Internet DRAFT - draft-penno-behave-rfc4787-5382-5508-bis
draft-penno-behave-rfc4787-5382-5508-bis
Port Control Protocol R. Penno
Internet-Draft Cisco
Intended status: BCP S. Perreault
Expires: July 11, 2013 Viagenie
S. Kamiset
Consultant
M. Boucadair
France Telecom
K. Naito
NTT
January 07, 2013
Network Address Translation (NAT) Behavioral Requirements Updates
draft-penno-behave-rfc4787-5382-5508-bis-04
Abstract
This document clarifies and updates several requirements of RFC4787,
RFC5382 and RFC5508 based on operational and development experience.
The focus of this document is NAPT44.
Requirements Language
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].
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on July 11, 2013.
Copyright Notice
Copyright (c) 2013 IETF Trust and the persons identified as the
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document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.1. Scope . . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. TCP Session Tracking . . . . . . . . . . . . . . . . . . . . . 3
3.1. TCP Transitory Connection Idle-Timeout . . . . . . . . . . 4
3.1.1. Port resources limited case . . . . . . . . . . . . . 5
3.1.2. Proposal: Apply RFC6191 and PAWS to NAT . . . . . . . 6
3.2. TCP RST . . . . . . . . . . . . . . . . . . . . . . . . . 9
4. Port Overlapping behavior . . . . . . . . . . . . . . . . . . 9
5. Address Pooling Paired (APP) . . . . . . . . . . . . . . . . . 10
6. EIF Security . . . . . . . . . . . . . . . . . . . . . . . . . 10
7. EIF Protocol Independence . . . . . . . . . . . . . . . . . . 10
8. EIF Mapping Refresh . . . . . . . . . . . . . . . . . . . . . 10
8.1. Outbound Mapping Refresh and Error Packets . . . . . . . . 11
9. EIM Protocol Independence . . . . . . . . . . . . . . . . . . 11
10. Port Parity . . . . . . . . . . . . . . . . . . . . . . . . . 11
11. Port Randomization . . . . . . . . . . . . . . . . . . . . . . 11
12. IP Identification (IP ID) . . . . . . . . . . . . . . . . . . 12
13. ICMP Query Mappings Timeout . . . . . . . . . . . . . . . . . 12
14. Hairpinning Support for ICMP Packets . . . . . . . . . . . . . 12
15. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12
16. Security Considerations . . . . . . . . . . . . . . . . . . . 12
17. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 13
18. References . . . . . . . . . . . . . . . . . . . . . . . . . . 13
18.1. Normative References . . . . . . . . . . . . . . . . . . . 13
18.2. Informative References . . . . . . . . . . . . . . . . . . 14
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 14
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1. Terminology
The reader should be familiar with all terms defined in RFC2663
[RFC2663],RFC4787 [RFC4787],RFC5382 [RFC5382],RFC5508 [RFC5508]
2. Introduction
[RFC4787], [RFC5382] and [RFC5508] greatly advanced NAT
interoperability and conformance. But with widespread deployment and
evolution of NAT more development and operational experience was
acquired some areas of the original documents need further
clarification or updates. This documents provides such
clarifications and updates.
2.1. Scope
This document focuses solely on NAPT44 and its goal is to clarify,
fill gaps or update requirements of [RFC4787], [RFC5382] and
[RFC5508]. It is out of the scope of this document the creation of
completely new requirements not associated with the documents cited
above. New requirements would be better served elsewhere and if they
are CGN specific in [I-D.ietf-behave-lsn-requirements]
3. TCP Session Tracking
[RFC5382] specifies TCP timers associated with various connection
states but does not specify the TCP state machine a NAPT44 should use
as a basis to apply such timers. The TCP state machine below,
adapted from [RFC6146], provides guidance on how TCP session tracking
could be implemented - it is non-normative.
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+-----------------------------+
| |
V |
+------+ CV4 |
|CLOSED|-----SYN------+ |
+------+ | |
^ | |
|TCP_TRANS T.O. | |
| V |
+-------+ +-------+ |
| TRANS | |V4 INIT| |
+-------+ +-------+ |
| ^ | |
data pkt | | |
| V4 or V4 RST | |
| TCP_EST T.O. | |
V | SV4 SYN |
+--------------+ | |
| ESTABLISHED |<---------+ |
+--------------+ |
| | |
CV4 FIN SV4 FIN |
| | |
V V |
+---------+ +----------+ |
|CV4 FIN | | SV4 FIN | |
| RCV | | RCV | |
+---------+ +----------+ |
| | |
SV4 FIN CV4 FIN TCP_TRANS
| | T.O.
V V |
+----------------------+ |
| CV4 FIN + SV4 FIN RCV|--------------------+
+----------------------+
(postamble)
3.1. TCP Transitory Connection Idle-Timeout
[RFC5382]:REQ-5 The transitory connection idle-timeout is defined as
the minimum time a TCP connection in the partially open or closing
phases must remain idle before the NAT considers the associated
session a candidate for removal. But the document does not clearly
states if these can be configured separately. This document
clarifies that a NAT device SHOULD provide different knobs for
configuring the open and closing idle timeouts. This document
further acknowledges that most TCP flows are very short (less than 10
seconds) [FLOWRATE][TCPWILD] and therefore a partially open timeout
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of 4 minutes might be excessive if security is a concern. Therefore
it MAY be configured to be less than 4 minutes in such cases. There
also may be a case that timeout of 4 minutes might be excessive. The
case and the solution are written below.
3.1.1. Port resources limited case
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.
To reuse resources effectively, reducing TIME_WAIT without making any
bad effect is important. To reduce TIME_WAIT, [RFC6191] is proposed
for clients and remote hosts. To prevent bad effects, there is a
PAWS mechanism, which prevent the old duplicate problem. We propose
mechanisms adopting to NAT, to change the TIME_WAIT behavior that
make it possible to save addresses and ports resources.
3.1.1.1. RFC6191 Reducing the TIME-WAIT State Using TCP Timestamps
[RFC6191] defines a mechanism for reducing the TIME_WAIT state using
TCP timestamps and sequence numbers. When a connection request is
received with a four-tuple that is in the TIME-WAIT state, the
connection request may be accepted if the sequence number or the
timestamp of the incoming SYN segment is greater than the last
sequence number seen on the previous incarnation of the connection
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3.1.1.2. TCP TIME_WAIT
The TCP TIME_WAIT state is described in [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.
3.1.1.3. 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].
3.1.2. Proposal: Apply RFC6191 and PAWS to NAT
This section proposes to apply [RFC6191] mechanism at NAT. This
mechanism MAY be adopted for both clients' and remote hosts' TCP
active close.
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client NAT remote host
| | |
| FIN | FIN |
|------------------------>|------------------------>|
| | |
| ACK | ACK |
|<------------------------|<------------------------|
| FIN | FIN |
|<------------------------|<------------------------|
| | |
| ACK(TSval=A) | ACK |
|------------------------>|------------------------>|
| | - |
| | | |
| | | |
| | | |
| | | TIME_WAIT |
| | | ->assassinated at x |
| | | |
| | | |
| | | |
| SYN(TSval>A) | x SYN |
|------------------------>|------------------------>|
| | - |
| | | |
| | | SYN_SENT |
| | | |
| | | |
(postamble)
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 value recently
received on the connection.
To make these mechanisms work, we should concern the case that 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 to solve this mechanism
and applying [RFC6191] and PAWS to NAT are described below. These
mechanisms are optional.
3.1.2.1. 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.
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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 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.
3.1.2.2. 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.
3.1.2.3. Resend the last ACK to the resended FIN
We should concern another case to make RFC6191 work at NAT. 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 receives an FIN packet of the previous connection, and
when the state of the previous connection is deleted from the NAT.
This mechanism MAY be used when clients starts closing process, and
the remote host could not receive the last ACK.
3.1.2.4. 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 [RFC6191] or the mechanism
written in 4.2.2.13 of [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 that they
implemented the server side behavior of the above two.
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3.2. TCP RST
[RFC5382] leaves the handling of TCP RST packets unspecified. This
document does not try standardize such behavior but clarifies based
on operational experience that a NAT that receives a TCP RST for an
active mapping and performs session tracking MAY immediately delete
the sessions and remove any state associated with it. If the NAT
device that performs TCP session tracking receives a TCP RST for the
first session that created a mapping, it MAY remove the session and
the mapping immediately.
4. Port Overlapping behavior
There may be another solution to the address resource restricted
environment written in 3.1.1. Also NAT are required to be maped
endpoint-independent in [RFC4787] and [RFC5382] REQ-1, the mechanism
below MAY be one optional implement to NAT.
If destination addresses and ports are different for outgoing
connections started by local clients, NAT MAY assign the same
external port as the source ports for the connections. The port
overlapping mechanism manages mappings between external packets and
internal packets by looking at and storing the 5-tuple (protocol,
source address, source port, destination address, destination port)
of them. This enables concurrent use of a single NAT external port
for multiple transport sessions, which enables NAT to work correctly
in IP address resource limited network.
Discussions:
[RFC4787]and[FC5382] requires "endpoint-independent mapping" at NAT,
and port overlapping NAT cannot meet the requirement. This mechanism
can degrade the transparency of NAT in that its mapping mechanism is
endpoint-dependent and makes NAT traversal harder. However, if a NAT
adopts endpoint-independent mapping together with endpoint-dependent
filtering, then the actual behavior of the NAT will be the same as
port overlapping NAT. It should also be noted that a lot of existing
NAT devices(e.g., SEIL, FITELnet Series) adopted this port
overlapping mechanism.
A: Reference URL for SEIL -> www.seil.jp
B: Reference URL for FITELnet -> www.furukawa.co.jp/fitelnet
The netfilter, which is a popular packet filtering mechanism for
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Linux, also adopts port overlapping behavior.
5. Address Pooling Paired (APP)
[RFC4787]: REQ-2 [RFC5382]:ND Address Pooling Paired behavior for NAT
is recommended in previous documents but behavior when a public IPv4
run out of ports is left undefined. This document clarifies that if
APP is enabled new sessions from a subscriber that already has a
mapping associated with a public IP that ran out of ports SHOULD be
dropped. The administrator MAY provide a knob that allows a NAT
device to starting using ports from another public IP when the one
that anchored the APP mapping ran out of ports. This is trade-off
between subscriber service continuity and APP strict enforcement.
(NE: It is sometimes referred as 'soft-APP')
6. EIF Security
[RFC4787]:REQ-8 and [RFC5382]:REQ-3 End-point independent filtering
could potentially result in security attacks from the public realm.
In order to handle this, when possible there MUST be strict filtering
checks in the inbound direction. A knob SHOULD be provided to limit
the number of inbound sessions and a knob SHOULD be provided to
enable or disable EIF on a per application basis. This is specially
important in the case of Mobile networks where such attacks can
consume radio resources and count against the user quota.
7. EIF Protocol Independence
[RFC4787]:REQ-8 and[RFC5382]: REQ-3 Current RFCs do not specify
whether EIF mappings are protocol independent. In other words, if a
outbound TCP SYN creates a mapping it is left undefined whether
inbound UDP packets create sessions and are forwarded. EIF mappings
SHOULD be protocol independent in order allow inbound packets for
protocols that multiplex TCP and UDP over the same IP: port through
the NAT and maintain compatibility with stateful NAT64 RFC6146
[RFC6146]. But the administrator MAY provide a configuration knob to
make it protocol dependent.
