Internet DRAFT - draft-amend-tcpm-mptcp-robe
draft-amend-tcpm-mptcp-robe
TCP Maintenance and Minor Extensions M. Amend
Internet-Draft DT
Intended status: Experimental J. Kang
Expires: 8 September 2022 Huawei
7 March 2022
Multipath TCP Extension for Robust Session Establishment
draft-amend-tcpm-mptcp-robe-02
Abstract
Multipath TCP extends the plain, single-path limited, TCP towards the
capability of multipath transmission. This greatly improves the
reliability and performance of TCP communication. For backwards
compatibility reasons the Multipath TCP was designed to setup
successfully an initial path first, after which subsequent paths can
be added for multipath transmission. For that reason the Multipath
TCP has the same limitations as the plain TCP during connection
setup, in case the selected path is not functional.
This document proposes a set of implementations and possible
combinations thereof, that provide a more Robust Establishment (RobE)
of MPTCP sessions. It includes RobE_TIMER, RobE_SIM, RobE_eSIM and
RobE_IPS.
RobE_TIMER is designed to stay close to MPTCP in that standard
functionality is used wherever possible. Resiliency against network
outages is achieved by modifying the SYN retransmission timer: If one
path is defective, another path is used.
RobE_SIM and RobE_eSIM provides the ability to simultaneously use
multiple paths for connection setup. They ensure connectivity if at
least one functional path out of a bunch of paths is given and offers
beside that the opportunity to significantly improve loading times of
Internet services.
RobE_IPS provides a heuristic to select properly an initial path for
connection establishment with a remote host based on empirical data
derived from previous connection information.
In practice, these independent solutions can be complementary used.
This document also presents the design and protocol procedure for
those combinations in addition to the respective stand-alone
solutions.
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Status of This Memo
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 7
2. Implementation without MPTCP protocol adaptation . . . . . . 8
2.1. Re-transmission Timer(RobE_TIMER) . . . . . . . . . . . . 8
2.2. Simultaneous Initial Paths Simple Version (RobE_SIM) . . 9
2.3. Heuristic Initial Path Selection (RobE_IPS) . . . . . . . 10
2.3.1. Architecture . . . . . . . . . . . . . . . . . . . . 10
2.3.2. Typical Scenarios . . . . . . . . . . . . . . . . . . 11
2.3.3. Path decision information . . . . . . . . . . . . . . 14
2.3.4. Initial Path Selection use local RTT information . . 15
2.4. Combination of RobE_SIM and RobE_IPS . . . . . . . . . . 15
2.5. Combination of RobE_TIMER and RobE_IPS . . . . . . . . . 16
3. Implementation with Bi-directional MPTCP Support . . . . . . 17
3.1. Simultaneous Initial Paths Extended Version
(RobE_eSIM) . . . . . . . . . . . . . . . . . . . . . . . 18
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3.1.1. RobE_eSIM implicit Negotiation and Procedure . . . . 18
3.1.2. RobE_eSIM explicit Negotiation and Procedure . . . . 20
3.1.3. Protocol Adaptation . . . . . . . . . . . . . . . . . 20
3.1.4. Fallback Mechanisms . . . . . . . . . . . . . . . . . 21
3.1.5. Comparison Robe_SIM and RobE_eSIM . . . . . . . . . . 23
3.1.6. Security Consideration . . . . . . . . . . . . . . . 24
3.2. Heuristic Initial Path Selection with remote RTT
Measurement . . . . . . . . . . . . . . . . . . . . . . . 24
3.2.1. Description . . . . . . . . . . . . . . . . . . . . . 24
3.2.2. Protocol Adaptation . . . . . . . . . . . . . . . . . 25
3.2.3. Fallback Mechanism . . . . . . . . . . . . . . . . . 26
3.2.4. Security Consideration . . . . . . . . . . . . . . . 26
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 26
5. References . . . . . . . . . . . . . . . . . . . . . . . . . 27
5.1. Normative References . . . . . . . . . . . . . . . . . . 27
5.2. Informative References . . . . . . . . . . . . . . . . . 27
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 27
1. Introduction
Multipath TCP Robust Session Establishment (MPTCP RobE) is a set of
extensions to regular MPTCP [RFC6824] and its next version [RFC8684],
which releases single path limitations during the initial connection
setup. Several scenarios require and benefit from a reliable and in
time connection setup which is not covered by [RFC6824] and [RFC8684]
so far. MPTCP was designed to be compliant with the TCP standard
[RFC0793] and introduced therefore the concept of an initial TCP flow
while adding subsequent flows after successful multipath negotiation
on the initial path. While fulfilling its purpose, MPTCP is however
fully dependent on the transmission characteristics of the
communication link selected for initiating MPTCP.
Figure 1 shows the traditional way of MPTCP handshaking with an
MP_CAPABLE exchanged first, followed when successfully negotiated by
additional flows engaging MP_JOIN. [RFC6824] and the next MPTCP
[RFC8684] differ in that a Key-A is sent with the first MP_CAPABLE or
not.
