Network Working Group M. Boucadair, Ed.
Internet-Draft C. Jacquenet, Ed.
Intended status: Standards Track Orange
Expires: April 30, 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
October 27, 2016
An MPTCP Option for Network-Assisted MPTCP
draft-boucadair-mptcp-plain-mode-09
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.
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This document specifies an MPTCP option that is used in the context
of Network-Assisted MPTCP: MP_CONVERT.
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 April 30, 2017.
Copyright Notice
Copyright (c) 2016 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.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5
3. Target Use Cases . . . . . . . . . . . . . . . . . . . . . . 5
3.1. Multipath Client . . . . . . . . . . . . . . . . . . . . 6
3.2. Multipath CPE . . . . . . . . . . . . . . . . . . . . . . 7
4. MP_CONVERT Option . . . . . . . . . . . . . . . . . . . . . . 7
5. MPTCP Connections from a Multipath TCP Client . . . . . . . . 10
5.1. Description . . . . . . . . . . . . . . . . . . . . . . . 10
5.2. Theory of Operation . . . . . . . . . . . . . . . . . . . 11
6. MPTCP Connections Between Single Path Client and Server . . . 13
6.1. Description . . . . . . . . . . . . . . . . . . . . . . . 13
6.2. Theory of Operation . . . . . . . . . . . . . . . . . . . 14
6.2.1. Downstream MCP . . . . . . . . . . . . . . . . . . . 14
6.2.2. Upstream MCP . . . . . . . . . . . . . . . . . . . . 14
7. Demux Native MPTCP Connections From Proxied MPTCP Connections 16
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 18
9. Security Considerations . . . . . . . . . . . . . . . . . . . 18
9.1. Privacy . . . . . . . . . . . . . . . . . . . . . . . . . 18
9.2. Denial-of-Service (DoS) . . . . . . . . . . . . . . . . . 19
9.3. Illegitimate MCP . . . . . . . . . . . . . . . . . . . . 19
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 19
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 19
11.1. Normative References . . . . . . . . . . . . . . . . . . 20
11.2. Informative References . . . . . . . . . . . . . . . . . 20
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 21
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
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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 8 SOCKS messages may have
to be exchanged between two MCPs before an MPTCP connection is
established, 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:
o No encapsulation required (no tunnels, whatsoever).
o No out-of-band signaling for each MPTCP subflow is required.
o Carries any protocol for the benefit of massive MPTCP adoption.
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o Avoids interference with native MPTCP connections.
o Targets both on-path and off-path MCPs.
o Accommodates various deployment contexts, such as those that
require the preservation of the source IP address and others
characterized by an address sharing design.
2. Terminology
The reader should be familiar with the terminology defined in
[RFC6824].
This document makes use of the following terms:
o Client: an endhost that initiates transport flows forwarded along
a single path. Such endhost is not assumed to support multipath
transport capabilities.
o Server: an endhost that communicates with a client. Such endhost
is not assumed to support multipath transport capabilities.
o Multipath Client: a Client that supports multipath transport
capabilities.
o Multipath Server: a Server that supports multipath transport
capabilities. Both the client and the server can be single-homed
or multi-homed. However, for the use cases discussed in this
document, the number of interfaces available at the endhosts is
not relevant.
o Transport flow: a sequence of packets that belong to a
unidirectional transport flow and which share at least one common
characteristic (e.g., the same destination address). TCP and SCTP
flows are composed of packets that have the same source and
destination addresses, the same protocol number and the same
source and destination ports.
o Multipath Conversion Point (MCP): a function that terminates a
transport flow and relays all data carried in the flow into
another transport flow.
MCP is a function that converts a multipath transport flow and
relays it over a single path transport flow and vice versa.
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 5 and
Section 6. The first use case is a Multipath Client that interacts
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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.
+--+ +-----+ +--+
|H1| | MCP | |RM|
+--+ +-----+ +--+
| | |
|<==================MPTCP Leg==============>|<---TCP -->|
| | |
Legend:
H1: Host 1
MCP: Multipath Conversion Point
RM: Remote Machine
Figure 1: Network-assisted MPTCP (Host-based Model)
In reference to 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. Because 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 proposes the MP_CONVERT option which is carried in the
SYN segment of the initial subflow. This SYN segment is sent towards
the MCP. The MP_CONVERT option contains the destination address (and
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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 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. MP_CONVERT Option
The MP_CONVERT (MC) option carries the source/destination IP
addresses and/or port numbers of the origin source and destination
nodes. It is also used to indicate whether the data carried in the
packet is relayed from a native TCP connection or refers to the use
of another transport protocol.
