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Versions: 00 01 02 draft-ietf-tcpm-fastopen

Internet Draft                                                  Y. Cheng
draft-cheng-tcpm-fastopen-00.txt                                  J. Chu
Intended status: Experimental                           S. Radhakrishnan
Creation date: March 7, 2011                                     A. Jain
Expiration date: September 8, 2011                          Google, Inc.


                             TCP Fast Open

Status of this Memo

   Distribution of this memo is unlimited.

   This Internet-Draft is submitted in full conformance with the
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   This Internet-Draft will expire on September 8, 2011.

Copyright Notice

   Copyright (c) 2011 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
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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.




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Abstract

   TCP Fast Open (TFO) allows data to be carried in the SYN or 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 [CDCM10].

   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 4 minutes but 35% of Chrome HTTP requests are made on new TCP
   connections.

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 allow 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 allow
       more than one valid cookie per client at any given time. But this
       is all server implementation dependent and transparent to the
       client. A client only needs to remember one valid cookie per
       server IP.

   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.

4.1.3. Client Cookie Handling



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   The client MUST cache cookies from different 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].

   Caching RTT allows seeding a more accurate SYN timeout than the
   default value [RFC2988]. 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.

4.2. Fast Open Protocol

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

   The server keeps two variables per listening 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 the system limit, the server
   SHOULD set FastOpenEnabled off. 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



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       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 valid 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. We also RECOMMEND
   the client to record servers that failed to respond to cookie
   requests and only attempt another cookie request after certain
   period.

4.2.2. TCP Fast Open

   Once the client obtains the cookie from the target server, 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.




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   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
       [RFC2988].

   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. If the cookie is invalid, i.e., the cookie does not authenticate
       the source IP address of the SYN packet, send a SYN-ACK packet
       acknowledging only the SYN sequence. In addition, include a Fast
       Open Cookie Option with a new cookie. Go to step 7.

   2. If PendingFastOpenRequests is over the system limit, reset
       FastOpenEnabled flag and send a SYN-ACK acknowledging only the
       SYN sequence. Go to step 7.

   3. Send the SYN-ACK packet acknowledging the SYN and data sequence.
       The server MAY include data in the SYN-ACK packet.

   4. Buffer the data and notify the application.

   5. Set FastOpened flag and increment PendingFastOpenRequests.

   6. 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].

   7. Advance to the SYN-RCVD state.

   If the SYN-ACK timer fires, the server SHOULD retransmit a SYN-ACK
   packet without data and 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.

   2. Send an ACK packet. Set acknowledgment number to RCV.NXT and



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       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 will retransmit it in
   the ACK packet. The data exchange will start after the handshake like
   a regular TCP connection.

   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. Another study
   [LANGLEY06] found that 6% of the probed paths on the Internet drop
   SYN packets with data. 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



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   against spoofed SYN floods with invalid cookies using existing
   techniques [RFC4987].

   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
   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.



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   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
   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 to not respond with data until
   handshake finishes, i.e., disallow step 6 in "Server receiving SYN-



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   ACK" section. 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. Related Work

7.1. 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.

7.2. 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.

7.3. 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



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   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.

   Without state kept on the server side, TCPCP relies on the client
   side to retransmit the SYN request with data in order to recover from
   possible loss of packet from server response. This may cause a lot
   more dubious connection requests. It also limits the response to only
   one packet, to fit completely within the SYN-ACK packet. For some TCP
   applications, in particular web applications, this does not provide
   enough latency benefit by sending one data packet one RTT earlier.

8. 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.

9. Acknowledgements

   The authors would like to thank Tom Herbert, Adam Langley, Roberto
   Peon, Mathew Mathis, and Barath Raghavan for their insightful
   comments.






















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10. References

10.1. 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.

   [RFC2988] Paxson, V. and M. Allman, "Computing TCP's Retransmission
             Timer", RFC 2988, November 2000.

   [RFC5681] Allman, M., Paxson, V. and E. Blanton, "TCP Congestion
             Control", RFC 5681, September 2009.

10.2. Informative References

   [RFC1644]  Braden, R., "T/TCP -- TCP Extensions for Transactions
             Functional Specification", RFC 1644, July 1994.

   [RFC4987] Eddy, W., "TCP SYN Flooding Attacks and Common
             Mitigations", RFC 4987, August 2007.

   [RFC6013] Simpson, W., "TCP Cookie Transactions (TCPCT)", RFC6013,
             January 2011.

   [CDCM10] Chu, J., Dukkipati, N., Cheng, Y. and M. Mathis, "Increasing
             TCP's Initial Window", Internet-Draft draft-ietf-tcpm-
             initcwnd-00.txt (work in progress), October 2010.

   [THK98] Touch, J., Heidemann, J., Obraczka, K., "Analysis of HTTP
             Performance", USC/ISI Research Report 98-463. December
             1998.

   [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

   [MAF04] Medina, A., Allman, M., and S. Floyd, "Measuring Interactions
             Between Transport Protocols and Middleboxes", Proceedings
             4th ACM SIGCOMM/USENIX Conference on Internet Measurement,
             October 2004.

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





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Author's Addresses

   Yuchung Cheng
   Google, Inc.
   1600 Amphitheatre Parkway
   Mountain View, CA 94043, USA
   EMail: ycheng@google.com

   H.K. 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@google.com

   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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