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Host A Host B
------------------------ ----------
Address A1 Address A2 Address B1
---------- ---------- ----------
| | |
| SYN + MP_CAPABLE(Key-A[*]) |
|--------------------------------------------->|
|<---------------------------------------------|
| SYN/ACK + MP_CAPABLE(Key-B) |
| | |
| ACK + MP_CAPABLE(Key-A, Key-B) |
|--------------------------------------------->|
| | |
| | SYN + MP_JOIN(Token-B, R-A) |
| |------------------------------->|
| |<-------------------------------|
| | SYN/ACK + MP_JOIN(HMAC-B, R-B) |
| | |
| | ACK + MP_JOIN(HMAC-A) |
| |------------------------------->|
| |<-------------------------------|
| | ACK |
[*] Key-A in the first MP-capable is related to
RFC6824 only and does not exist in RFC8684.
Figure 1: MPTCP connection setup
Multipath TCP itself enables hosts to exchange packets belonging to a
single connection over several paths. Implemented in mobile phones
(UEs), these paths are usually assigned to different network
interfaces within the UE and correspond to different access networks
such as cellular and WiFi. The path or network interface for
initiating the initial subflow setup is most often provided by the
operation system of the UE. For example, if both a cellular
connection and WiFi are present in a mobile phone, WiFi is usually
the interface offered to initiate the MPTCP session.
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This design falls short in situations where the default path does not
provide the best performance compared to other available paths. In a
worst case the default path is not even capable of setting up the
initial flow letting any other functional path unused. For example,
if the WiFi signal is weak, broken or cannot forward traffic to the
destination, the establishment of the subflow will be delayed or
impossible. This in turn, leads to a longer startup delay or no
communication at all for services using MPTCP even if other
functional paths are available. Even in scenarios where all paths
are functional but services would benefit from a setup over the path
with the lowest latency, MPTCP has no mean to support this demand.
It can be concluded, that sequential path establishment relying with
an initial path establishment over an externally given default route
will result in experience reduction when using MPTCP. So this
document proposes solutions to overcome the aforementioned
limitations and provides a more robust connection setup compared to
traditional MPTCP.
Introduction of RobE_SIM and RobE_eSIM aims to overcome the
limitations of [RFC6824] and [RFC8684], using one initial flow and
introduces the concept of multiple potential initial flows triggered
simultaneously.Potential initial flows give the freedom to use more
than one path to request multipath capability and select the initial
flow at a later point. Potential initial flow mechanisms and the
gain of robustness and performance over the traditional MPTCP
connection setup are evaluated in [RobE_slides] and [RobE_paper].
RobE_SIM is a break-before-make mechanism, guaranteeing at least the
robust connection establishment, however the RobE_eSIM reuses every
potential initial flow request to combine it with less overhead and
accelerated multipath availability, leveraging a new MPTCP option
MP_JOIN_CAP. From a standardization perspective, the RobE_SIM is
fully compliant with [RFC6824] and [RFC8684] and is herein more of a
descriptive and procedural nature. The RobE_eSIM requires a new
MPTCP option but offers the potential to significantly improve the
MPTCP experience.
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For the limitation of the default initial path, RobE_IPS makes no
changes to standard MPTCP procedure and improves the performance of
connection establishment by introducing an initial path selection
strategy and required algorithms. The input for strategy and
algorithms is the transmission status information which represents
the transmission performance of each available path or network
interface. The transmission status information is characterized by
at least one of the parameters: signal strength, throughput, round-
trip time (RTT), and link success rate. In this way, a path with
better transmission performance can be learned and determined and the
respective network interface can be used for connection
establishment.
The most simple approach for a robust MPTCP session establishment is
RobE_TIMER, iterating the process of initial path establishment over
all available paths, if the previous try has failed. Triggering a
new try on a next path is depending on an expiration timer,
preferably re-use TCP's in-built expiration timer.
Table 1 summarizes the impact of RobE_TIMER, RobE_SIM, RobE_eSIM, and
RobE_IPS compared to [RFC6824] and [RFC8684].
