Internet DRAFT - draft-ietf-rserpool-service
draft-ietf-rserpool-service
Network Working Group P. Lei
Internet-Draft Cisco Systems
Expires: April 9, 2006 P. Conrad
University of Delaware
October 6, 2005
Services Provided By Reliable Server Pooling
draft-ietf-rserpool-service-02.txt
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Copyright (C) The Internet Society (2005).
Abstract
The Reliable Server Pooling architecture (abbreviated "RSerPool", and
defined in [3]), provides a set of services and protocols for
building fault tolerant and highly available client/server
applications. This memo describes the semantics of the services that
RSerPool provides to upper layer protocols.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Conventions Used In This Document . . . . . . . . . . . . . . 4
3. Example Application Scenarios . . . . . . . . . . . . . . . . 4
3.1. Example Scenario for Failover Without RSerPool . . . . . . 4
3.2. Example Scenario Using RSerPool Basic Mode . . . . . . . . 5
3.3. Example Scenario Using RSerPool Enhanced Mode . . . . . . 7
4. Service Primitives . . . . . . . . . . . . . . . . . . . . . . 8
4.1. Initialization . . . . . . . . . . . . . . . . . . . . . . 9
4.2. PE Registration Services . . . . . . . . . . . . . . . . . 9
4.3. PE Selection Services . . . . . . . . . . . . . . . . . . 9
4.4. RSerPool Managed Data Channel . . . . . . . . . . . . . . 10
4.5. Failover Services . . . . . . . . . . . . . . . . . . . . 11
4.5.1. State Cookie Exchange . . . . . . . . . . . . . . . . 11
4.5.2. Failover Callback Function . . . . . . . . . . . . . . 11
4.5.3. Business Card . . . . . . . . . . . . . . . . . . . . 13
5. Transport Mappings . . . . . . . . . . . . . . . . . . . . . . 13
5.1. Defined Transport Mappings . . . . . . . . . . . . . . . . 13
5.2. Transport Mappings Requirements . . . . . . . . . . . . . 14
5.2.1. Mappings: Mandatory Requirements . . . . . . . . . . . 14
5.2.2. Mappings: Optional Requirements . . . . . . . . . . . 14
5.2.3. Mappings: Other Requirements . . . . . . . . . . . . . 15
6. Security Considerations . . . . . . . . . . . . . . . . . . . 15
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 15
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 15
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 16
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 17
Intellectual Property and Copyright Statements . . . . . . . . . . 18
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1. Introduction
The Reliable Server Pooling architecture is defined in [3]. The
central idea of this architecture is 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". The RSerPool architecture supports high-availabilty and
load balancing by enabling a pool user to identify the most
appropriate server from the server pool at a given time. The
architecture also supports failover to an alternate server when
needed.
This memo describes how an upper layer protocol or application for a
pool user or pool element uses the RSerPool architecture and
protocols. Specifically, it describes how the ASAP protocol [5] and
transport protocols (SCTP, TCP, etc.) can be utilized to realize
highly available services between pool users and pool elements.
The purpose of this document is to describe:
1. the precise services provided by RSerPool to the upper layer,
2. the tradeoffs in choosing which services to utilize,
3. how applications must be designed for each of these services,
4. how applications written over various transports (SCTP, TCP, and
others) can be mapped into these services.
RSerPool services can be used in one of two modes: "Basic Mode" and
"Enhanced Mode". Basic Mode provides a smaller set of services than
Enhanced Mode, but offers imposes fewer restrictions on the
application layer protocols that can be supported. Enhanced Mode
provides extra capabilities, including some features that require
applications to exchange application data messages via RSerPool
service primitives (a restriction not present in Basic Mode).
For Enhanced Mode, the RSerPool data exchange primitives are
implemented by multiplexing the ASAP messages and application data
over a single transport protocol connection or association. This
memo defines how to do this multiplexing over SCTP. This memo also
describes the requirments needed to extend support to other transport
protocols as required.
Note that while RSerPool services are divided into Basic and Enhanced
Modes, both modes assume a full implementation of the ASAP protocol.
The purpose of dividing RSerPool services into two modes is solely to
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provide more flexibility for applications to interact with RSerPool.
In particular, Basic Mode provides an easy migration path for legacy
applications to take advantage of many useful RSerPool services,
including load balancing and high availability. Enhanced Mode
extends the services provided by Basic Mode with enhanced failover
capabilities.
