Network Working Group M. Boucadair, Ed.
Internet-Draft C. Jacquenet, Ed.
Intended status: Informational Orange
Expires: June 10, 2017 O. Bonaventure, Ed.
Tessares
W. Henderickx, Ed.
Nokia/Alcatel-Lucent
R. Skog, Ed.
Ericsson
December 7, 2016
Network-Assisted MPTCP: Use Cases, Deployment Scenarios and Operational
Considerations
draft-nam-mptcp-deployment-considerations-01
Abstract
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. MPTCP Conversion Points (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 describes Network-Assisted MPTCP uses cases, deployment
scenarios, and operational considerations.
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/.
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Internet-Drafts are draft documents valid for a maximum of six months
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This Internet-Draft will expire on June 10, 2017.
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document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Network-Assisted MPTCP Use Cases . . . . . . . . . . . . . . 4
4. Deployment Scenarios . . . . . . . . . . . . . . . . . . . . 5
4.1. LTE/WLAN Aggregation . . . . . . . . . . . . . . . . . . 6
4.2. Fixed/Wireless Access Aggregation . . . . . . . . . . . . 6
4.3. Data Center . . . . . . . . . . . . . . . . . . . . . . . 7
5. Deployment & Operational Considerations . . . . . . . . . . . 7
5.1. MCP Location . . . . . . . . . . . . . . . . . . . . . . 8
5.2. MCP Insertion in a Multipath Communication . . . . . . . 8
5.2.1. Explicit Mode (Off-path) . . . . . . . . . . . . . . 8
5.2.2. Implicit Mode (On-path) . . . . . . . . . . . . . . . 10
5.3. Authorization . . . . . . . . . . . . . . . . . . . . . . 14
5.4. MCP Behaviors . . . . . . . . . . . . . . . . . . . . . . 15
5.4.1. Transparent MCP . . . . . . . . . . . . . . . . . . . 15
5.4.2. Non-Transparent MCP . . . . . . . . . . . . . . . . . 17
5.5. Address Family Considerations . . . . . . . . . . . . . . 19
5.6. Policies & Configuration Parameters . . . . . . . . . . . 19
5.6.1. Towards End-to-End MPTCP Connections . . . . . . . . 19
5.6.2. Traffic Distribution Scheme . . . . . . . . . . . . . 22
5.6.3. Flows Eligible to Multipath Service . . . . . . . . . 22
5.6.4. TCP Fragmentation . . . . . . . . . . . . . . . . . . 23
5.6.5. DSCP Preservation . . . . . . . . . . . . . . . . . . 23
5.6.6. Supported Transport Protocols . . . . . . . . . . . . 23
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5.6.7. Logging . . . . . . . . . . . . . . . . . . . . . . . 23
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 23
7. Security Considerations . . . . . . . . . . . . . . . . . . . 23
7.1. Privacy . . . . . . . . . . . . . . . . . . . . . . . . . 23
7.2. Denial-of-Service (DoS) . . . . . . . . . . . . . . . . . 24
7.3. Illegitimate MCP . . . . . . . . . . . . . . . . . . . . 24
7.4. High Rate Reassembly . . . . . . . . . . . . . . . . . . 24
8. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 24
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 26
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 26
10.1. Normative References . . . . . . . . . . . . . . . . . . 26
10.2. Informative References . . . . . . . . . . . . . . . . . 26
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 28
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, 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.
Such MCPs can be deployed in CPEs (Customer Premises Equipment), as
well in the network side. MCPs are responsible for establishing
multi-path communications on behalf of endpoints.
This document describes Network-Assisted MPTCP uses cases
(Section 3), deployment scenarios (Section 4), and operational
considerations (Section 5).
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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 on 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 received over it over another
transport flow.
MCP is a function provided by the network operator that converts a
multipath transport flow and relays it over a single path
transport flow and vice versa.
3. Network-Assisted MPTCP Use Cases
The first use case is a Multipath Client that interacts with a
Server. To benefit from the capabilities of its multipath transport
protocol, the Multipath Client will interact with a Multipath
Conversion Point (MCP) located in the network as illustrated in
Figure 1.
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C <===========>MCP <------------> S
+<============>+
Legend:
<===>: MPTCP leg
Figure 1: A Multipath Client interacts with a Server through a
Multipath Conversion Point
A similar approach consists of a Multipath Server that leverages its
multipath capabilities when interacting with a Client as shown in
Figure 2.
C <---------> MCP <===========> S
+<=============>+
Figure 2: A Client interacts with a Multipath Server through a
Multipath Conversion Point
The third use case is when a Client interacts with a Server and the
corresponding communication is deemed eligible to multi-path
forwarding. In this case, two Multipath Conversion Points are used.
The upstream MCP converts the (single path) transport flow initiated
by the Client into a multipath transport flow towards a downstream
MCP (called, network-located MCP). This downstream MCP converts the
multipath transport flow received from the upstream MCP in a single
path transport flow forwarded to the destination Server. The end-to-
end transport flow between the Client and the Server is thus
decomposed into three flows as shown in Figure 3: A (single path)
transport flow between the Client and the upstream MCP, a multipath
transport flow between the upstream and the downstream MCPs, and a
single path transport flow between the downstream MCP and the Server.
Upstream Downstream
C <---> MCP <===========> MCP <------------> S
+<=============>+
Figure 3: A Client interacts with a Server through two Multipath
Conversion Points
4. Deployment Scenarios
This section discusses some deployment scenarios related to Network-
Assisted MPTCP designs.
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4.1. LTE/WLAN Aggregation
This deployment scenario is considered by mobile operators so that
they can propose their customers to aggregate LTE resources with WLAN
resources.
As depicted in Figure 4, the mobile terminal (UE, User Equipment) is
MPTCP-capable. The MCP function is enabled in the network to
terminate MPTCP connections (e.g., in the PDN Gateway, a dedicated
service function located at the (S)Gi interface, co-located with a
TCP proxy, etc.).
This deployment scenario is an implementation of the use case
depicted in Figure 1.
+------------+ _--------_ +----------------+
| | ( LTE ) | |
| UE +=======+ +===+ Backbone |
| (MPTCP | (_ _) | Network |
| Client) | (_______) |+--------------+|
| | IP Network #1 || Concentrator ||------> Internet
| | || (MCP) ||
| | |+--------------+|
| | IP Network #2 | |
| | _--------_ | |
| | ( ) | |
| +=======+ WLAN +==+ |
| | (_ _) | |
+------------+ (_______) +----------------+
Figure 4: Network-Assisted MPTCP LTE/WLAN Aggregation (Host-based
model)
4.2. Fixed/Wireless Access Aggregation
One of the promising deployment scenarios for Multipath TCP is to
enable a CPE that is connected to multiple access networks (e.g.,
DSL, LTE, WLAN) to optimize the usage of such resources. This
deployment scenario, called Hybrid Access, relies upon MCPs located
in both the CPE and the network (Figure 5). The latter plays the
role of an MPTCP concentrator. Such concentrator terminates the
MPTCP sessions established from CPEs, before redirecting traffic into
legacy TCP sessions.
This deployment scenario is an implementation of the use case
depicted in Figure 2.
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+------------+ _--------_ +----------------+
| | ( LTE ) | |
| CPE +=======+ +===+ Backbone |
| (MCP) | (_ _) | Network |
| | (_______) |+--------------+|
| | IP Network #1 || Concentrator ||------> Internet
| | || (MCP) ||
| | |+--------------+|
| | IP Network #2 | |
| | _--------_ | |
| | ( DSL ) | |
| +=======+ +==+ |
| | (_ _) | |
+-----+------+ (_______) +----------------+
|
---- LAN ----
|
end-nodes
Figure 5: Network-Assisted MPTCP Fixed/Wireless Access Aggregation
For mobile operators that provide CPE-based mobile broadband
services, LTE and WLAN resources can be aggregated by means of MPTCP.
In such deployment scenario, the MCP function is enabled in the CPE
and in the network.
4.3. Data Center
As discussed in Section 2.1 of [I-D.ietf-mptcp-experience], MPTCP is
being contemplated for deployment within data centers. Such
deployments may adhere to the use cases defined in Figure 2 or
Figure 3. One or two MCPs may be enabled at the data center segment.
The objective is to optimize the resources usage within the data
center infrastructure.
