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The source tree adaptive routing (star) protocol, a link state routing algorithm that reduces the number of route updates by allowing packets to traverse paths that deviate from the optimum. Each node maintains a source routing tree and updates it only under certain conditions. The star protocol follows the least overhead routing approach and forms a 'partial topology graph' by aggregating the source trees sent by its neighbors.
Typology: Study notes
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Essentially involves disseminating
information with regards to each and every link
to each and every node in the network.
Dijkstra’s algorithm is often used for
achieving link state routing.
However, the bandwidth in a wireless network
is scarce.
Thus, the frequent dissemination of link state
(the frequency required would be higher if the
nodes were to move faster) is precarious to the
functioning of the network.
Each node maintains a source routing tree.
This tree is propagated to each of the
neighbors.
Updates may either be incremental or
atomic.
Updates to source routing trees only under
certain conditions.
SOME BORING (??) details.
The STAR Protocol follows what is called
the least overhead routing approach (LORA)
as opposed to the optimal routing approach
Remember !!! ORA is the used in DSDV and
The source tree that each node computes
(we talked about this earlier in an earlier
slide) – is not necessarily the minimum cost
tree.
The tree contains the preferred paths that
the node takes to each destination in the
Each node reports its source tree to its
neighbors through an update broadcast.
By aggregating the source trees sent by its
neighbors and the links on which these
source trees were sent, a node can form a
“Partial Topology Graph”.
Why is this partial?
Can you see some similarity with WRP?
A node sends a new update if :
If a new destination is discovered or if
one of its neighbors reports the existence
of a new destination.
At least one destination becomes
unreachable to the node or to one of its
neighbors.
Third and final condition is more
complex :
Needs one more slide !
We will defer it for later !
If a new destination is discovered or if one
of its neighbors reports the existence of a
new destination.
This is obvious is it not?
When the node is first initialized it sends a
broadcast to each of its neighbors.
The neighbors will then broadcast an
update (called Link State Update or LSU) to
inform this node of the existence of other
nodes, and in turn to inform other nodes
about the new node.
The third condition really has three parts:
Node A would transmit a packet to its
neighbors if:
The source tree of Node A has a path
which results in a loop. (How does A know ?)
Node A chooses a successor node Node B
for a given destination such that Node B has
a higher address than Node A.
The reported distance via Node B to the
destination node (say Node J) is longer than
the distance from the previous “successor”
node. HOWEVER ....
This is not true if previously Node J was a
direct neighbor of Node A, and Node B is a
neighbor to Node J.
In other words, no update is now
transmitted for Node J or any other node
that is reached via AB J.
Node A chooses a successor node Node B for
a given destination such that Node B has a
higher address than Node A.
This is in fact used to provide a further
robustness to loops.
Someone or the other has to have the lowest
address.
If you force that I broadcast an update
whenever I choose a successor with a higher
address, I provide further robustness to
routing.
Is it obvious why this is useful?
Well, if the other node chose me as a
The reported distance via Node B to the
destination node (say Node J) is longer than
the distance from the previous “successor”
node unless Condition A is satisfied.
This is primarily used to handle asymmetric
links – when a link in one direction has a
different cost from a link in another direction.
Better to explain this with an example –
which we consider next to fully make sure
that we understand STAR.
Hea
d
Nod
e
Tail
Nod
e
Link Cost
Hea
d
Nod
e
Tail
Nod
e
Link Cost
Global Link State
Let Link
between E
X fail.
This is bi-
directional.
Node E
tries to find
a new path
to X and Y.
Uses D’s
source tree.
Routes via
Link 6
through D.
Subsequent
ly, let the
link from
Node D to
Node Y, i.e.,
D Y fail.
What does
D do?
D has no
clue about
the fact the
link between
E and X has
in fact
failed.
Thanks to its ignorance, Node D will try to
relay packets to Nodes X and Y via Nodes C
and E ! – BUMMER I.
Also note that C’s address is smaller than
Node D’s.
So Node D does not broadcast any update
A loop is formed between nodes C, D, and
What happens if Condition 3 was
included?
