University of British Columbia
CICS 515 (Part 2) Computer Networks
Lecture 5 – IP (Ch 4)
Instructor: Dr. Son T. Vuong
Email: vuong@cs.ubc.ca
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The World Connected
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Ch 4: Network Layer and Routing
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The IP Protocol
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Routing
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IP Format, Addressing, fragmentation,
Internet Control Protocols (ICMP) (next lecture)
RIP (Routing Information Protocol)
OSPF (Open Shortest Path First)
The Interior Gateway Routing Protocol
BGP – The Exterior Gateway Routing Protocol
IPv6
Internet Multicasting
Mobile IP
CICS515 Summer 2012
Instructor: Dr. Son Vuong
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ISO Architecture
End host
End host
Application
Application
Presentation
Presentation
Session
Session
Transport
Transport
IP
Network
Network
Network
Network
Data link
Data link
Data link
Data link
Physical
Physical
Physical
Physical
One or more nodes
within the network
CICS515 Summer 2012
Instructor: Dr. Son Vuong
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Internet Architecture
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Defined by Internet Engineering Task Force (IETF)
Hourglass Design
Application vs Application Protocol (FTP, HTTP)
FTP
HTTP
NV
TFTP
UDP
TCP
TCP
IP
NET1
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NET2
Application
UDP
IP
…
NETn
Instructor: Dr. Son Vuong
Network
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Design Principles for Internet
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Make sure it works.
Keep it simple.
Make clear choices.
Exploit modularity.
Expect heterogeneity.
Avoid static options and parameters.
Look for a good design; it need not be perfect.
Be strict when sending and tolerant when
receiving.
Think about scalability.
Consider performance and cost.
CICS515 Summer 2012
Instructor: Dr. Son Vuong
5
Collection of Subnetworks
The Internet = interconnected collection of many networks.
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Instructor: Dr. Son Vuong
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Example TCP/IP internet
H2
H1
Network 1 (Ethernet)
H7
R3
H8
H3
Network 4
(point-to-point)
Network 2 (Ethernet)
R1
R2
H4
Network 3 (FDDI)
H1
H8
H5
TCP
R1
R2
IP
IP
ETH
H6
ETH
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R3
IP
FDDI
FDDI
Instructor: Dr. Son Vuong
IP
PPP
PPP
TCP
IP
ETH
ETH
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IP Service Model
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Packet Delivery Model
 Connectionless
(datagram-based)
 Best-effort delivery (unreliable service)
 Loss,
out-of-order, duplication
 long, variable delay
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Global Addressing Scheme
 IP
Addresses
 Routing info provided within header, no set up
phase.
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IP runs over any Layer 2/3 network
 Ethernet,
CICS515 Summer 2012
FDDI, ATM, Point to Point, etc.
Instructor: Dr. Son Vuong
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IP Packet Format
0
4
8
V ersion
HLen
16
TOS
31
Length
Ident
TTL
19
Flags
Protocol
Offset
Checksum
SourceIPAddr
DestinationIPAddr
Pad
(variable)
Options (variable)
Data
CICS515 Summer 2012
Instructor: Dr. Son Vuong
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The IP Protocol (2)
5-54
Some of the IP options.
CICS515 Summer 2012
Instructor: Dr. Son Vuong
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IP Packet Details
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Datagram format
(4) - Currently set to 4 (IPv4). We’ll
discuss IPv6.
 Hlen (4) - Number of 32-bit words in the header
(allows for a variable number of options)
 TOS (8) - Type of service (not widely used)
 Length (16) - Number of bytes in this datagram Maximum size is 64KB.
 Ident (16) - Used for fragmentation
 Flags(3)/Offset(13) (16) - Used for
fragmentation (offset in units of 8 bytes)
 Version
CICS515 Summer 2012
Instructor: Dr. Son Vuong
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IP Packet Details cont.
 TTL
(8) - Number of hops this datagram can travel
(defaults to 64). Originally was intended to count
seconds, but impossible without a central clock.
 Protocol (8) - Demultiplexing key for higher level
protocols (TCP=6, UDP=17)
 Checksum (16) - Of the header only, using Internet
Checksum method (as in UDP and TCP)
 DestAddr & SrcAddr (32) - See later.
 Options, e.g. timestamp, record route, (strict/loose)
source routing
CICS515 Summer 2012
Instructor: Dr. Son Vuong
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Fragmentation and Reassembly
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Each Layer 2/3 network has a Maximum
Transmission Unit (MTU) e.g. Ethernet is 1500,
FDDI is 4500
Unreasonable to make all IP packets small
enough to fit within all possible MTUs.
Strategy
 Fragment
only when necessary (MTU < Datagram)
 Try to avoid fragmentation at source host
 Fragments are self-contained IP datagrams
 Reassembly of fragments at destination host.
CICS515 Summer 2012
Instructor: Dr. Son Vuong
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Fragmentation Example
H1
R1
ETH IP (1400)
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R2
FDDI IP (1400)
Instructor: Dr. Son Vuong
R3
H8
PPP IP (512)
ETH IP (512)
PPP IP (512)
ETH IP (512)
PPP IP (376)
ETH IP (376)
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Fragmentation cont.
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If one fragment is lost, discard all other
fragments. Higher layers will recover.
The IP header has fields for handling this
type of fragmentation.
 Set
the M bit (in flags) to indicate that more
data is coming.
 Set the offset to indicate where each of the
fragmented blocks starts.
 Set the ident field to identify related packets.
