DPRIVE Working Group D. Wing
Internet-Draft T. Reddy
Intended status: Informational Cisco
Expires: September 16, 2016 March 15, 2016

DPRIVE TLS/DTLS Message Flows
draft-wing-dprive-profile-and-msg-flows-01

Abstract

Message flows for DNS-over-TLS and DNS-over-DTLS are discussed and compared.

Status of This Memo

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Table of Contents

1. Introduction

The DPRIVE working group has two active documents that provide DNS confidentiality, DNS over DTLS [I-D.ietf-dprive-dnsodtls] and DNS over TLS [I-D.ietf-dprive-dns-over-tls].

This document shows message flows for those two documents. Also shown is how TCP Fast Open (TFO) [RFC7413] eliminates a round-trip, which is especially helpful for DNS communication.

2. Server state lost

This section shows message flows after server state is lost, such as due to routing change (communicating to a different server, unbeknownst to the client) or due to server losing state (crash or software upgrade).

2.1. TLS

With TLS, the client is immediately informed of server state loss with a TCP RST, as shown in the diagram below. This costs one round trip, and this round trip is unavoidable. This is a TCP RST, and is not authenticated. After the RST, a new TCP connection and TLS state are established.

client                                                    server
   |                                                         |
   |<-----------------DPRIVE communications----------------->|
   |                                                         |
   |                         ...                             |
   |                                                         |
   |                                                  (state lost)
   |                                                         |
   |-DNS-over-TLS------------------------------------------->|
   |<------TCP RST-------------------------------------------|
   |--TCP SYN----------------------------------------------->|
   |<-TCP SYNACK---------------------------------------------|
   |--TCP ACK, TLS ClientHello w/Resumption ---------------->|
   |<-TLS ServerHello, ChangeCipherSpec, Finished -----------|
   |--TLS ChangeCipherSpec, Finished, DNS query------------->|
   |<-DNS response-------------------------------------------|
   |                                                         |

Figure 1: Server State Loss, TLS

2.2. DTLS

With DTLS, the client is immediately informed of the server state loss with a DTLS Alert, as shown in the diagram below. This DTLS Alert is not authenticated. This message costs one round trip, but can be avoided if the client anticipates this server state loss and consumes additional packet overhead, as discussed below Figure 2.

 client                                          server
    |                                               |              
    |<-----------DPRIVE communications------------->|
    |                                               |
    |                   ...                         |
    |                                               |
    |                                        (state lost)
    |                                               |
 1. |-----------DPRIVE query----------------------->|
 2. |<----------DTLS Alert--------------------------|
 3. |-DLTS ClientHello w/resumption---------------->|
    |                   ...                         |

Figure 2: Server State Loss, DTLS

An optimization of the above flow is possible, if the client believes the server is likely to have lost state, such as if the most recent DPRIVE communications was a long time ago (exact value of "long time" is debatable). In that situation, the client can send a DTLS handshake with TLS resumption -- effectively, it sends the DTLS handshake identical to packet (3) of Figure 2 (avoiding packets 1 and 2). This packet is larger, though, as it contains the TLS session resumption information. Thus, it is a trade-off of a larger message versus a (possible) round trip savings. This message flow is shown below.

client                                              server
   |                                                   |              
   |<----------DPRIVE communications------------------>|
   |                                                   |
   |                   ...                             |
   |                                                   |
   |                                             (state lost)
   |                                                   |
   |--DTLS ClientHello w/resumption ------------------>|
   |<-DTLS ServerHello, ChangeCipherSpec, Finished-----|
   |--DTLS ChangeCipherSpec, Finished, DNS query------>|
   |<-DNS response-------------------------------------|
   |                   ...                             |  

Figure 3: Server State Loss, DTLS False Start

2.3. TLS 1.3

Session resumption via identifiers and tickets is obsolete in TLS1.3 [I-D.ietf-tls-tls13]. Both methods are replaced by a pre-shared key (PSK) mode. A PSK is established on a previous connection after the handshake is completed, and can then be presented by the client on the next visit.

