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AX25.DOC AX.25 Amateur Packet-Radio Link-Layer Protocol Version 2.0 October 1984 Copyright (c) 1984 by The American Radio Relay Legue, Inc. Blanket permission to copy this publication by end users for noncommercial purposes is hereby granted. No part of this work may be reproduced in any form where such copy is offered in exchange for any payment unless written permission has first been secured from the publisher. 2. AX.25 Link-Layer Protocol Specification 2.1 Scope and Field of Operation In order to provide a mechanism for the reliable transport of data between two signaling terminals, it is necessary to define a protocol that can accept and deliver data over a variety of types of communications links. The AX.25 Link- Layer Protocol is designed to provide this service, independent of any other level that may or may not exist. This protocol conforms to ISO Recommendations 3309, 4335 (including DAD 1&2) and 6256 high-level data link control (HDLC) and uses some terminology found in these documents. It also conforms with ANSI X3.66, which describes ADCCP, balanced mode. This protocol follows, in principle, the CCITT X.25 Recommendation, with the exception of an extended address field and the addition of the Unnumbered Information (UI) frame. It also follows the principles of CCITT Recommendation Q.921 (LAPD) in the use of multiple links, distinguished by the address field, on a single shared channel. As defined, this protocol will work equally well in either half- or full-duplex Amateur Radio environments. This protocol has been designed to work equally well for direct connections between two individual amateur packet-radio stations or an individual station and a multiport controller. This protocol allows for the establishment of more than one link-layer connection per device, if the device is so capable. This protocol does not prohibit self-connections. A self-connection is considered to be when a device establishes a link to itself using its own address for both the source and destination of the frame. Most link-layer protocols assume that one primary (or master) device (generally called a DCE, or data circuit- terminating equipment) is connected to one or more secondary (or slave) device(s) (usually called a DTE, or data terminating equipment). This type of unbalanced operation is not practical in a shared-RF Amateur Radio environment. Instead, AX.25 assumes that both ends of the link are of the same class, thereby eliminating the two different classes of devices. The term DXE is used in this protocol specification to describe the balanced 2.2 Frame Structure Link layer packet radio transmissions are sent in small blocks of data, called frames. Each frame is made up of several smaller groups, called fields. Fig.1 shows the three basic types of frames. Note that the first bit to be transmitted is on the left side. First Bit Sent Flag Address Control FCS Flag 01111110 112/560 Bits 8 Bits 16 Bits 01111110 Fig. 1A -- U and S frame construction First Bit Sent Flag Address Control PID Info. FCS Flag 01111110 112/560 Bits 8 Bits 8 Bits N*8 Bits 16 Bits 01111110 Fig. 1B -- Information frame construction Each field is made up of an integral number of octets (or bytes), and serves a specific function as outlined below. 2.2.1 Flag Field The flag field is one octet long. Since the flag is used to delimit frames, it occurs at both the beginning and end of each frame. Two frames may share one flag, which would denote the end of the first frame, and the start of the next frame. A flag consists of a zero followed by six ones followed by another zero, or 01111110 (7E hex). As a result of bit stuffing (see 2.2.6, below), this sequence is not allowed to occur anywhere else inside a complete frame. 2.2.2 Address Field The address field is used to identify both the source of the frame and its destination. In addition, the address field contains the command/response information and facilities for level 2 repeater operation. The encoding of the address field is described in 2.2.13. The control field is used to identify the type of frame being passed and control several attributes of the level 2 connection. It is one octet in length, and its encoding is discussed in 2.3.2.1, below. 