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<title>Packet Protocols</title>
<meta name="author" content="Philip Levis" />
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<tr class="field"><th class="docinfo-name">Type:</th><td class="field-body">Documentary</td>
</tr>
<tr><th class="docinfo-name">Status:</th>
-<td>Draft</td></tr>
+<td>Final</td></tr>
<tr class="field"><th class="docinfo-name">TinyOS-Version:</th><td class="field-body">> 2.1</td>
</tr>
<tr><th class="docinfo-name">Author:</th>
<td>Philip Levis</td></tr>
-<tr class="field"><th class="docinfo-name">Draft-Created:</th><td class="field-body">10-Dec-2004</td>
-</tr>
-<tr class="field"><th class="docinfo-name">Draft-Version:</th><td class="field-body">1.10</td>
-</tr>
-<tr class="field"><th class="docinfo-name">Draft-Modified:</th><td class="field-body">2007-03-28</td>
-</tr>
-<tr class="field"><th class="docinfo-name">Draft-Discuss:</th><td class="field-body">TinyOS Developer List <tinyos-devel at mail.millennium.berkeley.edu></td>
-</tr>
</tbody>
</table>
<div class="note">
</pre>
<p>A component can obtain a pointer to its data region within a packet by
calling <tt class="docutils literal"><span class="pre">getPayload()</span></tt>. A call to this command includes the length
-the caller requires. The command <tt class="docutils literal"><span class="pre">maxPayloadLength</span></tt> returns the maximum
-length the payload can be: if the <tt class="docutils literal"><span class="pre">len</span></tt> parameter to <tt class="docutils literal"><span class="pre">getPayload</span></tt>
-is greater than the value <tt class="docutils literal"><span class="pre">maxPayloadLength``would</span> <span class="pre">return,</span>
-<span class="pre">``getPayload</span></tt> MUST return NULL.</p>
+the caller requires. The command <tt class="docutils literal"><span class="pre">maxPayloadLength</span></tt> returns the
+maximum length the payload can be: if the <tt class="docutils literal"><span class="pre">len</span></tt> parameter to
+<tt class="docutils literal"><span class="pre">getPayload</span></tt> is greater than the value <tt class="docutils literal"><span class="pre">maxPayloadLength</span></tt> would
+return, <tt class="docutils literal"><span class="pre">getPayload</span></tt> MUST return NULL.</p>
<p>A component can set the payload length with <tt class="docutils literal"><span class="pre">setPayLoadLength.</span></tt> A
component can obtain the size of the data region of packet in use with
a call to <tt class="docutils literal"><span class="pre">payloadLength</span></tt>. As Send interfaces always include a
can put in the packet. By definition, the return value of
<tt class="docutils literal"><span class="pre">payloadLength</span></tt> must be less than or equal to the return value of
<tt class="docutils literal"><span class="pre">maxPayloadLength</span></tt>.</p>
-<p>The Packet interface assumes that headers have a fixed size.
-It is difficult to return a pointer into the data region when its
-position will only be known once the header values are bound.</p>
-<p>Generally, an incoming call to the Packet interface of a protocol
-has an accompanying outgoing call to the Packet interface of the
-component below it. The one exception to this is the data link
-layer. For example, if there is a network that introduces
-16-bit sequence numbers to packets, it might look like this:</p>
+<p>The Packet interface assumes that headers have a fixed size. It is
+difficult to return a pointer into the data region when its position
+will only be known once the header values are bound.</p>
+<p>The <tt class="docutils literal"><span class="pre">clear</span></tt> command clears out all headers, footers, and metadata
+for lower layers. For example, calling <tt class="docutils literal"><span class="pre">clear</span></tt> on a routing
+component, such as CollectionSenderC[4]_, will clear out the
+collection headers and footers. Furthermore, CollectionSenderC will
+recursively call <tt class="docutils literal"><span class="pre">clear</span></tt> on the layer below it, clearing out the
+link layer headers and footers. Calling <tt class="docutils literal"><span class="pre">clear</span></tt> is typically
+necessary when moving a packet across two link layers. Otherwise, the
+destination link layer may incorrectly interpret metadata from the
+source link layer, and, for example, transmit the packet on the wrong
+RF channel. Because <tt class="docutils literal"><span class="pre">clear</span></tt> prepares a packet for a particular link
+layer, in this example correct code would call the command on the
+destination link layer, not the source link layer.</p>
+<p>Typically, an incoming call to the Packet interface of a protocol has
+an accompanying outgoing call to the Packet interface of the component
+below it. The one exception to this is the data link layer. For
+example, if there is a network that introduces 16-bit sequence numbers
+to packets, it might look like this:</p>
<pre class="literal-block">
generic module SequenceNumber {
provides interface Packet;
};
command void Packet.clear(message_t* msg) {
- uint8_t len;
void* payload = call SubPacket.getPayload(msg, call SubPacket.maxPayloadLength());
- memset(payload, len, 0);
+ call SubPacket.clear();
+ if (payload != NULL) {
+ memset(payload, sizeof(seq_header_t), 0);
+ }
}
command uint8_t Packet.payloadLength(message_t* msg) {
command bool isForMe(message_t* amsg);
command am_id_t type(message_t* amsg);
command void setType(message_t* amsg, am_id_t t);
+ command am_group_t group(message_t* amsg);
+ command void setGroup(message_t* amsg, am_group_t grp);
+ command am_group_t localGroup();
}
</pre>
<p>The command address() returns the local AM address of the
-node. AMPacket provides accessors for its two fields, destination and
-type. It also provides commands to set these fields, for the same
-reason that Packet allows a caller to set the payload length.
-Packet interfaces SHOULD provide accessors
-and mutators for all of their fields to enable queues and other
-buffering to store values in a packet buffer. Typically, a component
-stores these values in the packet buffer itself (where the field is),
-but when necessary it may use the metadata region of message_t or other
-locations.</p>
+node. AMPacket provides accessors for its four fields, destination,
+source, type and group. It also provides commands to set these
+fields, for the same
+reason that Packet allows a caller to set the payload length. Packet
+interfaces SHOULD provide accessors and mutators for all of their
+fields to enable queues and other buffering to store values in a
+packet buffer. Typically, a component stores these values in the
+packet buffer itself (where the field is), but when necessary it may
+use the metadata region of message_t or other locations.</p>
+<p>The group field refers to the AM group, a logical network identifier.
+Link layers will typically only signal reception for packets whose AM
+group matches the node's, which <tt class="docutils literal"><span class="pre">localGroup</span></tt> returns.</p>
</div>
<div class="section">
<h2><a id="sending-interfaces" name="sending-interfaces">2.2 Sending interfaces</a></h2>
and serialize them onto sending interface, or they can introduce
a new sending interface that supports multiple pending transmissions.</p>
<p>The cancel command allows a sender to cancel the current transmission.
-A call to cancel when there is no pending sendDone event MUST return FAIL.
-If there is a pending sendDone event and the cancel returns SUCCESS, then
-the packet layer MUST NOT transmit the packet and MUST signal sendDone
-with ECANCEL as its error code. If there is a pending sendDone event
-and cancel returns FAIL, then sendDone SHOULD occur as if the cancel
-was not called.</p>
+A call to cancel when there is no pending sendDone event MUST return
+FAIL. If there is a pending sendDone event and the cancel returns
+SUCCESS, then the packet layer MUST NOT transmit the packet and MUST
+signal sendDone with ECANCEL as its error code. If there is a pending
+sendDone event and cancel returns FAIL, then sendDone MUST occur as if
+the cancel was not called.</p>
</div>
<div class="section">
<h2><a id="receive-interface" name="receive-interface">2.3 Receive interface</a></h2>
event message_t* receive(message_t* msg, void* payload, uint8_t len);
}
</pre>
-<p>A call to <tt class="docutils literal"><span class="pre">Receive.getPayload()</span></tt> MUST behave identically to a call
-to <tt class="docutils literal"><span class="pre">Packet.getPayload()</span></tt>. The <tt class="docutils literal"><span class="pre">receive()</span></tt> event's <tt class="docutils literal"><span class="pre">payload</span></tt>
-parameter MUST be identical to what a call to <tt class="docutils literal"><span class="pre">getPayload()</span></tt> would
-return, and the <tt class="docutils literal"><span class="pre">len</span></tt> parameter MUST be identical to the length that
-a call to <tt class="docutils literal"><span class="pre">getPayload</span></tt> would return. These parameters are for
+<p>The <tt class="docutils literal"><span class="pre">receive()</span></tt> event's <tt class="docutils literal"><span class="pre">payload</span></tt> parameter MUST be identical to
+what a call to the corresponding <tt class="docutils literal"><span class="pre">Packet.getPayload()</span></tt> would return,
+and the <tt class="docutils literal"><span class="pre">len</span></tt> parameter MUST be identical to the length that a call
+to <tt class="docutils literal"><span class="pre">Packet.getPayload</span></tt> would return. These parameters are for
convenience, as they are commonly used by receive handlers, and their
-presence removes the need for a call to <tt class="docutils literal"><span class="pre">getPayload()</span></tt>, while
-<tt class="docutils literal"><span class="pre">getPayload()</span></tt> is a convenience so a component does not have to wire
-to <tt class="docutils literal"><span class="pre">Packet.</span></tt> The command <tt class="docutils literal"><span class="pre">payloadLength</span></tt> has a similar motivation
-and the same semantics as its twin in <tt class="docutils literal"><span class="pre">Packet</span></tt>.</p>
-<p>Receive has a <em>buffer-swap</em> policy. The handler of the event MUST return
-a pointer to a valid message buffer for the signaler to use. This
-approach enforces an equilibrium between upper and lower packet
-layers. If an upper layer cannot handle packets as quickly as they
-are arriving, it still has to return a valid buffer to the lower
+presence removes the need for a call to <tt class="docutils literal"><span class="pre">getPayload()</span></tt>. Unlike Send,
+Receive does not have a convenience <tt class="docutils literal"><span class="pre">getPayload</span></tt> call, because doing
+so prevents fan-in. As Receive has only a single event, users of
+Receive can be wired multiple times.</p>
+<p>Receive has a <em>buffer-swap</em> policy. The handler of the event MUST
+return a pointer to a valid message buffer for the signaler to
+use. This approach enforces an equilibrium between upper and lower
+packet layers. If an upper layer cannot handle packets as quickly as
+they are arriving, it still has to return a valid buffer to the lower
layer. This buffer could be the <tt class="docutils literal"><span class="pre">msg</span></tt> parameter passed to it: it
just returns the buffer it was given without looking at it. Following
-this policy means that a data-rate mismatch in an upper-level component
-will be isolated to that component. It will drop packets, but it will
-not prevent other components from receiving packets. If an upper
-layer did not have to return a buffer immediately, then when an
+this policy means that a data-rate mismatch in an upper-level
+component will be isolated to that component. It will drop packets,
+but it will not prevent other components from receiving packets. If an
+upper layer did not have to return a buffer immediately, then when an
upper layer cannot handle packets quickly enough it will end up
holding all of them, starving lower layers and possibly preventing
packet reception.</p>
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