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33 .Nd Alternate TCP Stack Framework
38 .Fn register_tcp_functions "struct tcp_function_block *blk" "int wait"
40 .Fn deregister_tcp_functions "struct tcp_function_block *blk"
44 framework allows a kernel developer to implement alternate TCP stacks.
45 The alternate stacks can be compiled in the kernel or can be implemented in
46 loadable kernel modules.
47 This functionality is intended to encourage experimentation with the TCP stack
48 and to allow alternate behaviors to be deployed for different TCP connections
51 A system administrator can set a system default stack.
52 By default, all TCP connections will use the system default stack.
53 Additionally, users can specify a particular stack to use on a per-connection
57 for details on setting the system default stack, or selecting a specific stack
58 for a given connection.)
60 This man page treats "TCP stacks" as synonymous with "function blocks".
62 A "TCP stack" is a collection of functions that implement a set of behavior.
63 Therefore, an alternate "function block" defines an alternate "TCP stack".
67 .Fn register_tcp_functions
68 function during initialization and successfully call the
69 .Fn deregister_tcp_functions
70 function prior to allowing the module to be unloaded.
73 .Fn register_tcp_functions
74 function requests that the system add a specified function block to the system.
77 .Fn deregister_tcp_functions
78 function requests that the system remove a specified function block from the
80 If the call fails because sockets are still using the specified function block,
81 the system will mark the function block as being in the process of being
83 This will prevent additional sockets from using the specified function block.
84 However, it will not impact sockets that are already using the function block.
88 argument is a pointer to a
89 .Vt "struct tcp_function_block" ,
90 which is explained below (see
91 .Sx Function Block Structure ) .
94 argument is used as the
98 and must be set to one of the valid values defined in that man page.
99 .Ss Function Block Structure
101 .Fa blk argument is a pointer to a
102 .Vt "struct tcp_function_block" ,
103 which has the following members:
104 .Bd -literal -offset indent
105 struct tcp_function_block {
106 char tfb_tcp_block_name[TCP_FUNCTION_NAME_LEN_MAX];
107 int (*tfb_tcp_output)(struct tcpcb *);
108 void (*tfb_tcp_do_segment)(struct mbuf *, struct tcphdr *,
109 struct socket *, struct tcpcb *,
112 int (*tfb_tcp_ctloutput)(struct socket *so,
113 struct sockopt *sopt,
114 struct inpcb *inp, struct tcpcb *tp);
115 /* Optional memory allocation/free routine */
116 void (*tfb_tcp_fb_init)(struct tcpcb *);
117 void (*tfb_tcp_fb_fini)(struct tcpcb *, int);
118 /* Optional timers, must define all if you define one */
119 int (*tfb_tcp_timer_stop_all)(struct tcpcb *);
120 void (*tfb_tcp_timer_activate)(struct tcpcb *,
122 int (*tfb_tcp_timer_active)(struct tcpcb *, uint32_t);
123 void (*tfb_tcp_timer_stop)(struct tcpcb *, uint32_t);
124 /* Optional functions */
125 void (*tfb_tcp_rexmit_tmr)(struct tcpcb *);
126 void (*tfb_tcp_handoff_ok)(struct tcpcb *);
128 volatile uint32_t tfb_refcnt;
134 .Va tfb_tcp_block_name
135 field identifies the unique name of the TCP stack, and should be no longer than
136 TCP_FUNCTION_NAME_LEN_MAX-1 characters in length.
140 .Va tfb_tcp_do_segment ,
142 .Va tfb_tcp_ctloutput
143 fields are pointers to functions that perform the equivalent actions
148 .Fn tcp_default_ctloutput
149 functions, respectively.
150 Each of these function pointers must be non-NULL.
152 If a TCP stack needs to initialize data when a socket first selects the TCP
153 stack (or, when the socket is first opened), it should set a non-NULL
157 Likewise, if a TCP stack needs to cleanup data when a socket stops using the
158 TCP stack (or, when the socket is closed), it should set a non-NULL pointer
165 argument is non-NULL, the function to which it points is called when the
166 kernel is destroying the TCP control block or when the socket is transitioning
167 to use a different TCP stack.
168 The function is called with arguments of the TCP control block and an integer
170 The flag will be zero if the socket is transitioning to use another TCP stack
171 or one if the TCP control block is being destroyed.
173 If the TCP stack implements additional timers, the TCP stack should set a
174 non-NULL pointer in the
175 .Va tfb_tcp_timer_stop_all ,
176 .Va tfb_tcp_timer_activate ,
177 .Va tfb_tcp_timer_active ,
179 .Va tfb_tcp_timer_stop
181 These fields should all be
183 or should all contain pointers to functions.
185 .Va tfb_tcp_timer_activate ,
186 .Va tfb_tcp_timer_active ,
188 .Va tfb_tcp_timer_stop
189 functions will be called when the
190 .Fn tcp_timer_activate ,
191 .Fn tcp_timer_active ,
194 functions, respectively, are called with a timer type other than the standard
196 The functions defined by the TCP stack have the same semantics (both for
197 arguments and return values) as the normal timer functions they supplement.
199 Additionally, a stack may define its own actions to take when the retransmit
200 timer fires by setting a non-NULL function pointer in the
201 .Va tfb_tcp_rexmit_tmr
203 This function is called very early in the process of handling a retransmit
205 However, care must be taken to ensure the retransmit timer leaves the
206 TCP control block in a valid state for the remainder of the retransmit
209 A user may select a new TCP stack before calling
213 Optionally, a TCP stack may also allow a user to begin using the TCP stack for
214 a connection that is in a later state by setting a non-NULL function pointer in
216 .Va tfb_tcp_handoff_ok
218 If this field is non-NULL and a user attempts to select that TCP stack after
223 for that socket, the kernel will call the function pointed to by the
224 .Va tfb_tcp_handoff_ok
226 The function should return 0 if the user is allowed to switch the socket to use
227 the TCP stack. Otherwise, the function should return an error code, which will
228 be returned to the user.
230 .Va tfb_tcp_handoff_ok
233 and a user attempts to select the TCP stack after calling
237 for that socket, the operation will fail and the kernel will return
244 fields are used by the kernel's TCP code and will be initialized when the
245 TCP stack is registered.
246 .Ss Requirements for Alternate TCP Stacks
247 If the TCP stack needs to store data beyond what is stored in the default
248 TCP control block, the TCP stack can initialize its own per-connection storage.
253 control block structure has been reserved to hold a pointer to this
254 per-connection storage.
255 If the TCP stack uses this alternate storage, it should understand that the
258 pointer may not be initialized to
260 Therefore, it should use a
262 function to initialize this field.
263 Additionally, it should use a
265 function to deallocate storage when the socket is closed.
267 It is understood that alternate TCP stacks may keep different sets of data.
268 However, in order to ensure that data is available to both the user and the
269 rest of the system in a standardized format, alternate TCP stacks must
270 update all fields in the TCP control block to the greatest extent practical.
273 .Fn register_tcp_functions
275 .Fn deregister_tcp_functions
276 functions return zero on success and non-zero on failure.
278 .Fn deregister_tcp_functions
281 until no more connections are using the specified TCP stack.
283 .Fn deregister_tcp_functions
284 must be prepared to wait until all connections have stopped using the
288 .Fn register_tcp_functions
289 function will fail if:
292 Any of the members of the
294 argument are set incorrectly.
296 The function could not allocate memory for its internal data.
298 A function block is already registered with the same name.
301 .Fn deregister_tcp_functions
302 function will fail if:
307 argument references the kernel's compiled-in default function block.
309 The function block is still in use by one or more sockets, or is defined as
310 the current default function block.
314 argument references a function block that is not currently registered.
322 This framework first appeared in
328 framework was written by
329 .An Randall Stewart Aq Mt rrs@FreeBSD.org .
331 This manual page was written by
332 .An Jonathan Looney Aq Mt jtl@FreeBSD.org .