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NSDI '06 Paper    [NSDI '06 Technical Program]

OCALA: An Architecture for Supporting Legacy Applications over Overlays

Dilip Joseph*   Jayanth Kannan*   Ayumu Kubota+   Karthik Lakshminarayanan*   Ion Stoica*   Klaus Wehrle^
*University of California at Berkeley       +KDDI Labs      ^RWTH Aachen University

In order for overlays and new network architectures to gain real user acceptance, users should be able to leverage overlay functionality without any modifications to their applications and operating systems. We present our design, implementation, and experience with OCALA, an overlay convergence architecture that achieves this goal. OCALA interposes an overlay convergence layer below the transport layer. This layer is composed of an overlay independent sub-layer that interfaces with legacy applications, and an overlay dependent sub-layer that delivers packets to the overlay. Unlike previous efforts, OCALA enables: (a) simultaneous access to multiple overlays (b) communication between hosts in different overlays (c) communication between overlay hosts and legacy hosts (d) extensibility, allowing researchers to incorporate their overlays into OCALA. We currently support five overlays, i3 [32], RON [1], HIP [19], DOA [39] and OverDoSe [31] on Linux, Windows XP/2000 and Mac OS X. We (and a few other research groups and end-users) have used OCALA for over a year with many legacy applications ranging from web browsers to remote desktop applications.

1  Introduction

Over the past two decades, researchers have proposed a plethora of solutions to extend the Internet's functionality, and to improve its resilience and security. After sustained efforts to add new functions such as mobility [25] and multicast [5] to IP, researchers have recently turned their attention to developing new network architectures (e.g., [1, 32, 35, 3, 19, 4, 29]) and using overlays to address the Internet's limitations.1 This trend has been fueled by the difficulty of changing IP, on one hand, and by the advent of the PlanetLab[26] testbed and the recent NSF GENI [23] initiative—which promises to create a worldwide testbed for evaluating new network architectures—on the other hand.

In order to evaluate the feasibility of these proposals and to ultimately bring them closer to reality, it is important to experiment with real users running real applications. Ideally, users should be able to opt into new experimental architectures without any changes to their legacy applications. (We use the term legacy applications to refer to existing applications like web browsers that assume IP semantics.) Supporting legacy applications on new network architectures is inherently a difficult task: legacy applications assume traditional semantics of IP addresses and DNS names, while a new network architecture may offer a substantially different interface. Existing solutions are in general tailored to a particular network architecture [1, 43, 19, 34], leading to duplication of effort across different implementations.

In this paper, we describe the design and implementation of our solution, OCALA (Overlay Convergence Architecture for Legacy Applications), that enables legacy applications to take advantage of the functionality provided by new network architectures. OCALA differs from existing solutions in that it enables (1) applications running on the same machine to access different overlays simultaneously, (2) stitching of multiple overlays so that users residing in different overlays can communicate with each other, (3) hosts to communicate through an overlay even if the other end-point understands only IP, and (4) extensibility so that a new overlay can be incorporated into OCALA with minimal effort.

In a nutshell, OCALA re-factors the protocol stack by imposing an Overlay Convergence (OC) layer. The OC layer is positioned below the transport layer in the IP stack. It is decomposed into the overlay-independent (OC-I) sub-layer, which interacts with the legacy applications by presenting an IP-like interface, and the overlay-dependent (OC-D) sub-layer, which tunnels the traffic of applications over overlays.

The main contributions of this paper are an overlay agnostic architecture for supporting legacy applications and an extensible implementation of this architecture as a proxy. Our implementation of OCALA as a proxy requires no changes to applications or operating systems.

In realizing our design, we borrow many techniques and protocols from the literature, such as address virtualization [12, 34, 36, 42, 43, 19], DNS capture and rewriting [42, 22, 9, 28], and SSL [10]. We have implemented the OC-D sub-layer for i3 and RON. In addition, OC-D modules for HIP [14], DOA [39] and OverDoSe [31] have been implemented by other research groups. To illustrate the utility of OCALA, we have provided services such as intrusion-detection, secure wireless access, secure Intranet access, and Network Address Translation (NAT) box traversal, to legacy applications.

OCALA does not come without limitations. The fact that OCALA is positioned below the transport layer makes it hard, if not impossible, for legacy applications to take advantage of network architectures that provide transport or application layer functionalities, such as multi-path congestion control and data storage [17].

2  Related Work

Supporting legacy applications over non-IP or IP-modified communication infrastructures has been addressed in many contexts. Examples include overlay networks and new network architectures (e.g., RON [1], i3 [32], HIP [19], DOA [39], WRAP [2], end-host support for mobility [34, 36, 42]), and mechanisms that enable end-hosts to use overlays without participating in them [21]. In contrast to these overlay-specific efforts, OCALA enables a user to simultaneously access different overlays and to communicate with hosts residing in overlays the user is not directly connected to.

A recent system, Oasis [18], enables legacy applications to route traffic through different overlays, and has a design similar to that of OCALA. However, Oasis's current implementation supports only overlays that are IP-addressable, and does not allow stitching together multiple overlays. In contrast, OCALA supports overlays that address hosts using a variety of identifiers and naming schemes (e.g., i3, DOA), and allows hosts on different overlays to communicate with each other. Oasis optimizes application performance by automatically selecting the “best” overlay. Furthermore, Oasis supports sandboxed code execution, a direct result of its Java-based implementation. The current implementation of OCALA does not support any of these features.

Our goal of stitching together multiple network architectures resembles the goal of AVES [22], TRIAD [3], UIP [7], IPNL [8], Plutarch [4], and IPv4/IPv6 transition schemes like [11]. In contrast to these proposals which provide universal connectivity, OCALA's focus is on exposing to users, functions that new architectures provide, both in isolation and when stitched together.

Layering is a widely-used principle in networking. Many architectures (e.g., HIP [19], WRAP [2]) hide the details of underlying layers by interposing a shim layer between the transport and network layers. More recently, Henderson generalized the HIP setup protocol to support other architectures that decouple location and identity of hosts [13]. OCALA's OC layer is similar to such a shim layer. OCALA is different from other architectures in that it explicitly splits the OC layer into an overlay independent sub-layer and an overlay dependent sub-layer, which respectively act as traditional network and link layers. This division enables OCALA to provide simultaneous access to and inter-operability across different network architectures.