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+========+==========+============+==========+============+==========+
|Scenario|MPTCP |RobE_TIMER |RobE_SIM |RobE_eSIM |RobE_IPS |
+========+==========+============+==========+============+==========+
|IP |Delayed |In the scope|No impact |No impact |Delayed |
|packet |connection|of timer | | |connection|
|loss | | | | | |
+--------+----------+------------+----------+------------+----------+
|IP |No |In the scope|No impact |No impact |No |
|broken |connection|of timer | | |connection|
+--------+----------+------------+----------+------------+----------+
|IP setup|Default |Default |Fastest |Fastest path|Selected |
|duration|route |route (+ |path | |path |
|de- | |path 1..n) | | | |
|pendency| | | | | |
+--------+----------+------------+----------+------------+----------+
|MP |MP_CAPABLE|sum_1..n( |MP_CAPABLE|max( |MP_CAPABLE|
|avail- |HS + |MP_CAPABLE_n|HS + |MP_CAPABLE_1|HS + |
|ability |MP_JOIN HS|HS) + |MP_JOIN HS|.. |MP_JOIN HS|
|duration| |MP_JOIN HS | |MP_CAPABLE_n| |
| | | | |HS) | |
+--------+----------+------------+----------+------------+----------+
|Guaran- |Depends on|Yes |Yes |Yes |Depends on|
|teeing |the | | | |selection |
|session |default | | | | |
|setup |route | | | | |
+--------+----------+------------+----------+------------+----------+
Table 1: Overview RobE features during initial connection setup
| IP: Initial Path; MP: Multi-Path; HS: Handshake
1.1. Terminology
This document makes use of a number of terms that are either MPTCP-
specific or have defined meaning in the context of MPTCP, as follows:
Path: A sequence of links between a sender and a receiver, defined
in this context by a 4-tuple of source and destination address/
port pairs.
Subflow: A flow of TCP segments operating over an individual path,
which forms part of a larger MPTCP connection. A subflow is
started and terminated similar to a regular TCP connection.
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2. Implementation without MPTCP protocol adaptation
RobE_TIMER, RobE_SIM, and RobE_IPS are compatible with the current
MPTCP protocol definitions in [RFC6824] and [RFC8684] but may lack of
the full optimization potential which requires protocol adaptation as
detailed in Section 3. Following sections will describe the newly
introduced mechanisms in detail.
2.1. Re-transmission Timer(RobE_TIMER)
In RobE_TIMER, a new connection is initiated by sending a
SYN+MP_CAPABLE along the initial path. If this path is functional,
the solution will perform in the same way as classic MPTCP: the
initial flow will be established, and subsequent flows can be created
afterwards. If however the initial path is faulty, the
retransmission will be triggered on another path. This path might
circumvent the dysfunctional network, and allow the client to create
an initial subflow. The first path is now seen as a subsequent path
and the client sends SYN+MP_JOIN messages to create a subsequent
flow.
In high latency networks, the initial SYN+MP_CAPABLE messages might
be delayed until the client retries sending them on another path.
Once the second SYN arrives at the server, it will try to complete
the three-way handshake. If the first SYN was delayed by more than
the retransmission time plus half a Round Trip Time (RTT) of the
second path, it will arrive at the server after the second SYN. The
server could now treat the segment as obsolete and drop it.
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Host A Host B
------------------------ ----------
Address A1 Address A2 Address B1
---------- ---------- ----------
| | |
| SYN + MP_CAPABLE(Key-A[*]) |
|Timer---------------------------------------->|
| | SYN + MP_CAPABLE(Key-A'[*]) |
| |------------------------------->|
| | SYN/ACK+MP_CAPABLE(Key-B') |
| |<-------------------------------|
| | ACK + MP_CAPABLE(Key-A',Key-B')|
| |------------------------------->|
| | SYN + MP_JOIN(Token-B',R-A) |
|--------------------------------------------->|
| Subflow will be set up as normal MPTCP |
| |
[*] Key-A in the first MP-capable is related to
RFC6824 only and does not exist in RFC8684.
Figure 2: The RobE_TIMER Solution
Immediately after sending the final ACK of the initial handshake,
subflows are established on the remaining paths as defined in
[RFC6824] and [RFC8684]
[Notes: How to set the Timer is TBD. If there is the case that the
first SYN on default path arrives earlier than that from the second
path, the MPTCP connection will be initialized on the path of the
first SYN. The server could treat the second SYN as obsolete and
drop it.]
2.2. Simultaneous Initial Paths Simple Version (RobE_SIM)
RobE_SIM is a sender only implementation and no prior negotiation
with the receiver side is required. In RobE_SIM, the MPTCP
connection setup benefits from the fastest path. As shown in
Figure 3, host A initiates the connection handshake on more than one
path independently (SA1 and SA2). The paths selected for RobE_SIM
and referred to as potential initial flows, can belong to the number
of interfaces on the device or a subset selected on experience. When
Host A receives the first SYN/ACK back from Host B (SA3), the path
carrying this message is identified as the normal initial path. Host
A sends then immediately a TCP RST message (SA6.1) on any other path
used for simultaneous connection setup causing an immediate
termination of assigned flows (break-before-make). The terminated
ones are merged as subsequent subflows following the JOIN procedure
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described in [RFC6824] and [RFC8684]. The process is equivalent to
any other scenario where the SYN/ACK arrives on an other path than
depicted in Figure 3.
Host A Host B
------------------------ ----------
Address A1 Address A2 Address B1
---------- ---------- ----------
| | |
| SYN + MP_CAPABLE(Key-A[*]) |
(SA1) |--------------------------------------------->| (SB1)
| | SYN + MP_CAPABLE(Key-A'[*]) |
(SA2) | |------------------------------->| (SB2)
| | |
(SA3) |<---------------------------------------------| (SB3)
| SYN/ACK + MP_CAPABLE(Key-B) |
(SA4) | |<-------------------------------| (SB4)
| | SYN/ACK + MP_CAPABLE(Key-B') |
| | |
| ACK + MP_CAPABLE(Key-A, Key-B) |
(SA5) |--------------------------------------------->| (SB5)
| | RST |
(SA6.1) | |------------------------------->| (SB6.1)
RobE SIM | | SYN + MP_JOIN(Token-B, R-A) |
(robust) | |------------------------------->|
| | MP_JOIN Process... |
[*] Key-A in the first MP-capable is related to
RFC6824 only and does not exist in RFC8684.
Figure 3: MPTCP RobE_SIM Connection Setup
2.3. Heuristic Initial Path Selection (RobE_IPS)
2.3.1. Architecture
Figure 4 provides the architecture for RobE_IPS and employs an
"Initial Path Selection" logic which can be integrated into the MPTCP
stack or exists as an isolated module in the terminal. The IPS logic
has access to a set of transmission status information for each
available path or its belonging network interfaces. When an
application starts a first communication, IPS selects based on the
available path transmission characteristics the path with the highest
probability to succeed.
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+-------------------+ +-------------------+
| Terminal | | Server |
| +-------------+ | | +-------------+ |
| |Application n| | | |Application n| |
| +-------------+ | | +-------------+ |
| | | | | |
| +-------------+ | | | |
| |Initial-path | |-------+ | | |
| | Selection | | | | | |
| +-------------+ | | | | |
| | | +--------+ | | |
| +-------------+ |--|Internet|--| +-------------+ |
| | MPTCP Stack | |--+--------+--| | MPTCP Stack | |
| +-------------+ | | +-------------+ |
+-------------------+ +-------------------+
Figure 4: Architecture for Initial-path Selection
2.3.2. Typical Scenarios
Two typical RobE_IPS scenarios are presented in this section.
Figure 5 shows the "Initial Path Selection" logic executed for each
MPTCP connection establishment. On the other hand Figure 6 describes
that "Initial Path Selection" in case no path information is
available. Considering the fact that no heuristics are given before
a recent MPTCP connection was established, the default initial path
can be adopted. Further combinations and implementations with more
or less sophisticated heuristics are possible.
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+---------------+
| Application |
| Request |
+---------------+
|
V
+---------------+
+--->| Initial-path |<---+
| | Selection | |
| +---------------+ |
| | |
| V |Info
| +---------------+ |
| | Set initial |----+
| | path |
| | for MPTCP |
| +---------------+
| |
| V
| +---------------+
|No |Establish MPTCP|
+----| Connection |
+---------------+
|Yes
V
Figure 5: RobE_IPS for each connection establishment
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+--------------+
| Application |
| Request |
+--------------+
|
V
+--------------+Yes
| First |------------+
| Connection? | |
+--------------+ |
|No |
V |
+--------------+ V
+----->| Initial-path |<-+ +-------+
| | Selection | | |Default|
| +--------------+ | |initial|
| | | | path |
| | | +-------+
| V |Info |
| +--------------+ | |
| | Set initial |--+ |
| | path | |
| | for MPTCP | |
| +--------------+ |
| | |
| V |
|No +--------------+ |
+------| Successful? |<-----------+
+--------------+
|Yes
V
Figure 6: RobE_IPS using default route when no meaningful
heuristic available
Figure 7 shows the process flow of "Initial Path Selection". Upon a
request from an application, the IPS logic will acquire transmission
status information which represents the transmission performance of
each available path or network interface and evaluate it. The
transmission status information is characterized by at least one of
the parameters: signal strength, throughput, round-trip time (RTT),
and link success rate. In this way, the path with the best
transmission performance can be determined and used for connection
establishment.
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|
V
+---------------------------+
|Acquire transmission status|
| info for available paths |
+---------------------------+
|
V
+---------------------------+
| Evaluating the status |
| for available paths |
+---------------------------+
|No
V
+---------------------------+
| Determining an available |
| path with better |
| transmission |
| performance |
+---------------------------+
|
V
+---------------------------+
| Using the network |
| interface |
|corresponding to the path |
| with better transmission |
|performance for connection |
| establishment |
+---------------------------+
|
V
Figure 7: Implementation process for Initial Path Selection
2.3.3. Path decision information
The level of heuristic can be mainly divided into three layers:
application level, transport-layer level and link-layer level based
on the information acquisition method. For example, RTT can be
calculated for each path within an MPTCP connection and belongs
thereof to the transport-layer level. The transmission status
information for each available path SHOULD be characterized by at
least one of the parameters: signal strength, throughput, RTT, and
link success rate. Application level information are more seen for
statistical purposes.
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* Application level: application name, domain name, port number, and
location.
* Transport-layer level: RTT, CWND, Error rate.
