1. 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

2. The Internet Network layer
Host, router network layer functions:
Transport layer: TCP, UDP
IP protocol
addressing conventions
datagram format
packet handling conventions
Routing protocols
path selection
RIP, OSPF, BGP
Network
layer
forwarding
table
ICMP protocol
error reporting
router “signaling”
Link layer
physical layer
Network Layer

3. 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. 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
Network Layer

5. 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
Network Layer

6. IP Fragmentation and ReassemblyID =x
offset =0
fragflag
length
=4000 =1
=1500
=185
=370
=1040
One large datagram becomes
several smaller datagrams
Example
4000 byte datagram
MTU = 1500 bytes
1480 bytes in data field
Why divide by 8?
offset =
1480/8
Network Layer

7. 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

8. What is an IP Address?
An IP address is a unique global address for a network interface
Exceptions:
Dynamically assigned IP addresses ( DHCP, Lab 7)
IP addresses in private networks ( NAT, Lab 7)
An IP address:
- is a 32 bit long identifier
- encodes a network number (network prefix)
and a host number
Network Layer

9. Network prefix and host number
The network prefix identifies a network and the host number identifies a specific host (actually, interface on the network).
How do we know how long the network prefix is?
Before 1993: The network prefix is implicitly defined (see class-based addressing) or
After 1993: The network prefix is indicated by a netmask.
network prefix
host number
Network Layer

10. Dotted Decimal Notation
IP addresses are written in a so-called dotted decimal notation
Each byte is identified by a decimal number in the range [0..255]:
Example:
1st Byte
= 128
2nd Byte
= 143
3rd Byte
= 137
4th Byte
= 144
Network Layer

11. Example 128.143 137.144 Example: ellington.cs.virginia.edu
Network address is: (or )
Host number is:
Netmask is: (or ffff0000)
Prefix or CIDR notation: /16
Network prefix is 16 bits long
Network Layer

12. Special IP Addresses Reserved or (by convention) special addresses:
Loopback interfaces
all addresses are reserved for loopback interfaces
Most systems use as loopback address
loopback interface is associated with name “localhost”
IP address of a network
Host number is set to all zeros, e.g.,
Broadcast address
Host number is all ones, e.g.,
Broadcast goes to all hosts on the network
Often ignored due to security concerns
Test / Experimental addresses Certain address ranges are reserved for “experimental use”. Packets should get dropped if they contain this destination address (see RFC 1918):
Convention (but not a reserved address)
Default gateway has host number set to ‘1’, e.g., e.g.,
Network Layer

13. Subnetting University Network Subnetting Engineering Medical School
Problem: Organizations have multiple networks which are independently managed
Solution 1: Allocate a separate network address for each network
Difficult to manage
From the outside of the organization, each network must be addressable.
Solution 2: Add another level of hierarchy to the IP addressing structure
Engineering
School
Medical
School
Library
Subnetting
Network Layer

14. Address assignment with subnetting
Each part of the organization is allocated a range of IP addresses (subnets or subnetworks)
Addresses in each subnet can be administered locally
University Network
/16
Engineering
School
Medical
School
/ /24
/24
Library
/24
Network Layer

15. Basic Idea of Subnetting
Split the host number portion of an IP address into a subnet number and a (smaller) host number.
Result is a 3-layer hierarchy
Then:
Subnets can be freely assigned within the organization
Internally, subnets are treated as separate networks
Subnet structure is not visible outside the organization
network prefix
host number
network prefix
subnet number
host number
extended network prefix
Network Layer

16. Subnetmask
Routers and hosts use an extended network prefix (subnetmask) to identify the start of the host numbers
Network Layer

17. Advantages of Subnetting
With subnetting, IP addresses use a 3-layer hierarchy:
Network
Subnet
Host
Reduces router complexity. Since external routers do not know about subnetting, the complexity of routing tables at external routers is reduced.
Note: Length of the subnet mask need not be identical at all subnetworks.
Network Layer

18. Example: Subnetmask 128.143.0.0/16 is the IP address of the network
/24 is the IP address of the subnet
is the IP address of the host
(or ffffff00) is the subnetmask of the host
When subnetting is used, one generally speaks of a “subnetmask” (instead of a netmask) and a “subnet” (instead of a network)
Use of subnetting or length of the subnetmask if decided by the network administrator
Consistency of subnetmasks is responsibility of administrator
Network Layer

19. No Subnetting
All hosts think that the other hosts are on the same network
Network Layer

20. With Subnetting
Hosts with same extended network prefix belong to the same network
Network Layer

21. With Subnetting
Different subnetmasks lead to different views of the size of the scope of the network
Network Layer

22. Classful IP Adresses (Until 1993)
When Internet addresses were standardized (early 1980s), the Internet address space was divided up into classes:
Class A: Network prefix is 8 bits long
Class B: Network prefix is 16 bits long
Class C: Network prefix is 24 bits long
Each IP address contained a key which identifies the class:
Class A: IP address starts with “0”
Class B: IP address starts with “10”
Class C: IP address starts with “110”
Network Layer