2.2.4 PID Field The Protocol Identifier (PID) field shall appear in information frames (I and UI) only. It identifies what kind of layer 3 protocol, if any, is in use. The PID itself is not included as part of the octet count of the information field. The encoding of the PID is as follows: M L S S B B yy01yyyy AX.25 layer 3 implemented. yy10yyyy AX.25 layer 3 implemented. 11001100 Internet Protocol datagram layer 3 implemented. 11001101 Address resolution protocol layer 3 implemented. 11110000 No layer 3 implemented. 11111111 Escape character. Next octet contains more Level 3 protocol information. Where: A y indicates all combinations used. Note: All forms of yy11yyyy and yy00yyyy other than those listed above are reserved at this time for future level 3 protocols. The assignment of these formats is up to amateur agreement. It is recommended that the creators of level 3 protocols contact the ARRL Ad Hoc Committee on Digital Communications for suggested encodings. 2.2.5 Information Field The information field is used to convey user data from one end of the link to the other. I fields are allowed in only three types of frames: the I frame, the UI frame, and the FRMR frame. The I field can be up to 256 octets long, and shall contain an integral number of octets. These constraints apply prior to the insertion of zero bits as specified in 2.2.6, below. Any information in the I field shall be passed along the link transparently, except for the zero-bit insertion (see 2.2.6) necessary to prevent flags from accidentally appearing in the I 2.2.6 Bit Stuffing In order to assure that the flag bit sequence mentioned above doesn't appear accidentally anywhere else in a frame, the sending station shall monitor the bit sequence for a group of five or more contiguous one bits. Any time five contiguous one bits are sent the sending station shall insert a zero bit after the first one bit. During frame reception, any time five contiguous one bits are received, a zero bit immediately following five one bits shall be discarded. 2.2.7 Frame-Check Sequence The frame-check sequence (FCS) is a sixteen-bit number calculated by both the sender and receiver of a frame. It is used to insure that the frame was not corrupted by the medium used to get the frame from the sender to the receiver. It shall be calculated in accordance with ISO 3309 (HDLC) Recommendations. 2.2.8 Order of Bit Transmission With the exception of the FCS field, all fields of an AX.25 frame shall be sent with each octet's least-significant bit first. The FCS shall be sent most-significant bit first. 2.2.9 Invalid Frames Any frame consisting of less than 136 bits (including the opening and closing flags), not bounded by opening and closing flags, or not octet aligned (an integral number of octets), shall be considered an invalid frame by the link layer. See also 2.4.4.4, below. 2.2.10 Frame Abort If a frame must be prematurely aborted, at least fifteen contiguous ones shall be sent with no bit stuffing added. 2.2.11 Interframe Time Fill Whenever it is necessary for a DXE to keep its transmitter on while not actually sending frames, the time between frames should be filled with contiguous flags. 2.2.12 Link Channel States Not applicable. 2.2.13 Address-Field Encoding The address field of all frames shall be encoded with both the destination and source amateur call signs for the frame. Except for the Secondary Station Identifier (SSID), the address characters only. If level 2 amateur "repeaters" are to be used, their call signs shall also be in the address field. The HDLC address field is extended beyond one octet by assigning the least-significant bit of each octet to be an "extension bit". The extension bit of each octet is set to zero, to indicate the next octet contains more address information, or one, to indicate this is the last octet of the HDLC address field. To make room for this extension bit, the amateur Radio call sign information is shifted one bit left. 2.2.13.1 Nonrepeater Address-Field Encoding If level 2 repeaters are not being used, the address field is encoded as shown in Fig. 2. The destination address is the call sign and SSID of the amateur radio station to which the frame is addressed, while the source address contains the amateur call sign and SSID of the station that sent the frame. These call signs are the call signs of the two ends of a level 2 AX.25 link only. First Octet Sent Address Field of Frame Destination Address Source Address A1 A2 A3 A4 A5 A6 A7 A8 A9 A10 A11 A12 A13 A14 Fig. 2 -- Nonrepeater Address-Field Encoding A1 through A14 are the fourteen octets that make up the two address subfields of the address field. The destination subaddress is seven octets long (A1 thru A7), and is sent first. This address sequence provides the receivers of frames time to check the destination address subfield to see if the frame is addressed to them while the rest of the frame is being received. The source address subfield is then sent in octets A8 through A14. Both of these subfields are encoded in the same manner, except that the last octet of the address field has the HDLC address extension bit set. There is an octet at the end of each address subfield that contains the Secondary Station Identifier (SSID). The SSID subfield allows an Amateur Radio operator to have more than one packet-radio station operating under the same call sign. This is useful when an amateur wants to put up a repeater in addition to a regular station, for example. The C bits (see 2.4.1.2, below) and H bit (see 2.2.13.2, below) are also contained in this octet, along with two bits which are reserved for future use. mode of operation. Octet ASCII Bin.Data Hex Data Flag 01111110 7E A1 K 10010110 96 A2 8 01110000 70 A3 M 10011010 9A A4 M 10011010 9A A5 O 10011110 9E A6 space 01000000 40 A7 SSID 11100000 E0 A8 W 10101110 AE A9 B 10000100 84 A10 4 01100100 68 A11 J 10010100 94 A12 F 10001100 8C A13 I 10010010 92 A14 SSID 01100001 61 Control I 00111110 3E PID none 11110000 F0 FCS part 1 XXXXXXXX HH FCS part 2 XXXXXXXX HH Flag 01111110 7E Bit position 76543210 Fig. 3A -- Nonrepeater AX.25 frame The frame shown is an I frame, not going through a level 2 repeater, from WB4JFI (SSID=0) to K8MMO (SSID=0), with no level 3 protocol. The P/F bit is set; the receive sequence number (N(R)) is 1; the send sequence number (N(S)) is 7. 2.2.13.1.1 Destination Subfield Encoding Fig. 3 shows how an amateur call sign is placed in the destination address subfield, occupying octets A1 thru A7. Octet ASCII Bin.Data Hex Data A1 W 10101110 AE A2 B 10000100 84 A3 4 01101000 68 A4 J 10010100 94 A5 F 10001100 8C A6 I 10010010 92 A7 SSID CRRSSID0 Bit Position--> 76543210 Fig. 3 -- Destination Field Encoding Where: 1. The top octet (A1) is the first octet sent, with bit 0 of each octet being the first bit sent, and bit 7 being the last bit sent. 2. The first (low-order or bit 0) bit of each octet is the HDLC address extension bit, which is set to zero on all but the last octet in the address field, where it is set to one. 3. The bits marked "r" are reserved bits. They may be used in an agreed-upon manner in individual networks. When not implemented, they should be set to one. 4. The bit marked "C" is used as the command/response bit of an AX.25 frame, as outlined in 2.4.1.2 below. 5. The characters of the call sign should be standard seven-bit ASCII (upper case only) placed in the leftmost seven bits of the octet to make room for the address extension bit. If the call sign contains fewer than six characters, it should be padded with ASCII spaces between the last call sign character and the SSID octet. 6. The 0000 SSID is reserved for the first personal AX.25 station. This establishes one standard SSID for "normal" stations to use for the first station. 2.2.13.2 Level 2 Repeater-Address Encoding If a frame is to go through level 2 amateur packet repeater(s), there is an additional address subfield appended to the end of the address field. This additional subfield contains the call sign(s) of the repeater(s) to be used. This allows more than one repeater to share the same RF channel. If this subfield exists, the last octet of the source subfield has its address extension bit set to zero, indicating that more address-field data follows. The repeater-address subfield is encoded in the same manner as the destination and source address subfields, except for the most-significant bit in the last octet, called the "H" bit. The H bit is used to indicate whether a frame has been repeated or not. In order to provide some method of indicating when a frame has been repeated, the H bit is set to zero on frames going to a repeater. The repeater will set the H bit to one when the discard any frames going to the repeater (uplink frames), while operating through a repeater. Fig. 4 shows how the repeater- address subfield is encoded. Fig. 4A is an example of a complete frame after being repeated. Octet ASCII Bin.Data Hex Data A15 W 10101110 AE A16 B 10000100 84 A17 4 01101000 68 A18 J 10010100 94 A19 F 10001100 8C A20 I 10010010 92 A21 SSID HRRSSID1 Bit Order --> 76543210 Fig. 4 -- Repeater-address encoding Where: 1. The top octet is the first octet sent, with bit 0 being sent first and bit 7 sent last of each octet. 2. As with the source and destination address subfields discussed above, bit 0 of each octet is the HDLC address extension bit, which is set to zero on all but the last address octet, where it is set to one. 3. The "R" bits are reserved in the same manner as in the source and destination subfields. 4. The "H" bit is the has-been-repeated bit. It is set to zero whenever a frame has not been repeated, and set to one by the repeater when the frame has been repeated. Octet ASCII Bin.Data Hex Data Flag 01111110 7E A1 K 10010110 96 A2 8 01110000 70 A3 M 10011010 9A A4 M 10011010 9A A5 O 10011110 9E A6 space 01000000 40 A7 SSID 11100000 E0 A8 W 10101110 AE A9 B 10000100 84 A11 J 10010100 94 A12 F 10001100 8C A13 I 10010010 92 A14 SSID 01100000 60 A15 W 10101110 AE A16 B 10000100 84 A17 4 01101000 68 A18 J 10010100 94 A19 F 10001100 8C A20 I 10010010 92 A21 SSID 11100011 E3 Control I 00111110 3F PID none 11110000 F0 FCS part 1 XXXXXXXX HH FCS part 2 XXXXXXXX HH Flag 01111110 7E Bit position 76543210 Fig. 4A -- AX.25 frame in repeater mode The above frame is the same as Fig. 3A, except for the addition of a repeater-address subfield (WB4JFI, SSID=1). The H bit is set, indicating this is from the output of the repeater. 2.2.13.3 Multiple Repeater Operation The link-layer AX.25 protocol allows operation through more than one repeater, creating a primitive frame routing mechanism. Up to eight repeaters may be used by extending the repeater-address subfield. When there is more than one repeater address, the repeater address immediately following the source address subfield will be considered the address of the first repeater of a multiple-repeater chain. As a frame progresses through a chain of repeaters, each successive repeater will set the H bit (has-been-repeated bit) in its SSID octet, indicating that the frame has been successfully repeated through it. No other changes to the frame are made (except for the necessary recalculation of the FCS). The destination station can determine the route the frame took to each it by examining the address field. The number of repeater addresses is variable. All but the last repeater address will have the address extension bits of all octets set to zero, as will all but the last octet (SSID octet) of the last repeater address. The last octet of the last repeater address will have the address extension bit set to one, indicating the end of the address field. It should be noted that various timers (see 2.4.7, below) may have to be adjusted to accommodate the additional delays encountered when a frame must pass through a multiple-repeater same path before reaching the source device. It is anticipated that multiple-repeater operation is a temporary method of interconnecting stations over large distances until such time that a layer 3 protocol is in use. Once this layer 3 protocol becomes operational, repeater chaining should be phased out. 2.3 Elements of Procedure 2.3.1 The elements of procedure are defined in terms of actions that occur on receipt of frames. 2.3.2 Control-Field Formats and State Variables 2.3.2.1 Control-Field Formats The control field is responsible for identifying the type of frame being sent, and is also used to convey commands and responses from one end of the link to the other in order to maintain proper link control. The control fields used in AX.25 use the CCITT X.25 control fields for balanced operation (LAPB), with an additional control field taken from ADCCP to allow connectionless and round- table operation. There are three general types of AX.25 frames. They are the Information frame (I frame), the Supervisory frame (S frame), and the Unnumbered frame (U frame). Fig. 5 shows the basic format of the control field associated with these types of frames. Control-Field Control-Field Bits Type 7 6 5 4 3 2 1 0 I Frame N(R) P N(S) 0 S Frame N(R) P/F S S 0 1 U Frame M M M P/F M M 1 1 Fig. 5 -- Control-field formats Where: 1. Bit 0 is the first bit sent and bit 7 is the last bit sent of the control field. 