2.3.4. Initial Path Selection use local RTT information
Figure 8 presents an "Initial Path Selection" logic based on RTT,
e.g. assuming two paths over LTE and WiFi access. RTT calculation on
the transport layer usually reflects the time when an information is
sent and a related acknowledgment received. For an asymmetric usage
(e.g. download only) of a communication it might happen that recent
RTT calculation is only available on sender side which is possibly
not the side which employs the IPS logic. A solution for this can be
found in Section 3.2. Instead of using the most recent RTT value of
a path a filtered value consisting of several measured RTTs can be
used. A RTT can also be derived from link layer information but may
have a limited meaning only when it does not represent the end-to-end
latency.
+-------------------+
| New Session |
+-------------------+
|
V
+-------------------+ No
|Running Connections|-----------+
|(LTE.RTT<WiFi.RTT) | |
+-------------------+ |
|Yes |
V V
+-------------------+ +----------------+
| Set LTE as | | Set WiFi as |
| initial path | | initial path |
+-------------------+ +----------------+
Figure 8: Initial-path Selection based on RTT
2.4. Combination of RobE_SIM and RobE_IPS
In an implementation, a single solution may not be sufficient to
achieve an expected behavior. Combination of approaches to improve
robustness is recommended therefore. Figure 9 shows the combination
of RobE_SIM and RobE_IPS. RobE_SIM can be used at the very beginning
when the sender is without any path information followed by RobE_IPS
for consecutive connections.
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+--------------+
| Application |
| Request |
+--------------+
|
V
+--------------+
+----->| Any data for | No
| | Initial Path |----------+
| | Selection? | |
| +--------------+ |
| | |
| V V
| +--------------+ +--------+
| | Initial-path | |RobE_SIM|
| | Selection |<-+ +--------+
| +--------------+ | |
| | | |
| V |Info |
| +---------------+ | |
|No |Establish MPTCP|-+ |
+------| Connection |<--------+
+---------------+
|
V
No +---------------+
<------| Successful ? |
Network+---------------+
Problem |Yes
V
Figure 9: Combination of RobE_SIM and RobE_IPS
2.5. Combination of RobE_TIMER and RobE_IPS
Since RobE_IPS solely does not guarantee that a session can be set up
based on the selection of initial path, it can also be combined with
RobE_TIMER which generates less overhead compared to the combination
with RobE_SIM in Section 2.4 and guarantees session setup.
RobE_TIMER can be introduced to optimize the control of path
switching when the initial path selected by RobE_IPS is
dysfunctional. When the system enables RobE_IPS and uses the
selected initial path for session establishment, it sets the timer
for path switching. When timer is expired, the system will change to
another path to re-establish connection according to Section 2.1.
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+---------------+
| Application |
| Request |
+---------------+
|
V
+---------------+
| Initial Path |
|----->| Selection |
| | and Set Timer |
| +---------------+
| |
| V
|Yes +---------------+
+------| Timer is up? |
+---------------+
|No
V
+---------------+
|Establish MPTCP|
| Connection |
+---------------+
|
V
No +---------------+
<------| Successful? |
Network+---------------+
Problem |Yes
V
Figure 10: Combination of RobE_Timer and RobE_IPS
3. Implementation with Bi-directional MPTCP Support
Solutions which requires bi-directional support between two MPTCP
hosts promise to have better and possibly more features. However,
they cannot be defined without extending current standards in
[RFC6824] and [RFC8684]. The RobE_SIM and RobE_IPS approach are both
capable of profiting from an explicit support of the remote end host
and will be defined within this section.
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3.1. Simultaneous Initial Paths Extended Version (RobE_eSIM)
RobE_eSIM extends RobE_SIM by reusing the potential initial flows.
This eliminates the overhead from RobE_SIM by introducing a new
option MP_JOIN_CAP and accelerate the transmission speed by early
availability of multiple paths. Further it relaxes the dependency on
a reliable third ACK of the 3-way handshake in [RFC8684]. Remote
endpoint support can be negotiated in two ways, an implicit one
described in Section 3.1.1 or an explicit on which is described in
Section 3.1.2.
3.1.1. RobE_eSIM implicit Negotiation and Procedure
Similar to RobE_SIM in Section 2.2, the establishment process of
[RFC6824] or [RFC8684] is applied independently on multiple paths
simultaneously. In Figure 11 this is shown in SA1 and SA2. The
first path which returns a SYN/ACK (e.g. SA3) is selected as the
initial path and proceeds with the traditional establishment process
(SA5). Any other path which has to send the final ACK of the 3-way
handshake includes a new option MP_JOIN_CAP (see definition in
Section 3.1.3.2) instead of an MP_CAPABLE (SA6.2).
Host A Host B
------------------------ ----------
Address A1 Address A2 Address B1
---------- ---------- ----------
| | |
| SYN + MP_CAPABLE(Key-A[*]) |
(SA1) |--------------------------------------------->| (SB1)
| | SYN + MP_CAPABLE(Key-A'[*]) |
(SA2) | |------------------------------->| (SB2)
| | |
(SA3) |<---------------------------------------------| (SB3)
| SYN/ACK + MP_CAPABLE(Key-B) |
(SA4) | |<-------------------------------| (SB4)
| | SYN/ACK + MP_CAPABLE(Key-B') |
| | |
| ACK + MP_CAPABLE(Key-A, Key-B) |
(SA5) |--------------------------------------------->| (SB5)
| | |
(SA6.2) | | | (SB6.2)
RobE EXT | | ACK + MP_JOIN_CAP(Key-A, HMAC) |
(+fast) | |------------------------------->|
[*] Key-A in the first MP-capable is related to
RFC6824 only and does not exist in RFC8684.
Figure 11: MPTCP RobE_eSIM implicit Connection Setup
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Following the possible process in Figure 11, two further
constellations are imaginable and elaborated below.
1. In the flow diagram Figure 11, A1<->B1 is assumed to be the
initial flow. A2<->B1 shall be recycled and the ACK is sent with
MP_JOIN_CAP. Furthermore, the MP_CAPABLE arrives first at Host B
(SB5) and the MP_JOIN_CAP afterwards (SB6.2). When the
MP_JOIN_CAP is received, Host B has to iterate over the
connection list once (like MP_JOIN) and check for Key-A
availability. If a Key-A connection is found, this one is
validated against the HMAC value. The validation has two
reasons: first, several Key-A can exist, because different hosts
may choose the same Key-A by accident. Furthermore, no one can
join a connection by just recording/brute-forcing Key-A and
duplicating the request.
2. Like above, but MP_JOIN_CAP arrives before last MP_CAPABLE at
Host B
* [RFC8684]; Based on Key-A, Host B will iterate over the
connection list, but it will not find a match, because Key-A
of the previous selected initial flow (SA3, SA5) has not
arrived yet. So it will continue with a fast iteration only
over the connections which are still in establishment phase
using the 10 bit Key-B fast hash (crc16(Key-B) & 0x3FF). If
it matches against a (precomputed) existing Key-B_fast_hash in
the connection list, it will validate the request using the
HMAC(Key-A+B+B') to ensure legitimation. If successful, both,
the initial flow and the MP_JOIN_CAP flow, can be immediately
established. This is true, because without the knowledge of
Key-B, Host A could not calculate the HMAC. So it is clear,
that Host A had received the SYN/ACK (SB3). This also
mitigates the exchange of a reliable ACK during the handshake
process. MPTCP sends the Key-A only with the last ACK and
therefore prevents subsequent flow establishment until
successful reception at Host B. Using RobE_EXT, the reception
of an MP_JOIN_CAP ([RFC8684]) is sufficient to establish both,
the path carrying Key-B and Key-B'.
* [RFC6824]; Can match based on Key-A, same effort as for an
MP_JOIN.
3. A2<->B1 is selected as initial flow, because the respective SYN/
ACK returns earlier at Host A. It is the same as above, just the
other way round.
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3.1.2. RobE_eSIM explicit Negotiation and Procedure
The process of an explicit negotiation of RobE_eSIM follows Figure 11
but uses the ROBE_eSIM_EN option Figure 13 additionally during the
handshake procedure.
Host A Host B
------------------------ ----------
Address A1 Address A2 Address B1
---------- ---------- ----------
| | |
| SYN+MP_CAPABLE+ROBE_eSIM_EN(Key-A[*]) |
|----------------------------------------------------->|
| | SYN+MP_CAPABLE+ROBE_eSIM_EN(Key-A'[*]) |
| |--------------------------------------->|
| SYN/ACK+MP_CAPABLE+ROBE_eSIM_EN(Key-B) |
|<---------------------------------------------------->|
| | SYN/ACK+MP_CAPABLE+ROBE_eSIM_EN(Key-B')|
| |<---------------------------------------|
| ACK+MP_CAPABLE(Key-A,Key-B) |
|----------------------------------------------------->|
| | |
| | ACK+MP_JOIN_CAP(Key-A,HMAC) |
| |--------------------------------------->|
| | |
[*] Key-A in the first MP-capable is related to
RFC6824 only and does not exist in RFC8684.
Figure 12: MPTCP RobE_eSIM explicit Connection Setup
3.1.3. Protocol Adaptation
3.1.3.1. ROBE_eSIM_EN Option
1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+---------------+---------------+-------+-------+---------------+
| Kind | Length |Subtype| (reserved) |
+---------------+---------------+-------+-------+---------------+
Figure 13: ROBE_eSIM_EN_OPTION
3.1.3.2. MP_JOIN_CAP Option
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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
+---------------+---------------+-------+-------+---------------+
| Kind | Length |Subtype| | ADDR_ID |
+---------------+---------------+-------+-------+---------------+
| Sender's Key-A (64 bits) |
| |
| |
+---------------------------------------------------------------+
| HMAC (>=96 bits) |
| |
| |
: :
+---------------------------------------------------------------+
Key-B_fast_hash = crc16(Key-B) & 0x3FF -> (10bit)
HMAC_keys = HMAC(Key-A+Key-B+Key-B') -> (>=96bit)
HMAC = (HMAC_keys & ~0x3FF) | Key-B_fast_hash -> (size HMAC_keys)
Figure 14: MP_JOIN_CAP
Computational effort on receiver side is most often expected to be
the same as with MP_JOIN. Key-A ensures identification of related
flows Key-B_fast_hash enables MP session even when selected initial
flow is not fully established yet (slight computational overhead).
HMAC authenticates relationship of initial and potential initial
flows.
3.1.4. Fallback Mechanisms
3.1.4.1. Fallback mechanism for implicit RobE_eSIM
[TBD]
3.1.4.2. Fallback mechanism for explicit RobE_eSIM
This mechanism considers that both sides support MPTCP capability but
the receiver is not equipped with RobE_eSIM. MPTCP session with
RobE_eSIM negotiation will seamlessly fallback to normal MPTCP
process.
[Requires further check how an unaware Host B reacts on possible
ROBE_eSIM_EN; Ignore or RST? See also RFC6824 Sec. 3.6 "Should
fallback [...] the path does not support the MPTCP options"]
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Host A Host B
------------------------ ----------
Address A1 Address A2 Address B1
---------- ---------- ----------
| | |
| SYN+MP_CAPABLE+ROBE_eSIM_EN |
|------------------------------------------>|
| | SYN+MP_CAPABLE+ROBE_eSIM_EN |
| |---------------------------->|
| SYN/ACK+MP_CAPABLE |
|<----------------------------------------->|
| | SYN/ACK+MP_CAPABLE |
| |<----------------------------|
| ACK+MP_CAPABLE |
|------------------------------------------>|
| | RST |
| |---------------------------->|
| | SYN+MP_JOIN |
| |---------------------------->|
| | MP_JOIN Process... |
| | |
Figure 15: Fallback to MPTCP when missing RobE_eSIM support
3.1.4.3. Fallback to regular TCP when missing MPTCP support
When the receiver is not MPTCP enabled, MPTCP session with RobE_eSIM
negotiation will seamlessly fallback to regular process which is
illustrated in this section.
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Host A Host B
------------------------ ----------
Address A1 Address A2 Address B1
---------- ---------- ----------
| | |
| SYN+MP_CAPABLE+ROBE_eSIM_EN |
|------------------------------------------>|
| | SYN+MP_CAPABLE+ROBE_eSIM_EN |
| |---------------------------->|
| SYN/ACK |
|<----------------------------------------->|
| | SYN/ACK |
| |<----------------------------|
| ACK |
|------------------------------------------>|
| | RST |
| |---------------------------->|
| | Regular TCP Process... |
| | |
Figure 16: Fallback to TCP without MPTCP support
3.1.5. Comparison Robe_SIM and RobE_eSIM
Potential initial flows in RobE_SIM Section 2.2 and RobE_eSIM
Section 3.1 guarantee MPTCP session establishment if at least one
selected path for session establishment is functional. Figure 17
makes the differences between both approaches visible and points to
the latest decision possibility during session setup when RobE_SIM or
RobE_eSIM can be selected. Until SA5 in Figure 17 traditional MPTCP
connection setup is independently applied on multiple paths
simultaneously and offers to select the initial flow later (potential
initial flows). The final decision which path is selected as the
main one and the handling of the remaining flow(s) differs in SA6.1
when RobE_SIM is applied or instead SA6.2 RobE_eSIM.
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Host A Host B
------------------------ ----------
Address A1 Address A2 Address B1
---------- ---------- ----------
| | |
| SYN + MP_CAPABLE(Key-A[*]) |
(SA1) |--------------------------------------------->| (SB1)
| | SYN + MP_CAPABLE(Key-A'[*]) |
(SA2) | |------------------------------->| (SB2)
| | |
(SA3) |<---------------------------------------------| (SB3)
| SYN/ACK + MP_CAPABLE(Key-B) |
(SA4) | |<-------------------------------| (SB4)
| | SYN/ACK + MP_CAPABLE(Key-B') |
| | |
| ACK + MP_CAPABLE(Key-A, Key-B) |
(SA5) |--------------------------------------------->| (SB5)
| | RST |
(SA6.1) | |------------------------------->| (SB6.1)
RobE SIM | | |
(robust) | | |
-------------------------------------------------------------------
RobE EXT | | |
(+fast) | | ACK + MP_JOIN_CAP(Key-A, HMAC) |
(SA6.2) | |------------------------------->| (SB6.2)
[*] Key-A in the first MP-capable is related to
RFC6824 only and does not exist in RFC8684.
Figure 17: MPTCP RobE_SIM and RobE_eSIM connection setup
3.1.6. Security Consideration
[Tbd, however no differences to [RFC6824] and [RFC8684] are expected]
3.2. Heuristic Initial Path Selection with remote RTT Measurement
3.2.1. Description
Usually the path RTT can be determined by a time difference between
sending a package and receiving an ACK and is integrated into the TCP
protocol. For asymmetric transmission, the latest RTT for TCP flows
is calculated by the side which sends data at latest and possible
does not correspond to the site which employs RobE_IPS. This problem
is already elaborated in Section 2.3.4 and can be solved by
transmitting the RTT information per subflow. The negotiation
procedure is depicted in Figure 18 and uses the MPTCP option L_RTT_EN
defined in Section 3.2.2.
