1. CSS432 Basic Internetworking Textbook Ch3.2
Professor: Munehiro Fukuda
CSS432: Basic Internetworking

2. CSS432: Basic Internetworking
IP Internet
Interconnected Collection of Networks
Viewed as a simple logical network
Routers: nodes interconnecting networks
Protocol Stack
IP on all nodes (both hosts and router)
TCP and UDP on top of IP R2 R1 H4 H5 H3 H2 H1
Network 2 (Ethernet)
Network 1 (Ethernet) H6
Network 3 (FDDI)
Network 4
(point-to-point) H7 R3 H8 R1
ETH
FDDI IP
TCP R2
PPP R3 H1 H8
Identical packet
Identical
datagram
Identical
datagram
Identical frame
Identical frame
CSS432: Basic Internetworking

3. CSS432: Basic Internetworking
Service Model
Global addressing
IP address
Best-effort delivery (unreliable service)
Connectionless (datagram-based)
packets are lost
packets are delivered out of order
duplicate copies of a packet are delivered
packets can be delayed for a long time
Datagram format
Version: IPv4/IPv6
HLen: header length in ints
Maximum header length?
TOS: type of service (priority queue in routers)
Length: packet length in bytes
Maximum packet size?
TTL: time to live (#hops)
Protocol: TCP, UDP
Checksum
SourceAddr: source IP address
DestinationAddr: destination IP address
frame type
Ex. Ethernet
preamble
dest addr
src addr
0x0800
CRC V
ersion
HLen
TOS
Length
Ident
Flags
Offset
TTL
Protocol
Checksum
SourceAddr
DestinationAddr
Options (variable)
Pad
(variable) 4 8 16 19 31
Data
CSS432: Basic Internetworking

4. Fragmentation and Reassembly
Each network has some MTU (maximum transmission unit)
To check each interface of your computer:
netstat –i
ifconfig
Strategy
fragment when necessary (MTU < Datagram)
try to avoid fragmentation at source host
re-fragmentation is possible
fragments are self-contained datagrams
use CS-PDU (not cells) for ATM
delay reassembly until destination host
do not recover from lost fragments
CSS432: Basic Internetworking

5. CSS432: Basic Internetworking
Example
Ident
= x
Offset
= 0
Start of header
Rest of header
1400 data bytes
Ident
= x
Offset
= 0
Start of header 1
Rest of header
512 data bytes
= 512
= 1024
376 data bytes
CSS432: Basic Internetworking

6. CSS432: Basic Internetworking
Discussions
How can we detect if a given datagram has lost some fragments?
Who will take care resending a datagram which could not be reassembled previously due to its fragment droppings?
CSS432: Basic Internetworking

7. CSS432: Basic Internetworking
Global Addresses
Properties
globally unique
hierarchical: network + host
Dot Notation
Class A –
( – , , ,
and – reserved)
Class B –
Class C –
Network
Host 7 24 A: 14 16 1 B: 21 8 C:
CSS432: Basic Internetworking

8. CSS432: Basic Internetworking
Datagram Forwarding
Algorithm
If ( datagram’s dest network# == network# of network interface x )
deliver it to the destination host over interface x
else
if ( datagram’s dest network# == network# of a next hop router y)
deliver it to the router y
deliver it to its default router
Example H8
Network 1 H1
Network 2
i/f 0 R3
i/f 1 R1 R2 R3
i/f 1 R1
Network 4
Network#
Next hop 1 R2 2
Interface1 3
Interface0 4
Network#
Next hop 1 R3 2 R1 3
Interface1 4
Interface0
Network#
Next hop 1
Interface0 2 R2 3 4
Interface1
i/f 0
i/f 0 R2
i/f 1
Network 3
CSS432: Basic Internetworking

9. Static Configuration for Cisco Routers
/24
/24
/24
/24
/24
Eth2:
Eth1:
Eth0:
Eth1:
Eth0:
Eht0:
Eth1:
hostname router1 !
interface ethernet 0
ip address
interface ethernet 1
ip address
ip route
ip route
ip route
CSS432: Basic Internetworking

10. CSS432: Basic Internetworking
Address Translation
Map IP addresses into physical addresses
destination host
next hop router
Techniques
encode physical address in host part of IP address
Pha = f( IPa) or IPa =f-1(Pha)
Pha: =>
Problems
Class C has only 8 bits to indicate a host
Ethernet has 48 bits to present a host address
table-based
Resolution through dynamic binding
Address Resolution Protocol
table of IP to physical address bindings
broadcast request if IP address not in table
target machine responds with its physical address
table entries are discarded if not refreshed
CSS432: Basic Internetworking

11. CSS432: Basic Internetworking
ARP Details
Reuqest I’m IPa and Pha. You’re IPb. How about Ph? A X B Y
table entries timeout in about 10 minutes
Update table with source when you are the target, otherwise no need to add an entry
update table if already have an entry IP Ph
IPa
Pha
IPb ?? IP Ph
IPb
Phb
Response I’m IPb and Phb. You’re IPa and Pha A X B Y IP Ph
IPa
Pha
IPb
Phb IP Ph
IPb
Phb
IPa
Pha
CSS432: Basic Internetworking

