routing protocols for ad-hoc wireless networks lecture#12
TRANSCRIPT
Routing Protocols for Ad-Hoc Wireless Networks
Lecture#12
Traditional Routing A routing protocol sets up a routing table in routers A node makes a local choice depending on global topology
Routing Table AT 1
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Traditional Distance Vector Routing
Based on periodic exchanges of neighbor information
(connectivity and cost)
Changes at one node propagate slowly through the network
Strategies needed to avoid the “count to infinity” problem
(problems with fast topology changes)
• Poisoned-reverse
• Split-horizon
Potential problems …
• Unreachable regions
• Routing loops
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Tables
CDest. Next Metric …
A A 1B B 0C C 2
Dest. Next Metric …A A 0B B 1C B 3
1 2
Dest. Next Metric …A B 3B B 2C C 0
BA
4
(A, 1)(B, 0)(C, 1)
(A, 1)(B, 0)(C, 1)
Update
CDest. Next Metric …
A A 1B B 0C C 1
Dest. Next Metric …A A 0B B 1C B 3 2
1 1
Dest. Next Metric …A B 3 2B B 1C C 0
BA
B broadcasts the new routing information to his neighbors
Routing table is updated
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(D, 0)
(A, 2)(B, 1)(C, 0)(D, 1)
(A, 1)(B, 0)(C, 1)(D, 2)
New Node
C1 1
BA D1
broadcasts to update tables of C, B, A with
new entry for D
Dest. Next Metric …A B 2B B 1C C 0D D 1
Dest. Next Metric …A A 1B B 0C C 1D C 2
Dest. Next Metric …A A 0B B 1C B 2D B 3
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Broken Link
C1 1
BA D1
Dest.c Next Metric …… … …D C 2
Dest. Next Metric …… … …D B 3
Dest. Next Metric …… … …D B 1
Dest. Next Metric …… … …D D
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(D, 2)(D, 2)
Loops
C1 1
BA D1
Dest. Next Metric …… … …D B 3
Dest. Next Metric …… … …D C 2
Dest. Next Metric …… … …D B 3
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(D,2)
(D,4)
(D,3)
(D,5)
(D,2)
(D,4)
Count to Infinity
C1 1
BA D1
Dest. Next Metric …… … …D B 3, 5, …
Dest. Next Metric …… … …D B 3, 5, …
Dest.c Next Metric …… … …D C 2, 4, 6…
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Interaction with Static Internet
Two network types
• Transit network: a datagram can enter and then leave the network
• Stub network: the network is either the sink or source for all datagrams
MANETs are envisioned as being only stub network (at least for
now)
• Reduces power and processing demands
• Simplifies routing interaction
• Mobile IP allows connectivity with the traditional static Internet
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MANET vs.. Traditional Routing
Traditional routing algorithms are likely to be …• Inefficient due to slow convergence times (e.g., using distance vector algorithms
as in RIP)
• Non-functional due to large amounts of data or inability to deal with asymmetric links
MANET routing must rely on data link information, not just network layer updates• Link layer determines connectivity and quality of links
Centralized approaches are too slow and not robust enough for MANET All (or almost all) nodes in a MANET may be routers Long-lived circuits cannot be used in MANETs Path length (hop count) may not be the best metric for routing in
MANETs
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Routing and Mobility
Goal of routing protocols
• decrease routing-related overhead
• find short routes
• find “stable” routes (despite mobility)
Issues
• Frequent route changes
• Amount of data transferred between route changes may be much smaller than traditional networks
• Route changes may be related to host movement
• Low bandwidth links
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Issues in Designing a Routing Protocol for Ad Hoc Wireless Network
Mobility• Frequent path loss due to the movement of intermediate nodes or
the end nodes• Requires faster convergence time
• Wired routing protocols have slow convergence, therefore, not suitable for ad hoc networks
Limited bandwidth• Imposes constraint on routing protocols in maintaining the
topological information
• Frequent changes in topology makes it difficult to maintain a consistent view of the topological information• Require more control overhead which results in more BW wastage
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Issues in Designing a Routing Protocol for Ad Hoc Wireless Network
Error-Prone Shared Broadcast Radio Channel
• Wireless links have time varying characteristics in terms of link capacity and link error probability
• Requires that routing protocols to interact with the MAC layer to find an alternate routes through better quality links
• Transmission results in collision of data packets and control packets
• Required to find paths with less congestion
Hidden and exposed terminal problems
• Collision of data and control packets and under utilization of BW is attributed to hidden and exposed node problems
Resource constraints
• Battery life and processing power
• Routing protocols must optimally used these resource14
Qualitative Characteristics of an Ideal Routing Protocol
Fully distributed• Centralized routing involves high control overhead and are not scalable• Distributed routing more tolerant• Avoids the risk of single point failure
Adaptive to frequent topology changes Minimum route computation and maintenances Localized
• Global state involves a huge state propagation control overhead
Loop free Free from stale route Unidirectional link support Packet collision minimum by limiting the broadcast Convergence must be quick Optimally use the resources Each node should try to store the stable route information only Provide certain level of QoS 15
Quantitative Performance Metrics End-to-end throughput
• External measure of routing effectiveness
End-to-end delay• External measure of routing effectiveness
Route acquisition time• Of particular concern with on-demand routing algorithms
Percentage of out-of-order datagram deliveries• Affects performance of higher-layer protocols like TCP
Efficiency (internal measures)• Ratio of average number of data bits transmitted to average number of data
bits delivered• Ratio of average number of control bits transmitted to average number of data
bits delivered• Ratio of average number of control and data packets transmitted to average
number of data packets delivered16
Performance Factors
Network size• Number of nodes
Network connectivity• Average degree of a node
Rate of change of the topology• Speed at which the network topology is changing
Link capacity• Effective data rate (bits/second) after affects of packet loss, coding, multiple access
overhead, etc
Fraction of unidirectional links Traffic patterns
• Uniform versus non-uniform destinations
• Bursty versus non-bursty traffic
Mobility• Model for node mobility
Fraction and frequency of sleeping nodes 17
Classification of Routing Protocols
Based in routing information update mechanisms• Proactive or table driven protocols
• Reactive or on-demand routing protocols
• Hybrid routing protocols
Based on the use of temporal information on routing• Use the temporal information regarding the lifetime of wireless links
and the lifetime of the paths selected
• Routing protocols using past temporal information• Use past status of the link or the status of the link at the time of routing
• Routing protocols that use future temporal information• Life time of wireless links, remaining battery life, prediction of location,
and prediction of link availability
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Classification of Routing Protocols
Based on the routing topology• Internet use hierarchical routing topology in order to reduce the state
information maintained at the core routers• Flat topology routing protocols
• Use flat addressing scheme• Presence of a unique addressing scheme in an ad hoc network
• Hierarchical topology routing protocols• Make use of a logical hierarchy in the network and an associated addressing scheme• Could be based on geographical information or it could be based on hop distance
Based on the utilization of specific resource• Power aware routing
• Aims at minimizing the battery power, routing decisions are based on either globally or locally
• Geographical information assisted routing• Reduce the control overhead by effectively utilizing the geographical information
available19
Classification of Routing Protocols
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Based in Routing Information Update Mechanisms
Table Driven Routing Protocols
(Proactive Protocols)DSDV, WRP, CGSR, STAR
Protocol Trade-offs
Proactive protocols• Always maintain routes
• Little or no delay for route determination
• Consume bandwidth to keep routes up-to-date
• Maintain routes which may never be used
Reactive protocols• Lower overhead since routes are determined on demand
• Significant delay in route determination
• Employ flooding (global search)
• Control traffic may be bursty
Which approach achieves a better trade-off depends on the traffic and mobility patterns !!
