RFC 8638
IPv4 Multicast over an IPv6 Multicast in Softwire Mesh Networks
RFC 8638: IPv4 Multicast over an IPv6 Multicast in Softwire Mesh Networks 🦋
Okay bestie, this one is from the Internet Engineering Task Force (IETF) 💖 It's numbered RFC 8638, dated September 2019, and it's a Standards Track doc, so it's an actual official spec and not just a vibe. ✨ It's an ISSN 2070-1721 document, which is the series ID for RFCs.
The authors are M. Xu, Y. Cui, and J. Wu from Tsinghua University, plus S. Yang from Shenzhen University and C. Metz from Cisco Systems 🎀 The header doesn't list anything it obsoletes or updates, so it's standing on its own.
Abstract
During the transition to IPv6 🌈 there are situations where a backbone network runs one IP address family internally (the internal IP or I-IP family) and connects client networks that run a different family (the external IP or E-IP family). In those situations, the I-IP backbone needs to offer both unicast and multicast transit services to the client E-IP networks 🫶
This doc describes a mechanism for supporting multicast across backbones where the I-IP and E-IP families are different. It focuses on the IPv4-over-IPv6 case because there aren't any real-world use cases for IPv6-over-IPv4 yet 💅
Status of This Memo
This is an Internet Standards Track document. 🔥
This document is a product of the Internet Engineering Task Force (IETF). It reflects the consensus of the IETF community. It went through public review and was approved for publication by the Internet Engineering Steering Group (IESG). If you want more on Internet Standards, check out Section 2 of RFC 7841 🧚
For the current status, any errata, and how to give feedback, go to https://www.rfc-editor.org/info/rfc8638 🌸
Copyright Notice
Copyright (c) 2019 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) as they stand on the date this document was published. Please read these carefully, because they spell out your rights and restrictions for this document. Code Components pulled out of this document must include the Simplified BSD License text described in Section 4.e of the Trust Legal Provisions, and they come with no warranty, as the Simplified BSD License describes 🎀
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Requirements Language . . . . . . . . . . . . . . . . . . . . 5
3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5
4. Scope . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
5. Mesh Multicast Mechanism . . . . . . . . . . . . . . . . . . 7
5.1. Mechanism Overview . . . . . . . . . . . . . . . . . . . 7
5.2. Group Address Mapping . . . . . . . . . . . . . . . . . . 7
5.3. Source Address Mapping . . . . . . . . . . . . . . . . . 8
5.4. Routing Mechanism . . . . . . . . . . . . . . . . . . . . 9
6. Control-Plane Functions of AFBR . . . . . . . . . . . . . . . 10
6.1. E-IP (*,G) and (S,G) State Maintenance . . . . . . . . . 10
6.2. I-IP (S',G') State Maintenance . . . . . . . . . . . . . 10
6.3. E-IP (S,G,rpt) State Maintenance . . . . . . . . . . . . 10
6.4. Inter-AFBR Signaling . . . . . . . . . . . . . . . . . . 10
6.5. SPT Switchover . . . . . . . . . . . . . . . . . . . . . 13
6.6. Other PIM Message Types . . . . . . . . . . . . . . . . . 13
6.7. Maintenance of Other PIM States . . . . . . . . . . . . . 13
7. Data-Plane Functions of the AFBR . . . . . . . . . . . . . . 13
7.1. Process and Forward Multicast Data . . . . . . . . . . . 13
7.2. TTL or Hop Count . . . . . . . . . . . . . . . . . . . . 14
7.3. Fragmentation . . . . . . . . . . . . . . . . . . . . . . 14
8. Packet Format and Translation . . . . . . . . . . . . . . . . 14
9. Softwire Mesh Multicast Encapsulation . . . . . . . . . . . . 16
10. Security Considerations . . . . . . . . . . . . . . . . . . . 16
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 16
12. Normative References . . . . . . . . . . . . . . . . . . . . 16
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 18
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 181. Introduction ✨
During the transition to IPv6 🌈 there are scenarios where a backbone network runs one IP address family internally (the internal IP or I-IP family) and connects client networks that run a different family (the external IP or E-IP family).
One way to solve this is to use the multicast features already built into the I-IP backbone so it can efficiently forward client E-IP multicast packets inside an I-IP core tree. 🦋 The I-IP tree is rooted at one or more ingress Address Family Border Routers (AFBRs) [RFC5565] and branches out to one or more egress AFBRs.
[RFC4925] lays out the requirements for the softwire mesh scenario, and it includes support for multicast traffic. It's likely that client E-IP multicast sources and receivers will live in different client E-IP networks hooked up to an I-IP backbone. That means the source-rooted or shared tree of the client E-IP has to cross the I-IP backbone 🧚
This could be done by reusing the multicast VPN (MVPN) approach described in [RFC6513]. MVPN-like schemes can support the softwire mesh scenario and get a "many-to-one" mapping between the E-IP client multicast trees and the transit-core multicast trees. 💜 The upside is that the number of trees in the I-IP backbone grows less than linearly with the number of E-IP client trees. Corporate enterprise networks, and by extension multicast VPNs, are known to run apps that create way too many (S,G) states, which are source-specific states tied to a given multicast group [RFC7761] [RFC7899]. Aggregation at the edge holds the (S,G) states for customers' VPNs, and the network operator still has to maintain them. The downside is that bandwidth and resources can get used inefficiently when multicast packets get delivered to a receiving AFBR with no attached E-IP receivers 💀
[RFC8114] offers a solution for delivering IPv4 multicast services over an IPv6 network, but it mainly focuses on the DS-Lite scenario [RFC6333], where IPv4 addresses given out by a broadband service provider are shared among customers. This document describes a detailed solution for the IPv4-over-IPv6 softwire mesh scenario, where client networks run IPv4 and the backbone network runs IPv6 🌸
Internet-style multicast is a bit different from the scenario in [RFC8114] because the trees are source-rooted and relatively sparse. The need for multicast aggregation at the edge (where lots of customer multicast trees map to one or more backbone multicast trees) doesn't exist, and so far nobody has identified it as a need. So alignment between the E-IP and I-IP multicast mechanisms is what's actually needed 🙈
[RFC5565] describes the "Softwire Mesh Framework". This document goes into more detail about how one-to-one mapping schemes ([RFC5565], Section 11.1) can be used for IPv4-over-IPv6 multicast 🍓
Figure 1 shows an example of how a softwire mesh network can carry multicast traffic. A multicast source S sits in one E-IP client network, and candidate E-IP group receivers sit in the same or different E-IP client networks, all sharing a common I-IP transit network. When E-IP sources and receivers aren't local to each other, they can only talk through the I-IP core. There may be several E-IP sources for a single multicast group, living in different client E-IP networks. With shared trees, the E-IP sources, receivers, and rendezvous points (RPs) might all be in different client E-IP networks. In the simplest case, a single operator manages the resources of the I-IP core, though an inter-operator setup is also possible and isn't ruled out 👑
+---------+ +---------+
| | | | +--------+
| E-IP | | E-IP +--+Source S|
| network | | network | +--------+
+---+-----+ +--+------+
| |
+-+--------+ +-------+--+
| | | upstream |
+-| AFBR +--+ AFBR |-+
| +----------+ +----------+ |
| | E-IP multicast
| I-IP transit core | packets are forwarded
| | across the I-IP
| +----------+ +----------+ | transit core
+-|downstream| |downstream|-+
| AFBR |--| AFBR |
+--+-------+ +--------+-+
| |
+---+----+ +---+----+
+--------+ | | | | +--------+
|Receiver+---+ E-IP | | E-IP +--+Receiver|
+--------+ |network | |network | +--------+
+--------+ +--------+
Figure 1: Softwire Mesh Multicast Framework2. Requirements Language
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. 🎀
3. Terminology 🎀
okay so before we dive in, we need the vocab locked. 💅 here's the glossary, bestie:
