Network Protocols: Design and Analysis-Internet Routing III

Network Protocols:

Design and Analysis

Polly Huang EE NTU

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

Internet Routing III

[Tsuchiya88a]

Landmark Routing

[Tsuchiya88a]

Context

• fairly early in the Internet life

– before BGP-3 – before CIDR

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 i f 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 t han area hierarchy

• could be more reliable if congestion because supports mult iple (?)

Landmark Routing

Disadvantages

• don’t always get shortest path [but true about all routing protoc ols that have aggregation/policy]

• admin control? (paper hints at approaches, but not fully explor ed)

• performance not fully explored?

– less info further away from destination, therefore more likely to get poo r 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

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

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

LM Hierarchy Definition

• Each LM (Li) associated with level (i) and radi us (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 entir e 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) y

a

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)

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

- Not shortest path

- Packet does not necessarily follow specified landmarks

Routing table for Router g

Landmark 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?

Evaluation

• analytic results

– but bounds not very helpful

• simulation

– routing table size (R) – mean path length

– distance to nearby landmark – (seems weak) r/d = radius/distance rt g ta bl e si ze m ea n pa th le n

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 c ase, 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

Methodology

• experiments over Internet: manually injected faults propagate across net

• simulation to study worst case behavior • theoretical analysis—helps understand

worst case bounds

Methodology Picture

Internet-scale experimentation. What kinds of complexities arise?

Have to be careful with real routes;

([Labovitz00a] Figure 1)

0 10 20 30 40 50 60 70 80 90 100 0 20 40 60 80 100 120 140 160

Seconds Until Convergence

C u m u la ti v e P e rc e n ta g e o f E v e n ts Tup Tshort Tlong Tdow n Shor t->Lo ng F ail-O ver ( Tlon g) New Rou te, Lon g->S hort Fai l-ov er (Tup and Tsh ort) Failu re (Tdo wn)

Observed Convergence Latency

Labovitz00a Figure 2a

Other Observations

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

convergence times

Affects on Traffic

([Labovitz00a] figure 4a)

Why does loss go up? There’s always a

direct path?

some people use old paths, 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 queue simple model that captures key details

What’s going on?

• there are many possible routes (indirect thro ugh 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

BGP Convergence Example

R

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

Does this explain measurements?

• Tup/Tshort converge quickly because they s horten path length and therefore are quickly accepted

• Tdown/Tlong converge slowly because BG P tries hard to find all alternatives

– Tlong actually sometimes goes quicker if it’s “n ot long enough” and can preempt some of the th rashing

Other Observations

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