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MapDistributionExplicit Routing Architectures


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

Map-Distribution/Explicit RoutingArchitectures
  • J. Noel Chiappa
  • Midnight sun routing Workshop
  • Luleå, Sweden
  • 17-19 June, 2002

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

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

  • 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

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

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

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

  • 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

Names contd
  • DNS names
  • Structured for human usability
  • Structure facilitates
  • Distribution
  • Lookup

  • 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

Maps contd
  • Node can represent some part of the physical
  • 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

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
  • E.g. the familiar hierarchical addressing
  • Creates hierarchical addresses
  • Interfaces
  • Topological aggregates

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

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

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

Example Routing Table
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

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

Non-Example of Routing Architecture
  • BGP
  • iBGP
  • eBGP
  • MP-BGP
  • OSPF
  • IS-IS
  • RIP

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

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

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.

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
  • Hierarchical Distributed Exit router based
  • At a given level of abstraction no node more
    responsible for selecting path then any other
  • Node at lower level cannot override higher
    layers choice of next high-layer abstraction to

Comparison ofPath Selection Mechanisms
  • Hop-by-hop architectures have the following
  • They effectively require global consistency in
    the routing databases to prevent routing loops.
  • Explicit architectures have the following
  • 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.

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

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

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

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
  • Constraint-based routing (which includes most QOS
    routing) is only practical with MD architectures.
  • Highly secure routing is only practical with MD

Combining Path Selection with Data Type
Path Selection
Data Type
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

Routing Architectures for QoS
  • QoS routing cannot be done with classic HbH
  • 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
  • MD-E architectures are the only ones suitable for

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
  • Overall, MD-E architectures are more suitable for

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).

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

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
  • Poor tools for doing abstraction, and controlling
  • The simplistic EGP/IGP split.

Recent Routing Work
  • The Internet engineering community has been
    experimenting with MD/E architectures for some
  • 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.
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