201.10.6.0/23 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.

26. Aggregation Not Always Possible 201.10.0.0/21 Provider 1 Provider 2 201.10.6.0/23 201.10.0.0/22 201.10.4.0/24 201.10.5.0/24 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

27. Dot-com implosion; Internet bubble bursts Advent of CIDR allows aggregation: linear growth Initial growth super-linear; no aggregation Back in business Internet boom: multihoming drives superlinear growth Growth in Routed Prefixes (1989-2005)

28. 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

29. 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

30. 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.

31. 5 Minute Break Questions Before We Proceed?

32. Address Allocation

33. 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 • Allocates large address blocks to Regional Internet Registries • ICANN is politically charged • Regional Internet Registries (RIRs) • E.g., ARIN (American Registry for Internet Numbers) • Allocates address blocks within their regions • Allocated to Internet Service Providers and large institutions ($$) • Internet Service Providers (ISPs) • Allocate address blocks to their customers (could be recursive) • Often w/o charge

34. 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 • …

35. 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)

36. src addr src port dest addr 5.6.7.8 1.2.3.4 80 1001 dst port 80 1001 5.6.7.8 1.2.3.4 Network Address Translation (NAT) Before NAT… • Every machine connected to Internet had unique IP address Server LAN 1.2.3.4 Internet 5.6.7.8 1.2.3.5 Clients

37. 80 1001 5.6.7.8 192.2.3.4 5.6.7.8 1.2.3.4 80 2000 5.6.7.8 192.2.3.4 80 1001 80 2000 5.6.7.8 1.2.3.4 192.2.3.4:1001 1.2.3.4:2000 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 Server NAT 192.2.3.4 Internet 5.6.7.8 1.2.3.4 192.2.3.5 Clients

38. 80 1001 5.6.7.8 192.2.3.5 5.6.7.8 1.2.3.4 80 2001 5.6.7.8 192.2.3.5 80 1001 80 2001 5.6.7.8 1.2.3.4 192.2.3.5:1001 1.2.3.4:2001 192.2.3.4:1001 1.2.3.4:2000 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 Server NAT 192.2.3.4 Internet 5.6.7.8 1.2.3.4 192.2.3.5 Clients

39. 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

40. 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)

41. Quick Security Analysis

42. 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

43. IP Packet Structure 4-bit Header Length 8-bit Type of Service (TOS) 4-bit Version 16-bit Total Length (Bytes) 3-bit Flags 16-bit Identification 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

44. 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?

45. 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 • 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

46. Security Implications of IP’s Design 4-bit Header Length 8-bit Type of Service (TOS) 4-bit Version 16-bit Total Length (Bytes) 3-bit Flags 16-bit Identification 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

47. Security Implications, con’t • Version field (4 bits) …. ? • Issue: fledgling IPv6 deployment means sometimes connectivity exceeds security enforcement • E.g., firewall rules only set up for IPv4 • Header length (4 bits) …. ? • Controls presence of IP options • 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 path • Allows attacker to stress router for denial-of-service • Often, today’s firewalls configured to drop packets with options.

48. IP Packet Structure 4-bit Header Length 8-bit Type of Service (TOS) 4-bit Version 16-bit Total Length (Bytes) 3-bit Flags 16-bit Identification 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

49. 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 • Discussed later in course

50. IP Packet Structure 4-bit Header Length 8-bit Type of Service (TOS) 4-bit Version 16-bit Total Length (Bytes) 3-bit Flags 16-bit Identification 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