Abstract â Flat Routing is subject of many proposals found in the literature. The majority of these proposals considers an underlay network providing ...
Flat Routing on a Binary Identity Space Rafael Pasquini, Maurício Ferreira Magalhães (Adviser) Department of Computer Engineering and Industrial Automation (DCA) School of Electrical and Computer Engineering (FEEC) State University of Campinas (Unicamp) C. P. 6.101 – 13.083-970 – Campinas – SP – Brazil {pasquini,mauricio}@dca.fee.unicamp.br Abstract – Flat Routing is subject of many proposals found in the literature. The majority of these proposals considers an underlay network providing communication between neighbors at the flat identity layer. Recently, investigations on Flat Routing with no underlay network have being conducted. These proposals perform routing directly on flat identifiers/names. Our work is aligned with these proposals due to the perspective of a new internetworking model in which the network layer has no information related to location. The focus of this paper is to briefly present a Flat Routing mechanism with no underlay network that has being developed in the context of this doctoral research. This Flat Routing proposal is based on tree main approaches: Landmarks, XOR Routing mechanisms and Bloom filters. Keywords –
Bloom filter, Flat Routing, Landmark, XOR Routing.
1. Introduction For a long time, routing has being defined as the process in which the packets are forwarded from the source to the destination node using information related to the nodes’ location. In the TCP/IP model, this process is performed at the network layer which is responsible for the management of all information related to nodes’ location. This approach is also present in the majority of the Flat Routing proposals found in the literature. Although these proposals consider the usage of flat identifiers for node identification purposes, they require a mapping from the flat identifier to the respective node locator for packet forwarding. More recently, routing has being researched in a scenario in which the network layer does not contain any location information to drive the packet forwarding. This investigative scenario is related to routing directly on flat identifiers/names and this doctoral research is in agreement with this approach. Examples of proposals in this scenario include Identity Based Routing (IBR) [5], Virtual Ring Routing (VRR) [3] and Routing on Flat Labels (ROFL) [4]. Some of the benefits introduced by this approach include: 1) native support for mobility and multi-homing; 2) simpler allocation of identifiers by requiring only uniqueness (the IP requires uniqueness and topological adherence); 3) no need for new mapping services since data is forwarded based on the flat identity (no need for identifier to locator mapping) and 4) better support for network access
controls which can be applied on the identifier. This entire flat scenario seems to be the ideal case by not requiring mapping functions but, at the same time, it introduces some challenges for dynamic scenarios in which mobility, multi-homing and failures are considered. In this line, our Flat Routing proposal is introduce with the concepts of Landmarks [10] and Bloom filters [2]. The employment of Landmark nodes is a first mechanism to circumvent the management difficulty imposed by the flat identity space. The Landmark nodes represent regions in which identifiers are grouped. To sum to the Landmark, Bloom filters are used in the inter-region routing decision. A Bloom filter is a simple space-efficient data structure that represents a set of elements and supports queries at the cost of false positive occurrences. We contribute in this work by briefly presenting our approach for the ongoing discussions around routing directly on flat labels. We combine ideas from the literature to construct routing tables based on XOR operations. This approach provides the fundamental basis for routing table growth control in the intra-region routing scenario. At the same time, the notion of Landmarks with Bloom filters is used to address the scalability issues for the interregion routing scenario. The remainder of this paper is organized as follows. Section 2 presents the related work. Section 3 briefly introduces our Flat Routing proposal. Section 4 brings the next steps and concludes the paper.
2. Related Work One pioneer related work is the IBR [5] which is organized in three main phases: 1) Routing on an abstract graph; 2) Flat (ad hoc) wireless network routing and 3) Flat Internet scale routing. In the first phase, IBR organizes nodes into a logical ring and a node’s position is determined by its identifier. Routing packets is a question of making progress along the ring through the usage of pointers to adjacent nodes. In the second phase (denominated VRR), the IBR was extended to work in an ad hoc wireless scenario. Such a scenario allows the investigation of the protocol under certain failure modes and resource constraints. In the third phase, the IBR was extended to work in the Internet scenario. Named ROFL, it uses a single identity space for naming hosts and introduces the concept of stable/ephemeral hosts tracked by their hosting routers. These hosting routers act on behalf of nodes while discovering successors, predecessors and transmitting data. The successor and predecessor requirements in this virtual ring scenario is a strong condition and, originally, none proximity relation is contemplated. In the VRR and ROFL scenarios, a cache mechanism was introduced aimed at reducing the stretch and considering proximity. Under this scenario, if a node does not have a successor or predecessor, the ring is broken and needs to be merged. Another related work is the Unmanaged Internet Protocol (UIP) [6]. It employs routing tables divided into columns (called buckets) and, based on XOR operations, it inserts neighbors in these buckets taking into account the number of bits returned from these XOR operations. UIP has a connectivity invariant that requires at least one neighbor in each bucket to assure traffic delivery. Regarding the layer-3 connectivity, the UIP adopts a hybrid approach. In this scenario, as always as possible, the UIP uses the connectivity provided by any underlay network. Conversely, in case of connectivity gaps left by addressbased protocols such as IP (for example, in case of NAT and/or firewalls discontinuities) the UIP circumvents these gaps at the identity layer. We consider the connectivity invariant requirement of UIP a strong requirement because, under this scenario, neighbors can be searched in the
complete network. Another restriction is related to the usage of the identity space. If it is completely allocated, it is certain that all nodes will fulfill their buckets. However, if the identity space is not completely allocated, nodes can search for non-existent neighbors on the whole network and, worst, never fulfill their routing table gaps. However, the UIP infrastructure seems to be more interesting for mesh networks instead of the virtual ring scenario of VRR and ROFL due to the possibility of establishing relations, at the identity layer, with physically near neighbors and not only with the predecessor and successor nodes. These relations bring the notion of topology to the (nontopological by nature) identity space. In order to prioritize the physically near neighbors, we relax the UIP connectivity invariant and, for our scenario, only neighbors topologically near are inserted in the routing tables, allowing the existence of empty buckets (gaps), contemplating proximity and simplifying the identity space usage.
3. Flat Routing Proposal This work totally eliminates the notion of location and prefix aggregation as well as the existence of an underlying routing infrastructure and a mapping service. The routing is uniquely done on the identifiers themselves by using three main approaches: Landmarks, XOR routing mechanisms and Bloom filters. The landmark is introduced to create the notion of a region and alleviate the overhead for fulfilling the routing tables. When a given node is not able to fulfill a bucket in its routing table, the landmark is used to act as a “default router” to satisfy the gap. The XOR routing mechanism encompasses the group of functions used to construct the routing tables and to route packets in the binary identity space. In our proposal, the XOR routing mechanism is used in the intra-region routing scenario. It has similarities with the UIP in what regards the routing tables’ format that are divided in buckets. However, our approach differs from the model found in UIP with respect to the underlay routing infrastructure since it does not consider the existence of any underlay network at all. This difference has impact on the neighborhood discovery process, in the number of entries present in the routing tables and leads to a new phase in the discovery process that we denom-
inated as being the Learning Process. Finally, the Bloom filter approach is the key mechanism for dealing with the scalability in our inter-region routing scenario. The main point behind the Bloom filters is the capability of offering a certain degree of aggregation to flat identifiers which by nature do not have any structured location and aggregation information associated to them. At the same time, while in TCP/IP routing an IP address gives the exact notion of where to find the destination, the Bloom filters give to our routing mechanism a routing hint on where a given destination is not located. This approach minimizes the routing stretch when compared to the flat routing proposals found today in the literature. The purpose of this section is to briefly introduce the three mechanisms mentioned above. Details about these mechanisms and the routing protocol developed in the context of our work are found in [8, 9]. It is also possible to find in the previous references extensive evaluations of our proposal under two different network scenarios, a regular mesh (ad hoc like) and a Power-law (Internet like) topology.
ble. In this case, the “default router” function of the landmark is used and the packet is forwarded towards the landmark to start the inter-region routing process based on Bloom filters.
3.2. XOR Routing Mechanism The second mechanism used in our proposal is the XOR routing mechanism. It is responsible for the routing tables’ construction and, as a consequence, for the intra-region routing that is based on the binary flat identity space. In Fig. 1 it is possible to verify an example of a routing table for node zero in an identity space of 4 bits. The figure shows the XOR operations necessary to introduce nodes 1, 3, 5, 10 and 13 in the routing table of node zero. The XOR operations find the bits in common between node zero and the other nodes. Depending on the quantity of bits in common, the nodes are inserted in each specific bucket.
Figure 1. Routing table for node 0 - Buckets and XOR operations.
3.1. Landmarks The first mechanism used in our proposal is the Landmark. The definition of Landmark found in [10] is extended in this work to create our Landmark region context as follows: “A Landmark is a node in the network whose neighbor nodes within a certain number of hops not only contain routing entries for that node but also are registered on it to constitute a landmark region”. Based on the definition above, in this work the landmark has two main functions: 1) to delimit regions in the network graph which are represented by the landmark ID and 2) to work as a “default router” inside a region for the inter-region communication cases. Both functions have impact on the routing tables that are based on the XOR routing mechanism. For the communication cases in which the nodes are located at different landmark regions, the packet will be forwarded inside the region using the XOR routing mechanism until it reaches a node with a gap in the correspondent bucket of the routing ta-
As our proposal does not consider any underlay network providing direct communication between nodes, it is only possible to provide communication at the flat identity layer. This proposal adopts a network centric approach in which the packet header needs only to carry one information regarding the source and destination node identifiers (fixed packet header size). The network centric approach is possible because during the neighborhood discovery process, the nodes perform two functions. The first function is to search for nodes aimed at filling their empty buckets. The second function is to learn information about nodes inside the region using the signaling messages that crosses the nodes during the discovery process. This learned information provides the basis for the communication between nodes without the usage of an underlay network.
3.3. Bloom filters After the execution of all XOR routing process, the nodes inside a landmark region have constructed their routing tables and are ready to perform the intra-region routing. The next step of our proposal
is to build the necessary infrastructure for the interregion routing. This process is mainly based on the usage of Bloom filters at the Landmark nodes. After the discovery process, each node sends a registry message to its Landmark containing its identifier. This information permits the Landmark to create a Bloom filter representing the nodes inside its region. This Bloom filter is diffused in the inter-region routing among the other Landmarks. The Bloom filter is an array of m bits, initially all set to 0. The size m of the array and the number k of independent hash functions are defined according to the number of expected elements to be inserted in the Bloom filter and the acceptable false positive rate. The elements in our proposal are all the identifiers contained inside a landmark region. To check if an item y is inserted in the Bloom filter, all the hi (y) positions are verified to see if they are set to 1. If not, y is not a member of the Bloom filter. On the other side, if all hi (y) are set to 1, y is present in the Bloom filter with some probability of being a false positive. After the creation of the Bloom filters, the landmarks present in the graph send their information to the others. After this process, the landmarks have a Landmark Information Base (LIB) in which the Bloom filters are stored. This information is used in the inter-region routing mechanism and helps to locate in which region a given identifier can be present through the precise information (no false negative occurrence) of which regions the identifier is, in fact, not present.
4. Conclusions This paper briefly introduces our Flat Routing proposal being developed in the context of a PhD thesis. In a certain way, instead of pointing a unique flat routing mechanism as the better solution, the objective of our work is to evaluate our protocol in a variety of scenarios and leave the decision of what is interesting or not to whom will deploy such mechanism. We consider that Landmarks and Bloom filters have an important contribution in this scenario. The dissemination of Bloom filters between the Landmarks opens up the possibility of knowing in which regions a certain destination is not present. As defined in NeTS-FIND Project [7], rout-
ing can be seen as the core element of any network architecture, but also the function with the greatest scaling problems. The project identified the biggest obstacle on the path to scalable routing: topology updates. Currently, to route efficiently, nodes must know where destinations are (global information) and, on dynamic scenarios, the routing algorithm must disseminate updates upon topology changes, requiring a not scalable amount of updates (churn) on realistic topologies. In our proposal, gaps are allowed in the XOR routing tables aimed at contemplating topological proximity. We consider that this topological proximity offers mechanisms to control churn and creates a scenario in which nodes do not need global information to route. In this context, the next step of our work is the formalization of our flat routing proposal according to the concepts found in the NeTS-FIND Project [7, 1].
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