Network Working Group A. Barbir
Request for Comments: 3897 Nortel Networks
Category: Informational September 2004
Open Pluggable Edge Services (OPES) Entities
and End Points Communication
Status of this Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2004).
Abstract
This memo documents tracing and non-blocking (bypass) requirements
for Open Pluggable Edge Services (OPES).
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . 2
2. OPES System . . . . . . . . . . . . . . . . . . . . . . . . . 2
3. Tracing Requirements . . . . . . . . . . . . . . . . . . . . . 3
3.1. Traceable entities . . . . . . . . . . . . . . . . . . . 3
3.2. System requirements . . . . . . . . . . . . . . . . . . 5
3.3. Processor requirements . . . . . . . . . . . . . . . . . 5
3.4. Callout server requirements . . . . . . . . . . . . . . 5
4. Bypass (Non-blocking feature) Requirements . . . . . . . . . . 6
4.1. Bypassable entities . . . . . . . . . . . . . . . . . . 7
4.2. System requirements . . . . . . . . . . . . . . . . . . 8
4.3. Processor requirements . . . . . . . . . . . . . . . . . 8
4.4. Callout server requirements . . . . . . . . . . . . . . 9
5. Protocol Binding . . . . . . . . . . . . . . . . . . . . . . . 9
6. Compliance Considerations . . . . . . . . . . . . . . . . . . 9
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
8. Security Considerations . . . . . . . . . . . . . . . . . . . 10
8.1. Tracing security considerations . . . . . . . . . . . . 10
8.2. Bypass security considerations . . . . . . . . . . . . . 11
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 12
9.1. Normative References . . . . . . . . . . . . . . . . . . 12
9.2. Informative References . . . . . . . . . . . . . . . . . 13
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 13
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11. Author's Address . . . . . . . . . . . . . . . . . . . . . . . 13
12. Full Copyright Statement . . . . . . . . . . . . . . . . . . . 14
1. Introduction
The Open Pluggable Edge Services (OPES) architecture [1] enables
cooperative application services (OPES services) between a data
provider, a data consumer, and zero or more OPES processors. The
application services under consideration analyze and possibly
transform application-level messages exchanged between the data
provider and the data consumer.
This work specifies OPES tracing and bypass functionality. The
architecture document [1] requires that tracing is supported in-band.
This design goal limits the type of application protocols that OPES
can support. The details of what a trace record can convey are also
dependent on the choice of the application level protocol. For these
reasons, this work only documents requirements for OPES entities that
are needed to support traces and bypass functionality. The task of
encoding tracing and bypass features is application protocol
specific. Separate documents will address HTTP and other protocols.
The architecture does not prevent implementers from developing out-
of-band protocols and techniques to address tracing and bypass. Such
protocols are out of scope of the current work.
1.1. Terminology
The keywords "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in BCP 14, RFC 2119 [2].
When used with the normative meanings, these keywords will be all
uppercase. Occurrences of these words in lowercase comprise normal
prose usage, with no normative implications.
2. OPES System
This section provides a definition of OPES System. This is needed in
order to define what is traceable (or bypassable) in an OPES Flow.
Definition: An OPES System is a set of all OPES entities authorized
by either the data provider or the data consumer application to
process a given application message.
The nature of the authorization agreement determines if authority
delegation is transitive (meaning an authorized entity is authorized
to include other entities).
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If specific authority agreements allow for re-delegation, an OPES
system can be formed by induction. In this case, an OPES system
starts with entities directly authorized by a data provider (or a
data consumer) application. The OPES system then includes any OPES
entity authorized by an entity that is already in the OPES system.
The authority delegation is always viewed in the context of a given
application message.
An OPES System is defined on an application message basis. Having an
authority to process a message does not imply being involved in
message processing. Thus, some OPES system members may not
participate in processing of a message. Similarly, some members may
process the same message several times.
The above definition implies that there can be no more than two OPES
systems [Client-side and server-side OPES systems can process the
same message at the same time] processing the same message at a given
time. This is based on the assumption that there is a single data
provider and a single data consumer as far as a given application
message is concerned.
For example, consider a Content Delivery Network (CDN) delivering an
image on behalf of a busy web site. OPES processors and services,
which the CDN uses to adapt and deliver the image, comprise an OPES
System. In a more complex example, an OPES System would contain
third party OPES entities that the CDN engages to perform adaptations
(e.g., to adjust image quality).
3. Tracing Requirements
The definition of OPES trace and tracing are given next.
