1. Scaling the Network: The Internet Protocol
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. Today Networking forwarding Internet Protocol (IP)
IP Classful Address Scheme
ARP!

4. 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

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
Complexity inside network

8. The University of Adelaide, School of Computer Science
10 June 2018
Internetworking
What is an internetwork?
An arbitrary collection of networks interconnected to provide some sort of host-host packet delivery service
Chapter 2 — Instructions: Language of the Computer

9. The University of Adelaide, School of Computer Science
10 June 2018 IP
IP stands for Internet Protocol
Key tool used today to build scalable, heterogeneous internetworks
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
PPP
Token Ring
802.11
... 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
10 June 2018
IP Service Model
Packet Delivery Model
Connectionless model for data delivery
Best-effort delivery (unreliable service)
What bad stuff can happen?
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
10 June 2018
Packet Format
Chapter 2 — Instructions: Language of the Computer

15. IP Fragmentation and Reassembly
The University of Adelaide, School of Computer Science
10 June 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

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

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

18. IP Addressing Globally unique logical address for a host
Address resolution
Logical to physical address mapping
What was the physical address?
Is possible to address any host in the network
Even if on different physical network

19. 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

20. 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

21. 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

22. 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.

23. 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.

24. 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

25. 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
Encapsulated in Ethernet data frame

26. 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?

27. ARP Example arp who-has 192.168.10.1 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

28. 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

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