[Docs] [txt|pdf] [Tracker] [WG] [Email] [Diff1] [Diff2] [Nits]
Versions: (draft-cheng-tcpm-fastopen) 00 01
02 03
Internet Draft Y. Cheng
draft-ietf-tcpm-fastopen-03.txt J. Chu
Intended status: Experimental S. Radhakrishnan
Expiration date: August, 2013 A. Jain
Google, Inc.
Feburary 25, 2013
TCP Fast Open
Status of this Memo
Distribution of this memo is unlimited.
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that other
groups may also distribute working documents as Internet-Drafts.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/1id-abstracts.html
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html
This Internet-Draft will expire in August, 2012.
Copyright Notice
Copyright (c) 2012 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Cheng, et. al. Expires August 2013 [Page 1]
Internet Draft TCP Fast Open February 2013
Abstract
TCP Fast Open (TFO) allows data to be carried in the SYN and SYN-ACK
packets and consumed by the receiving end during the initial
connection handshake, thus saving up to one full round trip time
(RTT) compared to standard TCP which requires a three-way handshake
(3WHS) to complete before data can be exchanged.
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.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Data In SYN . . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.1 Relaxing TCP semantics on duplicated SYNs . . . . . . . . . 4
2.2. SYNs with spoofed IP addresses . . . . . . . . . . . . . . 4
3. Protocol Overview . . . . . . . . . . . . . . . . . . . . . . . 5
4. Protocol Details . . . . . . . . . . . . . . . . . . . . . . . 7
4.1. Fast Open Cookie . . . . . . . . . . . . . . . . . . . . . 7
4.1.1. TCP Options . . . . . . . . . . . . . . . . . . . . . . 7
4.1.2. Server Cookie Handling . . . . . . . . . . . . . . . . 8
4.1.3. Client Cookie Handling . . . . . . . . . . . . . . . . 9
4.2. Fast Open Protocol . . . . . . . . . . . . . . . . . . . . 9
4.2.1. Fast Open Cookie Request . . . . . . . . . . . . . . . 10
4.2.2. TCP Fast Open . . . . . . . . . . . . . . . . . . . . . 11
5. Reliability and Deployment Issues . . . . . . . . . . . . . . . 13
6. Security Considerations . . . . . . . . . . . . . . . . . . . . 14
6.1. Server Resource Exhaustion Attack by SYN Flood with Valid
Cookies . . . . . . . . . . . . . . . . . . . . . . . . . . 14
6.2. Amplified Reflection Attack to Random Host . . . . . . . . 15
6.3 Attacks from behind sharing public IPs (NATs) . . . . . . . 16
7. TFO's Applicability . . . . . . . . . . . . . . . . . . . . . . 17
7.1 Duplicate data in SYNs . . . . . . . . . . . . . . . . . . . 17
7.2 Potential performance improvement . . . . . . . . . . . . . 17
7.3 Example: Web clients and servers . . . . . . . . . . . . . . 17
7.3.1 HTTP request replay . . . . . . . . . . . . . . . . . . 17
7.3.2 HTTP persistent connection . . . . . . . . . . . . . . . 18
8. Performance Experiments . . . . . . . . . . . . . . . . . . . . 18
9. Related Work . . . . . . . . . . . . . . . . . . . . . . . . . 19
9.1. T/TCP . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
9.2. Common Defenses Against SYN Flood Attacks . . . . . . . . . 19
Cheng, et. al. Expires August 2013 [Page 2]
Internet Draft TCP Fast Open February 2013
9.3. TCP Cookie Transaction (TCPCT) . . . . . . . . . . . . . . 20
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 20
11. Acknowledgement . . . . . . . . . . . . . . . . . . . . . . . 20
12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 20
12.1. Normative References . . . . . . . . . . . . . . . . . . . 20
12.2. Informative References . . . . . . . . . . . . . . . . . . 21
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 23
1. Introduction
TCP Fast Open (TFO) 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, and potential new security vulnerabilities and their
mitigation. It also includes discussion of deployment issues and
related proposals. TFO requires extensions to the socket API but this
document does not cover that.
TFO is motivated by the performance needs of today's Web
applications. Network latency is largely determined by a connection's
round-trip time (RTT) and the number of round trips required to
transfer application data. RTT consists of propagation delay and
queuing delay.
Network bandwidth has grown substantially over the past two decades,
potentially reducing queuing delay, while propagation delay is
largely constrained by the speed of light and has remained unchanged.
Therefore reducing the number of round trips has typically become the
most effective way to improve the latency of applications like the
Web [CDCM11].
Current TCP only permits data exchange after 3WHS [RFC793], which
adds one RTT to network latency. For short transfers (e.g., web
objects) this additional RTT is a significant portion of overall
network latency [THK98]. One widely deployed solution is HTTP
persistent connections. However, this solution is limited since hosts
and middle boxes terminate idle TCP connections due to resource
constraints. For example, the Chrome browser keeps TCP connections
idle for up to 5 minutes but 35% of Chrome HTTP requests are made on
new TCP connections [RCCJR11]. We discuss Web applications and TFO in
detail later in section 7.
Cheng, et. al. Expires August 2013 [Page 3]
Internet Draft TCP Fast Open February 2013
2. Data In SYN
Allowing data in SYN packets to be delivered raises two issues
discussed in the following subsections. These issues make TFO
undesirable for certain applications. Therefore TCP implementations
MUST NOT use TFO by default and only use TFO if requested explicitly
by the application on a per service port basis. Applications need to
evaluate TFO applicability (described in Section 7) before using TFO.
2.1 Relaxing TCP semantics on duplicated SYNs
[RFC793] (section 3.4) already allows data in SYN packets but forbids
the receiver from delivering the data to the application until 3WHS
is completed. This is because TCP's initial handshake serves to
capture old or duplicate SYNs.
TFO allows data to be delivered to the application before 3WHS is
completed, thus opening itself to a data integrity issue for the
applications in Section 2.1 in either of the following cases:
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 obsoleted T/TCP protocol employs a new TCP "TAO" option and
connection count to guard against old or duplicate SYNs [RFC1644].
However it is not widely used due to various vulnerabilities
[PHRACK98].
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 7) that are tolerant of duplicate
SYN packets with data. We believe this is the right design trade-off
balancing complexity with usefulness for certain applications.
2.2. SYNs with spoofed IP addresses
Standard TCP suffers from the SYN flood attack [RFC4987] because
bogus SYN packets, i.e., 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. Secondary damage comes from
these SYN requests taking up memory space. Though this is less of an
issue today as servers typically have plenty of memory.
Cheng, et. al. Expires August 2013 [Page 4]
Internet Draft TCP Fast Open February 2013
TFO goes one step further to allow server-side TCP to process and
send up data to the application layer before 3WHS is completed. This
opens up more 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 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
the primary security issues introduced by TFO in Section 3. We defer
further discussion of SYN flood attacks to the "Security
Considerations" section.
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 Cookie Request option.
2. The server generates a cookie and sends it through the Fast Open
Cookie 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 Fast Open Cookie option and data.
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 max amount is
governed by the TCP's congestion control [RFC5681].
Cheng, et. al. Expires August 2013 [Page 5]
Internet Draft TCP Fast Open February 2013
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
Cheng, et. al. Expires August 2013 [Page 6]
Internet Draft TCP Fast Open February 2013
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
security.
4.1.1. TCP Options
Fast Open Cookie Option
The server uses this option to grant a cookie to the client in the
SYN-ACK packet; the client uses it to pass the cookie back to the
server in the SYN packet.
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Kind | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ Cookie ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Kind 1 byte: constant TBD (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 4 to 16 bytes (Length - 2)
Options with invalid Length values or without SYN flag set MUST be
ignored. The minimum Cookie size is 4 bytes. Although the diagram
shows a cookie aligned on 32-bit boundaries, alignment is not
required.
Fast Open Cookie Request Option
The client uses this option in the SYN packet to request a cookie
from a TFO-enabled server
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Kind | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Cheng, et. al. Expires August 2013 [Page 7]
Internet Draft TCP Fast Open February 2013
Kind 1 byte: same as the Fast Open Cookie option
Length 1 byte: constant 2. This distinguishes the option
from the Fast Open cookie option.
Options with invalid Length values, without SYN flag set, or with ACK
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:
1. The cookie authenticates the client's (source) IP address of the
SYN packet. The IP address can 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
is all 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 often 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
Cheng, et. al. Expires August 2013 [Page 8]
Internet Draft TCP Fast Open February 2013
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-client 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.
The server MAY return a cookie request option, e.g., a null cookie,
to signal the support of Fast Open without generating cookies, for
probing or debugging purposes.
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 both client
and server IP dependent. Beside the cookie, we RECOMMEND that the
client caches the MSS and RTT to the server to enhance performance.
The MSS advertised by the server is stored in the cache to determine
the maximum amount of data that can be supported in the SYN packet.
This information is needed because data is sent before the server
announces its MSS in the SYN-ACK packet. Without this information,
the data size in the SYN packet is limited to the default MSS of 536
bytes [RFC1122]. The client SHOULD update the cache MSS value
whenever it discovers new MSS value, e.g., through path MTU
discovery.
Caching RTT allows seeding a more accurate SYN timeout than the
default value [RFC6298]. This lowers the performance penalty if the
network or the server drops the SYN packets with data or the cookie
options (See "Reliability and Deployment Issues" section below).
The cache replacement algorithm is not specified and is left for the
implementations.
Note that before TFO sees wide deployment, clients SHOULD cache
negative responses from servers in order to reduce the amount of
futile TFO attempts. Since TFO is enabled on a per-service port basis
but cookies are independent of service ports, clients' cache should
include remote port numbers too.
4.2. Fast Open Protocol
One predominant requirement of TFO is to be fully compatible with
Cheng, et. al. Expires August 2013 [Page 9]
Internet Draft TCP Fast Open February 2013
existing TCP implementations, both on the client and the server
sides.
The server keeps two variables per listening 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 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
SHOULD 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 TCB 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 Cookie Request
option.
2. The server SHOULD respond with a SYN-ACK based on the procedures
in the "Server Cookie Handling" section. This SYN-ACK SHOULD
contain a Fast Open Cookie option if the server currently supports
TFO for this listener port.
3. If the SYN-ACK contains a Fast Open Cookie option, 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 without valid Fast Open Cookie
Option, 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 causes SYN or SYN-ACK timeouts. We RECOMMEND
both the client and the server 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 penalties (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
Cheng, et. al. Expires August 2013 [Page 10]
Internet Draft TCP Fast Open February 2013
argue the delay in the data transmission till after 3WHS is justified
if the SYN drop is due to network congestion). Next section
describes a heuristic to detect such drops when the client receives
the SYN-ACK.
We also RECOMMEND the client to record servers that failed to respond
to cookie requests and only attempt another cookie request after
certain period. An alternate proposal is to request cookie in FIN
instead since FIN-drop by incompatible middle-box does not affect
latency. However such paths are likely to drop SYN packet with data
later, and many applications close the connections with RST instead,
so the actual benefit of this approach is not clear.
4.2.2. TCP Fast Open
Once the client obtains the cookie from the target server, the client
can perform subsequent TFO connections until the cookie is expired by
the server. The nature of TCP sequencing makes the TFO specific
changes relatively small in addition to [RFC793].
Client: Sending SYN
To open a TFO connection, the client MUST have obtained the cookie
from the server:
1. Send a SYN packet.
a. If the SYN packet does not have enough option space for the
Fast Open Cookie option, abort TFO and fall back to regular 3WHS.
b. Otherwise, include the Fast Open Cookie 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.
3. If RTT is available from the cache, seed SYN timer according to
[RFC6298].
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 Cookie options.
Server: Receiving SYN and responding with SYN-ACK
Cheng, et. al. Expires August 2013 [Page 11]
Internet Draft TCP Fast Open February 2013
Upon receiving the SYN packet with Fast Open Cookie 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
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 to the implementation.
6. Advance to the SYN-RCVD state. If the FastOpened flag is set, the
server MUST follow the congestion control [RFC5681], in particular
the initial congestion window [RFC3390], to send more data packets.
Note that if SYN-ACK is lost, regular TCP reduces the initial
congestion window before sending any data. In this case TFO is
slightly more aggressive in the first data round trip even though
it does not change the congestion control.
If the SYN-ACK timer fires, the server SHOULD retransmit a SYN-ACK
segment with neither data nor Fast Open Cookie 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. Update the cookie cache if the SYN-ACK has a Fast Open Cookie
Option or MSS option or both.
2. Send an ACK packet. Set acknowledgment number to RCV.NXT and
include the data after SND.UNA if data is available.
Cheng, et. al. Expires August 2013 [Page 12]
Internet Draft TCP Fast Open February 2013
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
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. Reliability and Deployment Issues
Network or Hosts Dropping SYN packets with data or unknown options
A study [MAF04] found that some middle-boxes and end-hosts may drop
packets with unknown TCP options incorrectly. 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 re-transmitting SYN without data or cookie
options and we recommend tracking these servers in the client.
Server Farms
A common server-farm setup is to have many physical hosts behind a
load-balancer sharing the same server IP. The load-balancer forwards
new TCP connections to different physical hosts based on certain
load-balancing algorithms. For TFO to work, the physical hosts need
to share the same key and update the key at about the same time.
Network Address Translation (NAT)
The hosts behind NAT sharing same IP address will get the same cookie
to 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 benefit. However, there is no performance
Cheng, et. al. Expires August 2013 [Page 13]
Internet Draft TCP Fast Open February 2013
penalty either as described in Section "Client: Receiving SYN-ACK".
6. 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 generating bogus cookies is
usually cheaper than validating them. But the additional cost of
validating the cookies, inherent to any authentication scheme, may
not be substantial compared to processing a regular SYN packet.
However, the 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). With DHCP, it's possible to
obtain cookies of past IP addresses without compromising any host.
Below we identify new vulnerabilities of TFO and describe the
countermeasures.
6.1. Server Resource Exhaustion Attack by SYN Flood with Valid Cookies
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.
For this reason it is crucial for the TFO server to limit the maximum
number of total pending TFO connection requests, i.e.,
PendingFastOpenRequests. 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.
There are other subtle but important differences in the vulnerability
between TFO and regular TCP handshake. Before the SYN flood attack
broke out in the late '90s, typical listener's max qlen was small,
enough to sustain the highest expected new connection rate and the
average RTT for the SYN-ACK packets to be acknowledged in time. E.g.,
if a server is designed to handle at most 100 connection requests per
second, and the average RTT is 100ms, a max qlen on the order of 10
will be sufficient.
This small max qlen made it very easy for any attacker, even equipped
with just a dailup modem to the Internet, to cause major disruptions
Cheng, et. al. Expires August 2013 [Page 14]
Internet Draft TCP Fast Open February 2013
to a web site by simply throwing a handful of "SYN bombs" at its
victim of choice. But for this attack scheme to work, the attacker
must pick a non-responsive source IP address to spoof with. Otherwise
the SYN-ACK packet will trigger TCP RST from the host whose IP
address has been spoofed, causing corresponding connection to be
removed from the server's listener queue hence defeating the attack.
In other words, the main damage of SYN bombs against the standard TCP
stack is not directly from the bombs 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 bombs 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 bombs as compared
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 qlen. 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.
6.2. Amplified Reflection Attack to Random Host
Limiting PendingFastOpenRequests with a system limit can be done
without Fast Open Cookies and would protect the server from resource
exhaustion. It would also limit how much damage an attacker can cause
through an amplified reflection attack from that server. However, it
would still be vulnerable to an amplified reflection attack from a
large number of servers. An attacker can easily cause damage by
tricking many servers to respond with data packets at once to any
spoofed victim IP address of choice.
With the use of Fast Open Cookies, the attacker would first have to
steal a valid cookie from its target victim. This likely requires the
attacker to compromise the victim host or network first.
The attacker here has little interest in mounting an attack on the
victim host that has already been compromised. But she may be
motivated to disrupt the victim's network. Since a stolen cookie is
only valid for a single server, she has to steal valid cookies from a
Cheng, et. al. Expires August 2013 [Page 15]
Internet Draft TCP Fast Open February 2013
large number of servers and use them before they expire to cause
sufficient damage without triggering the defense in the previous
section.
One can argue that if the attacker has compromised the target network
or hosts, she 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.
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.
6.3 Attacks from behind sharing public IPs (NATs)
An attacker behind NAT can easily obtain valid cookies to launch the
above attack to hurt other clients that share the path. [BOB12]
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 [RFC 1323] to
send non-zero Timestamp Echo Reply in SYN, potentially cause firewall
issues. Therefore we believe the benefit may not outweigh the
drawbacks.
Cheng, et. al. Expires August 2013 [Page 16]
Internet Draft TCP Fast Open February 2013
7. TFO's Applicability
This section is to help applications considering TFO to evaluate
TFO's benefits and drawbacks using a Web client and server
applications as an example throughout.
7.1 Duplicate data in SYNs
It is possible, though uncommon, that using TFO the first data
written to a socket is 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. Thus applications
MUST NOT use TFO unless they can tolerate this behavior.
7.2 Potential performance improvement
TFO is designed for latency-conscious applications that are sensitive
to TCP's initial connection setup delay. For example, many
applications perform short request and response message exchanges. 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 employ long-lived
connections to best take advantage of TCP's built-in congestion
control, and to reduce the impact from TCP's connection setup
overhead. Note that when an application employs too many short-lived
connections, it may negatively impact network stability, as these
connections often exit before TCP's congestion control algorithm
takes effect. Implementations supporting a large number of short-
lived connections should employ temporal sharing of TCB data as
described in [RFC2140].
