Routing
Routing protocol
Goal: determine “good” path
(sequence of routers) thru
network from source to dest.
5
2
A
Graph abstraction for
routing algorithms:
• graph nodes are routers
• graph edges are
physical links
– link cost: delay, $ cost, or
congestion level
B
2
1
D
3
C
3
1
5
F
1
E
2
• “good” path:
– typically means
minimum cost path
– other def’s possible
Routing Algorithm classification
Global or decentralized
information?
Global:
• all routers have complete
topology, link cost info
• “link state” algorithms
Decentralized:
• router knows physicallyconnected neighbors, link
costs to neighbors
• iterative process of
computation, exchange of
info with neighbors
• “distance vector” algorithms
Static or dynamic?
Static:
• routes change
slowly over time
Dynamic:
• routes change more
quickly
– periodic update
– in response to link
cost changes
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
• N: set of nodes whose least
cost path definitively known
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
Dijkstra’s algorithm: example
Step
0
1
2
3
4
5
start N
A
AD
ADE
ADEB
ADEBC
ADEBCF
D(B),p(B) D(C),p(C) D(D),p(D) D(E),p(E) D(F),p(F)
2,A
1,A
5,A
infinity
infinity
2,A
4,D
2,D
infinity
2,A
3,E
4,E
3,E
4,E
4,E
5
2
A
B
2
1
D
3
C
3
1
5
F
1
E
2
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
D
1
1
0
A
0 0
C
e
1+e
B
e
initially
2+e
D
0
1
A
1+e 1
C
0
B
0
… recompute
routing
0
D
1
A
0 0
2+e
B
C 1+e
… recompute
2+e
D
0
A
1+e 1
C
0
B
e
… recompute
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:
X
D (Y,Z)
distance from X to
= Y, via Z as next hop
= c(X,Z) + min {DZ(Y,w)}
w
Distance Table: example
7
A
B
1
C
E
cost to destination via
D ()
A
B
D
A
1
14
5
B
7
8
5
C
6
9
4
D
4
11
2
2
8
1
E
2
D
E
D (C,D) = c(E,D) + min {DD(C,w)}
= 2+2 = 4
w
E
D (A,D) = c(E,D) + min {DD(A,w)}
E
w
= 2+3 = 5
loop!
D (A,B) = c(E,B) + min {D B(A,w)}
= 8+6 = 14
w
loop!
Distance table gives routing table
E
cost to destination via
Outgoing link
D ()
A
B
D
A
1
14
5
A
A,1
B
7
8
5
B
D,5
C
6
9
4
C
D,4
D
4
11
2
D
D,4
Distance table
to use, cost
Routing table
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
Distance Vector Algorithm:
At all nodes, X:
1 Initialization:
2 for all adjacent nodes v:
3
D X(*,v) = infinity
/* the * operator means "for all rows" */
4
D X(v,v) = c(X,v)
5 for all destinations, y
6
send min D X(y,w) to each neighbor /* w over all X's neighbors */
w
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 X(y,V) = D X(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) */
w
20 /* call this received new value is "newval" */
21 for the single destination y: D X(Y,V) = c(X,V) + newval
22
23 if we have a new min DX(Y,w)for any destination Y
w
24
send new value of min D X(Y,w) to all neighbors
w
25
26 forever
Distance Vector Algorithm: example
X
2
Y
7
1
Z
Distance Vector Algorithm: example
X
2
Y
7
1
Z
X
Z
X
Y
D (Y,Z) = c(X,Z) + minw{D (Y,w)}
= 7+1 = 8
D (Z,Y) = c(X,Y) + minw {D (Z,w)}
= 2+1 = 3
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)
“good
news
travels
fast”
1
X
4
Y
50
1
Z
algorithm
terminates
Distance Vector: link cost changes
Link cost changes:
• good news travels fast
• bad news travels slow “count to infinity” problem!
60
X
4
Y
50
1
Z
algorithm
continues
on!
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
X
4
Y
50
1
Z
algorithm
terminates
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
Roadmap
• Routing in the Internet
– Routing algorithms
– Routing Protocols
• Intra-AS routing: RIP and OSPF
• Inter-AS routing: BGP
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
Internet AS Hierarchy
Inter-AS border (exterior gateway) routers
Intra-AS interior routers
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)
RIP ( Routing Information Protocol)
•
•
•
•
Distance vector algorithm
Included in BSD-UNIX Distribution in 1982
Distance metric: # of hops (max = 15 hops)
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
Dest
w
x
z
….
w
Next
C
…
hops
4
...
A
RIP:
Example
Advertisement
from A to D
x
D
z
B
y
C
Destination Network
w
y
z
x
….
Next Router
Num. of hops to dest.
….
....
A
B
B
--
Routing table in D
2
2
7
1
RIP: Example
Dest
w
x
z
….
Next
C
…
w
hops
4
...
A
Advertisement
from A to D
z
x
Destination Network
w
y
z
x
….
D
B
C
y
Next Router
Num. of hops to dest.
….
....
A
B
B A
--
Routing table in D
2
2
7 5
1
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)
RIP Table processing
• RIP routing tables managed by applicationlevel process called route-d (daemon)
• advertisements sent in UDP packets,
periodically repeated
routed
routed
Transprt
(UDP)
network
(IP)
link
physical
Transprt
(UDP)
forwarding
table
forwarding
table
network
(IP)
link
physical
RIP Table example (continued)
Router: giroflee.eurocom.fr
Destination
-------------------127.0.0.1
192.168.2.
193.55.114.
192.168.3.
224.0.0.0
default
Gateway
Flags Ref
Use
Interface
-------------------- ----- ----- ------ --------127.0.0.1
UH
0 26492 lo0
192.168.2.5
U
2
13 fa0
193.55.114.6
U
3 58503 le0
192.168.3.5
U
2
25 qaa0
193.55.114.6
U
3
0 le0
193.55.114.129
UG
0 143454
• Three attached 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)
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
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.
Hierarchical OSPF
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.
Chapter 4: Network Layer
• 4. 1 Introduction
• 4.2 Virtual circuit and
datagram networks
• 4.3 What’s inside a router
• 4.4 IP: Internet Protocol
–
–
–
–
Datagram format
IPv4 addressing
ICMP
IPv6
• 4.5 Routing algorithms
– Link state
– Distance Vector
– Hierarchical routing
• 4.6 Routing in the
Internet
– RIP
– OSPF
– BGP
• 4.7 Broadcast and
multicast routing
Internet inter-AS routing: BGP
• BGP (Border Gateway Protocol): the de facto
standard
• BGP provides each AS a means to:
1. Obtain subnet reachability information from
neighboring ASs.
2. Propagate the reachability information to all routers
internal to the AS.
3. Determine “good” routes to subnets based on
reachability information and policy.
• Allows a subnet to advertise its existence to
rest of the Internet: “I am here”
BGP basics
• Pairs of routers (BGP peers) exchange routing info over semipermanent TCP connections: BGP sessions
• Note that BGP sessions do not correspond to physical links.
• When AS2 advertises a prefix to AS1, AS2 is promising it will
forward any datagrams destined to that prefix towards the prefix.
– AS2 can aggregate prefixes in its advertisement
3c
3a
3b
AS3
1a
AS1
2a
1c
1d
1b
2c
AS2
2b
eBGP session
iBGP session
Distributing reachability info
• With eBGP session between 3a and 1c, AS3 sends prefix
reachability info to AS1.
• 1c can then use iBGP do distribute this new prefix reach info to
all routers in AS1
• 1b can then re-advertise the new reach info to AS2 over the 1bto-2a eBGP session
• When router learns about a new prefix, it creates an entry for the
prefix in its forwarding table.
3c
3a
3b
AS3
1a
AS1
2a
1c
1d
1b
2c
AS2
2b
eBGP session
iBGP session
external
internal
Path attributes & BGP routes
• When advertising a prefix, advert includes BGP
attributes.
– prefix + attributes = “route”
• Two important attributes:
– AS-PATH: contains the ASs through which the advert
for the prefix passed: AS 67 AS 17
– NEXT-HOP: Indicates the specific internal-AS router
to next-hop AS. (There may be multiple links from
current AS to next-hop-AS.)
• When gateway router receives route advert,
uses import policy to accept/decline.
BGP route selection
• Router may learn about more than 1
route to some prefix. Router must select
route.
• Elimination rules:
1. Local preference value attribute: policy
decision
2. Shortest AS-PATH
3. Closest NEXT-HOP router: hot potato routing
4. Additional criteria
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
BGP routing policy
legend:
B
W
provider
network
X
A
customer
network:
C
Y
Figure 4.5-BGPnew: a simple BGP scenario
• 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
BGP routing policy (2)
legend:
B
W
provider
network
X
A
customer
network:
C
Y
Figure 4.5-BGPnew: a simple BGP scenario
• 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!
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
Chapter 4: Network Layer
• 4. 1 Introduction
• 4.2 Virtual circuit and
datagram networks
• 4.3 What’s inside a router
• 4.4 IP: Internet Protocol
–
–
–
–
Datagram format
IPv4 addressing
ICMP
IPv6
• 4.5 Routing algorithms
– Link state
– Distance Vector
– Hierarchical routing
• 4.6 Routing in the
Internet
– RIP
– OSPF
– BGP
• 4.7 Broadcast and
multicast routing
Broadcast Routing
• Deliver packets from source to all other nodes
• Source duplication is inefficient:
duplicate
duplicate
creation/transmission
R1
R1
duplicate
R2
R2
R3
R4
source
duplication
R3
R4
in-network
duplication
• Source duplication: how does source
determine recipient addresses?
In-network duplication
• Flooding: when node receives brdcst pckt, sends
copy to all neighbors
– Problems: cycles & broadcast storm
• Controlled flooding: node only brdcsts pkt if it
hasn’t brdcst same packet before
– Node keeps track of pckt ids already brdcsted
– Or reverse path forwarding (RPF): only forward pckt if
it arrived on shortest path between node and source
• Spanning tree
– No redundant packets received by any node
Spanning Tree
• First construct a spanning tree
• Nodes forward copies only along spanning
tree
A
B
c
F
A
E
B
c
D
F
G
(a) Broadcast initiated at A
E
D
G
(b) Broadcast initiated at D
Spanning Tree: Creation
• Center node
• Each node sends unicast join message to
center node
– Message forwarded until it arrives at a node
already belonging to spanning tree
A
A
3
B
c
4
E
F
1
2
B
c
D
F
5
E
D
G
G
(a) Stepwise construction
of spanning tree
(b) Constructed spanning
tree
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
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
routers
forward unicast
datagrams
multicast receiver (red)
not a multicast receiver (red)
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
Multicast
routers (red) duplicate and
forward multicast datagrams
• Router actively participate
in multicast, making
copies of packets as
needed and forwarding
towards multicast
receivers
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
Internet Multicast Service Model
128.59.16.12
128.119.40.186
multicast
group
226.17.30.197
128.34.108.63
128.34.108.60
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
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
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
wide-area
multicast
routing
IGMP
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
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 groupspecific query to see if any
hosts left in group
– RFC 2236
IGMP v3: under development as
Internet draft
Approaches for building mcast
trees
Approaches:
• source-based tree: one tree per source
– shortest path trees
– reverse path forwarding
• group-shared tree: group uses one tree
– minimal spanning (Steiner)
– center-based trees
…we first look at basic approaches, then specific
protocols adopting these approaches
Shortest Path Tree
• mcast forwarding tree: tree of shortest
path routes from source to all receivers
– Dijkstra’s algorithm
S: source
LEGEND
R1
1
2
R4
R2
3
R3
router with attached
group member
5
4
R6
router with no attached
group member
R5
6
R7
i
link used for forwarding,
i indicates order link
added by algorithm
Reverse Path Forwarding
 rely on router’s knowledge of unicast
