Depth-First Forwarding in Unreliable Networks
draft-cardenas-dff-03
The information below is for an old version of the document.
| Document | Type |
This is an older version of an Internet-Draft that was ultimately published as RFC 6971.
|
|
|---|---|---|---|
| Authors | Ulrich Herberg , Alvaro Cardenas , Tadashige Iwao , Sandra Cespedes | ||
| Last updated | 2011-11-13 (Latest revision 2011-10-31) | ||
| RFC stream | (None) | ||
| Formats | |||
| Reviews | |||
| Stream | Stream state | (No stream defined) | |
| Consensus boilerplate | Unknown | ||
| RFC Editor Note | (None) | ||
| IESG | IESG state | Became RFC 6971 (Experimental) | |
| Telechat date | (None) | ||
| Responsible AD | (None) | ||
| Send notices to | (None) |
draft-cardenas-dff-03
Internet Engineering Task Force U. Herberg
Internet-Draft A. Cardenas
Intended status: Experimental T. Iwao
Expires: May 17, 2012 Fujitsu
M. Dow
Freescale
S. Cespedes
U. Icesi/U. of Waterloo
November 14, 2011
Depth-First Forwarding in Unreliable Networks
draft-cardenas-dff-03
Abstract
This document describes the Depth-First Forwarding (DFF) protocol for
IPv6 networks based on the LoWPAN adaptation layer. The protocol is
a mesh-under data forwarding mechanism that increases reliability of
data delivery.
DFF forwards data packets using a network-wide "depth-first search"
for the Final Destination of the packet. DFF may be used in
conjunction with a mesh-under routing protocol, which provides
"hints" for DFF in which order to try to send the packet to the
neighbors discovered by the neighborhood discovery mechanism. In
that case, DFF can be used as local repair mechanism.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on May 17, 2012.
Copyright Notice
Copyright (c) 2011 IETF Trust and the persons identified as the
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document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Motivation . . . . . . . . . . . . . . . . . . . . . . . . 4
1.2. Protocol Dependencies . . . . . . . . . . . . . . . . . . 5
2. Notation and Terminology . . . . . . . . . . . . . . . . . . . 5
2.1. Notation . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 6
3. Applicability Statement . . . . . . . . . . . . . . . . . . . 6
4. Protocol Overview and Functioning . . . . . . . . . . . . . . 7
5. Information Sets . . . . . . . . . . . . . . . . . . . . . . . 8
5.1. Processed Set . . . . . . . . . . . . . . . . . . . . . . 8
6. Packet Format . . . . . . . . . . . . . . . . . . . . . . . . 9
7. Protocol Parameters and Constants . . . . . . . . . . . . . . 11
8. Data Packet Generation and Processing . . . . . . . . . . . . 11
8.1. Data Packet Generation . . . . . . . . . . . . . . . . . . 11
8.2. Data Packet Processing . . . . . . . . . . . . . . . . . . 12
9. Missed Link Layer Acknowledgment when Sending a Packet . . . . 14
10. Getting the Next Hop for a Packet . . . . . . . . . . . . . . 15
11. Poisoning . . . . . . . . . . . . . . . . . . . . . . . . . . 16
12. Route Repair Mechanism . . . . . . . . . . . . . . . . . . . . 16
13. Sequence Numbers . . . . . . . . . . . . . . . . . . . . . . . 16
14. Proposed Values for Parameters and Constants . . . . . . . . . 16
15. Security Considerations . . . . . . . . . . . . . . . . . . . 17
16. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 17
17. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 17
18. References . . . . . . . . . . . . . . . . . . . . . . . . . . 17
18.1. Normative References . . . . . . . . . . . . . . . . . . . 17
18.2. Informative References . . . . . . . . . . . . . . . . . . 17
Appendix A. Examples . . . . . . . . . . . . . . . . . . . . . . 18
A.1. Example 1: Normal Delivery . . . . . . . . . . . . . . . . 18
A.2. Example 2: Forwarding with Link Failure . . . . . . . . . 18
A.3. Example 3: Forwarding with Missed Link Layer
Acknowledgment . . . . . . . . . . . . . . . . . . . . . . 19
A.4. Example 4: Forwarding with a Loop . . . . . . . . . . . . 20
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Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 21
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1. Introduction
This document describes the Depth-First Forwarding (DFF) protocol for
IPv6 networks based on the LoWPAN adaptation layer, as specified in
[RFC4944]. The protocol is a mesh-under data forwarding mechanism
that increases reliability of data delivery in networks with dynamic
topologies.
DFF forwards data packets using a network-wide "depth-first search"
for the final destination of the packet. DFF relies on a
neighborhood discovery mechanism which lists neighbors of a node for
the next hop of a data packet. In addition, DFF may be used in
conjunction with a mesh-under routing protocol, which provides
"hints" for DFF in which order to try to send the packet to the
neighbors discovered by the neighborhood discovery mechanism.
If the packet makes no forward progress using the selected next hop,
DFF will successively try all neighbors of the node (as determined by
an additional mechanism, e.g. a mesh-under routing protocol, ND,
HELLO message exchange). If none of the next hops successfully
receives the packet, DFF returns the packet to the previous hop,
which in turn tries to send it to alternate neighbors.
As network topologies do not necessarily form a tree, loops can
occur. Therefore, DFF contains a loop detection and avoidance
mechanism.
If DFF is used in conjunction with a mesh-under routing protocol, the
cost of routes provided by that routing protocol may be updated while
rerouting the packet through alternative next hops. Thus, DFF
provides an optional local route repair mechanism.
1.1. Motivation
In networks with dynamic topologies, even frequent exchanges of
control messages between nodes for updating the routing tables cannot
guarantee freshness of routes: packets may not be delivered to their
destination because the topology has changed since the last routing
protocol update.
While more frequent routing protocol updates could mitigate that
problem to a certain extent, that requires network bandwidth (e.g.
when flooding control messages through the network for route
discovery). This is an issue in networks with lossy links, where
further control traffic exchange can worsen the network stability
because of collisions (e.g. in the case of a network-wide flood of
Route Request messages or Link State Advertisements). Moreover,
additional control traffic exchange drains energy from battery-driven
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nodes.
The data-forwarding mechanism specified in this document allows for
forwarding data packets along alternate paths for increasing
reliability of data delivery, using a depth-first search. The
objective is to decrease the necessary control traffic overhead in
the network, and in the same time to increase delivery success rates.
If a mesh-under routing protocol is used in conjunction to DFF, that
protocol can be informed about the updated topology, and routes can
then be repaired.
1.2. Protocol Dependencies
DFF can be used as stand-alone forwarding mechanism, but may be used
in conjunction with a mesh-under routing protocol which allows for
providing an order of preference to which next hops a packet should
be forwarded (e.g. the packet may be forwarded first to neighbors
that are listed as next hops to the final destination, preferring
those with the lowest route cost).
DFF requires a list of bidirectional neighbors for each node, which
must be provided by an external mechanism, such as specified in
[RFC4861]. This specification assumes there is such a neighborhood
discovery protocol in place and outlines the requirements for that
protocol, as well as the interaction between DFF and a mesh-under
routing protocol if such is used in conjunction to DFF.
2. Notation and Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
[RFC2119].
Additionally, this document uses the notation in Section 2.1 and the
terminology in Section 2.2.
2.1. Notation
The following notations are used in this document:
List - A list of elements is defined as [] for an empty list,
[element] for a list with one element, and [element1, element2,
...] for a list with multiple elements.
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Concatenation of lists: If L1 and L2 are lists, then L1@L2 is a new
list with all elements of L2 concatenated to L1.
2.2. Terminology
All terms introduced in [RFC4944] are to be interpreted as described
therein, in particular Originator Address, Final Destination Address,
Source Address, and Destination Address.
Additionally, this document uses the following terminology:
Address - An address is either a 16-bit short or a EUI-64 link layer
address, as specified in [RFC4944].
Packet - An IPv6 packet encapsulated in a IEEE 802.15.4 frame, using
the LoWPAN adaption layer as specified in [RFC4944].
3. Applicability Statement
This protocol:
o Is applicable for use in LoWPAN based networks, using the frame
format for transmission of IPv6 packets defined in [RFC4944].
o Assumes addresses used in the network are either 16-bit short or
EUI-64 link layer address, as specified in [RFC4944].
o Operates as "mesh-under" forwarding protocol, i.e. on the link
layer. While the proposed mechanism could also be used as "route-
over", this is not specified in this document and thus out of
scope.
o Is designed to work in networks with lossy links or with a dynamic
topology.
o Relies on an external neighborhood discovery mechanism, which MUST
provide a list of bidirectional neighbors of a node. In addition,
DFF MAY use information from a mesh-under routing protocol used in
conjunction to DFF. Such a protocol provide provides "hints" to
DFF in which order the neighbors should be successively tried as
next hop for a packet.
o Increases reliability of data delivery by trying alternative
paths, using a "depth-first forwarding" approach.
o Provides a loop detection mechanism, and an optional local route
repair mechanism if a mesh-under routing protocol is used in
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conjunction to DFF.
o Is designed to work in a completely distributed manner, and does
not depend on any central entity.
4. Protocol Overview and Functioning
DFF is a mesh-under data forwarding mechanism responsible for finding
a path to a Final Destination of a packet, using a "depth-first
search" in the network. DFF operates on LoWPAN based networks (using
the frame format and the transmission of IPv6 packets defined in
[RFC4944]).
DFF requires an external mechanism to discover the bidirectional
neighborhood of a node. The specification of such a mechanism is out
of scope of this document. The list of neighbors is required because
DFF successively tries to forward a packet to all neighbors of a node
during the depth-first search, and eventually returns it to the
previous hop if the packet was not successfully received by any of
the neighbors.
In addition to the mandatory neighborhood information, DFF may use
information from a mesh-under routing protocol that runs in
conjunction to DFF. Such protocol can increase the efficiency of the
depth-first search, as it allows for providing an order of preference
which next hops to try first (e.g. by preferring neighbors that are
listed in the mesh-under routing protocol as next hops along the path
to the final destination, and by preferring next hops with a lower
route cost). If the topology as reflected by that mesh-under routing
protocol represents the effective topology of the network, then DFF
will forward the data packet along the path provided by the protocol.
However, if the topology has changed and the routing protocol has not
yet converged, then DFF will try alternate paths. Compared to the
typical forwarding mechanism in LoWPAN networks, a mesh-under routing
protocol thus only serves DFF to give recommendations (or "hints")
where to forward a packet. That also implies that if the mesh-under
routing protocol does not provide a route to a final destination, the
data packet would not be dropped but forwarded by DFF, trying to find
a path to that final destination.
In order to avoid loops when forwarding a data packet towards its
Final Destination, DFF stores a data packet identifier (i.e. a
sequence number) to detect loops. DFF lists for each recently
forwarded packet which neighbors that packet has already been sent
to, allowing for trying to forward the packet to all candidates for
the next hop.
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DFF requires additional header information for each data packet,
provided by a LoWPAN dispatch byte specified in this document. This
header contains a sequence number used for identifying a packet
uniquely, and two flags: RET and DUP. If none of the transmissions
of a data packet to the neighbors of a node has succeeded, the packet
is returned to the previous hop, indicated by setting the return
(RET) flag. The previous hop then continues to try other neighbors
in turn, resulting in a depth-first search in the network.
Whenever a packet has been sent to a neighbor and no link layer
acknowledgment (ACK) has been received, the duplicate (DUP) flag is
set in the packet header for the following transmissions. The
rationale is that the packet may have been successfully received by
the neighbor and only the ACK has been lost, resulting in duplicates
of the packet in the network. The DUP flag tags such a possible
duplicate.
Whenever a node receives a packet that it has already forwarded (as
identified by the sequence number of the packet), and which is not a
duplicate (i.e. DUP = 0), it will assume a loop and return the
packet to the previous hop (with the RET flag set). Optionally, if a
mesh-under routing protocol is used in conjunction to DFF, the route
using the next hop which resulted in the loop may be "poisoned" (i.e.
the route cost may be increased).
5. Information Sets
This section specifies the information sets used by DFF. In
addition, DFF requires access to a list of bidirectional neighbors of
the node, provided by an external neighborhood discovery mechanism,
which is not specified within this document.
5.1. Processed Set
Each node MUST maintain a Processed Set in order to support the loop
detection functionality. The Processed Set lists sequence numbers of
previously received packets, as well as a list of next hops to which
the packet has been sent successively as part of the depth-first
search mechanism. The set consists of Processed Tuples:
(P_orig_address, P_seq_number, P_prev_hop,
P_next_hop_neighbor_list, P_time)
where
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P_orig_address is is the Originator Address of the received
packet;
P_seq_number is the Sequence Number of the received packet;
P_prev_hop is the Source Address (i.e. the previous hop) of the
packet;
P_next_hop_neighbor_list is a list of addresses of next hops to
which the packet has been sent previously, as part of the depth-
first search mechanism, as explained in Section 8.2;
P_time specifies when this Tuple expires and MUST be removed.
6. Packet Format
This document assumes that the data forwarding is based on the LoWPAN
adaptation layer ("mesh-under"), and that data packets are conform
with the packet format specified in [RFC4944]. In particular,
[RFC4944] states that
Additional mesh routing capabilities, such as specifying the mesh
routing protocol, source routing, and so on may be expressed by
defining additional routing headers that precede the fragmentation
or addressing header in the header stack.
Hence, all data packets to be forwarded using DFF MUST be preceded by
the Mesh Addressing header defined in [RFC4944], and SHOULD be
preceded by a header that identifies the DFF data forwarding
mechanism, which is defined in the following.
After these two headers, any other LoWPAN header, e.g. hop-by-hop
options, header compression or fragmentation, MAY also be added
before the actual payload. Figure 1 depicts the Mesh Addressing
header defined in [RFC4944], and Figure 2 depicts the DFF header.
1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1 0|V|F|HopsLft| DeepHopsLeft |orig. address, final address...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: Mesh Addressing Header
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1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 1| Mesh Forw |D|R|x| Sequence Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2: Header for DFF data frames
Field definitions of the Mesh Addressing header are as specified in
[RFC4944].
Field definitions of the DFF header are as follows:
Mesh Forw - A 6-bit identifier that allows for the use of different
mesh forwarding mechanisms. As specified in [RFC4944], additional
mesh forwarding mechanisms should use the reserved dispatch byte
values following LOWPAN_BCO; therefore, 0 1 MUST precede Mesh
Forw. The value of Mesh Forw is LOWPAN_DFF.
Duplicate packet flag (D) - This flag is included in the DFF mesh
header to indicate that the packet has been re-sent as a
duplicate. The flag MUST be set to 1 by the node that re-sends
the packet after detecting link-layer failure to deliver through
the last attempted next-hop, as specified in Section 8.2. Once
the flag is set to 1, it MUST not be modified by nodes forwarding
the packet.
Return packet flag (R) - This flag is included in the DFF mesh
header to indicate that the packet has been returned to the
previous hop after failure to deliver to all the available next-
hops. The flag MUST be set to 1 prior to forwarding the packet
back to the previous hop and MUST be set to 0 prior to forwarding
the packet to the selected next-hop, as specified in Section 8.2.
This flag is modified in a hop-by-hop basis.
Reserved flag (x) - This bit is reserved for future flag
definitions.
Sequence Number - This is a sequence number generated by the
originator, unique on a node for each new generated packet, as
specified in Section 13. The Originator Address concatenated with
the Sequence Number represent an identifier of previously seen
data packets.
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7. Protocol Parameters and Constants
The parameters and constants used in this specification are described
in this section.
P_HOLD_TIME - is the time period after which a newly created
Processed Tuple expires and MUST be deleted.
MAX_HOPS_LEFT - is the initial value of Deep Hops Left in the mesh
header specified in [RFC4944].
8. Data Packet Generation and Processing
The following sections describe the process of handling a new packet,
generated on a node (Section 8.1), as well as forwarding a packet
from another node (Section 8.2). When DFF is used, the following
specification MUST be used instead of the default frame delivery,
specified in Section 11 of [RFC4944].
In the following, it is assumed that all data packets are preceded by
the Mesh Addressing header and the DFF header, as specified in
Section 6. In order to allow for interoperability with nodes not
using DFF as forwarding mechanism, packets that are preceded by the
Mesh Addressing header but not the DFF header are treated as
specified in Section 11 of [RFC4944].
8.1. Data Packet Generation
Before a new packet (denoted the "current packet") is transmitted on
a node, the following steps MUST be performed:
1. Add the Mesh Addressing header to the current packet, as
specified in [RFC4944] and described in Section 6, with:
* V and F are set according to the used address length;
* Hops Left := 0xF (i.e. reserved value indicating that the Deep
Hops Left field is following);
* Deep Hops Left := MAX_HOPS_LEFT;
* Originator Address := address of this node;
* Final Destination Address := address of the final destination
node.
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2. Add the DFF header to the current packet, as specified in
Section 6, with:
* Duplicate packet flag (D) := 0;
* Return packet flag (R) := 0;
* Sequence Number := a new Sequence Number of the packet (as
defined in Section 13).
3. Select the next hop (denoted "next_hop") for the current packet,
as specified in Section 10.
4. Add a Processed Tuple to the Processed Set with:
* P_orig_address := the Originator Address of the current
packet;
* P_seq_number := the Sequence Number of the current packet;
* P_prev_hop := the Originator Address of the current packet;
* P_next_hop_neighbor_list := [next_hop];
* P_time := current time + P_HOLD_TIME.
5. Set the Source Address in the 802.15.4 header to the node's own
address and the Destination Address to next_hop.
6. Transmit the current packet. If the transmission fails
(determined by missing link layer acknowledgments), the
procedures in Section 9 SHOULD be executed.
8.2. Data Packet Processing
If a packet (denoted the "current packet") is received on the node,
then the following tasks MUST be performed:
1. If the Final Destination Address from the Mesh Addressing header
matches the address of this node, consume the packet as per
normal delivery.
2. Otherwise, decrement the value of the Deep Hops Left field in the
mesh header. Drop the packet if Deep Hops Left is decremented to
zero.
3. If no Processed Tuple (denoted the "current tuple") exists in the
Processed Set, with:
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+ P_orig_address = the Originator Address of the current packet,
AND;
+ P_seq_number = the Sequence Number of the current packet,
then:
1. add a Processed Tuple (denoted the "current tuple") with:
+ P_orig_address := the Originator Address of the current
packet;
+ P_seq_number := the Sequence Number of the current packet;
+ P_prev_hop := the Source Address (i.e. the previous hop)
of the current packet;
+ P_next_hop_neighbor_list := [];
+ P_time := current time + P_HOLD_TIME.
2. Set RET to 0 in the packet DFF header.
3. Select the next hop (denoted "next_hop") for the current
packet, as specified in Section 10.
4. P_next_hop_neighbor_list := P_next_hop_neighbor_list@
[next_hop];
5. Set the Source Address in the 802.15.4 header to the node's
own address and the Destination Address field to next_hop.
6. Transmit the current packet. If the transmission fails
(determined by missing link layer acknowledgments), the
procedures in Section 9 SHOULD be executed.