CICS515 Summer 2012
Instructor: Dr. Son Vuong
15
IP Fragmentation and Reassembly
length ID fragflag offset
=4000 =x
=0
=0
Example
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4000 byte datagram One large datagram becomes
several smaller datagrams
 MTU = 1500 bytes
1480 bytes in
data field
offset =
1480/8
length ID fragflag offset
=1500 =x
=1
=0
length ID fragflag offset
=1500 =x
=1
=185
length ID fragflag offset
=1040 =x
=0
=370
length = 4000 – 2*1480
= 4000 - 2960 = 1040
CICS515 Summer 2012
Instructor: Dr. Son Vuong
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IP Fragmentation– Peer Instruction – Question 5.1
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A 1300-byte IP datagram sent through a network with
500-byte MTU must be fragmented into 3 fragments
with the following respective <length, offset> values
in the header:
A. <500, 0>, <500, 460>, <500, 920>
B. <500, 0>, <500, 480>, <300, 960>.
C. <500, 0>, <500, 500, <300, 1000>
D. <500, 0>, <500, 60>, <340, 120>
E. None of the above
CICS515 Summer 2012
Instructor: Dr. Son Vuong
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Global Addresses
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Properties of IP addresses.
 Globally
unique - No confusion about
where to send a packet.
 Hierarchical - Network component and
host number.
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Normally written in “Dot notation” (4 byte
values, total 32 bits)
 10.3.2.4
 128.96.33.81
 192.12.69.77
 142.103.7.7
CICS515 Summer 2012
(cascade.cs.ubc.ca)
Instructor: Dr. Son Vuong
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IP Addresses
IP address formats.
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Instructor: Dr. Son Vuong
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Address Notation
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Binary
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11000000 00000101 00110000 00000011
Hex Colon
 C0:05:30:03
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Dotted Decimal
 192.5.48.3
CICS515 Summer 2012
Instructor: Dr. Son Vuong
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Class Ranges
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Dotted Decimal w.x.y.z
Class A: w= 0 thru 127
Class B: w= 128 thru 191
Class C: w= 192 thru 223
Class D: w= 224 thru 239
Class E: w= 240 thru 255
CICS515 Summer 2012
Instructor: Dr. Son Vuong
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Class Formats
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Class A: 128 Networks, 16777216 hosts each
Class B: 16384 Networks, 65536 hosts each
Class C: 2097152 Networks, 256 hosts each
The plan was to give each organization
(company or university) a network number
that is appropriate for their size, and let them
allocate host numbers.
Example: UBC has several class B and C
addresses.
 E.g.
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142.103.7.7 and 198.162.33.12
In reality, variations on this method are used.
CICS515 Summer 2012
Instructor: Dr. Son Vuong
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IP Addresses (2)
Special IP addresses.
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Instructor: Dr. Son Vuong
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Subnets
A campus network consisting of LANs for
various departments.
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Instructor: Dr. Son Vuong
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Subnets (2)
A class B network subnetted into 64 subnets.
CICS515 Summer 2012
Instructor: Dr. Son Vuong
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CIDR – Classless InterDomain Routing
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address format: a.b.c.d/x subnet portion of
arbitrary length x
subnet
part
host
part
11001000 00010111 00010000 00000000
200.23.16.0/23
A set of IP address assignments
CICS515 Summer 2012
Instructor: Dr. Son Vuong
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NAT – Network Address Translation
Placement and operation of a NAT box.
CICS515 Summer 2012
Instructor: Dr. Son Vuong
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Datagram Forwarding
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Using these IP address, how do we route
messages?
Strategy
 every
datagram contains destination's address
 if directly connected to destination network, then
forward to host
 if not directly connected to destination network, then
forward to some router
 forwarding table maps network number into next hop
 each host has a default router
 each router maintains a forwarding table
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A forwarding table maps network numbers into
router addresses.
CICS515 Summer 2012
Instructor: Dr. Son Vuong
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Example: Forwarding Table for R2
Network 1 (Ethernet)
H7
H2
H1
R3
H8
H3
Network 4
(point-to-point)
Network 2 (Ethernet)
R1
For Router R2
R2
H4
Network
Next Hop
1
2
3
4
R3
R1
interface 1
interface 0
Network 3 (FDDI)
H5
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H6
Instructor: Dr. Son Vuong
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Examples
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Sending from H1 to H2:
 Same
network, so send an Ethernet frame to
the Ethernet address for H2
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Sending from H1 to H8:
 Send
an Ethernet frame from H1 to R1
 Send an FDDI packet from R1 to R2
 Send a point to point message from R2 to R3
 Send an Ethernet frame from R3 to H8
CICS515 Summer 2012
Instructor: Dr. Son Vuong
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Scalability
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In reality, it’s not possible to list an appropriate
router for every network on the internet. The
table will get too big.
Commonly we’ll have a list of well-known
networks, but use a default router for all other
networks.
For example: Network 3 could get to Network 2
via R1, and will use R2 for all other networks.
Sometimes, we only have a single default
router on each network.
CICS515 Summer 2012
Instructor: Dr. Son Vuong
31
Internet Control Message Protocol
(ICMP)
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If something goes wrong with an IP packet, a
control message is sent back to the sender:
Echo (ping) Request/Reply
 Timestamp Request/Reply
 Redirect (from router to source host)
 Source quench
 Destination unreachable (protocol, port, or host)
 TTL exceeded (so datagrams don't cycle forever)
 Checksum failed
 Reassembly failed
 Cannot fragment
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CICS515 Summer 2012
Instructor: Dr. Son Vuong
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Summary - What have we covered?
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CICS515 Summer 2012
internetworks
IP (Layer 3.5)
packets and fragmentation
addressing and address classes
packet forwarding
ICMP
Instructor: Dr. Son Vuong
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