Network Layer 4-1
Chapter 4
Network Layer
Computer Networking,
A Top Down Approach
Featuring the Internet,
2nd edition,
Jim Kurose,Keith Ross
Addison-Wesley,July
2002,
The PowerPoint Slides are based on the material
provided by
J.F Kurose and K.W,Ross.
Network Layer 4-2
Chapter 4,Network Layer
Chapter goals:
? understand principles
behind network layer
services:
? routing (path selection)
? dealing with scale
? how a router works
? advanced topics,IPv6,
mobility
? instantiation and
implementation in the
Internet
Overview:
? network layer services
? routing principles,path
selection
? hierarchical routing
? IP
? Internet routing protocols
? intra-domain
? inter-domain
? what?s inside a router?
? IPv6
? mobility
Network Layer 4-3
Chapter 4 roadmap
4.1 Introduction and Network Service Models
4.2 Routing Principles
4.3 Hierarchical Routing
4.4 The Internet (IP) Protocol
4.5 Routing in the Internet
4.6 What?s Inside a Router
4.7 IPv6
4.8 Multicast Routing
4.9 Mobility
Network Layer 4-4
Network layer functions
? transport packet from
sending to receiving hosts
? network layer protocols in
every host,router
three important functions:
? path determination,route
taken by packets from source
to dest,Routing algorithms
? forwarding,move packets
from router?s input to
appropriate router output
? call setup,some network
architectures require router
call setup along path before
data flows
network
data link
physical
network
data link
physical
network
data link
physical
network
data link
physical
network
data link
physical
network
data link
physical
network
data link
physical
network
data link
physical
application
transport
network
data link
physical
application
transport
network
data link
physical
Network Layer 4-5
Network service model
Q,What service model
for,channel”
transporting packets
from sender to
receiver?
? guaranteed bandwidth?
? preservation of inter-packet
timing (no jitter)?
? loss-free delivery?
? in-order delivery?
? congestion feedback to
sender?
virtual circuitor
datagram?
The most important
abstraction provided
by network layer:
Network Layer 4-6
Virtual circuits
? call setup,teardown for each call before data can flow
? each packet carries VC identifier (not destination host ID)
? every router on source-dest path maintains,state” for
each passing connection
? transport-layer connection only involved two end systems
? link,router resources (bandwidth,buffers) may be
allocated to VC
? to get circuit-like perf.
“source-to-dest path behaves much like telephone
circuit”
? performance-wise
? network actions along source-to-dest path
Network Layer 4-7
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
network
data link
physical
application
transport
network
data link
physical
1,Initiate call 2,incoming call
3,Accept call4,Call connected
5,Data flow begins 6,Receive data
Network Layer 4-8
Datagram networks,the Internet model
? 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
physical
application
transport
network
data link
physical
1,Send data 2,Receive data
Network Layer 4-9
Network layer service models:
Network
Architecture
Internet
ATM
ATM
ATM
ATM
Service
Model
best effort
CBR
VBR
ABR
UBR
Bandwidth
none
constant
rate
guaranteed
rate
guaranteed
minimum
none
Loss
no
yes
yes
no
no
Order
no
yes
yes
yes
yes
Timing
no
yes
yes
no
no
Congestion
feedback
no (inferred
via loss)
no
congestion
no
congestion
yes
no
Guarantees?
? Internet model being extended,Intserv,Diffserv
? Chapter 6
Network Layer 4-10
Datagram or VC network,why?
Internet
? data exchange among
computers
?,elastic” service,no strict
timing req,
?,smart” end systems
(computers)
? can adapt,perform
control,error recovery
? simple inside network,
complexity at,edge”
? many link types
? different characteristics
? uniform service difficult
ATM
? evolved from telephony
? human conversation,
? strict timing,reliability
requirements
? need for guaranteed
service
?,dumb” end systems
? telephones
? complexity inside
network
Network Layer 4-11
Chapter 4 roadmap
4.1 Introduction and Network Service Models
4.2 Routing Principles
? Link state routing
? Distance vector routing
4.3 Hierarchical Routing
4.4 The Internet (IP) Protocol
4.5 Routing in the Internet
4.6 What?s Inside a Router
4.7 IPv6
4.8 Multicast Routing
4.9 Mobility
Network Layer 4-12
Routing
Graph abstraction for
routing algorithms:
? graph nodes are
routers
? graph edges are
physical links
? link cost,delay,$ cost,
or congestion level
Goal,determine,good” path
(sequence of routers) thru
network from source to dest.
Routing protocol
A
ED
CB
F
2
2
1 3
1
1
2
5
3
5
?,good” path:
? typically means minimum
cost path
? other def?s possible
Network Layer 4-13
Routing Algorithm classification
Global or decentralized
information?
Global:
? all routers have complete
topology,link cost info
?,link state” algorithms
Decentralized:
? router knows physically-
connected 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 4-14
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 routing table for
that node
? iterative,after k
iterations,know least cost
path to k dest.?s
Notation:
? c(i,j),link cost from node i
to j,cost infinite 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,that is next v
? N,set of nodes whose
least cost path definitively
known
Network Layer 4-15
Dijsktra?s Algorithm
1 Initialization:
2 N = {A}
3 for all nodes v
4 if v adjacent to A
5 then D(v) = c(A,v)
6 else D(v) = infinity
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 4-16
Dijkstra?s algorithm,example
Step
0
1
2
3
4
5
start N
A
AD
ADE
ADEB
ADEBC
ADEBCF
D(B),p(B)
2,A
2,A
2,A
D(C),p(C)
5,A
4,D
3,E
3,E
D(D),p(D)
1,A
D(E),p(E)
infinity
2,D
D(F),p(F)
infinity
infinity
4,E
4,E
4,E
A
ED
CB
F
2
2
1 3
1
1
2
5
3
5
Network Layer 4-17
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(n**2)
? more efficient implementations possible,O(nlogn)
Oscillations possible:
? e.g.,link cost = amount of carried traffic
A
D
C
B
1 1+e
e0
e
1 1
0 0
A
D
C
B2+e 0
00 1+e
1
A
D
C
B0 2+e
1+e1
0 0
A
D
C
B2+e 0
e0 1+e
1
initially … recomputerouting … recompute … recompute
Network Layer 4-18
Distance Vector Routing Algorithm
iterative:
? continues until no
nodes exchange info.
? self-terminating,no
“signal” to stop
asynchronous:
? nodes need not
exchange info/iterate
in lock step!
distributed:
? each node
communicates only with
directly-attached
neighbors
Distance Table data structure
? each node has its own
? row for each possible destination
? column for each directly-
attached neighbor to node
? example,in node X,for dest,Y
via neighbor Z:
D (Y,Z)X
distance fromX to
Y,via Z as next hop
c(X,Z) + min {D (Y,w)}Z
w
=
=
Network Layer 4-19
Distance Table,example
A
E D
CB
7
8
1
2
1
2
D ()
A
B
C
D
A
1
7
6
4
B
14
8
9
11
D
5
5
4
2
E cost to destination via
D (C,D)E c(E,D) + min {D (C,w)}D
w== 2+2 = 4
D (A,D)E c(E,D) + min {D (A,w)}D
w== 2+3 = 5
D (A,B)E c(E,B) + min {D (A,w)}B
w== 8+6 = 14
loop!
loop!
Network Layer 4-20
Distance table gives routing table
D ()
A
B
C
D
A
1
7
6
4
B
14
8
9
11
D
5
5
4
2
E cost to destination via
A
B
C
D
A,1
D,5
D,4
D,2
Outgoing link
to use,cost
Distance table Routing table
Network Layer 4-21
Distance Vector Routing,overview
Iterative,asynchronous,
each local iteration caused
by,
? local link cost change
? message from neighbor,its
least cost path change
from neighbor
Distributed:
? each node notifies
neighbors only when its
least cost path to any
destination changes
? neighbors then notify
their neighbors if
necessary
wait for (change in local link
cost of msg from neighbor)
recompute distance table
if least cost path to any dest
has changed,notify
neighbors
Each node:
Network Layer 4-22
Distance Vector Algorithm:
1 Initialization,
2 for all adjacent nodes v,
3 D (*,v) = infinity /* the * operator means "for all rows" */
4 D (v,v) = c(X,v)
5 for all destinations,y
6 send min D (y,w) to each neighbor /* w over all X's neighbors */
X
X
X
w
At all nodes,X:
Network Layer 4-23
Distance Vector Algorithm (cont.):
8 loop
9 wait (until I see a link cost change to neighbor V
10 or until I receive update from neighbor V)
11
12 if (c(X,V) changes by d)
13 /* change cost to all dest's via neighbor v by d */
14 /* note,d could be positive or negative */
15 for all destinations y,D (y,V) = D (y,V) + d
16
17 else if (update received from V wrt destination Y)
18 /* shortest path from V to some Y has changed */
19 /* V has sent a new value for its min DV(Y,w) */
20 /* call this received new value is "newval" */
21 for the single destination y,D (Y,V) = c(X,V) + newval
22
23 if we have a new min D (Y,w)for any destination Y
24 send new value of min D (Y,w) to all neighbors
25
26 forever
w
XX
X
X
X
w
w
Network Layer 4-24
Distance Vector Algorithm,example
X Z
12
7
Y
Network Layer 4-25
Distance Vector Algorithm,example
X Z
12
7
Y
D (Y,Z)X c(X,Z) + min {D (Y,w)}w=
= 7+1 = 8
Z
D (Z,Y)X c(X,Y) + min {D (Z,w)}w=
= 2+1 = 3
Y
Network Layer 4-26
Distance Vector,link cost changes
Link cost changes:
? node detects local link cost change
? updates distance table (line 15)
? if cost change in least cost path,
notify neighbors (lines 23,24)
X Z
14
50
Y1
algorithm
terminates“good
news
travels
fast”
Network Layer 4-27
Distance Vector,link cost changes
Link cost changes:
? good news travels fast
? bad news travels slow -
“count to infinity” problem! X Z
14
50
Y60
algorithm
continues
on!
Network Layer 4-28
Distance Vector,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?
X Z
14
50
Y60
algorithm
terminates
Network Layer 4-29
Comparison of LS and DV algorithms
Message complexity
? LS,with n nodes,E links,
O(nE) msgs sent each
? 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 4-30
Chapter 4 roadmap
4.1 Introduction and Network Service Models
4.2 Routing Principles
4.3 Hierarchical Routing
4.4 The Internet (IP) Protocol
4.5 Routing in the Internet
4.6 What?s Inside a Router
4.7 IPv6
4.8 Multicast Routing
4.9 Mobility
Network Layer 4-31
Hierarchical Routing
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
Our routing study thus far - idealization
? all routers identical
? network,flat”
… not true in practice
Network Layer 4-32
Hierarchical Routing
? aggregate routers into
regions,“autonomous
systems” (AS)
? routers in same AS run
same routing protocol
?,intra-AS” routing
protocol
? routers in different AS
can run different intra-
AS routing protocol
? special routers in AS
? run intra-AS routing
protocol with all other
routers in AS
? also responsible for
routing to destinations
outside AS
? run inter-AS routing
protocol with other
gateway routers
gateway routers
Network Layer 4-33
Intra-AS and Inter-AS routing
Gateways:
?perform inter-AS
routing amongst
themselves
?perform intra-AS
routers with other
routers in their
AS
inter-AS,intra-AS
routing in
gateway A.c
network layer
link layer
physical layer
a
b
b
a
aC
A
B
d
A.a
A.c
C.b B.a
c
b
c
Network Layer 4-34
Intra-AS and Inter-AS routing
Host
h2a
b
b
a
aC
A
B
d c
A.a
A.c
C.b B.a
c
b
Host
h1
Intra-AS routing
within AS A
Inter-AS
routing
between
A and B
Intra-AS routing
within AS B
? We?ll examine specific inter-AS and intra-AS
Internet routing protocols shortly
Network Layer 4-35
Chapter 4 roadmap
4.1 Introduction and Network Service Models
4.2 Routing Principles
4.3 Hierarchical Routing
4.4 The Internet (IP) Protocol
? 4.4.1 IPv4 addressing
? 4.4.2 Moving a datagram from source to destination
? 4.4.3 Datagram format
? 4.4.4 IP fragmentation
? 4.4.5 ICMP,Internet Control Message Protocol
? 4.4.6 DHCP,Dynamic Host Configuration Protocol
? 4.4.7 NAT,Network Address Translation
4.5 Routing in the Internet
4.6 What?s Inside a Router
4.7 IPv6
4.8 Multicast Routing
4.9 Mobility
Network Layer 4-36
The Internet Network layer
forwarding
table
Host,router network layer functions:
Routing protocols
?path selection
?RIP,OSPF,BGP
IP protocol
?addressing conventions
?datagram format
?packet handling conventions
ICMP protocol
?error reporting
?router,signaling”
Transport layer,TCP,UDP
Link layer
physical layer
Network
layer
Network Layer 4-37
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 may have multiple
interfaces
? IP addresses
associated with each
interface
223.1.1.1
223.1.1.2
223.1.1.3
223.1.1.4 223.1.2.9
223.1.2.2
223.1.2.1
223.1.3.2223.1.3.1
223.1.3.27
223.1.1.1 = 11011111 00000001 00000001 00000001
223 1 11
Network Layer 4-38
IP Addressing
? IP address:
? network part (high
order bits)
? host part (low order
bits)
? What?s a network?