8. EIF Mapping Refresh
[RFC4787]: REQ-6 [RFC5382]: ND The NAT mapping Refresh direction MAY
have a "NAT Inbound refresh behavior" of "True" but it does not
clarifies how this applies to EIF mappings. The issue in question is
whether inbound packets that match an EIF mapping but do not create a
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new session due to a security policy should refresh the mapping
timer. This document clarifies that even when a NAT device has a
inbound refresh behavior of TRUE, that such packets SHOULD NOT
refresh the mapping. Otherwise a simple attack of a packet every 2
minutes can keep the mapping indefinitely.
8.1. Outbound Mapping Refresh and Error Packets
In the case of NAT outbound refresh behavior there might be certain
types of packets that should not refresh the mapping. For example,
if the mapping is kept alive by ICMP Error or TCP RST outbound
packets sent as response to inbound packets, these SHOULD NOT refresh
the mapping.
9. EIM Protocol Independence
[RFC4787] [RFC5382]: REQ-1 Current RFCs do not specify whether EIM
are protocol independent. In other words, if a outbound TCP SYN
creates a mapping it is left undefined whether outbound UDP can reuse
such mapping and create session. On the other hand, Stateful NAT64
[RFC6146] clearly specifies three binding information bases (TCP,
UDP, ICMP). This document clarifies that EIM mappings SHOULD be
protocol dependent . A knob MAY be provided in order allow protocols
that multiplex TCP and UDP over the same source IP and port to use a
single mapping.
10. Port Parity
A NAT devices MAY disable port parity preservation for dynamic
mappings. Nevertheless, A NAT SHOULD support means to explicitly
request to preserve port parity (e.g., [I-D.boucadair-pcp-rtp-rtcp]).
11. Port Randomization
A NAT SHOULD follow the recommendations specified in Section 4 of
[RFC6056] especially: "A NAPT that does not implement port
preservation [RFC4787] [RFC5382] SHOULD obfuscate selection of the
ephemeral port of a packet when it is changed during translation of
that packet. A NAPT that does implement port preservation SHOULD
obfuscate the ephemeral port of a packet only if the port must be
changed as a result of the port being already in use for some other
session. A NAPT that performs parity preservation and that must
change the ephemeral port during translation of a packet SHOULD
obfuscate the ephemeral ports. The algorithms described in this
document could be easily adapted such that the parity is preserved
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(i.e., force the lowest order bit of the resulting port number to 0
or 1 according to whether even or odd parity is desired)."
12. IP Identification (IP ID)
A NAT SHOULD handle the Identification field of translated IPv4
packets as specified in Section 9 of [I-D.ietf-intarea-ipv4-id-
update].
13. ICMP Query Mappings Timeout
Section 3.1 of [RFC5508] says that ICMP Query Mappings are to be
maintained by NAT device. However, RFC doesn't discuss about the
Query Mapping timeout values. Section 3.2 of that RFC only discusses
about ICMP Query Session Timeouts. ICMP Query Mappings MAY be
deleted once the last the session using the mapping is deleted.
14. Hairpinning Support for ICMP Packets
[RFC5508]:REQ-7 This requirement specifies that NAT devices enforcing
Basic NAT MUST support traversal of hairpinned ICMP Query sessions.
This implicitly means that address mappings from external address to
internal address (similar to Endpoint Independent Filters) MUST be
maintained to allow inbound ICMP Query sessions. If an ICMP Query is
received on an external address, NAT device can then translate to an
internal IP. [RFC5508]:REQ-7 This requirement specifies that all NAT
devices (i.e., Basic NAT as well as NAPT devices) MUST support the
traversal of hairpinned ICMP Error messages. This too requires NAT
devices to maintain address mappings from external IP address to
internal IP address in addition to the ICMP Query Mappings described
in section 3.1 of that RFC.
15. IANA Considerations
TBD
16. Security Considerations
In the case of EIF mappings due to high risk of resource crunch, a
NAT device MAY provide a knob to limit the number of inbound sessions
spawned from a EIF mapping.
[TCP-Security] contains a detailed discussion of the security
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implications of TCP Timestamps and of different timestamp generation
algorithms.
17. Acknowledgements
Thanks to Dan Wing, Suresh Kumar, Mayuresh Bakshi, Rajesh Mohan and
Senthil Sivamular for review and discussions
18. References
18.1. Normative References
[I-D.ietf-pcp-base]
Wing, D., Cheshire, S., Boucadair, M., Penno, R., and P.
Selkirk, "Port Control Protocol (PCP)",
draft-ietf-pcp-base-29 (work in progress), November 2012.
[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.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2663] Srisuresh, P. and M. Holdrege, "IP Network Address
Translator (NAT) Terminology and Considerations",
RFC 2663, August 1999.
[RFC3605] Huitema, C., "Real Time Control Protocol (RTCP) attribute
in Session Description Protocol (SDP)", RFC 3605,
October 2003.
[RFC4787] Audet, F. and C. Jennings, "Network Address Translation
(NAT) Behavioral Requirements for Unicast UDP", BCP 127,
RFC 4787, January 2007.
[RFC5382] Guha, S., Biswas, K., Ford, B., Sivakumar, S., and P.
Srisuresh, "NAT Behavioral Requirements for TCP", BCP 142,
RFC 5382, October 2008.
[RFC5508] Srisuresh, P., Ford, B., Sivakumar, S., and S. Guha, "NAT
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Behavioral Requirements for ICMP", BCP 148, RFC 5508,
April 2009.
[RFC6056] Larsen, M. and F. Gont, "Recommendations for Transport-
Protocol Port Randomization", BCP 156, RFC 6056,
January 2011.
[RFC6146] Bagnulo, M., Matthews, P., and I. van Beijnum, "Stateful
NAT64: Network Address and Protocol Translation from IPv6
Clients to IPv4 Servers", RFC 6146, April 2011.
[RFC6191] Gont, F., "Reducing the TIME-WAIT State Using TCP
Timestamps", BCP 159, RFC 6191, April 2011.
18.2. Informative References
[FLOWRATE]
Zhang, Y., Breslau, L., Paxson, V., and S. Shenker, "On
the Characteristics and Origins of Internet Flow Rates".
[I-D.boucadair-pcp-rtp-rtcp]
Boucadair, M. and S. Sivakumar, "Reserving N and N+1 Ports
with PCP", draft-boucadair-pcp-rtp-rtcp-05 (work in
progress), October 2012.
[I-D.ietf-behave-lsn-requirements]
Perreault, S., Yamagata, I., Miyakawa, S., Nakagawa, A.,
and H. Ashida, "Common requirements for Carrier Grade NATs
(CGNs)", draft-ietf-behave-lsn-requirements-10 (work in
progress), December 2012.
[I-D.naito-nat-resource-optimizing-extension]
Kengo, K. and A. Matsumoto, "NAT TIME_WAIT reduction",
draft-naito-nat-resource-optimizing-extension-02 (work in
progress), July 2012.
[TCPWILD] Qian, F., Subhabrata, S., Spatscheck, O., Morley Mao, Z.,
and W. Willinger, "TCP Revisited: A Fresh Look at TCP in
the Wild".
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Authors' Addresses
Reinaldo Penno
Cisco Systems, Inc.
170 West Tasman Drive
San Jose, California 95134
USA
Email: repenno@cisco.com
Simon Perreault
Viagenie
2875 boul. Laurier, suite D2-630
Quebec, QC G1V 2M2
Canada
Email: simon.perreault@viagenie.ca
Sarat Kamiset
Consultant
California
Phone:
Fax:
Mohamed Boucadair
France Telecom
Rennes, 35000
France
Email: mohamed.boucadair@orange.com
Kengo Naito
NTT
Tokyo
Japan
Email: kengo@lab.ntt.co.jp
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