Internet DRAFT - draft-cheshire-recursive-pcp
draft-cheshire-recursive-pcp
PCP working group S. Cheshire
Internet-Draft Apple
Intended status: Standards Track Mar 10, 2013
Expires: September 11, 2013
Recursive PCP
draft-cheshire-recursive-pcp-02
Abstract
The Port Control Protocol (PCP) allows clients to request explicit
dynamic inbound and outbound port mappings in their closest on-path
NAT, firewall, or other middlebox. However, in today's world, there
may be more than one NAT on the path between a client and the public
Internet. This document describes how the closest on-path middlebox
generates a corresponding upstream PCP request to the next closest
on-path middlebox, to request an appropriate explicit dynamic port
mapping in that middlebox too. Applied recursively, this generates
the necessary chain of port mappings in any number of middleboxes on
the path between the client and the public Internet.
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 September 11, 2013.
Copyright Notice
Copyright (c) 2013 IETF Trust and the persons identified as the
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
Cheshire Expires September 11, 2013 [Page 1]
Internet-Draft Recursive PCP Mar 2013
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.
1. Introduction
When NAT Port Mapping Protocol [NAT-PMP] was first created in 2004, a
common network configuration was that a residential customer received
a single public routable IPv4 address from their ISP, and had a
single NAT gateway serving multiple computers in their home.
Consequently, creating appropriate mappings in that single NAT
gateway was sufficient to provide full Internet connectivity.
In today's world, with public routable IPv4 addresses becoming less
readily available, it is increasingly common for customers to receive
a private address from their ISP, and the ISP uses a NAT gateway of
its own to translate those packets before sending them out onto the
public Internet. This means that there is likely to be more than on
NAT on the path between client machines and the public Internet:
o If a residential customer receives a translated address from their
ISP, and then installs their own residential NAT gateway to share
that address between multiple client devices in their home, then
there are at least two NAT gateways on the path between client
devices and the public Internet.
o If a mobile phone customer receives a translated address from
their mobile phone carrier, and uses "Personal Hotspot" or
"Internet Sharing" software on their mobile phone to make Wi-Fi
Internet access available to other client devices, then there are
at least two NAT gateways on the path between those client devices
and the public Internet.
o If a hotel guest connects a portable Wi-Fi gateway, such as an
Apple AirPort Express, to their hotel room Ethernet port to share
their room's Internet connection between their phone, their iPad,
and their laptop computer, then packets from the client devices
may traverse the hotel guest's portable NAT, the hotel network's
NAT, and the ISP's NAT before reaching the public Internet.
While it is possible, in theory, that client devices could somehow
discover all the NATs on the path, and communicate with each one
separately using Port Control Protocol [PCP] (NAT-PMP's IETF
Standards Track successor), in practice it's not clear how client
devices would reliably learn this information. Since the NAT
Cheshire Expires September 11, 2013 [Page 2]
Internet-Draft Recursive PCP Mar 2013
gateways are installed and operated by different individuals and
organizations, no single entity has knowledge of all the NATs on the
path. Also, even if a client device could somehow know all the NATs
on the path, requiring a client device to communicate separately with
all of them imposes unreasonable complexity on PCP clients, many of
which are expected to be simple low-cost devices.
In addition, this goes against the spirit of NAT gateways. The main
purpose of a NAT gateway is to make multiple downstream client
devices making outgoing TCP connections to appear, from the point of
view of everything upstream of the NAT gateway, to be a single client
device making outgoing TCP connections. In the same spirit, it makes
sense for a PCP-capable NAT gateway to make multiple downstream
client devices requesting port mappings to appear, from the point of
view of everything upstream of the NAT gateway, to be a single client
device requesting port mappings.
This document specifies how a PCP-capable NAT gateway uses Recursive
PCP to create the appearance of being a single device, from the point
of view of the upstream network.
1.1. Conventions and Terminology 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 "Key words for use in
RFCs to Indicate Requirement Levels" [RFC2119].
Where this document uses the terms "upstream" and "downstream", the
term "upstream" refers to the direction outbound packets travel
towards the public Internet, and the term "downstream" refers to the
direction inbound packets travel from the public Internet towards
client systems. Typically when a home user views a web site, their
computer sends an outbound TCP SYN packet upstream towards the public
Internet, and an inbound downstream TCP SYN ACK reply comes back from
the public Internet.
