0-RTT TCP Converter
draft-ietf-tcpm-converters-00
The information below is for an old version of the document.
| Document | Type |
This is an older version of an Internet-Draft that was ultimately published as RFC 8803.
|
|
|---|---|---|---|
| Authors | Olivier Bonaventure , Mohamed Boucadair , Bart Peirens , SungHoon Seo , Anandatirtha Nandugudi | ||
| Last updated | 2018-02-19 | ||
| RFC stream | Internet Engineering Task Force (IETF) | ||
| Formats | |||
| Reviews | |||
| Additional resources | Mailing list discussion | ||
| Stream | WG state | WG Document | |
| Document shepherd | (None) | ||
| IESG | IESG state | Became RFC 8803 (Experimental) | |
| Consensus boilerplate | Unknown | ||
| Telechat date | (None) | ||
| Responsible AD | (None) | ||
| Send notices to | (None) |
draft-ietf-tcpm-converters-00
TCPM Working Group O. Bonaventure
Internet-Draft Tessares
Intended status: Experimental M. Boucadair
Expires: August 20, 2018 Orange
B. Peirens
Proximus
S. Seo
Korea Telecom
A. Nandugudi
Tessares
February 16, 2018
0-RTT TCP Converter
draft-ietf-tcpm-converters-00
Abstract
This document specifies an application proxy, called Transport
Converter, to assist the deployment of Multipath TCP. This proxy is
designed to avoid inducing extra delay when involved in a network-
assisted connection (that is, 0-RTT). This specification assumes an
explicit model, where the proxy is explicitly configured on hosts.
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 August 20, 2018.
Copyright Notice
Copyright (c) 2018 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
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(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 . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Applicability Scope . . . . . . . . . . . . . . . . . . . . . 5
3. Architecture . . . . . . . . . . . . . . . . . . . . . . . . . 6
3.1. Sample Examples of Converter-Assisted Multipath TCP
Connections . . . . . . . . . . . . . . . . . . . . . . . 9
3.2. Sample Example of Incoming Converter-Assisted
Multipath TCP Connection . . . . . . . . . . . . . . . . . 11
3.3. Differences with SOCKSv5 . . . . . . . . . . . . . . . . . 12
4. The Converter Protocol . . . . . . . . . . . . . . . . . . . . 15
4.1. Requirements . . . . . . . . . . . . . . . . . . . . . . . 15
4.2. The Fixed Header . . . . . . . . . . . . . . . . . . . . . 15
4.3. Transport Converter TLVs . . . . . . . . . . . . . . . . . 16
4.3.1. Connect TLV . . . . . . . . . . . . . . . . . . . . . 16
4.3.2. Extended TCP Header TLV . . . . . . . . . . . . . . . 18
4.3.3. Error TLV . . . . . . . . . . . . . . . . . . . . . . 19
4.3.4. The Bootstrap TLV . . . . . . . . . . . . . . . . . . 21
4.3.5. Supported TCP Options TLV . . . . . . . . . . . . . . 22
5. Interactions with middleboxes . . . . . . . . . . . . . . . . 23
6. Security Considerations . . . . . . . . . . . . . . . . . . . 24
6.1. Privacy & Ingress Filtering . . . . . . . . . . . . . . . 24
6.2. Authorization . . . . . . . . . . . . . . . . . . . . . . 24
6.3. Denial of Service . . . . . . . . . . . . . . . . . . . . 24
6.4. Traffic Theft . . . . . . . . . . . . . . . . . . . . . . 25
6.5. Multipath TCP-specific Considerations . . . . . . . . . . 25
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 27
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 28
8.1. Contributors . . . . . . . . . . . . . . . . . . . . . . . 28
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 29
9.1. Normative References . . . . . . . . . . . . . . . . . . . 29
9.2. Informative References . . . . . . . . . . . . . . . . . . 29
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 33
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1. Introduction
Transport protocols like TCP evolve regularly [RFC7414]. TCP has
been improved in different ways. Some improvements such as changing
the initial window size or modifying the congestion control scheme
can be applied independently on clients and servers. Other
improvements such as Selective Acknowledgements [RFC2018] or large
windows [RFC7323] require a new TCP option or to change the semantics
of some fields in the TCP header. These modifications must be
deployed on both clients and servers to be actually used on the
Internet. Experience with the latter TCP extensions reveals that
their deployment can require many years. Fukuda reports in
[Fukuda2011] results of a decade of measurements showing the
deployment of Selective Acknowledgements, Window Scale and TCP
Timestamps. Trammel et al. provide in [ANRW17] measurements showing
that TCP Fast Open [RFC7413] (TFO) is still not widely deployed.
There are some situations where the transport stack used on clients
(resp. servers) can be upgraded at a faster pace than the transport
stack running on servers (resp. clients). In those situations,
clients would typically want to benefit from the features of an
improved transport protocol even if the servers have not yet been
upgraded and conversely. In the past, Performance Enhancing Proxies
have been proposed and deployed [RFC3135] as solutions to improve TCP
performance over links with specific characteristics.
Recent examples of TCP extensions include Multipath TCP [RFC6824] or
TCPINC [I-D.ietf-tcpinc-tcpcrypt]. Those extensions provide features
that are interesting for clients such as wireless devices. With
Multipath TCP, those devices could seamlessly use WLAN and cellular
networks, for bonding purposes, faster handovers, or better
resiliency. Unfortunately, deploying those extensions on both a wide
range of clients and servers remains difficult.