The presence of an MCP is transparent to the client side,
nevertheless the origin client's IP address may be hidden to the
ultimate server. Enforcing per-host policies at the server's side
may be problematic if all connections are bound to an MCP's IP
address. To mitigate this problem, MCPs may insert the TCP Option
for Host Identification [RFC7974].
5. Deployment & Operational Considerations
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5.1. MCP Location
The location of MCPs is deployment-specific. Network Providers may
choose to adopt centralized or distributed designs. Nevertheless, in
order to take advantage of MPTCP, the location of an MCP should not
jeopardize packet forwarding performance overall.
5.2. MCP Insertion in a Multipath Communication
Two deployment scenarios can be considered for involving an MCP in
the communication path. These scenarios are described below.
5.2.1. Explicit Mode (Off-path)
This scenario 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]. A device can be a CPE or a
host.
MPTCP connections are established explicitly using the address(es) of
the MCP (Figure 6). In order to forward packets to their ultimate
destination, the MCP is provided during the connection establishment
with the destination IP address (and optionally destination port
numbers). Typically, this is achieved thanks to the use of the
MP_CONVERT Information Element (IE) defined in
[I-D.boucadair-mptcp-plain-mode]. Figure 6 illustrates the flow
exchange to established a communication with a legacy server (RM).
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___________________
/ Host-based Model \
\___________________/
+---+ +-----+ +--+
|H1 | | MCP | |RM|
+---+ +-----+ +--+
h1@h2@ mcp@ rm@
| | | |
| |src:hi@ dst:mcp@| dst:rm@|
| |<=================MPTCP Leg=============>|<---TCP -->|
| | | |
___________________
/ CPE-based Model \
\___________________/
Upstream Downstream
+--+ +-----+ +-----+ +--+
|H1| | MCP | | MCP | |RM|
+--+ +-----+ +-----+ +--+
h1@ uMCP@1 uMCP@2 dMCP@ rm@
| | | | |
|src:h1@ |src:uMCP@1 dst:dmcp@| |
|<-TCP Leg->|<=========MPTCP Leg============>|<-TCP Leg->|
| dst:rm@| | | dst:rm@|
| | | | |
| | |src:uMCP@2 dst:dmcp@| |
| | |<===Secondary MPTCP subflow==>|<---TCP -->|
| | | ... | dst:rm@|
| | | | |
Legend:
RM: A remote machine.
Figure 6: Sample Connection Establishment (Explicit Mode)
This scenario aims to avoid any adherence of the Network-Assisted
MPTCP procedure and the underlying routing and forwarding policies.
Furthermore, this scenario allows for more flexibility in terms of
mounting MPTCP subflows as it does not require any specific order in
the establishment of subflows among available interfaces.
Because the MCP's reachability information is explicitly configured
on the device, means to guarantee successful inbound MPTCP
connections can be enabled in the device to instruct the MCP to
maintain active bindings so that incoming packets can be successfully
redirected towards the appropriate device.
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5.2.2. Implicit Mode (On-path)
Unlike the explicit mode, the implicit mode assumes that the MCP is
located on a default forwarding path (primary path). As such, 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). Figure 7 illustrates the flow exchange to
establish a communication with a legacy server (RM).
Note that a standard MPTCP implementation will try to establish other
subflows using rm@. In order to prevent such behavior, H1 is
instructed by some means to prevent establishing additional subflows
to rm@. For example, the MCP may set the C-bit in MP_CAPABLE option
([I-D.ietf-mptcp-rfc6824bis]) it returns to H1.
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___________________
/ Host-based Model \
\___________________/
+----+ +-----+ +--+
| H1 | | MCP | |RM|
+----+ +-----+ +--+
h1@h2@ mcp@ rm@
| | | |
|src:h1@ dst:rm@| dst:rm@|
|<============Initial MPTCP subflow========>|<---TCP -->|
| | ... | |
| |src:h2@ dst:mcp@| dst:rm@|
| |<=========Secondary MPTCP subflow=======>|<---TCP -->|
| | ... | |
___________________
/ CPE-based Model \
\___________________/
Upstream Downstream
+--+ +-----+ +-----+ +--+
|H1| | MCP | | MCP | |RM|
+--+ +-----+ +-----+ +--+
h1@ uMCP@1 uMCP@2 dMCP@ rm@
| | | | |
|src:h1@ |src:uMCP@1 dst:rm@|src:uMCP@1 |
|<-TCP Leg->|<=========MPTCP Leg============>|<-TCP Leg->|
| dst:rm@| | | dst:rm@|
| | | | |
| | |src:uMCP@2 dst:mcp@|src:uMCP@1 |
| | |<===Secondary MPTCP subflow==>|<---TCP -->|
| | | ... | dst:rm@|
| | | | |
Figure 7: Sample Connection Establishment (Implicit Mode)
Subsequent subflows are then sent directly to the MCP (Figure 7).
