TCP/IP Networking in UNIX - Day 1

Goals

• understand Network Access Layer - ARP
• understand the Internet Layer - IP, routing, ICMP
• understand the Transport Layer - UDP and TCP
• examine typical application layers: telnet, ftp, smtp, dns, RIP, NFS

Introductory notes

• tcp/ip as a generic term, or family of technologies
• DOD suite
• tcp/ip dominant for net/unix
• compare to IPX, AppleTalk, etc.

generic networks

• concerns
  • Naming/addressing: the ability to locate and talk to a particular node
  • Segmenting: busting up long packets for resource sharing and error correction (consider max size v. error rate).
  • Flow control: handling over-saturation
  • Synchronization: keeping distributed processes in sync
  • Prioritization: packet priority
  • Error control: checksums, reliability v. unreliability
• topology
  • bus - single wire
  • ring - good for large physical distances
  • star topology - most common, uses much wire
• layered protocols
  • modularity - cf. winsock
  • flexibility - change one layer
  • standardization

The OSI 7-layer burrito, err, model

1. "Physical": not the wire, but the impulse, current
2. Data-link: packet building, checksums, MAC level [bridges, switches]
3. Network: IP, IPX, routing tables [routers]
4. Transport: TCP
5. Session: TCP, NetBIOS, SPX
6. Presentation: na
7. Application layer: applications

TCP/IP networks

• twisted pair, rj-45, hubs, routers, switches
• collision: Carrier Sense with Multiple Access (CSMA): listen politely. Transmit. If node detects a collision, send jamming pulse. Random delay, timeout doubles, x10. Complicated by delays: .65C.
• What is the format of an address in TCP/IP?:
  • every network must have a unique name, and ever node must have a unique name. No publically-visible names may compete.
  • dotted quad: xxx.xxx.xxx.xxx.
  • Math: 4 byte segments, 0-255
  • class c= 2^8 = 256 theoretical nodes
  • class b= 2^16 = 65,536 theoretical nodes
  • class a= 2^32 = 4,294,967,296 (aka, the internet, hence IPv6)
  • network and node sections of the IP and network notation
  • multicast (usually 255.255)
  • routers must have a local IP for every network they are on
  • local delivery = peer to peer = data link layer.
    If both IPs are on the same network (see above), the node will try to deliver locally.
    1. on same wire
    2. find IP: ARP for MAC address. Cache arp.
    3. talk directly to MAC - others ignore

    //pseudocode for endnodes
    
    //	receiving packets
    if ((destAddress == myAddress) or (destAddress == broadcast)) {
    	
    	process the packet
    }
    
    
    else {
    
    	drop the packet
    
    }
    
    
    //	sending packets
    if (destNet == myNet) {
    
    	deliver locally (ARP or cache)
    
    }
    
    
    else {
    	send to the router
    }
    
    
    
    //	#############################################################
    //pseudocode for routers
    
    if ((destAddress == myAddress) or (destAddress == broadcast)) {
    	
    	process the packet
    }
    
    
    else if (destNet is directly connected) {
    
    	deliver (arp)
    }
    
    
    
    else if (destNet in table) {
    
    	deliver to next router
    
    }
    
    
    else {
    
    	drop the packet
    
    }
    
    
    
    
    
    

  • Question: what happens if destination is not on the network, and there is no router?
• How do devices get an address:
  • manual configuration
  • automagic assignment: dhcp/bootp
• How is the address mapped onto the physical address?:
  • MAC addresses: your isp, bridges, etc. vendor + serial: 08:00:20:0A:8C:6D
  • why? hardware hot swaps and local routing
  • how ARP works: FF:FF:FF:FF:FF:FF "What is the MAC for a particular IP?"
• How does an end node find a router?:
  • Gateway (default route)
  • other routing commands
  • sniffing RIP traffic
• How do routers learn network topology?: Cost; hops. Cost/hop of 0 means direct connection.
  
  • manual config
  • RIP or other learning algorithms
• How do users find services on the network? IP numbers are unfriendly...
  • HOST files
  • DNS = distributed name lookup

TCP and UDP

Homework

• browse the usenet groups mentioned in the course resources

Remember, your login is based on your machine's hostname, not on any other number.
~/[initials] refers to the subdirectory under your homedir, named after your initials. Everything except for .dotfiles will be stored in your ~/[initials] directory.