7/4/21
Chapter 4
Network Layer:
Data Plane
•
Computer
Networking: A Top
Down Approach
Note: This class lecture is based on Chapter 3 of
the textbook (Kurose and Ross) and the figures
provided by the authors.
7th Edition, Global Edition
Jim Kurose, Keith Ross
Pearson
April 2016
Network layer: our goals
§understand principles
behind network layer
services, focusing on data
plane:
•
•
•
•
•
•
network layer service models
forwarding versus routing
how a router works
addressing
generalized forwarding
Internet architecture
§ instantiation, implementation
in the Internet
• IP protocol
• NAT, middleboxes
Network Layer: 4-2
1
7/4/21
Network layer: “data plane” roadmap
4.1 Network layer: overview
• data plane
• control plane
4.2 What’s inside a router
• input ports, switching, output ports
• buffer management, scheduling
4.3 IP: the Internet Protocol
4.4 Generalized Forwarding, SDN
• datagram format
• Match+action
• addressing
• OpenFlow: match+action in action
• network address translation
4.5 Middleboxes
• IPv6
Network Layer: 4-3
Network-layer services and protocols
§ transport segment from sending to
receiving host
• sender: encapsulates segments into
datagrams, passes to link layer
• receiver: delivers segments to transport
layer protocol
§ network layer protocols in every
Internet device: hosts, routers.
§ routers:
• examines header fields in all IP
datagrams passing through it
• moves datagrams from input ports to
output ports to transfer datagrams
along end-end path
mobile network
national or global ISP
application
transport
network
link
physical
network
link
physical
network
link
physical
enterprise
network
network
link
physical
network
link
physical
network
link
physical
datacenter
network
application
transport
network
link
physical
Network Layer: 4-4
2
7/4/21
Two key network-layer functions
network-layer functions:
§ forwarding:
•Local decision
•Inside one router
•move packets from a router’s input
link to appropriate router output link
analogy: taking a trip
§ forwarding: process of getting
through single interchange
§ routing: process of planning trip
from source to destination
§ routing:
•Global process
•determine route taken by packets
from source to destination
•Several decisions / Several forwarding
forwarding
routing
• routing algorithms
Network Layer: 4-5
Network layer: data plane, control plane
Data plane:
Control plane
§ local, per-router function
§ determines how datagram
arriving on router input port
is forwarded to router
output port
§ forwarding function
§ network-wide logic
§ determines how datagram is
routed among routers along endend path from source host to
destination host
§ two control-plane approaches:
values in arriving
packet header
1
0111
3
2
• traditional routing algorithms:
implemented in routers
• software-defined networking (SDN):
implemented in (remote) servers
Network Layer: 4-6
3
7/4/21
Interplay between routing and forwarding
routing algorithm
local forwarding table
header value output link
0100
0101
0111
1001
3
2
2
1
value in arriving
packetʼs header
1
0111
3 2
Per-router control plane
Individual routing algorithm components in each and every
router interact in the control plane
4.1
•
OVERVIEW OF NETWORK LAYER
Routing
Algorithm
309
control
plane
Routing algorithm
Control plane
Data plane
data
plane
Local forwarding
table
header
output
0100
0110
0111
1001
3
2
2
1
Values in arriving
packet’s header
values in arriving
packet header
1
1101
0111
3
2
1
3
Figure 4.2
♦
2
Routing algorithms determine values in forward tables
tables. In this example, a routing algorithm runs in each and every router and both
forwarding and routing functions are contained within a router. As we’ll see in Sections 5.3 and 5.4, the routing algorithm function in one router communicates with
the routing algorithm function in other routers to compute the values for its forwarding table. How is this communication performed? By exchanging routing messages
containing routing information according to a routing protocol! We’ll cover routing
algorithms and protocols in Sections 5.2 through 5.4.
The distinct and different purposes of the forwarding and routing functions can
be further illustrated by considering the hypothetical (and unrealistic, but technically
feasible) case of a network in which all forwarding tables are configured directly by
human network operators physically present at the routers. In this case, no routing
protocols would be required! Of course, the human operators would need to interact
with each other to ensure that the forwarding tables were configured in such a way
that packets reached their intended destinations. It’s also likely that human configuration would be more error-prone and much slower to respond to changes in the network topology than a routing protocol. We’re thus fortunate that all networks have
both a forwarding and a routing function!
Network Layer: 4-8
4
7/4/21
Software-Defined Networking (SDN) control plane
Remote controller computes, installs forwarding tables in routers
Remote Controller
control
plane
data
plane
CA
CA
values in arriving
packet header
CA
CA
CA
1
0111
3
2
Network Layer: 4-9
Network service model
Q: What service model for “channel” transporting datagrams
from sender to receiver?
example services for
individual datagrams:
example services for a flow of
datagrams:
§ guaranteed delivery
§ guaranteed delivery with
less than 40 msec delay
§ in-order datagram delivery
§ guaranteed minimum bandwidth
to flow
§ restrictions on changes in interpacket spacing
Network Layer: 4-10
5
7/4/21
Network Service Models
Per Datagram
• Guaranteed Delivery: No packets lost
• Guaranteed Delivery with Bounded delay:
•
Maximum delay
Per Flow
• In-Order packet delivery: Some packets may be missing
• Guaranteed minimal throughput
•
minimum bandwidth to flow
• Guaranteed maximum jitter: Delay variation
• Security Services (optional in most networks) ATM
• IP offers none of these Þ Best effort service (Security is
optional)
Network-layer service model
Network
Architecture
Internet
ATM
ATM
Internet
Internet
Quality of Service (QoS) Guarantees ?
Service
Model
Bandwidth
Loss
Order
Timing
best effort
none
no
no
no
Constant Bit Rate
Constant rate
yes
yes
yes
Internet “best effort” service model
Bit Rate
yes
NoAvailable
guarantees
on: Guaranteed min no
i. successful
delivery to
Intserv
Guaranteeddatagram
yes
yesdestination
yes
(RFC
)
ii. 1633
timing
or order of delivery
Diffserv
(RFC 2475) available
possible
possibly
iii. bandwidth
to end-end
flow possibly
no
yes
no
Network Layer: 4-12
6
7/4/21
Network-layer service model
Network
Architecture
Quality of Service (QoS) Guarantees ?
Service
Model
Bandwidth
Loss
Order
Timing
best effort
none
no
no
no
ATM
Constant Bit Rate
Constant rate
yes
yes
yes
ATM
Available Bit Rate
Guaranteed min no
yes
no
Internet
Intserv Guaranteed
(RFC 1633)
yes
yes
yes
yes
Internet
Diffserv
possible
possibly
possibly
no
Internet
(RFC 2475)
Network Layer: 4-13
Reflections on best-effort service:
§ simplicity of mechanism has allowed Internet to be widely deployed
adopted
§ sufficient provisioning of bandwidth allows performance of real-time
applications (e.g., interactive voice, video) to be “good enough” for
“most of the time”
§ replicated, application-layer distributed services (datacenters, content
distribution networks) connecting close to clients’ networks, allow
services to be provided from multiple locations
§ congestion control of “elastic” services helps
It’s hard to argue with success of best-effort service model
Network Layer: 4-14
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7/4/21
Network layer: “data plane” roadmap
4.1 Network layer: overview
• data plane
• control plane
4.2 What’s inside a router
• input ports, switching, output ports
• buffer management, scheduling
4.3 IP: the Internet Protocol
4.4 Generalized Forwarding, SDN
• datagram format
• Match+action
• addressing
• OpenFlow: match+action in action
• network address translation
• IPv6
4.5 Middleboxes
Network Layer: 4-15
Router architecture overview
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7/4/21
Router architecture overview
Router architecture overview
§Two key router functions:
• run routing algorithms/protocol (RIP, OSPF, BGP)
• forwarding datagrams from incoming to outgoing link
routing
processor
routing, management
control plane (software)
operates in millisecond
time frame
forwarding data plane
(hardware) operates
in nanosecond
timeframe
high-speed
switching
fabric
router input ports
router output ports
Network Layer: 4-18
9
7/4/21
Input port functions
Input port functions
line
termination
physical layer:
bit-level reception
link layer:
e.g., Ethernet
(chapter 6)
link
layer
protocol
(receive)
lookup,
forwarding
switch
fabric
queueing
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’
§ input port queuing: if datagrams arrive faster than forwarding
rate into switch fabric
Network Layer: 4-20
10
7/4/21
Input port functions
line
termination
physical layer:
bit-level reception
link layer:
e.g., Ethernet
(chapter 6)
link
layer
protocol
(receive)
lookup,
forwarding
switch
fabric
queueing
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
Network Layer: 4-21
Destination-based forwarding
Q: but what happens if ranges don’t divide up so nicely?
Network Layer: 4-22
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7/4/21
Destination-based forwarding
3
Q: but what happens if ranges don’t divide up so nicely?
Network Layer: 4-23
Longest prefix matching
longest prefix match
when looking for forwarding table entry for given
destination address, use longest address prefix that
matches destination address.
Link interface
Destination Address Range
11001000
00010111
00010***
********
0
11001000
00010111
00011000
********
1
11001000
00010111
00011***
********
2
3
otherwise
examples:
11001000
00010111
00010110
10100001
which interface?
11001000
00010111
00011000
10101010
which interface?
Network Layer: 4-24
12
7/4/21
Longest prefix matching
longest prefix match
when looking for forwarding table entry for given
destination address, use longest address prefix that
matches destination address.
Link interface
Destination Address Range
11001000
00010111
00010***
********
0
11001000
00010111
00011000
********
1
11001000
00010111
match!
00011***
********
2
3
otherwise
examples:
11001000
00010111
00010110
10100001
which interface?
11001000
00010111
00011000
10101010
which interface?
Network Layer: 4-25
Longest prefix matching
longest prefix match
when looking for forwarding table entry for given
destination address, use longest address prefix that
matches destination address.
Link interface
Destination Address Range
11001000
00010111
00010***
********
0
11001000
00010111
00011000
********
1
11001000
00010111
00011***
********
2
3
otherwise
examples:
11001000
11001000
match!
00010111
00010110
10100001
which interface?
00010111
00011000
10101010
which interface?
Network Layer: 4-26
13
7/4/21
Longest prefix matching
longest prefix match
when looking for forwarding table entry for given
destination address, use longest address prefix that
matches destination address.
Link interface
Destination Address Range
examples:
11001000
00010111
00010***
********
0
11001000
00010111
00011000
********
1
11001000
00010111
00011***
********
2
otherwise
match!
11001000
00010111
00010110
10100001
which interface?
11001000
00010111
00011000
10101010
which interface?
3
Network Layer: 4-27
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: ~1M routing table entries in TCAM
Network Layer: 4-28
14
7/4/21
Longest Prefix Matching: Example 4-1
Consider a datagram network using 8-bit host addresses.
Suppose a router uses longest-prefix matching, and has the following
forwarding table:
1. Suppose a datagram arrives at the
router, with destination address
10000000. To which interface will
this datagram be forwarded using
longest-prefix matching?
2. Suppose a datagram arrives at the router,
with destination address 00011011. To
which interface will this datagram be
forwarded using longest-prefix matching?
3. Suppose a datagram arrives at the router,
with destination address 01001011. To
which interface will this datagram be
forwarded using longest-prefix matching?
Prefix Match
interface
00
1
11
2
001
3
111
4
100
5
otherwise
6
Longest Prefix Matching: Solution 4-1
Prefix Match
interface
00
1
11
2
001
3
111
4
100
5
otherwise
6
1. Since the address is 10000000, it will go to interface 5.
2. Since the address is 00011011, it will go to interface 1.
3. Since the address is 01001011, it will go to interface 6.
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Switching fabrics
§ transfer packet from input link to appropriate output link
§ 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
R
high-speed
switching
fabric
...
...
N input ports
R
(rate: NR,
ideally)
N output ports
R
R
Network Layer: 4-31
Switching fabrics
§ transfer packet from input link to appropriate output link
§ 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 major types of switching fabrics:
memory
memory
bus
interconnection
network
Network Layer: 4-32
16
7/4/21
Types of Switching Fabrics
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
Network Layer: 4-34
17
7/4/21
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 routers
bus
Network Layer: 4-35
Switching Via Interconnection Network
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Switching via interconnection network
§ Crossbar, Clos networks, other
interconnection nets initially
developed to connect processors in
multiprocessor
§ multistage switch: nxn switch from
multiple stages of smaller switches
§ exploiting parallelism:
• fragment datagram into fixed length cells on
entry
• switch cells through the fabric, reassemble
datagram at exit
3x3 crossbar
8x8 multistage switch
built from smaller-sized switches
Network Layer: 4-37
Switching via interconnection network
§ scaling, using multiple switching “planes” in parallel:
§ speedup, scaleup via parallelism
fabric plane 0
fabric plane 1
fabric plane 2
fabric plane 3
fabric plane 4
fabric plane 5
fabric plane 6
fabric plane 7
. . .. . .
. . .. . .
. . .. . .
. . .. . .
§ basic unit: 8
switching planes
§ each plane: 3-stage
interconnection
network
§ up to 100’s Tbps
switching capacity
. . .. . .
. . .. . .
. . .. . .
. . .. . .
§ Cisco CRS router:
Network Layer: 4-38
19
7/4/21
Router architecture overview
Input port queuing
§ If switch 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: a queued datagram waiting for service
at the front of a queue prevents others in queue from moving forward
in the queue
switch
fabric
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
Network Layer: 4-40
20
7/4/21
Output ports
Output port queuing
switch
fabric
(rate: NR)
datagram
buffer
queueing
link
layer
protocol
(send)
This is a really important slide
line
termination
§ Buffering required when datagrams
arrive from fabric faster than link
transmission rate. Drop policy: which
datagrams to drop if no free buffers?
§ Scheduling discipline chooses
among queued datagrams for
transmission
R
Datagrams can be lost
due to congestion, lack of
buffers
Priority scheduling – who
gets best performance,
network neutrality
Network Layer: 4-42
21
7/4/21
Output port queuing
switch
fabric
switch
fabric
at t, packets more
from input to output
one packet time later
§buffering when arrival rate via switch exceeds output line speed
§queueing (delay) and loss due to output port buffer overflow!
Network Layer: 4-43
Where Does Queuing Occur?
•
If switching fabric is slow, packets wait on the input port.
•
If switching fabric is fast, packets wait for output port
Queueing (Scheduling) and drop policies
•
Queueing: First Come First Served (FCFS)
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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 Gbps link: 2.5 Gbit buffer
§ more recent recommendation: with N flows, buffering equal to
RTT .C
N
§ but too much buffering can increase delays (particularly in home routers)
• long RTTs: poor performance for realtime apps, sluggish TCP response
• recall delay-based congestion control: “keep bottleneck link just full enough
(busy) but no fuller”
Destination
Source
ACK
Network Layer: 4-45
Buffer Management
switch
fabric
datagram
buffer
queueing
scheduling
link
layer
protocol
(send)
line
R
termination
R
queue
(waiting area)
drop when buffers are full
• tail drop: drop arriving
packet
• priority: drop/remove on
priority basis
§ marking: which packets to
Abstraction: queue
packet
arrivals
buffer management:
§ drop: which packet to add,
packet
departures
mark to signal congestion
(ECN, RED)
link
(server)
Network Layer: 4-46
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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
• Random Early Drop (RED): Drop arriving packets even before the
queue is full
• Routers measure average queue and drop incoming packet with
certain probability
packet
arrivals
queue
link
(waiting area) (server)
packet
departures
Packet Scheduling: FCFS
packet scheduling: deciding
which packet to send next on
link
• first come, first served
• priority
• round robin
• weighted fair queueing
FCFS: packets transmitted in
order of arrival to output
port
§ also known as: First-in-firstout (FIFO)
§ real world examples?
Abstraction: queue
packet
arrivals
R
queue
(waiting area)
packet
departures
link
(server)
Network Layer: 4-48
24
7/4/21
Scheduling policies: priority
Priority scheduling:
§ arriving traffic classified,
queued by class
high priority queue
arrivals
classify
• any header fields can be
used for classification
§ send packet from highest
priority queue that has
buffered packets
• FCFS within priority class
departures
link
low priority queue
2
1 3
4
5
arrivals
packet
in
service
1
4
2
3
5
departures
1
3
2
4
5
Network Layer: 4-49
Scheduling policies: round robin
Round Robin (RR) scheduling:
§arriving traffic classified,
queued by class
• any header fields can be
used for classification
§server cyclically, repeatedly
scans class queues,
sending one complete
packet from each class (if
available) in turn
R
classify
arrivals
link departures
Network Layer: 4-50
25
7/4/21
Scheduling policies: round robin
Round Robin (RR) scheduling:
§ multiple classes
§ cyclically scan class queues, sending one
complete packet from each class (if available)
§ real world example?
2
1 3
5
4
arrivals
packet
in
service
1
2
3
4
5
departures
1
3
2
4
5
Scheduling policies: weighted fair queueing
Weighted Fair Queuing (WFQ):
§ A generalized form Round Robin
queuing
§ each class, i, has weight, wi,
and gets weighted amount
of service in each cycle:
classify
wi
arrivals
Sjwj
§ minimum bandwidth
guarantee (per-traffic-class)
w1
w2
w3
R
link departures
Network Layer: 4-52
26
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Packet Scheduling Matching: Example 4-2
Packet Scheduling Matching: solution 4-2
1235476
27
7/4/21
Sidebar: Network Neutrality
What is network neutrality?
§ technical: how an ISP should share/allocation its resources
• packet scheduling, buffer management are the mechanisms
§ social, economic principles
• protecting free speech
• encouraging innovation, competition
§ enforced legal rules and policies
Different countries have different “takes” on network neutrality
Network Layer: 4-55
Sidebar: Network Neutrality
2015 US FCC Order on Protecting and Promoting an Open Internet: three
“clear, bright line” rules:
§no blocking … “shall not block lawful content, applications, services,
or non-harmful devices, subject to reasonable network management.”
§no throttling … “shall not impair or degrade lawful Internet traffic
on the basis of Internet content, application, or service, or use of a
non-harmful device, subject to reasonable network management.”
§no paid prioritization. … “shall not engage in paid prioritization”
Network Layer: 4-56
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ISP: telecommunications or information service?
Is an ISP a “telecommunications service” or an “information
service” provider?
§ the answer really matters from a regulatory standpoint!
US Telecommunication Act of 1934 and 1996:
• Title II: imposes “common carrier duties” on telecommunications
services: reasonable rates, non-discrimination and requires regulation
• Title I: applies to information services:
• no common carrier duties (not regulated)
• but grants FCC authority “… as may be necessary in the execution
of its functions”
4
Network Layer: 4-57
Review
1. Forwarding uses routing table to find output port for
datagrams using longest prefix match. Routing protocols
make the table.
2. IP provides only best effort service.
3. Routers consist of input/output ports, switching
fabric, and processors.
4. Datagrams may be dropped even if the queues are not full
(Random early drop).
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Network layer: “data plane” roadmap
4.1 Network layer: overview
• data plane
• control plane
4.2 What’s inside a router
• input ports, switching, output ports
• buffer management, scheduling
4.3 IP: the Internet Protocol
4.4 Generalized Forwarding, SDN
• datagram format
• Match+action
• addressing
• OpenFlow: match+action in action
• network address translation
• IPv6
4.5 Middleboxes
Network Layer: 4-59
Network Layer: Internet
host, router network layer functions:
transport layer: TCP, UDP
network
layer
Path-selection
algorithms:
implemented in
• routing protocols
(OSPF, BGP)
• SDN controller
IP protocol
forwarding
table
• datagram format
• addressing
• packet handling conventions
ICMP protocol
• error reporting
• router “signaling”
link layer
physical layer
Network Layer: 4-60
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IP Datagram format
32 bits
IP protocol version number
header length(bytes)
“type” of service:
§ diffserv (0:5)
§ ECN (6:7)
TTL: remaining max hops
(decremented at each router)
upper layer protocol (e.g., TCP or UDP)
6: TCP or 17: UDP
overhead
§ 20 bytes of TCP
§ 20 bytes of IP
§ = 40 bytes + app
layer overhead for
TCP+IP
ver head. type of
len service
16-bit identifier
upper
time to
layer
live
length
flgs
fragment
offset
header
checksum
source IP address
destination IP address
options (if any)
payload data
(variable length,
typically a TCP
or UDP segment)
total datagram
length (bytes)
fragmentation/
reassembly
header checksum
32-bit source IP address
32-bit destination IP address
e.g., timestamp, record
route taken
Maximum length: 64K bytes
Typically: 1500 bytes or less
Network Layer: 4-61
IP Fragmentation Fields
§ Header length: in units of 16-bit words
§ Data Unit Identifier (ID)
§ Sending host puts an identification number in each
datagram
§ Total length: Length of user data plus header in bytes
§ Fragment Offset - Position of fragment in original datagram
§ In multiples of 8 byte blocks
§ More fragments flag
§ Indicates that this is not the last fragment
§ Datagrams can be fragmented/refragmented at any router
§ Datagrams are reassembled only at the destination host
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IP fragmentation, reassembly
The maximum transmission unit (MTU) is the
size of the largest protocol data unit (PDU) that
can be communicated in a single network
layer transaction.
fragmentation:
in: one large datagram
out: 3 smaller datagrams
…
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 related
fragments
IP fragmentation, reassembly
example:
v
v
4000 byte datagram
MTU = 1500 bytes
1480 bytes in
data field
offset =
1480/8
offset =
2960/8
length ID fragflag
=4000 =x
=0
offset
=0
one large datagram becomes
several smaller datagrams
length ID fragflag
=1500 =x
=1
offset
=0
length ID fragflag
=1500 =x
=1
offset
=185
length ID fragflag
=1040 =x
=0
offset
=370
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IP fragmentation, reassembly
Example
q 4000 byte datagram
q Maximum Transmission Unit (MTU)
= 1500 bytes
length ID MoreFrag offset
=4000 =x
=0
=0
One large datagram becomes
several smaller datagrams
length ID MoreFrag offset
=1500 =x
=1
=0
1480 bytes in
data field
length ID MoreFrag offset
=1500 =x
=1
=185
offset =
1480/8
offset =
(1480+1480)/8
=2960/8
length ID MoreFrag offset
=1040 =x
=0
=370
20
1480
20
Len=1500
1480
Len=1500
20 1020
Len=1040
Len=4000
Fragmentation : Example 4-3
§ Consider sending a 1600-byte datagram into a link that has an MTU of 500 bytes.
Suppose the original datagram is stamped with the identification number 291.
§ How many fragments are generated? What are the values in the various fields in the IP
datagram(s) generated related to fragmentation?
§ 1600 byte datagram
§ Maximum Transmission Unit (MTU)= 500 bytes
§ Maximum size of data field in each fragment = 500 -20 = 480 bytes
§ The number of required fragments =
!"##$%#
=4
&'#
§ Each fragment will have Identification number 291
§ Each fragment except the last one will be of size 500 bytes (including IP header)
§ The last datagram will be of size 160 bytes
§ The offset of the 4 fragments will be 0,60,120,180
§ Each of the first fragments will have flag=1, the last fragment will have flag=0
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HW # 4-1
§ Consider sending a 2400-byte datagram into a link that has an MTU
of 720 bytes. Suppose the original datagram is stamped with the
identification number 422.
§ How many fragments are generated? What are the values in the
various fields in the IP datagram(s) generated related to
fragmentation?
HW # 4-1: solution
§ 2400 byte datagram
§ Maximum Transmission Unit (MTU)= 720 bytes
§ Maximum size of data field in each fragment = 720-20 = 700 bytes
§ The number of required fragments =
%&##$%#
(##
=4
§ Each fragment will have Identification number 422
§ Each fragment except the last one will be of size 720 bytes (including IP
header)
§ The last datagram will be of size 2400 – 700 – 700 – 700 = 300 bytes
§ The offset of the 4 fragments will be 0,88,176, 264
§ Each of the first fragments will have flag=1, the last fragment will have flag=0
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Network layer: “data plane” roadmap
4.1 Network layer: overview
• data plane
• control plane
4.2 What’s inside a router
• input ports, switching, output ports
• buffer management, scheduling
4.3 IP: the Internet Protocol
4.4 Generalized Forwarding, SDN
• datagram format
• Match+action
• addressing
• OpenFlow: match+action in action
• network address translation
• IPv6
4.5 Middleboxes
Network Layer: 4-69
IP addressing: introduction
223.1.1.1
§ IP address: 32-bit identifier
associated with each host or
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) each with a different
IP address
223.1.2.1
223.1.1.2
223.1.1.4
223.1.2.9
223.1.3.27
223.1.1.3
223.1.2.2
223.1.3.1
223.1.3.2
dotted-decimal IP address notation:
223.1.1.1 = 11011111 00000001 00000001 00000001
223
1
1
1
Network Layer: 4-70
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IP addressing: introduction
223.1.1.1
Q: how are interfaces
actually connected?
A: we’ll learn about
that in chapters 6, 7
223.1.2.1
223.1.1.2
A: wired
Ethernet interfaces
connected by
Ethernet switches
223.1.1.4
223.1.1.3
For now: don’t need to worry
about how one interface is
connected to another (with no
intervening router)
223.1.2.9
223.1.3.27
223.1.2.2
223.1.3.1
223.1.3.2
A: wireless WiFi interfaces
connected by WiFi base station
Network Layer: 4-71
IP addressing: introduction
§ IP address: 32-bit identifier for host, router interface
§ interface: connection between host/router and physical
link
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Binary IP Addresses
Dotted Decimal
37
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IP Addresses
IP Addresses
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Subnet
Subnets
223.1.1.1
§ What’s a subnet ?
• device interfaces that can
physically reach each other
without passing through an
intervening router
223.1.2.1
223.1.1.2
223.1.1.4
223.1.1.3
223.1.2.9
223.1.3.27
223.1.2.2
§ IP addresses have structure:
• subnet part: devices in same subnet
have common high order bits
• host part: remaining low order bits
223.1.3.1
223.1.3.2
network consisting of 3 subnets
Network Layer: 4-78
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Subnets
subnet 223.1.1.0/24
subnet 223.1.2.0/24
223.1.1.1
Recipe for defining subnets:
§detach each interface from its
host or router, creating
“islands” of isolated networks
§each isolated network is
subnet
called a subnet
223.1.3.0/24
223.1.2.1
223.1.1.2
223.1.1.4
223.1.2.9
223.1.3.27
223.1.1.3
223.1.2.2
223.1.3.2
223.1.3.1
subnet mask: /24
(high-order 24 bits: subnet part of IP address)
Network Layer: 4-79
Subnets
223.1.1.2
subnet 223.1.1/24
§ where are the
subnets?
§ what are the
/24 subnet
addresses?
223.1.1.1
223.1.1.4
223.1.1.3
223.1.9.2
223.1.7.0
subnet 223.1.9/24
223.1.9.1
223.1.7.1
223.1.8.1
subnet 223.1.2/24
223.1.2.1
223.1.2.6
subnet 223.1.7/24
223.1.8.0
subnet 223.1.8/24
223.1.2.2
223.1.3.27
223.1.3.1
subnet 223.1.3/24
223.1.3.2
Network Layer: 4-80
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Subnet Mask
CIDR
41
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IP addressing: CIDR
CIDR: Classless InterDomain Routing (pronounced “cider”)
• subnet portion of address of arbitrary length
• address format: a.b.c.d/x, where x is # bits in subnet portion
of address
host
part
subnet
part
11001000 00010111 00010000 00000000
200.23.16.0/23
Network Layer: 4-83
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
• All 1’s in the host part is used for subnet broadcast
• All 0’s in the host part was meant as “subnet
address” but not really used for anything. Some
implementation allow it to be used as host address.
Some don’t. Better to avoid it.
subnet
part
host
part
11001000 00010111 00010000 00000000
200.23.16.0/23
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IP addressing: CIDR
§ Classless Inter-domain Routing
§ Improve address space utilization
§ Routing scalability in the Internet
§ For example, if an ISP owns network 172.16.0.0/16, then
the ISP can offer 172.16.1.0/24, 172.16.2.0/24,and so on to
customers. Yet, when advertising to other providers, the ISP
only needs to advertise 172.16.0.0/16
IP Address Classes
43
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IP Address Classes
• Class A:
0 Network
1
7
Local
24
bits
10 Network
Local
16 bits
2
14
110
Network
Local
Class C:
8 bits
3
21
1110
Host Group (Multicast)
Class D:
4
28
bits
11110
Future use
Class E:
5
27
bits
Local = Subnet + Host (Variable length)
• Class B:
•
•
•
•
Router
Router
Subnet
IP Address Classes
44
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IP Address Classes
§ Class A: The first octet is the network portion.
Octets 2, 3, and 4 are for subnets/hosts
§ Class B: The first two octets are the network
portion. Octets 3 and 4 are for subnets/hosts
§ Class C: The first three octets are the network
portion. Octet 4 is for subnets/hosts
Private Address Range
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Network Masks
Subnetting
Creates multiple logical networks that
exist within a single Class A, B, or C
network.
— If you do not subnet, you will only be able
to use one network from your Class A, B,
or C network, which is unrealistic
— Each data link on a network must have a
unique network ID, with every node on
that link being a member of the same
network
—
46
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Benefits of Subnetting
Reduced network traffic
2) Optimized network performance
3) Simplified management
4) Facilitated spanning of large
geographical distances
1)
IP Subnet-Zero
§ This command allows you to use the first and last subnet in your
network design.
§ For example, the Class C mask of 192 provides subnets 64 and 128,
but with the IP subnet-zero command, you now get to use subnets 0,
64, 128, and 192
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How to create subnets
— Determine the number of required network IDs:
ØOne for each subnet
ØOne for each wide area network connection
— Determine the number of required host IDs per subnet:
ØOne for each TCP/IP host
ØOne for each router interface
— Based on the above requirements, create the following:
ØOne subnet mask for your entire network
ØA unique subnet ID for each physical segment
ØA range of host IDs for each subnet
Subnetting a Class A/B/C Address
§ How many subnets does the chosen subnet mask produce?
§ How many valid hosts per subnet are available?
§ What are the valid subnets?
§ What’s the broadcast address of each subnet?
§ What are the valid hosts in each subnet?
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Practice Example #1C: 255.255.255.128 (/25)
Network 192.168.10.0
— How many subnets? Since 128 is 1 bit on (10000000),
the answer would be 21= 2.
— How many hosts per subnet? We have 7 host bits off
(10000000), so the equation would be 27– 2 = 126
hosts.
— What are the valid subnets? 256 – 128 = 128.
Remember, we’ll start at zero and count in our block
size, so our subnets are 0, 128.
— What’s the broadcast address for each subnet? The
number right before the value of the next subnet is all
host bits turned on and equals the broadcast address.
For the zero subnet, the next subnet is 128, so the
broadcast of the 0 subnet is 127.
— What are the valid hosts? These are the numbers
between the subnet and broadcast address
DHCP
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How does a host get an IP address?
Q: How does a host get IP address?
§ Manually
• hard-coded by system admin in a file
• Windows: control-panel->network->configuration->tcp/ip->properties
• UNIX: /etc/rc.config
§ Automagically
• DHCP: Dynamic Host Configuration Protocol: dynamically get address
from a server
• “plug-and-play”
DHCP
§ Dynamic Host Control Protocol
§ Allows hosts to get an IP address automatically from a
server
§ Do not need to program each host manually
§ Each allocation has a limited “lease” time
§ Can reuse a limited number of addresses
§ Hosts broadcast “Is there a DHCP Server Here?”
§ Sent to 255.255.255.255
§ DHCP servers respond
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DHCP: Dynamic Host Configuration Protocol
goal: host dynamically obtains 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 join/leave network
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: 4-101
DHCP client-server scenario
DHCP server
223.1.1.1
223.1.2.1
Typically, DHCP server will be colocated in router, serving all subnets
to which router is attached
223.1.2.5
223.1.1.2
223.1.1.4
223.1.1.3
223.1.2.9
223.1.3.27
223.1.2.2
223.1.3.1
arriving DHCP client needs
address in this network
223.1.3.2
Network Layer: 4-102
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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
yiaddrr:Here’s
223.1.2.4
server!
an IP
transaction ID: 654
address you can use
lifetime: 3600 secs
DHCP request
src: 0.0.0.0, 68
dest:: 255.255.255.255, 67
Broadcast:
OK. I would
yiaddrr: 223.1.2.4
like totransaction
use this ID:
IP655
address!
lifetime: 3600 secs
DHCP ACK
src: 223.1.2.5, 67
dest: 255.255.255.255,
68
Broadcast:
OK. You’ve
yiaddrr: 223.1.2.4
got
that IPID:
address!
transaction
655
lifetime: 3600 secs
Network Layer: 4-103
DHCP client-server scenario
DHCP server: 223.1.2.5
DHCP discover
Arriving client
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
The two steps above can
be skipped “if a client
remembers and wishes to
reuse a previously
allocated network address”
[RFC 2131]
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
Network Layer: 4-104
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DHCP: more than IP addresses
DHCP can return more than just allocated IP address on
subnet:
§ address of first-hop router for client (Default Gateway)
§ name and IP address of DNS sever
§ network mask (indicating network versus host portion of address)
Network Layer: 4-105
DHCP: example
§ Connecting laptop will use DHCP
to get IP address, address of firsthop router, address of DNS server.
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
168.1.1.1
router with DHCP
server built into
router
§ DHCP REQUEST message encapsulated
in UDP, encapsulated in IP, encapsulated
in Ethernet
§ Ethernet frame broadcast (dest:
FFFFFFFFFFFF) on LAN, received at router
running DHCP server
§ Ethernet demux’ed to IP demux’ed,
UDP demux’ed to DHCP
Network Layer: 4-106
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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
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
§ encapsulated DHCP server reply
forwarded to client, demuxing up to
DHCP at client
router with DHCP
server built into
router
§ client now knows its IP address, name
and IP address of DNS server, IP
address of its first-hop router
Network Layer: 4-107
DHCP: Wireshark
output (home LAN)
Message type: Boot Request (1)
Hardware type: Ethernet
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)
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 Request
Option: (61) Client identifier
Length: 7; Value: 010016D323688A;
Hardware type: Ethernet
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
……
request
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
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How do organizatios get
an IP address?
How do organizatios get an IP address?
55
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IP addresses: how to get one?
Q: how does network get subnet part of IP address?
A: gets allocated portion of its provider ISP’s address space
ISP's block
11001000 00010111 00010000 00000000
200.23.16.0/20
ISP can then allocate out its address space in 8 blocks:
Organization 0
Organization 1
Organization 2
...
Organization 7
11001000 00010111 00010000 00000000
11001000 00010111 00010010 00000000
11001000 00010111 00010100 00000000
…..
….
11001000 00010111 00011110 00000000
200.23.16.0/23
200.23.18.0/23
200.23.20.0/23
….
200.23.30.0/23
Network Layer: 4-111
Hierarchical addressing: more specific routes
§ Organization 1 moves from Fly-By-Night-ISP to ISPs-R-Us
§ ISPs-R-Us now advertises a more specific route to Organization 1
Organization 0
200.23.16.0/23
Organization 2
200.23.20.0/23
Organization 7
.
.
.
.
.
.
Fly-By-Night-ISP
“Send me anything
with addresses
beginning
200.23.16.0/20”
Internet
200.23.30.0/23
ISPs-R-Us
Organization 1
200.23.18.0/23
“Send me anything
with addresses
beginning
199.31.0.0/16”
“or 200.23.18.0/23”
Network Layer: 4-112
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IP addressing: last words ...
Q: how does an ISP get block of
addresses?
A: ICANN: Internet Corporation for
Assigned Names and Numbers
http://www.icann.org/
• allocates IP addresses, through 5
regional registries (RRs) (who may
then allocate to local registries)
• manages DNS root zone, including
delegation of individual TLD (.com,
.edu , …) management
Q: are there enough 32-bit IP
addresses?
§ ICANN allocated last chunk of
IPv4 addresses to RRs in 2011
§ NAT (next) helps IPv4 address
space exhaustion
§ IPv6 has 128-bit address space
"Who the hell knew how much address
space we needed?" Vint Cerf (reflecting
on decision to make IPv4 address 32 bits
long)
Network Layer: 4-113
“Route Print” Command in Windows
MAC: netstat -rn
===========================================================================
Interface List
0x1 ........................... MS TCP Loopback interface
0x2 ...00 16 eb 05 af c0 ...... Intel(R) WiFi Link 5350 - Packet Scheduler Miniport
0x3 ...00 1f 16 15 7c 41 ...... Intel(R) 82567LM Gigabit Network Connection - Packet Scheduler Miniport
0x40005 ...00 05 9a 3c 78 00 ...... Cisco Systems VPN Adapter - Packet Scheduler Miniport
===========================================================================
===========================================================================
Active Routes:
Network Destination
Netmask
Gateway
Interface Metric
0.0.0.0
0.0.0.0
192.168.0.1
192.168.0.108
10
0.0.0.0
0.0.0.0
192.168.0.1
192.168.0.106
10
127.0.0.0
255.0.0.0
127.0.0.1
127.0.0.1
1
169.254.0.0
255.255.0.0
192.168.0.106
192.168.0.106
20
192.168.0.0
255.255.255.0
192.168.0.106
192.168.0.106
10
192.168.0.0
255.255.255.0
192.168.0.108
192.168.0.108
10
192.168.0.106 255.255.255.255
127.0.0.1
127.0.0.1
10
192.168.0.108 255.255.255.255
127.0.0.1
127.0.0.1
10
192.168.0.255 255.255.255.255
192.168.0.106
192.168.0.106
10
192.168.0.255 255.255.255.255
192.168.0.108
192.168.0.108
10
224.0.0.0
240.0.0.0
192.168.0.106
192.168.0.106
10
224.0.0.0
240.0.0.0
192.168.0.108
192.168.0.108
10
255.255.255.255 255.255.255.255
192.168.0.106
192.168.0.106
1
255.255.255.255 255.255.255.255
192.168.0.106
40005
1
255.255.255.255 255.255.255.255
192.168.0.108
192.168.0.108
1
Default Gateway:
192.168.0.1
===========================================================================
Persistent Routes:
None
Adr & mask = Dest
Þ Match
Longest Prefix match
is used
Metric: Lower is better
Note: 127.0.0.1 = Local Host, 224.x.y.z = Multicast on local LAN
57
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Practice Example #4: 255.255.255.224 (/27)
Network 192.168.10.0
— How many subnets? 224 is 11100000, so our
equation would be 23 = 8.
— How many hosts? 25– 2 = 30.
— What are the valid subnets? 256 – 224 = 32. We just
start at zero and count to the subnet mask value in
blocks (increments) of 32: 0, 32, 64, 96, 128, 160,
192, and 224.
— What’s the broadcast address for each subnet
(always the number right before the next subnet)?
— What are the valid hosts (the numbers between the
subnet number and the broadcast address)?
Practice Solution #4: 255.255.255.224 (/27)
Network 192.168.10.0
Subnet
Address
0
32
First Host
1
Last Host
30
Broadcast
Address
31
………….
192
224
33
193
225
62
222
254
63
223
255
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HW # 5: 255.255.128.0 (/17)
Network 172.16.0.0
— Subnets?
— Hosts?
— Valid subnets?
— Broadcast address for each subnet?
— Valid hosts?
HW Solution # 5: 255.255.128.0 (/17)
Network 172.16.0.0
— Subnets? 21 = 2
— Hosts? 215– 2 = 32,766 (7 bits in the third octet, and 8 in the fourth)
— Valid subnets? 256 – 128 = 128. 0, 128. Remember that subnetting is
performed in the third octet, so the subnet numbers are really 0.0
and 128.0, as shown in the next table
— Broadcast address for each subnet?
— Valid hosts?
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Practice Solution #5: 255.255.128.0 (/17)
Network 172.16.0.0
Subnet
0.0
128.0
First Host
0.1
128.1
Last Host
127.254
255.254
Broadcast
127.255
255.255
HW # 6: 255.255.240.0 (/20)
Network 172.16.0.0
§ Subnets?
§ Hosts?
§ Valid subnets?
§ Broadcast address for each subnet?
§ Valid hosts?
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Practice Solution #6: 255.255.240.0 (/20)
Network 172.16.0.0
§ Subnets? 24= 16.
§ Hosts? 212 – 2 = 4094.
§ Valid subnets? 256 – 240 = 0, 16, 32, 48, etc., up
to 240.
§ Broadcast address for each subnet?
§ Valid hosts?
Practice Solution #6: 255.255.240.0 (/20)
Network 172.16.0.0
Subnet
0.0
16.0
………..
240.0
First Host
0.1
16.1
240.1
Last Host
15.254
31.254
255.254
Broadcast
15.255
31.255
255.255
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Packet Scheduling Matching: Example 4-7
Consider the three subnets in the diagram below.
Consider the three subnets in the diagram below.
What is the maximum # of interfaces in the 223.1.2/24 network?
What is the maximum # of interfaces in the 223.1.3/29 network?
Packet Scheduling Matching: solution 4-7
Consider the three subnets in the diagram below.
What is the maximum # of interfaces in the 223.1.2/24 network? 256
What is the maximum # of interfaces in the 223.1.3/29 network? 8
Which of the following addresses can be used by an interface in the
223.1.3/29 network? 223.1.3.6
Which of the following addresses can not be used by an interface in the
223.1.3/29 network? 223.1.3.16, 223.1.2.6
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Subnet Addressing: Example 4-8
Consider the router and the two attached subnets below (A and B). The number of hosts is also shown
below. The subnets share the 24 high-order bits of the address space: 192.168.6.0/24
Assign subnet addresses to each of the subnets (A and B) so that the amount of address space assigned is
minimal, and at the same time leaving the largest possible contiguous address space available for
assignment if a new subnet were to be added. Then answer the questions below.
1.
2.
3.
4.
5.
6.
Is the address space public or private?
How many hosts can there be in this address space?
What is the subnet address of subnet A? (CIDR notation)
What is the broadcast address of subnet A?
What is the starting address of subnet A?
What is the ending address of subnet A?
Subnet Addressing: Example 4-8(cont.)
Consider the router and the two attached subnets below (A and B). The number of hosts is also shown
below. The subnets share the 24 high-order bits of the address space: 192.168.6.0/24
Assign subnet addresses to each of the subnets (A and B) so that the amount of address space assigned is
minimal, and at the same time leaving the largest possible contiguous address space available for
assignment if a new subnet were to be added. Then answer the questions below.
7.
8.
9.
10.
What is the subnet address of subnet B? (CIDR notation)
What is the broadcast address of subnet B?
What is the starting address of subnet B?
What is the ending address of subnet B?
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Subnet Addressing: Solution 4-8
1.
2.
3.
4.
5.
6.
The address 192.168.6.0/24 is private.
Maximum number of hosts = 2^x - 2 = 2^8 - 2 = 254. The reason we have to
subtract 2 from the final number is because there are always 2 addresses
allocated for each address block: the subnet ID (the first address) and the
broadcast address (the last address); for example, if you have 5 bits for hosts,
you can have 30 hosts, because 2 of the addresses are for the subnet ID and
the broadcast address which when added equals 32, which is 2^5.
Subnet A has 20 hosts, so it will need at least 22 addresses (for the subnet ID
and broadcast address). The least number of bits that satisfy this is 5 bits.
Knowing that, we take the prior subnet and add 32, the result of which is
192.168.6.64/27
The broadcast address of subnet A (192.168.6.64/27) is 192.168.6.95, because
it is the last address in the IP range.
The first IP address of subnet A (192.168.6.64/27) is 192.168.6.65, found by
adding 1 to the subnet address.
The last IP address of subnet A (192.168.6.64/27) is 192.168.6.94, found by
subtracting 1 from the broadcast address (192.168.6.95).
Subnet Addressing: Solution 4-8 (cont.)
7.
Similar to the prior subnet, subnet B has 62 hosts, so it will need at least 64
addresses (for the subnet ID and broadcast address). The least number of bits
that satisfy this is 6 bits. Knowing that, we take the prior subnet and add 64,
the result of which is 192.168.6.0/26
8.
The broadcast address of subnet B (192.168.6.0/26) is 192.168.6.63, because it
is the last address in the IP range.
9.
The first IP address of subnet B (192.168.6.0/26) is 192.168.6.1, found by
adding 1 to the subnet address.
10. The last IP address of subnet B (192.168.6.0/26) is 192.168.6.62, found by
subtracting 1 from the broadcast address (192.168.6.63).
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Network layer: “data plane” roadmap
4.1 Network layer: overview
• data plane
• control plane
4.2 What’s inside a router
• input ports, switching, output ports
• buffer management, scheduling
4.3 IP: the Internet Protocol
4.4 Generalized Forwarding, SDN
• datagram format
• Match+action
• addressing
• OpenFlow: match+action in action
• network address translation
• IPv6
4.5 Middleboxes
Network Layer: 4-129
NAT: network address translation
NAT: all devices in local network share just one IPv4 address as
far as outside world is concerned
rest of
Internet
138.76.29.7
local network (e.g., home
network) 10.0.0/24
10.0.0.4
10.0.0.1
10.0.0.2
10.0.0.3
all datagrams leaving local network have
same source NAT IP address: 138.76.29.7,
but different source port numbers
datagrams with source or destination in
this network have 10.0.0/24 address for
source, destination (as usual)
Network Layer: 4-130
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NAT: network address translation
§ all devices in local network have 32-bit addresses in a “private” IP
address space (10/8, 172.16/12, 192.168/16 prefixes) that can only
be used in local network
§ advantages:
§ just one IP address needed from provider ISP for all devices
§ can change addresses of host in local network without notifying
outside world
§ can change ISP without changing addresses of devices in local
network
§ security: devices inside local net not directly addressable, visible
by outside world
Network Layer: 4-131
NAT: network address translation
implementation: NAT router must (transparently):
§ 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 address
§ 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
destination fields of every incoming datagram with corresponding
(source IP address, port #) stored in NAT table
Network Layer: 4-132
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NAT: network address translation
NAT translation table
WAN side addr
LAN side addr
2: NAT router changes
datagram source address
from 10.0.0.1, 3345 to
138.76.29.7, 5001,
updates table
1: host 10.0.0.1 sends
datagram to
128.119.40.186, 80
138.76.29.7, 5001 10.0.0.1, 3345
……
……
S: 10.0.0.1, 3345
D: 128.119.40.186, 80
1
2
S: 138.76.29.7, 5001
D: 128.119.40.186, 80
138.76.29.7
S: 128.119.40.186, 80
D: 138.76.29.7, 5001
3
10.0.0.4
S: 128.119.40.186, 80
D: 10.0.0.1, 3345
10.0.0.1
10.0.0.2
4
10.0.0.3
3: reply arrives, destination
address: 138.76.29.7, 5001
Network Layer: 4-133
NAT: network address translation
§ NAT has been controversial:
•
•
•
•
routers “should” only process up to layer 3
address “shortage” should be solved by IPv6
violates end-to-end argument (port # manipulation by network-layer device)
NAT traversal: what if client wants to connect to server behind NAT?
§ but NAT is here to stay:
• extensively used in home and institutional nets, 4G/5G cellular nets
Network Layer: 4-134
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IPv6: motivation
§ initial motivation: 32-bit IPv4 address space would be
completely allocated
§ additional motivation:
• speed processing/forwarding: 40-byte fixed length header
• enable different network-layer treatment of “flows”
Network Layer: 4-135
IPv6 datagram format
priority: identify
priority among
datagrams in flow
128-bit
IPv6 addresses
32 bits
ver
pri
flow label
hop limit
payload len
next hdr
source address
(128 bits)
flow label: identify
datagrams in same
"flow.” (concept of
“flow” not well defined).
