draft-ietf-rserpool-comp-07.txt   draft-ietf-rserpool-comp-08.txt 
Network Working Group J. Loughney, Ed. Network Working Group J. Loughney, Ed.
Internet-Draft M. Stillman Internet-Draft M. Stillman
Expires: April 1, 2004 Nokia Expires: January 17, 2005 Nokia
Q. Xie Q. Xie
Motorola Motorola
R. Stewart R. Stewart
Cisco Cisco
A. Silverton A. Silverton
Motorola Motorola
October 02, 2003 July 16, 2004
Comparison of Protocols for Reliable Server Pooling Comparison of Protocols for Reliable Server Pooling
draft-ietf-rserpool-comp-07.txt draft-ietf-rserpool-comp-08.txt
Status of this Memo Status of this Memo
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and any of which I become aware will be disclosed, in accordance with
RFC 3668.
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Copyright Notice Copyright Notice
Copyright (C) The Internet Society (2003). All Rights Reserved. Copyright (C) The Internet Society (2004). All Rights Reserved.
Abstract Abstract
This document compares protocols that may be applicable for the This document compares protocols that may be applicable for the
Reliable Server Pooling problem space. This document discusses the Reliable Server Pooling problem space. This document discusses the
usage and applicability of these protocols for the Reliable Server usage and applicability of these protocols for the Reliable Server
Pooling architecture. Pooling architecture.
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 3 1.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.2 Terminology . . . . . . . . . . . . . . . . . . . . . . . . 3 1.2 Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
1.3 Abbreviations . . . . . . . . . . . . . . . . . . . . . . . 3 1.3 Abbreviations . . . . . . . . . . . . . . . . . . . . . . 3
2. Relation to Other Solutions . . . . . . . . . . . . . . . . 4 2. Relation to Other Solutions . . . . . . . . . . . . . . . . . 4
2.1 CORBA . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 2.1 CORBA . . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.2 DNS . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 2.2 DNS . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.2.1 Requirements . . . . . . . . . . . . . . . . . . . . . . . . 5 2.2.1 Requirements . . . . . . . . . . . . . . . . . . . . . 5
2.2.2 Technical Issues . . . . . . . . . . . . . . . . . . . . . . 6 2.2.2 Technical Issues . . . . . . . . . . . . . . . . . . . 6
2.3 Service Location Protocol (SLP) . . . . . . . . . . . . . . 9 2.3 Dynamic Delegation Discovery System (DDDS) and URI . . . . 9
2.3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . 9 2.4 Service Location Protocol (SLP) . . . . . . . . . . . . . 11
2.3.2 What to Use . . . . . . . . . . . . . . . . . . . . . . . . 9 2.4.1 Introduction . . . . . . . . . . . . . . . . . . . . . 11
2.3.3 Summary of SLP Issues . . . . . . . . . . . . . . . . . . . 11 2.4.2 What to Use . . . . . . . . . . . . . . . . . . . . . 12
2.4 L4/L7 Switching . . . . . . . . . . . . . . . . . . . . . . 12 2.4.3 Summary of SLP Issues . . . . . . . . . . . . . . . . 13
2.4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . 12 2.5 L4/L7 Switching . . . . . . . . . . . . . . . . . . . . . 15
2.4.2 L4 Switching . . . . . . . . . . . . . . . . . . . . . . . . 12 2.5.1 Introduction . . . . . . . . . . . . . . . . . . . . . 15
2.4.3 L7 Switching . . . . . . . . . . . . . . . . . . . . . . . . 14 2.5.2 L4 Switching . . . . . . . . . . . . . . . . . . . . . 15
2.4.4 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . 15 2.5.3 L7 Switching . . . . . . . . . . . . . . . . . . . . . 16
2.5 ASAP and ENRP . . . . . . . . . . . . . . . . . . . . . . . 16 2.5.4 Summary . . . . . . . . . . . . . . . . . . . . . . . 18
2.5.1 ASAP . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 2.6 ASAP and ENRP . . . . . . . . . . . . . . . . . . . . . . 19
2.5.2 ENRP . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 2.6.1 ASAP . . . . . . . . . . . . . . . . . . . . . . . . . 19
3. Comparison Against Requirements . . . . . . . . . . . . . . 17 2.6.2 ENRP . . . . . . . . . . . . . . . . . . . . . . . . . 19
4. Security Concerns . . . . . . . . . . . . . . . . . . . . . 18 3. Comparison Against Requirements . . . . . . . . . . . . . . . 20
5. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 18 4. Security Concerns . . . . . . . . . . . . . . . . . . . . . . 21
Normative References . . . . . . . . . . . . . . . . . . . . 18 5. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 21
Non-Normative References . . . . . . . . . . . . . . . . . . 19 6. References . . . . . . . . . . . . . . . . . . . . . . . . . . 21
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 19 6.1 Normative References . . . . . . . . . . . . . . . . . . . . 21
Intellectual Property and Copyright Statements . . . . . . . 21 6.2 Non-Normative References . . . . . . . . . . . . . . . . . . 21
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 22
Intellectual Property and Copyright Statements . . . . . . . . 24
1. Introduction 1. Introduction
1.1 Overview 1.1 Overview
In creating a solution to provide reliable server pools [1], there In creating a solution to provide reliable server pools [1], there
are a number of existing protocols, which appear to have similar are a number of existing protocols, which appear to have similar
properties as to what RSERPOOL is trying to accomplish. This document properties as to what RSERPOOL is trying to accomplish. This
discusses the applicability of these protocols in meeting the document discusses the applicability of these protocols in meeting
requirements of Reliable Server Pooling [2]. the requirements of Reliable Server Pooling [2].
This study does not intend to be complete, rather intends to This study does not intend to be complete, rather intends to
highlight several protocols which working group members have highlight several protocols which working group members have
suggested. suggested.
1.2 Terminology 1.2 Terminology
This document uses the following terms: This document uses the following terms:
Operational Scope: The part of the network visible to pool users by a Operational Scope: The part of the network visible to pool users by a
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isn't sufficient because the issues would still exist for all the isn't sufficient because the issues would still exist for all the
legacy systems, which will form the bulk of the host population for legacy systems, which will form the bulk of the host population for
years to come. The solution is not to use third party servers. years to come. The solution is not to use third party servers.
Additionally, if the client can contact the server directly, then the Additionally, if the client can contact the server directly, then the
server knows the real IP address of the client. Since there is no server knows the real IP address of the client. Since there is no
third party involved, the caching TTL can be set as low as desired third party involved, the caching TTL can be set as low as desired
(even to zero). That will increase load on the server, but nowhere (even to zero). That will increase load on the server, but nowhere
else. else.
Finally, DNS is based on a recursion. This recursion presents certain Finally, DNS is based on a recursion. This recursion presents
difficulties for RSERPOOL. Even if a host resolver is not a stub certain difficulties for RSERPOOL. Even if a host resolver is not a
resolver, it has to go to another full resolver where 2 possibilities stub resolver, it has to go to another full resolver where 2
exists: either the mapping name-IP address is found or it has to do possibilities exists: either the mapping name-IP address is found or
another recursive resolution of the name, staring from that it has to do another recursive resolution of the name, staring from
intermediate resolver, until there is a cache hit in one of the that intermediate resolver, until there is a cache hit in one of the
intermediate resolvers or it is resolved by its root resolver (or intermediate resolvers or it is resolved by its root resolver (or
home DNS server). home DNS server).
This process of recursion means that there is no end-to-end This process of recursion means that there is no end-to-end
communication between the host and its server where the name-to-IP communication between the host and its server where the name-to-IP
mapping resides. That also means that a lot of timers are running in mapping resides. That also means that a lot of timers are running in
intermediate systems. Any updating of the transient status of the intermediate systems. Any updating of the transient status of the
pool element or of the pool may need to be propagated through the pool element or of the pool may need to be propagated through the
DNS. DNS.
2.2.2.2 Dynamic Registration 2.2.2.2 Dynamic Registration
Registration / de-registration of servers is needed. It can be done Registration / de-registration of servers is needed. It can be done
with DNS by NOTIFY/IXFR. However, frequent updates and replication with DNS by NOTIFY/IXFR. However, frequent updates and replication
are incompatible. This is not a DNS problem per se, but it has an are incompatible. This is not a DNS problem per se, but it has an
effect on DNS as it is deployed. effect on DNS as it is deployed.
RSERPOOL MUST allow software server entities (i.e., PEs) to register RSERPOOL must allow software server entities (i.e., PEs) to register
themselves with a name server dynamically. They can also de-register themselves with a name server dynamically. They can also de-register
themselves for purposes of preventative maintenance or can be themselves for purposes of preventative maintenance or can be
de-registered by a name server that believes the server entity is no de-registered by a name server that believes the server entity is no
longer operational. This is a dynamic approach, which is coordinated longer operational. This is a dynamic approach, which is coordinated
through servers in the pool and among RSERPOOL name servers. through servers in the pool and among RSERPOOL name servers.
2.2.2.3 Load Balancing 2.2.2.3 Load Balancing
RFC 2782 [3] itself points out some of the limitations of using DNS RFC 2782 [3] itself points out some of the limitations of using DNS
SRV for load balancing between servers. SRV for load balancing between servers.
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redirecting clients to servers randomly as various caches in various redirecting clients to servers randomly as various caches in various
resolvers expire at random (although small) intervals. DNS offers resolvers expire at random (although small) intervals. DNS offers
excellent network scalability but poor control over load balance. excellent network scalability but poor control over load balance.
As mentioned previously, the issue of doing DNS-based dynamic load As mentioned previously, the issue of doing DNS-based dynamic load
balancing on short time scales will have impacts on third parties, balancing on short time scales will have impacts on third parties,
due to the presence of stub resolvers. due to the presence of stub resolvers.
2.2.2.4 Heartbeating & Status Monitoring 2.2.2.4 Heartbeating & Status Monitoring
DNS does not incorporate an application layer heartbeat. Heartbeating RSERPOOL working group has agreed that one of its main design goals
would dramatically boost traffic levels, and given the unavoidable for RSERPOOL is "...performance for supporting real-time
third party dependencies of DNS, the resulting loading would be applications", as reflected in RFC 3237 [2]. An example of such
unacceptable. It is passive in the sense that it does not monitor or real-time applications would be the IP-based call control
store information on the state of the host such as whether the host applications in a 3G cellular network.
is up or down or what kind of load it is currently experiencing.
RSERPOOL SHOULD monitor the state of each server entity on various To achieve this goal, it is felt critical that RSERPOOL monitors the
hosts on a continual basis and can collect several state variables state of each server entity on various hosts on a continual basis and
including up/down state and current load. If a server is no longer collects several state variables including up/down state and current
operational, eventually it will be dropped from the list of available load. If a server is no longer operational, eventually it will be
servers maintained by the name server, so that subsequent application dropped from the list of available servers maintained by the name
name queries will not resolve to this server address. server, so that subsequent application name queries will not resolve
to this server address.
DNS does not incorporate an application layer heartbeat.
Heartbeating would dramatically boost traffic levels, and given the
unavoidable third party dependencies of DNS, the resulting loading
would be unacceptable. It is passive in the sense that it does not
monitor or store information on the state of the host such as whether
the host is up or down or what kind of load it is currently
experiencing.
It is not entirely impossible to make DNS utilize the assistance of
an external heartbeat function/protocol for this monitoring purpose.
However, to achieve the degree of real-time performance RSERPOOL is
seeking, one would most likely need a tight coupling of this external
function to the DNS operation. This in turn would likely result in
substantial modification of the existing DNS, which is what we want
to avoid.
2.2.2.5 Name/Address Resolution Granularity 2.2.2.5 Name/Address Resolution Granularity
The technical requirement for DNS name/address resolution is The technical requirement for DNS name/address resolution is
basically that given a name, find a host associated with this name basically that given a name, find a host associated with this name
and return its IP address(es). In other words, in DNS we have the and return its IP address(es). In other words, in DNS we have the
following mapping: following mapping:
Name ; a host machine Name ; a host machine
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contains no such mechanism. contains no such mechanism.
2.2.2.7 Lack of Support for Redundancy Models 2.2.2.7 Lack of Support for Redundancy Models
Server pooling as defined in RSERPOOL requires support for different Server pooling as defined in RSERPOOL requires support for different
redundancy arrangements or models depending on the needs of the redundancy arrangements or models depending on the needs of the
specific application. Commonly used models in practice includes N+M, specific application. Commonly used models in practice includes N+M,
N-active, etc. These models basically define how a PE behaves when N-active, etc. These models basically define how a PE behaves when
another PE in the same pool fails and it is often critical for the another PE in the same pool fails and it is often critical for the
application to have full control over this behavior of each PE in the application to have full control over this behavior of each PE in the
pool. Without major extensions, it seems difficult for DNS to support pool. Without major extensions, it seems difficult for DNS to
such redundancy models. support such redundancy models.
2.3 Service Location Protocol (SLP) 2.3 Dynamic Delegation Discovery System (DDDS) and URI
2.3.1 Introduction In this section, we discuss the difficulties for RSERPOOL to make use
of DDDS and URI as building blocks for its distributed pool handle
database (i.e., RSERPOOL namespace).
RFC 3401 [5] defines DDDS as an abstract algorithm for applying
dynamically retrieved string transformation rules to an
application-unique string. DDDS has been found useful for URI
Resolution, ENUM telephone number to URI resolution, and the NAPTR
DNS resource record.
As stated in [5], DDDS is "used to implement lazy binding of strings
to data, in order to support dynamically configured delegation
systems. The DDDS functions by mapping some unique string to data
stored within a DDDS Database by iteratively applying string
transformation rules until a terminal condition is reached."
In order to discuss the applicability of DDDS/URI in RSERPOOL, we
need first talk about some fundamental characteristics of pool
handles or names in RSERPOOL.
It is important to note that, handles or names in RSERPOOL are meant
to identify pools comprising of _generic_ communications nodes.
Those nodes in reality will be some kind of servers - service
applications that runs on some networked machines. However, it is
very important to note that the working group has never had the
intention to go beyond the "server of generic IP applications" in its
pool definition. Nor has it seen the need to categorize the types of
the service applications for the purpose of RSERPOOL. This is
fundamentally different from the assumption behind URI as well as the
Service Location Protocol (to be discussed below).
With the above noted, here are some additional characteristics of
RSERPOOL handles/names:
1. RSERPOOL handles have only local significance, i.e., there is no
requirement for pool handles to be globally unique.
This is because the first tier of applications we envision for
RSERPOOL is those tightly coupled local systems that can use
RSERPOOL to make its components highly available. For example, a
3G radio access network that contains charging server, call
controller, media server, etc. where RSERPOOL can make those
currently singleton elements into pools and thus gain high
availability. This type of local systems can be as compact as a
bunch of server blades located in a single high performance
chassis or a group of closely located boxes in a central office
or in a few closely located buildings. The use of RSERPOOL in
such scenarios can be totally transparent to the outside world.
For example, a SIP phone may be talking to a softswitch without
knowing that the call control elements inside the softswitch are
a bunch of RSERPOOL-enabled pools. In such cases, the pool names
has no need to be globally unique (there is even no need for the
outside to know they exist). They only need to be unique within
the softswitch itself.
We also have considered the possibility of supporting larger
scale (even global) deployment cases of RSERPOOL. In the
requirement, we indicate we want RSERPOOL to be able to do that
but we have made it clear that RSERPOOL will not be able to do
that by itself. Instead, it will rely on existing external
infrastructures (e.g., DNS, possibly URN/DDDS) to bridge locally
scoped RSERPOOL clouds into a larger scale deployment.
2. RSERPOOL handles have no need for supporting any structure/
syntax.
As merely locally significant identifiers for distinguishing
pools of generic communication nodes, we consider adding
structure/syntax to RSERPOOL handle definition will buy us
nothing but will have real negative impact on the performance and
increase the implementation complexity. The only recommend we
have made so far is to use NULL-terminated ascii string for the
pool handles. This seems to meet our needs nicely.
3. RSERPOOL handles are relatively dynamic.
We consider that the pools may change relatively frequently; they
may come and go as the system re-adjusts its capacity or
configuration. We do not envision the handles to be long lived.
4. RSERPOOL handles are not for human readers.
Unlike UNIs (URN/URL), we do not envision RSERPOOL handles to
appear in e-mails, web documents, etc. for human viewing.
Probably the only case where RSERPOOL handles will be read by a
human is in a log file or configuration file, just like other
system configuration parameters.
Due to the aforementioned characteristics of RSERPOOL handles, we do
not see the benefit for directly using URNs/URIs for RSERPOOL
handles. The two rather fundamental requirements (per RFC 1737) that
brought us URNs - the global scope and persistence - do not apply to
RSERPOOL handles. Moreover, as mentioned above, we see little
benefit of making RSERPOOL handles human-readable or parsable in free
text.
In the future, there may be the possibility that URNs and the
associated infrastructure (e.g., DNS, DDDS) to play a vital role when
we start to consider wide area or global deployment scenarios for
RSERPOOL. For instance, a SIP client device that is looking for
certain network resource will start with a URN that is eventually
resolved to a pool handle that is then passed to an RSERPOOL
namespace server, which in turn will resolve the pool name to a list
of reachable/routable transport addresses of the server instances.
Similarly, since RSERPOOL handles are not global and have no
structure/syntax, we do not see that using DDDS inside RSERPOOL can
bring meaningful benefits.
2.4 Service Location Protocol (SLP)
2.4.1 Introduction
SLP [4] is comprised of three components: User Agents (UA), Service SLP [4] is comprised of three components: User Agents (UA), Service
Agents (SA) and Directory Agents (DA). User agents work on the user's Agents (SA) and Directory Agents (DA). User agents work on the
behalf to contact a service. The UA retrieves service information user's behalf to contact a service. The UA retrieves service
from service agents or directory agents. A service agent works on information from service agents or directory agents. A service agent
behalf of one or more services to advertise services. A directory works on behalf of one or more services to advertise services. A
agent collects service advertisements. directory agent collects service advertisements.
The directory agent of SLP simply acts as a cache and is passive in The directory agent of SLP simply acts as a cache and is passive in
this regard. The directory agent is optional and SLP can function this regard. The directory agent is optional and SLP can function
without it. It is incumbent upon the servers to update the cache as without it. It is incumbent upon the servers to update the cache as
necessary by reregistering. The directory server is not required in necessary by reregistering. The directory server is not required in
small networks as the user agents can contact service agents directly small networks as the user agents can contact service agents directly
using multicast. Unicast queries to SAs are possible subsequent to using multicast. Unicast queries to SAs are possible subsequent to
the UA having discovered them. User agents are encouraged to locate a the UA having discovered them. User agents are encouraged to locate
directory at regular intervals if they can't find one initially, a directory at regular intervals if they can't find one initially,
otherwise they can detect DAs by listening passively for DA otherwise they can detect DAs by listening passively for DA
advertisements. advertisements.
2.3.2 What to Use 2.4.2 What to Use
Figure 1 shows how SLP might be realized to provide RSERPOOL endpoint Figure 1 shows how SLP might be realized to provide RSERPOOL endpoint
name resolution (ENR) services: name resolution (ENR) services:
Pool User (PU) ENR Service Pool Endpoint (PE) Pool User (PU) ENR Service Pool Endpoint (PE)
+-------------+ +---------+ +-------------+ +---------+
| APPLICATION | | SERVICE | | APPLICATION | | SERVICE |
+-+-------------+-+ +---+---------+---+ +-+-------------+-+ +---+---------+---+
|ASAP/RSERPOOL API| <--------------------> |ASAP/RSERPOOL API| |ASAP/RSERPOOL API| <--------------------> |ASAP/RSERPOOL API|
+-+----+--------+-+ +----------+ +-+--------+----+-+ +-+----+--------+-+ +----------+ +-+--------+----+-+
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DAs in the mesh. For example, it will forward the registrations DAs in the mesh. For example, it will forward the registrations
the SLP SA made on behalf of the PE on right of Figure 1. the SLP SA made on behalf of the PE on right of Figure 1.
o SCTP is used for communication between entities. Multicast UDP is o SCTP is used for communication between entities. Multicast UDP is
used by SLP entities for active and passive discovery. While the used by SLP entities for active and passive discovery. While the
RSERPOOL architecture cannot rely upon multicast mechanisms, it RSERPOOL architecture cannot rely upon multicast mechanisms, it
can profit from them when these are present in the network can profit from them when these are present in the network
SLPv2 will be needed, but SLPv2 alone does not fulfill RSERPOOL SLPv2 will be needed, but SLPv2 alone does not fulfill RSERPOOL
update requirements for timeliness. This is achieved through mesh- update requirements for timeliness. This is achieved through mesh-
enhancements to the Service Location Protocol (mSLP) [5]. enhancements to the Service Location Protocol (mSLP) [6].
These enhancements make it possible for SAs to know of only a subset These enhancements make it possible for SAs to know of only a subset
of all DAs. Mesh-enhanced SAs need only forward their registrations of all DAs. Mesh-enhanced SAs need only forward their registrations
to only one mesh-enhanced DA. The mesh takes care of forwarding the to only one mesh-enhanced DA. The mesh takes care of forwarding the
message to the other DAs. message to the other DAs.
2.3.3 Summary of SLP Issues 2.4.3 Summary of SLP Issues
The most fundamental difference between SLP and RSERPOOL is that SLP A fundamental difference between SLP and RSERPOOL is that SLP is a
is service-oriented while RSERPOOL is communication-oriented. More protocol that focuses on the service level, while RSERPOOL is at the
specifically, what SLP provides to its user is a mapping function communication level. More specifically, what SLP provides to its
from a name of a service to the location of the service provider, in user is a mapping function from a name of a _service_ (e.g., b/w
the form of a URL string. The availability of the service provider is printing, color printing, faxing) to the location of the service
outside of the scope of SLP. How a service is accessible can be provider, in the form of a URL string. The availability of the
described by the SLP attribute list associated with the service URL. service provider is outside of the scope of SLP. How a service is
SLP is essentially a discovery protocol, not a transport protocol. accessible can be described by the SLP attribute list associated with
Therefore, the granularity of SLP operation is at application service the service URL. SLP is essentially a discovery protocol, not a
level. transport protocol. Therefore, the granularity of SLP operation is
at application service level.
In contrast, RSERPOOL provides to its user is a mapping function from In contrast, what RSERPOOL provides to its user is a mapping function
a communication destination name to a set of routable and reachable from a communication destination name (i.e., a pool handle) to a set
transport addresses that leads to a group of distributed software of routable and reachable transport addresses that leads to a group
server entities registered under that name that collectively of distributed software server entities registered under that name.
represent the named communication destination. With respect to SLP, RSERPOOL has NO intention to understand or convey to its user what is
this information could be represented in SLP attributes. RSERPOOL, the service (e.g., printing, faxing, document scanning) the named
however, also has the responsibility of reliably delivering a user pool is providing at the application level. In other words, the
message to one of these server entities. responsibility of RSERPOOL is only to reliably deliver a user message
to one of those server entities in the destination pool.
Currently, mSLP would need changes, for example it was designed to In theory, information such as transport addresses and their
scale to ~10 DAs not ~100 DAs. Additionally, SLP is currently reachability could be represented in SLP attributes. Currently, mSLP
designed to run on top of UDP and TCP. If SCTP support is needed, would need changes, for example it was designed to scale to ~10 DAs
some additional specification work would be needed. not ~100 DAs. Additionally, SLP is currently designed to run on top
of UDP and TCP. If SCTP support is needed, some additional
specification work would be needed.
SLP security makes no attempt to address the confidentiality of data SLP security makes no attempt to address the confidentiality of data
transmitted between SLP agents. To properly address this concern, SLP transmitted between SLP agents. To properly address this concern,
agents would need to establish secure communication with each other. SLP agents would need to establish secure communication with each
This would be achieved through the use of IPSec Encapsulating other. This would be achieved through the use of IPSec Encapsulating
Security Payload. Security Payload.
Server discovery, however, is something which SLP does well, and if Server discovery, however, is something which SLP does well, and if
used for RSERPOOL, this would be useful. used for RSERPOOL, this would be useful.
Other difficulties and shortcomings for using SLP to implement Other difficulties and shortcomings for using SLP to implement
RSERPOOL include: RSERPOOL include:
o Due to the fact that the resolution granularity of SLP is at the o Due to the fact that the resolution granularity of SLP is at the
service level, it relies on a syntax rich scheme to define service level, it relies on a syntax rich scheme to define
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space to identify which entities are to be included in the space to identify which entities are to be included in the
response to a specific query. This type of complicated processing response to a specific query. This type of complicated processing
and searching for each query may severely limit the performance of and searching for each query may severely limit the performance of
SLP in a real-time world which is a key requirement of RSERPOOL. SLP in a real-time world which is a key requirement of RSERPOOL.
o Without major extensions, SLP will not be able to provide a o Without major extensions, SLP will not be able to provide a
solution for real-time or semi-real-time fault detection and solution for real-time or semi-real-time fault detection and
recovery. This is partially because SLP is a discovery protocol, recovery. This is partially because SLP is a discovery protocol,
not a communication protocol. not a communication protocol.
2.4 L4/L7 Switching 2.5 L4/L7 Switching
2.4.1 Introduction 2.5.1 Introduction
This section discusses L4 and L7 switching techniques and their This section discusses L4 and L7 switching techniques and their
relation to the RSERPOOL architecture [2]. Since these technologies relation to the RSERPOOL architecture [2]. Since these technologies
are highly proprietary, it is difficult to discuss these techniques are highly proprietary, it is difficult to discuss these techniques
in a thorough manner. in a thorough manner.
In both cases, the deployment of these techniques is dependent upon In both cases, the deployment of these techniques is dependent upon
the type of switching equipment deployed and breaks the end-to-end the type of switching equipment deployed and breaks the end-to-end
communication model required by RSERPOOL. These devices provide a communication model required by RSERPOOL. These devices provide a
specialized service intended to address a few network challenges, specialized service intended to address a few network challenges,
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balancing, web server scaling, and streaming media load balancing. balancing, web server scaling, and streaming media load balancing.
They are not robust methods for providing network reliability or They are not robust methods for providing network reliability or
highly reliable and highly available location transparent server highly reliable and highly available location transparent server
clustering as required by RSERPOOL. clustering as required by RSERPOOL.
The following sections will provide an overview and example of each The following sections will provide an overview and example of each
technique and an accounting for key RSERPOOL architectural technique and an accounting for key RSERPOOL architectural
requirements not met. See Section 3 for a more detailed accounting requirements not met. See Section 3 for a more detailed accounting
of requirements compliance. of requirements compliance.
2.4.2 L4 Switching 2.5.2 L4 Switching
L4 devices make switching decisions based on the TCP or UDP port L4 devices make switching decisions based on the TCP or UDP port
numbers of the packet in transit. numbers of the packet in transit.
2.4.2.1 Example 2.5.2.1 Example
Web caching is an example of L4 switching. The topology requires the Web caching is an example of L4 switching. The topology requires the
introduction of an L4 capable switch in series with an existing introduction of an L4 capable switch in series with an existing
network connection and L2/L3 switch. This is of particular use to network connection and L2/L3 switch. This is of particular use to
web cache configurations where, for example, all traffic destined for web cache configurations where, for example, all traffic destined for
port 80 (HTTP) could be redirected to a web cache or distributed by port 80 (HTTP) could be redirected to a web cache or distributed by
the switch across a number of web caches to achieve load balancing. the switch across a number of web caches to achieve load balancing.
The L4 switch can react to a failed cache and cease to send traffic The L4 switch can react to a failed cache and cease to send traffic
to that device by automatically detecting that it is unreachable. to that device by automatically detecting that it is unreachable.
This is all accomplished without any configuration on a client This is all accomplished without any configuration on a client
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which packets belong to which connection and see that all such which packets belong to which connection and see that all such
packets are switched to the same firewall. packets are switched to the same firewall.
An L4 switch is incapable of differentiating between packets An L4 switch is incapable of differentiating between packets
containing cacheable objects and non-cacheable objects, therefore, L7 containing cacheable objects and non-cacheable objects, therefore, L7
devices, which look inside packets, are deployed where such devices, which look inside packets, are deployed where such
determinations must be made. In general, anytime that knowledge of determinations must be made. In general, anytime that knowledge of
the application level data is required to make a switching decision, the application level data is required to make a switching decision,
L7 devices must be deployed. L7 devices must be deployed.
2.4.2.2 Technical Issues 2.5.2.2 Technical Issues
The more general behavior of L4 switching, redirecting traffic based The more general behavior of L4 switching, redirecting traffic based
on destination UDP or TCP ports, is similar to a function provided by on destination UDP or TCP ports, is similar to a function provided by
RSERPOOL. Where it differs in this regard is that L4 switching is RSERPOOL. Where it differs in this regard is that L4 switching is
dependent upon the network infrastructure and not peer-to-peer or dependent upon the network infrastructure and not peer-to-peer or
end-to-end as required by RSERPOOL. end-to-end as required by RSERPOOL.
L4 switching meets the requirement of forwarding to active elements L4 switching meets the requirement of forwarding to active elements
only, as a switch will detect unreachable PEs, but does not provide only, as a switch will detect unreachable PEs, but does not provide
for the necessary registration and deregistration of PEs or for the necessary registration and deregistration of PEs or
resolution by name. L4 switches require the manual configuration of resolution by name. L4 switches require the manual configuration of
access control lists to determine switching behavior. This is access control lists to determine switching behavior. This is
achieved in RSERPOOL by more flexible means and without any achieved in RSERPOOL by more flexible means and without any
dependence on specialized network equipment. dependence on specialized network equipment.
Most of the features of ASAP [6] and ENRP [7] are not met by a device Most of the features of ASAP [7] and ENRP [8] are not met by a device
employing L4 switching techniques. See the comparison table in employing L4 switching techniques. See the comparison table in
Section 5. Section 5.
2.4.2.3 Security Issues 2.5.2.3 Security Issues
It is not clear that L4 switching introduces any new security It is not clear that L4 switching introduces any new security
concerns. In fact, in a two-port security model, where secure concerns. In fact, in a two-port security model, where secure
RSERPOOL services are provided on one port, and similar, but insecure RSERPOOL services are provided on one port, and similar, but insecure
services, on another, L4 switching could be used to direct traffic to services, on another, L4 switching could be used to direct traffic to
a secure or insecure PE or ENRP server as necessary. a secure or insecure PE or ENRP server as necessary.
2.4.3 L7 Switching 2.5.3 L7 Switching
As previously mentioned, L7 switching was developed to differentiate As previously mentioned, L7 switching was developed to differentiate
between the type of objects being directed by network switches. In between the type of objects being directed by network switches. In
the L4 case, the devices cannot differentiate between the types of the L4 case, the devices cannot differentiate between the types of
data, only the destination of the packets containing that data. L7 data, only the destination of the packets containing that data. L7
switches look at the application layer of a packet in transit to switches look at the application layer of a packet in transit to
determine what type of object is contained within. determine what type of object is contained within.
2.4.3.1 Example 2.5.3.1 Example
For an L7 switch to do this, it is necessary to intercept data For an L7 switch to do this, it is necessary to intercept data
midstream. In the case of HTTP, which is carried over TCP, the L7 midstream. In the case of HTTP, which is carried over TCP, the L7
switch must break the TCP handshake when a new request is made to the switch must break the TCP handshake when a new request is made to the
server attached to the switch. This process begins during the server attached to the switch. This process begins during the
initialization of the TCP connection and before the higher level initialization of the TCP connection and before the higher level
protocol, i.e., HTTP, sends its request. The switch acts as the protocol, i.e., HTTP, sends its request. The switch acts as the
server during the TCP SYN, SYN ACK, ACK handshake between that server server during the TCP SYN, SYN ACK, ACK handshake between that server
and the client. Once the HTTP request is issued by the client and and the client. Once the HTTP request is issued by the client and
the switch decides that this is non-cacheable data that should be the switch decides that this is non-cacheable data that should be
directed to the server as opposed to a web cache, the L7 switch sets directed to the server as opposed to a web cache, the L7 switch sets
up a second connection with the actual server through an additional up a second connection with the actual server through an additional
three-way handshake. The switch will forward the client's request to three-way handshake. The switch will forward the client's request to
the server and for the duration of this connection, must graft the the server and for the duration of this connection, must graft the
client-switch and switch-server connections together by modifying IP client-switch and switch-server connections together by modifying IP
addresses and TCP ports on the fly. Cacheable data is handled addresses and TCP ports on the fly. Cacheable data is handled
similarly, but is redirected to groups of web caches as opposed to similarly, but is redirected to groups of web caches as opposed to
the web servers. the web servers.
2.4.3.2 Technical Issues 2.5.3.2 Technical Issues
It is not clear that L7 switching adds anything, as a general It is not clear that L7 switching adds anything, as a general
mechanism, beyond what is provided by L4 switching, towards providing mechanism, beyond what is provided by L4 switching, towards providing
a sufficient RSERPOOL architecture. a sufficient RSERPOOL architecture.
While this technique can be very valuable as a means to scale web While this technique can be very valuable as a means to scale web
servers, it is apparent that it takes a significant amount of work on servers, it is apparent that it takes a significant amount of work on
the part of the switch to realize these gains. The nature of this the part of the switch to realize these gains. The nature of this
method also requires that for each type of application traffic method also requires that for each type of application traffic
handled, a custom software module must be written and added to the handled, a custom software module must be written and added to the
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and PEs may be attached. and PEs may be attached.
Also of concern is the compatibility of SCTP with L7 techniques. The Also of concern is the compatibility of SCTP with L7 techniques. The
interception and subsequent splicing of sessions may nullify some of interception and subsequent splicing of sessions may nullify some of
the inherent benefits of SCTP and certainly add additional and the inherent benefits of SCTP and certainly add additional and
unnecessary complexity and latency to the transport layer. ASAP and unnecessary complexity and latency to the transport layer. ASAP and
ENRP along with the multi-homing and stream based behavior of SCTP ENRP along with the multi-homing and stream based behavior of SCTP
provide more benefit than custom L7 switching would provide and at a provide more benefit than custom L7 switching would provide and at a
significantly lower cost. significantly lower cost.
2.4.3.3 Security Issues 2.5.3.3 Security Issues
While L7 switches do provide some robustness to TCP-based DoS attacks While L7 switches do provide some robustness to TCP-based DoS attacks
directed at servers by requiring a proper three-way handshake, and directed at servers by requiring a proper three-way handshake, and
they can be used to redirect encrypted traffic to certain servers they can be used to redirect encrypted traffic to certain servers
better capable of processing that traffic, they may break the better capable of processing that traffic, they may break the
security model of RSERPOOL. security model of RSERPOOL.
It may not be possible to make all the routing and switching It may not be possible to make all the routing and switching
decisions necessary to support RSERPOOL services without knowing more decisions necessary to support RSERPOOL services without knowing more
than just the destination address and port of a packet. The than just the destination address and port of a packet. The
necessary extended attributes are not elements of L4 or L7 switching, necessary extended attributes are not elements of L4 or L7 switching,
but are instead, parameters of ASAP and ENRP. As the ENRP traffic is but are instead, parameters of ASAP and ENRP. As the ENRP traffic is
encrypted in RSERPOOL, the L7 devices would not be able to extract encrypted in RSERPOOL, the L7 devices would not be able to extract
the necessary session layer data without becoming potential third the necessary session layer data without becoming potential third
party security liabilities. party security liabilities.
2.4.4 Summary 2.5.4 Summary
The L4/L7 switching techniques, being network oriented services, are The L4/L7 switching techniques, being network oriented services, are
not able to provide the communications session oriented behavior not able to provide the communications session oriented behavior
required by RSERPOOL. required by RSERPOOL.
Adequate support for naming, as well as registration and Adequate support for naming, as well as registration and
deregistration services, is not provided by these devices. RSERPOOL deregistration services, is not provided by these devices. RSERPOOL
requires a fault tolerant name service as well as the ability to requires a fault tolerant name service as well as the ability to
register and deregister PEs in real-time. To accomplish this with L4/ register and deregister PEs in real-time. To accomplish this with
L7 switching, one would need to define a standard protocol to allow L4/L7 switching, one would need to define a standard protocol to
the switches to communicate amongst themselves and, perhaps, allow the switches to communicate amongst themselves and, perhaps,
implement a co-resident name server on the switch. implement a co-resident name server on the switch.
The RSERPOOL communication model is broken as these mechanisms are The RSERPOOL communication model is broken as these mechanisms are
deployed on switch hardware as opposed to end devices such as PEs, deployed on switch hardware as opposed to end devices such as PEs,
PUs, and ENRP servers. This implies a significant requirement for PUs, and ENRP servers. This implies a significant requirement for
processing power and a lack of support for mobility. It is unlikely processing power and a lack of support for mobility. It is unlikely
that one could or would build L4/L7 behavior into end devices and that one could or would build L4/L7 behavior into end devices and
RSERPOOL requires peer-to-peer functionality. RSERPOOL requires peer-to-peer functionality.
The equipment needed to deploy such solutions can be an order of The equipment needed to deploy such solutions can be an order of
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ability to provide a robust framework for location transparent ability to provide a robust framework for location transparent
clustering capable of scaling in size and performance from web server clustering capable of scaling in size and performance from web server
or other Internet applications to real-time telecommunications or other Internet applications to real-time telecommunications
infrastructure. There are a host of concerns with the ability of infrastructure. There are a host of concerns with the ability of
these techniques to meet critical RSERPOOL requirements in the areas these techniques to meet critical RSERPOOL requirements in the areas
of flexibility, adaptability, timing, security, etc. The amount of of flexibility, adaptability, timing, security, etc. The amount of
effort required to achieve RSERPOOL functionality across L4/L7 effort required to achieve RSERPOOL functionality across L4/L7
switches would amount to implementing RSERPOOL, as it is currently switches would amount to implementing RSERPOOL, as it is currently
defined, on those very switches. defined, on those very switches.
2.5 ASAP and ENRP 2.6 ASAP and ENRP
ASAP [6] and ENRP [7] are being developed in the RSERPOOL working ASAP [7] and ENRP [8] are being developed in the RSERPOOL working
group. Even though they are separate protocols, they are designed to group. Even though they are separate protocols, they are designed to
work together. work together.
2.5.1 ASAP 2.6.1 ASAP
ASAP uses a name-based addressing model which isolates a logical ASAP uses a name-based addressing model which isolates a logical
communication endpoint from its IP address(es), thus effectively communication endpoint from its IP address(es), thus effectively
eliminating the binding between the communication endpoint and its eliminating the binding between the communication endpoint and its
physical IP address(es) which normally constitutes a single point of physical IP address(es) which normally constitutes a single point of
failure. In addition, ASAP defines each logical communication failure. In addition, ASAP defines each logical communication
destination as a pool, providing full transparent support for destination as a pool, providing full transparent support for
server-pooling and load sharing. If multiple endpoints register under server-pooling and load sharing. If multiple endpoints register
a the same name, a server pool is effectively created. It also allows under a the same name, a server pool is effectively created. It also
dynamic system scalability - members of a server pool can be added or allows dynamic system scalability - members of a server pool can be
removed at any time without interrupting the service. added or removed at any time without interrupting the service.
ASAP monitors the reachability of the Pool Elements in order to ASAP monitors the reachability of the Pool Elements in order to
provide fault tolerance. To support real-time or semi-real-time fault provide fault tolerance. To support real-time or semi-real-time
detection and recovery, ASAP makes use of the peer reachability fault detection and recovery, ASAP makes use of the peer reachability
feedback from either the transport layer (such as SCTP) or the upper feedback from either the transport layer (such as SCTP) or the upper
layer protocol and re-send (or failover) user messages to alternate layer protocol and re-send (or failover) user messages to alternate
PEs in the destination pool. Load sharing and redundancy model PEs in the destination pool. Load sharing and redundancy model
support is provided in ASAP at the message sender side. ASAP allows support is provided in ASAP at the message sender side. ASAP allows
extensions to be made in the future to accommodate new load sharing extensions to be made in the future to accommodate new load sharing
policies and redundancy models. policies and redundancy models.
