0. Link Reversal Algorithms
Jennifer L. Welch
DISC 2014 Tutorial

1. What is Link Reversal? Distributed algorithm design technique
Used in solutions for a variety of problems
routing, leader election, mutual exclusion, scheduling, resource allocation,…
Model problem as a directed graph and reverse the direction of links appropriately
Use local knowledge to decide which links to reverse

2. Outline Routing in a Graph: Correctness Routing in a Graph: Complexity
Routing and Leader Election in a Distributed System
Mutual Exclusion in a Distributed System
Scheduling in a Graph
Resource Allocation in a Distributed System
Distinction between a “graph” and a “distributed system”: in the former, more abstract, just worry about vertices and links between them, info about neighbors is magically available.
In the latter, take into account asynchronous message passing, and other real-world complications.

3. ROUTING IN A GRAPH: CORRECTNESS
Section 1
ROUTING IN A GRAPH: CORRECTNESS

4. Routing [Gafni & Bertsekas 1981]
Undirected connected graph represents communication topology of a system
Unique destination node D 1 2 3 4 5 6

5. Routing [Gafni & Bertsekas 1981]
Assign virtual directions to the graph edges (links) s.t.
if nodes forward messages over the links, they reach the destination
Directed version of the graph (orientation) must
be acyclic
have destination as only sink
Thus every node has path to destination. D 3 1 2 4 5 6

6. Mending Routes What happens if some edges go away?
Might need to change the virtual directions on some remaining edges (reverse some links)
More generally, starting with an arbitrary directed graph, each vertex should decide independently which of its incident links to reverse

7. Mending Routes Example3 1 2 4 5 6

8. Sinks A vertex with no outgoing links is a sink.
The property of being a sink can be detected locally.
A sink can then reverse some incident links
Basis of several algorithms…
sink

9. Full Reversal Routing Algorithm
Input: directed graph G with destination vertex D
Let S(G) be set of sinks in G other than D
while S(G) is nonempty do
reverse every link incident on a vertex in S(G)
G now refers to resulting directed graph

10. Full Reversal (FR) Routing ExampleD 1 2 3 4 5 6 D 1 2 3 4 5 6 D 1 2 3 4 5 6 D 1 2 3 4 5 6 D 1 2 3 4 5 6 D 1 2 3 4 5 6

11. Why Does FR Terminate? Suppose it does not.
Let W be vertices that take infinitely many steps
nonempty by assumption
Let X be vertices that take finitely many steps
nonempty since it includes D
Consider neighboring vertices w in W, x in X
exist since graph is connected
Consider first step by w after last step by x: link is w g x and stays that way forever.
Then w cannot take any more steps, contradiction. X
(stop) D w x W
(don’t
stop)

12. Why is FR Correct? Assume input graph is acyclic.
Acyclicity is preserved at each iteration:
Any new cycle introduced must include a vertex that just took a step, but such a vertex is now a source (has no incoming links)
When FR terminates, no vertex, except possibly D, is a sink.
A DAG must have at least one sink:
if no sink, then a cycle can be constructed
Thus output graph is acyclic and D is the unique sink.

13. Pair Algorithm
easy to argue
Can implement FR by having each vertex v keep an ordered pair (c,v), the height (or vertex label) of vertex v
c is an integer counter that can be incremented
v is the id of vertex v
View link between v and u as being directed from vertex with larger height to vertex with smaller height (compare pairs lexicographically)
If v is a sink then v sets c to be 1 larger than maximum counter of all v’s neighbors

14. Pair Algorithm Example
(1,0) 3 1 2
(0,1)
(0,2)
(2,3)
(2,1)

15. Pair Algorithm Example
(1,0) 3 1 2
(0,1)
(0,2)
(2,3)
(2,1)
(3,2)

16. Pair Algorithm Example
(1,0) 3 1 2
(0,1)
(0,2)
(2,3)
(2,1)
(3,2)

17. Partial Reversal Routing Algorithm
Try to avoid repeated reversals of the same link.
Vertices keep track of which incident links have been reversed recently.
Link (u,v) is reversed by v iff the link has not been reversed by u since the last iteration in which v took a step.