This document specifies an application proxy, called Transport
Converter (TC). A Transport Converter is a function that is
installed by a network operator to aid the deployment of TCP
extensions and to provide the benefits of such extensions to clients.
A Transport Converter supports one or more TCP extensions. The
Converter Protocol (CP) is an application layer protocol that uses a
TCP port number (see IANA section). The Transport Converter adheres
to the main principles as drawn in [RFC1919]. In particular, the
Converter achieves the following:
o Listen for client sessions;
o Receive from a client the address of the final target server;
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o Setup a session to the final server;
o Relay control messages and data between the client and the server;
o Perform access controls according to local policies.
The main advantage of network-assisted converters is that they enable
new TCP extensions to be used on a subset of the end-to-end path,
which encourages the deployment of these extensions. The Transport
Converter allows the client and the server to directly negotiate some
options between the endpoints. This document focuses on Multipath
TCP [RFC6824] and TCP Fast Open [RFC7413]. The support for other TCP
extensions will be discussed in other documents.
This document does not assume that all the traffic is eligible to the
network-assisted conversion service. Only a subset of the traffic
will be forwarded to a converter according to a set of policies.
Furthermore, it is possible to bypass the converter to connect to the
servers that already support the required TCP extension.
This document assumes that a client is configured with one or a list
of transport converters. Configuration means are outside the scope
of this document.
This document is organized as follows. We first provide a brief
explanation of the operation of Transport Converters in Section 3.
We compare them in Section 3.3 with SOCKS proxies that are already
used to deploy Multipath TCP in cellular networks [IETFJ16]. We then
describe the Converter Protocol in Section 4. We then discuss the
interactions with middleboxes (Section 5) and the security
considerations (Section 6).
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2. Applicability Scope
This specification is designed with Multipath TCP
[RFC6824][I-D.ietf-mptcp-rfc6824bis] and TCP Fast Open [RFC7413] in
mind. That is, the specification draws how network-assisted
Multipath TCP connections can be established even if the remote
server is not Multipath TCP-capable without inducing extra connection
delays (0-RTT proxy). Further, the specification allows the client
for end-to-end Multipath TCP connections with or without proxy
involvement. Assessing the applicability of the solution to other
use cases and other TCP extensions such as [I-D.ietf-tcpinc-tcpcrypt]
is outside the scope of this document. Future documents are required
to specify the exact behavior when the converter is deployed in other
contexts than Multipath TCP.
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3. Architecture
The architecture considers three types of endhosts:
o Client endhosts;
o Transport Converters;
o Server endhosts.
It does not mandate anything on the server side. The architecture
assumes that new software will be installed on the Client hosts and
on Transport Converters. Further, the architecture allows for making
use of TCP new extensions if those are supported by a given server.
A Transport Converter is a network function that relays all data
exchanged over one upstream connection to one downstream connection
and vice versa. A connection can be initiated from both interfaces
of the transport converter (Internet-facing interface, client-facing
interface). The converter, thus, maintains state that associates one
upstream connection to a corresponding downstream connection. One of
the benefits of this design is that different transport protocol
extensions can be used on the upstream and the downstream
connections. This encourages the deployment of new TCP extensions
until they are supported by many servers.
+------------+
<--- upstream --->| Transport |<--- downstream --->
| Converter |
+------------+
Figure 1: A Transport Converter relays data between pairs of TCP
connections
Transport converters can be operated by network operators or third
parties. The Client is configured, through means that are outside
the scope of this document, with the names and/or the addresses of
one or more Transport Converters. The packets belonging to the pair
of connections between the Client and Server passing through a
Transport Converter may follow a different path than the packets
directly exchanged between the Client and the Server. Deployments
should minimize the possible additional delay by carefully selecting
the location of the Transport Converter used to reach a given
destination.
A transport converter can be embedded in a standalone device or be
activated as a service on a router. How such function is enabled is
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deployement-specific.
+-+ +-+ +-+
Client - |R| -- |R| -- |R| - - - Server
+-+ +-+ +-+
|
Transport
Converter
Figure 2: A Transport Converter can be installed anywhere in the
network
When establishing a connection, the Client can, depending on local
policies, either contact the Server directly (e.g., by sending a TCP
SYN towards the Server) or create the connection via a Transport
Converter. In the latter case, which is the case we consider in this
document, the Client initiates a connection towards the Transport
Converter and indicates the address and port number of the ultimate
Server inside the connection establishment packet. Doing so enables
the Transport Converter to immediately initiate a connection towards
that Server, without experiencing an extra delay. The Transport
Converter waits until the confirmation that the Server agrees to
establish the connection before confirming it to the Client.
The client places the destination address and port number of the
target Server in the payload of the SYN sent to the Converter by
leveraging TCP Fast Open [RFC7413]. In accordance with [RFC1919],
the Transport Converter maintains two connections that are combined
together. The upstream connection is the one between the Client and
the Transport Converter. The downstream connection is between the
Transport Converter and the remote Server. Any user data received by
the Transport Converter over the upstream (resp., downstream)
connection is relayed over the downstream (resp., upstream)
connection.
At a high level, the objective of the Transport Converter is to allow
the Client to use a specific extension, e.g. Multipath TCP, on a
subset of the end-to-end path even if the Server does not support
this extension. This is illustrated in Figure 3 where the Client
initiates a Multipath TCP connection with the Converter (Multipath
packets are shown with =) while the Converter uses a regular TCP
connection with the Server.
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Transport
Client Converter Server
======================>
-------------------->
<--------------------
<======================
Multipath TCP packets Regular TCP packets
Figure 3: Different TCP variants can be used on the Client-Converter
path and on the Converter-Server path
Figure 4 illustrates the establishment of a TCP connection by the
Client through a Transport Converter. The information shown between
brackets is part of the Converter protocol described later in this
document.
The Client sends a SYN destined to the Transport Converter. This SYN
contains a TFO Cookie and inside its payload the addresses and ports
of the destination Server. The Transport Converter does not reply
immediately to this SYN. It first tries to create a TCP connection
towards the destination Server. If this second connection succeeds,
the Transport Converter confirms the establishment of the connection
to the Client by returning a SYN+ACK and the first bytes of the
bytestream contain information about the TCP Options that were
negotiated with the final Server. This information is sent at the
beginning of the bytestream, either directly in the SYN+ACK or in a
subsequent packet. For graphical reasons, the figures in this
section show that the Converter returns this information in the SYN+
ACK packet. An implementation could also place this information in a
packet that it sent shortly after the SYN+ACK.
Transport
Client Converter Server
-------------------->
SYN TFO [->Server:port]
-------------------->
SYN
<--------------------
SYN+ACK
<--------------------
SYN+ACK [ ]
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Figure 4: Establishment of a TCP connection through a Converter
The connection can also be established from the Internet towards a
client via a transport converter. This is typically the case when
the client embeds a server (video server, for example).
The procedure described in Figure 4 assumes that the Client has
obtained a TFO Cookie from the Transport Converter. This is part of
the Bootstrap procedure which is illustrated in Figure 5. The Client
sends a SYN with a TFO Request option to obtain a valid cookie from
the Converter. The Converter replies with a TFO cookie in the SYN+
ACK. Once this connection has been established, the Client sends a
Bootstrap message to request the list of TCP options supported by the
Transport Converter. Thanks to this procedure, the Client knows
which TCP options are supported by a given Transport Converter.
Transport
Client Converter Server
-------------------->
SYN TFO(empty)
<--------------------
SYN+ACK TFO(cookie)
-------------------->
[Bootstrap]
<--------------------
[Supported TCP Options]
Figure 5: Bootstrapping a Client connection to a Transport Converter
Note that the Converter may rely on local policies to decide whether
it can service a given requesting client. That is, the Converter may
not return a cookie for that client.
Also, the Converter may behave in a Cookie-less mode when appropriate
means are enforced at the converter and the network in-between to
protect against attacks such as spoofing and SYN flood. Under such
deployments, the use of TFO is not required.
3.1. Sample Examples of Converter-Assisted Multipath TCP Connections
As an example, let us consider how such a protocol can help the
deployment of Multipath TCP [RFC6824]. We assume that both the
Client and the Transport Converter support Multipath TCP, but
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consider two different cases depending
whether the Server supports Multipath TCP or not. A Multipath TCP
connection is created by placing the MP_CAPABLE (MPC) option in the
SYN sent by the Client.
Figure 6 describes the operation of the Transport Converter if the
Server does not support Multipath TCP.
Transport
Client Converter Server
-------------------->
SYN, MPC [->Server:port]
-------------------->
SYN, MPC
<--------------------
SYN+ACK
<--------------------
SYN+ACK,MPC [ ]
-------------------->
ACK,MPC
-------------------->
ACK
Figure 6: Establishment of a Multipath TCP connection through a
Converter
The Client tries to initiate a Multipath TCP connection by sending a
SYN with the MP_CAPABLE option (MPC in Figure 6). The SYN includes
the address and port number of the final Server and the Transport
Converter attempts to initiate a Multipath TCP connection towards
this Server. Since the Server does not support Multipath TCP, it
replies with a SYN+ACK that does not contain the MP_CAPABLE option.
The Transport Converter notes that the connection with the Server
does not support Multipath TCP and returns the TCP Options received
from the Server to the Client.
Figure 7 considers a Server that supports Multipath TCP. In this
case, it replies to the SYN sent by the Transport Converter with the
MP_CAPABLE option. Upon reception of this SYN+ACK, the Transport
Converter confirms the establishment of the connection to the Client
and indicates to the Client that the Server supports Multipath TCP.
With this information, the Client has discovered that the Server
supports Multipath TCP natively. This will enable it to bypass the
Transport Converter for the next Multipath TCP connection that it
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will initiate towards this Server.
Transport
Client Converter Server
-------------------->
SYN, MPC [->Server:port]
-------------------->
SYN, MPC
<--------------------
SYN+ACK, MPC
<--------------------
SYN+ACK, MPC [ MPC supported ]
-------------------->
ACK, MPC
-------------------->
ACK, MPC
Figure 7: Establishment of a Multipath TCP connection through a
converter
3.2. Sample Example of Incoming Converter-Assisted Multipath TCP
Connection
An example of an incoming converter-assisted Multipath TCP connection
is depicted in Figure 8. In order to support incoming connections
from remote hosts, the client may use PCP [RFC6887] to instruct the
converter to create dynamic mappings. Those mappings will be used by
the converter to intercept an incoming TCP connection destined to the
client and convert it into a Multipath TCP connection.
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Transport
H1 Converter Remote Host
<-------------------
SYN
<-------------------
SYN, MPC[Remote Host:port]
--------------------->
SYN+ACK, MPC
--------------------->
SYN+ACK
<---------------------
ACK
<-------------------
ACK, MPC
Figure 8: Establishment of an Incoming TCP Connection through a
Converter
3.3. Differences with SOCKSv5
The description above is a simplified description of the Converter
protocol. At a first glance, the proposed solution could seem
similar to the SOCKS v5 protocol [RFC1928]. This protocol is used to
proxy TCP connections. The Client creates a connection to a SOCKS
proxy, exchanges authentication information and indicates the
destination address and port of the final server. At this point, the
SOCKS proxy creates a connection towards the final server and relays
all data between the two proxied connections. The operation of an
implementation based on SOCKSv5 is illustrated in Figure 9.
