1. Chapter 4 Network Layer
Computer Networking: A Top Down Approach 6th edition Jim Kurose, Keith Ross Addison-Wesley March 2012
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Network Layer

2. Chapter 4: network layer
chapter 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 destination
routing algorithms
Network Layer

6. Interplay between routing and forwarding1 2 3
0111
value in arriving
packet’s header
routing algorithm
local forwarding table
header value
output link
0100
0101
1001
routing algorithm determines
end-to-end-path through network
forwarding table determines
local forwarding at this router
Network Layer

7. 3rd important function : Connection setup
3rd important function in some network architectures:
ATM, frame relay, X.25
before datagrams flow, two end hosts and intervening routers establish virtual connection (VC)
routers get involved
network vs transport layer connection service:
network: between two hosts (may also involve intervening routers in case of VCs)
transport: between two processes
Network Layer

8. Network service model
Q: What service model for “channel” transporting datagrams from sender to receiver?
example services for individual datagrams:
guaranteed delivery
guaranteed delivery with less than 40 msec delay
example services for a flow of datagrams:
in-order datagram delivery
guaranteed minimum bandwidth to flow
restrictions on “changes in inter-packet spacing (jitter)”
Security
Network Layer

9. Network layer service models:
Guarantees ?
Network
Architecture
Internet
ATM
Service
Model
best effort
CBR
ABR
Congestion
feedback
no (inferred
via loss) no
yes
Bandwidth
none
constant
rate
guaranteed
minimum
Loss no
yes
Order no
yes
Timing no
yes
CBR: Constant Bit Rate
ABR: Available Bit Rate
Network Layer

10. 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

11. Connection, connection-less service
datagram network provides network-layer connectionless service
virtual-circuit network provides network-layer connection service
analogous to TCP/UDP connection-oriented / connectionless transport-layer services, but:
Network layer - service: host-to-host
Network layer - no choice: network provides either one or the other, but not both
Network layer - implementation: in network core
Network Layer

12. Virtual circuits (VCs)
“source-to-dest path behaves much like telephone circuit”
Performance is almost guaranteed
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

13. VC implementation a VC consists of:
path from source to destination
VC numbers, one number for each link along path
entries in forwarding tables in routers along path
packet belonging to VC carries VC number (rather than dest address)
VC number can be changed on each link.
new VC number comes from forwarding table
Network Layer

14. VC forwarding table VC routers maintain connection state information!22 12 32 1 3 2
VC number
interface
number
forwarding table in
northwest router:
Incoming interface Incoming VC # Outgoing interface Outgoing VC #
… … … …
VC routers maintain connection state information!
Network Layer

15. 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

16. 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

17. Datagram forwarding table
4 billion IP addresses, so
rather than individual destination address list
“range of addresses” list
(aggregate table entries)
routing algorithm
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

18. Datagram forwarding table
Destination Address Range
through
otherwise
Link Interface 1 2 3
Q: but what happens if ranges don’t divide up so nicely?
Network Layer

19. Longest prefix matching
when looking for forwarding table entry for given destination address, use longest address prefix that matches destination address.
Destination Address Range
*** *********
*********
*** *********
otherwise
Link interface 1 2 3
examples:
DA:
which interface?
DA:
which interface?
Network Layer

20. Datagram or VC network: why?
Internet (datagram)
data exchange among computers
“elastic” service, no strict timing req.
“smart” end systems (computers)
can adapt, perform control, error recovery
simple inside network, complexity at “edge”
ATM (VC)
evolved from telephony
“dumb” end systems
telephones
complexity inside network
Network Layer

21. Chapter 4: outline 4.1 introduction
4.2 virtual circuit and datagram networks
4.3 what’s inside a router (skip)
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

22. 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

23. 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

24. IP datagram format
Network Layer

25. Link MTU vs. Path MTU vs. MSS
Maximum Transmission Unit (MTU) is defined by the maximum payload size of the Layer 2 frame.
Link MTU: The max packet size that can be transmitted over a link
Path MTU: The minimum link MTU of all links in a path between a source and a destination
Layer 3 payload determines Layer 4 Maximum Segment Size (MSS)
MAC Header
(Path MTU)
Transport Layer

26. IP fragmentation, reassembly
network links have MTU - 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

27. IP fragmentation, reassemblyID =x
offset =0
flags
=010
length
=4000
example:
4000 byte datagram
MTU = 1500 bytes ID =x
offset =0
flags
=001
length
=1500
=185
=370
=000
=1040
one large datagram becomes
several smaller datagrams
1480 bytes in data field
(20 bytes in head)
offset = 1480/8
offset = 2960/8
Network Layer

28. 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

29. 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 or two interfaces (e.g., wired Ethernet, wireless )
IP addresses associated with each interface =
223 1 1 1
Network Layer

30. 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 (or hubs)
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

31. 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

32. Subnets
/24
/24
/24
subnet
recipe
to determine the subnets, detach each interface from its router, creating islands of isolated networks
each isolated network is called a subnet
subnet mask: /24
Network Layer

33. Subnets
how many?
Network Layer

34. Classful Addressing 0~127 128~191 192~223 224~239 240~255
Network Layer

35. Classful Addressing
Class A, B and C addresses are divided into 2 parts : (fixed sized) Netid and Hostid.
Network Layer

36. Classful Addressing – blocks in class A
Class A addresses are wasted!!!
Network Layer

37. Classful Addressing – blocks in class B
Many class B addresses are wasted too.
Network Layer

38. Classful Addressing – blocks in class C
Class C blocks are too small for most businesses.
Network Layer

39. Masking Concept
Given an address from a block of addresses, we can find
the network address (netid) by ANDing with a mask.
Network Layer

40. Masking Concept - Default masks
- Network address (netid) can be found by applying the default mask to any of the addresses in the block (including itself).
- The masked address retains the netid of the block and sets the hostid to zero.
Network Layer

41. IP addressing: CIDR CIDR: Classless InterDomain Routing
subnet portion of address of “arbitrary length”
address format: a.b.c.d/x, where x is # bits in subnet portion of address
subnet
part
host
part
/23
Network Layer

42. IP addressing: CIDR Subnet mask
Used by every machine to determine which part of IP address is to be used for the “subnet address”
 /25
 /26 .
 /31
Network Layer