1. Reti di calcolatori e Sicurezza -- Network Layer ---
Part of these slides are adapted from the slides of the book:
Computer Networking: A Top Down Approach Featuring the Internet, 2nd edition. Jim Kurose, Keith Ross Addison-Wesley, July (copyright
J.F Kurose and K.W. Ross, All Rights Reserved)
Network Layer

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 (seminario)
hierarchical routing (seminario) IP
Internet routing protocols reliable transfer (seminario)
intra-domain
inter-domain
what’s inside a router?
IPv6 (seminario)
Mobility (seminario)
Network Layer

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. Network: Funzionalità
Trasportare pacchetti (datagram) dal sender al receiver
I protocolli del livello network “girano” sia sugli host che sui router
Funzionalità principali:
Determinazione del percorso dei pacchetti:. Routing
Switching: funzione che definisce le modalità di input/output dei pacchetti in un router
Call setup: attività di inizializzazione del percorso (solo in alcune architteture) (NO IP! .. Ma con QoS?)
application
transport
network
data link
physical
network
data link
physical
Network Layer

5. Il modello del servizio
Quale è il modello di servizio offerto dal livello network?
Viene assicurata una determinata banda di trasmissione?
loss-free delivery?
in-order delivery?
congestion feedback?
Due risposte
possibili ?
virtual circuit --
datagram ? ?
Servizi di rete
Network Layer

6. Circuiti Virtuali
“il cammino che viene stabilito tra il sender ed il receiver fornisce un comporta di tipo telefonico (a commutazione di circuito)”
call setup: per ogni attivazione del circuto prima di poter trasmettere dati
Ogni pacchetto trasmesso deve avere un tag di identificazione del circuito (non importa l’indirizzo di destinazione)
Network Layer

7. Circuito Virtuale (2)
Ogni router lungo il cammino deve mantenere le informazioni di stato per ogni connessione che passa attraverso il router.
Le risorse del router (bandwidth, buffer) devono essere allocate per il circuito virtuale.
Network Layer

8. Circuito Virtuale (3) Comportamento ideale di un circuto virtuale
application
transport
network
data link
physical
application
transport
network
data link
physical
5. Data flow begins
6. Receive data
4. Call connected
3. Accept call
1. Initiate call
2. incoming call
Network Layer

9. Circuito Virtuale (conclusioni)
Utilizzato in particolari applicazioni (ad esempio quando si vogliono avere dei collegamenti dedicati tra intranet aziendali)
ATM, frame-relay, X.25 utilizzano questo modello di servizio
Internet: no!!
Network Layer

10. Reti Datagram: Internet
Nessuna azione di attivazione (call set up)
router: non mantengono informazioni di stato sulle connessioni
Network: non esiste la nozione di “connessione”
I pacchetti sono caratterizzati dall’indirizzo di destinazione (pacchetti di una connessione possono seguire un percorso differente)
application
transport
network
data link
physical
application
transport
network
data link
physical
1. Send data
2. Receive data
Network Layer

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

12. Datagram vs VC Internet ATM Scambio dei dati tra le applicazioni
Servizi “elastic” rispetto ai requisiti temporali
Host (computers)
Possono implementare politiche per il controllo della congestione, etc
Rete semplice ma applicazioni evolute
Eterogeneità: difficile prevedere una nozione uniforme di servizio
ATM
Origine nella telefonia
Richiesta di affidabilità e grossi vincoli temporali
Host:terminali telefonici
La rete è di fatto complessa
Network Layer

13. 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

14. Routing Routing protocol Graph abstraction for routing algorithms:
Goal: determine “good” path
(sequence of routers) thru
network from source to dest. 5 3 B C 2 5 A 2 1 F 3
Graph abstraction for routing algorithms:
graph nodes are routers
graph edges are physical links
link cost: delay, $ cost, or congestion level 1 2 D E 1
“good” path:
typically means minimum cost path
other def’s possible
Network Layer

15. Caratteristiche del routing
Switching vs Routing
Switching: attività che seleziona una porta del router in base alle informazioni della tabella di routing
routing: attività di costruzione della tabella di routing
Network Layer

16. Routing Algorithm classification
JUMP
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

17. 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

18. Dijsktra’s Algorithm 1 Initialization: 2 N = {A} 3 for all nodes v
if v adjacent to A
then D(v) = c(A,v)
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:
D(v) = min( D(v), D(w) + c(w,v) )
13 /* new cost to v is either old cost to v or known
shortest path cost to w plus cost from w to v */
15 until all nodes in N
Network Layer

19. Dijkstra’s algorithm: example
Step 1 2 3 4 5
start N A AD
ADE
ADEB
ADEBC
ADEBCF
D(B),p(B)
2,A
D(C),p(C)
5,A
4,D
3,E
D(D),p(D)
1,A
D(E),p(E)
infinity
2,D
D(F),p(F)
infinity
4,E 5 3 B C 2 5 A 2 1 F 3 1 2 D E 1
Network Layer

20. DA: Esempio Selected edge Candidate edge Not visited B C A E F D G H 72 3 2 3 2 A E F D 1 2 2 4 4 G H
Selected edge
Candidate edge
Not visited
Network Layer

21. DA: Esempio Selected edge Candidate edge Not visited B C A E F D G H 72 3 2 3 2 A E F D 1 2 2 6 4 G H
Selected edge
Candidate edge
Not visited
Network Layer

22. DA: Esempio Selected edge Candidate edge Not visited B C A E F D G H 72 3 2 3 2 A E F D 1 2 2 6 4 G H
Selected edge
Candidate edge
Not visited
Network Layer

23. DA: Esempio Selected edge Candidate edge Not visited B C A E F D G H 72 3 2 3 2 A E F D 1 2 2 6 4 G H
Selected edge
Candidate edge
Not visited
Network Layer

24. DA: Esempio Selected edge Candidate edge Not visited B C A E F D G H 72 3 2 3 2 A E F D 1 2 2 6 4 G H
Selected edge
Candidate edge
Not visited
Network Layer

25. 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 A A 1 A
1+e
2+e
2+e
2+e D B D B D B D B
1+e 1
1+e 1 e 1 C
1+e e C C C 1 1 e
… recompute
routing
… recompute
… recompute
initially
Network Layer

26. 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 from X to
Y, via Z as next hop
c(X,Z) + min {D (Y,w)} Z w =
Network Layer

27. Distance Table: example1 7 6 4 14 8 9 11 5 2 E
cost to destination via
destination 1 B C 7 A 8 2 1 E D 2
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
loop!
D (A,B) E
c(E,B) + min {D (A,w)} B w =
8+6 = 14
loop!
Network Layer

28. Distance table gives routing table1 7 6 4 14 8 9 11 5 2 E
cost to destination via
destination
Outgoing link
to use, cost A B C D
A,1
D,5
D,4
destination
Distance table
Routing table
Network Layer

29. Routing: distribuito e asincrono
Algoritmo di Bellmann-Ford: vettore delle distanze (distance vector)
Network Layer

30. 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
Each node:
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
Network Layer

31. Distance Vector Algorithm:
At all nodes, X:
1 Initialization:
2 for all adjacent nodes v:
D (*,v) = infinity /* the * operator means "for all rows" */
D (v,v) = c(X,v)
5 for all destinations, y
send min D (y,w) to each neighbor /* w over all X's neighbors */ X X X w
Network Layer

32. Distance Vector Algorithm (cont.):
8 loop
9 wait (until I see a link cost change to neighbor V
or until I receive update from neighbor V) 11
12 if (c(X,V) changes by d)
/* change cost to all dest's via neighbor v by d */
/* note: d could be positive or negative */
for all destinations y: D (y,V) = D (y,V) + d 16
17 else if (update received from V wrt destination Y)
/* shortest path from V to some Y has changed */
/* V has sent a new value for its min DV(Y,w) */
/* call this received new value is "newval" */
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
send new value of min D (Y,w) to all neighbors 25
26 forever X X w X X w X w
Network Layer

