Subnet & Supernet - Seoul National...
Transcript of Subnet & Supernet - Seoul National...
Subnet & Supernet
Chong-kwon Kim
SNU SCONE lab.
Problems of Addressing IP addressing scheme is too rigid
– One network ID for each organization– Only three classes
Problems1. Large physical network (Large extended LAN)
2. Inefficient use of addresses• Need to allocate Class B address to a network with 255 hosts
– 255/65535 = 0.39% efficient
SNU SCONE lab.
Subnetting-1 Solution for large organizations A class A (or B) network may have tens of thousand
of hosts Problems? One solution is to assign many class C addresses Routing complexity increases
One Entry in Forwarding Table
One organization
One physical network
One Entry in Forwarding Table
One organization
Many physical networks
Many Entries in Forwarding Table
Subnetting
Subnetting-2 Partition a large network into multiple small physical
networks called subnet Use part of host ID for subnet identification
– How do you know What part of host ID is used for Subnet? Subnet mask
Routing– Outside, route based on network ID (prefix) only– Inside, route based on (network+subnet ID)
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SNU SCONE lab.
Subnet Example & Forwarding Procedure
Subnet Mask: 255.255.255.128Subnet Number: 128.96.34.0 (00100010 00000000)
Subnet Mask:255.255.255.128Subnet number:128.96.34.128 (00100010 10000000)
Subnet Mask:255.255.255.0Subnet number:128.96.33.0(00100001 00000000)
128.96.34.15 128.96.34.1
128.96.34.130
128.96.34.129 128.96.34.139
128.96.33.14 128.96.33.1
Router R0 Forwarding Table
SubnetNo SubnetMask NextHop
128.96.34.0 255.255.255.128 Interface 0128.96.34.128 255.255.255.128 Interface 1128.96.33.0 255.255.255.0 R1
R0
R1
IP Lookup procedure
Let D = Destination IP addressFor each forwarding entry
D1 = SubnetMask & Dif D1 = SubnetNumber
Deliver to the NextHopbreak
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CIDR Solution to efficient use of address Allocate multiple (small) network IDs to an
organization such that they can be aggregated into one prefix
CIDR(Classless Inter-Domain Routing), Supernetting– Ignore IP address class– Variable network ID length– Prefix: Network ID part of IP addresses
1010…..11 00
1010…..11 01
1010…..11 10
1010…..11 11
1010…..11
Prefix
Scenario:A company with 900 hosts
CIDR - Example
Network ID allocation & Aggregation– For a network with N hosts, host ID length should be at least n
where 2^n > N• Use 32 -n bits for network ID
– Example: • For an AS with 4,000 hosts, host ID part should be 12 bit long
– Network ID part is 20 bit long– Allocate contiguous network IDs from all 0’s to all I’s– Share a common prefix (network ID part) of desired length– Example
• 192.4.16.0-192.4.31.0 (11000000 00000100 0001xxxx xxxxxxxx)• 16 class C addresses
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SubnetNumber SubnetMask NextHop
128.96.34.0 255.255.255.128 Interface 0128.96.34.128 255.255.255.128 Interface 1128.96.33.0 255.255.255.0 R2
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CIDR Notation Notation
– IPAddress / length• Length specifies the network prefix• Similar to subnet mask• 185.21.16.0/20 = 255.255.240.0
Forwarding table entry– Use prefix length instead of subnet mask
SubnetNumber NextHop
128.96.34.0/25 Interface 0128.96.34.128/25 Interface 1128.96.33.0/24 R2
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CIDR & Routing Route aggregation
Longest matching prefix
Destination NH128.112.128.0/24 Int 0128.112.128.0/21 Int 1
How do you route a packet to 128.112.128.0?
How do you route a packet to 128.112.129.0?
ISP2
Routing
Chong-kwon Kim
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Routing & Forwarding Routing
– Collect network information and determine shortest paths– Path selection criteria
• Hop count, distance, reliability, QoS, …– As a result, generate forwarding tables
Forwarding– Move packets according to forwarding table
SNU SCONE lab.
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Routing Scalability Millions of networks in the Internet Scalability problem
– Control packet overhead– Processing overhead
Divide and conquer, Abstraction Partition the Internet into pieces called AS
(Autonomous system) or RD(Routing Domain)– Single authority unit over
• Address management & Routing inside the domain
– Examples• ISP, Large University/company, …
AS ID (16 bit)– Each AS has a unique ID
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Intra-/Inter-domain Routing Intradomain routing
– Routing within an AS where the owner has a complete control over the network operation
– Optimality > Reachability– Collect all information & find shortest paths– IGP (Interior Gateway Protocol)– RIP, OSPF
Interdomain routing– Routing across AS boundaries– AS would not disclose inside information– Reachability > Optimality– Exchange reachability info. between ASs– EGP (Exterior Gateway Protocol)– EGP, BGP-4
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Graph Model Represent a network as a graph
– Node: network or router– Link: network link
• Link cost
Find the shortest paths on the graph– Network conditions change dynamically– Shortest path algorithms
Point-to-point
Ethernet
FDDI
A
XY
Z B
x y z2 1 13
C=2
C=1
C=3
C=1
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Ethernet
FDDI
P2P
Intradomain Routing Algorithm & Protocol Routing algorithm = Shortest path algorithms
– Bellman-Ford algorithm– Dijkstra algorithm
Routing protocol– Distributed realization of shortest path algorithms
• What information should be exchanged for distributeimplementation of shortest path algorithms?
– Application layer protocol that exchange• Routing info.• Network topology• Network operating conditions
– Faults, congestion, estimated delay...
– RIP(Routing Information Protocol)– OSPF(Open Shortest Path First)
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Shortest Path Algorithm
Graph G = (N,E)– Link (i, j) is incident on node i and j
• Associated cost,– Path (i, j, k, l, ,m) is a series of links connecting two end
nodes i and m• Cost =
Shortest path algorithm– Find a path between two nodes with minimum cost
cij
lmkljkij cccc
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Bellman-Ford AlgorithmLet D(v) be the cost of current shortest path from node v to s
AlgorithmStep 1: D(s) = 0
D(v) = for all vStep 2: D’(v) = min [D(u) +
u ∈ N(v)
If D’(v) = D(v) for all v , StopO.W. D(v) = D’(v) for all vRepeat
cuv ]
sv
l
m
n
Distributed Implementation?
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DV (Distance-Vector) Algorithm
Based on distributed BF Algorithm Each node sends to neighbor nodes its own optimal path
costs as– Distance vector
• Shortest path cost to each destination network
Each node receives distance vector from all of its neighbor nodes and compute best routes
sv
l
m
n
1
5
2
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BF - ExampleDistance to network
Router4 8 12 16
A 0
B 0
C 0
D 0
Router4 8 12 16
A ?
B ?
C ?
D ?
192.168.0.4
8 12 16
A 0
B 0
C 0
D 0
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RIP Neighbor routers exchange RIP request/response
messages that contain distance vector
When to send RIP messages?
– Periodic– Triggered
Command Version 0Family of Net1 0
Distance to Net1
Subnet MaskNext hop
IP Address of Net1
Net2Net3...
Use UDP Port 520
How to limit the delivery only to directly connected routers?
Route Adaptation
(F)(A)(D)(G)(A)(G)
Next Hop
Suppose F notices that link (F, G) is broken① F advertises to A that its cost to G is ② A receives from B, C and E with cost = 3, 2, 3, respectively③ A updates its route to G via C with cost 3④ F receives advertisement from A and updates the route via A with cost 4
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RIP Problem Slow convergence
– Count to infinity
A
D
C
B
10
1
11
1
Routing Table Updates
A B C D
d nh d nh d nh d nh
2 B 1 D 2 B 0 dd
2 B ∞ ur 2 B 0 dd
3 C 3 C 3 A 0 dd
4 C 4 C 4 A 0 dd
Before Break
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Split Horizon & Poison Reverse Horizon
– Directions where to advertise distance-vector
Split horizon – Do not advertise a route to an interface from where the best
trigger (next hop) arrives
Split horizon with Poison reverse– Advertise a route with ∞ to an interface from where the best
trigger arrives
A
D
C
B
101
111
Routing Table Updates
A B C D
d nh d nh d nh d nh
2 B 1 D 2 B 0 dd
2 B ∞ ur 2 B 0 dd
Read: http://technet.microsoft.com/library/Cc940478
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Dijkstra Algorithm Find shortest paths from node s
d
3
s
a b
c
e1
2
22
1
13
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AlgorithmStep 1: F = {s}
D(v) =
Step 2: If F = N, StopO.W. Find u s.t. D(u) = min { D(x) }
x ∈ N-F
F = F + {u}D(v) = min [D(v), D(u) + ], ∀l v ∈ N(u)
Repeat
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Link State Routing Protocol - 1 Problem of distance-vector routing protocol
– Large overhead– Slow convergence– Not scalable
Search for a new intra-domain routing protocol– Starts in 1987– Multiple paths between a source-destination pair– Descriptive metric
Distributed database model– Each router maintains complete network information
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Link State Routing Protocol - 2 Dijkstra (or any other SP algorithms) Procedure
– Each router monitors the status of directly connected links– Announce the link state information to all routers
• LSA (Link State Advertisement)• Use flooding
– Collect LSAs into the local link state database and compute the shortest path tree rooted at the router
LSA contains– ID of the node that creates the LSA– List of directly connected neighbors (routers and networks) and
the cost(state) of each link– Sequence number (SEQNO)– LS Age
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LSA Flooding Reliable flooding
– Make sure LSA reaches to all routers• While maintaining the efficiency of forwarding
– Use seqno to detect duplicate
Procedure– A router generates new LSA periodically
• Increment SEQNO• Start SEQNO = 0 when reboot
– Flood to all links– When a router receives an LSP
• Check the LSA is new one• If new, store the LSA and after increment LSAge, flood to all
interfaces except the one from which the LSP was received• If not, ignore
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Flooding: Example
X
C
A
B D
X
C
A
B D
X
C
A
B
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OSPF - Area OSPF is very complex protocol
– Hierarchy• A large AS/RD is partitioned
into several areas
– Load balancing
A large AS has thousands of routers- Hierarchical structure
Area: a set of routers that exchange LSAArea 0 : Backbone areaABR(Area Border Router)- Router that is both the member of
backbone area and non-backbone area
Use R4-R5 link?Optimality vs. Scalability
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OSPF PDU – 1/2
Authentication
Version Type Message length
Checksum Authentication type
SourceAddr
AreaId
0 8 16 31
OSPF common header format
Incorrect routing may causelarge security problemsMake sure LSP is generated bylegitimate routers
Lowest IP address among theIP addresses assigned to a router
Network A
Network B
OSPF PDU – 2/2
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(LSA) Link-State Advertisement
Network A
Network B
Type 1 LSA
Type 2 LSA
Link-state ID = Advertising router Smallest IP address
Router ID
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Interdomain Routing Interdomain routing problems
– Large size– No centralized control or common metric– Trust, policy
Interdomain routing protocols– EGP (Exterior Gateway Protocol)– BGP (Border Gateway Protocol)
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EGP & BGP EGP is designed for tree structured networks
Old Internet topologyThere is only one ingress/egress point to/from an AS- Use the default route
Today’s multi-backbone Internet - Loops
BGP
EGP
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AS Types Stub AS
– an AS that has only a single connection to one other AS– Carry local traffic only
Multihomed AS– Connections to more than one AS– Carry local traffic only
Transit AS– Connections to more than one AS– Designed to provide transit services
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BGP - Configuration
• Each AS has a BGP speaker
• Neighbor BGP speakers exchange reachability information (TCP)
• Determine paths to prefixes from the collected reachability information
• Advertise the paths (reachability info) toother AS
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BGP BGP speakers advertise
– Local networks– (Transit AS only) Reachable networks with complete path
information
(128.96/16, AS2)
(128.96/16, AS1/AS2)
(128.96/16, AS3/AS1/AS2)
Why sending complete path info?
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iBGP (interior BGP) Distribute reachability info to all routers within the AS
– Each router learns the best BG to route a packet to a particular prefix
Routers also runs a intradomain routing to find paths to BGs
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BG(Border GW)
IPv6
Chong-kwon Kim
SNU SCONE lab.
