CSS432 Basic Internetworking Textbook Ch3.2

CSS432: Basic Internetworking 1

CSS432 Basic InternetworkingTextbook Ch3.2

Professor: Munehiro Fukuda


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

H3H2H1

Network 2 (Ethernet)

Network 1 (Ethernet)

H6

Network 3 (FDDI)

Network 4(point-to-point)

H7 R3 H8

R1

ETH FDDI

IPIP

ETH

TCP R2

FDDI PPP

IP

R3

PPP ETH

IP

H1

IP

ETH

TCP

H8

Identical frame Identical frame

Identicaldatagram

Identicaldatagram

Identical packet


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

Version HLen TOS Length

Ident Flags Offset

TTL Protocol Checksum

SourceAddr

DestinationAddr

Options (variable) Pad(variable)

0 4 8 16 19 31

Data

preamble dest addr src addr 0x0800 CRC

frame type

Ex. Ethernet


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


Example

H1 R1 R2 R3 H8

ETH IP (1400) FDDI IP (1400) PPP IP (512)

PPP IP (376)

PPP IP (512)

ETH IP (512)

ETH IP (376)

ETH IP (512)

Ident = x Offset = 0

Start of header

0

Rest of header

1400 data bytes


Start of header

1

Rest of header

512 data bytes


Start of header

1

Rest of header

512 data bytes


Start of header

0

Rest of header

376 data bytes


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?


Global Addresses Properties

globally uniquehierarchical: 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

Network Host

7 24

0A:

Network Host

14 16

1 0B:

Network Host

21 8

1 1 0C:


Datagram Forwarding

AlgorithmIf ( datagram’s dest network# == network# of network interface x )

deliver it to the destination host over interface xelse

if ( datagram’s dest network# == network# of a next hop router y)deliver it to the router y

elsedeliver it to its default router

Example

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

R1 R2 R3 R1

R2

R3

H1

H8

Network 4

Network 3

Network 2Network 1

i/f 0 i/f 0

i/f 0

i/f 1

i/f 1 i/f 1


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 172.16.3.0 255.255.255.0 172.16.1.2ip route 172.16.4.0 255.255.255.0 172.16.1.2ip route 172.16.5.0 255.255.255.0 172.16.1.2

Router 2

Router 1

Router 3

172.16.3.0/24

172.16.1.0/24

172.16.4.0/24172.16.2.0/24

172.16.5.0/24

Eth2: 172.16.5.1 Eth1: 172.16.3.1 Eth0: 172.16.3.2

Eth1: 172.16.4.1

Eth0: 172.16.1.2

Eht0: 172.16.1.1

Eth1: 172.16.2.1


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 => 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


ARP Details

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

A X B Y

Response I’m IPb and Phb. You’re IPa and Pha

Reuqest I’m IPa and Pha. You’re IPb. How about Ph?

A X B YIP Ph

IPb Phb

IPa Pha

IP Ph

IPa Pha

IPb Phb

IP Ph

IPb Phb

IP Ph

IPa Pha

IPb ??


ARP Packet Format

TargetHardwareAddr (bytes 2 – 5)

TargetProtocolAddr (bytes 0 – 3)

SourceProtocolAddr (bytes 2 – 3)

Hardware type = 1 ProtocolType = 0x0800

SourceHardwareAddr (bytes 4 – 5)

TargetHardwareAddr (bytes 0 – 1)

SourceProtocolAddr (bytes 0 – 1)

HLen = 48 PLen = 32 Operation

SourceHardwareAddr (bytes 0 – 3)

0 8 16 31

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

preamble dest addr src addr 0x0806 CRC

frame type

Ex. Ethernet


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.

A X B Y

Response You’re IPa and Pha

Request I’m Pha. What’s my IPa?

A X B Y

IP Ph

IPa Pha

IPb Phb

IP Ph

IPa Pha

IP Ph

?? Pha

RARP server

IP Ph

IPa Pha

IPb Phb

RARP server


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.


DHCP

Options

Boot file name

Server host name

Client Hardware addrRouter IP aaddrServer IP addrYour IP addrClient IP addr

FlagsSecsXid

HOPSHLENHtypeOP

BOOTP/DHCPUDP headerdatagramFrame addr

DHCPrelay

DHCPserver

Other network

Host

Broadcast

Unicast

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.


DHCP Client State TransitionINITIALIZE

Fig. 23.4 on p453 of Internetworking with TCP/IP

REQUEST

SELECT

RENEWREBIND

BOUND

Host boots

/ DHCPDISCOVER to all servers

Select offer / DHCPREQUEST to a specific server

Receive DHCPOFFERfrom all servers

Receive DHCPACK from the current server

Lease reaches 50% expiration/ DHCPREQUEST to the current server

DHCPACK

DHCPNACK

Lease reaches 87.5% expiration/ DHCPREQUEST to any server

DHCPACK

DHCPNACKOrLease expires


Two-Step bootstrap Procedure

Diskless Workstation

Step 1A: BOOTP request: BOOT FILE NAME=I want to boot “unix”

Step 1B:BOOTP reply: Server = mercury, BOOT FILE NAME=“/local/var/bootfiles/xncd19r”

Step 2A: TFTP request: request for the image

Step 2B: TFTP reply: image returned

BOOTP server

File server

OS Image:Unix

Windows


Internet Control Message Protocol (ICMP)

frame header

datagram heaader

ICMP header ICMP data

Src R1 R2 R3 RK

RE

Rcv

Error occurred

Mistakenly routed

An error reporting message (ICMP)

ICMP type 0-18

Is Src responsible for this ICMP message?


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


Discussions

How can traceroute be implemented using ICMP (type=11, code=0) messages?


Reviews IP Internet: Protocol stack,

fragmentation/reassembly, IP address, and datagram forwarding

Address translation: ARP, RAPR, and DHCP ICMP

Exercises in Chapter 3Ex. 36 (fragmentation)Ex. 44 (ARP)Ex. 45 (ARP)


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.

Documents

CSS432 Basic Internetworking Textbook Ch3.2