Chapter 2 Applications and Layered Architectures

Protocols, Services & Layering OSI Reference Model TCP/IP Architecture How the Layers Work Together Berkeley Sockets Application Layer Protocols & Utilities Chapter 2 Applications and Layered Architectures

Protocols, Services & Layering Layers, Services & Protocols z The overall communications process between two or more machines connected across one or more networks is very complex z Layering partitions related communications functions into groups that are manageable z Each layer provides a service to the layer above z Each layer operates according to a protocol z Let’s use examples to show what we mean Web Browsing Application z World Wide Web allows users to access resources (i.e. documents) located in computers connected to the Internet z Documents are prepared using HyperText Markup Language (HTML) z A browser application program is used to access the web z The browser displays HTML documents that include links to other documents z Each link references a Uniform Resource Locator (URL) that gives the name of the machine and the location of the given document z Let’s see what happens when a user clicks on a link 1. DNS A. 64.15.247.200

Q. www.nytimes.com?

z User clicks on http://www.nytimes.com/ z URL contains Internet name of machine (www.nytimes.com), but not Internet address z Internet needs Internet address to send information to a machine z Browser software uses Domain Name System (DNS) protocol to send query for Internet address z DNS system responds with Internet address 2. TCP ACK

ACK, TCP Connection Request From: 64.15.247.200 Port 80 To:128.100.11.13 Port 1127

TCP Connection Request From: 128.100.11.13 Port 1127 To: 64.15.247.200 Port 80 z Browser software uses HyperText Transfer Protocol (HTTP) to send request for document z HTTP server waits for requests by listening to a well-known port number (80 for HTTP) z HTTP client sends request messages through an “ephemeral port number,” e.g. 1127 z HTTP needs a Transmission Control Protocol (TCP) connection between the HTTP client and the HTTP server to transfer messages reliably 3. HTTP Content

200 OK

GET / HTTP/1.1 z HTTP client sends its request message: “GET …” z HTTP server sends a status response: “200 OK” z HTTP server sends requested file z Browser displays document z Clicking a link sets off a chain of events across the Internet! z Let’s see how protocols & layers come into play… Protocols z A protocol is a set of rules that governs how two or more communicating entities in a layer are to interact z Messages that can be sent and received z Actions that are to be taken when a certain event occurs, e.g. sending or receiving messages, expiry of timers z The purpose of a protocol is to provide a service to the layer above Layers z A set of related communication functions that can be managed and grouped together z Application Layer: communications functions that are used by application programs z HTTP, DNS, SMTP (email) z Transport Layer: end-to-end communications between two processes in two machines z TCP, User Datagram Protocol (UDP) z Network Layer: node-to-node communications between two machines z Internet Protocol (IP) Example: HTTP z HTTP is an application layer protocol z Retrieves documents on behalf of a browser application program z HTTP specifies fields in request messages and response messages z Request types; Response codes z Content type, options, cookies, … z HTTP specifies actions to be taken upon receipt of certain messages HTTP Protocol

GET HTTP HTTP Client Server Response

z HTTP assumes messages can be exchanged directly between HTTP client and HTTP server z In fact, HTTP client and server are processes running in two different machines across the Internet z HTTP uses the reliable stream transfer service provided by TCP Example: TCP z TCP is a transport layer protocol z Provides reliable byte stream service between two processes in two computers across the Internet z Sequence numbers keep track of the bytes that have been transmitted and received z Error detection and retransmission used to recover from transmission errors and losses z TCP is connection-oriented: the sender and receiver must first establish an association and set initial sequence numbers before data is transferred z Connection ID is specified uniquely by (send port #, send IP address, receive port #, receiver IP address) HTTP uses service of TCP

HTTP HTTP client server Response GET

Port 1127 Port 80

GETResponseTCPTCP80, 1127 1127, 80TCPGETbytesResponse Example: DNS Protocol z DNS protocol is an application layer protocol z DNS is a distributed database that resides in multiple machines in the Internet z DNS protocol allows queries of different types z Name-to-address or Address-to-name z Mail exchange z DNS usually involves short messages and so uses service provided by UDP z Well-known port 53 Local Authoritative Name Name Server Server 1 5 4 23 6

Root Name Server z Local Name Server: resolve frequently-used names z University department, ISP z Contacts Root Name server if it cannot resolve query z Root Name Servers: 13 globally z Resolves query or refers query to Authoritative Name Server z Authoritative Name Server: last resort z Every machine must register its address with at least two authoritative name servers Example: UDP z UDP is a transport layer protocol z Provides best-effort datagram service between two processes in two computers across the Internet z Port numbers distinguish various processes in the same machine z UDP is connectionless z Datagram is sent immediately z Quick, simple, but not reliable Summary z Layers: related communications functions z Application Layer: HTTP, DNS z Transport Layer: TCP, UDP z Network Layer: IP z Services: a protocol provides a communications service to the layer above z TCP provides connection-oriented reliable byte transfer service z UDP provides best-effort datagram service z Each layer builds on services of lower layers z HTTP builds on top of TCP z DNS builds on top of UDP z TCP and UDP build on top of IP Chapter 2 Applications and Layered Architectures

OSI Reference Model Why Layering? z Layering simplifies design, implementation, and testing by partitioning overall communications process into parts z Protocol in each layer can be designed separately from those in other layers z Protocol makes “calls” for services from layer below z Layering provides flexibility for modifying and evolving protocols and services without having to change layers below z Monolithic non-layered architectures are costly, inflexible, and soon obsolete Open Systems Interconnection z Network architecture: z Definition of all the layers z Design of protocols for every layer z By the 1970s every computer vendor had developed its own proprietary layered network architecture z Problem: computers from different vendors could not be networked together z Open Systems Interconnection (OSI) was an international effort by the International Organization for Standardization (ISO) to enable multivendor computer interconnection OSI Reference Model z Describes a seven-layer abstract reference model for a network architecture z Purpose of the reference model was to provide a framework for the development of protocols z OSI also provided a unified view of layers, protocols, and services which is still in use in the development of new protocols z Detailed standards were developed for each layer, but most of these are not in use z TCP/IP protocols preempted deployment of OSI protocols 7-Layer OSI Reference Model

Application Application End-to-End Protocols Application Application Layer Layer Presentation Presentation Layer Layer Session Session Layer Layer Transport Transport Layer Layer Network Network Network Network Layer Layer Layer Layer Data Link Data Link Data Link Data Link Layer Layer Layer Layer Physical Physical Physical Physical Layer Layer Layer Layer

Communicating End Systems One or More Network Nodes Physical Layer z Transfers bits across link z Definition & specification of the physical aspects of a communications link z Mechanical: cable, plugs, pins... z Electrical/optical: modulation, signal strength, voltage levels, bit times, … z functional/procedural: how to activate, maintain, and deactivate physical links… z Ethernet, DSL, cable modem, telephone modems… z Twisted-pair cable, coaxial cable optical fiber, radio, infrared, … Data Link Layer z Transfers frames across direct connections z Groups bits into frames z Detection of bit errors; Retransmission of frames z Activation, maintenance, & deactivation of data link connections z Medium access control for local area networks z Flow control frames Data Link Data Link Layer Layer

Physical bits Physical Layer Layer Network Layer z Transfers packets across multiple links and/or multiple networks z Addressing must scale to large networks z Nodes jointly execute routing algorithm to determine paths across the network z Forwarding transfers packet across a node z Congestion control to deal with traffic surges z Connection setup, maintenance, and teardown when connection-based Internetworking Ethernet LAN z Internetworking is part of network layer and provides zzz zz z transfer of packets across multiple possibly dissimilar ATM networks Network ATM Switch z Gateways (routers) direct packets across networks ATM ATM HSwitch Switch

ATM H Switch NetNet 33 Net 1 G Net 1 G G G NetNet 5 5 H Net 2 G Net 4 G H G = gateway H = host Transport Layer z Transfers data end-to-end from process in a machine to process in another machine z Reliable stream transfer or quick-and-simple single- block transfer z Port numbers enable multiplexing z Message segmentation and reassembly z Connection setup, maintenance, and release

Transport Transport Layer Layer

Network Network Network Network Layer Layer Layer Layer

Communication Network Application & Upper Layers z Application Layer: Provides services that are frequently Application required by applications: DNS, Application web acess, file transfer, email… Application ApplicationLayer z Presentation Layer: machine- Layer Presentation independent representation of TransportLayer data… Layer Session z Session Layer: dialog Layer management, recovery from Transport Layer errors, … Incorporated into Application Layer Headers & Trailers z Each protocol uses a header that carries addresses, sequence numbers, flag bits, length indicators, etc… z CRC check bits may be appended for error detection

Application APP DATA Application

Application AH APP DATA Application Layer Layer Transport Transport Layer TH AH APP DATA Layer