client                                                    server
   |                                                                        |
   |<-----------------DPRIVE communications-------------------------------->|
   |                                                                        |
   |                         ...                                            |
   |                                                                        |
   |                                                                     (state lost)
   |                                                                        |
   |-DNS-over-TLS---------------------------------------------------------->|
   |<------TCP RST----------------------------------------------------------|
   |--TCP SYN-------------------------------------------------------------->|
   |<-TCP SYNACK------------------------------------------------------------|
   |--TCP ACK, TLS ClientHello+key_share+pre_shared_key-------------------->|
   |<-TLS ServerHello+pre_shared_key, EncryptedExtensions, Finished --------|
   |--TLS Finished--------------------------------------------------------->|
   |<-DNS response----------------------------------------------------------|
   |                                                                        |

Figure 4: Session resumption

3. TCP Fast Open

If the client and server TCP stacks both support TCP Fast Open (TFO) [RFC7413], the TCP 3-way handshake can be done without a round trip, as shown below. Currently, TFO is supported in Linux 3.7 (TCP client and server), iOS 9, and OS X 10.11.

client                                                         server
   |                                                              |
   |<-------------------DPRIVE communications-------------------->|
   |                                                              |
   |                         ...                                  |
   |                                                              |
   |                                                     (state lost)
   |                                                              |
   |-DNS-over-TLS------------------------------------------------>|
   |<------TCP RST------------------------------------------------|
   |--TCP SYN, TLS ClientHello w/Resumption --------------------->|
   |<-TCP SYNACK, TLS ServerHello, ChangeCipherSpec, Finished-----|
   |--TLS ChangeCipherSpec, Finished, DNS query------------------>|
   |<-DNS response------------------------------------------------|

Figure 5: Server State Loss, TLS with TCP FastOpen

4. Probing for Server State Loss

In between normal DNS traffic while the communication to the DNS server is quiescent, the DNS client may want to probe the server to ensure it has maintained cryptographic state. Such probes can also keep alive firewall or NAT bindings. This probing reduces the frequency of needing a new handshake when a DNS query needs to be resolved, improving the user experience at the cost of bandwidth and processing time; cellular devices could even send the probes while in power-save state [I-D.isomaki-rtcweb-mobile].

If the server has lost state, a DTLS (or TLS) handshake needs to be initiated with the server.

4.1. DTLS

A DTLS heartbeat [RFC6520] verifies the server still has DTLS state by returning a DTLS message. If the server has lost state, it returns a DTLS Alert.

4.2. TLS

TLS runs over TCP, so a simple probe is a 0-length TCP packet (a "window probe"). This verifies the TCP connection is still working, which is also sufficient to prove the server has retained TLS state, because if the server loses TLS state it abandons the TCP connection. If the server has lost state, a TCP RST is returned immediately.

5. NAT or Firewall Pinhole Closed

A NAT or Firewall, on the path between the DPRIVE client and DPRIVE server, lose state -- either due to timing out the pinhole, exhausting its resources (and needing to prematurely close the pinhole), or crashing. This differs from the server losing state.

5.1. DTLS

With DTLS, the NAT or firewall will create a new mapping when it sees the new UDP packet. With a NAT, depending on its load (of other traffic) and its implmentation that mapping might be assigned to the same UDP port and IP address as the previous mapping, a different UDP port, and/or a different source IP address. The situation where the same mapping occurs is shown below.

 client            NAT or firewall                server
    |                     |                         |              
    |<-----------DPRIVE communications------------->|
    |                     |                         |
    |                (state loss)                   |
    |                     |                         |
    |-----------DPRIVE query----------------------->|
    |        (new state created in NAT/firewall)    |
    |                     |                         |
    |<----------DPRIVE response---------------------|
    |                   ...                         |

Figure 6: NAT/Firewall State Loss, DTLS

A different mapping can cause the server to reject the communication (DTLS Alert) cause the server to reject the communication (DTLS Alert) if the server was sensative to the client's source address or source port, consuming a round trip. This is shown below.

 client            NAT or firewall                server
    |                     |                         |              
    |<-----------DPRIVE communications------------->|
    |                     |                         |
    |                (state loss)                   |
    |                     |                         |
    |-----------DPRIVE query----------------------->|
    |        (new state created in NAT/firewall)    |
    |                     |                         |
    |<----------DTLS Alert--------------------------| 
    |                     |                         |
    |-DLTS ClientHello w/resumption---------------->|
    |                     |                         |
    |<----------DPRIVE response---------------------|
    |                    ...                        |

Figure 7: NAT/Firewall State Loss, DTLS, changed mapping

5.2. TLS

With a TCP connection when the NAT or firewall has lost state and sees a TCP packet without the SYN bit set, there are several possible reactions by the NAT or firewall:

  • send TCP RST, spoofing the source IP address of the original packet's destination address. This is shown in the following figure.
  • create state. A firewall is unlikely to create state when it sees an in-progress TCP packet, but some NATs may create state. However, if the NAT creates state for a different source TCP port than the previous connection, the server will reject the TCP packet as shown in Figure 5.
 client            NAT or firewall                             server
    |                     |                                        |
    |<-----------DPRIVE communications---------------------------->|
    |                     |                                        |
    |                (state loss)                                  |
    |                     |                                        |
    |----DPRIVE query---->X                                        |
    |        (no state exists for TCP flow)                        |
    |                     |                                        |
    |<---TCP RST----------|                                        |
    |                     |                                        |
  (client does            |                                        |
  TLS re-establishment with TCP FastOpen)                          |
    |                     |                                        |
    |--TCP SYN, TLS ClientHello w/Resumption --------------------->|
    |<-TCP SYNACK, TLS ServerHello, ChangeCipherSpec, Finished-----|
    |--TLS ChangeCipherSpec, Finished, DNS query------------------>|
    |<-DNS response------------------------------------------------|
    |                     |                          |

Figure 8: NAT/Firewall State Loss, TLS with TCP FastOpen

 client            NAT or firewall                server
    |                     |                          |              
    |<-----------DPRIVE communications-------------->|
    |                     |                          |
    |                (state loss)                    |
    |                     |                          |
    |----DPRIVE query---->X                          |
    |        (no state exists for TCP flow)          |
    |                     |                          |
    |<---TCP RST----------|                          |
    |                     |                          |
  (client does normal     |                          |
  TLS re-establishment)   |                          |
    |                     |                          |
    |--TCP SYN-------------------------------------->|
    |<-TCP SYNACK------------------------------------|
    |--TCP ACK, TLS ClientHello w/Resumption ------->|
    |<-TLS ServerHello, ChangeCipherSpec, Finished --|
    |--TLS ChangeCipherSpec, Finished, DNS query---->|
    |<-DNS response----------------------------------|
    |                     |                          |

Figure 9: NAT/Firewall State Loss, TLS

6. IANA Considerations

None.

7. Acknowledgements

Authors would like to thank Allison Mankin for comments and review.

8. References

8.1. Normative References

[I-D.ietf-dprive-dns-over-tls] Zi, Z., Zhu, L., Heidemann, J., Mankin, A., Wessels, D. and P. Hoffman, "Specification for DNS over TLS", Internet-Draft draft-ietf-dprive-dns-over-tls-07, March 2016.
[I-D.ietf-dprive-dnsodtls] Reddy, T., Wing, D. and P. Patil, "DNS over DTLS (DNSoD)", Internet-Draft draft-ietf-dprive-dnsodtls-04, January 2016.

8.2. Informative References

[I-D.ietf-tls-tls13] Rescorla, E., "The Transport Layer Security (TLS) Protocol Version 1.3", Internet-Draft draft-ietf-tls-tls13-11, December 2015.
[I-D.isomaki-rtcweb-mobile] Isomaki, M., "RTCweb Considerations for Mobile Devices", Internet-Draft draft-isomaki-rtcweb-mobile-00, July 2012.
[RFC6520] Seggelmann, R., Tuexen, M. and M. Williams, "Transport Layer Security (TLS) and Datagram Transport Layer Security (DTLS) Heartbeat Extension", RFC 6520, DOI 10.17487/RFC6520, February 2012.
[RFC7413] Cheng, Y., Chu, J., Radhakrishnan, S. and A. Jain, "TCP Fast Open", RFC 7413, DOI 10.17487/RFC7413, December 2014.

Authors' Addresses

Dan Wing Cisco Systems, Inc. 170 West Tasman Drive San Jose, California 95134 USA EMail: dwing@cisco.com
Tirumaleswar Reddy Cisco Systems, Inc. Cessna Business Park, Varthur Hobli Sarjapur Marathalli Outer Ring Road Bangalore, Karnataka 560103 India EMail: tireddy@cisco.com