ahmed helmy - uf1 ip-multicast (outline) -motivation and background -multicast vs. unicast...

Ahmed Helmy - UF 1

IP-Multicast (outline)- Motivation and Background

- Multicast vs. unicast

- Multicast Applications

- Delivery of Multicast- Local delivery and multicast addressing

- WAN delivery and its model

- Group Membership Protocol (IGMP)

- Multicast Algorithms and Concepts- Flooding, Spanning Tree, Reverse Path Broadcasting (RPB),

Truncated RPB, Reverse Path Multicasting, Center-Based Trees

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- Multicast Routing Protocols- Dense vs. Sparse Multicast- DVMRP- MOSPF- PIM (PIM-DM, PIM-SM)

- Multicast and the Internet- The MBONE- Recent deployment

Outline (Contd.)

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Unicast vs. Multicast

• Multicast provides multipoint-to-multipoint communication

• Today majority of Internet applications rely on point-to-point transmission (e.g., TCP).

• IP-Multicast conserves bandwidth by replicating packets in the network only when necessary

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Unicast vs. MulticastS

R1

R2

R3R4

S

R1

R2

R3R4

Multiple unicasts Multicast

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Example Multicast Applications

• One-to-Many– Scheduled audio/video distribution: lectures,

presentations– Push media: news headlines, weather updates– Caching: web site content & other file-based updates

sent to distributed replication/caching sites– Announcements: network time, configuration updates– Monitoring: stock prices, sensor equipment

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IP Multicast Applications (contd.)

• Many-to-One– Resource discovery

– Data collection and sensing

– Auctions

– Polling

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– Many-to-Many

• Multimedia Teleconferencing (audio, video, shared whiteboard, text editor)

• Collaboration

• Multi-Player Games

• Concurrent Processing

• Chat Groups

• Distributed Interactive Simulation

IP Multicast Applications (contd.)

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- Currently: The Multicast Backbone (MBONE) carries audio and video multicasts of IETF meetings, NASA space shuttle missions,.. etc.

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- How do hosts know about new groups?- The Session Directory (SD) tool lists active

multicast sessions on MBONE and allows to join a conference using MBONE tools:- vat (visual audio tool), rat (robust audio tool)

- vic (video tool)

- wb (shared white board)

- nte (network text editor), .. etc.

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• Resource Discovery– Multicast may be used (instead of broadcast) to

transmit to group members on the same LAN.

– Multicast may be used for resource discovery within a specific scope using the TTL field in the IP header.

More Applications ...

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Multicast Scope Control:TTL Expanding-Ring Search

to reach or find a nearby subset of a group

s

1

2

3

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Multicast Scope Control: Administrative TTL Boundaries

to keep multicast traffic within an administrative domain, e.g., for privacy reasons

an administrative domain

TTL threshold set oninterfaces to these links,greater than the diameterof the admin. domain

the rest of the Internet

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Multicast Scope Control: Administratively-Scoped Addresses

an administrative domain

address boundary set oninterfaces to these links

the rest of the Internet

– RFC 1112

– uses address range 239.0.0.0 — 239.255.255.255

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• Over the same (LAN): – The source addresses the IP packet to the multicast

group

– The network interface card maps the Class D address to the corresponding IEEE-802 multicast address

– Receivers notify their IP layer to receive datagrams addressed to the group.

– Key issue is ‘addressing’ & filtration

Transmission and Delivery of Multicast Datagrams

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• Over different subnets:– Routers implement a multicast routing protocol

that constructs the multicast delivery trees and supports multicast data packet forwarding.

– Routers implement a group membership protocol to learn about the existence of group members on directly attached subnets.

– Hosts implement the group membership protocol that provides the ‘IP-multicast host model’

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Addressing

• Types of IP addresses: – Unicast: used to transmit packets to one destination.

– Broadcast: used to send datagrams to entire subnet.

– Multicast: used to deliver datagrams to a set of hosts (members of a multicast group) in various scattered subnets.

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• IP-Multicast is a “best-effort” service. – Reliable/ordered delivery are not guaranteed.

– Reliability may be provided by upper-layer protocols (e.g., reliable multicast protocols).

• IP-Multicast packets include a "group address" (Class D) in the Destination field of the IP header.

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Multicast Addressing

• An IP multicast group is identified by a Class D address.

• Multicast group addresses range from (224.0.0.0) to (239.255.255.255).

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• The Internet Assigned Numbers Authority (IANA) registers IP multicast groups.

• The block of multicast addresses ranging from (224.0.0.1) to (224.0.0.255) is reserved for local LAN multicast: – used by routing protocols and other low-level topology

discovery or maintenance protocols

– E.g., "all-hosts" group (224.0.0.1), "all-routers” group (224.0.0.2), "all DVMRP routers", etc.

• The range (239.0.0.0) to (239.255.255.255) are used for site-local "administratively scoped" applications.

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• All multicast addresses in IANA's reserved block begin with 01-00-5E (hex)

• Mapping between a Class D and an Ethernet multicast address is obtained by:– placing the low-order 23 bits of the Class D

address into the low-order 23 bits of IANA's reserved address block.

Mapping Class D to Ethernet Address

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– How multicast group address 224.10.8.5 (E0-0A-08-05) is mapped into an Ethernet (IEEE-802) multicast address.

• The mapping may place up to 32 different IP groups into the same Ethernet address because the upper five bits of the IP multicast group ID are ignored.

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• Hosts can join or leave a group at any time

• A host may be a member of multiple groups

• Senders need not be members of the group

• Participants do not know about each other

• The two components of IP-multicast: – the group membership protocol

– the multicast routing protocol

The Multicast Host Model

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• Routers need to learn about the presence of group members on directly attached subnets

• When a host joins a group:– it transmits a group membership message for

the group(s) that it wishes to receive

– sets its IP process and network interface card to receive packets sent to those groups.

Group Membership Protocol

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• This receiver-initiated join process scales well:– as the group size increases, it becomes more

likely for a new member to locate a nearby branch of the multicast distribution tree.

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• Multicast Routing Protocols– Run on routers and establish the multicast distribution tree

to forward packets from sender(s) to group members.

• Based on unicast routing concepts:– DVMRP is a distance-vector routing protocol,

– MOSPF is an extension to the OSPF link-state unicast routing protocol.

• Center-based trees (e.g., CBT & PIM-SM) introduce the notion of the tree ‘core’.

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Internet Group Management Protocol

(IGMP)• IGMP runs between hosts and their

immediately neighboring multicast routers.

• The protocol allows a host to inform its ‘first-hop’ router that it wishes to receive packets destined to a specific group.

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• Routers periodically query the LAN to determine if group members are still active.

• One router per LAN is elected as "querier" to query for group members.

• Through IGMP a router determines which multicast traffic needs to be forwarded to each of its "leaf" subnets.

Router Operation in IGMP

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IGMP Version 1

• RFC-1112

• To determine local group membership:– Multicast routers periodically transmit ‘Host

Membership Query’ messages

– Queries are ddressed to the all-hosts group (224.0.0.1) with TTL = 1 (i.e., not forwarded by any other multicast router).

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• Upon receiving a Query, a host responds with a ‘Host Membership Report’ for each group that it wishes to Join

• Observation: The router only needs to know of at least one group member on the leaf subnet

Hosts Joining Groups

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Report Suppression Mechanism

• To avoid Report implosion:– Each host starts a random delay timer for its Reports. – If during the delay period another Report is heard for

the same group, the host resets its timer – Otherwise, the host transmits a Report causing other

group members to reset their timers

• Thus, Reports are spread out over time and Report traffic is minimized

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– IGMP-Query Message

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• The querier periodically transmits Queries to update local membership

• If no Report is received for a group after a number of Queries, the router assumes that members are no longer present on that LAN– the group is removed from the membership list

of that interface/subnet

Updating Local Membership

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Reducing Join Latency

• When a host first joins a group, it immediately transmits a Report for the group rather than waiting for a router Query.

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• IGMP V2 was part of IP-mcast (V3.3-3.8)– spec <draft-ietf-idmr-igmp-v2-01.txt>

• IGMP V2 enhances IGMP V1 – IGMP V2 elects one querier for each LAN, the

router with the lowest IP address.

– In V1, the querier election was done by the multicast routing protocol (different multicast routing protocol used different methods).

IGMP Version 2 (IGMP V2)

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• IGMP V2 defines a new Query message, the ‘Group-Specific Query’, to Query a specific group rather than all groups

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• To reduce ‘leave latency’ V2 defines a ‘Leave Group’ message – When a host leaves a group, it sends a ‘Leave Group’ to the

all-routers group (224.0.0.2) with the group field set to the group to be left.

– Upon receiving a Leave from a LAN, the querier sends Group-Specific Query on that LAN.

– If there are no Reports in response to the Group-Specific Query, the group is removed from the membership list of that subnet.

Reducing Leave Latency

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• Observation: With IGMP V1 and V2, if a host wants to receive any sources from a group, the traffic from all sources for the group has to be forwarded onto the subnet.

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• Spec: <draft-ietf-idmr-igmp-v3-03.txt>

• IGMP V3 supports Group-Source Reports: – A host can elect to receive traffic from specific

sources of a multicast group.

– An inclusion Group-Source Report specifies the sources a host wants to receive.

– An exclusion Group-Source Report identifies the sources a host does not want to receive.

IGMP Version 3 (IGMP v3)

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• IGMP v3 enhances support for Leave Group messages to support ‘Group-Source’ Leave messages:– A host can leave an entire group or specific

(source, group) pair(s).

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Multicast Forwarding Algorithms

• A multicast routing protocol is responsible for the establishment of the multicast distribution tree and for performing packet forwarding.

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• Several algorithms may be employed by multicast routing protocols: Flooding

Spanning Trees

Reverse Path Broadcasting (RPB)

Truncated Reverse Path Broadcasting (TRPB)

Reverse Path Multicasting (RPM)

Core-Based Trees

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• These algorithms are implemented in the most prevalent multicast routing protocols in the Internet today. Distance Vector Multicast Routing Protocol

(DVMRP)

Multicast OSPF (MOSPF)

Protocol-Independent Multicast (PIM) [PIM-DM and PIM-SM]

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Flooding

• The simplest technique for multicast delivery.

• When a router receives a multicast packet it determines whether or not this is the first time it has seen this packet. – On first reception, a packet is forwarded on all

interfaces except the one on which it arrived.

– If the router has seen the packet before, it is discarded.

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• A router does not maintain a routing table, but needs to keep track of recently seen packets.

• Flooding does not scale for Internet-wide application: - Generates a large number of duplicate packets and

uses all available paths across the internetwork.

- Routers maintain a distinct table entry for each recently seen packet (consumes memory).

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Spanning Tree • More effective than flooding

• Defines a tree structure where one active path connects any two routers on the Internet.

• Spanning Tree rooted at R

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• A router forwards each multicast packet to interfaces that are part of the spanning tree except the receiving interface.

• A spanning tree avoids looping of multicast packets and reaches all routers in the network.

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• A spanning tree algorithm is easy to implement

• However, a spanning tree solution:– may centralize traffic on small number of links

– may not provide the most efficient path between the source and the group members.

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Reverse Path Broadcasting (RPB)

• More efficient than building a single spanning tree for the entire Internet.

• Establishes source-rooted distribution trees for every source subnet.

• A different spanning tree is constructed for each active (source, group) pair.

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RPB Algorithm• For each (source, group) pair

– if a packet arrives on a link that the router considers to be the shortest path back to the source of the packet

• then the router forwards the packet on all interfaces except the incoming interface.

– Otherwise, the packet is discarded.

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• The interface over which a router accepts multicast packets from a particular source is called the "parent" link.

• The outbound links over which a router forwards the multicast packets are called the "child" links.

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• Reverse Path Broadcasting (RPB) Forwarding

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• Enhancement to reduce packet duplication:– A router determines if a neighboring router

considers it to be on the shortest path back to the source.

– If Yes, the packet is forwarded to the neighbor.

– Otherwise, the packet is not forwarded on that potential child link.

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• To derive the parent-child information:– link-state routing protocol already has it (since

each router maintains a topological database for the entire routing domain).

– distance-vector routing protocol uses ‘poison reverse’:

• a neighbor can either advertise its previous hop for the source subnet as part of its routing update messages or "poison reverse" the route.

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• Example of Reverse Path Broadcasting

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Benefits

• Reasonably efficient and easy to implement.

• Does not require keeping track of previous packets, as flooding does.

• Multicast packets follow the "shortest" path from the source to the group members.

• Avoids concentration over single spanning tree

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Limitations

• Does not take into account group membership when building the distribution tree.

• As a result, packets may be unnecessarily forwarded to subnets with no group members.

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Truncated Reverse Path Broadcasting (TRPB)

• Using IGMP, routers discover group members and avoid forwarding packets onto leaf subnets with no members.

• The spanning delivery tree is "truncated" if a leaf subnet has no group members.

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• Truncated Reverse Path Broadcasting (TRPB)

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• TRPB eliminates unnecessary traffic on leaf subnets

• But it does not consider group membership when building the branches of the distribution tree.

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Reverse Path Multicasting (RPM)

• RPM enhances TRPB.

• RPM creates a delivery tree that spans only: -Subnets with group members-Routers and subnets along the shortest path to group members

• In RPM, non-member branches are pruned

• Packets are forwarded only along branches leading to group members.

