netwoking from the start networking is the best .ppt

CS162
Operating Systems and
Systems Programming
Lecture 21
Networking
November 13, 2006
Prof. John Kubiatowicz
http://inst.eecs.berkeley.edu/~cs162

Lec 21.2
11/13/06 Kubiatowicz CS162 ©UCB Fall 2006
• Data stripped across
multiple disks
– Successive blocks
stored on successive
(non-parity) disks
– Increased bandwidth
over single disk
• Parity block (in green)
constructed by XORing
data bocks in stripe
– P0=D0D1D2D3
– Can destroy any one
disk and still
reconstruct data
– Suppose D3 fails,
then can reconstruct:
D3=D0D1D2P0
• Later in term: talk about spreading information widely
across internet for durability.
Review: RAID 5+: High I/O Rate Parity
Increasing
Logical
Disk
Addresses
Stripe
Unit
D0 D1 D2 D3 P0
D4 D5 D6 P1 D7
D8 D9 P2 D10 D11
D12 P3 D13 D14 D15
P4 D16 D17 D18 D19
D20 D21 D22 D23 P5
Disk 1 Disk 2 Disk 3 Disk 4 Disk 5

Lec 21.3
Review: Networking Definitions
• Network: physical connection that allows two computers
to communicate
• Packet: unit of transfer, sequence of bits carried over
the network
– Network carries packets from on CPU to another
– Destination gets interrupt when packet arrives
• Protocol: agreement between two parties as to how
information is to be transmitted

Lec 21.4
Review: Ethernet and CSMA/CD
• Ethernet (early 80’s): first practical local area network
– It is the most common LAN for UNIX, PC, and Mac
– Use wire instead of radio, but still broadcast medium
• Key advance was in arbitration called CSMA/CD:
Carrier sense, multiple access/collision detection
– Carrier Sense: don’t send unless idle
» Don’t mess up communications already in process
– Collision Detect: sender checks if packet trampled.
» If so, abort, wait, and retry.
– Backoff Scheme: Choose wait time before trying again
• Adaptive randomized waiting strategy:
– Adaptive and Random: First time, pick random wait time
with some initial mean. If collide again, pick random value
from bigger mean wait time. Etc.
– Randomness is important to decouple colliding senders
– Scheme figures out how many people are trying to send!

Lec 21.5
Goals for Today
• Networking
– Point-to-Point Networking
– Routing
– Internet Protocol (IP)
Note: Some slides and/or pictures in the following are
adapted from slides ©2005 Silberschatz, Galvin, and Gagne
Note: Some slides and/or pictures in the following are
adapted from slides ©2005 Silberschatz, Galvin, and Gagne.
Many slides generated from my lecture notes by Kubiatowicz.

Lec 21.6
Point-to-point networks
• Why have a shared bus at all? Why not simplify and
only have point-to-point links + routers/switches?
– Didn’t used to be cost-effective
– Now, easy to make high-speed switches and routers that
can forward packets from a sender to a receiver.
• Point-to-point network: a network in which every
physical wire is connected to only two computers
• Switch: a bridge that transforms a shared-bus
(broadcast) configuration into a point-to-point network.
• Router: a device that acts as a junction between two
networks to transfer data packets among them.
Router
Internet
Switch

Lec 21.7
Point-to-Point Networks Discussion
• Advantages:
– Higher link performance
» Can drive point-to-point link faster than broadcast link
since less capacitance/less echoes (from impedance
mismatches)
– Greater aggregate bandwidth than broadcast link
» Can have multiple senders at once
– Can add capacity incrementally
» Add more links/switches to get more capacity
– Better fault tolerance (as in the Internet)
– Lower Latency
» No arbitration to send, although need buffer in the switch
• Disadvantages:
– More expensive than having everyone share broadcast link
– However, technology costs now much cheaper
• Examples
– ATM (asynchronous transfer mode)
» The first commercial point-to-point LAN
» Inspiration taken from telephone network
– Switched Ethernet
» Same packet format and signaling as broadcast Ethernet,
but only two machines on each ethernet.

