Internet Engineering Task Force A. Ford, Ed. Internet-Draft Roke Manor Research Intended status: Informational C. Raiciu Expires: August 7, 2010 University College London S. Barre Universite catholique de Louvain J. Iyengar Franklin and Marshall College B. Ford Max Planck Institute for Software Systems February 3, 2010 Architectural Guidelines for Multipath TCP Development draft-ford-mptcp-architecture-01 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 improved resilience to network failure and higher throughput. 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 lists certain high level design decisions that provide foundations for the MPTCP design, based upon these architectural requirements. 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." Ford, et al. Expires August 7, 2010 [Page 1] Internet-Draft MPTCP Architecture February 2010 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 August 7, 2010. Copyright Notice Copyright (c) 2010 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the BSD License. Ford, et al. Expires August 7, 2010 [Page 2] Internet-Draft MPTCP Architecture February 2010 Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4 1.1. Motivation . . . . . . . . . . . . . . . . . . . . . . . . 4 1.2. Requirements Language . . . . . . . . . . . . . . . . . . 5 1.3. Terminology . . . . . . . . . . . . . . . . . . . . . . . 5 1.4. Reference Scenario . . . . . . . . . . . . . . . . . . . . 5 2. Goals . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 2.1. Functional Goals . . . . . . . . . . . . . . . . . . . . . 5 2.2. Compatibility Goals . . . . . . . . . . . . . . . . . . . 6 2.2.1. Application Compatibility . . . . . . . . . . . . . . 6 2.2.2. Network Compatibility . . . . . . . . . . . . . . . . 7 2.2.3. Compatibility with other network users . . . . . . . . 7 3. Multipath Architecture . . . . . . . . . . . . . . . . . . . . 7 3.1. Decomposing Transport Functions . . . . . . . . . . . . . 9 4. High-Level Design Decisions . . . . . . . . . . . . . . . . . 11 4.1. Sequence Numbering . . . . . . . . . . . . . . . . . . . . 11 4.2. Reliability . . . . . . . . . . . . . . . . . . . . . . . 12 4.3. Buffers . . . . . . . . . . . . . . . . . . . . . . . . . 13 4.4. Signalling . . . . . . . . . . . . . . . . . . . . . . . . 13 4.5. Path Management . . . . . . . . . . . . . . . . . . . . . 14 4.6. Connection Identification . . . . . . . . . . . . . . . . 14 4.7. Network Layer Compatibility . . . . . . . . . . . . . . . 15 4.8. Congestion Control . . . . . . . . . . . . . . . . . . . . 15 5. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 6. Security Considerations . . . . . . . . . . . . . . . . . . . 16 7. Interactions with Applications . . . . . . . . . . . . . . . . 16 8. Interactions with Middleboxes . . . . . . . . . . . . . . . . 16 9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 16 10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 16 11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 16 11.1. Normative References . . . . . . . . . . . . . . . . . . . 16 11.2. Informative References . . . . . . . . . . . . . . . . . . 17 Appendix A. Implementation Architecture . . . . . . . . . . . . . 17 A.1. Functional Separation . . . . . . . . . . . . . . . . . . 18 A.1.1. Application to default MPTCP protocol . . . . . . . . 18 A.1.2. Generic architecture for MPTCP . . . . . . . . . . . . 21 A.2. PM/MPS interface . . . . . . . . . . . . . . . . . . . . . 22 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 23 Ford, et al. Expires August 7, 2010 [Page 3] Internet-Draft MPTCP Architecture February 2010 1. Introduction Multipath TCP (MPTCP) is a set of extensions of regular TCP [2] that 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 [3], congestion control algorithms [4], and application-level considerations [5]. 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. 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 both on the end- systems and within the network. If these resources could instead be used concurrently, end user experience could be greatly improved. Such enhancements would also reduce the necessary expenditure on network infrastructure which would otherwise be needed to create an equivalent improvement in user experience. By the application of resource pooling [6], 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. Multipath TCP as presented in [3] addresses these aims, by achieving resource pooling through splitting a TCP session to run over multiple paths, and presenting it 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 Ford, et al. Expires August 7, 2010 [Page 4] Internet-Draft MPTCP Architecture February 2010 components can be used to create an alternative solution, while still achieving the goals of resource pooling. 1.2. 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]. 1.3. 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 either initiating or terminating a MPTCP connection. Multipath TCP (MPTCP): A modified version of the TCP [2] protocol that supports the simultaneous use of multiple paths between endpoints. Subflow: A flow of TCP packets operating over an individual path, which forms part of a larger MPTCP connection. MPTCP Connection: A set of one or more subflows combined to provide a single Multipath TCP service to an application at an endpoint. 1.4. Reference Scenario TBD - would this be useful? Endpoints, routes. Addresses/path selection mechanisms? 2. Goals This section outlines key goals for Multipath TCP. These are separated into functional goals, i.e. the behaviour that MPTCP must provide, and compatibility goals, i.e. the impact MPTCP must place on other entities. 2.1. Functional Goals The fundamental goal of MPTCP is to use multiple paths (which are not necessarily entirely disjoint) between two endpoints. There are two primary motivations for this goal, which themselves provide functional goals for the design. These are: Ford, et al. Expires August 7, 2010 [Page 5] Internet-Draft MPTCP Architecture February 2010 o Improve Throughput: To do this, MPTCP MUST support the use of multiple paths simultaneously. MPTCP SHOULD NOT reduce the throughput seen below that of legacy TCP operating on any one of the paths. o Improve Resilience: MPTCP MUST support the use of multiple paths interchangeably for resilience purposes, by permitting packets to be sent and re-sent on any available path. It follows that, in the worst case, the protocol MUST be no less resilient than legacy TCP. The secondary benefit of resource pooling is that, as MPTCP should be able to balance traffic among available paths, and respond to congestion appropriately, network utility should be optimized in a global sense by shifting load away from congested bottlenecks and taking advantage of spare capacity wherever it may be located. To support the goal of resource pooling as presented above, a MPTCP host must be able to detect and utilise multiple paths. Impacts on the design of such functions are derived later in Section 3. 2.2. Compatibility Goals In addition to the functional goals listed above, a Multipath TCP must meet a number of compatibility goals in order to support deployment in today's Internet. These goals fall into the following categories: 2.2.1. Application Compatibility Application compatibility refers to the appearance of MPTCP to the application both in terms of the API that can be used and the expected service model that is provided. A multipath-capable equivalent of TCP SHOULD retain backward compatibility with existing APIs, so that existing applications can use the newer transport merely by upgrading the operating systems of the end-hosts. This does not preclude the use of an advanced API to permit multipath-aware applications to specify preferences, nor for users to configure their systems in a different way from the default, for example switching on or off the automatic use of MPTCP. A Multipath TCP MUST follow the same service model as TCP: 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. Ford, et al. Expires August 7, 2010 [Page 6] Internet-Draft MPTCP Architecture February 2010 2.2.2. Network Compatibility In terms of compatibility with the network layer, and devices that operate at the network layer, Multipath TCP 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 [7]. This has an effect on protocol design, in terms of ensuring MPTCP still looks like TCP on the wire, and uses established TCP extensions where appropriate. Secondly, this may require the protocol extensions to feature functionality to allow it to detect and traverse such established middleboxes. 2.2.3. Compatibility with other network users As a corollary to both network and application compatibility, the architecture must enable new Multipath TCP flows to coexist gracefully with existing legacy TCP flows, competing for bandwidth neither unduly aggressively or unduly timidly (unless low-precedence operation is specifically requested by the application, such as with LEDBAT). The use of multiple paths MUST not significantly harm users using single path TCP at shared bottlenecks, beyond the impact that would occur from another single legacy TCP flow. Furthermore, MPTCP SHOULD feature automatic negotiation of its use. A host supporting Multipath TCP that requires the other endpoint to do so too must be able to detect reliably whether this endpoint does in fact support the next-generation protocol, using it if so, and otherwise automatically falling back to the legacy protocol. 3. Multipath Architecture Here we present an architectural view of multipath TCP. The architecture directly follows the protocol goals as presented above, and identifies the practical impact that these functional and compatibility goals will have on the design of the MPTCP solution. 