Internet Engineering Task Force (IETF) Y. Cheng
Request for Comments: 7413 J. Chu
Category: Experimental S. Radhakrishnan
ISSN: 2070-1721 A. Jain
December 2014 TCP Fast Open
This document describes an experimental TCP mechanism called TCP Fast
Open (TFO). TFO allows data to be carried in the SYN and SYN-ACK
packets and consumed by the receiving end during the initial
connection handshake, and saves up to one full round-trip time (RTT)
compared to the standard TCP, which requires a three-way handshake
(3WHS) to complete before data can be exchanged. However, TFO
deviates from the standard TCP semantics, since the data in the SYN
could be replayed to an application in some rare circumstances.
Applications should not use TFO unless they can tolerate this issue,
as detailed in the Applicability section.
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for examination, experimental implementation, and
This document defines an Experimental Protocol for the Internet
community. This document is a product of the Internet Engineering
Task Force (IETF). It represents the consensus of the IETF
community. It has received public review and has been approved for
publication by the Internet Engineering Steering Group (IESG). Not
all documents approved by the IESG are a candidate for any level of
Internet Standard; see Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
Copyright (c) 2014 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
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction ....................................................31.1. Terminology ................................................42. Data in SYN .....................................................42.1. Relaxing TCP Semantics on Duplicated SYNs ..................42.2. SYNs with Spoofed IP Addresses .............................53. Protocol Overview ...............................................54. Protocol Details ................................................74.1. Fast Open Cookie ...........................................74.1.1. Fast Open Option ....................................84.1.2. Server Cookie Handling ..............................84.1.3. Client Cookie Handling ..............................220.127.116.11. Client Caching Negative Responses .........104.2. Fast Open Protocol ........................................114.2.1. Fast Open Cookie Request ...........................114.2.2. TCP Fast Open ......................................125. Security Considerations ........................................145.1. Resource Exhaustion Attack by SYN Flood with Valid
Cookies ...................................................145.1.1. Attacks from behind Shared Public IPs (NATs) .......155.2. Amplified Reflection Attack to Random Host ................166. TFO Applicability ..............................................176.1. Duplicate Data in SYNs ....................................176.2. Potential Performance Improvement .........................176.3. Example: Web Clients and Servers ..........................186.3.1. HTTP Request Replay ................................186.3.2. HTTP over TLS (HTTPS) ..............................186.3.3. Comparison with HTTP Persistent Connections ........186.3.4. Load Balancers and Server Farms ....................19
7. Open Areas for Experimentation .................................197.1. Performance Impact Due to Middleboxes and NAT .............197.2. Impact on Congestion Control ..............................207.3. Cookie-less Fast Open .....................................208. Related Work ...................................................208.1. T/TCP .....................................................208.2. Common Defenses against SYN Flood Attacks .................218.3. Speculative Connections by the Applications ...............218.4. Fast Open Cookie-in-FIN ...................................218.5. TCP Cookie Transaction (TCPCT) ............................219. IANA Considerations ............................................2210. References ....................................................2210.1. Normative References .....................................2210.2. Informative References ...................................23Appendix A. Example Socket API Changes to Support TFO .............25A.1. Active Open .................................................25A.2. Passive Open ................................................25
Authors' Addresses ................................................261. Introduction
TCP Fast Open (TFO) is an experimental update to TCP that enables
data to be exchanged safely during TCP's connection handshake. This
document describes a design that enables applications to save a round
trip while avoiding severe security ramifications. At the core of
TFO is a security cookie used by the server side to authenticate a
client initiating a TFO connection. This document covers the details
of exchanging data during TCP's initial handshake, the protocol for
TFO cookies, potential new security vulnerabilities and their
mitigation, and the new socket API.
TFO is motivated by the performance needs of today's Web
applications. Current TCP only permits data exchange after the
three-way handshake (3WHS) [RFC793], which adds one RTT to network
latency. For short Web transfers this additional RTT is a
significant portion of overall network latency, even when HTTP
persistent connection is widely used. For example, the Chrome
browser [Chrome] keeps TCP connections idle for up to 5 minutes, but
35% of HTTP requests are made on new TCP connections [RCCJR11]. For
such Web and Web-like applications, placing data in the SYN can yield
significant latency improvements. Next we describe how we resolve
the challenges that arise upon doing so.
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].
"TFO" refers to TCP Fast Open. "Client" refers to TCP's active open
side, and "server" refers to TCP's passive open side.
2. Data in SYN
Standard TCP already allows data to be carried in SYN packets
([RFC793], Section 3.4) but forbids the receiver from delivering it
to the application until the 3WHS is completed. This is because
TCP's initial handshake serves to capture old or duplicate SYNs.