33. Distance Vector Algorithm: exampleZ 1 2 7 Y
Network Layer

34. Distance Vector Algorithm: exampleZ 1 2 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

35. Distance Vector: 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) 1 Y 4 1 X Z 50
algorithm
terminates
“good
news
travels
fast”
Network Layer

36. Distance Vector: link cost changes
good news travels fast
bad news travels slow - “count to infinity” problem! 60 Y 4 1 X Z 50
algorithm
continues
on!
Network Layer

37. 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? 60 Y 4 1 X Z 50
algorithm
terminates
Network Layer

38. 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

39. 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

40. Routing
I meccanismi di routing che abbiamo studiato sono neccanismi ideali
Rete è piatta
Tutti i router sono identici
Buona astrazione ma … irrealistica!!!
Network Layer

41. Internet Passato recente … P Network Layer NSFNET backbone Stanford
BARRNET
regional
Berkeley P
ARC
NCAR UA
UNM
Westnet
UNL KU
ISU
MidNet …
Network Layer

42. Internet Oggi Backbone service provider Peering point
Large corporation
Small
corporation “
Consumer ”
ISP
Network Layer

43. Internet scalabilità: 50 milioni di possibili host:
Non possono memorizzare tutte le possibili destinazioni nella tabella di routing!
Messaggi per la modifica delle tabelle di routing avrebbero un costo troppo elevato!
Domini Amministrativi:
internet = rete di reti
Ogni rete ha una sua autorità (network admin) che ha il controllo complete sul proprio dominio
Network Layer

44. Routing Gerarchico gateway router
I router vengono aggregati in regioni: “autonomous systems” (AS)
I routers nella stessa AS eseguono lo stesso protocollo di routing
“intra-AS” routing
Router in regioni differenti possono eseguire un protocollo diverso dal protocollo intra-AS routing
Router speciali in una AS
Eseguono il protocollo intra-AS routing con tutti gli altri router nella AS
Inoltre sono I responsabili del routing verso l’esterno della AS
Eseguono un protocollo inter-AS routing con gli altri gateway
Network Layer

45. Intra-AS vs Inter-AS routing
C.b
Gateway:
inter-AS routing
intra-AS routing
B.a
A.a b
A.c c a a C b a B d c A b
network layer
inter-AS, intra-AS routing nel
gateway A.c
link layer
physical layer
Network Layer

46. Intra-AS vs Inter-AS routing
between
A and B a b C A B d c
A.a
A.c
C.b
B.a
Host h2
Host h1
Intra-AS routing
within AS B
Intra-AS routing
within AS A
Network Layer

47. Routing Gerarchico Rete viene suddivisa in regioni
Router di una regione hanno informazione completa su quella regione
Analogia: Prefissi telefonici
Network Layer

48. Routing Gerarchico
Network Layer

49. 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

50. The Internet Network layer
Host, router network layer functions:
Transport layer: TCP, UDP
IP protocol
addressing conventions
datagram format
packet handling conventions
Routing protocols
path selection
RIP, OSPF, BGP
Network
layer
forwarding
table
ICMP protocol
error reporting
router “signaling”
Link layer
physical layer
Network Layer

51. 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
Network Layer

52. 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
LAN
network consisting of 3 IP networks
(for IP addresses starting with 223,
first 24 bits are network address)
Network Layer

53. IP Addressing How to find the networks?
How to find the networks?
Detach each interface from router, host
create “islands of isolated networks
Interconnected
system consisting
of six networks
Network Layer

54. IP Address Classes Class A: Class B: Class C Class D
For very large organizations
16 million hosts allowed
Class B:
For large organizations
65 thousand hosts allowed
Class C
For small organizations
255 hosts allowed
Class D
Multicast addresses
No network/host hierarchy
Network Layer

55. Indirizzamento per classe
Proprietà
unico
gerarchico: network + host
Dot Notation
Network
Host 7 24 A: 14 16 1 B: 21 8 C:
Network Layer

56. IP addressing: CIDR Indirizzamento via classi:
Uso poco efficiente dello spazio degli indirizzi: la classe B alloca indirizzi per 65K host anche se la rete ne richiede solamente 2k
CIDR: Classless InterDomain Routing
Reti IP hanno indirizzi di lunghezza arbitraria
formato: a.b.c.d/x, dove x è # di bit nella porzione dell’indirizzo che definisce la rete.
network
part
host
/23
Network Layer

57. Cammino di un datagram RoutingTable@A IP datagram:
Dest. Net. next router Nhops
IP datagram:
misc
fields
source
IP addr
dest
data A B E
Datagram non viene modificato durante il percorso
Ruolo basilare degli indirizzi IP
Network Layer

58. Cammino di un datagram RoutingTable@A A::send IP-datagram @ B:
misc
fields
Dest. Net. next router Nhops
data
A::send B:
lookup(IP(B)) nella tabella di routing di A
B è sulla stessa sottorete di A
Call link.services
B e A sono connessi direttamente A B E
Network Layer

59. Cammino di un datagram RoutingTable@A A:: send(datagram)@E:
misc
fields
Dest. Net. next router Nhops
data
A::
E è un host di una rete differente =
link invia il datagram al router
datagram
….. A B E
Network Layer

60. Cammino di un datagram Routing Table 223.1.1.4:: send @ 223.1.2.2
Dest. Net router Nhops interface
misc
fields
data ::
look up nella tabella di routing
E è un host sulla medesima interfaccia del router ( )
link invia il mediante l’interfaccia
datagram !!! (era ora!) A B E
Network Layer

61. 32 bit destination IP address
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
fragment
offset
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

62. IPv4
Tipo del servizio:
Differenza tra datagram di controllo e datagram dei dati
Router usano questo campo per differenziare i livelli di servizio offerti
TTL = 0 allora il datagram deve essere abbandonato
Protocol (simile al numero di porta del trasporto)
6 => TCP
17 => UDP
Network Layer

63. Checksum
Perchè la suite TCP/IP (IPv4) prevede checksum sia al livello del trasporto che al livello della rete?
Router non è richiesto fare questo calcolo
TCP potrebbe basarsi su di un livello di trasporto differente (esempio ATM)
Network Layer

64. MTU Datagram è incapsulato all’interno di un pacchetto del link
Quantità di dati che può trasportare un protocollo del livello link è denominata MTU (max.transfer size):
Link differenti possono avere, MTU distinte
Ethernet MTU = 1500 bytes
WAN MTU = 576 bytes
Datagram è incapsulato all’interno di un pacchetto del link
MTU limite alla dimensione del datagram
Network Layer

65. Problema
In un percorso sulla rete un datagram può passare lungo link che hanno una differente struttura del MTU
Soluzione: Frammentare i dati del datagram in piu’ datagram
Network Layer

66. 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

67. IP Fragmentation and ReassemblyID =x
offset =0
fragflag
length
=4000 =1
=1500
=1480
=2960
=1040
One large datagram becomes
several smaller datagrams
Example
4000 byte datagram
MTU = 1500 bytes
Network Layer

68. IP: Frammentazione & Riassemblaggio
F&R: Carico di lavoro sui router
Piccola frammentazione
MTU almeno 576 bytes
MSS 536 bytes
20 bytes = header del segmento
20 bytes = header del datagram
Network Layer

69. Network Protocols ICMP,
Network Layer

70. ICMP: Internet Control Message Protocol
Protocollo utilizzato da host, router, gateway per scambiarsi in formazioni relative al livello network
errori: unreachable host, network, port, protocol
echo request/reply (ping)
traceroute
Network Layer

71. ICMP: Internet Control Message Protocol
ICMP livello sopra IP:
ICMP msgs sono incapsulati in datagram IP
ICMP struttura dei messaggi:
Campo type,
Campo code
Contengono inoltre i primi 8 bytes del datagram IP che ha causato l’errore.
Type Code 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