IPv6
IPv6 is a new IP that will replace IPv4
Urgency of new protocol– Address space depletion
• IPv4 32 bit address can support only 4 billion nodes• Expected to be full by year 20XX
The final day has been extended many timesWhy?
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IPng Ipng (IP next generation) WG
– IETF - 1991– Developing a new protocol is once a lifetime opportunity– Add functions that is/will be useful for the future Internet
Requirements– Routing & addressing– QoS(Quality of Service)– Autoconfiguration– Security– Mobility– Smooth transition
Select SIPP with minor modifications– Called IPv6– 128 bit address
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Address Address types
Allocate addresses considering the ease of routing Scalable Small routing table Aggregation
(Hierarchy) Hierarchy
– Registry (Continent) > ISP (Backbone ISP > non-backbone ISP) > Subscriber (AS) > Subnet > Host
– CIDR-like aggregation• ISP obtains an address space and controls address allocation• Ideally an ISP advertises only one prefix
Prefix Address Type Space010 Provider based unicast 1/81111 1110 10 Link local 1/10241111 1110 11 Site local 1/10241111 1111 Multicast 1/256
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Addressing & Routing - 2 Geographic aggregation
– Hosts within a geographic region has the same prefix– Continent level– Registry ID
010 Registry ID ISP ID Subscriber ID Subnet ID Interface ID
3 m n o p 125-m-n-o-p
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IPv4 to IPv6 Transition & Address IPv4 to IPv6 transition is a difficult problem Co-existence of IPv4 & IPv6 Approaches
– Dual-stack– Tunneling
Dual-stack– Process both IPv4 & IPv6 packets
Tunneling– Encapsulate IPv6 packet with IPv4 header– Use IPv4-mapped IPv6 addresses for easy encapsulation
• 00..00 + IPv4-Address
V6Sdr
V6Rc
V4 Network
V6 packet V6 packet
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Address Notation Hexadecimal/2 byte separated by semicolons
– 47CD:1234:4422:AC02:0022:1234:A456:0124
Long contiguous 0 bits– 47CD:0000:0000:0000:0000:0000:A456:0124=> 47CD:: A456:0124
IPv4 part– Dotted decimal– ::FFFF:128.96.33.81
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IPv6 Packet Format - 1
Ves Class Flow Label
Payload length Next header Hop limit
SOURCE ADDRESS
DESTINATION ADDRESS
0 4 8 16 24 31
Hop-by-hop
Destination option
Routing header
Fragment header
Authentication header
ESP header
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IPv6 Packet Format - 2
Traffic class and Flow label– To support QoS
Next header– Types of header appeared next to the IP header
• Ex: TCP: 6
IPv6 header TCP header + DataNext = TCP(6)
IPv6 header Routing header TCP header + DataNext=routing(43) Next = TCP(6)
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Extension Header Options & Fragmentation info. are recorded in
extension headers For fast packet processing at intermediate routers,
extension headers appear in a specific order Fragmentation header
NextHeader Reserved Offset RES M
Ident
0 8 16 29 31
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Autoconfiguration Automatic configuration of IP address and other
information Two approaches
– Stateful: DHCP– Stateless
• Server-less autoconfiguration
How to create globally unique address?– Subnet prefix + Unique Interface ID– Uniqueness of Interface ID is guaranteed at HW level
How to obtain the subnet prefix?– Let the router advertise the subnet prefix– RS(Router Solicitation)
• Solicit the routing information– RA(Router Advertisement)
• Inform prefix information and etc.
Multicast
Chong-kwon Kim
SNU SCONE lab.
Multicasting
Types of communications– Unicast – Broadcast– Multicast– Anycast
Importance of multicast– Replicated data– Entertainment
• IPTV, VOD, ..
Multicast requirements– Efficiency– Scalability
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Multicasting Methods
Multiple unicast (Simulcast)– Unicast to each receiver– Inefficient
• Sender processing• Network traffic
– Management of (many) receivers is almost impossible
Router based multicast (IP level multicast)– Routers replicate packets and forward to multiple links
S
R
R
Scalability
Router overhead- Routing table- Packet processing
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Multicast Models SSM (Source Specific Multicast) vs ASM (Any Source
Multicast) Centralized
– Each sender manages group members• Hosts that wish to join/leave a multicast group should send
join/leave requests to the sender
– Difficult to implement IP-level multicast
Distributed– No centralized member management function– Hosts can join/leave multicast groups freely
• Host informs its router that it has joined a group
– Similar to broadcast & filtering• TV, Radio
– Any host can send to a multicast group
IGMP (Internet Group Mgmt Protocol) - 1 Two step multicast
– First distribute datagrams to multicast routers that have multicast group members
– A multicast router handles multicasting within its subnet
IGMP– A protocol to check the presence of group members within a
subnet– Transmission of group membership query and response
between a multicast router and hosts
IGMP - 2 When a host joins a group
– Broadcast its membership
A multicast router periodically broadcasts group membership queries
A host that is a member of a multicast group responds to the poll– After random delay between (0, 10) sec. Why?
MulticastRouter
Hosts
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Multicast Routing Problem To do
– Install directives (like a forwarding Table) at each router to duplicate packets and forward the packets
Steiner tree problem– Graph G = (V, E)– R (receiver set) is a subset of V– Find the best subtree of G that includes all R – NP-Complete– Compare to MST (Minimum Spanning Tree) problem
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Multicast Tree Types Source based
– Use a shortest path tree (union of shortest paths) rooted on the source
– Different multicast trees optimized to each source
Shared– Common tree used by all senders
SS
Steiner Tree
How to build a commonshared tree?
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Multicast Routing Protocols Source based tree protocols
– Per source and group (destination) overhead– Good performance– DVMRP, MOSPF, PIM-DM
Shared tree protocols– Per group overhead– Less efficient, traffic concentration– CBT, PIM-SM
Source-based Shared
Tree Maintenance OH
High Low
Efficiency Good Poor
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DVMRP
Distance Vector Multicast Routing Protocol RIP-dependent
– Uses RIP to exchange group membership information
Flood and prune protocol– Broadcast to all networks through a spanning tree rooted at
the source– Only routers w/ member accept packets– Prune branches (subtrees) w/o members
Mechanisms– RPB– RPM
• Pruning & grafting
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RPB(Reverse Path Broadcast) Dalal & Metcalfe(1978) Broadcasting
– Avoid flooding loops– Use shortest path from destination to source (reverse path)
Mechanism– Flood (relay) a packet if the packet arrives on the shortest path
link to the source– O.w. discard– Compare to flooding used in OSPF LSA distribution
RPB achieves shortest path broadcast
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Example
S
R
RSender
S
R
RSender
Problems of RPB 1. Broadcasts to subnets w/o group members2. Multiple broadcasts to the same link
RPM (Reverse Path Multicast) delivers to routers with group members only
Pruning– Cut branches w/o members– Start from leaf networks– Non-member routers send prune upstream
• An upstream router prunes itself when all downstream routers send prunes and sends prune upstream
Flood-and-pruning– Repeat flood periodically to restore whole shortest path tree
RPM
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Prune
S
R
RSender
Prune branches where no members and branches not on shortest paths of other members
Example - Detailed
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h1
h5
h4
h3
h2
R2 knows that it isresponsible to h1How?Mark this fact and forwardmulticast packet from D toh1How about Nx?
NxR1, R3 have the same cost to DDecide a parent for h2How?
NzNy
Diff. btw Nx and (Ny, Nz)? Ny is a leaf while Nx is not
How to know a network is leaf or not?
