Energy Analysis of Four Wireless Sensor Network MAC Protocols
Network Protocols: Design and Analysis
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Transcript of Network Protocols: Design and Analysis
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Network Protocols: Design and Analysis
Polly Huang
EE NTU
http://cc.ee.ntu.edu.tw/~phuang
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Internet Routing III
[Tsuchiya88a] [Labovitz00a]
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Landmark Routing [Tsuchiya88a]
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Polly Huang, NTU EE 4
Context
• fairly early in the Internet life– before BGP-3– before CIDR
• example of SIGCOMM “wild idea” paper
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Polly Huang, NTU EE 5
Key Idea
• Self-configuring hierarchy for routing with many routers
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Polly Huang, NTU EE 6
Why Landmark Routing?
• area routing requires knowledge of topology, maybe doesn’t get best aggregration possible
• LM knows about internal structure of nearby nodes, even if in different AS
• dynamic address assignment—easier to manage• reduce size of routing table… because address are automat
ic, and reassigned on-demand, can get better aggregation than area hierarchy
• could be more reliable if congestion because supports multiple (?)
• different approach than area routing
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Polly Huang, NTU EE 7
Landmark Routing Disadvantages
• don’t always get shortest path [but true about all routing protocols that have aggregation/policy]
• admin control? (paper hints at approaches, but not fully explored)
• performance not fully explored?– less info further away from destination, therefore more likely to get
poor quality routes to it [but no different from area routing]– performance of LM placement/config algorithms?
• combines routing and address (but so does area routing)• addressing
– address may not be stable– LM uses variable length address
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Polly Huang, NTU EE 8
Landmark hierarchy
• Details about things nearby and less information about things far away
• Not defined by arbitrary boundaries– thus, not well suited to the real world that does
have administrative boundaries– (although he says something about adding
admin boundaries)
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Polly Huang, NTU EE 9
A Landmark
1
2
3
4
56
7
89
10
11
Router 1 is a landmarkof radius 2
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Polly Huang, NTU EE 10
Landmark Overview
• Landmark routers have “height” which determines how far away they can be seen (visibility)
• Routers within Radius n can see a landmark router LM(n)
• See means that those routers have LM(n)’s address and know next hop to reach it. – Router x as an entry for router y if x is within radius of y
• Distance vector style routing with simple metric• Routing table: Landmark (LM2(d)), Level(2), Next hop
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Polly Huang, NTU EE 11
LM Hierarchy Definition
• Each LM (Li) associated with level (i) and radius (ri)
• Every node is an L0 landmark
• Recursion: some Li are also Li+1– Every Li is seen by at least one Li+1
• Terminating state when all level j LMs see entire network
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Polly Huang, NTU EE 12
LM addresses
• LM(2).LM(1).LM(0) (x.a.b and y.a.b)
• LM level maps to radius (part of configuration), e.g.:– LM level 0: radius 2
– LM level 1: radius 4
– LM level 2: radius 8
• If destination is more than two hops away, will not have complete routing information, refer to LM(1) portion of address, if not then refer to LM(2)..(c would forward based on y in y.a.b)
X
y
ab
c
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Polly Huang, NTU EE 13
LM Routing
• LM does not imply hierarchical forwarding
• It is not a source route
• En route to LM(1) may encounter router that is within LM(0) radius of destination address (like longest match)
• Paths may be asymmetric
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Polly Huang, NTU EE 14
LM self-configuration
• Bottom-up hierarchy construction algorithm– goal to bound number of children
• Every router is L0 landmark• All routers advertise themselves over a distance• All Li landmarks run election to self-promote
one or more Li+1 landmarks• Dynamic algorithm to adapt to topology
changes--Efficient hierarchy
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Polly Huang, NTU EE 15
Landmark Routing: Basic Idea
Source wants to reach LM0[a], whose address is c.b.a:•Source can see LM2[c], so sends packet towards c•Entering LM1[b] area, first router diverts packet to b•Entering LM0[a] area, packet delivered to a
- Not shortest path- Packet does not necessarily follow specified landmarks
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Polly Huang, NTU EE 16
Landmark Routing: Example
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Polly Huang, NTU EE 17
Routing table for Router gLandmark Level Next hop
LM2[d]
LM0[e]
LM1[i]
LM0[k]
LM0[f]
2
1
0
0
0
f
k
f
k
f
r0 = 2, r1 = 4, r2 = 8 hops
Router g
How to go from d.i.g to d.n.t?How does path length compare to shortest path?
