Download The Internet Network layer: IP Addressing

The Internet Network layer: IP
Addressing
Host, router network layer functions:
3 Major Components
Transport layer: TCP, UDP
Network
layer
IP protocol
•addressing conventions
•datagram format
•packet handling conventions
Routing protocols
•path selection
•RIP, OSPF, BGP
routing
table
ICMP protocol
•error reporting
•router
Link layer
physical layer
Addressing
1
IP Addressing: introduction
 IP address: 32-bit
223.1.1.1
identifier for host,
router interface
 interface: connection
between host, router
and physical link



Dotted-decimal notation
Routers typically have
multiple interfaces
host may have multiple
interfaces
IP addresses
associated with
interface, not host,
router
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
Must be globally unique
1
1
Addressing
1
2
IP Address
7
6
5
4
3
2
1
0
7
6
5
4
3
2
1
0
7
6
5
4
3
2
1
0
7
6
5
4
3
2
1
0
128
64
32
16
8
4
2
1
128
64
32
16
8
4
2
1
128
64
32
16
8
4
2
1
128
64
32
16
8
4
2
1
1
128
1
0
64
0
0
0
0
0
0
0
0
0
0
1
0
192
128
1
1
64
1
32
1
16
0
8
0
0
0
0
1
0
248
IP address
128
1
1
64
1
32
1
16
252
1
8
0
4
0
0
1
0
128
1
1
64
1
32
1
16
1
8
0
4
1
0
1
253
192.248.252.253
See Excel Sheet
Addressing
3
/24 – Network mask; leftmost 24 bits identify the
network address
IP Addressing
 IP address:
 network part (high
order bits)
 host part (low order
bits)
 What is a network ?
(from IP address
perspective)
 device interfaces with
same network part of
IP address
 can physically reach
each other without
intervening router
One IP Network:
223.1.1.0/24
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
network consisting of 3 IP networks
(for IP addresses starting with 223,
first 24 bits are network address)
Addressing
4
IP Addressing
How to find the
networks?
 Detach each
interface from
router, host
 create islands of
isolated networks
223.1.1.2
223.1.1.1
223.1.1.4
223.1.1.3
223.1.9.2
223.1.7.0
223.1.9.1
223.1.7.1
223.1.8.1
223.1.8.0
223.1.2.6
Interconnected
system consisting
of six networks
223.1.2.1
223.1.3.27
223.1.2.2
223.1.3.1
223.1.3.2
Addressing
5
IP Addressing
given notion of network, let's re-examine IP addresses:
Note: Reserving 216= 65K for
host addresses would be
wasteful for a 2K hosts
requirement
Classful addressing:
class
A
0 network
B
10
C
110
D
1110
1.0.0.0 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
Addressing
6
IP Addressing: CIDR
(1993) IETF
standardized on CIDR
 Classful addressing:


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
 CIDR: Classless Inter Domain Routing


network portion of address of arbitrary length
address format: a.b.c.d/x, where x is # bits in network portion of
address
network
part
host
part
11001000 00010111 00010000 00000000
200.23.16.0/23
Addressing
7
IP addressing: Masks
 Masks are commonly used in some configuration files
 Simply convert the mask to binary and check which is the
network part and which is the host part
 e.g., for a 23 bits network and 9 bits host, the mask would be