To enable applications to exchange data in a TCP handshake, TFO
removes the constraint and allows data in SYN packets to be delivered
to the application. This change to TCP semantic raises two issues
(discussed in the following subsections) that make TFO unsuitable for
Therefore, TCP implementations MUST NOT use TFO by default, but only
use TFO if requested explicitly by the application on a per-service-
port basis. Applications need to evaluate TFO applicability as
described in Section 6 before using TFO.
2.1. Relaxing TCP Semantics on Duplicated SYNs
TFO allows data to be delivered to the application before the 3WHS is
completed, thus opening itself to a data integrity issue in either of
the two cases below:
a) the receiver host receives data in a duplicate SYN after it has
forgotten it received the original SYN (e.g., due to a reboot);
b) the duplicate is received after the connection created by the
original SYN has been closed and the close was initiated by the
sender (so the receiver will not be protected by the TIME-WAIT 2
The now obsoleted T/TCP [RFC1644] (obsoleted by [RFC6247]) attempted
to address these issues. It was not successful and not deployed due
to various vulnerabilities as described in Section 8, "Related Work".
Rather than trying to capture all dubious SYN packets to make TFO
100% compatible with TCP semantics, we made a design decision early
on to accept old SYN packets with data, i.e., to restrict TFO use to
a class of applications (Section 6) that are tolerant of duplicate
SYN packets with data. We believe this is the right design trade-
off: balancing complexity with usefulness.
2.2. SYNs with Spoofed IP Addresses
Standard TCP suffers from the SYN flood attack [RFC4987] because SYN
packets with spoofed source IP addresses can easily fill up a
listener's small queue, causing a service port to be blocked
TFO goes one step further to allow server-side TCP to send up data to
the application layer before the 3WHS is completed. This opens up
serious new vulnerabilities. Applications serving ports that have
TFO enabled may waste lots of CPU and memory resources processing the
requests and producing the responses. If the response is much larger
than the request, the attacker can further mount an amplified
reflection attack against victims of choice beyond the TFO server
Numerous mitigation techniques against regular SYN flood attacks
exist and have been well documented [RFC4987]. Unfortunately, none
are applicable to TFO. We propose a server-supplied cookie to
mitigate these new vulnerabilities in Section 3 and evaluate the
effectiveness of the defense in Section 7.
3. Protocol Overview
The key component of TFO is the Fast Open Cookie (cookie), a message
authentication code (MAC) tag generated by the server. The client
requests a cookie in one regular TCP connection, then uses it for
future TCP connections to exchange data during the 3WHS:
Requesting a Fast Open Cookie:
1. The client sends a SYN with a Fast Open option with an empty
cookie field to request a cookie.
2. The server generates a cookie and sends it through the Fast Open
option of a SYN-ACK packet.
3. The client caches the cookie for future TCP Fast Open connections
Performing TCP Fast Open:
1. The client sends a SYN with data and the cookie in the Fast Open
2. The server validates the cookie:
a. If the cookie is valid, the server sends a SYN-ACK
acknowledging both the SYN and the data. The server then
delivers the data to the application.
b. Otherwise, the server drops the data and sends a SYN-ACK
acknowledging only the SYN sequence number.
3. If the server accepts the data in the SYN packet, it may send the
response data before the handshake finishes. The maximum amount
is governed by TCP's congestion control [RFC5681].
4. The client sends an ACK acknowledging the SYN and the server data.
If the client's data is not acknowledged, the client retransmits
the data in the ACK packet.
5. The rest of the connection proceeds like a normal TCP connection.
The client can repeat many Fast Open operations once it acquires a
cookie (until the cookie is expired by the server). Thus, TFO is
useful for applications that have temporal locality on client and
Requesting Fast Open Cookie in connection 1:
TCP A (Client) TCP B (Server)
#1 SYN-SENT ----- <SYN,CookieOpt=NIL> ----------> SYN-RCVD
#2 ESTABLISHED <---- <SYN,ACK,CookieOpt=C> ---------- SYN-RCVD
(caches cookie C)
Performing TCP Fast Open in connection 2:
TCP A (Client) TCP B (Server)
#1 SYN-SENT ----- <SYN=x,CookieOpt=C,DATA_A> ----> SYN-RCVD
#2 ESTABLISHED <---- <SYN=y,ACK=x+len(DATA_A)+1> ---- SYN-RCVD
#3 ESTABLISHED <---- <ACK=x+len(DATA_A)+1,DATA_B>---- SYN-RCVD
#4 ESTABLISHED ----- <ACK=y+1>--------------------> ESTABLISHED
#5 ESTABLISHED --- <ACK=y+len(DATA_B)+1>----------> ESTABLISHED
4. Protocol Details
4.1. Fast Open Cookie
The Fast Open Cookie is designed to mitigate new security
vulnerabilities in order to enable data exchange during a handshake.
