1. Chapter 4 Network Layer
Computer Networking: A Top Down Approach 6th edition Jim Kurose, Keith Ross Addison-Wesley March 2012
All material copyright
J.F Kurose and K.W. Ross, All Rights Reserved
Network Layer

2. Chapter 4: network layer
Lecture goals:
understand principles behind network layer services:
network layer service models
forwarding versus routing
how a router works
routing (path selection)
broadcast, multicast
instantiation, implementation in the Internet
Network Layer

3. Chapter 4: outline 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. Network layer transport segment from sending to receiving host
application
transport
network
data link
physical
transport segment from sending to receiving host
on sending side encapsulates segments into datagrams
on receiving side, delivers segments to transport layer
network layer protocols in every host, router
router examines header fields in all IP datagrams passing through it
network
data link
physical
application
transport
network
data link
physical
Network Layer

5. Two key network-layer functions
forwarding: move packets from router’s input to appropriate router output
routing: determine route taken by packets from source to dest.
routing algorithms
analogy:
routing: process of planning trip from source to dest
forwarding: process of getting through single interchange
Network Layer

6. Connection, connection-less service
datagram network provides network-layer connectionless service
virtual-circuit network provides network-layer connection service
analogous to TCP/UDP connecton-oriented / connectionless transport-layer services, but:
service: host-to-host
no choice: network provides one or the other
implementation: in network core
Network Layer

7. Virtual circuits
“source-to-dest path behaves much like telephone circuit”
performance-wise
network actions along source-to-dest path
call setup, teardown for each call before data can flow
each packet carries VC identifier (not destination host address)
every router on source-dest path maintains “state” for each passing connection
link, router resources (bandwidth, buffers) may be allocated to VC (dedicated resources = predictable service)
Network Layer

8. Virtual circuits: signaling protocols
used to setup, maintain teardown VC
used in ATM, frame-relay, X.25
not used in today’s Internet
application
transport
network
data link
physical
application
transport
network
data link
physical
5. data flow begins
6. receive data
4. call connected
3. accept call
1. initiate call
2. incoming call
Network Layer

9. Datagram networks no call setup at network layer
routers: no state about end-to-end connections
no network-level concept of “connection”
packets forwarded using destination host address
application
transport
network
data link
physical
application
transport
network
data link
physical
1. send datagrams
2. receive datagrams
Network Layer

10. Datagram or VC network: why?
Internet (datagram)
data exchange among computers
“elastic” service, no strict timing req.
many link types
different characteristics
uniform service difficult
“smart” end systems (computers)
can adapt, perform control, error recovery
simple inside network, complexity at “edge”
ATM (VC)
evolved from telephony
human conversation:
strict timing, reliability requirements
need for guaranteed service
“dumb” end systems
telephones
complexity inside network
Network Layer

11. 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

12. 32 bit destination IP address
IP datagram format
IP protocol version
number
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
head.
len
type of
service
flgs
fragment
offset
upper
layer
32 bit destination IP address
options (if any)
total datagram
length (bytes)
header length
(bytes)
“type” of data
for
fragmentation/
reassembly
max number
remaining hops
(decremented at
each router)
upper layer protocol
to deliver payload to
e.g. timestamp,
record route
taken, specify
list of routers
to visit.
how much overhead?
20 bytes of TCP
20 bytes of IP
= 40 bytes + app layer overhead
Network Layer

13. IP addressing: introduction
IP address: 32-bit identifier for host, router interface
interface: connection between host/router and physical link
routers typically have multiple interfaces
host typically has one active interface (e.g., wired Ethernet, wireless )
one IP address associated with each interface =
223 1 1 1
Network Layer

14. IP addressing: introduction
Q: how are interfaces actually connected?
A: wired Ethernet interfaces connected by Ethernet switches
A: wireless WiFi interfaces connected by WiFi base station
Network Layer

15. Subnets IP address: what’s a subnet ? 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
subnet
network consisting of 3 subnets
Network Layer

16. IP addressing: how to get a block?
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

17. 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
DHCP is also called as “plug-and-play” protocol
Network Layer

18. 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/“on”)
support for mobile users who want to join network (more shortly)
DHCP overview: DORA concept
host broadcasts “DHCP discover” msg [optional]
DHCP server responds with “DHCP offer” msg [optional]
host requests IP address: “DHCP request” msg
DHCP server sends address: “DHCP ack” msg
Network Layer

