Repository : ssh://git@diktynna/doc
On branches: backup-redmine/2020-07-12,master
commit c220099cb0d255ddfdbe26f8f83ea168715b9ef9 Author: Sven Eckelmann sven@narfation.org Date: Sat Jun 20 21:38:04 2020 +0000
doc: batman-adv/BATMAN_IV
c220099cb0d255ddfdbe26f8f83ea168715b9ef9 batman-adv/BATMAN_IV.textile | 10 +++++----- 1 file changed, 5 insertions(+), 5 deletions(-)
diff --git a/batman-adv/BATMAN_IV.textile b/batman-adv/BATMAN_IV.textile index 0c4430b..f017dc3 100644 --- a/batman-adv/BATMAN_IV.textile +++ b/batman-adv/BATMAN_IV.textile @@ -4,11 +4,11 @@ B.A.T.M.A.N. is a proactive routing protocol for Wireless Ad-hoc Mesh Networ
h2(#b.a.t.m.a.n.-iii-brief-overview). B.A.T.M.A.N. III (Brief Overview)
-On a regular basis every B.A.T.M.A.N. node broadcasts an originator message (or OGM), thereby informing its link-local neighbors about its existence (first step). Link-local neighbors which are receiving the Originator messages are relaying them by rebroadcasting it, according to specific B.A.T.M.A.N. forwarding rules. The B.A.T.M.A.N. mesh network is therefore flooded with Originator messages. This flooding process will be performed by single-hop neighbors in the second step, by two-hop neighbors in the third step, and so forth. OGMs are send and repeated as UDP broadcasts, therefore OGMs are flooded until every node has received it at least once, or until they got lost due to packet loss of communication links, or until their TTL (time to live) value has expired. In practise OGM packet loss caused by interference, collision or congestion is significant. The number of OGMs received from a given Originator via each link-local neighbor is used to estimate the qual ity of a (single-hop or multi-hop) route. In order to be able to find the best route to a certain originator, B.A.T.M.A.N counts the originator-messages received and logs which link-local neighbor relayed the message. Using this information B.A.T.M.A.N. maintains a table with the best link-local router towards every originator on the network. Unlike wired networks, WiFi setups often face the problem of asymetric links (Node A has a better connection towards Node B than vice versa). To ensure that the detected connections allow communication in both directions each B.A.T.M.A.N. node awaits rebroadcasts of its own OGMs from his neighbors within a certain timeframe (bidirectional link check). If the OGMs are not successfully retransmitted the connection is considered too asymetric (unusable) and therefore ignored. +On a regular basis every B.A.T.M.A.N. node broadcasts an originator message (or OGM), thereby informing its link-local neighbors about its existence (first step). Link-local neighbors which are receiving the Originator messages are relaying them by rebroadcasting it, according to specific B.A.T.M.A.N. forwarding rules. The B.A.T.M.A.N. mesh network is therefore flooded with Originator messages. This flooding process will be performed by single-hop neighbors in the second step, by two-hop neighbors in the third step, and so forth. OGMs are send and repeated as UDP broadcasts, therefore OGMs are flooded until every node has received it at least once, or until they got lost due to packet loss of communication links, or until their TTL (time to live) value has expired. In practise OGM packet loss caused by interference, collision or congestion is significant. The number of OGMs received from a given Originator via each link-local neighbor is used to estimate the qual ity of a (single-hop or multi-hop) route. In order to be able to find the best route to a certain originator, B.A.T.M.A.N counts the originator-messages received and logs which link-local neighbor relayed the message. Using this information B.A.T.M.A.N. maintains a table with the best link-local router towards every originator on the network. Unlike wired networks, WiFi setups often face the problem of asymetric links (Node A has a better connection towards Node B than vice versa). To ensure that the detected connections allow communication in both directions each B.A.T.M.A.N. node awaits rebroadcasts of its own OGMs from its neighbors within a certain timeframe (bidirectional link check). If the OGMs are not successfully retransmitted the connection is considered too asymetric (unusable) and therefore ignored.