4. Otherwise, if a tuple exists:
1. If the return flag of the current packet is not set (RET=0)
(i.e. a loop has been detected):
1. Set RET := 1
2. Set the Source Address in the 802.15.4 header to the
node's own address and the Destination Address field to
the Source Address of the current packet (i.e. the
previous hop).
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3. Transmit the current packet.
2. Otherwise, if the return flag of the current packet is set
(RET = 1):
1. Set RET := 0
2. Execute the "poisoning" procedure specified in
Section 11.
3. Select the next hop (denoted "next_hop") for the current
packet, as specified in Section 10.
4. Modify the current tuple:
- P_next_hop_neighbor_list := P_next_hop_neighbor_list@
[next_hop];
- P_time := current time + P_HOLD_TIME.
5. If the selected next hop is equal to P_prev_hop of the
current tuple (i.e. all neighbors have been
unsuccessfully tried), set the RET flag (RET := 1). If
this node has the same address as the Originator Address
of the current packet, drop the packet.
6. Set the Source Address in the 802.15.4 header to the
node's own address and the Destination Address field to
next_hop.
7. Transmit the current packet. If the transmission fails
(determined by missing link layer acknowledgments), the
procedures in Section 9 SHOULD be executed.
9. Missed Link Layer Acknowledgment when Sending a Packet
If a packet (the "current packet") has been sent to the next hop, as
specified in Section 8.1 and Section 8.2, and no link layer
acknowledgment has been received after several retries (as specified
in [ieee802.15.4]), then:
1. Set the duplicate flag (DUP) of the DFF header of the current
packet to 1.
2. Select the next hop (denoted "next_hop") for the current packet,
as specified in Section 10.
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3. Find the Processed Tuple (the "current tuple") in the Processed
Set, with:
+ P_orig_address = the Originator Address of the current packet,
AND;
+ P_seq_number = the Sequence Number of the current packet,
and modify the current tuple:
* P_next_hop_neighbor_list := P_next_hop_neighbor_list@
[next_hop];
* P_time := current time + P_HOLD_TIME.
4. If the selected next hop is equal to P_prev_hop of the current
tuple, set the RET flag (RET := 1), otherwise reset it (RET :=
0).
5. Set the Source Address in the 802.15.4 header to the node's own
address and the Destination Address field to next_hop.
6. Transmit the current packet. If the transmission fails
(determined by missing link layer acknowledgments), the
procedures in Section 9 SHOULD be executed.
10. Getting the Next Hop for a Packet
Before a packet (the "current packet") is sent from a node towards
the packet destination, a valid next hop along the path has to be
selected. This section describes how to select the next hop (denoted
"next_hop"). As a Processed Tuple was either existing when receiving
the packet, or otherwise was created, it can be assumed the a
Processed Tuple for that packet (the "current tuple") is available.
The next hop is chosen from a list of neighbors in order of
decreasing preference of the following conditions. This list is only
a suggestion, any other order of priority MAY be used, however,
P_prev_hop of the current tuple SHOULD be the last entry. The list
SHOULD NOT contain addresses which are listed in
P_next_hop_neighbor_list of the current tuple, and an address SHOULD
NOT appear more than once in the list.
1. If a mesh-under routing protocol is used, then a next hop along
the path to the Final Destination Address of the packet may be
added, where next hops with lower route costs have a higher
priority.
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2. All other neighbors from an external neighborhood discovery
process can be added.
3. P_prev_hop of the current tuple SHOULD be added last; this case
is only used for returning the packet to the previous hop, in
which case the RET flag MUST be set to 1.
11. Poisoning
When a packet is returned (i.e. a packet with RET = 1 is received by
a node) or a link layer acknowledgment (ACK) has not been received
for a forwarded packet, and if a mesh-under routing protocol is used
in conjunction to DFF, the cost for the route MAY be increased in
that routing protocol. Thus, future transmissions prefer other
routes. For the case of a missing link layer ACK, in addition to
increasing the route cost, the link cost to the neighbor may also be
increased if such is supported by the neighborhood discovery process.
It is up to the implementation to decide by how much the route and
link cost should be increased and out of scope of this document.
12. Route Repair Mechanism
If DFF is used in conjunction with a mesh-under routing protocol,
route repair mechanisms of that mesh-under routing protocol MAY be
disabled in order to avoid unnecessary flooding of the network. As
DFF finds alternate paths for the data traffic, and in addition may
"poison" unused routes of the mesh-under routing protocol, route
repair mechanisms may be unnecessary and even reduce the stability of
the network (e.g. because of collisions when Route Requests or Link
State Advertisements are flooded in the network).
13. Sequence Numbers
Whenever a node generates a packet, a sequence number is required in
the DFF header. This sequence number needs to be unique only locally
on each node. For example, a sequence number may start at 0 for the
first generated packet, and then increase in steps of 1 for each new
packet. The sequence number MUST not be greater than 65535 and
SHOULD wrap around to 0.
14. Proposed Values for Parameters and Constants
This section lists the parameters and constants used in the
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specification of the protocol, and proposed values of each that MAY
be used.
o P_HOLD_TIME := 5 seconds
o MAX_HOPS_LEFT := 255
15. Security Considerations
The security mechanisms for protecting the network can be provided by
link-layer technologies. Further details are presented in the
Security Considerations section of [RFC4944].
16. IANA Considerations
IANA is requested to allocate a value from the Dispatch Type Field
registry for LOWPAN_DFF.
17. Acknowledgements
Yuichi Igarashi (Hitachi), Kazuya Monden (Hitachi), Geoff Mulligan
(IPSO), Hiroki Satoh (Hitachi), Ganesh Venkatesh (UC San Diego), and
Jiazi Yi (Ecole Polytechnique) provided useful discussions which
helped to improve this document.
18. References
18.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC4944] Montenegro, G., Kushalnagar, N., Hui, J., and D. Culler,
"Transmission of IPv6 Packets over IEEE 802.15.4
Networks", RFC 4944, September 2007.
[ieee802.15.4]
Computer Society, IEEE., "IEEE Std. 802.15.4-2003",
October 2003.
18.2. Informative References
[RFC4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
"Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
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September 2007.
Appendix A. Examples
In this section, some example network topologies are depicted, using
the DFF mechanism for data forwarding. In these examples, it is
assumed that DFF is used in conjunction to a mesh-under routing
protocol. This protocol provides a list of neighbors of each node,
and a routing table with one or more next hops to a destination, if
the topology provides a path from the node to the destination.
A.1. Example 1: Normal Delivery
Figure 3 depicts a network topology with seven nodes A to G, with
links between them as indicated by lines. It is assumed that node A
sends a packet to G, through B and D, according to the mesh-under
routing protocol.
+---+
+---+ D +-----+
| +---+ |
+---+ | |
+---+ B +---+ |
| +---+ | |
+-+-+ | +---+ +-+-+
| A | +---+ E +---+ G +
+-+-+ +---+ +-+-+
| +---+ |
+---+ C +---+ |
+---+ | |
| +---+ |
+---+ F +-----+
+---+
Figure 3: Example 1: normal delivery
If no link fails in this topology, and no loop occurs, then DFF will
not modify the usual data forwarding mechanism. Each node adds a
Processed Tuple for the incoming packet, and selects the next hop
according to Section 10, i.e. it will first select the next hop for
node G as determined by the mesh-under routing protocol.
A.2. Example 2: Forwarding with Link Failure
Figure 4 depicts the same topology as the Example 1, but both links
between B and D and between B and E are unavailable (e.g. because of
wireless link characteristics).
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+---+
XXX-+ D +-----+
X +---+ |
+---+ X |
+---+ B +---+ |
| +---+ X |
+-+-+ X +---+ +-+-+
| A | XXXX+ E +---+ G +
+-+-+ +---+ +-+-+
| +---+ |
+---+ C +---+ |
+---+ | |
| +---+ |
+---+ F +-----+
+---+
Figure 4: Example 2: link failure
When B receives the packet from node a, it adds a Processed Tuple,
and then tries to forward the packet to D. Once B detects that the
packet cannot be successfully delivered to D because it does not
receive link layer ACKs, it will follow the procedures listed in
Section 9, by setting the DUP flag to 1, selecting E as new next hop,
adding E to the list of next hops in the Processed Tuple, and then
forwarding the packet to E.
As the link to E also fails, B will again follow the procedure in
Section 9. As all possible next hops (D and E) are listed in the
Processed Tuple, B will set the RET flag in the packet and return it
to A.
A determines that it already has a Processed Tuple for the returned
packet, reset the RET flag of the packet and select a new next hop
for the packet. As B is already in the list of next hops in the
Processed Tuple, it will select C as next hop and forward the packet
to it. C will then forward the packet to F, and F delivers the
packet to its destination G.
A.3. Example 3: Forwarding with Missed Link Layer Acknowledgment
Figure 5 depicts the same topology as the Example 1, but the link
layer acknowledgments from C to A are lost (e.g. because the link is
uni-directional). It is assumed that A prefers a path to G through C
and F.
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+---+
+---+ D +-----+
| +---+ |
+---+ | |
+---+ B +---+ |
| +---+ | |
+-+-+ | +---+ +-+-+
| A | +---+ E +---+ G +
+-+-+ +---+ +-+-+
. +---+ |
+...+ C +---+ |
+---+ | |
| +---+ |
+---+ F +-----+
+---+
Figure 5: Example 3: missed link layer acknowledgment
While C successfully receives the packet from A, A does not receive
the ACK and assumes the packet has not been delivered to C.
Therefore, it sets the DUP flag of the packet to 1, in order to
indicate that this packet is a duplicate. Then, it forwards the
packet to B.
A.4. Example 4: Forwarding with a Loop
Figure 6 depicts the same topology as the Example 1, but there is a
loop from D to A, and A sends the packet through B and D.
+-----------------+
| |
| +-+-+
| +---+ D +
| | +---+
\|/ +---+ |
+---+ B +---+
| +---+ |
+-+-+ | +---+ +-+-+
| A | +---+ E +---+ G +
+-+-+ +---+ +-+-+
| +---+ |
+---+ C +---+ |
+---+ | |
| +---+ |
+---+ F +-----+
+---+
Figure 6: Example 4: loop
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When A receives the packet through the loop from D, it will find a
Processed Tuple for the packet. Node A will set the RET flag and
return the packet to D, which in turn will return it to B. B will
then select E as next hop, which will then forward it to G.
Authors' Addresses
Ulrich Herberg
Fujitsu
1240 E. Arques Avenue, M/S 345
Sunnyvale, CA 94085
US
Phone: +1 408 530-4528
Email: ulrich.herberg@us.fujitsu.com
Alvaro A. Cardenas
Fujitsu
1240 E. Arques Avenue, M/S 345
Sunnyvale, CA 94085
US
Phone: +1 408 530-4516
Email: alvaro.cardenas-mora@us.fujitsu.com
Tadashige Iwao
Fujitsu
Shiodome City Center, 5-2, Higashi-shimbashi 1-chome, Minato-ku
Tokyo,
JP
Phone: +81-44-754-3343
Email: smartnetpro-iwao_std@ml.css.fujitsu.com
Michael L. Dow
Freescale
6501 William Cannon Drive West
Austin, TX 78735
USA
Phone: +1 512 895 4944
Email: m.dow@freescale.com
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Sandra L. Cespedes
U. Icesi/U. of Waterloo
Calle 18 No. 122-135 Pance
Cali, Valle
Colombia
Phone:
Email: slcesped@bbcr.uwaterloo.ca
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