Single transmission
Example - Detailed
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h1
h5
h2
Nx
NzNy
No memberNo member
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MOSPF Multicast extensions to OSPF Extend LSA to report the groups active on a subnet
– Group-Membership-LSA
Source/destination routing– Source-based shortest path trees
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PIM(Protocol Indep. Multicast) Motivation
– Independence from unicast routing – Group members may be sparsely/densely populated– Select shared/source-based trees flexibly– Also, consider traffic intensity
SM (Sparse Mode)Simplicity is important Shared tree & Source-specific tree
DM (Dense Mode)Efficiency is important Source based tree
Similar to DVMRP
PIM-SM
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Start w/Shared Tree
Convert toSource specific tree
RP(Rendezvous Point) of each groupis determined in advance
GA In Sndr Out
G RP-R2 * R2-R4
* R2-R5
RP sends JOIN message to the sender Create sender-specific forwarding state
(S,G) state
No encapsulationBut, shared treeinefficiency
GA In Sndr Out
G R1-R3 S R3-RP
Sender-specific state
R
R
R
R
R
R
R
R
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PIM-SM Transmission
DR (Designated Router)
OverheadHow to decrease the overhead?
Tunneling
REGISTER
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Interdomain Multicast DVMRP, MOSPF are for intradomain multicast Suppose to use PIM for interdomain multicast
– Location of RP– Triangle routing
How to build interdomain multicast? Interdomain
– Source-specific tree
Intradomain– Each domain operates PIM-SM with its own RPs
RPR
S
R
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MSDP (Multicast Source Discovery)
RPs are connected to MSDP peers in foreign domains
Informs active senders to MSDP peers
Foreign RPs send JOIN messages to active senders to form a sender-specific tree across domain boundary
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PIM-SSM Source-specific multicast for one-to-many applications Channel
– (S, G) combination
Mechanism– A receiver report membership (channel) to a local router– The local router sends a JOIN message to the sender
• Bypass shared tree construction– Forms sender-specific multicast tree
Can be used for interdomain milticast
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BIDIR-PIM (Bidirectional) For many-to-many
applications within a domain– Conference
Forward packets regardless of incoming interface– Note PIM-SM forwards a packet
only when it is arrived from the upstream (i.e. from the RP)
MIP (Mobile IP)
Chong-kwon Kim
Host Mobility Support The Internet uses network based routing for scalability
– Assume hosts do not change locations (attachments)
What happens if hosts roam changing physical networks?– IP addresses should be changed
Root Cause of the problemIP address is both Identifier & LocatorSeparate ID & Locator roles
CN
MN(Mobil Node)
Invariant IP Address Suppose we change the IP addresses of an MN
– To communicate with the MN, CNs should know the new address– Can it be done?
The Internet was designed w/ an assumption that IP address is fixed– TCP connection (Flow) is defined by
• Source address & port number• Destination address & port number
How to support mobility while not violating the Internet semantics?– And your solution should be scalable, simple, etc
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Architecture & Basic Mechanism
MN (Mobile Node) CN (Correspondent Node)
HA (Home Agent)FA (Foreign Agent)
HN (Home Network)VN (Visited Network)
MN
CN
TunnelHA
FA
VNHN
● Preparation1. MN obtains a new address (called CoA (care-of address) at the VN
2. MN informs the care-of address to the HA using BU (Binding Update)
Two types of CoA- Co-located CoA- FA CoA
Tunneling & Encapsulation
MN
CN
TunnelHA
FA
VNHN
HoA Data
CA HoA DataS? D? HoA (Home Address)
CoA (Care-of Address)
CA(CN’s Address)
● CN MN
1. CN transparently sends a datagram to the MN w/ the original IP address (HoA)
2. The datagram arrives at HN(Home Network) & ARP request (may) will be issued
3. HA intercepts packets to the MN (How?)
4. HA relays packets to the VN for the MN
● MN CN
Destination address? Source address?
Any problems?
CA HoA Data
CA
Route Optimization Problem - Triangle routing
– Packets from CN to MN are relayed through HA
Route optimization– Send packets directly to the MN– MN sends BU to the CN– CN maintains “Binding cache “& sends datagrams to the
MN directly
Is encapsulation still needed? Probably Yes!! Why?
CN
HA FA
MN
CA CoA DATA
CA HoA DATA
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Cache Consistency
MN moves to a new location– Cache consistency problem
• CNs may have old care-of-address
HA FA1
CN
FA2
MN
Registration
Bindingupdate
BindingAck
In case of MN’s registration to a new FA- Send BU to the previous FA- Old FA also maintains binding cache and replies with “Binding Ack”
Send warning message to CNs