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Computer Systems - Lecuture 11 - Subnets, NAT, Fragmentation

Subnets

  • Internet is made up of networks
  • We've run out of IP addresses since forever SO:
    • For every IP address we can host a few subnets or smaller networks with a subnet mask
    • This allows us to have a network within a network
      • i.e. a subnet

Subnet Masks

  • Written like this: 255.255.255.128 // dot notation or 192.238.120.32/25 // slash notation
  • Specifies subnet mask prefix
    • Dot notation can be converted to binary to figure out the mask
    • Slash notation is simply the number of bits that make up the mask
  • If you use a binary-AND operation on the original IP + subnet mask we get our internal network address

A mask works like this:

Computer Science/17
Electrical Engineering/18
Arts/19

Our first router (the one everything forks to) will be responsible for handling all external data.

  • Future changes can be made without any external impact
  • No need to request more external IP addresses
  • Routing on the internet does not change

Network Address Translation (NAT)

  • We've run out of IP addresses
  • Apparently this is a not very good solution but it works
  • Using subnet masks, if we give an ISP a /16 allocation that's still only 65534 hosts
    • We can try dynamic allocation (allocating IP addresses whilst connected)
      • Still limited to 65534 concurrent connections
      • ADSL requires always on connections
  • Now all internal IP addresses are private and are used to communicate within the LAN
    • Router NAT translates the private IP address to a external one with a translation table
    • Also changes the port
      • Now we have issues with communicating on ports
      • Fixed with port forwarding (set the outward port the same as the internal one)

Criticisms of NAT

  • Violates IP architectural model (every interface on the network no longer is unique)
  • Breaks end-to-end connectivity
  • Changes internet from connectionless to pseudo-connection-oriented
    • NAT maintains connection state
    • If it crashes everything screws up
  • Violates layer model by assuming nature of payload contents
    • Causes problems with FTP and other protocols
  • Limits number of outgoing connections

Why it works

  • Significant security advantage
    • Internal network is shielded from attacks from incoming unsolicited packets
  • Likely to remain after IPv6 is normal

Fragmentation

  • Process of breaking up larger packets into smaller packets
  • IP packets have a maximum size of 65535 determined by the total length header field
  • Most links can't handle this size
    • Due to OS
    • Hardware
    • Weird protocols
    • Reducing transmissions due to errors
  • Means lower layers should be able to fragment larger packets
  • Defined by a MTU - Maximum Transmission Unit
    • Path MTU - Maximum size for path through network
      • Reason why this is dynamic is cause the path can change after it's been sent

Non Transparent Fragmentation vs Transparent Fragmentation

  • Have to reassemble our fragmentated packets, we can do it two ways:
  • Non Transparent Fragmentation (Used by IP)
    • Reassembly happens at the destination host
  • Transparent Fragmentation
    • Reassembly is performed at the next router

Fragmentation and IP headers

  • IP headers have the following:
    • Identification
      • Indentifies a packet in some fragments
    • Flags
      • Don't Fragment or More Fragments (on the way)
    • Fragment Offset
      • Offset in 8-byte blocks
  • Use these to store info of fragmentation
  • Allows receiving host to reconstruct at buffer

Downsides

  • Overhead from fragmentation is incurred all the way to the host
  • If a single fragment is lost the entire packet has to be resent
  • Overhead on host is more than expected
Path MTU discovery
  • Basically send smaller and smaller packets until you can get to the host in one go
    • No fragmentation is required
      • Not guarenteed
IPv4 vs IPv6 Fragmentation
  • IPv4 allows for either nontransparent fragmentation or path MTU discovery
    • Min accept size 576 bytes
  • IPv6 expects hosts to discover the optimal path MTU, routers will not perform fragmentation in IPv6
    • IPv6 minimum accept size 1280 bytes