18. Partial Reversal (PR) Routing ExampleD 1 2 3 4 5 6 D 1 2 3 4 5 6 D 1 2 3 4 5 6 D 1 2 3 4 5 6 D 1 2 3 4 5 6 D 1 2 3 4 5 6
Green indicates links that were reversed in FR but not in PR.
Note that final DAG is different in PR than in FR.

19. Why is PR Correct?
Termination can be proved similarly as for FR: difference is that it might take two steps by w after last step by x until link is w g x . X
(stop) D w x W
(don’t
stop)

20. Why is PR Correct?
Preservation of acyclicity is more involved, deferred to later.
Alternate geometric proof due to Radeva and Lynch 2011

21. Triple Algorithm
not so easy to argue
Can implement PR by having each vertex v keep an ordered triple (a,b,v), the height (or vertex label) of vertex v
a and b are integer counters
v is the id of vertex v
View link between v and u as being directed from vertex with larger height to vertex with smaller height (compare triples lexicographically)
If v is a sink then v
sets a to be 1 greater than smallest a of all its neighbors
sets b to be 1 less than smallest b of all its neighbors with new value of a (if none, then leave b alone)
Actually it is not immediately obvious that the Triple algorithm implements Partial Reversal: requires some arguing.
This is in contrast to the previous situation, where it is easy to see that the Pair algorithm implements Full Reversal.

22. Triple Algorithm Example
(0,1,0) 3 1 2
(0,0,1)
(0,0,2)
(0,2,3)
(1,0,1)

23. Triple Algorithm Example
(0,1,0) 3 1 2
(0,0,1)
(0,0,2)
(0,2,3)
(1,0,1)
(1,-1,2)

24. Triple Algorithm Example
(0,1,0) 3 1 2
(0,0,1)
(0,0,2)
(0,2,3)
(1,0,1)
(1,-1,2)

25. General Vertex Label Algorithm
Generalization of Pair and Triple algorithms
Assign a label to each vertex s.t.
labels are from a totally ordered, countably infinite set
new label for a sink depends only on old labels for the sink and its neighbors
sequence of labels taken on by a vertex increases without bound

26. Correctness of General Vertex Label Algorithm
Termination: Similar to the arguments for LR and PR.
Difference is that it might take several steps by w after last step by x until link is w g x.
However it will happen because w’s label increases without bound and eventually is larger than x’s final label. X
(stop) D w x W
(don’t
stop)
Termination proof is along the same lines as for PR. Acyclicity preservation is easy because labels are from totally ordered set.

27. Correctness of General Vertex Label Algorithm
Acyclicity is preserved because labels are from a totally ordered set
Termination proof is along the same lines as for PR. Acyclicity preservation is easy because labels are from totally ordered set.

28. Relationship Between AlgorithmsFR PR ?
Pair Algorithm
Triple Algorithm
General Vertex Label Algorithm

29. Binary Link Labels Routing [Charron-Bost et al. 2013]
Alternate way to implement and generalize FR and PR
Instead of unbounded vertex labels, apply binary link labels to input DAG
link directions are independent of labels (in contrast to algorithms using vertex labels)
Algorithm for a sink:
if at least one incident link is labeled 0, then reverse all incident links labeled 0 and flip labels on all incident links
if no incident link is labeled 0, then reverse all incident links but change no labels

30. Binary Link Labels Example1 3 1 2 1

31. Binary Link Labels Example1 3 1 2 1

32. Binary Link Labels Example1 3 1 2 1

33. Why is BLL Correct?
Termination can be proved very similarly to termination for PR.
Takes 1 or 2 steps until link is w g x. X
(stop) D w x W
(don’t
stop)

34. Why is BLL Correct?
What about acyclicity preservation? Depends on initial labeling: 1 3 2 1 3 2

35. Conditions on Initial Labeling
All labels are the same
all 1’s => Full Reversal
all 0’s => Partial Reversal
Every vertex has all incoming links labeled the same (“uniform” labeling)
Both of the above are special cases of a more general condition that is necessary and sufficient for preserving acyclicity
Acyclicity Condition: for every circuit in the graph, (w+s)(r+s) > 0, where
r is number of right-way marked links,
w is number of wrong-way marked links
s is number of occurrences in the circuit s.t. both its incident links in the circuit are incoming and unmarked.

