draft-ietf-sip-fork-loop-fix-05.txt   draft-ietf-sip-fork-loop-fix-06.txt 
Network Working Group R. Sparks, Ed. Network Working Group R. Sparks, Ed.
Internet-Draft Estacado Systems Internet-Draft Estacado Systems
Updates: 3261 (if approved) S. Lawrence Updates: 3261 (if approved) S. Lawrence
Intended status: Standards Track Pingtel Corp. Intended status: Standards Track Bluesocket Inc.
Expires: September 8, 2007 A. Hawrylyshen Expires: May 6, 2008 A. Hawrylyshen
Ditech Networks Inc. Ditech Networks Inc.
March 7, 2007 B. Campen
Estacado Systems
November 3, 2007
Addressing an Amplification Vulnerability in Session Initiation Protocol Addressing an Amplification Vulnerability in Session Initiation Protocol
(SIP) Forking Proxies (SIP) Forking Proxies
draft-ietf-sip-fork-loop-fix-05 draft-ietf-sip-fork-loop-fix-06
Status of this Memo Status of this Memo
By submitting this Internet-Draft, each author represents that any By submitting this Internet-Draft, each author represents that any
applicable patent or other IPR claims of which he or she is aware applicable patent or other IPR claims of which he or she is aware
have been or will be disclosed, and any of which he or she becomes have been or will be disclosed, and any of which he or she becomes
aware will be disclosed, in accordance with Section 6 of BCP 79. aware will be disclosed, in accordance with Section 6 of BCP 79.
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and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
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The list of current Internet-Drafts can be accessed at The list of current Internet-Drafts can be accessed at
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This Internet-Draft will expire on September 8, 2007. This Internet-Draft will expire on May 6, 2008.
Copyright Notice Copyright Notice
Copyright (C) The IETF Trust (2007). Copyright (C) The IETF Trust (2007).
Abstract Abstract
This document normatively updates RFC 3261, the Session Initiation This document normatively updates RFC 3261, the Session Initiation
Protocol (SIP), to address a security vulnerability identified in SIP Protocol (SIP), to address a security vulnerability identified in SIP
proxy behavior. This vulnerability enables an attack against SIP proxy behavior. This vulnerability enables an attack against SIP
networks where a small number of legitimate, even authorized, SIP networks where a small number of legitimate, even authorized, SIP
requests can stimulate massive amounts of proxy-to-proxy traffic. requests can stimulate massive amounts of proxy-to-proxy traffic.
This document strengthens loop-detection requirements on SIP proxies This document strengthens loop-detection requirements on SIP proxies
when they fork requests (that is, forward a request to more than one when they fork requests (that is, forward a request to more than one
destination). It also corrects and clarifies the description of the destination). It also corrects and clarifies the description of the
loop-detection algorithm such proxies are required to implement. loop-detection algorithm such proxies are required to implement.
Additionally, this document defines a Max-Breadth mechanism for
limiting the number of concurrent branches pursued for any given
request.
Table of Contents Table of Contents
1. Conventions and Definitions . . . . . . . . . . . . . . . . . 3 1. Conventions and Definitions . . . . . . . . . . . . . . . . . 4
2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Vulnerability: Leveraging Forking to Flood a Network . . . . . 3 3. Vulnerability: Leveraging Forking to Flood a Network . . . . . 4
4. Normative changes to RFC 3261 . . . . . . . . . . . . . . . . 5 4. Updates to RFC 3261 . . . . . . . . . . . . . . . . . . . . . 8
4.1. Strengthening the requirement to perform loop-detection . 5 4.1. Strengthening the Requirement to Perform Loop-detection . 8
4.2. Correcting and clarifying the RFC 3261 loop-detection 4.2. Correcting and Clarifying the RFC 3261 Loop-detection
algorithm . . . . . . . . . . . . . . . . . . . . . . . . 6 Algorithm . . . . . . . . . . . . . . . . . . . . . . . . 8
4.2.1. Update to section 16.6 . . . . . . . . . . . . . . . . 6 4.2.1. Update to section 16.6 . . . . . . . . . . . . . . . . 8
4.2.2. Update to section 16.3 . . . . . . . . . . . . . . . . 7 4.2.2. Update to Section 16.3 . . . . . . . . . . . . . . . . 9
4.2.3. Note to Implementers . . . . . . . . . . . . . . . . . 7 4.2.3. Impact of Loop-detection on Overall Network
5. Impact on overall network performance . . . . . . . . . . . . 8 Performance . . . . . . . . . . . . . . . . . . . . . 10
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8 4.2.4. Note to Implementors . . . . . . . . . . . . . . . . . 10
7. Security Considerations . . . . . . . . . . . . . . . . . . . 8 5. Max-Breadth . . . . . . . . . . . . . . . . . . . . . . . . . 11
8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 10 5.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . . 11
9. Change Log . . . . . . . . . . . . . . . . . . . . . . . . . . 10 5.2. Examples . . . . . . . . . . . . . . . . . . . . . . . . . 12
9.1. -03 to -04 (addressing WGLC comments) . . . . . . . . . . 10 5.3. Formal Mechanism . . . . . . . . . . . . . . . . . . . . . 13
9.2. -02 to -03 . . . . . . . . . . . . . . . . . . . . . . . . 10 5.3.1. "Max-Breadth" Header . . . . . . . . . . . . . . . . . 13
9.3. -01 to -02 . . . . . . . . . . . . . . . . . . . . . . . . 10 5.3.2. Terminology . . . . . . . . . . . . . . . . . . . . . 13
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 11 5.3.3. Proxy Behavior . . . . . . . . . . . . . . . . . . . . 13
10.1. Normative References . . . . . . . . . . . . . . . . . . . 11 5.3.4. UAC Behavior . . . . . . . . . . . . . . . . . . . . . 14
10.2. Informative References . . . . . . . . . . . . . . . . . . 11 5.3.5. UAS behavior . . . . . . . . . . . . . . . . . . . . . 14
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 12 5.4. Implementor Notes . . . . . . . . . . . . . . . . . . . . 15
Intellectual Property and Copyright Statements . . . . . . . . . . 13 5.4.1. Treatment of CANCEL . . . . . . . . . . . . . . . . . 15
5.4.2. Reclamation of Max-Breadth on 2xx Responses . . . . . 15
5.4.3. Max-Breadth and Automaton UAs . . . . . . . . . . . . 15
5.5. Parallel and Sequential Forking . . . . . . . . . . . . . 15
5.6. Max-Breadth Split Weight Selection . . . . . . . . . . . . 16
5.7. Max-Breadth's Effect on Forking-based Amplification
Attacks . . . . . . . . . . . . . . . . . . . . . . . . . 16
5.8. Max-Breadh Header Field ABNF Definition . . . . . . . . . 16
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 16
6.1. Max-Forwards Header Field . . . . . . . . . . . . . . . . 16
6.2. 440 Max-Breadth Exceeded response . . . . . . . . . . . . 17
7. Security Considerations . . . . . . . . . . . . . . . . . . . 17
8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 18
9. Change Log . . . . . . . . . . . . . . . . . . . . . . . . . . 18
9.1. -05 to -06 . . . . . . . . . . . . . . . . . . . . . . . . 18
9.2. -04 to -05 . . . . . . . . . . . . . . . . . . . . . . . . 19
9.3. -03 to -04 . . . . . . . . . . . . . . . . . . . . . . . . 19
9.4. -02 to -03 . . . . . . . . . . . . . . . . . . . . . . . . 19
9.5. -01 to -02 . . . . . . . . . . . . . . . . . . . . . . . . 20
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 20
10.1. Normative References . . . . . . . . . . . . . . . . . . . 20
10.2. Informative References . . . . . . . . . . . . . . . . . . 20
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 21
Intellectual Property and Copyright Statements . . . . . . . . . . 22
1. Conventions and Definitions 1. Conventions and Definitions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC-2119 [RFC2119]. document are to be interpreted as described in RFC-2119 [RFC2119].
2. Introduction 2. Introduction
Interoperability testing uncovered a vulnerability in the behavior of Interoperability testing uncovered a vulnerability in the behavior of
forking SIP proxies as defined in [RFC3261]. This vulnerability can forking SIP proxies as defined in [RFC3261]. This vulnerability can
be leveraged to cause a small number of valid SIP requests to be leveraged to cause a small number of valid SIP requests to
generate an extremely large number of proxy-to-proxy messages. A generate an extremely large number of proxy-to-proxy messages. A
version of this attack demonstrates fewer than ten messages version of this attack demonstrates fewer than ten messages
stimulating potentially 2^70 messages. stimulating potentially 2^71 messages.
This document specifies normative changes to the SIP protocol to This document specifies normative changes to the SIP protocol to
address this vulnerability. According to this update, when a SIP address this vulnerability. According to this update, when a SIP
proxy forks a request to more than one destination, it is required to proxy forks a request to more than one destination, it is required to
ensure it is not participating in a request loop. ensure it is not participating in a request loop.
This normative update alone is insufficient to protect against
crafted variations of the attack described here involving multiple
AORs. To further address the vulnerability, this document defines
the Max-Breadth mechanism to limit the total number of concurrent
branches caused by a forked SIP request. The mechanism only limits
concurrency. It does not limit the total number of branches a
request can traverse over its lifetime.
3. Vulnerability: Leveraging Forking to Flood a Network 3. Vulnerability: Leveraging Forking to Flood a Network
This section describes setting up an attack with a simplifying This section describes setting up an attack with a simplifying
assumption, that two accounts on each of two different RFC 3261 assumption, that two accounts on each of two different RFC 3261
compliant proxy/registrar servers that do not perform loop-detection compliant proxy/registrar servers that do not perform loop-detection
are available to an attacker. This assumption is not necessary for are available to an attacker. This assumption is not necessary for
the attack, but makes representing the scenario simpler. The same the attack, but makes representing the scenario simpler. The same
attack can be realized with a single account on a single server. attack can be realized with a single account on a single server.
Consider two proxy/registrar services, P1 and P2, and four Addresses Consider two proxy/registrar services, P1 and P2, and four Addresses
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a@P2 b@P2 a@P2 b@P2 a@P2 b@P2 a@P2 b@P2 a@P2 b@P2 a@P2 b@P2 a@P2 b@P2 a@P2 b@P2
/\ /\ /\ /\ /\ /\ /\ /\ /\ /\ /\ /\ /\ /\ /\ /\
. .
. .
. .
Figure 2: Attack request propagation Figure 2: Attack request propagation
Requests will continue to propagate down this tree until Max-Forwards Requests will continue to propagate down this tree until Max-Forwards
reaches zero. If the endpoint and two proxies involved follow RFC reaches zero. If the endpoint and two proxies involved follow RFC
3261 recommendations, the tree will be 70 rows deep, representing 3260 recommendations, the tree will be 70 rows deep, representing
2^70 requests. The actual number of messages may be much larger if 2^71-1 requests. The actual number of messages may be much larger if
the time to process the entire tree worth of requests is longer than the time to process the entire tree worth of requests is longer than
Timer C at either proxy. In this case, a storm of 408s, and/or a Timer C at either proxy. In this case, a storm of 408s, and/or a
storm of CANCELs will also be propagating through the tree along with storm of CANCELs will also be propagating through the tree along with
the INVITEs. Remember that there are only two proxies involved in the INVITEs. Remember that there are only two proxies involved in
this scenario - each having to hold the state for all the this scenario - each having to hold the state for all the
transactions it sees (at least 2^69 simultaneously active transactions it sees (at least 2^70 simultaneously active
transactions near the end of the scenario). transactions near the end of the scenario).
The attack can be simplified to one account at one server if the The attack can be simplified to one account at one server if the
service can be convinced that contacts with varying attributes service can be convinced that contacts with varying attributes
(parameters, schemes, embedded headers) are sufficiently distinct, (parameters, schemes, embedded headers) are sufficiently distinct,
and these parameters are not used as part of AOR comparisons when and these parameters are not used as part of AOR comparisons when
forwarding a new request. Since RFC 3261 mandates that all URI forwarding a new request. Since RFC 3261 mandates that all URI
parameters must be removed from a URI before looking it up in a parameters must be removed from a URI before looking it up in a
location service and that the URIs from the Contact header are location service and that the URIs from the Contact header are
compared using URI equality, the following registration should be compared using URI equality, the following registration should be
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Max-Forwards value of 70, and assuming they performed linearly as the Max-Forwards value of 70, and assuming they performed linearly as the
state they held increases, it would have taken 3 trillion years to state they held increases, it would have taken 3 trillion years to
complete the processing of the single INVITE that initiated the complete the processing of the single INVITE that initiated the
attack. It is interesting to note that a few proxies rebooted during attack. It is interesting to note that a few proxies rebooted during
the scenario, and rejoined in the attack when they restarted (as long the scenario, and rejoined in the attack when they restarted (as long
as they maintained registration state across reboots). This points as they maintained registration state across reboots). This points
out that if this attack were launched on the Internet at large, it out that if this attack were launched on the Internet at large, it
might require coordination among all the affected elements to stop might require coordination among all the affected elements to stop
it. it.
4. Normative changes to RFC 3261 Loop-detection, as specified in this document, at any of the proxies
in the scenarios described so far would have stopped the attack
immediately. However, there is a variant of the attack that uses
multiple AORs where loop-detection alone is insufficient protection.
In this variation, each participating AOR forks to all the other
participating AORs. For small numbers of participating AORs (10
example), paths through the resulting tree will not loop until very
large numbers of messages have been generated. Acquiring a
sufficient number of AORs to launch such an attack on networks
currently available is quite feasible.
4.1. Strengthening the requirement to perform loop-detection In this scenario, requests will often take many hops to complete a
loop, and there are a very large number of different loops that will
occur during the attack. In fact, if N is the number of
participating AORs, and provided N is less than or equal to Max-
Forwards, the amount of traffic generated by the attack is greater
than N!, even if all proxies involved are performing loop-detection.
Suppose we have a set of N AORs, all of which are set up to fork
to the entire set. For clarity, assume AOR 1 is where the attack
begins. Every permutation of the remaining N-1 AORs will play
out, defining (N-1)! distinct paths, without repeating any AOR.
Then, each of these paths will fork N ways one last time, and a
loop will be detected on each of these branches. These final
branches alone total N! requests ((N-1)! paths, with N forks at
the end of each path).
Forwarded Requests vs. Number of Participating AORs
___N____Requests_
| 1 | 1 |
| 2 | 4 |
| 3 | 15 |
| 4 | 64 |
| 5 | 325 |
| 6 | 1956 |
| 7 | 13699 |
| 8 | 109600 |
| 9 | 986409 |
| 10 | 9864100 |
Forwarded Requests vs. Number of Participating AORs
In a network where all proxies are performing loop-detection, an
attacker is still afforded rapidly increasing returns on the number
of AORs they are able to leverage. The Max-Breadth mechanism defined
in this document is designed to limit the effectiveness of this
variation of the attack.
In all of the scenarios, it is important to notice that at each
forking proxy, an additional branch could be added pointing to a
single victim (that might not even be a SIP-aware element), resulting
in a massive amount of traffic being directed towards the victim from
potentially as many sources as there are AORs participating in the
attack.
4. Updates to RFC 3261
4.1. Strengthening the Requirement to Perform Loop-detection
The following requirements mitigate the risk of a proxy falling The following requirements mitigate the risk of a proxy falling
victim to the attack described in this document. victim to the attack described in this document.
When a SIP proxy forks a particular request to more than one When a SIP proxy forks a particular request to more than one
destination, it MUST ensure that request is not looping through this destination, it MUST ensure that request is not looping through this
proxy. It is RECOMMENDED that proxies meet this requirement by proxy. It is RECOMMENDED that proxies meet this requirement by
performing the Loop-Detection steps defined in this document. performing the Loop-Detection steps defined in this document.
The requirement to use this document's refinement of the loop- The requirement to use this document's refinement of the loop-
detection algorithm in RFC 3261 is set at should-strength to allow detection algorithm in RFC 3261 is set at should-strength to allow
for future standards track mechanisms that will allow a proxy to for future standards track mechanisms that will allow a proxy to
determine it is not looping. For example, a proxy forking to determine it is not looping. For example, a proxy forking to
destinations established using the sip-outbound mechanism destinations established using the sip-outbound mechanism
[I-D.ietf-sip-outbound] would know those branches will not loop. [I-D.ietf-sip-outbound] would know those branches will not loop.
A SIP proxy forwarding a request to only one location MAY perform A SIP proxy forwarding a request to only one location MAY perform
loop detection but is not required to. When forwarding to only one loop detection but is not required to. When forwarding to only one
location, the amplification risk being exploited is not present, and location, the amplification risk being exploited is not present, and
the Max-Forwards mechanism is sufficient to protect the network. A the Max-Forwards mechanism will protect the network to the extent it
proxy is not required to perform loop detection when forwarding a was designed to do (always keep the constant multiplier due to
request to a single location even if it happened to have previously exhausting Max-Forwards while not forking in mind.) A proxy is not
forked that request (and performed loop detection) in its progression required to perform loop detection when forwarding a request to a
through the network. single location even if it happened to have previously forked that
request (and performed loop detection) in its progression through the
network.
4.2. Correcting and clarifying the RFC 3261 loop-detection algorithm 4.2. Correcting and Clarifying the RFC 3261 Loop-detection Algorithm
4.2.1. Update to section 16.6 4.2.1. Update to section 16.6
This section replaces all of item 8 in section 16.6 of RFC 3261 (item This section replaces all of item 8 in section 16.6 of RFC 3261 (item
8 begins on page 105 and ends on page 106 of RFC 3261). 8 begins on page 105 and ends on page 106 of RFC 3261).
8. Add a Via header field value 8. Add a Via header field value
The proxy MUST insert a Via header field value into the copy before The proxy MUST insert a Via header field value into the copy before
the existing Via header field values. The construction of this value the existing Via header field values. The construction of this value
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Route header field values used when processing the received request. Route header field values used when processing the received request.
Implementers need to take care to include all fields used by the Implementers need to take care to include all fields used by the
location service logic in that particular implementation. location service logic in that particular implementation.
This second part MUST NOT vary with the request method. CANCEL and This second part MUST NOT vary with the request method. CANCEL and
non-200 ACK requests MUST have the same branch parameter value as the non-200 ACK requests MUST have the same branch parameter value as the
corresponding request they cancel or acknowledge. This branch corresponding request they cancel or acknowledge. This branch
parameter value is used in correlating those requests at the server parameter value is used in correlating those requests at the server
handling them (see Sections 17.2.3 and 9.2). handling them (see Sections 17.2.3 and 9.2).
4.2.2. Update to section 16.3 4.2.2. Update to Section 16.3
This section replaces all of item 4 in section 16.3 of RFC 3261 (item This section replaces all of item 4 in section 16.3 of RFC 3261 (item
4 appears on page 95 RFC 3261). 4 appears on page 95 RFC 3261).
4. Loop Detection Check 4. Loop Detection Check
Proxies required to perform loop-detection by RFC-XXXX (RFC-Editor: Proxies required to perform loop-detection by RFC-XXXX (RFC-Editor:
replace XXXX with the RFC number of this document) MUST perform the replace XXXX with the RFC number of this document) MUST perform the
following loop-detection test before forwarding a request. Each Via following loop-detection test before forwarding a request. Each Via
header field value in the request whose sent-by value matches a value header field value in the request whose sent-by value matches a value
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"second part" defined in Section 4.2.1 of RFC-XXXX. This second part "second part" defined in Section 4.2.1 of RFC-XXXX. This second part
will not be present if the message was not forked when that Via will not be present if the message was not forked when that Via
header field value was added. If the second field is present, the header field value was added. If the second field is present, the
proxy MUST perform the second part calculation described in proxy MUST perform the second part calculation described in
Section 4.2.1 of RFC-XXXX on this request and compare the result to Section 4.2.1 of RFC-XXXX on this request and compare the result to
the value from the Via header field. If these values are equal, the the value from the Via header field. If these values are equal, the
request has looped and the proxy MUST reject the request with a 482 request has looped and the proxy MUST reject the request with a 482
(Loop Detected) response. If the values differ, the request is (Loop Detected) response. If the values differ, the request is
spiraling and processing continues to the next step. spiraling and processing continues to the next step.
4.2.3. Note to Implementers 4.2.3. Impact of Loop-detection on Overall Network Performance
These requirements and the recommendation to use the loop-detection
mechanisms in this document make the favorable trade of exponential
message growth for work that is at worst case order n^2 as a message
crosses n proxies. Specifically, this work is order m*n where m is
the number of proxies in the path that fork the request to more than
one location. In practice, m is expected to be small.
The loop detection algorithm expressed in this document requires a
proxy to inspect each Via element in a received request. In the
worst case where a message crosses N proxies, each of which loop
detect, proxy k does k inspections, and the overall number of
inspections spread across the proxies handling this request is the
sum of k from k=1 to k=N which is N(N+1)/2.
4.2.4. Note to Implementors
A common way to create the second part of the branch parameter value A common way to create the second part of the branch parameter value
when forking a request is to compute a hash over the concatenation of when forking a request is to compute a hash over the concatenation of
the Request-URI, any Route header field values used during processing the Request-URI, any Route header field values used during processing
the request and any other values used by the location service logic the request and any other values used by the location service logic
while processing this request. The hash should be chosen so that while processing this request. The hash should be chosen so that
there is a low probability that two distinct sets of these parameters there is a low probability that two distinct sets of these parameters
will collide. Because the maximum number of inputs which need to be will collide. Because the maximum number of inputs which need to be
compared is 70 the chance of a collision is low even with a compared is 70 the chance of a collision is low even with a
relatively small hash value, such as 32 bits. CRC-32c as specified relatively small hash value, such as 32 bits. CRC-32c as specified
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A common point of failure to interoperate at SIPit events has been A common point of failure to interoperate at SIPit events has been
due to parsers objecting to the contents of other's Via header field due to parsers objecting to the contents of other's Via header field
values when inspecting the Via stack for loops. Implementers need to values when inspecting the Via stack for loops. Implementers need to
take care to avoid making assumptions about the format of another take care to avoid making assumptions about the format of another
element's Via header field value beyond the basic constraints placed element's Via header field value beyond the basic constraints placed
on that format by RFC 3261. In particular, parsing a header field on that format by RFC 3261. In particular, parsing a header field
value with unknown parameter names, parameters with no values, value with unknown parameter names, parameters with no values,
parameters values with and without quoted strings must not cause an parameters values with and without quoted strings must not cause an
implementation to fail. implementation to fail.
5. Impact on overall network performance Removing, obfuscating, or in any other way modifying the branch
parameter values in Via header fields in a received request before
forwarding it removes the ability for the node that placed that
branch parameter into the message to perform loop-detection. If two
elements in a loop modify branch parameters this way, a loop can
never be detected.
These requirements and the recommendation to use the loop-detection 5. Max-Breadth
mechanisms in this document make the favorable trade of exponential
message growth for work that is at worst case order n^2 as a message
crosses n proxies. Specifically, this work is order m*n where m is
the number of proxies in the path that fork the request to more than
one location. In practice, m is expected to be small.
The loop detection algorithm expressed in this document requires a 5.1. Overview
proxy to inspect each Via element in a received request. In the
worst case where a message crosses N proxies, each of which loop The Max-Breadth mechanism defined here limits the total number of
detect, proxy k does k inspections, and the overall number of concurrent branches caused by a forked SIP request. With this
inspections spread across the proxies handling this request is the mechanism, all proxyable requests are assigned a positive integral
sum of k from k=1 to k=N which is N(N+1)/2. Max-Breadth value, which denotes the maximum number of concurrent
branches this request may spawn through parallel forking as it is
forwarded from its current point. When a proxy forwards a request,
its Max-Breadth value is divided among the outgoing requests. In
turn, each of the forwarded requests has a limit on how many
concurrent branches they may spawn. As branches complete, thier
portion Max-Breadth value becomes available for subsequent branches,
if needed. If there is insufficient Max-Breadth to carry out a
desired parallel fork, a proxy can return the 440 Max-Breadth
Exceeded response defined in this document.
This mechanism operates independently from Max-Forwards. Max-
Forwards limits the depth of the tree a request may traverse as it is
forwarded from its origination point to each destination it may be
forked to. As Section 3 shows, the number of branches in a tree of
even limited depth can be made large (exponential with depth) by
leveraging forking. Each such branch has a pair of SIP transaction
state machines associated with it. The Max-Breadth mechanism limits
the number of branches that are active (those that have running
transaction state machines) at any given point in time.
Max-Breadth does not prevent forking. It only limits the number of
concurrent parallel forked branches. In particular, a Max-Breadth of
1 restricts a request to pure serial forking rather than restricting
it from being forked at all.
5.2. Examples
UAC Proxy A Proxy B Proxy C
| INVITE | | |
| Max-Breadth: 60 | INVITE | |
| Max-Forwards: 70 | Max-Breadth: 30 | |
|-------------------->| Max-Forwards: 69 | |
| |------------------->| |
| | INVITE | |
| | Max-Breadth: 30 | |
| | Max-Forwards: 69 | |
| |--------------------------------------->|
| | | |
Parallel forking
UAC Proxy A Proxy B Proxy C
| INVITE | | |
| Max-Breadth: 60 | INVITE | |
| Max-Forwards: 70 | Max-Breadth: 60 | |
|-------------------->| Max-Forwards: 69 | |
| |------------------->| |
| | some error response| |
| |<-------------------| |
| | INVITE | |
| | Max-Breadth: 60 | |
| | Max-Forwards: 69 | |
| |--------------------------------------->|
| | | |
Sequential forking
UAC Proxy A Proxy B Proxy C
| INVITE | | |
| Max-Breadth: 60 | INVITE | |
| Max-Forwards: 70 | Max-Breadth: 60 | INVITE |
|-------------------->| Max-Forwards: 69 | Max-Breadth: 60 |
| |------------------->| Max-Forwards: 68 |
| | |------------------>|
| | | |
| | | |
| | | |
No forking
MB == Max-Breadth MF == Max-Forwards
| MB: 4
| MF: 5
MB: 2 P MB: 2
MF: 4 / \ MF: 4
+---------------+ +------------------+
MB: 1 P MB: 1 MB: 1 P MB: 1
MF: 3 / \ MF: 3 MF: 3 / \ MF: 3
+---+ +-------+ +----+ +-------+
P P P P
MB: 1 | MB: 1 | MB: 1 | MB: 1 |
MF: 2 | MF: 2 | MF: 2 | MF: 2 |
P P P P
MB: 1 | MB: 1 | MB: 1 | MB: 1 |
MF: 1 | MF: 1 | MF: 1 | MF: 1 |
P P P P
.
.
.
Max-Breadth and Max-Forwards working together
5.3. Formal Mechanism
5.3.1. "Max-Breadth" Header
The Max-Breadth header takes a single positive integer as its value.
The Max-Breadth header takes no parameters.
5.3.2. Terminology
For each "response context" (see [RFC3261] Sec 16) in a proxy, this
mechanism defines two positive integral values; Incoming Max-Breadth
and Outgoing Max-Breadth. Incoming Max-Breadth is the value of the
Max-Breadth header field value in the request that formed the
response context. Outgoing Max-Breadth is the sum of the Max-Breadth
of all forwarded requests in the response context, that have not
received a final response.
5.3.3. Proxy Behavior
If a SIP proxy receives a request with no Max-Breadth header field
value, it MUST add one, with a value that is RECOMMENDED to be 60.
Proxies MUST have a maximum allowable Incoming Max-Breadth value,
which is RECOMMENDED to be 60. If this maximum is exceeded in a
received request, the proxy MUST overwrite it with a value that
SHOULD be no greater than its allowable maximum.
All proxied requests MUST contain a single Max-Breadth header field
value.
SIP proxies MUST NOT allow the Outgoing Max-Breadth to exceed the
Incoming Max-Breadth in a given response context.
If a SIP proxy determines a response context has insufficient
Incoming Max-Breadth to carry out a desired parallel fork, and the
proxy is unwilling/unable to compensate by forking serially or
sending a redirect, that proxy MUST return a 440 Max-Breadth Exceeded
response.
Notice that these requirements means a proxy receiving a request with
a Max-Breadth of 1 can only fork serially, but it is not required to
fork at all - it can return a 440 instead. Thus, this mechanism is
not a tool a user-agent can use to force all proxies in the path of a
request to fork serially.
A SIP proxy MAY distribute Max-Breadth in an arbitrary fashion
between active branches. A proxy SHOULD NOT use a smaller amount of
Max-Breadth than was present in the original request, unless the
Incoming Max-Breadth exceeded the proxy's maximum acceptable value.
A proxy MUST NOT decrement Max-Breadth for each hop or otherwise use
it to restrict the "depth" of a request's propagation.
5.3.3.1. Reusing Max-Breadth
Because forwarded requests that have received a final response do not
count towards the Outgoing Max-Breadth, whenever a final response
arrives, the Max-Breadth that was used on that branch becomes
available for reuse. Proxies SHOULD be prepared to reuse this Max-
Breadth in cases where there may be elements left in the target-set.
5.3.4. UAC Behavior
A UAC MAY place a Max-Breadth header field value in outgoing
requests. If so, this value is RECOMMENDED to be 60.
5.3.5. UAS behavior
This mechanism does not affect UAS behavior.
5.4. Implementor Notes
5.4.1. Treatment of CANCEL
Since CANCEL requests are never proxied, a Max-Breadth header-field-
value is meaningless in a CANCEL request. Sending a CANCEL in no way
effects the Outgoing Max-Breadth in the associated INVITE response
context. Receiving a CANCEL in no way effects the Incoming Max-
Breadth of the associated INVITE response context.
5.4.2. Reclamation of Max-Breadth on 2xx Responses
Whether 2xx responses free up Max-Breadth is mostly a moot issue,
since proxies are forbidden to start new branches in this case. But,
there is one caveat. For INVITE, we may receive multiple 2xx for a
single branch. Also, 2543 implementations may send back a 6xx
followed by a 2xx on the same branch. Implementations that subtract
from the Outgoing Max-Breadth when they receive an INVITE/2xx must be
careful to avoid bugs caused by subtracting multiple times for a
single branch.
5.4.3. Max-Breadth and Automaton UAs
Designers of automaton UAs (including B2BUAs, gateways, exploders,
and any other element that programmatically sends requests as a
result of incoming SIP traffic) should consider whether Max-Breadth
limitations should be placed on outgoing requests. For example, it
is reasonable to design B2BUAs to carry the Max-Breadth value from
incoming requests over into requests that are sent as a result.
Also, it is reasonable to place Max-Breadth constraints on sets of
requests sent by exploders, when they may be leveraged in an
amplification attack.
5.5. Parallel and Sequential Forking
Inherent in the definition of this mechanism is the ability of a
proxy to reclaim apportioned Max-Breadth while forking sequentially.
The limitation on outgoing Max-Breadth is applied to concurrent
branches only.
For example, if a proxy receives a request with a Max-Breadth of 4,
and has 8 targets to forward it to, that proxy may parallel fork to 4
of these targets initially (each with a Max-Breadth of 1, totaling an
Outgoing Max-Breadth of 4). If one of these transactions completes
with a failure response, the outgoing Max-Breadth drops to 3,
allowing the proxy to forward to one of the 4 remaining targets
(again, with a Max-Breadth of 1).
5.6. Max-Breadth Split Weight Selection
There are a variety of mechanisms for controlling the weight of each
fork branch. Fork branches that are given more Max-Breadth are more
likely to complete quickly (because it is less likely that a proxy
down the line will be forced to fork sequentially). By the same
token, if it is known that a given branch will not fork later on, a
Max-Breadth of 1 may be assigned with no ill effect. This would be
appropriate, for example, if a proxy knows the branch is using the
SIP outbound extension [I-D.ietf-sip-outbound].
5.7. Max-Breadth's Effect on Forking-based Amplification Attacks
Max-Breadth limits the total number of active branches spawned by a
given request at any one time, while placing no constraint on the
distance (measured in hops) that the request can propagate. (ie,
receiving a request with a Max-Breadth of 1 means that any forking
must be sequential, not that forking is forbidden)
This limits the effectiveness of any amplification attack that
leverages forking, because the amount of state/bandwidth needed to
process the traffic at any given point in time is capped.
5.8. Max-Breadh Header Field ABNF Definition
This specification extends the grammar for the Session Initation
Protocol by adding the following extension-header:
Max-Breadth = "Max-Breadth" HCOLON 1*DIGIT
6. IANA Considerations 6. IANA Considerations
None. This specification registers a new SIP header field and a new SIP
response according to the processes defined in [RFC3261].
6.1. Max-Forwards Header Field
This information should appear in the header sub-registry under
http://www.iana.org/assignments/sip-parameters.
RFC XXXX (this specification)
Header Field Name: Max-Breadth
Compact Form: none
6.2. 440 Max-Breadth Exceeded response
This information should appear in the response-code sub-registry
under http://www.iana.org/assignments/sip-parameters.
Response code: 440
Default Reason Phrase: Max-Breadth Exceeded
7. Security Considerations 7. Security Considerations
This document is entirely about documenting and addressing a This document is entirely about documenting and addressing a
vulnerability in SIP proxies as defined by RFC 3261 that can lead to vulnerability in SIP proxies as defined by RFC 3261 that can lead to
an exponentially growing message exchange attack. an exponentially growing message exchange attack.
Alternative solutions that were discussed included Alternative solutions that were discussed included
Doing nothing - rely on suing the offender: While systems that have Doing nothing - rely on suing the offender: While systems that have
skipping to change at page 9, line 32 skipping to change at page 17, line 46
and the effect of even a single successful instance of this kind and the effect of even a single successful instance of this kind
of attack would be devastating to a service-provider. of attack would be devastating to a service-provider.
Putting a smaller cap on Max-Forwards: The effect of the attack is Putting a smaller cap on Max-Forwards: The effect of the attack is
exponential with respect to the initial Max-Forwards value. exponential with respect to the initial Max-Forwards value.
Turning this value down limits the effect of the attack. This Turning this value down limits the effect of the attack. This
comes at the expense of severely limiting the reach of requests in comes at the expense of severely limiting the reach of requests in
the network, possibly to the point that existing architectures the network, possibly to the point that existing architectures
will begin to fail. will begin to fail.
Controlling the number of concurrent requests: Bounding the total
number branches to which the original request can be forwarded
simultaneously limits the impact of the attack at any given point
in time. Proposals for limiting mechanisms where considered, but
no consensus to adopt them currently exists.
Disallowing registration bindings to arbitrary contacts: The way Disallowing registration bindings to arbitrary contacts: The way
registration binding is currently defined is a key part of the registration binding is currently defined is a key part of the
success of the kind of attack documented here. The alternative of success of the kind of attack documented here. The alternative of
limiting registration bindings to allow only binding to the limiting registration bindings to allow only binding to the
network element performing the registration, perhaps to the network element performing the registration, perhaps to the
extreme of ignoring bits provided in the Contact in favor of extreme of ignoring bits provided in the Contact in favor of
transport artifacts observed in the registration request has been transport artifacts observed in the registration request has been
discussed (particularly in the context of the mechanisms being discussed (particularly in the context of the mechanisms being
defined in [I-D.ietf-sip-outbound]. Mechanisms like this may be defined in [I-D.ietf-sip-outbound]. Mechanisms like this may be
considered again in the future, but are currently insufficiently considered again in the future, but are currently insufficiently
developed to address the present threat. developed to address the present threat.
Deprecate forking: This attack does not exist in a system that Deprecate forking: This attack does not exist in a system that
relies entirely on redirection and initiation of new requests by relies entirely on redirection and initiation of new requests by
the original endpoint. Removing such a large architectural the original endpoint. Removing such a large architectural
component from the system at this time was deemed a too extreme component from the system at this time was deemed a too extreme
solution. solution.
The Max-Breadth mechanism defined here does not decrease the
aggregate traffic caused by the forking-loop attack. It only serves
to spread the traffic caused by the attack over a longer period, by
limiting the number of concurrent branches that are being processed
at the same time. An attacker could pump multple requests into a
network that uses the Max-Breadth mechanism and gradually build
traffic to unreasonable levels. Deployments should monitor carefully
and react to gradual increases in the number of concurrent
outstanding transactions related to a given resource to protect
against this possibility. An alternative design of the Max-Breadth
mechanism that was considered and rejected was to not allow the
breadth from completed branches to be reused Section 5.3.3.1. Under
this alternative, an introduced request would cause at most the
initial value of Max-Breadth transactions to be generated in the
network. While that approach limits any variant of the amplification
vulnerability described here to a constant multipler, it would
dramatically change the potential reach of requests and there is
beleif that it would break existing deployments.
8. Acknowledgments 8. Acknowledgments
Thanks go to the implementors that subjected their code to this Thanks go to the implementors that subjected their code to this
scenario and helped analyze the results at SIPit 17. Eric Rescorla scenario and helped analyze the results at SIPit 17. Eric Rescorla
provided guidance and text for the hash recommendation note. provided guidance and text for the hash recommendation note.
9. Change Log 9. Change Log
RFC Editor - Remove this section before publication RFC Editor - Remove this section before publication
9.1. -03 to -04 (addressing WGLC comments) 9.1. -05 to -06
Integrated Max-Breadth based on working group discussion of the
secdir review
Added a paragraph pointing out that removing or modifying other
node's branch parameters defeats their ability to loop detect
Moved the total number of messages from O(2^70) to O(2^71) based
on an observation by Jan Kolomaznik. To see this, note that the
total number of requests is the sum from i=0 to Max-Forwards of
2^i which is 2^(Max-Forwards+1) - 1. The point of the text
doesn't change - (the point being that the number is _big_).
Made the new 4xx concrete (choosing 440)
Added a sentence reinforcing that if you forward to only one
branch, you still potentially have a constant multiplier of
messages in the network as Max-Forwards runs out (based on
feedback from Thomas Cross.)
9.2. -04 to -05
Boilerplate update, editorial nits fixed
9.3. -03 to -04
Addressed WGLC comments
Changed the hash recommendation per list consensus Changed the hash recommendation per list consensus
Reintroduced Call-ID and CSeq (list discussion rediscovered one Reintroduced Call-ID and CSeq (list discussion rediscovered one
use for them in avoiding repeated hash collisions) use for them in avoiding repeated hash collisions)
9.2. -02 to -03 9.4. -02 to -03
Closed Open Issue 1 "Why are we including all of the Route headers Closed Open Issue 1 "Why are we including all of the Route headers
values?". The text has been modified to include only those values values?". The text has been modified to include only those values
used in processing the request. used in processing the request.
Closed Open Issues 2 and 3 "Why did 3261 include Call-ID To-tag, Closed Open Issues 2 and 3 "Why did 3261 include Call-ID To-tag,
and From-tag and CSeq?" and "Why did 3261 include Proxy-Require and From-tag and CSeq?" and "Why did 3261 include Proxy-Require
and Proxy-Authorization?". The group has not been able to and Proxy-Authorization?". The group has not been able to
identify why these fields would be included in the hash generally, identify why these fields would be included in the hash generally,
and successful interoperability tests have not included them. and successful interoperability tests have not included them.
Since they were not included in the text for -02, the text for Since they were not included in the text for -02, the text for
this version was not affected. this version was not affected.
Removed the word "cryptographic" from the hash description in the Removed the word "cryptographic" from the hash description in the
non-normative note to implementers (per list discussion) and added non-normative note to implementers (per list discussion) and added
characterization of the properties the hash chosen should have. characterization of the properties the hash chosen should have.
9.3. -01 to -02 9.5. -01 to -02
Integrated several editorial fixes suggested by Jonathan Rosenberg Integrated several editorial fixes suggested by Jonathan Rosenberg
Noted that the reduction of the attack to a single registration Noted that the reduction of the attack to a single registration
against a single URI as documented in previous versions, is, in against a single URI as documented in previous versions, is, in
fact, going to be effective against implementations conforming to fact, going to be effective against implementations conforming to
the standards before this repair. the standards before this repair.
Re-incorporated motivation from the original maxforwards-problem Re-incorporated motivation from the original maxforwards-problem
draft into the security considerations section based on feedback draft into the security considerations section based on feedback
from Cullen Jennings from Cullen Jennings
Introduced replacement text for the loop detection algorithm Introduced replacement text for the loop detection algorithm
skipping to change at page 11, line 40 skipping to change at page 20, line 45
[RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, [RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
A., Peterson, J., Sparks, R., Handley, M., and E. A., Peterson, J., Sparks, R., Handley, M., and E.
Schooler, "SIP: Session Initiation Protocol", RFC 3261, Schooler, "SIP: Session Initiation Protocol", RFC 3261,
June 2002. June 2002.
10.2. Informative References 10.2. Informative References
[I-D.ietf-sip-outbound] [I-D.ietf-sip-outbound]
Jennings, C. and R. Mahy, "Managing Client Initiated Jennings, C. and R. Mahy, "Managing Client Initiated
Connections in the Session Initiation Protocol (SIP)", Connections in the Session Initiation Protocol (SIP)",
draft-ietf-sip-outbound-08 (work in progress), March 2007. draft-ietf-sip-outbound-10 (work in progress), July 2007.
[RFC1321] Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321, [RFC1321] Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321,
April 1992. April 1992.
[RFC3309] Stone, J., Stewart, R., and D. Otis, "Stream Control [RFC3309] Stone, J., Stewart, R., and D. Otis, "Stream Control
Transmission Protocol (SCTP) Checksum Change", RFC 3309, Transmission Protocol (SCTP) Checksum Change", RFC 3309,
September 2002. September 2002.
Authors' Addresses Authors' Addresses
Robert Sparks (editor) Robert Sparks (editor)
Estacado Systems Estacado Systems
17210 Campbell Road 17210 Campbell Road
Suite 250 Suite 250
Dallas, Texas 75254-4203 Dallas, Texas 75254-4203
USA USA
Email: RjS@nostrum.com Email: RjS@nostrum.com
Scott Lawrence Scott Lawrence
Pingtel Corp. Bluesocket Inc.
400 West Cummings Park 10 North Ave.
Suite 2200 Burlington, MA 01803
Woburn, MA 01801
USA USA
Phone: +1 781 938 5306 Phone: +1 781 229 0533
Email: slawrence@pingtel.com Email: slawrence@bluesocket.com
Alan Hawrylyshen Alan Hawrylyshen
Ditech Networks Inc. Ditech Networks Inc.
1167 Kensington Rd NW 823 E. Middlefield Rd
Suite 200 Mountain View, CA 94043
Calgary, Alberta T2N 1X7
Canada Canada
Phone: +1 403 806 3366 Phone: +1 650 623 1300
Email: ahawrylyshen@ditechnetworks.com Email: alan.ietf@polyphase.ca
Byron Campen
Estacado Systems
17210 Campbell Road
Suite 250
Dallas, Texas 75254-4203
USA
Email: bcampen@estacado.net
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
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