TCP Fast Open
draft-ietf-tcpm-fastopen-03

Internet Draft                                                  Y. Cheng
draft-ietf-tcpm-fastopen-03.txt                                   J. Chu
Intended status: Experimental                           S. Radhakrishnan
Expiration date: August, 2013                                    A. Jain
                                                            Google, Inc.
                                                       Feburary 25, 2013

                             TCP Fast Open

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   Copyright (c) 2012 IETF Trust and the persons identified as the
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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 saving up to one full round trip time
   (RTT) compared to standard TCP which requires a three-way handshake
   (3WHS) to complete before data can be exchanged.

Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [RFC2119].
   TFO refers to TCP Fast Open. Client refers to the TCP's active open
   side and server refers to the TCP's passive open side.

Table of Contents

   1. Introduction  . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2. Data In SYN . . . . . . . . . . . . . . . . . . . . . . . . . .  4
     2.1 Relaxing TCP semantics on duplicated SYNs  . . . . . . . . .  4
     2.2. SYNs with spoofed IP addresses  . . . . . . . . . . . . . .  4
   3. Protocol Overview . . . . . . . . . . . . . . . . . . . . . . .  5
   4. Protocol Details  . . . . . . . . . . . . . . . . . . . . . . .  7
     4.1. Fast Open Cookie  . . . . . . . . . . . . . . . . . . . . .  7
       4.1.1. TCP Options . . . . . . . . . . . . . . . . . . . . . .  7
       4.1.2. Server Cookie Handling  . . . . . . . . . . . . . . . .  8
       4.1.3. Client Cookie Handling  . . . . . . . . . . . . . . . .  9
     4.2. Fast Open Protocol  . . . . . . . . . . . . . . . . . . . .  9
       4.2.1. Fast Open Cookie Request  . . . . . . . . . . . . . . . 10
       4.2.2. TCP Fast Open . . . . . . . . . . . . . . . . . . . . . 11
   5. Reliability and Deployment Issues . . . . . . . . . . . . . . . 13
   6. Security Considerations . . . . . . . . . . . . . . . . . . . . 14
     6.1. Server Resource Exhaustion Attack by SYN Flood with Valid
          Cookies . . . . . . . . . . . . . . . . . . . . . . . . . . 14
     6.2. Amplified Reflection Attack to Random Host  . . . . . . . . 15
     6.3 Attacks from behind sharing public IPs (NATs)  . . . . . . . 16
   7. TFO's Applicability . . . . . . . . . . . . . . . . . . . . . . 17
     7.1 Duplicate data in SYNs . . . . . . . . . . . . . . . . . . . 17
     7.2 Potential performance improvement  . . . . . . . . . . . . . 17
     7.3 Example: Web clients and servers . . . . . . . . . . . . . . 17
       7.3.1 HTTP request replay  . . . . . . . . . . . . . . . . . . 17
       7.3.2 HTTP persistent connection . . . . . . . . . . . . . . . 18
   8. Performance Experiments . . . . . . . . . . . . . . . . . . . . 18
   9. Related Work  . . . . . . . . . . . . . . . . . . . . . . . . . 19
     9.1. T/TCP . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
     9.2. Common Defenses Against SYN Flood Attacks . . . . . . . . . 19
 

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     9.3. TCP Cookie Transaction (TCPCT)  . . . . . . . . . . . . . . 20
   10. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 20
   11. Acknowledgement  . . . . . . . . . . . . . . . . . . . . . . . 20
   12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 20
     12.1. Normative References . . . . . . . . . . . . . . . . . . . 20
     12.2. Informative References . . . . . . . . . . . . . . . . . . 21
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 23

1. Introduction

   TCP Fast Open (TFO) enables data to be exchanged safely during TCP's
   connection handshake.

   This document describes a design that enables applications to save a
   round trip while avoiding severe security ramifications. At the core
   of TFO is a security cookie used by the server side to authenticate a
   client initiating a TFO connection. This document covers the details
   of exchanging data during TCP's initial handshake, the protocol for
   TFO cookies, and potential new security vulnerabilities and their
   mitigation. It also includes discussion of deployment issues and
   related proposals. TFO requires extensions to the socket API but this
   document does not cover that.

   TFO is motivated by the performance needs of today's Web
   applications. Network latency is largely determined by a connection's
   round-trip time (RTT) and the number of round trips required to
   transfer application data. RTT consists of propagation delay and
   queuing delay. 

   Network bandwidth has grown substantially over the past two decades,
   potentially reducing queuing delay, while propagation delay is
   largely constrained by the speed of light and has remained unchanged.
   Therefore reducing the number of round trips has typically become the
   most effective way to improve the latency of applications like the
   Web [CDCM11].

   Current TCP only permits data exchange after 3WHS [RFC793], which
   adds one RTT to network latency. For short transfers (e.g., web
   objects) this additional RTT is a significant portion of overall
   network latency [THK98]. One widely deployed solution is HTTP
   persistent connections. However, this solution is limited since hosts
   and middle boxes terminate idle TCP connections due to resource
   constraints. For example, the Chrome browser keeps TCP connections
   idle for up to 5 minutes but 35% of Chrome HTTP requests are made on
   new TCP connections [RCCJR11]. We discuss Web applications and TFO in
   detail later in section 7.

 

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2. Data In SYN

   Allowing data in SYN packets to be delivered raises two issues
   discussed in the following subsections. These issues make TFO
   undesirable for certain  applications. Therefore TCP implementations
   MUST NOT use TFO by default and only use TFO if requested explicitly
   by the application on a per service port basis. Applications need to
   evaluate TFO applicability (described in Section 7) before using TFO.

2.1 Relaxing TCP semantics on duplicated SYNs

   [RFC793] (section 3.4) already allows data in SYN packets but forbids
   the receiver from delivering the data to the application until 3WHS
   is completed. This is because TCP's initial handshake serves to
   capture old or duplicate SYNs.

   TFO allows data to be delivered to the application before 3WHS  is
   completed, thus opening itself to a data integrity issue for the
   applications in Section 2.1 in either of the following cases:

   a) the receiver host receives data in a duplicate SYN after it has   
   forgotten it received the original SYN (e.g. due to a reboot); b) the
   duplicate is received after the connection created by the    original
   SYN has been closed and the close was initiated by the    sender (so
   the receiver will not be protected by the 2MSL TIMEWAIT    state).

   The obsoleted T/TCP protocol employs a new TCP "TAO" option and
   connection count to guard against old or duplicate SYNs [RFC1644].
   However it is not widely used due to various vulnerabilities
   [PHRACK98].

   Rather than trying to capture all dubious SYN packets to make TFO
   100% compatible with TCP semantics, we made a design decision early
   on to accept old SYN packets with data, i.e., to restrict TFO use to
   a class of applications (Section 7) that are tolerant of duplicate
   SYN packets with data. We believe this is the right design trade-off
   balancing complexity with usefulness for certain applications.

2.2. SYNs with spoofed IP addresses

   Standard TCP suffers from the SYN flood attack [RFC4987] because
   bogus SYN packets, i.e., SYN packets with spoofed source IP addresses
   can easily fill up a listener's small queue, causing a service port
   to be blocked completely until timeouts. Secondary damage comes from
   these SYN requests taking up memory space. Though this is less of an
   issue today as servers typically have plenty of memory.

 

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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 more serious new vulnerabilities. Applications serving ports
   that have TFO enabled may waste lots of CPU and memory resources
   processing the requests and producing the responses. If the response
   is much larger than the request, the attacker can mount an amplified
   reflection attack against victims of choice beyond the TFO server
   itself.

   Numerous mitigation techniques against regular SYN flood attacks
   exist and have been well documented [RFC4987]. Unfortunately none are
   applicable to TFO. We propose a server-supplied cookie to mitigate
   the primary security issues introduced by TFO in Section 3. We defer
   further discussion of SYN flood attacks to the "Security
   Considerations" section.

3. Protocol Overview

   The key component of TFO is the Fast Open Cookie (cookie), a message
   authentication code (MAC) tag generated by the server. The client
   requests a cookie in one regular TCP connection, then uses it for
   future TCP connections to exchange data during 3WHS: Requesting a
   Fast Open Cookie:
   1. The client sends a SYN with a Fast Open Cookie Request option.

   2. The server generates a cookie and sends it through the Fast Open
      Cookie option of a SYN-ACK packet.

   3. The client caches the cookie for future TCP Fast Open connections
      (see below).

   Performing TCP Fast Open:

   1. The client sends a SYN with Fast Open Cookie option and data.

   2. The server validates the cookie:
      a. If the cookie is valid, the server sends a SYN-ACK
         acknowledging both the SYN and the data. The server then 
         delivers the data to the application.

      b. Otherwise, the server drops the data and sends a SYN-ACK
         acknowledging only the SYN sequence number.

   3. If the server accepts the data in the SYN packet, it may send the
      response data before the handshake finishes. The max amount is
      governed by the TCP's congestion control [RFC5681].

 

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   4. The client sends an ACK acknowledging the SYN and the server data.
      If the client's data is not acknowledged, the client retransmits
      the data in the ACK packet.

   5. The rest of the connection proceeds like a normal TCP connection.
   The client can repeat many Fast Open operations once it acquires a
   cookie (until the cookie is expired by the server). Thus TFO is
   useful for applications that have temporal locality on client and
   server connections.

