Transcript Presentation3

Department of Computer and IT Engineering
University of Kurdistan
Computer Networks II
Multicasting
By: Dr. Alireza Abdollahpouri
‫‪Multicast vs. Unicast‬‬
‫یکی از مهمترین دالیل استفاده از چندپخش ی‪ ،‬صرفه‬
‫جویی در پهنای باند است‬
‫‪2‬‬
IP multicast
DVMRP,PIM, MOSPF
Multicast routing
protocols
IGMP
Group management
protocol
3
‫‪Some applications of multicasting‬‬
‫•‬
‫•‬
‫•‬
‫•‬
‫•‬
‫•‬
‫‪4‬‬
‫دسترس ی به پایگاه داده های توزیع شده‬
‫انتشار اطالعات و اخبار‬
‫‪Teleconferencing‬‬
‫آموزش از راه دور‬
‫‪IPTV‬‬
‫بازيهای چندنفره‬
Multicast Applications
ConferenceXP: An Example of Multicast
application
Video Conference
Distance Learning
5
Multicast Issues
 Naming and addressing (IP class D)
‫(نحوه‬
)‫آدرسدهی‬
 Membership Management (IGMP)
)‫(نحوه مدیریت گروههای چندپخش ی‬
 Routing )DVMRP, PIM, MOSPF, …(
)‫(نحوه هدایت بسته های چندپخش ی‬
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
)‫(چندپخش ی در الیه شبکه‬
7
Note:
In network-layer multicasting, the
router may forward the received
packet through several of its
interfaces.
8
Multicast: one sender to many receivers
Application-layer multicast
 end systems involved in
multicast copy and
forward unicast datagrams
among themselves
)‫(چندپخش ی در الیه کاربرد‬
9
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
10
‫تحویلبستههایچندپخش یدرالیهپیوندداده‬
‫‪11‬‬
Example
Change the multicast IP address 230.43.14.7 to an Ethernet multicast physical
address.
Solution
We can do this in two steps:
1- We write the rightmost 23 bits of the IP address in hexadecimal. This can
be done by changing the rightmost 3 bytes to hexadecimal and then
subtracting 8 from the leftmost digit if it is greater than or equal to 8. In our
example, the result is 2B:0E:07.
2- We add the result of part a to the starting Ethernet multicast address,
which is 01:00:5E:00:00:00. The result is:
12
Group management protocol (IGMP)
DVMRP,PIM, MOSPF
Multicast routing
protocols
IGMP
Group management
protocol
13
Note:
IGMP is a group management
protocol. It helps a multicast router
create and update a list of loyal
members related to each router
interface.
14
IGMP message types
15
‫‪Encapsulation of IGMP packet‬‬
‫بسته بندی پیغامهای ‪ ICMP‬در قالب بسته های ‪ IP‬و سپس فریمهای الیه پایینتر‬
‫‪16‬‬
‫‪IGMP message format‬‬
‫حداکثر زمان پاسخ (بر حسب ‪ 1/0‬ثانیه) در پیغامهای ‪Query‬‬
‫‪8 bytes‬‬
‫‪17‬‬
IGMP type field
Type
Value
General or special query
0x11 or 00010001
Membership report
0x16 or 00010110
Leave report
0x17 or 00010111
18
IGMP operation
19
Membership report
20
Leave report
No Response
21
Note:
The general query message does not
define a particular group.
22
General query message
No Response
23
Example
Imagine there are three hosts in a network, as shown in Figure
below. A query message was received at time 0; the random delay
time (in tenths of seconds) for each group is shown next to the
group address. Show the sequence of report messages.
24
Solution
The events occur in this sequence:
1. Time 12. The timer for 228.42.0.0 in host A expires and a
membership report is sent, which is received by the router and every
host including host B which cancels its timer for 228.42.0.0.
2. Time 30. The timer for 225.14.0.0 in host A expires and a
membership report is sent, which is received by the router and every
host including host C which cancels its timer for 225.14.0.0.
3. Time 50. The timer for 251.71.0.0 in host B expires and a
membership report is sent, which is received by the router and every
host.
