Internet DRAFT - draft-nishida-tcpm-apaws
draft-nishida-tcpm-apaws
Network Working Group Y. Nishida
Internet-Draft GE Global Research
Intended status: Experimental October 17, 2015
Expires: April 19, 2016
A-PAWS: Alternative Approach for PAWS
draft-nishida-tcpm-apaws-02
Abstract
This documents describe a technique called A-PAWS which can provide
protection against old duplicates segments like PAWS. While PAWS
requires TCP to set timestamp options in all segments in a TCP
connection, A-PAWS supports the same feature without using
timestamps. A-PAWS is designed to be used complementary with PAWS.
TCP needs to use PAWS when it is necessary and activates A-PAWS only
when it is safe to use. Without impairing the reliability and the
robustness of TCP, A-PAWS can provide more option space to other TCP
extensions.
Status of This Memo
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This Internet-Draft will expire on April 19, 2016.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Conventions and Terminology . . . . . . . . . . . . . . . . . 3
3. The A-PAWS Design . . . . . . . . . . . . . . . . . . . . . . 3
3.1. Signaling Methods . . . . . . . . . . . . . . . . . . . . 4
3.2. A-PAWS Negotiation Logic for non-SYN Segment Signaling . 5
3.3. Sending Behavior . . . . . . . . . . . . . . . . . . . . 6
3.4. Receiving Behavior . . . . . . . . . . . . . . . . . . . 6
4. When To Activate A-PAWS . . . . . . . . . . . . . . . . . . . 6
5. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . 7
5.1. Protection Against Early Incarnations . . . . . . . . . . 7
5.2. Protection Against Security Threats . . . . . . . . . . . 7
5.3. Middlebox Considerations . . . . . . . . . . . . . . . . 8
5.4. Aggressive Mode in A-PAWS . . . . . . . . . . . . . . . . 8
6. Security Considerations . . . . . . . . . . . . . . . . . . . 9
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 9
8.1. Normative References . . . . . . . . . . . . . . . . . . 9
8.2. Informative References . . . . . . . . . . . . . . . . . 9
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 10
1. Introduction
PAWS (Protect Against Wrapped Sequences) defined in [RFC1323] is a
technique that can identify old duplicate segments in a TCP
connection. An old duplicate segment can be generated when it has
been delayed by queueing, etc. If such a segment has the sequence
number which falls within the receiver's current window, the receiver
will accept it without any warning or error. However, this segment
can be a segment created by an old connection that has the same port
and address pair, or a segments sent 2**32 bytes earlier on the same
connection. Although this situation rarely happens, it impairs the
reliability of TCP.
PAWS utilizes timestamp option in [RFC1323] to provide protection
against this. It is assumed that every received TCP segment contains
a timestamp. PAWS can identify old duplicate segments by comparing
the timestamp in the received segments and the timestamps from other
segments received recently. If both TCP endpoints agree to use PAWS,
all segments belong to this connection should have timestamp. Since
PAWS is the only standardized protection against old duplicate
segments, it has been implemented and used in most TCP
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implementations. However, as some TCP extensions such as [RFC2018],
[RFC5925] and [RFC6824] also requires a certain amount of option
space in non-SYN segments, using 10-12 bytes length in option space
for timestamp in all segments tends to be considered expensive in
recent discussions.
In addition, although PAWS is necessary for connections which
transmit more than 2**32 bytes, it is not very important for other
connections since [RFC0793] already has protection against segments
from old connections by using timers. Moreover, some research
results indicates that most of TCP flows tend to transmit small
amount of data, which means only small fraction of TCP connections
really need PAWS [QIAN11]. Timestamp option is also used for RTTM
(Round Trip Time Measurement) in [RFC1323]. Gathering many RTT
samples from the timestamp in every TCP segment looks useful approach
to improve RTO estimation. However, some research results shows the
number of samples per RTT does not affect the effectiveness of the
RTO [MALLMAN99]. Hence, we can think if PAWS is not used, sending a
few timestamps per RTT will be sufficient.
Based on these observations, we propose a new technique called A-PAWS
which can archive similar protection against old duplicates segments.
The basic idea of A-PAWS is to attain the same protection against old
all duplicate segments as PAWS while reducing the use of TS options
in segments. A-PAWS is designed to be used complementary with PAWS.
This means an implementation that supports A-PAWS is still required
to supports PAWS. A-PAWS is activated only when it is safe to use.
This sounds the applicability of A-PAWS is limited, however, we
believe TCP will have a lot of chances to save the option space if it
uses A-PAWS.
There are some discussions that PAWS can also be used to enhance
security, however, we still believe that A-PAWS can maintain the same
level of security as PAWS. Detailed discussions on this point are
provided in Section 5. A-PAWS is an experimental idea yet, but we
hope it will contribute to facilitating the use of TCP option space.
2. Conventions and Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
3. The A-PAWS Design
A-PAWS assumes PAWS as it is designed to be used complementary with
PAWS. Hence, a node which supports A-PAWS MUST support PAWS. The
following mechanisms are required in TCP in order to perform A-PAWS.
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3.1. Signaling Methods
An endpoint that supports A-PAWS can use the following signaling
methods to activate A-PAWS logic.
1) Option Exchange in SYN
This method uses a new experimental TCP option defined in
[RFC6994] and exchanges it during SYN negotiation. The format of
the option is depicted in Figure 1. The option does not have any
content as it simply indicates the endpoint supports A-PAWS. In
this signaling method, when an endpoint wants to use A-PAWS, it
MUST put A-PAWS option in SYN or SYN-ACK segment. If an endpoint
does not find A-PAWS option in received SYN or SYN-ACK segment,
it MUST not send segments with A-PAWS logic in Section 3.3.
However, it MUST activate A-PAWS receiver logic in Section 3.4 if
it has sent A-PAWS option in SYN or SYN-ACK segment. This is
because some middleboxes may remove A-PAWS option in SYN or SYN-
ACK segment. A-PAWS receiver logic in Section 3.4 can interact
with both A-PAWS and PAWS sender. This signaling requires
additional option space in SYN segments, hence non-SYN segment
signaling should be used when there is not enough space in SYN
option space.
2) Option Exchange in non-SYN Segments
This method uses the option in Figure 1 as well as the SYN
segment signaling. However, the options are not exchanged during
SYN negotiation. When a endpoint sets A-PAWS option in the
segments, it indicates that it can receive the segments from
A-PAWS senders. Hence, it MUST activate A-PAWS receiver logic in
Section 3.4 if it sends the options. However, it MUST not send
segments with A-PAWS logic in Section 3.3 until it receives
A-PAWS options. This approach does not require extra option
space or special timestamp value in SYN segments. However,
negotiating features in non-SYN segments will require to address
further arguments such as when to send the options or how to
retransmits the options. We discuss these points in the next
section and provide some recommended rules for implementations.
1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+---------------+---------------+------------------------------+
| Kind = 254 | Length = 4 | 16-bit ExID = TBD |
+---------------+---------------+------------------------------+
Figure 1: A-PAWS option format
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3.2. A-PAWS Negotiation Logic for non-SYN Segment Signaling
One important characteristic for A-PAWS is its signaling mechanism
does not require tight synchronization between endpoints since A-PAWS
receivers can interact with both A-PAWS senders and PAWS senders.
This allow us not to invent another three-way handshake like
mechanisms for non-SYN segments. This approach will require drastic
changes in the current TCP semantics. Instead, we propose a
relatively simple and easy mechanism for feature negotiation by using
the following rules on A-PAWS endpoints.
Rule 1: An endpoint MUST activate A-PAWS receiver logic in
Section 3.4 before it sends A-PAWS option.
Rule 2: An endpoint MUST not send segments with A-PAWS logic in
Section 3.3 until it receives A-PAWS option from the other
endpoint.
These rules can avoid situations where an endpoint sends segments by
A-PAWS logic to an endpoint that doesn't use A-PAWS logic.
Another discussion point for this signaling method is when to set
A-PAWS option in segments. As A-PAWS employs asynchronous signaling,
both endpoints basically can set A-PAWS option in segments anytime
they want. However, it is recommended to use the following rules for
setting A-PAWS options.
Rule 3: An endpoint SHOULD use a data segment when it sets A-PAWS
option in a segment.
Rule 4: When an endpoint receives a data segment with A-PAWS
option, it SHOULD set A-PAWS option for its ACK segment.
Rule 5: An endpoint MAY use A-PAWS options in retransmitted
segments.
These rules allow endpoints to have loose synchronized signaling so
that they can at least solicit responses from their peers. Of
course, even an endpoint solicit a response by setting A-PAWS option
in a data segment, it might not receive A-PAWS option in the ACK
segment. This can be caused by the lost of the ACK segment or
middleboxes that remove unknown options. In order to address these
cases, the following rules can be used.
Rule 6: As long as an endpoint does not violate the other rules,
it MAY set A-PAWS option in multiple data segments with a certain
interval in case no A-PAWS options has been sent from the peer.
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This rule can address the cases where A-PAWS options has been removed
by middleboxes or segments with A-PAWS options has been lost.
3.3. Sending Behavior
A-PAWS enabled TCP transmits segments, it needs to follow the rules
below.
1. TCP needs to check how many bytes has been transmitted in a
connection. If the transmitted bytes exceeds 2**32 -
'Sender.Offset', TCP migrates PAWS mode and MUST set timestamp
option in all segments to be transmitted. The value for
'Sender.Offset' is discussed in Section 5.
2. If the number of bytes transmitted in a TCP connection does not
exceeds 2**32 - 'Sender.Offset', TCP MAY omit timestamp option in
segments as long as it does not affect RTTM. This draft does not
define how much TCP can omit timestamps because it should be
determined by RTTM.
3.4. Receiving Behavior
A-PAWS enabled TCP receives segments, it needs to follow the rules
below.
1. TCP needs to check how many bytes has been received in a TCP
connection. If it exceeds 2**32 bytes, A-PAWS nodes SHOULD
discard the received segments which does not have timestamp
option. TCP MUST perform PAWS check when received bytes exceeds
2**32 bytes.
2. If the number of bytes received in a TCP connection does not
exceeds 2**32 bytes, A-PAWS nodes SHOULD accept the segments even
if it does not have timestamp option. A-PAWS nodes MAY skip PAWS
check until the received bytes exceeds 2**32 bytes.
4. When To Activate A-PAWS
In basic principal, A-PAWS capable nodes can always use A-PAWS logic
as long as the peers agree with them. However, the following cases
require special considerations to enable A-PAWS.
1. As "When To Keep Quiet" section in [RFC0793] suggests, it is
recommended that TCP keeps quiet for a MSL upon starting up or
recovering from a crash where memory of sequence numbers has been
lost. However, if timestamps are being used and if the timestamp
clock can be guaranteed to be increased monotonically, this quiet
time may be unnecessary. Because TCP can identify the segments
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from old connections by checking the timestamp. We think some
TCP implementations may disable the quiet time because of using
timestamps from this reason. However, since A-PAWS nodes does
not set timestamp options in all segments, TCP cannot rely on
this approach. To avoid decreasing the robustness of TCP
connection, TCP MUST NOT use A-PAWS for a MSL upon starting up or
recovering from a crash.
2. Various TCP implementations provide APIs such as setsockopt()
that can set SO_REUSEADDR flag on TCP connections. If this flag
is set, the TCP connection allows to reuse the same local port
without waiting for 2 MSL period. While this option is useful
when users want to relaunch applications immediately, it makes
the TCP connection a little vulnerable as TCP stack might receive
duplicate segments from earlier incarnations. It has been said
that PAWS can contribute to mitigate this risk by checking the
timestamps in segments. In order to keep the same level of
protection, TCP SHOULD NOT send A-PAWS option when SO_REUSEADDR
flag is set. This rule prevents the peer from sending segments
to this node with A-PAWS logic. However, the node can send
segments with A-PAWS logic as long as it received A-PAWS option
from the peer.
5. Discussion
As A-PAWS is an experimental logic, the following points need to be
considered and discussed.
5.1. Protection Against Early Incarnations
There are some discussions that timestamp can enhance the robustness
against early incarnations. Since A-PAWS does not set timestamps in
all segments, some may say that it degrades the robustness of TCP.
We believe that the degradation caused by A-PAWS on this point is
negligible. As long as TCP limits the usage of A-PAWS as described
in Section 4, duplicate segments from early incarnations should not
be received by TCP.
5.2. Protection Against Security Threats
A TCP connection can be identified by a 5-tuple: source address,
destination address, source port number, destination port number and
protocol. Crackers need to guess all these parameters when they try
malicious attacks on the connection. PAWS can enhance the protection
for this as it additionally requires timestamp checking. However, we
think the effect of PAWS against malicious attacks is limited due to
the simplicity of PAWS check. In PAWS, a segment can be considered
as an old duplicate if the timestamp in the segment less than some
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timestamps recently received on the connection. The "less than" in
this context is determined by processing timestamp values as 32 bit
unsigned integers in a modular 32-bit space. For example, if t1 and
t2 are timestamp values, t1 < t2 is verified when 0 < (t2 - t1) <
2**31 computed in unsigned 32-bit arithmetic. Hence, if crackers set
a random value in the timestamp option, there will be 50% chance for
them to trick PAWS check. Moreover, there will be more chances if
they send multiple segments with different timestamps, which will not
be difficult to perform.