shortest path from it to sender
 each router has simple forwarding behavior:
if (mcast datagram received on incoming
link on shortest path back to center)
then flood datagram onto all outgoing
links
else ignore datagram
Reverse Path Forwarding: example
S: source
LEGEND
R1
R4
router with attached
group member
R2
R5
R3
R6
R7
router with no attached
group member
datagram will be
forwarded
datagram will not be
forwarded
• result is a source-specific reverse SPT
– may be a bad choice with asymmetric links
Reverse Path Forwarding:
pruning
• forwarding tree contains subtrees with no mcast
group members
– no need to forward datagrams down subtree
– “prune” msgs sent upstream by router with
no downstream group members
LEGEND
S: source
R1
router with attached
group member
R4
R2
P
R5
R3
R6
P
R7
P
router with no attached
group member
prune message
links with multicast
forwarding
Shared-Tree: Steiner Tree
• Steiner Tree: minimum cost tree connecting all
routers with attached group members
• problem is NP-complete
• excellent heuristics exists
• not used in practice:
– computational complexity
– information about entire network needed
– monolithic: rerun whenever a router needs to
join/leave
Center-based trees
• single delivery tree shared by all
• one router identified as “center” of tree
• to join:
– edge router sends unicast join-msg addressed to
center router
– join-msg “processed” by intermediate routers and
forwarded towards center
– join-msg either hits existing tree branch for this
center, or arrives at center
– path taken by join-msg becomes new branch of tree
for this router
Center-based trees: an example
Suppose R6 chosen as center:
LEGEND
R1
3
R2
router with attached
group member
R4
2
R5
R3
1
R6
R7
1
router with no attached
group member
path order in which join
messages generated
Internet Multicasting Routing: DVMRP
• DVMRP: distance vector multicast routing
protocol, RFC1075
• flood and prune: reverse path forwarding,
source-based tree
– RPF tree based on DVMRP’s own routing tables
constructed by communicating DVMRP routers
– no assumptions about underlying unicast
– initial datagram to mcast group flooded everywhere
via RPF
– routers not wanting group: send upstream prune
msgs
DVMRP: continued…
• soft state: DVMRP router periodically (2 hours)
“forgets” branches are pruned:
– mcast data again flows down unpruned branch
– downstream router: reprune or else continue to
receive data
• routers can quickly regraft to tree
– following IGMP join at leaf
• odds and ends
– commonly implemented in commercial routers
– Mbone routing done using DVMRP
Tunneling
Q: How to connect “islands” of multicast
routers in a “sea” of unicast routers?
physical topology
logical topology
 mcast datagram encapsulated inside “normal” (non-multicast-
addressed) datagram
 normal IP datagram sent thru “tunnel” via regular IP unicast to
receiving mcast router
 receiving mcast router unencapsulates to get mcast datagram
PIM: Protocol Independent Multicast
• not dependent on any specific underlying
unicast routing algorithm (works with all)
• two different multicast distribution scenarios :
Dense:
Sparse:
 group members
densely packed, in
“close” proximity.
 bandwidth more
plentiful
 # networks with group
members small wrt #
interconnected networks
 group members “widely
dispersed”
 bandwidth not plentiful
Consequences of Sparse-Dense
Dichotomy:
Dense
Sparse:
• group membership by • no membership until
routers assumed until
routers explicitly join
routers explicitly prune • receiver- driven
• data-driven
construction of mcast
construction on mcast
tree (e.g., centertree (e.g., RPF)
based)
• bandwidth and non• bandwidth and nongroup-router
group-router
processing profligate
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)
R1
– intermediate routers update
state and forward join
• after joining via RP, router
can switch to sourcespecific tree
– increased performance:
less concentration, shorter
paths
R4
join
R2
R3
join
R5
join
R6
all data multicast
from rendezvous
point
R7
rendezvous
point
PIM - Sparse Mode
sender(s):
• unicast data to RP,
which distributes
down RP-rooted tree
• RP can extend mcast
tree upstream to
source
• RP can send stop
msg if no attached
receivers
– “no one is listening!”
R1
R4
join
R2
R3
join
R5
join
R6
all data multicast
from rendezvous
point
R7
rendezvous
point