AddressingAddressing

Jennifer RexfordJennifer Rexford

Advanced Computer NetworksAdvanced Computer Networkshttp://www.cs.princeton.edu/courses/archive/fall06/http://www.cs.princeton.edu/courses/archive/fall06/

cos561/cos561/Tuesdays/Thursdays 1:30pm-2:50pmTuesdays/Thursdays 1:30pm-2:50pm

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, …

• Today’s class: addressing

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

???

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

Overview of Today’s 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

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?

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

MAC Addresses

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

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

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

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

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

switch

A

B

C

D

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

A

B

C

D

Switch learns how to reach A.

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 won’t happen very often

A

B

C

D

Switch floods frame that is destined to C.

Switch Filtering/Forwarding

When switch receives a frame:

index switch table using MAC dest addressif entry found for destination

then { 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

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

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

addresseshost host DNS... host host DNS...

router router

1.2.3.0/24 5.6.7.0/24

1.2.3.7 1.2.3.156???

1.2.3.19

router

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)Frameheader

Packetheader

TCPheader

Userdata

Application gateway

Transport gateway

Router

Bridge, switch

Repeater, hub

IP Addresses

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)

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

LAN 1

... host host host

LAN 2

...

router router routerWAN WAN

LAN = Local Area NetworkWAN = Wide Area Network

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

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

Hierarchy Through Prefixes

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

addresses)

00001100 00100010 10011110 00000101

Network (24 bits) Host (8 bits)

12 34 158 5

Example IP Address and Subnet Mask

00001100 00100010 10011110 00000101

12 34 158 5

11111111 11111111 11111111 00000000

255 255 255 0

Address

Mask

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

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

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

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., AT&T 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…

04/21/23

Classless Inter-Domain Routing (CIDR)

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

Usually written as 12.4.0.0/15

04/21/23

CIDR = Hierarchy in Address Allocation

12.0.0.0/8

12.0.0.0/16

12.254.0.0/16

12.1.0.0/1612.2.0.0/1612.3.0.0/16

:::

12.253.0.0/16

12.3.0.0/2412.3.1.0/24

::

12.3.254.0/24

12.253.0.0/1912.253.32.0/1912.253.64.0/1912.253.96.0/1912.253.128.0/1912.253.160.0/1912.253.192.0/19

:::

• Prefixes are key to Internet scalability– Routing protocols and packet forwarding based on

prefixes– Today, routing tables contain ~150,000-200,000

prefixes

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…

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 AllocationRegDate: 1986-02-24

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

4.0.0.0/84.83.128.0/1712.0.0.0/812.34.158.0/24126.255.103.0/24

12.34.158.5destination

forwarding table

Serial0/0.1outgoing link

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)

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

NAT

inside

outside

10.0.0.1

10.0.0.2

138.76.29.7

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

arrivingclient

DHCP server233.1.2.5

DHCP discover(broadcast)

DHCP offer

DHCP request

DHCP ACK

(broadcast)

Growth in the Number of IP Prefixes

CIDR

pre-CIDR

Internetboom

Internetbust

recovery?

Vint Cerf and Bob Kahn

“A Protocol for Packet Network Intercommunication”

(IEEE Trans. on Communications, May 1974)

Written when Vint Cerf was an assistant professor at Stanford, and Bob Kahn was working at ARPA.

Life in the Early 1970s

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

ARPAnet satellite net

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, …

ARPAnet satellite net

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

Gateways Between Different Kinds of Networks

ARPAnet satellite net

Gateway • “Embed internetwork

packets in local packet format or extract them”

• Route (at internetwork level) to next gateway

gateway

Internetwork layer • Internetwork appears as

a single, uniform entity• Despite the

heterogeneity of the local networks

• Network of networks

Internetwork Packet Format

• 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

localheader

text checksumsourceaddress

dest.address

seq.#

bytecount

flagfield

internetwork header

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

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

Discussion

• What did they get right?– Which ideas were key to the Internet’s 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 wasn’t to support communication

between disparate packet-switched networks– Would you do anything differently?