   Requesting Fast Open Cookie in connection 1:

      TCP A (Client)                                    TCP B(Server)
      ______________                                    _____________
      CLOSED                                                   LISTEN

   #1 SYN-SENT       ----- <SYN,CookieOpt=NIL>  ---------->  SYN-RCVD

   #2 ESTABLISHED    <---- <SYN,ACK,CookieOpt=C> ----------  SYN-RCVD
       (caches cookie C)

   Performing TCP Fast Open in connection 2:

      TCP A (Client)                                    TCP B(Server)
      ______________                                    _____________
      CLOSED                                                   LISTEN

   #1 SYN-SENT       ----- <SYN=x,CookieOpt=C,DATA_A> ---->  SYN-RCVD

   #2 ESTABLISHED    <---- <SYN=y,ACK=x+len(DATA_A)+1> ----  SYN-RCVD

   #3 ESTABLISHED    <---- <ACK=x+len(DATA_A)+1,DATA_B>----  SYN-RCVD

   #4 ESTABLISHED    ----- <ACK=y+1>--------------------> ESTABLISHED

   #5 ESTABLISHED    --- <ACK=y+len(DATA_B)+1>----------> ESTABLISHED

 

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4. Protocol Details

4.1. Fast Open Cookie

   The Fast Open Cookie is designed to mitigate new security
   vulnerabilities in order to enable data exchange during handshake.
   The cookie is a message authentication code tag generated by the
   server and is opaque to the client; the client simply caches the
   cookie and passes it back on subsequent SYN packets to open new
   connections. The server can expire the cookie at any time to enhance
   security.

4.1.1. TCP Options

   Fast Open Cookie Option

   The server uses this option to grant a cookie to the client in the
   SYN-ACK packet; the client uses it to pass the cookie back to the
   server in the SYN packet.

                                   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                   |      Kind     |    Length     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   ~                            Cookie                             ~
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Kind            1 byte: constant TBD (assigned by IANA)
   Length          1 byte: range 6 to 18 (bytes); limited by
                           remaining space in the options field.
                           The number MUST be even.
   Cookie          4 to 16 bytes (Length - 2)
   Options with invalid Length values or without SYN flag set MUST be
   ignored.  The minimum Cookie size is 4 bytes. Although the diagram
   shows a cookie aligned on 32-bit boundaries, alignment is not
   required.

   Fast Open Cookie Request Option

   The client uses this option in the SYN packet to request a cookie
   from a TFO-enabled server
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      Kind     |    Length     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

 

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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 generation and verification are fast relative to the rest of  
   SYN and SYN-ACK processing.

   4. A server may encode other information in the cookie, and accept  
   more than one valid cookie per client at any given time. But this  
   is all server implementation dependent and transparent to the  
   client.

   5. The cookie expires after a certain amount of time. The reason for 
    cookie expiration is detailed in the "Security Consideration"  
   section. This can be done by either periodically changing the  
   server key used to generate cookies or including a timestamp when  
   generating the cookie. 

      To gradually invalidate cookies over time, the server can  
   implement key rotation to generate and verify cookies using  
   multiple keys. This approach is useful for large-scale servers to  
   retain Fast Open rolling key updates. We do not specify a  
   particular mechanism because the implementation is often server  
   specific.

   The server supports the cookie generation and verification
   operations:

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

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

   Example Implementation: a simple implementation is to use AES_128 to
 

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   encrypt the IPv4 (with padding) or IPv6 address and truncate to 64
   bits. The server can periodically update the key to expire the
   cookies. AES encryption on recent processors is fast and takes only a
   few hundred nanoseconds [RCCJR11].

   If only one valid cookie is allowed per-client and the server can
   regenerate the cookie independently, the best validation process is
   to simply regenerate a valid cookie and compare it against the
   incoming cookie. In that case if the incoming cookie fails the check,
   a valid cookie is readily available to be sent to the client. 

   The server MAY return a cookie request option, e.g., a null cookie,
   to signal the support of Fast Open without generating cookies, for
   probing or debugging purposes.

4.1.3. Client Cookie Handling

   The client MUST cache cookies from servers for later Fast Open
   connections. For a multi-homed client, the cookies are both client
   and server IP dependent. Beside the cookie, we RECOMMEND that the
   client caches the MSS and RTT to the server to enhance performance.

   The MSS advertised by the server is stored in the cache to determine
   the maximum amount of data that can be supported in the SYN packet.
   This information is needed because data is sent before the server
   announces its MSS in the SYN-ACK packet. Without this information,
   the data size in the SYN packet is limited to the default MSS of 536
   bytes [RFC1122]. The client SHOULD update the cache MSS value
   whenever it discovers new MSS value, e.g., through path MTU
   discovery.

   Caching RTT allows seeding a more accurate SYN timeout than the
   default value [RFC6298]. This lowers the performance penalty if the
   network or the server drops the SYN packets with data or the cookie
   options (See "Reliability and Deployment Issues" section below).

   The cache replacement algorithm is not specified and is left for the
   implementations.

   Note that before TFO sees wide deployment, clients SHOULD cache
   negative responses from servers in order to reduce the amount of
   futile TFO attempts. Since TFO is enabled on a per-service port basis
   but cookies are independent of service ports, clients' cache should
   include remote port numbers too.

4.2. Fast Open Protocol

   One predominant requirement of TFO is to be fully compatible with
 

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   existing TCP implementations, both on the client and the server
   sides.

   The server keeps two variables per listening port:

   FastOpenEnabled: default is off. It MUST be turned on explicitly by
   the application. When this flag is off, the server does not perform
   any TFO related operations and MUST ignore all cookie options.

   PendingFastOpenRequests: tracks number of TFO connections in SYN-RCVD
   state.  If this variable goes over a preset system limit, the server
   SHOULD disable TFO for all new connection requests until
   PendingFastOpenRequests drops below the system limit. This variable
   is used for defending some vulnerabilities discussed in the "Security
   Considerations" section.

   The server keeps a FastOpened flag per TCB to mark if a connection
   has successfully performed a TFO.

4.2.1. Fast Open Cookie Request

   Any client attempting TFO MUST first request a cookie from the server
   with the following steps:

   1. The client sends a SYN packet with a Fast Open Cookie Request  
   option.

   2. The server SHOULD respond with a SYN-ACK based on the procedures  
   in the "Server Cookie Handling" section. This SYN-ACK SHOULD  
   contain a Fast Open Cookie option if the server currently supports  
   TFO for this listener port.

   3. If the SYN-ACK contains a Fast Open Cookie option, the client  
   replaces the cookie and other information as described in the  
   "Client Cookie Handling" section. Otherwise, if the SYN-ACK is  
   first seen, i.e.,not a (spurious) retransmission, the client MAY  
   remove the server information from the cookie cache. If the SYN-  
   ACK is a spurious retransmission without valid Fast Open Cookie  
   Option, the client does nothing to the cookie cache for the   reasons
   below.

   The network or servers may drop the SYN or SYN-ACK packets with the
   new cookie options which causes SYN or SYN-ACK timeouts. We RECOMMEND
   both the client and the server retransmit SYN and SYN-ACK without the
   cookie options on timeouts. This ensures the connections of cookie
   requests will go through and lowers the latency penalties (of dropped
   SYN/SYN-ACK packets). The obvious downside for maximum compatibility
   is that any regular SYN drop will fail the cookie (although one can
 

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   argue the delay in the data transmission till after 3WHS is justified
   if the SYN drop is due to network congestion).  Next section
   describes a heuristic to detect such drops when the client receives
   the SYN-ACK.

   We also RECOMMEND the client to record servers that failed to respond
   to cookie requests and only attempt another cookie request after
   certain period. An alternate proposal is to request cookie in FIN
   instead since FIN-drop by incompatible middle-box does not affect
   latency. However such paths are likely to drop SYN packet with data
   later, and many applications close the connections with RST instead,
   so the actual benefit of this approach is not clear. 

4.2.2. TCP Fast Open

   Once the client obtains the cookie from the target server, the client
   can perform subsequent TFO connections until the cookie is expired by
   the server. The nature of TCP sequencing makes the TFO specific
   changes relatively small in addition to [RFC793].

   Client: Sending SYN

   To open a TFO connection, the client MUST have obtained the cookie
   from the server:

   1. Send a SYN packet.

      a. If the SYN packet does not have enough option space for the  
   Fast Open Cookie option, abort TFO and fall back to regular 3WHS.

      b. Otherwise, include the Fast Open Cookie option with the cookie 
    of the server. Include any data up to the cached server MSS or  
   default 536 bytes.

   2. Advance to SYN-SENT state and update SND.NXT to include the data  
   accordingly.

   3. If RTT is available from the cache, seed SYN timer according to  
   [RFC6298].

   To deal with network or servers dropping SYN packets with payload or
   unknown options, when the SYN timer fires, the client SHOULD
   retransmit a SYN packet without data and Fast Open Cookie options.

   Server: Receiving SYN and responding with SYN-ACK
 

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   Upon receiving the SYN packet with Fast Open Cookie option:

   1. Initialize and reset a local FastOpened flag. If FastOpenEnabled  
   is false, go to step 5.

   2. If PendingFastOpenRequests is over the system limit, go to step 5.

   3. If IsCookieValid() in section 4.1.2 returns false, go to step 5.

   4. Buffer the data and notify the application. Set FastOpened flag  
   and increment PendingFastOpenRequests.

   5. Send the SYN-ACK packet. The packet MAY include a Fast Open  
   Option. If FastOpened flag is set, the packet acknowledges the SYN  
   and data sequence. Otherwise it acknowledges only the SYN   sequence.
   The server MAY include data in the SYN-ACK packet if the   response
   data is readily available. Some application may favor   delaying the
   SYN-ACK, allowing the application to process the   request in order
   to produce a response, but this is left to the   implementation.

   6. Advance to the SYN-RCVD state. If the FastOpened flag is set, the 
    server MUST follow the congestion control [RFC5681], in particular  
   the initial congestion window [RFC3390], to send more data   packets.

      Note that if SYN-ACK is lost, regular TCP reduces the initial  
   congestion window before sending any data. In this case TFO is  
   slightly more aggressive in the first data round trip even though  
   it does not change the congestion control.

   If the SYN-ACK timer fires, the server SHOULD retransmit a SYN-ACK
   segment with neither data nor Fast Open Cookie options for
   compatibility reasons.

   A special case is simultaneous open where the SYN receiver is a
   client in SYN-SENT state. The protocol remains the same because
   [RFC793] already supports both data in SYN and simultaneous open. But
   the client's socket may have data available to read before it's
   connected. This document does not cover the corresponding API change.

   Client: Receiving SYN-ACK

   The client SHOULD perform the following steps upon receiving the SYN-
   ACK: 1. Update the cookie cache if the SYN-ACK has a Fast Open Cookie
     Option or MSS option or both.

   2. Send an ACK packet. Set acknowledgment number to RCV.NXT and  
   include the data after SND.UNA if data is available.
 

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   3. Advance to the ESTABLISHED state.

   Note there is no latency penalty if the server does not acknowledge
   the data in the original SYN packet. The client SHOULD retransmit any
   unacknowledged data in the first ACK packet in step 2. The data
   exchange will start after the handshake like a regular TCP
   connection.

   If the client has timed out and retransmitted only regular SYN
   packets, it can heuristically detect paths that intentionally drop
   SYN with Fast Open option or data. If the SYN-ACK acknowledges only
   the initial sequence and does not carry a Fast Open cookie option,
   presumably it is triggered by a retransmitted (regular) SYN and the
   original SYN or the corresponding SYN-ACK was lost.

   Server: Receiving ACK

   Upon receiving an ACK acknowledging the SYN sequence, the server
   decrements PendingFastOpenRequests and advances to the ESTABLISHED
   state. No special handling is required further.

5. Reliability and Deployment Issues

   Network or Hosts Dropping SYN packets with data or unknown options

   A study [MAF04] found that some middle-boxes and end-hosts may drop
   packets with unknown TCP options incorrectly. Studies [LANGLEY06,
   HNRGHT11] both found that 6% of the probed paths on the Internet drop
   SYN packets with data or with unknown TCP options. The TFO protocol
   deals with this problem by re-transmitting SYN without data or cookie
   options and we recommend tracking these servers in the client.

   Server Farms

   A common server-farm setup is to have many physical hosts behind a
   load-balancer sharing the same server IP. The load-balancer forwards
   new TCP connections to different physical hosts based on certain
   load-balancing algorithms. For TFO to work, the physical hosts need
   to share the same key and update the key at about the same time.

   Network Address Translation (NAT)

   The hosts behind NAT sharing same IP address will get the same cookie
   to the same server. This will not prevent TFO from working. But on
   some carrier-grade NAT configurations where every new TCP connection
   from the same physical host uses a different public IP address, TFO
   does not provide latency benefit. However, there is no performance
 

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   penalty either as described in Section "Client: Receiving SYN-ACK".

6. Security Considerations

   The Fast Open cookie stops an attacker from trivially flooding
   spoofed SYN packets with data to burn server resources or to mount an
   amplified reflection attack on random hosts. The server can defend
   against spoofed SYN floods with invalid cookies using existing
   techniques [RFC4987]. We note that generating bogus cookies is
   usually cheaper than validating them. But the additional cost of
   validating the cookies, inherent to any authentication scheme, may
   not be substantial compared to processing a regular SYN packet. 

   However, the attacker may still obtain cookies from some compromised
   hosts, then flood spoofed SYN with data and "valid" cookies (from
   these hosts or other vantage points). With DHCP, it's possible to
   obtain cookies of past IP addresses without compromising any host.
   Below we identify new vulnerabilities of TFO and describe the
   countermeasures.

6.1. Server Resource Exhaustion Attack by SYN Flood with Valid Cookies

   Like regular TCP handshakes, TFO is vulnerable to such an attack. But
   the potential damage can be much more severe. Besides causing
   temporary disruption to service ports under attack, it may exhaust
   server CPU and memory resources.

   For this reason it is crucial for the TFO server to limit the maximum
   number of total pending TFO connection requests, i.e.,
   PendingFastOpenRequests. When the limit is exceeded, the server
   temporarily disables TFO entirely as described in "Server Cookie
   Handling". Then subsequent TFO requests will be downgraded to regular
   connection requests, i.e., with the data dropped and only SYN
   acknowledged. This allows regular SYN flood defense techniques
   [RFC4987] like SYN-cookies to kick in and prevent further service
   disruption.

   There are other subtle but important differences in the vulnerability
   between TFO and regular TCP handshake. Before the SYN flood attack
   broke out in the late '90s, typical listener's max qlen was small,
   enough to sustain the highest expected new connection rate and the
   average RTT for the SYN-ACK packets to be acknowledged in time. E.g.,
   if a server is designed to handle at most 100 connection requests per
   second, and the average RTT is 100ms, a max qlen on the order of 10
   will be sufficient.

   This small max qlen made it very easy for any attacker, even equipped
   with just a dailup modem to the Internet, to cause major disruptions
 

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   to a web site by simply throwing a handful of "SYN bombs" at its
   victim of choice. But for this attack scheme to work, the attacker
   must pick a non-responsive source IP address to spoof with. Otherwise
   the SYN-ACK packet will trigger TCP RST from the host whose IP
   address has been spoofed, causing corresponding connection to be
   removed from the server's listener queue hence defeating the attack.

   In other words, the main damage of SYN bombs against the standard TCP
   stack is not directly from the bombs themselves costing TCP
   processing overhead or host memory, but rather from the spoofed SYN
   packets filling up the often small listener's queue.

