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