Path Layer UDP Substrate Specification
draft-trammell-plus-spec-00
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draft-trammell-plus-spec-00
Network Working Group B. Trammell
Internet-Draft M. Kuehlewind
Intended status: Experimental ETH Zurich
Expires: June 26, 2017 December 23, 2016
Path Layer UDP Substrate Specification
draft-trammell-plus-spec-00
Abstract
This document specifies a common Path Layer UDP Substrate (PLUS) wire
image for encrypted transport protocols carried over UDP. The base
PLUS header carries information for driving a minimal state machine
at middleboxes described in [I-D.trammell-plus-statefulness], and
provides optional exposure of additional information to devices along
the path using the mechanism described in
[I-D.trammell-plus-abstract-mech].
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
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and may be updated, replaced, or obsoleted by other documents at any
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material or to cite them other than as "work in progress."
This Internet-Draft will expire on June 26, 2017.
Copyright Notice
Copyright (c) 2016 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
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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
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. State Maintenance and Measurement: Basic Header . . . . . . . 3
2.1. Sender Behavior . . . . . . . . . . . . . . . . . . . . . 5
2.2. Receiver Behavior . . . . . . . . . . . . . . . . . . . . 5
2.3. On-Path State Maintenance using the Basic Header . . . . 6
2.3.1. State Establishment . . . . . . . . . . . . . . . . . 7
2.3.2. Bidirectional Stop Signaling . . . . . . . . . . . . 8
2.3.3. State Rebinding . . . . . . . . . . . . . . . . . . . 8
2.4. Measurement and Diagnosis using the Basic Header . . . . 9
3. Path Communication: Extended Header . . . . . . . . . . . . . 9
3.1. Measurement and Diagnostics using the Extended Header . . 11
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12
5. Security Considerations . . . . . . . . . . . . . . . . . . . 12
6. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 12
7. Informative References . . . . . . . . . . . . . . . . . . . 12
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 13
1. Introduction
This document defines a wire image for a Path Layer UDP Substrate
(PLUS), for limited exposure of information from encrypted, UDP-
encapsulated transport protocols. The wire image implements
signaling to drive the minimal state machine defined in
[I-D.trammell-plus-statefulness] as well as optional exposure of
additional information to devices along the path using the mechanism
described in [I-D.trammell-plus-abstract-mech].
As discussed in [I-D.hardie-path-signals], basic information about
flows currently exposed by TCP are missing from transport protocols
that encrypt their headers. Given the ossification of protocol wire
images due to the widespread deployment of stateful network devices
that rely on header inspection, this header encryption is necessary
to support transport protocol evolution. However, the loss of basic
information for on-path state maintenance as well as network
performance measurement, diagnostics, and troubleshooting via header
encryption makes network management more difficult. The PLUS wire
image defined by this document aims to mitigate this difficulty,
allowing deployment of encrypted protocols without loss of essential
in- network functionality.
This wire image is intended primarily to support state maintenance
and measurement; the principles of measurement and primitives we aim
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to support are drawn from recent work on explicit measurability in
protocol design [IPIM].
2. State Maintenance and Measurement: Basic Header
Every packet in each direction of a flow using PLUS MUST carry either
a PLUS basic or PLUS extended header. The PLUS basic header supports
multiplexing using a connection token; basic state maintenance using
association and confirmation signals, packet serial numbers, and a
two-way stop signal; and basic measurability using packet serial
number echo. The format of the basic header, together with the UDP
header, is shown in Figure 1.
The extended header is defined in Section 3.
3 2 1
1 0 9 8 7 6 5 4 3 2 1 0 9 8 7 6 5 4 3 2 1 0 9 8 7 6 5 4 3 2 1 0
+------------------------------+-------------------------------+
| UDP source port | UDP destination port |
+------------------------------+-------------------------------+
| UDP length | UDP checksum |
+------------------------------+-------------------------------+
| magic |
+--------------------------------------------------------------+
| |
+- connection/association token CAT -+
| |
+--------------------------------------------------------------+
| packet serial number PSN |
+--------------------------------------------------------------+
| packet serial echo PSE |
+-+-+-+-+-------+-----------------------------------------------+
|S|0|L|R| ign | \
+-+-+-+-+-------+ /
/ \
\ transport protocol header/payload (encrypted) /
/ \
Figure 1: PLUS header with basic exposure
Fields are encoded in network byte order and are defined as follows:
o magic: A 32-bit number identifying this packet as carrying a PLUS
header. This magic number is chosen to avoid collision with
possible values of the first four bytes of widely deployed
protocols on UDP. Should the QUIC [I-D.ietf-quic-transport]
header be defined to place the version number in the first four
bytes of the packet, this number should be compatible with the
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QUIC version numbering scheme. The value 0xd8007ffe has been
provisionally selected for the PLUS magic number based of
experience with the SPUD prototype, and a cursory survey of common
UDP protocols.