   On the other hand, TFO SYN bombs can cause damage directly if
   admitted without limit into the stack. The RST packets from the
   spoofed host will fuel rather than defeat the SYN bombs as compared
   to the non-TFO case, because the attacker can flood more SYNs with
   data to cost more data processing resources. For this reason, a TFO
   server needs to monitor the connections in SYN-RCVD being reset in
   addition to imposing a reasonable max qlen. Implementations may
   combine the two, e.g., by continuing to account for those connection
   requests that have just been reset against the listener's
   PendingFastOpenRequests until a timeout period has passed.

   Limiting the maximum number of pending TFO connection requests does
   make it easy for an attacker to overflow the queue, causing TFO to be
   disabled. We argue that causing TFO to be disabled is unlikely to be
   of interest to attackers because the service will remain intact
   without TFO hence there is hardly any real damage.

6.2. Amplified Reflection Attack to Random Host

   Limiting PendingFastOpenRequests with a system limit can be done
   without Fast Open Cookies and would protect the server from resource
   exhaustion. It would also limit how much damage an attacker can cause
   through an amplified reflection attack from that server. However, it
   would still be vulnerable to an amplified reflection attack from a
   large number of servers. An attacker can easily cause damage by
   tricking many servers to respond with data packets at once to any
   spoofed victim IP address of choice.

   With the use of Fast Open Cookies, the attacker would first have to
   steal a valid cookie from its target victim. This likely requires the
   attacker to compromise the victim host or network first.

   The attacker here has little interest in mounting an attack on the
   victim host that has already been compromised. But she may be
   motivated to disrupt the victim's network. Since a stolen cookie is
   only valid for a single server, she has to steal valid cookies from a
 

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   large number of servers and use them before they expire to cause
   sufficient damage without triggering the defense in the previous
   section.

   One can argue that if the attacker has compromised the target network
   or hosts, she could perform a similar but simpler attack by injecting
   bits directly. The degree of damage will be identical, but TFO-
   specific attack allows the attacker to remain anonymous and disguises
   the attack as from other servers.

   The best defense is for the server not to respond with data until
   handshake finishes. In this case the risk of amplification reflection
   attack is completely eliminated. But the potential latency saving
   from TFO may diminish if the server application produces responses
   earlier before the handshake completes.

6.3 Attacks from behind sharing public IPs (NATs)

   An attacker behind NAT can easily obtain valid cookies to launch the
   above attack to hurt other clients that share the path. [BOB12]
   suggested that the server can extend cookie generation to include the
   TCP timestamp---GetCookie(IP_Address, Timestamp)---and implement it
   by  encrypting the concatenation of the two values to generate the
   cookie. The client stores both the cookie and its corresponding
   timestamp, and echoes both in the SYN.  The server then implements
   IsCookieValid(IP_Address, Timestamp, Cookie) by encrypting the IP and
   timestamp data and comparing it with the cookie value.

   This enables the server to issue different cookies to clients that
   share the same IP address, hence can selectively discard those
   misused cookies from the attacker. However the attacker can simply
   repeat the attack with new cookies. The server would eventually need
   to throttle all requests from the IP address just like the current
   approach. Moreover this approach requires modifying [RFC 1323] to
   send non-zero Timestamp Echo Reply in SYN, potentially cause firewall
   issues. Therefore we believe the benefit may not outweigh the
   drawbacks.

 

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7. TFO's Applicability

   This section is to help applications considering TFO to evaluate
   TFO's benefits and drawbacks using a Web client and server
   applications as an example throughout.

7.1 Duplicate data in SYNs

   It is possible, though uncommon, that using TFO the first data
   written to a socket is delivered more than once to the application on
   the remote host(Section 2.1). This replay potential only applies to
   data in the SYN but not subsequent data exchanges. Thus applications
   MUST NOT use TFO unless they can tolerate this behavior.

7.2 Potential performance improvement

   TFO is designed for latency-conscious applications that are sensitive
   to TCP's initial connection setup delay. For example, many
   applications perform short request and response message exchanges. To
   benefit from TFO, the first application data unit (e.g., an HTTP
   request) needs to be no more than TCP's maximum segment size (minus
   options used in SYN). Otherwise the remote server can only process
   the client's application data unit once the rest of it is delivered
   after the initial handshake, diminishing TFO's benefit.

   To the extent possible, applications SHOULD employ long-lived
   connections to best take advantage of TCP's built-in congestion
   control, and to reduce the impact from TCP's connection setup
   overhead. Note that when an application employs too many short-lived
   connections, it may negatively impact network stability, as these
   connections often exit before TCP's congestion control algorithm
   takes effect. Implementations supporting a large number of short-
   lived connections should employ temporal sharing of TCB data as
   described in [RFC2140].

7.3 Example: Web clients and servers

   We look at Web client and server applications that use HTTP and TCP
   protocols and follow the guidelines above to evaluate if TFO is safe
   and useful for Web.

7.3.1 HTTP request replay

   We believe TFO is safe for the Web because even with standard TCP the
   Web browser may replay an HTTP request to the remote Web server
   multiple times. After sending an HTTP request, the browser could time
   out and retry the same request on another TCP connection. This
   scenario occurs far more frequently than the SYN duplication issue
 

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   presented by TFO. To ensure transactional behavior, Web sites employ
   application-specific mechanisms such as including unique identifiers
   in the data. 

