RFC 6456
Multi-Segment Pseudowires in Passive Optical Networks
Multi-Segment Pseudowires in Passive Optical Networks 🦋
okay so bestie, this is RFC 6456 💖 It's an Informational doc (so NOT a standards track thing, just here to share knowledge ✨), dated November 2011, with ISSN 2070-1721. The authors are H. Li and R. Zheng from Huawei Technologies, plus A. Farrel from Old Dog Consulting. The title block doesn't list anything it obsoletes or updates, so it's a fresh little drop 🌸
Abstract
okay so this doc is all about using MPLS multi-segment pseudowires (MS-PWs) in a dual-tech setup that mixes a Passive Optical Network (PON) with an MPLS Packet Switched Network (PSN) 🔥
PON tech can be used in mobile backhaul networks to cover the end segments closest to the aggregation devices. And in those setups there can be a huge number of pseudowire (PW) Terminating Provider Edge (T-PE) nodes 😭 The MPLS control plane could provision them, but supporting those protocols would make managing the T-PEs way more complicated and seriously more expensive. The other option is static or management plane configuration, but with that many segments in a PON, the network manager would be buried under the workload 💀
So this doc explains how to set up the end segment of an end-to-end MPLS PW over a Gigabit-capable Passive Optical Network (G-PON) or a 10 Gigabit-capable Passive Optical Network (XG-PON). It uses the G-PON and XG-PON management protocol, the Optical Network Termination Management and Control Interface (OMCI). The result is that PW provisioning is simpler and faster than doing it by hand 🚀
It also shows how an MS-PW can be built from an end segment running over a PON and then switched onto one or more segments running over an MPLS PSN. main character energy ✨
Status of This Memo
ngl this part is the legal-ish stuff, but here's the tea 🍓
This document is not an Internet Standards Track specification. It's published just for informational purposes 🧚
It is a product of the Internet Engineering Task Force (IETF) and represents the consensus of the IETF community. It went through public review and got approved for publication by the Internet Engineering Steering Group (IESG). Not every doc the IESG approves is a candidate for any level of Internet Standard, so check Section 2 of RFC 5741 for the details 💅
Info about the current status of this document, any errata, and how to send feedback lives at http://www.rfc-editor.org/info/rfc6456 🎀
Copyright Notice
Copyright (c) 2011 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 (http://trustee.ietf.org/license-info) as they stand on the date of publication. Please read these carefully, because they spell out your rights and restrictions for this document 🫶 Any Code Components pulled out of this document must include the Simplified BSD License text, as described in Section 4.e of the Trust Legal Provisions. They come without warranty, as described in the Simplified BSD License.
Table of Contents 🌈
1. Introduction ....................................................2
2. Terminology for G-PON/XG-PON ....................................5
3. Multi-Segment Pseudowire over PON Network Reference Model .......6
4. Label Provisioning for Pseudowires over PON .....................9
5. Security Considerations .........................................9
6. References .....................................................10
6.1. Normative References ......................................10
6.2. Informative References ....................................111. Introduction ✨
okay so PWs running over Packet Switched Networks (PSNs) are defined in [RFC3985] 🦋 That architecture gets extended in [RFC5659] for multi-segment pseudowires (MS-PWs), which satisfy the requirements in [RFC5254]. If you want more MS-PW detail, go check [RFC6073] 💖
MS-PWs are seriously useful for certain applications where lots of paths come together toward one common point in the network, like mobile backhaul 🔥 The segments can get bundled inside tunnels between PW switching points, which boosts scalability and cuts down on the number of control plane adjacencies, in cases where a control plane is in use 🧚
Segments of an MS-PW in a PSN can be set up with manual provisioning (static PWs) or with a dynamic control plane like the Label Distribution Protocol (LDP) [RFC5036] [RFC4447] 💅
