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IETF RFC 6013
TCP Cookie Transactions (TCPCT)
Last modified on Friday, January 21st, 2011
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Independent Submission W. Simpson
Request for Comments: 6013 DayDreamer
Category: Experimental January 2011
ISSN: 2070-1721
TCP Cookie Transactions (TCPCT)
 Abstract
TCP Cookie Transactions (TCPCT) deter spoofing of connections and
prevent resource exhaustion, eliminating Responder (server) state
during the initial handshake. The Initiator (client) has sole
responsibility for ensuring required delays between connections. The
cookie exchange may carry data, limited to inhibit amplification and
reflection denial of service attacks.
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for examination, experimental implementation, and
evaluation.
This document defines an Experimental Protocol for the Internet
community. This is a contribution to the RFC Series, independently
of any other RFC stream. The RFC Editor has chosen to publish this
document at its discretion and makes no statement about its value for
implementation or deployment. Documents approved for publication by
the RFC Editor are not a candidate for any level of Internet
Standard; see Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/RFC 6013.
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RFC 6013 TCP Cookie Transactions January 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
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document.
This document may not be modified, and derivative works of it may not
be created, except to format it for publication as an RFC or to
translate it into languages other than English.
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RFC 6013 TCP Cookie Transactions January 2011
Table of Contents
1. Introduction ....................................................4
1.1. Terminology ................................................4
2. Protocol Overview ...............................................4
2.1. Message Summary (Simplified) ...............................6
2.2. Compatibility and Transparency .............................7
2.3. Fully Loaded Cookies .......................................7
2.4. TCP Header Extension .......................................8
2.5. <SYN> Option Handling ......................................9
3. Protocol Details ................................................9
3.1. TCP Cookie Option .........................................10
3.2. TCP Cookie-Pair Standard Option ...........................10
3.3. TCP Cookie-less Option ....................................11
3.4. TCP Timestamps Extended Option ............................11
3.5. Cookie Generation .........................................13
4. Cookie Exchange ................................................16
4.1. Initiator <SYN> ...........................................16
4.2. Responder <SYN,ACK(SYN)> ..................................17
4.3. Initiator <ACK(SYN)> ......................................17
4.4. Responder <ACK> ...........................................18
4.5. Simultaneous Open .........................................18
5. Accelerated Close ..............................................19
5.1. Initiator Close ...........................................20
5.2. Responder Close ...........................................20
6. Accelerated Open ...............................................21
6.1. Initiator <SYN> Data ......................................21
6.2. Responder <SYN,ACK(SYN)> Data .............................22
6.3. Initiator <ACK(SYN)> Data .................................23
6.4. Responder <ACK> Data ......................................24
7. Advisory Reset .................................................24
8. Interactions with Other Options ................................24
8.1. TCP Selective Acknowledgment ..............................25
8.2. TCP Timestamps ............................................25
8.3. TCP Extensions for Transactions ...........................25
8.4. TCP MD5 Signature .........................................25
8.5. TCP Authentication ........................................25
9. History ........................................................26
10. Acknowledgments ...............................................27
11. IESG Considerations ...........................................27
12. Operational Considerations ....................................28
13. Security Considerations .......................................28
Appendix A. Example Headers .......................................30
A.1. Example <SYN> Options .....................................30
A.2. Example <ACK(SYN)> with Sack ..............................31
A.3. Example <ACK(SYN)> with 64-bit Timestamps .................32
Normative References ..............................................33
Informative References ............................................34
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RFC 6013 TCP Cookie Transactions January 2011
1. Introduction
TCP Cookie Transactions (TCPCT) provide a cryptologically secure
mechanism to guard against simple flooding attacks sent with bogus IP
[RFC 791] Sources or TCP [RFC 793] Ports. The initial TCP <SYN>
exchange is vulnerable to forged IP Addresses, predictable Ports, and
discoverable Sequence Numbers [Morris1985] [Gont2009]. (See also
[RFC 2827], [RFC 3704], and [RFC 4953].)
During connection establishment, the cookie (nonce) exchange
negotiates elimination of Responder (server) state. These cookies
are later used to inhibit premature closing of connections, and
reduce retention of state after the connection has terminated.
The cookie pair is much too large to fit with the other recommended
options in the maximal 60 byte TCP header (40 bytes of option space).
A successful option exchange signals availability of the TCP header
extension, adding space for additional options.
Also, implementations may optionally exchange limited amounts of
transaction data during the initial cookie exchange, reducing network
latency and host task context switching.
Finally, implementations may optionally rapidly recycle prior
connections. For otherwise stateless applications, this
transparently facilitates persistent connections and pipelining of
requests over each connection.
Many of these ideas have been previously proposed in one form or
another (see History and Acknowledgments sections). This
specification integrates these improvements into a coherent whole.
Further motivation and rationale were detailed in [MSV2009].
1.1. Terminology
The key words "MAY", "MUST, "MUST NOT", "OPTIONAL", "RECOMMENDED",
"REQUIRED", "SHOULD", and "SHOULD NOT" in this document are to be
interpreted as described in [RFC 2119].
byte An 8-bit quantity; also known as "octet" in standardese.
2. Protocol Overview
The TCPCT extensions consist of several simple phases:
1. Each party passes a "cookie" to the other. Due to limited space,
only the most basic options are included.
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The Cookie option also indicates that optional <SYN> data is
acceptable. This data MAY be ignored by either party.
A Responder that understands the Cookie option remains stateless.
2. During the remainder of the standard TCP three-way handshake, the
Timestamps and Cookie-Pair options guard the exchange.
Other options present in the original <SYN> that were successfully
returned in the <SYN,ACK(SYN)> MUST be included with the
<ACK(SYN)>. Additional options MAY also be included as desired.
As there is no Responder state, it has no record of acknowledging
previous data. Any optional <SYN> data MUST be retransmitted.
Upon verification of the Timestamps and Cookie-Pair, the Responder
creates its Transport Control Block (TCB) [RFC 793].
Note that the Responder returns the Cookie-Pair with its initial
data, but subsequent data segments need only the Timestamps.
3. During close (or reset) of the TCP connection, the Timestamps and
Cookie-Pair options guard the exchange.
Upon verification of the Timestamps and Cookie-Pair, the Responder
removes its TCB.
The sequence of messages is summarized in the diagram below.
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2.1. Message Summary (Simplified)
Initiator Responder
========= =========
<SYN> ->
base options
Timestamps
Cookie
[request data]
<- <SYN,ACK(SYN)>
base options
Timestamps
Cookie
[response data]
(stateless)
<ACK(SYN)> ->
full options
Timestamps
Cookie-Pair
[Sack(response)]
data
<- <ACK>
full options
Timestamps
Cookie-Pair
data
(TCB state created)
<- <ACK>
Timestamps
data
<- <FIN,ACK>
Timestamps
Cookie-Pair
<FIN,ACK(FIN)> ->
Timestamps
Cookie-Pair
<- <ACK(FIN)>
Timestamps
Cookie-Pair
(TCB state removed)
TIME-WAIT
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RFC 6013 TCP Cookie Transactions January 2011
2.2. Compatibility and Transparency
It is usually better that data arrive slowly, than not at all.
Many/most unmanaged middleboxes [RFC 3234] (such as stateless
firewalls, load balancers, intrusion detection systems, or network
address translators [RFC 3022]) cannot carry transport traffic other
than TCP and UDP.
Every TCP implementation MUST ignore without error any TCP option it
does not implement ([RFC 1122] section 4.2.2.5). In a study of the
effects of middleboxes on transport protocols [MAF2004], the vast
majority of modern TCP stacks correctly handle unknown TCP options.
But it is still prudent to follow the [RFC 793] "general principle of
robustness: be conservative in what you do, be liberal in what you
accept from others."
Therefore, for each of the extensions defined here, an extension
option will be sent in a <SYN,ACK(SYN)> segment only after the
corresponding option was received in the original <SYN> segment.
Furthermore, TCP options will be sent on later segments only after an
exchange of options has indicated that both parties understand the
extension (see [RFC 1323] [rfc1323bis] and its antecedents).
Unfortunately, not all middleware adheres to these long-standing
requirements. Instead, unknown <SYN> options are copied to the
<SYN,ACK(SYN)>. This is indistinguishable from a Monkey in the
Middle (MITM) reflection attack.
2.3. Fully Loaded Cookies
One Kind to aid them all, One Kind to find them,
One Kind to hold them all and in the header bind them.
The cookie exchange provides a singular opportunity to extend TCP
with backward compatibility. Semantics for the option have been
"overloaded" with a baker's dozen of capabilities and facilities.
A. First and foremost, the cookie exchange improves operational
security for vulnerable servers against flooding attacks. The
cookie exchange indicates that the Responder (server) will discard
its initial state. All other semantics are subordinate.
B. Together with Sequence and Timestamp values, Cookie values protect
against insertion and reflection attacks.
C. Cookie values allow applications to detect replay attacks.
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D. Cookie values MAY be used as an index or nonce for application
security protocols. This facility is beyond the scope of this
specification.
E. The <SYN> and <SYN,ACK(SYN)> MAY carry application data. This
feature is entirely optional, and data is not guaranteed to pass
successfully through middleware. Nor are the parties guaranteed
to process this data without changes to the Application Program
Interface (API). Such changes are beyond the scope of this
specification.
F. The size of the cookies precludes most other options in the
standard TCP header space. The cookie exchange negotiates TCP
header extension.
G. The cookie exchange and resulting TCP header extension permit
negotiation of larger 64-bit (or 128-bit) Timestamps for paths
with large bandwidth-delay products.
H. TCP header extension frees some space for additional options.
I. Previously SYN-only options can be updated.
J. The cookie exchange indicates agreement to use accelerated close.
K. The cookie exchange indicates agreement that only the Initiator
(client) handles TIME-WAIT state.
L. The Timestamps and Cookie-Pair combination inhibits third parties
from disrupting communications with <FIN> and <RST>.
M. The Timestamps and Cookie-Pair combination facilitates rapid reuse
of the TCP Source Port with a common destination.
2.4. TCP Header Extension
Once the Cookie option has been successfully exchanged, TCP header
extension is permitted. The Timestamps extended option (defined
below) indicates the presence of the header extension.
Validation of known timestamp values protects against data corruption
by misbehaving middleboxes.
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2.5. <SYN> Option Handling
As the Responder retains no TCB state after the initial TCP <SYN>
exchange, all options present in the original <SYN> MUST be repeated.
For example, an option defined in the [RFC 793] original specification
-- Maximum Segment Size (MSS) -- previously appeared only in a <SYN>
bearing segment (including <SYN,ACK(SYN)>). If present, MSS will be
repeated in the Initiator <ACK(SYN)>, together with any additional
options.
Generally, the Initiator MAY propose SYN-only options -- such as MSS
-- anytime both Timestamps and Cookie-Pair options are present.
These options are treated the same as with an original <SYN>. The
Responder acknowledges using a subsequent <ACK> segment containing
both Timestamps and Cookie-Pair options (similar to <SYN,ACK(SYN)>
processing).
This facility allows previously SYN-only options to be updated from
time to time. They take effect upon receipt.
However, <ACK> segments without data will not be delivered reliably.
Any otherwise SYN-only options sent without data MUST be
retransmitted with successive segments until sent with data (or
<FIN>), and an <ACK> is received.
3. Protocol Details
Another solution [RFC 5452] describes use of an unpredictable Source
Port. That is RECOMMENDED by this specification. See [RFC 6056] for
further information.
An earlier solution [RFC 1948] describes an unpredictable Initial
Sequence Number (ISN). That is REQUIRED by this specification.
Support for the (32-bit) TCP Timestamps Option [RFC 1323] is REQUIRED.
A TSoffset SHOULD be generated per connection [GO2010]. The Don't
Fragment (DF) bit MUST be set in the IP (v4) header.
The TCP User Timeout Option [RFC 5482] is RECOMMENDED.
Only one instance is permitted of any of the Cookie, Cookie-less, or
Cookie-Pair option(s). Segments with duplicative or mutually
exclusive options MUST be silently discarded.
For examples, see Appendix A.
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3.1. TCP Cookie Option
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Kind | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ Cookie ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Kind 1 byte: constant 253 (experimental).
Length 1 byte: range 10 to 18 (bytes); limited by remaining
space in the options field. The number MUST be
even; the cookie is a multiple of 16 bits.
Cookie 8 to 16 bytes (Length - 2): an unpredictable value.
Options with invalid Length values MUST be ignored. The minimum
Cookie size is 64 bits. If there is not sufficient space for a
64-bit cookie, this option MUST NOT be used.
The Responder Cookie MUST be the same size as the Initiator Cookie.
The cookie pair is a multiple of 32 bits.
Although the diagram shows a cookie aligned on 32-bit boundaries,
that is not required.
3.2. TCP Cookie-Pair Standard Option
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Kind | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ Initiator-Cookie ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ Responder-Cookie ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Kind 1 byte: constant 253 (experimental).
Length 1 byte: range 18 to 34 (bytes). The number MUST be
even; the cookie pair is a multiple of 32 bits.
Initiator-Cookie 8 to 16 bytes, from the original <SYN>.
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RFC 6013 TCP Cookie Transactions January 2011
Responder-Cookie 8 to 16 bytes, from the <SYN,ACK(SYN)>.
The Cookie-Pair standard option only appears after the Timestamps
extended option (below).
Options with invalid Length values MUST be ignored. As the minimum
Initiator-Cookie size is 64 bits, the minimum cookie pair is 128 bits
(64 bits followed by 64 bits), while the maximum is 256 bits (128
bits followed by 128 bits).
3.3. TCP Cookie-less Option
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Kind | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Kind 1 byte: constant 253 (experimental).
Length 1 byte: constant 2 (bytes). This distinguishes the
option from other Cookie options.
