network protocols: design and analysis

Network Protocols: Design and Analysis

Polly Huang

EE NTU

http://cc.ee.ntu.edu.tw/~phuang

[email protected]

Internet Routing III

[Tsuchiya88a] [Labovitz00a]

Landmark Routing [Tsuchiya88a]

Polly Huang, NTU EE 4

Context

• fairly early in the Internet life– before BGP-3– before CIDR

• example of SIGCOMM “wild idea” paper


Key Idea

• Self-configuring hierarchy for routing with many routers


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


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


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)


A Landmark

1

2

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4

56

7

89

10

11

Router 1 is a landmarkof radius 2


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


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


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


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


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


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


Landmark Routing: Example


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


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

Questions?

BGP Routing Convergence Times [Labovitz00a]


Context

• BGP widely deployed in the Internet

• but poorly understood


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]


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?


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


Methodology Picture

Internet-scale experimentation.What kinds of complexities arise?Have to be careful with real routes;

([Labovitz00a]Figure 1)


0

10

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60

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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


Other Observations

• No correlation between network distance (latency, router, or AS hops) and convergence times

• Why is long convergence bad?…


Affects on Traffic

([Labovitz00a] figure 4a)

Why does loss go up?There’s always a direct path?some people use oldpaths, routing loops


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


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

31Polly Huang, NTU EE

BGP Convergence ExampleR

AS0 AS1

AS2AS3

*B R via 3 B R via 03 B R via 23



AS0 AS1 AS2

** **B R via 203

*B R via 013 B R via 103


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!


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


Other Observations

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

• xxx

Questions?

network protocols: design and analysis

Documents

routing protocols

routerswhy landmark

size of routing table

complete routing information

landmarkof radius

radius n

router y

router x