The format of the option is shown in Figure 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|D|Flags| Protocol |
+---------------+---------------+-------+-+-----+---------------+
| Address (IPv4 - 4 octets / IPv6 - 16 octets) |
+-------------------------------+-------------------------------+
| Port (2 octets, optional) |
+-------------------------------+
Figure 3: MP_CONVERT Option
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The description of the fields is as follows:
o Kind and Length: are the same as those defined in Section 3 of
[RFC6824]. The minimum size of this option is 4 bytes.
o Subtype: to be defined by IANA (Section 8). Implementations may
use the "0xe" subtype encoding for early deployment purposes in
managed networks.
o 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.
o "Flags" 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.
o Protocol: conveys the protocol number as assigned by IANA
[proto_numbers].
o Address: includes a source or destination IP address. The address
family is determined by the "Length" field. Concretely, a
MP_CONVERT option that carries an IPv4 address has a Length field
of 8 bytes (or 10, if a port number is included). A MP_CONVERT
option that carries an IPv6 address has a Length of 20 bytes (or
22, if a port number is included). This field is optional.
o 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.
The MP_CONVERT option is a variable length MPTCP option that MUST NOT
be used in TCP segments whose SYN flag is reset. This option can
only appear in the SYN used to create the initial subflow of a
Multipath TCP connection (see the example in Figure 4).
Up to two MP_CONVERT options can appear inside a SYN segment. If two
MP_CONVERT options are included, these options MUST NOT have the same
D-bit value.
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+----+ +-----+ +--+
| C | | MCP | |S |
+----+ +-----+ +--+
| | ________________________________| |
| / Initial subflow \ |
| |========SYN(MP_CAPABLE+MC(S))===>| |
| | |--SYN------->|
| | |<--SYN/ACK---|
| |<====SYN/ACK(MP_CAPABLE)=========| |
| | ... | |
| \ ________________________________/ |
.... ....
| | ________________________________| |
| / Additional subflow \ |
| \ ________________________________/ |
Legend:
<===>: MPTCP leg
<--->: TCP leg
Figure 4: Carrying the MP_CONVERT Option
The MP_CONVERT option MUST be included in the SYN payload.
NOTE 1: Given the length of the MP_CONVERT option, especially when
IPv6 addresses are used, and the set of TCP options that are
likely to be included in a SYN message, it will not always be
possible to place the MP_CONVERT option inside the dedicated TCP
option space.
NOTE 2: Including data in a SYN payload is allowed as per
Section 3.4 of [RFC0793].
NOTE 3: Stateless approaches that rely on inserting the MP_CONVERT
option in all packets are out of scope.
DISCUSSION NOTE: ADD DETAILS ABOUT THE NEED FOR AN EXPLICIT SIGNAL
THAT MPTCP OPTIONS ARE INCLUDED IN THE SYN PAYLOAD?
If the MP_CONVERT option 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 and
"End of Options List" (EOL) options when generating the SYN in the
MPTCP leg. The EOL option serves as a marker to delineate the end of
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the TCP options and the beginning of the data included in the
original SYN .
An implementation MUST ignore MP_CONVERT options that include
multicast, broadcast, and host loopback addresses [RFC6890].
Concretely, an implementation that receives an MP_CONVERT option with
such addresses MUST silently tear down the MPTCP connection.
When an MCP creates an MPTCP connection triggered by an incoming
packet, it MUST copy in the 'Protocol' field of the MP_CONVERT option
the value of the 'Protocol' field (resp. type of the transport
header) of the IPv4 (resp. IPv6) header of this incoming packet.
The MCP may be configured to enable traffic aggregation for some
transport protocols because of the nature of the service they relate
to. By default, the 'Protocol' field MUST be set to 6 (TCP).
5. MPTCP Connections from a Multipath TCP Client
5.1. Description
The simplest usage of the MP_CONVERT option 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 5.
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.
