Addressing

1
Addressing

• Jennifer Rexford
• Advanced Computer Networks
• http//www.cs.princeton.edu/courses/archive/fall08
  /cos561/
• Tuesdays/Thursdays 130pm-250pm

2
What is Addressing?

• Providing suitable identifiers to nodes
• So you can direct data to a node
• So you know which node sent the data
• and how to send data back to that node
• Addressing in the U.S. mail
• Zip code 08540
• Street Olden Street
• Building on street 35
• Room in building 306
• Name of occupant Jennifer Rexford

???
3
Phone Numbers

• Hierarchical
• Country code (1)
• Area code (609)
• Local exchange (258)
• Subscriber number (5182)
• Some exceptions
• 800 indirection service (free for the caller)
• 900 indirection service (billed to the caller)
• Cell phone numbers, where the node is mobile
• ... blurring distinction between name and address

4
Overview of Todays Class

• Two widely-used addressing schemes
• Medium Access Control (MAC) addresses
• Internet Protocol (IP) addresses
• Key concepts in addressing
• Number of unique addresses
• Allocating addresses to nodes
• Flat vs. hierarchical structure
• Persistent vs. temporary identifiers
• Handling diminishing address space
• Spoofing of source addresses
• Discussion of Clark88 and Saltzer81 papers

5
Some Questions

• Could every host on the Internet have an
  arbitrary, unique numerical address?
• Would it scale?
• Is hierarchy necessary to make it scale?
• Tying the addressing to the topology routing?
• What about mobile hosts?
• Who should allocate the addresses?
• Network provider? Device manufacturer?
• Does the sender of the traffic need to
  authenticate itself? The destination?
• What about spoofing and impersonation?

6
Comparing MAC and IP Addresses
MAC IP
Assignment Hard-coded in the adaptor Configured or learned
Size 48 bits 32 bits (in v4)
Structure Flat Hierarchical
Portability Constant over life of the adapter Changes with time and location
Purpose Delivery within a single network Delivery across an inter-network
E.g., social security number vs. postal address
7
MAC Addresses
8
MAC Addresses

• Flat name space of 48 bits
• Typically written in six octets in hex
• E.g., 00-15-C5-49-04-A9 for my Ethernet
• Organizationally unique identifier
• Assigned by IEEE Registration Authority
• Determines the first 24 bits of the address
• E.g., 00-15-C5 corresponds to Dell Inc
• Remainder of the MAC address
• Allocated by the manufacturer
• E.g., 49-04-A9 for my Ethernet card

9
Scalability Challenges

• MAC addresses are flat
• Multiple hosts on the same network
• No relationship between MAC addresses
• Data plane
• Forwarding based on MAC address
• Table size? Look-up overhead?
• Control plane
• Determining where the host is located
• Keeping the information up-to-date

10
Forwarding Frames to Destination Adapter

• Shared media
• Forward all frames on the shared media
• Adapter grabs frames with matching dest address
• Multi-hop switched networks
• Flood every frame over every link?
• Learn where the MAC address is located?

...
host
host
host
host
host
host
host
11
When to Learn?

• When the adapter connects to the network?
• Requires adaptor to register its presence
• Overhead even when not sending/receiving
• Leading to control messages and large tables
• When the adapter sends a frame?
• Source MAC address is in the frame
• Allows switch to learn about the adapter
• When the adapter needs to receive a frame?
• Destination MAC address is in the frame
• Switch needs to figure out how to get there

12
Motivation For Self Learning

• Switches forward frames selectively
• Forward frames only on segments that need them
• Switch table
• Maps dest MAC address to outgoing interface
• Goal construct the switch table automatically

B
A
C
switch
D
13
Self Learning Building the Table

• When a frame arrives
• Inspect the source MAC address
• Associate the address with the incoming interface
• Store the mapping in the switch table
• Use a TTL field to eventually forget the mapping

B
Switch learns how to reach A.
A
C
D
14
Self Learning Handling Misses

• When frame arrives with unfamiliar dest
• Forward the frame out all of the interfaces
• except for the one where the frame arrived
• Hopefully, this case wont happen very often

Switch floods frame that is destined to C.
B
A
C
D
15
Switch Filtering/Forwarding

