Reliable Server Pooling Working L. Coene
group Siemens
Internet-Draft P. Conrad
Expires: April 15, 2004 University of Delaware
P. Lei
Cisco
October 16, 2003
Reliable Server pool applicability Statement
<draft-ietf-rserpool-applic-01.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."
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 15, 2004.
Copyright Notice
Copyright (C) The Internet Society (2003). All Rights Reserved.
Abstract
This document describes the applicability of the reliable server pool
architecture and protocols to applications which want to have High
availability services. This is accomplished by using redundant
servers and failover between servers of the same pool in case of
server failure. Processing load in a pool may de distributed/shared
between the members of the pool according to a certain policy. Also
some guidance is given on the choice of underlying transport protocol
(and corresponding transport protocol mapping) for transporting
application data and Rserpool specific control data.
Coene, et al. Expires April 15, 2004 [Page 1]
Internet-Draft Rspool applicability October 2003
Table of Contents
1. INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1 Scope . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.2 Terminology . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Reliable serverpool . . . . . . . . . . . . . . . . . . . . 4
2.1 Architecture . . . . . . . . . . . . . . . . . . . . . . . . 4
2.2 ASAP/ENRP applicability . . . . . . . . . . . . . . . . . . 4
2.2.1 Minimal Rserpool service . . . . . . . . . . . . . . . . . . 4
2.2.2 Full Rserpool service . . . . . . . . . . . . . . . . . . . 5
3. Application and Control data Transport . . . . . . . . . . . 6
3.1 Rserpool use between 2 pools . . . . . . . . . . . . . . . . 6
3.2 state sharing via the cookie . . . . . . . . . . . . . . . . 6
3.3 PE Registration Services . . . . . . . . . . . . . . . . . . 6
3.4 Failover Callback Function . . . . . . . . . . . . . . . . . 6
3.5 PE Selection Services . . . . . . . . . . . . . . . . . . . 7
3.6 Upper Layer/Application Level Acknowledgements . . . . . . . 8
3.7 RSerPool Managed Data Channel . . . . . . . . . . . . . . . 8
4. Transport protocols used by ENRP & ASAP . . . . . . . . . . 10
4.1 ASAP on top of UDP . . . . . . . . . . . . . . . . . . . . . 10
4.2 ASAP on top of TCP . . . . . . . . . . . . . . . . . . . . . 10
4.3 ASAP on top of SCTP . . . . . . . . . . . . . . . . . . . . 10
4.4 Address hiding . . . . . . . . . . . . . . . . . . . . . . . 10
5. Proxies and Rserpool . . . . . . . . . . . . . . . . . . . . 12
6. Issues for Reliable Server pooling . . . . . . . . . . . . . 13
6.1 State transfer accoss the server pool . . . . . . . . . . . 13
7. Security considerations . . . . . . . . . . . . . . . . . . 14
8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . 15
References . . . . . . . . . . . . . . . . . . . . . . . . . 16
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 16
Intellectual Property and Copyright Statements . . . . . . . 18
Coene, et al. Expires April 15, 2004 [Page 2]
Internet-Draft Rspool applicability October 2003
1. INTRODUCTION
Reliable server pooling provides protocols for providing higly
available services. The services are located in pool of redundant
servers and if a server fails, another server will take over. The
only requirement put on these servers belonging to the pool is that
if state is maintained by the server, this state must be transfered
to the other server taking over. The mechanism for transfering this
state information is NOT part of the Reliable server pooling
architecture and/or protocols and must be provided by other
protocols.
The goal is to provide server based redundancy. Transport and network
level redundancy are handled by the transport and network layer
protocols.
The application may choose to distribute its traffic over the servers
of the pool conforming to a certain policy.
The application wishing to make use of Rserpool protocols may use
different transport layers(such as UDP, TCP and SCTP). However some
transport layers may have restrictions build in in the way they might
be operating in the Rserpool architecture and its protocols.
1.1 Scope
The scope of this document is to explore the different ways that
Reliable server pool protocols can be used in order to provide a
highly available service towards applications with different
requirements.
1.2 Terminology
The terms are commonly identified in related work and can be found in
the Aggregate Server Access Protocol and Endpoint Name Resolution
Protocol Common Parameters documentRFC ARCH [2].
