Chapter 4

Network Layer:
The Data Plane

Computer
Networking: A Top
Down Approach
7th Edition, Global Edition
Jim Kurose, Keith Ross
Pearson
April 2016
Network Layer: Data Plane 4-1
Chapter 4: outline
4.1 Overview of Network 4.4 Generalized Forward and
layer SDN
• data plane • match
• control plane • action
4.2 What’s inside a router • OpenFlow examples
4.3 IP: Internet Protocol of match-plus-action in
• datagram format action
• fragmentation
• IPv4 addressing
• network address
translation
• IPv6

Network Layer: Data Plane 4-2

IP addressing: introduction
223.1.1.1
 IP address: 32-bit
identifier for host, router
223.1.2.1

interface 223.1.1.2
223.1.1.4 223.1.2.9
 interface: connection
between host/router and 223.1.3.27
physical link 223.1.1.3
223.1.2.2
• router’s typically have
multiple interfaces
• host typically has one or
two interfaces (e.g., wired 223.1.3.1 223.1.3.2

Ethernet, wireless 802.11)

 IP addresses associated
with each interface 223.1.1.1 = 11011111 00000001 00000001 00000001

223 1 1 1

Network Layer: Data Plane 4-3

 IP addressing: introduction
223.1.1.1
Q: how are interfaces
actually connected?
223.1.2.1

A: we’ll learn about that 223.1.1.2

223.1.1.4 223.1.2.9

in chapter 5, 6.
223.1.3.27
223.1.1.3
223.1.2.2

A: wired Ethernet interfaces

connected by Ethernet switches
223.1.3.1 223.1.3.2

For now: don’t need to worry

about how one interface is
connected to another (with no
A: wireless WiFi interfaces
intervening router)
connected by WiFi base station

Network Layer: Data Plane 4-4

Subnets
 IP address: 223.1.1.1
• subnet part - high order
bits 223.1.1.2 223.1.2.1
223.1.1.4 223.1.2.9
• host part - low order
bits 223.1.2.2
 what’s a subnet ? 223.1.1.3 223.1.3.27

• device interfaces with subnet

same subnet part of IP
address 223.1.3.1 223.1.3.2

• can physically reach

each other without
intervening router network consisting of 3 subnets

Network Layer: Data Plane 4-5

Subnets
223.1.1.0/24
223.1.2.0/24
recipe 223.1.1.1

 to determine the 223.1.1.2 223.1.2.1

subnets, detach each 223.1.1.4 223.1.2.9

interface from its host 223.1.2.2

or router, creating 223.1.1.3 223.1.3.27

islands of isolated subnet

networks
 each isolated network 223.1.3.1 223.1.3.2

is called a subnet
223.1.3.0/24

subnet mask: /24

Network Layer: Data Plane 4-6
Subnets 223.1.1.2

how many? 223.1.1.1 223.1.1.4

223.1.1.3

223.1.9.2 223.1.7.0

223.1.9.1 223.1.7.1
223.1.8.1 223.1.8.0

223.1.2.6 223.1.3.27

223.1.2.1 223.1.2.2 223.1.3.1 223.1.3.2

Network Layer: Data Plane 4-7

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 host
part part
11001000 00010111 00010000 00000000
200.23.16.0/23

Network Layer: Data Plane 4-8

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: Data Plane 4-9

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: Data Plane 4-10

DHCP client-server scenario

DHCP
223.1.1.0/24
server
223.1.1.1 223.1.2.1

223.1.1.2 arriving DHCP

223.1.1.4 223.1.2.9
client needs
address in this
223.1.3.27
223.1.2.2 network
223.1.1.3

223.1.2.0/24

223.1.3.1 223.1.3.2

223.1.3.0/24

Network Layer: Data Plane 4-11

DHCP client-server scenario
DHCP server: 223.1.2.5 DHCP discover arriving
client
src : 0.0.0.0, 68
Broadcast: is there a
dest.: 255.255.255.255,67
DHCPyiaddr:
server 0.0.0.0
out there?
transaction ID: 654

DHCP offer
src: 223.1.2.5, 67
Broadcast: I’m a DHCP
dest: 255.255.255.255, 68
server! Here’s an IP
yiaddrr: 223.1.2.4
address youID:can
transaction 654 use
lifetime: 3600 secs
DHCP request
src: 0.0.0.0, 68
Broadcast: OK. I’ll take
dest:: 255.255.255.255, 67
yiaddrr: 223.1.2.4
that IP address!
transaction ID: 655
lifetime: 3600 secs

DHCP ACK
src: 223.1.2.5, 67
Broadcast: OK. You’ve
dest: 255.255.255.255, 68
yiaddrr: 223.1.2.4
got that IPID:
transaction address!
655
lifetime: 3600 secs

Network Layer: Data Plane 4-12

Transport Layer 3-13
DHCP: more than IP addresses
DHCP can return more than just allocated IP
address on subnet:
• address of first-hop router for client
• name and IP address of DNS sever
• network mask (indicating network versus host portion
of address)

Network Layer: Data Plane 4-14

DHCP: example
 connecting laptop needs
its IP address, addr of
DHCP DHCP first-hop router, addr of
UDP
DHCP
DHCP IP
DNS server: use DHCP
DHCP Eth
Phy  DHCP request encapsulated
DHCP
in UDP, encapsulated in IP,
encapsulated in 802.1
DHCP DHCP 168.1.1.1
Ethernet
DHCP UDP
IP
 Ethernet frame broadcast
DHCP

DHCP Eth router with DHCP

Phy server built into (dest: FFFFFFFFFFFF) on LAN,
router received at router running
DHCP server

 Ethernet demuxed to IP
demuxed, UDP demuxed to
DHCP
Network Layer: Data Plane 4-15
DHCP: example
 DCP server formulates
DHCP ACK containing
DHCP
DHCP
DHCP UDP
client’s IP address, IP
DHCP IP address of first-hop
DHCP Eth router for client, name &
Phy IP address of DNS server
 encapsulation of DHCP
server, frame forwarded
DHCP DHCP to client, demuxing up to
UDP
DHCP
DHCP IP
DHCP at client
DHCP Eth router with DHCP
DHCP
Phy server built into  client now knows its IP
router address, name and IP
address of DSN server, IP
address of its first-hop
router

Network Layer: Data Plane 4-16

 DHCP: Wireshark Message type: Boot Reply (2)
reply
output (home LAN) Hardware type: Ethernet
Hardware address length: 6
Hops: 0
Transaction ID: 0x6b3a11b7
Seconds elapsed: 0
Message type: Boot Request (1) Bootp flags: 0x0000 (Unicast)
Hardware type: Ethernet Client IP address: 192.168.1.101 (192.168.1.101)
Hardware address length: 6 Your (client) IP address: 0.0.0.0 (0.0.0.0)
Hops: 0
Transaction ID: 0x6b3a11b7
request Next server IP address: 192.168.1.1 (192.168.1.1)
Relay agent IP address: 0.0.0.0 (0.0.0.0)
Seconds elapsed: 0 Client MAC address: Wistron_23:68:8a (00:16:d3:23:68:8a)
Bootp flags: 0x0000 (Unicast) Server host name not given
Client IP address: 0.0.0.0 (0.0.0.0) Boot file name not given
Your (client) IP address: 0.0.0.0 (0.0.0.0) Magic cookie: (OK)
Next server IP address: 0.0.0.0 (0.0.0.0) Option: (t=53,l=1) DHCP Message Type = DHCP ACK
Relay agent IP address: 0.0.0.0 (0.0.0.0) Option: (t=54,l=4) Server Identifier = 192.168.1.1
Client MAC address: Wistron_23:68:8a (00:16:d3:23:68:8a) Option: (t=1,l=4) Subnet Mask = 255.255.255.0
Server host name not given Option: (t=3,l=4) Router = 192.168.1.1
Boot file name not given Option: (6) Domain Name Server
Magic cookie: (OK) Length: 12; Value: 445747E2445749F244574092;
Option: (t=53,l=1) DHCP Message Type = DHCP Request IP Address: 68.87.71.226;
Option: (61) Client identifier IP Address: 68.87.73.242;
Length: 7; Value: 010016D323688A; IP Address: 68.87.64.146
Hardware type: Ethernet Option: (t=15,l=20) Domain Name = "hsd1.ma.comcast.net."
Client MAC address: Wistron_23:68:8a (00:16:d3:23:68:8a)
Option: (t=50,l=4) Requested IP Address = 192.168.1.101
Option: (t=12,l=5) Host Name = "nomad"
Option: (55) Parameter Request List
Length: 11; Value: 010F03062C2E2F1F21F92B
1 = Subnet Mask; 15 = Domain Name
3 = Router; 6 = Domain Name Server
44 = NetBIOS over TCP/IP Name Server
……

