Address Resolution
Mississippi College
Computer Science
Dr. Bennet
  1. Finding The Receiver
    1. Internet messages are sent to an IP address, which is virtual.
    2. But they are encapsulated in a hardware packet. Hardware doesn't know anything about IP addresses, it delivers to a hardware address.
      IP dst = 20.30.40.50
      IP Payload
      Frame dst = ?
      Frame Payload (IP datagram)
    3. The host that sends this packet must fill in the hardware address that will get the packet to 20.30.40.50.
    4. So there must be some way to map an IP address to the hardware address.
    5. This is called Address Resolution, for which we have an Address Resolution Protocol (ARP).
  2. This may be done is several ways. Here is the standard ARP used to support IP4.
    1. To look up 20.30.40.50, the sender first sends a broadcast message “who has 20.30.40.50”
    2. If some host does, it responds. The response is sent directly to the requestor'sMAC address; no need to broadcast.
    3. If the sender may repeat the request if there's not answer, but if no answer is received in a reasonable time, it fails the send.
    4. Caching
      1. The sender will cache the result for some time, since it is likely more messages will be sent.
      2. The “who has” message includes the IP address of the requestor, so the host that answers knows the IP ↔ MAC mapping for the sender, which it also caches.
      3. A exchange can proceed between the two for a while without having to compute the mapping again for a while.
    5. ARP messages are not IP messages; they are encapsulated directly into the hardware packet.
      ARP Message
      Frame Header
      Frame Payload (ARP Message)
    6. The Ethernet MAC header records frame type hex 0806, which is ARP, not IP.
    7. ARP message format
      Hardware address type
      Protocol address type
      Hdwr Addr Len
      Pcol Addr Len
      Operation
      Sender Hardware Address (first 4 bytes)
      Snd Hdwr Addr (last 2 bytes)
      Snd Ptcl Addr (first 2 bytes)
      Snd Ptcl Addr (last 2 bytes)
      Targ Hdwr Addr (first 2 bytes)
      Target Hardware Addr (last 4 bytes)
      Target Hardware Protocol Addr
      0
      8
      16
      24
      31
      1. Here, hardware address is a 48-bit MAC address, and protocol address is a 32-bit IP4 address. Typical use.
      2. The same format is used for requests and responses. If some info is unknown, it is filled with zeros.
      3. The format is designed to handle mapping between any addresses protocol and any hardware addresses.
        1. Hence, the type codes to say what type of address, and the hardware and protocol address lengths, since they may be of any size.
        2. For this usual case, the type codes are for MAC and IP4, and the lengths are 6 and 4 (bytes).
        3. If used with different sized addresses, the last part of the diagram would differ.
        4. The protocol could handle IP6 addresses, but IP6 does not use it.
    8. Resolution is at the top of the network interface layer.
      1. Above network interface, everything uses IP addresses.
      2. In network interface and below, we use hardware addresses, so the translation is at the top of network interface.
  3. IP6 resolution is part of IP6 itself, rather than a support protocol.
    1. Operation is similar. The host wishing to send first transmits a Neighbor Solicitation message, much like an ARP who has.
    2. The holder of the address answers with a neighbor solicitation response.
    3. Difference: The solicitation is not broadcast, it is sent to a multicast address called the solicited-node address.
    4. The IP6 solicited-node address is computed from the low 24 bits of the IP6 host address, with the prefix ff02::1:ff/104. (This prefix is in the IP6 multicast range.)
    5. Solicitation messages are sent to the solicited-node address built from the requested IP6 address.
    6. Each host listens on its solicited-node address built from its own IP6 address.
    7. The multicast MAC address for any IP6 multicast address is a 1 followed by the low 23 bits of the multicast address, which makes it a hardware multicast address.
    8. That all means that solicitations generally go only to the correct host. It may go to a few others, which can discard it.
    9. This is more efficient ARP, where pretty much all the hosts on a large network must discard large numbers of who has broadcasts which don't apply to them.
  4. Encapsulation, Resolution, and Routing. Here we cover a detail about IP routing that we avoided earlier.
    1. To the right is another version of the network described earlier. This adds the IP addresses bound to the router interfaces.
    2. Earlier, we gave the routing table for router X as
      NetworkMaskLine
      101.12.0.0255.255.0.0a
      140.64.0.0255.192.0.0b
      199.56.8.0255.255.255.0b
      201.10.6.0255.255.254.0c
    3. A better answer is this
      NetworkMaskDestination
      101.12.0.0255.255.0.0a local
      140.64.0.0255.192.0.0b local
      199.56.8.0255.255.255.0b gateway 140.64.0.2
      201.10.6.0255.255.254.0c local
    4. Local delivery means the router sends directly the recipient, which is on the same network.
    5. When it sends local, it ARPs the destination address and the hardware network takes it there directly.
    6. The table specifies that when sending to the 199.56.8.0/24 network, it must send the packet to another router first, the gateway.
    7. In fact, the router will ARP the address of the gateway, not of the recipient.
    8. A packet bound for 199.56.8.22 will be sent in a hardware packet bound for the MAC address of 140.64.0.2. That will receive the packet and consult its own routing table.
    9. The computer you are looking at this page on probably has a routing table that looks something like this:
      NetworkMaskDestination
      10.20.30.40255.255.0.0a local
      defaulta gateway 10.20.30.1
      1. Assumes you are on a network 10.20.30.40/16, and have only a single network connection that I called a.
      2. Assumes your organization or ISP has an external router at 10.20.30.1.
      3. If it's local, deliver it. If not, it's the gateway's problem.
    10. Routing tables get more complex and you move further from the edge nodes.