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CMPE 151: Network
Administration
Lecture 2
Spring 2004
Review?
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Computers
Operating Systems
Kernels
Distributed Systems
Spring 2004
What is an OS?
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Machine/resource manager.
Application programs
Compilers, Editors, etc.
Operating System
Instruction Set Architecture
Microarchitecture
Physical Devices
Spring 2004
Hardware
OS as extended machine…
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Top-down view.
Layer of software that hides hardware.
Provides programmer easier instructions.
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E.g., read block from file.
(Part of) the OS runs in supervisor
(privileged) mode: can execute priviledged
instructions (e.g., access to physical decices
through drivers).
Spring 2004
OS as resource manager
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Bottom-up view.
Provides orderly and controlled
allocation of resources.
Provides (“concurrent”) programs (fair)
access to resources (processor, disk,
printer).
Time (e.g., CPU) and space (e.g.,
memory) multiplexing.
Spring 2004
OS History
Spring 2004
First generation: 1945-1955
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Vacuum tubes: machines took whole
rooms!
Machine language programming
(plugboard wiring).
No OS.
Spring 2004
Second generation: 1955-1965
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Transistors made computers
commercially viable.
Builders, operators, users.
Mainframes: multimillion dollar
machines.
Punch cards, input and output tapes.
Batch systems.
Spring 2004
Third generation: 1965-1980
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ICs.
Multiprogramming.
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Machine shared by “concurrent” programs.
Memory partitions hold multiple jobs.
Timesharing.
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Multiprogramming still batch processing: scientific
computation and commercial data processing.
Cheap terminals: interactive use.
Interactive service + batch processing.
Spring 2004
Fourth generation: 1980-…
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High-scale circuit integration.
Computer miniaturization.
Mainframes -> minicomputers ->
microcomputers or PCs.
PC OSs: CP/M, DOS, MS-DOS.
GUI-based OSs: UNIX-based, MS
Windows-based, MAC OS, …
Spring 2004
Modern OSs
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Mainframe OSs: IBM’s OS/390, DEC’s
VMS.
Server OSs: Solaris, FreeBSD, etc.
PC OS: Linux, MacOS, Windows…
Real-time OSs: VxWorks.
Embedded OSs: Linux, PalmOS,
Windows CE
Smart card OSs
Spring 2004
Basic OS Concepts
Spring 2004
Processes
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Process: program in execution.
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Address space: memory usable by program
(text, data, stack).
State: registers.
OS uses process table to keep track of
processes.
Processes can create other (child)
processes.
Spring 2004
Inter-Process Communication
(IPC)
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Shared memory.
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Processes communicate/synchronize
through a shared data item.
Message passing.
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Processes communicate via messages.
Spring 2004
Shared Memory
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Processes must access shared data in a
mutual exclusive way.
Primitives:
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Semaphores: Dijkstra(1965)
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P(S) and V(S) operations.
Atomic (indivisible) operations.
(Conditional) Critical Regions
Monitors
Spring 2004
Message Passing
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Processes communicate/synchronize by
sending/receiving messages.
Primitives:
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Send(message), receive(message).
Issues:
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Synchronous versus asynchronous.
Reliable versus unreliable.
Spring 2004
Distributed Shared Memory
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Sharing data among computers that
don’t share physical memory.
DSM provides shared memory
abstraction.
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Read- and write-like primitives.
Needs message passing to convey
updates among physically disjoint
processing elements.
Spring 2004
Deadlocks
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Shared data/resource may lead to
deadlock: processes get “stuck”.
Example: v is using r1 and requests r2;
w is using r2 and requests r1.
v
w
Spring 2004
Memory Management
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Share memory among several processes.
Monoprogramming: memory sharing between
OS and program (embedded OSs).
Multiprogramming: multiple processes
(partially or totally) in memory.
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Swapping.
Virtual memory: paging.
Spring 2004
I/O
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OS I/O subsystem manages I/O devices.
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File system:
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File as an abstraction.
Basic operations: create, delete, read, write.
Hierarchical file systems.
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Device-dependent (device drivers) or independent.
Dynamically attach tree branches (e.g., mount
system call in UNIX).
Access control: permissions.
Spring 2004
System Calls
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Interface between OS and user program: set
of system calls.
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Like making a special procedure call.
