One document matched: draft-carpenter-referral-ps-02.xml
<?xml version="1.0" encoding="US-ASCII"?>
<!DOCTYPE rfc SYSTEM "rfc2629.dtd">
<!-- This is built from a template for a generic Internet Draft. Suggestions for
improvement welcome - write to Brian Carpenter, brian.e.carpenter @ gmail.com -->
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(which supports the latest, sometimes undocumented and under-tested, features.) -->
<?rfc toc="yes"?>
<!-- You want a table of contents -->
<?rfc symrefs="yes"?>
<!-- Use symbolic labels for references -->
<?rfc sortrefs="yes"?>
<!-- This sorts the references -->
<?rfc iprnotified="no" ?>
<!-- Change to "yes" if someone has disclosed IPR for the draft -->
<?rfc compact="yes"?>
<!-- This defines the specific filename and version number of your draft (and inserts the appropriate IETF boilerplate -->
<rfc category="info" docName="draft-carpenter-referral-ps-02"
ipr="trust200902">
<front>
<title abbrev="Referral Problem">Problem Statement for
Referral</title>
<author fullname="Brian Carpenter" initials="B. E." surname="Carpenter">
<organization abbrev="Univ. of Auckland"></organization>
<address>
<postal>
<street>Department of Computer Science</street>
<street>University of Auckland</street>
<street>PB 92019</street>
<city>Auckland</city>
<region></region>
<code>1142</code>
<country>New Zealand</country>
</postal>
<email>brian.e.carpenter@gmail.com</email>
</address>
</author>
<author fullname="Sheng Jiang" initials="S.J." surname="Jiang">
<organization>Huawei Technologies Co., Ltd</organization>
<address>
<postal>
<street>Huawei Building, No.3 Xinxi Rd.,</street>
<city>Shang-Di Information Industry Base, Hai-Dian District,
Beijing</city>
<country>P.R. China</country>
</postal>
<email>jiangsheng@huawei.com</email>
</address>
</author>
<author fullname="Zhen Cao" initials="Z" surname="Cao">
<organization abbrev="ChinaMobile">China Mobile</organization>
<address>
<postal>
<street>Unit2, 28 Xuanwumenxi Ave,Xuanwu District</street>
<city>Beijing</city>
<region></region>
<code>100053</code>
<country>P.R. China</country>
</postal>
<email>caozhenpku@gmail.com</email>
</address>
</author>
<date day="23" month="February" year="2011" />
<area></area>
<workgroup>Network Working Group</workgroup>
<abstract>
<t>The purpose of a referral is to enable a given entity in a multiparty
Internet application to pass information to another party. It enables a
communication initiator to be aware of relevant information of its
destination entity before launching the communication. This memo
discusses the problems involved in referral scenarios.</t>
</abstract>
</front>
<middle>
<section anchor="intro" title="Introduction">
<t>A frequently occurring situation is that one entity A connected to
the Internet (or to some private network using the Internet protocol
suite) needs to be aware of the information of another entity B in order
to reach it. The information can be obtained from B itself or some
third-party entity C. This is known as a referral.</t>
<t>Referral is the act whereby one entity informs another entity how to
contact a specific entity. It enables a communication initiator to be
aware of relevant information of its destination entity in order to
launch a communication channel. This referral information can be
obtained through an existing communication channel between these two
entities or from thrid-party entities.</t>
<t>In the original design of the Internet, IP addresses were global,
unique, and quasi-permanent. Also any differentiation beyond that
provided by an IP address was done by protocol and port numbers.
Referrals were therefore performed simply by passing an IP address and
possibly protocol and port numbers. In fact simple referrals (the first
case above, sometimes called first-party referrals) were never needed
since A could simply use B's address. Third-party referrals were
trivial: C would tell A about B's address. Thus, it became common
practice to pass raw addresses between entities. A classical example is
the FTP PORT command <xref target="RFC0959"></xref>.</t>
</section>
<section title="Terminology">
<t>This document makes use of the following terms:</t>
<list style="symbols">
<t>"Entity": we use this rather than "application" to describe any
software component embedded in an Internet host, not just a specific
application, that sends, receives or makes use of referrals. Also, in
case of dynamic load sharing or failover, an entity might even migrate
between hosts.</t>
<t>"Referral": the act of one entity informing another entity how to
contact a specific entity.</t>
<t>"Reference": the actual data (name, address, identitifier, locator,
pointer, etc.) that is the basis of a referral.</t>
<t>"Referring entity": the entity that sends a referral.</t>
<t>"Receiving entity": the entity that receives a referral.</t>
<t>"Referenced entity": the target entity of a reference.</t>
<t>"Scope": the region or regions of the Internet within which a given
reference is applicable to reach the referenced entity.</t>
</list>
</section>
<section title="Goals of Referral">
<t>The principal purpose of referral is to enable one entity in a
multi-party application to pass information to another party involved in
the same application. This document makes no assumptions about whether
the entities are acting as clients, servers, peers, super-nodes, relays,
proxies, etc., as far as the application is concerned. Neither does it
take a position as to how the various entities become aware of the need
to send a referral; this depends entirely on the structure of the
application.</t>
<section title="Reachability">
<t>The primary goals of referral is to enable a communication
initiator to reach its destination entity. Referral is a best effort
mechanism. It does not guarantee actual reachability, since the
referring entity has no general way of knowing which paths exist
between the receiving entity and the referenced entity. Even if a
reference is theoretically in scope, and within its defined lifetime,
it may have become unreachable since it was sent. A receiving entity
should always be prepared for reachability failures and associated
retry and failover mechanisms, which are out of scope for the referral
mechanism itself.</t>
</section>
<section title="Path Selection">
<t>A reference might carry multiple references for the same target.
These may lead to multiple possible paths from the receiving entity to
the referenced entity. This scenario is particularly generic when the
destination or/and source entity has multiple interfaces or is
multi-homed.</t>
<t>The referring entity is not likely to know which path is best. The
receiving entity will need to make a choice, possibly by local policy
(e.g. <xref target="RFC3484"></xref>) or possibly by trial and error
(e.g. <xref target="RFC4038"></xref>, <xref target="RFC5534"></xref>).
This choice is also out of scope for the referral mechanism
itself.</t>
<section title="An Example: Triangle Path Optimization">
<t>In application scenarios, the triangular path shown below is
common. Both Host A and Host B connect to an application server and
the application server forwards traffic as a relay agent. A slightly
more complicated scenario is when the two hosts connect to different
application servers individually and application servers talk to
each other's relay agents. In SIP, this is often called the "SIP
trapezoid".</t>
<t><artwork><![CDATA[
+------------------------------+
| application server |
+--+------------------------+--+
/ \
| |
| referral information |
| |
| |
+-+-+ +-+-+
| A +----------------------+ B |
+---+ direct communication +---+]]></artwork></t>
<t>By passing A's reference to B, B can try to communicate directly
with A, using the communication line at the bottom. If the direct
communication is established successfully, the triangle path gets
optimized. Both the application server and network bandwidth can be
benefit from this operation.</t>
</section>
</section>
<section title="Interface Selection">
<t>We also encounter multi-interfaced hosts whose reachability is
bound to a particular (logical/physical) interface. More information
is required to indicate which interface may be used under different
circumstances. The multi-interface problem is defined and studied by the IETF MIF WG.
Here referral can provide host A's multi-interface information to
host B; accordingly, host B can select one of the interfaces to
establish a connection.</t>
<t><artwork><![CDATA[
+------------+ Path 1 +------------+
|Interface A1+----------------+Interface B1|
| Host A | | Host B |
|Interface A2|----------------+Interface B2|
+------------+ Path 2 +-----------+]]></artwork></t>
<t>For example, as shown in the above figure, Host A has connected to
Host B through Path 1. They can exchange references through Path 1.
They may disciver that Path 2, using different interfaces, is better than
Path 1, maybe cheaper, faster or more stable. Then, they can switch to
Path 2. For example, Host A has interface A1 as broadband access, almost free; and
interface A2 is 3G access, which costs 0.1 $ per MB. Both of them are
avaible for incoming connections. If this information is passed to
host B, through referral, then host B should choose the A1 interface to
reach host A. Such information is useful to express a host's
status or preference.</t>
<t>In order to choose between different interfaces, not only the
connectivity information of these interfaces, but also some additonal
information may be helpful, such as bandwidth, financial cost, latency,
etc. This additional information may also be provided through
referral. However, this additional information, even if accurate when sent by the
referring entity, may nevertheless be invalid at the location of the receiving
entity.</t>
</section>
</section>
<section anchor="ps" title="Problem Statement">
<t>Unfortunately, the simple approach to referrals, passing an IP
address, often fails in today's Internet. As has been known for some
time <xref target="RFC2101"></xref>, hosts' IP addresses no longer all
have global scope. They often have limited reachability, and may have
limited lifetime. They are not sufficient to establish communication in
many cases of dynamic referrals, for a variety of reasons. FQDNs may be
used instead in some scenarios. However, FQDNs also have their own
limitations and may fail in some scenarios.</t>
<section anchor="ip" title="IP Addresses are not sufficient">
<t>It is no longer reasonable to assume that a host with a fixed
location has a fixed IP address, or even a stable IP address.</t>
<t>Furthermore, in the context of IPv4 address exhaustion, several
solutions have emerged to share a single public IPv4 address between
several customers simultaneously. Consequently, a public IPv4 address
often no longer identifies a single customer/user/host, while a private
IPv4 address is meaningless out of the private network scope. Other
information (e.g., port range) is required to identify unambiguously a
given customer/user/host. Both IP addresses and port numbers may be
different on either side of a NAT or some other middlebox <xref
target="RFC3234"></xref>, and firewalls may block them. It is no
longer reasonable to assume that an IP address for a host, which
allows a given peer to reach that host in one location, also works
from a different location - even if that host is reachable from the
second location.</t>
<t>Also, the Internet now has two co-existing address formats for IPv4
and IPv6. Direct communication can only be established when both peers
use the same IP version. Having the address of the source and
destination in the same IP version does not necessarily mean that the
path will be using that IP version. Simple approaches may cause
unnecessary double translation <xref
target="I-D.boucadair-softwire-cgn-bypass"></xref>. Some addresses may
even be the result of translation between IPv4 and IPv6, with severe
limitations on their scope and lifetime. Sending an out-of-scope or
expired address, or one of the wrong format, as a referral will
fail.</t>
<t>A specific problem of this kind may be caused by NAT64
<xref target="I-D.ietf-behave-v6v4-xlate-stateful"/>. If an IPv6-only
host behind a NAT64 obtains a synthetic IPv6 address for an IPv4-only
host, it can communicate successfully via the NAT64. However, if
the synthetic address is referred to another IPv6 host, it may or may
not work correctly. We can consider four cases: </t>
<t><list style="numbers">
<t>If the receiving entity is behind the same NAT64 as the
referring entity, all should be well. </t>
<t>If the referring entity and the receiving entity are
behind different NAT64 devices, both using the defined Well Known Prefix <xref target="RFC6052"/>,
all should be well, because the same synthetic address will work in both cases. </t>
<t>If the receiving entity is behind a different NAT64 that uses a
Network Specific Prefix <xref target="RFC6052"/>, the synthetic address will
be meaningless and communication will fail. The only way to avoid this
failure is for the original NAT64's Network Specific Prefix to be globally reachable,
which seems highly unlikely for operational and security reasons. </t>
<t>If the receiving entity is a dual stack node that is not behind
a NAT64, the synthetic address will be meaningless. Although
there is an IPv4 path to the target host, the receiving entity
will not know how to find it. Again, the only way to avoid this
failure is for the original NAT64's prefix to be globally reachable. </t>
</list></t>
<t>In the last two cases, even if connectivity failure is avoided, the path
taken by the packets will be far from optimal, traversing the original NAT64. </t>
<t>IP addresses today may have an implied "context" (VPN, VoIP VC, IP
TV, etc.): the reachability of such an address depends on that
context.</t>
<t>An implication of these issues is that there is no clean definition
of the scope of an address (especially an IPv4 address, due to the
prevalence of NAT). It is impossible to determine algorithmically, by
inspecting the bits of an address, what its scope of reachability is.
Resolving this problem would greatly clarify the general problem of
referrals.</t>
</section>
<section anchor="fqdn" title="FQDNs are not sufficient">
<t>In some cases, this problem may be readily solved by passing a
Fully Qualified Domain Name (FQDN) instead of an IP address. Indeed,
that is an architecturally preferred solution <xref
target="RFC1958" />. However, it is not sufficient in many cases of
dynamic referrals. Experience shows that an application cannot use a
domain name in order to reliably find usable address(es) of an
arbitrary peer. Domain names work fairly well to find the addresses of
public servers, as in web servers or SMTP servers, because operators
of such servers take pains to make sure that their domain names work.
But DNS records are not as reliably maintained for arbitrary hosts
such as might need to be contacted in peer-to-peer applications, or
for servers within corporate networks. Many small networks do not even
maintain DNS entries for their hosts, and for some networks that do
list local hosts in DNS, the listings may well be unusable from a
remote location, because of two-faced DNS, or because the A record
contains a private address. These cases may even be intentional as
part of a security ring-fence, where w3.example.com only resolves
within the corporate boundary, and/or resolves to IP addresses which
are only reachable within the corporate administrative boundaries. In
such contexts, incoming connections are usually filtered by the
corporate firewall.</t>
<t>An additional issue with FQDNs is the very common situation where
multiple hosts are hidden behind a NAT, but they share one FQDN which
is in fact a dummy name, created automatically by the ISP so that
reverse DNS lookup will succeed for the NAT's public IPv4 address.
Such FQDNs are useless for identifying specific hosts.</t>
<t>Furthermore, an FQDN may not be sufficient to establish successful
communications involving heterogeneous peers (i.e., IPv4 and IPv6)
since A and AAAA records may not be consistently provisioned. There
are known cases where a server has one name that produces an A record
(e.g., www.example.com) and another name that produces an AAAA record
(e.g., ipv6.example.com). An additional complication is that some
answers from DNS may be synthetic IP addresses, e.g., AAAA records
sent by DNS64. The host may have no means to detect that such an
address represents an IPv4 host. These addresses should not be
interpreted as native IPv6 address.</t>
<t>In such cases, an IP address either cannot be derived from an FQDN,
or if so derived, cannot be accessed from an arbitrary location in the
Internet.</t>
<t>A related problem is that an application does not have a reliable
way of knowing its own domain name - or to be more precise, a way of
knowing a domain name that will allow the application to be reached
from another location.</t>
<t>There are wider systemic problems with the DNS as a reliable way to
find a usable address, which are somewhat out of scope here, but can
be summarised:</t>
<list style="symbols">
<t>In large networks, it is now quite common that the DNS
administrator is out of touch with the applications user or
administrator, and as a result, that the DNS is out of sync with
reality.</t>
<t>DNS was never designed to accommodate mobile or roaming hosts,
whose locator may change rapidly.</t>
<t>DNS has never been satisfactorily adapted to isolated,
transiently-connected, or ad hoc networks.</t>
<t>It is no longer reasonable to assume that all addresses
associated with a DNS name are bound to a single host. One result is
that the DNS name might suffice for an initial connection, but a
specific address is needed to rebind to the same peer, say, to
recover from a broken connection.</t>
<t>It is no longer reasonable to assume that a DNS query will return
all usable addresses for a host.</t>
<t>Hosts may be identified by a different URI per service: no unique
URI scheme, meaning no single FQDN, will apply.</t>
</list>
<t>For all the above reasons, the problem of address referrals cannot
be solved simply by recommending the use of FQDNs instead. The
guideline in <xref target="RFC1958" /> is in fact too simple for
today's network. Something more elaborate than an IP address or an
FQDN appears to be needed in the general case of application
referrals.</t>
</section>
<section anchor="Information" title="Relevant Information is lacking">
<t>Neither an IP address nor an FQDN gives complete information about
the referenced entity. For example, IP addresses normally have
associated lifetimes (derived from DHCP, SLAAC or the relevant DNS
TTL), so they should be treated as invalid after their lifetimes
expire. A referral that does not convey the lifetime associated with
an address is problematic. As mentioned above, the scope of a
reference also affects its usefulness. These are examples of
additional information that is necessary to correctly interpret a
referral; therefore part of the problem is conveying such information
along with the reference.</t>
</section>
<section anchor="IDLocator"
title="Extra complexity from ID-Locator Split Mechanisms">
<t>Additional complexity for referrals would come from the deployment
of any technology that separates locators from identifiers, rather
than combining the two as an IP address. Since a very wide range of
such solutions have been proposed (e.g. HIP, LISP, ILNP and Name-based
Sockets) <xref target="I-D.ubillos-name-based-sockets"></xref>, it is
difficult to define the resulting problems precisely.</t>
<t>However, to consider the example of Name-based Sockets, if a
referral was made based on the IP address being used at a given
instant for a Name-based Socket, that address might be useless by the
time the referral was completed, because the socket suddenly migrated
to a different IP address.</t>
<t>The SHIM6 protocol <xref target="RFC5533"> </xref> and the Multiple
Interfaces (MIF) Working Group may produce similar difficulties, since
they also consider scenarios where the IP address in use for some
purpose may change unexpectedly.</t>
<t>Any referral mechanism must be able to deal with situations where
the locator corresponding to a given identifier is subject to
change.</t>
</section>
</section>
<section anchor="motivation"
title="A Generic Referral Mechanism is needed">
<t>The first motivation is the observation that unless the parties
involved have reached an understanding about the scope, lifetime, and
format of the elements in a referral through some other means, that
information must be passed with the referral. This is required so that
the receiving entity can determine whether or not the referral is
useful. The referral therefore needs to consist of a fully-fledged data
structure, or to be made using a mutually agreed referral protocol.</t>
<t>When an attempt to establish a communication channel based on certain
referral information fails, good design suggests that the receiving
entity should attempt to correct the situation. For example, if
communication fails to be established using an IP address, it would
often be appropriate to attempt a DNS lookup, despite the difficulties
mentioned above. The second motivating problem is that it may be helpful
to the entity receiving a reference to also receive information about
the source of the reference, such as an FQDN, if that is known to the
sender of the reference. The receiving entity can then attempt to
recover a valid address (and possibly port number) for the referred
entity.</t>
<t>The third motivating problem is to allow a reference to contain
alternatives to an IP address or an FQDN, when any such alternatives
exist.</t>
<t>Additional arguments for a generic referral mechanism include:</t>
<list style="numbers">
<t>Allow for general mechanisms that can be used by any application to
handle references and understand the meaning of referral information,
such as IP address, possibly protocol and port numbers. However, there
is an open question whether this standard referral design should be
used for new applications only, or extended to existing
applications.</t>
<t>Simplify ALG design during middlebox traversal. There are
middleboxes, like firewalls and translators, especially in the mobile
network, which require application layer gateways ALG. The cost of ALG
functions is huge for the mobile operator in terms of implementation,
performance. Standard references could simplify ALG implementation
during middlebox traversal in the mobile network.</t>
<t>Simplify packet inspection. Operators sometimes need to inspect
information or details during communication for administration
reasons. If referral mechanism is standardized, it is easier for an
operator to capture and investigate the required information.</t>
</list>
<t>We observe that we have identified two general requirements: the need
to define address scope more precisely, and the need to communicate
references in a generic way.</t>
<t>It should be noted that partial or application-specific solutions to
these problems abound, because any multi-party distributed application
must solve them. The best documented example is ICE <xref
target="RFC5245" />, which is an active protocol specific to
applications mediated by SDP <xref target="RFC4566" />. ICE "works by
including a multiplicity of IP addresses and ports in SDP offers and
answers, which are then tested for connectivity by peer-to-peer
connectivity checks." The question raised here is whether we can define
requirements for a generic solution that can be used by future
applications, and possibly be retro-fitted to existing applications.</t>
<t>One approach could be a "SuperICE" designed to be completely general
and not tied to the SDP model. Another approach is the idea of a generic
referral object. Such an object could be passed between the entities of
a multi-party application, but without defining a specific protocol for
that purpose. Some applications might choose to send it in-band as a raw
binary object, others might use a simple ASCII encoding, and still
others might prefer to encode it in XML, for example. Finally, it might
also be used as part of SuperICE.</t>
</section>
<section anchor="security" title="Security Considerations">
<t>It should be noted that referral should not function as a way to
nullify the effect of a firewall or any other security mechanism. If the
receiving entity chooses a particular reference and attempts to send
packets to the corresponding IP address, whether they are delivered or
not will depend on the existing security mechanisms, whatever they may
be.</t>
<t>Nevertheless, if a site security policy requires it, certain
references may be excluded from referral information sent to certain
destinations. This would require a security policy mechanism to be added
to the process of generating referral information.</t>
<t>Forged or intercepted referral information would enable a wide
variety of attacks. Although not fundamentally different from attacks
based on forged or observed IP addresses or FQDNs, no doubt referral
would allow such attacks to be more ingenious, simply because they
provide more information than an address or FQDN alone. Referral
information should be transmitted through authenticated and encrypted
channels. It is not further elaborated here.</t>
<t>Referral may raise potential privacy issues, which are not explored
in this document. For example, in the SIP context, mechanisms such as
<xref target="RFC3323"></xref> and <xref target="RFC5767"></xref> are
available to hide information that might identify end-points. Referral
usage scenarios must ensure that they do not unintentionally defeat
privacy solutions.</t>
</section>
<!-- security -->
<section anchor="iana" title="IANA Considerations">
<t>This document requests no action by IANA.</t>
</section>
<!-- iana -->
<section anchor="ack" title="Acknowledgements">
<t>Bo Zhou, formerly with China Mobile, was an original author of this document.
His contributions are gratefully acknowledged. </t>
<t>Valuable comments and contributions were made by Mohamed Boucadair,
Dan Wing, Keith Moore and others.</t>
<t>This document was produced using the xml2rfc tool <xref
target="RFC2629"></xref>.</t>
</section>
<!-- ack -->
<section anchor="changes" title="Change log">
<t>draft-carpenter-referral-ps-00: original version, 2010-06-21.</t>
<t>draft-carpenter-referral-ps-01: add content regarding to ID-Locator
Split Mechanisms, 2010-08-30.</t>
<t>draft-carpenter-referral-ps-02: add content regarding NAT64, 2011-02-23.</t>
</section>
<!-- changes -->
</middle>
<back>
<references title="Informative References">
<?rfc include="reference.RFC.5245"?>
<?rfc include="reference.RFC.5534"?>
<?rfc include="reference.RFC.5533"?>
<?rfc include="reference.RFC.3323"?>
<?rfc include="reference.RFC.4566"?>
<?rfc include="reference.RFC.3484"?>
<?rfc include="reference.RFC.0959"?>
<?rfc include="reference.RFC.2629"?>
<?rfc include="reference.RFC.2101"?>
<?rfc include="reference.RFC.2775"?>
-->
<?rfc include="reference.RFC.1958"?>
<?rfc include="reference.RFC.3234"?>
<?rfc include="reference.RFC.4038"?>
<?rfc include="reference.RFC.5767"?>
<?rfc include="reference.RFC.6052"?>
<?rfc include="reference.I-D.boucadair-softwire-cgn-bypass"?>
<?rfc include="reference.I-D.ubillos-name-based-sockets"?>
<?rfc include="reference.I-D.ietf-behave-v6v4-xlate-stateful"?>
</references>
</back>
</rfc>
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