7.3 Example: Web clients and servers
We look at Web client and server applications that use HTTP and TCP
protocols and follow the guidelines above to evaluate if TFO is safe
and useful for Web.
7.3.1 HTTP request replay
We believe TFO is safe for the Web because even with standard TCP the
Web browser may replay an HTTP request to the remote Web server
multiple times. After sending an HTTP request, the browser could time
out and retry the same request on another TCP connection. This
scenario occurs far more frequently than the SYN duplication issue
Cheng, et. al. Expires August 2013 [Page 17]
Internet Draft TCP Fast Open February 2013
presented by TFO. To ensure transactional behavior, Web sites employ
application-specific mechanisms such as including unique identifiers
in the data.
7.3.2 HTTP persistent connection
Next we evaluate if the Web can benefit from TFO given that HTTP
persistent connection support is already widely deployed.
TCP connection setup overhead has long been identified as a
performance bottleneck for web applications [THK98]. HTTP persistent
connection support was proposed to mitigate this issue and has been
widely deployed. However, 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.
Can the utilization rate of such connections increase by keeping idle
connections even longer? Unfortunately, such an approach is
problematic due to middle-boxes and the rapidly growing share of
mobile end hosts. Thus one major issue faced by persistent
connections is NAT. Studies [HNESSK10][MQXMZ11] show that the
majority of home routers and ISPs fail to meet the the 124-minute
idle timeout mandated in [RFC5382]. In [MQXMZ11], 35% of mobile ISPs
timeout idle connections within 30 minutes. The end hosts attempting
to use these broken connections are often forced to wait for a
lengthy TCP timeout, as they often receive no signal when middleboxes
break their connections. Thus browsers risk large performance
penalties when keeping idle connections open.
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].
Since TFO data duplication presents no new issues and HTTP persistent
connection support has many limitations, Web applications can safely
use TFO and will likely achieve performance gains. The next section
presents more empirical data of the potential performance benefit.
8. Performance Experiments
[RCCJR11] studied Chrome browser performance based on 28 days of
global statistics. Chrome browser keeps idle HTTP persistent
connections up to 5 to 10 minutes. However the average number of the
Cheng, et. al. Expires August 2013 [Page 18]
Internet Draft TCP Fast Open February 2013
transactions per connection is only 3.3. Due to the low utilization,
TCP 3WHS accounts up to 25% of the HTTP transaction network latency.
The authors tested a Linux TFO implementation with TFO enabled Chrome
browser on popular web sites in emulated environments such as
residential broadband and mobile networks. They showed that TFO
improves page load time by 10% to 40%. More details on the design
tradeoffs and measurement can be found at [RCCJR11].
9. Related Work
9.1. 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. Its TAO option and
connection count, besides adding complexity, require the server to
keep state per remote host, while still leaving it wide open for
attacks. It is trivial for an attacker to fake a CC value that will
pass the TAO test. Unfortunately, in the end its scheme is still not
100% bullet proof as pointed out by [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.
9.2. Common Defenses Against SYN Flood Attacks
TFO is still vulnerable to SYN flood attacks just like normal TCP
handshakes, but the damage may be much worse, thus deserves a careful
thought.
There have been plenty of studies on how to mitigate attacks from
regular SYN flood, i.e., SYN without data [RFC4987]. 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.
Cheng, et. al. Expires August 2013 [Page 19]
Internet Draft TCP Fast Open February 2013
9.3. 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. However, TCPCT and TFO are designed for
different goals and they are not compatible.
The TCPCT server does not keep any connection state during the
handshake, therefore the server application needs to consume the data
in SYN and (immediately) produce the data in SYN-ACK before sending
SYN-ACK. Otherwise the application's response has to wait until
handshake completes. In contrary, TFO allows server to respond data
during handshake. Therefore for many request-response style
applications, TCPCT may not achieve same latency benefit as TFO.
Rapid-Restart [SIMPSON11] is based on TCPCT and shares similar goal
as TFO. In Rapid-Restart, both the server and the client retain the
TCP control blocks after a connection is terminated in order to
allow/resume data exchange in next connection handshake. In contrary,
TFO does not require keeping both TCB on both sides and is more
scalable.
10. IANA Considerations
The Fast Open Cookie Option and Fast Open Cookie Request Option
define no new namespace. The options require IANA allocate one value
from the TCP option Kind namespace. Early implementation before the
allocation SHOULD follow [EXPOPT] and use experimental option 254 and
magic number 0xF989 (16 bits), and migrate to the new option after
the allocation according.
