| ROLL | R. Jadhav, Ed. |
| Internet-Draft | Huawei |
| Intended status: Standards Track | P. Thubert |
| Expires: December 5, 2019 | Cisco |
| R. Sahoo | |
| Z. Cao | |
| Huawei | |
| June 3, 2019 |
Efficient Route Invalidation
draft-ietf-roll-efficient-npdao-12
This document describes the problems associated with No-Path Destination Advertisement Object (NPDAO) messaging used in Routing Protocol for Low power and lossy networks (RPL) for route invalidation and signaling changes to improve route invalidation efficiency.
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RPL [RFC6550] (Routing Protocol for Low power and lossy networks) specifies a proactive distance-vector based routing scheme. RPL has an optional messaging in the form of DAO (Destination Advertisement Object) messages, which the 6LBR (6Lo Border Router) and 6LR (6Lo Router) can use to learn a route towards the downstream nodes. In storing mode, DAO messages would result in routing entries being created on all intermediate 6LRs from the node's parent all the way towards the 6LBR.
RPL allows the 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 NPDAO is a DAO message with route lifetime of zero, originates at the target node and always flows upstream towards the 6LBR. This document explains the problems associated with the current use of NPDAO messaging and also discusses the requirements for an optimized route invalidation messaging scheme. Further a new pro-active route invalidation message called as "Destination Cleanup Object" (DCO) is specified which fulfills requirements of an optimized route invalidation messaging.
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.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.
This specification requires readers to be familiar with all the terms and concepts that are discussed in "RPL: IPv6 Routing Protocol for Low-Power and Lossy Networks" [RFC6550].
RPL uses NPDAO messaging in the storing mode so that the node changing it routing adjacencies can invalidate the previous route. This is needed so that nodes along the previous path can release any resources (such as the routing entry) it maintains on behalf of target node.
For the rest of this document consider the following topology:
(6LBR)
|
|
|
(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 6LBR. Node (A) is the common ancestor for (D) for paths through (B)-(G) and (C)-(H). When (D) switches from (B) to (C), RPL allows sending NPDAO to (B) and regular DAO to (C).
Nodes in LLNs may be resource constrained. There is limited memory available and routing entry records are 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. Thus it becomes necessary to have an 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 updates needs to be done for the routing tables entries of (C),(H),(A),(G), and (B) with destination (D),(E) and (F). Without efficient route invalidation, a 6LR may have to hold a lot of stale route entries.
When a node switches its parent, the NPDAO is to be sent to its previous parent and a regular DAO to 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 does not specify how route invalidation will work for dependent nodes rooted at the switching node, resulting in stale routing entries of the dependent nodes. 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. There is no NPDAO generated by the dependent child nodes (E) and (F), through the previous path via (D) to (B) and (G), 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 impacting downward traffic for the switching node.
In the example topology, consider Node (D) switches from parent (B) to (C). An NPDAO sent via the previous route may invalidate the previous 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 (that's why the node decided to switch). Therefore, it is required that the route invalidation does not depend on the previous link which is prone to failure. The previous link referred here represents the link between the node and its previous parent (from whom the node is now disassociating).
It should be possible to do route invalidation for dependent nodes rooted at the switching node.
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 in downstream unreachability to the node switching paths. Therefore, it is desirable that the route invalidation is synchronized with the DAO to avoid the risk of route downtime.
As described in Section 1.2, the NPDAO originates at the node changing to a new parent and traverses upstream towards the root. In order to solve the problems as mentioned in Section 2, the document adds a new pro-active route invalidation message called "Destination Cleanup Object" (DCO) that originates at a common ancestor node and flows downstream 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 with updated Path Sequence for the target.
The 6LRs in the path for DCO take action such as route invalidation based on the DCO information and subsequently send another DCO with the same information downstream to the next hop. This operation is similar to how the DAOs are handled on intermediate 6LRs in storing MOP in [RFC6550]. Just like DAO in storing MOP, the DCO is sent using link-local unicast source and destination IPv6 address. Unlike DAO, which always travels upstream, the DCO always travels downstream.
In 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 an incremented Path Sequence. Node C will update the routing table based on the reachability information in the 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., 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), should generate a DCO which traverses downwards in the network.
Every RPL message is divided into base message fields and additional Options as described in Section 6 of [RFC6550]. 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 specify the reachability information for the given targets. Similarly, a Transit Information option may be associated with a set of RPL Target options.
This document specifies a change in the Transit Information Option to contain the "Invalidate previous route" (I) flag. This I-flag signals the common ancestor node to generate a DCO on behalf of the target node. The I-flag is carried in the Transit Information Option which augments the reachability information for a given set of RPL Target(s). Transit Information Option should be carried in the DAO message with I-flag set in case route invalidation is sought for the corresponding 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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2: Updated Transit Information Option (New I flag added)
I (Invalidate previous route) flag: The 'I' flag is set by the target node to indicate to the common ancestor node that it wishes to invalidate any previous route between the two paths.
[RFC6550] allows parent address to be sent in the Transit Information Option depending on the mode of operation. In case of storing mode of operation the field is usually not needed. In case of DCO, the parent address field MUST NOT be included.
The common ancestor node SHOULD generate a DCO message in response to this I-flag when it sees that the routing adjacencies have changed for the target. I-flag 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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| |
+ DODAGID(optional) +
| |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 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 of DCO message is expected to send a DCO-ACK back. If the DCO-ACK is not received even after setting the 'K' flag, an implementation may retry the DCO at a later time. The number of retries are implementation and deployment dependent. A node receiving a DCO message without the 'K' flag set MAY respond with a DCO-ACK, especially to report an error condition. An example error condition could be that the node sending the DCO-ACK does not find the routing entry for the indicated target.
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 future use. These bits 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. The initial DCOSequence can be chosen randomly by the node.
DODAGID (optional): 128-bit unsigned integer set by a DODAG root that uniquely identifies a DODAG. This field MUST be present when the 'D' flag is set. DODAGID is used when a local RPLInstanceID is in use, in order to identify the DODAGID that is associated with the RPLInstanceID.
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 MUST carry at least one RPL Target and the Transit Information Option and MAY carry other valid options. This specification allows for the DCO message to carry the following options:
The DCO carries an RPL 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 a DAO message. Thus if a DCO is received by a 6LR and subsequently a DAO is received with an old seqeunce number, then the DAO MUST be ignored.
The DCO-ACK message SHOULD be sent as a unicast packet by a DCO recipient in response to a unicast DCO message with 'K' flag set. If 'K' flag is not set then the receiver of the DCO message MAY send a DCO-ACK to signal an error condition.
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| |
+ DODAGID(optional) +
| |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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: The DCOSequence in DCO-ACK is copied from the DCOSequence received in the DCO message.
Status: Indicates the completion. Status 0 is defined as unqualified acceptance in this specification. Status 1 is defined as "No routing-entry for the Target found". 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 MUST be present when the 'D' flag is set. DODAGID is used when a local RPLInstanceID is in use, in order to identify the DODAGID that is associated with the RPLInstanceID.
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.
The scope of DCOSequence values is unique to each node.
A 6LR may generate an unsolicited DCO to unilaterally cleanup the path on behalf of the target entry. The 6LR has all the state information namely, the Target address and the Path Sequence, required for generating DCO in its routing table. The conditions why 6LR may generate an unsolicited DCO are beyond the scope of this document but some possible reasons could be:
Note that if the 6LR initiates a unilateral path cleanup using DCO and if it has the latest state for the target then the DCO would finally reach the target node. Thus the target node would be informed of its invalidation.
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-flag in the Transit Information Option as part of regular DAO so as to request invalidation of previous route from the common ancestor node.
Dependent nodes do not have any indication regarding if any of its parent nodes in turn have decided to switch their parent. Thus for route invalidation the dependent nodes may choose to always set the 'I' flag in all its DAO message's Transit Information Option. Note that setting the I-flag is not counter productive even if there is no previous route to be invalidated.
Even with the changed semantics, the current NPDAO mechanism in [RFC6550] can 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 DCO and the existing NPDAO mechanism. It is also possible that the same node supports both the NPDAO and DCO signaling.
Section 9.8 of [RFC6550] states, "When a node removes a node from its DAO parent set, it SHOULD send a No-Path DAO message to that removed DAO parent to invalidate the existing router". This document introduces an alternate and more optimized way of route invalidation but it also allows existing NPDAO messaging to work. Thus an implementation has two choices to make when a route invalidation is to be initiated:
This document recommends using option 2 for reasons specified in Section 3 in this document.
[RFC6550] allows a node to select multiple preferred parents for route establishment. Section 9.2.1 of [RFC6550] specifies, "All DAOs generated at the same time for the same Target MUST be sent with the same Path Sequence in the Transit Information". Subsequently when route invalidation has to be initiated, RPL mentions use of NPDAO which can be initiated with an updated Path Sequence to all the parent nodes through which the route is to be invalidated.
With DCO, the Target node itself does not initiate the route invalidation and it is left to the common ancestor node. A common ancestor node when it discovers an updated DAO from a new next-hop, it initiates a DCO. With multiple preferred parents, this handling does not change. But in this case it is recommended that an implementation initiates a DCO after a time period (DelayDCO) such that the common ancestor node may receive updated DAOs from all possible next-hops. This will help to reduce DCO control overhead i.e., the common ancestor can wait for updated DAOs from all possible directions before initiating a DCO for route invalidation. After timeout, the DCO needs to be generated for all the next-hops for whom the route invalidation needs to be done.
This documents recommends using a DelayDCO timer value of 1sec. This value is inspired by the default DelayDAO value of 1sec in [RFC6550]. Here the hypothesis is that the DAOs from all possible parent set would be received on the common ancestor within this time period.
