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MAC State Convergence Function. Authors:. Date: 2007-09-18. Abstract. This presentation provides one potential approach for providing 802.21 MIH support in 802.11:

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mac state convergence function
MAC State Convergence Function

Authors:

Date: 2007-09-18

Matthew Gast, Trapeze Networks

abstract
Abstract

This presentation provides one potential approach for providing 802.21 MIH support in 802.11:

  • Discussion of a “convergence function” that would supply MAC state information to higher layer applications such as 802.21 MIH, and
  • Discussion of required support of 802.21 primitive operations in terms of existing 802.11 primitives and potential new additions

Matthew Gast, Trapeze Networks

the problem in brief
The Problem, In Brief
  • The 802.11 MAC does not have much state
    • Example: MIH Link_Up event is the “first” time that a wireless LAN is available
    • Existing MLME operations are like the “lower” MAC – there is no high-level state, such as the ESSID a STA is associated with
  • Many higher layer protocols (e.g. 802.21 MIH) need context
  • Proposal: A “MAC state convergence function”
    • Snoops on MAC management (MLME) and PHY management (PLME) events, and can retrieve state from station management (SME)
    • Synthesizes events and state to create new logical operations/indications
    • Inputs: MLME and PLME events, state information retrieved from SME
    • Output: Operations and events useful to higher level protocols

Matthew Gast, Trapeze Networks

architectural overview
Architectural Overview

MIH

Other higher layers using

MAC state information

MAC_STATE_CONVERGENCE_SAP

MAC State

Convergence Function

MAC_SAP

MSCF-SME_SAP

MAC

MLME

SME

Mobility

Management

MLME_SAP

PHY_SAP

Interworking

Service

Management

PHY

PLME

PLME_SAP

Note: The base standard version of this diagram is Figure 5-10 on page 42 of 802.11-2007

Matthew Gast, Trapeze Networks

analysis of 802 21 primitive operations

Analysis of 802.21 Primitive Operations

Matthew Gast, Trapeze Networks

primitives required by 802 21
From 802.21-D7.1, Table L2:

Link_Event_Subscribe

Link_Event_Unsubscribe

Link_Configure_Thresholds

Link_Up

Link_Down

Link_Going_Down

Link_Event_Rollback

Link_Detected

Link_Parameters_Report

Link_PDU_Transmit_Status

Link_Handover_Imminent

Link_Handover_Complete

Link_Capability_Discover

Link_Get_Parameters

Link_Action

Primitives Required by 802.21

Matthew Gast, Trapeze Networks

classification of 802 21 primitives
Link Operations

Link_Up

Link_Down

Link_Going_Down

Link_Event_Rollback

Link_Detected

Link_Parameters_Report

Link_PDU_Transmit_Status

Mobility Operations

Link_Handover_Imminent

Link_Handover_Complete

MAC “API”

Link_Event_Subscribe

Link_Event_Unsubscribe

Link_Capability_Discover

Link_Action

Link Parameter operations

Link_Configure_Thresholds

Link_Get_Parameters

Link_Parameters_Report

Classification of 802.21 Primitives

Matthew Gast, Trapeze Networks

notes on analysis
Notes on Analysis
  • This analysis attempts to:
    • Describe each 802.21 operation in English words, and
    • Figure out the closest mapping to existing 802.11 primitive operations within the MLME or PLME, or, if there is no close primitive, what we would need to implement within the convergence function.
  • Unless otherwise specified, the “technical meaning in terms of 802.11” should be read as a list of alternative options (“or”) rather than a list of requirements (“and”)

Matthew Gast, Trapeze Networks

link operations

Link Operations

Link_Up

Link_Down

Link_Going_Down

Link_Event_Rollback

Link_Detected

Link_Parameters_Report

Link_PDU_Transmit_Status

Matthew Gast, Trapeze Networks

link up
Link_Up
  • English meaning for 802.11: A network (i.e., a set of APs offering identical data-link services) is now up at layer 2
  • Technical meaning in terms of 802.11
    • (1) the first MLME-Associate.confirm with a result code of success for unencrypted networks, or…
    • (2) the first MLME-Associate.confirm along with an MLME-SetKeys.confirm, both with result codes of success, for encrypted networks
  • Implications for 802.11u draft
    • MLME-Reassociate primitives are unimportant because they cannot be “first”
    • MLME-Associate.confirm primitives may need to be filtered, based on whether associations occur within a “small enough” time window to be considered “first” – perhaps this should be tied to whether a full 802.1X authentication was required?

Matthew Gast, Trapeze Networks

the meaning of link up
The Meaning of Link_Up
  • From the 802.21 perspective, the main consumer is applications that need to know when new application-layer addressing is necessary
    • Application examples: MIP, SIP, etc…
  • From the 802.11 perspective, a “link” is the ability to continue to send frames to the same destination
    • On an IP network, the destination of these frames is likely the default IP router for the subnet
    • A single “link” may move from AP to AP without the application being aware of it
  • Conclusion: LinkUp events only need to be generated when an 802.11 STA moves to an AP that does not offer identical service to its existing AP
    • Therefore, the term “network” means a collection of APs that offer common link-layer attachment

Matthew Gast, Trapeze Networks

link down
Link_Down
  • English meaning: The 802.11 link has gone down and can no longer send packets
  • Technical meaning in terms of 802.11
    • (1) the STA has received an MLME-Disassociate.indication
    • (2) the STA has received an MLME-Deauthenticate.indication
    • (3) frames to the STA are not acknowledged for a “long enough” period of time
  • Implications for 802.11u draft
    • The convergence function must access the state of Protected Management Frame MIB entry (dot11RSNAProtectedManagementFramesEnabled)
    • Need to investigate whether TGw authentication will suppress Disassociate and Deauthenticate primitives. If not, then unauthenticated disconnection attempts or disconnects that fail authentication must be filtered by the convergence function
    • Different applications may require different link performance metrics for #3, so the convergence function should store the state of metrics that are required to declare a link dead. Defaults may be vendor-specific.

Matthew Gast, Trapeze Networks

link going down
Link_Going_Down
  • English meaning: The 802.11 link seems to be getting weaker, and might go down soon. This requires some form of prediction for future link quality. It may also mean an administrative shutdown
  • Technical meaning in terms of 802.11
    • (1) According to indications received with frames, it is possible to predict the link will fail “soon”
    • (2) This may be received Data frames, or received Beacon frames
  • Implications for 802.11u draft
    • This is a new event that will be synthesized by the convergence function
    • No “best” prediction algorithm can be specified in the standard, and must be vendor-specific logic in the SME
    • There may be a need to provide configurable thresholds for the predictive algorithm through the SAP with between the CF and SME
    • An identifier and sequence number are required to support rollback (the next primitive)
    • RSSI is available for all of the radio PHYs (clauses 14, 15, 17, 18, and 19), but not the IR PHY (clause 16)

Matthew Gast, Trapeze Networks

link event rollback
Link_Event_Rollback
  • English meaning: “Oops! We were wrong about that impending link failure, so never mind.”
  • Technical meaning
    • The Link_Going_Down event no longer applies, so it needs to be cancelled
    • No tie to existing 802.11 primitive operations
  • Implications for 802.11u draft:
    • If the previous event is supported, this must also be supported
    • This event needs a tie to the previous event -- they operate as two sides of the same operation

Matthew Gast, Trapeze Networks

link detected
Link_Detected
  • English meaning: A new link of is available for potential use
    • This corresponds to detection of a Beacon frame for a new network, e.g. a new component of an MLME-SCAN result
    • It is not: “we have powered on the 802.11 interface”
  • Implication for 802.11u draft
    • The convergence function may need to issue period MLME-SCAN requests to learn about networks in the area, and report new networks as they appear in the scan result list
    • Many areas have lots of 802.11 networks of many different types available within small areas; it may be prudent to use the TGu network type classifications to help filter new entries in the scan result list so that only relevant links are detected

Matthew Gast, Trapeze Networks

link pdu transmit status
Link_PDU_Transmit_Status
  • English meaning: Gets the status of a protocol data unit transmission
  • Technical meaning in terms of 802.11
    • If this refers to a frame transmission: Maps to existing MA-UNITDATA.confirm primitive
    • If it refers to an application layer success indication (e.g. MIH information services), then this is out of scope for 802.11 because the 802.11 MAC cannot get access to higher-layer protocol information
  • Implication for 802.11u draft
    • MA-UNITDATA.confirm results are received by the convergence function and passed up as this event

Matthew Gast, Trapeze Networks

mobility operations

Mobility Operations

Link_Handover_Imminent

Link_Handover_Complete

Matthew Gast, Trapeze Networks

note on mih handover operations
Note on MIH “Handover” operations
  • According to 802.21-D7.1, clause 7.3.11:
    • “Link_Handover_Imminent is generated when a native link layer handover or switch decision has been made and its execution is imminent …”
    • Similar definition for handover complete operation in 7.3.12
  • This is an L2 event, indicating intra-ESS transitions in 802.11
    • This will be an extremely frequent report for 802.11 link layers
    • Question for 802.21 WG: why is it necessary to report intra-L2 handover, since 802.11 access networks are built with L2 mobility in mind?
    • These events probably do not need support in 802.11

Matthew Gast, Trapeze Networks

link handover imminent
Link_Handover_Imminent
  • English meaning: The non-AP STA is about to try to move to another AP, but has not completed the process
  • Technical meaning in terms of 802.11
    • (1) The non-AP STA has 802.11 authenticated (not 802.1X-authenticated) to a new AP, but has not moved its association. The mobility manager intends to move
    • (2) The non-AP STA has initiated an 802.11r fast transition; in 802.11r-D7.0, this means that the MLME-RESOURCE_REQUEST.request has been sent, or that the MLME-(RE)ASSOCIATE.request containing the Fast Transition IE has been sent
  • Implications for 802.11u draft
    • The “intention to move” can only be determined by the SME. Therefore, this event must be reported by the SME and passed to the convergence function
    • Or, TGu may choose not to support this, since intra-ESS handover is not an interesting or valuable event from the perspective of L3 mobility

Matthew Gast, Trapeze Networks

link handover complete
Link_Handover_Complete
  • English meaning: The non-AP STA has completed its move to a new AP
  • Technical meaning in terms of 802.11
    • (1) On an unencrypted network, the MLME has received the MLME-ASSOCIATE.confirm primitive
    • (2) On an encrypted network, the MLME-(RE)ASSOCIATE.confirm primitive has been followed by an MLME-SetKeys.confirm primitive in a “short” period of time
  • Implications for 802.11u draft
    • TGu may choose not to support this, since intra-ESS handover is not an interesting or valuable event from the perspective of L3 mobility

Matthew Gast, Trapeze Networks

link handover discussion points
Link Handover discussion points
  • Purpose of Link Handover, as described in September 19, 2007 joint meeting
    • Inform higher layers that there may be interruption in service, causing packet loss or TCP slow start delays
    • 802.11 already handles packet loss on AP-to-AP transition within an ESS
      • Frames are buffered during transition
      • Frames may be retransmitted rapidly without requiring network- or application-layer action (link layer retry counters)
  • Use cases for handover imminent events from Sep-19-2007 meeting
    • Potential use case #1: Use handover imminent events to filter Link_Up to network purposes within the MIH function
    • Potential use case #2: Inter-ESS mobility in combination with higher layer mobility protocols (e.g. Mobile IP) or other application protocols, so that a MIH-capable device will hold back on transmissions during the ESS switch

Matthew Gast, Trapeze Networks

mac api operations

MAC API Operations

Link_Event_Subscribe

Link_Event_Unsubscribe

Link_Capability_Discover

Link_Action

Matthew Gast, Trapeze Networks

link event subscribe
Link_Event_Subscribe
  • English meaning: MIH has requested to receive events from the lower level that will assist in its own operation
  • Technical meaning
    • Not relevant to the proposed 802.11 convergence function. The convergence function would always be available, and thus, there is no need to define a subscription model.
    • These functions are not defined for other IEEE 802 networks in 802.21-D7.1 Table L2.
  • Implications for 802.11u draft
    • None; does not need to be incorporated

Matthew Gast, Trapeze Networks

link event unsubscribe
Link_Event_Unsubscribe
  • English meaning: MIH no longer needs lower level events
  • Technical meaning
    • Not relevant to the proposed 802.11 convergence function. The convergence function would always be available, and thus, there is no need to define a subscription model.
    • These functions are not defined for other IEEE 802 networks in 802.21-D7.1 Table L2.
  • Implications for 802.11u draft
    • None; does not need to be incorporated

Matthew Gast, Trapeze Networks

link capability discover
Link_Capability_Discover
  • English meaning: MIH requests a list of supported events
  • Technical meaning in terms of 802.11
    • No primitive currently exists
  • Implications for 802.11u
    • Need to define two primitives in the convergence function: a request that comes in from higher layers, and a result that returns supported events

Matthew Gast, Trapeze Networks

link action
Link_Action
  • English meaning: Requests action on a link. Valid actions are listed in Table B4 on 802.21-D7.1, p. 178.
  • Technical meaning for 802.11
    • Disconnect: Deauthenticate/Disassociate peer MAC address (MLME-Deauthentate.request or MLME-Diassociate.request)
    • Low power: change power save status; in 802.11-2007, this translates to MLME-POWERMGT.request
    • Power down and power up link: no 802.11 primitives exist
    • Scan: issue MLME-SCAN.request
    • LINK_RESOURCE_RETAIN attribute is defined in Table B5 (p 184). It is used to reserve resources for future reconnection, which is not a concept that exists in 802.11
    • DATA_FORWARDING_REQUEST attribute asks that buffered data be sent to new point, which is already required by 802.11
  • Implications for 802.11u draft
    • Convergence function would need to map disconnect and low power actions to respective 802.11 primitives

Matthew Gast, Trapeze Networks

link parameter operations

Link Parameter Operations

Link_Configure_Thresholds

Link_Get_Parameters

Link_Parameters_Report

Matthew Gast, Trapeze Networks

what are link parameters
What are Link Parameters?
  • 802.21 defines generic parameters for all links (LINK_PARAM_GENERAL) in Table B2, page 181:
    • Speed, signal strength, SINR (“signal over interference plus noise ratio”), throughput, and packet Error Rate
  • 802.21 defines 2 additional parameters for 802.11 links (LINK_PARAM_802_11):
    • RSSI of the beacon channel
    • Flag to indicate that no QoS resources are available (i.e., request for capacity denied)
  • Implications for 802.11u
    • The 802.11 MAC reports, but does not persistently store, many of these parameters. They are stored in the SME, and the convergence function provides a “window” on to that new state that higher layers can access
    • The 802.21 parameter set is not as rich as the 802.11k framework, which includes noise, received power, signal/noise, antenna indication
    • TGu may wish to assist 802.21 in incorporating new 802.11k measurement parameters into the draft, along with additional parameters such as frame- or bit-error rates

Matthew Gast, Trapeze Networks

link configure thresholds
Link_Configure_Thresholds
  • Meaning in English: Sets thresholds for MIH operations so that events will be reported only when they exceed thresholds
  • Technical meaning for 802.11
    • This operation sets a trigger threshold. Even though 802.11k-D8.0 defines triggered reporting, its MLME primitives do not define primitives
  • Implications for TGu draft
    • Need to define operations of the converge function to store parameter values (from the SME)
    • Intent is to support the entire parameter list required by 802.21
    • Need to define behavior of events that are triggered when thresholds are passed; 802.11k revises clause 11.10.7 to create triggered measurement reports, and TGu will need to define something similar in its functional description

Matthew Gast, Trapeze Networks

link get parameters
Link_Get_Parameters
  • English meaning: Retrieve the parameters set by the previous operation so that a higher layer protocol can determine when events will be triggered
  • Technical meaning
    • This operation retrieves a trigger threshold set by the previous operation
  • Implication for 802.11u
    • Need to define operations of the converge function to retrieve parameter values (from the SME)

Matthew Gast, Trapeze Networks

link parameters report
Link_Parameters_Report
  • English meaning: Link parameters have crossed a specified threshold
  • Technical meaning in terms of 802.11
    • The MLME is stateless, so there is no existing 802.11 primitive. This type of event would have be generated by the SME and passed to the convergence function so it can be made available to higher layers
  • Implications for TGu draft
    • Generation of an event triggered by crossing a parameter threshold needs to be defined, perhaps in a similar fashion to 802.11k

Matthew Gast, Trapeze Networks

way forward

Way Forward

Matthew Gast, Trapeze Networks

next steps
Next Steps
  • TGu investigation required
    • Do MLME events for Deauthentication and Disassociation incorporate TGw protection, or do we need to filter them?
  • Joint TGu-802.21 technical investigation
    • Do the intra-L2 handover events need to be supported with an 802.11 link layer?
    • Is the current parameter list sufficient for 802.11, or should more types of parameters be defined?
  • Define convergence function for inclusion within TGu draft
    • Call for ad hoc meeting!

Matthew Gast, Trapeze Networks

references
References
  • 802.21-D7.1
  • 802.11-2007
  • 802.11k-D8.0
  • 802.11w-D2.3
  • 802.11u-D1.0

Matthew Gast, Trapeze Networks