- Address Family Border Router (AFBR) - a router that connects two or more networks running different IP address families. 🌉 In softwire mesh multicast, the AFBR also runs E-IP and I-IP control planes so it can keep E-IP and I-IP multicast state, and it does the encapsulation and decapsulation of client E-IP multicast packets for transport across the I-IP core. An AFBR can act as a source and/or receiver in an I-IP multicast tree. 🦋
- Upstream AFBR: an AFBR that's closer to the source of a multicast data flow. ⬆️
- Downstream AFBR: an AFBR that's closer to a receiver of a multicast data flow. ⬇️
- I-IP (Internal IP): the IP address family supported by the core network. In this document, I-IP is IPv6. 💜
- E-IP (External IP): the IP address family supported by the client network(s) attached to the I-IP transit core. In this document, E-IP is IPv4. 🌸
- I-IP core tree: a distribution tree rooted at one or more AFBR source nodes and branching out to one or more AFBR leaf nodes. It's built with standard IP or MPLS multicast signaling protocols (this document focuses on IP multicast) running entirely inside the I-IP core network. The I-IP core tree forwards E-IP multicast packets that belong to E-IP trees across the I-IP core. Another name for it is multicast or multipoint softwire. 🧚
- E-IP client tree: a distribution tree rooted at one or more hosts or routers inside a client E-IP network, branching out to one or more leaf nodes in the same or different client E-IP networks. 🌈
- uPrefix64: the /96 unicast IPv6 prefix used to build an IPv4-embedded IPv6 unicast address [RFC8114]. 🔥
- mPrefix64: the /96 multicast IPv6 prefix used to build an IPv4-embedded IPv6 multicast address [RFC8114]. ✨
- PIMv4, PIMv6: see [RFC8114]. 🍓
- Inter-AFBR signaling: the mechanism downstream AFBRs use to send PIMv6 messages to the upstream AFBR. 💌
4. Scope 🌍
okay so this doc is all about the IPv4-over-IPv6 setup. Here's the picture, it's giving architecture realness 🤩:
+---------+ +---------+
| IPv4 | | IPv4 | +--------+
| Client | | Client |--+Source S|
| Network | | Network | +--------+
+----+----+ +----+----+
| |
+--+-------+ +-------+--+
| | | Upstream |
+-+ AFBR +--+ AFBR |-+
| +----------+ +----------+ |
| |
| IPv6 transit core |
| |
| +----------+ +----------+ |
+-+Downstream+--+Downstream+-+
| AFBR | | AFBR |
+--+-------+ +-------+--+
| |
+----+----+ +----+----+
+--------+ | IPv4 | | IPv4 | +--------+
|Receiver+--+ Client | | Client +--+Receiver|
+--------+ | Network | | Network | +--------+
+---------+ +---------+
Figure 2: IPv4-over-IPv6 ScenarioIn Figure 2, the E-IP client networks run IPv4, and the I-IP core runs IPv6. 💖
Because the IPv6 group address space is so much bigger, the client E-IP tree can be mapped to a specific I-IP core tree. 🫶 This makes life easier on the AFBR, since it becomes possible to algorithmically map an IPv4 group/source address to an IPv6 group/source address and vice versa. 🧮
The IPv4-over-IPv6 scenario is an emerging requirement as network operators build out native IPv6 backbone networks. 🏗️ These networks support native IPv6 services and apps, but in lots of cases, legacy IPv4 unicast and multicast services will also need to be accommodated. 👑
5. Mesh Multicast Mechanism 🕸️
5.1. Mechanism Overview
okay this part is the main plot ngl 🎬 Routers in the client E-IP networks have routes to all the other client E-IP networks. Through PIMv4 messages, E-IP hosts and routers discover or learn about IPv4 addresses that are in (S,G) or (,G) state [RFC7761]. 🔍 Any I-IP multicast state set up in the core is called (S',G') or (,G') and it's kept separate from E-IP multicast state. 🧩
Say a downstream AFBR gets an E-IP PIM Join/Prune message from the E-IP network for either an (S,G) tree or a (*,G) tree. 📨 The AFBR translates the PIMv4 message into a PIMv6 message, and that PIMv6 message is aimed at the I-IP IPv6 address of the upstream AFBR. 🎯 When the PIMv6 message arrives at the upstream AFBR, it gets translated back into a PIMv4 message. 🔁 The result is that E-IP trees get built, along with a matching I-IP tree in the I-IP network. 🌳 An example of the packet format and translation is in Section 8. 👀
Here's the thing: it is incumbent upon the AFBRs to do the PIM message conversions in the control plane and the IP group address conversions or mappings in the data plane. ⚙️ The AFBRs do an algorithmic, one-to-one mapping of IPv4 to IPv6. 💯
5.2. Group Address Mapping
A simple algorithmic mapping between IPv4 multicast group addresses and IPv6 group addresses gets done here. 🧠 Here's Figure 3 as a little reminder of the format:
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
| 0-------------32--40--48--56--64--72--80--88--96-----------127|
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
| mPrefix64 | group address |
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
Figure 3: IPv4-Embedded IPv6 Multicast Address FormatAn IPv6 multicast prefix (mPrefix64) gets provisioned on each AFBR. 🛠️ AFBRs prepend that prefix to an IPv4 multicast group address whenever they translate it into an IPv6 multicast group address. 💫
The construction of the mPrefix64 for Source-Specific Multicast (SSM) is the same as the construction of the mPrefix64 described in Section 5 of [RFC8114]. 📖
With this scheme, every IPv4 multicast address can be mapped to an IPv6 multicast address (with the assigned prefix), and every IPv6 multicast address with the assigned prefix can be mapped back to an IPv4 multicast address. 🔄 The group address translation algorithm is specified in Section 5.2 of [RFC8114]. 🙌
5.3. Source Address Mapping
There are two kinds of multicast: Any-Source Multicast (ASM) and SSM. 🎶 The I-IP network and E-IP network might support different kinds of multicast, and then the source address translation rules needed to cover every possible scenario could get super complicated. 😵💫 But since SSM can be implemented with a strict subset of the PIM-SM protocol mechanisms [RFC7761], we can treat the I-IP core as SSM-only to keep things as simple as possible. ✂️ That leaves only two scenarios to talk through in detail:
- E-IP network supports SSM
One possible way to make sure the translated PIMv6 message reaches the upstream AFBR is to set S' to a virtual IPv6 address that leads to the upstream AFBR. 🛣️ The unicast address translation should be achieved according to [RFC6052]. 🪄
- E-IP network supports ASM
The (S,G) source list entry and the (,G) source list entry differ only in that the latter has both the WildCard (WC) and RPT bits of the Encoded-Source-Address set, while in the former, those bits are cleared. 🔬 (See Section 4.9.5.1 of [RFC7761].) So the source list entries in (,G) messages can be translated into source list entries in (S',G') messages by clearing both the WC and RPT bits at downstream AFBRs, and the reverse translation at upstream AFBRs works the other way around. 🔁✨
5.4. Routing Mechanism
With mesh multicast, PIMv6 messages coming from a downstream AFBR need to get propagated to the correct upstream AFBR, and every AFBR needs the /96 prefix in the IPv4-embedded IPv6 source address format [RFC6052]. 🗺️
So to make this happen, every AFBR MUST announce the address of one of its E-IPv4 interfaces in the "v4" field [RFC6052] alongside the corresponding uPrefix46. 📣 The announcement MUST be sent to the other AFBRs through Multiprotocol BGP (MBGP) [RFC4760]. 📬 Every uPrefix64 that an AFBR announces MUST be unique. 🔐 "uPrefix64" is an IPv6 prefix, and its distribution mechanism is the same as the traditional mesh unicast scenario. 🌐
Okay, plot twist 😳: the "v4" field is an E-IP address, and BGP messages are not tunneled through softwires or any other mechanism specified in [RFC5565]. So AFBRs MUST be able to transport and encode/decode BGP messages that are carried over the I-IP, and whose Network Layer Reachability Information (NLRI) and next hop (NH) are of the E-IP address family. 🧐
This is how it goes down: when a downstream AFBR receives an E-IP PIM (S,G) message, it can translate that into (S',G') by looking up the IP address of the corresponding AFBR's E-IP interface. 🔎 Since the uPrefix64 of S' is unique and known to every router in the I-IP network, the translated message gets forwarded to the corresponding upstream AFBR, and the upstream AFBR can translate it back to (S,G). 🎁
