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EECS 582Advanced Operating Systems

James PriestleyUniversity of Michigan

Day 4: InternetworkingJanuary 12, 2012

Credit for the majority of these slides goes to Professor Dutta

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What is the Objective of Networking?

•Communication between applications on different computers

•“To facilitate the sharing of computer resources"

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Networking Constraints

•Must understand application needs/demands•Traffic data rate•Traffic pattern (bursty or constant bit rate)•Traffic target (multipoint or single destination, mobile or

fixed)•Delay sensitivity•Loss sensitivity

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Back in the Old Days…

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Circuit Switching

R

R

R

RRH

H

H

H

R

RH

R

H: Hosts

R: Routers

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Circuit Switching

R

R

R

RRH

H

H

H

R

RH

R

H: Hosts

R: Routers

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Packet Switching (Internet)

Packets

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Packet Switching

•Interleave packets from different sources

•Efficient: resources used on demand•Statistical multiplexing

•General•Multiple types of applications

•Accommodates bursty traffic•Addition of queues

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Characteristics of Packet Switching

•Store and forward•Packets are self contained units•Can use alternate paths – reordering

•Contention•Congestion•Delay

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Internet[work]

Internet[work]

•A collection of interconnected networks

•Host: network endpoints (computer, PDA, light switch, …)

•Router: node that connects networks

•Internet vs. internet

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Networking Networks

A

B D

C

1

3

2

4

?

How can different networks communicate?

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Networking Networks

A

B D

C

1

3

2

4

GATEWAY

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Challenge

•Many differences between networks•Address formats•Performance – bandwidth/latency•Packet size•Loss rate/pattern/handling•Routing

•How to translate between various network technologies?

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How To Find Nodes?

Internet

Computer 1 Computer 2

Need naming and routing

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Naming

What’s the IP address for www.cmu.edu?

It is 128.2.11.43

Translates human readable names to logical endpoints

Local DNS ServerComputer 1

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Routing

R

R

R

RRH

H

H

H

R

RH

R

Routers send packet towards destination

H: Hosts

R: Routers

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Meeting Application Demands

•Reliability•Corruption•Lost packets

•Flow and congestion control

•Fragmentation

•In-order delivery

•Etc…

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What if the Data gets Corrupted?

InternetGET windex.htmlGET index.html

Solution: Add a checksum

Problem: Data Corruption

0,9 96,7,8

21

4,5 71,2,3

6X

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What if Network is Overloaded?

Problem: Network Overload

•Short bursts: buffer•What if buffer overflows?

•Packets dropped•Sender adjusts rate until load = resources → “congestion control”

Solution: Buffering and Congestion Control

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What if the Data gets Lost?

InternetGET index.html

Problem: Lost Data

InternetGET index.html

Solution: Timeout and Retransmit

GET index.htmlGET index.html

Acknowledgement

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Problem: Packet size

Solution: Fragment data across packets

What if the Data Doesn’t Fit?

•On Ethernet, max IP packet is 1.5kbytes

•Typical web page is 10kbytes

GETindex.html

GET index.html

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Solution: Add Sequence Numbers

Problem: Out of Order

What if the Data is Out of Order?

GETx.htindeml

GET x.htindeml

GET index.html

ml 4inde

2 x.ht 3GET

1

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Lots of Functions Needed

•Link

•Multiplexing

•Routing

•Addressing/naming (locating peers)

•Reliability

•Flow control

•Fragmentation

•Etc….

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What is Layering?

•Modular approach to network functionality

•Example:

Link hardware

Host-to-host connectivity

Application-to-application channels

Application

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Protocols

•Module in layered structure

•Set of rules governing communication between network elements (applications, hosts, routers)

•Protocols define:•Interface to higher layers (API) •Interface to peer

•Format and order of messages

•Actions taken on receipt of a message

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Layering Characteristics

•Each layer relies on services from layer below and exports services to layer above

•Interface defines interaction

•Hides implementation - layers can change without disturbing other layers (black box)

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Layering

Host Host

Application

Transport

Network

Link

User A User B

Layering: technique to simplify complex systems

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E.g.: OSI Model: 7 Protocol Layers

•Physical: how to transmit bits

•Data link: how to transmit frames

•Network: how to route packets

•Transport: how to send packets end2end

•Session: how to tie flows together

•Presentation: byte ordering, security

•Application: everything else

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OSI Layers and Locations

Switch RouterHost Host

Application

Transport

Network

Data Link

Presentation

Session

Physical

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Layer Encapsulation

Get index.html

Connection ID

Source/Destination

Link Address

User A User B

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Protocol Demultiplexing

•Multiple choices at each layer

FTP HTTP TFTPNV

TCP UDP

IP

NET1 NET2 NETn…

TCP/UDPIPIPX

Port Number

Network

Protocol Field

Type Field

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Is Layering Harmful?

•Sometimes …•Layer N may duplicate lower level functionality (e.g.,

error recovery)•Layers may need same info (timestamp, MTU)•Strict adherence to layering may hurt performance

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The need for an internet

Why did we need an internet?

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Connecting Networks

•How to internetwork various network technologies

•ARPANET, X.25 networks, LANs, satellite networks, packet networks, serial links…

•Many differences between networks•Address formats•Performance – bandwidth/latency•Packet size•Loss rate/pattern/handling•Routing

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Challenge 1: Address Formats

•Map one address format to another?•Bad idea → many translations needed

•Provide one common format•Map lower level addresses to common format

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Challenge 2: Different Packet Sizes

•Define a maximum packet size over all networks?•Either inefficient or high threshold to support

•Implement fragmentation/re-assembly•Who is doing fragmentation?•Who is doing re-assembly?

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Gateway Alternatives

•Translation•Difficulty in dealing with different features supported by

networks•Scales poorly with number of network types (N^2

conversions)

•Standardization•“IP over everything” (Design Principle 1)•Minimal assumptions about network•Hourglass design

Standardization

•Minimum set of assumptions for underlying net•Minimum packet size•Reasonable delivery odds, but not 100%•Some form of addressing unless point to point

•Important non-assumptions:•Perfect reliability•Broadcast, multicast•Priority handling of traffic•Internal knowledge of delays, speeds, failures, etc.

•Much engineering then only has to be done once

IP Hourglass

•Need to interconnect many existing networks

•Hide underlying technology from applications

•Decisions:•Network provides minimal functionality•“Narrow waist”

Tradeoff: No assumptions, no guarantees.

Technology

Applications email WWW phone...

SMTP HTTP RTP...

TCP UDP…

IP

ethernet PPP…

CSMA async sonet...

copper fiber radio...

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End-to-End Argument [Clark88]

•Deals with where to place functionality•Inside the network (in switching elements)•At the edges

•Argument•There are functions that can only be correctly

implemented by the endpoints – do not try to completely implement these elsewhere

•Guideline not a law

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Example: Reliable File Transfer

•Solution 1: make each step reliable, and then concatenate them

•Solution 2: end-to-end check and retry

OS

Appl.

OS

Appl.

Host A Host B

OK

E2E Example: File Transfer

•Even if network guaranteed reliable delivery•Need to provide end-to-end checks•E.g., network card may malfunction•The receiver has to do the check anyway!

•Full functionality can only be entirely implemented at application layer; no need for reliability from lower layers

•Does FTP look like E2E file transfer?•TCP provides reliability between kernels not disks

•Is there any need to implement reliability at lower layers?

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Discussion

•Yes, but only to improve performance

•If network is highly unreliable•Adding some level of reliability helps performance, not

correctness•Don’t try to achieve perfect reliability!•Implementing a functionality at a lower level should have

minimum performance impact on the applications that do not use the functionality

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Examples

•What should be done at the end points, and what by the network?

•Reliable/sequenced delivery?•Addressing/routing?•Security?•What about Ethernet collision detection?•Multicast?•Real-time guarantees?

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Fragmentation

•IP packets can be 64KB•Different link-layers have different MTUs •Split IP packet into multiple fragments

•IP header on each fragment•Various fields in header to help process•Intermediate router may fragment as needed

•Where to do reassembly?•End nodes – avoids unnecessary work•Dangerous to do at intermediate nodes

•Buffer space•Multiple paths through network

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Fragmentation is Harmful

•Uses resources poorly•Forwarding costs per packet•Best if we can send large chunks of data•Worst case: packet just bigger than MTU

•Poor end-to-end performance•Loss of a fragment

•Reassembly is hard•Buffering constraints

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Path MTU Discovery

•Hosts dynamically discover minimum MTU of path

•Algorithm:•Initialize MTU to MTU for first hop

•Send datagrams with Don’t Fragment bit set

•If ICMP “pkt too big” msg, decrease MTU

•What happens if path changes?•Periodically (>5mins, or >1min after previous

increase), increase MTU

•Some routers will return proper MTU

•MTU values cached in routing table

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IP Address Problem (1991)

•Address space depletion•In danger of running out of classes A and B

•Why?•Class C too small for most domains•Very few class A – IANA (Internet Assigned Numbers

Authority) very careful about giving•Class B – greatest problem

•Sparsely populated – but people refuse to give it back

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IPv4 Routing Problems

•Core router forwarding tables were growing large•Class A: 128 networks, 16M hosts•Class B: 16K networks, 64K hosts•Class C: 2M networks, 256 hosts

•32 bits does not give enough space encode network location information inside address – i.e., create a structured hierarchy

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Questions?

Comments?

Discussion?