CSC 290e Mobile Computing

Multicast and DNS
12 September 2001

------------------------------------- 

Last time we talked about TCP and UDP.  These are protocols which support one
process sending a message to one other process (or one machine to one other
machine).  This is referred to as **unicast** and is the same as
"point-to-point" communication.

Multicast: point-to-multipoint
           usually single source, multiple destinations
           usually unreliable
           easy on a LAN: 
             On an ethernet LAN, everything is broadcast by default, received
             by every host on the network, and filtered out depending on what
             the machine is listening to.  Always listen to broadcast. Listen
             to some multicast by default (e.g., appletalk), registered
             multicast addresses, and own unicast address
           hard on a WAN - can simulate with multiple-unicast, 
                           but this is noisy
                
  trivial solution is to broadcast, but this is noisy (too many packets)
  better idea is to set up a spanning tree of recipients 
    + less traffic.  Send message on all ports not received on 
    - not very fault tolerant
    - set up time to establish tree

  What is the MBONE (multicast backbone)? 
    - islands of multicast capable hosts connected by software tunneling
    - what is a tunnel?  In this case, it is a connection between two
      multicast enabled routers.  The first one takes in multicast packets,
      encapsulates the packets to make them look like normal IP packets.  They
      are then unicast to the next router which decapsulates them (turning
      them back into multicast packets) and sends them out on its local
      network.  
    - tools available include vic, vat, sdr

---------------------------

What are the addresses?
   mac address is unique to the ethernet card
   IP address is unique to the machine (usually)
      why have both?  IP addresses can be grouped

      IP address Classifications
         class   prefixbits + bits for NW + bits for host
         A       0   + 7  + 24    1.0.0.0 -> 127.255.255.255
         B       10  + 14 + 16  128.0.0.0 -> 191.255.255.255
         C       110 + 21 + 8   192.0.0.0 -> 223.255.255.255 (URCS)
         D       1110           224.0.0.0 -> 239.255.255.255 (mcast)
                                (some reserved in mcast RFC)
         E       11110          240.0.0.0 -> 247.255.255.255
         bcast                  255.255.255.255
      
---------------------------

DNS - Domain Name Service
  distributed Internet directory service
  Translate: IP address <--> name
  try it out...from a prompt, type:
     nslookup cnn.com
        (why are there multiple IP addresses?)
     nslookup 192.5.69.207
  Something else (unrelated) to try:  
     traceroute www.cs.rochester.edu
     traceroute (insert your favorite machine here)

  Getting an IP address
     1. ask the DNS server you know for the address
     2. if that DNS server does not have the address, it asks another
     3. your request propagates through DNS servers, until an authoritative
        answer is provided 

  Data is distributed among multiple servers, arranged generally as a
    hierarchy. 
    ?? why not directly ask the root each time??
  Data is replicated at many servers.  
  Each replication has a timeout period.