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Client SOCKS Proxy Server
-------------------->
SYN
<--------------------
SYN+ACK
-------------------->
ACK
-------------------->
Version=5, Auth Methods
<--------------------
Method
-------------------->
Auth Request (if "No auth" method negotiated)
<--------------------
Auth Response
-------------------->
Connect Server:Port -------------------->
SYN
<--------------------
SYN+ACK
<--------------------
Succeeded
-------------------->
Data1
-------------------->
Data1
<--------------------
Data2
<--------------------
Data2
Figure 9: Establishment of a TCP connection through a SOCKS proxy
without authentication
The Converter protocol also relays data between an upstream and a
downstream connection, but there are important differences with
SOCKSv5.
A first difference is that the Converter protocol leverages the TFO
option [RFC7413] to exchange all control information during the
three-way handshake. This reduces the connection establishment delay
compared to SOCKS that requires two or more round-trip-times before
the establishment of the downstream connection towards the final
destination. In today's Internet, latency is a important metric and
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various protocols have been tuned to reduce their latency
[I-D.arkko-arch-low-latency]. A recently proposed extension to SOCKS
also leverages the TFO option [I-D.olteanu-intarea-socks-6].
A second difference is that the Converter protocol explicitly takes
the TCP extensions into account. By using the Converter protocol,
the Client can learn whether a given TCP extension is supported by
the destination Server. This enables the Client to bypass the
Transport Converter when the destination supports the required TCP
extension. Neither SOCKS v5 [RFC1928] nor the proposed SOCKS v6
[I-D.olteanu-intarea-socks-6] provide such a feature.
A third difference is that a Transport Converter will only accept the
connection initiated by the Client provided that the downstream
connection is accepted by the Server. If the Server refuses the
connection establishment attempt from the Transport Converter, then
the upstream connection from the Client is rejected as well. This
feature is important for applications that check the availability of
a Server or use the time to connect as a hint on the selection of a
Server [RFC6555].
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4. The Converter Protocol
We now describe in details the messages that are exchanged between a
Client and a Transport Converter. The Converter Protocol (CP)
leverages the TCP Fast Open extension defined in [RFC7413].
The Converter Protocol uses a 32 bits long fixed header that is sent
by both the Client and the Transport Converter. This header
indicates both the version of the protocol used and the length of the
CP message.
4.1. Requirements
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
[RFC2119] [RFC8174] when, and only when, they appear in all capitals,
as shown here.
4.2. The Fixed Header
The Fixed Header is used to exchange information about the version
and length of the messages between the Client and the Transport
Converter. The Client and the Transport Converter MUST send the
fixed-sized header shown in Figure 10 as the first four bytes of the
bytestream.
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
+---------------+---------------+-------------------------------+
| Version | Total Length | Reserved |
+---------------+---------------+-------------------------------+
Figure 10: The fixed-sized header of the Converter protocol
The Version is encoded as an 8 bits unsigned integer value. This
document specifies version 1. The Total Length is the number of 32
bits word, including the header, of the bytestream that are consumed
by the Converter protocol messages. Since Total Length is also an 8
bits unsigned integer, those messages cannot consume more than 1020
bytes of data. This limits the number of bytes that a Transport
Converter needs to process. A Total Length of zero is invalid and
the connection MUST be reset upon reception of such a header. The
Reserved field MUST be set to zero in this version of the protocol.
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4.3. Transport Converter TLVs
The Converter protocol uses variable length messages that are encoded
using a TLV format to simplify the parsing of the messages and leave
room to extend the protocol in the future. A given TLV can only
appear once on a connection. If two or more copies of the same TLV
are exchanged over a Converter connection, the associated TCP
connections MUST be closed. All fields are encoded using the network
byte order.
Five TLVs are defined in this document. They are listed in Table 1.
+------+------+----------+---------------------------+
| Type | Hex | Length | Description |
+------+------+----------+---------------------------+
| 1 | 0x1 | 1 | Bootstrap TLV |
| | | | |
| 10 | 0xA | Variable | Connect TLV |
| | | | |
| 20 | 0x14 | Variable | Extended TCP Header TLV |
| | | | |
| 21 | 0x15 | Variable | Supported TCP Options TLV |
| | | | |
| 30 | 0x1E | Variable | Error TLV |
+------+------+----------+---------------------------+
Table 1: The TLVs used by the Converter protocol
To use a given Transport Converter, a Client MUST first obtain a
valid TFO cookie from it. This is the bootstrap procedure during
which the Client opens a connection to the Transport Converter with
an empty TFO option. According to [RFC7413], the Transport Converter
returns its cookie in the SYN+ACK. Then the Client sends a Bootstrap
TLV and the Transport Converter replies with the Supported TCP
Options TLV that lists the TCP options that it supports (section
Section 4.3.5).
With the TFO Cookie of the Transport Converter, the Client can
request the establishment of connections to remote servers with the
Connect TLV (see Section 4.3.1). If the connection can be
established with the final server, the Transport Converter replies
with the Extended TCP Header TLV and returns an Error TLV inside a
RST packet (see section Section 4.3.3).
