📄 rfc3040.txt
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presentation device for content consumers.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".Cooper, et al. Informational [Page 7] RFC 3040 Internet Web Replication & Caching Taxonomy January 2001 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 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 user agent to use them. The use of a proxy is transparent to the content consumer but not to the user agent. The term "automatic proxy configuration" is also used in this sense. traffic interception 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. interception proxy (a.k.a. "transparent proxy", "transparent cache") The term "transparent proxy" has been used within the caching community to describe proxies used with zero configuration within the user agent. Such use is somewhat transparent to user agents. Due to discrepancies with [1] (see definition of "proxy" above), and objections to the use of the word "transparent", we introduce the term "interception proxy" to describe proxies that receive redirected traffic flows from network elements performing traffic interception. Interception proxies receive inbound traffic flows through the process of traffic redirection. (Such proxies are deployed by network administrators to facilitate or require the use of appropriate services offered by the proxy). Problems associated with the deployment of interception proxies are described in theCooper, et al. Informational [Page 8] RFC 3040 Internet Web Replication & Caching Taxonomy January 2001 document "Known HTTP Proxy/Caching Problems" [23]. The use of interception proxies requires zero configuration of the user agent which act as though communicating directly with an origin server.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.Cooper, et al. Informational [Page 9] RFC 3040 Internet Web Replication & Caching Taxonomy January 2001 ------------------ ----------------- ------------------ | Replica Origin |-----| Master Origin |-----| Replica Origin | | Server | | Server | | Server | ------------------ ----------------- ------------------ Protocols used in this relationship can be found in Section 5.3.2 Proxy Relationships There are a variety of ways in which (caching) proxies and cache servers communicate with each other, and with user agents.3.2.1 Client to Non-Interception 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 client and (replica) origin server (see Section 3.1.1). ----------------- ----------------- ----------------- | Local | | Local | | Local | | Proxy | | Proxy | | Proxy | ----------------- ----------------- ----------------- \ | / \ | / ----------------------------------------- | ----------------- | Client | ----------------- In addition, a user agent may interact with an additional server - operated on behalf of a proxy for the purpose of automatic user agent proxy configuration. Schemes and protocols used in these relationships can be found in Section 6.3.2.2 Client to 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 used, the client communicates directly with an origin server. Where a surrogate is used the client communicates as if with an origin server. The surrogate fulfills the request from its internal cache, or acts as a gateway or tunnel to the origin server.Cooper, et al. Informational [Page 10] RFC 3040 Internet Web Replication & Caching Taxonomy January 2001 -------------- -------------- -------------- | 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 onlyCooper, et al. Informational [Page 11] RFC 3040 Internet Web Replication & Caching Taxonomy January 2001 An inbound request 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 already exists 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 request. The relationship between A & B only makes sense in a caching environment, while the relationships between A & D and A & E are also appropriate where D or E are non-caching proxies. Protocols used in these relationships can be found in Section 7.1.3.2.3.2 (Caching) Proxy Arrays Where a user agent 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 (replica) origin server, see Section 3.1.1.)Cooper, et al. Informational [Page 12] RFC 3040 Internet Web Replication & Caching Taxonomy January 2001 ----------------- ----------------- ----------------- | Caching Proxy | | Caching Proxy | | Caching Proxy | | Array | | Array | | Array | ----------------- ----------------- ----------------- \ | / ----------------------------------------- | -------------- | Network | | Element | -------------- | /// | ------------ | Client | ------------ The interception proxy may be directly in-line of the flow of traffic - in which case the intercepting network element and interception proxy form parts of the same hardware system - or may be out-of-path, requiring the intercepting network element to redirect traffic to another network segment. In this latter case, communication protocols enable the intercepting network element to stop and start redirecting traffic when the interception proxy becomes (un)available. Details of these protocols can be found in Section 8.4. Replica Selection This section describes the schemes and protocols used in the cooperation and communication between client 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 Best known reference: This memo. Description:⌨️ 快捷键说明
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