routing metrics:

going back over link state and distance vector protocols
and now we're learning how traceroute uses icmp (internet control message protocol) to generate a packet's path through the network
TTL == 0 -> router sends packet back to originator to let it know a packet was dropped
icmp is rfc 792
end to end protocols: tcp/udp, transport layer

application layer protocols - use transport layer protocols to get across the internet
typical net app has 2 pieces, client/server
client inits contact w/ server, typically requests services from server
server provides requested service to clients, sometimes auth is required

how does a proc ident the other procs with which it wants to comm?
	ip address, port number...i keel you
now talking about NAT, in all it's gory glory, internal IP:PORT mapped to an external IP:PORT for the world to understand

tcp services
	connection oriented (setup required)
	reliable transport
	flow control (sender can't overwhelm receiver)
	congestion control (throttle sender when net overloaded)
	no timing
	no minimum bw

in the flag field of the TCP header, many bits, one for ACK