wdiff rfc7413.original rfc7413.txt

Internet Draft Engineering Task Force (IETF) Y. Cheng
draft-ietf-tcpm-fastopen-10.txt
Request for Comments: 7413 J. Chu
Intended status:
Category: Experimental S. Radhakrishnan
Expiration date: April, 2015
ISSN: 2070-1721 A. Jain
Google
September 29,
December 2014

TCP Fast Open

Abstract

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 round-trip time (RTT)
compared to the standard TCP, which requires a three-way handshake
(3WHS) to complete before data can be exchanged. However However, TFO
deviates from the standard TCP semantics 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 issue,
as detailed in the Applicability section.

Status of this This Memo

Distribution of this memo is unlimited.

This Internet-Draft document is submitted in full conformance with the
provisions of BCP 78 not an Internet Standards Track specification; it is
published for examination, experimental implementation, and BCP 79.

Internet-Drafts are working documents
evaluation.

This document defines an Experimental Protocol for the Internet
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Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 ....................................................3
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . . 3 ................................................4
2. Data In in SYN . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.1 .....................................................4
2.1. Relaxing TCP Semantics on Duplicated SYNs . . . . . . . . . 4 ..................4
2.2. SYNs with Spoofed IP Addresses . . . . . . . . . . . . . . 4 .............................5
3. Protocol Overview . . . . . . . . . . . . . . . . . . . . . . . 5 ...............................................5
4. Protocol Details . . . . . . . . . . . . . . . . . . . . . . . 6 ................................................7
4.1. Fast Open Cookie . . . . . . . . . . . . . . . . . . . . . 6 ...........................................7
4.1.1. Fast Open option . . . . . . . . . . . . . . . . . . . 7 Option ....................................8
4.1.2. Server Cookie Handling . . . . . . . . . . . . . . . . 7 ..............................8
4.1.3. Client Cookie Handling . . . . . . . . . . . . . . . . 8
4.1.3.1 ..............................9
4.1.3.1. Client Caching Negative Responses . . . . . . . . . 9 .........10
4.2. Fast Open Protocol . . . . . . . . . . . . . . . . . . . . 9 ........................................11
4.2.1. Fast Open Cookie Request . . . . . . . . . . . . . . . 10 ...........................11
4.2.2. TCP Fast Open . . . . . . . . . . . . . . . . . . . . . 11 ......................................12
5. Security Considerations . . . . . . . . . . . . . . . . . . . . 13 ........................................14
5.1. Resource Exhaustion Attack by SYN Flood with Valid
Cookies . . . . . . . . . . . . . . . . . . . . . . . . . . 13
5.1.1 ...................................................14
5.1.1. Attacks from behind Shared Public IPs (NATs) . . . . . . 14 .......15
5.2. Amplified Reflection Attack to Random Host . . . . . . . . 14 ................16
6. TFO's TFO Applicability . . . . . . . . . . . . . . . . . . . . . . 15
6.1 ..............................................17
6.1. Duplicate Data in SYNs . . . . . . . . . . . . . . . . . . . 15
6.2 ....................................17
6.2. Potential Performance Improvement . . . . . . . . . . . . . 16 .........................17
6.3. Example: Web Clients and Servers . . . . . . . . . . . . . 16 ..........................18
6.3.1. HTTP Request Replay . . . . . . . . . . . . . . . . . . 16 ................................18
6.3.2. HTTP over TLS (HTTPS) . . . . . . . . . . . . . . . . . 16 ..............................18
6.3.3. Comparison with HTTP Persistent Connections . . . . . . 17 ........18
6.3.4. Load Balancers and Server farms . . . . . . . . . . . . 17 Farms ....................19
7. Open Areas for Experimentation . . . . . . . . . . . . . . . . 17 .................................19
7.1. Performance impact due Impact Due to middle-boxes Middleboxes and NAT . . . . . . 18 .............19
7.2. Impact on congestion control . . . . . . . . . . . . . . . 18 Congestion Control ..............................20
7.3. Cookie-less Fast Open . . . . . . . . . . . . . . . . . . . 18 .....................................20
8. Related Work . . . . . . . . . . . . . . . . . . . . . . . . . 19 ...................................................20
8.1. T/TCP . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 .....................................................20
8.2. Common Defenses Against against SYN Flood Attacks . . . . . . . . . 19 .................21
8.3. Speculative Connections by the Applications . . . . . . . . 19 ...............21
8.4. Fast Open Cookie in FIN . . . . . . . . . . . . . . . . . . 19 Cookie-in-FIN ...................................21
8.5. TCP Cookie Transaction (TCPCT) . . . . . . . . . . . . . . 20 ............................21
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 20 ............................................22
10. Acknowledgement . . . . . . . . . . . . . . . . . . . . . . . 20
11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 20
11.1. ....................................................22
10.1. Normative References . . . . . . . . . . . . . . . . . . . 20
11.2. .....................................22
10.2. Informative References . . . . . . . . . . . . . . . . . . 21 ...................................23
Appendix A. Example Socket API Changes to support Support TFO . . . . . . 22
A.1 .............25
A.1. Active Open . . . . . . . . . . . . . . . . . . . . . . . . 22
A.2 .................................................25
A.2. Passive Open . . . . . . . . . . . . . . . . . . . . . . . . 23 ................................................25
Acknowledgments ...................................................26
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 23 ................................................26

1. 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 3-way
three-way handshake (3WHS)[RFC793], (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 minutes, but
35% of HTTP requests are made on new TCP connections [RCCJR11]. For
such Web and Web-like
applications applications, placing data in the SYN can yield
significant latency improvements. Next we describe how we resolve
the challenges that arise upon doing so.

1.1. Terminology

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

"TFO" refers to TCP Fast Open. Client "Client" refers to the TCP's active open
side
side, and server "server" refers to the TCP's passive open side.

2. Data In in SYN

Standard TCP already allows data to be carried in SYN packets
([RFC793], section 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 of to TCP semantic raises two issues discussed
(discussed in the following subsections, making subsections) that make TFO unsuitable for
certain applications.

Therefore

Therefore, TCP implementations MUST NOT use TFO by default, but only
use TFO if requested explicitly by the application on a per service per-service-
port basis. Applications need to evaluate TFO applicability as
described in Section 6 before using TFO.

2.1

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. (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 2MSL TIMEWAIT TIME-WAIT 2
MSL state).

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 the Related Work section. 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 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
completely until timeouts.
completely.

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
itself.

Numerous mitigation techniques against regular SYN flood attacks
exist and have been well documented [RFC4987]. Unfortunately 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
(see below).

Performing TCP Fast Open:

1. The client sends a SYN with data and the cookie in the Fast Open
option.

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 the 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 Thus, TFO is
useful for applications that have temporal locality on client and
server connections.

Requesting Fast Open Cookie in connection 1:

TCP A (Client) TCP B(Server) B (Server)
______________ ______________ _____________
CLOSED LISTEN

#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) B (Server)
______________ ______________ _____________
CLOSED LISTEN

#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 message authentication code 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 Option

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Kind | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ Cookie ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Kind 1 byte: constant-TBD (to be assigned by IANA) 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
Open).

The minimum Cookie 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
be ignored.

4.1.2. Server Cookie Handling

The server is in charge of cookie generation and authentication. The
cookie SHOULD be a message authentication code MAC tag with the following properties. We use SHOULD because
"SHOULD" because, in some cases 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 can not cannot be
fabricated by any other parties 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, cookie and accept
more than one valid cookie per client at any given time. But this
is server implementation 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 Consideration"
section. 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
specific.

The server supports the cookie generation cookie-generation and verification
operations:

- GetCookie(IP_Address): returns a (new) cookie cookie.

- IsCookieValid(IP_Address, Cookie): checks if the cookie is valid,
i.e., it has not expired and it the cookie authenticates the client
IP address.

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 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 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 multi-homed multihomed client, the cookies are dependent on
the client and server IP addresses. Hence Hence, the client should cache
at most one (most recently received) cookie per client and server IP
addresses
address pair.

