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This page is not in sync with the actual implementation. Neither the packet format matches nor the use as unicast probe frames is explained.

Echo Location Protocol (ELP)


Image Source: Petteri Aimonen, Wikimedia Commons

1. Definitions

  • Node - A mesh router which utilizes the B.A.T.M.A.N. protocol as specified in this document on at least one network interface.
  • originator - A node broadcasting its own OGMs (see OGM for details) that is therefore addressable within the mesh network routing layer. It is uniquely identifiable by its originator address. B.A.T.M.A.N.-Advanced uses the MAC address of its primary hard interface.
  • hard-interface - Network interface utilized by B.A.T.M.A.N. for its own Ethernet frames.
  • Neighbor: An ELP sender within one hop distance (note, this is defined differently for the OGM protocol)

2. Conceptual Data Structures

2.1. Neighbor List

  • Neighbor Address: The Ethernet source address of the received ELP message.
  • Originator Address: The originator address of the node.
  • Last Received Sequence Number: The sequence number of the last received ELP message
  • Last Seen: The time when the last ELP message was received
  • metric: the EWMA of the metric towards this neighbor

2.2. Originator List

  • Originator Address: The originator address of the node.

3. Protocol Procedure

3.1 Broadcasting own Echo Location Protocol (ELP) Messages

Each node periodically (ELP interval) generates and broadcasts ELP messages for each interface B.A.T.M.A.N. is running on. A jitter may be applied to avoid collisions.

The Echo Location Protocol (ELP) Format:

  • Packet type: Initialize this field with the ELP packet type.
  • Version: Set your internal compatibility version.
  • TTL: not used.
  • Num Neigh: The number of neighbors that this neighbour already discovered with the interface where this packet was sent.
  • Sequence number: On first broadcast set the sequence number to an arbitrary value and increment the field by one for each following broadcast.
  • Interval: Set to the current ELP interval of this interface in milliseconds. The default interval is 500ms and it may be reconfigured during run-time.
  • Originator Address: Set this field to the primary MAC address of this B.A.T.M.A.N. node.

If this B.A.T.M.A.N. interface wants to announce neighboring nodes it should append a neighbor entry message for each neighbor to be announced and fill the "number of neighbors" field accordingly.

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | Packet Type   |    Version    |      TTL      |   Num Neigh   |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                       Sequence Number                         |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                          Interval                             |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                     Originator Address                        |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |      Originator Address       |                               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

3.3.1. ELP minimum packet size / padding

An ELP packet should be padded to at least 300 Bytes (excluding ethernet frame header) and may be padded to up to 1500 Bytes. Especially on wireless interfaces the packet size of broadcast packets can have quite an impact on the probability of arrival.

3.2. Receiving Echo Location Messages (ELP)

Upon receiving an ELP packet a node must perform the following preliminary checks before the packet is further processed:

  • If the ELP contains a version which is different to the own internal version the message must be silently dropped (thus, it must not be further processed).
  • If the sender address of the ELP message is an Ethernet multicast (including broadcast) address the message must be silently dropped.
  • If the destination address of the ELP message is a unicast address the message must be silently dropped.
  • If the originator address of the ELP message is our own the message must be silently dropped as this ELP message originated from this node.

3.3. Neighbor Ranking

For each ELP message having passed the preliminary checks the following actions must be performed:

  • The last seen time of this neighbor interface needs to be updated.
  • The last updated time of this neighbor interface needs to be updated.
  • The elp interval of this neighbor interface needs to be updated with the elp interval set in the received ELP message.
  • The last received sequence number from this neighbor needs to be set to the sequence number of the received ELP message.

4. Proposed Values for Constants

  • SEQNO_SIZE: 2^32
  • OUTDATED_MAX: 4