Network Working Group I. Melve Internet-Draft UNINETT Expires: April 21, 2000 G. Tomlinson Novell I. Cooper Mirror Image October 22, 1999 Internet Web Replication and Caching Taxonomy draft-ietf-wrec-taxonomy-02.txt Status of this Memo This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC2026. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet-Drafts. 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." To view the entire list of Internet-Draft Shadow Directories, see http://www.ietf.org/shadow.html. This Internet-Draft will expire on April 21, 2000. Copyright Notice Copyright (C) The Internet Society (1999). All Rights Reserved. Abstract This memo specifies standard terminology and the current taxonomy of web replication and caching infrastructure deployed today. It introduces standard concepts and protocols uses today within this application domain. Currently deployed solutions employing this technologies are presented to establish a standard taxonomy. Research issues and HTTP proxy caching known problems are covered in two accompanying document, and are not part of this document. This document presents open protocols and points to published RFCs for each protocol. Melve, et. al. Expires April 21, 2000 [Page 1] Internet-Draft WREC Taxonomy October 1999 Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . 4 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 5 2.1 Base Terms . . . . . . . . . . . . . . . . . . . . . . . . 5 2.2 First order derivative terms . . . . . . . . . . . . . . . 7 2.3 Second order derivatives . . . . . . . . . . . . . . . . . 7 2.4 Topological terms . . . . . . . . . . . . . . . . . . . . 8 2.5 Automatic use of proxies . . . . . . . . . . . . . . . . . 8 3. Distributed System Relationships . . . . . . . . . . . . . 10 3.1 Replication Relationships . . . . . . . . . . . . . . . . 10 3.1.1 Client to Replica . . . . . . . . . . . . . . . . . . . . 10 3.1.2 Inter-Replica . . . . . . . . . . . . . . . . . . . . . . 10 3.2 Proxy Relationships . . . . . . . . . . . . . . . . . . . 11 3.2.1 Client to Non-Network Transparent Proxy . . . . . . . . . 11 3.2.2 Surrogate to Origin Server . . . . . . . . . . . . . . . . 11 3.2.3 Inter-Proxy . . . . . . . . . . . . . . . . . . . . . . . 12 3.2.3.1 (Caching) Proxy Meshes . . . . . . . . . . . . . . . . . . 12 3.2.3.2 (Caching) Proxy Clusters . . . . . . . . . . . . . . . . . 13 3.2.4 Network Element to Caching Proxy . . . . . . . . . . . . . 13 4. Client to Replica Communication . . . . . . . . . . . . . 15 4.1 Navigation Hyperlinks . . . . . . . . . . . . . . . . . . 15 4.2 URL Redirection . . . . . . . . . . . . . . . . . . . . . 15 4.3 DNS Redirection . . . . . . . . . . . . . . . . . . . . . 16 5. Inter-Replica Communication . . . . . . . . . . . . . . . 17 5.1 Batch Driven Replication . . . . . . . . . . . . . . . . . 17 5.2 Demand Driven Replication . . . . . . . . . . . . . . . . 17 5.3 Synchronized Replication . . . . . . . . . . . . . . . . . 18 6. Client to Proxy Configuration . . . . . . . . . . . . . . 19 6.1 Manual Proxy Configuration . . . . . . . . . . . . . . . . 19 6.2 Proxy Auto Configuration (PAC) . . . . . . . . . . . . . . 19 6.3 Cache Array Routing Protocol (CARP) v1.0 . . . . . . . . . 20 6.4 Web Proxy Auto-Discovery Protocol (WPAD) . . . . . . . . . 20 7. Inter-Proxy Communication . . . . . . . . . . . . . . . . 22 7.1 Loosely coupled Inter-Proxy Communication . . . . . . . . 22 7.1.1 Internet Cache Protocol (ICP) . . . . . . . . . . . . . . 22 7.1.2 Hyper Text Caching Protocol (HTCP/0.0) . . . . . . . . . . 22 7.1.3 Cache Digest . . . . . . . . . . . . . . . . . . . . . . . 23 7.1.4 Cache Pre-filling . . . . . . . . . . . . . . . . . . . . 24 7.2 Tightly Coupled Inter-Cache Communication . . . . . . . . 24 7.2.1 Cache Array Routing Protocol (CARP) v1.0 . . . . . . . . . 25 8. Network Element Communication . . . . . . . . . . . . . . 26 8.1 Web Cache Coordination Protocol (WCCP) . . . . . . . . . . 26 8.2 SOCKS . . . . . . . . . . . . . . . . . . . . . . . . . . 26 9. Security Considerations . . . . . . . . . . . . . . . . . 28 9.1 Authentication . . . . . . . . . . . . . . . . . . . . . . 28 9.1.1 Man in the middle attacks . . . . . . . . . . . . . . . . 28 9.1.2 Trusted third party . . . . . . . . . . . . . . . . . . . 28 9.1.3 Authentication based on IP number . . . . . . . . . . . . 29 Melve, et. al. Expires April 21, 2000 [Page 2] Internet-Draft WREC Taxonomy October 1999 9.2 Privacy . . . . . . . . . . . . . . . . . . . . . . . . . 29 9.2.1 Trusted third party . . . . . . . . . . . . . . . . . . . 29 9.2.2 Logs and legal implications . . . . . . . . . . . . . . . 29 9.3 Service security . . . . . . . . . . . . . . . . . . . . . 30 9.3.1 Denial of service . . . . . . . . . . . . . . . . . . . . 30 9.3.2 Replay attack . . . . . . . . . . . . . . . . . . . . . . 30 9.3.3 Stupid configuration of proxies . . . . . . . . . . . . . 30 9.3.4 Copyrighted transient copies . . . . . . . . . . . . . . . 30 9.3.5 Application level access . . . . . . . . . . . . . . . . . 30 10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . 31 References . . . . . . . . . . . . . . . . . . . . . . . . 32 Authors' Addresses . . . . . . . . . . . . . . . . . . . . 33 Full Copyright Statement . . . . . . . . . . . . . . . . . 35 Melve, et. al. Expires April 21, 2000 [Page 3] Internet-Draft WREC Taxonomy October 1999 1. Introduction Since its introduction in 1990, the World-Wide Web has evolved from a simple client server model into a sophisticated distributed architecture. This evolution has been driven largely due to the scaling problems associated with exponential growth. Distinct paradigms and solutions have emerged to satisfy specific requirements. Two core infrastructural components being employed to meet the demands of this growth are replication and caching. In many cases, there is a need for web caches and replicated services to be able to coexist. There are many protocols, both open and proprietary, employed in web replication and caching today. A majority of the open protocols include DNS[13], Cache Digests [15][17], CARP[4], HTTP[1], ICP[5], PAC[2], SOCKS[12], WPAD[3], and WCCP[11]. Additional protocols are being planned to address emerging solution requirements. This memo specifies standard terminology and the taxonomy of web replication and caching infrastructure deployed in the Internet today. The principal goal of this document is to establish a common understanding and reference point of this application domain. We also expect that this document will be used in the creation of a standard architectural framework for efficient, reliable, and predictable service in a web which includes both replicas and caches. Melve, et. al. Expires April 21, 2000 [Page 4] Internet-Draft WREC Taxonomy October 1999 2. Terminology The following terminology provides definitions of common terms used within the web replication and caching community. Base terms are taken, where possible, from the HTTP/1.1 specification[1] and are included here for reference. First- and second-order derivatives are constructed from these base terms to help define the relationships that exist within this area. Terms that are in common usage and which are contrary to the definitions in RFC2616 and this document are highlighted. 2.1 Base Terms The majority of these terms are taken as-is from RFC 2616[1], and are included here for reference. client (as given in [1]) A program that establishes connections for the purpose of sending requests. server (as given in [1]) An application program that accepts connections in order to service requests by sending back responses. Any given program may be capable of being both a client and a server; our use of these terms refers only to the role being performed by the program for a particular connection, rather than to the program's capabilities in general. Likewise, any server may act as an origin server, proxy, gateway, or tunnel, switching behavior based on the nature of each request. proxy (as given in [1]) An intermediary program which acts as both a server and a client for the purpose of making requests on behalf of other clients. Requests are serviced internally or by passing them on, with possible translation, to other servers. A proxy MUST implement both the client and server requirements of this specification. A "transparent proxy" is a proxy that does not modify the request or response beyond what is required for proxy authentication and identification. A "non-transparent proxy" is a proxy that modifies the request or response in order to provide some added service to the user agent, such as group annotation services, media type transformation, protocol reduction, or anonymity filtering. Except where either transparent or non-transparent behavior is explicitly stated, the HTTP proxy requirements apply to both types of proxies. Note: The term "transparent proxy" refers to a semantically transparent proxy as described in [1], not what is commonly Melve, et. al. Expires April 21, 2000 [Page 5] Internet-Draft WREC Taxonomy October 1999 understood within the caching community. We recommend that the term "transparent proxy" is always prefixed to avoid confusion (e.g. "network transparent proxy"). The above condition requiring implementation of both the server and client requirements of HTTP/1.1 is only appropriate for a non-network transparent proxy. cache (as given in [1]) A program's local store of response messages and the subsystem that controls its message storage, retrieval, and deletion. A cache stores cacheable responses in order to reduce the response time and network bandwidth consumption on future, equivalent requests. Any client or server may include a cache, though a cache cannot be used by a server that is acting as a tunnel. Note: The term "cache" used alone often is meant as "caching proxy". Note: There are additional motivations for caching, for example reducing server load (as a further means to reduce response time). cacheable (as given in [1]) A response is cacheable if a cache is allowed to store a copy of the response message for use in answering subsequent requests. The rules for determining the cacheability of HTTP responses are defined in section 13. Even if a resource is cacheable, there may be additional constraints on whether a cache can use the cached copy for a particular request. tunnel (as given in [1]) An intermediary program which is acting as a blind relay between two connections. Once active, a tunnel is not considered a party to the HTTP communication, though the tunnel may have been initiated by an HTTP request. The tunnel ceases to exist when both ends of the relayed connections are closed. replication (as given in [19]) Creating and maintaining a duplicate copy of a database or file system on a different computer, typically a server. inbound/outbound (as given in [1]) Inbound and outbound refer to the request and response paths for messages: "inbound" means "traveling toward the origin server", and "outbound" means "traveling toward the user agent". network element A network device that introduces multiple paths between source and destination, transparent to HTTP. Melve, et. al. Expires April 21, 2000 [Page 6] Internet-Draft WREC Taxonomy October 1999 2.2 First order derivative terms The following terms are constructed taking the above base terms as foundation. origin server (as given in [1]) The server on which a given resource resides or is to be created. user agent (as given in [1]) The client which initiates a request. These are often browsers, editors, spiders (web-traversing robots), or other end user tools. caching proxy A proxy with a cache, acting as a server to clients, and a client to servers. Caching proxies are often referred to as "proxy caches" or simply "caches". The term "proxy" is also frequently mis-used when referring to caching proxies. surrogate (a.k.a. "reverse proxies", "server accelerators") An intermediary program which acts as a server or tunnel for the purpose of responding to requests on behalf of one or more origin servers. Requests are serviced internally from a cache or by tunnelling them on to origin servers. The implementation requirements for surrogates have not been standardized; depending on the implementation, surrogates may or may not respond to the cache directives defined in [1]. Surrogates are also known as "reverse proxies" and "(origin) server accelerators". 2.3 Second order derivatives The following terms further build on first order derivatives: master origin server An origin server on which the definitive version of a resource resides. replica origin server An origin server holding a replica of a resource, but which may act as an authoritative reference for client requests. content consumer The user or system that initiates inbound requests, through use of a user agent. browser A special instance of a user agent that acts as a content presentation device for content consumers. Melve, et. al. Expires April 21, 2000 [Page 7] Internet-Draft WREC Taxonomy October 1999 2.4 Topological terms The following definitions are added to describe caching device topology: user agent cache The cache within the user agent program. local caching proxy The caching proxy to which a user agent connects. intermediate caching proxy Seen from the content consumer's view, all caches participating in the caching mesh that are not the user agent's local caching proxy. cache server A server to requests made by local and intermediate caching proxies, but which does not act as a proxy. cache array A cluster of caching proxies, acting logically as one service and partitioning the resource name space across the array. Also known as "diffused array" or "cache cluster". caching mesh a loosely coupled set of co-operating proxy- and (optionally) caching-servers, or clusters, acting independently but sharing cacheable content between themselves using inter-cache communication protocols. 2.5 Automatic use of proxies Network administrators may wish to force or facilitate the use of proxies (typically caching proxies) by clients, enabling such configuration within the network itself or within automatic systems in user agents, such that the content consumer need not be aware of any such configuration issues. The terms that describe such configurations are given below. automatic user-agent proxy configuration The technique of discovering the availability of one or more proxies and the automated configuration of the client to use them. The use of a proxy is transparent to the user but not to the client. The term "automatic proxy configuration" is also used in this sense. traffic interception Melve, et. al. Expires April 21, 2000 [Page 8] Internet-Draft WREC Taxonomy October 1999 The process of using a network element to examine network traffic to determine whether it should be redirected. traffic redirection Redirection of client requests from a network element performing traffic interception to a proxy. Used to deploy (caching) proxies without the need to manually reconfigure individual user agents, or to force the use of a proxy where such use would not otherwise occur. (network) transparent proxy A proxy that receives traffic as a result of network traffic redirection. The term "transparent proxy" is typically used to refer to a network transparent proxy and the additional systems that perform traffic redirection. The use of this type of proxy is transparent to both user and client. Due to a conflicting definition in [1], caution should be exercised when referring to a "transparent proxy". As stated above, it is recommended that the phrase "transparent proxy" is prepended with appropriate terminology to avoid confusion. Melve, et. al. Expires April 21, 2000 [Page 9] Internet-Draft WREC Taxonomy October 1999 3. Distributed System Relationships This section identifies the relationships that exist in a distributed replication and caching environment. Having defined these relationships, later sections describe the communication protocols used in each relationship. 3.1 Replication Relationships The following sections describe relationships between clients and replicas and between replicas themselves. 3.1.1 Client to Replica A client may communicate with one or more replica origin servers, as well as with master origin servers. (In the absence of replica servers the client interacts directly with the origin server as is the normal case.) ------------------ ----------------- ------------------ | Replica Origin | | Master Origin | | Replica Origin | | Server | | Server | | Server | ------------------ ----------------- ------------------ \ | / \ | / ----------------------------------------- | Client to ----------------- Replica Server | Client | ----------------- Protocols used to enable the client to use one of the replicas can be found in Section 4. 3.1.2 Inter-Replica This is the relationship between master origin server(s) and replica origin servers, to replicate data sets that are accessed by clients in the relationship shown in Section 3.1.1. ------------------ ----------------- ------------------ | Replica Origin |-----| Master Origin |-----| Replica Origin | | Server | | Server | | Server | ------------------ ----------------- ------------------ Protocols used in this relationship can be found in Section 5. Melve, et. al. Expires April 21, 2000 [Page 10] Internet-Draft WREC Taxonomy October 1999 3.2 Proxy Relationships There are a variety of ways in which (caching) proxies and cache servers communicate with each other, and with clients. 3.2.1 Client to Non-Network Transparent Proxy A client may communicate with zero or more proxies for some or all requests. Where the result of communication results in no proxy being used, the relationship is between cache and origin server or replica origin server (see Section 3.1.1). ----------------- ----------------- ----------------- | Local | | Local | | Local | | Proxy | | Proxy | | Proxy | ----------------- ----------------- ----------------- \ | / \ | / ----------------------------------------- | ----------------- | Client | ----------------- In addition, a client may interact with an additional server - operated on behalf of a proxy - to aid the configuration of the client to use that proxy. Protocols used in this relationship can be found in Section 6. 3.2.2 Surrogate to Origin Server A client may communicate with zero or more surrogates for requests intended for one or more origin servers. Where a surrogate is not available, the client communicates directly with an origin server. Melve, et. al. Expires April 21, 2000 [Page 11] Internet-Draft WREC Taxonomy October 1999 -------------- -------------- -------------- | Origin | | Origin | | Origin | | Server | | Server | | Server | -------------- -------------- -------------- \ | / \ | / ----------------- | Surrogate | | | ----------------- | | ------------ | Client | ------------ 3.2.3 Inter-Proxy Inter-Proxy relationships exist as meshes (loosely coupled) and clusters (tightly coupled). 3.2.3.1 (Caching) Proxy Meshes Within a loosely coupled mesh of (caching) proxies, communication can happen at the same level between peers, and with one or more parents. --------------------- --------------------- -----------| Intermediate | | Intermediate | | | Caching Proxy (D) | | Caching Proxy (E) | |(peer) --------------------- --------------------- -------------- | (parent) / (parent) | Cache | | ------/ | Server (C) | | / -------------- | / (peer) | ----------------- --------------------- -------------| Local Caching |-------| Intermediate | | Proxy (A) | (peer)| Caching Proxy (B) | ----------------- --------------------- | | ---------- | Client | ---------- Client included for illustration purposes only Melve, et. al. Expires April 21, 2000 [Page 12] Internet-Draft WREC Taxonomy October 1999 An outbound request from a local (caching) proxy may be routed to one of a number of intermediate (caching) proxies based on a determination of whether that parent is better suited to resolving the request. For example, in the above figure, Cache Server C and Intermediate Caching Proxy B are peers of the Local Caching Proxy A, and may only be used when the resource requested by A is on either B or C. Intermediate Caching Proxies D & E are parents of A, and it is A's choice of which to use to resolve a particular query. The relationship between A & B only makes sense in a caching environment, while the relationships between A & D and A & E are also appropriate for cacheless proxies. Protocols used in these relationships can be found in Section 7.1. 3.2.3.2 (Caching) Proxy Clusters Where a client may have a relationship with a proxy, it is possible that it may instead have a relationship with an array of proxies arranged in a tightly coupled mesh. ---------------------- ---------------------- | --------------------- | | | (Caching) Proxy | |----- | Array |----- ^ ^ --------------------- ^ ^ | | ^ ^ | |--- | | |----- | -------------------------- Protocols used in this relationship can be found in Section 7.2. 3.2.4 Network Element to Caching Proxy A network element performing traffic interception may choose to redirect requests from a client to a specific proxy within an array. (It may also choose not to redirect the traffic, in which case the relationship is between client and origin server or replica origin server, see Section 3.1.1.) Melve, et. al. Expires April 21, 2000 [Page 13] Internet-Draft WREC Taxonomy October 1999 ----------------- ----------------- ----------------- | Caching Proxy | | Caching Proxy | | Caching Proxy | | Array | | Array | | Array | ----------------- ----------------- ----------------- \ | / ----------------------------------------- | -------------- | Network | | Element | -------------- | /// | ------------ | Client | ------------ The network transparent (caching) proxy may be directly in-line of the flow of traffic - in which case the intercepting network element and network transparent proxy form parts of the same hardware system - or may be out-of-path, requiring the intercepting network element to redirect traffic over another network segment. In this latter case, communication protocols enable the intercepting network element to stop and start redirecting traffic when the network transparent proxy becomes (un)available. Details of these protocols can be found in Section 8. Melve, et. al. Expires April 21, 2000 [Page 14] Internet-Draft WREC Taxonomy October 1999 4. Client to Replica Communication This section describes the cooperation and communication between clients and replica origin web servers. The ideal situation is to discover an optimal replica origin server for clients to communicate with. Optimality is a policy based decision, often based upon proximity, but may be based on other criteria such as load. 4.1 Navigation Hyperlinks Authoritative reference: This memo. Description: The simplest of client to replica communication mechanisms. This utilizes hyperlink URIs embedded in web pages that point to the mirror sites. The human user manually selects the link of the replica origin server they wish to use. Security: Relies on the protocol security associated with the appropriate URI scheme. Deployment: Probably the most commonly deployed client to replica communication mechanism. Ubiquitous interoperability with humans. Submitter: Document editors. 4.2 URL Redirection Authoritative reference: This memo. Description: A simple and commonly used mechanism to connect web clients with origin server replicas is to use URL redirection. Clients are redirected to a optimal web server replica via the use of the HTTP[1] protocol response codes, e.g. 302 "Found", or 307 "Temporary Redirect". A web client establishes HTTP communication with one of the web server replicas. The initially contacted replica origin web server can either choose to accept the service or redirect the client to the proper replica. Refer to section 10.3 in HTTP/1.1 RFC2616 for information on HTTP response codes. Security: Relies entirely upon HTTP security. Melve, et. al. Expires April 21, 2000 [Page 15] Internet-Draft WREC Taxonomy October 1999 Deployment: Observed at a number of large web sites. Extent of usage in the Internet is unknown at this time. Submitter: Document editors. 4.3 DNS Redirection Authoritative reference: * RFC1794 DNS Support for Load Balancing Proximity[13] * This memo Description: The Domain Name Service (DNS) provides a more sophisticated client to replica communication mechanism. This is accomplished by DNS servers that sort resolved IP addresses based upon quality of service policies. When a client resolves the name of an origin server, the enhanced DNS server sorts the available IP addresses of the replica origin servers starting with the most optimal replica and ending with the least optimal replica. Security: Relies entirely upon DNS security, and other protocols that may be used in determining the sort order. Deployment: Observed at a number of large web sites and large ISP web hosted services. Extent of usage in the Internet is unknown at this time. Submitter: Document editors. Melve, et. al. Expires April 21, 2000 [Page 16] Internet-Draft WREC Taxonomy October 1999 5. Inter-Replica Communication This section describes the cooperation and communication between master- and replica- origin servers. Used in replicating data sets between origin servers. 5.1 Batch Driven Replication Authoritative reference: This memo. Description: In this model, the replica origin server to be updated initiates communication with a master origin server. The communication is established at intervals based upon queued transactions which are scheduled for deferred processing. The scheduling mechanism policies vary, but generally are reoccuring at a specified time. Once communication is established, data sets are copied to the initiating replica origin server. Security: Relies upon the protocol being used to transfer the data set. FTP and RDIST are the most common protocols observed. Deployment: Very common for mirror synchronization in the Internet. Submitter: Document editors. 5.2 Demand Driven Replication Authoritative reference: This memo. Description: In this model the replica origin server acquires the content as needed due to client demand. This is generally done by a surrogate. When a client requests a resource that is not in the data set of the replica origin server/surrogate, the surrogate attempts to acquire it from the master origin server and then forwards it to the requesting client. Security: Relies upon the protocol being used to transfer the resources. FTP, Gopher, HTTP and ICP are the most common protocols observed. Deployment: Observed at several large web sites. Extent of usage in the Melve, et. al. Expires April 21, 2000 [Page 17] Internet-Draft WREC Taxonomy October 1999 Internet is unknown at this time. Submitter: Document editors. 5.3 Synchronized Replication Authoritative reference: This memo. Ed note: there is no IETF protocol specified at this time. The editors are aware of at least two open source protocols, AFS and CODA, along with one expired IETF draft and one proprietary protocol Novell NRS; none of which can be considered an authoritative reference Description: In this model, the replicated origin servers cooperate using synchronized strategies and specialized replica protocols to keep the replica data sets coherent. Synchronization strategies range from tightly coherent (a few minutes) to loosely coherent (a few or more hours). Updates occur between replicas based upon the synchronization time constraints of the coherency model employed and are generally in the form of deltas only. Security: All of the known protocols utilize strong cryptographic key exchange methods, which are either based upon the Kerberos shared secret model or the public/private key RSA model. Deployment: Observed at a few sites, primarily at university campuses. Submitter: Document editors. Melve, et. al. Expires April 21, 2000 [Page 18] Internet-Draft WREC Taxonomy October 1999 6. Client to Proxy Configuration This section describes the configuration, cooperation and communication between end user clients (browsers and applications) a proxy. 6.1 Manual Proxy Configuration Authoritative reference: This memo. Description: Each user needs to configure her user agent by supplying information pertaining to proxied protocols and local policies. Security: The potential for doing wrong is high; each user individually sets preferences. Deployment: Widely deployed, used in all current browsers. Most browsers also support additional options. Submitter: Document editors. 6.2 Proxy Auto Configuration (PAC) Authoritative reference: No RFC, no Internet-Draft; Navigator Proxy Auto-Config File Format[2]. Description: A JavaScript script retrieved from a web server is executed to determine an appropriate proxy (if any) for the resource being requested. User agents must be configured to request this JavaScript resource upon startup. No bootstrap mechanism, manual configuration necessary. Manual configuration is made easier by centralizing the script to one URI. Security: Common policy per organization possible but still requires initial manual configuration. PAC is better than "manual proxy configuration" since PAC administrators may update the proxy configuration without further user intervention. Interoperability of PAC files is not high, since different browsers have slightly different interpretations of the same script, possibly leading to undesired effects. Melve, et. al. Expires April 21, 2000 [Page 19] Internet-Draft WREC Taxonomy October 1999 Deployment: Implemented in most browsers. Submitter: Document editors. 6.3 Cache Array Routing Protocol (CARP) v1.0 Authoritative reference: Expired Internet-Draft: draft-vinod-carp-v1-03.txt[4] Note: Reference kept since there is known implementation. Description: Clients may use CARP directly as a hash function based proxy selection mechanism. They need to be configured with the location of the cluster information. Security: Security considerations are not covered in the specification drafts. Deployment: Implemented in Microsoft Proxy Server, Squid. Implemented in clients via PAC scripts. Submitter: Document editors. 6.4 Web Proxy Auto-Discovery Protocol (WPAD) Authoritative reference: Internet-Draft: draft-ietf-wrec-wpad-00.txt[3] Description: WPAD uses a collection of pre-existing Internet resource discovery mechanisms to perform web proxy auto-discovery. The only goal of WPAD is to locate the PAC URL[2]. WPAD does not specify which proxies will be used. WPAD gets you to the PAC URL, and the PAC script then operates as defined above to choose proxies per resource request. The WPAD protocol specifies the following: * how to use each mechanism for the specific purpose of web proxy auto-discovery * the order in which the mechanisms should be performed * the minimal set of mechanisms which must be attempted by a WPAD compliant web client Melve, et. al. Expires April 21, 2000 [Page 20] Internet-Draft WREC Taxonomy October 1999 The resource discovery mechanisms utilized by WPAD are as follows: * Dynamic Host Configuration Protocol DHCP * Service Location Protocol SLP * "Well Known Aliases" using DNS A records * DNS SRV records * "service: URLs" in DNS TXT records Security: Relies upon DNS and HTTP security. Deployment: Implemented in web clients and caching proxy servers. More than two independent implementations. Submitter: Josh Cohen, Microsoft, joshco@microsoft.com Melve, et. al. Expires April 21, 2000 [Page 21] Internet-Draft WREC Taxonomy October 1999 7. Inter-Proxy Communication 7.1 Loosely coupled Inter-Proxy Communication This section describes the cooperation and communication between caching proxies. 7.1.1 Internet Cache Protocol (ICP) Authoritative reference: RFC 2186 Internet Cache Protocol (ICP), version 2[5] Description: ICP is used by caches to query other caches about web objects, to see if a web object is present at the other cache. ICP uses UDP. Since UDP is an uncorrected network transport protocol, an estimate of network congestion and availability may be calculated by ICP loss. This rudimentary loss measurement does, together with round trip times provide a load balancing method for caches. Security: See RFC 2187[6] ICP does not convey information about HTTP headers associated with a web object. HTTP headers may include access control and cache directives, Since caches ask for objects, and then download the objects using HTTP, false cache hits may occur (object present in cache, but not accessible for sibling cache is one example). ICP suffers from all the security problems of UDP. Deployment: Widely deployed. Most current caching proxy implementations support ICP in some form. Submitter: Document editors. See also Internet-Draft draft-lovric-icp-ext-02.txt[7], ICP development Web page[8], ICP1.4 specification[9]. 7.1.2 Hyper Text Caching Protocol (HTCP/0.0) Authoritative reference: Internet-Draft: draft-vixie-htcp-proto-05.txt[16] Description: HTCP is a protocol for discovering HTTP caching proxies and cached data, managing sets of HTTP caching proxies, and Melve, et. al. Expires April 21, 2000 [Page 22] Internet-Draft WREC Taxonomy October 1999 monitoring cache activity. HTCP includes HTTP headers, while ICPv2 does not. HTTP headers are vital information for caching proxies. Security: Optionally uses HMAC-MD5[18]shared secret authentication. Protocol is subject to attack if authentication is not used. Deployment: HTCP is implemented in Squid and the Web Gateway Interceptor[20]. Submitter: Document editors. 7.1.3 Cache Digest Authoritative reference: * No RFC, no Internet-Draft; Cache Digest specification - version 5[15] * Summary Cache[17] (see note) Description: Cache Digests are a response to the problems of latency and congestion associated with previous inter-cache communications mechanisms such as the Internet Cache Protocol (ICP)[5] and the HyperText Cache Protocol[16]. Unlike most of these protocols, Cache Digests support peering between caching proxies and cache servers without a request-response exchange taking place. Instead, a summary of the contents of the server (the Digest) is fetched by other servers which peer with it. Using Cache Digests it is possible to determine with a relatively high degree of accuracy whether a given URL is cached by a particular server. Cache Digests are both an exchange protocol and a data format [15]. Security: If the contents of a Digest are sensitive, they should be protected from access by The Wrong People. Any methods which would normally be applied to secure an HTTP connection can be applied to Cache Digests. A 'Trojan horse' attack is currently possible in a mesh: Cache A can build a fake peer Digest for cache B and serve it to B's peers if requested. This way A can direct traffic toward/from B. The impact of this problem is minimized by the 'pull' model of transferring Cache Digests from one system to another. Cache Digests provide knowledge about peer cache content on a URL level. Hence, they do not dictate a particular level of policy Melve, et. al. Expires April 21, 2000 [Page 23] Internet-Draft WREC Taxonomy October 1999 management and can be used to implement various policies on any level (user, organization, etc.). Deployment: Cache Digests are supported in Squid. Cache Meshes: * NLANR Mesh * TF-CACHE mesh (European Academic networks) Submitter: Alex Rousskov, NLANR, rousskov@nlanr.net for [15] Pei Cao for [17] Note: The technology of Summary Cache[17] is patent pending by the University of Wisconsin-Madison. 7.1.4 Cache Pre-filling Authoritative reference: Expired Internet-Draft: draft-lovric-francetelecom-satellites-00.txt[14] Description: Cache pre-filling is a push-caching implementation. It is particularly well adapted to IP-multicast networks because it allows preselected URLs to be inserted in one single time within all the caches that belong to the targeted multicast group. Different implementations of cache pre-filling already exist, especially in satellite contexts. However, there is still no standard for this kind of push-caching and vendors propose solutions either based on dedicated equipments or public domain caches extended with a pre-filling module. Security: Relies on the inter cache protocols being employed. Deployment: Observed in two commercial content distribution service providers. Submitter: Ivan Lovric, France Telecom, ivan.lovric@cnet.francetelecom.fr 7.2 Tightly Coupled Inter-Cache Communication Melve, et. al. Expires April 21, 2000 [Page 24] Internet-Draft WREC Taxonomy October 1999 7.2.1 Cache Array Routing Protocol (CARP) v1.0 Also see Section 6.3 Authoritative reference: Expired Internet-Draft: draft-vinod-carp-v1-03.txt[4] Note: Reference kept since there is known deployment. Description: CARP is a hashing function for dividing URL-space among a cluster of proxy caches. Included in CARP is the definition of a Proxy Array Membership Table, and ways to download this information. An HTTP client agent (either a proxy server or a client browser) which implements CARP v1.0 can allocate and intelligently route requests for the correct URLs to any member of the Proxy Array. Due to the resulting sorting of requests through these proxies, duplication of cache contents is eliminated and global cache hit rates may be improved. Security: Security considerations are not covered in the specification drafts. Deployment: Implemented in caching proxy servers. More than two independent implementations. Submitter: Document editors. Melve, et. al. Expires April 21, 2000 [Page 25] Internet-Draft WREC Taxonomy October 1999 8. Network Element Communication This section describes the cooperation and communication between caching proxy and network elements. Examples include routers and switches. Generally used for transparent caching and/or diffused arrays. 8.1 Web Cache Coordination Protocol (WCCP) Authoritative reference: Internet-Draft: draft-ietf-wrec-web-pro-00.txt[11] Description: WCCP V1 runs between a router functioning as a redirecting network element and out-of-path transparent caching proxies. The protocol allows one or more caching proxies to register themselves with a single router to receive redirected web traffic. It also allows one of the proxies, the designated proxy, to dictate to the router how redirected web traffic is distributed across the caching proxies. Security: WCCP V1 has no security features. Deployment: Network elements: WCCP V1 is deployed on a wide range of Cisco routers. Caching proxies: WCCP V1 is deployed on a number of vendors' caches. Submitter: David Forster, CISCO, dforster@cisco.com 8.2 SOCKS Authoritative reference: RFC1928 SOCKS Protocol Version 5[12] Description: SOCKS is primarily used as a proxy cache to firewall protocol. Although, firewalls don't conform to the narrowly defined network element definition of routers and switches, they are a integral part of the network infrastructure. When used in conjunction with a firewall, SOCKS provides a authenticated tunnel between the proxy cache and the firewall. Security: A extensive framework provides for multiple authentication methods. Currently, SSL, CHAP, DES, 3DES are known to be Melve, et. al. Expires April 21, 2000 [Page 26] Internet-Draft WREC Taxonomy October 1999 available. Deployment: SOCKS is been widely deployed in the Internet. Submitter: Document editors. Melve, et. al. Expires April 21, 2000 [Page 27] Internet-Draft WREC Taxonomy October 1999 9. Security Considerations This document provides a taxonomy for web caching and replication. Recommended practice, architecture and protocols are not described in detail. Replication and caching means copying objects. There are legal implications of making and keeping transient or permanent copies; these are not covered in the security considerations. Information on security in each protocol is provided in the preceding description of the protocol, and in their accompanying documentation. HTTP security is discussed in section 15 of RFC2616[1], the HTTP/1.1 specification, and to a lesser extent in RFC1945[10], the HTTP/1.0 specification. RFC2616 contains security considerations for HTTP proxies. Caching proxies have the same security issues as other application level proxies. Application level proxies are not covered in these security considerations. Authentication based on client IP number is problematic when connecting through a proxy, details are not discussed here. 9.1 Authentication Requests for web objects and responses to such requests may go to replicas and/or flow through proxies. The integrity of the communication needs to be preserved, to ensure protection of access to the communication and protect the communication exchange from unintended change. In the case of security breach, the culprit needs to be identified 9.1.1 Man in the middle attacks HTTP proxies are men-in-the-middle, the perfect place for a man-in-the-middle-attack. A discussion of this is found in section 15 of RFC2616[1]. 9.1.2 Trusted third party A proxy must either be trusted to act on behalf of server and/or client, or it must act as a tunnel. When presenting cached objects to clients, the clients need to trust the caching proxy to act on behalf on the origin server. A replica may get accreditation from the origin server. Melve, et. al. Expires April 21, 2000 [Page 28] Internet-Draft WREC Taxonomy October 1999 9.1.3 Authentication based on IP number Authentication based on client IP number is problematic when connecting through a proxy, as the authenticating server sees the proxy's IP number. One (not recommended) solution to this is spoofing the client's IP number. Authentication based on IP number assumes that the end-to-end properties of the Internet are preserved. This is typically not the case for a network transparent proxy. 9.2 Privacy 9.2.1 Trusted third party When using a replication service, you need to trust both the replica and the object location service. A object location service is used to find the replicated object. Current examples include DNS round robin, manual mirror lists, URNs, HTTP redirecting. Redirection of traffic, either by redirecting to replicas or by redirection done by proxies, may introduce third parties the end user and/or origin server need to trust. In the case of network transparent proxies, such trusted third parties are often unknown to both end points of the communication. Unknown trusted third parties may have security implications. Both proxies and location services may have access to aggregated access information. A proxy typically knows about all access by all the clients using it, information that is more sensitive than the information held by one origin server. 9.2.2 Logs and legal implications Logs from proxies need to be kept secure, as they provide information about users and end user patterns. A proxy log is even more sensitive than a web server log, as all requests from the user population goes through the proxy. Logs from replication servers may need to be amalgamated to get aggregated statistics from a service, transporting logs across borders may have legal implications. Log handling is restricted by law in some countries. Requirements for object security and privacy are the same in a web replication and caching system as it is in the Internet at large. The only reliable solution is strong cryptography. End to end encryption does not necessarily make objects cacheable, as is the case of SSL encrypted web sessions. Melve, et. al. Expires April 21, 2000 [Page 29] Internet-Draft WREC Taxonomy October 1999 9.3 Service security 9.3.1 Denial of service Any redirection of traffic is susceptible to denial of service attacks at the redirect point, and both proxies and location services may redirect traffic. By attacking a proxy, access to all servers may be denied for a large set of clients. It has been argued that introduction of a network transparent proxy is denial of service since the end to end nature of the Internet is destroyed without the end users knowledge. 9.3.2 Replay attack A caching proxy is by definition a replay attack. 9.3.3 Stupid configuration of proxies It is quite easy to have a stupid configuration which will harm service for end users. This is the most common security problem with proxies. 9.3.4 Copyrighted transient copies The legislative forces of the world are considering the question of transient copies, like those kept in replication and caching system, being legal. Legal implications of replication and caching is subject to local law. Caching proxies need to preserve the protocol output, including headers. Replication services need to preserve the source of the objects. 9.3.5 Application level access Caching proxies are application level components in the traffic flow path, and may give intruders access to information that was only available at network level equipment in a proxy-free world. Some network level equipment may have required physical access to get sensitive information, and introducing application level components may require additional system security. Melve, et. al. Expires April 21, 2000 [Page 30] Internet-Draft WREC Taxonomy October 1999 10. Acknowledgements The editors would like to thank the following for their assistance: David Forster, Alex Rousskov, Josh Cohen, John Martin, John Dilley, Ivan Lovric, Joe Touch, Henrik Nordstrom, Patrick McManus, Duane Wessels, Wojtek Sylwestrzak, Ted Hardie, Misha Rabinovich, Larry Masinter, and Keith Moore. Melve, et. al. Expires April 21, 2000 [Page 31] Internet-Draft WREC Taxonomy October 1999 References [1] Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter, L., Leach, P. and T. Berners-Lee, "Hypertext Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999. [2] Netscape, Inc., "Navigator Proxy Auto-Config File Format", External reference http://www.netscape.com/eng/mozilla/2.0/ relnotes/demo/proxy-live.html, March 1996. [3] Gauthier, P., Cohen, J., Dunsmuir, M. and C. Perkins, "The Web Proxy Auto-Discovery Protocol", Internet Draft draft-ietf-wrec-wpad-01.txt, July 1999. [4] Valloppillil, V. and K.W. Ross, "Cache Array Routing Protocol", Expired Internet Draft draft-vinod-carp-v1-03.txt available at http://ircache.nlanr.net/Cache/ICP/carp.txt, February 1998. [5] Wessels, D. and K. Claffy, "Internet Cache Protocol (ICP), Version 2", RFC 2186, September 1997. [6] Wessels, D. and K. Claffy, "Application of Internet Cache Protocol (ICP), Version 2", RFC 2187, September 1997. [7] Lovric, I., "Internet Cache Protocol Extension", Internet Draft draft-lovric-icp-ext-02.txt, October 1999. [8] Wessels, D., "ICP Home Page", External reference http://ircache.nlanr.net/Cache/ICP/, July 1999. [9] University of Southern California and University of Colorado-Boulder, "Internet Cache Protocol Specification 1.4", External reference http://excalibur.usc.edu/icpdoc/icp.html, September 1994. [10] Berners-Lee, T., Fielding, R. and H. Frystyk, "Hypertext Transfer Protocol -- HTTP/1.0", RFC 1945, May 1996. [11] Cisco Systems, "Cisco Web Cache Coordination Protocol V1.0", Internet Draft draft-ietf-wrec-web-pro-00.txt, June 1999. [12] Leech, M., Ganis, M., Lee, Y., Kuris, R., Koblas, D. and L. Jones, "SOCKS Protocol Version 5", RFC 1928, March 1996. [13] Brisco, T., "DNS Support for Load Balancing", RFC 1794, April 1995. [14] Goutard, C., Lovric, I. and E. Maschio-Esposito, "Pre-filling a cache - A satellite overview", Expired Internet Draft Melve, et. al. Expires April 21, 2000 [Page 32] Internet-Draft WREC Taxonomy October 1999 draft-lovric-francetelecom-satellites-00.txt, February 1999. [15] Hamilton, M., Rousskov, A. and D. Wessels, "Cache Digest specification - version 5", External reference http://squid.nlanr.net/CacheDigest/cache-digest-v5.txt, December 1998. [16] Vixie, P. and D. Wessels, "Hyper Text Caching Protocol (HTCP/0.0)", Internet Draft draft-vixie-htcp-proto-05.txt, August 1999. [17] Fan, L., Cao, P., Almeida, J. and A. Broder, "Summary Cache: A Scalable Wide-Area Web Cache Sharing Protocol", Proceedings of ACM SIGCOMM'98 pp. 254-265, September 1998. [18] Krawczyk, H., Bellare, M. and R. Canetti, "HMAC: Keyed-Hashing for Message Authentication", RFC 2104, February 1997. [19] FOLDOC, "Free Online Dictionary of Computing: Replication", Online reference http://foldoc.doc.ic.ac.uk/foldoc/foldoc.cgi?replication, December 1997. [20] http://www.vix.com/vix/wgi.html Authors' Addresses Ingrid Melve UNINETT Tempeveien 22 Trondheim Norway Phone: +47 73 55 79 07 EMail: Ingrid.Melve@uninett.no Gary Tomlinson Novell Inc. 122 East 1700 South Provo, Utah 84606 USA Phone: +1 801 861 7021 EMail: garyt@novell.com Melve, et. al. Expires April 21, 2000 [Page 33] Internet-Draft WREC Taxonomy October 1999 Ian Cooper Mirror Image Internet, Inc. 49 Dragon Court 2nd floor Woburn, MA 01801 USA Phone: +1 781 939 0735 EMail: ian@mirror-image.com Melve, et. al. Expires April 21, 2000 [Page 34] Internet-Draft WREC Taxonomy October 1999 Full Copyright Statement Copyright (C) The Internet Society (1999). All Rights Reserved. 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