Chapter 4a, Network
Layer (IP Addresses)
Modified by John Copeland
Georgia Tech
for use in ECE3600
A note on the use of these ppt slides:
We’re making these slides freely available to all (faculty, students, readers).
They’re in PowerPoint form so you can add, modify, and delete slides
(including this one) and slide content to suit your needs. They obviously
represent a lot of work on our part. In return for use, we only ask the
following:
 If you use these slides (e.g., in a class) in substantially unaltered form,
that you mention their source (after all, we’d like people to use our book!)
 If you post any slides in substantially unaltered form on a www site, that
you note that they are adapted from (or perhaps identical to) our slides, and
note our copyright of this material.
Computer Networking:
A Top Down Approach
Featuring the Internet,
5th edition.
Jim Kurose, Keith Ross
Addison-Wesley, July
2009.
Thanks and enjoy! JFK/KWR
All material copyright 1996-2009
J.F Kurose and K.W. Ross, All Rights Reserved
Network Layer
4-1
Chapter 4: Network Layer
Chapter goals:
 understand principles behind network layer
services:
network layer service models
 forwarding versus routing
 how a router works
 routing (path selection)
 dealing with scale
 advanced topics: IPv6, mobility

 instantiation, implementation in the Internet
Network Layer
4-2
Chapter 4: Network Layer
 4. 1 Introduction
 4.2 Virtual circuit and
datagram networks
 4.3 What’s inside a
router
 4.4 IP: Internet
Protocol




Datagram format
IPv4 addressing
ICMP
IPv6
 4.5 Routing algorithms
 Link state
 Distance Vector
 Hierarchical routing
 4.6 Routing in the
Internet



RIP
OSPF
BGP
 4.7 Broadcast and
multicast routing
Network Layer
4-3
Network layer
 transport segment from




sending to receiving host
on sending side encapsulates
segments into datagrams
on receiving side, delivers
segments to transport layer
network layer protocols in
every host, router
Router examines IP header
fields in all IP datagrams
passing through it
application
transport
network
data link
physical
network
data link
physical
network
data link
physical
network
data link
physical
network
data link
physical
network
data link
physical
network
data link
physical
network
data link
physical
network
data link
physical
application
transport
network
data link
physical
Network Layer
4-4
Two Key Network-Layer Functions
 forwarding: move
packets from
router’s input to
appropriate router
output
 routing: determine
route taken by
packets from source
to dest.

routing algorithms
analogy:
 routing: process of
planning trip from
source to dest
 forwarding: process
of getting through
single interchange
Network Layer
4-5
Interplay between routing and forwarding
The Routing Algorithm is used
to calculate the link-ID’s in the
Forwarding Table.
routing algorithm
local forwarding table
header value output link
0100
0101
0111
1001
When a datagram arrives, the
destination IP address is used
to lookup the output link-ID.
3
2
2
1
value in arriving
packet’s header
0111
1
3 2
Network Layer
4-6
Connection setup
 3rd important function in
some network architectures:
ATM, frame relay, X.25 (but not IP)
 before datagrams flow, two end hosts and intervening
routers establish virtual connection
 routers get involved
 network vs transport layer connection service:
 network: between two hosts (may also involve
intervening routers in case of VCs)
 transport: between two processes

Network Layer
4-7
Network service model
Q: What service model for “channel” transporting
datagrams from sender to receiver?
Example services for
individual datagrams:
 guaranteed delivery
 guaranteed delivery
with less than 40 msec
delay
 “best effort” (e.g., IP)
Example services for a
flow of datagrams:
 in-order datagram
delivery
 guaranteed minimum
bandwidth to flow
 restrictions on
changes in interpacket spacing
Network Layer
4-8
ATM Network layer service models:
Network
Architecture
Internet
Service
Model
Guarantees ?
Congestion
Bandwidth Loss Order Timing feedback
best effort none
ATM
CBR
ATM
VBR
ATM
ABR
ATM
UBR
constant
rate
guaranteed
rate
guaranteed
minimum
none
no
no
no
yes
yes
yes
yes
yes
yes
no
yes
no
no (inferred
via loss)
no
congestion
no
congestion
yes
no
yes
no
no
ATM = Asynchronous Transfer Mode
Network Layer
4-9
Chapter 4: Network Layer
 4. 1 Introduction
 4.2 Virtual circuit and
datagram networks
 4.3 What’s inside a
router
 4.4 IP: Internet
Protocol




Datagram format
IPv4 addressing
ICMP
IPv6
 4.5 Routing algorithms
 Link state
 Distance Vector
 Hierarchical routing
 4.6 Routing in the
Internet



RIP
OSPF
BGP
 4.7 Broadcast and
multicast routing
Network Layer 4-10
Network layer connection and
connection-less service
 datagram network provides network-layer
connectionless service
 VC network provides network-layer
connection service
 analogous to the transport-layer services,
but:
service: host-to-host
 no choice: network provides one or the other
 implementation: in network core

Network Layer
4-11
Virtual circuits
“source-to-dest path behaves much like telephone
circuit”


performance-wise
network actions along source-to-dest path
 call setup, teardown for each call
before data can flow
 each packet carries VC identifier (not destination host
address)
 every router on source-dest path maintains “state” for
each passing connection
 link, router resources (bandwidth, buffers) may be
allocated to VC (dedicated resources = predictable service)
Network Layer 4-12
VC implementation
a VC consists of:
1.
2.
3.
path from source to destination
VC numbers, number may differ for each link
along path
entries in forwarding tables in routers along
path
 packet belonging to VC carries VC number
(rather than destination address)
 VC number can be changed on each link.

New VC number comes from forwarding table
Network Layer 4-13
Forwarding table
VC number
22
12
1
Forwarding table in
northwest router:
Incoming interface
1
2
3
1
…
2
32
3
interface
number
Incoming VC #
12
63
7
97
…
Outgoing interface
3
1
2
3
…
Outgoing VC #
22
18
17
87
…
Routers maintain connection state information!
Network Layer 4-14
Virtual circuits: signaling protocols
 used to setup, maintain teardown VC
 used in ATM, frame-relay, X.25
 not used in today’s Internet
application
transport 5. Data flow begins
network 4. Call connected
data link 1. Initiate call
physical
6. Receive data application
3. Accept call transport
2. incoming call network
data link
physical
Network Layer 4-15
Datagram networks (Internet)
 no call setup at network layer
 routers: no state about end-to-end connections
 no network-level concept of “connection”
 packets forwarded using destination host address
 packets between same source-dest pair may take
different paths (network congestion is busty)
application
transport
network
data link 1. Send data
physical
application
transport
2. Receive data network
data link
physical
Network Layer 4-16
Datagram or VC network: why?
Internet (IP, datagram)
 data exchange among
ATM (VC)
 evolved from telephony
computers
 human conversation:
 “elastic” service, no
 strict timing, reliability
strict timing req.
requirements
 “smart” end systems
(computers)
 need for guaranteed
 can adapt, perform
service
control, error recovery
 “dumb” end systems
 simple inside network,
 telephones
complexity at “edge”
 complexity inside
 many link types
network
 different characteristics
 uniform service difficult
Network Layer 4-17
Chapter 4: Network Layer
 4. 1 Introduction
 4.2 Virtual circuit and
datagram networks
 4.3 What’s inside a
router
 4.4 IP: Internet
Protocol




Datagram format
IPv4 addressing
ICMP
IPv6
 4.5 Routing algorithms
 Link state
 Distance Vector
 Hierarchical routing
 4.6 Routing in the
Internet



RIP
OSPF
BGP
 4.7 Broadcast and
multicast routing
Network Layer 4-18
Router Architecture Overview
Two key router functions:
 run routing algorithms/protocol (RIP, OSPF, BGP)

forwarding datagrams from incoming to outgoing link
Network Layer 4-19
Input Port Functions
Physical layer:
bit-level reception
Data link layer:
e.g., Ethernet
see chapter 5
Decentralized switching:
 given datagram dest., lookup output port
using forwarding table in input port
memory
 goal: complete input port processing at
‘line speed’
 queuing: if datagrams arrive faster than
forwarding rate into switch fabric
Network Layer 4-20
Three types of switching fabrics
Network Layer 4-21
Switching Via Memory
First generation routers:
 traditional computers with switching under direct
control of CPU
packet copied to system’s memory
 speed limited by memory bandwidth (2 bus
crossings per datagram)
Input
Port
Memory
Output
Port
System Bus
Network Layer 4-22
Switching Via a Bus
 datagram from input port memory
to output port memory via a shared
bus
 bus contention: switching speed
limited by bus bandwidth
 1 Gbps bus, Cisco 1900: sufficient
speed for access and enterprise
routers (not regional or backbone)
Network Layer 4-23
Switching Via An Interconnection
Network
 overcome bus bandwidth limitations
 Banyan networks, other interconnection nets
initially developed to connect processors in
multiprocessor
 Advanced design: fragmenting datagram into fixed
length cells, switch cells through the fabric.
 Cisco 12000: switches Gbps through the
interconnection network
Network Layer 4-24
Output Ports

Buffering required when datagrams arrive from
fabric faster than the transmission rate
 Scheduling discipline chooses among queued
datagrams for transmission
Network Layer 4-25
Output port queueing
 buffering when arrival rate via switch exceeds
output line speed

queueing (delay) and loss due to output port
buffer overflow!
Network Layer 4-26
Input Port Queuing
 Fabric slower than input ports combined -> queueing
may occur at input queues
 Head-of-the-Line (HOL) blocking: queued datagram
at front of queue prevents others in queue from
moving forward

queueing delay and loss due to input buffer overflow!
Network Layer 4-27
Chapter 4: Network Layer
 4. 1 Introduction
 4.2 Virtual circuit and
datagram networks
 4.3 What’s inside a
router
 4.4 IP: Internet
Protocol




Datagram format
IPv4 addressing
ICMP
IPv6
 4.5 Routing algorithms
 Link state
 Distance Vector
 Hierarchical routing
 4.6 Routing in the
Internet



RIP
OSPF
BGP
 4.7 Broadcast and
multicast routing
Network Layer 4-28
The Internet Network layer
Host, router network layer functions:
Transport layer: TCP, UDP
Network
layer
IP protocol
•addressing conventions
•datagram format
•packet handling conventions
Routing protocols
•path selection
•RIP, OSPF, BGP
forwarding
table
ICMP/IP protocol
•error reporting
•router “signaling”
Link layer
physical layer
Network Layer 4-29
Chapter 4: Network Layer
 4. 1 Introduction
 4.2 Virtual circuit and
datagram networks
 4.3 What’s inside a
router
 4.4 IP: Internet
Protocol




Datagram format
IPv4 addressing
ICMP
IPv6
 4.5 Routing algorithms
 Link state
 Distance Vector
 Hierarchical routing
 4.6 Routing in the
Internet



RIP
OSPF
BGP
 4.7 Broadcast and
multicast routing
Network Layer 4-30
IP datagram format (IPv4)
IP protocol version
number
header length
(bytes)
“type” of data
max number
remaining hops
(decremented at
each router)
upper layer protocol
to deliver payload to
how much overhead
with TCP?
 20 bytes of TCP*
 20 bytes of IP
 = 40 bytes + app
layer overhead
32 bits
type of
ver head.
len service
length
fragment
16-bit identifier flgs
offset
upper
time to
header
layer
live
checksum
total datagram
length (bytes)
for
fragmentation/
reassembly
32 bit source IP address
32 bit destination IP address
Options (if any)
data
(variable length,
typically a TCP
or UDP segment)
*plus options, usually 12-20 bytes
E.g. timestamp,
record route
taken, specify
list of routers
to visit.
Network Layer 4-31
IP Fragmentation and Reassembly
Example
 4000 byte
datagram
 MTU = 1500 bytes
1480 bytes in
data field
offset =
1480/8
length
=4000
ID
=x
fragflag
=0
offset
=0
One large datagram becomes
several smaller datagrams
length
=1500
ID
=x
fragflag
=1
offset
=0
length
=1500
ID
=x
fragflag
=1
offset
=185
length
=1040
ID
=x
fragflag
=0
offset
=370
Steps:
1. Subtract 20 from original length: 4000 -20 = 3980 (bytes of "IP data")
2. Subtract 20 from new MTU:
1500- 20 = 1480 (max. bytes of data in each fragment)
3. Divide "maximum data bytes" by 8: 1480/8 = 185 to get offset increment
4. Offset of each fragment "n" (n = 0, 1, 2, ...) = n x "offset increment": 0, 185, 370. ...
5. Length of each fragment (except last) = 20 + "max. data bytes" = 20 +1480 = 1500
Length of last fragment = 20 + remaining data bytes = 20 + 3980 - 2 x 1480 = 1040
Network Layer 4-32
IP Fragmentation & Reassembly
 network links have MTU
(max.transfer size) - largest
possible link-level frame.
 different link types,
different MTUs
 large IP datagram divided
(“fragmented”) within net
 one datagram becomes
several datagrams
 “reassembled” only at
final destination
 IP header bits used to
identify, order related
fragments
fragmentation:
in: one large datagram
out: 3 smaller datagrams
Blue: IP Header
reassembly
Another fragment flag, DNF (do not fragment) causes a ICMP response
(and dropped datagram) instead of fragmentation. The sender then
resends future datagrams with smaller size (may fragment itself or
Network Layer 4-33
reduce MSS for TCP).
Chapter 4: Network Layer
 4. 1 Introduction
 4.2 Virtual circuit and
datagram networks
 4.3 What’s inside a
router
 4.4 IP: Internet
Protocol




Datagram format
IPv4 addressing
ICMP
IPv6
 4.5 Routing algorithms
 Link state
 Distance Vector
 Hierarchical routing
 4.6 Routing in the
Internet



RIP
OSPF
BGP
 4.7 Broadcast and
multicast routing
Network Layer 4-34
IP Addressing: introduction
 IP address: 32-bit
identifier for host, and
router interface
 interface: connection
between host/router and
physical link (sometimes
called a "port").
 router’s typically have
multiple interfaces
 host typically has one
interface
 IP addresses associated
with each interface
Could advertise a
single route to
Inet, 223.1.0.0/22
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
223.1.3.2
223.1.3.1
223.1.1.1 = 11011111 00000001 00000001 00000001
223
1
1
1
Network Layer 4-35
Subnets
 IP address:
 subnet part (high
order bits)
 host part (low order
bits)

What’s a subnet ?



device interfaces with
same subnet part of IP
address
can physically reach
each other without
intervening router
(e.g., on the same
Ethernet LAN)
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
subnet
223.1.3.1
223.1.3.2
network consisting of 3 subnets
Network Layer 4-36
Subnets – have a
contiguous block of IP
addresses which have
the first N bits in
common (a "/N").
Recipe
 To determine the
subnets, detach each
interface from its host
or router, creating
islands of isolated
networks. Each isolated
network is called a
223.1.0.0/22
subnet.
223.1.1.0/24
223.1.2.0/24
223.1.3.0/24
Subnet mask: /24
Higher Order Subnet
Network Layer 4-37
Subnets
223.1.1.2
How many?
223.1.1.1
223.1.1.4
All hosts on the same
223.1.1.3
sub-net are connected
223.1.7.0
223.1.9.2
by a Local Area
Network (LAN) (e.g.,
Ethernet and/or WiFi)
to the same port on a 223.1.9.1
router (the Gateway
223.1.8.1
223.1.8.0
Router).
223.1.2.6
223.1.2.1
223.1.7.1
223.1.3.27
223.1.2.2
223.1.3.1
223.1.3.2
Network Layer 4-38
IP addressing: CIDR
CIDR: Classless InterDomain Routing
subnet portion of address of arbitrary length
 address format: a.b.c.d/x, where x is # bits in
subnet portion of address

subnet
part
host
part
11001000 00010111 00010000 00000000
200.23.16.0/23
Original scheme:
Class A = /8 = 2^24 (16,600,000) addresses
Class B = /16 = 2^16 (65,000) addresses
Class C = /24 = 2^8 (256) addresses
Network Layer 4-39
IP addresses: how to get one?
Q: How does host get assigned an IP address?
 hard-coded by system admin in a file
MS Widowsn: control-panel->network>configuration->tcp/ip->properties
 UNIX: /etc/rc.config file, or use 'ifconfig'
 DHCP: Dynamic Host Configuration Protocol:
dynamically get address from as server
 “plug-and-play”
(more in next chapter)

