Internet DRAFT - draft-cheshire-pcp-anycast
draft-cheshire-pcp-anycast
PCP working group S. Cheshire
Internet-Draft Apple
Intended status: Standards Track July 14, 2013
Expires: January 15, 2014
PCP Anycast Address
draft-cheshire-pcp-anycast-02
Abstract
The Port Control Protocol Anycast Address enables PCP clients to
transmit messages to their closest on-path NAT, Firewall, or other
middlebox, without having to learn the IP address of that middlebox
via some external channel.
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 January 15, 2014.
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
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.
Cheshire Expires January 15, 2014 [Page 1]
Internet-Draft PCP Anycast Address July 2013
1. Introduction
The Port Control Protocol document [RFC6887] specifies the message
formats used, but the address to which a client sends its request is
either assumed to be the default router (which is appropriate in a
typical single-link residential network) or has to be configured
otherwise via some external mechanism, such as DHCP.
One drawback of relying on external configuration is that it creates
an external dependency on another piece of network infrastructure
which must be configured with the right address for PCP to work. In
some environments the staff managing the DHCP servers may not be the
same staff managing the NAT gateways, and in any case, constantly
keeping the DHCP server address information up to date as NAT
gateways are added, removed, or reconfigured, is burdensome.
Another drawback of relying on DHCP for configuration is that one of
the target deployment environments for PCP -- 3GPP for mobile
telephones -- does not use DHCP.
One design option that was considered for Apple's NAT gateways was to
have the NAT gateway simply handle and respond to all packets
addressed to UDP port 5351, regardless of the destination address in
the packet. Since the device is a NAT gateway, it already examines
every packet in order to rewrite port numbers, so also detecting
packets addressed to UDP port 5351 is not a significant additional
burden. Also, since this device is a NAT gateway which rewrites port
numbers, any attempt by a client to talk *though* this first NAT
gateway to create mappings in some second upstream NAT gateway is
futile and pointless. Any mappings created in the second NAT gateway
are useful to the client only if there are also corresponding
mappings created in the first NAT gateway. Consequently, there is no
case where it is useful for PCP requests to pass transparently
through the first PCP-aware NAT gateway on their way to the second
PCP-aware NAT gateway. In all cases, for useful connectivity to be
established, the PCP request must be handled by the first NAT
gateway, and then the first NAT gateway generates a corresponding new
upstream request to establish a mapping in the second NAT gateway.
(This process can be repeated recursively for as many times as
necessary for the depth of nesting of NAT gateways; this is
transparent to the client device [Recurs].)
These two issues result in the following related observations: the
PCP client may not *know* what destination address to use in its PCP
request packets; the PCP server doesn't *care* what destination
address is in the PCP request packets.
Given that the devices neither need to know nor care what destination
Cheshire Expires January 15, 2014 [Page 2]
Internet-Draft PCP Anycast Address July 2013
address goes in the packet, all we need to do is pick one and use it.
It's little more than a placeholder in the IP header. Any globally
routable unicast address will do. Since this address is one that
automatically routes its packet to the closest on-path device that
implements the desired functionality, it is an anycast address.
In the simple case where the first-hop router is also the NAT gateway
(as is common in a typical single-link residential network), sending
to the PCP anycast address is equivalent to sending to the client's
default router, as specified in the PCP base document [RFC6887].
In the case of a larger corporate network, where there may be several
internal routed subnets and one or more border NAT gateway(s)
connecting to the rest of the Internet, sending to the PCP anycast
address has the interesting property that it magically finds the
right border NAT gateway for that client. Since we posit that other
network infrastructure does not need (and should not have) any
special knowledge of PCP (or its anycast address) this means that to
other non-NAT routers, the PCP anycast address will look like any
other unicast destination address on the public Internet, and
consequently the packet will be forwarded as for any other packet
destined to the public Internet, until it reaches a NAT or firewall
device that is aware of the PCP anycast address. This will result in
the packet naturally arriving the NAT gateway that handles this
client's outbound traffic destined to the public Internet, which is
exactly the NAT gateway that the client wishes to communicate with
when managing its port mappings.
2. Benefit of using a PCP Anycast Address
The benefit of using an anycast address is simplicity and
reliability. In an example deployment scenario:
1. A network administrator installs a PCP-capable NAT.
2. An end user (who may be the same person) runs a PCP-enabled
application. This application can implement PCP with purely
user-level code -- no operating system support is required.