In a lot of access and metro networks, a Passive Optical Network (PON) gives you longer distance, higher bandwidth, and better economy than stuff like point-to-point Ethernet or Digital Subscriber Line (DSL) 🌸 And mobile backhaul over PON is already being deployed in the real world, so this isn't hypothetical at all 🚀
Here's Figure A, the physical infrastructure of an Optical Distribution Network (ODN) 👑
| |
|<--Optical Distribution Network-->|
| |
| branch main |
+-----+ fibers fiber
Base ------| | | |
Stations ------| ONU |\ | |
------| | \ V |
+-----+ \ |
\ +----------+ |
+-----+ \| | | +-----+
Base ------| | | Optical | V | |
Stations ------| ONU |---------| Splitter |-------------| OLT |
------| | /| | | |
+-----+ / +----------+ +-----+
/
+-----+ /
Base ------| |/
Stations ------| ONU |
------| |
+-----+
Figure A: Typical PON System ArchitectureIn a PON, the Optical Network Unit (ONU) and the Optical Line Termination (OLT) are adjacent nodes, connected by an Optical Distribution Network (ODN). The ODN is made of optical fibers and optical splitters arranged in a tree topology 🌳 The link between each ONU and the OLT is simulated as a point-to-point link, and there's no path redundancy between them. The OLT lives in the central office, while the ONUs live at customer premises. ONUs get deployed in huge numbers, so they're super cost sensitive 💸 If you want more about ODNs, see [G.984.1] 🎀
In a mobile backhaul network, lots of 2G and 3G base stations still use legacy interfaces like Time-Division Multiplexing (TDM) and ATM 🙈 So these native services must be carried across the PON before they can be carried over the PSN using PWs. This doc explains how MS-PWs can be built with end segments that run over the PON and get switched onto further segments running over the PSN. In this setup, the base stations connect to the ONUs through access circuits (ACs), and the ONUs act as Terminating Provider Edge (T-PE) nodes. The OLT is a Switching Provider Edge (S-PE). This model is shown in Figure B 🦋
Routing protocols and dynamic label distribution protocols like LDP would significantly increase the ONUs' cost and complexity, since they put requirements on both hardware and software. On top of coding and maintaining those protocols, a much more powerful CPU and more memory are needed for them to run smoothly 🥺
Since there's no redundant path between each ONU and the OLT, routing and path selection aren't necessary in the PON 💖 That makes static provisioning of PW labels between the ONUs and the OLT simple, and it's the preferred option because it can greatly reduce the cost of an ONU acting as a T-PE 🤑 However, using a Network Management System (NMS) to provision PWs in a PON would mean the network manager has to configure each ONU, and configure the OLT once for each PW 😩 With potentially very many ONUs (and therefore very many PWs) in a PON, that's a huge amount of operational effort. There's also the problem that the configuration of each PW at the OLT and the ONU might end up inconsistent, since those nodes get configured separately 🙃
[G.988] defines the G-PON/XG-PON management protocol called the "ONT Management and Control Interface (OMCI)". OMCI is an implementation requirement for all G-PON/XG-PON systems. If OMCI is used to configure PWs on an ONU, you don't need to upgrade the ONU's hardware, and the extension to the OMCI implementation is negligible. That's a way to cut the cost and complexity of provisioning PWs in a G-PON/XG-PON 🤩
This doc shows how the two technologies (PON and PSN) can be combined to give you an end-to-end multi-segment MPLS PW ✨ The MPLS PWs also get carried over the PON inside MPLS Label Switched Path (LSP) tunnels. There's one MPLS LSP tunnel in each direction between each ONU and the OLT, in a one-to-one relationship with the underlying G-PON/XG-PON channel. The OLT and ONU perform penultimate hop popping (PHP) [RFC3031] on this single-hop LSP, so no labels are used on the wire for the MPLS LSP tunnel 🫧 Nothing changes about how MPLS PWs operate. MPLS packets are carried by the G-PON link layer according to ITU-T [G.984.3amd1], or by the XG-PON link layer according to ITU-T [G.987.3] 🌈
to be continued... 🎀
2. Terminology for G-PON/XG-PON ✨
okay so first we need the vocab 🎀 these terms come from [G.987], and we're using them the same way throughout:
- Gigabit-capable Passive Optical Network (G-PON). A flavor of PON access tech that supports rates above 1 Gbit/s. It's based on the ITU-T G.984.x series of Recommendations [G.984.1], [G.984.4amd2] and [G.984.3amd1]. 🌸
