Network Working Group Neil Hart
Internet Draft Mustapha Aissaoui
Expires: April 2007 Tiberiu Grigoriu
Matthew Boccci
Michael Hua
Alcatel
VCCV Extensions for Segmented Pseudo-Wire
draft-hart-pwe3-segmented-pw-vccv-01.txt
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Abstract
This document describes extensions to Single Hop Virtual Circuit
Connectivity Verification (SH-VCCV) procedures for segmented pseudo
wires to test the end-to-end forwarding datapath and to provide a MS-
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PW segment trace capability. This is accomplished by changing the
adaptation function for the Single Hop VCCV parameter at the
switching point between two distinct PW control planes.
Table of Contents
1. Terminology....................................................2
2. Introduction...................................................3
3. Adaptation of the Single-Hop CW VCCV for end to end verification4
3.1. VCCV Capability Signaling for MS-PW.......................4
3.2. End-to-end VCCV...........................................5
3.3. Partial tracing from T-PE.................................5
3.4. Partial Tracing between S-PEs.............................6
3.5. Automated VCCV Trace from T-PE............................6
4. Control Plane Processing of an VCCV Echo Message in a MS-PW....6
4.1. Sending a VCCV Echo Request...............................7
4.2. Receiving an VCCV Echo Request............................7
4.3. Receiving an VCCV Echo Reply..............................7
4.4. VCCV Trace Operations.....................................8
5. Security Considerations........................................8
6. IANA Considerations............................................9
7. Acknowledgments................................................9
8. References.....................................................9
8.1. Normative References......................................9
8.2. Informative References....................................9
Author's Addresses................................................9
Intellectual Property Statement..................................10
Disclaimer of Validity...........................................10
Copyright Statement..............................................11
Acknowledgment...................................................11
1. Terminology
- PW Terminating Provider Edge (T-PE). A PE where the customer-
facing attachment circuits (ACs) are bound to a PW forwarder. A
Terminating PE is present in the first and last segments of a MS-PW.
This incorporates the functionality of a PE as defined in [RFC3985].
- Single-Segment Pseudo Wire (SS-PW). A PW setup directly between two
T-PE devices. Each PW in one direction of a SS-PW traverses one PSN
tunnel that connects the two T-PEs.
- Multi-Segment Pseudo Wire (MS-PW). A static or dynamically
configured set of two or more contiguous PW segments that behave and
function as a single point-to-point PW. Each end of a MS-PW by
definition MUST terminate on a T-PE.
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- PW Segment. A part of a single-segment or multi-segment PW, which
is set up between two PE devices, T-PEs and/or S-PEs.
- PW Switching Provider Edge (S-PE). A PE capable of switching the
control and data planes of the preceding and succeeding PW segments
in a MS-PW. The S-PE terminates the PSN tunnels of the preceding and
succeeding segments of the MS-PW.
- PW switching point for a MS-PW. A PW Switching Point is never the
S-PE and the T-PE for the same MS-PW. A PW switching point runs
necessary protocols to setup and manage PW segments with other PW
switching points and terminating PEs
2. Introduction
Virtual Circuit Connectivity Verification (VCCV) allows network
operators to test the forwarding datapath of pseudo wire (PW)
services. As pseudo wires are extended to cover multiple segments, it
is required to be able to continue operating VCCV end-to-end across a
switching point and to provide the ability to trace the path of the
multi-segment PW (MS-PW) over any number of segments.. Figure 1
illustrates a multi-segment pseudo wire providing connectivity from
T-PE1 to T-PE2 through the switching point S-PE. By suitable
implementation at the S-PEs and with no modification to the T-PE
single-segment PW (SS-PW) implementation, VCCV can be simply extended
to provide both end to end and segment connection verification.
