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Simple Web Server HTTP/ OK Date: Wed, 29 Nov :14:10 GMT Server: Apache/1.1.1 Content-Length: 183 Content-Type: text/html Connection: close 410 Web Page 410 is my favorite class. GET /index.html HTTP/1.0 Web Client Request:: Web Server Reply:
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Lecture Topics: 11/29• IP• UDP• TCP• DNS
Homework 9• Write a simple web server
– not as hard as it sounds• Specification out tonight• Skeleton code out by tomorrow
morning• Base implementation (80%)
– return a requested file – report a few errors– <100 lines of code
Simple Web Server
HTTP/1.0 200 OKDate: Wed, 29 Nov 2000 20:14:10 GMTServer: Apache/1.1.1Content-Length: 183Content-Type: text/htmlConnection: close
<!DOCTYPE HTML PUBLIC "-//IETF//DTD HTML 2.0//EN"><HTML><HEAD><TITLE>410 Web Page</TITLE></HEAD><BODY><p>410 is my favorite class.</p></body></html>
GET /index.html HTTP/1.0
Web Client Request::
Web Server Reply:
Homework 9 Continued• Additional features (25%) do some of the following
– generate an HTML index file based on the contents of a directory
– create a web log that stores all accesses– keep “hit” counters for each file that are reported when
the user accesses /hits– many more options
• You will need to learn how to use sockets– there will be links on the homework page and sample
programs– not too hard
• You may need to read a little on HTTP and HTML– links on the web page
Open Issues• What happens to the L1 cache on system calls?
– the OS uses the L1 cache as usual– it only replaces invalid entries– user code will have to
• What is hub?– a hub logically only connects the wires between
multiple computers or networks– a packet received on any wire is immediately
forwarded to every other wire– it increases the size of the collision domain
• What is a switch?– a switch is like a hub except it only forwards packets
towards the destination– it learns where other computers are by listening to
where they send their packets from
Hubs vs. Switches• Hubs forward
packets everywhere (broadcast)– cheap– work best if the
network is small• Switches only
forward toward the destination– more expensive – self-configuring– good for large
networks
C3
HUB
C4
C1
C2
C5
C3
SWITCH
C4
C1
C2
C5
Protocol Stack• These layers comprise the protocol stack• Each layer of the stack adds extra info to the
packetApplication Data
IP Header Application DataTCP HeaderEthernet Header
Ethernet Checksum
Ethernet Frame
IP Header Application DataTCP Header
IP Datagram
Application DataTCP Header
TCP Segment
Application layerTransport layer
Network layer
Data link layer
IP• Ethernet allows packets to be sent
between nodes on the same network• Internet Protocol (IP) allows packets
to be sent between nodes on different networks
• Key idea: some machines (i.e. routers) are on multiple networks
• Ethernet is still used to send packets between routers
IP Routers
R
R
R
RRHH
H
H
H
R
RH
R
Routers send packet to next closest point
H: Hosts
R: Routers
YOUCNN
IP Addresses• IP addresses are 4 bytes (e.g.
128.95.8.137)– part of the address designates the network
(e.g. 128.95)– the rest of the address designates the
machine on the network (e.g. 8.137)• A router sends the packet to the correct
network, and the network sends the packet to the correct node– this greatly reduces the information a router
must store
How do routers know where to send packets?
• Forwarding tables at each router– Network == 128.95, then send to router X
• Original Internet: manual update• Now: automatic update based on
“cost”– exchange tables with neighbors– use neighbor with smallest hop count
– what if node says zero cost to everywhere?
IP Service Model• IP's service model is best-effort
unreliable data-gram delivery• IP does it's best to get a message from
source to destination• But it makes no guarantees that
– the packet will arrive– the packets will arrive in order– the contents of the packet won't be changed– only one copy of the packet will arrive
• Aren't you glad the designers of IP don't design cars?
What IP can do
• Some are uncommon, but they all happen
INTERNETHello World Hello WorldArrive in order
INTERNETHello World WorldLose a packet
INTERNETHello World World HelloReorder packets
INTERNETHello World Hello Hello WorldDuplicate packets
INTERNETHello World Goodbye WorldCorrupt Packets
Checksum
chksum hash
10101001011101001100101010110101010110111010001011010101011010101010110101001011101001100101010110101010110111010011000101110100
Sent Data & Checksum1010100101110100110010101011010101011011101000101101010101101010101011010100101110100110010101011010101011011101
Data to Send
10101001011101001100101010110101010110111010001011010100100010101010110101001011101001100101010110101010110111010011000101110100
Received Data & Checksum
chksum hash 1001010110101010
Calculated and stored checksum differ, so data is corrupt
INTERNET
Corrupted bits
User Datagram Protocol (UDP)
• UDP is basically IP with a data checksum and ports
• The checksum ensures that corrupted data isn't accepted
• Ports allow multiple connections to be de-multiplexed (TCP has ports too)– a port is a number between 0 and 65535– common applications such as web servers run on
"well-known ports" so web browsers know which port to contact
– a UDP/TCP "connection" is determined by the • Source IP address, Source port number,• Destination IP address and Destination port number
UDP• UDP characteristics (similar to IP)
– unreliable (no guarantees the packet arrives)– packets could be reordered or duplicated– corrupted packets are discarded, but not
retransmitted• Who would ever want this?
– common for low-latency applications where loss is tolerable (streaming media, games)
– because reliable protocols (TCP) can be slower
Transmission Control Protocol
• TCP is what you usually want– reliable (messages guaranteed to
eventually arrive)– dropped packets are retransmitted– duplicate packets are discarded– reordered packets are unreordered
• TCP is a bi-directional (i.e. full-duplex) stream based protocol– Both nodes can send and receive– Don't explicitly send packets, instead
send a stream of bytes
Ensuring Packets Arrive w/ UDP
Will you marry me?
INTERNET ?• No Answer--Did she get my proposal? Was
the packet lost or corrupted? I better send it again
Will you marry me?
INTERNET ?Will you
marry me?INTERNET ?
Will you marry me?
INTERNET ?Stalker!
Reliability with TCP• With TCP, the receiver is obligated to send an
acknowledgment (ACK), when it receives a packet
• The receiver is not obligated to respond with an answer, only an acknowledgement of receipt
• If the sender does not receive an ACK, it eventually retransmits the packet– can the receiver ever receive two copies of a
packet?
Will you marry me? INTERNET Will you marry me?
Got your messageGot your message
Timing Out
Packet
ACK
Tim
eou
t
Packet
ACK
Tim
eout
Packet
Tim
eout
Packet
ACK
Tim
eout
Packet
ACK
Tim
eou
t
Packet
ACK
Tim
eout
ACK lost Packet lost Early timeout
Tim
e
Sender Receiver Sender Receiver Sender Receiver
Eliminating Duplicate Packets
• Problem: The receiver can still receive duplicate packets (either due to the Internet or a retransmission) – how do we distinguish a duplicate from
another packet with the same contents?• Solution: Send a sequence
number with each packet– 1st packet gets sequence number 0– 2nd packet gets sequence number 1– retransmitted 1st packet has sequence
number 0
Sequence Numbers• Each new
packet gets a new sequence number
• Don't send the next packet until the previous packet is acknowledged
Pkt 0
ACK 0Pkt 0
ACK 1
Pkt 1ACK 0
Discard Duplicate
Sending Multiple Packets• Sending one packet at a time is
too slow– round trip time is 100ms– network has enough buffer space to
hold several packets• Idea: send lots of packets at once
– receiver acknowledges the packets they receive
– retransmit only unacked packets
Sending Multiple Packets• Sender sends multiple
packets at once• Each packet that was
not acknowledged is retransmitted
• Problem:– retransmit packets
even if only ACKs are lost (e.g. packet 2)
01234
23
2
ACK 0 ACK 1
ACK 4
ACK 3
ACK 2
TCP Sending Packets• In TCP, the receiver sends an acknowledgement
after every packet• The receiver acknowledges up to the last
consecutive packet it received– receive "0" send "ack 0" I've received 0-0– receive "1" send "ack 1" I've received 0-1– receive "2" send "ack 2" I've received 0-2– receive "4" send "ack 2" I've received 0-2– receive "5" send "ack 2" I've received 0-2
• It's okay if ACKs get lost– e.g. "ack 1" can be lost because "ack 2" covers it
• The sender can recognize if a packet was lost– Mmmmm? I've received 3 "ack 2"s, maybe packet 3 got
lost
TCP ACKing• The receiver acknowledges
up to the last consecutive packet it received
• Packet 3 was lost, so the receiver sends an "ACK 2" when it receives packet 4, not an "ACK 4"
• The receiver must remember which packets it has received– it sends an "ACK 4" when it
receives packet 3 because it received packet 4 earlier
01234
3
ACK 0 ACK 1
ACK 2
ACK 4
Handling Reordered Packets
• TCP ensures that the data stream is delivered in-order
• Packets may arrive out-of-order
• TCP must buffer out-of-order packets until the missing packets arrive
• Packets are delivered to the application in the order they were sent, not the order they arrived
01234
3
ACK 0 ACK 1
ACK 2
ACK 4
Deliver Packet 0Deliver Packet 1Deliver Packet 2Save Packet 4
Deliver Packets 3-4
Receive Window• From the receiver's perspective there are three
types of packets– packets received and acked – packets received out of order– packets not yet received
• The first missing packet through the highest received packet is known as the receive window– the receiver only has to buffer packets in the receive
windowPacket # 0 1 2 3 4 5 6 7 8 9Received? X X X X XAcknowledged?
X X X
Receive Window
Send Window• From the sender's perspective there are three types
of packets– packets sent and acked – packets sent and unacked– packets not yet sent
• All sent and unacked packets make up the send window– the sender must buffer all sent and unacked packets and all
packets not yet sent
Packet # 0 1 2 3 4 5 6 7 8 9Sent? X X X X X X XReceived ack? X X
Send Window
Sliding Window• TCP is known as a sliding window
protocol • As packets are sent/received/
acknowledged, the sender and receiver windows slide through sequence numbers
Domain Name Service (DNS)
• IP must use IP addresses, e.g. 128.95.4.112• You want to type
http://www.cs.washington.edu, not http://128.95.4.112
• DNS handles making this translation
• If the local DNS server doesn't know the translation, it asks one of the 13 root name-servers, which have known IP addresses
Local DNS server(128.95.1.4)
What’s the IP address for www.netscape.com?
Your computer(128.95.1.24)
Oh, you can find it at 207.200.89.226
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