Download Ch2. Application Layer

Ch 2. Application Layer
Myungchul Kim
[email protected]
Principles of application-layer protocols
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Distributed among two or more end systems
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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
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Browser (client) – server
Processes
How processes running on different end systems communicate vs
interprocess communication
Web: a network application
Web application = document formats, browsers, servers, and an
application protocol HTTP
An application-layer protocol defines:
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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.
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The host that initiates the session is labeled the client.
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Fig 2.2
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Socket
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Addressing processes
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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.
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.
User agents
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An interface between the user and the network application.
The browser also implements the client side of the HTTP protocols.
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An application’s service requirements
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Reliable data transfer: loss-tolerant applications?
Bandwidth: bandwidth-sensitive application vs elastic applications
Timing: end-to-end delays
Fig 2.4
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Services provided by the Internet Transport Protocols
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TCP services
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UDP, TCP
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.
UDP services
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Lightweight transport protocol
Connectionless, no handshaking, no guarantee, no congestion-control
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Fig 2.5
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The Web and HTTP
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HTTP
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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
Nonpersistent connections
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Only a single Web object is transferred over a TCP connection.
Most browsers open five to ten parallel TCP connections.
RTT (Round Trip Time)
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Fig 2.7
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Persistent connections
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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
HTTP message format
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ASCII text
HTTP request message, response message
Request line (GET, POST, HEAD) (pp. 94, 96)
Header line
Fig 2.8
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File Transfer: FTP
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On TCP
Out of band: A control connection and a data
connection, cf. in-band for HTTP
The FTP server must maintain state about the user.
Fig 2.11
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Electronic Mail
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components
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User agents
Mail servers (mailbox)
Simple Mail Transfer Protocol (SMTP)
On TCP
Fig 2.13
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Comparison with HTTP
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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 ContentTransfer-Encoding
Content-Type: text, image, and application
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Mail Access Protocols
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A mail server is always on and always connected to the Internet?
POP3, IMAP and HTTP
Fig 2.14
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DNS (Directory Service)
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Hostname -> IP address
Domain Name System (DNS)
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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
The problems with a centralized design -> doesn’t scale
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A single point of failure
Traffic volume
Distant centralized database
Maintenance
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The DNS with a large number of name servers
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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
Recursive queries
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Fig 2.17
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Recursive and iterative queries
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Fig 2.18
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DNS caching
DNS records
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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?
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