Chapter
h
6
Th Network
The
N
kL
Layer
1
Network Layer
y Design
g Isues
•
•
•
•
•
Store-and-Forward Packet Switching
Services Provided to the Transport Layer
Implementation of Connectionless Service
Implementation of Connection-Oriented Service
p
of Virtual-Circuit and Datagram
g
Subnets
Comparison
2
Store-and-Forward Packet Switching
g
fig 5-1
The environment of the network layer protocols.
protocols
3
4
5
Implementation
p
of Connectionless Service
Routing within a diagram subnet.
subnet
6
Implementation of Connection-Oriented Service
Routing within a virtual
virtual-circuit
circuit subnet.
subnet
7
8
Comparison of Virtual-Circuit and
Datagram Subnets
54
5-4
9
Routing algorithms can be grouped into two classes:
nonadaptive and adaptive
10
Routing
g Algorithms
g
•
•
•
•
•
•
•
•
•
The Optimality
p
y Principle
p
Shortest Path Routing
Flooding
Distance Vector Routing
Link State Routing
Hierarchical Routingg
Broadcast Routing
Multicast Routing
Routing for Mobile Hosts
11
Routing
g Algorithms
g
(2)
( )
Conflict between fairness and optimality.
optimality
12
13
The Optimality
p
y Principle
p
(a) A subnet. (b) A sink tree for router B.
14
15
Shortest Path Routing
g
The first 5 steps used in computing the shortest path from A to D.
D
The arrows indicate the working node.
16
Flooding
Flooding Algorithm: Every incoming packet is sent out on every
outgoing line except the one its arrived on
Methods for damming the flood
(a) hop count
(b) keeping track of which packets have been flooded (list of sequence
numbers)
Modified flooding algorithm: Selective flooding
Application of the flooding algorithm
(a) military
(b) distribution of database
(c) wireless networks
(d) bench mark for other algorithm (shortest path)
17
Distance Vector Routing
1.
Each router maintains a routing table, whose entry contains two
parts: the preferred outgoing line to use for that destination and an
estimate of the time or distance to that destination
2. The router is assumed to know the distance to each of its neighbors
3. Each router sends the routing table (destination, distance) to each
neighbor
i hb
4 Based
4.
B d on the
h collected
ll
d routing
i tables
bl from
f
the
h neighbors,
i hb
the
h router
computes the shortest distance for each destination and update the
routing table
18
Distance Vector Routing
g
(a) A subnet. (b) Input from A, I, H, K, and the new
routing table for J.
19
Distance Vector Routing
g ((2))
• Good news propagates fast
• Routers may take infinite number of exchanges
t know
to
k
bad
b d news ((a router
t was d
down))
The count-to-infinity problem.
20
Link State Routing
g
Each router must do the following:
g
1. Discover its neighbors, learn their network addresses.
2 Measure
2.
Meas re the delay
dela or cost to each of its neighbors.
neighbors
3. Construct a packet telling all it has just learned.
4. Send this packet to all other routers.
p the shortest ppath to every
y other router.
5. Compute
21
Learning
g about the Neighbors
g
• The LAN is modeled as a node ((artificial))
(a) Nine routers and a LAN.
LAN (b) A graph model of (a).
(a)
22
Measuring
g Line Cost
• A special ECHO packet is sent to the other side to measure the delay
((with / without taking
g the load into account))
A subnet in which the East and West parts are connected by two lines.
23
24
25
26
27
6
6-
6-
6 16
6-16
28
Distributing
g the Link State Packets
• Flooding is used to distribute the link state packet
Over a router has accumulates a full set of link state packet,
it can construct the entire subnet graph and compute the shortest path.
The packet buffer for router B in the previous slide (Fig. 6-13).
29
Hierarchical Routing
g
Hierarchical routing.
routing
30
31
6-21
6-21
6-21
6-21
32
Multicast Routing
g
(a) A network
network. (b) A spanning tree for the leftmost router
router.
