GARP (Part 2)

Oct 09, 2004 CS573: Network Protocols and Standards 1

GARP (Part 2)

Network Protocols and Standards

Autumn 2004-2005

Oct 09, 2004 CS573: Network Protocols and Standards 2

GARP Participant Components

A GARP Participant consists of: A GARP application component A GID component

GID: GARP Information Declaration Remember: There is one GARP

Participant For each port For each application

GMRP (Multicast) and GVRP (VLANs) are examples

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The Application Component Defines a set of one or more Attribute Types Defines for each Attribute Type

Their set of values Their semantics Their structure and encoding Their values in the GARP PDU

For each application, a unique Group MAC Address is specified Used as destination MAC address for all GARP

protocol exchanges pertaining to that application

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The GID Component Declares attribute information

among participants on the same segment

Contains two state machines for each attribute Registrar state machine Applicant state machine

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GARP Participant

There is an instance of the Applicant and the Registrar per Attribute

GARP Participant

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GARP ParticipantGARP Participant

There is an instance of the Applicant and the Registrar per Attribute

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Participant: Protocol Elements Registrar

Records attribute registration declared by other participants on the same segment

Required in bridges (Used in forwarding decisions!)

May be implemented in stations for source pruning

Applicant Ensures that this participant’s declarations are

registered by other participants

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Registrar State Machine IN

Someone registered this attribute value on this segment

MT (Empty) All declarations for this attribute value on this

segment have been withdrawn LV (Leaving)

Timing out the registration; if no re-declaration is received before the Leave Timer expires, transition will be made to the MT state

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Registrar State Machine

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Applicant State Machine Three levels of complexity defined:

For participants that do not include a registrar (stations)

Simple applicant Applicant only

For participants that implement a registrar (bridges and some stations)

Full applicant

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Simple Applicant Simplest type of applicant – no registrar Three states

V (very anxious) Applicant has no reason to believe that the other

registrars have registered this value (No JoinIn message has been received and no Join message has been sent)

A (anxious) If no message has been lost, other registrars will have

registered this attribute value A second Join still needs to be sent or seen

Q (Quiet) No need to send more messages

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Simple Applicant State Machine

Messages are labeled by the registrar state of the sender

The JoinIN sent when a Join timer expires is shown as sJoinIN

Other transitions happen in response to received messages (prefixed with r)

Note that even though the Participant doesn’t have a Registrar, it sends JoinIN and LeaveEmpty by default

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Applicant Only In order to save on Join and Leave messages when

there are several Members on the same segment, stations may track the GARP message activity on the segment for a given Attribute, even when they are not currently interested in registering the Attribute

For this purpose, a distinction is made within an applicant between being a Member or an Observer

An Observer is a Participant that tracks the Attribute state, but does not wish to make a declaration

A Member is also a Participant that is interested in maintaining a declaration for a given attribute value

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Applicant Only: Member Types Two types of Members

Passive member (P): attempts to maintain the registration of an attribute value, but hasn’t itself sent a Join message to declare it

Active member (A): attempts to maintain the registration of an attribute value and has sent a Join message to declare it

Note that the observer is always “passive” Tracks the attribute state, but … Does not maintain declaration Observer is NOT a member

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Applicant Only An observer can become a member without

sending a Join message if it is in the Quiet state A Passive member can quit without sending a

Leave message State Machine Acronyms (States)

VO: very anxious, observer AO: anxious, observer QO: quiet, observer VP: very anxious, passive VA: very anxious, active LA: Leaving, active (a substate where leave message is

generated)

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Applicant Only State Machine

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Applicant Only When a Participant becomes interested in

a given Attribute The Applicant transitions first from Observer

to Passive Member If the Observer was in the Quiet state, the

Applicant becomes Passive and Quiet Otherwise, it schedules a Join for the earliest

transmission occasion A Passive Member that sends a Join becomes

an Active Member

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Full Applicant Full applicant state machine

Used in bridges Also used in stations that include a registrar for source

pruning Differences with Applicant only

A sent Join message is labeled with the current state of the registrar

When registrar is in IN state, send JoinIN When registrar is not in IN state, send JoinEmpty

A JoinEmpty causes the Applicants on a segment to go to Very Anxious state. The goal is to force other members to update the sender’s state by not considering this Join message valid for that purpose

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Full Applicant An observer has an additional state

LO state: Leaving Observer Similar to LA (Leaving Active) Enters after hearing a LeaveEmpty message

(unless it hears a Join) Like JoinEmpty, LeaveEmpty forces the

Applicants to go to very anxious state Purpose: To protect against problems

incurred when other Participants fail to see a Leave

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Full Applicant What if Join Messages are not labeled with the

registrar state? Consider the scenario …

Two bridges A and B on the same segment Both interested in an attribute AT Both have registrars in the MT state

What happens when A sends 2 Joins? Registrar states: MT for A and IN for B

This is because A’s Join has no indication about it’s knowledge of B’s wish to register the attribute AT

Therefore, by labeling Join with registrar state, A will force B to send Joins for AT

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GIP GARP Information Propagation

Propagates attribute registration information between participants for the same application on all ports which are in the forwarding state in a bridge

A registration is propagated on a port if any other port in the bridge has seen a registration for the concerned attribute

A de-registration is propagated on a port if all other ports are de-registered for the concerned attribute

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VLANs Preview Motivation

Increased Bandwidth on LAN segments Larger LAN switches (more ports) Larger subnetworks

Geographical scope Number of users

Same bridged LAN (or extended LAN) capable of serving several logical groups of users

Groups defined according to attributes Corporate divisions Higher Layer protocols Collection of servers they share, etc.