Network Working Group K. Naito
Internet-Draft A. Matsumoto
Intended status: Informational NTT
Expires: March 28, 2013 September 24, 2012
NAT TIME_WAIT reduction
draft-naito-nat-time-wait-reduction-03
Abstract
When network address translation (NAT) is used in an address resource
restricted environment, or when a lot of users are located under a
NAT device, IP addresses and port resources may be eaten up, and this
affects user experiences very negatively. This situation can be
greatly mitigated by tweaking mapping behavior and session timer
handling in NAT functions. This document proposes extension for
optimizing NAT IP address and port resources in address resource
restricted environments. The extension makes use of TCP timestamps
and sequence numbers for TIME_WAIT assassination.
Status of this Memo
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This Internet-Draft will expire on March 28, 2013.
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1. Introduction
After IPv4 addresses run out, IPv4 address resources will be further
restricted site-by-site. If global IPv4 address are shared between
several clients, assignable port resources at each client will be
limited.
NAT is a tool that is widely used to deal with this IPv4 address
shortage problem. However, the demand for resources to provide
Internet access to users and devices will continue to increase. IPv6
is a fundamental solution to this problem, but the deployment of IPv6
will take time.
In some cases, e.g. browsing a dynamic web page for a map service, a
lot of sessions are used by the browser, and a number of ports are
eaten up in a short time. What is worse is that when a NAT is
between a PC and a server, TIME_WAIT state of each TCP connection is
kept for certain period, typically for four minutes, which consumes
port resources. Therefore, new connections cannot be established.
This problem is caused or worsened by the following behavior.
TIME_WAIT state assigned for a TCP connection remains active for
2MSL after the last ACK to the last FIN is transferred.
We propose mechanisms to change the above behavior that make it
possible to save addresses and ports resources.
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1.1. TCP TIME_WAIT
The TCP TIME_WAIT state is described in RFC793 [RFC0793]. The TCP
TIME_WAIT state needs to be kept for 2MSL before a connection is
CLOSED, for the reasons below.
1: In the event that packets from a session are delayed in the in-
between network, and delivered to the end relatively later, we
should prevent the packets from being transferred and interpreted
as a packet that belongs to a new session.
2: If the remote TCP has not received the acknowledgment of its
connection termination request, it will re-send the FIN packet
several times.
These points are important for the TCP to work without problems.
1.2. TIME_WAIT Assassination
A TCP server MAY accept a TCP SYN for the 5-tuple session that is
just finished and marked as TIME_WAIT state,as far as the TCP
sequence number is increased. This is known as TIME-WAIT
assassination. It should also be noted that some assassination
hazards are described in RFC1337 [RFC1337].
1.3. Protect Against Wrapped Sequence numbers (PAWS)
The TCP sequence number wraps frequently especially in a high
bandwidth session. PAWS is used to prevent old duplicate packets
that occurred in a previous session from being transferred to the new
session whose valid TCP sequence numbers happen to overlap with the
old duplicate packets. This is implemented by introducing TCP
timestamp option, and checking the timestamp option value of each
packet. PAWS is described in RFC1323 [RFC1323].
2. NAT resource optimizing extension proposal
2.1. Apply RFC6191 to NAT
RFC 6191 [RFC6191] defines a mechanism for reducing the TIME_WAIT
state using TCP timestamps and sequence numbers. This document
proposes to apply this RFC6191 [RFC6191]mechanism at NAT. In case
there are several clients with nonsuccessive timestamp or sequence
number values are connected to a NAT device (i.e. not monotonically
increasing among clients), two mechanisms for applying RFC6191 to NAT
are described below. Also, PAWS works to discard old duplicate
packets at NAT. A packet can be discarded as an old duplicate if it
is received with a timestamp or sequence number value less than a
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value recently received on the connection.
2.2. Rewrite timestamp and sequence number values at NAT
Rewrite timestamp and sequence number values of outgoings packets at
NAT to be monotonically increasing. This can be done by adopting
following mechanisms at NAT.
A: Store the newest rewritten value of timestamp and sequence number
as the 'max value at the time'
B: NAT rewrite timestamp and sequence number values of incoming
packets to be monotonically increasing.
When packets come back as replies from remote hosts, NAT rewrite
again the packets' timestamp and sequence number values to be the
original values. This can be done by adopting following mechanisms
at NAT.
C: Store the values of original timestamp and sequence number of
packets, and rewritten values of those.
2.3. Split an assignable number of port space to each client
Adopt following mechanisms at NAT.
A: Choose clients that can be assigned ports.
B: Split assignable port numbers between clients.
Packets from other clients which are not chosen by these mechanisms
are rejected at NAT, unless there is unassigned port left.
2.4. Resend the last ACK to the resended FIN
In case the remote TCP could not receive the acknowledgment of its
connection termination request, NAT, on behalf of clients. resends
the last ACK packet when it recieves an FIN packet of the previous
connection, and when the state of the previous connection is deleted
from the NAT. This mechanism should be used when clients starts
closing process, and the remote host could not receive the last ACK.
2.5. Remote host behavior of several implementations
To solve the port shortage problem on the client side, the behavior
of remote host should be compliant to RFC 6191 [RFC6191] or the
mechanism written in 4.2.2.13 of RFC1122 [RFC1122], since NAT may
reuse the same 5 tuple for a new connection.We have investigated
behaviors of OSes (e.g., Linux, FreeBSD, Windows, MacOS), and found
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that they implemented the server side behavior of the above two.
3. Security Considerations
Security issues are not discussed in this memo.
4. Normative References
[RFC0793] Postel, J., "Transmission Control Protocol", STD 7,
RFC 793, September 1981.
[RFC1122] Braden, R., "Requirements for Internet Hosts -
Communication Layers", STD 3, RFC 1122, October 1989.
[RFC1323] Jacobson, V., Braden, B., and D. Borman, "TCP Extensions
for High Performance", RFC 1323, May 1992.
[RFC1337] Braden, B., "TIME-WAIT Assassination Hazards in TCP",
RFC 1337, May 1992.
[RFC6191] Gont, F., "Reducing the TIME-WAIT State Using TCP
Timestamps", BCP 159, RFC 6191, April 2011.
Appendix A. Revision History
02: Changed intended status to "informational".
01: 'draft-naito-nat-resource-optimizing-extension-01' was divided
into two drafts after IETF83 meeting.
'draft-naito-nat-resource-optimizing-extension-01' containes two
mechanisms. One mechanism, TIME_WAIT reduction is written in this
draft, and the other is written in
'draft-naito-nat-port-overlapping'.
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Authors' Addresses
Kengo Naito
NTT NT Lab
3-9-11 Midori-Cho
Musashino-shi, Tokyo 180-8585
Japan
Phone: +81 422 59 4949
Email: naito.kengo@lab.ntt.co.jp
Arifumi Matsumoto
NTT NT Lab
3-9-11 Midori-Cho
Musashino-shi, Tokyo 180-8585
Japan
Phone: +81 422 59 3334
Email: arifumi@nttv6.net
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