Internet Draft Y. Cheng
draft-ietf-tcpm-fastopen-01.txt J. Chu
Intended status: Experimental S. Radhakrishnan
Expiration date: Feburary, 2013 A. Jain
Google, Inc.
July 16, 2012
TCP Fast Open
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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 providing a saving of up to one full round
trip time (RTT) compared to standard TCP requiring 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.
1. Introduction
TCP Fast Open (TFO) enables data to be exchanged safely during TCP
connection handshake.
This document describes a design that enables qualified applications
to attain a round trip saving 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. The
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 discussions on
deployment issues and related proposals. TFO requires extensions to
the existing socket API, which will be covered in a separate
document.
TFO is motivated by the performance need of today's Web applications.
Network latency is determined by the round-trip time (RTT) and the
number of round trips required to transfer application data. RTT
consists of transmission delay and propagation delay. Network
bandwidth has grown substantially over the past two decades, much
reducing the transmission delay, while propagation delay is largely
constrained by the speed of light and has remained unchanged.
Therefore reducing the number of round trips has become the most
effective way to improve the latency of Web applications [CDCM11].
Standard TCP only permits data exchange after 3WHS [RFC793], which
introduces one RTT delay to the network latency. For short transfers,
e.g., web objects, this additional RTT becomes a significant portion
of the 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
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constraints. E.g., the Chrome browser keeps TCP connections idle up
to 5 minutes but 35% of Chrome HTTP requests are made on new TCP
connections. More discussions on HTTP persistent connections are in
section 7.1.
2. Data In SYN
[RFC793] (section 3.4) already allows data in SYN packets but forbids
the receiver to deliver the data to the application until 3WHS is
completed. This is because TCP's initial handshake serves to capture
- Old or duplicate SYNs
- SYNs with spoofed IP addresses
TFO allows data to be delivered to the application before 3WHS is
completed, thus opening itself to a possible data integrity problem
caused by the dubious SYN packets above.
2.1. TCP Semantics and Duplicate SYNs
A past proposal called T/TCP employs a new TCP "TAO" option and
connection count to guard against old or duplicate SYNs [RFC1644].
The solution is complex, involving state tracking on per remote peer
basis, and is vulnerable to IP spoofing attack. Moreover, it has been
shown that even with all the complexity, T/TCP is still not 100%
bullet proof. Old or duplicate SYNs may still slip through and get
accepted by a T/TCP server [PHRACK98].
Rather than trying to capture all the dubious SYN packets to make TFO
100% compatible with TCP semantics, we've made a design decision
early on to accept old SYN packets with data, i.e., to restrict TFO
for a class of applications that are tolerant of duplicate SYN
packets with data, e.g., idempotent or query type transactions. We
believe this is the right design trade-off balancing complexity with
usefulness. There is a large class of applications that can tolerate
dubious transaction requests.
For this reason, TFO MUST be disabled by default, and only enabled
explicitly by applications on a per service port basis.
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
faked SYN requests taking up memory space. This is normally not an
issue today with typical servers having plenty of memory.
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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 much 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 the regular SYN flood attack
exist and have been well documented [RFC4987]. Unfortunately none are
applicable to TFO. We propose a server supplied cookie to mitigate
most of the security risks introduced by TFO. A more thorough
discussion on SYN flood attack against TFO is deferred 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 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].
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.
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The client can perform many TFO 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
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4. Protocol Details
4.1. Fast Open Cookie
The Fast Open Cookie is invented 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, that 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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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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 cookie expires after a certain amount of time. The reason 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 in the cookie.
4. The generation and verification are fast relative to the rest of
SYN and SYN-ACK processing.
5. 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.
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 [RCCJB11].
Note that if only one valid cookie is allowed per-client and the
server can regenerate the cookie independently, the best validation
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process may be for the server 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 without additional computation.
Also note the server may want to use special cookie values, e.g.,
"0", for specific scenarios. For example, the server wants to notify
the client the support of TFO, but chooses not to return a valid
cookie for security or performance reasons upon receiving a TFO
request.
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 are advised to
also 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
existing TCP implementations, both on the client and the server
sides.
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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
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
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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.
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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
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.
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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 MAY send more data packets before the handshake completes. The
maximum amount is ruled by the initial congestion window and the
receiver window [RFC3390].
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.
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.
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
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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 retransmitting 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
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 two 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
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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
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
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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
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.
7. Web Performance
7.1. HTTP persistent connection
TCP connection setup overhead has long been identified as a
performance bottleneck for web applications [THK98]. HTTP persistent
connection was proposed to mitigate this issue and has been widely
deployed. However, [RCCJR11][AERG11] show that the average number of
transactions per connection is between 2 and 4, based on large-scale
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measurements from both servers and clients. In these studies, the
servers and clients both kept the idle connections up to several
minutes, well into the human think time.
Can the utilization rate increase by keeping connections even longer?
Unfortunately, this is problematic due to middle-boxes and rapidly
growing mobile end hosts. One major issue is NAT. Studies
[HNESSK10][MQXMZ11] show that the majority of home routers and ISPs
fail to meet the the 124 minutes idle timeout mandated in [RFC5382].
In [MQXMZ11], 35% of mobile ISPs timeout idle connections within 30
minutes. NAT boxes do not possess a reliable mechanism to notify
endhosts when idle connections are removed from local tables, either
due to resource constraints such as mapping table size, memory, or
lookup overhead, or due to the limited port number and IP address
space. Moreover, unmapped packets received by NAT boxes are often
dropped silently. (TCP RST is not required by RFC5382.) The end host
attempting to use these broken connections are often forced to wait
for a lengthy TCP timeout. Thus the browser risks large performance
penalty 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 a prominent issue in modern LTE devices that
mobile browsers close the HTTP connections within seconds or even
immediately [SOUDERS11].
Idle connections also consume more memory resources. Due to the
complexity of today's web applications, the application layer often
needs orders of magnitude more memory than the TCP connection
footprint. As a result, servers need to implement advanced resource
management in order to support a large number of idle connections.
7.2 Case Study: Chrome Browser
[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
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 detailed on the design
tradeoffs and measurement can be found at [RCCJB11].
8. TFO's Applicability
TFO aims at latency conscious applications that are sensitive to
TCP's initial connection setup delay. These application protocols
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often employ short-lived TCP connections, or employ long-lived
connections but are more sensitive to the connection setup delay due
to, e.g., a more strict connection failover requirement.
Only transaction-type applications where RTT constitutes a
significant portion of the total end-to-end latency will likely
benefit from TFO. Moreover, the client request must fit in the SYN
packet. Otherwise there may not be any saving in the total number of
round trips required to complete a transaction.
To the extent possible applications protocols SHOULD employ long-
lived connections to best take advantage of TCP's built-in congestion
control algorithm, and to reduce the impact from TCP's connection
setup overhead. E.g., for the web applications, P-HTTP will likely
help and is much easier to deploy hence should be attempted first.
TFO will likely provide further latency reduction on top of P-HTTP.
But the additional benefit will depend on how much persistency one
can get from HTTP in a given operating environment.
One alternative to short-lived TCP connection might be UDP, which is
connectionless hence doesn't inflict any connection setup delay, and
is best suited for application protocols that are transactional.
Practical deployment issues such as middlebox and/or firewall
traversal may severely limit the use of UDP based application
protocols though.
Note that when the application employs too many short-lived
connections, it may negatively impact network stability, as these
connections often exit before TCP's congestion control algorithm
kicks in. Implementations supporting large number of short-lived
connections should employ temporal sharing of TCB data as described
in [RFC2140].
More discussion on TCP Fast Open and its projected performance
benefit can be found in [RCCJB11].
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
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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.
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.
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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.
11. Acknowledgements
The authors would like to thank Tom Herbert, Rick Jones, Adam
Langley, Mathew Mathis, Roberto Peon, and Barath Raghavan for their
insightful comments.
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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.
[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.
[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
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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
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.
[RCCJB11] Radhakrishnan, S., Cheng, Y., Chu, J., Jain, A. and B.
Raghavan, "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.
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Author's 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
Google, Inc.
1600 Amphitheatre Parkway
Mountain View, CA 94043, USA
EMail: sivasankar@cs.ucsd.edu
Arvind Jain
Google, Inc.
1600 Amphitheatre Parkway
Mountain View, CA 94043, USA
EMail: arvind@google.com
Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
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