============================
Packet Protocols
============================

:TEP: 116 
:Group: Core Working Group 
:Type: Documentary
:Status: Final
:TinyOS-Version: > 2.1
:Author: Philip Levis

.. 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
============================================================================

The memo documents the interfaces used by packet protocol components in  
TinyOS 2.x as well as the structure and implementation of ActiveMessageC, 
the basic data-link HIL component. It also documents the virtualized
active message interfaces AMSenderC and AMReceiverC.

1. Introduction
============================================================================

Sensor nodes are network-centric devices. Much of their software
complexity comes from network protocols and their interactions.
In TinyOS, the basic network abstraction is an *active message*,
a single-hop, unreliable packet. Active messages have a destination
address, provide synchronous acknowledgements, and can be of
variable length up to a fixed maximum size. They also have a 
type field, which is essentially a protocol identifier for
components built on top of this abstraction.

In TinyOS 1.x, the component GenericComm provides interfaces for
transmitting and receiving active messages::

  configuration GenericComm {
    provides {
      interface StdControl as Control;
      interface SendMsg[uint8_t id];
      interface ReceiveMsg[uint8_t id];
      command uint16_t activity();
    }
    uses {
      event result_t sendDone();
    }
  }

This component, while simple, has several issues. First, it has the
activity() commmand, which does not have a single caller in the entire
TinyOS tree. This command requires GenericComm to allocate a 
timer, wasting CPU cycles and RAM. 

Second, it does not allow a node to receive packets besides
those destined to it.  Several network
protocols (e.g., MintRoute [1]_, TAG [2]_) take advantage 
of snooping on these packets for a variety of improvements in efficiency or
performance. This has led to the creation of GenericCommPromiscuous,
whose Receive interface does not distinguish
between packets received that were addressed to the node and
packets received that were addressed to other nodes. Choosing
one of the two implementations is a global decision across
an application. There is a way to enable both reception
semantics at the same time for a different protocols, 
but they require a creative use of default event handlers.

Third, it assumes that components will directly access the packet
structure, the accepted approach in TinyOS 1.x. However, directly
accessing packet structures introduces unforseen dependencies:
a component that names a header field, for example, binds itself
to data link layers that have a field with that name. Similarly,
components on top of GenericComm directly access the data payload
of a packet.

TEP 111 documents the structure of a TinyOS 2.x packet buffer [3]_.
This TEP documents the interfaces used to access packet buffers,
as well as ActiveMessageC, the basic data-link packet communication
HIL.

2. Communication interfaces
============================================================================

Packet-level communication has three basic classes of interfaces.
*Packet* interfaces are for accessing message fields and payloads. 
*Send* interfaces are for transmitting packets, and are
distinguished by their addressing scheme. 
The *Receive* interface is for handling packet reception events.
Finally, depending on whether the protocol has a dispatch identifier
field, the Receive and Send interfaces may be parameterized in order
to support multiple higher-level clients.

2.1 Packet interfaces
--------------------------------------------------------------------

The basic TinyOS 2.x message buffer type is message_t, which is 
described in TEP 111. message_t right-justifies data-link
headers to the data payload so that higher-level components can 
pass buffers between different data link layers without having
to move data payloads. This means that the data payload of a 
data link frame is always at a fixed offset of a message_t.

Once protocols layer on top of each other, the data
payload for components on top of the data link layer are
no longer at a fixed offset. Where a component can put its 
header or data depends on what headers underlying components
introduce. Therefore, in order to be able to find out where
it can put its data, it must query the components below it.
The Packet interface defines this mechanism::

  interface Packet {
    command void clear(message_t* msg);
    command uint8_t payloadLength(message_t* msg);
    command void setPayLoadLength(message_t* msg, uint8_t len);
    command uint8_t maxPayloadLength();
    command void* getPayload(message_t* msg, uint8_t len);
  }

A component can obtain a pointer to its data region within a packet by
calling ``getPayload()``. A call to this command includes the length
the caller requires. The command ``maxPayloadLength`` returns the
maximum length the payload can be: if the ``len`` parameter to
``getPayload`` is greater than the value ``maxPayloadLength`` would
return, ``getPayload`` MUST return NULL.


A component can set the payload length with ``setPayLoadLength.`` A
component can obtain the size of the data region of packet in use with
a call to ``payloadLength``. As Send interfaces always include a
length parameter in their send call, ``setPayLoadLength`` is not
required for sending, and so is never called in common use
cases. Instead, it is a way for queues and other packet buffering
components to store the full state of a packet without requiring
additional memory allocation.

The distinction between ``payloadLength`` and ``maxPayloadLength``
comes from whether the packet is being received or sent. In the
receive case, determining the size of the existing data payload is
needed; in the send case, a component needs to know how much data it
can put in the packet. By definition, the return value of
``payloadLength`` must be less than or equal to the return value of
``maxPayloadLength``.

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.

The ``clear`` command clears out all headers, footers, and metadata
for lower layers. For example, calling ``clear`` on a routing
component, such as CollectionSenderC[4]_, will clear out the
collection headers and footers. Furthermore, CollectionSenderC will
recursively call ``clear`` on the layer below it, clearing out the
link layer headers and footers. Calling ``clear`` 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 ``clear`` 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.

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::

  generic module SequenceNumber {
    provides interface Packet;
    uses interface Packet as SubPacket;
  }
  implementation {
    typedef nx_struct seq_header {
      nx_uint16_t seqNo;
    } seq_header_t;

    enum {
      SEQNO_OFFSET = sizeof(seq_header_t),
    };
 
    command void Packet.clear(message_t* msg) {
      void* payload = call SubPacket.getPayload(msg, call SubPacket.maxPayloadLength());
      call SubPacket.clear();
      if (payload != NULL) {
        memset(payload, sizeof(seq_header_t), 0);
      }
    }

    command uint8_t Packet.payloadLength(message_t* msg) {
      return SubPacket.payloadLength(msg) - SEQNO_OFFSET;
    }

    command void Packet.setPayloadLength(message_t* msg, uint8_t len) {
      SubPacket.setPayloadLength(msg, len + SEQNO_OFFSET);
    }

    command uint8_t Packet.maxPayloadLength() {
      return SubPacket.maxPayloadLength(msg) - SEQNO_OFFSET;
    }

    command void* Packet.getPayload(message_t* msg, uint8_t len) {
      uint8_t* payload = call SubPacket.getPayload(msg, len + SEQNO_OFFSET);
      if (payload != NULL) {       
        payload += SEQNO_OFFSET;
      }
      return payload;
    } 
  }


The above example is incomplete: it does not include the code for
the send path that increments sequence numbers. 

In practice, calls to Packet are very efficient even if they 
pass through many components before reaching the data link
layer. nesC's inlining means that in almost all cases
there will not actually be any function calls, and since payload
position and length calculations all use constant offsets,
the compiler generally uses constant folding to generate a 
fixed offset. 

The Packet interface provides access to the one field all packet
layers have, the data payload. Communication layers can add additional
header and footer fields, and may need to provide access to these
fields. If a packet communication component provides access to header
and/or footer fields, it MUST do so through an interface. The interface 
SHOULD have a name of the form *XPacket*, where *X* is a name that
describes the communication layer. For example, active message components
provide both the Packet interface and the AMPacket interface. The latter
has this signature::

  interface AMPacket {
    command am_addr_t address();
    command am_addr_t destination(message_t* amsg);
    command am_addr_t source(message_t* amsg);
    command void setDestination(message_t* amsg, am_addr_t addr);
    command void setSource(message_t* amsg, am_addr_t addr);
    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();
  }


The command address() returns the local AM address of the
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.

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 ``localGroup`` returns.

2.2 Sending interfaces
--------------------------------------------------------------------

There are multiple sending interfaces, corresponding to different
addressing modes. For example, address-free protocols, such as
collection routing, provide the basic ``Send`` interface. Active
message communication has a destination of an AM address, so
it provides the ``AMSend`` interface.  This, for example, is the 
basic, address-free Send interface::

  interface Send {
    command error_t send(message_t* msg, uint8_t len);
    command error_t cancel(message_t* msg);
    event void sendDone(message_t* msg, error_t error);  

    command uint8_t maxPayloadLength();
    command void* getPayload(message_t* msg, uint8_t len);
  }

while this is the AMSend interface::

  interface AMSend {
    command error_t send(am_addr_t addr, message_t* msg, uint8_t len);
    command error_t cancel(message_t* msg);
    event void sendDone(message_t* msg, error_t error);

    command uint8_t maxPayloadLength();
    command void* getPayload(message_t* msg, uint8_t len); 
  }

Sending interfaces MUST include these four commands and one event.
The duplication of some of the commands in Packet is solely for ease
of use: ``maxPayloadLength`` and ``getPayload`` MUST behave
identically as ``Packet.maxPayloadLength`` and ``Packet.getPayload.``
Their inclusion is so that components do not have to wire to
both Packet and the sending interface for basic use cases.

When called with a length that is too long for the underlying
maximum transfer unit (MTU), the send command MUST return ESIZE.

The ``Send`` and ``AMSend`` interfaces have an explicit queue of
depth one. A call to ``send`` on either of these interfaces MUST 
return EBUSY if a prior call to ``send`` returned SUCCESS but no
``sendDone`` event has been signaled yet. More explicitly::

  if (call Send.send(...) == SUCCESS &&
      call Send.send(...) == SUCCESS) {
     // This block is unreachable.
  }

Systems that need send queues have two options. They can
use a QueueC (found in tos/system) to store pending packet pointers
and serialize them onto sending interface, or they can introduce
a new sending interface that supports multiple pending transmissions.

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 MUST occur as if
the cancel was not called.

2.3 Receive interface
--------------------------------------------------------------------

Receive is the interface for receiving packets. It has this signature::

  interface Receive {
    event message_t* receive(message_t* msg, void* payload, uint8_t len);
  }

The ``receive()`` event's ``payload`` parameter MUST be identical to
what a call to the corresponding ``Packet.getPayload()`` would return,
and the ``len`` parameter MUST be identical to the length that a call
to ``Packet.getPayload`` 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 ``getPayload()``. Unlike Send,
Receive does not have a convenience ``getPayload`` call, because doing
so prevents fan-in. As Receive has only a single event, users of
Receive can be wired multiple times.

Receive has a *buffer-swap* 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 ``msg`` 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
upper layer cannot handle packets quickly enough it will end up
holding all of them, starving lower layers and possibly preventing
packet reception.

A *user* of the Receive interface has three basic options when it
handles a receive event:

  1) Return ``msg`` without touching it.
  2) Copy some data out of ``payload`` and return ``msg``.
  3) Store ``msg`` in its local frame and return a different ``message_t*`` for the lower layer to use.

These are simple code examples of the three cases::

  // Case 1
  message_t* Receive.receive(message_t* msg, void* payload, uint8_t len) {
    return msg; 
  }  

  // Case 2
  uint16_t value;
  message_t* Receive.receive(message_t* msg, void* payload, uint8_t len) {
    if (len >= sizeof(uint16_t)) {
      nx_uint16_t* nval = (nx_uint16_t*)payload;
      value = *nval;
    }
    return msg;
  }

  //Case 3
  message_t buf;
  message_t* ptr = &buf;
  message_t* Receive.receive(message_t* msg, void* payload, uint8_t len) {
    message_t* tmp = ptr;
    ptr = msg;
    post processTask();
    return tmp;
  }


Because of case 3), a lower layer MUST respect the buffer swap semantics
and use the pointer returned from ``receive``. The pointer passed as
a parameter to ``receive`` MUST NOT be touched, used, or stored after
the signaling of ``receive.``

2.4 Dispatch
--------------------------------------------------------------------

A packet protocol MAY have a dispatch identifier. This generally manifests
as the protocol component providing parameterized interfaces (rather than
a single interface instance). A dispatch identifier allows multiple 
services to use a protocol independently. If a protocol provides a
dispatch mechanism, then each dispatch identifier SHOULD correspond to
a single packet format: if an identifier corresponds to multiple packet
formats, then there is no way to disambiguate them. Packets whose internal 
structure depends on their fields (for example,
a packet that has a control field which indicates which optional fields
are present) do not pose such problems.

3. HIL: ActiveMessageC
============================================================================

A platform MUST provide ActiveMessageC as a basic HIL to 
packet-level communication.  ActiveMessageC provides a best-effort, 
single-hop communication abstraction.  Every active message has a 
16-bit destination address and an 8-bit type. There is one reserved 
destination address, ``AM_BROADCAST_ADDR``, which has the value 
of ``0xffff``. ActiveMessageC has the following signature::

  configuration ActiveMessageC {
    provides {
      interface Init;
      interface SplitControl;  

      interface AMSend[uint8_t id];
      interface Receive[uint8_t id];
      interface Receive as Snoop[uint8_t id];
 
      interface Packet;
      interface AMPacket;
      interface PacketAcknowledgements;
    }
  }

The Receive interface is for packets destined to the node, while 
the Snoop interface is for packets destined to other nodes. A 
packet is destined for a node if its destination AM address is 
either the AM broadcast address or an address associated with 
the AM stack. Different link layers have different snooping 
capabilities. The Snoop interface does not assume always-on 
listening, for example, in the case of a TDMA or RTS/CTS data 
link layer. By separating out these two interfaces, ActiveMessageC
avoids the complications encountered in 1.x with regards to
GenericComm vs. GenericCommPromiscuous.