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Host A Host B
------------------------ ----------
Address A1 Address A2 Address B1
---------- ---------- ----------
| | |
| SYN+MP_CAPABLE+L_RTT_EN |
|------------------------------------------>|
| SYN/ACK+MP_CAPABLE+L_RTT_EN |
|<------------------------------------------|
| ACK+MP_CAPABLE |
|------------------------------------------>|
| ACK+DSS+L_RTT_EN(latest RTT)+Data |
|<------------------------------------------|
| | SYN+MP_JOIN |
| |---------------------------->|
| | MP_JOIN Process... |
| | |
Figure 18: Negotiation procedure for RTT exchange
A successful negotiation allows the exchange of the measured RTT
value from one subflow of an MPTCP host to another using the "Latest
RTT" field within the L_RTT_EN option.
3.2.2. Protocol Adaptation
Calculating the "Latest RTT" by a remote host in an asymmetry
transmission scenario should be transferred from remote host to the
client running RobE_IPS. So a new MPTCP subtype option named
L_RTT_EN is allocated for this function. During the three-way
handshake L_RTT_EN is used for negotiation of remote RTT measurement
capability between client and server (in Section 3.2.1). When both
parts support the usage of remote RTT measurement, the "Latest RTT"
field in L_RTT_EN is applied for carrying the value of latest RTT
computed by the remote host.
1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+---------------+---------------+-------+-------+---------------+
| Kind | Length |Subtype| (reserved) |
+---------------+---------------+-------+-------+---------------+
| Latest RTT (32 bits) |
| |
| |
+---------------------------------------------------------------+
Figure 19: ROBE_L_RTT_EN OPTION
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3.2.3. Fallback Mechanism
When the receiver is not L_RTT_EN capable, MPTCP session with
L_RTT_EN negotiation will seamlessly fallback to normal MPTCP
process.
[TBD, Need same checks as Section 3.1.4.2]
Host A Host B
------------------------ ----------
Address A1 Address A2 Address B1
---------- ---------- ----------
| | |
| SYN+MP_CAPABLE+L_RTT_EN |
|------------------------------------------>|
| SYN/ACK+MPTCP_CAPABLE |
|<------------------------------------------|
| ACK+MPTCP_CAPABLE |
|------------------------------------------>|
| | SYN+MP_JOIN |
| |---------------------------->|
| | MP_JOIN Process... |
| | |
Figure 20: Fallback to MPTCP without RobE_IPS
3.2.4. Security Consideration
[Tbd]
4. IANA Considerations
This document defines three new values to MPTCP Option Subtype as
following.
+=======+==============+=======================+=============+
| Value | Symbol | Name | Reference |
+=======+==============+=======================+=============+
| TBD | ROBE_eSIM_EN | RobE_eSIM enabled | Section 3.1 |
+-------+--------------+-----------------------+-------------+
| TBD | MP_JOIN_CAP | Join connection | Section 3.1 |
| | | directly in RobE_eSIM | |
+-------+--------------+-----------------------+-------------+
| TBD | L_RTT_EN | Server RTT enabled | Section 3.2 |
+-------+--------------+-----------------------+-------------+
Table 2: RobE Option Subtypes
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5. References
5.1. 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>.
[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>.
[RFC8684] Ford, A., Raiciu, C., Handley, M., Bonaventure, O., and C.
Paasch, "TCP Extensions for Multipath Operation with
Multiple Addresses", RFC 8684, DOI 10.17487/RFC8684, March
2020, <https://www.rfc-editor.org/info/rfc8684>.
5.2. Informative References
[RobE_paper]
Amend, M., Rakocevic, V., Matz, A.P., and E. Bogenfeld,
"RobE: Robust Connection Establishment for Multipath TCP",
ANRW '18 p. 76-82, 16 July 2018,
<http://doi.acm.org/10.1145/3232755.3232762>.
[RobE_slides]
Amend, M., Matz, A.P., and E. Bogenfeld, "A proposal for
MPTCP Robust session Establishment (MPTCP RobE)", IETF
99 Multipath TCP WG session, 18 July 2017,
<https://datatracker.ietf.org/meeting/99/materials/slides-
99-mptcp-a-proposal-for-mptcp-robust-session-
establishment-mptcp-robe-01>.
Authors' Addresses
Markus Amend
Deutsche Telekom
Deutsche-Telekom-Allee 9
64295 Darmstadt
Germany
Email: Markus.Amend@telekom.de
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Jiao Kang
Huawei
D2-03,Huawei Industrial Base
Shenzhen
Guangdong, 518129
China
Email: kangjiao@huawei.com
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