Internet Engineering Task Force A. Ford, Ed. Internet-Draft Roke Manor Research Intended status: Experimental C. Raiciu Expires: April 22, 2010 University College London S. Barre Universite catholique de Louvain J. Iyengar Franklin and Marshall College B. Ford Max Planck Institute for Software Systems October 19, 2009 Architectural Guidelines for Multipath TCP Development draft-ford-mptcp-architecture-00 Status of this Memo This Internet-Draft is submitted to IETF in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. 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." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. This Internet-Draft will expire on April 22, 2010. Copyright Notice Copyright (c) 2009 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 in effect on the date of Ford, et al. Expires April 22, 2010 [Page 1] Internet-Draft MPTCP Architecture October 2009 publication of this document (http://trustee.ietf.org/license-info). Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Abstract Often endpoints are connected by multiple paths, but the nature of TCP/IP restricts communications to a single path per socket. Resource usage within the network would be more efficient were these multiple paths able to be used concurrently. This should enhance user experience through higher throughput and improved resilience to network failure. This document outlines architectural guidelines for the development of a Multipath Transport Protocol, with references to how these architectural components come together in the Multipath TCP (MPTCP) protocol. This document also contains suggestions for functional separation within an implementation, maximising the flexibility that can be achieved with these architectural components. Ford, et al. Expires April 22, 2010 [Page 2] Internet-Draft MPTCP Architecture October 2009 Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4 1.1. Goals . . . . . . . . . . . . . . . . . . . . . . . . . . 4 1.1.1. Functional Goals . . . . . . . . . . . . . . . . . . . 4 1.1.2. Performance/Efficiency Goals . . . . . . . . . . . . . 5 1.2. Motivation . . . . . . . . . . . . . . . . . . . . . . . . 5 1.3. Requirements Language . . . . . . . . . . . . . . . . . . 6 2. Fundamentals . . . . . . . . . . . . . . . . . . . . . . . . . 6 2.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 7 2.2. Reference Scenario . . . . . . . . . . . . . . . . . . . . 7 2.3. Layered Representation . . . . . . . . . . . . . . . . . . 7 3. Multipath Architecture . . . . . . . . . . . . . . . . . . . . 8 3.1. Motivations . . . . . . . . . . . . . . . . . . . . . . . 8 3.2. Decomposing Transport Functions . . . . . . . . . . . . . 8 3.3. TCP Performance . . . . . . . . . . . . . . . . . . . . . 10 4. Implementation Architecture . . . . . . . . . . . . . . . . . 11 4.1. Functional Separation . . . . . . . . . . . . . . . . . . 11 4.1.1. Application to default MPTCP protocol . . . . . . . . 11 4.1.2. Generic architecture for MPCTP . . . . . . . . . . . . 14 4.2. PM/MPS interface . . . . . . . . . . . . . . . . . . . . . 15 5. Security Considerations . . . . . . . . . . . . . . . . . . . 16 6. Interactions with Applications . . . . . . . . . . . . . . . . 16 7. Interactions with Middleboxes . . . . . . . . . . . . . . . . 17 8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 17 9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 17 10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 17 10.1. Normative References . . . . . . . . . . . . . . . . . . . 17 10.2. Informative References . . . . . . . . . . . . . . . . . . 17 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 18 Ford, et al. Expires April 22, 2010 [Page 3] Internet-Draft MPTCP Architecture October 2009 1. Introduction Multipath TCP (MPTCP) will provide a set of extensions for regular TCP [2] to allow one TCP connection to be spread across multiple paths. This section describes the motivation behind the design of Multipath TCP. Companion documents to this architectural overview are those which provide details of the protocol extensions, congestion control algorithms, and application-level considerations. Put together, these components build a complete Multipath TCP implementation. Other components, however, could be introduced in place of these, in accordance with the architecture specified in this document. Please note this document is a work-in-progress and covers several topics, some of which may be more appropriately moved to separate documents as this work evolves. 1.1. Goals This section outlines what we perceive to be the most important goals for a multipath-capable transport protocol. We divide these goals into two categories: functional goals and performance/efficiency goals. 1.1.1. Functional Goals o Multihoming: The multipath transport protocol must allow for a logical transport endpoint as seen by the application to correspond to multiple physical network attachment points, such as multiple IP addresses on the same or different network interfaces. o Application Compatibility: Multipath-capable equivalents of existing transports such as concurrent multipath versions of TCP, SCTP, or DCCP, must retain backward compatibility with existing APIs, so that existing applications can use the newer transports, merely by upgrading the operating systems of the end-hosts. o Network Compatibility: Multipath transport protocols must remain backward compatible with the Internet as it exists today, including being able to traverse predominant existing middleboxes such as firewalls, NATs, and performance enhancing proxies [3]. o Automatic Negotiation: A host supporting a multipath-capable equivalent of an existing transport must be able to detect reliably whether a new communication partner supports the next- generation protocol, using it if so, and otherwise automatically falling back to the existing protocol (e.g., standard TCP, SCTP, Ford, et al. Expires April 22, 2010 [Page 4] Internet-Draft MPTCP Architecture October 2009 or DCCP). o End-to-End Reliability and Security: The multipath-capable equivalent of an existing transport must retain its end-to-end reliability properties and allow for end-to-end authentication and/or privacy protection in a network-compatible fashion, i.e., maintain compatibility with legacy middleboxes. 1.1.2. Performance/Efficiency Goals o Multihoming and Multipath Capable: The protocol mustbe able to detect and utilize multiple available paths between two logical endpoints, either one path at a time (fail-over multipath) or several at once (concurrent multipath). o Resource Pooling: Transports should be able to balance traffic among available paths, optimizing network utility in a global sense by shifting load away from congested bottlenecks and taking advantage of spare capacity wherever it may be located [4]. o TCP-Friendliness: The architecture must enable new multipath transport flows to coexist gracefully with predominant existing transport flows, competing for bandwidth neither unduly aggressively or unduly timidly (unless low-precedence operation is specifically requested by the application, such as with LEDBAT). o Congestion State Sharing: Since popular applications such as HTTP often use multiple transport instances between the same pair of hosts, the protocol must avoid multiplicative explosions in multipath congestion control contexts - i.e., N transport instances times M multipath flows each - by enabling a multipath "bundle" of congestion control contexts to be shared cleanly among application-visible transport instances. o Support Small Transactions: Recognizing that many applications today make heavy use of frequent small communications, such as HTTP conditional GET transactions or streaming media frames, next- generation transports should minimize the performance costs of supporting these common application behaviors, including by minimizing unnecessary protocol overhead on small packets and by unnecessary round-trip delays or state maintenance costs when applications use short transactions. 1.2. Motivation As the Internet evolves, demands on Internet resources are ever- increasing, but often these resources (in particular, bandwidth) cannot be fully utilised due to protocol constraints on both the end- Ford, et al. Expires April 22, 2010 [Page 5] Internet-Draft MPTCP Architecture October 2009 systems and within the network. These unused resources, if they could be used, would lead to an enhanced user experience. This would also reduce the necessary expenditure on network infrastructure which would otherwise be needed to create an equivalent experience improvement. By the application of resource pooling [4], these available resources can be 'pooled' such that they appear as a single logical resource to the user. The purpose of Multipath TCP, therefore, is to provide a TCP to the user that is able to make use of multiple available paths. The achievement of resource pooling through Multipath TCP bring two key benefits: o To increase the resilience of the connectivity by providing multiple paths, protecting end hosts from the failure of one. o To increase the efficiency of the resource usage, and thus increase the network capacity available to end hosts. A Multipath TCP MUST follow the same service model as TCP [2]: byte oriented, in order reliable delivery. To have a deployable protocol, MPTCP SHOULD adhere to the following "do no harm" philosophy: multipath TCP SHOULD behave no worse (throughput wise) than running a single TCP connection over any of its paths, and using multiple paths MUST not harm users using single path TCP at shared bottlenecks. In addition, it should aim to be backwards-compatible where possible with existing, regular TCP. Multipath TCP as presented in [5] follows these aims, and achieves resource pooling by combining multiple TCP sessions running over multiple paths, and presenting them as a single TCP connection to the application. This is not the only way of creating a Multipath TCP, however, and as such this architecture is designed so that other components can be used in place 1.3. Requirements Language The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 [1]. 2. Fundamentals Ford, et al. Expires April 22, 2010 [Page 6] Internet-Draft MPTCP Architecture October 2009 2.1. Terminology Path: A sequence of links between a sender and a receiver, defined in this context by a source and destination address pair. Endpoint: A host operating an MPTCP implementation, and either initiating or terminating a MPTCP connection. Subflow: A flow of TCP packets operating over an individual path, which forms part of a larger MPTCP connection. Multipath TCP (MPTCP): A modified version of the TCP [2] protocol that supports the simultaneous use of multiple paths between endpoints. 2.2. Reference Scenario TBD? Endpoints, routes. Addresses/path selection mechanisms? 2.3. Layered Representation A Multipath TCP operates at the transport layer, and its existence should be transparent to both higher and lower layers. It is a set of additional features on top of standard TCP, and as such the impact on applications should be minimal, or entirely transparent, and these application considerations are discussed in detail in [6]. Multipath-aware applications would be able to use an extended sockets API to have further influence on the behaviour of MPTCP, which is also specified in [6]. Figure 1 illustrates this architecture. +-------------------------------+ | Application | +---------------+ +-------------------------------+ | Application | | MPTCP | +---------------+ + - - - - - - - + - - - - - - - + | TCP | | Subflow (TCP) | Subflow (TCP) | +---------------+ +-------------------------------+ | IP | | IP | IP | +---------------+ +-------------------------------+ Figure 1: Comparison of Standard TCP and MPTCP Protocol Stacks This layered model could apply to any multipath transport protocol, with any multipath, transport and network layers. This is explored in more detail in Section 3.2. Ford, et al. Expires April 22, 2010 [Page 7] Internet-Draft MPTCP Architecture October 2009 This document now proceeds with a detailed discussion of an architecture for developing a multipath TCP implementation, especially regarding the functional separation by which different components should be developed, in Section 3. 3. Multipath Architecture This section describes the layered functional separation that drives the design of the MPTCP protocol. Its main goal is to separate MPTCP in two parts that communicate through a well defined interface. We first provide the motivations for this functional separation, then we describe in more details the two main components of the MPTCP architecture. 3.1. Motivations The major goal behind MPTCP is to send data over different paths in the same time. This assumes that an MPTCP implementation must be able to discover and use the multiple paths that connect two given hosts, when they exist. However, different mechanisms can be envisioned for multipath discovery and use. Examples are as follows: Use multiple addresses: This is the method currently proposed in the MPTCP protocol design [5] - if hosts are multi-addressed, different address pairs may take different routes. Use a path selector value: An end-host might be able to tag packets with a path selector value, or adjust existing packet metadata. If some network nodes are able to read this tag and use it as a path selector, the host can influence the outgoing path of the packet. Next-hop selection: In a network configuration where multiple next- hops can offer to forward packets, a host may decide to send some of its packets through one next-hop, and some through another. The above list is not exhaustive, and could grow as new network technologies are deployed. 3.2. Decomposing Transport Functions As shown in Figure 2, we first loosely separate functions within transports into "application-oriented" and "network-oriented" parts. We use this separation of functions as an architectural framework that a multipath transport must recognize, primarily to maintain backward compatibility with applications and with the network. The desire for network compatibility will impact design choices at the Ford, et al. Expires April 22, 2010 [Page 8] Internet-Draft MPTCP Architecture October 2009 subflow level, while the need for application compatibility will primarily impact design choices at the higher, application-facing MPTCP layer. The top application-oriented "Semantic" functions are whatever communication abstractions are to be made available to applications, including providing the end-to-end reliability and ordering properties of abstractions like TCP's byte streams or SCTP's message- based multi-streams; these functions essentially deal with concerns of application-visible semantics. We consider the bottom part "network-oriented" because they represent functions that, while traditionally located in the ostensibly "end- to-end" Transport Layer, have proven in practice to be of great concern to network operators and the middleboxes they deploy in the network to enforce network usage policies [7][8] or optimize communication performance [9]. The network-oriented functions include congestion control and other performance-management functions ("Flow" performance functions), and endpoint/service identification functions (e.g., port numbers) that network operators and their middleboxes require to enforce network access and security policies ("Endpoint" functions). These network-oriented transport functions are collectively labeled in figure Figure 2 as "Flow/Endpoint" functions. +-----------------+ | Application | +---------------+ ---> +-----------------+ | Application | / | Semantic | (Application-Oriented +---------------+ <-- | Functions | Functions) | Transport | |- - - - - -| +---------------+ <-- | Flow / Endpoint | (Network-Oriented | Network | \ | Functions | Functions) +---------------+ ---> +-----------------+ | Network | +-----------------+ Figure 2: Decomposition of Transport Functions Following from the discussion above, a multipath transport would have to manage Flow/Endpoint functions for every path in an end-to-end connection, while providing a transparent single interface to the application. In keeping with this architectural worldview, MPTCP divides the Transport Layer into two components: the MPTCP part, which is responsible for the Semantic functions of global ordering of application data and reliability; and the "legacy TCP" part, which Ford, et al. Expires April 22, 2010 [Page 9] Internet-Draft MPTCP Architecture October 2009 implements the Flow/Endpoint functions. The figure below shows how MPTCP implements this architecture: +--------------------------+ +-------------------------+ | Application | | Application | +--------------------------+ +-------------------------+ | Semantic | | MPTCP | |- - - - - - - - - | + - - - - - + - - - - - + | Flow/Endpt | Flow/Endpt | | TCP | TCP | +--------------------------+ +-------------------------+ | Network | Network | | IP | IP | +--------------------------+ +-------------------------+ Figure 3: Mapping Transport Architecture to MPTCP 3.3. TCP Performance To provide multipath transport, Multipath TCP must send data over multiple paths. A naive implementation could simply run a standard, unmodified TCP congestion control algorithm [10] on each subflow. As listed in Section 1.1.1, however, it is a goal that MPTCP does not cause harm to other TCP flows, and such a naive approach would lead to a multipath session taking a disproportionate amount of bandwidth at shared bottlenecks. Therefore, in addition to purely sending data over multiple paths, MPTCP must do it in a way that will not affect TCP performance of non-multipath users. This raises the need for an efficient multipath congestion control algorithm. While this specification does not mandate the use of any particular algorithm for congestion control, it ensures that the protocol is designed in such a way that any congestion control algorithm can be designed, independently of the other components in use in the MPTCP implementation. Consequently, our architecture for MPTCP decouples the policy from the mechanism. The policy is the decision of what path to use for each packet to send. It is mainly driven by the implementation-dependent congestion control algorithm. The mechanism is the technology used to ensure that a packet will be sent on the desired path. This separation is intended to be relatively future-proof by allowing these components to evolve at different speeds. The decomposition of the transport functions, as described in Section 3.2, places the congestion control functionality in the flow/ endpoint layer, which maps to an individual subflow in the MPTCP design. In order to meet the "do-no-harm" philosophy, however, there must be an interface to the congestion control implementation that permits the appropriate coupling of congestion windows at the upper Ford, et al. Expires April 22, 2010 [Page 10] Internet-Draft MPTCP Architecture October 2009 (MPTCP) layer. 4. Implementation Architecture This section provides suggestions for an architecture to implement an extensible, modular multipath transport protocol. 4.1. Functional Separation This section describes a generic view of the internal implementation of a Multipath TCP, through which the technical components specified in the companion documents can fit together. It shows how an implementation could be built that permits extensibility between components without changing the external representation. We first show the functional decomposition of an MPTCP solution that is completely contained in the transport layer. That solution is described in more details in [5]. Then we generalize the approach to allow good extensibility of that solution. 4.1.1. Application to default MPTCP protocol Although, in the default approach, MPTCP is fully contained in the transport layer, it can still be divided into two main modules. One manages the scheduling of packets as well as congestion control. The other one manages the control of paths. The interface between the two is dealt with thanks to a Path Index. As shown in Figure 4, the Path Manager announces to the MultiPath Scheduler what paths can be used trough path indices, and maintains the mapping between that value and the particular action that it must apply to use the path (an example of such a mapping is in Table 1). In the case of the built-in Path Manager, the action is to replace an address/port pair with another one, in such a way that another path is used across the Internet to forward that packet. Ford, et al. Expires April 22, 2010 [Page 11] Internet-Draft MPTCP Architecture October 2009 Control plane <-- | --> Data plane +---------------------------------------------------------------+ | Multipath Scheduler (MPS) | +---------------------------------------------------------------+ ^ | | | | [A1,B1,|pA1,pB1] |For conn_id | | | | +-------------+ |Paths 1->4 can be | | Data packet |<--Path idx:3 |used. | +-------------+ attached | | | by MPS | | V +--------------------------------------------\------------------+ | Path Manager (PM) \[A1,B1]->[A1,B2] | +--------------------------------------------------\------------+ / \ | \ /-----------------------------\ | /"\ /"\ /"\ /"\ | rewriting table: || | | | | | | | | | Subflow id <--> network_id || | | | | | | | | | || | | | | | | | | | [see table below] || | | | | | | | | | || \./ \./ \./ \./ +------------------------------+| path1 path2 path3 path4 Figure 4: Functional separation of MPTCP in the transport layer The MultiPath Scheduler only deals with abstract paths, represented by numbers. It only sees one address pair throughout the communication, that we call the connection identifier. However, the MultiPath Scheduler must be able to perform per-subflow congestion control, and thus to distinguish between the subflows. This leads to define a subflow identifier, that consists of the usual transport identifier extended with the path index: . The following options, described in [5], are managed by the MultiPath Scheduler. o MULTIPATH CAPABLE (MPC): Tell the peer that we support MPTCP. Note that the MPC option also holds a token, which is necessary only if the built-in Path Manager is used. In the next section we describe the generalized case, where the token can be ignored by the receiver if another path manager is used. o DATA SEQUENCE NUMBER (DSN): Identify the position of a set of bytes in the meta-flow. o DATA FIN (DFIN): Terminate a meta-flow. Ford, et al. Expires April 22, 2010 [Page 12] Internet-Draft MPTCP Architecture October 2009 An implementation MUST use those options even if another Path Manager than the default one is implemented. The Path manager applies a particular technology to give the MPS the possibility to use several paths. The built-in MPTCP Path Manager uses multiple IPv4 addresses as its mean to influence the forwarding of packets through the Internet. When the MPS starts a new connection, the PM chooses a token that will be used to identify the connection. This is necessary to allow the PM applying the correct path index to incoming packets. An example mapping table is given hereafter: +-----------------+------------------+---------+-----------------+ | connection id | subflow id | token | Network id | +-----------------+------------------+---------+-----------------+ | | | token_1 | | | | | token_1 | | | | | token_1 | | | | | token_1 | | | | | token_2 | | | | | token_2 | | +-----------------+------------------+---------+-----------------+ Table 1: Example mapping table for built-in PM Table 1 shows an example where two connections are ongoing. One is identified by token_1, the other one with token_2. Since addresses are rewritten by the path manager, the attachment to the right connection is achieved thanks to the token, which is used at connection establishment and subflow establishment. It is then remembered. The