SHIM6 WG E. Nordmark
Internet-Draft Sun Microsystems
Expires: March 31, 2006 September 27, 2005
Level 3 multihoming shim protocol
draft-ietf-shim6-proto-00.txt
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Copyright Notice
Copyright (C) The Internet Society (2005).
Abstract
The SHIM6 working group is exploring a layer 3 shim approach for
providing locator agility below the transport protocols, so that
multihoming can be provided for IPv6 with failover and load spreading
properties, without assuming that a multihomed site will have a
provider independent IPv6 address which is announced in the global
IPv6 routing table. The hosts in a site which has multiple provider
allocated IPv6 address prefixes, will use the shim6 protocol
specified in this document to setup state with peer hosts, so that
the state can later be used to failover to a different locator pair,
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should the original one stop working.
This document picks a particular approach to such a protocol and
tries to flush out a bunch of details, with the hope that the WG can
better understand the details in this proposal as well as discovering
and understanding alternative designs that might be better. Thus
this proposal is my no means cast in stone as the direction; quite to
the contrary it is a depth first exploration of the design space.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1 Placement of the shim . . . . . . . . . . . . . . . . . . 4
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . 5
2.1 Definitions . . . . . . . . . . . . . . . . . . . . . . . 5
2.2 Notational Conventions . . . . . . . . . . . . . . . . . . 6
3. Assumptions . . . . . . . . . . . . . . . . . . . . . . . . 7
4. Protocol Overview . . . . . . . . . . . . . . . . . . . . . 8
4.1 Context Tags and Use of Flow Label . . . . . . . . . . . . 9
4.2 Protocol type overloading . . . . . . . . . . . . . . . . 10
4.3 Securing shim6 . . . . . . . . . . . . . . . . . . . . . . 11
4.4 Overview of Shim Control Messages . . . . . . . . . . . . 11
5. Message Formats . . . . . . . . . . . . . . . . . . . . . . 14
5.1 Common Shim6 header . . . . . . . . . . . . . . . . . . . 14
5.2 I1 Message Format . . . . . . . . . . . . . . . . . . . . 15
5.3 R1 Message Format . . . . . . . . . . . . . . . . . . . . 16
5.4 I2 Message Format . . . . . . . . . . . . . . . . . . . . 17
5.5 R2 Message Format . . . . . . . . . . . . . . . . . . . . 19
5.6 No Context Error Message Format . . . . . . . . . . . . . 20
5.7 Locator List Update Message Format . . . . . . . . . . . . 21
5.8 Locator List Update Acknowledgement Message Format . . . . 22
5.9 Rehome Request Message Format . . . . . . . . . . . . . . 23
5.10 Rehome Acknowledgement Message Format . . . . . . . . . 23
5.11 Reachability Probe Message Format . . . . . . . . . . . 23
5.12 Reachability Probe Reply Message Format . . . . . . . . 24
5.13 Payload Message Format . . . . . . . . . . . . . . . . . 25
5.14 Keepalive Message Format . . . . . . . . . . . . . . . . 26
5.15 Locator Pair Test Message Format . . . . . . . . . . . . 27
5.16 Locator Pair Test Reply Message Format . . . . . . . . . 28
5.17 Context Locator Pair Explore Message Format . . . . . . 29
6. Option Formats . . . . . . . . . . . . . . . . . . . . . . . 31
6.1 Validator Option Format . . . . . . . . . . . . . . . . . 32
6.2 Locator List Option Format . . . . . . . . . . . . . . . . 32
6.3 Locator Preferences Option Format . . . . . . . . . . . . 33
6.4 CGA Parameter Data Structure Option Format . . . . . . . . 34
6.5 CGA Signature Option Format . . . . . . . . . . . . . . . 35
6.6 ULID Pair Option Format . . . . . . . . . . . . . . . . . 35
6.7 Packet In Error Option Format . . . . . . . . . . . . . . 36
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6.8 Explorer Results Option Format . . . . . . . . . . . . . . 36
7. Conceptual Model of a Host . . . . . . . . . . . . . . . . . 38
7.1 Conceptual Data Structures . . . . . . . . . . . . . . . . 38
8. Establishing Host Pair Contexts . . . . . . . . . . . . . . 40
8.1 Sending I1 messages . . . . . . . . . . . . . . . . . . . 40
8.2 Receiving I1 messages . . . . . . . . . . . . . . . . . . 40
8.3 Receiving R1 messages . . . . . . . . . . . . . . . . . . 40
8.4 Retransmitting I1 messages . . . . . . . . . . . . . . . . 40
8.5 Receiving I2 messages . . . . . . . . . . . . . . . . . . 40
8.6 Retransmitting I2 messages . . . . . . . . . . . . . . . . 40
8.7 Concurrent context establishment . . . . . . . . . . . . . 40
9. No Such Content Errors . . . . . . . . . . . . . . . . . . . 41
10. Handling ICMP Error Messages . . . . . . . . . . . . . . . . 42
11. Taredown of the Host Pair Context . . . . . . . . . . . . . 43
12. Updating the Locator Pairs . . . . . . . . . . . . . . . . . 44
13. Various Probe Mechanisms . . . . . . . . . . . . . . . . . . 45
14. Rehoming to a Different Locator Pair . . . . . . . . . . . . 46
15. Payload Packets before a Switch . . . . . . . . . . . . . . 47
16. Payload Packets after a Switch . . . . . . . . . . . . . . . 48
17. Open Issues . . . . . . . . . . . . . . . . . . . . . . . . 50
18. Design Alternatives . . . . . . . . . . . . . . . . . . . . 52
18.1 State Cleanup . . . . . . . . . . . . . . . . . . . . . 52
18.2 Not Overloading the Flow Label . . . . . . . . . . . . . 52
18.3 Detecting Context Loss . . . . . . . . . . . . . . . . . 53
19. Implications Elsewhere . . . . . . . . . . . . . . . . . . . 55
20. Security Considerations . . . . . . . . . . . . . . . . . . 57
21. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 58
22. References . . . . . . . . . . . . . . . . . . . . . . . . . 59
22.1 Normative References . . . . . . . . . . . . . . . . . . 59
22.2 Informative References . . . . . . . . . . . . . . . . . 59
Author's Address . . . . . . . . . . . . . . . . . . . . . . 60
Intellectual Property and Copyright Statements . . . . . . . 61
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1. Introduction
The SHIM6 working group, and the MULTI6 WG that preceded it, is
exploring a layer 3 shim approach for providing locator agility below
the transport protocols, so that multihoming can be provided for IPv6
with failover and load spreading properties, without assuming that a
multihomed site will have a provider independent IPv6 address which
is announced in the global IPv6 routing table. The hosts in a site
which has multiple provider allocated IPv6 address prefixes, will use
the shim6 protocol specified in this document to setup 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.
This document takes the outlines contained in [16] and [15] and
expands to an actual proposed protocol.
We assume that redirection attacks are prevented using the mechanism
specified in HBA [4].
The WG mailing list is discussing the scheme used for reachability
detection [5]. The schemes that are being discussed are Context
Unreachability Detection (CUD) or Force Bidirectional communication
Detection (FBD). This document doesn't discuss the tradeoffs between
the two, but it does suggest a set of keepalive and probe messages
that are sufficient to handle both. Once the WG has decided which
approach to take, we can remove the unneeded messages.
There is a related but slightly separate issue of how the hosts can
find which of the locator pairs is working after a failure. This is
discussed in [6]. We don't yet know how these details will be done,
but this draft specifies a separate "Explore" message as a
placeholder for such a protocol mechanism.
1.1 Placement of the shim
TBD: Copy material from [16].
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2. Terminology
This document uses the terms MUST, SHOULD, RECOMMENDED, MAY, SHOULD
NOT and MUST NOT defined in RFC 2119 [7]. The terms defined in RFC
2460 [1] are also used.
2.1 Definitions
This document introduces the following terms (taken from [16]):
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 IPX, AppleTalk, or IP
itself.
interface A node's attachment to a link.
address An IP layer name that contains both 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 a 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 (stack
name in [18]). The transport endpoint name is a
function of the transport protocol and would
typically include the IP identifier plus a port
number.
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 locator which 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
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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 this
fields carry "addresses". If identifiers and
locators are separated these fields will contain
locators for packets on the wire.
FQDN Fully Qualified Domain Name
Host-pair context The state that the multihoming shim maintains for
a particular peer. The peer is identified by one
or more ULIDs.
2.2 Notational Conventions
A, B, and C are hosts. X is a potentially malicious host.
FQDN(A) is the domain name for A.
Ls(A) is the locator set for A, which consists of the locators L1(A),
L2(A), ... Ln(A).
ULID(A) is an upper-layer ID for A. In this proposal, ULID(A) is
always one member of A's locator set.
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 7 for a
description of the conceptual data structures.
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3. Assumptions
The general approach of a level3 shim as well as this specific
proposal makes the following assumptions:
o When there is ingress filtering in the ISPs, that the use of all
{source, destination} locator pairs will cause the packets to exit
using different ISPs so that all exit ISPs can be tried. Since
there might be only one destination locator, when the peer
supports shim6 but is not multihomed, this implies that the
selection of the exit ISP should be related to the source address
in the packets.
o Even without ingress filtering, there is the assumption that if
the host tries all {source, destination} locator pairs, that it
has done a good enough job of trying to find a working path to the
peer. Since we want the protocol to provide benefits even if the
peer has a single locator, this seems to imply that the choice of
source locator needs to somehow affect the exit path from the
site.
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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 B using some upper-
layer protocol. This results in the ULP on A sending packets to
B. We call this the initial contact. Assuming the IP addresses
selected by Default Address Selection [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 might make
sense to make this communication robust against locator failures.
For instance, this heuristic might be that more than 50 packets
have been sent or received. This makes the shim initiate the
4-way context establishment exchange.
As a result of this exchange, both A and B will know a list of
locators for each other.
o Communication continues without any change for the ULP packets.
In addition, there might be some messages exchanged between the
shim sub-layers for (un)reachability detection.
o At some point in time something fails. Depending on the approach
to reachability detection, there might be some advise 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 explore the different alternate locator pairs until a working
pair is found, and rehome to using that pair.
o Once a working alternative locator pair has been found, the shim
will rewrite the packets on transmit, and tag the packets with a
shim context tag, and send them on the wire. The receiver will
use the <Source Locator, Destination Locator, 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.
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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 an error
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 complete state loss (crash and
reboot) of a peer.
The data packets in shim6 are 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" so that the receiver
can always determine the context to which they belong. For commonly
used IP protocols this is done by using a different value in the Flow
Label field, that is, there is no additional header added to the
packets. But for other IP protocol types there is an extra 8 byte
header inserted, which carries the next header value.
4.1 Context Tags and Use of Flow Label
A context between to 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, the
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 MUST uniquely identify one
context.
In addition, the non-shim6 messages, which we call payload packets,
will not contain the ULIDs after a failure. This introduces the
requirement that the <peer locator, local locator, local context tag>
MUST uniquely identify the context. Since the peer's set of locators
might be dynamic the simplest form of unique allocation of the local
context tag is to pick a number that is unique on the host. Hosts
which serve multiple ULIDs using disjoint sets of locators can
maintain the context tag allocation per such disjoint set.
As an optimization, to ensure that payload packets, even after the
locators have been switched from being the original ones, do not
require an extra shim extension header, the proposal uses the Flow
Label field in the IPv6 header to carry the context tag for common
upper layer protocols such as TCP and UDP. This works as follows:
o The allocation and usage of the flow label during the initial
communication is unchanged. Thus the TCP, UDP, etc packets are
sent with a flow label which is allocated according to [11].
o When the context is created, each endpoint picks an unused context
tag based on the constraints above, which is also not used as a
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flow label for the set of locators.
o The context tag is used by the shim to refer to the shim state in
the control messages (such as probes, and locator updates.
o The payload traffic (TCP, UDP, etc.) continue to flow unchanged.
o Should there be a need to switch to a different locator pair, then
the TCP, UDP, etc packets will be sent using an alternate locator
pair, and with a flow label that is the same as the (20 bit)
context tag.
The mechanism for detecting a loss of context state at the peer that
is currently proposed in this document 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). The next
section specifies how this can be done.
Note that there is no need for a single context to have more than one
context tag; whether the locator pair is <A1, B2>, <A1, B3> or <A2,
B3> the same context tag is used in the flow label field. Only the
payload packets using the original <A1, B1> locator pair use the flow
label (which is different than the context tag).
Whether we overload the flow label field to carry the context tag or
not, any protocol (such as RSVP or NSIS) which signals information
about flows from the host stack to devices in the path, need to be
made aware of the locator agility introduced by a layer 3 shim, so
that the signaling can be performed for the locator pairs that are
currently being used.
4.2 Protocol type overloading
The mechanism for detecting a loss of context state at the peer that
is currently proposed in this document 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). There is an
alternative to detection of lost state outlined in Section 18.
The idea is to steal a partial bit from the protocol type fields that
are used in the Next Header values, so that the common upper layer
protocols can be identified.
For example:
o TCP has protocol 6; TCP using alternate locators has protocol P.
