Chapter 5: Network Layer. CS455/555: Spring 2007. Network Layer: Design Issues. Services provided to the Transport Layer: Connection-oriented vs Connectionless service Underlying network: Internet (connectionless) and ATM ( Connection-oriented) Virtual circuits vs. datagrams.
CS455/555: Spring 2007
(1) Discover its neighbors and learn their network addresses.
(2) Measure the delay or cost to each of its neighbors.
(3) Construct a packet telling all it has learned.
(4) Send this packet to all other routers.
(5) Compare the shortest path to every other router.
(1) When a stream of input packets arrive to go on the same outgoing link, packets may be lost due to insufficient buffer size on that queue. But “Nagle discovered that infinite amount of memory at routers may lead to worsening of congestion.”
(2) Slow processors (Mismatch is always a problem)
(3) “Congestion tends to feed upon itself and become worse.”
(I) Data link layer: retransmission policy, out-of-order caching policy, ACK policy, flow control policy
(ii) Network layer: VC vs. datagrams inside the subnet, packet queueing and service policy, packet discard policy, routing algorithms, packet lifetime management
(iii) Transport layer: retransmission policy, out-of-order caching policy, ACK policy, flow control policy, timeout determination
Bucket capacity = 1 Mbytes
Token arrival rate = 2 Mbytes/sec
Network capacity: 10 Mbytes/sec
Application produces 0.5 Mbyte burst every 250 msec. For 3 seconds
1 + 2X = 10X; 8X = 1; X = 1/8 sec =125 milliseconds. During this time, it can transmit 1.25 Mbytes. But the burst size is not that much.
So this will continue until the 0.5 Mbytes is sent. This takes 0.05 seconds or 50 milliseconds
Output: 0-50 msec: 10 Mbytes/sec
50-250 msec: None
By the time the next burst arrives, how many tokens will be accumulated? 2*200/1000 = 0.4 Mbytes of tokens
How long with the next burst last? 0.4 + 2X = 10 X; X = 0.4/8= 50 msec.
Output: 250-300msec: 10 Mbytes/sec
This will continue for 3 seconds.
Static NAT: A type of NAT in which a private IP address is mapped to a public IP address, where the public address is always the same IP address (i.e., it has a static address). This allows an internal host, such as a Web server, to have an unregistered (private) IP address and still be reachable over the Internet.
Dynamic NAT: A type of NAT in which a private IP address is mapped to a public IP address drawing from a pool of registered (public) IP addresses.
Typically, the NAT router in a network will keep a table of registered IP addresses, and when a private IP address requests access to the Internet, the router chooses an IP address from the table that is not at the time being used by another private IP address.
Dynamic NAT helps to secure a network as it masks the internal configuration of a private network and makes it difficult for someone outside the network to monitor individual usage patterns.
Another advantage of dynamic NAT is that it allows a private network to use private IP addresses that are invalid on the Internet but useful as internal addresses.
10.0.0.0 to 10.255.255.255.255/8
172.16.0.0 – 172.31.255.255/12
NAT box translates the public addresses into private addresses and vice versa
Use of UDP/TCP source port address (Page 446, fig. 5-60)
Objections to NAT: (not on Internet):