HDLC DataLink Layer
Chapter 6
621
A node receiving REJ initiates sequential retransmission of data frames
starting with the data frame indicated by the N(R) contained in the REJ
frame.
Timeout Recovery
If, due to a transmission error, a node does not receive (or receives and
discards) a single data frame or the last frames in a sequence of data
frames, it will not detect an out-of-sequence exception; therefore, it will
not transmit a REJ frame. Following completion of a timeout period, the
node that transmitted the unacknowledged data frame(s) must use a
supervisory frame to enquire about the receive variable at the other node.
If the response does not acknowledge all outstanding data frames, the
node can then retransmit all of its unacknowledged data frames. The
N(R) of the response determines the N(S) at which the retransmission
must begin.
FCS Error
Any frame with an FCS error will not be accepted by the receiving node
and will be discarded. A node will take no action as a result of a frame
that it discards.
FrameReject Exception Condition
A frame-reject exception condition is established upon the receipt of an
FCS error-free frame which contains an invalid control field, an invalid
N(R), or a data field that has exceeded the maximum of 256 bytes. If a
frame-reject exception condition occurs at a node, it could do either of the
following:
Take recovery action without reporting the condition to the remote
node.
Report the condition to the remote node with a FRMR response. The
remote node should then take recovery action. If, after waiting an
appropriate time, no recovery action appears to have been taken, the
node reporting the frame-reject exception condition may take recovery
action.
Recovery action includes transmitting a mode-setting command.
High-level functions may also be involved in the recovery.