Download Chapter 4: Network Layer

Chapter 4: Network Layer
Chapter goals:
 understand principles behind network layer
services:
network layer service models
 forwarding versus routing
 how a router works
 routing (path selection)
 dealing with scale
 advanced topics: IPv6, mobility

 instantiation, implementation in the Internet
Network Layer
4-1
Chapter 4: Network Layer
 4. 1 Introduction
 4.2 Virtual circuit and
datagram networks
 4.3 What’s inside a
router
 4.4 IP: Internet
Protocol




Datagram format
IPv4 addressing
ICMP
IPv6
 4.5 Routing algorithms
 Link state
 Distance Vector
 Hierarchical routing
 4.6 Routing in the
Internet



RIP
OSPF
BGP
 4.7 Broadcast and
multicast routing
Network Layer
4-2
Network layer
 transport segment from




sending to receiving host
on sending side
encapsulates segments
into datagrams
on rcving side, delivers
segments to transport
layer
network layer protocols
in every host, router
router examines header
fields in all IP datagrams
passing through it
application
transport
network
data link
physical
network
data link
physical
network
data link
physical
network
data link
physical
network
data link
physical
network
data link
physical
network
network
data link
data link
physical
physical
network
data link
physical
network
data link
physical
network
data link
physical
network
data link
physical
Network Layer
application
transport
network
data link
physical
4-3
Application
Application protocol
Application
TCP
TCP protocol
TCP
IP
Data Link
Host
IP
IP protocol
Data
Link
Data
Link
IP
IP protocol
Data
Link
Router
Data
Link
Data
Link
IP protocol
Data
Link
Router
Data
Link
IP
Network
Access
Host
Network Layer
4-4
Two Key Network-Layer Functions
 forwarding: move
packets from router’s
input to appropriate
router output
 routing: determine
route taken by
packets from source
to dest.

analogy:
 routing: process of
planning trip from
source to dest
 forwarding: process
of getting through
single interchange
routing algorithms
Network Layer
4-5
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
packet’s header
0111
1
3 2
Network Layer
4-6
Connection setup
 3rd important function in some network architectures:
ATM, frame relay, X.25
 before datagrams flow, two end hosts and intervening
routers establish virtual connection
 routers get involved
 network vs transport layer connection service:
 network: between two hosts (may also involve
intervening routers in case of VCs)
 transport: between two processes

Network Layer
4-7
Network service model
Q: What service model for “channel” transporting
datagrams from sender to receiver?
Example services for
individual datagrams:
 guaranteed delivery
 guaranteed delivery
with less than 40 msec
delay
Example services for a
flow of datagrams:
 in-order datagram
delivery
 guaranteed minimum
bandwidth to flow
 restrictions on
changes in interpacket spacing
Network Layer
4-8
Network layer service models:
Network
Architecture
Internet
Service
Model
Guarantees ?
Congestion
Bandwidth Loss Order Timing feedback
best effort none
ATM
CBR
ATM
VBR
ATM
ABR
ATM
UBR
constant
rate
guaranteed
rate
guaranteed
minimum
none
no
no
no
yes
yes
yes
yes
yes
yes
no
yes
no
no (inferred
via loss)
no
congestion
no
congestion
yes
no
yes
no
no
Network Layer
4-9
Chapter 4: Network Layer
 4. 1 Introduction
 4.2 Virtual circuit and
datagram networks
 4.3 What’s inside a
router
 4.4 IP: Internet
Protocol




Datagram format
IPv4 addressing
ICMP
IPv6
 4.5 Routing algorithms
 Link state
 Distance Vector
 Hierarchical routing
 4.6 Routing in the
Internet



RIP
OSPF
BGP
 4.7 Broadcast and
multicast routing
Network Layer 4-10
Network layer connection and
connection-less service
 datagram network provides network-layer
connectionless service
 VC network provides network-layer
connection service
 analogous to the transport-layer services,
but:
service: host-to-host
 no choice: network provides one or the other
 implementation: in network core

Network Layer
4-11
Virtual circuits
“source-to-dest path behaves much like telephone
circuit”


performance-wise
network actions along source-to-dest path
 call setup, teardown for each call before data can flow
 each packet carries VC identifier (not destination host
address)
 every router on source-dest path maintains “state” for
each passing connection
 link, router resources (bandwidth, buffers) may be
allocated to VC (dedicated resources = predictable service)
Network Layer 4-12
VC implementation
a VC consists of:
1.
2.
3.
path from source to destination
VC numbers, one number for each link along
path
entries in forwarding tables in routers along
path
 packet belonging to VC carries VC number
(rather than dest address)
 VC number can be changed on each link.

New VC number comes from forwarding table
Network Layer 4-13
Forwarding table
VC number
22
12
1
Forwarding table in
northwest router:
Incoming interface
1
2
3
1
…
2
32
3
interface
number
Incoming VC #
12
63
7
97
…
Outgoing interface
3
1
2
3
…
Outgoing VC #
22
18
17
87
…
Routers maintain connection state information!
Network Layer 4-14
Virtual circuits: signaling protocols
 used to setup, maintain teardown VC
 used in ATM, frame-relay, X.25
 not used in today’s Internet
application
transport 5. Data flow begins
network 4. Call connected
data link 1. Initiate call
physical
6. Receive data application
3. Accept call transport
2. incoming call network
data link
physical
Network Layer 4-15
Datagram networks
 no call setup at network layer
 routers: no state about end-to-end connections
 no network-level concept of “connection”
 packets forwarded using destination host address
 packets between same source-dest pair may take
different paths
application
transport
network
data link 1. Send data
physical
application
transport
2. Receive data network
data link
physical
Network Layer 4-16
Forwarding table
Destination Address Range
4 billion
possible entries
Link Interface
11001000 00010111 00010000 00000000
through
11001000 00010111 00010111 11111111
0
11001000 00010111 00011000 00000000
through
11001000 00010111 00011000 11111111
1
11001000 00010111 00011001 00000000
through
11001000 00010111 00011111 11111111
2
otherwise
3
Network Layer 4-17
Longest prefix matching
Prefix Match
11001000 00010111 00010
11001000 00010111 00011000
11001000 00010111 00011
otherwise
Link Interface
0
1
2
3
Examples
DA: 11001000 00010111 00010110 10100001
Which interface?
DA: 11001000 00010111 00011000 10101010
Which interface?
Network Layer 4-18
Datagram or VC network: why?
Internet (datagram)
 data exchange among
ATM (VC)
 evolved from telephony
computers
 human conversation:
 “elastic” service, no strict
 strict timing, reliability
timing req.
requirements
 “smart” end systems
 need for guaranteed
(computers)
service
 can adapt, perform
 “dumb” end systems
control, error recovery
 telephones
 simple inside network,
 complexity inside
complexity at “edge”
network
 many link types
 different characteristics
 uniform service difficult
Network Layer 4-19
Chapter 4: Network Layer
 4. 1 Introduction
 4.2 Virtual circuit and
datagram networks
 4.3 What’s inside a
router
 4.4 IP: Internet
Protocol




Datagram format
IPv4 addressing
ICMP
IPv6
 4.5 Routing algorithms
 Link state
 Distance Vector
 Hierarchical routing
 4.6 Routing in the
Internet



RIP
OSPF
BGP
 4.7 Broadcast and
multicast routing
Network Layer 4-20
Router Architecture Overview
Two key router functions:
 run routing algorithms/protocol (RIP, OSPF, BGP)
 forwarding datagrams from incoming to outgoing link
Network Layer 4-21
Input Port Functions
Physical layer:
bit-level reception
Data link layer:
e.g., Ethernet
see chapter 5
Decentralized switching:
 given datagram dest., lookup output port
using forwarding table in input port
memory
 goal: complete input port processing at
‘line speed’
 queuing: if datagrams arrive faster than
forwarding rate into switch fabric
Network Layer 4-22
Three types of switching fabrics
Network Layer 4-23
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
Memory
Output
Port
System Bus
Network Layer 4-24
Switching Via a Bus
 datagram from input port memory
to output port memory via a shared
bus
 bus contention: switching speed
limited by bus bandwidth
 32 Gbps bus, Cisco 5600: sufficient
speed for access and enterprise
routers
Network Layer 4-25
Switching Via An Interconnection
Network
 overcome bus bandwidth limitations
 Banyan networks, other interconnection nets
initially developed to connect processors in
multiprocessor
 advanced design: fragmenting datagram into fixed
length cells, switch cells through the fabric.
 Cisco 12000: switches 60 Gbps through the
interconnection network
Network Layer 4-26
Output Ports
 Buffering required when datagrams arrive from
fabric faster than the transmission rate
 Scheduling discipline chooses among queued
datagrams for transmission
Network Layer 4-27
Output port queueing
 buffering when arrival rate via switch exceeds
output line speed
 queueing (delay) and loss due to output port
buffer overflow!
Network Layer 4-28
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 Gps link: 2.5 Gbit buffer
 Recent recommendation: with N flows,
buffering equal to RTT. C
N
 Scheduling: QoS
 AQM, RED
Network Layer 4-29
Input Port Queuing
 Fabric slower than input ports combined -> queueing
may occur at input queues
 Head-of-the-Line (HOL) blocking: queued datagram
at front of queue prevents others in queue from
moving forward
 queueing delay and loss due to input buffer overflow!
Network Layer 4-30
Scheduling Policies
 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
Network Layer 4-31
Scheduling Policies: more
Priority scheduling: transmit highest priority queued
packet
 multiple classes, with different priorities


class may depend on marking or other header info, e.g. IP
source/dest, port numbers, etc..
Real world example?
Network Layer 4-32
Scheduling Policies: still more
round robin scheduling:
 multiple classes
 cyclically scan class queues, serving one from each
class (if available)
 real world example?
Scheduling Policies: still more
Weighted Fair Queuing:
 generalized Round Robin
 each class gets weighted amount of service in each
cycle
 real-world example?
Network Layer 4-34
Chapter 4: Network Layer
 4. 1 Introduction
 4.2 Virtual circuit and
datagram networks
 4.3 What’s inside a
router
 4.4 IP: Internet
Protocol




Datagram format
IPv4 addressing
ICMP
IPv6
 4.5 Routing algorithms
 Link state
 Distance Vector
 Hierarchical routing
 4.6 Routing in the
Internet



RIP
OSPF
BGP
 4.7 Broadcast and
multicast routing
Network Layer 4-35
The Internet Network layer
Host, router network layer functions:
Routing protocols
•path selection
•RIP, OSPF, BGP
Network
layer
Transport layer: TCP, UDP
IP protocol
•addressing conventions
•datagram format
•packet handling conventions
forwarding
table
ICMP protocol
•error reporting
•router “signaling”
Link layer
physical layer
Network Layer 4-36
Chapter 4: Network Layer
 4. 1 Introduction
 4.2 Virtual circuit and
datagram networks
 4.3 What’s inside a
router
 4.4 IP: Internet
Protocol




Datagram format
IPv4 addressing
ICMP
IPv6
 4.5 Routing algorithms
 Link state
 Distance Vector
 Hierarchical routing
 4.6 Routing in the
Internet



RIP
OSPF
BGP
 4.7 Broadcast and
multicast routing
Network Layer 4-37
IP datagram format
IP protocol version
number
header length
(bytes)
“type” of data
max number
remaining hops
(decremented at
each router)
upper layer protocol
to deliver payload to
how much overhead
with TCP?
 20 bytes of TCP
 20 bytes of IP
 = 40 bytes + app
layer overhead
32 bits
type of
ver head.
len service
length
fragment
16-bit identifier flgs
offset
upper
time to
header
layer
live
checksum
total datagram
length (bytes)
for
fragmentation/
reassembly
32 bit source IP address
32 bit destination IP address
Options (if any)
data
(variable length,
typically a TCP
or UDP segment)
E.g. timestamp,
record route
taken, specify
list of routers
to visit.
Network Layer 4-38
IP datagram format
Don’t Fragment header.
The IPv4 (Internet Protocol)
20 bytes ≤ Header Length < (24 – 1) x 4 bytes = 60 bytes
 20 bytes ≤ Total Length < 216 – 1 bytes = 65535 bytes

Network Layer 4-39
IP datagram format
Some of the IP options.
5-54
Network Layer 4-40
IP Fragmentation & Reassembly
 network links have MTU (max.
transmission 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
fragmentation:
in: one large datagram
out: 3 smaller datagrams
reassembly
Network Layer 4-41
IP Fragmentation con.
 IP分组头中与分段有关的域（红色）：
version
header
length
Identification
time-to-live (TTL)
Total Length (in bytes)
TOS
protocol
0
DM
F F
Fragment Offset
header checksum
Total Length: 本分段的长度。
Identification: 属于同一分组的分段有相同的Identification。
DF: 如果置1，则不允许分段。如果分组长度大于MTU, 则抛弃。
MF: 如果置1，说明还有跟在后面的分段。
Fragment Offset: 本分段中的数据相对于分段前的分组中数据的
位移，单位是8个字节。
IP Fragmentation & Reassembly
Example
 4000 byte
datagram
 MTU = 1500 bytes
1480 bytes in
data field
offset =
1480/8
length ID fragflag offset
=4000 =x
=0
=0
One large datagram becomes
several smaller datagrams
length ID fragflag offset
=1500 =x
=1
=0
length ID fragflag offset
=1500 =x
=1
=185
length ID fragflag offset
=1040 =x
=0
=370
Network Layer 4-43
IP Fragmentation con.
 举例，请注意 Fragment Offset 的单位是8个字节。
Header length: 20
Total length:
3620
Identification:
0xa428
DF flag:
0
MF flag:
0
Fragment offset: 0
Header length: 20
Total length:
660
Identification:
0xa428
DF flag:
0
MF flag:
0
Fragment offset: 370
IP datagram
Header length: 20
Header length: 20
Total length:
1500
Total length:
1500
Identification:
0xa428 Identification:
0xa428
DF flag:
0
DF flag:
0
MF flag:
1
MF flag:
1
Fragment offset: 185
fragment offset: 0
Fragment 3
MTU: 4000
MTU: 1500
Router
Fragment 2
Fragment 1
IP Fragmentation con.
 发送主机或路由器都可能对IP分组进行分段。
FDDI
Ring
Host A
Ethernet
Router
MTU: 4352
 一个IP分组可能被多次分段。
 但重组 (reassembly)仅在目的主机进行。
Host B
MTU: 1500
IP Fragmentation & Reassembly
Costs of fragmentation?
Network Layer 4-46
Chapter 4: Network Layer
 4. 1 Introduction
 4.2 Virtual circuit and
datagram networks
 4.3 What’s inside a
router
 4.4 IP: Internet
Protocol




Datagram format
IPv4 addressing
ICMP
IPv6
 4.5 Routing algorithms
 Link state
 Distance Vector
 Hierarchical routing
 4.6 Routing in the
Internet



RIP
OSPF
BGP
 4.7 Broadcast and
multicast routing
Network Layer 4-47
Design Principles for Internet
 Make sure it works.
 Keep it simple.
 Make clear choices.
 Exploit modularity.
 Expect heterogeneity.
 Avoid static options and parameters.
 Look for a good design; it need not be perfect.
 Be strict when sending and tolerant when receiving.
 Think about scalability.
 Consider performance and cost.
Network Layer 4-48
Collection of Subnetworks
The Internet is an interconnected collection of many networks.
Network Layer 4-49
IP Address
10000000
1st Byte
= 128
10001111
2nd Byte
= 143
10001001
3rd Byte
= 137
10010000
4th Byte
= 144
128.143.137.144
Network Layer 4-50
IP Addressing: introduction
 IP address: 32-bit
identifier for host,
router interface
 interface: connection
between host/router
and physical link



router’s typically have
multiple interfaces
host typically has one
interface
IP addresses
associated with each
interface
223.1.1.1
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
223.1.3.2
223.1.3.1
223.1.1.1 = 11011111 00000001 00000001 00000001
223
1
1
1
Network Layer 4-51
Subnets
 IP address:
 subnet part (high
order bits)
 host part (low order
bits)
 What’s a subnet ?
 device interfaces with
same subnet part of IP
address
 can physically reach
each other without
intervening router
223.1.1.1
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
subnet
223.1.3.1
223.1.3.2
network consisting of 3 subnets
Network Layer 4-52
Subnets
A campus network consisting of LANs for various
departments.
Network Layer 4-53
Subnets
Recipe
 To determine the
subnets, detach each
interface from its
host or router,
creating islands of
isolated networks.
Each isolated network
is called a subnet.
223.1.1.0/24
223.1.2.0/24
223.1.3.0/24
Subnet mask: /24
Network Layer 4-54
Subnets
223.1.1.2
How many?
223.1.1.1
223.1.1.4
223.1.1.3
223.1.9.2
223.1.7.0
223.1.9.1
223.1.7.1
223.1.8.1
223.1.8.0
223.1.2.6
223.1.2.1
223.1.3.27
223.1.2.2
223.1.3.1
223.1.3.2
Network Layer 4-55
IP addressing: CIDR
CIDR: Classless InterDomain Routing
subnet portion of address of arbitrary length:
VLSM (Variable Length Subnet Masking )
 address format: a.b.c.d/x, where x is # bits in
subnet portion of address

subnet
part
host
part
11001000 00010111 00010000 00000000
200.23.16.0/23
Network Layer 4-56
IP addressing: CIDR
A set of IP address assignments.
5-59
Network Layer 4-57
e.g. 128.14.32.0/20 (212 addrs)
“min”
The
same
20-bit
prefix
“max”
10000000 00001110 00100000 00000000
10000000 00001110 00100000 00000001
10000000 00001110 00100000 00000010
10000000 00001110 00100000 00000011
10000000 00001110 00100000 00000100
10000000 00001110 00100000 00000101


10000000 00001110 00101111 11111011
10000000 00001110 00101111 11111100
10000000 00001110 00101111 11111101
10000000 00001110 00101111 11111110
10000000 00001110 00101111 11111111
VLSM
 # of IP addrs w.r.t. length of network mask:
mask length
/27
/26
/25
/24
/23
/22
/21
/20
/19
/18
/17
/16
/15
/14
/13
# of addrs
32
64
128
256
512
1,024
2,048
4,096
8,192
16,384
32,768
65,536
131,072
262,144
524,288
Network Layer 4-59
Classful addressing
Network Layer 4-60
Classful addressing
Network Layer 4-61
Special IP
Network Layer 4-62
Special IP

broadcast IP：
• The subnet broadcast
202.112.41.224 202.112.41.255 hosts in the same
subnet receive it.
• Limited broadcast
255.255.255.255, all hosts receive it.

multicast IP：
• D, 224.0.0.0 – 239.255.255.255
Network Layer 4-63
Special IP

“0”：
• Host + network
• Host =0, network address;
• Network =0, host address;

loopback:
• 127.0.0.0 ~ 127.255.255.255; 127.0.0.1
Network Layer 4-64
Special IP
 Private IP
10.0.0.0
172.16.0.0
192.168.0.0
- 10.255.255.255
- 172.31.255.255
- 192.168.255.255
Used in LAN
NAT: Network Address Translation
Network Layer 4-65
IP address
network address
host address
 How to identify?
 Explicit: VLSM
 Implicit: Classful
Network Layer 4-66
Subnets
network addr
network addr
host addr
subnet addr
host addr
A class B network subnetted into 64 subnets
 VLSM
Network Layer 4-67
Why subnets?
DUT
College2
College1
library
Network Layer 4-68
Subnets: one more example
145.13.3.11
…
145.13.3.10
R2
145.13.3.101
145.13.7.34
145.13.7.35
subnet 145.13.3.0
R3
R1
…
subnet
145.13.7.0
145.13.7.56
subnet 145.13.21.0
…
145.13.21.23
145.13.21.9
145.13.21.8
network
145.13.0.0
Problems
 mask, network addr, host addr?

e.g. 224.221.121.19/12
 IP addr classification?
 e.g. 125.2.156.7
 IP addr class choosing: A, B, C?

e.g. 3470 hosts/interfaces
 length of subnet/net/mask addr?
 e.g. 578 hosts/interfaces
Network Layer 4-70
Problems
Subnet mask is 255.255.255.224
Host IP: 202.112.41.241
Problems: 1, network address and host address?
2, how many subnets are created at most?
Problems
 224 1111…..11100000 (x=27)
 202.112.41.241: 24111110001 =17
 net: 202.112.41.224
 host: 0.0.0.17;
 255.255.255.2241111…..11100000
 Eight subnets:
202.112.41.0； 202.112.41.32； 202.112.41.64；
202.112.41.96； 202.112.41.128； 202.112.41.160；
202.112.41.192； 202.112.41.224；
IP addresses: how to get one?
Q: How does a host get IP address?
 hard-coded by system admin in a file
Windows: control-panel->network->configuration>tcp/ip->properties
 UNIX: /etc/rc.config
 DHCP: Dynamic Host Configuration Protocol:
dynamically get address from as server
 “plug-and-play”

Network Layer 4-73
DHCP: Dynamic Host Configuration Protocol
Goal: allow host to dynamically obtain its IP address
from network server when it joins network
Can renew its lease on address in use
Allows reuse of addresses (only hold address while connected
an “on”)
Support for mobile users who want to join network (more
shortly)
DHCP overview:
 host broadcasts “DHCP discover” msg
 DHCP server responds with “DHCP offer” msg
 host requests IP address: “DHCP request” msg
 DHCP server sends address: “DHCP ack” msg
Network Layer 4-74
DHCP client-server scenario
A
B
223.1.2.1
DHCP
server
223.1.1.1
223.1.1.2
223.1.1.4
223.1.2.9
223.1.2.2
223.1.1.3
223.1.3.1
223.1.3.27
223.1.3.2
E
arriving DHCP
client needs
address in this
network
Network Layer 4-75
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
time
src: 0.0.0.0, 68
dest:: 255.255.255.255, 67
yiaddrr: 223.1.2.4
transaction ID: 655
Lifetime: 3600 secs
DHCP ACK
src: 223.1.2.5, 67
dest: 255.255.255.255, 68
yiaddrr: 223.1.2.4
transaction ID: 655
Lifetime: 3600 secs
Network Layer 4-76
IP addresses: how to get one?
Q: How does network get subnet part of IP
addr?
A: gets allocated portion of its provider ISP’s
address space
ISP's block
11001000 00010111 00010000 00000000
200.23.16.0/20
Organization 0
Organization 1
Organization 2
...
11001000 00010111 00010000 00000000
11001000 00010111 00010010 00000000
11001000 00010111 00010100 00000000
…..
….
200.23.16.0/23
200.23.18.0/23
200.23.20.0/23
….
Organization 7
11001000 00010111 00011110 00000000
200.23.30.0/23
Network Layer 4-77
IP addressing: the last word...
Q: How does an ISP get block of addresses?
A: ICANN: Internet Corporation for Assigned
Names and Numbers
 allocates addresses
 manages DNS
 assigns domain names, resolves disputes
Network Layer 4-78
IP addresses are exhausted
 Solutions?
VLSM/CIDR IPv4
 NAT
 IPv6

Network Layer 4-79
NAT: Network Address Translation
rest of
Internet
local network
(e.g., home network)
10.0.0/24
10.0.0.4
10.0.0.1
10.0.0.2
138.76.29.7
10.0.0.3
All datagrams leaving local
network have same single source
NAT IP address: 138.76.29.7,
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 4-80
NAT: Network Address Translation
 Motivation: local network uses just one IP address as
far as outside world is concerned:
 range of addresses not needed from ISP: just one IP
address for all devices
 can change addresses of devices in local network
without notifying outside world
 can change ISP without changing addresses of
devices in local network
 devices inside local net not explicitly addressable,
visible by outside world (a security plus).
Network Layer 4-81
NAT: Network Address Translation
Implementation: NAT router must:



outgoing datagrams: replace (source IP address, port
#) of every outgoing datagram to (NAT IP address,
new port #)
. . . remote clients/servers will respond using (NAT
IP address, new port #) as destination addr.
remember (in NAT translation table) every (source
IP address, port #) to (NAT IP address, new port #)
translation pair
incoming datagrams: replace (NAT IP address, new
port #) in dest fields of every incoming datagram
with corresponding (source IP address, port #)
stored in NAT table
Network Layer 4-82
NAT: Network Address Translation
2: NAT router
changes datagram
source addr from
10.0.0.1, 3345 to
138.76.29.7, 5001,
updates table
2
NAT translation table
WAN side addr
LAN side addr
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
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: Reply arrives
dest. address:
138.76.29.7, 5001
3
1
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
4: NAT router
changes datagram
dest addr from
138.76.29.7, 5001 to 10.0.0.1, 3345
Network Layer 4-83
NAT: Network Address Translation
 16-bit port-number field:

60,000 simultaneous connections with a single
LAN-side address!
 NAT is controversial:
 port
# are for processes, not hosts
 routers should only process up to layer 3
 violates end-to-end argument
• NAT possibility must be taken into account by app
designers, eg, P2P applications

address shortage should instead be solved by
IPv6
Network Layer 4-84
NAT traversal problem
 client wants to connect to
server with address 10.0.0.1


server address 10.0.0.1 local
Client
to LAN (client can’t use it as
destination addr)
only one externally visible
NATted address: 138.76.29.7
 solution 1: statically
configure NAT to forward
incoming connection
requests at given port to
server

10.0.0.1
?
138.76.29.7
10.0.0.4
NAT
router
e.g., (123.76.29.7, port 2500)
always forwarded to 10.0.0.1
port 25000
Network Layer 4-85
NAT traversal problem
 solution 2: Universal Plug and
Play (UPnP) Internet Gateway
Device (IGD) Protocol. Allows
NATted host to:
 learn public IP address
(138.76.29.7)
 add/remove port mappings
(with lease times)
10.0.0.1
IGD
10.0.0.4
138.76.29.7
NAT
router
i.e., automate static NAT port
map configuration
Network Layer 4-86
NAT traversal problem
 solution 3: relaying (used in Skype)
NATed client establishes connection to relay
 External client connects to relay
 relay bridges packets between to connections

2. connection to
relay initiated
by client
Client
3. relaying
established
1. connection to
relay initiated
by NATted host
138.76.29.7
10.0.0.1
NAT
router
Network Layer 4-87
Chapter 4: Network Layer
 4. 1 Introduction
 4.2 Virtual circuit and
datagram networks
 4.3 What’s inside a
router
 4.4 IP: Internet
Protocol




Datagram format
IPv4 addressing
ICMP
IPv6
 4.5 Routing algorithms
 Link state
 Distance Vector
 Hierarchical routing
 4.6 Routing in the
Internet



RIP
OSPF
BGP
 4.7 Broadcast and
multicast routing
Network Layer 4-88
ICMP: Internet Control Message Protocol
 used by hosts & routers to
communicate network-level
information
 error reporting:
unreachable host, network,
port, protocol
 echo request/reply (used
by ping)
 network-layer “above” IP:
 ICMP msgs carried in IP
datagrams
 ICMP message: type, code plus
first 8 bytes of IP datagram
causing error
Type
0
3
3
3
3
3
3
4
Code
0
0
1
2
3
6
7
0
8
9
10
11
12
0
0
0
0
0
description
echo reply (ping)
dest. network unreachable
dest host unreachable
dest protocol unreachable
dest port unreachable
dest network unknown
dest host unknown
source quench (congestion
control - not used)
echo request (ping)
route advertisement
router discovery
TTL expired
bad IP header
Network Layer 4-89
Traceroute and ICMP
 Source sends series of
UDP segments to dest



First has TTL =1
Second has TTL=2, etc.
Unlikely port number
 When nth datagram arrives
to nth router:



Router discards datagram
And sends to source an
ICMP message (type 11,
code 0)
Message includes name of
router& IP address
 When ICMP message
arrives, source calculates
RTT
 Traceroute does this 3
times
Stopping criterion
 UDP segment eventually
arrives at destination host
 Destination returns ICMP
“host unreachable” packet
(type 3, code 3)
 When source gets this
ICMP, stops.
Network Layer 4-90
Chapter 4: Network Layer
 4. 1 Introduction
 4.2 Virtual circuit and
datagram networks
 4.3 What’s inside a
router
 4.4 IP: Internet
Protocol




Datagram format
IPv4 addressing
ICMP
IPv6
 4.5 Routing algorithms
 Link state
 Distance Vector
 Hierarchical routing
 4.6 Routing in the
Internet



RIP
OSPF
BGP
 4.7 Broadcast and
multicast routing
Network Layer 4-91
IPv6
 Initial motivation: 32-bit address space soon
to be completely allocated.

