Mobile Throughput Guidance Signaling Protocol
draft-flinck-mobile-throughput-guidance-00
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| Authors | Ankur Jain , Andreas Terzis , Nurit Sprecher , Peter Szilagyi, Hannu Flinck | ||
| Last updated | 2014-10-26 | ||
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draft-flinck-mobile-throughput-guidance-00
7
Internet Engineering Task Force A. Jain
Internet-Draft A. Terzis
Intended status: Informational Google
Expires: April 29, 2015 N. Sprecher
P. Szilagyi
H. Flinck
Nokia Networks
October 26, 2014
Mobile Throughput Guidance Signaling Protocol
draft-flinck-mobile-throughput-guidance-00.txt
Abstract
The behaviour of the Transmission Control Protocol (TCP), which
assumes that network congestion is the primary cause for packet loss
and high delay, can lead to inefficient use of a cellular network's
resources and degrade application performance. The root cause for
this inefficiency is that TCP has difficulty adapting to the rapidly
varying network conditions. In cellular networks, the bandwidth
available for end devices can vary by an order of magnitude over a
few seconds due to changes in the underlying radio channel
conditions, as devices move, as well as changes in system load as
other devices enter and leave the network.
This document proposes a mechanism and protocol elements that can be
used to assist TCP in cellular networks, ensuring high utilization
and high service delivery performance.
The document describes the applicability of the proposed mechanism
for video delivery over cellular networks; it also presents test
results.
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."
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This Internet-Draft will expire on April 29, 2015.
Copyright Notice
Copyright (c) 2014 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Contributing Authors . . . . . . . . . . . . . . . . . . 3
1.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
1.3. Acronyms and Abbreviations . . . . . . . . . . . . . . . 3
1.4. Problem statement . . . . . . . . . . . . . . . . . . . . 3
1.5. Mechanism Principles . . . . . . . . . . . . . . . . . . 4
2. Architecture . . . . . . . . . . . . . . . . . . . . . . . . 5
3. Applicability to Video Delivery Optimization . . . . . . . . 6
3.1. Test Results . . . . . . . . . . . . . . . . . . . . . . 7
4. Protocol . . . . . . . . . . . . . . . . . . . . . . . . . . 7
4.1. Common Kind-Length-Value header . . . . . . . . . . . . . 8
4.2. Plane text mode Throughput Guidance Options . . . . . . . 10
4.3. Encrypted mode . . . . . . . . . . . . . . . . . . . . . 11
4.4. Nonce (Initialization Vector) . . . . . . . . . . . . . 14
4.5. Authentication . . . . . . . . . . . . . . . . . . . . . 15
5. Manageability considerations . . . . . . . . . . . . . . . . 16
6. Security considerations . . . . . . . . . . . . . . . . . . . 16
7. IANA considerations . . . . . . . . . . . . . . . . . . . . . 17
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 17
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 17
9.1. Normative References . . . . . . . . . . . . . . . . . . 17
9.2. Informative References . . . . . . . . . . . . . . . . . 18
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 18
1. Introduction
The following sub-sections present the problem statement and the
solution principles.
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1.1. Contributing Authors
The editors gratefully acknowledge the following additional
contributors: Swaminathan Arunachalam and Csaba Vulkan.
1.2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
1.3. Acronyms and Abbreviations
This document uses the following acronyms:
ECGI E-UTRAN Cell Global Identifier format
ECN Explicit Congestion Notification
HMAC Hash-based Message Authentication Code
IP Internet Protocol
IV Initialization Vector
LTE Long Term Evolution
MTG Mobile Throughput Guidance
RAN Radio Access Network
RTT Round Trip Time
SACK Selective Acknowledgement
TCP Transmission Control Protocol
UE User Equipment
1.4. Problem statement
The bandwidth available for end devices in a cellular network can
vary by an order of magnitude over a few seconds. This variation is
due to changes in the underlying radio channel conditions, as devices
move, as well as system load variations driven by the arrival and
departure of devices to/from the network. On the other hand, packet
losses tend to be sporadic and temporary because retransmissions
mechanisms at the physical and link layers repair most packet
corruptions.
