diff options
Diffstat (limited to 'Documentation/networking/ppp_generic.txt')
| -rw-r--r-- | Documentation/networking/ppp_generic.txt | 428 |
1 files changed, 0 insertions, 428 deletions
diff --git a/Documentation/networking/ppp_generic.txt b/Documentation/networking/ppp_generic.txt deleted file mode 100644 index fd563aff5fc9..000000000000 --- a/Documentation/networking/ppp_generic.txt +++ /dev/null @@ -1,428 +0,0 @@ - PPP Generic Driver and Channel Interface - ---------------------------------------- - - Paul Mackerras - paulus@samba.org - 7 Feb 2002 - -The generic PPP driver in linux-2.4 provides an implementation of the -functionality which is of use in any PPP implementation, including: - -* the network interface unit (ppp0 etc.) -* the interface to the networking code -* PPP multilink: splitting datagrams between multiple links, and - ordering and combining received fragments -* the interface to pppd, via a /dev/ppp character device -* packet compression and decompression -* TCP/IP header compression and decompression -* detecting network traffic for demand dialling and for idle timeouts -* simple packet filtering - -For sending and receiving PPP frames, the generic PPP driver calls on -the services of PPP `channels'. A PPP channel encapsulates a -mechanism for transporting PPP frames from one machine to another. A -PPP channel implementation can be arbitrarily complex internally but -has a very simple interface with the generic PPP code: it merely has -to be able to send PPP frames, receive PPP frames, and optionally -handle ioctl requests. Currently there are PPP channel -implementations for asynchronous serial ports, synchronous serial -ports, and for PPP over ethernet. - -This architecture makes it possible to implement PPP multilink in a -natural and straightforward way, by allowing more than one channel to -be linked to each ppp network interface unit. The generic layer is -responsible for splitting datagrams on transmit and recombining them -on receive. - - -PPP channel API ---------------- - -See include/linux/ppp_channel.h for the declaration of the types and -functions used to communicate between the generic PPP layer and PPP -channels. - -Each channel has to provide two functions to the generic PPP layer, -via the ppp_channel.ops pointer: - -* start_xmit() is called by the generic layer when it has a frame to - send. The channel has the option of rejecting the frame for - flow-control reasons. In this case, start_xmit() should return 0 - and the channel should call the ppp_output_wakeup() function at a - later time when it can accept frames again, and the generic layer - will then attempt to retransmit the rejected frame(s). If the frame - is accepted, the start_xmit() function should return 1. - -* ioctl() provides an interface which can be used by a user-space - program to control aspects of the channel's behaviour. This - procedure will be called when a user-space program does an ioctl - system call on an instance of /dev/ppp which is bound to the - channel. (Usually it would only be pppd which would do this.) - -The generic PPP layer provides seven functions to channels: - -* ppp_register_channel() is called when a channel has been created, to - notify the PPP generic layer of its presence. For example, setting - a serial port to the PPPDISC line discipline causes the ppp_async - channel code to call this function. - -* ppp_unregister_channel() is called when a channel is to be - destroyed. For example, the ppp_async channel code calls this when - a hangup is detected on the serial port. - -* ppp_output_wakeup() is called by a channel when it has previously - rejected a call to its start_xmit function, and can now accept more - packets. - -* ppp_input() is called by a channel when it has received a complete - PPP frame. - -* ppp_input_error() is called by a channel when it has detected that a - frame has been lost or dropped (for example, because of a FCS (frame - check sequence) error). - -* ppp_channel_index() returns the channel index assigned by the PPP - generic layer to this channel. The channel should provide some way - (e.g. an ioctl) to transmit this back to user-space, as user-space - will need it to attach an instance of /dev/ppp to this channel. - -* ppp_unit_number() returns the unit number of the ppp network - interface to which this channel is connected, or -1 if the channel - is not connected. - -Connecting a channel to the ppp generic layer is initiated from the -channel code, rather than from the generic layer. The channel is -expected to have some way for a user-level process to control it -independently of the ppp generic layer. For example, with the -ppp_async channel, this is provided by the file descriptor to the -serial port. - -Generally a user-level process will initialize the underlying -communications medium and prepare it to do PPP. For example, with an -async tty, this can involve setting the tty speed and modes, issuing -modem commands, and then going through some sort of dialog with the -remote system to invoke PPP service there. We refer to this process -as `discovery'. Then the user-level process tells the medium to -become a PPP channel and register itself with the generic PPP layer. -The channel then has to report the channel number assigned to