<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
-<meta name="generator" content="Docutils 0.4: http://docutils.sourceforge.net/" />
+<meta name="generator" content="Docutils 0.3.6: http://docutils.sourceforge.net/" />
<title>The Collection Tree Protocol (CTP)</title>
<meta name="author" content="Rodrigo Fonseca, Omprakash Gnawali, Kyle Jamieson, Sukun Kim, Philip Levis, and Alec Woo" />
<style type="text/css">
</style>
</head>
<body>
-<div class="document" id="the-collection-tree-protocol-ctp">
<h1 class="title">The Collection Tree Protocol (CTP)</h1>
<table class="docinfo" frame="void" rules="none">
<col class="docinfo-name" />
<td>Rodrigo Fonseca, Omprakash Gnawali, Kyle Jamieson, Sukun Kim, Philip Levis, and Alec Woo</td></tr>
<tr class="field"><th class="docinfo-name">Draft-Created:</th><td class="field-body">3-Aug-2006</td>
</tr>
-<tr class="field"><th class="docinfo-name">Draft-Version:</th><td class="field-body">1.1.2.3</td>
+<tr class="field"><th class="docinfo-name">Draft-Version:</th><td class="field-body">1.8</td>
</tr>
-<tr class="field"><th class="docinfo-name">Draft-Modified:</th><td class="field-body">2006-10-25</td>
+<tr class="field"><th class="docinfo-name">Draft-Modified:</th><td class="field-body">2007-02-28</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="document" id="the-collection-tree-protocol-ctp">
<div class="note">
<p class="first admonition-title">Note</p>
<p class="last">This memo documents a part of TinyOS for the TinyOS Community, and
of this memo is unlimited. This memo is in full compliance with
TEP 1.</p>
</div>
-<div class="section">
-<h1><a id="abstract" name="abstract">Abstract</a></h1>
-<p>This memo documents the Collection Tree Protocol (CTP), which
-provides best-effort anycast datagram communication to one of the
+<div class="section" id="abstract">
+<h1><a name="abstract">Abstract</a></h1>
+<p>This memo documents the Collection Tree Protocol (CTP), which
+provides best-effort anycast datagram communication to one of the
collection roots in a network.</p>
</div>
-<div class="section">
-<h1><a id="introduction" name="introduction">1. Introduction</a></h1>
+<div class="section" id="introduction">
+<h1><a name="introduction">1. Introduction</a></h1>
</div>
-<div class="section">
-<h1><a id="assumptions-and-limitations" name="assumptions-and-limitations">2. Assumptions and Limitations</a></h1>
+<div class="section" id="assumptions-and-limitations">
+<h1><a name="assumptions-and-limitations">2. Assumptions and Limitations</a></h1>
<p>CTP is a tree-based collection protocol. Some number of nodes in a
network advertise themselves as tree roots. Nodes form a set of routing
trees to these roots. CTP is <strong>address-free</strong> in that a node does not
<li>Has single-hop source and destination fields.</li>
</ol>
</blockquote>
-<p>CTP assumes that it has link quality estimates of some number of nearby
-neighbors. These provide an estimate of the number of transmissions it
+<p>CTP assumes that it has link quality estimates of some number of nearby
+neighbors. These provide an estimate of the number of transmissions it
takes for the node to send a unicast packet whose acknowledgment is
successfully received.</p>
<p>CTP has several mechanisms in order to improve delivery reliability,
might benefit from a different protocol, which can, for example, pack
multiple small frames into a single data-link packet.</p>
</div>
-<div class="section">
-<h1><a id="collection-and-ctp" name="collection-and-ctp">3. Collection and CTP</a></h1>
-<p>CTP uses expected transmissions (ETX) as its routing gradient. A root has an
-ETX of 0. The ETX of a node is the ETX of its parent plus the ETX of its
-link to its parent. This additive measure assumes that nodes use link-level
-retransmissions. Given a choice of valid routes, CTP SHOULD choose the one with the
-lowest ETX value. CTP represents ETX values as 16-bit fixed-point real numbers
-with a precision of hundredths. An ETX value of 451, for example, represents
-an ETX of 4.51, while an ETX value of 109 represents an ETX of 1.09.</p>
-<p>Routing loops are a problem that can emerge in a CTP network. Routing loops
-generally occur when a node choose a new route that has a significantly higher
-ETX than its old one, perhaps in response to losing connectivity with a candidate
-parent. If the new route includes a node which was a descendant, then a loop
-occurs.</p>
+<div class="section" id="collection-and-ctp">
+<h1><a name="collection-and-ctp">3. Collection and CTP</a></h1>
+<p>CTP uses expected transmissions (ETX) as its routing gradient. A root
+has an ETX of 0. The ETX of a node is the ETX of its parent plus the
+ETX of its link to its parent. This additive measure assumes that
+nodes use link-level retransmissions. Given a choice of valid routes,
+CTP SHOULD choose the one with the lowest ETX value. CTP represents
+ETX values as 16-bit fixed-point real numbers with a precision of
+hundredths. An ETX value of 451, for example, represents an ETX of
+4.51, while an ETX value of 109 represents an ETX of 1.09.</p>
+<p>Routing loops are a problem that can emerge in a CTP network. Routing
+loops generally occur when a node choose a new route that has a
+significantly higher ETX than its old one, perhaps in response to
+losing connectivity with a candidate parent. If the new route includes
+a node which was a descendant, then a loop occurs.</p>
<p>CTP addresses loops through two mechanisms. First, every CTP packet
contains a node's current gradient value. If CTP receives a data frame with
a gradient value lower than its own, then this indicates that there
the same THL value, while a looped version of the packet is unlikely
to do so.</p>
</div>
-<div class="section">
-<h1><a id="ctp-data-frame" name="ctp-data-frame">4. CTP Data Frame</a></h1>
+<div class="section" id="ctp-data-frame">
+<h1><a name="ctp-data-frame">4. CTP Data Frame</a></h1>
<p>The CTP data frame format is as follows:</p>
<pre class="literal-block">
- 1
- 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6
+ 1
+ 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
-|C|P| reserved | THL |
+|P|C| reserved | THL |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ETX |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
-| origin |
+| origin |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
-| seqno | collect_id |
+| seqno | collect_id |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
-| data ...
+| data ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
</pre>
<p>Field definitions are as follows:</p>
<blockquote>
<ul class="simple">
-<li>C: Congestion notification. If a node is receiving packets faster than it can forward them, it MAY set the C field to notify other nodes. If a node hears a packet from node <em>N</em> with the C bit set, it MUST NOT transmit CTP data frames to <em>N</em> until it hears a packet from N with the C bit cleared.</li>
<li>P: Routing pull. The P bit allows nodes to request routing information from other nodes. If a node with a valid route hears a packet with the P bit set, it SHOULD transmit a routing frame in the near future.</li>
+<li>C: Congestion notification. If a node drops a CTP data frame, it MUST set the C field on the next data frame it transmits.</li>
<li>THL: Time Has Lived. When a node generates a CTP data frame, it MUST set THL to 0. When a node receives a CTP data frame, it MUST increment the THL. If a node receives a THL of 255, it increments it to 0.</li>
<li>ETX: The ETX routing metric of the single-hop sender. When a node transmits a CTP data frame, it MUST put the ETX value of its route through the single-hop destination in the ETX field. If a node receives a packet with a lower gradient than its own, then it MUST schedule a routing frame in the near future.</li>
<li>origin: The originating address of the packet. A node forwarding a data frame MUST NOT modify the origin field.</li>