23. The old way: Internet Address Classes
Network Layer

24. The old way: Internet Address Classes
We will learn about multicast addresses later in this course.
Network Layer

25. Problems with Classful IP Addresses
By the early 1990s, the original classful address scheme had a number of problems
Flat address space. Routing tables on the backbone Internet need to have an entry for each network address. When Class C networks were widely used, this created a problem. By the 1993, the size of the routing tables started to outgrow the capacity of routers.
Other problems:
Too few network addresses for large networks
Class A and Class B addresses were gone
Limited flexibility for network addresses:
Class A and B addresses are overkill (>64,000 addresses)
Class C address is insufficient (requires 40 Class C addresses)
Network Layer

26. Allocation of Classful Addresses
Network Layer

27. CIDR - Classless Interdomain Routing
IP backbone routers have one routing table entry for each network address:
With subnetting, a backbone router only needs to know one entry for each Class A, B, or C networks
This is acceptable for Class A and Class B networks
27 = 128 Class A networks
214 = 16,384 Class B networks
But this is not acceptable for Class C networks
221 = 2,097,152 Class C networks
In 1993, the size of the routing tables started to outgrow the capacity of routers
Consequence: The Class-based assignment of IP addresses had to be abandoned
Network Layer

28. CIDR - Classless Interdomain Routing
Goals:
New interpretation of the IP address space
Restructure IP address assignments to increase efficiency
Permits route aggregation to minimize route table entries
CIDR (Classless Interdomain routing)
abandons the notion of classes
Key Concept: The length of the network prefix in the IP addresses is kept arbitrary
Consequence: Size of the network prefix must be provided with an IP address
Network Layer

29. CIDR Notation CIDR notation of an IP address: 192.0.2.0/18
"18" is the prefix length. It states that the first 18 bits are the network prefix of the address (and 14 bits are available for specific host addresses)
CIDR notation can replace the use of subnetmasks (but is more general)
IP address and subnetmask becomes /24
CIDR notation allows to drop traling zeros of network addresses:
/18 can be written as /18
Network Layer

30. Why do people still talk about
CIDR eliminates the concept of class A, B, and C networks and replaces it with a network prefix
Existing classful network addresses are converted to CIDR addresses:
 /16
The change has not affected many (previously existing) enterprise networks
Many network administrators (especially on university campuses) have not noticed the change (and still talk about Class A,B,C networks)
(Note: CIDR was introduced with the role-out of BGPv4 as interdomain routing protocol. )
Network Layer

31. CIDR address blocks
CIDR notation can nicely express blocks of addresses
Blocks are used when allocating IP addresses for a company and for routing tables (route aggregation)
CIDR Block Prefix # of Host Addresses
/27 32
/26 64
/25 128
/24 256
/23 512
/22 1,024
/21 2,048
/20 4,096
/19 8,192
/18 16,384
/17 32,768
/16 65,536
/15 131,072
/14 262,144
/13 524,288
Network Layer

32. CIDR and Address assignments
Backbone ISPs obtain large block of IP addresses space and then reallocate portions of their address blocks to their customers.
Example:
Assume that an ISP owns the address block /18, which represents 16,384 (214) IP addresses
Suppose a client requires 800 host addresses
With classful addresses: need to assign a class B address (and waste ~64,700 addresses) or four individual Class Cs (and introducing 4 new routes into the global Internet routing tables)
With CIDR: Assign a /22 block, e.g., /22, and allocated a block of 1,024 (210) IP addresses.
Network Layer

33. CIDR and Routing Routing table Aggregation of routing table entries:
/16 and /16 are represented as /15
Longest prefix match: Routing table lookup finds the routing entry that matches the longest prefix
What is the outgoing interface
for
/24 ->
Route aggregation can be exploited
when IP address blocks are assigned
in an hierarchical fashion
Prefix
Interface /4
interface #5 /9
interface #2
/17
interface #1
Routing table
Network Layer

34. CIDR and Routing Information
Company X :
/22
ISP X owns:
/18
/15
/21
Internet Backbone
ISP y :
/24
Organization z1 :
/26
Organization z2 :
/26
Network Layer

35. CIDR and Routing Information
Backbone routers do not know anything about Company X, ISP Y, or Organizations z1, z2.
Company X :
/22
ISP X owns:
ISP y sends everything which matches the prefix: /26 to Organizations z /26 to Organizations z2
ISP X does not know about Organizations z1, z2.
/18
/15
/21
Internet Backbone
ISP X sends everything which matches the prefix: /22 to Company X,
/24 to ISP y
ISP y :
/24
Backbone sends everything which matches the prefixes /18, /15, /21 to ISP X.
Organization z1 :
/26
Organization z2 :
/26
Network Layer

