TLS/DTLS Optimizations for Internet of Things Deployments
draft-fossati-tls-iot-optimizations-00

Abstract

Internet protocols work well in a variety of environments, including Internet of Things (IoT) deployments. While there are some optimization possibilities to reduce code size, bandwidth utilization, and to improve battery lifetime, in general most Internet protocols are also applicable to constrained environments. TLS and DTLS are two such security protocols that can be used by many IoT devices since DTLS/TLS provide lot of flexiblity in terms credential choice, ciphersuite usage, etc. The DICE working group has developed a specification that profiles the use of TLS and DTLS for IoT environments, without changing the TLS/DTLS specifications.

This memo goes a step further and proposes changes to the DTLS/TLS protocol to introduce further optimizations. Since the ongoing work on TLS/DTLS 1.3 already offers several improvements (compared to previous versions) this document focuses on the use of version 1.3 and suggests further optimizations.

Status of This Memo

This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.

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This Internet-Draft will expire on January 9, 2017.

Copyright Notice

Copyright (c) 2016 IETF Trust and the persons identified as the document authors. All rights reserved.

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Table of Contents

• 1. Introduction
• 2. Requirements Language
• 3. Selective Fragment Retransmission
• 3.1. Problem Statement
• 3.2. Proposal
• 4. Transport Agnostic Security Associations
• 4.1. Problem Statement
• 4.2. Proposal
• 5. Reducing the DTLS Record Layer Header Overhead
• 5.1. Problem Statement
• 5.2. Proposal
• 6. Reducing Buffers
• 6.1. Problem Statement
• 6.2. Proposal
• 7. Acknowledgements
• 8. Security Considerations
• 9. References
• 9.1. Normative References
• 9.2. Informative References
• Authors' Addresses

1. Introduction

Internet protocols work well in a variety of environments, including Internet of Things (IoT) deployments. While there are some optimization possibilities to reduce code size, bandwidth utilization, and to improve battery lifetime, in general most Internet protocols are also applicable to constrained environments. TLS and DTLS are two such security protocols that can be used by many IoT devices since DTLS/TLS provide lot of flexiblity in terms credential choice, ciphersuite usage, etc. The DICE working group has developed a specification that profiles the use of TLS and DTLS for IoT environments, without changing the TLS/DTLS specifications.

This memo goes a step further and proposes changes to the DTLS/TLS protocol to introduce further optimizations. Since the ongoing work on TLS/DTLS 1.3 [I-D.ietf-tls-tls13] already offers several improvements (compared to previous versions) this document focuses on the use of version 1.3 and suggests further optimizations.

This document discusses four extensions, namely:

Selective Fragment Retransmission:

This extension improves retransmissions of lost handshake packets.

Transport Agnostic Security Associations:

Changes to a connection (such as an IP address change) requires a new handshake. This extension introduces a transport independent identifier.

Reducing the DTLS Record Layer Header Overhead:

This extension changes the record layer format to reduce the overhead.

Reducing Buffers:

This extension allows a DTLS/TLS server running on a constrained node to indicate its buffer size.

2. 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].

3. Selective Fragment Retransmission

3.1. Problem Statement

The unit of retransmission used by the DTLS handshake is a whole flight (see Section 4.2.4 of [RFC6347]. If the underlying media is inherently lossy, or shows high latency variance that might fire spurious retransmission, a single fragment that gets lost or is excessively delayed will force the whole flight to be retransmitted.

This is further exacerbated when the effective MTU is very low and therefore handshake messages have higher probability to be fragmented. For example, IEEE 802.15.4 networks have a 128-byte MTU size. In such an environment a very "ordinary" TLS_ECDHE_RSA_WITH_AES_256_CBC_SHA negotiation can take up to 30 individual fragments, 2/3 of which are sent in flight 4. The loss of a single fragment in flight 4 implies a retransmission that is 20x the magnitude of the original loss.

The retransmission timer settings suggested in Section 11 of [I-D.ietf-dice-profile] offer mitigation for the spurious retransmit issue and, in general, help with congestion. However, they do not solve the retransmission of the entire flight.

3.2. Proposal

A potential solution is to add a NACK-based retransmission scheme to the DTLS handshake and the granularity of retransmission would be a message fragment. We note that each fragment in a DTLS handshake is effectively associated to a unique identifier defined by the tuple Handshake.{message_seq,fragment_offset,fragment_length} that can be used in the NACK report to identify the exact geometry of the missing data in the current flight, together with the right-most received byte.

4. Transport Agnostic Security Associations

4.1. Problem Statement

In DTLS, the security context demultiplexing is done via the 5-tuple. This implies that the associated DTLS context needs to be re-negotiated from scratch whenever the IP address changes. For example, when moving the network attachment from WLAN to a cellular connection, or when the IP address of the IoT devices changes during a sleep cyle. A NAT device may also modify the source UDP port after an idle period. In such situations, there is not enough information in the DTLS record header for a DTLS server, which handles multiple clients, to associate the changed address to an existing client.

4.2. Proposal

A potential solution is to add the ability to negotiate, at handshake time, a transport independent identifier that is unique per security association. We call this identifier the 'Connection ID (CID)' in Figure 1. It decouples the DTLS session from the underlying transport protocol allowing the same DTLS security association to be migrated across different transport sessions.

            
                                     00
                                     /\
                                     :
 IP                    UDP           : DTLS Record Header
 +-----+-----+-------+ +-----+-----+ : +---------+-------+------
 | src | dst | proto | | src | dst | : | Seq#i   |  CID  | ...
 +-----+-----+-------+ +-----+-----+ : +---------+-------+------
 `----------------+----------------' :              ^
                   `................ : .............'
  <Handover event>                   :
                   GSM-SMS           : DTLS Record Header
                   +-------+-------+ : +---------+-------+-----
                   | tp-oa | tp-da | : | Seq#i+1 |  CID  | ...
                   +-------+-------+ : +---------+-------+-----
                                     :
                                     \/
                                     00
            
          

            
      struct {
           ContentType type;
           ProtocolVersion version;
           uint16 epoch;
           uint48 sequence_number;
           uint32 connection_id;                          // New field
           uint16 length;
           opaque fragment[DTLSPlaintext.length];
      } DTLSPlaintext;