Originator Message version 2 (OGMv2)

Image Source: Martin Hundebøll, Jeppe Ledet-Pedersen, Network Coding for Wireless Mesh Networks

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 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 originator within one hop distance.
  • Router: A neighbor which is a potential, loop-free next hop for forwarding data packets towards a specific originator.
  • link throughput: link throughput from one interface to a neighbor's interface (see ELP for details).
  • best link throughput: The best of all link throughput values towards a neighbor (see ELP for details).
  • path throughput: The incoming OGM's throughput combined with the best link throughput of the according neighbor.
  • Originator entry: Local data structure in the originator list (see 'Conceptual Data Structures' for details).

Throughputs are specified in units of 100 kbit/s.

2. Conceptual Data Structures

2.1. Originator List

An originator list holds all addressable and to a certain degree reachable originators within the mesh network. The Originator Address and Selected Router fields of this list are of special interest for the actual routing decisions upon incoming data packets.

  • Originator Address: The originator address of the node.
  • Originator's Sequence Number: The newest OGM Sequence Number that has been accepted from the given Originator.
  • Selected Router: A neighbor which is chosen as the next hop to forward data packets to for the according originator.

3. Protocol Procedure

3.1 Broadcasting own Originator Message 2 (OGMv2)

Each node periodically (OGM interval) generates a single OGM which is broadcasted on all hard interfaces. A jitter may be applied to avoid collisions. The default interval in batman-adv is 1 second.

The Originator Message 2 (OGMv2) Format:

  • Packet type: Initialize this field with the ELP packet type.
  • Version: Set your internal compatibility version.
  • TTL: Initialize with BATADV_TTL
  • Flags: not used
  • Sequence number: On first broadcast set the sequence number to an arbitrary value and increment the field by one for each following OGMv2.
  • Originator Address: Set this field to the primary MAC address of this B.A.T.M.A.N. node.
  • TVLV length: Length of the TLVL data appended to the OGM
  • Throughput: Throughput metric value in 100 kbit/s. Initialize with BATADV_THROUGHPUT_MAX_VALUE
  • TVLV data: Appended TVLV data for the originator. See TVLV for a detailed description.
 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      |   Flags       |
 |                       Sequence Number                         |
 |                     Originator Address                        |
 |  (cont'd) Originator Address  |  TVLV length                  |
 |                         Throughput                            |
 |                        TVLV data ...                          |

3.2. Receiving Originator Messages

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

3.2.1. Preliminary Checks

The following checks are simple sanity checks which don't affect the routing logic:

  • Version Check: If the OGM contains a version which is different to the own internal version the message must be silently dropped (thus, it must not be further processed).
  • Source Check: If the sender address of the OGM is an ethernet multicast (including broadcast) address the message must be silently dropped.
  • Destination Check If the destination address of the OGM is a unicast address which does not match the received interface MAC address, the message must be silently dropped.
  • Own Message Check: If the originator address of the OGM is our own the message must be silently dropped as this OGM originated from this node.

3.2.2. Metric Update

The following steps check whether the Neighbor we received the OGMv2 from is a potential Router.

Each step is performed per potential outgoing interface where the OGMv2 may be rebroadcasted to allow Multi Link Optimizations. This a default interface next to the configured hard interfaces, which is used for locally generated traffic.

The following checks are performed before updating the metric:

  • Protection window check: If the OGMv2s sequence number is older than BATADV_OGM_MAX_AGE or newer than the BATADV_EXPECTED_SEQNO_RANGE, and the protection window is active, the packet is silently dropped. If both conditions are met but the protection window is not active yet, the OGMv2 is allowed but the protection window gets activated.
  • Age check: If the sequence number is strictly older than the last OGMv2, the packet is silently dropped. The only exception is when the protection window has just been activated, then the OGMv2 can pass.

If the initial checks above have passed, the internal stats are updated:

  • the last seen timestamps of the router and the originator are updated
  • the last sequence number and ttl values are adopted
  • if the link throughput to the neighbor this OGMv2 was forwarded by is lower than the path throughput of the OGMv2, then this lower link throughput is adopted
  • Forward penalties are applied:
    • if the considered interface is the default interface, no penalty is applied
    • if the incoming and considered outgoing interface is the same half duplex interface and the reported throughput is larger than 1 MBit/s, the throughput is reduced by 50%
    • Otherwise, a hop penalty is applied and the throughput is reduced by the according value (default 5.8% or 15/255). This is especially useful for "perfect" networks to create a decreasing metric over multiple hops.
  • The throughput value with the penalties applied is stored for the router

3.2.3. Route Update

After that, we check the OGMv2 whether a router update should be done and the OGMv2 should be rebroadcasted

  • If the OGMv2 was received through a neighbor that is not (yet) a router, drop the OGMv2

The passing OGMv2 will be considered for a router update:

  • If the OGMv2 has been received from the best router, no change is necessary
  • If no router has been selected yet, the received router becomes the selected router immediately
  • If the throughput from the received router is higher than the throughput via the selected router, the received router becomes the selected router
  • Also, if the sequence number is by at least OGM_MAX_ORIG_DIFF higher than the last received sequence number from the selected router, the received router becomes the selected router.

If the OGMv2 has been received by the (now) selected router, the OGM is forwarded on the considered outgoing interface (except for the default interface). However, the OGMv2 is not forwarded if another OGMv2 has been forwarded with the same sequence number.

Furthermore, TVLV data is processed when this OGMv2 was newer than previously received OGMv2s.

4. Re-broadcasting other nodes' OGMv2s

When an OGMv2 is to be re-broadcasted some of the message fields must be changed others must be left unchanged. All fields not mentioned in the following section remain untouched:

  • The TTL must be decremented by one. If the TTL becomes zero (after the decrementation) the packet must be dropped.
  • The Path throughput for the considered outgoing interface is adopted

The OGMv2 is then rebroadcasted on the specific outgoing interface.

5. Values for Constants