The cookie is a MAC tag generated by the server and is opaque to the
client; the client simply caches the cookie and passes it back on
subsequent SYN packets to open new connections. The server can
expire the cookie at any time to enhance security.
4.1.1. Fast Open Option
| Kind | Length |
~ Cookie ~
Kind 1 byte: value = 34
Length 1 byte: range 6 to 18 (bytes); limited by
remaining space in the options field.
The number MUST be even.
Cookie 0, or 4 to 16 bytes (Length - 2)
The Fast Open option is used to request or to send a Fast Open
Cookie. When a cookie is not present or is empty, the option is used
by the client to request a cookie from the server. When the cookie
is present, the option is used to pass the cookie from the server to
the client or from the client back to the server (to perform a Fast
The minimum cookie size is 4 bytes. Although the diagram shows a
cookie aligned on 32-bit boundaries, alignment is not required.
Options with invalid Length values or without the SYN flag set MUST
4.1.2. Server Cookie Handling
The server is in charge of cookie generation and authentication. The
cookie SHOULD be a MAC tag with the following properties. We use
"SHOULD" because, in some cases, the cookie may be trivially
generated as discussed in Section 7.3.
1. The cookie authenticates the client's (source) IP address of the
SYN packet. The IP address may be an IPv4 or IPv6 address.
2. The cookie can only be generated by the server and cannot be
fabricated by any other parties, including the client.
3. The generation and verification are fast relative to the rest of
SYN and SYN-ACK processing.
4. A server may encode other information in the cookie and accept
more than one valid cookie per client at any given time. But this
is server-implementation dependent and transparent to the client.
5. The cookie expires after a certain amount of time. The reason for
cookie expiration is detailed in the "Security Considerations"
section (Section 5). This can be done by either periodically
changing the server key used to generate cookies or including a
timestamp when generating the cookie.
To gradually invalidate cookies over time, the server can
implement key rotation to generate and verify cookies using
multiple keys. This approach is useful for large-scale servers to
retain Fast Open rolling key updates. We do not specify a
particular mechanism because the implementation is server
The server supports the cookie-generation and verification
- GetCookie(IP_Address): returns a (new) cookie.
- IsCookieValid(IP_Address, Cookie): checks if the cookie is valid,
i.e., it has not expired and the cookie authenticates the client
Example Implementation: a simple implementation is to use AES_128 to
encrypt the IPv4 (with padding) or IPv6 address and truncate to 64
bits. The server can periodically update the key to expire the
cookies. AES encryption on recent processors is fast and takes only
a few hundred nanoseconds [RCCJR11].
If only one valid cookie is allowed per IP, and the server can
regenerate the cookie independently, the best validation process is
to simply regenerate a valid cookie and compare it against the
incoming cookie. In that case, if the incoming cookie fails the
check, a valid cookie is readily available to be sent to the client.
4.1.3. Client Cookie Handling
The client MUST cache cookies from servers for later Fast Open
connections. For a multihomed client, the cookies are dependent on
the client and server IP addresses. Hence, the client should cache
at most one (most recently received) cookie per client and server IP
When caching cookies, we recommend that the client also cache the
Maximum Segment Size (MSS) advertised by the server. The client can
cache the MSS advertised by the server in order to determine the
maximum amount of data that the client can fit in the SYN packet in
subsequent TFO connections. Caching the server MSS is useful
because, with Fast Open, a client sends data in the SYN packet before
the server announces its MSS in the SYN-ACK packet. If the client
sends more data in the SYN packet than the server will accept, this
will likely require the client to retransmit some or all of the data.
Hence, caching the server MSS can enhance performance.
Without a cached server MSS, the amount of data in the SYN packet is
limited to the default MSS of 536 bytes for IPv4 [RFC1122] and 1220
bytes for IPv6 [RFC2460]. Even if the client complies with this
limit when sending the SYN, it is known that an IPv4 receiver
advertising an MSS less than 536 bytes can receive a segment larger
than it is expecting.
If the cached MSS is larger than the typical size (1460 bytes for
IPv4 or 1440 bytes for IPv6), then the excess data in the SYN packet
may cause problems that offset the performance benefit of Fast Open.
For example, the unusually large SYN may trigger IP fragmentation and
may confuse firewalls or middleboxes, causing SYN retransmission and
other side effects. Therefore, the client MAY limit the cached MSS
to 1460 bytes for IPv4 or 1440 for IPv6.