Transport protocols that derive network capacity through implicit
signals, such as packet loss and delay variation, can have difficulty
adapting to such rapidly changing network conditions. In turn, this
difficulty leads to poorer quality of experience for applications
like video playback as well as inefficient use of the cellular
network's resources.
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1.5. Mechanism Principles
This document proposes that the cellular network provides information
on throughput guidance to the TCP server; this information will
indicate the throughput estimated to be available at the radio
downlink interface. The network SHOULD provide this information in
near real time in situations where the network conditions are
changing frequently or the user is moving.
While the implementation details will vary according to the access
technology, the resource allocation can be abstracted as the capacity
of the "radio link" between the network and the UE. For example, in
the case of an LTE network, the number of physical resource blocks
allocated to a UE, along with the modulation scheme and coding rate
used, can be translated into radio link capacity in Megabits per
second (Mbps).
The TCP server can use this explicit information to inform several
congestion control decisions. For example: (1) selecting the initial
window size, (2) deciding the value of the congestion window during
the congestion avoidance phase, and (3) adjusting the size of the
congestion window when the conditions on the "radio link"
deteriorate. In other words, with this additional information, TCP
does neither have to congest the network when probing for available
resource, nor rely on heuristics to reduce its sending rate after a
congestion episode.
The same explicit information can also be used to optimize
application behaviour given the available resources. For example,
when video is encoded in multiple bitrates, the application server
can select the highest encoding rate that the network can deliver.
This document proposes an in-band exposure mechanism where the
information elements are added to the TCP headers of the relevant
upstream packets. In particular, the throughput guidance information
is added into the Options field of the TCP header (see RFC 0793
[RFC0793]) of packets from the TCP client to the TCP server. An in-
band mechanism is proposed because it does not require a separate
interface, reference value, or correlation mechanism. Furthermore,
an in-band mechanism can keep up with the rapid changes in the
underlying radio link throughput.
Section 4 describes the definition details and semantics of the
Options field of the TCP header.
The proposed scheme is similar to existing mechanisms such as ECN,
where an ECN-aware router sets a mark in the IP header in order to
signal impending congestion (see [RFC3168]). Note, however, that the
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proposed scheme provides explicit information, (termed "Throughput
Guidance") about the estimated throughput available at the radio link
between the Radio Access Network (RAN) and the UE.
The following issues are not covered: (1)the throughput estimation
for the uplink between the UE and the RAN, and (2)the capacity of the
network path between the RAN and the server communicating with the
UE.
2. Architecture
A Mobile Throughput Guidance Signaling Protocol (MTGSP) is specified
to allow a functional entity that resides in the RAN to signal
throughput guidance information to the TCP server. The TCP server
resides behind the core network of the operator or in the Internet.
As Figure 1 depicts below, the functional element of the Throughput
Guidance Provider signals to the TCP server the information on the
(near-real time) throughput estimated to be available at the radio
downlink interface.
The TCP server MAY use the information to optimize the TCP behaviour.
The information MAY also be used to adapt the application behaviour
accordingly and to optimize service delivery performance.
TCP flow behaviour based on
+-+-+-+-+-+-+-+ Throughput Guidance Information +-+-+-+-+-+-+-+
| | <---------------------------------------- | |
| TCP client | +-+-+-+-+-+-+-+-+-+-+-+ | TCP Server |
| | | Througput Guidance | (x) | |
| | | Provider | ----------> | |
+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+
UE <--------------- RAN -------------------> x = Mobile Thourghput
Guidance Signaling
Figure 1
As described above, MTGSP SHALL use the Options field of the TCP
header of the same TCP flow to provide throughput guidance
information.
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The information source and the algorithm used by the Throughput
Guidance Provider to calculate the throughput guidance are beyond the
scope of this document.
The TCP server MAY use the throughput guidance information to assist
TCP in any of the following ways:
o Determine the size of the initial congestion window
o Determine when to exit the slow start phase
o Determine the size of the congestion window during the congestion
avoidance phase
o Determine the size of the window after a congestion event
3. Applicability to Video Delivery Optimization
The applicability of the protocol specified in this document to
mobile video delivery optimization has been evaluated and tested in
different network load scenarios.
In this use case, TCP traffic, for which throughput guidance
information is required, passes through a Radio Analytics application
which resides in a Mobile-edge Computing (MEC) server (see
[MEC_White_Paper]). This Radio Analytics application acts as the
Throughput Guidance Provider and sends throughput guidance
information for a TCP flow using the Options field in the TCP header
(according to the message specification provided below in section 4).
The TCP server MAY use this information to assist TCP congestion
control decisions as described above. The information MAY also be
used to select the application level coding so that it matches the
estimated capacity at the radio downlink.
All of these improvements aim to enhance the quality of experience of
the end user by reducing the time-to-start of the content as well as
video stall occurrences.
With Mobile-edge Computing, the Radio Analytics application can be
deployed on top of platforms implemented by different vendors and
across multi-operator networks. This means that efficient
utilization of the network resources can be expected as well as
enhanced quality of experience for the vast majority of the end
users.
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3.1. Test Results
Nokia Networks and Google tested the video delivery optimization use
case in a laboratory environment, simulating (as closely as possible)
a live production network. Different network load scenarios were
simulated.
All network level metrics showed an average improvement of 40-80%, as
detailed below:
o Reduction of end-to-end RTT by 40-60%
o TCP retransmissions reduced by 70-80%
o Buffer bloat reduced by 70-80%
The application-level metrics also improved, as detailed below:
o Click-to-play time reduced by 12-34%
o Stalling occurrences reduced by 46-100%
o Reduction in the number of format changes by 21-27%
4. Protocol
The Mobile Throughput Guidance Signaling message conveys information
on the throughput estimated to be available at the down link path of
a given TCP connection. The information is sent to the uplink end-
point of the connection (e.g., the TCP server). The TCP server MAY
use this information to optimize TCP behavior and to adjust
application-level behavior to the link conditions.
A good example is a content optimizer or a cache that can adapt the
application-level coding to match the indicated downlink radio
conditions. As radio link conditions may change rapidly, this
guidance information is best carried in-band using TCP options
headers rather than through an out-of-band protocol.
Using the TCP options to carry throughput guidance associates the
guidance information with an ongoing TCP connection and explicitly
avoids separate session identification information. The proposed
mechanism neither impacts the TCP state machine nor the congestion
control algorithms of the TCP protocol.
The Options field enables information elements to be inserted into
each packet with a 40-byte overall limit; this needs to be shared
with the standardized and widely-used option elements, such as the
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TimeStamp and SACK. The TCP Options field uses a Kind-Length-Value
structure that enables TCP implementations to interpret or ignore
information elements in the Options field based on the Kind.
In this draft, we define a Kind-Length-Value structure for encoding
information about the estimated capacity of a radio access link
between the RAN and the UE which is traversed by a TCP connection.
Note that the Mobile Throughput Guidance Signaling defines an
extendible framework that can convey confidential information to an
information receiver. The intention is to define a generic container
to convey in-band information within the limited TCP Option space
with optional authentication and/or encryption capabilities.
Throughput guidance is provided in this document. Additional
information can be specified in future documents.
The TCP options for Mobile Throughput Guidance Signaling are added by
the Throughput Guidance Provider functional element (which resides on
top of a radio network element), when there is enough space in the
TCP header.
Confidential information must be delivered in a secure way. The
information can be provided as plain text in a secure and closed
network. In other cases, the information should be authenticated and
encrypted at the TCP-header level (between the Throughput Guidance
Provider and the TCP server). An acceptable level of authentication
and encryption (according to best common practices) may require more
data than can be fitted into a single TCP header (maximum of 40 if no
other options are present). As described below, packet fragmentation
will be used is such a case.
Two transfer modes are defined to deal with data confidentiality in
this document; namely, plain-text mode and authenticated encryption
mode. A third mode, authentication-only mode, is equally feasible.
However, we do not currently provide the authentication-only mode but
will consider it for later updates. Both modes share a common Kind-
Length-Value "option header" structure with a flag field separating
the two cases.
4.1. Common Kind-Length-Value header
To make Mobile Throughput Guidance Signaling extendible to different
use cases a common Kind-Length-Value header is defined below.
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Kind | Length | Flags| variable length data |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2
Kind:
The Option Kind field indicates that the subsequent options are
part of Mobile Throughput Guidance Signaling. The size of this
field is 1 byte.
Length:
A 1 byte field, indicating the length of the kind and the length
fields, as well as the length of the following Options field(s):
Flags:
One byte of MTG protocol flag field as defined below.
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
| Seq |Frag |P|T|
+-+-+-+-+-+-+-+-+
Flag field of common Kind-Lenght-Value header
Figure 3
Seq:
Three-bit sequence number that maintains context across different
packet types as defined by P- and T-bits below. The scope of the
sequence number is to protect against packet reordering, not to
provide a globally unique identifier or sequence number. The use
of these bits are reserved for possible transfer mode extensions.
Frag:
Three bits that provide information about how to reassemble
information if fragmented into multiple packets. If no
fragmentation across multiple TCP packet headers is needed, these
bits are set to zero. Otherwise, Frag is a counter starting from
1 and incremented by 1 for each subsequent packet of the same type
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(see P- and T-bits below). For the last fragment, the Fragment is
always 7 (binary 111) to indicate that the information is
complete.
P and T bits:
These two bits encode the packet type: Plaintext (P=0, T= 0),
Cipher text (P=0, T=1), Nonce (IV) (P=1, T=0) or Authentication
(P=1, T=1). For Plaintext, the Fragment bits are always zero.
Variable length data:
The variable length content in type, value format. The content
depends of the transfer mode as defined in the following sections
of this document.
As an example for the use of Flag-field, consider a cipher text of a
single block. For it the T-bit is set one, P-bit is set to zero,
Fragment and Seq-fields are zero in the Flag-field. In case the
cipher text option that cannot fit into a single TCP packet option,
the cipher text is fragmented across multiple TCP headers. The first
fragment has value Frag= 001, and the value is incremented for each
subsequent fragment. The last fragment is marked with all Frag-bits
set to 1 (Frag= 111 for the last fragment). Details follow in the
next sections.
4.2. Plane text mode Throughput Guidance Options
The plain text mode can be used in secure and closed networks or with
information that has no confidentiality requirement. The plane text
mode is made of one or more type, value -pairs. The type defines
fixed the length of the following value.
Table of Type Value pairs of Throughput Guidance option data
+---------------------+------+----------+---------------------------+
| Name | Type | Length | Value Unit |
+---------------------+------+----------+---------------------------+
| Throughput Guidance | 1 | 2 bytes | kbits/s |
| Access point ID | 4 | 7 bytes | global identifier (ECGI) |
+---------------------+------+----------+---------------------------+
Table 1: MTG type-vale pairs
The Type 1 element carries the actual throughput estimate in the
16-bit value unit. The throughput value is encoded using a fixed-
point number representation. The 12 most significant bits are used
for the integer value while the bottom 4 bits correspond to the
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decimal portion of the throughput value. Throughput is expressed in
Megabits per second.
The type-value pair elements are laid out consecutively in the
header. At the end padding (i.e., the NO-OP TCP Option header with
kind equal to 1, or the End of Option List TCP Option header with
kind equal to 0) may be required to align the header size to the
multiple of 4 bytes (required by the TCP standard). All bits in the
Flag field are set to zero.
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Kind | Length | Flags| Type1|Value-1| Type2|Value-2| ...Padding|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Kind: Value for Mobile Throughput Guidance Signaling
Type: as defined in the table of Type Value pairs of MTG option data
Flags: P- and T-bits set to zero
Layout of plain text option data in the TCP header options space.
Figure 4
4.3. Encrypted mode
Encryption requires authentication for integrity protection, as it is
insecure to use encryption without it. Thus, the encrypted contains
authentication as well. Encryption and authentication must use
different keys. The following diagram shows the encryption process.
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+-+-+-+-+ +-+-+-+-+-+
+-+-+-+-+-+ | key1 | |IV(Nonce)|
|key index| --> +-+-+-+-+ +-+-+-+-+-+
+-+-+-+-+-+ | key 2 | |
+-+-+-+-+ key +-+-+-V-+-+-+-+
... ----> | AES 128-CNT|
+-+-+-+-+ +-+-+-+-+-+-+-+
| key n | |
+-+-+-+-+ +<------ Plain text
|
+-+-+-V-+-+-+-+
| Cipher text |
+-+-+-+-+-+-+-+
Encryption method
Figure 5
The encryption uses Advanced Encryption Standard (AES), 128 bits (16
bytes) block size, 128 bits (16 bytes) key size, Counter (CTR) block
cipher mode. Integrity protection with CTR mode is MUST; this is be
provided via HMAC based message authentication (see Authentication
section below).
The plaintext contains type-value pair elements of the variable
length data. The plaintext is divided into blocks of 16 bytes. A
block of plain text MUST not exceed 16 bytes in a single run.
Encryption takes a key (16 bytes), an IV or Nonce (16 bytes), the
plain-text (at most 16 bytes) and produces a cipher text of 16 bytes.
Note: multiple keys, at most 256, may be available (can be negotiated
via out-of-band key management; this out of scope of this document)
for encryption key index. The keys MUST be different from those used
for authentication.
The Nonce is 16 bytes. A unique Nonce is generated for each
encrypted block. The same Initialization Vector, IV or Nonce MUST
NOT be used with the same encryption key more than once. This is be
enforced by the Throughput Guidance Provider; otherwise security
scheme will be broken.
The resulting cipher text is in blocks of 16 bytes. The cipher text
blocks are packed into the option space together with the used Key
Index in a following way if they fit into singe option space of a
single TCP header.
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Kind | Length | Flags | Key Index |first block of 16 bytes | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Kind: Value for Mobile Throughput Guidance Signaling
Flags: Type of cipher text T-bit set to 1, only one block Frag= 000.
Key Index is the index used in encryption
Cipher text layout in the TCP options without fragmentation
Figure 6
The flag field of the common option header indicates that the content
is cipher text by having the T bit set to one. Since the ciphered
block is not fragmented the Frag-bits of the flag field are set to
zero (Frag= 000). (Use of Seq bits is left for later submissions).
If there are multiple cipher text blocks of 16 bytes, the flag field
shows the type of the option being cipher text with the T-bit set to
one, and by Frag-field showing the fragment number starting from 001
and incremented by one for each subsequent fragment of a packet of
the same type. For the last fragment, the Frag-field is always
binary 111 to indicate the last fragment.
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First fragment:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Kind |Length|Flags|KeyIndex| 1st blocks of 16 bytes | fragmented block |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Kind: Value for Mobile Throughput Guidance Signaling
Flags: Type of cipher text T-bit = 1, Frag field = 001 first fragment
Key Index is the index used in encryption
Second fragment:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Kind | Length | Flags | Key Index | Rest of the fragmented block |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Kind: Value for Mobile Throughput Guidance Signaling
Flags: Type of cipher text T-bit = 1, Frag field = 111 last fragment
Key Index is the index used in encryption
Cipher text layout extending to two consecutive headers
Figure 7
4.4. Nonce (Initialization Vector)
The 16 byte Nonce (or IV) is transmitted along with the cipher text
to protect against de-synchronization between the encryption-
decryption points.
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Kind | Length | Flags | Key Index | Nonce (IV) 16 bytes | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Kind: Value for Mobile Throughput Guidance Signaling
Flags: Type of IV/Nonce P-bit set to 1, only one block Frag= 000
Key Index is the index used in encryption
Nonce (IV) in a single header
Figure 8
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If the Nonce (IV) doesn't fit into the remaining free bytes of the
option field it needs to be fragmented using the Frag-field in the
same way as cipher text layout is extending across two or more
consecutive TCP headers but with the option type field set to
indicate Nonce/IV by P-bit set to 1.
4.5. Authentication
The authentication covers the cipher text, the Nonce (IV) and
includes additional TCP protocol header fields to protect against
replay attacks. The authentication uses HMAC codes (e.g. HMAC-
SHA2-224), 128 bits (16 bytes) key size, 224 bits (28 bytes) digest
size. Multiple keys (at most 256) for authentication with the same
information receiver can be used. The keys MUST be different from
those used for encryption. Truncation is possible but at least 160
bits (20 bytes) must be used from the digest to meet the typical
security level of mobile networks.
Authentication takes a key, the input (arbitrary length) and produces
a 28 byte long digest, which is truncated to 20 bytes (keeping the
most significant bytes). The HMAC algorithm and truncation can be
negotiated via key management (out of scope of this document).
The authentication covers the TCP sequence number, ACK number, and
TimeStamp (TSval, TSecr not the possible 2 bytes of padding) fields
of the TCP header as well as the Common Kind-Length-Value header with
its data in all cipher text option and IV/Nonce option packets. (The
Authentication type options itself cannot be covered by the
authentication.)
The order in which the fields are included into the message
authentication code is the following. From the TCP header: TCP Seq,
ACK, TSval, TSecr. Followed by the following fields from the
ciphered text: Kind, Length, Flags, Key Index, cipher text, and from
the IV/Nonce type of option packets TCP Seq, ACK, TSval, TSecr (note
cipher text and IV/Nonce type of options may be in different TCP
packets) followed by Kind, Length, Flags, Key Index, Nonce/IV.
In case the option packets used as input to the HMAC are fragmented
into multiple TCP headers, they are processed so that headers with
cipher text option are processed first, followed by IV/Nonce option
packets.
The options containing the result of the HMAC are marked by setting
both P- and T-bits of the flag-field to one. Key Index is set to
point to the used authentication key, followed by the resulting
authentication code. If the option doesn't fit into the free option
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space in the TCP header, it is fragmented across multiple TCP headers
in the same way as the cipher text options use the Frag-field.
5. Manageability considerations
There SHOULD be a mechanism to configure the application in the RAN
with a list of destinations to which throughput guidance should be
provided.
In addition, it SHOULD be possible to configure the frequency (in
milliseconds) at which throughput guidance needs to be signaled as
well as the required security level and parameters for the encryption
and the authentication if supported.
6. Security considerations
The introduction of explicit information from the cellular network
that can affect the behavior of a transport connection endpoint
introduces a set of security considerations.
First, the identity of the network entity that injects the throughput
guidance header must be explicitly known to the endpoint receiving
the information. Omitting such information would enable malicious
third parties to inject erroneous information.
Fortunately, the issue of malicious disinformation can be easily
addressed using well known techniques. First, the network entity
responsible for injecting the throughput guidance header can encrypt
the header and include a cryptographically secure message
authentication code. In this way the transport endpoint that
receives the throughput guidance header can check that the
information was sent by a legitimate entity and that the information
has not been tampered.
