25. UDP and TCP

UDP (Universal Datagram Protocol) and TCP (Transmission Control Protocol)

Properties

UDP - 1 unit, unidirectional, “stateless”, unreliable, less overhead than TCP and faster. Use for local info (i.e. staying within local area), such as: (Rip and rdisc use UDP)

• TFTP (Trivial File Transfer Protocol)
• Anything not connection oriented (all broadcasts and multicasts)
• Time Crucial (real-time video)
• Faster way to correct lost packets NFS

TCP - Continuous, bi-directional, confirms receipt of packet, Requests transmission to be faster or slower, if not received, retransmits, simple program, focuses on data not delivery, sequence number tells order of packet, acknowledges packets, fin (closes connection) and syn (start at seq 0)

Ports 0-1023 ports are reserved for root. These numbers are to tell who is sending requests for services. The computer receiving requests looks at .rhost (in local home dir) to find allowed users/hosts.

The receiving machine can verify packet by looking at IP layer, for hosts. The application layer claims to be a valid user, but it can be changed, so rsh says “if sending user/hosts port number is greater than 1024, it won’t use package”, rsh is very unsecure.

1. /etc/inet/inet.d This file listens for services on ports that are activated.
2. Server daemon starts /user/lib/sendmail (server on 25/TCP)
3. User types command “Telnet host 25”, client connects to 25/TCP
4. Communications begin /etc/inet/inetd.conf

Sample of inetd.conf: Service [protocol used] [owner] [what process to start] [cmd line] i.e. service stream tcp nowait root /usr/sbin/in.telnet.d in.telnetd

To start daemons

• Rc
  1. Server daemon in /etc/rc_.d/s_ Inet.d Server daemon is inet.d (rc start)
  2. Each port matches a program in /etc/inet/inetd.conf.
  3. When connection started, start program so a connection is begun and a process is spawned by forking (making an exact copy and then executed (process))
• RPC (Not at fixed port numbers)
  1. rpc bind (starts (rc))
  2. inetd for a program or program itself (rc) which registers with rpcbind and assigns port number
  3. client reaches rpcbind to learn port number of program itself
  4. client reaches program directly. Rpc can adjust to any free port to solve port conflict within software.

  Go to Chapter 24       Go to Index        Go to Chapter 26