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Destination Sequenced Distance Vector (DSDV)
Each route table at each of the nodes, lists all available destinations and the number of hops to each
Contains the shortest distance and the first node on the shortest path to every other node in the network
Each entry is tagged with a sequence number that is originated by the destination node
Each node periodically transmits updates • These updates indicate which nodes are accessible from each node and
number of hops to reach them
It incorporates table updates with increasing sequence number tags to prevent loops, to counter the count-to-infinity problems, and for faster convergence
Routing information is advertised by broadcasting or multicasting• Packets are transmitted periodically and incrementally
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Route Advertisements
Data is kept for a length of time between the arrival of the first and the arrival of the best route• Advertisement of possibly unstable routes is delayed to reduce the number of
rebroadcasts (damping fluctuations)
DSDV requires each mobile node to advertise its own routing table to each of its current neighbors
Entries may change fairly dynamically Each mobile computer agrees to relay data packets to other
computers upon request Mobile computer may exchange data with any other computer in a
group • Even if the target is not within the range of direct communication
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Route Establishment in DSDV
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Route Maintenance in DSDV
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Route Advertisements
Advertise to each neighbor own routing information• Destination Address• Metric = Number of Hops to Destination• Destination Sequence Number• Other info (e.g. hardware addresses)
Rules to set sequence number information• On each advertisement increase own destination sequence
number (use only even numbers)• If a node is no more reachable (timeout) increase sequence
number of this node by 1 (odd sequence number) and set metric = .
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Route Table Entry Structure
Data broadcast by each mobile computer contains its new sequence number and• Destination’s address• Number of hops required to reach destination• Sequence number of the information received regarding that
destination Within the header of the packet, the transmitted route
tables will also contain• Hardware address of transmitting node• Network address of transmitting node
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Table Entries
Sequence number originated from destination• Ensures loop freeness
Install time when entry was made• Used to delete stale entries from table
Stable data pointer to a table holding information on how stable a route is• Used to damp fluctuations in network
Destination Next Metric Seq. Nr Install Time Stable Data
A A 0 A-550 001000 Ptr_A
B B 1 B-102 001200 Ptr_B
C B 3 C-588 001200 Ptr_C
D B 4 D-312 001200 Ptr_D
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Route Table Entry Structure
Routes with more recent sequence numbers are always preferred as the basis of forwarding decisions• But they are not necessarily advertised
Problem • One way links
• Receiving a packet does not indicate the existence of path back to that neighbor
To avoid one way links• No mobile node inserts routing information received from a
neighbor unless that neighbor shows that it can receive packets from the mobile node
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Responding to Topology Change
Mobile nodes cause broken links as they move• Broken link is described by a metric of ∞
Any route through the broken link is immediately assigned an ∞ and an updated sequence number• Sequence number is incremented by one• Information describing a broken link is the only scenario in
which sequence number is generated by any node other than destination
Such a route change is immediately disclosed in broadcast routing information
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(D, 0, D-000)
DSDV (New Node)
CBA DDest. Next Metric Seq.
A A 0 A-550
B B 1 B-104
C B 2 C-590
Dest. Next Metric Seq.
A A 1 A-550
B B 0 B-104
C C 1 C-590
Dest. Next Metric Seq.
A B 2 A-550
B B 1 B-104
C C 0 C-590
D D 1 D-000
1. D broadcast for first timeSend Sequence number D-000
2. C inserts entry for D with sequence number D-000, then
immediately broadcast own table
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(A, 2, A-550)(B, 1, B-102)(C, 0, C-592)(D, 1, D-000)
(A, 2, A-550)(B, 1, B-102)(C, 0, C-592)(D, 1, D-000)
DSDV (New Node cont.)
CBA DDest. Next Metric Seq.
A A 1 A-550
B B 0 B-102
C C 1 C-592
D C 2 D-000
Dest. Next Metric Seq.
A A 0 A-550
B B 1 B-104
C B 2 C-590
Dest. Next Metric Seq.