Lec 21.8
Point-to-Point Network design
• Switches look like computers: inputs, memory, outputs
– In fact probably contains a processor
• Function of switch is to forward packet to output that gets it closer to
destination
• Can build big crossbar by combining smaller switches
• Can perform broadcast if necessary
Queue
Queue
Queue
Queue
Queue
Queue
Queue
Queue
Crossbar
Control
(processor)
Inputs Outputs
Switch
1
Switch
2
Switch
3

Lec 21.9
Flow control options
• What if everyone sends to the same output?
– Congestion—packets don’t flow at full rate
• In general, what if buffers fill up?
– Need flow control policy
• Option 1: no flow control. Packets get dropped if they
arrive and there’s no space
– If someone sends a lot, they are given buffers and packets
from other senders are dropped
– Internet actually works this way
• Option 2: Flow control between switches
– When buffer fills, stop inflow of packets
– Problem: what if path from source to destination is completely
unused, but goes through some switch that has buffers filled
up with unrelated traffic?
A,B
B,C,D
A
C
D

Lec 21.10
Flow Control (con’t)
• Option 3: Per-flow flow control.
– Allocate a separate set of buffers to each end-to-end
stream and use separate “don’t send me more” control on
each end-to-end stream
• Problem: fairness
– Throughput of each stream is entirely dependent on
topology, and relationship to bottleneck
• Automobile Analogy
– At traffic jam, one strategy is merge closest to the
bottleneck
» Why people get off at one exit, drive 50 feet, merge back
into flow
» Ends up slowing everybody else a huge emount
– Also why have control lights at on-ramps
» Try to keep from injecting more cars than capacity of road
(and thus avoid congestion)
aaaa
bbbb cccc
ababab
dddd
acbcac dadcdbdc

Lec 21.11
The Internet Protocol: “IP”
• The Internet is a large network of computers spread
across the globe
– According to the Internet Systems Consortium, there
were over 353 million computers as of July 2005
– In principle, every host can speak with every other one
under the right circumstances
• IP Packet: a network packet on the internet
• IP Address: a 32-bit integer used as the destination
of an IP packet
– Often written as four dot-separated integers, with each
integer from 0—255 (thus representing 8x4=32 bits)
– Example CS file server is: 169.229.60.83  0xA9E53C53
• Internet Host: a computer connected to the Internet
– Host has one or more IP addresses used for routing
» Some of these may be private and unavailable for routing
– Not every computer has a unique IP address
» Groups of machines may share a single IP address
» In this case, machines have private addresses behind a
“Network Address Translation” (NAT) gateway

Lec 21.12
Address Subnets
• Subnet: A network connecting a set of hosts with
related destination addresses
• With IP, all the addresses in subnet are related by a
prefix of bits
– Mask: The number of matching prefix bits
» Expressed as a single value (e.g., 24) or a set of ones in a
32-bit value (e.g., 255.255.255.0)
• A subnet is identified by 32-bit value, with the bits
which differ set to zero, followed by a slash and a
mask
– Example: 128.32.131.0/24 designates a subnet in which
all the addresses look like 128.32.131.XX
– Same subnet: 128.32.131.0/255.255.255.0
• Difference between subnet and complete network range
– Subnet is always a subset of address range
– Once, subnet meant single physical broadcast wire; now,
less clear exactly what it means (virtualized by switches)

Lec 21.13
Address Ranges in IP
• IP address space divided into prefix-delimited ranges:
– Class A: NN.0.0.0/8
» NN is 1–126 (126 of these networks)
» 16,777,214 IP addresses per network
» 10.xx.yy.zz is private
» 127.xx.yy.zz is loopback
– Class B: NN.MM.0.0/16
» NN is 128–191, MM is 0-255 (16,384 of these networks)
» 65,534 IP addresses per network
» 172.[16-31].xx.yy are private
– Class C: NN.MM.LL.0/24
» NN is 192–223, MM and LL 0-255
(2,097,151 of these networks)
» 254 IP addresses per networks
» 192.168.xx.yy are private
• Address ranges are often owned by organizations
– Can be further divided into subnets