11. Acknowledgement
We thank Rick Jones, Bob Briscoe, Adam Langley, Matt Mathis, Neal
Cardwell, Roberto Peon, and Tom Herbert for their feedbacks. We
especially thank Barath Raghavan for his contribution on the security
design of Fast Open.
12. References
12.1. Normative References
[RFC793] Postel, J. "Transmission Control Protocol", RFC 793,
September 1981.
[RFC1122] Braden, R., Ed., "Requirements for Internet Hosts -
Communication Layers", STD 3, RFC 1122, October 1989.
Cheng, et. al. Expires August 2013 [Page 20]
Internet Draft TCP Fast Open February 2013
[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.
[RFC6298] Paxson, V., Allman, M., Chu, J. and M. Sargent, "Computing
TCP's Retransmission Timer", RFC 6298, June 2011.
12.2. Informative References
[AERG11] M. Al-Fares, K. Elmeleegy, B. Reed, and I. Gashinsky,
"Overclocking the Yahoo! CDN for Faster Web Page Loads". In
Proceedings of Internet Measurement Conference, November
2011.
[CDCM11] Chu, J., Dukkipati, N., Cheng, Y. and M. Mathis,
"Increasing TCP's Initial Window", Internet-Draft draft-
ietf-tcpm-initcwnd-02.txt (work in progress), October 2011.
[EXPOPT] Touch, Joe, "Shared Use of Experimental TCP Options",
Internet-Draft draft-ietf-tcpm-experimental-options (work
in progress), October 2012.
[HNESSK10] S. Haetoenen, A. Nyrhinen, L. Eggert, S. Strowes, P.
Sarolahti, M. Kojo., "An Experimental Study of Home Gateway
Characteristics". In Proceedings of Internet Measurement
Conference. Octobor 2010
[HNRGHT11] M. Honda, Y. Nishida, C. Raiciu, A. Greenhalgh, M.
Handley, H. Tokuda, "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] Z. Mao, Z. Qian, Q. Xu, Z. Mao, M. Zhang. "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
Cheng, et. al. Expires August 2013 [Page 21]
Internet Draft TCP Fast Open February 2013
http://www.phrack.com/issues.html?issue=53&id=6
[QWGMSS11] F. Qian, Z. Wang, A. Gerber, Z. Mao, S. Sen, O.
Spatscheck. "Profiling Resource Usage for Mobile
Applications: A Cross-layer Approach", In Proceedings of
International Conference on Mobile Systems. April 2011.
[RCCJR11] Radhakrishnan, S., Cheng, Y., Chu, J., Jain, A. and
Raghavan, B., "TCP Fast Open". In Proceedings of 7th ACM
CoNEXT Conference, December 2011.
[RFC1644] Braden, R., "T/TCP -- TCP Extensions for Transactions
Functional Specification", RFC 1644, July 1994.
[RFC2140] Touch, J., "TCP Control Block Interdependence", RFC2140,
April 1997.
[RFC4987] Eddy, W., "TCP SYN Flooding Attacks and Common
Mitigations", RFC 4987, August 2007.
[RFC6013] Simpson, W., "TCP Cookie Transactions (TCPCT)", RFC6013,
January 2011.
[SIMPSON11] Simpson, W., "Tcp cookie transactions (tcpct) rapid
restart", Internet draft draft-simpson-tcpct-rr-02.txt
(work in progress), July 2011.
[SOUDERS11] S. Souders. "Making A Mobile Connection".
http://www.stevesouders.com/blog/2011/09/21/making-a-
mobile-connection/
[THK98] Touch, J., Heidemann, J., Obraczka, K., "Analysis of HTTP
Performance", USC/ISI Research Report 98-463. December
1998.
[BOB12] Briscoe, B., "Some ideas building on draft-ietf-tcpm-
fastopen-01", tcpm list,
http://www.ietf.org/mail-archive/web/tcpm/current/
msg07192.html
Cheng, et. al. Expires August 2013 [Page 22]
Internet Draft TCP Fast Open February 2013
Authors' Addresses
Yuchung Cheng
Google, Inc.
1600 Amphitheatre Parkway
Mountain View, CA 94043, USA
EMail: ycheng@google.com
Jerry Chu
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
Cheng, et. al. Expires August 2013 [Page 23]
Html markup produced by rfcmarkup 1.104, available from
http://tools.ietf.org/tools/rfcmarkup/