Coene, et al. Expires April 15, 2004 [Page 3]
Internet-Draft Rspool applicability October 2003
2. Reliable serverpool
2.1 Architecture
A overview of the reliable server pool architecture is given in the
Rserpool architecture document RFC ARCH [2].
The Rserpool architecture is made up of clients(Pool Users - PU) and
servers(Pool Elements - PE). Both PU and PE's can be grouped into a
pool in which a PE provides a service(File transfer, storage, bank
transaction) to a PU. The PU's may try to find out via the endpoint
resolution protocol(ENRP) which PE's are active. The PU can set up a
communication channel with a particular PE(chosen out of the server
pool) by using the Aggregate Server Access Protocol (ASAP) or by
using directly any of the transport protocols(UDP/TCP/SCTP/RTP). ASAP
may be running on top of UDP, TCP or SCTP.
The minimum mode of using Rserpool is to use only the ENRP for
Endpoint name resolution. The PU may setup the client - server
communication WITHOUT ASAP, but using present transport
protocols(such as UDP, TCP..)
The normal use of Rserpool is to use ENRP for Enpoint name resolution
and ASAP for client - server communication. ASAP may be using as
underlying transport protocol UDP, TCP or SCTP.
2.2 ASAP/ENRP applicability
2.2.1 Minimal Rserpool service
The minimum service provided by Rserpool is the use of ENRP for
Endpoint name resolution. The ENRP procol may be running over TCP or
SCTP.
o Endpoint name resolution
o no automatic failover from one PE to another, has to be done by
the application itself
o bussinesscard or cookie mechanism not possible
o May be used by already existing applications which do not want to
change the interface between PU and PE.
o Only PU-NS and PE-NS communication will use Rserpool protocols
Coene, et al. Expires April 15, 2004 [Page 4]
Internet-Draft Rspool applicability October 2003
2.2.2 Full Rserpool service
The fullservice provided by Rserpool is the use of ENRP for Endpoint
name resolution and the Use of ASAP for PU - PE communication . ENRP
may be running over TCP or SCTP while ASAP may be running over TCP,
SCTP, UDP or RTP.
o Endpoint name resolution
o automatic failover from one PE to another is transparent for the
application itself
o bussinesscard exhange for determining if a PU is a pool or not. It
allows the PE to treat the PU's as pool and use Rserpool protocols
for it
o cookie mechanism can be used for state transfer between PE's
o May be used by allready existing applications which do not want to
change the interface between PU and PE.
o All entities will use Rserpool protocols for communication with
their respective peers
Coene, et al. Expires April 15, 2004 [Page 5]
Internet-Draft Rspool applicability October 2003
3. Application and Control data Transport
3.1 Rserpool use between 2 pools
Bussinesscards will allow to detect if their peer is part of a pool
itself. Both the PU and the PE can be part of their own pools. If the
PU or PE would fails, then the businesscard will have informed the
respective peer to contact a alternative fellow PE/PU belonging to
the pool.
3.2 state sharing via the cookie
Every time a response is send back, a cookie could be send along the
response. The cookie is "encrypted" and is stored by the PU, no
modification at all it done to the cookie . If a PE fails then the
cookie is send to a alternate PE, the PE check if the cookie is
valid. The contents of the cookie is only provided and validated by
the PE. It can be used for state sharing between the PE.
3.3 PE Registration Services
Pool Elements ("server") must use the following services to add or
remove themselves from server pools: REGISTER, to add the pool
element into a server pool using {pool handle, mapping mode, protocol
or mapping id, port, policy info} where mapping mode is defined in
Section 5. A response result code is returned. DEREGISTER, to remove
the pool element from a server pool using {pool handle, mapping mode,
protocol or mapping id, port, policy info} where mapping mode is
defined in Section 5. A response result code is returned. TBD: if
REGISTER also returns an opaque instance id, the application can just
use that id for DEREGISTER, instead of passing in the (same)
parameters used in REGISTER.
3.4 Failover Callback Function
The charter of the RSerPool Working Group specifically states that
transaction failover is out of scope for RSerPool, i.e. "if a server
fails during processing of a transaction this transaction may be
lost. Some services may provide a way to handle the failure, but
this is not guaranteed." Accordingly, the RSerPool framework
provides a "hook" for applications to provide their own application-
specific failover mechanism(s).