36. Relationship Between Algorithms Revisited
Binary Link Label Algorithm FR PR ?
Pair Algorithm
Triple Algorithm
General Vertex Label Algorithm

37. ROUTING IN A GRAPH: COMPLEXITY
Section 2
ROUTING IN A GRAPH: COMPLEXITY

38. What About Complexity?
Busch et al. (2003,2005) initiated study of the performance of link reversal routing
Work complexity of a vertex: number of steps taken by the vertex
Global work complexity: sum of work complexity of all vertices

39. Worst-Case Work Complexity Bounds [Busch et al.]
bad vertex: has no (directed) path to dest.
Pair algorithm (Full Reversal):
for every input, global work complexity is O(n2), where n is number of initial bad vertices
for every n, there exists an input with n bad vertices with global work complexity Ω(n2)
Triple algorithm (~Partial Reversal): same as Pair algorithm (if appropriately initialized)
Perhaps unintuitive that PR is not better than FR, at least in worst case.

40. Exact Work Complexity Bounds
A more fine-grained question: Given any input graph and any vertex in that graph, exactly how many steps does that vertex take?
Busch et al. answered this question for FR.
Charron-Bost et al. answered this question for BLL (as long as labeling satisfies Acyclicity Condition): includes FR and PR.
Acyclicity Condition: For every circuit, (w+s)(r+s) > 0, where
w = number of marked wrong-way links
r = number of marked rightway links
s = number of sinks in the circuit with both links unmarked

41. Definitions
Let C = <v1, v2, …, vk> be a chain in the labeled input DAG (series of vertices s.t. either (vi,vi+1) or (vi+1,vi) is a link).
r: number of links that are labeled 1 and rightway ((vi,vi+1) is a link, i.e., directed away from D)
s: number of occurrences of vertices s.t. the two adjacent links are incoming and labeled 0
Res: 1 if last link in X is labeled 0 and rightway, else 0

42. Example of Definitions1 1 1 2 3 8 D 4 1 1 1 7 6 5
For chain <D,7,6,5>: r = 1, s = 0, Res = 1 For chain <D,1,2,3,4,5>: r = 2, s = 1, Res = 0

43. Grouping the Vertices group S group D
D is set of all vertices with an incoming link labeled 0 and either an incoming link labeled 1 or an outgoing link
S is all other vertices (all incoming links, if any, labeled 1 or a sink with all links labeled 0) 1
group S
group D

44. BLL Work Complexity Theorem: The work complexity of vertex v is
min{r(C) + s(C) + Res(C) : C is a chain from D to v}
if v is in S
min{2r(C) + 2s(C) + Res(C) : C is a chain from D to v}
if v is in D
Proof is by showing how different combinations of r, s, and Res are either invariant or increase by certain amounts throughout execution until a chain becomes a path from v to D.

45. Work Complexity for FR Recall: The work complexity of vertex v is
min{r(C) + s(C) + Res(C) : C is a chain from D to v} if v is in S
min{2r(C) + 2s(C) + Res(C) : C is a chain from D to v} if v is in D
For FR (all labels are 1), all vertices are in S, s(C) = 0, and Res(C) = 0. Thus formula becomes
minimum, over all chains from D to v, of number of links in the chain that are directed away from D
Worst-case graph for global work complexity:
D  1  2  …  n
vertex i has work complexity i
global work complexity then is Θ(n2)

46. Work Complexity for PR
For PR (all labels are 0 initially), work complexity of vertex v is
min, over all D-to-v chains, of s + Res if v is a sink or a source initially
min, over all D-to-v chains, of 2s + Res if v is neither a sink nor a source initially
Worst-case graph for global work complexity:
D  1  2  3  …  n
work complexity of vertex i is Θ(i)
global work complexity is Θ(n2)

47. Comparing FR and PR
Looking at worst-case global work complexity shows no difference – both are quadratic in number of bad vertices
Can use game theory to show some meaningful differences (Charron-Bost et al. 2013)
Consider class of uniform labelings: for each vertex, all incoming links have the same binary label
A vertex’s choice of initial label for its incoming links can be viewed as its strategy in a game 1 or
Should v play ? v

48. Comparing FR and PR with Game Theory
Compare FR and PR among all uniform labelings (every vertex initially has all its incoming links with same label)
Global work complexity of FR can be larger than optimal (w.r.t. all uniform labelings) by a factor of Θ(n)
Global work complexity of PR is never more than twice the optimal
Thus PR is a safer bet than than FR w.r.t. global work complexity
FR is a Nash equilibrium. PR is not necessarily a Nash equilibrium.

49. Time Complexity
Time complexity is number of iterations in greedy execution (all sinks take step in each iteration)
Busch et al. (2003, 2005) observed that time complexity cannot exceed global work complexity
Thus O(n2) iterations for both pair (FR) and triple (~PR) algorithms
Busch et al. also showed graphs on which pair (FR) and triple (~PR) algorithms require Ω(n2) iterations
BLL, as long as acyclicity condition holds

50. Time Complexity
Charron-Bost et al. (2011) derived an exact formula for the last iteration in which any vertex takes a step in any graph for BLL…
BLL, as long as acyclicity condition holds

51. FR Time Complexity: Recurrence for Work
Let Wi(t) be number of steps vertex i has taken by iteration t
Identify a recurrence relation for Wi(t) based on understanding how vertex i and its neighbors take turns being sinks: 1 3 2 o
t = 0
Wi(t+1) = min{Wj(t)+1,Wk(t) :
j is initially incoming to i,
k is initially outgoing from i} 1 3 2 o
t = 1 1 3 2 o
t = 2 1 3 2 o
t = 3 1 3 2 o
t = 4

52. Linear Recurrence
Wi(t+1) = min{Wj(t)+1,Wk(t) : j is initially incoming to i, k is initially outgoing from i}
This recurrence is linear in the min-plus algebra 
Represent system of equations as a matrix A
W(t+1) = A  W(t) = At  O (O is all-zero vector)
Classical duality arguments give us an expression for the time vector using max-plus algebra
Formula for time complexity of general BLL is rather technical

53. FR Time Complexity Formula
Theorem: For every bad vertex v, termination time of v is
1 + max{len(C): C is chain ending at v with r = σv – 1}
where σv is the work complexity of v
Worst-case graph for global time complexity: n
n-1 D 1 2
n/2
n/2+3
vertex n/2 has work complexity n/2;
consider chain that goes around the loop
counter-clockwise n/2-1 times starting and
ending at n/2: has r = n/2-1 and length Θ(n2)
n/2+1
n/2+2

54. BLL Time Complexity What about other link labelings? Transform to FR!
In more detail: for every labeled input graph G (satisfying the Acyclicity Condition), construct another graph T(G) s.t.
for every execution of BLL on G, there is a “corresponding” execution of FR on T(G)
time complexities of relevant vertices in the corresponding executions are the same

55. Idea of Transformation
If a vertex v is initially in the category D (a sink with some links labeled 0 and some labeled 1, or not a sink with an incoming link labeled 0), then its incident links are partitioned into two sets:
all links in one set reverse at odd-numbered steps by v
all links in the other set reverse at even-numbered steps by v
Transformation replaces each vertex in D with two vertices, one corresponding to odd steps by v and the other to even steps, and inserts appropriate links
Vertices initially in category S reverse all their links at every step they take, which is already FR-like behavior
Vertices initially in category S reverse all their links at every step they take, which is FR-like behavior

56. Example of LR->FR Transformation
LR input
FR input 1
Yellow vertex in graph on left is in D, so it is replaced
with two vertices in graph on right

57. PR Time Complexity
Theorem: For every vertex v with no path to D originally, termination time of v is
1 + max{len(C): C is a chain ending at v with s + Res = σv – 1} if v is a sink or a source initially
1 + max{len(C): C is a chain ending at v with 2s + Res = σv – 1} otherwise
Worst-case graph for global time complexity:
D  1  2  3  …  n/2  n/2+1  …  n
Vertex n/2 has work complexity n/2. Consider chain that starts at n/2, ends at n/2, and goes back and forth between n/2 and n making (n-2)/4 round trips. 2s+Res for this chain is n/2-1, and length is Θ(n2).
Not giving general formula for time complexity of vertex v in BLL – rather technical. Instead, just giving its specialization to PR.

58. FR vs. PR Again
On chain on previous slide, PR has quadratic time complexity.
But FR on that chain has linear time complexity
in fact, FR has linear time complexity on any tree
On chain on previous slide, PR has (slightly) better global work complexity than FR.

59. ROUTING AND LEADER ELECTION IN A DISTRIBUTED SYSTEM
Section 3
ROUTING AND LEADER ELECTION IN A DISTRIBUTED SYSTEM

60. From Graph to Distributed System
To adapt previous ideas to a distributed system:
Node (processor) is a vertex
Communication channel is an edge (link)
Issues to be overcome:
Neighboring nodes need to communicate to agree on which way the link between them should be directed: delays and losses
Topology can change due to movement and failures; might not always be connected

61. Routing in a Dynamic System
In any execution that experiences a finite number of topology changes, after the last topology change:
every node in same connected component as D (destination) should have a path to D
every node not in the same component as D should stop trying to find a route to D or forward a message to D

62. What’s Wrong with FR? D 2 1 3 D 2 1 3 D 2 1 3 D 2 1 3

63. TORA [Park & Corson 1997]
Modify the generalized algorithm of Gafni & Bertsekas using increasing vertex labels
Vertex labels, or heights, are 5-tuples
one entry is current time: Temporally Ordered Routing Algorithm
Every node in same connected component as D eventually has a path to D in the directed version of the communication graph induced by the heights
Clever use of additional entries in the heights allows node to tell when they are partitioned from D and should stop participating

64. Heights in TORA [ t , oid , r , d , i ] reference level delta
reflection
bit
orders
nodes with
same ref.
level
id of node
originating
this ref.
level
id of node,
breaks ties
time this
ref. level
was started

65. TORA Overview
Route Creation: use standard spanning tree construction ideas to set ref levels to (0,0,0) and deltas to distances from D
Route Maintenance and Partition Detection: see next slide
Route Erasure: When partition is detected, flood “clear” messages throughout component

66. TORA Route Maintenance
If node i loses last outgoing link:
due to a link failure (Case Generate):
set ref level to (current time, i, 0), a full reversal
due to a height change and
nbrs don’t have same ref level (Case Propagate):
adopt max ref level and set d to effect a partial reversal
nbrs have same ref level with r = 0 (Case Reflect):
adopt new ref level, set r to 1, set d to 0 (full reversal)
nbrs have same ref level with r = 1 and oid = i (Case Detect):
Partition! Start process of erasing routes.

67. TORA Example – PartitionD 1 2 3 4 5
Generate D 1 2 3 4 5
Propagate D 1 2 3 4 5
Reflect D 1 2 3 4 5
Detect D 1 2 3 4 5
Propagate

68. TORA Discussion Works best with perfectly synchronized clocks
How to prove correctness?
Gafni & Bertsekas result does not directly apply because of asynchronous delay in updating neighbors about new heights
Other issues remain: partition detection, route creation, route erasure
Can be adapted to solve leader election: when partition is detected, elect a new leader! (Cf. Ingram et al. 2013)

69. MUTUAL EXCLUSION IN A DISTRIBUTED SYSTEM
Section 4
MUTUAL EXCLUSION IN A DISTRIBUTED SYSTEM

70. Link Reversal for Mutual Exclusion [Snepscheut 1987]
Goal: no-lockout mutual exclusion in a message-passing system with a tree communication topology
Solution: Pass around a unique “token” message.

71. Link Reversal for Mutual Exclusion [Snepscheut 1987]
impose logical directions on communication channels s.t. token holder is unique sink
when a node has the token, it can enter critical section
When node needs the token, it sends “request” message on its unique outgoing link – toward token holder
When a node receives a request, it remembers it in a FIFO queue and forwards it toward the token-holder (if not already waiting)
when token holder responds to a request, it forwards the token to the neighbor at the head of the queue, and reverses direction of that link

72. From a Tree to a DAG
For a general communication topology, the previous algorithm can be run on a spanning tree overlay of the graph.
However, this does not take advantage of the redundancy offered by additional links.
Instead, direct all links in the graph:
request message can be forwarded on any outgoing link
when a node receives the token, all its outgoing links are reversed, to make it a sink

73. From a Tree to a DAG

74. Related Work
Naimi and Trehel 1987: analyzed expected message complexity
Raymond 1989: gave fault-tolerant extension
Dhamdhere and Kulkarni 1994: considered dynamic topogies
Walter et al. 2001: handled dynamic topologies by adding vertex labels

75. Section 5
SCHEDULING IN A GRAPH

76. Scheduling in a Graph [Barbosa & Gafni 1989]
What happens when the Full Reversal routing algorithm is executed without a destination?
I.e., every vertex in the graph does a reversal when it is a sink
Call this algorithm FRND (FR with No Destination).
When a vertex is a sink, it is said to be scheduled: can take some action with the guarantee that none of its neighbors are scheduled at the same time.

77. Behavior of FRND Thus every vertex is scheduled infinitely often.
Claim: FRND maintains acyclicity.
Same proof as for FR.
Claim: Every vertex is a sink infinitely often.
By Claim 1, at each iteration there is at least one sink, so FRND never terminates.
If some vertices take finitely many steps and some take infinitely many, then use same argument as for showing FR terminates to get a contradiction.
Thus every vertex is scheduled infinitely often.

78. FRND Example 2 1 3 4 5 2 1 3 4 5 2 1 3 4 5 2 1 3 4 5 2 1 3 4 5 2 1 3 4 5

79. Behavior of FRND
Claim: In the greedy execution of FRND from an initial DAG, eventually the pattern of sinks becomes periodic.
Let G1, G2,… be the sequence of graph states in the execution. Since finite number of vertices, Gi = Gj for some i and j. Thus Gi+1 = Gj+1, etc.
Note: every vertex appears at least once in every period.

80. Q&A about FRND How long until the execution becomes periodic?
At most polynomial number of iterations [Malka & Rajsbaum 1991]
How long is the period?
At least 2 iterations, since neighbors cannot be sinks simultaneously
Can be as bad as exponential [Malka et al. 1993]
How “fair” is the period?
every vertex takes same number of steps…

81. Period is Fair
Claim 1: Difference in total number of steps taken by u and v at any iteration is at most the distance between them.
Proof is by induction on the distance.
Claim 2: Every vertex takes same number of steps in the period.
Suppose u appears a times and v appears b times with b > a. After k-th execution of the period, u has taken ka steps and v has taken kb steps. Eventually kb – ka exceeds the distance between u and v, contradicting Claim 1.

82. Multiplicity and Concurrency
Multiplicity of the period is the number of times that each vertex takes a step in the period.
Concurrency is ratio of multiplicity m to the period length p, i.e., m/p.
Concurrency is fraction of iterations during which any given vertex takes steps.
at most 1/2
at least 1/n

83. Multiplicity and Concurrency
Multiplicity of the period is the number of times that each vertex takes a step in the period.
Concurrency is ratio of multiplicity m to the period length p, i.e., m/p.
Concurrency is fraction of iterations during which any given vertex takes steps.
at most 1/2
at least 1/n

84. Concurrency Example Period length is 5 multiplicity is 2
concurrency is 2/5

85. Exact Expression for Concurrency
Claim 1: For any initial orientation of a tree, the greedy execution of FRND reaches a periodic orientation with length 2 and multiplicity 1, so concurrency = 1/2.
Claim 2: For any periodic orientation G of a non-tree graph, the concurrency is equal to the minimum, over all (simple) circuits k in G, of the fraction of links in k that are right-way.
For Claim 2, look back at the pentagon examples:
clockwise circuit has 2 right-way links out of 5
counter-clockwise circuit has 3 right-way links out of 5