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E.g., access a file, create a process, etc.
System calls executed by kernel.
Calling program pushes parameters onto stack;
calls library; library routine (same name as system
call) executes TRAP, switching to kernel mode; OS
handles call; returns control to library; library
returns to user program.
Example system calls for file system
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open, close, read, write, mkdir,
Spring 2004
rmdir.
System Calls
System call
to access
physical
resources
User-level process
Kernel
Physical machine
System call: implemented by hardware interrupt (trap)
which puts processor in supervisor mode and kernel address
space; executes kernel-supplied handler routine (device driver)
executing with interrupts disabled.
Spring 2004
Kernels
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Executes in supervisor mode.
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Privilege to access machine’s physical
resources.
User-level process: executes in “user”
mode.
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Restricted access to resources.
Address space boundary restrictions.
Spring 2004
Kernel Functions
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Memory management.
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Process management.
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Address space allocation.
Memory protection.
Process creation, deletion.
Scheduling.
Resource management.
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Device drivers/handlers.
Spring 2004
Kernel and Distributed
Systems
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Inter-process communication: RPC, MP,
DSM.
Distributed (Networked) File systems.
Some parts may run as user-level and
some as kernel processes.
Spring 2004
What next?
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Brief overview:
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IP
TCP
DNS
FTP
HTTP
NFS…
Spring 2004
What are protocols?
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Set of rules governing communication
between network elements (applications,
hosts, routers).
Protocols define:
 Format and order of messages.
 Actions taken on receipt of a message.
Protocols are hard to design
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We need design guidelines!
Spring 2004
Protocol stack
User A
Teleconferencing
User B
Peers
Application
Transport
Network
Link
Host
Host
Layering: technique to simplify complex systems
Spring 2004
Layering Characteristics
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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).
Spring 2004
Encapsulation
Spring 2004
OSI Model: 7 Protocol Layers
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Physical: how to transmit bits
Data link: how to transmit frames
Network: how to route packets hop2hop
Transport: how to send packets end2end
Session: how to tie flows together
Presentation: byte ordering, security
Application: everything else!
Spring 2004
Layering Functionality
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Reliability
Flow control
Fragmentation
Multiplexing
Connection setup (handshaking)
Addressing/naming (locating peers)
Spring 2004
Example: Transport layer
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First end-to-end layer.
End-to-end state.
May provide reliability, flow and
congestion control.
Spring 2004
Example: Network Layer
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Point-to-point communication.
Network and host addressing.
Routing.
Spring 2004
Internetworking
Spring 2004
Internetworking
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Interconnection of 2 or more networks
forming an internetwork, or internet.
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LANs, MANs, and WANs.
Different networks mean different
protocols.
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TCP/IP, IBM’s SNA, DEC’s DECnet, ATM,
Novell and AppleTalk.
Spring 2004
Internetworks (cont’d)
Spring 2004
TCP/IP
• TCP/IP is the most widely used
internetworking protocol suite
– Initially funded through ARPA.
– Picked up by NSF.
– Used in the Internet.
• Other internetworking protocols exist but are
less used
– Example: AppleTalk, X.25, etc.
Spring 2004
IP
Spring 2004
The Internet Protocol: IP
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Glues Internet together.
Common network-layer protocol spoken
by all Internet participating networks.
Best effort datagram service:
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No reliability guarantees.
No ordering guarantees.
Spring 2004
IP (cont’d)
• IP is responsible for datagram routing.
• Important: each datagram is routed
independently!
– Two different datagrams from same source to same
destination can take different routes!
– Why?
– Implications?
Spring 2004
IP (cont’d)
• IP provides a best effort delivery mechanism
– Does not guarantee to prevent duplicate
datagrams, delayed and out-of-order delivery,
corruption of data or datagram loss
• Reliable delivery is provided by the transport
layer, not the network layer (IP)
• Network layer (IP) can detect and report errors
without actually fixing them
Spring 2004
The Internet Protocol
Host
Host
Application
Transport
IP
Router
Router
IP
IP
Network
IP
Network
Spring 2004
Datagrams
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Transport layer breaks data streams into
datagrams which are transmitted over
Internet, possibly being fragmented.
When all datagram fragments arrive at
destination, reassembled by network
layer and delivered to transport layer at
destination host.
Spring 2004
IP Datagram Format
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IP datagram consists of header and
