Network Working Group M. Boucadair, Ed.
Internet-Draft C. Jacquenet, Ed.
Intended status: Standards Track Orange
Expires: September 11, 2017 O. Bonaventure, Ed.
Tessares
D. Behaghel
OneAccess
S. Secci
UPMC
W. Henderickx, Ed.
Nokia/Alcatel-Lucent
R. Skog, Ed.
Ericsson
S. Vinapamula
Juniper
S. Seo
Korea Telecom
W. Cloetens
SoftAtHome
U. Meyer
Vodafone
LM. Contreras
Telefonica
B. Peirens
Proximus
March 10, 2017

Extensions for Network-Assisted MPTCP Deployment Models
draft-boucadair-mptcp-plain-mode-10

Abstract

Because of the lack of Multipath TCP (MPTCP) support at the server side, some service providers now consider a network-assisted model that relies upon the activation of a dedicated function called MPTCP Conversion Point (MCP). Network-Assisted MPTCP deployment models are designed to facilitate the adoption of MPTCP for the establishment of multi-path communications without making any assumption about the support of MPTCP by the communicating peers. MCPs located in the network are responsible for establishing multi-path communications on behalf of endpoints, thereby taking advantage of MPTCP capabilities to achieve different goals that include (but are not limited to) optimization of resource usage (e.g., bandwidth aggregation), of resiliency (e.g., primary/backup communication paths), and traffic offload management.

This document specifies extensions for Network-Assisted MPTCP deployment models.

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 [RFC2119].

Status of This Memo

This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.

Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at http://datatracker.ietf.org/drafts/current/.

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."

This Internet-Draft will expire on September 11, 2017.

Copyright Notice

Copyright (c) 2017 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 Simplified BSD License.


Table of Contents

1. Introduction

The overall quality of connectivity services can be enhanced by combining several access network links for various purposes - resource optimization, better resiliency, etc. Some transport protocols, such as Multipath TCP [RFC6824], can help achieve such better quality, but failed to be massively deployed so far.

The support of multipath transport capabilities by communicating hosts remains a privileged target design so that such hosts can directly use the available resources provided by a variety of access networks they can connect to. Nevertheless, network operators do not control end hosts while the support of MPTCP by content servers remains close to zero.

Network-Assisted MPTCP deployment models are designed to facilitate the adoption of MPTCP for the establishment of multi-path communications without making any assumption about the support of MPTCP capabilities by communicating peers. Network-Assisted MPTCP deployment models rely upon MPTCP Conversion Points (MCPs) that act on behalf of hosts so that they can take advantage of establishing communications over multiple paths. MCPs can be deployed in CPEs (Customer Premises Equipment), as well as in the provider's network. MCPs are responsible for establishing multi-path communications on behalf of endpoints. Further details about the target use cases are provided in Section 3.

Most of the current operational deployments that take advantage of multi-interfaced devices rely upon the use of an encapsulation scheme (such as [I-D.zhang-gre-tunnel-bonding], [TR-348]). The use of encapsulation is motivated by the need to steer traffic towards the concentrator and also to allow the distribution of any kind of traffic besides TCP (e.g., UDP) among the available paths without requiring any advanced traffic engineering tweaking technique in the network to intercept traffic and redirect it towards the appropriate MCP.

Current operational MPTCP deployments by network operators are focused on the forwarding of TCP traffic. The design of such deployments sometimes assumes the use of extra signalling provided by SOCKS [RFC1928], at the cost of additional management complexity and possible service degradation (e.g., up to 6 SOCKS messages may have to be exchanged between two MCPs before actual payload data to be transferred, thereby yielding several tens of milliseconds of extra delay before the connection is established) .

To avoid the burden of encapsulation and additional signalling between MCPs, this document explains how a plain transport mode is enabled, so that packets are exchanged between a device and its upstream MCP without requiring the activation of any encapsulation scheme (e.g., IP-in-IP [RFC2473], GRE [RFC1701]). This plain transport mode also avoids the need for out-of-band signalling, unlike the aforementioned SOCKS context.

The solution described in this document also works properly when NATs are present in the communication path between a device and its upstream MCP. In particular, the solution in this document accommodates deployments that involve CGN (Carrier Grade NAT) upstream the MCP.

Network-Assisted MPTCP deployment and operational considerations are discussed in [I-D.nam-mptcp-deployment-considerations].

The plain transport mode is characterized as follows:

2. Terminology

The reader should be familiar with the terminology defined in [RFC6824].