86. Choosing a Good Initial Orientation
For trees, the initial orientation is unimportant: they all lead to a period with concurrency 1/2
For non-trees, the initial orientation can make a big difference to the concurrency:
consider a ring of n vertices, where n is even
if initially there is just 1 sink, there will never be more than 1 sink in any orientation: concurrency is 1/n
if initially every other vertex is a sink, vertices keep alternating: concurrency is 1/2
Unfortunately, determining the best orientation is NP-complete!

87. RESOURCE ALLOCATION IN A DISTRIBUTED SYSTEM
Section 6
RESOURCE ALLOCATION IN A DISTRIBUTED SYSTEM

88. Resource Allocation in a Distributed System [Chandy & Misra 1984]
Dining philosophers (or resource allocation) problem is a generalization of the mutual exclusion problem:
conflict graph: vertices correspond to the nodes, edge between I and j means I and j compete for exclusive access to a resource
Ensure exclusion: no two neighbors in the conflict graph are in their critical sections simultaneously
Ensure fairness: every node requesting access to its critical section eventually is granted access
There are multiple resources,
each resource can be used by only one node at a time,
different resources can be used at the same time, and
each proc needs multiple resources (corresponding to all its incident edges) to enter its critical section

89. First Solution
Use FRND on the conflict graph: when a vertex is a sink, it can enter its critical section
Every vertex is a sink infinitely often
no two neighbors are sinks simultaneously
Issues:
How to adapt FRND to asynchronous message passing?
Why bother a node that is not interested in entering its critical section?

90. Chandy & Misra’s Solution
Key data structure is precedence graph, directed version of conflict graph
Precedence graph is represented in a distributed fashion by having each node keep a variable for each neighbor indicating who yields to whom
variables are initialized so that precedence graph is acyclic
Each pair of neighbors i and j share a token to ensure exclusion
if i doesn’t have token when it wants to enter C.S. it sends request to j
j sends back the token immediately if j is in its remainder section or if it is in its trying section and i has precedence over j, otherwise j defers the request from i

91. Chandy & Misra’s Solution
Thus precedence graph is used to arbitrate between contending neighbors, but otherwise is ignored.
Once i has all its tokens, it enters the C.S.
When i leaves the C.S., it satisfies all deferred requests and does a full reversal in the precedence graph

92. Correctness Ideas Management of tokens ensures exclusion.
By starting with an acyclic conflict graph and only modifying it with full reversal, the precedence graph is always acyclic:
no deadlock can be caused by a cycle of nodes waiting on each other

93. Conclusion Other applications of link reversal include:
distributed queueing
k-mutual exclusion
publish/subscribe
simulated annealing
graph coloring
neural networks
Appeal of the approach is using local knowledge to solve global problems

94. Open Questions
Explore consequences of work and time bounds for BLL on other applications of link reversal
Discover effects on bounds of distributed system realities:
message delays
failures
ongoing topology changes X X

95. Acknowledgements
Welch and Walter, “Link Reversal Algorithms,” Synthesis Lectures on Distributed Computing Theory #8, Morgan & Claypool Publishers, 2011.
Editor Nancy Lynch
My collaborators: Bernadette Charron-Bost, Matthias Fuegger, Antoine Gaillard, Rebecca Ingram, Navneet Malpani, Mira Radeva, Patrick Shields, Nitin Vaidya, Saira Viqar, Jennifer Walter, Josef Widder
Funding: National Science Foundation, Texas Higher Education Coordinating Board

96. References
Barbosa and Gafni, “Concurrency in Heavily Loaded Systems,” ACM TOPLAS 1989.
Busch, Surapaneni and Tirthapura, “Analysis of Link Reversal Routing Algorithms for Mobile Ad Hoc Networks,” SPAA 2003.
Busch and Tirthapura, “Analysis of Link Reversal Routing Algorithms,” SIAM JOC 2005.
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