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RFC 5925
TCPM WG J. Touch
Internet Draft USC/ISI
Obsoletes: 2385 A. Mankin
Intended status: Proposed Standard Johns Hopkins Univ.
Expires: September 2010 R. Bonica
Juniper Networks
March 23, 2010
The TCP Authentication Option
draft-ietf-tcpm-tcp-auth-opt-11.txt
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Abstract
This document specifies the TCP Authentication Option (TCP-AO), which
obsoletes the TCP MD5 Signature option of RFC-2385 (TCP MD5). TCP-AO
specifies the use of stronger Message Authentication Codes (MACs),
protects against replays even for long-lived TCP connections, and
provides more details on the association of security with TCP
connections than TCP MD5. TCP-AO is compatible with either static
master key tuple (MKT) configuration or an external, out-of-band MKT
management mechanism; in either case, TCP-AO also protects
connections when using the same MKT across repeated instances of a
connection, using traffic keys derived from the MKT, and coordinates
MKT changes between endpoints. The result is intended to support
current infrastructure uses of TCP MD5, such as to protect long-lived
connections (as used, e.g., in BGP and LDP), and to support a larger
set of MACs with minimal other system and operational changes. TCP-AO
uses a different option identifier than TCP MD5, even though TCP-AO
and TCP MD5 are never permitted to be used simultaneously. TCP-AO
supports IPv6, and is fully compatible with the proposed requirements
for the replacement of TCP MD5.
Table of Contents
1. Contributors...................................................3
2. Conventions used in this document..............................4
3. Introduction...................................................4
3.1. Applicability Statement...................................5
3.2. Executive Summary.........................................6
4. The TCP Authentication Option..................................7
4.1. Review of TCP MD5 Option..................................7
4.2. The TCP Authentication Option Format......................8
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5. TCP-AO Keys and Their Properties..............................10
5.1. Master Key Tuple.........................................10
5.2. Traffic Keys.............................................12
5.3. MKT Properties...........................................13
6. Per-Connection TCP-AO Parameters..............................14
7. Cryptographic Algorithms......................................15
7.1. MAC Algorithms...........................................15
7.2. Traffic Key Derivation Functions.........................19
7.3. Traffic Key Establishment and Duration Issues............22
7.3.1. MKT Reuse Across Socket Pairs.......................23
7.3.2. MKTs Use Within a Long-lived Connection.............23
8. Additional Security Mechanisms................................23
8.1. Coordinating Use of New MKTs.............................24
8.2. Preventing replay attacks within long-lived connections..25
9. TCP-AO Interaction with TCP...................................27
9.1. TCP User Interface.......................................27
9.2. TCP States and Transitions...............................28
9.3. TCP Segments.............................................28
9.4. Sending TCP Segments.....................................29
9.5. Receiving TCP Segments...................................30
9.6. Impact on TCP Header Size................................32
9.7. Connectionless Resets....................................33
9.8. ICMP Handling............................................34
10. Obsoleting TCP MD5 and Legacy Interactions...................35
11. Interactions with Middleboxes................................36
11.1. Interactions with non-NAT/NAPT Middleboxes..............36
11.2. Interactions with NAT/NAPT Devices......................36
12. Evaluation of Requirements Satisfaction......................36
13. Security Considerations......................................42
14. IANA Considerations..........................................44
15. References...................................................45
15.1. Normative References....................................45
15.2. Informative References..................................46
16. Acknowledgments..............................................48
1. Contributors
This document evolved as the result of collaboration of the TCP
Authentication Design team (tcp-auth-dt), whose members were
(alphabetically): Mark Allman, Steve Bellovin, Ron Bonica, Wes Eddy,
Lars Eggert, Charlie Kaufman, Andrew Lange, Allison Mankin, Sandy
Murphy, Joe Touch, Sriram Viswanathan, Brian Weis, and Magnus
Westerlund. The text of this document is derived from a proposal by
Joe Touch and Allison Mankin [To06] (originally from June 2006),
which was both inspired by and intended as a counterproposal to the
revisions to TCP MD5 suggested in a document by Ron Bonica, Brian
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Weis, Sriran Viswanathan, Andrew Lange, and Owen Wheeler [Bo07]
(originally from Sept. 2005) and in a document by Brian Weis [We05].
Russ Housley suggested L4/application layer management of the master
key tuples. Steve Bellovin motivated the KeyID field. Eric Rescorla
suggested the use of TCP's initial sequence numbers (ISNs) in the
traffic key computation and SNEs to avoid replay attacks, and Brian
Weis extended the computation to incorporate the entire connection ID
and provided the details of the traffic key computation. Mark Allman,
Wes Eddy, Lars Eggert, Ted Faber, Russ Housley, Gregory Lebovitz, Tim
Polk, Eric Rescorla, Joe Touch, and Brian Weis developed the master
key coordination mechanism.
2. Conventions used in this document
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].
In this document, these words will appear with that interpretation
only when in ALL CAPS. Lower case uses of these words are not to be
interpreted as carrying RFC-2119 significance.
In this document, the characters ">>" preceeding an indented line(s)
indicates a compliance requirement statement using the key words
listed above. This convention aids reviewers in quickly identifying
or finding the explicit compliance requirements of this RFC.
3. Introduction
The TCP MD5 Signature (TCP MD5) is a TCP option that authenticates
TCP segments, including the TCP IPv4 pseudoheader, TCP header, and
TCP data. It was developed to protect BGP sessions from spoofed TCP
segments which could affect BGP data or the robustness of the TCP
connection itself [RFC2385][RFC4953].
There have been many recent concerns about TCP MD5. Its use of a
simple keyed hash for authentication is problematic because there
have been escalating attacks on the algorithm itself [Wa05]. TCP MD5
also lacks both key management and algorithm agility. This document
adds the latter, and provides a simple key coordination mechanism
giving the ability to move from one key to another within the same
connection. It does not however provide for complete cryptographic
key management to be handled in-band of TCP, because TCP SYN segments
lack sufficient remaining space to handle such a negotiation (see
Section 9.6). This document obsoletes the TCP MD5 option with a more
general TCP Authentication Option (TCP-AO). This new option supports
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the use of other, stronger hash functions, provides replay protection
for long-lived connections and across repeated instances of a single
connection, coordinates key changes between endpoints, and provides a
more explicit recommendation for external key management. The result
is compatible with IPv6, and is fully compatible with proposed
requirements for a replacement for TCP MD5 [Be07].
TCP-AO obsoletes TCP MD5, although a particular implementation may
support both mechanisms for backward compatibility. For a given
connection, only one can be in use. TCP MD5-protected connections
cannot be migrated to TCP-AO because TCP MD5 does not support any
changes to a connection's security algorithm once established.
3.1. Applicability Statement
TCP-AO is intended to support current uses of TCP MD5, such as to
protect long-lived connections for routing protocols, such as BGP and
LDP. It is also intended to provide similar protection to any long-
lived TCP connection, as might be used between proxy caches, e.g.,
and is not designed solely or primarily for routing protocol uses.
TCP-AO is intended to replace (and thus obsolete) the use of TCP MD5.
TCP-AO enhances the capabilities of TCP MD5 as summarized in Section
3.2. This document recommends overall that:
>> TCP implementations that support TCP MD5 MUST support TCP-AO.
>> TCP-AO SHOULD be implemented where the protection afforded by TCP
authentiation is needed, either because IPsec is not supported, or
because TCP-AO's particular properties are needed (e.g., per-
connection keys).
>> TCP-AO MAY be implemented elsewhere.
TCP-AO is not intended to replace the use of the IPsec suite (IPsec
and IKE) to protect TCP connections [RFC4301][RFC4306]. Specific
differences are noted in Section 3.2. In fact, we recommend the use
of IPsec and IKE, especially where IKE's level of existing support
for parameter negotiation, session key negotiation, or rekeying are
desired. TCP-AO is intended for use only where the IPsec suite would
not be feasible, e.g., as has been suggested is the case to support
some routing protocols [RFC4953], or in cases where keys need to be
tightly coordinated with individual transport sessions [Be07].
TCP-AO is not intended to replace the use of Transport Layer Security
(TLS) [RFC5246], sBGP or soBGP [Le09], or any other mechanisms that
protect only the TCP data stream. TCP-AO protects the transport
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layer, preventing attacks from disabling the TCP connection itself
[RFC4953]. Data stream mechanisms protect only the contents of the
TCP segments, and can be disrupted when the connection is affected.
Some of these data protection protocols - notably TLS - offer a
richer set of key management and authentication mechanisms than TCP-
AO, and thus protect the data stream in a different way. TCP-AO may
be used together with these data stream protections to complement
each others' strengths.
3.2. Executive Summary
This document replaces TCP MD5 as follows [RFC2385]:
o TCP-AO uses a separate option Kind (TBD-IANA-KIND).
o TCP-AO allows TCP MD5 to continue to be used concurrently for
legacy connections.
o TCP-AO replaces TCP MD5's single MAC algorithm with MACs specified
in a separate document and can be extended to include other MACs.
o TCP-AO allows rekeying during a TCP connection, assuming that an
out-of-band protocol or manual mechanism provides the new keys.
The option includes a 'key ID' which allows the efficient
concurrent use of multiple keys, and a key coordination mechanism
using a 'receive next key ID' manages the key change within a
connection. Note that TCP MD5 does not preclude rekeying during a
connection, but does not require its support either. Further,
TCP-AO supports key changes with zero segment loss, whereas key
changes in TCP MD5 can lose segments in transit during the
changeover or require trying multiple keys on each received
segment during key use overlap because it lacks an explicit key
ID. Although TCP recovers lost segments through retransmission,
loss can have a substantial impact on performance.
o TCP-AO provides automatic replay protection for long-lived
connections using sequence number extensions.
o TCP-AO ensures per-connection traffic keys as unique as the TCP
connection itself, using TCP's initial sequence numbers (ISNs) for
differentiation, even when static master key tuples are used
across repeated instances of connections on a single socket pair.
o TCP-AO specifies the details of how this option interacts with
TCP's states, event processing, and user interface.
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o TCP-AO is 2 bytes shorter than TCP MD5 (16 bytes overall, rather
than 18) in the initially specified default case (using a 96-bit
MAC).
TCP-AO differs from an IPsec/IKE solution in as follows
[RFC4301][RFC4306]:
o TCP-AO does not support dynamic parameter negotiation.
o TCP-AO includes TCP's socket pair (source address, destination
address, source port, destination port) as a security parameter
index (together with the KeyID), rather than using a separate
field as an index (IPsec's SPI).
o TCP-AO forces a change of computed MACs when a connection
restarts, even when reusing a TCP socket pair (IP addresses and
port numbers) [Be07].
o TCP-AO does not support encryption.
o TCP-AO does not authenticate ICMP messages (some ICMP messages may
be authenticated when using IPsec, depending on the
configuration).
4. The TCP Authentication Option
The TCP Authentication Option (TCP-AO) uses a TCP option Kind value
of TBD-IANA-KIND. The following sections describe TCP-AO and provide
a review of TCP MD5 for comparison.
4.1. Review of TCP MD5 Option
For review, the TCP MD5 option is shown in Figure 1.
+---------+---------+-------------------+
| Kind=19 |Length=18| MD5 digest... |
+---------+---------+-------------------+
| ...digest (con't)... |
+---------------------------------------+
| ... |
+---------------------------------------+
| ... |
+-------------------+-------------------+
| ...digest (con't) |
+-------------------+
Figure 1 The TCP MD5 Option [RFC2385]
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In the TCP MD5 option, the length is fixed, and the MD5 digest
occupies 16 bytes following the Kind and Length fields (each one
byte), using the full MD5 digest of 128 bits [RFC1321].
The TCP MD5 option specifies the use of the MD5 digest calculation
over the following values in the following order:
1. The IP pseudoheader (IP source and destination addresses, protocol
number, and segment length).
2. The TCP header excluding options and checksum.
3. The TCP data payload.
4. A key.
4.2. The TCP Authentication Option Format
TCP-AO provides a superset of the capabilities of TCP MD5, and is
minimal in the spirit of SP4 [SDNS88]. TCP-AO uses a new Kind field,
and similar Length field to TCP MD5, a KeyID field, and a RNextKeyID
field as shown in Figure 2.
+------------+------------+------------+------------+
| Kind | Length | KeyID | RNextKeyID |
+------------+------------+------------+------------+
| MAC ...
+-----------------------------------...
...-----------------+
... MAC (con't) |
...-----------------+
Figure 2 The TCP Authentication Option (TCP-AO)
TCP-AO defines these fields as follows:
o Kind: An unsigned 1-byte field indicating TCP-AO. TCP-AO uses a
new Kind value of TBD-IANA-KIND.
>> An endpoint MUST NOT use TCP-AO for the same connection in
which TCP MD5 is used. When both options appear, TCP MUST silently
discard the segment.
>> A single TCP segment MUST NOT have more than one TCP-AO in its
options sequence. When multiple TCP-AOs appear, TCP MUST discard
the segment.
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o Length: An unsigned 1-byte field indicating the length of the
option in bytes, including the Kind, Length, KeyID, RNextKeyID,
and MAC fields.
>> The Length value MUST be greater than or equal to 4. When the
Length value is less than 4, TCP MUST discard the segment.
>> The Length value MUST be consistent with the TCP header length.
When the Length value is invalid, TCP MUST discard the segment.
This Length check implies that the sum of the sizes of all
options, when added to the size of the base TCP header (5 words),
matches the TCP Offset field exactly. This full verification can
be computed because RFC 793 specifies the size of the required
options, and RFC 1122 requires that all new options follow a
common format with a fixed length field location
[RFC793][RFC1122]. A partial verification can be limited to check
only TCP-AO, so that the TCP-AO length, when added to the TCP-AO
offset from start of the TCP header, does not exceed the TCP
header size as indicated in the TCP header Offset field.
Values of 4 and other small values larger than 4 (e.g., indicating
MAC fields of very short length) are of dubious utility but are
not specifically prohibited.
o KeyID: An unsigned 1-byte field indicating the master key tuple
(MKT, as defined in Section 5.1) used to generate the traffic keys
which were used to generate the MAC that authenticates this
segment.
It supports efficient key changes during a connection and/or to
help with key coordination during connection establishment, to be
discussed further in Section 8.1. Note that the KeyID has no
cryptographic properties - it need not be random, nor are there
any reserved values.
>> KeyID values MAY be the same in both directions of a
connection, but do not have to be and there is no special meaning
when they are.
This allows MKTs to be installed on a set of devices without
coordinating the KeyIDs across an entire in advance, and allows
new devices to be added to the set using a group of MKTs later
without requiring renumbering of KeyIDs. These two capabilities
are particularly important when used with wildcards in the TCP
socket pair of the MKT, i.e., when a MKT is used among a set of
devices specified by a pattern (as noted in Section 5.1).
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o RNextKeyID: An unsigned 1-byte field indicating the MKT that is
ready at the sender to be used to authenticate received segments,
i.e., the desired 'receive next' keyID.
It supports efficient key change coordination, to be discussed
further in Section 8.1. Note that the RNextKeyID has no
cryptographic properties - it need not be random, nor are there
any reserved values.
o MAC: Message Authentication Code. Its contents are determined by
the particulars of the security association. Typical MACs are 96-
128 bits (12-16 bytes), but any length that fits in the header of
the segment being authenticated is allowed. The MAC computation is
described further in Section 7.1.
>> Required support for TCP-AO MACs are defined in [Le09]; other
MACs MAY be supported.
TCP-AO fields do not indicate the MAC algorithm either implicitly (as
with TCP MD5) or explicitly. The particular algorithm used is
considered part of the configuration state of the connection's
security and is managed separately (see Section 5).
Please note that the use of TCP-AO does not affect TCP's advertised
maximum segment size (MSS), as is the case for all TCP options
[Bo09].
The remainder of this document explains how TCP-AO is handled and its
relationship to TCP.
5. TCP-AO Keys and Their Properties
TCP-AO relies on two sets of keys to authenticate incoming and
outgoing segments: master key tuples (MKTs) and traffic keys. MKTs
are used to derive unique traffic keys, and include the keying
material used to generate those traffic keys, as well as indicating
the associated parameters under which traffic keys are used. Such
parameters include whether TCP options are authenticated, and
indicators of the algorithms used for traffic key derivation and MAC
calculation. Traffic keys are the keying material used to compute the
MAC of individual TCP segments.
5.1. Master Key Tuple
A Master Key Tuple (MKT) describes TCP-AO properties to be associated
with one or more connections. It is composed of the following:
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o TCP connection identifier. A TCP socket pair, i.e., a local IP
address, a remote IP address, a TCP local port, and a TCP remote
port. Values can be partially specified using ranges (e.g., 2-30),
masks (e.g., 0xF0), wildcards (e.g., "*"), or any other suitable
indication.
o TCP option flag. This flag indicates whether TCP options other
than TCP-AO are included in the MAC calculation. When options are
included, the content of all options, in the order present, are
included in the MAC, with TCP-AO's MAC field zeroed out. When the
options are not included, all options other than TCP-AO are
excluded from all MAC calculations (skipped over, not zeroed).
Note that TCP-AO, with its MAC field zeroed out, is always
included in the MAC calculation, regardless of the setting of this
flag; this protects the indication of the MAC length as well as
the key ID fields (KeyID, RNextKeyID). The option flag applies to
TCP options in both directions (incoming and outgoing segments).
o IDs. The values used in the KeyID or RNextKeyID of TCP-AO; used to
differentiate MKTs in concurrent use (KeyID), as well as to
indicate when MKTs are ready for use in the opposite direction
(RNextKeyID).
Each MKT has two IDs - a SendID and a RecvID. The SendID is
inserted as the KeyID of the TCP-OP option of outgoing segments,
and the RecvID is matched against the TCP-AO KeyID of incoming
segments. These and other uses of these two IDs are described
further in Section 9.4 and 9.5.
>> MKT IDs MUST support any value, 0-255 inclusive. There are no
reserved ID values.
ID values are assigned arbitrarily, i.e., the values are not
monotonically increasing, have no reserved values, and are
otherwise not meaningful. They can be assigned in sequence, or
based on any method mutually agreed by the connection endpoints
(e.g., using an external MKT management mechanism).
>> IDs MUST NOT be assumed to be randomly assigned.
o Master key. A byte sequence used for generating traffic keys, this
may be derived from a separate shared key by an external protocol
over a separate channel. This sequence is used in the traffic key
generation algorithm described in Section 7.2.
Implementations are advised to keep master key values in a
private, protected area of memory or other storage.
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o Key Derivation Function (KDF). Indicates the key derivation
function and its parameters, as used to generate traffic keys from
master keys. Explained further in Section 7.1 of this document and
specified in detail in [Le09].
o Message Authentication Code (MAC) algorithm. Indicates the MAC
algorithm and its parameters as used for this connection,
explained further in Section 7.1 of this document and specified in
detail in [Le09].
>> Components of a MKT MUST NOT change during a connection.
MKT component values cannot change during a connection because TCP
state is coordinated during connection establishment. TCP lacks a
handshake for modifying that state after a connection has been
established.
>> The set of MKTs MAY change during a connection.
MKT parameters are not changed. Instead, new MKTs can be installed,
and a connection can change which MKT it uses.
>> The IDs of MKTs MUST NOT overlap where their TCP connection
identifiers overlap.
This document does not address how MKTs are created by users or
processes. It is presumed that a MKT affecting a particular
connection cannot be destroyed during an active connection - or,
equivalently, that its parameters are copied to an area local to the
connection (i.e., instantiated) and so changes would affect only new
connections. The MKTs can be managed by a separate application
protocol.
5.2. Traffic Keys
A traffic key is a key derived from the MKT and the local and remote
IP address pairs and TCP port numbers, and, for established
connections, the TCP Initial Sequence Numbers (ISNs) in each
direction. Segments exchanged before a connection is established use
the same information, substituting zero for unknown values (e.g.,
ISNs not yet coordinated).
A single MKT can be used to derive any of four different traffic
keys:
o Send_SYN_traffic_key
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o Receive_SYN_traffic_key
o Send_other_traffic_key
o Receive_other_traffic_key
Note that the keys are unidirectional. A given connection typically
uses only three of these keys, because only one of the SYN keys is
typically used. All four are used only when a connection goes through
'simultaneous open' [RFC793].
The relationship between MKTs and traffic keys is shown in Figure 3.
Traffic keys are indicated with a "*". Note that every MKT can be
used to derive any of the four traffic keys, but only the keys
actually needed to handle the segments of a connection need to be
computed. Section 7.2 provides further details on how traffic keys
are derived.
MKT-A MKT-B
+---------------------+ +------------------------+
| SendID = 1 | | SendID = 5 |
| RecvID = 2 | | RecvID = 6 |
| MAC = HMAC-SHA1 | | MAC = AES-CMAC |
| KDF = KDF-HMAC-SHA1 | | KDF = KDF-AES-128-CMAC |
+---------------------+ +------------------------+
| |
+----------+----------+ |
| | |
v v v
Connection 1 Connection 2 Connection 3
+------------------+ +------------------+ +------------------+
| * Send_SYN_key | | * Send_SYN_key | | * Send_SYN_key |
| * Recv_SYN_key | | * Recv_SYN_key | | * Recv_SYN_key |
| * Send_Other_key | | * Send_Other_key | | * Send_Other_key |
| * Recv_Other_key | | * Recv_Other_key | | * Recv_Other_key |
+------------------+ +------------------+ +------------------+
Figure 3 Relationship between MKTs and traffic keys
5.3. MKT Properties
TCP-AO requires that every protected TCP segment match exactly one
MKT. When an outgoing segment matches an MKT, TCP-AO is used. When no
match occurs, TCP-AO is not used. Multiple MKTs may match a single
outgoing segment, e.g., when MKTs are being changed. Those MKTs
cannot have conflicting IDs (as noted elsewhere), and some mechanism
must determine which MKT to use for each given outgoing segment.
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>> An outgoing TCP segment MUST match at most one desired MKT,
indicated by the segment's socket pair. The segment MAY match
multiple MKTs, provided that exactly one MKT is indicated as desired.
Other information in the segment MAY be used to determine the desired
MKT when multiple MKTs match; such information MUST NOT include
values in any TCP option fields.
We recommend that the mechanism used to select from among multiple
MKTs use only information that TCP-AO would authenticate. Because
MKTs may indicate that options other than TCP-AO are ignored in the
MAC calculation, we recommend that TCP options should not be used to
determine MKTs.
>> An incoming TCP segment including TCP-AO MUST match exactly one
MKT, indicated solely by the segment's socket pair and its TCP-AO
KeyID.
Incoming segments include an indicator inside TCP-AO to select from
among multiple matching MKTs - the KeyID field. TCP-AO requires that
the KeyID alone be used to differentiate multiple matching MKTs, so
that MKT changes can be coordinated using the TCP-AO key change
coordination mechanism.
>> When an outgoing TCP segment matches no MKTs, TCP-AO is not used.
TCP-AO is always used when outgoing segments match an MKT, and is not
used otherwise.
6. Per-Connection TCP-AO Parameters
TCP-AO uses a small number of parameters associated with each
connection that uses TCP-AO, once instantiated. These values can be
stored in the Transport Control Block (TCP) [RFC793]. These values
are explained in subsequent sections of this document as noted; they
include:
1. Current_key - the MKT currently used to authenticate outgoing
segments, whose SendID is inserted in outgoing segments as KeyID
(see Section 9.4, step 5). Incoming segments are authenticated
using the MKT corresponding to the segment and its TCP-AO KeyID
(see Section 9.5, step 5), as matched against the MKT TCP
connection identifier and the MKT RecvID. There is only one
current_key at any given time on a particular connection.
>> Every TCP connection in a non-IDLE state MUST have at most one
current_key specified.
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2. Rnext_key -the MKT currently preferred for incoming (received)
segments, whose RecvID is inserted in outgoing segments as
RNextKeyID (see Section 9.5, step 5).
>> Each TCP connection in a non-IDLE state MUST have at most one
rnext_key specified.
3. A pair of Sequence Numbers Extensions (SNEs). SNEs are used to
prevent replay attacks, as described in Section 8.2. Each SNE is
initialized to zero upon connection establishment. Its use in the
MAC calculation is described in Section 7.1.
4. One or more MKTs. These are the MKTs that match this connection's
socket pair.
MKTs are used, together with other parameters of a connection, to
create traffic keys unique to each connection, as described in
Section 7.2. These traffic keys can be cached after computation, and
can be stored in the TCB with the corresponding MKT information. They
can be considered part of the per-connection parameters.
7. Cryptographic Algorithms
TCP-AO uses cryptographic algorithms to compute the MAC (Message
Authentication Code) that is used to authenticate a segment and its
headers; these are called MAC algorithms and are specified in a
separate document to facilitate updating the algorithm requirements
independently from the protocol [Le09]. TCP-AO also uses
cryptographic algorithms to convert MKTs, which can be shared across
connections, into unique traffic keys for each connection. These are
called Key Derivation Functions (KDFs), and are specified [Le09].
This section describes how these algorithms are used by TCP-AO.
7.1. MAC Algorithms
MAC algorithms take a variable-length input and a key and output a
fixed-length number. This number is used to determine whether the
input comes from a source with that same key, and whether the input
has been tampered in transit. MACs for TCP-AO have the following
interface:
MAC = MAC_alg(traffic_key, message)
INPUT: MAC_alg, traffic_key, message
OUTPUT: MAC
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where:
o MAC_alg - the specific MAC algorithm used for this computation.
The MAC algorithm specifies the output length, so no separate
output length parameter is required. This is specified as
described in [Le09].
o Traffic_key - traffic key used for this computation. This is
computed from the connection's current MKT as described in Section
7.2.
o Message - input data over which the MAC is computed. In TCP-AO,
this is the TCP segment prepended by the IP pseudoheader and TCP
header options, as described in Section 7.1.
o MAC - the fixed-length output of the MAC algorithm, given the
parameters provided.
At the time of this writing, the algorithms' definitions for use in
TCP-AO, as described in [Le09] are each truncated to 96 bits. Though
the algorithms each output a larger MAC, 96 bits provides a
reasonable tradeoff between security and message size. Though could
change in the future, so TCP-AO size should not be assumed as fixed
length.
The MAC algorithm employed for the MAC computation on a connection is
done so by definition in the MKT, per [Le09]'s definitions.
The mandatory-to-implement MAC algorithms for use with TCP-AO are
described in a separate RFC [Le09]. This allows the TCP-AO
specification to proceed along the IETF standards track even if
changes are needed to its associated algorithms and their labels (as
might be used in a user interface or automated MKT management
protocol) as a result of the ever evolving world of cryptography.
>> Additional algorithms, beyond those mandated for TCP-AO, MAY be
supported.
The data input to the MAC is the following fields in the following
sequence, interpreted in network-standard byte order:
1. The sequence number extension (SNE), in network-standard byte
order, as follows (described further in Section 8.2):
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+--------+--------+--------+--------+
| SNE |
+--------+--------+--------+--------+
Figure 4 Sequence number extension
The SNE for transmitted segments is maintained locally in the
SND.SNE value; for received segments, a local RCV.SNE value is
used. The details of how these values are maintained and used is
described in Sections 8.2, 9.4, and 9.5.
2. The IP pseudoheader: IP source and destination addresses, protocol
number and segment length, all in network byte order, prepended to
the TCP header below. The IP pseudoheader is exactly as used for
the TCP checksum in either IPv4 or IPv6 [RFC793][RFC2460]:
+--------+--------+--------+--------+
| Source Address |
+--------+--------+--------+--------+
| Destination Address |
+--------+--------+--------+--------+
| zero | Proto | TCP Length |
+--------+--------+--------+--------+
Figure 5 TCP IPv4 pseudoheader [RFC793]
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+--------+--------+--------+--------+
| |
+ +
| |
+ Source Address +
| |
+ +
| |
+ +
+--------+--------+--------+--------+
| |
+ +
| |
+ Destination Address +
| |
+ +
| |
+--------+--------+--------+--------+
| Upper-Layer Payload Length |
+--------+--------+--------+--------+
| zero | Next Header |
+--------+--------+--------+--------+
Figure 6 TCP IPv6 pseudoheader [RFC2460]
3. The TCP header, by default including options, and where the TCP
checksum and TCP-AO MAC fields are set to zero, all in network
byte order.
The TCP option flag of the MKT indicates whether the TCP options
are included in the MAC. When included, only the TCP-AO MAC field
is zeroed.
When TCP options are not included, all TCP options except for TCP-
AO are omitted from MAC processing. Again, the TCP-AO MAC field is
zeroed for the MAC processing.
4. The TCP data, i.e., the payload of the TCP segment.
Note that the traffic key is not included as part of the data; the
MAC algorithm indicates how to use the traffic key, e.g., as HMACs do
[RFC2104][RFC2403]. The traffic key is derived from the current MKT
as described in Sections 7.2.
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7.2. Traffic Key Derivation Functions
TCP-AO's traffic keys are derived from the MKTs using Key Derivation
Functions (KDFs). The KDFs used in TCP-AO have the following
interface:
traffic_key = KDF_alg(master_key, context, output_length)
INPUT: KDF_alg, master_key, context, output_length
OUTPUT: traffic_key
where:
o KDF_alg - the specific key derivation function (KDF) that is the
basic building block used in constructing the traffic key, as
indicated in the MKT. This is specified as described in [Le09].
o Master_key - The master_key string, as will be stored into the
associated MKT.
o Context - The context used as input in constructing the
traffic_key, as specified in [Le09]. The specific way this context
is used, in conjunction with other information, to create the raw
input to the KDF is also explained further in [Le09].
o Output_length - The desired output length of the KDF, i.e., the
length to which the KDF's output will be truncated. This is
specified as described in [Le09].
o Traffic_key - The desired output of the KDF, of length
output_length, to be used as input to the MAC algorithm, as
described in Section 7.1.
