Internet Engineering Task Force A. Cardenas Internet-Draft Fujitsu Laboratories Intended status: Informational S. Cespedes Expires: September 9, 2011 University of Waterloo T. Iwao Fujitsu Limited March 8, 2011 Depth-First Forwarding in Unreliable Networks draft-cardenas-dff-00 Abstract Routing protocols are generally composed of two independent phases, the control plane and the data forwarding plane. The control plane is responsible for route discovery and maintenance. The data forwarding plane performs a table lookup operation to set the packet on the right path. In unreliable networks, the routing process incurs a large control overhead when is constantly repairing routes, detecting loops, and finding alternate paths due to frequent link failures. This document describes the Depth-First Forwarding (DFF) protocol; a data forwarding mechanism that can be used to minimize the burden and control overhead of a control plane used in unreliable networks. DFF offers reliability and low control overhead by supporting in the data plane loop detection, updates to the routing tables, and rerouting of data packets through alternate paths. DFF can be integrated with different types of control plane mechanisms and can be used in mesh- under and route-over specifications. In this draft, we describe a sample DFF implementation as a 6LoWPAN mesh-under data forwarding protocol. 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 http://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." Cardenas, et al. Expires September 9, 2011 [Page 1] Internet-Draft DFF March 2011 This Internet-Draft will expire on September 9, 2011. Copyright Notice Copyright (c) 2011 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 (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. 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 Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License. Cardenas, et al. Expires September 9, 2011 [Page 2] Internet-Draft DFF March 2011 Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4 1.1. Requirements notation . . . . . . . . . . . . . . . . . . 4 1.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 4 2. Protocol Overview . . . . . . . . . . . . . . . . . . . . . . 5 3. Hop-by-Hop Implementation Options . . . . . . . . . . . . . . 5 4. Depth-First Forwarding Operation . . . . . . . . . . . . . . . 6 5. Message Formats . . . . . . . . . . . . . . . . . . . . . . . 8 6. Data Structures . . . . . . . . . . . . . . . . . . . . . . . 9 7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 10 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10 9. Security Considerations . . . . . . . . . . . . . . . . . . . 10 10. Appendix A: Example Implementation of a Control Plane for DFF . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 11. Appendix B: Implementing DFF without requesting new dispatch bytes . . . . . . . . . . . . . . . . . . . . . . . . 11 12. Normative References . . . . . . . . . . . . . . . . . . . . . 12 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 12 Cardenas, et al. Expires September 9, 2011 [Page 3] Internet-Draft DFF March 2011 1. Introduction Networks with dynamic links present a challenge for typical routing protocols because the reliability of links may be different at the time when the route was discovered, and at the time when data is forwarded. In these unreliable networks, the control overhead for detecting routing errors and for fixing paths happens often, so it is important to avoid expensive control plane mechanisms that might overreact in the presense of instability. Because a lightweight control plane mechanism cannot guarantee the construction and maintenance of error- free routes, a data forwarding protocol designed for these conditions should be able to detect errors and find backup paths to survive link failures. This document describes Depth-First Forwarding (DFF), a data forwarding mechanism that can detect loops, update the routing tables, and reroute data packets via alternate paths. DFF is compatible with light-weight control plane mechanisms supporting routing tables that maintain more than one possible next hop for each final destination. 1.1. Requirements notation 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 [RFC2119]. 1.2. Terminology Readers are expected to be familiar with all the terms and concepts that are discussed in "Transmission of IPv6 Packets over IEEE 802.15.4 Networks" [RFC4944]. Other terms used: Final Destination: This is the final destination of the data packet within the mesh network. Local destination: The local destination of the data packet refers to the next-hop neighbor to which the packet is forwarded on its way to the final destination. Originator: This is the source node that created the 6lowpan data packet. Cardenas, et al. Expires September 9, 2011 [Page 4] Internet-Draft DFF March 2011 2. Protocol Overview DFF is a data forwarding strategy responsible for loop detection, choosing alternate next hops, and updating the cost metrics in the routing tables to reflect information gathered by forwarding data packets. DFF is intended to work in a network where nodes maintain proactively a routing table with multiple candidate next hops for each final destination. An example of a control plane satisfying these conditions is described in the Appendix. DFF provides an advantage in networks where the reliability of links changes rapidly. It assumes that the control plane mechanism cannot guarantee up to date routing tables, nor the absence of loops. Therefore, whenever a data packet is forwarded, DFF can keep a data packet identifier to detect loops, update routing tables if a loop is detected, and use alternate paths to reroute the packet around the failed path. DFF achieves this functionality by implementing a distributed depth- first search over the network graph as defined by the routing table. If the routing tables are up to date, the search will only involve the default route. However, if the routing table is not up to date and forwarding of a data packet results in a loop, or if the link layer fails to successfully transmit the packet to the next hop, the data packet is then sent to an alternate next hop neighbor. A distributed depth-first search mechanism is implemented in order to keep track of the nodes that have participated in the forwarding of the data packet. Although DFF can be used without a control plane by performing a blind (i.e., without a routing table) depth-first search of the network, this configuration will incurr in increased latency because data packets are forwarded by intermediate nodes to a random next-hop neighbor. Therefore, it is recommended to implement DFF in combination with a proactive control plane protocol, in order to efficiently guide the depth-first search by using information stored in the routing tables. 3. Hop-by-Hop Implementation Options While DFF can be used in a route-over or mesh-under protocol, this document provides a sample implementation of a mesh-under forwarding solution for 6LoWPAN networks; therefore, all addresses referenced in this document are either 16-bit short or EUI-64 link layer addresses. Cardenas, et al. Expires September 9, 2011 [Page 5] Internet-Draft DFF March 2011 DFF requires the use of hop-by-hop options, and this document describes how these hop-by-hop options can be implemented by allocating a new dispatch byte from the reserved values for mesh forwarding in [RFC4944]. To avoid the request of a new dispatch byte, the appendix describes a way to implement DFF by overloading the fragmentation header in [RFC4944]. This implementation has the advantage of using headers already defined by the standard; however, the implementation by overloading the fragmentation header only allows rerouting a packet on loop detection and not when a link fails. The reason behind this loss of functionality is that rerouting when a link fails requires the use of a duplicate flag in the header of the packet. This is a hop-by-hop option that can be dynamically updated by intermediate nodes, and the fragmentation header of [RFC4944] cannot be changed by intermediate nodes. Similarly, a route over implementation of DFF would need to obtain new fields in the hop-by-hop options of IPv6 packets. 4. Depth-First Forwarding Operation The operation procedure described in this section relies on the existence of a routing table in every node. This table SHOULD be filled by a proactive control plane that stores multiple candidate next hops for final destinations. An example of a proactive distance vector control plane that could be integrated to DFF is provided in the Appendix. In order for an Originator to send packets based on depth-first forwarding, it encapsulates the data packet using the standard mesh header defined in [RFC4944] and the DFF mesh header (Figure 1). The DFF mesh header is employed to detect loops and reroute packets in the forwarding path. The Originator then checks the routing table to select the next hop with the lowest cost to reach the destination. Before forwarding the packet, an entry is created in the loop detection table (Figure 2), where information such as the Originator's address, data packet identifier, previous hop (it points to the node itself when the node is the Originator of packet), and the selected next hop are stored. Upon reception of a data packet at an intermediate node (which might be the Originator if there is a loop in the path), the node checks if an entry with the same (Originator,DID) exists in the loop detection table. If there is no such entry, intermediate nodes SHOULD create a new entry in a similar way to that described for the originator of the data packet; however, in the Previous hop field, they store the Cardenas, et al. Expires September 9, 2011 [Page 6] Internet-Draft DFF March 2011 address of the router from which the packet was received. After the entry has been created in the loop detection table, the node forwards the packet to the selected candidate next hop. For those cases in which an entry already exists in the loop detection table, the node checks which one was the last attemped node, and poisons the routing table entry that uses that particular node to reach the destination. By poisoning failed paths, DFF updates the routing table based on the results from the data plane. Then, in order to reroute the data packet, the node selects a new next hop among the list of candidates stored in the routing table. The selected node MUST not be registered as a previous attempt in the list of attempted neighbours in the loop detection table. I also MUST be a different node from that registered in the Previous Hop field. In this way, DFF effectively makes data forwarding with loops a depth-first search guided by the routing table stored in each node. If the node has attempted all the candidate next hops, then the packet is returned to Previous Hop. If the Previous hop address is the same address of the current node, that means the node is the originator of the packet. If in addition, the node has already attempted all next-hop options, this means that routing has failed; therefore, the originator must drop the packet and delete the entry in the loop table. In addition to rerouting packets when a loop is detected, nodes reroute packets when the link layer fails to receive ACK from the neighbor they sent the data packet to. As soon as the link layer gives up on the transmission, DFF proceeds to reroute the packet through a different candidate. In this case, nodes set a duplicate detection flag in the DFF mesh header, identifying whether or not the packet is a potential duplicate. Duplicate packets can appear in the network when the link layer reports a failed transmission due to a failed reception of ACKs from the recipient of the packet. This situation may appear on links that are lossy only in one direction. Duplicate packets do not alter the depth-first search logic: if a packet with a duplicate flag is received by a node who has already sent a packet with the same (Originator,DID) to Next Hop n (Last Next Hop attempted), it assumes that this resulted in a loop, and the node then attempts to reroute the packet to Next Hop n+1 (if available), or to send it back to Previous Hop if no other candidate next hop are available. This, however, may be a false loop detection, therefore the node does not poison entries in the routing table whenever the forwarded packet has the duplicate flag activated. For packets encapsulated according to [RFC4944] that do not include a DFF mesh header, the DFF node processes them with a simple forwarding Cardenas, et al. Expires September 9, 2011 [Page 7] Internet-Draft DFF March 2011 mechanism that selects the next hop with the lowest cost to reach the final destination. In this case, the node does node create any entries in the loop detection table, and it does not attempt to reroute such packets through alternate paths. This forwarding option allows for the coexistence of DFF nodes with nodes that do not follow the message formats defined in this document (Figure 1). A 6lowpan mesh header [RFC4944] is still required for the operation of this basic forwarding mechanism. 5. Message Formats This document assumes that multi-hop forwarding occurs in the adaptation layer following the message format of [RFC4944]. [RFC4944] indicates that hop-by-hop processing headers with additional mesh routing capabilities may be expressed by defining additional headers that precede fragmentation or addressing headers. Hence, all data packets to be forwarded using DFF MUST be preceded by the standard mesh (L2) addressing header defined in [RFC4944], and MAY be preceded by a header that identifies the data forwarding mechanism (in this case DFF). After these two headers, other LoWPAN headers such as hop-by-hop options, header compression or fragmentation can also be included before the actual payload. (Figure 1) shows the mesh headers of a data frame to be forwarded with DFF. 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Mesh type and header +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |0 1|Mesh Forw|Flags| DID | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 1: Mesh Header for DFF data frames Field definitions are as follows: Mesh type and header: The mesh (L2) addressing header and its associated dispatch byte as defined in [RFC4944]. Mesh Forw: is a 6-bit identifier that allows for the use of different mesh forwarding mechanisms. As specified in [RFC4944], additional mesh forwarding mechanisms should use the reserved dispatch byte values following LOWPAN_BCO; therefore, 01 SHOULD precede Mesh Forw. A possible value to use as a mesh forwarding identifier based on the reserved ranges defined in [RFC4944] is Cardenas, et al. Expires September 9, 2011 [Page 8] Internet-Draft DFF March 2011 010001. In this case the dispatch byte would be 01010001. Flags: Bits in this field are used by DFF to set control flags. 0xx means that this current packet is not a duplicate, and 1xx is used to identify a potential duplicate. The last two bits are reserved to define possible new options for data forwarding. DID: This is Data Frame Identifier. It is a sequence number generated by the Originator. The originator address concatenated with the DID sequence number form an identifier of previously seen data packets. 6. Data Structures The loop detection option is based on the idea of storing the DID and originator-ID of a data packet, so that if a packet containing the same DID identifier and originator is received, DFF detects it as a loop. After the loop is detected, DFF follows a distributed depth-first search for the destination through the candidate next hops kept in the routing table. In order to do a Depth-First search, nodes need to keep a list of their children (i.e., the candidate next hops that have been used to forward the packet), and the previous hop (the node who sent the data packet for the first time to the current router). A Loop Detection Table (Figure 2) needs to be kept by the nodes to support the loop detection functionality. The candidate next hop field does not need to be pre-stored, it can be filled dynamically as soon as the node attempts to send the packet to a next-hop neighbor. Cardenas, et al. Expires September 9, 2011 [Page 9] Internet-Draft DFF March 2011 +---------+-------------------------------------------------------+ |Parameter| Description | +---------+-------------------------------------------------------+ |(O,DID) | Source Address concatenated with a sequential | | | number. Used to identify previously seen data packets | +---------+-------------------------------------------------------+ |Previous | Address of the router who sent the data packet for the| |Hop | first time to the current router. If forwarding fails,| | | return data packet to this router | +---------+-------------------------------------------------------+ |TTL | Time to live for the current DID entry | +---------+-------------------------------------------------------+ |Next Hop | First neighbor selected to forward the packet | | 1 | | +---------+-------------------------------------------------------+ | ... | ... | +---------+-------------------------------------------------------+ |Next hop | Neighbor selected the k-th time | | K | | +---------+-------------------------------------------------------+ Figure 2: Basic Elements of a Loop Detection Table 7. Acknowledgements Ganesh Venkatesh, and Geoff Mulligan provided useful discussions which helped shape this document. 