<li>data: the data payload, of zero or more bytes. A node forwarding a data frame MUST NOT modify the data payload.</li>
</ul>
</blockquote>
-<p>Together, the origin, seqno and collect_id fields denote a unique
+<p>Together, the origin, seqno and collect_id fields denote a unique
<strong>*origin packet.*</strong> Together, the origin, seqno, collect_id, and
THL denote a unique <strong>*packet instance*</strong> within the network. The
distinction is important for duplicate suppression in the presence
drop the packet. However, if it suppresses packet instances, then it
will route succesfully in the presence of transient loops unless the
THL happens to wrap around to a forwarded packet instance.</p>
-<p>A node MUST send CTP data frames as unicast messages with link-layer
+<p>A node MUST send CTP data frames as unicast messages with link-layer
acknowledgments enabled.</p>
</div>
-<div class="section">
-<h1><a id="ctp-routing-frame" name="ctp-routing-frame">5. CTP Routing Frame</a></h1>
+<div class="section" id="ctp-routing-frame">
+<h1><a name="ctp-routing-frame">5. CTP Routing Frame</a></h1>
<p>The CTP routing frame format is as follows:</p>
<pre class="literal-block">
- 1
- 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6
+ 1
+ 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
-|C|P| reserved | parent |
+|P|C| reserved | parent |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
-| parent | ETX |
+| parent | ETX |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
-| ETX |
+| ETX |
+-+-+-+-+-+-+-+-+
</pre>
<p>The fields are as follows:</p>
<blockquote>
<ul class="simple">
-<li>C: Same as data frame.</li>
<li>P: Same as data frame.</li>
+<li>C: Congestion notification. If a node drops a CTP data frame, it MUST set the C field on the next routing frame it transmits.</li>
<li>parent: The node's current parent.</li>
<li>metric: The node's current routing metric value.</li>
</ul>
own, it MUST schedule a routing frame for transmission in the near
future.</p>
</div>
-<div class="section">
-<h1><a id="implementation" name="implementation">6. Implementation</a></h1>
+<div class="section" id="implementation">
+<h1><a name="implementation">6. Implementation</a></h1>
<p>An implementation of CTP can be found in the tos/lib/net/ctp directory
of TinyOS 2.0. This section describes the structure of that implementation
and is not in any way part of the specification of CTP.</p>
to send. It decides when and if to send them. The name is a little
misleading: the forwarding engine is responsible for forwarded traffic
as well as traffic generated on the node.</p>
-<div class="section">
-<h2><a id="link-estimation" name="link-estimation">6.1 Link Estimation</a></h2>
-<p>The link estimator estimates the ETX to single-hop neighbors.
-The implementation uses two mechanisms to estimate the quality of a link:
-periodic broadcast packets and data packets. The estimator itself
-only generates broadcast packets. For data traffic, it depends on
-other components telling it about acknowledged and unacknowledged
-transmissions.</p>
-<p>The periodic broadcast packets have sequence numbers, which the
-estimator uses to estimate the sender-to-receiver packet reception
-rate (PRR). The data payload of periodic broadcast packets contain
-these estimates. Therefore, when node A receives a link estimation
-broadcast message from node B, it can use the packet header to
-estimate the B-to-A PRR and the packet payload to update B's
-estimate of the A-to-B PRR.</p>
-<p>Multiplying these two values gives a <em>bidirectional</em> PRR, or
-an estimate of the probability that if A transmits a packet to B,
-B will successfully hear and acknowledge the packet and A will
-hear the acknowledgment. The inverse of the bidirecitonal PRR
-is the ETX.</p>
-<p>CTP link estimation adapts its beaconing rate to be slow when
-its routing table is stable and faster when changes occur.
