Chapter 4 Network Layer

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Network Layer

4
-
1

Chapter 4

Network Layer

Part 2:


IP: Internet Protocol

Computer Networking:
A Top Down Approach

5
th

edition.

Jim Kurose, Keith Ross

Addison
-
Wesley, April
2009.


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

The Internet Network layer

forwarding

table

Host, router network layer functions:

Routing protocols


path selection


RIP, OSPF, BGP

IP protocol


addressing conventions


datagram format


packet handling conventions

ICMP protocol


error reporting


router “signaling”

Transport layer: TCP, UDP

Link layer

physical layer

Network

layer

Network Layer

4
-
4

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
-
5

IP datagram format

ver

length

32 bits

data

(variable length,

typically a TCP

or UDP segment)

16
-
bit identifier

header


checksum

time to

live

32 bit source IP address

IP protocol version

number (4 bits)

header length


( in bytes)(4 bits)

max number

remaining hops

(decremented

by 1 at

each
router; die at 0)

for

fragmentation/

Reassembly;
see next slide

total datagram

length (bytes
)
(this field is 16
bits)

upper layer protocol

to deliver payload
to;

value of 6 = TCP, 17 = UDP

head.

len

type of

service

“type” of data

flgs

fragment


offset

upper


layer

32 bit destination IP address

Options (if any)

E.g. timestamp,

record route

taken, specify

list of routers

to visit.

how much overhead
with TCP?


20 bytes of TCP


20 bytes of
IP (no
options)


= 40 bytes + app
layer overhead

16 bits, so max data is
65,535 bytes. Typical
size is 1,500bytes

Header checksum


Used by routers


Computed by treating each 2 bytes in the header
as a number and sum these using 1s complement.


Store in the checksum field


Router computes header checksum for
each

IP
datagram


Discard datagram if checksum is wrong


Checksum must be recomputed and stored again at
every router; TTL and maybe options fields change

Network Layer

4
-
6

Header checksum


Why are there checksums at both TCP and
IP level?


IP only check IP header, TCP/UDP checksum
entire TCP/UDP segment


TCP/UDP do not have to run on IP (
eg

could run
on ATM)

Network Layer

4
-
7

Network Layer

4
-
8

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

reassembly

*Example: Ethernet up to 1,500 bytes, some wide
-
area links up to 576

Problem
: different links from host
-
to
-
host may by diff types!

Network Layer

4
-
9

IP Fragmentation and Reassembly

ID

=x

offset

=0

fragflag

=0

length

=4000

ID

=x

offset

=0

fragflag

=1

length

=1500

ID

=x

offset

=185

fragflag

=1

length

=1500

ID

=x

offset

=370

fragflag

=0

length

=1040

One large datagram becomes

several smaller datagrams

Example


4000 byte
datagram


MTU = 1500 bytes


1480 bytes in

data field

offset
= multiple of 8 bytes
so

1480/
8 = 185

ID: set by sending host IP layer; typically increments ID num for each
datagram it sends. Last fragment sent has
flag

field set to 0 to indicate
it’s the last fragment; all other fragments have
flag

set to 1

If one fragment is
lost,,IP

discards all
fragments

Fragmentation costs


Complicates routers and end systems


DoS

attacks:
Attacker sends series of bizarre fragments


Jolt2 attack: attacker sends a stream of small fragments
to target host. None has offset of 0. Target collapses as it
attempts to rebuild
datagrams
.


Another attack: send overlapping fragments. OS can crash
attempting to reassemble.


IP v6 has no fragments.


Fragmentation animation:
http://www.awl.com/kurose
-
ross

Network Layer

4
-
10

Fragmentation animation


See:
http://wps.aw.com/aw_kurose_network_4/
63/16303/4173752.cw/index.html

Network Layer

4
-
11

Network Layer

4
-
12

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
-
13

IP Addressing: introduction


IP address:

32
-
bit
identifier for host,
router
interface



interface:

connection
between host/router
and physical link


router’s typically have
multiple interfaces


host typically has one
interface


IP addresses
associated with
each
interface

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

223.1.1.1 = 11011111 00000001 00000001 00000001

223

1

1

1

IPv4 address = 32 bits = 2
32
≈ 4 billion addresses

NAT’s discussed
later

Network Layer

4
-
14

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

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

network consisting of 3 subnets

subnet

Subnet: a network with no routers (e.g., ethernet)

Network Layer

4
-
15

Subnets



223.1.1.0/24

223.1.2.0/24

223.1.3.0/24

Recipe


To determine the
subnets, detach each
interface from its
host or router,
creating islands of
isolated networks with
interfaces terminating
the end points of the
isolated networks.
Each isolated network
is called a
subnet
.

