[PATCH nft] doc: describe behaviour of {ip,ip6} length

[Pablo Neira Ayuso <pablo@netfilter.org>]

This field exposes internal kernel GRO/GSO packet aggregation
implementation details to userspace, provide a hint to the user to
understand better when matching on this field.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
---
 doc/payload-expression.txt | 16 ++++++++++++++++
 1 file changed, 16 insertions(+)

diff --git a/doc/payload-expression.txt b/doc/payload-expression.txt
index 06538832ec52..d12a7df78b08 100644
--- a/doc/payload-expression.txt
+++ b/doc/payload-expression.txt
@@ -134,6 +134,14 @@ Destination address |
 ipv4_addr
 |======================
 
+Careful with matching on *ip length*: If GRO/GSO is enabled, then the kernel
+might aggregate several packets into one big packet that is larger than MTU.
+If GRO/GSO maximum size is larger than 65535 (see man ip-link(8), specifically
+gro_ipv6_max_size and gso_ipv6_max_size), then *ip length* might be 0 for such
+jumbo packets.  *meta length* allows you to match on the packet length
+including the IP header size.  If you want to perform heuristics on the
+*ip length* field, then disable GRO/GSO.
+
 ICMP HEADER EXPRESSION
 ~~~~~~~~~~~~~~~~~~~~~~
 [verse]
@@ -244,6 +252,14 @@ Destination address |
 ipv6_addr
 |=======================
 
+Careful with matching on *ip6 length*: If GRO/GSO is enabled, then the kernel
+might aggregate several packets into one big packet that is larger than MTU.
+If GRO/GSO maximum size is larger than 65535 (see man ip-link(8), specifically
+gro_ipv6_max_size and gso_ipv6_max_size), then *ip6 length* might be 0 for such
+jumbo packets.  *meta length* allows you to match on the packet length
+including the IP header size.  If you want to perform heuristics on the
+*ip6 length* field, then disable GRO/GSO.
+
 .Using ip6 header expressions
 -----------------------------
 # matching if first extension header indicates a fragment
-- 
2.30.2