Lec 21.14
Administrivia
• Exam reminders:
– MIDTERM II: Dec 4th
» All material from last midterm and up to Wednesday
11/29
» Lectures #13 – 26
– Final Exam
» Sat Dec 16th
, 8:00am-11:00am, Bechtel Auditorium
» All Material
• Project 3 due Thursday (11/13)

Lec 21.15
Hierarchical Networking: The Internet
• How can we build a network with millions of hosts?
– Hierarchy! Not every host connected to every other one
– Use a network of Routers to connect subnets together
» Routing is often by prefix: e.g. first router matches first
8 bits of address, next router matches more, etc.
subnet1
subnet2
Router
Other
subnets
Router
Router
Transcontinental
Link
subnet3
Other
subnets

Lec 21.16
Simple Network Terminology
• Local-Area Network (LAN) – designed to cover small
geographical area
– Multi-access bus, ring, or star network
– Speed  10 – 1000 Megabits/second
– Broadcast is fast and cheap
– In small organization, a LAN could consist of a single
subnet. In large organizations (like UC Berkeley), a LAN
contains many subnets
• Wide-Area Network (WAN) – links geographically
separated sites
– Point-to-point connections over long-haul lines (often
leased from a phone company)
– Speed  1.544 – 45 Megabits/second
– Broadcast usually requires multiple messages

Lec 21.17
Routing
• Routing: the process of forwarding packets hop-by-hop
through routers to reach their destination
– Need more than just a destination address!
» Need a path
– Post Office Analogy:
» Destination address on each letter is not
sufficient to get it to the destination
» To get a letter from here to Florida, must route to local
post office, sorted and sent on plane to somewhere in
Florida, be routed to post office, sorted and sent with
carrier who knows where street and house is…
• Internet routing mechanism: routing tables
– Each router does table lookup to decide which link to use
to get packet closer to destination
– Don’t need 4 billion entries in table: routing is by subnet
– Could packets be sent in a loop? Yes, if tables incorrect
• Routing table contains:
– Destination address range  output link closer to
destination
– Default entry (for subnets without explicit entries)

Lec 21.18
Setting up Routing Tables
• How do you set up routing tables?
– Internet has no centralized state!
» No single machine knows entire topology
» Topology constantly changing (faults, reconfiguration, etc)
– Need dynamic algorithm that acquires routing tables
» Ideally, have one entry per subnet or portion of address
» Could have “default” routes that send packets for unknown
subnets to a different router that has more information
• Possible algorithm for acquiring routing table
– Routing table has “cost” for each entry
» Includes number of hops to destination, congestion, etc.
» Entries for unknown subnets have infinite cost
– Neighbors periodically exchange routing tables
» If neighbor knows cheaper route to a subnet, replace your
entry with neighbors entry (+1 for hop to neighbor)
• In reality:
– Internet has networks of many different scales
– Different algorithms run at different scales

Lec 21.19
Network Protocols
• Protocol: Agreement between two parties as to how
– Example: system calls are the protocol between the
operating system and application
– Networking examples: many levels
» Physical level: mechanical and electrical network (e.g. how
are 0 and 1 represented)
» Link level: packet formats/error control (for instance, the
CSMA/CD protocol)
» Network level: network routing, addressing
» Transport Level: reliable message delivery
• Protocols on today’s Internet:
Ethernet ATM Packet radio
IP
UDP TCP
RPC
NFS WWW e-mail
ssh
Physical/Link
Network
Transport