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 (application considerations are discussed in detail in [5]). Although the standard TCP API will still be provided to the application layer, 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 [5]. Ford, et al. Expires August 7, 2010 [Page 7] Internet-Draft MPTCP Architecture February 2010 The MPTCP layer relies upon (what appear to the network to be) standard TCP sessions, termed "subflows", to provide the underlying transport per path, and as such these retain the network compatibility desired. MPTCP as described in [3] carries MPTCP- specific information in a TCP-compatible manner, although this mechanism is separate from the actual information being transferred so could evolve in future revisions. Figure 1 illustrates the layered architecture. +-------------------------------+ | Application | +---------------+ +-------------------------------+ | Application | | MPTCP | +---------------+ + - - - - - - - + - - - - - - - + | TCP | | Subflow (TCP) | Subflow (TCP) | +---------------+ +-------------------------------+ | IP | | IP | IP | +---------------+ +-------------------------------+ Figure 1: Comparison of Standard TCP and MPTCP Protocol Stacks Within the new MPTCP layer, a number of functions are provided that can be identified and, if necessary, implemented separately within a modular architecture. These functions are those for: o Path Management: This is the function to detect and use multiple paths between two endpoints. In the case of the MPTCP design [3], this feature is implemented using multiple IP addresses at least one of the endpoints. Although this does not guarantee path diversity, and there may be shared bottlenecks, this is a simple mechanism that can be used with no additional features in the network. The path management features of the MPTCP protocol are the mechanisms to signal alternative addresses to endpoints, and mechanisms to set up new subflows attached to an existing MPTCP connection. o Packet Scheduling: This function breaks the bytestream received from the application layer into segments which are transmitted on one of the available lower (subflow) layers. The MPTCP design makes use of a data sequence mapping, associating packets sent on different subflows to a connection-level sequence numbering, thus allowing packets sent on different subflows to be correctly re- ordered at the receiver. The packet scheduler is dependent upon information about the availability of paths exposed by the path management component, and then makes use of the subflow layers to transmit these packets. Ford, et al. Expires August 7, 2010 [Page 8] Internet-Draft MPTCP Architecture February 2010 o Subflow (single-path TCP) Interface: The subflow layer takes segments from the packet scheduling component and transmits them over the specified path, ensuring detectable delivery to the endpoint. Detection of delivery is necessary to allow the congestion control protocol to attribute packet delivery or loss to the right path. Note that the packet scheduling layer does not embed enough information in packets to allow this to happen: segments with the same connection-level sequence number can be transmitted over multiple paths, i.e. as retransmissions or just to increase redundancy. MPTCP uses TCP at this layer for network compatibility; TCP ensures in-order, reliable delivery. TCP adds its of sequence numbers to the segments; these are used to detect and retransmit lost packets. o Congestion Control: This function manages congestion control across the subflows. As specified, this congestion control algorithm must ensure that a MPTCP connection does not unfairly take more bandwidth than a single path TCP flow would take at a shared bottlneck. An algorithm to support this is specified in [4]. These functions fit together as follows. The Path Management looks after the discovery (and if necessary, initialisation) of multiple paths between two endpoints. The Packet Scheduler then receives packets from the application for the network and does the necessary operations on them (such as adding a data-level sequence number) before sending to the subflow layer. The subflow layer adds its own sequence number, acks, and passes them to network. The receiving subflow re-orders data and passes it to the multipath layer, which performs connection level re-ordering, removes the segment boundaries and sends it to the application. Finally, the congestion control component exists as part of the packet scheduling, in order to schedule which packets should be sent at what rate on which subflow. 3.1. Decomposing Transport Functions This section provides a generic view of the above functional separation, presenting an extensible model by which transport layer functions can be analysed and developed in a modular fashion. 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 subflow level, while the need for application compatibility will primarily impact design choices at the higher, application-facing Ford, et al. Expires August 7, 2010 [Page 9] Internet-Draft MPTCP Architecture February 2010 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 [8][9] or optimize communication performance [10]. 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 implements the Flow/Endpoint functions. The figure below shows how MPTCP implements this architecture: Ford, et al. Expires August 7, 2010 [Page 10] Internet-Draft MPTCP Architecture February 2010 +--------------------------+ +-------------------------+ | Application | | Application | +--------------------------+ +-------------------------+ | Semantic | | MPTCP | |- - - - - - - - - | + - - - - - + - - - - - + | Flow/Endpt | Flow/Endpt | | TCP | TCP | +--------------------------+ +-------------------------+ | Network | Network | | IP | IP | +--------------------------+ +-------------------------+ Figure 3: Mapping Transport Architecture to MPTCP 4. High-Level Design Decisions There is seemingly a wide range of choices when designing a multipath extension to TCP. However, the goals as discussed earlier in this document constrain the possible solutions, leaving relative little choice in many areas. Here, we outline high-level design choices derived from the architectural requirements, and their implications for complete protocol design. 4.1. Sequence Numbering MPTCP uses two layers of sequence spaces: a connection level sequence number, and another sequence number for each subflow. This permits connection-level segmentation and reassembly, and retransmission of the same part of connection-level sequence space on different subflow-level sequence space. The alternative approach would be to use a single connection level sequence number, which gets sent on multiple subflows. This has two problems: first, the individual subflows will appear to the network as TCP sessions with gaps in the sequence space; this in turn may upset certain middleboxes such as intrusion detection systems, or certain transparent proxies, and would go against the network compatibility goal. Second, the sender cannot attribute packet losses or receptions to the correct path when the same packet is sent on multiple paths, in the case of retransmissions. The sender must be able to tell the receiver how to reorder the data, for delivery to the application. The sender does so by telling the receiver how subflow-level data (carying subflow sequence numbers) maps at connection level, which we refer to as Data Sequence Mapping. This mapping takes the form (data seq, subflow seq, length), i.e. for a given number of bytes (the length), the subflow sequence space beginning at the given sequence number maps to the connection-level sequence space (beginning at the given data seq number). Ford, et al. Expires August 7, 2010 [Page 11] Internet-Draft MPTCP Architecture February 2010 This architecture does not mandate a mechanism for signalling such information, and it could conceivably have various sources. One option would be to use existing fields in the TCP segment (such as subflow seqno, length) and only add the data sequence number to each segment, for instance as a TCP option. This is, however, vulnerable to middleboxes that resegment or assemble data, since there is no specified behaviour for coalescing TCP options. If one signalled (data seqno, length), this would still be vulnerable to middleboxes that coalesce segments and do not correctly coalesce the options. Because of these potential issues, the current specification of MPTCP mandates that the full mapping should be sent to the other end. To reduce the overhead, it would be permissable for the mapping to be sent periodically and cover more than a single segment. It could also be excluded entirely in the case of a connection before more than one subflow is used, where the data-level and subflow-level sequence space is the same. 4.2. Reliability MPTCP uses the data sequence mapping and subflow ACKs to decide when a connection-level segment was received. There are currently no connection-level acks; this decision was made to reduce network overheads. This has certain implications on end-to-end semantics. It means that, once a packet is acked at subflow level it cannot be discarded in the re-order buffer at the connection level. Connection-level MPTCP ACKs are not cumulative, as in TCP. As such, the emergent behaviour is different from standard TCP, where the receiver can simply drop out-of-order segments if needed (for instance, due to memory pressure). It is possible to conceive of some cases where not adding data-level acks could be detrimental to robustness. Consider a subflow traversing a transparent proxy; if the proxy acks a segment and then crashes, the sender will not retransmit the lost segment on another subflow, as it thinks the segment has been received. The connection grinds to a halt despite having other working subflows, and the sender would be unable to determine the cause of the problem. To deal with this case we are considering adding "informative" data- level acks. Regarding retransmissions, it must be possible for a packet to be retransmitted on a different subflow to that on which it was originally sent. This is one of MPTCP's core goals, in order to maintain integrity during temporary or permanent subflow failure, and this is enabled by the dual sequence number space. Ford, et al. Expires August 7, 2010 [Page 12] Internet-Draft MPTCP Architecture February 2010 The scheduling of retransmissions will have significant impact on MPTCP user experience. The current MPTCP specification suggests that data outstanding on subflows that have timed out should be rescheduled for transmission on different subflows. This behaviour aims to minimize disruption when a path breaks, and uses the first timeout as