Note that there is no requirement of synchronization between DCO and DAOs. The DelayDCO timer simply ensures that the DCO control overhead can be reduced and is only needed when the network contains nodes using multiple preferred parent.
Many thanks to Alvaro Retana, Cenk Gundogan, Simon Duquennoy, Georgios Papadopoulous, Peter Van Der Stok for their review and comments. Alvaro Retana helped shape this document's final version with critical review comments.
IANA is requested to allocate new codes for the DCO and DCO-ACK messages from the RPL Control Codes registry.
| Code | Description | Reference |
|---|---|---|
| TBD1 | Destination Cleanup Object | This document |
| TBD2 | Destination Cleanup Object Acknowledgment | This document |
| TBD3 | Secure Destination Cleanup Object | This document |
| TBD4 | Secure Destination Cleanup Object Acknowledgment | This document |
IANA is requested to allocate bit 1 from the Transit Information Option Flags registry for the I-flag (Section 4.2)
IANA is requested to create a registry for the 8-bit Destination Cleanup Object (DCO) Flags field. This registry should be located in existing category of "Routing Protocol for Low Power and Lossy Networks (RPL)".
New bit numbers may be allocated only by an IETF Review. Each bit is tracked with the following qualities:
The following bits are currently defined:
| Bit number | Description | Reference |
|---|---|---|
| 0 | DCO-ACK request (K) | This document |
| 1 | DODAGID field is present (D) | This document |
IANA is requested to create a registry for the 8-bit Destination Cleanup Object Acknowledgment (DCO-ACK) Status field. This registry should be located in existing category of "Routing Protocol for Low Power and Lossy Networks (RPL)".
New Status values may be allocated only by an IETF Review. Each value is tracked with the following qualities:
The following bits are currently defined:
| Status Code | Description | Reference |
|---|---|---|
| 0 | Unqualified acceptance | This document |
| 1 | No routing-entry for the indicated Target found | This document |
IANA is requested to create a registry for the 8-bit Destination Cleanup Object (DCO) Acknowledgment Flags field. This registry should be located in existing category of "Routing Protocol for Low Power and Lossy Networks (RPL)".
New bit numbers may be allocated only by an IETF Review. Each bit is tracked with the following qualities:
The following bits are currently defined:
| Bit number | Description | Reference |
|---|---|---|
| 0 | DODAGID field is present (D) | This document |
This document introduces the ability for a common ancestor node to invalidate a route on behalf of the target node. The common ancestor node is directed to do so by the target node using the 'I' flag in DCO's Transit Information Option. However, the common ancestor node is in a position to unilaterally initiate the route invalidation since it possesses all the required state information, namely, the Target address and the corresponding Path Sequence. Thus a rogue common ancestor node could initiate such an invalidation and impact the traffic to the target node.
This document also introduces an I-flag which is set by the target node and used by the ancestor node to initiate a DCO if the ancestor nodes sees an update in the route adjacency. However, this flag could be spoofed by a malicious 6LR in the path and can cause invalidation of an existing active path. Note that invalidation will happen only if the other conditions such as Path Sequence condition is also met. Having said that a malicious 6LR may spoof a DAO on behalf of the (sub) child with the I-flag set and can cause route invalidation on behalf of the (sub) child node.
This document assumes that the security mechanisms as defined in [RFC6550] are followed, which means that the common ancestor node and all the 6LRs are part of the RPL network because they have the required credentials. A non-secure RPL network needs to take into consideration the risks highlighted in this section.
All RPL messages support a secure version of messages which allows integrity protection using either a MAC or a signature. Optionally, secured RPL messages also have encryption protection for confidentiality.
The document adds new messages (DCO, DCO-ACK) which are syntactically similar to existing RPL messages such as DAO, DAO-ACK. Secure versions of DCO and DCO-ACK are added similar to other RPL messages (such as DAO, DAO-ACK).
RPL supports three security modes as mentioned in Section 10.1 of [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. |
| [RFC8174] | Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017. |
In Figure 1, node (D) switches its parent from (B) to (C). This example assumes that Node D has already established its own route via Node B-G-A-6LBR using pathseq=x. The example uses DAO and DCO messaging convention and specifies only the required parameters to explain the example namely, the parameter 'tgt', which stands for Target Option and value of this parameter specifies the address of the target node. The parameter 'pathseq', which specifies the Path Sequence value carried in the Transit Information Option. The parameter 'I_flag' specifies the 'I' flag in the Transit Information Option. sequence of actions is as follows:
(6LBR)
|
|
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(N11)
/ \
/ \
/ \
(N21) (N22)
/ / \
/ / \
/ / \
(N31) (N32) (N33)
: | /
: | /
: | /
(N41)
Figure 5: Sample topology 2
In Figure 5, node (N41) selects multiple preferred parents (N32) and (N33). The sequence of actions is as follows: