Network Working Group Y. Gu
Internet-Draft Huawei
Expires: April 24, 2013 M. Shore
No Mountain Software
S. Sivakumar
Cisco Systems
October 21, 2012
A Framework and Problem Statement for Flow-associated Middlebox State
Migration
draft-gu-statemigration-framework-03
Abstract
This document presents an initial framework and discussion of the
problem of transferring middlebox (for example, firewall or NAT)
flow-coupled state from one middlebox to another while the flow is
still active. This has most recently come up in the context of
virtual machine (VM) migration between hypervisors, but it is a
problem that has appeared in other situations, as well. We present
some of the parameters of the problem, define some language for
discussing the problem, and begin to identify a path forward for
addressing it.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
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Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on April 24, 2013.
Copyright Notice
Copyright (c) 2012 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5
3. Goals . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
4. Middlebox state . . . . . . . . . . . . . . . . . . . . . . . 7
4.1. What state is associated with a flow on a middlebox? . . . 7
4.2. State vs policy . . . . . . . . . . . . . . . . . . . . . 8
4.3. Mechanisms for instantiating middlebox state . . . . . . . 9
5. "Moving" endpoints . . . . . . . . . . . . . . . . . . . . . . 10
5.1. A few words about addresses . . . . . . . . . . . . . . . 10
5.2. Scenarios . . . . . . . . . . . . . . . . . . . . . . . . 10
5.2.1. Virtual machine migration . . . . . . . . . . . . . . 10
5.2.2. SCTP NAT . . . . . . . . . . . . . . . . . . . . . . . 10
6. "Directionality" . . . . . . . . . . . . . . . . . . . . . . . 12
7. Problems . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
7.1. Recognizing when an endpoint has moved . . . . . . . . . . 13
7.2. Topology discovery . . . . . . . . . . . . . . . . . . . . 13
7.3. Copying state from a middlebox . . . . . . . . . . . . . . 14
7.4. Installing state on the new middlebox . . . . . . . . . . 15
8. Related and prior work . . . . . . . . . . . . . . . . . . . . 16
8.1. SOCKS . . . . . . . . . . . . . . . . . . . . . . . . . . 16
8.2. RSIP . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
8.3. midcom . . . . . . . . . . . . . . . . . . . . . . . . . . 16
8.4. NSIS NAT/Firewall signaling layer . . . . . . . . . . . . 16
8.5. STUN . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
8.6. TURN . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
8.7. ICE . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
8.8. PCP . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
8.9. UPnP IGD . . . . . . . . . . . . . . . . . . . . . . . . . 17
9. Security Considerations . . . . . . . . . . . . . . . . . . . 18
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 19
11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 20
12. Informative References . . . . . . . . . . . . . . . . . . . . 21
Appendix A. On the applicability of the Context Transfer
Protocol . . . . . . . . . . . . . . . . . . . . . . 23
A.1. Topology awareness . . . . . . . . . . . . . . . . . . . . 23
A.2. Triggers . . . . . . . . . . . . . . . . . . . . . . . . . 24
A.3. Copying state . . . . . . . . . . . . . . . . . . . . . . 24
A.4. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . 25
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 26
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1. Introduction
An end-to-end network flow typically traverses one or more
"middlebox," which may retain state about the flow. These include,
for example, firewalls, NATs, traffic optimizers, and similar. The
flow-associated state is usually instantiated through a combination
of traffic inspection and broad policies, but may also be created by
the use of an explicit request or signaling mechanism.
When an endpoint changes its point of attachment to a network, it
retains its IP address, and the standard 5-tuple used to describe a
flow (source and destination addresses, source and destination ports,
protocol) stay the same. Because of this it is possible to move
existing middlebox state containing these elements.
The problem of how to handle transfering flow-associated middlebox
state when one flow endpoint moves is not a new one, but with some
exceptions it remains largely unaddressed. For example, situations
in which one endpoint or another "move" (we define what it means to
move an endpoint in more detail in Section 5) include mobile IP
[RFC5944], failover in a high-availability deployment, and VM
(virtual machine) migration. Related problems include multihomed
endpoints in SCTP and load balancing.
Note that while there is a hope that NAT will no longer be with us in
purely IPv6 networks, there is every expectation that firewalls will
continue to be deployed and consequently there will continue to be a
need to migrate firewall state with moving endpoints in IPv6
environments.
