draft-ietf-rtgwg-rfc3682bis-10.txt   rfc5082.txt 
Routing WG V. Gill Network Working Group V. Gill
Internet-Draft J. Heasley Request for Comments: 5082 J. Heasley
Obsoletes: 3682 (if approved) D. Meyer Obsoletes: 3682 D. Meyer
Intended status: Standards Track P. Savola, Ed. Category: Standards Track P. Savola, Ed.
Expires: December 27, 2007 C. Pignataro C. Pignataro
June 25, 2007 October 2007
The Generalized TTL Security Mechanism (GTSM) The Generalized TTL Security Mechanism (GTSM)
draft-ietf-rtgwg-rfc3682bis-10.txt
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Copyright (C) The IETF Trust (2007). This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
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and status of this protocol. Distribution of this memo is unlimited.
Abstract Abstract
The use of a packet's Time to Live (TTL) (IPv4) or Hop Limit (IPv6) The use of a packet's Time to Live (TTL) (IPv4) or Hop Limit (IPv6)
to verify whether the packet was originated by an adjacent node on a to verify whether the packet was originated by an adjacent node on a
connected link has been used in many recent protocols. This document connected link has been used in many recent protocols. This document
generalizes this technique. This document obsoletes Experimental RFC generalizes this technique. This document obsoletes Experimental RFC
3682. 3682.
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Assumptions Underlying GTSM . . . . . . . . . . . . . . . . . 3 2. Assumptions Underlying GTSM . . . . . . . . . . . . . . . . . 3
2.1. GTSM Negotiation . . . . . . . . . . . . . . . . . . . . . 4 2.1. GTSM Negotiation . . . . . . . . . . . . . . . . . . . . . 4
2.2. Assumptions on Attack Sophistication . . . . . . . . . . . 4 2.2. Assumptions on Attack Sophistication . . . . . . . . . . . 4
3. GTSM Procedure . . . . . . . . . . . . . . . . . . . . . . . . 5 3. GTSM Procedure . . . . . . . . . . . . . . . . . . . . . . . . 5
4. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 7 4. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 6
5. Security Considerations . . . . . . . . . . . . . . . . . . . 7 5. Security Considerations . . . . . . . . . . . . . . . . . . . 6
5.1. TTL (Hop Limit) Spoofing . . . . . . . . . . . . . . . . . 7 5.1. TTL (Hop Limit) Spoofing . . . . . . . . . . . . . . . . . 7
5.2. Tunneled Packets . . . . . . . . . . . . . . . . . . . . . 7 5.2. Tunneled Packets . . . . . . . . . . . . . . . . . . . . . 7
5.2.1. IP Tunneled over IP . . . . . . . . . . . . . . . . . 8 5.2.1. IP Tunneled over IP . . . . . . . . . . . . . . . . . 8
5.2.2. IP Tunneled over MPLS . . . . . . . . . . . . . . . . 9 5.2.2. IP Tunneled over MPLS . . . . . . . . . . . . . . . . 9
5.3. Onlink Attackers . . . . . . . . . . . . . . . . . . . . . 11 5.3. Onlink Attackers . . . . . . . . . . . . . . . . . . . . . 11
5.4. Fragmentation Considerations . . . . . . . . . . . . . . . 11 5.4. Fragmentation Considerations . . . . . . . . . . . . . . . 11
5.5. Multi-Hop Protocol Sessions . . . . . . . . . . . . . . . 12 5.5. Multi-Hop Protocol Sessions . . . . . . . . . . . . . . . 12
6. Applicability Statement . . . . . . . . . . . . . . . . . . . 12 6. Applicability Statement . . . . . . . . . . . . . . . . . . . 12
6.1. Backwards Compatibility . . . . . . . . . . . . . . . . . 13 6.1. Backwards Compatibility . . . . . . . . . . . . . . . . . 12
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 13 7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 13
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 13 7.1. Normative References . . . . . . . . . . . . . . . . . . . 13
8.1. Normative References . . . . . . . . . . . . . . . . . . . 13 7.2. Informative References . . . . . . . . . . . . . . . . . . 14
8.2. Informative References . . . . . . . . . . . . . . . . . . 14 Appendix A. Multi-Hop GTSM . . . . . . . . . . . . . . . . . . . 15
Appendix A. Multi-hop GTSM . . . . . . . . . . . . . . . . . . . 15
Appendix B. Changes Since RFC3682 . . . . . . . . . . . . . . . 15 Appendix B. Changes Since RFC3682 . . . . . . . . . . . . . . . 15
Appendix C. Draft Changelog . . . . . . . . . . . . . . . . . . 15
Appendix C.1. Changes between -09 and -10 . . . . . . . . . . . . 15
Appendix C.2. Changes between -08 and -09 . . . . . . . . . . . . 16
Appendix C.3. Changes between -07 and -08 . . . . . . . . . . . . 16
Appendix C.4. Changes between -06 and -07 . . . . . . . . . . . . 16
Appendix C.5. Changes between -05 and -06 . . . . . . . . . . . . 17
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 17
Intellectual Property and Copyright Statements . . . . . . . . . . 18
1. Introduction 1. Introduction
The Generalized TTL Security Mechanism (GTSM) is designed to protect The Generalized TTL Security Mechanism (GTSM) is designed to protect
a router's IP based control plane from CPU-utilization based attacks. a router's IP-based control plane from CPU-utilization based attacks.
In particular, while cryptographic techniques can protect the router- In particular, while cryptographic techniques can protect the router-
based infrastructure (e.g., BGP [RFC4271], [RFC4272]) from a wide based infrastructure (e.g., BGP [RFC4271], [RFC4272]) from a wide
variety of attacks, many attacks based on CPU overload can be variety of attacks, many attacks based on CPU overload can be
prevented by the simple mechanism described in this document. Note prevented by the simple mechanism described in this document. Note
that the same technique protects against other scarce-resource that the same technique protects against other scarce-resource
attacks involving a router's CPU, such as attacks against processor- attacks involving a router's CPU, such as attacks against processor-
line card bandwidth. line card bandwidth.
GTSM is based on the fact that the vast majority of protocol peerings GTSM is based on the fact that the vast majority of protocol peerings
are established between routers that are adjacent. Thus most are established between routers that are adjacent. Thus, most
protocol peerings are either directly between connected interfaces or protocol peerings are either directly between connected interfaces
at the worst case, are between loopback and loopback, with static or, in the worst case, are between loopback and loopback, with static
routes to loopbacks. Since TTL spoofing is considered nearly routes to loopbacks. Since TTL spoofing is considered nearly
impossible, a mechanism based on an expected TTL value can provide a impossible, a mechanism based on an expected TTL value can provide a
simple and reasonably robust defense from infrastructure attacks simple and reasonably robust defense from infrastructure attacks
based on forged protocol packets from outside the network. Note, based on forged protocol packets from outside the network. Note,
however, that GTSM is not a substitute for authentication mechanisms. however, that GTSM is not a substitute for authentication mechanisms.
In particular, it does not secure against insider on-the-wire In particular, it does not secure against insider on-the-wire
attacks, such as packet spoofing or replay. attacks, such as packet spoofing or replay.
Finally, the GTSM mechanism is equally applicable to both TTL (IPv4) Finally, the GTSM mechanism is equally applicable to both TTL (IPv4)
and Hop Limit (IPv6), and from the perspective of GTSM, TTL and Hop and Hop Limit (IPv6), and from the perspective of GTSM, TTL and Hop
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3. Use of GTSM is OPTIONAL, and can be configured on a per-peer 3. Use of GTSM is OPTIONAL, and can be configured on a per-peer
(group) basis. (group) basis.
4. The peer routers both implement GTSM. 4. The peer routers both implement GTSM.
5. The router supports a method to use separate resource pools 5. The router supports a method to use separate resource pools
(e.g., queues, processing quotas) for differently classified (e.g., queues, processing quotas) for differently classified
traffic. traffic.
Note that this document does not prescribe further restrictions that Note that this document does not prescribe further restrictions that
a router may apply to the packets not matching the GTSM filtering a router may apply to packets not matching the GTSM filtering rules,
rules, such as dropping packets that do not match any configured such as dropping packets that do not match any configured protocol
protocol session and rate-limiting the rest. This document also does session and rate-limiting the rest. This document also does not
not suggest the actual means of resource separation, as those are suggest the actual means of resource separation, as those are
hardware and implementation-specific. hardware and implementation-specific.
The possibility of denial-of-service (DoS) attack prevention, However, the possibility of denial-of-service (DoS) attack prevention
however, is based on the assumption that classification of packets is based on the assumption that classification of packets and
and separation of their paths are done before they go through a separation of their paths are done before the packets go through a
scarce resource in the system. In practice, this means that, the scarce resource in the system. In practice, the closer GTSM
closer GTSM processing is done to the line-rate hardware, the more processing is done to the line-rate hardware, the more resistant the
resistant the system is to the DoS attacks. system is to DoS attacks.
2.1. GTSM Negotiation 2.1. GTSM Negotiation
This document assumes that, when used with existing protocols, GTSM This document assumes that, when used with existing protocols, GTSM
will be manually configured between protocol peers. That is, no will be manually configured between protocol peers. That is, no
automatic GTSM capability negotiation, such as is provided by RFC automatic GTSM capability negotiation, such as is provided by RFC
3392 [RFC3392] is assumed or defined. 3392 [RFC3392], is assumed or defined.
If a new protocol is designed with built-in GTSM support, then it is If a new protocol is designed with built-in GTSM support, then it is
recommended that procedures are always used for sending and recommended that procedures are always used for sending and
validating received protocol packets (GTSM is always on, see for validating received protocol packets (GTSM is always on, see for
example [RFC2461]). If, however, dynamic negotiation of GTSM support example [RFC2461]). If, however, dynamic negotiation of GTSM support
is necessary, protocol messages used for such negotiation MUST be is necessary, protocol messages used for such negotiation MUST be
authenticated using other security mechanisms to prevent DoS attacks. authenticated using other security mechanisms to prevent DoS attacks.
Also note that this specification does not offer a generic GTSM Also note that this specification does not offer a generic GTSM
capability negotiation mechanism, so messages of the protocol capability negotiation mechanism, so messages of the protocol
augmented with the GTSM behavior will need to be used if dynamic augmented with the GTSM behavior will need to be used if dynamic
negotiation is deemed necessary. negotiation is deemed necessary.
2.2. Assumptions on Attack Sophistication 2.2. Assumptions on Attack Sophistication
Throughout this document, we assume that potential attackers have Throughout this document, we assume that potential attackers have
evolved in both sophistication and access to the point that they can evolved in both sophistication and access to the point that they can
send control traffic to a protocol session, and that this traffic send control traffic to a protocol session, and that this traffic
appears to be valid control traffic (i.e., has the source/destination appears to be valid control traffic (i.e., it has the source/
of configured peer routers). destination of configured peer routers).
We also assume that each router in the path between the attacker and We also assume that each router in the path between the attacker and
the victim protocol speaker decrements TTL properly (clearly, if the victim protocol speaker decrements TTL properly (clearly, if
either the path or the adjacent peer is compromised, then there are either the path or the adjacent peer is compromised, then there are
worse problems to worry about). worse problems to worry about).
For maximal protection, ingress filtering should be applied before For maximal protection, ingress filtering should be applied before
the packet goes through the scarce resource. Otherwise an attacker the packet goes through the scarce resource. Otherwise an attacker
directly connected to one interface could disturb a GTSM-protected directly connected to one interface could disturb a GTSM-protected
session on the same or another interface. Interfaces which aren't session on the same or another interface. Interfaces that aren't
configured with this filtering (e.g., backbone links) are assumed to configured with this filtering (e.g., backbone links) are assumed to
not have such attackers (i.e., are trusted). not have such attackers (i.e., are trusted).
As a specific instance of such interfaces, we assume that tunnels are As a specific instance of such interfaces, we assume that tunnels are
not a back-door for allowing TTL-spoofing on protocol packets for a not a back-door for allowing TTL-spoofing on protocol packets to a
GTSM-protected peering session with a directly connected neighbor. GTSM-protected peering session with a directly connected neighbor.
We assume that: 1) there are no tunneled packets terminating on the We assume that: 1) there are no tunneled packets terminating on the
router, 2) tunnels terminating on the router are assumed to be secure router, 2) tunnels terminating on the router are assumed to be secure
and endpoints are trusted, 3) tunnel decapsulation includes source and endpoints are trusted, 3) tunnel decapsulation includes source
address spoofing prevention [RFC3704], or 4) the GTSM-enabled session address spoofing prevention [RFC3704], or 4) the GTSM-enabled session
does not allow protocol packets coming from a tunnel. does not allow protocol packets coming from a tunnel.
Since the vast majority of peerings are between adjacent routers, we Since the vast majority of peerings are between adjacent routers, we
can set the TTL on the protocol packets to 255 (the maximum possible can set the TTL on the protocol packets to 255 (the maximum possible
for IP) and then reject any protocol packets that come in from for IP) and then reject any protocol packets that come in from
configured peers which do NOT have an inbound TTL of 255. configured peers that do NOT have an inbound TTL of 255.
GTSM can be disabled for applications such as route-servers and other GTSM can be disabled for applications such as route-servers and other
multi-hop peerings. In the event that an attack comes in from a multi-hop peerings. In the event that an attack comes in from a
compromised multi-hop peering, that peering can be shut down. compromised multi-hop peering, that peering can be shut down.
3. GTSM Procedure 3. GTSM Procedure
If GTSM is not built into the protocol and is used as an additional If GTSM is not built into the protocol and is used as an additional
feature (e.g., for BGP, LDP, or MSDP), it SHOULD NOT be enabled by feature (e.g., for BGP, LDP, or MSDP), it SHOULD NOT be enabled by
default in order to be backward-compatible with the unmodified default in order to remain backward-compatible with the unmodified
protocol. However, if the protocol defines a built-in dynamic protocol. However, if the protocol defines a built-in dynamic
capability negotiation for GTSM, a protocol peer MAY suggest the use capability negotiation for GTSM, a protocol peer MAY suggest the use
of GTSM provided that GTSM would only be enabled if both peers agree of GTSM provided that GTSM would only be enabled if both peers agree
to use it. to use it.
If GTSM is enabled for a protocol session, the following steps are If GTSM is enabled for a protocol session, the following steps are
added to the IP packet sending and reception procedures: added to the IP packet sending and reception procedures:
Sending protocol packets: Sending protocol packets:
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any registered GTSM-enabled session, and hence GTSM cannot any registered GTSM-enabled session, and hence GTSM cannot
make any judgment on the level of risk associated with them. make any judgment on the level of risk associated with them.
+ Trusted: these are packets that have been identified as + Trusted: these are packets that have been identified as
belonging to one of the GTSM-enabled sessions, and their TTL belonging to one of the GTSM-enabled sessions, and their TTL
values are within the expected range. values are within the expected range.
+ Dangerous: these are packets that have been identified as + Dangerous: these are packets that have been identified as
belonging to one of the GTSM-enabled sessions, but their TTL belonging to one of the GTSM-enabled sessions, but their TTL
values are NOT within the expected range, and hence GTSM values are NOT within the expected range, and hence GTSM
believes there is a risk that the packets have been spoofed. believes there is a risk that these packets have been
spoofed.
The exact policies applied to packets of different The exact policies applied to packets of different
classifications are not postulated in this document and are classifications are not postulated in this document and are
expected to be configurable. Configurability is likely expected to be configurable. Configurability is likely
necessary in particular with the treatment of related messages necessary in particular with the treatment of related messages
(ICMP errors). It should be noted that fragmentation may (ICMP errors). It should be noted that fragmentation may
restrict the amount of information available to the restrict the amount of information available for
classification. classification.
However, by default, the implementations: However, by default, the implementations:
+ SHOULD ensure that packets classified as Dangerous do not + SHOULD ensure that packets classified as Dangerous do not
compete for resources with packets classified as Trusted or compete for resources with packets classified as Trusted or
Unknown. Unknown.
+ MUST NOT drop (as part of GTSM processing) packets + MUST NOT drop (as part of GTSM processing) packets
classified as Trusted or Unknown. classified as Trusted or Unknown.
+ MAY drop packets classified as Dangerous. + MAY drop packets classified as Dangerous.
4. Acknowledgments 4. Acknowledgments
The use of the TTL field to protect BGP originated with many The use of the TTL field to protect BGP originated with many
different people, including Paul Traina and Jon Stewart. Ryan different people, including Paul Traina and Jon Stewart. Ryan
McDowell also suggested a similar idea. Steve Bellovin, Jay McDowell also suggested a similar idea. Steve Bellovin, Jay
Borkenhagen, Randy Bush, Alfred Hoenes, Vern Paxon, Robert Raszuk and Borkenhagen, Randy Bush, Alfred Hoenes, Vern Paxon, Robert Raszuk,
Alex Zinin also provided useful feedback on earlier versions of this and Alex Zinin also provided useful feedback on earlier versions of
document. David Ward provided insight on the generalization of the this document. David Ward provided insight on the generalization of
original BGP-specific idea. Alex Zinin, Alia Atlas, and John Scudder the original BGP-specific idea. Alex Zinin, Alia Atlas, and John
provided significant amount of feedback for the newer versions of the Scudder provided a significant amount of feedback for the newer
document. During and after the IETF Last Call, Francis Dupont, Sam versions of the document. During and after the IETF Last Call,
Hartman, Lars Eggert, and Ross Callon. useful comments were provided by Francis Dupont, Sam Hartman, Lars
Eggert, and Ross Callon.
5. Security Considerations 5. Security Considerations
GTSM is a simple procedure that protects single hop protocol GTSM is a simple procedure that protects single-hop protocol
sessions, except in those cases in which the peer has been sessions, except in those cases in which the peer has been
compromised. In particular, it does not protect against the wide compromised. In particular, it does not protect against the wide
range of on-the-wire attacks; protection from these attacks requires range of on-the-wire attacks; protection from these attacks requires
more rigorous security mechanisms. more rigorous security mechanisms.
5.1. TTL (Hop Limit) Spoofing 5.1. TTL (Hop Limit) Spoofing
The approach described here is based on the observation that a TTL The approach described here is based on the observation that a TTL
(or Hop Limit) value of 255 is non-trivial to spoof, since as the (or Hop Limit) value of 255 is non-trivial to spoof, since as the
packet passes through routers towards the destination, the TTL is packet passes through routers towards the destination, the TTL is
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authentication at the tunnel endpoints, as well as how the tunneled authentication at the tunnel endpoints, as well as how the tunneled
packets are protected in flight. Such mechanisms are, however, packets are protected in flight. Such mechanisms are, however,
beyond the scope of this memo. beyond the scope of this memo.
An exception to the observation that a packet with TTL of 255 is An exception to the observation that a packet with TTL of 255 is
difficult to spoof may occur when a protocol packet is tunneled and difficult to spoof may occur when a protocol packet is tunneled and
the tunnel is not integrity-protected (i.e., the lower layer is the tunnel is not integrity-protected (i.e., the lower layer is
compromised). compromised).
When the protocol packet is tunneled directly to the protocol peer When the protocol packet is tunneled directly to the protocol peer
(the protocol peer is the decapsulator), the GTSM provides some (i.e., the protocol peer is the decapsulator), the GTSM provides some
limited added protection as the security depends entirely on the limited added protection as the security depends entirely on the
integrity of the tunnel. integrity of the tunnel.
For protocol adjacencies over a tunnel, if the tunnel itself is For protocol adjacencies over a tunnel, if the tunnel itself is
deemed secure (e.g., the underlying infrastructure is deemed secure, deemed secure (i.e., the underlying infrastructure is deemed secure,
and the tunnel offers degrees of protection against spoofing such as and the tunnel offers degrees of protection against spoofing such as
keys or cryptographic security), the GTSM can serve as a check that keys or cryptographic security), the GTSM can serve as a check that
the protocol packet did not originate beyond the head-end of the the protocol packet did not originate beyond the head-end of the
tunnel. In addition, if the protocol peer can receive packets for tunnel. In addition, if the protocol peer can receive packets for
the GTSM-protected protocol session from outside the tunnel, the GTSM the GTSM-protected protocol session from outside the tunnel, the GTSM
can help thwart attacks from beyond the adjacent router. can help thwart attacks from beyond the adjacent router.
When the tunnel tail-end decapsulates the protocol packet and then When the tunnel tail-end decapsulates the protocol packet and then
IP-forwards the packet to a directly connected protocol peer, TTL is IP-forwards the packet to a directly connected protocol peer, the TTL
decremented as described below. This means that the tunnel is decremented as described below. This means that the tunnel
decapsulator is the penultimate node from the GTSM-protected protocol decapsulator is the penultimate node from the GTSM-protected protocol
peer's perspective. As a result, the GTSM check protects from peer's perspective. As a result, the GTSM check protects from
attackers encapsulating packets to your peers. However, specific attackers encapsulating packets to your peers. However, specific
cases arise when the connection from the tunnel decapsulator node to cases arise when the connection from the tunnel decapsulator node to
the protocol peer is not an IP forwarding hop, where TTL-decrementing the protocol peer is not an IP forwarding hop, where TTL-decrementing
does not happen (e.g., layer-2 tunneling, bridging, etc). In the does not happen (e.g., layer-2 tunneling, bridging, etc). In the
IPsec architecture [RFC4301], another example is the use of Bump-in- IPsec architecture [RFC4301], another example is the use of Bump-in-
the-Wire (BITW) [BITW]. the-Wire (BITW) [BITW].
5.2.1. IP Tunneled over IP 5.2.1. IP Tunneled over IP
Protocol packets may be tunneled over IP directly to a protocol peer, Protocol packets may be tunneled over IP directly to a protocol peer,
or to a decapsulator (tunnel endpoint) that then forwards the packet or to a decapsulator (tunnel endpoint) that then forwards the packet
to a directly connected protocol peer. Examples of tunneling IP over to a directly connected protocol peer. Examples of tunneling IP over
IP include IP-in-IP [RFC2003], GRE [RFC2784], or various forms of IP include IP-in-IP [RFC2003], GRE [RFC2784], and various forms of
IPv6-in-IPv4 (e.g., [RFC4213]). These cases are depicted below. IPv6-in-IPv4 (e.g., [RFC4213]). These cases are depicted below.