Lec 21.20
Network Layering
• Layering: building complex services from simpler ones
– Each layer provides services needed by higher layers by
utilizing services provided by lower layers
• The physical/link layer is pretty limited
– Packets are of limited size (called the “Maximum Transfer
Unit or MTU: often 200-1500 bytes in size)
– Routing is limited to within a physical link (wire) or perhaps
through a switch
• Our goal in the following is to show how to construct a
secure, ordered, message service routed to anywhere:
Physical Reality: Packets Abstraction: Messages
Limited Size Arbitrary Size
Unordered (sometimes) Ordered
Unreliable Reliable
Machine-to-machine Process-to-process
Only on local area net Routed anywhere
Asynchronous Synchronous
Insecure Secure

Lec 21.21
Building a messaging service
• Handling Arbitrary Sized Messages:
– Must deal with limited physical packet size
– Split big message into smaller ones (called fragments)
» Must be reassembled at destination
– Checksum computed on each fragment or whole message
• Internet Protocol (IP): Must find way to send packets
to arbitrary destination in network
– Deliver messages unreliably (“best effort”) from one
machine in Internet to another
– Since intermediate links may have limited size, must be
able to fragment/reassemble packets on demand
– Includes 256 different “sub-protocols” build on top of IP
» Examples: ICMP(1), TCP(6), UDP (17), IPSEC(50,51)

Lec 21.22
IP Packet Format
• IP Packet Format:
16-bit identification
ToS
4
13-bit frag off
Total length(16-bits)
protocol
TTL 16-bit header checksum
32-bit source IP address
32-bit destination IP address
IHL
flags
options (if any)
Data
0 15 16 31
IP Ver4
IP Header
Length
Size of datagram
(header+data)
Flags &
Fragmentation
to split large
messages
Time to
Live (hops)
Type of
transport
protocol
IP header
20 bytes

Lec 21.23
Building a messaging service
• Process to process communication
– Basic routing gets packets from machinemachine
– What we really want is routing from processprocess
» Example: ssh, email, ftp, web browsing
– Several IP protocols include notion of a “port”, which is a
16-bit identifiers used in addition to IP addresses
» A communication channel (connection) defined by 5 items:
[source address, source port, dest address, dest port,
protocol]
• UDP: The User Datagram Protocol
– UDP layered on top of basic IP (IP Protocol 17)
» Unreliable, unordered, user-to-user communication
UDP Data
16-bit UDP length 16-bit UDP checksum
16-bit source port 16-bit destination port
IP Header
(20 bytes)

Lec 21.24
Building a messaging service (con’t)
• UDP: The Unreliable Datagram Protocol
– Datagram: an unreliable, unordered, packet sent from
source user  dest user (Call it UDP/IP)
– Important aspect: low overhead!
» Often used for high-bandwidth video streams
» Many uses of UDP considered “anti-social” – none of the
“well-behaved” aspects of (say) TCP/IP
• But we need ordered messages
– Create ordered messages on top of unordered ones
» IP can reorder packets! P0,P1 might arrive as P1,P0
– How to fix this? Assign sequence numbers to packets
» 0,1,2,3,4…..
» If packets arrive out of order, reorder before delivering
to user application
» For instance, hold onto #3 until #2 arrives, etc.
– Sequence numbers are specific to particular connection

Lec 21.25
Performance Considerations
• Before continue, need some performance metrics
– Overhead: CPU time to put packet on wire
– Throughput: Maximum number of bytes per second
» Depends on “wire speed”, but also limited by slowest router
(routing delay) or by congestion at routers
– Latency: time until first bit of packet arrives at receiver
» Raw transfer time + overhead at each routing hop
• Contributions to Latency
– Wire latency: depends on speed of light on wire
» about 1–1.5 ns/foot
– Router latency: depends on internals of router
» Could be < 1 ms (for a good router)
» Question: can router handle full wire throughput?
Router Router
LR1 LR2
LW1 LW2 Lw3