2. Conventions Used In This Document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [2].
3. Example Application Scenarios
To illustrate the differences between Basic and Enhanced Mode, this
section decribes example failover scenarios for:
an application written directly over a transport layer protocol,
not utilizing RSerPool
an application using RSerPool Basic Mode, and
an application using RSerPool Enhanced Mode.
3.1. Example Scenario for Failover Without RSerPool
Consider a typical client/server application that does not use a
reliable server pooling framework of any kind. Typically, the server
is specified by a DNS name. At some point, the application
translates this name to an IP address (via DNS), and subsequently
makes initial contact with the server to begin a session, via SCTP,
TCP, UDP, or other transport protocol. If the client loses contact
or fails to make contact with the server (either due to server
failure, or a failure in the network) the client must either abandon
the session, or try to contact another server.
In this scenario, the client must first determine that a failure took
place. There are several ways that a client application may
determine that a server failed, including the following:
1. The client may have sent a request to the server, and may time
out waiting for a response, or may receive a message such as "no
route to host", "port not available", or "connection refused".
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2. The client may have sent a request to the server, or may have
tried to initiate a connection or association and may have
received a connection/association failure error.
3. The client may already have established a connection to server,
but at some point receives an indication from the transport layer
that the connection failed.
Suppose that the client application has a feature by which the user
can enter the hostname of a secondary server to contact in the event
of failure. Once the application determines that a failure took
place on the primary server, the application can then attempt to
resolve the hostname of the secondary server, and contact the
secondary server to establish a session there. This process can be
iterated to a tertiary server, and so forth.
In this scenario, the identification of these alternate servers is an
additional burden placedd on the end user. Furthermore, there is no
capability in this model to dynamically update the identity of the
alternate servers based on current availablity or reachability. DNS
has some capabilities that can be used to help, but there are
significant limitations to these capabilities (See [4] for a
discussion of this point).
3.2. Example Scenario Using RSerPool Basic Mode
Now consider the same client/server application mentioned in
Section 3.1. First we describe what the application programmer must
do to modify the code to use RSerPool Basic Mode. We then describe
the benefits that these modifications provide.
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.
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3. Without the use of additional RSerPool services, failure
detection is application specific just as in the previous
scenario. 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, described in
Section 3.3), but nevertheless quite useful as compared to not using
RSerPool at all (as in Section 3.1). 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 [3]). Finally, when failures
occur, these are reported to the pool via signaling present in ASAP
[5]) and ENRP [6], other clients will eventually know (once this
failure is confirmed by other elements of the RSerPool architecture)
that this server has failed.
Utilizing this subset of services is useful for:
applications built over connectionless protocols such as UDP that
cannot easily be adapted to the transport layer requirements
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required for enhanced services (see section Section 5)
applications running on systems which do not provide an
appropriate mapping layer for the desired transport protcol
an expedient way to provide some of the benefits of RSerPool to
legacy applications (regardless of the transport protocol used)
However, to take full advantage of the RSerPool framework,
utilization of the complete set of Enhanced Mode services as
described in the next section is recommended.
3.3. Example Scenario Using RSerPool Enhanced Mode
Finally, consider the same client/server application as in
Section 3.1, but this time, modified to take advantage of RSerPool
Enhanced Mode services. As in the Section 3.1, we first describe the
modifications needed, then we describe the benefits provided.
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:
The RSerPool framework control channel provides maintainance
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 4.5.1) 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
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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 4.5.2) 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 4.5.3)).
Retrofitting an existing application for Enhanced Mode requires more
application programmer effort than retrofitting an application for
Basic Mode. In particular, all use of the transport layer's
primitives (e.g. the calls to the sockets API) must be replaced by
the use of the RSerPool primitives (e.g. the RSerPool API). This can
be mitigated by making the RSerPool API as close to existing
transport APIs as possible. However, the benefit is that failure
detection and failover is automated in this case. This automatic
failure detection takes advantage of heartbeat mechanisms that are
provided either in the underlying transport protocol, or in a mapping
defined on top of that protocol (see Section 4.5).
Provided that developers of APIs for RSerPool stay close to familiar
APIs for existing transport protocols, the effort of writing a new
applications over RSerPool Enhanced Mode need not be significantly
different from writing the same application directly over a supported
transport protocol or mapping.
4. Service Primitives
Upper layer protocols and applications may "choose" to use these
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primitive services as needed. By selecting and using the appropriate
set of service primitives, a range of failover scenarios may be
supported. These service primitives are described in the sub-
sections that follow.
