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Path Selection. An Extensible Framework. Extensible Path Selection Framework. Selection of paths and forwarding of data frames in 802.11 mesh networks are both accomplished at the link layer Path selection messages are also transported at the link layer using MMPDUs

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path selection

Path Selection

An Extensible Framework

extensible path selection framework
Extensible Path Selection Framework
  • Selection of paths and forwarding of data frames in 802.11 mesh networks are both accomplished at the link layer
    • Path selection messages are also transported at the link layer using MMPDUs
  • IEEE 802.11 uses an extensible framework to enable flexible implementation of path selection protocols and link metrics to meet special application needs
    • Vendor specific protocol and metric
    • Special requirements (high mobility of MPs)
extensible path selection framework1
Extensible Path Selection Framework
  • An MP may include multiple protocol and metric implementations
    • However only one path selection protocol and one link metric shall be active in a mesh at a time
    • In a given mesh network, all MPs must select and use the same protocol and metric to find paths
    • MPs use Mesh Configuration Elements to announce the active protocol and metric
  • IEEE 802.11 standard specifies default mandatory path selection protocol (Hybrid Wireless Mesh Protocol) and metric (Airtime)
airtime link metric
Airtime Link Metric
  • r is the rate at which the MP would transmit a frame of length Bt
  • ef is the frame error rate
link metric reporting
Link Metric Reporting
  • Procedure used to determine the link metric associated with a particular link
  • If bidirectional link metrics are required,
    • Each MP may request a link metric report from a neighbor peer MP by sending a request frame
    • Or may voluntarily submit a metric report to a neighbor peer MP by sending a report frame
  • Upon receiving a report, MP updates its local link metric information
link metric report
Link Metric Report
  • Metric M: Value of the link metric associated with the mesh link between the neighbor peer MP sending the link metric report and the local MP
path selection1

Path Selection

Hybrid Wireless Mesh Protocol


  • Routing Protocol that selects paths from a source MP (Mesh SA) to a destination MP (mesh DA), and thus between STAs
  • Combines two known flavors of routing in multihop wireless networks (hence hybrid):
    • Proactive: routes are maintained at all times
    • Reactive: routes are established on-demand
  • Efficient: optimizes path selection by minimizing a radio aware cost metric (air time)
  • Based on Distance Vector routing concepts
  • Two modes of operation:
    • On-Demand Path Selection Mode:
      • MPs use peer-to-peer paths to communicate
      • Based on Ad Hoc On Demand Distance Vector (AODV) (IETF RFC 3561)
    • Proactive Tree Building Mode:
      • An MP is configured to be a root MP
      • Paths between MPs and the root are continuously maintained
  • Both modes may be used concurrently
on demand path selection mode
On-demand Path Selection Mode

Path discovery Procedure

  • A source MP finds a path to a destination MP by broadcasting a path request (PREQ) with
    • Source MP
    • Destination MP (or list of destination MPs)
    • Sequence number (to distinguish PREQs from each other)
    • Metric field initialized to initial value of the active metric
path request preq information element
Path Request (PREQ) Information Element
  • Hop Count:
    • Number of hops from originator to the MP transmitting the request
  • Time to Live:
    • Maximum number of hops allowed for the PREQ
path request preq information element1
Path Request (PREQ) Information Element
  • Originator Address:
    • Originating MP’s MAC address
  • Originator Sequence Number:
    • Sequence Number specific to the originating MP
  • Proxied Address:
    • Address of a proxied entity (e.g., non-mesh STA); this field is present when the PREQ is generated for a frame received from a proxied entity
  • Lifetime:
    • Time that receiving MPs consider forwarding information to be valid
path request preq information element2
Path Request (PREQ) Information Element
  • Metric:
    • Cumulative metric from the originator to the MP transmitting the PREQ
  • Destination Count:
    • Number of destinations contained in the PREQ
  • Destination address:
    • MAC address of destination
  • Destination Sequence number:
    • Latest sequence number received in the past by the originator for any path towards the destination
path request preq information element3
Path Request (PREQ) Information Element
  • Per-destination flags:
    • Destination Only (DO):
      • If DO = 1, only destination can respond with a unicast PREP, else, an intermediate MP with active forwarding information to the destination responds with a unicast PREP
    • Reply and Forward (RF):
      • Controls forwarding of PREQ at intermediate MPs when DO = 0 and intermediate MP replies with PREP. PREQ is not forwarded if RF = 0, and forwarded if RF = 1
on demand path selection mode1
On-demand Path Selection Mode
  • When an MP receives a PREQ
    • It creates a path to the source
    • If such a path already exists, then it updates the path if
      • the PREQ Originator Sequence Number is higher, or
      • the sequence number is the same but the metric value is lower
    • If a path is created or updated, then the PREQ is forwarded and the metric field updated to reflect the cumulative metric for the entire path
    • If the MP is a destination MP, then the MP sends a Path Reply (PREP) back to the source MP
on demand path selection mode2
On-demand Path Selection Mode
  • Transmission of PREP back to the source
    • Transmitted along the path identified so far as the best path from the source MP to the destination MP
    • At each intermediate MP on the path, the path metric in the PREP is updated
  • To speed up path discovery
    • Intermediate MPs may be allowed to return a PREP to the source, thus taking advantage of existing paths to the destination
    • This is accomplished by means of 2 flag bits in PREQ
      • “Destination Only (DO)” and “Reply and Forward”
path reply prep information element1
Path Reply (PREP) Information Element
  • Hop Count:
    • Number of hops from the path destination to MP transmitting the PREP
  • Time to Live:
    • Maximum number of hops allowed for the PREP
  • Destination MP address:
    • MAC address of the MP for which a path is supplied
  • Destination Sequence Number:
    • Destination Sequence Number (DSN) of the target of the destination MP
path reply prep information element2
Path Reply (PREP) Information Element
  • Destination Proxied Address:
    • Proxied address on behalf of which the PREP is sent
  • Lifetime:
    • Lifetime in TUs of the PREQ this PREP corresponds to
  • Metric:
    • Cumulative metric from the path destination to the MP transmitting the PREP
  • Originator Address:
    • MAC address of the originator of the PREQ
  • Originator Sequence Number:
    • Sequence Number of the originator of the PREQ
forwarding information
Forwarding Information
  • Information maintained at an MP used in forwarding frames to a destination MP
    • Destination MP address
    • Destination Sequence Number
    • Next Hop Address
    • Path metric
    • Precursor list (list of MPs using this MP as next hop)
    • Lifetime of this forwarding information
  • Stored forwarding information may be active or inactive (e.g., lifetime exceeded, PERR received)
path error perr information element
Path Error (PERR) Information Element
  • Used when an MP is unable to forward data frames to next hop MP (thus detecting a broken link)
    • Contains list of unreachable destinations
  • Sent to source MPs that have an active path over this broken link
    • PERR is sent to precursors (as unicast or broadcast frames)
    • An MP receiving a PERR, propagates PERR to its precursors
  • An MP generating or receiving a PERR may decide to reestablish paths
  • Active forwarding information associated with unreachable destinations should become inactive (no longer be used)
path error perr information element1
Path Error (PERR) Information Element
  • Destination Address:
    • Detected unreachable MAC address
  • Destination Sequence Number:
    • Sequence number of detected unreachable destination MP
proactive tree building mode
Proactive Tree Building Mode
  • Two mechanisms for proactively disseminating path selection information for reaching the root MP
    • Proactive PREQ intended to create paths between the root MP and all MPs
    • Root Announcement (RANN) intended to distribute path (metric) information for reaching the root MP, but the actual paths to the root MP can be built on demand
  • Root MP would send either proactive PREQ or RANN periodically
proactive preq mechanism
Proactive PREQ mechanism
  • Proactive PREQs are sent periodically by the root MP with the destination address set to all 1
    • Processing of Proactive PREQs is the same as in on-demand mode
  • The transmission of proactive PREP by MPs receiving a proactive PREQ depends on the “proactive PREP” bit in the PREQ:
    • If set to 0, then MP sends a proactive PREP only if a bidirectional path needs to be established
    • If set to 1, then MP shall send proactive PREP; (PREP is also sent when route from MP to root MP changes)
proactive rann mechanism
Proactive RANN Mechanism
  • Proactive RANN are sent periodically by root MP
    • Announce the existence of root MP
    • allow MPs to establish paths to the root MP on-demand
  • An MP that has to create (or refresh) a path to the root MP
    • MP sends unicast PREQ to root MP via the MP through which it receives the RANN
    • Root MP sends a PREP in response to each PREQ
root announcement rann information element
Root Announcement (RANN) Information Element
  • Flags:
    • Bit 0: Portal Role (0 = Non-portal, 1 = Portal). Bits 1-7: Reserved
  • Hop Count:
    • Number of hops from the originator (Root MP) to the MP transmitting the request
root announcement rann information element1
Root Announcement (RANN) Information Element
  • Time to Live:
    • Remaining number of times the RANN may be forwarded
  • Originator Address:
    • MAC address of originating root MP
  • Destination Sequence Number:
    • Sequence number specific to the originator (root MP)
  • Metric:
    • Cumulative metric from the originator (root MP) to the MP transmitting the announcement