4.3.1. Connect TLV
This TLV (Figure 11) is used to request the establishment of a
connection via a Transport Converter.
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The 'Remote Peer Port' and 'Remote Peer IP Address' fields contain
the destination port and IP address of the target server for an
outgoing connection towards a server located on the Internet. For
incoming connections destined to a client serviced via a Converter,
these fields convey the source port and IP address.
The Remote Peer IP Address MUST be encoded as an IPv6 address. IPv4
addresses MUST be encoded using the IPv4-Mapped IPv6 Address format
defined in [RFC4291].
The optional 'TCP Options' field is used to specify how specific TCP
Options should be advertised by the Transport Converter to the final
destination of a connection. If this field is not supplied, the
Transport Converter MUST use the default TCP options that correspond
to its local policy.
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
+---------------+---------------+-------------------------------+
| Type | Length | Remote Peer Port |
+---------------+---------------+-------------------------------+
| |
| Remote Peer IP Address (128 bits) |
| |
| |
+---------------------------------------------------------------+
| TCP Options (Variable) |
| ... |
+---------------------------------------------------------------+
Figure 11: The Connect TLV
The 'TCP Options' field is a variable length field that carries a
list of TCP Option fields (Figure 12). Each TCP Option field is
encoded as a block of 2+n bytes where the first byte is the TCP
Option Type and the second byte is the length of the TCP Option as
specified in [RFC0793]. The minimum value for the TCP Option Length
is 2. The TCP Options that do not include a length subfield, i.e.,
option types 0 (EOL) and 1 (NOP) defined in [RFC0793] cannot be
placed inside the TCP Options field of the Connect TLV. The optional
Value field contains the variable-length part of the TCP option. A
length of two indicates the absence of the Value field. The TCP
Options field always ends on a 32 bits boundary after being padded
with zeros.
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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
+---------------+---------------+---------------+---------------+
| TCPOpt type | TCPOpt Length | Value (opt) | .... |
+---------------+---------------+---------------+---------------+
| .... |
+---------------------------------------------------------------+
| ... |
+---------------------------------------------------------------+
Figure 12: The TCP Options field
If a Transport Converter receives a Connect TLV with a non-empty TCP
Options field, it shall present those options to the destination peer
in addition to the TCP Options that it would have used according to
its local policies. For the TCP Options that are listed without an
optional value, the Converter MUST generate its own value. For the
TCP Options that are included in the 'TCP Options' field with an
optional value, it shall copy the entire option for use in the
connection with the destination peer. This feature is required to
support TCP Fast Open.
4.3.2. Extended TCP Header TLV
The Extended TCP Header TLV is used by the Transport Converter to
send to the Client the extended TCP header that was returned by the
Server in the SYN+ACK packet. This TLV is only sent if the Client
sent a Connect TLV to request the establishment of a connection.
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
+---------------+---------------+-------------------------------+
| Type | Length | Reserved |
+---------------+---------------+-------------------------------+
| Returned Extended TCP header |
| ... |
+---------------------------------------------------------------+
Figure 13: The Extended TCP Header TLV
The Returned Extended TCP header field is a copy of the extended
header that was received in the SYN+ACK by the Transport Converter.
The Reserved field is set to zero by the transmitter and ignored by
the receiver.
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4.3.3. Error TLV
This optional TLV can be used by the Transport Converter to provide
information about some errors that occurred during the processing of
a request to convert a connection. This TLV appears after the
Converter header in a RST segment returned by the Transport Converter
if the error is fatal and prevented the establishment of the
connection. If the error is not fatal and the connection could be
established with the final destination, then the error TLV will be
carried in the payload.
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
+---------------+---------------+----------------+--------------+
| Type | Length | Error | Value |
+---------------+---------------+----------------+--------------+
Figure 14: The Error TLV
Different types of errors can occur while processing Converter
protocol messages. Each error is identified by a code represented as
an unsigned integer. Four classes of errors are defined:
o Message validation and processing errors (0<= error code <= 31):
returned upon reception of an an invalid message (including valid
messages but with invalid or unknown TLVs).
o Client-side errors (32<= error code <= 63): the Client sent a
request that could not be accepted by the Converter (e.g.,
unsupported operation).
o Converter-side errors (64<= error code <96) : problems encountered
on the Converter (e.g., lack of ressources) which prevent it from
fulfilling the Client's request.
o Errors caused by destination server (96<= error code <= 127) : the
final destination could not be reached or it replied with a reset
message.
The following errors are defined in this document:
o Unsupported Version (0): The version number indicated in the fixed
header of a message received from a peer is not supported. This
error code MUST be generated by a Converter when it receives a
request having a version number that it does not support. The
value field MUST be set to the version supported by the Converter.
When multiple versions are supported by the converter, it includes
the list of supported version in the value field; each version is
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encoded in 8 bits. Upon receipt of this error code, the client
checks whether it supports one of the versions returned by the
Converter. The highest common supported version MUST be used by
the client in subsequent exchanges with the Converter.
o Malformed Message (1): This error code is sent to indicate that a
message can not be successfully parsed. To ease troubleshooting,
the value field MUST echo the received message. The Converter and
the Client MUST send a RST containing this error upon reception of
a malformed message.
o Unsupported Message (2): This error code is sent to indicate that
a message type is not supported by the converter. To ease
troubleshooting, the value field MUST echo the received message.
The Converter and the Client MUST send a RST containing this error
upon reception of an unsupported message.
o Not Authorized (32): This error code indicates that the Converter
refused to create a connection because of a lack of authorization
(e.g., administratively prohibited, authorization failure, etc.).
