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: we’ll learn about that in chapter 5, 6.
A: wired Ethernet interfaces connected by Ethernet switches
A: wireless WiFi interfaces connected by WiFi base station
For now: don’t need to worry about how one interface is connected to another (with no intervening router)
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
“plug-and-play”
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:
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. 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

29. IPv6 datagram format
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
ver
pri
flow label
payload len
next hdr
hop limit
source address
(128 bits)
destination address
(128 bits)
data
32 bits
Network Layer

30. 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

31. 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

32. 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

33. Graph abstraction z x u y w v 5 2 3 1 graph: G = (N,E)
N = set of routers = { u, v, w, x, y, z }
E = set of links ={ (u,v), (u,x), (v,x), (v,w), (x,w), (x,y), (w,y), (w,z), (y,z) }
aside: graph abstraction is useful in other network contexts, e.g.,
P2P, where N is set of peers and E is set of TCP connections
Network Layer

34. Graph abstraction: costsu y x w v z 2 1 3 5
c(x,x’) = cost of link (x,x’)
e.g., c(w,z) = 5
cost could always be 1, or
inversely related to bandwidth,
or inversely related to
congestion
cost of path (x1, x2, x3,…, xp) = c(x1,x2) + c(x2,x3) + … + c(xp-1,xp)
key question: what is the least-cost path between u and z ?
routing algorithm: algorithm that finds that least cost path
Network Layer

35. Routing algorithm classification
Q: global or decentralized information?
global:
all routers have complete topology, link cost info
“link state” algorithms
decentralized:
router knows physically-connected neighbors, link costs to neighbors
iterative process of computation, exchange of info with neighbors
“distance vector” algorithms
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

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. 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
This example is to find the forwarding table of router U or the shortest paths from router U to every routers(X,W,V,Y,Z)
Network Layer

38. Dijkstra’s algorithm: example
The result will be the forwarding table for router U:
Any packet arrives to U, it will be forwarded based on this forwarding table. w 3 4 v x u 5 7 y 8 z 2 9 v x y w z
(u,w)
(u,x)
destination
link
Network Layer

39. Distance vector algorithm
Bellman-Ford equation (dynamic programming)
let
dx(y) := cost of least-cost path from x to y
then
dx(y) = min {c(x,v) + dv(y) } v
cost from neighbor v to destination y
cost to neighbor v
min taken over all neighbors v of x
Network Layer

40. Distance vector algorithm
key idea:
from time-to-time, each node sends its own distance vector estimate to neighbors
when x receives new DV estimate from neighbor, it updates its own DV using B-F equation:
Dx(y) ← minv{c(x,v) + Dv(y)} for each node y ∊ N
under minor, natural conditions, the estimate Dx(y) converge to the actual least cost dx(y)
Network Layer

41. z y x Dx(z) = min{c(x,y) + Dy(z), c(x,z) + Dz(z)} = min{2+1 , 7+0} = 3
Dx(y) = min{c(x,y) + Dy(y), c(x,z) + Dz(y)} = min{2+0 , 7+1} = 2
node x
table
cost to
cost to
x y z
x y z x x 2 3 y
from ∞ ∞ ∞
from y z z ∞ ∞ ∞
node y
table
cost to x z 1 2 7 y
x y z x ∞ ∞ ∞ y
from z ∞ ∞ ∞
node z
table
cost to
x y z x
∞ ∞ ∞
from y ∞ ∞ ∞ z 7 1
time
Network Layer

42. z y x Dx(z) = min{c(x,y) + Dy(z), c(x,z) + Dz(z)} = min{2+1 , 7+0} = 3
Dx(y) = min{c(x,y) + Dy(y), c(x,z) + Dz(y)} = min{2+0 , 7+1} = 2
node x
table
cost to
cost to
cost to
x y z
x y z
x y z x x 2 3 x y y
from ∞ ∞ ∞
from y
from z z ∞ ∞ ∞ z
node y
table
cost to
cost to x z 1 2 7 y
cost to
x y z
x y z
x y z x ∞ ∞ ∞ x x y y
from
from y
from z z ∞ ∞ ∞ z
node z
table
cost to
cost to
cost to
x y z
x y z
x y z x x x
∞ ∞ ∞
from y y
from y
from ∞ ∞ ∞ z z z 7 1
time
time
Network Layer

43. Comparison of LS and DV algorithms
message complexity
LS: with n nodes, E links, O(nE) msgs sent
DV: exchange between neighbors only
convergence time varies
speed of convergence
LS: O(n2) algorithm requires O(nE) msgs
may have oscillations
DV: convergence time varies
may be routing loops
count-to-infinity problem
robustness: what happens if router malfunctions?
LS:
node can advertise incorrect link cost
each node computes only its own table
DV:
DV node can advertise incorrect path cost
each node’s table used by others
error propagate thru network
Network Layer

44. Hierarchical routing
collect routers into regions, “autonomous systems” (AS)
Each AS within an ISP
ISP may consist of one or more ASes
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

45. Interconnected ASes 3b 1d 3a 1c 2a
AS3
AS1
AS2 1a 2c 2b 1b
Intra-AS
Routing
algorithm
Inter-AS
Forwarding
table 3c
forwarding table configured by both intra- and inter-AS routing algorithm.
Network Layer

46. Inter-AS tasks
suppose router in AS1 receives datagram destined outside of AS1:
router should forward packet to gateway router 3c 3a 3b 2c
AS3
other
networks
AS1 1c 1a 1d 1b 2a 2b
other
networks
AS2
Network Layer

47. 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
IGRP: Interior Gateway Routing Protocol (Cisco proprietary)
Network Layer

48. RIP ( Routing Information Protocol)
Uses distance vector algorithm
distance-vector routing protocols which employ the hop count as a routing metric D C B A u v w x y z
from router A to destination subnets:
subnet hops u v w x y z
Network Layer

49. OSPF (Open Shortest Path First)
uses link state algorithm
route computation using Dijkstra’s algorithm
Network Layer

50. Internet inter-AS routing: BGP
BGP (Border Gateway Protocol):
“glue that holds the Internet together”
The Border Gateway Protocol (BGP) is the protocol backing the core routing decisions on the Internet
allows subnet to advertise its existence to rest of Internet: “I am here”
Network Layer

51. creation/transmission
Broadcast routing
deliver packets from source to all other nodes
source duplication is inefficient: R1 R2 R3 R4
source duplication
in-network
duplication
duplicate
creation/transmission
source duplication: how does source determine recipient addresses?
Network Layer

52. In-network duplication
flooding: when node receives broadcast packet, sends copy to all neighbors
problems: cycles & broadcast storm
spanning tree:
no redundant packets received by any node
Network Layer

53. Spanning tree
Spanning Tree Protocol (STP) is a network protocol that builds a logical loop-free topology for Ethernet networks. A B G D E c F
(a) broadcast initiated at A
(b) broadcast initiated at D
Network Layer

54. Shortest path tree
mcast 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

55. 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