CS 241 Section Week #12(04/23/09)

Outline

• LMP2 Overview• Brief intro to Networking• What is a protocol?• OSI Model• TCP/IP Model• UDP and TCP• Socket Programming Library Functions• Hypertext Transfer Protocol

LMP2 Overview

LMP2 Overview

• LMP2 encodes or decodes a number of files• It has the following parameters:

– the filenames – the number of bytes (rw_units) for each

read/write from the file;• But, process files in “round robin” fashion

• encrypt rw_unit of file1• encrypt rw_unit of file2• encrypt rw_unit of file3• encrypt rw_unit of file1• ………

LMP2 Overview

• For the output in 2nd part, – print in get_frame() will suffice– you may print in my_munmap.

• Most csil machines, use signed char, whose range is from -128 to 127. So use unsigned char if you compare with 255.

LMP2 Overview• For N files,

-The Table of File-Mappings has exactly N entries -The Virtual Page Table has enough entries to fit each file's length divided by 4096B -The Physical Memory Frame Table has 16 entries

• Example: 3 files of length 3072bytes, 2048bytes, and 8192bytes: -The Table of File-Mappings has 3 entries-The Virtual Page Table has:

(3072/4096=>1) + (2048/4096=>1) + (8192/4096=>2) = 4 entries -The Physical Memory Frame Table has 16 entries and four of them are filled

Brief intro to Networking

Networking

• Allows computers and other networked devices to talk to each other– How can we tell what the packet we just received means?

• Interactions between applications on different machines are governed by protocols– Protocols dictate the format and sequence of the

information exchange

Names and Addresses

• A network address identifies a specific computer on the network– Several kinds of addresses exist: MAC address (for LANs),

IP address (for the Internet), etc.

• Domain or DNS names are used for convenience, so we don’t have to remember numerical addresses

Ports• Ports are numbers that represent an end-point for

communication– Ports are logical, not physical, entities– All packets arrive to the same physical interface, and are

then differentiated based on the port number (and other contents of the packet header)

• Usually, distinct ports are used for communication via different protocols– E.g., port 80 is for HTTP, 22 is for SSH, etc.– See /etc/services for a list

What is a protocol?

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What is a protocol?

• It is a formal description of message formats and the rules that two computers must follow in order to exchange messages.

• This set of rules describes how data is transmitted over a network.

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Why are protocols needed?

• Protocols are needed for communication between any two devices. – In what format will the messages be transmitted?– At what speed should messages be transmitted?– What to do if errors take place?– What to do if parts of a message are lost?

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Network Model• What is a model? – A hypothetical description of a

complex entity or process.

• Network model - A method of describing and analyzing data communications networks by breaking the entire set of communications process into a number of layers

• Each layer has a specific function

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Open Systems Interconnect (OSI) Model

• Who made: – International Standards Organization (ISO)

• A Model of How Protocols and Networking Components Could be Made

• “Open” means the concepts are non-proprietary; can be used by anyone.

• OSI is not a protocol. It is a model for understanding and designing a network architecture that is flexible and robust.

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7-layer OSI model

• Why so many layers?– To reduce complexity, networks are organized as

a stack of layers, one below the other

– Each layer performs a specific task. It provides services to an adjacent layer

– This is similar to the concept of a function in programming languages – function do a specific task

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The Layers of the OSI Model

Application

Presentation

Session

Transport

Network

Data Link

Physical

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The Layers of the OSI ModelSome Mnemonics

Application

Presentation

Session

Transport

Network

Data Link

Physical

All

People

Seem

To

Need

Data

Processing

Please

Do

Not

Tell

Secret

Passwords

Anytime

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Physical layer• Specifications for the physical

components of the network• Bit representation: encode bits into electrical or

optical signals• Transmission rate: the number of bits sent each

second

Application

Presentation

Session

Transport

Network

Data Link

Physical

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Data Link LayerResponsible for delivery of data between two systems on the same network. Main functions are:

• Framing: divides the stream of bits received from network layer into manageable data units called frames.

• Physical Addressing: add a header to the frame to define the physical address of the source and the destination machines.

Application

Presentation

Session

Transport

Network

Data Link

Physical

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Network LayerMain functions of this layer are:

• Responsible for delivery of packets across multiple networks.

• Routing: Provide mechanisms to transmit data over independent networks that are linked together.

Application

Presentation

Session

Transport

Network

Data Link

Physical

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Transport LayerMain functions of this layer are:

• Responsible for source-to-destination delivery of the entire message.

• Segmentation and reassembly: divide message into smaller segments, number them and transmit. Reassemble these messages at the receiving end.

Application

Presentation

Session

Transport

Network

Data Link

Physical

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Session LayerMain functions of this layer are:• Dialog control: allows two systems

to enter into a dialog, keep a track of whose turn it is to transmit.

Application

Presentation

Session

Transport

Network

Data Link

Physical

H5synsyn syn

From Presentation Layer

To Transport Layer

Session Layer

From Transport Layer

To Presentation Layer

H5synsyn syn

Session Layer

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Presentation LayerResponsibilities of this layer are:

• Translation since different computers use different encoding systems (bit order translation

Application

Presentation

Session

Transport

Network

Data Link

Physical

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Application Layer•Contains protocols that allow the users

to access the network (FTP, HTTP, SMTP, etc)

•DOES NOT include application programs such as email, browsers, word processing applications, etc.

Application

Presentation

Session

Transport

Network

Data Link

PhysicalTo Presentation Layer From Presentation Layer

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Summary of Functions of Layers

Application

Presentation

Session

Transport

Network

Data Link

Physical

To allow access to network resources

To establish, manage & terminate sessions

To move packets from source to destination

To transmit bits over a medium & provide electrical specs.

To translate, encrypt and compress data

To provide reliable end-to-end message

delivery

To organise bits into frames

TCP/IP Model

• Layers in the TCP/IP model– Physical Layer– Data Link Layer (MAC)– Network Layer (IPv4, IPv6)– Transport Layer (UDP, TCP)– Application Layer (HTTP, SSH)

Networking

• Each layer is (traditionally) self-contained.– The data-link layer doesn’t care if it’s running IPv4

or IPv6, doesn’t care it it’s TCP or UDP.– When processing the data-link layer, no other

layer is (traditionally) considered.

Networking

Networking• Data Link Layer:

– Purpose: To transfer packets in a flat addressing space.

• Multiple packets go into a hub.• The hub bundles packets together to send them

“upstream” if necessary, based only on the MAC address.

– Not significantly covered in this class.

Networking• Network Layer:

– Purpose: To transfer packets in a hierarchical addressing space.

• Streams of packets go into switches.• Switches determine if the address is within it’s

“subnet” and will route based on “subnets” (IPv4/IPv6).

– Also, not significantly covered in this class.

Networking• Transport Layer:

– After the data link and network layer, packets have arrived at the destination computer.

– The transport layer:• Provides “port multiplexing”, allowing for multiple

programs all communicate without receiving every packet.

• Protocol-specific assurances.

UDP

• User Datagram Protocol (UDP)– The simplest commonly used transport protocol.– Provides:

• Port multiplexing• Error-checking (via simple checksum)

UDP

• User Datagram Protocol (UDP)– Does NOT provide:

• Ordering– Packet A may be sent before Packet B, but Packet B may arrive

after Packet A• Reliability

– The sender has no way to know that Destination received some packet sent by Sender (assured by retransmissions)

• Flow / Congestion Control– There’s no mechanism to “tweak” packet size

UDP• User Datagram Protocol (UDP)

– Only a few parameters are needed for UDP:• Source Port• Destination Port

– From the network layer (IP), you have two more parameters:

• Source IP• Destination IP

UDP

• UDP Packet:

Source Port Destination Port

Data Length Checksum

Data

TCP

• Transmission Control Protocol– TCP provides an alternative to UDP to allow for

more assurances– Provides:

• Port multiplexing• Ordered data / no duplicates• Error-free transmission• Flow control• Congestion control

TCP

• TCP Packet:

Source Port Destination Port

ACK Number

SEQ Number

Data

Offset / Flags Window Size

Checksum Urgent Pointer

Optional Fields

Socket Programming

Socket

• Standard APIs for sending and receiving data across computer networks

• Introduced by BSD operating systems in 1983

• POSIX incorporated 4.3BSD sockets and XTI in 2001

• #include <sys/socket.h>

Typical TCP Server-Client

Typical UDP Server-Client

Programming Sockets

• Lucky for us, the OS does most the work between UDP and TCP!– TCP Socket:

• socket(PF_INET, SOCK_STREAM, IPPROTO_TCP);

– UDP Socket:• socket(PF_INET, SOCK_DGRAM, 0);

Programming Sockets

• To create a socket in C, you need to run two commands:– socket()

socket

int socket(int domain, int type, int protocol);

• Creates a communication endpoint

• Parameters– domain: AF_INET (IPv4)– type: SOCK_STREAM (TCP) or SOCK_DGRAM (UDP)– protocol: 0 (socket chooses the correct protocol based on

type)

• Returns a nonnegative integer corresponding to a socket file descriptor if successful, -1 with errno set if unsuccessful

Programming Sockets

• To create a socket in C, you need to run two commands:– socket()– bind()

bind

int bind(int socket, const struct sockaddr *address, socklen_t address_len);

