Network Working Group A. Langley
Internet-Draft Google Inc
Expires: February 6, 2009 August 5, 2008
Faster application handshakes with SYN/ACK payloads
draft-agl-tcpm-sadata-01
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Abstract
This document advocates the usage of small, mostly constant payloads
in the SYN+ACK frame of the 3-way TCP [RFC0793] handshake. We show
how this can have immediate benefits for some protocols.
Additionally, we describe a new TCP option that enables a wider range
of protocols to gain from it.
Table of Contents
1. Requirements Notation . . . . . . . . . . . . . . . . . . . . 3
2. Changes since 00 . . . . . . . . . . . . . . . . . . . . . . . 4
3. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 5
4. Example One: Opportunistic HTTP encryption . . . . . . . . . . 7
5. Example Two: Faster SSH connections . . . . . . . . . . . . . 9
6. Example Three: Compressed HTTP headers . . . . . . . . . . . . 11
7. The SYNACK Payload Processed Option . . . . . . . . . . . . . 12
8. Security Considerations . . . . . . . . . . . . . . . . . . . 15
9. Comparison to T/TCP . . . . . . . . . . . . . . . . . . . . . 16
10. Middlebox Interactions . . . . . . . . . . . . . . . . . . . . 17
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 18
12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 19
13. References . . . . . . . . . . . . . . . . . . . . . . . . . . 20
13.1. Normative References . . . . . . . . . . . . . . . . . . 20
13.2. Informative References . . . . . . . . . . . . . . . . . 20
Appendix A. Changes . . . . . . . . . . . . . . . . . . . . . . . 23
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 24
Intellectual Property and Copyright Statements . . . . . . . . . . 25
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1. Requirements Notation
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].
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2. Changes since 00
To be removed by the RFC Editor before publication.
o Greatly expanded on the introduction
o Fixed the wording around retransmissions which mistakenly
suggested that no packets of any type could be transmitted without
payloads.
o Renamed the flag to SYNACK Payloads Processed.
o Required that the flag be echoed in resulting SYNACK frames.
o Added discussion of simultaneous open.
o Added discussion of SYNACKs with payloads that are nothing to do
with this spec, noting that they are still permitted.
o Changed the option to a standard, 2 byte, flags option.
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3. Introduction
At the current time, almost no stacks will send payloads in the
SYNACK frame of a TCP handshake even though RFC 793 [RFC0793] permits
it. This springs from a handful of reasons:
1. Processing time for a SYN must be minimal to mitigate the effects
of SYN floods. Even waking up an application to process a SYN
would greatly increase the costs.
2. Replies to SYNs must be small, otherwise it provides a way to
amplify a DDoS attacks using false source IP addresses.
3. The ubiquitous sockets API doesn't make it easy to do so.
This document proposes that a semi-constant payload (a payload such
that it's trivial for the kernel to compute) overcomes the first and
third reasons. Additionally, limiting that payload to 64-bytes
overcomes the second.
There are protocols that could immediately benefit from a gradual
deployment of hosts which supported a "setsockopt" to set a constant
payload and hosts that would ACK and enqueue such a payload. SMTP
would be one such protocol: clients wait for a 200 code banner from
the server before starting their part of the exchange and the banner
is small and constant. SMTP is also a protocol which ends up making
many, short lived connections.
Since such behaviour is already permitted by TCP it requires no
standards work. It would also be easy to deploy. Active open hosts
that don't enqueue payloads in SYNACK frames will ACK only the SYN
flag and the passive open host then knows to retransmit the payload
immediately after.
However, this common lack of carrying data in SYNACK frames, and the
sockets API which reflects it, has guided the design of many
application layer protocols. These protocols are often designed such
that:
1. The client starts the exchange. For example, the first
application layer bytes sent on an HTTP connection are the
client's request.
2. The exchange is large since there is little space pressure. SSH
algorithm agreement uses strings like
""diffie-hellman-group14-sha1"" (28 bytes) because of this.
In these cases we suggest that, had a general ability to send
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payloads in SYNACK frames existed at the time that these protocols
were written, they may have ended up differently. However, the
ability for a passive open host to send a payload with no latency
overhead is of value: we outline three motivating examples in next
sections.
Modifications to take advantage of SYNACK payloads would then require
changes to the application level protocol. This could be managed by
assigning new ports, trying connections on the new ports first,
backing off etc. However, given that SYNACK payloads are partly a
latency optimisation, that would utterly negate any gains.
Because of this, we also describe a TCP option that lets the
application layer on both sides know that their respective stacks
support at least the limited SYNACK payloads described herein, and
also to agree to use an alternative protocol which takes advantage of
it.
Fundamentally, any protocol which used payloads in SYNACK frames
could achieve the same effect without them, at the cost of an extra
round trip. Thus, this should only be used where latency is
important. None the less, the advantage of avoiding a round trip
should not be discounted. Round trip times are often in excess of
100ms for distant hosts, or in poorly networked areas of the world.
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4. Example One: Opportunistic HTTP encryption
Here we assume that both HTTP client and server implement this
specification.
The client, before calling "connect", calls "setsockopt" to instruct
the kernel to include an option to advertise support for this
specification. The server has already configured its listening
socket to include a Diffie-Hellman public value in the SYNACK
payloads elicited from SYN frames carrying this option.
Additionally, the server's stack generates an 8-byte random nonce and
includes it in the payload.
The client is aware that the server implements this specification
because the advertising option is echoed back in the SYNACK frame.
Thus, it expects to read the nonce and public value from the
connection. It then sends its own nonce and public value to the
server. Both sides can calculate a shared key and use a cipher to
encrypt the remaining data in both directions.
Choosing the correct cryptographic primitives can make this
particularly cheap. Curve25519 [curve25519] is an elliptic curve
Diffie-Hellman function that can be calculated in 240 microseconds on
a 2.33GHz Intel Core2. Salsa20/8 [salsa20] is a stream cipher that
can encrypt data in 2 cycles/byte on the same hardware.
The resulting key could also be used to establish integrity using the
forthcoming TCP Auth Option specification [1].
This example demonstrates a number of salient features of this
specification:
o By using the correct primitive (curve25519), a constant payload
can be used to establish cryptographic connections.
o We can add significant extensions to latency sensitive protocols
without affecting latency. Previous attempts [RFC2817] to do the
same have required an extra round trip weather the server side
supported the protocol or not.
o We can do in a backwards compatible fashion, affording a gradual
deployment.
The above is cursorary in order not to distract from the topic of
this document, however enquiring readers are welcome to continue
reading this section if they still have questions.
*Why Elliptic Curves?*: The payload must be short otherwise SYN-
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floods could use this as an amplification to backscatter DDoS another
host. The reduced computation cost (as compared to Diffie-Hellman
over a multiplicative finite field) is very nice.
Most importantly, curve25519 is specifically designed to allow a
constant public value to be used for multiple key agreements. If a
new public value had to be generated for every SYN, not only would
the stack have to be able to perform that operation, a SYN flood
would be very effective.
*Can't the client's public value fit in the SYN?*: A SYN generally
has twenty bytes of free option space these days. (We can't use the
payload space in a SYN). Since we wouldn't want to define the last
option ever, we need to leave four bytes spare. Two bytes for the
option header means fourteen bytes (or 112 bits) for the public
value. The closest prime is then 2^112-75.
The best, general algorithm currently known for breaking the Diffie-
Hellman problem on elliptic curves is Pollard's Rho. The work
involved in this attack is sqrt(n), which is 2^56 in this case.
Critically, once you have solved a single instance you can precompute
tables to speed up breaking more instances. With a petabyte of
storage, you could break 112-bit curves in only 2^12 operations.
*Can't a smaller field be used?*: Some speedup could be gained by
using an elliptic curve with a field size around 200 bits. However
the effort of defining such a curve is pretty huge. The standard
NIST curves around that size are slower than curve25519 [2].
*What about man-in-the-middle attacks*: All opportunistic schemes are
open to man-in-the-middle and downgrade attacks. This is no
exception, it's a trade off and for real security, TLS [RFC4346]
should be used. It has been suggested that the HTTP server include a
header in replies giving a URL on the same domain, using the "https"
scheme, which contains the server's public value and an expiry time.
