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Q-construct
If you ever wondered what are the weird characters you see after your callsign on aprs.fi and things like qAr and qAR. Well here is the explanation in what we call :The "q-construct"
The "q-construct" is implemented on the APRS internet server to add the following capabilities to the Internet APRS transport mechanism.
- APRS-IS Entry Identification
- Support for a Future Authorization Algorithm
- Support for Loop Detection
- Support for Automatic Loop Protection
- Compatibility with Existing IGate and Client Software
- Only Server Support Needed for Implementation
The currently defined q constructs are:
Server Generated:
- qAC - Packet was received from the client directly via a verified connection (FROMCALL=login). The callSSID following the qAC is the server's callsign-SSID.
- qAX - Packet was received from the client directly via a unverified connection (FROMCALL=login). The callSSID following the qAX is the server's callsign-SSID. This construct is in addition to the TCPIP*/TCPXX* construct currently in place. TCPXX and qAX have been depricated.
- qAU - Packet was received from the client directly via a UDP connection. The callSSID following the qAU is the server's callsign-SSID.
- qAo - (letter O) Packet was received via a client-only port, the FROMCALL does not match the login, and the packet contains either a ,I or qAR construct where the indicated IGate matches the login.
- qAO - (letter O) Packet was received via a client-only port and the FROMCALL does not match the login.
- qAS - Packet was received from another server or generated by this server. The latter case would be for a beacon generated by the server. Due to the virtual nature of APRS-IS, use of beacon packets by servers is strongly discouraged. The callSSID following the qAS is the login or IP address of the first identifiable server (see algorithm).
- qAr - Packet was received indirectly (via an intermediate server) from an IGate using the ,I construct. The callSSID following the qAr it the callSSID of the IGate.
- qAR - Packet was received directly (via a verified connection) from an IGate using the ,I construct. The callSSID following the qAR it the callSSID of the IGate.
Client Generated:
- qAR - Packet is placed on APRS-IS by an IGate from RF. The callSSID following the qAR it the callSSID of the IGate.
- qAZ - Packet is generated by the server/client/IGate and should not be propagated. The callSSID following the qAZ is the callSSID of the server/client/IGate. This is normally used for connection messages such as messages to USERLIST.
- qAI - Trace packet. This packet tells each server to add login identification to the packet. This packet starts with the callSSID of the originating station following the qAI. See algorithm for more details.
Client generated q constructs will be verified if a new authorization algorithm is created.
Servers MUST have unique logins from any other server/IGate/client that insert data onto the APRS internet server. This is to prevent packets from being erroneously detected as looping. For instance, if my server's login is AE5PL and my weather client is AE5PL, my server will see ,qAC,AE5PL and consider this packet a looped packet. As logins can be any combination of 9 alphanumeric characters, this should not pose a problem.
IGates which append IGATECALL,I to the end of packets and which are directly connected to a server which supports the q construct will have the IGATECALL,I converted to qAR,IGATECALL, qAr,IGATECALL, or qAo,IGATECALL to support the extended capabilities of the q construct.
Servers will have the ability to selectively enable tracing on all packets through server configuration. This must be used judiciously and only when a loop condition is suspected due to the increased bandwidth demands that tracing creates.
q constructs will only appear on the server and are not to be used elsewhere due to bandwidth considerations.
For example, this is what happens to a packet without a q construct which enters the system via a verified connection:
Original packet:
OD5RW>APRS,TCPIP*:payload
Packet leaving the server if trace is off:
OD5RW>APRS,TCPIP*,qAC,OD5RW-JS:payload
or, if trace is on:
OD5RW>APRS,TCPIP*,qAI,OD5RW,OD5RW-JS:payload
Here is a similar example where the packet is gated to APRS-IS from RF:
Original packet:
OD5RW>APRS,WIDE1*:payload
Packet leaving the server if trace is off:
OD5RW>APRS,WIDE1*,qAR,OD5RW-10:payload
or, if trace is on:
OD5RW>APRS,WIDE1*,qAI,OD5RW-10,OD5RW-JS:payload
Logins used must not consist of exactly 8 characters from 0 to 9 or A to F as this would indicate a server generated IP address for the q construct.
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