Transcript Basic Internetworking

CSS432 Basic Internetworking
Textbook Ch 3.2
Instructor: Joe McCarthy
(based on Prof. Fukuda’s slides)
CSS432: Basic Internetworking
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You
are
here
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IP: Internet Protocol

Interconnected Collection of Networks





Bridged networks (link layer): homogenous
IP networks (network layer): heterogeneous
Viewed as a single logical network
Routers: nodes interconnecting networks
Protocol Stack

IP on all nodes (both hosts and router)
 TCP and UDP on top of IP
H1
TCP
R1
IP
IP
ETH
H8
Identical packet
ETH
Identical frame
R2
Identical
datagram
FDDI
FDDI
IP
R3
Identical
datagram
PPP
PPP
IP
TCP
IP
ETH
ETH
Identical frame
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Service Model

Global addressing


IP address (e.g., 128.95.155.134)
Best-effort delivery (unreliable service)




Connectionless (datagram-based)
Packets may be lost, reordered, duplicated, delayed
Lowest common denominator: “run over anything”
Simplifies routing (& routers)
IP Packet format:
Ex. Ethernet preamble









dest addr
src addr
0x0800
CRC
0
4
Version
8
HLen
Minimum / maximum header length?
TOS: type of service (priority queue in routers)
Length: packet length in bytes


frame type
Version: IPv4/IPv6
HLen: header length in 32-bit words

http://bpastudio.csudh.edu/fac/lpress/471/hout/netech/stackinaction.htm
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TOS
Minimum / maximum packet size?
TTL: “time” to live (# hops); default: 64
Protocol: TCP, UDP
Checksum (header, 1’s complement)
SourceAddr: source IP address
DestinationAddr: destination IP address
Options: optional & variable
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Length
Ident
TTL
19
Flags
Protocol
Offset
Checksum
SourceAddr
DestinationAddr
Options (variable)
Pad
(variable)
Data
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Fragmentation & Reassembly

Each network has some MTU
(Maximum Transmission Unit)

To check each interface of your computer (Linux/Mac OS X):



netstat –i
[/sbin/]ifconfig
Strategy






Fragment / split when necessary (MTU < Datagram size)
try to avoid fragmentation at source host
Re-fragmentation is possible
Each fragment is a self-contained datagram
Where should reassembly occur?
What about lost fragments?
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Fragmentation & Reassembly

Each physical network has its MTU
(Maximum Transmission Unit)

To check each interface of your computer (Linux/Mac OS X):



netstat –i
[/sbin/]ifconfig
Strategy






Fragment / split when necessary (MTU < Datagram size)
try to avoid fragmentation at source host
Re-fragmentation is possible
Each fragment is a self-contained datagram
Reassemble at destination
Lost fragment(s)  discard packet
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Fragmentation Example
Send a 1400 byte segment from H5 to H8
MTUs:
• 802.11:
• Ethernet:
• PPP: 532 bytes
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Fragmentation Example
Send a 1400 byte segment from H5 to H8
MTUs:
• 802.11: 1500 bytes
• Ethernet: 1500 bytes
• PPP: 532 bytes
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Fragmentation Example
Send a 1400 byte segment from H5 to H8
376
(a)
376
(b)
MTUs:
• 802.11: 1500 bytes
• Ethernet: 1500 bytes
• PPP: 532 bytes
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Fragmentation Example
Send a 1400 byte segment from H5 to H8
376
(a)
376
(b)
MTUs:
• 802.11: 1500 bytes
• Ethernet: 1500 bytes
• PPP: 532 bytes
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Global Addresses

Properties
 globally
unique
 hierarchical: network + host

Dotted Decimal Notation

Class A
 1.0.0.1 – 126.255.255.254

A:
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Network
Host
(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
B:
192.0.0.1 – 223.255.255.254
C:

0
7
1 0
1 1 0
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16
Network
Host
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8
Network
Host
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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
else
deliver it to its default router

Example
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Static Configuration for
Cisco Routers
172.16.5.0/24
Eth2: 172.16.5.1
Router 2
Eth0: 172.16.1.2
172.16.3.0/24
Eth1: 172.16.3.1
Eth0: 172.16.3.2
Router 3
Eth1: 172.16.4.1
172.16.2.0/24
Eth1: 172.16.2.1
Router 1
hostname router1
Eht0: 172.16.1.1
!
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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172.16.4.0/24
172.16.1.0/24
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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)


Problems



Pha: 0010 0001 0100 1001 => IPa: 128.96.33.81
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 (ARP)




table of IP  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
Request (from A): I’m IPa and Pha. You’re IPb. How about Phb?