Subnet mask: /
24

(leftmost 24 bits define the subnet address)

Any host on this net must have address 223.1.1.xxx

Network Layer

4
-
16

Subnets

How many?

223.1.1.1

223.1.1.3

223.1.1.4

223.1.2.2

223.1.2.1

223.1.2.6

223.1.3.2

223.1.3.1

223.1.3.27

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

6

Network Layer

4
-
17

IP addressing: CIDR

CIDR:

C
lassless
I
nter
D
omain
R
outing


The internet’s address assignment strategy


subnet portion of address of arbitrary length


address format:
a.b.c.d/x
, where x is # bits in subnet
portion of address


The x bits are called the
prefix
or
network prefix

11001000 00010111

0001000
0 00000000

subnet

part

host

part

200.23.16.0/23

Network Layer

4
-
18

IP addressing: CIDR

CIDR:

C
lassless
I
nter
D
omain
R
outing


An organization is assigned a block of contiguous addresses
with a common prefix


The
prefix
(leading
x
bits) are only part of address
considered by routers outsie the organization.


Remaining bits (32
-
x
) distinguish among thedevices
within

the organization.


These bits may (or may not) also be subnetted

11001000 00010111

0001000
0 00000000

subnet

part

host

part

200.23.16.0/23

Network Layer

4
-
19

IP addressing: classes

classful addressing


Used before CIDR


Three classes (A, B, C) of 8, 16, or 24 bits respectively


Problem: not very flexible. Class C subnet could
accommodate only up to 2
8

or 254 (2 addresses were
reserved for special uses) devices!


Class B subnets could accommodate 65,634 hosts (too large
for most small businesses)

11001000 00010111

0001000
0 00000000

subnet

part

host

part

200.23.16.0/23

Network Layer

4
-
20

IP addressing: classes

broadcasting


Special address 255.255.255.255


When a host sends a datagram with this address, it is
received by all hosts on the subnet.


Routers optionally forward message into neighboring
subnets (but usually don’t)

11001000 00010111

0001000
0 00000000

subnet

part

host

part

200.23.16.0/23

Network Layer

4
-
21

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

ISP's block
11001000 00010111 0001
0000 00000000 200.23.16.0/20


Organization 0
11001000 00010111 0001000
0 00000000 200.23.16.0/23

Organization 1
11001000 00010111 0001001
0 00000000 200.23.18.0/23

Organization 2
11001000 00010111 0001010
0 00000000 200.23.20.0/23


... ….. …. ….

Organization 7
11001000 00010111 0001111
0 00000000 200.23.30.0/23



Example


ISP: Fly
-
by
-
Night
-
ISP advertises to
outside world that it should be sent any
datagrams whose first 20 address bits
match 200.23.26.0/20


There are 8 organizations using FbN
-
ISP
each with own subnet


Address aggregation (or route aggregation
or route summarization)

Network Layer

4
-
22

Network Layer

4
-
23

Hierarchical addressing: route aggregation

“Send me anything

with addresses

beginning

200.23.16.0/20”

200.23.16.0/23

200.23.18.0/23

200.23.30.0/23

Fly
-
By
-
Night
-
ISP

Organization 0

Organization 7

Internet

Organization 1

ISPs
-
R
-
Us

“Send me anything

with addresses

beginning

199.31.0.0/16”

200.23.20.0/23

Organization 2

.

.

.

.

.

.

Hierarchical addressing allows efficient advertisement of routing

information:

FbN
-
ISP buys ISPs
-
R
-
Us which owns 199.31.0.0/16


Example


Now FbN
-
ISP wants to transfer Organization 1 to
ISPs
-
R
-
Us.


Problem: Organization 1’s IP are outside of ISPs
-
R
-
Us
address block


Don’t want to make Org 1 change all it’s addresses!


Solution: ISPs
-
R
-
Us will advertise more
addresses.


Also advertise 200.23.28.0/23


Since 200.23.28.0/23 is a longer prefix than
200.23.16.0/20, when routers see 200.23.28.0 they will
send to ISPs
-
R
-
Us

Network Layer

4
-
24

Network Layer

4
-
25

Hierarchical addressing: more specific
routes

ISPs
-
R
-
Us has a more specific route to Organization 1

“Send me anything

with addresses

beginning

200.23.16.0/20”

200.23.16.0/23

200.23.18.0/23

200.23.30.0/23

Fly
-
By
-
Night
-
ISP

Organization 0

Organization 7

Internet

Organization 1

ISPs
-
R
-
Us

“Send me anything

with addresses

beginning 199.31.0.0/16

or 200.23.18.0/23”

200.23.20.0/23

Organization 2

.

.

.

.

.

.

Network Layer

4
-
26

IP addressing: the last word...

Q:

How does an ISP get block of addresses?

A:

ICANN
:
I
nternet
C
orporation for
A
ssigned


N
ames and
N
umbers


allocates addresses


manages DNS


assigns domain names, resolves disputesICANN


allocates addresses to regional internet
registries (e.g., ARIN, RIPE, APNIC, and
LACNIC)



Network Layer

4
-
27

IP addresses: how to get one?

Q:

How does an
ISP
get IP addresses?



ISP gets a set of IP addresses from a
local Internet registry (LIR) or
national Internet registry (NIR), or from their appropriate Regional
Internet Registry (RIR)


ISP is allocated an address block, e.g., 200.23.16.0/20


ISP could then divides its address block into small contiguous blocks
and sells these


Example: could divide its block into 8 equal sized contiguous address
blocks (subnet is underlined):

ISP’s block

200.23.16.0/20

11001000 00010111 0001
0000 00000000

Organization 0

200.23.16.0/23

11001000 00010111 0001000
0 00000000

Organization 1

200.23.18.0/23

11001000 00010111 0001001
0 00000000

Organization 2

200.23.20.0/23

11001000 00010111 0001010
0 00000000



Organization 7

200.23.30.0/23

11001000 00010111 0001111
0 00000000




Network Layer

4
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28

IP addresses: how to get one?

Q:

How does a
host

get IP address?



hard
-
coded by system admin in a file


Windows: control
-
panel
-
>network
-
>configuration
-
>tcp/ip
-
>properties


UNIX: /etc/rc.config


Routers are also hard
-
coded with a network
management system


DHCP:

D
ynamic
H
ost
C
onfiguration
P
rotocol:
dynamically get address from as server


“plug
-
and
-
play”



Network Layer

4
-
29

DHCP: Dynamic Host Configuration Protocol

Goal:

allow host to
dynamically
obtain its IP address from network
server when it joins network


Can renew its lease on address in use


Can configure server to give a host the same address each time it
connects


Allows reuse of addresses (only hold address while connected an “on”)


Support for mobile users who want to join network (more shortly)

Network Layer

4
-
30

DHCP: Dynamic Host Configuration Protocol

Example:



Student carries laptop from dorm room to library to Williams 309.


In each location may connect to a different subnet and will need a new IP address


Example:



ISP has 2,000 customers but at most only 400 are connected at one time


Only needs a block of 512 addresses (e.g., a block of the form a.b.c.c/23)


Each time host joins, assign an unused IP; when a host leaves, put it’s IP into a pool of free
addresses

Network Layer

4
-
31

DHCP: Dynamic Host Configuration Protocol

DHCP overview:


Client
-
server protocol


Could have a DHCP server for each subnet


or a DHCP relay agent (typically a router) that sends request to a
DHCP server that it knows



Network Layer

4
-
32

DHCP client
-
server scenario

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

A

B

E



DHCP



server





arriving
DHCP

client

needs

address in this

network

Router acts as relay agent for subnets 223.1.1.x and 223.1.3.x

Network Layer

4
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DHCP: Dynamic Host Configuration Protocol

DHCP overview:


host broadcasts “
DHCP discover

msg

[optional
] UDP packet to port
67.using broadcast address 255.255.255.255 (source address 0.0.0.0).


DHCP server responds with “
DHCP offer

msg

[optional
] using
broadcast
address 255.255.255.255


This message contains a transaction ID of the received discover message


Also contains the proposed IP address for the client, the network mask


and an IP address lease time (amt of time the IP address is valid; hours to days)


host requests IP address: “
DHCP request

msg


Echos back the received configuration parameters


The host may have received an offer from several servers; must choose 1


DHCP server sends address: “
DHCP
ack

msg


Address is now confirmed!


There is also a mechanism for lease renewal!

Network Layer

4
-
34

DHCP client
-
server scenario

DHCP server: 223.1.2.5

arriving


client

time

DHCP discover

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

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
-
35

DHCP: more than IP address

DHCP can return more than just allocated IP
address on subnet:


address of first
-
hop router for client


name and IP address of DNS sever


network mask (indicating network versus host
portion of address)

Network Layer

4
-
36

DHCP: example


connecting laptop needs its
IP address, addr of first
-
hop router, addr of DNS
server: use DHCP

router

(runs DHCP)

DHCP

UDP

IP

Eth

Phy

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP

UDP

IP

Eth

Phy

DHCP

DHCP

DHCP

DHCP

DHCP


DHCP request encapsulated
in UDP, encapsulated in IP,
encapsulated in 802.1
Ethernet



Ethernet frame broadcast
(dest:
FFFFFFFFFFFF
) on LAN,
received at router running
DHCP server


Ethernet demux’ed to IP
demux’ed, UDP demux’ed to
DHCP

168.1.1.1


Network Layer

4
-
37


DCP server formulates
DHCP ACK containing
client’s IP address, IP
address of first
-
hop
router for client, name &
IP address of DNS server


router

(runs DHCP)

DHCP

UDP

IP

Eth

Phy

DHCP

DHCP

DHCP

DHCP

DHCP

UDP

IP

Eth

Phy

DHCP

DHCP

DHCP

DHCP

DHCP


encapsulation of DHCP
server, frame forwarded
to client, demux’ing up to
DHCP at client


client now knows its IP
address, name and IP
address of DSN server, IP
address of its first
-
hop
router


DHCP: example

Network Layer

4
-
38

DHCP: wireshark
output
(home LAN)

Message type:
Boot Reply (2)

Hardware type: Ethernet

Hardware address length: 6

Hops: 0

Transaction ID: 0x6b3a11b7

Seconds elapsed: 0

Bootp flags: 0x0000 (Unicast)

Client IP address: 192.168.1.101 (192.168.1.101)

Your (client) IP address: 0.0.0.0 (0.0.0.0)

Next server IP address: 192.168.1.1 (192.168.1.1)

Relay agent IP address: 0.0.0.0 (0.0.0.0)

Client MAC address: Wistron_23:68:8a (00:16:d3:23:68:8a)

Server host name not given

Boot file name not given

Magic cookie: (OK)

Option: (t=53,l=1) DHCP Message Type = DHCP ACK

Option: (t=54,l=4) Server Identifier = 192.168.1.1

Option: (t=1,l=4) Subnet Mask = 255.255.255.0

Option: (t=3,l=4) Router = 192.168.1.1

Option: (6) Domain Name Server


Length: 12; Value: 445747E2445749F244574092;


IP Address: 68.87.71.226;


IP Address: 68.87.73.242;


IP Address: 68.87.64.146

Option: (t=15,l=20) Domain Name = "hsd1.ma.comcast.net."


reply

Message type:
Boot Request (1)

Hardware type: Ethernet

Hardware address length: 6

Hops: 0

Transaction ID: 0x6b3a11b7

Seconds elapsed: 0

Bootp flags: 0x0000 (Unicast)

Client IP address: 0.0.0.0 (0.0.0.0)

Your (client) IP address: 0.0.0.0 (0.0.0.0)

Next server IP address: 0.0.0.0 (0.0.0.0)

Relay agent IP address: 0.0.0.0 (0.0.0.0)

Client MAC address: Wistron_23:68:8a (00:16:d3:23:68:8a)

Server host name not given

Boot file name not given

Magic cookie: (OK)

Option: (t=53,l=1)
DHCP Message Type = DHCP Request

Option: (61) Client identifier


Length: 7; Value: 010016D323688A;


Hardware type: Ethernet


Client MAC address: Wistron_23:68:8a (00:16:d3:23:68:8a)

Option: (t=50,l=4) Requested IP Address = 192.168.1.101

Option: (t=12,l=5) Host Name = "nomad"

Option: (55) Parameter Request List


Length: 11; Value: 010F03062C2E2F1F21F92B


1 = Subnet Mask; 15 = Domain Name


3 = Router; 6 = Domain Name Server


44 = NetBIOS over TCP/IP Name Server


……

request

DHCP


Advantage: no setting IP addresses by
hand!


Disadvantage: Cannot maintain a TCP
connection to a remote application when
move from one subnet to another


The IP address will change


Why is this a problem with TCP? Hint: think
about sockets.


A recent extension to IP (mobile IP) solves this
problem.

Network Layer

4
-
39

NAT: Network Address
Translation


Problem:


In your home you have several computers,
laptops, mobile phones, networked game
-
boys,
X
-
Boxes, tablets, even a networked
refridgerator.


What if the ISP has already allocated the
contiguous portions of the SOHO network’s
current address range?


Solution: NAT

Network Layer

4
-
40

Network Layer

4
-
41

NAT: Network Address Translation

10.0.0.1

10.0.0.2

10.0.0.3

10.0.0.4

138.76.29.7

local network

(e.g., home network)

10.0.0/24

rest of

Internet

Datagrams with source or

destination in this network

have 10.0.0/24 address for

source, destination (as usual)


(devices get their IP from a DHCP
server running within the router!)

All

datagrams
leaving

local

network have
same

single source
NAT IP address: 138.76.29.7,

different source
port
numbers


(so the router does not look like a
“router” but as a
single

device)

NAT realms


RFC 1918 reserves 3 portions of the IP address
space for private networks or
realms


A
realm with private addresses



Refers to a network whose addresses only have meaning
to devices within that network


One of these realms is 10.0.0.0/8


Private realms may use the address space 10.0.0/24


These addresses are
not

known to the world, only within
the realm

Network Layer

4
-
42

Network Layer

4
-
43

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
-
44

NAT: Network Address Translation

Implementation:

NAT router must:



outgoing datagrams:

replace

(source IP address, port
#) of every outgoing datagram to (NAT IP address,
new port #)

. . . remote clients/servers will respond using (NAT
IP address, new port #) as destination addr.



remember (in NAT translation table)
every (source
IP address, port #) to (NAT IP address, new port #)
translation pair



incoming datagrams:

replace

(NAT IP address, new
port #) in dest fields of every incoming datagram
with corresponding (source IP address, port #)
stored in NAT table


Network Layer

4
-
45

NAT: Network Address Translation

10.0.0.1

10.0.0.2

10.0.0.3

S: 10.0.0.1, 3345

D: 128.119.40.186, 80

1

10.0.0.4

138.76.29.7

1:

host 10.0.0.1

sends datagram to

128.119.40.186, 80

NAT translation table

WAN side addr LAN side addr

138.76.29.7, 5001 10.0.0.1, 3345

…… ……

S: 128.119.40.186, 80

D: 10.0.0.1, 3345


4

S: 138.76.29.7, 5001

D: 128.119.40.186, 80

2

2:

NAT router

changes datagram

source addr from

10.0.0.1, 3345 to

138.76.29.7, 5001,

updates table

S: 128.119.40.186, 80

D: 138.76.29.7, 5001


3

3:

Reply arrives


dest. address:


138.76.29.7, 5001

4:

NAT router

changes datagram

dest addr from

138.76.29.7, 5001 to 10.0.0.1, 3345



Network Layer

4
-
46

NAT: Network Address Translation


16
-
bit port
-
number field:


60,000 simultaneous connections with a single LAN
-
side
address!


NAT is controversial:


routers should only process up to layer 3


Messes with application servers running
within

the lan


violates end
-
to
-
end argument


NAT possibility must be taken into account by app
designers, eg, P2P applications


Must use a hack to get around this (
connection reversal
)


address shortage should instead be solved by IPv6

Network Layer

4
-
47

NAT traversal problem


client wants to connect to
server with address 10.0.0.1


server address 10.0.0.1 local
to LAN (client can’t use it as
destination addr)


only one externally visible
NATted address: 138.76.29.7


solution 1: statically
configure NAT to forward
incoming connection
requests at given port to
server


e.g., (123.76.29.7, port 2500)
always forwarded to 10.0.0.1
port 25000

10.0.0.1

10.0.0.4

NAT

router

138.76.29.7

Client

?

Network Layer

4
-
48

NAT traversal problem


solution 2: Universal Plug and
Play (UPnP) Internet Gateway
Device (IGD) Protocol. Allows
NATted host to:


learn public IP address
(138.76.29.7)


add/remove port mappings
(with lease times)


i.e., automate static NAT port
map configuration

10.0.0.1

10.0.0.4

NAT

router

138.76.29.7

IGD

Network Layer

4
-
49

NAT traversal problem


solution 3: relaying (used in Skype)


NATed client establishes connection to relay


External client connects to relay


relay bridges packets between to connections


138.76.29.7

Client

10.0.0.1

NAT

router

1.

connection to

relay initiated

by NATted host

2.

connection to

relay initiated

by client

3.

relaying

established

Network Layer

4
-
50

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
-
51

ICMP: Internet Control Message Protocol


used by hosts & routers to
communicate network
-
level
information


error reporting:
unreachable host, network,
port, protocol


echo request/reply (used
by ping)


network
-
layer “above” IP:


ICMP msgs carried in IP
datagrams


ICMP has an “upper layer”
code just as “TCP” does


ICMP message:

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


ICMP


Ping


Sends ICMP type 8
code 0 to specified
host


Destination host
sends back a ICMP
type 0 code 0 echo
reply


Quench message
(seldom used)


Congestion control


Congested router
sends ICMP source
quench message to
host to force it to
reduce transmission
rate


TCP uses it’s own
mechanism

Network Layer

4
-
52

ICMP


Tracerout


Traces a route from a host to any other host


Uses ICMP messages

Network Layer

4
-
53

Network Layer

4
-
54

Traceroute and ICMP


Source sends series of
UDP segments to dest


First has TTL =1


Second has TTL=2, etc.


Unlikely

port number


When nth datagram arrives
to nth router:


TTL reaches 0


Router discards datagram


And sends to source an
ICMP message (type 11,
code 0)


Message includes name of
router& IP address


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.

Network Layer

4
-
55

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
-
56

IPv6


Initial motivation:

32
-
bit address space soon
to be completely allocated.


Additional motivation:


header format helps speed processing/forwarding


header changes to facilitate QoS

IPv6 datagram format:



fixed
-
length 40 byte header


no fragmentation allowed

Network Layer

4
-
57

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

Network Layer

4
-
58

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


ICMPv6:

new version of ICMP


additional message types, e.g. “Packet Too Big”


multicast group management functions

Network Layer

4
-
59

Transition From IPv4 To IPv6


Not all routers can be upgraded simultaneous


no “flag days”


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
-
60

Tunneling

A

B

E

F

IPv6

IPv6

IPv6

IPv6

tunnel

Logical view:

Physical view:

A

B

E

F

IPv6

IPv6

IPv6

IPv6

IPv4

IPv4

Network Layer

4
-
61

Tunneling

A

B

E

F

IPv6

IPv6

IPv6

IPv6

tunnel

Logical view:

Physical view:

A

B

E

F

IPv6

IPv6

IPv6

IPv6

C

D

IPv4

IPv4

Flow: X

Src: A

Dest: F



data

Flow: X

Src: A

Dest: F



data

Flow: X

Src: A

Dest: F



data

Src:B

Dest: E

Flow: X

Src: A

Dest: F



data

Src:B

Dest: E

A
-
to
-
B:

IPv6

E
-
to
-
F:

IPv6

B
-
to
-
C:

IPv6 inside

IPv4

B
-
to
-
C:

IPv6 inside

IPv4