The Value field is set to zero. This error code MUST be sent by
the Converter when a request cannot be successfully processed
because the authorization failed.
o Unsupported TCP Option (33). A TCP Option that the Client
requested to advertise to the final Server is not supported by the
Transport Converter. The Value field is set to the type of the
unsupported TCP Option. If several unsupported TCP Options were
specified in the Connect TLV, only one of them is returned in the
Value.
o Resource Exceeded (64): This error indicates that the Transport
Converter does not have enough resources to perform the request.
This error MUST be sent by the Converter when it does not have
sufficient resources to handle a new connection.
o Network Failure (65): This error indicates that the converter is
experiencing a network failure to relay the request. The
converter MUST send this error code when it experiences forwarding
issues to relay a connection.
o Connection Reset (96): This error indicates that the final
destination responded with a RST packet. The Value field is set
to zero.
o Destination Unreachable (97): This error indicates that an ICMP
destination unreachable, port unreachable, or network unreachable
was received by the Converter. The Value field contains the Code
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field of the received ICMP message. This error message MUST be
sent by the Converter when it receives an error message that is
bound to a message it relayed previously.
Table 2 summarizes the different error codes.
+-------+------+-------------------------+
| Error | Hex | Description |
+-------+------+-------------------------+
| 0 | 0x00 | Unsupported version |
| | | |
| 1 | 0x01 | Malformed Message |
| | | |
| 2 | 0x02 | Unsupported Message |
| | | |
| 32 | 0x20 | Not Authorized |
| | | |
| 33 | 0x21 | Unsupported TCP Option |
| | | |
| 64 | 0x40 | Resource Exceeded |
| | | |
| 65 | 0x41 | Network Failure |
| | | |
| 96 | 0x60 | Connection Reset |
| | | |
| 97 | 0x61 | Destination Unreachable |
+-------+------+-------------------------+
Table 2: The different error codes
4.3.4. The Bootstrap TLV
The Bootstrap TLV is sent by a Client to request the TCP Extensions
that are supported by a Transport Converter. It is typically sent on
the first connection that a Client establishes with a Transport
Converter to learn its capabilities. The Transport Converter replies
with the Supported TCP Options TLV described in Section 4.3.5.
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
+---------------+---------------+-------------------------------+
| Type | Length | Zero |
+---------------+---------------+-------------------------------+
Figure 15: The Bootstrap TLV
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4.3.5. Supported TCP Options TLV
The Supported TCP Options TLV is used by a Converter to announce the
TCP options that it supports. Each supported TCP Option is encoded
with its TCP option Kind listed in the TCP Parameters registry
maintained by IANA. TCP option Kinds 0, 1, and 2 defined in
[RFC0793] are supported by all TCP implementations and thus cannot
appear in this list. The list of supported TCP Options is padded
with 0 to end on a 32 bits boundary.
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
+---------------+---------------+-------------------------------+
| Type | Length | Reserved |
+---------------+---------------+-------------------------------+
| Kind #1 | Kind #2 | ... |
+---------------+---------------+-------------------------------+
/ ... /
/ /
+---------------------------------------------------------------+
Figure 16: The Supported TCP Options TLV
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5. Interactions with middleboxes
The Converter protocol was designed to be used in networks that do
not contain middleboxes that interfere with TCP. We describe in this
section how a Client can detect middlebox interference and stop using
the Transport Converter affected by this interference.
Internet measurements [IMC11] have shown that middleboxes can affect
the deployment of TCP extensions. In this section, we only discuss
the middleboxes that modify SYN and SYN+ACK packets since the
Converter protocol places its messages in such packets.
Let us first consider a middlebox that removes the TFO Option from
the SYN packet. This interference will be detected by the Client
during the bootstrap procedure shown in Figure 5. A Client should
not use a Transport Converter that does not reply with the TFO option
during the Bootstrap.
Consider a middlebox that removes the SYN payload after the bootstrap
procedure. The Client can detect this problem by looking at the
acknowledgement number field of the SYN+ACK returned by the Transport
Converter. The Client should stop to use this Transport Converter
given the middlebox interference.
As explained in [RFC7413], some carrier-grade NATs can affect the
operation of TFO if they assign different IP addresses to the same
end host. Such carrier-grade NATs could affect the operation of the
TFO Option used by the Converter protocol. See also the discussion
in section 7.1 of [RFC7413].
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6. Security Considerations
6.1. Privacy & Ingress Filtering
The Converter may have access to privacy-related information (e.g.,
subscriber credentials). The Converter MUST NOT leak such sensitive
information outside a local domain.
Given its function and its location in the network, a Transport
Converter has access to the payload of all the packets that it
processes. As such, it must be protected as a core IP router.
Furthermore, ingress filtering policies MUST be enforced at the
network boundaries [RFC2827].
This document assumes that all network attachements are managed by
the same administrative entity. Therefore, enforcing anti-spoofing
filters at these network ensures that hosts are not sending traffic
with spoofed source IP addresses.
6.2. Authorization
The Converter protocol is intended to be used in managed networks
where end hosts can be identified by their IP address. Thanks to the
Bootstrap procedure (Figure 5), the Transport Converter can verify
that the Client correctly receives packets sent by the Converter.
Stronger authentication schemes should be defined to use the
Converter protocol in more open network environments.
See below for authorization considerations that are specific for
Multipath TCP.