indicators. More conservative versions would be to use second or third timeouts for the same packet. When packet loss is detected and corrected with fast retransmit, retransmission on different subflows may still be desirable in certain cases, for instance to reduce the receive buffer requirements. However, the lost packets MUST still be sent on the path that lost them (this is dictated by our network compatiblity goal), so throughput will be wasted. It is unclear at this point what the optimal retransmit strategy is. 4.3. Buffers Receive Buffer: ideally, a subflow failing should not affect the throughput of other working subflows. However, the receive buffer has limited size: if a flow times out, the other subflows will quickly fill the receive buffer with out-of-order data, and will stall. Hence, receive buffer sizing is important for both robustness and throughput. The smallest receive buffer we need to avoid stalling under any circumstances is max(RTO)*sum(BW). This is, for most multipath connections, too expensive. A more reasonable size is proportional to max(RTT)*sum(BW) which ensures subflows don't stall when fast retransmit works. Also, depending on how the implementation behaves, an additional sum(RTT*BW) might be needed for the individual re-order buffers of the TCP subflows. Send Buffer: the smallest send buffer we need is sum(BDP) across all paths; this is to hold data until it's acked at subflow level. If we didn't use a subflow level ack, and relied on a data-level ack, the send buffer would need to be as big as the receive buffer of the connection, max(RTT)*sum(BW). In practice, the senders will be web servers and receivers will be desktops or mobile servers. The send buffer size matters particularly for servers, which must be able to maintain a large number of ongoing connections. 4.4. Signalling Since MPTCP will use regular TCP streams as its transport mechanism, a MPTCP connection will also begin as a single TCP stream. Nevertheless, it must signal to the peer that it supports MPTCP and wishes to use it on this connection. As such, a TCP Option will be Ford, et al. Expires August 7, 2010 [Page 13] Internet-Draft MPTCP Architecture February 2010 used to transmit this information, since this is the established mechanism for indicating additional functionality on a TCP session. On top of this, however, is signalling required during the operation of an MPTCP session, such as that for reassembly for multiple subflows, and for informing the other endpoint about potential other available addresses. It is not mandated by the architecture in what format this signalling should be transmitted. The current MPTCP protocol proposal suggests the use of TCP options for this signalling, however another approach would be to embed such information in the payload, and use type-length-value (TLV) encoding to separate signalling and payload data. 4.5. Path Management Currently, the network does not expose multiple paths between endpoints. Multipath TCP will use multiple addresses at one or both endpoints to get different paths to the destination. The hope is that these paths, whilst not necesarily entirely non-overlapping, will be sufficiently disjoint to allow multipath achieve improved throughput and robustness. Multiple different (source, destination) address pairs will thus be used as path selectors. For increased chance of successfully setting up additional subflows (such as when one end is behind a firewall, NAT, or other restrictive middlebox), either endpoint should be able to add new subflows to a MPTCP connection. The modularity of path management will permit alternative mechanisms to be employed if appropriate in the future. 4.6. Connection Identification Since an MPTCP connection may not be bound to a traditional 5-tuple (source addr and port, destination addr and port, protocol number) for the entirity of its existance, it is desirable to provide a new mechanism for connection identification. This will be useful for MPTCP-aware applications, and for the MPTCP implementation (and MPTCP-aware middleboxes) to have a unique identifier with which to associate the multiple subflows. Therefore, each MPTCP connection should have a connection identifier at each endpoint, which is locally unique within that endpoint. This is analogous to a port number in regular TCP. The manifestation and purpose of such an identifier is out of the scope of this Ford, et al. Expires August 7, 2010 [Page 14] Internet-Draft MPTCP Architecture February 2010 architecture document. For legacy applications, however, a MPTCP connection will be identified by the 5-tuple of the first TCP subflow. [TBD: This will continue to be the case even if that subflow closes / even if an address disappears / the connection will close in that case unless the extended API has been used / etc]. 4.7. Network Layer Compatibility MPTCP's modifications remain at the TCP layer, although some knowledge of the underlying IP layer is required. MPTCP MUST work with IPv4 and IPv6 interchangeably, i.e. one MPTCP connection may operate over both IPv4 and IPv6 networks. 