IP

• Stallings lists the following functions IP performs
  • Encapsulation
    • We have seen this all along.
    • 
      • Version number, (4 or 6 now, we are looking at 4 right now.)
      • Length of the header in 32 bit words.
      • DS/ECN differentiated services, congestion control
      • Total packet length (data + header)
      • Sequence number
      • Flags, (don't fragment)
      • Fragment offset (we will discuss this)
      • TTL - a hop count
      • Protocol (TCP, UDP, ICMP, ...)
      • Header checksum
      • Source/destination address
  • Fragmentation/reassembly
    • Fragmentation is sometimes required because
      • Lower level networks don't support size of PDU we are sending
      • Error control is sometimes easier
      • More fair access to the network to all packets.
      • Smaller buffers on receiving side.
      • Possible checkpoint/recovery issues.
    • But
      • More overhead (headers/footers) is required
      • More OS interruption
      • More time spent processing smaller pdus
    • We can set a don't fragment flag (in the header above)
      • This indicates that the receiver has no buffer space and can't reassemble the packet.
      • But it means packets will be discarded if they need to be fragmented.
    • Fragmentation occurs at routers when required.
    • Reassembly occurs at the destination.
      • Reduces buffer requirements on routers.
      • No need for all fragments to travel the same route.
    • How to fragment:
      • Use the following fields:
        • ID - identifies a group of fragments that make up an higher level PDU
        • Data length - length of local data
        • Offset - position in main PDU of local data
        • More flag - 0 -> last fragment, 1-> additional fragments.
      • To fragment a PDU
        • Create n new datagrams, copy the field header of source to all of them.
        • Divide data field of source along 64 bit boundary's. The last need not be. Data + header must fit into new PDU data field.
        • Set data length of all new datagrams appropriately
        • Set more flag of all but last to be 1
        • Set offset field approprately.
          • Add offest to existing value (remember, we may be a fragement)
        • 
    • To reassemle
      • All fragements with the same ID are part of the same final PDU
      • A buffer is allocated, and fragments are stored at the proper position as they arrive.
      • This is computed by the more and offset flags.
      • There is a time to live on a reassembled fragment
      • If this is exceeded the partial PDU is discarded and the packet is declared lost.
  • IP is connectionless
    • Advantages:
      • It is more flexible
      • It is very robust
      • TCP can supply these requriements.
  • IP does not guarentee delivery.
  • It does not guarentee ordered delivery
  • IP does does report back if a PDU is discarded when possible. (ICMP)
  • ICMP provides some level of flow coontrol.
  • IP implements a packet lifetime via a ttl field.
  • IP Does provide an addressing scheme:
    • A 32 bit address, seperated into 4 octets.
    • 147.64.242.52
    • This contains a network part and a host part.
    • Originally IPV4 provided classes.
      • If the first bit is a 0, this is a class A netork.
        • The next 7 bits are network bits.
        • The last 24 bits are the host address.
        • 2²⁴ = 16,777,216
        • The standard netmask is FF.0.0.0
        • Networks 1 to 126, 125 of these
      • If the first two bits are 10 it is a class B network.
        • The next 14 bits (16 total) are network information
        • The last 16 bits are host information.
        • 2¹⁶ = 65,536
        • The standard netmask is FF.FF.0.0
        • Networks 128 to 191, 16K of these
      • If the first three bits are 110 this is a class C network.
        • The next 21 bits (24 total) are network
        • The last 8 bits are host
        • 2⁸ = 256
        • The standard netmask is FF.FF.FF.0
        • Networks 192 to 223, 2M of these
      • Other networks are reserved for
        • Multicast
        • Future use
      • Within each class is a non-routed network.
        • Class A: 10.
        • Class B: 172.16 -> 172.32
        • Class C: 192.168.
      • 127.0.0.0 is reserved for local loopback.
    • CIDR
      • The reason we didn't rou out of IPV4 network addresses when we though.
      • Classless Inter-Domain Routing
      • This redefines the netmask.
      • Network addresses have an associated CIDR block length.
      • This indicates the length of the network portion.
      • EUP has 147.64 which is a class B.
      • They have assigned 147.64.242 and 147.64.243 to CS
      • Our netmask is FF.FF.FE (242 = 72₁₆)
      • Or 147.64.242/23
      • This means 23 bits of the address are network part
      • And 9 bits (or 512 machines) are host part.
      • And the netmask with the IP to get network part
      • And !netmask with IP to get host part.
• Look at ifconfig.