36. IPv6 - IP Version 6 IP Version 6
Is the successor to the currently used IPv4
Specification completed in 1994
Makes improvements to IPv4 (no revolutionary changes)
One (not the only !) feature of IPv6 is a significant increase in of the IP address to 128 bits (16 bytes)
IPv6 will solve – for the foreseeable future – the problems with IP addressing
1024 addresses per square inch on the surface of the Earth.
Network Layer

37. IPv6 Header
Network Layer

38. IPv6 vs. IPv4: Address Comparison
IPv4 has a maximum of
232  4 billion addresses
IPv6 has a maximum of
2128 = (232)4  4 billion x 4 billion x 4 billion x 4 billion addresses
Network Layer

39. Notation of IPv6 addresses
Convention: The 128-bit IPv6 address is written as eight 16-bit integers (using hexadecimal digits for each integer)
CEDF:BP76:3245:4464:FACE:2E50:3025:DF12
Short notation:
Abbreviations of leading zeroes:
CEDF:BP76:0000:0000:009E:0000:3025:DF12  CEDF:BP76:0:0:9E :0:3025:DF12
“:0000:0000:0000” can be written as “::”
CEDF:BP76:0:0:FACE:0:3025:DF12  CEDF:BP76::FACE:0:3025:DF12
IPv6 addresses derived from IPv4 addresses have 96 leading zero bits. Convention allows to use IPv4 notation for the last 32 bits.
::80:8F:89:90  ::
Network Layer

40. IPv6 Provider-Based Addresses
The first IPv6 addresses will be allocated to a provider-based plan
Type: Set to “010” for provider-based addresses
Registry: identifies the agency that registered the address
The following fields have a variable length (recommeded length in “()”)
Provider: Id of Internet access provider (16 bits)
Subscriber: Id of the organization at provider (24 bits)
Subnetwork: Id of subnet within organization (32 bits)
Interface: identifies an interface at a node (48 bits)
010
Registry ID
Provider ID
Subscriber ID
Subnetwork ID
Interface ID
Network Layer

41. 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
DHCP: Dynamic Host Configuration Protocol: dynamically get address from as server
A “plug-and-play” protocol
Network Layer

42. 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
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
Network Layer

43. DHCP client-server scenario
server B
arriving DHCP
client needs
address in this
network E
Network Layer

44. 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
Network Layer

45. 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 /20
Organization /23
Organization /23
Organization /23
… … ….
Organization /23
Network Layer

46. Hierarchical addressing: route aggregation
Hierarchical addressing allows efficient advertisement of routing
information:
Organization 0
/23
Organization 1
/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”
ISPs-R-Us
Network Layer

47. 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
assigns domain names, resolves disputes
Network Layer

48. 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)
Network Layer

49. 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

50. 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

51. 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
Network Layer

52. 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
address shortage should instead be solved by IPv6
Network Layer

53. NAT traversal problem
client wants to connect to server with address
server address local to LAN (client can’t use it as destination addr)
only one externally visible NATted address:
solution 1: statically configure NAT to forward incoming connection requests at given port to server
e.g., ( , port 2500) always forwarded to port 25000
Client ?
NAT
router
Network Layer

54. NAT traversal problem
solution 2: Universal Plug and Play (UPnP) Internet Gateway Device (IGD) Protocol. Allows NATted host to:
learn public IP address ( )
add/remove port mappings (with lease times)
i.e., automate static NAT port map configuration
IGD
NAT
router
Network Layer

55. 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
2. connection to
relay initiated
by client
NAT
router
1. connection to
relay initiated
by NATted host
3. relaying
established
Client
Network Layer

56. 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

57. 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 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
Network Layer

58. 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:
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

59. 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

60. 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

61. 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

62. 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

63. 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

64. Tunneling A B E F Logical view: Physical view: A B E F tunnel IPv6
Network Layer

65. Tunneling A B E F Logical view: A B C D E F Physical view: Src:B
IPv6
IPv6
IPv6
IPv6 A B C D E F
Physical view:
IPv6
IPv6
IPv4
IPv4
IPv6
IPv6
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
Flow: X
Src: A
Dest: F
data
A-to-B:
IPv6
E-to-F:
IPv6
B-to-C:
IPv6 inside
IPv4
B-to-C:
IPv6 inside
IPv4
Network Layer

66. Acknowledgements Slides by Kames F. Kurose & Keith W. Ross
Additional material by Jorg Liebeherr, Magda El Zarki
Network Layer