CSS432 Basic Internetworking Textbook Ch3.2

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CSS432 Basic InternetworkingTextbook Ch3.2

• Professor Munehiro Fukuda

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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

Identical packet
Identical datagram
Identical datagram
Identical frame
Identical frame
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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

frame type
Ex. Ethernet
preamble
dest addr
src addr
0x0800
CRC
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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 lt 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

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Example
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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?

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Global Addresses

• Properties
• globally unique
• hierarchical network host
• Dot Notation
• Class A
• 1.0.0.1 126.255.255.254
• (0.0.0.0 0.255.255.255, 1.0.0.0,
  126.255.255.255,
• and 127.0.0.0 127.255.255.255 reserved)
• Class B
• 128.0.0.1 191.255.255.254
• Class C
• 192.0.0.1 223.255.255.254

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Datagram Forwarding

• Algorithm
• If ( datagrams dest network network of
  network interface x )
• deliver it to the destination host over
  interface x
• else
• if ( datagrams dest network network of a
  next hop router y)
• deliver it to the router y
• else
• 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 Interface0
2 R2
3 R2
4 Interface1
Network Next hop
1 R3
2 R1
3 Interface1
4 Interface0
Network Next hop
1 R2
2 Interface1
3 Interface0
4 R2
i/f 0
i/f 0
R2
i/f 1
Network 3
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Static Configuration for Cisco Routers
hostname router1 ! interface ethernet 0 ip
address 172.16.1.1 255.255.255.0 ! interface
ethernet 1 ip address 172.16.2.1
255.255.255.0 ! ip route 172.16.3.0 255.255.255.0
172.16.1.2 ip route 172.16.4.0 255.255.255.0
172.16.1.2 ip route 172.16.5.0 255.255.255.0
172.16.1.2
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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 0010 0001 0100 1001 gt 128.96.33.81
• 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

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ARP Details
Reuqest Im IPa and Pha. Youre 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
IPb Phb

IP Ph
IPa Pha
IPb ??
Response Im IPb and Phb. Youre IPa and Pha
A
X
B
Y
IP Ph
IPb Phb
IPa Pha
IP Ph
IPa Pha
IPb Phb
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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
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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 Im Pha. Whats my IPa?
A
X
B
Y
RARP server
IP Ph
?? Pha
IP Ph
IPa Pha
IPb Phb
Response Youre IPa and Pha
A
X
B
Y
RARP server
IP Ph
IPa Pha
IP Ph
IPa Pha
IPb Phb
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RARP Disadvantage

• Using a computers 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.

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DHCP

• 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.

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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. 23.4 on p453 of Internetworking with TCP/IP
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Two-Step bootstrap Procedure
BOOTP server
Step 1A BOOTP request BOOT FILE NAMEI 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
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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
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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 0 TTL exceeded Trace route,
11 1 Fragment reassemble failed
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Discussions

• How can traceroute be implemented using ICMP
  (type11, code0) messages?

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• Reviews
• IP Internet Protocol stack, fragmentation/reassem
  bly, 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)

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• 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 DHCPs
  minium lease causes inconvenience? Explain.