In this document we establish terminology (Section 2), describe the
problem, and lay out the components of the problem that would need to
be addressed in a solution. We also review past and current work on
middlebox communication and identify gaps in existing technology.
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2. Terminology
flow: "Traffic flow" is defined in [RFC2722] as an artificial
logical equivalent of a call or connection. It is delimited by a
start and a stop time.
middlebox: A middlebox was defined in [RFC3234] as "any intermediary
device performing functions other than the normal, standard
functions of an IP router on the datagram path between a source
host and a destination host." RFC 3234 provides an older but
excellent and still-relevant taxonomy of middlebox types.
move: When we talk about an endpoint "moving" what we are describing
is the endpoint changing its point of attachment to the network.
For the purpose of this discussion we assume that it retains the
same IP address after the move that it had before the move.
policy: See Section 4.2
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3. Goals
The problem we are interested in solving is the question of how to
keep longer-lived network flows "alive" when an endpoint's point of
attachment to a network changes. The particular piece of this we
intend to address is how to move the middlebox (in this case,
firewall or NAT) state associated with a network flow to new
middleboxes.
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4. Middlebox state
4.1. What state is associated with a flow on a middlebox?
To date, we haven't been able to find a normative definition of the
term 'state' in IETF documents. More generally it tends to be
considered to be a set of observable properties associated with an
object. This is (largely) distinct from automata theory, in which
"state" refers to the condition of an object (or automaton). The
observable things which might be associated by a middlebox with a
network flow are described below.
Transport-layer middleboxes which keep flow-associated state through
the duration of the flow typically keep, at a minimum, the standard
IP 5-tuple:
{s_addr, d_addr, s_port, d_port, protocol}
where
s_addr is the source address
d_addr is the destination address
s_port is the source port
d_port is the destination port
protocol is the IP protocol (TCP, UDP, SCTP, RSVP, etc.)
Other data elements often associated with a network flow include
timers.
Over the lifetime of a flow, it is not expected that elements of the
standard 5-tuple will change, but there may be other pieces of state,
such as timers, or data extracted from stateful inspection, which may
be expected to change before a flow terminates.
As mentioned above, when an endpoint "moves" it retains its IP
address(es) and the sockaddr information associated with a flow on an
endpoint does not change.
Middlebox state is almost always associated with a specific interface
(rather than the interface being an attribute of the flow). Some
"stateful inspection" firewalls may keep state from higher layers in
the networking stack: everything from TCP sequence numbers to entire
SIP dialogues.
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Note that the state associated with a flow may be left up when the
flow is torn down in some implementations, such as those NATs that
put the state on an activity-based timer as an efficiency mechanism,
to avoid reinstantiating state should a new flow be created which
shares the attributes of the flow which just ended. This is often
the case with HTTP, for example.
It should also be noted that it is possible that a given
bidirectional network flow (say, TCP) may have each flow (to and from
its peer) follow different routes, commonly referred to as
"asymmetric routing." When an endpoint moves, it is possible that
o both flows traverse the same middlebox before the move and after
the move,
o both flows traverse the same middlebox before the move and
different middleboxes after the move,
o both flows traverse different middleboxes before the move but the
same middlebox after the move, or
o both flows traverse different middleboxes before the move and
different middleboxes after the move
4.2. State vs policy
We would like to draw a clear distinction between state and policy.
'Policy' is a set of statements that define how traffic (in this
case) is to be treated by the middlebox. In some sense policy is a
description of what state should be applied to a network flow; that
is to say, state includes the instantiation of policy. When a flow
first arrives at a middlebox, it consults its policy to determine
what state (if any) is to be created and then associated with that
flow
As a general rule of thumb, policy is provisioned while state
represents run-time responses to environmental conditions (in this
case, network flows). Because policy is provisioned and because we
assume that the middleboxes between which state would be migrated are
under the administrative control of the same organization, we will
make another assumption that there is consistent policy configured
across middleboxes. We are aware that this is not always a correct
assumption.
Note that implicit in this description is the notion of policy
definition having an administrative scope. That is to say, there is
an assumption that state must only be migrated between middleboxes in
the same administrative policy domain. There are several risks
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associated with migrating state between middleboxes in different
administrative domains, prominent among which is the possibility of
installing local state on the "new" middlebox which violates its
policy. We feel that migrating state between middleboxes in
different administrative policy domains should be considered out of
scope for the time being.
4.3. Mechanisms for instantiating middlebox state
State is created on middleboxes using a small number of mechanisms,
sometimes in combination.
The most common means by which middlebox state is created is that the
middlebox examines traffic and compares it against its own policies,
which have typically been configured or provisioned by a systems or
network administrator but in very simple cases can come
preprovisioned, for example on commodity consumer equipment. It then
creates middlebox state, in the form of a firewall pinhole, a NAT
table mapping, QoS table entry, etc.
Another means is through explicit request. An endpoint or its proxy
sends a request for resources (again, firewall pinhole, NAT table
mapping, and so on) to the middlebox using some sort of "signaling"
protocol to request the resource. The middlebox compares the request
to its policy and grants or denies the request based on that policy.
Examples of explicit request include RSVP [RFC2205], midcom
[RFC3303], TURN [RFC5766], and the work being done by the IETF
pcp [1] working group.
It is worth mention that there are mechanisms that are essentially
hybrids of the previous two approaches, using expected effects of
sending traffic across a middlebox to trigger hoped-for state
instantiation. STUN [RFC5389] is probably the best-known example of
this.
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5. "Moving" endpoints
Moving an endpoint, in the context of this internet draft, refers to
changing its point of attachment to a network. Doing so may cause
traffic to cross different middleboxes from the ones the traffic
traversed when the middlebox state was created.
5.1. A few words about addresses
One question that comes up from time to time in discussions of VM
migration is whether or not the IP address will change as a result of
the migration. We believe that this is out of scope for the time
being, not the least because host operating system support is
potentially difficult. If our goal is to keep a given network flow
up and alive during a migration, not only would the endpoint
operating system need to be aware that its address has changed, it
would also need to to be able to signal the other end of the flow,
which would have to respond by modifying open sockets' sockaddrs,
etc. There are also some obvious security problems that would need
to be addressed.
5.2. Scenarios
In this section we introduce a few scenarios. We believe the problem
characteristics are fundamentally the same in these scenarios and
that what we're describing is a general problem.
5.2.1. Virtual machine migration
The live migration (i.e. the VM appears to remain "up" and available
during the migration - that is to say, TCP or other connection-
oriented flows are not dropped) of virtual machines between
hypervisors in the same data center has been established practice for
several years now, but there's been a move towards live migration of
VMs between geographically disparate data centers (see, for example
this collaboration [2] between Cisco and VMWare). This provides the
ability to perform data center maintenance without downtime, data
center migration or consolidation, data center expansion, and
workload balancing. There is a compelling use case for VM migration.
5.2.2. SCTP NAT
The SCTP [RFC4960] protocol supports multihomed endpoints. Any NAT
that is port-aware (and these days it is nearly all of them) will
need to have SCTP support in order to be able to handle extracting
the port numbers even for flows that are single-homed on each end.
This provides a mechanism for transparent failover when one path
taken by the network flow fails (see section 6.4 in [RFC4960]
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The upshot of this is that if a NAT is maintaining state related to a
flow on the primary path and the primary path fails, that state may
need to be transferred to the NAT being traversed by the secondary
path.
This problem is being addressed in the IETF behave [3] working group.
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6. "Directionality"
One of the questions that comes up when considering an overall
architecture to solve this set of problems is who initiates the state
migration and how the data "flow" from place to place.
One approach is to have the middleboxes communicate directly with
each other. In this case having all middleboxes poll all other
middleboxes for copies of their state seems wasteful and inefficient,
suggesting that communication between middleboxes would need a
specific trigger. The "old" middlebox could send its state to the
"new" middlebox or the new middlebox could send a request to the old
middlebox for a copy of its state. In either case one middlebox
would need to know the location of the other and be able to
communicate with it (both parties would need to authenticate to each
other). Note that if a catastrophic network event caused the old
middlebox to become unreachable, it would be impossible to
successfully query it for its state. [Note that this approach was
considered for SCTP NAT traversal and discarded as impossible, since
there was no way for one NAT to know about other NATs.]
