Homework Assignment #1 1. Homework Assignment Part 1: LAN setup –All nodes are hosts (including...

Homework Assignment #1

1

Homework Assignment

• Part 1: LAN setup–All nodes are hosts (including middle nodes)–Each link is its own LAN, with its own IP subnet–Can ARP and ping only to directly-connected hosts

2

H

H

H

H

H

HH H

Homework Assignment

• Part 2: Writing your own switch–Middle nodes are switches and know nothing about IP–Switches transit Ethernet frames between interfaces–All hosts belong to a common IP subnet

3

H

H

H

H

H

HS S

Homework Assignment

• Part 3: Fun with OSPF–All nodes are routers running Quagga–Each link is its own (say, /30) subnet–Each node has an OSPF adjacency with each neighbor

4

R

R

R

R

R

RR R

Homework Suggestions

• Automation with scripts–Generate the host, Click, and OSPF configuration–Faster, less error-prone, and saves your work

• Checking the host configuration– ifconfig–arp -a

• Passive monitoring–Tcpdump on host interfaces–ListenEther on switches

• Start simple–E.g., two hosts connected to a single switch or router

5

Jennifer Rexford

Fall 2010 (TTh 1:30-2:50 in COS 302)

COS 561: Advanced Computer Networks

http://www.cs.princeton.edu/courses/archive/fall10/cos561/

Enterprise Configuration

Outline

• Enterprise network components–Repeaters/hubs, bridges/switches, and routers

• Enterprise network design–Hubs and switches, with DHCP server–Ethernet subnets interconnected by routers

• Flexible connectivity–Virtual Local Area Networks (VLANs)–Multi-homing to multiple ISPs– Interconnecting multiple enterprise locations

• Discussion of papers–VLAN survey and SEATTLE architecture

7

Enterprise Network Components

8

9

Physical Layer: Repeaters

• Distance limitation in local-area networks–Electrical signal becomes weaker as it travels– Imposes a limit on the length of a LAN

• Repeaters join LANs together–Analog electronic device–Continuously monitors electrical signals on each LAN–Transmits an amplified copy

Repeater

10

Physical Layer: Hubs

• Joins multiple input lines electrically–Do not necessarily amplify the signal–Very similar to repeaters

• Disadvantages–Limited aggregate throughput due to shared link–Cannot support multiple rates or formats

(e.g., 10 Mbps vs. 100 Mbps Ethernet)–Limitations on maximum # of

nodes and physical distance

hubhub

hub

11

Link Layer: Bridges

• Connects two or more LANs at the link layer–Extracts destination address from the frame–Looks up the destination in a table–Forwards the frame to the appropriate LAN segment

• Each segment can carry its own traffic

host host host host host

host host host host host

host

host

Bridge

12

Link Layer: Switches• Typically connects individual computers–A switch is essentially the same as a bridge–Supports concurrent communication

• Cut-through switching–Start forwarding a frame while it is still arriving

hub hub hub

switch/bridge

segment segment

segment

Hubs, Switches, and Routers

13

Hub/

Repeater

Bridge/

Switch

Router

Protocol layer physical link network

Traffic isolation no yes yes

Plug and play yes yes no

Efficient routing no no yes

Cut through yes yes no

Enterprise Network Design

14

Simple Enterprise Design

• A single layer-two subnet–Hubs and switches–Gateway router connecting to the Internet– ISP announces the address block into BGP

• Local services: DHCP and DNS

15

S

S

S

G

SDHCP server

DNS server

1.2.3.1

1.2.3.76

1.2.3.51.2.3.150

Internet

1.2.3.0/24

0.0.0.0/0

Scalability Limitations

• Spanning tree–Paths that are longer than necessary–Heavy load on the root bridge –Bandwidth wasted for links not in the tree

• Forwarding tables–Bridge tables grow with number of hosts

• Broadcast traffic–ARP and DHCP–Applications that broadcast (e.g., iTunes)

• Flooding–Frames sent to unknown destinations

16

Hybrid of Switches and Routers• Layer-two subnets interconnected by routers–No plug-and-play and mobility between layer-2 subnets–Need consistent configuration of IP routing and DHCP

17

R

R

R

R

Ethernet Bridging - Flat addressing - Self-learning

- Flooding - Forwarding along a tree

IP Routing - Hierarchical addressing - Subnet configuration - Host configuration

- Forwarding along shortest paths

R

1.2.3.192/26

1.2.3.128/26

1.2.3.0/26

1.2.3.64/26

Internet

Virtual Local Area Networks (VLANs)

18

19

Evolution Toward Virtual LANs• In the olden days…–Thick cables snaked through cable ducts in buildings–Every computer they passed was plugged in–All people in adjacent offices were put on the same LAN– Independent of whether they belonged together or not

• More recently…–Hubs and switches changed all that–Every office connected to central wiring closets–Often multiple LANs (k hubs) connected by switches–Flexibility in mapping offices to different LANs

Group users based on organizational structure, rather than the physical

layout of the building.

