Network Working Group B. Trammell, Ed.
Internet-Draft M. Kuehlewind, Ed.
Intended status: Informational ETH Zurich
Expires: September 12, 2016 March 11, 2016

Requirements for the design of a Substrate Protocol for User Datagrams (SPUD)


The Substrate Protocol for User Datagrams (SPUD) BoF session at the IETF 92 meeting in Dallas in March 2015 identified the potential need for a UDP-based encapsulation protocol to allow explicit cooperation with middleboxes while using new, encrypted transport protocols. This document proposes an initial set of requirements for such a protocol, and discusses tradeoffs to be made in further refining these requirements.

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Table of Contents

1. Motivation

A number of efforts to create new transport protocols or experiment with new network behaviors in the Internet have been built on top of UDP, as it traverses firewalls and other middleboxes more readily than new protocols do. Each such effort must, however, either manage its flows within common middlebox assumptions for UDP or train the middleboxes on the new protocol (thus losing the benefit of using UDP). A common Substrate Protocol for User Datagrams (SPUD) would allow each effort to re-use a set of shared methods for notifying middleboxes of the flows’ semantics, thus avoiding both the limitations of current flow semantics and the need to re-invent the mechanism for notifying the middlebox of the new semantics.

As a concrete example, it is common for some middleboxes to tear down required state (such as NAT bindings) very rapidly for UDP flows. By notifying the path that a particular transport using UDP maintains session state and explicitly signals session start and stop using the substrate, the using protocol may reduce or avoid the need for heartbeat traffic.

This document defines a specific set of requirements for a SPUD facility, based on analysis on a target set of applications to be developed on SPUD developing experience with a prototype described in [I-D.hildebrand-spud-prototype]. It is intended as the basis for determining the next steps to make progress in this space, including possibly chartering a working group for specific protocol engineering work.

Within this document, requirements are presented as for a facility implementable as an encapsulation protocol, atop which new transports (“superstrates”) can be built. Alternately, these could be viewed as a set of requirements for future transport protocol development without a layer separation between the transport and the superstrate.

The final intention of this work is to make it possible to define and deploy new transport protocols that use encryption to protect their own operation as well as the confidentiality, authenticity, integrity, and linkability resistance of their payloads. The accelerating deployment of encryption will render obsolete network operations techniques that rely on packet inspection and modification based upon assumptions about the protocols in use. This work will allow the replacement the current regime of middlebox inspection and modification of transport and application- layer headers and payload with one that allows inspection only of information explicitly exposed by the endpoints, and modification of such information only under endpoint control.

2. History

An outcome of the IAB workshop on Stack Evolution in a Middlebox Internet (SEMI) [RFC7663], held in Zurich in January 2015, was a discussion on the creation of a substrate protocol to support the deployment of new transport protocols in the Internet. Assuming that a way forward for transport evolution in user space would involve encapsulation in UDP datagrams, the workshop noted that it may be useful to have a facility built atop UDP to provide minimal signaling of the semantics of a flow that would otherwise be available in TCP. At the very least, indications of first and last packets in a flow may assist firewalls and NATs in policy decision and state maintenance. This facility could also provide minimal application-to- path and path-to-application signaling, though there was less agreement about what should or could be signaled here. Further transport semantics would be used by the protocol running atop this facility, but would only be visible to the endpoints, as the transport protocol headers themselves would be encrypted, along with the payload, to prevent inspection or modification. This encryption might be accomplished by using DTLS [RFC6347] as a subtransport [I-D.huitema-tls-dtls-as-subtransport] or by other suitable methods.

The Substrate Protocol for User Datagrams (SPUD) BoF was held at IETF 92 in Dallas in March 2015 to develop this concept further. It is clear from discussion before and during the SPUD BoF that any selective exposure of traffic metadata outside a relatively restricted trust domain must be advisory, non-negotiated, and declarative rather than imperative. This conclusion matches experience with previous endpoint-to-middle and middle-to-endpoint signaling approaches. As with other metadata systems, exposure of specific elements must be carefully assessed for privacy risks and the total of exposed elements must be so assessed. Each exposed parameter should also be independently verifiable, so that each entity can assign its own trust to other entities. Basic transport over the substrate must continue working even if signaling is ignored or stripped, to support incremental deployment. These restrictions on vocabulary are discussed further in [I-D.trammell-stackevo-explicit-coop]. This discussion includes privacy and trust concerns as well as the need for strong incentives for middlebox cooperation based on the information that are exposed.

3. Terminology

This document uses the following terms:

4. Use Cases

The primary use case for endpoint to path signaling in the Internet, making use of packet grouping, is the binding of limited related semantics (start, ack, and stop) to a flow or a group of packets within a flow which are semantically related in terms of the application or superstrate. By explicitly signaling start and stop semantics, a flow allows middleboxes to use those signals for setting up and tearing down their relevant state (NAT bindings, firewall pinholes), rather than requiring the middlebox to infer this state from continued traffic. At best, this would allow the application to refrain from sending heartbeat traffic, which might result in reduced radio utilization and thus greater battery life on mobile platforms.

SPUD could also be used to provide information relevant for network treatment for middleboxes as a replacement for deep packet inspection for traffic classification purposes, rendered ineffective by superstrate encryption. In this application, properties would be expressed in terms of network-relevant parameters (intended bandwidth, latency and loss sensitivity, etc.) as opposed to application-relevant semantics. See [I-D.trammell-stackevo-explicit-coop] for discussion on limitations in signaling in untrusted environments.

SPUD may also provide some facility for SPUD-aware nodes on the path to signal some property of the path relative to a tube to the endpoints and other SPUD- aware nodes on the path. The primary use case for path to application signaling is parallel to the use of ICMP [RFC0792], in that it describes a set of conditions (including errors) that applies to the datagrams as they traverse the path. This usage is, however, not a pure replacement for ICMP but a “5-tuple ICMP” for error messages which should be application-visible; these would traverse NATs in the same way as the traffic related to it, and be deliverable to the application with appropriate tube information.

Link-layer characteristics of use to the transport layer (e.g., whether a high-transient-delay, highly-buffered link such as LTE is present on the path) could also be signaled using this path-to-endpoint facility.

Further use cases are outlined in more detail in [I-D.kuehlewind-spud-use-cases].

5. Functional Requirements

The following requirements detail the services that SPUD must provide to superstrates, endpoints, and middleboxes using SPUD.

5.1. Grouping of Packets (into “tubes”)

Transport semantics and many properties of communication that endpoints may want to expose to middleboxes are bound to flows or groups of flows (five- tuples). SPUD must therefore provide a basic facility for associating packets together (into what we call a “tube”, for lack of a better term) and associate information to these groups of packets. Each packet in a SPUD “flow” (determined by 5-tuple) belongs to exactly one tube. Notionally, a tube consists of a set of packets with a set of common properties, that should therefore receive equivalent treatment from the network; these tubes may or may not be related to separate semantic entities in the superstrate (e.g. SCTP streams).

The simplest mechanisms for association involve the addition of an identifier to each packet in a tube. Other mechanisms that don’t directly encode the identifier in a packet header, but instead provide it in a way that it is simple to derive from other information available in the packet at the endpoints and along the path, are also possible. In any cases, for the purposes of this requirement we treat this identifier as a simple vector of N bits. The properties of the tube identifier are subject to tradeoffs on the requirements for privacy, security, ease of implementation, and header overhead efficiency.

In determining the optimal size and scope for this tube identifier, we first assume that the 5-tuple of source and destination IP address, UDP port, and IP protocol identifier (17 for UDP) is used in the Internet as an existing flow identifier, due to the widespread deployment of network address and port translation. We conclude that SPUD tube IDs should be scoped to this 5-tuple.

While a globally-unique identifier would allow easier state comparison and migration for mobility use cases, it would have two serious disadvantages. First, N would need to be sufficiently large to minimize the probability of collision among multiple tubes having the same identifier along the same path during some period of time. A 128-bit UUID [RFC4122] or an identifier of equivalent size generated using an equivalent algorithm would probably be sufficient, at the cost of 128 bits of header space in every packet. Second, globally unique tube identifiers would also introduce new possibilities for user and node tracking, with a serious negative impact on privacy. We note that global identifiers for mobility, when necessary to expose to the path, can be supported separately from the tube identification mechanism, by using a generic tube-grouping application-to-path signaling bound to the tube.

Even when tube IDs are scoped to 5-tuples, N must still be sufficiently large, and the bits in the identifier sufficiently random, that possession of a valid tube ID implies that a node can observe packets belonging to the tube (see Section 6.6). This reduces the chances of success of blind packet injection attacks of packets with guessed valid tube IDs.

When scoped to 5-tuples, the forward and backward directions of a bidirectional connection will have different tube IDs, since these will necessarily take different paths and may interact with a different set of middleboxes due to asymmetric routing. SPUD will therefore require some facility to note that one tube is the “reverse” direction of another, a general case of the tube grouping signal above.

5.2. Endpoint to Path Signaling

SPUD must be able to provide information scoped to a tube from the end- point(s) to all SPUD-aware nodes on the path about the packets in that tube. We note that in-band signaling would meet this requirement.

5.3. Path to Endpoint Signaling

SPUD must be able to provide information from a SPUD-aware middlebox to the endpoint. This information is associated with a tube, in terms of “the properties of the path(s) the packets in this tube will traverse”. Path-to- endpoint signaling must be made available to the superstrate and/or the application at the endpoint. We note that in-band signaling would meet this requirement.

5.4. Tube Start and End Signaling

SPUD must provide a facility for endpoints to signal that a tube has started, that the start of the tube has been acknowledged and accepted by the remote endpoint(s), and that a tube has ended and its state can be forgotten by the path. Given unreliable signaling (see Section 7.10) both endpoints and devices on the path must be resilient to the loss of any of these signals. Specifically, timeouts are still necessary to clean up stale state. See Section 8.6 and Section 8.7 for more discussion on tube start and end signaling.

5.5. Declarative signaling

All information signaled via SPUD is defined to be declarative (as opposed to imperative). A SPUD endpoint must function correctly if no middlebox along the path understands the signals it sends, or if sent signals from middleboxes it does not understand. Likewise, a SPUD-aware middlebox must function correctly if sent signals from endpoints it does not understand, or in the absence of expected signals from endpoints.

5.6. Extensibility

SPUD must enable multiple new transport semantics and application/path declarations without requiring updates to SPUD implementations in middleboxes.

The use of SPUD for experimental signaling must be possible either without the registration of codepoints or namespaces with IANA, or with trivially easy (First Come, First Served [RFC5226] registration of such codepoints.

6. Security Requirements

6.1. Privacy

SPUD must allow endpoints to control the amount of information exposed to middleboxes, with the default being the minimum necessary for correct functioning. This includes the cryptographic protection of transport layer headers from inspection by devices on path, in order to prevent ossification of these headers.

6.2. Authentication

The basic SPUD protocol must not require any authentication or a priori trust relationship between endpoints and middleboxes to function. However, SPUD should interoperate with the presentation/exchange of authentication information in environments where a trust relationship already exists, or can be easily established, either in-band or out-of-band, and use this information where possible and appropriate.

Given the advisory nature of the signaling it supports, SPUD may also support eventual authentication: authentication of a signal after the reception of a packet after that containing the signal.

6.3. Integrity

SPUD must provide integrity protection of exposed information in SPUD- encapsulated packets, though the details of this integrity protection are still open; see Section 8.2.

Endpoints should be able to detect changes to headers SPUD uses for its own signaling (whether due to error, accidental modification, or malicious modification), as well as the injection of packets into a SPUD flow (defined by 5-tuple) or tube by nodes other than the remote endpoints. Errors and accidental modifications can be detected using a simple checksum over the SPUD header, while detecting malicious modifications requires cryptographic integrity protection. Similar to Section 6.2, cryptographic integrity protection may also be eventual.

Integrity protection of the superstrate is left up to the superstrate.

6.4. Encrypted Feedback

Some use cases involve collecting information along a forward path from a sending endpoint to a receiving endpoint. In cases where this information is also useful to the sending endpoint, SPUD must provide a feedback channel to communicate this information back to the sender. As this information does not need to be exposed to the path, this feedback channel should be encrypted for confidentiality and authenticity, when available (see Section 6.2).

6.5. Preservation of Security Properties

The use of SPUD must not weaken the essential security properties of the superstrate: confidentiality, integrity, authenticity, and defense against linkability. If the superstrate includes payload encryption for confidentiality, for example, the use of SPUD must not allow deep packet inspection systems to have access to the plaintext. Likewise, the use of SPUD must not create additional opportunities for linkability not already existing in the superstrate.

With respect to access control, SPUD itself must not be used to negotiate the means to lift administrative prohibition of certain traffic, although it could be used to provide more useful information why it is prohibited.

6.6. Proof a device is on-path

Devices may make assertions of network characteristics relevant to a flow. One way these assertions can be assessed is by a demonstration that the device making it is on-path to the flow and so could adjust the characteristics to match the assertion. SPUD must therefore allow endpoints to distinguish on- path devices from devices not on the path. Network elements may also need to confirm that application-to-path assertions are made by the source indicated in the flow. In both cases, return routability (as in Section 6.7) may offer one incrementally deployable method of testing the topology to make this confirmation.

6.7. Protection against trivial abuse

Malicious background traffic is a serious problem for UDP-based protocols due to the ease of forging source addresses in UDP together with only limited deployment of network egress filtering [RFC2827]. Trivial abuse includes flooding and state exhaustion attacks, as well as reflection and amplification attacks. SPUD must provide minimal protection against this trivial abuse. This probably implies that SPUD should provide:

We note that using a “magic number” or other pattern of bits in an encapsulation-layer header not used in any widely deployed protocol has the nice property that no existing node in the Internet can be induced to reflect traffic containing it. This allows the magic number to provide probabilistic assurance that a given packet is not reflected, assisting in meeting this requirement.

7. Technical Requirements

The following requirements detail the constraints on how the SPUD facility must meet its functional requirements.

7.1. Middlebox Traversal

SPUD, including all path-to-endpoint and endpoint-to-path signaling as well as superstrate and superstrate payload, must be able to traverse middleboxes and firewalls, including those that are not SPUD-aware. Therefore SPUD must be encapsulated in a transport protocol that is known to be accepted on a large fraction of paths in the Internet, or implement some form of probing to determine in advance which transport protocols will be accepted on a certain path. This encapsulation will require port numbers to support endpoints connected via network address and port translation (NAPT). We note that UDP encapsulation would meet these requirements.

7.2. Low Overhead in Network Processing

SPUD must be low-overhead, specifically requiring very little effort to recognize that a packet is a SPUD packet and to determine the tube it is associated with. We note that a magic number as in Section 6.7 would also have a low probability of colliding with any non- SPUD traffic, therefore meeting the recognition requirement. Tube identifiers appearing directly in the encapsulation-layer header would meet the tube association requirement.

7.3. Implementability in User-Space

To enable fast deployment SPUD and superstrates must be implementable without requiring kernel replacements or modules on the endpoints, and without having special privilege (such as is required for raw packet transmission, i.e. root or “jailbreak”) on the endpoints. We note here that UDP would meet this requirement, as nearly all operating systems and application development platforms allow a userspace application to open UDP sockets.

7.4. Incremental Deployability in an Untrusted, Unreliable Environment

SPUD must operate in the present Internet. In order to maximize deployment, it should also be useful between endpoints even before the deployment of middleboxes that understand it. The information exposed over SPUD must provide incentives for adoption by both endpoints and middleboxes, and must maximize privacy (by minimizing information exposed). Further, SPUD must be robust to packet loss, duplication and reordering by the underlying network service. SPUD must work in multipath, multicast, and endpoint multi- homing environments.

Incremental deployability likely requires limitations of the vocabulary used in signaling, to ensure that each actor in a non-trusted environment has incentives to participate in the signaling protocol honestly; see [I-D.trammell-stackevo-explicit-coop] for more.

7.5. No unnecessary restrictions on the superstrate

Beyond those restrictions deemed necessary as common features of any secure, responsible transport protocol (see Section 6.7), SPUD must impose only minimal restrictions on the transport protocols it encapsulates. However, to serve as a substrate, it is necessary to factor out the information that middleboxes commonly rely on and endpoints are commonly willing to expose. This information should be included in SPUD, and might itself impose additional restrictions to the superstrate.

7.6. Minimal additional start-up latency

SPUD should not introduce additional start-up latency for superstrates.

7.7. Minimal header overhead

To avoid reducing network performance, the information and coding used in SPUD should be designed to use the minimum necessary amount of additional space in encapsulation headers.

7.8. Minimal non-productive traffic

SPUD should minimize additional non-productive traffic (e.g. keepalives), and should provide mechanisms to allow its superstrates to minimize their reliance on non-productive traffic.

7.9. Endpoint control over reverse-path middlebox signaling

In some cases, a middlebox may need to send a packet directly in response to a sending endpoint, e.g. to signal an error condition. In this case, the direct return packet generated by the middlebox uses the reversed end-to-end 5-tuple in order to receive equivalent NAT treatment, though the reverse path might not be the same as the forward path. Endpoints have control over this feature: A SPUD-aware middlebox must not emit a direct return packet unless it is in direct response to a packet from a receiving endpoint, and must not forward a packet for which it has sent a direct return packet.

7.10. Reliability, Fragmentation, MTU, and Duplication

As any information provided by SPUD is anyway opportunistic, SPUD need not provide reliable signaling for the information associated with a tube. Signals must be idempotent; all middleboxes and endpoints must gracefully handle receiving duplicate signal information. SPUD must continue working in the presence of IPv4 fragmentation on path, but in order to reduce the impact of requiring fragments reassembly at middleboxes for signals to be intelligible, endpoints using SPUD should attempt to fit all signals into single MTU-sized packets.

Given the importance of good path MTU information to SPUD’s own signaling, SPUD should implement packetization layer path MTU discovery [RFC4821].

Any facilities requiring more than an MTU’s worth of data in a single signal should use an out-of-band method which does provide reliability – this method may be an existing transport or superstrate/SPUD combination, or a “minimal transport” defined by SPUD for its own use.

7.11. Interoperability with non-encapsulated superstrates

It is presumed that “superstrate X with SPUD” is a distinct entity on the wire from “superstrate X”. The APIs the superstrate presents to the application should be equivalent, and the two wire protocols should be freely transcodeable between each other, with the caveat that the variant without SPUD would not necessarily support features enabling communication with the path. However, there is no requirement that the headers the superstrate uses be the same in the SPUD and non-SPUD variants. Headers that the superstrate chooses always to expose to the path can therefore be encoded in the SPUD layer but not appear in an upper-layer header.

8. Open questions and discussion

The preceding requirements reflect the present best understanding of the authors of the functional and technical requirements on an encapsulation-based protocol for common middlebox-endpoint cooperation for superstrates. There remain a few large open questions and points for discussion, detailed in the subsections below.

8.1. Property binding

Related to identifier scope is the scope of properties bound to SPUD packets by endpoints. SPUD may support both per-tube properties as well as per-packet properties. Properties signaled per packet reduce state requirements at middleboxes, but also increase per-packet overhead. Small signal size (in bits of entropy) and encoding efficiency (in bits on the wire) is therefore more important for per-packet signaling that per-tube signaling.

It is likely that both types of property binding are useful, but the selection of which properties to bind how must be undertaken carefully. It is also possible that SPUD will provide a very limited set of per-packet signals (such as ECN) using flags in the SPUD header, and require all more complicated properties to be bound per-tube.

8.2. Tradeoffs in integrity protection

In order to protect the integrity of information carried by SPUD against forging by malicious devices along the path, it would be necessary to be able to authenticate the originator of that information. We presume that the authentication of endpoints is a generally desirable property, and to be handled by the superstrate; in this case, SPUD may be able borrow that authentication to protect the integrity of endpoint-originated information.

However, in the Internet, it is not in the general case possible for the endpoint to authenticate every middlebox that might see packets it sends and receives. In this case information produced by middleboxes may enjoy less integrity protection than that produced by endpoints. In addition, endpoint authentication of middleboxes and vice-versa may be better conducted out-of- band (treating the middlebox as an endpoint for the authentication protocol) than in-band (treating the middlebox as a participant in a 3+ party communication).

8.3. Piggybacked, interleaved, and reflected signaling

The requirements in Section 5.2 and Section 5.3 are best met by in-band signaling: packets carrying the same 6-tuple as packets containing superstrate headers and payload.

Path-to-endpoint signaling can be piggybacked and/or interleaved (where SPUD and the superstrate each have their own packets).

In-band signaling has the advantage that it does not require foreknowledge of the identity and addresses of devices along the path by endpoints and vice versa, but does add complexity to the signaling protocol. Piggybacked signaling uses some number of bits in each packet generated by the overlying transport. It requires either reducing the MTU available to the encapsulated transport and/or opportunistically using “headroom” as it is available: bits between the network-layer MTU and the bits actually used by the transport. For use cases that accumulate information from devices on path in the SPUD header, piggybacked signaling also requires a mechanism for endpoints to create “scratch space” for potential use of the on-path devices. In contrast, interleaved signaling uses signaling packets on the same 5-tuple and tube ID, which don’t carry any superstrate data. These interleaved packets can also contain scratch space for on-path device use. This reduces complexity and sidesteps MTU problems, at the cost of sending more packets per flow.

8.4. Continuum of trust among endpoints and middleboxes

There are different security considerations for different security contexts. The end-to-end context is one; anything that only needs to be seen by the path shouldn’t be exposed in SPUD, but rather by the superstrate. There are multiple different types of end-to-middle context based on levels of trust between end and middle – is the middlebox on the same network as the endpoint, under control of the same owner? Is there some contract between the application user and the middlebox operator? SPUD should support different levels of trust than the default (“untrusted, but presumed honest due to limitations on the signaling vocabulary”) and fully-authenticated; how these points along the continuum are to be implemented and how they relate to each other needs to be explored further.

8.5. Discovery and capability exposure

There are two open issues in discovery and capability exposure. First, endpoints might want to be able to discover SPUD-aware middleboxes along the path, and to discover which parts of the vocabulary that can be spoken by the endpoints are supported by those middleboxes as well as the other communication endpoint, and vice versa. Second, SPUD coult assist endpoints in discovering and negotiate which superstrates are available for a given interaction, though it is explicitly not a goal of SPUD to expose information about the details of the superstrate to middleboxes.

8.6. Hard state vs. soft state

The initial thinking on signaling envisions “hard state” in middleboxes that is established when the middlebox observes the start of a SPUD tube and is torn down when the middlebox observes the end (stop) of a SPUD tube. Such state can be abandoned as a result of network topology changes (e.g., routing update in response to link or node failure). An alternative is a “soft state” approach that requires periodic refresh of state in middleboxes, but cleanly times out and discards abandoned state. SPUD has the opportunity to use different timeouts than the defaults that are required for current NAT and firewall pinhole maintenance. Of course, applications will still have to detect non-SPUD middleboxes that use shorter timers.

8.7. Tube vs. superstrate association lifetime

The requirements as presently defined use tube start and stop signaling for two things: (1) setting up and tearing down state along the path, and (2) signaling superstrate such as association startup, acceptance, and teardown, which may have security implications. These may require separate signaling. Specifically, if tube start acknowledgment is to be used to provide explicit guarantees to the path about the acceptability of a tube to a remote endpoint, it cannot be a completely unreliable signal. Second, the lifetime of a tube may be much shorter than the lifetime of a superstrate association, and the creation of a new tube over an existing association may need to be treated differently by endpoints and path devices than a tube creation coincident with an association creation.

8.8. SPUD Support Discovery

If SPUD is not usable on a path to an endpoint, a SPUD sender needs to be able to fall back to some other approach to achieve the goals of the superstrate; a SPUD endpoint must be able to easily determine whether a remote endpoint with which it wants to communicate using SPUD as a substrate can support SPUD, and whether path to the remote endpoint as well as the return path from the remote endpoint will pass SPUD packets.

It is not clear whether this is a requirement of SPUD, or a requirement of the superstrate / application over SPUD.

9. Security Considerations

The security-relevant requirements for SPUD are outlined in Section 6. In addition, security-relevant open issues are discussed in Section 8.2 and Section 8.4. These will be further addressed in protocol definition work following from these requirements.

10. IANA Considerations

This document has no actions for IANA.

11. Contributors

In addition to the editors, this document is the work of David Black, Ken Calvert, Ted Hardie, Joe Hildebrand, Jana Iyengar, and Eric Rescorla.

12. Acknowledgments

Thanks to Ozgu Alay, Roland Bless, Cameron Byrne, Toerless Eckert, Gorry Fairhurst, Daniel Kahn Gillmor, Tom Herbert, Christian Huitema, Iain Learmonth, Diego Lopez, and Matteo Varvelli for feedback and comments on these requirements, as well as to the participants at the SPUD BoF at IETF 92 meeting in Dallas and the IAB SEMI workshop in Zurich for the discussions leading to this work.

This work is 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.

13. Informative References

[RFC0792] Postel, J., "Internet Control Message Protocol", STD 5, RFC 792, DOI 10.17487/RFC0792, September 1981.
[RFC2827] Ferguson, P. and D. Senie, "Network Ingress Filtering: Defeating Denial of Service Attacks which employ IP Source Address Spoofing", BCP 38, RFC 2827, DOI 10.17487/RFC2827, May 2000.
[RFC3234] Carpenter, B. and S. Brim, "Middleboxes: Taxonomy and Issues", RFC 3234, DOI 10.17487/RFC3234, February 2002.
[RFC4122] Leach, P., Mealling, M. and R. Salz, "A Universally Unique IDentifier (UUID) URN Namespace", RFC 4122, DOI 10.17487/RFC4122, July 2005.
[RFC4821] Mathis, M. and J. Heffner, "Packetization Layer Path MTU Discovery", RFC 4821, DOI 10.17487/RFC4821, March 2007.
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA Considerations Section in RFCs", BCP 26, RFC 5226, DOI 10.17487/RFC5226, May 2008.
[RFC6347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347, January 2012.
[RFC7510] Xu, X., Sheth, N., Yong, L., Callon, R. and D. Black, "Encapsulating MPLS in UDP", RFC 7510, DOI 10.17487/RFC7510, April 2015.
[RFC7663] Trammell, B. and M. Kuehlewind, "Report from the IAB Workshop on Stack Evolution in a Middlebox Internet (SEMI)", RFC 7663, DOI 10.17487/RFC7663, October 2015.
[I-D.hildebrand-spud-prototype] Hildebrand, J. and B. Trammell, "Substrate Protocol for User Datagrams (SPUD) Prototype", Internet-Draft draft-hildebrand-spud-prototype-03, March 2015.
[I-D.kuehlewind-spud-use-cases] Kuehlewind, M. and B. Trammell, "SPUD Use Cases", Internet-Draft draft-kuehlewind-spud-use-cases-00, July 2015.
[I-D.huitema-tls-dtls-as-subtransport] Huitema, C., Rescorla, E. and J. Jana, "DTLS as Subtransport protocol", Internet-Draft draft-huitema-tls-dtls-as-subtransport-00, March 2015.
[I-D.trammell-stackevo-explicit-coop] Trammell, B., "Architectural Considerations for Transport Evolution with Explicit Path Cooperation", Internet-Draft draft-trammell-stackevo-explicit-coop-00, September 2015.

Authors' Addresses

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