TCP连接建立系列 — TCP选项解析

本文主要分析：在收到客户端的SYN包时，服务器端是如何解析它所携带的TCP选项，并结合本端情况决定是否予以支持。

内核版本：3.6

Author：zhangskd @ csdn blog

 

概述

 

收到客户端的SYN包时，需要全面的解析它携带的TCP选项，这样我们就知道客户端支持哪些选项，如果本端也支持，

那么连接就支持这些TCP选项。这些信息在连接建立的过程中，是保存在连接请求块的(request_sock、inet_request_sock、

tcp_request_sock)。

 

函数调用路径：

tcp_v4_conn_request

        |--> tcp_parse_options

 

3.6版本Linux内核支持的TCP选项包括：

NOP，用于填充。

Max Segment Size，最大分段大小。

Window Scaling，窗口扩大因子。

SACK Permit，是否允许使用SACK。

SACK，携带SACK块。

Timestamp，时间戳选项。

MD5SIG，MD5签名。

Cookie，Cookie extension，2013年3月已从内核移除。

EXP，Experimental，处于实验阶段的选项，新版本用于Fast Open。

 

实现

 

全面解析数据包的TCP选项，并保存到指定的tcp_options_received实例中。

/* Look for tcp options. Normally only called on SYN and SYNACK packets. * But, this can also be called on packets in the established flow when the fast version below fails. */void tcp_parse_options(const struct sk_buff *skb, struct tcp_options_received *opt_rx, const u8 **hvpp,                       int estab, struct tcp_fastopen_cookie *foc){    const unsigned char *ptr;    const struct tcphdr *th = tcp_hdr(skb);    int length = (th->doff * 4) - sizeof(struct tcphdr); /* 选项的总长度 */    ptr = (const unsigned char *) (th + 1); /* 选项的起始地址 */    opt_rx->saw_tstamp = 0; /* Saw TIMESTAMP on last packet */    while(length > 0) {        int opcode = *ptr++; /* 选项kind */        int opsize; /* 选项length */        switch(opcode) {        case TCPOPT_EOL: /* End of options */            return;         case TCPOPT_NOP: /* Padding，填充选项 */            length--; /* 此选项只占一个字节 */            continue;        default:            opsize = *ptr++; /* 选项长度 */            if (opsize < 2) /* silly options */                return; /* 选项长度过小 */            if (opsize > length)                return; /* don't parse partial options */            switch(opcode) {            case TCPOPT_MSS: /* MSS选项 */                if (opsize == TCPOLEN_MSS && th->syn && ! estab) {                    u16 in_mss = get_unaligned_be16(ptr);                    if (in_mss) {                        /* 如果用户指定了MSS，且比对端通告的小 */                        if (opt_rx->user_mss && opt_rx->user_mss < in_mss)                            in_mss = opt_rx->user_mss;                        opt_rx->mss_clamp = in_mss; /* Maximal mss */                    }                }                break;            case TCPOPT_WINDOW: /* Window scaling */                if (opsize == TCPOLEN_WINDOW && th->syn && ! estab &&                    sysctl_tcp_window_scaling) {                    __u8 snd_wscale = *(__u8 *)ptr; /* 对端的接收窗口扩大因子 */                    opt_rx->wscale_ok = 1; /* 连接使用窗口扩大选项 */                    if (snd_wscale > 14) {                        net_info_ratelimited("%s: Illegal window scaling value %d >14 received\n",                               __func__, snd_wscale);                        snd_wscale = 14;                    }                    opt_rx->snd_wscale = snd_wscale; /* 保存对端的接收窗口扩大因子 */                }                break;            case TCPOPT_TIMESTAMP: /* Better RTT estimations/PAWS */                if ((opsize == TCPOLEN_TIMESTAMP) && ((estab && opt_rx->tstamp_ok) ||                     (!estab && sysctl_tcp_timestamps))) {                    opt_rx->saw_tstamp = 1; /* 连接支持TIMESTAMP选项 */                    opt_rx->rcv_tsval = get_unaligned_be32(ptr); /* Time stamp value */                    opt_rx->rcv_tsecr = get_unaligned_be32(ptr + 4); /* Time stamp echo reply */                }                break;            case TCPOPT_SACK_PERM: /* SACK Permitted */                if (opsize == TCPOLEN_SACK_PERM && th->syn && !estab && sysctl_tcp_sack) {                    opt_rx->sack_ok = TCP_SACK_SEEN; /* 连接支持SACK选项 */                    tcp_sack_reset(opt_rx); /* 清空dsack和num_sacks变量 */                }                break;            case TCPOPT_SACK: /* SACK Block */                if ((opsize >= (TCPOLEN_SACK_BASE + TCPOLEN_SACK_PERBLOCK)) &&                    !((opsize - TCPOLEN_SACK_BASE) % TCPOLEN_SACK_PERBLOCK) && opt_rx->sack_ok) {                    /* 保存SACK选项的起始偏移地址 */                    TCP_SKB_CB(skb)->sacked = (ptr - 2) - (unsigned char *) th;                 }                break;#ifdef CONFIG_TCP_MD5SIG            case TCPOPT_MD5SIG:                /* The MD5 Hash has already been checked                 * (see tcp_v{4, 6}_do_rcv()).                 */                break;#endif            /* 注意，在新版内核中此选项已被删除。Cookie extension (experimental) */            case TCPOPT_COOKIE:                 /* This option is variable length. */                switch(opsize) {                case TCPOLEN_COOKIE_BASE:                    /* not yet implemented */                    break;                case TCPOLEN_COOKIE_PAIR:                    /* not yet implemented */                    break;                case TCPOLEN_COOKIE_MIN + 0:                case TCPOLEN_COOKIE_MIN + 2:                case TCPOLEN_COOKIE_MIN + 4:                case TCPOLEN_COOKIE_MIN + 6:                case TCPOLEN_COOKIE_MAX:                    /* 16-bit multple */                    opt_rx->cookie_plus = opsize;                    *hvpp = ptr;                    break;                default:                    /* ignore option */                    break;                }                break;             case TCPOPT_EXP: /* Experimental */                /* Fast Open option shares code 254 using a 16 bits magic number.                 * It's valid only in SYN or SYN-ACK with an even size.                 */                if (opsize < TCPOLEN_EXP_FASTOPEN_BASE || get_unaligned_be16(ptr) != TCPOPT_FASTOPEN_MAGIC                    || foc == NULL || ! th->syn || (opsize & 1))                    break;                foc->len = opsize - TCPOLEN_EXP_FASTOPEN_BASE;                if (foc->len >= TCP_FASTOPEN_COOKIE_MIN && foc->len <= TCP_FASTOPEN_COOKIE_MAX)                    memcpy(foc->val, ptr + 2, foc->len);                else if (foc->len != 0)                    foc->len = -1;                break;            }            ptr += opsize - 2;            length -= opsize;        }    }}

/* TCP options */#define TCPOPT_NOP 1 /* Padding */#define TCPOPT_EOL 0 /* End of options */#define TCPOPT_MSS 2 /* Segment size negotiating */#define TCPOPT_WINDOW 3 /* Window scaling */#define TCPOPT_TIMESTAMP 8 /* Better RTT estimations/PAWS */#define TCPOPT_SACK_PERM 4 /* SACK Permitted */#define TCPOPT_SACK 5 /* SACK Block */#define TCPOPT_MD5SIG 19 /* MD5 Signature (RFC2385) */#define TCPOPT_COOKIE 253 /* Cookie extension (experimental) */#define TCPOPT_EXP 254 /* Experimental *//* Magic number to be after the option value for sharing TCP experimental options. * See draft-ietf-tcpm-experimental-options-00.txt */#define TCPOPT_FASTOPEN_MAGIC 0xF989/* TCP option lengths */#define TCPOLEN_MSS 4#define TCPOLEN_WINDOW 3#define TCPOLEN_TIMESTAMP 10#define TCPOLEN_SACK_PERM 2#define TCPOLEN_COOKIE_BASE 2 /* Cookie-less header extension */#define TCPOLEN_COOKIE_PAIR 3 /* Cookie pair header extension */#define TCPOLEN_COOKIE_MIN (TCPOLEN_COOKIE_BASE + TCP_COOKIE_MIN)#define TCPOLEN_COOKIE_MAX (TCPOLEN_COOKIE_BASE + TCP_COOKIE_MAX)#define TCPOLEN_EXP_FASTOPEN_BASE 4/* These are used to set the sack_ok field in struct tcp_options_received */#define TCP_SACK_SEEN (1 << 0) /* peer is SACK capable */#define TCP_FACK_ENABLED (1 << 1) /* FACK is enabled locally */#define TCP_DSACK_SEEN (1 << 2) /* DSACK was received from peer *//* for TCP_COOKIE_TRANSACTIONS (TCPCT) socket option */#define TCP_COOKIE_MIN 8 /* 64-bits */#define TCP_COOKIE_MAX 16 /* 128-bits *//* TCP Fast Open */#define TCP_FASTOPEN_COOKIE_MIN 4 /* Min Fast Open Cookie size in bytes */#define TCP_FASTOPEN_COOKIE_MAX 16 /* Max Fast Open Cookie size in bytes *//* TCP Fast Open Cookie as stored in memory */struct tcp_fastopen_cookie {    s8 len;    u8 val[TCP_FASTOPEN_COOKIE_MAX];};static inline void tcp_sack_reset(struct tcp_options_received *rx_opt){    rx_opt->dsack = 0;    rx_opt->num_sacks = 0;}

 

用于判断连接是否使用ECN。要注意的是在建立连接时，要求IP报的ECN域不能被设置，否则就禁用ECN。

/* RFC3168: 6.1.1 SYN packets must not have ECT/ECN bits set. * * If we receive a SYN packet with these bits set, it means a network is playing  bad games * with TOS bits. In order to avoid possible false congestion notifications, we disable * TCP ECN negociation. */static inline void TCP_ECN_create_request(struct request_sock *req, const struct sk_buff *skb){    const struct tcphdr *th = tcp_hdr(skb);    if (sysctl_tcp_ecn && th->ece && th->cwr && INET_ECN_is_not_ect(TCP_SKB_CB(skb)->ip_dsfield))        inet_rsk(req)->ecn_ok = 1; /* 连接支持ECN */}

 

清零TCP选项。

static inline void tcp_clear_options(struct tcp_options_received *rx_opt){    rx_opt->tstamp_ok = rx_opt->sack_ok = 0;    rx_opt->wscale_ok = rx_opt->snd_wscale = 0;    rx_opt->cookie_plus = 0;}