3. This PCP-enabled application sends its PCP request to the PCP
anycast address. This packet travels through the network like
any other, without any special support from DNS, DHCP, other
routers, or anything else, until it reaches the PCP-capable NAT,
which receives it, handles it, and sends back a reply.
Using the PCP anycast address, the only two things that need to be
deployed in the network are the two things that actually use PCP: The
Cheshire Expires January 15, 2014 [Page 3]
Internet-Draft PCP Anycast Address July 2013
PCP-capable NAT, and the PCP-enabled application. Nothing else in
the network needs to be changed or upgraded, and nothing needs to be
configured, including the PCP client.
3. Historical Objections to Anycast
In March 2001 a draft document entitled "Analysis of DNS Server
Discovery Mechanisms for IPv6" [DNSDisc] proposed using anycast to
discover DNS servers, a proposal that was subsequently abandoned in
later revisions of that draft document.
There are legitimate reasons why using anycast to discover DNS
servers is not compelling, mainly because it requires explicit
configuration of routing tables to direct those anycast packets to
the desired DNS server. However, DNS server discovery is very
different to NAT gateway discovery. A DNS server is something a
client explicitly talks to, via IP address. The DNS server may be
literally anywhere on the Internet. Various reasons make anycast an
uncompelling technique for DNS server discovery:
o DNS is a pure application-layer protocol, running over UDP.
o On an operating system without appropriate support for configuring
anycast addresses, a DNS server would have to use something like
Berkeley Packet Filter (BPF) to snoop on received packets to
intercept DNS requests, which is inelegant and inefficient.
o Without appropriate routing changes elsewhere in the network,
there's no reason to assume that packets sent to that anycast
address would even make it to the desired DNS server machine.
This places an addition configuration burden on the network
administrators, to install approprate routing table entries to
direct packets to the desired DNS server machine.
In contrast, a NAT gateway is something a client's packets stumble
across as they try to leave the local network and head out onto the
public Internet. The NAT gateway has to be on the path those packets
naturally take or it can't perform its NAT functions. As a result,
the objections to using anycast for DNS server discovery do not apply
to PCP:
o No routing changes are needed (or desired) elsewhere in the local
network, because the whole *point* of using anycast is that we
want the client's PCP request packet to take the same forwarding
path through the network as a TCP SYN to any other remote
destination address, because we want the *same* NAT gateway that
would have made a mapping in response to receiving an outbound TCP
Cheshire Expires January 15, 2014 [Page 4]
Internet-Draft PCP Anycast Address July 2013
SYN packet from the client to be the the one that makes a mapping
in response to receiving a PCP request packet from the client.
o A NAT engine is already snooping on (and rewriting) every packet
it forwards. As part of that snooping it could trivially look for
packets addressed to the PCP UDP port and process them locally
(just like the local processing it already does when it sees an
outbound TCP SYN packet).
4. IANA Considerations
IANA should allocate an IPv4 and an IPv6 well-known PCP anycast
address.
192.0.0.0/24 and 2001:0000::/23 are reserved for IETF Protocol
Assignments, as listed at
<http://www.iana.org/assignments/iana-ipv4-special-registry/> and
<http://www.iana.org/assignments/iana-ipv6-special-registry/>
Suitable addresses in these ranges, such as 192.0.0.8, and a
corresponding suitable IPv6 address, should be allocated.
5. Security Considerations
In a network without any border gateway, NAT or firewall that is
aware of the PCP anycast address, outgoing PCP requests could leak
out onto the external Internet, possibly revealing information about
internal devices.
Using an IANA-assigned well-known PCP anycast address enables border
gateways to block such outgoing packets. In the default-free zone,
routers should be configured to drop such packets. Such
configuration can occur naturally via BGP messages advertising that
no route exists to said address.
Sensitive clients that do not wish to leak information about their
presesence can set an IP TTL on their PCP requests that limits how
far they can travel into the public Internet.
Cheshire Expires January 15, 2014 [Page 5]
Internet-Draft PCP Anycast Address July 2013
6. References
6.1. Normative References
[RFC6887] Wing, D., Cheshire, S., Boucadair, M., Penno, R., and P.
Selkirk, "Port Control Protocol (PCP)", RFC 6887,
April 2013.
6.2. Informative References
[DNSDisc] Hagino, J. and D. Thaler, "Analysis of DNS Server
Discovery Mechanisms for IPv6",
draft-ietf-ipngwg-dns-discovery-01 (work in progress),
November 2001.
[Recurs] Cheshire, S., "Recursive PCP",
draft-cheshire-recursive-pcp-02 (work in progress),
Mar 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 January 15, 2014 [Page 6]