The context used as input to the KDF combines TCP socket pair with
the endpoint initial sequence numbers (ISNs) of a connection. This
data is unique to each TCP connection instance, which enables TCP-AO
to generate unique traffic keys for that connection, even from a MKT
used across many different connections or across repeated connections
that share a socket pair. Unique traffic keys are generated without
relying on external key management properties. The KDF context is
defined in Figure 7 and Figure 8.
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+--------+--------+--------+--------+
| Source Address |
+--------+--------+--------+--------+
| Destination Address |
+--------+--------+--------+--------+
| Source Port | Dest. Port |
+--------+--------+--------+--------+
| Source ISN |
+--------+--------+--------+--------+
| Dest. ISN |
+--------+--------+--------+--------+
Figure 7 KDF Context for an IPv4 connection
+--------+--------+--------+--------+
| |
+ +
| |
+ Source Address +
| |
+ +
| |
+ +
+--------+--------+--------+--------+
| |
+ +
| |
+ Destination Address +
| |
+ +
| |
+--------+--------+--------+--------+
| Source Port | Dest. Port |
+--------+--------+--------+--------+
| Source ISN |
+--------+--------+--------+--------+
| Dest. ISN |
+--------+--------+--------+--------+
Figure 8 KDF Context for an IPv6 connection
Traffic keys are directional, so "source" and "destination" are
interpreted differently for incoming and outgoing segments. For
incoming segments, source is the remote side, whereas for outgoing
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segments source is the local side. This further ensures that
connection keys generated for each direction are unique.
For SYN segments (segments with the SYN set, but the ACK not set),
the destination ISN is not known. For these segments, the connection
key is computed using the context shown above, in which the
Destination ISN value is zero. For all other segments, the ISN pair
is used when known. If the ISN pair is not known, e.g., when sending
a RST after a reboot, the segment should be sent without
authentication; if authentication was required, the segment cannot
have been MAC'd properly anyway and would have been dropped on
receipt.
>> TCP-AO SYN segments (SYN set, no ACK set) MUST use a destination
ISN of zero (whether sent or received); all other segments use the
known ISN pair.
Overall, this means that each connection will use up to four distinct
traffic keys for each MKT:
o Send_SYN_traffic_key - the traffic key used to authenticate
outgoing SYNs. The source ISN known (the TCP connection's local
ISN), and the destination (remote) ISN is unknown (and so the
value 0 is used).
o Receive_SYN_traffic_key - the traffic key used to authenticate
incoming SYNs. The source ISN known (the TCP connection's remote
ISN), and the destination (remote) ISN is unknown (and so the
value 0 is used).
o Send_other_traffic_key - the traffic key used to authenticate all
other outgoing TCP segments.
o Receive_other_traffic_key - the traffic key used to authenticate
all other incoming TCP segments.
The following table describes how each of these traffic keys is
computed, where the TCP-AO algorithms refer to source (S) and
destination (D) values of the IP address, TCP port, and ISN, and each
segment (incoming or outgoing) has a values that refer to the local
side of the connection (l) and remote side (r):
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S-IP S-port S-ISN D-IP D-port D-ISN
----------------------------------------------------------------
Send_SYN_traffic_key l-IP l-port l-ISN r-IP r-port 0
Receive_SYN_traffic_key r-IP r-port r-ISN l-IP l-port 0
Send_other_traffic_key l-IP l-port l-ISN r-IP r-port r-ISN
Receive_other_traffic_key r-IP r-port r-ISN l-IP l-port l-ISN
The use of both ISNs in the traffic key computations ensures that
segments cannot be replayed across repeated connections reusing the
same socket, their 32-bit space avoids repeated use except under
reboot, and reuse assumes both sides repeat their use on the same
connection). We do expect that:
>> Endpoints should select ISNs pseudorandomly, e.g., as in [RFC1948]
A SYN is authenticated using a destination ISN of zero (whether sent
or received), and all other segments would be authenticated using the
ISN pair for the connection. There are other cases in which the
destination ISN is not known, but segments are emitted, such as after
an endpoint reboots, when it is possible that the two endpoints would
not have enough information to authenticate segments. This is
addressed further in Section 9.7.
7.3. Traffic Key Establishment and Duration Issues
TCP-AO does not provide a mechanism for traffic key negotiation or
parameter negotiation (MAC algorithm, length, or use of TCP-AO on a
connection), or for coordinating rekeying during a connection. We
assume out-of-band mechanisms for MKT establishment, parameter
negotiation, and rekeying. This separation of MKT use from MKT
management is similar to that in the IPsec security suite
[RFC4301][RFC4306].
We encourage users of TCP-AO to apply known techniques for generating
appropriate MKTs, including the use of reasonable master key lengths,
limited traffic key sharing, and limiting the duration of MKT use
[RFC3562]. This also includes the use of per-connection nonces, as
suggested in Section 7.2.
TCP-AO supports rekeying in which new MKTs are negotiated and
coordinated out-of-band, either via a protocol or a manual procedure
[RFC4808]. New MKT use is coordinated using the out-of-band mechanism
to update both TCP endpoints. When only a single MKT is used at a
time, the temporary use of invalid MKTs could result in segments
being dropped; although TCP is already robust to such drops, TCP-AO
uses the KeyID field to avoid such drops. A given connection can have
multiple matching MKTs, where the KeyID field is used to identify the
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MKT that corresponds to the traffic key used for a segment, to avoid
the need for expensive trial-and-error testing of MKTs in sequence.
TCP-AO provides an explicit MKT coordination mechanism, described in
Section 8.1. Such a mechanism is useful when new MKTs are installed,
or when MKTs are changed, to determine when to commence using
installed MKTs.
Users are advised to manage MKTs following the spirit of the advice
for key management when using TCP MD5 [RFC3562], notably to use
appropriate key lengths (12-24 bytes) and to avoid sharing MKTs among
multiple BGP peering arrangements.
7.3.1. MKT Reuse Across Socket Pairs
MKTs can be reused across different socket pairs within a host, or
across different instances of a socket pair within a host. In either
case, replay protection is maintained.
MKTs reused across different socket pairs cannot enable replay
attacks because the TCP socket pair is included in the MAC, as well
as in the generation of the traffic key. MKTs reused across repeated
instances of a given socket pair cannot enable replay attacks because
the connection ISNs are included in the traffic key generation
algorithm, and ISN pairs are unlikely to repeat over useful periods.
7.3.2. MKTs Use Within a Long-lived Connection
TCP-AO uses sequence number extensions (SNEs) to prevent replay
attacks within long-lived connections. Explicit MKT rollover,
accomplished by external means and indexed using the KeyID field, can
be used to change keying material for various reasons (e.g.,
personnel turnover), but is not required to support long-lived
connections.
8. Additional Security Mechanisms
TCP-AO adds mechanisms to support efficient use, especially in
environments where only manual keying is available. These include the
previously described mechanisms for supporting multiple concurrent
MKTs (via the KeyID field) and for generating unique per-connection
traffic keys (via the KDF). This section describes additional
mechanisms to coordinate MKT changes and to prevent replay attacks
when a traffic key is not changed for long periods of time.
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8.1. Coordinating Use of New MKTs
At any given time, a single TCP connection may have multiple MKTs
specified for each segment direction (incoming, outgoing). TCP-AO
provides a mechanism to indicate when a new MKT is ready, to allow
the sender to commence use of that new MKT. This mechanism allows new
MKT use to be coordinated, to avoid unnecessary loss due to sender
authentication using a MKT not yet ready at the receiver.
Note that this is intended as an optimization. Deciding when to start
using a key is a performance issue. Deciding when to remove an MKT is
a security issue. Invalid MKTs are expected to be removed. TCP-AO
provides no mechanism to coordinate their removal, as we consider
this a key management operation.
New MKT use is coordinated through two ID fields in the header:
o KeyID
o RNextKeyID
KeyID represents the outgoing MKT information used by the segment
sender to create the segment's MAC (outgoing), and the corresponding
incoming keying information used by the segment receiver to validate
that MAC. It contains the SendID of the MKT in active use in that
direction.
RNextKeyID represents the preferred MKT information to be used for
subsequent received segments ('receive next'). I.e., it is a way for
the segment sender to indicate a ready incoming MKT for future
segments it receives, so that the segment receiver can know when to
switch MKTs (and thus their KeyIDs and associated traffic keys). It
indicates the RecvID of the MKT desired to for incoming segments.
There are two pointers kept by each side of a connection, as noted in
the per-connection information (see Section 6):
o Currently active outgoing MKT (Current_key)
o Current preference for incoming MKT (rnext_key)
Current_key indicates a MKT that is used to authenticate outgoing
segments. Upon connection establishment, it points to the first MKT
selected for use.
Rnext_key points to an incoming MKT that is ready and preferred for
use. Upon connection establishment, this points to the currently
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active incoming MKT. It can be changed when new MKTs are installed
(e.g., either by automatic MKT management protocol operation or by
user manual selection).
Rnext_key is changed only by manual user intervention or MKT
management protocol operation. It is not manipulated by TCP-AO.
Current_key is updated by TCP-AO when processing received TCP
segments as discussed in the segment processing description in
Section 9.5. Note that the algorithm allows the current_key to change
to a new MKT, then change back to a previously used MKT (known as
"backing up"). This can occur during a MKT change when segments are
received out of order, and is considered a feature of TCP-AO, because
reordering does not result in drops. The only way to avoid reuse of
previously used MKTs is to remove the MKT when it is no longer
considered permitted.
8.2. Preventing replay attacks within long-lived connections
TCP uses a 32-bit sequence number which may, for long-lived
connections, roll over and repeat. This could result in TCP segments
being intentionally and legitimately replayed within a connection.
TCP-AO prevents replay attacks, and thus requires a way to
differentiate these legitimate replays from each other, and so it
adds a 32-bit sequence number extension (SNE) for transmitted and
received segments.
The SNE extends TCP's sequence number so that segments within a
single connection are always unique. When TCP's sequence number rolls
over, there is a chance that a segment could be repeated in total;
using an SNE differentiates even identical segments sent with
identical sequence numbers at different times in a connection. TCP-AO
emulates a 64-bit sequence number space by inferring when to
increment the high-order 32-bit portion (the SNE) based on
transitions in the low-order portion (the TCP sequence number).
TCP-AO thus maintains SND.SNE for transmitted segments, and RCV.SNE
for received segments, both initialized as zero when a connection
begins. The intent of these SNEs is, together with TCP's 32-bit
sequence numbers, to provide a 64-bit overall sequence number space.
For transmitted segments SND.SNE can be implemented by extending
TCP's sequence number to 64-bits; SND.SNE would be the top (high-
order) 32 bits of that number. For received segments, TCP-AO needs to
emulate the use of a 64-bit number space, and correctly infer the
appropriate high-order 32-bits of that number as RCV.SNE from the
received 32-bit sequence number and the current connection context.
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The implementation of SNEs is not specified in this document, but one
possible way is described here that can be used for either RCV.SNE,
SND.SNE, or both.
Consider an implementation with two SNEs as required (SND.SNE, RCV.
SNE), and additional variables as listed below, all initialized to
zero, as well as a current TCP segment field (SEG.SEQ):
o SND.PREV_SEQ, needed to detect rollover of SND.SEQ
o RCV.PREV_SEQ, needed to detect rollover of RCV.SEQ
o SND.SNE_FLAG, which indicates when to increment the SND.SNE
o RCV.SNE_FLAG, which indicates when to increment the RCV.SNE
When a segment is received, the following algorithm (in C-like
pseudocode) computes the SNE used in the MAC; this is the "RCV" side,
and an equivalent algorithm can be applied to the "SND" side:
/* set the flag when the SEG.SEQ first rolls over */
if ((RCV.SNE_FLAG == 0)
&& (RCV.PREV_SEQ > 0x7fff) && (SEG.SEQ < 0x7fff)) {
RCV.SNE = RCV.SNE + 1;
RCV.SNE_FLAG = 1;
}
/* decide which SNE to use after incremented */
if ((RCV.SNE_FLAG == 1) && (SEG.SEQ > 0x7fff)) {
SNE = RCV.SNE - 1; # use the pre-increment value
} else {
SNE = RCV.SNE; # use the current value
}
/* reset the flag in the *middle* of the window */
if ((RCV.PREV_SEQ < 0x7fff) && (SEG.SEQ > 0x7fff)) {
RCV.SNE_FLAG = 0;
}
/* save the current SEQ for the next time through the code */
RCV.PREV_SEQ = SEG.SEQ;
In the above code, the first line when the sequence number first
rolls over, i.e., when the new number is low (in the bottom half of
the number space) and the old number is high (in the top half of the
number space). The first time this happens, the SNE is incremented
and a flag is set.
If the flag is set and a high number is seen, it must be a reordered
segment, so use the pre-increment SNE, otherwise use the current SNE.
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The flag will be cleared by the time the number rolls all the way
around.
The flag prevents the SNE from being incremented again until the flag
is reset, which happens in the middle of the window (when the old
number is in the bottom half and the new is in the top half). Because
the receive window is never larger than half of the number space, it
is impossible to both set and reset the flag at the same time -
outstanding segments, regardless of reordering, cannot straddle both
regions simultaneously.
9. TCP-AO Interaction with TCP
The following is a description of how TCP-AO affects various TCP
states, segments, events, and interfaces. This description is
intended to augment the description of TCP as provided in RFC-793,
and its presentation mirrors that of RFC-793 as a result [RFC793].
9.1. TCP User Interface
The TCP user interface supports active and passive OPEN, SEND,
RECEIVE, CLOSE, STATUS and ABORT commands. TCP-AO does not alter this
interface as it applies to TCP, but some commands or command
sequences of the interface need to be modified to support TCP-AO.
TCP-AO does not specify the details of how this is achieved.
TCP-AO requires the TCP user interface be extended to allow the MKTs
to be configured, as well as to allow an ongoing connection to manage
which MKTs are active. The MKTs need to be configured prior to
connection establishment, and the set of MKTs may change during a
connection:
>> TCP OPEN, or the sequence of commands that configure a connection
to be in the active or passive OPEN state, MUST be augmented so that
a MKT can be configured.
>> A TCP-AO implementation MUST allow the set of MKTs for ongoing TCP
connections (i.e., not in the CLOSED state) to be modified.
The MKTs associated with a connection needs to be available for
confirmation; this includes the ability to read the MKTs:
>> TCP STATUS SHOULD be augmented to allow the MKTs of a current or
pending connection to be read (for confirmation).
Senders may need to be able to determine when the outgoing MKT
changes (KeyID) or when a new preferred MKT (RNextKeyID) is
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indicated; these changes immediately affect all subsequent outgoing
segments:
>> TCP SEND, or a sequence of commands resulting in a SEND, MUST be
augmented so that the preferred outgoing MKT (Current_key) and/or the
preferred incoming MKT rnext_key of a connection can be indicated.
It may be useful to change the outgoing active MKT (Current_key) even
when no data is being sent, which can be achieved by sending a zero-
length buffer or by using a non-send interface (e.g., socket options
in Unix), depending on the implementation.
It is also useful to indicate recent segment KeyID and RNextKeyID
values received; although there could be a number of such values,
they are not expected to change quickly so any recent sample should
be sufficient:
>> TCP RECEIVE, or the sequence of commands resulting in a RECEIVE,
MUST be augmented so that the KeyID and RNextKeyID of a recently
received segment is available to the user out-of-band (e.g., as an
additional parameter to RECEIVE, or via a STATUS call).
9.2. TCP States and Transitions
TCP includes the states LISTEN, SYN-SENT, SYN-RECEIVED, ESTABLISHED,
FIN-WAIT-1, FIN-WAIT-2, CLOSE-WAIT, CLOSING, LAST-ACK, TIME-WAIT, and
CLOSED.
>> A MKT MAY be associated with any TCP state.
9.3. TCP Segments
TCP includes control (at least one of SYN, FIN, RST flags set) and
data (none of SYN, FIN, or RST flags set) segments. Note that some
control segments can include data (e.g., SYN).
>> All TCP segments MUST be checked against the set of MKTs for
matching TCP connection identifiers.
>> TCP segments whose TCP-AO does not validate MUST be silently
discarded.
>> A TCP-AO implementation MUST allow for configuration of the
behavior of segments with TCP-AO but that do not match an MKT. The
initial default of this configuration SHOULD be to silently accept
such connections. If this is not the desired case, an MKT can be
included to match such connections, or the connection can indicate
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that TCP-AO is required. Alternately, the configuration can be
changed to discard segments with the AO option not matching an MKT.
>> Silent discard events SHOULD be signaled to the user as a warning,
and silent accept events MAY be signaled to the user as a warning.
Both warnings, if available, MUST be accessible via the STATUS
interface. Either signal MAY be asynchronous, but if so they MUST be
rate-limited. Either signal MAY be logged; logging SHOULD allow rate-
limiting as well.
All TCP-AO processing occurs between the interface of TCP and IP; for
incoming segments, this occurs after validation of the TCP checksum.
For outgoing segments, this occurs before computation of the TCP
checksum.
Note that use of TCP-AO on a connection not negotiated within TCP. It
is the responsibility of the receiver to determine when TCP-AO is
required via other means (e.g., out of band, manually or with an key
management protocol) and to enforce that requirement.
9.4. Sending TCP Segments
The following procedure describes the modifications to TCP to support
inserting TCP-AO when a segment departs.
>> Note that TCP-AO MUST be the last TCP option processed on outgoing
segments, because its MAC calculation may include the values of other
TCP options.
1. Find the per-connection parameters for the segment:
a. If the segment is a SYN, then this is the first segment of a
new connection. Find the matching MKT for this segment based
on the segment's socket pair.
i. If there is no matching MKT, omit TCP-AO. Proceed with
transmitting the segment.
ii. If there is a matching MKT, then set the per-connection
parameters as needed (see Section 6). Proceed with the
step 2.
b. If the segment is not a SYN, then determine whether TCP-AO is
being used for the connection and use the MKT as indicated by
the current_key value from the per-connection parameters (see
Section 6) and proceed with the step 2.
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2. Using the per-connection parameters:
a. Augment the TCP header with TCP-AO, inserting the appropriate
Length and KeyID based on the MKT indicated by current_key
(using the current_key MKT's SendID as the TCP-AO KeyID).
Update the TCP header length accordingly.
b. Determine SND.SNE as described in Section 8.2.
c. Determine the appropriate traffic key, i.e., as pointed to by
current_key (as noted in Section 8.1, and as probably cached
in the TCB). I.e., use the send_SYN_traffic_key for SYN
segments, and the send_other_traffic_key for other segments.
d. Determine the RNextKeyID as indicated by the rnext_key
pointer, and insert it in the TCP-AO RNextKeyID field (using
the rnext_key MKT's RecvID as the TCP-AO KeyID) (as noted in
Section 8.1).
e. Compute the MAC using the MKT (and cached traffic key) and
data from the segment as specified in Section 7.1.
f. Insert the MAC in the TCP-AO MAC field.
g. Proceed with transmitting the segment.
9.5. Receiving TCP Segments
The following procedure describes the modifications to TCP to support
TCP-AO when a segment arrives.
>> Note that TCP-AO MUST be the first TCP option processed on
incoming segments, because its MAC calculation may include the values
of other TCP options which could change during TCP option processing.
This also protects the behavior of all other TCP options from the
impact of spoofed segments or modified header information.
>> Note that TCP-AO checks MUST be performed for all incoming SYNs to
avoid accepting SYNs lacking TCP-AO where required. Other segments
can cache whether TCP-AO is needed in the TCB.
