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Subj: AX.25 Doc : Part 6/9
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AX25.DOC PART 6/9
AX.25 Amateur Packet-Radio Link-Layer Protocol
Version 2.0 October 1984
2.3.4.3.3 Frame Reject (FRMR) Response
2.3.4.3.3.1
The FRMR response frame is sent to report that the receiver
of a frame cannot successfully process that frame and that the
error condition is not correctable by sending the offending frame
again. Typically this condition will appear when a frame without
an FCS error has been received with one of the following
conditions:
1. The reception of an invalid or not implemented command or
response frame.
2. The reception of an I frame whose information field exceeds
the agreed-upon length. (See 2.4.7.3, below.)
3. The reception of an improper N(R). This usually happens
when the N(R) frame has already been sent and acknowledged,
or when N(R) is out of sequence with what was expected.
4. The reception of a frame with an information field where one
is not allowed, or the reception of a U or S frame whose
length is incorrect. Bits W and Y described in 2.3.4.3.3.2
should both be set to one to indicate this condition.
5. The reception of a supervisory frame with the F bit set
to one, except during a timer recovery condition (see
2.4.4.9), or except as a reply to a command frame sent with
the P bit set to one. Bit W (described in 2.3.4.3.3.2)
should be set to one.
6. The reception of an unexpected UA or DM response frame. Bit
W should be set to one.
7. The reception of a frame with an invalid N(S). Bit W should be
set to one.
An invalid N(R) is defined as one which points to an I
frame that previously has been transmitted and acknowledged, or
an I frame which has not been transmitted and is not the next
sequential I frame pending transmission.
An invalid N(S) is defined as an N(S) that is equal to
the last transmitted N(R)+k and is equal to the received state
variable V(R), where k is the maximum number of outstanding
information frames as defined in 2.4.7.4 below.
An invalid or not implemented command or response is
defined as a frame with a control field that is unknown to the
receiver of this frame.
2.3.4.3.3.2
When a FRMR frame is sent, an information field is added to
the frame that contains additional information indicating where
the problem occurred. This information field is three octets
long and is shown in Fig. 9.
Information Field Bits
23 22 21 20 19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
0 0 0 0 Z Y X W V(R) C V(S) 0 Rejected Frame
R Control Field
Fig. 9 -- FRMR frame information field
Where:
1. The rejected frame control field carries the control field
of the frame that caused the reject condition. It is in
bits 0-7 of the information field.
2. V(S) is the current send state variable of the device
reporting the rejection (bit 9 is the low bit).
3. The CR bit is set to zero to indicate the rejected frame
was a command, or one if it was a response.
4. V(R) is the current receive state variable of the device
reporting rejection (bit 13 is the low bit).
5. If W is set to 1, the control field received was invalid or
not implemented.
6. If X is set to 1, the frame that caused the reject
condition was considered invalid because it was a U or S
frame that had an information field that is not allowed.
Bit W must be set to 1 in addition to the X bit.
7. If Y is set to 1, the control field received and returned
in bits exceeded the maximum allowed under this
recommendation in 2.4.7.3, below.
8. If A is set to 1, the control field received and returned
in bits 1 to 8 contained an invalid N(R).
9. Bits 8, and 20 to 23 are set to 0.
2.3.4.3.4 Unnumbered Acknowledge (UA) Response
The UA response frame is sent to acknowledge the
reception and acceptance of a SABM or DISC command frame. A
received command is not actually processed until the UA response
frame is sent. Information fields are not permitted in a UA
frame.
2.3.4.3.5 Disconnected Mode (DM) Response
The disconnected mode response is sent whenever a DXE
receives a frame other than a SABM or UI frame while in a
disconnected mode. It is also sent to request a set mode
command, or to indicate it cannot accept a connection at the
moment. The DM response does not have an information field.
Whenever a SABM frame is a received, and it is determined
that a connection is not possible, a DM frame shall be sent.
This will indicate that the called station cannot accept a
connection at that time.
While a DXE is in the disconnected mode, it will respond
to any command other than a SABM or UI frame with a DM response
with the P/F bit set to 1.
2.3.4.3.6 Unnumbered Information (UI) Frame
The Unnumbered Information frame contains PID and
information fields and is used to pass information along the link
outside the normal information controls. This allows information
fields to go back and forth on the link bypassing flow control.
Since these frames are not acknowledgeable, if one gets
obliterated, there is no way to recover it. A received UI frame
with the P bit set shall cause a response to be transmitted.
This response shall be a DM frame when in the disconnected state
or a RR (or RNR, if appropriate) frame in the information
transfer state.
2.3.5 Link Error Reporting and Recovery
There are several link-layer errors that are recoverable
without terminating the connection. These error situations may
occur as a result of malfunctions within the DXE, or if
transmission errors occur.
2.3.5.1 DXE Busy Condition
When a DXE becomes temporarily unable to receive I
frames, such as when receive buffers are full, it will send a
Receive Not Ready (RNR) frame. This informs the other DXE that
this DXE cannot handle any more I frames at the moment. This
condition is usually cleared by the sending of a UA, RR, REJ, or
SABM command frame.
2.3.5.2 Send Sequence Number Error
If the send sequence number, N(S), of an otherwise error-
free received frame does not match the receive state variable,
V(R), a send sequence error has occurred, and the information
field will be discarded. The receiver will not acknowledge this
frame, or any other I frames, until N(S) matches V(R).
The control field of the erroneous I frame(s) will be
accepted so that link supervisory functions such as checking the
P/F bit can still be performed. Because of this updating, the
retransmitted I frame may have an updated P bit and N(R).
2.3.5.3 Reject (REJ) Recovery
REJ is used to request a retransmission of I frames
following the detection of a N(S) sequence error. Only one
outstanding "sent REJ" condition is allowed at a time. This
condition is cleared when the requested I frame has been
received.
A DXE receiving the REJ command will clear the condition
by resending all outstanding I frames (up to the window size),
starting with the one indicated in N(R) of the REJ frame.
END AX25.DOC PART 6/9
----------------------
73's Nacho EA2CMN @EA2CMN.EAZ.ESP.EU
QTH: Zaragoza (Spain)
[27-Sep-1994 - 15:54]
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