This document makes use of the following terms:

3. Target Use Cases

We consider two important use cases in this document. We briefly introduce them in this section and leave the details to Section 6 and Section 7. The first use case is a Multipath Client that interacts with a remote Server through a MCP (Section 3.1). The second use case is a multi-homed CPE that includes a MCP and interacts with a remote Server through a downstream MCP (Section 3.2).

3.1. Multipath Client

In this use case, the Multipath Client would like to take advantage of MPTCP even if the Server does not support MPTCP. A typical example is a smartphone that could use both WLAN and LTE access networks to reach a server in order to achieve higher bandwidth or better resilience.

+--+                                      +-----+      +--+
|C |                                      | MCP |      |S |
+--+                                      +-----+      +--+
 |                                           |           |
 |<==================MPTCP Leg==============>|<---TCP -->|
 |                                           |           |

Legend:
    C: Client
  MCP: Multipath Conversion Point
    S: Server

Figure 1: Network-assisted MPTCP (Host-based Model)

Figure 1, the MCP terminates the MPTCP connection established by the client and binds it to a TCP connection towards the remote server. Two deployments of this use case are possible.

A first deployment is when the MCP is on the path between the Multipath Client and the Server. In this case, the MCP can terminate the MPTCP connection initiated by the Client and binds it to a TCP connection that the MCP establishes with the Server. When the MCP is not located on all default forwarding paths, the MPTCP connection must be initiated by using the path where the MCP is located.

A second deployment is when the MCP is not on the path between the Multipath Client and the Server. In this case, the Client must first initiate a connection towards the MCP and request it to initiate a TCP connection towards the Server. This is what the SOCKS protocol performs by exchanging control messages to create appropriate mappings to handle the connection. Unfortunately, this requires additional round-trip-time that affects the performance of the end-to-end data transfer, in particular for short-lived connections.

This document specifies the MP_CONVERT Information Element that is carried in the SYN segment of the initial subflow. This SYN segment is sent towards the MCP. The MP_CONVERT Information Element contains the destination address (and optionally a port number) of the Server. Thanks to this information, the MCP can immediately establish the TCP connection with the Server without any additional round-trip-time, unlike a SOCKS-based MPTCP design.

3.2. Multipath CPE

In this use case, neither the Client nor the Server support MPTCP. Two MCPs are used as illustrated in Figure 2. The upstream MCP is embedded in the CPE while the downstream MCP is located in the provider's network. The CPE is attached to multiple access networks (e.g., xDSL and LTE). The upstream MCP transparently terminates the TCP connections initiated by the Client and converts them into MPTCP connections.

          Upstream                      Downstream
+--+      +-----+                         +-----+      +--+
|H1|      | MCP |                         | MCP |      |RM|
+--+      +-----+                         +-----+      +--+
 |           |                              |           |
 |<---TCP--->|<========MPTCP Leg===========>|<---TCP--->|
 |           |                              |           |

Figure 2: Network-assisted MPTCP (CPE-based Model)

The same considerations detailed in Section 3.1 apply for the insertion of the downstream MCP in an MPTCP connection.

4. The MP_PREFER_PROXY MPTCP Option

The implicit mode assumes that the MCP is located on a default forwarding path (Section 5.2.2 of [I-D.nam-mptcp-deployment-considerations]). In such mode, the first subflow must always be placed over that primary path so that the MCP can intercept MPTCP flows. Once intercepted, the MCP advertises its reachability information by means of MPTCP signals (MP_JOIN or ADD_ADDR).

In order to distinguish native MPTCP connections from proxied ones, a new MPTCP option, called MP_PREFER_PROXY, is defined. This option is meant to inform an on-path MCP that the connection should be proxied. The absence of the MP_PREFER_PROXY option is an indication that the corresponding MPTCP connection is native: an on-path MCP must not be involved in such connection. If no explicit signal is included in the initial SYN message, the MCP cannot distinguish "native" MPTCP connections from "proxied" ones.

4.1. Option Format

The format of the MP_PREFER_PROXY is shown in Figure 3.

                           1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +---------------+---------------+-------+-----------------------+
      |     Kind      |    Length     |Subtype|         Reserved      |
      +---------------+---------------+-------+-----------------------+

Figure 3: MP_PREFER_PROXY MPTCP Option

  • Kind and Length: are the same as those defined in Section 3 of [RFC6824]. The size of this option is 4 bytes.
  • Subtype: must be allocated by IANA (Section 9).
  • "Reserved" bits: are reserved bits for future assignment as additional flag bits. These additional flag bits MUST each be set to zero and MUST be ignored upon receipt.

4.2. Option Processing

The MP_PREFER_PROXY option MUST only appear in the SYN message used to create the initial subflow of a Multipath TCP connection.

If the MP_PREFER_PROXY appears in either a SYN segment that does not include the MP_CAPABLE option or a segment whose SYN flag is unset, it MUST be ignored. An implementation MAY log this event since it likely indicates an operational issue.

The sender inserts the MP_PREFER_PROXY option for MPTCP connections that it wants to be proxied by an on-path MCP. Such insertion is possible only when there is enough space left in the dedicated TCP option space.

Upon receipt of a SYN message with an MP_CAPABLE, the MCP MUST check whether an MP_PREFER_PROXY option is present:[I-D.nam-mptcp-deployment-considerations] details the use of the MP_PREFER_PROXY option.

  • If no such option is included, the MCP MUST NOT interfere with that MPTCP connection (that is, it must not track this MPTCP connection). Processing subsequent subflows of this connection will be handled directly by the endpoints.
  • If the MP_PREFER_PROXY option is present, the MCP MUST track the establishment of the connection. That means that the MCP must be prepared to insert itself for the establishment of subsequent subflows, in particular.

Section 5.2.2.1 of

5. Supplying Data to MCPs

This section focuses mainly on th explicit mode (Section 5.2.1 of [I-D.nam-mptcp-deployment-considerations]) which assumes that the IP reachability information of an MCP is explicitly configured on a device, e.g., by means of a specific DHCP option [I-D.boucadair-mptcp-dhc].

5.1. The MP_CONVERT Information Element

In order to avoid extra delays when establishing a proxied MPTCP connection, specific information are provided to an MCP during the 3WHS. Such information is meant to help the MCP instantiate the required states to process the connection upstream. The supply of such information is achieved by means of an object called the MP_CONVERT (MC) Information Element (IE). This information element typically carries the source/destination IP addresses and/or port numbers of the used by the source and destination endpoints. Other information may also be supplied to an MCP; future extensions may be defined.

The format of the MP_CONVERT Information Element is shown in Figure 4.

0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+---------------------------------------------------------------+
|                        Magic Number                           |
+---------------+---------------+---------------------------+-+-+
|     Type      |     Length    |         Reserved          |D|M|
+---------------+---------------+---------------------------+-+-+
|          Address (IPv4 - 4 octets / IPv6 - 16 octets)         |
+-------------------------------+-------------------------------+
|   Port (2 octets, optional)   |
+-------------------------------+

Figure 4: MP_CONVERT Information Element

  • Magic Number: This field MUST be set to "0xFAA8 0xFAA8" to indicate this is an MP_CONVERT Information Element. This field is meant to unambiguously distinguish any data supplied by an application from the one injected by an MCP. Other magic numbers are considered by the authors (e.g., 64 bits that include in addition to "0xFAA8 0xFAA8" 32 bits to enclose the RFC number).
  • Type: This field indicates the type of the MP_CONVERT Information Element. It MUST be set to 0 to indicate this element includes an IP address and, eventually, a port number. Other type values MAY be defined in the future.
  • Length: Indicates, in bytes, the length of MP_CONVERT Information Element. The minimum size of this option is 4 bytes.
  • "Reserved" bits: are reserved bits for future assignment as additional flag bits. These additional flag bits MUST each be set to zero and MUST be ignored upon receipt.
  • D-bit (Direction bit): this flag indicates whether the enclosed IP address (and port number) reflects the source or the destination IP address (and port number). When the D-bit is set, the enclosed IP address must be interpreted as the source IP address. When the D-bit is unset, the enclosed IP address must be interpreted as the destination IP address.
  • M-bit (More bit): When the M-bit is unset, it indicates that another MP_CONVERT IE is included. When the M-bit is set, it indicates this is the last MP_CONVERT IE included in the payload; if any data is placed right after this MP_CONVERT IE, it is application data.
  • Address: includes a source or destination IP address. The address family is determined by the "Length" field. Concretely, a MP_CONVERT Information Element that carries an IPv4 address has a Length field of 8 bytes (or 10, if a port number is included). A MP_CONVERT Information Element that carries an IPv6 address has a Length of 20 bytes (or 22, if a port number is included).
  • Port: If the D-bit is set (resp. unset), a source (resp. destination) port number may be associated with the IP address. This field is valid for protocols that use a 16 bit port number (e.g., UDP, TCP, SCTP). This field is optional.

If the length of MP_CONVERT Information Element is not a multiple of 4 bytes, padding MUST be added to preserve 32 bits boundaries.

5.2. Processing an MP_CONVERT Information Element

The MP_CONVERT Information Element is a variable length object that MUST NOT be used in TCP segments whose SYN flag is unset. This IE can only appear in the TCP control messages with SYN flag set. The information carried in the MP_CONVERT IE is used by an MCP to create the initial subflow of a Multipath TCP connection (see the example in Figure 5).

Up to two MP_CONVERT Information Elements with type set to zero can appear inside a SYN segment. If two MP_CONVERT Information Elements with type zero are included, these options MUST NOT have the same D-bit value.

+----+                              +-----+       +--+
|  C |                              | MCP |       |S |
+----+                              +-----+       +--+
 |  | ________________________________|             |
 |  /       Initial subflow           \             |
 |  |========SYN(MP_CAPABLE+MC(S))===>|             |
 |  |                                 |--SYN------->|
 |  |                                 |<--SYN/ACK---|
 |  |<====SYN/ACK(MP_CAPABLE)=========|             |
 |  |             ...                 |             |
 |  \ ________________________________/             |
                 ....                      ....
 |  | ________________________________|             |
 |  /       Additional subflow        \             |
 |  \ ________________________________/             |

Legend:
     <===>: MPTCP leg
     <--->: TCP leg
      MC(): MP_CONVERT Information Element

Figure 5: Carrying the MP_CONVERT Information Element

If the MP_CONVERT Information Element appears in either a SYN segment that does not include the MP_CAPABLE option or a segment whose SYN flag is reset, it MUST be ignored. An implementation MAY log this event since it likely indicates an operational issue.

If the original SYN message contains data in its payload (e.g., [RFC7413]), that data MUST be placed right after the MP_CONVERT IEs when generating the SYN in the MPTCP leg.

An implementation MUST ignore MP_CONVERT Information Elements that include multicast, broadcast, and host loopback addresses [RFC6890]. Concretely, an implementation that receives an MP_CONVERT Information Element with such addresses MUST silently tear down the MPTCP connection.

An implementation that supports the MP_CONVERT Information Element with type zero MUST echo in the SYN/ACK the instances of the MP_CONVERT Information Elements included in a SYN received from the sender. A sender that does not receive in a SYN/ACK a copy of the MP_CONVERT Information Elements it included in a SYN message MUST terminate the MPTCP connection and falls back to TCP or native MPTCP connection. Furthermore, the sender MUST add an entry to its local cache to record the MCPs that do not support the MP_CONVERT Information Element. This cache MUST be flushed out under the following conditions: a new network attachment is detected by the host, a new MCP is configured, the host gets a new IP address/prefix, or a TTL has expired. Subsequent connections to an MCP in the cache MUST NOT be placed using the explicit proxy mode. This procedure is denoted as MCP capability discovery.

In the following sections, MP_CONVERT Information Element is used to refer to the MP_CONVERT Information Element with the type field set to zero. Future documents will specify the exact behavior of processing MP_CONVERT Information Elements with a non zero type field.

6. MPTCP Connections from a Multipath TCP Client

6.1. Description

The simplest usage of the MP_CONVERT Information Element is when a Multipath TCP Client wants to use MPTCP to efficiently utilise different network paths (e.g., WLAN and LTE from a smartphone) to reach a server that does not support Multipath TCP. The basic operation is illustrated in Figure 6.

To use its multipath capabilities to establish an MPTCP connection over the available networks, the Client splits its end-to-end connection towards the TCP Server into two:

(1)
An MPTCP connection, that typically relies upon the establishment of one subflow per network path, is established between the client and the MCP.
(2)
A TCP connection that is established by the MCP with the server.

Any data that is eligible to be transported over the MPTCP connection is sent by the Client towards the MCP over the MPTCP connection. The MCP then forwards these data over the regular TCP connection until they reach the server. The same forwarding principle applies for the data sent by the Server over the TCP connection with the MCP.

     C <===========>MCP <------------> S
     +<============>+

Legend:
  <===>: subflows of the upstream MPTCP connection
  <--->: downstream TCP connection

Figure 6: A Multipath TCP Client interacts with a Server through a Multipath Conversion Point

6.2. Theory of Operation

We assume in this section that the Multipath TCP Client has been configured with the IP address of one or more MCPs which convert the Multipath TCP connection into a regular TCP connection. The address of such MCPs can be statically configured on the Client, dynamically provisioned to the MPTCP Client by means of a DHCP option [I-D.boucadair-mptcp-dhc], or by any other means that are outside the scope of this document.

Conceptually, the MCP acts as a relay between an upstream MPTCP connection and a downstream TCP connection. The MCP has at least a single IP address that is reachable from the Multipath TCP Client. It may be assigned other IP addresses. For the sake of simplicity, we assume in this section that the MCP has a single IP address denoted MCP@. Similarly, we assume that the client has two addresses C@1 and C@2 while address S@ is assigned to the server.

The MCP maps an upstream MPTCP connection (and its associated subflows) onto a downstream TCP connection. On the MCP, an established Multipath TCP connection can be identified by the local Token that was assigned upon reception of the SYN segment.

This Token is guaranteed to be unique on the MCP (provided that it has a single IP address) during the entire lifetime of the MPTCP connection. The 4-tuple (IP src, IP dst, Port src, Port dst) is used to identify the downstream TCP connection.

To initiate a connection to a remote server S, the Multipath TCP Client sends a SYN segment towards the MCP that includes the MP_CONVERT Information Element described in Figure 4. The destination address of the SYN segment is the IP address of the MCP. The MP_CONVERT Information Element included in the SYN contains the IP address and optionally the destination port of the Server (see Figure 7).

+----+                              +-----+    +--+
|  C |                              | MCP |    |S |
+----+                              +-----+    +--+
C@1 C@2                              MCP@       S@
 |  | ________________________________|          |
 |  /       Initial subflow           \          |
 |  |=======SYN(MP_CAPABLE+MC(S@))===>|          |
 |  |                                 |--SYN---->|
 |  |                                 |<-SYN/ACK-|
 |  |<====SYN/ACK(MP_CAPABLE)=========|          |
 |  |             ...                 |          |
 |  \ ________________________________/          |
                 ....                    ....
 |  |________________________________ |          |
 | /       Additional subflow        \|          |
 | \ ________________________________/           |

Legend:
     <===>: MPTCP leg
     <--->: TCP leg 

Figure 7: Single-ended MCP Flow Example

The MCP processes this SYN segment as follows. First, it generates the local key and a unique Token for the Multipath TCP connection. This Token identifies the MPTCP connection. It is passed to the MCP together with the contents of the MP_CONVERT Information Element (i.e., the address of the destination server) and the destination port.

The MCP then establishes a TCP connection with the destination server. If the received MP_CONVERT Information Element contains a port number, it is used as the destination port of the outgoing TCP connection that is being established by the MCP. Otherwise, the destination port of the upstream MPTCP connection is used as the destination port of the downstream TCP connection. The MCP creates a flow entry for the downstream TCP connection and maps the upstream MPTCP connection onto the downstream TCP connection.

The downstream TCP connection is considered to be active upon reception of the SYN/ACK segment sent by the destination server. The reception of this segment triggers the MCP that confirms the establishment of the upstream MPTCP connection by sending a SYN/ACK segment towards the Multipath TCP Client (including MP_Convert).

At this point, there are two established connections. The endpoints of the upstream Multipath TCP connection are the Multipath TCP Client and the MCP. The endpoints of the downstream TCP connection are the MCP and the Server. These two connections are bound by the MCP.

All the techniques defined in [RFC6824] can be used by the upstream Multipath TCP connection. In particular, the subflows established over the different network paths can be controlled by either the Multipath TCP Client or the MCP. It is likely that the network operators that deploy MCPs will define policies for the utilisation of the MCP. These policies are discussed in Section 5.6 of [I-D.nam-mptcp-deployment-considerations].

Any data received by the MCP on the upstream Multipath TCP connection will be forwarded by the MCP over the bound downstream TCP connection. The same applies for data received over the downstream TCP connection which will be forwarded by the MCP over the upstream Multipath TCP connection.

One of the functions of the MCP is to maintain the binding between the upstream Multipath TCP connection and the downstream TCP connection. If the downstream TCP connection fails for some reason (excessive retransmissions, reception of a RST segment, etc.), then the MCP SHOULD force the teardown of the upstream Multipath TCP connection by transmitting a FASTCLOSE. Similarly, if the upstream Multipath TCP connection fails for some reason (e.g., reception of a FASTCLOSE), the MCP SHOULD tear the downstream TCP connection down and remove the flow entries.

The same reasoning applies when the upstream Multipath TCP connection ends with the transmission of DATA_FINs. In this case, the MCP SHOULD also terminate the bound downstream TCP connection by using FIN segments. If the downstream TCP connection terminates with the exchange of FIN segments, the MCP SHOULD initiate a graceful termination of the bound upstream Multipath TCP connection.

An MCP SHOULD associate a lifetime with the Multipath TCP and TCP flow entries. In this case, it SHOULD use the same lifetime for each pair of bounded connections.

