1. Datagram Networks: Internet Protocol (IPv4)

2. The Internet Network layer: IP
Internet Network Layer Components:
IP protocol (addressing, datagram format and handling), Routing Protocols and ICMP protocol
forwarding
table
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

3. 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 may have multiple interfaces
IP addresses associated with each interface =
223 1 1 1

4. network consisting of 3 IP networks:
IP Addressing
IP address:
network part (high order bits)
host part (low order bits)
NetworkID.HostID
What’s an IP network ?
device interfaces with same network part of IP address
Hosts within the same IP network can reach each other without intervening router
network consisting of 3 IP networks:
, ,

5. IP Addressing IP networks are not restricted to Ethernet segments
IP networks are not restricted to Ethernet segments
Here we have 3 point-to-point links and each have a different IP network defined over them.
Interconnected
system consisting
of 6 IP networks

6. IP Addressing How to find IP networks?
How to find IP networks?
Detach each interface from router, host
Create “islands of isolated IP networks
Each island defines an IP network
Internet consists of millions of such IP networks
Interconnected
system consisting
of 6 IP networks

7. IP Addresses
given notion of “IP network”, let’s re-examine IP addresses:
We have “class-full” addressing
Original Internet design
class to A
network
host B to 10
network
host to C
110
network
host to D
1110
multicast address
32 bits

8. IP addressing: 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
No longer used in the current Internet
Solution? Classless Inter Domain Routing (CIDR)
CIDR: Classless InterDomain Routing
Standardized in 1993
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
/23

9. Netmask
With CIDR a new way is needed to determine the IP network given an IP address:
Solution: Define a netmask
Given an IP address of the form networkID.hostID, the netmask of the IP address is obtained by putting all “1”s in the networkID portion and all “0”s in the hostID portion
network
part
host
/23 =
Given an IPAddr and a netmask, we bit-wise AND IPAddr and netmask to obtain the IP network. The rest is the hostID.
NetworkID = IPAddr & Netmask
HostID = IPAddr & ~Netmask

10. IP Layer Broadcast
Recall that a host can send a LL broadcast message by putting FF-FF-FF-FF-FF-FF in destination MAC address
How can a host send IP-layer broadcast packet? 2 ways:
Put in destination IP
Means that all IP hosts within the same LL broadcast domain will receive this IP datagram
Make <hostID> all 1’s
Means that all hosts within the same IP subnet will receive this datagram
Example: If IP subnet is , then a packet with a destination IP: will be received by all hosts whose IP subnet is

11. IP addresses: how to get one?
Q: How does a host get an IP address?
hard-coded by system admin in a file
Wintel: control-panel->network->configuration->tcp/ip->properties
UNIX: /etc/rc.config
DHCP: Dynamic Host Configuration Protocol: dynamically get address from as server
“plug-and-play” (more shortly)

12. IP addresses: how to get one?
Q: How does network get network part of IP addr?
A: gets allocated portion of its provider ISP’s address space
ISP's block /20
Organization /23
Organization /23
Organization /23
… … ….
Organization /23

13. Hierarchical addressing: route aggregation
CIDRized addresses facilitate hierarchical routing
Fly-By-Night-ISP advertises that any IP datagram whose addresses begin with /20 should be sent to it. The rest of the world need not that there are 8 other organizations each with its own IP network. This is called route aggregation
Dividing an IP network into smaller IP networks as done in here is called subnetting. Each organization can further divide their IP address range into smaller IP subnets
Organization 0
/23
Organization 1
“Send me anything
with addresses
beginning
/20”
/23
Organization 2
/23 .
Fly-By-Night-ISP .
Internet
Organization 7
/23
“Send me anything
with addresses
beginning
/16”
ISPs-R-Us

14. Hierarchical addressing: more specific routes
ISPs-R-Us has a more specific route to Organization 1. By the longest prefix matching rule, packets with destination addresses beginning with /23 are sent to ISPs-R-Us
Organization 0
/23
“Send me anything
with addresses
beginning
/20”
Organization 2
/23 .
Fly-By-Night-ISP .
Internet
Organization 7
/23
“Send me anything
with addresses
beginning /16
or /23”
ISPs-R-Us
Organization 1
/23

15. IP addressing: the last word...
Q: How does an ISP get block of addresses?
A: ICANN: Internet Corporation for Assigned
Names and Numbers
allocates addresses
manages DNS root servers
assigns domain names, resolves disputes

16. 32 bit destination IP address
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
header length
(bytes)
max number
remaining hops
(decremented at
each router)
for
fragmentation/
reassembly
total datagram
length (bytes)
upper layer protocol
to deliver payload to
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
= 40 bytes + app layer overhead

17. Forwarding: Getting a datagram from source to dest.
forwarding table in A
Dest. Net. Next Hop Nhops
IP datagram:
misc
fields
source
IP addr
dest
data A B E
datagram remains unchanged, as it travels from source to destination

18. Forwarding: Getting a datagram from source to dest.
forwarding table in A
misc
fields
Dest. Net. next router Nhops
data
Starting at A, send IP datagram addressed to B:
look up network address of B in forwarding table
find B is on same net. as A
B and A are directly connected
link layer will send datagram directly to B inside link-layer frame. How? A B E

19. Delivering the packet from A to B A B E
Starting at A, given IP datagram addressed to B:
look up net. address of B, find B on same net. as A
link layer send datagram to B inside link-layer frame
How does A know the MAC address of B?  ARP protocol
frame source,
dest address
datagram source,
dest address
B’s MAC
addr
A’s MAC
addr
A’s IP
addr
B’s IP
addr
IP payload
datagram
frame

20. ARP: Address Resolution Protocol
Question: how to determine
MAC address of B
given B’s IP address?
Each IP node (Host, Router) on LAN has ARP module, table
ARP Table: IP/MAC address mappings for some LAN nodes
< IP address; MAC address; TTL>
< ………………………….. >
TTL (Time To Live): time after which address mapping will be forgotten (typically 20 min)

21. ARP protocol A knows B's IP address, wants to learn MAC address of B
A broadcasts ARP query pkt, containing B's IP address
all machines on LAN receive ARP query
B receives ARP packet, replies to A with its (B's) MAC address
A caches (saves) IP-to-MAC address pairs until information becomes old (times out)
soft state: information that times out (goes away) unless refreshed

22. Forwarding: Getting a datagram from source to dest.
forwarding table in A
misc
fields
Dest. Net. next router Nhops
data
Starting at A, dest. E:
look up network address of E in forwarding table
E on different network
A, E not directly attached
routing table: next hop router to E is
link layer sends datagram to router inside link-layer frame
datagram arrives at
continued….. A B E

23. Forwarding: Getting a datagram from source to dest.
forwarding table in router
Dest. Net router Nhops interface
misc
fields
data
Arriving at , destined for
look up network address of E in router’s forwarding table
E on same network as router’s interface
router, E directly attached
link layer sends datagram to inside link-layer frame via interface
datagram arrives at A B E

24. Another IP Packet Forwarding Example
A creates IP packet with source A, destination E
A uses ARP to get R’s MAC address for
A creates Ethernet frame with R's MAC as dest, Ethernet frame contains A-to-E IP datagram
A’s data link layer sends Ethernet frame to R
R’s data link layer receives Ethernet frame
R removes IP datagram from Ethernet frame, sees its destined to E
R uses ARP to get E’s MAC address
R creates frame containing A-to-E IP datagram sends to E A R E

25. IP packet forwarding algorithm
D = destination IP address
Bool found = false;
For each forwarding table entry (SubnetNumber, SubnetMask, NextHop) do {
D1 = SubnetMask & D;
if (D1 == SubnetNumber) {
if (NextHop is an Interface) {
Deliver the datagram directly to the destination within a LL frame
} else {
Deliver the datagram to NextHop (a router)
} //end-else
found = true;
break;
} //end-if
} //end-for
If (!found) {
if (there is a <default> router) Deliver the datagram to the <default> router
else “Report: Destination unreachable”

