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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. 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 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 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. 2.3.5.4 Time-out Error Recovery 2.3.5.4.1 T1 Timer Recovery If a DXE, due to a transmission error, does not receive in a sequence of I frames, it will not detect a send-sequence- number error, and therefore will not transmit a REJ. The DXE which transmitted the unacknowledged I frame(s) shall, following the completion of time-out period T1, take appropriate recovery action to determine when I frame retransmission should begin as described in 2.4.4.9, below. This condition is cleared by the reception of an acknowledgement for the sent frame(s), or by the link being reset. See 2.4.6. 2.3.5.4.2 Timer T3 Recovery Timer T3 is used to assure the link is still functional during periods of low information transfer. Whenever T1 is not running (no outstanding I frames), T3 is used to periodically poll the other DXE of a link. When T3 times out, a RR or RNR frame is transmitted as a command and with the P bit set. The waiting acknowledgement procedure (2.4.4.9, below) is then executed. 2.3.5.5 Invalid Frame or FCS Error If an invalid frame is received, or a frame is received with an FCS error, that frame will be discarded with no action taken. 2.3.5.6 Frame Rejection Condition A frame rejection condition occurs when an otherwise error-free frame has been received with one of the conditions listed in 2.3.4.3.3 above. Once a rejection error occurs, no more I frames are accepted (except for the examination of the P/F bit) until the error is resolved. The error condition is reported to the other DXE by sending a FRMR response frame. See 2.4.5. 2.4 Description of AX.25 Procedures The following describes the procedures used to setup, use, and disconnect a balanced link between two DXE stations. 2.4.1 Address Field Operation 2.4.1.1 Address Information All transmitted frames shall have address fields conforming to 2.2.13, above. All frames shall have both the destination device and the source device addresses in the address field, with the destination address coming first. This allows many links to share the same RF channel. The destination address is always the address of the station(s) to receive the frame, while the source address contains the address of the device that sent the frame. sign if the point-to-multipoint operation is allowed. Operation with destination addresses other than actual amateur call signs is a subject for further study. 2.4.1.2 Command/Response Procedure AX.25 Version 2.0 has implemented the command/response information in the address field. In order to maintain compatibility with previous versions of AX.25, the command/response information is conveyed using two bits. An upward-compatible AX.25 DXE can determine whether it is communicating with a DXE using an older version of this protocol by testing the command/response bit information located in bit 7 of the SSID octets of both the destination and source address subfields. If both C bits are set to zero, the device is using the older protocol. The newer version of the protocol always has one of these two bits set to one and the other set to zero, depending on whether the frame is a command or a response. The command/response information is encoded into the address field as shown in Fig. 10. Frame Type Dest. SSID C-Bit Source SSID C-Bit Previous versions 0 0 Command (V.2.0) 1 0 Response (V.2.0) 0 1 Previous versions 1 1 Fig. 10 -- Command/Response encoding Since all frames are considered either commands or responses, a device shall always have one of the bits set to one, and the other bit set to zero. The use of the command/response information in AX.25 allows S frames to be either commands or responses. This aids maintenance of proper control over the link during the information transfer state. 