7. MPTCP Connections Between Single Path Client and Server

7.1. Description

There are situations where neither the client nor the server can use multipath transport protocols albeit network providers would want to optimize network resource usage by means of multi-path communication techniques. Hybrid access service offerings are typical business incentives for such situations, where network operators combine a fixed network (e.g., xDSL) with a wireless network (e.g., LTE). In this case, as illustrated in Figure 8, two MCPs are used for each flow. The first MCP, located downstream of the client, converts the single path TCP connection originated from the client into a Multipath TCP connection established with a second MCP. The latter will then establish a TCP connection with the destination server.

          Upstream        Downstream
     C <---> MCP <===========> MCP <------------> S
               +<=============>+

Legend:
     <===>: MPTCP leg
     <--->: TCP leg 

Figure 8: A Client interacts with a Server through an upstream and a downstream Multipath Conversion Points

7.2. Theory of Operation

7.2.1. Downstream MCP

The downstream MCP can be deployed on-path or off-path. If the downstream MCP is deployed off-path, its behavior is described in Section 6.2.

If the downstream MCP is deployed on-path, it only terminates MPTCP connections that carry an empty MP_PREFER_PROXY option inside their SYN (i.e., no address is conveyed). If the MCP receives a SYN segment that contains the MP_CAPABLE option but no MP_PREFER_PROXY, it MUST forward the SYN to its final destination without any modification.

7.2.2. Upstream MCP

The upstream and downstream MCPs cooperate. The upstream MCP may be configured with the addresses of downstream MCPs. If the downstream MCP is deployed on-path, the upstream MCP inserts an MP_PREFER_PROXY option.

In this section, we assume that the upstream MCP has been configured with one address of the downstream MCP. This address can be configured statically, dynamically distributed by means of a DHCP option [I-D.boucadair-mptcp-dhc], or by any other means that are outside the scope of this document.

We assume that the upstream MCP has two addresses uMCP@1 and uMCP@2 while the downstream MCP is assigned a single IP address dMCP@.

The upstream MCP maps an upstream TCP connection onto a downstream MPTCP connection (and its associated subflows) . On the upstream MCP, an established MPTCP connection can be identified by the local Token that was assigned upon reception of the SYN segment from the Client.

The Client sends a SYN segment addressed to the Server and it is intercepted by the upstream MCP which in turns initiates an MPTCP connection towards its downstream MCP that includes the MP_CONVERT Information Element described in Figure 4. The destination address of the SYN segment is the IP address of the downstream MCP. The MP_CONVERT Information Element included in the SYN contains the IP address and optionally the destination port of the Server; this information is extracted from the SYN message received over the upstream TCP connection.

Concretely, the upstream MCP processes the SYN segment received from the Client as follows.

First, it generates the local key and a unique Token for the Multipath TCP connection to identify the MPTCP connection. It extracts the destination IP address and, optionally, the destination port that will then be carried in a MP_CONVERT Information Element. The upstream MCP establishes an MPTCP connection with the downstream MCP. The upstream MCP creates a flow entry for the downstream MPTCP connection and maps the upstream TCP connection onto the downstream MPTCP connection.

The downstream MPTCP connection is considered to be active upon reception of the SYN+ACK segment from the downstream MCP. The reception of this segment triggers the upstream MCP that confirms the establishment of the upstream TCP connection by sending a SYN+ACK segment towards the TCP Client.

At this point, there are two established connections maintained by the upstream MCP:

(1)
The endpoints of the upstream TCP connection are the Client and the upstream MCP.
(2)
The endpoints of the downstream MPTCP connection are the upstream MCP and the downstream MCP.

These two connections are bound by the upstream MCP. An example is shown in Figure 9.

          Upstream                     Downstream
+--+      +-----+                        +-----+      +--+
|C1|      | MCP |                        | MCP |      |S1|
+--+      +-----+                        +-----+      +--+
 C@1   uMCP@1 uMCP@2                       dMCP@       S@
  |         | |______________________________|          |
  |--SYN--->|/       Initial subflow         \          |
  |         |=======SYN(MP_CAPABLE+MC(S@))==>|          |
  |         |                                |--SYN---->|
  |         |                                |<-SYN/ACK-|
  |         |<====SYN/ACK(MP_CAPABLE)========|          |
  |<SYN/ACK-|              ...               |          |
  |          \ ______________________________/          |
                         ....                    ....
  |         | | ____________________________ |          |
  |         | |/       Additional subflow   \|          |
  |         | |\ ___________________________/|          |
                             ....

Figure 9: Dual-Ended MCP Flow Example

[RFC6824] can be used by the MPTCP connection. In particular, the utilisation of the different network paths can be controlled by one MCP or the other.

Any data received by the upstream MCP over the upstream TCP connection will be forwarded by the MCP over the bound downstream MPTCP connection, assuming such data are eligible to MPTCP transport. The same applies for data received over the downstream MPTCP connection which will be forwarded by the upstream MCP over the upstream TCP connection.

