E1D protocol sets the maximum permitted size of an HDLC frame to 264 bytes of useful payload, not counting the 2 bytes of FCS. This limit is fair enough, as it matches GSM TS 08.56, the primary intended application. However, osmo_isdnhdlc engine used internally by the daemon requires an Rx buffer 2 bytes larger than the maximum payload length, for temporary storage of FCS bytes: by the nature of HDLC, a receiver has no way of identifying FCS bytes as such until after they have been received and stored in the Rx buffer.
This bug was caught when bringing up OsmoBSC with Nokia Flexi Multiradio BTS. This BTS emits some long OML messages that get broken into segments, with each not-last segment traveling over HDLC as a maximum-sized frame - and these frames then get dropped by osmo-e1d, causing further breakage downstream.