255.255.254.0
Or 1111 1111 1111 1111 1111 1110 0000 0000
 Possible values for masks are combinations where there are
only 1's at the left side and 0's on the right side of the mask
network
part
host
part
11001000 00010111 00010000 00000000
200.23.16.0/23
Addressing
8
IP addresses: how to get one?
Network (network portion):
 get allocated portion of ISP's address space for
use within an organisation’s subnet:
ISP's block
11001000 00010111 00010000 00000000
200.23.16.0/20
Organization 0
11001000 00010111 00010000 00000000
200.23.16.0/23
Organization 1
11001000 00010111 00010010 00000000
200.23.18.0/23
Organization 2
...
11001000 00010111 00010100 00000000
…..
….
200.23.20.0/23
….
Organization 7
11001000 00010111 00011110 00000000
200.23.30.0/23
Addressing
9
Hierarchical addressing: route aggregation
Hierarchical addressing allows efficient advertisement of routing
information:
Ability to use single
network prefix to
advertise multiple
networks
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
Addressing
10
Hierarchical addressing: more specific routes
ISPs-R-Us has a more specific route to Organization 1
Organization 0
200.23.16.0/23
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
Organization 1
200.23.18.0/23
Send me anything
with addresses
beginning 199.31.0.0/16
or 200.23.18.0/23
Uses longest prefix matching rule – longest most specific address matching the
11
Addressing
destination address
IP addressing: the last word...
Q: How does an ISP get block of addresses?
A: ICANN: Internet Corporation for Assigned
Names and Numbers
Global authority
 allocates addresses
 manages DNS
 assigns domain names, resolves disputes
Based on guidelines in RFC 2050
Addressing
12
IP addresses: how to get one?
Obtaining Host Addresses:
 hard-coded by system admin (in a file)
 DHCP: Dynamic Host Configuration Protocol:
dynamically get address: plug-and-play
 host
broadcasts DHCP discover msg
 DHCP server responds with DHCP offer msg
 host requests IP address: DHCP request msg
 DHCP server sends address: DHCP ack msg
Addressing
13
DHCP client-server scenario
DHCP server: 223.1.2.5
DHCP discover
Within a UDP packet,
to port 67
arriving
client
src : 0.0.0.0, 68
dest.: 255.255.255.255,67
yiaddr: 0.0.0.0
transaction ID: 654
DHCP offer
src: 223.1.2.5, 67
dest: 255.255.255.255, 68
yiaddrr: 223.1.2.4
transaction ID: 654
Lifetime: 3600 secs
DHCP request
time
src: 0.0.0.0, 68
dest:: 255.255.255.255, 67
yiaddrr: 223.1.2.4
transaction ID: 655
Lifetime: 3600 secs
DHCP ACK
src: 223.1.2.5, 67
dest: 255.255.255.255, 68
yiaddrr: 223.1.2.4
transaction ID: 655
Lifetime: 3600 secs
Network Layer 4-14
Network Address Translation (NAT)
An approach to address allocation (RFC 2663, 3022)
NAT Translation Table
WAN Side
138.76.29.7,
5001
LAN Side
10.0.0.1, 3345
IP, Port
10.0.0.1
S=10.0.0.1, 3345
D=128.119.40.186, 80
10.0.0.2
S=138.76.29.7, 5001
Multiplying
the
D=128.119.40.186,
80
number of devices
sharing the same IP
S=128.119.40.186, 80 Address
S=128.119.40.186, 80
D=138.76.29.7, 5001
Router’s IP Address
– taken from ISP’s
DHCP server
D=10.0.0.1, 3345
10.0.0.3
Address for devices – from DHCP
server run by the router
NAT-enabled router hides details of the home network from the outside world; behaves like a
15
Addressing
single device with a single IP address (does not appear as a router anymore)
Network Address Translation (NAT)
An approach to address allocation (RFC 2663, 3022)
NAT Translation Table
WAN Side
138.76.29.7,
5001
LAN Side
10.0.0.1, 3345
IP, Port
10.0.0.1
S=10.0.0.1, 3345
D=128.119.40.186, 80
10.0.0.2
10.0.0.3
NAT router generates a new source
port source number for each
datagram it receives from the private
network (realm of private addresses)
Address space 10.0.0.0/8 is one of three portions of the IP address space that is reserved in
Addressing
RFC1918 for a private network
16
Network Address Translation (NAT)
An approach to address allocation (RFC 2663, 3022)
NAT Translation Table
WAN Side
138.76.29.7,
5001
LAN Side
10.0.0.1, 3345
IP, Port
10.0.0.1
S=10.0.0.1, 3345
D=128.119.40.186, 80
10.0.0.2
10.0.0.3
PROBLEMS with NAT:
• Violates the use of port numbers
• Routers are supposed to process packets only up to layer 3
• Violates End-to-End argument; Host addresses should not be modified
• Interferes with P2P applications. A host behind a NAT-enabled router cannot act as a server.
• Suggestion by purists in the IETF: IPv6 should be used instead!
17
Addressing
Example #1
Getting a datagram from source to dest.
routing table in A
Dest. Net. next router Nhops
223.1.1
223.1.2
223.1.3
IP datagram:
misc source dest
fields IP addr IP addr
data
A
 datagram remains
unchanged, as it travels
from source to destination
 Addresses are the fields
of interest here
Host A learns that Host B can be
reached directly via its outgoing
interface. In turn, the Link-Layer
protocol delivers the datagram to
Host B. (details on next slide)
223.1.1.4
223.1.1.4
1
2
2
223.1.1.1
223.1.2.1
B
223.1.1.2
223.1.1.4
223.1.1.3
223.1.3.1
223.1.2.9
223.1.3.27
223.1.2.2
E
223.1.3.2
Addressing
18
Example #1
Getting a datagram from source to dest.
B is on the same network as A
misc
data
fields 223.1.1.1 223.1.1.3
Dest. Net. next router Nhops
223.1.1
223.1.2
223.1.3
Starting at A, given IP
datagram addressed to B:
 look up net. address of B
A
223.1.1.4
223.1.1.4
223.1.1.1
 finds B is on same net. as A
223.1.2.1
 link layer will send datagram
directly to B inside link-layer
frame
 B and A are directly
connected
1
2
2
B
223.1.1.2
223.1.1.4
223.1.1.3
223.1.3.1
223.1.2.9
223.1.3.27
223.1.2.2
E
223.1.3.2
Addressing
19
Example #2
Getting a datagram from source to dest.
misc
data
fields 223.1.1.1 223.1.2.2
Dest. Net. next router Nhops
223.1.1
223.1.2
223.1.3
Starting at A, dest. E:
 look up network address of E
 E on different network
A, E not directly attached
 routing table: next hop
router to E is 223.1.1.4
 link layer sends datagram to
router 223.1.1.4 inside linklayer frame
 datagram arrives at 223.1.1.4
 continued…..
Continued
on next slide…
A
223.1.1.4
223.1.1.4
1
2
2
223.1.1.1

