Internet Engineering Task Force D. Dolson
Internet-Draft J. Snellman
Intended status: Informational Sandvine
Expires: August 4, 2017 January 31, 2017

Beneficial Functions of Middleboxes


At IETF97, at a meeting regarding the Path Layer UDP Substrate (PLUS) protocol, a request was made for documentation about the benefits that might be provided by permitting middleboxes to have some visibility to transport-layer information.

This document summarizes benefits provided to the Internet by middleboxes -- intermediary devices that provide functions apart from normal IP routing between a source and destination host [RFC3234].

RFC3234 defines a taxonomy of middleboxes and issues in the internet circa 2002. Most of those middleboxes utilized or modified application-layer data. This document will focus primarily on devices that observe and act on information found in the transport layer, most commonly TCP at this time.

A primary goal of this document is to provide information to working groups developing new transport protocols, in particular the PLUS and QUIC working groups, to aid understanding of what might be gained or lost by design decisions.

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

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."

This Internet-Draft will expire on August 4, 2017.

Copyright Notice

Copyright (c) 2017 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 ( 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

From RFC3234 [RFC3234], "A middlebox is defined as any intermediary device performing functions other than the normal, standard functions of an IP router on the datagram path between a source host and destination host."

Middleboxes are usually (but not exclusively) deployed at locations permitting observation of bidirectional traffic flows. This is typically at the point a stub network connects to the internet:

The QUIC working group and PLUS BoF are debating the appropriate amount of information that end-points should expose to on-path network middleboxes and human operators. This document itemizes a variety of features provided by middleboxes and by ad hoc analysis performed by operators using packet analyzers.

Many of the techniques described in this document require stateful analysis of transport streams. A generic state machine is described in [I-D.trammell-plus-statefulness].

Although many middleboxes observe and manipulate application-layer content they are out of scope for this document, the aim being to describe benefits of transport-layer features. Application-layer content should be encrypted and/or authenticated, whereas we hope to provide motivation to make transport connections managable from the network.

1.1. Requirements Language

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].

2. Measurements

A number of measurements can be made by network devices that are either in-line with the traffic (responsible for forwarding) or receiving off-line copy of traffic from a tap or file capture. These measurements can be used either in automated systems, or for manual network troubleshooting (e.g., using packet analysis tools such as Wireshark). The automated devices can further be classified as monitoring devices that compute these metrics for large amounts of connections and generate aggregated reports from them, and active devices that make decisions on how to handle specific packets based on these metrics.

Long-term trends in these measurements can aid an operator in capacity planning. Short-term anomalies in these measurements can identify network breakages, attacks in progress or misbehaving devices/applications.

2.1. Packet Loss

Network problems and under-provisioning can be detected if packet loss is measurable. TCP packet loss can be detected by observing gaps in sequence numbers, retransmitted sequence numbers, and SACK options. Packet loss can be detected per direction.

Gaps indicate loss upstream of the tap point; retransmissions indicate loss downstream of the tap. Selective acknowledgements (SACKs) can be used to detect either form of packet loss (although some care needs to be taken to avoid mis-identifying packet reordering as packet loss), and to distinguish between upstream vs. downstream losses.

Packet loss measurements on both sides of the measurement point are an important component in precisely diagnosing insufficiently dimensioned devices or links in networks. Additionally since packet losses are one of the two main ways for congestion to manifest, packet loss is an important measurement for any middlebox that needs to make traffic handling decisions based on observed levels of congestion.

2.2. Round Trip Times

A TCP packet stream can be used to measure the round-trip time on each side of the measurement point. During the connection handshake, the SYN, SYNACK, and ACK timings can be used to establish a baseline RTT in each direction. Once the connection is established, the RTT between the server and the measurement point can only reliably be determined using TCP timestamps. On the side between the measurement point and the client, the exact timing of data segments and ACKs can be used as an alternative. For this latter method to be accurate when packet loss is present, the connection must use selective acknowledgements.

In many kinds of networks, congestion will show up as queueing, and congestion-induced packet loss will only happen in extreme cases. RTTs will also show up as a much smoother signal than the discrete packet loss events. This makes RTTs a good way to identify individual subscribers for whom the network is a bottleneck at a given time, or geographical sites (such as cellular towers) that are experiencing large scale congestion.

The main limit of RTT measurement as a congestion signal is the difficulty of reliably distinguishing between the data segments being queued vs. the ACKs being queued.

2.3. Measuring Packet Reordering

If a network is reordering packets of transport connections, caused perhaps by ECMP misconfiguration (e.g., described in [RFC2991] and [RFC7690]) the end-points may react as though packet loss is occurring and retransmit packets or reduce forwarding rates. It is therefore beneficial to be able to diagnose packet reordering from within a network.

For TCP, packet reordering can be detected by observing TCP sequence numbers per direction. See, for example a number of standard packet reordering metrics in [RFC4737] and informational metrics in [RFC5236].

2.4. Throughput and Bottleneck Identification

Although throughput to or from an IP address can be measured without transport-layer measurements, the transport layer provides clues about what the end-points were attempting to do.

