Download Lesson 11

Internetwork Protocols
Lesson 11
NETS2150/2850
http://www.ug.cs.usyd.edu.au/~nets2150/
School of Information Technologies
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Lesson Outcomes
The needs for internetworking and not
standalone network!
Design issues in a connection-less model
Understand the intricacies of IP addressing
What’s missing in IPv4?
The importance of ICMP, the
companion protocol of IP
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Internetworking Terms
Communications Network
Facility that provides data transfer service
An internet
Collection of communications networks
interconnected by routers
The Internet - note upper case I
The global collection of thousands of individual
machines and networks
An intranet
Corporate internet operating within the organization
Uses Internet (TCP/IP) technology to deliver
documents and resources
Can be isolated internet, or can have links to the
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Internet
The Internet/Network layer
Host, router network layer functions:
Transport layer: TCP, UDP
Network
layer
IP protocol
•addressing conventions
•packet format
•packet handling conventions
Routing protocols
•path selection
•RIP, OSPF
routing
table
ICMP protocol
•error reporting
•router “signalling”
Data link layer
physical layer
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Architectural Approaches
Mode of Operation:
Connection oriented
Connectionless
application
application
transport
transport
network
network
data
link
data
link
physical
physical
application
application
transport
transport
network
network
data
link
data
link
physical
physical
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Connection Oriented
Assume that each network is connection
oriented
Intermediate System (IS1) connect two
or more networks
Logical connection set up between ESs
• Concatenation of logical connections across
networks
Individual network virtual circuits joined by
IS
1: Bridges and routers are examples of ISs.
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Connectionless Operation
Corresponds to datagram mechanism in
packet switched network
Each NPDU treated separately
Network layer protocol common to all ESs
and routers
Known generically as the internet protocol
Internet Protocol (IP)
One such internet protocol was developed for
ARPANET (Advanced Research Projects Agency Network )
RFC 791
Lower layer protocol needed to access
particular network
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Connectionless Internetworking
Pros:
Flexibility
Robust
No connection setup overhead
Cons:
Unreliable, not guaranteed delivery
Not guaranteed order of delivery
• Packets can take different routes
Reliability is responsibility of next layer up (e.g.
TCP)
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Ordered Delivery
PDUs may arrive out of order
Different paths through network
But, PDU order must be maintained
Number PDUs sequentially
Easy to reorder received PDUs
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IP Operation
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IP Design Issues
Routing
Datagram lifetime
Fragmentation and re-assembly
Error control
Flow control
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Routing
End systems and routers maintain routing tables
Indicate next router to which packet should be sent,
for all possible destination network
Could be static
• But, may contain alternative routes
Or Dynamic
• Flexible response to congestion and errors
Source routing
Source specifies route as sequential list of routers to
be followed because of:
• Security
• Priority
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Datagram Lifetime
Datagram could loop indefinitely
Consumes resources
Transport protocol may need upper bound on
datagram life
Datagram marked with lifetime
TTL field in IP
Decrement TTL on passing through each
router
Once lifetime expires, datagram discarded
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Fragmentation and Reassembly
Network links have MTU
(max. transmission unit) largest possible data linklevel frame
Different link types,
different MTUs
ATM  53 octets
Ethernet  1518 octets
Pros:
More efficient error
control
fragmentation:
in: one large packet
out: 3 smaller packets
reassembly
• Smaller retransmission
Fairer
• Prevent monopolising
Smaller buffers at rcvr
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Disadvantages of Fragmentation
Make PDUs as large as possible because
PDU contains some control information
Smaller block, larger overhead to data ratio
PDU arrival generates interrupt to CPU
Waste CPU time
Smaller blocks, more interrupts!
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Fragmentation and Reassembly
When to re-assemble??
At destination?
• Results in packets getting smaller as data
traverses internet
Intermediate re-assembly?
• Need large buffers at routers
• Buffers may fill with fragments
• All fragments must go through same router
– Inhibits dynamic routing
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IP Fragmentation (1)
IP reassembles at destination only
It uses these fields in header
Identifies end system originated packet
• Source and destination address
• Protocol layer generating data (e.g. TCP)
• Identification supplied by IP layer
Total Length
• Length of packet in octets
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IP Fragmentation (2)
Offset
• Position of fragment of user data in original
packet
• In multiples of 64-bit (8-octet) units
More fragment flag
• Indicates that this is not the last fragment
• 0 – last or the only packet
• 1 – not last
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IP Fragmentation and Reassembly
Example
4000-octet
packet (with 20octet header)
MTU = 1500
octets
Data in each is
1480 octets
Fragments =
3980/1480 = 3
length ID Moreflag offset
=4000 =x
=0
=0
One large packet becomes
3 smaller packets
length ID Moreflag offset
=1500 =x
=1
=0
length ID Moreflag offset
=1500 =x
=1
=185
length ID Moreflag offset
=1040 =x
=0
=370
Offset in 1st fragment = 0, 2nd fragment = (1480/8)
= 185 and 3rd fragment = (185+185) = 370
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Dealing with Failure
Reassembly may fail if some fragments get
lost
So, need to detect failure
Reassembly time out
Assigned to first fragment to arrive
If timeout expires before all fragments arrive,
discard partial data
Use packet lifetime (time to live)
If TTL runs out, kill partial data
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Error Control
IP do NOT guarantee delivery
IP uses checksum for error detection
Router should attempt to inform source
if packet discarded
e.g. for TTL expiring or destination
unreachable
But, datagram identification needed
Handled by ICMP protocol (see later)
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IP Checksum – 2 steps
1. Add the 16-bit values up. Each time a
carry-out (17th bit) is produced, swing
that bit around and add it back into
the lsb
2. Once all the values are added in this
manner, invert all the bits in the result
- called its “one's complement”
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Example:
msb
1000 0110 0101 1110
1010 1100 0110 0000
0111 0001 0010 1010
1000 0001 1011 0101
lsb
First, we add the 16-bit values 2 at a time:
1000 0110 0101 1110 First 16-bit value
+ 1010 1100 0110 0000 Second 16-bit value
------------------------1 0011 0010 1011 1110 Produced a carry-out, which gets added
+ \----------------------> 1 back into lsb
---------------------------0011 0010 1011 1111
+ 0111 0001 0010 1010 Third 16-bit value
---------------------------0 1010 0011 1110 1001 No carry to swing around (**)
+ 1000 0001 1011 0101 Fourth 16-bit value
---------------------------1 0010 0101 1001 1110 Produced a carry-out, which gets added
+ \----------------------> 1 back into lsb
---------------------------0010 0101 1001 1111 Our sum
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Example (Cont.)
Then we have to take the one's complement of the
sum.
We do this by simply inverting all the bits in the final
result from above:
0010 0101 1001 1111
Our sum
1101 1010 0110 0000
The "one's complement"
So the checksum stored in the header would be
1101 1010 0110 0000.
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Flow Control
Allows routers and/or stations to limit
rate of incoming data
Difficult in connectionless systems
Not done here, left to higher layer (i.e.
transport)
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Internet Protocol (IP) Version 4
Part of TCP/IP
Specified in RFC 791
Will (eventually) be replaced by IPv6
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IP Services
Send primitive
Request transmission of data unit onto the
network
Deliver primitive
Notify user of arrival of data unit from the
network
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IPv4 Header
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Header Fields (1)
Version (4 bits)
Currently 4
Internet header length (4 bits)
In 32-bit units
Including options
Type of service (before) – Differentiated
Service (now) (8 bits)
Allows classification of packets
Total length (16 bits)
in octets
Header plus data
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Header Fields (2)
Identification (16 bits)
Sequence number
Used with addresses and user protocol to identify
packet uniquely
Flags (3 bits)
More bit (1 bit)
Don’t fragment (1 bit)
Fragmentation offset (13 bits)
Time to live (8 bits)
Protocol (8 bits)
Next higher layer to receive data field at
destination
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Header Fields (3)
Header checksum (16 bits)
Reverified and recomputed at each router
Uses 16-bit ones complement sum
Source address (32 bits)
Destination address (32 bits)
Options (variable)
See next slide
Padding (variable)
To fill to multiple of 32 bits long
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Options
Security Label
Allows secured handling of packets
Source routing
A list of router addresses specifies a route to
follow
Route recording
Records route taken by a packet
Stream identification
Allows special handling of streaming traffic
Timestamping
Intermediate routers add timestamp on packet
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Data Field
Carries user data from next layer up
Integer multiple of 8 bits long (octet)
Max length of packet (header plus data)
65,535 octets
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IP Addressing: Introduction
IP address: 32-bit identifier
for host, router interface
interface: connection between
host/router and physical link
router’s have multiple
interfaces
IP addresses associated with
each interface
223.1.1.1
223.1.2.1
223.1.1.2
223.1.1.4
223.1.1.3
223.1.2.9
223.1.3.27
223.1.2.2
Dotted-decimal notation
Decimal number representing
each of the octets, separated
by a decimal point
223.1.3.2
223.1.3.1
223.1.1.1 = 11011111 00000001 00000001 00000001
223
1
1
1
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IP Addressing
IP address: 2 parts
network part (high order
bits)
host part (low order
bits)
What’s a network ?