2.4.2 P/F Bit Procedures The next response frame returned by the DXE to a SABM or DISC command with the P bit set to 1 will be a UA or DM response with the F bit set to 1. The next response frame returned to an I frame with the P bit set to 1, received during the information transfer state, will be a RR, RNR, or REJ response with the F bit set to 1. The next response frame returned to a supervisory command frame with the P bit set to 1, received during the information transfer state, will be a RR, RNR, or REJ response frame with the F bit set to 1. The next response frame returned to a S or I command frame with the P bit set to 1, received in the disconnected state, will be a DM response frame with the F bit set to 1. The P bit is used in conjunction with the time-out recovery condition discussed in 2.3.5.4, above. When not used, the P/F bit is set to zero. 2.4.3 Procedures For Link Set-Up and Disconnection 2.4.3.1 LAPB Link Connection Establishment When one DXE wishes to connect to another DXE, it will send a SABM command frame to that device and start timer (T1). If the other DXE is there and able to connect, it will respond with a UA response frame, and reset both of its internal state variables (V(S) and V(R)). The reception of the UA response frame at the other end will cause the DXE requesting the connection to cancel the T1 timer and set its internal state variables to 0. If the other DXE doesn't respond before T1 times out, the device requesting the connection will re-send the SABM frame, and start T1 running again. The DXE should continue to try to establish a connection until it has tried unsuccessfully N2 times. N2 is defined in 2.4.7.2, below. If, upon reception of a SABM command, the DXE decides that it cannot enter the indicated state, it should send a DM frame. When receiving a DM response, the DXE sending the SABM should cancel its T1 timer, and not enter the information- transfer state. The DXE sending a SABM command will ignore and discard any frames except SABM, DISC, UA, and DM frames from the other DXE. Frames other than UA and DM in response to a received SABM will be sent only after the link is set up and if no outstanding SABM exists. 2.4.3.2 Information-Transfer Phase After establishing a link connection, the DXE will enter the information-transfer state. In this state, the DXE will accept and transmit I and S frames according to the procedure When receiving a SABM command while in the information- transfer state, the DXE will follow the resetting procedure outlined in 2.4.6 below. 2.4.3.3 Link Disconnection 2.4.3.3.1 While in the information-transfer state, either DXE may indicate a request to disconnect the link by transmitting a DISC command frame and starting timer T1 (see 2.4.7). 2.4.3.3.2 A DXE, upon receiving a valid DISC command, shall send a UA response frame and enter the disconnected state. A DXE, upon receiving a UA or DM response to a sent DISC command, shall cancel timer T1, and enter the disconnected state. 2.4.3.3.3 If a UA or DM response is not correctly received before T1 times out, the DISC frame should be sent again and T1 restarted. If this happens N2 times, the DXE should enter the disconnected state. 2.4.3.4 Disconnected State 2.4.3.4.1 A DXE in the disconnected state shall monitor received commands and react upon the reception of a SABM as described in 2.4.3.1 above and will transmit a DM frame in response to a DISC command. 2.4.3.4.2 In the disconnected state, a DXE may initiate a link set-up as outlined in connection establishment above (2.4.3.1). It may also respond to the reception of a SABM and establish a connection, or it may ignore the SABM and send a DM instead. 2.4.3.4.3 Any DXE receiving a command frame other than a SABM or UI frame with the P bit set to one should respond with a DM frame with the F bit set to one. The offending frame should be ignored. 2.4.3.4.4 When the DXE enters the disconnected state after an error condition or if an internal error has resulted in the DXE being in the disconnected state, the DXE should indicate this by sending a DM response rather than a DISC frame and follow the link disconnection procedure outlined in 2.4.3.3.3, above. The DXE may then try to re-establish the link using the link set-up procedure outlined in 2.4.3.1, above. 2.4.3.5 Collision Recovery 2.4.3.5.1 Collisions in a Half-Duplex Environment Collisions of frames in a half-duplex environment are taken care of by the retry nature of the T1 timer and retransmission count variable. No other special action needs to be taken. 2.4.3.5.2 Collisions of Unnumbered Commands If sent and received SABM or DISC command frames are the same, both DXEs should send a UA response at the earliest opportunity, and both devices should enter the indicated state. If sent and received SABM or DISC commands are different, both DXEs should enter the disconnected state and transmit a DM frame at the earliest opportunity. 2.4.3.5.3 Collision of a DM with a SABM or DISC When an unsolicited DM response frame is sent, a collision between it and a SABM or DISC may occur. In order to prevent this DM from being misinterpreted, all unsolicited DM frames should be transmitted with the F bit set to zero. All SABM and DISC frames should be sent with the P bit set to one. This will prevent any confusion when a DM frame is received. 2.4.3.6 Connectionless Operation In Amateur Radio, there is an additional type of operation that is not feasible using level 2 connections. This operation is the round table, where several amateurs may be engaged in one conversation. This type of operation cannot be accommodated by AX.25 link-layer connections. The way round-table activity is implemented is technically outside the AX.25 connection, but still using the AX.25 frame structure. AX.25 uses a special frame for this operation, called the Unnumbered Information (UI) frame. When this type of operation is used, the destination address should have a code word installed in it to prevent the users of that particular round table from seeing all frames going through the shared RF medium. An example of this is if a group of amateurs are in a round-table discussion about packet radio, they could put PACKET in the destination address, so they would receive frames only from others in the same discussion. An added advantage of the use of AX.25 in this manner is that the source of each frame is in the source address subfield, so software could be written to automatically display who is making what comments. Since this mode is connectionless, there will be no requests for retransmissions of bad frames. Collisions will also occur, with the potential of losing the frames that collided. 2.4.4 Procedures for Information Transfer Once a connection has been established, as outlined above, both devices are able to accept I, S, and U frames. 2.4.4.1 Sending I Frames Whenever a DXE has an I frame to transmit, it will send the I frame with N(S) of the control field equal to its current send state variable V(S). Once the I frame is sent, the send state variable is incremented by one. If timer T1 is not running, it should be started. If timer T1 is running, it should be restarted. The DXE should not transmit any more I frames if its send state variable equals the last received N(R) from the other side of the link plus seven. If it were to send more I frames, the flow control window would be exceed, and errors could result. If a DXE is in a busy condition, it may still send I frames as long as the other device is not also busy. If a DXE is in the frame-rejection mode, it will stop sending I frames. 2.4.4.2 Receiving I Frames 2.4.4.2.1 If a DXE receives a valid I frame (one with a correct FCS and whose send sequence number equals the receiver's receive state variable) and is not in the busy condition, it will accept the received I frame, increment its receive state variable, and act in one of the following manners: 1. If it has an I frame to send, that I frame may be sent with the transmitted N(R) equal to its receive state variable V(R) (thus acknowledging the received frame). Alternately, the device may send a RR frame with N(R) equal to V(R), and then send the I frame. 2. If there are no outstanding I frames, the receiving device will send a RR frame with N(R) equal to V(R). The receiving DXE may wait a small period of time before sending the RR frame to be sure additional I frames are not being transmitted. 2.4.4.2.2 If the DXE is in a busy condition, it may ignore any received I frames without reporting this condition other than repeating the indication of the busy condition. If a busy condition exists, the DXE receiving the busy condition indication should poll the sender of the busy indication periodically until the busy condition disappears. frames with the P bit set to one. The reception of I frames that contain zero-length information fields shall be reported to the next level but no information field will be transferred. 2.4.4.3 Reception of Out of Sequence Frames When an I frame is received with a correct FCS, but its send sequence number, N(S), does not match the current receiver's receive state variable, the frame should be discarded. A REJ frame shall be sent with a receive sequence number equal to one higher (modulo 8) than the last correctly received I frame if an uncleared N(S) sequence error condition has not been previously established. The received state variable and poll bit of the discarded frame should be checked and acted upon, if necessary, before discarding the frame. 2.4.4.4 Reception of Incorrect Frames When a DXE receives a frame with an incorrect FCS, an invalid frame, or a frame with an improper address, that frame shall be discarded. 