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MapDistributionExplicit Routing Architectures

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Midnight sun routing Workshop. Lule , Sweden. 17-19 June, 2002. 18 ... Prepared by Avri Doria for Midnight Sun Routing Workshop. 3. Architectural Fundamentals ... – PowerPoint PPT presentation

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Title: MapDistributionExplicit Routing Architectures

1
Map-Distribution/Explicit RoutingArchitectures

J. Noel Chiappa
Midnight sun routing Workshop
Luleå, Sweden
17-19 June, 2002

2
Overview

Architectural fundamentals
Classes of routing architecture
Reason why a different class of routing
architecture from the one in use today is
necessary

3
Architectural Fundamentals

Terminology and Concepts
Names
Objects
Routing Fundamentals
Maps
Abstraction hierarchies
Attributes
Routing Tables
What is a Routing Architecture?

4
Terminology

I am far from thinking that nomenclature is a
remedy for every defect in art and sciencestill
I cannot but feel that confusion of terms
generally springs from, and always leads to,
confusion of ideas.
John Louis Petit, Architectural Studies in
France, 1854

5
Names and Objects

Must separate
name for a thing
object, i.e. the thing in itselfto which those
names refer
Must separate
Form of the name for a thing
Generic concept of a name for a thing

6
Namespaces

1 or more names of types of names per object
Single namespace used to refer to different
objects
Namespaces may or may not have structure

7
Objects

Intrinsic to discussion of routing architecture
Network
Route or path
Node
Interface
Not intrinsic to discussion of routing
architecture
Host
Router
Switch

8
Names

Address
Structured
Topologically
Minimizes destination tracked
Allows quick location on a map
Refer to either
Interface
Topology aggregate
One namespace 2 referent objectsis a source of
potential confusion

9
Names contd

DNS names
Structured for human usability
Structure facilitates
Distribution
Lookup

10
Maps

Representations of the fundamental underlying
topology with which routing has to deal
In one simple model, terminology of graph theory
is used
Nodes open-ended list of attributes
Arcs uni-directional link which connect nodes
and have no attributes

11
Maps contd

Node can represent some part of the physical
network
As large as an ISP
As small as an interface
Contiguous portion of a graph representing entire
physical network can be represented as
Single node
Set of nodes
Either of which is an abstraction for that part
of the network
Abstractions can be recursive

12
Abstraction hierarchies

Name for the naming structure that creates a
nested set of abstractions for parts of a network
Consists of naming boundaries around parts of
network
E.g. the familiar hierarchical addressing
structure
Creates hierarchical addresses
Interfaces
Topological aggregates

13
Example Physical Topology
14
Example Abstraction Hierarchy

For any single Physical Topology there can be
many Abstraction Hierarchies
Physical Topology and Abstraction Hierarchy are
different things

X
Y
A
B
15
Attributes

Nodes characterized by list of optional
attributese.g.
Bandwidth
Delay
Delay variance
Error Rate
Cost
Allowed Users
Nodes have required inherent attributes
Connectivity of node

16
Routing Tables

A database used to make decisions about where to
send traffic
Often a linear array
Indexed by destination address
Provides pieces of information
Outbound interface
Next entity to send packet to
Other information used to calculate paths

17
Example Routing Table
18
What is aRouting and Addressing Architecture?

Addressing architecture includes
System of naming
Networks
Interfaces
Topology aggregates
Routing Architecture
A way of exchanging information as to where named
things are
A way of computing and selecting path between
sets of communicating entities
A way of causing traffic to take these paths

19
What is aRouting ArchitectureNot?

It is not simply a protocol, i.e.
A set of packet formats
Instructions on how to process items
It considers the entire question of how the
network organizes the handling of user traffic

20
Non-Example of Routing Architecture

BGP
iBGP
eBGP
MP-BGP
OSPF
IS-IS
RIP

21
Example of Routing Architecture

Entire collection of BGP variants and all IGPs,
including specifications on their interaction.
Nimrod
Islay

22
Classes of routing architecture

Currently 2 fundamentally different ways of
classifying routing architectures.
Path Selection
Data Types

23
Class Path Selection

Fully distributed
Commonly known as Hop by Hop (HbH)
Each node along path makes completely independent
decision of path data is to follow
Explicit
One node selects entire path
When source chooses path, commonly known as
Source routing
In more general case it is not necessary that
source include entire route in packet, therefore
term Explicit (E) is preferred.

24
Spectrum of Path Selection Alternatives

Spectrum between E and HbH includes
Hierarchical Explicit One node picks entire path
But in terms of higher level abstractions, paths
across those abstractions selected by other
entities.
Hierarchical Distributed Exit router based
routing
At a given level of abstraction no node more
responsible for selecting path then any other
node
Node at lower level cannot override higher
layers choice of next high-layer abstraction to
transit

25
Comparison ofPath Selection Mechanisms

Hop-by-hop architectures have the following
disadvantages
They effectively require global consistency in
the routing databases to prevent routing loops.
Explicit architectures have the following
advantages
They trivially allow the testing and deployment
of new path-selection algorithms.
They can allow the users much more control over
the path of their traffic.
They are more naturally immune to loops.
They can be made much more robust.
They can be secured against almost all attacks.
Some optimization problems cannot be solved by
local optimization algorithms, which means no HbH
architecture can handle them.