72. ICMP: esempi di uso Echo request reply Address mask request/reply
Controllo se un host è ancora “vivo”
Address mask request/reply
determinara subnet mask
Destination unreachable
Indirizzo non valido
TTL expired
… troppo lontano
Network Layer

73. Ping ICMP echo request/reply
Source sends ICMP echo request message to the destination address
Echo request packet contains sequence number and timestamp
Destination replies with an ICMP echo reply message containing the data in the original echo request message
Source can calculate round trip time (RTT) of packets
If no echo reply comes back then the destination is unreachable
Network Layer

74. Ping (2) A R1 R2 R3 B
Echo request
Time
Echo reply
Network Layer

75. Traceroute Traceroute records the route that packets take
A clever use of the TTL field
When a router receives a packet, it decrements TTL
If TTL=0, it sends an ICMP time exceeded message back to the sender
To determine the route, progressively increase TTL
Every time an ICMP time exceeded message is received, record the sender’s (router’s) address
Repeat until the destination host is reached or an error message occurs
Network Layer

76. Traceroute (2) Time A B R1 R2 R3 Te = Time exceeded
Pu = Port unreachable A R1 R2 R3 B
TTL=1, Dest = B,
port = invalid
Te (R1)
TTL=2, Dest = B
Time
Te (R2)
TTL=3, Dest = B
Te (R3)
TTL=4, Dest = B
Pu (B)
Network Layer

77. Come si ottiene l’indirizzo IP?
Hosts
File di configurazione
Win: control-panel->network->configuration->tcp/ip->properties
Linux: /etc/rc.config
DHCP: Dynamic Host Configuration Protocol: ottiene l’indirizzo in modo dinamico: “plug-and-play”
host broadcast “DHCP discover” msg
DHCP server risponde “DHCP offer” msg
host invia la richiesta per IP address: “DHCP request” msg
DHCP server invia l’indirizzo: “DHCP ack” msg
Network Layer

78. 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

79. DHCP client-server scenario A B E
DHCP
server
arriving DHCP
client needs
address in this
network
Network Layer

80. DHCP client-server scenario
DHCP server:
arriving
client
DHCP discover
src : , 68
dest.: ,67
yiaddr:
transaction ID: 654
DHCP offer
src: , 67
dest: , 68
yiaddrr:
transaction ID: 654
Lifetime: 3600 secs
DHCP request
src: , 68
dest:: , 67
yiaddrr:
transaction ID: 655
Lifetime: 3600 secs
time
DHCP ACK
src: , 67
dest: , 68
yiaddrr:
transaction ID: 655
Lifetime: 3600 secs
Network Layer

81. NAT: Network Address Translation
rest of
Internet
local network
(e.g., home network)
10.0.0/24
All datagrams leaving local
network have same single source NAT IP address: ,
different source port numbers
Datagrams with source or
destination in this network
have /24 address for
source, destination (as usual)
Network Layer

82. 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

83. 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

84. NAT: Network Address Translation
NAT translation table
WAN side addr LAN side addr
1: host
sends datagram to
, 80
2: NAT router
changes datagram
source addr from
, 3345 to
, 5001,
updates table
, , 3345
…… ……
S: , 3345
D: , 80 1
S: , 80
D: , 3345 4
S: , 5001
D: , 80 2
S: , 80
D: , 5001 3
4: NAT router
changes datagram
dest addr from
, 5001 to , 3345
3: Reply arrives
dest. address:
, 5001
Network Layer

85. 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

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

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

88. 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

89. 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

90. Internet AS Hierarchy Intra-AS border (exterior gateway) routers
Inter-AS interior (gateway) routers
Network Layer

91. 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

92. 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

93. RIP: Example z w x y A D B C y B 2 z B 7 x -- 1
Destination Network Next Router Num. of hops to dest.
w A 2
y B 2
z B 7 x
…. …
Routing table in D
Network Layer

94. RIP: Example w x y z A C D B y B 2 z B A 7 5 x -- 1 Advertisement
Dest Next hops w x
z C 4
…. …
Advertisement
from A to D w x y z A C D B
Destination Network Next Router Num. of hops to dest.
w A 2
y B 2
z B A 7 5 x
…. …
Routing table in D
Network Layer

95. 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

96. 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)
Transprt
(UDP)
network forwarding
(IP) table
network
(IP)
forwarding
table
link
link
physical
physical
Network Layer

97. RIP Table example (continued)
Router: giroflee.eurocom.fr
Destination Gateway Flags Ref Use Interface
UH lo0
U fa0
U le0
U qaa0
U le0
default UG
Three attached class C networks (LANs)
Router only knows routes to attached LANs
Default router used to “go up”
Route multicast address:
Loopback interface (for debugging)
Network Layer

98. 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

99. 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

100. Hierarchical OSPF
Network Layer

101. 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

102. Inter-AS routing in the Internet: BGP
Network Layer

103. 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

104. 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

105. BGP: controlling who routes to you
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

106. BGP: controlling who routes to you
A advertises to B the path AW
B advertises to W 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

107. 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

108. 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

109. 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

110. 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

111. Router Architecture Overview
Two key router functions:
run routing algorithms/protocol (RIP, OSPF, BGP)
switching datagrams from incoming to outgoing link
Network Layer

112. Input Port Functions Decentralized switching: Physical layer:
bit-level reception
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
Data link layer:
e.g., Ethernet
see chapter 5
Network Layer

113. 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

114. Three types of switching fabrics
Network Layer

115. 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
Memory
System Bus
Modern routers:
input port processor performs lookup, copy into memory
Cisco Catalyst 8500
Network Layer

116. 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

117. 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

118. Output Ports
Buffering required when datagrams arrive from fabric faster than the transmission rate
Scheduling discipline chooses among queued datagrams for transmission
Network Layer

119. 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

120. 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

121. 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

122. 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

123. 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

124. 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

125. Dual Stack Approach A B C D E F IPv6 IPv6 IPv4 IPv4 IPv6 IPv6 A-to-B:
Flow: X
Src: A
Dest: F
data
Src:A
Dest: F
data
Src:A
Dest: F
data
Flow: ??
Src: A
Dest: F
data
A-to-B:
IPv6
B-to-C:
IPv4
B-to-C:
IPv4
B-to-C:
IPv6
Network Layer

126. Tunneling A B E F Logical view: A B C D E F Physical view: Src:B
IPv6
IPv6
IPv6
IPv6 A B C D E F
Physical view:
IPv6
IPv6
IPv4
IPv4
IPv6
IPv6
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
Flow: X
Src: A
Dest: F
data
A-to-B:
IPv6
E-to-F:
IPv6
B-to-C:
IPv6 inside
IPv4
B-to-C:
IPv6 inside
IPv4
Network Layer

127. 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

128. 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

129. 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 receivers
Multicast
routers (red) duplicate and
forward multicast datagrams
Network Layer

130. 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

131. Internet Multicast Service Model
multicast
group
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
Notes:
The Internet multicast service model and addressing/group management ideas have their foundation in the PhD thesis of S. Deering: “Multicast Routing in a Datagram Network,” Dept. of Computer Science, Stanford University, 1991.
3.1 Network Layer: Introduction
Network Layer

132. Multicast groups class D Internet addresses reserved for multicast:
host group semantics:
anyone can “join” (receive) multicast group
anyone can send to multicast group
no network-layer identification to hosts of members
needed: infrastructure to deliver mcast-addressed datagrams to all hosts that have joined that multicast group
Notes:
3.1 Network Layer: Introduction
Network Layer

133. 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
Notes:
3.2 Network Layer: Multicast Addressing and Group Management
wide-area
multicast
routing
IGMP
Network Layer

134. 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
Notes:
3.2 Network Layer: Multicast Addressing and Group Management
query
report
Network Layer

135. IGMP IGMP version 1 IGMP v2: additions include
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
Notes:
RFC-1112: S. Deering, “Host Extension for IP Multicasting,” August 1989
RFC 2236: R. Fenner, “Internet Group Management Protocol, Version 2”, November 1997.
B. Cain, S. Deering, A. Thyagarajan, “Internet Group Management Protocol, Version 3,” work in progress, draft-ietf-idmr-igmp-v3-00.txt
3.2 Network Layer: Multicast Addressing and Group Management
Network Layer

136. 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
Source-based trees
Notes:
3.3 Network Layer: Multicast Routing Algorithms
Shared tree

137. Approaches for building mcast trees
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
Notes:
3.3 Network Layer: Multicast Routing Algorithms
…we first look at basic approaches, then specific protocols adopting these approaches

138. Shortest Path Tree
mcast forwarding tree: tree of shortest path routes from source to all receivers
Dijkstra’s algorithm
S: source
LEGEND R1 2 R4
router with attached
group member 1 R2 5
Notes:
3.3 Network Layer: Multicast Routing Algorithms
router with no attached
group member 3 4 R5 6
link used for forwarding,
i indicates order link
added by algorithm R3 i R6 R7

139. 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
Notes:
3.3 Network Layer: Multicast Routing Algorithms

140. Reverse Path Forwarding: example
S: source
LEGEND R1 R4
router with attached
group member R2
router with no attached
group member R5 R3
datagram will be forwarded R6 R7
datagram will not be
forwarded
Notes:
3.3 Network Layer: Multicast Routing Algorithms
result is a source-specific reverse SPT
may be a bad choice with asymmetric links

141. 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
router with no attached
group member R2
Notes:
3.3 Network Layer: Multicast Routing Algorithms P P R5
prune message P
links with multicast
forwarding R3 R6 R7

142. 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
Notes:
1. See L. Wei and D. Estrin, “A Comparison of multicast trees and algorithms,” TR USC-CD , Dept. Computer Science, University of California, Sept 1993 for a comparison of heuristic approaches.
3.3 Network Layer: Multicast Routing Algorithms

143. 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
Notes:
1. It’s always nice to see a PhD dissertation with impact. The earliest discussion of center-based trees for multicast appears to be D. Wall, “Mechanisms for Broadcast and Selective Broadcast,” PhD dissertation, Stanford U., June 1980.
3.3 Network Layer: Multicast Routing Algorithms

144. Center-based trees: an example
Suppose R6 chosen as center:
LEGEND R1
router with attached
group member R4 3
router with no attached
group member R2 2 1 R5
path order in which join messages generated
Notes:
3.3 Network Layer: Multicast Routing Algorithms R3 1 R6 R7

145. 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
Notes:
D. Waitzman, S. Deering, C. Partridge, “Distance Vector Multicast Routing Protocol,” RFC 1075, Nov The version of DVMRP in use today is considerably enhanced over the RFC1075 spec.
A more up-to-date “work-in-progress” defines a version 3 of DVMRP: T. Pusateri, “Distance Vector Multicast Routing Protocol,” work-in-progress, draft-ietf-idmr-v3-05.ps
3.4 Network Layer: Internet Multicast Routing Algorithms

146. 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
Notes:
1. See for a (slightly outdatet) list of multicast capable routers (supporting DVMPR as well as other protocols) from various vendors.
2. ftp://parcftp.xerox.com/pub/net-research/ipmulti for circa 1996 public copy “mrouted” v3.8 of DVMRP routing software for various workstation routing platforms.
3.4 Network Layer: Internet Multicast Routing Algorithms

147. 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
Notes:
For a general discussion of IP encapsulation, see C. Perkins, “IP Encapsulation within IP,” RFC 2003, Oct
The book S. Bradner, A Mankin, “Ipng: Internet protocol next generation,” Addison Wesley, 1995 has a very nice discussion of tunneling
Tunneling can also be used to connect islands of IPv6 capable routers in a sea IPv4 capable routers. The long term hope is that the sea evaporates leaving only lands of IPv6!
3.4 Network Layer: Internet Multicast Routing Algorithms

