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本地IP包分片--local_df,ignore_df

 3 years ago
source link: http://abcdxyzk.github.io/blog/2018/01/04/kernel-net-df/
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本地IP包分片--local_df,ignore_df

2018-01-04 01:41:00

local_df 和 ignore_df 是一个意思，在某个版本rename了

ip_queue_xmit 函数中有：

if (ip_dont_fragment(sk, &rt->dst) && !skb->ignore_df)
	iph->frag_off = htons(IP_DF);
else
	iph->frag_off = 0;

ip_dont_fragment

static inline
int ip_dont_fragment(struct sock *sk, struct dst_entry *dst)
{
	return  inet_sk(sk)->pmtudisc == IP_PMTUDISC_DO ||
		(inet_sk(sk)->pmtudisc == IP_PMTUDISC_WANT &&
		 !(dst_metric_locked(dst, RTAX_MTU)));
}

一般情况下都是开启pmtu、skb->ignore_df = 0, 所以 iph->frag_off = htons(IP_DF);

ip_queue_xmit -> ip_finish_output -> ip_fragment :

static int ip_fragment(struct sock *sk, struct sk_buff *skb,
	       unsigned int mtu, 
	       int (*output)(struct sock *, struct sk_buff *))
{
	struct iphdr *iph = ip_hdr(skb);

	// 如果需要分片，直接进入分片函数
	if ((iph->frag_off & htons(IP_DF)) == 0)
		return ip_do_fragment(sk, skb, output);

	// 如果没设置分片，或手动设置的分片过大，则直接丢弃
	if (unlikely(!skb->ignore_df ||
		     (IPCB(skb)->frag_max_size &&
		      IPCB(skb)->frag_max_size > mtu))) {
		struct rtable *rt = skb_rtable(skb);
		struct net_device *dev = rt->dst.dev;

		IP_INC_STATS(dev_net(dev), IPSTATS_MIB_FRAGFAILS);
		icmp_send(skb, ICMP_DEST_UNREACH, ICMP_FRAG_NEEDED,
			  htonl(mtu));
		kfree_skb(skb);
		return -EMSGSIZE;
	}

	// 所以设置 skb->ignore_df = 1 且 skb->len > mtu 则执行到这里
	return ip_do_fragment(sk, skb, output);
}

所以设置 skb->ignore_df = 1 且 skb->len > mtu 则执行 ip_do_fragment 进行IP分片,

分片是按网卡mtu进行，如果mss小于网卡mtu-40，则需要设置 IPCB(skb)->frag_max_size

iph->frag_off 定义

#define IP_DF           0x4000          /* Flag: "Don't Fragment"       */
#define IP_MF           0x2000          /* Flag: "More Fragments"       */
#define IP_OFFSET       0x1FFF          /* "Fragment Offset" part       */

1) 不分片的包 iph->frag_off = htons(IP_DF)
2) 最后一个分片包 ((ntohs(iph->frag_off) & IP_OFFSET) > 0)
3) 其余分片包 ((ntohs(iph->frag_off) & IP_OFFSET) > 0 && (iph->frag_off & htons(IP_MF)) > 0)


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