19. DHCP client-server scenario
/24
arriving DHCP
client needs
address in this
network
/24
/24
Network Layer

20. DHCP client-server scenario
DHCP server:
DHCP discover
src : , 68
dest.: ,67
yiaddr:
transaction ID: 654
arriving
client
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
DHCP ACK
src: , 67
dest: , 68
yiaddrr:
transaction ID: 655
lifetime: 3600 secs
Network Layer

21. DHCP: example
DHCP
UDP IP
Eth
Phy
DHCP
DHCP
connecting laptop needs its IP address, addr of first-hop router, addr of DNS server: use DHCP
DHCP
DHCP request encapsulated in UDP, encapsulated in IP, encapsulated in Ethernet
DHCP
DHCP
UDP IP
Eth
Phy
DHCP
Ethernet frame broadcast (dest: FFFFFFFFFFFF) on LAN, received at router running DHCP server
router with DHCP
server built into
router
Ethernet demuxed to IP demuxed, UDP demuxed to DHCP
Network Layer

22. DHCP: example
DHCP
DCP server formulates DHCP ACK containing client’s IP address, IP address of first-hop router for client, name & IP address of DNS server
DHCP
UDP IP
Eth
Phy
encapsulation of DHCP server, frame forwarded to client, demuxing up to DHCP at client
DHCP
UDP IP
Eth
Phy
DHCP
DHCP
router with DHCP
server built into
router
client now knows its IP address, name and IP address of DSN server, IP address of its first-hop router
DHCP
Network Layer

23. 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

24. 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

25. 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

26. 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

27. 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

28. Traceroute and ICMP source sends series of UDP segments to dest
first set has TTL =1
second set has TTL=2, etc.
unlikely port number
when nth set of datagrams arrives to nth router:
router discards datagrams
and sends source ICMP messages (type 11, code 0)
ICMP messages includes name of router & IP address
when ICMP messages arrives, source records RTTs
stopping criteria:
UDP segment eventually arrives at destination host
destination returns ICMP “port unreachable” message (type 3, code 3)
source stops
3 probes
3 probes
3 probes
Network Layer

29. IPv6: motivation
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 (20 byte in IPv4)
no fragmentation allowed
Network Layer

30. IPv6 datagram format
priority: identify priority among datagrams in flow
flow Label: identify datagrams in same “flow.”
next header: identify upper layer protocol for data
ver
pri
flow label
payload len
next hdr
hop limit
source address
(128 bits)
destination address
(128 bits)
data
32 bits
Network Layer

31. 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”
Network Layer

32. Transition from IPv4 to IPv6
not all routers can be upgraded simultaneously
how will network operate with mixed IPv4 and IPv6 routers?
tunneling: IPv6 datagram carried as payload in IPv4 datagram among IPv4 routers
IPv4 header fields
UDP/TCP payload
IPv6 source dest addr
IPv6 header fields
IPv4 payload
IPv4 source, dest addr
IPv6 datagram
IPv4 datagram
Network Layer

33. Interplay between routing, forwarding
routing algorithm determines
end-end-path through network
routing algorithm
forwarding table determines
local forwarding at this router
local forwarding table
dest address
output link
address-range 1
address-range 2
address-range 3
address-range 4 3 2 1
IP destination address in
arriving packet’s header 1 2 3
Network Layer

34. Routing algorithm Routing Algorithm Q: static or dynamic? static:
The main function of the network layer is routing packets from the source machine to the destination machine.
Q: static or dynamic?
static:
routes change slowly over time
dynamic:
routes change more quickly
periodic update
in response to link cost changes
Network Layer

35. A Link-State Routing Algorithm
Link State Routing: Each router must do the following:
Discover its neighbors and learn their network addresses.
Measure the delay or cost to each of its neighbors
Construct & send this packet to all other routers.
Choose the shortest path to every other router.
Network Layer

36. A Link-State Routing Algorithm
Dijkstra’s algorithm
net topology, link costs known to all nodes
accomplished via “link state broadcast”
all nodes have same info
computes least cost paths from one node (‘source”) to all other nodes
gives forwarding table for that node
iterative: after k iterations, know least cost path to k destinations
notation:
c(x,y): link cost from node x to y; = ∞ if not direct neighbors
D(v): current value of cost of path from source to dest. v
p(v): predecessor node along path from source to v
N': set of nodes whose least cost path definitively known
Network Layer

37. Dijsktra’s Algorithm 1 Initialization: 2 N' = {u} 3 for all nodes v
if v adjacent to u
then D(v) = c(u,v)
else D(v) = ∞ 7
8 Loop
9 find w not in N' such that D(w) is a minimum
10 add w to N'
11 update D(v) for all v adjacent to w and not in N' :
D(v) = min( D(v), D(w) + c(w,v) )
13 /* new cost to v is either old cost to v or known
shortest path cost to w plus cost from w to v */
15 until all nodes in N'
Network Layer

38. Dijkstra’s algorithm: example
D(v)
p(v)
D(w)
p(w)
D(x)
p(x)
D(y)
p(y)
D(z)
p(z)
Step N' u ∞
7,u
3,u
5,u 1 uw ∞
11,w
6,w
5,u 2
uwx
14,x
11,w
6,w 3
uwxv
14,x
10,v 4
uwxvy
12,y 5
uwxvyz w 3 4 v x u 5 7 y 8 z 2 9
notes:
construct shortest path tree by tracing predecessor nodes
ties can exist (can be broken arbitrarily)
Network Layer

39. Dijkstra’s algorithm: examplew 3 4 v x u 5 7 y 8 z 2 9
resulting forwarding
table in u: v x y w z
(u,w)
(u,x)
destination
link
Network Layer

40. Distance vector algorithm
Distance Vector algorithm: Distance vector routing algorithms operate by having each router maintain a table (i.e., a vector) giving the best known distance to each destination and which line to use to get there. These tables are updated by exchanging information with the neighbors
Network Layer

41. Hierarchical routing
The hierarchical routing is used, the routers are divided into what we will call regions, with each router knowing all the details about how to route packets.
routers in same AS run same routing protocol
“intra-AS” routing protocol
routers in different AS can run different intra-AS routing protocol
gateway router:
at “edge” of its own AS
has link to router in another AS
Network Layer

42. Intra-AS Routing also known as interior gateway protocols (IGP)
most common intra-AS routing protocols:
RIP: Routing Information Protocol
OSPF: Open Shortest Path First
Network Layer

43. RIP ( Routing Information Protocol)
Routing Information Protocol (RIP):
The Routing Information Protocol (RIP) is one of the oldest distance-vector routing protocols which employ the hop count as a routing metric. 
RIP prevents routing loops by implementing limit on the number of hops allowed in a path from source to destination.
Network Layer

44. OSPF (Open Shortest Path First)
OSPF supports three kinds of connections and networks:
Point-to-point lines between exactly two routers.
Multi-access networks with broadcasting (e.g., most LANs).
Multi-access networks without broadcasting (e.g., most packet-switched WANs).
Network Layer

45. Internet inter-AS routing: BGP
Border Gateway Protocol (BGP) :
The Border Gateway Protocol (BGP) is the protocol backing the core routing decisions on the Internet.
It maintains a table of IP networks or 'prefixes' which designate network reach ability among autonomous system(AS). It is described as a path vector protocol.
Network Layer

46. Broadcast routing
Sending a packet to all destinations simultaneously is called broadcasting; various methods have been proposed for doing it. deliver packets from source to all other nodes
flooding: in which every incoming packet is sent out on every outgoing line except the one it arrived on.
when node receives broadcast packet, sends copy to all neighbors
Network Layer

47. Spanning tree
Spanning Tree Protocol (STP) is a network protocol that builds a logical loop-free topology for Ethernet networks. The basic function of STP is to prevent bridge loops and the broadcast radiation that results from them.
Network Layer

48. Shortest path tree
multicast forwarding tree: tree of shortest path routes from source to all receivers
Dijkstra’s algorithm R1 R2 R3 R4 R5 R6 R7 2 1 6 3 4 5
s: source
LEGEND
router with attached
group member
router with no attached
group member
Notes:
3.3 Network Layer: Multicast Routing Algorithms
link used for forwarding,
i indicates order link
added by algorithm i
Network Layer

49. Chapter 4: done! 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
understand principles behind network layer services:
network layer service models, forwarding versus routing how a router works, routing (path selection), broadcast, multicast
instantiation, implementation in the Internet
Network Layer