shim6 Working Group J. Abley
Internet-Draft ICANN
Intended status: Informational M. Bagnulo
Expires: April 28, 2011 A. Garcia-Martinez
UC3M
October 25, 2010
Applicability Statement for the Level 3 Multihoming Shim Protocol
(Shim6)
draft-ietf-shim6-applicability-08
Abstract
This document discusses the applicability of the Shim6 IPv6 protocol
and associated support protocols and mechanisms to provide site
multihoming capabilities in IPv6.
Status of this Memo
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This Internet-Draft will expire on April 28, 2011.
Copyright Notice
Copyright (c) 2010 IETF Trust and the persons identified as the
document authors. All rights reserved.
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described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Deployment Scenarios . . . . . . . . . . . . . . . . . . . . . 3
3. Address Configuration . . . . . . . . . . . . . . . . . . . . 5
3.1. Protocol Version (IPv4 vs. IPv6) . . . . . . . . . . . . . 5
3.2. Prefix Lengths . . . . . . . . . . . . . . . . . . . . . . 6
3.3. Address Generation . . . . . . . . . . . . . . . . . . . . 6
3.4. Use of CGA vs. HBA . . . . . . . . . . . . . . . . . . . . 6
4. Shim6 and Ingress Filtering . . . . . . . . . . . . . . . . . 7
5. Shim6 Capabilities . . . . . . . . . . . . . . . . . . . . . . 9
5.1. Fault Tolerance . . . . . . . . . . . . . . . . . . . . . 9
5.1.1. Establishing Communications After an Outage . . . . . 9
5.1.2. Short-Lived Communications . . . . . . . . . . . . . . 9
5.1.3. Long-Lived Communications . . . . . . . . . . . . . . 10
5.2. Load Balancing . . . . . . . . . . . . . . . . . . . . . . 10
5.3. Traffic Engineering . . . . . . . . . . . . . . . . . . . 11
6. Application Considerations . . . . . . . . . . . . . . . . . . 11
7. Interaction with Other Protocols . . . . . . . . . . . . . . . 12
7.1. Shim6 and Mobile IPv6 . . . . . . . . . . . . . . . . . . 12
7.1.1. Multihomed Home Network . . . . . . . . . . . . . . . 12
7.1.2. Shim6 Between the HA and the MN . . . . . . . . . . . 15
7.2. Shim6 and SEND . . . . . . . . . . . . . . . . . . . . . . 15
7.3. Shim6 and SCTP . . . . . . . . . . . . . . . . . . . . . . 16
7.4. Shim6 and NEMO . . . . . . . . . . . . . . . . . . . . . . 16
7.5. Shim6 and HIP . . . . . . . . . . . . . . . . . . . . . . 17
8. Security Considerations . . . . . . . . . . . . . . . . . . . 17
8.1. Privacy Considerations . . . . . . . . . . . . . . . . . . 18
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 19
10. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 19
11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 19
12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 20
12.1. Normative References . . . . . . . . . . . . . . . . . . . 20
12.2. Informative References . . . . . . . . . . . . . . . . . . 21
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 22
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1. Introduction
Site multihoming is an arrangement by which a site may use multiple
paths to the rest of the Internet to provide better reliability for
traffic passing in and out of the site than would be possible with a
single path. Some of the motivations for operators to multi-home
their network are described in [RFC3582].
In IPv4, site multihoming is achieved by injecting into the global
Internet routing system (sometimes referred to as the Default-Free
Zone, or DFZ) the additional state required to allow session
resilience over re-homing events [RFC4116]. There is concern that
this approach will not scale [RFC3221], [RFC4984].
In IPv6, site multihoming in the style of IPv4 is not generally
available to end sites due to a strict policy of route aggregation in
the DFZ. Site multihoming for sites without provider-independent
(PI) addresses is achieved by assigning multiple addresses to each
host, one or more from each provider. This multihoming approach
provides no transport-layer stability across re-homing events.
Shim6 provides layer-3 support for making re-homing events
transparent to the transport layer by means of a shim approach.
State information relating to the multihoming of two endpoints
exchanging unicast traffic is retained on the endpoints themselves,
rather than in the network. Communications between Shim6-capable
hosts and Shim6-incapable hosts proceed as normal, but without the
benefit of transport-layer stability. The Shim6 approach is thought
to have better scaling properties with respect to the state held in
the DFZ than the IPv4 approach.
This note describes the applicability of the Level 3 multihoming
(hereafter Shim6) protocol defined in [RFC5533] and the failure
detection mechanisms defined in [RFC5534].
The terminology used in this document, including terms like locator,
and ULID, is defined in [RFC5533].
2. Deployment Scenarios
The goal of the Shim6 protocol is to support locator agility in
established communications: different layer-3 endpoint addresses may
be used to exchange packets belonging to the same transport-layer
session, all the time presenting a consistent identifier pair to
upper-layer protocols.
