Internet Engineering Task Force (IETF)                         M. Scharf
Request for Comments: 6897                      Alcatel-Lucent Bell Labs
Category: Informational                                          A. Ford
ISSN: 2070-1721                                                    Cisco
                                                              March 2013


       Multipath TCP (MPTCP) Application Interface Considerations

Abstract

   Multipath TCP (MPTCP) adds the capability of using multiple paths to
   a regular TCP session.  Even though it is designed to be totally
   backward compatible to applications, the data transport differs
   compared to regular TCP, and there are several additional degrees of
   freedom that applications may wish to exploit.  This document
   summarizes the impact that MPTCP may have on applications, such as
   changes in performance.  Furthermore, it discusses compatibility
   issues of MPTCP in combination with non-MPTCP-aware applications.
   Finally, the document describes a basic application interface that is
   a simple extension of TCP's interface for MPTCP-aware applications.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for informational purposes.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Not all documents
   approved by the IESG are a candidate for any level of Internet
   Standard; see Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc6897.














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Copyright Notice

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   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
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   described in the Simplified BSD License.

Table of Contents

   1. Introduction ....................................................3
   2. Terminology .....................................................4
   3. Comparison of MPTCP and Regular TCP .............................5
      3.1. Effect on Performance ......................................5
           3.1.1. Throughput ..........................................5
           3.1.2. Delay ...............................................6
           3.1.3. Resilience ..........................................7
      3.2. Potential Problems .........................................8
           3.2.1. Impact of Middleboxes ...............................8
           3.2.2. Dealing with Multiple Addresses inside
                  Applications ........................................9
           3.2.3. Security Implications ..............................10
   4. Operation of MPTCP with Legacy Applications ....................10
      4.1. Overview of the MPTCP Network Stack .......................10
      4.2. Address Issues ............................................11
           4.2.1. Specification of Addresses by Applications .........11
           4.2.2. Querying of Addresses by Applications ..............12
      4.3. MPTCP Connection Management ...............................13
           4.3.1. Reaction to Close Call by Application ..............13
           4.3.2. Other Connection Management Functions ..............13
      4.4. Socket Option Issues ......................................13
           4.4.1. General Guideline ..................................13
           4.4.2. Disabling of the Nagle Algorithm ...................13
           4.4.3. Buffer Sizing ......................................14
           4.4.4. Other Socket Options ...............................14
      4.5. Default Enabling of MPTCP .................................14
      4.6. Summary of Advice to Application Developers ...............15







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RFC 6897                        MPTCP API                     March 2013


   5. Basic API for MPTCP-Aware Applications .........................15
      5.1. Design Considerations .....................................15
      5.2. Requirements on the Basic MPTCP API .......................16
      5.3. Sockets Interface Extensions by the Basic MPTCP API .......17
           5.3.1. Overview ...........................................17
           5.3.2. Enabling and Disabling of MPTCP ....................19
           5.3.3. Binding MPTCP to Specified Addresses ...............19
           5.3.4. Querying the MPTCP Subflow Addresses ...............20
           5.3.5. Getting a Unique Connection Identifier .............20
   6. Other Compatibility Issues .....................................21
      6.1. Usage of TLS over MPTCP ...................................21
      6.2. Usage of the SCTP Sockets API .............................21
      6.3. Incompatibilities with Other Multihoming Solutions ........21
      6.4. Interactions with DNS .....................................22
   7. Security Considerations ........................................22
   8. Conclusion .....................................................23
   9. Acknowledgments ................................................23
   10. References ....................................................24
      10.1. Normative References .....................................24
      10.2. Informative References ...................................