COMP 431 Application-Layer Protocols Internet Services & Protocols Overview application application transport ◆ Application-layer protocols define: network Client/Server Computing & link » The types of messages exchanged physical » The syntax and semantics of messages Socket Programming » The rules for when and how messages are sent ◆ Public protocols (defined in RFCs) regional ISP Jasleen Kaur » HTTP, FTP, SMTP, POP, IMAP, DNS ◆ Proprietary protocols » RealAudio, RealVideo » Skype January 28, 2020 » … Institutional network 1 2
Application-Layer Protocols Network Working Group R. Fielding UC Irvine Request for Comments: 2616 J. Gettys Compaq/W3C Obsoletes: 2068 J. Mogul Compaq Overview application Category: Standards Track H. Frystyk W3C/MIT application L. Masinter Xerox transport ◆ Application-layer protocols define: June 1999 P. Leach Microsoft network T. Berners-Lee W3C/MIT link » The types of messages exchanged physical » The syntax and semantics of messages Hypertext Transfer Protocol -- HTTP/1.1 » The rules for when and how messages are sent Abstract ◆ Public protocols (defined in RFCs) regional ISP The Hypertext Transfer Protocol (HTTP) is an application-level » HTTP, FTP, SMTP, POP, IMAP, DNS protocol for distributed, collaborative, hypermedia information systems. It is a generic, stateless, protocol which can be used for many tasks beyond its use for hypertext, such as name servers and ◆ Proprietary protocols distributed object management systems, through extension of its » RealAudio, RealVideo request methods, error codes and headers [47]. A feature of HTTP is the typing and negotiation of data representation, allowing systems » Skype to be built independently of the data being transferred. » … HTTP has been in use by the World-Wide Web global information Institutional network initiative since 1990. This specification defines the protocol referred to as "HTTP/1.1", and is an update to RFC 2068 [33]. 3 4
Application-Layer Protocols Application-Layer Protocols application Overview Outline application transport application application network transport ◆ The architecture of distributed systems link ◆ Application developers write programs network physical » Client/Server computing that: link » Peer-to-Peer computing physical » Run on (different) end systems » Content delivery networks » Communicate over network network regional ISP ◆ The programming model used in link ◆ Note: application developers don’t constructing distributed systems physical regional ISP need to write code for network-core » Socket programming devices » Network devices do not run user applications or application layer protocols application application Institutional network transport network link Institutional network physical 5 6
Application-Layer Protocols The Application Layer application application Outline The client-server paradigm transport network Client application application link ◆ Typical network application has two transport ◆ Example client/server systems and physical pieces: client and server network their application-level protocols: link ◆ Client: » The World-Wide Web (HTTP) physical » Initiates contact with server ( “ speaks regional ISP » Reliable file transfer (FTP) reply first ” ) » E-mail (SMTP & POP) » Requests service from server » Internet Domain Name System (DNS) » For W eb, client is implemented in regional ISP request ◆ Example p2p applications systems: browser; for e-mail, in mail reader » BitTorrent ◆ Server: ◆ Other protocols and systems: » Provides requested service to client » Streaming media — DASH » “ Always ” running Institutional network » Content delivery networks (CDNs) » May also include a “ client interface ” application application transport » A server may be a logical machine network » Implemented by one of thousands of link Institutional network Server physical servers in a data center physical 7 8
The Application Layer Application-Layer Protocols application The peer-to-peer-paradigm application Outline transport network application application ◆ Example client/server systems and link transport ◆ No “always-on” server their application-level protocols physical network ◆ Arbitrary end systems directly » The World-Wide Web (HTTP) link physical communicate » Reliable file transfer (FTP) » E-mail (SMTP & POP) » Peers request service from other peers, regional ISP provide service in return to other peers » Internet Domain Name System (DNS) ◆ Self scalability – new peers bring new regional ISP service capacity, as well as new service ◆ Protocol design issues: demands » In-band vs. out-of-band control signaling ◆ Peers are intermittently » Push vs. pull protocols connected and change IP addresses » Persistent vs. non-persistent connections ◆ Complex management application application ◆ Client/server service architectures Institutional network transport » Contacted server responds vs . forwards request network link Institutional network physical 9 10
Client/Server Paradigm Client/Server Paradigm Socket programming A quick aside on processes socket ◆ A process is the OS term for a ◆ Sockets are the fundamental clients, servers a host-local , application program running within a host building block for client/server created/released , OS-controlled systems client process : the executing ◆ On the same host, two processes interface into which an program that initiates the communicate using inter-process application process can both ◆ Sockets are created and managed communication send and receive messages communication by applications to/from another (remote or local) » A service defined by the OS server process : the executing » Strong analogies with files application process program waits to be contacted ◆ Processes on different hosts communicate by exchanging ◆ Two types of transport services messages are available via the socket API: » By using some protocol! » UDP sockets: unreliable, datagram-oriented communications » TCP sockets: reliable, stream-oriented communications 11 12
Client/Server Paradigm Client/Server Paradigm application Socket-programming using TCP Addressing processes application transport network ◆ A socket is an application created, OS-controlled interface link ◆ To receive messages, a process must physical into which an application can both send and receive messages have an identifier to and from another application » How does a client identify a server » A “ door ” between application processes and end-to-end transport process protocols regional ISP ◆ We know that a host device has unique 32-bit IP address ◆ But does the IP address of host controlled by controlled by application application suffice for identifying the destination process process developer developer process? socket socket » No! Many processes can be (and are!) TCP with TCP with controlled by controlled by Institutional network app1 … app9 running on the same host buffers, buffers, operating operating transport Internet variables variables system system network link Host Host physical (end system) (end system) 13 14
Client/Server Paradigm Socket-programming using TCP application Addressing processes TCP socket programming model application transport ◆ Processes are identified by a “port network ◆ A TCP socket provides a reliable, bi-directional, byte-stream number” link physical communications channel from one process to another » Sort of like a socket identifier » A “ pair of pipes ” abstraction ◆ The “server” identifier includes both an IP address and port numbers associated with the server process on the host regional ISP ◆ Example port numbers: » HTTP server: 80 » mail server: 25 Process Process Internet ◆ For a browser to send an HTTP read write bytes message to www.cs.unc.edu the request socket socket read write is addressed to IP address bytes Institutional network app1 … app9 152.2.131.244 and port 80 transport network Host Host link physical (end system) (end system) 15 16
Socket-programming using TCP Socket-programming using TCP Network addressing for sockets Socket programming in Python ◆ Sockets are addressed using an IP address and port number Client Server Internet write read bytes socket socket read write bytes process process Local port numbers socket socket ( e.g ., 6500) ◆ Client creates a local TCP socket ◆ When the client creates a socket, the specifying the host and port number client’s TCP establishes connection to TCP with TCP with of server process server’s TCP buffers, buffers, variables variables » Python resolves host names to IP ◆ When contacted by a client, server addresses using DNS creates a new socket for server process Internet addresses of hosts ◆ Client contacts server to communicate with client ( e.g ., 152.2.131.245) End System End System » Server process must be running » This allows the server to talk with » Server must have created socket t hat multiple clients Internet domain “ welcomes ” client’s contact name of host DNS e.g ., classroom.cs.unc.edu 17 18
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