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o UDP has protocol 17; TCP using alternate locators has protocol
P+1.
o ICMPv6 has protocol 58; ICMPv6 using alternate locators has
protocol P+2.
o SCTP has protocol 132; SCTP using alternate locators has protocol
P+3.
o DCCP has protocol ??; DCCP using alternate locators has protocol
P+4.
o ESP has protocol 50; ESP using alternate locators has protocol
P+5.
o AH has protocol 51; AH using alternate locators has protocol P+6.
o FRAG has protocol 44; FRAG using alternate locators has protocol
P+7.
Thus with 7 or so additional protocol field values we can do a
reasonable job of overloading the flow label field and get detection
of lost context state.
4.3 Securing shim6
The mechanisms are secured using a combination of techniques:
o The HBA technique [4] for validating the locators to prevent an
attacker from redirecting the packet stream to somewhere else.
o Requiring a Reachability Probe+Reply before a new locator is used
as the destination, in order to prevent 3rd party flooding
attacks.
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 of 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.
4.4 Overview of Shim Control Messages
The shim context establishment is accomplished using four messages;
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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 [17].]
There is a No Contex error message defined, when a control or payload
packet arrives and there is no matching context state at the
receiver. When such a message is received, it will result in the
destruction of the shim context and a re-establishment.
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 is a Locator List Update message and
acknowledgement.
Even though 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. Currently this mechanism has a
separate message pair (Rehome Request and acknowledgement), but
perhaps this can be encoded using the Locator List Update message
pair with a preference option and no change to the list of locators.
At least two approaches (CUD and FBD) have been discussed for the
shim (un)reachability detection [5]. This document attempt to define
messages for both cases; once the WG has picked an approach we can
delete any unneeded messages.
The CUD approach uses a probe message and acknowledgement, which can
be suppressed e.g. using positive advise from the ULP. This message
pair also seems needed to verify that the host is indeed present at a
new locator before the data stream is redirected to that locator, in
order to prevent 3rd party DoS attacks.
The FBD approach uses a keepalive message, which is sent when a host
has received packets from the peer, but the ULP has not given the
host an opportunity to send any payload packet to the peer.
The above probe and keepalive messages assume we have an established
host-pair context. However, communication might fail during the
initial context (that is, when the application or transport protocol
is trying to setup some communication). If we want the shim to be
able to optimize discovering a working locator pair in that case, we
need a mechanism to test the reachability of locators independent of
some context. We define a locator pair test message and
acknowledgement for this purpose, even though it isn't yet clear
whether we need such a thing.
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Finally, when the context is established and there is a failure there
needs to be a way to explore the set of locator pairs to efficiently
find a working pair. We define an explore message as a placeholder
for some mechanism in this space [6].
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5. Message Formats
The shim6 messages are all carried using a new IP protocol number TBD
[to be assigned by IANA]. The shim6 messages have a common header,
defined below, with some fixed fields, followed by type specific
fields.
5.1 Common Shim6 header
The common part of the header has a next header and header extension
length field which is consistent with the other IPv6 extension
headers, even if the next header value is only used for data payload
which is carried with a shim6 header on the front.
The shim6 headers must be a multiple of 8 octets, hence the minimum
size is 8 octets.
The common 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 | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | |
+-+-+-+-+-+-+-+-+ |
| Type specific format |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Fields:
Next Header: 8-bit selector. Normally set to NO_NXT_HDR (59).
Indicates the next header value for the shim6 payload
messages.
Hdr Ext Len: 8-bit unsigned integer. Length of the shim6 header in
8-octet units, not including the first 8 octets.
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
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.
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Type: 8-bit unsigned integer. Identifies the actual message
from the table below.
+------------+-----------------------------------------------------+
| Type Value | Message |
+------------+-----------------------------------------------------+
| 1 | I1 (first establishment message from the initiator) |
| | |
| 2 | R1 (first establishment message from the responder) |
| | |
| 3 | I2 (2nd establishment message from the initiator) |
| | |
| 4 | R2 (2nd establishment message from the responder) |
| | |
| 5 | No Context Error |
| | |
| 6 | Locator List Update |
| | |
| 7 | Locator List Update Acknowledgement |
| | |
| 8 | Rehome Request |
| | |
| 9 | Rehome Acknowledgement |
| | |
| 10 | Reachability Probe |
| | |
| 11 | Reachability Probe Reply |
| | |
| 12 | Payload |
| | |
| 13 | Keepalive |
| | |
| 14 | Locator Pair Test |
| | |
| 15 | Locator Pair Test Reply |
| | |
| 16 | Context Locator pair explore |
+------------+-----------------------------------------------------+
Table 1
5.2 I1 Message Format
The I1 message is the first message in the context establishment
exchange.
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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 | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 1 | Res | Initiator Context Tag |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Initiator Nonce |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ Options +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Fields:
Next Header: NO_NXT_HDR (59).
Type: 1
Res: 4-bit field. Reserved for future use. Zero on
transmit. MUST be ignored on receipt.
Initiator Context Tag: 20-bit field. The Context Tag 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 allowed in the message:
ULID pair: TBD Do we need to carry the ULIDs, or assume they are
the same as the address fields in the IPv6 header?
Depends on how we handle failures during initial
contact.
5.3 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.
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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 | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 2 | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Initiator Nonce |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Responder Nonce |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ Options +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Fields:
Next Header: NO_NXT_HDR (59).
Type: 2
Reserved: 24-bit field. Reserved for future use. Zero on
transmit. MUST be ignored on receipt.
Initiator Nonce: 32-bit unsigned integer. Copied from the I1
message.
Responder Nonce: 32-bit unsigned integer. A number picked by the
initiator which the initiator will return in the I2
message.
The following options are allowed in the message:
Responder Validator: Variable length mandatory option. Typically 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 a R1
message, and that the parameters in the I2 message are
the same as those in the I1 message.
5.4 I2 Message Format
The I2 message is the third message in the context establishment
exchange. The initiator sends this in response to a R1 message,
after checking the Initiator Nonce, etc.
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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 | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 3 | Res | Initiator Context Tag |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Initiator Nonce |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Responder Nonce |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ Options +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Fields:
Next Header: NO_NXT_HDR (59).
Type: 3
Res: 4-bit field. Reserved for future use. Zero on
transmit. MUST be ignored on receipt.
Initiator Context Tag: 20-bit field. The Context Tag 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.
The following options are allowed in the message:
Responder Validator: Variable length mandatory option. Copied from
the Validator in the R1 message.
ULID pair: TBD Do we need to carry the ULIDs, or assume they are
the same as the address fields in the IPv6 header?
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
validating the locator list MUST also be included.
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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 the some of the locators in the list use
CGA (and not HBA) for validation.
5.5 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.
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 | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 4 | Res | Responder Context Tag |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Initiator Nonce |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ Options +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Fields:
Next Header: NO_NXT_HDR (59).
Type: 4
Res: 4-bit field. Reserved for future use. Zero on
transmit. MUST be ignored on receipt.
Responder Context Tag: 20-bit field. The Context Tag the responder
has allocated for the context
Initiator Nonce: 32-bit unsigned integer. Copied from the I2
message.
The following options are allowed in the message:
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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
validating 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 the some of the locators in the list use
CGA (and not HBA) for validation.
5.6 No Context Error Message Format
Should a host receive a packet (payload packet or shim6 control
message such a a locator update) and the host does not have any
context state for the locators (in the IPv6 source and destination
fields) and the context tag, then it will generate a No Context
Error. The error includes the packet that was received, subject to
the packet not exceeding 1280 octets.
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 | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 5 | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ Options +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Fields:
Next Header: NO_NXT_HDR (59).
Type: 5
Reserved: 24-bit field. Reserved for future use. Zero on
transmit. MUST be ignored on receipt.
The following options are allowed in the message:
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Packet in Error: Variable length mandatory option containing the IPv6
packet that was in error, starting with the IPv6
header, and normally containing the full packet. If
the resulting No Context Error message would exceed
1280 octets, the Packet In Error option will not
include the full packet in error in order to limit the
error to 1280 octets.
5.7 Locator List Update Message Format
The Locator List Update (LLU) Message contains a complete replacement
of the senders locator list, and the options 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.
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 | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 6 | Res | Initiator Context Tag |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Request Nonce |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ Options +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Fields:
Next Header: NO_NXT_HDR (59).
Type: 6
Res: 4-bit field. Reserved for future use. Zero on
transmit. MUST be ignored on receipt.
Initiator Context Tag: 20-bit field. The Context Tag the initiator
has allocated for the context.
Request Nonce: 32-bit unsigned integer. A random number picked by
the initiator which the responder will return in the
acknowledgement message.
The following options are allowed in the message:
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Locator List: The list of the senders (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: Included so the receiver can verify the
locator list.
CGA Signature: Included when the some of the locators in the list use
CGA (and not HBA) for validation.
5.8 Locator List Update Acknowledgement Message Format
This message is sent in response to a LLU 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 | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 7 | Res | Responder Context Tag |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Request Nonce |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ Options +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Fields:
Next Header: NO_NXT_HDR (59).
Type: 7
Res: 4-bit field. Reserved for future use. Zero on
transmit. MUST be ignored on receipt.
Initiator Context Tag: 20-bit field. The Context Tag the responder
has allocated for the context.
Request Nonce: 32-bit unsigned integer. Copied from the LLU message.
The following options are allowed in the message:
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TBD any options?:
5.9 Rehome Request Message Format
TBD Is there any use to have a separate Rehome pair of messages? The
sender can indicates its new knowledge of one of its locators (such
as it no longer working) using the LLU message. Would it be useful
to be able to specify just failure or preference changes without
listing the actual locators? This would require that the locator
list is ordered so that a Rehome Request can refer to the locators by
some short index.
Perhaps this functionality can be accomplished by sending a Locator
Update message and only including new Locator Preferences, without
including any Locator List option? If so, we don't need a separate
message.
5.10 Rehome Acknowledgement Message Format
TBD: See above.
5.11 Reachability Probe Message Format
The Reachability Probe message is used to prevent 3rd party DoS
attacks, and can also be used to verify whether a context is
reachable at a given locator should that be needed for the general
reachability detection mechanism (e.g., if we pick the CUD mechanism
where one end sends probes and expects a reply).
Before a host uses a locator for the peer that is different than the
ULID, it needs to verify that the peer is indeed present at that
locator by sending a Context Verify and receiving an acknowledgement.
This message includes the ULID pair as well as the context tag, so
that the peer can indeed verify that it has that ULID and that the
context tag is correct.
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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 | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 10 | Res | Receiver Context Tag |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Request Nonce |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ Options +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Fields:
Next Header: NO_NXT_HDR (59).
Type: 10
Res: 4-bit field. Reserved for future use. Zero on
transmit. MUST be ignored on receipt.
Receiver Context Tag: 20-bit field. The Context Tag the peer has
allocated for the context.
Request Nonce: 32-bit unsigned integer. A random number picked by
the initiator which the responder will return in the
acknowledgement message.
The following options are allowed in the message:
ULID pair: The ULID pair that is being probed.
5.12 Reachability Probe Reply Message Format
This is sent in response to a Reachability Probe message. Although,
if the receiver of the RT does not have a matching context it will
send a No Context Error message.
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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 | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 11 | Res | Receiver Context Tag |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Request Nonce |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ Options +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Fields:
Next Header: NO_NXT_HDR (59).
Type: 11
Res: 4-bit field. Reserved for future use. Zero on
transmit. MUST be ignored on receipt.
Receiver Context Tag: 20-bit field. The Context Tag the peer has
allocated for the context.
Request Nonce: 32-bit unsigned integer. Copied from the request
message.
The following options are allowed in the message:
ULID pair: The ULID pair that is being probed. Copied from the
Probe message.
5.13 Payload Message Format
The payload message is used for payload which do not have a
designated "foo-inside-shim6" protocol type, as specified in
Section 4.2.
Since the shim is placed between the IP endpoint sub-layer and the IP
routing sub-layer in the host, 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, 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
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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 effects the location of the shim6 header.
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 | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 12 | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Fields:
Next Header: The payload which follows this header.
Hdr Ext Len: 0 (since the header is 8 octets).
Checksum: The checksum of the 8 octets.
Type: 12
Reserved: Reserved for future use. Zero on transmit. MUST be
ignored on receipt.
5.14 Keepalive Message Format
The keepalive message would be used if we decide to do the Force
Bidirectional communication as a way to get verification that the
locator pair continues to work. If we are not going to do FBD we
probably will not need this 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 | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 13 | Res | Receiver Context Tag |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ Options +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Fields:
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Next Header: NO_NXT_HDR (59).
Type: 13
Res: 4-bit field. Reserved for future use. Zero on
transmit. MUST be ignored on receipt.
Receiver Context Tag: 20-bit field. The Context Tag the peer has
allocated for the context.
The following options are allowed in the message:
TBD any options?:
5.15 Locator Pair Test Message Format
The above Reachability Probe message probes a context. This message
just probes a locator. If we are going to handle failure during
initial contact using the shim, then the shim needs to be able to
find out what locators are working (and that they correspond to a
desirable ULID) without assuming there is a context setup, and
without knowing the actual ULID. The latter is needed so that we can
handle the case when the AAAA RRset contains any combination of
multiple hosts and multiple IP addresses for a given host. Having
the responder send back the ULID that corresponds to a particular
locator allows the initiator to take the AAAA RRset and determine
which IPv6 addresses therein are for different hosts.
Once we understand how the shim will be involved in locator failures
during initial contact, then we can determine whether we need this
mechanism, and whether it can be overloaded on the Probe Message
(e.g., by making the Receiver Context tag optional in the
Reachability Probe message).
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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 | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 14 | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Request Nonce |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ Target ULID +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ Options +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Fields:
Next Header: NO_NXT_HDR (59).
Type: 14
Reserved: 24-bit field. Reserved for future use. Zero on
transmit. MUST be ignored on receipt.
Request Nonce: 32-bit unsigned integer. A random number picked by
the sender which the target will return in the reply
message.
Target ULID: 128-bit IPv6 address.
The following options are allowed in the message:
TBD any options?:
5.16 Locator Pair Test Reply Message Format
If a host receives a Locator Pair Test message, and the Target ULID
is one of its IP addresses, then it will send this reply.
TBD: If ULID doesn't match, does it just ignore the test message? Or
send some error?
TBD: Should the responder instead return its ULID, so that it is
easier for the sender to determine which of the IPv6 addresses from
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the DNS correspond to different hosts vs. different locators for the
same host?
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 | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 15 | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Request Nonce |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ Target ULID +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ Options +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Fields:
Next Header: NO_NXT_HDR (59).
Type: 15
Reserved: 24-bit field. Reserved for future use. Zero on
transmit. MUST be ignored on receipt.
Request Nonce: 32-bit unsigned integer. Copied from the test
message.
Target ULID: 128-bit IPv6 address. Copied from the test message.
TBD: Or should the host be able to fill this in to
make it easier for the peer to determine which
locators refer to the same host?
The following options are allowed in the message:
TBD any options?:
5.17 Context Locator Pair Explore Message Format
This is a placeholder for the protocol mechanism outlined in [6].
The idea behind that mechanism is to be able to handle the case when
one locator pair works in from A to B, and another locator pair works
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from B to A, but there is no locator pair which works in both
directions. The protocol mechanism is that as A is sending explore
packets to B, B will observe which locator pairs it has received from
and report that back in explore packets it is sending to A.
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 | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 16 | Res | Receiver Context Tag |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sequence Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ Options +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Fields:
Next Header: NO_NXT_HDR (59).
Type: 16
Res: 4-bit field. Reserved for future use. Zero on
transmit. MUST be ignored on receipt.
Receiver Context Tag: 20-bit field. The Context Tag the peer has
allocated for the context.
Sequence Number: 32-bit unsigned integer. Used to determine which
packets have been received by the peer.
The following options are allowed in the message:
Explorer Results: Indication of what Explorer messages the sender has
recently received from the peer.
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6. Option Formats
The options follow the same layout as in RFC 2461 [2]. Thus they all
are a multiple of 8 octets.
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 | Length | ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ ... ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Fields:
Type: 8-bit identifier of the type of option. The options
defined in this document are below.
Length: 8-bit unsigned integer. The length of the option
(including the type and length fields) in units of 8
octets. The value 0 is invalid. Nodes MUST silently
discard an ND packet that contains an option with
length zero.
+------------------------------+------+
| Option Name | Type |
+------------------------------+------+
| Validator | 1 |
| | |
| Locator List | 2 |
| | |
| Locator Preferences | 3 |
| | |
| CGA Parameter Data Structure | 4 |
| | |
| CGA Signature | 5 |
| | |
| ULID Pair | 6 |
| | |
| Packet In Error | 7 |
| | |
| Explorer Results | 8 |
+------------------------------+------+
Table 2
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6.1 Validator Option Format
The responder can choose exactly what input uses to compute the
validator, and what one-way function (MD5, SHA1) it uses, as long as
the reponder can verify that the validator it receives back in the I2
packet is indeed one that 1) it computed, 2) it computed for the
particular context, and 3) that it isn't a replayed I2 message.
One way for the responder to do this is to maintain a single secret
(S) and a running counter for the Responder Nonce. For each I1
message, the responder can then increase the counter, use the counter
value as the responder nonce, and use the following information as
input to the one-way function:
o The the secret S
o That Responder Nonce
o The Initiator Context Tag from the I1 message
o The ULIDs from the I1 message
o The locators from the I1 message (strictly only needed if they are
different from the ULIDs)
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 | Length | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
~ Validator ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Fields:
Validator: Variable length content whose interpretation is local
to the responder.
6.2 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 and the Explorer packet refers to the locators by
using the index in the list.
TBD: Do we need this when all the locators are contained in the PDS?
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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 | Length | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| Reserveds |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ Locators ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Fields:
Reserved: 48-bit field. Reserved for future use. Zero on
transmit. MUST be ignored on receipt.
Locators: A variable number of 128-bit locators. The number of
locators present can be determined by the option
length field.
6.3 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 has 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 [8] for how
SRV defines the interaction of priority and weight.
As of this draft we define the preferences to include three 8-bit
fields: a priority, a weight, and 8-bits of flags. The intent is
that the TBD flags can carry information such as "this locator is
not working", and "this locator is temporary". The latter allows
making the distinction between more stable addresses and less stable
addresses when shim6 is combined with IP mobility, when we might have
more stable home locators, and less stable care-of-locators.
The locators are not included in the preference list. Instead, the
first element refers to locator that was in the first element in the
Locator List option. This assumes that the Locator List option is
stable. See Section 17.
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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 | Length | Pri[1] | Weight[1] |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Flags[1] | Pri[2] | Weight[2] | Flags[2] |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ ... ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Fields:
Pri[i]: 8-bit unsigned integer. The Priority associated with
the i'th locator in the Locator List option that is in
use.
Weight[i]: 8-bit unsigned integer. The Weight associated with
the i'th locator in the Locator List option that is in
use.
Flags[i]: 8-bit unsigned integer. The flags associated with the
i'th locator in the Locator List option that is in
use.
The set of flags is TBD: Assume there will be two initially: BROKEN
and TEMPORARY.
6.4 CGA Parameter Data Structure Option Format
This option contains the CGA parameter data structure (hereafter
called the PDS). When HBA is used to validate the locators, the PDS
contains the HBA multiprefix extension. When CGA is used to validate
the locators, in addition to the CGA PDS, the signature will need to
be included as 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 | Length | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
~ CGA Parameter Data Structure ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Fields:
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CGA Parameter Data Structure: Variable length content. Content
defined in [4].
6.5 CGA Signature Option Format
When CGA is used for validation of one or more of the locators in the
PDS, 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 | Length | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
~ CGA Signature ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Fields:
CGA Signature: Variable length content. Content defined in [4].
6.6 ULID Pair Option Format
It isn't clear whether we need this option. It depends whether we
want to be able to setup a context for a ULID pair when that ULID
pair can't be used to communicate. Thus the IPv6 addresses in the
context establishment would not be the ULIDs.
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 | Length | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| Reserveds |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ Sender ULID +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ Receiver ULID +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Fields:
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Reserved: 48-bit field. Reserved for future use. Zero on
transmit. MUST be ignored on receipt.
Sender ULID: A 128-bit IPv6 address.
Receiver ULID: A 128-bit IPv6 address.
6.7 Packet In Error 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 | Length | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| Reserveds |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ IPv6 header, shim6/TCP/UDP header, etc ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Fields:
Reserved: 48-bit field. Reserved for future use. Zero on
transmit. MUST be ignored on receipt.
Packet: A variable length field which contains the packet in
error starting with the IPv6 header.
6.8 Explorer Results Option Format
TBD: This needs to indicate which explorer packets (sequence numbers,
source and destination locators) that have been recently received, in
order to detect which locator pairs work when there is no locator
pair which works in both directions. When indicating locators it
makes sense to use the offset in the Locator List (that was carries
in the LLU option), since this takes less space than including the
locators themselves.
p
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 = 8 | Length | TBD |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ ... ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Fields:
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TBD:
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7. 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.
7.1 Conceptual Data Structures
The key conceptual data structure for the shim6 protocol is the host
pair context. This is a data structures which contains the following
information:
o The peer ULID; ULID(peer)
o The local ULID; ULID(local)
o The list of peer locators, with their preferences; Ls(peer)
o For each peer locator, a bit whether it has been validated using
HBA, and a bit whether the locator has been probed to verify that
the ULID is present at that location.
o The preferred peer locator - used as destination; Lp(peer)
o The set of local locators and the preferences; Ls(local)
o The preferred local locator - used as source; Lp(local)
o The context tag used to transmit packets - allocated by the peer;
CT(peer)
o The context to expect in received packets - allocated by the local
host; CT(local)
o Reachability state for the locator pairs.
o During pair exploration, information about the explore packets
that have been sent and received.
The receiver finds the context by looking it up using <Source
Locator, Destination Locator, CT(local)>, where the context tag is in
the Flow Label field for ULP payload packets, and in the shim headers
for control messages. The sender needs to be able to find the
context state when a packet is passed down from the ULP. In that
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case the lookup key is the pair of ULIDs.
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8. Establishing Host Pair Contexts
TBD
8.1 Sending I1 messages
8.2 Receiving I1 messages
8.3 Receiving R1 messages
8.4 Retransmitting I1 messages
8.5 Receiving I2 messages
8.6 Retransmitting I2 messages
8.7 Concurrent context establishment
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9. No Such Content Errors
TBD
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10. Handling ICMP Error Messages
The routers in the path as well as the destination might generate
various ICMP error messages, such as host unreachable, packet too
big, and payload type unknown. It is critical that these packets
make it back up to the ULPs so that they can take appropriate action.
When the ULP packets are sent unmodified, that is, while the initial
locators=ULIDs are working, this introduces no new concerns; an
implementation's existing mechanism for delivering these errors to
the ULP will work. But when the shim on the transmitting side
replaces the ULIDs in the IP address fields with some other locators,
then an ICMP error coming back will have a "packet in error" which is
not a packet that the ULP sent. Thus the implementation will have to
apply the reverse mapping to the "packet in error" before passing the
ICMP error up to the ULP.
This mapping is different than when receiving ULP packets from the
peer, because in that case the packets contain CT(local). But the
ICMP errors have a "packet in error" with CT(peer) since they were
intended to be received by the peer. In any case, since the <Source
Locator, Destination Locator, CT(peer)> has to be unique when
received by the peer, the local host should also only be able to find
one context that matches this tuple.
Should the ULP packet have been conveyed using the protocol type
encoding (Section 4.2), then that encoding must be undone for the
packet in error before it is delivered to the ULP. If the ULP packet
had been encapsulated in a shim6 payload message, then this extension
header must be removed. The result needs to be that the ULP receives
an ICMP error where the contained "packet in error" looks as if the
shim did not exist.
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11. Taredown of the Host Pair Context
Each host can unilaterally decide when to tare down a host-pair
context. It is RECOMMENDED that hosts not tare down the context when
they know that there is some upper layer protocol that might use the
context. For example, an implementation might know this is there is
an open socket which is connected to the ULID(peer). However, there
might be cases when the knowledge is not readily available to the
shim layer, for instance for UDP applications which not not connect
their sockets, or any application which retains some higher level
state across (TCP) connections and UDP packets.
Thus it is RECOMMENDED that implementations minimize premature
taredown by observing the amount of traffic that is sent and received
using the context, and only after it appears quiescent, tare down the
state.
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12. Updating the Locator Pairs
TBD
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13. Various Probe Mechanisms
TBD
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14. Rehoming to a Different Locator Pair
TBD
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15. Payload Packets before a Switch
When there is no context state for the ULID pair on the sender, there
is no effect on how ULP packets are sent. If the host is using some
heuristic for determining when to perform a deferred context
establishment, then the host might need to do some accounting (count
the number of packets sent and received) even before there is a host-
pair context. This need to count packets might also appear on the
receive side, depending on what heuristics the implementation has
chosen.
If there is a host-pair context for the ULID pair, then the sender
needs to verify whether context uses the ULIDs as locators, that is,
whether Lp(peer) == ULID(peer) and Lp(local) == ULID(local).
If this is the case, then packets will be sent unmodified by the
shim. If it is not the case, then the logic in Section 16 will need
to be used.
There will also be some maintenance activity relating to
(un)reachability detection, whether packets are sent with the
original locators or not. The details of this is out of scope for
this document and will be covered is follow-ons to [5].
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16. Payload Packets after a Switch
When sending packets, if there is a host-pair context for the ULID
pair, and the ULID pair is no longer used as the locator pair, then
the sender needs to transfer the packet. The transformation depends
on the payload type, since some protocol values can be carried
without adding a shim6 extension header, and others need an 8-octet
header.
Before the payload dependent transformation, the IP address fields
are replaced. The IPv6 source address field is set to Lp(local) and
the destination address field is set to Lp(peer). NOTE that this
MUST NOT cause any recalculation of the ULP checksums, since the ULP
checksums are carried end-to-end and the ULP pseudo-header contains
the ULIDs which are preserved end-to-end.
The sender skips any "routing sub-layer extension headers", thus it
skips any hop-by-hop extension header, any routing header, and any
destination options header that is followed by a routing header. The
(extension) header that follows after that is viewed as the ULP
header.
If the ULP header is of a type listed in Section 4.2, then it is
replaced by the "foo-in-shim6 value for that protocol type. And in
this case, the context tag CT(peer) is placed in the flow label field
in the IPv6 header. Then the packet can be passed to the IP routing
sub-layer.
If the ULP header type is not listed in that section, then a shim6
Payload extension header is inserted in the packet before the ULP
header. In this case the context tag CT(peer) is also placed in the
flow label field, and the packet is passed down to the routing sub-
layer. TBD: We could use the Reserved field in the payload message
instead of using flow label in this case.
The receiver parses the (extension) headers in order. Should it find
a shim6 extension header it will look at the type field in that
header. If the type is Payload message, then the packet must be
passed to the shim6 payload handling for rewriting. If the receiver
finds one of the eight additional payload type (for "foo-inside-
shim6"), then it treats this analogous to the case of a shim6 payload
extension header.
In both cases the receiver extracts the context tag from the IPv6
flow label field, and uses this together with the IPv6 source and
destination address fields to find a host-pair context. If no
context is found, the receiver SHOULD generate a No Such Context
error message (see Section 9).
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With the context in hand, the receiver can now replace the IP address
fields with the ULIDs kept in the context. Finally, the traces of
the shim are removed from the packet; any payload extension header is
removed, and the next header value in the preceding header is set to
be the actual protocol number for the payload. Then the packet can
be passed to the ULP.
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17. Open Issues
The following open issues are known:
o Is there need for keeping the list of locators private between the
two communicating endpoints? We can potentially accomplish that
when using CGA but not with HBA, but it comes at the cost of doing
some public key encryption and decryption operations as part of
the context establishment.
o Forking the context state. On the mailing list we've discussed
the need to fork the context state, so that different ULP streams
can be sent using different locator pairs. No protocol extensions
are needed if any forking is done independently by each endpoint.
But if we want A to be able to tell B that certain traffic (a
5-tuple?) should be forked, then we need a way to convey this in
the shim6 protocol. The hard part would be defining what
selectors can be specified for the filter which determines which
traffic uses which of the forks. So the question is whether we
really need signaling for forking, or whether it is sufficient to
allow each endpoint to do its own selection of which locator pair
it is using for which traffic.
o If we allow forking, it seems like the mechanism for reachability
detection, whether it is CUD or FBD, must be applied separately
for each locator pair that is in use. Without forking a single
locator pair will be in use for each host-pair context, hence
things would be simpler.
o Having the Locator List option contain all the prefixes implies
extra bytes when the locators are also in the CGA Parameter Data
Structure option. To optimize this will still need to provide an
ordered list, so that the Locator Preferences can refer to the
locators by "index". (The Explore Results option might need to
refer to them by index as well.)
o The index to a locator might get out of synch between the two ends
if messages with a new Locator List option is lost. It might make
sense to include a "generation" or "locator list version" number
in the Locator List option so that the Locator Preference (and
Explorer Result) options can refer to a particular version of the
list.
o The specified mechanism (of relying on No Such Context errors)
doesn't always detect the loss of the context on the peer when the
original ULID=locators are used. See Section 18 for other
options.
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o Which messages need sequence numbers to prevent parts of the
protocol to operate on stale information should the shim6
information get out of date? Just the Locator List Update
message?
o The CGA PDS might not need to be included in every LLU message.
If it is associated with the ULID, it is sufficient to exchange it
once. Then a HBA-protected LLU would not need anything (it can
just change the preferences for the locators in any case), and a
CGA-protected LLU would just need the signature option.
o In the LLU do we need to indicate which locators need to be
validated using HBA vs. CGA? Or it could tell which locators are
in the HBA extension in the PDS, and assume any others need CGA
validation.
o What happens when a host runs out of 20 bit context tags? When is
it safe for a host to reuse a context tag? With the unilateral
taredown one end might discard the context state long before the
other end.
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18. Design Alternatives
This document has picked a certain set of design choices in order to
try to work out a bunch of the details, and stimulate discussion.
But as has been discussed on the mailing list, there are other
choices that make sense. This section tries to enumerate some
alternatives.
18.1 State Cleanup
This document uses a timer based cleanup mechanism, as specified in
Section 11.
An alternative would be to use an explicit CLOSE mechanism, akin to
the one specified in HIP [17]. If an explicit CLOSE handshake and
associated timer is used, then there would no longer be a need for
the No Context Error message due to a peer having garbage collected
its end of the context. However, there is still potentially a need
to have a No Context Error message in the case of a complete state
loss of the peer (also known as a crash followed by a reboot). Only
if we assume that the reboot takes at least the CLOSE timer, or that
it is ok to not provide complete service until CLOSE timer minutes
after the crash, can we completely do away with the No Context Error
message.
If there is no need for the No Context Error message, this also means
that it might be possible to remove the need to explicitly
identifying the shim6 payload packets after a locator switch, neither
using the foo-inside-shim6 protocol number nor using the shim6
Payload message. In essence, the receiver could identify the context
based on the locator pair and the Flow Label in the received packets.
There might be some debugging and operational issues with removing
the explicit identification of the shim6 packets after a locator
switch. Should the receiver have lost the context state, then there
will be no indication that something is going wrong. The shim on the
receiver would happily pass up the packets unmodified to the ULP, and
the ULP would most likely see a checksum error. The checksum error
is caused by the ULP packet having different IP addresses than the
packet that the sending ULP passed down to its shim.
18.2 Not Overloading the Flow Label
This document overloads the Flow Label field as a context tag for
packets that are sent after the locators have been switched, that is,
the packets where the sending shim has replaced the ULIDs with some
other locator pair.
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An alternative would be to not do this, and instead always use the
shim6 Payload message to encapsulate the payloads when the locators
are different than the ULIDs. While this doesn't remove the need to
have any QoS signaling protocol be aware of the shim6 architectural
implications Section 19, it does offer some other simplifications to
the protocol, namely that there would no longer be a need to use
designated protocol number values for the "foo-inside-shim6"; the
cases when those protocol numbers are used would instead use the
Payload message. The downside of always using the Payload message
after a failure is that the path MTU usable by the ULP would be 8
octets less.
18.3 Detecting Context Loss
This document specifies that context loss is detected by receiving a
No Such Context error message from the peer. Such messages are
generated in response to a shim6 message that contain a the peer's
context tag, including the shim6 Payload messages, when the receiver
doesn't have matching context. They are also generated in response
to data packets after a locator switch (because such payload packets
are identified as such using the overloaded protocol field specified
in Section 4.2).
This approach has the disadvantage of the overloaded protocol type,
and it also doesn't detect the loss of context state when the
original ULIDs are used as locators, because there might be no shim6
messages exchanged if the reachability detection manages to suppress
any extra packets.
Discussion: it isn't clear we could remove the protocol type
overloading with this approach, because without protocol type
overloading it is undefined in what order the receiver would do
things. Normally the receiver follows the next header chain and
processes things in order. This also works with an overloaded
protocol type; that next header value is basically indicating that
there is a zero length shim payload header. Thus the sender can
control whether this happens before the processing of some other
extension header or after. Without any such indication in the
packet, the receiver would find a shim context based on the <Source
Locator, Destination Locator, Flow Label>. But would it process this
before or after some other extension header, such as a MIPv6 Home
Address Option, or IP-in-IP encapsulation header?
An alternative would be to remove the protocol field overloading and
mandate that there be some low-frequency periodic Reachability Probe/
Reply messages, even when there is bidirectional communication and
the ULPs report that they are doing fine. Such an approach would be
able to detect state loss even before there is a locator switch.
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Presumably such probes can be suppressed when there are no ULP
packets being sent to the peer.
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19. Implications Elsewhere
The general shim6 approach, as well as the specifics of this proposed
solution, has implications elsewhere. The key implications are:
o Applications that perform referals, or callbacks using IP
addresses as the 'identifiers' can still function in limited ways,
as described in [12]. But in order for such applications to be
able to take advantage of the multiple locators for redundancy,
the applications need to be modified to either use fully qualified
domain names as the 'identifiers', or they need to pass all the
locators as the 'identifiers' i.e., the 'identifier' from the
applications perspective becomes a set of IP addresses instead of
a single IP address.
o Firewalls that today pass limited traffic, e.g., outbound TCP
connections, would presumably block the shim6 protocol. This
means that even when shim6 capable hosts are communicating, the I1
packets would be dropped, hence the hosts would not discover that
their peer is shim6 capable. This is in fact a feature, since if
the hosts managed to establish a host-pair context, then the
firewall would probably drop the "different" packets that are sent
after a failure (either using a "TCP-inside-shim6" protocol
number, or using the shim6 payload packet with a TCP packet inside
it). Thus stateful firewalls that are modified to allow shim6
packets through should also be modified to allow the payload
packets through after a failure. This presumably implies that the
firewall needs to track the set of locators in use by looking at
the shim6 exchanges.
o Signaling protocols for QoS or other things that involve having
devices in the network path look at IP addresses and port numbers,
or IP addresses and Flow Labels, need to be invoked on the hosts
when the locator pair changes due to a failure. At that point in
time those protocols need to inform the devices that a new pair of
IP addresses will be used for the flow, as well as a new Flow
Label being used.
o MTU implications. The path MTU mechanisms we use are robust
against different packets taking different paths through the
Internet, by computing a minimum over the recently observed path
MTUs. When shim6 fails over from using one locator pair to
another pair, this means that packets might travel over a
different path through the Internt, hence the path MTU might be
quite different. Perhaps such a path change would be a good hint
to the path MTU mechanism to try a larger MTU?
The fact that the shim, at least for uncommon payload types, will
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add an 8 octet extension header (the payload message) after a
locator switch, can also affect the usable path MTU for the ULPs.
In this case the MTU change is local to the sending host, thus
conveying the change to the ULPs is an implementation matter.
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20. Security Considerations
Some of the residual threats in this proposal are:
o An attacker which arrives late on the path (after the context has
been established) can use the No Such Context error to cause one
peer to recreate the context, and at that point in time the
attacker can observe all of the exchange. But this doesn't seem
to open any new doors for the attacker since such an attacker can
observe the Context tags that are being used, and once known it
can use those to send bogus messages.
o An attacker which is present on the path so that it can find out
the context tags, can generate a No Such Context error after it
has moved off the path. For this packet to be effective it needs
to have a source locator which belongs to the context, thus there
can not be "too much" ingress filtering between the attackers new
location and the communicating peers. But this doesn't seem to be
that severe, because once the error causes the context to be torn
down and re-established, a new pair of context tags will be used,
which will not be known to the attacker. If this is still a
concern, we could require a 2-way handshake "did you really loose
the state?" in response to the error message.
o It might be possible for an attacker to try random 24-bit context
tags and see if they can cause disruption for communication
between two hosts. We can make this harder by using a larger
context tag (64-bits?) in the shim6 control messages, and use the
low-order 24 bits as the flow label.
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21. Acknowledgements
Over the years many people active in the multi6 and shim6 WGs have
contributed ideas a suggestions that are reflected in this draft.
Thanks to Marcelo Bagnulo for providing comments on earlier versions
of this draft.
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22. References
22.1 Normative References
[1] Deering, S. and R. Hinden, "Internet Protocol, Version 6 (IPv6)
Specification", RFC 2460, December 1998.
[2] Narten, T., Nordmark, E., and W. Simpson, "Neighbor Discovery
for IP Version 6 (IPv6)", RFC 2461, December 1998.
[3] Thomson, S. and T. Narten, "IPv6 Stateless Address
Autoconfiguration", RFC 2462, December 1998.
[4] Bagnulo, M., "Hash Based Addresses (HBA)",
draft-ietf-shim6-hba-00 (work in progress), July 2005.
[5] Beijnum, I., "Shim6 Reachability Detection",
draft-ietf-shim6-reach-detect-00 (work in progress), July 2005.
[6] Arkko, J., "Failure Detection and Locator Selection Design
Considerations", draft-ietf-shim6-failure-detection-00 (work in
progress), July 2005.
22.2 Informative References
[7] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[8] Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
specifying the location of services (DNS SRV)", RFC 2782,
February 2000.
[9] Draves, R., "Default Address Selection for Internet Protocol
version 6 (IPv6)", RFC 3484, February 2003.
[10] Abley, J., Black, B., and V. Gill, "Goals for IPv6 Site-
Multihoming Architectures", RFC 3582, August 2003.
[11] Rajahalme, J., Conta, A., Carpenter, B., and S. Deering, "IPv6
Flow Label Specification", RFC 3697, March 2004.
[12] Nordmark, E., "Shim6 Application Referral Issues",
draft-ietf-shim6-app-refer-00 (work in progress), July 2005.
[13] Abley, J., "Shim6 Applicability Statement",
draft-ietf-shim6-applicability-00 (work in progress),
July 2005.
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[14] Huston, G., "Architectural Commentary on Site Multi-homing
using a Level 3 Shim", draft-ietf-shim6-arch-00 (work in
progress), July 2005.
[15] Bagnulo, M. and J. Arkko, "Functional decomposition of the
multihoming protocol", draft-ietf-shim6-functional-dec-00 (work
in progress), July 2005.
[16] Nordmark, E. and M. Bagnulo, "Multihoming L3 Shim Approach",
draft-ietf-shim6-l3shim-00 (work in progress), July 2005.
[17] Moskowitz, R., "Host Identity Protocol", draft-ietf-hip-base-03
(work in progress), June 2005.
[18] Lear, E. and R. Droms, "What's In A Name:Thoughts from the
NSRG", draft-irtf-nsrg-report-10 (work in progress),
September 2003.
Author's Address
Erik Nordmark
Sun Microsystems
17 Network Circle
Menlo Park, CA 94043
USA
Phone: +1 650 786 2921
Email: erik.nordmark@sun.com
Nordmark Expires March 31, 2006 [Page 60]
Internet-Draft Shim6 Protocol September 2005
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