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MPTCP 64bit seq

一、snd_high_order, rcv_high_order

发送和接收都将seq映射到64位上,这样能防止不同子流之间seq造成的歧义。

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# 发送
static inline __be32 mptcp_get_highorder_sndbits(const struct sk_buff *skb, const struct mptcp_cb *mpcb)
{
	return htonl(mpcb->snd_high_order[(TCP_SKB_CB(skb)->mptcp_flags &
			MPTCPHDR_SEQ64_INDEX) ? 1 : 0]);
}

static inline void mptcp_check_sndseq_wrap(struct tcp_sock *meta_tp, int inc)
{
	if (unlikely(meta_tp->snd_nxt > meta_tp->snd_nxt + inc)) {
		struct mptcp_cb *mpcb = meta_tp->mpcb;
		mpcb->snd_hiseq_index = mpcb->snd_hiseq_index ? 0 : 1;
		mpcb->snd_high_order[mpcb->snd_hiseq_index] += 2;
	}
}

# 接收
static inline u64 mptcp_get_data_seq_64(const struct mptcp_cb *mpcb, int index, u32 data_seq_32)
{
	return ((u64)mpcb->rcv_high_order[index] << 32) | data_seq_32;
}

static inline u64 mptcp_get_rcv_nxt_64(const struct tcp_sock *meta_tp)
{
	struct mptcp_cb *mpcb = meta_tp->mpcb;
	return mptcp_get_data_seq_64(mpcb, mpcb->rcv_hiseq_index,
				     meta_tp->rcv_nxt);
}

static inline void mptcp_check_rcvseq_wrap(struct tcp_sock *meta_tp, u32 old_rcv_nxt)
{
	if (unlikely(old_rcv_nxt > meta_tp->rcv_nxt)) {
		struct mptcp_cb *mpcb = meta_tp->mpcb;
		mpcb->rcv_high_order[mpcb->rcv_hiseq_index] += 2;
		mpcb->rcv_hiseq_index = mpcb->rcv_hiseq_index ? 0 : 1;
	}
}

1. 发送端 MPTCPHDR_SEQ64_INDEX

MPTCPHDR_SEQ64_INDEX 在发送和接收上有不同用法,在发送上

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static bool mptcp_skb_entail(struct sock *sk, struct sk_buff *skb, int reinject)
{
	...
	if (!reinject) // 如果是第一次发送的包, MPTCPHDR_SEQ64_INDEX 只是作为 snd_hiseq_index 的替代
		TCP_SKB_CB(skb)->mptcp_flags |= (mpcb->snd_hiseq_index ? MPTCPHDR_SEQ64_INDEX : 0);
	...

2. wrap

在 mptcp_check_sndseq_wrap 中 snd_hiseq_index ^= 1, 然后 snd_high_order[i] += 2; 所以 snd_high_order使用 snd_high_order[i] 和 snd_high_order[i-1]。

在 mptcp_check_rcvseq_wrap 中 rcv_high_order[i] += 2; rcv_hiseq_index ^= 1; 所以 rcv_high_order 使用 rcv_high_order[i] 和 rcv_high_order[i+1]。

为什么?

因为发送的时候只需要用到最高seq(snd_nxt),但接收的时候会超高最高seq(rcv_nxt)。在 mptcp_detect_mapping 中指明了:

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if (unlikely(after(data_seq, meta_tp->rcv_nxt) && data_seq < meta_tp->rcv_nxt)) {
	tp->mptcp->map_data_seq = mptcp_get_data_seq_64(mpcb, !mpcb->rcv_hiseq_index, data_seq);

二、64bit OR 33bit

1. 接收端 MPTCPHDR_SEQ64_INDEX

在 mptcp_write_dss_data_ack() 中 mdss->m = 0; 所以 MPTCPHDR_SEQ64_SET 永远不启用。 接收端只有在 MPTCPHDR_SEQ64_SET 启用时 MPTCPHDR_SEQ64_INDEX, MPTCPHDR_SEQ64_OFO 才有用, 见 mptcp_get_64_bit

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static inline u8 mptcp_get_64_bit(u64 data_seq, struct mptcp_cb *mpcb)
{
	u64 data_seq_high = (u32)(data_seq >> 32);

	if (mpcb->rcv_high_order[0] == data_seq_high)
		return 0;
	else if (mpcb->rcv_high_order[1] == data_seq_high)
		return MPTCPHDR_SEQ64_INDEX;
	else
		return MPTCPHDR_SEQ64_OFO;
}

static inline __u32 *mptcp_skb_set_data_seq(const struct sk_buff *skb, u32 *data_seq, struct mptcp_cb *mpcb)
{
	__u32 *ptr = (__u32 *)(skb_transport_header(skb) + TCP_SKB_CB(skb)->dss_off);

	if (TCP_SKB_CB(skb)->mptcp_flags & MPTCPHDR_SEQ64_SET) {
		u64 data_seq64 = get_unaligned_be64(ptr);

		if (mpcb)
			TCP_SKB_CB(skb)->mptcp_flags |= mptcp_get_64_bit(data_seq64, mpcb);

		*data_seq = (u32)data_seq64;
		ptr++;
	} else {
		*data_seq = get_unaligned_be32(ptr);
	}

	return ptr;
}

2. bug??

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if (mpcb->rcv_high_order[0] == data_seq_high)
	return 0;
else if (mpcb->rcv_high_order[1] == data_seq_high)
	return MPTCPHDR_SEQ64_INDEX;

这四句应该改成:

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i = mpcb->rcv_hiseq_index;
if (mpcb->rcv_high_order[i] == data_seq_high)
	return 0;
else if (mpcb->rcv_high_order[i^1] == data_seq_high)
	return MPTCPHDR_SEQ64_INDEX;

3. 33bit

rcv_high_order[i1] = rcv_high_order[i] + 1; 所以所谓的64bit,其实是33bit。

4. MPTCPHDR_SEQ64_OFO

33bit seq 超过了 rcv_high_order[i1],判定为无效数据,不收取