A B 2 A-550
B B 1 B-102
C C 0 C-592
D D 1 D-000
………………
3. C increases its sequence number to C-592 then
broadcasts its new table.4. B gets this new information and updates its table…….
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(D, 2, D-100)(D, 2, D-100)
DSDV (no loops, no count to infinity)
CBA D1
Dest.c Next Metric Seq.
… … …
D C 2 D-100
Dest. Next Metric Seq.
… … …
D B 3 D-100
Dest. Next Metric Seq.
… … …
D D D-101
1. Node C detects broken Link:-> Increase Seq. Nr. by 1
(only case where not the destination sets the sequence number -> odd
number)
2. B does its broadcast-> no affect on C (C knows that B has stale information because C has higher seq. number for destination D)
-> no loop -> no count to infinity
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(D, , D-101)(D, , D-101)
DSDV (Immediate Advertisement)
CBA D
Dest.c Next Metric Seq.
… … …
D C 3 D-100
Dest. Next Metric Seq.
… … …
D B 4 D-100
Dest. Next Metric Seq.
… … …
D B 1 D-100
Dest. Next Metric Seq.
… … …
D D 1 D-100
D D D-101
1. Node C detects broken Link:-> Increase Seq. Nr. by 1
(only case where not the destination sets the sequence number -> odd
number)
3. Immediate propagation B to A:
(update information has higher Seq. Nr. -> replace table entry)
2. Immediate propagationC to B:
(update information has higher Seq. Nr. -> replace table entry)
Dest.c Next Metric Seq.
… … … ...
D C 2 D-100
D C D-101
Dest. Next Metric Seq.
… … … ...
D B 3 D-100
D B D-101
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Full/Incremental Update
Full Update• Send all of the routing information from own table• Requires multiple Network Protocol Data Units (NPDU)
A full update is scheduled• When movements become frequent and the incremental update size
approaches the size of a NPDU• To make the next incremental update smaller
Incremental Update• Send only entries that have changed since last full Update• Make it fit into one single packet
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Route Selection Criteria
On receiving a new routing information• Information is compared to the information already available• Metric is incremented by one• New information is scheduled for advertisement
Any route with more recent sequence number is used• Routes with older sequence numbers are discarded
A route with sequence number equal to an existing route is chosen if it has a better metric• Older route may be discarded or stored as less preferable
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Operating DSDV at Layer 2
Addresses stored in the route tables correspond to the layer at which the DSDV is operated• e.g., Operation at layer 3 uses network layer addresses
For operation at layer 2• Layer 3 protocol information along with layer 2 information
should be included• Each destination node would advertise which layer 3 protocols it
supports• Each node advertising reachability would also include the
supported layer 3 protocols• Changes would be part of incremental update
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DSDV (Problem of Fluctuations)
What are Fluctuations• Entry for D in A: [D, Q, 14, D-100]
• D makes Broadcast with Seq. No. D-102
• A receives from P Update (D, 15, D-102)-> Entry for D in A: [D, P, 15, D-102] A must propagate this route immediately
• A receives from Q Update (D, 14, D-102)-> Entry for D in A: [D, Q, 14, D-102]A must propagate this route immediately
This can happen every time D or any other node does its broadcast and lead to unnecessary route advertisements in the network, so called fluctuations
A
D
QP
10 Hops11 Hops
(D,0,D-102)
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DSDV (Damping Fluctuations)
A
D
QP
10 Hops11 Hops
How to damp fluctuations• Record last and avg. Settling Time of every Route in a
separate table. (Stable Data)Settling Time = Time between arrival of first route and the best route with a given seq. no
• A still must update his routing table on the first arrival of a route with a newer seq. no., but it can wait to advertising it. Time to wait is proposed to be 2*(avg. Settling Time)
• Like this, fluctuations in larger networks can be damped to avoid unnecessary advertisement, thus saving bandwidth
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Summary
Advantages• Simple (almost like Distance Vector)
• Loop free through destination seq. numbers
• No latency caused by route discovery
Disadvantages• No sleeping nodes
• Bi-directional links required
• Overhead: most routing information never used
• Scalability is a major problem
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Source Tree Adaptive Routing(STAR) A bandwidth efficient routing protocol for ad hoc networks A variation of table-driven routing protocols, with the Least
Overhead Routing Approach (LORA) as the key concept rather than the Optimum Routing Approach (ORA) that was employed by the most of the table-driven routing protocols