1. Find the per-connection parameters for the segment:
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a. If the segment is a SYN, then this is the first segment of a
new connection. Find the matching MKT for this segment, using
the segment's socket pair and its TCP-AO KeyID, matched
against the MKT's TCP connection identifier and the MKT's
RecvID.
i. If there is no matching MKT, remove TCP-AO from the
segment. Proceed with further TCP handling of the
segment.
NOTE: this presumes that connections that do not match
any MKT should be silently accepted, as noted in Sec 9.3.
ii. If there is a matching MKT, then set the per-connection
parameters as needed (see Section 6). Proceed with the
step 2.
2. Using the per-connection parameters:
a. Check that the segment's TCP-AO Length matches the length
indicated by the MKT.
i. If lengths differ, silently discard the segment. Log
and/or signal the event as indicated in Section 9.3.
b. Determine the segment's RCV.SNE as described in Section 8.2.
c. Determine the segment's traffic key from the MKT as described
in Section 7.1 (and as likely cached in the TCB). I.e., use
the receive_SYN_traffic_key for SYN segments, and the
receive_other_traffic_key for other segments.
d. Compute the segment's MAC using the MKT (and its derived
traffic key) and portions of the segment as indicated in
Section 7.1.
i. If the computed MAC differs from the TCP-AO MAC field
value, silently discard the segment. Log and/or signal
the event as indicated in Section 9.3.
e. Compare the received RNextKeyID value to the currently active
outgoing KeyID value (Current_key MKT's SendID).
i. If they match, no further action is required.
ii. If they differ, determine whether the RNextKeyID MKT is
ready.
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1. If the MKT corresponding to the segment's socket
pair and RNextKeyID is not available, no action is
required (RNextKeyID of a received segment needs to
match the MKT's SendID).
2. If the matching MKT corresponding to the segment's
socket pair and RNextKeyID is available:
a. Set Current_key to the RNextKeyID MKT.
f. Proceed with TCP processing of the segment.
It is suggested that TCP-AO implementations validate a segment's
Length field before computing a MAC, to reduce the overhead incurred
by spoofed segments with invalid TCP-AO fields.
Additional reductions in MAC validation overhead can be supported in
the MAC algorithms, e.g., by using a computation algorithm that
prepends a fixed value to the computed portion and a corresponding
validation algorithm that verifies the fixed value before investing
in the computed portion. Such optimizations would be contained in the
MAC algorithm specification, and thus are not specified in TCP-AO
explicitly. Note that the KeyID cannot be used for connection
validation per se, because it is not assumed random.
9.6. Impact on TCP Header Size
TCP-AO, using the initially required 96-bit MACs, uses a total of 16
bytes of TCP header space [Le09]. TCP-AO is thus 2 bytes smaller than
the TCP MD5 option (18 bytes).
Note that TCP option space is most critical in SYN segments, because
flags in those segments could potentially increase the option space
area in other segments. Because TCP ignores unknown segments,
however, it is not possible to extend the option space of SYNs
without breaking backward-compatibility.
TCP's 4-bit data offset requires that the options end 60 bytes (15
32-bit words) after the header begins, including the 20-byte header.
This leaves 40 bytes for options, of which 15 are expected in current
implementations (listed below), leaving at most 25 for other uses.
TCP-AO consumes 16 bytes, leaving 9 bytes for additional SYN options
(depending on implementation dependant alignment padding, which could
consume another 2 bytes at most).
o SACK permitted (2 bytes) [RFC2018][RFC3517]
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o Timestamps (10 bytes) [RFC1323]
o Window scale (3 bytes) [RFC1323]
After a SYN, the following options are expected in current
implementations of TCP:
o SACK (10bytes) [RFC2018][RFC3517] (18 bytes if D-SACK [RFC2883])
o Timestamps (10 bytes) [RFC1323]
TCP-AO continues to consume 16 bytes in non-SYN segments, leaving a
total of 24 bytes for other options, of which the timestamp consumes
10. This leaves 14 bytes, of which 10 are used for a single SACK
block. When two SACK blocks are used, such as to handle D-SACK, a
smaller TCP-AO MAC would be required to make room for the additional
SACK block (i.e., to leave 18 bytes for the D-SACK variant of the
SACK option) [RFC2883]. Note that D-SACK is not supportable in TCP
MD5 in the presence of timestamps, because TCP MD5's MAC length is
fixed and too large to leave sufficient option space.
Although TCP option space is limited, we believe TCP-AO is consistent
with the desire to authenticate TCP at the connection level for
similar uses as were intended by TCP MD5.
9.7. Connectionless Resets
TCP-AO allows TCP resets (RSTs) to be exchanged provided both sides
have established valid connection state. After such state is
established, if one side reboots, TCP-AO prevents TCP's RST mechanism
from clearing out old state on the side that did not reboot. This
happens because the rebooting side has lost its connection state, and
thus its traffic keys.
It is important that implementations are capable of detecting
excesses of TCP connections in such a configuration and can clear
them out if needed to protect its memory usage [Ba09]. To protect
against such state from accumulating and not being cleared out, a
number of recommendations are made:
>> Connections using TCP-AO SHOULD also use TCP keepalives [RFC1122].
The use of TCP keepalives ensures that connections whose keys are
lost are terminated after a finite time; a similar effect can be
achieved at the application layer, e.g., with BGP keepalives
[RFC4271]. Either kind of keepalive helps ensure the TCP state is
cleared out in such a case; the alternative, of allowing
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unauthenticated RSTs to be received, would allow one of the primary
vulnerabilities that TCP-AO is intended to protect against.
Keepalives ensure that connections are dropped across reboots, but
this can have a detrimental effect on some protocols. In specific,
BGP reacts poorly to such connection drops, even if caused by the use
of BGP keepalives; "graceful restart" was introduced to address this
effect [RFC4724], and extended to support BGP with MPLS [RFC4781]. As
a result:
>> BGP connections SHOULD require support for graceful restart when
using TCP-AO.
We recognize that support for graceful restart is not always
feasible. As a result:
>> When BGP without graceful restart is used with TCP-AO, both sides
of the connection SHOULD save traffic keys in storage that persists
across reboots and restore them after a reboot, and SHOULD limit any
performance impacts that result from this storage/restoration.
9.8. ICMP Handling
TCP can be attacked both in-band, using TCP segments, or out-of-band
using ICMP. ICMP packets cannot be protected using TCP-AO mechanisms,
however; in this way, both TCP-AO and IPsec do not directly solve the
need for protected ICMP signaling. TCP-AO does make specific
recommendations on how to handle certain ICMPs, beyond what IPsec
requires, and these are made possible because TCP-AO operates inside
the context of a TCP connection.
IPsec makes recommendations regarding dropping ICMPs in certain
contexts, or requiring that they are endpoint authenticated in others
[RFC4301]. There are other mechanisms proposed to reduce the impact
of ICMP attacks by further validating ICMP contents and changing the
effect of some messages based on TCP state, but these do not provide
the level of authentication for ICMP that TCP-AO provides for TCP
[Go09]. As a result, we recommend a conservative approach to
accepting ICMP messages as summarized in [Go09]:
>> A TCP-AO implementation MUST default to ignore incoming ICMPv4
messages of Type 3 (destination unreachable) Codes 2-4 (protocol
unreachable, port unreachable, and fragmentation needed - 'hard
errors') and ICMPv6 Type 1 (destination unreachable) Code 1
(administratively prohibited) and Code 4 (port unreachable) intended
for connections in synchronized states (ESTABLISHED, FIN-WAIT-1, FIN-
WAIT-2, CLOSE-WAIT, CLOSING, LAST-ACK, TIME-WAIT) that match MKTs.
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>> A TCP-AO implementation SHOULD allow whether such ICMPs are
ignored to be configured on a per-connection basis.
>> A TCP-AO implementation SHOULD implement measures to protect ICMP
"packet too big" messages, some examples of which are discussed in
[Go09]
>> An implementation SHOULD allow ignored ICMPs to be logged.
This control affects only ICMPs that currently require 'hard errors',
which would abort the TCP connection [RFC1122]. This recommendation
is intended to be similar to how IPsec would handle those messages,
with an additional default assumed [RFC4301].
10. Obsoleting TCP MD5 and Legacy Interactions
TCP-AO obsoletes TCP MD5. As we have noted earlier:
>> TCP implementations that support TCP MD5 MUST support TCP-AO.
Systems implementing TCP MD5 only are considered legacy, and ought to
be upgraded when possible. In order to support interoperation with
such legacy systems until upgrades are available:
>> TCP MD5 SHOULD be supported where interactions with legacy systems
is needed.
>> A system that supports both TCP-AO and TCP MD5 MUST use TCP-AO for
connections unless not supported by its peer, at which point it MAY
use TCP MD5 instead.
>> A TCP implementation MUST NOT use both TCP-AO and TCP MD5 for a
particular TCP connection, but MAY support TCP-AO and TCP MD5
simultaneously for different connections (notably to support legacy
use of TCP MD5).
The Kind value explicitly indicates whether TCP-AO or TCP MD5 is used
for a particular connection in TCP segments.
It is possible that MKTs could be augmented to support TCP MD5,
although use of MKTs is not described in RFC2385.
It is possible to require TCP-AO for a connection or TCP MD5, but it
is not possible to require 'either'. When an endpoint is configured
to require TCP MD5 for a connection, it must be added to all outgoing
segments and validated on all incoming segments [RFC2385]. TCP MD5's
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requirements prohibit the speculative use of both options for a given
connection, e.g., to be decided by the other end of the connection.
11. Interactions with Middleboxes
TCP-AO may interact with middleboxes, depending on their behavior
[RFC3234]. Some middleboxes either alter TCP options (such as TCP-AO)
directly or alter the information TCP-AO includes in its MAC
calculation. TCP-AO may interfere with these devices, exactly where
the device modifies information TCP-AO is designed to protect.