The handling of these subsequent subflows is identical to the one of
the explicit mode; only the establishment of the initial subflow
differs. Concretely, in reference to Figure 8, once the upstream MCP
intercepts an initial subflow, it adds itself to the MPTCP connection
by sending ADD_ADDR on the primary subflow. Then, MP_JOIN is sent to
the IP address conveyed in ADD_ADDR to create the secondary subflow.
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+----+ +-----+ +--+
| H1 | | MCP | |RM|
+----+ +-----+ +--+
h1@h2@ mcp@ rm@
| | | |
|<==============ADD_ADDR====================| |
| | _______________________________________ | |
| |/ Secondary subflow \| |
| |================SYN+MP_JOIN=============>| |
| |<============SYN/ACK(MPJOIN)=============| |
| |============ACK(MP_JOIN)================>| |
| | ... | |
Figure 8: Secondary Subflow Creation (Host-based Implicit Mode)
5.2.2.1. Demux Native MPTCP Flows from Proxied MPTCP Flows
If no explicit signal is included in the initial SYN message, the MCP
cannot distinguish "native" MPTCP connections from "proxied" ones.
An operator that deploys MCP resources would like to control the
MPTCP connections it terminate. For example, an operator may want to
enforce a policy that consists in assisting only MPTCP connections
that are established by an upstream MCP, not those that are
established by an MPTCP host located behind a CPE.
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 if the MCP is instructed to always relay MPTCP
connections to TCP ones. In this example, we suppose that C2 and S2
are MPTCP-capable, 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_PREFER_PROXY option only for the MPTCP connections it establishes
[I-D.boucadair-mptcp-plain-mode]. The absence of MP_PREFER_PROXY
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-capable client and server,
while Network-Assisted MPTCP connections are established with the
help of MCPs.
Concretely, if the upstream MCP receives a SYN that includes the
MP_PREFER_PROXY 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_PREFER_PROXY, 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(PP)=======>|<-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
Legend:
PP: MP_PREFER_PROXY
Figure 10: Example of a Successful E2E MPTCP Connection (On-path)
5.3. Authorization
The Network Provider that manages the various network attachments
(including the MCPs) may enforce authentication and authorization
policies using appropriate mechanisms. For example, a non-exhaustive
list of methods to achieve authorization is provided hereafter:
o The network provider may enforce a policy based on the
International Mobile Subscriber Identity (IMSI) to verify that a
user is allowed to benefit from the aggregation service. If that
authorization fails, the Packet Data Protocol (PDP) context
/bearer will not be mounted. This method does not require any
interaction with the MCP.
o The network provider may enforce a policy based upon Access
Control Lists (ACLs), e.g., at a Broadband Network Gateway (BNG)
to control the CPEs that are authorized to communicate with an
MCP. These ACLs may be installed as a result of RADIUS exchanges,
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for instance ([I-D.boucadair-mptcp-radius]). This method does not
require any interaction with the MCP.
o The MCP may implement an Ident interface [RFC1413] to retrieve an
identifier that will be used to assess whether that client is
entitled to make use of the aggregation service. Ident exchanges
will take place only when receiving the first subflow from a given
source IP address.
o The device that embeds the MCP may also host a RADIUS client that
will solicit an AAA server to check whether connections received
from a given source IP address are authorized or not
([I-D.boucadair-mptcp-radius]).
A first safeguard against the misuse of MCP resources by illegitimate
users (e.g., users with access networks that are not managed by the
same service provider that operates the MCP) is to reject MPTCP
connections received on the Internet-facing interfaces. Only MPTCP
connections received on the customer-facing interfaces of an MCP will
be accepted.
Because only the CPE is entitled to establish MPTCP connections with
an MCP, ACLs may be installed on the CPE to avoid that internal
terminals issue MPTCP connections towards one of the MCPs.
