I would be happy to open a ticket for this, but given that I'm a newb when it comes to the source, I'd like some confirmation of what I've found before I do. According to RFC3261 8.1.1.5 "The sequence number value MUST be expressible as a 32-bit unsigned integer and MUST be less than 2**31" I also checked RFC2543 just for fun and similar wording is there. A client just informed me that he is seeing errors in the field where FreeSWITCH is generating CSeq headers with a negative sequence number and those requests are getting rejected by other endpoints. ===================== In digging through the code, I found MASTER/src/mod/endpoints/mod_sofia/sofia_presence.c line 2134 defines a function sofia_presence_get_cseq() which appears to be responsible for creating the sequence value that ends up in the CSeq headers. A cursory glance at this logic leads me to believe that it is definitely capable of creating a sequence number with bit 31 set, which would be in violation of the RFC. I believe a simple fix with little side-effect would be to add the following line right before last_cseq is set ( line 2149 in MASTER ): } } + callsequence &= 0x7FFFFFFF; // RFC3261 8.1.1.5 prohibits bit 31 to be set profile->last_cseq = callsequence; ===================== Another small thing is that FreeSWITCH is generating a negative value in certain scenarios when outputting a CSeq header. This means that the sequence value is improperly being interpreted as a signed integer. This shouldn't matter because the high order bit isn't supposed to be set, but regardless the code is still technically wrong. In MASTER/libs/sofia-sip/libsofia-sip-ua/sip/sip_basic.c I found a definition of sip_cseq_e() which correctly interprets the value as unsigned. There are a few logging examples that are wrong that I found using a simple grep, although these line #'s are probably off because it's from an old checkout. The "CSeq %d" in each of the following probably needs to be a "CSeq %u" nta\sl_utils_log.c:76 "%s%s "URL_FORMAT_STRING" (CSeq %d %s)\n", nta\sl_utils_log.c:100 "%s%03u %s (CSeq %d %s)\n", nta\sl_utils_print.c:69 "%s%s "URL_FORMAT_STRING" (CSeq %d %s)\n", nta\sl_utils_print.c:86 "%s%03u %s (CSeq %d %s)\n", However in MASTER/src/mod/endpoints/mod_sofia/sofia.c I found the following: line 381: -switch_snprintf(sip_cseq, sizeof(sip_cseq), "%d", sip->sip_cseq->cs_seq); +switch_snprintf(sip_cseq, sizeof(sip_cseq), "%u", sip->sip_cseq->cs_seq); line 1707: -switch_snprintf(sip_cseq, sizeof(sip_cseq), "%d", sip->sip_cseq->cs_seq); +switch_snprintf(sip_cseq, sizeof(sip_cseq), "%u", sip->sip_cseq->cs_seq); ===================== Something like the following might also be a consideration, but probably only in 1.8: In MASTER/libs/sofia-sip/libsofia-sip-ua/sip/sip_basic.c line 1288 add something like the following: if ( seq & 0x80000000 ) { SU_DEBUG_1(("%s: CSeq header creation failed - sequence value %u violates RFC3261/8.1.1.5", __func__, seq )); return NULL; } P.S. I'd really like to see at least the first 2 changes proposed above ( callsequence masking and format string fixes ) back-ported to 1.6. Royce Mitchell, IT Consultant ITAS Solutions royce3@itas-solutions.com