Network Working Group E. Nordmark
Request for Comments: 5533 Sun Microsystems
Category: Standards Track M. Bagnulo
UC3M
June 2009
Shim6: Level 3 Multihoming Shim Protocol for IPv6
Status of This Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (c) 2009 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
Provisions Relating to IETF Documents in effect on the date of
publication of this document (http://trustee.ietf.org/license-info).
Please review these documents carefully, as they describe your rights
and restrictions with respect to this document.
Abstract
This document defines the Shim6 protocol, a layer 3 shim for
providing locator agility below the transport protocols, so that
multihoming can be provided for IPv6 with failover and load-sharing
properties, without assuming that a multihomed site will have a
provider-independent IPv6 address prefix announced in the global IPv6
routing table. The hosts in a site that has multiple provider-
allocated IPv6 address prefixes will use the Shim6 protocol specified
in this document to set up state with peer hosts so that the state
can later be used to failover to a different locator pair, should the
original one stop working.
Nordmark & Bagnulo Standards Track [Page 1]
RFC 5533 Shim6 Protocol June 2009
Table of Contents
1. Introduction ....................................................4
1.1. Goals ......................................................5
1.2. Non-Goals ..................................................5
1.3. Locators as Upper-Layer Identifiers (ULID) .................6
1.4. IP Multicast ...............................................7
1.5. Renumbering Implications ...................................8
1.6. Placement of the Shim ......................................9
1.7. Traffic Engineering .......................................11
2. Terminology ....................................................12
2.1. Definitions ...............................................12
2.2. Notational Conventions ....................................15
2.3. Conceptual ................................................15
3. Assumptions ....................................................15
4. Protocol Overview ..............................................17
4.1. Context Tags ..............................................19
4.2. Context Forking ...........................................19
4.3. API Extensions ............................................20
4.4. Securing Shim6 ............................................20
4.5. Overview of Shim Control Messages .........................21
4.6. Extension Header Order ....................................22
5. Message Formats ................................................23
5.1. Common Shim6 Message Format ...............................23
5.2. Shim6 Payload Extension Header Format .....................24
5.3. Common Shim6 Control Header ...............................25
5.4. I1 Message Format .........................................26
5.5. R1 Message Format .........................................28
5.6. I2 Message Format .........................................29
5.7. R2 Message Format .........................................31
5.8. R1bis Message Format ......................................33
5.9. I2bis Message Format ......................................34
5.10. Update Request Message Format ............................37
5.11. Update Acknowledgement Message Format ....................38
5.12. Keepalive Message Format .................................40
5.13. Probe Message Format .....................................40
5.14. Error Message Format .....................................40
5.15. Option Formats ...........................................42
5.15.1. Responder Validator Option Format .................44
5.15.2. Locator List Option Format ........................44
5.15.3. Locator Preferences Option Format .................46
5.15.4. CGA Parameter Data Structure Option Format ........48
5.15.5. CGA Signature Option Format .......................49
5.15.6. ULID Pair Option Format ...........................49
5.15.7. Forked Instance Identifier Option Format ..........50
5.15.8. Keepalive Timeout Option Format ...................50
6. Conceptual Model of a Host .....................................51
6.1. Conceptual Data Structures ................................51
Nordmark & Bagnulo Standards Track [Page 2]
RFC 5533 Shim6 Protocol June 2009
6.2. Context STATES ............................................52
7. Establishing ULID-Pair Contexts ................................54
7.1. Uniqueness of Context Tags ................................54
7.2. Locator Verification ......................................55
7.3. Normal Context Establishment ..............................56
7.4. Concurrent Context Establishment ..........................56
7.5. Context Recovery ..........................................58
7.6. Context Confusion .........................................60
7.7. Sending I1 Messages .......................................61
7.8. Retransmitting I1 Messages ................................62
7.9. Receiving I1 Messages .....................................62
7.10. Sending R1 Messages ......................................63
7.10.1. Generating the R1 Validator .......................64
7.11. Receiving R1 Messages and Sending I2 Messages ............64
7.12. Retransmitting I2 Messages ...............................65
7.13. Receiving I2 Messages ....................................66
7.14. Sending R2 Messages ......................................67
7.15. Match for Context Confusion ..............................68
7.16. Receiving R2 Messages ....................................69
7.17. Sending R1bis Messages ...................................69
7.17.1. Generating the R1bis Validator ....................70
7.18. Receiving R1bis Messages and Sending I2bis Messages ......71
7.19. Retransmitting I2bis Messages ............................72
7.20. Receiving I2bis Messages and Sending R2 Messages .........72
8. Handling ICMP Error Messages ...................................74
9. Teardown of the ULID-Pair Context ..............................76
10. Updating the Peer .............................................77
10.1. Sending Update Request Messages ..........................77
10.2. Retransmitting Update Request Messages ...................78
10.3. Newer Information while Retransmitting ...................78
10.4. Receiving Update Request Messages ........................79
10.5. Receiving Update Acknowledgement Messages ................81
11. Sending ULP Payloads ..........................................81
11.1. Sending ULP Payload after a Switch .......................82
12. Receiving Packets .............................................83
12.1. Receiving Payload without Extension Headers ..............83
12.2. Receiving Shim6 Payload Extension Headers ................83
12.3. Receiving Shim Control Messages ..........................84
12.4. Context Lookup ...........................................84
13. Initial Contact ...............................................86
14. Protocol Constants ............................................87
15. Implications Elsewhere ........................................88
15.1. Congestion Control Considerations ........................88
15.2. Middle-Boxes Considerations ..............................88
15.3. Operation and Management Considerations ..................89
15.4. Other Considerations .....................................90
16. Security Considerations .......................................91
16.1. Interaction with IPSec ...................................93
Nordmark & Bagnulo Standards Track [Page 3]
RFC 5533 Shim6 Protocol June 2009
16.2. Residual Threats .........................................94
17. IANA Considerations ...........................................95
18. Acknowledgements ..............................................97
19. References ....................................................97
19.1. Normative References .....................................97
19.2. Informative References ...................................97
Appendix A. Possible Protocol Extensions ........................100
Appendix B. Simplified STATE Machine ............................101
B.1. Simplified STATE Machine Diagram ........................108
Appendix C. Context Tag Reuse ...................................109
C.1. Context Recovery ........................................109
C.2. Context Confusion .......................................109
C.3. Three-Party Context Confusion .........................110
C.4. Summary .................................................110
Appendix D. Design Alternatives .................................111
D.1. Context Granularity .....................................111
D.2. Demultiplexing of Data Packets in Shim6 Communications ..111
D.2.1. Flow Label .........................................112
D.2.2. Extension Header ...................................115
D.3. Context-Loss Detection ................................115
D.4. Securing Locator Sets ...................................117
D.5. ULID-Pair Context-Establishment Exchange ............120
D.6. Updating Locator Sets ...................................121
D.7. State Cleanup ...........................................122
1. Introduction
This document describes a layer 3 shim approach and protocol for
providing locator agility below the transport protocols, so that
multihoming can be provided for IPv6 with failover and load-sharing
properties [11], without assuming that a multihomed site will have a
provider-independent IPv6 address announced in the global IPv6
routing table. The hosts in a site that has multiple provider-
allocated IPv6 address prefixes will use the Shim6 protocol specified
in this document to set up state with peer hosts so that the state
can later be used to failover to a different locator pair, should the
original one stop working (the term locator is defined in Section 2).
The Shim6 protocol is a site-multihoming solution in the sense that
it allows existing communication to continue when a site that has
multiple connections to the Internet experiences an outage on a
subset of these connections or further upstream. However, Shim6
processing is performed in individual hosts rather than through site-
wide mechanisms.
We assume that redirection attacks are prevented using Hash-Based
Addresses (HBA) as defined in [3].
Nordmark & Bagnulo Standards Track [Page 4]
RFC 5533 Shim6 Protocol June 2009
The reachability and failure-detection mechanisms, including how a
new working locator pair is discovered after a failure, are specified
in RFC 5534 [4]. This document allocates message types and option
types for that sub-protocol, and leaves the specification of the
message and option formats, as well as the protocol behavior, to RFC
5534.
1.1. Goals
The goals for this approach are to:
o Preserve established communications in the presence of certain
classes of failures, for example, TCP connections and UDP streams.
o Have minimal impact on upper-layer protocols in general and on
transport protocols and applications in particular.
o Address the security threats in [15] through a combination of the
HBA/CGA approach specified in RFC 5535 [3] and the techniques
described in this document.
o Not require an extra roundtrip up front to set up shim-specific
state. Instead, allow the upper-layer traffic (e.g., TCP) to flow
as normal and defer the set up of the shim state until some number
of packets have been exchanged.
o Take advantage of multiple locators/addresses for load spreading
so that different sets of communication to a host (e.g., different
connections) might use different locators of the host. Note that
this might cause load to be spread unevenly; thus, we use the term
"load spreading" instead of "load balancing". This capability
might enable some forms of traffic engineering, but the details
for traffic engineering, including what requirements can be
satisfied, are not specified in this document, and form part of
potential extensions to this protocol.
1.2. Non-Goals
The problem we are trying to solve is site multihoming, with the
ability to have the set of site prefixes change over time due to site
renumbering. Further, we assume that such changes to the set of
locator prefixes can be relatively slow and managed: slow enough to
allow updates to the DNS to propagate (since the protocol defined in
this document depends on the DNS to find the appropriate locator
sets). However, note that it is an explicit non-goal to make
communication survive a renumbering event (which causes all the
locators of a host to change to a new set of locators). This
proposal does not attempt to solve the related problem of host
Nordmark & Bagnulo Standards Track [Page 5]
RFC 5533 Shim6 Protocol June 2009
mobility. However, it might turn out that the Shim6 protocol can be
a useful component for future host mobility solutions, e.g., for
route optimization.
Finally, this proposal also does not try to provide a new network-
level or transport-level identifier name space distinct from the
current IP address name space. Even though such a concept would be
useful to upper-layer protocols (ULPs) and applications, especially
if the management burden for such a name space was negligible and
there was an efficient yet secure mechanism to map from identifiers
to locators, such a name space isn't necessary (and furthermore
doesn't seem to help) to solve the multihoming problem.
The Shim6 proposal doesn't fully separate the identifier and locator
functions that have traditionally been overloaded in the IP address.
However, throughout this document the term "identifier" or, more
specifically, upper-layer identifier (ULID), refers to the
identifying function of an IPv6 address. "Locator" refers to the
network-layer routing and forwarding properties of an IPv6 address.
1.3. Locators as Upper-Layer Identifiers (ULID)
The approach described in this document does not introduce a new
identifier name space but instead uses the locator that is selected
in the initial contact with the remote peer as the preserved upper-
layer identifier (ULID). While there may be subsequent changes in
the selected network-level locators over time (in response to
failures in using the original locator), the upper-level protocol
stack elements will continue to use this upper-level identifier
without change.
This implies that the ULID selection is performed as today's default
address selection as specified in RFC 3484 [7]. Some extensions are
needed to RFC 3484 to try different source addresses, whether or not
the Shim6 protocol is used, as outlined in [9]. Underneath, and
transparently, the multihoming shim selects working locator pairs
with the initial locator pair being the ULID pair. If communication
subsequently fails, the shim can test and select alternate locators.
A subsequent section discusses the issues that arise when the
selected ULID is not initially working, which creates the need to
switch locators up front.
Using one of the locators as the ULID has certain benefits for
applications that have long-lived session state or that perform
callbacks or referrals, because both the Fully Qualified Domain Name
(FQDN) and the 128-bit ULID work as handles for the applications.
Nordmark & Bagnulo Standards Track [Page 6]
RFC 5533 Shim6 Protocol June 2009
However, using a single 128-bit ULID doesn't provide seamless
communication when that locator is unreachable. See [18] for further
discussion of the application implications.
There has been some discussion of using non-routable addresses, such
as Unique-Local Addresses (ULAs) [14], as ULIDs in a multihoming
solution. While this document doesn't specify all aspects of this,
it is believed that the approach can be extended to handle the non-
routable address case. For example, the protocol already needs to
handle ULIDs that are not initially reachable. Thus, the same
mechanism can handle ULIDs that are permanently unreachable from
outside their site. The issue becomes how to make the protocol
perform well when the ULID is known a priori to be unreachable (e.g.,
the ULID is a ULA), for instance, avoiding any timeout and retries in
this case. In addition, one would need to understand how the ULAs
would be entered in the DNS to avoid a performance impact on
existing, non-Shim6-aware IPv6 hosts potentially trying to
communicate to the (unreachable) ULA.
1.4. IP Multicast
IP multicast requires that the IP Source Address field contain a
topologically correct locator for the interface that is used to send
the packet, since IP multicast routing uses both the source address
and the destination group to determine where to forward the packet.
In particular, IP multicast routing needs to be able to do the
Reverse Path Forwarding (RPF) check. (This isn't much different than
the situation with widely implemented ingress filtering [6] for
unicast.)
While in theory it would be possible to apply the shim re-mapping of
the IP address fields between ULIDs and locators, the fact that all
the multicast receivers would need to know the mapping to perform
makes such an approach difficult in practice. Thus, it makes sense
to have multicast ULPs operate directly on locators and not use the
shim. This is quite a natural fit for protocols that use RTP [10],
since RTP already has an explicit identifier in the form of the
synchronization source (SSRC) field in the RTP headers. Thus, the
actual IP address fields are not important to the application.
In summary, IP multicast will not need the shim to remap the IP
addresses.
This doesn't prevent the receiver of multicast to change its
locators, since the receiver is not explicitly identified; the
destination address is a multicast address and not the unicast
locator of the receiver.
Nordmark & Bagnulo Standards Track [Page 7]
RFC 5533 Shim6 Protocol June 2009
1.5. Renumbering Implications
As stated above, this approach does not try to make communication
survive renumbering in the general case.
When a host is renumbered, the effect is that one or more locators
become invalid, and zero or more locators are added to the host's
network interface. This means that the set of locators that is used
in the shim will change, which the shim can handle as long as not all
the original locators become invalid at the same time; the shim's
ability to handle this also depends on the time that is required to
update the DNS and for those updates to propagate.
But IP addresses are also used as ULIDs, and making the communication
survive locators becoming invalid can potentially cause some
confusion at the upper layers. The fact that a ULID might be used
with a different locator over time opens up the possibility that
communication between two ULIDs might continue to work after one or
both of those ULIDs are no longer reachable as locators, for example,
due to a renumbering event. This opens up the possibility that the
ULID (or at least the prefix on which it is based) may be reassigned
to another site while it is still being used (with another locator)
for existing communication.
In the worst case, we could end up with two separate hosts using the
same ULID while both of them are communicating with the same host.
This potential source for confusion is avoided by requiring that any
communication using a ULID MUST be terminated when the ULID becomes
invalid (due to the underlying prefix becoming invalid). This
behavior can be accomplished by explicitly discarding the shim state
when the ULID becomes invalid. The context-recovery mechanism will
then make the peer aware that the context is gone and that the ULID
is no longer present at the same locator(s).
Nordmark & Bagnulo Standards Track [Page 8]
RFC 5533 Shim6 Protocol June 2009
1.6. Placement of the Shim
-----------------------
| Transport Protocols |
-----------------------
-------------- ------------- IP endpoint
| Frag/reass | | Dest opts | sub-layer
-------------- -------------
---------------------
| Shim6 shim layer |
---------------------
------ IP routing
| IP | sub-layer
------
Figure 1: Protocol Stack
The proposal uses a multihoming shim layer within the IP layer, i.e.,
below the ULPs, as shown in Figure 1, in order to provide ULP
independence. The multihoming shim layer behaves as if it is
associated with an extension header, which would be placed after any
routing-related headers in the packet (such as any hop-by-hop
options). However, when the locator pair is the ULID pair, there is
no data that needs to be carried in an extension header; thus, none
is needed in that case.
Layering the Fragmentation header above the multihoming shim makes
reassembly robust in the case that there is broken multi-path routing
that results in using different paths, hence potentially different
source locators, for different fragments. Thus, the multihoming shim
layer is placed between the IP endpoint sublayer (which handles
fragmentation and reassembly) and the IP routing sublayer (which
selects the next hop and interface to use for sending out packets).
Applications and upper-layer protocols use ULIDs that the Shim6 layer
maps to/from different locators. The Shim6 layer maintains state,
called ULID-pair context, per ULID pair (that is, such state applies
to all ULP connections between the ULID pair) in order to perform
this mapping. The mapping is performed consistently at the sender
and the receiver so that ULPs see packets that appear to be sent
using ULIDs from end to end. This property is maintained even though
the packets travel through the network containing locators in the IP
address fields, and even though those locators may be changed by the
transmitting Shim6 layer.
Nordmark & Bagnulo Standards Track [Page 9]
RFC 5533 Shim6 Protocol June 2009
The context state is maintained per remote ULID, i.e., approximately
per peer host, and not at any finer granularity. In particular, the
context state is independent of the ULPs and any ULP connections.
However, the forking capability enables Shim6-aware ULPs to use more
than one locator pair at a time for a single ULID pair.
---------------------------- ----------------------------
| Sender A | | Receiver B |
| | | |
| ULP | | ULP |
| | src ULID(A)=L1(A) | | ^ |
| | dst ULID(B)=L1(B) | | | src ULID(A)=L1(A) |
| v | | | dst ULID(B)=L1(B) |
| multihoming shim | | multihoming shim |
| | src L2(A) | | ^ |
| | dst L3(B) | | | src L2(A) |
| v | | | dst L3(B) |
| IP | | IP |
---------------------------- ----------------------------
| ^
------- cloud with some routers -------
Figure 2: Mapping with Changed Locators
The result of this consistent mapping is that there is no impact on
the ULPs. In particular, there is no impact on pseudo-header
checksums and connection identification.
Conceptually, one could view this approach as if both ULIDs and
locators are present in every packet, and as if a header-compression
mechanism is applied that removes the need for the ULIDs to be
carried in the packets once the compression state has been
established. In order for the receiver to re-create a packet with
the correct ULIDs, there is a need to include some "compression tag"
in the data packets. This serves to indicate the correct context to
use for decompression when the locator pair in the packet is
insufficient to uniquely identify the context.
There are different types of interactions between the Shim6 layer and
other protocols. Those interactions are influenced by the usage of
the addresses in these other protocols and the impact of the Shim6
mapping on these usages. A detailed analysis of the interactions of
different protocols, including the Stream Control Transmission
Protocol (SCTP), mobile IP (MIP), and Host Identity Protocol (HIP),
can be found in [19]. Moreover, some applications may need to have a
richer interaction with the Shim6 sublayer. In order to enable that,
an API [23] has been defined to enable greater control and
information exchange for those applications that need it.
Nordmark & Bagnulo Standards Track [Page 10]
RFC 5533 Shim6 Protocol June 2009
1.7. Traffic Engineering
At the time of this writing, it is not clear what requirements for
traffic engineering make sense for the Shim6 protocol, since the
requirements must both result in some useful behavior as well as be
implementable using a host-to-host locator agility mechanism like
Shim6.
Inherent in a scalable multihoming mechanism that separates the
locator function of the IP address from identifying function of the
IP address is that each host ends up with multiple locators. This
means that, at least for initial contact, it is the remote peer
application (or layer working on its behalf) that needs to select an
initial ULID, which automatically becomes the initial locator. In
the case of Shim6, this is performed by applying RFC 3484 address
selection.
This is quite different than the common case of IPv4 multihoming
where the site has a single IP address prefix, since in that case the
peer performs no destination address selection.
Thus, in "single prefix multihoming", the site (and in many cases its
upstream ISPs) can use BGP to exert some control of the ingress path
used to reach the site. This capability does not by itself exist in
"multiple prefix multihoming" approaches such as Shim6. It is
conceivable that extensions allowing site or provider guidance of
host-based mechanisms could be developed. But it should be noted
that traffic engineering via BGP, MPLS, or other similar techniques
can still be applied for traffic on each individual prefix; Shim6
does not remove the capability for this. It does provide some
additional capabilities for hosts to choose between prefixes.
These capabilities also carry some risk for non-optimal behaviour
when more than one mechanism attempts to correct problems at the same
time. However, it should be noted that this is not necessarily a
situation brought about by Shim6. A more constrained form of this
capability already exists in IPv6, itself, via its support of
multiple prefixes and address-selection rules for starting new
communications. Even IPv4 hosts with multiple interfaces may have
limited capabilities to choose interfaces on which they communicate.
Similarly, upper layers may choose different addresses.
In general, it is expected that Shim6 is applicable in relatively
small sites and individual hosts where BGP-style traffic engineering
operations are unavailable, unlikely, or if run with provider-
independent addressing, possibly even harmful, considering the growth
rates in the global routing table.
Nordmark & Bagnulo Standards Track [Page 11]
RFC 5533 Shim6 Protocol June 2009
The protocol provides a placeholder, in the form of the Locator
Preferences option, that can be used by hosts to express priority and
weight values for each locator. This option is merely a placeholder
when it comes to providing traffic engineering; in order to use this
in a large site, there would have to be a mechanism by which the host
can find out what preference values to use, either statically (e.g.,
some new DHCPv6 option) or dynamically.
Thus, traffic engineering is listed as a possible extension in
Appendix A.
2. Terminology
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 [1].
2.1. Definitions
This document introduces the following terms:
upper-layer protocol (ULP)
A protocol layer immediately above IP. Examples
are transport protocols such as TCP and UDP;
control protocols such as ICMP; routing protocols
such as OSPF; and Internet or lower-layer
protocols being "tunneled" over (i.e.,
encapsulated in) IP, such as the Internet Packet
Exchange (IPX), AppleTalk, or IP itself.
interface A node's attachment to a link.
address An IP-layer name that both contains topological
significance and acts as a unique identifier for
an interface. 128 bits. This document only uses
the "address" term in the case where it isn't
specific whether it is a locator or an
identifier.
locator An IP-layer topological name for an interface or
a set of interfaces. 128 bits. The locators are
carried in the IP address fields as the packets
traverse the network.
identifier An IP-layer name for an IP-layer endpoint. The
transport endpoint name is a function of the
transport protocol and would typically include
the IP identifier plus a port number.
Nordmark & Bagnulo Standards Track [Page 12]
RFC 5533 Shim6 Protocol June 2009
NOTE: This proposal does not specify any new form
of IP-layer identifier, but still separates the
identifying and locating properties of the IP
addresses.
upper-layer identifier (ULID)
An IP address that has been selected for
communication with a peer to be used by the
upper-layer protocol. 128 bits. This is used for
pseudo-header checksum computation and connection
identification in the ULP. Different sets of
communication to a host (e.g., different
connections) might use different ULIDs in order
to enable load spreading.
Since the ULID is just one of the IP locators/
addresses of the node, there is no need for a
separate name space and allocation mechanisms.
address field The Source and Destination Address fields in the
IPv6 header. As IPv6 is currently specified,
these fields carry "addresses". If identifiers
and locators are separated, these fields will
contain locators for packets on the wire.
FQDN Fully Qualified Domain Name
ULID-pair context The state that the multihoming shim maintains
between a pair of upper-layer identifiers. The
context is identified by a Context Tag for each
direction of the communication and also by a
ULID-pair and a Forked Instance Identifier (see
below).
Context Tag Each end of the context allocates a Context Tag
for the context. This is used to uniquely
associate both received control packets and Shim6
Payload Extension headers as belonging to the
context.
current locator pair
Each end of the context has a current locator
pair that is used to send packets to the peer.
However, the two ends might use different current
locator pairs.
Nordmark & Bagnulo Standards Track [Page 13]
RFC 5533 Shim6 Protocol June 2009
default context At the sending end, the shim uses the ULID pair
(passed down from the ULP) to find the context
for that pair. Thus, normally, a host can have
at most one context for a ULID pair. We call
this the "default context".
context forking A mechanism that allows ULPs that are aware of
multiple locators to use separate contexts for
the same ULID pair, in order to be able use
different locator pairs for different
communication to the same ULID. Context forking
causes more than just the default context to be
created for a ULID pair.
Forked Instance Identifier (FII)
In order to handle context forking, a context is
identified by a ULID pair and a Forked Context
Identifier. The default context has an FII of
zero.
initial contact We use this term to refer to the pre-shim
communication when a ULP decides to start
communicating with a peer by sending and
receiving ULP packets. Typically, this would not
invoke any operations in the shim, since the shim
can defer the context establishment until some
arbitrary, later point in time.
Hash-Based Addresses (HBA)
A form of IPv6 address where the interface ID is
derived from a cryptographic hash of all the
prefixes assigned to the host. See [3].
Cryptographically Generated Addresses (CGA)
A form of IPv6 address where the interface ID is
derived from a cryptographic hash of the public
key. See [2].
CGA Parameter Data Structure (PDS)
The information that CGA and HBA exchange in
order to inform the peer of how the interface ID
was computed. See [2] and [3].
Nordmark & Bagnulo Standards Track [Page 14]
RFC 5533 Shim6 Protocol June 2009
2.2. Notational Conventions
A, B, and C are hosts. X is a potentially malicious host.
FQDN(A) is the Fully Qualified Domain Name for A.
Ls(A) is the locator set for A, which consists of the locators L1(A),
L2(A), ... Ln(A). The locator set is not ordered in any particular
way other than maybe what is returned by the DNS. A host might form
different locator sets containing different subnets of the host's IP
addresses. This is necessary in some cases for security reasons.
See Section 16.1.
ULID(A) is an upper-layer identifier for A. In this proposal,
ULID(A) is always one member of A's locator set.
CT(A) is a Context Tag assigned by A.
STATE (in uppercase) refers to the specific state of the state
machine described in Section 6.2
2.3. Conceptual
This document also makes use of internal conceptual variables to
describe protocol behavior and external variables that an
implementation must allow system administrators to change. The
specific variable names, how their values change, and how their
settings influence protocol behavior are provided to demonstrate
protocol behavior. An implementation is not required to have them in
the exact form described here, so long as its external behavior is
consistent with that described in this document. See Section 6 for a
description of the conceptual data structures.
3. Assumptions
The design intent is to ensure that the Shim6 protocol is capable of
handling path failures independently of the number of IP addresses
(locators) available to the two communicating hosts, and
independently of which host detects the failure condition.
Consider, for example, the case in which both A and B have active
Shim6 state and where A has only one locator while B has multiple
locators. In this case, it might be that B is trying to send a
packet to A, and has detected a failure condition with the current
locator pair. Since B has multiple locators, it presumably has
multiple ISPs, and (consequently) likely has alternate egress paths
Nordmark & Bagnulo Standards Track [Page 15]
RFC 5533 Shim6 Protocol June 2009
toward A. B cannot vary the destination address (i.e., A's locator),
since A has only one locator. However, B may need to vary the source
address in order to ensure packet delivery.
In many cases, normal operation of IP routing may cause the packets
to follow a path towards the correct (currently operational) egress.
In some cases, it is possible that a path may be selected based on
the source address, implying that B will need to select a source
address corresponding to the currently operating egress. The details
of how routing can be accomplished is beyond the scope of this
document.
Also, when the site's ISPs perform ingress filtering based on packet
source addresses, Shim6 assumes that packets sent with different
source and destination combinations have a reasonable chance of
making it through the relevant ISP's ingress filters. This can be
accomplished in several ways (all outside the scope of this
document), such as having the ISPs relax their ingress filters or
selecting the egress such that it matches the IP source address
prefix. In the case that one egress path has failed but another is
operating correctly, it may be necessary for the packet's source
(node B in the previous paragraph) to select a source address that
corresponds to the operational egress, in order to pass the ISP's
ingress filters.
The Shim6 approach assumes that there are no IPv6-to-IPv6 NATs on the
paths, i.e., that the two ends can exchange their own notion of their
IPv6 addresses and that those addresses will also make sense to their
peer.
The security of the Shim6 protocol relies on the usage of Hash-Based
Addresses (HBA) [3] and/or Cryptographically Generated Addresses
(CGA) [2]. In the case that HBAs are used, all the addresses
assigned to the host that are included in the Shim6 protocol (either
as a locator or as a ULID) must be part of the same HBA set. In the
case that CGAs are used, the address used as ULID must be a CGA, but
the other addresses that are used as locators do not need to be
either CGAs or HBAs. It should be noted that it is perfectly
acceptable to run the Shim6 protocol between a host that has multiple
locators and another host that has a single IP address. In this
case, the address of the host with a single address does not need to
be an HBA or a CGA.
Nordmark & Bagnulo Standards Track [Page 16]
RFC 5533 Shim6 Protocol June 2009
4. Protocol Overview
The Shim6 protocol operates in several phases over time. The
following sequence illustrates the concepts:
o An application on host A decides to contact an application on host
B using some upper-layer protocol. This results in the ULP on
host A sending packets to host B. We call this the initial
contact. Assuming the IP addresses selected by default address
selection [7] and its extensions [9] work, then there is no action
by the shim at this point in time. Any shim context establishment
can be deferred until later.
o Some heuristic on A or B (or both) determine that it is
appropriate to pay the Shim6 overhead to make this host-to-host
communication robust against locator failures. For instance, this
heuristic might be that more than 50 packets have been sent or
received, or that there was a timer expiration while active packet
exchange was in place. This makes the shim initiate the 4-way,
context-establishment exchange. The purpose of this heuristic is
to avoid setting up a shim context when only a small number of
packets is exchanged between two hosts.
As a result of this exchange, both A and B will know a list of
locators for each other.
If the context-establishment exchange fails, the initiator will
then know that the other end does not support Shim6, and will
continue with standard (non-Shim6) behavior for the session.
o Communication continues without any change for the ULP packets.
In particular, there are no Shim6 Extension headers added to the
ULP packets, since the ULID pair is the same as the locator pair.
In addition, there might be some messages exchanged between the
shim sublayers for (un)reachability detection.
o At some point in time, something fails. Depending on the approach
to reachability detection, there might be some advice from the
ULP, or the shim (un)reachability detection might discover that
there is a problem.
At this point in time, one or both ends of the communication need
to probe the different alternate locator pairs until a working
pair is found, and then switch to using that locator pair.
o Once a working alternative locator pair has been found, the shim
will rewrite the packets on transmit and tag the packets with the
Shim6 Payload Extension header, which contains the receiver's
Nordmark & Bagnulo Standards Track [Page 17]
RFC 5533 Shim6 Protocol June 2009
Context Tag. The receiver will use the Context Tag to find the
context state, which will indicate which addresses to place in the
IPv6 header before passing the packet up to the ULP. The result
is that, from the perspective of the ULP, the packet passes
unmodified end-to-end, even though the IP routing infrastructure
sends the packet to a different locator.
o The shim (un)reachability detection will monitor the new locator
pair as it monitored the original locator pair, so that subsequent
failures can be detected.
o In addition to failures detected based on end-to-end observations,
one endpoint might know for certain that one or more of its
locators is not working. For instance, the network interface
might have failed or gone down (at layer 2), or an IPv6 address
might have become deprecated or invalid. In such cases, the host
can signal its peer that trying this address is no longer
recommended. This triggers something similar to a failure
handling, and a new working locator pair must be found.
The protocol also has the ability to express other forms of
locator preferences. A change in any preference can be signaled
to the peer, which will have made the peer record the new
preferences. A change in the preferences might optionally make
the peer want to use a different locator pair. In this case, the
peer follows the same locator switching procedure as after a
failure (by verifying that its peer is indeed present at the
alternate locator, etc).
o When the shim thinks that the context state is no longer used, it
can garbage collect the state; there is no coordination necessary
with the peer host before the state is removed. There is a
recovery message defined to be able to signal when there is no
context state, which can be used to detect and recover from both
premature garbage collection as well as from complete state loss
(crash and reboot) of a peer.
The exact mechanism to determine when the context state is no
longer used is implementation dependent. For example, an
implementation might use the existence of ULP state (where known
to the implementation) as an indication that the state is still
used, combined with a timer (to handle ULP state that might not be
known to the shim sublayer) to determine when the state is likely
to no longer be used.
NOTE 1: The ULP packets in Shim6 can be carried completely unmodified
as long as the ULID pair is used as the locator pair. After a switch
to a different locator pair, the packets are "tagged" with a Shim6
Nordmark & Bagnulo Standards Track [Page 18]
RFC 5533 Shim6 Protocol June 2009
Extension header so that the receiver can always determine the
context to which they belong. This is accomplished by including an
8-octet Shim6 Payload Extension header before the (extension) headers
that are processed by the IP endpoint sublayer and ULPs. If,
subsequently, the original ULIDs are selected as the active locator
pair, then the tagging of packets with the Shim6 Extension header is
no longer necessary.
4.1. Context Tags
A context between two hosts is actually a context between two ULIDs.
The context is identified by a pair of Context Tags. Each end gets
to allocate a Context Tag, and once the context is established, most
Shim6 control messages contain the Context Tag that the receiver of
the message allocated. Thus, at a minimum, the combination of <peer
ULID, local ULID, local Context Tag> have to uniquely identify one
context. But, since the Shim6 Payload Extension headers are
demultiplexed without looking at the locators in the packet, the
receiver will need to allocate Context Tags that are unique for all
its contexts. The Context Tag is a 47-bit number (the largest that
can fit in an 8-octet extension header), while preserving one bit to
differentiate the Shim6 signaling messages from the Shim6 header
included in data packets, allowing both to use the same protocol
number.
The mechanism for detecting a loss of context state at the peer
assumes that the receiver can tell the packets that need locator
rewriting, even after it has lost all state (e.g., due to a crash
followed by a reboot). This is achieved because, after a rehoming
event, the packets that need receive-side rewriting carry the Shim6
Payload Extension header.
4.2. Context Forking
It has been asserted that it will be important for future ULPs -- in
particular, future transport protocols -- to be able to control which
locator pairs are used for different communication. For instance,
host A and host B might communicate using both Voice over IP (VoIP)
traffic and ftp traffic, and those communications might benefit from
using different locator pairs. However, the basic Shim6 mechanism
uses a single current locator pair for each context; thus, a single
context cannot accomplish this.
For this reason, the Shim6 protocol supports the notion of context
forking. This is a mechanism by which a ULP can specify (using some
API not yet defined) that a context, e.g., the ULID pair <A1, B2>,
Nordmark & Bagnulo Standards Track [Page 19]
RFC 5533 Shim6 Protocol June 2009
should be forked into two contexts. In this case, the forked-off
context will be assigned a non-zero Forked Instance Identifier, while
the default context has FII zero.
The Forked Instance Identifier (FII) is a 32-bit identifier that has
no semantics in the protocol other than being part of the tuple that
identifies the context. For example, a host might allocate FIIs as
sequential numbers for any given ULID pair.
No other special considerations are needed in the Shim6 protocol to
handle forked contexts.
Note that forking as specified does NOT allow A to be able to tell B
that certain traffic (a 5-tuple?) should be forked for the reverse
direction. The Shim6 forking mechanism as specified applies only to
the sending of ULP packets. If some ULP wants to fork for both
directions, it is up to the ULP to set this up and then instruct the
shim at each end to transmit using the forked context.
4.3. API Extensions
Several API extensions have been discussed for Shim6, but their
actual specification is out of scope for this document. The simplest
one would be to add a socket option to be able to have traffic bypass
the shim (not create any state and not use any state created by other
traffic). This could be an IPV6_DONTSHIM socket option. Such an
option would be useful for protocols, such as DNS, where the
application has its own failover mechanism (multiple NS records in
the case of DNS) and using the shim could potentially add extra
latency with no added benefits.
Some other API extensions are discussed in Appendix A. The actual
API extensions are defined in [23].
4.4. Securing Shim6
The mechanisms are secured using a combination of techniques:
o The HBA technique [3] for verifying the locators to prevent an
attacker from redirecting the packet stream to somewhere else.
o Requiring a Reachability Probe+Reply (defined in [4]) before a new
locator is used as the destination, in order to prevent 3rd party
flooding attacks.
Nordmark & Bagnulo Standards Track [Page 20]
RFC 5533 Shim6 Protocol June 2009
o The first message does not create any state on the responder.
Essentially, a 3-way exchange is required before the responder
creates any state. This means that a state-based DoS attack
(trying to use up all memory on the responder) at least provides
an IPv6 address that the attacker was using.
o The context-establishment messages use nonces to prevent replay
attacks and to prevent off-path attackers from interfering with
the establishment.
o Every control message of the Shim6 protocol, past the context
establishment, carries the Context Tag assigned to the particular
context. This implies that an attacker needs to discover that
Context Tag before being able to spoof any Shim6 control message.
Such discovery probably requires any potential attacker to be
along the path in order to sniff the Context Tag value. The
result is that through this technique, the Shim6 protocol is
protected against off-path attackers.
4.5. Overview of Shim Control Messages
The Shim6 context establishment is accomplished using four messages;
I1, R1, I2, R2. Normally, they are sent in that order from initiator
and responder, respectively. Should both ends attempt to set up
context state at the same time (for the same ULID pair), then their
I1 messages might cross in flight, and result in an immediate R2
message. (The names of these messages are borrowed from HIP [20].)
R1bis and I2bis messages are defined; they are used to recover a
context after it has been lost. An R1bis message is sent when a
Shim6 control or Shim6 Payload Extension header arrives and there is
no matching context state at the receiver. When such a message is
received, it will result in the re-creation of the Shim6 context
using the I2bis and R2 messages.
The peers' lists of locators are normally exchanged as part of the
context-establishment exchange. But the set of locators might be
dynamic. For this reason, there are Update Request and Update
Acknowledgement messages as well as a Locator List option.
Even when the list of locators is fixed, a host might determine that
some preferences might have changed. For instance, it might
determine that there is a locally visible failure that implies that
some locator(s) are no longer usable. This uses a Locator
Preferences option in the Update Request message.
Nordmark & Bagnulo Standards Track [Page 21]
RFC 5533 Shim6 Protocol June 2009
The mechanism for (un)reachability detection is called Forced
Bidirectional Communication (FBD). FBD uses a Keepalive message
which is sent when a host has received packets from its peer but has
not yet sent any packets from its ULP to the peer. The message type
is reserved in this document, but the message format and processing
rules are specified in [4].
In addition, when the context is established and there is a
subsequent failure, there needs to be a way to probe the set of
locator pairs to efficiently find a working pair. This document
reserves a Probe message type, with the packet format and processing
rules specified in [4].
The above Probe and Keepalive messages assume we have an established
ULID-pair context. However, communication might fail during the
initial contact (that is, when the application or transport protocol
is trying to set up some communication). This is handled using the
mechanisms in the ULP to try different address pairs as specified in
[7] and [9]. In future versions of the protocol, and with a richer
API between the ULP and the shim, the shim might be able to help
optimize discovering a working locator pair during initial contact.
This is for further study.
4.6. Extension Header Order
Since the shim is placed between the IP endpoint sublayer and the IP
routing sublayer, the Shim header will be placed before any Endpoint
Extension headers (Fragmentation headers, Destination Options header,
AH, ESP) but after any routing-related headers (Hop-by-Hop Extensions
header, Routing header, and a Destinations Options header, which
precedes a Routing header). When tunneling is used, whether IP-in-IP
tunneling or the special form of tunneling that Mobile IPv6 uses
(with Home Address options and Routing header type 2), there is a
choice whether the shim applies inside the tunnel or outside the
tunnel, which affects the location of the Shim6 header.
In most cases, IP-in-IP tunnels are used as a routing technique;
thus, it makes sense to apply them on the locators, which means that
the sender would insert the Shim6 header after any IP-in-IP
encapsulation. This is what occurs naturally when routers apply IP-
in-IP encapsulation. Thus, the packets would have:
o Outer IP header
o Inner IP header
Nordmark & Bagnulo Standards Track [Page 22]
RFC 5533 Shim6 Protocol June 2009
o Shim6 Extension header (if needed)
o ULP
But the shim can also be used to create "shimmed tunnels", i.e.,
where an IP-in-IP tunnel uses the shim to be able to switch the
tunnel endpoint addresses between different locators. In such a
case, the packets would have:
o Outer IP header
o Shim6 Extension header (if needed)
o Inner IP header
o ULP
In any case, the receiver behavior is well-defined; a receiver
processes the Extension headers in order. However, the precise
interaction between Mobile IPv6 and Shim6 is for further study; it
might make sense to have Mobile IPv6 operate on locators as well,
meaning that the shim would be layered on top of the MIPv6 mechanism.
5. Message Formats
The Shim6 messages are all carried using a new IP protocol number
(140). The Shim6 messages have a common header (defined below) with
some fixed fields, followed by type-specific fields.
The Shim6 messages are structured as an IPv6 Extension header since
the Shim6 Payload Extension header is used to carry the ULP packets
after a locator switch. The Shim6 control messages use the same
extension header formats so that a single "protocol number" needs to
be allowed through firewalls in order for Shim6 to function across
the firewall.
5.1. Common Shim6 Message Format
The first 17 bits of the Shim6 header is common for the Shim6 Payload
Extension header and for the control messages. It looks as follows:
0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Next Header | Hdr Ext Len |P|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Nordmark & Bagnulo Standards Track [Page 23]
RFC 5533 Shim6 Protocol June 2009
Fields:
Next Header: The payload that follows this header.
Hdr Ext Len: 8-bit unsigned integer. Length of the Shim6 header in
8-octet units, not including the first 8 octets.
P: A single bit to distinguish Shim6 Payload Extension
headers from control messages.
Shim6 signaling packets may not be larger than 1280 bytes, including
the IPv6 header and any intermediate headers between the IPv6 header
and the Shim6 header. One way to meet this requirement is to omit
part of the locator address information if, with this information
included, the packet would become larger than 1280 bytes. Another
option is to perform option engineering, dividing into different
Shim6 messages the information to be transmitted. An implementation
may impose administrative restrictions to avoid excessively large
Shim6 packets, such as a limitation on the number of locators to be
used.
5.2. Shim6 Payload Extension Header Format
The Shim6 Payload Extension header is used to carry ULP packets where
the receiver must replace the content of the Source and/or
Destination fields in the IPv6 header before passing the packet to
the ULP. Thus, this extension header is required when the locator
pair that is used is not the same as the ULID pair.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Next Header | 0 |1| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| Receiver Context Tag |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Fields:
Next Header: The payload that follows this header.
Hdr Ext Len: 0 (since the header is 8 octets).
P: Set to one. A single bit to distinguish this from the
Shim6 control messages.
Nordmark & Bagnulo Standards Track [Page 24]
RFC 5533 Shim6 Protocol June 2009
Receiver Context Tag:
47-bit unsigned integer. Allocated by the receiver to
identify the context.
5.3. Common Shim6 Control Header
The common part of the header has a Next Header field and a Header
Extension Length field that are consistent with the other IPv6
Extension headers, even if the Next Header value is always "NO NEXT
HEADER" for the control messages.
The Shim6 headers must be a multiple of 8 octets; hence, the minimum
size is 8 octets.
The common Shim6 Control message header is as follows:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Next Header | Hdr Ext Len |P| Type |Type-specific|S|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Checksum | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| Type-specific format |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Fields:
Next Header: 8-bit selector. Normally set to NO_NXT_HDR (59).
Hdr Ext Len: 8-bit unsigned integer. Length of the Shim6 header in
8-octet units, not including the first 8 octets.
P: Set to zero. A single bit to distinguish this from
the Shim6 Payload Extension header.
Type: 7-bit unsigned integer. Identifies the actual message
from the table below. Type codes 0-63 will not
trigger R1bis messages on a missing context, while
codes 64-127 will trigger R1bis.
S: A single bit set to zero that allows Shim6 and HIP to
have a common header format yet still distinguishes
between Shim6 and HIP messages.
Checksum: 16-bit unsigned integer. The checksum is the 16-bit
one's complement of the one's complement sum of the
entire Shim6 header message, starting with the Shim6
Nordmark & Bagnulo Standards Track [Page 25]
RFC 5533 Shim6 Protocol June 2009
Next Header field and ending as indicated by the Hdr
Ext Len. Thus, when there is a payload following the
Shim6 header, the payload is NOT included in the Shim6
checksum. Note that, unlike protocols like ICMPv6,
there is no pseudo-header checksum part of the
checksum; this provides locator agility without having
to change the checksum.
Type-specific: Part of the message that is different for different
message types.
+------------+----------------------------------------------------+
| Type Value | Message |
+------------+----------------------------------------------------+
| 1 | I1 (1st establishment message from the initiator) |
| 2 | R1 (1st establishment message from the responder) |
| 3 | I2 (2nd establishment message from the initiator) |
| 4 | R2 (2nd establishment message from the responder) |
| 5 | R1bis (Reply to reference to non-existent context) |
| 6 | I2bis (Reply to an R1bis message) |
| 64 | Update Request |
| 65 | Update Acknowledgement |
| 66 | Keepalive |
| 67 | Probe Message |
| 68 | Error Message |
+------------+----------------------------------------------------+
Table 1
5.4. I1 Message Format
The I1 message is the first message in the context-establishment
exchange.
Nordmark & Bagnulo Standards Track [Page 26]
RFC 5533 Shim6 Protocol June 2009
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 59 | Hdr Ext Len |0| Type = 1 | Reserved1 |0|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Checksum |R| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| Initiator Context Tag |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Initiator Nonce |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ Options +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Fields:
Next Header: NO_NXT_HDR (59).
Hdr Ext Len: At least 1, since the header is 16 octets when there
are no options.
Type: 1
Reserved1: 7-bit field. Reserved for future use. Zero on
transmit. MUST be ignored on receipt.
R: 1-bit field. Reserved for future use. Zero on
transmit. MUST be ignored on receipt.
Initiator Context Tag:
47-bit field. The Context Tag that the initiator has
allocated for the context.
Initiator Nonce:
32-bit unsigned integer. A random number picked by
the initiator, which the responder will return in the
R1 message.
The following options are defined for this message:
ULID pair: When the IPv6 source and destination addresses in the
IPv6 header does not match the ULID pair, this option
MUST be included. An example of this is when
recovering from a lost context.
Nordmark & Bagnulo Standards Track [Page 27]
RFC 5533 Shim6 Protocol June 2009
Forked Instance Identifier:
When another instance of an existent context with the
same ULID pair is being created, a Forked Instance
Identifier option MUST be included to distinguish this
new instance from the existent one.
Future protocol extensions might define additional options for this
message. The C-bit in the option format defines how such a new
option will be handled by an implementation. See Section 5.15.
5.5. R1 Message Format
The R1 message is the second message in the context-establishment
exchange. The responder sends this in response to an I1 message,
without creating any state specific to the initiator.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 59 | Hdr Ext Len |0| Type = 2 | Reserved1 |0|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Checksum | Reserved2 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Initiator Nonce |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Responder Nonce |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ Options +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Fields:
Next Header: NO_NXT_HDR (59).
Hdr Ext Len: At least 1, since the header is 16 octets when there
are no options.
Type: 2
Reserved1: 7-bit field. Reserved for future use. Zero on
transmit. MUST be ignored on receipt.
Reserved2: 16-bit field. Reserved for future use. Zero on
transmit. MUST be ignored on receipt.
Nordmark & Bagnulo Standards Track [Page 28]
RFC 5533 Shim6 Protocol June 2009
Initiator Nonce:
32-bit unsigned integer. Copied from the I1 message.
Responder Nonce:
32-bit unsigned integer. A number picked by the
responder, which the initiator will return in the I2
message.
The following options are defined for this message:
Responder Validator:
Variable length option. This option MUST be included
in the R1 message. Typically, it contains a hash
generated by the responder, which the responder uses
together with the Responder Nonce value to verify that
an I2 message is indeed sent in response to an R1
message, and that the parameters in the I2 message are
the same as those in the I1 message.
Future protocol extensions might define additional options for this
message. The C-bit in the option format defines how such a new
option will be handled by an implementation. See Section 5.15.
5.6. I2 Message Format
The I2 message is the third message in the context-establishment
exchange. The initiator sends this in response to an R1 message,
after checking the Initiator Nonce, etc.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 59 | Hdr Ext Len |0| Type = 3 | Reserved1 |0|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Checksum |R| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| Initiator Context Tag |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Initiator Nonce |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Responder Nonce |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved2 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ Options +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Nordmark & Bagnulo Standards Track [Page 29]
RFC 5533 Shim6 Protocol June 2009
Fields:
Next Header: NO_NXT_HDR (59).
Hdr Ext Len: At least 2, since the header is 24 octets when there
are no options.
Type: 3
Reserved1: 7-bit field. Reserved for future use. Zero on
transmit. MUST be ignored on receipt.
R: 1-bit field. Reserved for future use. Zero on
transmit. MUST be ignored on receipt.
Initiator Context Tag:
47-bit field. The Context Tag that the initiator has
allocated for the context.
Initiator Nonce:
32-bit unsigned integer. A random number picked by
the initiator, which the responder will return in the
R2 message.
Responder Nonce:
32-bit unsigned integer. Copied from the R1 message.
Reserved2: 32-bit field. Reserved for future use. Zero on
transmit. MUST be ignored on receipt. (Needed to
make the options start on a multiple of 8 octet
boundary.)
The following options are defined for this message:
Responder Validator:
Variable length option. This option MUST be included
in the I2 message and MUST be generated by copying the
Responder Validator option received in the R1 message.
ULID pair: When the IPv6 source and destination addresses in the
IPv6 header do not match the ULID pair, this option
MUST be included. An example of this is when
recovering from a lost context.
Nordmark & Bagnulo Standards Track [Page 30]
RFC 5533 Shim6 Protocol June 2009
Forked Instance Identifier:
When another instance of an existent context with the
same ULID pair is being created, a Forked Instance
Identifier option MUST be included to distinguish this
new instance from the existent one.
Locator List: Optionally sent when the initiator immediately wants
to tell the responder its list of locators. When it
is sent, the necessary HBA/CGA information for
verifying the locator list MUST also be included.
Locator Preferences:
Optionally sent when the locators don't all have equal
preference.
CGA Parameter Data Structure:
This option MUST be included in the I2 message when
the locator list is included so the receiver can
verify the locator list.
CGA Signature: This option MUST be included in the I2 message when
some of the locators in the list use CGA (and not HBA)
for verification.
Future protocol extensions might define additional options for this
message. The C-bit in the option format defines how such a new
option will be handled by an implementation. See Section 5.15.
5.7. R2 Message Format
The R2 message is the fourth message in the context-establishment
exchange. The responder sends this in response to an I2 message.
The R2 message is also used when both hosts send I1 messages at the
same time and the I1 messages cross in flight.
Nordmark & Bagnulo Standards Track [Page 31]
RFC 5533 Shim6 Protocol June 2009
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 59 | Hdr Ext Len |0| Type = 4 | Reserved1 |0|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Checksum |R| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| Responder Context Tag |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Initiator Nonce |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ Options +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Fields:
Next Header: NO_NXT_HDR (59).
Hdr Ext Len: At least 1, since the header is 16 octets when there
are no options.
Type: 4
Reserved1: 7-bit field. Reserved for future use. Zero on
transmit. MUST be ignored on receipt.
R: 1-bit field. Reserved for future use. Zero on
transmit. MUST be ignored on receipt.
Responder Context Tag:
47-bit field. The Context Tag that the responder has
allocated for the context.
Initiator Nonce:
32-bit unsigned integer. Copied from the I2 message.
The following options are defined for this message:
Locator List: Optionally sent when the responder immediately wants
to tell the initiator its list of locators. When it
is sent, the necessary HBA/CGA information for
verifying the locator list MUST also be included.
Locator Preferences:
Optionally sent when the locators don't all have equal
preference.
Nordmark & Bagnulo Standards Track [Page 32]
RFC 5533 Shim6 Protocol June 2009
CGA Parameter Data Structure:
Included when the locator list is included so the
receiver can verify the locator list.
CGA Signature: Included when some of the locators in the list use CGA
(and not HBA) for verification.
Future protocol extensions might define additional options for this
message. The C-bit in the option format defines how such a new
option will be handled by an implementation. See Section 5.15.
5.8. R1bis Message Format
Should a host receive a packet with a Shim6 Payload Extension header
or Shim6 control message with type code 64-127 (such as an Update or
Probe message), and the host does not have any context state for the
received Context Tag, then it will generate a R1bis message.
This message allows the sender of the packet referring to the non-
existent context to re-establish the context with a reduced context-
establishment exchange. Upon the reception of the R1bis message, the
receiver can proceed with re-establishing the lost context by
directly sending an I2bis message.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 59 | Hdr Ext Len |0| Type = 5 | Reserved1 |0|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Checksum |R| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| Packet Context Tag |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Responder Nonce |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ Options +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Fields:
Next Header: NO_NXT_HDR (59).
Hdr Ext Len: At least 1, since the header is 16 octets when there
are no options.
Type: 5
Nordmark & Bagnulo Standards Track [Page 33]
RFC 5533 Shim6 Protocol June 2009
Reserved1: 7-bit field. Reserved for future use. Zero on
transmit. MUST be ignored on receipt.
R: 1-bit field. Reserved for future use. Zero on
transmit. MUST be ignored on receipt.
Packet Context Tag:
47-bit unsigned integer. The Context Tag contained in
the received packet that triggered the generation of
the R1bis message.
Responder Nonce:
32-bit unsigned integer. A number picked by the
responder which the initiator will return in the I2bis
message.
The following options are defined for this message:
Responder Validator:
Variable length option. Typically, a hash generated
by the responder, which the responder uses together
with the Responder Nonce value to verify that an I2bis
message is indeed sent in response to an R1bis
message.
Future protocol extensions might define additional options for this
message. The C-bit in the option format defines how such a new
option will be handled by an implementation. See Section 5.15.
5.9. I2bis Message Format
The I2bis message is the third message in the context-recovery
exchange. This is sent in response to an R1bis message, after
checking that the R1bis message refers to an existing context, etc.
Nordmark & Bagnulo Standards Track [Page 34]
RFC 5533 Shim6 Protocol June 2009
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 59 | Hdr Ext Len |0| Type = 6 | Reserved1 |0|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Checksum |R| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| Initiator Context Tag |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Initiator Nonce |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Responder Nonce |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved2 |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| Packet Context Tag |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ Options +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Fields:
Next Header: NO_NXT_HDR (59).
Hdr Ext Len: At least 3, since the header is 32 octets when there
are no options.
Type: 6
Reserved1: 7-bit field. Reserved for future use. Zero on
transmit. MUST be ignored on receipt.
R: 1-bit field. Reserved for future use. Zero on
transmit. MUST be ignored on receipt.
Initiator Context Tag:
47-bit field. The Context Tag that the initiator has
allocated for the context.
Initiator Nonce:
32-bit unsigned integer. A random number picked by
the initiator, which the responder will return in the
R2 message.
Nordmark & Bagnulo Standards Track [Page 35]
RFC 5533 Shim6 Protocol June 2009
Responder Nonce:
32-bit unsigned integer. Copied from the R1bis
message.
Reserved2: 49-bit field. Reserved for future use. Zero on
transmit. MUST be ignored on receipt. (Note that 17
bits are not sufficient since the options need to
start on a multiple-of-8-octet boundary.)
Packet Context Tag:
47-bit unsigned integer. Copied from the Packet
Context Tag field contained in the received R1bis.
The following options are defined for this message:
Responder Validator:
Variable length option. Just a copy of the Responder
Validator option in the R1bis message.
ULID pair: When the IPv6 source and destination addresses in the
IPv6 header do not match the ULID pair, this option
MUST be included.
Forked Instance Identifier:
When another instance of an existent context with the
same ULID pair is being created, a Forked Instance
Identifier option is included to distinguish this new
instance from the existent one.
Locator List: Optionally sent when the initiator immediately wants
to tell the responder its list of locators. When it
is sent, the necessary HBA/CGA information for
verifying the locator list MUST also be included.
Locator Preferences:
Optionally sent when the locators don't all have equal
preference.
CGA Parameter Data Structure:
Included when the locator list is included so the
receiver can verify the locator list.
CGA Signature: Included when some of the locators in the list use CGA
(and not HBA) for verification.
Future protocol extensions might define additional options for this
message. The C-bit in the option format defines how such a new
option will be handled by an implementation. See Section 5.15.
Nordmark & Bagnulo Standards Track [Page 36]
RFC 5533 Shim6 Protocol June 2009
5.10. Update Request Message Format
The Update Request message is used to update either the list of
locators, the locator preferences, or both. When the list of
locators is updated, the message also contains the option(s)
necessary for HBA/CGA to secure this. The basic sanity check that
prevents off-path attackers from generating bogus updates is the
Context Tag in the message.
The Update Request message contains options (the Locator List and the
Locator Preferences) that, when included, completely replace the
previous locator list and locator preferences, respectively. Thus,
there is no mechanism to just send deltas to the locator list.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 59 | Hdr Ext Len |0| Type = 64 | Reserved1 |0|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Checksum |R| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| Receiver Context Tag |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Request Nonce |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ Options +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Fields:
Next Header: NO_NXT_HDR (59).
Hdr Ext Len: At least 1, since the header is 16 octets when there
are no options.
Type: 64
Reserved1: 7-bit field. Reserved for future use. Zero on
transmit. MUST be ignored on receipt.
R: 1-bit field. Reserved for future use. Zero on
transmit. MUST be ignored on receipt.
Receiver Context Tag:
47-bit field. The Context Tag that the receiver has
allocated for the context.
Nordmark & Bagnulo Standards Track [Page 37]
RFC 5533 Shim6 Protocol June 2009
Request Nonce:
32-bit unsigned integer. A random number picked by
the initiator, which the peer will return in the
Update Acknowledgement message.
The following options are defined for this message:
Locator List: The list of the sender's (new) locators. The locators
might be unchanged and only the preferences have
changed.
Locator Preferences:
Optionally sent when the locators don't all have equal
preference.
CGA Parameter Data Structure (PDS):
Included when the locator list is included and the PDS
was not included in the I2/ I2bis/R2 messages, so the
receiver can verify the locator list.
CGA Signature: Included when some of the locators in the list use CGA
(and not HBA) for verification.
Future protocol extensions might define additional options for this
message. The C-bit in the option format defines how such a new
option will be handled by an implementation. See Section 5.15.
5.11. Update Acknowledgement Message Format
This message is sent in response to an Update Request message. It
implies that the Update Request has been received and that any new
locators in the Update Request can now be used as the source locators
of packets. But it does not imply that the (new) locators have been
verified to be used as a destination, since the host might defer the
verification of a locator until it sees a need to use a locator as
the destination.
Nordmark & Bagnulo Standards Track [Page 38]
RFC 5533 Shim6 Protocol June 2009
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 59 | Hdr Ext Len |0| Type = 65 | Reserved1 |0|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Checksum |R| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| Receiver Context Tag |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Request Nonce |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ Options +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Fields:
Next Header: NO_NXT_HDR (59).
Hdr Ext Len: At least 1, since the header is 16 octets when there
are no options.
Type: 65
Reserved1: 7-bit field. Reserved for future use. Zero on
transmit. MUST be ignored on receipt.
R: 1-bit field. Reserved for future use. Zero on
transmit. MUST be ignored on receipt.
Receiver Context Tag:
47-bit field. The Context Tag the receiver has
allocated for the context.
Request Nonce: 32-bit unsigned integer. Copied from the Update
Request message.
No options are currently defined for this message.
Future protocol extensions might define additional options for this
message. The C-bit in the option format defines how such a new
option will be handled by an implementation. See Section 5.15.
Nordmark & Bagnulo Standards Track [Page 39]
RFC 5533 Shim6 Protocol June 2009
5.12. Keepalive Message Format
This message format is defined in [4].
The message is used to ensure that when a peer is sending ULP packets
on a context, it always receives some packets in the reverse
direction. When the ULP is sending bidirectional traffic, no extra
packets need to be inserted. But for a unidirectional ULP traffic
pattern, the shim will send back some Keepalive messages when it is
receiving ULP packets.
5.13. Probe Message Format
This message and its semantics are defined in [4].
The goal of this mechanism is to test whether or not locator pairs
work in the general case. In particular, this mechanism is to be
able to handle the case when one locator pair works from A to B and
another locator pair works from B to A, but there is no locator pair
that works in both directions. The protocol mechanism is that, as A
is sending Probe messages to B, B will observe which locator pairs it
has received and report that back in Probe messages it sends to A.
5.14. Error Message Format
The Error message is generated by a Shim6 receiver upon the reception
of a Shim6 message containing critical information that cannot be
processed properly.
In the case that a Shim6 node receives a Shim6 packet that contains
information that is critical for the Shim6 protocol and that is not
supported by the receiver, it sends an Error Message back to the
originator of the Shim6 message. The Error message is
unacknowledged.
In addition, Shim6 Error messages defined in this section can be used
to identify problems with Shim6 implementations. In order to do so,
a range of Error Code types is reserved for that purpose. In
particular, implementations may generate Shim6 Error messages with
Code types in that range, instead of silently discarding Shim6
packets during the debugging process.
Nordmark & Bagnulo Standards Track [Page 40]
RFC 5533 Shim6 Protocol June 2009
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 59 | Hdr Ext Len |0| Type = 68 | Error Code |0|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Checksum | Pointer |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ Packet in error +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Fields:
Next Header: NO_NXT_HDR (59).
Hdr Ext Len: At least 1, since the header is 16 octets. Depends on
the specific Error Data.
Type: 68
Error Code: 7-bit field describing the error that generated the
Error message. See Error Code list below.
Pointer: 16-bit field. Identifies the octet offset within the
invoking packet where the error was detected.
Packet in error:
As much of invoking packet as possible without the
Error message packet exceeding the minimum IPv6 MTU.
The following Error Codes are defined:
+---------+---------------------------------------------------------+
| Code | Description |
| Value | |
+---------+---------------------------------------------------------+
| 0 | Unknown Shim6 message type |
| 1 | Critical option not recognized |
| 2 | Locator verification method failed (Pointer to the |
| | inconsistent verification method octet) |
| 3 | Locator List Generation number out of sync. |
| 4 | Error in the number of locators in a Locator Preference |
| | option |
| 120-127 | Reserved for debugging purposes |
+---------+---------------------------------------------------------+
Table 2
Nordmark & Bagnulo Standards Track [Page 41]
RFC 5533 Shim6 Protocol June 2009
5.15. Option Formats
The format of the options is a snapshot of the current HIP option
format [20]. However, there is no intention to track any changes to
the HIP option format, nor is there an intent to use the same name
space for the option type values. But using the same format will
hopefully make it easier to import HIP capabilities into Shim6 as
extensions to Shim6, should this turn out to be useful.
All of the TLV parameters have a length (including Type and Length
fields) that is a multiple of 8 bytes. When needed, padding MUST be
added to the end of the parameter so that the total length becomes a
multiple of 8 bytes. This rule ensures proper alignment of data. If
padding is added, the Length field MUST NOT include the padding. Any
added padding bytes MUST be zeroed by the sender, and their values
SHOULD NOT be checked by the receiver.
Consequently, the Length field indicates the length of the Contents
field (in bytes). The total length of the TLV parameter (including
Type, Length, Contents, and Padding) is related to the Length field
according to the following formula:
Total Length = 11 + Length - (Length + 3) mod 8;
The total length of the option is the smallest multiple of 8 bytes
that allows for the 4 bytes of the Option header and option, itself.
The amount of padding required can be calculated as follows:
padding = 7 - ((Length + 3) mod 8)
And:
Total Length = 4 + Length + padding
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type |C| Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ ~
~ Contents ~
~ +-+-+-+-+-+-+-+-+
~ | Padding |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Nordmark & Bagnulo Standards Track [Page 42]
RFC 5533 Shim6 Protocol June 2009
Fields:
Type: 15-bit identifier of the type of option. The options
defined in this document are below.
C: Critical. One, if this parameter is critical and MUST
be recognized by the recipient; zero otherwise. An
implementation might view the C-bit as part of the
Type field by multiplying the type values in this
specification by two.
Length: Length of the Contents, in bytes.
Contents: Parameter-specific, defined by Type.
Padding: Padding, 0-7 bytes, added if needed.
+------+------------------------------+
| Type | Option Name |
+------+------------------------------+
| 1 | Responder Validator |
| 2 | Locator List |
| 3 | Locator Preferences |
| 4 | CGA Parameter Data Structure |
| 5 | CGA Signature |
| 6 | ULID Pair |
| 7 | Forked Instance Identifier |
| 10 | Keepalive Timeout Option |
+------+------------------------------+
Table 3
Future protocol extensions might define additional options for the
Shim6 messages. The C-bit in the option format defines how such a
new option will be handled by an implementation.
If a host receives an option that it does not understand (an option
that was defined in some future extension to this protocol) or that
is not listed as a valid option for the different message types
above, then the Critical bit in the option determines the outcome.
o If C=0, then the option is silently ignored, and the rest of the
message is processed.
o If C=1, then the host SHOULD send back a Shim6 Error message with
Error Code=1, with the Pointer field referencing the first octet
in the Option Type field. When C=1, the rest of the message MUST
NOT be processed.
Nordmark & Bagnulo Standards Track [Page 43]
RFC 5533 Shim6 Protocol June 2009
5.15.1. Responder Validator Option Format
The responder can choose exactly what input is used to compute the
validator and what one-way function (such as MD5 or SHA1) it uses, as
long as the responder can check that the validator it receives back
in the I2 or I2bis message is indeed one that:
1) computed,
2) computed for the particular context, and
3) isn't a replayed I2/I2bis message.
Some suggestions on how to generate the validators are captured in
Sections 7.10.1 and 7.17.1.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = 1 |0| Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ Validator ~
~ +-+-+-+-+-+-+-+-+
~ | Padding |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Fields:
Validator: Variable length content whose interpretation is local
to the responder.
Padding: Padding, 0-7 bytes, added if needed. See
Section 5.15.
5.15.2. Locator List Option Format
The Locator List option is used to carry all the locators of the
sender. Note that the order of the locators is important, since the
Locator Preferences option refers to the locators by using the index
in the list.
Note that we carry all the locators in this option even though some
of them can be created automatically from the CGA Parameter Data
Structure.
Nordmark & Bagnulo Standards Track [Page 44]
RFC 5533 Shim6 Protocol June 2009
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = 2 |0| Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Locator List Generation |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Num Locators | N Octets of Verification Method |
+-+-+-+-+-+-+-+-+ |
~ ~
~ +-+-+-+-+-+-+-+-+
~ | Padding |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ Locators 1 through N ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Fields:
Locator List Generation:
32-bit unsigned integer. Indicates a generation
number that is increased by one for each new locator
list. This is used to ensure that the index in the
Locator Preferences refers to the right version of the
locator list.
Num Locators: 8-bit unsigned integer. The number of locators that
are included in the option. We call this number "N"
below.
Verification Method:
N octets. The ith octet specifies the verification
method for the ith locator.
Padding: Padding, 0-7 bytes, added if needed so that the
Locators start on a multiple-of-8-octet boundary.
Note that for this option, there is never a need to
pad at the end since the Locators are a multiple-of-8-
octets in length. This internal padding is included
in the Length field.
Locators: N 128-bit locators.
The defined verification methods are:
Nordmark & Bagnulo Standards Track [Page 45]
RFC 5533 Shim6 Protocol June 2009
+---------+----------------------------------+
| Value | Method |
+---------+----------------------------------+
| 0 | Reserved |
| 1 | HBA |
| 2 | CGA |
| 3-200 | Allocated using Standards action |
| 201-254 | Experimental use |
| 255 | Reserved |
+---------+----------------------------------+
Table 4
5.15.3. Locator Preferences Option Format
The Locator Preferences option can have some flags to indicate
whether or not a locator is known to work. In addition, the sender
can include a notion of preferences. It might make sense to define
"preferences" as a combination of priority and weight, the same way
that DNS SRV records have such information. The priority would
provide a way to rank the locators, and, within a given priority, the
weight would provide a way to do some load sharing. See [5] for how
SRV defines the interaction of priority and weight.
The minimum notion of preferences we need is to be able to indicate
that a locator is "dead". We can handle this using a single octet
flag for each locator.
We can extend that by carrying a larger "element" for each locator.
This document presently also defines 2-octet and 3-octet elements,
and we can add more information by having even larger elements if
need be.
The locators are not included in the preference list. Instead, the
first element refers to the locator that was in the first element in
the Locator List option. The generation number carried in this
option and the Locator List option is used to verify that they refer
to the same version of the locator list.
Nordmark & Bagnulo Standards Track [Page 46]
RFC 5533 Shim6 Protocol June 2009
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = 3 |0| Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Locator List Generation |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Element Len | Element[1] | Element[2] | Element[3] |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ ... ~
~ +-+-+-+-+-+-+-+-+
~ | Padding |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Case of Element Len = 1 is depicted.
Fields:
Locator List Generation:
32-bit unsigned integer. Indicates a generation
number for the locator list to which the elements
should apply.
Element Len: 8-bit unsigned integer. The length in octets of each
element. This specification defines the cases when
the length is 1, 2, or 3.
Element[i]: A field with a number of octets defined by the Element
Len field. Provides preferences for the ith locator
in the Locator List option that is in use.
Padding: Padding, 0-7 bytes, added if needed. See
Section 5.15.
When the Element length equals one, then the element consists of only
a one-octet Flags field. The currently defined set of flags are:
BROKEN: 0x01
TRANSIENT: 0x02
The intent of the BROKEN flag is to inform the peer that a given
locator is known to be not working. The intent of TRANSIENT is to
allow the distinction between more stable addresses and less stable
addresses when Shim6 is combined with IP mobility, and when we might
have more stable home locators and less stable care-of-locators.
Nordmark & Bagnulo Standards Track [Page 47]
RFC 5533 Shim6 Protocol June 2009
When the Element length equals two, then the element consists of a
one-octet Flags field followed by a one-octet Priority field. This
Priority field has the same semantics as the Priority field in DNS
SRV records.
When the Element length equals three, then the element consists of a
one-octet Flags field followed by a one-octet Priority field and a
one-octet Weight field. This Weight field has the same semantics as
the Weight field in DNS SRV records.
This document doesn't specify the format when the Element length is
more than three, except that any such formats MUST be defined so that
the first three octets are the same as in the above case, that is, a
one-octet Flags field followed by a one-octet Priority field, and a
one-octet Weight field.
5.15.4. CGA Parameter Data Structure Option Format
This option contains the CGA Parameter Data Structure (PDS). When
HBA is used to verify the locators, the PDS contains the HBA
multiprefix extension in addition to the PDS mandatory fields and
other extensions unrelated to Shim6 that the PDS might have. When
CGA is used to verify the locators, in addition to the PDS option,
the host also needs to include the signature in the form of a CGA
Signature option.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = 4 |0| Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ CGA Parameter Data Structure ~
~ +-+-+-+-+-+-+-+-+
~ | Padding |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Fields:
CGA Parameter Data Structure:
Variable length content. Content defined in [2] and
[3].
Padding: Padding, 0-7 bytes, added if needed. See
Section 5.15.
Nordmark & Bagnulo Standards Track [Page 48]
RFC 5533 Shim6 Protocol June 2009
5.15.5. CGA Signature Option Format
When CGA is used for verification of one or more of the locators in
the Locator List option, then the message in question will need to
contain this option.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = 5 |0| Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ CGA Signature ~
~ +-+-+-+-+-+-+-+-+
~ | Padding |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Fields:
CGA Signature: A variable-length field containing a PKCS#1 v1.5
signature, constructed by using the sender's private
key over the following sequence of octets:
1. The 128-bit CGA Message Type tag [CGA] value for
Shim6: 0x4A 30 5662 4858 574B 3655 416F 506A 6D48.
(The tag value has been generated randomly by the
editor of this specification.).
2. The Locator List Generation number of the
correspondent Locator List option.
3. The subset of locators included in the
correspondent Locator List option whose
verification method is set to CGA. The locators
MUST be included in the order in which they are
listed in the Locator List Option.
Padding: Padding, 0-7 bytes, added if needed. See
Section 5.15.
5.15.6. ULID Pair Option Format
I1, I2, and I2bis messages MUST contain the ULID pair; normally, this
is in the IPv6 Source and Destination fields. In case the ULID for
the context differs from the address pair included in the Source and
Destination Address fields of the IPv6 packet used to carry the I1/
I2/I2bis message, the ULID Pair option MUST be included in the I1/I2/
I2bis message.
Nordmark & Bagnulo Standards Track [Page 49]
RFC 5533 Shim6 Protocol June 2009
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = 6 |0| Length = 36 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved2 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ Sender ULID +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ Receiver ULID +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Fields:
Reserved2: 32-bit field. Reserved for future use. Zero on
transmit. MUST be ignored on receipt. (Needed to
make the ULIDs start on a multiple-of-8-octet
boundary.)
Sender ULID: A 128-bit IPv6 address.
Receiver ULID: A 128-bit IPv6 address.
5.15.7. Forked Instance Identifier Option Format
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = 7 |0| Length = 4 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Forked Instance Identifier |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Fields:
Forked Instance Identifier:
32-bit field containing the identifier of the
particular forked instance.
5.15.8. Keepalive Timeout Option Format
This option is defined in [4].
Nordmark & Bagnulo Standards Track [Page 50]
RFC 5533 Shim6 Protocol June 2009
6. Conceptual Model of a Host
This section describes a conceptual model of one possible data
structure organization that hosts will maintain for the purposes of
Shim6. The described organization is provided to facilitate the
explanation of how the Shim6 protocol should behave. This document
does not mandate that implementations adhere to this model as long as
their external behavior is consistent with that described in this
document.
6.1. Conceptual Data Structures
The key conceptual data structure for the Shim6 protocol is the ULID-
pair context. This is a data structure that contains the following
information:
o The state of the context. See Section 6.2.
o The peer ULID: ULID(peer).
o The local ULID: ULID(local).
o The Forked Instance Identifier: FII. This is zero for the default
context, i.e., when there is no forking.
o The list of peer locators with their preferences: Ls(peer).
o The generation number for the most recently received, verified
peer locator list.
o For each peer locator, the verification method to use (from the
Locator List option).
o For each peer locator, a flag specifying whether it has been
verified using HBA or CGA, and a bit specifying whether the
locator has been probed to verify that the ULID is present at that
location.
o The current peer locator is the locator used as the destination
address when sending packets: Lp(peer).
o The set of local locators and the preferences: Ls(local).
o The generation number for the most recently sent Locator List
option.
o The current local locator is the locator used as the source
address when sending packets: Lp(local).
Nordmark & Bagnulo Standards Track [Page 51]
RFC 5533 Shim6 Protocol June 2009
o The Context Tag used to transmit control messages and Shim6
Payload Extension headers; this is allocated by the peer:
CT(peer).
o The context to expect in received control messages and Shim6
Payload Extension headers; this is allocated by the local host:
CT(local).
o Timers for retransmission of the messages during context-
establishment and update messages.
o Depending how an implementation determines whether a context is
still in use, there might be a need to track the last time a
packet was sent/received using the context.
o Reachability state for the locator pairs as specified in [4].
o During pair exploration, information about the Probe messages that
have been sent and received as specified in [4].
o During context-establishment phase, the Initiator Nonce, Responder
Nonce, Responder Validator, and timers related to the different
packets sent (I1,I2, R2), as described in Section 7.
6.2. Context STATES
The STATES that are used to describe the Shim6 protocol are as
follows:
Nordmark & Bagnulo Standards Track [Page 52]
RFC 5533 Shim6 Protocol June 2009
+---------------------+---------------------------------------------+
| STATE | Explanation |
+---------------------+---------------------------------------------+
| IDLE | State machine start |
| | |
| I1-SENT | Initiating context-establishment exchange |
| | |
| I2-SENT | Waiting to complete context-establishment |
| | exchange |
| | |
| I2BIS-SENT | Potential context loss detected |
| | |
| ESTABLISHED | SHIM context established |
| | |
| E-FAILED | Context-establishment exchange failed |
| | |
| NO-SUPPORT | ICMP Unrecognized Next Header type |
| | (type 4, code 1) received, indicating |
| | that Shim6 is not supported |
+---------------------+---------------------------------------------+
In addition, in each of the aforementioned STATES, the following
state information is stored:
Nordmark & Bagnulo Standards Track [Page 53]
RFC 5533 Shim6 Protocol June 2009
+---------------------+---------------------------------------------+
| STATE | Information |
+---------------------+---------------------------------------------+
| IDLE | None |
| | |
| I1-SENT | ULID(peer), ULID(local), [FII], CT(local), |
| | INIT Nonce, Lp(local), Lp(peer), Ls(local) |
| | |
| I2-SENT | ULID(peer), ULID(local), [FII], CT(local), |
| | INIT Nonce, RESP Nonce, Lp(local), Lp(peer),|
| | Ls(local), Responder Validator |
| | |
| ESTABLISHED | ULID(peer), ULID(local), [FII], CT(local), |
| | CT(peer), Lp(local), Lp(peer), Ls(local), |
| | Ls(peer), INIT Nonce?(to receive late R2) |
| | |
| I2BIS-SENT | ULID(peer), ULID(local), [FII], CT(local), |
| | CT(peer), Lp(local), Lp(peer), Ls(local), |
| | Ls(peer), CT(R1bis), RESP Nonce, |
| | INIT Nonce, Responder Validator |
| | |
| E-FAILED | ULID(peer), ULID(local) |
| | |
| NO-SUPPORT | ULID(peer), ULID(local) |
+---------------------+---------------------------------------------+
7. Establishing ULID-Pair Contexts
ULID-pair contexts are established using a 4-way exchange, which
allows the responder to avoid creating state on the first packet. As
part of this exchange, each end allocates a Context Tag and shares
this Context Tag and its set of locators with the peer.
In some cases, the 4-way exchange is not necessary -- for instance,
when both ends try to set up the context at the same time, or when
recovering from a context that has been garbage collected or lost at
one of the hosts.
7.1. Uniqueness of Context Tags
As part of establishing a new context, each host has to assign a
unique Context Tag. Since the Shim6 Payload Extension headers are
demultiplexed based solely on the Context Tag value (without using
the locators), the Context Tag MUST be unique for each context.