Reliability Considerations of Path-Aware Semantic Addressing
draft-li-6lo-pasa-reliability-01
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| Authors | Guangpeng Li , Zhe Lou , Luigi Iannone | ||
| Last updated | 2023-03-08 | ||
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draft-li-6lo-pasa-reliability-01
6lo Working Group G. Li
Internet-Draft D. Lou
Intended status: Informational L. Iannone
Expires: 9 September 2023 Huawei
8 March 2023
Reliability Considerations of Path-Aware Semantic Addressing
draft-li-6lo-pasa-reliability-01
Abstract
Path-Aware Semantic Address (PASA)
[I-D.li-6lo-path-aware-semantic-addressing]), proposes to
algorithmically assign addresses to nodes in a 6lo environment so to
achieve stateless forwarding, hence, allowing to avoid using a
routing protocol. PASA is more suitable for stable and static
wireline connectivity, in order to avoid renumbering due to topology
changes. Even in such kind of scenarios, reliability remains a
concern. This memo tackles specifically reliability in PASA
deployments, analyzing possible broad solution categories to solve
the issue.
Status of This Memo
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This Internet-Draft will expire on 9 September 2023.
Copyright Notice
Copyright (c) 2023 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 (https://trustee.ietf.org/
license-info) in effect on the date of publication of this document.
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Please review these documents carefully, as they describe your rights
and restrictions with respect to this document. Code Components
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Table of Contents
1. Introduction and Problem Statement . . . . . . . . . . . . . 2
2. Solution Alternatives . . . . . . . . . . . . . . . . . . . . 3
3. Multi-Address Approach . . . . . . . . . . . . . . . . . . . 3
3.1. Topology Building . . . . . . . . . . . . . . . . . . . . 4
3.2. Link Failures . . . . . . . . . . . . . . . . . . . . . . 7
3.3. Node Failures . . . . . . . . . . . . . . . . . . . . . . 11
3.4. Node Forwarding Procedure . . . . . . . . . . . . . . . . 13
3.4.1. Forwarder Node . . . . . . . . . . . . . . . . . . . 13
3.4.2. Root Node . . . . . . . . . . . . . . . . . . . . . . 14
4. Single-Address Approach . . . . . . . . . . . . . . . . . . . 15
4.1. Topology Building . . . . . . . . . . . . . . . . . . . . 15
4.2. Link Failures . . . . . . . . . . . . . . . . . . . . . . 17
4.3. Node Failures . . . . . . . . . . . . . . . . . . . . . . 19
4.4. Node Forwarding Procedure . . . . . . . . . . . . . . . . 19
4.4.1. Forwarder Node . . . . . . . . . . . . . . . . . . . 19
4.4.2. Root Node . . . . . . . . . . . . . . . . . . . . . . 20
5. Links/Nodes Failure Detection and Recovery . . . . . . . . . 21
6. Resiliency . . . . . . . . . . . . . . . . . . . . . . . . . 22
7. Security Considerations . . . . . . . . . . . . . . . . . . . 22
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 23
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 23
9.1. Normative References . . . . . . . . . . . . . . . . . . 23
9.2. Informative References . . . . . . . . . . . . . . . . . 23
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 24
1. Introduction and Problem Statement
The common characteristic of various topological addressing schemes
([I-D.daniel-6lowpan-hilow-hierarchical-routing],
[I-D.li-6lo-path-aware-semantic-addressing], [KIM07]) is the
possibility of nodes to forward packets without the need of routing
protocols. In such context the addresses are build in such a way
that a node is capable of forwarding a packet to the next hop by
comparing the destination address with its own address. It is not
required to build a routing table for the whole topology, on which to
execute look-up algorithms, only neighbor awareness is sufficient.
However, this kind of stateless forwarding typically works in a
simple topology with static paths, where higher communication
resiliency is hard to achieve. Once a link (or a node) fails, the
traffic may become not routable anymore, and packets are dropped,
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even in the presence of alternative physical paths. Indeed, in order
to use these alternative paths renumbering is necessary to (re)build
an alternative logical topology. Such a solution, while looking as a
simple operation, may be not enough, and is complicate in practice,
since it implies to put the system offline during the renumbering
process. What is desirable is to have some mechanisms to quickly
enable the usage of alternative paths with little extra effort,
without the need to put the system offline, hence providing higher
resiliency. The present memo analyzes two possible approaches to
increase the resilience of PASA networks.
2. Solution Alternatives
In order to improve the reliability of the system, the pre-requisite
is to have redundant links. This means that nodes are likely
connected in a meshed fashion, where some of the links are actively
used, and others not. In a normal situation, in the context of PASA,
the actively used links form a tree. This is the same concept of
spanning trees used in layer 2 technologies (e.g. [IEEE802.1W]).