2. N(S) is the send sequence number (bit 1 is the LSB). LSB). 4. The "S" bits are the supervisory function bits, and their encoding is discussed in 2.3.4.2. 5. The "M" bits are the unnumbered frame modifier bits and their encoding is discussed in 2.3.4.3. 6. The P/F bit is the Poll/Final bit. Its function is described in 2.3.3. The distinction between command and response, and therefore the distinction between P bit and F bit, is made by addressing rules discussed in 2.4.1.2. 2.3.2.1.1 Information-Transfer Format All I frames have bit 0 of the control field set to zero. N(S) is the sender's send sequence number (the send sequence number of this frame). N(R) is the sender's receive sequence number (the sequence number of the next expected received frame). These numbers are described in 2.3.2.4. In addition, the P/F bit is to be used as described in 2.4.2. 2.3.2.1.2 Supervisory Format Supervisory frames are denoted by having bit 0 of the control field set to one, and bit 1 of the control field set to zero. S frames provide supervisory link control such as acknowledging or requesting retransmission of I frames, and link- level window control. Since S frames do not have an information field, the sender's send variable and the receiver's receive variable are not incremented for S frames. In addition, the P/F bit is used as described in 2.4.2. 2.3.2.1.3 Unnumbered Format Unnumbered frames are distinguished by having both bits 0 and 1 of the control field set to one. U frames are responsible for maintaining additional control over the link beyond what is accomplished with S frames. They are also responsible for establishing and terminating link connections. U frames also allow for the transmission and reception of information outside of the normal flow control. Some U frames may contain information and PID fields. The P/F bit is used as described in 2.4.2. 2.3.2.2 Control-Field Parameters 2.3.2.3 Sequence Numbers Every AX.25 I frame shall be assigned, modulo 8, a sequential number from 0 to 7. This will allow up to seven outstanding I frames per level 2 connection at a time. 2.3.2.4 Frame Variables and Sequence Numbers 2.3.2.4.1 Send State Variable V(S) The send state variable is a variable that is internal to the DXE and is never sent. It contains the next sequential number to be assigned to the next transmitted I frame. This variable is updated upon the transmission of each I frame. 2.3.2.4.2 Send Sequence Number N(S) The send sequence number is found in the control field of all I frames. It contains the sequence number of the I frame being sent. Just prior to the transmission of the I frame, N(S) is updated to equal the send state variable. 2.3.2.4.3 Receive State Variable V(R) The receive state variable is a variable that is internal to the DXE. It contains the sequence number of the next expected received I frame. This variable is updated upon the reception of an error-free I frame whose send sequence number equals the present received state variable value. 2.3.2.4.4 Received Sequence Number N(R) The received sequence number is in both I and S frames. Prior to sending an I or S frame, this variable is updated to equal that of the received state variable, thus implicitly acknowledging the proper reception of all frames up to and including N(R)-1. 2.3.3 Functions of Poll/Final (P/F) Bit The P/F bit is used in all types of frames. It is used in a command (poll) mode to request an immediate reply to a frame. The reply to this poll is indicated by setting the response (final) bit in the appropriate frame. Only one outstanding poll condition per direction is allowed at a time. The procedure for P/F bit operation is described in 2.4.2. 2.3.4 Control Field Coding for Commands and Responses The following commands and responses, indicated by their control field encoding, are to be use by the DXE: 2.3.4.1 Information Command Frame Control Field The function of the information (I) command is to transfer across a data link sequentially numbered frames containing an information field. The information-frame control field is encoded as shown subfield to maintain control of their passage over the link-layer connection. Control Field Bits 7 6 5 4 3 2 1 0 N(R) P N(S) 0 Fig. 6 -- I frame control field 2.3.4.2 Supervisory Frame Control Field The supervisory frame control fields are encoded as shown in Fig. 7. Control Field Bits 7 6 5 4 3 2 1 0 Receive Ready RR N(R) P/F 0 0 0 1 Receive Not Ready RNR N(R) P/F 0 1 0 1 Reject REJ N(R) P/F 1 0 0 1 Fig. 7 -- S frame control fields The Frame identifiers: C or SABM Layer 2 Connect Request D or DISC Layer 2 Disconnect Request I Information Frame RR Receive Ready. System Ready To Receive RNR or NR Receive Not Ready. TNC Buffer Full RJ or REJ Reject Frame. Out of Sequence or Duplicate FRMR Frame Reject. Fatal Error UI Unnumbered Information Frame. "Unproto" DM Disconnect Mode. System Busy or Disconnected. 2.3.4.2.1 Receive Ready (RR) Command and Response Receive Ready is used to do the following: 1. to indicate that the sender of the RR is now able to receive more I frames. 2. to acknowledge properly received I frames up to, and including N(R)-1, and 3. to clear a previously set busy condition created by an RNR command having been sent. requested by sending a RR command frame with the P-bit set to one. 2.3.4.2.2 Receive Not Ready (RNR) Command and Response Receive Not Ready is used to indicate to the sender of I frames that the receiving DXE is temporarily busy and cannot accept any more I frames. Frames up to N(R)-1 are acknowledged. Any I frames numbered N(R) and higher that might have been caught between states and not acknowledged when the RNR command was sent are not acknowledged. The RNR condition can be cleared by the sending of a UA, RR, REJ, or SABM frame. The status of the DXE at the other end of the link can be requested by sending a RNR command frame with the P bit set to one. 2.3.4.2.3 Reject (REJ) Command and Response The reject frame is used to request retransmission of I frames starting with N(R). Any frames that were sent with a sequence number of N(R)-1 or less are acknowledged. Additional I frames may be appended to the retransmission of the N(R) frame if there are any. Only one reject frame condition is allowed in each direction at a time. The reject condition is cleared by the proper reception of I frames up to the I frame that caused the reject condition to be initiated. The status of the DXE at the other end of the link can be requested by sending a REJ command frame with the P bit set to one. 2.3.4.3 Unnumbered Frame Control Fields Unnumbered frame control fields are either commands or responses. Fig. 8 shows the layout of U frames implemented within this protocol. Control Field Type Control Field Bits 7 6 5 4 3 2 1 0 Set Asynchronous Balanced Mode-SABM Cmd 0 0 1 P 1 1 1 1 Disconnect-DISC Cmd 0 1 0 P 0 0 1 1 Disconnected Mode-DM Res 0 0 0 F 1 1 1 1 Unnumbered Acknowledge-UA Res 0 1 1 F 0 0 1 1 Frame Reject-FRMR Res 1 0 0 F 0 1 1 1 Fig. 8 -- U frame control fields 2.3.4.3.1 Set Asynchronous Balanced Mode (SABM) Command The SABM command is used to place 2 DXEs in the asynchronous balanced mode. This is a balanced mode of operation known as LAPB where both devices are treated as equals. Information fields are not allowed in SABM commands. Any outstanding I frames left when the SABM command is issued will remain unacknowledged. The DXE confirms reception and acceptance of a SABM command by sending a UA response frame at the earliest opportunity. If the DXE is not capable of accepting a SABM command, it should respond with a DM frame if possible. 2.3.4.3.2 Disconnect (DISC) Command The DISC command is used to terminate a link session between two stations. No information field is permitted in a DISC command frame. Prior to acting on the DISC frame, the receiving DXE confirms acceptance of the DISC by issuing a UA response frame at its earliest opportunity. The DXE sending the DISC enters the disconnected state when it receives the UA response. Any unacknowledged I frames left when this command is acted upon will remain unacknowledged.