Hierarchy of routing knowledge
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IP Routing : All routers within domains that carry transit traffic have to maintain both interior and exterior routing information. Problem: Additional resource required by the routers Increasing routing convergence time. Tag Switching :

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Hierarchy of Routing Knowledge

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Hierarchy of routing knowledge

IP Routing:

All routers within domains that carry transit traffic have to maintain both interior and exterior routing information.

Problem:

Additional resource required by the routers

Increasing routing convergence time

Tag Switching:

Complete decoupling of interior and exterior routing

Solution:

Tag switches within the domain just maintain routing information provided by interior routing.

Tag switches on the border of a domain maintain both exterior and interior routes.

Reduces the routing load on nonborder switches and shortens routing convergence time.

Hierarchy of Routing Knowledge


Hierarchy of routing knowledge

Hierarchy of Routing Knowledge

Tag Switching

domain1

domain2

domain3

exterior

exterior

eb switch

eb switch

ib switch

ib switch: ingress border tag switch

eb switch: egress border tag switch


Hierarchy of routing knowledge

Hierarchy of Routing Knowledge

Tag Switches

ingress

border switch

egress

border switch

local switch

Packet Tag Stack

incoming

outgoing

Tag2: interior

Tag2: interior

Tag1: exterior

Tag1: exterior

Tag1: exterior


Hierarchy of routing knowledge

Multicast Routing

  • To support multicast forwarding with tag switching, we need to associate a tag with a multicast tree.

  • each multicast entry in the TFIB of the tag switch may have one or more<outgoing tag,outgoing interface, outgoing link level information> subentries.

  • The tag switching architecture assumes that:

  • multicast tags are associated with interfaces on a tag switch.(rather than with a tag switch as a whole);

  • tag space that a tag switch could use for allocating tags for multicast is partitioned into nonoverlapping regions among all tag switches connected to a common data-link subnetwork;

  • There are procedures by which tag switches that belong to a common multicast tree and are on a common data-link subnetwork agree on

  • the tag switch that is responsible for allocating a tag for the tree.


Hierarchy of routing knowledge

Multicast Routing

upstream allocation

downstream allocation

extended

discovery

basic

discovery

M1

M2

tag binding direction

M1, M2:

multicast group

packet travel direction


Hierarchy of routing knowledge

Multicast Routing

  • Two possible ways to create binding between tags and multicast trees

  • Upstream allocation:

  • simple

  • may create uneven distribution of allocated tags

  • changes in topology could result in tag rebinding

  • inconsistent with the direction of multicast routing information distribution

  • Downstream allocation:

  • more complex

  • consistent with the distribution of multicast routing information

  • allow piggy-backing of tag binding information on existing multicast

  • routing protocols

  • avoid the need of rebinding when there are changes in the upstream neighbor

  • more likely to provide the more even distribution of allocated tags


Hierarchy of routing knowledge

QoS

  • Two mechanism are needed to provide a range of QoS

  • 1) Classify packets into different classes.

  • 2) Ensure that the handling of packets is such that the appropriate QoS

  • characteristics are provided to each class.

  • On ATM tag switches

  • additional tags can be allocated to differentiate the different classes.

  • Example: two tags per prefix, one used by premium traffic and one by standard. A tag binding in this case is consisting of <Prefix, QoS class, tag>.

  • Tag switching with RSVP

  • A tag object can be carried in an RSVP reservation message and thus

  • associated with a session.

  • Each tag capable router assigns a tag to the session and passes it upstream

  • with the reservation message, like the binding of tags to routes with

  • downstream allocation.


Flexible routing

Destination Based Routing

limits the ability to influence the actual paths taken by packets

limits the ability to distribute traffic evenly

limits ISP’s ability to segregate different classes with respect to the links used by those classes.

not adequate to routing with resource reservations

Tag Switching

flexible granularity.

routing with resource reservations (Qos routing)

allows installation of tag binding in tag switches that do not corresponding to the destination-based routing paths

Flexible Routing


Hierarchy of routing knowledge

Tag Forwarding Equivalence Classes

  • FEC’s: Forwarding Equivalence Classes

  • A group of IP packets which are forwarded in the same manner, in tag

  • switching, if a pair of tag switches are adjacent along a tag switched

  • path, they must agree on an assignment of tags to FEC’s.

  • Different tag switches could use different procedures to classify packets into FEC’s. 

  • Network could be organized around a hierarchy of FEC’s, different packets mapped into the same FEC are indistinguishable.

  • No FEC identifier.


Hierarchy of routing knowledge

Tag Forwarding Equivalence Classes

  • To partition a set of packets into FEC’s. Some examples:

  • Consider two packets to be in the same FEC if these packets have to traverse through a common router/tag switch.

  • * the FEC can be identified by the address of a tag switch, useful for binding tags to unicast routes.  

  • Consider two packets to be in the same FEC if they have the same source address and the same destination address.

  • * useful for binding tags to multicast trees that are constructed by protocols such as PIM

  • 3)  Consider two packets to be in the same FEC if they have the same source address, the same destination address, the same transport protocol, the same source port, the same destination port.

  • * useful for binding individual flows using RSVP


Hierarchy of routing knowledge

Tag Forwarding Equivalence Classes

  • Much of the power of tag switching arises from the fact that:

  • many different ways to partition the packets into FEC’s;

  • different tag switches can partition the untagged packets in

  • different ways;

  • the route to be used for a particular FEC can be chosen in

  • different ways;

  • a hierarchy of tags, organized as a stack, can be used to

  • represent the network’s hierarchy of FEC’s.


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