7.3.2 HTTP persistent connection

   Next we evaluate if the Web can benefit from TFO given that HTTP
   persistent connection support is already widely deployed.

   TCP connection setup overhead has long been identified as a
   performance bottleneck for web applications [THK98]. HTTP persistent
   connection support was proposed to mitigate this issue and has been
   widely deployed. However, studies [RCCJR11][AERG11] show that the
   average number of transactions per connection is between 2 and 4,
   based on large-scale measurements from both servers and clients. In
   these studies, the servers and clients both kept idle connections up
   to several minutes, well into "human think" time.

   Can the utilization rate of such connections increase by keeping idle
   connections even longer?  Unfortunately, such an approach is
   problematic due to middle-boxes and the rapidly growing share of
   mobile end hosts. Thus one major issue faced by persistent
   connections is NAT. Studies [HNESSK10][MQXMZ11] show that the
   majority of home routers and ISPs fail to meet the the 124-minute
   idle timeout mandated in [RFC5382]. In [MQXMZ11], 35% of mobile ISPs
   timeout idle connections within 30 minutes. The end hosts attempting
   to use these broken connections are often forced to wait for a
   lengthy TCP timeout, as they often receive no signal when middleboxes
   break their connections. Thus browsers risk large performance
   penalties when keeping idle connections open.

   To circumvent this problem, some applications send frequent TCP keep-
   alive probes.  However, this technique drains power on mobile devices
   [MQXMZ11]. In fact, power has become such a prominent issue in modern
   LTE devices that mobile browsers close HTTP connections within
   seconds or even immediately [SOUDERS11].

   Since TFO data duplication presents no new issues and HTTP persistent
   connection support has many limitations, Web applications can safely
   use TFO and will likely achieve performance gains. The next section
   presents more empirical data of the potential performance benefit.

8. Performance Experiments

   [RCCJR11] studied Chrome browser performance based on 28 days of
   global statistics. Chrome browser keeps idle HTTP persistent
   connections up to 5 to 10 minutes. However the average number of the
 

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   transactions per connection is only 3.3. Due to the low utilization,
   TCP 3WHS accounts up to 25% of the HTTP transaction network latency.
   The authors tested a Linux TFO implementation with TFO enabled Chrome
   browser on popular web sites in emulated environments such as
   residential broadband and mobile networks. They showed that TFO
   improves page load time by 10% to 40%. More details on the design
   tradeoffs and measurement can be found at [RCCJR11].

9. Related Work

9.1. T/TCP

   TCP Extensions for Transactions [RFC1644] attempted to bypass the
   three-way handshake, among other things, hence shared the same goal
   but also the same set of issues as TFO. It focused most of its effort
   battling old or duplicate SYNs, but paid no attention to security
   vulnerabilities it introduced when bypassing 3WHS. Its TAO option and
   connection count, besides adding complexity, require the server to
   keep state per remote host, while still leaving it wide open for
   attacks. It is trivial for an attacker to fake a CC value that will
   pass the TAO test. Unfortunately, in the end its scheme is still not
   100% bullet proof as pointed out by [PHRACK98].

   As stated earlier, we take a practical approach to focus TFO on the
   security aspect, while allowing old, duplicate SYN packets with data
   after recognizing that 100% TCP semantics is likely infeasible. We
   believe this approach strikes the right tradeoff, and makes TFO much
   simpler and more appealing to TCP implementers and users.

9.2. Common Defenses Against SYN Flood Attacks

   TFO is still vulnerable to SYN flood attacks just like normal TCP
   handshakes, but the damage may be much worse, thus deserves a careful
   thought.

   There have been plenty of studies on how to mitigate attacks from
   regular SYN flood, i.e., SYN without data [RFC4987]. But from the
   stateless SYN-cookies to the stateful SYN Cache, none can preserve
   data sent with SYN safely while still providing an effective defense.

   The best defense may be to simply disable TFO when a host is
   suspected to be under a SYN flood attack, e.g., the SYN backlog is
   filled. Once TFO is disabled, normal SYN flood defenses can be
   applied. The "Security Consideration" section contains a thorough
   discussion on this topic.

 

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9.3. TCP Cookie Transaction (TCPCT)

   TCPCT [RFC6013] eliminates server state during initial handshake and
   defends spoofing DoS attacks. Like TFO, TCPCT allows SYN and SYN-ACK
   packets to carry data. However, TCPCT and TFO are designed for
   different goals and they are not compatible.

   The TCPCT server does not keep any connection state during the
   handshake, therefore the server application needs to consume the data
   in SYN and (immediately) produce the data in SYN-ACK before sending
   SYN-ACK. Otherwise the application's response has to wait until
   handshake completes. In contrary, TFO allows server to respond data
   during handshake. Therefore for many request-response style
   applications, TCPCT may not achieve same latency benefit as TFO.

   Rapid-Restart [SIMPSON11] is based on TCPCT and shares similar goal
   as TFO. In Rapid-Restart, both the server and the client retain the
   TCP control blocks after a connection is terminated in order to
   allow/resume data exchange in next connection handshake. In contrary,
   TFO does not require keeping both TCB on both sides and is more
   scalable.

10. IANA Considerations

   The Fast Open Cookie Option and Fast Open Cookie Request Option
   define no new namespace. The options require IANA allocate one value
   from the TCP option Kind namespace. Early implementation before the
   allocation SHOULD follow [EXPOPT] and use experimental option 254 and
   magic number 0xF989 (16 bits), and migrate to the new option after
   the allocation according.

11. Acknowledgement

   We thank Rick Jones, Bob Briscoe, Adam Langley, Matt Mathis, Neal
   Cardwell, Roberto Peon, and Tom Herbert for their feedbacks. We
   especially thank Barath Raghavan for his contribution on the security
   design of Fast Open.

12. References

12.1. Normative References
   [RFC793]  Postel, J. "Transmission Control Protocol", RFC 793,
             September 1981.

   [RFC1122] Braden, R., Ed., "Requirements for Internet Hosts -
             Communication Layers", STD 3, RFC 1122, October 1989.

 

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   [RFC5382] S. Guha, Ed., Biswas, K., Ford B., Sivakumar S., Srisuresh,
             P., "NAT Behavioral Requirements for TCP", RFC 5382

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

   [RFC6298] Paxson, V., Allman, M., Chu, J. and M. Sargent, "Computing
             TCP's Retransmission Timer", RFC 6298, June 2011.

12.2. Informative References

   [AERG11]  M. Al-Fares, K. Elmeleegy, B. Reed, and I. Gashinsky,
             "Overclocking the Yahoo! CDN for Faster Web Page Loads". In
             Proceedings of Internet Measurement Conference, November
             2011.

   [CDCM11]  Chu, J., Dukkipati, N., Cheng, Y. and M. Mathis,
             "Increasing TCP's Initial Window", Internet-Draft draft-
             ietf-tcpm-initcwnd-02.txt (work in progress), October 2011.

   [EXPOPT] Touch, Joe, "Shared Use of Experimental TCP Options",
             Internet-Draft draft-ietf-tcpm-experimental-options (work
             in progress), October 2012.

   [HNESSK10] S. Haetoenen, A. Nyrhinen, L. Eggert, S. Strowes, P.
             Sarolahti, M. Kojo., "An Experimental Study of Home Gateway
             Characteristics". In Proceedings of Internet Measurement
             Conference. Octobor 2010

   [HNRGHT11] M. Honda, Y. Nishida, C. Raiciu, A. Greenhalgh, M.
             Handley, H. Tokuda, "Is it Still Possible to Extend TCP?".
             In Proceedings of Internet Measurement Conference. November
             2011.

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

   [MAF04]   Medina, A., Allman, M., and S. Floyd, "Measuring
             Interactions Between Transport Protocols and Middleboxes",
             In Proceedings of Internet Measurement Conference, October
             2004.

   [MQXMZ11] Z. Mao, Z. Qian, Q. Xu, Z. Mao, M. Zhang. "An Untold Story
             of Middleboxes in Cellular Networks", In Proceedings of
             SIGCOMM. August 2011.

   [PHRACK98] "T/TCP vulnerabilities", Phrack Magazine, Volume 8, Issue
             53 artical 6. July 8, 1998. URL
 

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             http://www.phrack.com/issues.html?issue=53&id=6

   [QWGMSS11] F. Qian, Z. Wang, A. Gerber, Z. Mao, S. Sen, O.
             Spatscheck. "Profiling Resource Usage for Mobile
             Applications: A Cross-layer Approach", In Proceedings of
             International Conference on Mobile Systems. April 2011.

   [RCCJR11] Radhakrishnan, S., Cheng, Y., Chu, J., Jain, A. and 
             Raghavan, B., "TCP Fast Open". In Proceedings of 7th ACM 
             CoNEXT Conference, December 2011.

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

   [RFC2140] Touch, J., "TCP Control Block Interdependence", RFC2140,
             April 1997.

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

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

   [SIMPSON11] Simpson, W., "Tcp cookie transactions (tcpct) rapid
             restart", Internet draft draft-simpson-tcpct-rr-02.txt
             (work in progress), July 2011.

   [SOUDERS11] S. Souders. "Making A Mobile Connection".
             http://www.stevesouders.com/blog/2011/09/21/making-a-
             mobile-connection/

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

   [BOB12] Briscoe, B., "Some ideas building on draft-ietf-tcpm-
             fastopen-01", tcpm list,
             http://www.ietf.org/mail-archive/web/tcpm/current/
             msg07192.html

 

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Authors' Addresses

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

   Jerry Chu
   Google, Inc.
   1600 Amphitheatre Parkway
   Mountain View, CA 94043, USA
   EMail: hkchu@google.com

   Sivasankar Radhakrishnan
   Department of Computer Science and Engineering
   University of California, San Diego
   9500 Gilman Dr
   La Jolla, CA 92093-0404
   EMail: sivasankar@cs.ucsd.edu

   Arvind Jain
   Google, Inc.
   1600 Amphitheatre Parkway
   Mountain View, CA 94043, USA
   EMail: arvind@google.com

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