• When switch receives a frame
• index switch table using MAC dest address
• if entry found for destinationthen
• if dest on segment from which frame arrived
  then drop the frame
• else forward the frame on interface
  indicated
• else flood

forward on all but the interface on which the
frame arrived
16
MAC Addresses

• Disadvantages
• Large forwarding tables in the data plane
• Flooding overhead to learn location information
• Lack of privacy
• Advantages
• Persistent identifier (well, except for spoofing)
• Mobile hosts are easy to handle
• Forwarding-table look-up is a simple match

17
COS 461 Internet Control Protocols (8)

• Dynamic Host Configuration Protocol (DHCP)
• End host learns how to send packets
• Learn IP address, DNS servers, and gateway
• Address Resolution Protocol (ARP)
• Others learn how to send packets to the end host
• Learn mapping between IP and MAC addresses

???
1.2.3.7
1.2.3.156
...
...
host
DNS
host
DNS
host
host
5.6.7.0/24
1.2.3.0/24
1.2.3.19
router
router
router
18
COS 461 Hubs and Switches (10)

• Different devices switch different things
• Physical layer electrical signals (repeaters,
  hubs)
• Link layer frames (bridges, switches)
• Network layer packets (routers)
• Key ideas in switches
• Self learning of the switch table
• Cut-through switching
• Spanning trees
• Virtual LANs (VLANs)

Application gateway
Transport gateway
Router
Bridge, switch
Repeater, hub
Frameheader
Packetheader
TCPheader
User data
19
IP Addresses
20
IP Addressing Scalability Through Hierarchy

• Hierarchy through IP prefixes
• Routing between networks
• Allocation of address blocks
• Non-uniform hierarchy
• More efficient address allocation
• More complex packet forwarding
• Dealing with limited address space
• Larger address space (IPv6 with 128 bits)
• Sharing a small set of addresses (NAT)
• Dynamic assignment of addresses (DHCP)

21
Grouping Related Hosts

• The Internet is an inter-network
• Used to connect networks together, not hosts
• Needs a way to address a group of hosts

...
...
host
host
host
host
host
host
LAN 2
LAN 1
router
router
router
WAN
WAN
LAN Local Area Network WAN Wide Area Network
22
Scalability Challenge

• Suppose hosts had arbitrary IP 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
23
Hierarchy Through Prefixes

• Divided into network and host portions
• 12.34.158.0/24 is 24-bit prefix (28 addresses)

12
34
158
5
Network (24 bits)
Host (8 bits)
24
Example IP Address and Subnet Mask
Address
12
34
158
5
255
255
255
0
Mask
25
Scalability Improved

• Number related hosts from a common subnet
• 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
26
Easy to Add New Hosts

• No need to update the routers
• E.g., adding a new host 5.6.7.213 on the right
• Doesnt 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
27
Classful Addressing (and 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

28
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
29
CIDR Hierarchy in Address Allocation

• Prefixes are key to Internet scalability
• Routing protocols and packet forwarding based on
  prefixes
• Today, routing tables contain 150,000-200,000
  prefixes

30
Obtaining a Block of Addresses

• Separation of control
• Prefix assigned to an institution
• Addresses assigned to nodes by the institution
• Who assigns prefixes?
• Internet Corp. for Assigned Names and Numbers
• Allocates large blocks to Regional Internet
  Registries
• Regional Internet Registries (RIRs)
• E.g., ARIN (American Registry for Internet
  Numbers)
• Allocated to ISPs and large institutions in a
  region
• Internet Service Providers (ISPs)
• Allocate address blocks to their customers
• Who may, in turn, allocate to their customers

31
whois h whois.arin.net 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

32
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
• Pushing complexity into forwarding decisions

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
33
Are 32-bit Addresses Enough?

• Not all that many unique addresses
• 232 4,294,967,296 (just over four billion)
• Plus, some are 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 a larger address space
• IPv6 has 128-bit addresses (2128 3.403 1038)

34
Short-Term Solutions Limping Along

• Network Address Translation (COS 461 lecture 9)
• Allowing multiple hosts to share an IP address
• IP addresses not unique and not end-to-end

35
Short-Term Solutions Limping Along

• Dynamic Host Configuration Protocol (lecture 8)
• Share a pool of addresses among many hosts
• Dynamically assign an IP address upon request

36
Growth in the Number of IP Prefixes
Internet bust
Internet boom
recovery?
CIDR
pre-CIDR
37
Continued Growth in the Number of Prefixes

