Ch 2. Application Layer Myungchul Kim

2 Principles of application-layer protocols o Distributed among two or more end systems – Browser (client) – server – Processes – How processes running on different end systems communicate vs interprocess communication o Networking applications have application-layer protocols that define the format and order of the messages exchanged between processes, as well as define the actions taken on the transmission or receipt of a message – Web: a network application – Web application = document formats, browsers, servers, and an application protocol HTTP o An application-layer protocol defines: – Types of messages exchanged – Syntax of the various message types – Semantics of the fields – Rules for determining when and how a process sends messages and responds to messages.

3 o The host that initiates the session is labeled the client. o Fig 2.2

4 o Socket – A process’s socket can be thought of as the process’s door. – API (application programmers’ interface) – Application developer has (1) the choice of transport protocols and (2) the ability to fix a few transport-layer parameters such as max buffer and max segment sizes. o Addressing processes – To identify a process: (1) the name or address of the host and (2) an identifier that specifies the process in the host. – IP address globally uniquely identifies the network interface. – Destination port number: a web server process with port number 80, a mail server process with port number 25, etc. o User agents – An interface between the user and the network application. – The browser also implements the client side of the HTTP protocols.

5 o An application’s service requirements – Reliable data transfer: loss-tolerant applications? – Bandwidth: bandwidth-sensitive application vs elastic applications – Timing: end-to-end delays – Fig 2.4

6 o Services provided by the Internet Transport Protocols – UDP, TCP o TCP services – Connection-oriented service: handshaking procedure, a TCP connection, a full-duplex – Reliable transport service: without error and in the proper order – Congestion-control mechanisms – Real-time applications usually run over UDP rather than TCP – TCP does not guarantee a min transmission rate – TCP does not provide any delay guarantee – TCP guarantee delivery of all data. o UDP services – Lightweight transport protocol – Connectionless, no handshaking, no guarantee, no congestion-control

7 – Fig 2.5

8 The Web and HTTP o HTTP – A client and a server – Web page: objects (an HTML file, a JPEG image, a Java Applet,…) – TCP – A stateless protocol: an HTTP server maintains no information about the clients. – No relation with a Web page o Nonpersistent connections – Only a single Web object is transferred over a TCP connection. – Most browsers open five to ten parallel TCP connections. – RTT (Round Trip Time)

9 – Fig 2.7

10 o Persistent connections – Without pipelining and with pipelining – Without pipelining: the client issues a new request only when the previous response has been received: one RTT – HTTP/1.1: persistent connections with pipelining o HTTP message format – ASCII text – HTTP request message, response message – Request line (GET, POST, HEAD) (pp. 94, 96) – Header line – Fig 2.8

11 File Transfer: FTP o On TCP o Out of band: A control connection and a data connection, cf. in-band for HTTP o The FTP server must maintain state about the user. o Fig 2.11

12 Electronic Mail o components – User agents – Mail servers (mailbox) – Simple Mail Transfer Protocol (SMTP) o On TCP o Fig 2.13

13 o Comparison with HTTP – HTTP: a pull protocol, SMTP: a push protocol – SMTP: in seven-bit ASCII format – HTTP encapsulates each object in its own HTTP response message. Internet mail places all of the message’s objects into one message. – MIME extension for non-ASCII data: Content-Type and Content- Transfer-Encoding – Content-Type: text, image, and application

14 o Mail Access Protocols – A mail server is always on and always connected to the Internet? – POP3, IMAP and HTTP – Fig 2.14

15 DNS (Directory Service) o Hostname -> IP address o Domain Name System (DNS) – A distributed database in a hierarchy of name servers – An application-layer protocol that allows host and name servers to communicate for the translation service. – On UDP – Host aliasing – Mail server aliasing – Load distribution: a set of IP addresses is associated with one canonical hostname. – Gethostbyname() in UNIX o The problems with a centralized design -> doesn’t scale – A single point of failure – Traffic volume – Distant centralized database – Maintenance

16 o The DNS with a large number of name servers – Local name servers: configured by hand in a host – Root name servers (pp. 126) – \Authoritative name servers: each host is registered with an authoritative name server o Recursive queries – Fig 2.17

17 o Recursive and iterative queries – Fig 2.18

18 o DNS caching o DNS records – Resource record (name, value, type, TTL0 – Type = A, NS, CNAME, and MX – Fig 2.19 – Flag: query or reply, authoritative, recursion, … – How data gets into the database in the first place?