5.4. MCP Behaviors
The MCP MUST be provided with instructions about the behavior to
adopt with regards to the processing of source addresses. The
following sub-sections elaborate on various schemes.
5.4.1. Transparent MCP
Transparent Network-Assisted MPTCP deployment is a deployment where
the visible source address of a packet forwarded by an MCP to a
remote machine located in the Internet is the IP address of the
endhost, not an address that is provisioned to the MCP. In order to
intercept incoming traffic, specific IPv4/IPv6 routes are injected so
that traffic is redirected towards the MCP.
No dedicated IP address pool is required to the MCP for the Network-
Assisted MPTCP service.
5.4.1.1. IPv4 Address Preservation
The MCP can be tweaked to behave in the IPv4 address preservation
mode. This is the IPv4 address assigned to the endhost (typically,
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within a mobile deployment context as discussed in Section 4.1) or a
WAN address of the CPE for the wired case (Section 4.2).
5.4.1.2. Source IPv6 Prefix Preservation at Network-located MCP
Some IPv6 deployments may require the preservation of the source IPv6
prefix (Figure 11).
This model requires the MCP to support ALGs to accommodate
applications with IPv6 address referrals.
+--+ +-----+ +---+ +--+
|H1| | MCP | |MCP| |RM|
+--+ +--+--+ +---+ +--+
IP@s IP@1, IP@2 IP@mcf IP@d
| || | |
|src:IP@s ||src:IP@1 dst:IP@mcf|src:IP@1 |
|---------->||------------------------------------>|---------->|
| Dst:IP@d|| | dst:IP@d|
| || | |
|<--SYN/ACK-||<---------------SYN/ACK--------------|<-SYN/ACK--|
|---ACK---->||----------------ACK----------------->|---ACK---->|
| || | |
Legend:
mcf: MCP Customer-facing Interface
Figure 11: Example of Source IPv6 Prefix Preservation at Network-
located MCP (Initial subflow)
5.4.1.3. Source IPv6 Address Preservation at Network-located MCP
Some IPv6 deployments may require the preservation of the source IPv6
address (Figure 12).
This model avoids the need for the MCP to support ALGs to accommodate
applications with IPv6 address referrals.
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+--+ +-----+ +---+ +--+
|H1| | MCP | |MCP| |RM|
+--+ +--++-+ +---+ +--+
IP@s IP@1, IP@2 IP@mcf IP@d
| || | |
|src:IP@s ||src:IP@1 dst:IP@mcf|src:IP@s |
|---------->||------------------------------------>|---------->|
| Dst:IP@d|| | dst:IP@d|
| || | |
|<--SYN/ACK-||<---------------SYN/ACK--------------|<-SYN/ACK--|
|---ACK---->||----------------ACK----------------->|---ACK---->|
| || | |
| || | |
Figure 12: Example of Outgoing SYN with Source Address Preservation
5.4.2. Non-Transparent MCP
Unlike the transport mode, this section focuses on deployments where
a dedicated IP address pool is provisioned to the MCP for the
Network-Assisted MPTCP service.
5.4.2.1. IPv4 Address Sharing at the at the Network-located MCP
Because of global IPv4 address depletion, optimization of the IPv4
address usage is mandatory. This obviously includes the IPv4
addresses that are assigned by the MCP at its Internet-facing
interfaces (Figure 13 and Figure 14). A pool of global IPv4
addresses is provisioned to the MCP along with possible instructions
about the address sharing ratio to apply (see Appendix B of
[RFC6269]). Adequate forwarding policies are enforced so that
traffic destined to an address of such pool is intercepted by the
appropriate MCP.