The same considerations as in Section 6.2 apply for the maintenance of the connections by the upstream MCP.

8. Interaction with TFO

This section discusses the implications of using MP_CONVERT Information Elements with TCP Fast Open (TFO). We distinguish between TFO negotiation (i.e., a Fast Open option with an empty cookie field to request a cookie) and TFO data (i.e., SYN with data and the cookie in the Fast Open option).

This section focuses on the implications of using MP_CONVERT Information Element on TFO efficiency. Implications related to MPTCP options and TFO negotiation are not specific to this document; the reader may refer to [I-D.barre-mptcp-tfo].

Distinct implications are assessed depending whether TFO negotiation and usage occurs before MCP capability discovery phase is completed or not (Section 5.2). Concretely, the following cases are discussed:

  1. MCP capability discovery was already completed prior to receiving a message with TFO negotiation or TFO data: For this case, the host has already contacted its MCP in the context of a prior connection. The outcome of such connections is used to determine the capabilities of its MCP (Section 5.2).
    1. The MCP supports MP_CONVERT Information Element: Any information provided to an MCP to facilitate MPTCP operation is unambiguously distinguished from TFO data that are also included in the SYN payload. An upstream MCP will remove the MP_CONVERT Information Elements before relaying the SYN message (with TFO data) to the next hop.
    2. The MCP does not support MP_CONVERT Information Element: No additional issue is raised for obvious reasons.
  2. MCP capability discovery is not completed prior to receiving a message with TFO negotiation or TFO data.
    1. If the same message is used to negotiate TFO and to retrieve the capabilities of the MCP, extra delay may be observed before negotiating TFO if the MCP does not support the MP_CONVERT Information Element. Obviously, no concern is raised when the MCP supports the MP_CONVERT Information Element.
    2. If the same message includes TFO data and is used to retrieve the capabilities of the MCP, extra delay may be observed before negotiating TFO if the MCP does not support the MP_CONVERT Information Element. Obviously, no concern is raised when the MCP supports the MP_CONVERT Information Element.

To mitigate cases where extra delays are experienced when TFO is present, it is RECOMMENDED to not proxy connections with TFO before the MCP capability discovery procedure is completed.

9. IANA Considerations

This document requests an MPTCP subtype code for this option:

  • MP_PREFER_PROXY

10. Security Considerations

MPTCP-related security threats are discussed in [RFC6181] and [RFC6824]. Additional considerations are discussed in the following sub-sections.

10.1. Privacy

The MCP may have access to privacy-related information (e.g., IMSI, link identifier, subscriber credentials, etc.). The MCP MUST NOT leak such sensitive information outside a local domain.

10.2. Denial-of-Service (DoS)

Means to protect the MCP against Denial-of-Service (DoS) attacks MUST be enabled. Such means include the enforcement of ingress filtering policies at the network boundaries [RFC2827].

In order to prevent the exhaustion of MCP resources by establishing a great number of simultaneous subflows for each MPTCP connection, the MCP administrator SHOULD limit the number of allowed subflows per CPE for a given connection. Means to protect against SYN flooding attacks MUST also be enabled ([RFC4987]).

Attacks that originate outside of the domain can be prevented if ingress filtering policies are enforced. Nevertheless, attacks from within the network between a host and an MCP instance are yet another actual threat. Means to ensure that illegitimate nodes cannot connect to a network should be implemented.

10.3. Illegitimate MCP

Traffic theft is a risk if an illegitimate MCP is inserted in the path. Indeed, inserting an illegitimate MCP in the forwarding path allows traffic intercept and can therefore provide access to sensitive data issued by or destined to a host. To mitigate this threat, secure means to discover an MCP should be enabled.

11. Acknowledgements

Many thanks to Chi Dung Phung, Mingui Zhang, Rao Shoaib, Yoshifumi Nishida, and Christoph Paasch for their valuable comments.

Thanks to Ian Farrer, Mikael Abrahamsson, Alan Ford, Dan Wing, and Sri Gundavelli for the fruitful discussions in IETF#95 (Buenos Aires).

Special thanks to Pierrick Seite, Yannick Le Goff, Fred Klamm, and Xavier Grall for their inputs.

Thanks also to Olaf Schleusing, Martin Gysi, Thomas Zasowski, Andreas Burkhard, Silka Simmen, Sandro Berger, Michael Melloul, Jean-Yves Flahaut, Adrien Desportes, Gregory Detal, Benjamin David, Arun Srinivasan, and Raghavendra Mallya for the discussion.

The design approach adopted in -10 is the outcome of fruitful discussions with Alan Ford. Many thanks Alan.

12. References

12.1. Normative References

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997.
[RFC6824] Ford, A., Raiciu, C., Handley, M. and O. Bonaventure, "TCP Extensions for Multipath Operation with Multiple Addresses", RFC 6824, DOI 10.17487/RFC6824, January 2013.
[RFC6890] Cotton, M., Vegoda, L., Bonica, R. and B. Haberman, "Special-Purpose IP Address Registries", BCP 153, RFC 6890, DOI 10.17487/RFC6890, April 2013.

12.2. Informative References

[I-D.barre-mptcp-tfo] Barre, S., Detal, G. and O. Bonaventure, "TFO support for Multipath TCP", Internet-Draft draft-barre-mptcp-tfo-01, January 2015.
[I-D.boucadair-mptcp-dhc] Boucadair, M., Jacquenet, C. and T. Reddy, "DHCP Options for Network-Assisted Multipath TCP (MPTCP)", Internet-Draft draft-boucadair-mptcp-dhc-06, October 2016.
[I-D.nam-mptcp-deployment-considerations] Boucadair, M., Jacquenet, C., Bonaventure, O., Henderickx, W. and R. Skog, "Network-Assisted MPTCP: Use Cases, Deployment Scenarios and Operational Considerations", Internet-Draft draft-nam-mptcp-deployment-considerations-01, December 2016.
[I-D.zhang-gre-tunnel-bonding] Leymann, N., Heidemann, C., Zhang, M., Sarikaya, B. and M. Cullen, "Huawei's GRE Tunnel Bonding Protocol", Internet-Draft draft-zhang-gre-tunnel-bonding-05, December 2016.
[RFC1701] Hanks, S., Li, T., Farinacci, D. and P. Traina, "Generic Routing Encapsulation (GRE)", RFC 1701, DOI 10.17487/RFC1701, October 1994.
[RFC1928] Leech, M., Ganis, M., Lee, Y., Kuris, R., Koblas, D. and L. Jones, "SOCKS Protocol Version 5", RFC 1928, DOI 10.17487/RFC1928, March 1996.
[RFC2473] Conta, A. and S. Deering, "Generic Packet Tunneling in IPv6 Specification", RFC 2473, DOI 10.17487/RFC2473, December 1998.
[RFC2827] Ferguson, P. and D. Senie, "Network Ingress Filtering: Defeating Denial of Service Attacks which employ IP Source Address Spoofing", BCP 38, RFC 2827, DOI 10.17487/RFC2827, May 2000.
[RFC4987] Eddy, W., "TCP SYN Flooding Attacks and Common Mitigations", RFC 4987, DOI 10.17487/RFC4987, August 2007.
[RFC6181] Bagnulo, M., "Threat Analysis for TCP Extensions for Multipath Operation with Multiple Addresses", RFC 6181, DOI 10.17487/RFC6181, March 2011.
[RFC7413] Cheng, Y., Chu, J., Radhakrishnan, S. and A. Jain, "TCP Fast Open", RFC 7413, DOI 10.17487/RFC7413, December 2014.
[TR-348] BBF, "Hybrid Access Broadband Network Architecture", July 2016.

Authors' Addresses

Mohamed Boucadair (editor) Orange Rennes, 35000 France EMail: mohamed.boucadair@orange.com
Christian Jacquenet (editor) Orange Rennes, France EMail: christian.jacquenet@orange.com
Olivier Bonaventure (editor) Tessares Belgium EMail: olivier.bonaventure@tessares.net
Denis Behaghel OneAccess EMail: Denis.Behaghel@oneaccess-net.com
Stefano Secci UPMC EMail: stefano.secci@lip6.fr
Wim Henderickx (editor) Nokia/Alcatel-Lucent Belgium EMail: wim.henderickx@alcatel-lucent.com
Robert Skog (editor) Ericsson EMail: robert.skog@ericsson.com
Suresh Vinapamula Juniper 1137 Innovation Way Sunnyvale, CA 94089 USA EMail: Sureshk@juniper.net
SungHoon Seo Korea Telecom Seoul, Korea EMail: sh.seo@kt.com
Wouter Cloetens SoftAtHome Vaartdijk 3 701 3018 Wijgmaal, Belgium EMail: wouter.cloetens@softathome.com
Ullrich Meyer Vodafone Germany EMail: ullrich.meyer@vodafone.com
Luis M. Contreras Telefonica Spain EMail: luismiguel.contrerasmurillo@telefonica.com
Bart Peirens Proximus EMail: bart.peirens@proximus.com