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
because, with Fast Open 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
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 1240 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,
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 Therefore, the client MAY limit the cached MSS
to 1460 bytes for IPv4 or 1440 for IPv6.

4.1.3.1

4.1.3.1. 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 (most importantly) no response (SYN/ACK) (SYN-ACK) from the server at all,
i.e., connection timeout. The last case is likely due to
incompatible
middle-boxes middleboxes or firewall blocking the connection
completely after it
sees data in SYN. processing the SYN packet with data. If the client
does not react to these negative responses and continue continues to retry
Fast Open, the client may never be able to connect to the specific
server.

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 per-service-port
basis, but cookies are independent of service ports, the client's
cache should include remote port numbers numbers, too.

4.2. Fast Open Protocol

One predominant requirement of TFO is to be fully compatible with
existing TCP implementations, both on both the client and the server sides.

The server keeps two variables per listening socket (IP address & and
port):

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 TFO-related operations and MUST ignore all cookie
options.

PendingFastOpenRequests: tracks the number of TFO connections in SYN-RCVD 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 (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
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 (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 (Section 4.1.3). Otherwise, if
the SYN-ACK is first seen, i.e., 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
till
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
server.

Client: Sending SYN

To open a TFO connection, the client MUST have obtained a cookie from
the server:

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
accordingly.

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 (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.
option. If the FastOpened flag is set, the packet acknowledges
the SYN and data sequence. Otherwise 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 application 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
the implementation.

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
reasons.

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
change.

Client: Receiving SYN-ACK

The client SHOULD perform the following steps upon receiving the SYN-
ACK:

1. If the SYN-ACK has a Fast Open option or option, an MSS option option, or both,
update the corresponding cookie and MSS information in the cookie
cache.

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
connection.

If the client has timed out and retransmitted only regular SYN
packets, it can heuristically detect paths that intentionally drop
SYN
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 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, 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
application-level DoS from Bot-nets, botnets, triggering other defenses and
alerts.

To protect the server 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". Then Handling"
(Section 4.1.2). Then, subsequent TFO requests will be downgraded to
regular connection requests, i.e., with the data dropped and only SYN
SYNs acknowledged. This allows regular SYN flood defense techniques
[RFC4987] like SYN- SYN cookies to kick in and prevent further service
disruption.

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 RST 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 to cost 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 TFO; hence, there is hardly any real damage.

5.1.1

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 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
value.

This enables the server to issue different cookies to clients that
share the same IP address, hence address; hence, it can selectively discard those
misused cookies from the attacker. However 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 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 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 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, 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 But, in
some
cases 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 example, with DHCP 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. The Then, the attacker then actively or passively
releases his IP address. When the IP address is re-assigned reassigned to a victim,
another host (victim) via DHCP, the attacker
now owning a different IP address, then floods spoofed Fast
Open requests with valid cookies to perform an 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 attack on the victim. 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 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's 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
example socket -- for
example, a JavaScript process that sends data to the server. A
proposed socket API change is provided in the Appendix.

6.1

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

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 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 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
about the probability of receiving duplicated SYN or SYN-ACK
packets with data in non-Tier 1 networks.

6.2 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
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
header).

On the other hand, TFO is particularly useful for GET requests. GET
requests
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 timeout 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 TLS Transport Layer Security (TLS) over TCP, it is safe and useful to
include a TLS CLIENT_HELLO client_hello in the SYN packet to save one RTT in the
TLS handshake. There is no concern about violating idem-potency. idempotency. In particular
particular, it can be used alone with the speculative connection
above.

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] ([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][MQXMZ11] [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
timeout
time out idle connections within 30 minutes. End hosts, unaware of
silent middle-box 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
LTE 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] performance
based on 28 days of global statistics. The Chrome browser keeps idle
HTTP persistent connections for 5 to 10 minutes. However However, the
average number of the transactions per connection is only 3.3 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 Farms

Servers behind a 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 cookies for a particular client IP
address. Otherwise Otherwise, a client may get different cookies across
connections; its Fast Open attempts would fall back to the regular
3WHS.