Specifically, an application can specify a callback function that is
invoked whenever a failover has taken place. This callback function
is invoked immediately after the new transport layer connection/
association is established with a new server, and gives the
application the opportunity to send one or more messages that may
Coene, et al. Expires April 15, 2004 [Page 6]
Internet-Draft Rspool applicability October 2003
help the server to resume any transaction or session that was in
progress when the first server failed.
As a simple example of how such a callback is useful, consider a file
transfer service built using RSerPool. Let us assume that some FTP
mirroring software is used to maintain mirrored sites, and that the
actual mirroring is out of scope. However, we would like to use
RSerPool to select a server from among the available mirror sites,
and to failover in the middle of a file transfer if a primary server
fails.
For this example, assume that a simple request/response protocol is
used, where one request message results in one or more response
messages. Each request message contains the filename, and the offset
desired within the file, (default zero.) Each response message
contains some portion of the file, along with the offset, length of
the portion in this message, and the length of the entire file.
A single request results is sufficient to result in a sequence of
response messages from the requested offset to the end of the file.
For simplicity, assume that the response messages are delivered by
the underlying transport strictly in order (although this requirement
could be relaxed if a small amount of extra complexity were
introduced.)
In this protocol, all that is needed for failover is for the
application to keep track of the number of bytes that it has read
from the server, and to provide a callback function that reissues the
request to the new server, replacing the offset with this number.
When there is no failover, only one request message is sent and the
minimum number of response messages are returned; in the event of
failover(s), single new request message is sent for each failover
that occurs.
While this is a simple example, for more complex application
requirements, the failover callback could be used in a variety of
ways: The client might send security credentials for authentication
by the server, and/or to provide a "key" by which the server could
locate and setup state by accessing some application-specific (and
out-of-scope) state sharing mechanism used by the servers. The client
might keep track of various synchronization points in the
transaction, and use the failover callback to replay message from a
recent synchronization point.
3.5 PE Selection Services
When automatic failover is enabled, selection of a new pool element
according to the pool policy in place is automatically performed by
Coene, et al. Expires April 15, 2004 [Page 7]
Internet-Draft Rspool applicability October 2003
the RSerPool framework in case of a detected failure (e.g. provides
automatic failover). No application intervention is required.
Automatic failover may be enabled by setting the appropriate send
flag when used in conjuction with data channel services (described in
Section 4.6) or explicitly during initialization when data channel
services are not used. FAILOVER_INDICATION, delivered by callback,
indicates that a failover has occurred and that any required
application level state recovery should be performed. The newly
selected pool element handle is provided. Business Card services:
when automatic failover is used, the exchange of business cards for
rendezvous services is automatically performed by the RSerPool
framework (e.g. no application intervention is required. When
automatic failover is not enabled, failover detection and selection
of an alternate PE must be done by the upper layer/ application. The
following primitives are provided: GET_PRIMARY_SERVER, takes as input
a pool handle and returns the {IP address, transport protocol,
transport protocol port} of the primary server. GET_NEXT_SERVER has a
dual meaning. First, it indicates to the RSerPool layer the failure
of the server returned by a previous GET_PRIMARY_SERVER or
GET_NEXT_SERVER call. Second, it provides the {IP address, transport
protocol, transport protocol port} of the next server that should be
contacted, according to the best information available to the
RSerPool layer at the present time. The appropriate pool policy for
server selection for the pool should be used for selecting the next
server.
3.6 Upper Layer/Application Level Acknowledgements
The RSerPool framework provides an upper layer/application level ack
service. The upper layer protocol may request that the peer
acknowledge receipt and successful processing of its sent data,
providing an additional degree of confidence over transport level
message retrieval. When used in conjuction with the data channel
services (described in Section 4.6), any unacknowledged data will be
automatically sent to a new pool element in case of failover, if
desired (e.g. automatic failover is enabled). The following service
primitive is used to acknowledge an upper layer acknowledgement
request. ULP_ACK, responds to a received upper layer acknowledgement
request.
3.7 RSerPool Managed Data Channel
The RSerPool framework provides these services to send and receive
application layer data, which are used in place of the direct call of
transport level system functions (e.g. send/sendto, recv/recvfrom)
and provides additional functionality to those calls.