• Associates the socket with a port on your local machine

• struct sockaddr_in used for struct sockaddrsa_family_t sin_family; /* AF_INET */in_port_t sinport; /* port number */struct in_addr sin_addr; /* IP address */

• Returns 0 if successful, -1 with errno set if unsuccessful

Programming Sockets

• Since UDP is packet-based and TCP is connection-based, you need to establish a connection in TCP:– Server: listen(), accept()

listen

int listen(int socket, int backlog);

• Puts the socket into the passive state to accept incoming requests

• Internally, it causes the network infrastructure to allocate queues to hold pending requests– backlog: number of connections allowed on the incoming

queue

• bind should be called beforehand

• Returns 0 if successful, -1 with errno set if unsuccessful

acceptint accept(int socket, struct sockaddr *restrict

address, socklen_t *restrict address_len);

• Accepts the pending requests in the incoming queue

• *address is used to return the information about the client making the connection. – sin_addr.s_addr holds the Internet address

• listen should be called beforehand

• Returns nonnegative file descriptor corresponding to the accepted socket if successful, -1 with errno set if unsuccessful

Programming Sockets

• Since UDP is packet-based and TCP is connection-based, you need to establish a connection in TCP:– Server: listen(), accept()– Client: connect()

connect

int connect(int socket, const struct sockaddr *address, socklen_t address_len);

• Establishes a link to the well-known port of the remote server

• Initiates the TCP 3-way handshake– Cannot be restarted even if interrupted

• Returns 0 if successful, -1 with errno set if unsuccessful

Programming Sockets

• In both TCP and UDP, you send and receive by using the same calls:– recv() / recvfrom()

recv and recvfromint recv(int socket, void *buf, int len, int flags);

int recvfrom(int socket, void *buf, int len, int flags, const struct sockaddr *from, socklet_t fromlen);

• receives data into the buffer buf

• recvfrom is used for unconnected datagram sockets. If used in connection-mode, last two parameters are ignored.

• Returns the number of bytes actually read if successful, -1 with errno set if unsuccessful

Programming Sockets

• In both TCP and UDP, you send and receive by using the same calls:– recv() / recvfrom()– send() / sendto()

send and sendtoint send(int socket, const void *msg, int len, int flags);

int sendto(int socket, const void *msg, int len, int flags, const struct sockaddr *to, socklet_t tolen);

• sends data pointed by msg

• sendto is used for unconnected datagram sockets. If used in connection-mode, last two parameters are ignored.

• Returns the number of bytes actually sent out if successful, -1 with errno set if unsuccessful

close and shutdownint close(int socket);

int shutdown(int socket, int how);

• close– Prevents any more reads and writes– same function covered in file systems

• shutdown– provides a little more control– how

• 0 – Further receives are disallowed• 1 – Further sends are disallowed• 2 – same as close

• Returns 0 if successful, -1 with errno set if unsuccessful

Using Sockets in C

#include <sys/socket.h>#include <sys/types.h>#include <netinet/in.h>#include <unistd.h>

On csil-core:gcc –o test test.c

On some systems, e.g., Solaris:gcc –o test test.c –lsocket -lnsl

TCP Client/Server Example

Run the provided test-server and test-client executables in two separate windows.test-client sends the string “Hello World!” to IP address

127.0.0.1 port 10000test-server listens on port 10000 and prints out any text

received

Next week, we will reproduce this behavior with codes client.c and server.c

HyperText Transfer Protocol

HTTP

• Hypertext Transfer Protocol– Delivers virtually all files and resources on the

World Wide Web– Uses Client-Server Model

• HTTP transaction– HTTP client opens a connection and sends a

request to HTTP server– HTTP server returns a response message

HTTP (continued)

• Request– GET /path/to/file/index.html HTTP/1.0– Other methods (POST, HEAD) possible for request

• Response– HTTP/1.0 200 OK– Common Status Codes

• 200 OK• 404 Not Found• 500 Server Error

Sample HTTP exchange

• Scenario– Client wants to retrieve the file at the following URL

(http://www.somehost.com/path/file.html)

• What a client does– Client opens a socket to the host www.somehost.com, port

80– Client sends the following message through the socket

GET /path/file.html HTTP/1.0From: someuser@uiuc.eduUser-Agent: HTTPTool/1.0[blank line here]

Sample HTTP exchange

• What a server does– Server responds through the same socket

HTTP/1.0 200 OKDate: Mon, 17 Apr 2006 23:59:59 GMTContent-Type: text/htmlContent-Length: 1354

<html><body>(more file contents) . . .</body></html>

Reference

• Beej's Guide to Network Programming– http://beej.us/guide/bgnet/

Summary

• Protocols

• Socket Programming– Library Functions

• Hypertext Transfer Protocol– Request– Response