Section 4 described how the TCP Header information can be signed and
encrypted for security purposes. An out-of-band mechanism is
currently used to agree upon the set of keys used to encrypt and
authenticate the messages exchanged between the endpoint and the
network element that generate the throughput guidance headers.
Furthermore, the throughput guidance information should be treated
only as a hint to the congestion control algorithm running at the
transport endpoint. The endpoint that receives this information
should not assume that it is always correct and accurate.
Specifically, endpoints should check the validity of the information
received and if they find it erroneous they should discard it and
possibly take other corrective actions (e.g., discard all future
throughput guidance information from a particular IP prefix).
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One way to check if the throughput guidance information overestimates
the capacity available on the radio link is to check whether any
packet losses or other signs of congestion (e.g., increasing RTT)
occur after the guidance is used. Notably, the same mechanism can be
used to deal with bottlenecks in other parts of the the end-to-end
network path. To check if the throughput guidance underestimates the
available network capacity, the source can periodically attempt to
send faster and then check for signs of congestion.
7. IANA considerations
In the current version of the document and for field tests, the
experimental value 253 is used for the "Throughput Guidance" TCP
option kind.
Note that in this case, following RFC 6994 [RFC6994], a two-byte
experiment ID field SHOULD follow immediately after the Length field
to allow for shared use of the experimental values. The figure below
shows how the throughput guidance option header will look in this
case. ExpID SHOULD be set to 0x6006 (16 bits).
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Kind | Length | ExpID | Flags | Variable Data |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 9
In future versions of the document, a code point should be assigned
for the MTGSP Kind field.
8. Acknowledgements
9. References
9.1. Normative References
[RFC0793] Postel, J., "Transmission Control Protocol", STD 7, RFC
793, September 1981.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC6994] Touch, J., "Shared Use of Experimental TCP Options", RFC
6994, August 2013.
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9.2. Informative References
[I-D.narten-iana-considerations-rfc2434bis]
Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", draft-narten-iana-
considerations-rfc2434bis-09 (work in progress), March
2008.
[MEC_White_Paper]
"Mobile-Edge Computing - Introductory Technical White
Paper", September 2014,
<http://portal.etsi.org/Portals/0/TBpages/MEC/Docs/Mobile-
edge_Computing_-
_Introductory_Technical_White_Paper_V1%2018-09-14.pdf>.
[RFC2629] Rose, M., "Writing I-Ds and RFCs using XML", RFC 2629,
June 1999.
[RFC3552] Rescorla, E. and B. Korver, "Guidelines for Writing RFC
Text on Security Considerations", BCP 72, RFC 3552, July
2003.
[RFC4413] West, M. and S. McCann, "TCP/IP Field Behavior", RFC 4413,
March 2006.
Authors' Addresses
Ankur Jain
Google
1600 Amphitheatre Parkway
Mountain View, CA 94043
US
Phone: +1-925-526-5879
Email: jankur@google.com
Andreas Terzis
Google
1600 Amphitheatre Parkway
Mountain View, CA 94043
US
Phone: +1-650-214-5270
Email: aterzis@google.com
Jain, et al. Expires April 29, 2015 [Page 18]
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Nurit Sprecher
Nokia Networks
Hod HaSharon
IL
Phone: +97297751229
Email: nurit.sprecher@nsn.com
Peter Szilagyi
Nokia Networks
Budapest
Hungary
Phone: +36209777797
Email: peter.1.szilagyi@nsn.com
Hannu Flinck
Nokia Networks
Helsinki
FI
Phone: +358504839522
Email: hannu.flinck@nsn.com
Swaminathan Arunachalam
Nokia Networks
Irving
US
Phone: +19723303204
Email: swaminathan.arunachalam@nsn.com
Csaba Vulkan
Nokia Networks
Budapest
Hungary
Phone: +36209777797
Email: csaba.vulkan@nsn.com
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