Reliable Server Pooling Working L. Coene
group Siemens
Internet-Draft P. Conrad
Expires: January 16, 2005 University of Delaware
P. Lei
Cisco
July 18, 2004
Reliable Server pool applicability Statement
<draft-ietf-rserpool-applic-02.txt>
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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
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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.
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 . . . . . . . . . . . . . . . . 4
2.2.3 Endpoint name resolution . . . . . . . . . . . . . . . 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 TCP . . . . . . . . . . . . . . . . . . . . 10
4.2 ASAP on top of SCTP . . . . . . . . . . . . . . . . . . . 10
4.3 Address hiding . . . . . . . . . . . . . . . . . . . . . . 10
5. Proxies and Rserpool . . . . . . . . . . . . . . . . . . . . . 11
6. Issues for Reliable Server pooling . . . . . . . . . . . . . . 12
6.1 State transfer accoss the server pool . . . . . . . . . . 12
6.2 ENRP Name servers . . . . . . . . . . . . . . . . . . . . 12
7. Security considerations . . . . . . . . . . . . . . . . . . . 13
8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 14
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 14
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 15
Intellectual Property and Copyright Statements . . . . . . . . 16
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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 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].
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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 name
resolution servers using the Aggregate Server Access Protcol (ASAP)
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 ASAP or
by using directly any of the transport protocols(UDP/TCP/SCTP/
RTP...). ASAP may be running on top of TCP or SCTP.
The name resolution servers may communicate among themselves via the
use of the Endpoint Name Resolution protocol(ENRP). This will allow
for name resolution and synchronisation of the namespace used.
The minimum mode of using Rserpool is to use only ASAP 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 ASAP for Enpoint name resolution
AND for client - server communication. ASAP may be using as
underlying transport protocol TCP or SCTP.
2.2 ASAP/ENRP applicability
2.2.1 Minimal Rserpool service
The minimum service provided by Rserpool is the use of ASAP for
Endpoint name resolution. The ASAP 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
2.2.2 Full Rserpool service
The fullservice provided by Rserpool is the use of ASAP for both
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Endpoint name resolution and for PU - PE communication . ASAP may
be running over TCP or SCTP.
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
2.2.3 Endpoint name resolution
Resolving a pool name towards the pool handle is the function of the
ENRP servers.
o Endpoint name resolution for PU's and PE's
o Home ENRP server for the PE's: de/registration
o Discovery, maintenance and synchronisation of the ENRP namespace
o
o
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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.
3.4 Failover Callback Function
Session/transaction failover is not performed by ASAP themselves.
"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." Tthe RSerPool
implementation may provide 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
help the server to resume any transaction or session that was in
progress when the first server failed.
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As a simple example of how such a callback is useful, consider a file
transfer service built using RSerPool. Assume that some FTP
mirroring software is used to maintain mirrored sites, and that the
actual mirroring is out of scope. RSerPool would be used 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.
In 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:
o 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.
o 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
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
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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,
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
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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.
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4. Transport protocols used by ENRP & ASAP
4.1 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.
ASAP uses the TCP mapping layer RFC TCPM [7]
4.2 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 [10].
4.3 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,
but must be done by the client software itself in minimal Rserpool
service.
This may require some buffering in the client during the failover.
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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.
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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.
6.2 ENRP Name servers
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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 [9].
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.
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8. Acknowledgments
The authors wish to thank H. Hazewinkel, M. Urena and M. Stillman
and many others for their invaluable comments.
9 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] Conrad, P. and P. Lei, "TCP Mapping for Reliable Server Pooling
Failover Mode", Draft in progress , June 2003.
[8] 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.
[9] 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.
[10] Coene, L., ""Stream Control Transmission Protocol Applicability
statement"", RFC 3257, April 2002.
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Authors' Addresses
Lode Coene
Siemens
Atealaan 32
Herentals 2200
Belgium
Phone: +32-14-252081
EMail: lode.coene@siemens.com
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
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