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Based on kernel version 4.13.3. Page generated on 2017-09-23 13:55 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		RXRPC_EXCLUSIVE_CALL	s-- n/a		Make an exclusive client call
329		RXRPC_UPGRADE_SERVICE	s-- n/a		Client call can be upgraded
330		RXRPC_TX_LENGTH		s-- data len	Total length of Tx data
331	
332		(SRT = usable in Sendmsg / delivered by Recvmsg / Terminal message)
333	
334	 (*) RXRPC_USER_CALL_ID
335	
336	     This is used to indicate the application's call ID.  It's an unsigned long
337	     that the app specifies in the client by attaching it to the first data
338	     message or in the server by passing it in association with an RXRPC_ACCEPT
339	     message.  recvmsg() passes it in conjunction with all messages except
340	     those of the RXRPC_NEW_CALL message.
341	
342	 (*) RXRPC_ABORT
343	
344	     This is can be used by an application to abort a call by passing it to
345	     sendmsg, or it can be delivered by recvmsg to indicate a remote abort was
346	     received.  Either way, it must be associated with an RXRPC_USER_CALL_ID to
347	     specify the call affected.  If an abort is being sent, then error EBADSLT
348	     will be returned if there is no call with that user ID.
349	
350	 (*) RXRPC_ACK
351	
352	     This is delivered to a server application to indicate that the final ACK
353	     of a call was received from the client.  It will be associated with an
354	     RXRPC_USER_CALL_ID to indicate the call that's now complete.
355	
356	 (*) RXRPC_NET_ERROR
357	
358	     This is delivered to an application to indicate that an ICMP error message
359	     was encountered in the process of trying to talk to the peer.  An
360	     errno-class integer value will be included in the control message data
361	     indicating the problem, and an RXRPC_USER_CALL_ID will indicate the call
362	     affected.
363	
364	 (*) RXRPC_BUSY
365	
366	     This is delivered to a client application to indicate that a call was
367	     rejected by the server due to the server being busy.  It will be
368	     associated with an RXRPC_USER_CALL_ID to indicate the rejected call.
369	
370	 (*) RXRPC_LOCAL_ERROR
371	
372	     This is delivered to an application to indicate that a local error was
373	     encountered and that a call has been aborted because of it.  An
374	     errno-class integer value will be included in the control message data
375	     indicating the problem, and an RXRPC_USER_CALL_ID will indicate the call
376	     affected.
377	
378	 (*) RXRPC_NEW_CALL
379	
380	     This is delivered to indicate to a server application that a new call has
381	     arrived and is awaiting acceptance.  No user ID is associated with this,
382	     as a user ID must subsequently be assigned by doing an RXRPC_ACCEPT.
383	
384	 (*) RXRPC_ACCEPT
385	
386	     This is used by a server application to attempt to accept a call and
387	     assign it a user ID.  It should be associated with an RXRPC_USER_CALL_ID
388	     to indicate the user ID to be assigned.  If there is no call to be
389	     accepted (it may have timed out, been aborted, etc.), then sendmsg will
390	     return error ENODATA.  If the user ID is already in use by another call,
391	     then error EBADSLT will be returned.
392	
393	 (*) RXRPC_EXCLUSIVE_CALL
394	
395	     This is used to indicate that a client call should be made on a one-off
396	     connection.  The connection is discarded once the call has terminated.
397	
398	 (*) RXRPC_UPGRADE_SERVICE
399	
400	     This is used to make a client call to probe if the specified service ID
401	     may be upgraded by the server.  The caller must check msg_name returned to
402	     recvmsg() for the service ID actually in use.  The operation probed must
403	     be one that takes the same arguments in both services.
404	
405	     Once this has been used to establish the upgrade capability (or lack
406	     thereof) of the server, the service ID returned should be used for all
407	     future communication to that server and RXRPC_UPGRADE_SERVICE should no
408	     longer be set.
409	
410	 (*) RXRPC_TX_LENGTH
411	
412	     This is used to inform the kernel of the total amount of data that is
413	     going to be transmitted by a call (whether in a client request or a
414	     service response).  If given, it allows the kernel to encrypt from the
415	     userspace buffer directly to the packet buffers, rather than copying into
416	     the buffer and then encrypting in place.  This may only be given with the
417	     first sendmsg() providing data for a call.  EMSGSIZE will be generated if
418	     the amount of data actually given is different.
419	
420	     This takes a parameter of __s64 type that indicates how much will be
421	     transmitted.  This may not be less than zero.
422	
423	The symbol RXRPC__SUPPORTED is defined as one more than the highest control
424	message type supported.  At run time this can be queried by means of the
425	RXRPC_SUPPORTED_CMSG socket option (see below).
426	
427	
428	==============
429	SOCKET OPTIONS
430	==============
431	
432	AF_RXRPC sockets support a few socket options at the SOL_RXRPC level:
433	
434	 (*) RXRPC_SECURITY_KEY
435	
436	     This is used to specify the description of the key to be used.  The key is
437	     extracted from the calling process's keyrings with request_key() and
438	     should be of "rxrpc" type.
439	
440	     The optval pointer points to the description string, and optlen indicates
441	     how long the string is, without the NUL terminator.
442	
443	 (*) RXRPC_SECURITY_KEYRING
444	
445	     Similar to above but specifies a keyring of server secret keys to use (key
446	     type "keyring").  See the "Security" section.
447	
448	 (*) RXRPC_EXCLUSIVE_CONNECTION
449	
450	     This is used to request that new connections should be used for each call
451	     made subsequently on this socket.  optval should be NULL and optlen 0.
452	
453	 (*) RXRPC_MIN_SECURITY_LEVEL
454	
455	     This is used to specify the minimum security level required for calls on
456	     this socket.  optval must point to an int containing one of the following
457	     values:
458	
459	     (a) RXRPC_SECURITY_PLAIN
460	
461		 Encrypted checksum only.
462	
463	     (b) RXRPC_SECURITY_AUTH
464	
465		 Encrypted checksum plus packet padded and first eight bytes of packet
466		 encrypted - which includes the actual packet length.
467	
468	     (c) RXRPC_SECURITY_ENCRYPTED
469	
470		 Encrypted checksum plus entire packet padded and encrypted, including
471		 actual packet length.
472	
473	 (*) RXRPC_UPGRADEABLE_SERVICE
474	
475	     This is used to indicate that a service socket with two bindings may
476	     upgrade one bound service to the other if requested by the client.  optval
477	     must point to an array of two unsigned short ints.  The first is the
478	     service ID to upgrade from and the second the service ID to upgrade to.
479	
480	 (*) RXRPC_SUPPORTED_CMSG
481	
482	     This is a read-only option that writes an int into the buffer indicating
483	     the highest control message type supported.
484	
485	
486	========
487	SECURITY
488	========
489	
490	Currently, only the kerberos 4 equivalent protocol has been implemented
491	(security index 2 - rxkad).  This requires the rxkad module to be loaded and,
492	on the client, tickets of the appropriate type to be obtained from the AFS
493	kaserver or the kerberos server and installed as "rxrpc" type keys.  This is
494	normally done using the klog program.  An example simple klog program can be
495	found at:
496	
497		http://people.redhat.com/~dhowells/rxrpc/klog.c
498	
499	The payload provided to add_key() on the client should be of the following
500	form:
501	
502		struct rxrpc_key_sec2_v1 {
503			uint16_t	security_index;	/* 2 */
504			uint16_t	ticket_length;	/* length of ticket[] */
505			uint32_t	expiry;		/* time at which expires */
506			uint8_t		kvno;		/* key version number */
507			uint8_t		__pad[3];
508			uint8_t		session_key[8];	/* DES session key */
509			uint8_t		ticket[0];	/* the encrypted ticket */
510		};
511	
512	Where the ticket blob is just appended to the above structure.
513	
514	
515	For the server, keys of type "rxrpc_s" must be made available to the server.
516	They have a description of "<serviceID>:<securityIndex>" (eg: "52:2" for an
517	rxkad key for the AFS VL service).  When such a key is created, it should be
518	given the server's secret key as the instantiation data (see the example
519	below).
520	
521		add_key("rxrpc_s", "52:2", secret_key, 8, keyring);
522	
523	A keyring is passed to the server socket by naming it in a sockopt.  The server
524	socket then looks the server secret keys up in this keyring when secure
525	incoming connections are made.  This can be seen in an example program that can
526	be found at:
527	
528		http://people.redhat.com/~dhowells/rxrpc/listen.c
529	
530	
531	====================
532	EXAMPLE CLIENT USAGE
533	====================
534	
535	A client would issue an operation by:
536	
537	 (1) An RxRPC socket is set up by:
538	
539		client = socket(AF_RXRPC, SOCK_DGRAM, PF_INET);
540	
541	     Where the third parameter indicates the protocol family of the transport
542	     socket used - usually IPv4 but it can also be IPv6 [TODO].
543	
544	 (2) A local address can optionally be bound:
545	
546		struct sockaddr_rxrpc srx = {
547			.srx_family	= AF_RXRPC,
548			.srx_service	= 0,  /* we're a client */
549			.transport_type	= SOCK_DGRAM,	/* type of transport socket */
550			.transport.sin_family	= AF_INET,
551			.transport.sin_port	= htons(7000), /* AFS callback */
552			.transport.sin_address	= 0,  /* all local interfaces */
553		};
554		bind(client, &srx, sizeof(srx));
555	
556	     This specifies the local UDP port to be used.  If not given, a random
557	     non-privileged port will be used.  A UDP port may be shared between
558	     several unrelated RxRPC sockets.  Security is handled on a basis of
559	     per-RxRPC virtual connection.
560	
561	 (3) The security is set:
562	
563		const char *key = "AFS:cambridge.redhat.com";
564		setsockopt(client, SOL_RXRPC, RXRPC_SECURITY_KEY, key, strlen(key));
565	
566	     This issues a request_key() to get the key representing the security
567	     context.  The minimum security level can be set:
568	
569		unsigned int sec = RXRPC_SECURITY_ENCRYPTED;
570		setsockopt(client, SOL_RXRPC, RXRPC_MIN_SECURITY_LEVEL,
571			   &sec, sizeof(sec));
572	
573	 (4) The server to be contacted can then be specified (alternatively this can
574	     be done through sendmsg):
575	
576		struct sockaddr_rxrpc srx = {
577			.srx_family	= AF_RXRPC,
578			.srx_service	= VL_SERVICE_ID,
579			.transport_type	= SOCK_DGRAM,	/* type of transport socket */
580			.transport.sin_family	= AF_INET,
581			.transport.sin_port	= htons(7005), /* AFS volume manager */
582			.transport.sin_address	= ...,
583		};
584		connect(client, &srx, sizeof(srx));
585	
586	 (5) The request data should then be posted to the server socket using a series
587	     of sendmsg() calls, each with the following control message attached:
588	
589		RXRPC_USER_CALL_ID	- specifies the user ID for this call
590	
591	     MSG_MORE should be set in msghdr::msg_flags on all but the last part of
592	     the request.  Multiple requests may be made simultaneously.
593	
594	     An RXRPC_TX_LENGTH control message can also be specified on the first
595	     sendmsg() call.
596	
597	     If a call is intended to go to a destination other than the default
598	     specified through connect(), then msghdr::msg_name should be set on the
599	     first request message of that call.
600	
601	 (6) The reply data will then be posted to the server socket for recvmsg() to
602	     pick up.  MSG_MORE will be flagged by recvmsg() if there's more reply data
603	     for a particular call to be read.  MSG_EOR will be set on the terminal
604	     read for a call.
605	
606	     All data will be delivered with the following control message attached:
607	
608		RXRPC_USER_CALL_ID	- specifies the user ID for this call
609	
610	     If an abort or error occurred, this will be returned in the control data
611	     buffer instead, and MSG_EOR will be flagged to indicate the end of that
612	     call.
613	
614	A client may ask for a service ID it knows and ask that this be upgraded to a
615	better service if one is available by supplying RXRPC_UPGRADE_SERVICE on the
616	first sendmsg() of a call.  The client should then check srx_service in the
617	msg_name filled in by recvmsg() when collecting the result.  srx_service will
618	hold the same value as given to sendmsg() if the upgrade request was ignored by
619	the service - otherwise it will be altered to indicate the service ID the
620	server upgraded to.  Note that the upgraded service ID is chosen by the server.
621	The caller has to wait until it sees the service ID in the reply before sending
622	any more calls (further calls to the same destination will be blocked until the
623	probe is concluded).
624	
625	
626	====================
627	EXAMPLE SERVER USAGE
628	====================
629	
630	A server would be set up to accept operations in the following manner:
631	
632	 (1) An RxRPC socket is created by:
633	
634		server = socket(AF_RXRPC, SOCK_DGRAM, PF_INET);
635	
636	     Where the third parameter indicates the address type of the transport
637	     socket used - usually IPv4.
638	
639	 (2) Security is set up if desired by giving the socket a keyring with server
640	     secret keys in it:
641	
642		keyring = add_key("keyring", "AFSkeys", NULL, 0,
643				  KEY_SPEC_PROCESS_KEYRING);
644	
645		const char secret_key[8] = {
646			0xa7, 0x83, 0x8a, 0xcb, 0xc7, 0x83, 0xec, 0x94 };
647		add_key("rxrpc_s", "52:2", secret_key, 8, keyring);
648	
649		setsockopt(server, SOL_RXRPC, RXRPC_SECURITY_KEYRING, "AFSkeys", 7);
650	
651	     The keyring can be manipulated after it has been given to the socket. This
652	     permits the server to add more keys, replace keys, etc. whilst it is live.
653	
654	 (3) A local address must then be bound:
655	
656		struct sockaddr_rxrpc srx = {
657			.srx_family	= AF_RXRPC,
658			.srx_service	= VL_SERVICE_ID, /* RxRPC service ID */
659			.transport_type	= SOCK_DGRAM,	/* type of transport socket */
660			.transport.sin_family	= AF_INET,
661			.transport.sin_port	= htons(7000), /* AFS callback */
662			.transport.sin_address	= 0,  /* all local interfaces */
663		};
664		bind(server, &srx, sizeof(srx));
665	
666	     More than one service ID may be bound to a socket, provided the transport
667	     parameters are the same.  The limit is currently two.  To do this, bind()
668	     should be called twice.
669	
670	 (4) If service upgrading is required, first two service IDs must have been
671	     bound and then the following option must be set:
672	
673		unsigned short service_ids[2] = { from_ID, to_ID };
674		setsockopt(server, SOL_RXRPC, RXRPC_UPGRADEABLE_SERVICE,
675			   service_ids, sizeof(service_ids));
676	
677	     This will automatically upgrade connections on service from_ID to service
678	     to_ID if they request it.  This will be reflected in msg_name obtained
679	     through recvmsg() when the request data is delivered to userspace.
680	
681	 (5) The server is then set to listen out for incoming calls:
682	
683		listen(server, 100);
684	
685	 (6) The kernel notifies the server of pending incoming connections by sending
686	     it a message for each.  This is received with recvmsg() on the server
687	     socket.  It has no data, and has a single dataless control message
688	     attached:
689	
690		RXRPC_NEW_CALL
691	
692	     The address that can be passed back by recvmsg() at this point should be
693	     ignored since the call for which the message was posted may have gone by
694	     the time it is accepted - in which case the first call still on the queue
695	     will be accepted.
696	
697	 (7) The server then accepts the new call by issuing a sendmsg() with two
698	     pieces of control data and no actual data:
699	
700		RXRPC_ACCEPT		- indicate connection acceptance
701		RXRPC_USER_CALL_ID	- specify user ID for this call
702	
703	 (8) The first request data packet will then be posted to the server socket for
704	     recvmsg() to pick up.  At that point, the RxRPC address for the call can
705	     be read from the address fields in the msghdr struct.
706	
707	     Subsequent request data will be posted to the server socket for recvmsg()
708	     to collect as it arrives.  All but the last piece of the request data will
709	     be delivered with MSG_MORE flagged.
710	
711	     All data will be delivered with the following control message attached:
712	
713		RXRPC_USER_CALL_ID	- specifies the user ID for this call
714	
715	 (9) The reply data should then be posted to the server socket using a series
716	     of sendmsg() calls, each with the following control messages attached:
717	
718		RXRPC_USER_CALL_ID	- specifies the user ID for this call
719	
720	     MSG_MORE should be set in msghdr::msg_flags on all but the last message
721	     for a particular call.
722	
723	(10) The final ACK from the client will be posted for retrieval by recvmsg()
724	     when it is received.  It will take the form of a dataless message with two
725	     control messages attached:
726	
727		RXRPC_USER_CALL_ID	- specifies the user ID for this call
728		RXRPC_ACK		- indicates final ACK (no data)
729	
730	     MSG_EOR will be flagged to indicate that this is the final message for
731	     this call.
732	
733	(11) Up to the point the final packet of reply data is sent, the call can be
734	     aborted by calling sendmsg() with a dataless message with the following
735	     control messages attached:
736	
737		RXRPC_USER_CALL_ID	- specifies the user ID for this call
738		RXRPC_ABORT		- indicates abort code (4 byte data)
739	
740	     Any packets waiting in the socket's receive queue will be discarded if
741	     this is issued.
742	
743	Note that all the communications for a particular service take place through
744	the one server socket, using control messages on sendmsg() and recvmsg() to
745	determine the call affected.
746	
747	
748	=========================
749	AF_RXRPC KERNEL INTERFACE
750	=========================
751	
752	The AF_RXRPC module also provides an interface for use by in-kernel utilities
753	such as the AFS filesystem.  This permits such a utility to:
754	
755	 (1) Use different keys directly on individual client calls on one socket
756	     rather than having to open a whole slew of sockets, one for each key it
757	     might want to use.
758	
759	 (2) Avoid having RxRPC call request_key() at the point of issue of a call or
760	     opening of a socket.  Instead the utility is responsible for requesting a
761	     key at the appropriate point.  AFS, for instance, would do this during VFS
762	     operations such as open() or unlink().  The key is then handed through
763	     when the call is initiated.
764	
765	 (3) Request the use of something other than GFP_KERNEL to allocate memory.
766	
767	 (4) Avoid the overhead of using the recvmsg() call.  RxRPC messages can be
768	     intercepted before they get put into the socket Rx queue and the socket
769	     buffers manipulated directly.
770	
771	To use the RxRPC facility, a kernel utility must still open an AF_RXRPC socket,
772	bind an address as appropriate and listen if it's to be a server socket, but
773	then it passes this to the kernel interface functions.
774	
775	The kernel interface functions are as follows:
776	
777	 (*) Begin a new client call.
778	
779		struct rxrpc_call *
780		rxrpc_kernel_begin_call(struct socket *sock,
781					struct sockaddr_rxrpc *srx,
782					struct key *key,
783					unsigned long user_call_ID,
784					s64 tx_total_len,
785					gfp_t gfp);
786	
787	     This allocates the infrastructure to make a new RxRPC call and assigns
788	     call and connection numbers.  The call will be made on the UDP port that
789	     the socket is bound to.  The call will go to the destination address of a
790	     connected client socket unless an alternative is supplied (srx is
791	     non-NULL).
792	
793	     If a key is supplied then this will be used to secure the call instead of
794	     the key bound to the socket with the RXRPC_SECURITY_KEY sockopt.  Calls
795	     secured in this way will still share connections if at all possible.
796	
797	     The user_call_ID is equivalent to that supplied to sendmsg() in the
798	     control data buffer.  It is entirely feasible to use this to point to a
799	     kernel data structure.
800	
801	     tx_total_len is the amount of data the caller is intending to transmit
802	     with this call (or -1 if unknown at this point).  Setting the data size
803	     allows the kernel to encrypt directly to the packet buffers, thereby
804	     saving a copy.  The value may not be less than -1.
805	
806	     If this function is successful, an opaque reference to the RxRPC call is
807	     returned.  The caller now holds a reference on this and it must be
808	     properly ended.
809	
810	 (*) End a client call.
811	
812		void rxrpc_kernel_end_call(struct socket *sock,
813					   struct rxrpc_call *call);
814	
815	     This is used to end a previously begun call.  The user_call_ID is expunged
816	     from AF_RXRPC's knowledge and will not be seen again in association with
817	     the specified call.
818	
819	 (*) Send data through a call.
820	
821		int rxrpc_kernel_send_data(struct socket *sock,
822					   struct rxrpc_call *call,
823					   struct msghdr *msg,
824					   size_t len);
825	
826	     This is used to supply either the request part of a client call or the
827	     reply part of a server call.  msg.msg_iovlen and msg.msg_iov specify the
828	     data buffers to be used.  msg_iov may not be NULL and must point
829	     exclusively to in-kernel virtual addresses.  msg.msg_flags may be given
830	     MSG_MORE if there will be subsequent data sends for this call.
831	
832	     The msg must not specify a destination address, control data or any flags
833	     other than MSG_MORE.  len is the total amount of data to transmit.
834	
835	 (*) Receive data from a call.
836	
837		int rxrpc_kernel_recv_data(struct socket *sock,
838					   struct rxrpc_call *call,
839					   void *buf,
840					   size_t size,
841					   size_t *_offset,
842					   bool want_more,
843					   u32 *_abort)
844	
845	      This is used to receive data from either the reply part of a client call
846	      or the request part of a service call.  buf and size specify how much
847	      data is desired and where to store it.  *_offset is added on to buf and
848	      subtracted from size internally; the amount copied into the buffer is
849	      added to *_offset before returning.
850	
851	      want_more should be true if further data will be required after this is
852	      satisfied and false if this is the last item of the receive phase.
853	
854	      There are three normal returns: 0 if the buffer was filled and want_more
855	      was true; 1 if the buffer was filled, the last DATA packet has been
856	      emptied and want_more was false; and -EAGAIN if the function needs to be
857	      called again.
858	
859	      If the last DATA packet is processed but the buffer contains less than
860	      the amount requested, EBADMSG is returned.  If want_more wasn't set, but
861	      more data was available, EMSGSIZE is returned.
862	
863	      If a remote ABORT is detected, the abort code received will be stored in
864	      *_abort and ECONNABORTED will be returned.
865	
866	 (*) Abort a call.
867	
868		void rxrpc_kernel_abort_call(struct socket *sock,
869					     struct rxrpc_call *call,
870					     u32 abort_code);
871	
872	     This is used to abort a call if it's still in an abortable state.  The
873	     abort code specified will be placed in the ABORT message sent.
874	
875	 (*) Intercept received RxRPC messages.
876	
877		typedef void (*rxrpc_interceptor_t)(struct sock *sk,
878						    unsigned long user_call_ID,
879						    struct sk_buff *skb);
880	
881		void
882		rxrpc_kernel_intercept_rx_messages(struct socket *sock,
883						   rxrpc_interceptor_t interceptor);
884	
885	     This installs an interceptor function on the specified AF_RXRPC socket.
886	     All messages that would otherwise wind up in the socket's Rx queue are
887	     then diverted to this function.  Note that care must be taken to process
888	     the messages in the right order to maintain DATA message sequentiality.
889	
890	     The interceptor function itself is provided with the address of the socket
891	     and handling the incoming message, the ID assigned by the kernel utility
892	     to the call and the socket buffer containing the message.
893	
894	     The skb->mark field indicates the type of message:
895	
896		MARK				MEANING
897		===============================	=======================================
898		RXRPC_SKB_MARK_DATA		Data message
899		RXRPC_SKB_MARK_FINAL_ACK	Final ACK received for an incoming call
900		RXRPC_SKB_MARK_BUSY		Client call rejected as server busy
901		RXRPC_SKB_MARK_REMOTE_ABORT	Call aborted by peer
902		RXRPC_SKB_MARK_NET_ERROR	Network error detected
903		RXRPC_SKB_MARK_LOCAL_ERROR	Local error encountered
904		RXRPC_SKB_MARK_NEW_CALL		New incoming call awaiting acceptance
905	
906	     The remote abort message can be probed with rxrpc_kernel_get_abort_code().
907	     The two error messages can be probed with rxrpc_kernel_get_error_number().
908	     A new call can be accepted with rxrpc_kernel_accept_call().
909	
910	     Data messages can have their contents extracted with the usual bunch of
911	     socket buffer manipulation functions.  A data message can be determined to
912	     be the last one in a sequence with rxrpc_kernel_is_data_last().  When a
913	     data message has been used up, rxrpc_kernel_data_consumed() should be
914	     called on it.
915	
916	     Messages should be handled to rxrpc_kernel_free_skb() to dispose of.  It
917	     is possible to get extra refs on all types of message for later freeing,
918	     but this may pin the state of a call until the message is finally freed.
919	
920	 (*) Accept an incoming call.
921	
922		struct rxrpc_call *
923		rxrpc_kernel_accept_call(struct socket *sock,
924					 unsigned long user_call_ID);
925	
926	     This is used to accept an incoming call and to assign it a call ID.  This
927	     function is similar to rxrpc_kernel_begin_call() and calls accepted must
928	     be ended in the same way.
929	
930	     If this function is successful, an opaque reference to the RxRPC call is
931	     returned.  The caller now holds a reference on this and it must be
932	     properly ended.
933	
934	 (*) Reject an incoming call.
935	
936		int rxrpc_kernel_reject_call(struct socket *sock);
937	
938	     This is used to reject the first incoming call on the socket's queue with
939	     a BUSY message.  -ENODATA is returned if there were no incoming calls.
940	     Other errors may be returned if the call had been aborted (-ECONNABORTED)
941	     or had timed out (-ETIME).
942	
943	 (*) Allocate a null key for doing anonymous security.
944	
945		struct key *rxrpc_get_null_key(const char *keyname);
946	
947	     This is used to allocate a null RxRPC key that can be used to indicate
948	     anonymous security for a particular domain.
949	
950	 (*) Get the peer address of a call.
951	
952		void rxrpc_kernel_get_peer(struct socket *sock, struct rxrpc_call *call,
953					   struct sockaddr_rxrpc *_srx);
954	
955	     This is used to find the remote peer address of a call.
956	
957	 (*) Set the total transmit data size on a call.
958	
959		void rxrpc_kernel_set_tx_length(struct socket *sock,
960						struct rxrpc_call *call,
961						s64 tx_total_len);
962	
963	     This sets the amount of data that the caller is intending to transmit on a
964	     call.  It's intended to be used for setting the reply size as the request
965	     size should be set when the call is begun.  tx_total_len may not be less
966	     than zero.
967	
968	
969	=======================
970	CONFIGURABLE PARAMETERS
971	=======================
972	
973	The RxRPC protocol driver has a number of configurable parameters that can be
974	adjusted through sysctls in /proc/net/rxrpc/:
975	
976	 (*) req_ack_delay
977	
978	     The amount of time in milliseconds after receiving a packet with the
979	     request-ack flag set before we honour the flag and actually send the
980	     requested ack.
981	
982	     Usually the other side won't stop sending packets until the advertised
983	     reception window is full (to a maximum of 255 packets), so delaying the
984	     ACK permits several packets to be ACK'd in one go.
985	
986	 (*) soft_ack_delay
987	
988	     The amount of time in milliseconds after receiving a new packet before we
989	     generate a soft-ACK to tell the sender that it doesn't need to resend.
990	
991	 (*) idle_ack_delay
992	
993	     The amount of time in milliseconds after all the packets currently in the
994	     received queue have been consumed before we generate a hard-ACK to tell
995	     the sender it can free its buffers, assuming no other reason occurs that
996	     we would send an ACK.
997	
998	 (*) resend_timeout
999	
1000	     The amount of time in milliseconds after transmitting a packet before we
1001	     transmit it again, assuming no ACK is received from the receiver telling
1002	     us they got it.
1003	
1004	 (*) max_call_lifetime
1005	
1006	     The maximum amount of time in seconds that a call may be in progress
1007	     before we preemptively kill it.
1008	
1009	 (*) dead_call_expiry
1010	
1011	     The amount of time in seconds before we remove a dead call from the call
1012	     list.  Dead calls are kept around for a little while for the purpose of
1013	     repeating ACK and ABORT packets.
1014	
1015	 (*) connection_expiry
1016	
1017	     The amount of time in seconds after a connection was last used before we
1018	     remove it from the connection list.  Whilst a connection is in existence,
1019	     it serves as a placeholder for negotiated security; when it is deleted,
1020	     the security must be renegotiated.
1021	
1022	 (*) transport_expiry
1023	
1024	     The amount of time in seconds after a transport was last used before we
1025	     remove it from the transport list.  Whilst a transport is in existence, it
1026	     serves to anchor the peer data and keeps the connection ID counter.
1027	
1028	 (*) rxrpc_rx_window_size
1029	
1030	     The size of the receive window in packets.  This is the maximum number of
1031	     unconsumed received packets we're willing to hold in memory for any
1032	     particular call.
1033	
1034	 (*) rxrpc_rx_mtu
1035	
1036	     The maximum packet MTU size that we're willing to receive in bytes.  This
1037	     indicates to the peer whether we're willing to accept jumbo packets.
1038	
1039	 (*) rxrpc_rx_jumbo_max
1040	
1041	     The maximum number of packets that we're willing to accept in a jumbo
1042	     packet.  Non-terminal packets in a jumbo packet must contain a four byte
1043	     header plus exactly 1412 bytes of data.  The terminal packet must contain
1044	     a four byte header plus any amount of data.  In any event, a jumbo packet
1045	     may not exceed rxrpc_rx_mtu in size.
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