(from IP address
perspective)
device interfaces with
same network part of IP
address
can physically reach
each other without
intervening router
223.1.1.1
223.1.2.1
223.1.1.2
223.1.1.4
223.1.1.3
223.1.2.9
223.1.3.27
223.1.2.2
LAN
223.1.3.1
223.1.3.2
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IP Addresses
given notion of “network”, let’s re-examine IP
addresses:
“classful” addressing:
class
A
0 network
B
10
C
110
D
1110
Address range
0.0.0.0 to
127.255.255.255
host
network
128.0.0.0 to
191.255.255.255
host
network
multicast address
host
192.0.0.0 to
223.255.255.255
224.0.0.0 to
239.255.255.255
32 bits
All host ids.
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Private IP addresses
Also called non-routable IP addresses
IP blocks reserved for private networks are:
10.0.0.0 through 10.255.255.255
172.16.0.0 through 172.31.255.255
192.168.0.0 through 192.168.255.255
Network Address Translation (NAT) protocol
could be used to map private IP addresses to
external IP address space (see RFC 1631)
Use to hide internal network structure from
the outside world (Security measure!)
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IP addressing: CIDR
Classful addressing contributed to:
inefficient use of address space, address space
exhaustion
e.g., class B net allocated enough addresses for 65K
hosts, even if only 2K hosts in that network
Solution  classless addressing
CIDR: Classless InterDomain Routing
network portion of address is of arbitrary length
address format: a.b.c.d/x, where x is # bits in
network portion of address (Slash notation)
network
part
host
part
11001000 00010111 00010000 00000000
200.23.16.0/23
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IP addresses: how to get one?
Q: How does host get IP address?
hard-coded by network admin:
MS Windows: control-panel->network>configuration->tcp/ip->properties
Red-Hat LINUX: /etc/sysconfig
DHCP: Dynamic Host Configuration Protocol:
dynamically gets address from a server
Client “plug-and-play”
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Who assigns IP addresses?
Q: How does network get network part of IP
addr?
A: gets allocated portion of its provider ISP’s
address space
ISP's block
11001000 00010111 00010000 00000000
200.23.16.0/20
Organization 0
Organization 1
Organization 2
...
11001000 00010111 00010000 00000000
11001000 00010111 00010010 00000000
11001000 00010111 00010100 00000000
…..
….
200.23.16.0/23
200.23.18.0/23
200.23.20.0/23
….
Organization 7
11001000 00010111 00011110 00000000
200.23.30.0/23
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Subnets and Subnet Masks
Allow arbitrary complexity of
internetworked LANs within organisation
Each LAN segment assigned subnet
number
Host portion of address partitioned into
subnet number and host number
With the help of subnet mask
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IP addressing: the last word...
Q: How does an ISP get block of
addresses?
A: ICANN: Internet Corporation for Assigned
Names and Numbers
allocates addresses
manages DNS
assigns domain names, resolves disputes
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ICMP: Internet Control Message Protocol
Used by hosts, routers to
communicate network-level
information (RFC 792)
error reporting:
unreachable host,
network, port, protocol
query: echo request/reply
(used by ping)
In network-layer “above” IP:
ICMP msgs carried in IP
packets
ICMP message: type, code
plus IP header and first 8
octets of data causing error
Type Code description
0
0 echo reply (ping)
3
0 dest. network unreachable
3
1 dest host unreachable
3
2 dest protocol unreachable
3
3 dest port unreachable
3
6 dest network unknown
3
7 dest host unknown
4
0 source quench (congestion
control - not used)
8
0 echo request (ping)
9
0 route advertisement
10
0 router discovery
11
0 TTL expired
12
0 bad IP header
ICMP
IP
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IP v6 - Version Number
IP v 1-3 defined and replaced
IP v4 - current version
IP v5 - non-IP real-time streaming
protocol
IP v6 - replacement for IP v4
During development it was called IPng
Overall spec in RFC 2460
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Why Change IP?
Address space exhaustion
Two level addressing (network and host)
wastes space
Network addresses used even if not
connected to Internet
Exponential growth of the Internet
Single address per host
Requirements for new types of service
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Network sniffer or a protocol analyzer: Ethereal
Ethereal: A Packet Sniffer
A basic tool for observing messages exchanges between protocol
entities
It captures messages being sent/received from/by your computer
Other packet sniffers are tcpdump, Zx Sniffer & AnalogX PacketMon
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Summary
IP enables host-to-host delivery of packets,
unreliably
Allows a flexible approach
Some assistance by ICMP when error
Who looks at process-to-process delivery??
Transport layer (next lesson!)
Read Stallings Chapter 18
Download RFC 791, a classic (1981)!
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