(from IP address
perspective)
? device interfaces with
same network part of
IP address
? can physically reach
each other without
intervening router
223.1.1.1
223.1.1.2
223.1.1.3
223.1.1.4 223.1.2.9
223.1.2.2
223.1.2.1
223.1.3.2223.1.3.1
223.1.3.27
network consisting of 3 IP networks
(for IP addresses starting with 223,
first 24 bits are network address)
LAN
Network Layer 4-39
IP Addressing
How to find the
networks?
? Detach each
interface from
router,host
? create,islands of
isolated networks
223.1.1.1
223.1.1.3
223.1.1.4
223.1.2.2223.1.2.1
223.1.2.6
223.1.3.2223.1.3.1
223.1.3.27
223.1.1.2
223.1.7.0
223.1.7.1
223.1.8.0223.1.8.1
223.1.9.1
223.1.9.2
Interconnected
system consisting
of six networks
Network Layer 4-40
IP Addresses
0network host
10 network host
110 network host
1110 multicast address
A
B
C
D
class
1.0.0.0 to
127.255.255.255
128.0.0.0 to
191.255.255.255
192.0.0.0 to
223.255.255.255
224.0.0.0 to
239.255.255.255
32 bits
given notion of,network”,let?s re-examine IP addresses:
“class-full” addressing:
Network Layer 4-41
IP addressing,CIDR
?Classful addressing,
? inefficient use of address space,address space exhaustion
? e.g.,class B net allocated enough addresses for 65K hosts,
even if only 2K hosts in that network
?CIDR,Classless InterDomain Routing
? network portion of address of arbitrary length
? address format,a.b.c.d/x,where x is # bits in network
portion of address
11001000 00010111 00010000 00000000
network
part
host
part
200.23.16.0/23
Network Layer 4-42
IP addresses,how to get one?
Q,How does host get IP address?
? hard-coded by system admin in a file
? Wintel,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”
(more shortly)
Network Layer 4-43
IP addresses,how to get one?
Q,How does network get network part of IP
addr?
A,gets allocated portion of its provider ISP?s
address space
ISP's block 11001000 00010111 00010000 00000000 200.23.16.0/20
Organization 0 11001000 00010111 00010000 00000000 200.23.16.0/23
Organization 1 11001000 00010111 00010010 00000000 200.23.18.0/23
Organization 2 11001000 00010111 00010100 00000000 200.23.20.0/23
..,….,…,….
Organization 7 11001000 00010111 00011110 00000000 200.23.30.0/23
Network Layer 4-44
Hierarchical addressing,route aggregation
“Send me anything
with addresses
beginning
200.23.16.0/20”
200.23.16.0/23
200.23.18.0/23
200.23.30.0/23
Fly-By-Night-ISP
Organization 0
Organization 7
Internet
Organization 1
ISPs-R-Us,Send me anythingwith addresses
beginning
199.31.0.0/16”
200.23.20.0/23
Organization 2
..
.
..
.
Hierarchical addressing allows efficient advertisement of routing
information:
Network Layer 4-45
Hierarchical addressing,more specific
routes
ISPs-R-Us has a more specific route to Organization 1
“Send me anything
with addresses
beginning
200.23.16.0/20”
200.23.16.0/23
200.23.18.0/23
200.23.30.0/23
Fly-By-Night-ISP
Organization 0
Organization 7
Internet
Organization 1
ISPs-R-Us,Send me anythingwith addresses
beginning 199.31.0.0/16
or 200.23.18.0/23”
200.23.20.0/23
Organization 2
..
.
..
.
Network Layer 4-46
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 4-47
Getting a datagram from source to dest.
IP datagram:
223.1.1.1
223.1.1.2
223.1.1.3
223.1.1.4 223.1.2.9
223.1.2.2
223.1.2.1
223.1.3.2223.1.3.1
223.1.3.27
A
B
E
misc
fields
source
IP addr
dest
IP addr data
? datagram remains
unchanged,as it travels
source to destination
? addr fields of interest
here
Dest,Net,next router Nhops
223.1.1 1
223.1.2 223.1.1.4 2
223.1.3 223.1.1.4 2
forwarding table in A
Network Layer 4-48
Getting a datagram from source to dest.
Starting at A,send IP
datagram addressed to B:
? look up net,address of B in
forwarding table
? find B is on same net,as A
? link layer will send datagram
directly to B inside link-layer
frame
? B and A are directly
connected
Dest,Net,next router Nhops
223.1.1 1
223.1.2 223.1.1.4 2
223.1.3 223.1.1.4 2
misc
fields 223.1.1.1 223.1.1.3 data
223.1.1.1
223.1.1.2
223.1.1.3
223.1.1.4 223.1.2.9
223.1.2.2
223.1.2.1
223.1.3.2223.1.3.1
223.1.3.27
A
B
E
forwarding table in A
Network Layer 4-49
Getting a datagram from source to dest.
Dest,Net,next router Nhops
223.1.1 1
223.1.2 223.1.1.4 2
223.1.3 223.1.1.4 2Starting at A,dest,E:
? look up network address of E
in forwarding table
? E on different network
? A,E not directly attached
? routing table,next hop
router to E is 223.1.1.4
? link layer sends datagram to
router 223.1.1.4 inside link-
layer frame
? datagram arrives at 223.1.1.4
? continued…..
misc
fields 223.1.1.1 223.1.2.3 data
223.1.1.1
223.1.1.2
223.1.1.3
223.1.1.4 223.1.2.9
223.1.2.2
223.1.2.1
223.1.3.2223.1.3.1
223.1.3.27
A
B
E
forwarding table in A
Network Layer 4-50
Getting a datagram from source to dest.
Arriving at 223.1.4,
destined for 223.1.2.2
? look up network address of E
in router?s forwarding table
? E on same network as router?s
interface 223.1.2.9
? router,E directly attached
? link layer sends datagram to
223.1.2.2 inside link-layer
frame via interface 223.1.2.9
? datagram arrives at
223.1.2.2!!! (hooray!)
misc
fields 223.1.1.1 223.1.2.3 data Dest,Net router Nhops interface223.1.1 - 1
223.1.1.4
223.1.2 - 1 223.1.2.9
223.1.3 - 1 223.1.3.27
223.1.1.1
223.1.1.2
223.1.1.3
223.1.1.4 223.1.2.9
223.1.2.2
223.1.2.1
223.1.3.2223.1.3.1
223.1.3.27
A
B
E
forwarding table in router
Network Layer 4-51
IP datagram format
ver length
32 bits
data
(variable length,
typically a TCP
or UDP segment)
16-bit identifier
Internet
checksum
time to
live
32 bit source IP address
IP protocol version
number
header length
(bytes)
max number
remaining hops
(decremented at
each router)
for
fragmentation/
reassembly
total datagram
length (bytes)
upper layer protocol
to deliver payload to
head.
len
type of
service“type” of data
flgs fragmentoffset
upper
layer
32 bit destination IP address
Options (if any) E.g,timestamp,record route
taken,specify
list of routers
to visit.
how much overhead
with TCP?
? 20 bytes of TCP
? 20 bytes of IP
? = 40 bytes + app
layer overhead
Network Layer 4-52
IP Fragmentation & Reassembly
? network links have MTU
(max.transfer size) - largest
possible link-level frame.
? different link types,
different MTUs
? large IP datagram divided
(“fragmented”) within net
? one datagram becomes
several datagrams
?,reassembled” only at final
destination
? IP header bits used to
identify,order related
fragments
fragmentation,
in,one large datagram
out,3 smaller datagrams
reassembly
Network Layer 4-53
IP Fragmentation and Reassembly
ID
=x
offset
=0
fragflag
=0
length
=4000
ID
=x
offset
=0
fragflag
=1
length
=1500
ID
=x
offset
=1480
fragflag
=1
length
=1500
ID
=x
offset
=2960
fragflag
=0
length
=1040
One large datagram becomes
several smaller datagrams
Example
? 4000 byte
datagram
? MTU = 1500 bytes
Network Layer 4-54
ICMP,Internet Control Message Protocol
? used by hosts,routers,
gateways to communication
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 Code description
0 0 echo reply (ping)
3 0 dest,network unreachable
3 1 dest host unreachable
3 2 dest protocol unreachable
3 3 dest port unreachable
3 6 dest network unknown
3 7 dest host unknown
4 0 source quench (congestion
control - not used)
8 0 echo request (ping)
9 0 route advertisement
10 0 router discovery
11 0 TTL expired
12 0 bad IP header
Network Layer 4-55
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 4-56
DHCP client-server scenario
223.1.1.1
223.1.1.2
223.1.1.3
223.1.1.4 223.1.2.9
223.1.2.2
223.1.2.1
223.1.3.2223.1.3.1
223.1.3.27
A
B
E
DHCP
server
arriving DHCP
client needs
address in this
network
Network Layer 4-57
DHCP client-server scenario
DHCP server,223.1.2.5 arriving
client
time
DHCP discover
src, 0.0.0.0,68
dest.,255.255.255.255,67
yiaddr,0.0.0.0
transaction ID,654
DHCP offer
src,223.1.2.5,67
dest,255.255.255.255,68
yiaddrr,223.1.2.4
transaction ID,654
Lifetime,3600 secs
DHCP request
src,0.0.0.0,68
dest:,255.255.255.255,67
yiaddrr,223.1.2.4
transaction ID,655
Lifetime,3600 secs
DHCP ACK
src,223.1.2.5,67
dest,255.255.255.255,68
yiaddrr,223.1.2.4
transaction ID,655
Lifetime,3600 secs
Network Layer 4-58
NAT,Network Address Translation
10.0.0.1
10.0.0.2
10.0.0.3
10.0.0.4
138.76.29.7
local network
(e.g.,home network)
10.0.0/24
rest of
Internet
Datagrams with source or
destination in this network
have 10.0.0/24 address for
source,destination (as usual)
All datagrams leaving local
network have same single source
NAT IP address,138.76.29.7,
different source port numbers
Network Layer 4-59
NAT,Network Address Translation
? Motivation,local network uses just one IP address as
far as outside word is concerned:
? no need to be allocated range of addresses from ISP,
- just one IP address is used 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 4-60
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 4-61
NAT,Network Address Translation
10.0.0.1
10.0.0.2
10.0.0.3
S,10.0.0.1,3345
D,128.119.40.186,80
1
10.0.0.4
138.76.29.7
1,host 10.0.0.1
sends datagram to
128.119.40,80
NAT translation table
WAN side addr LAN side addr
138.76.29.7,5001 10.0.0.1,3345
…… ……
S,128.119.40.186,80
D,10.0.0.1,3345 4
S,138.76.29.7,5001
D,128.119.40.186,802
2,NAT router
changes datagram
source addr from
10.0.0.1,3345 to
138.76.29.7,5001,
updates table
S,128.119.40.186,80
D,138.76.29.7,5001 3
3,Reply arrives
dest,address:
138.76.29.7,5001
4,NAT router
changes datagram
dest addr from
138.76.29.7,5001 to 10.0.0.1,3345
Network Layer 4-62
NAT,Network Address Translation
?16-bit port-number field,
? 60,000 simultaneous connections with a single
LAN-side address!
?NAT is controversial:
? routers should only process up to layer 3
? 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 4-63
Chapter 4 roadmap
4.1 Introduction and Network Service Models
4.2 Routing Principles
4.3 Hierarchical Routing
4.4 The Internet (IP) Protocol
4.5 Routing in the Internet
? 4.5.1 Intra-AS routing,RIP and OSPF
? 4.5.2 Inter-AS routing,BGP
4.6 What?s Inside a Router?
4.7 IPv6
4.8 Multicast Routing
4.9 Mobility
Network Layer 4-64
Routing in the Internet
? The Global Internet consists of Autonomous Systems
(AS) interconnected with each other:
? Stub AS,small corporation,one connection to other AS?s
? Multihomed AS,large corporation (no transit),multiple
connections to other AS?s
? Transit AS,provider,hooking many AS?s together
? Two-level routing,
? Intra-AS,administrator responsible for choice of routing
algorithm within network
? Inter-AS,unique standard for inter-AS routing,BGP
Network Layer 4-65
Internet AS Hierarchy
Intra-AS border (exterior gateway) routers
Inter-AS interior (gateway) routers
Network Layer 4-66
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 4-67
RIP ( Routing Information Protocol)
? Distance vector algorithm
? Included in BSD-UNIX Distribution in 1982
? Distance metric,# of hops (max = 15 hops)
? Can you guess why?
? Distance vectors,exchanged among neighbors every
30 sec via Response Message (also called
advertisement)
? Each advertisement,list of up to 25 destination nets
within AS
Network Layer 4-68
RIP,Example
Destination Network Next Router Num,of hops to dest.
w A 2
y B 2
z B 7
x -- 1
…,….,...
w x y
z
A
C
D B
Routing table in D
Network Layer 4-69
RIP,Example
Destination Network Next Router Num,of hops to dest.
w A 2
y B 2
z B A 7 5
x -- 1
…,….,...
Routing table in D
w x y
z
A
C
D B
Dest Next hops
w - -
x - -
z C 4
…,…,..
Advertisement
from A to D
Network Layer 4-70
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 4-71
RIP Table processing
? RIP routing tables managed by application-level
process called route-d (daemon)
? advertisements sent in UDP packets,periodically
repeated
physical
link
network forwarding
(IP) table
Transprt
(UDP)
routed
physical
link
network
(IP)
Transprt
(UDP)
routed
forwarding
table
Network Layer 4-72
RIP Table example (continued)
Router,giroflee.eurocom.fr
? Three attached class C networks (LANs)
? Router only knows routes to attached LANs
? Default router used to,go up”
? Route multicast address,224.0.0.0
? Loopback interface (for debugging)
Destination Gateway Flags Ref Use Interface
-------------------- -------------------- ----- ----- ------ ---------
127.0.0.1 127.0.0.1 UH 0 26492 lo0
192.168.2,192.168.2.5 U 2 13 fa0
193.55.114,193.55.114.6 U 3 58503 le0
192.168.3,192.168.3.5 U 2 25 qaa0
224.0.0.0 193.55.114.6 U 3 0 le0
default 193.55.114.129 UG 0 143454
Network Layer 4-73
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 4-74
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 4-75
Hierarchical OSPF
Network Layer 4-76
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 4-77
Inter-AS routing in the Internet,BGP
F igur e 4, 5, 2 - ne w 2, B G P u s e f o r i n t e r - d o m a i n r o u ti n g
A S 2
( O S PF
i ntr a - AS
r o ut i ng )
A S 1
( R IP i n t r a - AS
r o ut i ng )
B G P
A S 3
( O S PF i ntr a - AS
r o ut i ng )
B G P
R1 R2
R3
R4
R5
Network Layer 4-78
Internet inter-AS routing,BGP
? BGP (Border Gateway Protocol),the de facto
standard
? Path Vector protocol:
? similar to Distance Vector protocol
? each Border Gateway broadcast to neighbors
(peers) entire path (i.e.,sequence of AS?s) to
destination
? BGP routes to networks (ASs),not individual
hosts
? E.g.,Gateway X may send its path to dest,Z:
Path (X,Z) = X,Y1,Y2,Y3,…,Z
Network Layer 4-79
Internet inter-AS routing,BGP
Suppose,gateway X send its path to peer gateway W
? W may or may not select path offered by X
?cost,policy (don?t route via competitors AS),loop
prevention reasons.
? If W selects path advertised by X,then:
Path (W,Z) = w,Path (X,Z)
? Note,X can control incoming traffic by controlling it
route advertisements to peers:
?e.g.,don?t want to route traffic to Z -> don?t
advertise any routes to Z
Network Layer 4-80
BGP,controlling who routes to you
F igur e 4,5 - B GP ne w, a s i m p l e B G P s c e n a r i o
A
B
C
W
X
Y
l e ge nd,
c us t o m e r
n e t w o r k,
pr o vi de r
n e t w o r k
? 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 4-81
BGP,controlling who routes to you
F igur e 4,5 - B GP ne w, a s i m p l e B G P s c e n a r i o
A
B
C
W
X
Y
l e ge nd,
c us t o m e r
n e t w o r k,
pr o vi de r
n e t w o r k
? A advertises to B the path AW
? B advertises to X the path BAW
? Should B advertise to C the path BAW?
? 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 4-82
BGP operation
Q,What does a BGP router do?
? Receiving and filtering route advertisements from
directly attached neighbor(s),
? Route selection,
? To route to destination X,which path )of
several advertised) will be taken?
? Sending route advertisements to neighbors,
Network Layer 4-83
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 4-84
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 4-85
Chapter 4 roadmap
4.1 Introduction and Network Service Models
4.2 Routing Principles
4.3 Hierarchical Routing
4.4 The Internet (IP) Protocol
4.5 Routing in the Internet
4.6 What?s Inside a Router?
4.7 IPv6
4.8 Multicast Routing
4.9 Mobility
Network Layer 4-86
Router Architecture Overview
Two key router functions:
? run routing algorithms/protocol (RIP,OSPF,BGP)
? switching datagrams from incoming to outgoing link
Network Layer 4-87
Input Port Functions
Decentralized switching:
? given datagram dest.,lookup output port
using routing table in input port memory
? goal,complete input port processing at
?line speed?
? queuing,if datagrams arrive faster than
forwarding rate into switch fabric
Physical layer:
bit-level reception
Data link layer:
e.g.,Ethernet
see chapter 5
Network Layer 4-88
Input Port Queuing
? Fabric slower that 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 4-89
Three types of switching fabrics
Network Layer 4-90
Switching Via Memory
First generation routers:
?packet copied by system?s (single) CPU
? speed limited by memory bandwidth (2 bus
crossings per datagram)
Input
Port
Output
Port
Memory
System Bus
Modern routers:
? input port processor performs lookup,copy into
memory
? Cisco Catalyst 8500
Network Layer 4-91
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
? 1 Gbps bus,Cisco 1900,sufficient
speed for access and enterprise
routers (not regional or backbone)
Network Layer 4-92
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 Gbps through the
interconnection network
Network Layer 4-93
Output Ports
? Buffering required when datagrams arrive from
fabric faster than the transmission rate
? Scheduling discipline chooses among queued
datagrams for transmission
Network Layer 4-94
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 4-95
Chapter 4 roadmap
4.1 Introduction and Network Service Models
4.2 Routing Principles
4.3 Hierarchical Routing
4.4 The Internet (IP) Protocol
4.5 Routing in the Internet
4.6 What?s Inside a Router?
4.7 IPv6
4.8 Multicast Routing
4.9 Mobility
Network Layer 4-96
IPv6
?Initial motivation,32-bit address space
completely allocated by 2008,
?Additional motivation:
? header format helps speed processing/forwarding
? header changes to facilitate QoS
?new,anycast” address,route to,best” of several
replicated servers
?IPv6 datagram format:
? fixed-length 40 byte header
? no fragmentation allowed
Network Layer 4-97
IPv6 Header (Cont)
Priority,identify priority among datagrams in flow
Flow Label,identify datagrams in same,flow.”
(concept of“flow” not well defined).
Next header,identify upper layer protocol for data
Network Layer 4-98
Other Changes from IPv4
?Checksum,removed entirely to reduce
processing time at each hop
?Options,allowed,but outside of header,
indicated by,Next Header” field
?ICMPv6,new version of ICMP
?additional message types,e.g.,Packet Too Big”
? multicast group management functions
Network Layer 4-99
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?
?Two proposed approaches:
? Dual Stack,some routers with dual stack (v6,v4)
can,translate” between formats
? Tunneling,IPv6 carried as payload in IPv4
datagram among IPv4 routers
Network Layer 4-100
Dual Stack Approach
A B E F
IPv6 IPv6 IPv6 IPv6
C D
IPv4 IPv4
Flow,X
Src,A
Dest,F
data
Flow,
Src,A
Dest,F
data
Src:A
Dest,F
data
A-to-B:
IPv6
Src:A
Dest,F
data
B-to-C:
IPv4
B-to-C:
IPv4
B-to-C:
IPv6
Network Layer 4-101
Tunneling
A B E F
IPv6 IPv6 IPv6 IPv6
tunnelLogical view:
Physical view:
A B E F
IPv6 IPv6 IPv6 IPv6
C D
IPv4 IPv4
Flow,X
Src,A
Dest,F
data
Flow,X
Src,A
Dest,F
data
Flow,X
Src,A
Dest,F
data
Src:B
Dest,E
Flow,X
Src,A
Dest,F
data
Src:B
Dest,E
A-to-B:
IPv6
E-to-F:
IPv6B-to-C:IPv6 inside
IPv4
B-to-C:
IPv6 inside
IPv4
Network Layer 4-102
Chapter 4 roadmap
4.1 Introduction and Network Service Models
4.2 Routing Principles
4.3 Hierarchical Routing
4.4 The Internet (IP) Protocol
4.5 Routing in the Internet
4.6 What?s Inside a Router?
4.7 IPv6
4.8 Multicast Routing
4.9 Mobility
Network Layer 4-103
Multicast,one sender to many receivers
? Multicast,act of sending datagram to multiple
receivers with single,transmit” operation
? analogy,one teacher to many students
? Question,how to achieve multicast
Multicast via unicast
? source sends N
unicast datagrams,
one addressed to
each of N receivers
multicast receiver (red)
not a multicast receiver (red)
routers
forward unicast
datagrams
Network Layer 4-104
Multicast,one sender to many receivers
? Multicast,act of sending datagram to multiple
receivers with single,transmit” operation
? analogy,one teacher to many students
? Question,how to achieve multicast
Network multicast
? Router actively
participate in multicast,
making copies of packets
as needed and
forwarding towards
multicast receiversMulticastrouters (red) duplicate and
forward multicast datagrams
Network Layer 4-105
Multicast,one sender to many receivers
? Multicast,act of sending datagram to multiple
receivers with single,transmit” operation
? analogy,one teacher to many students
? Question,how to achieve multicast
Application-layer
multicast
? end systems involved in
multicast copy and
forward unicast
datagrams among
themselves
Network Layer 4-106
Internet Multicast Service Model
multicast group concept,use of indirection
? hosts addresses IP datagram to multicast group
? routers forward multicast datagrams to hosts that
have,joined” that multicast group
128.119.40.186
128.59.16.12
128.34.108.63
128.34.108.60
multicast
group
226.17.30.197
Network Layer 4-107
Multicast groups
?class D Internet addresses reserved for multicast:
?host group semantics:
o anyone can,join” (receive) multicast group
o anyone can send to multicast group
o no network-layer identification to hosts of
members
?needed,infrastructure to deliver mcast-addressed
datagrams to all hosts that have joined that multicast
group
Network Layer 4-108
Joining a mcast group,two-step process
? local,host informs local mcast router of desire to join
group,IGMP (Internet Group Management Protocol)
? wide area,local router interacts with other routers to
receive mcast datagram flow
? many protocols (e.g.,DVMRP,MOSPF,PIM)
IGMP
IGMP
IGMP
wide-area
multicast
routing
Network Layer 4-109
IGMP,Internet Group Management
Protocol
? host,sends IGMP report when application joins
mcast group
? IP_ADD_MEMBERSHIP socket option
?host need not explicitly,unjoin” group when
leaving
? router,sends IGMP query at regular intervals
? host belonging to a mcast group must reply to
query
query report
Network Layer 4-110
IGMP
IGMP version 1
? router,Host
Membership Query
msg broadcast on LAN
to all hosts
? host,Host
Membership Report
msg to indicate group
membership
? randomized delay
before responding
? implicit leave via no
reply to Query
? RFC 1112
IGMP v2,additions
include
? group-specific Query
? Leave Group msg
? last host replying to Query
can send explicit Leave
Group msg
? router performs group-
specific query to see if any
hosts left in group
? RFC 2236
IGMP v3,under development
as Internet draft
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
R1
R2
R3
R4
R5
R6 R7
2
1
6
3 4
5
i
router with attached
group member
router with no attached
group member
link used for forwarding,
i indicates order link
added by algorithm
LEGENDS,source
Reverse Path Forwarding
if (mcast datagram received on incoming link
on shortest path back to center)
then flood datagram onto all outgoing links
else ignore datagram
?rely on router?s knowledge of unicast
shortest path from it to sender
?each router has simple forwarding behavior:
Reverse Path Forwarding,example
? result is a source-specific reverse SPT
– may be a bad choice with asymmetric links
R1
R2
R3
R4
R5
R6 R7
router with attached
group member
router with no attached
group member
datagram will be
forwarded
LEGENDS,source
datagram will not be
forwarded
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
R1
R2
R3
R4
R5
R6 R7
router with attached
group member
router with no attached
group member
prune message
LEGENDS,source
links with multicast
forwarding
P
P
P
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:
R1
R2
R3
R4
R5
R6 R7
router with attached
group member
router with no attached
group member
path order in which join
messages generated
LEGEND
2
1
3
1
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?
? 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
physical topology logical topology
PIM,Protocol Independent Multicast
? not dependent on any specific underlying unicast
routing algorithm (works with all)
? two different multicast distribution scenarios,
Dense:
? group members
densely packed,in
“close” proximity.
? bandwidth more
plentiful
Sparse:
? # networks with group
members small wrt #
interconnected networks
? group members,widely
dispersed”
? bandwidth not plentiful
Consequences of Sparse-Dense Dichotomy:
Dense
? group membership by
routers assumed until
routers explicitly prune
? data-driven construction
on mcast tree (e.g.,RPF)
? bandwidth and non-
group-router processing
profligate
Sparse:
? no membership until
routers explicitly join
? receiver- driven
construction of mcast
tree (e.g.,center-based)
? bandwidth and non-group-
router processing
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)
? intermediate routers
update state and
forward join
? after joining via RP,
router can switch to
source-specific tree
? increased performance,
less concentration,
shorter paths
R1
R2
R3
R4
R5
R6
R7
join
join
join
all data multicast
from rendezvous
point
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
R2
R3
R4
R5
R6
R7
join
join
join
all data multicast
from rendezvous
point
rendezvous
point
Network Layer 4-128
Chapter 4 roadmap
4.1 Introduction and Network Service Models
4.2 Routing Principles
4.3 Hierarchical Routing
4.4 The Internet (IP) Protocol
4.5 Routing in the Internet
4.6 What?s Inside a Router?
4.7 IPv6
4.8 Multicast Routing
4.9 Mobility
Network Layer 4-129
What is mobility?
? spectrum of mobility,from the network perspective:
no mobility high mobility
mobile user,using
same access point
mobile user,passing
through multiple
access point while
maintaining ongoing
connections (like cell
phone)
mobile user,
connecting/
disconnecting
from network
using DHCP,
Network Layer 4-130
Mobility,Vocabulary
home network,permanent
“home” of mobile
(e.g.,128.119.40/24)
Permanent address:
address in home
network,can always be
used to reach mobile
e.g.,128.119.40.186
home agent,entity that will
perform mobility functions on
behalf of mobile,when mobile
is remote
wide area
network
correspondent
Network Layer 4-131
Mobility,more vocabulary
Care-of-address,address
in visited network.
(e.g.,79,129.13.2)
wide area
network
visited network,network
in which mobile currently
resides (e.g.,79.129.13/24)
Permanent address,remains
constant (e.g.,128.119.40.186)
home agent,entity in
visited network that
performs mobility
functions on behalf
of mobile,
correspondent,wants
to communicate with
mobile
Network Layer 4-132
How do you contact a mobile friend:
? search all phone
books?
? call her parents?
? expect her to let you
know where he/she is?
I wonder where
Alice moved to?Consider friend frequently changing addresses,how do you find her?
Network Layer 4-133
Mobility,approaches
? Let routing handle it,routers advertise permanent
address of mobile-nodes-in-residence via usual
routing table exchange.
? routing tables indicate where each mobile located
? no changes to end-systems
? Let end-systems handle it,
? indirect routing,communication from
correspondent to mobile goes through home
agent,then forwarded to remote
? direct routing,correspondent gets foreign
address of mobile,sends directly to mobile
Network Layer 4-134
Mobility,approaches
? Let routing handle it,routers advertise permanent
address of mobile-nodes-in-residence via usual
routing table exchange.
? routing tables indicate where each mobile located
? no changes to end-systems
? let end-systems handle it,
? indirect routing,communication from
correspondent to mobile goes through home
agent,then forwarded to remote
? direct routing,correspondent gets foreign
address of mobile,sends directly to mobile
not
scalable
to millions of
mobiles
Network Layer 4-135
Mobility,registration
End result:
? Foreign agent knows about mobile
? Home agent knows location of mobile
wide area
network
home network
visited network
1
mobile contacts
foreign agent on
entering visited
network
2
foreign agent contacts home
agent home:,this mobile is
resident in my network”
Network Layer 4-136
Mobility via Indirect Routing
wide area
network
home
network
visited
network
3
2
4
1
correspondent
addresses packets
using home address
of mobile
home agent intercepts
packets,forwards to
foreign agent
foreign agent
receives packets,
forwards to mobile
mobile replies
directly to
correspondent
Network Layer 4-137
Indirect Routing,comments
? Mobile uses two addresses:
? permanent address,used by correspondent (hence
mobile location is transparent to correspondent)
? care-of-address,used by home agent to forward
datagrams to mobile
? foreign agent functions may be done by mobile itself
? triangle routing,correspondent-home-network-
mobile
? inefficient when
correspondent,mobile
are in same network
Network Layer 4-138
Forwarding datagrams to remote mobile
Permanent address,
128.119.40.186
Care-of address,
79.129.13.2dest,128.119.40.186
packet sent by
correspondent
dest,79.129.13.2 dest,128.119.40.186
packet sent by home agent to foreign
agent,a packet within a packet
dest,128.119.40.186
foreign-agent-to-mobile packet
Network Layer 4-139
Indirect Routing,moving between networks
?suppose mobile user moves to another
network
? registers with new foreign agent
? new foreign agent registers with home agent
? home agent update care-of-address for mobile
? packets continue to be forwarded to mobile (but
with new care-of-address)
?Mobility,changing foreign networks
transparent,on going connections can be
maintained!
Network Layer 4-140
Mobility via Direct Routing
wide area
network
home
network
visited
network
4
2
41correspondent
requests,receives
foreign address of
mobile
correspondent forwards
to foreign agent
foreign agent
receives packets,
forwards to mobile
mobile replies
directly to
correspondent
3
Network Layer 4-141
Mobility via Direct Routing,comments
?overcome triangle routing problem
?non-transparent to correspondent:
correspondent must get care-of-address
from home agent
? What happens if mobile changes networks?
Network Layer 4-142
Mobile IP
?RFC 3220
?has many features we?ve seen,
? home agents,foreign agents,foreign-agent
registration,care-of-addresses,encapsulation
(packet-within-a-packet)
?three components to standard:
? agent discovery
? registration with home agent
? indirect routing of datagrams
Network Layer 4-143
Mobile IP,agent discovery
? agent advertisement,foreign/home agents advertise
service by broadcasting ICMP messages (typefield = 9)
RB HFM G V
b i ts
rese rv e d
t y p e = 1 6
t y p e = 9 cod e = 0
= 9
c h e ck su m
= 9
rou te r a d d ress
sta n d a rd
IC M P f i e l d s
m o b i l i t y a g e n t
a d verti sem e n t
e xte n sio n
l e n g th seq u e n ce #
reg i strat i o n l i f e t i m e
0 o r m o re ca re - of -
a d d r e ss e s
0 8 16 24
R bit,registration
required
H,F bits,home
and/or foreign agent
Network Layer 4-144
Mobile IP,registration example
v i s i t e d n e t w o r k,7 9,1 2 9,1 3 / 2 4
hom e a g e n t
HA, 1 2 8,1 1 9,4 0,7
f o r e i gn a g e n t
CO A, 7 9,1 2 9,1 3,2
C O A,79,12 9,1 3,2
…,
I C M P a ge n t a dv,
M o b i l e a g e n t
M A, 1 2 8,1 1 9,4 0,1 8 6
r e gi st r a t io n r e q,
C O A,79,12 9,13, 2
H A,128,1 19,4 0.7
M A,128,11 9.4 0,18 6
Li fet i m e, 99 99
i de nt i fi c ati on,7 14
…,
r e gi st r a t io n r e q,
C O A,79,12 9,13, 2
H A,128,1 19,4 0.7
M A,128,11 9.4 0,18 6
Li fet i m e, 99 99
i de nt i fi c ati on, 71 4
en c a ps u l at i on f orm at
…,
r e gi st r a t io n r e p ly
H A,128,1 19,4 0.7
M A,128,11 9.4 0,18 6
Li fet i m e, 49 99
Ide nt i fi c ati on, 7 14
en c a ps u l at i on f orm at
…,
r e gi st r a t io n r e p ly
H A,128,1 19,4 0.7
M A,128,11 9.4 0,18 6
Li fet i m e, 49 99
Ide nt i fi c ati on, 7 14
…,
t im e
Network Layer 4-145
Network Layer,summary
Next stop:
the Data
link layer!
What we?ve covered:
? network layer services
? routing principles,link state and
distance vector
? hierarchical routing
? IP
? Internet routing protocols RIP,
OSPF,BGP
? what?s inside a router?
? IPv6
? mobility