Although no cookie is attached, this indicates that other features of
this specification are available, including TCP header extension,
Accelerated Close, Accelerated Open, and Advisory Reset. This is
intended for use with TCP authentication options, beyond the scope of
this specification.
3.4. TCP Timestamps Extended Option
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Kind | Length | Extend | R | S |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| |
~ TS Value ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ TS Echo Reply ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Kind 1 byte: constant 254 (experimental).
Length 1 byte: constant 4 (bytes).
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RFC 6013 TCP Cookie Transactions January 2011
Extend 1 byte: range 9 to 255; the data offset (in 32-bit
words) following the standard TCP header. Note this
value MUST include the timestamp pair indicated by
(S)ize.
(R)eserved 5 bits: default zero. Reserved for future use.
(S)ize 3 bits:
1. 32-bit timestamps.
2. 64-bit timestamps.
4. 128-bit timestamps.
Other values are beyond the scope of this
specification.
TS Value 4, 8, or 16 bytes. The current value of the
timestamp for the sender.
TS Echo Reply 4, 8, or 16 bytes. A copy of the most recently
received TS Value.
The full timestamp pair follows the TCP header (indicated by +=+
delimiters) and maintains 32-bit alignment.
This TCP header extension is ignored for sequence number
computations. The Sequence Number of the first byte of segment data
will be the Initial Sequence Number (ISN) plus one (1) for the <SYN>.
Every TCPCT implementation MUST recognize a Timestamps extended
option. The larger 64-bit (or 128-bit) timestamps only appear in an
extended option.
Segments with invalid Extend values MUST be silently discarded.
Only one instance is permitted of either the (32-bit) Timestamps
standard option or this Timestamps extended option. Segments with
duplicative or mutually exclusive options MUST be silently discarded.
Implementation Notes:
Serendipitous alignment allows simple loads and stores, instead of
slower byte by byte iterations.
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When the TCP header is aligned on a 32-bit boundary and this is
the only option, the timestamps in the extended header SHOULD be
aligned on a 64-bit boundary. For both 32-bit and 64-bit
timestamps, any data following the extended header will be aligned
on a 64-bit boundary.
However, the 128-bit timestamps are not 128-bit aligned.
3.5. Cookie Generation
The technique by which a party generates a cookie is implementation
dependent. The method chosen must satisfy some basic requirements:
1. The cookie MUST depend on the specific parties. This prevents an
attacker from obtaining a cookie using a real IP address and TCP
port, and then using it to swamp the victim with requests from
randomly chosen IP addresses or ports.
2. It MUST NOT be possible for anyone other than the issuing entity
to generate cookies that will be accepted by that entity. This
implies that the issuing entity will use local secret information
in the generation and subsequent verification of a cookie. It
must not be possible to deduce this secret information from any
particular cookie.
3. The cookie generation and verification methods MUST be fast to
thwart attacks intended to sabotage CPU resources.
A recommended technique is to use a cryptographic hashing function.
An incoming cookie can be verified at any time by regenerating it
locally from values contained in the incoming datagram and the local
secret random value.
3.5.1. Initiator Cookie
The Initiator secret value that affects its cookie SHOULD change for
each new exchange, and is thereafter internally cached per TCB. This
provides improved synchronization and protection against replay
attacks.
An alternative is to cache the cookie instead of the secret value.
Incoming cookies can be compared directly without the computational
cost of regeneration.
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RFC 6013 TCP Cookie Transactions January 2011
It is RECOMMENDED that the cookie be calculated over the secret
value, the IP Source and Destination addresses, the TCP Source and
Destination ports, and any (optional) Initiator <SYN> segment data.
Implementation Notes:
Although the recommendation includes the TCP Source Port, this is
very implementation specific. For example, it might not be
included when the value is constant or unknown.
Likewise, segment data might not be included directly. For
example, a pointer to the data could be included instead, with
care taken to ensure the pointer changes anytime the data changes.
However, it is important that the implementation protect mutually
suspicious users of the same system from generating the same
cookie.
3.5.2. Responder Cookie
The Responder secret value that affects its cookies remains the same
for many different Initiators. However, this secret SHOULD be
changed periodically to limit the time for use of its cookies
(typically each 600 seconds).
The Responder-Cookie calculation MUST include its own TCP Sequence
and Acknowledgment Numbers (after updating values), its own TCP
Timestamps value, and the Initiator-Cookie value. This provides
improved synchronization and protection against replay attacks.
It is RECOMMENDED that the cookie be calculated over the secret
value, the IP Source and Destination addresses, its own TCP
Destination Port (that is, the incoming Source Port), and the
required values (above), followed by the secret value again.
The cookie is not cached per Initiator to avoid saving state during
the initial TCP <SYN> exchange. On receipt of a TCP <ACK(SYN)>, the
Responder regenerates its cookie for validation.
Implementation Notes:
Although the recommendation does not include the TCP Source Port,
this is very implementation specific. It might be successfully
included in some variants.
The Responder Cookie depends on the TCP Sequence and
Acknowledgment Numbers as they will appear for future
verification. The Sequence Number will be the Initial Sequence
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RFC 6013 TCP Cookie Transactions January 2011
Number (ISN) plus one (1) for its <SYN> that will be acknowledged.
The Acknowledgment Number will be the Initial Sequence Number
(ISN) plus one (1) for the <SYN> that it is now acknowledging.
The (32-bit) TCP Timestamps standard option MAY change to the
larger 64-bit (or 128-bit) extended form; only the least
significant 32 bits are included. The Initiator Timestamp field
value MAY increment during the exchange; it MUST NOT be included.
The secret value is included twice to better protect against pre-
calculated attacks using substitutions for variable length data.
Some examples using this technique are IP-MAC and H-MAC, and it is
likely that existing code could be shared.
The Responder SHOULD designate a (fixed or randomly selected) bit
of its cookie to distinguish each changed secret value. The bit
is set to a (fixed or randomly selected) constant 0 or 1, and
checked upon receipt before further verification. This ensures
that only one verification calculation is necessary (on average)
during Denial of Service (DoS) attacks.
If a Responder Cookie is identical to the Initiator Cookie, the
Responder SHOULD change one or more bits of its cookie to prevent
its accidental appearance as a reflection attack.
3.5.3. Responder Secret Value
Each Responder maintains up to two secret values concurrently for
efficient secret rollover. Each secret value has 4 states:
Generating
Generates new Responder-Cookies, but not yet used for primary
verification. This is a short-term state, typically lasting only
one Round Trip Time (RTT).
Primary
Used both for generation and primary verification.
Retiring
Used for verification, until the first failure that can be
verified by the newer Generating secret. At that time, this
cookie's state is changed to Secondary, and the Generating
cookie's state is changed to Primary. This is a short-term state,
typically lasting only one RTT.
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Secondary
Used for secondary verification, after primary verification
failures. This state lasts no more than twice the Maximum Segment
Lifetime (2MSL). Then, the secret is discarded.
Implementation Notes:
Care MUST be taken to ensure that any expired secrets are promptly
wiped from memory, and secrets are never saved to external
storage.
