Inter-layer state / references / control buffer

Hi! I'm getting to a point where we need to associate more and more state and/or references with every individual msgb. In the circuit switched BSC case, every message (msgb) is associated with a logical channel (lchan) through which it is sent. However, in the packet switched case, this is not the case. Rather, a msgb is received on a particular NS-VC, inside a particular BSSGP-VC, inside a LLC session, etc. When we pass a msgb through the protocol layers and back, we somehow need to identify where to send the response to. Once we actually reach the 04.08/04.11 layer, things become a bit less difficult. In the end, all messages are at least somehow associated with a 'subscriber'. If a message passes from the UDP socket into the NS protocol and into BSSGP, we need to know the BSVCI and NSVCI if we want to send a response back. One option would be to pass those values as explicit function call arguments, which is partially what my current code is doing. But the more layers we traverse, the longer the function arguments get. Furthermore, there are parts of our protocol (04.08 and 04.11 particularly) that can either be transported on top of 08.58 (RSL) or LLC/BSSGP/NS. Those upper layers shouldn't really care what the underlying transport looks like. One possible option is to somehow identify the NS-VC and BSSGP-VC by putting an identifier or pointer into a 'struct msgb' member. This would require libosmocore changes every time we add some bits here or there. Another option is to introduce something similar to the skb->cb in the linux kernel: A 'control buffer' area of unspecified content provide by the core networking code as part of every sk_buff, which gets type-casted to the IPCB or TCB-CB when the msgb enters the respective protocol layer. The advantage is that we could have one msgb structure definition with the typecasting functions/macros being part of the application (e.g. openbsc) The problem with this is: Its scope is limited to whatever is the current layer of the protocol stack. The IPCB is overwritten with the TCBCB once the message enters TCP. In our case, we would actually want to have the associated data to persist even while the msgb passes from one layer into another. This would mean that there is a more-or-less static structure containing all the various identifiers, which is then typecasted to the control buffer. While this might not be ideal, it definitely keeps the details hidden away from libosmocore, which is good. Any comments are welcome, Harald -- - Harald Welte <laforge(a)gnumonks.org> http://laforge.gnumonks.org/ ============================================================================ "Privacy in residential applications is a desirable marketing option." (ETSI EN 300 175-7 Ch. A6)