destination address
(128 bits)
payload (data)
What’s missing (compared with IPv4):
§ no checksum (to speed processing at routers)
§ no fragmentation/reassembly
§ no options (available as upper-layer, next-header protocol at router)
Network Layer: 4-136
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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 (“packet within a packet”)
• tunneling used extensively in other contexts (4G/5G)
IPv4 header fields
IPv4 source, dest addr
IPv6 header fields
IPv6 source dest addr
IPv4 payload
UDP/TCP payload
IPv6 datagram
IPv4 datagram
Network Layer: 4-137
Tunneling and encapsulation
Ethernet connecting
two IPv6 routers:
A
B
IPv6
IPv6
Link-layer frame
Ethernet connects two
IPv6 routers
E
F
IPv6
IPv6
IPv6 datagram
The usual: datagram as payload in link-layer frame
Network Layer: 4-138
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Tunneling and encapsulation
Ethernet connecting
two IPv6 routers:
A
B
IPv6
IPv6
Link-layer frame
IPv4 network
connecting two
IPv6 routers
Ethernet connects two
IPv6 routers
E
F
IPv6
IPv6
IPv6 datagram
The usual: datagram as payload in link-layer frame
A
B
E
F
IPv6
IPv6/v4
IPv6/v4
IPv6
IPv4 network
Network Layer: 4-139
Tunneling and encapsulation
Ethernet connecting
two IPv6 routers:
A
B
IPv6
IPv6
Link-layer frame
IPv4 tunnel
connecting two
IPv6 routers
IPv4 datagram
Ethernet connects two
IPv6 routers
E
F
IPv6
IPv6
IPv6 datagram
The usual: datagram as payload in link-layer frame
A
B
IPv6
IPv6/v4
IPv4 tunnel
connecting IPv6 routers
E
F
IPv6/v4
IPv6
IPv6 datagram
tunneling: IPv6 datagram as payload in a IPv4 datagram
Network Layer: 4-140
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Tunneling
logical view:
physical view:
IPv4 tunnel
connecting IPv6 routers
A
B
IPv6
IPv6/v4
A
B
C
D
E
F
IPv6
IPv6/v4
IPv4
IPv4
IPv6/v4
IPv6
flow: X
src: A
dest: F
data
A-to-B:
IPv6
E
F
IPv6/v4
IPv6
src:B
dest: E
src:B
dest: E
src:B
dest: E
Flow: X
Src: A
Dest: F
Flow: X
Src: A
Dest: F
Flow: X
Src: A
Dest: F
data
data
data
B-to-C:
IPv6 inside
IPv4
B-to-C:
IPv6 inside
IPv4
flow: X
src: A
dest: F
data
B-to-C:
IPv6 inside
IPv4
E-to-F:
IPv6
Network Layer: 4-141
Tunneling
logical view:
physical view:
B
IPv6
IPv6/v4
A
B
C
D
E
F
IPv6
IPv6/v4
IPv4
IPv4
IPv6/v4
IPv6
flow: X
src: A
dest: F
Note source and
destination
addresses!
IPv4 tunnel
connecting IPv6 routers
A
data
A-to-B:
IPv6
E
F
IPv6/v4
IPv6
src:B
dest: E
src:B
dest: E
src:B
dest: E
Flow: X
Src: A
Dest: F
Flow: X
Src: A
Dest: F
Flow: X
Src: A
Dest: F
data
data
data
B-to-C:
IPv6 inside
IPv4
B-to-C:
IPv6 inside
IPv4
B-to-C:
IPv6 inside
IPv4
flow: X
src: A
dest: F
data
E-to-F:
IPv6
Network Layer: 4-142
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IPv6: adoption
§ Google1: ~ 30% of clients access services via IPv6
§ NIST: 1/3 of all US government domains are IPv6 capable
1
https://www.google.com/intl
/en/ipv6/statistics.html
Network Layer: 4-143
IPv6: adoption
§ Google1: ~ 30% of clients access services via IPv6
§ NIST: 1/3 of all US government domains are IPv6 capable
§ Long (long!) time for deployment, use
• 25 years and counting!
• think of application-level changes in last 25 years: WWW, social
media, streaming media, gaming, telepresence, …
• Why?
1
https://www.google.com/intl/en/ipv6/statistics.html
Network Layer: 4-144
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Network layer: “data plane” roadmap
4.1 Network layer: overview
• data plane
• control plane
4.2 What’s inside a router
• input ports, switching, output ports
• buffer management, scheduling
4.3 IP: the Internet Protocol
4.4 Generalized Forwarding, SDN
• datagram format
• Match+action
• addressing
• OpenFlow: match+action in action
• network address translation
• IPv6
4.5 Middleboxes
Network Layer: 4-145
Generalized forwarding: match plus action
Review: each router contains a forwarding table (aka: flow table)
§ “match plus action” abstraction: match bits in arriving packet, take action
values in arriving
• destination-based
forwarding: forward based on dest. IP address
packet header
0111
• generalized forwarding: 1 2
3
• many header fields can determine action
• many action possible: drop/copy/modify/log packet
forwarding table
(aka: flow table)
Network Layer: 4-146
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Flow table abstraction
§ flow: defined by header field values (in link-, network-, transport-layer fields)
§ generalized forwarding: simple packet-handling rules
• match: pattern 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
Router’s flow table define
router’s match+action rules
Flow table
match action
Network Layer: 4-147
Flow table abstraction
§ flow: defined by header fields
§ generalized forwarding: simple packet-handling rules
• match: pattern 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
src = *.*.*.*, dest=3.4.*.*
src=1.2.*.*, dest=*.*.*.*
src=10.1.2.3, dest=*.*.*.*
Flow table
match action
* : wildcard
4
1
2
forward(2)
drop
send to controller
3
Network Layer: 4-148
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OpenFlow: flow table entries
Match
Action
Stats
Packet + byte counters
1.
2.
3.
4.
Forward packet to port(s)
Drop packet
Modify fields in header(s)
Encapsulate and forward to controller
Header fields to match:
Ingress
Port
Src
MAC
Dst
MAC
Eth
Type
VLAN
ID
VLAN
Pri
IP Src
IP Dst
IP
Proto
IP
ToS
Network layer
Link layer
TCP/UDP
Src Port
TCP/UDP
Dst Port
Transport layer
Network Layer: 4-149
OpenFlow: examples
Destination-based forwarding:
Switch MAC
src
Port
*
*
MAC
dst
*
Eth VLAN VLAN
type
ID
Pri
*
*
*
IP
Src
IP
Dst
*
51.6.0.8
IP
Src
IP
Dst
*
*
IP
Src
IP
Dst
128.119.1.1
*
IP
Prot
IP
ToS
*
*
IP
Prot
IP
ToS
*
*
IP
Prot
IP
ToS
*
*
TCP TCP
s-port d-port Action
port6
*
*
IP datagrams destined to IP address 51.6.0.8 should be forwarded to router output port 6
Firewall:
Switch MAC
src
Port
*
*
MAC
dst
*
Eth VLAN VLAN
type
ID
Pri
*
*
*
TCP TCP
s-port d-port Action
drop
22
*
Block (do not forward) all datagrams destined to TCP port 22 (ssh port #)
Switch MAC
src
Port
*
*
MAC
dst
*
Eth VLAN VLAN
type
ID
Pri
*
*
*
Block (do not forward) all datagrams sent by host 128.119.1.1
TCP TCP
s-port d-port Action
drop
*
*
Network Layer: 4-150
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OpenFlow: examples
Layer 2 destination-based forwarding:
Switch MAC
src
Port
*
*
MAC
dst
22:A7:23:
11:E1:02
Eth VLAN VLAN
type
ID
Pri
*
*
*
IP
Src
IP
Dst
IP
Prot
IP
ToS
*
*
*
*
TCP TCP
s-port d-port Action
*
*
port3
layer 2 frames with destination MAC address 22:A7:23:11:E1:02 should be forwarded to
output port 3
Network Layer: 4-151
OpenFlow abstraction
§ match+action: abstraction unifies different kinds of devices
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
Network Layer: 4-152
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OpenFlow example
Orchestrated tables can create
network-wide behavior, e.g.,:
Host h6
10.3.0.6
1
2
s3
controller
§ datagrams from hosts h5 and
h6 should be sent to h3 or h4,
via s1 and from there to s2
4
3
Host h5
10.3.0.5
s1
1
Host h1
10.1.0.1
2
s2
1
4
Host h4
10.2.0.4
4
2
3
3
Host h3
10.2.0.3
Host h2
10.1.0.2
Network Layer: 4-153
OpenFlow example
match
action
Orchestrated tables can create
network-wide behavior, e.g.,:
Host h6
10.3.0.6
IP Src = 10.3.*.*
IP Dst = 10.2.*.* forward(3)
1
2
3
s3
controller
§ datagrams from hosts h5 and
h6 should be sent to h3 or h4,
via s1 and from there to s2
4
Host h5
10.3.0.5
1
Host h1
10.1.0.1
s1
2
4
3
match
ingress port = 1
IP Src = 10.3.*.*
IP Dst = 10.2.*.*
action
forward(4)
Host h2
10.1.0.2
s2
1
4
2
3
Host h3
10.2.0.3
match
Host h4
10.2.0.4
action
ingress port = 2
forward(3)
IP Dst = 10.2.0.3
ingress port = 2
forward(4)
IP Dst = 10.2.0.4
Network Layer: 4-154
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Generalized forwarding: summary
§ “match plus action” abstraction: match bits in arriving packet header(s) in
any layers, take action
• matching over many fields (link-, network-, transport-layer)
• local actions: drop, forward, modify, or send matched packet to
controller
• “program” network-wide behaviors
§ simple form of “network programmability”
• programmable, per-packet “processing”
• historical roots: active networking
• today: more generalized programming:
P4 (see p4.org).
Network Layer: 4-155
Network layer: “data plane” roadmap
4.1 Network layer: overview
4.2 What’s inside a router
4.3 IP: the Internet Protocol
4.4 Generalized Forwarding
4.5 Middleboxes
• middlebox functions
• evolution, architectural principles of
the Internet
Network Layer: 4-156
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Middleboxes
Middlebox (RFC 3234)
“any intermediary box performing functions apart
from normal, standard functions of an IP router on
the data path between a source host and
destination host”
Middleboxes everywhere!
national or global ISP
NAT: home,
Firewalls, IDS: corporate,
institutional, service providers,
ISPs
cellular,
institutional
Load balancers:
corporate, service
provider, data center,
mobile nets
Applicationspecific: service
providers,
institutional,
CDN
datacenter
network
enterprise
network
Caches: service
provider, mobile, CDNs
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Middleboxes
§ initially: proprietary (closed) hardware solutions
§ move towards “whitebox” hardware implementing open API
§ move away from proprietary hardware solutions
§ programmable local actions via match+action
§ move towards innovation/differentiation in software
§ SDN: (logically) centralized control and configuration management
often in private/public cloud
§ network functions virtualization (NFV): programmable services over
white box networking, computation, storage
The IP hourglass
Internet’s “thin waist”:
§ one network layer
protocol: IP
§ must be implemented
by every (billions) of
Internet-connected
devices
HTTP SMTP RTP
QUIC
DASH
TCP UDP
IP
Ethernet PPP …
PDCP WiFi Bluetooth
…
many protocols
in physical, link,
transport, and
application
layers
copper radio fiber
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The IP hourglass, at middle age
HTTP SMTP RTP
QUIC
DASH
Internet’s middle age
“love handles”?
§ middleboxes,
operating inside the
network
…
TCP UDP
caching
IP
Firewalls
Ethernet PPP …
PDCP WiFi Bluetooth
copper radio fiber
Architectural Principles of the Internet
RFC 1958
“Many members of the Internet community would argue that there is no architecture, but only a tradition,
which was not written down for the first 25 years (or at least not by the IAB). However, in very general terms,
the community believes that
the goal is connectivity, the tool is the Internet
Protocol, and the intelligence is end to end rather than hidden in the
network.”
Three cornerstone beliefs:
§ simple connectivity
§ IP protocol: that narrow waist
§ intelligence, complexity at network edge
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The end-end argument
§ some network functionality (e.g., reliable data transfer, congestion)
can be implemented in network, or at network edge
application
transport
network
data link
physical
application
transport
network
data link
physical
end-end implementation of reliable data transfer
application
transport
network
data link
physical
hop-by-hop (in-network) implementation of reliable data transfer
network
link
physical
network
link
physical
network
link
physical
network
link
physical
network
link
physical
network
link
physical
application
transport
network
data link
physical
The end-end argument
§ some network functionality (e.g., reliable data transfer, congestion)
can be implemented in network, or at network edge
“The function in question can completely and correctly be implemented only
with the knowledge and help of the application standing at the end points of the
communication system. Therefore, providing that questioned function as a
feature of the communication system itself is not possible. (Sometimes an
incomplete version of the function provided by the communication system may
be useful as a performance enhancement.)
We call this line of reasoning against low-level function implementation the “endto-end argument.”
Saltzer, Reed, Clark 1981
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Where’s the intelligence?
20th century phone net:
• intelligence/computing at
network switches
Internet (pre-2005)
• intelligence, computing at
edge
Internet (post-2005)
• programmable network devices
• intelligence, computing, massive
application-level infrastructure at edge
Chapter 4: done!
§ Network layer: overview
§ What’s inside a router
§ IP: the Internet Protocol
§ Generalized Forwarding, SDN
§ Middleboxes
Question: how are forwarding tables (destination-based forwarding)
or flow tables (generalized forwarding) computed?
Answer: by the control plane (next chapter)
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Network Layer Data Plane: Summary
1. Forwarding consists of matching the destination address to a list of
entries in a table. Routing consists of making that table.
2. IP is a forwarding protocol. IPv4 uses 32 bit addresses in dotdecimal notation. IPv6 uses 128 bit addresses in Hex-Colon
notation.
3. DHCP is used to assign addresses dynamically.
4. Private addresses are used inside an enterprise network. NAT
allows a single public address to be used by many internal hosts
with private addresses.
5. OpenFlow separates data plane from control plane and centralizes
the control plane
Acronyms
ACK
ACM
AQM
ARP
ATM
BGP
BUM
CAMs
CBR
CCR
CIDR
CPU
DHCP
DNS
Acknowledgement
Automatic Computing Machinery
Active Queue Management
Address Resolution Protocol
Asynchronous Transfer Mode
Border Gateway Protocol
Broadcast, Unknown, and Multicast
Content Addressable Memories
Constant bit rate
Computer Communications Review
Classless Inter-Domain Routing
Central Processing Unit
Dynamic Host Control Protocol
Domain Name Service
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Acronyms (Cont)
FCAPS
FCFS
FTP
GFR
HTTP
ICMP
ID
IP
IPv4
IPv6
ISP
KISS
LAN
MAC
Fault, Configuration, Accounting, Performance and Security
First Come First Served
File Transfer Protocol
Guaranteed Frame Rate
Hyper-Text Transfer Protocol
IP Control Message Protocol
Identifier
Inter-Network Protocol
IP Version 4
IP Version 6
Internet Service Provider
Keep it simple stupid
Local Area Network
Media Access Control
Acronyms (Cont)
MS
MTU
NAT
PBX
PHY
QoS
RED
RFC
RIP
RTT
SDN
SMTP
SSL
TCAM
Microsoft
Maximum Transmission Unit
Network Address Translation
Private Branch Exchange
Physical Layer
Quality of Service
Random Early Drop
Request for Comment
Routing Information Protocol
Round Trip Time
Software Defined Networking
Simple Mail Transfer Protocol
Secure Socket Layer
Ternary Content Addressable Memory
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Acronyms (Cont)
TCP
TLS
ToS
TTL
UBR
UPnP
VBR
VCI
VLAN
VPN
WAN
WiFi
Transmission Control Protocol
Transport Level Security
Type of Service
Time to live
Unspecified bit rate
Universal Plug and Play
Variable bit rate
Virtual Circuit Identifiers
Virtual Local Area Network
Virtual Private Network
Wide Area Network
Wireless Fidelity
Additional Chapter 4 slides
Network Layer: 4-172
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IP fragmentation/reassembly
fragmentation:
in: one large datagram
out: 3 smaller datagrams
…
§ network links have MTU (max.
transfer size) - largest possible
link-level frame
• different link types, different MTUs
§ large IP datagram divided
(“fragmented”) within net
reassembly
…
• one datagram becomes several
datagrams
• “reassembled” only at destination
• IP header bits used to identify, order
related fragments
Network Layer: 4-173
IP fragmentation/reassembly
example:
§ 4000 byte datagram
§ MTU = 1500 bytes
1480 bytes in
data field
offset =
1480/8
length ID fragflag
=4000 =x
=0
offset
=0
one large datagram becomes
several smaller datagrams
length ID fragflag
=1500 =x
=1
offset
=0
length ID fragflag
=1500 =x
=1
offset
=185
length ID fragflag
=1040 =x
=0
offset
=370
Network Layer: 4-174
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DHCP: Wireshark output (home LAN)
Message type: Boot Request (1)
Hardware type: Ethernet
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)
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 Request
Option: (61) Client identifier
Length: 7; Value: 010016D323688A;
Hardware type: Ethernet
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
……
request
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
Network Layer: 4-175
Subnets
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Subnets
Subnets
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Subnets
90