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C <===========>MCP <------------> S
+<============>+
Legend:
<===>: subflows of the upstream MPTCP connection
<--->: downstream TCP connection
Figure 5: A Multipath TCP Client interacts with a Server through a
Multipath Conversion Point
5.2. 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 option described in Figure 3. The destination address of
the SYN segment is the IP address of the MCP. The MP_CONVERT option
included in the SYN contains the IP address and optionally the
destination port of the Server (see Figure 6).
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+----+ +-----+ +--+
| 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 6: 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 option (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 option 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.
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.
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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
[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.
6. MPTCP Connections Between Single Path Client and Server
6.1. 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 7, 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
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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 7: A Client interacts with a Server through an upstream and a
downstream Multipath Conversion Points
6.2. Theory of Operation
6.2.1. 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 5.2.
If the downstream MCP is deployed on-path, it only terminates MPTCP
connections that carry an empty MP_CONVERT 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_CONVERT option, it MUST
forward the SYN to its final destination without any modification.
6.2.2. 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_CONVERT
option that carries no IP address.
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.
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To initiate a connection with a remote server S, the Client sends a
SYN segment that is intercepted by the upstream MCP which in turns
initiates an MPTCP connection towards its downstream MCP that
includes the MP_CONVERT option described in Figure 3. The
destination address of the SYN segment is the IP address of the
downstream MCP. The MP_CONVERT option 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 option. 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 8.
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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 8: Dual-Ended MCP Flow Example
All the techniques defined in [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 5.2 apply for the maintenance
of the connections by the upstream MCP.
7. Demux Native MPTCP Connections From Proxied MPTCP Connections
Section 6 assumes that the Clients that use the upstream MCP do not
support MPTCP. If a Multipath Client is attached to the upstream
MCP, it should be possible for this client to establish an MPTCP
connection with a Multipath Server without using the MCPs.
Because MPTCP connections are not destined explicitly to an MCP, an
on-path MCP instance will need extra means to distinguish "native"
MPTCP connections from "proxied" ones. The subsequent risk is that
native MPTCP communications will be reverted to TCP connections as
shown in Figure 9. In this example, we suppose that C2 and S2 are
MPTCP-compatible, but C1 and S1 are not.
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Upstream Downstream
+--+ +-----+ +-----+ +--+
|C1| | MCP | | MCP | |S1|
+--+ +-----+ +-----+ +--+
| | | |
|src:c1@ |src:uMCP@1 dst:s1@|src:uMCP@1 |
|<-TCP Leg->|<========MPTCP Leg===========>|<-TCP Leg->|
| dst:s1@| | dst:s1@|
| | | |
\_________________SINGLE PATH CLIENT/SERVER_____________/
_________________MULTIPLE PATH CLIENT/SERVER___________
/ \
| |
| | | |
|src:c2@ |src:uMCP@1 dst:s1@|src:uMCP@1 |
|<=MPTCP====|=============================>|<-TCP Leg->|
| dst:s2@| | dst:s2@|
| | | |
+--+ +-----+ +-----+ +--+
|C2| | MCP | | MCP | |S2|
+--+ +-----+ +-----+ +--+
Upstream Downstream
Figure 9: Example of a Broken E2E MPTCP Connection (On-path)
To mitigate this, the upstream MCP may be instructed to insert a
MP_CONVERT option only for the MPTCP connections it establishes. The
absence of MP_CONVERT option instances is an explicit indication that
this MPTCP connection is a native one. As such, an on-path MCP will
not revert this connection into a TCP connection, but will forward
packets without any modification to the next hop.
Figure 10 illustrates the results of such procedure: native MPTCP
connections are established between MPTCP-compliant client and
server, while Networok-Assisted MPTCP connections are established
with the help of MCPs.
Concretely, if the upstream MCP receives a SYN that includes the
MP_CAPABLE option, it MAY decide to forward it towards its final
destination without modifying it. When the downstream MCP receives a
SYN that does not include an MP_CONVERT option, it forwards it
towards its final destination.
