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annotate docs/extension-virtualports.txt @ 9:a11b330771e0

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Fix buffer pointer handling when logging received bytes Actually save the read buffer pointer so we can log the actual protocol bytes read in all cases correctly instead of displaying out of bounds memory in the case where the packet arrives in multiple pieces.
author William Astle <lost@l-w.ca>
date Sat, 30 Jul 2016 10:35:14 -0600
parents 3a4c972c92ec
children
Ignore whitespace changes - Everywhere: Within whitespace: At end of lines:
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1 This document describes an extension to the LWWire protocol which provides
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2 virtual serial ports. The protocol comes in two parts. The first is the low
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3 level signalling protocol to transfer data on the ports and the second is
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4 the higher level "modem" protocol.
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5
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6 A low level port is simply an eight bit clean channel that allows traffic to
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7 pass over it bidirectionally. The low level signalling protocol does not
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8 care what that data is.
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9
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10 A port must be opened before it can be used. Once it is opened, it is
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11 connected with some sort of endpoint. That may be a modem simulation or some
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12 other endpoint specification. When communication is finished, the port is
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13 then closed. A port can be closed by either the client or the server.
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14
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15 LWWire Protocol Extension
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16 =========================
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17
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18 This extension is called VPORT and is assigned extension number 00. It
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19 defines the following operations in its extension space:
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20
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21 00 VPORT_OPEN
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22
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23 This request opens a virtual port. It takes the following parameters:
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24
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25 Octet Meaning
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26 ----- -------
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27 0-2 F3 00 00
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28 3 endpoint type
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29
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30 The endpoint type is 0 for a basic virtual modem as defined by this
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31 extension. Any other value is an extension number which may accept VPORT
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32 connections. In particular, the CCHAN extension is of interest here.
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33
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34 Octet Meaning
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35 ----- -------
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36 0 status
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37 1 port number assigned
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38 F0-FF reserved for private endpoint types
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39
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40 If the status is 0, it means the open was successful and a port was
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41 successfully allocated. Communication over the port can now begin.
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42
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43 Any other status is a failure and the port number value must be regarded as
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44 invalid. The server MUST send 0 for this port number in this instance. The
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45 status will be usually be one 1 if no ports are available to assign or 2 if
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46 ports are available but the requested endpoint type is not available.
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47
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48
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49 01 VPORT_CLOSE
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50
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51 This call informs the server that the client is finished with the specified
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52 port. The call looks as follows:
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53
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54 Octet Meaning
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55 ----- -------
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56 0-2 F3 00 01
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57 3 port number
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58
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59 The return from this call is exactly one octet, a status. If the status is
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60 zero, the port has been closed successfully. If it is 1, it means the port
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61 was not open in the first place.
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62
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63 When a port is closed, all data currently buffered on the server is cleared.
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64 It may or may not be delivered to its intended destination.
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65
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66
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67 02 VPORT_POLL
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68
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69 This call requests the server to provide information on the specified port.
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70 The call looks as follows:
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71
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72 Octet Meaning
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73 ----- -------
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74 0-2 F3 00 02
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75 03 the port to poll
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76
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77 The return looks as follows:
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78
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79 Octet Meaning
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80 ----- -------
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81 0 status
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82 1 write max
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83 2 read max
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84
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85 If the status is nonzero, then the port is not open and the write max and
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86 read max items should be ignored.
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87
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88 Otherwise, write max will be set to the maximum number of octets the server
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89 is willing to handle on a write operation and read max will be the number of
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90 bytes the server can provide in response to a read operation. If either
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91 number exceeds 255, then the server will set the respective value to 255. A
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92 value of 0 for the read max means the server has no data to send. A value of
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93 0 for the write max means the server's buffer is full and it is waiting for
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94 the endpoint to do something with it.
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95
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96
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97 03 VPORT_POLLMULTI
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98
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99 This operation is similar to VPORT_POLL except it allows polling multiple
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100 ports simultaneously. It will return 3 bytes for each port polled, organized
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101 as in the VPORT_POLL call. The ports are returned in the same order as
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102 specified in the request.
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103
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104 The request looks as follows:
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105
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106 Octet Meaning
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107 ----- -------
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108 0-2 F3 00 03
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109 3 number of ports to query
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110 4-n port nubmers to query
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111
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112 This operation can easily have quite a large request size and quite a large
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113 response size (maximum request size is 260 bytes and the maximum response is
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114 768 bytes). This call should only be used for a small number of active
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115 ports.
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116
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117 04 VPORT_READ
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118
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119 This request reads bytes from a virtual port. The request looks as follows:
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120
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121 Octet Meaning
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122 ----- -------
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123 0-2 F3 00 04
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124 3 port number
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125 4 maximum size to read in octets
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126
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127 The response is an LWWire variable length packet as follows:
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128
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129 Octet Meaning
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130 ----- -------
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131 0 status
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132 1 number of bytes returned (n)
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133 2-n returned data
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134
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135 If status is nonzero, then the read failed. This will usually be due to a
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136 lack of data (1) OR the port not being open (2). In this case, any
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137 subsequent octets in the packet can be ignored.
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138
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139 Otherwise, the packet will contain n bytes of data. The client must be
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140 certain not to request more bytes than it can handle.
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141
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142
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143 05 VPORT_WRITE
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144
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145 This request is used to send octets to a port. The request looks as follows:
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146
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147 Octet Meaning
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148 ----- -------
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149 0-2 F3 00 05
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150 3 port number
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151 4 number of bytes
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152 5-n the data to write
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153
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154 The response is as follows:
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155
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156 Octet Meaning
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157 ----- -------
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158 0 status
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159 1 bytes written
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160
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161 If status is zero, than the bytes written value will be valid. If all bytes
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162 were written, then it will match the number in the request. If this value is
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163 less than the requested number, it means some bytes were not written for
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164 whatever reason and the client must deal with queueing or buffering as
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165 appropriate.
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166
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167
6
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168 06 VPORT_READFIXED
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169
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170 This request reads bytes from a virtual port. The request looks as follows:
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171
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172 Octet Meaning
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173 ----- -------
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174 0-2 F3 00 04
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175 3 port number
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176 4 size to read in octets (N)
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177
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178 The response is looks as follows:
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179
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180 Octet Meaning
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181 ----- -------
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182 0 status
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183 1 number of bytes returned (n)
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184 2-... returned data
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185
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186 In all cases, there will be exactly N octets in the returned data. Any bytes
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187 not filled by actual data are undefined but should be zero.
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188
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189 If status is nonzero, then the read failed. This will usually be due to a
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190 lack of data (1) OR the port not being open (2). In this case, any
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191 subsequent octets in the packet can be ignored.
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192
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193 If (n) is less than (N), then there will be extra octets in the returned
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194 data section which must be ignored.
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195
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196 This option is provided as an alternative to the VPORT_READ call for systems
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197 which cannot easily handle a variable length packet safely. In particular, a
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198 115k "bitbanger" implementation on the Coco itself might choose to use this
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199 option to adding another 500+ bytes to its lwwire client driver. Ohter
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200 communication channels that do not have this limitation should prefer the
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201 variable length packet option with larger request sizes to minimize
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202 transaction count.
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203