Week 6: Oct 3

Cerner visit will push midterm back to Oct 17
IP router table construction
UDP
ARP/DHCP

4.1.5: ARP
        Basic ideas: broadcast request/unicast reply, cache

        Timeout
                Alternatives: polling, link-layer notice, IP-layer notice
        finer points: If A arps "where is B?"
                1. B always puts A in its cache
                2. All hosts with A in their cache update the entry
        self-arp: detects duplicates, ethernet addr changes

        Flooding: 
            what if A tries to send 100 packets to B; how many ARPs? 
            A b'casts, everyone replies & needs to ARP to get A's addr
        ARP and networks without b'cast [eg ATM]
                Failure in presence of looping
        security implications of ARP
        proxy arp

DHCP; once known as Reverse ARP (RARP)

===============================================================

    4.2.2: distance vector routing (Routing Information Protocol, RIP)
        Table exchange: updates in three cases:
                new node reported
                decreased distance to existing node reported
                NOTE: these are the "no-bad-news" cases for which convergence
                happens because the distances are monotonically decreasing (or the same).
                If all hopcounts are 1, shortest path is the first discovered.
                
                *increased* distance reported by NextHop for that node

        Slow convergence problem
        
        small infinity, triggered updates
        	RIP: infinity=16, updates every 30 sec
        	Cisco IGRP: infinity <= 256; default = 100
        	
        	
        Fixes:  small infinity, 
                split horizon,
                     (don't report reachability to your own NextHop for a node)
                poison reverse,
                     (claim a node to be at distance infinity, when
                      reporting to your own NextHop for that node)
                triggered updates: send report immed. on any change
                hold down,
                    (*receiver* doesn't use new alternate routes for ~60 sec 
                     following discovery of unreachability)
                route poisoning: 
                    hold down only for changes that increase hop count

                

===========================================

Chapter 5: transport
UDP - P&D 5.1
        unreliable, unconnected (stateless). General uses: NFS, boot, RPC
        checksum & algorithm
            checksum can also be updated; eg in IP header
            0000 checksum never occurs naturally, can be used as "checksum not used"
        pseudo-header: include some IP header fields in UDP checksum
        ports & final destinations
        lost packets due to queue overflow	 (skipped)
                at router
                at host

        socket programming interface (P&D 1.3)  // done in java
                udpclient/udpserver
                simpletalk client, server: TCP and UDP versions
                socket, bind, sockaddr_in, gethostbyname,