it back -to the user-level process. From that point, the PPP negotiation code -in the PPP daemon (pppd) can take over and perform the PPP -negotiation, accessing the channel through the /dev/ppp interface. - -At the interface to the PPP generic layer, PPP frames are stored in -skbuff structures and start with the two-byte PPP protocol number. -The frame does *not* include the 0xff `address' byte or the 0x03 -`control' byte that are optionally used in async PPP. Nor is there -any escaping of control characters, nor are there any FCS or framing -characters included. That is all the responsibility of the channel -code, if it is needed for the particular medium. That is, the skbuffs -presented to the start_xmit() function contain only the 2-byte -protocol number and the data, and the skbuffs presented to ppp_input() -must be in the same format. - -The channel must provide an instance of a ppp_channel struct to -represent the channel. The channel is free to use the `private' field -however it wishes. The channel should initialize the `mtu' and -`hdrlen' fields before calling ppp_register_channel() and not change -them until after ppp_unregister_channel() returns. The `mtu' field -represents the maximum size of the data part of the PPP frames, that -is, it does not include the 2-byte protocol number. - -If the channel needs some headroom in the skbuffs presented to it for -transmission (i.e., some space free in the skbuff data area before the -start of the PPP frame), it should set the `hdrlen' field of the -ppp_channel struct to the amount of headroom required. The generic -PPP layer will attempt to provide that much headroom but the channel -should still check if there is sufficient headroom and copy the skbuff -if there isn't. - -On the input side, channels should ideally provide at least 2 bytes of -headroom in the skbuffs presented to ppp_input(). The generic PPP -code does not require this but will be more efficient if this is done. - - -Buffering and flow control --------------------------- - -The generic PPP layer has been designed to minimize the amount of data -that it buffers in the transmit direction. It maintains a queue of -transmit packets for the PPP unit (network interface device) plus a -queue of transmit packets for each attached channel. Normally the -transmit queue for the unit will contain at most one packet; the -exceptions are when pppd sends packets by writing to /dev/ppp, and -when the core networking code calls the generic layer's start_xmit() -function with the queue stopped, i.e. when the generic layer has -called netif_stop_queue(), which only happens on a transmit timeout. -The start_xmit function always accepts and queues the packet which it -is asked to transmit. - -Transmit packets are dequeued from the PPP unit transmit queue and -then subjected to TCP/IP header compression and packet compression -(Deflate or BSD-Compress compression), as appropriate. After this -point the packets can no longer be reordered, as the decompression -algorithms rely on receiving compressed packets in the same order that -they were generated. - -If multilink is not in use, this packet is then passed to the attached -channel's start_xmit() function. If the channel refuses to take -the packet, the generic layer saves it for later transmission. The -generic layer will call the channel's start_xmit() function again -when the channel calls ppp_output_wakeup() or when the core -networking code calls the generic layer's start_xmit() function -again. The generic layer contains no timeout and retransmission -logic; it relies on the core networking code for that. - -If multilink is in use, the generic layer divides the packet into one -or more fragments and puts a multilink header on each fragment. It -decides how many fragments to use based on the length of the packet -and the number of channels which are potentially able to accept a -fragment at the moment. A channel is potentially able to accept a -fragment if it doesn't have any fragments currently queued up for it -to transmit. The channel may still refuse a fragment; in this case -the fragment is queued up for the channel to transmit later. This -scheme has the effect that more fragments are given to higher- -bandwidth channels. It also means that under light load, the generic -layer will tend to fragment large packets across all the channels, -thus reducing latency, while under heavy load, packets will tend to be -transmitted as single fragments, thus reducing the overhead of -fragmentation. - - -SMP safety ----------- - -The PPP generic layer has been designed to be SMP-safe. Locks are -used around accesses to the internal data structures where necessary -to ensure their integrity. As part of this, the generic layer -requires that the channels adhere to certain requirements and in turn -provides certain guarantees to the channels. Essentially the channels -are required to provide the appropriate locking on the ppp_channel -structures that form the basis of the communication between the -channel and the generic layer. This is because the channel provides -the storage for the ppp_channel structure, and so the channel is -required to provide the guarantee that this storage exists and is -valid at the appropriate times. - -The generic layer requires these guarantees from the channel: - -* The ppp_channel object must exist from the time that - ppp_register_channel() is called until after the call to - ppp_unregister_channel() returns. - -* No thread may be in a call to any of ppp_input(), ppp_input_error(), - ppp_output_wakeup(), ppp_channel_index() or ppp_unit_number() for a - channel at the time that ppp_unregister_channel() is called for that - channel. - -* ppp_register_channel() and ppp_unregister_channel() must be called - from process context, not interrupt or softirq/BH context. - -* The remaining generic layer functions may be called at softirq/BH - level but must not be called from a hardware interrupt handler. - -* The generic layer may call the channel start_xmit() function at - softirq/BH level but will not call it at interrupt level. Thus the - start_xmit() function may not block. - -* The generic layer will only call the channel ioctl() function in - process context. - -The generic layer provides these guarantees to the channels: - -* The generic layer will not call the start_xmit() function for a - channel while any thread is already executing in that function for - that channel. - -* The generic layer will not call the ioctl() function for a channel - while any thread is already executing in that function for that - channel. - -* By the time a call to ppp_unregister_channel() returns, no thread - will be executing in a call from the generic layer to that channel's - start_xmit() or ioctl() function, and the generic layer will not - call either of those functions subsequently. - - -Interface to pppd ------------------ - -The PPP generic layer exports a character device interface called -/dev/ppp. This is used by pppd to control PPP interface units and -channels. Although there is only one /dev/ppp, each open instance of -/dev/ppp acts independently and can be attached either to a PPP unit -or a PPP channel. This is achieved using the file->private_data field -to point to a separate object for each open instance of /dev/ppp. In -this way an effect similar to Solaris' clone open is obtained, -allowing us to control an arbitrary number of PPP interfaces and -channels without having to fill up /dev with hundreds of device names. - -When /dev/ppp is opened, a new instance is created which is initially -unattached. Using an ioctl call, it can then be attached to an -existing unit, attached to a newly-created unit, or attached to an -existing channel. An instance attached to a unit can be used to send -and receive PPP control frames, using the read() and write() system -calls, along with poll() if necessary. Similarly, an instance -attached to a channel can be used to send and receive PPP frames on -that channel. - -In multilink terms, the unit represents the bundle, while the channels -represent the individual physical links. Thus, a PPP frame sent by a -write to the unit (i.e., to an instance of /dev/ppp attached to the -unit) will be subject to bundle-level compression and to fragmentation -across the individual links (if multilink is in use). In contrast, a -PPP frame sent by a write to the channel will be sent as-is on that -channel, without any multilink header. - -A channel is not initially attached to any unit. In this state it can -be used for PPP negotiation but not for the transfer of data packets. -It can then be connected to a PPP unit with an ioctl call, which -makes it available to send and receive data packets for that unit. - -The ioctl calls which are available on an instance of /dev/ppp depend -on whether it is unattached, attached to a PPP interface, or attached -to a PPP channel. The ioctl calls which are available on an -unattached instance are: - -* PPPIOCNEWUNIT creates a new PPP interface and makes this /dev/ppp - instance the "owner" of the interface. The argument should point to - an int which is the desired unit number if >= 0, or -1 to assign the - lowest unused unit number. Being the owner of the interface means - that the interface will be shut down if this instance of /dev/ppp is - closed. - -* PPPIOCATTACH attaches this instance to an existing PPP interface. - The argument should point to an int containing the unit number. - This does not make this instance the owner of the PPP interface. - -* PPPIOCATTCHAN attaches this instance to an existing PPP channel. - The argument should point to an int containing the channel number. - -The ioctl calls available on an instance of /dev/ppp attached to a -channel are: - -* PPPIOCCONNECT connects this channel to a PPP interface. The - argument should point to an int containing the interface unit - number. It will return an EINVAL error if the channel is already - connected to an interface, or ENXIO if the requested interface does - not exist. - -* PPPIOCDISCONN disconnects this channel from the PPP interface that - it is connected to. It will return an EINVAL error if the channel - is not connected to an interface. - -* All other ioctl commands are passed to the channel ioctl() function. - -The ioctl calls that are available on an instance that is attached to -an interface unit are: - -* PPPIOCSMRU sets the MRU (maximum receive unit) for the interface. - The argument should point to an int containing the new MRU value. - -* PPPIOCSFLAGS sets flags which control