OS Support for Networking

Spectrum of Distributed Capabilities

1. Communications architecture (e.g., TCP/IP)
   running on individual OS's supports communication between systems
2. Network operating system
   individual OS's on each computer coordinated to provide remote services (e.g., file and printer sharing), supported by a common OS "adjunct"; individual systems are still visible
3. Distributed operating system
   single integrated OS manages collection of networked resources so as to appear to be a single system

Terminology

• Computer communictions
  enables the exchange of information between computers
• Computer network
  collection of interconnected computers
• Protocol
  rules/conventions enabling communication, including:
  • syntax: how is data formatted? what are signal levels?
  • semantics: control/coordination information
  • timing: speed, sequencing
• Protocol architecture
  breaks down implementation of protocol into tasks and modules

Example: File Transfer (a simplified architecture)

TCP/IP Protocol Architecture

• is a protocol suite
• can be divided into layers:
  • physical
  • network access
  • internet
  • host-to-hoste, or transport
  • application

Physical Layer

• characteristics of the transmission medium
• nature of the signals
• data rate
• bit encoding
• etc.

Network Access Layer

• how data is exchanged between end system and network
• alternate standards, e.g.
  • circuit switching
  • packet switching (frame relay)
  • LANs (Ethernet)

Internet Layer

• how is data routed across the network, and between networks
• implemented by both end systems and routers
• this is the layer of the Internet Protocol (IP)

Transport Layer

• how is integrity and transmission order of data preserved
• implemented by end systems
• this is the layer of the Transmission Control Protocol (TCP)
  TCP uses a "sliding window" to manage retransmissions

Application Layer

examples:

• file transfer (FTP)
• remote shell (SSH)
• remote procedure call (RPC)
• browser service (HTTP)

TCP/IP Concepts

TCP Header

UDP Header

IPv4 Header

IPv6 Header

IPv6

• extends and improves on IPv4
• accomodates higher speeds, mixture of data streams, graphics and video
• larger address space (128 vs. 32 bits)

DS = Differentiated services field (formerly Type of Service)

ECN = Explicit congestion notification field (formerly Traffic Class)

This is only the first header. Other information (including some of what is contained in the IPv4 header) may be contained in additional (chained) headers.

Items in the Header

• destination network address
• source network address
• length
• facilities requests (e.g., priority)
• etc.

Protocols and Headers (packet nesting)

Protocol Data Units (PDUs) in the TCP/IP Architecture

TCP/IP Applications (examples)

• Simple Mail Transfer Protocol (SMTP)
• File Transfer Protocol (FTP)
• Hypertext Transfer Protocol (HTTP)
• TELNET
• Secure Shell (SSH) Protocol

Most protocols are standardized via RFC's (Requests for Comment) by the IETF (Internet Engineering Task Force). They starts out as requests for comment (proposals) and some are eventually widely enough aceepted that they becomes standards.

Sockets

• enable communications between logical endpoints
• identified via host address + port number
• two types:
  • stream: use TCP, connection-oriented, provide reliable data transfer
    • server: accepts connections (passively waits)
    • client: requests connection (actively initiates)
  • datagram: use UDP, delivery is not guaranteed

Socket Setup

• created via socket command
• parameters:
  • protocol family: PF_INET foro TCP/IP
  • type: stream or datagram
  • protocol: TCP or UDP

TCP Socket Usage: Server Side

• socket: creates socket
• bind: associates local port with socket
• listen: allow client connections
• accept: accept one connect (creates new socket per connection)
• recv/read, send/write: transfer data between sockets
• close/shutdown: shut down connection

TCP Socket Usage: Client Side

• socket: creates socket
• connect: connects to server identified by host address and port
• send/write, recv/read: transfer data between sockets
• close/shutdown: shut down connection

Socket System Calls for Connection-Oriented Protocol

Linux Kernel Components for TCP/IP Processing

Example: Trivial File Transfer Protocol (TFTP)

• simplifications: no acess control or user identification
• initiated by client sending to server via UDP to port 69
• client provides TID (client port number for transaction)
• server responds with TID (server port number for transaction)
• data is transferred via UPD using the TIDs
• errors handled by sender timing out and restransmitting if no ACK received prior to timeout

TFTP Packet Formats

TFTP Error Codes

A TFTP Packet in Context

TFTP Operation