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CMPE 151: Network
Administration
Lecture 2
Winter 2005
Review?
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Network protocols.
TCP/IP.
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Outline
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Network protocols.
IP
TCP
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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

We need design guidelines!
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Protocol stack
User A
Teleconferencing
User B
Peers
Application
Transport
Network
Link
Host
Host
Layering: technique to simplify complex systems
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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).
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Encapsulation
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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!
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Layering Functionality
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Reliability
Flow control
Fragmentation
Multiplexing
Connection setup (handshaking)
Addressing/naming (locating peers)
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Example: Transport layer
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First end-to-end layer.
End-to-end state.
May provide reliability, flow and
congestion control.
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Example: Network Layer
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Point-to-point communication.
Network and host addressing.
Routing.
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Internetworking
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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.
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Internetworks (cont’d)
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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.
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IP
Winter 2005
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.
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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?
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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
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The Internet Protocol
Host
Host
Application
Transport
IP
Router
Router
IP
IP
Network
IP
Network
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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.
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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.
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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).
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Payload
Header
IP(v4) Header Format
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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.
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Encapsulation - Example
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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.
Winter 2005
Encapsulation Across
Multiple Hops - Example
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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).
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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.
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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.
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Fragmentation - Example
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Addressing
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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.
Winter 2005
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”).
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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.
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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.
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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.
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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.
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IP Address Format (cont’d)
• How can we recognize to which class an IP
address belongs to?
– Look at the first 4 bits!
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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.
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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
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Networks and Hosts
in Each Class
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Understanding IP Addresses
• Examples:
– 10.0.0.37 (class A)
– 128.10.0.1 (class B)
– 192.5.48.3 (class C)
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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”.
Winter 2005
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.
Winter 2005
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.
Winter 2005
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.
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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.
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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.
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TCP Reliability
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Reliable delivery.
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ACKs.
Timeouts and retransmissions.
Ordered delivery.
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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.
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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
Winter 2005
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.
Winter 2005
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.
Winter 2005
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.
Winter 2005
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)
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Host 2
TCP Connection Release 1
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Abrupt release:
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Send RESET.
May cause data loss.
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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.
Winter 2005
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.
Winter 2005
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.
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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.
Winter 2005
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.
Winter 2005
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
Winter 2005
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.
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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.
Winter 2005
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).
Winter 2005
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.
Winter 2005
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.
Winter 2005
cwin
TCP Congestion Control:
Example
timeout
threshold
threshold
time
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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.
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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.
Winter 2005
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.
Winter 2005
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.
Winter 2005
Client-Server Model
Client File Server
Kernel
Kernel
Winter 2005
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.
Winter 2005
The Web and HTTP
Winter 2005
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.
Winter 2005
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.
Winter 2005
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.
Winter 2005
Domain Name System (DNS)
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Basic function: translation of names
(ASCII strings) to network (IP)
addresses and vice-versa.
Example:
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zephyr.isi.edu <-> 128.9.160.160
Winter 2005
DNS
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Hierarchical name space.
Distributed database.
RFCs 1034 and 1035.
Winter 2005
How is it used?
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Client-server model.
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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.
Winter 2005
Name Resolution 1
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Application wants to resolve name.
Resolver sends query to local name server.
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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.
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Authoritative RR comes from authority managing the
RR and is always correct.
Cached RRs may be out of date.
Winter 2005
Name Resolution 2
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If information not available locally (not
even cached), local NS will have to ask
someone else.
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It asks the server of the top-level domain
of the name requested.
Winter 2005
Electronic Mail
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Non-interactive.
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Deferred mail (e.g., destination temporarily
unavailable).
Spooling:
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Message delivery as background activity.
Mail spool: temporary storage area for
outgoing mail.
Winter 2005
Mail system
User
sends mail
User
interface
User
reads mail
Outgoing
mail
spool
Mailboxes
incoming
mail
Winter 2005
Client
(send)
TCP
connection
(outgoing)
Server TCP
(receive) connection
(incoming)
Observations
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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).
Winter 2005
Remote access
Winter 2005
Telnet
User’s
machine
Telnet
client
Telnet
server
OS
OS
TCP connection
over Internet
Winter 2005
Telnet basic operation
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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.
Winter 2005