| Negative AcKnowledgement layer (NAKs), paired with positive ACKs. The default is to send a message
using NAKs: the sender sends messages with monotonically increasing seqnos, receiver requests
retransmissions of missing messages (gaps). When a SWITCH_NAK_ACK event is received, the mode
is switched to using NAK_ACKS: the sender still uses monotonically increasing seqnos, but the receiver
acknowledges every message. NAK and NAK_ACK seqnos are the same, when switching the mode, the current
seqno is reused. Both NAK and NAK_ACK messages use the current view ID in which the message is sent to
queue messages destined for an upcoming view, or discard messages sent in a previous view. Both modes
reset their seqnos to 0 when receiving a view change. The NAK_ACK scheme is used for broadcasting
view changes.
The third mode is for out-of-band control messages (activated by SWITCH_OUT_OF_BAND): this mode does
neither employ view IDs, nor does it use the same seqnos as NAK and NAK_ACK. It uses its own seqnos,
unrelated to the ones used by NAK and NAK_ACK, and never resets them. In combination with the sender's
address, this makes every out-of-band message unique. Out-of-band messages are used for example for
broadcasting FLUSH messages.
Once a mode is set, it remains in effect until exactly 1 message has been sent, afterwards the default
mode NAK is used again.
The following communication between 2 peers exists (left side is initiator,
right side receiver):
send_out_of_band
--------------> synchronous (1)
<-------------
ack
send_nak
--------------> asynchronous (2)
send_nak_ack
--------------> synchronous (3)
<--------------
ack
retransmit
<-------------- asynchronous (4)
When a message is sent, it will contain a header describing the type of the
message, and containing additional data, such as sequence number etc. When a
message is received, it is fed into either the OutOfBander or NAKer, depending on the
header's type.
Note that in the synchronous modes, ACKs are sent for each request. If a reliable unicast protocol layer
exists somewhere underneath this layer, then even the ACKs are transmitted reliably, thus increasing
the number of messages exchanged. However, since it is envisaged that ACK/OUT_OF_BAND are not used
frequently, this problem is currently not addressed.
author:
Bela Ban |