Solutions?
 Additional motivation:
 header format helps speed processing/forwarding
 header changes to facilitate QoS
IPv6 datagram format:
 fixed-length 40 byte header
 no fragmentation allowed
Network Layer 4-92
IPv6 – overview
 IPv6 (IP Version 6), 又称为 IPng (IP Next
Generation)，是 IPv4的下一个版本。
 IPv6的标准文档是RFC 2460
 RFC 2461 – RFC 2466 讨论更多的有关IPv6的细节
IPv6 – overview
 IPv6 把IP地址的长度增加到了16个字节
 IPv6简化了IP分组的首部格式
 IPv6增强了对进一步扩展的支持
 IPv6增强了对QoS (Quality of Service)的支持
 IPv6增强了对安全的支持
 IPv6增加了对 Anycast 通信方式的支持
IPv6的基本首部
 IPv6 仍支持无连接的传送所引进的主要变化如下
 更大的地址空间。IPv6 将地址从 IPv4 的 32 位 增
大到了 128 位。



地址空间>3.4*1038.如果整个地球覆盖计算机，IPv6允许
每平方米拥有7*1023个IP地址。
如果每微妙分配100万个地址，需要1019年的时间才能将所
有地址分配完毕。
“地球上的每一粒沙子都可以有一个IP地址”。
IPv6的基本首部
 IPv6 仍支持无连接的传送所引进的主要变化如下
 更大的地址空间。IPv6 将地址从 IPv4 的 32 位 增
大到了 128 位。
 扩展的地址层次结构。
 灵活的首部格式。
 改进的选项。
 允许协议继续扩充。
 支持即插即用（即自动配置）
 支持资源的预分配。
IPv6数据报的首部
 IPv6 将首部长度变为固定的 40 字节，称为基
本首部(base header)。
 将不必要的功能取消了，首部的字段数减少到
只有 8 个。
 取消了首部的检验和字段，加快了路由器处理
数据报的速度。
 在基本首部的后面允许有零个或多个扩展首部
。
 所有的扩展首部和数据合起来叫做数据报的有
效载荷(payload)或净负荷。
IPv6数据报的一般形式
有效载荷
选项
基本
首部
扩展
首部 1
…
扩展
首部 N
IPv6 数据报
数 据 部 分
位
0
4
版本
12
通信量类
IPv6 的
有效载荷
（至 64 KB）
24
流
有 效 载 荷 长 度
IPv6 的
基本首部
（40 B）
16
标
31
号
下一个首部
源 地 址
（128 位）
目 的 地 址
（128 位）
有效载荷（扩展首部 / 数据）
跳数限制
位
0
4
版本
12
通信量类
IPv6 的
有效载荷
（至 64 KB）
24
流
有 效 载 荷 长 度
IPv6 的
基本首部
（40 B）
16
标
31
号
下一个首部
源 地 址
（128 位）
目 的 地 址
（128 位t）
扩展首部 / 数据
有效载荷（扩展首部
/ 数据）
跳数限制
位
0
4
版本
12
流量类别
24
流
有 效 载 荷 长 度
IPv6
的
基
本
首
部
40 B
16
标
31
号
下一个首部
跳数限制
源 地 址
（128 位）
目 的 地 址
（128 位）
版本(version)—— 4 位。它指明了协议的版本，对 IPv6
该字段总是 6。
位
0
4
版本
12
24
流
流量类别
有 效 载 荷 长 度
IPv6
的
基
本
首
部
40 B
16
标
31
号
下一个首部
跳数限制
源 地 址
（128 位）
目 的 地 址
（128 位）
流量类别(traffic class)—— 8 位。这是为了区分不同的
IPv6 数据报的类别或优先级。目前正在进行不同的流量
类别性能的实验。
位
0
4
版本
12
流量类别
24
流
有 效 载 荷 长 度
IPv6
的
基
本
首
部
40 B
16
标
下一个首部
31
号
跳数限制
源 地 址
（128 位）
目 的 地 址
（128 位）
流标号(flow label)—— 20 位。 “流”是互联网络上从特定源
点到特定终点的一系列数据报， “流”所经过的路径上的路由
器都保证指明的服务质量。
所有属于同一个流的数据报都具有同样的流标号。
位
0
4
版本
12
16
流量类别
流
有 效 载 荷 长 度
IPv6
的
基
本
首
部
40 B
24
标
31
号
下一个首部
跳数限制
源 地 址
（128 位）
目 的 地 址
（128 位）
有效载荷长度(payload length)—— 16 位。它指明 IPv6 数据报
除基本首部以外的字节数（所有扩展首部都算在有效载荷之内
），其最大值是 64 KB。
位
0
4
版本
12
流量类别
24
流
有 效 载 荷 长 度
IPv6
的
基
本
首
部
40 B
16
标
31
号
下一个首部
跳数限制
源 地 址
（128 位）
目 的 地 址
（128 位）
下一个首部(next header)—— 8 位。它相当于 IPv4 的协议字段
或可选字段。
位
0
4
版本
12
流量类别
24
流
有 效 载 荷 长 度
IPv6
的
基
本
首
部
40 B
16
标
31
号
下一个首部
跳数限制
源 地 址
（128 位）
目 的 地 址
（128 位）
跳数限制(hop limit)—— 8 位。源站在数据报发出时即设定跳数
限制。路由器在转发数据报时将跳数限制字段中的值减1。
当跳数限制的值为零时，就要将此数据报丢弃。
位
0
4
版本
12
流量类别
24
流
有 效 载 荷 长 度
IPv6
的
基
本
首
部
40 B
16
标
31
号
下一个首部
跳数限制
源 地 址
（128 位）
目 的 地 址
（128 位）
源地址—— 128 位。是数据报的发送站的 IP 地址。
位
0
4
版本
12
16
流量类别
流
有 效 载 荷 长 度
IPv6
的
基
本
首
部
40 B
24
标
31
号
下一个首部
跳数限制
源 地 址
（128 位）
目 的 地 址
（128 位）
目的地址—— 128 位。是数据报的接收站的 IP 地址。
IPv6的扩展首部
 IPv6 把原来 IPv4 首部中选项的功能都放在扩
展首部中，并将扩展首部留给路径两端的源站
和目的站的主机来处理。
 数据报途中经过的路由器都不处理这些扩展首
部（只有一个首部例外，即逐跳选项扩展首部
）。
 这样就大大提高了路由器的处理效率。
六种扩展首部
在 RFC 2460 中定义了六种扩展首部：
 逐跳选项
 路由选择
 分片
 鉴别
 封装安全有效载荷
 目的站选项
IPv6的扩展首部
无扩展首部
基本首部
下一个首部
= TCP/UDP
TCP/UDP 首部
和数据
(TCP/UDP 报文段）
有效载荷
有扩展首部
基本首部
路由选择首部
分片首部
下一个首部
= 路由选择
下一个首部
= 分片
下一个首部
= TCP/UDP
有效载荷
TCP/UDP 首部
和数据
(TCP/UDP 报文段）
IPv6的地址空间
IPv6 数据报的目的地址可以是以下三种基本类型
地址之一：
(1) 单播(unicast)
单播就是传统的点对点通信。
(2) 组播(multicast) 组播是一点对多点的通信。
(3) 任播(anycast) 这是 IPv6 增加的一种类型。
任播的目的站是一组计算机，但数据报在交付时
只交付其中的一个，通常是距离最近的一个。
冒号十六进制记法
(colon hexadecimal notation)
 每个 16 位的值用十六进制值表示，各值之间用
冒号分隔。
68E6:8C64:FFFF:FFFF:0:1180:960A:FFFF
 零压缩(zero compression)，即一连串连续的零
可以为一对冒号所取代。
 FF05:0:0:0:0:0:0:B3
可以写成：
 FF05::B3
点分十进制记法的后缀
 0:0:0:0:0:0:128.10.2.1
再使用零压缩即可得出： ::128.10.2.1
 CIDR 的斜线表示法仍然可用。
 60 位的前缀 12AB00000000CD3 可记为：
12AB:0000:0000:CD30:0000:0000:0000:0000/
60
或12AB::CD30:0:0:0:0/60
或12AB:0:0:CD30::/60
前缀为0000 0000的地址
 前缀为 0000 0000 是保留一小部分地址与 IPv4
兼容的，这是因为必须要考虑到在比较长的时期
IPv4和 IPv6 将会同时存在，而有的结点不支持
IPv6。“IPv4映射的IPv6地址”
 因此数据报在这两类结点之间转发时，就必须进
行地址的转换。
80 位
IPv4 映射的 0000..................0000
IPv6 地址
16 位
32 位
FFFF
IPv4 地址
Changes from IPv4
 Header Length

使用 Fixed Header : 40 bytes
 3 Flag Bits (0, DF, MF), Fragment Offset

分段有关信息被移动到扩展首部
 Checksum: removed entirely to reduce
processing time at each hop
 Options: allowed, but outside of header,
indicated by “Next Header” field

改变为扩展首部
Network Layer 4-116
ICMPv6
 ICMPv6 的报文格式和 IPv4 使用的 ICMP 的相
似，即前 4 个字节的字段名称都是一样的。但
ICMPv6 将第 5 个字节起的后面部分作为报文主
体。
additional message types, e.g. “Packet Too Big”
 multicast group management functions

 ICMPv6 的报文划分为四大类
差错报告报文
 提供信息的报文
 多播听众发现报文


邻站发现报文
Transition From IPv4 To IPv6
 Not all routers can be upgraded simultaneous
no “flag days”
 How will the network operate with mixed IPv4 and
IPv6 routers?

Network Layer 4-118
IPv6 – Transition
 RFC 2893 中提出了两种由IPv4向IPv6转变的
方法：
Dual IP Layer (又称Dual Stack，双协议栈): 在主机
和路由器上同时实现IPv4和IPv6两种协议.
 Tunneling (隧道技术): 把IPv6分组封装在IPv4分组
中传送。

IPv6 – Dual IP Layer
应用程序
TCP/UDP协议
IPv6协议
IPv4协议
数据链路层
物理层
Dual Stack (双协议栈)：
如果一台主机同时支持IPv6和IPv4两种协议，那么该
主机既能与支持IPv4协议的主机通信，又能与支持IPv6协
议的主机通信，这就是双协议栈技术的工作机理。
Tunneling
Logical view:
Physical view:
E
F
IPv6
IPv6
IPv6
A
B
E
F
IPv6
IPv6
IPv6
IPv6
A
B
IPv6
tunnel
IPv4
IPv4
Network Layer 4-121
Tunneling
Logical view:
Physical view:
A
B
IPv6
IPv6
A
B
C
IPv6
IPv6
IPv4
Flow: X
Src: A
Dest: F
data
A-to-B:
IPv6
E
F
IPv6
IPv6
D
E
F
IPv4
IPv6
IPv6
tunnel
Src:B
Dest: E
Src:B
Dest: E
Flow: X
Src: A
Dest: F
Flow: X
Src: A
Dest: F
data
data
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 4-122
Chapter 4: Network Layer
 4. 1 Introduction
 4.2 Virtual circuit and
datagram networks
 4.3 What’s inside a
router
 4.4 IP: Internet
Protocol




Datagram format
IPv4 addressing
ICMP
IPv6
 4.5 Routing algorithms
 Link state
 Distance Vector
 Hierarchical routing
 4.6 Routing in the
Internet



RIP
OSPF
BGP
 4.7 Broadcast and
multicast routing
Network Layer 4-123
Interplay between routing, forwarding
routing algorithm
local forwarding table
header value output link
0100
0101
0111
1001
3
2
2
1
value in arriving
packet’s header
0111
1
3 2
Network Layer 4-124
Graph abstraction
5
2
u
2
1
Graph: G = (N,E)
v
x
3
w
3
1
5
z
1
y
2
N = set of routers = { u, v, w, x, y, z }
E = set of links ={ (u,v), (u,x), (v,x), (v,w), (x,w), (x,y), (w,y), (w,z), (y,z) }
Remark: Graph abstraction is useful in other network contexts
Example: P2P, where N is set of peers and E is set of TCP connections
Network Layer 4-125
Graph abstraction: costs
5
2
u
v
2
1
x
• c(x,x’) = cost of link (x,x’)
3
w
3
1
5
z
1
y
- e.g., c(w,z) = 5
2
• cost could always be 1, or
inversely related to bandwidth,
or inversely related to
congestion
Cost of path (x1, x2, x3,…, xp) = c(x1,x2) + c(x2,x3) + … + c(xp-1,xp)
Question: What’s the least-cost path between u and z ?
Routing algorithm: algorithm that finds least-cost path
Network Layer 4-126
Routing Algorithm classification
Global or decentralized
information?
Global:
 all routers have complete
topology, link cost info
 “link state” algorithms
Decentralized:
 router knows physicallyconnected neighbors, link
costs to neighbors
 iterative process of
computation, exchange of
info with neighbors
 “distance vector” algorithms
Static or dynamic?
Static:
 routes change slowly
over time
Dynamic:
 routes change more
quickly
 periodic update
 in response to link
cost changes
Network Layer 4-127
Chapter 4: Network Layer
 4. 1 Introduction
 4.2 Virtual circuit and
datagram networks
 4.3 What’s inside a
router
 4.4 IP: Internet
Protocol




Datagram format
IPv4 addressing
ICMP
IPv6
 4.5 Routing algorithms
 Link state
 Distance Vector
 Hierarchical routing
 4.6 Routing in the
Internet



RIP
OSPF
BGP
 4.7 Broadcast and
multicast routing
Network Layer 4-128
A Link-State Routing Algorithm
Dijkstra’s algorithm
 net topology, link costs
known to all nodes
 accomplished via “link
state broadcast”
 all nodes have same info
 computes least cost paths
from one node (‘source”) to
all other nodes
 gives forwarding table
for that node
 iterative: after k
iterations, know least cost
path to k dest.’s
Notation:
 c(x,y): link cost from node
x to y; = ∞ if not direct
neighbors
 D(v): current value of cost
of path from source to
dest. v
 p(v): predecessor node
along path from source to v
 N': set of nodes whose
least cost path definitively
known
Network Layer 4-129
Dijsktra’s Algorithm
1 Initialization:
2 N' = {u}
3 for all nodes v
4
if v adjacent to u
5
then D(v) = c(u,v)
6
else D(v) = ∞
7
8 Loop
9 find w not in N' such that D(w) is a minimum
10 add w to N'
11 update D(v) for all v adjacent to w and not in N' :
12
D(v) = min( D(v), D(w) + c(w,v) )
13 /* new cost to v is either old cost to v or known
14 shortest path cost to w plus cost from w to v */
15 until all nodes in N'
Network Layer 4-130
Dijkstra’s algorithm: example
Step
0
1
2
3
4
5
N'
u
ux
uxy
uxyv
uxyvw
uxyvwz
D(v),p(v) D(w),p(w)
2,u
5,u
2,u
4,x
2,u
3,y
3,y
D(x),p(x)
1,u
D(y),p(y)
∞
2,x
D(z),p(z)
∞
∞
4,y
4,y
4,y
5
2
u
v
2
1
x
3
w
3
1
5
z
1
y
2
Network Layer 4-131
Dijkstra’s algorithm: example (2)
Resulting shortest-path tree from u:
v
w
u
z
x
y
Resulting forwarding table in u:
destination
link
v
x
(u,v)
(u,x)
y
(u,x)
w
(u,x)
z
(u,x)
Network Layer 4-132
Dijkstra algorithm illustration
[2 / s]
5
[5 / s]
a
b
3
2
4
[0 / s]
3
2
s
1
e
2
1
1
c
[1 / s]
d
[∞ / ]
[∞ / ]
Dijkstra algorithm illustration
[2 / s]
5
[4 / c]
a
b
3
2
4
[0 / s]
3
2
s
1
e
2
1
1
c
d
[1 / s]
[2 / c]
[∞ / ]
Dijkstra algorithm illustration
[2 / s]
5
[4 / c]
a
b
3
2
4
[0 / s]
3
2
s
1
e
2
1
1
c
d
[1 / s]
[2 / c]
[∞ / ]
Dijkstra algorithm illustration
[2 / s]
5
[3 / d]
a
b
3
2
4
[0 / s]
3
2
s
1
e
2
1
1
c
[1 / s]
d
[2 / c]
[4 / d]
Dijkstra algorithm illustration
[2 / s]
5
[3 / d]
a
b
3
2
4
[0 / s]
3
2
s
1
e
2
1
1
c
d
[1 / s]
[2 / c]
[4 / d]
Dijkstra algorithm illustration
[2 / s]
5
[3 / d]
a
b
3
2
4
[0 / s]
3
2
s
1
e
2
1
1
c
d
[1 / s]
[2 / c]
[4 / d]
Dijkstra algorithm summary
• 复杂度 (Complexity) – O(n2)
注意: 计算所有的最短路径和计算一条最短路径具有相同的复杂度。
• 输出结果给出了网络上的一棵生成树 (Spanning Tree)。
5
a
b
3
2
4
s
e
3
2
1
1
2
c
d
1
Dijkstra’s algorithm, discussion
Algorithm complexity: n nodes
 each iteration: need to check all nodes, w, not in N
 n(n+1)/2 comparisons: O(n2)
 more efficient implementations possible: O(nlogn)
Oscillations possible:
 e.g., link cost = amount of carried traffic
D
1
1
0
A
0 0
C
e
1+e
e
initially
B
1
2+e
A
0
D 1+e 1 B
0
0
C
… recompute
routing
0
D
1
A
0 0
C
2+e
B
1+e
… recompute
2+e
A
0
D 1+e 1 B
e
0
C
… recompute
Network Layer 4-140
Chapter 4: Network Layer
 4. 1 Introduction
 4.2 Virtual circuit and
datagram networks
 4.3 What’s inside a
router
 4.4 IP: Internet
Protocol




Datagram format
IPv4 addressing
ICMP
IPv6
 4.5 Routing algorithms
 Link state
 Distance Vector
 Hierarchical routing
 4.6 Routing in the
Internet



RIP
OSPF
BGP
 4.7 Broadcast and
multicast routing
Network Layer 4-141
Distance Vector Algorithm
Bellman-Ford Equation (dynamic programming)
Define
dx(y) := cost of least-cost path from x to y
Then
dx(y) = min
{c(x,v) + dv(y) }
v
where min is taken over all neighbors v of x
Network Layer 4-142
Bellman-Ford example
5
2
u
v
2
1
x
3
w
3
1
5
z
1
y
Clearly, dv(z) = 5, dx(z) = 3, dw(z) = 3
2
B-F equation says:
du(z) = min { c(u,v) + dv(z),
c(u,x) + dx(z),
c(u,w) + dw(z) }
= min {2 + 5,
1 + 3,
5 + 3} = 4
Node that achieves minimum is next
hop in shortest path ➜ forwarding table
Network Layer 4-143
Distance Vector Algorithm
 Dx(y) = estimate of least cost from x to y
 Node x knows cost to each neighbor v:
c(x,v)
 Node x maintains distance vector Dx =
[Dx(y): y є N ]
 Node x also maintains its neighbors’
distance vectors
 For
each neighbor v, x maintains
Dv = [Dv(y): y є N ]
Network Layer 4-144
Distance vector algorithm (4)
Basic idea:
 From time-to-time, each node sends its own
distance vector estimate to neighbors
 Asynchronous
 When a node x receives new DV estimate from
neighbor, it updates its own DV using B-F equation:
Dx(y) ← minv{c(x,v) + Dv(y)}
for each node y ∊ N
 Under minor, natural conditions, the estimate
Dx(y) converge to the actual least cost dx(y)
Network Layer 4-145
Distance Vector Algorithm (5)
Iterative, asynchronous:
each local iteration caused
by:
 local link cost change
 DV update message from
neighbor
Distributed:
 each node notifies
neighbors only when its DV
changes

neighbors then notify
their neighbors if
necessary
Each node:
wait for (change in local link
cost or msg from neighbor)
recompute estimates
if DV to any dest has
changed, notify neighbors
Network Layer 4-146
Dx(y) = min{c(x,y) + Dy(y), c(x,z) + Dz(y)}
= min{2+0 , 7+1} = 2
node x table
cost to
x y z
cost to
x y z
from
from
x 0 2 7
y ∞∞ ∞
z ∞∞ ∞
node y table
cost to
x y z
Dx(z) = min{c(x,y) +
Dy(z), c(x,z) + Dz(z)}
= min{2+1 , 7+0} = 3
x 0 2 3
y 2 0 1
z 7 1 0
x ∞ ∞ ∞
y 2 0 1
z ∞∞ ∞
node z table
cost to
x y z
from
from
x
x ∞∞ ∞
y ∞∞ ∞
z 71 0
time
2
y
7
1
z
Network Layer 4-147
Dx(y) = min{c(x,y) + Dy(y), c(x,z) + Dz(y)}
= min{2+0 , 7+1} = 2
node x table
cost to
x y z
x ∞∞ ∞
y ∞∞ ∞
z 71 0
from
from
from
from
x 0 2 7
y 2 0 1
z 7 1 0
cost to
x y z
x 0 2 7
y 2 0 1
z 3 1 0
x 0 2 3
y 2 0 1
z 3 1 0
cost to
x y z
x 0 2 3
y 2 0 1
z 3 1 0
x
2
y
7
1
z
cost to
x y z
from
from
from
x ∞ ∞ ∞
y 2 0 1
z ∞∞ ∞
node z table
cost to
x y z
x 0 2 3
y 2 0 1
z 7 1 0
cost to
x y z
cost to
x y z
from
from
x 0 2 7
y ∞∞ ∞
z ∞∞ ∞
node y table
cost to
x y z
cost to
x y z
Dx(z) = min{c(x,y) +
Dy(z), c(x,z) + Dz(z)}
= min{2+1 , 7+0} = 3
x 0 2 3
y 2 0 1
z 3 1 0
time
Network Layer 4-148
Distance Vector: link cost changes
Link cost changes:
 node detects local link cost change
 updates routing info, recalculates
distance vector
 if DV changes, notify neighbors
“good
news
travels
fast”
1
x
4
y
50
1
z
At time t0, y detects the link-cost change, updates its DV,
and informs its neighbors.
At time t1, z receives the update from y and updates its table.
It computes a new least cost to x and sends its neighbors its DV.
At time t2, y receives z’s update and updates its distance table.
y’s least costs do not change and hence y does not send any
message to z.
Network Layer 4-149
Distance Vector: link cost changes
Good news spreads fast
1
X
4
Y
50
1
Z
算法
收敛
Distance Vector: link cost changes
Link cost changes:
 good news travels fast
 bad news travels slow -
“count to infinity” problem!
 44 iterations before
algorithm stabilizes: see
text
60
x
4
y
50
1
z
Poisoned reverse:
 If Z routes through Y to
get to X :

Z tells Y its (Z’s) distance
to X is infinite (so Y won’t
route to X via Z)
 will this completely solve
count to infinity problem?
Network Layer 4-151
Distance Vector: link cost changes
Bad news spreads slowly, so called “Count
to Infinity Problem”:
60
X
4
Y
50
1
Z
算法仍
不收敛
Distance Vector–Poisoned Reverse
Poisoned Reverse: 如果 Z 到 X 的最短路径
经过 Y，那么 Z 告诉 Y “Z 到 X 的最短距离
是 ∞ “，这样 Y 就不会选择经由 Z 到达 X 的
路线。
60
X
4
Y
50
1
Z
算法
收敛
∞
立即修改
Comparison of LS and DV algorithms
Message complexity
 LS: with n nodes, E links,
O(nE) msgs sent
 DV: exchange between
neighbors only
 convergence time varies
Speed of Convergence
 LS: O(n2) algorithm requires
O(nE) msgs
 may have oscillations
 DV: convergence time varies
 may be routing loops
 count-to-infinity problem
Robustness: what happens
if router malfunctions?
LS:


node can advertise
incorrect link cost
each node computes only
its own table
DV:


DV node can advertise
incorrect path cost
each node’s table used by
others
• error propagate thru
network
Network Layer 4-154
DV versus LS
Distance Vector
 仅与邻居节点交换消息
 消息包括到所有节点的
最短距离
 收敛速度比较慢
 能够广播不正确的路径
信息
 有Count to Infinity
Problem
Link State
 向网络上所有其它节
点广播消息
 消息仅包括到邻居节
点的距离
 收敛速度比较快
 能够广播不正确的链
路信息
 没有Count to
Infinity Problem
Chapter 4: Network Layer
 4. 1 Introduction
 4.2 Virtual circuit and
datagram networks
 4.3 What’s inside a
router
 4.4 IP: Internet
Protocol




Datagram format
IPv4 addressing
ICMP
IPv6
 4.5 Routing algorithms
 Link state
 Distance Vector
 Hierarchical routing
 4.6 Routing in the
Internet



RIP
OSPF
BGP
 4.7 Broadcast and
multicast routing
Network Layer 4-156
Hierarchical Routing
Our routing study thus far - idealization
 all routers identical
 network “flat”
… not true in practice
scale: with 200 million
destinations:
 can’t store all dest’s in
routing tables!
 routing table exchange
would swamp links!
administrative autonomy
 internet = network of
networks
 each network admin may
want to control routing in its
own network
Network Layer 4-157
Hierarchical Routing
 aggregate routers into
regions, “autonomous
systems” (AS)
 routers in same AS run
same routing protocol


Gateway router
 Direct link to router in
another AS
“intra-AS” routing
protocol
routers in different AS
can run different intraAS routing protocol
Network Layer 4-158
Interconnected ASes
3c
3a
3b
AS3
1a
2a
1c
1d
1b
Intra-AS
Routing
algorithm
2c
AS2
AS1
Inter-AS
Routing
algorithm
Forwarding
table
2b
 forwarding table
configured by both
intra- and inter-AS
routing algorithm


intra-AS sets entries
for internal dests
inter-AS & intra-As
sets entries for
external dests
Network Layer 4-159
Inter-AS tasks
AS1 must:
1. learn which dests are
reachable through
AS2, which through
AS3
2. propagate this
reachability info to all
routers in AS1
Job of inter-AS routing!
 suppose router in AS1
receives datagram
destined outside of
AS1:
 router should
forward packet to
gateway router, but
which one?
3c
3b
3a
AS3
1a
2a
1c
1d
1b
2c
AS2
2b
AS1
Network Layer 4-160
Example: Setting forwarding table in router 1d
 suppose AS1 learns (via inter-AS protocol) that subnet
x reachable via AS3 (gateway 1c) but not via AS2.
 inter-AS protocol propagates reachability info to all
internal routers.
 router 1d determines from intra-AS routing info that
its interface I is on the least cost path to 1c.
 installs forwarding table entry (x,I)
x
3c
3a
3b
AS3
1a
2a
1c
1d
1b AS1
2c
2b
AS2
Network Layer 4-161
Example: Choosing among multiple ASes
 now suppose AS1 learns from inter-AS protocol that
subnet x is reachable from AS3 and from AS2.
 to configure forwarding table, router 1d must
determine towards which gateway it should forward
packets for dest x.
 this is also job of inter-AS routing protocol!
x
3c
3a
3b
AS3
1a
2a
1c
1d
1b
2c
AS2
2b
AS1
Network Layer 4-162
Example: Choosing among multiple ASes
 now suppose AS1 learns from inter-AS protocol that
subnet x is reachable from AS3 and from AS2.
 to configure forwarding table, router 1d must
determine towards which gateway it should forward
packets for dest x.
 this is also job of inter-AS routing protocol!
 hot potato routing: send packet towards closest of
two routers.
Learn from inter-AS
protocol that subnet
x is reachable via
multiple gateways
Use routing info
from intra-AS
protocol to determine
costs of least-cost
paths to each
of the gateways
Hot potato routing:
Choose the gateway
that has the
smallest least cost
Determine from
forwarding table the
interface I that leads
to least-cost gateway.
Enter (x,I) in
forwarding table
Network Layer 4-163
Intra-AS Routing
 also known as Interior Gateway Protocols (IGP)
 most common Intra-AS routing protocols:

RIP: Routing Information Protocol

OSPF: Open Shortest Path First

IGRP: Interior Gateway Routing Protocol (Cisco
proprietary)
Network Layer 4-164
Chapter 4: Network Layer
 4. 1 Introduction
 4.2 Virtual circuit and
datagram networks
 4.3 What’s inside a
router
 4.4 IP: Internet
Protocol




Datagram format
IPv4 addressing
ICMP
IPv6
 4.5 Routing algorithms
 Link state
 Distance Vector
 Hierarchical routing
 4.6 Routing in the
Internet



RIP
OSPF
BGP
 4.7 Broadcast and
multicast routing
Network Layer 4-165
RIP ( Routing Information Protocol)
 distance vector algorithm
 included in BSD-UNIX Distribution in 1982
 distance metric: # of hops (max = 15 hops)
From router A to subnets:
u
v
A
z
C
B
D
w
x
y
destination hops
u
1
v
2
w
2
x
3
y
3
z
2
Network Layer 4-166
RIP advertisements
 distance vectors: exchanged among
neighbors every 30 sec via Response
Message (also called advertisement)
 each advertisement: list of up to 25
destination subnets within AS
Network Layer 4-167
RIP: Example
z
w
A
x
D
B
y
C
Destination Network
w
y
z
x
….
Next Router
Num. of hops to dest.
….
....
A
B
B
--
2
2
7
1
Routing/Forwarding table in D
Network Layer 4-168
RIP: Example
Dest
w
x
z
….
Next
C
…
w
hops
1
1
4
...
A
Advertisement
from A to D
z
x
Destination Network
w
y
z
x
….
D
B
C
y
Next Router
Num. of hops to dest.
….
....
A
B
B A
--
Routing/Forwarding table in D
2
2
7 5
1
Network Layer 4-169
RIP: Link Failure and Recovery
If no advertisement heard after 180 sec -->
neighbor/link declared dead
 routes via neighbor invalidated
 new advertisements sent to neighbors
 neighbors in turn send out new advertisements (if
tables changed)
 link failure info quickly (?) propagates to entire net
 poison reverse used to prevent ping-pong loops
(infinite distance = 16 hops)
Network Layer 4-170
RIP Table processing
 RIP routing tables managed by application-level
process called route-d (daemon)
 advertisements sent in UDP packets, periodically
repeated
routed
routed
Transprt
(UDP)
network
(IP)
link
physical
Transprt
(UDP)
forwarding
table
forwarding
table
network
(IP)
link
physical
Network Layer 4-171
Chapter 4: Network Layer
 4. 1 Introduction
 4.2 Virtual circuit and
datagram networks
 4.3 What’s inside a
router
 4.4 IP: Internet
Protocol




Datagram format
IPv4 addressing
ICMP
IPv6
 4.5 Routing algorithms
 Link state
 Distance Vector
 Hierarchical routing
 4.6 Routing in the
Internet



RIP
OSPF
BGP
 4.7 Broadcast and
multicast routing
Network Layer 4-172
OSPF (Open Shortest Path First)
 “open”: publicly available
 uses Link State algorithm
 LS packet dissemination
 topology map at each node
 route computation using Dijkstra’s algorithm
 OSPF advertisement carries one entry per neighbor
router
 advertisements disseminated to entire AS (via
flooding)

carried in OSPF messages directly over IP (rather than TCP
or UDP
Network Layer 4-173
OSPF “advanced” features (not in RIP)
 security: all OSPF messages authenticated (to




prevent malicious intrusion)
multiple same-cost paths allowed (only one path in
RIP)
For each link, multiple cost metrics for different
TOS (e.g., satellite link cost set “low” for best effort;
high for real time)
integrated uni- and multicast support:
 Multicast OSPF (MOSPF) uses same topology data
base as OSPF
hierarchical OSPF in large domains.
Network Layer 4-174
Hierarchical OSPF
Network Layer 4-175
Hierarchical OSPF
 two-level hierarchy: local area, backbone.
Link-state advertisements only in area
 each nodes has detailed area topology; only know
direction (shortest path) to nets in other areas.
 area border routers: “summarize” distances to nets
in own area, advertise to other Area Border routers.
 backbone routers: run OSPF routing limited to
backbone.
 boundary routers: connect to other AS’s.

Network Layer 4-176
Chapter 4: Network Layer
 4. 1 Introduction
 4.2 Virtual circuit and
datagram networks
 4.3 What’s inside a
router
 4.4 IP: Internet
Protocol




Datagram format
IPv4 addressing
ICMP
IPv6
 4.5 Routing algorithms
 Link state
 Distance Vector
 Hierarchical routing
 4.6 Routing in the
Internet



RIP
OSPF
BGP
 4.7 Broadcast and
multicast routing
Network Layer 4-177
Internet inter-AS routing: BGP
 BGP (Border Gateway Protocol): the de
facto standard
 BGP provides each AS a means to:
1.
2.
3.
Obtain subnet reachability information from
neighboring ASs.
Propagate reachability information to all ASinternal routers.
Determine “good” routes to subnets based on
reachability information and policy.
 allows subnet to advertise its existence to
rest of Internet: “I am here”
Network Layer 4-178
BGP basics
 pairs of routers (BGP peers) exchange routing info
over semi-permanent TCP connections: BGP sessions
 BGP sessions need not correspond to physical
links.
 when AS2 advertises a prefix to AS1:
 AS2 promises it will forward datagrams towards
that prefix.
 AS2 can aggregate prefixes in its advertisement
eBGP session
3c
3a
3b
AS3
1a
AS1
iBGP session
2a
1c
1d
1b
2c
AS2
2b
Network Layer 4-179
Distributing reachability info
 using eBGP session between 3a and 1c, AS3 sends
prefix reachability info to AS1.
 1c can then use iBGP do distribute new prefix
info to all routers in AS1
 1b can then re-advertise new reachability info
to AS2 over 1b-to-2a eBGP session
 when router learns of new prefix, it creates entry
for prefix in its forwarding table.
eBGP session
3c
3a
3b
AS3
1a
AS1
iBGP session
2a
1c
1d
1b
2c
AS2
2b
Network Layer 4-180
Path attributes & BGP routes
 advertised prefix includes BGP attributes.
 prefix + attributes = “route”
 two important attributes:
 AS-PATH: contains ASs through which prefix
advertisement has passed: e.g, AS 67, AS 17
 NEXT-HOP: indicates specific internal-AS router
to next-hop AS. (may be multiple links from
current AS to next-hop-AS)
 when gateway router receives route
advertisement, uses import policy to
accept/decline.
Network Layer 4-181
BGP route selection
 router may learn about more than 1 route
to some prefix. Router must select route.
 elimination rules:
1.
2.
3.
4.
local preference value attribute: policy
decision
shortest AS-PATH
closest NEXT-HOP router: hot potato routing
additional criteria
Network Layer 4-182
BGP messages
 BGP messages exchanged using TCP.
 BGP messages:
OPEN: opens TCP connection to peer and
authenticates sender
 UPDATE: advertises new path (or withdraws old)
 KEEPALIVE keeps connection alive in absence of
UPDATES; also ACKs OPEN request
 NOTIFICATION: reports errors in previous msg;
also used to close connection

Network Layer 4-183
BGP routing policy
legend:
B
W
X
A
provider
network
customer
network:
C
Y
 A,B,C are provider networks
 X,W,Y are customer (of provider networks)
 X is dual-homed: attached to two networks
X does not want to route from B via X to C
 .. so X will not advertise to B a route to C

Network Layer 4-184
BGP routing policy (2)
legend:
B
W
X
A
provider
network
customer
network:
C
Y
 A advertises path AW to B
 B advertises path BAW to X
 Should B advertise path BAW to C?
 No
way! B gets no “revenue” for routing CBAW
since neither W nor C are B’s customers
 B wants to force C to route to w via A
 B wants to route only to/from its customers!
Network Layer 4-185
Why different Intra- and Inter-AS routing ?
Policy:
 Inter-AS: admin wants control over how its traffic
routed, who routes through its net.
 Intra-AS: single admin, so no policy decisions needed
Scale:
 hierarchical routing saves table size, reduced update
traffic
Performance:
 Intra-AS: can focus on performance
 Inter-AS: policy may dominate over performance
Network Layer 4-186
Static routing table
 Destination subnet/Mask, Next hop/router, Output
interface, # of hops to destination
 typical problems:
 Given net structure, to produce the routing table
 Given routing table, to produce the net structure
Network Layer 4-187
Static routing table: Problem 1
R1, R2, R3: Destination subnet/Mask, Next hop/router,
Interface, # of hops to destination ?
Network Layer 4-188
Static routing table: Problem 2
Destination
Mask
Next hop
Interface # of hops
202.204.65.0
255.255.255.0
C
Vlan160
1
202.204.64.0
255.255.255.0
C
Valn159
1
202.14.71.0
255.255.255.0 202.124.254.9
Vlan2
4
202.38.70.0
255.255.255.0 202.124.254.9
Vlan2
5
C
Vlan2
1
202.204.65.1
Vlan160
2
202.124.254.0 255.255.255.0
176.20.0.0
255.255.0.0
net structure?
Network Layer 4-189
Chapter 4: Network Layer
 4. 1 Introduction
 4.2 Virtual circuit and
datagram networks
 4.3 What’s inside a
router
 4.4 IP: Internet
Protocol




Datagram format
IPv4 addressing
ICMP
IPv6
 4.5 Routing algorithms
 Link state
 Distance Vector
 Hierarchical routing
 4.6 Routing in the
Internet



RIP
OSPF
BGP
 4.7 Broadcast and
multicast routing
Network Layer 4-190
Broadcast Routing
 deliver packets from source to all other nodes
 source duplication is inefficient:
duplicate
duplicate
creation/transmission
R1
R1
duplicate
R2
R2
R3
R4
source
duplication
R3
R4
in-network
duplication
 source duplication: how does source
determine recipient addresses?
Network Layer 4-191
In-network duplication
 flooding: when node receives brdcst pckt,
sends copy to all neighbors

Problems: cycles & broadcast storm
 controlled flooding: node only brdcsts pkt
if it hasn’t brdcst same packet before
Node keeps track of pckt ids already brdcsted
 Or reverse path forwarding (RPF): only forward
pckt if it arrived on shortest path between
node and source

 spanning tree
 No redundant packets received by any node
Network Layer 4-192
Spanning Tree
 First construct a spanning tree
 Nodes forward copies only along spanning
tree
A
B
c
F
A
E
B
c
D
F
G
(a) Broadcast initiated at A
E
D
G
(b) Broadcast initiated at D
Network Layer 4-193
Spanning Tree: Creation
 Center node
 Each node sends unicast join message to center
node

Message forwarded until it arrives at a node already
belonging to spanning tree
A
A
3
B
c
4
E
F
1
2
B
c
D
F
5
E
D
G
G
(a) Stepwise construction
of spanning tree
(b) Constructed spanning
tree
Network Layer 4-194
Multicast Routing: Problem Statement
 Goal: find a tree (or trees) connecting
routers having local mcast group members



tree: not all paths between routers used
source-based: different tree from each sender to rcvrs
shared-tree: same tree used by all group members
Shared tree
Source-based trees
Approaches for building mcast trees
Approaches:
 source-based tree: one tree per source
shortest path trees
 reverse path forwarding

 group-shared tree: group uses one tree
 minimal spanning (Steiner)
 center-based trees
…we first look at basic approaches, then specific
protocols adopting these approaches
Shortest Path Tree
 mcast forwarding tree: tree of shortest
path routes from source to all receivers

Dijkstra’s algorithm
S: source
LEGEND
R1
1
2
R4
R2
3
R3
router with attached
group member
5
4
R6
router with no attached
group member
R5
6
R7
i
link used for forwarding,
i indicates order link
added by algorithm
Reverse Path Forwarding
 rely on router’s knowledge of unicast
shortest path from it to sender
 each router has simple forwarding behavior:
if (mcast datagram received on incoming link
on shortest path back to center)
then flood datagram onto all outgoing links
else ignore datagram
Reverse Path Forwarding: example
S: source
LEGEND
R1
R4
router with attached
group member
R2
R5
R3
R6
R7
router with no attached
group member
datagram will be
forwarded
datagram will not be
forwarded
• result is a source-specific reverse SPT
– may be a bad choice with asymmetric links
Reverse Path Forwarding: pruning
 forwarding tree contains subtrees with no mcast
group members
 no need to forward datagrams down subtree
 “prune” msgs sent upstream by router with no
downstream group members
LEGEND
S: source
R1
router with attached
group member
R4
R2
P
R5
R3
R6
P
R7
P
router with no attached
group member
prune message
links with multicast
forwarding
Shared-Tree: Steiner Tree
 Steiner Tree: minimum cost tree
connecting all routers with attached group
members
 problem is NP-complete
 excellent heuristics exists
 not used in practice:
computational complexity
 information about entire network needed
 monolithic: rerun whenever a router needs to
join/leave

Center-based trees
 single delivery tree shared by all
 one router identified as “center” of tree
 to join:
edge router sends unicast join-msg addressed
to center router
 join-msg “processed” by intermediate routers
and forwarded towards center
 join-msg either hits existing tree branch for
this center, or arrives at center
 path taken by join-msg becomes new branch of
tree for this router

Center-based trees: an example
Suppose R6 chosen as center:
LEGEND
R1
3
R2
router with attached
group member
R4
2
R5
R3
1
R6
R7
1
router with no attached
group member
path order in which join
messages generated
Internet Multicasting Routing: DVMRP
 DVMRP: distance vector multicast routing
protocol, RFC1075
 flood and prune: reverse path forwarding,
source-based tree
RPF tree based on DVMRP’s own routing tables
constructed by communicating DVMRP routers
 no assumptions about underlying unicast
 initial datagram to mcast group flooded
everywhere via RPF
 routers not wanting group: send upstream prune
msgs

DVMRP: continued…
 soft state: DVMRP router periodically (1 min.)
“forgets” branches are pruned:
mcast data again flows down unpruned branch
 downstream router: reprune or else continue to
receive data

 routers can quickly regraft to tree

following IGMP join at leaf
 odds and ends
 commonly implemented in commercial routers
 Mbone routing done using DVMRP
Tunneling
Q: How to connect “islands” of multicast
routers in a “sea” of unicast routers?
physical topology
logical topology
 mcast datagram encapsulated inside “normal” (non-multicast-
addressed) datagram
 normal IP datagram sent thru “tunnel” via regular IP unicast to
receiving mcast router
 receiving mcast router unencapsulates to get mcast datagram
PIM: Protocol Independent Multicast
 not dependent on any specific underlying unicast
routing algorithm (works with all)
 two different multicast distribution scenarios :
Dense:
Sparse:
 group members
 # networks with group
densely packed, in
“close” proximity.
 bandwidth more
plentiful
members small wrt #
interconnected networks
 group members “widely
dispersed”
 bandwidth not plentiful
Consequences of Sparse-Dense Dichotomy:
Dense
 group membership by
Sparse:
 no membership until
routers assumed until
routers explicitly join
routers explicitly prune  receiver- driven
 data-driven construction
construction of mcast
on mcast tree (e.g., RPF)
tree (e.g., center-based)
 bandwidth and non bandwidth and non-groupgroup-router processing
router processing
profligate
conservative
PIM- Dense Mode
flood-and-prune RPF, similar to DVMRP but
 underlying unicast protocol provides RPF info
for incoming datagram
 less complicated (less efficient) downstream
flood than DVMRP reduces reliance on
underlying routing algorithm
 has protocol mechanism for router to detect it
is a leaf-node router
PIM - Sparse Mode
 center-based approach
 router sends join msg
to rendezvous point
(RP)

router can switch to
source-specific tree
increased performance:
less concentration,
shorter paths
R4
join
intermediate routers
update state and
forward join
 after joining via RP,

R1
R2
R3
join
R5
join
R6
all data multicast
from rendezvous
point
R7
rendezvous
point
PIM - Sparse Mode
sender(s):
 unicast data to RP,
which distributes down
RP-rooted tree
 RP can extend mcast
tree upstream to
source
 RP can send stop msg
if no attached
receivers

“no one is listening!”
R1
R4
join
R2
R3
join
R5
join
R6
all data multicast
from rendezvous
point
R7
rendezvous
point
Chapter 4: summary
 4. 1 Introduction
 4.2 Virtual circuit and
datagram networks
 4.3 What’s inside a
router
 4.4 IP: Internet
Protocol




Datagram format
IPv4 addressing
ICMP
IPv6
 4.5 Routing algorithms
 Link state
 Distance Vector
 Hierarchical routing
 4.6 Routing in the
Internet



RIP
OSPF
BGP
 4.7 Broadcast and
multicast routing
Network Layer 4-212
Chapter 4: 3rd Homework
 1. P22
 2. 给定子网掩码255.255.255.224，请问与主机
210.30.97.245对应的网络地址和主机地址分别是多少？
 3. 列举IPv6和IPv4的5项不同点
Hand in at once by Monitors ONE week later.
No print! Hand-writing with you ID and name.
Review Questions
See the textbook (P.442-)
 R2, R3, R8, R18, R21, R33
 P11, P16, P23
 虚电路网络与数据报网络的区别?
 链路状态路由算法与距离向量路由算法的区别?
 广播、组播、单播、任播？
单播
 主机之间“一对一”的通讯模式，网络中的交换机和路由器
对数据只进行转发不进行复制。但由于其能够针对每个客户
的及时响应，所以现在的网页浏览全部都是采用IP单播协议。
网络中的路由器和交换机根据其目标地址选择传输路径，将
IP单播数据传送到其指定的目的地。
215
单播
 单播的优点：
1. 服务器及时响应客户机的请求
 2. 服务器针对每个客户不通的请求发送不通的数据，容
易实现个性化服务。
 单播的缺点：
 1. 服务器针对每个客户机发送数据流，在客户数量大、
每个客户机流量大的流媒体应用中服务器不堪重负。
 2. 现有的网络带宽是金字塔结构，城际省际主干带宽仅
仅相当于其所有用户带宽之和的5％。如果全部使用单播
协议，将造成网络主干不堪重负。

216
广播
 主机之间“一对所有”的通讯模式，网络对其中每一台主机
发出的信号都进行无条件复制并转发，所有主机都可以接收
到所有信息（不管你是否需要），由于其不用路径选择，所
以其网络成本可以很低廉。有线电视网就是典型的广播型网
络。在数据网络中也允许广播的存在，但其被限制在二层交
换机的局域网范围内，禁止广播数据穿过路由器，防止广播
数据影响大面积的主机。
217
广播
 广播的优点：
1. 网络设备简单，维护简单，布网成本低廉
 2. 由于服务器不用向每个客户机单独发送数据，所以服
务器流量负载极低。
 广播的缺点：
 1.无法针对每个客户的要求和时间及时提供个性化服务。
 2. 网络允许服务器提供数据的带宽有限，客户端的最大
带宽＝服务总带宽。
 3. 广播禁止在Internet宽带网上传输。

218
组播
 主机之间“一对一组”的通讯模式，也就是加入了同一个组
的主机可以接受到此组内的所有数据，网络中的交换机和路
由器只向有需求者复制并转发其所需数据。
219
组播
 组播的优点：
1. 需要相同数据流的客户端加入相同的组共享一条数据
流，节省了服务器的负载。具备广播所具备的优点。
 2. 由于组播协议是根据接受者的需要对数据流进行复制
转发，所以服务端的服务总带宽不受客户接入端带宽的
限制。
 3. 此协议和单播协议一样允许在Internet宽带网上传输。
 组播的缺点：
 1．与单播协议相比没有纠错机制，发生丢包错包后难以
弥补，但可以通过一定的容错机制和QoS加以弥补。
 2．现行网络虽然都支持组播的传输，但在客户认证、
QoS等方面还需要完善，这些缺点在理论上都有成熟的
解决方案，只是需要逐步推广应用到现存网络当中。

220