When a problem is detected, various possibilities arise in order to
logically guarantee connectivity by starting using previously unused
links. In the specific case of PASA
[I-D.li-6lo-path-aware-semantic-addressing], the assumption is that
all nodes, except the root, have at least one secondary parent, which
will only be used if the primary one is not reachable. In this way,
when the link toward the primary parent is broken, an alternative
link toward a secondary parent can be used. In such context two
different approaches can be identified:
* Multi-Address: using multiple addresses per node, one for each
alternative parent (logically creating multiple topologies).
* Single-Address: using one single address per node, even if an
alternative parent is present. The single address of the node
comes from his primary parent.
Both approaches, with pros and cons, are described and analyzed
hereafter.
3. Multi-Address Approach
In the multi-address case, multiple logical topologies are built by
using different addresses and different links. This is equivalent of
using several contexts of Virtual Routing and Forwarding (VRF). In
the following it is assumed that two logical topologies are built on
top of the physical connectivity, however, the principles can be
easily extended to more than two topologies.
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3.1. Topology Building
In the multi-address case, two root nodes are used. Each root node
is the root of a different tree covering all the nodes. The
Allocation Function (AF) used to assign addresses in the two parallel
topologies might differ. However, attention should be given to
guarantee that addresses in the two topologies are different. This
can be easily achieved by using two different addresses for the root
nodes. Indeed such addresses will be the prefix of the whole tree,
which also means that the address of the root nodes can be used to
actually identify the different topologies. For both topologies, the
address allocation procedure works in the exact same way as described
in [I-D.li-6lo-path-aware-semantic-addressing], the only additional
action to be taken is that a node cannot choose the same parent node
in both topologies. This can be easily achieved by imposing that two
parents must not have the same "node-id".
Let us make a simple example with the topology depicted in Figure 1,
where there are two root nodes, named "R-1" and "R-2" and a set of
few nodes N-XY, where X represent the depth in the tree and Y a
unique number for that level of the tree. Physical links are not
depicted in the figure but, as already mentioned, the assumption is
that each node is connected at least to two potential parents.
+---+ +---+
|R-1| |R-2|
+---+ +---+
+----+ +----+ +----+ +----+
|N-11| |N-12| |N-12| |N-12|
+----+ +----+ +----+ +----+
+----+ +----+ +----+
|N-21| |N-22| |N-23|
+----+ +----+ +----+
+----+ +----+ +----+
|N-31| |N-32| |N-33|
+----+ +----+ +----+
Figure 1: Simple Topology example.
Let us also assume that R-1 has the address 1, which is used to
allocate the address to other nodes. After applying the allocation
function presented in [I-D.li-6lo-path-aware-semantic-addressing], a
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possible outcome is the one presented in Figure 2, where the links
selected to form the logical topology are shown, as well as the
assigned addresses.
+---+ +---+
| 1|----------+ |R-2|
+---+-----+ \ +---+
/ \ \ \
/ \ \ \
+---+ +---+ +----+ +----+
| 10| | 11| | 111| |1111|
+---+ +---+ +----+ +----+
/ \ \
/ \ +------+
/ \ \
+----+ +----+ +-----+
| 100| | 101| | 1011|
+----+ +----+ +-----+
/ \ \
/ \ +-----------+
/ \ \
+----+ +-----+ +------+
|1001| |10011| |100111|
+----+ +-----+ +------+
Figure 2: Possible PASA assignment and logical topology using R-1
as root.
In a similar way, assuming root R-2 has the address 01, and again
applying the allocation function presented in
[I-D.li-6lo-path-aware-semantic-addressing], a possible outcome is
the one presented in Figure 3, where the links selected to form the
logical topology are shown, as well as the assigned addresses.
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+---+ +---+
|R-1| +-----------| 01|
+---+ / +------+---+
/ / / \
/ / / \
+---+ +----+ +---+ +-----+
|011| |0111| |010| |01111|
+---+ +----+ +---+ +-----+
/ / |
+----------+ / |
/ +---+ |
/ / |
+-----+ +-----+ +------+
| 0101| |01011| | 0100 |
+-----+ +-----+ +------+
/ / |
+----------+ / |
/ +---+ |
/ / |
+-----+ +------+ +-------+
|01001| |010011| |0100111|
+-----+ +------+ +-------+
Figure 3: Possible PASA assignment and logical topology using R-2
as root.
When everything is working without problem, one of the logical
topologies can be used as primary topology, while using the second
one only in case of link/node failures. A simple selection can be
done for example with the rule:
* Interpreting root nodes' addresses as integers, and choosing the
tree with the smallest value.
Another approach could be trying to use some load balancing
approaches, where sockets open on the various nodes are bound to one
of the available addresses based on some algorithms. The algorithm
can be as simple as a random choice. However it has to be
considered, that random local choices can uniformly distribute
connections on different addresses, but it does not mean that the
traffic is uniformly distributed on the network as a whole [SINGH20].
Such kind of optimization algorithms are out of the scope of this
document. In the following, it is assumed that a primary/secondary
approach is used, where the topology in Figure 2 is the primary one.
In [I-D.li-6lo-path-aware-semantic-addressing], the root node has
always address 1. Such a simple approach allows to easily switch
between IPv6 and PASA addresses. Rebuilding the full IPv6 address
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from the PASA address is simply done via a coalescence operation with
the PASA prefix, as described in Section 4.3.1 of [RFC8138]. The
opposite operation, obtaining the corresponding PASA address from an
IPv6 address is done by removing the /64 PASA prefix and then, in the
remaining suffix, all leading zeros are also removed.
The latter procedure is not anymore sufficient. Taking the example
in Figure 3, nodes have to be aware that the root node has actually a
two-bits address, namely "01". In order to maintain the simplicity
of the design of PASA, the addresses of root nodes can assigned as
follows:
* Each root has an address where the least significant bit is set to
1 and all the others to zero.
* Each root has a different address length that has to be known.
* An address length of 1, means no leading zeros.
* An address length of n, means n-1 leading zeros followed by 1.
Coming back to the example, root R-1, has an address length equal to
1, hence its address is "1", as depicted in Figure 2, while R-2 has
an address length equal to 2, hence its address is "01", as depicted
in Figure 3. Leafs and forwarders need to be aware of the root
address length. For instance, the leafs at the bottom of Figure 3
need to know that the root address length is 2, so that their
addresses start with 01. Hence, the only requirement imposed by this
solution on the nodes is to allow addresses that start with zeros and
explicitly know the root's address length
([I-D.li-6lo-path-aware-semantic-addressing] only specifies addresses
starting with 1 and the root's address length is implicitly always
one).
3.2. Link Failures
In case of link failure, there are three actions that need to be
taken in order to ensure connectivity. 1. The parent node, with
respect to the link in object (the one failed), has to inform all the
nodes between itself and the root that a certain sub-tree is not
reachable anymore through it, by using the primary topology. This
can be achieved by sending an ICMPv6 message announcing the sub-tree
unreachability status. To this end, the link's parent node sends
such a message to its parent, which will not only store the
unreachable status of the sub-tree prefix, but also forward the same
message to its own parent. Recursively, eventually the root will
receive the same message and store the unreachable status of the sub-
tree refix. In this way, packets destined to that sub-tree are
actually re-directed toward the root. After this procedure, when a
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node sees a packet that is destined to the node in the unreachable
sub-tree, it sends it up to the root.
1. The child node, with respect to the failed link, has to inform
the root that its sub-tree can still be reached, if traffic is
sent through the secondary topology, by using the secondary
address of this node. This can be achieved by sending an ICMPv6
message toward the root of the secondary tree, hence using the
secondary address as a source of the message. The secondary root
will then forward the message, after decompression, to the root
of the primary tree. With this operation the root of the primary
tree is now aware that to reach a certain sub-tree, traffic has
to be sent through the secondary tree to a specific address (the
secondary address of the child on the broken link). In order to
actually ship a packet destined to an address in the primary tree
through the secondary tree, two options are possible:
encapsulation or routing.
* Encapsulation is pretty simple. Whenever there is a packet
destined to the sub-tree with a redirect entry on the primary
root, the root encapsulates (tunnels) the packet to the
secondary address of the child node of the broken link and
sends it to the secondary root. The packet will be forwarded
according to the stateless PASA procedure until it reaches the
intended node. There, it is decapsulated and the original
packet is routed in the sub-tree until its final destination.
In the other direction, all packets coming from the sub-tree
can be encapsulated toward the secondary root, using the
procedure described in Section 7.2 of
[I-D.li-6lo-path-aware-semantic-addressing]. The secondary
root will forward the packet to the primary root if the
destination address belongs to it is in the primary topology.
In this way the broken link is circumvented.
* Routing relies on some forwarding entries stored on the nodes
along the path on the secondary tree. Basically, when the
ICMPv6 message, sent by the child node of the broken link, is
forwarded along the secondary tree using the same recursive
approach previously described, each node along the path stores
the information that they are part of a forwarding path toward
the sub-tree specified in the ICMPv6 message itself. In this
way, no additional encapsulation is necessary, since the
packet can be forwarded from the primary root to the secondary
root, who in turn will forward it to the child from which it
received the ICMPv6 message, and so on until the message
reaches the sub-tree where it is forwarded using the normal
PASA stateless forwarding. In the opposite direction, for
packets coming from the sub-tree, nodes along the alternate
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path on the secondary tree will simply forward the packets to
the secondary root, who will forward them to the primary root.
The first solution (encapsulation) may increase the likelihood
to have MTU issues. Indeed, an additional encapsulation will
increase the packet size. Furthermore, packets need as well
to undergo several header compression/decompression operations
which will increase the latency and consume more energy. The
second solution does not create overhead, but needs to store
state in nodes along the alternative paths. The number of
entries is certainly limited, because it is just the number of
sub-trees unreachable through the primary tree using the node
as part of the alternative path. However, this may be an
issue on devices with strong memory constraints. Yet, if the
state grows bigger, it is the symptom of massive failures in
the network, which may be a far bigger/more urgent problem.
In both cases the root nodes have to keep some states: the
redirection rules for all unreachable sub-trees. This is not
a problem since root gateways are usually more powerful than
the other nodes and do not run on batteries. However, if the
number of entries grows large, this is again a symptom of
massive failures.
