Operating System Meets Network

Using a bottom-up approach

Course so far

•  Concentrated on standard OS concepts •  General OS architecture •  Process - threads - micro-kernels •  Memory Management

–  Algorithms and VM •  IO and File Systems

– RAID, Files, Directories

What we left out

•  Process and thread synchronization issues •  Avoiding deadlock, non-exclusion and

starvation •  High level scheduling concepts •  Multiprocessor scheduling •  Interprocess communication (IPC)

Where is the course going?

You are here!!

Topics we want to cover

•  The Effect of Networks on Operating Systems

•  Networks – Ethernet, Gigabit Ethernet, WLAN, HIPERLAN

•  Network Protocols •  Network Processing •  Sockets •  Socket programming

Course Cont’d

•  Client Server Systems - – Remote procedure call (RPC)

•  Network Filing System (NFS) •  X Windowing System (X) •  Virtual Network Computer (VNC) •  Web Server Design Issues

– Supporting Apache

Network Evolution

•  First use of the Network was to share expensive resources on a Local Area network –  e.g. printer, fileserver – For PCs we had Novell Netware, Appletalk – For Unix, IP LAN with postscript printers and

Network File System (NFS)

Network Operating System

•  Functions normally provided by an OS now being provided using servers across the network

•  Users and applications are unaware of the network

•  Host Operating system is changed to be clients of Network Servers

Network Servers

•  Servers that are needed to make LANs work •  Boot Servers

–  provide a way to boot machines over the network

•  DHCP Servers – assign IP addresses and give configuration info

•  DNS – Resolves names into IP addresses •  Gateways - provide access to other LANs

Net-Centric Applications

•  Assume network is there and visible to applications and users

•  Use the services of LAN servers – DNS, Gateway, etc

•  FTP – file Transfer Protocol •  Telnet – network Terminal •  Email – mh program

Ubiquitous Applications

•  Applications’ input and output can be sent over the network

•  Applications can run anywhere on the network

•  X-Windowing System – Part of Athena Project

•  VNC – Virtual Network Computer

Global Servers

•  Clients are not on local network •  Just be serving files

–  FTP server •  Execution of code on behalf of the client

–  Web Server – PERL scripts or Java code •  Increased Network Capacity •  Security Concerns

–  If you are hacked you will lose money and business big time!

Application Server

•  Runs a particular commercial application – Selling cheap airline tickets, or books etc

•  Web Server front end •  Database back end •  Glue in the middle

–  Java, CORBA, Perl scripts

•  Big site may have 100s or 1000s of Application Servers

Blade Server Technology

•  A server is a very thin piece of electronics called a Blade

•  Consists of CPU, memory, NIC and some storage (flash or small disk)

•  Very densely packed, 100+ in 19 inch rack •  Complicated management software •  Pioneers – Egenera and RLX Technologies

–  IBM, SUN and HP have Blade Server products

GriD Computing

•  The Network is the Computer •  All processing handled by the network •  1000s of servers available over the globe •  Lots of Middleware required •  Commercial reality still years away

Implications for Network and Operating Systems

•  Network is now centre-stage •  Lots of money being poured into getting e-

commerce going •  OS needs to efficient, reliable and capable

of handling network demands •  More OS functions dedicated to supporting

network related activity –  e.g. the sendfile command

LAN Network Technologies

•  Ethernet – Dominant around 90% of the market

•  Token Ring – Developed by IBM and Madge Networks – Good for Factory Floors

•  finite delay before getting the token

•  Token Bus – good in theory – Never really made it in practice

Ethernet

•  Developed by Xerox Parc •  Originally ran at 3 Mbps •  10Mbps -> 100Mbps -> 1Gbps -> 10Gbps •  Good wireless support

–  802.11b - 11 Mbps –  802.11a - 55 Mbps

Why has Ethernet been so successful

•  Electronics is cheap to build •  Moved away from CSMA-CD access

technology to support half-duplex and full duplex switched environments

•  Switches not HUBs •  Kept the Link Layer the same no matter

which access technology is being used

Why Ethernet has been so successful

•  Excellent Standardisation Process – Supported by IEEE, 802.3 standards

•  Intel, Xerox and DEC were early adopters and standardized Ethernet systems

•  Standards afforded an easy way to upgrade to faster systems

•  Also good wireless standards

Ethernet 802.3 Frames

Dst Src Type or Length

Payload F C S

6 6 2 46 - 1500 4

Pre amble

8

Ethernet Addresses

•  48 bits long; Destination and Source •  Transmitted with lowest bit first •  First two bits significant.

U G

U: 0 = unicast, 1 = multicast G: 0 = global, 1 = local

Type or Length

•  If field less than or equal to 1500 then it is treated as a length field

•  If the field is greater than 1500 bytes it is treated as a type field

•  The Type field is used to indicate different types of payloads

Different Payload Types

PAYLOAD VALUE IN HEX

Loopback 0x0060 Echo 0x0200

IP packet 0x0800

X25 packet 0x0805

ARP packet 0x0806

Appletalk 0x809B IPX 0x8137 IPv6 0x86DD

Different Payload Types

•  Provides a good medium to do a number of things

•  User Data (IP and IPX) •  Diagnostics (DEC protocols) •  Network Services (ARP) •  Different Networks (Appletalk, X.25, etc)

Ethernet uses different media

•  Originally coax cable – awkward for large segments

•  Shielded twisted pair – Cat 5 cable – Up to 1000 Mbps

Ethernet Limits 10 –100 Mbits

•  Minimum Packet Size 64 bytes = 512 bits •  Required for the collision detection stuff to

work •  Maximum Packet size = 1518 bytes •  Total Ethernet Frame Size = Maximum

Packet Size + Preamble = 1518 + 8 = 1526 bytes

Interframe Gap

•  Ethernet must allow a minimum idle period between frames. This is referred to as the Interframe Gap. – No frames are transmitted during this sequence. – Allows the hardware to prepare for the arrival

of the next set of packets.

Interframe Gap cont’d •  Minimum Interframe Gap is 96 bit times. So

the actual time is dependent on the the speed of the network

•  For 10 Mbps = 9.6 microseconds •  For 100 Mbps = 960 nanoseconds •  For 1Gbps = 96 nanoseconds •  For 10 Gbps = 9.6 nanoseconds

•  Some NIC cards use a shorter IFG which can lead to more collisions

Gigabit Ethernet Limits

•  Because clock is so much faster: •  Minimum packet size 512 bytes = 4096 bits •  Packets shorter than 512 bytes have an

extension to make sure they are 512 bytes •  Maximum length of 1500 bytes is too small

for stations that want to transmit a lot of data

Burst Extension

•  Station does not have to release the carrier after transmitting a packet

•  Sender inserts a special sequence in the Gap between frames – Interframe Gap (IFG) sequence but does not drop the carrier

•  Sender can transmit a number of Ethernet frames in one burst up to 8192 bytes

Burst Extension

Pre amble

Frame No 1

I F G

Pre amble

Frame No 2

I F G

Pre amble

Frame No 3

Ethernet NICs

•  Cost dominated by bus interface and network connectors

•  Bus Interfaces: – PCI, EISA, Micro-Channel

•  Network Connectors – RJ45, Twisted Pair, Copper and Fibre

IEEE 802.11 Wireless LAN Standards

•  Uses CSMA/CA –  CSMA:

•  The sender listens to the medium before transmitting and defers transmission if the medium is busy

–  CA: Collision Avoidance •  A station wanting to transmit sends out an RTS frame •  If the medium is free, the destination node sends a CTS

response, and then the sender sends the data packet •  The sender waits for an ACK packet from the receiver before

discarding the packet

Inter-frame Spaces (IFS) •  Short IFS (SIFS)

•  used for quick interaction such as RTS and CTS exchanges or sending an ACK

•  Point Coordinate IFS (PIFS) •  used by the Access Point to the medium ahead of other

stations

•  Distributed IFS (DIFS) •  time before starting a new transmission

•  Extended IFS (EFS) •  Used by a station that receives a packet it does not

understand

Backoff

•  Stations must backoff from the channel: –  If when the station senses the medium before

the transmission of the first packet and the medium is busy (DIFS)

– After each successful transmission – After each retransmission

•  Uses Exponential Backoff Algorithm

Synchronization

•  Need to keep all mobile stations synchronized to local Access Point (AP)

•  AP sends special frames called Beacon Frames

•  AP transmits its current clock value in the Beacon Frames

•  The MS sets its clock value to the AP’s clock value

Security Issues

•  Authentication –  Ensuring that only authorized people can join a

particular WLAN •  Preventing eavesdropping

–  Uses encryption techniques •  WEP based on RSA’s RC4 Algorithm •  WiFi Protected Access (WPA2) •  Security Issues being looked at by 802.11i group

•  Traffic Analysis •  Active Attacks •  Passive Attacks

IEEE 802.11 WLANs •  802.11a

•  Operates at 5 GHz •  Maximum data rate 54 Mbps •  Typical data rate 23 Mbps

•  802.11b •  Operates at 2.4 GHz •  Maximum data rate 11 Mbps •  Typical data rate 4.5 Mbps

•  802.11g •  Operates at 2.4 GHz •  Maximum data rate 54 Mbps •  Typical date rate 23 Mbits

HIPERLAN

•  HIPERLAN stands for High Performance Radio Local Area Network

•  HIPERLAN was developed by European Telecommunications Standards Institute (ETSI)

•  HIPERLAN/2 operates at 5 GHz and offer data rates of 54 Mbps. Similar to 802.11

Key Differences

•  802.11 uses CSMA/CA while HIPERLAN uses TDMA – this is more efficient for audio and video applications

•  Typical data flow from HIPERLAN 42 Mbps while from 802.11 a/g 23 Mbits

•  HIPERLAN implemented QoS protocols for different sorts of connections. 802.11e developed QoS for 802.11 much later (2005)

Linux Ethernet Device Drivers

•  Supports quite a variety •  Most can be loaded as modules •  Most written by or use drivers written by

Donald Becker – Actually worked for NASA

Ethernet Network Device Structure

•  Made up of 2 levels •  Lower level

–  Interrupt driven – Receive ISR puts packet on General Network

Receive Queue (GNRQ) – Transmit may use several Qs –  ISR – short and sweet

Linux Network Driver Structure Lower Level

Device

ISR

Network

Receive Q Transmit Q

Linux Network Processing: Upper Level

•  Carried out as part of the tasks of the OS – When the OS is not in interrupt mode

•  Receive Packet Processing – Determining packet Type – Process Network Protocol (IP, IPX, X.25) – Process Transport Protocols (TCP, UDP) –  Identify socket structure for network

connection

Packet Processing: Upper Level

Type processing

Network Protocols

Transport Protocol Processing

Socket Structure for Connection

Sources for this lecture •  Gigabit Ethernet – Rich Seifert – Addison-

Wesley 1998 •  TCP/IP and Linux Protocol Implementation

– Jon Crowcroft and Iain Phillips – Wiley 2002