the operation of the - interface. The argument should be a pointer to an int containing - the new flags value. The bits in the flags value that can be set - are: - SC_COMP_TCP enable transmit TCP header compression - SC_NO_TCP_CCID disable connection-id compression for - TCP header compression - SC_REJ_COMP_TCP disable receive TCP header decompression - SC_CCP_OPEN Compression Control Protocol (CCP) is - open, so inspect CCP packets - SC_CCP_UP CCP is up, may (de)compress packets - SC_LOOP_TRAFFIC send IP traffic to pppd - SC_MULTILINK enable PPP multilink fragmentation on - transmitted packets - SC_MP_SHORTSEQ expect short multilink sequence - numbers on received multilink fragments - SC_MP_XSHORTSEQ transmit short multilink sequence nos. - - The values of these flags are defined in <linux/ppp-ioctl.h>. Note - that the values of the SC_MULTILINK, SC_MP_SHORTSEQ and - SC_MP_XSHORTSEQ bits are ignored if the CONFIG_PPP_MULTILINK option - is not selected. - -* PPPIOCGFLAGS returns the value of the status/control flags for the - interface unit. The argument should point to an int where the ioctl - will store the flags value. As well as the values listed above for - PPPIOCSFLAGS, the following bits may be set in the returned value: - SC_COMP_RUN CCP compressor is running - SC_DECOMP_RUN CCP decompressor is running - SC_DC_ERROR CCP decompressor detected non-fatal error - SC_DC_FERROR CCP decompressor detected fatal error - -* PPPIOCSCOMPRESS sets the parameters for packet compression or - decompression. The argument should point to a ppp_option_data - structure (defined in <linux/ppp-ioctl.h>), which contains a - pointer/length pair which should describe a block of memory - containing a CCP option specifying a compression method and its - parameters. The ppp_option_data struct also contains a `transmit' - field. If this is 0, the ioctl will affect the receive path, - otherwise the transmit path. - -* PPPIOCGUNIT returns, in the int pointed to by the argument, the unit - number of this interface unit. - -* PPPIOCSDEBUG sets the debug flags for the interface to the value in - the int pointed to by the argument. Only the least significant bit - is used; if this is 1 the generic layer will print some debug - messages during its operation. This is only intended for debugging - the generic PPP layer code; it is generally not helpful for working - out why a PPP connection is failing. - -* PPPIOCGDEBUG returns the debug flags for the interface in the int - pointed to by the argument. - -* PPPIOCGIDLE returns the time, in seconds, since the last data - packets were sent and received. The argument should point to a - ppp_idle structure (defined in <linux/ppp_defs.h>). If the - CONFIG_PPP_FILTER option is enabled, the set of packets which reset - the transmit and receive idle timers is restricted to those which - pass the `active' packet filter. - Two versions of this command exist, to deal with user space - expecting times as either 32-bit or 64-bit time_t seconds. - -* PPPIOCSMAXCID sets the maximum connection-ID parameter (and thus the - number of connection slots) for the TCP header compressor and - decompressor. The lower 16 bits of the int pointed to by the - argument specify the maximum connection-ID for the compressor. If - the upper 16 bits of that int are non-zero, they specify the maximum - connection-ID for the decompressor, otherwise the decompressor's - maximum connection-ID is set to 15. - -* PPPIOCSNPMODE sets the network-protocol mode for a given network - protocol. The argument should point to an npioctl struct (defined - in <linux/ppp-ioctl.h>). The `protocol' field gives the PPP protocol - number for the protocol to be affected, and the `mode' field - specifies what to do with packets for that protocol: - - NPMODE_PASS normal operation, transmit and receive packets - NPMODE_DROP silently drop packets for this protocol - NPMODE_ERROR drop packets and return an error on transmit - NPMODE_QUEUE queue up packets for transmit, drop received - packets - - At present NPMODE_ERROR and NPMODE_QUEUE have the same effect as - NPMODE_DROP. - -* PPPIOCGNPMODE returns the network-protocol mode for a given - protocol. The argument should point to an npioctl struct with the - `protocol' field set to the PPP protocol number for the protocol of - interest. On return the `mode' field will be set to the network- - protocol mode for that protocol. - -* PPPIOCSPASS and PPPIOCSACTIVE set the `pass' and `active' packet - filters. These ioctls are only available if the CONFIG_PPP_FILTER - option is selected. The argument should point to a sock_fprog - structure (defined in <linux/filter.h>) containing the compiled BPF - instructions for the filter. Packets are dropped if they fail the - `pass' filter; otherwise, if they fail the `active' filter they are - passed but they do not reset the transmit or receive idle timer. - -* PPPIOCSMRRU enables or disables multilink processing for received - packets and sets the multilink MRRU (maximum reconstructed receive - unit). The argument should point to an int containing the new MRRU - value. If the MRRU value is 0, processing of received multilink - fragments is disabled. This ioctl is only available if the - CONFIG_PPP_MULTILINK option is selected. - -Last modified: 7-feb-2002 |