Transcript IP Multicast - Universitas Brawijaya

Multimedia Networking
#6 IP Multicast
Semester Ganjil 2012
PTIIK Universitas Brawijaya
Schedule of Class Meeting
1.
2.
3.
4.
Introduction
Applications of MN
Requirements of MN
Coding and
Compression
5. RTP
6. IP Multicast
7. IP Multicast (cont’d)
8. Overlay Multicast
9. CDN: Solutions
10. CDN: Case Studies
11. QoS on the Internet:
Constraints
12. QoS on the Internet:
Solutions
13. Discussion
14. Summary
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Today’s Outline
• IP Multicast
– Concept and components
– Addressing architecture
– IP Multicast Protocols
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IP Multicast
Multicast
• Many receivers
– Receiving the same content
• Applications
– Video conferencing
– Online gaming
– IP television (IPTV)
– Financial data feeds
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Iterated Unicast
• Unicast message to each recipient
• Advantages
– Simple to implement
– No modifications to network
• Disadvantages
– High overhead on sender
– Redundant packets on links
– Sender must maintain list of receivers
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IP Multicast
• Embed receiver-driven tree in network layer
– Sender sends a single packet to the group
– Receivers “join” and “leave” the tree
• Advantages
– Low overhead on the sender
– Effective use of network resources
• Avoids redundant network traffic
• Disadvantages
– Control-plane protocols for multicast groups
– Overhead of duplicating packets in the routers
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Multicast Communication
10 flows
of the same packet
1 flows
of a packet
sender
sender
Multicast
Unicast
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IP Multicast Communication
• Concept
– Multicast data sender sends the data only once, and
only the intended recipients (who want to receive the
data) receive the data
• IP multicast provides one-to-many or many-to-many
communication effectively
– Each data (i.e. multicast stream) is classified by
multicast address (and source address if SSM is used)
– Non-reliable communication (i.e. on top of UDP)
• IP multicast is basically applied to real-time applications
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Data Flow
• Data sender
– Sender sends data once
• Data receiver
– Receiver that has requested getting the data receives the data
• Multicast routers
– Router copies and forwards the data only toward the data receivers
Multicast
Router
Data
Data
Sender
Receiver
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Communication Flow
• Control messages
– Sender announces the session information or receivers discover the
session information
– Each receiver requests to start and stop receiving data by “join and
leave” operations
– Multicast routers maintains membership state by having reports
Multicast
Router
Routing
protocol
Multicast
Router
Query
Join/Leave
request
Announcement
Sender
Receiver
Discovery
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Multicast Addresses
• Multicast “group” defined by IP address
– Multicast addresses look like unicast addresses
– 224.0.0.0 to 239.255.255.255
• Using multicast IP addresses
– Sender sends to the IP address
– Receivers join the group based on IP address
– Network sends packets along the tree
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Example Multicast Protocol
• Receiver sends a “join” messages to the sender
– And grafts to the tree at the nearest point
A
B
c
D
F
E
G
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IP Multicast is Best Effort
• Sender sends packet to IP multicast address
– Loss may affect Amultiple receivers
B
c
D
F
E
G
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Terminologies
• Group address (or multicast address)
– Used for destination address
• Join and leave
– Data reception state requested by receiver hosts
• Join and prune
– Data reception state requested by routers
• (*,G) and (S,G)
– Notation of source address and group address in
join-and-leave (or join-and-prune) state
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Terminologies
• Scope
– Expected data distribution area
– Classified by multicast address or TTL
• TTL (Time To Live) or Hop limit
– Expected maximum hop count of each packet
• IIF and OIF
– IIF: Incoming interface from which data is received
– OIF: Outgoing interface to which data is sent
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Terminologies
• Multicast session
– Multicast data stream classified by the “multicast
address” is called “multicast session”
• Multicast channel
– Multicast data stream explicitly classified by the
pair of “multicast address” and “source address” is
called “multicast channel”
• Used for SSM
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Multicast Protocols
Multicast
Router
Multicast
Router
Query
Sender
host
Join/Leave
request
Receiver
host
• Host-to-Router Protocols
– IGMPv1, IGMPv2, IGMPv3, MLDv1, MLDv2
• Router-to-Router Protocols
– DVMRP, MOSPF, CBT, PIM-DM, PIM-SM, PIM-SSM
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Multicast Address Assignment
IP Multicast
• Simple to use in applications
– Multicast “group” defined by IP multicast address
• IP multicast addresses look similar to IP unicast addrs
• 224.0.0.0 to 239.255.255.255 (RPC 3171)
– 265 M multicast groups at most
– Best effort delivery only
• Sender issues single datagram to IP multicast address
• Routers delivery packets to all subnetworks that have a
receiver “belonging” to the group
• Receiver-driven membership
– Receivers join groups by informing upstream routers
– Internet Group Management
Protocol (v3: RFC 3376)
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IP Multicast Address
• IP multicast address
–
–
–
–
IPv4: 224.0.0.0 – 239.255.255.255
IPv6: FFx0::1
MUST be specified as a destination address
MUST NOT be specified as a source address
• Dynamic address assignment
– Regular applications select their multicast addresses
dynamically
• Some multicast addresses are assigned by IANA for special
uses
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IPv4 Multicast Addresses
• IPv4 multicast address
– 224.0.0.0 (0xe0000000) - 239.255.255.255 (0xefffffff)
+----+----+---------+------------+
| 4 |
28 bits
|
+----+----+---------+------------+
|1110|
group address
|
+----+----+---------+------------+
– Administrative scope [RFC2365]
– Local address (224/24)
– Administrative scope (239/8)
– Organization-Local (239.192/14)
– GLOP address (233/8) [RFC3180]
– EGLOP (233.252.0.0 - 233.255.255.255) [RFC3138]
– SSM address (232/8) [RFC]
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IPv6 Multicast Addresses
• IPv6 multicast address: FFxx::
+--------+----+----+---------------------------------------------+
|
8
| 4 | 4 |
112 bits
|
+--------+----+----+---------------------------------------------+
|11111111|flgs|scop|
group ID
|
+--------+----+----+---------------------------------------------+
– Flags
• 000T (T=1: transient, T=0: well-known)
– Scope
•
•
•
•
•
•
•
0x1: Interface Local
0x2: Link-Local
0x3: Subnet-Local
0x4: Admin-Local
0x5: Site-Local
0x8: Organization-Local
0xE: Global
– SSM address (FF3x::/32 (or 96)) [RFC3306]
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Host-to-Router
Multicast Protocols
IGMP v1
• Two types of IGMP msgs (both have IP TTL of 1)
– Host membership query: Routers query local networks to
discover which groups have members
– Host membership report: Hosts report each group (e.g.,
multicast addr) to which belong, by broadcast on net
interface from which query was received
• Routers maintain group membership
– Host senders an IGMP “report” to join a group
– Multicast routers periodically issue host membership query
to determine liveness of group members
– Note: No explicit “leave” message from clients
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IGMP: Improvements
• IGMP v2 added:
– If multiple routers, one with lowest IP elected querier
– Explicit leave messages for faster pruning
– Group-specific query messages
• IGMP v3 added:
– Source filtering: Join specifies multicast “only from”
or “all but from” specific source addresses
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IGMP: Parameters and Design
• Parameters
– Maximum report delay: 10 sec
– Membership query internal default: 125 sec
– Time-out interval: 270 sec = 2 * (query interval + max delay)
• Is a router tracking each attached peer?
– No, only each network, which are broadcast media
• Should clients respond immediately to queries?
– Random delay (from 0..D) to minimize responses to queries
– Only one response from single broadcast domain needed
• What if local networks are layer-2 switched?
– L2 switches typically broadcast multicast traffic out all ports
– Or, IGMP snooping (sneak peek into layer-3 contents),
Cisco’s proprietary protocols, or static forwarding tables
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Router-to-Router
Multicast Protocols
Multicast Tree
A
B
c
D
F
E
G
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Multicast routing: problem statement
goal: find a tree (or trees) connecting routers having
legend
local mcast group members
• tree: not all paths between routers used
• shared-tree: same tree used by all group members

source-based: different tree from each sender to rcvrs
group
member
not group
member
router
with a
group
member
router
without
group
member
shared tree
source-based trees
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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
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Single vs. Multiple Senders
• Source-based tree
– Separate tree for
each sender
– Tree is optimized for
that sender
– But, requires
multiple trees for
multiple senders
• Shared tree
– One common tree
– Spanning tree that
reaches all
participants
– Single tree may be
inefficient
– But, avoids having
many different trees
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Shortest path tree
• mcast forwarding tree: tree of shortest path
routes from source to all receivers
– Dijkstra’s algorithm
LEGEND
s: source
R1
1
2
R2
3
router with attached
group member
R4
5
4
R3
R6
router with no attached
group member
R5
6
R7
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link used for forwarding,
i indicates order link
added by algorithm
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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
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Reverse path forwarding: example
s: source
LEGEND
R1
R4
router with attached
group member
R2
R5
router with no attached
group member
datagram will be forwarded
R3
R6
R7
datagram will not be
forwarded

result is a source-specific reverse SPT
 may be a bad choice with asymmetric links
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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
router with no attached
group member
P
R5
R3
P
R6
P
prune message
links with multicast
forwarding
R7
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Internet Multicasting Routing: DVMRP
• DVMRP: distance vector multicast routing
protocol, RFC1075
• flood and prune: reverse path forwarding, sourcebased 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
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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 router
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Tunneling
Q: how to connect “islands” of multicast
routers in a “sea” of unicast routers?
physical topology



logical topology
mcast datagram encapsulated inside “normal” (nonmulticast-addressed) datagram
normal IP datagram sent thru “tunnel” via regular IP unicast
to receiving mcast router (recall IPv6 inside IPv4 tunneling)
receiving mcast router unencapsulates to get mcast
datagram
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PIM: Protocol Independent Multicast
• not dependent on any specific underlying unicast
routing algorithm (works with all)
• two different multicast distribution scenarios :
sparse:
dense:


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
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Consequences of sparse-dense dichotomy:
dense
sparse:
• group membership by
• no membership until routers
routers assumed until
explicitly join
routers explicitly prune
• receiver- driven construction
• data-driven construction on
of mcast tree (e.g., centermcast tree (e.g., RPF)
based)
• bandwidth and non-group- • bandwidth and non-grouprouter processing profligate
router processing conservative
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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

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PIM - sparse mode
• explicit join and prune:
center-based approach, SPT
to the source
• 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
R6
all data multicast
from rendezvous
point
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rendezvous
point
43
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
R1
R4
join
R2
join
R5
join
R3
R6
all data multicast
from rendezvous
point
R7
rendezvous
point
– “no one is listening!”
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Conclusion of Today’s Lecture
• IP Multicast runs on top of UDP under best-effort
IP network
– suitable for real-time applications
• IP Multicast is efficient-use of network resources
– suitable for one-many or many-many communication
• IP Multicast requires
– specific address assignment
– host-to-router protocols
– router-to-router protocols
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