223.1.2.1
B
223.1.1.2
223.1.1.4
223.1.1.3
223.1.3.1
223.1.2.9
223.1.3.27
223.1.2.2
E
223.1.3.2
Addressing
20
Example #2
Getting a datagram from source to dest.
misc
data
fields 223.1.1.1 223.1.2.2
Arriving at 223.1.1.4,
destined for 223.1.2.2
 look up network address of E
Dest.
next
network router Nhops interface
223.1.1
223.1.2
223.1.3
A
-
1
1
1
223.1.2.1
interface 223.1.2.9
Router & E are directly
attached to each other
 link layer sends datagram to
223.1.2.2 inside link-layer
frame via interface 223.1.2.9
 datagram arrives at
223.1.2.2!!!
223.1.3.27
223.1.1.1
 E on same network as router’s

223.1.1.4
223.1.2.9
B
223.1.1.2
223.1.1.4
223.1.1.3
223.1.3.1
223.1.2.9
223.1.3.27
223.1.2.2
E
223.1.3.2
Addressing
21
Exercise#1
Show the forwarding process if a packet arrives at R1 in the
figure with the destination address 180.70.65.140.
subnet:
180.70.65.128/25
180.70.65.135/25
m0
subnet:
201.4.16.0/22
m1
m3
R1
201.4.16.2/22
m2
201.4.22.3/24
subnet:
201.4.22.0/24
180.70.65.194/26
subnet:
180.70.65.192/26
R2
180.70.65.200/26
rest of the
Internet
Addressing
22
(continuation...)
Show the forwarding process if a packet arrives at R1 in the
figure with the destination address 180.70.65.140.
Exercise#1
Routing Table for Router 1 (R1)
Mask
Network Address
Next Hop
Interface
/26
180.70.65.192
-
M2
/25
180.70.65.128
-
M0
/24
201.4.22.0
-
M3
/22
201.4.16.0
…
M1
Any
Any
180.70.65.200
m2
See the Excel worksheet to find the solution.
Addressing
23
Exercise#2
Show the forwarding process if a packet arrives at R1 in the
figure with the destination address 18.24.32.78.
subnet:
180.70.65.128/25
180.70.65.135/25
m0
subnet:
201.4.16.0/22
m1
m3
R1
201.4.16.2/22
m2
201.4.22.3/24
subnet:
201.4.22.0/24
180.70.65.194/26
subnet:
180.70.65.192/26
R2
180.70.65.200/26
rest of the
Internet
Addressing
24
Exercise#2
Show the forwarding process if a packet arrives at R1 in the
figure with the destination address 18.24.32.78.
SOLUTION:
All masks are applied, one by one, to the destination address, but no
matching network address is found. When it reaches the end of the
table, the module gives the next-hop router’s address 180.70.65.200
and interface number m2 to ARP (link-layer protocol). This is
probably an out-going packet that needs to be sent, via the default
router, to someplace else in the internet.
Addressing
25
IP datagram format
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
32 bits
ver
header
len
type of
service
Some header fields are
optional. This helps to
indicate where data actually
begins
length
fragment
16-bit identifier flgs
offset
time to upper
Internet
layer
live
checksum
32 bit source IP address
32 bit destination IP address
Options (if any)
data
(variable length,
typically a TCP
or UDP segment)
e.g. IP Broadcast address:
255.255.255.255 – message is delivered
to all hosts on the same network
total datagram
(header + data)
length (bytes)
for
fragmentation/
reassembly
Calculated based
on the header only
(treated as
sequence of 16bits)
E.g. timestamp,
record route
taken, specify
list of routers
to visit.
Addressing
26
IP Fragmentation & Reassembly
Performed by
DESTINATION
HOST
 network links have MTU
(max. transmission unit) largest possible link-level
frame.
 different link types,
different MTUs
 large IP datagram divided
(“fragmented”) within net
 one datagram becomes
several datagrams
(FRAGMENTS)
 “reassembled” only at
final destination
 IP header bits used to
identify, order related
fragments
fragmentation:
in: one large datagram
out: 3 smaller datagrams
reassembly
Supported by IP: MTUs of at least 576 bytes
MSS=536 bytes, TCP segment header=20 bytes, IP datagram header
= 20 bytes27
Addressing
IP Fragmentation and Reassembly
length ID fragflag offset
=4000 =x
=0
=0
One large datagram becomes
several smaller datagrams
payload
1,480 bytes
[0,1479]
1,480 bytes
Multiple of 8 bytes
length
=1500
ID
=x
flag
=1
Offset
0
length
=1500
ID
=x
flag
=1
Offset
185 (i.e. 185 * 8 =1480)
length
=1040
ID
=x
flag
=0
Offset
370 (i.e. 370*8=2960)
[1480,2959]
1,020 bytes
[2960, 3979]
Total Size of Datagram =
4,000 bytes
20 bytes of IP header,
3,980 bytes of IP Payload
Addressing
28
ICMP: Internet Control Message Protocol
 used by hosts, routers,
gateways to communicate
network-level information
 error reporting: unreachable
host, network, port, protocol
 echo request/reply (used by
ping)
 Part of IP, but architecturally lies
“above” IP:
 ICMP msgs are carried as
IP payload
 ICMP message: comprised of
type, code plus first 8 bytes of
IP datagram 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
Addressing
29
End of Session
Addressing
30