Internet DRAFT - draft-cardenas-dff
draft-cardenas-dff
Internet Engineering Task Force U. Herberg, Ed.
Internet-Draft Fujitsu
Intended status: Experimental A. Cardenas
Expires: November 8, 2013 University of Texas at Dallas
T. Iwao
Fujitsu
M. Dow
Freescale
S. Cespedes
U. Icesi
May 7, 2013
Depth-First Forwarding in Unreliable Networks (DFF)
draft-cardenas-dff-14
Abstract
This document specifies the "Depth-First Forwarding" (DFF) protocol
for IPv6 networks, a data forwarding mechanism that can increase
reliability of data delivery in networks with dynamic topology and/or
lossy links. The protocol operates entirely on the forwarding plane,
but may interact with the routing plane. DFF forwards data packets
using a mechanism similar to a "depth-first search" for the
destination of a packet. The routing plane may be informed of
failures to deliver a packet or loops. This document specifies the
DFF mechanism both for IPv6 networks (as specified in RFC2460) and in
addition also for LoWPAN "mesh-under" networks (as specified in
RFC4944). The design of DFF assumes that the underlying link layer
provides means to detect if a packet has been successfully delivered
to the next hop or not. It is applicable for networks with little
traffic, and is used for unicast transmissions only.
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."
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This Internet-Draft will expire on November 8, 2013.
Copyright Notice
Copyright (c) 2013 IETF Trust and the persons identified as the
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
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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.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.1. Motivation . . . . . . . . . . . . . . . . . . . . . . . . 5
1.2. Experiments to be conducted . . . . . . . . . . . . . . . 6
2. Notation and Terminology . . . . . . . . . . . . . . . . . . . 7
2.1. Notation . . . . . . . . . . . . . . . . . . . . . . . . . 7
2.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 8
3. Applicability Statement . . . . . . . . . . . . . . . . . . . 9
4. Protocol Overview and Functioning . . . . . . . . . . . . . . 11
4.1. Information Sets Overview . . . . . . . . . . . . . . . . 12
4.2. Signaling Overview . . . . . . . . . . . . . . . . . . . . 12
5. Protocol Dependencies . . . . . . . . . . . . . . . . . . . . 13
6. Information Sets . . . . . . . . . . . . . . . . . . . . . . . 14
6.1. Symmetric Neighbor List . . . . . . . . . . . . . . . . . 14
6.2. Processed Set . . . . . . . . . . . . . . . . . . . . . . 14
7. Packet Header Fields . . . . . . . . . . . . . . . . . . . . . 15
8. Protocol Parameters . . . . . . . . . . . . . . . . . . . . . 16
9. Data Packet Generation and Processing . . . . . . . . . . . . 16
9.1. Data Packets Entering the DFF Routing Domain . . . . . . . 17
9.2. Data Packet Processing . . . . . . . . . . . . . . . . . . 18
10. Unsuccessful Packet Transmission . . . . . . . . . . . . . . . 20
11. Determining the Next Hop for a Packet . . . . . . . . . . . . 21
12. Sequence Numbers . . . . . . . . . . . . . . . . . . . . . . . 22
13. Modes of Operation . . . . . . . . . . . . . . . . . . . . . . 22
13.1. Route-Over . . . . . . . . . . . . . . . . . . . . . . . . 23
13.1.1. Mapping of DFF Terminology to IPv6 Terminology . . . 23
13.1.2. Packet Format . . . . . . . . . . . . . . . . . . . . 23
13.2. Mesh-Under . . . . . . . . . . . . . . . . . . . . . . . . 25
13.2.1. Mapping of DFF Terminology to LoWPAN Terminology . . 25
13.2.2. Packet Format . . . . . . . . . . . . . . . . . . . . 26
14. Scope Limitation of DFF . . . . . . . . . . . . . . . . . . . 27
14.1. Route-Over MoP . . . . . . . . . . . . . . . . . . . . . . 29
14.2. Mesh-Under MoP . . . . . . . . . . . . . . . . . . . . . . 30
15. MTU Exceedance . . . . . . . . . . . . . . . . . . . . . . . . 32
16. Security Considerations . . . . . . . . . . . . . . . . . . . 32
16.1. Attacks Out of Scope . . . . . . . . . . . . . . . . . . . 32
16.2. Protection Mechanisms of DFF . . . . . . . . . . . . . . . 32
16.3. Attacks In Scope . . . . . . . . . . . . . . . . . . . . . 33
16.3.1. Denial of Service . . . . . . . . . . . . . . . . . . 33
16.3.2. Packet Header Modification . . . . . . . . . . . . . 33
16.3.2.1. Return Flag Tampering . . . . . . . . . . . . . . 34
16.3.2.2. Duplicate Flag Tampering . . . . . . . . . . . . 34
16.3.2.3. Sequence Number Tampering . . . . . . . . . . . . 34
17. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 35
18. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 35
19. References . . . . . . . . . . . . . . . . . . . . . . . . . . 35
19.1. Normative References . . . . . . . . . . . . . . . . . . . 35
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19.2. Informative References . . . . . . . . . . . . . . . . . . 36
Appendix A. Examples . . . . . . . . . . . . . . . . . . . . . . 37
A.1. Example 1: Normal Delivery . . . . . . . . . . . . . . . . 37
A.2. Example 2: Forwarding with Link Failure . . . . . . . . . 37
A.3. Example 3: Forwarding with Missed Link Layer
Acknowledgment . . . . . . . . . . . . . . . . . . . . . . 38
A.4. Example 4: Forwarding with a Loop . . . . . . . . . . . . 39
Appendix B. Deployment Experience . . . . . . . . . . . . . . . . 40
B.1. Deployments in Japan . . . . . . . . . . . . . . . . . . . 40
B.2. Kit Carson Electric Cooperative . . . . . . . . . . . . . 40
B.3. Simulations . . . . . . . . . . . . . . . . . . . . . . . 40
B.4. Open Source Implementation . . . . . . . . . . . . . . . . 40
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 41
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1. Introduction
This document specifies the Depth-First Forwarding (DFF) protocol for
IPv6 networks, both for IPv6 forwarding ([RFC2460], henceforth
denoted "route-over"), and also for "mesh-under" forwarding using the
LoWPAN adaptation layer ([RFC4944]). The protocol operates entirely
on the forwarding plane, but may interact with the routing plane.
The purpose of DFF is to increase reliability of data delivery in
networks with dynamic topologies and/or lossy links.
DFF forwards data packets using a "depth-first search" for the
destination of the packets. DFF relies on an external neighborhood
discovery mechanism which lists neighbors of a router that may be
attempted as next hops for a data packet. In addition, DFF may use
information from the Routing Information Base (RIB) for deciding in
which order to try to send the packet to the neighboring routers.
If the packet makes no forward progress using the first selected next
hop, DFF will successively try all neighbors of the router. If none
of the next hops successfully receives or forwards 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 trees, loops can occur.
Therefore, DFF contains a loop detection and avoidance mechanism.
DFF may provide information, which may - by a mechanism outside of
this specification - be used for updating cost of routes in the RIB
based on failed or successful delivery of packets through alternative
next hops. Such information may also be used by a routing protocol.
DFF assumes that the underlying link layer provides means to detect
if a packet has been successfully delivered to the next hop or not,
is designed for networks with little traffic, and is used for unicast
transmissions only.
1.1. Motivation
In networks with dynamic topologies and/or lossy links, even frequent
exchanges of control messages between routers for updating the
routing tables cannot guarantee that the routes correspond to the
effective topology of the network at all times. Packets may not be
delivered to their destination because the topology has changed since
the last routing protocol update.
More frequent routing protocol updates can mitigate that problem to a
certain extent, however this requires additional signaling, consuming
channel and router resources (e.g., when flooding control messages
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through the network). This is problematic in networks with lossy
links, where further control traffic exchange can worsen the network
stability because of collisions. Moreover, additional control
traffic exchange may drain energy from battery-driven routers.
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 at the same time to increase delivery success rates.
As this specification is intended for experimentation, the mechanism
is also specified for forwarding on the LoWPAN adaption layer
(according to Section 11 of [RFC4944]), in addition to IPv6
forwarding as specified in [RFC2460]. Other than different header
formats, the DFF mechanism for route-over and mesh-under is similar,
and is therefore first defined in general, and then more specifically
for both IPv6 route-over forwarding (as specified in Section 13.1),
and for LoWPAN adaptation layer mesh-under (as specified in
Section 13.2).
1.2. Experiments to be conducted
This document is presented as an Experimental specification that can
increase reliability of data delivery in networks with dynamic
topology and/or lossy links. It is anticipated that, once sufficient
operational experience has been gained, this specification will be
revised to progress it on to the Standards Track. This experiment is
intended to be tried in networks that meet the applicability
described in Section 3, and with the scope limitations set out in
Section 14. While experimentation is encouraged in such networks,
operators should exercise caution before attempting this experiment
in other types of network as the stability of interaction between DFF
and routing in those networks has not been established.
Experience reports regarding DFF implementation and deployment are
encouraged particularly with respect to:
o Optimal values for the parameter P_HOLD_TIME, depending on the
size of the network, the topology and the amount of traffic
originated per router. The longer a Processed Tuple is hold, the
more memory is consumed on a router. Moreover, if a tuple is hold
too long, a sequence number wrap-around may occur, and a new
packet may have the same sequence number as one indicated in an
old Processed Tuple. However, if the tuple is expired too soon
(before the packet has been completed its path to the
destination), it may be mistakenly detected as new packet instead
of one already seen.
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o Optimal values for the parameter MAX_HOP_LIMIT, depending on the
size of the network, the topology, and the lossyness of the link
layer. MAX_HOP_LIMIT makes sure that packets do not unnecessarily
traverse in the network; it may be used to limit the "detour" of
packets that is acceptable. The value may also be based on a per-
packet-basis if hop-count information is available from the RIB or
routing protocol. In such a case, the hop-limit for the packet
may be a percentage (e.g., 200%) of the hop-count value indicated
in the routing table.
o Optimal methods to increase cost of a route when a loop or lost L2
ACK is detected by DFF. While this is not specified as a
normative part of this document, it may be of interest in an
experiment to find good values of how much to increase link cost
in the RIB or routing protocol.
o Performance of using DFF in combination with different routing
protocols, such as reactive and proactive protocols. This also
implies how routes are updated by the RIB / routing protocol when
informed by DFF about loops or broken links.
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.
Concatenation of lists: If L1 and L2 are lists, then L1@L2 is a new
list with first all elements of L1, followed by all elements of L2
in that order.
Byte order: All packet formats in this specification use network
byte order (most significant octet first) for all fields. The
most significant bit in an octet is numbered bit 0, and the least
significant bit of an octet is numbered bit 7.
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Assignment: a := b
An assignment operator, whereby the left side (a) is assigned the
value of the right side (b).
Comparison: c = d
A comparison operator, returning true if the value of the left
side (c) is equal to the value of the right side (d).
Flags: This specification uses multiple 1-bit flags. A value of '0'
of a flag means 'false', a value of '1' means 'true'.
2.2. Terminology
The terms "route-over" and "mesh-under", introduced in [RFC6775] are
used in this document, where "route-over" is not only limited to
6LoWPANs but applies to general IPv6 networks:
Mesh-under: A topology where nodes are connected to a [6LoWPAN
Border Router] 6LBR through a mesh using link-layer forwarding.
Thus, in a mesh-under configuration, all IPv6 hosts in a LoWPAN
are only one IP hop away from the 6LBR. This topology simulates
the typical IP-subnet topology with one router with multiple nodes
in the same subnet.
Route-over: A topology where hosts are connected to the 6LBR through
the use of intermediate layer-3 (IP) routing. Here, hosts are
typically multiple IP hops away from a [6LoWPAN Router] 6LBR. The
route-over topology typically consists of a 6LBR, a set of 6LRs,
and hosts.
The following terms are used in this document. As the DFF mechanism
is specified both for route-over IPv6 and for mesh-under LoWPAN
adaptation layer, the terms are generally defined in this section,
and then specifically mapped for each of the different modes of
operation in Section 13.
Depth-first search: "Depth-first search (DFS) is an algorithm for
traversing or searching a tree, tree structure, or graph. One
starts at the root (selecting some node as the root in the graph
case) and explores as far as possible along each branch before
backtracking" [DFS_wikipedia]. In this document, the algorithm
for traversing a graph is applied to forwarding packets in a
computer network, with nodes being routers.
Routing Information Base (RIB): A table stored in the user-space of
an operating system of a router or host. The table lists routes
to network destinations, as well as associated metrics with these
routes.
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Mode of Operation (MoP): The DFF mechanism specified in this
document can either be used as "route-over" IPv6 forwarding
mechanism (Mode of Operation: "route-over"), or as "mesh-under"
LoWPAN adaptation layer (Mode of Operation: "mesh-under").
Packet: An IPv6 Packet (for "route-over" MoP) or a "LoWPAN
encapsulated packet" (for "mesh-under" MoP) containing an IPv6
Packet as payload.
Packet Header: An IPv6 extension header (for "route-over" MoP) or a
LoWPAN header (for "mesh-under" MoP).
Address: An IPv6 address (for "route-over" MoP), or a 16-bit short
or EUI-64 link layer address (for "mesh-under" MoP).
Originator: The router which added the DFF header (specified in
Section 7) to a Packet.
Originator Address: An Address of the Originator. This Address
SHOULD be an Address configured on the interface which transmits
the Packet, selected according to [RFC6724].
Destination: The router or host to which a Packet is finally
destined. In case this router or host is outside of the routing
domain in which DFF is used, the Destination is the router that
removes the DFF header (specified in Section 7) from the Packet.
This case is described in Section 14.1.
Destination Address: An Address to which the Packet is sent.
Next Hop: An Address of the next hop router to which the Packet is
sent along the path to the Destination.
Previous Hop: The Address of the previous hop router from which a
Packet has been received. In case the Packet has been received by
a router from outside of the routing domain where DFF is used
(i.e., no DFF header is contained in the Packet), the Originator
Address of the router adding the DFF header to the Packet is used
as the Previous Hop.
Hop Limit: An upper bound how many times the Packet may be
forwarded.
3. Applicability Statement
This document specifies DFF, a packet forwarding mechanism intended
for use in networks with dynamic topology and/or lossy links with the
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purpose of increasing reliability of data delivery. The protocol's
applicability is determined by its characteristics, which are that
this protocol:
o Is applicable for use in IPv6 networks, either as "route-over"
forwarding mechanism using IPv6 ([RFC2460]), or as "mesh-under"
forwarding mechanism using the frame format for transmission of
IPv6 packets defined in [RFC4944].
o Assumes addresses used in the network are either IPv6 addresses
(if the protocol is used as "route-over"), or 16-bit short or
EUI-64 link layer addresses, as specified in [RFC4944] if the
protocol is used as "mesh-under". In "mesh-under" mode, mixed 16-
bit and EUI-64 addresses within one DFF routing domain are allowed
(if conform with [RFC4944]), as long as DFF is limited to be used
within one PAN (Personal Area Network). It is assumed that the
"route-over" mode and "mesh-under" mode are mutually exclusive in
the same routing domain.
o Assumes that the underlying link layer provides means to detect if
a Packet has been successfully delivered to the Next Hop or not
(e.g., by L2 ACK messages). Examples for such underlying link
layers are IEEE 802.15.4 or IEEE 802.11.
o Is applicable in networks with lossy links and/or with a dynamic
topology. In networks with very stable links and fixed topology,
DFF will not bring any benefit (but also not be harmful, other
than the additional overhead for the Packet header).
o Works in a completely distributed manner, and does not depend on
any central entity.
o Is applicable for networks with little traffic in terms of numbers
of Packets per second, since each recently forwarded Packet
increases the state on a router. The amount of traffic per time
that is supported by DFF depends on the memory resources of the
router running DFF, on the density of the network, on the loss
rate of the channel, and the maximum hop limit for each Packet:
for each recently seen Packet, a list of Next Hops that the Packet
has been sent to is stored in memory. The stored entries can be
deleted after an expiration time, so that only recently received
Packets require storage on the router. Implementations are
advised to measure and report rates of packets in the network, and
also to report memory usage. Thus, operators can determine memory
exhaustion because of growing Information Sets or problems because
of too rapid sequence number wrap-around.