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RPM Operation

• The first multicast packet is forwarded (using TRPB) to all routers in the network.

• Routers at edges of the network with no downstream routers are called ‘leaf’routers.

• A leaf router with no downstream members sends a "prune" message on its parent link to stop packet flow down that branch.

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• Prune messages are sent hop-by-hop back toward the source.

• A router receiving a prune message stores the prune state in memory.

• A router with no local members that receives prunes on all child interfaces sends a prune one hop back toward the source.

• This succession of prune messages creates a multicast forwarding tree that contains only branches that lead to group members.

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• Reverse Path Multicasting (RPM)

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• To adapt to membership/network dynamics, the prune state is timed out periodically, and packets are broadcast throughout the network.

• This may result in a burst of prune messages.

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Limitations

• Despite improvements over RPM, there are scaling issues and limitations:– Multicast packets are periodically forwarded to

every router in the network.

– Routers maintain prune state off-tree for all (source,group) pairs.

• These limitations are amplified with increase in number of sources and groups.

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Center/Core-Based Trees (CBT)

• Earlier algorithms build source-based trees

• CBT builds a single delivery tree (rooted at the core) that is shared by all group members.

• Multicast traffic for each group is sent and received over the shared tree, regardless of the source.

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• A core-based tree involves one or more cores in the CBT domain.

• Each leaf-router of a group sends a hop-by-hop "join" message toward the "core tree" of that group.

• Routers need to know the group core to send the join request.

CBT Operation

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• Intermediate routers process the join request:– The interface on which the join was received is added to the

delivery tree.

• Intermediate routers forward join requests toward the core until the join reaches a core or a router on the distribution tree.

• Senders unicast their packets toward the core.

• When the unicast packet reaches a member of the delivery tree, the packet is multicast to all outgoing interfaces that are part of the tree.

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Benefits

• Advantages over RPM, in terms of scalability: – A router maintains state information for each

group, not for each (source, group) pair.

– Multicast packets only flow down branches leading to members (not periodically broadcast).

– Only join state is kept on-tree

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Limitations• CBT may result in traffic concentration near the

core since traffic from all sources traverses the same set of links as it approaches the core.

• A single shared delivery tree may create sub-optimal routes resulting in increased delay.

• Core management issues– dynamic core selection

– core placement strategies

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Multicast Routing Protocols

• In general, there are two classes of multicast routing protocols:– Dense-mode protocols (broadcast-and-prune)

• DVMRP, PIM-DM, (MOSPF!)

– Sparse-mode protocols (explicit-join)• PIM-SM, CBT, BGMP

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Dense vs. Sparse Mode Multicast

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R2

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Dense-Mode Multicast

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Dense vs. Sparse Mode Multicast

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R2

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Dense-Mode Multicast Sparse-Mode Multicast

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Distance Vector Multicast Routing Protocol (DVMRP)

• DVMRP constructs source-rooted trees using variants of RPM.

• Many MBONE routers run DVMRP

• DVMRP was first defined in RFC-1075.– The original spec was derived from the Routing

Information Protocol (RIP) and used TRPB.

– Mrouted version 3.8 uses RPM.

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DVMRP Basic Operation

• DVMRP implements RPM.

• The first packet for any (source, group) pair is broadcast to the entire network, providing the packet's TTL permits.

• Leaf routers with no local members send prune messages back toward the source.

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• Prune messages cause the removal of branches that do not lead to group members

• The result is source-specific shortest path tree with all leaves having group members.

• After a period of time, the pruned branches grow back and the packets are broadcast throughout the network.

• A “graft” mechanism helps to quickly re-establish previously pruned branches.

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• A new member joining the group causes the first-hop router to send a graft message to the group's previous-hop router.

• When an upstream router receives a graft message, it removes the prune state.

• Graft messages may cascade back toward the source allowing previously pruned branches to be restored.

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Example DVMRP Scenario

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s

g

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Initial Broadcast using Truncated Broadcast

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s

g

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Prune non-member branches

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prune (s,g)

prune (s,g)

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graft (s,g)

graft (s,g)

Graft new membersg g

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report (g)

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DVMRP Distribution Treeg g

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Avoiding duplicates on LANs

• To avoid duplicates, one router per LAN is elected the Dominant Router

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• The router with lowest metric to the source subnet (with the lowest IP address as tie breaker) becomes the Dominant router

• A dominant router is ‘the’ forwarder for the LAN for traffic from the source subnet

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• Entries in a typical DVMRP forwarding table: – Source Subnet

– Multicast Group

– InPort - The parent port for the (S, G) pair. A "Pr" indicates that a prune was sent to upstream.

– OutPorts - The child ports over which packets for the (S, G) pair are forwarded. A ‘p’ indicates prune message received on that interface.

DVMRP Forwarding Table

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DVMRP Forwarding Table

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Multicast Extensions to OSPF (MOSPF)

• OSPF V2 is defined in RFC-1583.

• OSPF is a unicast routing protocol that distributes topology information and calculates routes for a single domain.

• MOSPF is defined in RFC-1584.

• MOSPF routers maintain a current image of the network topology through the unicast OSPF link-state routing protocol.

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• MOSPF does not support tunnels

• Basic MOSPF runs in a single OSPF domain

• MOSPF uses IGMP to discover members on directly attached subnets.

• The Designated Router (DR) is responsible for sending membership information to all routers in the OSPF domain.

• The DR floods Group-Membership Link State Advertisements (LSAs) throughout the OSPF domain.

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• The shortest path tree for (S, G) pair is built "on demand" when a router receives the first packet for (S,G).

• When the initial packet arrives, the source subnet is located in MOSPF link state database. – MOSPF LS-DB = OSPF LS-DB + Group-Membership LSAs

Building the Shortest Path Tree

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• Source-rooted shortest-path tree is constructed using Dijkstra's algorithm.

• To forward packets to downstream members, each router determines its position in the shortest path tree

• After the tree is built, Group-Membership LSAs are used to prune those branches that do not lead to group members.

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• All routers within an OSPF domain calculate the same shortest path trees.

MOSPF LS-DB allow a router to perform RPM computation "in memory".

No need for broadcast and prune.

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• Forwarding cache entry contains the (source, group) pair, the upstream node, and the downstream interfaces.

• MOSPF Forwarding Cache

Forwarding Cache

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• The forwarding cache contains: – Destination: The group address

– Source: The packet’s source subnet.

– Upstream: The interface from which (S,G) packets are received.

– Downstream: The interfaces to which (S,G) packets are forwarded

– TTL: min. number of hops a packet needs to reach the group members. [This allows the router to discard packets with no chance of reaching the members.]

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• The forwarding cache is not aged. The forwarding cache will change when: The topology of the OSPF network changes,

forcing all of the datagram shortest-path trees to be recalculated.

There is a change in the Group-Membership LSAs indicating that the distribution of individual group members has changed.

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Limitations

Limited to OSPF domains Flooding membership information does not

scale well for Internet-wide multicsat


Protocol-Independent Multicast (PIM)

• Design Rationale:– Broadcast and prune keeps state off-tree and is

suitable when members are densely distributed

– Explicit join/center-based approach keeps state on-tree and is suitable when members are sparsely distributed

– PIM attempts to combine the best of both worlds


Design Choices

• Shared trees or shortest path trees?– Both: use shared trees to ‘Rendezvous’ then

switch to shortest path to deliver

• DV or LS for routing?– Use routing tables regardless of which protocol

created them (hence the name ‘Protocol Independent’)


PIM Operation Modes

• PIM provides both dense-mode (DM) and sparse-mode (SM) protocols

• PIM-DM: similar to DVMRP but does not build its own routing table

• PIM-SM: similar to CBT but provides switching to SPT and bootstrap mechanism for electing the tree center dynamically


How PIM-DM works• Packets initially flow on broadcast tree

• Forwarded away from source using the RPF algorithm– A router forwards a multicast datagram if received on

the interface used to send unicast datagrams to the source

• Then, Prunes are sent up the tree to remove branches with no members


How PIM-DM worksSource

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How PIM-SM works

• A Rendezvous Point (RP) is chosen as tree center per group to enable members and senders to “meet”

• Members send their explicit joins toward the RP

• Senders send their packets to the RP

• Packets flow only where there is join state

• (*,G) [any-source,group] state is kept in routers between receivers and the RP


How PIM-SM works• When should we use shared-trees versus source-

trees?– Source-trees tradeoff low-delay from source with

more router state– Shared-trees tradeoff higher-delay from source with

less router state

• Switch to the source-tree if the data rate is above a certain threshold


How PIM-SM worksSource

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Link

(*,G) Data

(S,G) Data

Control


How PIM-SM works

BA DRP


Source

EC Link

(*,G) Data

(S,G) Data

Control

ahmed helmy - uf1 ip-multicast (outline) -motivation and background -multicast vs. unicast...

Documents

multicast traffic

multicast scope control

ipmulticast outlinemotivation

multicast backbone mbone

resource discovery multicast

active multicast sessions

specific scope

ip header