• Since 2005
• Now up to 250,000-300,000 prefixes
• Increased concern about scalability
• Data plane longest-prefix match lookup times
• Control plane memory and messages for routing
  protocols
• http//trac.tools.ietf.org/group/irtf/trac/wiki/Ro
  utingResearchGroup
• Exploration of architectural alternatives
• Avoid routers needing to know all prefixes
• Routing on ASes, using tunnels, caching, etc.

http//trac.tools.ietf.org/group/irtf/trac/wiki/Ro
utingResearchGroup
38
Design Philosophy of the DARPA Internet Protocols

• David Clark
• Proc. ACM SIGCOMM, 1988

39
Fundamental Goal

• Effective technique for multiplexed utilization
  of existing interconnected networks
• Concrete objective connect the ARPAnet and the
  ARPA packet radio network
• Must grapple with
• Diverse technologies, including legacy networks
• Separate administrative control

40
Second-Level Goals

• Main goals
• Survivability in the face of failure
• Multiple types of communication service
• Wide variety of network technologies
• Other goals
• Distributed management of resources
• Cost effectiveness
• Host attachment with low level of effort
• Accountability of resources

41
Design Consequences of the Goals

• Effective multiplexed utilization of existing
  networks
• Packet switching, not circuit switching
• Continued communication despite network failures
• Routers dont store state about ongoing transfers
• End hosts provide key communication services
• Support for multiple types of communication
  service
• Multiple transport protocols (e.g., TCP and UDP)
• Accommodation of a variety of different networks
• Simple, best-effort packet delivery service
• Packets may be lost, corrupted, or delivered out
  of order
• Distributed management of network resources
• Multiple institutions managing the network
• Intradomain and interdomain routing protocols

42
Different Goals, Different Outcomes

• What about the unique needs of
• Network operators for commercial carriers
• Secure, mission-critical networks (e.g.,
  military)
• Different goals, and different priorities
• How would the goals differ?
• How would the priorities differ?
• Different outcomes
• What design decisions would change?

43
Mismatch With Network Operators

• Accountability of network resources
• But, routers dont maintain state about transfers
• But, measurement isnt part of the infrastructure
• Reliability/predictability of services
• But, IP doesnt provide performance guarantees
• But, equipment is not very reliable (no
  five-9s)
• Fine-grain control over the network
• But, routers dont do fine-grain resource
  allocation
• But, network self-configures after failures
• End-to-end control over communication
• But, end hosts adapt to congestion
• But, traffic may traverse multiple domains

44
Mismatch With Security

• The Internet must support multiplexed utilization
  of existing interconnected networks
• Doesnt consider the need to balance trade-offs
  between interconnectivity and security
• Required security mechanisms are driven by the
  limitations of the least capable legacy network
• Internet communication must continue despite loss
  of networks or gateways
• Oversimplifies the nature of modern threats by
  not including cyberattacks, signals intelligence,
  

http//www.darpa.mil/STO/solicitations/AGN/index.h
tml
45
Mismatch With Security

• The Internet must support multiple types of
  communications service
• Mission-specific secure networks are not an
  appropriate arena for experimentation/innovation
• Lack of separation between user applications and
  network services needlessly exposes essential
  services to easy attack by users
• The Internet architecture must accommodate a
  variety of networks
• Cross-domain security solutions are particularly
  difficult to design

46
Mismatch With Security

• The Internet architecture must permit distributed
  management of its resources
• Existing distributed management is largely based
  on assumptions of trust, allowing a single inept
  or malicious user or administrator to create
  chaos
• Protocols do not have ways to limit damage from
  errors or malicious users
• Defensive systems are layered upon protocols at
  additional cost and complexity, instead of being
  jointly designed with those protocols
• Configuration complexity and human error are the
  largest source of vulnerability in many networks

47
Mismatch With Security

• Internet architecture must be cost effective
• To the extent that economic considerations may be
  a root cause of poor security, a criterion
  concerning the cost of network defense should
  perhaps have a higher priority
• Internet architecture must permit host attachment
  with a low level of effort
• It is unclear what minimum requirements must be
  imposed on end systems in a secure network
• Resources used must be accountable
• Authentication and accountability are central to
  availability, integrity, and confidentiality

48
Trade-Offs in Goals

• Is it possible to address these problems
• Decentralized management of the Internet
• Diverse layer-2 technologies like wireless
• Naïve, selfish, or malicious hosts
• Without sacrificing the other goals?
• Without a major change to the architecture?