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+--+ +---+ +--+
|H1| |MCP| |RM|
+--+ +---+ +--+
|| | |
||src:IP@s MP_CONVERT(D=0,IP@d) dst:IP@mcf|src:IP@mif |
||-----------------------SYN--------------------->|---SYN---->|
|| | dst:IP@d|
|| | |
||<--------------------SYN/ACK--------------------|<-SYN/ACK--|
||-----------------------ACK--------------------->|---ACK---->|
|| | |
Legend:
mcf: MCP Customer-facing Interface
mif: MCP Internet-facing Interface
Figure 13: Example of Outgoing SYN without Source Address
Preservation (Single-ended MCP)
+--+ +-----+ +---+ +--+
|H1| | MCP | |MCP| |RM|
+--+ +--++-+ +---+ +--+
| || | |
|src:IP@s ||src:IP@1 MP_CONVERT(D=0,IP@d) |src:IP@mif |
|---SYN---->||----------------SYN----------------->|---SYN---->|
| dst:IP@d|| dst:IP@mcf| dst:IP@d|
| || | |
|<--SYN/ACK-||<---------------SYN/ACK--------------|<-SYN/ACK--|
|---ACK---->||----------------ACK----------------->|---ACK---->|
| || | |
Figure 14: Example of Outgoing SYN without Source Address
Preservation (Dual-ended MCPs)
5.4.2.2. IPv4 Address 1:1 Translation at the MCP
For networks that do not face global IPv4 address depletion yet, the
MCP can be configured so that source IPv4 addresses of the CPE are
replaced with other (public) IPv4 addresses. A pool of global IPv4
addresses is then provisioned to the MCP for this purpose. Rewriting
source IPv4 addresses may be used as a means to redirect incoming
traffic towards the appropriate MCP.
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5.4.2.3. IPv6 Prefix Sharing (NPTv6) at the Network-located MCP
Rewriting the source IPv6 prefix ([RFC6296]) may be needed to
redirect incoming traffic towards the appropriate MCP. A pool of
IPv6 prefixes is then provisioned to the MCP for this purpose.
5.5. Address Family Considerations
Subflows of a given MPTCP connection can be associated to the same
address family or may be established with different address families.
Also, the Network-Assisted MPTCP using MP_CONVERT IE, regardless of
the addressing scheme enforced by each CPE network attachment. In
particular, the plain transport mode indifferently accommodates the
following combinations.
LAN Leg MPTCP Legs TCP Leg towards RM
------- ----------- ------------------
IPv4 IPv4 IPv4
IPv4 IPv6 IPv4
IPv4 IPv6 & IPv4 IPv4
IPv6 IPv6 IPv6
IPv6 IPv4 IPv6
IPv6 IPv6 & IPv4 IPv6
5.6. Policies & Configuration Parameters
5.6.1. Towards End-to-End MPTCP Connections
In order to promote the use of MPTCP end-to-end, the MCP MUST NOT
strip MP_CPABALE from the SYN segments it forwards to their final
destination (i.e., server). The communication leg between an MCP and
the server may be placed using MPTCP if that server is MPTCP-capable,
or falls back to TCP otherwise.
There may be cases where remote servers will not respond to SYN
segments with the MP_CAPABLE option, and therefore it is desirable to
provide a mechanism to disable relaying MP_CAPABLE to some or all
remote hosts.
Also, an MCP SHOULD maintain a cache to record the servers that are
known to cause excessive delays. Any subsequent connection to a
server that is present in this cache MUST be placed using TCP. Cache
entries MUST be flushed out after the expiry of a configurable timer;
this timer is set by default to 24h.
This default behavior would lead to the establishment of end-to-end
MPTCP connections if the client and servers are both MPTCP-capable
with or without the withdrawal of MCPs from the communication.
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Whether an MCP must be maintained in the processing of an MPTCP
connection that involve MPTCP-capable clients and server is a
configurable parameter. The default mode MUST be to maintain the MCP
because its presence may solve several complications that may arise
in some deployments, such:
1. The use of private IPv4 addresses in some access networks.
Typically, additional subflows can not be added to the MPTCP
connection without the help of an MCP.
2. The assignment of IPv6 prefix only by some networks. If the
server is IPv4-only, subflows that are placed over IPv6 cannot be
added to an MPTCP connection towards that server.
3. Some of the access networks are subject to subscription with
volume quota.
Figure 15 shows an example of a communication with MCP withdrawal.
In this example, the MCP tracks the initial subflow. Upon receipt of
MP_CAPABLE in the SYN/ACK from the server, this connection is not
tracked any more by the proxy. Subsequent subflows are handled
directly by the endhost and the server.
For cases where some access networks are subject to a volume quota,
it is desirable to support a signal to indicate to a remote peer how
it must place data into available subflows.