o Connection/Association Token (CAT): A 64-bit token identifying
this association. The CAT should be chosen randomly by the
connection initiator. The CAT performs two functions in the PLUS
header:
* Multiplexing: PLUS packets on the same 5-tuple with a different
CAT value are taken to belong to a separate flow, with
completely separate state.
* Rebinding: A PLUS packet sharing one endpoint (source address/
port pair, or destination address/port pair) and the CAT with
an existing flow is taken to belong to that flow, since the
other endpoint identifier has changed due to a mobility event
or address translation change.
o Packet Serial Number (PSN): A 32-bit serial number for this
packet. The first PSN for each direction in a flow is chosen
randomly, and subsequent packets increment the PSN by one. The
PSN wraps around.
o Packet Serial Echo (PSE): The most recent PSN seen by the sender
in the opposite direction before this packet was sent.
o Flags byte: eight bits carrying additional flags:
* Stop flag (S): Packet carries a stop or stop confirmation when
set.
* Extended Header bit: Flag bit 0x40 is set to zero in packets
with a Basic Header.
* LoLa flag (L): Packet is latency sensitive and prefers drop to
delay when set.
* RoI flag (R): Packet is not sensitive to reordering when set.
* Ignored: Bits 0-3 are ignored, and available for use by the
overlying transport.
Since PLUS is designed to be used for UDP-encapsulated, encrypted
transport protocols, overlying transports are presumed to provide
encryption and integrity protection for their own headers. For the
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sake of efficiency, it is also assumed that this integrity protection
can be extended to the bits in the PLUS Basic Header.
2.1. Sender Behavior
When a sender has a packet ready to send using PLUS, it determines
the values in the Basic Header as follows:
o The magic number is set to the constant 0xd8007ffe.
o If the sender is the flow initiator, and the packet is the first
packet in the flow, the sender selects a cryptographically random
64-bit number for the CAT. When multiplexing, it must ensure the
PSN is not already in use for the 5-tuple. Otherwise, the sender
uses the CAT associated with the flow.
o If the packet is the first packet in the flow in this direction,
the sender selects a cryptographically random 32-bit number for
the PSN. Otherwise, the sender adds one to the PSN on the last
packet it sent in this flow, and uses that value for the PSN. If
the last PSN is 0xffffffff, it wraps around, setting the PSN to
0x00000000.
o If the packet is the first packet in the flow in this direction,
the sender sets the PSE to 0x00000000. Otherwise it sets the PSE
to the PSN of the last packet seen in the opposite direction.
o If the overlying transport determines that this packet is the last
to be sent in this direction, the sender sets the S flag; see
Section 2.3.2 for details.
o If the overlying transport determines that this packet is loss-
insensitive but latency-sensitive, the sender sets the L flag.
o If the overlying transport determines that this packet may be
freely reordered, the sender sets the R flag.
o The overlying transport may freely use the four ignored flag bits
for its own purposes.
2.2. Receiver Behavior
When a receiver receives a packet containing a PLUS Basic Header, it
processes the values in the Basic Header as follows:
o It verifies that the magic number is the constant 0xd800fffe.
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o It verifies the integrity of the information in the PLUS Basic
Header, using information carried in the overlying transport.
Packets failing integrity checks SHOULD be dropped, but MAY be
further analyzed by the receiver to determine the likely cause of
verification failure; reaction to the failure is transport and
implementation specific.
o It stores the PSN to be sent as the PSE on the the next packet it
sends in the opposite direction.
2.3. On-Path State Maintenance using the Basic Header
The basic header provides all the signals necessary to drive the
transport- independent state machine described in
[I-D.trammell-plus-statefulness], as shown in Figure 2.
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`- - - - - - - - - - - - - - - - - - - - - - - - - - - -'
` +============+ a->b +============+ '
` / \--------->/ \<-+ '
+--->( zero ) ( uniflow ) | a->b '
^ ` \ /<---------\ /--+ '
| ` +============+ TO_IDLE +============+ '
| `- - - - - - - - - - - - - - - | b->a - - - - - - - - -'
| V
| +============+
| TO_IDLE / \
+<-----------------------( associating )
| \ /
| +============+
| | a->b
| V
| +============+
| TO_ASSOCIATED / \<-+
+<-----------------------( associated ) | a<->b
| \ /--+
| +============+
| | stop y->z
| V
| +============+
| TO_ASSOCIATED / \<-+
+<-----------------( half-close ) | a<->b
| \ /--+
| +============+
| | stop z->y
| V
| +============+
| TO_CLOSING / \
+------------( closing )
\ /
+============+
Figure 2: Transport-independent state machine as implemented by PLUS
2.3.1. State Establishment
A PLUS-aware on-path device forwarding a packet with a PLUS Basic
Header with a 5-tuple and CAT it does not have state for moves that
flow to the uniflow state. It will move the flow back to zero state
after not seeing a packet on the same flow in the same direction with
the same CAT within a timeout interval TO_IDLE, and continues
forwarding packets in that direction (the a->b direction in
Figure 2).
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A PLUS-aware on-path device forwarding a packet with a PLUS Basic
Header with a matching 5-tuple and CAT as a flow in the uniflow
state, but in the opposite direction (the b->a direction in
Figure 2), moves that flow to the associating state. It stores the
PSN of the packet that caused this transition, and waits for a packet
in the a->b direction containing a PSE indicating that that packet
has been received. When it sees that packet, it transitions the flow
to associated state. Otherwise, it drops state after a timeout
interval TO_IDLE.
Once a flow has moved to the associated state, it will remain in that
state for a timeout interval TO_ASSOCIATED. The on-path device
forwards any packet with a PLUS Basic Header in either direction for
this flow. It resets the TO_ASSOCIATED timer for every packet it
forwards in this state.
2.3.2. Bidirectional Stop Signaling
A PLUS-aware on-path device forwarding a packet for a flow in the
associated state with an S flag set moves that flow to half-close
state. It stores the PSN on the packet causing the transition, and
continues forwarding packets as if in associated state, dropping
state on timeout interval TO_ASSOCIATED.
When it sees a packet in the opposite direction with the S flag set
and the PSE set to exactly the stored PSN, it transitions the flow to
closing state. The device will forward packets in both directions
for flows in the closing state within a timeout interval TO_CLOSING;
these packets will not reset the timer. See Section 2.3.2 for
details.
Note that even though the S flag is integrity-protected end to end, a
packet with the S flag set could be forged by one on-path device to
drive the flow into half-close state on all downstream devices.
However, this attack is of severely limited utility. First, it would
require coordination between attackers on both sides of a given on-
path device in order to forge a confirmation of the stop signal - a
flag with the S bit set and a valid PSE corresponding to the PSN of
the first stop signal to drive the flow into closing state. Second,
the information in the Basic Header on each packet will drive the
state machine into associated state even in the middle of a flow,
enabling fast recovery even in the case of such a coordinated attack.
2.3.3. State Rebinding
A PLUS-aware on-path device forwarding a packet for a flow in the
zero state, where one of the endpoint identifiers (address and port)
and the CAT, but not the other endpoint identifier, match a flow in a
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non-zero state, accounts that packet to the existing flow, updating
the changed endpoint identifier. This allows fast rebinding of state
in case of changes in network address translation or connectivity of
the sender.
2.4. Measurement and Diagnosis using the Basic Header
The basic header trivially supports passive two-way delay measurement
as well as partial loss estimation at a single observation point.
To calculate two-way delay, an observation point calculates the delay
between seeing a PSN and a corresponding PSE in each direction, then
adds the delays from each direction together. The fact that the PSN
increments by one for every packet, including packets carrying
retransmitted data or only control traffic, makes this measurement
much simpler than the equivalent measurement using TCP sequence and
acknowledgment numbers.
To calculate loss upstream from an observation point in each
direction, the observation point simply counts skips in the PSN
number space. Since PLUS does not expose information about
retransmissions (and, indeed, may not even carry a transport that
uses retransmission for loss recovery), loss downstream from the
observation point cannot be observed.
3. Path Communication: Extended Header
Additional facilities for communicating with on-path devices under
endpoint control are provided by the PLUS Extended Header. The
extended header shares the layout of its first 21 bytes with the PLUS
Basic Header, except the Extended Header bit (0x40 on byte 20) is
set. As with the Basic Header, overlying transports are presumed to
provide encryption and integrity protection for the PLUS Extended
Header.
The Extended Header shown in Figure 3 provides for a single Sender to
Path or Path to Receiver information element, as in
[I-D.trammell-plus-abstract-mech], to appear on the packet, within a
Path Communication Field. PCF Type information is carried in Byte 21
of the header, with the length of the PCF value to be determined by
its type.
Further details of PCF encoding are not yet defined in this revision
of the specification; the remainder of this section discusses the
types of information elements to be supported. The exact encoding of
PCF type and value information are to be derived from an analysis of
the requirements of these Information Elements.
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3 2 1
1 0 9 8 7 6 5 4 3 2 1 0 9 8 7 6 5 4 3 2 1 0 9 8 7 6 5 4 3 2 1 0
+------------------------------+-------------------------------+
| UDP source port | UDP destination port |
+------------------------------+-------------------------------+
| UDP length | UDP checksum |
+------------------------------+-------------------------------+
| magic |
+--------------------------------------------------------------+
| |
+- connection/association token CAT -+
| |
+--------------------------------------------------------------+
| packet serial number PSN |
+--------------------------------------------------------------+
| packet serial echo PSE |
+-+-+-+-+-------+---------------+------------------------------+
|S|1|L|R| ign | PCF Type | /
+-+-+-+-+-------+---------------+ \
\ /
/ PCF value (variable-length) \
\ /
+--------------------------------------------------------------+
/ \
\ transport protocol header/payload (encrypted) /
/ \
Figure 3: PLUS extended header (conceptual; details TBD)
As described in [I-D.trammell-plus-abstract-mech], there are two
types of signals: Path to Receiver signals, which allow devices along
the path to provide information about the path or its treatment of
the flow to the receiver; and Sender to Path signals, which allow the
sender to expose information about itself or the flow to the path.
Path to Receiver signals are treated specially by header integrity
protection, as their values, but not length or type, may be changed
by devices on path: the value of a given path- to-receiver signal is
assumed to be an appropriately sized array of zero bytes by the
integrity protection facility.
Path to Receiver signals generally take the form of accumulators:
initialized to some value by the sender, and subject to some
aggregation function by each on-path device that understands them.
Sender to Path signals are generally used to expose information about
the traffic for measurement or diagnostic purposes. In any case, the
information sent and received is to be treated as advisory only.
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3.1. Measurement and Diagnostics using the Extended Header
We have identified the following sender to path signals as
potentially useful for measurement and diagnostic purposes. These
signals are advisory only, and should not be presumed by either the
endpoints or devices along the path to affect forwarding behavior.
o Timestamp and timestamp echo. Similar to TCP timestamps in
[RFC7323], also encoding a delta between receipt of last timestamp
and transmission of echo as in section 4.1.2 of [IPIM]. Allows
constant-rate clock exposure to devices on path. Note that this
is less necessary for RTT measurement of one-sided flows than it
is in TCP, due to the properties of the PSN and PSE values in the
Basic Header.
We have identified the following path to receiver signals as
potentially useful. Note that accumulated values for use at the
sender must be fed back to the sender by the overlying transport, and
that the presence of non-PLUS aware devices on path at breaks in MTU
mean that the accumulated value can only be used as a hint to
processes for measurement and discovery of the accumulated values at
the sender.
o MTU accumulator. This signal allows measurement of MTU
information from PLUS-aware devices. The sender sets the initial
value to the sender's MTU. A PLUS-aware forwarding device on path
receiving this value fills in the minimum of the received value
and the MTU of the next hop into this field.
o State timeout accumulator. This signal allows measurement of
timeouts from PLUS-aware devices. It is initialized to a maximum
("no information") value by the sender. A PLUS-aware forwarding
device on path receiving this value fills in the minimum of the
received value and the configured timeout for the flow's present
state into this field.
o Rate limit accumulator. This signal allows exposure of rate
limiting along the path. It is initialized to a maximum ("no
information") value by the sender. A PLUS-aware forwarding device
on path receiving this value fills in the minimum of the received
value and the rate limit to which this flow is subject into this
field.
o Trace accumulator. This signal allows exposure of a trace of
PLUS-aware devices on path, similar to the Path Changes mechanism
in section 4.3 of [IPIM]. The sender initializes the value to a
value chosen randomly for the flow; all packets in the flow using
path trace accumulator must use the same initial value. A PLUS-
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aware forwarding device on path receiving this value fills in the
result of XORing the received value with a randomly chosen device
identifier, which it must use for all path trace accumulator
signals it participates in. Packets traversing the same set of
PLUS-aware forwarding devices in the same flow therefore arrive at
the receiver with the same accumulated value, and changes to the
set of devices on path can be detected by the receiver.
4. IANA Considerations
This document has no actions for IANA. Path communication field
types and PLUS magic numbers may be moved to a Standards Action
registry in a future revision.
5. Security Considerations
[EDITOR'S NOTE: write me]
6. Acknowledgments
This work is partially supported by the European Commission under
Horizon 2020 grant agreement no. 688421 Measurement and Architecture
for a Middleboxed Internet (MAMI), and by the Swiss State Secretariat
for Education, Research, and Innovation under contract no. 15.0268.
This support does not imply endorsement.
7. Informative References
[I-D.hardie-path-signals]
Hardie, T., "Path signals", draft-hardie-path-signals-00
(work in progress), October 2016.
[I-D.ietf-quic-transport]
Iyengar, J. and M. Thomson, "QUIC: A UDP-Based Multiplexed
and Secure Transport", draft-ietf-quic-transport-00 (work
in progress), November 2016.
[I-D.trammell-plus-abstract-mech]
Trammell, B., "Abstract Mechanisms for a Cooperative Path
Layer under Endpoint Control", draft-trammell-plus-
abstract-mech-00 (work in progress), September 2016.
[I-D.trammell-plus-statefulness]
Kuehlewind, M., Trammell, B., and J. Hildebrand,
"Transport-Independent Path Layer State Management",
draft-trammell-plus-statefulness-02 (work in progress),
December 2016.
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[IPIM] Allman, M., Beverly, R., and B. Trammell, "In-Protocol
Internet Measurement (arXiv preprint 1612.02902)",
December 2016.
[RFC0793] Postel, J., "Transmission Control Protocol", STD 7,
RFC 793, DOI 10.17487/RFC0793, September 1981,
<http://www.rfc-editor.org/info/rfc793>.
[RFC2474] Nichols, K., Blake, S., Baker, F., and D. Black,
"Definition of the Differentiated Services Field (DS
Field) in the IPv4 and IPv6 Headers", RFC 2474,
DOI 10.17487/RFC2474, December 1998,
<http://www.rfc-editor.org/info/rfc2474>.
[RFC7323] Borman, D., Braden, B., Jacobson, V., and R.
Scheffenegger, Ed., "TCP Extensions for High Performance",
RFC 7323, DOI 10.17487/RFC7323, September 2014,
<http://www.rfc-editor.org/info/rfc7323>.
[RFC7675] Perumal, M., Wing, D., Ravindranath, R., Reddy, T., and M.
Thomson, "Session Traversal Utilities for NAT (STUN) Usage
for Consent Freshness", RFC 7675, DOI 10.17487/RFC7675,
October 2015, <http://www.rfc-editor.org/info/rfc7675>.
Authors' Addresses
Brian Trammell
ETH Zurich
Gloriastrasse 35
8092 Zurich
Switzerland
Email: ietf@trammell.ch
Mirja Kuehlewind
ETH Zurich
Gloriastrasse 35
8092 Zurich
Switzerland
Email: mirja.kuehlewind@tik.ee.ethz.ch
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