2. Optionally, for optimization purposes, the child node, with
respect to the link in object, may inform all the nodes of its
sub-tree that they should start using the secondary tree (i.e.
the secondary address). This can be achieved by sending specific
ICMPv6 messages to all of its children, who will do the same
recursively. In this way communications will take advantage from
the stateless forwarding. However, communication using the
primary address, with the mechanism described in the previous
points must still be supported, for ongoing communications that
would otherwise break and for any communication initiated from
the Internet toward and address in the primary tree. For
instance because only primary addresses are shared publicly (via
DNS or other means).
All of the above-mentioned ICMPv6 messages are forwarded using PASA
stateless forwarding procedure as for
[I-D.li-6lo-path-aware-semantic-addressing].
Using the example previously introduced, let us assume that the link
between N-11 and N-21 breaks (cf. Figure 1). This means that in the
primary topology (see Figure 2) the link between nodes 10 and 100 is
broken. According the procedure presented above, the following
action are taken:
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1. 10 sends an ICMPv6 message to the root. Root will register that
100-sub-tree is not reachable through 10. Messages has to be
redirected.
2. 100 sends an ICMPv6 message to 01 (root of the secondary tree)
using 01100 as source address (see Figure 3) using the recursive
procedure through node 0110. Once this message reaches 01, the
root of the secondary tree, it will be forwarded to 1, the root
of the primary tree. Now root 1 has an entry stating:
* For 100-sub-tree encapsulate to 01100 and forward to 01
3. 100 will send an ICMPv6 message to its children suggesting to use
their secondary addresses.
At this point connection is guaranteed. Let us assume the in the
primary tree (see Figure 2) nodes 11 and 1001 where communicating to
each other. Packets will flow in the following way:
* From 11 to 1001:
1. Packet is transmitted from 11 to 1 (on the primary tree).
2. Because of the redirect entry, 1 encapsulates packet toward
100 and transmits it to 01 (root secondary).
3. 01 will use PASA stateless forwarding to transmit the packet
to 010 (on the secondary tree).
4. 010 will use PASA stateless forwarding to transmit the packet
to 0101 (on the secondary tree).
5. 0101 will decapsulate, note the destination is on the primary
tree, use the PASA stateless forwarding to transmit the packet
to 1001 (on the primary tree).
* From 1001 to 11:
1. Packet is transmitted from 1001 to 100 (on the primary tree).
2. Because 100 knows the upstream link is broken it encapsulates
the packet with source 0101 and destination 01 (root primary
tree) then transmits the packet to 010 (on the secondary
tree).
3. 010 will use PASA stateless forwarding to transmit the packet
to 01 (on the secondary tree).
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4. 01 will decapsulate and see that packet is destined a node in
the primary tree and transmits it to 1.
5. 1 will use the PASA stateless forwarding to transmit the
packet to 11 (on the primary tree).
In case of communication toward/from the public Internet the
procedure is similar. For outgoing packets the primary root will
forward the packet upstream. For incoming packets, the root will
firstly reduce the IPv6 header to an PASA header, then forwards it as
described above. PASA header expansion and IPv6 header reduction are
operations described in [I-D.li-6lo-path-aware-semantic-addressing].
3.3. Node Failures
In case that an entire node fails, several links will not be usable
anymore. Nevertheless, the procedure described in the previous
section can be still applied, what may change is who is performing
the action. More specifically:
1. The parent of the failed node, has to inform all the nodes
between itself and the root that a certain sub-tree is not
reachable anymore through it. This is the exact same procedure
like in Section 3.2.
2. All of the children of the failed node, have to independently
inform the root that its sub-tree can still be reached if traffic
is sent through the secondary topology, using the secondary
address of the node that is the root of the sub-tree. This is
the exact same procedure like in Section 3.2, just done by all
children.
3. All of the children of the node, optionally, for optimization
purposes, may inform all the nodes of their sub-trees that they
should start use the secondary tree (i.e. the secondary address).
This is the exact same procedure like in Section 3.2, just done
by all children.
Using again the example previously introduced, let us assume that
node N-21 fails (cf. Figure 1). This means that in the primary
topology (see Figure 2) the links between nodes 10 and 100 is
unusable, as well as the links between 100 and its three children,
namely 1001, 10011, and 100111. According the procedure presented
above, the following action are taken:
1. 10 sends an ICMPv6 message to the root. Root will register that
100-sub-tree is not reachable through 10 but has to be
redirected.
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2. The three children of 100 will perform the following:
* 1001 sends an ICMPv6 message to 01 (root of secondary tree)
using 01001 as source address (see Figure 3). This message
will then be forwarded to 1, the root of the primary tree.
Now root 1 has an entry stating:
- For 1001-sub-tree encapsulate to 01001 and forward to 01
* 10011 sends an ICMPv6 message to 01 (root of secondary tree)
using 010011 as source address (see Figure 3). This message
will then be forwarded to 1, the root of the primary tree.
