1. NAT – Network Address Translation
IP address is a scarce resource.
So, give a company only one or a few IP addresses used by the gateway router.
Within the company, each machine has an unique IP address, chosen from /8
/12
/16
These addresses can only appear within a company but never on the outside Internet

2. NAT
Whenever a machine wants to send a packet to the outside, the packet will be sent to the NAT box.
The NAT box will convert the internal IP address to the real IP address of the company, and pass the packet to the gateway router.
When there is a packet destined for an internal machine arrived at the router, what should the router and NAT box do?
For IP packets carrying TCP or UDP, use port number. Other protocols are much more complicated.

3. NAT For IP packets carrying TCP or UDP, use port number.
When an outgoing packet arrives at the NAT box,
The IP address is replaced
The source port number is replaced
Header checksum is recomputed
When a reply came for this process, use the replaced source port number as index to find the correct IP address and original port number.

4. ICMP ICMP – Internet Control Message Protocol
Each ICMP message is encapsulated in an IP packet
Treated like any other datagram, but no error message sent if ICMP message causes error
Some interesting messages:
Time exceeded: When an IP packet arrived at a router is dropped because the TTL field becomes 0, the router will send an ICMP TIME EXCEEDED message back to the source. Used in traceroute.
Echo and Echo reply: ping.

5. Address Resolution IP address is virtual
Not understood by underlying the hardware of physical networks
IP packets need to be transmitted by the underlying physical network
Address resolution
Translating IP address to physical address
Address Resolution Protocol (ARP)
As mentioned earlier, IP address is virtual and need to be mapped to a physical address for delivering packets by a physical network. This translation of IP to physical addresses is called address resolution and this is achieved by ARP.
Computer Science, FSU

6. ARP Example
Here is an illustration of ARP broadcast by W requesting hardware address of Y. As you can see, every node in the local physical network receive this request. Only Y sends the reply and also only to W.
Computer Science, FSU

7. ARP Cache Each computer maintains a cache table Exchanges ARP messages
IP address  hardware address mapping
Only about computers on the same network
Exchanges ARP messages
To resolve IP addresses with unknown hardware addresses
Computer Science, FSU

8. ARP Protocol
When a node sends an IP packet to another node on the same physical network
Look up destination address in the ARP table
If not found
Broadcast a request to the local network
Whose IP address is this?
When we want to deliver an IP packet to another node on the same physical network, we need to map the destination IP address to corresponding hardware address. First, we look up the arp cache. If an entry is found, then we can encapsulate the IP packet into local link layer packet with destination hardware address and transmit. If there is no entry found, in that case we need to broadcast an ARP request specifying the target IP address for which we don’t know the corresponding hardware address.
Computer Science, FSU

9. ARP Response The target node responds to sender (unicast?)
With its physical address
Adds the requester into its ARP table (why?)
On receiving the response
Requester updates its table
Other nodes upon receiving the request
Refresh the requester entry if already there
No action otherwise (why?)
Table entries deleted if not refreshed for a while
We can categorize the nodes in the local network into requester node, target node and all others. Lets see what each of them do in turn.
The target node responds only to the sender with its hardware address. Why not broadcast the reply also? All others may not necessarily be interested in communicating with target node and a broadcast incurs processing overhead at every node in the network. The target node adds the requester’s IP and hardware addresses into its ARP cache (if not already there). Why? It is likely that target node would also send IP packets to the requester node later and so it makes sense to avoid an ARP request broadcast by the target node that time.
The requester node on receiving the response, updates its ARP table.
All other nodes, receive only the request not the reply. They check if the requester has an entry in their caches. If found, they refresh that entry. Otherwise, no action taken. Why not add an entry for the requester? We don’t want to grow the ARP table unnecessarily.
Finally, a lifetime is associated with each entry in the ARP cache and an entry is deleted if it not refreshed within that time.
Computer Science, FSU

10. TRY
tcpdump -ennqti eth0 \( arp or icmp \)

11. DHCP DHCP – Dynamic Host Configuration Protocol
A new machine asks for an IP address
Broadcast DHCP DISCOVER packet
A DHCP relay agent got this packet and relay it to the DHCP server
The DHCP server assigns an IP address
Periodically renew

12. Hierarchical Routing gateway routers
aggregate routers into regions, “autonomous systems” (AS)
routers in same AS run same routing protocol
“intra-AS” routing protocol
routers in different AS can run different intra-AS routing protocol
special routers in AS
run intra-AS routing protocol with all other routers in AS
also responsible for routing to destinations outside AS
run inter-AS routing protocol with other gateway routers

13. Intra-AS and Inter-AS routing
C.b
Gateways:
perform inter-AS routing amongst themselves
perform intra-AS routing with other routers in their AS
B.a
A.a b
A.c c a a C b a B d c A b
network layer
inter-AS, intra-AS routing in
gateway A.c
link layer

14. Intra-AS and Inter-AS routing
between
A and B a b C A B d c
A.a
A.c
C.b
B.a
Host h2
Host h1
Intra-AS routing
within AS B
Intra-AS routing
within AS A

15. Why different Intra- and Inter-AS routing ?
Policy:
Inter-AS: admin wants control over how its traffic routed, who routes through its net.
Intra-AS: single admin, so no policy decisions needed
Scale:
hierarchical routing saves table size, reduced update traffic
Performance:
Intra-AS: can focus on performance
Inter-AS: policy may dominate over performance

16. Intra-AS Routing Also known as Interior Gateway Protocols (IGP)
Most common IGPs:
RIP: Routing Information Protocol
OSPF: Open Shortest Path First
IGRP: Interior Gateway Routing Protocol (Cisco proprietary)

17. OSPF
Represents the network as a graph, and runs the shortest path algorithm to find the path to any router.
Divide the network into areas for scalability.
The backbone area is called area 0
Route: local area  backbone  local area

18. OSPF Each area computes shortest paths.
Backbone routers also accept information from area border routers to compute the shortest path to reach other routers. Then advertise this information to the border routers, who tells routers inside the area – To be able to select the best exit router in an area

19. Inter-AS routing

20. BGP From BGP point of view, three types of networks
Stub network (only one connection to the network)
Multiconnected network (multiple connection, but refuse to transmit traffic)
Transit network (backbone)

21. Internet Inter-AS routing: BGP
BGP (Border Gateway Protocol): the de facto standard
Path Vector protocol:
similar to Distance Vector protocol
each Border Gateway broadcast to neighbors (peers) entire path (i.e, sequence of ASs) to destination
E.g., Gateway X may send its path to dest. Z:
Path (X,Z) = X,Y1,Y2,Y3,…,Z

22. Internet Inter-AS routing: BGP
BGP messages exchanged using TCP.
BGP messages:
OPEN: opens TCP connection to peer and authenticates sender
UPDATE: advertises new path (or withdraws old)
KEEPALIVE keeps connection alive in absence of UPDATES; also ACKs OPEN request
NOTIFICATION: reports errors in previous msg; also used to close connection

23. Internet Inter-AS routing: BGP
Suppose gateway X send its path to peer gateway W
W may or may not select path offered by X
cost, policy (don’t route via competitors AS), loop prevention reasons.
If W selects path advertised by X, then:
Path (W,Z) = W, Path (X,Z)
Note: X can control incoming traffic by controlling its route advertisements to peers:
e.g., don’t want to route traffic to Z  don’t advertise any routes to Z

24. BGP: an example [3210]* [4210] [7610] NLRI=128.186.0.0/16 ASPATH=[10]