| ROLL | R. Jadhav, Ed. |
| Internet-Draft | Huawei |
| Intended status: Standards Track | P. Thubert |
| Expires: January 20, 2019 | Cisco |
| R. Sahoo | |
| Z. Cao | |
| Huawei | |
| July 19, 2018 |
Efficient Route Invalidation
draft-ietf-roll-efficient-npdao-04
This document describes the problems associated with the use of No-Path DAO messaging in RPL and signaling changes to improve route invalidation efficiency.
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RPL [RFC6550] specifies a proactive distance-vector based routing scheme. The specification has an optional messaging in the form of DAO messages using which the 6LBR can learn route towards any of the nodes. In storing mode, DAO messages would result in routing entries been created on all intermediate hops from the node's parent all the way towards the 6LBR.
RPL allows use of No-Path DAO (NPDAO) messaging to invalidate a routing path corresponding to the given target, thus releasing resources utilized on that path. A No-Path DAO is a DAO message with route lifetime of zero, originates at the target node and always flows upstream towards the 6LBR, signaling route invalidation for the given target. This document explains the problems associated with the current use of NPDAO messaging and also discusses the requirements for an optimized No-Path DAO messaging scheme. Further a new pro-active route invalidation message called as "Destination Cleanup Object (DCO)" is specified which fulfills all mentioned requirements of an optimized route invalidation messaging.
6TiSCH architecture [I-D.ietf-6tisch-architecture] leverages RPL and specifies use of non-storing and storing MOP for its routing operation. Thus an improvement in route invalidation will help optimize 6TiSCH based networks.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119.
The document only caters to the RPL's storing mode of operation (MOP). The non-storing MOP does not require use of NPDAO for route invalidation since routing entries are not maintained on 6LRs.
Common Ancestor node: 6LR node which is the first common node on the old and new path for the child node.
NPDAO: No-Path DAO. A DAO message which has target with lifetime 0.
DCO: Destination Cleanup Object, A new RPL control message type defined by this draft.
Regular DAO: A DAO message with non-zero lifetime.
This document also uses terminology described in [RFC6550].
RPL introduced No-Path DAO messaging in the storing mode so that the node switching its current parent can inform its parents and ancestors to invalidate the existing route. Subsequently parents or ancestors would release any resources (such as the routing entry) it maintains on behalf of target node. The NPDAO message always traverses the RPL tree in upward direction, originating at the target node itself.
For the rest of this document consider the following topology:
(6LBR)
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|
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(A)
/ \
/ \
/ \
(G) (H)
| |
| |
| |
(B) (C)
\ ;
\ ;
\ ;
(D)
/ \
/ \
/ \
(E) (F)
Figure 1: Sample topology
Node (D) is connected via preferred parent (B). (D) has an alternate path via (C) towards the BR. Node (A) is the common ancestor for (D) for paths through (B)-(G) and (C)-(H). When (D) switches from (B) to (C), [RFC6550] suggests sending No-Path DAO to (B) and regular DAO to (C).
There are following cases in which a node switches its parent and may employ No-Path DAO messaging:
Case I: Current parent becomes unavailable because of transient or permanent link or parent node failure.
Case II: The node finds a better parent node i.e. the metrics of another parent is better than its current parent.
Case III: The node switches to a new parent whom it "thinks" has a better metric but does not in reality.
The usual steps of operation when the node switches the parent is that the node sends a No-Path DAO message via its current parent to invalidate its current route and subsequently it tries to establish a new routing path by sending a new DAO via its new parent.
Nodes in LLNs may be resource constrained. There is limited memory available and routing entry records are the one of the primary elements occupying dynamic memory in the nodes. Route invalidation helps 6LR nodes to decide which entries could be discarded to better achieve resource utilization in case of contention. Thus it becomes necessary to have efficient route invalidation mechanism. Also note that a single parent switch may result in a "sub-tree" switching from one parent to another. Thus the route invalidation needs to be done on behalf of the sub-tree and not the switching node alone. In the above example, when Node (D) switches parent, the route invalidation needs to be done for (D), (E) and (F). Thus without efficient route invalidation, a 6LR may have to hold a lot of unwanted route entries.
When a node switches its parent, the NPDAO is to be sent via its previous parent and a regular DAO via its new parent. In cases where the node switches its parent because of transient or permanent parent link/node failure then the NPDAO message is bound to fail. RPL assumes communication link with the previous parent for No-Path DAO messaging.
RPL allows use of route lifetime to remove unwanted routes in case the routes could not be refreshed. But route lifetimes in case of LLNs could be substantially high and thus the route entries would be stuck for longer times.
No-path DAO is sent by the node who has switched the parent but it does not work for the dependent child nodes below it. The specification does not specify how route invalidation will work for sub-childs, resulting in stale routing entries on behalf of the sub-childs on the previous route. The only way for 6LR to invalidate the route entries for dependent nodes would be to use route lifetime expiry which could be substantially high for LLNs.
In the example topology, when Node (D) switches its parent, Node (D) generates an NPDAO on its behalf. Post switching, Node (D) transmits a DIO with incremented DTSN so that child nodes, node (E) and (F), generate DAOs to trigger route update on the new path for themselves. There is no NPDAO generated by these child nodes through the previous path resulting in stale entries on nodes (B) and (G) for nodes (E) and (F).
A switching node may generate both an NPDAO and DAO via two different paths at almost the same time. There is a possibility that an NPDAO generated may invalidate the previous route and the regular DAO sent via the new path gets lost on the way. This may result in route downtime thus impacting downward traffic for the switching node. In the example topology, consider Node (D) switches from parent (B) to (C) because the metrics of the path via (C) are better. Note that the previous path via (B) may still be available (albeit at relatively bad metrics). An NPDAO sent from previous route may invalidate the existing route whereas there is no way to determine whether the new DAO has successfully updated the route entries on the new path.
When the switching node sends the NPDAO message to the previous parent, it is normal that the link to the previous parent is prone to failure. Therefore, it is required that the NPDAO message MUST be tolerant to the link failure during the switching. The link referred here represents the link between the node and its previous parent (from whom the node is now disassociating).
While switching the parent node and sending NPDAO message, it is required that the routing entries to the dependent nodes of the switching node will be updated accordingly on the previous parents and other relevant upstream nodes.
While sending the NPDAO and DAO messages, it is possible that the NPDAO successfully invalidates the previous path, while the newly sent DAO gets lost (new path not set up successfully). This will result into downstream unreachability to the current switching node. Therefore, it is desirable that the NPDAO is synchronized with the DAO to avoid the risk of route downtime.
As described in Section 1.2, the NPDAO originates at the node switching the parent and traverses upstream towards the root. In order to solve the problems as mentioned in Section 2, the draft adds new pro-active route invalidation message called as "Destination Cleanup Object" (DCO) that originates at a common ancestor node between the new and old path. The common ancestor node generates a DCO in response to the change in the next-hop on receiving a regular DAO for the target.
In the Figure 1, when node D decides to switch the path from B to C, it sends a regular DAO to node C with reachability information containing target as address of D and a incremented path sequence number. Node C will update the routing table based on the reachability information in DAO and in turn generate another DAO with the same reachability information and forward it to H. Node H also follows the same procedure as Node C and forwards it to node A. When node A receives the regular DAO, it finds that it already has a routing table entry on behalf of the target address of node D. It finds however that the next hop information for reaching node D has changed i.e. the node D has decided to change the paths. In this case, Node A which is the common ancestor node for node D along the two paths (previous and new), may generate a DCO which traverses downwards in the network. The document in the subsequent section will explain the message format changes to handle this downward flow of NPDAO.
Every RPL message is divided into base message fields and additional Options. The base fields apply to the message as a whole and options are appended to add message/use-case specific attributes. As an example, a DAO message may be attributed by one or more "RPL Target" options which specifies the reachability information for the given targets. Similarly, a Transit Information option may be associated with a set of RPL Target options.
The draft proposes a change in DAO message to contain "Invalidate previous route" (I) bit. This I-bit which is carried in regular DAO message, signals the common ancestor node to generate a DCO on behalf of the target node. The I-bit is carried in the transit container option which augments the reachability information for a given set of RPL Target(s).
0 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = 0x06 | Option Length |E|I| Flags | Path Control |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Path Sequence | Path Lifetime | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
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+ +
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+ Parent Address* +
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+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2: Updated Transit Information Option (New I flag added)
I (Invalidate previous route) bit: 1 bit flag. The 'I' flag is set by the target node to indicate that it wishes to invalidate the previous route by a common ancestor node between the two paths.
The common ancestor node SHOULD generate a DCO message in response to this I-bit when it sees that the routing adjacencies have changed for the target. I-bit governs the ownership of the DCO message in a way that the target node is still in control of its own route invalidation.
A new ICMPv6 RPL control message type is defined by this specification called as "Destination Cleanup Object" (DCO), which is used for proactive cleanup of state and routing information held on behalf of the target node by 6LRs. The DCO message always traverses downstream and cleans up route information and other state information associated with the given target.
0 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| RPLInstanceID |K|D| Flags | Reserved | DCOSequence |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
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+ DODAGID(optional) +
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+ +
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Option(s)...
+-+-+-+-+-+-+-+-+
Figure 3: DCO base object
RPLInstanceID: 8-bit field indicating the topology instance associated with the DODAG, as learned from the DIO.
K: The 'K' flag indicates that the recipient is expected to send a DCO-ACK back.
D: The 'D' flag indicates that the DODAGID field is present. This flag MUST be set when a local RPLInstanceID is used.
Flags: The 6 bits remaining unused in the Flags field are reserved for flags. The field MUST be initialized to zero by the sender and MUST be ignored by the receiver.
Reserved: 8-bit unused field. The field MUST be initialized to zero by the sender and MUST be ignored by the receiver.
DCOSequence: Incremented at each unique DCO message from a node and echoed in the DCO-ACK message.
DODAGID (optional): 128-bit unsigned integer set by a DODAG root that uniquely identifies a DODAG. This field is only present when the 'D' flag is set. This field is typically only present when a local RPLInstanceID is in use, in order to identify the DODAGID that is associated with the RPLInstanceID. When a global RPLInstanceID is in use, this field need not be present. Unassigned bits of the DCO Base are reserved. They MUST be set to zero on transmission and MUST be ignored on reception.
A Secure DCO message follows the format in [RFC6550] figure 7, where the base message format is the DCO message shown in Figure 3.
The DCO message MAY carry valid options. This specification allows for the DCO message to carry the following options:
The DCO carries a Target option and an associated Transit Information option with a lifetime of 0x00000000 to indicate a loss of reachability to that Target.
A DCO message may contain a Path Sequence in the transit information option to identify the freshness of the DCO message. The Path Sequence in the DCO MUST use the same Path Sequence number present in the regular DAO message when the DCO is generated in response to DAO message.
The DCO-ACK message may be sent as a unicast packet by a DCO recipient in response to a unicast DCO message.
0 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| RPLInstanceID |D| Reserved | DCOSequence | Status |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
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+ DODAGID(optional) +
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+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 4: DCO-ACK base object
RPLInstanceID: 8-bit field indicating the topology instance associated with the DODAG, as learned from the DIO.
D: The 'D' flag indicates that the DODAGID field is present. This flag MUST be set when a local RPLInstanceID is used.
Reserved: 7-bit unused field. The field MUST be initialized to zero by the sender and MUST be ignored by the receiver.
DCOSequence: Incremented at each unique DCO message from a node and echoed in the DCO-ACK message.
Status: Indicates the completion. Status 0 is defined as unqualified acceptance in this specification. The remaining status values are reserved as rejection codes.
DODAGID (optional): 128-bit unsigned integer set by a DODAG root that uniquely identifies a DODAG. This field is only present when the 'D' flag is set. This field is typically only present when a local RPLInstanceID is in use, in order to identify the DODAGID that is associated with the RPLInstanceID. When a global RPLInstanceID is in use, this field need not be present. Unassigned bits of the DCO-Ack Base are reserved. They MUST be set to zero on transmission and MUST be ignored on reception.
A Secure DCO-ACK message follows the format in [RFC6550] figure 7, where the base message format is the DCO-ACK message shown in Figure 4.
Current RPL [RFC6550] does not provide a mechanism for route invalidation for dependent nodes. This document allows the dependent nodes invalidation. Dependent nodes will generate their respective DAOs to update their paths, and the previous route invalidation for those nodes should work in the similar manner described for switching node. The dependent node may set the I-bit in the transit information container as part of regular DAO so as to request invalidation of previous route from the common ancestor node.
Even with the changed semantics, the current NPDAO mechanism in [RFC6550] can still be used. There are certain scenarios where current NPDAO signalling may still be used, for example, when the route lifetime expiry of the target happens or when the node simply decides to gracefully terminate the RPL session on graceful node shutdown. Moreover a deployment can have a mix of nodes supporting the proposed DCO and the existing NPDAO mechanism.
Many thanks to Cenk Gundogan, Simon Duquennoy, and Georgios Papadopoulous for their review and comments.
IANA is requested to allocate new ICMPv6 RPL control codes in RPL [RFC6550] for DCO and DCO-ACK messages.
| Code | Description | Reference |
|---|---|---|
| 0x04 | Destination Cleanup Object | This document |
| 0x05 | Destination Cleanup Object Acknowledgement | This document |
| 0x84 | Secure Destination Cleanup Object | This document |
| 0x85 | Secure Destination Cleanup Object Acknowledgement | This document |
IANA is requested to allocate bit 18 in the Transit Information Option defined in RPL [RFC6550] section 6.7.8 for Invalidate route 'I' flag.
This document handles security considerations inline to base RPL. Secure versions of DCO and DCO-ACK are added similar to other RPL messages. For general RPL security considerations, see [RFC6550].
| [RFC2119] | Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997. |
| [RFC6550] | Winter, T., Thubert, P., Brandt, A., Hui, J., Kelsey, R., Levis, P., Pister, K., Struik, R., Vasseur, JP. and R. Alexander, "RPL: IPv6 Routing Protocol for Low-Power and Lossy Networks", RFC 6550, DOI 10.17487/RFC6550, March 2012. |
| [I-D.ietf-6tisch-architecture] | Thubert, P., "An Architecture for IPv6 over the TSCH mode of IEEE 802.15.4", Internet-Draft draft-ietf-6tisch-architecture-14, April 2018. |
In Figure 1, node (D) switches its parent from (B) to (C). The sequence of actions is as follows: