1			    ======================
  2			    RxRPC NETWORK PROTOCOL
  3			    ======================
  4
  5The RxRPC protocol driver provides a reliable two-phase transport on top of UDP
  6that can be used to perform RxRPC remote operations.  This is done over sockets
  7of AF_RXRPC family, using sendmsg() and recvmsg() with control data to send and
  8receive data, aborts and errors.
  9
 10Contents of this document:
 11
 12 (*) Overview.
 13
 14 (*) RxRPC protocol summary.
 15
 16 (*) AF_RXRPC driver model.
 17
 18 (*) Control messages.
 19
 20 (*) Socket options.
 21
 22 (*) Security.
 23
 24 (*) Example client usage.
 25
 26 (*) Example server usage.
 27
 28 (*) AF_RXRPC kernel interface.
 29
 30 (*) Configurable parameters.
 31
 32
 33========
 34OVERVIEW
 35========
 36
 37RxRPC is a two-layer protocol.  There is a session layer which provides
 38reliable virtual connections using UDP over IPv4 (or IPv6) as the transport
 39layer, but implements a real network protocol; and there's the presentation
 40layer which renders structured data to binary blobs and back again using XDR
 41(as does SunRPC):
 42
 43		+-------------+
 44		| Application |
 45		+-------------+
 46		|     XDR     |		Presentation
 47		+-------------+
 48		|    RxRPC    |		Session
 49		+-------------+
 50		|     UDP     |		Transport
 51		+-------------+
 52
 53
 54AF_RXRPC provides:
 55
 56 (1) Part of an RxRPC facility for both kernel and userspace applications by
 57     making the session part of it a Linux network protocol (AF_RXRPC).
 58
 59 (2) A two-phase protocol.  The client transmits a blob (the request) and then
 60     receives a blob (the reply), and the server receives the request and then
 61     transmits the reply.
 62
 63 (3) Retention of the reusable bits of the transport system set up for one call
 64     to speed up subsequent calls.
 65
 66 (4) A secure protocol, using the Linux kernel's key retention facility to
 67     manage security on the client end.  The server end must of necessity be
 68     more active in security negotiations.
 69
 70AF_RXRPC does not provide XDR marshalling/presentation facilities.  That is
 71left to the application.  AF_RXRPC only deals in blobs.  Even the operation ID
 72is just the first four bytes of the request blob, and as such is beyond the
 73kernel's interest.
 74
 75
 76Sockets of AF_RXRPC family are:
 77
 78 (1) created as type SOCK_DGRAM;
 79
 80 (2) provided with a protocol of the type of underlying transport they're going
 81     to use - currently only PF_INET is supported.
 82
 83
 84The Andrew File System (AFS) is an example of an application that uses this and
 85that has both kernel (filesystem) and userspace (utility) components.
 86
 87
 88======================
 89RXRPC PROTOCOL SUMMARY
 90======================
 91
 92An overview of the RxRPC protocol:
 93
 94 (*) RxRPC sits on top of another networking protocol (UDP is the only option
 95     currently), and uses this to provide network transport.  UDP ports, for
 96     example, provide transport endpoints.
 97
 98 (*) RxRPC supports multiple virtual "connections" from any given transport
 99     endpoint, thus allowing the endpoints to be shared, even to the same
100     remote endpoint.
101
102 (*) Each connection goes to a particular "service".  A connection may not go
103     to multiple services.  A service may be considered the RxRPC equivalent of
104     a port number.  AF_RXRPC permits multiple services to share an endpoint.
105
106 (*) Client-originating packets are marked, thus a transport endpoint can be
107     shared between client and server connections (connections have a
108     direction).
109
110 (*) Up to a billion connections may be supported concurrently between one
111     local transport endpoint and one service on one remote endpoint.  An RxRPC
112     connection is described by seven numbers:
113
114	Local address	}
115	Local port	} Transport (UDP) address
116	Remote address	}
117	Remote port	}
118	Direction
119	Connection ID
120	Service ID
121
122 (*) Each RxRPC operation is a "call".  A connection may make up to four
123     billion calls, but only up to four calls may be in progress on a
124     connection at any one time.
125
126 (*) Calls are two-phase and asymmetric: the client sends its request data,
127     which the service receives; then the service transmits the reply data
128     which the client receives.
129
130 (*) The data blobs are of indefinite size, the end of a phase is marked with a
131     flag in the packet.  The number of packets of data making up one blob may
132     not exceed 4 billion, however, as this would cause the sequence number to
133     wrap.
134
135 (*) The first four bytes of the request data are the service operation ID.