26. IP packet forwarding: Example
Subnet Number SubnetMask NextHop
Interface0
Interface1 R2
<default> R3
Assume Destination IP =
D1 = & =  Deliver the datagram to the destination over Interface0
Assume Destination IP =
D1 = & =  Deliver datagram to the destination over Interface1
Assume Destination IP =
IP subnet will not match any of the known IP subnetsDeliver the packet to the <default> router, R3

27. IP Fragmentation & Reassembly
network links have MTU (max.transfer unit) - 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

28. IP Fragmentation & ReassemblyID =x
offset =0
fragflag
length
=4000 =1
=1500
=1480
=2960
=1040
One large datagram becomes
several smaller datagrams
Example
4000 byte datagram
3980 byte payload
MTU = 1500 bytes
1st packet payload: 1480 bytes
2nd packet payload: 1480 bytes
3rd packet payload: 1020 bytes

29. ICMP: Internet Control Message Protocol
Used by hosts, routers, gateways to communication network-level information
error reporting: unreachable host, network, port, protocol
echo request/reply (used by ping)
ICMP runs over IP:
ICMP msgs carried in IP datagrams
ICMP message: type, code plus first 8 bytes of IP datagram causing error
Type Code description
echo reply (ping)
dest. network unreachable
dest host unreachable
dest protocol unreachable
dest port unreachable
dest network unknown
dest host unknown
source quench (congestion
control - not used)
echo request (ping)
route advertisement
router discovery
TTL expired
bad IP header

30. Host IP Address Configuration
Q: How does a host get an IP address?
hard-coded by system admin in a file
Wintel: control-panel->network->configuration->tcp/ip->properties
UNIX: /etc/rc.config
DHCP: Dynamic Host Configuration Protocol: dynamically get address from as server
“plug-and-play” (more shortly)

31. 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
Allows reuse of addresses (only hold address while connected an “on”
Support for mobile users who want to join network (more shortly)
DHCP overview:
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

32. DHCP client-server scenario
server B
arriving DHCP
client needs
address in this
network E

33. DHCP client-server scenario
DHCP server:
arriving
client
DHCP discover
src : , 68
dest.: ,67
yiaddr:
transaction ID: 654
DHCP offer
src: , 67
dest: , 68
yiaddrr:
transaction ID: 654
Lifetime: 3600 secs
DHCP request
src: , 68
dest:: , 67
yiaddrr:
transaction ID: 655
Lifetime: 3600 secs
time
DHCP ACK
src: , 67
dest: , 68
yiaddrr:
transaction ID: 655
Lifetime: 3600 secs

34. NAT: Network Address Translation
rest of
Internet
local network
(e.g., home network)
10.0.0/24
All datagrams leaving local
network have same single source NAT IP address: ,
different source port numbers
Datagrams with source or
destination in this network
have /24 address for
source, destination (as usual)

35. NAT: Network Address Translation
Motivation: local network uses just one IP address as far as outside word is concerned:
no need to be allocated range of addresses from ISP: - just one IP address is used 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).

36. NAT: Network Address Translation
NAT translation table
WAN side addr LAN side addr
1: host
sends datagram to
, 80
2: NAT router
changes datagram
source addr from
, 3345 to
, 5001,
updates table
, , 3345
…… ……
S: , 3345
D: , 80 1
S: , 80
D: , 3345 4
S: , 5001
D: , 80 2
S: , 80
D: , 5001 3
4: NAT router
changes datagram
dest addr from
, 5001 to , 3345
3: Reply arrives
dest. address:
, 5001

37. 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

38. 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
violates end-to-end argument
NAT possibility must be taken into account by app designers, eg, P2P applications
What about embedded IP addresses?
FTP, SMTP, SIP etc.
address shortage should instead be solved by IPv6