8.1 最大报文段（MSS）选项
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MSS选项用于通知对端本端能接受的每个TCP分段中的最大数据长度，发送端TCP用接收到的MSS值作为决定所发送分段的最大大小的最大值。MSS选项的总长度为4字节，MSS值为16bit，最大为65535。MSS选项只能在有SYN标记的包中携带。
在初始化socket的时候就需要设置MSS相关信息，connect时：
2752 void tcp_connect_init(struct sock *sk)2753 {2754 const struct dst_entry *dst = __sk_dst_get(sk);2755 struct tcp_sock *tp = tcp_sk(sk);2756 __u8 rcv_wscale;...2778 tp->advmss = dst_metric_advmss(dst); //dst_metric_advmss到路由表中查询MSS，会利用到路径MTU探测的结果2779 if (tp->rx_opt.user_mss && tp->rx_opt.user_mss < tp->advmss)//tp->rx_opt.user_mss是TCP_MAXSEG socket选项设置的值2780 tp->advmss = tp->rx_opt.user_mss; //取二者中最小的... 发送SYN时： 828 static int tcp_transmit_skb(struct sock *sk, struct sk_buff *skb, int clone_it, 829 gfp_t gfp_mask) 830 { 831 const struct inet_connection_sock *icsk = inet_csk(sk); 832 struct inet_sock *inet; 833 struct tcp_sock *tp; 834 struct tcp_skb_cb *tcb; 835 struct tcp_out_options opts; 836 unsigned int tcp_options_size, tcp_header_size; 837 struct tcp_md5sig_key *md5; 838 struct tcphdr *th; 839 int err;... 868 memset(&opts, 0, sizeof(opts)); 869 870 if (unlikely(tcb->tcp_flags & TCPHDR_SYN)) 871 tcp_options_size = tcp_syn_options(sk, skb, &opts, &md5);//将选项信息写入opts局部变量中 872 else 873 tcp_options_size = tcp_established_options(sk, skb, &opts, 874 &md5);... 925 tcp_options_write((__be32 *)(th + 1), tp, &opts);//将opts中的选项信息写入报文的TCP头中...构建SYN|ACK时：
2654 struct sk_buff *tcp_make_synack(struct sock *sk, struct dst_entry *dst,2655 struct request_sock *req,2656 struct tcp_fastopen_cookie *foc)2657 {2658 struct tcp_out_options opts;2659 struct inet_request_sock *ireq = inet_rsk(req);2660 struct tcp_sock *tp = tcp_sk(sk);2661 struct tcphdr *th;2662 struct sk_buff *skb;2663 struct tcp_md5sig_key *md5; 2664 int tcp_header_size;2665 int mss;...2678 mss = dst_metric_advmss(dst);2679 if (tp->rx_opt.user_mss && tp->rx_opt.user_mss < mss)2680 mss = tp->rx_opt.user_mss;...2703 memset(&opts, 0, sizeof(opts));2704 #ifdef CONFIG_SYN_COOKIES2705 if (unlikely(req->cookie_ts))2706 TCP_SKB_CB(skb)->when = cookie_init_timestamp(req);2707 else2708 #endif2709 TCP_SKB_CB(skb)->when = tcp_time_stamp;2710 tcp_header_size = tcp_synack_options(sk, req, mss, skb, &opts, &md5,2711 foc) + sizeof(*th);...2735 tcp_options_write((__be32 *)(th + 1), tp, &opts);... SYN包的tcp_syn_options函数用于生成选项信息： 498 static unsigned int tcp_syn_options(struct sock *sk, struct sk_buff *skb, 499 struct tcp_out_options *opts, 500 struct tcp_md5sig_key **md5) 501 { 502 struct tcp_sock *tp = tcp_sk(sk); 503 unsigned int remaining = MAX_TCP_OPTION_SPACE; 504 struct tcp_fastopen_request *fastopen = tp->fastopen_req;... 525 opts->mss = tcp_advertise_mss(sk);... 用tcp_advertise_mss函数得到MSS的值： 118 static __u16 tcp_advertise_mss(struct sock *sk) 119 { 120 struct tcp_sock *tp = tcp_sk(sk); 121 const struct dst_entry *dst = __sk_dst_get(sk); 122 int mss = tp->advmss; 123 124 if (dst) { 125 unsigned int metric = dst_metric_advmss(dst); //再获取一次路由表中的MSS 126 127 if (metric < mss) { 128 mss = metric; 129 tp->advmss = mss; //刷新MSS 130 } 131 } 132 133 return (__u16)mss; 134 }SYN|ACK报文的选项信息由tcp_synack_options构建：
560 static unsigned int tcp_synack_options(struct sock *sk, 561 struct request_sock *req, 562 unsigned int mss, struct sk_buff *skb, 563 struct tcp_out_options *opts, 564 struct tcp_md5sig_key **md5, 565 struct tcp_fastopen_cookie *foc) 566 { 567 struct inet_request_sock *ireq = inet_rsk(req);... 588 opts->mss = mss; 589 remaining -= TCPOLEN_MSS_ALIGNED;...tcp_options_write用于将选项信息写入TCP报头：