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o Is applicable for dense topologies with multiple paths between
each source and each destination. Certain topologies are less
suitable for DFF: topologies that can be partitioned by the
removal of a single router or link, topologies with multiple stub
routers that each have a single link to the network, topologies
with only a single path to a destination, or topologies where the
"detour" that a Packet makes during the depth-first search in
order to reach the destination would be too long. Note that the
number of retransmissions of a Packet that stipulate a "too long"
path depends on the underlying link layer (capacity and
probability of Packet loss), as well as how much bandwidth is
required for data traffic by applications running in the network.
In such topologies, the Packet may never reach the Destination,
and therefore unnecessary transmissions of data Packets may occur
until the Hop Limit of the Packet reaches zero and the Packet is
dropped. This may consume channel and router resources.
o Is used for unicast transmissions only (not for anycast or
multicast).
o Is for use within stub networks, and for traffic between a router
inside the routing domain in which DFF is used and a known border
router. Examples of such networks are LoWPANs. Scope limitations
are described in Section 14.
4. Protocol Overview and Functioning
When a Packet is to be forwarded by a router using DFF, the router
creates a list of candidate Next Hops for that Packet. This list
(created per packet) is ordered, and Section 11 provides
recommendations of how to order the list, e.g., first listing Next
Hops listed in the RIB, if available, ordered in increasing cost,
followed by other neighbors provided by an external neighborhood
discovery. DFF proceeds to forward the Packet to the Next Hop listed
first in the list. If the transmission was not successful (as
determined by the underlying link layer) or if the Packet was
"returned" by a Next Hop to which it had been sent before, the router
will try to forward the Packet to the next Next Hop on the list. A
router "returns" a Packet to the router from which it was originally
received, once it has unsuccessfully tried to forward the Packet to
all elements in the candidate Next Hop list. If the Packet is
eventually returned to the Originator of the Packet, and after the
Originator has exhausted all of its Next Hops for the Packet, the
Packet is dropped.
For each recently forwarded Packet, a router running DFF stores
information about the Packet as entry in an information set, denoted
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Processed Set. Each entry in the Processed Set contains a sequence
number, included in the Packet Header, identifying the Packet (refer
to Section 12 for further details on the sequence number).
Furthermore, the entry contains a list of Next Hops to which the
Packet has been sent. This list of recently forwarded Packets also
allows for avoiding loops when forwarding a Packet. Entries in the
Processed Set expire after a given expiration timeout, and are
removed.
4.1. Information Sets Overview
This specification requires a single set on each router, the
Processed Set. The Processed Set stores the sequence number, the
Originator Address, the Previous Hop and a list of Next Hops, to
which the Packet has been sent, for each recently seen Packet.
Entries in the set are removed after a predefined time-out. Each
time a Packet is forwarded to a Next Hop, that Next Hop is added to
the list of Next Hops of the entry for the Packet.
Note that an implementation of this protocol may maintain the
information of the Processed Set in the indicated form, or in any
other organization which offers access to this information. In
particular, it is not necessary to remove Tuples from a Set at the
exact time indicated, only to behave as if the Tuples were removed at
that time.
In addition to the Processed Set, a list of symmetric neighbors must
be provided by an external neighborhood discovery mechanism, or may
be determined from the RIB (e.g., if the RIB provides routes to
adjacent routers, and if these one-hop routes are verified to be
symmetric).
4.2. Signaling Overview
Information is needed on a per-packet basis by a router running DFF
that receives a Packet. This information is encoded in the Packet
Header that is specified in this document as IPv6 Hop-by-Hop Options
extension header and as LoWPAN header respectively, for the intended
"route-over" and "mesh-under" Modes of Operations. This DFF header
contains a sequence number used for uniquely identifying a Packet,
and two flags: RET (for "return") and DUP (for "duplicated").
While a router successively tries sending a data Packet to one or
more of its neighbors, RET = 0. If none of the transmissions of the
Packet to the neighbors of a router have succeeded, the Packet is
returned to the router from which the Packet has been first received,
indicated by setting the return flag (RET := 1). The RET flag is
required to discern between a deliberately returned Packet and a
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looping Packet: if a router receives a Packet with RET = 1 (and DUP =
0 or DUP = 1) that it has already forwarded, the Packet was
deliberately returned, and the router will continue to successively
send the Packet to routers from the candidate Next Hop list. If that
Packet has RET = 0, the router assumes that the Packet is looping and
returns it to the router from which it last received it. An external
mechanism may use this information for increasing the route cost of
the route to the Destination using the Next Hop which resulted in the
loop in the RIB or the routing protocol. It is out of scope of this
document to specify such a mechanism. Note that once DUP is set to
1, loop detection is not possible any more as the flag is not reset
any more. Therefore, a Packet may loop if the RIBs of routers in the
domain are inconsistent, until hop limit has reached 0.
Whenever a Packet transmission to a neighbor has failed (as
determined by the underlying link layer, e.g., using L2 ACKs), 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 L2 ACK has been
lost, resulting in possible duplicates of the Packet in the network.
The DUP flag tags such a possible duplicate. The DUP flag is
required to discern between a duplicated Packet and a looping Packet:
if a router receives a Packet with DUP = 1 (and RET = 0) that it has
already forwarded, the Packet is not considered looping, and
successively forwarded to the next router from the candidate Next Hop
list. If the received Packet has DUP = 0 (and RET = 0), the router
assumes that the Packet is looping, sets RET := 1, and returns it to
the Previous Hop. Again, an external mechanism may use this
information for increasing route costs and/or informing the routing
protocol.
The reason for not dropping received duplicated Packets (with DUP =
1) is that a duplicated Packet may during its path again be
duplicated if another L2 ACK is lost. However, when DUP is already
set to 1, it is not possible discerning the duplicate from the
duplicate of the duplicate. As a consequence, loop detection is not
possible after the second lost L2 ACK on the path of a Packet.
However, if duplicates are simply dropped, it is possible that the
Packet was actually a looping packet (and not a duplicate), and so
the Depth First Search would be interrupted.
5. Protocol Dependencies
DFF MAY use information from the Routing Information Base (RIB),
specifically for determining an order of preference for to which next
hops a packet should be forwarded (e.g., the packet may be forwarded
first to neighbors that are listed in the RIB as next hops to the
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destination, preferring those with the lowest route cost).
Section 11 provides recommendations about the order of preference for
the next hops of a packet.
DFF MUST have access to a list of symmetric neighbors for each
router, provided by a mechanism such as, e.g., NHDP [RFC6130]. That
neighborhood discovery protocol is not specified in this document.
6. Information Sets
This section specifies the information sets used by DFF.
6.1. Symmetric Neighbor List
DFF MUST have access to a list of Addresses of symmetric neighbors of
the router. This list can be provided by an external neighborhood
discovery mechanism, or alternatively may be determined from the RIB
(e.g., if the RIB provides routes to adjacent routers, and if these
one-hop routes are verified to be symmetric). The list of Addresses
of symmetric neighbors is not specified within this document. The
Addresses in the list are used to construct a list of candidate Next
Hops for a Packet, as specified in Section 11.
6.2. Processed Set
Each router maintains 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
forwarding mechanism. To protect against this situation, it is
recommended that an implementation retains the Processed Set in non-
volatile storage if such is provided by the router.
The set consists of Processed Tuples
(P_orig_address, P_seq_number, P_prev_hop,
P_next_hop_neighbor_list, P_time)
where
P_orig_address is the Originator Address of the received Packet;
P_seq_number is the sequence number of the received Packet;
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P_prev_hop is the Address of 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 forwarding mechanism, as specified in Section 9.2;
P_time specifies when this Tuple expires and MUST be removed.
The consequences when no or not enough non-volatile storage is
available on a router (e.g., because of limited resources) or when an
implementation chooses not to make the Processed Set persistent, are
that Packets that are already in a loop caused by the routing
protocol may continue to loop until the hop limit is exhausted. Non-
looping Packets may be sent to Next Hops that have already received
the Packet previously and will return the Packet, leading to some
unnecessary retransmissions. This effect is only temporary and
applies only for Packets already traversing the network.
7. Packet Header Fields
This section specifies the information required by DFF in the Packet
Header. Note that, depending on whether DFF is used in the "route-
over" MoP or in the "mesh-under" MoP, the DFF header is either an
IPv6 Hop-by-Hop Options extension header (as specified in
Section 13.1.2) or a LoWPAN header (as specified in Section 13.2.2).
Section 13.1.2 and Section 13.2.2 specify the precise order, format
and encoding of the fields that are listed in this section.
Version (VER) - This 2-bit value indicates the version of DFF that
is used. This specification defines value 00. Packets with other
values of the version MUST be forwarded using forwarding as
defined in [RFC2460] and [RFC4944] for route-over and mesh-under
MoP respectively.
Duplicate Packet Flag (DUP) - This 1-bit flag is set in the DFF
header of a Packet, when that Packet is being re-transmitted due
to a signal from the link-layer that the original transmission
failed, as specified in Section 9.2. Once the flag is set to 1,
it MUST NOT be modified by routers forwarding the Packet.
Return Packet Flag (RET) - The 1-bit flag MUST be set to 1 prior to
sending the Packet back to the Previous Hop. Upon receiving a
packet with RET = 1, and before sending it to a new Candidate Next
Hop, that flag MUST be set to 0 as specified in Section 9.2.
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Sequence Number - A 16-bit field, containing an unsigned integer
sequence number generated by the Originator, unique to each router
for each Packet to which the DFF has been added, as specified in
Section 12. The Originator Address concatenated with the sequence
number represents an identifier of previously seen data Packets.
Refer to Section 12 for further information about sequence
numbers.
8. Protocol Parameters
The parameters used in this specification are listed in this section.
These parameters are configurable, do not need to be stored in non-
volatile storage, and can be varied by implementations at run time.
Default values for the parameters depend on the network size,
topology, link layer and traffic patterns. Part of the
experimentation described in Section 1.2 is to determine suitable
default values.
P_HOLD_TIME - Is the time period after which a newly created or
modified Processed Tuple expires and MUST be deleted. An
implementation SHOULD use a value for P_HOLD_TIME that is high
enough that the Processed Tuple for a Packet is still in memory on
all forwarding routers while the Packet is transiting the routing
domain. The value SHOULD at least be MAX_HOP_LIMIT times the
expected time to send a Packet to a router on the same link. The
value MUST be lower than the time it takes until the same sequence
number is reached again after a wrap-around on the router
identified by P_orig_address of the Processed Tuple.
MAX_HOP_LIMIT - Is the initial value of Hop Limit, and therefore the
maximum number of times that a Packet is forwarded in the routing
domain. When choosing the value of MAX_HOP_LIMIT, the size of the
network, the distance between source and destination in number of
hops, and the maximum possible "detour" of a Packet SHOULD be
considered (compared to the shortest path). Such information MAY
be used from the RIB, if provided.
9. Data Packet Generation and Processing
The following sections specify the process of handling a Packet
entering the DFF routing domain (i.e., without DFF header) in
Section 9.1, as well as forwarding a data Packet from another router
running DFF in Section 9.2.
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9.1. Data Packets Entering the DFF Routing Domain
This section applies for any data Packets upon their first entry into
a routing domain, in which DFF is used. This occurs when a new data
Packet is generated on this router, or when a data Packet is
forwarded from outside the routing domain (i.e., from a host attached
to this router or from a router outside the routing domain in which
DFF is used). Before such a data Packet (henceforth denoted "current
Packet") is transmitted, the following steps MUST be executed:
1. If required, encapsulate the Packet as specified in Section 14.
2. Add the DFF header to the current Packet (to the outer header if
the Packet has been encapsulated), with:
* DUP := 0;
* RET := 0;
* Sequence Number := a new sequence number of the Packet (as
specified in Section 12).
3. Check that the Packet does not exceed the MTU as specified in
Section 15. In case it does, execute the procedures listed in
Section 15 and do not further process the Packet.
4. Select the Next Hop (henceforth denoted "next_hop") for the
current Packet, as specified in Section 11.
5. 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.
6. Pass the current Packet to the underlying link layer for
transmission to next_hop. If the transmission fails (as
determined by the link layer), the procedures in Section 10 MUST
be executed.
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9.2. Data Packet Processing
When a Packet (henceforth denoted the "current Packet") is received
by a router, then the following tasks MUST be performed:
1. If the Packet Header is malformed (i.e., the header format is not
as expected by this specification), drop the Packet.
2. Otherwise, if the Destination Address of the Packet matches an
Address of an interface of this router, deliver the Packet to
upper layers and do not further process the Packet as specified
below.
3. Decrement the value of the Hop Limit field by one (1).
4. Drop the Packet if Hop Limit is decremented to zero and do not
further process the Packet as specified below.
5. If no Processed Tuple (henceforth denoted the "current tuple")
exists 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.
Then:
1. Add a Processed Tuple (henceforth 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 Previous Hop Address of the current
Packet;
+ P_next_hop_neighbor_list := [];
+ P_time := current time + P_HOLD_TIME.
2. Set RET to 0 in the DFF header.
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3. Select the Next Hop (henceforth denoted "next_hop") for the
current Packet, as specified in Section 11.
4. P_next_hop_neighbor_list := P_next_hop_neighbor_list@
[next_hop].
5. Pass the current Packet to the underlying link layer for
transmission to next_hop. If the transmission fails (as
determined by the link layer), the procedures in Section 10
MUST be executed.
6. 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. Pass the current Packet to the underlying link layer for
transmission to the Previous Hop.
2. Otherwise, if the return flag of the current Packet is set
(RET = 1):
1. If the Previous Hop of the Packet is not contained in
P_next_hop_neighbor_list of the current tuple, drop the
Packet.
2. If the Previous Hop of the Packet (i.e., the address of
the router from which the current Packet has just been
received) is equal to P_prev_hop of current tuple (i.e.,
the address of the router from which the current Packet
has been first received), drop the Packet.
3. Set RET := 0.
4. Select the Next Hop (henceforth denoted "next_hop") for
the current Packet, as specified in Section 11.
5. Modify the current tuple:
- P_next_hop_neighbor_list := P_next_hop_neighbor_list@
[next_hop];
- P_time := current time + P_HOLD_TIME.
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6. If the selected Next Hop is equal to P_prev_hop of the
current tuple, as specified in Section 11, (i.e., all
Candidate Next Hops have been unsuccessfully tried), set
RET := 1. If this router (i.e., the router receiving the
current packet) has the same Address as the Originator
Address of the current Packet, drop the Packet.
7. Pass the current Packet to the underlying link layer for
transmission to next_hop. If transmission fails (as
determined by the link layer), the procedures in
Section 10 MUST be executed.
10. Unsuccessful Packet Transmission
DFF requires that the underlying link layer provides information as
to whether a Packet is successfully received by the Next Hop. Absence
of such a signal is interpreted as delivery failure of the Packet
(henceforth denoted the "current Packet"). Note that the underlying
link layer MAY retry sending the Packet multiple times (e.g., using
exponential back-off) before determining that the Packet has not been
successfully received by the Next Hop. Whenever Section 9 explicitly
requests it in case of such a delivery failure, the following steps
are executed:
1. Set the duplicate flag (DUP) of the DFF header of the current
Packet to 1.
2. Select the Next Hop (henceforth denoted "next_hop") for the
current Packet, as specified in Section 11.
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,
4. If no current tuple is found, drop the Packet.
5. Otherwise, modify the current tuple:
* P_next_hop_neighbor_list := P_next_hop_neighbor_list@
[next_hop];
* P_time := current time + P_HOLD_TIME.
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6. If the selected next_hop is equal to P_prev_hop of the current
tuple, as specified in Section 11 (i.e., all neighbors have been
unsuccessfully tried):
* RET := 1
* Decrement the value of the Hop Limit field by one (1). Drop
the Packet if Hop Limit is decremented to zero.
7. Otherwise
* RET := 0
8. Transmit the current Packet to next_hop. If transmission fails
(determined by the link layer), and if the next_hop does not
equal P_prev_hop from the current tuple, the procedures in
Section 10 MUST be executed.
11. Determining the Next Hop for a Packet
When forwarding a Packet, a router determines a valid Next Hop for
that Packet as specified in this section. As a Processed Tuple was
either existing when receiving the Packet (henceforth denoted the
"current Packet"), or otherwise was created, it can be assumed the a
Processed Tuple for that Packet (henceforth denoted the "current
tuple") is available.
The Next Hop is chosen from a list of candidate Next Hops in order of
decreasing priority. This list is created per Packet. The maximum
candidate Next Hop List for a Packet contains all the neighbors of
the router (as determined from an external neighborhood discovery
process), except for the Previous Hop of the current Packet. A
smaller list MAY be used, if desired, and the exact selection of the
size of the candidate Next Hop List is a local decision in each
router, which does not affect interoperability. Selecting a smaller
list may reduce the path length of a Packet traversing the network
and reduce required state in the Processed Set, but may result in
valid paths that are not explored. If information from the RIB is
used, then the candidate Next Hop list MUST contain at least the Next
Hop, indicated in the RIB as the Next Hop on the shortest path to the
destination, and SHOULD contain all Next Hops, indicated to the RIB
as Next Hops on paths to the destination. If a Next Hop from the RIB
equals the Previous Hop of the current Packet, it MUST NOT be added
to the candidate Next Hop list.
The list MUST NOT contain Addresses which are listed in
P_next_hop_neighbor_list of the current tuple, in order to avoid
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sending the Packet to the same neighbor multiple times. Moreover, an
Address MUST NOT appear more than once in the list, for the same
reason. Also, Addresses of an interface of this router MUST NOT be
added to the list.
The list has an order of preference, where Next Hops at the top of
the list being the ones that Packets are sent to first in the depth-
first processing specified in Section 9.1 and Section 9.2. The
following order is RECOMMENDED, with the elements listed on top
having the highest preference:
1. The neighbor that is indicated in the RIB as the Next Hop on the
shortest path to the destination of the current Packet;
2. Other neighbors indicated in the RIB as Next Hops on path to the
destination of the current Packet;
3. All other symmetric neighbors (except the Previous Hop of the
current Packet).
Additional information from the RIB or the list of symmetric
neighbors MAY be used for determining the order, such as route cost
or link quality.
If the candidate Next Hop list created as specified in this section
is empty, the selected Next Hop MUST be P_prev_hop of the current
tuple; this case applies when returning the Packet to the Previous
Hop.
12. Sequence Numbers
Whenever a router generates a Packet or forwards a Packet on behalf
of a host or a router outside the routing domain where DFF is used, a
sequence number MUST be created and included in the DFF header. This
sequence number MUST be unique locally on each router where it is
created. A sequence number MUST start at 0 for the first Packet to
which the DFF header is added, and then increase in steps of 1 for
each new Packet. The sequence number MUST NOT be greater than 65535
and MUST wrap around to 0.
13. Modes of Operation
DFF can be used either as "route-over" IPv6 forwarding protocol, or
alternatively as "mesh-under" data forwarding protocol for the LoWPAN
adaptation layer ([RFC4944]). Previous sections have specified the
DFF mechanism in general; specific differences for each MoP are
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specified in this section.
13.1. Route-Over
This section maps the general terminology from Section 2.2 to the
specific terminology when using the "route-over" MoP.
13.1.1. Mapping of DFF Terminology to IPv6 Terminology
The following terms are those listed in Section 2.2, and their
meaning is explicitly defined when DFF is used in the "route-over"
MoP:
Packet - An IPv6 packet, as specified in [RFC2460].
Packet Header - An IPv6 extension header, as specified in [RFC2460].
Address - An IPv6 address, as specified in [RFC4291].
Originator Address - The Originator Address corresponds to the
Source address field of the IPv6 header as specified in [RFC2460].
Destination Address - The Destination Address corresponds to the
Destination field of the IPv6 header as specified in [RFC2460].
Next Hop - The Next Hop is the IPv6 address of the next hop to which
the Packet is sent; the link layer address from that IP address is
resolved by a mechanism such as ND [RFC4861]. The link layer
address is then used by L2 as destination.
Previous Hop - The Previous Hop is the IPv6 address from the
interface of the previous hop from which the Packet has been
received.
Hop Limit - The Hop Limit corresponds to the Hop Limit field in the
IPv6 header as specified in [RFC2460].
13.1.2. Packet Format
In the "route-over" MoP, all IPv6 Packets MUST conform with the
format specified in [RFC2460].
The DFF header, as specified below, is an IPv6 Extension Hop-by-Hop
Options header, and is depicted in Figure 1 (where DUP is abbreviated
to D, and RET is abbreviated to R because of the limited space in the
figure). This document specifies a new option to be used inside the
Hop-by-Hop Options header, which contains the DFF fields (DUP and RET
flags and sequence number, as specified in Section 7).
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[RFC6564] specifies:
New options for the existing Hop-by-Hop Header SHOULD NOT be
created or specified unless no alternative solution is feasible.
Any proposal to create a new option for the existing Hop-by-Hop
Header MUST include a detailed explanation of why the hop-by-hop
behavior is absolutely essential in the document proposing the new
option with hop-by-hop behavior.
[RFC6564] recommends to use Destination Headers instead of Hop-by-Hop
Option headers. Destination Headers are only read by the destination
of an IPv6 packet, not by intermediate routers. However, the
mechanism specified in this document relies on intermediate routers
reading and editing the header. Specifically, the sequence number
and the DUP and RET flags are read by each router running the DFF
protocol. Modifying the DUP flag and RET flag is essential for this
protocol to tag duplicate or returned Packets. Without the DUP flag,
a duplicate Packet cannot be discerned from a looping Packet, and
without the RET flag, a returned Packet cannot be discerned from a
looping Packet.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Next Header | Hdr Ext Len | OptTypeDFF | OptDataLenDFF |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|VER|D|R|0|0|0|0| Sequence Number | Pad1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: IPv6 DFF Header
Field definitions of the DFF header are as follows:
Next Header - 8-bit selector. Identifies the type of header
immediately following the Hop-by-Hop Options header. As specified
in [RFC2460].
Hdr Ext Len - 8-bit unsigned integer. Length of the Hop-by-Hop
Options header in 8-octet units, not including the first 8 octets.
As specified in [RFC2460]. This value is set to 0 (zero).
OptTypeDFF - 8-bit identifier of the type of option as specified in
[RFC2460]. This value is set to IP_DFF. The two high order bits
of the option type MUST be set to '11' and the third bit is equal
to '1'. With these bits, according to [RFC2460], routers that do
not understand this option on a received Packet discard the packet
and, only if the packet's Destination Address was not a multicast
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address, send an ICMP Parameter Problem, Code 2, message to the
packet's Source Address, pointing to the unrecognized Option Type.
Also, according to [RFC2460], the values within the option are
expected to change en route.
OptDataLenDFF - 8-bit unsigned integer. Length of the Option Data
field of this option, in octets as specified in [RFC2460]. This
value is set to 2 (two).
DFF fields - A 2-bit version field (abbreviated as VER), the DUP
(abbreviated as D) and RET (abbreviated as R) flags follow after
Mesh Forw, as specified in Section 7. The version specified in
this document is 00. All other bits (besides VER, DUP, and RET)
of this octet are reserved and MUST be set to 0.
Sequence Number - A 16-bit field, containing an unsigned integer
sequence number, as specified in Section 7.
Pad1 - Since the Hop-by-Hop Options header must have a length of
multiples of 8 octets, a Pad1 option is used, as specified in
[RFC2460]. All bits of this octet are 0.
13.2. Mesh-Under
This section maps the general terminology from Section 2.2 to the
specific terminology when using the "mesh-under" MoP.
13.2.1. Mapping of DFF Terminology to LoWPAN Terminology
The following terms are those listed in Section 2.2 (besides "Mode of
Operation"), and their meaning is explicitly defined when DFF is used
in the "mesh-under" MoP:
Packet - A "LoWPAN encapsulated packet" (as specified in [RFC4944],
which contains an IPv6 packet as payload.
Packet Header - A LoWPAN header, as specified in [RFC4944].
Address - A 16-bit short or EUI-64 link layer address, as specified
in [RFC4944].
Originator Address - The Originator Address corresponds to the
Originator Address field of the Mesh Addressing header as
specified in [RFC4944].
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Destination Address - The Destination Address corresponds to the
Final Destination field of the Mesh Addressing header as specified
in [RFC4944].
Next Hop - The Next Hop is the destination address of a frame
containing a LoWPAN encapsulated packet, as specified in
[RFC4944].
Previous Hop - The Previous Hop is the source address of the frame
containing a LoWPAN encapsulated packet, as specified in
[RFC4944].
Hop Limit - The Hop Limit corresponds to the Deep Hops Left field in
the Mesh Addressing header as specified in [RFC4944].
13.2.2. Packet Format
In the "mesh-under" MoP, all IPv6 Packets MUST conform with the
format specified in [RFC4944]. All data Packets exchanged by routers
using this specification MUST contain the Mesh Addressing header as
part of the LoWPAN encapsulation, as specified in [RFC4944].
The DFF header, as specified below, MUST follow the Mesh Addressing
header. After these two headers, any other LoWPAN header, e.g.,
header compression or fragmentation headers, MAY also be added before
the actual payload. Figure 2 depicts the Mesh Addressing header
defined in [RFC4944], and Figure 3 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 2: Mesh Addressing 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 1| Mesh Forw |VER|D|R|0|0|0|0| sequence number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 3: Header for DFF data Packets
Field definitions of the Mesh Addressing header are as specified in
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[RFC4944]. When adding that header to the LoWPAN encapsulation on
the Originator, the fields of the Mesh Addressing header MUST be set
to the following values:
o V := 0 if the Originator Address is an IEEE extended 64-bit
address (EUI-64); otherwise, V := 1 if it is a short 16-bit
address.
o F := 0 if the Final Destination Address is an IEEE extended 64-bit
address (EUI-64); otherwise, F := 1 if it is a short 16-bit
address.
o Hops Left := 0xF (i.e., reserved value indicating that the Deep
Hops Left field is following);
o Deep Hops Left := MAX_HOP_LIMIT.
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.
DFF fields - A 2-bit version field (abbreviated as VER), the DUP
(abbreviated as D) and RET (abbreviated as R) flags follow after
Mesh Forw, as specified in Section 7. The version specified in
this document is 00. All other bits (besides VER, DUP, and RET)
of this octet are reserved and MUST be set to 0.
Sequence Number - A 16-bit field, containing an unsigned integer
sequence number, as specified in Section 7.
14. Scope Limitation of DFF
The forwarding mechanism specified in this document MUST be limited
in scope to the routing domain in which DFF is used. That also
implies that any headers specific to DFF do not traverse the
boundaries of the routing domain. This section specifies, both for
the "route-over" MoP and the "mesh-under" MoP, how to limit the scope
of DFF to the routing domain in which it is used.
Figure 4 to Figure 7 depict four different cases for source and
destination of traffic with regards to the scope of the routing
domain in which DFF is used. Section 14.2 and Section 14.1 specify
how routers limit the scope of DFF for the "route-over" MoP and the
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"mesh-under" MoP respectively for these cases. In these sections,
all nodes "inside the routing domain" are routers and use DFF, and
may also be sources or destinations. Sources or destinations
"outside the routing domain" do not run DFF and are either hosts
attached to a router in the routing domain that is running DFF, or
are themselves routers but outside the routing domain and not running
DFF.
+-----------------+
| |
| (S) ----> (D) |
| |
+-----------------+
Routing Domain
Figure 4: Traffic within the routing domain (from S to D)
+-----------------+
| |
| (S) --------> (R) --------> (D)
| |
+-----------------+
Routing Domain
Figure 5: Traffic from within the routing domain to outside of the
domain (from S to D)
+-----------------+
| |
(S) --------> (R) --------> (D) |
| |
+-----------------+
Routing Domain
Figure 6: Traffic from outside the routing domain to inside the
domain (from S to D)
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+-----------------+
| |
(S) --------> (R1) -----------> (R2) --------> (D)
| |
+-----------------+
Routing Domain
Figure 7: Traffic from outside the routing domain, traversing the
domain and then to the outside of the domain (from S to D)
14.1. Route-Over MoP
In Figure 4, both the source and destination of the traffic are
routers within the routing domain. If traffic is originated at S,
the DFF header is added to the IPv6 header (as specified in
Section 13.1.2). The Originator Address is set to S and the
Destination Address is set to D. The Packet is forwarded to D using
this specification. When router D receives the Packet, it processes
the payload of the IPv6 Packet in upper layers. This case assumes
that S has knowledge that D is in the routing domain, e.g., because
of administrative setting based on the IP address of the Destination.
If S has no knowledge about whether D is in the routing domain, IPv6-
in-IPv6 tunnels as specified in [RFC2473] MUST be used. These cases
are described in the following paragraphs.
In Figure 5, the source of the traffic (S) is within the routing
domain, and the destination (D) is outside of the routing domain.
The IPv6 Packet, originated at S, MUST be encapsulated according to
[RFC2473] (IPv6-in-IPv6 tunnels), and the DFF header added to the
outer IPv6 header. S chooses the next router that should process the
Packet as the tunnel exit-point (R). Administrative setting, as well
as information from a routing protocol may be used to determine the
tunnel exit-point. If no information is available which router to
choose as tunnel exit-point, the Next Hop MUST be used as tunnel
exit-point. In some cases, the tunnel exit-point will be the final
router along a path towards the Packet's Destination, and the Packet
will only traverse a single tunnel (e.g., if R is a known border
router then S can choose R as tunnel exit- point). In other cases,
the tunnel exit-point will not be the final router along the path to
D, and the Packet may traverse multiple tunnels to reach the
Destination; note that in this case, the DFF mechanism is only used
inside each IPv6-in-IPv6 tunnel. The Originator Address of the
Packet is set to S and the Destination Address is set to the tunnel
exit-point (in the outer IPv6 header). The Packet is forwarded to
the tunnel exit-point using this specification (potentially using
multiple consecutive IPv6-in-IPv6 tunnels). When router R receives
the Packet, it decapsulates the IPv6 Packet and forwards the inner
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IPv6 Packet to D, using normal IPv6 forwarding as specified in
[RFC2460].
In Figure 6, the source of the traffic (S) is outside of the routing
domain, and the destination (D) is inside of the routing domain. The
IPv6 Packet, originated at S, is forwarded to R using normal IPv6
forwarding as specified in [RFC2460]. Router R MUST encapsulate the
IPv6 Packet according to [RFC2473], and add the DFF header (as
specified in Section 13.1.2) to the outer IPv6 header. Like in the
previous case, R has to select a tunnel exit-point; if it knows that
D is in the routing domain (e.g., based on administrative settings),
it SHOULD select D as the tunnel exit-point. In case it does not
have any information which exit-point to select, it MUST use the Next
Hop as tunnel exit-point, limiting the effectiveness of DFF to inside
each IPv6-in-IPv6 tunnel. The Originator Address of the Packet is
set to R, the Destination Address to the tunnel exit-point (both in
the outer IPv6 header), the sequence number in the DFF header is
generated locally on R. The Packet is forwarded to D using this
specification. When router D receives the Packet, it decapsulates
the inner IPv6 Packet and processes the payload of the inner IPv6
Packet in upper layers.
This mechanism is typically not used in transit networks; therefore,
this case is discouraged, but described nevertheless for
completeness: In Figure 7, both the source of the traffic (S) and the
destination (D) are outside of the routing domain. The IPv6 Packet,
originated at S, is forwarded to R1 using normal IPv6 forwarding as
specified in [RFC2460]. Router R1 MUST encapsulate the IPv6 Packet
according to [RFC2473] and add the DFF header (as specified in
Section 13.1.2). R1 selects a tunnel exit-point like in the previous
cases; if R2 is, e.g., a known border router, then R1 can select R2
as tunnel exit-point. The Originator Address is set to R1, the
Destination Address to the tunnel exit-point (both in the outer IPv6
header), the sequence number in the DFF header is generated locally
on R1. The Packet is forwarded to the tunnel exit-point using this
specification (potentially traversing multiple consecutive IPv6-in-
IPv6 tunnels). When router R2 receives the Packet, it decapsulates
the inner IPv6 Packet and forwards the inner IPv6 Packet to D, using
normal IPv6 forwarding as specified in [RFC2460].
14.2. Mesh-Under MoP
In Figure 4, both the source and destination of the traffic are
routers within the routing domain. If traffic is originated at
router S, the LoWPAN encapsulated Packet is created from the IPv6
packet as specified in [RFC4944]. Then, the Mesh Addressing header
and the DFF header (as specified in Section 13.2.2) are added to the
LoWPAN encapsulation on router S. The Originator Address is set to S
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and the Destination Address is set to D. The Packet is then forwarded
using this specification. When router D receives the Packet, it
processes the payload of the Packet in upper layers.
In Figure 5, the source of the traffic (S) is within the routing
domain, and the destination (D) is outside of the routing domain
(which is known by S to be outside the routing domain because D uses
a different IP prefix from the PAN). The LoWPAN encapsulated Packet,
originated at router S, is created from the IPv6 packet as specified
in [RFC4944]. Then, the Mesh Addressing header and the DFF header
(as specified in Section 13.2.2) are added to the LoWPAN
encapsulation on router S. The Originator Address is set to S and the
Destination Address is set to R, which is a known border router of
the PAN. The Packet is then forwarded using this specification.
When router R receives the Packet, it restores the IPv6 packet from
the LoWPAN encapsulated Packet and forwards it to D, using normal
IPv6 forwarding as specified in [RFC2460].
In Figure 6, the source of the traffic (S) is outside of the routing
domain, and the destination (D) is inside of the routing domain. The
IPv6 packet, originated at S, is forwarded to R using normal IPv6
forwarding as specified in [RFC2460]. Router R (which is a known
border router to the PAN) creates the LoWPAN encapsulated Packet from
the IPv6 packet as specified in [RFC4944]. Then, R adds the Mesh
Addressing header and the DFF header (as specified in
Section 13.2.2). The Originator Address is set to R, the Destination
Address to D, the sequence number in the DFF header is generated
locally on R. The Packet is forwarded to D using this specification.
When router D receives the Packet, it restores the IPv6 packet from
the LoWPAN encapsulated Packet and processes the payload in upper
layers.
As LoWPANs are typically no transit networks, this case is
discouraged, but described nevertheless for completeness: In
Figure 7, both the source of the traffic (S) and the destination (D)
are outside of the routing domain. The IPv6 packet, originated at S,
is forwarded to R1 using normal IPv6 forwarding as specified in
[RFC2460]. Router R1 (which is a known border router of the PAN)
creates the LoWPAN encapsulated Packet from the IPv6 Packet as
specified in [RFC4944]. Then, it adds the Mesh Addressing header and
the DFF header (as specified in Section 13.2.2). The Originator
Address is set to R1, the Destination Address to R2 (which is another
border router towards the Destination), the sequence number in the
DFF header is generated locally on R1. The Packet is forwarded to R2
using this specification. When router R2 receives the Packet, it
restores the IPv6 packet from the LoWPAN encapsulated Packet and
forwards the IPv6 packet to D, using normal IPv6 forwarding as
specified in [RFC2460].
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15. MTU Exceedance
When adding the DFF header as specified in Section 9.1 or when
encapsulating the Packet as specified in Section 14, the Packet size
may exceed the MTU. This is described in Section 5 of [RFC2460].
When the Packet size of a Packet to be forwarded by DFF exceeds the
MTU, the following steps are executed:
1. The router MUST discard the Packet.
2. The router MAY log the event locally (depending on the storage
capabilities of the router).
3. The router MUST send back an ICMP Packet Too Big to the source of
the Packet reporting back the Next Hop MTU considering the
additional overhead of adding the headers.
16. Security Considerations
Based on the recommendations in [RFC3552], this section describes
security threats to DFF, lists which attacks are out of scope, which
attacks DFF is susceptible to, and which attacks DFF protects
against.
16.1. Attacks Out of Scope
As DFF is a data forwarding protocol, any security issues concerning
the payload of the Packets are not considered in this section.
It is the responsibility of upper layers to use appropriate security
mechanisms (IPsec, TLS, ...) according to application requirements.
As DFF does not modify the contents of IP datagrams, other than the
DFF header (which is a Hop-by-Hop Options extension header in the
"route-over" MoP, and therefore not protected by IPsec), no special
considerations for IPsec have to be addressed.
Any attack that is not specific to DFF, but that applies in general
to the link layer (e.g., wireless, PLC), is out of scope. In
particular, these attacks are: Eavesdropping, Packet insertion,
Packet replaying, Packet deletion, and man-in-the-middle attacks.
Appropriate link-layer encryption can mitigate part of these attacks
and is therefore RECOMMENDED.
16.2. Protection Mechanisms of DFF
DFF itself does not provide any additional integrity, confidentiality
or authentication. Therefore, the level of protection of DFF depends
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on the underlying link layer security as well as protection of the
payload by upper layer security (e.g., IPsec).
In the following sections, whenever encrypting or digitally signing
Packets is suggested for protecting DFF, it is assumed that routers
are not compromised.
16.3. Attacks In Scope
This section discusses security threats to DFF, and for each
describes whether (and how) DFF is affected by the threat. DFF is
designed to be used in lossy and unreliable networks. Predominant
examples of lossy networks are wireless networks, where routers send
Packets via broadcast. The attacks listed below are easier to
exploit in wireless media, but can also be observed in wired
networks.
16.3.1. Denial of Service
Denial of Service attacks are possible when using DFF by either
exceeding the storage on a router, or by exceeding the available
bandwidth of the channel. As DFF does not contain any algorithms
with high complexity, it is unlikely that the processing power of the
router could be exhausted by an attack on DFF.
The storage of a router can be exhausted by increasing the size of
the Processed Set, i.e., by adding new tuples, or by increasing the
size of each tuple. New tuples can be added by injecting new Packets
in the network, or by forwarding overheard Packets.
Another possible DoS attack is to send Packets to a non-existing
Address in the network. DFF would perform a depth-first search until
the Hop Limit has reached zero. Is is therefore RECOMMENDED to set
the Hop Limit to a value that limits the path length.
If security provided by the link layer is used, this attack can be
mitigated if the malicious router does not possess valid credentials,
since other routers would not forward data through the malicious
router.
16.3.2. Packet Header Modification
The following attacks can be exploited by modifying the Packet Header
information, unless additional security (such as link layer security)
is used:
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16.3.2.1. Return Flag Tampering
A malicious router may tamper the "return" flag of a DFF Packet, and
send it back to the previous hop, but only if that router had been
selected as next hop by the receiving router before (as specified in
Section 9.2). If the malicious router had not been selected as next
hop, then a returned Packet is dropped by the receiving router. If,
otherwise, the malicious router had been selected as next hop by the
receiving router, and the malicious router has set the return flag,
the receiving router would then try alternative neighbors. This may
lead to Packets never reaching their Destination, as well as
unnecessary depth-first search in the network (bandwidth exhaustion /
energy drain).
This attack can be mitigated by using appropriate security of the
underlying link layer.
16.3.2.2. Duplicate Flag Tampering
A malicious router may modify the Duplicate Flag of a Packet that it
forwards.
If it changes the flag from 0 to 1, the Packet would be detected as
duplicate by other routers in the network and not as looping packet.
If the Duplicate Flag is set from 1 to 0, and a router receives that
Packet for the second time (i.e., it has already received a Packet
with the same Originator Address and sequence number before), it will
wrongly detect a loop.
This attack can be mitigated by using appropriate security of the
underlying link layer.
16.3.2.3. Sequence Number Tampering
A malicious router may modify the sequence number of a Packet that it
forwards.
In particular, if the sequence number is modified to a number of
another, previously sent, Packet of the same Originator, this Packet
may wrongly be perceived as looping packet.
This attack can be mitigated by using appropriate security of the
underlying link layer.
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17. IANA Considerations
IANA is requested to allocate a value from the Dispatch Type Field
registry for LOWPAN_DFF.
IANA is requested to allocate a value from the Destination Options
and Hop-by-Hop Options registry for IP_DFF. The first three bits of
that value MUST be 111.
18. Acknowledgements
Jari Arkko (Ericsson), Antonin Bas (Ecole Polytechnique), Thomas
Clausen (Ecole Polytechnifque), Yuichi Igarashi (Hitachi), Kazuya
Monden (Hitachi), Geoff Mulligan (Proto6), Hiroki Satoh (Hitachi),
Ganesh Venkatesh (Mobelitix), and Jiazi Yi (Ecole Polytechnique)
provided useful reviews of the draft and discussions which helped to
improve this document.
The authors also would like to thank Ralph Droms, Adrian Farrel,
Stephen Farrell, Ted Lemon, Alvaro Retana, Dan Romascanu, and Martin
Stiemerling for their reviews during IETF LC and IESG evaluation.
19. References
19.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", RFC 2460, December 1998.
[RFC2473] Conta, A. and S. Deering, "Generic Packet Tunneling in
IPv6 Specification", RFC 2473, December 1998.
[RFC4291] Hinden, R. and S. Deering, "IP Version 6 Addressing
Architecture", RFC 4291, February 2006.
[RFC4944] Montenegro, G., Kushalnagar, N., Hui, J., and D. Culler,
"Transmission of IPv6 Packets over IEEE 802.15.4
Networks", RFC 4944, September 2007.
[RFC6130] Clausen, T., Dearlove, C., and J. Dean, "Mobile Ad Hoc
Network (MANET) Neighborhood Discovery Protocol (NHDP)",
RFC 6130, April 2011.
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[RFC6564] Krishnan, S., Woodyatt, J., Kline, E., Hoagland, J., and
M. Bhatia, "A Uniform Format for IPv6 Extension Headers",
RFC 6564, April 2012.
[RFC6724] Thaler, D., Draves, R., Matsumoto, A., and T. Chown,
"Default Address Selection for Internet Protocol Version 6
(IPv6)", RFC 6724, September 2012.
19.2. Informative References
[DFF_paper1]
Cespedes, S., Cardenas, A., and T. Iwao, "Comparison of
Data Forwarding Mechanisms for AMI Networks", 2012 IEEE
Innovative Smart Grid Technologies Conference (ISGT),
January 2012.
[DFF_paper2]
Iwao, T., Iwao, T., Yura, M., Nakaya, Y., Cardenas, A.,
Lee, S., and R. Masuoka, "Dynamic Data Forwarding in
Wireless Mesh Networks", First IEEE International
Conference on Smart Grid Communications (SmartGridComm),
October 2010.
[DFS_wikipedia]
"Dynamic Data Forwarding in Wireless Mesh Networks", http
://en.wikipedia.org/w/
index.php?title=Depth-first_search&oldid=549733112,
March 2013.
[KCEC_press_release]
Kit Carson Electric Cooperative (KCEC), "DFF deployed by
KCEC (Press Release)", http://www.kitcarson.com/
index.php?option=com_content&view=article&id=45&Itemid=1,
2011.
[RFC3552] Rescorla, E. and B. Korver, "Guidelines for Writing RFC
Text on Security Considerations", BCP 72, RFC 3552,
July 2003.
[RFC4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
"Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
September 2007.
[RFC6775] Shelby, Z., Chakrabarti, S., Nordmark, E., and C. Bormann,
"Neighbor Discovery Optimization for IPv6 over Low-Power
Wireless Personal Area Networks (6LoWPANs)", RFC 6775,
November 2012.
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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 a routing protocol is running which adds or inserts
entries into the RIB.
A.1. Example 1: Normal Delivery
Figure 8 depicts a network topology with seven routers A to G, with
links between them as indicated by lines. It is assumed that router
A sends a Packet to G, through B and D, according to the routing
protocol.
+---+
+---+ D +-----+
| +---+ |
+---+ | |
+---+ B +---+ |
| +---+ | |
+-+-+ | +---+ +-+-+
| A | +---+ E +---+ G +
+-+-+ +---+ +-+-+
| +---+ |
+---+ C +---+ |
+---+ | |
| +---+ |
+---+ F +-----+
+---+
Figure 8: Example 1: normal delivery
If no link fails in this topology, and no loop occurs, then DFF
forward the Packet along the Next Hops listed in each of the routers
RIB along the path towards the destination. Each router adds a
Processed Tuple for the incoming Packet, and selects the Next Hop as
specified in Section 11, i.e., it will first select the next hop for
router G as determined by the routing protocol.
A.2. Example 2: Forwarding with Link Failure
Figure 9 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 9: Example 2: link failure
When B receives the Packet from router 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 10, 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 10. 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 10 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 10: Example 3: missed link layer acknowledgment
While C successfully receives the Packet from A, A does not receive
the L2 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 may be a duplicate. Then, it forwards the
Packet to B.
A.4. Example 4: Forwarding with a Loop
Figure 11 depicts the same topology as the Example 1, but there is a
loop from D to A, and A sends the Packet to G through B and D.
+-----------------+
| |
| +-+-+
| +---+ D +
| | +---+
\|/ +---+ |
+---+ B +---+
| +---+ |
+-+-+ | +---+ +-+-+
| A | +---+ E +---+ G +
+-+-+ +---+ +-+-+
| +---+ |
+---+ C +---+ |
+---+ | |
| +---+ |
+---+ F +-----+
+---+
Figure 11: 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. Router 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.
Appendix B. Deployment Experience
DFF has been deployed and experimented with both in real deployments
and in network simulations, as described in the following.
B.1. Deployments in Japan
The majority of the large Advanced Metering Infrastructure (AMI)
deployments using DFF are located in Japan, but the data of these
networks is property of Japanese utilities and cannot be disclosed.
B.2. Kit Carson Electric Cooperative
DFF has been deployed at Kit Carson Electric Cooperative (KCEC), a
non-profit organization distributing electricity to about 30,000
customers in New Mexico. As described in a press release
[KCEC_press_release], DFF is running on currently about 2000 electric
meters. All meters are connected through a mesh network using an
unreliable, wireless medium. DFF is used together with a distance
vector routing protocol. Metering data from each meter is sent
towards a gateway periodically every 15 minutes. The data delivery
reliability is over 99%.
B.3. Simulations
DFF has been evaluated in Ns2 and OMNEST simulations, in conjunction
with a distance vector routing protocol. The performance of DFF has
been compared to using only the routing protocol without DFF. The
results published in peer-reviewed academic papers
([DFF_paper1][DFF_paper2]) show significant improvements of the
Packet delivery ratio compared to using only the distance vector
protocol.
B.4. Open Source Implementation
Fujitsu Laboratories of America is currently working on an open
source implementation of DFF, which is to be released in early 2013,
and which allows for interoperability testings of different DFF
implementations. The implementation is written in Java, and can be
used both on real machines and in the Ns2 simulator.
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Authors' Addresses
Ulrich Herberg (editor)
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
University of Texas at Dallas
School of Computer Science, 800 West Campbell Rd, EC 31
Richardson, TX 75080-3021
US
Email: alvaro.cardenas@me.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
Calle 18 No. 122-135 Pance
Cali, Valle
Colombia
Phone:
Email: scespedes@icesi.edu.co
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