1.2. Recursive Application
The protocol specified is described as "recursive" because of the
following properties:
o When the text refers to the upstream PCP server as if it were the
final outermost NAT gateway, in fact that upstream PCP server
could itself be another Recursive PCP server making requests to
its own upstream PCP server, and relaying back the corresponding
replies. That distinction is invisible to the PCP client making
the request.
Cheshire Expires September 11, 2013 [Page 3]
Internet-Draft Recursive PCP Mar 2013
o When the text refers to an incoming PCP request being received
from a downstream PCP client, that downstream PCP client could
itself be a Recursive PCP server relaying a request on behalf of
one of its own downstream PCP clients (which could itself be
another Recursive PCP server, and so on). The fact that the
Recursive PCP server receiving the request does not need to be
aware of this or take any special action, is an important
simplifying property of the protocol. The purpose of a NAT
gateway is to make many downstream client devices appear to be a
single client device, and the purpose of a Recursive PCP server is
to make many downstream client devices making PCP requests appear
to be a single client device making PCP requests.
This recursive operation is an important simplifying property of the
design.
When a PCP client talks to a PCP server, that PCP server behaves
*exactly* as if it were the one and only NAT gateway on the path to
the public Internet. If the PCP server is not in fact the final
outermost NAT gateway, it is the PCP server's responsibility to hide
that fact. The client should never have to be aware of the
difference between talking to a single NAT gateway, and talking to a
NAT gateway which is itself behind one or more other NAT gateways.
This simplifying property applies both when the PCP client is a
simple end-host client, and when the PCP client is itself the client
face of a Recursive PCP server.
Similarly, when a PCP server receives a request from a PCP client,
that PCP client behaves exactly as if it were a simple end-host PCP
client requesting mappings for itself. If the client is not in fact
a simple end-host PCP client, it is the PCP client's responsibility
to hide that fact. The server should never have to be aware of the
difference between talking to an end-host PCP client, and talking to
the client face of a Recursive PCP server that is requesting mappings
on behalf of its own downstream clients. If the PCP client is a
firewall device, and it chooses to use the PCP THIRD_PARTY Option to
make mappings on behalf of its downstream clients, then it should
still behave like any other PCP client using the THIRD_PARTY Option.
2. Operation of Recursive PCP
Upon receipt of a PCP mapping-creation request from a downstream PCP
client, a Recursive PCP server first examines its local mapping table
to see if it already has a valid active mapping matching the Internal
Address and Internal Port (and in the case of PEER requests, remote
peer) given in the request.
Cheshire Expires September 11, 2013 [Page 4]
Internet-Draft Recursive PCP Mar 2013
If the Recursive PCP server does not already have a valid active
mapping for this mapping-creation request, then it allocates an
available port on its external interface. We assume for the sake of
this description that the address of its external interface is itself
a private address, subject to translation by an upstream NAT. The
Recursive PCP server then constructs an appropriate corresponding PCP
request of its own (described below), and sends it to its upstream
NAT, and the newly-created local mapping is considered temporary
until a confirming reply is received from the upstream PCP server.
If the Recursive PCP server does already have a valid active mapping
for this mapping-creation request, and the lifetime remaining on the
local mapping is at least 3/4 of the lifetime requested by the PCP
client, then the Recursive PCP server SHOULD send an immediate reply
giving the outermost External Address and Port (previously learned
using Recursive PCP, as described below), and the actual lifetime
remaining for this mapping. If the lifetime remaining on the local
mapping is less than 3/4 of the lifetime requested by the PCP client,
then the Recursive PCP server MUST generate an upstream request as
described below.
For mapping-deletion requests (Lifetime = 0), the local mapping, if
any, is deleted, and then (regardless of whether a local mapping
existed) a corresponding upstream request is generated.
How the Recursive PCP server knows the destination IP address for its
upstream PCP request is outside the scope of this document, but this
may be achieved in a zero-configuration manner using PCP Anycast
[Anycast]. In the upstream PCP request:
o The PCP Client's IP Address and Internal Port are the Recursive
PCP server's own external address and port just allocated for this
mapping.
o The Suggested External Address and Port in the upstream PCP
request SHOULD be copied from the original PCP request.
o The Requested Lifetime is as requested by the client if it falls
within the acceptable range for this PCP server; otherwise it
SHOULD be capped to appropriate minimum and maximum values
configured for this PCP server.
o The Mapping Nonce is copied from the original PCP request.
o For PEER requests, the Remote Peer IP Address and Port are copied
from the original PCP request.
Cheshire Expires September 11, 2013 [Page 5]
Internet-Draft Recursive PCP Mar 2013
o Any options in the original PCP request are handled or rejected
locally. No options are blindly copied from the original PCP
request to the upstream PCP request. Options in the original PCP
request pertain to the transaction between the client and its
Recursive PCP server. In the new upstream PCP request PCP options
may also be used if necessary to create the desired mapping, but
they are best thought of as new options pertaining to the
transaction between the Recursive PCP server and its upstream PCP
server, rather than as pre-existing options that were "copied"
from the original PCP request (even if, in some cases, the content
of those new options may be similar or identical to the options in
the original PCP request).
Upon receipt of a PCP reply giving the outermost (i.e. publicly
routable) External Address, Port and Lifetime, the Recursive PCP
server records this information in its own mapping table and relays
the information to the requesting downstream PCP client in a PCP
reply. The Recursive PCP server therefore records, among other
things, the following information in its mapping table:
o Client's Internal Address and Port.
o External Address and Port allocated by this Recursive PCP server.
o Outermost External Address and Port allocated by the upstream PCP
server.
o Mapping lifetime (also dictated by the upstream PCP server).
o Mapping nonce.
In the downstream PCP reply:
o The Lifetime is as granted by the upstream PCP server, or less, if
the granted lifetime exceeds the maximum lifetime this PCP server
is configured to grant. If the downstream Lifetime is more than
the Lifetime granted by the upstream PCP server (which is NOT
RECOMMENDED) then this Recursive PCP server MUST take
responsibility for renewing the upstream mapping itself.
o The Epoch Time is *this* Recursive PCP server's Epoch Time, not
the Epoch Time of the upstream PCP server. Each PCP server has
its own independent Epoch Time. However, if the Epoch Time
received from the upstream PCP server indicates a loss of state in
that PCP server, the Recursive PCP server can either recreate the
lost mappings itself, or it can reset its own Epoch Time to cause
its downstream clients to perform such state repairs themselves.
A Recursive PCP server MUST NOT simply copy the upstream PCP
Cheshire Expires September 11, 2013 [Page 6]
Internet-Draft Recursive PCP Mar 2013
server's Epoch Time into its downstream PCP replies, since if it
suffers its own state loss it needs the ability to communicate
that state loss to clients. Thus each PCP server has its own
independent Epoch Time. However, as a convenience, a downstream
Recursive PCP server may simply choose to reset its own Epoch Time
whenever it detects that its upstream PCP server has lost state.
Thus, in this case, the Recursive PCP server's Epoch Time always
resets whenever its upstream PCP server loses state; it may also
reset at other times too.
o The Mapping Nonce is copied from the reply received from the
upstream PCP server.
o The Assigned External Port and Assigned External IP Address are
copied from the reply received from the upstream PCP server.
(I.e. they are the outermost External IP Address and Port, not the
locally-assigned external address and port.)
o For PEER requests, the Remote Peer IP Address and Port are copied
from the reply received from the upstream PCP server.
o Any options in the reply received from the upstream PCP server are
handled locally as appropriate to the options in question. No
options are blindly copied from the upstream PCP reply to the
downstream PCP reply. If the original PCP request contained
options which necessitate a corresponding option in the reply,
then appropriate reply options should be generated and inserted
into the downstream PCP reply by the Recursive PCP server. These
downstream reply options are best thought of as data pertaining to
the transaction between the Recursive PCP server and its
downstream client, rather than as pre-existing options that were
"copied" from the upstream PCP reply into the downstream PCP reply
(even if, in some cases, the content of those new options in the
downstream PCP reply may be similar or identical to the options
received in the reply from the upstream PCP server).
2.1. Optimized Hairpin Routing
A Recursive PCP server SHOULD implement Optimized Hairpin Routing.
What this means is the following:
o If a Recursive PCP server observes an outgoing packet arriving on
its internal interface that is addressed to an External Address
and Port appearing in the NAT gateway's own mapping table, then
the NAT gateway SHOULD (after creating a new outbound mapping if
one does not already exist) rewrite the packet appropriately and
deliver it to the internal client currently allocated that
External Address and Port.
Cheshire Expires September 11, 2013 [Page 7]
Internet-Draft Recursive PCP Mar 2013
o If a Recursive PCP server observes an outgoing packet arriving on
its internal interface which is addressed to an Outermost External
Address and Port appearing in the NAT gateway's own mapping table,
then the NAT gateway SHOULD do likewise: create a new outbound
mapping if one does not already exist, and then rewrite the packet
appropriately and deliver it to the internal client currently
allocated that Outermost External Address and Port. This is not
necessary for successful communication, but for efficiency.
Without this Optimized Hairpin Routing, the packet will be
delivered all the way to the outermost NAT gateway, which will
then perform standard hairpin translation and send it back. Using
knowledge of the Outermost External Address and Port, this
rewriting can be anticipated and performed locally, which will
typically offer higher throughput and lower latency than sending
it all the way to the outermost NAT gateway and back.
2.2. Termination of Recursion
Any recursive algorithm needs a mechanism to terminate the recursion
at the appropriate point. This termination of recursion can be
achieved in a variety of ways:
o An ISP's NAT gateway could be configured to know that it is the
outermost NAT gateway, and consequently does not need to relay PCP
requests upstream. In fact, it may be the case that many large-
scale NATs of the kind used by ISPs may simply not implement
Recursive PCP, thereby naturally terminating the recursion at that
point.
o A NAT gateway could determine automatically that if its external
address is not one of the known private addresses
[RFC1918][RFC6598] then its external address is a public routable
IP address, and consequently it does not need to relay PCP
requests upstream.
o A NAT gateway could attempt sending PCP requests upstream, and
upon failing to receive any positive reply (e.g. receiving ICMP
host unreachable, ICMP port unreachable, or a timeout) conclude
that it does not need to relay PCP requests upstream.
2.3. Recursive PCP with Firewalls
When a Recursive PCP server is a NAT gateway, it sends out upstream
PCP requests using its own external IP address. When a Recursive PCP
server is a firewall, it still needs to install upstream mappings on
behalf of its downstream clients. It should do this either by using
the downstream client's IP address as the source IP address in its
upstream PCP request, or by using the PCP THIRD_PARTY Option in its
Cheshire Expires September 11, 2013 [Page 8]
Internet-Draft Recursive PCP Mar 2013
upstream PCP request.
3. IANA Considerations
No IANA actions are required by this document.
4. Security Considerations
No new security concerns are raised by use of Recursive PCP. Since
the purpose of a NAT gateway is to enable multiple client devices to
appear as a single client device to the upstream network, a NAT
gateway implementing Recursive PCP maintains this property, appearing
to the upstream network to be a single client device using PCP to
request port mappings for itself. Whether those port mappings are
for multiple processes running on multiple CPUs connected via an
internal bus in a single computer, or multiple processes running on
multiple CPUs connected via an IP network, is transparent to the
external network.
5. References
5.1. Normative References
[PCP] 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.
[RFC1918] Rekhter, Y., Moskowitz, R., Karrenberg, D., Groot, G., and
E. Lear, "Address Allocation for Private Internets",
BCP 5, RFC 1918, February 1996.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC6598] Weil, J., Kuarsingh, V., Donley, C., Liljenstolpe, C., and
M. Azinger, "IANA-Reserved IPv4 Prefix for Shared Address
Space", BCP 153, RFC 6598, April 2012.
5.2. Informative References
[Anycast] Cheshire, S., "PCP Anycast Address",
draft-cheshire-pcp-anycast-00 (work in progress),
February 2013.
[NAT-PMP] Cheshire, S., "NAT Port Mapping Protocol (NAT-PMP)",
Cheshire Expires September 11, 2013 [Page 9]
Internet-Draft Recursive PCP Mar 2013
draft-cheshire-nat-pmp-07 (work in progress),
January 2013.
Author's Address
Stuart Cheshire
Apple Inc.
1 Infinite Loop
Cupertino, California 95014
USA
Phone: +1 408 974 3207
Email: cheshire@apple.com
Cheshire Expires September 11, 2013 [Page 10]
