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.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Server state lost . . . . . . . . . . . . . . . . . . . . . . 2
2.1. TLS . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
2.2. DTLS . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.3. TLS 1.3 . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. TCP Fast Open . . . . . . . . . . . . . . . . . . . . . . . . 5
4. Probing for Server State Loss . . . . . . . . . . . . . . . . 6
4.1. DTLS . . . . . . . . . . . . . . . . . . . . . . . . . . 6
4.2. TLS . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
5. NAT or Firewall Pinhole Closed . . . . . . . . . . . . . . . 6
5.1. DTLS . . . . . . . . . . . . . . . . . . . . . . . . . . 6
5.2. TLS . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 9
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 9
8.1. Normative References . . . . . . . . . . . . . . . . . . 9
8.2. Informative References . . . . . . . . . . . . . . . . . 10
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 10
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.
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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
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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.
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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
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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.
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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:
o send TCP RST, spoofing the source IP address of the original
packet's destination address. This is shown in the following
figure.
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o 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
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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", draft-
ietf-dprive-dns-over-tls-07 (work in progress), March
2016.
[I-D.ietf-dprive-dnsodtls]
Reddy, T., Wing, D., and P. Patil, "DNS over DTLS
(DNSoD)", draft-ietf-dprive-dnsodtls-04 (work in
progress), January 2016.
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8.2. Informative References
[I-D.ietf-tls-tls13]
Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", draft-ietf-tls-tls13-11 (work in progress),
December 2015.
[I-D.isomaki-rtcweb-mobile]
Isomaki, M., "RTCweb Considerations for Mobile Devices",
draft-isomaki-rtcweb-mobile-00 (work in progress), 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,
<http://www.rfc-editor.org/info/rfc6520>.
[RFC7413] Cheng, Y., Chu, J., Radhakrishnan, S., and A. Jain, "TCP
Fast Open", RFC 7413, DOI 10.17487/RFC7413, December 2014,
<http://www.rfc-editor.org/info/rfc7413>.
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
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