Internet DRAFT - draft-naito-nat-resource-optimizing-extension
draft-naito-nat-resource-optimizing-extension
Network Working Group K. Naito
Internet-Draft A. Matsumoto
Intended status: Standards Track NTT
Expires: January 10, 2013 July 9, 2012
NAT TIME_WAIT reduction
draft-naito-nat-resource-optimizing-extension-02
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 January 10, 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. By
tracing timestamp and sequence number values in NAT that manages
states of traversing TCP sessions, a TIME_WAIT remaining wait-time
can be reduced to zero, when a TCP-SYN packet carrying a larger
timestamp or sequence number value arrives. In this case, 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
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number value less than a value recently received on the connection.
When there are several clients with nonsuccessive timestamp or
sequence number values are connected to a NAT device (i.e. not
monotonically increasing among clients), it prevents some clients
from getting a port to start a connection for a long time because
other clients with larger timestamp or sequence number values are
preferred. Two workarounds for this issue are described below.
2.1.1. Rewrite timestamp and sequence number values at NAT
Rewrite timestamp and sequence number values of outgoings packets at
NAT to be monotonically increasing.
2.1.2. Split an assignable number of port space to each client
Set some rules among clients connecting to NAT, e.g., split
assignable ports between clients. This MAY be done by distributing
rules to clients via NAT equipment.
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.
[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.
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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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