4.1. Initialization
The INITIALIZE service is used to establish a service access point to
communicate with the ASAP layer on the local host. This is the first
service accessed by either a PU or a PE.
4.2. 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.
4.3. 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
flag when used in conjuction with data channel services (described in
Section 4.4) 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.
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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.
4.4. 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_REQUEST, to send data to a pool element by using a pool
handle, specific pool element handle, or by transport address.
When sending to a pool handle, the specific pool element handle
chosen is returned. In the case that data is sent to a pool
handle, or specific pool element handle, the user can request
automatic resending (on a best-effort basis) if the original pool
element selected is unreachable. (However, it is ultimately the
application's responsibility to detect and recover from errors,
using acknowledgements at the application layer if needed.)
When sending to a specific transport address, this primitive is
considered a "pass thru" to the underlying transport, and no
failover services are performed.
In each case, appropriate error code(s) are returned in the event
of failure. (see [5] for more detail).
DATA_RECEIVED_NOTIFICATION, 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.
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4.5. Failover Services
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 three "hooks" for applications to provide their own
application-specific failover mechanism(s), one on the PE side (State
Cookie Exchange), one on the PU side (Failover Callback), and one for
entities that are combination of PU/PE (business card).
4.5.1. State Cookie Exchange
SET_COOKIE: This is invoked by a PE to set the state cookie that
is sent periodically over the control channel, when present, from
a PE to a PU. The most recently received cookie is cached by the
PU; in the event of failover, it is forwarded to the new PE.
COOKIE_INDICATION: This is invoked by callback at a PE, when that
PE receives a cookie from a PU. This cookie is an indication that
the PU has failed over to the current PE from some other PE. The
contents of the cookie are provided to the PE prior to any
data.indication for messages arriving from the PU that sent the
cookie. This provides a hook by which a PE "X" can send a "hint"
to its successor PE "Y", in the event that one of X's PU's fails
over from X to Y. PE "Y" can use the contents of the cookie to
establish application layer state prior to processing resent or
new messages from the PU. The PU application layer is not
involved in any way in this exchange; it is handled automatically
by the ASAP layer.
4.5.2. Failover Callback Function
AUTHOR'S NOTE (PTC): the service defined in this section is not a
part of section 4 of the current version ASAP draft. It should
either be added to the ASAP draft, or this service should be removed
from the services draft, after discussion on the list. Open
question: does the cookie feature eliminate the need for this
feature?
An PU that establishing a session with a PE 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. In essence, this
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allows an application designed to put the reestablishment of state
into the PU side instead of the PE side, if desired.
This service that complements the cookie feature, in the following
way: the cookie feature provides failover hooks on the PE side, where
the callback is a failover hook for the PU side. The on-the-wire
impact is that it is important that the ASAP entity should invoke the
failover callback (if any is registered) prior to resending any
messages from previous DATA_SEND_REQUEST primitives.
Note that if both a state cookie from a PU and a failover callback
are present, the state cookie should be sent before the failover
callback is issued.
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 is sufficient to result in a sequence of response
messages from the requested offset to the end of the file.
In this protocol, all that is needed for failover is for the
application to:
keep track of the lowest byte that it has not yet received from
the server,
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
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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.
4.5.3. Business Card
This section TBD... describe a service primitive
Set.BusinessCard.PE.List. What should the parameters be? This
service primitive should also be defined in Section 4 of the ASAP
draft.
5. Transport Mappings
While SCTP is the preferred transport layer protocol for applications
built for RSerPool failover mode (for reasons explained shortly), it
is also possible to use other transport protocols as well (e.g. TCP)
if an SCTP implementation is not available on the client and/or
server. However, there are certain features present in SCTP that are
required if the RSerPool framework is to function in failover mode.
When a transport protocol other than SCTP is used, these features
must be provided by an "adaption layer" (also called a "shim
protocol") that sits between the base transport protocol (e.g. TCP)
and the RSerPool layer. We refer to these "adaptation layers" or
"shim protocols" as "mappings" as the idea is that the requirements
of the RSerPool framework are "mapped" onto the capabilities of the
underlying protocol (e.g. SCTP or TCP).
5.1. Defined Transport Mappings
In order to support the RSerPool framework over a variety of
transport protocols and configurations, several mappings are defined
to provide RSerPool services over a given transport protocol. Each
mapping translates the requirements of the RSerPool framework onto
the capabilities of the transport protocol desired (e.g. SCTP, TCP,
etc.). Initially, three mappings are defined:
NO_MAPPING (0x00): With this mapping, no RserPool control channel
is provided and the application specific communication between a
pool user and the pool element (e.g. data channel) is out of scope
of RSerPool. However, pool elements can register the application
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specific communication "protocol" and "port", and thus can be
provided to pool users.
SCTP (0x01): SCTP transport is used for the RSerPool control
channel. The data channel MAY be multiplexed onto the same SCTP
association, if desired. This mapping is the preferred mapping.
TCP (0x02): TCP transport is used for the RSerPool control
channel. The data channel MAY be multiplexed onto the same TCP
connection, if desired.
A particular pool element might support any combination of these
mappings in order to support a variety of pool users with different
capabilities (i.e. different mapping support). In this case, pool
elements should register each mapping that it supports with its
pool(s).
5.2. Transport Mappings Requirements
5.2.1. Mappings: Mandatory Requirements
These features MUST be present in any mapping of the RSerPool
framework mode to TCP (or any other transport protocol):
1. Message orientation, which facilitates application re-
synchronization during failover. Messages must be "framed" in
order to allow for undelivered message retrieval from the
transport protocol.
2. A heartbeat mechanism to monitor the health of an association or
connection.
3. A mechanism to transport and differentiate between control
channel messages (e.g. ASAP messages) and data channel messages.
For example in SCTP, the payload protocol identifier (PPID) may
be used.
4. [NOTE: retrieval was eliminated here as a requirement, now that
failover is best effort.]
5.2.2. Mappings: Optional Requirements
There are several additional features that are present in SCTP that
are lacking in TCP. While these features are not crucial to
RSerPool, providing them in the mapping layer makes it easier for an
application layer programmer to write to a single API. This single
API can then be mapped over both SCTP and TCP, as well as any other
transport protocol for which a mapping is provided. Since these
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features are not essential for RSerPool, they are optional in any
defined mapping. However, appropriate error messages or indications
should be provided when these features are not available. These
features include:
1. Support for multiple streams
2. Support for unordered delivery of messages
5.2.3. Mappings: Other Requirements
There are some features of SCTP that a mapping may not be able to
provide, because they would require access to transport layer
internals, or modifications in the transport layer itself. The
services provided by the RSerPool layer to the application should
therefore provide mechanisms for the upper layer to access these
features when present (e.g. in SCTP), but also provide appropriate
error messages or indications that these features are not available
when they cannot be provided. These features include:
1. Application access to the RTT and RTO estimates
2. Application access to the Path MTU value
3. Application access to set the lifetime parameter on outgoing SCTP
messages
6. Security Considerations
[Open Issue TBD: Security issues are not discussed in this memo at
this time, but will be added in a later version of this draft.]
7. IANA Considerations
[Open Issue TBD: Will there be an enumeration of the various
transport layer mappings that must be registered with IANA?]
8. Acknowledgements
The authors wish to thank Maureen Stillman, Qiaobing Xie, Michael
Tuexen, Randall Stewart, and many others for their invaluable
comments.
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9. References
[1] Bradner, S., "The Internet Standards Process -- Revision 3",
BCP 9, RFC 2026, October 1996.
[2] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[3] Tuexen, M., "Architecture for Reliable Server Pooling",
draft-ietf-rserpool-arch-10 (work in progress), July 2005.
[4] Loughney, J., "Comparison of Protocols for Reliable Server
Pooling", draft-ietf-rserpool-comp-10 (work in progress),
July 2005.
[5] Stewart, R., "Aggregate Server Access Protocol (ASAP)",
draft-ietf-rserpool-asap-12 (work in progress), July 2005.
[6] Stewart, R., "Endpoint Handlespace Redundancy Protocol (ENRP)",
draft-ietf-rserpool-enrp-12 (work in progress), July 2005.
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Authors' Addresses
Peter Lei
Cisco Systems
8735 W Higgins Rd, Suite 300
Chicago, IL 60631
US
Phone: +1 773 695 8201
Email: peterlei@cisco.com
Phillip T. Conrad
University of Delaware
Dept. of Computer and Information Sciences
103 Smith Hall
Newark, DE 19716
US
Phone: +1 302 831 8622
Email: conrad@acm.org
URI: http://udel.edu/~pconrad
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