DATA_SEND, to send data to a pool element by using a pool handle,
Coene, et al. Expires April 15, 2004 [Page 8]
Internet-Draft Rspool applicability October 2003
specific pool element handle, or by transport address. An upper
layer acknowledgement may be requested with this service. Appropriate
error code(s) are returned. When sending to a pool handle, the
specific pool element handle is returned.
DATA_INDICATION, delivered by callback, to indicate that data has
been received from a pool element and to pass that data to the
application layer protocol. An application layer acknowledgement
request can be indicated along with the data.
The application MAY direct that the RSerPool framework multiplex both
the control and data channels onto the same SCTP association/TCP
connection/ etc., if desired.
Coene, et al. Expires April 15, 2004 [Page 9]
Internet-Draft Rspool applicability October 2003
4. Transport protocols used by ENRP & ASAP
4.1 ASAP on top of UDP
UDP is a unreliable message transport delivery protocol, so if a
message gets lost due to a changeover of server(or client), then the
message will not be retransmitted after changeover has occured. New
messages will be sent to alternate server/client within the
serverpool.
This service may be of some importance to services where realtime
constraints apply.(Example video servers: a few lost message ain't
that important as long as the big bulk of messages get through). No
congestion control is done and as such no real measure of the
congestion status on the server(or client) is taken into account,
thus making loadsharing harder. Only the ENRP server responsible for
that particular server pool will have an up to date view of the load
distribution in the pool.
4.2 ASAP on top of TCP
TCP provides full reliable delivery with congestion control of the
message to its peer node. It provides for a single homed, single
stream delivery of a byte stream from or to the server. Change over
will retrieve the unsent messages and send them on another TCP
connection to a different server of the server pool.
4.3 ASAP on top of SCTP
PR-SCTP is the only know protocol which allows the choice of full,
partial or no reliable delivery with congestion control of the
message to its peer node. If the no-reliable delivery option is
selected of SCTP, then ASAP will function as described in ASAP over
UDP and including congestion control.
if multihoming, streams, unsequenced and/or assured delivery are
required for the application, then SCTP should be used for ASAP. See
SCTP aplicability statement RFC 3257 [9].
4.4 Address hiding
If an application requires only a single address(due to memory
constraints) to reach a pool element of a pool , then ASAP can
provide one address at a time when quering the ENRP server. If that
pool element fails, then the client must request a new address from
the ENRP server, before it can fail-over(as it has no information
about the other pool elements of the same pool except the pool
handle). This is done by ASAP itself in the full Rserpool service,
Coene, et al. Expires April 15, 2004 [Page 10]
Internet-Draft Rspool applicability October 2003
but must be done by the client software itself in minimal Rserpool
service.
This may require some buffering in the client during the failover.
Coene, et al. Expires April 15, 2004 [Page 11]
Internet-Draft Rspool applicability October 2003
5. Proxies and Rserpool
Application which require absolutely no protocol changes to their
clients, may be able to use Rserpool protocols by using a proxy
between the client and the server pool. Neither ASAP nor ENRP is used
by the client application, but the proxy employs ENRP and ASAP. The
client will only know the IP address and portnumbers of the proxy to
contact. This can be accomplished via normal DNS queries.
The main drawback is that the proxy becomes the single point of
failure for the connection between the client and the server.
Coene, et al. Expires April 15, 2004 [Page 12]
Internet-Draft Rspool applicability October 2003
6. Issues for Reliable Server pooling
6.1 State transfer accoss the server pool
Rserpool protocols(ENRP and ASAP) do NOT provide any service for
directly transfering state information of a application from one
Processing Element(PE) to another PE.
However by using the ASAP cookie mechanims, the PU may be able to
transfer some state provided by the PE to the PU, to the new PE in
case of failover. This is the responsability of the PU to do this.
Coene, et al. Expires April 15, 2004 [Page 13]
Internet-Draft Rspool applicability October 2003
7. Security considerations
The protocols used in the Reliable server pool architecture only
tries to increase the availability of the servers in the network.
Rserpool protocols does not contain any protocol mechanisms which are
directly related to user message authentication, integrity and
confidentiality functions. For such features, it depends on the IPSEC
protocols or on Transport Layer Security(TLS) protocols for its own
security and on the architecture and/or security features of its user
protocols.
A overview of possible treats to Reliable Server pooll protcols is
detailed in RFC TREAT [8].