(c) A multicast tree for group 1. (d) A multicast tree for group 2.
33
Broadcast Routing
g
Reverse path forwarding. (a) A subnet. (b) a Sink tree. (c) The
tree built by reverse path forwarding.
34
6-18
35
36
37
Routing
g for Mobile Hosts ((2))
Packet routing for mobile users.
users
38
Routing
g in Ad Hoc Networks
Possibilities
P
ibili i when
h the
h routers are mobile:
bil
1. Military
y vehicles on battlefield.
– No infrastructure.
2. A fleet of ships at sea.
–
All moving all the time
3. Emergency works at earthquake .
–
The infrastructure destroyed.
destroyed
4. A gathering of people with notebook computers.
–
I an area lacking
In
l ki 802.11.
802 11
39
Route Discovery
y
a) (a) Range of A's broadcast.
b) (b) After B and D have received A's broadcast.
c) (c) After C, F, and G have received A's broadcast.
d) (d) After E, H, and I have received A's broadcast.
Shaded nodes are new recipients. Arrows show possible reverse routes.
40
Route Discovery
y ((2))
Format of a ROUTE REQUEST packet.
41
Route Discovery
y ((3))
Format of a ROUTE REPLY packet.
42
Route Maintenance
(a) D
D'ss routing table before G goes down.
(b) The graph after G has gone down.
43
Node Lookup
p in Peer-to-Peer Networks
( ) A sett off 32 node
(a)
d identifiers
id tifi arrangedd in
i a circle.
i l The
Th shaded
h d d ones
correspond to actual machines. The arcs show the fingers from
nodes 1,, 4,, and 12. The labels on the arcs are the table indices.
(b) Examples of the finger tables.
44
Congestion
g
Control Algorithms
g
•
•
•
•
•
•
General Principles of Congestion Control
Congestion Prevention Policies
Congestion Control in Virtual-Circuit Subnets
C
Congestion
ti Control
C t l in
i Datagram
D t
Subnets
S b t
Load Shedding
g
Jitter Control
45
Congestion
g
When too much traffic is offered,
offered congestion sets in and
performance degrades sharply.
46
General Principles
p of Congestion
g
Control
1. Monitor the system .
– detect when and where congestion occurs
occurs.
2. Pass information to where action can be taken.
3. Adjust system operation to correct the problem.
47
Congestion
g
Prevention Policies
(timeout interval)
5-26
(input /output queue)
(timeout interval)
(selective repeat)
(response time)
Policies that affect congestion.
congestion
48
Congestion Control in Virtual-Circuit
Subnets
a. Admission control (no more new VC will be setup)
b New VCs are allowed only to bypass the congestion areas
b.
c. Resources (bandwidth, buffer,..) reservation
(a) A congested subnet.
subnet (b) A redrawn subnet
subnet, eliminates
congestion and a virtual circuit from A to B.
49
Congestion control in datagram subnets
a.
Each router estimates the availability of each output line.
unew = auold + (1 − a ) f
a: updating period
f: utilization of the line
b Whenever
b.
Wh
u moves above
b
the
th threshold,
th h ld some action
ti will
ill
be taken
c. The warning bit method
d. Choke p
packets ((to source))
e. hop-by-hop choke packets
50
Hop-by-Hop
Hop
by Hop
Choke
C
o e Packets
ac ets
(a) A choke packet that affects
only the source.
(b) A choke packet that affects
eachh hop
h it passes through.
th
h
51
Load shedding (discarding packets)
a.
b.
c.
d.
wine
i policy
li (minimize
( i i i retransmission)
t
i i )
milk policy (for multimedia, new is better)
priority (discarding low priority packets)
( wired subnet,, the router
Random Earlyy Detection (for
discards the packets randomly, and the source should
response
p
lost ppackets byy slowingg down the transmission
rate)
52
Jitter Control
• Jitter control is important for audio and video applications
a. Routers adjust the transmission order of output packets
b. For video on demand packets can be buffered at the receiver in advance
(a) High jitter.
(b) Low jitter.
53
Quality
Q
y of Service
•
•
•
•
•
Requirements
q
for Achievingg Good Q
Quality
y of Service
Techniques
Integrated Services
Differentiated Services
Label Switching and MPLS
54
Requirements
q
5-30
How stringent the quality
quality-of-service
of service requirements are.
are
55
ATM networks classify flows into four categories:
1. Constant bit rate (telephony)
1
2. Real-time variable bit rate ( compressed video)
3. Non-real-time variable bit rate (movie over Internet)
4. Available bit rate ((file transfer))
56
Techniques
q
for Achieving
g Good QOS
1.
2.
3
3.
4.
5.
6.
7.
8
8.
9.
Over provisioning (over supply)
Buffering
T ffi shapping
Traffic
h i (agreement
(
t on certain
t i traffic
t ffi pattern)
tt )
The leaky bucket algorithm
The token bucket algorithm
Resource reservation
Admission control
P
Proportional
ti l routing
ti
Packet scheduling
57
Buffering
g
Smoothing the output stream by buffering packets.
packets
58
59
60
61
The Leaky
Bucket
Algorithm
(a) Input to a leaky bucket.
(b) Output from a leaky
bucket Output from a token
bucket.
bucket with capacities of (c)
250 KB,, ((d)) 500 KB,, ((e))
750 KB, (f) Output from a
500KB token bucket feeding
a 10-MB/sec
/
leaky
l k bucket.
b k
62
63
Resource reservation
Th different
Three
diff
t ki
kinds
d off resources can be
b reservedd
•
•
•
Bandwidth
p
Buffer space
CPU cycles
64
Because of random arrivals, and random service time distribution,
queues can build
b ild up andd delays
d l
can occur.
Consider that packets arrives with Poisson distribution with mean λ
packet/sec,
k t/
the
th service
i time
ti is
i exponential
ti l with
ith μ packet/sec.
k t/
The CPU can be modeled as an M/M/1 queue.
Th delay
The
d l is
i given
i
as
ρ
λ
T =
ρ =
λ (1 − ρ )
μ
1
1
=
μ 1− ρ
For example = λ = 950000 packets / sec
μ = 1000000 packets / sec
ρ = 0.95
T = 20 μ sec onds
65
Admission Control
When the incomingg traffic from some flow is well shaped,
p , the
capacity can be reserved in advance on the routers along
the path.
p
When a flow with flow specification (produced by the sender)
is offered
offered, the router along the router examines the flow
specification and modifies the parameters as need be.
Finally the acceptable flow specification (acceptable
parameters) can be established.
66
An example of flow specification
5-34
maximum sustained rate average
g
over a long time interval
maximum capacity of the bucket
maximum
i
ttolerated
l t d
transmission rate
⟩
related to protocols and
processing time
A router will map the parameters to a set of
specific
ifi resource reservations
ti
67
Proportional Routing
To provide a high quality of service
service, the traffic can be split
over multiple routes.
A simple method is to divide the traffic equally or in
proportion to the capacities of the outgoing links.
68
Packet Scheduling
g
1. Fair queueing algorithm
The router has separate queues for each output line, one for each flow. When a
li b
line
becomes idl
idle, th
the router
t scans th
the queues round
d robins,
bi
ttaking
ki th
the fifirstt
packet on the nest queue.
2. Weighted fair queueing algorithm
* for priority service (e.g. giving some servers more bandwidth)
(a) A router with five packets queued for line O.
O
(b) Finishing times for the five packets.
69
Integrated
g
Services
I was aimed
It
i d at both
b h unicast
i
(streaming
(
i video)
id ) andd multicast
li
(digital television broadcasting) applications
In many multicast applications groups can change membership
dynamically. Under these conditions, the approach of
having the senders reserve bandwidth in advance dose not
work well. If the number of receivers are large, the
dynamical grouping dose not work at all.
70
Integrated Services (flow-based algorithms)
RSVP-The ReSerVation Protocol
(a) A network
network, (b) The multicast spanning tree for host 1.
1
(c) The multicast spanning tree for host 2.
71
RSVP-The ReSerVation Protocol (2)
( )
Already
reserved
When Host5
requests, new
reservation is
needed
d d
(a) Host 3 requests a channel to host 1.
1 (b) Host 3 then requests a second
channel, to host 2. (c) Host 5 requests a channel to host 1.
72
Differentiated services (class-based)
(
)
Differentiated Service (DS) can be offered by a set of routers
forming an administrative domain (e.g.
(e g an ISP or a Telco).
Telco)
The administration defined a set of service classes with
corresponding forwarding rules
rules. Some classes (e.g.
(e g
premium service) with higher requirements may has better
service than others.
others DS scheme does not require advance
setup, resource reservation or time consuming end-to-end
negotiation for each flow.
flow This makes DS relatively easy
to implement.
For example
example, the operator reserves resources for all IP-phone
IP phone
users (the IP-phone class), on contrary, a flow-based
scheme each phone call gets its own resources and
scheme,
73
guarantees.
Differentiated Services (class-based service)
Expedited Forwarding
• Two queues for each output line, one for expedited packets
and one for regular packets
Expedited packets experience a traffic-free network.
74
Assured Forwardingg
The assured forwarding scheme which manages the service
classes,
l
specifies
ifi three
h packet
k di
discard
d probabilities
b bili i andd
four priority classes, each class having its own resources.
Taken together, three two factors define 12 service classes.
p
procedure
p
Implementation
1. Classify the packets into one of the four classes (on the
sending host or the ingress router)
2. Mark the packets (type A service in IP, which is specified in
the type of service field)
3. Pass the packets through a shaper/dropper filter (e.g. leaky
b k t or token
buckets
t k buckets)
b k t)
75
Assured Forwarding
g
A possible implementation of the data flow for assured forwarding.
forwarding
76
Label Switchingg and MPLS
This scheme adds a label in front each packet and does the
routing based on the label rather than on the destination
address.
address
Making the label an index into an internal table makes finding
the correct output line becomes just a matter of table
lookup.
( 225, A
(X,
ATM, fframe relay
l andd virtual
i
l circuits
i i did the
h similar
i il
thing)
77
Label Switching
g and MPLS
• A new MPLS header is added in front of the IP header.
• On a router-to-router line using PPP as the framing protocol
• MPLS is layer 2.5
s: stacking multiple labels in hierarchical networks
TTL: Time To Live
Transmitting a TCP segment using IP, MPLS, and PPP.
78
1. MPLS headers are not p
part of the network layer
y packet
p
or
the data link layer frame
2 It is possible to build MPLS switches that can forward
2.
both IP packets and ATM cells
3 In the MPLS router
3.
router, the label is used as an index into a
table to determine the outgoing line to use and the new
label to be used (label swapping)
4. The router may group multiple flows that end at a
particular
ti l router
t (LAN) andd use a single
i l label
l b l (Forwarding
(F
di
Equivalence Class, FEC)
5. No setup is needed for MPLS routing
79
6. Two ways to create the forwarding table.
D t di
Data-driven:
The
Th hit router
t asks
k the
th router
t downstream
d
t
to
t
generate a label for the flow (for ATM subnet)
C
Control-driven:
ldi
The
Th router checks
h k to see for
f which
hi h routers it
i is
i
the final destination. It then creates one or more FECs for them,
allocates a label for each one,
one and passes the labels in their
forwarding table and send new ones to their neighbors.
7 A packet may carry an entire stack of labels.
7.
labels The S bit is set to 1
for the bottom label and 0 for all the other labels.
80
Internetworking
g
•
•
•
•
•
•
•
How Networks Differ
How Networks Can Be Connected
Concatenated Virtual Circuits
Connectionless Internetworking
Tunneling
Internetwork Routing
Fragmentation
81
82
Connecting
g Networks
A collection of interconnected networks.
networks
83
How Networks Differ
5-43
Some of the many ways networks can differ.
84
85
How Networks Can Be Connected
(a) Two Ethernets connected by a switch.
(b) Two Ethernets connected by routers.
86
87
88
89
90
Each network in an internetwork is independent and is often referred to as
an Autonomous System (AS)
91
92
Fragmentation
g
(a) Transparent fragmentation. (b) Nontransparent fragmentation.
93
Fragmentation
g
((2))
Fragmentation when the elementary data size is 1 byte.
(a) Original packet, containing 10 data bytes.
(b) Fragments after passing through a network with maximum
packet size of 8 payload bytes plus header.
header
(c) Fragments after passing through a size 5 gateway.
94
The Network Layer
y in the Internet
•
•
•
•
•
•
•
•
The IP Protocol
IP Addresses
Internet Control Protocols
OSPF – The Interior Gateway Routing Protocol
BGP – The Exterior Gateway Routing Protocol
Internet Multicasting
Mobile IP
IPv6
95
96
Design
g Principles
p for Internet
1.
2.
3
3.
4.
5
5.
6.
7.
8
8.
9.
10
10.
Make sure it works.
Keep it simple.
Make clear choices.
choices
Exploit modularity.
E
Expect
heterogeneity.
h
i
Avoid static options and parameters. (negotiation is preferred)
Look for a good design; it need not be perfect.
Be strict when sending and tolerant when receiving
receiving.
Think about scalability.
C id performance
Consider
f
andd cost.
97
Collection of Subnetworks
The Internet is an interconnected collection of many networks.
98
Fragment
g
offset: Tell where in the current datagram
g
this fragment
g
belongs
g ((A
13
maximum of 2 =8192 fragments per datagram)
99
Time to line: Packet lifetime ≤ 255 seconds
The IP Protocol (2)
( )
5-54
Some of the IP options.
options
100
ICANN (I
(Internet
t
tC
Corporation
ti ffor A
Assigned
i
dN
Names
and Numbers)
101
102
Subnets
Gettingg a new network address would be hard to do since
network addresses are scarce. The problem is the rule that
a single
g class A,, B or C address refers to on network,, not
to a collection of LANs. To solve this problem is to allow
a network to be split
p into several parts
p
for internal use but
still act like a single network to the outside world. In the
Internet literature,, the pparts of the network ((e.g.
g Ethernets))
are called subnets. (Do not confuse with subnet used
elsewhere))
103
Subnets
A campus network consisting of LANs for various departments.
departments
104
Subnets (2)
( )
To implement subnetting, the main router needs a subnet mask that indicates
the split
p between network +subnet number and host. Subnet masks are also
written in dotted decimal notation or (/number of bits). For example, the
original 16-bit host number is split into 6-bit subnet number and 10-bit host
number allowing for up to 64 Ethernets
number,
Ethernets, each with maximum of 1022 hosts
(0 and -1 are not available)
A class B network subnetted into 64 subnets.
subnets
105
106
CDR – Classless InterDomain Routing
g
The Internet is rapidly running out of IP addresses. One solution is CIDR which
allocates the remaining IP addresses in variable-singed blocks.
If a site needs 2000 addresses, it is given a block of 2048 addresses.
For example, a set of IP addresses assignments is given as follows.
A set of IP address assignments
assignments.
107
Dropping the classes makes forwarding more complicated. The network-based
forwarding scheme is no longer applicable.
applicable A table look
look-up
up with address
matching forwarding algorithm is devised.