first column holds the information that is exposed to the applications, while the last column shows the information that is actually written in packets that will fly through the network. We note that additionnally to the addresses, ports can be rewritten, which contributes to supporting NATs. The table also shows the role of the token, which is to attach various combinations of ports and addresses to a single connection. The token is specific to the built-in path manager, and can be ignored if another path manager is used. An implementation of the built-in path manager MUST implement the following options (defined in more details in [5]): o Add Address (ADDR): Announce a new address we own o Remove Addresse (REMADDR): Withdraw a previously announced address o Join Connection (JOIN): Attach a new subflow to the current connection Ford, et al. Expires April 22, 2010 [Page 13] Internet-Draft MPTCP Architecture October 2009 Those options form the default MPTCP Path Manager, based on declaring IP addresses, and carries control information in TCP options. An implementation of Multipath TCP can use any Path Manager, but it MUST be able to fallback to the default PM in case the other end does not support the custom PM. Alternative Path Managers may be specified in separate documents in the future. 4.1.2. Generic architecture for MPCTP Now that the functional decomposition has been shown for MPTCP with the built-in Path Manager, we show how that architecture can be generalized to allow the implementation of other Path Managers for MPTCP. A general overview of the architecture is provided in Figure 5. The Multipath Scheduler (MPS) learns about the number of available paths through notifications received from the Path Manager (PM). From the point of view of the Multipath Scheduler, a path is just a number, called a Path Index. Notifications from the PM to the MPS MAY contain supporting information about the paths, if relevant, so that the MPS can make more intelligent decisions about where to route traffic. When the Multipath Scheduler initiates a communication to a new host, it can only send the packets to the default path. But since the Path manager is layered below the MPS, it can detect that a new communication is happening, and tell the MPS about the other paths it knows about. Ford, et al. Expires April 22, 2010 [Page 14] Internet-Draft MPTCP Architecture October 2009 Control plane <-- | --> Data plane +---------------------------------------------------------------+ | Multipath Scheduler (MPS) | +---------------------------------------------------------------+ ^ | | | | [A1,B1,|pA1,pB1] | | | |Announcing new | +-------------+ |paths. (referred | | Data packet |<--Path idx:3 |to as path indices) | +-------------+ attached | | | by MPS | | V +--------------------------------------------\------------------+ | Path Manager (PM) \__________zzzzz | +--------------------------------------------------------\------+ / \ | \ /---------------------------\ | /"\ /"\ /"\ | subflow_id Action | | | | | | | | | xxxxx | | | | | | | | | yyyyy | | \./ \./ \./ | zzzzz | | path1 path2 path3 +---------------------------+ Figure 5: Overview of MPTCP architecture From then on, it is possible for the MPS to attach a Path Index to the control structure of its packets (internal to the MPTCP implementation), so that the Path Manager can map this Path Index to the corresponding action. (see table in the lower left part of Figure 5). The particular action depends on the network mechanism used to select a path. Examples are address rewriting, tunnelling or setting a path selector value inside the packet. The applicability of the architecture is not limited to the MPTCP protocol. While we define in this document an MPTCP MPS (MPTCP Multipath Scheduler), other Multipath Schedulers can be defined. For example, if an appropriate socket interface is designed, applications could behave as a Multipath Scheduler and decide where to send any particular data. In this document we concentrate on the MPTCP case, however. 4.2. PM/MPS interface The minimal set of requirement for a Path Manager is as follows: o Outgoing untagged packets: Any outgoing packet flowing through the Path Manager is either tagged or untagged (by the MPS) with a path index. If it is untagged, the packet is sent normally to the Ford, et al. Expires April 22, 2010 [Page 15] Internet-Draft MPTCP Architecture October 2009 Internet, as if no multi-path support were present. Untagged packets can be used to trigger a path discovery procedure, that is, a Path Manager can listen to untagged packets and decide at some time to find if any other path than the default one is useable for the corresponding host pair. Note that any other criteria could be used to decide when to start discovering available