Rserpool architecture allows the use of different Transport protocols
for its application and control data exchange. Those transport
protocols may have mechanisms for reducing the risk of blind
denial-of-service attacks and/or masquerade attacks. If such measures
are required by the applications, then it is advised to check the
SCTP applicability statement[RFC3057] for guidance on this issue.
Coene, et al. Expires April 15, 2004 [Page 14]
Internet-Draft Rspool applicability October 2003
8. Acknowledgments
The authors wish to thank H. Hazewinkel, M. Urena and M. Stillman and
many others for their invaluable comments.
Coene, et al. Expires April 15, 2004 [Page 15]
Internet-Draft Rspool applicability October 2003
References
[1] Tuexen, M., Stewart, R., Shore, M., Xie, Q., Ong, L., Loughney,
J. and M. Stillman, "Requirements for Reliable Server Pooling",
RFC 3237, January 2002.
[2] Tuexen, M., Stewart, R., Shore, M., Xie, Q., Ong, L., Loughney,
J. and M. Stillman, "Architecture for Reliable Server Pooling",
Draft in progress , October 2002.
[3] Stewart, R., Xie, Q., Stillman, M. and M. Tuexen, "Aggregate
Server Access Protocol (ASAP)", Draft in progress , October
2002.
[4] Xie, Q., Stewart, R. and M. Stillman, "Endpoint Name Resolution
Protocol (ENRP)", Draft in progress , October 2002.
[5] Stewart, R., Xie, Q., Stillman, M. and M. Tuexen, "Aggregate
Server Access Protocol and Endpoint Name Resolution Protocol
Common Parameters", Draft in progress , October 2002.
[6] Conrad, P. and P. Lei, ""Services Provided By Reliable Server
Pooling", Draft in progress , January 2003.
[7] Stewart, R., Xie, Q., Morneault, K., Sharp, C., Schwarzbauer,
H., Taylor, T., Rytina, I., Kalla, M., Zhang, L. and V. Paxson,
""Stream Control Transmission Protocol"", RFC 2960, October
2000.
[8] Stillman, M., Gopal, R., Sengodan, S., Guttman, E. and M.
Holdrege, ""Threats Introduced by Rserpool and Requirements for
Security in response to Threats"", RFC zzzz, Nov 2002.
[9] Coene, L., ""Stream Control Transmission Protocol Applicability
statement"", RFC 3257, April 2002.
Authors' Addresses
Lode Coene
Siemens
Atealaan 32
Herentals 2200
Belgium
Phone: +32-14-252081
EMail: lode.coene@siemens.com
Coene, et al. Expires April 15, 2004 [Page 16]
Internet-Draft Rspool applicability October 2003
Phil Conrad
University of Delaware
USA
Phone: +
EMail: pconrad@acm.org
Peter Lei
Cisco
8735 W Higgins Rd, Suite 300
Chicago, IL 60631
USA
Phone: +1 847 870 7201
EMail: peter.lei@ieee.org
Coene, et al. Expires April 15, 2004 [Page 17]
Internet-Draft Rspool applicability October 2003
Intellectual Property Statement
The IETF takes no position regarding the validity or scope of any
intellectual property or other rights that might be claimed to
pertain to the implementation or use of the technology described in
this document or the extent to which any license under such rights
might or might not be available; neither does it represent that it
has made any effort to identify any such rights. Information on the
IETF's procedures with respect to rights in standards-track and
standards-related documentation can be found in BCP-11. Copies of
claims of rights made available for publication and any assurances of
licenses to be made available, or the result of an attempt made to
obtain a general license or permission for the use of such
proprietary rights by implementors or users of this specification can
be obtained from the IETF Secretariat.
The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary
rights which may cover technology that may be required to practice
this standard. Please address the information to the IETF Executive
Director.
Full Copyright Statement
Copyright (C) The Internet Society (2003). All Rights Reserved.
This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph are
included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of
developing Internet standards in which case the procedures for
copyrights defined in the Internet Standards process must be
followed, or as required to translate it into languages other than
English.
The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assignees.
This document and the information contained herein is provided on an
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
Coene, et al. Expires April 15, 2004 [Page 18]
Internet-Draft Rspool applicability October 2003
HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Acknowledgment
Funding for the RFC Editor function is currently provided by the
Internet Society.
Coene, et al. Expires April 15, 2004 [Page 19]
