Osmocom Libraries » libosmo-sccp + libosmo-sigtran

code paths we have to cover:

• ss7_asp_xua_srv_conn_cb()
  • uses osmo_stream_srv_recv(), which in turn calls stream_sctp_recvmsg_wrapper() which in turn calls sctp_recvmsg()
    • sets msgb_sctp_ppid/msgb_sctp_stream to data from sctp_sndrcvinfo
    • performs processing (well, actually really just logging) of flags / MSG_NOTIFICATION
• xua_cli_read_cb()
  • uses osmo_stream_cli_recv(), which in turn calls stream_sctp_recvmsg_wrapper() which in turn calls sctp_recvmsg() [see above]
• osmo_ss7_asp_send()
  • set msgb_sctp_ppid [and possibly stream?]
  • uses osmo_stream_{srv,cli})_send(), which in turn uses (via write_queue) osmo_stream_cli_write()
    • osmo_stream_{cli,srv}_write(), which uses sctp_send() with sctp_streaminfo

sctp_send¶

sctp_send() in libsctp is turning the data+len bytes and the sctp_sndrecvinfo into a sendmsg() call whose msghdr contains one io_vector and one cmsg header. In order to do the cmsg trick, it needs to have a memory buffer of CMSG_SPACE(sizeof(struct sctp_sndrcvinfo)) - a macro that aligns/pads to size_t.

• extend struct iofd_msghdr with a char cmdsg[CMSG_SPACE(sizeof(struct sctp_sndrcvinfo))] element
• implement the sctp_send() function semantics on top of that

sctp_recvmsg¶

The naming is really confusing. Normally, recvmsg is something that deals with struct msghdr but in the case of sctp_recvmsg it actually does not. The user can only pass one buffer, and not an io_vector to it.

sctp_recvmsg is turning the data+len bytes and the sctp_sndrcvinfo into a recvmsg call whose msghdr contains one io_vector and one cmsg header. In order to do the cmsg trick, it needs to have a memory buffer of CMSG_SPACE(sizeof(struct sctp_sndrcvinfo)) - a macro that aligns/pads to size_t.

• extend struct iofd_msghdr with a char cmdsg[CMSG_SPACE(sizeof(struct sctp_sndrcvinfo))] element
• implement the sctp_recvmsg() function semantics on top of that