Another approach is to have some controlling entity involved, either
mediating communication between middleboxes or directing
communication between middleboxes. In a VM migration scenario, a VM
manager, or a network manager communicating with a VM manager, is an
obvious candidate. As described in Section 4.2, the migration must
stay within an administrative policy boundary, which may eliminate
the need for multiple mediators.
The orthogonal question to whether or not there's a mediating entity
is who initiates the communication - does the old middlebox respond
to a catastrophic event by dumping state before shutting down (not
always possible, obviously) or is it polled by a mediating device or
a new middlebox? Another possibility is to periodically transfer
incremental information so that a non-recoverable error can save most
of the flows, if not all.
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7. Problems
The problems that must be solved in order to move middlebox state
along with a moving endpoint include:
o Recognizing when an endpoint has moved
o Locating middleboxes along the original path
o Locating middleboxes along the new path
o Getting a copy of state from middleboxes along the old path
o Installing that state in middleboxes along the new path
7.1. Recognizing when an endpoint has moved
As touched upon in Section 5.2, there are various circumstances that
could cause an endpoint to change its point of attachment to a
network. They fall into two broad categories: planned and unplanned.
In the planned case, some entity knows that an endpoint is about to
move and the move can happen in a controlled fashion. There may be
time to send network queries, learn topology, and gather state.
The unplanned case is typically a response to the failure of some
element in the network. A monitoring heartbeat is missed, a
connection times out, or some other indication of catastraphic
failure is received by an endpoint or by a monitoring service. Not
only does this interfere with the notion of an organized transfer
from one path to the new one, it also means that there may be cases
where the old middlebox is not reachable and it's not possible to
query its state.
7.2. Topology discovery
Somehow or other the state migration mechanism needs to be able to
locate and communicate with both the middleboxes on the old path and
the middleboxes on the new path. This is not a trivial problem; IP
was designed to have the network itself be largely opaque to
endpoints, and very often systems and network administrators prefer
not to expose network topology, feeling that it would introduce
security threats.
There are several options, including configuration, discovery, and
notification. In configuration, someone with knowledge of the
network topology would be able to construct a table describing
middleboxes associated with certain routes. In discovery, a network
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mechanism would be used to query for the middleboxes along a path,
similar to traceroute or to a PATH message in RSVP [RFC2205].
A configuration mechanism would have the disadvantage of being not
particularly responsive to changes in the network, as well as being
somewhat error-prone. However, it would not involve inventing a new
network mechanism or requiring changes on every participating
middlebox (although the state migration mechanism itself would nearly
certainly require changes).
[Note that an architecture that had the middlebox copying its own
state out to some third party would almost certainly have to be
configuration-base.]
A discovery-based approach would require putting new software on
every middlebox, an approach that is intuitively unappealing and that
has been repeatedly shown to inhibit adoption of newer technologies.
There is no such thing as incremental deployment using this approach.
It also introduces security problems, since without the appropriate
protections it would allow attackers to probe and discover not just
network topology but specifically the location of security devices/
middleboxes in a given network. On the other hand it's robust
against configuration errors and highly responsive to changes in the
underlying network.
A third option, notification, relies on a middlebox announcing its
presence to the network, typically using anycast or broadcast. This
also requires changes to both the middlebox and a controlling entity,
and a an announcement/notification protocol. It has the advantage of
being responsive to new middleboxes coming up in the network,
although a mechanism (such as a heartbeat) would be needed to detect
outages and drops.
The primary security consideration in a notification scenario is that
the network must be tightly controlled to prevent announcements from
being eavesdropped upon by adversaries.
7.3. Copying state from a middlebox
Another problem to be solved is the one of copying state from a
middlebox, encoding it, and transferring it over the network.
It may be the case that the middleboxes are from different
manufacturers/vendors, and so the problem of representing the state
we wish to transfer includes the question of presenting it in a
vendor-neutral format, including both state semantics and state
syntax.
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A somewhat more challenging aspect of this problem is how to
transport the encoded state. For one thing, it may be that the event
that triggered the endpoint migration has also rendered the middlebox
in question unreachable. For another, what sort of load this imposes
on the middlebox depends, among other things, on the "directionality"
of the state migration. It may be that an external device, such as a
session controller, a hypervisor, or another middlebox queries the
old middlebox for a copy of its state. In high-availability and
other scenarios the middlebox may end up "pushing" copies of its
state out to some controlling or intermediate entity, such as a
hypervisor.
Among the transport characteristics that need to be considered is
reliability, and being able to recognize when a copy of the source
middlebox state has not been transferred correctly, whether it's
because it's incomplete, damaged, or inauthentic.
7.4. Installing state on the new middlebox
The problem of installing state on the new middlebox is closely
related to the one of copying state from the old middlebox. In both
cases we're facing the problems of representation and encoding, a
transport protocol to/from the middlebox, and questions about
reachability.
Reliability is a question here, as well, with the additional concern,
beyond what is described in the previous section, of whther or not
the state is installed correctly on the new middlebox. Issues that
could interfere with installation include resource limitations, and
authority/authorization.
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8. Related and prior work
There has been substantial prior work on instantiating middlebox
state, but very little on topology problems, particularly on topology
problems related to moving middlebox state with an endpoint as it
changes its point of attachment to the network.
For more complete, if dated, overviews of middlebox control
protocols, see [RFC3234], and Lars Eggerts's expired internet draft
[I-D.eggert-middlebox-control-survey] surveying middlebox
communication options available at the time the draft was written
(2007).
8.1. SOCKS
SOCKS [RFC1928] is probably among the earliest mechanism used to
create state for firewall traversal. It provides a mechanism to
create a per-stream tunnel (optionally authenticated) between an
endpoint and a firewall (or SOCKS proxy), and assumes that the
location of the SOCKS proxy is configured into the endpoint It was
never extended to include communicating with NAT devices and has
fallen out of favor.
8.2. RSIP
Realm-Specific IP (RSIP) [RFC3103] was a NAT-specific protocol for
requesting address/port mappings from a NAT. Although there was
initially enormous enthusiasm for RSIP it never saw wide deployment,
nor was it extended to support other types of middleboxes. It also
did not specify any discovery mechanism.
8.3. midcom
The Middlebox Communication (midcom) [RFC3303] working group
developed a mechanism based on SNMPv3 to request and manage middlebox
(firewall and NAT) resources. It was largely driven by requirements
for firewall and NAT traversal from the VoIP community.
midcom assumes that an endpoint's agent (or possibly an endpoint
itself) submits requests directly to the middlebox, the middlebox's
location having been configured into the agent or endpoint in
advance.
8.4. NSIS NAT/Firewall signaling layer
The NSIS [RFC5971] working group specified a NAT and firewall
signaling (control) layer to run on top of its GIST transport. Tne
signaling runs end-to-end between signaling endpoints and has no
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mechanism for discovery of middleboxes not on the signaling path.
8.5. STUN
Session Traversal Utilities for NAT (STUN) [RFC5389] is a protocol
originally developed in the gaming community and adapted by the IETF
for use in helping VoIP protocols get their media streams across NAT
devices. Note that it does not establish a direct relationship with
a NAT device and does not send requests directly to it, nor does it
work with firewalls. It is unauthenticated, relying instead on
security mechanisms in the application protocol to provide protection
against spoofed responses.
8.6. TURN
Traversal Using Relays around NAT (TURN) [RFC5766] is a protocol that
essentially extends STUN, but rather than having the application
traffic (in the STUN/TURN case, typically RTP-encapsulated media)
flow end-to-end, it is relayed between endpoints. Like STUN it does
not support communicating firewall control messages.
8.7. ICE
Interactive Connectivity Establishment (ICE) [RFC5245] is not an on-
the-wire protocol, but rather a procedure for using STUN and TURN to
collect a set of candidate addresses for endpoints, and then
determine the 'best' from that set.
Like TURN and ICE, it does not support firewall traversal, and is not
a general middlebox mechanism.
8.8. PCP
The IETF chartered yet another working group to specify a protocol
for requesting firewall and NAT resources, the Port Control
Protocol(PCP) [1] working group. The deployment context for PCP is
Carrier Grade NAT (CGN). PCP does not specify a discovery mechanism,
nor do its documents include text addressing topology issues.
8.9. UPnP IGD
The UPnP Forum [4] is responsible for the development and maintenance
of a suite of protocols for device control, providing services in the
network similar to the services provided by "Plug and Play" across a
hardware device's backplane. One of the protocols they've developed
is the Internet Gateway Device (IGD) [5] protocol. While it provides
the ability to communicate directly between an endpoint and a NAT,
its discovery and topology awareness is limited to the local link.
