Addressing

1
Addressing

• Jennifer Rexford
• Advanced Computer Networks
• http//www.cs.princeton.edu/courses/archive/fall06
  /cos561/
• Tuesdays/Thursdays 130pm-250pm

2
Goals of the Course

• Study networked systems
• Large in size and scope
• Heterogeneous components
• Decentralized control
• Explore design trade-offs
• Scalability, performance, reliability,
  flexibility,
• And design principles
• Layering, end-to-end argument, late binding,
  hierarchy, randomization, indirection, caching,
• Todays class addressing

3
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

???
4
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

5
Overview of Todays Lecture

• 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 the Cerf/Kahn 1974 paper

6
Some Questions

• Could every host on the Internet have an
  arbitrary, unique numerical address?
• Would it scale?
• If hierarchy is necessary, how to do it?
• Tying the addressing to the topology routing?
• What about mobile hosts? Temporary addresses?
• 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?

7
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
8
MAC Addresses
9
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

10
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

11
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
12
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

13
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
14
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
15
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
16
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
17
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

18
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
19
COS 461 Hubs and Switches (11)

• 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
20
IP Addresses
21
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)

22
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
23
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
24
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)
25
Example IP Address and Subnet Mask
Address
12
34
158
5
255
255
255
0
Mask
26
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
27
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
28
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

29
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
30
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

31
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

32
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

33
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
34
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)

35
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

36
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

37
Growth in the Number of IP Prefixes
Internet bust
Internet boom
recovery?
CIDR
pre-CIDR
38
A Protocol for Packet Network Intercommunication
(IEEE Trans. on Communications, May 1974)

• Vint Cerf and Bob Kahn

Written when Vint Cerf was an assistant professor
at Stanford, and Bob Kahn was working at ARPA.
39
Life in the Early 1970s

• Multiple unconnected networks
• ARPAnet
• Data-over-cable
• Packet satellite (Aloha)
• Packet radio

satellite net
ARPAnet
40
Differences Across Packet-Switched Networks

• Addressing
• Maximum packet size
• Timing for handling success/failure of delivery
• Handling of lost or corrupted data
• Routing, fault detection, status information,

satellite net
ARPAnet
41
Where to Handle Heterogeneity?

• Application process?
• End host?
• Packet switches?
• Someplace else?
• Compatible process and host conventions
• Obviate the need to support all combinations
• Retain the unique features of each network
• Avoid changing the local network components
• Introduce the notion of a gateway

42
Gateways Between Different Kinds of Networks

• Gateway
• Embed internetwork packets in local packet
  format or extract them
• Route (at internetwork level) to next gateway

• Internetwork layer
• Internetwork appears as a single, uniform entity
• Despite the heterogeneity of the local networks
• Network of networks

gateway
satellite net
ARPAnet
43
Internetwork Packet Format
internetwork header
local header
text
checksum

• Internetwork header in standard format
• Interpreted by the gateways and end hosts
• Source and destination addresses
• Uniformly and uniquely identify every host
• Ensure proper sequencing of the data
• Include a sequence number and byte count
• Enable detection of corrupted text
• Checksum for an end-to-end check on the text

44
Process-Level Communication

• Enable pairs of processes to communicate
• Full duplex
• Unbounded but finite-length messages
• E.g., keystrokes or a file
• Key ideas
• Port numbers to (de)multiplex packets
• Breaking messages into segments
• Sequence numbers and reassembly
• Retransmission and duplicate detection
• Window-based flow control

45
Differences in Max Packet Size

• Select smallest packet size as the new max?
• Coordinate to determine max size on a path?
• Enable gateway to fragment a large packet?
• Reassembly by the next gateway? The receiver?
• Design trade-offs
• Coordination overhead for identifying the max
• Overhead of sending many small packets
• Overhead of buffering packets for reassembly

46
Discussion

• What did they get right?
• Which ideas were key to the Internets success?
• Which decisions still seem right today?
• What did they miss?
• Which ideas had to be added later?
• Which decisions seem wrong in hindsight?
• What would you do in a clean-slate design?
• If your goal wasnt to support communication
  between disparate packet-switched networks
• Would you do anything differently?