ActiveMessageC is usually just a configuration that has
pass-through wiring to a chip-specific HAL active message
implementation. The definition of ActiveMessageC is left
to the platform for when a node has more than one
radio. In this case, the platform decides how to map the
basic packet abstraction to the hardware underneath. Approaches 
include choosing one radio or having some form of address-based
dispatch.


4. AM Services: AMSenderC, AMReceiverC, AMSnooperC, AMSnoopingReceiverC
============================================================================

TinyOS 2.x provides four component single-hop communication
virtualizations to applications:
AMReceiverC, AMSnooperC, AMSnoopingReceiverC, and AMSenderC. Each is a
generic component that takes an active message ID as a
parameter. These components assume the existence of ActiveMessageC.

4.1 Dispatch: ``am_id_t``
--------------------------------------------------------------------

Active messages have an 8-bit type field, which allows multiple
protocols to all use AM communication without conflicting. Following
the guidelines for protocol dispatch identifiers, each
am_id_t used in a network SHOULD have a single packet format, so
that the am_id_t, combined with the packet contents, are sufficient
to determine the exact packet format.

4.2 AMReceiverC
--------------------------------------------------------------------

AMReceiverC has the following signature::

  generic configuration AMReceiverC(am_id_t t) {
    provides{
      interface Receive;
      interface Packet;
      interface AMPacket;
    }
  }

AMReceiver.Receive.receive is signalled whenever the packet layer
receives an active message of the corresponding AM type whose
destination address is the local address or the broadcast
address. Note that since Receive.receive swaps buffers, a program MUST
NOT instantiate two AMReceivers with the same am_id_t and MUST NOT
instantiate an AMReceiver and an AMSnoopingReceiver with the same
am_id_t.

4.3 AMSnooperC
--------------------------------------------------------------------

AMSnooper has an identical signature to AMReceiver::

  generic configuration AMSnooperC(am_id_t t) {
    provides{
      interface Receive;
      interface Packet;
      interface AMPacket;
    }
  }

AMSnooper.Receive.receive is signalled whenever the packet layer
receives an active message of the corresponding AM type whose
destination address is neither to the local address nor the broadcast
address. Note that since Receive.receive swaps buffers, a program MUST
NOT instantiate two AMSnoopers with the same am_id_t and MUST NOT
instantiate an AMSnooper and an AMSnoopingReceiver with the same
am_id_t.

4.4 AMSnoopingReceiverC
--------------------------------------------------------------------

AMSnoopingReceiverC has an identical signature to AMReceiverC::

  generic configuration AMSnoopingReceiverC(am_id_t t) {
    provides{
      interface Receive;
      interface Packet;
      interface AMPacket;
    }
  }

AMSnoopingReceiverC.Receive.receive is signalled whenever the packet
layer receives an active message of the corresponding AM type,
regardless of destination address. Note that since Receive.receive
swaps buffers, a program that instantiates an AMSnoopingReceiverC with
a certain am_id_t MUST NOT instantiate another AMSnoopingReceiverC,
AMSnooperC, or AMReceiverC with the same am_id_t.

4.5 AMSenderC 
--------------------------------------------------------------------

AMSenderC has the following signature::

  generic configuration AMSenderC(am_id_t AMId) {
    provides {
      interface AMSend;
      interface Packet;
      interface AMPacket;
      interface PacketAcknowledgements as Acks;
    }
  }

Because this is a send virtualization, AMSenderC.AMSend.send returns
EBUSY only if there is a send request outstanding on this particular
AMSenderC. That is, each AMSenderC has a queue of depth one. The exact
order in which pending AMSenderC requests are serviced is undefined,
but it MUST be fair, where fair means that each client with outstanding
packets receives a reasonable approximation of an equal share of the 
available transmission bandwidth.

5. Power Management and Local Address
============================================================================

In addition to standard datapath interfaces for sending and
receiving packets, an active message layer also has control interfaces.


5.1 Power Management
--------------------------------------------------------------------

The communication virtualizations do not support power management.
ActiveMessageC provides SplitControl for explicit power control.
For packet communication to operate properly, a component in an 
application has to call ActiveMessageC.SplitControl.start().
The HAL underneath ActiveMessageC  MAY employ power management 
techniques, such as TDMA scheduling or low power listening, when
"on."