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___________________
/ Host-based Model \
\___________________/
+----+ +-----+ +--+
| H1 | | MCP | |RM|
+----+ +-----+ +--+
h1@h2@ mcp@ rm@
| | | |
| |src:h1 dst:rm@|src:h1 dst:rm@|
| |=====SYN+MP_CAPABLE======>|====SYN+MP_CAPABLE=======>|
| |<=====SYN/ACK+MP_CAPABLE==|<====SYN/ACK+MP_CAPABLE===|
| |=====ACK+MP_CAPABLE======>|====ACK+MP_CAPABLE=======>|
| | __________________________________________________ |
| |/ Secondary subflow \|
| |=====================SYN+MP_JOIN====================>|
| |<===================SYN/ACK(MPJOIN)==================|
| |=====================ACK(MP_JOIN)===================>|
| | ... |
Figure 15: Sample Connection Establishment with MCP Withdrawal
(Implicit Mode)
If the MCP is instructed to be involved in the communication even if
the server is MPTCP-capable, the flow exchange depicted in Figure 16
will be observed.
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___________________
/ Host-based Model \
\___________________/
+----+ +-----+ +--+
| H1 | | MCP | |RM|
+----+ +-----+ +--+
h1@h2@ mcp@ rm@
| | | |
|src:h1 dst:rm@|src:h1 dst:rm@|
|=======SYN+MP_CAPABLE======>|====SYN+MP_CAPABLE=======>|
|<=======SYN/ACK+MP_CAPABLE==|<====SYN/ACK+MP_CAPABLE===|
|=======ACK+MP_CAPABLE======>|====ACK+MP_CAPABLE=======>|
|<==============ADD_ADDR=====| |
| | ________________________ | |
| |/ Secondary subflow \| |
| |src:h2 dst=mcp@| |
| |========SYN+MP_JOIN======>| .... |
| |<======SYN/ACK(MPJOIN)====| |
| |======ACK(MP_JOIN)=======>| |
| | ... | |
Figure 16: Sample Connection Establishment with MCP Withdrawal
(Implicit Mode)
5.6.2. Traffic Distribution Scheme
The logic of traffic distribution over multiple paths is deployment-
specific. This document does not require nor preclude any particular
traffic distribution scheme. Nevertheless, MCPs MUST be configurable
with a parameter to indicate which traffic distribution scheme to
enable. Indeed, policies can be enforced by an MCP instance operated
by the Network Provider to manage both upstream and downstream
traffic. These policies may be subscriber-specific, connection-
specific, system-wise, or else.
5.6.3. Flows Eligible to Multipath Service
The Multipath Client and MCPs may be provided with a set of
classification policies to help electing flows for the MPTCP service.
These policies may be provisioned either statically and dynamically
(or a combination thereof).
Also, multiple MCPs may serve a given end-user, as a function of the
nature of the service or the traffic to be forwarded over MPTCP
connections. For example, an MCP may be used by a service provider
to proceed with CPE-targeted maintenance operations, whereas another
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MCP may be configured to service multi-path communications initiated
by a set of end-users.
5.6.4. TCP Fragmentation
Methods to avoid TCP fragmentation, such as rewriting the TCP Maximum
Segment Size (MSS) option, must be supported by MCPs.
5.6.5. DSCP Preservation
The MCP MAY be configured to preserve the same DSCP marking or
enforce DSCP re-marking policies. DSCP preservation MUST be enabled
by default.
5.6.6. Supported Transport Protocols
The MCP supports TCP by design. Additional transport protocols
SHOULD be supported. A configuration parameter MUST be supported by
the MCP to indicate which transport protocols can be relayed into an
MPTCP connection.
5.6.7. Logging
If the MCP is used in global IPv4 address sharing environments, the
logging recommendations discussed in Section 4 of [RFC6888] need to
be considered. Security-related issues encountered in address
sharing environments are documented in Section 13 of [RFC6269]. A
configuration parameter should be supported to enable/disable the
logging function.
6. IANA Considerations
This document does not request any action from IANA.
7. Security Considerations
MPTCP-related security threats are discussed in [RFC6181] and
[RFC6824]. Additional considerations are discussed in the following
sub-sections.
7.1. 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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7.2. 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's resources by establishing
a large 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.
7.3. 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.
7.4. High Rate Reassembly
The MCP may perform packet reassembly. Some security-related issues
are discussed in [RFC4963][RFC1858][RFC3128].
8. Contributors
The following individuals contributed to this document:
Denis Behaghel
OneAccess
Email: Denis.Behaghel@oneaccess-net.com
Stefano Secci
Universite Pierre et Marie Curie
Paris
France
Email: stefano.secci@lip6.fr
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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
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9. Acknowledgements
Many thanks to Chi Dung Phung, Mingui Zhang, Rao Shoaib, Yoshifumi
Nishida, and Christoph Paasch for the comments.
Thanks to Ian Farrer, Mikael Abrahamsson, Alan Ford, Dan Wing, and
Sri Gundavelli for the fruitful discussions held during the IETF#95
meeting.
Special thanks to Pierrick Seite, Yannick Le Goff, Fred Klamm, and
Xavier Grall for their valuable comments.
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.
10. References
10.1. Normative References
[I-D.boucadair-mptcp-plain-mode]
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, "An MPTCP Option for
Network-Assisted MPTCP", draft-boucadair-mptcp-plain-
mode-09 (work in progress), October 2016.
[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>.
10.2. 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.
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[I-D.boucadair-mptcp-radius]
Boucadair, M. and C. Jacquenet, "RADIUS Extensions for
Network-Assisted Multipath TCP (MPTCP)", draft-boucadair-
mptcp-radius-03 (work in progress), October 2016.
[I-D.ietf-mptcp-experience]
Bonaventure, O., Paasch, C., and G. Detal, "Use Cases and
Operational Experience with Multipath TCP", draft-ietf-
mptcp-experience-07 (work in progress), October 2016.
[I-D.ietf-mptcp-rfc6824bis]
Ford, A., Raiciu, C., Handley, M., Bonaventure, O., and C.
Paasch, "TCP Extensions for Multipath Operation with
Multiple Addresses", draft-ietf-mptcp-rfc6824bis-07 (work
in progress), October 2016.
[RFC1413] St. Johns, M., "Identification Protocol", RFC 1413,
DOI 10.17487/RFC1413, February 1993,
<http://www.rfc-editor.org/info/rfc1413>.
[RFC1858] Ziemba, G., Reed, D., and P. Traina, "Security
Considerations for IP Fragment Filtering", RFC 1858,
DOI 10.17487/RFC1858, October 1995,
<http://www.rfc-editor.org/info/rfc1858>.
[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>.
[RFC3128] Miller, I., "Protection Against a Variant of the Tiny
Fragment Attack (RFC 1858)", RFC 3128,
DOI 10.17487/RFC3128, June 2001,
<http://www.rfc-editor.org/info/rfc3128>.
[RFC4963] Heffner, J., Mathis, M., and B. Chandler, "IPv4 Reassembly
Errors at High Data Rates", RFC 4963,
DOI 10.17487/RFC4963, July 2007,
<http://www.rfc-editor.org/info/rfc4963>.
[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>.
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[RFC6269] Ford, M., Ed., Boucadair, M., Durand, A., Levis, P., and
P. Roberts, "Issues with IP Address Sharing", RFC 6269,
DOI 10.17487/RFC6269, June 2011,
<http://www.rfc-editor.org/info/rfc6269>.
[RFC6296] Wasserman, M. and F. Baker, "IPv6-to-IPv6 Network Prefix
Translation", RFC 6296, DOI 10.17487/RFC6296, June 2011,
<http://www.rfc-editor.org/info/rfc6296>.
[RFC6888] Perreault, S., Ed., Yamagata, I., Miyakawa, S., Nakagawa,
A., and H. Ashida, "Common Requirements for Carrier-Grade
NATs (CGNs)", BCP 127, RFC 6888, DOI 10.17487/RFC6888,
April 2013, <http://www.rfc-editor.org/info/rfc6888>.
[RFC7974] Williams, B., Boucadair, M., and D. Wing, "An Experimental
TCP Option for Host Identification", RFC 7974,
DOI 10.17487/RFC7974, October 2016,
<http://www.rfc-editor.org/info/rfc7974>.
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
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Wim Henderickx (editor)
Nokia/Alcatel-Lucent
Belgium
Email: wim.henderickx@alcatel-lucent.com
Robert Skog (editor)
Ericsson
Email: robert.skog@ericsson.com
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