Internet Engineering Task Force (IETF) C. Pignataro
Request for Comments: 6720 R. Asati
Updates: 5036 Cisco Systems
Category: Standards Track August 2012
The Generalized TTL Security Mechanism (GTSM) for
the Label Distribution Protocol (LDP)
The Generalized TTL Security Mechanism (GTSM) describes a generalized
use of a packet's 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 the
Label Distribution Protocol (LDP).
This specification uses a bit reserved in RFC 5036 and therefore
updates RFC 5036.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
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Table of Contents
1. Introduction ....................................................21.1. Specification of Requirements ..............................31.2. Scope ......................................................32. GTSM Procedures for LDP .........................................42.1. GTSM Flag in the Common Hello Parameter TLV ................42.2. GTSM Sending and Receiving Procedures for LDP Link Hello ...52.3. GTSM Sending and Receiving Procedures for LDP
Initialization .............................................53. LDP Peering Scenarios and GTSM Considerations ...................54. Security Considerations .........................................65. Acknowledgments .................................................76. References ......................................................76.1. Normative References .......................................76.2. Informative References .....................................81. 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 Label Switching Router (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
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 protect an 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
1. The 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 Initialization message
GTSM specifies that "it SHOULD NOT be enabled by default in order to
remain backward compatible with the unmodified protocol" (see Section
3 of [RFC5082]). This document specifies a "built-in dynamic GTSM
capability negotiation" for LDP to suggest the use of GTSM. GTSM
will be used as specified in this document provided both peers on an
LDP session can detect each others' support for GTSM procedures and
agree to use it. That is, the desire to use GTSM (i.e., its
negotiation mechanics) is enabled by default without any
This specification uses a bit reserved in Section 3.5.2 of [RFC5036]
and therefore updates [RFC5036].
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].
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 [LDP-IPV6].
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 (such as LDP IGP
Synchronization [RFC5443] or LDP Session Protection [LDP-SPROT]) that
relies on multi-hop LDP peering sessions would not work with GTSM and
will require (statically or dynamically) disabling the GTSM
capability. See Section 3.
2. GTSM Procedures for LDP
2.1. GTSM Flag in the Common Hello Parameter TLV
A new flag in the Common Hello Parameter TLV, named G flag (for
GTSM), is defined by this document in a previously reserved bit. 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].
A value of 1 specifies that this LSR supports GTSM procedures,
where a value of 0 specifies that this LSR does not support GTSM.
This field is reserved. It MUST be set to zero on transmission
and ignored on receipt.
Figure 1: GTSM Flag in the Common Hello Parameter TLV
The G flag is meaningful only if the T flag is set to 0 (which must
be the case for Basic Discovery); otherwise, the value of the G flag
is ignored on receipt.
Any LSR not supporting GTSM for LDP as defined in this document
(i.e., an LSR that does not recognize the G flag) would continue to
ignore the G flag, independent of the values of the T and R flags, as
per Section 3.5.2 of [RFC5036]. Similarly, an LSR that does
recognize the G flag but that does not support GTSM (either because
it is not implemented or because it is so configured) would not set
the G flag (i.e., G=0) when sending LDP Link Hellos and would
effectively ignore the G flag when receiving LDP Link Hello messages.
2.2. GTSM Sending and Receiving Procedures for LDP Link Hello
First, LSRs using LDP Basic Discovery [RFC5036] send LDP Hello
messages to link-level multicast address (220.127.116.11 or "all
routers"). Such messages are never forwarded beyond one hop and are
RECOMMENDED to have their IP TTL or Hop Count = 1.
Unless configured otherwise, an LSR that supports GTSM procedures
MUST set the G flag (for GTSM) to 1 in the Common Hello Parameter TLV
in the LDP Link Hello message [RFC5036].
If an LSR that supports GTSM and is configured to use it recognizes
the presence of the G flag (in the Common Hello Parameter TLV) 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 the G flag (in the
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 deactivation.
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, then the LSR MUST enforce GTSM
procedures, as defined in Section 3 of [RFC5082], in the forthcoming
TCP Transport Connection with that neighbor. 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 [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,
then the LSR MUST NOT enforce GTSM procedures, as defined in Section
3 of [RFC5082], in the forthcoming TCP Transport Connection with that
3. LDP Peering Scenarios and GTSM Considerations
This section discusses GTSM considerations arising from the LDP
peering scenarios used, including single-hop versus multi-hop LDP
neighbors, as well as the use of LDP Basic Discovery versus Extended
The reason that the GTSM capability negotiation is enabled for Basic
Discovery by default (i.e., G=1) but not for Extended Discovery is
that the usage of Basic Discovery typically relates to a single-hop
LDP peering session, whereas the usage of Extended Discovery
typically relates to 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
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 set up
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
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; hence, it would
prohibit the LDP peering session from getting established (note that
this may impact features such as LDP IGP Synchronization [RFC5443] or
LDP Session Protection [LDP-SPROT]). In the third case (c), GTSM is
enabled by default for Basic Discovery and enforced on the subsequent
LDP peering, and is 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 that would be enabled with GTSM. Otherwise, GTSM would not
be enforced on the second LDP peering session corresponding to the
This document allows for the implementation to provide an option to
statically (e.g., via configuration) and/or dynamically override the
default behavior and enable/disable GTSM on a per-peer basis. This
would address all the exceptions listed above.
4. Security Considerations
This document increases the security for LDP, making it more
resilient to off-link attacks. Security considerations for GTSM are
detailed in Section 5 of [RFC5082].
As discussed in Section 3, it is possible that
o GTSM for LDP may not always be enforced on a single-hop LDP
peering session, and LDP may still be susceptible to forged/
spoofed protocol packets, if a single-hop LDP peering session is
set up using Extended Discovery.
o GTSM for LDP may cause the LDP peering session to not get
established (or may be torn down), if IP routing ever declares
that the directly connected peer is more than one IP hop away.
Suffice to say, use of cryptographic integrity (e.g., [RFC5925])
is recommended as an alternate solution for detecting forged
protocol packets (especially for the multi-hop case).
The GTSM specification [RFC5082] says that protocol messages used for
dynamic negotiation of GTSM support MUST be authenticated. However,
LDP discovery [RFC5036] uses UDP transport and does not have an
authentication mechanism. The GTSM specification further elaborates
by saying that GTSM is not a substitute for authentication and does
not secure against insider on-the-wire attacks. LDP Basic Discovery
uses link-level multicast address (18.104.22.168 or "all routers") that
are never forwarded beyond the link, and this acts as a basic
protection against off-the-wire attacks.
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, Bin Mo, Mach Chen,
Vero Zheng, Adrian Farrel, Eric Rosen, Eric Gray, and Brian Weis for
their thorough reviews and useful comments and suggestions.
6.1. Normative References
[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.