h1(#b.a.t.m.a.n.-iv-tq). B.A.T.M.A.N. IV (TQ)
-The B.A.T.M.A.N. III algorithm has serious problems when it comes to asymetric links. The bidirectional link check tries to limit its impact but the result is far from being perfect. The timeframe in which B.A.T.M.A.N. accepts his own OGMs being rebroadcasted by its neighbor allows to tweak the behaviour. If this timeframe is rather short B.A.T.M.A.N. is very strict on choosing links. This may lead to many ignored links which might be usable in one direction. Only symetric connections will be considered. If the timeframe value is less strict B.A.T.M.A.N. will accept more links but tends to route in the wrong direction. +The B.A.T.M.A.N. III algorithm has serious problems when it comes to asymetric links. The bidirectional link check tries to limit its impact but the result is far from being perfect. The timeframe in which B.A.T.M.A.N. accepts its own OGMs being rebroadcasted by its neighbor allows to tweak the behaviour. If this timeframe is rather short B.A.T.M.A.N. is very strict on choosing links. This may lead to many ignored links which might be usable in one direction. Only symetric connections will be considered. If the timeframe value is less strict B.A.T.M.A.N. will accept more links but tends to route in the wrong direction.
Example: OGMs from Node A propagate to B. The link is asymetric, therefore B receives all packets from A in contrast to A which receives almost nothing from B. As all the packets from A get to B the packet count at B's side goes up. B will assume that it has a perfect link towards A which is not the case.
@@ -36,7 +36,7 @@ As explained in the previous section the packet counting floods the network with
* B.A.T.M.A.N. knows the receiving link quality (RQ) by counting the packets of its neighbors. !rq.png(Receive Quality (RQ))! -* B.A.T.M.A.N. knows the echo link quality (EQ) by counting rebroadcasts of its own OGMs from his neighbors. +* B.A.T.M.A.N. knows the echo link quality (EQ) by counting rebroadcasts of its own OGMs from its neighbors. !eq_alt1.png(Echo Link Quality (EQ))! * B.A.T.M.A.N. can calculate the transmit link quality (TQ) by dividing the echo link quality by the receiving link quality. !tq.png(Transmit Link Quality (TQ))! @@ -89,7 +89,7 @@ The global TQ is an average of all received TQ values from one originator via a
h2(#handling-asymetric-links). Handling Asymetric Links
-Although the transmit link quality is most important decision factor B.A.T.M.A.N. IV also keeps track of the receiving link quality. On the WiFi layer every unicast packet has to be acknowledged by the neighbor node to approve the transmission. If this neighbor is not able to sucessfully send his ACKs the WiFi layer considers this transmission to be failed and tries to retransmit until it gives up. +Although the transmit link quality is most important decision factor B.A.T.M.A.N. IV also keeps track of the receiving link quality. On the WiFi layer every unicast packet has to be acknowledged by the neighbor node to approve the transmission. If this neighbor is not able to sucessfully send its ACKs the WiFi layer considers this transmission to be failed and tries to retransmit until it gives up.
!asym_link1.png(Asymetry situation without asymetry penalty)!
@@ -117,7 +117,7 @@ Example: 3 nodes (A, B and C) in a row (A can hear B but not C). Node A emits an
!echo_cancel1.png(communication without echo cancellation)!
-To detect echos (messages that already passed through a node) B.A.T.M.A.N. IV introduces a new protocol field called "previous sender" which contains the IP address of the node rebroadcasting the OGM. Whenever a node receives a message from a neighbor it will fill the "previous sender" field with the address of the sending neighbor before rebroadcasting it. If a node detects his own IP address in the "previous sender" field the packet will be ignored. +To detect echos (messages that already passed through a node) B.A.T.M.A.N. IV introduces a new protocol field called "previous sender" which contains the IP address of the node rebroadcasting the OGM. Whenever a node receives a message from a neighbor it will fill the "previous sender" field with the address of the sending neighbor before rebroadcasting it. If a node detects its own IP address in the "previous sender" field the packet will be ignored.
Back to the example: Node B will ignore (drop) the packet coming back from node C as node C wrote the IP address of node B in the "previous sender" field.