Native |<-----------Pseudo Wire----------->| Native
Layer2 | | Layer2
Service | |<-PSN1-->| |<--PSN2->| | Service
(AC) V V V V V V (AC)
| +----+ +-----+ +----+ |
+----+ | | |=========| |=========| | | +----+
| |----------|........PW1.........|...PW3........|----------| |
| CE1| | | | | | | | | |CE2 |
| |----------|........PW2.........|...PW4........|----------| |
+----+ | |=========| |=========| | +----+
^ +----+ +-----+ +----+ ^
| T-PE1 S-PE T-PE2 |
| |
|<------------------- Emulated Service -------->|
Figure 1 MS-PW Service
The method proposed in this document adapts the SS-PW Control Word
method such that S-PE nodes decrement the TTL in the PW label and
pass forward the VCCV packet to the next segment of the PW. Packets
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for which the TTL expires have their payload examined and are
identified as VCCV packets due to the presence of the CW. They are
then diverted to the control plane for further processing. This
method is a variant to the one described in [PWE3-MS] which is based
on defining a new VCCV sub-TLV in the optional PW switching point
TLV. The advantages of the proposed method are that it limits the
processing of the VCCV packet payload only to the S-PE/T-PE node
which is the target for the message. All other S-PE nodes in between
are not required to inspect the VCCV CW and are only required to
decrement the TTL of the PW label. Also, this method does not require
upgrading the SS-PW CW method supported by T-PE nodes. Finally, it
provides a model of operation consistent with the operation of MPLS
LSP Ping and LSP Trace.
3. Adaptation of the Single-Hop CW VCCV for end to end verification
3.1. VCCV Capability Signaling for MS-PW
In Figure 1 T-PE1 uses the VCCV parameter included in the interface
parameter field of the PW ID FEC TLV or the sub-TLV interface
parameter of the Generalized PW ID FEC TLV to indicate to the far end
T-PE2 what VCCV capabilities T-PE1 supports. This is the same VCCV
parameter as would be used if T-PE1 and T-PE2 were connected directly
by T-LDP. S-PE, which is a PW switching point, as part of the
adaptation function for interface parameters, processes locally the
VCCV parameter then passes it to T-PE2. If there were multiple S-PEs
on the path between T-PE1 and T-PE2, each would carry out the same
processing, passing along the VCCV parameter.
The local processing of the VCCV parameter removes CC Types specified
by the originating T-PE, except PWE3 Control Word that is passed
unchanged. In other words, VCCV for MS-PW will not support other CC
types, i.e., TTL=1 and Router Alert. For example, if T-PE1 indicates
as supported CC Types both Control Word and Router Alert then the S-
PE removes the Router Alert CC Type, leaving Control Word CC Type
unchanged and then passes the modified VCCV parameter to the next S-
PE along the path.
The far end T-PE (T-PE2) receives the VCCV parameter indicating the
Control Word CC Type only if that is supported by the initial T-PE
(T-PE1) and all S-PEs along the PW path. In that case, T-PE2 knows
that T-PE1 is capable of receiving the CW CC type and that each S-PE
node in the path of the MS-PW is capable of relaying and generating/
terminating the VCCV messages.
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3.2. End-to-end VCCV
In Figure 1, if T-PE1, S-PE and T-PE2 support Control Word for VCCV,
then as described in section 3.1. the control plane negotiates the
common use of Control Word for VCCV end to end.
Thus, the end-to-end connectivity of the multi-segment pseudowire can
be verified as follows:
a) T-PE forms an MPLS echo request message with the FEC matching
that of the last segment PW to the destination T-PE. In Figure 1,
this would be the FEC of segment PW3 for example. See Section 4.
for details on how this FEC is obtained by T-PE1.
b) T-PE sets inner PW label TTL to a large enough value to allow the
packet to reach the far end T-PE
c) T-PE sends a VCCV packet that will follow the exact same data
path at each S-PE as that taken by data packets,
d) S-PE performs an outer label pop, an inner label swap with TTL
decrement, and new outer label push.
e) there is no requirement for the S-PE to inspect the CW
f) VCCV packet is diverted to VCCV control processing at the
destination T-PE
g) Destination T-PE replies using the specified reply mode, i.e.,
reverse PW path or IGP path
3.3. Partial tracing from T-PE
In order to trace part of the multi-segment pseudowire, the TTL of
the PW label may be used to force the VCCV message to 'pop out' at an
intermediate node. When the TTL expires, the S-PE can determine that
the packet is a VCCV packet by checking the control word. If the
control word format matches that specified in [VCCV], the packet
should be diverted to VCCV processing.
In Figure 1, if T-PE1 sends a VCCV message with the TTL of the PW
label equal to 1, the TTL will expire at the S-PE. T-PE1 can thus
verify the first segment of the pseudo wire.
Note that this use of the TTL is subject to the caution expressed in
[VCCV]. If a penultimate LSR between S-PEs or between an S-PE and a
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T-PE manipulates the PW label TTL, the VCCV message may not emerge
from the MS-PW at the correct S-PE.
3.4. Partial Tracing between S-PEs
Assuming that all nodes along an MS-PW support the Control Word CC
Type, VCCV between S-PEs may be accomplished using the PW label TTL
as in section 3.3. In Figure-1, the S-PE may verify the path between
it and T-PE2 by sending a VCCV message with the PW label TTL set to
1. Given a more complex network with multiple S-PEs, an S-PE may
verify the connectivity between it and an S-PE two segments away by
sending a VCCV message with the PW label TTL set to 2. Thus, an S-PE
can diagnose connectivity problems by successively increasing the TTL.
3.5. Automated VCCV Trace from T-PE
Although tracing of the MS-PW path is possible using the methods
explained in sections 3.3. and 3.4. , these require multiple manual
iterations and that the FEC of the last PW segment to the target T-
PE/S-PE be known a priori at the node originating the echo request
message for each iteration. This mode of operation will be referred
to as the "ping" mode.
A full automated path tracing capability that iteratively probes the
segments the MS-PW to learn the target FEC information is required.
This will be referred to as the "trace" mode of operation. The
details of this method are described in Section 4.4.
4. Control Plane Processing of an VCCV Echo Message in a MS-PW
The challenge for the control plane is to be able to build the VCCV
echo request packet with the necessary information such as the target
FEC 128 PW sub-TLV (FEC128) of the downstream PW segment which the
packet is destined for. This could be even more difficult in
situations in which the MS-PW spans different providers and
Autonomous Systems.
For example, in Figure 1, T-PE1 has the required information to
compose the FEC128 of the PW1 segment but it does not readily have
the information required to compose the FEC128 of the PW3 segment if
a VCCV echo request is supposed to be destined to T-PE2. This
challenge can be overcome by the methods described in the following
subsections 4.1. , 4.2. and 4.4. .
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4.1. Sending a VCCV Echo Request
When in the "ping" mode of operation, the sender of the echo request
message requires the FEC of the last segment to the target S-PE/T-PE
node. This information can either be configured manually or be
obtained by inspecting the corresponding sub-TLV's of the PW switching point
TLV. However, the PW switching point TLV is optional and there is no guarantee
that all S-PE nodes will populate it with their system address and the PWid of the
last PW segment traversed by the label mapping message. Thus a manual
configuration is always preferred.
When in the "trace" mode operation, the T-PE will automatically learn the target
FEC by probing one by one the hops of the MS-PW path. Each S-PE node includes the
FEC to the downstream node in the echo reply message in a similar way that LSP
trace will have the probed node return the downstream interface and label stack in
the echo reply message. The details of this method are described in the following
sections.
4.2. Receiving an VCCV Echo Request
Upon receiving a VCCV echo request the control plane on S-PEs (or the
target node of each segment of the MS-PW) validates the request and
responds to the request with an echo reply consisting of the FEC128
of the next downstream segment and a return code of 8 (label switched
at stack-depth) indicating that it is an S-PE and not the egress
router for the MS-PW.
If the node is the T-PE or the egress node of the MS-PW, it responds
to the echo request with an echo reply with a return code of 3
(egress router) and no FEC128 is included.
4.3. Receiving an VCCV Echo Reply
The operation to be taken by the node that receives the echo reply in
response to its echo request depends on its current mode of operation
such as "ping" or "trace".
In "ping" mode, the node may choose to ignore the target FEC128 in
the echo reply and report only the return code to the operator.
However, in "trace" mode, the node builds and sends the subsequent
VCCV echo request with a incrementing TTL and the information (such
as the downstream FEC128) it received in the echo request to the next
downstream PW segment.
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4.4. VCCV Trace Operations
As an example, in Figure 1, VCCV trace can be performed on the MS-PW
originating from T-PE1 by a single operational command. The following
process ensues:
1. T-PE1 sends a VCCV echo request with TTL set to 1 and a FEC128
containing the pseudo-wire information of the first segment (PW1
between T-PE1 and S-PE) to S-PE for validation.
2. S-PE validates the echo request with the FEC128. Since it is a
switching point between the first and second segment it builds an
echo reply with a return code of 8 and includes the FEC128 of the
second segment (PW3 between S-PE and T-PE2) and sends the echo
reply back to T-PE1.
3. T-PE1 builds a second VCCV echo request based on the FEC128 in
the echo reply from the S-PE. It increments the TTL and sends the
next echo request out to T-PE2. Note that the VCCV echo request
packet is switched at the S-PE datapath and forwarded to the next
downstream segment without any involvement from the control
plane.
4. T-PE2 receives and validates the echo request with the FEC128 of
the PW3 from T-PE1. Since T-PE2 is the destination node or the
egress node of the MS-PW it replies to T-PE1 with an echo reply
with a return code of 3 (Egress Router) and no FEC128 is
included.
5. T-PE1 receives the echo reply from T-PE2. T-PE1 is made aware
that T-PE2 is the destination of the MS-PW because the echo reply
does not contain the FEC128 and because its return code is 3. The
trace process is completed.
Note that the above example assumes only FEC128 sub-TLV is exchanged
but it is possible that the exchanged information could also involve
other TLV or Target FEC sub-TLVs (such as FEC129, LDP Prefix or RSVP
LSP). For more detail on the format of the VCCV echo packet, refer to
[VCCV] and [RFC4379]. The TTL here refers to that of the inner (VC)
label TTL but it is also possible that the methods described in this
section could use the MH-TTL as described in [PW3-MS].
5. Security Considerations
Same as security concerns described in [VCCV].
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6. IANA Considerations
All the values of the CC, CV and channel types are as described in
[VCCV].
7. Acknowledgments
The authors would like to thank Vach Kompella and Kendall Harvey for
their valuable comments and suggestions.
8. References
8.1. Normative References
[VCCV] Nadeau, T., et al."Pseudo Wire Virtual Circuit Connection
Verification (VCCV)", Internet Draft
[PWE3-MS] Martini L., "Segmented Pseudo Wire", Internet Draft
[RFC4379] Kompella, K., et al., "Detecting Multi-Protocol Label
Switched (MPLS) Data Plane Failures", RFC4379, February
2006.
8.2. Informative References
Author's Addresses
Neil Hart
Alcatel
600 March Rd
Kanata, ON, Canada. K2K 2E6
Email: neil.hart@alcatel.com
Tiberiu Grigoriu
Alcatel
600 March Rd
Kanata, ON, Canada. K2K 2E6
Email: tiberiu.grigoriu@alcatel.com
Mustapha Aissaoui
Alcatel
600 March Rd
Kanata, ON, Canada. K2K 2E6
Email: mustapha.aissaoui@alcatel.com
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Matthew Bocci
Alcatel
Voyager Place, Shoppenhangers Rd
Maidenhead, Berks, UK SL6 2PJ
Email: matthew.bocci@alcatel.co.uk
Michael Hua
Alcatel
600 March Rd
Kanata, ON, Canada. K2K 2E6
Email: michael.hua@alcatel.com
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