data (or payload).
Header:
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20-byte fixed (mandatory) part.
Variable length optional part.
Spring 2004
IP Versions
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IPv4: IP version 4.
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Current, predominant version.
32-bit long addresses.
IPv6: IP version 6.
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Evolution of IPv4.
Longer addresses (16-byte long).
Spring 2004
Payload
Header
IP(v4) Header Format
Spring 2004
Encapsulation
• Each datagram is encapsulated within a data link
layer frame
– The whole datagram is placed in the data area of
the frame.
– The data link layer addresses for source and
destination included in the frame header.
Spring 2004
Encapsulation - Example
Spring 2004
Encapsulation Across
Multiple Hops
• Each router in the path from source to
destination:
– Decapsulates datagram from incoming frame.
– Forwards datagram - determines next hop.
– Encapsulate datagram in outgoing frame.
Spring 2004
Encapsulation Across
Multiple Hops - Example
Spring 2004
Maximum Transfer Unit
• Each data link layer technology specifies the
maximum size of a frame.
– Called the Maximum Transfer Unit (MTU).
• Ethernet: 1,500 bytes.
• Token Ring: 2048 or 4096 bytes.
• What happens when large packet wants to travel
through network with smaller MTU?
• Maximum payloads (data portion of datagram)
range from 48 bytes (ATM cells) to 64Kbytes (IP
packets).
Spring 2004
Fragmentation
• Another solution (used by IP): fragmentation.
• Gateways break packets into fragments to fit the
network’s MTU; each sent as separate datagram.
• Gateway on the other side have to reassemble
fragments into original datagram.
Spring 2004
Keeping Track of Fragments
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Fragments must be numbered so that
original data stream can be reconstructed.
Define elementary fragment size that can pass
through every network.
When packet fragmented, all pieces equal to
elementary fragment size, except last one (may
be smaller).
Datagram may contain several fragments.
Spring 2004
Fragmentation - Example
Spring 2004
Addressing
Spring 2004
Universal Addressing
• One key aspect of internetworks is unique
addresses.
• Sending host puts destination internetworking
address in the packet.
• Destination addresses can be interpreted by
any intermediate router/gateway.
• Router/gateway examines address and
forwards packet on to the destination.
Spring 2004
IP Addresses
• Each machine on the Internet has a unique IP address.
• The IP address is different from the “physical” /“MAC”
address.
– The “physical address” is the address of a computer
(actually, of a NIC) in the LAN.
• It is only know within the LAN.
– The IP address is a universal address.
– When a packet arrives in a LAN, there needs to be a
conversion from IP to MAC address (local “address
resolution”).
Spring 2004
IP Addresses (cont’d)
• An IP address is represented by a binary
number with 32 bits (in IPv4).
– Meaning that there are around 4 billion
addresses.
– Often IP addresses are represented in “dotted
decimal”, such as 128.114.144.4.
• Each group of numbers can go from 0 to 255.
Spring 2004
IP Address Organization
• Each IP address is divided into a prefix and a
suffix
– Prefix identifies network to which computers
are attached.
– Suffix identifies computers within that
network.
Spring 2004
Network and Host Numbers
• Every network in a TCP/IP internet is assigned a
unique network number.
• Each host on a specific network is assigned a host
address that is unique within that network.
• Host’s IP address is the combination of the network
number (prefix) and host address (suffix).
• Assignment of network numbers must be coordinated
globally; assignment of host addresses can be
managed locally.
Spring 2004
IP Address Format
• IP address are 32 bits long.
• There are different classes of addresses,
corresponding to different subdivisions of the 32
bits into prefix and suffix.
– Some address classes have large prefix, small
suffix.
• Many such networks, few hosts per network.
– Other address classes have small prefix, large
suffix.
• Few such networks, many hosts per network.
Spring 2004
IP Address Format (cont’d)
• How can we recognize to which class an IP
address belongs to?
– Look at the first 4 bits!
Spring 2004
IP Address Format (cont’d)
• Class A, B and C are primary classes.
– Used for ordinary addressing.
• Class D is used for multicast, which is a
limited form of broadcast.
– Internet hosts join a multicast group.
– Packets are delivered to all members of the
group.
– Routers manage delivery of single packets
from source to all members of multicast group.
• Class E is reserved.
Spring 2004
IP Addresses (cont’d)
• Another way to determine the address class
is by looking at the first group of numbers in
the dotted decimal notation
Spring 2004
Networks and Hosts
in Each Class
Spring 2004
Understanding IP Addresses
• Examples:
– 10.0.0.37 (class A)
– 128.10.0.1 (class B)
– 192.5.48.3 (class C)
Spring 2004
IP addresses: how to get one?
• ICANN (Internet Corporation for Assigned Names
and Numbers) coordinate IP address
assignment.
• How does host get its IP address in the network?
2 possibilities:
– 1: Hard-coded by system administrator in a file
inside the host.
– 2: DHCP: “Dynamic Host Configuration Protocol”
• Dynamically get address: “plug-and-play”.
Spring 2004
DHCP
• DHCP allows a computer to join a new network
and automatically obtain an IP address The
network administrator establishes a pool of
addresses for DHCP to assign.
• When a computer boots, it broadcasts a DHCP
request to which a server sends a DHCP reply.
Spring 2004
DHCP (Cont’d)
• DHCP allows non-mobile computers that run
server software to be assigned a
permanent address (won’t change when the
computer reboots).
– The permanent address actually needs to be
re-negotiated after a certain period of time.
Spring 2004
The Internet Transport
Protocols: TCP and UDP
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UDP: user datagram protocol (RFC
768).
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Connection-less protocol.
TCP: transmission control protocol
(RFCs 793, 1122, 1323).
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Connection-oriented protocol.
Spring 2004
UDP
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Provides connection-less, unreliable service.
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Low overhead.
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No delivery guarantees.
No ordering guarantees.
No duplicate detection.
No connection establishment/teardown.
Suitable for short-lived connections.
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Example: client-server applications.
Spring 2004
TCP
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Reliable end-to-end communication.
TCP transport entity:
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Runs on machine that supports TCP.
Interfaces to the IP layer.
Manages TCP streams.
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Accepts user data, breaks it down and sends it
as separate IP datagrams.
At receiver, reconstructs original byte stream
from IP datagrams.
Spring 2004
TCP Reliability
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Reliable delivery.
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ACKs.
Timeouts and retransmissions.
Ordered delivery.
Spring 2004
TCP Service Model 1
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Obtained by creating TCP end points.
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Example: UNIX sockets.
Socket number or address: IP address + 16-bit
port number (TSAP).
Multiple connections can terminate at same
socket.
Connections identified by socket ids at both ends.
Port numbers below 1024: well-known ports
reserved for standard services.
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List of well-known ports in RFC 1700.
Spring 2004
TCP Service Model 2
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TCP connections are full-duplex and
point-to-point.
Byte stream (not message stream).
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A
Message boundaries are not preserved
e2e.
B
C
4 512-byte segments sent as
separate IP datagrams
D
ABCD
2048 bytes of data delivered
to application in single READ
Spring 2004
TCP Byte Stream
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When application passes data to TCP, it may
send it immediately or buffer it.
Sometimes application wants to send data
immediately.
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Example: interactive applications.
Use PUSH flag to force transmission.
TCP could still bundle PUSH data together (e.g., if it
cannot transmit it right away).
URGENT flag.
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Also forces TCP to transmit at once.
Spring 2004
TCP Protocol Overview 1
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TCP’s TPDU: segment.
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20-byte header + options.
Data.
TCP entity decides the size of segment.
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2 limits: 64KByte IP payload and MTU.
Segments that are too large are
fragmented.
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More overhead by addition of IP header.
Spring 2004
TCP Protocol Overview 2
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Sequence numbers.
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Reliability, ordering, and flow control.
Assigned to every byte.
32-bit sequence numbers.
Spring 2004
TCP Connection Setup
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3-way handshake.
Host 1
SYN (SEQ=x)
SYN(SEQ=y,ACK=x+1)
(SEQ=x+1, ACK=y+1)
Spring 2004
Host 2
TCP Connection Release 1
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Abrupt release:
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Send RESET.
May cause data loss.
Spring 2004
TCP Connection Release 2
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Graceful release:
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Each side of the connection released
independently.
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Either side send TCP segment with FIN=1.
When FIN acknowledged, that direction is shut down for
data.
Connection released when both sides shut down.
4 segments: 1 FIN and 1 ACK for each
direction; 1st. ACK+2nd. FIN combined.
Spring 2004
TCP Connection Release 3
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Timers to avoid 2-army problem.
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If response to FIN not received within
2*MSL (maximum segment lifetime), FIN
sender releases connection.
After connection released, TCP waits for
2*MSL (e.g., 120 sec) to ensure all old
segments have aged.
Spring 2004
TCP Transmission
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Sender process initiates connection.
Once connection established, TCP can
start sending data.
Sender writes bytes to TCP stream.
TCP sender breaks byte stream into
segments.
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Each byte assigned sequence number.
Segment sent and timer started.
Spring 2004
TCP Transmission (cont’d)
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If timer expires, retransmit segment.
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After retransmitting segment for maximum
number of times, assumes connection is dead
and closes it.
If user aborts connection, sending TCP
flushes its buffers and sends RESET
segment.
Receiving TCP decides when to pass
received data to upper layer.
Spring 2004
TCP Flow Control
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Sliding window.
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Receiver’s advertised window.
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Size of advertised window related to receiver’s
buffer space.
Sender can send data up to receiver’s
advertised window.
Spring 2004
TCP Flow Control: Example
App. writes
2K of data
App. does
3K write
Sender
blocked
Sender
may send up
to 2K
4K
2K;SEQ=0
2K
ACK=2048; WIN=2048
2K; SEQ=2048
0
App. reads
2K of data
ACK=4096; WIN=0
ACK=4096; WIN=2048
1K; SEQ=4096
2K
1K
Spring 2004
TCP Flow Control:
Observations
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TCP sender not required to transmit
data as soon as it comes in from
application.
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Example: when first 2KB of data comes in,
could wait for more data since window is
4KB.
Receiver not required to send ACKs as
soon as possible.
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Wait for data so ACK is piggybacked.
Spring 2004
Congestion Control
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Why do it at the transport layer?
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Use law of “conservation of packets”.
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Real fix to congestion is to slow down sender.
Keep number of packets in the network constant.
Don’t inject new packet until old one leaves.
Congestion indicator: packet loss.
Spring 2004
TCP Congestion Control
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Like, flow control, also window based.
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Sender keeps congestion window (cwin).
Each sender keeps 2 windows: receiver’s
advertised window and congestion window.
Number of bytes that may be sent is
min(advertised window, cwin).
Spring 2004
TCP Congestion Control
(cont’d)
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Slow start [Jacobson 1988]:
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Connection’s congestion window starts at 1
segment.
If segment ACKed before time out,
cwin=cwin+1.
As ACKs come in, current cwin is increased
by 1.
Exponential increase.
Spring 2004
TCP Congestion Control
(cont’d)
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Congestion Avoidance:
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Third parameter: threshold.
Initially set to 64KB.
If timeout, threshold=cwin/2 and cwin=1.
Re-enters slow-start until cwin=threshold.
Then, cwin grows linearly until it reaches
receiver’s advertised window.
Spring 2004
cwin
TCP Congestion Control:
Example
timeout
threshold
threshold
time
Spring 2004
TCP Retransmission Timer
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When segment sent, retransmission
timer starts.
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If segment ACKed, timer stops.
If time out, segment retransmitted and
timer starts again.
Spring 2004
How to set timer?
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Based on round-trip time: time between
a segment is sent and ACK comes back.
If timer is too short, unnecessary
retransmissions.
If timer is too long, long retransmission
delay.
Spring 2004
Jacobson’s Algorithm 1
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Determining the round-trip time:
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TCP keeps RTT variable.
When segment sent, TCP measures how
long it takes to get ACK back (M).
RTT = alpha*RTT + (1-alpha)M.
alpha: smoothing factor; determines
weight given to previous estimate.
Typically, alpha=7/8.
Spring 2004
Jacobson’s Algorithm 2
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Determining timeout value:
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Measure RTT variation, or |RTT-M|.
Keeps smoothed value of cumulative
variation D=alpha*D+(1-alpha)|RTT-M|.
Alpha may or may not be the same as
value used to smooth RTT.
Timeout = RTT+4*D.
Spring 2004
Client-Server Model
Client File Server
Kernel
Kernel
Spring 2004
Printer Server
Kernel
File Transfer
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Sharing remote files: “on-line” access
versus “file transfer”.
“On-line” access transparent access to
shared files, e.g., distributed file
system.
Sharing through file transfer: user
copies file then operates on it.
Spring 2004
The Web and HTTP
Spring 2004
The Web
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WWW, or the world-wide web is a
resource discovery service.
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Resource space is organized hierarchically,
and resources are linked to one another
according to some relation.
Hypertext organization: link “granularity”;
allows links within documents.
Graphical user interface.
Spring 2004
The Client Side
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Users perceive the Web as a vast collection
of information.
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Page is the Web’s information transfer unit.
Each page may contain links to other pages.
Users follow links by clicking on them which takes
them to the corresponding page.
This process can go on indefinetly, traversing
several pages located in different places.
Spring 2004
The Browser
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Program running on client that retrieves and
displays pages.
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Interacts with server of page.
Interprets commands and displays page.
Examples: Mosaic, Netscape’s Navigator and
Communicator, Microsoft Internet Explorer.
Other features: back, forward, bookmark,
caching, handle multimedia objects.
Spring 2004
Domain Name System (DNS)