409 static void tcp_options_write(__be32 *ptr, struct tcp_sock *tp, 410 struct tcp_out_options *opts) 411 { 412 u16 options = opts->options; /* mungable copy */... 422 if (unlikely(opts->mss)) { 423 *ptr++ = htonl((TCPOPT_MSS << 24) | //MSS选项标识号：2 424 (TCPOLEN_MSS << 16) | //MSS选项长度：4 425 opts->mss); //MSS值 426 }... 收到SYN或SYN|ACK后，TCP会使用tcp_parse_options函数解析选项信息：3481 void tcp_parse_options(const struct sk_buff *skb,3482 struct tcp_options_received *opt_rx, int estab,3483 struct tcp_fastopen_cookie *foc)3484 {3485 const unsigned char *ptr;3486 const struct tcphdr *th = tcp_hdr(skb);3487 int length = (th->doff * 4) - sizeof(struct tcphdr);34883489 ptr = (const unsigned char *)(th + 1);3490 opt_rx->saw_tstamp = 0;34913492 while (length > 0) {3493 int opcode = *ptr++;3494 int opsize;34953496 switch (opcode) {3497 case TCPOPT_EOL:3498 return;3499 case TCPOPT_NOP: /* Ref: RFC 793 section 3.1 */3500 length--;3501 continue;3502 default:3503 opsize = *ptr++;3504 if (opsize < 2) /* "silly options" */3505 return;3506 if (opsize > length)3507 return; /* don't parse partial options */3508 switch (opcode) {3509 case TCPOPT_MSS:3510 if (opsize == TCPOLEN_MSS && th->syn && !estab) {3511 u16 in_mss = get_unaligned_be16(ptr); //得到报文中MSS选项的值3512 if (in_mss) {3513 if (opt_rx->user_mss &&3514 opt_rx->user_mss < in_mss)3515 in_mss = opt_rx->user_mss; //不能超过用户设定的值3516 opt_rx->mss_clamp = in_mss; //记录MSS3517 }3518 }3519 break;... 收到SYN时tcp_v4_conn_request函数会调用tcp_openreq_init函数将MSS信息记录到request_sock中：075 static inline void tcp_openreq_init(struct request_sock *req,1076 struct tcp_options_received *rx_opt,1077 struct sk_buff *skb)1078 { 1079 struct inet_request_sock *ireq = inet_rsk(req);...1086 req->mss = rx_opt->mss_clamp;... Server端在收到三次握手的ACK时TCP会由tcp_v4_syn_recv_sock调用tcp_create_openreq_child创建sock：381 struct sock *tcp_create_openreq_child(struct sock *sk, struct request_sock *req, struct sk_buff *skb)382 {383 struct sock *newsk = inet_csk_clone_lock(sk, req, GFP_ATOMIC);...481 newtp->rx_opt.mss_clamp = req->mss;... tcp_v4_syn_recv_sock中接下来会利用对端MSS选项信息计算rcv_mss：
1642 struct sock *tcp_v4_syn_recv_sock(struct sock *sk, struct sk_buff *skb,1643 struct request_sock *req,1644 struct dst_entry *dst)1645 {...1658 newsk = tcp_create_openreq_child(sk, req, skb); //在这里会转存SYN包中的MSS信息...1692 tcp_sync_mss(newsk, dst_mtu(dst)); //使用SYN包中的MSS信息计算当前MSS1693 newtp->advmss = dst_metric_advmss(dst);1694 if (tcp_sk(sk)->rx_opt.user_mss &&1695 tcp_sk(sk)->rx_opt.user_mss < newtp->advmss)1696 newtp->advmss = tcp_sk(sk)->rx_opt.user_mss;1697 1698 tcp_initialize_rcv_mss(newsk); //计算rcv_mss的值... rcv_mss是本端所估计的对端使用的MSS。之所以需要估计是因为对端的MSS信息也是在不断变化的。Client端在收到SYN|ACK时会调用tcp_rcv_synsent_state_process函数处理MSS信息：
5373 static int tcp_rcv_synsent_state_process(struct sock *sk, struct sk_buff *skb,5374 const struct tcphdr *th, unsigned int len)5375 {5376 struct inet_connection_sock *icsk = inet_csk(sk);5377 struct tcp_sock *tp = tcp_sk(sk);...5381 tcp_parse_options(skb, &tp->rx_opt, 0, &foc);...5470 tcp_sync_mss(sk, icsk->icsk_pmtu_cookie);5471 tcp_initialize_rcv_mss(sk);... 看来无论是收到SYN还是SYN|ACK，MSS信息都会保存在tp->rx_opt.mss_clamp中，且都会调用tcp_sync_mss函数。 tcp_sync_mss用来计算当前MSS信息：
1296 unsigned int tcp_sync_mss(struct sock *sk, u32 pmtu)1297 {1298 struct tcp_sock *tp = tcp_sk(sk);1299 struct inet_connection_sock *icsk = inet_csk(sk);1300 int mss_now;1301 1302 if (icsk->icsk_mtup.search_high > pmtu)1303 icsk->icsk_mtup.search_high = pmtu;1304 1305 mss_now = tcp_mtu_to_mss(sk, pmtu);1306 mss_now = tcp_bound_to_half_wnd(tp, mss_now); //根据最大窗口大小计算mss_now1307 1308 /* And store cached results */1309 icsk->icsk_pmtu_cookie = pmtu;1310 if (icsk->icsk_mtup.enabled)1311 mss_now = min(mss_now, tcp_mtu_to_mss(sk, icsk->icsk_mtup.search_low));1312 tp->mss_cache = mss_now; //将计算结果保存在mss_cache中，以便发送数据时使用1313 1314 return mss_now;1315 } tcp_mtu_to_mss用于将MTU值转换为MSS值：1198 static inline int __tcp_mtu_to_mss(struct sock *sk, int pmtu)1199 {1200 const struct tcp_sock *tp = tcp_sk(sk);1201 const struct inet_connection_sock *icsk = inet_csk(sk);1202 int mss_now;1203 1204 /* Calculate base mss without TCP options:1205 It is MMS_S - sizeof(tcphdr) of rfc11221206 */1207 mss_now = pmtu - icsk->icsk_af_ops->net_header_len - sizeof(struct tcphdr);1208 1209 /* IPv6 adds a frag_hdr in case RTAX_FEATURE_ALLFRAG is set */1210 if (icsk->icsk_af_ops->net_frag_header_len) {1211 const struct dst_entry *dst = __sk_dst_get(sk);1212 1213 if (dst && dst_allfrag(dst))1214 mss_now -= icsk->icsk_af_ops->net_frag_header_len;1215 }1216 1217 /* Clamp it (mss_clamp does not include tcp options) */1218 if (mss_now > tp->rx_opt.mss_clamp) //mss_clamp是从对端发送的带SYN标记的包的MSS选项中获得1219 mss_now = tp->rx_opt.mss_clamp; //不能超过对端通过的MSS值1220 1221 /* Now subtract optional transport overhead */1222 mss_now -= icsk->icsk_ext_hdr_len;1223 1224 /* Then reserve room for full set of TCP options and 8 bytes of data */1225 if (mss_now < 48)1226 mss_now = 48;1227 return mss_now;1228 }1229 1230 /* Calculate MSS. Not accounting for SACKs here. */1231 int tcp_mtu_to_mss(struct sock *sk, int pmtu)1232 {1233 /* Subtract TCP options size, not including SACKs */1234 return __tcp_mtu_to_mss(sk, pmtu) -1235 (tcp_sk(sk)->tcp_header_len - sizeof(struct tcphdr));1236 } 当前最新的MSS记录在tp->mss_cache中，tcp_sendmsg函数构建报文段时会用到：1016 int tcp_sendmsg(struct kiocb *iocb, struct sock *sk, struct msghdr *msg,1017 size_t size)1018 { ...1067 mss_now = tcp_send_mss(sk, &size_goal, flags);... tcp_send_mss会调用tcp_current_mss得到当前MSS：1321 unsigned int tcp_current_mss(struct sock *sk)1322 {1323 const struct tcp_sock *tp = tcp_sk(sk);1324 const struct dst_entry *dst = __sk_dst_get(sk);1325 u32 mss_now;1326 unsigned int header_len;1327 struct tcp_out_options opts; 1328 struct tcp_md5sig_key *md5;1329 1330 mss_now = tp->mss_cache; //得到MSS记录1331 1332 if (dst) { //如果有路由表项，则根据路由表中的信息更新MSS1333 u32 mtu = dst_mtu(dst);1334 if (mtu != inet_csk(sk)->icsk_pmtu_cookie)1335 mss_now = tcp_sync_mss(sk, mtu);1336 }1337 1338 header_len = tcp_established_options(sk, NULL, &opts, &md5) +1339 sizeof(struct tcphdr);1340 /* The mss_cache is sized based on tp->tcp_header_len, which assumes1341 * some common options. If this is an odd packet (because we have SACK1342 * blocks etc) then our calculated header_len will be different, and1343 * we have to adjust mss_now correspondingly */1344 if (header_len != tp->tcp_header_len) {1345 int delta = (int) header_len - tp->tcp_header_len; //TCP报头长度发生了变换1346 mss_now -= delta; //修正MSS1347 }1348 1349 return mss_now;1350 } 以上便是MSS选项从MSS值获取、选项构建、选项解析、MSS值记录到MSS值的使用的全过程。而发送数据时最终使用的当前MSS（mss_now）则是由通告MSS（SYN中MSS选项的值）、最大窗口大小、报头长度及其变化、路径MTU等多种因素共同来决定的。 0 0
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