The first secret after initialization begins in Primary state.
The system might have shutdown and restarted rapidly during the
previous first secret. Thus, the first secret MUST be partially
time dependent, to ensure that it differs from previous first
secrets, usually by appending a time to lengthen the first secret.
Those that are not the first secret SHOULD NOT include the time.
At the same time, there is no TCP TIME-WAIT requirement before
accepting connections, and there may be pent up demand for a busy
service. Also, there may be outstanding datagrams attempting to
complete an earlier cookie exchange. The first secret is likely
to be the weakest, as no recent entropy has been included.
Therefore, while terminating outstanding exchanges with the first
secret, a new Generating secret SHOULD be created after no more
than one Maximum Segment Lifetime (1MSL). Subsequent secrets
SHOULD be generated at the usual rate (typically 600 seconds).
The implementation SHOULD continually gather additional entropy
from checksums, cookies, timestamps, and packet arrival timing.
4. Cookie Exchange
A successful option exchange signals availability of additional
features.
4.1. Initiator <SYN>
The Cookie exchange MAY be initiated at any time, limited only by the
frequency of the timestamp clock.
If the TCB exists from a prior (or ongoing) connection, the timestamp
MUST be incremented in the option.
The Initiator generates its unpredictable cookie value, and includes
the Cookie option.
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During the initial exchange, the Initiator is solely responsible for
retransmission. Although the cookie and sequence have not changed,
each retransmission appears to the Responder as another original
<SYN>.
Implementation Notes:
Sending the <SYN> SHOULD NOT affect any existing TCB. This allows
an additional RTT for duplicate or out-of-sequence segments to
drain.
The new TCB information SHOULD be temporarily cached until a valid
matching <SYN,ACK(SYN)> arrives. Then, any old TCB values are
replaced.
4.2. Responder <SYN,ACK(SYN)>
Upon receipt of the <SYN> with a Cookie option, the Responder
determines whether there are sufficient resources to begin another
connection.
If the TCB exists from a prior (or ongoing) connection, the timestamp
MUST be incremented in the option.
Each Sequence Number MUST be randomized [RFC 1948].
The Responder generates its unpredictable cookie value, and includes
the Cookie option.
As the Responder retains no TCB state, retransmission timers are not
available. Arrival of an Initiator's retransmission appears to be an
original <SYN> transmission. There are no differences in processing.
Implementation Notes:
Sending the <SYN,ACK(SYN)> MUST NOT affect any existing TCB. This
allows an additional RTT for duplicate or out-of-sequence segments
to drain.
This also inhibits third parties from disrupting communications.
4.3. Initiator <ACK(SYN)>
Upon receipt of the <SYN,ACK(SYN)> with a Cookie option, the
Initiator validates its cookie, timestamp, and corresponding
Acknowledgment Number. The existing TCB is updated as necessary.
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All Initiator <SYN> options are always retransmitted on this first
<ACK(SYN)>, allowing the Responder to validate its cookie and
establish its state.
This segment contains both Timestamps and Cookie-Pair options.
The Initiator sends the Timestamps extended option with an
appropriate Size -- chosen by a configurable parameter, or
automatically based on its analysis of the bandwidth-delay product
discovered through the RTT of its <SYN> timestamp. When the chosen
Size is greater than 32 bits, the Initiator adds a random prefix to
its own timestamp, and a random prefix to the Responder timestamp
echo reply.
Implementation Notes:
A Responder Cookie identical to the Initiator Cookie MUST be
discarded. This is usually an indication of a Monkey in the
Middle (MITM) reflection attack or a seriously misconfigured
network, and SHOULD be logged.
4.4. Responder <ACK>
Upon receipt of the <ACK(SYN)> with a Cookie-Pair option, the
Responder validates its cookie, timestamp, and corresponding
Acknowledgment Number, and establishes state for the connection. Any
existing TCB is updated as necessary.
This segment contains both Timestamps and Cookie-Pair options.
However, the Responder MAY refuse to negotiate the larger 64-bit (or
128-bit) Timestamps extended option by returning the least
significant bits in a smaller Timestamps extended option.
Implementation Notes:
An <ACK(SYN)> that fails to validate MUST be discarded, and SHOULD
be logged.
4.5. Simultaneous Open
TCP allows two parties to simultaneously initiate the connection.
Both parties send and receive an original <SYN> without an
intervening <SYN,ACK(SYN)> (see [RFC 793] section 3.4 and Figure 8).
Each party receives a Cookie for a <Source Address, Source Port,
Destination Address, Destination Port> connection that has also
issued a Cookie.
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This condition will be unusual. The Source Port SHOULD be randomized
[RFC 5452], and SHOULD be chosen to differ from the Destination Port.
In particular, the Source Port SHOULD be greater than 1024,
preventing intervening network equipment from incorrectly classifying
the return traffic. The Destination Port is most likely to be a
well-known port less than 1024 [RFC 3232].
In the event that these protections are insufficient, the conflict is
resolved in an orderly fashion:
a. The lesser TCP Port number becomes the Responder;
b. The lesser IP Address becomes the Responder;
c. The lesser Cookie becomes the Responder;
d. All of the above being equal, there is an egregiously insufficient
source of randomness, but both Initiators are probably present on
the same host: the lesser TCB memory address becomes the
Responder.
The Initiator silently discards the simultaneous <SYN>. The
Responder revises its Cookie option, and sends the <SYN,ACK(SYN)> as
usual, but without removing its existing TCB.
Implementation Notes:
This is usually an indication of a Monkey in the Middle (MITM)
reflection attack or a seriously misconfigured network, and SHOULD
be logged.
5. Accelerated Close
Support for accelerated close is REQUIRED. Accelerated close relies
on the presence of cookies and timestamps. This provides improved
synchronization and protection against replay attacks.
Either party MAY close with <FIN> at any time. This <FIN> SHOULD be
sent with the final data segment.
This segment contains both Timestamps and Cookie-Pair options.
When all segments preceding the <FIN> have been processed and
acknowledged, each party SHOULD acknowledge the <FIN>.
In general, <FIN> is treated as advisory. A persistent connection
can be rapidly re-established. This also inhibits third parties from
disrupting communications.
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RFC 6013 TCP Cookie Transactions January 2011
Rapidly closing the connection expedites removing Responder state.
Any <FIN> bearing segment SHOULD terminate delayed <ACK> [RFC 5681].
Retransmit at the latest Timestamps estimated Smoothed Round Trip
Time (SRTT). Backoff SHOULD NOT be used for <FIN> bearing
retransmissions [RFC 2988].
As the Responder retains no TCB state after closing, a successful
option exchange signals the Initiator will be responsible for
handling TIME-WAIT state. (For previous proposal and rationale, see
[FTY1999] section 3.)
A new Cookie exchange MAY be initiated at any time. This facilitates
persistent connections through intervening network equipment.