5.2 Local Active Message Address
--------------------------------------------------------------------

An application can change ActiveMessageC's local AM address 
at runtime. This will change which packets a node receives and
the source address it embeds in packets. To change the local AM
address at runtime, a component can wire to the component
``ActiveMessageAddressC``. This component only changes the
AM address of the default radio stack (AMSenderC, etc.); if
a radio has multiple stacks those may have other components
for changing their addresses in a stack-specific fashion.

5. HAL Requirements
============================================================================

A radio chip *X* MUST have a packet abstraction with the following
signature::

  provides interface Init;
  provides interface SplitControl;
  provides interface AMSend[am_id_t type];
  provides interface Receive[am_id_t type];
  provides interface Receive as Snoop[am_id_t type];
  provides interface Packet;
  provides interface AMPacket;
  provides interface PacketAcknowledgments;


The component SHOULD be named *XActiveMessageC*, where *X* is 
the name of the radio chip. The component MAY have additional interfaces. 
These interfaces can either be chip-specific or chip-independent. 

6. message_t
============================================================================

Active messages are a basic single-hop packet abstraction. Therefore,
following TEP 111 [3]_, all data link and active message headers
MUST be in the ``message_header_t`` structure of message_t. This ensures
that an active message received from one data link layer (e.g., the radio)
can be passed to another data link layer (e.g., the UART) without
shifting the data payload. This means that the ``message_header_t`` must
include all data needed for AM fields, which might introduce headers
in addition to those of the data link. For example, this is an example 
structure for a CC2420 (802.15.4) header::

  typedef nx_struct cc2420_header_t {
    nx_uint8_t length;
    nx_uint16_t fcf;
    nx_uint8_t dsn;
    nx_uint16_t destpan;
    nx_uint16_t dest;
    nx_uint16_t src;
    nx_uint8_t type;
  } cc2420_header_t;


The first six fields (length through src) are all 802.15.4 headers. The
type field, however, has been added to the header structure in order
to support AM dispatch.

7. Implementation
============================================================================

The following files in ``tinyos-2.x/tos/system`` provide reference 
implementations of the abstractions described in this TEP.

  * ``AMSenderC.nc``, ``AMReceiverC.nc``, ``AMSnooperC.nc``,
    and ``AMSnoopingReceiverC.nc`` are implementations of 
    virtualized AM services.
  * ``AMQueueP`` provides a send queue of *n* entries for *n*
    AMSenderC clients, such that each client has a dedicated entry.
  * ``AMQueueImplP`` is the underlying queue implementation,
    which is reusable for different clients (it is also used
    in the serial stack [4]_).
  * ``AMQueueEntryP`` sits on top of ``AMQueueP`` and stores
    the parameters to ``AMSend.send`` in an outstanding
    packet with the ``AMPacket`` interface.

The following files in ``tinyos-2.x/tos/interfaces`` contain
example implementations of packet protocol interfaces:

  * ``Packet.nc`` is the basic interface that almost all
    packet protocols provide.
  * ``Send.nc`` is the transmission interface for address-free 
     protocols.
  * ``AMSend.nc`` is the transmission interface for AM address
     send protocols.
  * ``AMPacket.nc`` is the packet interface for AM-specific
    fields.

An active messaging implementation for the CC2420 radio chip
can be found in ``tos/chips/CC2420/CC2420ActiveMessageC.nc``.
The micaz platform and telos family have an ``ActiveMessageC.nc``
which exports the interfaces of ``CC2420ActiveMessageC``.

8. Author's Address
============================================================================

| Philip Levis
| 358 Gates Hall
| Computer Science Laboratory
| Stanford University
| Stanford, CA 94305
|
| phone - +1 650 725 9046

9. Citations
============================================================================

.. [1] The MintRoute protocol. ``tinyos-1.x/tos/lib/MintRoute``. Also, A. Woo, T. Tong, and D. Culler. "Taming the Underlying Challenges of Reliable Multihop Routing in Sensor Networks." SenSys 2003.

.. [2] Tiny AGgregation, one protocol of the TinyDB system.  ``tinyos-1.x/tos/lib/TinyDB``. Also, S. Madden and M. Franklin and J. Hellerstein and W. Hong. "TinyDB: An Acquisitional Query Processing System for Sensor Networks." Transactions on Database Systems (TODS) 2005.

.. [3] TEP 111: message_t.

.. [4] TEP 113: Serial Communication.