Basic function: translation of names
(ASCII strings) to network (IP)
addresses and vice-versa.
Example:

zephyr.isi.edu <-> 128.9.160.160
Spring 2004
DNS



Hierarchical name space.
Distributed database.
RFCs 1034 and 1035.
Spring 2004
How is it used?

Client-server model.




Client DNS (running on client hosts), or
resolver.
Application calls resolver with name.
Resolver contacts local DNS server (using
UDP) passing the name.
Server returns corresponding IP address.
Spring 2004
Name Resolution 1


Application wants to resolve name.
Resolver sends query to local name server.



Resolver configured with list of local name servers.
Select servers in round-robin fashion.
If name is local, local name server returns
matching authoritative RRs.


Authoritative RR comes from authority managing the
RR and is always correct.
Cached RRs may be out of date.
Spring 2004
Name Resolution 2

If information not available locally (not
even cached), local NS will have to ask
someone else.

It asks the server of the top-level domain
of the name requested.
Spring 2004
Electronic Mail

Non-interactive.


Deferred mail (e.g., destination temporarily
unavailable).
Spooling:


Message delivery as background activity.
Mail spool: temporary storage area for
outgoing mail.
Spring 2004
Mail system
User
sends mail
User
interface
User
reads mail
Outgoing
mail
spool
Mailboxes
incoming
mail
Spring 2004
Client
(send)
TCP
connection
(outgoing)
Server TCP
(receive) connection
(incoming)
Observations




When user sends mail, message stored
is system spool area.
Client transfer runs on background.
Initiates transfer to remote machine.
If transfer succeeds, local copy of
message removed; otherwise, tries
again later (30 min) for a maximum
interval (3 days).
Spring 2004
Remote access
Spring 2004
Telnet
User’s
machine
Telnet
client
Telnet
server
OS
OS
TCP connection
over Internet
Spring 2004
Telnet basic operation




When user invokes telnet, telnet client on
user machine establishes TCP connection to
specified server.
TCP connection established; user’s keystrokes
sent to remote machine.
Telnet server sends back response, echoed on
user’s terminal.
Telnet server can accept multiple concurrent
connections.
Spring 2004