11.1. Interactions with non-NAT/NAPT Middleboxes
TCP-AO supports middleboxes that do not change the IP addresses or
ports of segments. Such middleboxes may modify some TCP options, in
which case TCP-AO would need to be configured to ignore all options
in the MAC calculation on connections traversing that element.
Note that ignoring TCP options may provide less protection, i.e., TCP
options could be modified in transit, and such modifications could be
used by an attacker. Depending on the modifications, TCP could have
compromised efficiency (e.g., timestamp changes), or could cease
correct operation (e.g., window scale changes). These vulnerabilities
affect only the TCP connections for which TCP-AO is configured to
ignore TCP options.
11.2. Interactions with NAT/NAPT Devices
TCP-AO cannot interoperate natively across NAT/NAPT devices, which
modify the IP addresses and/or port numbers. We anticipate that
traversing such devices may require variants of existing NAT/NAPT
traversal mechanisms, e.g., encapsulation of the TCP-AO-protected
segment in another transport segment (e.g., UDP), as is done in IPsec
[RFC2663][RFC3947]. Such variants can be adapted for use with TCP-AO,
or IPsec with NAT traversal can be used instead of TCP-AO in such
cases [RFC3947].
An alternate proposal for accommodating NATs extends TCP-AO
independently of this specification [To10].
12. Evaluation of Requirements Satisfaction
TCP-AO satisfies all the current requirements for a revision to TCP
MD5, as summarized below [Be07].
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1. Protected Elements
A solution to revising TCP MD5 should protect (authenticate) the
following elements.
This is supported - see Section 7.1.
a. IP pseudoheader, including IPv4 and IPv6 versions.
Note that we do not allow optional coverage because IP
addresses define a connection. If they can be coordinated
across a NAT/NAPT, the sender can compute the MAC based on the
received values; if not, a tunnel is required, as noted in
Section 11.2.
b. TCP header.
Note that we do not allow optional port coverage because ports
define a connection. If they can be coordinated across a
NAT/NAPT, the sender can compute the MAC based on the received
values; if not, a tunnel is required, as noted in Section
11.2.
c. TCP options.
Note that TCP-AO allows exclusion of TCP options from
coverage, to enable use with middleboxes that modify options
(except when they modify TCP-AO itself). See Section 11.
d. TCP payload data.
2. Option Structure Requirements
A solution to revising TCP MD5 should use an option with the
following structural requirements.
This is supported - see Section 7.1.
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a. Privacy.
The option should not unnecessarily expose information about
the TCP-AO mechanism. The additional protection afforded by
keeping this information private may be of little value, but
also helps keep the option size small.
TCP-AO exposes only the MKT IDs, MAC, and overall option
length on the wire. Note that short MACs could be obscured by
using longer option lengths but specifying a short MAC length
(this is equivalent to a different MAC algorithm, and is
specified in the MKT). See Section 4.2.
b. Allow optional per connection.
The option should not be required on every connection; it
should be optional on a per connection basis.
This is supported because the set of MKTs can be installed to
match some connections and not others. Connections not
matching any MKT do not require TCP-AO. Further, incoming
segments with TCP-AO are not discarded solely because they
include the option, provided they do not match any MKT.
c. Require non-optional.
The option should be able to be specified as required for a
given connection.
This is supported because the set of MKTs can be installed to
match some connections and not others. Connections matching
any MKT require TCP-AO.
d. Standard parsing.
The option should be easily parseable, i.e., without
conditional parsing, and follow the standard RFC 793 option
format.
This is supported - see Section 4.2.
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e. Compatible with Large Windows and SACK.
The option should be compatible with the use of the Large
Windows and SACK options.
This is supported - see Section 9.6. The size of the option is
intended to allow use with Large Windows and SACK. See also
Section 3.2, which indicates that TCP-AO is 2 bytes shorter
than TCP MD5 in the default case, assuming a 96-bit MAC.
3. Cryptography requirements
A solution to revising TCP MD5 should support modern cryptography
capabilities.
a. Baseline defaults.
The option should have a default that is required in all
implementations.
TCP-AO uses a default required algorithm as specified in
[Le09], as noted in Section 7.1.
b. Good algorithms.
The option should use algorithms considered accepted by the
security community, which are considered appropriately safe.
The use of non-standard or unpublished algorithms should be
avoided.
TCP-AO uses MACs as indicated in [Le09]. The KDF is also
specified in [Le09]. The KDF input string follows the typical
design (see [Le09]).
c. Algorithm agility.
The option should support algorithms other than the default,
to allow agility over time.
TCP-AO allows any desired algorithm, subject to TCP option
space limitations, as noted in Section 4.2. The use of set of
MKTs allows separate connections to use different algorithms,
both for the MAC and the KDF.
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d. Order-independent processing.
The option should be processed independently of the proper
order, i.e., they should allow processing of TCP segments in
the order received, without requiring reordering. This avoids
the need for reordering prior to processing, and avoids the
impact of misordered segments on the option.
This is supported - see Sections 9.3, 9.4, and 9.5. Note that
pre-TCP processing is further required, because TCP segments
cannot be discarded solely based on a combination of
connection state and out-of-window checks; many such segments,
although discarded, cause a host to respond with a replay of
the last valid ACK, e.g. [RFC793]. See also the derivation of
the SNE, which is reconstituted at the receiver using a
demonstration algorithm that avoids the need for reordering
(in Section 8.2).
e. Security parameter changes require key changes.
The option should require that the MKT change whenever the
security parameters change. This avoids the need for
coordinating option state during a connection, which is
typical for TCP options. This also helps allow "bump in the
stack" implementations that are not integrated with endpoint
TCP implementations.
Parameters change only when a new MKT is used. See Section 5.
4. Keying requirements.
A solution to revising TCP MD5 should support manual keying, and
should support the use of an external automated key management
system (e.g., a protocol or other mechanism).
Note that TCP-AO does not specify a MKT management system.
a. Intraconnection rekeying.
The option should support rekeying during a connection, to
avoid the impact of long-duration connections.
This is supported by the use of IDs and multiple MKTs; see
Section 5.
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b. Efficient rekeying.
The option should support rekeying during a connection without
the need to expend undue computational resources. In
particular, the options should avoid the need to try multiple
keys on a given segment.
This is supported by the use of the KeyID. See Section 8.1.
c. Automated and manual keying.
The option should support both automated and manual keying.
The use of MKTs allows external automated and manual keying.
See Section 5. This capability is enhanced by the generation
of unique per-connection keys, which enables use of manual
MKTs with automatically generated traffic keys as noted in
Section 7.2.
d. Key management agnostic.
The option should not assume or require a particular key
management solution.
This is supported by use of a set of MKTs. See Section 5.
5. Expected Constraints
A solution to revising TCP MD5 should also abide by typical safe
security practices.
a. Silent failure.
Receipt of segments failing authentication must result in no
visible external action and must not modify internal state,
and those events should be logged.
This is supported - see Sections 9.3, 9.4, and 9.5.
b. At most one such option per segment.
Only one authentication option can be permitted per segment.
This is supported by the protocol requirements - see Section
4.2.
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c. Outgoing all or none.
Segments out of a TCP connection are either all authenticated
or all not authenticated.
This is supported - see Section 9.4.
d. Incoming all checked.
Segments into a TCP connection are always checked to determine
whether their authentication should be present and valid.
This is supported - see Section 9.5.
e. Non-interaction with TCP MD5.
The use of this option for a given connection should not
preclude the use of TCP MD5, e.g., for legacy use, for other
connections.
This is supported - see Section 9.7.
f. "Hard" ICMP discard.
The option should allow certain ICMPs to be discarded, notably
Type 3 (destination unreachable), Codes 2-4 (transport
protocol unreachable, port unreachable, or fragmentation
needed and IP DF field set), i.e., the ones indicating the
failure of the endpoint to communicate.
This is supported - see Section 13.
g. Maintain TCP connection semantics, in which the socket pair
alone defines a TCP association and all its security
parameters.
This is supported - see Sections 5 and 11.
13. Security Considerations
Use of TCP-AO, like use of TCP MD5 or IPsec, will impact host
performance. Connections that are known to use TCP-AO can be attacked
by transmitting segments with invalid MACs. Attackers would need to
know only the TCP connection ID and TCP-AO Length value to
substantially impact the host's processing capacity. This is similar
to the susceptibility of IPsec to on-path attacks, where the IP
addresses and SPI would be visible. For IPsec, the entire SPI space
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(32 bits) is arbitrary, whereas for routing protocols typically only
the source port (16 bits) is arbitrary (typically with less than 16
bits of randomness [La09]). As a result, it would be easier for an
off-path attacker to spoof a TCP-AO segment that could cause receiver
validation effort. However, we note that between Internet routers
both ports could be arbitrary (i.e., determined a-priori out of
band), which would constitute roughly the same off-path antispoofing
protection of an arbitrary SPI.
TCP-AO, like TCP MD5, may inhibit connectionless resets. Such resets
typically occur after peer crashes, either in response to new
connection attempts or when data is sent on stale connections; in
either case, the recovering endpoint may lack the connection key
required (e.g., if lost during the crash). This may result in time-
outs, rather than more responsive recovery after such a crash.
Recommendations for mitigating this effect are discussed in Section
9.7.
TCP-AO does not include a fast decline capability, e.g., where a SYN-
ACK is received without an expected TCP-AO and the connection is
quickly reset or aborted. Normal TCP operation will retry and
timeout, which is what should be expected when the intended receiver
is not capable of the TCP variant required anyway. Backoff is not
optimized because it would present an opportunity for attackers on
the wire to abort authenticated connection attempts by sending
spoofed SYN-ACKs without TCP-AO.
TCP-AO is intended to provide similar protections to IPsec, but is
not intended to replace the use of IPsec or IKE either for more
robust security or more sophisticated security management. TCP-AO is
intended to protect the TCP protocol itself from attacks that TLS,
sBGP/soBGP, and other data stream protection mechanism cannot. Like
IPsec, TCP-AO does not address the overall issue of ICMP attacks on
TCP, but does limit the impact of ICMPs, as noted in Section 9.8.
TCP-AO includes the TCP connection ID (the socket pair) in the MAC
calculation. This prevents different concurrent connections using the
same MKT (for whatever reason) from potentially enabling a traffic-
crossing attack, in which segments to one socket pair are diverted to
attack a different socket pair. When multiple connections use the
same MKT, it would be useful to know that segments intended for one
ID could not be (maliciously or otherwise) modified in transit and
end up being authenticated for the other ID. That requirement would
place an additional burden of uniqueness on MKTs within endsystems,
and potentially across endsystems. Although the resulting attack is
low probability, the protection afforded by including the received ID
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warrants its inclusion in the MAC, and does not unduly increase the
MAC calculation or MKT management.
The use of any security algorithm can present an opportunity for a
CPU DOS attack, where the attacker sends false, random segments that
the receiver under attack expends substantial CPU effort to reject.
In IPsec, such attacks are reduced by the use of a large Security
Parameter Index (SPI) and Sequence Number fields to partly validate
segments before CPU cycles are invested validated the Integrity Check
Value (ICV). In TCP-AO, the socket pair performs most of the function
of IPsec's SPI, and IPsec's Sequence Number, used to avoid replay
attacks, isn't needed due to TCP's Sequence Number, which is used to
reorder received segments (provided the sequence number doesn't wrap
around, which is why TCP-AO adds the SNE in Section 8.2). TCP already
protects itself from replays of authentic segment data as well as
authentic explicit TCP control (e.g., SYN, FIN, ACK bits, but even
authentic replays could affect TCP congestion control [Sa99]. TCP-AO
does not protect TCP congestion control from this last form of attack
due to the cumbersome nature of layering a windowed security sequence
number within TCP in addition to TCP's own sequence number; when such
protection is desired, users are encouraged to apply IPsec instead.
Further, it is not useful to validate TCP's Sequence Number before
performing a TCP-AO authentication calculation, because out-of-window
segments can still cause valid TCP protocol actions (e.g., ACK
retransmission) [RFC793]. It is similarly not useful to add a
separate Sequence Number field to TCP-AO, because doing so could
cause a change in TCP's behavior even when segments are valid.
14. IANA Considerations
[Paragraphs below in braces should be removed by the RFC Editor upon
publication]
[TCP-AO requires that IANA allocate a value from the TCP option Kind
namespace, to be replaced for TCP-IANA-KIND throughout this
document.]
[The entry for the TCP MD5 option should be listed as "Obsoleted by
TCP-AO in IANA tables.]
The TCP Authentication Option (TCP-AO) was assigned TCP option TCP-
IANA-KIND by IANA action.
This document defines no new namespaces.
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To specify MAC and KDF algorithms, TCP-AO refers to a separate
document that may involve IANA actions [Le09].
15. References
15.1. Normative References
[Le09] Lebovitz, G., E. Rescorla, "Cryptographic Algorithms for
TCP's Authentication Option, TCP-AO", draft-ietf-tcpm-tcp-
ao-crypto-02, Oct. 2009.
[RFC793] Postel, J., "Transmission Control Protocol," STD-7,
RFC-793, Standard, Sept. 1981.
[RFC1122] Braden, R., "Requirements for Internet Hosts --
Communication Layers," RFC-1122, Oct. 1989.
[RFC2018] Mathis, M., J. Mahdavi, S. Floyd, A. Romanow, "TCP
Selective Acknowledgement Options", RFC-2018, Proposed
Standard, April 1996.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP-14, RFC-2119, Best Current
Practice, March 1997.
[RFC2385] Heffernan, A., "Protection of BGP Sessions via the TCP MD5
Signature Option," RFC-2385, Proposed Standard, Aug. 1998.
[RFC2403] Madson, C., R. Glenn, "The Use of HMAC-MD5-96 within ESP
and AH," RFC-2403, Proposed Standard, Nov. 1998.
[RFC2460] Deering, S., R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification," RFC-2460, Proposed Standard, Dec.
1998.
[RFC2883] Floyd, S., J. Mahdavi, M. Mathis, M. Podolsky, "An
Extension to the Selective Acknowledgement (SACK) Option
for TCP", RFC-2883, Proposed Standard, July 2000.
[RFC3517] Blanton, E., M. Allman, K. Fall, L. Wang, "A Conservative
Selective Acknowledgment (SACK)-based Loss Recovery
Algorithm for TCP", RFC-3517, Proposed Standard, April
2003.
[RFC4306] Kaufman, C., "Internet Key Exchange (IKEv2) Protocol,"
RFC-4306, Proposed Standard, Dec. 2005.
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[RFC4724] Sangli, S., E. Chen, R. Fernando, J. Scudder, Y. Rekhter,
"Graceful Restart Mechanism for BGP," RFC-4724, Jan. 2007.
[RFC4271] Rekhter, Y, T. Li, S. Hares, "A Border Gateway Protocol 4
(BGP-4)," RFC-4271, Jan. 2006.
[RFC4781] Rekhter, Y., R. Aggarwal, "Graceful Restart Mechanism for
BGP with MPLS," RFC-4781, Jan. 2007.
15.2. Informative References
[Ba09] Bashyam, M., M. Jethanandani,, A. Ramaiah "Clarification of
sender behaviour in persist condition," draft-ananth-tcpm-
persist-02, (work in progress), Jan. 2010.
[Be07] Eddy, W., (ed), S. Bellovin, J. Touch, R. Bonica, "Problem
Statement and Requirements for a TCP Authentication
Option," draft-bellovin-tcpsec-01, (work in progress), Jul.
2007.
[Bo07] Bonica, R., B. Weis, S. Viswanathan, A. Lange, O. Wheeler,
"Authentication for TCP-based Routing and Management
Protocols," draft-bonica-tcp-auth-06, (work in progress),
Feb. 2007.
[Bo09] Borman, D., "TCP Options and MSS," draft-ietf-tcpm-tcpmss-
02, Jul. 2009.
[La09] Larsen, M., F. Gont, "Port Randomization," draft-ietf-
tsvwg-port-randomization-06, Feb. 2010.
[Go09] Gont, F., "ICMP attacks against TCP," draft-ietf-tcpm-icmp-
attacks-11, (work in progress), Feb. 2010.
[Le09] Lepinski, M., S. Kent, "An Infrastructure to Support Secure
Internet Routing," draft-ietf-sidr-arch-09, (work in
progress), Oct. 2009.
[RFC1321] Rivest, R., "The MD5 Message-Digest Algorithm," RFC-1321,
Informational, April 1992.
[RFC1323] Jacobson, V., R. Braden, D. Borman, "TCP Extensions for
High Performance," RFC-1323, May 1992.
[RFC1948] Bellovin, S., "Defending Against Sequence Number Attacks,"
RFC-1948, Informational, May 1996.
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[RFC2104] Krawczyk, H., M. Bellare, R. Canetti, "HMAC: Keyed-Hashing
for Message Authentication," RFC-2104, Informational, Feb.
1997.
[RFC2663] Srisuresh, P. and M. Holdrege, "IP Network Address
Translator (NAT) Terminology and Considerations", RFC 2663,
August 1999.
[RFC3234] Carpenter, B., S. Brim, "Middleboxes: Taxonomy and Issues,"
RFC-3234, Informational, Feb. 2002.
[RFC3562] Leech, M., "Key Management Considerations for the TCP MD5
Signature Option," RFC-3562, Informational, July 2003.
[RFC3947] Kivinen, T., B. Swander, A. Huttunen, V. Volpe,
"Negotiation of NAT-Traversal in the IKE," RFC-3947,
Proposed Standard, Jan. 2005.
[RFC4301] Kent, S., K. Seo, "Security Architecture for the Internet
Protocol," RFC-4301, Proposed Standard, Dec. 2005.
[RFC4808] Bellovin, S., "Key Change Strategies for TCP-MD5,"
RFC-4808, Informational, Mar. 2007.
[RFC4953] Touch, J., "Defending TCP Against Spoofing Attacks,"
RFC-4953, Informational, Jul. 2007.
[RFC5246] Dierks, T., E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2," RFC-5246, Aug. 2008.
[Sa99] Savage, S., N. Cardwell, D. Wetherall, T. Anderson, "TCP
Congestion Control with a Misbehaving Receiver," ACM
Computer Communications Review, V29, N5, pp71-78, October
1999.
[SDNS88] Secure Data Network Systems, "Security Protocol 4 (SP4),"
Specification SDN.401, Revision 1.2, July 12, 1988.
[To06] Touch, J., A. Mankin, "The TCP Simple Authentication
Option," draft-touch-tcpm-tcp-simple-auth-03, (expired work
in progress), Oct. 2006.
[To10] Touch, J., "A TCP Authentication Option NAT Extension,"
draft-touch-tcp-ao-nat-01, Jan. 2010.
[Wa05] Wang, X., H. Yu, "How to break MD5 and other hash
functions," Proc. IACR Eurocrypt 2005, Denmark, pp.19-35.
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[We05] Weis, B., "TCP Message Authentication Code Option," draft-
weis-tcp-mac-option-00, (expired work in progress), Dec.
2005.
16. Acknowledgments
Alfred Hoenes, Charlie Kaufman, Adam Langley, and numerous other
members of the TCPM WG provided substantial feedback on this
document.
This document was prepared using 2-Word-v2.0.template.dot.
Authors' Addresses
Joe Touch
USC/ISI
4676 Admiralty Way
Marina del Rey, CA 90292-6695
U.S.A.
Phone: +1 (310) 448-9151
Email: touch@isi.edu
URL: http://www.isi.edu/touch
Allison Mankin
Johns Hopkins Univ.
Washington, DC
U.S.A.
Phone: 1 301 728 7199
Email: mankin@psg.com
URL: http://www.psg.com/~mankin/
Ronald P. Bonica
Juniper Networks
2251 Corporate Park Drive
Herndon, VA 20171
U.S.A.
Email: rbonica@juniper.net
Touch, Mankin, Bonica Expires September 23, 2010 [Page 48]
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