| ICN Research Group | C. Gundogan |
| Internet-Draft | T. Schmidt |
| Intended status: Experimental | HAW Hamburg |
| Expires: September 14, 2017 | M. Waehlisch |
| link-lab & FU Berlin | |
| March 13, 2017 |
Publish-Subscribe Deployment Option for NDN in the Constrained Internet of Things
draft-gundogan-icnrg-pub-iot-00
Constrained IoT devices often operate more efficiently in a loosely coupled environment without maintaining end-to-end connectivity between nodes. Information Centric Networking naturally supports this demand by replicated data distribution and hop wise forwarding. This document outlines a deployment option for NDN in low-power and lossy networks (LLNs) that follows a publish-subscribe pattern. The proposed protocol scheme simplifies name-based routing significantly and facilitates even large off-duty cycles for constrained nodes.
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In the emerging Internet of Things (IoT), it is expected that large quantities of very constrained sensors and actuators collect, communicate, and process massive amounts of machine data. Early experiments with constrained nodes show promising results for different deployments of ICN communication [NDN-EXP].
Characteristics of constrained nodes:
Challenges of NDN deployment [RFC7927]
Multiple scenarios have been discussed in [RFC7476] and [IWMT] that evaluate the applicability of ICN in IoT.
We consider two characteristic constrained IoT scenarios with the enumerated challenges:
IoT scenarios usually impose routing requirements to support mobile nodes, handle failing links and to be resilient against attacks. A secure and autonomous bootstrapping is essential, especially for large-scale IoT deployments.
ICN decouples content consumers from data producers (decoupling in space). A more sophisticated decoupling can be provided with the publish-subscribe messaging pattern that further adds a decoupling in time and synchronization. Constrained devices in LLNs can leverage this loose coupling to increase sleep cycles and delegate the authority over as much information as possible to more powerful devices that act as content proxies. In Figure 1, once content is published to the content proxy (CP) by a producer (P), consumers (C) can retrieve this content from (CP) without interacting with the producer. This indirection when retrieving information allows (P) to align sleep cycles accordingly to the period of generating new sensor readings, instead of handling content requests from any consumers (C).
(CP)
/ | \
/ | `-----.
/ | | | \
(P) (C) ...... (C)
Figure 1: Content Proxy (CP) - Producer (P) - Consumer (C)
TODO: The problem of PUSH
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 [RFC2119]. The use of the term, "silently ignore" is not defined in RFC 2119. However, the term is used in this document and can be similarly construed.
This document uses the terminology of [RFC7476], [RFC7927], and [RFC7945] for ICN entities.
The following terms are used in the document and defined as follows:
The publish-subscribe system is centered around prefix-specific content proxies (CPs) that are deployed in IoT edge networks. Such proxy function can be hosted on the Cloud- or Internet Gateway, or may reside on a stable, less constrained node within the IoT infrastructure. It is assumed that a CP is present for each prefix covering publishable content.
Implementing a pub-sub NDN involves several steps that are bound to the tight requirements of resource-constrained devices. These steps include:
A (sensor) node that wants to publish a data item needs to rely on path information towards the Content Proxy. Following the approach of PANINI [PANINI], default routes will be established as follows.
Each CP in the local IoT sub-network advertises the prefix(es) it represents to the routing system. It does so by broadcasting Prefix Advertisement Messages (PAMs) on the link layer (see Section 4 for the corresponding protocol details). Nodes that newly receive PAM advertisements will add or refresh a prefix-specific default route in their FIB. Intermediate nodes in a multi-hop environment also re-broadcast PAMs, so that the entire sub-network is flooded and default route entries build a shortest path tree (SPT) towards the CP as shown in Table 1 (alternatively, a DODAG w.r.t. a gateway for redundant CPs).
(CP)
PAM / \
/ \ PAM
(A) (B)
/|\ /|\
: : : : : : PAM
Figure 2: SPT building by Prefix Advertisement Messages (PAMs)
Information flowing from constrained sensor nodes towards a gateway is the prevalent communication pattern in the IoT (converge cast). The publish-subscribe system hence establishes a default routing (see sample FIB in Table 1) and uses the tree (DODAG) topology with default routes towards the CP as a first step of content aggregation. Content replication towards other CPs, an Internet gateway, or into a cloud can follow subsequently.
| Prefix | Face | Lifetime |
|---|---|---|
| / | Fx | Ft |
| ... | ... | ... |
It is noteworthy that the role of the new PAM message remains orthogonal to the existing Interest or Data semantics. A PAM never carries data nor requests, but persists on the control plane of name-based routing. User applications stay unaffected, and continue to rely on the NDN-specific request-response paradigm.