Network Layer: Data Plane 4-17

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 11001000 00010111 00010000 00000000 200.23.16.0/20

Organization 0 11001000 00010111 00010000 00000000 200.23.16.0/23

Organization 1 11001000 00010111 00010010 00000000 200.23.18.0/23
Organization 2 11001000 00010111 00010100 00000000 200.23.20.0/23
... ….. …. ….
Organization 7 11001000 00010111 00011110 00000000 200.23.30.0/23

Network Layer: Data Plane 4-18

Hierarchical addressing: route aggregation

hierarchical addressing allows efficient advertisement of routing

information:

Organization 0
200.23.16.0/23
Organization 1
“Send me anything
200.23.18.0/23 with addresses
Organization 2 beginning
200.23.20.0/23 . Fly-By-Night-ISP 200.23.16.0/20”
.
. . Internet
.
Organization 7 .
200.23.30.0/23
“Send me anything
ISPs-R-Us
with addresses
beginning
199.31.0.0/16”

Network Layer: Data Plane 4-19

Hierarchical addressing: more specific routes

ISPs-R-Us has a more specific route to Organization 1

Organization 0
200.23.16.0/23

“Send me anything
with addresses
Organization 2 beginning
200.23.20.0/23 . Fly-By-Night-ISP 200.23.16.0/20”
.
. . Internet
.
Organization 7 .
200.23.30.0/23
“Send me anything
ISPs-R-Us
with addresses
Organization 1 beginning 199.31.0.0/16
or 200.23.18.0/23”
200.23.18.0/23

Network Layer: Data Plane 4-20

IP addressing: the last word...

Q: how does an ISP get block of addresses?

A: ICANN: Internet Corporation for Assigned
Names and Numbers http://www.icann.org/
• allocates addresses
• manages DNS
• assigns domain names, resolves disputes

Network Layer: Data Plane 4-21

 NAT: network address translation

rest of local network

Internet (e.g., home network)
10.0.0/24 10.0.0.1

10.0.0.4
10.0.0.2
138.76.29.7

10.0.0.3

all datagrams leaving local datagrams with source or

network have same single destination in this network
source NAT IP address: have 10.0.0/24 address for
138.76.29.7,different source source, destination (as usual)
port numbers
Network Layer: Data Plane 4-22
 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: Data Plane 4-23

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: Data Plane 4-24

 NAT: network address translation
NAT translation table 1: host 10.0.0.1
2: NAT router WAN side addr LAN side addr
changes datagram sends datagram to
source addr from 138.76.29.7, 5001 10.0.0.1, 3345 128.119.40.186, 80
10.0.0.1, 3345 to …… ……
138.76.29.7, 5001,
updates table S: 10.0.0.1, 3345
D: 128.119.40.186, 80
10.0.0.1
1
S: 138.76.29.7, 5001
2 D: 128.119.40.186, 80 10.0.0.4
10.0.0.2
138.76.29.7 S: 128.119.40.186, 80
D: 10.0.0.1, 3345
4
S: 128.119.40.186, 80
D: 138.76.29.7, 5001 3 10.0.0.3
4: NAT router
3: reply arrives changes datagram
dest. address: dest addr from
138.76.29.7, 5001 138.76.29.7, 5001 to 10.0.0.1, 3345

* Check out the online interactive exercises for more

examples: http://gaia.cs.umass.edu/kurose_ross/interactive/ Network Layer: Data Plane 4-25
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
• address shortage should be solved by IPv6
• violates end-to-end argument
• NAT possibility must be taken into account by app
designers, e.g., P2P applications
• NAT traversal: what if client wants to connect
to server behind NAT?
Network Layer: Data Plane 4-26
Chapter 4: outline
4.1 Overview of Network 4.4 Generalized Forward and
layer SDN
• data plane • match
• control plane • action
4.2 What’s inside a router • OpenFlow examples
4.3 IP: Internet Protocol of match-plus-action in
• datagram format action
• fragmentation
• IPv4 addressing
• network address
translation
• IPv6

Network Layer: Data Plane 4-27

 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
• no fragmentation allowed

Network Layer: Data Plane 4-28

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: Data Plane 4-29
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: Data Plane 4-30

Transition from IPv4 to IPv6
 not all routers can be upgraded simultaneously
• no “flag days”
• 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 IPv6 header fields
IPv4 payload
IPv4 source, dest addr IPv6 source dest addr
UDP/TCP payload

IPv6 datagram
IPv4 datagram
Network Layer: Data Plane 4-31
Tunneling
A B IPv4 tunnel E F
connecting IPv6 routers
logical view:
IPv6 IPv6 IPv6 IPv6

A B C D E F
physical view:
IPv6 IPv6 IPv4 IPv4 IPv6 IPv6

Network Layer: Data Plane 4-32

Tunneling
A B IPv4 tunnel E F
connecting IPv6 routers
logical view:
IPv6 IPv6 IPv6 IPv6

A B C D E F
physical view:
IPv6 IPv6 IPv4 IPv4 IPv6 IPv6

flow: X src:B src:B flow: X

src: A dest: E src: A
dest: F
dest: E
dest: F
Flow: X Flow: X
Src: A Src: A
data Dest: F Dest: F data

data data

A-to-B: E-to-F:
IPv6 B-to-C: B-to-C: IPv6
IPv6 inside IPv6 inside
IPv4 IPv4 Network Layer: Data Plane 4-33
 IPv6: adoption
 Google: 8% of clients access services via IPv6
 NIST: 1/3 of all US government domains are IPv6
capable

 Long (long!) time for deployment, use

•20 years and counting!
•think of application-level changes in last 20 years: WWW,
Facebook, streaming media, Skype, …
•Why?

Network Layer: Data Plane 4-34

Chapter 4: outline
4.1 Overview of Network 4.4 Generalized Forward and
layer SDN
• data plane • match
• control plane • action
4.2 What’s inside a router • OpenFlow examples
4.3 IP: Internet Protocol of match-plus-action in
• datagram format action
• fragmentation
• IPv4 addressing
• network address
translation
• IPv6

Network Layer: Data Plane 4-35

Generalized Forwarding and SDN
Each router contains a flow table that is computed and
distributed by a logically centralized routing controller

logically-centralized routing controller

control plane

data plane
local flow table
headers counters actions

1
0100 1101

3 2
values in arriving
packet’s header
Network Layer: Data Plane 4-36
OpenFlow data plane abstraction
 flow: defined by header fields
 generalized forwarding: simple packet-handling rules
• Pattern: match values in packet header fields
• Actions: for matched packet: drop, forward, modify, matched
packet or send matched packet to controller
• Priority: disambiguate overlapping patterns
• Counters: #bytes and #packets

Flow table in a router (computed and distributed by

controller) define router’s match+action rules
Network Layer: Data Plane 4-37
OpenFlow data plane abstraction
 flow: defined by header fields
 generalized forwarding: simple packet-handling rules
• Pattern: match values in packet header fields
• Actions: for matched packet: drop, forward, modify, matched
packet or send matched packet to controller
• Priority: disambiguate overlapping patterns
• Counters: #bytes and #packets

* : wildcard
1. src=1.2.*.*, dest=3.4.5.*  drop
2. src = *.*.*.*, dest=3.4.*.*  forward(2)
3. src=10.1.2.3, dest=*.*.*.*  send to controller
OpenFlow: Flow Table Entries
Rule Action Stats

Packet + byte counters

1. Forward packet to port(s)
2. Encapsulate and forward to controller
3. Drop packet
4. Send to normal processing pipeline
5. Modify Fields

Switch MAC MAC Eth VLAN IP IP IP TCP TCP

Port src dst type ID Src Dst Prot sport dport

Link layer Network layer Transport layer

Examples
Destination-based forwarding:
Switch MAC MAC Eth VLAN IP IP IP TCP TCP
Action
Port src dst type ID Src Dst Prot sport dport
* * * * * * 51.6.0.8 * * * port6
IP datagrams destined to IP address 51.6.0.8 should
be forwarded to router output port 6
Firewall:
Switch MAC MAC Eth VLAN IP IP IP TCP TCP
Forward
Port src dst type ID Src Dst Prot sport dport
* * * * * * * * * 22 drop
do not forward (block) all datagrams destined to TCP port 22

Switch MAC MAC Eth VLAN IP IP IP TCP TCP

Forward
Port src dst type ID Src Dst Prot sport dport
* * * * * 128.119.1.1* * * * drop
do not forward (block) all datagrams sent by host 128.119.1.1
Examples
Destination-based layer 2 (switch) forwarding:
Switch MAC MAC Eth VLAN IP IP IP TCP TCP
Action
Port src dst type ID Src Dst Prot sport dport
22:A7:23:
* 11:E1:02 * * * * * * * * port3
layer 2 frames from MAC address 22:A7:23:11:E1:02
should be forwarded to output port 6

Network Layer: Data Plane 4-41

OpenFlow abstraction
 match+action: unifies different kinds of devices
 Router  Firewall
• match: longest • match: IP addresses
destination IP prefix and TCP/UDP port
• action: forward out numbers
a link • action: permit or
 Switch deny
• match: destination  NAT
MAC address • match: IP address
• action: forward or and port
flood • action: rewrite
address and port

Network Layer: Data Plane 4-42

OpenFlow example Example: datagrams from
hosts h5 and h6 should
be sent to h3 or h4, via s1
match action and from there to s2
IP Src = 10.3.*.* Host h6
forward(3)
IP Dst = 10.2.*.* 10.3.0.6
1 s3 controller
2

3 4
Host h5
10.3.0.5

1 s1 1 s2
2 Host h4
4 2 4
Host h1 10.2.0.4
3 3
10.1.0.1
Host h2
10.1.0.2 match action
match action Host h3
ingress port = 2
10.2.0.3 forward(3)
ingress port = 1 IP Dst = 10.2.0.3
IP Src = 10.3.*.* forward(4) ingress port = 2
forward(4)
IP Dst = 10.2.*.* IP Dst = 10.2.0.4
Chapter 4: done!
4.1 Overview of Network 4.4 Generalized Forward and
layer: data plane and SDN
control plane • match plus action
4.2 What’s inside a router • OpenFlow example
4.3 IP: Internet Protocol
• datagram format
• fragmentation Question: how do forwarding tables
• IPv4 addressing (destination-based forwarding) or
• NAT flow tables (generalized
• IPv6 forwarding) computed?
Answer: by the control plane (next
chapter)

Network Layer: Data Plane 4-44