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This document includes comments and recommendations by the IAB one architectural issues related to Open Pluggable Edge Services (OPES) from IAB RFC 3238, and how OPES addresses those considerations (RFC 3914).

RFC 3238

This document includes comments and recommendations by the IAB on some architectural and policy issues related to the chartering of Open Pluggable Edge Services (OPES) in the IETF. OPES are services that would be deployed at application-level intermediaries in the network, for example, at a web proxy cache between the origin server and the client. These intermediaries would transform or filter content, with the explicit consent of either the content provider or the end user.

1. Introduction

Open Pluggable Edge Services (OPES) are services that would be deployed in the network, for example, at a web proxy cache between the origin server and the client, that would transform or filter content. Examples of proposed OPES services include assembling personalized web pages, adding user-specific regional information to web pages, virus scanning, content adaptation for clients with limited bandwidth, language translation, and the like [OPES].

The question of chartering OPES in the IETF ([OPESBOF1], [OPESBOF2], [OPESBOF3]) and the related controversy in the IETF community ([Carr01], [CDT01], [Morris01], [Orman01], [Routson01]) have raised to the fore several architectural and policy issues about robustness and the end-to-end integrity of data (in terms of the disparities between what the "origin server" makes available and what the client receives). In particular, questions have been raised about the possible requirements, for a protocol to be developed and standardized in the IETF, for that protocol to protect the end-to-end privacy and integrity of data. This document attempts to address some of the architectural and policy issues that have been unresolved in the chartering of OPES, and to come to some common recommendations from the IAB regarding these issues.

The purpose of this document is not to recommend specific solutions for OPES, or even to mandate specific functional requirements. This is also not a recommendation to the IESG about whether or not OPES should be chartered. Instead, these are recommendations on issues that any OPES solutions standardized in the IETF should be required to address, similar to the "Security Considerations" currently required in IETF documents [RFC2316]. As an example, one way to address security issues is to show that appropriate security mechanisms have been provided in the protocol, and another way to address security issues is to demonstrate that no security issues apply to this particular protocol. (Note however that a blanket sentence that "no security issues are involved" is never considered sufficient to address security concerns in a protocol with known security issues.) This document will try to make our concerns underlying integrity, privacy, and security as clear as possible. We recommend that the IESG require that OPES documents address integrity, privacy, and security concerns in one way or another, either directly by demonstrating appropriate mechanisms, or by making a convincing case that there are no integrity or privacy concerns relevant to a particular document.

In particular, it seems unavoidable that at some point in the future some OPES service will perform inappropriately (e.g., a virus scanner rejecting content that does not include a virus), and some OPES intermediary will be compromised either inadvertently or with malicious intent. Given this, it seems necessary for the overall architecture to help protect end-to-end data integrity by addressing, from the beginning of the design process, the requirement of helping end hosts to detect and respond to inappropriate behavior by OPES intermediaries.

One of the goals of the OPES architecture must be to maintain the robustness long cited as one of the overriding goals of the Internet architecture [Clark88]. Given this, we recommend that the IESG require that the OPES architecture protect end-to-end data integrity by supporting end-host detection and response to inappropriate behavior by OPES intermediaries. We note that in this case by "supporting end-host detection", we are referring to supporting detection by the humans responsible for the end hosts at the content provider and client. We would note that many of these concerns about the ability of end hosts to detect and respond to the inappropriate behavior of intermediaries could be applied to the architectures for web caches and content distribution infrastructures even without the additional complication of OPES.

Each section of the document contains a set of IAB Considerations that we would recommend be addressed by the OPES architecture.

Section 6 summarizes by listing all of these considerations in one place.

In this document we try to use terminology consistent with RFC 3040 [RFC 3040] and with OPES works in progress.

2. Some history of the controversy about chartering OPES

One view on OPES has been that "OPES is deeply evil and the IETF should stay far, far away from this hideous abomination" [ODell01].

Others have suggested that "OPES would reduce both the integrity, and the perception of integrity, of communications over the Internet, and would significantly increase uncertainly about what might have been done to content as it moved through the network", and that therefore the risks of OPES outweigh the benefits [CDT01]. This view of the risks of OPES was revised in later email, based on the proposals from [Carr01], "assuming that certain privacy and integrity protections can be incorporated into the goals of the working group" [Morris01].

One issue concerns the one-party consent model. In the one-party consent model, one of the end-nodes (that is, either the content provider or the end user) is required to explicitly authorize the OPES service, but authorization is not required from both parties.

[CDT01] comments that relying only on a one-party consent model in the OPES charter "could facilitate third-party or state-sponsored censorship of Internet content without the knowledge or consent of end users", among other undesirable scenarios.

A natural first question is whether there is any architectural benefit to putting specific services inside the network (e.g., at the application-level web cache) instead of positioning all services either at the content provider or the end user. (Note that we are asking here whether there is architectural benefit, which is not the same as asking if there is a business model.) Client-centric services suggested for OPES include virus scanning, language translation, limited client bandwidth adaptation, request filtering, and adaptation of streaming media, and suggested server-centric services include location-based services and personalized web pages.

It seems clear that there can indeed be significant architectural benefit in providing some OPES services inside the network at the application-level OPES intermediary. For example, if some content is already available from a local or regional web cache, and the end user requires some transformation (such as adaptation to a limited- bandwidth path) applied to that data, providing that service at the web cache itself can prevent the wasted bandwidth of having to retrieve more data from the content provider, and at the same time avoid unnecessary delays in providing the service to the end user.

A second question is whether the architectural benefits of providing services in the middle of the network outweigh the architectural costs, such as the potential costs concerning data integrity. This is similar to the issues considered in RFC 3135 [RFC 3135] of the relative costs and benefits of placing performance-enhancing proxies (PEPs) in the middle of a network to address link-related degradations. In the case of PEPs, the potential costs include disabling the end-to-end use of IP layer security mechanisms; introducing a new possible point of failure that is not under the control of the end systems; adding increased difficulty in diagnosing and dealing with failures; and introducing possible complications with asymmetric routing or mobile hosts. RFC 3135 carefully considers these possible costs, the mitigations that can be introduced, and the cases when the benefits of performance-enhancing proxies to the user are likely to outweigh the costs. A similar approach could be applied to OPES services (though we do not attempt that here).

A third question is whether an OPES service, designed primarily for a single retrieval action, has an impact on the application layer addressing architecture. This is related to the integrity issue above, but is independent of whether these services are applied in the middle of the network or at either end.

Most of this document deals with the specific issue of data integrity with OPES services, including the goal of enabling end hosts to detect and respond to inappropriate behavior from broken or compromised OPES intermediaries.

We agree that one-party consent, with one of the end-hosts explicitly authorizing the OPES service, must be a requirement for OPES to be standardized in the IETF.

However, as we discuss in the next section of this document, we agree with [CDT01] that the one-party consent model by itself (e.g., with one of the end-hosts authorizing the OPES service, and the other end-host perhaps being unaware of the OPES service) is insufficient for protecting data integrity in the network. We also agree with [CDT01] that, regardless of the security and authorization mechanisms standardized for OPES in the IETF, OPES implementations could probably be modified to circumvent these mechanisms, resulting in the unauthorized modification of content. Many of the protocols in the IETF could be modified for anti-social purposes - transport protocols could be modified to evade end-to-end congestion control, routing protocols could be modified to inject invalid routes, web proxy caches could be used for the unauthorized modification of content even without OPES, and so on. None of these seem like compelling reasons not to standardize transport protocols, routing protocols, web caching protocols, or OPES itself. In our view, it means instead that the infrastructure needs, as much as possible, to be designed to detect and defend itself against compromised implementations, and misuses of protocols need to be addressed directly, each in the appropriate venue.

Mechanisms such as digital signatures, which help users to verify for themselves that content has not been altered, are a first step towards the detection of the unauthorized modification of content in the network. However, in the case of OPES, additional protection to ensure the end-to-end integrity of data is desirable as well, for example, to help end-users to detect cases where OPES intermediaries were authorized to modify content, but perform inappropriate modifications. We would note that mechanisms can *help* end-users to detect compromised OPES intermediaries in some cases even if they do not *guarantee* that end-users will be able to detect compromised OPES intermediaries in all cases.

If OPES is chartered, the OPES working group will also have to explicitly decide and document whether the OPES architecture must be compatible with the use of end-to-end encryption by one or more ends of an OPES-involved session. If OPES was compatible with end-to-end encryption, this would effectively ensure that OPES boxes would be restricted to ones that are known, trusted, explicitly addressed at the IP layer, and authorized (by the provision of decryption keys) by at least one of the ends. Compatibility with end-to-end encryption would also help to prevent the widespread deployment of yet another set of services that, to benefit from, require one to keep one's packet contents in the clear for all to snoop.

IAB Considerations:

(2.1) One-party consent: An OPES framework standardized in the IETF must require that the use of any OPES service be explicitly authorized by one of the application-layer end-hosts (that is, either the content provider or the client).

(2.2) IP-layer communications: For an OPES framework standardized in the IETF, the OPES intermediary must be explicitly addressed at the IP layer by the end user.

We note that (2.2) is not intended to preclude a chain of intermediaries, with the first intermediary in the chain explicitly addressed at the IP layer by the end user.

3. End-to-end Integrity

The proposed OPES services have several possible forms, including server-centric services, such as the dynamic assembling of web pages, explicitly authorized by the content provider; client-centric services such as virus scanning or language translation explicitly authorized by the end user to act on the response from the content provider; and client-centric services such as privacy-based services or content-filtering explicitly authorized by the end user to act on the request from the end user to the content provider. We consider the issue of the end-to-end integrity of data separately for these different classes of services.

For each specific service, the question arises of whether it is necessary for both the content provider and the end user to be able to detect and respond to inappropriate behavior by OPES intermediaries, or if it is sufficient for just one of the two end- hosts to have this ability. We don't attempt a general answer, but we do discuss the issues further in the sections below.

3.1. Data integrity with client-centric OPES services on responses

Why is there any concern about the end-to-end integrity of data in a client-centric OPES service acting on a response from a content provider? If the client requests a service such as virus scanning or language translation, why is that of any concern to the content provider one way or another? One answer is that one of the proper concerns of the IETF is to design architectures that enable end-hosts to detect and respond to inappropriate actions in the network. This seems of particular importance for powerful devices in the network such as OPES intermediaries, which are authorized by one of the end- nodes to act on or transform data in the network, but other than that are not under the direct control of that end-node.

Consider as an example the services of virus scanning or language translation. The end user has reasonable power in detecting and dealing with imperfect or corrupted virus scanners or language translators that are under her direct control (e.g., on her own machine). The end user knows exactly what program is installed, and has direct access to the content before and after the service is applied. The end user would have less control over similar services offered by OPES in the network itself, where the end user's only control might be the binary one of authorizing or not authorizing the service. (We also note that services deployed on the end host in a self-contained fashion, such as a local virus scanning program, are not a service in the network, and therefore are not in the province of the IETF one way or another.) For a OPES service such as virus scanning or language translation, the end user could detect a corrupted intermediary, but only through a "black-box" approach of comparing the input with the output. This is also imprecise and requires some effort, compared to the effort required to detect a corrupted virus scanner installed on one's own machine. For example, the user could retrieve the "non-OPES" version of the content directly from the content provider, if there is a "non-OPES" version, and compare this with the "OPES" version of the content available from the OPES intermediary. However, in the case of dynamic content, the "non-OPES" version of the content retrieved by the user directly from the content provider might not necessarily be the same as the "non-OPES" version of the content considered by the OPES intermediary. This limited control by the end user of the OPES service, and the limited ability of the end user to detect imperfect or corrupted intermediaries, argues for an architecture that helps the content provider to detect and respond to imperfect or corrupted OPES intermediaries as well.

We consider the specific example of virus scanning, authorized by the end user as an OPES service. One could imagine virus scanning as a widely deployed OPES service, augmenting the virus scanning done on the end host itself. If I ask for, say, a paper by Steve Bellovin on security and viruses in the network, and am informed by my authorized OPES virus-scanning service that this content does not pass the virus-scan, there are a number of possibilities:

(1) Unknown to Steve, the content (that is, Steve's paper) contains a harmful virus.

(2) Steve inserted a harmful virus in the content on purpose, with playful or malicious intent.

(3) The OPES virus scanner can't distinguish between a true harmful virus, and Steve's paper about harmful viruses.

(4) My local OPES virus scanner has been hacked, with malicious intent, to reject all content from Steve Bellovin.

At some point, for some content, some widely-deployed implementation of some OPES virus scanner is likely to result in problem (3), and some OPES implementation is likely to be corrupted to result in problem (4). Because the end user has limited control over the OPES virus scanner, the end user also is limited in its ability to detect problems (3) or (4) in the OPES virus scanner. In addition, the content provider is probably the one with the strongest incentive to detect problems (3) or (4) in the OPES virus scanner. (The content provider generally has a strong incentive to detect problem (1) as well.) In this case, it seems prudent that the overall OPES architecture should be carefully designed to prevent the OPES service of virus scanning, as authorized by the client, from unnecessarily preventing the distribution of content that in fact does not have viruses.

Obviously, it is not viable to propose that content providers simply indicate that some content should be passed to the end user without virus scanning - the point of virus scanning is for the end user to exercise control in this regard. However, if some form of end-system notification allows the content provider to find out that the content is being rejected by a virus scanning service instead of being delivered to the end user, then the content provider (Steve, in this case) might want to inform end users that this content is known by the content provider not to pass some OPES virus scanning services.

End users could then make their own decisions about whether or not to retrieve that content bypassing the OPES virus scanning service, relying on their own virus scanner or an alternate virus scanning service for this particular content. Such end-system notification to the content provider, if requested, cannot be enforced, and cannot be relied upon from corrupted intermediaries, but it seems important nevertheless.

Of course, malicious users can also use their awareness of the virus scanning service to perfect their ability to construct malicious viruses that can evade the virus scanning service. This will be done anyway, with any virus scanning service, and seems like an acceptable cost to allow content providers some protection against the vagaries of imperfect or corrupted OPES services in the network.

Thus, for client-requested services such as virus scanning and language translation, it is clearly desirable for the origin server to have notification, if it requests it, that these services are being performed on its content before the content is sent to the client. Any such end-system notification might be accompanied by reduced performance (in terms of overhead, delays, etc.) for the OPES service applied to that content. But some form of end-system notification is clearly necessary if content providers are to be able to detect and respond to actions by OPES intermediaries that are deemed inappropriate by the content provider.

Similarly for a client-based OPES service of language translation, it is clearly desirable for content providers to be able to inform end users when some content is deemed by the content provider to be incompatible with language translation. In this case, the important issue is not to prevent the OPES language translation from being performed on the content, but instead to give the content provider some mechanism to discover the language translation, and to inform the end user (or more precisely, to inform the end user's host computer) if the content provider believes that this language translation is incompatible with this particular content.

IAB Considerations:

(3.1) Notification: The overall OPES framework needs to assist content providers in detecting and responding to client-centric actions by OPES intermediaries that are deemed inappropriate by the content provider.

3.2. Data integrity with server-centric OPES services

What are the concerns, if any, with the end-to-end integrity of data in a server-centric OPES service such as location-based services?

For example, CNN could authorize a location-based OPES service, where the OPES intermediary inserts the weather report or news headline of regional interest into the requested web page. The same issue of the detection and response to broken or modified OPES intermediaries occurs with server-centric OPES as with client-centric OPES services.

We only consider server-centric services on responses, as we are not aware of any proposals for server-centric OPES services on requests from the client to the content provider.

How are the end-nodes to detect inappropriate actions from OPES services authorized by the content provider? The OPES service is being performed at an OPES intermediary in the network itself, and not under the direct control of the content provider; in particular, the content provider might not have the ability to monitor directly the output of the OPES intermediary. One could argue that the content provider and server-centric OPES intermediary are part of a single distributed application, and can be responsible on their own for detecting and dealing with broken or modified OPES intermediaries, without involving the end user. But this is unconvincing, basically arguing that standardizing protocols for performing OPES services is a network issue properly in the domain of the IETF, but the ensuring the overall integrity of the service is a distributed application matter, and not in the province of the IETF at all. It would seem to us that you can't have it both ways.

Simply labeling the content provider and the OPES intermediary as part of the same distributed application does not give the content provider the ability to monitor the actions of the OPES intermediary.

However, if the end user receives some form of notification that these OPES services have been provided, and has some mechanism for receiving the "non-OPES" content from the content provider without the OPES intermediary's modifications (if there is such a thing as a non-OPES version of the content), then the end user is in a better position to detect and react to inappropriate actions from compromised or poorly-designed OPES intermediaries. Thus, it is clear that some form of end-system notification is required to allow the end user to detect and respond to broken or modified OPES intermediaries. If the end user has notification of action by OPES intermediaries, it could "veto" an OPES service simply by throwing the OPES-modified content away. And if the client wants to talk directly to the origin server to receive the "non-OPES" version, and the origin server is configured to allow this, then the OPES intermediary must be designed to permit this end-to-end communication.

In addition to concerns about detecting and responding to faulty or compromised OPES intermediaries, there are purely policy-based concerns about the integrity of data. If the content provider looks at the source IP address from the HTTP request, or tosses a coin, in order to decide what content to provide, then that is the content provider's business. But if there exists a "non-OPES" version of some content available from the content provider, and also modified versions available from OPES intermediaries, then it is important that end users would be able to discover that they are receiving a modified version from the network, and not the "non-OPES" version that is also available from the content provider directly.

IAB Considerations:

(3.2) Notification: The overall OPES framework should assist end users in detecting the behavior of OPES intermediaries, potentially allowing them to identify imperfect or compromised intermediaries.

(3.3) Non-blocking: If there exists a "non-OPES" version of content available from the content provider, the OPES architecture must not prevent users from retrieving this "non-OPES" version from the content provider.

3.3. Data integrity with client-centric OPES services on requests

There have also been proposals for OPES services authorized by the client on requests from the client to the content provider. Examples include services that remove fields from the HTTP header for added privacy, and content-filtering services that filter requests based on the requested URL. For such services, there is still a need for end hosts to be assisted in detecting and responding to imperfect or corrupted intermediaries, but it seems less clear to what extent this applies to the content provider, and to what extent it applies to the end user that authorized the service. The requirements will probably have to be determined by the OPES and wider IETF communities on a case-by-case basis for each specific service.

4. Application Layer Addresses

Most application layer addressing revolves around URIs, which, for the most part, give a structured method to refer to a single data entity on a remote server. URIs are universal in that, in principle, the same result is obtained irrespective of the location of the client performing the resolution.

Practice often differs from this theory -- ad-strippers remove data from pages at the client end; web server farms redirect clients to one of several potential target machines for load-balancing or to give the user "localized" content.

However, from an architectural standpoint, it is important to be clear about what is being done here. In all cases, URI resolution standards (as defined for individual URI schemes, such as HTTP) apply unchanged between the client and the OPES intermediary. What the intermediary does to fulfill the request is not material to the discussion, and must produce a result that is compliant with the applicable URI scheme definition. In this sense, the OPES intermediary is the "endpoint" of URI resolution.

In client-centric OPES, the intermediary is resolving the URI on behalf of the client, and then applying client-requested services to provide a data response to the client. The client gets the data it wanted, but it did not carry out the URI resolution.

In server-centric OPES, the "origin server" cedes its authority to the intermediary to determine what is the "appropriate" content to supply for a given URI. The client may well perform standard URI resolution, but that reaches no further than the intermediary.

With those distinctions firmly in mind, there are two particular areas of concern for OPES-like services.

The first is the consideration of the effect of a series of interactions, over time and location (i.e., not just one document retrieval). Potential problems include inconsistencies in intra- and inter-document references -- depending on what content is changed, references from one version of a document might not exist in a modified target, etc.

The other concern is whether this leads to the creation of content that is exclusively accessible through the use of an intermediary.

That is, there is no "non-OPES" version. Either this should not be allowed, or this would argue for an extension to the Internet application layer addressing architecture.

IAB Considerations:

(4.1) URI resolution: OPES documentation must be clear in describing these services as being applied to the result of URI resolution, not as URI resolution itself.

(4.2) Reference validity: All proposed services must define their impact on inter- and intra-document reference validity.

(4.3) Any services that cannot be achieved while respecting the above two considerations may be reviewed as potential requirements for Internet application addressing architecture extensions, but must not be undertaken as ad hoc fixes.

5. Privacy

Intermediaries in the middle of the network increase the number of locations where the privacy of an end-to-end transaction could be compromised. Some of these privacy concerns apply to web caches and CDNs in general as well as specifically to OPES intermediaries. It seems a reasonable requirement, for OPES to be chartered in the IETF, that the issue of providing mechanisms for end users to determine the privacy policies of OPES intermediaries should be addressed. These mechanisms could be quite different for client-centric and server- centric OPES services.

For a complex issue such as an OPES architecture, which interacts with protocols from other standards bodies as well as from other IETF working groups, it seems necessary to keep in mind the overall picture while, at the same time, breaking out specific parts of the problem to be standardized in particular working groups. Thus, a requirement that the overall OPES architecture address privacy concerns does not necessarily mean that the mechanisms for this need to be developed in the IETF, or in the OPES working group (if it is chartered).

IAB Considerations:

(5.1) Privacy: The overall OPES framework must provide for mechanisms for end users to determine the privacy policies of OPES intermediaries.

6. Summary of IAB Considerations