18.104.22.168. Client Caching Negative Responses
The client MUST cache negative responses from the server in order to
avoid potential connection failures. Negative responses include the
server not acknowledging the data in the SYN, ICMP error messages,
and (most importantly) no response (SYN-ACK) from the server at all,
i.e., connection timeout. The last case is likely due to
incompatible middleboxes or firewall blocking the connection
completely after processing the SYN packet with data. If the client
does not react to these negative responses and continues to retry
Fast Open, the client may never be able to connect to the specific
For any negative responses, the client SHOULD disable Fast Open on
the specific path (the source and destination IP addresses and ports)
at least temporarily. Since TFO is enabled on a per-service-port
basis, but cookies are independent of service ports, the client's
cache should include remote port numbers, too.
4.2. Fast Open Protocol
One predominant requirement of TFO is to be fully compatible with
existing TCP implementations, on both the client and server sides.
The server keeps two variables per listening socket (IP address and
FastOpenEnabled: default is off. It MUST be turned on explicitly by
the application. When this flag is off, the server does not
perform any TFO-related operations and MUST ignore all cookie
PendingFastOpenRequests: tracks the number of TFO connections in SYN-
RCVD state. If this variable goes over a preset system limit, the
server MUST disable TFO for all new connection requests until
PendingFastOpenRequests drops below the system limit. This
variable is used for defending some vulnerabilities discussed in
the "Security Considerations" section (Section 5).
The server keeps a FastOpened flag per connection to mark if a
connection has successfully performed a TFO.
4.2.1. Fast Open Cookie Request
Any client attempting TFO MUST first request a cookie from the server
with the following steps:
1. The client sends a SYN packet with a Fast Open option with a
Length field of 0 (empty cookie field).
2. The server responds with a SYN-ACK based on the procedures in the
"Server Cookie Handling" section (Section 4.1.2). This SYN-ACK
may contain a Fast Open option if the server currently supports
TFO for this listener port.
3. If the SYN-ACK has a Fast Open option with a cookie, the client
replaces the cookie and other information as described in the
"Client Cookie Handling" section (Section 4.1.3). Otherwise, if
the SYN-ACK is first seen and not a (spurious) retransmission, the
client MAY remove the server information from the cookie cache.
If the SYN-ACK is a spurious retransmission, the client does
nothing to the cookie cache for the reasons below.
The network or servers may drop the SYN or SYN-ACK packets with the
new cookie options, which will cause SYN or SYN-ACK timeouts. We
RECOMMEND both the client and the server to retransmit SYN and SYN-
ACK packets without the cookie options on timeouts. This ensures the
connections of cookie requests will go through and lowers the latency
penalty (of dropped SYN/SYN-ACK packets). The obvious downside for
maximum compatibility is that any regular SYN drop will fail the
cookie (although one can argue the delay in the data transmission
until after the 3WHS is justified if the SYN drop is due to network
congestion). The next section describes a heuristic to detect such
drops when the client receives the SYN-ACK.
We also RECOMMEND the client to record the set of servers that failed
to respond to cookie requests and only attempt another cookie request
after a certain period.
4.2.2. TCP Fast Open
Once the client obtains the cookie from the target server, it can
perform subsequent TFO connections until the cookie is expired by the
Client: Sending SYN
To open a TFO connection, the client MUST have obtained a cookie from
1. Send a SYN packet.
a. If the SYN packet does not have enough option space for the
Fast Open option, abort TFO and fall back to the regular 3WHS.
b. Otherwise, include the Fast Open option with the cookie of the
server. Include any data up to the cached server MSS or
default 536 bytes.
2. Advance to SYN-SENT state and update SND.NXT to include the data
To deal with network or servers dropping SYN packets with payload or
unknown options, when the SYN timer fires, the client SHOULD
retransmit a SYN packet without data and Fast Open options.
Server: Receiving SYN and responding with SYN-ACK
Upon receiving the SYN packet with Fast Open option:
1. Initialize and reset a local FastOpened flag. If FastOpenEnabled
is false, go to step 5.
2. If PendingFastOpenRequests is over the system limit, go to step 5.
3. If IsCookieValid() (in Section 4.1.2) returns false, go to step 5.
4. Buffer the data and notify the application. Set the FastOpened
flag and increment PendingFastOpenRequests.
5. Send the SYN-ACK packet. The packet MAY include a Fast Open
option. If the FastOpened flag is set, the packet acknowledges
the SYN and data sequence. Otherwise, it acknowledges only the
SYN sequence. The server MAY include data in the SYN-ACK packet
if the response data is readily available. Some applications may
favor delaying the SYN-ACK, allowing the application to process
the request in order to produce a response, but this is left up to
6. Advance to the SYN-RCVD state. If the FastOpened flag is set, the
server MUST follow [RFC5681] (based on [RFC3390]) to set the
initial congestion window for sending more data packets.
If the SYN-ACK timer fires, the server SHOULD retransmit a SYN-ACK
segment with neither data nor Fast Open options for compatibility
A special case is simultaneous open where the SYN receiver is a
client in SYN-SENT state. The protocol remains the same because
[RFC793] already supports both data in the SYN and simultaneous open.
But the client's socket may have data available to read before it's
connected. This document does not cover the corresponding API
Client: Receiving SYN-ACK
The client SHOULD perform the following steps upon receiving the SYN-
1. If the SYN-ACK has a Fast Open option, an MSS option, or both,
update the corresponding cookie and MSS information in the cookie
2. Send an ACK packet. Set acknowledgment number to RCV.NXT and
include the data after SND.UNA if data is available.
3. Advance to the ESTABLISHED state.
Note there is no latency penalty if the server does not acknowledge
the data in the original SYN packet. The client SHOULD retransmit
any unacknowledged data in the first ACK packet in step 2. The data
exchange will start after the handshake like a regular TCP
If the client has timed out and retransmitted only regular SYN
packets, it can heuristically detect paths that intentionally drop
SYNs with the Fast Open option or data. If the SYN-ACK acknowledges
only the initial sequence and does not carry a Fast Open cookie
option, presumably it is triggered by a retransmitted (regular) SYN
and the original SYN or the corresponding SYN-ACK was lost.
Server: Receiving ACK
Upon receiving an ACK acknowledging the SYN sequence, the server
decrements PendingFastOpenRequests and advances to the ESTABLISHED
state. No special handling is required further.
5. Security Considerations
The Fast Open Cookie stops an attacker from trivially flooding
spoofed SYN packets with data to burn server resources or to mount an
amplified reflection attack on random hosts. The server can defend
against spoofed SYN floods with invalid cookies using existing
techniques [RFC4987]. We note that although generating bogus cookies
is cost free, the cost of validating the cookies, inherent to any
authentication scheme, may be substantial compared to processing a
regular SYN packet. We describe these new vulnerabilities of TFO and
the countermeasures in detail below.
5.1. Resource Exhaustion Attack by SYN Flood with Valid Cookies
An attacker may still obtain cookies from some compromised hosts,
then flood spoofed SYN packets with data and "valid" cookies (from
these hosts or other vantage points). Like regular TCP handshakes,
TFO is vulnerable to such an attack. But the potential damage can be
much more severe. Besides causing temporary disruption to service
ports under attack, it may exhaust server CPU and memory resources.
Such an attack will show up on application server logs as an
application-level DoS from botnets, triggering other defenses and
To protect the server, it is important to limit the maximum number of
total pending TFO connection requests, i.e., PendingFastOpenRequests
(Section 4.2). When the limit is exceeded, the server temporarily
disables TFO entirely as described in "Server Cookie Handling"
(Section 4.1.2). Then, subsequent TFO requests will be downgraded to
regular connection requests, i.e., with the data dropped and only
SYNs acknowledged. This allows regular SYN flood defense techniques
[RFC4987] like SYN cookies to kick in and prevent further service
The main impact of SYN floods against the standard TCP stack is not
directly from the floods themselves costing TCP processing overhead
or host memory, but rather from the spoofed SYN packets filling up
the often small listener's queue.
On the other hand, TFO SYN floods can cause damage directly if
admitted without limit into the stack. The reset (RST) packets from
the spoofed host will fuel rather than defeat the SYN floods as
compared to the non-TFO case, because the attacker can flood more
SYNs with data and incur more cost in terms of data processing
resources. For this reason, a TFO server needs to monitor the
connections in SYN-RCVD being reset in addition to imposing a
reasonable max queue length. Implementations may combine the two,
e.g., by continuing to account for those connection requests that
have just been reset against the listener's PendingFastOpenRequests
until a timeout period has passed.
Limiting the maximum number of pending TFO connection requests does
make it easy for an attacker to overflow the queue, causing TFO to be
disabled. We argue that causing TFO to be disabled is unlikely to be
of interest to attackers because the service will remain intact
without TFO; hence, there is hardly any real damage.
5.1.1. Attacks from behind Shared Public IPs (NATs)
An attacker behind a NAT can easily obtain valid cookies to launch
the above attack to hurt other clients that share the path.
[BRISCOE12] suggested that the server can extend cookie generation to
include the TCP timestamp -- GetCookie(IP_Address, Timestamp) -- and
implement it by encrypting the concatenation of the two values to
generate the cookie. The client stores both the cookie and its
corresponding timestamp, and it echoes both in the SYN. The server
then implements IsCookieValid(IP_Address, Timestamp, Cookie) by
encrypting the IP and timestamp data and comparing it with the cookie
This enables the server to issue different cookies to clients that
share the same IP address; hence, it can selectively discard those
misused cookies from the attacker. However, the attacker can simply
repeat the attack with new cookies. The server would eventually need
to throttle all requests from the IP address just like the current
approach. Moreover, this approach requires modifying [RFC1323]
(obsoleted by [RFC7323]) to send a non-zero Timestamp Echo Reply in
the SYN, potentially causing firewall issues. Therefore, we believe
the benefit does not outweigh the drawbacks.