2.4.4.5 Receiving Acknowledgement Whenever an I or S frame is correctly received, even in a busy condition, the N(R) of the received frame should be checked to see if it includes an acknowledgement of outstanding sent I frames. The T1 timer should be cancelled if the received frame actually acknowledges previously unacknowledged frames. If the T1 timer is cancelled and there are still some frames that have been sent that are not acknowledged, T1 should be started again. If the T1 timer runs out before an acknowledgement is received, the device should proceed to the retransmission procedure in 2.4.4.9. 2.4.4.6 Receiving Reject Upon receiving a REJ frame, the transmitting DXE will set its send state variable to the same value as the REJ frame's received sequence number in the control field. The DXE will then retransmit any I frame(s) outstanding at the next available opportunity conforming to the following: 1. If the DXE is not transmitting at the time, and the channel is open, the device may commence to retransmit the I frame(s) immediately. 2. If the DXE is operating on a full-duplex channel transmitting a UI or S frame when it receives a REJ frame, it may finish sending the UI or S frame and then retransmit the I frame(s). transmitting another I frame when it receives a REJ frame, it may abort the I frame it was sending and start retransmission of the requested I frames immediately. 4. The DXE may send just the one I frame outstanding, or it may send more than the one indicated if more I frames followed the first one not acknowledged, provided the total to be sent does not exceed the flow-control window (7 frames). If the DXE receives a REJ frame with the poll bit set, it should respond with either a RR or RNR frame with the final bit set before retransmitting the outstanding I frame(s). 2.4.4.7 Receiving a RNR Frame Whenever a DXE receives a RNR frame, it shall stop transmission of I frames until the busy condition has been cleared. If timer T1 runs out after the RNR was received, the waiting acknowledgement procedure listed in 2.4.4.9, below, should be performed. The poll bit may be used in conjunction with S frames to test for a change in the condition of the busied-out DXE. 2.4.4.8 Sending a Busy Indication Whenever a DXE enters a busy condition, it will indicate this by sending a RNR response at the next opportunity. While the DXE is in the busy condition, it may receive and process S frames, and if a received S frame has the P bit set to one, the DXE should send a RNR frame with the F bit set to one at the next possible opportunity. To clear the busy condition, the DXE should send either a RR or REJ frame with the received sequence number equal to the current receive state variable, depending on whether the last received I frame was properly received or not. 2.4.4.9 Waiting Acknowledgement If timer T1 runs out waiting the acknowledgement from the other DXE for an I frame transmitted, the DXE will restart timer T1 and transmit an appropriate supervisory command frame (RR or RNR) with the P bit set. If the DXE receives correctly a supervisory response frame with the F bit set and with an N(R) within the range from the last N(R) received to the last N(S) sent plus one, the DXE will restart timer T1 and will set its send state variable V(S) to the received N(R). It may then resume with I frame transmission or retransmission, as appropriate. If, on the other hand, the DXE receives correctly a supervisory response frame with the F bit not set, or an I frame or supervisory command frame, and with an N(R) within the range from the last N(R) received to the last N(S) sent plus one, the DXE will not restart timer T1, but will use the received N(R) as an indication of acknowledgement of transmitted I frames up to and including I frame numbered N(R)-1. with the F bit set is received, the DXE will retransmit an appropriate supervisory command frame (RR or RNR) with the P bit set. After N2 attempts to get a supervisory response frame with the F bit set from the other DXE, the DXE will initiate a link resetting procedure as described in 2.4.6, below. 2.4.5 Frame Rejection Conditions A DXE shall initiate the frame-reset procedure when a frame is received with the correct FCS and address field during the information-transfer state with one or more of the conditions in 2.3.4.3.3, above. Under these conditions, the DXE will ask the other DXE to reset the link by transmitting a FRMR response as outlined in 2.4.6.3, below. After sending the FRMR frame, the sending DXE will enter the frame reject condition. This condition is cleared when the DXE that sent the FRMR frame receives a SABM or DISC command, or a DM response frame. Any other command received while the DXE is in the frame reject state will cause another FRMR to be sent out with the same information field as originally sent. In the frame rejection condition, additional I frames will not be transmitted, and received I frames and S frames will be discarded by the DXE. The DXE that sent the FRMR frame shall start the T1 timer when the FRMR is sent. If no SABM or DISC frame is received before the timer runs out, the FRMR frame shall be retransmitted, and the T1 timer restarted as described in the waiting acknowledgement section (2.4.4.9) above. If the FRMR is sent N2 times without success, the link shall be reset. 