In addition, we think there might be a case where using PAWS
increases security risks. PAWS recommends to increase timestamp over
a system when TCP waives the "quiet time" described in [RFC0793].
However, if timestamps are generated from a global counter, it may
leak some information such as system uptime as discussed in
[SILBERSACK05]. A-PAWS might be able to allows TCP to use random
timestamp values per connections.
5.3. Middlebox Considerations
A-PAWS is designed to be robust against middleboxes. This means that
endpoints will not be messed up even if middleboxes discard A-PAWS
option. This is because A-PAWS sender logic is activated only when
TCP receives a segment with A-PAWS options. A-PAWS receiver logic
does not need to know whether the sender is using PAWS or A-PAWS.
Activating A-PAWS receiving logic for PAWS sender might be redundant
as it requires additional overheads. However, we believe the
overhead will be acceptable in most cases because of the simplicity
of A-PAWS logic.
Another concern on middleboxes is that they can insert or delete some
bytes in TCP connections. If a middlebox inserts extra bytes into a
TCP connections, there might be a situation where an A-PAWS sender
can transmit segments without timestamp, while an A-PAWS receiver
perform PAWS check on them as it already has received 2**32 bytes.
In order to avoid discarding segments unnecessarily, we recommend
that A-PAWS sender should have a certain amount of offset bytes in
order to migrate PAWS mode before the receiver receives 2**32 bytes.
We call this protocol parameter 'Sender.Offset'. The proper value
for 'Sender.Offset' needs to be discussed.
5.4. Aggressive Mode in A-PAWS
The current A-PAWS requires TCP to migrate PAWS mode after sending/
receiving 2**32 bytes. However, if both nodes check if 2 MSL has
already passed during sending/receiving 2**32 bytes, it is safe to
continue using A-PAWS. We call this Aggressive mode. The use of
Aggressive mode will be explored in future versions.
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6. Security Considerations
We believe A-PAWS can maintain the same level of security as PAWS
does, but further discussions will be needed. Some security aspects
of A-PAWS are discussed in Section 5.
7. IANA Considerations
This document uses the Experimental Option Experiment Identifier. An
application for this codepoint in the IANA TCP Experimental Option
ExID registry will be submitted.
8. References
8.1. Normative References
[RFC0793] Postel, J., "Transmission Control Protocol", STD 7, RFC
793, September 1981.
[RFC1323] Jacobson, V., Braden, R., and D. Borman, "TCP Extensions
for High Performance", RFC 1323, DOI 10.17487/RFC1323, May
1992, <http://www.rfc-editor.org/info/rfc1323>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
8.2. Informative References
[MALLMAN99]
Allman, M. and V. Paxson, "On Estimating End-to-End
Network Path Properties", Proceedings of the ACM SIGCOMM ,
September 1999.
[QIAN11] Qian, L. and B. Carpenter, "A Flow-Based Performance
Analysis of TCP and TCP Applications", 3rd International
Conference on Computer and Network Technology (ICCNT 2011)
, February 2011.
[RFC2018] Mathis, M., Mahdavi, J., Floyd, S., and A. Romanow, "TCP
Selective Acknowledgment Options", RFC 2018, DOI 10.17487/
RFC2018, October 1996,
<http://www.rfc-editor.org/info/rfc2018>.
[RFC5925] Touch, J., Mankin, A., and R. Bonica, "The TCP
Authentication Option", RFC 5925, DOI 10.17487/RFC5925,
June 2010, <http://www.rfc-editor.org/info/rfc5925>.
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[RFC6824] Ford, A., Raiciu, C., Handley, M., and O. Bonaventure,
"TCP Extensions for Multipath Operation with Multiple
Addresses", RFC 6824, DOI 10.17487/RFC6824, January 2013,
<http://www.rfc-editor.org/info/rfc6824>.
[RFC6994] Touch, J., "Shared Use of Experimental TCP Options", RFC
6994, August 2013.
[SILBERSACK05]
Silbersack, M., "Improving TCP/IP security through
randomization without sacrificing interoperability.",
EuroBSDCon 2005 , November 2005.
Author's Address
Yoshifumi Nishida
GE Global Research
2623 Camino Ramon
San Ramon, CA 94583
USA
Email: nishida@wide.ad.jp
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