Router t
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Polly Huang, NTU EE 18
Evaluation• analytic results
– but bounds not very helpful
• simulation– routing table size (R)
– mean path length
– distance to nearby landmark
– (seems weak)
[Figure 6 from Tsuchiya88a]
r/d =radius/distance
rtg
tabl
e si
zem
ean
path
len
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Questions?
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BGP Routing Convergence Times [Labovitz00a]
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Polly Huang, NTU EE 21
Context
• BGP widely deployed in the Internet
• but poorly understood
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Polly Huang, NTU EE 22
Key Idea
• convergence time takes longer we expected
• observes 2-3 minute convergence times (6x longer than expected!)
• bounds on BGP convergence: O(n!) worst case, O((n-3)*30s) [n is number of ASes]
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Polly Huang, NTU EE 23
Why is Convergence Important?
• robustness– PSTN (telephone) failover times are in
milliseconds– Internet failover times are in 10s of seconds– open research question: how can Internet
routing do much better?
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Polly Huang, NTU EE 24
Methodology
• experiments over Internet: manually injected faults propagate across net
• simulation to study worst case behavior
• theoretical analysis—helps understand worst case bounds
• traces of 2 years of convergence times
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Polly Huang, NTU EE 25
Methodology Picture
Internet-scale experimentation.What kinds of complexities arise?Have to be careful with real routes;
([Labovitz00a]Figure 1)
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Polly Huang, NTU EE 26
0
10
20
30
40
50
60
70
80
90
100
0 20 40 60 80 100 120 140 160
Seconds Until Convergence
Cu
mu
lati
ve P
erce
nta
ge
of
Eve
nts
Tup
Tshort
Tlong
Tdow n
Shor
t->Lon
g Fa
il-O
ver (
Tlong
)
New
Rou
te,
Lon
g->S
hort
Fai
l-ov
er
(Tup
and
Tsh
ort)
Failu
re(T
dow
n)
Long tailed distribution (up to 15 minutes); more msgs in longer waits; long absolute times
Observed Convergence LatencyLabovitz00aFigure 2a
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Polly Huang, NTU EE 27
Other Observations
• No correlation between network distance (latency, router, or AS hops) and convergence times
• Why is long convergence bad?…
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Polly Huang, NTU EE 28
Affects on Traffic
([Labovitz00a] figure 4a)
Why does loss go up?There’s always a direct path?some people use oldpaths, routing loops
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Polly Huang, NTU EE 29
How To Tell What’s Going On?
• Simulate BGP– model one router per AS– assume full routing mesh– ignore latency– synchronous processing via global queuesimple model that captures key details
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Polly Huang, NTU EE 30
What’s going on?
• there are many possible routes (indirect through other ASes) and it takes a long time w/BGP to figure out that none work– BGP can try all paths of length 2, then 3, then 4
=> O(n!) steps– even with min-route-adver it still can take O(n)
steps
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31Polly Huang, NTU EE
BGP Convergence ExampleR
AS0 AS1
AS2AS3
*B R via 3 B R via 03 B R via 23
*B R via 3 B R via 03 B R via 13
*B R via 3 B R via 13 B R via 23
AS0 AS1 AS2
** **B R via 203
*B R via 013 B R via 103
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Polly Huang, NTU EE 32
What about MinRouteAdver?
• BGP has a minimum advertisement interval timer– designed to limit updates
– and to encourage aggregation
• How does it affect convergence?– by delaying announcements, routers figure out the pain
sooner
– see section 5.2
• result: n-3 rounds rather than n!
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Polly Huang, NTU EE 33
Does this explain measurements?
• Tup/Tshort converge quickly because they shorten path length and therefore are quickly accepted
• Tdown/Tlong converge slowly because BGP tries hard to find all alternatives– Tlong actually sometimes goes quicker if it’s “n
ot long enough” and can preempt some of the thrashing
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Polly Huang, NTU EE 34
Other Observations
• Could do loop detection at sender side and not just receiver side
• xxx
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Questions?