When a downstream AFBR receives an E-IP PIM (,G) message, S' can be generated with the "source address" field set to * (the wildcard value). 🃏 The translated message gets forwarded to the corresponding upstream AFBR. Every PIM router within a PIM domain MUST be able to map a particular multicast group address to the same RP when the source address is set to the wildcard value. (See Section 4.7 of [RFC7761].) 🏠 So when the upstream AFBR checks the "source address" field of the message, it finds the IPv4 address of the RP and figures out that this was originally a (,G) message. 🕵️♀️ It then gets translated back to the (*,G) message and processed. 🥳
6. Control-Plane Functions of AFBR 🧠
AFBRs are responsible for the functions detailed in the subsections that follow. 📋
6.1. E-IP (*,G) and (S,G) State Maintenance
E-IP (,G) and (S,G) state maintenance for an AFBR is the same as E-IP (,G) and (S,G) state maintenance for a multicast AFTR (mAFTR) described in Section 7.2 of [RFC8114]. 🤝
6.2. I-IP (S',G') State Maintenance
It's possible that the I-IP transit core runs another, non-transit, I-IP PIM-SSM instance. 🤔 Since the translated source address starts with the unique "Well-Known" prefix or the ISP-defined prefix that MUST NOT be used by another service provider, mesh multicast won't influence non-transit PIM-SSM multicast at all. 🛡️ When an AFBR receives an I-IP (S',G') message, it MUST check S'. If S' starts with the unique prefix, then the message is actually a translated E-IP (S,G) or (*,G) message, and the AFBR translates it back to a PIMv4 message and processes it. 🔙💅
6.3. E-IP (S,G,rpt) State Maintenance
When an AFBR wants to propagate a Join/Prune(S,G,rpt) message [RFC7761] to an I-IP upstream router, the AFBR MUST operate as specified in Sections 6.5 and 6.6. 🫡🌸
6.4. Inter-AFBR Signaling
Okay so picture this: one downstream AFBR has joined the RPT of (,G) AND the shortest path tree of (S,G), and it's decided to do an SPT switchover 🚀 (see Section 4.2.1 of [RFC7761]). According to [RFC7761], it SHOULD send a Prune(S,G,rpt) message along with the periodic Join(,G) message upstream towards the RP. But here's the tea 🫖: routers in the I-IP transit core don't process (S,G,rpt) messages, because the I-IP transit core is treated as SSM only 😭. So the downstream AFBR can't prune S off this RPT, and it ends up getting two copies of the same data for (S,G). Not it! 🙅♀️ So we introduce a brand new mechanism so downstream AFBRs can tell upstream AFBRs to prune any given S from an RPT 💅.
When a downstream AFBR wants to propagate an (S,G,rpt) message upstream, it SHOULD encapsulate the (S,G,rpt) message, then send the encapsulated unicast message to the corresponding upstream AFBR, which we call "RP'" 🎀.
When RP' gets this encapsulated message, it MUST decapsulate it the same way as in the unicast scenario and pull out the original (S,G,rpt) message 🧚. Now, the incoming interface of this message might be different from the outgoing interface that sends multicast data down to the corresponding downstream AFBR. There might also be other downstream AFBRs that still need multicast data for (S,G) coming in on that incoming interface. So RP' should NOT just process this message the way [RFC7761] describes on the incoming interface 🙈.
To handle this, we introduce an "interface agent" that processes all the encapsulated (S,G,rpt) messages the upstream AFBR receives 🦋. The interface agent's RP' SHOULD prune S from the RPT of group G when no downstream AFBR is subscribed to receive multicast data for (S,G) along the RPT. Wait, hold on, that's not quite what the spec says, let me be precise: the original says the interface agent's RP' "should prune S from the RPT of group G when no downstream AFBR is subscribed to receive multicast data for (S,G) along the RPT." So it's lowercase "should" here, not a key word, and I'm keeping it exactly as the original wording: the RP' should prune S from the RPT of group G when no downstream AFBR is subscribed to receive multicast data for (S,G) along the RPT.
This way, downstream AFBRs won't miss any multicast data they need ✨. The tradeoff is that multicast data for (S,G) gets duplicated along the RPT received by the AFBRs affected by the SPT switchover, as long as at least one downstream AFBR exists that hasn't yet sent Prune(S,G,rpt) messages to the upstream AFBR 💀.
In certain deployment scenarios (e.g., if there's only a single downstream router), the interface agent function is NOT required 🙌.
The mechanism used to achieve this is left to the implementation. Here's one possible solution for an "interface agent" implementation 👑:
+----------------------------------------+
| |
| +-----------+----------+ |
| | PIM-SM | UDP | |
| +-----------+----------+ |
| ^ | |
| | | |
| | v |
| +----------------------+ |
| | I/F Agent | |
| +----------------------+ |
| PIM ^ | multicast |
| messages | | data |
| | +-------------+---+ |
| +--+--|-----------+ | |
| | v | v |
| +--------- + +----------+ |
| | I-IP I/F | | I-IP I/F | |
| +----------+ +----------+ |
| ^ | ^ | |
| | | | | |
+--------|-----|----------|-----|--------+
| v | v
Figure 4: Interface Agent Implementation ExampleFigure 4 shows an example of an interface agent using UDP encapsulation 🌸. The interface agent has two jobs. In the control plane, it works like a real interface that has joined (,G), standing in for all the I-IP interfaces that are outgoing interfaces of the (,G) state machine. It also processes the (S,G,rpt) messages received from all the I-IP interfaces 💜.
The interface agent keeps downstream (S,G,rpt) state machines for every downstream AFBR, and it only submits Prune(S,G,rpt) messages to the PIM-SM module when EVERY (S,G,rpt) state machine is in the Prune(P) or PruneTmp(P') state. That means no downstream AFBR is subscribed to receive multicast data for (S,G) along the RPT of G 🫶. Once a (S,G,rpt) state machine moves to NoInfo (NI) state, which means that AFBR has switched back to receiving multicast data for (S,G) along the RPT, the interface agent MUST send a Join(S,G,rpt) to the PIM-SM module immediately 🚨.
In the data plane, when a multicast data packet comes in, the interface agent MUST encapsulate it first, then propagate the encapsulated packet out of every I-IP interface 🍓.
NOTICE: It's possible that an E-IP neighbor of RP' has joined the RPT of G, so the per-interface state machine for receiving E-IP Join/Prune(S,G,rpt) messages SHOULD be preserved 📣.
6.5. SPT Switchover
After a new AFBR requests traffic for a multicast group, it receives all the data from the RPT at first 🌈. At this point, every downstream AFBR gets multicast data from any source on this RPT, whether or not they've already switched over to an SPT 😩.
To cut down on this redundancy, it is RECOMMENDED that every AFBR's SwitchToSptDesired(S,G) function uses the "switch on first packet" policy 🏃♀️. This keeps the delay before switching to the SPT as small as possible. Once every AFBR has done the SPT switchover for every S of group G, no data gets forwarded on the RPT of G, so no more unnecessary duplication happens 🎉.
6.6. Other PIM Message Types
Besides Join and Prune, other message types exist, including Register, Register-Stop, Hello and Assert. Register and Register-Stop messages are sent by unicast, while Hello and Assert messages are only used between directly linked routers to negotiate with each other 🤝. It's not necessary to translate these for forwarding, so processing them is out of scope for this document 🙃.
6.7. Maintenance of Other PIM States
Besides the states mentioned above, other states exist, including (,,RP) and I-IP (*,G') state. Since we treat the I-IP core as SSM only, maintaining these states is out of scope for this document 🧁.
- Data-Plane Functions of the AFBR
7.1. Process and Forward Multicast Data
Refer to Section 7.4 of [RFC8114]. If there's at least one outgoing interface whose IP address family is different from the incoming interface, the AFBR MUST encapsulate this packet with mPrefix64-derived and uPrefix64-derived IPv6 addresses to form an IPv6 multicast packet 💖.
7.2. TTL or Hop Count
When encapsulating, the TTL and hop count in the outer header SHOULD be set by policy. When decapsulating, the TTL and hop count in the inner header SHOULD be modified by policy; it MUST NOT be incremented, and it MAY be decremented to reflect the cost of tunnel forwarding 🕰️. Also, how TTL and hop count info in protocol headers is processed depends on the tunneling technology, which is out of scope of this document 🙈.
7.3. Fragmentation
The encapsulation done by an upstream AFBR makes packets bigger 📦. So the outgoing I-IP link MTU might not fit the larger packet size. Core operators can't always raise the MTU of every link, so source fragmentation after encapsulation and reassembly of encapsulated packets MUST be supported by AFBRs [RFC5565] 🧩. Path MTU Discovery (PMTUD) [RFC8201] SHOULD be enabled, and ICMPv6 packets MUST NOT be filtered in the I-IP network 🚫. Fragmentation and tunnel configuration considerations are in Section 8 of [RFC5565]. The detailed procedure can be found in Section 7.2 of [RFC2473] 📖.
- Packet Format and Translation
Because the PIM-SM specification is independent of the underlying unicast routing protocol, the packet format in Section 4.9 of [RFC7761] stays the same, except that the group address and source address MUST be translated when traversing an AFBR 🔁.
For example, Figure 5 shows the register-stop message format in the IPv4 and IPv6 address families 👀.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|PIM Ver| Type | Reserved | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| IPv4 Group Address (Encoded-Group format) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| IPv4 Source Address (Encoded-Unicast format) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
(a) IPv4 Register-Stop Message Format
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|PIM Ver| Type | Reserved | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| IPv6 Group Address (Encoded-Group format) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| IPv6 Source Address (Encoded-Unicast format) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
(b) IPv6 Register-Stop Message Format
Figure 5: Register-Stop Message FormatIn Figure 5, the semantics of the fields "PIM Ver", "Type", "Reserved", and "Checksum" are specified in Section 4.9 of [RFC7761] 🧠.
IPv4 Group Address (Encoded-Group format): The encoded-group format of the IPv4 group address described in Section 4.9.1 of [RFC7761] 💫.
IPv4 Source Address (Encoded-Group format): The encoded-unicast format of the IPv4 source address described in Section 4.9.1 of [RFC7761] 💫.
IPv6 Group Address (Encoded-Group format): The encoded-group format of the IPv6 group address described in Section 5.2 🌟.
IPv6 Source Address (Encoded-Group format): The encoded-unicast format of the IPv6 source address described in Section 5.3 ⭐.
- Softwire Mesh Multicast Encapsulation
Softwire mesh multicast encapsulation doesn't require any one particular encapsulation mechanism. Instead, it MUST accommodate a variety of different encapsulation mechanisms and allow the use of encapsulation mechanisms mentioned in [RFC4925] 🎁. Also, all of the AFBRs attached to the I-IP network MUST implement the same encapsulation mechanism and follow the requirements mentioned in Section 8 of [RFC5565] 🔐.
10. Security Considerations 💀
okay so security is where we get SERIOUS bestie 🫶 The security concerns raised in [RFC4925] and [RFC7761] apply here too, so go read those as well 📚
The extra workload that comes with some schemes, like interface agents, could be abused by an attacker to run a DDoS attack 😭🚨
Compared with [RFC4925], this one needs even more careful thought 👀 An attacker could potentially set up lots of multicast trees in the edge networks, which would cause too many multicast states in the core network 🙈 To defend against this, BGP policies SHOULD be carefully configured, for example AFBRs only accepting Well-Known prefix advertisements from trusted peers 🛡️ On top of that, cryptographic methods for authenticating BGP sessions [RFC7454] could be used 🔐
11. IANA Considerations 🎀
This document has no IANA actions. Nothing to register, nothing to update, we're chilling 🌸
12. Normative References 📚
Here's the full reading list, every single reference exactly as written 💅
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC2473] Conta, A. and S. Deering, "Generic Packet Tunneling in
IPv6 Specification", RFC 2473, DOI 10.17487/RFC2473,
December 1998, <https://www.rfc-editor.org/info/rfc2473>.
[RFC4760] Bates, T., Chandra, R., Katz, D., and Y. Rekhter,
"Multiprotocol Extensions for BGP-4", RFC 4760,
DOI 10.17487/RFC4760, January 2007,
<https://www.rfc-editor.org/info/rfc4760>.
[RFC4925] Li, X., Ed., Dawkins, S., Ed., Ward, D., Ed., and A.
