1. CS 457 – Lecture 10 Internetworking and IP
Spring 2012

2. The Network layer Network layer Transport layer: TCP, UDP
IP protocol
addressing conventions
datagram format
packet handling conventions
Routing protocols
path selection
RIP, OSPF, BGP
Network
layer
Forwarding table
ICMP protocol
error reporting
router “signaling”
Link layer
Physical layer

3. 32 bit destination IP address
IP datagram format
ver
length
32 bits
data
(variable length,
typically a TCP
or UDP segment)
16-bit identifier
Internet
checksum
time to
live
32 bit source IP address
IP protocol version
number
header length
(bytes)
max number
remaining hops
(decremented at
each router)
for
fragmentation/
reassembly
total datagram
length (bytes)
upper layer protocol
to deliver payload to
head.
len
type of
service
“type” of data
flgs
fragment
offset
upper
layer
32 bit destination IP address
Options (if any)
E.g. timestamp,
record route
taken, specify
list of routers
to visit.

4. IP Fragmentation & Reassembly
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

5. IP Fragmentation and Reassembly
ID =x
Offset =0
Fragflag =0
Length =4000
Fragflag =1
Length =1500
Offset =185
Offset =370
Length =1040
One large datagram becomes
several smaller datagrams
Example
4000 byte datagram
MTU = 1500 bytes
1480 bytes in data field
offset =
1480/8

6. Addressing Already have MAC layer addresses
Ethernet provides addresses
And so do other link layers
But can’t use Ethernet address at IP
Ethernet addresses are flat
Ethernet address assignment is not related to the network topology
Ethernet is not the only link layer!

7. IP Address (IPv4) A unique 32-bit number (i.e., 4B addresses)
Identifies an interface (on a host, on a router, …)
Represented in dotted-quad notation 12 34
158 5

8. Scalability Challenge
Suppose hosts had arbitrary addresses
Then every router would need a lot of information
…to know how to direct packets toward the host
...
...
host
host
host
host
host
host
LAN 1
LAN 2
router
router
router
WAN
WAN
forwarding table

9. Grouping Related Hosts
The Internet is an “inter-network”
Used to connect networks together, not hosts
Needs a way to address a network (i.e., group of hosts)
...
...
host
host
host
host
host
host
LAN 1
LAN 2
router
router
router
WAN
WAN
LAN = Local Area Network
WAN = Wide Area Network

10. Hierarchical Addressing: IP Prefixes
Divided into network & host portions (left and right)
/24 is a 24-bit prefix with 28 addresses
More on Hierarchy (CIDR) in next lectures 12 34
158 5
Network (24 bits)
Host (8 bits)

11. IP Address and 24-bit Subnet Mask12 34
158 5
255
255
255
Mask

12. Scalability Improved Number related hosts from a common subnet
/24 on the left LAN
/24 on the right LAN
...
...
host
host
host
host
host
host
LAN 1
LAN 2
router
router
router
WAN
WAN
/24
/24
forwarding table

13. Easy to Add New Hosts No need to update the routers
E.g., adding a new host on the right
Doesn’t require adding a new forwarding entry
...
...
host
host
host
host
host
host
LAN 1
LAN 2
router
router
router
host
WAN
WAN
/24
/24
forwarding table

14. Avoiding Manual Configuration
Address Resolution Protocol (ARP)
Learn mapping between IP address and MAC address
Dynamic Host Configuration Protocol (DHCP)
End host learns IP address, DNS servers, and gateway
???
...
...
host
host
DNS
host
host
DNS
/24
/24
router
router
router

15. Key Ideas in ARP and DHCP
Broadcasting: when in doubt, shout!
Broadcast query to all hosts in the local-area-network
… when you don’t know how to identify the right one
Caching: remember the past for a while
Store the information you learn to reduce overhead
Remember your own address & other host’s addresses
Soft state: eventually forget the past
Associate a time-to-live field with the information
… and either refresh or discard the information
Key for robustness in the face of unpredictable change

16. Broadcasting Broadcasting: sending to everyone
Special destination address: FF-FF-FF-FF-FF-FF
All adapters on the LAN receive the packet
Delivering a broadcast packet
Easy on a “shared media”
Like shouting in a room – everyone can hear you
E.g., Ethernet, wireless, and satellite links

17. MAC Address vs. IP Address
MAC addresses
Hard-coded in read-only memory when adaptor is built
Like a social security number
Flat name space of 48 bits (e.g., 00-0E-9B-6E-49-76)
Portable, and can stay the same as the host moves
Used to get packet between interfaces on same network
IP addresses
Configured, or learned dynamically
Like a postal mailing address
Hierarchical name space of 32 bits (e.g., )
Not portable, and depends on where the host is attached
Used to get a packet to destination IP subnet