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
• phuang_at_cc.ee.ntu.edu.tw

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Internet Routing III

• Tsuchiya88a
• Labovitz00a

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Landmark Routing Tsuchiya88a
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Context

• fairly early in the Internet life
• before BGP-3
• before CIDR
• example of SIGCOMM wild idea paper

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Key Idea

• Self-configuring hierarchy for routing with many
  routers

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Why Landmark Routing?

• area routing requires knowledge of topology,
  maybe doesnt get best aggregration possible
• LM knows about internal structure of nearby
  nodes, even if in different AS
• dynamic address assignmenteasier to manage
• reduce size of routing table because address are
  automatic, 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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Landmark Routing Disadvantages

• dont 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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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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A Landmark
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Router 1 is a landmark of radius 2
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2
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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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LM Hierarchy Definition

• Each LM (Li) associated with level (i) and radius
  (ri)
• Every node is an L0 landmark
• Recursion some Li are also Li1
• Every Li is seen by at least one Li1
• Terminating state when all level j LMs see entire
  network

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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)

c
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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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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 Li1 landmarks
• Dynamic algorithm to adapt to topology
  changes--Efficient hierarchy

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Landmark Routing Basic Idea
- Not shortest path - Packet does not
necessarily follow specified landmarks

• Source wants to reach LM0a, whose address is
  c.b.a
• Source can see LM2c, so sends packet towards c
• Entering LM1b area, first router diverts packet
  to b
• Entering LM0a area, packet delivered to a

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Landmark Routing Example
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Routing table for Router g
Router g
r0 2, r1 4, r2 8 hops
Router t
How to go from d.i.g to d.n.t? How does path
length compare to shortest path?
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Evaluation

• analytic results
• but bounds not very helpful
• simulation
• routing table size (R)
• mean path length
• distance to nearby landmark
• (seems weak)

rtg table size
r/d radius/distance
mean path len
Figure 6 from Tsuchiya88a
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Questions?
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BGP Routing Convergence Times Labovitz00a
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Context

• BGP widely deployed in the Internet
• but poorly understood

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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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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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Methodology

• experiments over Internet manually injected
  faults propagate across net
• simulation to study worst case behavior
• theoretical analysishelps understand worst case
  bounds
• traces of 2 years of convergence times

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Methodology Picture
(Labovitz00a Figure 1)
Internet-scale experimentation. What kinds of
complexities arise? Have to be careful with real
routes
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Observed Convergence Latency
Labovitz00a Figure 2a
Long tailed distribution (up to 15 minutes) more
msgs in longer waits long absolute times
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Other Observations

• No correlation between network distance (latency,
  router, or AS hops) and convergence times
• Why is long convergence bad?

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Affects on Traffic
(Labovitz00a figure 4a)
Why does loss go up? Theres always a direct
path? some people use old paths, routing loops
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How To Tell Whats Going On?

• Simulate BGP
• model one router per AS
• assume full routing mesh
• ignore latency
• synchronous processing via global queue
• simple model that captures key details

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Whats 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
  gt O(n!) steps
• even with min-route-adver it still can take O(n)
  steps

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BGP Convergence Example
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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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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 its
  not long enough and can preempt some of the
  thrashing

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Other Observations

• Could do loop detection at sender side and not
  just receiver side
• xxx

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Questions?