In order to be useful, the Shim6 protocol requires that at least one
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of the peers has more than one address which could be used on the
wire (as locators). In the event of communications failure between
an active pair of addresses, the Shim6 protocol will attempt to
reestablish communication by trying different combinations of
locators.
While other multi-addressing scenarios are not precluded, the
scenario in which the Shim6 protocol is expected to operate is that
of a multihomed site which is connected to multiple transit
providers, and which receives an IPv6 prefix from each of them. This
configuration is intended to provide protection for the end-site in
the event of a failure in some subset of the available transit
providers, without requiring the end-site to acquire PI address space
or requiring any particular cooperation between the transit
providers.
,------------------------------------. ,----------------.
| Rest of the Internet +-------+ Remote Host R |
`--+-----------+------------------+--' `----------------'
| | | LR[1] ... LR[m]
,---+----. ,---+----. ,----+---.
| ISP[1] | | ISP[2] | ...... | ISP[n] |
`---+----' `---+----' `----+---'
| | |
,---+-----------+------------------+---.
| Multi-Homed Site S assigned |
| prefixes P[1], P[2], ..., P[n] |
| |
| ,--------. L[1] = P[1]:iid[1], |
| | Host H | L[2] = P[2]:iid[2], ... |
| `--------' L[n] = P[n]:iid[n] |
`--------------------------------------'
Figure 1
In the scenario illustrated in Figure 1 host H communicates with some
remote host R. Each of the addresses L[i] configured on host H in the
multihomed site S can be reached through provider ISP[i] only, since
ISP[i] is solely responsible for advertising a covering prefix for
P[i] to the rest of the Internet.
The use of locator L[i] on H hence causes inbound traffic towards H
to be routed through ISP[i]. Changing the locator from L[i] to L[j]
will have the effect of re-routing inbound traffic to H from ISP[i]
to ISP[j]. This is the central mechanism by which the Shim6 protocol
aims to provide multihoming functionality: by changing locators, host
H can change the upstream ISP used to route inbound packets towards
itself. Regarding the outbound traffic to H, the path taken in this
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case depends on both the actual locator LR[j] chosen by R, and the
administrative exit selection policy of site S.
The Shim6 protocol has other potential applications beyond site
multihoming. For example, since Shim6 is a host-based protocol, it
can also be used to support host multihoming. In this case, a
failure in communication between a multihomed host and some other
remote host might be repaired by selecting a locator associated with
a different interface.
3. Address Configuration
3.1. Protocol Version (IPv4 vs. IPv6)
The Shim6 protocol is defined only for IPv6. However, there is no
fundamental reason why a Shim6-like approach could not support IPv4
addresses as locators, either to provide multihoming support to IPv4-
numbered sites, or as part of an IPv4/IPv6 transition strategy. Some
extensions to the Shim6 protocol for supporting IPv4 locators have
been proposed in [I-D.nordmark-shim6-esd].
The Shim6 protocol, as specified for IPv6, incorporates cryptographic
elements in the construction of locators (see [RFC3972], [RFC5535]).
Since IPv4 addresses are insufficiently large to contain addresses
constructed in this fashion, direct implementation of Shim6 as
specified for IPv6 for use with IPv4 addresses might require protocol
modifications.
In addition, there are other factors to take into account when
considering the support of IPv4 addresses, in particular IPv4
locators. Using multiple IPv4 addresses in a single host in order to
support Shim6 style of multihoming would result in an increased IPv4
address consumption, which with the current rate of IPv4 addresses
would be problematic. Besides, Shim6 may suffer additional problems
if locators become translated on the wire. Address translation is
more likely to involve IPv4 addresses. IPv4 addresses can be
translated to other IPv4 addresses (for example, private IPv4 address
into public IPv4 address and vice versa) or to/from IPv6 addresses
(for example, as defined by NAT64
[I-D.ietf-behave-v6v4-xlate-stateful]). When address translation
occurs, a locator exchanged by Shim6 could be different to the
address needed to reach the corresponding host, either because the
translated version of the locator exchanged by Shim6 is not known or
because the translation state does not exist any more in the
translator device. Supporting these scenarios would require NAT
traversal mechanisms which are not defined yet and which would imply
additional complexity (as any other NAT traversal mechanism).
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3.2. Prefix Lengths
The Shim6 protocol does not assume that all the prefixes assigned to
the multihomed site have the same prefix length.
However, the use of CGA [RFC3972] and HBA [RFC5535] involve encoding
information in the lower 64 bits of the locators. This imposes the
requirement on address assignment to Shim6-capable hosts that all
interface addresses should be able to accommodate 64-bit interface
identifiers. It should be noted that this is imposed by RFC4291
[RFC4291].
3.3. Address Generation
The security of the Shim6 protocol is based on the use of CGA and HBA
addresses.