26
Class Data Type

Routing architecture depends on kind of data
passed between nodes.
Destination Vector (DV)
Node passes sets of (designation, information)
n-tuples to immediate neighbors
Information used to build routing tables
Map distribution (MP)
Nodes distribute information about immediate
surrounding through the network
Information is used to construct topological
maps.

27
More on DV

Selection of path from a given destination is
usually evenly distributed
For any level of abstraction no node has more
significant responsibility in selecting the path
In the course of running the algorithm to select
the path, intermediate results in that
computation are passed between independent nodes

28
More on MP

Selection can be localized or on an a HbH basis,
but the actual computation is not fully
distributed
No intermediate results passed around network
Computation can be delayed until the path is
needed
Computation can be partial
Path selection algorithm can inspect map directly
to select path
Alternatively a classical routing table can be
built.

29
Comparison of Data Exchanges

DV architectures have the following advantages
They require less computing and storage overhead
than MD architectures.
MD architectures have the following advantages
They are much more practical to secure against
many attacks.
They can react more quickly to changes (in
topology, etc.).
Their stabilization time is not only shorter on
average, but also much more bounded.
They require less bandwidth to react to
significant changes in network topology.
They allow the incremental deployment of new
attributes.
Constraint-based routing (which includes most QOS
routing) is only practical with MD architectures.
Highly secure routing is only practical with MD
architectures.

30
Combining Path Selection with Data Type
DV
MD
HbH
Path Selection
E
Data Type
31
QoS Routing and Traffic Engineering?

QoS Routing means
selecting paths on multiple constraints,
using constraints related to service levels (e.g.
bandwidth, delay), not just on a single simple
metric like the number of hops.
Traffic Engineering means
allocating paths for traffic aggregates in such a
way that an optimal use is made of network
resources.

32
Routing Architectures for QoS

QoS routing cannot be done with classic HbH
architectures
Some constraints (e.g. overall delay) cannot be
calculated in a hop-by-hop fashion.
Unless path selection decisions are tightly
coordinated (i.e. no partially-deployed
attributes), loops can result.
QoS routing cannot be done with DV architectures
Each combinations of constraints requires a
separate routing table, leading to combinatorial
explosion.
MD-E architectures are the only ones suitable for
QoS.

33
Load-Based Routing

QoS can also imply load-based routing, where
traffic patterns change in response to offered
load. The increased dynamicity of LBR make it
more susceptible to oscillatory behaviour
Distributed computations of DV architectures are
harder to stabilize.
Generally, with Explicit architectures, paths are
fixed, again increasing resistance to oscillatory
behaviour.
Overall, MD-E architectures are more suitable for
LBR.

34
Routing Architectures for TE

TE is an optimization process, and getting good
optimization with local traffic placing
algorithms can be difficult global algorithms
can do better. There are several different
aspects of local
Local knowledge only (i.e. routing tables, as
opposed to a complete map).
Local control only (i.e. the ability to select
the next hop only, not the complete path).
MD-E architectures, which are capable of being
more global along both axes, are thus much better
suited to TE.
Simply routing each aggregate independently may
not produce a pattern of traffic flows which the
available network resources can support.
For such cases, a centralized allocation
algorithm, not a distributed one, is needed.
Such a centralized algorithm must have both
complete knowledge (i.e. MD) and complete control
(i.e. Explicit).

35
The Current Internet Routing Architecture

The current Internet routing architecture has the
following key characteristics
Destination Vector type
Hop-by-hop path selection
Do not be misled by the deployment of IGP's which
use MD architectures, such as OSPF and IS-IS, in
limited areas of the network.
The overall operation of the current Internet
routing architecture is DV
The data passed between AS's consists of routing
tables, not maps.
The path across a number of AS's is selected
piecemeal, not in a unitary fashion. (I.e. if
downstream ISP Y has two paths to destination
site D, upstream ISP X can't select which of
those two it wants to use all it can do is give
the traffic to Y, which picks which one it will
use.)

36
Problems With The Current Routing Architecture

The overall DV-HbH model does not allow a lot of
desirable features
User control of paths
QoS Routing
Load-Based Routing
Traffic Engineering
Not very robust.
Secured only by extensive configuration, which
limits the flexibility, especially in response to
failure.
Poor tools for doing abstraction, and controlling
it.
The simplistic EGP/IGP split.

37
Recent Routing Work

The Internet engineering community has been
experimenting with MD/E architectures for some
time.
They have not gotten wide-spread deployment, for
two likely reasons
They are very different from existing routing
protocols, particularly the DV-based system which
people are most familiar with, and with the
incredible pressure to provide service, it was
easier to stay with what was familiar, and known
to work.
The MD/E systems which were done are more
complex, because they provide greater
capabilities, but there was apparently not enough
need for the capabilities they provide.
Recently, work in the MPLS community has started
to explore MD/E architectures (e.g. QOSPF
together with an LSP setup protocol).
The work is poorly integrated with the
internetwork layer.