Lec 21.26
Sample Computations
• E.g.: Ethernet within Soda
– Latency: speed of light in wire is 1.5ns/foot, which
implies latency in building < 1 μs (if no routers in path)
– Throughput: 10-1000Mb/s
– Throughput delay: packet doesn’t arrive until all bits
» So: 4KB/100Mb/s = 0.3 milliseconds (same order as disk!)
• E.g.: ATM within Soda
– Latency (same as above, assuming no routing)
– Throughput: 155Mb/s
– Throughput delay: 4KB/155Mb/s = 200μ
• E.g.: ATM cross-country
– Latency (assuming no routing):
» 3000miles * 5000ft/mile  15 milliseconds
– How many bits could be in transit at same time?
» 15ms * 155Mb/s = 290KB
– In fact, BerkeleyMIT Latency ~ 45ms
» 872KB in flight if routers have wire-speed throughput
• Requirements for good performance:
– Local area: minimize overhead/improve bandwidth
– Wide area: keep pipeline full!

Lec 21.27
Reliable Message Delivery: the Problem
• All physical networks can garble and/or drop packets
– Physical media: packet not transmitted/received
» If transmit close to maximum rate, get more throughput –
even if some packets get lost
» If transmit at lowest voltage such that error correction
just starts correcting errors, get best power/bit
– Congestion: no place to put incoming packet
» Point-to-point network: insufficient queue at switch/router
» Broadcast link: two host try to use same link
» In any network: insufficient buffer space at destination
» Rate mismatch: what if sender send faster than receiver
can process?
• Reliable Message Delivery
– Reliable messages on top of unreliable packets
– Need some way to make sure that packets actually make
it to receiver
» Every packet received at least once
» Every packet received only once
– Can combine with ordering: every packet received by
process at destination exactly once and in order

Lec 21.28
Using Acknowledgements
• How to ensure transmission of packets?
– Detect garbling at receiver via checksum, discard if bad
– Receiver acknowledges (by sending “ack”) when packet
received properly at destination
– Timeout at sender: if no ack, retransmit
• Some questions:
– If the sender doesn’t get an ack, does that mean the
receiver didn’t get the original message?
» No
– What it ack gets dropped? Or if message gets delayed?
» Sender doesn’t get ack, retransmits. Receiver gets
message twice, acks each.
B
A
Packet
ack
B
A
Packet
ack
Packet
Timeout

Lec 21.29
B
A
Pkt #0
Ack #0
Pkt #1
Ack #1
Pkt #0
Ack #0
How to deal with message duplication
• Solution: put sequence number in message to identify
re-transmitted packets
– Receiver checks for duplicate #’s; Discard if detected
• Requirements:
– Sender keeps copy of unack’ed messages
» Easy: only need to buffer messages
– Receiver tracks possible duplicate messages
» Hard: when ok to forget about received message?
• Simple solution: Alternating-bit protocol
– Send one message at a time; don’t send
next message until ack received
– Sender keeps last message; receiver
tracks sequence # of last message received
• Pros: simple, small overhead
• Con: Poor performance
– Wire can hold multiple messages; want to
fill up at (wire latency  throughput)
• Con: doesn’t work if network can delay
or duplicate messages arbitrarily

Lec 21.30
Conclusion
• Network: physical connection that allows two
computers to communicate
– Packet: sequence of bits carried over the network
• Broadcast Network: Shared Communication Medium
– Transmitted packets sent to all receivers
– Arbitration: act of negotiating use of shared medium
» Ethernet: Carrier Sense, Multiple Access, Collision Detect
• Point-to-point network: a network in which every
physical wire is connected to only two computers
– Switch: a bridge that transforms a shared-bus
(broadcast) configuration into a point-to-point network.
• Protocol: Agreement between two parties as to how
• Internet Protocol (IP)
– Used to route messages through routes across globe
– 32-bit addresses, 16-bit ports
• Reliable, Ordered, Arbitrary-sized Messaging:
– Built through protocol layering on top of unreliable,
limited-sized, non-ordered packet transmission links

netwoking from the start networking is the best .ppt

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