Now root 1 has an entry stating:
- For 10011-sub-tree encapsulate to 010011 and forward to 01
* 100111 sends an ICMPv6 message to 01 (root of secondary tree)
using 0100111 as source address (see Figure 3). This message
will then be forwarded to 1, the root of the primary tree.
Now root 1 has an entry stating:
- For 100111-sub-tree encapsulate to 0100111 and forward to
01
At this point connection is guaranteed. Let us assume, like in the
example for the link failure, that in the primary tree (see Figure 2)
nodes 11 and 1001 where communicating to each other. Packets will
flow in the following path:
* From 11 to 1001:
1. Packet is transmitted from 11 to 1 (on the primary tree).
2. Because of the redirect entry, 1 encapsulates packet toward
100 and transmits it to 01 (root secondary).
3. 01 will use PASA stateless forwarding to transmit the packet
to 01001 (on the secondary tree).
4. 01001 will decapsulate, note the destination is its own
primary address, the packet will be decapsulate once more and
delivered to the upper layer.
* From 1001 to 11:
1. Because 1001 knows the upstream link is broken it encapsulates
the packet with source 01001 and destination 01 (root
secondary tree).
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2. 01 will see that packet is destined to a node in the primary
tree and transmits it to 1.
3. 1 will use the PASA stateless forwarding to transmit the
packet to 11 (on the primary tree).
In case of communication toward/from the public Internet the
procedure is the same as described in Section 3.2.
3.4. Node Forwarding Procedure
Nodes, have to forward packets according to the procedures described
in the previous sections. Nevertheless, compared to the original
specification the modifications are very limited. Hereafter, the
forwarding procedure for both forwarder and root nodes is provided.
The mention "PASA Native Forwarding" is used where the original
procedure described in [I-D.li-6lo-path-aware-semantic-addressing] is
employed.
3.4.1. Forwarder Node
As described in Figure 4, in the context of multiple topologies, when
a a forwarder node receives a packet, it needs first to verify if
there is any rule that redirects the packet. If it is not the case,
it needs to check if there is an encapsulation rule, if it is the
case then the packets needs to be encapsulated accordingly. Then
normal PASA forwarding is applied.
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+-----------------+
| Packet Received |
+-----------------+
|
V
+---------------+ +-----------------+
/ Is there a \ Yes | Forward |
| redirect rule |------->| according |---+
\ that applies? / | to rule | |
+--------------+ +-----------------+ |
| No |
| |
V |
+---------------+ +-----------------+ |
/ Is there an \ Yes | Encapsulate | |
| encap. rule |------->| according | |
\ that applies? / | to rule | |
+--------------+ +-----------------+ |
| No | |
|<--------------------------+ |
V |
+-----------------+ |
| PASA | |
|Native Forwarding| |
+-----------------+ |
| <--------------------------------------+
V
+------------+
| END |
+------------+
Figure 4: Forwarding procedure in case of multiple topologies.
3.4.2. Root Node
In the case of a root node, and in the context of multiple
topologies, the PASA native forwarding is always applied for outward
packets. Only in case of inward packets, the node has to check
whether there is an encapsulation rule through an alternative
topology to bypass a failed link/node. Figure 5 show this simple
case.
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+-----------------+
| Packet Received |
+-----------------+
|
V
+---------------+ +-----------------+
/ Is there an \ Yes | Encapsulate |
| encap. rule |------->| according |
\ that applies? / | to rule |
+--------------+ +-----------------+
| No |
| |
V V
+-----------------+ +-----------------+
| PASA | | Forward to |
|Native Forwarding| | Alternative Root|
+-----------------+ +-----------------+
| |
| <--------------------------+
V
+------------+
| END |
+------------+
Figure 5: Root node forwarding procedure in case of multiple
topologies.
4. Single-Address Approach
4.1. Topology Building
In this approach, starting from the root node, we can assign a single
address to each node in the PASA network based on the address
allocation function described in
[I-D.li-6lo-path-aware-semantic-addressing]. All nodes with assigned
addresses will send a message to the root to register themselves so
that the root has an overview of the nodes and the topology in the
PASA network. The nodes with the links used to assign the addresses
form the tree topology. By default, a node forwards the packet via
the tree by using the native PASA forwarding method defined in
[I-D.li-6lo-path-aware-semantic-addressing].
The root will have a backup with the same address 1, and Virtual
Router Redundancy Protocol (VRRP [RFC5798]) could be used to
implement same address root redundancy. In order to increase the
resilience of the network, each node will have at least one
alternative parent for redundancy. This alternative uplink is added
to the already existed Neighbor Discovery table. For leaf nodes,
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there will be only alternative uplink entries. For forwarder nodes,
there will be alternative uplink(s) and alternative downlink(s)
stored in the ND table. All the alternative links will be reported
to the root by using ICMPv6 messages. An example of ND table which
includes the alternative parent(s)/children is shown in Figure 6).
In particular, it shows the entries in the ND table of node 100 in
the topology shown in Figure 7.
+-------------+-------+
| Destination | Flags |
+-------------+-------+
| 100 | I | I = Current Node
+-------------+-------+
| 10 | PP | PP = Primary Parent
+-------------+-------+
| 1000 | PFC | PFC = Primary Forwarder Child
+-------------+-------+
| 10010 | PFC |
+-------------+-------+
| 1001 | PLC | PLC = Primary Leaf Child
+-------------+-------+
| 10011 | PLC |
+-------------+-------+
| 110 | AP | AP = Alternative Parent
+-------------+-------+
| 10100 | AFC | AFC = Alternative Forwarder Child whose
+-------------+-------+ alternative parent is the current node
| 10101 | ALC | ALC = Alternative Leaf Child whose
+-------------+-------+ alternative parent is the current node
Figure 6: Example of a ND Table of a forwarder node with address
of '100'.
There could be more than one alternative forwarder and leaf children.
"Primary" here means that they belong to the PASA topology, to
differentiate them from the backup alternative role. The first entry
of Figure 6 shows the address of the node itself '100'. This node's
parent on the tree is '10' that is recorded in second entry and
marked accordingly a Primary Parent (PP). There are two Primary
Forwarder Children (PFC), namely '1000' and '10010, followed by two
Primary Leaf Children (PLC), namely '1001' and '10011'. Then one
alternative parent (AP) follows, namely '110'. Finally, for the sake
of clarity, two alternative children have been added to complete the
table (not depicted in Figure 7), an Alternative Forwarder Child
(AFC) with address 10100, and an Alternative Leaf Child (ALC) with
address 10101.
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As there is only one tree, in general, the packet forwarding will
follow the normal PASA forwarding method by using the primary PASA
topology if there is no link or node failure. Even when there are
failures on the alternative links, the normal PASA forwarding method
is not impacted. However, if there is a link failure on the PASA
tree, the forwarding behavior will change as described in the
following.
4.2. Link Failures
Upon a link failure an ICMPv6 message will be generated to report the
event to the root. The root will then compute a new forwarding path
based on the current state and encapsulate (tunnel) the packet to
nodes where broken links could be avoided.
1 '1'
/-\ /-\
| |\----------\...| |.......
\-/\ \--\......\...\-/. .
/ | \....\- \...\. . .
/ .|.... \...... \_._\-----.-----\ .
/.... | .... \ . \__ . \ .
10 /.. 11| . \ 110 \__ .111 \ . 1110
/-\ . /-\ /-\ \ /-\ \- /-\
| |. | | .| | | | | |
/\-/\.... \-/ .. .\-\ \-/ \-/
/ | \ \ ... ..... .. . \
/ | \ \.. ..... .. . \
/ | \.. \ .. .. ... \
Failure X | .... ...\ . . . .. \
/ ... ..\ .. .\ . .. \
/ ... | ... \. \ . ..\
100 /-\..101/-\. /-\1010 \/-\ 1011 .\/-\ 1101
| | | | .| | | | | |
\-/ \ \-/ . . \-/ \-/ \-/
/ | \ \ . . . .
/ | \ \ .. . . .
/ | \..\ . .. .
/ | .\ . \ . .
/ | . . \.. .
/ ... . \ .. \ .
/ ... | . \. \ .
/-\.. /-\.. /-\ \/-\
| | | | | | | |
\-/ \-/ \-/ \-/
1000 1001 10010 10011
Figure 7: An example of link failure in single address topology.
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In order to give an example illustrating what happens to packets
flowing downlink, let us assume a packet initiated from node 1101
destined to node 1001. And the link between node 10 and node 100 is
broken.
When the link fails, upon detection of the failure, node 10 will send
an ICMPv6 message to the root, to make it aware of the failure. The
packet forwarding will happen as follows:
1. The packet is transmitted from node 1101 to the root 1, using
PASA stateless forwarding.
2. Root 1 is aware that the path to destinations in the 100 sub-tree
is not reachable through normal PASA forwarding because of the
link failure, hence computing an alternative path. In this
example: 1 -> 110 -> 100 -> 1001. Since normal PASA forwarding
does not allow to go first through node 110 and then node 100,
the root 1 will encapsulate the addresses of node 110 and node
100 in an extension header so to perform segmented routing
[I-D.geng-spring-sr-redundancy-protection].
3. Once the packet reaches 100, the segment routing extension is
dropped, and the packet is sent to its destination 1001 by using
PASA native forwarding.
In the unlikely case that the root is not yet aware of the link
failure during the packet transmission, the packet forwarding will
happen as follows:
1. Packet is transmitted from node 1101 to the root 1, using PASA
native forwarding.
2. Packet is transmitted from root 1 to node 10, following the
normal PASA forwarding method.
3. Node 10, which is aware about the link failure, redirects the
packet back to the root with SRv6 encapsulation.
4. Root 1, which should in the meantime have received an ICMPv6 link
failure notification message, receives the encapsulated packet
and, after decapsulation, it operates like in the previous
example. Since it is now aware that the path to destinations in
the 100 sub-tree are not reachable through normal PASA forwarding
because of the link failure, it computes an alternative path. In
this example: 1 -> 110 -> 100 -> 1001. As normal PASA forwarding
does not allow to go first through node 110 and then node 100,
the root 1 will encapsulate the addresses of node 110 and node
100 in an extension header so to perform segmented routing
[I-D.geng-spring-sr-redundancy-protection].
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5. Once packet reaches 100, segment routing extension dropped, and
packet is sent to its destination 1001 by using PASA native
forwarding.
Let us now look at what happens to packets flowing in the opposite
direction, for instance from 1001 to 1101, with the same link failed,
namely the link between 100 and 10. Upon link failure detection by
100, the node will send an ICMPv6 message through an alternative
parent, toward the root, to report the link failure, The packet will
be handled as follows:
1. The packet is transmitted from node 1001 to node 100 using PASA
native forwarding.
2. Because of the failed link, node 100 sends the packet to an
alternative parent node.
3. PASA native forwarding is used then. If the alternative parent
is in the same sub-tree like the destination, the packet is
forwarded downward to the right child, otherwise it is sent
upward to the its own parent. This goes on recursively until the
packet reaches the root in the worst case, where it is then sent
downward to the correct forwarder child, until it reaches the
destination. In this example, the path is: 100 -> 110 -> 1101.
4.3. Node Failures
As for the multiple-address case, a node failure can be seen as
multiple link failures, basically all links the node connects to. In
this case, the parent of the failed node and its children will simply
apply the same procedure described in the previous section.
4.4. Node Forwarding Procedure
4.4.1. Forwarder Node
As describe in Figure 8, in the context of single-address approach,
when a forwarder node receives a packet, it performs the normal PASA
native forwarding (after decapsulation, if needed). If case of link
failure, the forwarder will take different actions depending on
downlink or uplink failure, as depicted in the Section 4.2.
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+----------------+
| Received Packet|
+-------+--------+
|
V
+------------------------+
| Perform PASA Forwarding |
+-----------+------------+
|
V
+---------------------+
/ \
| Outgoing Link working?|---------------------------------+
\ / Yes |
+---------------------+ |
| |
| No |
V |
+---------------------+ V
/ \ Down +-------------------+ +-----+
| Down/Up Link Failure? |----->| Redirect to Root |--->| END |
\ / +-------------------+ +-----+
+--------------------+ ^
| |
| Up |
V |
+---------------------+ |
| Send the Packet to | |
| the Alternative |----------------------------------+
| Parent |
+---------------------+
Figure 8: Forwarding Procedure of Forwarder Node
4.4.2. Root Node
In the case of a root node, and in the context of single-address
approach, the PASA native forwarding is always applied, for outward
packets. Only in case of inward packets, the node has to check
whether there is a redirection needed. If it is the case, it will
compute the path and define the segment routing header in order to
forward the packet to avoid the broken link(s).
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+----------------+
| Received Packet|
+-------+--------+
|
V
+---------------------+
/ Is the a \ No
| redirect rule due to |-----------+
\ broken links / |
+---------------------+ |
|Yes |
V |
+---------------------+ |
| Encapsulate to | |
| alternative path | |
+---------------------+ |
| |
V |
+------------------------+ |
| PASA Native Forwarding |<---------+
+------------------------+
|
V
+-------+
| END |
+-------+
Figure 9: Forwarding Procedure on root node.
5. Links/Nodes Failure Detection and Recovery
Previous sections describe actions and possible solutions to failure
events, but didn't touch how failures are detected. This memo
assumes that depending on the specific technology in use and the
level of desired reliability, the most suitable failure detection
mechanism is used to trigger the above-described actions. It is
considered not desirable to define one single failure detection
technique to be used in the context of PASA, neither to define new
ones.
The link failure could be detected by leveraging layer 2 feedbacks,
like for instance the lack of acknowledgement upon packet
transmission. It can also be detected using by existing network
layer solutions, like for instance the Bidirectional Forwarding
Detection (BFD) [RFC7130] or IPv6 specific mechanisms [RFC5534].
Another aspect of the general failure management is to recover from
failures, going back to the original state. In the context of PASA
there are a couple of possible approaches that can be used, e.g. by
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using native addresses lifetime. Addresses can be assigned
associated with a lifetime. When such lifetime expires, node have to
undergo the same initial procedure for address allocation. This is
also a good moment to check whether a certain link or node is back to
normal functioning. If it is not the case, the algorithmic procedure
will anyway create topologies that do not take into account failed
links/nodes. A faster alternative approach could be based, like in
the case of failure detection, on periodic checks that may leverage
on layer 2 features or on some neighbor discovery messages. The
former method is more effective, while the latter introduces
communication overhead.
6. Resiliency
Real resiliency provided by the different approaches depends from the
specific topology.
The single-address solution may introduce more state. Indeed, the
root has the overview of the PASA network. It knows all nodes'
addresses, the alternative links and the broken links. It is able to
compute a usable path towards a destination. This comes with the
benefit of potentially being able to find a higher number of
alternative paths, hence, in the end providing a stronger protection
against multiple failures. The forwarder node and the leaf node are
rather dummy, performing PASA stateless forwarding. They only are
aware of link state toward their direct neighbors, and take action
accordingly. However, the use of source routing may create MTU
issues if the path is too long.
The multi-address approach leverages more on the stateless forwarding
of PASA. The root is in general unaware of nodes' addresses, and the
network topology. In case of failure, a redirection rule is set on
the root, hence the amount of states is proportional to the number of
failures. This means less state overall, but may be less robust to
multiple failures. Differently from the single address solution, a
small amount states are also required on forwarder nodes, because if
a link fails a redirect rule has to be used.
The above mentioned pros and cons need to be pondered when choosing a
reliability solution to be deployed in an PASA domain.
7. Security Considerations
TBD
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8. IANA Considerations
This document contains no requests to IANA.
9. References
9.1. Normative References
[I-D.li-6lo-path-aware-semantic-addressing]
Li, G., Lou, Z., Iannone, L., Liu, P., Long, R.,
Makhijani, K., and P. Thubert, "Path-Aware Semantic
Addressing (PASA) for Low power and Lossy Networks", Work
in Progress, Internet-Draft, draft-li-6lo-path-aware-
semantic-addressing-01, 16 December 2022,
<https://datatracker.ietf.org/doc/html/draft-li-6lo-path-
aware-semantic-addressing-01>.
[RFC5534] Arkko, J. and I. van Beijnum, "Failure Detection and
Locator Pair Exploration Protocol for IPv6 Multihoming",
RFC 5534, DOI 10.17487/RFC5534, June 2009,
<https://www.rfc-editor.org/info/rfc5534>.
[RFC5798] Nadas, S., Ed., "Virtual Router Redundancy Protocol (VRRP)
Version 3 for IPv4 and IPv6", RFC 5798,
DOI 10.17487/RFC5798, March 2010,
<https://www.rfc-editor.org/info/rfc5798>.
[RFC7130] Bhatia, M., Ed., Chen, M., Ed., Boutros, S., Ed.,
Binderberger, M., Ed., and J. Haas, Ed., "Bidirectional
Forwarding Detection (BFD) on Link Aggregation Group (LAG)
Interfaces", RFC 7130, DOI 10.17487/RFC7130, February
2014, <https://www.rfc-editor.org/info/rfc7130>.
[RFC8138] Thubert, P., Ed., Bormann, C., Toutain, L., and R. Cragie,
"IPv6 over Low-Power Wireless Personal Area Network
(6LoWPAN) Routing Header", RFC 8138, DOI 10.17487/RFC8138,
April 2017, <https://www.rfc-editor.org/info/rfc8138>.
9.2. Informative References
[I-D.daniel-6lowpan-hilow-hierarchical-routing]
Park, S. D., "Hierarchical Routing over 6LoWPAN (HiLow)",
Work in Progress, Internet-Draft, draft-daniel-6lowpan-
hilow-hierarchical-routing-01, 18 June 2007,
<https://datatracker.ietf.org/doc/html/draft-daniel-
6lowpan-hilow-hierarchical-routing-01>.
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[I-D.geng-spring-sr-redundancy-protection]
Geng, X., Chen, M., Yang, F., Camarillo, P., and G. S.
Mishra, "SRv6 for Redundancy Protection", Work in
Progress, Internet-Draft, draft-geng-spring-sr-redundancy-
protection-05, 2 August 2021,
<https://datatracker.ietf.org/doc/html/draft-geng-spring-
sr-redundancy-protection-05>.
[IEEE802.1W]
"IEEE Std 802.1w-2001, IEEE Std for Local and metropolitan
are networks - Common specifications Part 3; Media Access
Control (MAC) Bridges - Amendment 2; Rapid
Reconfiguration", n.d.,
<https://standards.ieee.org/ieee/802.1w/1046/>.
[KIM07] Kim, Y.-S., Lee, E. J., Kim, B. S., and H. S. Kim,
"Extended Tree-Based Routing Algorithm in IPv6-enabled
Wireless Sensor Networks", IEEE 2007 International
Conference on Convergence Information Technology (ICCIT
2007), pp. 1269-1274, 2007.
[SINGH20] Singh, S. K. and J. Prakash, "Energy Efficiency and Load
Balancing in MANET: A Survey", IEEE 2020 6th International
Conference on Advanced Computing and Communication Systems
(ICACCS), 2020, pp. 832-837, 2020.
Authors' Addresses
Guangpeng Li
Huawei Technologies
Beiqing Road, Haidian District
Beijing
100095
China
Email: liguangpeng@huawei.com
David Lou
Huawei Technologies
Riesstrasse 25
80992 Munich
Germany
Email: zhe.lou@huawei.com
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Luigi Iannone
Huawei Technologies France S.A.S.U.
18, Quai du Point du Jour
92100 Boulogne-Billancourt
France
Email: luigi.iannone@huawei.com
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