136
137 (*) Security is negotiated on a per-connection basis.  The connection is
138     initiated by the first data packet on it arriving.  If security is
139     requested, the server then issues a "challenge" and then the client
140     replies with a "response".  If the response is successful, the security is
141     set for the lifetime of that connection, and all subsequent calls made
142     upon it use that same security.  In the event that the server lets a
143     connection lapse before the client, the security will be renegotiated if
144     the client uses the connection again.
145
146 (*) Calls use ACK packets to handle reliability.  Data packets are also
147     explicitly sequenced per call.
148
149 (*) There are two types of positive acknowledgment: hard-ACKs and soft-ACKs.
150     A hard-ACK indicates to the far side that all the data received to a point
151     has been received and processed; a soft-ACK indicates that the data has
152     been received but may yet be discarded and re-requested.  The sender may
153     not discard any transmittable packets until they've been hard-ACK'd.
154
155 (*) Reception of a reply data packet implicitly hard-ACK's all the data
156     packets that make up the request.
157
158 (*) An call is complete when the request has been sent, the reply has been
159     received and the final hard-ACK on the last packet of the reply has
160     reached the server.
161
162 (*) An call may be aborted by either end at any time up to its completion.
163
164
165=====================
166AF_RXRPC DRIVER MODEL
167=====================
168
169About the AF_RXRPC driver:
170
171 (*) The AF_RXRPC protocol transparently uses internal sockets of the transport
172     protocol to represent transport endpoints.
173
174 (*) AF_RXRPC sockets map onto RxRPC connection bundles.  Actual RxRPC
175     connections are handled transparently.  One client socket may be used to
176     make multiple simultaneous calls to the same service.  One server socket
177     may handle calls from many clients.
178
179 (*) Additional parallel client connections will be initiated to support extra
180     concurrent calls, up to a tunable limit.
181
182 (*) Each connection is retained for a certain amount of time [tunable] after
183     the last call currently using it has completed in case a new call is made
184     that could reuse it.
185
186 (*) Each internal UDP socket is retained [tunable] for a certain amount of
187     time [tunable] after the last connection using it discarded, in case a new
188     connection is made that could use it.
189
190 (*) A client-side connection is only shared between calls if they have have
191     the same key struct describing their security (and assuming the calls
192     would otherwise share the connection).  Non-secured calls would also be
193     able to share connections with each other.
194
195 (*) A server-side connection is shared if the client says it is.
196
197 (*) ACK'ing is handled by the protocol driver automatically, including ping
198     replying.
199
200 (*) SO_KEEPALIVE automatically pings the other side to keep the connection
201     alive [TODO].
202
203 (*) If an ICMP error is received, all calls affected by that error will be
204     aborted with an appropriate network error passed through recvmsg().
205
206
207Interaction with the user of the RxRPC socket:
208
209 (*) A socket is made into a server socket by binding an address with a
210     non-zero service ID.
211
212 (*) In the client, sending a request is achieved with one or more sendmsgs,
213     followed by the reply being received with one or more recvmsgs.
214
215 (*) The first sendmsg for a request to be sent from a client contains a tag to
216     be used in all other sendmsgs or recvmsgs associated with that call.  The
217     tag is carried in the control data.
218
219 (*) connect() is used to supply a default destination address for a client
220     socket.  This may be overridden by supplying an alternate address to the
221     first sendmsg() of a call (struct msghdr::msg_name).
222
223 (*) If connect() is called on an unbound client, a random local port will
224     bound before the operation takes place.
225
226 (*) A server socket may also be used to make client calls.  To do this, the
227     first sendmsg() of the call must specify the target address.  The server's
228     transport endpoint is used to send the packets.
229
230 (*) Once the application has received the last message associated with a call,
231     the tag is guaranteed not to be seen again, and so it can be used to pin
232     client resources.  A new call can then be initiated with the same tag
233     without fear of interference.
234
235 (*) In the server, a request is received with one or more recvmsgs, then the
236     the reply is transmitted with one or more sendmsgs, and then the final ACK
237     is received with a last recvmsg.
238
239 (*) When sending data for a call, sendmsg is given MSG_MORE if there's more
240     data to come on that call.
241
242 (*) When receiving data for a call, recvmsg flags MSG_MORE if there's more
243     data to come for that call.
244
245 (*) When receiving data or messages for a call, MSG_EOR is flagged by recvmsg
246     to indicate the terminal message for that call.
247
248 (*) A call may be aborted by adding an abort control message to the control
249     data.  Issuing an abort terminates the kernel's use of that call's tag.
250     Any messages waiting in the receive queue for that call will be discarded.
251
252 (*) Aborts, busy notifications and challenge packets are delivered by recvmsg,
253     and control data messages will be set to indicate the context.  Receiving
254     an abort or a busy message terminates the kernel's use of that call's tag.
255
256 (*) The control data part of the msghdr struct is used for a number of things:
257
258     (*) The tag of the intended or affected call.
259
260     (*) Sending or receiving errors, aborts and busy notifications.
261
262     (*) Notifications of incoming calls.
263
264     (*) Sending debug requests and receiving debug replies [TODO].
265
266 (*) When the kernel has received and set up an incoming call, it sends a
267     message to server application to let it know there's a new call awaiting
268     its acceptance [recvmsg reports a special control message].  The server
269     application then uses sendmsg to assign a tag to the new call.  Once that
270     is done, the first part of the request data will be delivered by recvmsg.
271
272 (*) The server application has to provide the server socket with a keyring of
273     secret keys corresponding to the security types it permits.  When a secure
274     connection is being set up, the kernel looks up the appropriate secret key
275     in the keyring and then sends a challenge packet to the client and
276     receives a response packet.  The kernel then checks the authorisation of
277     the packet and either aborts the connection or sets up the security.
278
279 (*) The name of the key a client will use to secure its communications is
280     nominated by a socket option.
281
282
283Notes on recvmsg:
284
285 (*) If there's a sequence of data messages belonging to a particular call on
286     the receive queue, then recvmsg will keep working through them until:
287
288     (a) it meets the end of that call's received data,
289
290     (b) it meets a non-data message,
291
292     (c) it meets a message belonging to a different call, or
293
294     (d) it fills the user buffer.
295
296     If recvmsg is called in blocking mode, it will keep sleeping, awaiting the
297     reception of further data, until one of the above four conditions is met.
298
299 (2) MSG_PEEK operates similarly, but will return immediately if it has put any
300     data in the buffer rather than sleeping until it can fill the buffer.
301
302 (3) If a data message is only partially consumed in filling a user buffer,
303     then the remainder of that message will be left on the front of the queue
304     for the next taker.  MSG_TRUNC will never be flagged.
305
306 (4) If there is more data to be had on a call (it hasn't copied the last byte
307     of the last data message in that phase yet), then MSG_MORE will be
308     flagged.
309
310
311================
312CONTROL MESSAGES
313================
314
315AF_RXRPC makes use of control messages in sendmsg() and recvmsg() to multiplex
316calls, to invoke certain actions and to report certain conditions.  These are:
317
318	MESSAGE ID		SRT DATA	MEANING
319	=======================	=== ===========	===============================
320	RXRPC_USER_CALL_ID	sr- User ID	App's call specifier
321	RXRPC_ABORT		srt Abort code	Abort code to issue/received
322	RXRPC_ACK		-rt n/a		Final ACK received
323	RXRPC_NET_ERROR		-rt error num	Network error on call
324	RXRPC_BUSY		-rt n/a		Call rejected (server busy)
325	RXRPC_LOCAL_ERROR	-rt error num	Local error encountered
326	RXRPC_NEW_CALL		-r- n/a		New call received
327	RXRPC_ACCEPT		s-- n/a		Accept new call
328
329	(SRT = usable in Sendmsg / delivered by Recvmsg / Terminal message)
330
331 (*) RXRPC_USER_CALL_ID
332
333     This is used to indicate the application's call ID.  It's an unsigned long
334     that the app specifies in the client by attaching it to the first data
335     message or in the server by passing it in association with an RXRPC_ACCEPT
336     message.  recvmsg() passes it in conjunction with all messages except
337     those of the RXRPC_NEW_CALL message.
338
339 (*) RXRPC_ABORT
340
341     This is can be used by an application to abort a call by passing it to
342     sendmsg, or it can be delivered by recvmsg to indicate a remote abort was
343     received.  Either way, it must be associated with an RXRPC_USER_CALL_ID to
344     specify the call affected.  If an abort is being sent, then error EBADSLT
345     will be returned if there is no call with that user ID.
346
347 (*) RXRPC_ACK
348
349     This is delivered to a server application to indicate that the final ACK
350     of a call was received from the client.  It will be associated with an
351     RXRPC_USER_CALL_ID to indicate the call that's now complete.
352
353 (*) RXRPC_NET_ERROR
354
355     This is delivered to an application to indicate that an ICMP error message
356     was encountered in the process of trying to talk to the peer.  An
357     errno-class integer value will be included in the control message data
358     indicating the problem, and an RXRPC_USER_CALL_ID will indicate the call
359     affected.
360
361 (*) RXRPC_BUSY
362
363     This is delivered to a client application to indicate that a call was
364     rejected by the server due to the server being busy.  It will be
365     associated with an RXRPC_USER_CALL_ID to indicate the rejected call.
366
367 (*) RXRPC_LOCAL_ERROR
368
369     This is delivered to an application to indicate that a local error was
370     encountered and that a call has been aborted because of it.  An
371     errno-class integer value will be included in the control message data
372     indicating the problem, and an RXRPC_USER_CALL_ID will indicate the call
373     affected.
374
375 (*) RXRPC_NEW_CALL
376
377     This is delivered to indicate to a server application that a new call has
378     arrived and is awaiting acceptance.  No user ID is associated with this,
379     as a user ID must subsequently be assigned by doing an RXRPC_ACCEPT.
380
381 (*) RXRPC_ACCEPT
382
383     This is used by a server application to attempt to accept a call and
384     assign it a user ID.  It should be associated with an RXRPC_USER_CALL_ID
385     to indicate the user ID to be assigned.  If there is no call to be
386     accepted (it may have timed out, been aborted, etc.), then sendmsg will
387     return error ENODATA.  If the user ID is already in use by another call,
388     then error EBADSLT will be returned.
389
390
391==============
392SOCKET OPTIONS
393==============
394
395AF_RXRPC sockets support a few socket options at the SOL_RXRPC level:
396
397 (*) RXRPC_SECURITY_KEY
398
399     This is used to specify the description of the key to be used.  The key is
400     extracted from the calling process's keyrings with request_key() and
401     should be of "rxrpc" type.
402
403     The optval pointer points to the description string, and optlen indicates
404     how long the string is, without the NUL terminator.
405
406 (*) RXRPC_SECURITY_KEYRING
407
408     Similar to above but specifies a keyring of server secret keys to use (key
409     type "keyring").  See the "Security" section.
410
411 (*) RXRPC_EXCLUSIVE_CONNECTION
412
413     This is used to request that new connections should be used for each call
414     made subsequently on this socket.  optval should be NULL and optlen 0.
415
416 (*) RXRPC_MIN_SECURITY_LEVEL
417
418     This is used to specify the minimum security level required for calls on
419     this socket.  optval must point to an int containing one of the following
420     values:
421
422     (a) RXRPC_SECURITY_PLAIN
423
424	 Encrypted checksum only.
425
426     (b) RXRPC_SECURITY_AUTH
427
428	 Encrypted checksum plus packet padded and first eight bytes of packet
429	 encrypted - which includes the actual packet length.
430
431     (c) RXRPC_SECURITY_ENCRYPTED
432
433	 Encrypted checksum plus entire packet padded and encrypted, including
434	 actual packet length.
435
436
437========
438SECURITY
439========
440
441Currently, only the kerberos 4 equivalent protocol has been implemented
442(security index 2 - rxkad).  This requires the rxkad module to be loaded and,
443on the client, tickets of the appropriate type to be obtained from the AFS
444kaserver or the kerberos server and installed as "rxrpc" type keys.  This is
445normally done using the klog program.  An example simple klog program can be
446found at:
447
448	http://people.redhat.com/~dhowells/rxrpc/klog.c
449
450The payload provided to add_key() on the client should be of the following
451form:
452
453	struct rxrpc_key_sec2_v1 {
454		uint16_t	security_index;	/* 2 */
455		uint16_t	ticket_length;	/* length of ticket[] */
456		uint32_t	expiry;		/* time at which expires */
457		uint8_t		kvno;		/* key version number */
458		uint8_t		__pad[3];
459		uint8_t		session_key[8];	/* DES session key */
460		uint8_t		ticket[0];	/* the encrypted ticket */
461	};
462
463Where the ticket blob is just appended to the above structure.
464
465
466For the server, keys of type "rxrpc_s" must be made available to the server.
467They have a description of "<serviceID>:<securityIndex>" (eg: "52:2" for an
468rxkad key for the AFS VL service).  When such a key is created, it should be
469given the server's secret key as the instantiation data (see the example
470below).
471
472	add_key("rxrpc_s", "52:2", secret_key, 8, keyring);
473
474A keyring is passed to the server socket by naming it in a sockopt.  The server
475socket then looks the server secret keys up in this keyring when secure
476incoming connections are made.  This can be seen in an example program that can
477be found at:
478
479	http://people.redhat.com/~dhowells/rxrpc/listen.c
480
481
482====================
483EXAMPLE CLIENT USAGE
484====================
485
486A client would issue an operation by:
487
488 (1) An RxRPC socket is set up by:
489
490	client = socket(AF_RXRPC, SOCK_DGRAM, PF_INET);
491
492     Where the third parameter indicates the protocol family of the transport
493     socket used - usually IPv4 but it can also be IPv6 [TODO].
