Network Working Group M. Shore
Internet-Draft D. McGrew
Intended status: Standards Track K. Biswas
Expires: January 14, 2009 Cisco Systems
July 13, 2008
Network-Layer Signaling: Transport Layer
draft-shore-nls-tl-06.txt
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Abstract
The RSVP model for communicating requests to network devices along a
datapath has proven useful for a variety of applications beyond what
the protocol designers envisioned, and while the architectural model
generalizes well the protocol itself has a number of features that
limit its applicability to applications other than IntServ. Network
Layer Signaling uses the RSVP on-path communication model and
provides a lightweight transport layer for non-QoS signaling
applications, such as discovery or diagnostics. It is based on a
"two-layer" architecture that divides protocol function into
transport and application. This document describes the transport
protocol.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Transport layer . . . . . . . . . . . . . . . . . . . . . 5
2. NLS-TL Messages . . . . . . . . . . . . . . . . . . . . . . . 7
2.1. Message Processing Overview . . . . . . . . . . . . . . . 7
2.1.1. Congestion Considerations . . . . . . . . . . . . . . 8
2.2. NAT Traversal Support . . . . . . . . . . . . . . . . . . 8
2.3. NLS-TL Message Format . . . . . . . . . . . . . . . . . . 9
2.3.1. The NLS-TL Message Header . . . . . . . . . . . . . . 9
2.3.2. NLS-TL TLVs . . . . . . . . . . . . . . . . . . . . . 10
2.4. Cryptographic Datatypes . . . . . . . . . . . . . . . . . 18
3. Sending NLS-TL Messages . . . . . . . . . . . . . . . . . . . 20
4. Messaging and state maintenance . . . . . . . . . . . . . . . 21
4.1. BUILD-ROUTE . . . . . . . . . . . . . . . . . . . . . . . 21
4.1.1. Route state deletion . . . . . . . . . . . . . . . . . 21
4.2. HOP-BY-HOP . . . . . . . . . . . . . . . . . . . . . . . . 21
4.3. BIDIRECTIONAL . . . . . . . . . . . . . . . . . . . . . . 22
4.4. Path Teardown Messages . . . . . . . . . . . . . . . . . . 22
4.5. Network Address Translation . . . . . . . . . . . . . . . 23
4.6. Authentication Exchange . . . . . . . . . . . . . . . . . 23
4.6.1. Authentication Exchange Messages . . . . . . . . . . . 24
4.6.2. Authentication TLV calculation . . . . . . . . . . . . 27
4.6.3. Security state transition table . . . . . . . . . . . 28
5. Application Interface . . . . . . . . . . . . . . . . . . . . 30
6. NAT Interactions . . . . . . . . . . . . . . . . . . . . . . . 31
7. Using NLS-TL as a stand-alone NAT traversal protocol . . . . . 32
8. Discovery of non-NLS NATs, and recovery . . . . . . . . . . . 33
9. Endpoint Processing . . . . . . . . . . . . . . . . . . . . . 35
9.1. Sending . . . . . . . . . . . . . . . . . . . . . . . . . 35
9.2. Receiving . . . . . . . . . . . . . . . . . . . . . . . . 36
10. Intermediate node processing . . . . . . . . . . . . . . . . . 37
11. Using NLS-TL to support bidirectional reservations . . . . . . 38
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12. Security Considerations . . . . . . . . . . . . . . . . . . . 39
12.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . . 39
12.2. Security Model . . . . . . . . . . . . . . . . . . . . . . 39
12.3. Cryptography . . . . . . . . . . . . . . . . . . . . . . . 40
12.3.1. Keys . . . . . . . . . . . . . . . . . . . . . . . . . 40
12.3.2. Reflection Attacks . . . . . . . . . . . . . . . . . . 41
13. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 42
13.1. NLS Application Identifiers . . . . . . . . . . . . . . . 42
13.2. NLS TLVs . . . . . . . . . . . . . . . . . . . . . . . . . 43
13.3. Error codes and values . . . . . . . . . . . . . . . . . . 44
14. References . . . . . . . . . . . . . . . . . . . . . . . . . . 45
14.1. Normative References . . . . . . . . . . . . . . . . . . . 45
14.2. Informative References . . . . . . . . . . . . . . . . . . 45
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 46
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 47
Intellectual Property and Copyright Statements . . . . . . . . . . 48
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1. Introduction
RSVP is based on a "path-coupled" signaling model, in which signaling
messages between two endpoints follow a path that is tied to the data
path between the same endpoints, and in which the signaling messages
are intercepted and interpreted by RSVP-capable routers along the
path. While RSVP was originally designed to support QoS signaling
for Integrated Services [RFC1633], this model has proven to
generalize to other problems extremely well. Some of these problems
include topology discover, communicating with firewalls and NATs,
discovery of IPSec tunnel endpoints, test applications, diagnostic
triggers, and so on.
This document describes the core protocol for a generalized on-path
request protocol that is being used today to carry topology discovery
and other requests -- one that is not tied directly to IntServ or
other QoS mechanisms and in which the protocol machinery itself is
sufficiently generalized to be able to support a variety of
applications (this protocol is referred to as "Network Layer
Signaling", or "NLS"). What this means in practice is that there
will be different signaling applications, all of which share a base
NLS transport layer. This architecture is based on work done by Bob
Braden and Bob Lindell, and described in [braden]. It is also
similar to the concepts used in secsh, where authentication and
connection protocols run on top of a secsh transport protocol (see
[RFC4251] for details).
The protocol machinery was originally based somewhat on RSVP
[RFC2205] without refresh overhead reduction extensions [RFC2961],
but in the process of generalization has lost many of the features
that define RSVP, such as necessary receiver-oriented reservations
and processing requirements at each node.
NLS differs from RSVP in several important ways. One of the most
significant of these is that the transport protocol described in this
document (NLS-TL) does not itself trigger reservations in network
nodes. Reservations will be installed and managed by NLS
applications, however some NLS applcations may not carry reservation
requests at all (discovery protocols, for example). Because of this
NLS-TL does not support reservation styles (those would be also be
attributes of an application). Another significant difference is
that that reservations may be installed by a NLS application in
either a forward (from the sender toward the receiver) or backward
(from the receiver toward the sender) direction -- this is
application-specific.
Other possibly significant differences include that NAT traversal
support is integrated into the message transport, and that NLS allows
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an application to install reservations for paths that are
bidirectional and asymmetric.Multicast is not supported in this
version of the protocol.
NLS shares some basic design features with NSIS [RFC4080], which is
another path-coupled protocol. However, unlike NLS, the NSIS
transport provides reliable delivery of request messages (in NLS this
is left to the application rather than the transport layer), and NLS
was not designed with QoS signaling support in mind. We leave the
question of reliable delivery up to the NLS application. Not every
signaling application requires reliable delivery and by moving
reliable delivery into the application layer we are able to keep
complexity in the transport layer to a minimum, providing only the
services which tend to need to be available in all on-path signaling
applications. It is also the case that by providing a light-weight,
datagram-based protocol we are able to rely on IP layer routing and
forwarding for delivery and do not require a separate routing
process.
The NLS Transport Layer is being used by PacketCable and the ITU-T to
carry topology discovery requests, in a protocol called "Control
Point Discovery" (CPD).
1.1. Transport layer
This document describes the transport layer. The NLS transport layer
is as simple as we could make it, supporting two basic functions:
routing and NAT traversal. The sources of complexity in signaling
protocols tend to be the signaling applications themselves. Those
applications have varying performance and reliability requirements,
and consequently we feel that application-specific functions belong
in the application layer.
The NLS transport layer is also relatively stateless. By "stateless"
we mean that the transport layer does not itself create or manipulate
state in participating nodes. By "relatively" we take exception to
the previous assertion, in that the transport layer provides
facilities for route identification and route pinning. This is an
optimization, albeit a significant one, which allows NLS to be used
without a separate route discovery process. Another source of state
is in the case of NATs, where an NLS-TL request may trigger the
creation of a NAT table mapping. However, this latter case does not
create NLS-TL maintenance state.
An application may wish to support summary refreshes or other
performance enhancements; that type of function is application-
specific and requires no support from the transport layer.
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The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
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2. NLS-TL Messages
2.1. Message Processing Overview
Unlike RSVP, NLS-TL has only one fundamental message type, and
directionality is significant to the NLS application only. Three new
attributes, HOP-BY-HOP, BUILD-ROUTE, and BIDIRECTIONAL, have been
added in support of greater flexibility in the NLS application. For
example, some applications which already know network topology or
which run a separate routing protocol may choose to route hop-by-hop
in a forward direction. Conversely, a topology discovery protocol
may choose to route end-to-end in the return direction. Both of
these would be departures from the Path/Resv message handling
specified in RSVP.
The BUILD-ROUTE flag has been added to allow route discovery to be
overloaded on top of basic messaging, much like the RSVP Path
message. If the BUILD-ROUTE flag is present, NLS nodes store routing
information carried in incoming HOP objects. They also overwrite
routing information into the HOP TLV in outgoing NLS messages.
The HOP-BY-HOP flag is used to instruct an NLS-TL node to address an
outgoing message to the address stored in routing state associated
with the message flow. Typically this will be state accumulated
through the use of the BUILD-ROUTE flag but it may be manually
configured, installed through an external routing or discovery
process, etc.
The BIDIRECTIONAL flag may be used to indicate that the application
for which this NLS-TL message carries a payload must be executed in
each direction. It may be used in combination with the HOP-BY-HOP
flag in some circumstances, but typically it will be used with the
HOP-BY-HOP flag set to 0.
Even with these departures, the basic operation of the protocol may
made be similar to RSVP with the appropriate use of the new
attributes. For example, a message may be injected into a network by
the sender towards a receiver, routed end-to-end with the receiver's
address in the destination address in the IP header. If the BUILD-
ROUTE bit is set in the NLS header, entities along the path the
message traverses will intercept it, store path state, act on (or
not) the application payload data, and forward the message towards
its destination. In NLS-TL, "path state" refers specifically to the
unicast IP address of the previous hop node along with locally-
relevant path information (for example, interface identifier).
When the message arrives at the receiver (or its proxy), the receiver
may generate another NLS message in response, this time back towards
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the original sender. As with the message in the forward direction,
this message may be routed either end-to-end or hop- by-hop,
depending on the requirements of the application. In order to
emulate an RSVP Resv message, the HOP-BY-HOP is set to 1 and the
BUILD-ROUTE bit is set to 0.
BUILD-ROUTE and HOP-BY-HOP must not be set in the same NLS-TL
message, and BUILD-ROUTE and TEARDOWN MUST NOT be set in the same
NLS-TL message.
2.1.1. Congestion Considerations
Transmission, loss response, and resend timings are out-of-scope for
this document. Different NLS applications will have different
transmission timing and resend characteristics and will need to be
specified in a manner appropriate to each application. For example,
a discovery application will need to behave differently from an
application which requests and maintains state in middleboxes.
However, each NLS application MUST specify how it will handle message
loss and MUST specify a backoff mechanism in the case where messages
are retransmitted as a response to message loss.
Loss response for stand-alone NAT traversal is described in
Section 7.
2.2. NAT Traversal Support
NAT traversal poses a particular challenge to a layered protocol like
NLS. If we assume the use of discrete, opaque applications, one of
which is NAT, interactions between other applications that make use
of addresses (for example, firewall rules or QoS filter specs) and
the NAT application are complicated. Either every application will
need to be able to peek into NAT payloads and identify which address
mapping is the one they need, or NATs supporting NLS will need to be
able to parse and write into every application payload type. Neither
approach is particularly robust, reintroducing a type of stateful
inspection and constraining how applications can be secured.
Because of the desire to be able to have a variety of NLS
applications successfully interact with NATs and because of the
constraints described above, in NLS NAT is supported in the transport
layer rather than in a separate application. Each address that needs
translation is tagged, put into a NAT_ADDRESS TLV, and passed to the
appropriate application at each NLS node. Application identification
is based on tag contents. Note that NLS supports Network Address
Translation for IPv4 only.
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2.3. NLS-TL Message Format
NLS messages consist of an NLS-TL header followed by optional TLV
fields followed by an optional application payload.
2.3.1. The NLS-TL Message Header
All NLS-TL messages (and by implication, all NLS messages) start with
an NLS header. The header is formatted as follows:
0 1 2 3
+-------------+-------------+-------------+-------------+
| Version | (Reserved) | Message Length |
+-------------+-------------+-------------+-------------+
| Flags | Checksum |
+-------------+-------------+-------------+-------------+
| Flow ID |
+-------------+-------------+-------------+-------------+
Figure 1
where the fields are as follows:
Version: 8 bits. The protocol version number; in this case 0x01.
Reserved: 8 bits. MUST be set to 0x0 when sending and ignored on
receipt.
Message Length: 16 bits. The total number of octets in the
message, including the NLS-TL header and complete payload.
Flags: 16 bits. Flag bits include
0x01 HOP-BY-HOP
0x02 BUILD-ROUTE
0X04 TEARDOWN
0x08 AX_CHALLENGE
0x10 AX_RESPONSE
0x20 BIDIRECTIONAL
Checksum: 16 bits. The one's complement of the one's complement
sum of the entire message. The checksum field is set to zero for
the purpose of computing the checksum. This may optionally be set
to all zeros. If a message is received in which this field is all
zeros, no checksum was sent.
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Flow ID: 32 bits. This is a value which, combined with the source
IP address of the message, provides unique identification of a
flow of NLS-TL messages, which may be used for later reference for
actions such as quick teardowns, status queries, etc. The
mechanism used for generating the value is implementation-
specific.
Throughout, we assume the use of 8-bit bytes, or octets.
2.3.2. NLS-TL TLVs
NLS-TL carries additional transport-layer information and requests as
type-length-value fields, which are inserted after the header and
before the application payload. The TLV format is as follows:
0 1 2 3
+-------------+-------------+-------------+-------------+
|M|R| Type | Length |
+-------------+-------------+-------------+-------------+
| |
// Value //
| |
+-------------+-------------+-------------+-------------+
Figure 2
where the fields are as follows:
Mandatory: 1 bit. If this bit is set, this TLV MUST NOT be ignored
silently, even if the recipient does not understand the type code.
If the recipient does not understand the TLV, it MUST return an
IPv4_ERROR_CODE or an IPv6_ERROR_CODE, as appropriate, with the
error code set to 0x02 (Unrecognized TLV). If it is not set then
the recipient MAY ignore the TLV.
Reserved: 1 bit. This bit is reserved for future use. It MUST be
set to 0 before sending and ignored on receipt.
Type: 14 bits. The type of information or request. Defined below.
Length: 16 bits. Total TLV length in octets, including the type
type and length fields. It must always be at least 4 and be a
multiple of 4.
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Value: Variable length. At least 4 octets and a multiple of 4
octets). The TLV semantic content. The format of the Value field
is determined by the value of the Type field
2.3.2.1. NAT_ADDRESS, TYPE=1
+-------------+-------------+-------------+-------------+
| Application ID | Flags | Proto |
+-------------+-------------+-------------+-------------+
| Address ID Tag |
+-------------+-------------+-------------+-------------+
| Original IPv4 Address |
+-------------+-------------+-------------+-------------+
| Mapped IPv4 Address |
+-------------+-------------+-------------+-------------+
| Original Port | Mapped Port |
+-------------+-------------+-------------+-------------+
where the fields are as follows:
Application ID: 16 bits. This is the same as the value that's used
for identifying application payloads. The Application ID field is
set by the sender.
Flags: 16 bits. Flag bits include
0x01 = NO_TRANSLATE
0x02 = NO_REWRITE
NO_TRANSLATE indicates that a NAT device handling the packet
should not create a NAT table entry for the original address. If
the NO_TRANSLATE bit is set, the NAT does nothing.
NO_REWRITE indicates that when the reply message is being returned
towards the sender, any NATs along the path MUST NOT overwrite the
Mapped Address.
Proto: IP protocol for this translation (TCP, UDP, SCTP, etc.).
Address ID: 32 bits. An value that's unique within the set of
Address IDs used with a particular Application ID; used to
uniquely identify a particular address (i.e. provide a tag).
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Original IPv4 Address: The original address for which a translation
is being requested.
Mapped IPv4 Address: The address created by the NAT -- i.e. the
"external" address.
Original Port: The original port for which a translation is being
requested
Mapped Port: The port number created by the NAT for this mapping.
The mandatory bit in the TLV header MUST always be set to 1 for this
TLV.
2.3.2.2. APPLICATION PAYLOAD, TYPE=2
+-------------+-------------+-------------+-------------+
| Application ID | Payload |
+-------------+-------------+-------------+-------------+
| |
// Payload //
| |
+-------------+-------------+-------------+-------------+
The application payload TLV carries the NLS application data. It
MUST follow any NAT TLVs. It consists of a 16-bit Application ID,
which uniquely identifies the NLS application for which the TLV is
intended, and the application payload itself. The application
payload is transparent to the NLS Transport Layer.
The APPLICATION_PAYLOAD TLV MUST be a multiple of 4 bytes in length.
2.3.2.3. TIMEOUT, TYPE=3
+-------------+-------------+-------------+-------------+
| Timeout Value |
+-------------+-------------+-------------+-------------+
The TIMEOUT TLV carries the number of milliseconds for which state
associated with a particular flow should be retained, with the
expectation that the state will be deleted when the timeout expires.
"State" in this case refers to routing state and to NAT state; NLS
application state will be managed by its application.
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2.3.2.4. IPV4_HOP, TYPE=4
+-------------+-------------+-------------+-------------+
| IPv4 Hop Address |
+-------------+-------------+-------------+-------------+
The IPv4_HOP TLV carries the IPv4 address of the interface through
which the last NLS entity forwarded the message.
2.3.2.5. IPv6_HOP, TYPE=5
+-------------+-------------+-------------+-------------+
| |
+ +
| |
+ IPv6 Next/Previous Hop Address +
| |
+ +
| |
+-------------+-------------+-------------+-------------+
The IPv6_HOP TLV carries the IPv6 address of the interface through
which the last NLS entity forwarded the message.
2.3.2.6. IPv4_ERROR_CODE, TYPE=6
+-------------+-------------+-------------+-------------+
| IPv4 Error Node Address (4 octets) |
+-------------+-------------+-------------+-------------+
| Flags | Error Code | Error Value |
+-------------+-------------+-------------+-------------+
The IPv4_ERROR_CODE TLV carries the address of a node at which an
NLS-TL error occurred, along with an error code and error value.
When no Error Value is defined, the Error Value field MUST be set to
0 by its sender and ignored by its receiver.
If the high-order bit of the Error Code is not set, the TLV carries
an error message. If it is set, the TLV carries an informational
message. Therefore Error Codes with values between 0 and 127 contain
error messages and Error Codes with values between 128 and 255
contain informational messages.
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IPv4 Error Node Address: 4 octets. The IPv4 address of the
interface on the node that generated the error message.
Flags: 8 bits. None currently defined.
Error Code: 8 bits. The type of error or informational message,
with values as follows:
Error Code = 0: No error
Error Code = 1: Bad parameters
Error Value = 1: HOP-BY-HOP and BUILD-ROUTE both present
Error Value = 2: BUILD-ROUTE present but no HOP TLV
Error Value = 3: HOP-BY-HOP present but no local stored
routing state
Error Value = 4: Message length not a multiple of 4
Error Code = 2: Unrecognized TLV
Error Value = TLV number
Error Code = 3: Unrecognized application
Error Value = Application ID
Error Code = 4: Non-NLS NAT detected in path
Error Code = 5: Security error
Error Value = 1: AGID not found
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Error Value = 2: Insufficient authorization
Error Value = 3: Request/reply mismatch
Error Value = 4: Authentication Failure
Error Code = 128: No message
Error Code = 129: Sending node has detected a route change
2.3.2.7. IPv6_ERROR_CODE, TYPE=7
+-------------+-------------+-------------+-------------+
| |
+ +
| |
+ IPv6 Error Node Address (16 octets) +
| |
+ +
| |
+-------------+-------------+-------------+-------------+
| Flags | Error Code | Error Value |
+-------------+-------------+-------------+-------------+
The IPv6_ERROR_CODE TLV carries the address of a node at which an
NLS-TL error occurred, along with an error code and error value.
"IPv6 Error Node Address:" 16 octets. The IPv6 address of the
interface on the node that generated the error message.
Flags: 8 bits. None currently defined.
The Error Code and Error value fields are the same as those used in
the IPv4_ERROR_CODE.
2.3.2.8. AGID, TYPE=8
The AGID is the authentication group ID, used in the authentication
dialogue to identify the group key.
+-------------+-------------+-------------+-------------+
// AGID //
+-------------+-------------+-------------+-------------+
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2.3.2.9. A_CHALLENGE, TYPE=9
The A_CHALLENGE TLV is used to carry a 16-octet random nonce to be
used as an authentication challenge. It MUST be generated using a
strong random or pseudorandom source.
For a description of why we need A_CHALLENGE and B_CHALLENGE (as
opposed to just a single CHALLENGE type), see Section 12.3.2.
+-------------+-------------+-------------+-------------+
| |
+ +
| |
+ Nonce +
| |
+ +
| |
+-------------+-------------+-------------+-------------+
2.3.2.10. A_RESPONSE, TYPE=10
The A_RESPONSE TLV carries the response to the authentication
challenge. It is a variable length TLV with the length dependent on
the transform being used.
For a description of why we need A_RESPONSE and B_RESPONSE (as
opposed to just a single RESPONSE type), see Section 12.3.2.
+-------------+-------------+-------------+-------------+
| |
// MAC //
| |
+-------------+-------------+-------------+-------------+
2.3.2.11. B_CHALLENGE, TYPE=11
The B_CHALLENGE TLV is used to carry a 16-octet random nonce to be
used as an authentication challenge. It MUST be generated using a
strong random or pseudorandom source.
For a description of why we need A_CHALLENGE and B_CHALLENGE (as
opposed to just a single CHALLENGE type), see Section 12.3.2.
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+-------------+-------------+-------------+-------------+
| |
+ +
| |
+ Nonce +
| |
+ +
| |
+-------------+-------------+-------------+-------------+
2.3.2.12. B_RESPONSE, TYPE=12
The B_RESPONSE TLV carries the response to the authentication
challenge. It is a variable length TLV with the length dependent on
the transform being used.
For a description of why we need A_RESPONSE and B_RESPONSE (as
opposed to just a single RESPONSE type), see Section 12.3.2.
+-------------+-------------+-------------+-------------+
| |
// MAC //
| |
+-------------+-------------+-------------+-------------+
2.3.2.13. AUTHENTICATION, TYPE=13
The AUTHENTICATION TLV carries a cryptographic hash over the entire
packet, as well as a 32-bit sequence number. In order to use this
TLV, the peer must first have passed a challenge/response exchange to
negotiate the approprite agid to use. It is a variable length TLV
with the length of the MAC dependent on the transform being used (as
determined by the agid). Details of computing the MAC are described
in Section 4.6.
+-------------+-------------+-------------+-------------+
| Sequence Number |
+-------------+-------------+-------------+-------------+
| |
// MAC //
| |
+-------------+-------------+-------------+-------------+
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2.3.2.14. ECHO, TYPE=14
A device can include an ECHO element in the messages that it sends.
A device receiving a message containing such a element must send the
element back, verbatim, in the following response.
+-------------+-------------+-------------+-------------+
| |
| |
// ECHO data //
| |
| |
+-------------+-------------+-------------+-------------+
An ECHO TLV SHOULD only appear during an Authentication Exchange, and
SHOULD NOT appear in any other message.
2.4. Cryptographic Datatypes
This section provides further detail on message formats for the
authentication exchange.
An NLS-TL message MSG has the following format:
MSG :== HDR OPT* APP* SEC*
where HDR, OPT, APP, and SEC are as follows:
HDR is the NLS header
OPT is an NLS optional TLV
APP is the optional Application Object
SEC is an AGID, A_CHALLENGE, A_RESPONSE, B_CHALLENGE, B_RESPONSE,
or AUTHENTICATION TLV's. These datatypes are defined below.
Note that though both OPT and APP are optional, one or the other MUST
exist (or both together).
The security TLVs are always last in order to avoid data-formatting
issues with the inputs to the message authentication codes, and to
minimize the amount of data movement needed during the Authentication
Exchange.
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Authorization Group Identifier (AGID): The AGID TLV identifies a
particular group key. The Value field carries an identifier;
there is no defined format. The length of this field is variable,
and MUST be a multiple of four octets. If it is generated at
random, then it SHOULD be at least 16 octets.
A_CHALLENGE: The A_CHALLENGE contains a 16-octet random nonce.
This TLV is put into a message whenever outbound authentication is
desired. When this TLV is received, then the next message sent
MUST contain either an A_RESPONSE TLV or an error message
indicating that no authentication is possible.
B_CHALLENGE: The B_CHALLENGE contains a 16-octet random nonce.
This TLV is put into a message whenever inbound authentication is
desired. When this TLV is received, then the following message
MUST contain either a B_RESPONSE TLV or an error message
indicating that no authentication is possible.
A_RESPONSE: The A_RESPONSE TLV is sent in response to a message
containing an A_CHALLENGE TLV. It contains a message
authentication code (MAC) value computed over the complete NLS
message containing the A_CHALLENGE, including the NLS header.
B_RESPONSE: The B_RESPONSE is sent in response to a message
containing a B_CHALLENGE TLV. It contains a message
authentication code (MAC) value computed over the complete NLS
message containing the B_CHALLENGE, including the NLS header.
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3. Sending NLS-TL Messages
When an endhost or its proxy wishes to initiate a NLS session, it
creates an NLS-TL message. If the message is being sent end-to-end
the destination address in the IP header is the address of the device
interface that is expected to terminate the path along which
signaling is expected to be sent (n.b. multicast is not supported in
this version of the protocol). It may be a application peer host or
terminal, or it may be a proxy. If the message is being sent hop-by-
hop the destination address in the IP header is the address of the
device interface that is the next hop along the path. That address
will have been discovered either through a separate routing process
or through RSVP-style soft-state messaging.
NLS-TL messages are UDP-encapsulated and sent on UDP port 7549. They
MAY be sent with the router alert bit set in IPv4 headers or with the
IPv6 router alert option [RFC2711], but it is not required. If the
message is end-to-end and needs route discovery and pinning, the
BUILD-ROUTE bit in the NLS-TL flags header MUST be set to 1 and the
HOP-BY-HOP bit MUST be set to 0. If the message is being routed hop-
by-hop, the HOP-BY-HOP bit MUST be set to 1 and the BUILT-ROUTE bit
MUST be set to 0. (Note that there may be applications in which both
the HOP-BY-HOP and the BUILD- ROUTE bit will be set to 0.)
If the NLS application wishes to support bidirectional reservations,
the BIDIRECTIONAL flag must be set to 1, the BUILD-ROUTE flag should
be set to 1, and the HOP-BY-HOP flag should be set to 0, at least in
the initial message. If the application makes use of periodic
refreshes it may optionally choose to route some number of them hop-
by-hop along the discovered path before sending out another message
to refresh the route state; that is an application design issue.
In this version of the protocol, each NLS message must fit in one
datagram. An NLS-TL message originator should perform PMTU discovery
in order to avoid exceeding path MTU size.
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4. Messaging and state maintenance
Message handling and state maintenance are determined by the presence
(or absence) of two flags in the NLS-TL header: the HOP-BY-HOP bit
and the BUILD-ROUTE bit. They also involve, and are involved by, NAT
processing.
4.1. BUILD-ROUTE
The BUILD-ROUTE bit in the flags field of the NLS-TL header allows
NLS-TL to function as a discovery and routing protocol, much like the
Path message described in RFC 2205.
If the BUILD-ROUTE flag is present in a NLS-TL message, upon receipt
a NLS node MUST check for the presence of an IPv4_HOP or IPv6_HOP TLV
in the NLS-TL payload. If one is not present, the message MUST be
discarded and an error returned to the sender. If both are present,
the message MUST be discarded and an error returned to the sender.
Otherwise, if there is no installed soft state associated with the
Flow ID, the node stores the HOP information, Flow ID, and other
state information it chooses to retain, and forwards the message
towards the address in the destination field of its IP header. If
there is installed soft state associated with the Flow ID, the node
compares the contents of the HOP field with the installed state. If
they are identical nothing needs to be done; if they are different
the HOP information in the node is overwritten with the information
in the current message. This allows the protocol to be responsive to
route changes, endpoint mobility, and so on.
A NLS node MAY send notification of a routing change back to the
sender.
4.1.1. Route state deletion
Routing state accumulated through BUILDROUTE MAY be deleted in one of
the following circumstances: 1) when the state is explicitly torn
down in a message with the TEARDOWN bit set; 2) following the timeout
period as described in Section 2.3.2.3, or 3) as a function of
implementation-specific resource management.
4.2. HOP-BY-HOP
If the HOP-BY-HOP bit is set in the flags field of the NLS-TL header,
a NLS node MUST forward the message to the address stored in
associated local soft state. That is to say, the node MUST write the
address in the local HOP information associated with the
MESSAGE_IDFlow ID into the destination field in the IP header on the
outbound message. This is like message processing in the Resv
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message in RFC 2205.
The HOP information may have been acquired using a routing process
based on HOP-BY-HOP processing, but it may have been acquired using
an external routing mechanism. If there is no HOP information stored
locally, the node MUST drop the message and return an error to the
sender.
4.3. BIDIRECTIONAL
If the BIDIRECTIONAL flag is set, the receiver must send the
answering message to the sender (that is to say, the destination
address in the IP header must be set to the address of the sender)
with the BUILD_ROUTE flag set and the HOP_BY_HOP flag set to 0. As
with the message sent from the sender to the receiver, the HOP TLV
contains information used to install routing state. If the nodes are
already authenticated to one another (they were already traversed in
the forward direction) it is unnecessary for the authentication
dialogue to be performed again. If the nodes are not already
authenticated to one another then the route is asymmetric and the
authentication dialogue must be performed.
Note that the sender and receiver should retain knowledge that the
session is bidirectional, as it may affect subsequent messaging and
error processing.
Because a complete authentication dialogue may take place in each
direction, with each node being authenticated to its adjacent node
(i.e. the dialogue takes care of authenticating both A to B and B to
A), this proposal neither changes the authentication dialogue nor
should it undermine the security of the protocol.
4.4. Path Teardown Messages
Receipt of a NLS message with the TEARDOWN bit set indicates that
matching path state must be deleted. Note that this is independent
of directionality, and the teardown message may be sent in either
direction. The applications which have reservations that were
installed by a message containing a matching Flow ID must be
notified, and they are responsible for managing (in this case,
deleting) their own flow-related state. TEARDOWN and HOP-BY-HOP MUST
NOT be set in the same message.
Unlike RFC 2205, if there is no matching path state the teardown
message must be forwarded. There may be path state in support of an
NLS application that is not running on every node, and the teardown
message must not be lost.
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4.5. Network Address Translation
If there is one or more NAT_ADDRESS TLVs present, an NLS-capable NAT
must process each one that has does not have the NO_TRANSLATE bit set
in the flags field. Processing takes place as follows:
o The originator (sender) of the message creates a NAT_ADDRESS TLV
for each address/port/protocol tuple requiring NAT mappings. It
also creates a random 32- bit tag, which is used to identify the
address in application payloads and to tag the mapping in the
NAT_ADDRESS TLV in the NLS-TL header. It also sets the TRANSLATE
bit in the flags field and zeros the Mapped Address field.
o When an NLS-capable NAT receives a request, for each NAT_ADDRESS
TLV in which the NO_TRANSLATE bit is not set and the Mapped
Address is all nulls, it creates a NAT table mapping for the
Original Address and Original Port and inserts the "external"
address and port into the Mapped Address and Mapped Port fields.
o When an NLS-capable NAT receives a request, for each NAT_ADDRESS
TLV in which the NO_TRANSLATE bit is not set and the Mapped
Address is not nulls, it creates a NAT table mapping for the
Mapped Address and Mapped port and overwrites those values with
the new external addresses and ports.
o When an NLS-capable node receives a request, for reach NAT_ADDRESS
TLV in which the Application ID matches an NLS application payload
ID and the application is supported by the node, the TLV is passed
to the application with the application payload, allowing the
application module on the node to correlate and use the address
based on the tag [and the Original Address?]
Note that this approach to NAT requires that participants be
sensitive to directional issues in cases where ordering matters, such
as the need to find the outermost NAT address. API support is
required in order to turn the NO_TRANSLATE bit on and off as needed
by a particular application.
Also note that in cases where the only function required is NAT table
mapping requests, there may be no application payloads, or it may be
desirable to create a rudimentary NAT NLS application that does
nothing other than allow the receiver, or other nodes, to turn the
NO_TRANSLATE bit on.
4.6. Authentication Exchange
NLS provides its own message authentication mechanism, based on a
dialogue between adjacent nodes. We refer to this as the
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"Authentication Exchange," or AX.
4.6.1. Authentication Exchange Messages
In the following, we consider only the Security TLVs, and we use
REQUEST and REPLY to represent the body of the messages.
1. A -> B : HDR1, REQUEST, AGID*, A_CHALLENGE
2. B -> A : HDR2, REQUEST, AGID, B_CHALLENGE, A_RESPONSE
3. A -> B : HDR3, REQUEST, AGID, B_RESPONSE
/* at this point, B might forward the REQUEST onward */
4. B -> A : HDR4, REPLY, AUTHENTICATION
Note that the Flow ID in each message in the Authentication Exchange
MUST be the Flow ID that appeared in the original request.
Note as well that the fields MUST be presented in the order
specified, or the MAC calculations will fail.
Message 1 (outbound). When device A sends a message, it constructs
the message as follows:
o It consults the policy associated with that interface to determine
which AGID values should be included in that message. For each
AGID in the policy that is associated with the Application ID in
the message, it includes in that message an AGID TLV containing
the AGID value.
o After the AGID TLVs have been included, an A_CHALLENGE TLV is
constructed and included in the message.
o In the NLS-TL header for message 1, the AX_CHALLENGE flag must be
set.
Message 1 (inbound). Device B receives (or intercepts) Message 1 and
processes it as follows:
o The local policy associated with the interface on which the
message arrived is checked to determine which AGIDs are associated
with the Application ID in the message. If the AGID set in the
message intersects with the locally derived AGID set, then one of
the AGID values is chosen to be 'active'; this choice is
arbitrary. Otherwise, the AX cannot be successfully completed,
and an "AGID not found" error message SHOULD be returned.
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Message 2 (outbound). Device B constructs Message 2 as follows:
o The NLS header is identical to that of Message 1, except that the
AX_CHALLENGE and AX_RESPONSE flags are now set. The TLVs from
Message 1 are copied verbatim into Message 2, in order, except for
the AGID TLVs and the A_CHALLENGE TLV. A single AGID TLV
containing the active AGID is appended to Message 2, followed by a
B_CHALLENGE TLV and an A_RESPONSE TLV. The B_CHALLENGE TLV is
constructed by generating its Nonce field uniformly at random.
The A_RESPONSE TLV contains a message authentication code (MAC)
value computed over the complete Message 1, also containing the
A_CHALLENGE, the B_CHALLENGE, the A_RESPONSE (set to zero for the
purpose of the MAC calculation) and including the NLS header,
using the secret key associated with the AGID. Device B then
sends Message 2 to Device A.
o In the NLS-TL header in Message 2, the AX_CHALLENGE and
AX_RESPONSE flags must be set
o If the optional ECHO TLV is used, it must be placed after the AGID
(i.e. between the AGID and the A_CHALLENGE)
For the purpose of the MAC calculation for A_RESPONSE, the "entire
NLS message" is:
HDR1||REQUEST||AGID||A_CHALLENGE||A_RESPONSE||B_CHALLENGE
Message 2 (inbound). Device A processes Message 2 by performing the
following checks:
o Verifying that the AGID in the message is associated with the
Application ID in the NLS message. If it is not, then the
Authentication Exchange cannot be successfully completed, an error
message of "Insufficient authorization" SHOULD be returned, and
the connection MUST be abandoned.
o Verifying that the TLVs other than the security TLVs in Message 2
match the non-security TLVs in Message 1. The two messages should
be bitwise identical, besides the security TLVs (and the transport
headers below the NLS header). If the messages do not match, then
the Authentication Exchange cannot be successfully completed, an
error message of "Request/reply mismatch" SHOULD be returned, and
the connection MUST be abandoned.
o If the other checks pass, then Device A computes its own value of
the A_RESPONSE TLV, using as input the key associated with the
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AGID in the message, and the locally cached copy of Message 2.
Note that it may be necessary to make a temporary copy of the
value of the A_RESPONSE MAC field before setting that field to
zero, in order to compare the locally computed value to the
received value.
o If the locally constructed A_RESPONSE does not match the
A_RESPONSE in Message 2, then the Authentication Exchange cannot
be successfully completed, an error message of "Authentication
failure" SHOULD be returned, and the connection MUST be abandoned.
If all of those those steps are passed, then Message 3 is computed as
described below.
o Message 3 (outbound): Device A constructs Message 3 as follows.
The NLS header is identical to that of Message 2, except that the
AX_RESPONSE flag is set, and the AX_CHALLENGE flag is not set.
The TLVs from Message 2 are copied verbatim into Message 3, in
order, except for the A_RESPONSE TLV. An A_CHALLENGE and
B_RESPONSE TLV are appended to Message 3. The B_RESPONSE TLV is
constructed by computing the MAC over the entire NLS message from
the header up to and including the B_RESPONSE TLV (with the MAC
field set to zero), using the secret key associated with the AGID.
Device A then sends Message 3 to Device B.
o In the Message 3 NLS-TL header, the AX_RESPONSE flag must be set
o If the optional ECHO TLV is used, it MUST follow the AGID (i.e.
between the AGID and the B_RESPONSE).
For the purpose of the MAC calculation for B_RESPONSE, the "entire
NLS message" is:
HDR2||REQUEST||AGID||B_RESPONSE||A_CHALLENGE||B_CHALLENGE
Message 3 (inbound): Device B processes Message 3 by performing the
following checks:
o Verifying that the AGID in the message is associated with the
Application ID in the NLS message. If it is not, then the
Authentication Exchange cannot be successfully completed, an error
message of "Insufficient authorization" SHOULD be returned, and
the connection MUST be abandoned.
o Computing its own value of B_RESPONSE, by computing the MAC over
the entire NLS message from the header up to and including the
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B_RESPONSE TLV (with the MAC field set to zero), using the secret
key associated with the AGID. If the locally constructed
B_RESPONSE does not match the one received in Message 3, then the
message is rejected, and an error message of "Authentication
failure" SHOULD be returned. Note that it may be necessary to
make a temporary copy of the value of the B_RESPONSE MAC field
before setting that field to zero, in order to compare the locally
computed value to the received value.
After an authentication exchange has completed sucessfully and a
single AGID has been negotiated, the nonces sent to and received from
the peer MUST both be saved for use with the AUTHENTICATION TLV.
Also, the sequence number associated with the nonces MUST be set to 0
immediately after finishing the exchange successfully, and before
using an AUTHENTICATION TLV. Should any previous state exist (i.e.
previous nonces and sequence numbers), these MUST be replaced by the
new nonces (and the sequence numbers reset to 0).
After these checks pass, then the body of the NLS message, with the
B_RESPONSE TLV removed, is proccessed by the NLS application, that
is, it is processed in the manner determined by the Application ID.
4.6.2. Authentication TLV calculation
The AUTHENTICATION TLV is calculated over the entire packet (as
described in the next paragraph) as well as the nonce from the last
challenge received from (or sent to, in the case of the receiver) the
peer (from either A_CHALLENGE or B_CHALLENGE). The nonce is
associated with a sequence number, which helps guard against replay
attacks. The AUTHENTICATION TLV MUST be at the end of the TLV
stream.
The appropriate MAC algorithm to be used is negotiated in a previous
Challenge/Response exchange, where AGID TLV's were exchanged and one
single AGID was agreed upon.
The sender:
o Add an empty AUTHENTICATION TLV to the end of the TLV stream (i.e.
with the HMAC field set to all 0's).
o Find the last nonce received from the peer in either an
A_CHALLENGE or B_CHALLENGE.
o Increment the sequence number associated with the nonce by one,
and write it into the 'sequence number' field of the
AUTHENTICATION TLV
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o Calculate the HMAC from the concatenation of the entire packet
(with header and the incomplete AUTHENTICATION TLV) and the nonce.
Write the resulting HMAC value into the HMAC field of the
AUTHENTICATION TLV.
o Send the packet.
The receiver:
o Copy the HMAC field from the AUTHENTICATION TLV into local
storage, and overwrite the HMAC value in the packet with all 0's.
Find the last nonce sent to the peer in either an A_CHALLENGE or
B_CHALLENGE
o Calculate the HMAC from the concatenation of the entire packet
(with header and the incomplete AUTHENTICATION TLV) and the nonce.
o Compare the calculated value to the value copied out int he first
step.
o If the values match, check to see if the sequence number falls
into the range of valid sequence number (as determined by a
sliding window), and if so, the sliding window is updated.
o If the values do NOT match, the packet MUST be discarded and an
error message SHOULD be returned to the sender (rate-limited to
prevent DoS attacks).
The sliding window SHALL be done according to [RFC4303] Appendix A2.
4.6.3. Security state transition table
The security state transitions are as follows:
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+---------------------+-----------------------+---------------------+
| State name | Event | Transition next |
| | | state |
+---------------------+-----------------------+---------------------+
| Closed | Send unprotected | Closed |
| | message | |
| | | |
| Closed | Send Message 1 | Waiting for Message |
| | | 2 |
| | | |
| Closed | Accept Message 1, | Waiting for Message |
| | send Message 2 | 3 |
| | | |
| Waiting for Message | Timeout expired | Closed |
| 2 | | |
| | | |
| Waiting for Message | Reject invalid | Closed |
| 2 | message | |
| | | |
| Waiting for Message | Accept Message 2 | Secure connection |
| 2 | | established |
| | | |
| Waiting for Message | Timeout expired | Closed |
| 3 | | |
| | | |
| Waiting for Message | Reject invalid | Closed |
| 3 | message | |
| | | |
| Waiting for Message | Accept Message 3 | Secure connection |
| 3 | | established |
| | | |
| Secure connection | Send authenticated | Secure connection |
| established | message | established |
+---------------------+-----------------------+---------------------+
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5. Application Interface
Application payloads are encapsulated within NLS-TL TLVs, and MUST
follow any NAT TLVs.
The Application Payload TLV carries includes the Application ID
field, which is used to vector the requests off to the correct
application on the router upon receipt. It is also used to identify
NAT_ADDRESS TLVs to be passed to the application. In a nutshell, if
the Application ID in a NAT_ADDRESS TLV matches the Application ID in
an Application TLV, the NAT_ADDRESS TLV must be passed to the
application along with the application payload.
The Length field carries the total application payload length,
excluding the header, in octets. The length must be at least 4 and
be a multiple of 4. It may be necessary for an application to pad
its payload to accomplish that.
Note that there is no identifier in the TLV other than the
Application ID. If there is a need for an application-specific
identifer for reservations or other applications requiring retained
state, those must be added to the application payload.
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6. NAT Interactions
NLS uses IP addresses for routing, both end-to-end and hop-by-hop.
Given the applications which NLS-TL will be transporting, it is
highly likely that those applications will be using payload-embedded
addresses and there will be some interactions. The use of a NAT
application together with other applications can mitigate this, but
there will be problems transiting non-NLS-capable NATs.
When an NLS entity receives an TL message travelling in the forward
direction, it writes the address in the IPv4_HOP or IPv6_HOP, as
appropriate, from the packet into local per-session state and
replaces the HOP data in the message with the address of the outgoing
interface. When the entity is a NAT, it will write the translated-to
address. Note that while it is usually the case that payload
integrity protection breaks in the presence of NATs if embedded
addresses are being rewritten, this is not substantially different
from the rewriting of the HOP field which occurs within NLS anyway.
However, if an NLS message crosses a non-NLS-capable NAT, several
problems may occur. The first is that if the message is being
dropped in a raw IP packet, the NAT may simply drop the packet
because it doesn't know how to treat it. Another is that the address
in the HOP field will be incorrect. NLS and the applications it
carries cannot be expected to function properly across non-
participating NATs. Discovery of a non-NLS-capable NAT is described
in Section 8.
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7. Using NLS-TL as a stand-alone NAT traversal protocol
Using the NLS Transport Layer as a stand-alone NAT traversal protocol
is straightforward -- simply use the TL without application payloads,
but set the NO_REWRITE flag in the NAT_ADDRESS TLV to 1. This
provides two functions: 1) installation of new NAT table mappings,
and 2) allowing the sender to learn what the "external" mappings are.
The Application ID field in the NAT_ADDRESS TLV must be set to 0.
The TL header flags in the forward direction must be
HOP-BY-HOP = 0
BUILD-ROUTE = 1
TEARDOWN = 0
The TL header flags in the reverse direction (i.e. in the response
message) must be
HOP-BY-HOP = 1
BUILD-ROUTE = 0
TEARDOWN = 0
The NAT table mappings are kept fresh through the retransmission of
the request every refresh period. The refresh messages are identical
to the original request message.
If a response message is not received, the retransmission and backoff
procedures described in Section 6 of [RFC2961] MUST be used.
When the NAT table mappings are no longer required, the sender must
send a teardown message containing the Flow ID of the installed
mappings and with the TL flags set to
HOP-BY-HOP = 0
BUILD-ROUTE = 0
TEARDOWN = 1
An acknowledgement response message is not required. If there has
been no refresh message received prior to the expiration of the
timeout period, the NAT table mappings must be deleted when the
timeout period ends.
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8. Discovery of non-NLS NATs, and recovery
This section describes a method of discovering non-NLS NATs in the
path, and a recovery-mechanism if one is discovered.
When there are non-NLS-capable NATs in the path, they will only be
able to process or modify the IP/UDP header of the NLS-TL message and
will not be able to understand or modify the NLS-TL message itself
(including the NAT_ADDRESS_TLV inside).
If there are non-NLS NATs in the path the sender needs to be made
aware of this, and it should be able to fall back to processing
without NLS, using any other mechanisms that may be available. Also,
the NLS_ NATs in the path which have allocated the NAT mappings based
on NLS NAT_ADDRESS_TLV processing, need to be able to release these
mappings.
The following algorithm can be applied for non-NLS NAT detection by
NLS nodes :
if (NAT_TL NAT_ADDRESS_TLV's mapped_addr == 0) {
This NLS_TL NAT is first NLS_TL NAT in path
if (NLS_TL packet's source IP address != NAT_ADDRESS_TLV's
original_address) {
This NLS_TL NAT is not the first in the path, and
some non-NLS_TL NAT has touched this packet;
send NLS_TL error message back to the sender
with NLS_TL error-code = 4 (non-nls-nat in path)
} else {
This NLS_TL NAT is the first in the path, and no non-
NLS_TL NAT has touched this packet;
proceed with NLS_TL processing.
}
} else {
This NLS_TL NAT is not the first NLS_TL NAT in path.
if (NLS_TL packet's source IP address != NAT_ADDRESS_TLV's
mapped_address) {
Some non-NLS_TL NAT has touched this packet, send
NLS_TL error message back to the sender with NLS_TL
error-code = 4 (non-nls-nat in path)
} else {
No non-NLS_TL NAT has touched this packet; proceed
with regular NLS_TL processing.
}
}
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The NLS_TL error message will be relayed back to the sender.
Intermediate NLS nodes should not be processing the NLS error
message, but let this NLS packet be routed back to the sender.
Once the sender sees an NLS_TL error-message with Error-Code = 4
(non-nls-nat in path), it should resend the same NLS_TL message as
earlier with the NAT_ADDRESS_TLV's Original IPv4 Address/Port/
Protocol as earlier and the Mapped IPv4 Address/Port as NULL, but
should set the TEARDOWN flag in the NLS-TL header.
The intermediate NLS NATs in the path, upon seeing an NLS_TL message
with the TEARDOWN bit set, should delete its local NAT mapping
corresponding to the Flow ID and send the message on towards the
receiver, traversing other NLS-capable NATs along the path which will
also process the TEARDOWN message.
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9. Endpoint Processing
This section describes the procedures used in the endpoints (that is
to say, the sender and the receiver) for processing NLS packets.
Note that these are the endpoints for the purposes of describing an
end-to-end NLS path; they may actually be network entities or
proxies.
9.1. Sending
When a host or its proxy wishes to send an NLS application request,
it puts together the application payload and encapsulates it in a
transport layer packet.
If the application needs to request NAT service because of its use of
addresses for reservations, etc., it must create a random 32-bit tag
for use as an address token in the application payload, and it must
create a NAT_ADDRESS TLV in which it inserts the address and port for
which it is requesting NAT service, as well as the 32-bit tag.
For example, in a hypothetical QoS application that needed NAT
services for the address 192.0.2.110, TCP port 6603 in the flow
description, it would generate the random tag 0x24924924, use that in
the application payload instead of an address, and create a
NAT_ADDRESS TLV with the following values:
Application ID = QoS
Flags = TRANSLATE
Proto = TCP
Address ID = 0x24924924
Original IPv4 Address = 192.0.2.110
Original Port = 6603
The endpoint also needs to set the flags that determine how path
establishment and routing are to be handled on intermediate nodes.
In some cases the application requires no stored state in NLS nodes
or it simply requires a single NLS pass. Examples of this kind of
application include topology discovery, tunnel endpoint discovery, or
diagnostic triggers. In this case, in the NLS-TL header both the
HOP-BY-HOP flag and the BUILD-ROUTE flag are set to 0.
If an application is establishing per-node state and wants the NLS
transport layer to establish and pin NLS routing for it, as might be
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the case with a QoS application or a firewall pinholing application,
the sending endpoint must set the BUILD-ROUTE flag to 1 and the HOP-
BY-HOP flag to 0.
The endhost then UDP encapsulates the NLS-TL packet, and transmits it
on UDP port 7549.
9.2. Receiving
An NLS node "knows" that it's an endpoint or proxy when the following
conditions are satisfied:
if (IP destination address == my address) {
if (HOP_BY_HOP)
if (next hop data available)
forward it on;
else
it's mine;
}
When an endpoint receives a packet and identifies it as terminating
there, it demultiplexes the payload and passes the payload and
associated NAT_ADDRESS data to the appropriate application.
If an application in the payload is not supported by the endpoint,
the endpoint must return a message to the sender with an ERROR_CODE
TLV with the error value set to 3 (Unrecognized application).
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10. Intermediate node processing
The processing of NLS-TL packets at intermediate nodes is
substantially the same as processing at endpoints. Upon the arrival
of a request, the node demultiplexes the packet contents and vectors
the application payloads off to their respective applications.
One major difference from endpoint processing is the handling of NAT
requests by NAT intermediate nodes. When an NLS-capable NAT receives
an NLS request, it checks for the presence of NAT_ADDRESS TLVs. For
each NAT_TLV, it executes the process described in Section 4.5.
For state maintenance and forwarding, the node must follow the
processes described in Section 4.1, Section 4.2, and Section 4.4.
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11. Using NLS-TL to support bidirectional reservations
When an application that uses NLS-TL to transport reservation
requests (for example, QoS reservations or firewall pinholes) and it
wishes to make the request for a bidirectional data stream, the
reservations should be made when the message is received in the
"forward" direction. Note that this is a significant departure from
the model used in RSVP and assumed in previous versions of NLS-TL.
The reason for this should be apparent -- if the route between the
sender and receiver is asymmetric, it is possible that a device
traversed by a PATH message may not be traversed by a RESV message,
and vice-versa.
It may be desirable to have different characteristics for the
reservation in one direction than for the other. In this case the
NLS application designer should make provision for identifying
reservation specifications to be used in each direction.
It should also not be assumed, as is done in RSVP, that error
messages will traverse all affected nodes unless care is taken by the
sender, or the "owner" of the reservation, to ensure that error
messages are propagated correctly. So, for example, if a reservation
fails at a particular node, it may not be sufficient to return the
error message towards the sender.
An application that manages reservations may wish to refresh
application state more frequently than it wishes to refresh route
state. In that case it should send the message with the
BIDIRECTIONAL and HOP_BY_HOP flags set, and the BUILD_ROUTE flag set
to 0.
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12. Security Considerations
12.1. Overview
This section describes a method for providing cryptographic
authentication to the Network Layer Signaling (NLS) transport layer
protocol. The method incorporates a peer discovery mechanism.
Importantly, there is no provision for confidentiality. This fact
simplifies the protocol, and removes the need for export control on
products implementing it. NLS applications which require
confidentiality may provide it themselves.
This mechanism provides both entity and message authentication along
a single hop. In other words, the device on each end of the hop is
assured that the identity of the other device, and the content of the
message from that device, are correct. These security services are
provided only on a hop-by-hop basis. That is, there are no
cryptogrpahic services provided across multiple hops, and each hop
can independently use or not use authentication. In the following,
we restrict our discussion to a single hop along an NLS path.
In order to support authentication, we introduce an optional two-
message exchange into NLS called the Authentication Exchange, or AX.
This exchange is needed in order to carry the challenge-response
information, and is described in detail in Section 4.6.
12.2. Security Model
Authenticated NLS-TL provides both authorization and entity
authentication using a group model. Authorizations correspond to
particular applications. An Authorization Group (AG) is a set of
network interfaces that share the following information:
o a list of NLS Application IDs; these correspond to applications
which the group is authorized to use,
o a group authentication key,
o a Message Authentication Code (MAC) algorithm type
Note that AGs are associated with interfaces and not devices since in
many situations there are different trust levels associated with
different interfaces.
For each device implementing Authenticated NLS-TL, each interface is
associated with a list of Application IDs, each of which is
associated with:
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o a list of AGIDs that authorize the corresponding application, or
o the symbol ALLOW, which indicates that the application has been
explictly allowed on the associated interface, or
o the symbol DROP, which indicates that the application has been
explicitly disallowed on the associated interface.
This model provides authorization at the NLS-TL layer. For example,
it is impossible to authorize VoIP traversal of a firewall while
still disallowing telnet across the firewall. The model can be
expanded to accomodate finer grained authorizations, but this issue
is not considered further in this draft. Sensitive applications,
such as firewall pinholing, as well as applications requiring finer-
grained authorizations, must provide their own authentication and
authorization.
12.3. Cryptography
Authenticated NLS-TL uses a single cryptographic function: a
pseudorandom function that accepts arbitrary-length inputs and
produces fixed-length outputs. This function is used as a message
authentication code (MAC).
The default MAC algorithm is HMAC SHA1, with a length truncated to 96
bits. No other message authentication code is defined. Other MACs
MAY be implemented. Each key used in NLS is associated with a single
MAC algorithm; thus crypto algorithm agility is supported by the same
protocol mechanisms that support key agility. In particular, an NLS
device can determine the MAC algorithm used by referencing the Value
field of the Authorization Group ID, or AGID, (defined in
Section 2.3.2.8).
12.3.1. Keys
Authenticated NLS-TL uses group keys, in order to reduce the amount
of protocol state and to mitigate the peer-discovery problem.
Keys may be acquired or provisioned one of three ways: 1) through
manual provisioning, 2) by being fetched from a central data store,
or 3) through an automated key management system.
Implementations MUST provide a way to set and delete keys manually.
However, they SHOULD also provide an automated group key management
system such as GDOI [RFC3547], so that efficient revocation is
possible.
When using an automated key management system, the AGID MAY be used
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to differentiate key versions as well as authorization groups, to
support rekeying. It can be expected to change over the lifetime of
a message flow, while when using manually-provisioned keys the AGID
may be expected to remain static.
12.3.2. Reflection Attacks
NLS is designed to resist reflection attacks. That family of attacks
works against poorly designed mutual authentication systems by
tricking one party into providing the response for its own challenge.
In order to resist reflection attacks, distinct TLV types are defined
for the first and second challenges, the A_CHALLENGE and B_CHALLENGE.
This fact ensures that the two invocations of the MAC during a single
challenge/response exchange will necessarily have different inputs,
thus thwarting reflection attacks.
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13. IANA Considerations
There are several parameters for which NLS-TL will need registry
services. These include
o a registry for NLS Application IDs (NLS Application Identifiers)
and for
o NLS-TL TLV identifiers (NLS TLVs).
o NLS-TL Error codes and error values
Initial values are given below. Future assignments are to be made
through expert review.
NLS-TL also uses UDP port number 7549.
13.1. NLS Application Identifiers
NAME VALUE DEFINITION
Network Address Translation 0 NAT
Control Point Discovery 1 PacketCable CDP
Firewall Traversal 2
Note that while there is no application payload for NAT, we define an
application ID to support NLS-TL authentication when using an
automated key management system. The details of the interface to a
key management system are out of the scope of this document.
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13.2. NLS TLVs
+---------------------+-------+----------------------+
| NAME | VALUE | DEFINITION |
+---------------------+-------+----------------------+
| NAT_ADDRESS | 1 | See Section 2.3.2.1 |
| | | |
| APPLICATION_PAYLOAD | 2 | See Section 2.3.2.2 |
| | | |
| TIMEOUT | 3 | See Section 2.3.2.3 |
| | | |
| IPV4_HOP | 4 | See Section 2.3.2.4 |
| | | |
| IPV6_HOP | 5 | See Section 2.3.2.5 |
| | | |
| IPV4_ERROR_CODE | 6 | See Section 2.3.2.6 |
| | | |
| IPV6_ERROR_CODE | 7 | See Section 2.3.2.7 |
| | | |
| AGID | 8 | See Section 2.3.2.8 |
| | | |
| A_CHALLENGE | 9 | See Section 2.3.2.9 |
| | | |
| A_RESPONSE | 10 | See Section 2.3.2.10 |
| | | |
| B_CHALLENGE | 11 | See Section 2.3.2.11 |
| | | |
| B_RESPONSE | 12 | See Section 2.3.2.12 |
| | | |
| AUTHENTICATION | 13 | See Section 2.3.2.13 |
| | | |
| ECHO | 14 | See Section 2.3.2.14 |
+---------------------+-------+----------------------+
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13.3. Error codes and values
NAME VALUE DEFINITION
Error Code 0 No error
Error Code 1 Bad parameters
Error Value 1 HOP-BY-HOP and BUILD-ROUTE both
present
Error Value 2 BUILD-ROUTE present but no HOP TLV
Error Value 3 HOP-BY-HOP present but
no local stored routing state
Error Value 4 Message length not a multiple of 4
Error Code 2 Unrecognized TLV
Error Code 3 Unrecognized application
Error Code 4 Non-NLS NAT detected in path
Error Code 5 Security error
Error Value 1 AGID not found
Error Value 2 Insufficient authorization
Error Value 3 Request/reply mismatch
Error Value 4 Authentication Failure
Error Code 128 No message
Error Code 129 Sending node has
detected a route change
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14. References
14.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2205] Braden, B., Zhang, L., Berson, S., Herzog, S., and S.
Jamin, "Resource ReSerVation Protocol (RSVP) -- Version 1
Functional Specification", RFC 2205, September 1997.
[RFC2961] Berger, L., Gan, D., Swallow, G., Pan, P., Tommasi, F.,
and S. Molendini, "RSVP Refresh Overhead Reduction
Extensions", RFC 2961, April 2001.
[RFC3547] Baugher, M., Weis, B., Hardjono, T., and H. Harney, "The
Group Domain of Interpretation", RFC 3547, July 2003.
[RFC4303] Kent, S., "IP Encapsulating Security Payload (ESP)",
RFC 4303, December 2005.
14.2. Informative References
[RFC1633] Braden, B., Clark, D., and S. Shenker, "Integrated
Services in the Internet Architecture: an Overview",
RFC 1633, June 1994.
[RFC2711] Partridge, C. and A. Jackson, "IPv6 Router Alert Option",
RFC 2711, October 1999.
[RFC4080] Hancock, R., Karagiannis, G., Loughney, J., and S. Van den
Bosch, "Next Steps in Signaling (NSIS): Framework",
RFC 4080, June 2005.
[RFC4251] Ylonen, T. and C. Lonvick, "The Secure Shell (SSH)
Protocol Architecture", RFC 4251, January 2006.
[braden] Braden, R. and R. Lindell, "A Two-Level Architecture for
Internet Signaling", draft-braden-2level-signaling-01.txt
(work in progress), November 2002.
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Appendix A. Acknowledgements
The authors would like to express their gratitude to Jan Vilhuber,
Senthil Sivakumar, Bill Foster, and Dan Wing for their careful review
and feedback. Special thanks to Jan for his text on challenge/
response calculations, and to Rajesh Karnik and Lisa Fang for their
help in clarifying the text describing the authentication exchange.
Brian Weis and Sheela Rowles provided invaluable discussion of
automated group key management.
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Authors' Addresses
Melinda Shore
Cisco Systems
809 Hayts Road
Ithaca, New York 14850
USA
Email: mshore@cisco.com
David A. McGrew
Cisco Systems
510 McCarthy Blvd
Milpitas, California 95035
USA
Email: mcgrew@cisco.com
Kaushik Biswas
Cisco Systems
510 McCarthy Blvd
Milpitas, California 95035
USA
Email: kbiswas@cisco.com
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