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9. Security Considerations
Any time we introduce new mechanisms to query and manipulate
middleboxes, we also introduce potentially very serious security
exposures.
In this case, because we're planning on discovering the location of
middleboxes, querying the middleboxes for their state, and installing
state on middleboxes, we face a very broad range indeed of potential
threats.
Network and systems administrators typically want to conceal network
topology from outsiders, and it may be necessary to use authenticated
discovery (packet filtering may be adequate for some deployments but
not all). This introduces problems around credentials management and
keying for participants, and may suggest that we would want to
minimize the number of network elements talking with one another.
Cleary the ability to copy data from a middlebox introduces the
ability to discovery yet more network topology, and in particular to
identify specific firewall pinholes and NAT table mappings, and their
associated state.
Similarly, the ability to install state on a middlebox can introduce
both Denial of Service (DoS) vulnerabilities but also the ability of
an attacker to penetrate a middlebox, or to disable it completely.
In all cases, protections must be designed with sensitivity to
performance, since middleboxes often are processing very heavy
traffic loads. This means keeping an eye on cryptographic processing
demands, key and other credentials management, etc.
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10. IANA Considerations
This document has no actions for IANA.
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11. Acknowledgments
Many thanks to David Black for his careful review and suggestions for
improvements.
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12. Informative References
[RFC1928] Leech, M., Ganis, M., Lee, Y., Kuris, R., Koblas, D., and
L. Jones, "SOCKS Protocol Version 5", RFC 1928,
March 1996.
[RFC2205] Braden, B., Zhang, L., Berson, S., Herzog, S., and S.
Jamin, "Resource ReSerVation Protocol (RSVP) -- Version 1
Functional Specification", RFC 2205, September 1997.
[RFC2722] Brownlee, N., Mills, C., and G. Ruth, "Traffic Flow
Measurement: Architecture", RFC 2722, October 1999.
[RFC3103] Borella, M., Grabelsky, D., Lo, J., and K. Taniguchi,
"Realm Specific IP: Protocol Specification", RFC 3103,
October 2001.
[RFC3234] Carpenter, B. and S. Brim, "Middleboxes: Taxonomy and
Issues", RFC 3234, February 2002.
[RFC3303] Srisuresh, P., Kuthan, J., Rosenberg, J., Molitor, A., and
A. Rayhan, "Middlebox communication architecture and
framework", RFC 3303, August 2002.
[RFC4067] Loughney, J., Nakhjiri, M., Perkins, C., and R. Koodli,
"Context Transfer Protocol (CXTP)", RFC 4067, July 2005.
[RFC4960] Stewart, R., "Stream Control Transmission Protocol",
RFC 4960, September 2007.
[RFC5245] Rosenberg, J., "Interactive Connectivity Establishment
(ICE): A Protocol for Network Address Translator (NAT)
Traversal for Offer/Answer Protocols", RFC 5245,
April 2010.
[RFC5389] Rosenberg, J., Mahy, R., Matthews, P., and D. Wing,
"Session Traversal Utilities for NAT (STUN)", RFC 5389,
October 2008.
[RFC5766] Mahy, R., Matthews, P., and J. Rosenberg, "Traversal Using
Relays around NAT (TURN): Relay Extensions to Session
Traversal Utilities for NAT (STUN)", RFC 5766, April 2010.
[RFC5944] Perkins, C., "IP Mobility Support for IPv4, Revised",
RFC 5944, November 2010.
[RFC5971] Schulzrinne, H. and R. Hancock, "GIST: General Internet
Signalling Transport", RFC 5971, October 2010.
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[I-D.eggert-middlebox-control-survey]
Eggert, L., Sarolahti, P., Denis-Courmont, R., Stirbu, V.,
and H. Tschofenig, "A Survey of Protocols to Control
Network Address Translators and Firewalls",
draft-eggert-middlebox-control-survey-01 (work in
progress), July 2007.
[1] <http://datatracker.ietf.org/wg/pcp/charter/>
[2] <http://www.cisco.com/en/US/solutions/collateral/ns340/ns517/
ns224/ns836/white_paper_c11-557822.pdf>
[3] <http://datatracker.ietf.org/wg/behave/>
[4] <http://www.upnp.org/>
[5] <http://upnp.org/specs/gw/igd2/>
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Appendix A. On the applicability of the Context Transfer Protocol
In this section we examine the applicability of the Context Transfer
Protocol [RFC4067] to the state migration problem, given the problems
outlined in Section 7. In Section 7, we identify the following
components of the overall state migration problem:
o Recognizing when an endpoint has moved
o Locating middleboxes along the original path
o Locating middleboxes along the new path
o Getting a copy of state from middleboxes along the old path
o Installing that state in middleboxes along the new path
A.1. Topology awareness
The Context Transfer Protocol was designed to support node mobility
-- to minimize disruptions when a mobile node attaches to a new
access router. In a typical scenario, when a mobile node moves from
one access router to another, CXTP provides a means to move
associated state (or context) to the new access router to which the
node becomes attached.
In the CXTP scenario, the mobile node "knows" that the access router
is there and has direct communication with it, by virtue of the
underlying network mobility mechanisms. A context transfer may be
initiated by the mobile node when it "knows" that it will be
attaching to a new access router, or it may be initiated by the
existing access router when it receives a link-layer trigger.
Alternatively, a context transfer may be initiated by the new access
router when it receives a link-layer trigger. In short, the context
transfer request is generated by a first party in the network, either
the mobile node itself or one of its access routers.
This contrasts rather starkly with the usual middlebox scenario,
where the middlebox is typically invisible to the endpoint (the
mobile node analogue). A mobile node has an explicit relationship
with an access router; a network endpoint has no such relationship
with a firewall or NAT, except in those cases in which the firewall
or NAT is doing double-duty as a proxy.
Topology awareness has been one of the most persistent and difficult
problems associated with middlebox communication issues. In the CXTP
case topology awareness is pre-existing in the network and the
relationship between the mobile node and the access router.
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A.2. Triggers
The question of the triggers initiating a context transfer or state
migration is very closely tied to the question of topology awareness,
since in the CXTP case the mobile node "knows" the access router is
there and has an explicit relationship with it, while in the state
migration case the middlebox is opaque to the endpoint.
The mechanisms underlying a mobile node attach/detach differ
significantly from those underlying, say, a virtual machine
migration. At the most basic level, a mobile node knows that it is
moving between access routers. A virtual machine typically does not
know that it's being moved - the VM migration is triggered by a third
party and is opaque to the VM itself, since its own state is
maintained intact across the migration. A network access device may
detect a change, but it will not have knowledge of the other
(previous) middlebox nor will it be able to request that information
from the migrated VM, since the VM itself will not know whether or
not there were middleboxes present, or where they were, as described
in the previous section.
A.3. Copying state
CXTP has been designed to transfer state between a source access
router and a destination access router -- that is to say, they must
know about each other, know that a given mobile node is associated
with the other access router, and have a network path between the two
access routers.
That is not the case when migrating virtual machines. The network
element which triggers a VM migration does not necessarily have
network topology awareness and does not have sufficient information
to be able to request a migration of associated state.
That said, CXTP looks highly suitable for actually transferring the
middlebox state, once the topology/ middlebox discovery problems are
solved. Security issues would need an extra level of scrutiny, not
only because, as described in [RFC4067], the threats in a handover
were not well understood at the time the document was published, but
also because the network elements involved are different and the
relationships among those network elements are different. Having a
third party (the element requesting the VM migration) request a
migration of network middlebox state requires different security
properties from having a network element (a mobile node or its access
routers) request a context transfer on its own behalf.
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A.4. Conclusion
Based on the previous discussion we believe that CXTP may be directly
useful for the actual transfer of middlebox state but that it does
not address some core problems which would need to be solved in order
to successfully migrate that state. These problems are topology
discovery (i.e. locating the correct middleboxes), and generating
triggers.
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Authors' Addresses
Yingjie Gu
Huawei
Phone: +86-25-56624760
Fax: +86-25-56624702
Email: guyingjie@huawei.com
Melinda Shore
No Mountain Software
PO Box 16271
Two Rivers, AK 99716
US
Phone: +1 907 322 9522
Email: melinda.shore@nomountain.net
Senthil Sivakumar
Cisco Systems
7100-8 Kit Creek Road
Research Triangle Park, NC
US
Email: ssenthil@cisco.com
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