From 6821ad8770340825f17962cf5ef64ebaffee7fd7 Mon Sep 17 00:00:00 2001 From: Maciej Żenczykowski Date: Wed, 18 May 2022 14:05:48 -0700 Subject: xfrm: do not set IPv4 DF flag when encapsulating IPv6 frames <= 1280 bytes. One may want to have DF set on large packets to support discovering path mtu and limiting the size of generated packets (hence not setting the XFRM_STATE_NOPMTUDISC tunnel flag), while still supporting networks that are incapable of carrying even minimal sized IPv6 frames (post encapsulation). Having IPv4 Don't Frag bit set on encapsulated IPv6 frames that are not larger than the minimum IPv6 mtu of 1280 isn't useful, because the resulting ICMP Fragmentation Required error isn't actionable (even assuming you receive it) because IPv6 will not drop it's path mtu below 1280 anyway. While the IPv4 stack could prefrag the packets post encap, this requires the ICMP error to be successfully delivered and causes a loss of the original IPv6 frame (thus requiring a retransmit and latency hit). Luckily with IPv4 if we simply don't set the DF flag, we'll just make further fragmenting the packets some other router's problems. We'll still learn the correct IPv4 path mtu through encapsulation of larger IPv6 frames. I'm still not convinced this patch is entirely sufficient to make everything happy... but I don't see how it could possibly make things worse. See also recent: 4ff2980b6bd2 'xfrm: fix tunnel model fragmentation behavior' and friends Cc: Lorenzo Colitti Cc: Eric Dumazet Cc: Lina Wang Cc: Steffen Klassert Signed-off-by: Maciej Zenczykowski Signed-off-by: Steffen Klassert --- net/xfrm/xfrm_output.c | 3 ++- 1 file changed, 2 insertions(+), 1 deletion(-) (limited to 'net/xfrm') diff --git a/net/xfrm/xfrm_output.c b/net/xfrm/xfrm_output.c index d4935b3b9983..555ab35cd119 100644 --- a/net/xfrm/xfrm_output.c +++ b/net/xfrm/xfrm_output.c @@ -273,6 +273,7 @@ static int xfrm4_beet_encap_add(struct xfrm_state *x, struct sk_buff *skb) */ static int xfrm4_tunnel_encap_add(struct xfrm_state *x, struct sk_buff *skb) { + bool small_ipv6 = (skb->protocol == htons(ETH_P_IPV6)) && (skb->len <= IPV6_MIN_MTU); struct dst_entry *dst = skb_dst(skb); struct iphdr *top_iph; int flags; @@ -303,7 +304,7 @@ static int xfrm4_tunnel_encap_add(struct xfrm_state *x, struct sk_buff *skb) if (flags & XFRM_STATE_NOECN) IP_ECN_clear(top_iph); - top_iph->frag_off = (flags & XFRM_STATE_NOPMTUDISC) ? + top_iph->frag_off = (flags & XFRM_STATE_NOPMTUDISC) || small_ipv6 ? 0 : (XFRM_MODE_SKB_CB(skb)->frag_off & htons(IP_DF)); top_iph->ttl = ip4_dst_hoplimit(xfrm_dst_child(dst)); -- cgit v1.2.3