8. IANA Considerations This memo includes the request of a new dispatch byte to identify DFF headers. In the Appendix there is an implementation that avoids the use of new dispatch bytes. 9. Security Considerations The security of a mesh forwarding protocol depends on the integrity, authentication, and confidentiality of the messages. The security mechanisms for protecting the network can be provided by link-layer technologies. Further details are presented in the Security Considerations section of [RFC4944]. Cardenas, et al. Expires September 9, 2011 [Page 10] Internet-Draft DFF March 2011 10. Appendix A: Example Implementation of a Control Plane for DFF There are many route discovery protocols compatible with DFF. The final selection of which control plane to use depends on the particular constraints and requirements of the network. For example, if nodes have tight memory constraints and the network is large, managing the size of the routing table is important. Therefore, a control plane that builds a network with a routing table that grows at a slower rate than the size of the network--e.g., via hierarchical routing, or clustering--is important. If minimizing the routing stretch of the network is a priority, then the control plane needs to keep larger routing tables. The only condition for a control plane in order to leverage an implementation of DFF is that, nodes should maintain a number of alternate routes, which are being advertised by multiple neighbors and which can be used immediately if the selected route were to fail, or if a loop is detected through a previous route. While a number of routing protocols satisfy the above constraint, they tend to include extra overhead for preventing loops or dealing with routing inconsistencies or failures. One of the primary goals of DFF is to avoid the use of these extra control messages. This appendix presents a basic control plane compatible with DFF. TBD. 11. Appendix B: Implementing DFF without requesting new dispatch bytes DFF can be implemented as a full standard conforming to [RFC4944] without requesting any new dispatch bytes. In this way, nodes implementing DFF can interoperate with other nodes that only implement headers defined in [RFC4944]. A possible way to avoid the DFF mesh header is by overloading the datagram_tag and datagram_offset fields of the fragmentation header defined in [RFC4944]. Because each source maintains a sequence number for the datagram_tag, and the datagram_offset can be used to differentiate between fragmented packets with the same value in datagram_tag, the DID value required by DFF can be generated by the concatenation of the datagram_tag and datagram_offset values of a fragmented data frame. Nonetheless, an implementation of DFF that avoids the request of a new dispatch byte will prevent the use of flags, and without the existence of a duplicate flag, duplicate packets will not be Cardenas, et al. Expires September 9, 2011 [Page 11] Internet-Draft DFF March 2011 detected. Therefore, it is RECOMMENDED that nodes that implement DFF by using the datagram_tag and datagram_offset fields for storing the DID value, do not reroute on link-layer ACK failures, but only on loop detections. In this case, all previously seen (Originator,DID) values can be assumed to correspond to loop detections, and the routing table cost to reach the final destination via the last attempted neighbor can be safely poisoned, without the risk of poisoning valid routes taken by duplicate packets. This implementation of DFF assumes the existence of fragmentation headers within the LoWPAN encapsulation. This works well if data packets are fragmented, but if the entire payload datagram fits within a single 802.15.4 frame, then [RFC4944] states that the LoWPAN encapsulation should not contain a fragmentation header. However, the use of a fragmentation header for a packet that does not need to be fragmented should, in principle, not affect the operation of nodes implementing [RFC4944]. Therefore, even if a packet does not need to be fragmented, the originator node can append the fragmentation header so DFF nodes can use it for extracting the DID identifier. The control plane used to populate the routing tables can also avoid the need to request a new dispatch byte by encapsulating routing updates in UDP packets. 12. Normative References [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC4944] Montenegro, G., Kushalnagar, N., Hui, J., and D. Culler, "Transmission of IPv6 Packets over IEEE 802.15.4 Networks", RFC 4944, September 2007. Authors' Addresses Alvaro A. Cardenas Fujitsu Laboratories 1240 E. Arques Avenue, M/S 345 Sunnyvale, CA 94085 US Phone: +1 408 530-4516 Email: alvaro.cardenas-mora@us.fujitsu.com Cardenas, et al. Expires September 9, 2011 [Page 12] Internet-Draft DFF March 2011 Sandra Cespedes University of Waterloo 200 University Ave. W. Waterloo, ON N2L 3G1 Canada Phone: +1 (519) 8884567 x37448 Email: slcesped@bbcr.uwaterloo.ca Tadashige Iwao Fujitsu Limited Fujitsu Kyushu R and D Center, 2-1, Momochihama 2-chome, Sawara-ku. Fukuoka, JP Phone: +81-92-821-8030 Email: smartnetpro-iwao_std@ml.css.fujitsu.com Cardenas, et al. Expires September 9, 2011 [Page 13]