494
495 (2) A local address can optionally be bound:
496
497	struct sockaddr_rxrpc srx = {
498		.srx_family	= AF_RXRPC,
499		.srx_service	= 0,  /* we're a client */
500		.transport_type	= SOCK_DGRAM,	/* type of transport socket */
501		.transport.sin_family	= AF_INET,
502		.transport.sin_port	= htons(7000), /* AFS callback */
503		.transport.sin_address	= 0,  /* all local interfaces */
504	};
505	bind(client, &srx, sizeof(srx));
506
507     This specifies the local UDP port to be used.  If not given, a random
508     non-privileged port will be used.  A UDP port may be shared between
509     several unrelated RxRPC sockets.  Security is handled on a basis of
510     per-RxRPC virtual connection.
511
512 (3) The security is set:
513
514	const char *key = "AFS:cambridge.redhat.com";
515	setsockopt(client, SOL_RXRPC, RXRPC_SECURITY_KEY, key, strlen(key));
516
517     This issues a request_key() to get the key representing the security
518     context.  The minimum security level can be set:
519
520	unsigned int sec = RXRPC_SECURITY_ENCRYPTED;
521	setsockopt(client, SOL_RXRPC, RXRPC_MIN_SECURITY_LEVEL,
522		   &sec, sizeof(sec));
523
524 (4) The server to be contacted can then be specified (alternatively this can
525     be done through sendmsg):
526
527	struct sockaddr_rxrpc srx = {
528		.srx_family	= AF_RXRPC,
529		.srx_service	= VL_SERVICE_ID,
530		.transport_type	= SOCK_DGRAM,	/* type of transport socket */
531		.transport.sin_family	= AF_INET,
532		.transport.sin_port	= htons(7005), /* AFS volume manager */
533		.transport.sin_address	= ...,
534	};
535	connect(client, &srx, sizeof(srx));
536
537 (5) The request data should then be posted to the server socket using a series
538     of sendmsg() calls, each with the following control message attached:
539
540	RXRPC_USER_CALL_ID	- specifies the user ID for this call
541
542     MSG_MORE should be set in msghdr::msg_flags on all but the last part of
543     the request.  Multiple requests may be made simultaneously.
544
545     If a call is intended to go to a destination other than the default
546     specified through connect(), then msghdr::msg_name should be set on the
547     first request message of that call.
548
549 (6) The reply data will then be posted to the server socket for recvmsg() to
550     pick up.  MSG_MORE will be flagged by recvmsg() if there's more reply data
551     for a particular call to be read.  MSG_EOR will be set on the terminal
552     read for a call.
553
554     All data will be delivered with the following control message attached:
555
556	RXRPC_USER_CALL_ID	- specifies the user ID for this call
557
558     If an abort or error occurred, this will be returned in the control data
559     buffer instead, and MSG_EOR will be flagged to indicate the end of that
560     call.
561
562
563====================
564EXAMPLE SERVER USAGE
565====================
566
567A server would be set up to accept operations in the following manner:
568
569 (1) An RxRPC socket is created by:
570
571	server = socket(AF_RXRPC, SOCK_DGRAM, PF_INET);
572
573     Where the third parameter indicates the address type of the transport
574     socket used - usually IPv4.
575
576 (2) Security is set up if desired by giving the socket a keyring with server
577     secret keys in it:
578
579	keyring = add_key("keyring", "AFSkeys", NULL, 0,
580			  KEY_SPEC_PROCESS_KEYRING);
581
582	const char secret_key[8] = {
583		0xa7, 0x83, 0x8a, 0xcb, 0xc7, 0x83, 0xec, 0x94 };
584	add_key("rxrpc_s", "52:2", secret_key, 8, keyring);
585
586	setsockopt(server, SOL_RXRPC, RXRPC_SECURITY_KEYRING, "AFSkeys", 7);
587
588     The keyring can be manipulated after it has been given to the socket. This
589     permits the server to add more keys, replace keys, etc. whilst it is live.
590
591 (2) A local address must then be bound:
592
593	struct sockaddr_rxrpc srx = {
594		.srx_family	= AF_RXRPC,
595		.srx_service	= VL_SERVICE_ID, /* RxRPC service ID */
596		.transport_type	= SOCK_DGRAM,	/* type of transport socket */
597		.transport.sin_family	= AF_INET,
598		.transport.sin_port	= htons(7000), /* AFS callback */
599		.transport.sin_address	= 0,  /* all local interfaces */
600	};
601	bind(server, &srx, sizeof(srx));
602
603 (3) The server is then set to listen out for incoming calls:
604
605	listen(server, 100);
606
607 (4) The kernel notifies the server of pending incoming connections by sending
608     it a message for each.  This is received with recvmsg() on the server
609     socket.  It has no data, and has a single dataless control message
610     attached:
611
612	RXRPC_NEW_CALL
613
614     The address that can be passed back by recvmsg() at this point should be
615     ignored since the call for which the message was posted may have gone by
616     the time it is accepted - in which case the first call still on the queue
617     will be accepted.
618
619 (5) The server then accepts the new call by issuing a sendmsg() with two
620     pieces of control data and no actual data:
621
622	RXRPC_ACCEPT		- indicate connection acceptance
623	RXRPC_USER_CALL_ID	- specify user ID for this call
624
625 (6) The first request data packet will then be posted to the server socket for
626     recvmsg() to pick up.  At that point, the RxRPC address for the call can
627     be read from the address fields in the msghdr struct.
628
629     Subsequent request data will be posted to the server socket for recvmsg()
630     to collect as it arrives.  All but the last piece of the request data will
631     be delivered with MSG_MORE flagged.
632
633     All data will be delivered with the following control message attached:
634
635	RXRPC_USER_CALL_ID	- specifies the user ID for this call
636
637 (8) The reply data should then be posted to the server socket using a series
638     of sendmsg() calls, each with the following control messages attached:
639
640	RXRPC_USER_CALL_ID	- specifies the user ID for this call
641
642     MSG_MORE should be set in msghdr::msg_flags on all but the last message
643     for a particular call.
644
645 (9) The final ACK from the client will be posted for retrieval by recvmsg()
646     when it is received.  It will take the form of a dataless message with two
647     control messages attached:
648
649	RXRPC_USER_CALL_ID	- specifies the user ID for this call
650	RXRPC_ACK		- indicates final ACK (no data)
651
652     MSG_EOR will be flagged to indicate that this is the final message for
653     this call.
654
655(10) Up to the point the final packet of reply data is sent, the call can be
656     aborted by calling sendmsg() with a dataless message with the following
657     control messages attached:
658
659	RXRPC_USER_CALL_ID	- specifies the user ID for this call
660	RXRPC_ABORT		- indicates abort code (4 byte data)
661
662     Any packets waiting in the socket's receive queue will be discarded if
663     this is issued.
664
665Note that all the communications for a particular service take place through
666the one server socket, using control messages on sendmsg() and recvmsg() to
667determine the call affected.
668
669
670=========================
671AF_RXRPC KERNEL INTERFACE
672=========================
673
674The AF_RXRPC module also provides an interface for use by in-kernel utilities
675such as the AFS filesystem.  This permits such a utility to:
676
677 (1) Use different keys directly on individual client calls on one socket
678     rather than having to open a whole slew of sockets, one for each key it
679     might want to use.
680
681 (2) Avoid having RxRPC call request_key() at the point of issue of a call or
682     opening of a socket.  Instead the utility is responsible for requesting a
683     key at the appropriate point.  AFS, for instance, would do this during VFS
684     operations such as open() or unlink().  The key is then handed through
685     when the call is initiated.
686
687 (3) Request the use of something other than GFP_KERNEL to allocate memory.
688
689 (4) Avoid the overhead of using the recvmsg() call.  RxRPC messages can be
690     intercepted before they get put into the socket Rx queue and the socket
691     buffers manipulated directly.
692
693To use the RxRPC facility, a kernel utility must still open an AF_RXRPC socket,
694bind an address as appropriate and listen if it's to be a server socket, but
695then it passes this to the kernel interface functions.
696
697The kernel interface functions are as follows:
698
699 (*) Begin a new client call.
700
701	struct rxrpc_call *
702	rxrpc_kernel_begin_call(struct socket *sock,
703				struct sockaddr_rxrpc *srx,
704				struct key *key,
705				unsigned long user_call_ID,
706				gfp_t gfp);
707
708     This allocates the infrastructure to make a new RxRPC call and assigns
709     call and connection numbers.  The call will be made on the UDP port that
710     the socket is bound to.  The call will go to the destination address of a
711     connected client socket unless an alternative is supplied (srx is
712     non-NULL).
713
714     If a key is supplied then this will be used to secure the call instead of
715     the key bound to the socket with the RXRPC_SECURITY_KEY sockopt.  Calls
716     secured in this way will still share connections if at all possible.
717
718     The user_call_ID is equivalent to that supplied to sendmsg() in the
719     control data buffer.  It is entirely feasible to use this to point to a
720     kernel data structure.
721
722     If this function is successful, an opaque reference to the RxRPC call is
723     returned.  The caller now holds a reference on this and it must be
724     properly ended.
725
726 (*) End a client call.
727
728	void rxrpc_kernel_end_call(struct socket *sock,
729				   struct rxrpc_call *call);
730
731     This is used to end a previously begun call.  The user_call_ID is expunged
732     from AF_RXRPC's knowledge and will not be seen again in association with
733     the specified call.
734
735 (*) Send data through a call.
736
737	int rxrpc_kernel_send_data(struct socket *sock,
738				   struct rxrpc_call *call,
739				   struct msghdr *msg,
740				   size_t len);
741
742     This is used to supply either the request part of a client call or the
743     reply part of a server call.  msg.msg_iovlen and msg.msg_iov specify the
744     data buffers to be used.  msg_iov may not be NULL and must point
745     exclusively to in-kernel virtual addresses.  msg.msg_flags may be given
746     MSG_MORE if there will be subsequent data sends for this call.
747
748     The msg must not specify a destination address, control data or any flags
749     other than MSG_MORE.  len is the total amount of data to transmit.
750
751 (*) Receive data from a call.
752
753	int rxrpc_kernel_recv_data(struct socket *sock,
754				   struct rxrpc_call *call,
755				   void *buf,
756				   size_t size,
757				   size_t *_offset,
758				   bool want_more,
759				   u32 *_abort)
760
761      This is used to receive data from either the reply part of a client call
762      or the request part of a service call.  buf and size specify how much
763      data is desired and where to store it.  *_offset is added on to buf and
764      subtracted from size internally; the amount copied into the buffer is
765      added to *_offset before returning.
766
767      want_more should be true if further data will be required after this is
768      satisfied and false if this is the last item of the receive phase.
769
770      There are three normal returns: 0 if the buffer was filled and want_more
771      was true; 1 if the buffer was filled, the last DATA packet has been
772      emptied and want_more was false; and -EAGAIN if the function needs to be
773      called again.
774
775      If the last DATA packet is processed but the buffer contains less than
776      the amount requested, EBADMSG is returned.  If want_more wasn't set, but
777      more data was available, EMSGSIZE is returned.
778
779      If a remote ABORT is detected, the abort code received will be stored in
780      *_abort and ECONNABORTED will be returned.
781
782 (*) Abort a call.
783
784	void rxrpc_kernel_abort_call(struct socket *sock,
785				     struct rxrpc_call *call,
786				     u32 abort_code);
787
788     This is used to abort a call if it's still in an abortable state.  The
789     abort code specified will be placed in the ABORT message sent.
790
791 (*) Intercept received RxRPC messages.
792
793	typedef void (*rxrpc_interceptor_t)(struct sock *sk,
794					    unsigned long user_call_ID,
795					    struct sk_buff *skb);
796
797	void
798	rxrpc_kernel_intercept_rx_messages(struct socket *sock,
799					   rxrpc_interceptor_t interceptor);
800
801     This installs an interceptor function on the specified AF_RXRPC socket.
802     All messages that would otherwise wind up in the socket's Rx queue are
803     then diverted to this function.  Note that care must be taken to process
804     the messages in the right order to maintain DATA message sequentiality.
805
806     The interceptor function itself is provided with the address of the socket
807     and handling the incoming message, the ID assigned by the kernel utility
808     to the call and the socket buffer containing the message.
809
810     The skb->mark field indicates the type of message:
811
812	MARK				MEANING
813	===============================	=======================================
814	RXRPC_SKB_MARK_DATA		Data message
815	RXRPC_SKB_MARK_FINAL_ACK	Final ACK received for an incoming call
816	RXRPC_SKB_MARK_BUSY		Client call rejected as server busy
817	RXRPC_SKB_MARK_REMOTE_ABORT	Call aborted by peer
818	RXRPC_SKB_MARK_NET_ERROR	Network error detected
819	RXRPC_SKB_MARK_LOCAL_ERROR	Local error encountered
820	RXRPC_SKB_MARK_NEW_CALL		New incoming call awaiting acceptance
821
822     The remote abort message can be probed with rxrpc_kernel_get_abort_code().
823     The two error messages can be probed with rxrpc_kernel_get_error_number().
824     A new call can be accepted with rxrpc_kernel_accept_call().
825
826     Data messages can have their contents extracted with the usual bunch of
827     socket buffer manipulation functions.  A data message can be determined to
828     be the last one in a sequence with rxrpc_kernel_is_data_last().  When a
829     data message has been used up, rxrpc_kernel_data_consumed() should be
830     called on it.