CGA and HBA generation process can use the information provided by
the stateless auto-configuration mechanism defined in [RFC4862] with
the additional considerations presented in [RFC3972] and [RFC5535].
Stateful address auto-configuration using DHCP [RFC3315] is not
currently supported, because there is no defined mechanism to convey
the CGA Parameter Data Structure and other relevant information from
the DHCP server to the host. The definition of such mechanism seems
to be quite straightforward in the case of the HBA, since only the
CGA Parameter Data Structure needs to be delivered from the DHCP
server to the Shim6 host, and this data structure does not contain
any secret information. In the case of CGAs, the difficulty is
increased, since private key information should be exchanged as well
as the CGA Parameter Data Structure. However, with appropriate
extensions a DHCP server could inform to a host about the SEC value
to use when generating an address, or DHCP could even be used by the
host to delegate to the server the CPU-intensive task of computing a
Modifier for a given <prefix, public key, SEC> combination
[I-D.ietf-csi-dhcpv6-cga-ps].
3.4. Use of CGA vs. HBA
The choice between CGA and HBA is a trade-off between flexibility and
performance.
The use of HBA is more efficient in the sense that addresses require
less computation than CGA, involving only hash operations for both
the generation and the verification of locator sets. However, the
locators of an HBA set are determined during the generation process,
and cannot be subsequently changed; the addition of new locators to
that initial set is not supported, except by re-generation of the
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entire set which will in turn cause all addresses to change.
The use of CGA is more computationally expensive, involving public
key cryptography in the verification of locator sets. However, CGAs
are more flexible in the sense that they support the dynamic
modification of locator sets.
Therefore, CGAs are well suited to support dynamic environments such
as mobile hosts, where the locator set must be changed frequently.
HBAs are better suited for sites where the prefix set remains
relatively stable.
It should be noted that, since HBAs are defined as a CGA extension,
it is possible to generate hybrid HBA/CGA structures that incorporate
the strengths of both: i.e. that a single address can be used as an
HBA, enabling computationally-cheap validation amongst a fixed set of
addresses, and also as a CGA, enabling dynamic manipulation of the
locator set. For additional details, see [RFC5535].
4. Shim6 and Ingress Filtering
Ingress filtering [RFC2827] prevents address spoofing by dropping
packets which come from customer networks with source addresses not
belonging to the prefix assigned to them. The problem of deploying
ingress filters with multihomed customers is discussed in [RFC3704],
in particular considering the case in which non-PI addresses are used
by customer networks. This is the case for IPv6 hosts in multihomed
networks with PA, and also for a Shim6 host in a multihomed network.
Note that this is also the case for other solutions supporting
multihoming, such as SCTP [RFC4960], HIP [RFC4423], etc.
One solution to this problem is to make the providers aware of the
alternative prefixes that can be used by a multihomed site, so that
ingress filtering would not be applied to packets with source
addresses belonging to these prefixes. This may be possible in some
cases, but it cannot be assumed as the general case.
[RFC3704] requires that sites using non-PI addresses should ensure
that each packet is delivered to the provider whose prefix matches
its source address. To deliver packets to the appropriate outgoing
ISP, some routers of the site must consider source addresses in their
forwarding decisions, in addition to the usual destination-based
forwarding. These routers maintain as many parallel routing tables
as there are valid source prefixes, and choose a route that is a
function of both the source and the destination address. The way
these routing tables are populated is out of the scope of this
document.
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Site exit routers are required (at least) to be part of a single
connected source based routing domain:
Multiple site exits
| | | |
-----+-----+-----+-----+-----
( )
( Source based routing domain )
( )
----+----+----+----+----+----
( )
( Generic routing domain )
( )
-----------------------------
Figure 2
In this way, packets arriving to this connected source based routing
domain would be delivered to the appropriate exit router.
Some particular cases of this generic deployment scenario are:
- a single exit router, in which the router chooses the exit provider
according to the source address of the packet to be forwarded
- a site in which all routers perform source address based forwarding
- a site in which only site-exit routers perform source address based
forwarding, and these site-exit routers are connected through point-
to-point tunnels, so that packets can be tunneled to the appropriate
exit router according to its source address
For hosts attached directly to networks of different providers, a
host solution to ensure that packets are forwarded to the appropriate
interface according to its source address must be provided. This
problem is discussed in the Multiple Interfaces (MIF) IETF Working
Group.
Shim6 has no means to enforce neither host nor network forwarding for
a given locator to be used as source address. If any notification is
received from the router dropping the packets with legitimate source
addresses as a result of ingress filtering, the affected locator
could be associated to a low preference (or not being used at all).
But even if such notification is not received, or not processed by
the Shim6 layer, defective ingress filtering configuration will be
treated as a communication failure, and Shim6 re-homing would finally
select a working path in which packets are not filtered, if this path
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exists. Note that this behavior results from the powerful end-to-end
resilience properties exhibited by REAP.