20

Why Group by Organizational Structure?• Privacy–Ethernet is a shared media–Any interface card can be put into “promiscuous” mode–… and get a copy of any flooded/broadcast traffic–So, isolating traffic on separate LANs improves privacy

• Load–Some LAN segments are more heavily used than others–E.g., researchers running experiments get out of hand–… can saturate their own segment and not the others–Plus, there may be natural locality of communication–E.g., traffic between people in the same research group

21

People Move, and Roles Change• Organizational changes are frequent–E.g., faculty office becomes a grad-student office–E.g., graduate student becomes a faculty member

• Physical rewiring is a major pain–Requires unplugging the cable from one port–… and plugging it into another–… and hoping the cable is long enough to reach–… and hoping you don’t make a mistake

• Would like to “rewire” the building in software–The resulting concept is a Virtual LAN (VLAN)

22

Example: Two Virtual LANs

Red VLAN and Orange VLANSwitches forward traffic as needed

R

ORO

R

R

R

OOOR OR R RO

O

O

23

Making VLANs Work

• Changing the Ethernet header–Adding a field for a VLAN tag– Implemented on the bridges/switches–… but can still interoperate with old Ethernet cards

• Bridges/switches trunk links–Saying which VLANs are accessible via which interfaces

• Approaches to mapping access links to VLANs–Each interface has a VLAN color

Only works if all hosts on same segment belong to same VLAN

–Each MAC address has a VLAN color Useful when hosts on same segment belong to different VLANs Useful when hosts move from one physical location to another

Multi-Homing

24

Motivation for Multi-Homing

• Benefits of multi-homing–Extra reliability, e.g., survive single ISP failure–Financial leverage through competition–Better performance by selecting better path–Gaming the 95th-percentile billing model

25

ISP 1 ISP 2

1.2.3.0/24

Multi-Homing Without BGP

Inbound Traffic

• Ask each ISP to originate the IP prefix

• … to rest of the Internet

Outbound Traffic

• One ISP as a primary, the other as a backup

• Or simple load balancing of all traffic

26

ISP 1 ISP 2

1.2.3.0/24

Multi-Homing With BGP

• Inbound traffic–Originate the prefix to both providers–Do not allow traffic from one ISP to another

• Outbound traffic–Select the “best” route for each remote prefix–Define BGP policies based on load, performance, cost

27

ISP 1 ISP 2

1.2.3.0/24

BGP sessions“Intelligent route control” or “multi-homed traffic engineering”.

Interconnecting Multiple Enterprise Sites

28

Challenges

• Challenges of interconnecting multiple sites–Performance–Reliability–Security–Privacy

• Solutions–Connecting via the Internet using secure tunnels–Virtual Private Network (VPN) service–Dedicated backbone between sites

29

Connecting Via the Internet

• Each site connects to the Internet–Encrypted tunnel between each pair of sites–Packet filtering to block unwanted traffic–But, no performance or reliability guarantees

30

Internet

Site 2

Site 3

Site 1

Virtual Private Network (VPN)• Each site connects to a common VPN provider–Provider allows each site to announce IP prefixes–Separate routing/forwarding table for each customer–Performance guarantees by overprovisioning resources

31

VPN Provider

Site 2

Site 3

Site 1

Conclusions

• Simple enterprise network is (mostly) plug and play–Ethernet with MAC learning and spanning tree–DHCP server to assign IP addresses from single subnet–Gateway router with default route to the Internet

• Quickly starts to require configuration–Choosing the root bridge in the spanning tree–Consistent configuration of DHCP and IP routers–VLAN access and trunk link configuration–Access control for traffic between VLANs–BGP sessions and routing policy

• Discussion of the two papers 32

Discussion

• Flat vs. hierarchical addressing?

• Roles of the end host vs. the network?

• How to best support flexible policies?

• Alternatives or extensions to VLANs?

33

Backup Slides on VLAN Survey

34

Uses of VLANs

• Scoping broadcast traffic

• Simplifying access control policies

• Decentralizing network management

• Enabling host mobility

35

Problem: Limited Granularity

• Limited number of VLANs–Placing multiple groups in the same VLAN–Reusing limited VLAN

• Limited number of hosts per VLAN–Divide a large group into multiple VLANs

• One VLAN per access port–Supporting VLANs on the end host–Supporting multiple groups at the router

36

Problem: Complex Configuration• Host address assignment–Wasting IP addresses–Complex host address assignment

• Spanning tree computation–Limitation of automated trunk configuration–Enabling extra links to survive failures–Distributing load over the root bridges

37

Backup Slides on SEATTLE

38

Avoiding Flooding

• Bridging uses flooding as a routing scheme–Unicast frames to unknown destinations are flooded

–Does not scale to a large network

• Objective #1: Unicast unicast traffic–Need a control-plane mechanism to discover and

disseminate hosts’ location information

“Send it everywhere! At least, they’ll learn where the source is.”

“Don’t know where destination is.”

Restraining Broadcasting

• Liberal use of broadcasting for bootstrapping(DHCP and ARP)–Broadcasting is a vestige of

shared-medium Ethernet–Very serious overhead in

switched networks

• Objective #2: Support unicast-based bootstrapping –Need a directory service

• Sub-objective #2.1: Yet, support general broadcast–Nonetheless, handling broadcast should be more scalable

Keeping Forwarding Tables Small• Flooding and self-learning lead to unnecessarily

large forwarding tables–Large tables are not only inefficient, but also dangerous

• Objective #3: Install hosts’ location information only when and where it is needed–Need a reactive resolution scheme–Enterprise traffic patterns are better-suited to reactive

resolution

Ensuring Optimal Forwarding Paths• Spanning tree avoids broadcast storms.

But, forwarding along a single tree is inefficient.–Poor load balancing and longer paths–Multiple spanning trees are insufficient

and expensive

• Objective #4: Utilize shortest paths–Need a routing protocol

• Sub-objective #4.1: Prevent broadcast storms–Need an alternative measure to prevent broadcast

storms

Backwards Compatibility• Objective #5: Do not modify end-hosts–From end-hosts’ view, network must work the same way

–End hosts should Use the same protocol stacks and applications Not be forced to run an additional protocol

SEATTLE in a Slide• Flat addressing of end-hosts

– Switches use hosts’ MAC addresses for routing– Ensures zero-configuration and backwards-compatibility (Obj # 5)

• Automated host discovery at the edge– Switches detect the arrival/departure of hosts– Obviates flooding and ensures scalability (Obj #1, 5)

• Hash-based on-demand resolution– Hash deterministically maps a host to a switch– Switches resolve end-hosts’ location and address via hashing– Ensures scalability (Obj #1, 2, 3)

• Shortest-path forwarding between switches– Switches run link-state routing to maintain only switch-level topology

(i.e., do not disseminate end-host information)– Ensures data-plane efficiency (Obj #4)

How does it work?

Host discovery or registration

B

D

x y

Hash(F(x) = B)

Store<x, A> at B

Traffic to x

Hash(F(x) = B)

Tunnel to egress node, A

Deliver to x

Switches

End-hosts

Control flowData flow

Notifying<x, A> to D

Entire enterprise(A large single IP subnet) LS core

E

Optimized forwarding directly from D to AC

A

Tunnel to relay switch, B

Terminology

Ingress

Relay (for x)

Egress

xy

B

A

Dst Src< x, A >

< x, A >

< x, A >

D

Ingress appliesa cache eviction

policyto this entry

shortest-path forwarding

47

Responding to Topology Changes• The quality of hashing matters!

A

B

CD

E

F

hh

h

h

hh

h

h

h

h

Consistent Hash minimizes

re-registration overhead

48

Single Hop Look-up

A

B

CD

F(x)

xy

y sends traffic to x

E

Every switch on a ring is

logically one hop away

49

Responding to Host Mobility

Relay (for x)

xy

B

A

Src< x, A >

< x, A >

< x, A >

D

when shortest-pathforwarding is used

G

< x, G >

Old Dst

New Dst

< x, G >

< x, G >

< x, G >

Unicast-based Bootstrapping: ARP• ARP –Ethernet: Broadcast requests –SEATTLE: Hash-based on-demand address resolution

1. Host discovery

2. Hashing F(IPa) = ra

3. Storing (IPa ,maca ,

sa)

4. BroadcastARP req

for a

5. HashingF(IPa) = ra

Switch

End-host

Control msgsARP msgs

sa

a

b

ra

sb

6. UnicastARP req

to ra 7. Unicast ARP reply

(IPa , maca , sa) to ingress

Owner of(IPa ,maca

)

Unicast Bootstrapping: DHCP

• DHCP–Ethernet: Broadcast requests and replies–SEATTLE: Utilize DHCP relay agent (RFC 2131)

Proxy resolution by ingress switches via unicasting

1. Host discovery

2. Hashing F(macd) = r

3. Storing (macd , sd)

4. BroadcastDHCP discovery

5. HashingF(0xDHCP) = r

Switch

End-host

Control msgsDHCP msgs

sd

d

h

r

sh

6. DHCP msg to r

DHCP server(macd=0xDHCP

)

7. DHCP msg to sd

8. Deliver DHCP

msg to d

Prototype Implementation• Link-state routing: eXtensible Open Router Platform

• Host information management and traffic forwarding: Click

Host info. registrationand notification messages

User/Kernel Click

XORP

OSPFDaemon

RingManager

Host Info

Manager

SeattleSwitch

Link-state advertisementsfrom other switches

Data Frames Data Frames

RoutingTable

NetworkMap

ClickInterfa

ce