1. Computer Communication Networks
Data Transmission, Media
Signal Encoding Techniques
Data Communication Techniques
Data Link Control, ATM
Multiplexing, Switching, Routing
Spread Spectrum, Wireless Networks
Local and Wide Area Networks
Network Layer (Addressing and Sub netting)

2. Understand principles behind network layer services:
Lecture Goals
Understand principles behind network layer services:
Internetworking concepts
The network layer IP
Routing (path selection)

3. network layer services IP Routing principle: path selection
Overview
Internetworking
network layer services IP
Routing principle: path selection
Hierarchical routing
Internet routing protocols reliable transfer
intra-domain
inter-domain

4. The Internetworking Problem
Two nodes communicating across a “network of networks”…
How to transport packets through this heterogeneous infrastructure ? A B

5. The Internetworking Problem
Problems: heterogeneity and scaling
Heterogeneity:
How to interconnect a large number of disparate networks ? (lower layers)
How to support a wide variety of applications ? (upper layers)
Scaling:
How to support a large number of end-nodes and applications in this interconnected network ?

6. The Internet Network layer
Host, router network layer functions:
Transport layer: TCP, UDP
IP protocol
addressing conventions
datagram format
packet handling conventions
Routing protocols
path selection
RIP, OSPF, BGP
Network
layer
routing
table
ICMP protocol
error reporting
router “signaling”
Link layer
physical layer

7. IP Addressing: introduction
IP address: 32-bit identifier for host, router interface
interface: connection between host, router and physical link
router’s typically have multiple interfaces
host may have multiple interfaces
IP addresses associated with interface, not host, router =
223 1 1 1

8. IP Addressing - 1 IP address:
network part (high order bits)
host part (low order bits)
What’s a network ? (from IP address perspective)
device interfaces with same network part of IP address
can physically reach each other without intervening router
LAN
network consisting of 3 IP networks
(for IP addresses starting with 223,
first 24 bits are network address)

9. IP Addressing - 2 How to find the networks?
Detach each interface from router, host
create “islands of isolated networks
Interconnected
system consisting
of six networks

10. IP Addresses
Given notion of “network”, let us re-examine IP addresses:
“class-full” addressing:
class to A
network
host B to 10
network
host to C
110
network
host to D
1110
multicast address
32 bits

11. Some Special IP Addresses
All-1s  All hosts on this net (limited broadcast: don’t forward out of this net)
All-0 host suffix  Network Address (‘0’ means ‘this’)
All-1 host suffix  All hosts on the destination net (directed broadcast).
127.*.*.*  Loopback through IP layer

12. Classful Addressing: IP addressing: CIDR - 1
inefficient use of address space, address space exhaustion
e.g., class B net allocated enough addresses for 65K hosts, even if only 2K hosts in that network

13. CIDR: Classless InterDomain Routing
IP addressing: CIDR - 2
CIDR: Classless InterDomain Routing
network portion of address of arbitrary length
address format: a.b.c.d/x, where x is # bits in network portion of address
network
part
host
/23

14. Subnet Addressing
External routers need to store entries only for the “network ID”
Internal routers & hosts use subnet mask to identify “subnet ID” and route packets between “subnets” within the “network”.
Eg: Mask: => subnet ID = 8 bits with upto 254 hosts/subnet

15. Subnet Addressing (Continued)
Route table lookup:
IF ((Mask[i] & Destination Addr) = =
Destination[i]) Forward to NextHop[i]
Subnet mask can end on any bit.
Mask must have contiguous 1s followed by contiguous zeros. Routers do not support other types of masks.

16. Route Table Lookup: Example
Destination
Mask
Next Hop
Deliver direct

17. IP addresses: how to get one?
Hosts (host portion):
hard-coded by system admin in a file
DHCP: Dynamic Host Configuration Protocol: dynamically get address: “plug-and-play”
host broadcasts “DHCP discover” msg
DHCP server responds with “DHCP offer” msg
host requests IP address: “DHCP request” msg
DHCP server sends address: “DHCP ack” msg

18. IP addresses: how to get one?
Network (network portion):
Get allocated portion of ISP’s address space:
ISP's block /20
Organization /23
Organization /23
Organization /23
… … ….
Organization /23

19. Hierarchical addressing: route aggregation
Hierarchical addressing allows efficient advertisement of routing
information:
Organization 0
/23
Organization 1
“Send me anything
with addresses
beginning
/20”
/23
Organization 2
/23 .
Fly-By-Night-ISP .
Internet
Organization 7
/23
“Send me anything
with addresses
beginning
/16”
ISPs-R-Us

20. Hierarchical addressing: more specific routes
ISPs-R-Us has a more specific route to Organization 1
Organization 0
/23
“Send me anything
with addresses
beginning
/20”
Organization 2
/23 .
Fly-By-Night-ISP .
Organization 7
Internet
/23
“Send me anything
with addresses
beginning /16
or /23”
ISPs-R-Us
Organization 1
/23

21. IP addressing: the last word...
Q: How does an ISP get block of addresses?
A: ICANN: Internet Corporation for Assigned
Names and Numbers
Allocates addresses
Manages DNS
Assigns domain names, resolves disputes

22. Getting a datagram from source to dest. - 1
routing table in A
Dest. Net. next router Nhops
IP datagram:
misc
fields
source
IP addr
dest
data A B E
datagram remains unchanged, as it travels source to destination
addr fields of interest here

23. Getting a datagram from source to dest. - 2
misc
fields
Dest. Net. next router Nhops
data
Starting at A, given IP datagram addressed to B:
look up net. address of B
find B is on same net. as A
link layer will send datagram directly to B inside link-layer frame
B and A are directly connected A B E

24. Getting a datagram from source to dest. - 3
misc
fields
Dest. Net. next router Nhops
data
Starting at A, dest. E:
look up network address of E
E on different network
A, E not directly attached
routing table: next hop router to E is
link layer sends datagram to router inside link-layer frame
datagram arrives at
continued….. A B E

25. Getting a datagram from source to dest. – 4
network router Nhops interface
Dest next
misc
fields
data
Arriving at , destined for
look up network address of E
E on same network as router’s interface
router, E directly attached
link layer sends datagram to inside link-layer frame via interface
datagram arrives at !!! (hooray!) A B E

26. IP Features Connectionless service Addressing Data forwarding
Fragmentation and reassembly
Supports variable size datagrams
Best-effort delivery
Provides only “Send” and “Delivery” services. Error and control messages generated by Internet Control Message Protocol (ICMP)

27. What IP does NOT provide
End-to-end data reliability & flow control (done by TCP or application layer protocols)
Sequencing of packets (like TCP)
Error detection in payload (TCP, UDP or other transport layers)
Error reporting (ICMP)

28. What IP does NOT provide (Continued)
Setting up route tables (RIP, OSPF, BGP etc)
Connection setup (it is connectionless)
Address/Name resolution (ARP, RARP, DNS)
Configuration (BOOTP, DHCP)
Multicast (IGMP, MBONE)

29. IP datagram format data (variable length, typically a TCP
IP protocol version
number
32 bits
total datagram
length (bytes)
header length
(bytes)
head.
len
type of
service
ver
length
for
fragmentation/
reassembly
“type” of data
fragment
offset
16-bit identifier
flgs
max number
remaining hops
(decremented at
each router)
time to
live
upper
layer
Internet
checksum
32 bit source IP address
32 bit destination IP address
upper layer protocol
to deliver payload to
Options (if any)
E.g. timestamp,
record route
taken, pecify
list of routers
to visit.
data
(variable length,
typically a TCP
or UDP segment)

30. IP Fragmentation & Reassembly - 1
network links have MTU (max.transfer size) - largest possible link-level frame.
different link types, different MTUs
large IP datagram divided (“fragmented”) within net
one datagram becomes several datagrams
“reassembled” only at final destination
IP header bits used to identify, order related fragments
fragmentation:
in: one large datagram
out: 3 smaller datagrams
reassembly

31. IP Fragmentation and Reassembly - 2ID =x
offset =0
fragflag
length
=4000
One large datagram becomes
several smaller datagrams ID =x
offset =0
fragflag =1
length
=1500 ID =x
offset
=1480
fragflag =1
length
=1500 ID =x
offset
=2960
fragflag =0
length
=1040

32. Summary Internet architectural principles IP addressing and header
Fragmentation/Reassembly, Path MTU discovery

33. Q&A ?