INTERNET-DRAFT Erik Nordmark
Oct 27, 2003 Sun Microsystems
Multihoming without IP Identifiers
<draft-nordmark-multi6-noid-01.txt>
Status of this Memo
This document is an Internet-Draft and is subject to all provisions
of Section 10 of RFC2026.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet-
Drafts.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
This Internet Draft expires April 27, 2004.
Abstract
This document outlines a potential solution to IPv6 multihoming in
order to stimulate discussion.
This proposed solution relies on verification using the existing DNS
to prevent redirection attacks, while allowing locator rewriting by
(border) routers, with no per-packet overhead. The solution does not
introduce a "stack name" type of identifier, instead it ensures that
all upper layer protocols can operate unmodified in a multihomed
setting while still seeing a stable IPv6 address.
DISCLAIMER: This work has been discussed extensively in a design
team. The design team is still exploring multiple approaches and
draft-nordmark-multi6-noid-01.txt [Page 1]
INTERNET-DRAFT Multihoming without IDs Oct 27, 2003
this is an attempt to capture one such approach on paper. Because of
this and due to lack of time to review the document one can not say
that this is a product of the DT; errors and confusions should be
attributed to the scribe and not to the DT.
draft-nordmark-multi6-noid-01.txt [Page 2]
INTERNET-DRAFT Multihoming without IDs Oct 27, 2003
Contents
1. INTRODUCTION............................................. 4
1.1. Non-Goals........................................... 4
1.2. Assumptions......................................... 5
2. TERMINOLOGY.............................................. 5
2.1. Notational Conventions.............................. 6
3. PROTOCOL OVERVIEW........................................ 6
3.1. Host-Pair Context................................... 10
3.2. Message Formats..................................... 11
4. PROTOCOL WALKTHROUGH..................................... 13
4.1. Initial Context Establishment....................... 13
4.2. Locator Change...................................... 15
4.3. Handling Locator Failures........................... 16
4.4. Locator Set Changes................................. 17
4.5. Preventing Premeditated Redirection Attacks......... 17
5. HANDLING STATE LOSS...................................... 18
6. ENCODING BITS IN THE IPv6 HEADER?........................ 19
7. COMPATIBILITY WITH STANDARD IPv6......................... 21
8. APPLICATION USAGE OF IDENTIFIERS......................... 21
9. CHECKSUM ISSUES.......................................... 22
10. IMPLICATIONS FOR PACKET FILTERING....................... 23
11. IPSEC INTERACTIONS...................................... 23
12. SECURITY CONSIDERATIONS................................. 24
13. DESIGN ALTERNATIVES..................................... 24
14. OPEN ISSUES............................................. 24
14.1. Handling Hosts without a FQDN...................... 25
14.2. Locator Set Inconsistencies........................ 25
14.3. Renumbering Considerations......................... 26
14.4. Initiator Confusion vs. "Virtual Hosting".......... 26
15. ACKNOWLEDGEMENTS........................................ 27
16. REFERENCES.............................................. 27
16.1. Normative References............................... 27
draft-nordmark-multi6-noid-01.txt [Page 3]
INTERNET-DRAFT Multihoming without IDs Oct 27, 2003
16.2. Informative References............................. 28
1. INTRODUCTION
The goal of the IPv6 multihoming work is to allow a site to take
advantage of multiple attachments to the global Internet without
having a specific entry for the site visible in the global routing
table. Specifically, a solution should allow users to use multiple
attachments in parallel, or to switch between these attachment points
dynamically in the case of failures, without an impact on the upper
layer protocols.
This proposed solution uses existing DNS mechanisms to perform enough
validation to prevent redirection attacks.
The goals for this proposed solution is to:
o Have no impact on upper layer protocols in general and on
transport protocols in particular.
o Address the security threats in [M6SEC].
o Allow routers rewriting the (source) locators as a means of
quickly detecting which locator is likely to work for return
traffic.
o No per-packet overhead.
o No extra roundtrip for setup.
o Take advantage of multiple locators/addresses for load spreading.
1.1. Non-Goals
The assumption is that the problem we are trying to solve is site
multihoming, with the ability to have the set of site locator
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. This proposal does not attempt to solve, perhaps
related, problems such as host multihoming or host mobility.
draft-nordmark-multi6-noid-01.txt [Page 4]
INTERNET-DRAFT Multihoming without IDs Oct 27, 2003
This proposal also does not try to provide an IP identifier. Even
though such a concept would be useful to ULPs and applications,
especially if the management burden for such a name space was zero
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) when using the DNS to verify the
locator relationships.
1.2. Assumptions
The main technical assumptions this proposal makes is that the DNS
infrastructure can be used for verification of the relationship
between locators on both the initiator of communication and the
responding peer. In particular, it assumes that getting DNS reverse
maps (ip6.arpa) populated for the hosts that wish to take advantage
of multihoming will not be a significant problem.
2. TERMINOLOGY
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.
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 identifier for an IP layer endpoint
(stack name in [NSRG]). The transport endpoint is a
function of the transport protocol and would
typically include the IP identifier plus a port
number. NOTE: This proposal does not contain any IP
layer identifiers.
draft-nordmark-multi6-noid-01.txt [Page 5]
INTERNET-DRAFT Multihoming without IDs Oct 27, 2003
Application identifier (AID)
- 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
communication to a host (e.g., different
connections) might use different AIDs in order to
enable load spreading.
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.
FQDN - Fully Qualified Domain Name
2.1. 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 L1(A), L2(A), ...
Ln(A).
AID(A) is an application ID for A. In this proposal, AID(A) is
always one member of Ls(A).
3. PROTOCOL OVERVIEW
In order to prevent redirection attacks this protocol relies on the
DNS (for the hosts which support this protocol) being maintained with
consistent forward and reverse maps. This allows any host, given one
locator, to determine the corresponding FQDN and the set of locators
for the host. Once those lookups have been performed, and the
original locator is indeed part of the set, the host can happily
allow any of those locators without being subject to redirection
draft-nordmark-multi6-noid-01.txt [Page 6]
INTERNET-DRAFT Multihoming without IDs Oct 27, 2003
attacks. Keeping the FQDN around allows the solution to handle
graceful renumbering by being able to redo the DNS lookups (e.g.,
based on the TTL on the resource records).
DNS is also used to provide an indication of multihoming capability
of a host. The details of this is TBD but a simple example would be
to introduce a new M6 RR type in the DNS which has no RDATA; thus the
mere existence of such a record at a FQDN would imply that the host
supports the M6 protocol.
-----------------------
| Transport Protocols |
-----------------------
------ ------- -------------- -------------
| AH | | ESP | | Frag/reass | | Dest opts |
------ ------- -------------- -------------
-----------------
| M6 shim layer |
-----------------
------
| IP |
------
Figure 1: Protocol stack
The proposal uses an M6 shim layer between IP and the ULPs as shown
in figure 1, in order to provide ULP independence. Conceptually the
M6 shim layer behaves as if it is an extension header, which would be
ordered immediately after any hop-by-hop options in the packet.
However, the amount of data that needs to be carried in an actual M6
extension header is close to zero. By using some encoding of the
nexthdr value it is possible to carry the common protocols/extension
headers without making the packets larger. The nexthdr encodings are
discussed later in this document. We refer to packets that use this
encoding to indicate to the receiver that M6 processing should be
applied as "M6 packets" (analogous to "ESP packets" or "TCP
packets").
Layering AH and ESP above the M6 shim means that IPsec can be made to
be unaware of locator changes the same way that transport protocols
can be unaware. Thus the IPsec security associations remain stable
even though the locators are changing. Layering the fragmentation
header above the M6 shim makes reassembly robust in the case that
there is broken multi-path routing which results in using different
paths, hence potentially different source locators, for different
draft-nordmark-multi6-noid-01.txt [Page 7]
INTERNET-DRAFT Multihoming without IDs Oct 27, 2003
fragments.
The proposal uses router rewriting of (source) locators as one way to
determine which is the preferred (or only working) locator to use for
return traffic. But not all packets can have their locators
rewritten. In addition to existing IPv6 packets, the packets
exchanged before M6 host-pair context state is established at the
receiver can not have their locators rewritten. Thus a simple
mechanism is needed to indicate to the routers on the path whether or
not it is ok to rewrite the locators in the packet. Conceptually
this is a single bit in the IPv6 header (we call it the "rewrite ok"
bit) but there is no spare bit available. Later in the document we
show how we solve this by allocating a range of next header values to
denote this semantic bit.
Applications and upper layer protocols use AIDs which the M6 layer
will map to/from different locators. The M6 layer maintains state,
called host-pair context, in order to perform this mapping. The
mapping is performed consistently at the sender and the receiver,
thus from the perspective of the upper layer protocols packets appear
to be sent using AIDs from end to end, even though the packets travel
through the network containing locators in the IP address fields, and
even though those locators might be rewritten in flight.
---------------------- ----------------------
| Sender A | | Receiver B |
| | | |
| ULP | | ULP |
| | src AID(A) | | ^ |
| | dst AID(B) | | | src AID(A) |
| v | | | dst AID(B) |
| M6 | | M6 |
| | src L1(A) | | ^ |
| | dst L1(B) | | | src L2(A) |
| v | | | dst L1(B) |
| IP | | IP |
---------------------- ----------------------
| ^
-- cloud with some routers -------
src L2(A) [Rewritten]
dst L1(B)
Figure 2: Mapping with router rewriting of 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 AIDs and
draft-nordmark-multi6-noid-01.txt [Page 8]
INTERNET-DRAFT Multihoming without IDs Oct 27, 2003
locators being present in every packet, but with a header compression
mechanism applied that removes the need for the AIDs once the state
has been established. As we will see below the flowid will be used
akin to a "compression tag" i.e., to indicate the correct context to
use for decompression.
The need for some "compression tag" is because the desire to allow
load spreading and handle site renumbering. Without those desires it
could have been possible to e.g. designate one fixed locator as the
AID for a host and storing that in the DNS. But instead different
connections between two hosts are allowed to use different AIDs and
on reception of a M6 packet the correct AIDs must be inserted into
the IP address fields before passing the packet to the ULP. The
flowid serves as a convenient "compression tag" without increasing
the packet size, and this usage doesn't conflict with other flowid
usage.
In addition to the zero overhead data messages, there are four
different M6 message types introduced (which could be defined as new
ICMPv6 messages). Three types are used to perform a 3-way handshake
to create state at both endpoints without creating state on the first
received packet (which would introduce a memory consumption DoS
attack), and finally a single message type to signal that state has
been lost. The four message types are called:
o Context request message; first message of the 3-way context
establishment. Sent by the responder when a data packet arrives
with no context state. An ULP packet can be piggybacked on this
message.
o Context response message; second message of the 3-way context
establishment. Sent in response to a context request. An ULP
packet can be piggybacked on this message.
o Context confirm message; third message of the 3-way context
establishment. Sent in response to a context response. An ULP
packet can be piggybacked on this message.
o Unknown context message; error which is sent when no state is
found.
Similar to MAST [MAST] the above exchange can be performed
asynchronously with data packets flowing between the two hosts; until
context state has been established at both ends the packets would
flow without allowing router rewriting of locators and without the
ability for the hosts to switch locators.
Once the 3-way state creation exchange has completed there is host-
draft-nordmark-multi6-noid-01.txt [Page 9]
INTERNET-DRAFT Multihoming without IDs Oct 27, 2003
pair context state at both hosts. At that point in time the
responder (which didn't use DNS before the setup) can asynchronously
start retrieving and verifying additional locators using the DNS.
Once a peer locator has been verified it will be a candidate
destination locator including the ability to dynamically switch to
using the last received source locator (that is already verified) as
the destination locator for return traffic.
3.1. Host-Pair Context
The host-pair context is established on the initiator of
communication based on information learned from the DNS (either by
starting with a FQDN or with an IP address -> FQDN lookup). The
responder will establish some initial state using the context
creation 3-way handshake and later discover and verify the peer's
locators using the DNS.
The context state contains the following information:
- the peer locator which the ULP uses as ID; AID(peer)
- the local locator which the ULP uses as ID; AID(local)
- the set of peer locators; Ls(peer)
- for each peer locator, a bit whether it has been verified with the
DNS (by doing reverse + forward lookup)
- the preferred peer locator - used as destination; Lp(peer)
- the set of local locators; Ls(local)
- the preferred local locator - used as source; Lp(local)
- the flowid used to transmit packets; F(local)
- the flowid to expect in receive packets; F(peer)
- the fully qualified domain name for the peer; FQDN(peer)
- State about peer locators that are in the process of being
verified in the DNS
This state is accessed differently in the transmit and receive paths.
In the transmit path when the ULP passes down a packet the key to the
context state is the tuple <AID(local), AID(peer)>; this key must
draft-nordmark-multi6-noid-01.txt [Page 10]
INTERNET-DRAFT Multihoming without IDs Oct 27, 2003
identify at most one state record. In the receive path it is the
F(peer) plus one of the locators in each of Ls(local) and Ls(peer)
that are used to identify at most one state record. Thus the sender
allocated flowid is part of the key for looking up the context state
at the receiver.
These uniqueness requirements imposed by those lookup keys uniquely
identifying one state record means that one can not create multiple
records (e.g. with different FQDN or locator sets) that have the same
AID pair, and the peer must pick a flowid so that host-pair contexts
which have at least one common members in Ls(local) and in the
Ls(peer) sets, but with different AID pair, gets a different
F(local). The context state at both ends must be consistent for this
to be completely robust. One way of ensuring this is to have each
host perform a periodic DNS lookup of its own FQDN in order to have a
current Ls(local) that is the same as the Ls(peer) that the peer
would find in the DNS.
Note that the flowids could be selected to be finer grain than above;
for instance having a different flowid for each connection. Doing so
requires some efficient data structure organization at the receiver
to map multiple F(peer) to the same context.
3.2. Message Formats
These message formats are largely the same as in [CB128] but the
context request, response, and confirm are sent in the opposite
direction.
The base M6 header is an ICMPv6 header 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Code | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| <code specific fields> |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
ICMPv6 Fields:
Type
TBD [IANA]
Code
8-bit field. The type of M6 message. The M6
draft-nordmark-multi6-noid-01.txt [Page 11]
INTERNET-DRAFT Multihoming without IDs Oct 27, 2003
header carries 4 different types of messages:
o Context request message; first message of the
3-way context establishment. An ULP packet can
be piggybacked on this message.
o Context response message; second message of the
3-way context establishment. An ULP packet can
be piggybacked on this message.
o Context confirm message; third message of the
3-way context establishment. An ULP packet can
be piggybacked on this message.
o Unknown context message; error which is sent
when no context state found.
Checksum The ICMPv6 checksum.
Future versions of this protocol may define message codes.
Receivers MUST silently ignore? Reject? [TBD] any message code
they do not recognize.
This drafts doesn't contain actual message layout for code
specific part. However, the content of these messages is
specified below.
The Context request message contains:
- Sender Nonce
- Sender AID
- Receiver AID
- Sender flowid (20 bits)
The Context response message contains:
- Receiver Nonce (copied from Sender Nonce in request)
- Context state consisting of: the two AIDs, the two flowids, and
the initial locators
- A timestamp or nonce (for sender's benefit)
- A hash over the context state and timestamp (to prevent
modification)
draft-nordmark-multi6-noid-01.txt [Page 12]
INTERNET-DRAFT Multihoming without IDs Oct 27, 2003
The Context confirm message contains:
- The context state, timestamp/nonce, and hash copied from the
context response.
The Unknown context message contains:
- The 20-bit flowid from the triggering packet.
4. PROTOCOL WALKTHROUGH
4.1. Initial Context Establishment
Here is the sequence of events when A starts talking to B:
1. A looks up FQDN(B) in the DNS which returns Ls(B) plus "B is
M6 capable". One locator is selected to be returned to the
application: AID(B) = L1(B). The others are installed in the
M6 layer on the host with AID(B) being the key to find that
state.
To make sure that the lookup from AID(B) returns a single
state record it appears that one needs to do a reverse lookup
AID(B)->FQDN and check that the result is FQDN(B). Whether
this check can be deferred until two entities try to use the
same AID(B) for a different Ls is for further study. Always
doing the reverse lookup would be more predictable in any
case. See section 14.4 for some more discussion.
2. The ULP creates "connection" state between AID(A)=L1(A) and
AID(B) and sends the first packet. L1(A) was picked using
regular source address selection mechanisms.
3. The M6 layer matches on AID(B) and finds the proto-context
state (setup in step #1) with Ls(B). The existence of that
state will make the M6 layer send a M6 packet. The M6 layer
selects a flowid F(local) consistent with the uniqueness
requirements in section 3.1 (which ensure that the receiver
will map to the correct AID pair).
draft-nordmark-multi6-noid-01.txt [Page 13]
INTERNET-DRAFT Multihoming without IDs Oct 27, 2003
4. The packet (TCP SYN or whatever) is sent to peer with
locators L1(A) to L1(B) i.e., the same as the AIDs. Since B
doesn't have any context state yet, A will not set the
"rewrite ok" bit in the header.
5. Host B receives packet and sees it is a "M6 packet". Passes
the packet to the M6 shim layer. The M6 layer can't create
state on the first packet, but since the rewrite bit is not
set in the packet it can pass the packet unmodified to the
ULP. The ULP sees a packet identified by AID(A), AID(B).
The M6 layer initiates a state creation 3-way exchange by
forming a context request message. The same technique as in
[MIPv6] can be used to securely do this exchange without any
local state; use a local key which is never shared with
anybody and pass the context state, a timestamp, and the
keyed hash of the state+timestamp in the context request
packet. When the state, timestamp, and keyed hash value is
returned in the context response message, the hash is used to
verify that the state hasn't been modified.
The 3-way exchange is done asynchronously with ULP packets,
but it is possible (assuming the MTU allows) to piggyback ULP
packets on this exchange.
Should ULP packets be passed down to the M6 layer on B before
the context response message has been received there will be
no context state and no state installed as a result of a DNS
lookup (unlike on A). This will indicate that the ULP
message should be passed as-is (not as an M6 message) to the
peer. Thus during the 3-way exchange packets can flow in
both directions using the original locators=AIDs. (However,
this has some interactions with the suggestions in section
5.)
6. Host A receives the context request message. It verifies
that the message is related to something it sent by looking
at the locators (should match the AIDs) and the flowid it
sent (which is in the state in the context request message).
If a ULP packet was piggybacked A will pass that to the ULP.
Then A sends a content response which has the same
information as the context request plus a nonce/timestamp
that A selected.
7. Host B receives the context response message. It verifies
that the hash of the state is correct using its per-host key
draft-nordmark-multi6-noid-01.txt [Page 14]
INTERNET-DRAFT Multihoming without IDs Oct 27, 2003
and verifies that the timestamp is recent. At this point in
time it knows that A is at least not just blasting out
packets as a DoS - A is also responding to context request
messages. Thus B goes ahead and allocates state at this
point in time using the state that is in the context response
message.
The M6 layer selects a flowid F(B) consistent with the
uniqueness requirements in section 3.1 (which ensure that the
receiver will map to the correct AID pair). At this point in
time B has enough information to handle M6 packets from A,
even though it hasn't yet determined and verified any
additional peer locators from the DNS. It has also the state
(F(B) mainly) necessary send data packets to A with "rewrite
ok" set. Thus B sends a context confirm message to A which
contains A's nonce/timestamp from the context response and
F(B).
If a ULP packet was piggybacked on the context response B
will pass that to the ULP.
At this point in time B can start asynchronously and
incrementally extracting and verifying Ls(A) from the DNS.
The first lookup consists of finding L1(A)=AID(A) in ip6.arpa
to get the FQDN and record it, and lookup the AAAA RR set for
that FQDN to get Ls(A). Then verify (also incrementally)
that each member of Ls(A) is indeed assigned to A by doing a
reverse lookup of each one (except L1(A) which was already
looked up). Only when the reverse lookup of a given peer
locator has completed is that locator marked as verified.
This reverse lookup of each locator prevents 3rd party DoS
attacks as described in [M6SEC].
8. Host A receives the context confirm message, verifies the
nonce/timestamp, and records F(peer) from the packet.
If a ULP packet was piggybacked on the context confirm A will
pass that to the ULP.
At this point in time A knows that B has context state, thus
it can start sending packets with "rewrite ok" set.
4.2. Locator Change
This is the sequence of events when B receives a packet with a
previously unused source locator for A, for instance L2(A).
draft-nordmark-multi6-noid-01.txt [Page 15]
INTERNET-DRAFT Multihoming without IDs Oct 27, 2003
Host B receives M6 packet with source L2(A) and destination L1(B).
Looks up context state using the flowid and the locators. If this
lookup succeeds then the locator is acceptable for incoming
packets (even though it might not have been verified for use as
return traffic) and the packet is rewritten to contain the AIDs
from the context state and passed to the ULP.
If L2(A) has not been verified then it would make sense for B to
put that first in the list of asynchronous DNS verifications that
are needed. If/once L2(A) has been verified B can make it the
preferred peer locator for use when sending packets to AID(A).
The verification needs to complete before using the locator as a
destination in order to prevent 3rd party DoS attacks [M6SEC].
If a host receives a packet with a known flowid but where the
locators (source and destination) are not part of the locator sets
it drops the packet and sends an Unknown context error as
specified in section 5.
4.3. Handling Locator Failures
Should not all locators be working when the communication is
initiated some extra complexity arises, because the ULP has
already been told which AIDs to use. If the locators that where
selected to be AIDs are not working it isn't possible to send a
zero-overhead initial packet from A to B. Instead both the AIDs
and the working locators need to be conveyed. This could be done
by either reusing IP-in-IP encapsulating or defining another M6
message type which carries both. Details TBD.
After context setup the sender can use retransmit hints from the
ULP to get the M6 layer to try a different verified locator.
If one outbound path from the site fails and the border routers
rewrite source locators then the peer in another site will see
packets with the working source locators. Once that locator has
been verified, the return path will switch to use the working
locator. As long as both ends are transmitting packets this will
relatively quickly switch to working locators except when both
hosts experience a failing locator at the same time.
Without locator rewriting one would need to add some notification
e.g., by defining a new bit in the router advertisement prefixes
(IMHO this is semantically different than the preferred vs.
deprecated stuff), but we also need some mechanism to carry this
draft-nordmark-multi6-noid-01.txt [Page 16]
INTERNET-DRAFT Multihoming without IDs Oct 27, 2003
info from the border routers to the routers on each subnet.
4.4. Locator Set Changes
Due to events like site renumbering the set of locators assigned
to a host might change at a slow rate. Since this proposal uses
the locators in the DNS as the credible source for which locators
are assigned there is some coordination necessary to ensure that
before a host, or the border routers for a site doing rewriting,
start using a new source locator, that locator has propagated
through the DNS so that the peer could have discovered it.
Due to concerns about having packets with unknown, hence
potentially bogus, source locators triggering DNS lookups this
proposal instead uses the DNS TTL as an indication that the set of
locators need to be refreshed. One could also envision a
combination of receiving a packet *and* the DNS TTL having expired
as the trigger to redo the DNS lookups.
When DNS TTL expires on either host it performs a new FQDN->Ls
lookup to get the new set of locators. (Presumably failures to
redo the lookup shouldn't have a negative effect.)
When a host sees (based on router advertisements [DISCOVERY]) that
one of its locators has become deprecated and it has additional
locators that are still preferred, it is recommended that the host
start using the preferred locator(s) with the contexts that have
already been established. This ensures that should the deprecated
locator become invalid the peers have already verified other
locator(s) for the host.
4.5. Preventing Premeditated Redirection Attacks
The threats document [M6SEC] talks of premeditated redirection
attacks that is where an attacker claims to be a host before the
real host appears. The absence of an actual IP layer identifier
in this proposal makes that a non-issue; the attacker could only
claim to be host A if the attacker is reachable at one of A's
locators. Thus by definition the attacker would have to be on the
path between the communicating peers and such attackers can
perform redirection attacks in today's Internet.
draft-nordmark-multi6-noid-01.txt [Page 17]
INTERNET-DRAFT Multihoming without IDs Oct 27, 2003
5. HANDLING STATE LOSS
The protocol needs to handle two forms of state loss:
- a peer loosing all state,
- the M6 layer garbage collecting state too early due to not
being aware of what all ULPs do.
The first case is the already existing case of a host crashing and
"rebooting" and as a result loosing transport and application
state. In this case there are some added complications from the
M6 layer since a peer will continue to send packets assuming the
context still exists and due to the loss of state on the receiver
it isn't even able to pass the correct packet up to the ULP (e.g.,
to be able to get TCP to generate a reset packet) since it doesn't
know what AIDs to use when replacing the locators.
The second case is a bit more subtle. Ideally an implementation
shouldn't discard the context state when there is some ULP that
still depends on this state. While this might be possible for
some implementations with a fixed set of applications, it doesn't
appear to be possible for implementations which provide the socket
API; there can be things like user-level "connections" on top of
UDP as well as application layer "session" above TCP which retain
the identifiers from some previous communication and expect to use
those identifiers at a later date. But the M6 layer has no
ability to be aware of this.
Thus an implementation shouldn't discard context state when it
knows it has ULP connection state (which can be checked in e.g.,
Unix for TCP), or when there is active communication (UDP packets
being sent to AID(A) recently), but when there is an infrequently
communicating user-level "connection" over UDP or "session" over
TCP the context state might be garbage collected even though it
shouldn't.
For instance, if B crashes and rebooted and A retransmits a packet
with flowid, L3(B), L2(A) then what is needed is a packet to L1(B)
from L1(A) passed to the ULP so that the ULP can send an error
(such as a TCP reset). But B has no matching state thus it needs
to send an Unknown context error back. (Should the packet not
have "rewrite ok" set host B can pass it to the ULP since it knows
that such packets contain locators that are AIDs. But once the
context has been established the peer is likely to send all
packets with "rewrite ok" set.)
If host B instead only lost (garbage collected too early) the M6
draft-nordmark-multi6-noid-01.txt [Page 18]
INTERNET-DRAFT Multihoming without IDs Oct 27, 2003
context state things are a bit more complicated for packets passed
down from the ULP. Without without any context state the M6 layer
on B can not determine whether packets to AID(A) coming from the
ULP are destinated to a standard IPv6 host or a host which
supports multihoming. Either B can determine this by doing a
reverse lookup of AID(A)->FQDN(A) followed by a FQDN(A) lookup to
see of there is an M6 record (and get the locator set of A as
well). Or, if DNS reverse lookups are undesirable or do not work,
perhaps a packet could be exchanged with A to ask it whether it
supports multihoming.
If B is communicating with both standard IPv6 hosts and hosts
which support multihoming then it has to avoid doing these DNS
lookups or peer queries for every packet sent to a standard IPv6
host. Implementation tricks (such as "has this socket ever used
M6" flag at the socket layer, and "negative caching" of peers that
do not support M6) can be useful to avoid performance overhead.
If as part of this B determines that A is M6 capable it has the
same information as the initiator during the initial context
establishment thus it can follow that procedure. If A didn't
garbage collect its end of the state this will require some extra
work to come up with a single host-pair context for a pair of AIDs
at both ends with consistent flowids in the two hosts (i.e.,
F(local) needs to match F(peer) at the other host). Specifying
this is for further study.
6. ENCODING BITS IN THE IPv6 HEADER?
The idea is to pick extra IP protocol values for common
combinations, and have a designated protocol value to capture the
uncommon IP protocols which might use M6. The uncommon IP
protocol values would require an additional extension header when
used over M6.
We pick two unused ranges of IP protocol values with 8 numbers
each (assuming we will not need more than 7 common transport
protocols). The ranges start at P1 and P2, respectively:
P1 TCP over M6 - rewrite ok
P1+1 UDP over M6 - rewrite ok
P1+2 SCTP over M6 - rewrite ok
P1+3 RDDP over M6 - rewrite ok
P1+4 ESP over M6 - rewrite ok
draft-nordmark-multi6-noid-01.txt [Page 19]
INTERNET-DRAFT Multihoming without IDs Oct 27, 2003
(...)
P1+7 escape - any protocol over M6 - rewrite ok
In this case we spend another 8 bytes (minimum IPv6
extension header size due to alignment rule) to carry the
actual IP protocol. This causes some mtu concerns for those
protocols, but they aren't very likely to be used with M6?
P2 TCP over M6 - no rewrite
P2+1 UDP over M6 - no rewrite
P2+2 SCTP over M6 - no rewrite
P2+3 RDDP over M6 - no rewrite
P2+4 ESP over M6 - no rewrite
(...)
P2+7 escape - any protocol over M6 - no rewrite
In this case we spend another 8 bytes (minimum IPv6
extension header size due to alignment rule) to carry the
actual IP protocol. This causes some mtu concerns for those
protocols, but they aren't very likely to be used with M6?
Thus a router would check if the protocol is in the P1 range and
if so, it can rewrite the locator(s). A host would check a
received packet against both P1 and P2 ranges and if so pass it to
the M6 shim layer.
Some possible alternatives to the above encoding is to:
- use some combination of the universal/local and group bit in
the interface id of the source address field to indicate
"rewrite ok".
- steal the ECN bits from the traffic class before ECN becomes a
proposed standard? Don't think this will be popular!
- always have a shim header - adds 8 bytes overhead per packet.
draft-nordmark-multi6-noid-01.txt [Page 20]
INTERNET-DRAFT Multihoming without IDs Oct 27, 2003
7. COMPATIBILITY WITH STANDARD IPv6
A host can easily implement M6 in a way that interoperates with
current IPv6 as follows.
When the DNS lookup routines do not find an M6 record for the peer
they will return the AAAA resource record set to the application;
those would be the IPv6 addresses. When the ULP passes down these
addresses the M6 layer will not have any state generated by the
DNS lookup code, thus no M6 processing will take place on the
sender. (Note that this relates to the M6 layer state recovery in
section 5.)
The receive side handles both standard IPv6 and M6 since it
demultiplexing on whether a packet is an M6 packet.
8. APPLICATION USAGE OF IDENTIFIERS
The upper level protocols will operate on AIDs which are mere
locators. Thus as long as a site hasn't renumbered the AID can be
used to either send packets to the host, or (e.g. if that locator
isn't working), it is possible for an application to do a reverse
lookup plus forward lookup of the AID to get the set of locators
for the peer.
Once a site has been renumbered the AIDs which contain the old
prefix will no longer be useful. Hence applications must try to
honor the DNS TTL somehow.
Applications which use to map the peer's IP address to a domain
name today perform a reverse lookup in the DNS (e.g., using the
getnameinfo() API). This proposal doesn't add or subtract to the
benefits of performing such reverse lookups.
draft-nordmark-multi6-noid-01.txt [Page 21]
INTERNET-DRAFT Multihoming without IDs Oct 27, 2003
9. CHECKSUM ISSUES
The IPv6 header does not have a checksum field; the IPv6 address
fields are assumed to be protected by the ULP pseudo-header
checksum. The general approach of an M6 shim which replaces
locators with identifiers (where only the identifiers are covered
by the ULP checksum) raises the potential issue of robustly
handling bit errors in the address fields.
With the definition of the M6 shim there can be undetectable bit
errors in the flowid field or the nexthdr field which might
adversely affect the operation of the protocol. And since the
AIDs are what's covered by the ULP's pseudo-header checksum the
locators in the address fields are without checksum protection.
An undetected bit error in the source locator would look like an
unverified source locator to the receiver. Thus the packet would
(after replacing locators with identifiers based on the context)
be passed to the ULP and a challenge response exchange be
triggered. In the case of a bit error in the locator this
challenge isn't likely to receive a response; and if there is a
response by someone it wouldn't be from the actual peer thus the
verification would fail. Thus such an undetected bit error is
harmless.
Except for the obscure case when Ls(A) contains multiple verified
locators, one or more of those are not working, and the bit error
causes L1(A) to be replaced by L2(A). That would make the return
traffic go to L2(A), but that might be a non-functioning locator.
In this case the mistake will be corrected when a subsequent
packet is received from A.
An undetected bit error in the destination address field is also
harmless; it might cause misdelivery of the packet to a host which
has no context but the reception of the resulting Unknown context
error message will show that it arrives from the incorrect locator
thus it will be ignored.
An undetected bit error in the IPv6 next header field can
potentially make a M6 packet appear as a non-M6 packet and vice
versa. This isn't any different than undetected bit errors in
IPv6 next header field without multihoming support.
An undetected bit error in the flowid in a data message could have
two possible effects: not finding any context state, or finding
the incorrect context state. In the first case the Unknown
context error message would be dropped by the peer since the
flowid included in the error message doesn't match the flowid that
was originally sent. In the second case this will result in a
draft-nordmark-multi6-noid-01.txt [Page 22]
INTERNET-DRAFT Multihoming without IDs Oct 27, 2003
packet with incorrect identifiers being delivered to the ULP which
most like will drop it due to ULP checksums not matching.
10. IMPLICATIONS FOR PACKET FILTERING
Ingress filtering should be replaced by locator rewrite when the
"rewrite ok" bit is set.
Locator rewriting (when the bit is set) can be applied at places
where ingress filtering isn't currently performed (e.g., due to
multihoming issues).
Firewall filtering potentially require modifications to be aware
of M6. All the packets contain locator thus a firewall would need
to be aware of the context state to let the correct packets
through. Such firewalls could optionally perform their own
verification by issuing DNS lookups the same way as the endpoint.
However, the firewalls probably has to be more careful not
exposing themselves to DoS attacks by doing too much DNS lookups.
11. IPSEC INTERACTIONS
As specified all of ESP, AH, and key management is layered above
the M6 layer. Thus they benefit from the stable identifiers
provided above the M6 layer. This means the IPsec security
associations are unaffected by switching locators.
The alternative would be to layer M6 above IPsec, but that doesn't
seem to provide any benefits. Since we want to allow routers
performing locator rewriting it wouldn't be possible to take
advantage of for instance AH to protect the integrity of the IP
headers.
draft-nordmark-multi6-noid-01.txt [Page 23]
INTERNET-DRAFT Multihoming without IDs Oct 27, 2003
12. SECURITY CONSIDERATIONS
This analysis is far from complete. Early analysis indicates this
addresses the issues in [M6SEC].
Just as in today's Internet hosts on the path can inject bogus
packets; in this proposal they need to extract the flowids from
the packets in order to do this which wouldn't be hard. Packet
injection from off-path places becomes harder since it requires
guessing the 20 bit flowid together with locators that are in the
locator sets.
DNS verification implications TBD
13. DESIGN ALTERNATIVES
Use an actual extension header for M6 and use a context tag in
that header instead of using the flowid. This would make the
packets 8 bytes larger since the minimum extension header size is
8 bytes due to the alignment rules for extension headers in IPv6.
14. OPEN ISSUES
DNS lookup fails or times out on the receiver; what should one do?
Send error?
Is it possible to facilitate transition to M6 using some "M6
proxy" at site boundaries until all important hosts in a site have
been upgraded to support M6? Would would be the properties of
such a proxy? Would it place any additional requirements on the
protocol itself?
One of the issues with FQDNs mapping to AAAA records is that in
some cases multiple AAAA records mean a multihomed host and in
other cases it means multiple hosts providing the same service.
If we need to introduce a new RR type for M6, would it be useful
to try to make this host/service distinction more clear at the
same time? An example solution would be that the M6 record would
draft-nordmark-multi6-noid-01.txt [Page 24]
INTERNET-DRAFT Multihoming without IDs Oct 27, 2003
by its existence indicate the M6 capability, and its RDATA would
contain a list of host names which would be used to resolve the
AAAA records for each host implementing the service.
Would destination locator rewriting be a useful way for the
routing system to pass some information to the host? Or is source
locator rewriting sufficient?
Understanding the performance of DNS verification with and without
DNSsec. With DNSsec how many public key signature verifications
are likely to be needed for the reverse lookup of each locator?
14.1. Handling Hosts without a FQDN
As specified in this document each host (including the initiating
one) whether or not multihomed needs to have a FQDN.
However, it isn't hard to allow hosts without a FQDN to
communicate with multihomed hosts that have a FQDN; as a result
the hosts without a FQDN would not benefit from "rehoming".
This requires that when a responder tries to verify the peer by
performing DNS lookups (reverse and forward) if it fails to
perform a reverse lookup on the peer AID then it will assume that
the peer has no FQDN. In this case the Ls(peer) will contain only
the AID(peer) i.e., the peer locator can not change.
Whether the reverse lookup on the AID should be repeated (in order
to handle transient failures) is TBD.
14.2. Locator Set Inconsistencies
Due to transient failures of the DNS lookups, misconfigured DNS
(returning different information "locally" than in remote
lookups), or changes to the resource record sets during a
renumbering event, the two ends of a context host-pair might have
conflicting views on each others locator sets.
This can result in black holes if the sender uses a source locator
which the receiver has not discovered using DNS lookups. It is
unclear whether the error messages sent back could be used to
detect and recover from this type of inconsistency.
But it is possible to add an additional protocol mechanism to make
the two ends converge on the set of locators which is the
intersection of what the two ends know. This could be done any
draft-nordmark-multi6-noid-01.txt [Page 25]
INTERNET-DRAFT Multihoming without IDs Oct 27, 2003
time after the context has been established.
For example, A could send some new message type to B containing
what it thinks is Ls(A) and Ls(B). When B receives this message
it calculates the intersection between the received sets and its
knowledge of the locator sets. The result is used both in B's
context state and sent back to A. When A receives the response it
can verify that the result is in fact a subset of its existing
locator sets (simply by forming the intersection between its state
and the received sets) and use that. As a sanity check the AIDs
should not be removed from the locator sets as part of this
exchange.
Verifying the flowids in this exchange guards against off-path
attackers artificially reducing the locator sets.
14.3. Renumbering Considerations
Need to write down any special coordination needed when a locator
is added to a locator set or when one is removed; this can happen
when a site is renumbered.
14.4. Initiator Confusion vs. "Virtual Hosting"
When A wants to communicate with host B and host X at the same
time there can be some confusion since the DNS could return
partially overlapping locator sets for the two remote hosts. For
example,
The lookup of FQDN(B) returns Ls(B) which contains L1(B), L2(B),
... Ln(B).
The lookup of FQDN(X) returns L1(B), L1(X)
The result is that connections that could be intended to go to B
and to X could both end up with an AID=L1(B), but the multihoming
shim layer would have two separate locator sets associated with
L1(B). Thus at a minimum when the second of the two
communications starts there has to be some way to resolve this
conflict.
In section 4.1 this is resolved by the initiator performing a
reverse lookup on the AID. Thus looking up L1(B) in the ip6.arpa
tree in the above example. That works because it would return
FQDN(B) thus X could be safely declared as being bogus. As a
result communication with X would not be possible.
draft-nordmark-multi6-noid-01.txt [Page 26]
INTERNET-DRAFT Multihoming without IDs Oct 27, 2003
However, in many (IPv4) hosting setups today multiple domain names
(www.foo.com, www.bar.com) are served by a single IP address. In
this case the reverse lookup can't point back at both names unless
the PTR resource record contains multiple records with different
names. Per [RFC2181] section 10.2 this is allowed but it doesn't
appear to be commonly used.
Can we depend on this little used feature of the PTR usage? If
not it would seems to mean that each locator can only be used with
one FQDN which would be more restrictive than we have with IPv4
today.
15. ACKNOWLEDGEMENTS
This document is the result of discussions in a MULTI6 design team
but is not the "product" of that design team. The scribe wishes
to acknowledge the contributions of (in alphabetical order):
Iljitsch van Beijnum, Brian Carpenter, Tony Li, Mike O'Dell, and
Pekka Savola.
The idea to allow locator rewriting by routers was first presented
by Mike O'Dell [ODELL96]. The techniques for avoiding state DoS
attacks on the first packet are patterned after [MIPv6].
16. REFERENCES
16.1. Normative References
[M6SEC] Nordmark, E., and T. Li, "Threats relating to IPv6
multihoming solutions", draft-nordmark-multi6-threats-
00.txt, October 2003.
[ADDR-ARCH] S. Deering, R. Hinden, Editors, "IP Version 6
Addressing Architecture", RFC 3513, April 2003.
[IPv6] S. Deering, R. Hinden, Editors, "Internet Protocol,
Version 6 (IPv6) Specification", RFC 2461.
draft-nordmark-multi6-noid-01.txt [Page 27]
INTERNET-DRAFT Multihoming without IDs Oct 27, 2003
16.2. Informative References
[NSRG] Lear, E., and R. Droms, "What's In A Name: Thoughts from
the NSRG", draft-irtf-nsrg-report-09.txt (work in
progress), March 2003.
[MIPv6] Johnson, D., C. Perkins, and J. Arkko, "Mobility Support
in IPv6", draft-ietf-mobileip-ipv6-24.txt (work in
progress), June 2003.
[AURA02] Aura, T. and J. Arkko, "MIPv6 BU Attacks and Defenses",
draft-aura-mipv6-bu-attacks-01 (work in progress), March
2002.
[NIKANDER03] Nikander, P., T. Aura, J. Arkko, G. Montenegro, and
E. Nordmark, "Mobile IP version 6 Route Optimization
Security Design Background", draft-nikander-mobileip-
v6-ro-sec-01 (work in progress), June 2003.
[ODELL96] O'Dell M., "8+8 - An Alternate Addressing Architecture
for IPv6", draft-odell-8+8-00.txt, October 1996,
[MAST] D. Crocker, "MULTIPLE ADDRESS SERVICE FOR TRANSPORT
(MAST): AN EXTENDED PROPOSAL", draft-crocker-mast-
protocol-01.txt, October, 2003.
[CB128] E. Nordmark, "Strong Identity Multihoming using 128 bit
Identifiers (SIM/CBID128)", draft-nordmark-multi6-sim-
00.txt, October 2003.
[DISCOVERY] T. Narten, E. Nordmark, and W. Simpson, "Neighbor
Discovery for IP Version 6 (IPv6)", RFC 2461, December
1998.
[IPv6-SA] R. Atkinson. "Security Architecture for the Internet
Protocol". RFC 2401, November 1998.
[IPv6-AUTH] R. Atkinson. "IP Authentication Header", RFC 2402,
November 1998.
[IPv6-ESP] R. Atkinson. "IP Encapsulating Security Payload
(ESP)", RFC 2406, November 1998.
[RFC2181] R. Elz, and R. Bush, "Clarifications to the DNS
Specification", RFC 2181, July 1997.
draft-nordmark-multi6-noid-01.txt [Page 28]
INTERNET-DRAFT Multihoming without IDs Oct 27, 2003
AUTHORS' ADDRESSES
Erik Nordmark
Sun Microsystems, Inc.
17 Network Circle
Mountain View, CA
USA
phone: +1 650 786 2921
fax: +1 650 786 5896
email: erik.nordmark@sun.com
Full Copyright Statement
Copyright (C) The Internet Society (2003). All Rights Reserved.
This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph are
included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of
developing Internet standards in which case the procedures for
copyrights defined in the Internet Standards process must be
followed, or as required to translate it into languages other than
English.
The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assignees.
This document and the information contained herein is provided on an
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
draft-nordmark-multi6-noid-01.txt [Page 29]