Internet DRAFT - draft-ietf-tcpm-fastopen
draft-ietf-tcpm-fastopen
Internet Draft Y. Cheng
draft-ietf-tcpm-fastopen-10.txt J. Chu
Intended status: Experimental S. Radhakrishnan
Expiration date: April, 2015 A. Jain
Google
September 29, 2014
TCP Fast Open
Abstract
This document describes an experimental TCP mechanism 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 detailed in the
Applicability section.
Status of this Memo
Distribution of this memo is unlimited.
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Copyright Notice
Copyright (c) 2014 IETF Trust and the persons identified as the
document authors. All rights reserved.
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This document is subject to BCP 78 and the IETF Trust's Legal
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Data In SYN . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.1 Relaxing TCP Semantics on Duplicated SYNs . . . . . . . . . 4
2.2. SYNs with Spoofed IP Addresses . . . . . . . . . . . . . . 4
3. Protocol Overview . . . . . . . . . . . . . . . . . . . . . . . 5
4. Protocol Details . . . . . . . . . . . . . . . . . . . . . . . 6
4.1. Fast Open Cookie . . . . . . . . . . . . . . . . . . . . . 6
4.1.1. Fast Open option . . . . . . . . . . . . . . . . . . . 7
4.1.2. Server Cookie Handling . . . . . . . . . . . . . . . . 7
4.1.3. Client Cookie Handling . . . . . . . . . . . . . . . . 8
4.1.3.1 Client Caching Negative Responses . . . . . . . . . 9
4.2. Fast Open Protocol . . . . . . . . . . . . . . . . . . . . 9
4.2.1. Fast Open Cookie Request . . . . . . . . . . . . . . . 10
4.2.2. TCP Fast Open . . . . . . . . . . . . . . . . . . . . . 11
5. Security Considerations . . . . . . . . . . . . . . . . . . . . 13
5.1. Resource Exhaustion Attack by SYN Flood with Valid
Cookies . . . . . . . . . . . . . . . . . . . . . . . . . . 13
5.1.1 Attacks from behind Shared Public IPs (NATs) . . . . . . 14
5.2. Amplified Reflection Attack to Random Host . . . . . . . . 14
6. TFO's Applicability . . . . . . . . . . . . . . . . . . . . . . 15
6.1 Duplicate Data in SYNs . . . . . . . . . . . . . . . . . . . 15
6.2 Potential Performance Improvement . . . . . . . . . . . . . 16
6.3. Example: Web Clients and Servers . . . . . . . . . . . . . 16
6.3.1. HTTP Request Replay . . . . . . . . . . . . . . . . . . 16
6.3.2. HTTP over TLS (HTTPS) . . . . . . . . . . . . . . . . . 16
6.3.3. Comparison with HTTP Persistent Connections . . . . . . 17
6.3.4. Load Balancers and Server farms . . . . . . . . . . . . 17
7. Open Areas for Experimentation . . . . . . . . . . . . . . . . 17
7.1. Performance impact due to middle-boxes and NAT . . . . . . 18
7.2. Impact on congestion control . . . . . . . . . . . . . . . 18
7.3. Cookie-less Fast Open . . . . . . . . . . . . . . . . . . . 18
8. Related Work . . . . . . . . . . . . . . . . . . . . . . . . . 19
8.1. T/TCP . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
8.2. Common Defenses Against SYN Flood Attacks . . . . . . . . . 19
8.3. Speculative Connections by the Applications . . . . . . . . 19
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8.4. Fast Open Cookie in FIN . . . . . . . . . . . . . . . . . . 19
8.5. TCP Cookie Transaction (TCPCT) . . . . . . . . . . . . . . 20
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 20
10. Acknowledgement . . . . . . . . . . . . . . . . . . . . . . . 20
11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 20
11.1. Normative References . . . . . . . . . . . . . . . . . . . 20
11.2. Informative References . . . . . . . . . . . . . . . . . . 21
Appendix A. Example Socket API Changes to support TFO . . . . . . 22
A.1 Active Open . . . . . . . . . . . . . . . . . . . . . . . . 22
A.2 Passive Open . . . . . . . . . . . . . . . . . . . . . . . . 23
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 23
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
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.
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 refers to TCP Fast Open. Client refers to the TCP's active open
side and server refers to the TCP's passive open side.
2. Data In SYN
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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 3WHS is completed. This is because TCP's
initial handshake serves to capture old or duplicate SYNs.
To enable applications exchange data in TCP handshake, TFO removes
the constraint and allows data in SYN packets to be delivered to the
application. This change of TCP semantic raises two issues discussed
in the following subsections, making TFO unsuitable for certain
applications.
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 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 2MSL TIMEWAIT
state).
The now obsoleted T/TCP [RFC1644] attempted to address these issues.
It was not successful and not deployed due to various vulnerabilities
as described in the Related Work section. 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
completely until timeouts.
TFO goes one step further to allow server-side TCP to send up data to
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the application layer before 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 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 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].
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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
server connections.
Requesting Fast Open Cookie in connection 1:
TCP A (Client) TCP 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)
______________ _____________
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 handshake.
The cookie is a message authentication code 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
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security.
4.1.1. Fast Open option
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Kind | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ Cookie ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Kind 1 byte: constant-TBD (to be assigned by IANA)
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 cookie is not present or 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 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 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 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 can not 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
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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 Consideration"
section. 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 and verification
operations:
- GetCookie(IP_Address): returns a (new) cookie
- IsCookieValid(IP_Address, Cookie): checks if the cookie is valid,
i.e., it has not expired and it 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 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 multi-homed 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
addresses 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
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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 1240
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.
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
server not acknowledging the data in 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
middle-boxes or firewall blocking the connection completely after it
sees data in SYN. If the client does not react to these negative
responses and continue 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 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, both on the client and the server
sides.
The server keeps two variables per listening socket (IP address &
port):
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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 options.
PendingFastOpenRequests: tracks 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.
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. 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. Otherwise, if the SYN-ACK is
first seen, i.e., 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 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 after 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.
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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 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 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 returns false, go to step 5.
4. Buffer the data and notify the application. Set FastOpened flag
and increment PendingFastOpenRequests.
5. Send the SYN-ACK packet. The packet MAY include a Fast Open
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Option. If 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 application 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 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 MSS 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 with 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
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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 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 Bot-nets, triggering other defenses and alerts.
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". Then
subsequent TFO requests will be downgraded to regular connection
requests, i.e., with the data dropped and only SYN acknowledged. This
allows regular SYN flood defense techniques [RFC4987] like 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 packets from the
spoofed host will fuel rather than defeat the SYN floods as compared
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to the non-TFO case, because the attacker can flood more SYNs with
data to cost more 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 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 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] to send
non-zero Timestamp Echo Reply in 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
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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 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 collect valid
cookies. The attacker then actively or passively releases his IP
address. When the IP address is re-assigned to a victim, the attacker
now owning a different IP address, floods spoofed Fast Open requests
to perform an amplified reflection attack on the victim.
The best defense is for the server not to respond with data until
handshake finishes. In this case the risk of 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 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 a JavaScript process that sends data to the server. A
proposed socket API change is 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
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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
about the probability of receiving duplicated SYN or SYN-ACK with
data in non-Tier 1 networks.
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 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
header).
On the other hand, TFO is particularly useful for GET requests. GET
requests 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 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)
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For TLS over TCP, it is safe and useful to include TLS CLIENT_HELLO
in the SYN packet to save one RTT in TLS handshake. There is no
concern about violating idem-potency. In 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] 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] 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 idle connections within 30 minutes. End hosts, unaware of
silent middle-box 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 devices that mobile browsers close HTTP connections within
seconds or even immediately [SOUDERS11].
[RCCJR11] studied 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 the average
number of the transactions per connection is only 3.3 and 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 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 for a particular client IP
address. Otherwise a client may get different cookies across
connections; its Fast Open attempts would fall back to 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
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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 to middle-boxes and NAT
[MAF04] found that some middle-boxes and end-hosts may drop packets
with unknown TCP options. Studies [LANGLEY06, HNRGHT11] both 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 regular TCP handshake and re-transmitting
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 behaviors.
Another interesting study is the loss of TFO performance benefit
behind certain carrier-grade NAT. 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 carrier-
grade NAT 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 "Client: Receiving SYN-ACK".
7.2. Impact on congestion control
Although TFO does not directly change the congestion control, there
are subtle cases that it may. When 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 useful.
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
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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, among other things, hence 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 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, and makes TFO much
simpler and more appealing to TCP implementers and users.
8.2. Common Defenses Against SYN Flood Attacks
[RFC4987] studies on mitigating attacks from regular SYN flood, i.e.,
SYN without data. But from the stateless SYN-cookies to the stateful
SYN Cache, none can preserve data sent with SYN safely while still
providing an effective defense.
The best defense may be to simply 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" section 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
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An alternate proposal is to request a TFO cookie in the FIN instead,
since FIN-drop by incompatible middle-boxes 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 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 such as Web may be
worse than with TFO.
9. IANA Considerations
IANA is requested to allocate one value from the TCP Option Kind
Numbers: The constant-TBD in Section 4.1.1 has to be replaced with
the newly assigned value. The length of the new TCP option Kind is
variable and the Meaning should be set to "TCP Fast Open Cookie".
Early implementation before the IANA allocation SHOULD follow
[RFC6994] and use experimental option 254 and magic number 0xF989 (16
bits), then migrate to the new option after the allocation
accordingly.
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. Normative References
[RFC793] Postel, J. "Transmission Control Protocol", RFC 793,
September 1981.
[RFC1122] Braden, R., Ed., "Requirements for Internet Hosts -
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Communication Layers", STD 3, RFC 1122, October 1989.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC5382] S. Guha, Ed., Biswas, K., Ford B., Sivakumar S., Srisuresh,
P., "NAT Behavioral Requirements for TCP", RFC 5382
[RFC5681] Allman, M., Paxson, V. and E. Blanton, "TCP Congestion
Control", RFC 5681, September 2009
[RFC6994] Touch, Joe, "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. Informative References
[AERG11] Al-Fares, M., Elmeleegy, K., Reed, B., Gashinsky, I.,
"Overclocking the Yahoo! CDN for Faster Web Page Loads".
In Proceedings of Internet Measurement Conference,
November 2011.
[HNESSK10] Haetoenen, S., Nyrhinen, A., Eggert, L., Strowes, S.,
Sarolahti, P., Kojo., M., "An Experimental Study of Home
Gateway Characteristics". In Proceedings of Internet
Measurement Conference. October 2010
[HNRGHT11] Honda, M., Nishida, Y., Raiciu, C., Greenhalgh, A.,
Handley, M., Tokuda, H., "Is it Still Possible to
Extend TCP?". In Proceedings of Internet Measurement
Conference. November 2011.
[LANGLEY06] Langley, A, "Probing the viability of TCP extensions",
URL http://www.imperialviolet.org/binary/ecntest.pdf
[MAF04] Medina, A., Allman, M., and S. Floyd, "Measuring
Interactions Between Transport Protocols and
Middleboxes". In Proceedings of Internet Measurement
Conference, October 2004.
[MQXMZ11] Wang, Z., Qian, Z., Xu, Q., Mao, Z., Zhang, M.,
"An Untold Story of Middleboxes in Cellular Networks".
In Proceedings of SIGCOMM. August 2011.
[PHRACK98] "T/TCP vulnerabilities", Phrack Magazine, Volume 8, Issue
53 artical 6. July 8, 1998. URL
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http://www.phrack.com/issues.html?issue=53&id=6
[RCCJR11] Radhakrishnan, S., Cheng, Y., Chu, J., Jain, A.,
Raghavan, B., "TCP Fast Open". In Proceedings of 7th
ACM CoNEXT Conference, December 2011.
[RFC1323] Jacobson, V., Braden, R., Borman, D., "TCP Extensions for
High Performance", RFC 1323, May 1992.
[RFC1644] Braden, R., "T/TCP -- TCP Extensions for Transactions
Functional Specification", RFC 1644, July 1994.
[RFC2460] Deering, S., Hinden, R., "Internet Protocol, Version 6
(IPv6) Specification", RFC 2460, December 1998.
[RFC4987] Eddy, W., "TCP SYN Flooding Attacks and Common
Mitigations", RFC 4987, August 2007.
[RFC6013] Simpson, W., "TCP Cookie Transactions (TCPCT)", RFC6013,
January 2011.
[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/
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 connect() call to minimize API changes and hence
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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 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 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 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() 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 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 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 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().
Authors' Addresses
Yuchung Cheng
Google, Inc.
1600 Amphitheatre Parkway
Mountain View, CA 94043, USA
EMail: ycheng@google.com
Jerry Chu
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Google, Inc.
1600 Amphitheatre Parkway
Mountain View, CA 94043, USA
EMail: hkchu@google.com
Sivasankar Radhakrishnan
Department of Computer Science and Engineering
University of California, San Diego
9500 Gilman Dr
La Jolla, CA 92093-0404
EMail: sivasankar@cs.ucsd.edu
Arvind Jain
Google, Inc.
1600 Amphitheatre Parkway
Mountain View, CA 94043, USA
EMail: arvind@google.com
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