4.8. Congestion Control As already documented in network-layer compatibility requirements, the congestion control algorithms used by an MPTCP implementation must not harm other legacy users on shared bottlenecks. To achieve this, the congestion control algorithms on use on each subflow must be coupled in some way - a proposal for this is given in [4]. 5. Summary This document has provided a summary of the components that have been identified to provide a Multipath TCP solution, and described the high-level design decisions that have been used as a basis of the MPTCP specification. The suite of drafts that specify a complete MPTCP implementation, on top of this architectural overview, are as follows: o A specification of the MPTCP protocol [3], describing the on- and off-the-wire differences to regular TCP. o A specification of a coupled congestion control algorithm [4], that can be applied to the above protocol while meeting the goals for such an algorithm as specified in this document. o A document [5] that builds upon the application compatibility issues discussed in this document, explaining in more detail what if any changes an application may experience through the use of MPTCP. This document also provides a proposed API through which an application can influence the behaviour of the MPTCP protocol, as specified in the above drafts. Ford, et al. Expires August 7, 2010 [Page 15] Internet-Draft MPTCP Architecture February 2010 6. Security Considerations Please see [11] for a threat analysis of Multipath TCP. The threats analysed in this companion document are addressed as appropriate in the protocol design [3]. 7. 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 [5]. 8. 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. (Not sure we really need this section any more) 9. 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. 10. IANA Considerations None. 11. References 11.1. Normative References [1] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. Ford, et al. Expires August 7, 2010 [Page 16] Internet-Draft MPTCP Architecture February 2010 11.2. Informative References [2] Postel, J., "Transmission Control Protocol", STD 7, RFC 793, September 1981. [3] Ford, A., Raiciu, C., and M. Handley, "TCP Extensions for Multipath Operation with Multiple Addresses", draft-ford-mptcp-multiaddressed-02 (work in progress), October 2009. [4] Raiciu, C., Handley, M., and D. Wischik, "Coupled Multipath- Aware Congestion Control", draft-raiciu-mptcp-congestion-00 (work in progress), October 2009. [5] Scharf, M. and A. Ford, "MPTCP Application Interface Considerations", draft-scharf-mptcp-api-00 (work in progress), October 2009. [6] Wischik, D., Handley, M., and M. Bagnulo Braun, "The Resource Pooling Principle", ACM SIGCOMM CCR vol. 38 num. 5, pp. 47-52, October 2008, . [7] Carpenter, B. and S. Brim, "Middleboxes: Taxonomy and Issues", RFC 3234, February 2002. [8] Srisuresh, P. and K. Egevang, "Traditional IP Network Address Translator (Traditional NAT)", RFC 3022, January 2001. [9] Freed, N., "Behavior of and Requirements for Internet Firewalls", RFC 2979, October 2000. [10] Border, J., Kojo, M., Griner, J., Montenegro, G., and Z. Shelby, "Performance Enhancing Proxies Intended to Mitigate Link-Related Degradations", RFC 3135, June 2001. [11] Bagnulo, M., "Threat Analysis for Multi-addressed/Multi-path TCP", draft-bagnulo-mptcp-threat-00 (work in progress), October 2009. [12] Allman, M., Paxson, V., and W. Stevens, "TCP Congestion Control", RFC 2581, April 1999. Appendix A. Implementation Architecture This section provides suggestions for an architecture to implement an extensible, modular multipath transport protocol. Ford, et al. Expires August 7, 2010 [Page 17] Internet-Draft MPTCP Architecture February 2010 A.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 [3]. Then we generalize the approach to allow good extensibility of that solution. A.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 August 7, 2010 [Page 18] Internet-Draft MPTCP Architecture February 2010 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 [3], 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 August 7, 2010 [Page 19] Internet-Draft MPTCP Architecture February 2010 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 [3]): 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 August 7, 2010 [Page 20] Internet-Draft MPTCP Architecture February 2010 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. A.1.2. Generic architecture for MPTCP 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 August 7, 2010 [Page 21] Internet-Draft MPTCP Architecture February 2010 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. A.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 August 7, 2010 [Page 22] Internet-Draft MPTCP Architecture February 2010 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. 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 Ford, et al. Expires August 7, 2010 [Page 23] Internet-Draft MPTCP Architecture February 2010 Costin Raiciu University College London Gower Street London WC1E 6BT UK Email: c.raiciu@cs.ucl.ac.uk 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 August 7, 2010 [Page 24]