IP TOS选项是指示IP报文转发的优先级，QOS控制的一种，常规的IP协议TOS都为0，就是普通报文。

设置：

IPV4/TOS设置（Socket），IPTOS_LOWDELAY 表示FLASH优先级（一般用在游戏，即时通信这块，因为需要包转发要足够快）

        bool Socket::SetTypeOfService(int fd, int tos) noexcept {if (fd == -1) {return false;}if (tos < 0) {tos = IPTOS_LOWDELAY; }Byte b = tos; /* IPTOS_LOWDELAY */
#if defined(_MACOS)return ::setsockopt(fd, IPPROTO_IP, IP_TOS, (char*)&b, sizeof(b)) == 0;
#elsereturn ::setsockopt(fd, SOL_IP, IP_TOS, (char*)&b, sizeof(b)) == 0;
#endif}

设置

IPV4/IPV6 TOS设置（Socket）

        void Socket::AdjustDefaultSocketOptional(int sockfd, bool in4) noexcept {if (sockfd != -1) {uint8_t tos = IPTOS_LOWDELAY;if (in4) {if (SOCKET_RESTRICTIONS_.IP_TOS_ON) {
#if defined(_MACOS)::setsockopt(sockfd, IPPROTO_IP, IP_TOS, (char*)&tos, sizeof(tos));
#else::setsockopt(sockfd, SOL_IP, IP_TOS, (char*)&tos, sizeof(tos));
#endif}#if IP_DONTFRAGMENTint dont_frag = IP_PMTUDISC_DO;::setsockopt(sockfd, IPPROTO_IP, IP_DONTFRAGMENT, (char*)&dont_frag, sizeof(dont_frag));
#elif IP_PMTUDISC_WANTint dont_frag = IP_PMTUDISC_WANT;::setsockopt(sockfd, IPPROTO_IP, IP_MTU_DISCOVER, &dont_frag, sizeof(dont_frag));
#endif}else {// linux-user: Add missing IP_TOS, IPV6_TCLASS and IPV6_RECVTCLASS sockopts// QEMU:// https://patchwork.kernel.org/project/qemu-devel/patch/20170311195906.GA13187@ls3530.fritz.box/
#if IPV6_TCLASSif (SOCKET_RESTRICTIONS_.IPV6_TCLASS_ON) {::setsockopt(sockfd, IPPROTO_IPV6, IPV6_TCLASS, (char*)&tos, sizeof(tos)); /* SOL_IPV6 */}
#endif#if IPV6_MTU_DISCOVER && IPV6_PMTUDISC_WANTint dont_frag = IPV6_PMTUDISC_WANT;::setsockopt(sockfd, IPPROTO_IPV6, IPV6_MTU_DISCOVER, &dont_frag, sizeof(dont_frag));
#endif}#if defined(SO_NOSIGPIPE)int no_sigpipe = 1;::setsockopt(sockfd, SOL_SOCKET, SO_NOSIGPIPE, &no_sigpipe, sizeof(no_sigpipe));
#endif}}