4. Time 70. The timer for 230.43.0.0 in host C expires and a
membership report is sent, which is received by the router and every
host including host A which cancels its timer for 230.43.0.0.
25
Multicast routing protocols
DVMRP,PIM, MOSPF
Multicast routing
protocols
IGMP
Group management
protocol
26
Note:
In unicast routing, each router in the
domain has a table that defines a
shortest path tree to possible
destinations.
27
Shortest path tree in unicast routing
28
Note:
In multicast routing, each involved
router needs to construct a tree for
each group.
29
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
30
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
Shortest Path Tree
 mcast forwarding tree: tree of shortest path
routes from source to all receivers
 Dijkstra’s algorithm
LEGEND
S: source
R1
1
R2
3
R3
2
4
R6
router with attached
group member
R4
router with no attached
group member
5
6
R7
R5
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
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
Multicast Forwarding
A closer look: RPF Check Fails
X
Multicast Packet from
Source 151.10.3.21
S0
RPF Check Fails!
Unicast Route Table
Network
Interface
151.10.0.0/16 S1
198.14.32.0/24 S0
204.1.16.0/24
E0
S1
S2
E0
Packet Arrived on Wrong Interface!
Discard Packet!
Multicast Forwarding
A closer look: RPF Check Succeeds
Multicast Packet from
Source 151.10.3.21
S0
S1
RPF Check Succeeds!
Unicast Route Table
Network
Interface
151.10.0.0/16 S1
198.14.32.0/24 S0
204.1.16.0/24
E0
S2
E0
Packet Arrived on Correct Interface!
Forward out all outgoing interfaces.
(i. e. down the distribution tree)
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
S: source
R1
R4
R2
R3
router with attached
group member
P
R6
P
R7
R5
P
router with no attached
group member
prune message
links with multicast
forwarding
Problem with RPF
38
‫‪RPF versus RPB‬‬
‫برای اینکه هر شبکه یک کپی از بسته چندپخش ی را دریافت کند‬
‫برای آن یک روتر به عنوان والد تعریف می کنیم‪.‬‬
‫‪39‬‬
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
3
R2
R3
R4
2
1
R6
router with attached
group member
R7
R5
1
router with no attached
group member
path order in which join
messages generated
Group-shared tree with rendezvous router
42
Sending a multicast packet to the rendezvous
router
43
Note:
In CBT, the source sends the multicast
packet (encapsulated in a unicast packet)
to the core router.
The core router decapsulates the packet
and forwards it to all interested
interfaces.
44
Taxonomy of common multicast protocols
45
DVMRP




Distance Vector Multicast Routing Protocol
DV + RPF + Pruning
DV vector carries distance to multicast sources
Pruning carries a timeout (prune lifetime)
 Afterwards, traffic delivery is resumed
 Explicit graft message to reverse pruning
 Done upon join
‫ برای ایجاد شاخه جدید در درخت‬Graft ‫ جهت هرس کردن درخت و‬Prune
MOSPF
 Multicast Extensions to OSPF
 Link-state advertisements include multicast
group membership
 Only report directly connected hosts
 Compute shortest-path spanning tree rooted
at source
 On demand, when receiving packet from source
for the first time
 Forward multicast traffic along tree
PIM
Protocol Independent Multicast (PIM) is the
name given to two independent multicast
routing protocols:
Protocol Independent Multicast, Dense Mode
(PIM-DM) and
Protocol Independent
Mode (PIM-SM).
Multicast,
Sparse
48
Note:
PIM-DM is used in a dense multicast
environment, such as a LAN.
49
Note:
PIM-DM uses RPF and
pruning/grafting strategies to handle
multicasting.
However, it is independent from the
underlying unicast protocol.
50
Note:
PIM-SM is used in a sparse multicast
environment such as a WAN.
PIM-SM is similar to CBT but uses a
simpler procedure.
51
MBONE
A multicast router may not find another
multicast router in the neighborhood to
forward the multicast packet. A solution
for this problem is tunneling. We make a
multicast backbone (MBONE) out of these
isolated routers using the concept of
tunneling.
52
Logical tunneling
53
MBONE
54
Questions
55