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5. Example Two: Faster SSH connections
SSH [RFC4253] connection latency is a small, but quotidian
frustration for those who use it. Current efforts to address it
involve multiplexing interactive sessions over long-term, persistent
connections.
Consider the following, diagrammatic representation of the beginning
of an SSH [RFC4253] connection:
0 SYN ------------> 0.5
1 <------------ SYNACK 0.5
1 Ident ------------> 1.5
1 NList ------------> 1.5
2 <------------ Ident 1.5
2 <------------ NList 1.5
2 <------------ KX 1.5
2 KX ------------> 2.5
Key:
Ident: A string which contains the SSH implementation name
NList: Name list: the list of supported algorithms
KX: Key exchange data, usually Diffie-Hellman
Standard SSH protocol
Figure 1
Here, arrows from the left to the right are frames from client to
server. Times on the left are the times that the client either
transmits or receives a packet (and vice versa). Times are measured
in round trip times (RTT), so that it takes 0.5 units for a frame to
pass between the hosts.
The above diagram is for a latency tuned implementation of SSH,
specifically, the client doesn't wait for the server's identity
string to be received. And yet, in this ideal scenario, the client
can only start transmitting useful data after 2 RTT and the server
can only start transmitting after 2.5 RTT. As a rule of thumb, the
RTT from San Francisco to London is 150ms, so this means a 300ms
latency, at least, when setting up this connection.
(To keep the discussion simple, we assume there is no packet loss,
that the path is symmetrical and that the client's ACK of the 3-way
handshake carries a data payload.)
Now, let us consider the situation when SYNACK payloads are
available. First we compact the name-list (which is part of the
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algorithm negotiation) and put it in the SYNACK.
0 SYN ------------> 0.5
1 <------------ SA+NList 0.5
1 NList ------------> 1.5
1 KX ------------> 1.5
2 <------------ KX 1.5
SSH protocol with a compact name list carried in the SYN+ACK frame
Figure 2
In this situation, the client knows the results of the algorithm
negotiation as soon as the SYNACK comes back and can include the
correct key exchange with the first ACK packet. This reduces the
server's latency by a full RTT since it can transmit as soon as the
3-way handshake completes.
As a final optimisation, we could assume either that the server takes
a successful guess at the key exchange algorithm to use, or that the
application level protocol specifies a single key exchange algorithm:
0 SYN ------------> 0.5
1 <------------ SA+KX 0.5
1 KX ------------> 1.5
A protocol which includes key exchange information in the SYN+ACK
frame.
Figure 3
Here the client's latency is 1 RTT and the server's is 1.5 RTT, which
is equal to the minimum required by the 3-way handshake, saving a
full RTT of latency from the initial diagram.
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6. Example Three: Compressed HTTP headers
So that all the examples aren't cryptography based, we consider a
third example.
There are many HTTP resources that are very small, or even empty.
Consider that clicking on Google results involves requesting a
resource from the Google server to redirect to the true result. Or
OCSP [RFC2560] revocation servers which serve small ASN.1 documents.
For these services the size of HTTP headers might dominate the
bandwidth requirements: Firefox 3 transmits over 350 bytes to request
the shortest URL possible ("/")
HTTP headers, however are highly compressible. They are highly
structured, and many strings are very common (such as "Keep-Alive").
Careful examination of the current patterns in both client requests
and server replies would probably yield a range coding [rangecoding]
model that achieved significant savings.
However, there is no easy method to deploy such a scheme. Obviously
the first client request on a connection could not use a scheme. A
server could advertise support in its reply headers for subsequent
requests on the same connection, although that could only affect
requests that haven't already been pipelined.
A SYNACK payload could serve to advertise support for this, and any
other extensions, allowing every request on a connection to use such
a scheme when both ends support it.
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7. The SYNACK Payload Processed Option
Alternative application protocols that take advantage of data in a
SYNACK frame necessarily require the application level to know when
this specification is in effect. To that end, we define an option
which signifies compliance with this specification to be carried in
the SYN and SYNACK frames:
1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+---------------+---------------+
| Kind = | Length = 2 |
| TDB-IANA-KIND1| |
+---------------+---------------+
SYNACK Payload Processed Option
Figure 4
It is required that both endpoints reach agreement about when this
option is in effect since it affects the application layer. The next
five paragraphs deal with this. This specification considers the
option to be an optimisation however, and a valid agreement might be
that the option is not in effect even in the case that both endpoints
support it. This is to allow implementations to back off in the case
of possible middleware interactions and overload.
Hosts MUST NOT include the SYNACK Payload Processed option unless an
application has requested it for the current socket. If SYNACK
Payload Processed is requested for a socket, the host SHOULD include
the SYNACK Payload Processed option. For example, it may choose not
to in the case of having to retransmit the SYN frame as middleware
may be filtering the extra option.
Upon receipt of a SYN frame with a SYNACK Payload Processed option,
to a valid, passive open socket, that socket will have either been
configured by an application to take advantage of this specification
or not. In the case that it has not, the host MUST NOT include the
SYNACK Payload Processed option in any SYNACK. In the case that it
has been so configured, the host SHOULD include the configured
payload in the SYNACK. Iff it chooses to do so, it MUST include the
SYNACK Payload Processed option.
For a given connection, for all resulting SYNACK frames, the presence
of the SYNACK Payload Processed option MUST NOT differ.
If a host has alternative mechanisms which involve sending payloads
in a SYNACK frame, they MUST NOT be used concurrently with this
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specification for a given connection. This specification does not
prohibit SYNACK frames with payloads generated by other means as long
as the SYNACK Payload Processed option is not included.
It's expected that a host will make a best effort to include a SYNACK
payload when the application has set one. It may choose not to for a
number of reasons including: the SYN frame didn't request it, the
host is under heavy SYN load, is using SYN cookies or that the host
is having to retransmit the SYNACK.
The next four paragraphs seek to establish a minimal basis for
application protocols to build upon. An implementation may allow
applications to set arbitrary payloads on a per connection basis, but
we expect that most will wish to expose a more limited scope.
Obviously some of these capabilities, such as the inclusion of random
bytes, are motivated by the examples above.
In the case that the SYNACK Payload Processed option is in effect:
The data payload MUST affect the SEQ/ACK numbers like any other data.
Any ACK frame resulting from such a SYNACK frame MUST acknowledge the
whole SYNACK frame, including the SYN flag. If a frame is the final
ACK in a 3-way handshake, a host MUST reject it unless it
acknowledges the whole SYNACK frame.
A host MUST provide a method for applications to set a SYNACK
payload, to determine if a passive-open connection sent a SYNACK
payload and to determine if an active open connection received the
SYNACK Payload Processed option in the SYNACK frame.
A host MUST support configuring passive open sockets with at least
64-bytes of data. (See "Security Considerations", below).
A host SHOULD support including at least 8 random bytes in the SYNACK
payload, at any arbitrary (but within range) byte offset. If it
does, the random bytes MUST be consistent between retransmissions of
the SYNACK frame and the host MUST support a method for the
application to learn the value of the random bytes included in any
resulting connection.
What follows is clarification on some corner cases:
In the case of a simultaneous open where one or both SYN frames
include the SYNACK Payload Processed flag, this specification is not
in effect. The connection continues as usual.
In the case of a frame carrying the SYNACK Payload Processed option
and with both SYN and FIN flags set, the host MAY support this
specification. In practice, many stacks with ignore a FIN flag and
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any payload in a SYN frame, in which case such a packet is no
different from any other SYN frame.
In the case that the MTU makes transmitting the larger SYNACKs
problematic, the host MAY choose to fragment the packet or it MAY
choose not to echo the SYNACK Payload Processed option, resulting in
a smaller SYNACK frame.
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8. Security Considerations
Any payload in a SYNACK packet must be as frugal as possible since a
host will be transmitting it to an unconfirmed address. If a 40 byte
frame could elicit a 1500 byte reply to an attacker controlled
address, this would be readily used to hide and amplify distributed
denial of service attacks.
Thus we specify a maximum size of 64 bytes for the payload. This is
sufficient to include a strong elliptic curve key (256 bits), a 64-
bit nonce and a small amount of overhead (24 bytes).
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9. Comparison to T/TCP
The idea of including data in frames which also carry a SYN flag
isn't new: it was included in the experimental T/TCP RFCs 1379
[RFC1379] and 1644 [RFC1644]. T/TCP suffered because it broke the
assumption that the source address of a new connection from a
passive-open socket had been verified by a 3-way handshake. This was
a critical security issue for applications like RSH which often used
source address whitelists.
This draft doesn't break any such assumptions that applications may
be depending on. Source addresses for new connections are still
validated by a 3-way handshake for passive-open sockets.
Additionally, this draft is dramatically simpler than T/TCP: it
doesn't introduce any additional TCP states nor does it deal with the
complexity of including payloads in a SYN frame. Nor does this draft
apply to any application which is unaware of it since applications
are required to explicitly configure SYNACK payloads before they come
into effect.
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10. Middlebox Interactions
The large number of middleboxes (firewalls, proxies, protocol
scrubbers, etc) currently present in the Internet pose some
difficulty for deploying new TCP options. Some firewalls may block
segments that carry unknown options. For instance, if the flags
option is not understood by a firewall, incoming SYNs advertising
SYNACK payload support may be dropped, preventing connection
establishment. This is similar to the ECN blackhole problem, where
certain faulty hosts and routers throw away packets with ECN bits set
[RFC3168]. Some recent results indicate that for new TCP options,
this may not be a significant threat, with only 0.2% of web requests
failing when carrying an unknown option [transport-middlebox].
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11. IANA Considerations
This document requires IANA to update values in its registry of TCP
options numbers to assign a new entry, referred herein as
"TBD-IANA-KIND1".
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12. Acknowledgements
Wesley Eddy kindly reviewed initial versions of this draft.
Joe Touch provided many helpful comments.
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13. References
13.1. Normative References
[RFC0793] Postel, J., "Transmission Control Protocol", STD 7,
RFC 793, September 1981.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
13.2. Informative References
[RFC1379] Braden, B., "Extending TCP for Transactions -- Concepts",
RFC 1379, November 1992.
[RFC1644] Braden, B., "T/TCP -- TCP Extensions for Transactions
Functional Specification", RFC 1644, July 1994.
[RFC2560] Myers, M., Ankney, R., Malpani, A., Galperin, S., and C.
Adams, "X.509 Internet Public Key Infrastructure Online
Certificate Status Protocol - OCSP", RFC 2560, June 1999.
[RFC2817] Khare, R. and S. Lawrence, "Upgrading to TLS Within
HTTP/1.1", RFC 2817, May 2000.
[RFC3168] Ramakrishnan, K., Floyd, S., and D. Black, "The Addition
of Explicit Congestion Notification (ECN) to IP",
RFC 3168, September 2001.
[RFC4253] Ylonen, T. and C. Lonvick, "The Secure Shell (SSH)
Transport Layer Protocol", RFC 4253, January 2006.
[RFC4346] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.1", RFC 4346, April 2006.
[curve25519]
Bernstein, D., "Curve25519: new Diffie-Hellman speed
records".
[salsa20] Bernstein, D., "Salsa20/8 and Salsa20/12".
[transport-middlebox]
Medina, A., Allman, M., and S. Floyd, "Measuring
Interactions Between Transport Protocols and Middleboxes",
ACM SIGCOMM/USENIX Internet Measurement Conference,
October 2004.
[rangecoding]
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Martin, G., "Range encoding: an algorithm for removing
redundancy from a digitized message", Video and Data
Recording Conference, July 1979.
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URIs
[1] <http://www.ietf.org/internet-drafts/
draft-ietf-tcpm-tcp-auth-opt-01.txt>
[2] <http://cr.yp.to/ecdh/reports.html>
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Appendix A. Changes
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Author's Address
Adam Langley
Google Inc
Email: agl@imperialviolet.org
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