PWE3 S. Bryant, Ed.
Internet-Draft C. Filsfils
Intended status: Standards Track Cisco Systems
Expires: January 10, 2009 U. Drafz
Deutsche Telekom
July 9, 2008
Load Balancing Fat MPLS Pseudowires
draft-bryant-filsfils-fat-pw-02
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Abstract
Where the payload carried over a pseudowire carries a number of
identifiable flows it can in some circumstances be desirable to carry
those flows over the equal cost multiple paths that exist in the
packet switched network. This draft describes a method of
identifying the flows, or flow groups, to the label switched routers
by including an additional label in the label stack.
Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
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"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC2119 [1].
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. ECMP in Label Switched Routers . . . . . . . . . . . . . . 3
1.2. Load Balance Label . . . . . . . . . . . . . . . . . . . . 4
2. Native Service Processing Function . . . . . . . . . . . . . . 4
3. Pseudowire Forwarder . . . . . . . . . . . . . . . . . . . . . 4
3.1. Encapsulation . . . . . . . . . . . . . . . . . . . . . . 5
4. Signaling Load Balance Label . . . . . . . . . . . . . . . . . 6
4.1. Structure of Load Balance Label TLV . . . . . . . . . . . 7
5. OAM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
6. Applicability . . . . . . . . . . . . . . . . . . . . . . . . 8
7. Security Considerations . . . . . . . . . . . . . . . . . . . 9
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
9. Congestion Considerations . . . . . . . . . . . . . . . . . . 10
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 10
11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 10
11.1. Normative References . . . . . . . . . . . . . . . . . . . 10
11.2. Informative References . . . . . . . . . . . . . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 11
Intellectual Property and Copyright Statements . . . . . . . . . . 13
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1. Introduction
A pseudowire is defined as a mechanism that carries the essential
elements of an emulated service from one provider edge (PE) to one or
more other PEs over a packet switched network (PSN) [10].
A pseudowire is normally transported over one single network path,
even if multiple Equal Cost Multiple Paths (ECMP) exit between the
ingress and egress PEs[2] [3]. This is required to preserve the
characteristics of the emulated service (e.g. avoid misordering for
example for SAToP pseudowire's [4]). Except in the extreme case
described in Section 6, the new capability proposed in this draft
does not change this default property of pseudowires.
Some pseudowires are used to transport large volumes of IP traffic
between routers at two locations. One example of this is the use of
an Ethernet pseudowire to create a virtual direct link between a pair
of routers. Such pseudowire's may carry from hundred's of Mbps to
Gbps of traffic. Such pseudowire's do not require strict ordering to
be preserved between packets of the pseudowire. They only require
ordering to be preserved within the context of each individual
transported IP flow. Some operators have requested the ability to
explicitly configure such a pseudowire to leverage the availability
of multiple ECMP paths. This allows for better capacity planning as
the statistical multiplexing of a larger number of smaller flows is
more efficient than with a smaller set of larger flows. Although
Ethernet is used as an example above, the mechanisms described in
this draft are general mechanisms that may be applied to any
pseudowire type in which there are identifiable flows, and in which
the there is no requirement to preserve the order between flows.
1.1. ECMP in Label Switched Routers
Label switched routers commonly hash the label stack or some elements
of the label stack as a method of discriminating between flows, in
order to distribute those flows over the available equal cost
multiple paths that exist in the network. Since the label at the
bottom of stack is usually the label most closely associated with the
flow, this normally provides the greatest entropy and hence is
normally included in the hash. This draft describes a method of
adding an additional label at the bottom of stack in order to
facilitate the load balancing of the flows within a pseudowire over
the available ECMPs. A similar design for general MPLS use has also
been proposed [11].
An alternative method of load balancing by creating a number of
pseudowires and distributing the flows amongst them was considered,
but was rejected because:
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o It did not introduce as much entropy as the load balance label
method.
o It required additional pseudowires to be set up and maintained.
1.2. Load Balance Label
An additional label is interposed between the pseudowire label and
the control word, or if the control word is not present, between the
pseudowire label and the pseudowire payload. This additional label
is called the pseudowire load balancing label (LB label).
Indivisible flows within the pseudowire MUST be mapped to the same
pseudowire LB label by the ingress PE. The pseudowire load balancing
label stimulates the correct ECMP load balancing behaviour in the
PSN. On receipt of the pseudowire packet at the egress PE (which
knows this additional label is present) the label is discarded
without processing.
Note that the LB label MUST NOT be an MPLS reserved label [5], but is
otherwise unconstrained by the protocol.
To ensure that the load balance label is not used inadvertently used
for forwarding the load balance label MUST have a TTL of 0.
2. Native Service Processing Function
The Native Service Processing (NSP) function is a component of a PE
that has knowledge of the structure of the emulated service and is
able to take action on the service outside the scope of the
pseudowire. In this case it is required that the NSP in the ingress
PE identify flows, or groups of flows within the service, and
indicate the flow (group) identity of each packet as it is passed to
the pseudowire forwarder. Since this is an NSP function, by
definition, the method used to identify a flow is outside the scope
of the pseudowire design. Similarly, since the NSP is internal to
the PE, the method of flow indication to the pseudowire forwarder is
outside the scope of this document
3. Pseudowire Forwarder
The pseudowire forwarder must be provided with a method of mapping
flows to load balanced paths.
The forwarder must generate a label for the flow or group of flows.
How the load balance label values are determined is outside the scope
of this document, however the load balance label allocated to a flow
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MUST NOT be an MPLS reserved label and SHOULD remain constant. It is
recommended that the method chosen to generate the load balancing
labels introduces a high degree of entropy in their values, to
maximise the entropy presented to the ECMP path selection mechanism
in the LSRs in the PSN, and hence distribute the flows as evenly as
possible over the available PSN ECMP paths. The forwarder at the
ingress PE prepends the pseudowire control word (if applicable), then
prepends either the pseudowire load balancing label, followed by the
pseudowire label. Alternatively it prepends the pseudowire control
word (if applicable), then selects and appends one of the allocated
pseudowire labels.
The forwarder at the egress PE uses the pseudowire label to identify
the pseudowire. If the label block approach is used operation is
identical to the current non-load balanced case. Alternatively, from
the pseudowire context, the egress PE can determine whether a
pseudowire load balancing label is present, and if one is present,
the label is discarded.
All other pseudowire forwarding operations are unmodified by the
inclusion of the pseudowire load balancing label.
3.1. Encapsulation
The PWE3 Protocol Stack Reference Model modified to include
pseudowire LB label is shown in Figure 1 below
+-------------+ +-------------+
| Emulated | | Emulated |
| Ethernet | | Ethernet |
| (including | Emulated Service | (including |
| VLAN) |<==============================>| VLAN) |
| Services | | Services |
+-------------+ +-------------+
| Load balance| | Load balance|
+-------------+ Pseudowire +-------------+
|Demultiplexer|<==============================>|Demultiplexer|
+-------------+ +-------------+
| PSN | PSN Tunnel | PSN |
| MPLS |<==============================>| MPLS |
+-------------+ +-------------+
| Physical | | Physical |
+-----+-------+ +-----+-------+
Figure 1: PWE3 Protocol Stack Reference Model
The encapsulation of a pseudowire with a pseudowire LB label is shown
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in Figure 2 below
+-------------------------------+
| MPLS Tunnel label(s) | n*4 octets (four octets per label)
+-------------------------------+
| PW label | 4 octets
+-------------------------------+
| Load Balance label | 4 octets
+-------------------------------+
| Optional Control Word | 4 octets
+-------------------------------+
| Payload |
| |
| | n octets
| |
+-------------------------------+
Figure 2: Encapsulation of a pseudowire with a pseudowire load
balancing label
4. Signaling Load Balance Label
When using the signalling procedures in [6], there is a Pseudowire
Interface Parameter Sub-TLV type used to signal the desire to include
the load balance label when advertising a VC label.
The presence of this parameter indicates that the egress PE requests
that the ingress PE place a load balance label between the pseudowire
label and the control word (or is the control word is not present
between the pseudowire label and the pseudowire payload).
If the ingress PE recognises load balance label indicator parameter
but does not wish to include the load balance label, it need only
issue its own label mapping message for the opposite direction
without including the load balance label Indicator. This will
prevent inclusion of the load balance label in either direction.
If PWE3 signalling [6] is not in use for a pseudowire, then whether
the load balance label is used MUST be identically provisioned in
both PEs at the pseudowire endpoints. If there is no provisioning
support for this option, the default behaviour is not to include the
load balance label.
Note that what is signalled is the desire to include the load balance
label in the label stack. The value of the label is a local matter
for the ingress PE, and the label value itself is not signalled.
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4.1. Structure of Load Balance Label TLV
The structure of the load balance label TLV is shown in Figure 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LBL | Length | must be zero |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 3: Multiple VC TLV
Where:
o LBL is the load balance label TLV identifier assigned by IANA.
o Length is the length of the TLV in octets and is 4.
5. OAM
The following OAM considerations apply to this method of load
balancing.
Where the OAM is only to be used to perform a basic test that the
pseudowires have been configured at the PEs, VCCV [12] messages may
be sent using any load balance pseudowire path, i.e. over any of the
multiple pseudowire labels, or using any pseudowire load balance
label.
Where it is required to verify that a pseudowire is fully functional
for all flows, VCCV [12] connection verification message MUST be sent
over each ECMP path to the pseudowire egress PE. This problem is
difficult to solve and scales poorly. We believe that this problem
is addressed by the following two methods:
1. If a failure occurs within the PSN, this failure will normally be
detected by the PSN's IGP (link/node failure, link or BFD or IGP
hello detection), and the IGP convergence will naturally modify
the ECMP set of network paths between the Ingress and Egress
PE's. Hence the PW is only impacted during the normal IGP
convergence time.
2. If the failure is related to the individual corruption of an LFIB
entry in a router, then only the network path using that specific
entry is impacted. If the PW is load balanced over multiple
network paths, then this failure can only be detected if, by
chance, the transported OAM flow is mapped onto the impacted
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network path, or all paths are tested. This type of error may be
better solved be solved by other means such as LSP self test
[13].
To troubleshoot the MPLS PSN, including multiple paths, the
techniques described in [7] and [8] can be used.
6. Applicability
The requirement to load-balance over multiple PSN paths occurs when
the ratio between the PW access speed and the PSN's core link
bandwidth is large (e.g. >= 0.1). ATM and FR are unlikely to meet
this property. Ethernet does and this is the reason why this
document focuses on Ethernet. Applications for other high-access-
bandwidth PW's (fiber-channel) may be defined in the future.
This design applies to MPLS pseudowires where it is meaningful to
deconstruct the packets presented to the ingress PE into flows. The
mechanism described in this document promotes the distribution of
flows within the pseudowire over different network paths. This in
turn means that whilst packets within a flow are delivered in order
(subject to normal IP delivery perturbations due to topology
variation), order is not maintained amongst packets of different
flows. It is not proposed to associate a different sequence number
with each flow. If sequence number support is required this
mechanism is not applicable.
Where it is known that the traffic carried by the Ethernet pseudowire
is IP the method of identifying the flows are well known and can be
applied. Such methods typically include hashing on the source and
destination addresses, the protocol ID and higher-layer flow-
dependent fields such as TCP/UDP ports, L2TPv3 Session ID's etc.
Where it is known that the traffic carried by the Ethernet pseudowire
is non-IP, techniques used for link bundling between Ethernet
switches may be reused. In this case however the latency
distribution would be larger than is found in the link bundle case.
The acceptability of the increased latency is for further study. Of
particular importance the Ethernet control frames SHOULD always be
mapped to the same PSN path to ensure in-order delivery.
If the payload of an Ethernet PW is made of a single inner flow (i.e.
an encrypted connection between two routers), then the functionality
described in this document does not give any benefits, though neither
does it give any drawbacks. This is unlikely to be a show-stopper
for two reasons:
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o Firstly, the customer of a high-bandwidth PW service has incentive
to get the best transport service because an inefficient use of
the PSN leads to jitter and eventually to loss to the PW's
payload.
o Secondly, the customer is usually able to tailor their
applications to generate many flows in the PSN. A well-known
example is massive data transport between servers which use many
parallel TCP sessions. This same technique can be used by any
transport protocol: multiple UDP ports, multiple L2TPv3 Session
ID's, multiple GRE keys may be used to decompose a large flow into
smaller components. This approach may be applied to IPsec where
multiple SPI's may be allocated to the same security association.
A node within the PSN is not able to perform deep-packet-inspection
(DPI) of the PW as the PW technology is not self-describing: the
structure of the PW payload is only known to the ingress and egress
PE devices. The two methods proposed in this document solve this
limitation.
The methods describe in this document are transparent to the PSN and
as such do not require any new capability from the PSN.
7. Security Considerations
The pseudowire generic security considerations described in [10] and
the security considerations applicable to a specific pseudowire type
(for example, in the case of an Ethernet pseudowire [9] apply.
The ingress PE should take steps to ensure that the load-balance
label is not used as a covert channel.
8. IANA Considerations
IANA is requested to allocate the next available values from the IETF
Consensus range in the Pseudowire Interface Parameters Sub-TLV type
Registry as a Load Balance Label indicator.
Parameter Length Description
TBD 4 Load Balancing Label
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9. Congestion Considerations
The congestion considerations applicable to pseudowires as described
in [10] and any additional congestion considerations developed at the
time of publication apply to this design.
The ability to explicitly configure a PW to leverage the availability
of multiple ECMP paths is beneficial to capacity planning as, all
other parameters being constant, the statistical multiplexing of a
larger number of smaller flows is more efficient than with a smaller
number of larger flows.
Note that if the classification into flows is only performed on IP
packets the behaviour of those flows in the face of congestion will
be as already defined by the IETF for packets of that type and no
additional congestion processing is required.
Where flows that are not IP are classified pseudowire congestion
avoidance must be applied to each non-IP load balance group.
10. Acknowledgements
The authors wish to thank Joerg Kuechemann, Wilfried Maas, Luca
Martini, Mark Townsley, Kireeti Kompella and Shane Amante for
valuable comments and contributions to this design.
11. References
11.1. Normative References
[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[2] Bryant, S., Swallow, G., Martini, L., and D. McPherson,
"Pseudowire Emulation Edge-to-Edge (PWE3) Control Word for Use
over an MPLS PSN", RFC 4385, February 2006.
[3] Swallow, G., Bryant, S., and L. Andersson, "Avoiding Equal Cost
Multipath Treatment in MPLS Networks", BCP 128, RFC 4928,
June 2007.
[4] Vainshtein, A. and YJ. Stein, "Structure-Agnostic Time Division
Multiplexing (TDM) over Packet (SAToP)", RFC 4553, June 2006.
[5] Rosen, E., Tappan, D., Fedorkow, G., Rekhter, Y., Farinacci,
D., Li, T., and A. Conta, "MPLS Label Stack Encoding",
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RFC 3032, January 2001.
[6] Martini, L., Rosen, E., El-Aawar, N., Smith, T., and G. Heron,
"Pseudowire Setup and Maintenance Using the Label Distribution
Protocol (LDP)", RFC 4447, April 2006.
[7] Allan, D. and T. Nadeau, "A Framework for Multi-Protocol Label
Switching (MPLS) Operations and Management (OAM)", RFC 4378,
February 2006.
[8] Kompella, K. and G. Swallow, "Detecting Multi-Protocol Label
Switched (MPLS) Data Plane Failures", RFC 4379, February 2006.
[9] Martini, L., Rosen, E., El-Aawar, N., and G. Heron,
"Encapsulation Methods for Transport of Ethernet over MPLS
Networks", RFC 4448, April 2006.
11.2. Informative References
[10] Bryant, S. and P. Pate, "Pseudo Wire Emulation Edge-to-Edge
(PWE3) Architecture", RFC 3985, March 2005.
[11] Kompella, K. and S. Amante, "The Use of Entropy Labels in MPLS
Forwarding", draft-kompella-mpls-entropy-label-00 (work in
progress), July 2008.
[12] Nadeau, T. and C. Pignataro, "Pseudowire Virtual Circuit
Connectivity Verification (VCCV) A Control Channel for
Pseudowires", draft-ietf-pwe3-vccv-15 (work in progress),
September 2007.
[13] Swallow, G., "Label Switching Router Self-Test",
draft-ietf-mpls-lsr-self-test-07 (work in progress), May 2007.
Authors' Addresses
Stewart Bryant (editor)
Cisco Systems
250 Longwater Ave
Reading RG2 6GB
United Kingdom
Phone: +44-208-824-8828
Email: stbryant@cisco.com
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Clarence Filsfils
Cisco Systems
Brussels
Belgium
Email: cfilsfil@cisco.com
Ulrich Drafz
Deutsche Telekom
Muenster,
Germany
Phone:
Fax:
Email: Ulrich.Drafz@t-com.net
URI:
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