1. Scaling the Network Chapters 3-4 Part 1
Networking
CS 3470, Section 1

2. Network Layer: Introduction
How does it all fit?
Data link layer
Delivers frames between two physically connected hosts
Network layer
Delivery of packets from machine to machine
Hop by hop between hosts and routers
Transport layer
Between two end hosts

3. Network Layer Functions
Addressing
Globally unique address for each routable device
Logical address, unlike MAC address
Assigned by network operator
Need to map to MAC address
Forwarding
From input port to appropriate output port in a router
Routing
Which path to use to forward packets from src to dest

4. Today Finish network forwarding Internet Protocol (IP)
Start addressing

5. Network Layer Forwarding
Forwarding input port to appropriate output port in a router
We already talked about forwarding over the network layer
Datagram / Connectionless
Virtual Circuit Switching
Which one is used with IP?

6. Datagram vs Virtual Circuit
Datagram (example: IP)
Data exchange among computers
“Elastic” service, no strict timing req.
“Smart” end systems (computers)
Can adapt, perform control, error recovery
Simple inside network, complexity at “edge”
Many link types
Different characteristics
Uniform service difficult

7. Datagram vs Virtual Circuit
Virtual Circuit (Example: ATM)
Evolved from telephony
Human conversation:
Strict timing, reliability requirements
Need for guaranteed service
Quality of service
“Dumb” end systems
Telephones
Complexity inside network

8. The University of Adelaide, School of Computer Science
27 April 2018
Internetworking
What is an internetwork?
An arbitrary collection of networks interconnected to provide some sort of host-host packet delivery service
A simple internetwork where H represents hosts and R represents routers
Chapter 2 — Instructions: Language of the Computer

9. The University of Adelaide, School of Computer Science
27 April 2018 IP
IP stands for Internet Protocol
Key tool used today to build scalable, heterogeneous internetworks
It runs on all the nodes in a collection of networks and defines the infrastructure that allows these nodes and networks to function as a single logical internetwork
Chapter 2 — Instructions: Language of the Computer

10. IP A IP router is a gateway from one network to another
Can interface with many network types
Ethernet
Token Ring
FDDI
Sonet
Wireless
... more

11. [TCP/UDP]/IP
Two well-known transport level protocols that run on top of IP at the hosts are UDP and TCP
TCP
Connection-based protocol
Error recovery
Packets arrive in order
UDP
Connectionless protocol
No error recovery
Packets can arrive in any order, or not at all

12. Internetworking A simple internetwork showing the protocol layers
IP connects them all!

13. The University of Adelaide, School of Computer Science
27 April 2018
IP Service Model
Packet Delivery Model
Connectionless model for data delivery
Best-effort delivery (unreliable service)
packets are lost
packets are delivered out of order
duplicate copies of a packet are delivered
packets can be delayed for a long time
Global Addressing Scheme
Provides a way to identify all hosts in the network
Chapter 2 — Instructions: Language of the Computer

14. The University of Adelaide, School of Computer Science
27 April 2018
Packet Format
Version (4): currently 4
Hlen (4): number of 32-bit words in header
TOS (8): type of service (not widely used)
Length (16): number of bytes in this datagram
Chapter 2 — Instructions: Language of the Computer

15. The University of Adelaide, School of Computer Science
27 April 2018
Packet Format
Ident (16): used by fragmentation
Flags/Offset (16): used by fragmentation
TTL (8): number of hops this datagram has traveled
Protocol (8): (TCP=6, UDP=17)
Checksum (16): of the header only
DestAddr & SrcAddr (32)
TTL commonly set to 64.
Chapter 2 — Instructions: Language of the Computer

16. IP Fragmentation and Reassembly
The University of Adelaide, School of Computer Science
27 April 2018
IP Fragmentation and Reassembly
Each network has some MTU (Maximum Transmission Unit)
Ethernet (1500 bytes), FDDI (4500 bytes)
Strategy
Fragmentation occurs in a router when it receives a datagram that it wants to forward over a network which has (MTU < datagram)
Reassembly is done at the receiving host
All the fragments carry the same identifier in the Ident field
Fragments are self-contained datagrams
IP does not recover from missing fragments
Chapter 2 — Instructions: Language of the Computer

17. IP Fragmentation and Reassembly
The University of Adelaide, School of Computer Science
27 April 2018
IP Fragmentation and Reassembly
IP datagrams traversing the sequence of physical networks
Chapter 2 — Instructions: Language of the Computer

18. IP Fragmentation and Reassembly
The University of Adelaide, School of Computer Science
27 April 2018
IP Fragmentation and Reassembly
Header fields used in IP fragmentation. (a) Unfragmented packet; (b) fragmented packets.
Chapter 2 — Instructions: Language of the Computer

19. IP Addressing Globally unique logical address for a host
Address resolution
Logical to physical address mapping
Is possible to address any host in the network
Even if on different physical network
There are so many networking technologies such as Ethernet, Token Ring etc. No single technology is best for all needs. Just like traffic control example, Ethernet may be good in some cases and Token Ring in some. But the problem is these technologies may not be compatible and may not be able to communicate with each other. They may not even be able to address each other. The job of IP is to interconnect different network technologies and provide universal connectivity in a heterogeneous world.
IP creates the illusion of a single uniform network. There is no such physical network. Actual network consists of many networks with different technologies. IP overlays a virtual network on top of physical network and hides the details of underlying network from users and application programs.
It assigns a globally unique logical address to each host. This makes it possible to address any host in the network even if it is on a different physical network. Also, this logical addressing helps in finding a path to a host based on its address. However, it has to map from these logical addresses to physical addresses to actually deliver the packet. We will see how this is done.

20. IP Addressing A 32-bit number that uniquely identifies a location
Written using dotted decimal notation
Common form:
Binary representation:
Two-level hierarchy: network id and host id
Network IDs administered by Internet Assigned Number Authority (IANA)
Host IDs administered locally
IP address is a 32-bit number. We can use an IP address to uniquely identify a location. Normally we write an IP address as dotted decimals instead of integer values. IP addresses are organized in a two-level hierarchy, that is, each IP address has two parts: network id part and host id part. A network gets the network id part from a central authority (IANA), individual networks then allocate the host id part to their own machines.

21. IP Addressing IP address is assigned to each network interface (NIC)
Routers connect two or more physical networks
Each interface has its own address
Multi-homed host
A host having multiple connections to Internet
Multiple addresses identify the same host
Does not forward packets between its interfaces
IP address is a 32-bit, 4-byte identifier for an interface of a host. An interface is the connection between host and the physical link. This is commonly referred to as network interface card (NIC). Though we normally refer to IP address of a machine, actually the address is associated with the interface, not the machine.
Routers typically have multiple interfaces since they connect to more than one network and transfer packets between networks. Some hosts may have multiple interfaces but they do not route packets from one interface to the other.

22. IP Addressing
Classful addressing scheme separates groups of addresses into classes
Class A
8 bits used for network (256)
24 bits used for hosts and network devices (16,777,216)
Binary address starts with 0
Class B
16 bits for networks (65,536)
16 bits for hosts and network devices (65,536)
binary address starts with 10
Class C
24 bits for the network (16,777,216)
8 bits for the host (256)
Binary address starts with 110

23. IP “Classful” Addressing Scheme
Three unicast address classes: A, B, and C
One multicast: class D
network
host 10
110
1110
multicast address A B C D
class to to to to
32 bits
In the early days of the Internet development, a so-called classful addressing scheme was used on the Internet.
In classful addressing, we can determine the class and hence the network id from the first few bits of the address. The network part can only be 8, 16 or 24 bits. Depending on the how big a network is and will be, a suitable class network id is allotted for that network.

24. Classless Inter-Domain Routing
Classful addressing scheme wasteful
IP address space exhaustion
Class B net allocated enough for 65K hosts
Even if only 2K hosts in that network
Solution: Classless Inter Domain Routing (CIDR)
Eliminate class distinction
No A,B,C
Keep multicast class D
Good thing about classful address is that we can separate the address into network part and host part by inspecting the address alone. But it wastes address space. Suppose we want to assign a network id to a network of size 2000 hosts. We can not assign class C network id, because in class C host part is only 8 bits and so we can not have more than 256 hosts in a class C network. So we will be forced to assign a class B address with 16 bits for host part that can have around (216). So we end up wasting addresses.
Allocating more class C address blocks is also not a good choice, as each class C address block will occupy one routing table entry, which will inflate the routing table, which forwarding lookup longer.
This might have been ok if we had lots of address space. Due to rapid growth of internet, the IP address space was getting exhausted quickly. So classless inter domain routing (CIDR) is proposed to address this problem. As the name suggests, under CIDR there is no class distinction.