-It adjusts the beaconing interval using an algorithm similar
-to the trickle dissemination protocol[<a href="#id1" name="id2"><span class="problematic" id="id2">2_</span></a>]. CTP sends beacons
-more often when one of three conditions occurs:</p>
+<div class="section" id="link-estimation">
+<h2><a name="link-estimation">6.1 Link Estimation</a></h2>
+<p>The implementation uses two mechanisms to estimate the quality of a link:
+periodic LEEP <a class="footnote-reference" href="#id4" id="id1" name="id1">[1]</a> packets and data packets. The implementation sends
+routing beacons as LEEP packets. These packets seed the neighbor table
+with bidirectional ETX values. The implementation adapts its beaconing
+rate based on network dynamics using an algorithm similar to the
+trickle dissemination protocol <a class="footnote-reference" href="#id5" id="id2" name="id2">[2]</a>. Beacons are sent on an exponentially
+increasing randomized timer. The implementation resets the timer to a
+small value when one or more of the following conditions are met:</p>
<blockquote>
<ol class="arabic simple">
<li>The routing table is empty (this also sets the P bit)</li>
<li>The node hears a packet with the P bit set</li>
</ol>
</blockquote>
-<p>CTP also estimates link quality using data transmissions. This
-is a direct measure of ETX. Whenever the data path transmits a
-packet, it tells the link estimator the destimation and whether
-it was successfully acknowledged. The estimator produces an ETX
-estimate every 5 such transmissions, where 0 successes has an
-ETX of 6.</p>
+<p>The implementation augments the LEEP link estimates with data
+transmissions. This is a direct measure of ETX. Whenever the data path
+transmits a packet, it tells the link estimator the destimation and
+whether it was successfully acknowledged. The estimator produces an
+ETX estimate every 5 such transmissions, where 0 successes has an ETX
+of 6.</p>
<p>The estimator combines the beacon and data estimates by incorporating
them into an exponentially weighted moving average. Beacon-based
estimates seed the neighbor table. The expectation is that the low
-beacon rate in a stable network means that for a selected route,
+beacon rate in a stable network means that for a selected route,
data estimates will outweigh beacon estimates. Additionally, as
the rate at which CTP collects data estimates is proportional to
the transmission rate, then it can quickly detect a broken link and
switch to another candidate neighbor.</p>
+<p>The component <tt class="docutils literal"><span class="pre">tos/lib/net/le/LinkEstimatorP</span></tt> implements the
+link estimator. It couples LEEP-based and data-based estimates.</p>
+</div>
+<div class="section" id="routing-engine">
+<h2><a name="routing-engine">6.2 Routing Engine</a></h2>
+<p>The implementation's routing engine is responsible for picking the next
+hop for a data transmission. It keeps track of the path ETX values of
+a subset of the nodes maintained by the link estimation table. The minimum
+cost route has the smallest sum the path ETX from that node and the link
+ETX of that node. The path ETX is therefore the sum of link ETX values
+along the entire route. The component <tt class="docutils literal"><span class="pre">tos/lib/net/ctp/CtpRoutingEngineP</span></tt>
+implements the routing engine.</p>
+</div>
+<div class="section" id="forwarding-engine">
+<h2><a name="forwarding-engine">6.3 Forwarding Engine</a></h2>
+<p>The component <tt class="docutils literal"><span class="pre">tos/lib/net/ctp/CtpForwardingEngineP</span></tt> implements the
+forwarding engine. It has five repsonsibilities:</p>
+<blockquote>
+<ol class="arabic simple">
+<li>Transmitting packets to the next hop, retransmitting when necessary, and
+passing acknowledgment based information to the link estimator</li>
+<li>Deciding <em>when</em> to transmit packets to the next hop</li>
+<li>Detecting routing inconsistencies and informing the routing engine</li>
+<li>Maintaining a queue of packets to transmit, which are a mix of locally
+generated and forwarded packets</li>
+<li>Detecting single-hop transmission duplicates caused by lost acknowledgments</li>
+</ol>
+</blockquote>
+<p>The four key functions of the forwading engine are packet reception
+(<tt class="docutils literal"><span class="pre">SubReceive.receive()</span></tt>), packet forwarding (<tt class="docutils literal"><span class="pre">forward()</span></tt>), packet
+transmission (<tt class="docutils literal"><span class="pre">sendTask()</span></tt>) and deciding what to do after a packet
+transmission (<tt class="docutils literal"><span class="pre">SubSend.sendDone()</span></tt>).</p>
+<p>The receive function decides whether or not the node should forward a
+packet. It checks for duplicates using a small cache of recently received
+packets. If it decides a packet is not a duplicate, it calls the
+forwading function.</p>
+<p>The forwarding function formats the packet for forwarding. It checks the
+received packet to see if there is possibly a loop in the network.
+It checks if there is space in the transmission queue.
+If there is no space, it drops the packet and sets the C bit. If the
+transmission queue was empty, then it posts the send task.</p>
+<p>The send task examines the packet at the head of the transmission
+queue, formats it for the next hop (requests the route from the
+routing layer, etc.), and submits it to the AM layer.</p>
+<p>When the send completes, sendDone examines the packet to see the result.
