Network programming

Overview

Teaching: 0 min
Exercises: 0 min

Questions

• Introduction to network programming

Objectives

• First learning objective. (FIXME)

1. In the beginning

• 1966: Advanced Research Projects Agency Network (ARPANET) - DOD
• 1969: Four sites:
  • SRI: Stanford Research Institutte
  • Utah: University of Utah
  • UCLA
  • UCSB

• 1977: East and West
  • MITRE Corporation: managed federally funded research and development centers (FFRDC) for a number of U.S. agencies (DOD, FAA, IRS, DVA, DHS, AO, CMMS, NIST)
  • BNN: now a subsidiary of Raytheon
  • Burroughs: now part of Unisys

2. Now

• 2012: the Carna Botnet was unleashed.
  • An ethical hacking experiment in 2012 that used Nmap Scripting Engine (NSE) to scan for random devices with default telnet login username and password.
  • Over 100,000 devices had these features and could easily be accessed.
  • A spider-like crawling approach was used to have the vulnerable devices to scan for other vulnerable devices.
  • In the end, a total of 420,000 devices were assisting the internet search, and of the 4.3 billion IP addresses possible, the Carna Botnet found 1.3 billion.
  • What came from the Carna Botnet was a massive census of the internet in 2012.

3. A client-server transaction

• Most network applications are based on the client-server model:
  • A server process and one or more client processes
  • Server manages some resource
  • Server provides service by manipulating resource for clients
  • Server activated by request from client (vending machine analogy)
• Clients and servers are processes running on hosts (can be the same or different hosts).

4. Computer networks

• A network is a hierarchical system of boxes and wires organized by geographical proximity
  • BAN (Body Area Network) spans devices carried / worn on body
  • SAN* (System Area Network) spans cluster or machine room
  • LAN (Local Area Network) spans a building or campus
  • WAN (Wide Area Network) spans country or world
• An internetwork (internet) is an interconnected set of networks

5. From the ground up

• Lowest level: Ethernet segments
  • consists of a collection of hosts connected by wires (twisted pairs) to a hub (replaced by switches and routers today).
  • Spans room or floor in a building
  • Each Ethernet adapter has a unique 48-bit address called MAC address: 00:16:ea:e3:54:e6
  • Hosts send bits to any other host in chunks called frames
  • Hub copies each bit from each port to every other port
    • Every host sees every bit
• Next level: bridged Ethernet segments.
  • Spans building or campus
  • Bridges cleverly learn which hosts are reachable from which ports and then selectively copy frames from port to port.
• Next level: Internet
  • Multiple incompatible LANs can be physically connected by specialized computers called routers.
  • The connected networks are called an internet (lower case)

6. Logical structure of an internet

• Ad hoc interconnection of networks
• No particular topology
• Vastly different router & link capacities
• Send packets from source to destination by hopping through networks
  • Router forms bridge from one network to another
  • Different packets may take different routes

7. The notion of an internet protocol

• How is it possible to send bits across incompatible LANs and WANs?
• Solution: protocol software running on each host and router
  • Protocol is a set of rules that governs how hosts and routers should cooperate when they transfer data from network to network.
  • Smooths out the differences between the different networks

8. What does an internet protocol do?

• Provides a naming scheme
  • An internet protocol defines a uniform format for host addresses.
  • Each host (and router) is assigned at least one of these internet addresses that uniquely identifies it.
• Provides a delivery mechanism
  • An internet protocol defines a standard transfer unit (packet)
  • Packet consists of header and payload:
    • Header: contains info such as packet size, source and destination addresses
    • Payload: contains data bits sent from source host

9. Transferring internet data via encapsulation

• Ad hoc interconnection of networks
• No particular topology
• Vastly different router & link capacities
• Send packets from source to destination by hopping through networks
  • Router forms bridge from one network to another
  • Different packets may take different routes

10. A trip down memory lane

11. Other issues

• We are glossing over a number of important questions:
  • What if different networks have different maximum frame sizes? (segmentation)
  • How do routers know where to forward frames?
  • How are routers informed when the network topology changes?
  • What if packets get lost?
• These (and other) questions are addressed by the area of systems known as computer networking

12. Global IP Internet (upper case)

• Most famous example of an internet
• Based on the TCP/IP protocol family
  • IP (Internet Protocol)
    • Provides basic naming scheme and unreliable delivery capability of packets (datagrams) from host-to-host
  • UDP (Unreliable Datagram Protocol)
    • Uses IP to provide unreliable datagram delivery from process-to-process.
  • TCP (Transmission Control Protocol)
    • Uses IP to provide reliable byte streams from process-to-process over connections.
• Accessed via a mix of Unix file I/O and functions from the sockets interface.

13. Hardware and software organization of an Internet Application

14. A programmer’s view of the Internet

• Hosts are mapped to a set of 32-bit IP addresses (lookout for IPv6 in the future)
  • 128.2.203.179
  • 127.0.0.1 (always localhost)
• The set of IP addresses is mapped to a set of identifiers called Internet domain names: 144.26.2.9 is mapped to www.wcupa.edu
• A process on one Internet host can communicate with a process on another Internet host over a connection.

15. IP addresses

• 32-bit IP addresses are stored in an IP address struct.
  • IP addresses are always stored in memory in network byte order (big-endian byte order)
  • True in general for any integer transferred in a packet header from one machine to another.
    • E.g., the port number used to identify an Internet connection

/* Internet address structure */
struct in_addr {
  uint32_t s_addr; /* network byte order (big-endian) */
};

• Dotted decimal notation
  • By convention, each byte in a 32-bit IP address is represented by its decimal value and separated by a period.
  • IP address: 0x8002C2F2 = 128.2.194.242
• Use getaddrinfo and getnameinfo functions (described later) to convert between IP addresses and dotted decimal format.
• Domain Naming System (DNS)
  • The Internet maintains a mapping between IP addresses and domain names in a huge worldwide distributed database called DNS.
  • Conceptually, programmers can view the DNS database as a collection of millions of host entries.
    • Each host entry defines the mapping between a set of domain names and IP addresses.
    • In a mathematical sense, a host entry is an equivalence class of domain names and IP addresses.

$ nslookup localhost
$ hostname -f
$ nslookup www.facebook.com
$ nslookup www.twitter.com

16. Internet connections

• Clients and servers communicate by sending streams of bytes over connections. Each connection is:
  • Point-to-point: connects a pair of processes.
  • Full-duplex: data can flow in both directions at the same time,
  • Reliable: stream of bytes sent by the source is eventually received by the destination in the same order it was sent.
• A socket is an endpoint of a connection
  • Socket address is an IPaddress:port pair
• A port is a 16-bit integer that identifies a process:
  • Ephemeral port: Assigned automatically by client kernel when client makes a connection request.
  • Well-known port: Associated with some service provided by a server (e.g., port 80 is associated with Web servers)

17. Well known service names and ports

• Popular services have permanently assigned well-known ports and corresponding well-known service names:
  • echo servers: echo 7
  • ftp servers: ftp 21
  • ssh servers: ssh 22
  • email servers: smtp 25
  • Web servers: http 80
• Mappings between well-known ports and service names is contained in the file /etc/services on each Linux machine.

18. Socket interface

• Set of system-level functions used in conjunction with Unix I/O to build network applications.
• Created in the early 80’s as part of the original Berkeley distribution of Unix that contained an early version of the Internet protocols.
• Available on all modern systems: Unix variants, Windows, OS X, IOS, Android, ARM
• What is a socket?
  • To the kernel, a socket is an endpoint of communication
  • To an application, a socket is a file descriptor that lets the application read/write from/to the network.
  • Remember: All Unix I/O devices, including networks, are modeled as files.
• Clients and servers communicate with each other by reading from and writing to socket descriptors.
• The main distinction between regular file I/O and socket I/O is how the application opens the socket descriptors.

19. Hands on: client/server network programming

• Create a directory called 07-networks.
• Create the following files server.c and client.c.

• Compile the two programs.
• Open a dual panel tmux and run server and client on each panel.
• Type strings into client and observe how it shows up on server.
• Type a corresponding string on server and observe how it shows on client.
• Type exit on each side to stop client and server.

$ gcc -o server server.c
$ gcc -o client client.c

Key Points

• First key point. Brief Answer to questions. (FIXME)