5.1. Initiator Close
Upon receipt of the Initiator <FIN> (and verification of the
Timestamps and Cookie-Pair options), the Responder sends its
<FIN,ACK(FIN)> unless there is additional data pending. In the
latter case, the <FIN> is ignored until the data has been processed
and acknowledged.
Upon receipt of the Responder <FIN,ACK(FIN)> (and verification of the
Timestamps and Cookie-Pair options), the Initiator sends its final
<ACK(FIN)> unless there is additional data pending. The Initiator
enters TIME-WAIT state.
This segment contains both Timestamps and Cookie-Pair options.
Upon receipt of the Initiator <ACK(FIN)> (and verification of the
Timestamps and Cookie-Pair options), the Responder removes its TCB.
Upon arrival of more data prompting a new Cookie exchange, the
Initiator SHOULD NOT send a final <ACK(FIN)> and/or SHOULD NOT wait
the remaining TIME-WAIT interval. Any existing TSoffset SHOULD be
incremented. TSoffset will be removed (with the TCB itself) at the
conclusion of a future TIME-WAIT state.
5.2. Responder Close
Upon receipt of the Responder <FIN> (and verification of the
Timestamps and Cookie-Pair options), the Initiator sends its
<FIN,ACK(FIN)> unless there is additional data pending. In the
latter case, the <FIN> is ignored until the data has been processed
and acknowledged.
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RFC 6013 TCP Cookie Transactions January 2011
Upon receipt of the Initiator <FIN,ACK(FIN)> (and verification of the
Timestamps and Cookie-Pair options), the Responder sends its final
<ACK(FIN)> and removes its TCB.
This segment contains both Timestamps and Cookie-Pair options.
If the Responder's final <ACK(FIN)> is lost, the Responder is likely
to send a <RST> (as the Responder retains no TCB state). This
distinguished <RST> SHOULD copy both Timestamps and Cookie-Pair
options.
Upon receipt of the Responder's final <ACK(FIN)> (and verification of
the Timestamps and Cookie-Pair options), the Initiator enters TIME-
WAIT state.
Upon arrival of more data prompting a new Cookie exchange, the
Initiator SHOULD NOT send a <FIN,ACK(FIN)> and/or SHOULD NOT wait the
remaining TIME-WAIT interval. Any existing TSoffset SHOULD be
incremented. TSoffset will be removed (with the TCB itself) at the
conclusion of a future TIME-WAIT state.
6. Accelerated Open
Support for accelerated open is OPTIONAL.
When an application is capable of idempotent transactions (such as a
query that returns a consistent result or service response heading),
the application sets the appropriate limit separately for each port
or connection. Applications are responsible for ensuring that
retransmissions do not cause duplication of data.
This facility allows single data segment transactions without
establishing TCB state at the Responder (server). For longer
transactions, a short look-ahead of upcoming data allows the
Initiator (client) to select alternatives for further processing.
6.1. Initiator <SYN> Data
By default, the Initiator <SYN> does not contain data. The
application sets the TCP_SYN_DATA_LIMIT to indicate that the <SYN>
MAY be sent with data.
The Responder Maximum Segment Size (MSS) is unknown, and the default
MSS (536 bytes) MUST be used instead ([RFC 1122] section 4.2.2.6).
This is further reduced by the total length of the TCP options (in
this case, commonly 496 bytes). Applications MAY specify a shorter
limit.
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RFC 6013 TCP Cookie Transactions January 2011
If the data will not entirely fit within the initial segment, data
MUST NOT be sent until after the Responder's <SYN,ACK(SYN)> is
received.
Unlike T/TCP [RFC 1644], <FIN> SHOULD NOT be sent with <SYN> data.
This facilitates persistent connections.
Likewise, <PSH> SHOULD NOT be set. Although the application might
use push to indicate that its data is ready to send, the push is
implied for <SYN> data segments.
During the initial exchange, the Initiator is solely responsible for
retransmission. Although the cookie and sequence have not changed,
each retransmission appears to the Responder as another original
<SYN>.
Implementation Notes:
Initiator <SYN,FIN> with the Cookie option and no segment data is
permitted in a test environment. This combination SHOULD be
silently discarded.
Initiator <SYN,FIN> with both the Cookie option and segment data
is similar to T/TCP [RFC 1644]. However, whenever the Responder
<SYN,ACK(SYN),FIN> has been sent with data (there is no further
data expected), TCB state has not been saved at the Responder.
There is no need to send <FIN> to close the connection.
6.2. Responder <SYN,ACK(SYN)> Data
By default, the Responder <SYN,ACK(SYN)> does not contain data. The
application sets the TCP_SYN_ACK_DATA_LIMIT to indicate that the
<SYN,ACK(SYN)> MAY be sent with data.
Segment data is limited to the Maximum Transmission Unit (MTU).
Applications MAY specify a shorter limit to prevent spoofed
amplification and reflection attacks [RFC 5358].
Upon receipt of the <SYN> with a Cookie option, the Responder MAY
process any data present. If the initial data is not accepted, the
Acknowledgment Number will be the received Sequence Number plus one
(1) for the <SYN>.
If the segment data is the entire response (there is no further data
expected), <FIN> MAY be set.
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RFC 6013 TCP Cookie Transactions January 2011
However, <PSH> SHOULD NOT be set. Although the application might use
push to indicate that its data is ready to send, the push is implied
for <FIN> data segments (see [RFC 793] section 3.7, page 41).
As the Responder retains no TCB state, retransmission timers are not
available. Arrival of an Initiator's retransmission appears to be an
original <SYN> transmission. There are no differences in processing.
Implementation Notes:
The Responder Cookie depends on the TCP Sequence and
Acknowledgment Numbers after processing <SYN>. Therefore, neither
will include data.
6.3. Initiator <ACK(SYN)> Data
Upon receipt of the <SYN,ACK(SYN)> with a Cookie option, the
Initiator MAY process any data present. In this case, the internal
RCV.NXT is advanced to provide at-most-once semantics.
If the segment data is the entire response (there is no further data
expected), the Initiator enters TIME-WAIT state.
Otherwise, original <SYN> data is retransmitted in <ACK(SYN)>, as its
processing is optional. The Acknowledgment Number will be the
received Sequence Number plus one (1) for the <SYN>. The Sequence
Number will be the Initial Sequence Number (ISN) plus one (1) for the
<SYN>.
Unlike T/TCP [RFC 1644], there is no implicit acknowledgment.
If the Selective Acknowledgment (Sack) option [RFC 2018] has been
successfully negotiated, a short Sack acknowledging the response data
MAY be sent following the Cookie-Pair in the extended header.
At this time, any second segment may be sent without awaiting an
<ACK>, according to the usual [RFC 5681] TCP congestion control
process.
Implementation Notes:
Upon arrival of more data prompting a new Cookie exchange, there
is no need to increment the previous timestamp; TCB state has not
been saved at the Responder. Instead, use the saved RCV.NXT, plus
one (1) for the (actual or implied) <FIN>.
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RFC 6013 TCP Cookie Transactions January 2011
Initiator <ACK(SYN),FIN> with the Cookie-Pair option and no
segment data is never required; TCB state has not been saved at
the Responder. This combination MUST be silently discarded.
6.4. Responder <ACK> Data
Upon receipt of the <ACK(SYN)> with a Cookie-Pair option (and
verification of the Timestamps and Cookie-Pair options), the
Responder SHOULD process any data present.
Since the TCP Sequence and Acknowledgment Numbers have not advanced,
the Responder will process the same incoming data, and transmit the
same response.
If the Selective Acknowledgment (Sack) option [RFC 2018] has been
successfully negotiated, with a short Sack covering earlier response
data, only additional unacknowledged response data is sent.
At this time, any second segment may be sent without awaiting an
<ACK>, according to the usual [RFC 5681] TCP congestion control
process.
7. Advisory Reset
When a TCB with matching Addresses and Ports is found, but the
Cookie-Pair fails to verify, the datagram MUST be silently discarded.
When no TCB with matching Addresses and Ports is found, a <RST> is
sent as usual. The Timestamps option SHOULD be copied [RFC 1323]. A
Cookie-Pair option MUST also be copied. The Cookie option (or
Cookie-less option) MUST NOT be copied.
Any <RST> is always treated as advisory. A <RST> without a matching
Cookie-Pair option could be caused by antique duplicates. Receipt
has no effect on the operation of the protocol. The implementation
SHOULD continue until a USER TIMEOUT expires. (See [RFC 5482] for
additional information.)
This also inhibits third parties from disrupting communications.
8. Interactions with Other Options
A successful Cookie (or Cookie-less) option exchange signals
availability of the TCP header extension. Other options with large
data portions MAY also use this feature. The extended option data is
processed in the order that the options appear.
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RFC 6013 TCP Cookie Transactions January 2011
8.1. TCP Selective Acknowledgment
(Kind 5 [RFC 2018].) The pairs of 32-bit fields are well suited to
the header extension. Because of its variable size, this is
RECOMMENDED as the final extended option.
During the cookie exchange, the <ACK(SYN)> MAY include this option to
acknowledge any optional transaction response data.
8.2. TCP Timestamps
(Kind 8 [RFC 1323].) Support is REQUIRED. See also section 3.
When a segment needs no header extension, and 32-bit timestamps have
been negotiated, this option MUST be sent.
8.3. TCP Extensions for Transactions
(Kinds 11-13 [RFC 1644].) Incompatible with this specification, and
MUST be ignored on receipt.
8.4. TCP MD5 Signature
(Kind 19 [RFC 2385].) This option is beyond the scope of this
specification. Because specific configuration is required, sending
is under the complete control of the operator. Segments lacking this
option will be silently discarded.
The size of the option itself precludes use with the Cookie option in
the <SYN>. Regardless of the system default, the Cookie option MUST
NOT be sent, and MUST be ignored on receipt. Instead, the Cookie-
less extension option indicates that other features of this
specification are available.
8.5. TCP Authentication
(Kind 29 [RFC 5925].) This option is beyond the scope of this
specification. Because specific configuration is required, sending
is under the complete control of the operator. Segments lacking this
option will be silently discarded.
The size of the option itself precludes use with the Cookie option in
the <SYN>. Regardless of the system default, the Cookie option MUST
NOT be sent, and MUST be ignored on receipt. Instead, the Cookie-
less extension option indicates that other features of this
specification are available.
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RFC 6013 TCP Cookie Transactions January 2011
9. History
T/TCP [RFC 1379] [RFC 1644] permits lightweight TCP transactions for
applications that traditionally have used UDP. However, T/TCP has
unacceptable security issues [Hannum1996] [Phrack1998].
The initial specification [KS1995] of Photuris [RFC 2522], now called
version 1 (December 1994 to March 1995), was based on a short list of
design requirements, and simple experimental code by Phil Karn. A
"Cookie" Exchange guards against simple flooding attacks sent with
bogus IP Sources or UDP Ports.
During 1995, the Photuris efficient secret rollover and many other
extensions were specified. Multiple interoperable implementations
were produced.
By September 1996, the long anticipated Denial of Service (DoS)
attacks in the form of TCP SYN floods were devastating popular (and
unpopular) servers and sites. Phil Karn informally mentioned
adapting anti-clogging cookies to TCP. Perry Metzger proposed adding
Karn's cookies as part of a "TCP++" effort [Metzger1996].
Later in 1996, Daniel J. Bernstein implemented "SYN cookies", small
cookies embedded in the TCP SYN Initial Sequence Number (ISN). This
technique was exceptionally clever, because it did not require
cooperation of the remote party and could be deployed unilaterally.
However, SYN cookies can only be used in emergencies; they are
incompatible with most TCP options. As there is insufficient space
in the Sequence Number, the cookie is not considered cryptologically
secure. Therefore, the mechanism remains inactive until the system
is under attack, and thus is not well tested in operation. SYN
cookies were not accepted for publication until recently [RFC 4987].
In 1998, Perry Metzger proposed adding Karn's cookies as part of a
"TCPng" discussion [Metzger1998].
In 1999, Faber, Touch, and Yue [FTY1999] proposed using an option to
negotiate the party that would maintain TIME-WAIT state. This
permits a server to entirely eliminate state after closing a
connection.
In 2000, the Stream Control Transmission Protocol (SCTP) [RFC 2960]
was published with an inadequate partial cookie mechanism claiming to
be based upon Photuris. It featured a deficient checksum (replaced
in 2002 by [RFC 3309] without graceful transition), and has undergone
subsequent revisions [RFC 4960].
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RFC 6013 TCP Cookie Transactions January 2011
In 2006, the Datagram Congestion Control Protocol (DCCP) [RFC 4340]
was published with a mechanism analogous to SYN cookies.
10. Acknowledgments
Andre Broido informally described utilizing cookies for Transport
Layer Security (TLS) session identifiers, in place of the [RFC 5077]
ticket. Rapid TLS session resumption would improve both latency and
privacy, but is beyond the scope of this specification. Also, he
provided numerous helpful comments and additional references, such as
[KBC2005].
H. K. Jerry Chu and Arvind Jain informally described retaining
existing cookies for accelerated open on subsequent connections.
That feature was subsumed by this specification.
Wesley M. Eddy and Adam Langley previously proposed another pair of
options [EL2008] extending the TCP header option space.
Adam Langley previously proposed another option [Langley2008]
permitting <SYN,ACK(SYN)> constant payload data. His (August 2008)
code was a base for the initial TCPCT implementation.
Joe Touch postulated a (hopefully hypothetical) failure mode: options
re-ordered by middleware. This caused a change in specifications,
and has considerably complicated option interactions and processing.
His helpful comments were appreciated.
Many thanks to Fernando Gont for suggestions, and Rick Jones for
performance testing.
11. IESG Considerations
Two TCP Option numbers are reserved for general experimental use
under the rules laid out in [RFC 4727] and [RFC 3692] section 1. Such
values reserved for experimental use are never to be made permanent;
permanent assignments should be obtained through standard processes.
Experimental numbers are intended for experimentation and testing and
are not intended for wide or general deployments.
For further information, contact the author.
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RFC 6013 TCP Cookie Transactions January 2011
12. Operational Considerations
Any implementation of this specification SHOULD be configurable,
separately for each port or connection.
TCPCT_COOKIE_DESIRED
Values: 0 (disabled), 8, 10, 12, 14, 16. Default: 16. Send the
Cookie option with the <SYN>.
TCPCT_EXTEND_TS[32|64|128]
Default: off. If defined, may designate 32-bit, 64-bit, or
128-bit timestamps extension.
TCPCT_IN_ALWAYS
Default: off. Silently discard any incoming <SYN> that is missing
the Cookie option.
TCPCT_OUT_NEVER
Default: off. Refuse to send (override) the Cookie option.
TCP_SYN_DATA_LIMIT
Default: 0. Maximum: 496. The maximum amount of data transmitted
with the <SYN>. Wait for data before sending.
TCP_SYN_ACK_DATA_LIMIT
Default: 0. Maximum: 1220. The maximum amount of data
transmitted with the <SYN,ACK(SYN)>. Wait for data before
sending.
13. Security Considerations
TCPCT was based on currently available tools, by experienced network
protocol designers with an interest in cryptography, rather than by
cryptographers with an interest in network protocols. This
specification is intended to be readily implementable without
requiring an extensive background in cryptology.
Therefore, only minimal background cryptologic discussion and
rationale is included in this document. Although some review has
been provided by the general cryptologic community, it is anticipated
that design decisions and tradeoffs will be thoroughly analysed in
subsequent dissertations and debated for many years to come.
Cryptologic details are reserved for separate documents that may be
more readily and timely updated with new analysis.
Simpson Experimental PAGE 28
RFC 6013 TCP Cookie Transactions January 2011
The security depends on the quality of the random numbers generated
by each party. Generating cryptographic quality random numbers on a
general purpose computer without hardware assistance is a very tricky
problem (see [RFC 4086] for discussion).
TCPCT is not intended to prevent or recover from all possible
security threats. Rather, it is designed to inhibit inadvertent
middlebox interference, while protecting against Denial of Service
(DoS) attacks. (See [RFC 4732], and [RFC 3552] section 4.6.3 et seq.)
The cookie exchange does not protect against an interloper that can
race to substitute another value, nor an interceptor that can modify
and/or replace a value. These attacks are considerably more
difficult than passive vacuum-cleaner monitoring.
Note that each incoming <SYN,ACK(SYN)> replaces the Responder cookie.
The initial exchange is most fragile, as protection against spoofing
relies entirely upon the sequence and timestamp. This replacement
strategy allows the correct pair to pass through, while any others
will be filtered via Responder verification later.
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RFC 6013 TCP Cookie Transactions January 2011
Appendix A. Example Headers
A.1. Example <SYN>
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Kind=MSS | Length=4 | (value) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Kind=UTO | Length=4 | (timeout) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Kind=SackOK | Length=2 | Kind=TS | Length=10 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TS Value |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TS Echo Reply |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Kind=Cookie | Length=16 | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
| |
+ Cookie +
| |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Kind=wscale | Length=3 | (value) | Kind=EOL |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
A 14 byte (112-bit) Cookie barely fits with the other recommended
options in the maximal 60 byte TCP header (40 bytes of option space).
Since the cookies are required to be the same size and meet a 32-bit
alignment requirement, the implementor recognizes that this order
provides optimal packing.
The UserTimeOut (UTO) option can appear in other locations instead,
such as following the Cookie option. Because some middleboxes are
sensitive to the order of options, UTO should not appear before MSS
nor between the TS and Cookie.
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RFC 6013 TCP Cookie Transactions January 2011
A.2. Example <ACK(SYN)> with Sack
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Kind=TSX | Length=4 | Extend=16 | 0 | S=1 |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| TS Value |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TS Echo Reply |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Kind=nop | Kind=nop | Kind=Cookie | Length=30 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| |
+ Initiator-Cookie +
| |
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
| |
+ Responder-Cookie +
| |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Kind=MSS | Length=4 | (value) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Kind=UTO | Length=4 | (timeout) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Kind=nop | Kind=nop | Kind=Sack | Length=10 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Starting Value |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Ending Value |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Kind=wscale | Length=3 | (value) | Kind=EOL |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Sack implies SackOK.
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RFC 6013 TCP Cookie Transactions January 2011
A.3. Example <ACK(SYN)> with 64-bit Timestamps
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Kind=TSX | Length=4 | Extend=15 | 0 | S=2 |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| |
+ TS Value +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ TS Echo Reply +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Kind=SackOK | Length=2 | Kind=Cookie | Length=30 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| |
+ Initiator-Cookie +
| |
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
| |
+ Responder-Cookie +
| |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Kind=MSS | Length=4 | (value) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Kind=UTO | Length=4 | (timeout) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Kind=wscale | Length=3 | (value) | Kind=EOL |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The larger 64-bit (or 128-bit) Timestamps extended option MUST be
recognized, although the Responder MAY return a smaller Timestamps
extended option.
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RFC 6013 TCP Cookie Transactions January 2011
Normative References
[RFC 791] Postel, J., "Internet Protocol", STD 5, RFC 791, September
1981.
[RFC 793] Postel, J., "Transmission Control Protocol", STD 7, RFC
793, September 1981.
[RFC 1122] Braden, R., Ed., "Requirements for Internet Hosts -
Communication Layers", STD 3, RFC 1122, October 1989.
[RFC 1323] Jacobson, V., Braden, R., and D. Borman, "TCP Extensions
for High Performance", RFC 1323, May 1992.
[RFC 1948] Bellovin, S., "Defending Against Sequence Number Attacks",
RFC 1948, May 1996.
[RFC 2018] Mathis, M., Mahdavi, J., Floyd, S., and A. Romanow, "TCP
Selective Acknowledgment Options", RFC 2018, October 1996.
[RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC 2988] Paxson, V. and M. Allman, "Computing TCP's Retransmission
Timer", RFC 2988, November 2000.
[RFC 3232] Reynolds, J., Ed., "Assigned Numbers: RFC 1700 is Replaced
by an On-line Database", RFC 3232, January 2002.
[RFC 5452] Hubert, A. and R. van Mook, "Measures for Making DNS More
Resilient against Forged Answers", RFC 5452, January 2009.
[RFC 5482] Eggert, L. and F. Gont, "TCP User Timeout Option", RFC
5482, March 2009.
[RFC 5681] Allman, M., Paxson, V., and E. Blanton, "TCP Congestion
Control", RFC 5681, September 2009.
Simpson Experimental PAGE 33
RFC 6013 TCP Cookie Transactions January 2011
Informative References
[EL2008] Eddy, W. and A. Langley, "Extending the Space Available
for TCP Options", Work in Progress, July 2008.
[FTY1999] Faber, T., Touch, J., and W. Yue, "The TIME-WAIT state in
TCP and Its Effect on Busy Servers", IEEE INFOCOM 99, pp.
1573-1584.
[Gont2009] Gont, F., "Security assessment of the Transmission Control
Protocol (TCP)", February 2009.
https://www.cpni.gov.uk/Docs/tn-03-09-security-
assessment-TCP.pdf
[GO2010] Gont, F. and A. Oppermann, "On the generation of TCP
timestamps", Work in Progress, June 2010.
[Hannum1996]
Hannum, C., "Security Problems Associated With T/TCP",
unpublished work in progress, September 1996.
http://www.mid-way.org/doc/ttcp-sec.txt
[KBC2005] Kohno, T., Broido, A., and K. C. Claffy, "Remote physical
device fingerprinting", IEEE Symposium on Security and
Privacy, May 2005. http://www.caida.org/
outreach/papers/2005/fingerprinting/
KohnoBroidoClaffy05-devicefingerprinting.pdf
[KS1995] Karn, P. and W. Simpson, "The Photuris Session Key
Management Protocol", March 1995.
Published as: "Photuris: Design Criteria", Proceedings of
Sixth Annual Workshop on Selected Areas in Cryptography,
LNCS 1758, Springer-Verlag. August 1999.
[Langley2008]
Langley, A., "Faster application handshakes with SYN/ACK
payloads", Work in Progress, August 2008.
[MAF2004] 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.
http://www.icsi.berkeley.edu/pubs/networking/tbit-
Aug2004.pdf
Simpson Experimental PAGE 34
RFC 6013 TCP Cookie Transactions January 2011
[Metzger1996]
Metzger, P., "Re: SYN floods (was: does history repeat
itself?)", September 9, 1996.
http://www.merit.net/mail.archives/nanog/
1996-09/msg00235.html
[Metzger1998]
Metzger, P., "Re: what a new TCP header might look like",
May 12, 1998. ftp://ftp.isi.edu/end2end/end2end-
interest-1998.mail
[Morris1985]
Morris, R., "A Weakness in the 4.2BSD Unix TCP/IP
Software", Technical Report CSTR-117, AT&T Bell
Laboratories, February 1985.
http://pdos.csail.mit.edu/~rtm/papers/117.pdf
[MSV2009] Metzger, P., Simpson, W., and P. Vixie, "Improving TCP
Security With Robust Cookies", Usenix ;login:, December
2009. http://www.usenix.org/publications/login/
2009-12/openpdfs/metzger.pdf
[Phrack1998]
route|daemon9, "T/TCP vulnerabilities", Phrack Magazine,
Volume 8, Issue 53, July 8, 1998.
http://www.phrack.org/issues.html?issue=53&id=6
[RFC 1379] Braden, R., "Extending TCP for Transactions -- Concepts",
RFC 1379, November 1992.
[RFC 1644] Braden, R., "T/TCP -- TCP Extensions for Transactions
Functional Specification", RFC 1644, July 1994.
[RFC 2385] Heffernan, A., "Protection of BGP Sessions via the TCP MD5
Signature Option", RFC 2385, August 1998.
[RFC 2522] Karn, P. and W. Simpson, "Photuris: Session-Key Management
Protocol", RFC 2522, March 1999.
[RFC 2827] Ferguson, P. and D. Senie, "Network Ingress Filtering:
Defeating Denial of Service Attacks which employ IP Source
Address Spoofing", BCP 38, RFC 2827, May 2000.
[RFC 2960] Stewart, R., Xie, Q., Morneault, K., Sharp, C.,
Schwarzbauer, H., Taylor, T., Rytina, I., Kalla, M.,
Zhang, L., and V. Paxson, "Stream Control Transmission
Protocol", RFC 2960, October 2000.
Simpson Experimental PAGE 35
RFC 6013 TCP Cookie Transactions January 2011
[RFC 3022] Srisuresh, P. and K. Egevang, "Traditional IP Network
Address Translator (Traditional NAT)", RFC 3022, January
2001.
[RFC 3234] Carpenter, B. and S. Brim, "Middleboxes: Taxonomy and
Issues", RFC 3234, February 2002.
[RFC 3309] Stone, J., Stewart, R., and D. Otis, "Stream Control
Transmission Protocol (SCTP) Checksum Change", RFC 3309,
September 2002.
[RFC 3552] Rescorla, E. and B. Korver, "Guidelines for Writing RFC
Text on Security Considerations", BCP 72, RFC 3552, July
2003.
[RFC 3692] Narten, T., "Assigning Experimental and Testing Numbers
Considered Useful", BCP 82, RFC 3692, January 2004.
[RFC 3704] Baker, F. and P. Savola, "Ingress Filtering for Multihomed
Networks", BCP 84, RFC 3704, March 2004.
[RFC 4086] Eastlake 3rd, D., Schiller, J., and S. Crocker,
"Randomness Requirements for Security", BCP 106, RFC 4086,
June 2005.
[RFC 4340] Kohler, E., Handley, M., and S. Floyd, "Datagram
Congestion Control Protocol (DCCP)", RFC 4340, March 2006.
[RFC 4727] Fenner, B., "Experimental Values In IPv4, IPv6, ICMPv4,
ICMPv6, UDP, and TCP Headers", RFC 4727, November 2006.
[RFC 4732] Handley, M., Ed., Rescorla, E., Ed., and Internet
Architecture Board, "Internet Denial-of-Service
Considerations", RFC 4732, November 2006.
[RFC 4953] Touch, J., "Defending TCP Against Spoofing Attacks", RFC
4953, July 2007.
[RFC 4960] Stewart, R., Ed., "Stream Control Transmission Protocol",
RFC 4960, September 2007.
[RFC 4987] Eddy, W., "TCP SYN Flooding Attacks and Common
Mitigations", RFC 4987, August 2007.
[RFC 5077] Salowey, J., Zhou, H., Eronen, P., and H. Tschofenig,
"Transport Layer Security (TLS) Session Resumption without
Server-Side State", RFC 5077, January 2008.
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RFC 6013 TCP Cookie Transactions January 2011
[RFC 5358] Damas, J. and F. Neves, "Preventing Use of Recursive
Nameservers in Reflector Attacks", BCP 140, RFC 5358,
October 2008.
[RFC 5925] Touch, J., Mankin, A., and R. Bonica, "The TCP
Authentication Option", RFC 5925, June 2010.
[RFC 6056] Larson, M. and F. Gont, "Recommendations for Transport-
Protocol Port Randomization", BCP 156, RFC 6056, January
2011.
[rfc1323bis]
Borman, D., Braden, R., and V. Jacobson., "TCP Extensions
for High Performance", Work in Progress, March 2009.
Author's Address
Questions about this document can be directed to:
William Allen Simpson
DayDreamer
Computer Systems Consulting Services
1384 Fontaine
Madison Heights, Michigan 48071
EMail: William.Allen.Simpson@Gmail.com
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TCP Cookie Transactions (TCPCT)
RFC TOTAL SIZE: 76505 bytes
PUBLICATION DATE: Friday, January 21st, 2011
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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