Peer router ---------- Tunnel endpoint router and peer Peer router ---------- Tunnel endpoint router and peer
TTL=255 [tunnel] [TTL=255 at ingress] TTL=255 [tunnel] [TTL=255 at ingress]
[TTL=255 at processing] [TTL=255 at processing]
Peer router -------- Tunnel endpoint router ----- On-link peer Peer router -------- Tunnel endpoint router ----- On-link peer
TTL=255 [tunnel] [TTL=255 at ingress] [TTL=254 at ingress] TTL=255 [tunnel] [TTL=255 at ingress] [TTL=254 at ingress]
[TTL=254 at egress] [TTL=254 at egress]
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header is decremented by one if the tunneling is being header is decremented by one if the tunneling is being
done as part of forwarding the datagram; otherwise, the done as part of forwarding the datagram; otherwise, the
inner header TTL is not changed during encapsulation. inner header TTL is not changed during encapsulation.
In the first case, the encapsulated packet is tunneled directly to In the first case, the encapsulated packet is tunneled directly to
the protocol peer (also a tunnel endpoint), and therefore the the protocol peer (also a tunnel endpoint), and therefore the
encapsulated packet's TTL can be received by the protocol peer with encapsulated packet's TTL can be received by the protocol peer with
an arbitrary value, including 255. an arbitrary value, including 255.
In the second case, the encapsulated packet is tunneled to a In the second case, the encapsulated packet is tunneled to a
decapsulator (tunnel endpoint) which then forwards it to a directly decapsulator (tunnel endpoint), which then forwards it to a directly
connected protocol peer. For IP-in-IP tunnels, RFC 2003 specifies connected protocol peer. For IP-in-IP tunnels, RFC 2003 specifies
the following decapsulator behavior: the following decapsulator behavior:
The TTL in the inner IP header is not changed when decapsulating. The TTL in the inner IP header is not changed when decapsulating.
If, after decapsulation, the inner datagram has TTL = 0, the If, after decapsulation, the inner datagram has TTL = 0, the
decapsulator MUST discard the datagram. If, after decapsulation, decapsulator MUST discard the datagram. If, after decapsulation,
the decapsulator forwards the datagram to one of its network the decapsulator forwards the datagram to one of its network
interfaces, it will decrement the TTL as a result of doing normal interfaces, it will decrement the TTL as a result of doing normal
IP forwarding. See also Section 4.4. IP forwarding. See also Section 4.4.
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the iTTL: For Uniform Model LSPs, the iTTL is the value of the TTL the iTTL: For Uniform Model LSPs, the iTTL is the value of the TTL
field from the popped MPLS header (encapsulating header), whereas for field from the popped MPLS header (encapsulating header), whereas for
Pipe Model LSPs, the iTTL is the value of the TTL field from the Pipe Model LSPs, the iTTL is the value of the TTL field from the
exposed header (encapsulated header). exposed header (encapsulated header).
For Uniform Model LSPs, RFC 3443 states that at ingress: For Uniform Model LSPs, RFC 3443 states that at ingress:
For each pushed Uniform Model label, the TTL is copied from the For each pushed Uniform Model label, the TTL is copied from the
label/IP-packet immediately underneath it. label/IP-packet immediately underneath it.
From this point, the inner TTL (TTL of the tunneled IP datagram) From this point, the inner TTL (i.e., the TTL of the tunneled IP
represents non-meaningful information, and at the egress node or datagram) represents non-meaningful information, and at the egress
during PHP, the ingress TTL (iTTL) is equal to the TTL of the popped node or during PHP, the ingress TTL (iTTL) is equal to the TTL of the
MPLS header (see Section 3.1 of RFC 3443). In consequence, for popped MPLS header (see Section 3.1 of RFC 3443). In consequence,
Uniform Model LSPs of more than one hop, the TTL at ingress (iTTL) for Uniform Model LSPs of more than one hop, the TTL at ingress
will be less than 255 (x <= 254), and as a result the check described (iTTL) will be less than 255 (x <= 254), and as a result the check
in Section 3 of this document will fail. described in Section 3 of this document will fail.
The TTL treatment is identical between Short Pipe Model LSPs without The TTL treatment is identical between Short Pipe Model LSPs without
PHP and Pipe Model LSPs (without PHP only). For these cases, RFC PHP and Pipe Model LSPs (without PHP only). For these cases, RFC
3443 states that: 3443 states that:
For each pushed Pipe Model or Short Pipe Model label, the TTL For each pushed Pipe Model or Short Pipe Model label, the TTL
field is set to a value configured by the network operator. In field is set to a value configured by the network operator. In
most implementations, this value is set to 255 by default. most implementations, this value is set to 255 by default.
In these models, the forwarding treatment at egress is based on the In these models, the forwarding treatment at egress is based on the
skipping to change at page 11, line 12 skipping to change at page 10, line 42
also the protocol peer, it is possible to deliver the protocol packet also the protocol peer, it is possible to deliver the protocol packet
with a TTL of 255 (x = 255). with a TTL of 255 (x = 255).
Finally, for Short Pipe Model LSPs with PHP, the TTL of the tunneled Finally, for Short Pipe Model LSPs with PHP, the TTL of the tunneled
packet is unchanged after the PHP operation. Therefore, the same packet is unchanged after the PHP operation. Therefore, the same
conclusions drawn regarding the Short Pipe Model LSPs without PHP and conclusions drawn regarding the Short Pipe Model LSPs without PHP and
Pipe Model LSPs (without PHP only) apply to this case. For Short Pipe Model LSPs (without PHP only) apply to this case. For Short
Pipe Model LSPs, the TTL at egress has the same value with or without Pipe Model LSPs, the TTL at egress has the same value with or without
PHP. PHP.
In conclusion, GTSM-checks are possible for IP tunneled over Pipe In conclusion, GTSM checks are possible for IP tunneled over Pipe
model LSPs, but not for IP tunneled over Uniform model LSPs. model LSPs, but not for IP tunneled over Uniform model LSPs.
Additionally, for all tunneling modes, if the LSP termination router Additionally, for all tunneling modes, if the LSP termination router
needs to forward the protocol packet to a directly connected protocol needs to forward the protocol packet to a directly connected protocol
peer, it is not possible to deliver the protocol packet to the peer, it is not possible to deliver the protocol packet to the
protocol peer with a TTL of 255. If the packet is further forwarded, protocol peer with a TTL of 255. If the packet is further forwarded,
the outgoing TTL (oTTL) is calculated by decrementing iTTL by one. the outgoing TTL (oTTL) is calculated by decrementing iTTL by one.
5.3. Onlink Attackers 5.3. Onlink Attackers
As described in Section 2, an attacker directly connected to one As described in Section 2, an attacker directly connected to one
interface can disturb a GTSM-protected session on the same or another interface can disturb a GTSM-protected session on the same or another
interface (by spoofing a GTSM peer's address) unless ingress interface (by spoofing a GTSM peer's address) unless ingress
filtering has been applied on the connecting interface. As a result, filtering has been applied on the connecting interface. As a result,
interfaces which do not include such protection need to be trusted interfaces that do not include such protection need to be trusted not
not to originate attacks on the router. to originate attacks on the router.
5.4. Fragmentation Considerations 5.4. Fragmentation Considerations
As already mentioned, fragmentation may restrict the amount of As mentioned, fragmentation may restrict the amount of information
information available for classification. Since non-initial IP available for classification. Since non-initial IP fragments do not
fragments do not contain Layer 4 information, it is highly likely contain Layer 4 information, it is highly likely that they cannot be
that they cannot be associated with a registered GTSM-enabled associated with a registered GTSM-enabled session. Following the
session. Following the receiving protocol procedures described in receiving protocol procedures described in Section 3, non-initial IP
Section 3, non-initial IP fragments would likely be classified with fragments would likely be classified with Unknown trustworthiness.
Unknown trustworthiness. And since the IP packet would need to be And since the IP packet would need to be reassembled in order to be
reassembled in order to be processed, the end result is that the processed, the end result is that the initial-fragment of a GTSM-
initial-fragment of a GTSM-enabled session effectively receives the enabled session effectively receives the treatment of an Unknown-
treatment of an Unknown-trustworthiness packet, and the complete trustworthiness packet, and the complete reassembled packet receives
reassembled packet receives the aggregate of the Unknowns. the aggregate of the Unknowns.
In principle an implementation could remember the TTL of all received In principle, an implementation could remember the TTL of all
fragments, and then when reassembling the packet verify that the TTL received fragments. Then when reassembling the packet, verify that
of all fragments matches the required value for an associated GTSM- the TTL of all fragments match the required value for an associated
enabled session. In the likely common case that the implementation GTSM-enabled session. In the likely common case that the
does not do this check on all fragments, then it is possible for a implementation does not do this check on all fragments, then it is
legitimate first fragment (which passes the GTSM check) to be possible for a legitimate first fragment (which passes the GTSM
combined with spoofed non-initial fragments, implying that the check) to be combined with spoofed non-initial fragments, implying
integrity of the received packet is unknown and unprotected. If this that the integrity of the received packet is unknown and unprotected.
check is performed on all fragments at reassembly, and some fragment If this check is performed on all fragments at reassembly, and some
does not pass the GTSM check for a GTSM-enabled session, the fragment does not pass the GTSM check for a GTSM-enabled session, the
reassembled packet is categorized as a Dangerous-trustworthiness reassembled packet is categorized as a Dangerous-trustworthiness
packet and receives the corresponding treatment. packet and receives the corresponding treatment.
Further, reassembly requires to wait for all the fragments and Further, reassembly requires to wait for all the fragments and
therefore likely invalidates or weakens the fifth assumption therefore likely invalidates or weakens the fifth assumption
presented in Section 2: it may not be possible to classify non- presented in Section 2: it may not be possible to classify non-
initial fragments before going through a scarce resource in the initial fragments before going through a scarce resource in the
system, when fragments need to be buffered for reassembly and later system, when fragments need to be buffered for reassembly and later
processed by a CPU. That is, when classification cannot be done with processed by a CPU. That is, when classification cannot be done with
the required granularity, non-initial fragments of GTSM-enabled the required granularity, non-initial fragments of GTSM-enabled
session packets would not use different resource pools. session packets would not use different resource pools.
Consequently, to get practical protection from fragment attacks, Consequently, to get practical protection from fragment attacks,
operators may need to rate-limit or discard all received fragments. operators may need to rate-limit or discard all received fragments.
As such, it is highly RECOMMENDED for GTSM-protected protocols to As such, it is highly RECOMMENDED for GTSM-protected protocols to
avoid fragmentation and reassembly by manual MTU tuning, using avoid fragmentation and reassembly by manual MTU tuning, using
adaptive measures such as Path MTU Discovery (PMTUD) or any other adaptive measures such as Path MTU Discovery (PMTUD), or any other
available method [RFC1191], [RFC1981], [RFC4821]. available method [RFC1191], [RFC1981], or [RFC4821].
5.5. Multi-Hop Protocol Sessions 5.5. Multi-Hop Protocol Sessions
GTSM could possibly offer some small though difficult to quantify GTSM could possibly offer some small, though difficult to quantify,
degree of protection when used with multi-hop protocol sessions (see degree of protection when used with multi-hop protocol sessions (see
Appendix A). In order to avoid having to quantify the degree of Appendix A). In order to avoid having to quantify the degree of
protection and the resulting applicability of multi-hop, we only protection and the resulting applicability of multi-hop, we only
describe the single-hop case because its security properties are describe the single-hop case because its security properties are
clearer. clearer.
6. Applicability Statement 6. Applicability Statement
GTSM is only applicable to environments with inherently limited GTSM is only applicable to environments with inherently limited
topologies (and is most effective in those cases where protocol peers topologies (and is most effective in those cases where protocol peers
skipping to change at page 12, line 49 skipping to change at page 12, line 34
GTSM will not protect against attackers who are as close to the GTSM will not protect against attackers who are as close to the
protected station as its legitimate peer. For example, if the protected station as its legitimate peer. For example, if the
legitimate peer is one hop away, GTSM will not protect from attacks legitimate peer is one hop away, GTSM will not protect from attacks
from directly connected devices on the same interface (see from directly connected devices on the same interface (see
Section 2.2 for more). Section 2.2 for more).
Experimentation on GTSM's applicability and security properties is Experimentation on GTSM's applicability and security properties is
needed in multi-hop scenarios. The multi-hop scenarios where GTSM needed in multi-hop scenarios. The multi-hop scenarios where GTSM
might be applicable is expected to have the following might be applicable is expected to have the following
characteristics: the topology between peers is fairly static and well characteristics: the topology between peers is fairly static and
known, and in which the intervening network (between the peers) is well-known, and in which the intervening network (between the peers)
trusted. is trusted.
6.1. Backwards Compatibility 6.1. Backwards Compatibility
RFC 3682 [RFC3682] did not specify how to handle "related messages" RFC 3682 [RFC3682] did not specify how to handle "related messages"
(ICMP errors). This specification mandates setting and verifying (ICMP errors). This specification mandates setting and verifying
TTL=255 of those as well as the main protocol packets. TTL=255 of those as well as the main protocol packets.
Setting TTL=255 in related messages does not cause issues for RFC Setting TTL=255 in related messages does not cause issues for RFC
3682 implementations. 3682 implementations.
skipping to change at page 13, line 30 skipping to change at page 13, line 14
as Dangerous and treated as described in Section 3. This is not as Dangerous and treated as described in Section 3. This is not
believed to be a significant problem because protocols do not depend believed to be a significant problem because protocols do not depend
on related messages (e.g., typically having a protocol exchange for on related messages (e.g., typically having a protocol exchange for
closing the session instead of doing a TCP-RST), and indeed the closing the session instead of doing a TCP-RST), and indeed the
delivery of related messages is not reliable. As such, related delivery of related messages is not reliable. As such, related
messages typically provide an optimization to shorten a protocol messages typically provide an optimization to shorten a protocol
keepalive timeout. Regardless of these issues, given that related keepalive timeout. Regardless of these issues, given that related
messages provide a significant attack vector to e.g., reset protocol messages provide a significant attack vector to e.g., reset protocol
sessions, making this further restriction seems sensible. sessions, making this further restriction seems sensible.
7. IANA Considerations 7. References
This document requires no action from IANA.
8. References
8.1. Normative References 7.1. Normative References
[RFC0791] Postel, J., "Internet Protocol", STD 5, RFC 791, [RFC0791] Postel, J., "Internet Protocol", STD 5, RFC 791,
September 1981. September 1981.
[RFC2003] Perkins, C., "IP Encapsulation within IP", RFC 2003, [RFC2003] Perkins, C., "IP Encapsulation within IP", RFC 2003,
October 1996. October 1996.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997. Requirement Levels", BCP 14, RFC 2119, March 1997.
skipping to change at page 14, line 25 skipping to change at page 14, line 5
[RFC4213] Nordmark, E. and R. Gilligan, "Basic Transition Mechanisms [RFC4213] Nordmark, E. and R. Gilligan, "Basic Transition Mechanisms
for IPv6 Hosts and Routers", RFC 4213, October 2005. for IPv6 Hosts and Routers", RFC 4213, October 2005.
[RFC4271] Rekhter, Y., Li, T., and S. Hares, "A Border Gateway [RFC4271] Rekhter, Y., Li, T., and S. Hares, "A Border Gateway
Protocol 4 (BGP-4)", RFC 4271, January 2006. Protocol 4 (BGP-4)", RFC 4271, January 2006.
[RFC4301] Kent, S. and K. Seo, "Security Architecture for the [RFC4301] Kent, S. and K. Seo, "Security Architecture for the
Internet Protocol", RFC 4301, December 2005. Internet Protocol", RFC 4301, December 2005.
8.2. Informative References 7.2. Informative References
[BITW] "Thread: 'IP-in-IP, TTL decrementing when forwarding and [BITW] "Thread: 'IP-in-IP, TTL decrementing when forwarding and
BITW' on int-area list, Message-ID: BITW' on int-area list, Message-ID:
<Pine.LNX.4.64.0606020830220.12705@netcore.fi>", <Pine.LNX.4.64.0606020830220.12705@netcore.fi>",
June 2006, <http://www1.ietf.org/mail-archive/web/ June 2006, <http://www1.ietf.org/mail-archive/web/
int-area/current/msg00267.html>. int-area/current/msg00267.html>.
[RFC1191] Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191, [RFC1191] Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191,
November 1990. November 1990.
skipping to change at page 15, line 6 skipping to change at page 15, line 5
[RFC3704] Baker, F. and P. Savola, "Ingress Filtering for Multihomed [RFC3704] Baker, F. and P. Savola, "Ingress Filtering for Multihomed
Networks", BCP 84, RFC 3704, March 2004. Networks", BCP 84, RFC 3704, March 2004.
[RFC4272] Murphy, S., "BGP Security Vulnerabilities Analysis", [RFC4272] Murphy, S., "BGP Security Vulnerabilities Analysis",
RFC 4272, January 2006. RFC 4272, January 2006.
[RFC4821] Mathis, M. and J. Heffner, "Packetization Layer Path MTU [RFC4821] Mathis, M. and J. Heffner, "Packetization Layer Path MTU
Discovery", RFC 4821, March 2007. Discovery", RFC 4821, March 2007.
Appendix A. Multi-hop GTSM Appendix A. Multi-Hop GTSM
NOTE: This is a non-normative part of the specification. NOTE: This is a non-normative part of the specification.
The main applicability of GTSM is for directly connected peers. GTSM The main applicability of GTSM is for directly connected peers. GTSM
could be used for non-directly connected sessions as well, where the could be used for non-directly connected sessions as well, where the
recipient would check that the TTL is within a configured number of recipient would check that the TTL is within a configured number of
hops from 255 (e.g., check that packets have 254 or 255). As such hops from 255 (e.g., check that packets have 254 or 255). As such
deployment is expected to have a more limited applicability and deployment is expected to have a more limited applicability and
different security implications, it is not specified in this different security implications, it is not specified in this
document. document.
skipping to change at page 15, line 36 skipping to change at page 15, line 35
o Explicitly require that related messages (ICMP errors) must also o Explicitly require that related messages (ICMP errors) must also
be sent and checked to have TTL=255. See Section 6.1 for be sent and checked to have TTL=255. See Section 6.1 for
discussion on backwards compatibility. discussion on backwards compatibility.
o Clarifications relating to fragmentation, security with tunneling, o Clarifications relating to fragmentation, security with tunneling,
and implications of ingress filtering. and implications of ingress filtering.
o A significant number of editorial improvements and clarifications. o A significant number of editorial improvements and clarifications.
Appendix C. Draft Changelog
NOTE to the RFC-editor: please remove this section before
publication.
Appendix C.1. Changes between -09 and -10
o Editorial updates from IETF LC and IESG review.
o Clarify fragmentation and integrity issues, from Sam Hartman.
o Repeat the hop limit's protection implication in Applicability
Statement (Ron Bonica).
o Remove "TCP RSTs" from "related messages" examples (Lars Eggert).
o Only define ICMP errors as related messages (Ross Callon).
Appendix C.2. Changes between -08 and -09
o Clarify that GTSM may be enabled on existing protocols if the
protocols include a capability negotiation feature and both
parties support GTSM.
o Editorial updates.
Appendix C.3. Changes between -07 and -08
o Describe the assumption of ingress filtering to protect against
on-link attacks.
o Rewrite the IP over MPLS section based on the new MPLS TTL
handling procedure (from Carlos Pignataro) to get the details of
new MPLS architecture right.
o Rephrase IP over IP tunneling section a bit, to make distinction
between encapsulation and decapsulation behavior clearer.
o Make it clearer in the tunneling section that unless the tunnel
peer is also the protocol peer, GTSM should be able to offer
protection.
o Describe better the applicability of GTSM when tunneling.
o Rephrase Multi-hop GTSM section to mainly refer to the difficult-
to-quantify security properties as a reason for exclusion at this
point.
o Some editorial updates.
Appendix C.4. Changes between -06 and -07
o Be more reserved about multi-hop security properties in section
'Multi-Hop Protocol Sessions'.
o Clarify IP-in-IP tunnel decapsulation/forwarding as decrementing
TTL.
o Add text on related messages backwards compatibility.
o Editorial updates.
Appendix C.5. Changes between -05 and -06
o Clarify the assumptions wrt. resource separation and protection
based on comments from Alex Zinin.
o Rewrite the GTSM procedure based on text from Alex Zinin.
o Reduce TrustRadius and multi-hop scenarios to a mention in an
Appendix.
o Describe TCP-RST, ICMP error and "related messages" handling.
o Update the tunneling security considerations text.
o Editorial updates (e.g., shortening the abstract).
Authors' Addresses Authors' Addresses
Vijay Gill Vijay Gill
EMail: vijay@umbc.edu
Email: vijay@umbc.edu
John Heasley John Heasley
EMail: heas@shrubbery.net
Email: heas@shrubbery.net
David Meyer David Meyer
EMail: dmm@1-4-5.net
Email: dmm@1-4-5.net
Pekka Savola (editor) Pekka Savola (editor)
Espoo Espoo
Finland Finland
EMail: psavola@funet.fi
Email: psavola@funet.fi
Carlos Pignataro Carlos Pignataro
EMail: cpignata@cisco.com
Email: cpignata@cisco.com
Full Copyright Statement Full Copyright Statement
Copyright (C) The IETF Trust (2007). Copyright (C) The IETF Trust (2007).
This document is subject to the rights, licenses and restrictions This document is subject to the rights, licenses and restrictions
contained in BCP 78, and except as set forth therein, the authors contained in BCP 78, and except as set forth therein, the authors
retain all their rights. retain all their rights.
This document and the information contained herein are provided on an This document and the information contained herein are provided on an
skipping to change at page 18, line 44 skipping to change at line 710
attempt made to obtain a general license or permission for the use of attempt made to obtain a general license or permission for the use of
such proprietary rights by implementers or users of this such proprietary rights by implementers or users of this
specification can be obtained from the IETF on-line IPR repository at specification can be obtained from the IETF on-line IPR repository at
http://www.ietf.org/ipr. http://www.ietf.org/ipr.
The IETF invites any interested party to bring to its attention any The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary copyrights, patents or patent applications, or other proprietary
rights that may cover technology that may be required to implement rights that may cover technology that may be required to implement
this standard. Please address the information to the IETF at this standard. Please address the information to the IETF at
ietf-ipr@ietf.org. ietf-ipr@ietf.org.
Acknowledgment
Funding for the RFC Editor function is provided by the IETF
Administrative Support Activity (IASA).
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