2.4.6 Resetting Procedure 2.4.6.1 The resetting procedure is used to initialize both directions of data flow after a nonrecoverable error has occurred. This resetting procedure is used in the information- transfer state of an AX.25 link only. 2.4.6.2 A DXE shall initiate a reset procedure whenever it receives an unexpected UA response frame or an unsolicited response frame with the F bit set to one. A DXE may also initiate the reset procedure upon receipt of a FRMR frame. Alternatively, the DXE may respond to a FRMR by terminating the connection with a DISC frame. 2.4.6.3 A DXE shall reset the link by sending a SABM frame and starting timer T1. Upon receiving a SABM frame from the DXE a UA frame back at the earliest opportunity, set its send and receive state variables, V(S) and V(R), to zero and stop T1 unless it has sent a SABM or DISC itself. If the UA is correctly received by the initial DXE, it resets its send and receive state variables, V(S) and V(R), and stops timer T1. Any busy condition that previously existed will also be cleared. If a DM response is received, the DXE will enter the disconnected state and stop timer T1. If timer T1 runs out before a UA or DM response frame is received, the SABM will be retransmitted and timer T1 restarted. If timer T1 runs out N2 times, the DXE will enter the disconnected state, and any previously existing link conditions will be cleared. Other commands or responses received by the DXE before completion of the reset procedure will be discarded. 2.4.6.4 One DXE may request that the other DXE reset the link by sending a DM response frame. After the DM frame is sent, the sending DXE will then enter the disconnected state. 2.4.7 List of System Defined Parameters 2.4.7.1 Timers To maintain the integrity of the AX.25 level 2 connection, use of these timers is recommended. 2.4.7.1.1 Acknowledgement Timer T1 The first timer, T1, is used to make sure a DXE doesn't wait forever for a response to a frame it sends. This timer cannot be expressed in absolute time, since the time required to send frames varies greatly with the signaling rate used at level 1. T1 should take at least twice the amount of time it would take to send maximum length frame to the other DXE, and get the proper response frame back from the other DXE. This would allow time for the other DXE to do some processing before responding. If level 2 repeaters are to be used, the value of T1 should be adjusted according to the number of repeaters the frame is being transferred through. 2.4.7.1.2 Response Delay Timer T2 The second timer, T2, may be implemented by the DXE to specify a maximum amount of delay to be introduced between the time an I frame is received, and the time the resulting response frame is sent. This delay may be introduced to allow a receiving DXE to wait a short period of time to determine if there is more than one frame being sent to it. If more frames are received, the DXE can acknowledge them at once (up to seven), rather than acknowledge each individual frame. The use of timer T2 is not Note that, on full-duplex channels, acknowledgements should not be delayed beyond k/2 frames to achieve maximum throughput. The k parameter is defined in 2.4.7.4, below. 2.4.7.1.3 Inactive Link Timer T3 The third timer, T3, is used whenever T1 isn't running to maintain link integrity. It is recommended that whenever there are no outstanding unacknowledged I frames or P-bit frames (during the information-transfer state), a RR or RNR frame with the P bit set to one be sent every T3 time units to query the status of the other DXE. The period of T3 is locally defined, and depends greatly on level 1 operation. T3 should be greater than T1, and may be very large on channels of high integrity. 2.4.7.2 Maximum Number of Retries (N2) The maximum number of retries is used in conjunction with the T1 timer. 2.4.7.3 Maximum Number of Octets in an I Field (N1) The maximum number of octets allowed in the I field will be 256. There shall also be an integral number of octets. 2.4.7.4 Maximum Number of I Frames Outstanding (k) The maximum number of outstanding I frames at a time is seven. END. --------------------------------------------------------------------------