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Based on kernel version 3.15.4. Page generated on 2014-07-07 09:04 EST.

1				    ======================
2				    RxRPC NETWORK PROTOCOL
3				    ======================
4	
5	The RxRPC protocol driver provides a reliable two-phase transport on top of UDP
6	that can be used to perform RxRPC remote operations.  This is done over sockets
7	of AF_RXRPC family, using sendmsg() and recvmsg() with control data to send and
8	receive data, aborts and errors.
9	
10	Contents 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	========
34	OVERVIEW
35	========
36	
37	RxRPC is a two-layer protocol.  There is a session layer which provides
38	reliable virtual connections using UDP over IPv4 (or IPv6) as the transport
39	layer, but implements a real network protocol; and there's the presentation
40	layer 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	
54	AF_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	
70	AF_RXRPC does not provide XDR marshalling/presentation facilities.  That is
71	left to the application.  AF_RXRPC only deals in blobs.  Even the operation ID
72	is just the first four bytes of the request blob, and as such is beyond the
73	kernel's interest.
74	
75	
76	Sockets 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	
84	The Andrew File System (AFS) is an example of an application that uses this and
85	that has both kernel (filesystem) and userspace (utility) components.
86	
87	
88	======================
89	RXRPC PROTOCOL SUMMARY
90	======================
91	
92	An 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	=====================
166	AF_RXRPC DRIVER MODEL
167	=====================
168	
169	About 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	
207	Interaction 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	
283	Notes 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	================
312	CONTROL MESSAGES
313	================
314	
315	AF_RXRPC makes use of control messages in sendmsg() and recvmsg() to multiplex
316	calls, 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	==============
392	SOCKET OPTIONS
393	==============
394	
395	AF_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	========
438	SECURITY
439	========
440	
441	Currently, only the kerberos 4 equivalent protocol has been implemented
442	(security index 2 - rxkad).  This requires the rxkad module to be loaded and,
443	on the client, tickets of the appropriate type to be obtained from the AFS
444	kaserver or the kerberos server and installed as "rxrpc" type keys.  This is
445	normally done using the klog program.  An example simple klog program can be
446	found at:
447	
448		http://people.redhat.com/~dhowells/rxrpc/klog.c
449	
450	The payload provided to add_key() on the client should be of the following
451	form:
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	
463	Where the ticket blob is just appended to the above structure.
464	
465	
466	For the server, keys of type "rxrpc_s" must be made available to the server.
467	They have a description of "<serviceID>:<securityIndex>" (eg: "52:2" for an
468	rxkad key for the AFS VL service).  When such a key is created, it should be
469	given the server's secret key as the instantiation data (see the example
470	below).
471	
472		add_key("rxrpc_s", "52:2", secret_key, 8, keyring);
473	
474	A keyring is passed to the server socket by naming it in a sockopt.  The server
475	socket then looks the server secret keys up in this keyring when secure
476	incoming connections are made.  This can be seen in an example program that can
477	be found at:
478	
479		http://people.redhat.com/~dhowells/rxrpc/listen.c
480	
481	
482	====================
483	EXAMPLE CLIENT USAGE
484	====================
485	
486	A 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	====================
564	EXAMPLE SERVER USAGE
565	====================
566	
567	A 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	
665	Note that all the communications for a particular service take place through
666	the one server socket, using control messages on sendmsg() and recvmsg() to
667	determine the call affected.
668	
669	
670	=========================
671	AF_RXRPC KERNEL INTERFACE
672	=========================
673	
674	The AF_RXRPC module also provides an interface for use by in-kernel utilities
675	such 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	
693	To use the RxRPC facility, a kernel utility must still open an AF_RXRPC socket,
694	bind an address as appropriate and listen if it's to be a server socket, but
695	then it passes this to the kernel interface functions.
696	
697	The 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 rxrpc_call *call);
729	
730	     This is used to end a previously begun call.  The user_call_ID is expunged
731	     from AF_RXRPC's knowledge and will not be seen again in association with
732	     the specified call.
733	
734	 (*) Send data through a call.
735	
736		int rxrpc_kernel_send_data(struct rxrpc_call *call, struct msghdr *msg,
737					   size_t len);
738	
739	     This is used to supply either the request part of a client call or the
740	     reply part of a server call.  msg.msg_iovlen and msg.msg_iov specify the
741	     data buffers to be used.  msg_iov may not be NULL and must point
742	     exclusively to in-kernel virtual addresses.  msg.msg_flags may be given
743	     MSG_MORE if there will be subsequent data sends for this call.
744	
745	     The msg must not specify a destination address, control data or any flags
746	     other than MSG_MORE.  len is the total amount of data to transmit.
747	
748	 (*) Abort a call.
749	
750		void rxrpc_kernel_abort_call(struct rxrpc_call *call, u32 abort_code);
751	
752	     This is used to abort a call if it's still in an abortable state.  The
753	     abort code specified will be placed in the ABORT message sent.
754	
755	 (*) Intercept received RxRPC messages.
756	
757		typedef void (*rxrpc_interceptor_t)(struct sock *sk,
758						    unsigned long user_call_ID,
759						    struct sk_buff *skb);
760	
761		void
762		rxrpc_kernel_intercept_rx_messages(struct socket *sock,
763						   rxrpc_interceptor_t interceptor);
764	
765	     This installs an interceptor function on the specified AF_RXRPC socket.
766	     All messages that would otherwise wind up in the socket's Rx queue are
767	     then diverted to this function.  Note that care must be taken to process
768	     the messages in the right order to maintain DATA message sequentiality.
769	
770	     The interceptor function itself is provided with the address of the socket
771	     and handling the incoming message, the ID assigned by the kernel utility
772	     to the call and the socket buffer containing the message.
773	
774	     The skb->mark field indicates the type of message:
775	
776		MARK				MEANING
777		===============================	=======================================
778		RXRPC_SKB_MARK_DATA		Data message
779		RXRPC_SKB_MARK_FINAL_ACK	Final ACK received for an incoming call
780		RXRPC_SKB_MARK_BUSY		Client call rejected as server busy
781		RXRPC_SKB_MARK_REMOTE_ABORT	Call aborted by peer
782		RXRPC_SKB_MARK_NET_ERROR	Network error detected
783		RXRPC_SKB_MARK_LOCAL_ERROR	Local error encountered
784		RXRPC_SKB_MARK_NEW_CALL		New incoming call awaiting acceptance
785	
786	     The remote abort message can be probed with rxrpc_kernel_get_abort_code().
787	     The two error messages can be probed with rxrpc_kernel_get_error_number().
788	     A new call can be accepted with rxrpc_kernel_accept_call().
789	
790	     Data messages can have their contents extracted with the usual bunch of
791	     socket buffer manipulation functions.  A data message can be determined to
792	     be the last one in a sequence with rxrpc_kernel_is_data_last().  When a
793	     data message has been used up, rxrpc_kernel_data_delivered() should be
794	     called on it..
795	
796	     Non-data messages should be handled to rxrpc_kernel_free_skb() to dispose
797	     of.  It is possible to get extra refs on all types of message for later
798	     freeing, but this may pin the state of a call until the message is finally
799	     freed.
800	
801	 (*) Accept an incoming call.
802	
803		struct rxrpc_call *
804		rxrpc_kernel_accept_call(struct socket *sock,
805					 unsigned long user_call_ID);
806	
807	     This is used to accept an incoming call and to assign it a call ID.  This
808	     function is similar to rxrpc_kernel_begin_call() and calls accepted must
809	     be ended in the same way.
810	
811	     If this function is successful, an opaque reference to the RxRPC call is
812	     returned.  The caller now holds a reference on this and it must be
813	     properly ended.
814	
815	 (*) Reject an incoming call.
816	
817		int rxrpc_kernel_reject_call(struct socket *sock);
818	
819	     This is used to reject the first incoming call on the socket's queue with
820	     a BUSY message.  -ENODATA is returned if there were no incoming calls.
821	     Other errors may be returned if the call had been aborted (-ECONNABORTED)
822	     or had timed out (-ETIME).
823	
824	 (*) Record the delivery of a data message and free it.
825	
826		void rxrpc_kernel_data_delivered(struct sk_buff *skb);
827	
828	     This is used to record a data message as having been delivered and to
829	     update the ACK state for the call.  The socket buffer will be freed.
830	
831	 (*) Free a message.
832	
833		void rxrpc_kernel_free_skb(struct sk_buff *skb);
834	
835	     This is used to free a non-DATA socket buffer intercepted from an AF_RXRPC
836	     socket.
837	
838	 (*) Determine if a data message is the last one on a call.
839	
840		bool rxrpc_kernel_is_data_last(struct sk_buff *skb);
841	
842	     This is used to determine if a socket buffer holds the last data message
843	     to be received for a call (true will be returned if it does, false
844	     if not).
845	
846	     The data message will be part of the reply on a client call and the
847	     request on an incoming call.  In the latter case there will be more
848	     messages, but in the former case there will not.
849	
850	 (*) Get the abort code from an abort message.
851	
852		u32 rxrpc_kernel_get_abort_code(struct sk_buff *skb);
853	
854	     This is used to extract the abort code from a remote abort message.
855	
856	 (*) Get the error number from a local or network error message.
857	
858		int rxrpc_kernel_get_error_number(struct sk_buff *skb);
859	
860	     This is used to extract the error number from a message indicating either
861	     a local error occurred or a network error occurred.
862	
863	 (*) Allocate a null key for doing anonymous security.
864	
865		struct key *rxrpc_get_null_key(const char *keyname);
866	
867	     This is used to allocate a null RxRPC key that can be used to indicate
868	     anonymous security for a particular domain.
869	
870	
871	=======================
872	CONFIGURABLE PARAMETERS
873	=======================
874	
875	The RxRPC protocol driver has a number of configurable parameters that can be
876	adjusted through sysctls in /proc/net/rxrpc/:
877	
878	 (*) req_ack_delay
879	
880	     The amount of time in milliseconds after receiving a packet with the
881	     request-ack flag set before we honour the flag and actually send the
882	     requested ack.
883	
884	     Usually the other side won't stop sending packets until the advertised
885	     reception window is full (to a maximum of 255 packets), so delaying the
886	     ACK permits several packets to be ACK'd in one go.
887	
888	 (*) soft_ack_delay
889	
890	     The amount of time in milliseconds after receiving a new packet before we
891	     generate a soft-ACK to tell the sender that it doesn't need to resend.
892	
893	 (*) idle_ack_delay
894	
895	     The amount of time in milliseconds after all the packets currently in the
896	     received queue have been consumed before we generate a hard-ACK to tell
897	     the sender it can free its buffers, assuming no other reason occurs that
898	     we would send an ACK.
899	
900	 (*) resend_timeout
901	
902	     The amount of time in milliseconds after transmitting a packet before we
903	     transmit it again, assuming no ACK is received from the receiver telling
904	     us they got it.
905	
906	 (*) max_call_lifetime
907	
908	     The maximum amount of time in seconds that a call may be in progress
909	     before we preemptively kill it.
910	
911	 (*) dead_call_expiry
912	
913	     The amount of time in seconds before we remove a dead call from the call
914	     list.  Dead calls are kept around for a little while for the purpose of
915	     repeating ACK and ABORT packets.
916	
917	 (*) connection_expiry
918	
919	     The amount of time in seconds after a connection was last used before we
920	     remove it from the connection list.  Whilst a connection is in existence,
921	     it serves as a placeholder for negotiated security; when it is deleted,
922	     the security must be renegotiated.
923	
924	 (*) transport_expiry
925	
926	     The amount of time in seconds after a transport was last used before we
927	     remove it from the transport list.  Whilst a transport is in existence, it
928	     serves to anchor the peer data and keeps the connection ID counter.
929	
930	 (*) rxrpc_rx_window_size
931	
932	     The size of the receive window in packets.  This is the maximum number of
933	     unconsumed received packets we're willing to hold in memory for any
934	     particular call.
935	
936	 (*) rxrpc_rx_mtu
937	
938	     The maximum packet MTU size that we're willing to receive in bytes.  This
939	     indicates to the peer whether we're willing to accept jumbo packets.
940	
941	 (*) rxrpc_rx_jumbo_max
942	
943	     The maximum number of packets that we're willing to accept in a jumbo
944	     packet.  Non-terminal packets in a jumbo packet must contain a four byte
945	     header plus exactly 1412 bytes of data.  The terminal packet must contain
946	     a four byte header plus any amount of data.  In any event, a jumbo packet
947	     may not exceed rxrpc_rx_mtu in size.
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