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NAME
tcp_functions — Alternate TCP Stack Framework
SYNOPSIS
#include <netinet/tcp.h>
#include <netinet/tcp_var.h>
int
register_tcp_functions(struct tcp_function_block *blk, int wait);
int
deregister_tcp_functions(struct tcp_function_block *blk);
DESCRIPTION
The tcp_functions framework allows a kernel developer to implement alternate TCP stacks.
The alternate stacks can be compiled in the kernel or can be implemented in loadable kernel
modules. This functionality is intended to encourage experimentation with the TCP stack and
to allow alternate behaviors to be deployed for different TCP connections on a single
system.
A system administrator can set a system default stack. By default, all TCP connections will
use the system default stack. Additionally, users can specify a particular stack to use on
a per-connection basis. (See tcp(4) for details on setting the system default stack, or
selecting a specific stack for a given connection.)
This man page treats "TCP stacks" as synonymous with "function blocks". This is
intentional. A "TCP stack" is a collection of functions that implement a set of behavior.
Therefore, an alternate "function block" defines an alternate "TCP stack".
tcp_functions modules must call the register_tcp_functions() function during initialization
and successfully call the deregister_tcp_functions() function prior to allowing the module
to be unloaded.
The register_tcp_functions() function requests that the system add a specified function
block to the system.
The deregister_tcp_functions() function requests that the system remove a specified function
block from the system. If the call fails because sockets are still using the specified
function block, the system will mark the function block as being in the process of being
removed. This will prevent additional sockets from using the specified function block.
However, it will not impact sockets that are already using the function block.
The blk argument is a pointer to a struct tcp_function_block, which is explained below (see
Function Block Structure). The wait argument is used as the flags argument to malloc(9),
and must be set to one of the valid values defined in that man page.
Function Block Structure
The blk argument is a pointer to a struct tcp_function_block, which has the following
members:
struct tcp_function_block {
char tfb_tcp_block_name[TCP_FUNCTION_NAME_LEN_MAX];
int (*tfb_tcp_output)(struct tcpcb *);
void (*tfb_tcp_do_segment)(struct mbuf *, struct tcphdr *,
struct socket *, struct tcpcb *,
int, int, uint8_t,
int);
int (*tfb_tcp_ctloutput)(struct socket *so,
struct sockopt *sopt,
struct inpcb *inp, struct tcpcb *tp);
/* Optional memory allocation/free routine */
void (*tfb_tcp_fb_init)(struct tcpcb *);
void (*tfb_tcp_fb_fini)(struct tcpcb *);
/* Optional timers, must define all if you define one */
int (*tfb_tcp_timer_stop_all)(struct tcpcb *);
void (*tfb_tcp_timer_activate)(struct tcpcb *,
uint32_t, u_int);
int (*tfb_tcp_timer_active)(struct tcpcb *, uint32_t);
void (*tfb_tcp_timer_stop)(struct tcpcb *, uint32_t);
void (*tfb_tcp_rexmit_tmr)(struct tcpcb *);
volatile uint32_t tfb_refcnt;
uint32_t tfb_flags;
};
The tfb_tcp_block_name field identifies the unique name of the TCP stack, and should be no
longer than TCP_FUNCTION_NAME_LEN_MAX-1 characters in length.
The tfb_tcp_output, tfb_tcp_do_segment, and tfb_tcp_ctloutput fields are pointers to
functions that perform the equivalent actions as the default tcp_output(), tcp_do_segment(),
and tcp_default_ctloutput() functions, respectively. Each of these function pointers must
be non-NULL.
If a TCP stack needs to initialize data when a socket first selects the TCP stack (or, when
the socket is first opened), it should set a non-NULL pointer in the tfb_tcp_fb_init field.
Likewise, if a TCP stack needs to cleanup data when a socket stops using the TCP stack (or,
when the socket is closed), it should set a non-NULL pointer in the tfb_tcp_fb_fini field.
If the TCP stack implements additional timers, the TCP stack should set a non-NULL pointer
in the tfb_tcp_timer_stop_all, tfb_tcp_timer_activate, tfb_tcp_timer_active, and
tfb_tcp_timer_stop fields. These fields should all be NULL or should all contain pointers
to functions. The tfb_tcp_timer_activate, tfb_tcp_timer_active, and tfb_tcp_timer_stop
functions will be called when the tcp_timer_activate(), tcp_timer_active(), and
tcp_timer_stop() functions, respectively, are called with a timer type other than the
standard types. The functions defined by the TCP stack have the same semantics (both for
arguments and return values) as the normal timer functions they supplement.
Additionally, a stack may define its own actions to take when the retransmit timer fires by
setting a non-NULL function pointer in the tfb_tcp_rexmit_tmr field. This function is
called very early in the process of handling a retransmit timer. However, care must be
taken to ensure the retransmit timer leaves the TCP control block in a valid state for the
remainder of the retransmit timer logic.
The tfb_refcnt and tfb_flags fields are used by the kernel's TCP code and will be
initialized when the TCP stack is registered.
Requirements for Alternate TCP Stacks
If the TCP stack needs to store data beyond what is stored in the default TCP control block,
the TCP stack can initialize its own per-connection storage. The t_fb_ptr field in the
struct tcpcb control block structure has been reserved to hold a pointer to this per-
connection storage. If the TCP stack uses this alternate storage, it should understand that
the value of the t_fb_ptr pointer may not be initialized to NULL. Therefore, it should use
a tfb_tcp_fb_init function to initialize this field. Additionally, it should use a
tfb_tcp_fb_fini function to deallocate storage when the socket is closed.
It is understood that alternate TCP stacks may keep different sets of data. However, in
order to ensure that data is available to both the user and the rest of the system in a
standardized format, alternate TCP stacks must update all fields in the TCP control block to
the greatest extent practical.
RETURN VALUES
The register_tcp_functions() and deregister_tcp_functions() functions return zero on success
and non-zero on failure. In particular, the deregister_tcp_functions() will return EBUSY
until no more connections are using the specified TCP stack. A module calling
deregister_tcp_functions() must be prepared to wait until all connections have stopped using
the specified TCP stack.
ERRORS
The register_tcp_functions() function will fail if:
[EINVAL] Any of the members of the blk argument are set incorrectly.
[ENOMEM] The function could not allocate memory for its internal data.
[EALREADY] A function block is already registered with the same name.
The deregister_tcp_functions() function will fail if:
[EPERM] The blk argument references the kernel's compiled-in default function
block.
[EBUSY] The function block is still in use by one or more sockets, or is defined
as the current default function block.
[ENOENT] The blk argument references a function block that is not currently
registered.
SEE ALSO
malloc(9), tcp(4)
HISTORY
This framework first appeared in FreeBSD 11.0.
AUTHORS
The tcp_functions framework was written by Randall Stewart <rrs@FreeBSD.org>.
This manual page was written by Jonathan Looney <jtl@FreeBSD.org>.