AddressingAddressing

Jennifer RexfordJennifer Rexford

Advanced Computer NetworksAdvanced Computer Networkshttp://www.cs.princeton.edu/courses/archive/fall08/http://www.cs.princeton.edu/courses/archive/fall08/

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

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 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

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?

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 (#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)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…

04/18/23

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

04/18/23

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?

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/

RoutingResearchGroup

• 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/RoutingResearchGroup

Design Philosophy of the DARPA Internet Protocols

David ClarkProc. ACM SIGCOMM, 1988

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

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

Design Consequences of the Goals

• Effective multiplexed utilization of existing networks– Packet switching, not circuit switching

• Continued communication despite network failures– Routers don’t 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

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?

Mismatch With Network Operators

• Accountability of network resources– But, routers don’t maintain state about transfers– But, measurement isn’t part of the infrastructure

• Reliability/predictability of services– But, IP doesn’t provide performance guarantees– But, equipment is not very reliable (no “five-9s”)

• Fine-grain control over the network– But, routers don’t 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

Mismatch With Security

• The Internet must support multiplexed utilization of existing interconnected networks– Doesn’t 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.html

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

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

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

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?