1.\" 2.\" Copyright (c) 2016 Jonathan Looney <jtl@FreeBSD.org> 3.\" All rights reserved. 4.\" 5.\" Redistribution and use in source and binary forms, with or without 6.\" modification, are permitted provided that the following conditions 7.\" are met: 8.\" 1. Redistributions of source code must retain the above copyright 9.\" notice, this list of conditions and the following disclaimer. 10.\" 2. Redistributions in binary form must reproduce the above copyright 11.\" notice, this list of conditions and the following disclaimer in the 12.\" documentation and/or other materials provided with the distribution. 13.\" 14.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND 15.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 16.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 17.\" ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE FOR 18.\" ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 19.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 20.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 21.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 22.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 23.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 24.\" SUCH DAMAGE. 25.\" 26.\" $FreeBSD$ 27.\" 28.Dd June 28, 2016 29.Dt TCP_FUNCTIONS 9 30.Os 31.Sh NAME 32.Nm tcp_functions 33.Nd Alternate TCP Stack Framework 34.Sh SYNOPSIS 35.In netinet/tcp.h 36.In netinet/tcp_var.h 37.Ft int 38.Fn register_tcp_functions "struct tcp_function_block *blk" "int wait" 39.Ft int 40.Fn deregister_tcp_functions "struct tcp_function_block *blk" 41.Sh DESCRIPTION 42The 43.Nm 44framework allows a kernel developer to implement alternate TCP stacks. 45The alternate stacks can be compiled in the kernel or can be implemented in 46loadable kernel modules. 47This functionality is intended to encourage experimentation with the TCP stack 48and to allow alternate behaviors to be deployed for different TCP connections 49on a single system. 50.Pp 51A system administrator can set a system default stack. 52By default, all TCP connections will use the system default stack. 53Additionally, users can specify a particular stack to use on a per-connection 54basis. 55(See 56.Xr tcp 4 57for details on setting the system default stack, or selecting a specific stack 58for a given connection.) 59.Pp 60This man page treats "TCP stacks" as synonymous with "function blocks". 61This is intentional. 62A "TCP stack" is a collection of functions that implement a set of behavior. 63Therefore, an alternate "function block" defines an alternate "TCP stack". 64.Pp 65.Nm 66modules must call the 67.Fn register_tcp_functions 68function during initialization and successfully call the 69.Fn deregister_tcp_functions 70function prior to allowing the module to be unloaded. 71.Pp 72The 73.Fn register_tcp_functions 74function requests that the system add a specified function block to the system. 75.Pp 76The 77.Fn deregister_tcp_functions 78function requests that the system remove a specified function block from the 79system. 80If the call fails because sockets are still using the specified function block, 81the system will mark the function block as being in the process of being 82removed. 83This will prevent additional sockets from using the specified function block. 84However, it will not impact sockets that are already using the function block. 85.Pp 86The 87.Fa blk 88argument is a pointer to a 89.Vt "struct tcp_function_block" , 90which is explained below (see 91.Sx Function Block Structure ) . 92The 93.Fa wait 94argument is used as the 95.Fa flags 96argument to 97.Xr malloc 9 , 98and must be set to one of the valid values defined in that man page. 99.Ss Function Block Structure 100The 101.Fa blk argument is a pointer to a 102.Vt "struct tcp_function_block" , 103which has the following members: 104.Bd -literal -offset indent 105struct 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 *, 110 int, int, uint8_t, 111 int); 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 *); 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 *, 121 uint32_t, u_int); 122 int (*tfb_tcp_timer_active)(struct tcpcb *, uint32_t); 123 void (*tfb_tcp_timer_stop)(struct tcpcb *, uint32_t); 124 void (*tfb_tcp_rexmit_tmr)(struct tcpcb *); 125 volatile uint32_t tfb_refcnt; 126 uint32_t tfb_flags; 127}; 128.Ed 129.Pp 130The 131.Va tfb_tcp_block_name 132field identifies the unique name of the TCP stack, and should be no longer than 133TCP_FUNCTION_NAME_LEN_MAX-1 characters in length. 134.Pp 135The 136.Va tfb_tcp_output , 137.Va tfb_tcp_do_segment , 138and 139.Va tfb_tcp_ctloutput 140fields are pointers to functions that perform the equivalent actions 141as the default 142.Fn tcp_output , 143.Fn tcp_do_segment , 144and 145.Fn tcp_default_ctloutput 146functions, respectively. 147Each of these function pointers must be non-NULL. 148.Pp 149If a TCP stack needs to initialize data when a socket first selects the TCP 150stack (or, when the socket is first opened), it should set a non-NULL 151pointer in the 152.Va tfb_tcp_fb_init 153field. 154Likewise, if a TCP stack needs to cleanup data when a socket stops using the 155TCP stack (or, when the socket is closed), it should set a non-NULL pointer 156in the 157.Va tfb_tcp_fb_fini 158field. 159.Pp 160If the TCP stack implements additional timers, the TCP stack should set a 161non-NULL pointer in the 162.Va tfb_tcp_timer_stop_all , 163.Va tfb_tcp_timer_activate , 164.Va tfb_tcp_timer_active , 165and 166.Va tfb_tcp_timer_stop 167fields. 168These fields should all be 169.Dv NULL 170or should all contain pointers to functions. 171The 172.Va tfb_tcp_timer_activate , 173.Va tfb_tcp_timer_active , 174and 175.Va tfb_tcp_timer_stop 176functions will be called when the 177.Fn tcp_timer_activate , 178.Fn tcp_timer_active , 179and 180.Fn tcp_timer_stop 181functions, respectively, are called with a timer type other than the standard 182types. 183The functions defined by the TCP stack have the same semantics (both for 184arguments and return values) as the normal timer functions they supplement. 185.Pp 186Additionally, a stack may define its own actions to take when the retransmit 187timer fires by setting a non-NULL function pointer in the 188.Va tfb_tcp_rexmit_tmr 189field. 190This function is called very early in the process of handling a retransmit 191timer. 192However, care must be taken to ensure the retransmit timer leaves the 193TCP control block in a valid state for the remainder of the retransmit 194timer logic. 195.Pp 196The 197.Va tfb_refcnt 198and 199.Va tfb_flags 200fields are used by the kernel's TCP code and will be initialized when the 201TCP stack is registered. 202.Ss Requirements for Alternate TCP Stacks 203If the TCP stack needs to store data beyond what is stored in the default 204TCP control block, the TCP stack can initialize its own per-connection storage. 205The 206.Va t_fb_ptr 207field in the 208.Vt "struct tcpcb" 209control block structure has been reserved to hold a pointer to this 210per-connection storage. 211If the TCP stack uses this alternate storage, it should understand that the 212value of the 213.Va t_fb_ptr 214pointer may not be initialized to 215.Dv NULL . 216Therefore, it should use a 217.Va tfb_tcp_fb_init 218function to initialize this field. 219Additionally, it should use a 220.Va tfb_tcp_fb_fini 221function to deallocate storage when the socket is closed. 222.Pp 223It is understood that alternate TCP stacks may keep different sets of data. 224However, in order to ensure that data is available to both the user and the 225rest of the system in a standardized format, alternate TCP stacks must 226update all fields in the TCP control block to the greatest extent practical. 227.Sh RETURN VALUES 228The 229.Fn register_tcp_functions 230and 231.Fn deregister_tcp_functions 232functions return zero on success and non-zero on failure. 233In particular, the 234.Fn deregister_tcp_functions 235will return 236.Er EBUSY 237until no more connections are using the specified TCP stack. 238A module calling 239.Fn deregister_tcp_functions 240must be prepared to wait until all connections have stopped using the 241specified TCP stack. 242.Sh ERRORS 243The 244.Fn register_tcp_functions 245function will fail if: 246.Bl -tag -width Er 247.It Bq Er EINVAL 248Any of the members of the 249.Fa blk 250argument are set incorrectly. 251.It Bq Er ENOMEM 252The function could not allocate memory for its internal data. 253.It Bq Er EALREADY 254A function block is already registered with the same name. 255.El 256The 257.Fn deregister_tcp_functions 258function will fail if: 259.Bl -tag -width Er 260.It Bq Er EPERM 261The 262.Fa blk 263argument references the kernel's compiled-in default function block. 264.It Bq Er EBUSY 265The function block is still in use by one or more sockets, or is defined as 266the current default function block. 267.It Bq Er ENOENT 268The 269.Fa blk 270argument references a function block that is not currently registered. 271.Sh SEE ALSO 272.Xr malloc 9 , 273.Xr tcp 4 274.Sh HISTORY 275This framework first appeared in 276.Fx 11.0 . 277.Sh AUTHORS 278.An -nosplit 279The 280.Nm 281framework was written by 282.An Randall Stewart Aq Mt rrs@FreeBSD.org . 283.Pp 284This manual page was written by 285.An Jonathan Looney Aq Mt jtl@FreeBSD.org . 286