One way of quickly excluding the network as the bottleneck during troubleshooting is to check whether the speed is limited by the endpoints. For example the connection speed might instead be limited by suboptimal TCP options, the sender's congestion window, the sender temporarily running out of data to send, the sender waiting for the receiver to send another request, or the receiver closing the receive window.

This data is also useful for middleboxes used to measure network quality of service. Connections, or portions of connections, that are limited by the endpoints do not provide an accurate measure of network's speed, and can be discounted or completely excluded in such analyses.

2.5. DDoS Detection

When an application or network resource is under attack, it is useful to identify this situation from the network perspective, upstream of the attacked resource.

Although detection methods tend to be proprietary, DDoS attack detection is fundamentally one of:

Two trends in protocol design will make DDoS detection more difficult:

Those desires assist in the worthy goal of improved privacy, but also serve to defeat DDoS detection.

2.6. Packet Corruption

One notable source of packet loss is packet corruption. This corruption will generally not be detected until the checksums are validated by the endpoint, and the packet is dropped. This means that detecting the exact location where packets are lost is not sufficient when troubleshooting networks. It should also be possible to find out where packets are being corrupted. IP and TCP checksum verification allows a measurement device to correctly distinguish between upstream packet corruption and normal downstream packet loss.

QUIC and PLUS designers should consider whether a middlebox will be able to detect corrupted or tampered packets.

2.7. Application-Layer Measurements

Network health may also be gleaned from application-layer diagnosis. E.g.,

Could this type of information be provided in a transport layer?

3. Actions

This section describes features provided by in-line devices that modify, discard, delay, or prioritize traffic.

3.1. NAT

Network Address Translators (NATs) allow multiple devices to share a public address by dividing the transport-layer port space among the devices.

NAT behavior recommendations are found for UDP in BCP 127 [RFC4787] and for TCP in BCP 142 [RFC7857].

To support NAT, there must be transport-layer port numbers that can be modified by the network. The application-layer must not assume the port number was unchanged (e.g., by including it in a checksum or signing it).

3.2. Firewall

Firewalls are a pervasive and essential component of making a network secure. Arguably many users within various types of organizations would not have been granted internet access if not for firewalls.

An important aspect of firewall policy is differentiating internally-initiated from externally-initiated communications.

A firewall functions better when it can observe the protocol state machine, described generally by Transport-Independent Path Layer State Management [I-D.trammell-plus-statefulness].

3.3. DDoS Scrubbing

In the context of a distributed denial-of-service (DDoS) attack, the purpose of a scrubber is to discard attack traffic while permitting useful traffic.

When attacks occur against constrained resources, there is obviously a huge benefit in being able to scrub well.

Futhermore, this is solely a task for an on-path network device because neither end-point of a legitimate connection has any control over the source of the attack traffic.

Source-spoofed DDoS attacks can be mitigated at the source using BCP 38 ([RFC2827]), but it is more difficult if source address filtering cannot be applied.

In contrast to devices in the core of the Internet, middleboxes statefully observing bidirectional transport connections can reject source-spoofed TCP traffic based on inability to provide sensible acknowledgement numbers to complete the three-way handshake. Obviously this requires middlebox visibility into transport-layer state machine.

Middleboxes may also scrub on the basis of statistical classification: testing how likely a given packet is legitimate. As protocol designers add more entropy to headers and lengths, this test becomes less useful and the best scrubbing strategy becomes random drop.

3.4. Performance-Enhancing Proxies

Performance-Enhancing Proxies (PEPs) can improve network performance by improving packet spacing or generating local acknowledgements, and are most commonly used in satellite and cellular networks. Transport-Layer PEPs are described in section 2.1.1 of [RFC3135].

PEPs allow central deployment of congestion control algorithms more suited to the specific network, most commonly use of delay-based congestion control. More advanced TCP PEPs deploy congestion control systems that treat all of a single subscriber's TCP connections as a single unit, improving fairness and allowing faster reaction to changing network conditions.

Local acknowledgements generated by PEPs speed up TCP slow start by splitting the effective latency, and allow for retransmissions to be done from the PEP rather than from the actual sender, saving downlink bandwidth on retransmissions. Local acknowledgements will also allow a PEP to maintain a local buffer of data appropriate to the actual network conditions, whereas the actual endpoints would often send too much or too little.

A PEP function requires transport-layer fields that allow chunks of data to be identified (e.g., TCP sequence numbers), acknowledgements to be identified (e.g., TCP ack numbers), and acknowledgements to be created from the PEP.

3.5. Bandwidth Aggregation

The Hybrid Access Aggregation Point (HAAP) is a middlebox that allows customers to aggregate the bandwidth of multiple access technologies [I-D.zhang-banana-problem-statement].

One of the approaches uses MPTCP proxies to divide the traffic along multiple paths. The MPTCP proxy operates at the transport layer while being located in the operator's network.

3.6. Prioritization

Bulk traffic may be served with a higher latency than interactive traffic with no reduction in throughput. This fact allows a middlebox function that improves response time in interactive applications by prioritizing interactive transport connections over bulk traffic transport connections. E.g., gaming traffic may be prioritized above email or software updates.

3.7. Measurement-Based Shaping

Basic traffic shaping functionality requires no transport-layer information. All that is needed is a way of mapping each packet to a traffic shaper quota. For example, there may be a rate limit per 5-tuple or per subscriber IP address. However, such fixed traffic shaping rules are wasteful as they end up rate limiting traffic even when the network has free resources available.

More advanced traffic shaping devices use transport layer metrics described in Section 2 to detect congestion on either a per-site or per-user level, and use different traffic shaping rules when congestion is detected. This type of device can overcome limitations of down-stream devices that behave poorly (e.g., by excessive buffering or sub-optimzally dropping packets).

4. IANA Considerations

This memo includes no request to IANA.

5. Security Considerations

5.1. Confidentiality

This document intentionally excludes middleboxes that observe or manipulate application-layer data.

The benefits described in this document can all be implemented without violating confidentiality. However, there is always the question of whether the fields and packet properties used to achieve these benefits may also be used for harm.

In particular, we want to ask what confidentiality is lost by exposing transport-layer fields beyond what can be learned by observing IP-layer fields.

Sequence numbers: an observer can learn how much data is transferred.

Start/Stop indicators: an observer can count transactions for some applications.

Device fingerprinting: an observer may be more easily able to identify a device type when different devices use different default field values or options.

5.2. Active Attacks

Being able to observe sequence numbers or session identifiers may make it easier to modify or terminate a transport connection. E.g., observing TCP sequence numbers allows generation of a RST packet that terminates the connection. However, signing transport fields mitigates this attack. The attack and solution are described for the TCP authentication option [RFC5925].

5.3. More Information Can Improve Security

Proposition: network maintainability and security can be improved by providing firewalls and DDoS mechanisms with some information about transport connections. In contrast, it would be very difficult to secure a network in which every packet appears unique and filled with random bits.

For denial-of-service (DoS) attacks on bandwidth, the receiving end-point is usually on the wrong side of the constrained network link. This fact makes it seem reasonable to give some clues to allow a middlebox device to help out before the constrained link.

E.g., in a blind attack, an attacker cannot receive data from the target of the attack (section of [RFC3552]). In the case of TCP, the blind attacker cannot complete the three-way handshake.

In the balance, some features providing the ability to mitigate/filter attacks and fix broken networks will improve security vs. the scenario when all packets are completely opaque.

6. References

6.1. Normative References

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997.
[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.
[RFC3552] Rescorla, E. and B. Korver, "Guidelines for Writing RFC Text on Security Considerations", BCP 72, RFC 3552, DOI 10.17487/RFC3552, July 2003.
[RFC4737] Morton, A., Ciavattone, L., Ramachandran, G., Shalunov, S. and J. Perser, "Packet Reordering Metrics", RFC 4737, DOI 10.17487/RFC4737, November 2006.
[RFC4787] Audet, F. and C. Jennings, "Network Address Translation (NAT) Behavioral Requirements for Unicast UDP", BCP 127, RFC 4787, DOI 10.17487/RFC4787, January 2007.
[RFC5925] Touch, J., Mankin, A. and R. Bonica, "The TCP Authentication Option", RFC 5925, DOI 10.17487/RFC5925, June 2010.
[RFC7857] Penno, R., Perreault, S., Boucadair, M., Sivakumar, S. and K. Naito, "Updates to Network Address Translation (NAT) Behavioral Requirements", BCP 127, RFC 7857, DOI 10.17487/RFC7857, April 2016.

6.2. Informative References

[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.
[I-D.zhang-banana-problem-statement] Cullen, M., Leymann, N., Heidemann, C., Boucadair, M., Hui, D., Zhang, M. and B. Sarikaya, "Problem Statement: Bandwidth Aggregation for Internet Access", Internet-Draft draft-zhang-banana-problem-statement-03, October 2016.
[RFC2991] Thaler, D. and C. Hopps, "Multipath Issues in Unicast and Multicast Next-Hop Selection", RFC 2991, DOI 10.17487/RFC2991, November 2000.
[RFC3135] Border, J., Kojo, M., Griner, J., Montenegro, G. and Z. Shelby, "Performance Enhancing Proxies Intended to Mitigate Link-Related Degradations", RFC 3135, DOI 10.17487/RFC3135, June 2001.
[RFC3234] Carpenter, B. and S. Brim, "Middleboxes: Taxonomy and Issues", RFC 3234, DOI 10.17487/RFC3234, February 2002.
[RFC5236] Jayasumana, A., Piratla, N., Banka, T., Bare, A. and R. Whitner, "Improved Packet Reordering Metrics", RFC 5236, DOI 10.17487/RFC5236, June 2008.
[RFC7690] Byerly, M., Hite, M. and J. Jaeggli, "Close Encounters of the ICMP Type 2 Kind (Near Misses with ICMPv6 Packet Too Big (PTB))", RFC 7690, DOI 10.17487/RFC7690, January 2016.

Authors' Addresses

David Dolson Sandvine 408 Albert Street Waterloo, ON N2L 3V3 Canada Phone: +1 519 880 2400 EMail:
Juho Snellman Sandvine Seestrasse 5 Zurich, 8002 Switzerland EMail: