Layer 2 Retransmission Protocol (L2RP)

General idea: a generic and optional mechanism for IEEE 802 series protocols to detect and retransmit lost unicast packets between stations. While potentially useful on all media, the target use case is wireless networks where individual stations talk directly to each other without a controller, such as 802.11 ad-hoc networks. This would allow for error correction on a level that happens in wifi infrastructure mode networks in a self-constructing wireless network without defined access points.

How it works:

The sender attaches a sequence number to each transmitted unicast frame and keeps a local copy until the receiver acknowledges it. If the receiver indicates it has not received the frame, the sender sends another copy.

Data frame:

Each frame sent by a particular sender (MAC address) contains an incrementing sequence number. Received sequence numbers are gathered and an ack message is sent back to the sender indicating sequences received or missing. The sender holds copies of the packets until successful receipt is acknowledged.

If transmission is idle for 2 seconds without receiving an ack frame from the recipient, resend the most recently transmitted frame (only). Double the timeout and try again. Any additional frame generated by layer 3 activity resets the timeout to zero since it causes a frame with a new sequence number that the recipient can respond to with a nack if it lost anything. [retransmit timeout = 2]

After 30 seconds of retransmission attempts, give up and discard all unacked frames from the buffer. Report discarded frames to layer 3 for further action (such as sending unreachable messages back to the packet origins). [give up timeout = 30]

If there's not enough memory to keep a copy of the most recent frame, discard the second most recent frame to make space. The logic behind discarding the second most recent instead of the oldest is that it avoids a pathological case in which the recipient on a long-latency link keeps requesting lost frames older than what the sender has.

If a recipient does not know what L2RP version it should be using with a sender and it receives an L2RP data frame, the frame must be discarded and must not be acknowledged. Instead, send an L2RP offer message indicating supported L2RP versions.

Deduplication:

By default, received data frames should be passed up to layer 3 even if they appear to be repeats of already-transmitted frames. If locally configured by the operator, frames with sequence numbers close to but less than the current sequence number should be discarded but should still result in an ack message. This allows deduplication on networks where signal echoes duplicate data packets.

[I forget the brand, but there's a MANET radio system which trades capacity for range by repeating every received packet with a decrementing counter. It's easy and stateless for the radio to implement but it duplicates packets which the upper protocol layers have to handle. L2RP can implement this deduplication by setting the always flag and discarding packets for already-received sequence numbers.]

Ack frame:

Completion flag: this frame contains the entire set of acks and nacks. If false (0) then the sender cannot discard frames that the message does not explicitly state have been received.

Ack up to is the sequence number before which all frames have been received or accounted for.

Ack latest is the most recent sequence number received when the ack was sent.

Ack frames must be sent once per second and only if data has been received since the last ack frame.

Detail acks components:

Nacks are negative acknowledgements. The describe a frame or frames prior to ack-latest which the recipient has not received from the sender. The sender should retransmit these frames or send an abandonment control message if the frames are no longer available.

Congest flags indicate that the recipient successfully received the frames but discarded them due to lack of memory. The sender must not resend these frames. L2RP should report congestion loss to layer 3 for further action.

Resequencing and abandonment:

There are several cases in which a sender may not possess frames indicated in a NACK message. Memory pressure may have required an early discard. The sender may have started a new session, forgetting previous frames. The sender may have rebooted. In these cases, the recipient sends an ack which the sender needs to correct.

Oldest is the sequence number of the oldest frame the sender still retains. The recipient should not send acks or nacks for any older frame; they are lost.

Latest is the sequence number of the most recently sent frame. If the recipient believes it has received a more recent frame, it must discard that information, set latest as it's most recently received frame for ack purposes and propagate received frames with sequence numbers after latest even if it believes it has already sent them.

Note that latest may be < oldest if sequence numbers have wrapped around maxint.

Negotiating L2RP:

Tell a neighbor that you support L2RP and which versions you support (currently = 0).

Always flag: you'd prefer that neighbors who support L2RP always use it when sending you packets even if the detected network conditions to not appear to require it.

Avoid flag: while you support L2RP you'd prefer that neighbors avoid using it unless network conditions suggest that it's needed. This will prevent neighbors from periodically turning it on just to check current loss rates.

This frame should be sent via multicast [multicast address] upon link initialization and twice more after 30 and 60 seconds.

This frame must be received via unicast with an exact min version = max version before L2RP data frames may be sent back to the negotiation frame's sender. A sender may send this control frame to any recipient with to which it has sent an unrelated data frame, but it may send them to a particular recipient no more than once every 10 minutes and no more than 10 times in 6 hours. However, if it receives a multicast control frame from the recipient, these limits are reset to zero.

After receiving a unicast negotiation frame where min version = max version, the recipient must send the next frame using that version of L2RP. The initial sequence number is selected randomly. The recipient will return an ack with nacks to which the sender will respond with a resequence control bringing the two into alignment. The recipient is not required to send any further frames to the recipient using L2RP. It may continue to send them bare based on its local configuration preferences and detected link conditions.

If no data frame is available within 1 second, the recipient should return a data frame with a following ethertype of 0 and 0 bytes of data.

Upon receiving a unicast negotiation frame where min version < max version, the recipient must return a negotiation frame selecting a specific version with which to communicate. It must not send data frames using L2RP until its partner sends the first L2RP message. If it has previously been sending data frames with L2RP, it must stop doing so and should resend any unacknowledged frames bare.

Typical communication process: