diff options
Diffstat (limited to 'Documentation')
| -rw-r--r-- | Documentation/networking/rxrpc.txt | 663 |
1 files changed, 663 insertions, 0 deletions
diff --git a/Documentation/networking/rxrpc.txt b/Documentation/networking/rxrpc.txt new file mode 100644 index 000000000000..fb809b738a0d --- /dev/null +++ b/Documentation/networking/rxrpc.txt @@ -0,0 +1,663 @@ + ====================== + RxRPC NETWORK PROTOCOL + ====================== + +The RxRPC protocol driver provides a reliable two-phase transport on top of UDP +that can be used to perform RxRPC remote operations. This is done over sockets +of AF_RXRPC family, using sendmsg() and recvmsg() with control data to send and +receive data, aborts and errors. + +Contents of this document: + + (*) Overview. + + (*) RxRPC protocol summary. + + (*) AF_RXRPC driver model. + + (*) Control messages. + + (*) Socket options. + + (*) Security. + + (*) Example client usage. + + (*) Example server usage. + + +======== +OVERVIEW +======== + +RxRPC is a two-layer protocol. There is a session layer which provides +reliable virtual connections using UDP over IPv4 (or IPv6) as the transport +layer, but implements a real network protocol; and there's the presentation +layer which renders structured data to binary blobs and back again using XDR +(as does SunRPC): + + +-------------+ + | Application | + +-------------+ + | XDR | Presentation + +-------------+ + | RxRPC | Session + +-------------+ + | UDP | Transport + +-------------+ + + +AF_RXRPC provides: + + (1) Part of an RxRPC facility for both kernel and userspace applications by + making the session part of it a Linux network protocol (AF_RXRPC). + + (2) A two-phase protocol. The client transmits a blob (the request) and then + receives a blob (the reply), and the server receives the request and then + transmits the reply. + + (3) Retention of the reusable bits of the transport system set up for one call + to speed up subsequent calls. + + (4) A secure protocol, using the Linux kernel's key retention facility to + manage security on the client end. The server end must of necessity be + more active in security negotiations. + +AF_RXRPC does not provide XDR marshalling/presentation facilities. That is +left to the application. AF_RXRPC only deals in blobs. Even the operation ID +is just the first four bytes of the request blob, and as such is beyond the +kernel's interest. + + +Sockets of AF_RXRPC family are: + + (1) created as type SOCK_DGRAM; + + (2) provided with a protocol of the type of underlying transport they're going + to use - currently only PF_INET is supported. + + +The Andrew File System (AFS) is an example of an application that uses this and +that has both kernel (filesystem) and userspace (utility) components. + + +====================== +RXRPC PROTOCOL SUMMARY +====================== + +An overview of the RxRPC protocol: + + (*) RxRPC sits on top of another networking protocol (UDP is the only option + currently), and uses this to provide network transport. UDP ports, for + example, provide transport endpoints. + + (*) RxRPC supports multiple virtual "connections" from any given transport + endpoint, thus allowing the endpoints to be shared, even to the same + remote endpoint. + + (*) Each connection goes to a particular "service". A connection may not go + to multiple services. A service may be considered the RxRPC equivalent of + a port number. AF_RXRPC permits multiple services to share an endpoint. + + (*) Client-originating packets are marked, thus a transport endpoint can be + shared between client and server connections (connections have a + direction). + + (*) Up to a billion connections may be supported concurrently between one + local transport endpoint and one service on one remote endpoint. An RxRPC + connection is described by seven numbers: + + Local address } + Local port } Transport (UDP) address + Remote address } + Remote port } + Direction + Connection ID + Service ID + + (*) Each RxRPC operation is a "call". A connection may make up to four + billion calls, but only up to four calls may be in progress on a + connection at any one time. + + (*) Calls are two-phase and asymmetric: the client sends its request data, + which the service receives; then the service transmits the reply data + which the client receives. + + (*) The data blobs are of indefinite size, the end of a phase is marked with a + flag in the packet. The number of packets of data making up one blob may + not exceed 4 billion, however, as this would cause the sequence number to + wrap. + + (*) The first four bytes of the request data are the service operation ID. + + (*) Security is negotiated on a per-connection basis. The connection is + initiated by the first data packet on it arriving. If security is + requested, the server then issues a "challenge" and then the client + replies with a "response". If the response is successful, the security is + set for the lifetime of that connection, and all subsequent calls made + upon it use that same security. In the event that the server lets a + connection lapse before the client, the security will be renegotiated if + the client uses the connection again. + + (*) Calls use ACK packets to handle reliability. Data packets are also + explicitly sequenced per call. + + (*) There are two types of positive acknowledgement: hard-ACKs and soft-ACKs. + A hard-ACK indicates to the far side that all the data received to a point + has been received and processed; a soft-ACK indicates that the data has + been received but may yet be discarded and re-requested. The sender may + not discard any transmittable packets until they've been hard-ACK'd. + + (*) Reception of a reply data packet implicitly hard-ACK's all the data + packets that make up the request. + + (*) An call is complete when the request has been sent, the reply has been + received and the final hard-ACK on the last packet of the reply has + reached the server. + + (*) An call may be aborted by either end at any time up to its completion. + + +===================== +AF_RXRPC DRIVER MODEL +===================== + +About the AF_RXRPC driver: + + (*) The AF_RXRPC protocol transparently uses internal sockets of the transport + protocol to represent transport endpoints. + + (*) AF_RXRPC sockets map onto RxRPC connection bundles. Actual RxRPC + connections are handled transparently. One client socket may be used to + make multiple simultaneous calls to the same service. One server socket + may handle calls from many clients. + + (*) Additional parallel client connections will be initiated to support extra + concurrent calls, up to a tunable limit. + + (*) Each connection is retained for a certain amount of time [tunable] after + the last call currently using it has completed in case a new call is made + that could reuse it. + + (*) Each internal UDP socket is retained [tunable] for a certain amount of + time [tunable] after the last connection using it discarded, in case a new + connection is made that could use it. + + (*) A client-side connection is only shared between calls if they have have + the same key struct describing their security (and assuming the calls + would otherwise share the connection). Non-secured calls would also be + able to share connections with each other. + + (*) A server-side connection is shared if the client says it is. + + (*) ACK'ing is handled by the protocol driver automatically, including ping + replying. + + (*) SO_KEEPALIVE automatically pings the other side to keep the connection + alive [TODO]. + + (*) If an ICMP error is received, all calls affected by that error will be + aborted with an appropriate network error passed through recvmsg(). + + +Interaction with the user of the RxRPC socket: + + (*) A socket is made into a server socket by binding an address with a + non-zero service ID. + + (*) In the client, sending a request is achieved with one or more sendmsgs, + followed by the reply being received with one or more recvmsgs. + + (*) The first sendmsg for a request to be sent from a client contains a tag to + be used in all other sendmsgs or recvmsgs associated with that call. The + tag is carried in the control data. + + (*) connect() is used to supply a default destination address for a client + socket. This may be overridden by supplying an alternate address to the + first sendmsg() of a call (struct msghdr::msg_name). + + (*) If connect() is called on an unbound client, a random local port will + bound before the operation takes place. + + (*) A server socket may also be used to make client calls. To do this, the + first sendmsg() of the call must specify the target address. The server's + transport endpoint is used to send the packets. + + (*) Once the application has received the last message associated with a call, + the tag is guaranteed not to be seen again, and so it can be used to pin + client resources. A new call can then be initiated with the same tag + without fear of interference. + + (*) In the server, a request is received with one or more recvmsgs, then the + the reply is transmitted with one or more sendmsgs, and then the final ACK + is received with a last recvmsg. + + (*) When sending data for a call, sendmsg is given MSG_MORE if there's more + data to come on that call. + + (*) When receiving data for a call, recvmsg flags MSG_MORE if there's more + data to come for that call. + + (*) When receiving data or messages for a call, MSG_EOR is flagged by recvmsg + to indicate the terminal message for that call. + + (*) A call may be aborted by adding an abort control message to the control + data. Issuing an abort terminates the kernel's use of that call's tag. + Any messages waiting in the receive queue for that call will be discarded. + + (*) Aborts, busy notifications and challenge packets are delivered by recvmsg, + and control data messages will be set to indicate the context. Receiving + an abort or a busy message terminates the kernel's use of that call's tag. + + (*) The control data part of the msghdr struct is used for a number of things: + + (*) The tag of the intended or affected call. + + (*) Sending or receiving errors, aborts and busy notifications. + + (*) Notifications of incoming calls. + + (*) Sending debug requests and receiving debug replies [TODO]. + + (*) When the kernel has received and set up an incoming call, it sends a + message to server application to let it know there's a new call awaiting + its acceptance [recvmsg reports a special control message]. The server + application then uses sendmsg to assign a tag to the new call. Once that + is done, the first part of the request data will be delivered by recvmsg. + + (*) The server application has to provide the server socket with a keyring of + secret keys corresponding to the security types it permits. When a secure + connection is being set up, the kernel looks up the appropriate secret key + in the keyring and then sends a challenge packet to the client and + receives a response packet. The kernel then checks the authorisation of + the packet and either aborts the connection or sets up the security. + + (*) The name of the key a client will use to secure its communications is + nominated by a socket option. + + +Notes on recvmsg: + + (*) If there's a sequence of data messages belonging to a particular call on + the receive queue, then recvmsg will keep working through them until: + + (a) it meets the end of that call's received data, + + (b) it meets a non-data message, + + (c) it meets a message belonging to a different call, or + + (d) it fills the user buffer. + + If recvmsg is called in blocking mode, it will keep sleeping, awaiting the + reception of further data, until one of the above four conditions is met. + + (2) MSG_PEEK operates similarly, but will return immediately if it has put any + data in the buffer rather than sleeping until it can fill the buffer. + + (3) If a data message is only partially consumed in filling a user buffer, + then the remainder of that message will be left on the front of the queue + for the next taker. MSG_TRUNC will never be flagged. + + (4) If there is more data to be had on a call (it hasn't copied the last byte + of the last data message in that phase yet), then MSG_MORE will be + flagged. + + +================ +CONTROL MESSAGES +================ + +AF_RXRPC makes use of control messages in sendmsg() and recvmsg() to multiplex +calls, to invoke certain actions and to report certain conditions. These are: + + MESSAGE ID SRT DATA MEANING + ======================= === =========== =============================== + RXRPC_USER_CALL_ID sr- User ID App's call specifier + RXRPC_ABORT srt Abort code Abort code to issue/received + RXRPC_ACK -rt n/a Final ACK received + RXRPC_NET_ERROR -rt error num Network error on call + RXRPC_BUSY -rt n/a Call rejected (server busy) + RXRPC_LOCAL_ERROR -rt error num Local error encountered + RXRPC_NEW_CALL -r- n/a New call received + RXRPC_ACCEPT s-- n/a Accept new call + + (SRT = usable in Sendmsg / delivered by Recvmsg / Terminal message) + + (*) RXRPC_USER_CALL_ID + + This is used to indicate the application's call ID. It's an unsigned long + that the app specifies in the client by attaching it to the first data + message or in the server by passing it in association with an RXRPC_ACCEPT + message. recvmsg() passes it in conjunction with all messages except + those of the RXRPC_NEW_CALL message. + + (*) RXRPC_ABORT + + This is can be used by an application to abort a call by passing it to + sendmsg, or it can be delivered by recvmsg to indicate a remote abort was + received. Either way, it must be associated with an RXRPC_USER_CALL_ID to + specify the call affected. If an abort is being sent, then error EBADSLT + will be returned if there is no call with that user ID. + + (*) RXRPC_ACK + + This is delivered to a server application to indicate that the final ACK + of a call was received from the client. It will be associated with an + RXRPC_USER_CALL_ID to indicate the call that's now complete. + + (*) RXRPC_NET_ERROR + + This is delivered to an application to indicate that an ICMP error message + was encountered in the process of trying to talk to the peer. An + errno-class integer value will be included in the control message data + indicating the problem, and an RXRPC_USER_CALL_ID will indicate the call + affected. + + (*) RXRPC_BUSY + + This is delivered to a client application to indicate that a call was + rejected by the server due to the server being busy. It will be + associated with an RXRPC_USER_CALL_ID to indicate the rejected call. + + (*) RXRPC_LOCAL_ERROR + + This is delivered to an application to indicate that a local error was + encountered and that a call has been aborted because of it. An + errno-class integer value will be included in the control message data + indicating the problem, and an RXRPC_USER_CALL_ID will indicate the call + affected. + + (*) RXRPC_NEW_CALL + + This is delivered to indicate to a server application that a new call has + arrived and is awaiting acceptance. No user ID is associated with this, + as a user ID must subsequently be assigned by doing an RXRPC_ACCEPT. + + (*) RXRPC_ACCEPT + + This is used by a server application to attempt to accept a call and + assign it a user ID. It should be associated with an RXRPC_USER_CALL_ID + to indicate the user ID to be assigned. If there is no call to be + accepted (it may have timed out, been aborted, etc.), then sendmsg will + return error ENODATA. If the user ID is already in use by another call, + then error EBADSLT will be returned. + + +============== +SOCKET OPTIONS +============== + +AF_RXRPC sockets support a few socket options at the SOL_RXRPC level: + + (*) RXRPC_SECURITY_KEY + + This is used to specify the description of the key to be used. The key is + extracted from the calling process's keyrings with request_key() and + should be of "rxrpc" type. + + The optval pointer points to the description string, and optlen indicates + how long the string is, without the NUL terminator. + + (*) RXRPC_SECURITY_KEYRING + + Similar to above but specifies a keyring of server secret keys to use (key + type "keyring"). See the "Security" section. + + (*) RXRPC_EXCLUSIVE_CONNECTION + + This is used to request that new connections should be used for each call + made subsequently on this socket. optval should be NULL and optlen 0. + + (*) RXRPC_MIN_SECURITY_LEVEL + + This is used to specify the minimum security level required for calls on + this socket. optval must point to an int containing one of the following + values: + + (a) RXRPC_SECURITY_PLAIN + + Encrypted checksum only. + + (b) RXRPC_SECURITY_AUTH + + Encrypted checksum plus packet padded and first eight bytes of packet + encrypted - which includes the actual packet length. + + (c) RXRPC_SECURITY_ENCRYPTED + + Encrypted checksum plus entire packet padded and encrypted, including + actual packet length. + + +======== +SECURITY +======== + +Currently, only the kerberos 4 equivalent protocol has been implemented +(security index 2 - rxkad). This requires the rxkad module to be loaded and, +on the client, tickets of the appropriate type to be obtained from the AFS +kaserver or the kerberos server and installed as "rxrpc" type keys. This is +normally done using the klog program. An example simple klog program can be +found at: + + http://people.redhat.com/~dhowells/rxrpc/klog.c + +The payload provided to add_key() on the client should be of the following +form: + + struct rxrpc_key_sec2_v1 { + uint16_t security_index; /* 2 */ + uint16_t ticket_length; /* length of ticket[] */ + uint32_t expiry; /* time at which expires */ + uint8_t kvno; /* key version number */ + uint8_t __pad[3]; + uint8_t session_key[8]; /* DES session key */ + uint8_t ticket[0]; /* the encrypted ticket */ + }; + +Where the ticket blob is just appended to the above structure. + + +For the server, keys of type "rxrpc_s" must be made available to the server. +They have a description of "<serviceID>:<securityIndex>" (eg: "52:2" for an +rxkad key for the AFS VL service). When such a key is created, it should be +given the server's secret key as the instantiation data (see the example +below). + + add_key("rxrpc_s", "52:2", secret_key, 8, keyring); + +A keyring is passed to the server socket by naming it in a sockopt. The server +socket then looks the server secret keys up in this keyring when secure +incoming connections are made. This can be seen in an example program that can +be found at: + + http://people.redhat.com/~dhowells/rxrpc/listen.c + + +==================== +EXAMPLE CLIENT USAGE +==================== + +A client would issue an operation by: + + (1) An RxRPC socket is set up by: + + client = socket(AF_RXRPC, SOCK_DGRAM, PF_INET); + + Where the third parameter indicates the protocol family of the transport + socket used - usually IPv4 but it can also be IPv6 [TODO]. + + (2) A local address can optionally be bound: + + struct sockaddr_rxrpc srx = { + .srx_family = AF_RXRPC, + .srx_service = 0, /* we're a client */ + .transport_type = SOCK_DGRAM, /* type of transport socket */ + .transport.sin_family = AF_INET, + .transport.sin_port = htons(7000), /* AFS callback */ + .transport.sin_address = 0, /* all local interfaces */ + }; + bind(client, &srx, sizeof(srx)); + + This specifies the local UDP port to be used. If not given, a random + non-privileged port will be used. A UDP port may be shared between + several unrelated RxRPC sockets. Security is handled on a basis of + per-RxRPC virtual connection. + + (3) The security is set: + + const char *key = "AFS:cambridge.redhat.com"; + setsockopt(client, SOL_RXRPC, RXRPC_SECURITY_KEY, key, strlen(key)); + + This issues a request_key() to get the key representing the security + context. The minimum security level can be set: + + unsigned int sec = RXRPC_SECURITY_ENCRYPTED; + setsockopt(client, SOL_RXRPC, RXRPC_MIN_SECURITY_LEVEL, + &sec, sizeof(sec)); + + (4) The server to be contacted can then be specified (alternatively this can + be done through sendmsg): + + struct sockaddr_rxrpc srx = { + .srx_family = AF_RXRPC, + .srx_service = VL_SERVICE_ID, + .transport_type = SOCK_DGRAM, /* type of transport socket */ + .transport.sin_family = AF_INET, + .transport.sin_port = htons(7005), /* AFS volume manager */ + .transport.sin_address = ..., + }; + connect(client, &srx, sizeof(srx)); + + (5) The request data should then be posted to the server socket using a series + of sendmsg() calls, each with the following control message attached: + + RXRPC_USER_CALL_ID - specifies the user ID for this call + + MSG_MORE should be set in msghdr::msg_flags on all but the last part of + the request. Multiple requests may be made simultaneously. + + If a call is intended to go to a destination other then the default + specified through connect(), then msghdr::msg_name should be set on the + first request message of that call. + + (6) The reply data will then be posted to the server socket for recvmsg() to + pick up. MSG_MORE will be flagged by recvmsg() if there's more reply data + for a particular call to be read. MSG_EOR will be set on the terminal + read for a call. + + All data will be delivered with the following control message attached: + + RXRPC_USER_CALL_ID - specifies the user ID for this call + + If an abort or error occurred, this will be returned in the control data + buffer instead, and MSG_EOR will be flagged to indicate the end of that + call. + + +==================== +EXAMPLE SERVER USAGE +==================== + +A server would be set up to accept operations in the following manner: + + (1) An RxRPC socket is created by: + + server = socket(AF_RXRPC, SOCK_DGRAM, PF_INET); + + Where the third parameter indicates the address type of the transport + socket used - usually IPv4. + + (2) Security is set up if desired by giving the socket a keyring with server + secret keys in it: + + keyring = add_key("keyring", "AFSkeys", NULL, 0, + KEY_SPEC_PROCESS_KEYRING); + + const char secret_key[8] = { + 0xa7, 0x83, 0x8a, 0xcb, 0xc7, 0x83, 0xec, 0x94 }; + add_key("rxrpc_s", "52:2", secret_key, 8, keyring); + + setsockopt(server, SOL_RXRPC, RXRPC_SECURITY_KEYRING, "AFSkeys", 7); + + The keyring can be manipulated after it has been given to the socket. This + permits the server to add more keys, replace keys, etc. whilst it is live. + + (2) A local address must then be bound: + + struct sockaddr_rxrpc srx = { + .srx_family = AF_RXRPC, + .srx_service = VL_SERVICE_ID, /* RxRPC service ID */ + .transport_type = SOCK_DGRAM, /* type of transport socket */ + .transport.sin_family = AF_INET, + .transport.sin_port = htons(7000), /* AFS callback */ + .transport.sin_address = 0, /* all local interfaces */ + }; + bind(server, &srx, sizeof(srx)); + + (3) The server is then set to listen out for incoming calls: + + listen(server, 100); + + (4) The kernel notifies the server of pending incoming connections by sending + it a message for each. This is received with recvmsg() on the server + socket. It has no data, and has a single dataless control message + attached: + + RXRPC_NEW_CALL + + The address that can be passed back by recvmsg() at this point should be + ignored since the call for which the message was posted may have gone by + the time it is accepted - in which case the first call still on the queue + will be accepted. + + (5) The server then accepts the new call by issuing a sendmsg() with two + pieces of control data and no actual data: + + RXRPC_ACCEPT - indicate connection acceptance + RXRPC_USER_CALL_ID - specify user ID for this call + + (6) The first request data packet will then be posted to the server socket for + recvmsg() to pick up. At that point, the RxRPC address for the call can + be read from the address fields in the msghdr struct. + + Subsequent request data will be posted to the server socket for recvmsg() + to collect as it arrives. All but the last piece of the request data will + be delivered with MSG_MORE flagged. + + All data will be delivered with the following control message attached: + + RXRPC_USER_CALL_ID - specifies the user ID for this call + + (8) The reply data should then be posted to the server socket using a series + of sendmsg() calls, each with the following control messages attached: + + RXRPC_USER_CALL_ID - specifies the user ID for this call + + MSG_MORE should be set in msghdr::msg_flags on all but the last message + for a particular call. + + (9) The final ACK from the client will be posted for retrieval by recvmsg() + when it is received. It will take the form of a dataless message with two + control messages attached: + + RXRPC_USER_CALL_ID - specifies the user ID for this call + RXRPC_ACK - indicates final ACK (no data) + + MSG_EOR will be flagged to indicate that this is the final message for + this call. + +(10) Up to the point the final packet of reply data is sent, the call can be + aborted by calling sendmsg() with a dataless message with the following + control messages attached: + + RXRPC_USER_CALL_ID - specifies the user ID for this call + RXRPC_ABORT - indicates abort code (4 byte data) + + Any packets waiting in the socket's receive queue will be discarded if + this is issued. + +Note that all the communications for a particular service take place through +the one server socket, using control messages on sendmsg() and recvmsg() to +determine the call affected. |
