Flow labels used to be 24-bit back in 1995 until the IPv6 header format changed in 1998 in which the field siz was reduced to 20-bit.
https://tools.ietf.org/html/rfc1883#section-6 https://tools.ietf.org/html/rfc2460#section-6 There since import at 26.10.2000. OK? Index: print-ip6.c =================================================================== RCS file: /cvs/src/usr.sbin/tcpdump/print-ip6.c,v retrieving revision 1.26 diff -u -p -r1.26 print-ip6.c --- print-ip6.c 10 Feb 2018 10:00:32 -0000 1.26 +++ print-ip6.c 29 Sep 2018 15:24:46 -0000 @@ -224,19 +224,10 @@ ip6_print(const u_char *bp, u_int length end: flow = ntohl(ip6->ip6_flow); -#if 0 - /* rfc1883 */ - if (flow & 0x0f000000) - (void)printf(" [pri 0x%x]", (flow & 0x0f000000) >> 24); - if (flow & 0x00ffffff) - (void)printf(" [flowlabel 0x%x]", flow & 0x00ffffff); -#else - /* RFC 2460 */ if (flow & 0x0ff00000) (void)printf(" [class 0x%x]", (flow & 0x0ff00000) >> 20); if (flow & 0x000fffff) (void)printf(" [flowlabel 0x%x]", flow & 0x000fffff); -#endif if (ip6->ip6_hlim <= 1) (void)printf(" [hlim %d]", (int)ip6->ip6_hlim);