6.3. Denial of Service
Another possible risk is the amplification attacks since a Transport
Converter sends a SYN towards a remote Server upon reception of a SYN
from a Client. This could lead to amplification attacks if the SYN
sent by the Transport Converter were larger than the SYN received
from the Client or if the Transport Converter retransmits the SYN.
To mitigate such attacks, the Transport Converter SHOULD rate limit
the number of pending requests for a given Client. It SHOULD also
avoid sending to remote Servers SYNs that are significantly longer
than the SYN received from the Client. In practice, Transport
Converters SHOULD NOT advertise to a Server TCP Options that were not
specified by the Client in the received SYN. Finally, the Transport
Converter SHOULD only retransmit a SYN to a Server after having
received a retransmitted SYN from the corresponding Client.
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Upon reception of a SYN that contains a valid TFO Cookie and a
Connect TLV, the Transport Converter attempts to establish a TCP
connection to a remote Server. There is a risk of denial of service
attack if a Client requests too many connections in a short period of
time. Implementations SHOULD limit the number of pending connections
from a given Client. Means to protect against SYN flooding attacks
MUST also be enabled [RFC4987].
6.4. Traffic Theft
Traffic theft is a risk if an illegitimate Converter is inserted in
the path. Indeed, inserting an illegitimate Converter in the
forwarding path allows traffic interception and can therefore provide
access to sensitive data issued by or destined to a host. Converter
discovery and configuration are out of scope of this document.
6.5. Multipath TCP-specific Considerations
Multipath TCP-related security threats are discussed in [RFC6181] and
[RFC6824].
The operator that manages the various network attachments (including
the Converters) can 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 Converter.
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 hosts that are authorized to communicate with a
Converter. These ACLs may be installed as a result of RADIUS
exchanges, e.g. [I-D.boucadair-mptcp-radius]. This method does
not require any interaction with the Converter.
o A device that embeds the Converter 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 Converter resources by
illegitimate users (e.g., users with access networks that are not
managed by the same provider that operates the Converter) is the
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Converter to reject Multipath TCP connections received on its
Internet-facing interfaces. Only Multipath PTCP connections received
on the customer-facing interfaces of a Converter will be accepted.
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7. IANA Considerations
This document requests the allocation of a reserved service name and
port number for the converter protocol at https://www.iana.org/
assignments/service-names-port-numbers/
service-names-port-numbers.xhtml.
This documents specifies version 1 of the Converter protocol. Five
types of Converter messages are defined:
o 1: Bootstrap TLV
o 10: Connect TLV
o 20: Extended TCP Header TLV
o 21: Supported TCP Options TLV
o 30: Error TLV
Furthermore, it also defines the following error codes:
+-------+------+-------------------------+
| Error | Hex | Description |
+-------+------+-------------------------+
| 0 | 0x00 | Unsupported version |
| | | |
| 1 | 0x01 | Malformed Message |
| | | |
| 2 | 0x02 | Unsupported Message |
| | | |
| 32 | 0x20 | Not Authorized |
| | | |
| 33 | 0x21 | Unsupported TCP Option |
| | | |
| 64 | 0x40 | Resource Exceeded |
| | | |
| 65 | 0x41 | Network Failure |
| | | |
| 96 | 0x60 | Connection Reset |
| | | |
| 97 | 0x61 | Destination Unreachable |
+-------+------+-------------------------+
Table 3: The different error codes
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8. Acknowledgements
Although they could disagree with the contents of the document, we
would like to thank Joe Touch and Juliusz Chroboczek whose comments
on the MPTCP mailing list have forced us to reconsider the design of
the solution several times.
We would like to thank Raphael Bauduin and Stefano Secci for their
help in preparing this draft. Sri Gundavelli and Nandini Ganesh
provided valuable feedback about the handling of TFO and the error
codes. Thanks to them.
This document builds upon earlier documents that proposed various
forms of Multipath TCP proxies [I-D.boucadair-mptcp-plain-mode],
[I-D.peirens-mptcp-transparent] and [HotMiddlebox13b].
From [I-D.boucadair-mptcp-plain-mode]:
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.
8.1. Contributors
As noted above, this document builds on two previous documents.
The authors of [I-D.boucadair-mptcp-plain-mode] were: - Mohamed
Boucadair - Christian Jacquenet - Olivier Bonaventure - Denis
Behaghel - Stefano Secci - Wim Henderickx - Robert Skog - Suresh
Vinapamula - SungHoon Seo - Wouter Cloetens - Ullrich Meyer - Luis M.
Contreras - Bart Peirens
The authors of [I-D.peirens-mptcp-transparent] were: - Bart Peirens -
Gregory Detal - Sebastien Barre - Olivier Bonaventure
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9. References
9.1. Normative References
[RFC0793] Postel, J., "Transmission Control Protocol", STD 7,
RFC 793, DOI 10.17487/RFC0793, September 1981,
<https://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,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC4291] Hinden, R. and S. Deering, "IP Version 6 Addressing
Architecture", RFC 4291, DOI 10.17487/RFC4291,
February 2006, <https://www.rfc-editor.org/info/rfc4291>.
[RFC4987] Eddy, W., "TCP SYN Flooding Attacks and Common
Mitigations", RFC 4987, DOI 10.17487/RFC4987, August 2007,
<https://www.rfc-editor.org/info/rfc4987>.
[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,
<https://www.rfc-editor.org/info/rfc6824>.
[RFC7413] Cheng, Y., Chu, J., Radhakrishnan, S., and A. Jain, "TCP
Fast Open", RFC 7413, DOI 10.17487/RFC7413, December 2014,
<https://www.rfc-editor.org/info/rfc7413>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
9.2. Informative References
[ANRW17] Trammell, B., Kuhlewind, M., De Vaere, P., Learmonth, I.,
and G. Fairhurst, "Tracking transport-layer evolution with
PATHspider", Applied Networking Research Workshop 2017
(ANRW17) , July 2017.
[Fukuda2011]
Fukuda, K., "An Analysis of Longitudinal TCP Passive
Measurements (Short Paper)", Traffic Monitoring and
Analysis. TMA 2011. Lecture Notes in Computer Science, vol
6613. , 2011.
[HotMiddlebox13b]
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Detal, G., Paasch, C., and O. Bonaventure, "Multipath in
the Middle(Box)", HotMiddlebox'13 , December 2013, <http:/
/inl.info.ucl.ac.be/publications/multipath-middlebox>.
[I-D.arkko-arch-low-latency]
Arkko, J. and J. Tantsura, "Low Latency Applications and
the Internet Architecture",
draft-arkko-arch-low-latency-02 (work in progress),
October 2017.
[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, "Extensions for Network-
Assisted MPTCP Deployment Models",
draft-boucadair-mptcp-plain-mode-10 (work in progress),
March 2017.
[I-D.boucadair-mptcp-radius]
Boucadair, M. and C. Jacquenet, "RADIUS Extensions for
Network-Assisted Multipath TCP (MPTCP)",
draft-boucadair-mptcp-radius-05 (work in progress),
October 2017.
[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-09 (work
in progress), July 2017.
[I-D.ietf-tcpinc-tcpcrypt]
Bittau, A., Giffin, D., Handley, M., Mazieres, D., Slack,
Q., and E. Smith, "Cryptographic protection of TCP Streams
(tcpcrypt)", draft-ietf-tcpinc-tcpcrypt-11 (work in
progress), November 2017.
[I-D.olteanu-intarea-socks-6]
Olteanu, V. and D. Niculescu, "SOCKS Protocol Version 6",
draft-olteanu-intarea-socks-6-01 (work in progress),
October 2017.
[I-D.peirens-mptcp-transparent]
Peirens, B., Detal, G., Barre, S., and O. Bonaventure,
"Link bonding with transparent Multipath TCP",
draft-peirens-mptcp-transparent-00 (work in progress),
July 2016.
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[IETFJ16] Bonaventure, O. and S. Seo, "Multipath TCP Deployment",
IETF Journal, Fall 2016 , n.d..
[IMC11] Honda, K., Nishida, Y., Raiciu, C., Greenhalgh, A.,
Handley, M., and T. Hideyuki, "Is it still possible to
extend TCP ?", Proceedings of the 2011 ACM SIGCOMM
conference on Internet measurement conference , 2011.
[RFC1919] Chatel, M., "Classical versus Transparent IP Proxies",
RFC 1919, DOI 10.17487/RFC1919, March 1996,
<https://www.rfc-editor.org/info/rfc1919>.
[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,
<https://www.rfc-editor.org/info/rfc1928>.
[RFC2018] Mathis, M., Mahdavi, J., Floyd, S., and A. Romanow, "TCP
Selective Acknowledgment Options", RFC 2018, DOI 10.17487/
RFC2018, October 1996,
<https://www.rfc-editor.org/info/rfc2018>.
[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, <https://www.rfc-editor.org/info/rfc2827>.
[RFC3135] Border, J., Kojo, M., Griner, J., Montenegro, G., and Z.
Shelby, "Performance Enhancing Proxies Intended to
Mitigate Link-Related Degradations", RFC 3135,
DOI 10.17487/RFC3135, June 2001,
<https://www.rfc-editor.org/info/rfc3135>.
[RFC6181] Bagnulo, M., "Threat Analysis for TCP Extensions for
Multipath Operation with Multiple Addresses", RFC 6181,
DOI 10.17487/RFC6181, March 2011,
<https://www.rfc-editor.org/info/rfc6181>.
[RFC6555] Wing, D. and A. Yourtchenko, "Happy Eyeballs: Success with
Dual-Stack Hosts", RFC 6555, DOI 10.17487/RFC6555,
April 2012, <https://www.rfc-editor.org/info/rfc6555>.
[RFC6887] Wing, D., Ed., Cheshire, S., Boucadair, M., Penno, R., and
P. Selkirk, "Port Control Protocol (PCP)", RFC 6887,
DOI 10.17487/RFC6887, April 2013,
<https://www.rfc-editor.org/info/rfc6887>.
[RFC7323] Borman, D., Braden, B., Jacobson, V., and R.
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Scheffenegger, Ed., "TCP Extensions for High Performance",
RFC 7323, DOI 10.17487/RFC7323, September 2014,
<https://www.rfc-editor.org/info/rfc7323>.
[RFC7414] Duke, M., Braden, R., Eddy, W., Blanton, E., and A.
Zimmermann, "A Roadmap for Transmission Control Protocol
(TCP) Specification Documents", RFC 7414, DOI 10.17487/
RFC7414, February 2015,
<https://www.rfc-editor.org/info/rfc7414>.
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Authors' Addresses
Olivier Bonaventure
Tessares
Email: Olivier.Bonaventure@tessares.net
Mohamed Boucadair
Orange
Email: mohamed.boucadair@orange.com
Bart Peirens
Proximus
Email: bart.peirens@proximus.com
SungHoon Seo
Korea Telecom
Email: sh.seo@kt.com
Anandatirtha Nandugudi
Tessares
Email: anand.nandugudi@tessares.net
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