foundations of inter-domain routing ph.d. dissertation defense vijay ramachandran dissertation...

Foundations ofInter-Domain RoutingPh.D. Dissertation DefenseVijay RamachandranDissertation Director: Joan Feigenbaum

Committee Members: Jim Aspnes, Paul Hudak,Tim Griffin (University of Cambridge)

V. Ramachandran — Ph.D. Dissertation Defense 2April 20, 2005

Overview This dissertation develops a theoretical

framework for the design and analysis of path-vector protocols primarily used for Internet inter-domain routing.

The framework can be used to understand the interactions of local routing policies and their effects on protocol behavior.

It can also be used to understand the design space of path-vector protocols and inherent trade-offs among desirable protocol properties.


Background: Internet Routing


Apply Policy =filter routes & tweak attributes

BGP Route Processing

Routing Table

Apply Import Policies

Best Route Selection

Apply Export Policies

Install forwardingentries for best

routes

ReceiveBGPupdates

Storageof routes

TransmitBGP updates

Based onattributevalues

IP Forwarding Table

Apply Policy =filter routes & tweak attributes Open-ended programming:

constrained only by vendor configuration language


BGP Route-Selection Procedure1. Highest local preference2. Shortest AS-path length3. For each AS next-hop, lowest MED

value4. eBGP routes over iBGP routes5. Shortest iBGP distance to egress

point


Motivation (1) Given certain policy inputs, BGP will oscillate

or converge nondeterministically.[VGE ’00, GSW ’02, MGWR ’02, Cisco ’01]

These anomalies are difficult for operatorsto debug because the problems traverse autonomously administered networks.

New features are often implemented without testing resulting worst-case scenarios.


Motivation (2) The BGP specification contains no

guidance on how to provide “good” routing policies.

Policies are unconstrained.Can policies be constrained to guarantee

convergence, and how can those constraintsbe described?

What is lost, if anything? Formal models allow rigorous analysis and

design at different levels of abstraction.


Prefer sendingtraffic throughneighbor 2

Prefer sendingtraffic throughneighbor 1

Protocol-Divergence Example

0

1 2

0

20

0

10

20120

10 21010 20

120 210


Related Work:Formally Modeling Policy Semantics [GSW ’02] introduced the Stable Paths

Problem (SPP) as the underlying theoretical problem that BGP is trying to solve.

SPP is NP-hard; solvability convergence.

An SPP instance is a graph in which each node represents one AS and has a policy in

the form of a linear preference ordering on

paths.


SPP Results [GSW ’02]

DISAGREE (multiple solutions)

BAD GADGET (no solution)

Dispute Wheel

No dispute wheel impliesrobust convergence.


Related Work:Local and Global Constraints [GR ’01] showed that Hierarchical

BGP (HBGP) is robust.Neighbors are divided into three classes:

customers, providers, and peers.Preference and scoping rules apply to

routes learned from different types of neighbors.

No customer/provider cycles. [GGR ’01] added an attribute to HBGP

to allow safe back-up routing.

Localconstrai

nt

Globalconstrai

nt


The Design Space of Path-Vector Protocols [GJR ’03] Robustness: Does the protocol predictably converge,

even after node and link failures?

Expressiveness: What routing policies are permitted?

Autonomy: What degree of independence do operators have in local-policy configuration?

Policy Opaqueness: Can local route settings be kept private?

Transparency: How directly does the protocol apply local-policy transformations to route data?

Global Constraint: What network assumptions are needed?


Three Levels of Abstraction [JR ’05]

Path-Vector Algebras [Sob. ’03]A description of the most important criteria involved in determiningbest routes. Does not include implementation details, e.g., a routeadvertisement is considered an atomic action.

Path-Vector Policy Systems (PVPS) [GJR ’03]A combination of message-passing system (protocol), policy language,and global constraint. The underlying path-vector system modelsimport & export policies, path selection, and route data structures.

Instances of the Stable Paths Problem (SPP) [GSW ’02]A routing configuration, indicating the preference order of permittedpaths on a given network. Solutions are consistent assignments;unique solutions give predictable convergence to a stable assignment.

Sets ofProtocols

Protocols

Networks


Path-Vector Policy Systems [GJR ’03]

( PV , PL , K )

Policy Language:

How can policies be described?PL acts as a local constraint on the expressiveness of policies.

Policy Language:

How can policies be described?PL acts as a local constraint on the expressiveness of policies.

Path-Vector System:

The underlying message-exchange system for route information. Whatis exchanged and how?

Path-Vector System:

The underlying message-exchange system for route information. Whatis exchanged and how?

Global Constraint:

What assumptions about the network must be true to achieve robustness?

Global Constraint:

What assumptions about the network must be true to achieve robustness?

Question:

What role do these components play in achieving protocol design goals?

Question:

What role do these components play in achieving protocol design goals?

Formal model of path-vector routing:


Linear Best-Route Selection Model Ignore iBGP and MED-attribute values. Assume that the route-selection procedure,

at each node, for each destination:1. maps each route to a rank in some totally

ordered set based on its attribute values; and2. chooses as best the path with minimal rank.

Rank is influenced by local policy, but the ranking criteria are the same at each node.


Robustness Condition[GJR ’03, Sob. ’03]Conjecture: No path-vector policy system

can exactly capture all robust configurations.

Theorem: A protocol in which a path’s rank monotonically increases as it is extended (imported by a neighbor) is robust.

This is the broadest-known sufficient condition for robustness, equivalent to dispute-wheel freeness on SPP instances.


Trade-Offs in Implementation[GJR ’03]Theorem. A globally unconstrained PVPS

expressive enough to capture all increasing configurations either does not support autonomy of neighbor ranking or is not transparent, or both.

Theorem. A transparent, robust PVPS that supports autonomy of neighbor ranking and is at least as expressive as shortest paths must have a non-trivial global constraint.


Algebras and PVPSes (1) [JR ’05]

Protocolsusinglength

Protocolsusing localpreference

Both,primarily

length

Both,primarilyloc. pref.

Robust protocols

ShortestPaths

Shortest Paths withpreference tie-breaking

Monotone preferences with length tie-breaking

Strictly monotonepreferences

BGP

Monotone(or arbitrary)preferences

For both,some

network instances

are convergent


Algebras and PVPSes (2) [JR ’05] The expressiveness of an algebra or PVPS

is the set of SPP equivalence classes permitted as legal routing configurations.

Given an algebra, we can construct a canonical PVPS that is exactly as expressive.

Given a PVPS, we can construct a canonical algebra that describes the same rank criteria.


Class-Based Systems [JR’ 04] The PVPS framework can be used to generalize

the HBGP constraints from [GR’ 01, GGR’ 01]. A class-based PVPS is described by:

A set of classes (types of neighbor assignments, e.g., customer/provider/peer) and consistency relationships

Class relative-preference and scoping rules These systems are transparent and have

“some” autonomy of neighbor ranking; they requirea nontrivial global constraint.


Relative Preference and Scope

Relative Preference:

If class i is to be preferred over class j, then node v should prefer routes from node w over those from node x.

Relative Preference:

If class i is to be preferred over class j, then node v should prefer routes from node w over those from node x.

Scope:

If class i routes cannot be exported to a class-k neighbor, then node u will only learn about the path uvxQ.

Scope:

If class i routes cannot be exported to a class-k neighbor, then node u will only learn about the path uvxQ.


Class-Based Robustness [JR’ 04] From the class description alone, we can

construct a global constraint involving a check on pairs of class assignments. Networks obeying this constraint are robust. Networks violating this constraint allow nodes to

write policies that induce routing anomalies. We give two types of enforcement algorithms:

a centralized algorithm that detects a set of nodes whose class assignments permit a policy-induced anomaly; and

a distributed algorithm that detects whether two specific nodes’ class assignments could induce an anomaly.


Nonlinear Route-Selection Model Recent work generalizes the PVPS

framework to include protocols that do not assume linear route-selection procedures.This permits modeling the MED attribute and

both iBGP and eBGP sessions.Because previous convergence constraints

depend on a notion of rank, these do not applyin the generalized case.

Relies on generalized SPP [GW ’02].


Generalized SPP [GW ’02] Recall BGP selection:

lowest MED value from paths to the same AS; then

shortest IGP distance. IGP distances are shown

near intra-domain links. MED values are shown

in parentheses near inter-domain links.

This example oscillates.MED-EVIL (no solution)


Independent Route Ranking

MED-EVIL (condensed)


Generalized Path Relations


Generalized Dispute Digraphs Given a GSPP

instance, form its generalized dispute digraph: nodes are paths; edges correspond to

the four relations. Theorem. If a GSPP

is not robust, this graph contains a cycle.

MED-EVIL Dispute Digraph


Proof Method

Given a protocol oscillation, choose a path whose first node is the last oscillating node on the path.

Follow the oscillation until the selection changes; this change occurred because of a linear or nonlinear selection. This corresponds to some relation between two paths; repeat with the ‘related’ path. Choose a subpath to find the last oscillating node.

Because the oscillation is finite, we must re-visit a path.We have just traced a cycle in the dispute digraph.

Cycle in MED-EVIL protocol-selection states.


Protocol-Design Applications Multiple-Path Broadcast

[B+ ’02] and [MC ’04] propose changing BGP to broadcast additional routes to avoid MED-induced oscillations.

We can prove the effect of this behavior using ourformal model.

Improvement: Detect an IRR violation on-the-fly and request the needed route.

“Compare-all-MEDs” and “Set AS-distinct local preferences” [MGWR ’02] can be proven correct.


Summary The PVPS framework allows for a study of

path-vector-protocol design—most importantly, a rigorous way to prove: what balance of local and global constraints are

needed for robustness; and what else is lost when these constraints are

implemented. The framework has provided concrete and

reasonable guidelines for class-based systems. The framework has been extended to include

protocols with IRR-violating selection procedures.


Open Questions Analogous local constraints for the

generalized case Real, deployable policy-configuration

languages More examples of exact trade-offs

between local and global constraints (to date, only class-based systems give this)

Full characterization of robust systems?

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