# Network Layer (4)

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28 Οκτ 2013 (πριν από 4 χρόνια και 6 μήνες)

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Network Layer (4)

Number of addresses assigned always power of 2

Network portion of address is of
arbitrary
length

a.b.c.d/x

x is number of bits in network portion of address

11001000 00010111

0001000
0 00000000

network

part

host

part

200.23.16.0/23

Example.

Cambridge 194.24.0.0/21 194.24.0.0
--

194.24.7.255

Edinburgh 194.24.8.0/22 194.24.8.0
--

194.24.11.255

(Available) 194.24.12.0/22 194.24.12.0
--

194.24.15.255

Oxford 194.24.16.0/20 194.24.16.0
--

194.24.31.255

223.1.1.1

223.1.1.2

223.1.1.3

223.1.1.4

223.1.2.9

223.1.2.2

223.1.2.1

223.1.3.2

223.1.3.1

223.1.3.27

LAN

first 24 bits are network address

223.1.3.2

223.1.3.1

223.1.3.27

223.1.1.1

223.1.1.3

223.1.1.4

223.1.2.2

223.1.2.1

223.1.2.6

223.1.1.2

223.1.7.0

223.1.7.1

223.1.8.0

223.1.8.1

223.1.9.1

223.1.9.2

Interconnected

system consisting

of six networks

CIDR

A router keeps routing table with entries

-

When an IP packet arrives, the router checks
its routing table to find the longest match.

CIDR

Example.

Cambridge 194.24.0.0/21 194.24.0.0
--

194.24.7.255

Edinburgh 194.24.8.0/22 194.24.8.0
--

194.24.11.255

(Available) 194.24.12.0/22 194.24.12.0
--

194.24.15.255

Oxford 194.24.16.0/20 194.24.16.0
--

194.24.31.255

When a packet addressing to 194.24.17.1
arrives, where should it be sent to?

CIDR

Entry aggregation

How does a router
in Tallahassee route
packet to C,E and
O, assuming that
he has only two

All to New York.

Can he reduce the
size of his routing
table?

C

E

O

N

T

H

CIDR Entry Aggregation

From 194.24.0.0 to
194.24.31.255, all to
N.

So aggregate the three
entries into one
194.24.0.0/19.

The N router can do
the same thing.

C

E

O

N

T

H

CIDR

If later the free
194.24.12.0/22
194.24.12.0
--

194.24.15.255 is
assigned to Pittsburgh
and has to go through
Houston, what should
the router at
Tallahassee do?

C

E

O

N

T

H

P

CIDR

When a packet arrives addressing 194.24.15.8,
the router checks the routing table and there
will be two matches: 194.24.12.0/22 and
194.24.0.0/19. Pick the longest match.

NAT

IP address is a scarce resource.

So, give a company only one or a few IP
addresses used by the gateway router.

Within the company, each machine has an unique

10.0.0.0/8

172.16.0.0/12

192.168.0.0/16

These addresses can only appear within a company
but never on the outside Internet

NAT

Whenever a machine wants to send a packet to the
outside, the packet will be sent to the NAT box.

The NAT box will convert the internal IP address to the
real IP address of the company, and pass the packet to
the gateway router.

When there is a packet destined for an internal
machine arrived at the router, what should the router
and NAT box do?

For IP packets carrying TCP or UDP, use port number.
Other protocols are much more compliated.

NAT

For IP packets carrying TCP or UDP, use port
number.

When an outgoing packet arrives at the NAT box,

The source port number is replaced

When a reply came for this process, use the
replaced source port number as index to find the
correct IP address and original port number.

ICMP

ICMP

Internet Control Message Protocol

Each ICMP message is encapsulated in an IP
packet

Treated like any other datagram, but no error message
sent if ICMP message causes error

Some interesting messages:

Time exceeded: When an IP packet arrived at a router
is dropped because the TTL field becomes 0, the
router will send an ICMP TIME EXCEEDED message
back to the source. Used in
traceroute
.

Computer Science, FSU

16

virtual

Not understood by underlying the hardware of physical networks

IP packets need to be transmitted by the
underlying physical network

Computer Science, FSU

17

ARP Example

Computer Science, FSU

18

ARP Cache

Each computer maintains a
cache
table

same

network

Exchanges ARP messages

To resolve IP addresses with unknown hardware

Computer Science, FSU

19

ARP Protocol

When a node sends an IP packet

To another node on the
same

physical network

Look up destination address in the ARP table

a request to the local network

Computer Science, FSU

20

ARP Response

The target node responds to sender (
unicast
?)

Adds the requester into its ARP table (why?)

On receiving the response

Other nodes upon receiving the request

Refresh the requester entry if already there

No action otherwise (why?)

Table entries deleted if not refreshed for a while

DHCP

DHCP

Dynamic Host Configuration Protocol

A DHCP relay agent got this packet and relay it to
the DHCP server

The DHCP server assigns an IP address

Periodically renew

Hierarchical Routing

aggregate routers into regions,
“autonomous systems” (AS)

routers in same AS run same
routing protocol

“intra
-
AS” routing

protocol

routers in different AS can run
different intra
-
AS routing
protocol

special routers in AS

run intra
-
AS routing
protocol with all other
routers in AS

also

responsible for routing
to destinations outside AS

run
inter
-
AS routing

protocol with other
gateway routers

gateway routers

Intra
-
AS and Inter
-
AS routing

Gateways:

perform inter
-
AS
routing amongst
themselves

perform intra
-
AS
routing with other
routers in their AS

inter
-
AS, intra
-
AS routing
in

gateway A.c

network layer

a

b

b

a

a

C

A

B

d

A.a

A.c

C.b

B.a

c

b

c

Inter
-
AS

routing

between

A and B

Intra
-
AS and Inter
-
AS routing

Host

h2

a

b

b

a

a

C

A

B

d

c

A.a

A.c

C.b

B.a

c

b

Host

h1

Intra
-
AS routing

within AS A

Intra
-
AS routing

within AS B

Why different Intra
-

and Inter
-
AS
routing ?