When the set of IP address is assigned to the three universities: Cambridge, Edinburgh
and
dO
Oxford.
f d
The router uses the mask to get the value which only matches Oxford address as
11000010 00011000 00010000 00000000
The Oxford entry is used and the packet is sent along the line going to Oxford.
A remote router, say Omaha, Nebraska, may use a single aggregate entry with a binary
address
dd
and
d submask.
b
k
108
NAT – Network Address Translation
• IP addresses are scarce. NAT is a quick fix. Each company is a assigned a few IP addresses for
Internet traffic. Within the company, every computer gets a unique IP address for intramural traffic
No p
packets containing
g these addresses may
y appear
pp
on the Internet.
• There are three reserved ranges. Most IP packets carry TCP or UPD payload, which have source
port and destination port. NAT box will establish translation table based on the TCP or UPD source
port and destination port.
Placement and operation of a NAT box.
109
NAT can quickly solve the IP address shortage problem (even for
ADSL cable modem application)
ADSL,
Here are some of the objections
1 NAT violates the architectural model of IP,
1.
IP i.e.
i e every IP
address identifies a single machine worldwide
2 NAT establish the translation table,
2.
table which changes the Internet
from connectionless to a kind of connection-oriented network
3. NAT violates the layer structure
4. NAT can only handle IP packets with TCP or UDP payload,
not others
5. NAT will not work when IP addresses are inserted in the body
of the text (e.g. FTP, H.323)
6. Each IP address can only map up to 65536-4096=61440
machines
7. Delay the implantation of IPV6
110
Internet Control Protocols (ICP)
ICP includes ICMP
ICMP, ARP
ARP, RARP
RARP, BOOTP
BOOTP, and DHCP
Internet Control Message Protocol (I CMP)
ICMP is used to test the internet and to report unexpected events.
5-61
Each ICMP message type is encapsulated in an IP packet.
The principal ICMP message types.
111
112
When Host1 (E1) sends a packet to a user on Host2 (192.31.65.5) by e-mail. The Domain Name
System (DNS) will find the e-mail address corresponding to IP address 192.31.65.5.
send all traffic for host 4 to the router, and this solution is called proxy ARP.
114
RARP, BOOTP, and DHCP
Given an Ethernet address, what is the corresponding IP
address?
Reverse Address Resolution Protocol (RARP) allows a newlybooted workstation to broadcast its Ethernet address. The
RARP server sees the request, looks up the file and sends
back the corresponding IP address. RARP server is
needed on each network, because the router will not
forward the broadcost (all 1’s) packet.
BOOTP uses UDP messages, however BOOTP requires
manual configuration of tables mapping IP address to
Ethernet address.
115
Dynamic Host Configuration Protocol (DHCP)
DHCP allows both manual IP assignment and automatic assignment.
agent
Operation of DHCP.
116
OSPF-The Interior Gateway Routing Protocol
Design
g pprinciples
p
1. published algorithm
2 support variety of distance metrics
2.
metrics, including physical
distance, delay, etc.
3 dynamic,
3.
d
i adapting
d ti to
t changes
h
4. support routing based on type of service
5. Do load balancing (load splitting)
6. support for hierarchical systems
7. security
117
6
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
Flow label will be used to allow a source and destination to setup
pap
pseudo connection
Next header tells which of the six extension headers
Fig.
The IPv6 fixed header (required)
133
IPv6 address
134
135
Extension Headers (2)
( )
To support of datagram exceeding 64KB
The hop-by-hop extension header for large datagrams (jumbograms).
136
Extension Headers (3)
( )
The routing header lists one or more routers that must be visited
•The next header tells which of the six extension headers follow this one. If this header is
the last IP header, then the Next header tells which transport protocol handler to pass
the packet to
•The routing type field gives the format of the rest of the header. The segments left field
keeps track of how many addresses have not yet been visited. When it hits 0, the packet
i on it
is
its own ((no more guidance).
id
)
The extension header for routing.
137