5. Shim6 Capabilities
5.1. Fault Tolerance
5.1.1. Establishing Communications After an Outage
If a host within a multihomed site attempts to establish a
communication with a remote host and selects a locator which
corresponds to a failed transit path, bidirectional communication
between the two hosts will not succeed. In order to establish a new
communication, the initiating host must try different combinations of
(source, destination) locator pairs until it finds a pair that works.
The mechanism for this default address selection is described in
[RFC3484]. As a result of the use of this mechanism, some failures
may not be recovered even if a valid alternative path exists between
two communicating hosts. For example, assuming a failure in ISP[1]
(see figure 1), and host H initiating a communication with host R,
the source address selection algorithm described in [RFC3484] may
result in the selection of the source address corresponding to ISP[1]
for every destination address being tried by the application.
However, note that if R is the node initiating the communication, it
will find a valid path provided that the application at R tries every
available address for H.
Since a Shim6 context is normally established between two hosts only
after initial communication has been set up, there is no opportunity
for Shim6 to participate in the discovery of a suitable, initial
(source, destination) locator pair. The same consideration holds for
referrals, as it is described in Section 6.
5.1.2. Short-Lived Communications
The Shim6 context establishment operation requires a 4-way packet
exchange, and involves some overhead on the participating hosts in
memory and CPU.
For short-lived communications between two hosts, the benefit of
establishing a Shim6 context might not exceed the cost, perhaps
because the protocols concerned are fault tolerant and can arrange
their own recovery (e.g. DNS) or because the frequency of re-homing
events is sufficiently low that the probability of such a failure
occurring during a short-lived exchange is not considered
significant.
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It is anticipated that the exchange of Shim6 context will provide
most benefit for exchanges between hosts which are long-lived. For
this reason the default behaviour of Shim6-capable hosts is expected
to employ deferred context-establishment. This default behaviour
will be able to be overridden by applications which prefer immediate
context establishment regardless of transaction longevity.
It must be noted that all the above considerations refer to the
lifetime of the interaction between the peers and not about the
lifetime of a particular connection (e.g. TCP connection). In other
words, the Shim6 context is established between ULID pairs and it
affects all the communication between these ULIDs. So, two nodes
with multiple short-lived communications using the same ULID pair
would benefit as much from the Shim6 features as two nodes having a
single long-lived communication. One example of such scenario would
be a web client software downloading web contents from a server over
multiple TCP connections. Each TCP connection is short-lived, but
the communication/contact between the two ULID could be long-lived.
5.1.3. Long-Lived Communications
As discussed in Section 5.1.2, hosts engaged in long-lived
communications will suffer lower proportional overhead, and greater
probability of benefit than those performing brief transactions.
Deferred context setup ensures that session establishment time will
not be increased by the use of Shim6.
5.2. Load Balancing
The Shim6 protocol does not support load balancing within a single
context: all packets associated with a particular context are
exchanged using a single locator pair per direction, with the
exception of forked contexts, which are created upon explicit
requests from the upper-layer protocol.
It may be possible to extend the Shim6 protocol to use multiple
locator pairs in a single context, but the impact of such an
extension on upper-layer protocols (e.g. on TCP congestion control)
should be considered carefully.
When many contexts are considered together in aggregation, e.g. on a
single host which participates in many simultaneous contexts or in a
site full of hosts, some degree of load sharing should occur
naturally due to the selection of different locator pairs in each
context. However, there is no mechanism defined to ensure that this
natural load sharing is arranged to provide a statistical balance
between transit providers.
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5.3. Traffic Engineering
The Shim6 protocol provides some lightweight traffic engineering
capabilities in the form of the Locator Preferences option, which
allows a host to inform a remote host of local preferences for
locator selection.
This mechanism is only available after a Shim6 context has been
established, and it is a host-based capability rather than a site-
based capability. There is no defined mechanism which would allow
use of the Locator Preferences option amongst a site full of hosts to
be managed centrally.
6. Application Considerations
Shim6 provides multihoming support without forcing changes in the
applications running on the host. The fact that an address has been
generated according to the CGA or HBA specification does not require
any specific action from the application, e.g. it can obtain remote
CGA or HBA addresses as a result of a getaddrinfo() call to trigger a
DNS Request. The storage of CGA or HBA addresses in DNS does not
require also any modification of this protocol, since they are
recorded using AAAA records. Moreover, neither the ULID/locator
management [RFC5533] nor the failure detection and recovery [RFC5534]
functions require application awareness.
However, a specific API [I-D.ietf-shim6-multihome-shim-api] is
developed for those applications which might require additional
capabilities in ULID/locator management, such as the locator pair in
use for a given context, or the set of local or remote locators
available for it. This API can also be used to disable Shim6
operation when required.
It is worth noting that callbacks can benefit naturally from Shim6
support. In a callback, an application in B retrieves IP_A, the IP
address of a peer A, and B uses IP_A to establish a new communication
with A. As long as the address exchanged, IP_A is the ULID for the
initial communication between A and B, and B uses the same address as
in the initial communication, and this initial communication is alive
(or the context has not been deleted), the new communication could
use the locators exchanged by Shim6 for the first communication. In
this case, communication could proceed even if the ULID of A is not
reachable.
However, Shim6 does not provide specific protection to current
applications when they use referrals. A referral is the exchange of
the IP address IP_A of a party A by party B to party C, so that party
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C could use IP_A to communicate with party A. In a normal case, the
ULID IP_A would be the only information sent by B to C as referral.
But if IP_A is no longer valid as locator in A, C could have trouble
in establishing a communication with A. Increased failure protection
for referrals could be obtained if B exchanged the whole list of
alternative locators of A, although in this case the application
protocol should be modified. Note that B could send to C the current
locator of A, instead of the ULID of A, as a way of using the most
recent reachability information about A. While in this case no
modification of the application protocol is required, some concerns
arise: host A may not accept one of its locator as ULID for
initiating a communication, and if CGA are used, the locator may not
be a CGA so a Shim6 context among A and C could not be created.
7. Interaction with Other Protocols
7.1. Shim6 and Mobile IPv6
We next consider some scenarios in which the Shim6 protocol and the
MIPv6 protocol [RFC3775] might be used simultaneously.
7.1.1. Multihomed Home Network
In this case, the Home Network of the Mobile Node (MN) is multihomed.
This implies the availability of multiple Home Network prefixes,
resulting on multiple HoAs for each MN. Since the MN is a node
within a multihomed site, it seems reasonable to expect that the MN
should be able to benefit from the multihoming capabilities provided
by the Shim6 protocol. Moreover, the MN needs to be able to obtain
the multihoming benefits even when it is roaming away from the Home
Network: if the MN is away from the Home Network while the Home
Network suffers a failure in a transit path, the MN should be able to
continue communicating using alternate paths to reach the Home
Network.
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The resulting scenario is the following:
+------------------------------------+
| Internet |
+------------------------------------+
| |
+----+ +----+
|ISP1| |ISP2|
+----+ +----+
| |
+------------------------------------+
| Multihomed Home Network |
| Prefixes: P1 and P2 |
| |
| Home Agent |
| // |
+------------------//----------------+
//
//
+-----+
| MN | HoA1, HoA2
+-----+
Figure 3
So, in this configuration, the Shim6 protocol is used to provide
multiple communication paths to all the nodes within the multihomed
sites (including the mobile nodes) and the MIPv6 protocol is used to
support mobility of the mobile nodes of the multihomed site.
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The proposed protocol architecture would be the following:
+--------------+
| Application |
+--------------+
| Transport |
+--------------+
| IP |
| +----------+ |
| | IPSec | |
| +----------+<--ULIDs
| | Shim6 | |
| +----------+<--HoAs
| | MIPv6 | |
| +----------+<--CoAs
| |
+--------------+
Figure 4
In this architecture, the upper layer protocols and IPSec would use
ULIDs of the Shim6 protocol. Only the HoAs will be presented by the
upper layers to the Shim6 layer as potential ULIDs. Two Shim6
entities will exchange their own available HoAs as locators.
Therefore, Shim6 provides failover between different HoAs and allows
preserving established communications when an outage affects the path
through the ISP that has delegated the HoA used for initiating the
communication (similarly to the case of a host within a multihomed
site). The CoAs are not presented to the Shim6 layer and are not
included in the local locator set in this case. The CoAs are managed
by the MIPv6 layer, which binds each HoA to a CoA.
So, in this case, the upper layer protocols select a ULID pair for
the communication. The Shim6 protocol translates the ULID pair to an
alternative locator in case that is needed. Both the ULIDs and the
alternative locators are HoAs. Next, the MIPv6 layer maps the
selected HoA to the corresponding CoA, which is the actual address
included in the wire.
The Shim6 context is established between the MN and the CN, and it
would allow the communication to use all the available HoAs to
provide fault tolerance. The MIPv6 protocol is used between the MN
and the HA in the case of the bidirectional tunnel mode, and between
the MN and the CN in case of the RO (Route Optimization) mode.
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7.1.2. Shim6 Between the HA and the MN
Another scenario where a Shim6-MIPv6 interaction may be useful is the
case where a Shim6 context is established between the MN and the HA
in order to provide fault tolerance capabilities to the bidirectional
tunnel between them.
Consider the case where the HA has multiple addresses (whether
because the Home Network is multihomed or because the HA has multiple
interfaces) and/or the MN has multiple addresses (whether because the
visited network is multihomed or because the MN has multiple
interfaces). In this case, if a failure affects the address pair
that is being used to run the tunnel between the MN and HA,
additional mechanisms need to be used to preserve the communication.
One possibility would be to use MIPv6 capabilities, by simply
changing the CoA used as the tunnel endpoint. However, MIPv6 lacks
of failure detection mechanisms that would allow the MN and/or the HA
to detect the failure and trigger the usage of an alternative
address. Shim6 provides such failure detection protocol, so one
possibility would be re-using the failure detection function from the
Shim6 failure detection protocol in MIPv6. In this case, the Shim6
protocol wouldn't be used to create Shim6 context and provide fault
tolerance, but just its failure detection functionality would be re-
used.
The other possibility would be to use the Shim6 protocol to create a
Shim6 context between the HA and the MN so that the Shim6 detects any
failure and re-homes the communication in a transparent fashion to
MIPv6. In this case, the Shim6 protocol would be associated to the
tunnel interface.
7.2. Shim6 and SEND
Secure Neighbor Discovery (SEND) [RFC3971] uses CGAs to prove address
ownership for Neighbor Discovery [RFC4861]. The Shim6 protocol can
use either CGAs or HBAs to protect locator sets included in Shim6
contexts. It is expected that some hosts will need to participate in
both SEND and Shim6 simultaneously.
In the case that both the SEND and Shim6 protocols are using the CGA
technique to generate addresses, then there is no conflict: the host
will generate addresses for both purposes as CGAs, and since it will
be in control of the associated private key, the same CGA can be used
for the different protocols.
In the case that a Shim6-capable host is using HBAs to protect its
locator sets, the host will need to generate hybrid HBA/CGA addresses
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as defined in [RFC5535] and discussed briefly in Section 3.4. In
this case, the CGA Parameter Data Structure containing a valid public
key and the Multi-Prefix extension are included as inputs to the hash
function.
7.3. Shim6 and SCTP
The SCTP [RFC4960] protocol provides a reliable, stream-based
communications channel between two hosts which provides a superset of
the capabilities of TCP. One of the notable features of SCTP is that
it allows the exchange of endpoint addresses between hosts, and is
able to recover from the failure of a particular endpoint pair in a
manner which is conceptually similar to locator selection in Shim6.
SCTP is a transport-layer protocol, higher in the protocol stack than
Shim6, and hence there is no fundamental incompatibility which would
prevent a Shim6-capable host from communicating using SCTP.
However, since SCTP and Shim6 both aim to exchange addressing
information between hosts in order to meet the same generic goal, it
is possible that their simultaneous use might result in unexpected
behaviour, e.g. lead to race conditions.
The capabilities of SCTP with respect to path maintenance of a
reliable, connection-oriented stream protocol are more extensive than
the more general layer-3 locator agility provided by Shim6.
Therefore, It is recommended that Shim6 is not used for SCTP
sessions, and that path maintenance is provided solely by SCTP.
There are at least two ways to enforce this behaviour. One option
would be to make the stack, and in particular the Shim6 sublayer,
aware of SCTP sockets and in this case refrain from creating a Shim6
context. The other option is that the upper layer, SCTP in this
case, informs using a Shim6-capable API like the one proposed in
[I-D.ietf-shim6-multihome-shim-api] that no Shim6 context must be
created for this particular communication.
Note that the issues described here for SCTP may also arise for a
multipath TCP solution.
7.4. Shim6 and NEMO
The NEMO [RFC3963] protocol extensions to MIPv6 allow a Mobile
Network to communicate through a bidirectional tunnel via a Mobile
Router (MR) to a NEMO-compliant Home Agent (HA) located in a Home
Network.
If either or both of the MR or HA are multihomed, then a Shim6
context established preserves the integrity of the bidirectional
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tunnel between them in the event that a transit failure occurs in the
connecting path.
Once the tunnel between MR and HA is established, hosts within the
Mobile Network which are Shim6-capable can establish contexts with
remote hosts in order to receive the same multihoming benefits as any
host located within the Home Network.
7.5. Shim6 and HIP
Shim6 and the Host Identity Protocol (HIP [RFC4423]) are
architecturally similar in the sense that both solutions allow two
hosts to use different locators to support communications between
stable ULIDs. The signaling exchange to establish the demultiplexing
context on the hosts is very similar for both protocols. However,
there are a few key differences. First, Shim6 avoids defining a new
namespace for ULIDs, preferring instead to use a routable locator as
a ULID, while HIP uses public keys and hashes thereof as ULIDs. The
use of a routable locator as ULID better supports deferred context
establishment, application callbacks, and application referrals, and
avoids management and resolution costs of a new namespace, but
requires additional security mechanisms to securely bind the ULID
with the locators. Second, Shim6 uses an explicit context header on
data packets for which the ULIDs differ from the locators in use
(this header is only needed after a failure/rehoming event occurs),
while HIP may compress this context-tag function into the ESP SPI
field [RFC5201]. Third, HIP as presently defined requires the use of
public-key operations in its signaling exchange and ESP encryption in
the data plane, while the use of Shim6 requires neither (if only HBA
addresses are used). HIP by default provides data protection, while
this is a non-goal for Shim6.
The Shim6 working group was chartered to provide a solution to a
specific problem, multihoming, which minimizes deployment disruption,
while HIP is considered more of an experimental approach intended to
solve several more general problems (mobility, multihoming and loss
of end-to-end addressing transparency) through an explicit
identifier/locator split. Communicating hosts that are willing and
interested to run HIP (perhaps extended with Shim6's failure
detection protocol) likely have no reason to also run Shim6. In this
sense, HIP may be viewed as a possible long-term evolution or
extension of the Shim6 architecture, or one possible implementation
of the extended Shim6 design ESD [I-D.nordmark-shim6-esd].
8. Security Considerations
This section considers the applicability of the Shim6 protocol from a
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security perspective, i.e. which security features can expect
applications and users of the Shim6 protocol.
First of all, it should be noted that the Shim6 protocol is not a
security protocol, like for instance HIP. This means that as opposed
to HIP, it is an explicit non-goal of the Shim6 protocol to provide
enhanced security for the communications that use the Shim6 protocol.
The goal of the Shim6 protocol design in terms of security is not to
introduce new vulnerabilities that were not present in the current
non-Shim6 enabled communications. In particular, it is an explicit
non-goal of the Shim6 protocol security to provide protection from
on-path attackers. On-path attackers are able to sniff and spoof
packets in the current Internet, and they are able to do the same in
Shim6 communications (as long as the communication flows through the
path they are located on). So, summarizing, the Shim6 protocol does
not provide data packet protection from on-path attackers.
However, the Shim6 protocol does use several security techniques.
The goal of these security measures is to protect the Shim6 signaling
protocol from new attacks resulting from the adoption of the Shim6
protocol. In particular, the use of HBA/CGA prevents on-path and
off-path attackers injecting new locators into the locator set of a
Shim6 context, thus preventing redirection attacks [RFC4218].
Moreover, the usage of probes before re-homing to a different locator
as a destination address prevents flooding attacks from off-path
attackers.
In addition, the usage of a 4-way handshake for establishing the
Shim6 context protects against DoS attacks, so hosts implementing the
Shim6 protocol should not be more vulnerable to DoS attacks than
regular IPv6 hosts.
Finally, many Shim6 signaling messages contain a Context Tag, meaning
that only attackers that know the Context Tag can forge them. As a
consequence, only on-path attackers can generate false Shim6
signaling packets for an established context. The impact of these
attacks would be limited since they would not be able to add
additional locators to the locator set (because of the HBA/CGA
protection). In general the possible attacks have similar effects to
the ones that an on-path attacker can launch on any regular IPv6
communication. The residual threats are described in the Security
Considerations of the Shim6 protocol specification [RFC5533].
8.1. Privacy Considerations
The Shim6 protocol is designed to provide some basic privacy
features. In particular, HBAs are generated in such a way, that the
different addresses assigned to a host cannot be trivially linked
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together as belonging to the same host, since there is nothing in
common in the addresses themselves. Similar features are provided
when the CGA protection is used. This means that it is not trivial
to determine that a set of addresses is assigned to a single Shim6
host.
However, the Shim6 protocol does exchange the locator set in clear
text and it also uses a fixed Context Tag when using different
locators in a given context. This implies that an attacker observing
the Shim6 context establishment exchange or seeing different payload
packets exchanged through different locators, but with the same
Context Tag, can determine the set of addresses assigned to a host.
However, this requires that the attacker is located along the path
and that it can capture the Shim6 signaling packets.
9. IANA Considerations
This document has no actions for IANA.
10. Contributors
The analysis on the interaction between the Shim6 protocol and the
other protocols presented in this note benefited from the advice of
various people including Tom Henderson, Erik Nordmark, Hesham
Soliman, Vijay Devarpalli, John Loughney and Dave Thaler.
11. Acknowledgements
Joe Abley's work was supported in part by the US National Science
Foundation (research grant SCI-0427144) and DNS-OARC.
Marcelo Bagnulo worked on this document while visiting Ericsson
Research Laboratory Nomadiclab.
Shinta Sugimoto reviewed this document and provided comments and
text.
Iljitsch van Beijnum, Brian Carpenter, Sam Xia reviewed this document
and provided comments.
12. References
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12.1. Normative References
[I-D.ietf-behave-v6v4-xlate-stateful]
Bagnulo, M., Matthews, P., and I. Beijnum, "Stateful
NAT64: Network Address and Protocol Translation from IPv6
Clients to IPv4 Servers",
draft-ietf-behave-v6v4-xlate-stateful-12 (work in
progress), July 2010.
[I-D.ietf-shim6-multihome-shim-api]
Komu, M., Bagnulo, M., Slavov, K., and S. Sugimoto,
"Socket Application Program Interface (API) for
Multihoming Shim", draft-ietf-shim6-multihome-shim-api-14
(work in progress), August 2010.
[RFC2827] Ferguson, P. and D. Senie, "Network Ingress Filtering:
Defeating Denial of Service Attacks which employ IP Source
Address Spoofing", BCP 38, RFC 2827, May 2000.
[RFC3315] Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C.,
and M. Carney, "Dynamic Host Configuration Protocol for
IPv6 (DHCPv6)", RFC 3315, July 2003.
[RFC3484] Draves, R., "Default Address Selection for Internet
Protocol version 6 (IPv6)", RFC 3484, February 2003.
[RFC3704] Baker, F. and P. Savola, "Ingress Filtering for Multihomed
Networks", BCP 84, RFC 3704, March 2004.
[RFC3775] Johnson, D., Perkins, C., and J. Arkko, "Mobility Support
in IPv6", RFC 3775, June 2004.
[RFC3963] Devarapalli, V., Wakikawa, R., Petrescu, A., and P.
Thubert, "Network Mobility (NEMO) Basic Support Protocol",
RFC 3963, January 2005.
[RFC3971] Arkko, J., Kempf, J., Zill, B., and P. Nikander, "SEcure
Neighbor Discovery (SEND)", RFC 3971, March 2005.
[RFC3972] Aura, T., "Cryptographically Generated Addresses (CGA)",
RFC 3972, March 2005.
[RFC4291] Hinden, R. and S. Deering, "IP Version 6 Addressing
Architecture", RFC 4291, February 2006.
[RFC4423] Moskowitz, R. and P. Nikander, "Host Identity Protocol
(HIP) Architecture", RFC 4423, May 2006.
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[RFC4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
"Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
September 2007.
[RFC4862] Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless
Address Autoconfiguration", RFC 4862, September 2007.
[RFC4960] Stewart, R., "Stream Control Transmission Protocol",
RFC 4960, September 2007.
[RFC5201] Moskowitz, R., Nikander, P., Jokela, P., and T. Henderson,
"Host Identity Protocol", RFC 5201, April 2008.
[RFC5533] Nordmark, E. and M. Bagnulo, "Shim6: Level 3 Multihoming
Shim Protocol for IPv6", RFC 5533, June 2009.
[RFC5534] Arkko, J. and I. van Beijnum, "Failure Detection and
Locator Pair Exploration Protocol for IPv6 Multihoming",
RFC 5534, June 2009.
[RFC5535] Bagnulo, M., "Hash-Based Addresses (HBA)", RFC 5535,
June 2009.
12.2. Informative References
[I-D.ietf-csi-dhcpv6-cga-ps]
Jiang, S., "DHCPv6 and CGA Interaction: Problem
Statement", draft-ietf-csi-dhcpv6-cga-ps-06 (work in
progress), October 2010.
[I-D.nordmark-shim6-esd]
Nordmark, E., "Extended Shim6 Design for ID/loc split and
Traffic Engineering", draft-nordmark-shim6-esd-01 (work in
progress), February 2008.
[RFC3221] Huston, G., "Commentary on Inter-Domain Routing in the
Internet", RFC 3221, December 2001.
[RFC3582] Abley, J., Black, B., and V. Gill, "Goals for IPv6 Site-
Multihoming Architectures", RFC 3582, August 2003.
[RFC4116] Abley, J., Lindqvist, K., Davies, E., Black, B., and V.
Gill, "IPv4 Multihoming Practices and Limitations",
RFC 4116, July 2005.
[RFC4218] Nordmark, E. and T. Li, "Threats Relating to IPv6
Multihoming Solutions", RFC 4218, October 2005.
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[RFC4984] Meyer, D., Zhang, L., and K. Fall, "Report from the IAB
Workshop on Routing and Addressing", RFC 4984,
September 2007.
Authors' Addresses
Joe Abley
ICANN
4676 Admiralty Way, Suite 330
Marina del Rey, CA 90292
USA
Phone: +1 519 670 9327
Email: joe.abley@icann.org
Marcelo Bagnulo
U. Carlos III de Madrid
Av. Universidad 30
Leganes, Madrid 28911
Spain
Phone: +34 91 6248814
Email: marcelo@it.uc3m.es
URI: http://www.it.uc3m.es/
Alberto Garcia Martinez
U. Carlos III de Madrid
Av. Universidad 30
Leganes, Madrid 28911
Spain
Phone: +34 91 6248782
Email: alberto@it.uc3m.es
URI: http://www.it.uc3m.es/
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