Network Working Group P. Lei Internet-Draft Cisco Systems, Inc. Intended status: Informational L. Ong Expires:
April 18,October 30, 2007 Ciena Corporation M. Tuexen Muenster Univ. of Applied Sciences October 15, 2006April 28, 2007 An Overview of Reliable Server Pooling Protocols draft-ietf-rserpool-overview-00.txtdraft-ietf-rserpool-overview-01.txt Status of this Memo By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she is aware have been or will be disclosed, and any of which he or she becomes aware will be disclosed, in accordance with Section 6 of BCP 79. 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." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. This Internet-Draft will expire on April 18,October 30, 2007. Copyright Notice Copyright (C) The Internet Society (2006).IETF Trust (2007). Abstract The Reliable Server Pooling effort (abbreviated "RSerPool"), provides an application-independent set of services and protocols for building fault tolerant and highly available client/server applications. This document provides an overview of the protocols and mechanisms in the reliable server pooling suite. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. ASAPAggregate Server Access Protocol (ASAP) Overview . . . . . . . . . . . . . . . . . . . . . . . .5 2.1. Pool Initialization . . . . . . . . . . . . . . . . . . . 5 2.2. Pool Entity Registration . . . . . . . . . . . . . . . . . 5 2.3. Pool Entity Selection . . . . . . . . . . . . . . . . . . 56 2.4. Endpoint Keepalive . . . . . . . . . . . . . . . . . . . . 6 2.5. Failover Services . . . . . . . . . . . . . . . . . . . . 6 2.5.1. Cookie Mechanism . . . . . . . . . . . . . . . . . . . 6 2.5.2. Business Card Mechanism . . . . . . . . . . . . . . . 6 2.5.3. Failover Callback Mechanism . . . . . . . . . .7 3. Endpoint Nameserver Redundancy Protocol (ENRP) Overview . . . 7 3. ENRP .3.1. Initialization . . . . . . . . . . . . . . . . . . . . . . 7 3.2. Server Discovery and Home Server Selection . . . . . . 7 3.1. Initialization. . 7 3.3. Server Pool Maintenance . . . . . . . . . . . . . . . . . 8 4. Example Scenarios . . . 7 3.2. Server Discovery. . . . . . . . . . . . . . . . . . . 8 4.1. Example Scenario Using RSerPool Resolution Service . . 7 3.3. Server Pool Maintenance. . 8 4.1.1. Standalone Mode . . . . . . . . . . . . . . . 8 4. Example Scenarios. . . . 8 4.1.2. Pool Mode . . . . . . . . . . . . . . . . . . 8 4.1. Example Scenario Using RSerPool Resolution Service. . . . 8 4.2. Example Scenario Using RSerPool Session Services . . . . . 9 5. Security Considerations . . . . . . . . . . . . . . . . . . . 1011 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 1011 7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 11 8. Normative References . . . . . . . . . . . . . . . . . . . . . 11 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 1112 Intellectual Property and Copyright Statements . . . . . . . . . . 13 1. Introduction The requirements for theReliable Server Pooling effort are defined in RFC3237 . The central idea of this architecture(RSerPool) protocol suite is designed to provide client applications ("pool users") with the ability to select a server (a "pool element") from among a group of servers providing equivalent service (a "pool"). The pool is accessed via an identifier called a "pool handle", which is a location-independent name separate from the IP address of any pool server. TheRSerPool architecture supports high-availability and load balancing by enabling a pool user to identify the most appropriate server from the server pool at a given time. The architecture is defined to support a set of basic goals: o application-independent protocol mechanisms o separation of server naming from IP addressing o use of the end-to-end principle to avoid dependancies on intermediate equipment o separation of session availability/failover functionality from application itself o facilitate different server selection policies o facilitate a set of application-independent failover capabilities o peer-to-peer structure The basic components of the Rserpool architecture are shown in Figure 1 below: ....................... ------ . +-------+ . / ENRP \ . | | . /---->| Server | . | PE 1 | . | /--- \______/ . | | . | | . +-------+ . | | . . | | . server pool . | V . . +-------+ . +-------+ . | | . | | . | PU 1 |-----------------.------| PE 2 | . | | . | | . +-------+ . +-------+ . . . . +-------+ . . | | . . | PE 3 | . . | | . . +-------+ . ....................... Figure 1 A server pool is defined as a set of one or more servers providing the same application functionality. The servers are called Pool Elements (PEs). Multiple PEs in a server pool can be used to provide fault tolerance or load sharing, for example. The PEs register into and deregisters out of the pool using the Aggregate Server Access Protocol ASAP .. Each server pool is identified by a unique byte string called the pool handle. The pool handle allows a mapping from the pool to a specific Pool Element located by its IP address and port. The pool handle is what is specified by the Pool User (PU) when it attempts to access a server in the pool, again using ASAP. Both IPv4 and IPv6 PE addresses are supported. To resolve the pool handle to the address necessary to access a Pool Element, the PU consults an entity called the Endpoint haNdlespace Redundancy Protocol (ENRP) server. This server may be a standalone server supporting many PUs or a part