draft-ietf-mpls-ecmp-bcp-03.txt   rfc4928.txt 
Network Working Group George Swallow Network Working Group G. Swallow
Internet Draft Cisco Systems, Inc. Request for Comments: 4928 S. Bryant
Category: Standards Track BCP: 128 Cisco Systems, Inc.
Expiration Date: August 2007 Category: Best Current Practice L. Andersson
Stewart Bryant Acreo AB
Cisco Systems, Inc.
Loa Andersson
Acreo
February 2007
Avoiding Equal Cost Multipath Treatment in MPLS Networks Avoiding Equal Cost Multipath Treatment in MPLS Networks
draft-ietf-mpls-ecmp-bcp-03.txt Status of This Memo
Status of this Memo
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Abstract Abstract
This document describes the Equal Cost Multipath (ECMP) behavior of This document describes the Equal Cost Multipath (ECMP) behavior of
currently deployed MPLS networks. This document makes best practice currently deployed MPLS networks. This document makes best practice
recommendations for anyone defining an application to run over an recommendations for anyone defining an application to run over an
MPLS network that wishes to avoid the reordering that can result from MPLS network that wishes to avoid the reordering that can result from
transmission of different packets from the same flow over multiple transmission of different packets from the same flow over multiple
different equal cost paths. different equal cost paths. These recommendations rely on inspection
of the IP version number field in packets. Despite the heuristic
nature of the recommendations, they provide a relatively safe way to
operate MPLS networks, even if future allocations of IP version
numbers were made for some purpose.
Contents Table of Contents
1 Introduction .............................................. 3 1. Introduction ....................................................2
1.1 Terminology ............................................... 3 1.1. Terminology ................................................2
2 Current ECMP Practices .................................... 3 2. Current ECMP Practices ..........................................2
3 Recommendations for Avoiding ECMP Treatment ............... 5 3. Recommendations for Avoiding ECMP Treatment .....................4
4 Security Considerations ................................... 6 4. Security Considerations .........................................5
5 References ................................................ 6 5. IANA Considerations .............................................5
5.1 Normative References ...................................... 6 6. References ......................................................6
5.2 Informative References .................................... 7 6.1. Normative References .......................................6
6 Authors' Addresses ........................................ 8 6.2. Informative References .....................................6
1. Introduction 1. Introduction
This document describes the Equal Cost Multipath (ECMP) behavior of This document describes the Equal Cost Multipath (ECMP) behavior of
currently deployed MPLS networks. We discuss cases where multiple currently deployed MPLS networks. We discuss cases where multiple
packets from the same top-level LSP might be transmitted over differ- packets from the same top-level LSP might be transmitted over
ent equal cost paths, resulting in possible mis-ordering of packets different equal cost paths, resulting in possible mis-ordering of
which are part of the same top-level LSP. This document also makes packets that are part of the same top-level LSP. This document also
best practice recommendations for anyone defining an application to makes best practice recommendations for anyone defining an
run over an MPLS network that wishes to avoid the resulting potential application to run over an MPLS network that wishes to avoid the
for mis-ordered packets. While disabling ECMP behavior is an option resulting potential for mis-ordered packets. While disabling ECMP
open to most operators, few (if any) have chosen to do so, and the behavior is an option open to most operators, few (if any) have
application designer does not have control over the behavior of the chosen to do so, and the application designer does not have control
networks that the application may run over. Thus ECMP behavior is a over the behavior of the networks that the application may run over.
reality that must be reckoned with. Thus, ECMP behavior is a reality that must be reckoned with.
1.1. Terminology 1.1. Terminology
ECMP Equal Cost Multipath ECMP Equal Cost Multipath
FEC Forwarding Equivalence Class FEC Forwarding Equivalence Class
IP ECMP A forwarding behavior in which the selection of the IP ECMP A forwarding behavior in which the selection of the
next-hop between equal cost routes is based on the next-hop between equal cost routes is based on the
header(s) of an IP packet header(s) of an IP packet
Label ECMP A forwarding behavior in which the selection of the Label ECMP A forwarding behavior in which the selection of the
next-hop between equal cost routes is based on the next-hop between equal cost routes is based on the label
label stack of an MPLS packet stack of an MPLS packet
LSP Label Switched Path LSP Label Switched Path
LSR Label Switching Router LSR Label Switching Router
2. Current ECMP Practices 2. Current ECMP Practices
The MPLS label stack and Forwarding Equivalence Classes are defined The MPLS label stack and Forwarding Equivalence Classes are defined
in [RFC3031]. The MPLS label stack does not carry a Protocol Identi- in [RFC3031]. The MPLS label stack does not carry a Protocol
fier. Instead the payload of an MPLS packet is identified by the Identifier. Instead the payload of an MPLS packet is identified by
Forwarding Equivalence Class (FEC) of the bottom most label. Thus it the Forwarding Equivalence Class (FEC) of the bottom most label.
is not possible to know the payload type if one does not know the Thus, it is not possible to know the payload type if one does not
label binding for the bottom most label. Since an LSR which is pro- know the label binding for the bottom most label. Since an LSR,
cessing a label stack need only know the binding for the label(s) it which is processing a label stack, need only know the binding for the
must process, it is very often the case that LSRs along an LSP are label(s) it must process, it is very often the case that LSRs along
unable to determine the payload type of the carried contents. an LSP are unable to determine the payload type of the carried
contents.
As a means of potentially reducing delay and congestion, IP networks As a means of potentially reducing delay and congestion, IP networks
have taken advantage of multiple paths through a network by splitting have taken advantage of multiple paths through a network by splitting
traffic flows across those paths. The general name for this practice traffic flows across those paths. The general name for this practice
is Equal Cost Multipath or ECMP. In general this is done by hashing is Equal Cost Multipath or ECMP. In general, this is done by hashing
on various fields on the IP or contained headers. In practice, on various fields on the IP or contained headers. In practice,
within a network core, the hashing is based mainly or exclusively on within a network core, the hashing is based mainly or exclusively on
the IP source and destination addresses. The reason for splitting the IP source and destination addresses. The reason for splitting
aggregated flows in this manner is to minimize the re-ordering of aggregated flows in this manner is to minimize the re-ordering of
packets belonging to individual flows contained within the aggregated packets belonging to individual flows contained within the aggregated
flow. Within this document we use the term IP ECMP for this type of flow. Within this document, we use the term IP ECMP for this type of
forwarding algorithm. forwarding algorithm.
For packets that contain both a label stack and an encapsulated IPv4 For packets that contain both a label stack and an encapsulated IPv4
(or IPv6) packet, current implementations in some cases may hash on (or IPv6) packet, current implementations in some cases may hash on
any combination of labels and IPv4 (or IPv6) source and destination any combination of labels and IPv4 (or IPv6) source and destination
labels. addresses.
In the early days of MPLS, the payload was almost exclusively IP. In the early days of MPLS, the payload was almost exclusively IP.
Even today the overwhelming majority of carried traffic remains IP. Even today the overwhelming majority of carried traffic remains IP.
Providers of MPLS equipment sought to continue this IP ECMP behavior. Providers of MPLS equipment sought to continue this IP ECMP behavior.
As shown above, it is not possible to know whether the payload of an As shown above, it is not possible to know whether the payload of an
MPLS packet is IP at every place where IP ECMP needs to be performed. MPLS packet is IP at every place where IP ECMP needs to be performed.
Thus vendors have taken the liberty of guessing what the payload is. Thus vendors have taken the liberty of guessing the payload. By
By inspecting the first nibble beyond the label stack, existing inspecting the first nibble beyond the label stack, existing
equipment infers that a packet is not IPv4 or IPv6 if the value of equipment infers that a packet is not IPv4 or IPv6 if the value of
the nibble (where the IP version number would be found) is not 0x4 or the nibble (where the IP version number would be found) is not 0x4 or
0x6 respectively. Most deployed LSRs will treat a packet whose first 0x6 respectively. Most deployed LSRs will treat a packet whose first
nibble is equal to 0x4 as if the payload were IPv4 for purposes of IP nibble is equal to 0x4 as if the payload were IPv4 for purposes of IP
ECMP. ECMP.
A consequence of this is that any application which defines a FEC A consequence of this is that any application that defines an FEC
which does not take measures to prevent the values 0x4 and 0x6 from that does not take measures to prevent the values 0x4 and 0x6 from
occurring in the first nibble of the payload may be subject to IP occurring in the first nibble of the payload may be subject to IP
ECMP and thus having their flows take multiple paths and arriving ECMP and thus having their flows take multiple paths and arriving
with considerable jitter and possibly out of order. While none of with considerable jitter and possibly out of order. While none of
this is in violation of the basic service offering of IP, it is this is in violation of the basic service offering of IP, it is
detrimental to the performance of various classes of applications. detrimental to the performance of various classes of applications.
It also complicates the measurement, monitoring and tracing of those It also complicates the measurement, monitoring, and tracing of those
flows. flows.
New MPLS payload types are emerging such as those specified by the New MPLS payload types are emerging, such as those specified by the
IETF PWE3 and AVT working groups. These payloads are not IP and, if IETF PWE3 and AVT working groups. These payloads are not IP and, if
specified without constraint might be mistaken for IP. specified without constraint, might be mistaken for IP.
It must also be noted that LSRs which correctly identify a payload as It must also be noted that LSRs that correctly identify a payload as
not being IP, most often will load-share traffic across multiple not being IP most often will load-share traffic across multiple
equal-cost paths based on the label stack. Any reserved label, no equal-cost paths based on the label stack. Any reserved label, no
matter where it is located in the stack, may be included in the com- matter where it is located in the stack, may be included in the
putation for load balancing. Modification of the label stack between computation for load balancing. Modification of the label stack
packets of a single flow could result in re-ordering that flow. That between packets of a single flow could result in re-ordering that
is, were an explicit null or a router-alert label to be added to a flow. That is, were an explicit null or a router-alert label to be
packet, that packet could take a different path through the network. added to a packet, that packet could take a different path through
the network.
Note that for some applications, being mistaken for IPv4 may not be Note that for some applications, being mistaken for IPv4 may not be
detrimental. The trivial case where the payload behind the top label detrimental. The trivial case being where the payload behind the top
is a packet belonging to an MPLS IPv4 VPN. Here the real payload is label is a packet belonging to an MPLS IPv4 VPN. Here the real
IP and most (if not all) deployed equipment will locate the end of payload is IP and most (if not all) deployed equipment will locate
the label stack and correctly perform IP ECMP. the end of the label stack and correctly perform IP ECMP.
A less obvious case is when the packets of a given flow happen to A less obvious case is when the packets of a given flow happen to
have constant values in the fields upon which IP ECMP would be per- have constant values in the fields upon which IP ECMP would be
formed. For example if an ethernet frame immediately follows the performed. For example, if an Ethernet frame immediately follows the
label and the LSR does not do ECMP on IPv6, then either the first label and the LSR does ECMP on IPv4, but does not do ECMP on IPv6,
nibble will be 0x4 or it will be something else. If the nibble is then either the first nibble will be 0x4, or it will be something
not 0x4 then no IP ECMP is performed, but Label ECMP may be per- else. If the nibble is not 0x4 then no IP ECMP is performed, but
formed. If it is 0x4, then the constant values of the MAC addresses Label ECMP may be performed. If it is 0x4, then the constant values
overlay the fields that would have been occupied by the source and of the MAC addresses overlay the fields that would have been occupied
destination addresses of an IP header. As a result the ECMP algo- by the source and destination addresses of an IP header. In this
rithm would be feed a constant value and thus would always return the case, the input to the ECMP algorithm would be a constant value and
same result. thus the algorithm would always return the same result.
3. Recommendations for Avoiding ECMP Treatment 3. Recommendations for Avoiding ECMP Treatment
We will use the term "Application Label" to refer to a label that has We will use the term "Application Label" to refer to a label that has
been allocated with a FEC Type that is defined (or simply used) by an been allocated with an FEC Type that is defined (or simply used) by
application. Such labels necessarily appear at the bottom of the an application. Such labels necessarily appear at the bottom of the
label stack, that is, below labels associated with transporting the label stack, that is, below labels associated with transporting the
packet across an MPLS network. The FEC Type of the Application label packet across an MPLS network. The FEC Type of the Application label
defines the payload that follows. Anyone defining an application to defines the payload that follows. Anyone defining an application to
be transported over MPLS is free to define new FEC Types and the for- be transported over MPLS is free to define new FEC Types and the
mat of the payload which will be carried. format of the payload that will be carried.
0 1 2 3 0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Label | Exp |0| TTL | | Label | Exp |0| TTL |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
. . . . . . . . . .
. . . . . . . . . .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Label | Exp |0| TTL | | Label | Exp |0| TTL |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Application Label | Exp |1| TTL | | Application Label | Exp |1| TTL |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1st Nbl| | |1st Nbl| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
In order to avoid IP ECMP treatment it is necessary that an applica- In order to avoid IP ECMP treatment, it is necessary that an
tion take precautions to not be mistaken as IP by deployed equipment application take precautions to not be mistaken as IP by deployed
that snoops on the presumed location of the IP Version field. Thus, equipment that snoops on the presumed location of the IP Version
at a minimum, the chosen format must disallow the values 0x4 and 0x6 field. Thus, at a minimum, the chosen format must disallow the
in the first nibble of their payload. values 0x4 and 0x6 in the first nibble of their payload.
It is strongly recommended, however, that applications restrict the It is REQUIRED, however, that applications depend upon in-order
first nibble values to 0x0 and 0x1. This will ensure that that their packet delivery restrict the first nibble values to 0x0 and 0x1.
traffic flows will not be affected if some future routing equipment This will ensure that their traffic flows will not be affected if
does similar snooping on some future version of IP. some future routing equipment does similar snooping on some future
version(s) of IP.
This behavior implies that if in the future an IP version is defined
with a version number of 0x0 or 0x1, then equipment complying with
this BCP would be unable to look past one or more MPLS headers, and
loadsplit traffic from a single LSP across multiple paths based on a
hash of specific fields in the IPv0 or IPv1 headers. That is, IP
traffic employing these version numbers would be safe from
disturbances caused by inappropriate loadsplitting, but would also
not be able to get the performance benefits.
For an example of how ECMP is avoided in Pseudowires, see [RFC4385]. For an example of how ECMP is avoided in Pseudowires, see [RFC4385].
4. Security Considerations 4. Security Considerations
This memo discusses the conditions under which MPLS traffic associ- This memo discusses the conditions under which MPLS traffic
ated with a single top-level LSP either does or does not have the associated with a single top-level LSP either does or does not have
possibility of being split between multiple paths, implying the pos- the possibility of being split between multiple paths, implying the
sibility of mis-ordering between packets belonging to the same top- possibility of mis-ordering between packets belonging to the same
level LSP. From a security point of view, the worse that could result top-level LSP. From a security point of view, the worse that could
from a security breach of the mechanisms described here would be mis- result from a security breach of the mechanisms described here would
ordering of packets, and possible corresponding loss of throughput be mis-ordering of packets, and possible corresponding loss of
(for example, TCP connections may in some cases reduce the window throughput (for example, TCP connections may in some cases reduce the
size in response to mis-ordered packets). However, in order to create window size in response to mis-ordered packets). However, in order
even this limited result, a hacker would need to either change the to create even this limited result, an attacker would need to either
configuration or implementation of a router, or change the bits on change the configuration or implementation of a router, or change the
the wire as transmitted in a packet. bits on the wire as transmitted in a packet.
Other security issues in the deployment of MPLS are outside of the Other security issues in the deployment of MPLS are outside the scope
scope of this document, but are discussed in other MPLS specifica- of this document, but are discussed in other MPLS specifications,
tions such as RFCs 3031, 3036, 3107, 3209, 3478, 3479, 4206, 4220, such as [RFC3031], [RFC3036], [RFC3107], [RFC3209], [RFC3478],
4221, 4378, AND 4379. [RFC3479], [RFC4206], [RFC4220], [RFC4221], [RFC4378], AND [RFC4379].
5. References 5. IANA Considerations
5.1. Normative References IANA has marked the value 0x1 in the IP protocol version number space
as "Reserved" and placed a reference to this document to both values
0x0 and 0x1.
[RFC3031] Rosen, E. et al., "Multiprotocol Label Switching Note that this document does not in any way change the policies
Architecture", RFC 3031, January 2001. regarding the allocation of version numbers, including the possible
use of the reserved numbers for some future purpose.
5.2. Informative References 6. References
[RFC3036] Andersson, L., et. al., "LDP Specification", RFC 3036, 6.1. Normative References
January 2001.
[RFC3031] Rosen, E., Viswanathan, A., and R. Callon, "Multiprotocol
Label Switching Architecture", RFC 3031, January 2001.
6.2. Informative References
[RFC3036] Andersson, L., Doolan, P., Feldman, N., Fredette, A., and
B. Thomas, "LDP Specification", RFC 3036, January 2001.
[RFC3107] Rekhter, Y. and E. Rosen, "Carrying Label Information in [RFC3107] Rekhter, Y. and E. Rosen, "Carrying Label Information in
BGP-4", RFC 3107, May 2001. BGP-4", RFC 3107, May 2001.
[RFC3209] Awduche, D., et. al., "RSVP-TE: Extensions to RSVP for [RFC3209] Awduche, D., Berger, L., Gan, D., Li, T., Srinivasan, V.,
LSP Tunnels", RFC 3209, December 2001. and G. Swallow, "RSVP-TE: Extensions to RSVP for LSP
Tunnels", RFC 3209, December 2001.
[RFC3478] Leelanivas, M., et. al., "Graceful Restart Mechanism for [RFC3478] Leelanivas, M., Rekhter, Y., and R. Aggarwal, "Graceful
Label Distribution Protocol", RFC 3478, February 2003. Restart Mechanism for Label Distribution Protocol", RFC
3478, February 2003.
[RFC3479] Farrel, A., "Fault Tolerance for the Label Distribution [RFC3479] Farrel, A., Ed., "Fault Tolerance for the Label
Protocol (LDP)", RFC 3479, February 2003. Distribution Protocol (LDP)", RFC 3479, February 2003.
[RFC4206] Kompella, K. and Y. Rekhter, "Label Switched Paths (LSP) [RFC4206] Kompella, K. and Y. Rekhter, "Label Switched Paths (LSP)
Hierarchy with Generalized Multi-Protocol Label Switching Hierarchy with Generalized Multi-Protocol Label Switching
(GMPLS) Traffic Engineering (TE)", RFC 4206, October 2005. (GMPLS) Traffic Engineering (TE)", RFC 4206, October 2005.
[RFC4220] Dubuc, M., et. al., "Traffic Engineering Link Management [RFC4220] Dubuc, M., Nadeau, T., and J. Lang, "Traffic Engineering
Information Base", RFC 4220, November 2005. Link Management Information Base", RFC 4220, November
2005.
[RFC4221] Nadeau, T., et. al., "Multiprotocol Label Switching (MPLS) [RFC4221] Nadeau, T., Srinivasan, C., and A. Farrel, "Multiprotocol
Management Overview", RFC 4221, November 2005. Label Switching (MPLS) Management Overview", RFC 4221,
November 2005.
[RFC4378] Allan, D. and T. Nadeau, "A Framework for Multi-Protocol [RFC4378] Allan, D., Ed., and T. Nadeau, Ed., "A Framework for
Label Switching (MPLS) Operations and Management (OAM)", Multi-Protocol Label Switching (MPLS) Operations and
RFC 4378, February 2006. Management (OAM)", RFC 4378, February 2006.
[RFC4379] Kompella, K. and G. Swallow, "Detecting Multi-Protocol [RFC4379] Kompella, K. and G. Swallow, "Detecting Multi-Protocol
Label Switched (MPLS) Data Plane Failures", RFC 4379, Label Switched (MPLS) Data Plane Failures", RFC 4379,
February 2006. February 2006.
[RFC4385] Bryant, S., et. al., "Pseudowire Emulation Edge-to-Edge [RFC4385] Bryant, S., Swallow, G., Martini, L., and D. McPherson,
(PWE3) Control Word for Use over an MPLS PSN", RFC 4385, "Pseudowire Emulation Edge-to-Edge (PWE3) Control Word for
February 2006. Use over an MPLS PSN", RFC 4385, February 2006.
6. Authors' Addresses Authors' Addresses
Loa Andersson Loa Andersson
Acreo Acreo AB
Electrum 236
SE-146 40 Kista
Sweden
Email: loa@pi.se EMail: loa@pi.se
Stewart Bryant Stewart Bryant
Cisco Systems Cisco Systems
250, Longwater, 250, Longwater,
Green Park, Green Park,
Reading, RG2 6GB, UK Reading, RG2 6GB, UK
Email: stbryant@cisco.com EMail: stbryant@cisco.com
George Swallow George Swallow
Cisco Systems, Inc. Cisco Systems, Inc.
1414 Massachusetts Ave 1414 Massachusetts Ave
Boxborough, MA 01719 Boxborough, MA 01719
Email: swallow@cisco.com EMail: swallow@cisco.com
Full Copyright Statement
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This document is subject to the rights, licenses and restrictions
contained in BCP 78, and except as set forth therein, the authors
retain all their rights.
This document and the information contained herein are provided on an
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Copyright (C) The IETF Trust (2007). This document is subject to the Acknowledgement
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This document and the information contained herein are provided on an Funding for the RFC Editor function is currently provided by the
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OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND
THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS
OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF
THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
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