Policy:

Inter
-
AS: admin wants control over how its traffic
routed, who routes through its net.

Intra
-
AS: single admin, so no policy decisions needed

Scale:

hierarchical routing saves table size, reduced update
traffic

Performance:

Intra
-
AS: can focus on performance

Inter
-
AS: policy may dominate over performance

Intra
-
AS Routing

Also known as Interior Gateway Protocols (IGP)

Most common IGPs:

RIP: Routing Information Protocol

OSPF: Open Shortest Path First

IGRP: Interior Gateway Routing Protocol (Cisco
proprietary)

OSPF

Represents the network as a graph, and runs
the shortest path algorithm to find the path to
any router.

Divide the network into areas for scalability.

The backbone area is called area 0

Within one area, a router has the same link state
database as all other routers. Routers belonging to
two areas keeps two databases. Link state in one
area is not told to other areas.

Route: local area

backbone

local area

OSPF

Each router knows the shortest path to reach
routers within his area.

Backbone routers also accept information
from area border routers to compute the
shortest path to reach other routers. Then
advertise this information to the border
routers, who tells routers inside the area.

To
be able to select the best exit router in an area

OSPF

To learn the link state, use flooding

Inefficient to talk to every router on the same LAN

So, select a designated router and let it to be
adjacent to all other routers on the same LAN.
routers

Messages include

Inter
-
AS routing

Internet Inter
-
AS routing: BGP

BGP (Border Gateway Protocol):
the

de facto standard

Path Vector

protocol:

similar to Distance Vector protocol

each Border Gateway broadcast to neighbors
(peers)
entire path

(I.e, sequence of ASs) to
destination

E.g., Gateway X may send

its path to dest. Z:

Path (X,Z) = X,Y1,Y2,Y3,…,Z

Internet Inter
-
AS routing: BGP

BGP messages exchanged using TCP.

BGP messages:

OPEN:

opens TCP connection to peer and
authenticates sender

UPDATE:

advertises new path (or withdraws old)

KEEPALIVE

keeps connection alive in absence of

reports errors in previous msg;
also used to close connection

Internet Inter
-
AS routing: BGP

Suppose:

gateway X send its path to peer gateway W

W may or may not select path offered by X

cost, policy (don’t route via competitors AS), loop
prevention reasons
.

If W selects path advertised by X, then:

Path (W,Z) = W, Path (X,Z)

Note: X can control incoming traffic by controlling its route advertisements to peers:

e.g., don’t want to route traffic to Z

routes to Z

BGP: an example

NLRI=128.186.0.0/16

ASPATH=[0]

128.186.0.0/16

NLRI=128.186.0.0/16

ASPATH=[10]

NLRI=128.186.0.0/16

ASPATH=[10]

NLRI=128.186.0.0/16

ASPATH=[210]

NLRI=128.186.0.0/16

ASPATH=[610]

NLRI=128.186.0.0/16

ASPATH=[610]

NLRI=128.186.0.0/16

ASPATH=[210]

NLRI=128.186.0.0/16

ASPATH=[7610]

NLRI=128.186.0.0/16

ASPATH=[4210]

NLRI=128.186.0.0/16

ASPATH=[3210]

[3210]*

[4210]

[7610]

BGP

Line FG goes
down.

Node F wants to
find path to D.

B: BCD

E: EFGCD

I: IFGCD

F can quickly
decide to ignore
the paths from I
and E.

A

G

F

E

I

J

H

D

C

B

Virtual Circuit

Destination information is large and the table
is large

Consider 32 bit IP address. A full table will have 4G
entries.

If an IP packet is 1250 byte long and the link speed is
10Gbps, how much time do you have for this lookup?

(1. You don’t have to implement the full table. 2. You
can also use pipeline.)

Virtual Circuit

Circuit means a path between the source and the
destination.

Real circuit switching has a physical path set up between
the source and the destination, like telephone network

When you dial, a request is sent to the network, network finds if
there are free links on the path and reserve that link for you.

Virtual circuit is different

used in packet switching
networks.

No real path set up, because it is packet switching (although link
bandwidth can be reserved).

But still has the connection phase. The purpose is to let the
routers know how to route the packets of this virtual circuit.

Virtual Circuits

When setting up the virtual
circuit, a VC identifier is picked.
The router knows where to
forward a packet with a certain
VC identifier.

Each packet will carry the VC
identifier, which is much
shorter than the full destination
efficient table lookup.

Resources can also be reserved.
QoS.

A practical problem in a
distributed environment

different stations may pick the
same VC identifier.

Labels can be swapped without
causing confusion.

B

E

F

D

C

A

H1

H2

A’s Table

In Out

H1, 1 C, 1

H2, 1 C, 2

H3

C’s Table

In Out

A, 1 E, 1

A, 2 D, 1