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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   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.


   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   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

   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

   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

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

   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

   The server supports the cookie generation and verification

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

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

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

   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

   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

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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

   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

   3. If RTT is available from the cache, seed SYN timer according to

   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-
   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

   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

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

   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

   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

   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

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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

   [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

   [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

   [LANGLEY06] Langley, A, "Probing the viability of TCP extensions",


   [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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   [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

   [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".

   [THK98]   Touch, J., Heidemann, J., Obraczka, K., "Analysis of HTTP
             Performance", USC/ISI Research Report 98-463. December

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Author's Addresses

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

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

   Sivasankar Radhakrishnan
   Google, Inc.
   1600 Amphitheatre Parkway
   Mountain View, CA 94043, USA

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


   Funding for the RFC Editor function is currently provided by the
   Internet Society.

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