148. 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
Notes:
a very readable discussion of the PIM architecture is S. Deering, D. Estrin, D. Faranacci, V. Jacobson, C. Liu, L. Wei, “The PIM Architecture for Wide Area Multicasting,” IEEE/ACM Transactions on Networking, Vol. 4, No. 2, April 1996.
D. Estrin et al, PIM-SM: Protocol Specification, RFC 2117, June 1997
S. Deering et al, PIM Version 2, Dense Mode Specification, work in progress, draft-ietf-idmr-pim-dm-05.txt
PIM is implemented in Cisco routers and has been deployed in UUnet as part of their streaming multimedia delivery effort. See S. LaPolla, “IP Multicast makes headway among ISPs,” PC Week On-Line,
3.4 Network Layer: Internet Multicast Routing Algorithms

149. Consequences of Sparse-Dense Dichotomy:
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
Notes:
3.4 Network Layer: Internet Multicast Routing Algorithms

150. 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
Notes:
3.4 Network Layer: Internet Multicast Routing Algorithms

151. 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 R4
join R2
join R5
join R3 R7 R6
Notes:
3.4 Network Layer: Internet Multicast Routing Algorithms
all data multicast
from rendezvous
point
rendezvous
point

152. 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
join R5
join R3 R7 R6
Notes:
3.4 Network Layer: Internet Multicast Routing Algorithms
all data multicast
from rendezvous
point
rendezvous
point

153. 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

154. What is mobility? spectrum of mobility, from the network perspective:
no mobility
high mobility
mobile user, using
same access point
mobile user, connecting/ disconnecting from network using DHCP.
mobile user, passing through multiple access point while maintaining ongoing connections (like cell phone)
Network Layer

155. Mobility: Vocabulary home network: permanent “home” of mobile
(e.g., /24)
home agent: entity that will perform mobility functions on behalf of mobile, when mobile is remote
wide area network
Permanent address: address in home network, can always be used to reach mobile
e.g.,
correspondent
Network Layer

156. Mobility: more vocabulary
visited network: network in which mobile currently resides (e.g., /24)
Permanent address: remains constant (e.g., )
Care-of-address: address in visited network.
(e.g., 79, )
wide area network
home agent: entity in visited network that performs mobility functions on behalf of mobile.
correspondent: wants to communicate with mobile
Network Layer

157. How do you contact a mobile friend:
I wonder where Alice moved to?
Consider friend frequently changing addresses, how do you find her?
search all phone books?
call her parents?
expect her to let you know where he/she is?
Network Layer

158. 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

159. 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

160. Mobility: registration
visited network
home network 1
mobile contacts foreign agent on entering visited network 2
foreign agent contacts home agent home: “this mobile is resident in my network”
wide area network
End result:
Foreign agent knows about mobile
Home agent knows location of mobile
Network Layer

161. Mobility via Indirect Routing
foreign agent receives packets, forwards to mobile
home agent intercepts packets, forwards to foreign agent
visited
network
home
network 3 4
wide area network 1 2
correspondent addresses packets using home address of mobile
mobile replies directly to correspondent
Network Layer

162. 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

163. Forwarding datagrams to remote mobile
dest:
foreign-agent-to-mobile packet
dest:
dest:
packet sent by home agent to foreign agent: a packet within a packet
Permanent address:
Care-of address:
dest:
packet sent by correspondent
Network Layer

164. 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

165. Mobility via Direct Routing
foreign agent receives packets, forwards to mobile
correspondent forwards to foreign agent
visited
network
home
network 4
wide area network 2 3 1 4
correspondent requests, receives foreign address of mobile
mobile replies directly to correspondent
Network Layer

166. 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

167. 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

168. Mobile IP: agent discovery
agent advertisement: foreign/home agents advertise service by broadcasting ICMP messages (typefield = 9)
H,F bits: home and/or foreign agent
R bit: registration required
Network Layer

169. Mobile IP: registration example
Network Layer

170. Network Layer: summary
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
Next stop:
the Data
link layer!
Network Layer