diff options
| -rw-r--r-- | openbsc/src/gprs/gprs_sgsn.c | 28 |
1 files changed, 26 insertions, 2 deletions
diff --git a/openbsc/src/gprs/gprs_sgsn.c b/openbsc/src/gprs/gprs_sgsn.c index f3da0387a..6f706642d 100644 --- a/openbsc/src/gprs/gprs_sgsn.c +++ b/openbsc/src/gprs/gprs_sgsn.c @@ -403,10 +403,34 @@ uint32_t sgsn_alloc_ptmsi(void) { struct sgsn_mm_ctx *mm; uint32_t ptmsi; - int max_retries = 23; + int max_retries = 100; restart: - ptmsi = rand() | 0xC0000000; + ptmsi = rand(); + /* Enforce that the 2 MSB are set without loosing the distance between + * identical values. Since rand() has no duplicate values within a + * period (because the size of the state is the same like the size of + * the random value), this leads to a distance of period/4 when the + * distribution of the 2 MSB is uniform. This approach fails with a + * probability of (3/4)^max_retries, only 1% of the approaches will + * need more than 16 numbers (even distribution assumed). + * + * Alternatively, a freeze list could be used if another PRNG is used + * or when this approach proves to be not sufficient. + */ + if (ptmsi >= 0xC0000000) { + if (!max_retries--) + goto failed; + goto restart; + } + ptmsi |= 0xC0000000; + + if (ptmsi == GSM_RESERVED_TMSI) { + if (!max_retries--) + goto failed; + goto restart; + } + llist_for_each_entry(mm, &sgsn_mm_ctxts, list) { if (mm->p_tmsi == ptmsi) { if (!max_retries--) |