Network Working Group B. Trammell
Internet-Draft M. Kuehlewind
Intended status: Experimental ETH Zurich
Expires: September 14, 2017 March 13, 2017

Path Layer UDP Substrate Specification


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].

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Table of Contents

1. Introduction

This document defines a wire image for a Path Layer UDP Substrate (PLUS), for limited exposure of information from encrypted, UDP-encapsulated [RFC0768] 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 to support are drawn from recent work on explicit measurability in protocol design [IPIM].

1.1. Notational Conventions

The words “MUST”, “MUST NOT”, “SHOULD”, and “MAY” are used in this document. It’s not shouting; when these words are capitalized, they have a special meaning as defined in [RFC2119].

2. State Maintenance and Measurement: Basic Header

Every packet in each direction of a flow using PLUS carries a PLUS header. This can be either a basic header, or an 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                     |L|R|S|0|
|                                                              |
+-             connection/association token CAT               -+
|                                                              |
|                 packet serial number  PSN                    |
|                 packet serial echo    PSE                    |
/                                                              \
\         transport protocol header/payload (encrypted)        /
/                                                              \

Figure 1: PLUS header with basic exposure

Fields are encoded in network byte order and are defined as follows:

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 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:

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:

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.

    `- - - - - - - - - - - - - - - - - - - - - - - - - - - -'
    `    +============+    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     /              \<-+     
  +<-----------------(    stop-wait   ) | a<->b
  |                   \              /--+          
  |                   +============+       
  |                       | stop z->y
  |                       V
  |              +============+
  | TO_STOPPING /              \
  +------------(    stopping     )
                \              /

Figure 2: Transport-independent state machine as implemented by PLUS

2.3.1. State Establishment

On the first packet with a PLUS header forwarded by an on-path device for a given 5-tuple plus CAT, the device moves that flow from the zero state to the uniflow state. The device retuens the flow 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. Otherwise, it stays in uniflow state and continues forwarding packets, as long as it only observes packets in the same direction as the initial packet. (the a->b direction in Figure 2).

A PLUS-aware on-path device forwarding a packet with a PLUS header with a reversed 5-tuple and identical CAT (the b->a direction in Figure 2) to a flow in the uniflow state, moves that flow to the associating state. It then waits to see a packet with a PSE in the a->b direction equal to the PSN on the first reverse packet; on receipt of this packet, the device moves 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 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 stop-wait 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 stopping state. The device will forward packets in both directions for flows in the stopping state within a timeout interval TO_STOPPING; these packets will not reset the timer.

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 stop-wait state on all downstream devices. However, this forgery 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 stopping 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

One end of a PLUS association may change its address while maintaining on-path state; e.g. due to a NAT change. A PLUS-aware on-path device that forwards 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 non-zero state, treats that packet as belonging to the existing flow, and updates the endpoint identifier.

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 on path.

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 makes this measurement much simpler than the equivalent measurement using TCP sequence and acknowledgment numbers.

[EDITOR’S NOTE: specify this fully.]

Loss and reordering upstream from an observation point in each direction can be estimated through examination of the PSN sequence observed. A skipped PSN not seen within a specified interval can be counted as a lost packet, and the extent of reordering estimated by the degree of skipping seen in those skipped PSNs that are later observed. 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 20 bytes with the PLUS Basic Header, except the Extended Header bit (0x01 on byte 11) 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 has a 1-byte type field, a 6-bit length field, a 2-bit Integrity indicator, and variable-length value field for the Path Communication Function (PCF):

  • PCF Type: a 1-byte value defining the type and semantics of the PCF value. Types 0x00 and 0xff are special and further explained below.
  • PDF Integrity indication field: a 2-bit field indicating how much of the PCF value field is integrity protected:
    • 00: the PCF field is not integrity protected.
    • 01: the first quarter of the PCF value field is integrity protected, rounded up to the nearest bit.
    • 10: the first half of the PCF value field is integrity protected, rounded up to the nearest bit.
    • 11: the whole PCF field is integrity protected.
  • PCF Length: a 6-bit field indicating the length of the variable length value field.
  • PCF Value: variable-length field containing a value of the type described in the PCF Type field.
  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                     |L|R|S|1|
|                                                              |
+-             connection/association token CAT               -+
|                                                              |
|                 packet serial number  PSN                    |
|                 packet serial echo    PSE                    |
|   PCF Type    |  PCF Len  | I |                              \
+---------------+-----------+---+    PCF value (variable)      /
/                                                              \
+- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - +
\                                                              /
/         transport protocol header/payload (encrypted)        \
\                                                              /

Figure 3: PLUS extended header with 1-byte PCF

The extended header is generally used to expose or request information from/to the path. If the value field is integrity protected it MUST NOT be changed by an element on the network path and provides only consumable information. Non-integrity protected parts of the value field carry a scratch space for path elements to provide the requested information defined by the PCF type. The type of the PCF determines the semantics of the value field and limits the intended use of the scratch space. A middlebox only needs to check for PCF types that are relevant for the function that middlebox provides. Knowing the type, also means that the semantics of the value field are fully known. Therefore a middlebox does not need to check the length and integrity field. These field are provided for receiver-side handling on unknown PCF types.

If a sender requests information from a path, these are generally used to expose information about the traffic for measurement or diagnostic purposes. These 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. In any case, the information sent and received is to be treated as advisory only, given its integrity cannot be checked.

A PLUS receiver that receives a PLUS packet with an extended header and an unknown PCF type where the integrity indication is smaller than 11, MUST feed the non-integrity-protected part of the PCF value field back to the sender, using an encrypted feedback channel provided by the upper layer protocol. The sender usually consumes this data as it has requested it previously from the path. The sender SHOULD check the reasonability of the received information, as follows:

  • If the information was intended to be consumed by the receiver instead of the sender, reflecting the receiver indicates by reflecting this information that the PFC is unknown.
  • If the value field is unchanged from its initial value, this may indicate that there is no path element on the current path that supports the requested PCF. However, as the path can change, a PLUS sender might re-try to requested the information at a later point of time or if any indication is received that the path might have changed.
  • If the PCF value field has an invalid or unreasonable value regarding the requested PCF type, this may be an indication that the scratch space is misused, and the sender SHOULD NOT use this extended header PCF on this path anymore.

The PCF types 0x00 and 0xff are used for special purposes. 0x00 indicates that another 1-byte type field is followed (before the length and integrity indication) to provide future extensibility. The type 0xff indicates that that PLUS payload data follows (without a PDF length and integrity indication). The semantics of this PLUS payload are not specified in this revision of this document but e.g. can be used to carry ICMP messages over PLUS.

If the length field is zero, the integrity indication field is not specified and reserved for future use. It MUST be set to zero and ignored by the receiver.

3.1. Measurement and Diagnostics using the Extended Header

We have identified the following signals that can be exposed by the sender 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. Details of type and encoding for these information

  • Packet number echo delta time. Exposes the interval between the receipt of the packet whose number appears in the PSE and the transmission of this packet, as in section 4.1.2 of [IPIM]. Together with analysis of the PSN and PSE sequence, this allows high-precision RTT estimation. The encoding of this field is TBD.
  • Timestamp (Type 0x22, 3-byte sender to path). Similar to TCP timestamps in [RFC7323], 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. [EDITOR’S NOTE: is this useful enough to keep?]
  • Timestamp Echo. Echo of the last received timestamp, as above. [EDITOR’S NOTE: as above, is this useful enough to keep?]
  • Congestion Exposure. The sender exposes the number of observed losses and ECN marks [RFC3168]. The path observes the information over time and derive information about the current whole-path congestion, as currently provided by counting retransmission on TCP, the RTCP Extended Report (XR) block for periodic ECN feedback [RFC6679], or ConEx for IPv6 [RFC7837]. The encoding of this field is TBD.

We have identified the following signals for request from the path 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.

  • 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. The encoding of this field is TBD.
  • 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. The encoding of this field is TBD.
  • 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, in bytes into this field. The information, when fed back to the sender, can be used as a hint for a running PLPMTUD [RFC4821] process.
  • 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-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. Implementation and Deployment Considerations

Here we discuss considerations for the implementation of overlying transports on PLUS, and their deployment in the Internet

4.1. Interface to Overlying Transport

[EDITOR’S NOTE: Since the overlying transport is responsible for providing the cryptographic context used for integrity protection and feedback, and may be responsible for feedback as well, the “northbound” interface needs to be concretely specified in a future revision of this document.]

4.2. Discovery

[EDITOR’S NOTE: Discovery is currently explicitly out of band. Should there be a way for a PLUS-aware overlying transport to discover whether its peer wants to use a PLUS- or non-PLUS variant of the transport? This is a specific case of the generalized multi-transport discovery and negotiation problem, so whatever happens here should be interoperable with deployed/deployable approaches.]

5. 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.

6. Security Considerations

This document describes the PLUS Basic and Extended Headers, and the protocol they support. This protocol can be used to expose information to devices along the path to replace the analysis of transport- and application-layer headers when those headers are encrypted. Care must be taken in the exposure of such information to ensure no irrelevant application and/or user confidential information is exposed.

PLUS itself contains some security-relevant features. In concert with an encrypted overlying transport, the PLUS Basic and Extended Headers are integrity-protected to prevent manipulation on-path of any value except Path to Receiver values; this integrity protection prevents path elements from injecting values without explicit sender involvement, or from being stripped from the PLUS Extended Header.

The CAT and PSE described in Section 2 taken together, provide entropy to prevent on-path devices from being driven into incorrect states by off-path attackers. Bidirectional stop signaling as in Section 2.3.2 requires an on-path attacker of a given middlebox to forge traffic on both of the middlebox’s interfaces to drive a middlebox to inappropriately drop state for a flow.

7. 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. Thanks to Ted Hardie, Joe Hildebrand, Mark Nottingham, and the participants of the PLUS BoF at IETF 96 in Berlin for input leading to this design; and to Gorry Fairhurst for the detailed review.

8. References

8.1. Normative References

[RFC0768] Postel, J., "User Datagram Protocol", STD 6, RFC 768, DOI 10.17487/RFC0768, August 1980.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997.

8.2. Informative References

[I-D.hardie-path-signals] Hardie, T., "Path signals", Internet-Draft draft-hardie-path-signals-00, October 2016.
[I-D.ietf-quic-transport] Iyengar, J. and M. Thomson, "QUIC: A UDP-Based Multiplexed and Secure Transport", Internet-Draft draft-ietf-quic-transport-01, January 2017.
[I-D.trammell-plus-abstract-mech] Trammell, B., "Abstract Mechanisms for a Cooperative Path Layer under Endpoint Control", Internet-Draft draft-trammell-plus-abstract-mech-00, September 2016.
[I-D.trammell-plus-statefulness] Kuehlewind, M., Trammell, B. and J. Hildebrand, "Transport-Independent Path Layer State Management", Internet-Draft draft-trammell-plus-statefulness-02, December 2016.
[IPIM] Allman, M., Beverly, R. and B. Trammell, "Principles for Measurability in Protocol Design (arXiv preprint 1612.02902)", December 2016.
[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.
[RFC3168] Ramakrishnan, K., Floyd, S. and D. Black, "The Addition of Explicit Congestion Notification (ECN) to IP", RFC 3168, DOI 10.17487/RFC3168, September 2001.
[RFC4821] Mathis, M. and J. Heffner, "Packetization Layer Path MTU Discovery", RFC 4821, DOI 10.17487/RFC4821, March 2007.
[RFC6679] Westerlund, M., Johansson, I., Perkins, C., O'Hanlon, P. and K. Carlberg, "Explicit Congestion Notification (ECN) for RTP over UDP", RFC 6679, DOI 10.17487/RFC6679, August 2012.
[RFC7323] Borman, D., Braden, B., Jacobson, V. and R. Scheffenegger, "TCP Extensions for High Performance", RFC 7323, DOI 10.17487/RFC7323, September 2014.
[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.
[RFC7837] Krishnan, S., Kuehlewind, M., Briscoe, B. and C. Ralli, "IPv6 Destination Option for Congestion Exposure (ConEx)", RFC 7837, DOI 10.17487/RFC7837, May 2016.
[RFC8035] Holmberg, C., "Session Description Protocol (SDP) Offer/Answer Clarifications for RTP/RTCP Multiplexing", RFC 8035, DOI 10.17487/RFC8035, November 2016.

Authors' Addresses

Brian Trammell ETH Zurich Gloriastrasse 35 8092 Zurich, Switzerland EMail:
Mirja Kuehlewind ETH Zurich Gloriastrasse 35 8092 Zurich, Switzerland EMail: