当然对于元组的理解，需要考虑上下文比如：
https://access.redhat.com/documentation/en-us/red_hat_enterprise_linux/9/html/configuring_and_managing_networking/linux-traffic-control_configuring-and-managing-networking
这个链接里的这一句话里的tuple，是指的packet专有的tuple，源地址，目的地址，源port，目的port。这就需要做记忆。尤其是作为唯一标识的tuple。
The connection tracking entry stores a Netfilter mark and tracks the connection state information in the memory table in which a new packet tuple maps with an existing entry.
/* A `tuple' is a structure containing the information to uniquely
identify a connection. ie. if two packets have the same tuple, they
are in the same connection; if not, they are not.
We divide the structure along "manipulatable" and
"non-manipulatable" lines, for the benefit of the NAT code.
*/
/* This contains the information to distinguish a connection. */
struct nf_conntrack_tuple {
struct nf_conntrack_man src;
/* These are the parts of the tuple which are fixed. */
struct {
union nf_inet_addr u3;
union {
/* Add other protocols here. */
__be16 all;
struct {
__be16 port;
} tcp;
struct {
__be16 port;
} udp;
struct {
u_int8_t type, code;
} icmp;
struct {
__be16 port;
} dccp;
struct {
__be16 port;
} sctp;
struct {
__be16 key;
} gre;
} u;
/* The protocol. */
u_int8_t protonum;
/* The direction (for tuplehash) */
u_int8_t dir;
} dst;
};