OPES trace: application message information about OPES entities
that adapted the message.
OPES tracing: the process of creating, manipulating, or
interpreting an OPES trace.
Note that the above trace definition assumes in-band tracing. This
dependency can be removed if desired. Tracing is performed on per
message basis. Trace format is dependent on the application protocol
that is being adapted. A traceable entity can appear multiple times
in a trace (for example, every time it acts on a message).
3.1. Traceable entities
This section focuses on identifying traceable entities in an OPES
Flow.
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Tracing information provides an "end" with information about OPES
entities that adapted the data. There are two distinct uses of OPES
traces. First, a trace enables an "end" to detect the presence of
OPES System. Such "end" should be able to see a trace entry, but
does not need to be able to interpret it beyond identification of the
OPES System and location of certain required OPES-related disclosures
(see Section 3.2).
Second, the OPES System administrator is expected to be able to
interpret the contents of an OPES trace. The trace can be relayed to
the administrator by an "end" without interpretation, as opaque data
(e.g., a TCP packet or an HTTP message snapshot). The administrator
can use the trace information to identify the participating OPES
entities. The administrator can use the trace to identify the
applied adaptation services along with other message-specific
information.
Since the administrators of various OPES Systems can have various
ways of looking into tracing, they require the freedom in what to put
in trace records and how to format them.
At the implementation level, for a given trace, an OPES entity
involved in handling the corresponding application message is
traceable or traced if information about it appears in that trace.
This work does not specify any order to that information. The order
of information in a trace can be OPES System specific or can be
defined by application bindings documents.
OPES entities have different levels of traceability requirements.
Specifically,
o An OPES System MUST add its entry to the trace.
o An OPES processor SHOULD add its entry to the trace.
o An OPES service MAY add its entry to the trace.
o An OPES entity MAY delegate addition of its trace entry to another
OPES entity. For example, an OPES System can have a dedicated
OPES processor for adding System entries; an OPES processor can
use a callout service to manage all OPES trace manipulations
(since such manipulations are OPES adaptations).
In an OPES context, a good tracing approach is similar to a trouble
ticket ready for submission to a known address. The address is
printed on the ticket. The trace in itself is not necessarily a
detailed description of what has happened. It is the responsibility
of the operator to decode trace details and to resolve the problems.
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3.2 System requirements
The following requirements document actions when forming an OPES
System trace entry:
o OPES system MUST include its unique identification in its trace
entry. Here, uniqueness scope is all OPES Systems that may adapt
the message being traced.
o An OPES System MUST define its impact on inter- and intra-document
reference validity.
o An OPES System MUST include information about its privacy policy,
including identity of the party responsible for setting and
enforcing the policy.
o An OPES System SHOULD include information that identifies, to the
technical contact, the OPES processors involved in processing the
message.
o When providing required information, an OPES System MAY use a
single URI to identify a resource containing several required
items. For example, an OPES System can point to a single web page
with a reference to System privacy policy and technical contact
information.
This specification does not define the meaning of the terms privacy
policy, policy enforcement, or reference validity or technical
contact and contains no requirements regarding encoding, language,
format, or any other aspects of that information. For example, a URI
used for an OPES System trace entry may look like "http://
www.examplecompany.com/opes/?client=example.com" where the identified
web page is dynamically generated and contains the all OPES System
information required above.
3.3. Processor requirements
The following requirements document actions when forming an OPES
System trace entry:
o OPES processor SHOULD add its unique identification to the trace.
Here, uniqueness scope is the OPES System containing the
processor.
3.4. Callout server requirements
In an OPES system, it is the task of an OPES processor to add trace
records to application messages. The OPES System administrator
decides if and under what conditions callout servers may add trace
information to application messages.
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4. Bypass (Non-blocking feature) Requirements
IAB recommendation (3.3) [6] requires that the OPES architecture does
not prevent a data consumer application from retrieving non-OPES
version of content from a data provider application, provided that
the non-OPES content exists. IAB recommendation (3.3) suggests that
the Non-blocking feature (bypass) be used to bypass faulty OPES
intermediaries (once they have been identified, by some method).
In addressing IAB consideration (3.3), one need to specify what
constitutes non-OPES content. In this work the definition of "non-
OPES" content is provider dependent. In some cases, the availability
of "non-OPES" content can be a function of the internal policy of a
given organization that has contracted the services of an OPES
provider. For example, Company A has as contract with an OPES
provider to perform virus checking on all e-mail attachments. An
employee X of Company A can issue a non-blocking request for the
virus scanning service. The request could be ignored by the OPES
provider since it contradicts its agreement with Company A.
The availability of non-OPES content can be a function of content
providers (or consumers or both) policy and deployment scenarios [5].
For this reason, this work does not attempt to define what is an OPES
content as opposed to non-OPES content. The meaning of OPES versus
non-OPES content is assumed to be determined through various
agreements between the OPES provider, data provider and/or data
consumer. The agreement determines what OPES services can be
bypassed and in what order (if applicable).
This specification documents bypassing of an OPES service or a group
of services identified by a URI. In this context, to "bypass the
service" for a given application message in an OPES Flow means to
"not invoke the service" for that application message. A bypass URI
that identifies an OPES system (processor) matches all services
attached to that OPES system (processor). However, bypassing of OPES
processors and OPES Systems themselves requires non-OPES mechanisms
and is out of this specification scope. A bypass request an
instruction to bypass, usually embedded in an application message.
The current specification does not provide for a good mechanism that
allow and "end" to specify to "bypass this service but only if it is
a part of that OPES system" or "bypass all services of that OPES
system but not of this OPES system". Furthermore, if an OPES
processor does not know for sure that a bypass URI does not match its
service, it must bypass that service.
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If no non-OPES content is available without the specified service,
the bypass request for that service must be ignored. This design
implies that it may not be possible to detect non-OPES content
existence or to detect violations of bypass rules in the environments
where the tester does not know whether non-OPES content exists. This
design assumes that most bypass requests are intended for situations
where serving undesirable OPES content is better than serving an
error message that no preferred non-OPES content exists.
Bypass feature is to malfunctioning OPES services as HTTP "reload"
request is to malfunctioning HTTP caches. The primary purpose of the
bypass is to get usable content in the presence of service failures
and not to provide the content consumer with more information on what
is going on. OPES trace should be used for the latter instead.
While this work defines a "bypass service if possible" feature, there
are other related bypass features that can be implemented in OPES
and/or in application protocols being adapted. For example, a
"bypass service or generate an error" or "bypass OPES entity or
generate an error". Such services would be useful for debugging
broken OPES systems and may be defined in other OPES specifications.
This work concentrates on documenting a user-level bypass feature
addressing direct IAB concerns.
4.1. Bypassable entities
In this work, the focus is on developing a bypass feature that allows
a user to instruct the OPES System to bypass some or all of its
services. The collection of OPES services that can be bypassed is a
function of the agreement of the OPES provider with either (or both)
the content provider or the content consumer applications. In the
general case, a bypass request is viewed as a bypass instruction that
contains a URI that identifies an OPES entity or a group of OPES
entities that perform a service (or services) to be bypassed. An
instruction may contain more than one such URI. A special wildcard
identifier can be used to represent all possible URIs.
In an OPES Flow, a bypass request is processed by each involved OPES
processor. This means that an OPES processor examines the bypass
instruction and if non-OPES content is available, the processor then
bypasses the indicated services. The request is then forwarded to
the next OPES processor in the OPES Flow. The next OPES processor
would then handle all bypass requests, regardless of the previous
processor actions. The processing chain continues throughout the
whole processors that are involved in the OPES Flow.
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4.2. System requirements
In an OPES System, bypass requests are generally client centric
(originated by the data consumer application) and go in the opposite
direction of tracing requests. This work requires that the bypass
feature be performed in-band as an extension to an application
specific protocol. Non-OPES entities should be able to safely ignore
these extensions. The work does not prevent OPES Systems from
developing their own out of band protocols.
The following requirements apply for bypass feature as related to an
OPES System (the availability of a non-OPES content is a
precondition):
o An OPES System MUST support a bypass feature. This means that the
OPES System bypasses services whose URIs are identified by an OPES
"end".
o An OPES System MUST provide OPES version of the content if non-
OPES version is not available.
In order to facilitate the debugging (or data consumer user
experience) of the bypass feature in an OPES System, it would be
beneficial if non-bypassed entities included information related to
why they ignored the bypass instruction. It is important to note
that in some cases the tracing facility itself may be broken and the
whole OPES System (or part) may need to be bypassed through the issue
of a bypass instruction.
4.3. Processor requirements
Bypass requirements for OPES processors are (the availability of a
non-OPES content is a precondition):
o OPES processor SHOULD be able to interpret and process a bypass
instruction. This requirement applies to all bypass instructions,
including those that identify unknown-to-recipient services.
o OPES processors MUST forward bypass request to the next
application hop provided that the next hop speaks application
protocol with OPES bypass support.
o OPES processor SHOULD be able to bypass it's service(s) execution.
OPES processors that know how to process and interpret a bypass
instruction have the following requirements:
o The recipient of a bypass instruction with a URI that does not
identify any known-to-recipient OPES entity MUST treat that URI as
a wildcard identifier (meaning bypass all applicable services).
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4.4. Callout server requirements
In an OPES system, it is the task of an OPES processor to process
bypass requests. The OPES System administrator decides if and under
what conditions callout servers process bypass requests.
5. Protocol Binding
The task of encoding tracing and bypass features is application
protocol specific. Separate documents will address HTTP and other
protocols. These documents must address the ordering of trace
information as needed.
6. Compliance Considerations
This specification defines compliance for the following compliance
subjects: OPES System, processors, entities and callout servers.
A compliance subject is compliant if it satisfies all applicable
"MUST" and "SHOULD" level requirements. By definition, to satisfy a
"MUST" level requirement means to act as prescribed by the
requirement; to satisfy a "SHOULD" level requirement means to either
act as prescribed by the requirement or have a reason to act
differently. A requirement is applicable to the subject if it
instructs (addresses) the subject.
Informally, compliance with this document means that there are no
known "MUST" violations, and all "SHOULD" violations are conscious.
In other words, a "SHOULD" means "MUST satisfy or MUST have a reason
to violate". It is expected that compliance claims are accompanied
by a list of unsupported SHOULDs (if any), in an appropriate format,
explaining why preferred behavior was not chosen.
Only normative parts of this specification affect compliance.
Normative parts are: parts explicitly marked using the word
"normative", definitions, and phrases containing unquoted capitalized
keywords from RFC 2119 [2]. Consequently, examples and illustrations
are not normative.
7. IANA Considerations
This specification contains no IANA considerations. Application
bindings MAY contain application-specific IANA considerations.
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8. Security Considerations
Security considerations for OPES are documented in [4]. Policy and
authorization issues are documented in [3]. It is recommended that
designers consult these documents before reading this section.
This document is a requirement document for tracing and bypass
feature. The requirements that are stated in this document can be
used to extend an application level protocol to support these
features. As such, the work has security precautions.
8.1. Tracing security considerations
The tracing facility for OPES architecture is implemented as a
protocol extension. Inadequate implementations of the tracing
facility may defeat safeguards built into the OPES architecture. The
tracing facility by itself can become a target of malicious attacks
or used to lunch attacks on an OPES System.
Threats caused by or against the tracing facility can be viewed as
threats at the application level in an OPES Flow. In this case, the
threats can affect the data consumer and the data provider
application.
Since tracing information is a protocol extension, these traces can
be injected in the data flow by non-OPES entities. In this case,
there are risks that non-OPES entities can be compromised in a
fashion that threat the overall integrity and effectiveness of an
OPES System. For example, a non-OPES proxy can add fake tracing
information into a trace. This can be done in the form of wrong, or
unwanted, or non existent services. A non-OPES entity can inject
large size traces that may cause buffer overflow in a data consumer
application. The same threats can arise from compromised OPES
entities. An attacker can control an OPES entity and inject wrong,
or very large trace information that can overwhelm an end or the next
OPES entity in an OPES flow. Similar threats can result from bad
implementations of the tracing facility in trusted OPES entities.
Compromised tracing information can be used to launch attacks on an
OPES System that give the impression that unwanted content
transformation was performed on the data. This can be achieved by
inserting wrong entity (such OPES processor) identifiers. A
compromised trace can affect the overall message integrity structure.
This can affect entities that use message header information to
perform services such as accounting, load balancing, or reference-
based services.
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Compromised trace information can be used to launch DoS attacks that
can overwhelm a data consumer application or an OPES entity in an
OPES Flow. Inserting wrong tracing information can complicate the
debugging tasks performed by system administrator during trouble
shooting of OPES System behavior.
As a precaution, OPES entities ought to be capable of verifying that
the inserted traces are performed by legal OPES entities. This can
be done as part of the authorization and authentication face. Policy
can be used to indicate what trace information can be expected from a
peer entity. Other application level related security concerns can
be found in [4].
8.2. Bypass security considerations
The bypass facility for OPES architecture is implemented as a
protocol extension. Inadequate implementations of the bypass
facility may defeat safeguards built into the OPES architecture. The
bypass facility by itself can become a target of malicious attacks or
used to lunch attacks on an OPES System.
Threats caused by or against the bypass facility can be viewed as
threats at the application level in an OPES Flow. In this case, the
threats can affect the data consumer and the data provider
application.
There are risks for the OPES System by non-OPES entities, whereby,
these entities can insert bypass instructions into the OPES Flow.
The threat can come from compromised non-OPES entities. The threat
might affect the overall integrity and effectiveness of an OPES
System. For example, a non-OPES proxy can add bypass instruction to
bypass legitimate OPES entities. The attack might result in
overwhelming the original content provider servers, since the attack
essentially bypass any load balancing techniques. In addition, such
an attack is also equivalent to a DoS attack, whereby, a legitimate
data consumer application may not be able to access some content from
a content provider or its OPES version.
Since an OPES Flow may include non-OPES entities, it is susceptible
to man-in-the-middle attacks, whereby an intruder may inject bypass
instructions into the data path. These attacks may affect content
availability or disturb load balancing techniques in the network.
The above threats can also arise by compromised OPES entities. An
intruder can compromise an OPES entities and then use man-in-the-
middle techniques to disturb content availability to a data consumer
application or overload a content provider server (essentially, some
form of a DoS attack).
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Attackers can use the bypass instruction to affect the overall
integrity of the OPES System. The ability to introduce bypass
instructions into a data flow may effect the accounting of the OPES
System. It may also affect the quality of content that is delivered
to the data consumer applications. Similar threats can arise from
bad implementations of the bypass facility.
Inconsistent or selective bypass is also a threat. Here, one end can
try to bypass a subset of OPES entities so that the resulting content
is malformed and crashes or compromises entities that process that
content (and expect that content to be complete and valid). Such
exceptions are often not tested because implementers do not expect a
vital service to disappear from the processing loop.
Other threats can arise from configuring access control policies for
OPES entities. It is possible that systems implementing access
controls via OPES entities may be incorrectly configured to honor
bypass and, hence, give unauthorized access to intruders.
Tap bypass can also be a threat. This is because systems
implementing wiretaps via OPES entities may be incorrectly configured
to honor bypass and, hence, ignore (leave undetected) traffic with
bypass instructions that should have been tapped or logged. It is
also possible for one end to bypass services such as virus scanning
at the receiving end. This threat can be used by hackers to inject
viruses throughout the network. Following an IETF policy on
Wiretapping [7], OPES communication model does not consider
wiretapping requirements. Nevertheless, the documented threat is
real, not obvious, and OPES technology users operating in wiretapping
or similar logging environments should be aware of it.
Other application level related security concerns can be found in
[4].
9. References
9.1. Normative References
[1] Barbir, A., Penno, R., Chen, R., Hofmann, M., and H. Orman, "An
Architecture for Open Pluggable Edge Services (OPES)", RFC 3835,
August 2004.
[2] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
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[3] Barbir, A., Batuner, O., Beck, A., Chan, T., and H. Orman,
"Policy, Authorization, and Enforcement Requirements of Open
Pluggable Edge Services (OPES)", RFC 3838, August 2004.
[4] Barbir, A., Batuner, O., Srinivas, B., Hofmann, M., and H.
Orman, "Security Threats and Risks for Open Pluggable Edge
Services (OPES)", RFC 3837, August 2004.
9.2 Informative References
[5] Barbir A., Burger, E., Chen, R., McHenry, S., Orman, H., and R.
Penno, "Open Pluggable Edge Services (OPES) Use Cases and
Deployment Scenarios", RFC 3752, April 2004.
[6] Floyd, S. and L. Daigle, "IAB Architectural and Policy
Considerations for Open Pluggable Edge Services", RFC 3238,
January 2002.
[7] IAB and IESG, "IETF Policy on Wiretapping", RFC 2804, May 2000.
10. Acknowledgements
Several people has contributed to this work. Many thanks to: Alex
Rousskov, Hilarie Orman, Oscar Batuner, Markus Huffman, Martin
Stecher, Marshall Rose and Reinaldo Penno.
11. Author's Address
Abbie Barbir
Nortel Networks
3500 Carling Avenue
Nepean, Ontario K2H 8E9
Canada
Phone: +1 613 763 5229
EMail: abbieb@nortelnetworks.com
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12. Full Copyright Statement
Copyright (C) The Internet Society (2004).
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