A
X
B
Y
IP
Ph
IP
Ph
IPa
Pha
IPb
Phb
IPb
??

Response (from B): I’m IPb and Phb. You’re IPa and Pha
A
X
B

table entries timeout after 15
minutes
Update table with source
info when you are the target,
otherwise no need to add an
entry
update table if already have
an entry
Y
IP
Ph
IP
Ph
IPa
Pha
IPb
Phb
IPb
Phb
IPa
Pha
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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
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CRC
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RARP: Reverse Address
Resolution Protocol
Request I’m Pha. What’s my IPa?
A
X
IP
Ph
??
Pha

B
Y


RARP server
IP
Ph
IPa
Pha

IPb
Phb

Response You’re IPa and Pha
A
X
IP
Ph
IPa
Pha
B
Y
RARP server
IP
Ph
IPa
Pha
IPb
Phb

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 & secondary
server.
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DHCP: Dynamic Host Configuration Protocol
Broadcast
DHCP
relay
Host
Other network
Unicast
Frame addr


DHCP
server
datagram
UDP header
IP addresses must be configured for each network
DHCP server:

Centralized repository for available IP addresses
 Pre-assigned or drawn from a pool
 Accessible by sending a DHCPDISCOVER message
to an IP broadcast address (255.255.255.255)
 May receive a DHCPDISCOVER from a Relay agent
connected to a different network

Client:



Receives a leased IP address
May renew the lease periodically
When lease expires, IP address can be reassigned
(by DHCP server)
CSS432: Basic Internetworking
BOOTP/DHCP
OP
Htype
HLEN HOPS
Xid
Secs
Flags
Client IP addr
Your IP addr
Server IP addr
Router IP aaddr
Client Hardware addr
Server host name
Boot file name
Options
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DHCP Client State Transition
Host boots
INITIALIZE
/ DHCPDISCOVER to all servers
SELECT
Receive DHCPOFFER
from all servers
Select offer / DHCPREQUEST to a specific server
DHCPNACK
Or
Lease expires
REBIND
DHCPNACK
Lease reaches 87.5% expiration
/ DHCPREQUEST to any server
RENEW
DHCPACK
DHCPACK
REQUEST
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 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
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Internet Control Message
Protocol
http://www.borella.net/content/MITP432/ICMP/img4.html
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ICMP Message Types
Type field
Code field ICMP Message Type
0 and 8
Echo reply/request
3
Destination unreachable
5
Redirect (change a route)
11
0
TTL exceeded
11
1
Fragment reassemble failed
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Applications
ping
Trace route,
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ICMP Message Header
http://nmap.org/book/tcpip-ref.html
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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
ICMP type 0-18
ICMP header
ICMP data
ICMP
IP
datagram heaader
Data Link
frame header
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
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)
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Ex 36
36. Suppose a TCP message that contains 1024 bytes of data
and 20 bytes of TCP header is passed to IP for delivery across
two networks interconnected by a router (i.e., it travels from
the source host to a router to the destination host).
The first network has an MTU of 1024 bytes; the second has a MTU of 576 bytes.
Each network’s MTU gives the size of the largest IP datagram
that can be carried in a link-layer frame.
Give the sizes and offsets of the sequence of fragments delivered to the network
layer at the destination host. Assume all IP headers are 20 bytes.
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Ex 44
44. Suppose hosts A and B have been assigned the same IP address
on the same Ethernet, on which ARP is used. B starts up after A.
What will happen to A’s existing connections?
Explain how “self-ARP” (querying the network on start-up
for one’s own IP address) might help with this problem.
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Ex 45
45. Suppose an IP implementation adheres literally to the following algorithm
on receipt of a packet, P, destined for IP address D:
if ( Ethernet address for D is in ARP cache )
send P
else
send out an ARP Query for D
put P into a queue until the response comes back
(a) If the IP layer receives a burst of packets destined for D,
how might this algorithm waste resources unnecessarily?
(b) Sketch an improved version.
(c) Suppose we simply drop P, after sending out a query,
when cache lookup fails. How would this behave?
(Some early ARP implementations allegedly did this)
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
More Exercises (not from our textbook)
 Q1.

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)
DHCP mandates a minimum address lease of one
hour. Can you imagine a situation in which DHCP’s
minimum lease causes inconvenience? Explain.
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