+If the packet was acknowledged, it pulls the packet off the transmission
+queue. If the packet was locally generated, it signals sendDone() to the
+client above. If it was forwarded, it returns the packet to the forwarding
+message pool. If there are packets remaining in the queue (e.g., the
+packet was not acknowledged), it starts a randomized timer that reposts
+this task. This timer essentially rate limits CTP so that it does not
+stream packets as quickly as possible, in order to prevent self-collisions
+along the path.</p>
</div>
-<div class="section">
-<h2><a id="routing-engine" name="routing-engine">6.2 Routing Engine</a></h2>
-<p>The</p>
</div>
+<div class="section" id="citations">
+<h1><a name="citations">7. Citations</a></h1>
+<div class="line-block">
+<div class="line">Rodrigo Fonseca </div>
+<div class="line">473 Soda Hall</div>
+<div class="line">Berkeley, CA 94720-1776</div>
+<div class="line"><br /></div>
+<div class="line">phone - +1 510 642-8919</div>
+<div class="line">email - <a class="reference" href="mailto:rfonseca@cs.berkeley.edu">rfonseca@cs.berkeley.edu</a></div>
+<div class="line"><br /></div>
+<div class="line"><br /></div>
+<div class="line">Omprakash Gnawali</div>
+<div class="line">Ronald Tutor Hall (RTH) 418 </div>
+<div class="line">3710 S. McClintock Avenue</div>
+<div class="line">Los Angeles, CA 90089 </div>
+<div class="line"><br /></div>
+<div class="line">phone - +1 213 821-5627</div>
+<div class="line">email - <a class="reference" href="mailto:gnawali@usc.edu">gnawali@usc.edu</a></div>
+<div class="line"><br /></div>
+<div class="line"><br /></div>
+<div class="line">Kyle Jamieson</div>
+<div class="line">The Stata Center</div>
+<div class="line">32 Vassar St.</div>
+<div class="line">Cambridge, MA 02139 </div>
+<div class="line"><br /></div>
+<div class="line">email - <a class="reference" href="mailto:jamieson@csail.mit.edu">jamieson@csail.mit.edu</a></div>
+<div class="line"><br /></div>
+<div class="line"><br /></div>
+<div class="line">Philip Levis</div>
+<div class="line">358 Gates Hall</div>
+<div class="line">Computer Science Laboratory</div>
+<div class="line">Stanford University</div>
+<div class="line">Stanford, CA 94305</div>
+<div class="line"><br /></div>
+<div class="line">phone - +1 650 725 9046</div>
+<div class="line">email - <a class="reference" href="mailto:pal@cs.stanford.edu">pal@cs.stanford.edu</a></div>
</div>
-<div class="system-messages section">
-<h1>Docutils System Messages</h1>
-<div class="system-message" id="id1">
-<p class="system-message-title">System Message: <a name="id1">ERROR/3</a> (<tt class="docutils">txt/tep123.txt</tt>, line 232); <em><a href="#id2">backlink</a></em></p>
-Unknown target name: "2".</div>
+</div>
+<div class="section" id="id3">
+<h1><a name="id3">8. Citations</a></h1>
+<table class="docutils footnote" frame="void" id="id4" rules="none">
+<colgroup><col class="label" /><col /></colgroup>
+<tbody valign="top">
+<tr><td class="label"><a class="fn-backref" href="#id1" name="id4">[1]</a></td><td>TEP 124: Link Estimation Extension Protocol</td></tr>
+</tbody>
+</table>
+<table class="docutils footnote" frame="void" id="id5" rules="none">
+<colgroup><col class="label" /><col /></colgroup>
+<tbody valign="top">
+<tr><td class="label"><a class="fn-backref" href="#id2" name="id5">[2]</a></td><td>Philip Levis, Neil Patel, David Culler and Scott Shenker. "A
+Self-Regulating Algorithm for Code Maintenance and Propagation
+in Wireless Sensor Networks." In Proceedings of the First USENIX
+Conference on Networked Systems Design and Implementation (NSDI), 2004.</td></tr>
+</tbody>
+</table>
</div>
</div>
</body>
projects / tinyos-2.x.git / blobdiff