In classical publish-subscribe systems, a Publish is typically implemented as a push mechanism on the data plane. However, this contradicts the request-response paradigm employed by NDN. To adapt the Publish operation to NDN semantics, it is split into two phases and the required push mechanism is moved into the control plane. The two phases consist of:
The first phase is the actual announcement of names in the upwards direction towards the CP. Because of NDN's name-based routing approach, the announcement of names is subject of the routing protocol and therefore belongs to the control plane. For this purpose, the control message type Name Advertisement Message (NAM) is adapted from PANINI [PANINI]. Similarly to the PAM, a NAM message utilizes a push mechanism in the control plane without interfering with the request-response mechanism on the data plane. NAMs are directed towards the (prefix-specific) parent of a node and traverse hop-by-hop along the gradient towards the CP. Each intermediate hop on the gradient installs forwarding states in the downward direction by using the announced names in the NAM and the incoming face. Typically, states are short-lived for content replication, only. NAMs contain one or multiple names encoded as TLV elements in the payload.
Figure 3 (a) depicts the propagation of the NAM towards the (CP). In this example, the name /HAW/temp123 is announced by (C) via (A) to (CP).
+-----------------------------------------------------------------+
| |
| +----------------------+ +-------------------------------+ |
| | | | | |
| | Mt: /HAW/temp123 | | Mt: /HAW/temp123 = 23C | |
| | ,->(CP) | | ,---(CP)<--, | |
| | NAM | / | | | / | | |
| | | / | | | / ,-' | |
| | (A)<-, | | Interest | (A) | Data(23C) | |
| | \ | NAM | | | \ `-, | |
| | \ | | | | \ | | |
| | (C) | | `--->(C)---' | |
| | /HAW/temp123 | | /HAW/temp123 = 23C | |
| | | | | |
| +----------------------+ +-------------------------------+ |
| (a) Phase 1 (b) Phase 2 |
+-----------------------------------------------------------------+
Figure 3: Publish
In addition to a FIB, the (CP) maintains another data structure Mt (Meta-Table) to store all announced names annotated with additional context information and a lifetime. Upon receipt of a NAM, the Mt is updated accordingly. Context information consists of generic properties attached to a name (e.g. topic names and content freshness indicators) and are out of scope of this document. The Mt has its own name and can be requested by other devices.
In the second phase, the (CP) requests the content of newly learned names from the first phase. For content requests, the regular NDN Interest-Data exchange on the data plane is used and is depicted in Figure 3 (b). Upon receipt, the content is cached on the (CP).
In the proposed publish-subscribe system, the Subscribe operation is equivalent to an Interest-based request of previously learned content names. A device can learn about new content by (a) Name Advertisements of the CP via dedicated prefix path (TODO) or broadcast. It may as well poll the Mt in order to learn about general updates at the CP. Context information in the Mt may give indications about periodic sensor readings, so that a periodic polling of the Mt can be aligned with the sensor reading period.
+--------------------------------------------------------+
| |
| +----------------------+ +----------------------+ |
| | | | | |
| | Data (Mt) | | Data (Nx) | |
| | ,-------->(S) | | ,-------->(S) | |
| | (CP)<--------' | | (CP)<--------' | |
| | / \ Int. (Mt) | | / \ Int. (Nx) | |
| | / \ | | / \ | |
| | (A) (B) | | (A) (B) | |
| | /|\ /|\ | | /|\ /|\ | |
| | : : : : : : | | : : : : : : | |
| | | | | |
| +----------------------+ +----------------------+ |
| (a) Request Mt (b) Request Content |
+--------------------------------------------------------+
Figure 4: Subscribe
In Figure 4 (a), a subscriber (S) requests the Mt to learn about new names. A new name (Nx) is then requested via the regular Interest/Data request-response paradigm in Figure 4 (b).
The majority of constrained devices at the IoT edge are mostly content producers, but not consumers. Subscribers do not necessarily need to be part of the distribution tree, but may reach the gateway (CP) by other means.
TODO
TODO
TODO
| [RFC2119] | Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997. |
This work was stimulated by fruitful discussions ... We would like to thank all active members for constructive thoughts and feedback. In particular, the authors would like to thank (in alphabetical order) Emmanuel Baccelli, Michael Frey, Oliver Hahm, Peter Kietzmann, Dirk Kutscher, Martine Lenders, Joerg Ott, Hauke Petersen, and Felix Shzu-Juraschek. This work was partly funded by the German Federal Ministry of Education and Research, project I3.