ece544: communication networks-ii spring 2011 d. raychaudhuri lecture 6 includes teaching materials...
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ECE544: Communication Networks-II Spring 2011
D. RaychaudhuriLecture 6
Includes teaching materials from L. Peterson, J. Kurose, K. Almeroth
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IP Multicast
•Introduction •Internet Group Management Protocol (IGMP)
•Routing Protocols–Intra-domain (DVMRP, MOSPF, PIM)–Inter-domain (MBGP, MSDP) – not covered here
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Multicast: one sender to many receivers
• Multicast: act of sending datagram to multiple receivers with single “transmit” operation– One-to-many, many-to-many
• Question: how to achieve multicast
Multicast via unicast• source sends N unicast
datagrams, one addressed to each of N receivers– Redundant traffic
around sender– Keep track of all the IP
addresses to send tomulticast receiver (red)
not a multicast receiver
Routers forward unicast datagrams
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Multicast: one sender to many receivers
• Multicast: act of sending datagram to multiple receivers with single “transmit” operation– One-to-many, many-to-many
• Question: how to achieve multicast
Network multicast (IP Multicast)• Routers actively
participate in multicast, making copies of packets as needed and forwarding towards multicast receivers
Multicast routers (red) duplicate and forward multicast datagrams
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Multicast: one sender to many receivers
• Multicast: act of sending datagram to multiple receivers with single “transmit” operation– One-to-many, many-to-many
• Question: how to achieve multicast
Application-layer multicast (P2P)
• end systems (“hosts”) involved in multicast copy and forward unicast datagrams among themselves
• “host” becomes routerP2P hosts duplicate and forward multicast datagrams
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Internet Multicast Service Model
multicast group concept: – Each group has its own IP multicast address– A host can join or leave freely– Routers forward multicast datagrams (with destination address
of the group’s multicast address) to hosts that have “joined” that multicast group
128.119.40.186
128.59.16.12
128.34.108.63
128.34.108.60
multicast group
226.17.30.197
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Multicast groupsclass D Internet addresses reserved for
multicast:
host group semantics:o anyone can “join” (receive) or leave multicast groupo anyone (not even a member) can send to multicast
groupo no network-layer identification of hosts members
needed: infrastructure to deliver mcast-addressed datagrams to all hosts that have joined that multicast group
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Mapping IP Multicast Address to Ethernet Address
• Ethernet MAC Addresses: 48 bits– broadcast: all 1s, ff:ff:ff:ff:ff:ff– multicast: multicast flag (the lowest bit of the 1st
octet)= 1• 01-00-5E-00-00-00 to 01-00-5E-7F-FF-FF for IP
multicast• IP multicast group address mapped to the
lower order 23 bits of MAC address • not one-to-one mapping, one Ethernet mcast
addr 32 IP mcast addrs
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IPv6 Multicast Addresses (RFC 2375)
• low-order flag indicates permanent / transient group; three other flags reserved
• scope field: 1 - node local–2 - link-local–5 - site-local–8 - organization-local–B - community-local–E - global–(all other values reserved)
4 112 bits8
group IDscopeflags11111111
4
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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)
IGMPIGMP
IGMP
wide-areamulticast
routing
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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
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How IGMP Works
• on each link, one router is elected the “querier”• querier periodically sends a Membership Query message
to the all-systems group (224.0.0.1), with TTL = 1• on receipt, hosts start random timers (between 0 and 10
seconds) for each multicast group to which they belong
Qrouters:
hosts:
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How IGMP Works (cont.)
• when a host’s timer for group G expires, it sends a Membership Report to group G, with TTL = 1
• other members of G hear the report and stop their timers• routers hear all reports, and time out non-responding groups
Q
G G G G
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Source Specific Multicast
• Source Specific Multicast: a receiving host specifies (source, mcast group) to join– receive multicast packets addressed
to the group and only if they are from the specific sender (one-to-many)
• Any source multicast (ASM): many-to-many
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IGMPIGMP 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 group-specific query to see if any hosts left in group
– RFC 2236
IGMP v3: – Join/Leave specific S in G– RFC 3376
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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 Shared tree
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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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Shortest Path Tree
• mcast forwarding tree: tree of shortest path routes from source to all receivers– Dijkstra’s algorithm
R1
R2
R3
R4
R5
R6 R7
21
6
3 4
5
i
router with attachedgroup member
router with no attachedgroup member
link used for forwarding,i indicates order linkadded by algorithm
LEGENDS: source
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Reverse Path Forwarding
if (mcast datagram received on incoming link on shortest path back to source)
then flood datagram onto all outgoing links else ignore datagram
rely on router’s knowledge of unicast shortest path from it to sender
each router has simple forwarding behavior:
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source
Building the Reverse Path
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source
Building a Reverse Path Tree
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Reverse Path Forwarding: example
• result is a source-specific reverse SPT– may be a bad choice with asymmetric links
R1
R2
R3
R4
R5
R6 R7
router with attachedgroup member
router with no attachedgroup member
datagram will be forwarded
LEGENDS: source
datagram will not be forwarded
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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
R1
R2
R3
R4
R5
R6 R7
router with attachedgroup member
router with no attachedgroup member
prune message
LEGENDS: source
links with multicastforwarding
P
P
P
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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
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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
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Center-based trees: an example
Suppose R6 chosen as center:
R1
R2
R3
R4
R5
R6 R7
router with attachedgroup member
router with no attachedgroup member
path order in which join messages generated
LEGEND
21
3
1
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Current Intra-Domain Multicast Routing
ProtocolsDVMRP — Distance-Vector Multicast Routing Protocol
flood-and-prune,unidirectional per-source trees,builds own routing table
MOSPF — Multicast Extensions to Open Shortest-Path First Protocol
broadcast membership,unidirectional per-source trees,uses OSPF routing table
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Current Intra-Domain Multicast Routing Protocols
(cont.)PIM-DM — Protocol-Independent Multicast, Dense-Mode
broadcast-and-prune,unidirectional per-source trees,uses unicast routing table (Protocol Independent)
PIM-SM — Protocol-Independent Multicast, Sparse-Modeuses meeting places (“rendezvous points”),unidirectional per-group or shared trees,uses unicast routing table (Protocol Independent)
CBT — Core-Based Treesuses meeting places (“cores”),bidirectional shared trees,uses unicast routing table
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The First Intra-Domain Routing Protocol:
DVMRP
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Distance-Vector Multicast Routing Protocol (DVMRP)
DVMRP consists of two major components:(1)a conventional distance-vector routing protocol (like RIP) which builds, in each router, a routing table like this:
(2) a protocol for determining how to forward multicast packets, based on the routing table and routing messages
Subnet(Destination)
shortest dist(cost)
via interface(NextHop)
a 1 i1
b 5 i1
c 3 i2… … …
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Example Topology
g g
s
g
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Phase 1: Truncated Broadcast
g g
s
g
flood iff the packet arrives over the link that is on the shortest path to S
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Phase 2: Pruning
g g
s
prune (s,g)
prune (s,g)
g
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Steady State
g g
s
g
g
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graft (s,g)
graft (s,g)
Grafting on New Receivers
g g
s
g
g
report (g)
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Steady State after Grafting
g g
s
g
g
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Multicast Routing: MOSPF
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Multicast OSPF (MOSPF)
• an extension to OSPF (Open Shortest-Path First),a link-state, intra-domain routing protocolspecified in RFCs 1584 & 1585
• multicast-capable routers indicate that capability with a flag in their link-state messages
• routers include in their link-state messages a list of all groups that have members on the router’s directly-attached links (as learned through IGMP)
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S1
R1
R2
X
Y
Link state: each router floods link-state advertisementMulticast: add membership information to “link state”
Each router then has a complete map of the topology, includingwhich links have members of which multicast groups
Z
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S1
R1
R2
X
Y
Z has network map, including membership at X and YZ computes shortest path tree from S1 to X and YZ builds multicast entry with one outgoing interfaceW, Q, R, each build multicast entries
Z
W
Q
R
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R1
R2
X
Y
Z
W
Q
R
S1
Link-state advertisement with new topology (may be due to link failure) may require recomputation of tree and forwarding entry.Link WZ failed in the diagram below.
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R1
R2
X
Y
Z
W
Q
R
S1
T
R3
Link state advertisement (T) with new membership (R3) may require incremental computation and addition of interface to outgoing interface list (Z) (Similarly, disappearance of a membership may cause deletionan interface from an outgoing interface list). Link WZ is back to normal.
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Multicast Routing: PIM
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Protocol Independent Multicast (PIM)
• “Protocol Independent”– does not perform its own routing information
exchange – uses unicast routing table made by any of the existing
unicast routing protocols
• PIM-DM (Dense Mode) - similar to DVMRP, but:– without the routing information exchange part– differs in some minor details
• PIM-SM (Sparse Mode), or just PIM - instead of directly building per-source, shortest-path trees:– initially builds a single (unidirectional) tree per group
, shared by all senders to that group – once data is flowing, the shared tree can be converted to a
per-source, shortest-path tree if needed
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PIM Protocol Overview• Basic protocol steps
– routers with local members send Join messages towards a Rendezvous Point (RP) to join shared tree
– routers with local sources encapsulate data to RP
– routers with local members may initiate data-driven switch to source-specific, shortest-path tree
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RP
R1
R2 R3
R4
Join messagetoward RP
Shared tree after R1,R2,R3 join
Phase 1: Build Shared Tree
Join G
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Phase 2: Sources Send to RP
RP
R1
R2 R3
R4
S1
unicast encapsulateddata packet to RP
RP decapsulates,forwards downShared treeS2
PIMRegister
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Phase 3: Stop Encapsulation
RP
R1
R2 R3
R4
S1
Join G for S1Join G for S2
S2
(S1,G)
(S1,G)
(*.G)
(S2,G)
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Phase 4: Switch to Shortest Path Tree
R1
R2 R3
R4
Join messagestoward S2
shared tree
S1
S2
RP
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Phase 5: Prune (S2 off) Shared Tree
R1
R2 R3
R4
S1
S2 distribution treeShared tree
Prune S2 off Shared tree where iif of S2 andRP entries differS2
RP
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Today’s Homework• Peterson & Davie, Chap 4
4.46 (3rd & 4th eds)4.52 (3rd & 4th eds)4.56 – 3rd ed and 4.58 – 4th ed
Due on Friday 3/4Reminder: Midterm 3/11
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Review Items for Mid-Term (1)
• Top-down design – how to evaluate high-level requirements
and map to a network topology & major components
– drawing a network diagram with switches, routers, etc.
– drawing the protocol stacks at each network element
– Evaluating basic traffic flows at different links/nodes in the network
– Analysis of wireless coverage requirements in terms of physical coverage and traffic load
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Review Items for Mid-Term (2)
• Shared Media Protocols– Basic Principles– ALOHA, Slotted ALOHA, CSMA– Throughput equations for simple cases– Details of Ethernet/802.3– Details of WiFi/802.11– Drawing time diagrams for 802.3 and .11
channels
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Review Items for Mid-Term (3)
• Switching– Switching vs shared media– Ethernet bridging and switching – Self learning Ethernet switch– Spanning tree algorithm & protocol– ATM switching principles– Virtual circuits and related signaling protocols– ATM AAL’s– ATM performance calculations
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Review Items for Mid-Term (4)
• IP Routing Basics– IPv4 principles and protocol structure– DHCP, ARP etc– Routing algorithms: DV and Dijkstra– Routing algorithm numerical problems– RIP and OSPF protocols– Loop removal in RIF– OSPF LSP flooding algorithm– Calculating routing overheads
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Review Items for Mid-Term (5)
• IP Multicast– IP multicast principles– IGMP basics– Source trees and reverse path routing– Shared trees– Tree setup and join/leave events– Rendezvous Point (RP) and related methods– MOSPF and PIM/SM protocols as examples
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Review Items for Mid-Term (5)
• IP Routing Advanced– CIDR addressing– Numerical examples with CIDR routing table– BGP routing principles– BGP protocol specifics; forwarding tables,
reachability advertisements, longest prefix match
– IPv6 main features and differences with IPv4