831
832     Messages should be handled to rxrpc_kernel_free_skb() to dispose of.  It
833     is possible to get extra refs on all types of message for later freeing,
834     but this may pin the state of a call until the message is finally freed.
835
836 (*) Accept an incoming call.
837
838	struct rxrpc_call *
839	rxrpc_kernel_accept_call(struct socket *sock,
840				 unsigned long user_call_ID);
841
842     This is used to accept an incoming call and to assign it a call ID.  This
843     function is similar to rxrpc_kernel_begin_call() and calls accepted must
844     be ended in the same way.
845
846     If this function is successful, an opaque reference to the RxRPC call is
847     returned.  The caller now holds a reference on this and it must be
848     properly ended.
849
850 (*) Reject an incoming call.
851
852	int rxrpc_kernel_reject_call(struct socket *sock);
853
854     This is used to reject the first incoming call on the socket's queue with
855     a BUSY message.  -ENODATA is returned if there were no incoming calls.
856     Other errors may be returned if the call had been aborted (-ECONNABORTED)
857     or had timed out (-ETIME).
858
859 (*) Allocate a null key for doing anonymous security.
860
861	struct key *rxrpc_get_null_key(const char *keyname);
862
863     This is used to allocate a null RxRPC key that can be used to indicate
864     anonymous security for a particular domain.
865
866 (*) Get the peer address of a call.
867
868	void rxrpc_kernel_get_peer(struct socket *sock, struct rxrpc_call *call,
869				   struct sockaddr_rxrpc *_srx);
870
871     This is used to find the remote peer address of a call.
872
873
874=======================
875CONFIGURABLE PARAMETERS
876=======================
877
878The RxRPC protocol driver has a number of configurable parameters that can be
879adjusted through sysctls in /proc/net/rxrpc/:
880
881 (*) req_ack_delay
882
883     The amount of time in milliseconds after receiving a packet with the
884     request-ack flag set before we honour the flag and actually send the
885     requested ack.
886
887     Usually the other side won't stop sending packets until the advertised
888     reception window is full (to a maximum of 255 packets), so delaying the
889     ACK permits several packets to be ACK'd in one go.
890
891 (*) soft_ack_delay
892
893     The amount of time in milliseconds after receiving a new packet before we
894     generate a soft-ACK to tell the sender that it doesn't need to resend.
895
896 (*) idle_ack_delay
897
898     The amount of time in milliseconds after all the packets currently in the
899     received queue have been consumed before we generate a hard-ACK to tell
900     the sender it can free its buffers, assuming no other reason occurs that
901     we would send an ACK.
902
903 (*) resend_timeout
904
905     The amount of time in milliseconds after transmitting a packet before we
906     transmit it again, assuming no ACK is received from the receiver telling
907     us they got it.
908
909 (*) max_call_lifetime
910
911     The maximum amount of time in seconds that a call may be in progress
912     before we preemptively kill it.
913
914 (*) dead_call_expiry
915
916     The amount of time in seconds before we remove a dead call from the call
917     list.  Dead calls are kept around for a little while for the purpose of
918     repeating ACK and ABORT packets.
919
920 (*) connection_expiry
921
922     The amount of time in seconds after a connection was last used before we
923     remove it from the connection list.  Whilst a connection is in existence,
924     it serves as a placeholder for negotiated security; when it is deleted,
925     the security must be renegotiated.
926
927 (*) transport_expiry
928
929     The amount of time in seconds after a transport was last used before we
930     remove it from the transport list.  Whilst a transport is in existence, it
931     serves to anchor the peer data and keeps the connection ID counter.
932
933 (*) rxrpc_rx_window_size
934
935     The size of the receive window in packets.  This is the maximum number of
936     unconsumed received packets we're willing to hold in memory for any
937     particular call.
938
939 (*) rxrpc_rx_mtu
940
941     The maximum packet MTU size that we're willing to receive in bytes.  This
942     indicates to the peer whether we're willing to accept jumbo packets.
943
944 (*) rxrpc_rx_jumbo_max
945
946     The maximum number of packets that we're willing to accept in a jumbo
947     packet.  Non-terminal packets in a jumbo packet must contain a four byte
948     header plus exactly 1412 bytes of data.  The terminal packet must contain
949     a four byte header plus any amount of data.  In any event, a jumbo packet
950     may not exceed rxrpc_rx_mtu in size.