LORA provides all the feasible paths with respect to some metric which may not be guaranteed optimal but with much less control overhead
Every node broadcasts its source tree information• The set of links used by a router in its preferred path to a destination is
called a source tree
Routers in STAR communicate their source routing trees to neighbors for all possible destinations
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Every node using its adjacent links, and the source tree broadcast by its neighbor links, builds a partial graph of its topology
During initialization, a node sends its update messages about new destinations, the chances of routing loops, and costs of paths exceeding a given threshold
Hence, each node will have a path to every destination node • The path, in most cases, would be sub-optimal
Such updates are communicated only when:• Routers determine long-term loops can be created• New destinations are found• When a destination becomes unavailable
Source Tree Adaptive Routing(STAR)
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Source Tree Adaptive Routing(STAR)
The basic update unit used to communicate changes to source trees is the Link State Update (LSU)
STAR uses sequence numbers to validate the LSUs A sequence number associated with a link consists of a counter that
can only be incremented by the head node (originator) of that link As LSUs for operational links never age out, there is no need for
the head node of a link to send periodic LSUs to update link’s sequence number
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Path Finding Approach When a nodes s has data packets to send to a particular destination
d, for which no path exists in its source-tree, its originates an update message to all its neighbors indicating the absence of a path to d
This update message triggers another update message from a neighbor which has path to d
Node s retransmits the update message as long as it does not have a path to d with increasing intervals between successive retransmissions
After getting the source-tree update from a neighbor, the node s updates its source-tree and, using this, finds a path to all nodes in the network
The data path contains information about the path to be traversed in order to prevent the possibility of routing loop formation
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Route Maintenance
In the presence of a reliable broadcast mechanism, START assumes implicit route maintenance
Path breaks• The link update message about the unavailability of a next-hop
node triggers an update message from a neighbor which has an alternate source tree indicating and alternate next-hop node to the destination
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Route Maintenance Handling the routing loops
• When an intermediate node k receives a data packet to destination d, and one of the nodes in the packet`s traversed is present in node k`s path to the destination d, then it discards the packet and a RouteRepair update message is reliably sent to the node in the head of the route repair path
• The route repair path corresponds to the path k to x, where x is the last router in the data packet`s traversed path that is first found in the path k to d, that belongs to the source tree of k
• The RouteRepair packet contains the complete source tree of node k and the traversed path of the packet
• When an intermediate node receives a RouterRepair update message, it removes itself from the top of the route repair path and reliably sends it to the head of the route repair path
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STAR Exchanging Update Messages Update messages exchange depends on the routing approach
used(ORA or LORA) For ORA to be supported in STAR, a router must send an update
message whenever its source tree changes For LORA in STAR, a router reports updates to its source trees in
event of unreachable nodes, new destinations, loops or cost exceeding a given threshold
If the new source tree includes neighbors other than those present in the source tree that was last updated, the router must send its entire source tree in the update
If the two source trees contain the same neighbors, the router sends only the updates needed to obtain the new tree from the old one
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STAR Impact of the Link Layer If the underlying MAC protocol does not provide collision-free
transmission for reliable broadcasting, then in STAR, the router broadcasts to neighbors unreliably
A packet must specify the path it has traversed A router must send its update reliably to the neighbor that caused it
to change its source routing tree
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Advantages Low communication overhead among all the table-driven routing
protocols The use of LORA approach in this table-driven routing protocol
reduces the average control overhead compared to several other on-demand routing protocols
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