Networking►Stanford CS144

network layer

The Network layer

Walk through the services provided by IP protocol

The IP Service Model

Datagram

IP works datagrams and forward the packages hop-by-hop

IP datagram

Router use destination part in datagram to lookup in a forwarding table for the next target it should send the datagram to

Unreliable

IP packages may be lost, duplicated, corrupted and so on, IP has NO guarantee

Best effort

IP only drop datagram if necessary, for example the congestion in a router force the router to drop the next incoming datagram

Connectionless

It doesn’t establish an end-to-end connection, it simply router the packages

Why the IP Service so simple?

  • Simple and minimal gives faster, more streamlined and lower cost network
  • The end-to-end principle: If possible implement features in the end hosts(the source and destination computers) rather than the network
  • Allow a variety of services, both reliable and unreliable to build on the top of IP.(Not all services need a reliable connection, for example, in real-time application, the resent package may be to old to use when it finally arrive the destionation)
  • Work over any link layer : IP make very little assumption of the below layer, make it easy to build the hardware for transmission

More on IP Service Model

  • IP tries to prevent packages looping forever

    • This can happen when the forwarding table went wrong in some router

    • It doesn’t prevent the loop from happening but catch and delete the looping packages

      It is implemented with the following idea

      There is a hop count (TTL,Time-to-live)field in IP header

      IP layers

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      #define max_allow 128
      //every package's TTL is initalized to max_allow
      void route(Package p){
          if(p.TTL == 0){
              drop(p);
          }
          else{
              p.TTL--;
              forward_to_next(p);
          }
      }

  • IP will fragment packages if they are so long

    • Different link layer can carry different max amount of data, for example, ethernet can only carry packages shorter than 500 bytes

    • The IP header provide router with information it need to fragment big packages into smaller ones.

      These small packages also contain information for laterly reassemble

  • IP use header checksum to reduce chance of delivering the packages to the wrong destination

  • Versions of IP

    • IPv4 : 32 bit address
    • IPv6 : 128 bit address

  • allow new options to be added to header.

    This feature violent the simple design and the end-to-end principle, so it is hardly used. Moreover, adding new field to header requires adding hardware to routers

IP header explanation

IPv4 header

  • Source address : package from

  • Destination address : package to

  • Protocol : what protocol is used for the data field(list)

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    IPnumbers = {
        1 : "ICMP",
        ...,
        6 : "TCP",
        ...,
        17: "UDP",
        ...
    }
    Package p
    protocol_to_use = IPnumbers[p.protocal]
    process_data(p.data , protocol_to_use)

  • Version : specify IPv4 or IPv6

  • Identification(aka Package ID), Flags, Frgment offset are used for package fragmentation

  • TOS(Type of Service) gives how important the package is

  • IHL(IP header length)

IPv4 header

Tips

  • Network layer only send package between computers, it doesn’t care about which application need the package, this is the job of Transport layer.

  • TCP port is like an address for applications

  • There are chances that the router is the destination of a package, for example, when you log in a router using TCP, that is the case.

  • * * * in traceroute means the router doesn’t response to traceroute, indicating that the router doesn’t want others to know it.