ASAP supports the "keepalive" monitoring of PEs by the name server ASAP supports the "keepalive" monitoring of PEs by the name server
and session failover, in which a set of application messages are and session failover, in which a set of application messages are
defined as a "session" and ASAP provides best-effort transmission of defined as a "session" and ASAP provides best-effort transmission of
all the messages in the "session" to the same PE in the destination all the messages in the "session" to the same PE in the destination
pool. For some classes of service, ASAP can provide failover for the pool. For some classes of service, ASAP can provide failover for the
remaining message in the "session" to an alternate PE if the first PE remaining message in the "session" to an alternate PE if the first PE
fails. fails.
2.5.2 ENRP 2.6.2 ENRP
ENRP defines procedures and message formats of a pool registry ENRP defines procedures and message formats of a pool registry
service (or name service) for storing, bookkeeping, retrieving, and service (or name service) for storing, bookkeeping, retrieving, and
distributing pool operation and membership information. It allows distributing pool operation and membership information. It allows
Pool Elements to be dynamically added, updated and removed from Pool Elements to be dynamically added, updated and removed from
service. There are also protocol mechanisms for detecting and service. There are also protocol mechanisms for detecting and
removing unreachable Pool Elements. removing unreachable Pool Elements.
Within the operational scope of RSERPOOL, ENRP defines the procedures Within the operational scope of RSERPOOL, ENRP defines the procedures
and message formats of a distributed, fault-tolerant registry service and message formats of a distributed, fault-tolerant registry service
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Table1: Comparison Against Requirements Table1: Comparison Against Requirements
4. Security Concerns 4. Security Concerns
This type of non-protocol document does not directly affect the This type of non-protocol document does not directly affect the
security of the Internet. security of the Internet.
5. Acknowledgements 5. Acknowledgements
The authors would like to thank Bernard Aboba, Erik Guttman, Matt The authors would like to thank Bernard Aboba, Erik Guttman, Matt
Holdrege, Lyndon Ong, Christopher Ross, Micheal Tuexen and Werner Holdrege, Lyndon Ong, Jon Peterson, Christopher Ross, Micheal Tuexen
Vogels for their invaluable comments and suggestions. and Werner Vogels for their invaluable comments and suggestions.
Normative References 6. References
6.1 Normative References
[1] Tuexen, M., Xie, Q., Stewart, R., Shore, M. and J. Loughney, [1] Tuexen, M., Xie, Q., Stewart, R., Shore, M. and J. Loughney,
"Architecture for Reliable Server Pooling", "Architecture for Reliable Server Pooling",
draft-ietf-rserpool-arch-07 (work in progress), Oct 2003. draft-ietf-rserpool-arch-07 (work in progress), Oct 2003.
[2] Tuexen, M., Xie, Q., Stewart, R., Shore, M., Ong, L., Loughney, [2] Tuexen, M., Xie, Q., Stewart, R., Shore, M., Ong, L., Loughney,
J. and M. Stillman, "Requirements for Reliable Server Pooling", J. and M. Stillman, "Requirements for Reliable Server Pooling",
RFC 3237, January 2002. RFC 3237, January 2002.
Non-Normative References 6.2 Non-Normative References
[3] Gulbrandsen, A., Vixie, P. and L. Esibov, "A DNS RR for [3] Gulbrandsen, A., Vixie, P. and L. Esibov, "A DNS RR for
specifying the location of services (DNS SRV)", RFC 2782, specifying the location of services (DNS SRV)", RFC 2782,
February 2000. February 2000.
[4] Guttman, E., Perkins, C., Veizades, J. and M. Day, "Service [4] Guttman, E., Perkins, C., Veizades, J. and M. Day, "Service
Location Protocol, Version 2", RFC 2608, June 1999. Location Protocol, Version 2", RFC 2608, June 1999.
[5] Zhao, W., Schulzrinne, H. and E. Guttman, "Mesh-enhanced Service [5] Mealling, M., "Dynamic Delegation Discovery System (DDDS) Part
One: The Comprehensive DDDS", RFC 3401, October 2002.
[6] Zhao, W., Schulzrinne, H. and E. Guttman, "Mesh-enhanced Service
Location Protocol (mSLP)", RFC 3528, April 2003. Location Protocol (mSLP)", RFC 3528, April 2003.
[6] Stewart, R., Xie, Q., Stillman, M. and M. Tuexen, "Aggregate [7] Stewart, R., Xie, Q., Stillman, M. and M. Tuexen, "Aggregate
Server Access Protocol (ASAP)", draft-ietf-rserpool-asap-07 Server Access Protocol (ASAP)", draft-ietf-rserpool-asap-09
(work in progress), May 2003. (work in progress), June 2004.
[7] Xie, Q., Stewart, R. and M. Stillman, "Enpoint Name Resolution [8] Xie, Q., Stewart, R. and M. Stillman, "Enpoint Name Resolution
Protocol (ENRP)", draft-ietf-rserpool-enrp-06 (work in Protocol (ENRP)", draft-ietf-rserpool-enrp-09 (work in
progress), May 2003. progress), July 2004.
[8] Bradner, S., "The Internet Standards Process -- Revision 3", BCP [9] Bradner, S., "The Internet Standards Process -- Revision 3", BCP
9, RFC 2026, October 1996. 9, RFC 2026, October 1996.
Authors' Addresses Authors' Addresses
John Loughney (editor) John Loughney (editor)
Nokia Research Center Nokia Research Center
PO Box 407 PO Box 407
Nokia Group FIN-00045 Nokia Group FIN-00045
Finland Finland
skipping to change at page 21, line 8 skipping to change at page 24, line 8
Mail Drop 2246 Mail Drop 2246
Schaumburg, IL 60196 Schaumburg, IL 60196
USA USA
Phone: +1-847-576-8747 Phone: +1-847-576-8747
EMail: aron.j.silverton@motorola.com EMail: aron.j.silverton@motorola.com
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