| Internet-Draft | STAMP for MPLS PWs and LSPs | December 2024 |
| Gandhi, et al. | Expires 7 June 2025 | [Page] |
Pseudowires (PWs) and Label Switched Paths (LSPs) are used in MPLS networks for various services including carrying layer 2 and layer 3 data packets. This document describes the procedure for encapsulation of the Simple Two-Way Active Measurement Protocol (STAMP) defined in RFC 8762 and its optional extensions defined in RFC 8972 for PWs and LSPs in MPLS networks. The procedure uses Generic Associated Channel (G-ACh) to encapsulate the STAMP test packets with or without adding an IP/UDP header.¶
This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.¶
Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at https://datatracker.ietf.org/drafts/current/.¶
Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress."¶
This Internet-Draft will expire on 7 June 2025.¶
Copyright (c) 2024 IETF Trust and the persons identified as the document authors. All rights reserved.¶
This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Revised BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Revised BSD License.¶
The Simple Two-way Active Measurement Protocol (STAMP) provides capabilities for the measurement of various metrics in IP networks [RFC8762] without the use of a control channel to pre-signal session parameters. [RFC8972] defines optional extensions for STAMP.¶
Pseudowires (PWs) are used in MPLS networks for various services including carrying layer 2 and layer 3 data packets [RFC6658]. The PWs are bidirectional in nature. The PWs can be point-to-point or point-to-multipoint. The PWs may use optional Control Word (CW) as defined in the Section 3, "Generic PW MPLS Control Word" of [RFC4385].¶
A Generic Associated Channel (G-ACh) [RFC5586] provides a mechanism to transport Operations, Administration, and Maintenance (OAM) and other control messages over MPLS data plane. The G-ACh types identify the various OAM messages being transported over the channel.¶
Virtual Circuit Connectivity Verification (VCCV) is used as Control Channel for PWs as described in [RFC5085]. A G-ACh can be used as a VCCV channel as described in [RFC7708].¶
MPLS Transport Profile (MPLS-TP) [RFC5960] was designed to use the MPLS data plane without any changes, therefore when we specify STAMP over an MPLS data plane, it is equally applicable to the MPLS-TP networks. When using STAMP for MPLS-TP PW, there are unique aspects that need to be considered concerning the PW CW, and these aspects are addressed in this document.¶
This document describes the procedure for encapsulation of the STAMP defined in [RFC8762] and its optional extensions defined in [RFC8972] for point-to-point PWs and LSPs in MPLS networks. The procedure uses G-ACh to encapsulate STAMP test packets with or without an IP/UDP header. This document defines two new G-ACh types when using STAMP without IP/UDP header, those are PW demultiplexer agnostic and hence applicable to both MPLS PWs and Layer 2 Tunneling Protocol version 3 (L2TPv3) PW demultiplexers. This document uses existing G-ACh types for IPv4 and IPv6 when using STAMP with IP/UDP header.¶
The STAMP test packets need to be transmitted with the same MPLS label stack that is used by the PW and LSP data traffic to ensure proper validation of underlay path taken by the actual PW and LSP data traffic. Also, the test packets need to follow the same ECMP underlay path taken by the PW and LSP data traffic in the network. The PW data traffic may be encapsulated using Control Word [RFC4385] with an IP header. As such, the STAMP test packets need to be transmitted over the PW using G-ACh and an IP/UDP header.¶
The data traffic over L2-Specific Sublayer (L2SS) as used in L2TP PW carry Control Word but do not carry IP/UDP header. As such, the STAMP packets need to be transmitted over L2-Specific Sublayer (L2SS) as used in L2TP PW using G-ACh without any IP/UDP header (as raw STAMP payload).¶
The Private Line Emulation (PLE) [I-D.ietf-pals-ple] traffic is sent over a Packet Switched Network (PSN) as Virtual Private Wire Services (VPWS) using PWs. The data packets are encapsulated with PLE Control Word, but they do not carry any IP header. As such, the STAMP test packets need to be transmitted using the same MPLS label stack including VPWS PW Label [I-D.ietf-pals-ple] as the PLE traffic and encapsulated using G-ACh but without any IP/UDP header. This allows the STAMP test packets to experience the same forwarding behaviour, follow the same underlay path as the PLE traffic and avoid different ECMP behavior on intermediate nodes.¶
The G-ACh type allows to demultiplex VCCV Control Channel for PWs [RFC7708]. For example, G-ACh types for BFD packets with or without IP/UDP headers allow to demultiplex VCCV Control Channel as described in [RFC5885]. Similarly, STAMP G-ACh types with or withour IP/UDP headers allow to demultiplex VCCV Control Channel. Signaling extensions for PW VCCV for STAMP are outside the scope of this document.¶
The G-ACh provides support for OAM Control Channel associated with the MPLS Transport Profile (MPLS-TP) [RFC5960] LSPs and PWs. The OAM Control Channel for MPLS-TP needs to be extended to encapsulate STAMP test packets (just like the delay and loss measurement packets defined in [RFC6374]. The G-ACh types for STAMP allow to demultiplex OAM Control Channel for MPLS-TP. Similarly, the VCCV Control Channel is also applicable to MPLS-TP PWs [RFC7189]. The G-ACh types for STAMP allow to demultiplex VCCV Control Channel for MPLS-TP PWs.¶
The requirements for the encapsulation of the STAMP test packets for the PWs and LSPs in MPLS networks can be summarized as follows:¶
o The G-ACh MUST support STAMP test packets with IP/UDP header.¶
o The G-ACh MUST support STAMP test packets without IP/UDP header.¶
o The G-ACh MUST support STAMP to demultiplex Control Channel.¶
o The Session-Sender test packets MUST follow the underlay path taken by the data traffic that is using Control Word.¶
o The Session-Sender test packets MUST follow the same ECMP underlay path taken by the data traffic that is using Control Word and Entropy Label defined in [RFC6790].¶
o The Session-Sender test packets MUST follow the same ECMP underlay path taken by the data traffic that is using Control Word but not using Entropy Label defined in [RFC6790].¶
o The Session-Reflector test packets MAY follow the reverse underlay path taken by Session-Sender test packets.¶
o The Session-Reflector test packets MAY follow the same reverse ECMP underlay path taken by Session-Sender test packets.¶
This document concerns with the STAMP operation for the Single-Segment PWs (SS-PWs).¶
The procedure for STAMP operation for point-to-multipoint (P2MP) PWs is outside the scope of this document.¶
Examples of MPLS data traffic use cases for STAMP test packets with IP/UDP headers:¶
Examples of MPLS data traffic use cases for STAMP test packets without IP/UDP headers:¶
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.¶
ECMP: Equal Cost Multi-Path.¶
G-ACh: Generic Associated Channel.¶
GAL: G-ACh Label.¶
HMAC: Hashed Message Authentication Code.¶
MPLS: Multiprotocol Label Switching.¶
OAM: Operations, Administration, and Maintenance.¶
PLE: Private Line Emulation.¶
PW: Pseudowire.¶
SHA: Secure Hash Algorithm.¶
STAMP: Simple Two-way Active Measurement Protocol.¶
TC: Traffic Class.¶
TTL: Time-To-Live.¶
In the STAMP Reference Topology shown in Figure 1, there exists a packet PW or LSP to transport data between Provider Edge (PE) Endpoints S1 and R1. The STAMP Session-Sender on PE S1 initiates a Session-Sender test packet and the STAMP Session-Reflector on PE R1 transmits a reply test packet. The Session-Reflector reply test packet may be transmitted to the STAMP Session-Sender on the same path (same set of links and nodes) in the reverse direction of the path taken towards the Session-Reflector. The T1 is a transmit timestamp and T4 is a receive timestamp added by S1. The T2 is a receive timestamp and T3 is a transmit timestamp added by R1.¶
|<------ MPLS Pseudowire ---->|
|<------ MPLS LSP ----------->|
| |
| T1 T2 |
| / \ |
+-------+ Test Packet +-------+
| | - - - - - - - - - ->| |
| S1 |=====================| R1 |
| |<- - - - - - - - - - | |
+-------+ Reply Test Packet +-------+
\ /
T4 T3
STAMP Session-Sender STAMP Session-Reflector
Provider Edge Endpoint Provider Edge Endpoint
The STAMP Session-Sender and Session-Reflector test packets defined in [RFC8972] are encapsulated and transmitted over the PWs in MPLS networks. The base STAMP test packets can be encapsulated using an IP/UDP header and may use destination UDP port 862 [RFC8762]. The source UDP port is chosen by the Session-Sender.¶
The STAMP test packets are encapsulated with MPLS header using the same label stack as the PW data traffic (including PW Label) and G-ACh header (instead of Control Word header used by the data traffic). The encapsulation allows the STAMP test packets to follow the same path as the PW data traffic, and provide the same ECMP behaviour on the intermediate nodes.¶
The IPv4 Time to Live (TTL), IPv6 Hop Limit and Generalized TTL Security Mechanism (GTSM) procedures from [RFC5082] also apply to the encapsulation of STAMP test packets, and hence the IPv4 and MPLS TTL and IPv6 Hop Limit MUST be set to 255.¶
The OAM Control Channel traffic between two Provider Edge (PE) endpoints is not forwarded past the PE endpoints towards the Customer Edge (CE) devices; instead, the OAM messages are intercepted at the PE endpoints for exception processing in control-plane. [RFC5085] defines mechanisms for VCCV Control Channel to carry OAM messages for MPLS PWs. Specifically, the method for "TTL Expiry VCCV (Type 3)" defined in Section 5.1.3 of [RFC5085] allows to terminate the OAM messages on the remote PE endpoint nodes. This method is also applicable to the STAMP test packets to force test packets to be processed on Session-Sender and Session-Reflector control-planes. As such, the PW Labels MUST be added with TTL value 1 in the STAMP Session-Sender and Session-Reflector test packets.¶
The VCCC Type 2 is also referred to as 'MPLS Router Alert Label" [RFC5085]. This approach could result in a different Equal Cost Multi-Path (ECMP) hashing behavior than pseudowire PDUs, and thus result in the VCCV control channel traffic taking a path which differs from that of the actual data traffic under test [RFC5085]. Hence, the VCCV Type 2 is not supported for STAMP for monitoring MPLS PW and MPLS LSP traffic.¶
There are two ways in which STAMP test packets may be encapsulated over a G-ACh, either using an IP/UDP header or without using an IP/UDP header.¶
For encapsulating the STAMP test packets over a G-ACh with IP/UDP headers (Format1), IPv4 and IPv6 channel types [RFC4385] are used for both Session-Sender and Session-Reflector test packets. The destination UDP port number in the Session-Sender and Session-Reflector test packets discriminate the test packets. The IP version (IPv4 or IPv6) MUST match the IP version used for signaling for dynamically established PWs or IP version MUST be configured for statically provisioned PWs.¶
For encapsulating the STAMP test packets over a G-ACh without adding IP/UDP headers (Format2), two new channel types are defined in this document, one for the Session-Sender test packets and one for the Session-Reflector test packets. The different channel types are required for the Session-Sender and Session-Reflector test packets as the STAMP test packets do not have a way to discriminate them.¶
The procedure defined in this document to encapsulate STAMP test packets is also aplicable to MPLS LSPs and MPLS-TP LSPs. For monitoring data traffic over MPLS LSPs (no Control Word case) using IP header, STAMP test packet Format1 (with IP/UDP headers) is used. For monoitoring data traffic over MPLS-TP LSPs (with Control Word), not using IP header, STAMP test packets with Format2 (without IP/UDP headers) are transmitted with TTL value 1 with the ultimate label in the MPLS header.¶
The G-ACh label (GAL) [RFC5586] along with Generic Associated Channel (G-ACh) types defined in this document can be used with STAMP test packets Format2 (without IP/UDP header), for example, in case of MPLS-TP LSPs (STAMP headers similar to [RFC6374]).¶
Control Channel Types defined in [RFC5085] are applicable to STAMP Test Packets for MPLS as follows:¶
| Control Channel Type | Control Channel Name | STAMP Header Format | G-ACh Type |
|---|---|---|---|
| Type 1 | In-band: Control Word with 0001b as first nibble | Format2 (No IP/UDP Headers) | G-ACH Type STAMP G-ACH (TBD1/TBD2) |
| Type 2 | Out-of-band: MPLS Router Alert Label | Not supported | Not supported |
| Type 3 | TTL Expiry: MPLS PW Label with TTL as 1 | Format 1 (IP/UDP Headets) | IPv4 G-ACH (0x21) and IPv6 G-ACH (0x57) |
| Type 3 | TTL Expiry: MPLS PW Label with TTL as 1 | Format2 (No IP/UDP Headers) | G-ACH Type STAMP G-ACH (TBD1/TBD2) |
The content of an example STAMP Session-Sender test packet for a PW encapsulated using a G-ACh and an IP/UDP header is shown in Figure 2.¶
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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Label(1) | TC |S| TTL | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ . . . . . . +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | PW Label | TC |1| 1 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |0 0 0 1|Version| Reserved | IPv4 (0x0021) or IPv6 (0x0057)| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | IP Header | . Source IP Address = Session-Sender IPv4 or IPv6 Address . . Destination IP Address=Session-Reflector IPv4 or IPv6 Address. . IPv4 Protocol or IPv6 Next header = UDP (17) . . . +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | UDP Header | . Source Port = As chosen by Session-Sender . . Destination Port = User-configured Destination Port | 862 . . . +---------------------------------------------------------------+ | Payload = Test Packet as specified in Section 3 of RFC 8972 | . in Figure 1 and Figure 3 . . . +---------------------------------------------------------------+ | Optional STAMP TLVs defined in RFC 8972 | . . +---------------------------------------------------------------+
The G-ACh header [RFC5586] with channel type for IPv4 or IPv6 MUST immediately follow the bottom of the MPLS label stack. The payload contains the STAMP Session-Sender test packet defined in [RFC8972].¶
The STAMP Session-Sender test packet G-ACh header contains following fields:¶
The content of an example STAMP Session-Sender test packet for a PW encapsulated using a G-ACh without an IP/UDP header 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Label(1) | TC |S| TTL | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ . . . . . . +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | PW Label | TC |1| 1 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |0 0 0 1|Version| Reserved | STAMP Sender G-ACh (TBD1) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Payload = Test Packet as specified in Section 3 of RFC 8972 | . in Figure 1 and Figure 3 . . . +---------------------------------------------------------------+ | Optional STAMP TLVs defined in RFC 8972 | . . +---------------------------------------------------------------+
The G-ACh header [RFC5586] with new STAMP Session-Sender channel type (value TBD1) MUST immediately follow the bottom of the MPLS label stack. The payload contains the STAMP Session-Sender test packet defined in [RFC8972].¶
The STAMP channel type allows the identification of the encased STAMP payload when demultiplexing G-ACh.¶
The STAMP Session-Sender test packet G-ACh header contains following fields:¶
The STAMP Session-Reflector reply test packet is sent on the same path in the reverse direction of a bidirectional PW. The STAMP test packet can be sent using an MPLS header with or without IP/UDP header. The Session-Reflector test packet is sent with an IP/UDP header if the Session-Sender test packet is received with an IP/UDP header, otherwise, it is sent without an IP/UDP header.¶
The content of an example STAMP Session-Reflector test packet for a PW encapsulated using a G-ACh and an IP/UDP header is shown in Figure 4.¶
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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Label(1) | TC |S| TTL | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ . . . . . . +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | PW Label | TC |1| 1 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |0 0 0 1|Version| Reserved | IPv4 (0x0021) or IPv6 (0x0057)| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | IP Header | . Source IP Address . . = Destination IP Address from Session-Sender Test Packet . . Destination IP Address . . = Source IP Address from Session-Sender Test Packet . . IPv4 Protocol or IPv6 Next header = UDP (17) . . . +---------------------------------------------------------------+ | UDP Header | . Source Port . . = Destination Port from Session-Sender Test Packet . . Destination Port . . = Source Port from Session-Sender Test Packet . . . +---------------------------------------------------------------+ | Payload = Test Packet as specified in Section 3 of RFC 8972 | . in Figure 2 and Figure 4 . . . +---------------------------------------------------------------+ | STAMP TLVs from Session-Sender Test Packet | . . +---------------------------------------------------------------+
The G-ACh header [RFC5586] with channel type IPv4 or IPv6 MUST immediately follow the bottom of the MPLS label stack. The payload contains the STAMP Session-Reflector test packet defined in [RFC8972].¶
The STAMP Session-Reflector reply test packet MUST use the IP/UDP information from the received test packet when an IP/UDP header is present in the received test packet.¶
The STAMP Session-Reflector test packet G-ACh header contains following fields:¶
The content of an example STAMP Session-Reflector test packet for a PW encapsulated using a G-ACh without an IP/UDP header is shown in Figure 5.¶
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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Label(1) | TC |S| TTL | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ . . . . . . +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | PW Label | TC |1| 1 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |0 0 0 1|Version| Reserved | STAMP Reflector G-ACh (TBD2) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Payload = Test Packet as specified in Section 3 of RFC 8972 | . in Figure 2 and Figure 4 . . . +---------------------------------------------------------------+ | STAMP TLVs from Session-Sender Test Packet | . . +---------------------------------------------------------------+
The G-ACh header [RFC5586] with new STAMP Session-Reflector channel type (value TBD2) MUST immediately follow the bottom of the MPLS label stack. The payload contains the STAMP Session-Reflector test packet defined in [RFC8972].¶
The STAMP channel type allows the identification of the encased STAMP payload when demultiplexing G-ACh.¶
The STAMP Session-Reflector reflects the test packet back to the Session-Sender using the same channel in the reverse direction of the PW on which it was received. The Session-Reflector has enough information to reflect the test packet received by it to the Session-Sender using the PW context.¶
The STAMP Session-Reflector test packet G-ACh header contains following fields:¶
The procedures defined in this document is intended for deployment in a single network administrative domain. As such, the Session-Sender address, Session-Reflector address, and IP and MPLS forward and return paths are provisioned by the operator for the STAMP session. It is assumed that the operator has verified the integrity of the IP and MPLS forward and return paths used to transmit STAMP test packets.¶
The security considerations specified in [RFC8762] and [RFC8972] also apply to the procedure described in this document. Specifically, the message integrity protection using HMAC, as defined in Section 4.4 of [RFC8762], also apply to the procedure described in this document.¶
Routers that support G-ACh are subject to the same security considerations as defined in [RFC4385] and [RFC5586].¶
The message throttling mechanisms described in Security Section of [RFC5085] also apply to the procedure described in this document.¶
STAMP uses the well-known UDP port number that could become a target of denial of service (DoS) or could be used to aid on-path attacks. Thus, the security considerations and measures to mitigate the risk of the attack documented in Section 6 of [RFC8545] equally apply to the STAMP extensions in this document.¶
If desired, attacks can be mitigated by performing basic validation checks of the timestamp fields (such as T2 is later than T1 in the STAMP Reference Topology shown in Figure 1 in received reply test packets at the Session-Sender. The minimal state associated with these protocols also limit the extent of measurement disruption that can be caused by a corrupt or invalid test packet to a single test cycle.¶
IANA maintains G-ACh Type Registry (see https://www.iana.org/assignments/g-ach-parameters/g-ach-parameters.xhtml). IANA is requested to allocate values for the G-ACh Types for STAMP from "MPLS Generalized Associated Channel (G-ACh) Types (including Pseudowire Associated Channel Types)" registry.¶
| Value | Description | Reference |
|---|---|---|
| TBD1 | STAMP Session-Sender G-ACh Type | This document |
| TBD2 | STAMP Session-Reflector G-ACh Type | This document |
The authors would like to thank Bharath Vasudevan, Ali Sianati, and Parag Jain for the discussions on method to punt STAMP test packets to control-plane for processing. The authors would also like to thank Greg Mirsky, Loa Andersson, and Stewart Bryant for reviewing this document and providing useful comments and suggestions.¶