Network Layer 4-40
IP addresses: how to get one?
Q: How does network get subnet part of IP addr?
A: gets allocated portion of its provider ISP’s address
space (or space assigned to organization*).
Autonomous Systems (AS) buy connectivity from ISPs.
Small companies may lease IP addresses from ISP as
well.
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
* see http://www.iana.org/ - Internet Assigned Numbers Authority
Network Layer 4-41
Hierarchical addressing: route aggregation
Hierarchical addressing allows efficient advertisement of routing
information:
Organization 0
200.23.16.0/23
Organization 1
200.23.18.0/23
Organization 2
200.23.20.0/23
Organization 7
.
.
.
.
.
.
Fly-By-Night-ISP
“Send me anything
with addresses
beginning
200.23.16.0/20”
Internet
200.23.30.0/23
ISPs-R-Us
“Send me anything
with addresses
beginning
199.31.0.0/16”
Network Layer 4-42
Hierarchical addressing: more specific
routes
ISPs-R-Us has a more specific route to Organization 1 (who switched ISPs)
“Send me anything
with addresses
beginning
200.23.16.0/20”
1101000 00011001 0001 xxxx xxxxxxxx
Organization 0
200.23.16.0/23
Organization 2
200.23.20.0/23
Organization 7
.
.
.
.
.
.
Fly-By-Night-ISP
Internet
200.23.30.0/23
Organization 1
200.23.18.0/23
“Send me anything
with addresses
beginning 199.31.0.0/16
or 200.23.18.0/23
1101000 00011001 0001 001x xxxxxxxx
ISPs-R-Us
Network Layer 4-43
Textbook refers to /20 in the network designator
200.23.16.0/20 as the “subnet mask”.
/20 represents a 32-bit binary number that has
20 “1” bits at left and 12 “0”s at the right:
11111111 11111111 11110000 00000000
This number in dotted decimal format is:
255.255.240.0
A network designator is incomplete without the
network mask (either the above form or
“/20”).
Network Layer 4-44
The (sub)network mask can change:
• an IP address into the corresponding
network address (for comparison in a
router forwarding table).
Match[i] = {(IP & mask[i] == Network_addr[i]}
“==“ means “TRUE if equals”
•
• an IP address (or network address) into
the network Broadcast Address:
Broadcast_addr = IP | ~mask
“&” bitwise AND
“|” bitwise OR
“~” bitwise inversion (0->1, 1->0)
Network Layer
Analogy to Telephone Numbers
(before "number portability")
Block of No.s (CIDR)
Block Mask
Area Covered
404-000-0000 /3
111-000-0000
Atlanta Area
404-894-0000 /6
111-111-0000
Georgia Tech
404-894-5000 /7
111-111-1000
ECE
Atlanta No.s
404-000-0000 to 404-999-9999
10^7
Georgia Tech 404-894-0000 to 404-894-9999
10^4
ECE No.s
10^3
404-894-5000 to 404-894-5999
Network Layer 4-46
Forwarding table
2^32 = 4 billion
possible addresses
Destination Address Range
Link Interface
11001000 00010111 00010000 00000000
200.23.16.0 / 21
through
11001000 00010111 00010111 11111111 (2^11 = 2048 addresses)
0
11001000 00010111 00011000 00000000
200.23.16.0 / 24
through
11001000 00010111 00011000 11111111 (2^8 = 256 addresses)
1
11001000 00010111 00011000 00000000
200.23.24.0 / 21
through
11001000 00010111 00011111 11111111 (2^11 = 2048 addresses)
2
(non-prefix bits shown in red)
otherwise (default route)
3
Network Layer 4-47
Longest prefix matching
Prefix Match
11001000 00010111 0001 0
11001000 00010111 0001 1000
11001000 00010111 0001 1
otherwise
Size
/21
/24
/21
Link Interface
0
1
2
3
Examples
DA: 11001000 00010111 0001 0110 1010 0001
Which interface?
DA: 11001000 00010111 0001 1000 1010 1010
Which interface?
DA: 11001000 00010111 0001 1100 1010 1010
Why do we use prefixes of different lengths?
Why do some IP addresses match more than one prefix?
Network Layer 4-48
IP Address Bitwise Calculations
200.23.16.0/20
1101000 00011001 0001xxxx xxxxxxxx
Network Mask, 0 or 1 -> 1, x -> 0
255.255.240.0
11111111 11111111 11110000 00000000
Minimum Host Address: x -> 0
200.23.16.0
1101000 00011001 00010000 00000000
Maximum Host Address: x -> 1
200.23.31.25
1101000 00011001 00011111 11111111
Minimum host address is the “Network Address”
Maximum host address is the “Broadcast Addr.”
Network Layer 4-49
IP Address Dotted-Decimal Calculations
200.23.16.0/20
1101000 00011001 0001xxxx xxxxxxxx
Byte 3 is the “Split Byte
Byte 4 is “Host Only”, 1 & 2 are “Network Only”
No. Network Bits
in the "Split Byte
Mask Value of
Split Byte
1
2
3
4
5
6
7
128
192
224
240
248
252
254
Network Part
Host Part
is Multiple
from Zero to:
of:
128
127
64
63
32
31
16
15
8
7
4
3
2
1
<--------- Sum to 256 -------------->
Network Layer 4-50
IP Address Dotted-Decimal Calculations - 2
200.23.16.0/20
1101000 00011001 0001xxxx xxxxxxxx
Byte 3 is the “Split Byte (4 network bits)
Byte 4 is “Host Only”, 1 & 2 are “Network Only”
No. Network Bits Mask Value of Network Part
Host Part
in the "Split Byte
Split Byte
is Multiple from Zero to:
(11111111)
(11110000)
of:
255
4
240
16
15
Network Mask = 255.255.240.0
Min. Host Addr. (Network Addr.) = 200.23.16.0
Maximum Host Address = 200.23.(16+15).255
(Broadcast Address = 200.23.31.255)
Number of Host Addresses = 2^12 = (15+1)*256
Network Layer 4-51
IP addressing: the last word...
Q: How does an ISP (or organization) get a
block of addresses?
A: ICANN: Internet Corporation for Assigned
Names and Numbers (www.icann.org)
 allocates addresses (no, IANA* does this)
 manages DNS (domain names can be registered
through several dozen registries (e.g., verisign.com)

assigns domain names, resolves disputes
* Internet Assigned Numbers Authority - www.iana.org
Network Layer 4-52
NAT: Network Address Translation
rest of
Internet
local network
(e.g., home network)
192.168.2.0/24
192.168.2.1
192.168.2.2
192.168.2.3
138.76.29.7
192.168.2.4
All datagrams leaving local
network have same single source
NAT IP address: 138.76.29.7,
different source port numbers
Datagrams with source or
destination in this network
have 192.168.2.0/24 address for
source, destination (as usual)
Network Layer 4-53
NAT: Network Address Translation
 Motivation: local network uses just one IP address as
far as outside world is concerned:
 range of addresses not needed from ISP: just one IP
address for all devices
 can change addresses of devices in local network
without notifying outside world
 can change ISP without changing addresses of
devices in local network
 devices inside local net not explicitly addressable,
visible by outside world (a security plus).
Network Layer 4-54
NAT: Network Address Translation
Implementation: NAT router must:

outgoing datagrams: replace (source IP address, port

remember (in NAT translation table) every (source

incoming datagrams: replace (NAT IP address, new
#) of every outgoing datagram to (NAT IP address,
new port #)
. . . remote clients/servers will respond using (NAT
IP address, new port #) as destination addr.
IP address, port #) to (NAT IP address, new port #)
translation pair
port #) in dest fields of every incoming datagram
with corresponding (source IP address, port #)
stored in NAT table
Network Layer 4-55
NAT: Network Address Translation
NAT translation table
WAN side: Server addr LAN side addr
& port, Client port
and Client port
1: host 10.0.0.1
2: NAT router
sends datagram to
changes datagram
128.119.40.186, 80
source addr from 128.119.40.186, 80, 5001 10.0.0.1, 3345
……
……
10.0.0.1, 3345 to
138.76.29.7, 5001,
S: 10.0.0.1, 3345
updates table
D: 128.119.40.186, 80
10.0.0.1
(Server IP, port are not changed)
1
S: 138.76.29.7, 5001
2 D: 128.119.40.186, 80
10.0.0.4
10.0.0.2
138.76.29.7
S: 128.119.40.186, 80
D: 138.76.29.7, 5001
3
3: Reply arrives
dest. address:
138.76.29.7, 5001
from 128.119.40.186, 80
Slide modified 10/19/2008 by JAC
S: 128.119.40.186, 80
D: 10.0.0.1, 3345
4
10.0.0.3
4: NAT router
changes datagram
dest addr from
138.76.29.7, 5001 to 10.0.0.1, 3345
Network Layer 4-56
NAT: Network Address Translation
 16-bit port-number field:
 60,000 simultaneous connections with a single LAN-side
address! [Actually more if the translation table has
outside IP,port as a factor. Many such implementations
do not change the client port number, since duplication is
not likely].
 NAT is controversial [?]:
 routers should only process up to layer 3
 violates end-to-end argument
• NAT possibility must be taken into account by app
designers, eg, P2P applications

address shortage should instead be solved by IPv6
Network Layer 4-57
169.254.0.0/16 is reserved for temporary IP’s (DHCP+)
224.0.0.0/4 is reserved for multicast
240.0.0.0/4 is reserved (but not used)
Source: http://en.wikipedia.org/wiki/IP_Addresses
Network Layer 4-58
Chapter 4: Network Layer
 4. 1 Introduction
 4.2 Virtual circuit and
datagram networks
 4.3 What’s inside a
router
 4.4 IP: Internet
Protocol




Datagram format
IPv4 addressing
ICMP
IPv6
 4.5 Routing algorithms
 Link state
 Distance Vector
 Hierarchical routing
 4.6 Routing in the
Internet



RIP
OSPF
BGP
 4.7 Broadcast and
multicast routing
Network Layer 4-59
ICMP: Internet Control Message Protocol
 used by hosts & routers to
Type Code description
communicate network-level
0
0
echo reply (ping reply)
information
3
0
dest. network unreachable
 error reporting: unreachable
3
1
dest host unreachable
host, network, port,
3
2
dest protocol unreachable
protocol
3
3
dest port unreachable
 echo request/reply (used by
3
6
dest network unknown
ping)
3
7
dest host unknown
4
0
source quench (congestion
 network-layer “above” IP:
control - not used)
 ICMP messages carried in
8
0
echo request (ping)
IP datagrams
9
0
route advertisement
 ICMP message: type=3, 4, 11, or
10
0
router discovery
12: code plus IP header and
11
0
TTL expired
following 8 bytes of IP
12
0
bad IP header
datagram causing error (would
include UDP or TCP port
numbers)
Slide modified 10/19/2008 by JAC
Network Layer 4-60
Traceroute and ICMP
 Source sends series of UDP
segments* to dest host



First has TTL =1
Second has TTL=2, etc.
Unlikely port number
 When nth datagram arrives
to nth router:



Router discards datagram
And sends to source an
ICMP message (type 11,
code 0)
Datagram includes router IP
address. Traceroute does
DNS lookup to find name of
router (if any)
* or ICMP pings
 When ICMP message
arrives, source calculates
RTT
 Traceroute does this 3
times
Stopping criterion
 UDP segment eventually
arrives at destination host
 Destination returns ICMP
“host unreachable” packet
(type 3, code 3)
 When source gets this
ICMP, stops.
Slide modified 10/19/2008 by JAC
Network Layer 4-61
Chapter 4: Network Layer
 4. 1 Introduction
 4.2 Virtual circuit and
datagram networks
 4.3 What’s inside a
router
 4.4 IP: Internet
Protocol




Datagram format
IPv4 addressing
ICMP
IPv6
 4.5 Routing algorithms
 Link state
 Distance Vector
 Hierarchical routing
 4.6 Routing in the
Internet



RIP
OSPF
BGP
 4.7 Broadcast and
multicast routing
Network Layer 4-62
IPv6
 Initial motivation: 32-bit address space now
(2012)completely allocated (NAT and CIDR* fixed
the problem for for a dozen years). IPv6 has 128
bits.
 Additional motivation:
header format helps speed processing/forwarding
 header changes to facilitate QoS
IPv6 datagram format:
 fixed-length 40 byte header (versus 20 for IPv4)
 no fragmentation allowed (6 bytes removed)
*Before CIDR (Classless Internet Domain Routing), there were only
three subnet sizes (classes):

Class A= /8 (4M), B = /16 (65k), C = /24 (255 addresses)
If an org needed 260 addresses, a Class B (65,535) was allocated.
Network Layer 4-63
IPv6 Header (Cont)
Priority: identify priority among datagrams in flow
Flow Label: identify datagrams in same “flow.”
(concept of“flow” not well defined).
Next header: identify upper layer protocol for data
“6to4 Translation”
4-byte IPv4 -> 16-byte IPv6
A.B.C.D -> :2002:aabb:ccdd/80
“:aa:”=“A in 2-char hex”
“:bb:”=“B in 2-char hex”
etc.
IPv4 address can become an
IPv6 sub-net with 80 bits for
“host” addresses (1e24 hosts)
http://en.wikipedia.org/wiki/6to4
Slide modified 10/23/2012 by JAC
Network Layer 4-64
Example – 6to4 convert 130.207.17.25 to IPv6 address
Convert the decimal byte-representations to hex:
130 = 0x82 207 = 0xCF 17 = 0x11 25 = 0x19
IPv6 addresses are written with colons separating every
16 bits (4 hex characters).
:0000: can be written ::
The first 16 bits 0x2002 are a reserved /16 block of
addresses reserved for IPv4 translations.
:2002::::::::/16
“6to4 Translation”
Next add the IPv4 32 bits:
4-byte IPv4 -> 16-byte IPv6
:2002:82CF:1119::::::/48
A.B.C.D -> :2002:aabb:ccdd/32
This is not just a single IPv6
“:aa:”=“A in 2-char hex”
“:bb:”=“B in 2-char hex”
address, but a block of 2^80
etc.
possible host addresses that
IPv4 address can become an IPv6
can replace private subnet
sub-net with 80 bits for “host”
addresses, like 192.168.0.0/16's. addresses (1e24 hosts)
http://en.wikipedia.org/wiki/6to4
Other Changes from IPv4
 Checksum: removed entirely to reduce
processing time at each hop
 Options: allowed, but outside of header,
indicated by “Next Header” field

(segmentation is done in Options Header,
only if needed)
 ICMPv6: new version of ICMP
 additional
message types, e.g. “Packet Too Big”
 multicast group management functions
Network Layer 4-66
Transition From IPv4 To IPv6
 Not all routers can be upgraded simultaneous
no “flag day”
 How will the network operate with mixed IPv4 and
IPv6 routers?

 Tunneling: IPv6 carried as payload in IPv4
datagram among IPv4 routers
Network Layer 4-67
Tunneling
Logical view:
Physical view:
E
F
IPv6
IPv6
IPv6
A
B
E
F
IPv6
IPv6
IPv6
IPv6
A
B
IPv6
tunnel
IPv4
IPv4
Network Layer 4-68
Tunneling
Logical view:
Physical view:
A
B
IPv6
IPv6
A
B
C
IPv6
IPv6
IPv4
Flow: X
Src: A
Dest: F
data
A-to-B:
IPv6
E
F
IPv6
IPv6
D
E
F
IPv4
IPv6
IPv6
tunnel
Src:B
Dest: E
Src:B
Dest: E
Flow: X
Src: A
Dest: F
Flow: X
Src: A
Dest: F
data
data
B-to-C:
IPv6 inside
IPv4
B-to-C:
IPv6 inside
IPv4
Flow: X
Src: A
Dest: F
data
E-to-F:
IPv6
Network Layer 4-69