Chapter 4 - Computer Networking a top-down Approach 7th

Computer
Networking: A Top
Down Approach
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All material copyright 1996-2016
J.F Kurose and K.W. Ross, All Rights Reserved
7th edition
Jim Kurose, Keith Ross
Pearson/Addison Wesley
April 2016
Chapter 4
Network Layer:
The Data Plane
4-1Network Layer: Data Plane

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

Chapter 4: network layer
chapter goals:
 understand principles behind network layer
services, focusing on data plane:
• network layer service models
• forwarding versus routing
• how a router works
• generalized forwarding
 instantiation, implementation in the Internet

Network layer
 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
application
transport
network
data link
physical
application
transport
network
data link
physical
network
data link
physical network
data link
physical
network
data link
physical
network
data link
physical
network
data link
physical
network
data link
physical
network
data link
physical
network
data link
physical
network
data link
physical
network
data link
physicalnetwork
data link
physical

Two key network-layer
functions
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
analogy: taking a trip
 forwarding: process of
getting through single
interchange
 routing: process of
planning trip from
source to destination

Network layer: data plane, control
plane
Data plane
 local, per-router function
 determines how
datagram arriving on
router input port is
forwarded to router
output port
 forwarding function
Control plane
 network-wide logic
 determines how datagram is
routed among routers along
end-end path from source
host to destination host
 two control-plane
approaches:
• traditional routing
algorithms: implemented
in routers
• software-defined
networking (SDN):
implemented in (remote)
servers
1
2
3
0111
values in arriving
packet header

Per-router control plane
Routing
Algorithm
Individual routing algorithm components in each and every
router interact in the control plane
data
plane
control
plane
5-7Network Layer: Control Plane
1
2
0111
values in arriving
packet header
3

data
plane
control
plane
Logically centralized control plane
A distinct (typically remote) controller interacts with local
control agents (CAs)
Remote Controller
CA
CA CA CA CA
5-8Network Layer: Control Plane
1
2
0111
3
values in arriving
packet header

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

Network layer service models:
Network
Architecture
Internet
ATM
ATM
ATM
ATM
Service
Model
best effort
CBR
VBR
ABR
UBR
Bandwidth
none
constant
rate
guaranteed
rate
guaranteed
minimum
none
Loss
no
yes
yes
no
no
Order
no
yes
yes
yes
yes
Timing
no
yes
yes
no
no
Congestion
feedback
no (inferred
via loss)
no
congestion
no
congestion
yes
no
Guarantees ?

layer
• data plane
• control plane
• datagram format
• fragmentation
• IPv4 addressing
• network address
translation
• IPv6
and SDN
• match
• action
in action
Chapter 4: outline

Router architecture overview
high-seed
switching
fabric
routing
processor
router input ports router output ports
forwarding data plane
(hardware) operttes in
nanosecond
timeframe
routing, management
control plane (software)
operates in millisecond
time frame
 high-level view of generic router architecture:

line
termination
link
layer
protocol
(receive)
lookup,
forwarding
queueing
Input port functions
decentralized switching:
 using header field values, lookup output
port using forwarding table in input port
memory (“match plus action”)
 goal: complete input port processing at
‘line speed’
 queuing: if datagrams arrive faster than
forwarding rate into switch fabric
physical layer:
bit-level reception
data link layer:
e.g., Ethernet
see chapter 5
switch
fabric

line
termination
link
layer
protocol
(receive)
lookup,
forwarding
queueing
Input port functions
decentralized switching:
 using header field values, lookup output
port using forwarding table in input port
memory (“match plus action”)
 destination-based forwarding: forward
based only on destination IP address
(traditional)
 generalized forwarding: forward based
on any set of header field values
physical layer:
bit-level reception
data link layer:
e.g., Ethernet
see chapter 5
switch
fabric

Destination Address Range
11001000 00010111 00010000 00000000
through
11001000 00010111 00010111 11111111
11001000 00010111 00011000 00000000
through
11001000 00010111 00011000 11111111
11001000 00010111 00011001 00000000
through
11001000 00010111 00011111 11111111
otherwise
Link Interface
0
1
2
3
Q: but what happens if ranges don’t divide up so nicely?
Destination-based
forwarding
forwarding table

Longest prefix matching
Destination Address Range
11001000 00010111 00010*** *********
11001000 00010111 00011000 *********
11001000 00010111 00011*** *********
otherwise
DA: 11001000 00010111 00011000 10101010
examples:
DA: 11001000 00010111 00010110 10100001 which interface?
which interface?
when looking for forwarding table entry for given
destination address, use longest address prefix
that matches destination address.
longest prefix matching
Link interface
0
1
2
3

Longest prefix matching
 we’ll see why longest prefix matching is used
shortly, when we study addressing
 longest prefix matching: often performed
using ternary content addressable memories
(TCAMs)
• content addressable: present address to TCAM:
retrieve address in one clock cycle, regardless of
table size
• Cisco Catalyst: can up ~1M routing table entries in
TCAM

Switching fabrics
 transfer packet from input buffer to
appropriate output buffer
 switching rate: rate at which packets can be
transfer from inputs to outputs
• often measured as multiple of input/output line rate
• N inputs: switching rate N times line rate desirable
 three types of switching fabrics
memory
memory
bus crossbar

Switching via memory
first generation routers:
 traditional computers with switching under direct
control of CPU
 packet copied to system’s memory
 speed limited by memory bandwidth (2 bus crossings
per datagram)
input
port
(e.g.,
Ethernet)
memory
output
port
(e.g.,
Ethernet)
system bus

Switching via a bus
 datagram from input port
memory
to output port memory via a
shared bus
 bus contention: switching
speed limited by bus bandwidth
 32 Gbps bus, Cisco 5600:
sufficient speed for access and
enterprise routers
bus

Switching via interconnection
network
 overcome bus bandwidth
limitations
 banyan networks, crossbar, other
interconnection nets initially
developed to connect processors
in multiprocessor
 advanced design: fragmenting
datagram into fixed length cells,
switch cells through the fabric.
 Cisco 12000: switches 60 Gbps
through the interconnection
network
crossbar

Input port queuing
 fabric slower than input ports combined -> queueing
may occur at input queues
• queueing delay and loss due to input buffer overflow!
 Head-of-the-Line (HOL) blocking: queued datagram at
front of queue prevents others in queue from moving
forward
output port contention:
only one red datagram can be
transferred.
lower red packet is blocked
switch
fabric
one packet time
later: green packet
experiences HOL
blocking
switch
fabric

Output ports
 buffering required when datagrams
arrive from fabric faster than the
transmission rate
 scheduling discipline chooses among
queued datagrams for transmission
line
termination
link
layer
protocol
(send)
switch
fabric
datagram
buffer
queueing
This slide in HUGELY important!
Datagram (packets) can be lost
due to congestion, lack of buffers
Priority scheduling – who gets best
performance, network neutrality

Output port queueing
 buffering when arrival rate via switch exceeds
output line speed
 queueing (delay) and loss due to output port
buffer overflow!
at t, packets more
from input to output
one packet time later
switch
fabric
switch
fabric

How much buffering?
 RFC 3439 rule of thumb: average buffering
equal to “typical” RTT (say 250 msec) times
link capacity C
• e.g., C = 10 Gpbs link: 2.5 Gbit buffer
 recent recommendation: with N flows,
buffering equal to
RTT C.
N

Scheduling mechanisms
 scheduling: choose next packet to send on link
 FIFO (first in first out) scheduling: send in order
of arrival to queue
• real-world example?
• discard policy: if packet arrives to full queue: who to
discard?
• tail drop: drop arriving packet
• priority: drop/remove on priority basis
• random: drop/remove randomly
queue
(waiting area)
packet
arrivals
packet
departureslink
(server)

Scheduling policies: priority
priority scheduling:
send highest
priority queued
packet
 multiple classes,
with different
priorities
• class may depend
on marking or other
header info, e.g. IP
source/dest, port
numbers, etc.
• real world example?
high priority queue
(waiting area)
low priority queue
(waiting area)
arrivals
classify
departures
link
(server)
1 3 2 4 5
5
5
2
2
1
1
3
3 4
4
arrivals
departures
packet
in
service

Scheduling policies: still more
Round Robin (RR) scheduling:
 multiple classes
 cyclically scan class queues, sending one
complete packet from each class (if available)
 real world example?
1 23 4 5
5
5
2
3
1
1
3
3 4
4
arrivals
departures
packet
in
service

Weighted Fair Queuing (WFQ):
 generalized Round Robin
 each class gets weighted amount of service
in each cycle
 real-world example?
Scheduling policies: still more

layer
• data plane
• control plane
• datagram format
• fragmentation
• IPv4 addressing
• network address
translation
• IPv6
and SDN
• match
• action
in action
Chapter 4: outline

The Internet network layer
forwarding
table
host, router network layer functions:
routing protocols
• path selection
• RIP, OSPF, BGP
IP protocol
• addressing conventions
• datagram format
• packet handling conventions
ICMP protocol
• error reporting
• router
“signaling”
transport layer: TCP, UDP
link layer
physical layer
network
layer

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)
IP datagram format
IP protocol version
number
header length
(bytes)
upper layer protocol
to deliver payload to
total datagram
length (bytes)
“type” of data
for
fragmentation/
reassemblymax number
remaining hops
(decremented at
each router)
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

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
fragmentation:
in: one large datagram
out: 3 smaller datagrams
reassembly
…
…

ID
=x
offset
=0
fragflag
=0
length
=4000
ID
=x
offset
=0
fragflag
=1
length
=1500
ID
=x
offset
=185
fragflag
=1
length
=1500
ID
=x
offset
=370
fragflag
=0
length
=1040
one large datagram becomes
several smaller datagrams
example:
 4000 byte datagram
 MTU = 1500 bytes
1480 bytes in
data field
offset =
1480/8
IP fragmentation, reassembly

layer
• data plane
• control plane
• datagram format
• fragmentation
• IPv4 addressing
• network address
translation
• IPv6
and SDN
• match
• action
in action
Chapter 4: outline

IP addressing: introduction
 IP address: 32-bit
identifier for host,
router interface
 interface: connection
between host/router
and physical link
• router’s typically have
multiple interfaces
• host typically has one or
two interfaces (e.g.,
wired Ethernet, wireless
802.11)
 IP addresses
associated with each
interface
223.1.1.1
223.1.1.2
223.1.1.3
223.1.1.4 223.1.2.9
223.1.2.2
223.1.2.1
223.1.3.2223.1.3.1
223.1.3.27
223.1.1.1 = 11011111 00000001 00000001 00000001
223 1 11

IP addressing: introduction
Q: how are interfaces
actually connected?
A: we’ll learn about
that in chapter 5, 6.
223.1.1.1
223.1.1.2
223.1.1.3
223.1.1.4 223.1.2.9
223.1.2.2
223.1.2.1
223.1.3.2223.1.3.1
223.1.3.27
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)

Subnets
 IP address:
• 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 network consisting of 3 subnets
223.1.1.1
223.1.1.3
223.1.1.4 223.1.2.9
223.1.3.2223.1.3.1
subnet
223.1.1.2
223.1.3.27
223.1.2.2
223.1.2.1

recipe
 to determine the
subnets, detach
each interface from
its host or router,
creating islands of
isolated networks
 each isolated
network is called a
subnet
subnet mask: /24
Subnets
223.1.1.0/24
223.1.2.0/24
223.1.3.0/24
223.1.1.1
223.1.1.3
223.1.1.4 223.1.2.9
223.1.3.2223.1.3.1
subnet
223.1.1.2
223.1.3.27
223.1.2.2
223.1.2.1

how many? 223.1.1.1
223.1.1.3
223.1.1.4
223.1.2.2223.1.2.1
223.1.2.6
223.1.3.2223.1.3.1
223.1.3.27
223.1.1.2
223.1.7.0
223.1.7.1
223.1.8.0223.1.8.1
223.1.9.1
223.1.9.2
Subnets

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
11001000 00010111 00010000 00000000
subnet
part
host
part
200.23.16.0/23

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”

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

DHCP client-server scenario
223.1.1.0/24
223.1.2.0/24
223.1.3.0/24
223.1.1.1
223.1.1.3
223.1.1.4 223.1.2.9
223.1.3.2223.1.3.1
223.1.1.2
223.1.3.27
223.1.2.2
223.1.2.1
DHCP
server
arriving DHCP
client needs
address in this
network

DHCP server: 223.1.2.5 arriving
client
DHCP discover
src : 0.0.0.0, 68
dest.: 255.255.255.255,67
yiaddr: 0.0.0.0
transaction ID: 654
DHCP offer
src: 223.1.2.5, 67
dest: 255.255.255.255, 68
yiaddrr: 223.1.2.4
transaction ID: 654
lifetime: 3600 secs
DHCP request
src: 0.0.0.0, 68
dest:: 255.255.255.255, 67
yiaddrr: 223.1.2.4
transaction ID: 655
lifetime: 3600 secs
DHCP ACK
src: 223.1.2.5, 67
dest: 255.255.255.255, 68
yiaddrr: 223.1.2.4
transaction ID: 655
lifetime: 3600 secs
DHCP client-server
scenario
Broadcast: is there a
DHCP server out there?
Broadcast: I’m a DHCP
server! Here’s an IP
address you can use
Broadcast: OK. I’ll take
that IP address!
Broadcast: OK. You’ve
got that IP address!

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)

 connecting laptop
needs its IP address,
addr of first-hop router,
addr of DNS server: use
DHCP
router with DHCP
server built into
router
 DHCP request
encapsulated in UDP,
encapsulated in IP,
encapsulated in 802.1
Ethernet Ethernet frame broadcast
(dest: FFFFFFFFFFFF) on LAN,
received at router running
DHCP server
 Ethernet demuxed to IP
demuxed, UDP demuxed
to DHCP
168.1.1.1
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP: example

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

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

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

Hierarchical addressing: route
aggregation
“Send me anything
with addresses
beginning
200.23.16.0/20”
200.23.16.0/23
200.23.18.0/23
200.23.30.0/23
Fly-By-Night-ISP
Organization 0
Organization 7
Internet
Organization 1
ISPs-R-Us
“Send me anything
with addresses
beginning
199.31.0.0/16”
200.23.20.0/23
Organization 2
.
.
.
.
.
.
hierarchical addressing allows efficient advertisement of routing
information:

ISPs-R-Us has a more specific route to Organization 1
“Send me anything
with addresses
beginning
200.23.16.0/20”
200.23.16.0/23
200.23.18.0/23
200.23.30.0/23
Fly-By-Night-ISP
Organization 0
Organization 7
Internet
Organization 1
ISPs-R-Us
“Send me anything
with addresses
beginning 199.31.0.0/16
or 200.23.18.0/23”
200.23.20.0/23
Organization 2
.
.
.
.
.
.
Hierarchical addressing: more specific
routes

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

NAT: network address
translation
10.0.0.1
10.0.0.2
10.0.0.3
10.0.0.4
138.76.29.7
local network
(e.g., home network)
10.0.0/24
rest of
Internet
datagrams with source or
destination in this network
have 10.0.0/24 address for
source, destination (as usual)
all datagrams leaving
local
network have same
single source NAT IP
address:
138.76.29.7,different 4-54Network Layer: Data Plane

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)
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
translation

10.0.0.1
10.0.0.2
10.0.0.3
S: 10.0.0.1, 3345
D: 128.119.40.186, 80
1
10.0.0.4
138.76.29.7
1: host 10.0.0.1
sends datagram to
128.119.40.186, 80
NAT translation table
WAN side addr LAN side addr
138.76.29.7, 5001 10.0.0.1, 3345
…… ……
S: 128.119.40.186, 80
D: 10.0.0.1, 3345
4
S: 138.76.29.7, 5001
D: 128.119.40.186, 802
2: NAT router
changes datagram
source addr from
10.0.0.1, 3345 to
138.76.29.7, 5001,
updates table
S: 128.119.40.186, 80
D: 138.76.29.7, 5001 3
3: reply arrives
dest. address:
138.76.29.7, 5001
4: NAT router
changes datagram
dest addr from
138.76.29.7, 5001 to 10.0.0.1, 3345
translation
* Check out the online interactive exercises for more
examples: http://gaia.cs.umass.edu/kurose_ross/interactive/

 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?
translation

layer
• data plane
• control plane
• datagram format
• fragmentation
• IPv4 addressing
• network address
translation
• IPv6
and SDN
• match
• action
in action
Chapter 4: outline

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

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
data
destination address
(128 bits)
source address
(128 bits)
payload len next hdr hop limit
flow labelpriver
32 bits

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

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 source, dest addr
IPv4 header fields
IPv4 datagram
IPv6 datagram
IPv4 payload
UDP/TCP payload
IPv6 source dest addr
IPv6 header fields

Tunneling
physical view:
IPv4 IPv4
A B
IPv6 IPv6
E
IPv6 IPv6
FC D
logical view:
IPv4 tunnel
connecting IPv6 routers
E
IPv6 IPv6
FA B
IPv6 IPv6

flow: X
src: A
dest: F
data
A-to-B:
IPv6
Flow: X
Src: A
Dest: F
data
src:B
dest: E
B-to-C:
IPv6 inside
IPv4
E-to-F:
IPv6
flow: X
src: A
dest: F
data
B-to-C:
IPv6 inside
IPv4
Flow: X
Src: A
Dest: F
data
src:B
dest: E
physical view:
A B
IPv6 IPv6
E
IPv6 IPv6
FC D
logical view:
IPv4 tunnel
connecting IPv6 routers
E
IPv6 IPv6
FA B
IPv6 IPv6
Tunneling
IPv4 IPv4

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?

layer
• data plane
• control plane
• datagram format
• fragmentation
• IPv4 addressing
• network address
translation
• IPv6
and SDN
• match
• action
in action
Chapter 4: outline

Generalized Forwarding and
SDN
23
0100 1101
values in arriving
packet’s header
logically-centralized routing controller
1
control plane
data plane
Each router contains a flow table that is computed and
distributed by a logically centralized routing controller
local flow table
headers counters actions

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

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
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
* : wildcard

OpenFlow: Flow Table Entries
Switch
Port
MAC
src
MAC
dst
Eth
type
VLAN
ID
IP
Src
IP
Dst
IP
Prot
TCP
sport
TCP
dport
Rule Action Stats
1. Forward packet to port(s)
2. Encapsulate and forward to controller
3. Drop packet
4. Send to normal processing pipeline
5. Modify Fields
Packet + byte counters
Link layer Network layer Transport layer

Destination-based forwarding:
*
Switch
Port
MAC
src
MAC
dst
Eth
type
VLAN
ID
IP
Src
IP
Dst
IP
Prot
TCP
sport
TCP
dport
Action
* * * * * 51.6.0.8 * * * port6
Examples
IP datagrams destined to IP address
51.6.0.8 should be forwarded to router output
port 6
*
Switch
Port
MAC
src
MAC
dst
Eth
type
VLAN
ID
IP
Src
IP
Dst
IP
Prot
TCP
sport
TCP
dport
Forward
* * * * * * * * 22 drop
Firewall:
do not forward (block) all datagrams destined to TCP
port 22
*
Switch
Port
MAC
src
MAC
dst
Eth
type
VLAN
ID
IP
Src
IP
Dst
IP
Prot
TCP
sport
TCP
dport
Forward
* * * * 128.119.1.1
* * * * drop
do not forward (block) all datagrams sent by host
128.119.1.1

Destination-based layer 2 (switch) forwarding:
*
Switch
Port
MAC
src
MAC
dst
Eth
type
VLAN
ID
IP
Src
IP
Dst
IP
Prot
TCP
sport
TCP
dport
Action
* * * * * * * * port3
Examples
layer 2 frames from MAC address
22:A7:23:11:E1:02 should be forwarded to
output port 6
22:A7:23:
11:E1:02

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

IP Src = 10.3.*.*
IP Dst = 10.2.*.*
forward(3)
match action
ingress port = 2
IP Dst = 10.2.0.3
ingress port = 2
IP Dst = 10.2.0.4
forward(3)
match action
forward(4)
ingress port = 1
IP Src = 10.3.*.*
IP Dst = 10.2.*.*
forward(4)
match action
OpenFlow example
Host h1
10.1.0.1
Host h2
10.1.0.2
Host h4
10.2.0.4
Host h3
10.2.0.3
Host h5
10.3.0.5
s1 s2
s31
2
3 4
1
2
3
4
1
2
3
4
Host h6
10.3.0.6
controller
Example: datagrams from
hosts h5 and h6 should
be sent to h3 or h4, via s1
and from there to s2

Chapter 4: done!
Question: how do forwarding
tables (destination-based
forwarding) or flow tables
(generalized forwarding)
computed?
Answer: by the control plane
(next chapter)
layer: data plane and
control plane
• datagram format
• fragmentation
• IPv4 addressing
• NAT
• IPv6
and SDN
• match plus action
• OpenFlow example

Chapter 4 - Computer Networking a top-down Approach 7th

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Chapter 4 - Computer Networking a top-down Approach 7th