For implementing OCALA, we rely on techniques and protocols previously proposed in different contexts. Intercepting DNS requests for interposing proxies on the data path has been used in AVES [22], Coral [9], and for improving web browsing performance over wireless networks [28]. Local-scope addresses have been utilized in the context of supporting mobility [34, 36, 42, 19], redirection [12], process migration [34, 33] and server availability [33]. Our address-negotiation protocol is similar to that in Yalagandula et. al. [42], while the OC-I sub-layer's security protocol is a generalization of SSL [10].

3  Design Overview

We focus on network architectures and overlays that offer a service model of end-to-end packet delivery similar to IP's, as opposed to those that provide transport- or application-layer functions, such as data storage (e.g., Oceanstore  [17]). Some examples are overlays that improve Internet's resilience and performance (e.g., RON [1], Detour [29], OverQoS [35]), overlays that provide new functions (e.g., mobility [43, 42]), overlays that bridge multiple address spaces [2, 22], as well as recent architectures such as i3 [32], HIP [19], and DOA [39]. Although not all architectures are realized as overlays, for convenience, in the remainder of this paper, we will use the term overlay to also refer to network architectures that are implemented as overlays.

Each end-host E in an overlay has an overlay-specific identifier (ID), which is used by other end-hosts to contact E through the overlay. While in the simplest case an overlay ID can be the host's IP address (e.g., RON), many overlays use other forms of identifiers (e.g., i3 and DOA use flat IDs, HIP uses hashes of public keys). Since overlay IDs may not be human-readable, end-hosts may also be assigned human-readable names for convenience.

3.1   Goals

Our design is centered around four main goals:
  • Transparency: Legacy applications should not break despite the fact that their traffic is relayed over an overlay instead of over IP.
  • Inter-operability: Hosts in different overlays should be able to communicate with one another, and further, hosts that do not participate in any overlay should also be accessible through overlays.
  • Expose overlay functionality: Users should have control in choosing the overlay used to send their traffic, and should be able to leverage the overlay functions despite using overlay-unaware (legacy) applications.
  • Factor out common functions: Instead of relying on the security provided by overlays, the architecture should provide basic security features such as host authentication and encryption.
Figure 1: The overlay convergence (OC) layer.

3.2   Overlay Convergence Layer

OCALA interposes a layer, called the overlay convergence (OC) layer, between the transport and the network layers (see Figure 1). The OC layer replaces the IP layer in the Internet's stack, and consists of two sub-layers: an overlay independent (OC-I) sub-layer, and an overlay dependent (OC-D) sub-layer.
The main functions of the OC-I sub-layer are to present a consistent IP-like interface to legacy applications and to multiplex/demultiplex traffic between applications and various overlays. In addition, the OC-I sub-layer provides common functions, such as authentication and encryption, that are useful across overlays.

The OC-D sub-layer consists of modules for various overlays, which are responsible for setting up overlay-specific state and for sending/receiving packets to/from the particular overlay. For example, the i3 OC-D module maintains private triggers at both end-points, while the OverQoS module performs resource reservation. Note that IP can be viewed as a “default” overlay module.
Figure 2: Three applications on host (A) which establish connections via IP and two overlays: RON and i3.

Figure 2 shows an example in which three applications on host A open connections over IP and two overlays: a web browser (Firefox) uses IP to connect to a CNN server, a chat client (IRC) uses i3 to preserve its anonymity, and ssh uses RON for improved resilience. The design also enables hosts in different overlays to communicate with each other. Figure 3 shows how two hosts on different overlays can communicate using a gateway host (B) that is connected to both overlays.

We refer to the end-to-end communication channel between two end-hosts at the OC-I sub-layer as a path, and to the communication channel between two end-hosts at the OC-D sub-layer as a tunnel. In Figure 3 the path between the two end-hosts is (A, B, C), and consists of two tunnels, (A, B) and (B, C).

3.3  Layering in OCALA: Discussion

The services implemented by the OC-I and OC-D sub-layers on the data plane are analogous to the services provided by the network and data-link layers in the OSI protocol stack respectively. Like the data-link layer which provides communication between any two nodes in the same link layer domain, OC-D provides communication between any two nodes in the same overlay. Similarly, like the network layer which provides communication across different link layer domains, the OC-I sub-layer provides communication across different overlays.

However, OCALA does not enforce strict layering within its sub-layers. Unlike traditional layering, where a layer uses only the services provided by the layer below, OCALA allows legacy applications to access the services provided by the OC-D sub-layer, by passing overlay-specific names or IDs to OC-D through the OC-I sub-layer. These names are resolved at the OC-D sub-layer, and their semantics is opaque to the OC-I sub-layer. This allows us to achieve the main goal of OCALA—enable legacy applications to take advantage of the functions provided by overlays—while keeping the OC-I sub-layer agnostic of the overlays.

4  Detailed Architecture

We present a goal-driven description of OCALA, by showing how our design achieves the four goals we laid out in  3.1. Achieving these design goals is challenging as they have conflicting requirements. For instance, on one hand, we want to expose the rich functionality provided by overlays to users, while on the other, we have to preserve the narrow IP interface exposed to the legacy applications. Our design aims to find a sweet spot in achieving these opposing goals.
Figure 3: Bridging multiple overlays.

4.1  Goal 1: Achieving Transparency

Our main goal is to ensure that legacy applications are oblivious to the existence of overlays. Ideally, applications should work without any changes or re-configuration when IP is replaced by the OC layer. Our design is fundamentally constrained by how a legacy application interacts with the external world. Most legacy applications make a DNS request, and then send/receive IP packets to/from the IP address returned in the DNS reply. Thus, legacy applications identify Internet hosts using names and IP addresses, where names are resolved using DNS to IP addresses.

We now describe and justify the following design decisions regarding names and IP addresses exposed to the legacy application:
  • Overlay hosts are identified primarily using names. These names are resolved using overlay-specific resolution protocols. Each overlay can implement its resolution protocol, which may differ from a DNS lookup.

  • The IP address returned to the application by the resolution protocol has only local meaning. This address serves as an OC-I handle to retrieve state corresponding to the remote host. Similarly, a tunnel descriptor is used by the OC-D sub-layer to maintain hop-by-hop state, and a path descriptor is used at the OC-I sub-layer to maintain end-to-end state.

4.1.1    Overlay Names

Users can exercise control over the overlay used for delivering their traffic by using: (a) fields in the IP headers, e.g., IP addresses, port numbers, or (b) DNS-like names.

In the first approach, a user can specify rules on how packets should be processed using fields in the IP header. For example, the user can specify that packets sent to address and port 80 should be forwarded through RON, while packets sent to 207.188.7.x should be forwarded through OverQoS.

In the second approach, users can encode the overlay to be used in the DNS names. We refer to the unique name associated with each overlay host as its overlay name. An overlay name is of the form foo.ov, where ov specifies the overlay, and foo is a name unique to that overlay. On receiving a DNS request for an overlay name, the OC layer sets up state which allows it to intercept and forward all the subsequent packets from the application to host foo.ov through overlay ov.

The main advantage in relying solely on the information in the IP headers is that it works with all Internet applications, since at the very least, an application sends and receives IP packets. On the other hand, using overlay names has several advantages. First, overlay names can be used to identify hosts (for example, NATed hosts) without routable IP addresses. This property is fundamental to overlays that bridge multiple address spaces [3, 22]. Second, names are human-readable and hence easier to use. Third, the user does not need to know the IP address of the destination in advance, which is not feasible in some cases. Indeed, when an overlay provides support for content replication, the IP address of the server that ultimately serves the content may not be known to the users.

Our implementation chooses DNS-like names as the primary method for overlay selection. For supporting applications that do not make DNS requests, we also support the use of IP header fields for overlay selection.
Figure 4: Path setup protocol.

4.1.2    Overlay-specific Resolution

Our second design decision is to resolve overlay names using overlay-specific mechanisms. A name of the form foo.ov, is resolved by the OC-D module for overlay ov. This design choice has two main advantages over DNS-based resolution. First, this allows multiple namespaces to co-exist with each other and with the DNS namespace, thus enabling a fully extensible namespace. Each overlay is allowed to implement its name allocation and resolution, without requiring a global infrastructure. Second, this allows OCALA to support network architectures that do not assume global IP address allocation. Examples include MetaNet [41] and IPNL [8] wherein names are the only way to refer to hosts. Other examples include architectures that leverage name resolution to implement different functions (e.g., DoA [39]).

In the remainder of the section, we describe how the control plane and data plane operations of OCALA transparently set up an end-to-end path and tunnel the legacy applications' data across the overlay.

4.1.3    Control Plane: End-to-End Path Setup

A new connection setup is triggered by the receipt of a DNS request for a previously unseen overlay destination or the receipt of the first data packet of a connection configured to use a particular overlay. The final result of these operations is establishing an end-to-end path at the OC-I sub-layer and setting up the state required to handle the application's traffic. While a path could consist of several tunnels at the OC-D sub-layer, in this section we consider a single-tunnel path. We generalize the description to multi-tunnel paths in  4.2.

Consider a legacy application on host A that wants to communicate with a remote legacy application at host B, called foo.ov (see Figure 4). The application first issues a DNS request for foo.ov, which is intercepted by the OC-I sub-layer. On receiving such a request, the OC-I sub-layer associates a globally unique path descriptor, pdAB, and remembers the mapping between the name and the descriptor (foo.ovpdAB) in order to service future requests for foo.ov. We minimize collisions by randomly choosing the path descriptor from a 128-bit number space.

The OC-I sub-layer then invokes the corresponding module in the OC-D sub-layer to setup a tunnel to foo.ov through overlay ov. In turn, the OC-D sub-layer invokes a resolution service to obtain the overlay ID (IDB) of foo.ov. Examples of resolution services are DNS (used in RON), OpenDHT [16] (used in DOA), and implicit name-to-identifier hashing (used in i3). After the OC-D sub-layer resolves the name, it instantiates the necessary state for communicating with foo.ov, and returns a pointer to this state, the tunnel descriptor, tdAB, to OC-I. For example, in i3, the setup phase involves negotiating a pair of private triggers with the remote end-host, and instantiating the mapping state between foo.ov and the private trigger IDs.

On receiving the tunnel descriptor tdAB from OC-D, the OC-I sub-layer at A then performs an OC-I sub-layer setup with its peer sub-layer at B. The OC-I sub-layer at host B allocates a descriptor for the tunnel at the OC-D sub-layer (tdBA), and an OC handle (IPBA). When OC-I sub-layer setup is completed, the OC-I sub-layer at host A stores the mapping (pdABtdAB), and returns an OC handle (oc_handle) to the application in the form of a local-scope IP address, IPAB. To maintain compatibility with IP, IPAB belongs to an unallocated address space (e.g., 1.x.x.x [15]). Figure 5 shows the state instantiated at hosts A and B during the setup protocol. The setup operations when the first data packet of a connection is intercepted are similar; we do not describe it here.

4.1.4    Data Plane: Packet Forwarding

The application at host A addresses packets destined to foo.ov to IPAB, the OC handle returned by the OC-I sub-layer (see Figure 5). The OC-I sub-layer retrieves the state associated with this handle, and appends the path descriptor, pdAB, to the packet, before handing it off to the OC-D sub-layer to be sent over tunnel tdAB. The OC-D sub-layer, using its tunnel state, sends the packets to foo.ov using the overlay ID, IDB. At the destination, the packet is handed to the OC-I sub-layer, which uses the path descriptor in the header to demultiplex the packet. Before sending the packet to the application, the OC-I sub-layer rewrites the source address to IPBA, the OC handle associated with the A to B path at B. The destination address is rewritten to the local IP address at B.

As evident from this description, the constraint imposed by supporting unmodified applications leaves us with little choice but to overload the semantics of application-level names and IP addresses. We discuss the limitations of overloading names and addresses on transparency in  4.5.
Figure 5: Forwarding a data packet from host A (with IP address IPA) to B (with IP address IPB). The mappings used to modify the packet are in bold.

4.2  Goal 2: Bridging Multiple Overlays

When multiple overlays are deployed, a potential undesirable side-effect is that hosts in different overlays may not be able to reach one another. For example, i3 allows NATed hosts to act as servers, but such servers will be unreachable through RON. Even in the Internet, hosts in different IP address spaces cannot communicate with one another [22]. Moreover, it is likely that some of the Internet hosts will not participate in overlays.

Our architecture addresses these problems by allowing remote resolution of names, a mechanism borrowed from other architectural proposals such as DOA [39]. When a host belonging to overlay ov1 resolves an overlay name foo.ov2, the OC-I sub-layer resolves the name by forwarding the request to a gateway which participates in the overlay ov2. We provide interoperability between overlay and legacy hosts by designing special OC-D modules that send and receive IP traffic to and from legacy hosts.

When performing remote resolution, path descriptors are used as state handles across intermediate hops (such as gateways). The tunnel descriptor is a handle passed between the OC-I and the OC-D sub-layers at the same host; the path descriptor is used as a handle between the OC-I sub-layers at different hosts. Thus, intermediate hops can use the path descriptor to retrieve state required to relay the packet further. Further, decoupling path and tunnel descriptors allows different paths to share the same tunnel. For example, paths (A,B,C) and (A,B,D) can share the tunnel (A,B).

We now describe our mechanisms to bridge different overlays in more detail.

4.2.1     Overlay Gateways

Consider a host A that uses the i3 overlay wishing to contact a host C in the RON overlay (See Figure 3). To enable this communication, we deploy a host (gateway) B that resides on both i3 and RON, and runs the OC-D modules for both overlays. Host A then sets up a two-hop path to C by using the gateway as an intermediate hop. For a multi-hop path, the setup protocol creates tunnels between consecutive hops and sets up the routing state at the OC-I sub-layer of the intermediate hop to create an end to end path. We now give the details of the protocol.

Assume that the overlay name of host C is foo.ron. Configuration files at host A (described in  4.3) indicate that connections to foo.ron should go through a gateway B in i3 with the name bar.i3. To communicate with host C, an application at host A issues a DNS request for foo.ron. The OC-I sub-layer, upon intercepting this request, instructs the i3 OC-D module to set up a tunnel to bar.i3. This operation is identical to the tunnel setup in  4.1.3. Once this tunnel is setup, the OC-I at A asks its peer at B to set up the rest of the path to the destination C recursively.

At the end of the setup protocol, an end-to-end path is established from A to C with the unique path descriptor pd. A common path descriptor helps identify a path so that any path breakages can be dynamically detected and quickly repaired. Our gateway, as in the case of a NAT, maintains per-path state.
Figure 6: (a) An overlay client connecting to a legacy server. (b) A legacy client connecting to an overlay server.

4.2.2     Legacy Gateways

Legacy gateways are similar to overlay gateways except that one of the tunnels is over IP to a legacy host that does not participate in any overlay natively and does not run the OC-I sub-layer. Thus, overlay functionality, such as improved routing, will be available only on the tunnel established over the overlay (between an overlay host and the gateway). There are two types of legacy gateways:

Legacy server gateway. The legacy server (LS) gateway allows an overlay-enabled client to contact a legacy server (see Figure 6(a)). Functionally, the LS gateway runs an OC-I sub-layer over an OC-D module (say i3) and a special OC-D module called LegacyServerIP (or LSIP). The setup protocol is similar to that for an overlay gateway. Consider a overlay host connecting to through the LS gateway. The OC-I sub-layer at the LS gateway forwards such setup requests to the LSIP module. The LSIP module now behaves like a NAT box with respect to the server. It first resolves the name using DNS and allocates a local port for this tunnel. Packets sent to the server are rewritten by changing the source address to that of the LS gateway, and altering the source port to be the allocated local port. The local port is then used to multiplex incoming packets, which are then sent to the OC-I sub-layer with the appropriate handle.

Legacy client gateway. The legacy client (LC) gateway enables overlay servers to offer their services to legacy clients; legacy clients are not overlay enabled, nor do they run the OC-I sub-layer (see Figure 6(b)). The LC gateway runs the OC-I sub-layer over an OC-D module (say i3) and a special OC-D module called LegacyClientIP (or LCIP). In addition, the client is configured to use the LC gateway as its DNS server. The LCIP module intercepts DNS queries from the client, and dispatches them to the OC-I sub-layer which initiates a tunnel over the overlay. The LCIP module then sends a DNS reply with an Internet routable address to the client, captures packets sent by the legacy client to that address, and sends them over the overlay. Any client can now contact the machine foo.i3 from any machine provided that its DNS server is set to the address of the LC gateway. The design of our LC gateway is similar to that of AVES [22]. The fact that the addresses returned by the gateway should be routable considerably limits the number of clients that can connect simultaneously [22]. HTTP traffic does not suffer from this limitation since gateways can use DNS names in the HTTP requests for demultiplexing.

4.3     Goal 3: Exposing Overlay Functionality

Different new architectures and overlays provide different functions. Users should be able to choose the overlay best suited for a particular application. The overlay selected might allow further customization of the functions it offers. For example, RON allows users to choose the metric based on which the paths are optimized, OverQoS allows users to specify QoS parameters, and architectures like i3 and DOA allow users to explicitly interpose middleboxes on the path. For flexibility, users should be able to customize their preferences for each tunnel along a path. Preferences include both overlay-specific (e.g., use latency optimized paths for RON) and overlay-independent options (e.g., identity of gateways, end-to-end authentication).

Given the limited options available to a legacy application for communicating its preferences to the OC layer, our initial design was to encode the user preferences in the DNS name. For example, a DNS name foo.delay50ms.overqos was used to identify a connection to the host with name foo using a path of less than 50 ms delay in OverQoS. However, overloading DNS names to include preferences had multiple disadvantages, from highly restrictive syntax to being plain cumbersome to utilize. Although this approach is implemented in OCALA, we do not use it.

            <Match urlPattern="*.ron" />
            <Match protocol="tcp" dstPort="22" />
Figure 7: Configuration snippet indicating that ssh traffic or connections to all DNS names ending in .ron should go over an instance of RON running on PlanetLab, using the minimum latency metric.

Instead, we opted for expressing user preferences using XML configuration files. On receiving a setup request for an overlay name, the OC-D sub-layer reads the preferences associated with the name (if any) from the configuration file, before proceeding with the setup operation. A snippet from a configuration file is shown in Figure 7. Though directly manipulating the configuration files offers great flexibility, we expect users to rely on our graphical user interface described in  7.

4.3.1     Support For Middleboxes

OCALA also allows users to customize their data path by redirecting traffic through specific middleboxes using the configuration files. Several new network architectures [32, 39] provide support for such middleboxes, by allowing both the sender and the receiver to explicitly insert middleboxes on the data path. OCALA's support for middleboxes is similar to that for gateways. Consider the case of a sender-imposed middlebox where a host A wishes to contact a host B through a middlebox M (see Figure 8). The only difference from the operation of a gateway is that the middlebox module running at M should be allowed to perform arbitrary transformations on the data sent by one end-point before forwarding it to the other. In OCALA, the middlebox module implements a single function call that is used by the OC-I sub-layer to pass packets to it. A configuration file at M specifies the middlebox operations to be applied to connections traversing the middlebox. The protocol for receiver-imposed middleboxes is similar.
Figure 8: Interfacing a middlebox.

4.4     Goal 4: Factoring Out Common Functions

A second-order goal aimed at reducing the effort of overlay developers, is to leverage the OC-I sub-layer to implement generic functions, such as security and data compression, that can be used by different overlays.

Security and authentication of data connections are important requirements for many overlays, especially in cases where flat names are employed. OCALA incorporates basic security mechanisms at the OC-I sub-layer. In particular, the OC-I sub-layer offers encryption and authentication, both of which operate agnostic of the overlay used for the traffic. The OC-I sub-layer's authentication mechanism is based on human-readable names and relies on the existence of a certification and name-allocation authority from which users can obtain certificates associating their overlay name to their public key.2 OCALA's protocol for securely communicating with a host known by its name alone is very similar to the Secure Sockets Layer protocol (SSL) [10] which relies on certificate authorities like VeriSign. We designed our own custom protocol rather than reusing SSL since in general middleboxes need to operate on unencrypted data, which is not possible under the existing end-to-end model of SSL.

4.5     Limitations

The primary goal of our design is to achieve transparency for legacy applications while providing complete access to overlay functions. We review how well our design meets this goal.

4.5.1     Access to Overlay Functions

While the OC layer enables legacy applications to take advantage of most overlay functions such as mobility, anycast, QoS, route optimizations and middleboxes, there are two important limitations. First, the fact that OCALA is positioned below the transport layer makes it hard, if not impossible, for legacy applications to take advantage of overlay networks that provide transport- or application-layer functionalities (e.g., multi-path congestion control, or data storage [17]). Second, the current instantiations of OCALA support only unicast legacy applications; it provides no support for legacy applications using IP multicast; we are currently designing a multicast abstraction at the OC-I sub-layer.

4.5.2     Transparency

The OC-I sub-layer overloads IP addresses in ways that might break assumptions made by some applications. In contrast to usual IP semantics, the scope of addresses returned by the OC-I sub-layer to applications is local. Firstly, the use of local scope addresses implies that addresses returned to legacy applications will not be valid at other hosts. In our experience, this does not break several common applications like ssh, Internet Explorer, remote desktop, and ftp servers. However, peer-to-peer applications and SIP may not work under OCALA (unless all hosts run OCALA). Secondly, applications like ftp that encode addresses in data packets will potentially not work since the OC-I sub-layer performs IP header rewriting before delivering packets to the application. Our implementation avoids address rewriting to some extent by negotiating the local addresses at the OC-I sub-layer, a technique borrowed from [42]. However, for legacy gateways, address rewriting cannot be avoided.

Local-scope addresses have been used before in several contexts and their limitations and workarounds are well-known [42]. In supporting overlays where hosts may not have routable IP addresses, we are left with little choice but to work around the limitations of local-scope addresses using mechanisms like address negotiation.

5     The Overlay Dependent Layer

The overlay dependent sub-layer implements the functions specific to an overlay. We first present the interface that is exported by an OC-D module to the OC-I sub-layer. We then describe the working of the OC-D modules for two overlays, i3 [32] and RON [1], which we developed in-house. This description serves not only as a validation of our architecture but also as a blueprint for implementing OC-D modules for other overlays.

5.1     OC-D Module API

Table 1 shows the basic API functions that every OC-D module needs to implement and expose to the OC-I sub-layer. For simplicity of exposition, we omit error-related functions here.
Function calls: OC-I → OC-D
setup(name,pref, path_d) setup path to host name using preferences pref
close(tunnel_d) close tunnel
send(tunnel_d, IP_pkt) send IP packet via tunnel
Callbacks: OC-D → OC-I
setup_done(path_d, tunnel_d) callback invoked when tunnel (tunnel_d) was established
recv(path_d, IP_pkt) receive IP packet from tunnel

Table 1: OC-D Module API.

The basic API consists of three functions and two callbacks. The setup function sets up a tunnel between the local host and a remote host according to the user's preferences. The user preferences pref and the overlay name of the remote host name are passed in the setup call. The path_d field represents the path descriptor at the OC-I sub-layer and is used by the OC-D sub-layer in the setup_done callback. Once the OC-D sub-layer creates the tunnel it returns the tunnel descriptor (tunnel_d) to the OC-I sub-layer using callback setup_done. The close function call is invoked by the OC-I sub-layer to close the specified tunnel. This function is usually called when a path's state at the OC-I sub-layer expires. We discuss the timeout values for this state in the implementation section ( 7.1).

The send function call, invoked by the OC-I sub-layer, includes a handle to the OC-D's state for that tunnel (i.e. the tunnel descriptor) and the packet itself. The recv call, is invoked by an OC-D module to the OC-I sub-layer, upon receiving a packet from the overlay.

5.2     The RON Module

RON [1] aims to improve the resilience of the Internet by using alternate routes in the overlay. RON offers an interface similar to IP, and not surprisingly, it requires very little effort to implement the OC-D module for RON. RON uses IP addresses and DNS names as overlay IDs and overlay names, respectively.

When the OC-I sub-layer asks the RON module to setup a connection to a RON host (identified by a name such as, this name is resolved using the DNS infrastructure to obtain an IP address. The RON module then sets up state associating the preferences and the destination IP address with the tunnel and passes its handle to the OC-I sub-layer. Data plane operations involve simple encapsulation and decapsulation.

5.3     i3 Module

i3 [32] is a network architecture that uses a rendezvous-based communication abstraction to support services like mobility, multicast, anycast and service composition. We now describe how the i3 module works when host A contacts host B over i3.

On receiving the setup request for B.i3 from the OC-I sub-layer, the i3 OC-D module at A first resolves the name to a 256-bit i3 identifier by using implicit mapping: the identifier of a host is derived by simply hashing its name. The identifier obtained by hashing B.i3 corresponds to B's public trigger identifier idB. Thus, i3 does not require any resolution infrastructure.

After the name is resolved, the i3 module at A initiates private trigger negotiation by contacting host B through its public trigger [idB|B]. Both hosts exchange a pair of private triggers [idAB|A] and [idBA|B], respectively, after which they communicate exclusively through these triggers: host A sends packets to host B using ID idBA, and host B sends packets to A using ID idAB. Once the control protocol sets up the required state, the i3 module sends packets captured by the OC-I sub-layer by encapsulating the payload with i3 headers that include the private triggers identifying the flow.

The i3 OC-D module allows receiver-imposed middleboxes by using i3's stack of IDs. An i3 host B that wishes to impose the middlebox M on all hosts contacting it, inserts a public trigger of the form [idB|(idM,B)]. When a client A sends a trigger negotiation request via the ID idB, i3 delivers it to M along with the stack (idM,B). The i3 OC-D module thus obtains the identity of the next hop and automatically proceeds to set up the tunnel to B through its OC-I sub-layer.

6     Applications

Legacy applications benefit from OCALA in two different ways. Firstly, OCALA enables applications to leverage the new functionality offered by overlays. Secondly, the OC-I sub-layer of OCALA allows a path to traverse multiple overlays thus composing their functionalities. We now describe some applications that demonstrate these two types of benefits.

6.1     Functions Enabled by Overlays

NAT Traversal: Since i3 enables access to machines behind NATs, a user can run legacy servers behind NATs by using the i3 OC-D module. In addition to allowing external hosts to contact these servers, OCALA also enables users to securely access their home machines from anywhere by using the human-readable i3 name of the home machine. When persuading users to deploy OCALA, we found NAT traversal to be a very attractive feature from the users' perspective.

Receiver Imposed Middleboxes: i3 enables hosts to redirect all incoming traffic to a middle-box which can be located anywhere in the network. We used this ability to force all traffic sent to a legacy web server to pass through an intrusion detection middlebox which was not located on the physical path to the server. We used the popular Bro [24] intrusion detection program in our implementation by writing a 200-line middlebox shim layer through which the OC-I sub-layer relays packets that are to be analyzed by Bro.

Observe that Bro is itself a legacy application, and thus packets sent to Bro should have valid IP headers. For this reason, the shim layer assigns virtual addresses to both end points and rewrites the IP headers appropriately, before sending the packets to Bro. To Bro, communication between the remote hosts looks like a conversation between two virtual hosts, and it can perform stateful analysis (e.g., TCP analysis by matching the data packets of a TCP connection with the corresponding acknowledgments). Since Bro sees only virtual addresses, it cannot perform certain analysis like address-scan detection that looks for several unsuccessful connection attempts to hosts within the same network.

Secure Mobility: HIP enables hosts to securely communicate with each other even when the hosts are mobile. We leverage this functionality of HIP to support ssh connections that remain alive even when one of the hosts changes its IP address.

6.2     Functions Enabled by the OC-I Sub-layer

Secure Intranet Access: We implemented a flexible and secure version of Virtual Private Networks (VPNs) [37] by using the OC-I sub-layer to contact legacy hosts using an overlay. A legacy server gateway runs inside the organization and hence has unrestricted access to all intranet hosts. To access intranet machines, external hosts relay packets through the legacy gateway. Authentication and encryption are important requirements in this scenario, and we leverage the OC-I sub-layer's security mechanisms. Any routing overlay, including vanilla IP, can be used for communicating between the user's machine and the legacy gateway. The main advantage of our system over VPN-based systems is that a client can access multiple intranets at the same time even if all intranets use the same address range. Users specify their preference through the configuration file—e.g., all connections to * should go through the gateway1 of company 1 while connections to * should use the gateway of company 2. Another distinguishing feature of our system is that a client is not assigned an IP address from the intranet address space. This improves the security of our system by making it difficult for a client infected by a scanning worm to directly attack other hosts within the intranet.

Overlay Composition : Overlay composition allows an application to explicitly stitch together different network overlays. Apart from enabling inter-operability, stitching allows a user to merge the functions offered by different overlays. For example, a user who connects to the Internet through a wireless hop, may use i3 for uninterrupted communication while switching between various wireless networks. In addition, the user may wish to optimize wide-area performance using RON. We achieve this by using i3 to connect to a close-by i3-to-RON gateway, which will then relay packets over a RON-optimized path over the wide-area.

7     Implementation

We have implemented the OC-I sub-layer as a user-level proxy. Although OCALA inserts a new layer into the network protocol stack, our implementation avoids modifications to the operating system by using the tun  [38, 40] packet capture device. The OC-I sub-layer reads from the tun device to capture packets sent by legacy applications and writes to it to send back replies.

The OCALA proxy and the configuration GUI consist of approximately 30,000 source lines of code (SLOC) in C++ and 6,000 SLOC in Java respectively. The software, which currently works on Linux, Windows XP/2000 and Mac OS X, is available at

We have implemented OC-D modules for RON and i3 using source code available from their project websites. The HIP, DOA and OverDoSe OC-D modules were independently implemented by external research groups. An OC-D module is a C++ class implementing the API of the OC-D base class, compiled into .so, .dll and .dylib dynamically loaded libraries in Linux, Windows and OS X respectively. OC-D modules are dynamically loaded and plugged into the proxy based on user configuration. In its simplest form, an OC-D module translates between OC-I API calls and overlay-specific functions. In our experience, implementing an OC-D module is a simple task requiring less than 200 lines of code. We only count the code used to interface the OC-D to the OC-I, and not the code used to implement overlay-specific functionality.

Users control the proxy and express their preferences (e.g., ssh traffic should go over RON, Internet Relay Chat should use i3) through a set of XML configuration files. We have implemented a graphical user interface that enables users to set their preferences without manually editing XML files. The GUI has a modular design which enables developers to plug in components which expose overlay-specific configuration options to users.

Our implementation requires administrative privileges for using the tun device and forces all users on the same machine to share the same configuration. These limitations can be avoided by a dynamic library-based implementation.

In the remainder of the section, we describe the implementation of the control plane, data plane and gateway operations in detail.

7.1     Control Plane: State Maintenance

Control plane setup begins when the OC-I sub-layer intercepts a DNS request for a previously unseen destination. The OC-I sub-layer initializes state, such as path descriptors, and communicates with its peer OC-I sub-layer(s) to set up the end-to-end path requested by the application. If the application requires, the same local-scope address is negotiated at both end points. If security is enabled, the protocol authenticates the nodes on the path and establishes 256-bit symmetric keys for each tunnel. These protocols are piggybacked on top of path setup to reduce latency. After setup completion, the OC-I sub-layer sends the DNS reply containing the local-scope address to the application. The local-scope addresses are allocated from the unused address range To prevent caching, the Time To Live (TTL) of the DNS reply is set to zero. The state associated with a path times out and is removed if no data packets are sent or received on that path for 7200s. This large timeout period was chosen to deal with applications like Internet Explorer which we found to cache DNS replies beyond their specified TTL. When the path is alive, periodic keep-alive messages are exchanged between the sender and the receiver to quickly detect and repair any breaks in the end-to-end path.

7.2     Data Plane: Packet Forwarding

Packets sent by the application are addressed to the local-scope addresses returned by the OC-I sub-layer after path setup. The OC-I sub-layer intercepts packets sent to local-scope addresses, as well as packets which match patterns (based on addresses and ports) that are explicitly specified in the configuration file. Depending on user preference, the OC-I sub-layer may compress or encrypt the packet before dispatching to the OC-D sub-layer. The headers added by the OC-I and OC-D sub-layers may lead to packet fragmentation; fragmentation can be avoided if applications perform end-to-end MTU discovery. Rewriting of addresses at the destination occurs only if local-scope address negotiation between the end points had failed during path setup.

7.3     Legacy Gateways

Our LSIP implementation includes packet-rewriting support for several applications such as FTPH.323PPTP and SNMP. The legacy server gateway does not support ICMP since there is no information in an ICMP packet (such as port numbers) to permit multiplexing of a single IP address among multiple hosts. The LCIP implementation is very similar to AVES [22], and a legacy client can connect to a name of the form in order to communicate to the webserver at foo.i3.

8     Evaluation

The purpose of our evaluation is to demonstrate that the overhead of packet capturing and tunneling in our implementation is not large. The real benefit of our architecture and implementation should be evaluated by the applications it enables, and eventually, the user acceptance it gains. We first micro-benchmark the data and control paths of the proxy, and then present local-area and wide-area experiments.

8.1    Micro-benchmarks

Micro-benchmarks were conducted on a 2.4 GHz Pentium IV PC with 512 MB RAM running Linux 2.6.9. An in-house tool that sends packets at a specified rate played the role of a legacy client. Both the proxy and the tool were instrumented to record the timestamps at relevant checkpoints. Each timing statistic reported here is a median of 100 runs.

Data Path Overhead. In comparison to a legacy application running over the host IP stack, the proxy adds two memory copies: from kernel to user space and back, both while sending and receiving packets. Table 2 reports the send and receive times of a single packet of size 1200 bytes 3 for i3 and RON4. The total send and receive times are split into three phases: (a) time to move a packet between the application and the proxy (using tun), (b) overhead at OC-I sub-layer, and (c) overhead at OC-D sub-layer.

Send Receive
(μs) i3 RON i3 RON
OC-I 19 18 8 6
OC-D 20 28 44 36
tun 24 24 16 15

Table 2: Split-up of per-packet overhead for send and receive.
As expected, the processing time of the OC-I sub-layer is independent of whether we use i3 or RON. The percentage of time spent in the OC-I sub-layer is not large—28% for send and 11% for receive (on enabling OC-I features like encryption, the overhead rises to more than 67%). The remaining overhead is almost equally split between OC-D processing and transferring the packet from the application to the proxy. Although the i3 and RON OC-D modules are very different, the processing times associated with them are similar. A dynamic library implementation can reduce the overhead of packet transfer between the application and proxy, by avoiding extra packet copying. The total processing time indicates that the proxy can sustain a throughput of about 15000 packets per second (for 1200-byte packets).

Control Path Overhead. Path setup is triggered when a DNS request made by an application is captured. If a path for the requested name was previously set up, the proxy immediately answers the DNS query with a small processing overhead of 15 μs. Otherwise, it performs additional operations to set up the path and hence takes longer (169 μs) to respond to the application.

8.2     LAN Experiments

In order to study the effect of the proxy overhead on end-to-end behavior, we measured (Table 3) the latency and TCP throughput between two clients communicating over i3, i3-shortcut5, RON and plain IP, within the same LAN. In a LAN environment, the overhead of the proxy can be localized without wide-area artifacts affecting the measurements.

i3 i3-shortcut RON IP
Latency (ms) 1.42 0.788 0.762 0.488
Tput. (KBps) 9589 10504 10022 11749

Table 3: LAN experiments for latency and throughput.
Latencies under i3-shortcut and RON are a few hundred microseconds larger than IP latency. Since LAN latencies are themselves very small, even a single intermediate server on the data path causes significant relative increase in latency for i3. The throughput results (average over 10 measurements) indicate that the performance hit due to proxy and overlay overheads is only about 10%. The throughput and latency of RON is not better than IP since in this simple experimental setup, all RON and IP packets traverse the same LAN. Since the i3 servers were also located on the same LAN, relaying packets through i3 did not cause significant throughput degradation.

8.3     Wide-area Experiments

We measured OCALA's performance over i3, i3-shortcut, RON and plain IP in the wide area. We also measured the performance when traffic traversed i3-RON, i3-IP and RON-IP gateways. Difficulty in obtaining hosts with root privileges limited our experiments to just three machines at Berkeley, Stanford and Boston, which we refer to as A, B and C respectively. Latency was measured using ping, and throughput was measured using ttcp. i3 and RON networks were deployed on PlanetLab. The i3 OC-D module on the end-host used the closest i3 server, while the end-host itself joined the RON network using its RON OC-D module.
Figure 9: Wide-area experiments: (a) latency (b) throughput.

We first consider the latency and throughput results for the single network scenario. Figure 9(a) shows that latencies for i3-shortcut and IP are nearly equal. This is not surprising, as in both cases, packets follow the direct IP path between the end-points. Although we configured RON to choose latency-optimized paths, we observed no significant improvements in latency compared to the direct IP path. Due to the limited size of our experiment, the path with the best latency was always the direct IP path. Plain i3 incurs larger latency as packets are forwarded via an intermediate i3 server. In a few experiments, IP incurred a higher latency than i3 and RON; we attribute this to UDP packets getting preferential treatment over ICMP ping packets (note that packets are encapsulated in UDP when i3 or RON are used). We confirmed this by measuring latencies using the UDP Echo[27] protocol, wherever permitted by firewalls.

Throughput measurements in Figure 9(b) indicate that i3 performs much worse than the direct IP path. Throughput over i3 and RON vary between 62% and 95% of the direct IP throughput. We attribute this performance degradation to the extra headers added to each packet and the proxy processing overheads. We further suspect that TCP packets are getting preferential treatment over UDP in the wide area.

i3-RON bridge. We measured throughput and latency between each pair of machines, with one of the machines in the pair connected only to i3 while the other was connected only to RON. A second machine (D) at Berkeley acted as an i3-RON gateway. As shown by Figure 9(a), the increase in latency for the bridged path over the direct IP path is small. However, the presence of the i3-RON gateway on the path resulted in lower throughput. The adverse effect of bridging is dominant when nodes are very close to each other. For example, throughput between the Berkeley and Stanford nodes under bridging is approximately one-third of the direct IP path, while for distant nodes (Berkeley-Boston, Stanford-Boston), the throughput drop is less than 20%.

Legacy Server Proxy. We ran i3-IP and RON-IP legacy server proxies on machine D. The proxies at A, B and C were configured to relay connections to mozilla mirrors ( through the server proxies, with the first hop using i3 or RON. The server proxies connect to the mozilla mirror on behalf of A, B or C. To measure throughput, we downloaded 10 different files from 10 different mozilla mirrors. The average throughput while using the i3-IP and RON-IP gateways was within 85% of the throughput obtained while directly downloading the same set of files.

The main reasons for reduced throughput in both wide area and LAN experiments are the overheads due to extra headers and relaying through intermediate hops (for bridging). These are inherent limitations of tunneling.

8.4     Number of Simultaneous Connections

OCALA proxy can handle a large number of simultaneous connections. Due to the difficulty in procuring machines with root privileges, our experiment is limited to the legacy server proxy scenario where one of the end-points of a connection is a legacy host not running OCALA. We used an 8 machine cluster (Intel Xeon 3.06 GHz, 2GB RAM each) for running the OCALA proxies. Machine 1 of the cluster ran a legacy server proxy. Machines 2 to 8 ran normal client proxies configured to use the legacy server proxy at 1 for relaying all legacy traffic. From each of machines 2 to 8, we accessed 175 legacy websites in parallel. The legacy server proxy on 1 was able to service over 1000 simultaneous connections with 26% CPU utilization and 0.4% memory utilization.

8.5     Path Robustness

OCALA's periodic keep-alive messages enable broken paths to be quickly detected and repaired. If no keep-alive responses are received for 5s, OCALA invokes path re-establishment. This mechanism retries every 10s till a path is set up. Again, due to difficulty in obtaining machines with root privileges, we measured the time taken to detect and repair a broken path between just two machines (at Berkeley and Boston). We experimented with paths consisting of one and two gateways. The average time to detect a path break was about 3s and the average time to repair the path was about 5s. These numbers agree with the reply wait timer and the path setup retry timer values in our implementation. (Due to difficulties in synchronizing the time across the different wide-area machines, we are unable to report results with finer time granularity.)

9     Discussion

In this section, we summarize our experiences with the OCALA deployment. We (and other groups) have used various versions of the proxy since March 2004. Over this time interval, the OCALA proxy has attracted interest from both overlay developers and end-users. Developers of various routing overlays and network architectures, such as Delay Tolerant Networks [6], OverQoS [35], Tetherless Computing [30], and QoS Middleware project [20], have expressed interest in leveraging the OCALA proxy for their own overlays.

The proxy has been used for supporting a variety of applications including ssh, ftp, web browsing, and virtual network computing (VNC) applications. Most end-users have typically used the proxy for accessing their home machines to get around NAT boxes and dynamic IP address allocation by their ISPs.

Based on our own experience and the feedback from other end-users and developers, we have learned a few lessons, some of which are obvious in retrospect. These lessons emphasize what is arguably the main benefit of OCALA: the ability to “open” the overlays to real users and real applications. The feedback received from such users has been invaluable in improving the OCALA design, and in some cases, the overlay design.

Efficiency matters. When using legacy applications, the users expect their applications to perform the “same” way no matter whether they run directly on top of IP or on top of an overlay. In particular, more often than not, we found the users unwilling to trade the performance for more functionality. This feedback led not only to proxy optimizations, but also to overlay optimizations. For example, the developers of i3 have added shortcuts to improve the end-to-end latency, and added the ability to share a private trigger among multiple tunnels to decrease the setup cost.

Security matters. Security was not part of our original design agenda. However, we found that the users expected at least the same level of security from the OC-D name resolution mechanism as they get from today's DNS (where impersonation while possible, is not trivial). In the area of mobility, the users and developers argued for even much stronger security guarantees such as authentication and encryption. In the end, this feedback led us to implement security in the OC-I sub-layer.

Usage is unexpected. Initially, we expected mobility to be the most popular application. However, this was not the case. Instead the users were more interested in using OCALA for such “mundane” tasks as accessing home machines behind NATs or firewalls, and getting around various connectivity constraints. In one instance, users leveraged the fact that the proxy communicates with i3 via UDP to browse the web through an access point that was configured to block TCP web traffic! The unexpected usage led us to provide better support for applications over NATs. In particular, we have implemented an OC-I handle negotiation mechanism that preserves the addresses in the IP headers. This allows us to support some applications that otherwise do not work over NATs (e.g., ftp).

10     Conclusion

Overlay networks have been the focus of much research in recent years due to their promise of introducing new functionality without changing the Internet infrastructure. Surprisingly little attention has been devoted to achieving the same desirable property at the end-host: provide access to new network architectures without any changes to legacy software such as operating systems, network applications, and middlebox applications.

Our work is a preliminary step in this direction and aims to improve the inter-operability between legacy applications and new network architectures, and between different network architectures. Currently, we (and others) are in the process of extending the OC-D sub-layer to support other overlay networks. Ultimately, we plan to enlarge our user base and gather more feedback to improve the proxy. As our experience showed, users often find unexpected uses to the system, which can push the design in new directions.


We thank David Andersen, Tom Anderson, Dennis Geels, Boon Thau Loo, Sridhar Machiraju, Sean Rhea, Mukund Seshadri, Scott Shenker, Arun Venkataramani, Mythili Vutukuru, and Michael Walfish for useful discussions and comments about the paper. We thank Stefan Goetz for his crucial contributions in porting the OCALA proxy to Windows and Mac OS X. We are grateful to Keith Sklower and Mike Howard for their help in deploying the OCALA legacy server and client proxies on a big scale.


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We focus on the interface provided by the network substrate, and not on how this substrate is implemented. Thus, we do not distinguish between the implementation of a network architecture and an overlay; an overlay is one way to implement a new network architecture on top of IP.
We do not know of any mechanism for eliminating the centralized authority for a human-readable and secure naming scheme. It is easy to extend our model to hierarchical name-allocation schemes.
We used this packet size in order to avoid fragmentation due to addition of headers.
We benchmark only the two OC-D modules that we implemented; the others were implemented external to our research group.
Shortcut is an i3 optimization that eliminates the inefficiency of relaying packets through intermediate i3 servers.

This document was translated from LATEX by HEVEA.
This paper was originally published in the Proceedings of the 3rd Symposium on Networked Systems Design and Implementation (NSDI '06)
May 8–10, 2006, San Jose, CA

Last changed: 3 May 2006 jel