MPLS Working Group C. Pignataro
Internet-Draft R. Asati
Intended status: Standards Track Cisco Systems
Expires: February 23, 2012 August 22, 2011
The Generalized TTL Security Mechanism (GTSM) for Label Distribution
Protocol (LDP)
draft-ietf-mpls-ldp-gtsm-02
Abstract
The Generalized TTL Security Mechanism (GTSM) describes a generalized
use of a packets Time to Live (TTL) (IPv4) or Hop Limit (IPv6) to
verify that the packet was sourced by a node on a connected link,
thereby protecting the router's IP control-plane from CPU utilization
based attacks. This technique improves security and is used by many
protocols. This document defines the GTSM use for Label Distribution
Protocol (LDP).
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on February 23, 2012.
Copyright Notice
Copyright (c) 2011 IETF Trust and the persons identified as the
document authors. All rights reserved.
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include Simplified BSD License text as described in Section 4.e of
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described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Specification of Requirements . . . . . . . . . . . . . . . 3
1.2. Scope . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. GTSM Procedures for LDP . . . . . . . . . . . . . . . . . . . . 4
2.1. GTSM Flag in Common Hello Parameter TLV . . . . . . . . . . 4
2.2. GTSM Sending and Receiving Procedures for LDP Link
Hello . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.3. GTSM Sending and Receiving Procedures for LDP
Initialization . . . . . . . . . . . . . . . . . . . . . . 5
3. Multi-Hop LDP peering Considerations for GTSM . . . . . . . . . 6
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 6
5. Security Considerations . . . . . . . . . . . . . . . . . . . . 7
6. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . 7
7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 7
7.1. Normative References . . . . . . . . . . . . . . . . . . . 7
7.2. Informative References . . . . . . . . . . . . . . . . . . 7
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 8
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1. Introduction
LDP [RFC5036] specifies two peer discovery mechanisms, a Basic one
and an Extended one, both using UDP transport. The Basic Discovery
mechanism is used to discover LDP peers that are directly connected
at the link level, whereas the Extended Discovery mechanism is used
to locate LSR neighbors that are not directly connected at the link
level. Once discovered, the LSR neighbors can establish the LDP
peering session, using the TCP transport connection.
The Generalized TTL Security Mechanism (GTSM) [RFC5082] is a
mechanism based on IPv4 Time To Live (TTL) or (IPv6) Hop Limit value
verification so as to provide a simple and reasonably robust defense
from infrastructure attacks using forged protocol packets from
outside the network. GTSM can be applied to any protocol peering
session that is established between routers that are adjacent.
Therefore, GTSM can fully benefit LDP protocol peering session
established using Basic Discovery.
This document specifies LDP enhancements to accommodate GTSM. In
particular, this document specifies the enhancements in the following
areas:
1. Common Hello Parameter TLV of LDP Link Hello message
2. Sending and Receiving procedures for LDP Link Hello message
3. Sending and Receiving procedures for LDP Initilization message
GTSM specifies that it SHOULD NOT be enabled by default in order to
remain backward-compatible with the unmodified protocol; this
document specifies having a built-in dynamic GTSM capability
negotiation for LDP to suggest the use of GTSM, provided GTSM is not
enabled unless both peers can detect each others' support for GTSM
procedures and agree on its usage as described in this document.
1.1. Specification of Requirements
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].
1.2. Scope
This document defines procedures for LDP using IPv4 routing, but not
for LDP using IPv6 routing, since the latter has GTSM built into the
protocol definition [I-D.ietf-mpls-ldp-ipv6].
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Additionally, the GTSM for LDP specified in this document applies
only to single-hop LDP peering sessions, and not to multi-hop LDP
peering sessions, in line with Section 5.5 of [RFC5082].
Consequently, any LDP method or feature that relies on multi-hop LDP
peering sessions would not work with GTSM and will require
(statically or dynamically) disabling GTSM. See Section 3.
2. GTSM Procedures for LDP
2.1. GTSM Flag in Common Hello Parameter TLV
A new flag in Common Hello Parameter TLV, named G flag (for GTSM), is
defined by this document. An LSR indicates that it is capable of
applying GTSM procedures, as defined in this document, to the
subsequent LDP peering session, by setting the GTSM flag to 1. The
Common Hello Parameters TLV, defined in Section 3.5.2 of [RFC5036],
is updated as shown in Figure 1.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0|0| Common Hello Parms(0x0400)| Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Hold Time |T|R|G| Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
T, Targeted Hello
As specified in [RFC5036].
R, Request Send Targeted Hellos
As specified in [RFC5036].
G, GTSM
A value of 1 specifies that this LSR supports GTSM procedures,
where a value of 0 specifies that this LSR does not support GTSM.
Reserved
This field is reserved. It MUST be set to zero on transmission
and ignored on receipt.
Figure 1: GTSM Flag in Common Hello Parameter TLV
The G flag is meaingful only if T and R flags are set to 0 (which
must be the case for Basic Discovery), otherwise, the value of G flag
SHOULD be ignored on receipt.
Any LSR not supporting GTSM for LDP, as defined in this document,
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would continue to ignore the G flag, independent of T and R flags'
value, as per Section 3.5.2 of [RFC5036].
2.2. GTSM Sending and Receiving Procedures for LDP Link Hello
Firstly, LSRs using LDP Basic Discovery [RFC5036] send LDP Hello
messages to link-level multicast address (224.0.0.2 or "all
routers"). Such messages are never forwarded beyond one hop and
assumed to have their IP TTL or Hop Count = 1.
An LSR that is capable of applying GTSM procedures to the subsequent
TCP/LDP peering session MUST set the G flag (for GTSM) to 1 in Common
Hello Parameter TLV in the LDP Link Hello message [RFC5036].
An LSR, upon receiving an LDP Link Hello message, would recognize the
presence of G flag (in Common Hello Parameter TLV) only if it
supports GTSM for LDP, as specified in this document. If an LSR
recognizes the presence of G flag with the value =1 in the received
LDP Link Hello message, then it MUST enforce GTSM for LDP in the
subsequent TCP/LDP peering session with the neighbor that sent the
Hello message, as specified in Section 2.3 of this document.
If an LSR does not recognize the presence of G flag (in Common Hello
Parameter TLV of Link Hello message), or recognizes the presence of G
flag with the value = 0, then the LSR MUST NOT enforce GTSM for LDP
in the subsequent TCP/LDP peering session with the neighbor that sent
the Hello message. This ensures backward compatibility as well as
automatic GTSM de-activation.
If an LSR that has sent the LDP Link Hello with G flag = 1, then the
LSR MUST set IP TTL or Hop Count = 255 in the forthcoming TCP
Transport Connection(s) with that neighbor (e.g., LSR2). Please see
Section 2.3 for more details about the TCP transport connection
specifics.
2.3. GTSM Sending and Receiving Procedures for LDP Initialization
If an LSR that has sent and received LDP Link Hello with G flag = 1
from the directly-connected neighbor (LSR2), then the LSR MUST
enforce GTSM procedures, as defined in Section 3 of [RFC5082], in the
forthcoming TCP Transport Connection with that neighbor (LSR2). This
means that the LSR MUST check for the incoming unicast packets' TTL
or Hop Count to be 255 for the particular LDP/TCP peering session and
decide the further processing as per the [RFC5082].
If an LSR that has sent LDP Link Hello with G flag = 1, but received
LDP Link Hello with G flag = 0 from the directly-connected neighbor
(LSR2), then the LSR MUST NOT enforce GTSM procedures, as defined in
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Section 3 of [RFC5082], in the forthcoming TCP Transport Connection
with that neighbor (LSR2).
3. Multi-Hop LDP peering Considerations for GTSM
The reason GTSM is enabled for Basic Discovery by default, but not
for Extended Discovery is that the usage of Basic Discovery typically
results in a single-hop LDP peering session, whereas the usage of
Extended Discovery typically results in a multi-hop LDP peering
session. GTSM protection for multi-hop LDP sessions is outside the
scope of this specification (see Section 1.2). However, it is worth
clarifying the following exceptions that may occur with Basic or
Extended Discovery usage:
a. Two adjacent LSRs (i.e., back-to-back PE routers) forming a
single-hop LDP peering session after doing an Extended Discovery
(e.g., for Pseudowire signaling)
b. Two adjacent LSRs forming a multi-hop LDP peering session after
doing a Basic Discovery, due to the way IP routing is setup
between them (either temporarily or permanently)
c. Two adjacent LSRs (i.e. back-to-back PE routers) forming a
single-hop LDP peering session after doing both Basic and
Extended Discovery.
In the first case (a), GTSM is not enabled for the LDP peering
session by default. In the second case (b), GTSM is actually enabled
by default and enforced for the LDP peering session, and hence, it
would prohibit the LDP peering session from getting established. In
the third case (c), GTSM is enabled by default for Basic Discovery
and enforced on the subsequent LDP peering, and not for Extended
Discovery. However, if each LSR uses the same IPv4 transport address
object value in both Basic and Extended discoveries, then it would
result in a single LDP peering session and that would be enabled with
GTSM. Otherwise, GTSM would not be enforced on the second LDP
peering session corresponding to the Extended Discovery.
This document allows for the implementation to provide an option to
statically (e.g., via configuration) and/or dynamically override the
default behavior and disable GTSM on a per-peer basis. This would
address all the exceptions listed above.
4. IANA Considerations
IANA is requested to assign the G, GTSM bit in the Common Hello
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Parameters TLV (see Figure 1 in Section 2.1), as per allocation
policy defined at [I-D.ietf-mpls-ldp-iana].
5. Security Considerations
This document increases the security for LDP, making it more
resilient to off-link attacks.
6. Acknowledgments
The authors of this document do not make any claims on the
originality of the ideas described. The concept of GTSM for LDP has
been proposed a number of times, and is documented in both the
Experimental and Standards Track specifications of GTSM. Among other
people, we would like to acknowledge Enke Chen and Albert Tian for
their document "TTL-Based Security Option for the LDP Hello Message".
The authors would like to thank Loa Andersson and Bin Mo for a
thorough review and most useful comments and suggestions.
7. References
7.1. Normative References
[I-D.ietf-mpls-ldp-iana]
Pignataro, C. and R. Asati, "Label Distribution Protocol
(LDP) Internet Assigned Numbers Authority (IANA)
Considerations Update", draft-ietf-mpls-ldp-iana-01 (work
in progress), May 2011.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC5036] Andersson, L., Minei, I., and B. Thomas, "LDP
Specification", RFC 5036, October 2007.
[RFC5082] Gill, V., Heasley, J., Meyer, D., Savola, P., and C.
Pignataro, "The Generalized TTL Security Mechanism
(GTSM)", RFC 5082, October 2007.
7.2. Informative References
[I-D.ietf-mpls-ldp-ipv6]
Manral, V., Papneja, R., Asati, R., and C. Pignataro,
"Updates to LDP for IPv6", draft-ietf-mpls-ldp-ipv6-04
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(work in progress), May 2011.
Authors' Addresses
Carlos Pignataro
Cisco Systems
7200-12 Kit Creek Road
Research Triangle Park, NC 27709
US
Email: cpignata@cisco.com
Rajiv Asati
Cisco Systems
7025-6 Kit Creek Road
Research Triangle Park, NC 27709
US
Email: rajiva@cisco.com
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