Network Network Layer NH TH AH APP DATA Layer

Data Link Data Link Layer DH NH TH AH APP DATA CRC Layer

Physical Physical Layer bits Layer OSI Unified View: Protocols z Layer n in one machine interacts with layer n in another machine to provide a service to layer n +1 z The entities comprising the corresponding layers on different machines are called peer processes. z The machines use a set of rules and conventions called the layer-n protocol. z Layer-n peer processes communicate by exchanging Protocol Data Units (PDUs) n-PDUs

n n Entity Entity

Layer n peer protocol OSI Unified View: Services z Communication between peer processes is virtual and actually indirect z Layer n+1 transfers information by invoking the services provided by layer n z Services are available at Service Access Points (SAP’s) z Each layer passes data & control information to the layer below it until the physical layer is reached and transfer occurs z The data passed to the layer below is called a Service Data Unit (SDU) z SDU’s are encapsulated in PDU’s Layers, Services & Protocols

n+1 n+1 entity entity

n-SDU n-SDU n-SAP n-SAP

n-SDU H n entity n entity H n-SDU n-PDU Interlayer Interaction

layer

N+1 user N provider N provider N+1 user

Request Indication

nse spo Re

m nfir Co

System A System B Connectionless & Connection- Oriented Services z Connection-Oriented z Connectionless z Three-phases: z Immediate SDU 1. Connection setup transfer between two SAPs z No connection setup to initialize state z E.g. UDP, IP information z Layered services need 2. SDU transfer not be of same type 3. Connection release z TCP operates over IP z E.g. TCP, ATM z IP operates over ATM Segmentation & Reassembly z A layer may impose a limit on the size of a data block (a) Segmentation that it can transfer for n-SDU implementation or other reasons z Thus a layer-n SDU may be n-PDU n-PDU n-PDU too large to be handled as a single unit by layer-(n-1) z Sender side: SDU is Reassembly segmented into multiple (b) PDUs n-SDU z Receiver side: SDU is reassembled from sequence of PDUs n-PDU n-PDU n-PDU Multiplexing z Sharing of layer n service by multiple layer n+1 users z Multiplexing tag or ID required in each PDU to determine which users an SDU belongs to n+1 n+1 entity entity n+1 n+1 entity entity

n-SDU n-SDU n-SDU H n entity n entity

H n-SDU n-PDU Summary z Layers: related communications functions z Application Layer: HTTP, DNS z Transport Layer: TCP, UDP z Network Layer: IP z Services: a protocol provides a communications service to the layer above z TCP provides connection-oriented reliable byte transfer service z UDP provides best-effort datagram service z Each layer builds on services of lower layers z HTTP builds on top of TCP z DNS builds on top of UDP z TCP and UDP build on top of IP Chapter 2 Applications and Layered Architectures

TCP/IP Architecture How the Layers Work Together Why Internetworking? z To build a “network of networks” or internet z operating over multiple, coexisting, different network technologies z providing ubiquitous connectivity through IP packet transfer z achieving huge economies of scale H

H NetNet 5 3 NetNet 5 1 G G G G NetNet 5 5 H G G NetNet 5 2 NetNet 5 4 H Why Internetworking? z To provide universal communication services z independent of underlying network technologies z providing common interface to user applications

H Reliable Stream Service H NetNet 5 3 NetNet 5 1 G G G G NetNet 5 5 H G G NetNet 5 2 NetNet 5 4 H User Datagram Service Why Internetworking? z To provide distributed applications z Any application designed to operate based on Internet communication services immediately operates across the entire Internet z Rapid deployment of new applications z Email, WWW, Peer-to-peer z Applications independent of network technology z New networks can be introduced below z Old network technologies can be retired Internet Protocol Approach z IP packets transfer information across Internet Host A IP → router→ router…→ router→ Host B IP z IP layer in each router determines next hop (router) z Network interfaces transfer IP packets across networks

Host A Router Host B Router Transport Internet Transport Layer Layer Internet Layer Layer Network Internet Internet NetNet 5 1 Layer Interface Network Layer Router Interface Network Network Interface Internet Interface Layer NetNet 5 4 Net 2 Network NetNet 5 3 Net 5 Interface TCP/IP Protocol Suite

HTTP SMTP DNS RTP Distributed applications Reliable User TCP UDP stream datagram service service

Best-effort IP (ICMP, ARP) connectionless packet transfer

Network Network Network interface 1 interface 2 interface 3

Diverse network technologies Internet Names & Addresses

Internet Names Internet Addresses z Each host a a unique name z Each host has globally unique z Independent of physical logical 32 bit IP address location z Separate address for each z Facilitate memorization by physical connection to a network humans z Routing decision is done based z Domain Name on destination IP address z Organization under single administrative unit z IP address has two parts: z Host Name z netid and hostid z Name given to host z netid unique computer z z User Name netid facilitates routing z Name assigned to user z Dotted Decimal Notation: int1.int2.int3.int4 [email protected] (intj = jth octet) DNS resolves IP name to IP128.100.10.13 address Physical Addresses z LANs (and other networks) assign physical addresses to the physical attachment to the network z The network uses its own address to transfer packets or frames to the appropriate destination z IP address needs to be resolved to physical address at each IP network interface z Example: Ethernet uses 48-bit addresses z Each Ethernet network interface card (NIC) has globally unique Medium Access Control (MAC) or physical address z First 24 bits identify NIC manufacturer; second 24 bits are serial number z 00:90:27:96:68:07 12 hex numbers

Intel Example internet Server PC Router (2,1) PPP (1,1) s (1,3) r Netid=2 (2,2) w Ethernet *PPP does not use addresses (netid=1) Workstation

(1,2)

Physical netid hostid address server 1 1 s workstation 1 2 w router 1 3 r router 2 1 - PC 2 2 - Encapsulation

IP header IP Payload

Ethernet IP FCS header header IP Payload

z Ethernet header contains: z source and destination physical addresses z network protocol type (e.g. IP) IP packet from workstation to server

Server PC Router (2,1) PPP (1,1) s (1,3) r (2,2) w Ethernet w, s (1,2), (1,1)

Workstation (1,2)

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 IP packet from server to PC

Server PC Router (2,1) (1,1), (2,2)

(1,1) s (1,3) r (2,2) w s, r (1,1), (2,2)

Workstation (1,2) 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. Router’s 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 How the layers work together

(a) Server PC Router (2,1) (1,1) PPP s (1,3) r (2,2) Ethernet HTTP uses process-to-process Reliable byte stream transfer of TCP connection: Server socket: (IP Address, 80) (b) Server PC socket (IP Address, Eph. #) PC HTTP TCP uses node-to-node HTTP Unreliable packet transfer of IP TCP Server IP address & PC IP address TCP

IP IP IP

Network interface Network interface Network interface Internet Router Ethernet PPP Encapsulation

TCP Header contains source & destination HTTP Request port numbers

IP Header contains source and destination TCP IP addresses; header HTTP Request transport protocol type

Ethernet Header contains IP TCP source & destination MAC HTTP Request addresses; header header network protocol type

Ethernet IP TCP FCS header header header HTTP Request How the layers work together: Network Analyzer Example

Internet

z User clicks on http://www.nytimes.com/ z Ethereal network analyzer captures all frames observed by its Ethernet NIC z Sequence of frames and contents of frame can be examined in detail down to individual bytes Top Pane Middle Pane Etherealshows windows shows frame/packet encapsulation for sequence a given frame

Bottom Pane shows hex & text Top pane: frame sequence TCP DNS Connection HTTP Query Setup Request & Response Middle pane: Encapsulation

Ethernet Frame

Ethernet Protocol Type Destination and Source Addresses Middle pane: Encapsulation And a lot of other stuff!

IP Packet

IP Source and Destination Addresses

Protocol Type Middle pane: Encapsulation

TCP Segment

Source and Destination Port Numbers

GET

HTTP Request Summary z Encapsulation is key to layering z IP provides for transfer of packets across diverse networks z TCP and UDP provide universal communications services across the Internet z Distributed applications that use TCP and UDP can operate over the entire Internet z Internet names, IP addresses, port numbers, sockets, connections, physical addresses Chapter 2 Applications and Layered Architectures

Sockets Socket API z API (Application Programming Interface) z Provides a standard set of functions that can be called by applications z Berkeley UNIX Sockets API z Abstraction for applications to send & receive data z Applications create sockets that “plug into” network z Applications write/read to/from sockets z Implemented in the kernel z Facilitates development of network applications z Hides details of underlying protocols & mechanisms z Also in Windows, Linux, and other OS’s Communications through Socket Interface

Client Server Socket Socket Application 1 Application 2 interface interface

User User descriptor descriptor Kernel Kernel Socket Socket • Application references a socket through a descriptor port number • Socket bound to a port number port number

Underlying Underlying communication communication protocols protocols

Communications network Stream mode of service

Connection-oriented z Connectionless z First, setup connection z Immediate transfer of one between two peer block of information application processes (boundaries preserved) z No setup overhead & delay z Then, reliable bidirectional in-sequence transfer of byte z Destination address with each block stream (boundaries not preserved in transfer) z Send/receive to/from multiple peer processes z Multiple write/read between z Best-effort service only peer processes z Possible out-of-order z Finally, connection release z Possible loss z Uses TCP z Uses UDP Client & Server Differences z Server z Specifies well-known port # when creating socket z May have multiple IP addresses (net interfaces) z Waits passively for client requests z Client z Assigned ephemeral port # z Initiates communications with server z Needs to know server’s IP address & port # z DNS for URL & server well-known port # z Server learns client’s address & port # Socket Calls for Connection- Oriented Mode

Server does Passive Open Server z socket creates socket to listen for connection requests socket() z Server specifies type: TCP (stream) bind() z socket call returns: non-negative integer descriptor; or -1 if unsuccessful listen() Client accept() socket() Blocks Connect negotiation connect()

Data read() write()

Data write() read()