6lo Working Group G. Li Internet-Draft D. Lou Intended status: Informational L. Iannone Expires: 11 March 2024 Huawei 8 September 2023 Reliability Considerations of Path-Aware Semantic Addressing draft-li-6lo-pasa-reliability-02 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 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 https://datatracker.ietf.org/drafts/current/. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." This Internet-Draft will expire on 11 March 2024. 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. Li, et al. Expires 11 March 2024 [Page 1] Internet-Draft PASA Reliability September 2023 Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Revised BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Revised BSD License. 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. PASA Router Operation . . . . . . . . . . . . . . . . 13 3.4.2. PASA Root Operation . . . . . . . . . . . . . . . . . 14 4. Single-Address Approach . . . . . . . . . . . . . . . . . . . 15 4.1. Topology Building . . . . . . . . . . . . . . . . . . . . 15 4.2. Link Failures . . . . . . . . . . . . . . . . . . . . . . 18 4.3. Node Failures . . . . . . . . . . . . . . . . . . . . . . 19 4.4. Node Forwarding Procedure . . . . . . . . . . . . . . . . 19 4.4.1. PASA Router Operation . . . . . . . . . . . . . . . . 19 4.4.2. PASA Root Operation . . . . . . . . . . . . . . . . . 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 impossible to forward, and packets are dropped, Li, et al. Expires 11 March 2024 [Page 2] Internet-Draft PASA Reliability September 2023 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 logical topologies. Li, et al. Expires 11 March 2024 [Page 3] Internet-Draft PASA Reliability September 2023 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-13| |N-14| +----+ +----+ +----+ +----+ +----+ +----+ +----+ |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 Li, et al. Expires 11 March 2024 [Page 4] Internet-Draft PASA Reliability September 2023 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. Li, et al. Expires 11 March 2024 [Page 5] Internet-Draft PASA Reliability September 2023 +---+ +---+ |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. As described in [I-D.li-6lo-path-aware-semantic-addressing], rebuilding the full IPv6 address from the PASA address is simply done via a coalescence operation with the PASA prefix (cf. Section 4.3.1 Li, et al. Expires 11 March 2024 [Page 6] Internet-Draft PASA Reliability September 2023 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. PASA Hosts and PASA Routers need to be aware of the PASA Root address length. For instance, the PASA Hosts 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 PASA 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, when 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 store the unreachable status of the sub-tree prefix, and additionally it will 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 Li, et al. Expires 11 March 2024 [Page 7] Internet-Draft PASA Reliability September 2023 actually re-directed toward the root. After this procedure, when a node sees a packet that is destined to the node in the unreachable sub-tree, it sends it up to the root. 2. 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, 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 [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 Li, et al. Expires 11 March 2024 [Page 8] Internet-Draft PASA Reliability September 2023 packets coming from the sub-tree, nodes along the alternate path on the secondary tree will simply forward the packets to the secondary PASA Root, who will forward them to the primary PASA 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 state, namely 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. 3. 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: Li, et al. Expires 11 March 2024 [Page 9] Internet-Draft PASA Reliability September 2023 1. 10 sends an ICMPv6 message to the PASA Root. The latter 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 0101 as source address (see Figure 3) using the recursive procedure through node 010. Once this message reaches 01, the PASA Root of the secondary tree, it will be forwarded to 1, the PASA Root of the primary tree. Now PASA Root 1 has an entry stating: * For 100-sub-tree encapsulate to 0101 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 using its secondary address 0101 and then 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). Li, et al. Expires 11 March 2024 [Page 10] Internet-Draft PASA Reliability September 2023 3. 010 will use PASA stateless forwarding to transmit the packet to 01 (on the secondary tree). 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 PASA Root will forward the packet upstream. For incoming packets, the PASA 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: Li, et al. Expires 11 March 2024 [Page 11] Internet-Draft PASA Reliability September 2023 1. 10 sends an ICMPv6 message to the primary PASA Root. The latter will register that 100-sub-tree is not reachable through 10 but has to be redirected. 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 PASA Root of the primary tree. Now PASA 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 PASA Root of the primary tree. Now PASA 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 PASA Root of the primary tree. Now PASA 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 1001 using its secondary address 01001 and then 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. Li, et al. Expires 11 March 2024 [Page 12] Internet-Draft PASA Reliability September 2023 * 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). 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 PASA Routers and PASA Root 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. PASA Router Operation 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. Li, et al. Expires 11 March 2024 [Page 13] Internet-Draft PASA Reliability September 2023 +-----------------+ | 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. PASA Root Operation In the case of a PASA Root, 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. Li, et al. Expires 11 March 2024 [Page 14] Internet-Draft PASA Reliability September 2023 +-----------------+ | 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 PASA Root to register themselves so that the PASA Root has an overview of the nodes and the topology in the PASA network. By default, a PASA Routers forward packets via the tree by using the native PASA forwarding method defined in [I-D.li-6lo-path-aware-semantic-addressing]. The PASA 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 PASA Hosts, there will be only alternative uplink entries. For PASA Routers, Li, et al. Expires 11 March 2024 [Page 15] Internet-Draft PASA Reliability September 2023 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. +---+ +---+ | 1 | |'1'| / +---+____________ .+---+ / .|..\___ .......\. / . | .\...... .\ / . | . \ . \_______ / . | . \ . \ +---+. +---+ +---+ +---+ | 10| | 11| |110| |111| +---+ ... +---+ .+---+ +---+ / | \\__ ............... \ / | \ \ .. . \ / | \ \____ . . . \ Failure X .|.....\.......\. . . \ / . | . \ . \ . . \ / . | . \ . \ . . \ +---+ +---+ +----+ +----+ +----+ |100| |101| |1010| |1011| |1101| +---+ +---+ +----+ +----+ +----+ / | \\. \\ / | \ \. \ \______ / | \ \ .. \ \ / | \ \ . . \ \ / | \ \ . .......\ ..... \ / | \ \ ........ \ .. \ / | \ \ .\ .\ +----+ +----+ +-----+ +-----+ +-----+ +-----+ |1000| |1001| |10010| |10011| |10100| |10101| +----+ +----+ +-----+ +-----+ +-----+ +-----+ Figure 6: An example of link failure in single address topology. Let us make an example using as reference the topology depicted in Figure 6. The figure show main links used in the PASA topology and the alternative links relevant to our example (depicting all alternative links would make the picture too cluttered). Figure 7 shows the corresponding ND table for the PASA Router 100. Li, et al. Expires 11 March 2024 [Page 16] Internet-Draft PASA Reliability September 2023 +-------------+-------+ | Destination | Flags | +-------------+-------+ | 100 | I | I = Current Node +-------------+-------+ | 10 | PP | PP = Primary Parent +-------------+-------+ | 1000 | PPRC | PPRC = Primary PASA Router Child +-------------+-------+ | 10010 | PPRC | +-------------+-------+ | 1001 | PPHC | PPHC = Primary PASA Host Child +-------------+-------+ | 10011 | PPHC | +-------------+-------+ | 110 | AP | AP = Alternative Parent +-------------+-------+ | 10100 | APRC | APRC = Alternative PASA Router Child whose +-------------+-------+ alternative parent is the current node | 10101 | APHC | APHC = Alternative PASA Host Child whose +-------------+-------+ alternative parent is the current node Figure 7: 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 7 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 PASA Router Children (PPRC), namely '1000' and '10010, followed by two Primary PASA Host Children (PPHC), 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, an Alternative PASA Router Child (APRC) with address 10100, and an Alternative PASA Host Child (APHC) with address 10101. 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. Li, et al. Expires 11 March 2024 [Page 17] Internet-Draft PASA Reliability September 2023 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. In order to give an example illustrating what happens to packets flowing downlink, let us assume a packet initiated from node 1101 and 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 point 2 of the the previous example. Li, et al. Expires 11 March 2024 [Page 18] Internet-Draft PASA Reliability September 2023 5. Once packet reaches 100, segment routing extension is 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, 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 then used. 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 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. PASA Router Operation 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. Li, et al. Expires 11 March 2024 [Page 19] Internet-Draft PASA Reliability September 2023 +----------------+ | 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. PASA Root Operation 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). Li, et al. Expires 11 March 2024 [Page 20] Internet-Draft PASA Reliability September 2023 +----------------+ | 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 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 Li, et al. Expires 11 March 2024 [Page 21] Internet-Draft PASA Reliability September 2023 using PASA 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 PASA Router and PASA Host are rather dummy, performing PASA stateless forwarding. They only are aware of the 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 state is also required on PASA Routers, 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 Li, et al. Expires 11 March 2024 [Page 22] Internet-Draft PASA Reliability September 2023 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, . [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, . [RFC5798] Nadas, S., Ed., "Virtual Router Redundancy Protocol (VRRP) Version 3 for IPv4 and IPv6", RFC 5798, DOI 10.17487/RFC5798, March 2010, . [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, . [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, . 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, . Li, et al. Expires 11 March 2024 [Page 23] Internet-Draft PASA Reliability September 2023 [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, . [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., . [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 Li, et al. Expires 11 March 2024 [Page 24] Internet-Draft PASA Reliability September 2023 Luigi Iannone Huawei Technologies France S.A.S.U. 18, Quai du Point du Jour 92100 Boulogne-Billancourt France Email: luigi.iannone@huawei.com Li, et al. Expires 11 March 2024 [Page 25]