5.2. Amplified Reflection Attack to Random Host
Limiting PendingFastOpenRequests with a system limit can be done
without Fast Open cookies and would protect the server from resource
exhaustion. It would also limit how much damage an attacker can
cause through an amplified reflection attack from that server.
However, it would still be vulnerable to an amplified reflection
attack from a large number of servers. An attacker can easily cause
damage by tricking many servers to respond with data packets at once
to any spoofed victim IP address of choice.
With the use of Fast Open cookies, the attacker would first have to
steal a valid cookie from its target victim. This likely requires
the attacker to compromise the victim host or network first. But, in
some cases, it may be relatively easy.
The attacker here has little interest in mounting an attack on the
victim host that has already been compromised. But it may be
motivated to disrupt the victim's network. Since a stolen cookie is
only valid for a single server, it has to steal valid cookies from a
large number of servers and use them before they expire to cause
sufficient damage without triggering the defense.
One can argue that if the attacker has compromised the target network
or hosts, it could perform a similar but simpler attack by injecting
bits directly. The degree of damage will be identical, but a TFO-
specific attack allows the attacker to remain anonymous and disguises
the attack as from other servers.
For example, with DHCP, an attacker can obtain cookies when he (or
the host he has compromised) owns a particular IP address by
performing regular Fast Open to servers supporting TFO and he can
collect valid cookies. Then, the attacker actively or passively
releases his IP address. When the IP address is reassigned to
another host (victim) via DHCP, the attacker then floods spoofed Fast
Open requests with valid cookies to the servers. Since the cookies
are valid, these servers accept the requests and respond with a SYN-
ACK plus data packets to the victim instead of the attacker. Thus,
the attacker is able to launch amplified reflection attacks to other
hosts that share IP addresses.
The best defense is for the server not to respond with data until the
handshake finishes. In this case, the risk of an amplification
reflection attack is completely eliminated. But the potential
latency saving from TFO may diminish if the server application
produces responses earlier before the handshake completes.
6. TFO Applicability
This section is to help applications considering TFO to evaluate
TFO's benefits and drawbacks using the Web client and server
applications as an example throughout. Applications here refer
specifically to the process that writes data into the socket -- for
proposed socket API change is provided in the Appendix.
6.1. Duplicate Data in SYNs
It is possible that using TFO results in the first data written to a
socket to be delivered more than once to the application on the
remote host (Section 2.1). This replay potential only applies to
data in the SYN but not subsequent data exchanges.
Empirically, [JIDKT07] showed the packet duplication on a Tier-1
network is rare. Since the replay only happens specifically when the
SYN data packet is duplicated and also the duplicate arrives after
the receiver has cleared the original SYN's connection state, the
replay is thought to be uncommon in practice. Nevertheless, a client
that cannot handle receiving the same SYN data more than once MUST
NOT enable TFO to send data in a SYN. Similarly, a server that
cannot accept receiving the same SYN data more than once MUST NOT
enable TFO to receive data in a SYN. Further investigation is needed
to judge the probability of receiving duplicated SYN or SYN-ACK
packets with data in networks that are not Tier 1.
6.2. Potential Performance Improvement
TFO is designed for latency-conscious applications that are sensitive
to TCP's initial connection setup delay. To benefit from TFO, the
first application data unit (e.g., an HTTP request) needs to be no
more than TCP's maximum segment size (minus options used in the SYN).
Otherwise, the remote server can only process the client's
application data unit once the rest of it is delivered after the
initial handshake, diminishing TFO's benefit.
To the extent possible, applications SHOULD reuse the connection to
take advantage of TCP's built-in congestion control and reduce
connection setup overhead. An application that employs too many
short-lived connections will negatively impact network stability, as
these connections often exit before TCP's congestion control
algorithm takes effect.
6.3. Example: Web Clients and Servers
6.3.1. HTTP Request Replay
While TFO is motivated by Web applications, the browser should not
use TFO to send requests in SYNs if those requests cannot tolerate
replays. One example is POST requests without application-layer
transaction protection (e.g., a unique identifier in the request
On the other hand, TFO is particularly useful for GET requests. GET
request replay could happen across striped TCP connections: after a
server receives an HTTP request but before the ACKs of the requests
reach the browser, the browser may time out and retry the same
request on another (possibly new) TCP connection. This differs from
a TFO replay only in that the replay is initiated by the browser, not
by the TCP stack.
6.3.2. HTTP over TLS (HTTPS)
For Transport Layer Security (TLS) over TCP, it is safe and useful to
include a TLS client_hello in the SYN packet to save one RTT in the
TLS handshake. There is no concern about violating idempotency. In
particular, it can be used alone with the speculative connection
6.3.3. Comparison with HTTP Persistent Connections
Is TFO useful given the wide deployment of HTTP persistent
connections? The short answer is yes. Studies ([RCCJR11] [AERG11])
show that the average number of transactions per connection is
between 2 and 4, based on large-scale measurements from both servers
and clients. In these studies, the servers and clients both kept
idle connections up to several minutes, well into "human think" time.
Keeping connections open and idle even longer risks a greater
performance penalty. [HNESSK10] and [MQXMZ11] show that the majority
of home routers and ISPs fail to meet the 124-minute idle timeout
mandated in [RFC5382]. In [MQXMZ11], 35% of mobile ISPs silently
time out idle connections within 30 minutes. End hosts, unaware of
silent middlebox timeouts, suffer multi-minute TCP timeouts upon
using those long-idle connections.
To circumvent this problem, some applications send frequent TCP keep-
alive probes. However, this technique drains power on mobile devices
[MQXMZ11]. In fact, power has become such a prominent issue in
modern Long Term Evolution (LTE) devices that mobile browsers close
HTTP connections within seconds or even immediately [SOUDERS11].
[RCCJR11] studied the performance of the Chrome browser [Chrome]
based on 28 days of global statistics. The Chrome browser keeps idle
HTTP persistent connections for 5 to 10 minutes. However, the
average number of the transactions per connection is only 3.3, and
the TCP 3WHS accounts for up to 25% of the HTTP transaction network
latency. The authors estimated that TFO improves page load time by
10% to 40% on selected popular Web sites.
6.3.4. Load Balancers and Server Farms
Servers behind load balancers that accept connection requests to the
same server IP address should use the same key such that they
generate identical Fast Open cookies for a particular client IP
address. Otherwise, a client may get different cookies across
connections; its Fast Open attempts would fall back to the regular
7. Open Areas for Experimentation
We now outline some areas that need experimentation in the Internet
and under different network scenarios. These experiments should help
evaluate Fast Open benefits and risks and its related protocols.
7.1. Performance Impact Due to Middleboxes and NAT
[MAF04] found that some middleboxes and end hosts may drop packets
with unknown TCP options. Studies ([LANGLEY06] [HNRGHT11]) have
found that 6% of the probed paths on the Internet drop SYN packets
with data or with unknown TCP options. The TFO protocol deals with
this problem by falling back to the regular TCP handshake and
retransmitting the SYN without data or cookie options after the
initial SYN timeout. Moreover, the implementation is recommended to
negatively cache such incidents to avoid recurring timeouts. Further
study is required to evaluate the performance impact of these drop
Another interesting study is the loss of TFO performance benefit
behind certain Carrier-Grade NAT (CGN). Typically, hosts behind a
NAT sharing the same IP address will get the same cookie for the same
server. This will not prevent TFO from working. But, on some CGN
configurations where every new TCP connection from the same physical
host uses a different public IP address, TFO does not provide latency
benefits. However, there is no performance penalty either, as
described in the "Client: Receiving SYN-ACK" text in Section 4.2.2.
7.2. Impact on Congestion Control
Although TFO does not directly change TCP's congestion control, there
are subtle cases where it could do so. When a SYN-ACK times out,
regular TCP reduces the initial congestion window before sending any
data [RFC5681]. However, in TFO, the server may have already sent up
to an initial window of data.
If the server serves mostly short connections, then the losses of
SYN-ACKs are not as effective as regular TCP on reducing the
congestion window. This could result in an unstable network
condition. The connections that experience losses may attempt again
and add more load under congestion. A potential solution is to
temporarily disable Fast Open if the server observes many SYN-ACK or
data losses during the handshake across connections. Further
experimentation regarding the congestion control impact will be
7.3. Cookie-less Fast Open
The cookie mechanism mitigates resource exhaustion and amplification
attacks. However, cookies are not necessary if the server has
application-level protection or is immune to these attacks. For
example, a Web server that only replies with a simple HTTP redirect
response that fits in the SYN-ACK packet may not care about resource
For such applications the server may choose to generate a trivial or
even a zero-length cookie to improve performance by avoiding the
cookie generation and verification. If the server believes it's
under a DoS attack through other defense mechanisms, it can switch to
regular Fast Open for listener sockets.
8. Related Work
TCP Extensions for Transactions [RFC1644] attempted to bypass the
3WHS, among other things; hence, it shared the same goal but also the
same set of issues as TFO. It focused most of its effort battling
old or duplicate SYNs, but paid no attention to security
vulnerabilities it introduced when bypassing the 3WHS [PHRACK98].
As stated earlier, we take a practical approach to focus TFO on the
security aspect, while allowing old, duplicate SYN packets with data
after recognizing that 100% TCP semantics is likely infeasible. We
believe this approach strikes the right trade-off and makes TFO much
simpler and more appealing to TCP implementers and users.
8.2. Common Defenses against SYN Flood Attacks
[RFC4987] studies the mitigation of attacks from regular SYN floods,
i.e., SYNs without data. But from the stateless SYN cookies to the
stateful SYN Cache, none can preserve data sent with SYNs safely
while still providing an effective defense.
The best defense may be simply to disable TFO when a host is
suspected to be under a SYN flood attack, e.g., the SYN backlog is
filled. Once TFO is disabled, normal SYN flood defenses can be
applied. The "Security Considerations" section (Section 5) contains
a thorough discussion on this topic.
8.3. Speculative Connections by the Applications
Some Web browsers maintain a history of the domains for frequently
visited Web pages. The browsers then speculatively pre-open TCP
connections to these domains before the user initiates any requests
for them [BELSHE11]. While this technique also saves the handshake
latency, it wastes server and network resources by initiating and
maintaining idle connections.
8.4. Fast Open Cookie-in-FIN
An alternate proposal is to request a TFO cookie in the FIN instead,
since FIN-drop by incompatible middleboxes does not affect latency.
However, paths that block SYN cookies may be more likely to drop a
later SYN packet with data, and many applications close a connection
with RST instead anyway.
Although cookie-in-FIN may not improve robustness, it would give
clients using a single connection a latency advantage over clients
opening multiple parallel connections. If experiments with TFO find
that it leads to increased connection-sharding, cookie-in-FIN may
prove to be a useful alternative.
8.5. TCP Cookie Transaction (TCPCT)
TCPCT [RFC6013] eliminates server state during the initial handshake
and defends spoofing DoS attacks. Like TFO, TCPCT allows SYN and
SYN-ACK packets to carry data. But the server can only send up to
MSS bytes of data during the handshake instead of the initial
congestion window, unlike TFO. Therefore, the latency of
applications (e.g., Web applications) may be worse than with TFO.
Appendix A. Example Socket API Changes to Support TFO
A.1. Active Open
The active open side involves changing or replacing the connect()
call, which does not take a user data buffer argument. We recommend
replacing the connect() call to minimize API changes, and, hence,
applications to reduce the deployment hurdle.
One solution implemented in Linux 3.7 is introducing a new flag,
MSG_FASTOPEN, for sendto() or sendmsg(). MSG_FASTOPEN marks the
attempt to send data in the SYN like a combination of connect() and
sendto(), by performing an implicit connect() operation. It blocks
until the handshake has completed and the data is buffered.
For a non-blocking socket, it returns the number of bytes buffered
and sent in the SYN packet. If the cookie is not available locally,
it returns -1 with errno EINPROGRESS, and sends a SYN with a TFO
cookie request automatically. The caller needs to write the data
again when the socket is connected. On errors, it returns the same
errno as connect() if the handshake fails.
An implementation may prefer not to change the sendmsg() call because
TFO is a TCP-specific feature. A solution is to add a new socket
option, TCP_FASTOPEN, for TCP sockets. When the option is enabled
before a connect() operation, sendmsg() or sendto() will perform a
Fast Open operation similar to the MSG_FASTOPEN flag described above.
This approach, however, requires an extra setsockopt() system call.
A.2. Passive Open
The passive open side change is simpler compared to the active open
side. The application only needs to enable the reception of Fast
Open requests via a new TCP_FASTOPEN setsockopt() socket option
The option enables Fast Open on the listener socket. The option
value specifies the PendingFastOpenRequests threshold, i.e., the
maximum length of pending SYNs with data payload. Once enabled, the
TCP implementation will respond with TFO cookies per request.
Traditionally, accept() returns only after a socket is connected.
But, for a Fast Open connection, accept() returns upon receiving a
SYN with a valid Fast Open cookie and data, and the data is available
to be read through, e.g., recvmsg(), read().
We thank Bob Briscoe, Michael Scharf, Gorry Fairhurst, Rick Jones,
Roberto Peon, William Chan, Adam Langley, Neal Cardwell, Eric
Dumazet, and Matt Mathis for their feedback. We especially thank
Barath Raghavan for his contribution on the security design of Fast
Open and proofreading this document numerous times.
1600 Amphitheatre Parkway
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1600 Amphitheatre Parkway
Mountain View, CA 94043
Department of Computer Science and Engineering
University of California, San Diego
9500 Gilman Drive
La Jolla, CA 92093-0404
1600 Amphitheatre Parkway
Mountain View, CA 94043