25. Classless Addressing Addresses allocated in contiguous blocks
Number of addresses assigned always power of 2
Network portion of address is of arbitrary length
Address format: a.b.c.d/x
x is number of bits in network portion of address
Unlike in classfull addressing, under classless addressing, the length of the network portion of an address can be arbitrary. Consequently, it is not possible to figure out the network part from the address alone. We need to supply an additional information (network address mask) for that purpose. Now going back to the previous example, by allotting 11 bits for the host portion and 21 bits for the network portion, we can accommodate 2000 hosts without wasting much address space.
network
part
host
/23

26. IP Addressing first 24 bits are network address LAN 223.1.1.1
LAN
Here is an example to illustrate how to decide who belongs to which network. Suppose that first 24 bits are the network part. Then these four machines are on the same network These three are on and similarly these three
Here router has three interfaces and belongs to three networks. All the other machines are only on a single network.

27. IP Addressing Interconnected system consisting of six networks
first 24 bits are network address
Interconnected
system consisting
of six networks
Here is another example to illustrate IP addressing. Once again let us assume that first 24 bits correspond to network part. How many networks are there? Six networks. What happens if the network part is only 16 bits. This whole thing belongs to a single network as per IP addressing. It is important to remember that IP addressing is logical and it is not necessary that they all belong to same physical network.

28. Special IP Addresses Network address: host id = all 0’s
Directed broadcast address: host id = all 1’s
Local broadcast address: all 1’s
Local host address (this computer): all 0’s
Loopback address
network id = 127, any host id (e.g )
When a packet is sent to a network’s directed broadcast address, a single copy of the packet travels across the internet until it reaches the specified network. The packet is then delivered to all hosts on the network. Local broadcast is limited to a local physical network and is used during system startup by a computer that does not yet know the network number. Address of all 0’s is used to refer to this computer when using startup protocols to figure out its own address. Loopback address is used for testing network applications without sending packets into the network.

29. Private IP Addresses Some addresses are not globally routable
IP packets created by these addresses cannot be transmitted into the public domain
Commonly used for home, office, and enterprise LANS

30. Private IP Addresses Address Range CIDR Number of Addresses– /8
16,777,216 –
/12
1,048,576 –
/16
65,535

31. Private IP addresses
Router uses Network Address Translation (NAT) to send IP packets from private IP addresses onto public networks
Router places it’s own IP address as destination
Maintains table, knows which host to route addresses
Router keeps translation table

32. Address Resolution IP address is virtual
Not understood by underlying 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.

33. ARP A router has to know where to deliver packets on the local network
ARP is used to discover MAC addresses based on IP addresses
arp who-has tell node31.ceee.lab
arp reply is-at 00:60:08:ce:9d:3b
arp who-has node31.ceee.lab tell
arp reply node31.ceee.lab is-at 00:02:3f:b4:cd:87

34. ARP Cache Each computer maintains a cache table Exchanges ARP messages
IP address  hardware address mapping
Only about computers on the same network
Try out “/usr/sbin/arp –a” command
Exchanges ARP messages
To resolve IP addresses with unknown hardware addresses
Encapsulated in DLL frame (e.g., Ethernet data frame)
Let’s start the study of ARP by first introducing the concept of ARP cache. ARP cache is essentially a mapping table between IP address and the corresponding MAC address. In the simplest form, it is maintained by each individual machine. It is important to note that, the ARP cache only contains the IP addresses on the same network (and the similarly, ARP is only used to translate IP address on the same network to its corresponding MAC address.
When a host needs to translate an IP address to the corresponding MAC address, it firsts check the ARP cache to see if the table contains the mapping, if it does, then the work is done. Otherwise, the machine needs to use the ARP protocol to find out the corresponding MAC address.

35. 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?
What info should the request message contain?
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.

36. 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.

37. ARP Example ARP broadcast by W requesting hardware address of Y
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.