- G-PON Encapsulation Method (GEM). A data frame transport scheme for G-PON systems. It's connection oriented and can chop user data frames into variable-sized transmission fragments. ✂️
- GEM port. An abstraction of the G-PON adaptation layer. It's a logical connection tied to one specific client packet flow between the OLT and the ONU. 🔗
- 10-gigabit-capable Passive Optical Network (XG-PON). A PON system with nominal rates on the order of 10 Gbit/s in at least one direction. It implements the protocol suite from the ITU-T G.987.x series Recommendations. 💜
- XG-PON encapsulation method (XGEM). The XG-PON version of the frame transport scheme. It's connection oriented and fragments user data frames into variable-sized transmission fragments, same energy as GEM. 🦋
- XGEM port. An abstraction in the XG-PON transmission convergence (XGTC) service adaptation sublayer. It's a logical connection for one specific client packet flow. 🧚
- Optical Distribution Network (ODN). In PON land, this is a tree of optical fibers in the access network, plus power or wavelength splitters, filters, and other passive optical devices. 🌳
- Optical Line Termination (OLT). The device that terminates the common (root) endpoint of an ODN. It implements a PON protocol (like the one in the ITU-T G.984 series) and adapts PON PDUs for uplink over the provider service interface. It also handles management and maintenance for the ODN and the ONUs under it. In this document, the OLT is a network element with multiple PON ports and uplinks that give it switching capability toward the PSN. 👑
- Optical Network Termination (ONT). A single subscriber device that terminates any one of the distributed (leaf) endpoints of an ODN. It implements a PON protocol and adapts PON PDUs to subscriber service interfaces. An ONT is a special case of an ONU. 🏠
- Optical Network Unit (ONU). The generic term for a device that terminates any one of the leaf endpoints of an ODN, implements a PON protocol, and adapts PON PDUs to subscriber service interfaces. In some contexts, an ONU implies a multiple subscriber device. In this document, an ONU is a Provider Edge (PE) node with one or more ACs that map to the service interfaces, and it acts as a T-PE. 🎀
- ONT Management and Control Interface (OMCI). The management and control channel between OLT and ONT in PON. OMCI runs between the OLT Controller and the ONT Controller across a GEM connection that's set up when the ONT initializes. It's asymmetric: the OLT's Controller is the master and the ONT's is the slave. One OLT Controller can manage multiple ONTs by running multiple instances of the protocol over separate control channels. OMCI manages the ONT for configuration, fault management, performance, and security. 🛠️
- Passive Optical Network (PON). An OLT connected, through an ODN, to one or more ONUs or ONTs. 🌈
3. Multi-Segment Pseudowire over PON Network Reference Model
okay this is where it gets spicy 🌶️ [RFC5659] gives us several pseudowire emulation edge-to-edge (PWE3) reference architectures for the multi-segment case. They're general models extended from [RFC3985] so you can run point-to-point pseudowires through multiple PSN tunnels. 💅
so a G-PON/XG-PON is made of an OLT, an ODN, and multiple ONUs. The ODN is literally a fiber tree giving you the physical connections between the OLT and the ONUs. G-PON/XG-PON has its own physical layer and link layer. A GEM/XGEM port is a logical point-to-point connection between the OLT and each ONU, sitting over the GPON Transmission Convergence (GTC) layer or the XG-PON transmission convergence (XGTC) layer. An ONU can have more than one GEM/XGEM port to the OLT, and each port can get its own Quality of Service (QoS) and bandwidth. It's giving customizable 🦋
Figure B shows how the MS-PW architecture gets applied to a network with a PON and a PSN. TPE1 is an ONU and S-PE1 is an OLT. One or more PWs run between the ONU and the remote end system (TPE2) to provide service emulation between Customer Edges (CEs) (CE1 and CE2). 🧚
Native |<------Multi-Segment Pseudowire------>| Native
Service | GEM/XGEM | Service
(AC) | |<--Port-->| | (AC)
| | | | | |
| | | PSN | PSN | |
| | |<-Tunnel->| |<-Tunnel->| | |
| V V V V V V |
| +----+ +-----+ +----+ |
+----+ | |TPE1|===========|S-PE1|==========|TPE2| | +----+
| |------|..... PW.Seg't1....X....PW.Seg't3.....|-------| |
| CE1| | | | | | | | | |CE2 |
| |------|..... PW.Seg't2....X....PW.Seg't4.....|-------| |
+----+ | | |===========| |==========| | | +----+
Base ^ +----+ +-----+ +----+ ^
Station | Provider Edge 1 ^ Provider Edge 2 |
| ONU | |
| PW switching point |
| OLT |
| |
|<------------------ Emulated Service --------------->|Figure B: MS-PW over PON Network Reference Model
Base ----AC-- TPE1--PW over PON--SPE1--PW over PSN--TPE2--AC------
Station
---------- ----------
-------- |Packetized| |Packetized| --------
|Native | |Native | |Native | |Native |
|Service | |Service | |Service | |Service |
-------- |----------| |----------| --------
|Control | |Control |
|Word | |Word |
|----------| |----------|
|PW Label | |PW Label |
|----------| |----------|
|GEM/XGEM | |MPLS |
|----------| |Tunnel |
|GPON/XGPON| |Label |
|-Phy | | |
---------- |----------|
|Link Layer|
|----------|
|Phy |
----------Figure C: On-Wire Data Encapsulations for MS-PWs
in each of the PON and the PSN, the PW segments get carried in PSN tunnels. In the PSN, the tunnel is set up and run like normal for PWs (see [RFC3985]). In the PON, the tunnel is a single-hop MPLS LSP tunnel, so the OLT and ONU are label edge routers. The OLT and ONU both use PHP on that MPLS LSP tunnel. Since it's a single-hop LSP (no MPLS-capable nodes between the OLT and ONU), there's no MPLS encapsulation for the tunnel on the wire. No label, no shim header. That's why the on-wire encapsulations look like Figure C. 🌸
Quick note: all PW segments are the same technology, which is packet encapsulated. ✨
The PW label lets multiple PWs share a single GEM/XGEM port inside the MPLS LSP tunnel. So traffic for multiple base stations stays separate, and one base station can run different services on separate ACs. On top of that, the multiple ACs at one ONU can belong to different native services. 💖
Each ONU can also run more than one GEM/XGEM port (each carrying a single MPLS LSP tunnel) back to the OLT. That gives you more bandwidth, and so more PWs. It can also be a simple way to bundle PWs that are supposed to route across different PSN tunnels in the core network, or even across different core networks. 🍓
Right now, Ethernet over GEM/XGEM is the dominant encapsulation in G-PON/XG-PON. For a quick rollout of MPLS over G-PON/XG-PON, you can put MPLS PWs over Ethernet over GEM/XGEM. It's an alternative way to transport them that works with existing hardware. 🫶
4. Label Provisioning for Pseudowires over PON
so here's the setup 🧚 for an MS-PW with a segment running over a PON, the OLT acts as an S-PE and the ONU acts as a T-PE. PW provisioning can be done with static configuration, e.g., from an NMS. The catch? Every ONU has to be configured as each PW gets set up. With the huge number of ONUs (and PWs), that's quite forbidding. 😭
Thankfully, the labor of provisioning static labels at the ONUs can be cut down a lot by using a management protocol over PON. This keeps the ONU simple, since it doesn't need a new dynamic control protocol. 💅
The usual management protocol in a G-PON/XG-PON system for managing and controlling ONUs is OMCI. It handles all configuration of the G-PON/XG-PON physical layer and the data GTC/XGTC layer on ONUs. Per [G.984.4amd2] and [G.988], OMCI can also set up PWs and the MPLS LSP Tunnels from ONUs to the OLT. When you use OMCI to provision PWs in a G-PON/XG-PON, the network manager only sends configuration information to the OLT. The OLT picks suitable PW labels and sends all the PW and MPLS LSP tunnel parameters to the ONUs through OMCI. The AC can be identified in the OMCI signaling, so the network manager doesn't need to configure the PWs at each ONU. The network manager really said "I'm out" 🙌
OMCI supports configuring a number of PW types, including TDM, ATM, and Ethernet. The protocol can also let the ONU tell the OLT the status of the AC. 🦋
5. Security Considerations 🔐✨
okay so bestie, this section is the serious one, but it's also kind of iconic in how it explains why the security stuff holds up. Let's get into it 💅
This document describes a variation of a multi-segment pseudowire running over an MPLS PSN, where one (or both) of the MPLS PSNs that connects a T-PE to its associated S-PE is swapped out for a G-PON/XG-PON PSN. The security considerations that apply to the PW itself [RFC3985] [RFC4385] are unchanged by this change in PSN type 🦋. For more PW security tea, check the security considerations section of whichever specific PW type you're deploying 🫶.
G-PON/XG-PON [G.987.3] [G.984.3amd1] has security mechanisms that are just as strong as the ones in a well-secured MPLS PSN 🔥. So using a G-PON/XG-PON PSN instead of an MPLS PSN does not increase the security risk of a multi-segment pseudowire 💖.
Protecting against an attack at the physical or data link layer of the PON is out of the scope of this document 🙈.
The MPLS control plane and management plane mechanisms are unchanged by this document 🌸. This document introduces OMCI as a provisioning mechanism that runs between the OLT Controller and the ONT Controller across a GEM connection that is established at ONT initialization ✨. In other words, the protocol runs on an in-fiber control channel 🧚. That means injection and modification of OMCI messages would be very hard 💀. It's even harder than injection or modification in an MPLS Associated Channel Header (ACH), which is accepted as providing adequate security by isolation ([RFC4385] and [RFC5586]) 👑.
6. References 📚
6.1. Normative References 💜
[G.984.1] ITU-T, "Gigabit-capable passive optical networks (GPON):
General characteristics", March 2008,
<http://www.itu.int/rec/T-REC-G.984.1-200803-I>.
[G.984.3amd1]
ITU-T, "Gigabit-capable Passive Optical Networks (G-PON):
Transmission convergence layer specification", February
2009, <http://www.itu.int/rec/T-REC-
G.984.3-200902-I!Amd1>.
[G.987] ITU-T, "10-Gigabit-capable passive optical network (XG-
PON) systems: Definitions, abbreviations, and acronyms",
October 2010, <http://www.itu.int/rec/T-REC-
G.987-201010-I>.
[G.987.3] ITU-T, "10-Gigabit-capable passive optical networks (XG-
PON): Transmission convergence (TC) layer specification",
October 2010, <http://www.itu.int/rec/T-REC-
G.987.3-201010-I/en>.
[G.988] ITU-T, "ONU management and control interface (OMCI)
specification", October 2010, <http://www.itu.int/rec/T-
REC-G.988-201010-I>.
[RFC3031] Rosen, E., Viswanathan, A., and R. Callon, "Multiprotocol
Label Switching Architecture", RFC 3031, January 2001.
[RFC3985] Bryant, S., Ed., and P. Pate, Ed., "Pseudo Wire Emulation
Edge-to-Edge (PWE3) Architecture", RFC 3985, March 2005.
[RFC4385] 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.
[RFC4447] Martini, L., Ed., Rosen, E., El-Aawar, N., Smith, T., and
G. Heron, "Pseudowire Setup and Maintenance Using the
Label Distribution Protocol (LDP)", RFC 4447, April 2006.
[RFC5036] Andersson, L., Ed., Minei, I., Ed., and B. Thomas, Ed.,
"LDP Specification", RFC 5036, October 2007.
[RFC5254] Bitar, N., Ed., Bocci, M., Ed., and L. Martini, Ed.,
"Requirements for Multi-Segment Pseudowire Emulation
Edge-to-Edge (PWE3)", RFC 5254, October 2008.
[RFC5586] Bocci, M., Ed., Vigoureux, M., Ed., and S. Bryant, Ed.,
"MPLS Generic Associated Channel", RFC 5586, June 2009.
[RFC5659] Bocci, M. and S. Bryant, "An Architecture for
Multi-Segment Pseudowire Emulation Edge-to-Edge", RFC
5659, October 2009.6.2. Informative References 🧠
[G.984.4amd2]
ITU-T, "Gigabit-capable passive optical networks (G-PON):
ONT management and control interface specification",
November 2009, <http://www.itu.int/rec/T-REC-
G.984.4-200911-I!Amd2>.
[RFC6073] Martini, L., Metz, C., Nadeau, T., Bocci, M., and M.
Aissaoui, "Segmented Pseudowire", RFC 6073, January 2011.Authors' Addresses 🎀
Hongyu Li
Huawei Technologies
Huawei Industrial Base
Shenzhen
China
EMail: hongyu.lihongyu@huawei.com
Ruobin Zheng
Huawei Technologies
Huawei Industrial Base
Shenzhen
China
EMail: robin@huawei.com
Adrian Farrel
Old Dog Consulting
EMail: adrian@olddog.co.ukThat's the whole RFC, bestie! 🌈💖 Thanks for hanging with me through all three parts ✨
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- ietf: rfc6456 2011-11
- 💖 girlified by claude-haiku-5-5 on : a language model's rewrite of the original, under its terms