tep123.txt

tinyos-2.x.rar

==============================================================================
The Collection Tree Protocol (CTP) 
==============================================================================

:TEP: 123
:Group: Network Working Group 
:Type: Documentary
:Status: Final
:TinyOS-Version: > 2.1
:Author: Rodrigo Fonseca, Omprakash Gnawali, Kyle Jamieson, Sukun Kim, Philip Levis, and Alec Woo

:Draft-Created: 3-Aug-2006
:Draft-Version: $Revision: 1.15 $
:Draft-Modified: $Date: 2009/01/16 19:50:47 $
:Draft-Discuss: TinyOS Developer List <tinyos-devel at mail.millennium.berkeley.edu>

.. Note::

   This memo documents a part of TinyOS for the TinyOS Community, and
   requests discussion and suggestions for improvements.  Distribution
   of this memo is unlimited. This memo is in full compliance with
   TEP 1.

Abstract
==============================================================================

This memo documents the Collection Tree Protocol (CTP), which 
provides best-effort anycast datagram communication to one of the 
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 and to probe topology
inconsistencies.

All fields in this specification are in network byte order.

2. Assumptions and Limitations
==============================================================================

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 **address-free** in that a node does not
send a packet to a particular root; instead, it implicitly chooses a
root by choosing a next hop. Nodes generate routes to roots using
a routing gradient.

The CTP protocol assumes that the data link layer provides four things:

 1) Provides an efficient local broadcast address.
 2) Provides synchronous acknowledgments for unicast packets.
 3) Provides a protocol dispatch field to support multiple higher-level
    protocols.
 4) Has single-hop 16-bit source and destination fields.

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.

CTP has several mechanisms in order to achieve high delivery
reliability, but it does not promise 100\% reliable delivery. It is a
best effort protocol.

CTP is designed for relatively low traffic rates such that there is
enough space in the channel to transmit and receive routing frames
even when the network is forwarding collection data
frames. Bandwidth-limited systems or high data rate applications might
benefit from a different protocol, which can, for example, pack
multiple small frames into a single data-link packet or employ rate
control mechanisms.


3. Collection and CTP
==============================================================================

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 decimal fixed-point real numbers with a precision
of tenths. An ETX value of 45, for example, represents an ETX of 4.5,
while an ETX value of 10 represents an ETX of 1.

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.

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
is an inconsistency in the tree. CTP tries to resolve the inconsistency
by broadcasting a beacon frame, with the hope that the node which sent
the data frame will hear it and adjust its routes accordingly. If a
collection of nodes is separated from the rest of the network, then they
will form a loop whose ETX increases forever. CTP's second mechanism
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. 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,
if a node suppresses duplicates based solely on originating address and
sequence number, packets in routing loops may be dropped. CTP data frames
therefore have an additional time has lived (THL) field, which the
routing layer increments on each hop. A link-level retransmission has
the same THL value, while a looped version of the packet is unlikely
to do so.

4. CTP Data Frame 
==============================================================================

The CTP data frame format is as follows::

                           1            
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |P|C| reserved  |      THL      |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |              ETX              |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |             origin            |                 
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |     seqno     |   collect_id  |    
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |    data ... 
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Field definitions are as follows:

  * P: Routing pull. The P bit allows nodes to request routing information from other nodes. If the unicast destination of the data frame with a valid route hears a packet with the P bit set, it SHOULD transmit a routing frame in the near future. Nodes other than the link-layer destination of the data frame MAY respond 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.
  * origin: The originating address of the packet. A node forwarding a data frame MUST NOT modify the origin field.
  * seqno: Origin sequence number. The originating node sets this field, and a node forwarding a data frame MUST NOT modify it. 
  * collect_id: Higher-level protocol identifier. The origin sets this field, and a node forwarding a data frame MUST NOT modify it.
  * data: the data payload, of zero or more bytes. A node forwarding a data frame MUST NOT modify the data payload. The length of the data field is computed by subtracting the size of the CTP header from the size of the link layer payload provided by the link layer.

Together, the origin, seqno and collect_id fields denote a unique 
***origin packet.*** Together, the origin, seqno, collect_id, and
THL denote a unique ***packet instance*** within the network. The
distinction is important for duplicate suppression in the presence
of routing loops. If a node suppresses origin packets, then if
asked to forward the same packet twice due to a routing loop, it will
drop the packet. However, if it suppresses packet instances, then it
will route successfully in the presence of transient loops unless the
THL happens to wrap around to a forwarded packet instance.

A node MUST send CTP data frames as unicast messages with link-layer 
acknowledgments enabled. 

5. CTP Routing Frame
==============================================================================

The CTP routing frame format is as follows::

                           1            
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |P|C| reserved  |     parent    |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |     parent    |      ETX      |    
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |      ETX      |          
      +-+-+-+-+-+-+-+-+

The fields are as follows:
  
  * P: Same as data frame with one difference: Routing frames are broadcast so multiple nodes respond to the P bit in the routing frame.