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
the community
evaluate Fast Open benefits and risks towards further
standardization and implementation of Fast Open and its related protocols.

7.1. Performance impact due Impact Due to middle-boxes Middleboxes and NAT

[MAF04] found that some middle-boxes middleboxes and end-hosts end hosts may drop packets
with unknown TCP options. Studies [LANGLEY06, HNRGHT11] both ([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 re-transmitting
retransmitting the SYN without data or cookie options after the
initial SYN timeout.
Moreover 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
behaviors.

Another interesting study is the loss of TFO performance benefit
behind certain carrier-grade NAT. Typically 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 But, on some carrier-
grade NAT 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 Section the "Client: Receiving SYN-ACK". SYN-ACK" text in Section 4.2.2.

7.2. Impact on congestion control Congestion Control

Although TFO does not directly change the TCP's congestion control, there
are subtle cases that where it may. When could do so. When a SYN-ACK times out,
regular TCP reduces the initial congestion window before sending any
data [RFC5681]. However However, in TFO TFO, the server may have already sent up
to an initial window of data.

If the server serves mostly short connections connections, then the losses of SYN-
ACKs
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
useful.

7.3. Cookie-less Fast Open

The cookie mechanism mitigates resource exhaustion and amplification
attacks. However However, cookies are not necessary if the server has
application-level protection or is immune to these attacks. For
example
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
exhaustion.

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

8.1. T/TCP

TCP Extensions for Transactions [RFC1644] attempted to bypass the
three-way handshake,
3WHS, among other things, hence 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 tradeoff, trade-off and makes TFO much
simpler and more appealing to TCP implementers and users.

8.2. Common Defenses Against against SYN Flood Attacks

[RFC4987] studies on mitigating the mitigation of attacks from regular SYN flood, floods,
i.e.,
SYN SYNs without data. But from the stateless SYN-cookies SYN cookies to the
stateful SYN Cache, none can preserve data sent with SYN SYNs safely
while still providing an effective defense.

The best defense may be to 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 Consideration" 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 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 Cookie-in-FIN

An alternate proposal is to request a TFO cookie in the FIN instead,
since FIN-drop by incompatible middle-boxes middleboxes does not affect latency.
However
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 window, unlike TFO. Therefore Therefore, the latency of
applications such as (e.g., Web applications) may be worse than with TFO.

9. IANA Considerations

IANA is requested to allocate has allocated one value from value, 34, in the TCP "TCP Option Kind
Numbers: The constant-TBD in Numbers"
registry. See Section 4.1.1 has to be replaced with
the newly assigned value. 4.1.1. The length of the this new TCP option Kind is
variable
variable, and the Meaning should be as shown in the "TCP Option Kind Numbers"
registry is set to "TCP Fast Open Cookie".
Early implementation before the IANA allocation SHOULD follow
[RFC6994] Current and new
implementations SHOULD use option (34). Existing implementations
that are using experimental option 254 and per [RFC6994] with magic
number 0xF989 (16
bits), then bits) as allocated in the IANA "TCP Experimental
Option Experiment Identifiers (TCP ExIDs)" registry by this document,
SHOULD migrate to the use this new option after the allocation
accordingly. (34) by default.

10. Acknowledgement

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 feedbacks. We especially thank
Barath Raghavan for his contribution on the security design of Fast
Open and proofreading this draft numerous times.

11. References

11.1.

10.1. Normative References

[RFC793] Postel, J. J., "Transmission Control Protocol", STD 7, RFC
793, September 1981. 1981,
<http://www.rfc-editor.org/info/rfc793>.

[RFC1122] Braden, R., Ed., "Requirements for Internet Hosts -
Communication Layers", STD 3, RFC 1122, October 1989. 1989,
<http://www.rfc-editor.org/info/rfc1122>.

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997. 1997,
<http://www.rfc-editor.org/info/rfc2119>.

[RFC3390] Allman, M., Floyd, S., and C. Partridge, "Increasing
TCP's Initial Window", RFC 3390, October 2002,
<http://www.rfc-editor.org/info/rfc3390>.

[RFC5382] S. Guha, S., Ed., Biswas, K., Ford Ford, B., Sivakumar Sivakumar, S., and
P. Srisuresh,
P., "NAT Behavioral Requirements for TCP", BCP
142, RFC 5382 5382, October 2008,
<http://www.rfc-editor.org/info/rfc5382>.

[RFC5681] Allman, M., Paxson, V. V., and E. Blanton, "TCP Congestion
Control", RFC 5681, September 2009 2009,
<http://www.rfc-editor.org/info/rfc5681>.

[RFC6994] Touch, Joe, J., "Shared Use of Experimental TCP Options",
RFC6994, August 2013.

[RFC3390] Allman, M., Floyd, S., and C. Partridge, "Increasing TCP's
Initial Window", RFC 3390, October 2002.

11.2.
6994, August 2013,
<http://www.rfc-editor.org/info/rfc6994>.

10.2. Informative References

[AERG11] Al-Fares, M., Elmeleegy, K., Reed, B., and I. Gashinsky, I.,
"Overclocking the Yahoo! CDN for Faster Web Page Loads".
In Loads",
in Proceedings of Internet Measurement Conference,
November 2011.

[BELSHE11] Belshe, M., "The Era of Browser Preconnect", February
2011, <http://www.belshe.com/2011/02/10/the-era-of-
browser-preconnect/>.

[BRISCOE12] Briscoe, B., "Some ideas building on draft-ietf-tcpm-
fastopen-01", message to the tcpm mailing list, July
2012, <http://www.ietf.org/mail-
archive/web/tcpm/current/msg07192.html>.

[Chrome] Google Chrome,
<https://www.google.com/intl/en-US/chrome/browser/>.

[HNESSK10] Haetoenen, S., Nyrhinen, A., Eggert, L., Strowes, S.,
Sarolahti, P., Kojo., M., and M. Kojo, "An Experimental Study of
Home Gateway Characteristics". In Characteristics", in Proceedings of Internet
Measurement Conference. Conference, October 2010 2010.

[HNRGHT11] Honda, M., Nishida, Y., Raiciu, C., Greenhalgh, A.,
Handley, M., and H. Tokuda, H., "Is it Still Possible to
Extend TCP?". In TCP?", in Proceedings of Internet Measurement
Conference.
Conference, November 2011.

[JIDKT07] Jaiswal, S., Iannaccone, G., Diot, C., Kurose, J., and D.
Towsley, "Measurement and Classification of Out-of-
Sequence Packets in a Tier-1 IP Backbone" IEEE/ACM
Transactions on Networking (TON), Volume 15, Issue 1, pp
54-66.

[LANGLEY06] Langley, A, A., "Probing the viability of TCP extensions",
URL http://www.imperialviolet.org/binary/ecntest.pdf
<http://www.imperialviolet.org/binary/ecntest.pdf>.

[MAF04] Medina, A., Allman, M., and S. Floyd, "Measuring
Interactions Between Transport Protocols and
Middleboxes". In
Middleboxes", in Proceedings of Internet Measurement
Conference, October 2004.

[MQXMZ11] Wang, Z., Qian, Z., Xu, Q., Mao, Z., and M. Zhang, M., "An
Untold Story of Middleboxes in Cellular Networks".
In Networks", in
Proceedings of SIGCOMM. SIGCOMM, August 2011.

[PHRACK98] "T/TCP vulnerabilities", Phrack Magazine, Volume 8, Issue
53 artical 6.
53, Article 6, July 8, 1998. URL
http://www.phrack.com/issues.html?issue=53&id=6 1998,
<http://www.phrack.com/issues.html?issue=53&id=6>.

[RCCJR11] Radhakrishnan, S., Cheng, Y., Chu, J., Jain, A., and B.
Raghavan, B., "TCP Fast Open". In Open", in Proceedings of the 7th ACM
CoNEXT Conference, December 2011.

[RFC1323] Jacobson, V., Braden, R., and D. Borman, D., "TCP Extensions
for High Performance", RFC 1323, May 1992. 1992,
<http://www.rfc-editor.org/info/rfc1323>.

[RFC1644] Braden, R., "T/TCP -- TCP Extensions for Transactions
Functional Specification", RFC 1644, July 1994. 1994,
<http://www.rfc-editor.org/info/rfc1644>.

[RFC2460] Deering, S., S. and R. Hinden, R., "Internet Protocol, Version 6
(IPv6) Specification", RFC 2460, December 1998. 1998,
<http://www.rfc-editor.org/info/rfc2460>.

[RFC4987] Eddy, W., "TCP SYN Flooding Attacks and Common
Mitigations", RFC 4987, August 2007. 2007,
<http://www.rfc-editor.org/info/rfc4987>.

[RFC6013] Simpson, W., "TCP Cookie Transactions (TCPCT)", RFC6013, RFC 6013,
January 2011. 2011, <http://www.rfc-editor.org/info/rfc6013>.

[RFC6247] Eggert, L., "Moving the Undeployed TCP Extensions RFC
1072, RFC 1106, RFC 1110, RFC 1145, RFC 1146, RFC 1379,
RFC 1644, and RFC 1693 to Historic Status", RFC 6247, May
2011, <http://www.rfc-editor.org/info/rfc6247>.

[RFC7323] Borman, D., Braden, B., Jacobson, V., and R.
Scheffenegger, Ed., "TCP Extensions for High
Performance", RFC 7323, September 2014,
<http://www.rfc-editor.org/info/rfc7323>.

[SOUDERS11] Souders, S., "Making A Mobile Connection".
http://www.stevesouders.com/blog/2011/09/21/making-a-
mobile-connection/

[BRISCOE12] Briscoe, B., "Some ideas building on draft-ietf-tcpm-
fastopen-01", tcpm list,
http://www.ietf.org/mail-archive/web/tcpm/
current/msg07192.html

[BELSHE11] Belshe, M., "The era of browser preconnect.",
http://www.belshe.com/2011/02/10/
the-era-of-browser-preconnect/

[JIDKT07] Jaiswal, S., Iannaccone, G., Diot, C., Kurose, J.,
Towsley, D., "Measurement and classification of
out-of-sequence packets in a tier-1 IP backbone.".
IEEE/ACM Transactions on Networking (TON), 15(1), 54-66.

[Chrome] Chrome. https://www.google.com/intl/en-US/chrome/browser/ Connection",
<http://www.stevesouders.com/blog/2011/09/21/making-a-
mobile-connection/>.

Appendix A. Example Socket API Changes to support Support TFO

A.1

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 changes, and, hence,
applications to reduce the deployment hurdle.

One solution implemented in Linux 3.7 is introducing a new flag
MSG_FASTOPEN 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 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 TCP-specific feature. A solution is to add a new socket option
TCP_FASTOPEN
option, TCP_FASTOPEN, for TCP sockets. When the option is enabled
before a
connect connect() operation, sendmsg() or sendto() will perform a
Fast Open operation similar to the MSG_FASTOPEN flag described above.
This
approach however approach, however, requires an extra setsockopt() system call.

A.2

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
before listen().

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

Traditionally, accept() returns only after a socket is connected. But
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().

Acknowledgments

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.

Authors' Addresses

Yuchung Cheng
Google, Inc.
1600 Amphitheatre Parkway
Mountain View, CA 94043, USA 94043
United States

EMail: ycheng@google.com

Jerry Chu
Google, Inc.
1600 Amphitheatre Parkway
Mountain View, CA 94043, USA 94043
United States

EMail: hkchu@google.com

Sivasankar Radhakrishnan
Department of Computer Science and Engineering
University of California, San Diego
9500 Gilman Dr Drive
La Jolla, CA 92093-0404
United States

EMail: sivasankar@cs.ucsd.edu

Arvind Jain
Google, Inc.
1600 Amphitheatre Parkway
Mountain View, CA 94043, USA 94043
United States

EMail: arvind@google.com