Chapter 2 Applications and Layered Architectures Protocols, - - PowerPoint PPT Presentation
Chapter 2 Applications and Layered Architectures Protocols, Services & Layering Layers, Services & Protocols l The overall communications process between two or more machines connected across one or more networks is very complex l
l The overall communications process
l Layering partitions related communications
l Each layer provides a service to the layer
l Each layer operates according to a protocol
l A set of related communication functions that can be
l Application Layer: communications functions that
l HTTP (Browsing), DNS (URL to IP address), SMTP (email)
l Transport Layer: end-to-end communications
l TCP Reliable Stream Service, UDP Datagram Service
l Network Layer: communications between machines
l Internet Protocol (IP) best effort service
l A protocol is a set of rules that governs how
l Messages that can be sent and received l Actions that are to be taken when a certain
l The purpose of a protocol is to provide a
l HTTP is an application layer protocol l Retrieves documents on behalf of a browser
l HTTP specifies fields in request messages
l Request types; Response codes l Content type, options, cookies, …
l HTTP specifies actions to be taken upon
GET Response
l HTTP provides service to a browsing application l HTTP messages exchanged between HTTP client
l HTTP client and server are processes running in two
l HTTP uses the reliable stream transfer service
Response GET
80, 1127 GET 1127, 80 bytes Response
GET
Response
l TCP is a transport layer protocol l Provides reliable byte stream service between two
l Sequence numbers keep track of the bytes that have
l Error detection and retransmission used to recover
l TCP is connection-oriented: the sender and receiver
l Connection ID is specified uniquely by
(send port #, send IP address, receive port #, receiver IP address)
l HTTP server has well-known port # 80
l DNS (Domain Name System) is an application
l DNS is a distributed database that resides in
l DNS protocol allows queries of different types
l Name-to-address or Address-to-name l Mail exchange
l DNS usually involves short messages and so
l Well-known port 53
l Local Name Server: resolve frequently-used names
l University department, ISP l Contacts Root Name server if it cannot resolve query
l Root Name Servers: 13 globally (actually 100’s)
l Resolves query or refers query to Authoritative Name
Server
l Authoritative Name Server: last resort
l Every machine must register its address with at least two
authoritative name servers
1 2 3 4 5 6 Local Name Server Root Name Server Authoritative Name Server
l World Wide Web allows users to access resources
l Documents prepared using HyperText Markup
l A browser application program is used to access the
l The browser displays HTML documents that include
l Each link references a Uniform Resource Locator
l Lets see what happens when a user clicks on a link
l User clicks on http://www.nytimes.com/ l URL contains Internet name of machine
l Internet needs Internet address to send packets to a
l Browser software uses DNS protocol to send query
l DNS system responds with Internet address
l Browser uses HTTP to send request for document l HTTP server waits for requests by listening to a
l HTTP client sends request messages through an
l HTTP needs a TCP connection between the HTTP
TCP Connection Request From: 128.100.11.13 Port 1127 To: 64.15.247.200 Port 80
ACK, TCP Connection Request From: 64.15.247.200 Port 80 To:128.100.11.13 Port 1127 ACK
l HTTP client sends its request message: GET … l HTTP server sends a status response: 200 OK l HTTP server sends requested file l Browser displays document l Clicking a link sets off a chain of events across the
l DNS, TCP, HTTP, Browser display
GET / HTTP/1.1 200 OK
Content
l UDP is a transport layer protocol l Provides best-effort datagram service
l Port numbers distinguish various processes
l UDP is connectionless l Datagram is sent immediately l Quick, simple, but not reliable l Used in DNS, real-time voice,…
l Layers: related communications functions
l Application Layer: HTTP, DNS l Transport Layer: TCP, UDP l Network Layer: IP
l Services: a protocol provides a communications
l TCP provides connection-oriented reliable stream
l UDP provides connectionless datagram service
l Each layer builds on services of lower layers
l HTTP builds on top of TCP l DNS builds on top of UDP l TCP and UDP build on top of IP
l A seven-layer abstract reference model for a
l Provides a framework for the development of
l Unified view of layers, protocols, and services which
l Detailed standards were developed for each layer,
l TCP/IP protocols preempted deployment of OSI
Application Layer Presentation Layer Session Layer Transport Layer Network Layer Data Link Layer Physical Layer Application Layer Presentation Layer Session Layer Transport Layer Network Layer Data Link Layer Physical Layer Network Layer
Application Application
Data Link Layer Physical Layer Network Layer Data Link Layer Physical Layer
Communicating End Systems
One or More Network Nodes
End-to-End Protocols
l Transfers bits across link l Definition & specification of the physical
l Mechanical: cable, plugs, pins... l Electrical/optical: modulation, signal strength,
l functional/procedural: how to activate, maintain,
l Ethernet, DSL, cable modem, cell phone,… l Twisted-pair cable, coaxial cable optical fiber,
l Transfers frames between directly connected machines l Groups bits into frames l Detection of bit errors; Retransmission of frames l Activation, maintenance, & deactivation of data link
l Access control for local area networks (WIFI, Ethernet) l Flow control between machines of different speeds
Data Link Layer Physical Layer Data Link Layer Physical Layer
frames bits
l Transfers packets across multiple links and/or
l Addressing must scale to large networks l Nodes jointly execute routing algorithm to
l Forwarding transfers packet along hops in path l Congestion control to deal with traffic surges l Connection setup, maintenance, and teardown
l Internetworking is part of network layer and provides
l Gateways (routers) direct packets across networks G = gateway H = host Net 1 Net 5 Net 3 Net 2
H
Net 3
G
H H H
G G G G G
Net 1 Net 2 Net 4 Net 5
l l l l l l
Ethernet LAN
ATM Switch ATM Switch ATM Switch ATM Switch
ATM Network
l Transfers data end-to-end from process in a
l Reliable stream transfer or quick-and-simple single-
l Port numbers enable multiplexing l Message segmentation and reassembly l Connection setup, maintenance, and release
Transport Layer Network Layer Transport Layer Network Layer Network Layer Network Layer Communication Network
l Application Layer: Provides
l Presentation Layer: machine-
l Session Layer: dialog
Application Layer Presentation Layer Session Layer Transport Layer
Application
Application Layer Transport Layer
Application
l Each protocol uses a header that carries addresses,
l CRC check bits may be appended for error detection
Application Layer Transport Layer Network Layer Data Link Layer Physical Layer Application Layer Transport Layer Network Layer Data Link Layer Physical Layer
Application Application APP DATA AH APP DATA TH AH APP DATA NH TH AH APP DATA DH NH TH AH APP DATA CRC bits
l To build a network of networks or internet
l operating over multiple, coexisting, different network
technologies
l providing ubiquitous connectivity through IP packet transfer l achieving huge economies of scale
G G G G G G
H
Net 5 Net 5
H
Net 5 Net 2
H
Net 5 Net 3
H
Net 5 Net 1 Net 5 Net 4
l To provide universal communication services
l independent of underlying network technologies l providing common interface to user applications
G G G G G G H
Net 5 Net 5
H
Net 5 Net 2
H
Net 5 Net 3
H
Net 5 Net 1 Net 5 Net 4
Reliable Stream Service User Datagram Service
l To provide distributed applications
l Any application designed to operate based on
l Rapid deployment of new applications l Email, WWW, Peer-to-peer
l Applications independent of network
l New networks can be introduced below l Old network technologies can be retired
l IP packets transfer information across Internet
l IP layer in each router determines next hop (router) l Network interfaces transfer IP packets across networks Router
Internet Layer Network Interface
Transport Layer Internet Layer Network Interface
Transport Layer Internet Layer Network Interface
Host A
Host B Net 5 Net 1 Net 5 Net 2 Net 5 Net 3
Router
Internet Layer Network Interface
Router
Internet Layer Network Interface
Net 5 Net 4
HTTP SMTP RTP TCP UDP IP Network interface 1 Network interface 3 Network interface 2 DNS
Internet Names
l
Each host a a unique name
l Independent of physical
location
l Facilitate memorization by
humans
l Domain Name l Organization under single
administrative unit
l
Host Name
l Name given to host
computer
l
User Name
l Name assigned to user
leongarcia@comm.utoronto.ca
Internet Addresses
l
Each host has globally unique logical 32 bit IP address
l
Separate address for each physical connection to a network
l
Routing decision is done based
l
IP address has two parts:
l
netid and hostid
l
netid unique
l
netid facilitates routing
l
Dotted Decimal Notation: int1.int2.int3.int4 (intj = jth octet) 128.100.10.13 DNS resolves IP name to IP address
l LANs (and other networks) assign physical
l The network uses its own address to transfer
l IP address needs to be resolved to physical address
l Example: Ethernet uses 48-bit addresses
l Each Ethernet network interface card (NIC) has globally
unique Medium Access Control (MAC) or physical address
l First 24 bits identify NIC manufacturer; second 24 bits are
serial number
l 00:90:27:96:68:07 12 hex numbers
Intel
(1,1) s (1,2) w (2,1) (1,3) r (2,2)
PPP Netid=2 Ethernet (netid=1)
PC Server Router Workstation
netid hostid Physical address server 1 1 s workstation 1 2 w router 1 3 r router 2 1
2 2
l Ethernet header contains: l source and destination physical addresses l network protocol type (e.g. IP)
IP header
IP Payload
Ethernet header
FCS
IP header
IP Payload
1.
IP packet has (1,2) IP address for source and (1,1) IP address for destination
2.
IP table at workstation indicates (1,1) connected to same network, so IP packet is encapsulated in Ethernet frame with addresses w and s
3.
Ethernet frame is broadcast by workstation NIC and captured by server NIC
4.
NIC examines protocol type field and then delivers packet to its IP layer
(1,1) s (1,2)
w
(2,1) (1,3) r (2,2) PPP Ethernet PC Server Router Workstation
(1,2), (1,1) w, s
(1,1) s (1,2)
w
(2,1) (1,3) r (2,2) PC Server Router Workstation
1.
IP packet has (1,1) and (2,2) as IP source and destination addresses
2.
IP table at server indicates packet should be sent to router, so IP packet is encapsulated in Ethernet frame with addresses s and r
3.
Ethernet frame is broadcast by server NIC and captured by router NIC
4.
NIC examines protocol type field and then delivers packet to its IP layer
5.
IP layer examines IP packet destination address and determines IP packet should be routed to (2,2)
6.
Routers table indicates (2,2) is directly connected via PPP link
7.
IP packet is encapsulated in PPP frame and delivered to PC
8.
PPP at PC examines protocol type field and delivers packet to PC IP layer
(1,1), (2,2) s, r (1,1), (2,2)
Network interface
IP TCP
HTTP
Network interface
IP
Network interface
IP TCP HTTP Ethernet PPP Router (1,1) s (2,1) (1,3) r (2,2) PPP Ethernet (a) (b) Server PC PC Server Router TCP uses node-to-node Unreliable packet transfer of IP Server IP address & PC IP address
Internet
HTTP uses process-to-process Reliable byte stream transfer of TCP connection: Server socket: (IP Address, 80) PC socket (IP Address, Eph. #)
TCP Header contains source & destination port numbers IP Header contains source and destination IP addresses; transport protocol type Ethernet Header contains source & destination MAC addresses; network protocol type
HTTP Request
TCP header
HTTP Request
IP header TCP header
HTTP Request
Ethernet header IP header TCP header
HTTP Request
FCS
l User clicks on http://www.nytimes.com/ l Ethereal network analyzer captures all frames
l Sequence of frames and contents of frame can be
l Encapsulation is key to layering l IP provides for transfer of packets across
l TCP and UDP provide universal
l Distributed applications that use TCP and
l Internet names, IP addresses, port numbers,
l HTTP servers use well-known port 80 l Client request / Server reply l Stateless: server does not keep any
l HTTP 1.0 new TCP connection per
l HTTP 1.1 persistent operation is default
l HTTP messages written in ASCII text l Request Message Format
l Request Line (Each line ends with carriage return)
l Method URL HTTP-Version \r\n l Method specifies action to apply to object l URL specifies object
l Header Lines (Ea. line ends with carriage return)
l Attribute Name: Attribute Value l E.g. type of client, content, identity of requester, … l Last header line has extra carriage return)
l Entity Body (Content)
l Additional information to server
Request method Meaning GET Retrieve information (object) identified by the URL. HEAD Retrieve meta-information about the object, but do not transfer the object; Can be used to find out if a document has changed. POST Send information to a URL (using the entity body) and retrieve result; used when a user fills out a form in a browser. PUT Store information in location named by URL DELETE Remove object identified by URL TRACE Trace HTTP forwarding through proxies, tunnels, etc. OPTIONS Used to determine the capabilities of the server, or characteristics of a named resource.
l Absolute URL
l scheme://hostname[:port]/path l http://www.nytimes.com/
l Relative URL
l /path l /
l Response Message Format
l Status Line
l HTTP-Version Status-Code Message l Status Code: 3-digit code indicating result l E.g. HTTP/1.0 200 OK
l Headers Section
l Information about object transferred to client l E.g. server type, content length, content type, …
l Content
l Object (document)
l Cookies are data exchanged by clients & servers as
l Since HTTP stateless, cookies can provide context
l Set cookie header line in reply message from server
l If client accepts cookie, cookie added to clients
l Henceforth client requests include ID l Server site can track client interactions, store these
l Sample analog signal in time and amplitude l Find closest binary representation
D/2 3D/2 5D/2 7D/2
Original signal Sample value Approximation
3 bits / sample
l Ws = 4 kHz → 8000
l 8 bits/sample l Rs=8 x 8000 = 64 kbps l Cellular phones use
l Ws = 22 kHertz → 44000
l 16 bits/sample l Rs=16 x 44000= 704 kbps
l MP3 uses more powerful
Network
Synchronous source sends periodic information blocks Network output not periodic
l
Voice, audio, or video generate a synchronous information stream
l
Information carried by equally-spaced fixed-length packets
l
Packet switching introduces random delays
l
Timing recovery is needed to re-establish the synchronous nature of the stream
Packet Arrivals Packet Playout
Sequence numbers help order packets
Send times Playout times Arrival times
Tplayout time Time
Receiver too slow; buffer fills and
Tplayout time Time
Receiver too fast buffer starvation Many late packets
Tplayout time Time
Receiver speed just right
Recovered clock t4 t3 t2 t1 Timestamps Add Smoothing filter Adjust frequency Counter +
Error signal
Playout command
l
Counter attempts to replicate transmitter clock
l
Frequency of counter is adjusted according to arriving timestamps
l
Jitter introduced by network causes fluctuations in buffer & in local clock
Timestamps inserted in packet payloads indicate when info was produced
l RTP designed to support real-time applications
l RTP provides means to carry:
l Type of information source l Sequence numbers l Timestamps
l Timing recovery done by higher layer protocol
l MPEG for video, MP3 for audio
l RTP runs over UDP
Time Stamp CSRC Identifier V P X CC M Payload Type Sequence Number SSRC Identifier
0 8 16 31
l Payload Type: e.g. PCM, MPEG2, … l Sequence Number: detect packet loss l Timestamp: sampling instant of first byte l Synchronization Source: ID for synch source l CSRC List: contributing sources to payload
l RTP companion protocol l Monitors quality of service at receivers l Conveys monitored info to senders l Canonical Name CNAME for each participant l RTCP Packets
l Sender Report Packet l Receiver Report Packet l Source Description (SDES) l BYE: end of participation by sender l APP: application specific functions
l Session: association involving exchange of
l Internet telephone call; multimedia
l Session Initiation Protocol
l Setting up, maintaining, terminating session l People & media devices l Multicast or mesh of unicast connections l Support for user mobility l Over UDP or TCP
(1)
INVITE sip: tom@startup.com c= IN IPv4 192.168.12.5 m=audio 35092 RTP/AVP 0 INVITE sip: tom@192.168.15.17 c= IN IPv4 192.168.12.5 m=audio 35092 RTP/AVP 0
(2) (3)
(4) (5) ringing (6)
SIP/2.0 200 OK SIP/2.0 200 OK
(7)
ACK
(8)
Media flow
(9)
proxy registrar
l API (Application Programming Interface)
l Provides a standard set of functions that can be
l Berkeley UNIX Sockets API
l Abstraction for applications to send & receive data l Applications create sockets that plug into network l Applications write/read to/from sockets l Implemented in the kernel l Facilitates development of network applications l Hides details of underlying protocols & mechanisms
l Also in Windows, Linux, and other OSs
Client Server
descriptor port number descriptor port number
socket through a descriptor
Application 1 Socket Socket interface User Kernel Application 2 User Kernel Underlying communication protocols Underlying communication protocols Communications network Socket Socket interface
Connection-oriented
l First, setup connection
between two peer application processes
l Then, reliable bidirectional
in-sequence transfer of byte stream (boundaries not preserved in transfer)
l Multiple write/read between
peer processes
l Finally, connection release l Uses TCP l Connectionless l Immediate transfer of one
block of information (boundaries preserved)
l No setup overhead & delay l Destination address with
each block
l Send/receive to/from
multiple peer processes
l Best-effort service only
l
Possible out-of-order
l
Possible loss
l Uses UDP
l Server
l Specifies well-known port # when creating socket l May have multiple IP addresses (net interfaces) l Waits passively for client requests
l Client
l Assigned ephemeral port # l Initiates communications with server l Needs to know servers IP address & port #
l DNS for URL & server well-known port #
l Server learns clients address & port #
socket() socket() bind() read() close() Data Data
Server Client
listen() accept() Blocks write() connect() Connect negotiation write() read()
Server does Passive Open
l
socket creates socket to listen for connection requests
l
Server specifies type: TCP (stream)
l
socket call returns: non-negative integer descriptor;
close()
socket() socket() bind() read() close() Data Data
Server Client
listen() accept() Blocks write() connect() Connect negotiation write() read()
Server does Passive Open
l
bind assigns local address & port # to socket with specified descriptor
l
Can wildcard IP address for multiple net interfaces
l
bind call returns: 0 (success); or -1 (failure)
l
Failure if port # already in use or if reuse option not set
close()
socket() socket() bind() read() close() Data Data
Server Client
listen() accept() Blocks write() connect() Connect negotiation write() read()
Server does Passive Open
l
listen indicates to TCP readiness to receive connection requests for socket with given descriptor
l
Parameter specifies max number of requests that may be queued while waiting for server to accept them
l
listen call returns: 0 (success); or -1 (failure)
close()
socket() socket() bind() read() close() Data Data
Server Client
listen() accept() Blocks write() connect() Connect negotiation write() read()
Server does Passive Open
l
Server calls accept to accept incoming requests
l
accept blocks if queue is empty
close()
socket() socket() bind() read() close() Data Data
Server Client
listen() accept() Blocks write() connect() Connect negotiation write() read() close()
Client does Active Open
l
socket creates socket to connect to server
l
Client specifies type: TCP (stream)
l
socket call returns: non-negative integer descriptor;
socket() socket() bind() read() close() Data Data
Server Client
listen() accept() Blocks write() connect() Connect negotiation write() read() close()
Client does Active Open
l
connect establishes a connection on the local socket with the specified descriptor to the specified remote address and port #
l
connect returns 0 if successful; -1 if unsuccessful
Note: connect initiates TCP three-way handshake
socket() socket() bind() read() close() Data Data
Server Client
listen() accept() Blocks write() connect() Connect negotiation write() read()
l
accept wakes with incoming connection request
l
accept fills client address & port # into address structure
l
accept call returns: descriptor of new connection socket (success); or -1 (failure)
l
Client & server use new socket for data transfer
l
Original socket continues to listen for new requests
close()
socket() socket() bind() read() close() Data Data
Server Client
listen() accept() Blocks write() connect() Connect negotiation write() read()
Data Transfer
l
Client or server call write to transmit data into a connected socket
l
write specifies: socket descriptor; pointer to a buffer; amount of data; flags to control transmission behavior
l
write call returns: # bytes transferred (success); or -1 (failure); blocks until all data transferred
close()
socket() socket() bind() read() close() Data Data
Server Client
listen() accept() Blocks write() connect() Connect negotiation write() read()
Data Transfer
l
Client or server call read to receive data from a connected socket
l
read specifies: socket descriptor; pointer to a buffer; amount of data
l
read call returns: # bytes read (success); or -1 (failure); blocks if no data arrives
close()
Note: write and read can be called multiple times to transfer byte streams in both directions
socket() socket() bind() read() close() Data Data
Server Client
listen() accept() Blocks write() connect() Connect negotiation write() read()
Connection Termination
l
Client or server call close when socket is no longer needed
l
close specifies the socket descriptor
l
close call returns: 0 (success); or -1 (failure)
Note: close initiates TCP graceful close sequence
close()
socket()
sendto() socket() recvfrom() Data
Server Client
bind() Blocks until server receives data from client recvfrom() close() close() Data sendto()
Server started
l
socket creates socket of type UDP (datagram)
l
socket call returns: descriptor; or -1 if unsuccessful
l
bind assigns local address & port # to socket with specified descriptor; Can wildcard IP address
socket()
sendto() socket() recvfrom() Data
Server Client
bind() Blocks until server receives data from client recvfrom() close() close() Data sendto()
l
recvfrom copies bytes received in specified socket into a specified location
l
recvfrom blocks until data arrives
sendto() socket()
sendto() socket() recvfrom() Data
Server Client
bind() Blocks until server receives data from client recvfrom() close() close() Data
Client started
l
socket creates socket of type UDP (datagram)
l
socket call returns: descriptor; or -1 if unsuccessful
sendto() socket()
sendto() socket() recvfrom() Data
Server Client
bind() Blocks until server receives data from client recvfrom() close() close() Data
Client started
l
sendto transfer bytes in buffer to specified socket
l
sendto specifies: socket descriptor; pointer to a buffer; amount of data; flags to control transmission behavior; destination address & port #; length of destination address structure
l
sendto returns: # bytes sent; or -1 if unsuccessful
socket()
sendto() socket() recvfrom() Data
Server Client
bind() Blocks until server receives data from client recvfrom() close() close() Data sendto()
l
recvfrom wakes when data arrives
l
recvfrom specifies: socket descriptor; pointer to a buffer to put data; max # bytes to put in buffer; control flags; copies: sender address & port #; length of sender address structure
l
recvfrom returns # bytes received or -1 (failure) Note: receivefrom returns data from at most one send, i.e. from one datagram
socket()
sendto() socket() recvfrom() Data
Server Client
bind() Blocks until server receives data from client recvfrom() close() close() Data sendto()
Socket Close
l
Client or server call close when socket is no longer needed
l
close specifies the socket descriptor
l
close call returns: 0 (success); or -1 (failure)