12. CSS432: Basic Internetworking
ARP Packet Format
An ARP packet is carried in a frame header
HardwareType: type of physical network (e.g., Ethernet)
ProtocolType: type of higher layer protocol (e.g., IP)
HLEN & PLEN: length of physical and protocol addresses
Operation: request or response
Source/Target-Physical/Protocol addresses
frame type
Ex. Ethernet
preamble
dest addr
src addr
0x0806
CRC T
argetHardwareAddr (bytes 2 – 5)
argetProtocolAddr (bytes 0 3)
SourceProtocolAddr (bytes 2
Hardware type = 1
ProtocolT
ype = 0x0800
SourceHardwareAddr (bytes 4
argetHardwareAddr (bytes 0 1)
SourceProtocolAddr (bytes 0
HLen = 48
PLen = 32
Operation
SourceHardwareAddr (bytes 0 8 16 31
CSS432: Basic Internetworking

13. RARP: Reverse Address Resolution Protocol
Use RARP if a client host is diskless workstation
Use the unique MAC address
Ask an RARP server about a client IP.
Works in the same LAN
Retransmit an RARP message after a large delay if it has been lost.
Prepare a primary and secondary server.
Request I’m Pha. What’s my IPa? A X B Y
RARP server IP Ph ??
Pha IP Ph
IPa
Pha
IPb
Phb
Response You’re IPa and Pha A X B Y
RARP server IP Ph
IPa
Pha IP Ph
IPa
Pha
IPb
Phb
CSS432: Basic Internetworking

14. CSS432: Basic Internetworking
RARP Disadvantage
Using a computer’s MAC address which does not allow computers to move to another network
Operating at data-link level which requires direct access to the network hardware
Not working beyond routers, because routers are IP-based but RAPS are not IP.
CSS432: Basic Internetworking

15. CSS432: Basic Internetworking
DHCP
DHCP
relay
server
Other network
Host
Broadcast
Unicast
Options
Boot file name
Server host name
Client Hardware addr
Router IP aaddr
Server IP addr
Your IP addr
Client IP addr
Flags
Secs
Xid
HOPS
HLEN
Htype OP
BOOTP/DHCP
UDP header
datagram
Frame addr
DHCP server:
Works as a centralized repository for IPs
Pools available IP addresses
Hands out one to a client on demand
Accessible by sending a DHCPDISCOVER message to an IP broadcast address
Receives a DHCPDISCOVER from a Relay agent connected to a different network
Advantages:
Works at a user level
Automatic IP configuration
Save IP addresses
Works across networks.
CSS432: Basic Internetworking

16. DHCP Client State Transition
INITIALIZE
Host boots
/ DHCPDISCOVER to all servers
SELECT
DHCPNACK
DHCPNACK Or
Lease expires
Receive DHCPOFFER
from all servers
Lease reaches 87.5% expiration
/ DHCPREQUEST to any server
REBIND
RENEW
Select offer / DHCPREQUEST to a specific server
DHCPACK
REQUEST
DHCPACK
Lease reaches 50% expiration
/ DHCPREQUEST to the current server
Receive DHCPACK from the current server
BOUND
Fig on p453 of Internetworking with TCP/IP
CSS432: Basic Internetworking

17. Two-Step bootstrap Procedure
BOOTP server
Step 1A: BOOTP request: BOOT FILE NAME=I want to boot “unix”
Diskless Workstation
Step 1B:
BOOTP reply: Server = mercury, BOOT FILE NAME=“/local/var/bootfiles/xncd19r”
Step 2A: TFTP request: request for the image
File server
Step 2B: TFTP reply: image returned
OS Image:
Unix
Windows
CSS432: Basic Internetworking

18. Internet Control Message Protocol (ICMP)
An error reporting message (ICMP)
Error occurred
Src R1 R2 R3 RK
Rcv
Is Src responsible for this ICMP message? RE
Mistakenly routed
frame header
datagram heaader
ICMP header
ICMP data
ICMP type 0-18
CSS432: Basic Internetworking

19. CSS432: Basic Internetworking
ICMP Message Types
Type field
Code field
ICMP Message Type
Applications
0 and 8
Echo reply/request
ping 3
Destination unreachable 5
Redirect (change a route) 11
TTL exceeded
Trace route, 1
Fragment reassemble failed
CSS432: Basic Internetworking

20. CSS432: Basic Internetworking
Discussions
How can traceroute be implemented using ICMP (type=11, code=0) messages?
CSS432: Basic Internetworking

21. CSS432: Basic Internetworking
Reviews
IP Internet: Protocol stack, fragmentation/reassembly, IP address, and datagram forwarding
Address translation: ARP, RAPR, and DHCP
ICMP
Exercises in Chapter 3
Ex. 36 (fragmentation)
Ex. 44 (ARP)
Ex. 45 (ARP)
CSS432: Basic Internetworking

22. CSS432: Basic Internetworking
More Exercises (not from Our Textbook)
Q1. (DHCP)
Consider a host that has a disk and uses DHCP to obtain an IP address. If the host stores its address on disk along with the data the lease expires, and then reboots within the lease period, can it use the same address? Why or why not?
Q2. (DHCP)
DHCP mandates a minimum address lease of one hour. Can you imagine a situation in which DHCP’s minium lease causes inconvenience? Explain.
CSS432: Basic Internetworking