Seamoby Working Group Ram Gopal.L
INTERNET DRAFT Govind Krishnamurthi
November 2001 Senthil Sengodan
Expires May 2002 Nokia
Issues in IPSec Context Transfer
draft-gopal-seamoby-ipsecctxt-issues-00.txt
Status of this Memo
This document is an Internet-Draft and is in full conformance with
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Copyright Notice
Copyright (C) The Internet Society (2001). All Rights Reserved.
1. Abstract
The reasons and the motivation for transferring context have been
described in [1]. The requirements for a context transfer protocol
can be found in [2]. In this document, we describe issues that need
to be considered for transferring security (specifically IPsec)
related context between access routers. While [3] has addressed
issues involved in IPSec feature context transfer, the scope and
aspects included here are broader than in [3]. Some of the aspects
that are addressed here but not in [3] are the various types of SAs
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whose context may be transferred, utilizing the user's static
profile when available at the new access router, etc.
2. Conventions used in this document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in
this document are to be interpreted as described in [RFC-2119].
3. Terminology
In addition to the terms used in [1],[2],[3], we define the
following
terms:
o MN - Mobile Node
o PR - Previous AR.
o NR - New AR.
o FP - Forwarding Path. The path traversed by packets when they
go from their source to destination. FP may correspond to the
control, data or management plane.
o PFP - Previous FP. The FP when the access router that the MN
is connected to is PR.
o NFP - New FP. The FP when the access router that the MN is
connected to is NR. The destination itself may be different
along the NFP compared to that along the PFP.
o Data FP - The FP for packets in the data plane.
o Control FP - The FP for packets in the control plane.
o Management FP - The FP for packets in the management plane.
o G - Gateway. This is an entity from which point onwards the
FP
to the CN remains unchanged.
o SA - Security Association. This may be either a Data SA, a
Control SA or a Management SA.
o Data SA - SA that provides security services to packets in
the
Data FP. Such an SA may be between any two entities along the
Data FP.
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o Control SA - SA that provides security services to packets in
the Control FP. Such an SA may be between any two entities
along
the Control FP.
o Management SA - SA that provides security services to packets
in the Management FP. Such an SA may be between any two
entities
along the Management FP.
4. Introduction
The purpose of this document is to describe issues that need to be
considered for IPSec security context transfer. While [3] has
addressed issues involved in IPSec feature context transfer, the
scope and aspects included here are broader than in [3]. Some of the
aspects that are addressed here but not in [3] are the various types
of SAs whose context may be transferred, utilizing the user's static
profile when available at the new access router, etc.
The organization of the document is as follows. Section 5 describes
several types of Security Associations (SA) and a model for
illustrating where/how these SAs fit in. The coverage in this
section is illustrative, and is not meant to be exhaustive.
Irrespective of the type of SA (whether it is described in Section 5
or not), Section 6 describes the issues involved in IPSec security
context for this SA.
5. Illustrating Various Types of SAs
While discussing IPSec context transfer between ARs, in order to
describe several types of IPSec SAs for which the IPSec context
needs
to be transferred, the following model is illustrative:
+- Access Network -+ +--------------+
+--------+ | +-----+ | | +----+ | +--------------+
| Mobile |===|=====| PR |======|==|=======| G |=|===| |
| Node | | +-----+ | | +----+ | | Remote |
| (MN) | | | | Internet * | | Node |
| | | | | * | | (RN) |
| | | +-----+ | | * | | |
| |***|*****| NR |******|**|*********** | | |
| | | +-----+ | | | | |
+--------+ +------------------+ +--------------+ +--------------+
Figure 1: Model for Problem Statement
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In Figure 1, the line from an MN to RN denoted using "=" represents
the previous forwarding path (PFP), while the line from the MN to RN
denoted using "*" represents the new forwarding path (NFP). "G"
denotes the network entity beyond which the forwarding path to RN
remains unchanged. The case of RN representing different physical
nodes along PFP and NFP is also subsumed by the above model.
It may be noted that when the AR changes from PR to NR, the change
in the forwarding path (FP) may either be marginal or significant.
At one extreme (marginal case), G is collocated with PR, while at
the other extreme (significant case), G is collocated with RN. The
FP may correspond to Data, Control or Management FP. When the Data
FP is being considered, RN corresponds to the correspondent node
(CN). When the Control FP is being considered, RN corresponds to an
entity with which MN exchanges control messages. Similarly, when the
Management FP is being considered, RN corresponds to an entity with
which MN exchanges messages in the Management plane.
Note:
1. The model has been generalized to accommodate data, control and
management plane messages between the MN and a remote node. While it
may be the case that management plane messages between MN-RN may
either not exist or may not be of interest, the framework covers
such
aspects as well should they be deemed necessary in certain
scenarios.
2. The model may be extended to cover the case of data, control and
management plane messages between any two entities, and not
restricting itself to the case where one of the entities is MN. It
may be reiterated that the model is not meant to be exhaustive, but
is merely intended to illustrate many types of SAs whose context may
be transferred.
5.1 Types of SAs
For the scenario depicted in Figure 1, several types of SAs are
possible:
(1) Type 1 SA: Each endpoint of such an SA lies between G
(inclusive)
and RN (inclusive). There is no change in such SAs when the FP
changes from PFP to NFP.
(2) Type 2 SA: One endpoint of such an SA is at MN, while the other
endpoint is between G (inclusive) and RN (inclusive). There is no
change in such an SA when the FP changes from PFP to NFP.
(3) Type 3 SA: One endpoint of such an SA is between PR (inclusive)
and G (exclusive), while the other endpoint is between G (inclusive)
and RN (inclusive).
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(4) Type 4 SA: Each endpoint of such an SA lies between PR
(inclusive) and G (exclusive). When the FP changes from PFP to NFP,
then both endpoints of such SAs change.
(5) Type 5 SA: One endpoint of such an SA is at MN, while the other
lies between PR (inclusive) and G (exclusive). When the FP changes
from PFP to NFP, then one endpoint of such SAs change.
Along any data FP, control FP or management FP, there may be zero,
one or more SAs of any given SA type. While the SAs discussed here
are with respect to the PFP, it may be noted that an SA type may
only
be known after the NFP is determined and any SGs along this NFP are
discovered. A reason for this is that "G" is not known until the NFP
is known.
5.2 Illustrating SA types
The notation used in the illustrations below is as follows. S1
represents one endpoint of the SA, while S2 represents the other. It
is immaterial if the SA itself is directed from S1 towards S2, or
vice versa. When an endpoint of the SA (either S1 or S2) may not be
collocated within a certain entity (i.e., exclusive case) then this
is indicated by lines with "%". For example, in Figure 4, since S1
or S1' may not be collocated with G, two sides of the box have a "%"
Type 1 SA: As illustrated in Figure 2, the two endpoints of the
SA (S1 and S2) lie between G (inclusive) and RN (inclusive).
Action: No security context needs to be transferred for such SAs
during context transfer.
+--------+ +----+ +---+ +--+ +--+ +--+
| Mobile |==| PR |=====| G |====|S1|======|S2|======|RN|
| Node | +----+ +---+ +--+ +--+ +--+
| (MN) | *
| | *
| | +-----+ *
| |**| NR |*******
| | +-----+
+--------+
Figure 2: Type 1 SA between S1 and S2
Type 2 SA: As illustrated in Figure 3, S1 is collocated with MN,
while S2 lies between G (inclusive) and RN (inclusive).
Action: No security context needs to be transferred for such SAs
during context transfer.
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+--------+ +----+ +---+ +--+ +--+
| Mobile |==| PR |=====| G |==========|S2|======|RN|
| Node | +----+ +---+ +--+ +--+
| (MN) | *
| | *
| | +-----+ *
| S1 |**| NR |*******
| | +-----+
+--------+
Figure 3: Type 2 SA between S1 and S2
Type 3 SA: As illustrated in Figure 4, S1 lies between PR
(inclusive) and G (exclusive), while S2 lies between G (inclusive)
and RN (inclusive).
Action: In this case, S1' may represent the new entity to which
context at S1 is moved. Note that one special case of Type 3 SAs is
the case where S1 is collocated with PR, while S1' is collocated
with
NR.
+--------+ +----+ +--+ %---+ +--+ +--+
| Mobile |==| PR |====|S1|======% G |=====|S2|======|RN|
| Node | +----+ +--+ %%%%% +--+ +--+
| (MN) | *
| | *
| | +----+ +---+ *
| |**| NR |****|S1'|********
| | +----+ +---+
+--------+
Figure 4: Type 3 SA between S1 and S2
Type 4 SA: As illustrated in Figure 5, both S1 and S2 lie
between PR (inclusive) and G (exclusive).
Action: In this case, S1' may represent the new entity to which
context at S1 is moved, while S2' may represent the new entity to
which context at S2 is moved. Note that one special case of Type 4
SAs is the case where S1 is collocated with PR, while S1' is
collocated with NR.
+--------+ +----+ +--+ +--+ %---+ +--+
| Mobile |==| PR |====|S1|====|S2|=====% G |==========|RN|
| Node | +----+ +--+ +--+ %%%%% +--+
| (MN) | *
| | *
| | +----+ +---+ +---+ *
| |**| NR |****|S1'|***|S2'|*******
| | +----+ +---+ +---+
+--------+
Figure 5: Type 4 SA between S1 and S2
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Type 5 SA: As illustrated in Figure 6, S1 is collocated with MN,
while S2 lies between PR (inclusive) and G (exclusive).
Action: In this case, S2' may represent the new entity to which
context at S2 is moved.
+--------+ +----+ +--+ %---+ +--+
| Mobile |==| PR |====|S2|======% G |===========|RN|
| Node | +----+ +--+ %%%%% +--+
| (MN) | *
| | *
| (S1) | +----+ +---+ *
| |**| NR |****|S2'|********
| | +----+ +---+
+--------+
Figure 6: Type 5 SA between S1 and S2
5.3 Grouping SA types: Multi-lateral and Multi-level SAs
The five SA types may be grouped in a variety of fashions within the
actual topology. These are illustrated below:
o SAs may be grouped in a multi-lateral fashion, such that one
or more of the same or different types of SAs are present in a
sequential fashion. Figure 7 illustrates this scenario, where S1-
S2 represents one SA, while Sa-Sb represents another.
+--------+ +----+ +--+ +--+ %---+ +--+
| Mobile |==| PR |====|Sa|====|Sb|=====% G |==========|RN|
| Node | |(S2)| +--+ +--+ %%%%% +--+
| (MN) | +----+ *
| | *
| | +-----+ +---+ +---+ *
| (S1) |**| NR |***|Sa'|***|Sb'|*******
| | |(S2')| +---+ +---+
+--------+ +-----+
Figure 7: Illustrating multi-lateral SAs
o SAs may be grouped in a multi-level fashion, such that one or
more of the same or different types of SAs are stacked one upon
the other. Figure 8 illustrates this scenario, where S1-S2
represents one SA, while Sa-Sb represents another.
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+--------+ +----+ +--+ +--+ %----+ +--+
| Mobile |==| PR |====|Sa|====|Sb|=====% G |==========|RN|
| Node | | | +--+ +--+ %(S2)| +--+
| (MN) | +----+ %%%%%%
| | *
| | +-----+ +---+ +---+ *
| (S1) |**| NR |***|Sa'|***|Sb'|********
| | | | +---+ +---+
+--------+ +-----+
Figure 8: Illustrating multi-level SAs
5.4 Some Specific Scenarios
Some typical scenarios where security context needs to be
transferred
are now discussed.
5.4.1 MN-PR control FP SA
Figure 9 illustrates the case where a control SA that exists between
MN-PR is replaced by a control SA that exists between MN-NR.
+--------+ +----+
| Mobile |==| PR |
| Node | |(S2)|
| (MN) | +----+
| |
| | +-----+
| (S1) |**| NR |
| | |(S2')|
+--------+ +-----+
Figure 9: Illustrating MN-PR Control FP SA
5.4.2 MN-SG data FP SA
Figure 10 illustrates the case where a data SA that exists between
MN-SG1 is replaced by a data SA that exists between MN-SG2.
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+--------+ +----+ +-----+ %---+ +--+
| Mobile |==| PR |====| SG1 |=========% G |==========|RN|
| Node | |(S2)| |(S2) | %---+ +--+
| (MN) | +----+ +-----+ *
| | *
| | +-----+ +-----+ *
| (S1) |**| NR |***| SG2 |************
| | |(S2')| |(S2')|
+--------+ +-----+ +-----+
Figure 10: Illustrating MN-SG Data FP SA
6. Context Transfer Requirements
6.1 Entities involved in context transfer
It was seen that no security context needs to be moved for Type 1 or
Type 2 SAs, but security context may need to be moved for Type 3, 4
or 5 SAs. Irrespective of the type of SA (Type 3, 4 or 5) whose
context needs to be transferred or the number of such SAs for which
context needs to be transferred, the context is always transferred
from the PR to the NR. This has the following implications:
o All IPSec security context for any SA in the PFP may need to be
made available to the PR.
o After the IPSec security context for one or more SAs in the PFP
has been transferred from PR to NR, a mechanism is needed to move
these contexts to the appropriate SA endpoint in the NFP.
As an illustration of this approach, consider a Type 4 SA in Figure
5, where context at S1 needs to be moved to S1', and context at S2
needs to be moved to S2'. In this case, the contexts at S1 and S2
are
made available at PR, which then transfers them to NR, and NR
subsequently moves these contexts to S1' and S2' respectively.
For the case where the security context already exists at PR and
where the new location of the security context is NR (for instance,
the case of Section 4.4.1), no context needs to be moved either from
a different entity to PR along PFP or from NR to a different entity
along NFP.
6.2 Discovery of SGs along FP
Along any FP (PFP or NFP), the SGs have to be discovered and the SAs
that need to be established have to be identified. When the FP
changes from PFP to NFP, there exists two possibilities:
o One or more security contexts associated with SAs along the
PFP,
may be transferred from PR to NR "prior" to discovering either
the
NFP and/or SGs along the NFP.
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o One or more security contexts associated with SAs along the
PFP,
are transferred from PR to NR "only after" the NFP is known and
the SGs along the NFP have been discovered.
There are trade-offs to each of these two approaches. In the former
case, the knowledge of the PFP, SGs and their associated security
context along the PFP at the NR may help in establishing the NFP.
The
NFP, of course, would still have to be in conformance with the
user's
static profile. In the latter case, knowing the NFP, the NR may
selectively request security context transfer (or may even not have
to request any security context transfer) from PR.
Such discovery of SGs along any FP is not within the scope of this
document.
6.3 IPv4 and IPv6 address support
This requirement fits within the framework of general context
transfer requirements. Context transfer should be possible
irrespective of whether one or both ARs have either an IPv4 or an
IPv6 address.
6.4 Private Addressing support
This requirement fits within the framework of general context
transfer requirements. Context transfer should be possible
irrespective of whether one or both ARs have a private IPv4 address.
One mechanism that may be used to handle the case of private
addresses is as described in [9].
7. IPSec Security Context
The IPSec feature context itself is comprised of several components:
o Part or whole of the static profile associated with the 'user'.
o Selector fields of an SA
o SPI value
o The static attributes of an SA
o The dynamic attributes of an SA
o Replay window parameters
Each of these is discussed in greater detail below.
7.1 User's static profile
Security requirements pertaining to a user are stored within the
user's static profile. Among other things, such a profile may
specify
the range of security mechanisms, security algorithms, key lengths
etc. that may be used for security associations pertaining to the
user. The various options provided in the profile may also be in a
certain preferred order, so that a certain option is only chosen if
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an option of higher preference is not available. For instance, a
decreasing order of preference for encryption algorithms could be
"AES, 3-DES, DES", implying that 3-DES is to be used only if AES is
cannot be used, and that DES is to be used only if both AES and 3-
DES
cannot be used. Similarly, a decreasing order of preference for key
lengths used within AES could be "256, 192, 128", implying that a
192-bit key is to be used only when a 256-bit key cannot be used,
and
that a 128-bit key is to be used only when both 256-bit and 192-bit
keys cannot be used.
The reason that the user's static profile may have to be known at
the
NR is that along the NFP, it may be possible to use a certain option
in the static profile that has a higher preference than which is
used
within an SA along the PFP. For instance, if we consider a Type 5 SA
(as shown in Figure 6), it may be possible that 3-DES is used
between
S1-S2 because S2 does not support AES, but that S2' supports AES. In
this case, although not always, it may be desirable to support the
more preferred option along the NFP. If the user's static profile is
not sent, then the NR would not have this choice.
7.2 Selector fields of an SA
Selector fields are used to determine which packets are provided the
services of a particular SA. While some selector fields are always
sent, others are optional. Selectors that are always sent are the
source IP address and the destination IP address. Optional selectors
are the source port, destination port, TOS byte etc.
7.3 SPI value
The SPI value is sent so that it could be reused at the endpoint of
the SA along the NFP.
7.4 Static attributes of an SA
Static attributes of an SA refer to those attributes that are
instantiated at the time of SA establishment, and which do not
change
with time. Examples of such attributes include:
o Authentication/encryption algorithm
o Key length
o Block size
o Algorithm mode
7.5 Dynamic attributes of an SA
Dynamic attributes of an SA refer to those attributes that change
with time. Examples of such attributes include:
o SA duration (in terms of seconds or bytes transmitted) before
key refresh
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7.6 Replay window parameters
A receiver of an IPSec SA may decide to activate anti-replay
protection. Parameters relevant to anti-replay protection are:
o window size
o highest sequence number of an authenticated packet
o indication of whether packets within window have been
successfully received or not
When the IPSec security context is transparently sent from PR to NR,
the replay window parameters are also sent. Using these parameters,
the entity that receives this security context from the NR in the
NFP, may transparently start providing anti-replay protection.
8. Security Considerations
The IPSec security feature context that is to be transferred from AR
to PR involves several security considerations:
o Discovery of SGs along a forwarding path needs to be done in a
secure fashion.
o Transmission of security context from an endpoint of an SA to
the PR needs to be done in a secure fashion.
o Transmission of security context from PR to NR needs to be
secure.
o Transmission of security context from NR to the endpoint of the
SA along the NFP needs to be secure.
9. References
[1] Levkowetz et al., "Context transfer: problem
statement", draft-ietf-seamoby-context-transfer-problem-stat-
00.txt.
[2] Syed et al., "General Requirements for a Context
Transfer Framework", draft-ietf-seamoby-ct-reqs-00.txt.
[3] L-N. Hamer, B. Kosinski, "IPSec Context Transfer," work-in-
progress, draft-hk-seamoby-ct-ipsec-00.txt, May 2001.
[4] S. Bradner, "keywords for use in RFCs to Indicate Requirement
Levels", RFC2119 (BCP), IETF, March 1997.
[5] S. Kent et. Al., "Security Architecture for the Internet
Protocol" RFC-2401, November 1998
[6] Kent, S., and R. Atkinson, "IP Authentication Header", RFC
2402, November 1998.
[7] Kent, S., and R. Atkinson, "IP Encapsulating Security
Payload (ESP)", RFC 2406, November 1998.
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[8] Harkins, D., and D. Carrel, "The Internet Key Exchange
(IKE)", RFC 2409, November 1998.
[9] D. Trossen et. al., "Protocol for Anticipated Candidate Access
Router Discovery for Seamless IP-level Handovers," draft-
trossen-seamoby-CARdiscovery-anticipated-00.txt, Aug 2001.
10. Author's Addresses
Ram Gopal.L
Govind Krishnamurthi
Senthil Sengodan
Nokia Research Center
5 Wayside Road
Burlington, MA 01803
USA
email: {ram.gopal,govind.krishnamurthi,senthil.sengodan}@nokia.com
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