Razvans comments

diff --git a/doc/txt/tep123.txt b/doc/txt/tep123.txt
index 477d87dc30f3645e81918cea8f646445705e6db2..942a24f9b8962b37081d48376cb46f8ea0444de7 100644 (file)
--- a/doc/txt/tep123.txt
+++ b/doc/txt/tep123.txt
@@ -31,6 +31,18 @@ collection roots in a network.
 1. Introduction
 ==============================================================================
 
+A collection protocol delivers data to one of possibly several data
+sinks, providing a many-to-one network layer. Collection is a
+fundamental component of most sensor network applications. The
+Collection Tree Protocol (CTP) is a reference Collection protocol in
+TinyOS 2.x. The users use Collection interfaces described in TEP 119
+[3]_ to use CTP in their applications.
+
+In this TEP, after a brief discussion of Collection and CTP, we
+specify the CTP routing and data frames. CTP uses routing frames to
+update and build collection tree in the network. CTP uses data frames
+to deliver application payload to the sink.
+
 2. Assumptions and Limitations
 ==============================================================================
 
@@ -93,13 +105,17 @@ is to not consider routes with an ETX higher than a reasonable constant.
 The value of this constant is implementation dependent.
 
 Packet duplication is an additional problem that can occur in CTP.
-Packet duplication occurs when a node receives a data frame successfully
-and transmits an ACK, but the ACK is not received. The sender retransmits
-the packet, and the receiver receives it a second time. This can have
-disasterous effects over multiple hops, as the duplication is exponential.
-For example, if each hop on average produces one duplicate, then on the
-first hop there will be two packets, on the second there will be four,
-on the third there will be eight, etc.
+Packet duplication occurs when a node receives a data frame
+successfully and transmits an ACK, but the ACK is not received. The
+sender retransmits the packet, and the receiver receives it a second
+time. This can have disasterous effects over multiple hops, as the
+duplication is exponential.  For example, if each hop on average
+produces one duplicate, then on the first hop there will be two
+packets, on the second there will be four, on the third there will be
+eight, etc. CTP keeps a small cache of packet signature for the
+packets it has seen to detect packet duplicates. When a new packet
+arrives, if its signature results in cache hit, CTP drops the packet
+because it is a duplicate.
 
 Routing loops complicate duplicate suppression, as a routing loop may
 cause a node to legitimately receive a packet more than once. Therefore,
@@ -131,7 +147,7 @@ The CTP data frame format is as follows::
 
 Field definitions are as follows:
 
-  * 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.
+  * 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. Data frames are unicast so only the destination of the unicast transmission responds to the P bit in the data frame.
   * C: Congestion notification. If a node drops a CTP data frame, it MUST set the C field on the next data frame it transmits.
   * 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.
   * 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.
@@ -170,7 +186,7 @@ The CTP routing frame format is as follows::
 
 The fields are as follows:
   
-  * P: Same as data frame.
+  * P: Same as data frame with one difference: Routing frames are broadcast so multiple nodes respond to the P bit in the routing frame.
   * C: Congestion notification. If a node drops a CTP data frame, it MUST set the C field on the next routing frame it transmits.
   * parent: The node's current parent.
   * metric: The node's current routing metric value.
@@ -185,6 +201,8 @@ below its own. When a parent hears a child advertise an ETX below its
 own, it MUST schedule a routing frame for transmission in the near
 future.
 
+A node MUST send CTP routing frames as broadcast messages.
+
 6. Implementation
 ==============================================================================
 
@@ -341,7 +359,4 @@ along the path.
        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.
-       
- 
-
-
+.. [3] TEP 119: Collection.