paths. Note also that MPS scheduling will not be possible until the Path Manager has notified the available paths. The PM is thus the first entity coming into action. o Outgoing tagged packets: The Path Manager maintains a table mapping path indices to actions. The action is the operation that allows using a particular path. Examples of possible actions are route selection, interface selection or packet transformation. When the PM sees a packet tagged with a path index, it looks up its table to find the appropriate action for that packet. The tag is purely local. It is removed before the packet is transmitted. o Incoming packets: A Path Manager MUST ensure that each incoming path is mapped unambiguously to exactly one outgoing path. Note that this requirement implies that the same number of incoming/ outgoing paths must be established. Moreover, a PM MUST tag any incoming path with the same Path Index as the one used for the corresponding outgoing path. This is necessary for MPTCP to know what outgoing path is acknowledged by an incoming packet. o Module interface: A PM MUST be able to notify the MPS about the number of available paths. Such notifications MUST contain the path indices that are legal for use by the MPS. In case the PM decides to stop providing service for one path, it MUST notify the MPS about path removal. Additionnaly, a PM MAY provide complementary path information when available, such as link quality or preference level. 5. Security Considerations Please see [11] for a threat analysis of Multipath TCP. 6. Interactions with Applications Interactions with applications - incuding, but not limited to performances changes that may be expected, semantic changes, and new features that may be requested of an API, are presented in [6]. Ford, et al. Expires April 22, 2010 [Page 16] Internet-Draft MPTCP Architecture October 2009 7. Interactions with Middleboxes TBD? List of issues that may arise with NATs, firewalls, proxies, etc? This will be an overview only, and protocol-specific solutions to this will be given in the companion docments. 8. Acknowledgements Alan Ford, Costin Raiciu and Sebastien Barre are supported by Trilogy (http://www.trilogy-project.org), a research project (ICT-216372) partially funded by the European Community under its Seventh Framework Program. The views expressed here are those of the author(s) only. The European Commission is not liable for any use that may be made of the information in this document. 9. IANA Considerations None. 10. References 10.1. Normative References [1] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. 10.2. Informative References [2] Postel, J., "Transmission Control Protocol", STD 7, RFC 793, September 1981. [3] Carpenter, B. and S. Brim, "Middleboxes: Taxonomy and Issues", RFC 3234, February 2002. [4] Wischik, D., Handley, M., and M. Bagnulo Braun, "The Resource Pooling Principle", ACM SIGCOMM CCR vol. 38 num. 5, pp. 47-52, October 2008, . [5] Ford, A., Raiciu, C., Handley, M., and S. Barre, "TCP Extensions for Multipath Operation with Multiple Addresses", draft-ford-mptcp-multiaddressed-01 (work in progress), Ford, et al. Expires April 22, 2010 [Page 17] Internet-Draft MPTCP Architecture October 2009 July 2009. [6] Scharf, M. and A. Ford, "MPTCP Application Interface Considerations", draft-scharf-mptcp-api-00 (work in progress), October 2009. [7] Srisuresh, P. and K. Egevang, "Traditional IP Network Address Translator (Traditional NAT)", RFC 3022, January 2001. [8] Freed, N., "Behavior of and Requirements for Internet Firewalls", RFC 2979, October 2000. [9] Border, J., Kojo, M., Griner, J., Montenegro, G., and Z. Shelby, "Performance Enhancing Proxies Intended to Mitigate Link-Related Degradations", RFC 3135, June 2001. [10] Allman, M., Paxson, V., and W. Stevens, "TCP Congestion Control", RFC 2581, April 1999. [11] Bagnulo, M., "Threat Analysis for Multi-addressed/Multi-path TCP", draft-bagnulo-mptcp-threat-00 (work in progress), October 2009. Authors' Addresses Alan Ford (editor) Roke Manor Research Old Salisbury Lane Romsey, Hampshire SO51 0ZN UK Phone: +44 1794 833 465 Email: alan.ford@roke.co.uk Costin Raiciu University College London Gower Street London WC1E 6BT UK Email: c.raiciu@cs.ucl.ac.uk Ford, et al. Expires April 22, 2010 [Page 18] Internet-Draft MPTCP Architecture October 2009 Sebastien Barre Universite catholique de Louvain Pl. Ste Barbe, 2 Louvain-la-Neuve 1348 Belgium Phone: +32 10 47 91 03 Email: sebastien.barre@uclouvain.be Janardhan Iyengar Franklin and Marshall College Mathematics and Computer Science PO Box 3003 Lancaster, PA 17604-3003 USA Phone: 717-358-4774 Email: jiyengar@fandm.edu Bryan Ford Max Planck Institute for Software Systems Saarbrucken, Germany Email: baford@mpi-sws.org Ford, et al. Expires April 22, 2010 [Page 19]