Computer network (top-down) - application layer and common protocols

1. Principles of application layer protocols

1.1. Architecture of network applications

Possible application architecture:

• Customer-service area model (C/S)
  • The unequal model is mainly server-based and has poor scalability.
  • Example: web application
• Peer-to-peer model (P2P)
  • Self-expandability;
  • Example: Thunder
• Hybrid: Client-Server and Peer-to-Peer Architecture
  • Napster：
    • The host registers its resources on the central server
    • The host queries the central server for resource location
  • Example: Instant messaging (chat room) :

process communication

Process: An application running on the host

• Form: Access through the services provided by the lower layer of socket API (source language)
• Address: Corresponding SAP at the host level

Addressing : At which address (IP), port

1.2. Socket ( socket )

A process sends a message to a socket or receives a message from a socket

TCP socket: (represents the local IP and port, the other party’s IP and port)

• For applications using connection-oriented services (TCP), a socket is a locally meaningful identifier of a 4-tuple
  • 4-tuple: (source IP, source port, destination IP, destination port)
  • Uniquely specifies a session (TCP socket actually represents the session relationship between two processes )
  • Applications use this flag to communicate with remote application processes
  • It is not necessary to specify these 4-tuples in every message sent.
  • Just like using the operating system to open a file, the OS returns a file handle, and this file handle will be used later instead of the directory name and file name of the file
  • Simple and easy to manage
  • So transmitting information only requires passing
    • data, socket

UDP socket：

• UDP service, communication between two processes requires no prior connection establishment
  • Each message is transmitted independently
  • The preceding and following messages are sent to different distributed processes.
• Therefore, only an integer can be used to identify the identifier of this application entity.
  • Because this message may be sent to another distributed process
• The minimum size of information passing through an inter-layer interface
• UDP socket: local IP, local port
• But when transmitting a message: you must provide the other party’s IP and port
  • When receiving a message: the transport layer needs to upload the IP and port of the other party
• So three things need to be passed to transmit information:
  • socket, the data itself, the other party’s address (IP and port)

Performance indicators provided by the transport layer to the application layer:

• Latency, throughput, data loss rate, security

• Both TCP and UDP are transmitted in clear text and do not provide security.

• Therefore, if the transmission is secure, the security of the transport layer service needs to be carried out through the SSL protocol.

  • The application uses the SSL library, which uses TCP communication
  • SSL
    • Implemented on TCP to provide encrypted TCP connections
    • Privacy
    • data integrity
    • End-to-end authentication
  • SSL socket API
    • The application provides an API to pass the plain text to the socket, and SSL encrypts it for transmission over the Internet.

2、WEB and HTTP

Version 1.0 of HTTP is a non-persistent HTTP connection.

After 1.1, the HTTP connection was changed to a persistent HTTP connection;

2.1. HTTP request message: 80 (default port)

• Two types of request messages: request and response;
• HTTP request message:
  • ASCII
  • Request line (GET, POST)

2.2、FTP：21

ftp: file transfer protocol

Dual-channel connection, performed on two TCP connections, out-of-band (commands sent) and in-band (data sent), stateful protocol

2.3、Email：25

SMTP protocol: mailbox server

• Manage and maintain emails sent to users in the mailbox
• Output message queue holds pending email messages
• SMTP protocol between mail servers
  • Send email message
  • Client: Sending mail server
  • Server: receiving mail server

Three stages of transmission:

• Handshake
• response message
• closure

• SMTP uses persistent connections
• SMTP requires that the message (header and body) be 7 ASCII encoded
• SMTP server uses CRLE, CRLE determines the tail of the message

3、DNS

UDP:57

Domain name resolution system

Problems solved by DNS:

• How to name;
• How to parse;
• How to maintain;

The main ideas of DNS:

• Hierarchical , domain-based naming scheme
• Several distributed databases complete name to IP address conversion
• Application service running on UDP with port 53
• Core Internet functionality, but implemented as application layer protocols
  • Handling complexity at the network edge

Main purpose of DNS:

• Implement hostname-IP address conversion (name/IP translate)
• Other purposes
  • Host alias to canonical name conversion
  • Conversion of mail server alias to mail server's formal name
  • load balancing

The general working process of DNS:

• Application calls the resolver
• The parser sends a query message (encapsulated in a UDP segment) to the Name Server as a client.
• Name Server returns response message (name/ip)
• Query domain name method
  • recursive query
    
  • Iterative query
    

4. P2P applications

4.1. Pure P2P architecture

• No (or very few) servers running all the time

4.2. File distribution: C/S vs P2P

• Unstructured P2P
  • centralized directory
  • Fully distributed
  • hybrid
• DHT (Structured) P2P
  • Hash table
  • tree shape

5. TCP socket programming

5.1. Socket programming

• Application processes use the services provided by the transport layer to exchange messages and implement application protocols to implement applications.
  • TCP/IP: The application process uses the Socket API to access the transport protocol
  • Where: SAP on the interface (Socket) How: Socket API
• socket: the door between distributed application processes, the end-to-end service interface provided by the transport layer protocol

5.2. TCP socket programming

The server runs first, waiting for the connection to be established

• The server process must be running first
  • Create welcome socket
  • Bundled with local port
  • Blocking wait to receive user connection on welcome socket

The client actively establishes a connection with the server

• Create client local socket (implicitly bound to local port)
  • Specify the IP address and port number of the server process to connect to the server process
• When a connection request comes with the client
  • The server accepts the request from the client, releases the blocking wait, returns a new socket, and communicates with the client.
  • Allows the server to communicate with multiple clients
  • Use source IP and source port to differentiate between different clients
• When the connection API call is valid, the client establishes a TCP connection with the server

socket structure

socket interaction

黑色的代表UCP交互的过程，红色的代表应用报文交互的过程

Two processes can guard the same port, but the sockets of the two processes are different.

• The essence of socket is the memory space address of a four-tuple structure.
  • The four-tuple represented by socket is target ip/source ip/target port/source port.

UDP Socket Programming

UDP socket exchange, no handshake is required before establishing communication, the connection can be established directly