of the PU itself, however it is envisioned that ENRP servers provide a fully distributed and fault- tolerant registry service using ENRP  to maintain synchronization of data concerning the pool handle mapping space. Rserpool provides a number of tools to aid client migration between servers on server failure: it allows the client to identify alternative servers, either on initial discovery or in real time; it also allows the original server to provide a state cookie to the client that can be forwarded to an alternative server to provide application-specific state information. The requirements for the Reliable Server Pooling effort are defined in RFC3237 . This document provides an overview of the RSerPool protocol suite, specifically the Aggregate Server Access Protocol ASAP  and the Endpoint Nameserver Redundancy Protocol ENRP .. In addition to the protocol specifications, there is a common parameter format specification COMMON  for both protocols, as well as a security threat analysis SEC .. 2. ASAPAggregate Server Access Protocol (ASAP) Overview ASAP is a straight-forward implementation of a set of mechanisms identified as necessary for support of the creation and maintenance of pools of redundant servers. These mechanisms include: o registration of a new server for the server pool o deregistration of an existing server infrom the pool o resolution of a pool 'handle' to a server or list of servers o liveness detection for servers in the pool o failover mechanisms for handling server failure 2.1. Pool Initialization Pools come into existence when a PE registers the first instance of the pool name.name with an ENRP server. They disappear when the last PE deregisters. In other words, the starting of the first PE on some machine causes the creation of the pool when the registration reaches the ENRP server. 2.2. Pool Entity Registration A new server joins an existing pool by sending a Registration message in ASAP to an ENRP server, indicating the 'handle' of the pool that it wishes to join, a pool identifier for itself (chosen randomly) information about it's lifetime in the pool, and what transport protocols and selection policies it supports. The Registration messageENRP server that it first contacts is sent tocalled its Home ENRP server.server, and maintains a list of subscriptions by the PE as well as performing periodic audits to confirm that the PE is still responsive. Similar procedures are applied to de-register itself from the server pool (or alternatively the server may simply let its previously statethe lifetime expire and bethat it previously registered with expire, after which it is gracefully removed from the pool. 2.3. Pool Entity Selection When an endpoint wishes to be connected to a server in the pool, it genereates a Handle Resolution message in ASAP and sends this requiresto its home ENRP server. The ENRP server resolves the resolutionhandle based on its knowledge of pool servers and returns a server 'handle' toHandle Resolution Response in ASAP. The Resolution Response contains a list of the IP addresses of a serverone or list ofmore servers in the pool. Thispool that can be contacted. The process by which the list of servers is created may involve a number of policies for server selection, for which theselection. The RSerPool protocol suite supports a few simply defined policies and allows the use of external server selection input for more complex policies. The endpoint generates a Handle Resolution message in ASAP and sends this to its home ENRP server to start the resolution process. The ENRP server resolves the handle based on its knowledge of pool servers and returns a Handle Resolution Response in ASAP.2.4. Endpoint Keepalive In order to maintain status information for members of the server pool, theENRP server may auditservers monitor the status of a particularpool elementelements using anthe ASAP Keep Alive message. When received by the pool element, itA Pool Element responds by verifying its membership into the pool inASAP Keep Alive message with an Ack message. When a PE is found to be unreachable, for example,response. In addition, an endpoint conversing with the pool element finds that it can no longer be reached by its transport connection, the endpointcan also informnotify its home ENRP server that the PE the endpoint was using has become unresponsive by sending the ENRP server an Endpoint Unreachable message. 2.5. Failover Services While maintaining application-independence, the RSerPool protocol suite provides some simple hooks for supporting failover of an individual session with a pool element. Generally, mechanisms for failover that rely on application state or transaction status cannot be supporteddefined without more specific knowledge of the application being supported. However, some simple mechanisms supported by RSerPool allow some level of failover that any application can use. 2.5.1. Cookie Mechanism Cookies may optionally be generated by the ASAP layer and periodically sent from the PE to the PU. The PU only stores the last received cookie. In case of fail over the PU sends this last received cookie to the new PE. This method provides a simple way of state sharing between the PEs. Please note that the old PE should sign the cookie and the receiving PE should verify the signature. For the PU, the cookie has no structure and is only stored and transmitted to the new PE. 2.5.2. Business Card Mechanism A PE can send a business card to its peer (PE or PU) containing its pool handle and guidance concerning which other PEs the peer should use for failover. This gives a PE a means of telling a PU what it identifies as the "next best" PE to use in case of failure, which may be based on pool considerations, such as load balancing, or user considerations, such as PEs that have the most up-to-date state information. 2.5.3. Failover Callback Mechanism TBD3. Endpoint Nameserver Redundancy Protocol (ENRP) Overview A server pool can be supported by one or more ENRP servers. If multiple ENRP servers are used to support a single pool then the ENRP protocol is used between the ENRP servers in order to maintain a distributed, fault-tolerant real-time registry service. ENRP servers communicate with each other in order to exchange information such as pool membership changes, handlespace data synchronization, etc. 3.1. Initialization Each ENRP server initially generates a 32-bit server ID that it uses in subsequent messaging and remains unchanged over the lifetime of the server. It then attempts to learn all of the other ENRP servers within the scope of the server pool, either by using a pre-defined Mentor server or by sending out Presence messages on a well-known multicast channel to determine other ENRP servers from the responses and select one as Mentor. A Mentor can be any peer ENRP server that it selects to provide current data about the pool. It then requests the most current data about the pool handlespace from its Mentor server and unpacks received Handle Table Response messages into its local database. It is then ready to provide ENRP services. 3.2. Server Discovery and Home Server Selection PEs can now register their presence with the newly functioning ENRP server by using ASAP messages. They discover the new ENRP server after the server sends out an ASAP Server Announce message on the well-known ASAP multicast channel. PEs need only register with one ENRP server, as other ENRP servers supporting the pool will synchronize their knowledge about pool elements using the ENRP protocol. The PE may have a configured list of ENRP servers to talk to, in the form of a list of IP addresses, in which case it will start to setup associations with some number of them and assign the first one that responds to it as its Home ENRP Server. Alternatively it can listen on the multicast channel for a set period and when it hears an ENRP server, start an association. The first server it gets up can then become its Home ENRP Server. 3.3. Server Pool Maintenance PE failure detection, keepalive, etc. TBD 4. Example Scenarios 4.1. Example Scenario Using RSerPool Resolution Service 4.1.1. Standalone Mode RSerPool can be used in a 'standalone' manner, where the application uses RSerPool to determine the address of a primary server in the pool, and then interacts directly with that server without further use of RSerPool services. If the initial server fails, the application uses RSerPool again to find the next server in the pool. 4.1.2. Pool Mode For pool user ("client") applications, if an ASAP implementation is available on the client system, there are typically only three modifications required to the application source code: 1. Instead of specifying the hostnames of primary, secondary, tertiary servers, etc., the application user specifies a pool handle. 2. Instead of using a DNS based service (e.g. the Unix library function gethostbyname()) to translate from a hostname to an IP address, the application will invoke an RSerPool service primitive GETPRIMARYSERVER that takes as input a pool handle, and returns the IP address of the primary server. The application then uses that IP address just as it would have used the IP address returned by the DNS in the previous scenario. 3. Without the use of additional RSerPool services, failure detection and failover procedures must be designed into each application. However, when failure is detected on the primary server, instead of invoking DNS translation again on the hostname of a secondary server, the application invokes the service primitive GETNEXTSERVER, which performs two functions in a single operation. 1. First it indicates to the RSerPool layer the failure of the server returned by a previous GETPRIMARYSERVER or GETNEXTSERVER call. 2. Second, it provides the IP address of the next server that should be contacted, according to the best information available to the RSerPool layer at the present time (e.g. set of available pool elements, pool element policy in effect for the pool, etc.). For pool element ("server") applications where an ASAP implementation is available, two changes are required to the application source code: 1. The server should invoke the REGISTER service primitive upon startup to add itself into the server pool using an appropriate pool handle. This also includes the address(es) protocol or mapping id, port (if required by the mapping), and pooling policy(s). 2. The server should invoke the DEREGISTER service primitive to remove itself from the server pool when shutting down. When using these RSerPool services, RSerPool provides benefits that are limited (as compared to utilizing all services), but nevertheless quite useful as compared to not using RSerPool at all. First, the client user need only supply a single string, i.e. the pool handle, rather than a list of servers. Second, the decision as to which server is to be used can be determined dynamically by the server selection mechanism (i.e. a "pool policy" performed by ASAP; see ASAP ).). Finally, when failures occur, these are reported to the pool via signaling present in ASAP  and ENRP ,, other clients will eventually know (once this failure is confirmed by other elements of the RSerPool architecture) that this server has failed. 4.2. Example Scenario Using RSerPool Session Services When the full suite of RSerPool services are used, all communication between the pool user and the pool element is mediated by the RSerPool framework, including session establishment and teardown, and the sending and receiving of data. Accordingly, it is necessary to modify the application to use the service primitives (i.e. the API) provided by RSerPool, rather than the transport layer primitives provided by TCP, SCTP, or whatever transport protocol is being used. As in the previous case, sessions (rather than connections or associations) are established, and the destination endpoint is specified as a pool handle rather than as a list of IP addresses with a port number. However, failover from one pool element to another is fully automatic, and can be transparent to the application:application (so long as the application has saved enough state in a state cookie): The RSerPool framework control channel provides maintainancemaintenance functions to keep pool element lists, policies, etc. current. Since the application data (e.g. data channel) is managed by the RSerPool framework, unsent data (data not yet submitted by RSerPool to the underlying transport protocol) is automatically redirected to the newly selected pool element upon failover. If the underlying transport layer supports retrieval of unsent data (as in SCTP), retrieved unsent data can also be automatically re- sent to the newly selected pool element. An application server (pool element) can provide a state cookie (described in Section 2.5.1) that is automatically passed on to another pool element (by the ASAP layer at the pool user) in the event of a failover. This state cookie can be used to assist the application at the new pool element in recreating whatever state is needed to continue a session or transaction that was interrupted by a failure in the communication between a pool user and the original pool element. The application client (pool user) can provide a callback function (described in Section 2.5.2) that is invoked on the pool user side in the case of a failover. This callback function can execute any application specific failover code, such as generating a special message (or sequence of messages) that helps the new pool element construct any state needed to continue an in-process session. Suppose in a particular peer-to-peer application, PU A is communicating with PE B, and it so happens that PU A is also a PE in pool X. PU A can pass a "business card" to PE B identifying it as a member of pool X. In the event of a failure at A, or a failure in the communication link between A and B, PE B can use the information in the business card to contact an equivalent PE to PU A from pool X. Additionally, if the application at PU A is aware of some particular PEs of pool X that would be preferred for B to contact in the event that A becomes unreachable from B, PU A can provide that list to the ASAP layer, and it will be included in A's business card. (See Section 2.5.2). 5. Security Considerations This document does not identify security requirements beyond those already documented in the ENRP and ASAP protocol specifications. 6. IANA Considerations This document does not require additional IANA actions beyond those already identified in the ENRP and ASAP protocol specifications. 7. Acknowledgements The authors wish to thank Maureen Stillman, Qiaobing Xie, Randall Stewart, Scott Bradner, and many others for their invaluable comments. 8. Normative References  Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. Tuexen, M., Xie, Q., Stewart, R., Shore, M., Ong, L., Loughney, J., and M. Stillman, "Requirements for Reliable Server Pooling", RFC 3237, January 2002.  Stewart, R., "Aggregate Server Access Protocol (ASAP)", draft-ietf-rserpool-asap-13draft-ietf-rserpool-asap-15 (work in progress), February 2006. January 2007.  Stewart, R., "Endpoint Handlespace Redundancy Protocol (ENRP)", draft-ietf-rserpool-enrp-13draft-ietf-rserpool-enrp-15 (work in progress), February 2006. January 2007.  Stewart, R., "Aggregate Server Access Protocol (ASAP) and Endpoint Handlespace Redundancy Protocol (ENRP) Parameters", draft-ietf-rserpool-common-param-10draft-ietf-rserpool-common-param-11 (work in progress), FebruaryOctober 2006.  Stillman, M., "Threats Introduced by Rserpool and Requirements for Security in response to Threats", draft-ietf-rserpool-threats-05draft-ietf-rserpool-threats-06 (work in progress), July 2005.November 2006. Authors' Addresses Peter Lei Cisco Systems, Inc. 955 Happfield Dr. Arlington Heights, IL 60004 US Phone: +1 773 695-8201 Email: email@example.com Lyndon Ong Ciena Corporation PO Box 308 Cupertino, CA 95015 US Email: Lyong@Ciena.com Michael Tuexen Muenster Univ. of Applied Sciences Stegerwaldstr. 39 48565 Steinfurt Germany Email: firstname.lastname@example.org Full Copyright Statement Copyright (C) The Internet Society (2006).IETF Trust (2007). This document is subject to the rights, licenses and restrictions contained in BCP 78, and except as set forth therein, the authors retain all their rights. 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