1. ni306 cm addressing

7/31/2019 1. NI306 CM Addressing

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AddressingRoutage et routeurs

Promthe Spathis

Chargs de TD : Timur Freidman et Mohamed Diallo

{promethee.spathis, tim ur.freidman, mohamed.diallo}

@{lip6,upmc}.fr

http://www-rp.lip6.fr/~spathis/rout

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 French mail

Zip code: 94110

Street: place Jussieu

Building on street: 4

Room in building: 109

Name of occupant: Promthe Spathis

???

4

What is Routing?

A famous quotation from RFC 791A nameindicates what we seek.An addressindicates where it is.A routeindicates how we get there.

-- Jon Postel

5

Forwarding vs. Routing

Forwarding:data planeDirecting a data packet to an outgoing link

Individual router usinga forwarding table

Routing: control planeComputing paths the packets wil l follow

Routers talking amongst themselves

Individual router creatinga forwardingtable


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6

Why Does Routing Matter?

End-to-end performance

Quality of the path affects user performance

Propagation delay, throughput, and packet loss Use of network resources

Balance of the traff ic over the routers and links

Avoiding congestion by directing traffic to lightly-

loaded links

Transient disruptions during changes

Failures, maintenance, and load balancing

Limiting packet loss and delay during changes

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

Spoof ing of source addresses

Some Questions

Could every host on the Internet have anarbitrary, 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 sh ould allocate the addresses? Network provider? Device manufacturer?

Does the sender of the traffic need toauthenticate itself? The destination? What about spoofing and impersonation?

Comparing MAC and IP Addresses

MAC IP

Assignment Hard-coded inthe adaptor

Configured orlearned

Size 48 bits 32 bits (in v4)

Structure Flat Hierarchical

Portability Constant over lifeof the adapter

Changes withtime and location

Purpose Delivery within asingle network

Delivery acrossan inter-network

E.g., social security number vs. postal address


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MAC Addresses

MAC Addresses

Flat name space of 48 bitsTypically 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 addressAllocated 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 planeForwarding 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


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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 f rame

Allows switch to learn about the adapter

When the adapter needs to receive a frame?

Destination MAC address is in the f rame

Switch needs to f igure out how to get there

Motivation For Self Learning

Switches forward frames selectively

Forward f rames 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 sourceMAC address

Associate the address with the incominginterface

Store the mapping in the switch table Use a TTL f ield to eventually forget the mapping

A

B

C

D

Switch learnshow to reach A.

Self Learning: Handling Misses

When frame arrives with unfamiliar dest

Forward the f rame out all of the interfaces

except for the one where the frame arrived

Hopefully, this case wont happen very often

A

B

C

D

Switch floods framethat is destined to C.


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Switch Filtering/Forwarding

When switch receives a frame:

index switch table using MAC dest addressif entry found for destination

then {

if dest on segment f rom which frame arrivedthen drop the frame

else forward the frame on interface indicated

}

else floodforward on all but the interface

on which the frame arrived20

Flooding Can Lead to Loops

Switches sometimes need to broadcast frames

Upon receiving a frame with an unfamiliar destination

Upon receiving a frame sent to the broadcast address Broadcasting is implemented by f looding

Transmi tting frame out every interface

except the one where the fram e arrived

Flooding can lead to forwarding loops

E.g., if the network contains a cycle of switches

Either accidentally, or by design for higher reliability

21

Solution: Spanning Trees

Ensure the topology has no loops

Avoid using som e of the links when f looding

to avoid forming a loop Spanning tree

Sub-graph that covers all vertices but contains nocycles

Links not in the spanning tree do not forward frames

22

Constructing a Spanning Tree

Need a distributed algorithm

Switches cooperate to build the spanning tree

and adapt automatically when failures occur

Key ingredients of the algorithm

Switches need to elect a root The switch with the smallest identifier

Each switch identifies if its interfaceis on the shortest path from the root

And it exclude from the tree if not

Messages (Y, d, X)

From node X

Claiming Y is th e root

And the distance is d

root

One hop

Three hops


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Steps in Spanning Tree Algorithm

Initially, each switch thinks it is the root

Switch sends a message out every interface

identif ying itself as the root with distance 0

Example: switch X announces (X, 0, X)

Switches update their view of the root

Upon receiving a message, check the root id

If the new id is smaller, start viewing that switch as root

Switches compute their distance from the root

Add 1 to the distance received from a neighbor

Identify interfaces not on a shortest path to the root

and exclude them from the spanning tree

24

Example From Switch #4s Viewpoint

Switch #4 thinks it is the root

Sends (4, 0 , 4) message to 2 and 7

Then, switch #4 hears from #2

Receives (2, 0 , 2) message from 2

and thinks that #2 is the root

And realizes it is just one hop away

Then, switch #4 hears from #7 Receives (2, 1 , 7) from 7

And realizes this is a longer path

So, prefe rs its own one-hop path

And removes 4-7 link from the tree

1

2

3

4

5

67

25

Example From Switch #4s Viewpoint

Switch #2 hears about switch #1

Switch 2 hears (1, 1, 3) from 3

Switch 2 starts treating 1 as root

And send s (1, 2, 2 ) to neighbors

Switch #4 hears from switch #2 Switch 4 starts treating 1 as root And send s (1, 3, 4 ) to neighbors

Switch #4 hears from switch #7 Switch 4 receives (1, 3, 7) from 7

And realizes this is a long er path

So, prefe rs its own three-hop path

And removes 4 -7 Iink from the tree

1

2

3

4

5

67

26

Robust Spanning Tree Algorithm

Algorithm must react to failures

Failure of the root node

Need to e lect a ne w root, with the next lowest identifier

Failure of other switches and links

Need to recompute the spanning tree

Root switch continues sending messages

Periodically reannouncing itself as the root (1, 0, 1)

Other switches continue forwarding messages

Detecting f ailures through timeout (soft state!)

Switch waits to hear from others

Eventually times out and claims to be the root


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8/14

IP Addressing: Scalability Through Hierarchy

Hierarchy through IP prefixes

Routing between networks

Allocation of address blocks Non-uniform h ierarchy

More ef ficient 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 networkstogether, 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

...


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


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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 su bnet

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

...


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

Doesnt require adding a new forwarding entry

host host host

LAN 1

...host host host

LAN 2

...


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


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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 hostsNetwork Prefix

Use two 32-bit numbers to represent a network.Network number = IP address + Mask

Usually written as 12.4.0.0/15

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

41

Scalability: Address Aggregation

Provider is given 201.10.0.0/21

201.10.0.0/22 201.10 .4 .0/24 201 .10.5.0/24 201.10 .6.0/23

Provider

Routers in the rest of the Internet just need to know how toreach 201.10.0.0/21. The provider can direct the IP

packets to the appropriate customer. 42

But, Aggregation Not Always Possible

201.10.0.0/21

201.10.0.0/22 201.10.4.0/24 201.10.5.0/24 201.10.6.0/23

Provider 1 Provider 2

Multi-homedcustomer with 201.10.6.0/23 has twoproviders. Other parts of the Internet need to know how to

reach these destina tions through bothproviders.


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Scalability Through Hierarchy

Hierarchical addressing

Critical for scalab le system

Dont require everyone to know everyone else

Reduces amount of updating when something changes

Non-unif orm hierarchy

Usefu l for heterogeneous networks of dif ferent sizes

Initial class-based addressing was far too coarse

Classless InterDomain Routing (CIDR) helps

Next few slides Plots are # of prefixes vs. time

44Growth faster than improvements in equipment capability

Pre-CIDR (1988-1994): Steep Growth

45Efforts to aggregate (even decreases after IETF meetings!)

CIDR Deployed (1994-1996): Much Flatter

46Good use of aggregation, and peer pressure in CIDR report

CIDR Growth (1996-1998): Roughly Linear


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47 Internet boom and increased multi-homing

Boom Period (1998-2001): Steep Growth

48

Long-Term View (1989-2005): Post-Boom

Obtaining a Block of Addresses

Separation of control

Prefix: assigned toan institution

Addresses: assigned to nodes bythe 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

Longest Prefix Match Forwarding

Forwarding tables in IP routers

Maps each IP pref ix to next-hop link(s)

Destination-based forwarding

Packet has a destination address

Router identifies longest-matching prefix

Pushing complexity into f orwarding decisions

4.0.0.0/84.83.128.0/17

12.0.0.0/8

12.34.158.0/24126.255.103.0/24

12.34.158.5

destination

forwarding table

Serial0/0.1

outgoing link


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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)

53

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 with IPv4 Private addresses

Network address translation (NAT)

Dynamically-assigned addresses (DHCP)

Short-Term Solutions: Limping Along

Network Address Translation (ARES) 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 Share a pool of addresses among many hosts

Dynamically assign an IP address upon request

arrivingclient

DHCP se rver233.1.2.5


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Hard Policy Questions

How much address space per geographic region?

Equal am ount 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 horribly out of date70

Conclusions

IP address

A 32-bit number

Allocated in pref ixes

Non-uniform hierarchy for scalability and flexibility

Packet forwarding

Based on IP prefixes

Longest-prefix-match forwarding

Next lecture

IP routers

Well cover some topics later

Routing protocols, DHCP, and ARP

Growth in the Number of IP Prefixes

CIDR

pre-CIDR

Internet

boom

Internetbust

recovery?

1. ni306 cm addressing

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