Durand, Ed., "Softwire Problem Statement", RFC 4925,
DOI 10.17487/RFC4925, July 2007,
<https://www.rfc-editor.org/info/rfc4925>.
[RFC5565] Wu, J., Cui, Y., Metz, C., and E. Rosen, "Softwire Mesh
Framework", RFC 5565, DOI 10.17487/RFC5565, June 2009,
<https://www.rfc-editor.org/info/rfc5565>.
[RFC6052] Bao, C., Huitema, C., Bagnulo, M., Boucadair, M., and X.
Li, "IPv6 Addressing of IPv4/IPv6 Translators", RFC 6052,
DOI 10.17487/RFC6052, October 2010,
<https://www.rfc-editor.org/info/rfc6052>.
[RFC6333] Durand, A., Droms, R., Woodyatt, J., and Y. Lee, "Dual-
Stack Lite Broadband Deployments Following IPv4
Exhaustion", RFC 6333, DOI 10.17487/RFC6333, August 2011,
<https://www.rfc-editor.org/info/rfc6333>.
[RFC6513] Rosen, E., Ed. and R. Aggarwal, Ed., "Multicast in MPLS/
BGP IP VPNs", RFC 6513, DOI 10.17487/RFC6513, February
2012, <https://www.rfc-editor.org/info/rfc6513>.
[RFC7454] Durand, J., Pepelnjak, I., and G. Doering, "BGP Operations
and Security", BCP 194, RFC 7454, DOI 10.17487/RFC7454,
February 2015, <https://www.rfc-editor.org/info/rfc7454>.
[RFC7761] Fenner, B., Handley, M., Holbrook, H., Kouvelas, I.,
Parekh, R., Zhang, Z., and L. Zheng, "Protocol Independent
Multicast - Sparse Mode (PIM-SM): Protocol Specification
(Revised)", STD 83, RFC 7761, DOI 10.17487/RFC7761, March
2016, <https://www.rfc-editor.org/info/rfc7761>.
[RFC7899] Morin, T., Ed., Litkowski, S., Patel, K., Zhang, Z.,
Kebler, R., and J. Haas, "Multicast VPN State Damping",
RFC 7899, DOI 10.17487/RFC7899, June 2016,
<https://www.rfc-editor.org/info/rfc7899>.
[RFC8114] Boucadair, M., Qin, C., Jacquenet, C., Lee, Y., and Q.
Wang, "Delivery of IPv4 Multicast Services to IPv4 Clients
over an IPv6 Multicast Network", RFC 8114,
DOI 10.17487/RFC8114, March 2017,
<https://www.rfc-editor.org/info/rfc8114>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8201] McCann, J., Deering, S., Mogul, J., and R. Hinden, Ed.,
"Path MTU Discovery for IP version 6", STD 87, RFC 8201,
DOI 10.17487/RFC8201, July 2017,
<https://www.rfc-editor.org/info/rfc8201>.Acknowledgements 🫶
Huge shoutout to Wenlong Chen, Xuan Chen, Alain Durand, Yiu Lee, Jacni Qin, and Stig Venaas, who provided useful input to this document 🌈
Authors' Addresses 🦋
Here are the authors, with their contact details exactly as listed ✨
Mingwei Xu
Tsinghua University
Department of Computer Science
Beijing 100084
China
Phone: +86-10-6278-5822
Email: xumw@tsinghua.edu.cn
Yong Cui
Tsinghua University
Department of Computer Science
Beijing 100084
China
Phone: +86-10-6278-5822
Email: cuiyong@tsinghua.edu.cn
Jianping Wu
Tsinghua University
Department of Computer Science
Beijing 100084
China
Phone: +86-10-6278-5983
Email: jianping@cernet.edu.cn
Shu Yang
Shenzhen University
South Campus
Shenzhen 518060
China
Phone: +86-755-2653-4078
Email: yang.shu@szu.edu.cn
Chris Metz
Cisco Systems
170 West Tasman Drive
San Jose, CA 95134
United States of America
Phone: +1-408-525-3275
Email: chmetz@cisco.com📦 where this page comes from
- ietf: rfc8638 2019-09
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