ece544: communication networks-ii spring 2009 hang liu lecture 6 includes teaching materials from d....
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ECE544: Communication Networks-II Spring 2009
Hang LiuLecture 6
Includes teaching materials from D. Raychaudhuri, L. Peterson, J. Kurose, K. Almeroth
Today’s Lecture• Recap
• Sub-netting• Super-netting (CIDR)• BGP• IPv6
• IP Multicast• Introduction • Internet Group Management Protocol
(IGMP)• Routing Protocols
– Intra-domain (DVMRP, MOSPF, PIM)– Inter-domain (BGMP, MSDP, PIM-SSM)
Internet StructureToday
Backbone service provider
Peeringpoint
Peeringpoint
Large corporation
Large corporation
Smallcorporation
“Consumer ” ISP
“Consumer” ISP
“ Consumer” ISP
Subnetting• Add another level to address/routing
hierarchy: subnet• Subnet masks define variable partition of host
part• Subnets visible only within site
Network number Host number
Class B address
Subnet mask (255.255.255.0)
Subnetted address
111111111111111111111111 00000000
Network number Host IDSubnet ID
Subnet Example
Forwarding table at router R1Subnet Number Subnet Mask Next
Hop128.96.34.0 255.255.255.128
interface 0128.96.34.128 255.255.255.128
interface 1128.96.33.0 255.255.255.0 R2
Subnet mask: 255.255.255.128Subnet number: 128.96.34.0
128.96.34.15 128.96.34.1
H1R1
128.96.34.130Subnet mask: 255.255.255.128Subnet number: 128.96.34.128
128.96.34.129128.96.34.139
R2H2
128.96.33.1128.96.33.14
Subnet mask: 255.255.255.0Subnet number: 128.96.33.0
H3
Classless Inter Domain Routing (CIDR) or Super-netting
Class B:
Class C:
Net ID Host ID
Host IDNet ID Problem: Class B addresses are running out Solution: Allocate multiple Class C addresses Problem: Random allocation of Class C addresses need multiple routing table entries Solution: Allocate “contiguous” Class C addresses Routing entry: [IP Address of Network and Net Mask]
IP Address: 195.201.3.5 = 11000011 11001001 00000011 00000101Net Mask: 254.0.0.0 = 11111110 00000000 00000000 00000000-----------------------------------------------------------------------------------------Network IP: 194.0.0.0 = 11000010 00000000 00000000 00000000
CIDR (continued)
Organization’s Requirements Assignment
Fewer than 256 addresses 1 Class C network
Fewer than 512 addresses 2 Contiguous Class C networks
Fewer than 1024 addresses 4 Contiguous Class C networks
Fewer than 2048 addresses 8 Contiguous Class C networks
Fewer than 4096 addresses 16 Contiguous Class C networks
Fewer than 8192 addresses 32 Contiguous Class C networks
Fewer than 16384 addresses 64 Contiguous Class C networks
How many Class C addresses ?
Contiguous Class C network addresses allow a “single” entry in the routing table for all the above organizations
Route Aggregation Examples
Q: How does network get network part of IP addr?
A: gets allocated portion of its provider ISP’s address space
ISP's block 11001000 00010111 00010000 00000000 200.23.16.0/20
Organization 0 11001000 00010111 00010000 00000000 200.23.16.0/23 Organization 1 11001000 00010111 00010010 00000000 200.23.18.0/23 Organization 2 11001000 00010111 00010100 00000000 200.23.20.0/23 ... ….. …. ….
Organization 7 11001000 00010111 00011110 00000000 200.23.30.0/23
Hierarchical addressing: more specific routes
ISPs-R-Us has a more specific route to Organization 1
“Send me anythingwith addresses beginning 200.23.16.0/20”
200.23.16.0/23
200.23.18.0/23
200.23.30.0/23
Fly-By-Night-ISP
Organization 0
Organization 7Internet
Organization 1
ISPs-R-Us“Send me anythingwith addresses beginning 199.31.0.0/16or 200.23.18.0/23”
200.23.20.0/23Organization 2
...
...
Coordinated Address Allocation
Multi-regional 192.0.0.0 -- 193.255.255.255
Europe 194.0.0.0 -- 195.255.255.255
Others 196.0.0.0 -- 197.255.255.255
North America 198.0.0.0 -- 199.255.255.255
Central/South America 200.0.0.0 -- 201.255.255.255
Pacific Rim 202.0.0.0 -- 203.255.255.255
Others 204.0.0.0 -- 205.255.255.255
Others 206.0.0.0 -- 207.255.255.255
Address aggregation using Geographic scope
European networks will have a single entry in routing tables of routers in other continents: [Network IP =194.0.0.0; mask = 254.0.0.0]194.0.0.0 = 10110010 00000000 00000000 00000000195.255.255.255 = 10110011 11111111 11111111 11111111
Same 7 high-order bits implies Mask = 11111110 00000000 00000000 00000000 = 254.0.0.0
Address Matching in CIDR
• Routing table uses “longest prefix match”– 171.69 (16 bit prefix) = routing table entry #1– 171.69.10 (24 bit prefix) = routing table entry #2– then DA=171.69.10.5 matches routing table entry #2– and DA = 171.69.20.3 matches routing table entry
#1
CIDR (Summary)
• Continuous block of 2N addresses• [Base address, Mask]• Lookup algorithm:
– Masks destination address against mask in routing table entry
– Match means route is found – May be multiple matchings! – Longest mask breaks “ties” (longest prefix
match)
IP addressing (Summary)• Classful addressing:
– inefficient use of address space, address space exhaustion
• e.g., class B net allocated enough addresses for 65K hosts, even if only 2K hosts in that network
• CIDR: Classless InterDomain Routing– network portion of address of arbitrary length– address format: a.b.c.d/x, where x is # bits in
network portion of address
11001000 00010111 00010000 00000000
networkpart
hostpart
200.23.16.0/23
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 (RIP, OSPF)– Inter-AS: unique standard for inter-AS routing: BGP
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.
Inter-AS routing in the Internet: BGP
Figure 4.5.2-new2: BGP use for inter-domain routing
AS2 (OSPF
intra-AS routing)
AS1 (RI P intra-AS
routing) BGP
AS3 (OSPF intra-AS
routing)
BGP
R1 R2
R3
R4
R5
Internet inter-AS routing: BGP• BGP (Border Gateway Protocol): the de facto
standard• Path Vector protocol:
– similar to Distance Vector protocol– each Border Gateway broadcast to neighbors
(peers) entire path (i.e., sequence of AS’s) to destination
– BGP routes to networks (ASs), not individual hosts– E.g., Gateway X may send its path to dest. Z:
Path (X,Z) = X,Y1,Y2,Y3,…,Z
Internet inter-AS routing: BGP
Suppose: gateway X send its path to peer gateway W
• W may or may not select path offered by X– cost, policy (don’t route via competitors AS),
loop prevention reasons.
• If W selects path advertised by X, then:Path (W,Z) = w, Path (X,Z)
• Note: X can control incoming traffic by controlling its route advertisements to peers:– e.g., don’t want to route traffic to Z -> don’t
advertise any routes to Z
BGP: controlling who routes to you
Figure 4.5-BGPnew: a simple BGP scenario
A
B
C
W X
Y
legend:
customer network:
provider network
• 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: controlling who routes to you
Figure 4.5-BGPnew: a simple BGP scenario
A
B
C
W X
Y
legend:
customer network:
provider network
• 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!
BGP operation
Q: What does a BGP router do?• Receiving and filtering route
advertisements from directly attached neighbor(s).
• Route selection. – To route to destination X, which path (of
several advertised) will be taken?– Policy
• Sending route advertisements to neighbors.
22
BGP Operations (Simplified)
Establish session on TCP port 179
Exchange all active routes
Exchange incremental updates
AS1
AS2
While connection is ALIVE exchangeroute UPDATE messages
BGP session
23
Four Types of BGP Messages
• Open : Establish a peering session.
• Keep Alive : Handshake at regular intervals.
• Notification : Shuts down a peering session.
• Update : Announcing new routes or withdrawing previously announced routes.
announcement = prefix + attributes values
Attributes are Used to Select Best Routes
192.0.2.0/24pick me!
192.0.2.0/24pick me!
192.0.2.0/24pick me!
192.0.2.0/24pick me!
Given multipleroutes to the sameprefix, a BGP speakermust pick at mostone best route(Note: it could reject them all!)
25
ASPATH Attribute
AS7018135.207.0.0/16AS Path = 6341
AS 1239Sprint
AS 1755Ebone
AT&T
AS 3549Global Crossing
135.207.0.0/16AS Path = 7018 6341
135.207.0.0/16AS Path = 3549 7018 6341
AS 6341
135.207.0.0/16
AT&T Research
Prefix Originated
AS 12654RIPE NCCRIS project
AS 1129Global Access
135.207.0.0/16AS Path = 7018 6341
135.207.0.0/16AS Path = 1239 7018 6341
135.207.0.0/16AS Path = 1755 1239 7018 6341
135.207.0.0/16AS Path = 1129 1755 1239 7018 6341
AS Graphs Do Not Show Topology!
The AS graphmay look like this. Reality may be closer to this…
BGP was designed to throw away information!
In fairness: could you do this “right” and still scale?Exporting internalstate would dramatically increase global instability and amount of routingstate
Shorter Doesn’t Always Mean Shorter
AS 4
AS 3
AS 2
AS 1
BGP says that path 4 1 is better than path 3 2 1
??
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 trafficPerformance: • Intra-AS: can focus on performance• Inter-AS: policy may dominate over
performance
IP Version 6• Features
– 128-bit addresses (classless)– multicast– real-time service– authentication and security – autoconfiguration – end-to-end fragmentation– protocol extensions
• Header– 40-byte “base” header– extension headers (fixed order, mostly fixed length)
• fragmentation• source routing• authentication and security• other options
IPv4 & IPv6 Header Comparison
Version IHLType of Service
Total Length
Identification FlagsFragment
Offset
Time to Live
Protocol Header Checksum
Source Address
Destination Address
Options Padding
Version Traffic Class Flow Label
Payload LengthNext
HeaderHop Limit
Source Address
Destination Address
IPv4 HeaderIPv4 Header IPv6 HeaderHeader
- field’s name kept from IPv4 to IPv6
- fields not kept in IPv6
- Name & position changed in IPv6
- New field in IPv6Leg
end
IP Multicast
•Introduction •Internet Group Management Protocol (IGMP)
•Routing Protocols–Intra-domain (DVMRP, MOSPF, PIM)–Inter-domain (MBGP, MSDP)
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
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
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
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
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
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
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
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
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
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:
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
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
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
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
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
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
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:
source
Building the Reverse Path
source
Building a Reverse Path Tree
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
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
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:
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
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
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
The First Intra-Domain Routing Protocol:
DVMRP
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… … …
Example Topology
g g
s
g
Phase 1: Truncated Broadcast
g g
s
g
flood iff the packet arrives over the link that is on the shortest path to S
Phase 2: Pruning
g g
s
prune (s,g)
prune (s,g)
g
Steady State
g g
s
g
g
graft (s,g)
graft (s,g)
Grafting on New Receivers
g g
s
g
g
report (g)
Steady State after Grafting
g g
s
g
g
Multicast Routing: MOSPF
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)
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
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
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.
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.
Multicast Routing: PIM
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
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
RP
R1
R2 R3
R4
Join messagetoward RP
Shared tree after R1,R2,R3 join
Phase 1: Build Shared Tree
Join G
Phase 2: Sources Send to RP
RP
R1
R2 R3
R4
S1
unicast encapsulateddata packet to RP
RP decapsulates,forwards downShared treeS2
PIMRegister
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)
Phase 4: Switch to Shortest Path Tree
R1
R2 R3
R4
Join messagestoward S2
shared tree
S1
S2
RP
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
Bidirectional Trees (BIDIR-PIM)
RP Address’s Link
Join
Join
Bidirectional Trees (BIDIR-PIM)• Bidirectional PIM (BIDIR-PIM): Enhancement to
PIM for many-to-many• A router in shared tree:
– Receive a mcast packet from a down-stream branch– Forward it both up the tree and down other branches
• BIDIR-PIM vs. PIM-SM shared tree– more direct but more state information
• BIDIR-PIM vs. PIM-SM source tree– Longer than PIM-SM source tree, but less state (no
source-specific tree)• What to optimize: bandwidth usage, router
state, path length, etc.• What sort of application to support: one-to-
many, many-to-many, etc.
Inter-domain Multicast Routing
What Exactly is Needed?
• inter-domain route exchange protocol
• mechanism for connecting domains– two models:
• discover sources using “source announcing” protocol
• know the source(s) a priori
Inter-Domain Route Exchange
• Exchange multicast reachability between Autonomous Systems (AS)– Just like unicast routes are exchanged with BGP– Protocol is “Multiprotocol extensions to BGP” (RFC
2283)• Also known as “Multicast” BGP (MBGP)• Also known as BGP4+
• MBGP is available and deployed today.– Multiple vendors: Juniper, Cisco, Nortel, etc.
What Exactly is Needed?
• inter-domain route exchange protocol
• mechanism for connecting domains– two models:
• discover sources using “source announcing” protocol
• know the source(s) a priori
The Internet Solution
• Re-use existing protocols/solutions– Use PIM-SM in the inter-domain
• The challenge is to avoid “root dependencies”– A root/RP/core is one domain but no active
group participants (sources or receivers) in the domain
– Root dependencies can lead to political problems and inefficiencies
The Internet Solution (cont)
• The key: Establish a root/RP/core per domain– No “root dependencies”
• Remember the problem:– Connecting sources and receivers– Solution: Multicast Source Discovery Protocol
(MSDP)
• MSDP is the last piece of the puzzle; is simple to implement; and yields an interim solution to inter-domain multicast
MSDP -- Basic Idea
• MSDP advertises multicast sources to other domains
• Other domains decide if group members are active and find a way to get the data
• “MSDP connects shared-trees together”
• MSDP typically runs in the RP
MSDP - Elements of Operation
• Receivers in a domain join the shared-tree
• The RP is known only to routers in the domain
• When a source goes active in a domain, it’s packets get to the RP in that domain
• The RP sends a Source-Active (SA) message identifying the source and group it sends to
MSDP - Elements of Operation (cont)
• How to get SA messages to all MSDP peers?
– Need MSDP topology flooding protocol
– The RP’s address is also in the SA message to accommodate “peer-RPF” like flooding
– Each MSDP peer receives SA message and forwards away from the originating RP
MSDP - Elements of Operation (cont)
• Each MSDP speaking RP will examine SA message to see if any local members are joined to the group
• If so, the RP joins to source described in SA message
• Otherwise, the SA message is ignored (Flood-and-Join model)
How MSDP works with PIM-SM
RP
RP
RP
RP
MSDP peer
Physical link
A
B
C D
Receiver
Source
PIM message
MSDP message
2b: SA
2b: SA
2b: SA
3:Join3:Join3:Join
3:Join
1: Register
2a:Join
What Exactly is Needed?
• inter-domain route exchange protocol
• mechanism for connecting domains– two models:
• discover sources using “source announcing” protocol
•know the source(s) a priori—SSM model
Source Specific Multicast (SSM)
• Basic idea:– Assumes receiver knows the
source(s)– Reverse SPT join to source
• No RPs or MSDP
– About as straightforward as you can get!
How SSM Works
Physical link
A
B
C D
Receiver
Source
PIM message
Join
JoinJoin
Join
Join
Join
Source Specific Multicast
• Advantages– Minor changes to existing infrastructure—still use
PIM-SM– No PIM-SM RP, or MSDP
• Limitations– Requires modifications (last hop routers) and
IGMPv3– May be difficult to support some applications
• Thoughts– Works for 9x% of killer-apps -- need mechanism
(WWW) to let receivers know who sources are– Success will depend on seamless migration strategy
104
Today’s Homework• Peterson & Davie, Chap 4
4.464.564.61
Due next Friday 3/6Reminder: Midterm 3/13
Office hour: 3:30PM-4:40PM, 3/6(CoRE 510)