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Upstream Downstream
+--+ +-----+ +-----+ +--+
|C1| | MCP | | MCP | |S1|
+--+ +-----+ +-----+ +--+
| | | |
|src:c1@ |src:uMCP@1 dst:s1@|src:uMCP@1 |
|<-TCP Leg->|<========MPTCP Leg(MC)=======>|<-TCP Leg->|
| dst:s1@| | dst:s1@|
| | | |
\_________________SINGLE PATH CLIENT/SERVER_____________/
_________________MULTIPLE PATH CLIENT/SERVER___________
/ \
| |
| | | |
|src:c2@ |src:uMCP@1 dst:s1@|src:uMCP@1 |
|<==========|============MPTCP========================>|
| dst:s2@| | dst:s2@|
| | | |
+--+ +-----+ +-----+ +--+
|C2| | MCP | | MCP | |S2|
+--+ +-----+ +-----+ +--+
Upstream Downstream
Figure 10: Example of a Successful E2E MPTCP Connection (On-path)
8. IANA Considerations
This document requests an MPTCP subtype code for this option:
o MP_CONVERT option
NOTE: Implementations may use "0xe" subtype encoding for early
deployment purposes in managed networks.
9. Security Considerations
MPTCP-related security threats are discussed in [RFC6181] and
[RFC6824]. Additional considerations are discussed in the following
sub-sections.
9.1. 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.
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9.2. 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.
9.3. 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.
10. 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.
11. References
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11.1. Normative References
[proto_numbers]
http://www.iana.org/assignments/protocol-numbers,
"Protocol Numbers".
[RFC0793] Postel, J., "Transmission Control Protocol", STD 7,
RFC 793, DOI 10.17487/RFC0793, September 1981,
<http://www.rfc-editor.org/info/rfc793>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
[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,
<http://www.rfc-editor.org/info/rfc6824>.
[RFC6890] Cotton, M., Vegoda, L., Bonica, R., Ed., and B. Haberman,
"Special-Purpose IP Address Registries", BCP 153,
RFC 6890, DOI 10.17487/RFC6890, April 2013,
<http://www.rfc-editor.org/info/rfc6890>.
11.2. Informative References
[I-D.boucadair-mptcp-dhc]
Boucadair, M., Jacquenet, C., and T. Reddy, "DHCP Options
for Network-Assisted Multipath TCP (MPTCP)", draft-
boucadair-mptcp-dhc-06 (work in progress), October 2016.
[I-D.nam-mptcp-deployment-considerations]
Boucadair, M., Jacquenet, C., Bonaventure, O., Behaghel,
D., stefano.secci@lip6.fr, s., Henderickx, W., Skog, R.,
Vinapamula, S., Seo, S., Cloetens, W., Meyer, U.,
Contreras, L., and B. Peirens, "Network-Assisted MPTCP:
Use Cases, Deployment Scenarios and Operational
Considerations", draft-nam-mptcp-deployment-
considerations-00 (work in progress), October 2016.
[I-D.zhang-gre-tunnel-bonding]
Leymann, N., Heidemann, C., Zhang, M., Sarikaya, B., and
M. Cullen, "Huawei's GRE Tunnel Bonding Protocol", draft-
zhang-gre-tunnel-bonding-03 (work in progress), May 2016.
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[RFC1701] Hanks, S., Li, T., Farinacci, D., and P. Traina, "Generic
Routing Encapsulation (GRE)", RFC 1701,
DOI 10.17487/RFC1701, October 1994,
<http://www.rfc-editor.org/info/rfc1701>.
[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,
<http://www.rfc-editor.org/info/rfc1928>.
[RFC2473] Conta, A. and S. Deering, "Generic Packet Tunneling in
IPv6 Specification", RFC 2473, DOI 10.17487/RFC2473,
December 1998, <http://www.rfc-editor.org/info/rfc2473>.
[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, <http://www.rfc-editor.org/info/rfc2827>.
[RFC4987] Eddy, W., "TCP SYN Flooding Attacks and Common
Mitigations", RFC 4987, DOI 10.17487/RFC4987, August 2007,
<http://www.rfc-editor.org/info/rfc4987>.
[RFC6181] Bagnulo, M., "Threat Analysis for TCP Extensions for
Multipath Operation with Multiple Addresses", RFC 6181,
DOI 10.17487/RFC6181, March 2011,
<http://www.rfc-editor.org/info/rfc6181>.
[RFC7413] Cheng, Y., Chu, J., Radhakrishnan, S., and A. Jain, "TCP
Fast Open", RFC 7413, DOI 10.17487/RFC7413, December 2014,
<http://www.rfc-editor.org/info/rfc7413>.
[TR-348] BBF, "Hybrid Access Broadband Network Architecture", July
2016.
Authors' Addresses
Mohamed Boucadair (editor)
Orange
Rennes 35000
France
Email: mohamed.boucadair@orange.com
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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
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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
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