| Internet-Draft | 1st nibble | February 2024 |
| Kompella, et al. | Expires 12 August 2024 | [Page] |
The goal of this memo is to create a new IANA registry (called the Post-stack First Nibble registry) for the first nibble (4-bit field) immediately following an MPLS label stack. The memo offers a rationale for such a registry, describes how the registry should be managed, and provides some initial entries. Furthermore, this memo sets out some documentation requirements for registering new values. Finally, it provides some recommendations that make processing MPLS packets easier and more robust.¶
The relationship between the IANA IP Version Numbers (RFC 2780) and the Post-stack First Nibble registry is described in this document.¶
This memo, if published, would update RFC 4928.¶
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/.¶
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This Internet-Draft will expire on 12 August 2024.¶
Copyright (c) 2024 IETF Trust and the persons identified as the document authors. All rights reserved.¶
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An MPLS packet consists of a label stack, an optional "post-stack header" (PSH) and an optional embedded packet (in that order). By PSH, we mean existing artifacts such as Control Words, BIER headers and the like, as well as new types of PSH being discussed by the MPLS Working Group. However, in the data plane, there are scant clues regarding the PSH, and no clue as to the type of embedded packet; this information is communicated via other means, such as the routing protocols that signal the labels in the stack. Nonetheless, in order to better handle an MPLS packet in the data plane, it is common practice for network equipment to "guess" the type of embedded packet. Such equipment may also need to process the PSH. Both of these require parsing the data after the label stack. To do this, the "first nibble" (the top four bits of the first octet following the label stack) is often used. Although some existing network devices may use such a method, it needs to be stressed that the correct interpretation of the Post-stack First Nibble (PFN) in a PSH can be made only in the context of the Label Stack Element (LSE) or group of LSEs in the preceding label stack that characterize the type of the PSH, and that any attempt to rely on the value in any other context is unreliable.¶
The semantics and usage of the first nibble are not well documented, nor are the assignments of values. This memo serves four purposes:¶
This memo introduces a requirement and a recommendation, the first building on the Section 2.1.1; the second deprecating the use of the heuristic in Section 2.1.1.1. The intent of both of these is that legacy routers continue to operate as they have, with no new problems introduced as a result of this memo. However, new implementations SHOULD follow these recommendations for a more robust operation.¶
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 BCP14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.¶
one whose Layer 2 header declares the type to be MPLS. For Ethernet, that means the Ethertype is 0x8847 or 0x8848.¶
(of an MPLS packet) all labels (four-octet fields) after the Layer 2 header, up to and including the label with the BoS bit set ([RFC3032]).¶
the most significant four bits of the first octet following the label stack.¶
all data after the label stack, including the PFN, an optional post-stack header, and the embedded packet.¶
optional field of interest to the egress LSR (and possibly to transit LSRs). Examples include a control word or an associated channel. The PSH MUST indicate its length, so that a parser knows where the embedded packet starts.¶
all octets beyond the PSH (if any). That could be an IPv4 or IPv6 packet , an Ethernet packet (for VPLS ([RFC4761], [RFC4762]) or EVPN [RFC7432]), or some other type of Layer 2 frame [RFC4446].¶
regardless of how the deprecation is understood in other IETF documents, the interpretation in this document is that if a practice has been deprecated, that practice should not be included in the new implementations or deployed in deployments.¶
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+\
X | Layer 2 Header | |
| | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+/
TC S TTL
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+\
Y | Label-1 | TC |0| TTL | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Label-2 | TC |0| TTL | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... | TC |0| TTL | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Label-n | TC |1| TTL | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+/
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+\
A | (PFN) | IP packet | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| data | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| end of IP packet | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+/
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+\
B | (PFN) | non-IP packet | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| data | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| end of non-IP packet | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+/
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+\
C | (PFN) | PSH | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| PSH | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| end of PSH | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| embedded packet | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+/
Figure 1 shows an MPLS packet with Layer 2 header X and a label stack Y ending with Label-n. Then, there are three examples of an MPLS payload. The complete MPLS packet thus would consist of [X Y A], or [X Y B], or [X Y C].¶
A. The first payload is a bare IP packet, i.e., no PSH. The PFN in this case overlaps with the IP version number.¶
B. The next payload is a bare non-IP packet; again, no PSH. The PFN here is the first nibble of the payload, whatever it happens to be.¶
C. The last example is an MPLS Payload that starts with a PSH followed by the embedded packet. Here, the embedded packet could be IP or non-IP.¶
An MPLS packet can contain many types of embedded packets. The most common types are:¶
Many other packet types are possible, and in principle, any Layer 2 embedded packet is permissible; indeed, in the past, PPP, Frame Relay and ATM packets were reasonably common.¶
In addition, there may be a PSH ahead of the embedded packet, and it needs to be parsed. The PFN is currently used for both of these purposes.¶
There are four common ways to load balance an MPLS packet:¶
Load balancing based on just the top label means that all packets with that top label will go the same way -- this is far from ideal. Load balancing based on the entire label stack (not including SPLs) is better, but it may still be uneven. If, however, the embedded packet is an IP packet, then the combination of (<source IP address>, <dest IP address>, <transport protocol>, <source port>, and <dest port>) from the IP header of the embedded packet forms an excellent basis for load-balancing. This is what is typically used for load balancing IP packets.¶
An MPLS packet doesn't, however, carry a payload type identifier. There is a simple (but dangerous) heuristic that is commonly used to guess the type of the embedded packet. The first nibble, i.e., the four most significant bits of the first octet, of an IP header contains the IP version number. That, in turn, indicates where to find the relevant fields for load-balancing. The heuristic goes roughly as follows:¶
This heuristic has been implemented in many (legacy) routers, and performs well in the case of Figure 2, A. However, this heuristic can work very badly for Figure 2, B. For example, if payload B is an Ethernet frame, then the PFN is the first nibble of the OUI of the destination MAC address, which can be 0x4 or 0x6, and if so would lead to very bad load-balancing. This behavior can happen to other types of non-IP payloads as well.¶
That, in turn, led to the idea of inserting a PSH (e.g., a pseudowire control word [RFC4385], a DetNet control word [RFC8964] or a BIER header [RFC8296]) where the PFN is not 0x4 or 0x6, to explicitly prevent forwarding engines from confusing the MPLS payload with an IP packet. [RFC8469] recommends the use of a control word when the embedded packet is an Ethernet frame. RFC 8469 was published at the request of the operator community and the IEEE RAC as a result of operational difficulties with pseudowires that did not contain the control word.¶
It is RECOMMENDED that where load-balancing of MPLS packets is desired, the load-balancing mechanism uses the value of a dedicated label, for example, either an Entropy Label [RFC6790] or a FAT Pseudowire Label [RFC6391]. Furthermore, the heuristic of guessing the type of the embedded packet, as discussed above, SHOULD NOT be used.¶
A consequence of the latter approach is that, while legacy routers may look for an PFN of 0x4 [RFC0791] or 0x6 [RFC8200], no router will look for a PFN of 0x7 (or whatever the next IP version number will be) for load-balancing purposes. This means that the values 0x4 and 0x6 are used to (sometimes incorrectly) identify IPv4 and IPv6 packets, but no other First Nibble values will be used to identify IP packets.¶
This also expands the PFN Registry to all 16 possible values, not just 0x0 and 0x1.¶
Paragraph 3 in Section 3 of RFC 4928 [RFC4928] states that:¶
It is REQUIRED, however, that applications depend upon in-order packet delivery restrict the first nibble values to 0x0 and 0x1. This will ensure that their traffic flows will not be affected if some future routing equipment does similar snooping on some future version(s) of IP.¶
The text in RFC 4928 [RFC4928] concerning the first nibble after the MPLS Label Stack has been updated by [I-D.ietf-mpls-1stnibble] and the heuristic for snooping this nibble has been deprecated. RFC 4928 is now updated as follows:¶
Network equipment that complies with this update MUST use a PSH (Post-Stack Header) with a PFN (Post-stack First Nibble) value that is neither 0x4 nor 0x6 in all cases when the MPLS payload is not an IP packet.¶
The recommendation (see Section 2.1.1.1) obviates the need for paragraph 4 in Section 3 of RFC 4928 [RFC4928]:¶
"This behavior implies that if in the future an IP version is defined with a version number of 0x0 or 0x1, then equipment complying with this BCP would be unable to look past one or more MPLS headers, and load-split traffic from a single LSP across multiple paths based on a hash of specific fields in the IPv0 or IPv1 headers. That is, IP traffic employing these version numbers would be safe from disturbances caused by inappropriate load-splitting, but would also not be able to get the performance benefits."¶
Furthermore, the following text is appended to Section 1.1 of RFC 4928 [RFC4928]:¶
PSH: Post-Stack Header¶
PFN: Post-stack First Nibble¶
The MPLS WG is currently engaged in updating the MPLS architecture; part of this work may involve the use of PSHs. That might be more challenging if PSH values are allocated on an ad hoc basis, and their parsing and semantics are ill-specified. Consider that the PFN value of 0x0 has two different formats, depending on whether the PSH is a pseudowire control word or a DetNet control word; disambiguation requires the context of the service label. This was a considered decision; documenting this would be helpful to future implementors.¶
With a registry, PSHs become easier to parse; not needing means outside the data plane to interpret them correctly; and their semantics and usage are documented. (Thank you, IANA!)¶
The use of the PFN stemmed from the desire to heuristically identify IP packets for load-balancing purposes. It was then discovered that non-IP packets, misidentified as IP when the heuristic failed, were being badly load balanced, leading to [RFC4928]. This situation may confuse some as to the relationship between the Post-stack First Nibble Registry and the IP Version Numbers registry. These registries are quite different:¶
The only intersection points between the two registries is for values 0x4 and 0x6 (for backward compatibility). There is no need to track future IP version number allocations in the Post-stack First Nibble registry.¶
This memo recommends the creation of an IANA registry called "The Post-stack First Nibble Registry" with the following values:¶
+=======================================================================+ | Code points allocated for BIER | +=======================================================================+ | Value | PSH Type | Reference | +-------+---------------------------------+-----------------------------+ | 0x5 | BIER header - Normal traffic | RFC 8296 | +=======================================================================+ | Code points allocated for DETNET | +=======================================================================+ | Value | PSH Type | Reference | +-------+---------------------------------+-----------------------------+ | 0x0 | DetNet Control Word | RFC 8964 | +-------+---------------------------------+-----------------------------+ | 0x1 | DetNet Associated Channel | draft-ietf-detnet-mpls-oam | +=======================================================================+ | Code points allocated for Network Service Header (NSH) | +=======================================================================+ | Value | PSH Type | Reference | +-------+---------------------------------+-----------------------------+ | 0x0 | NSH Base Header, Payload | RFC 8300 | +-------+---------------------------------+-----------------------------+ | 0x2 | NSH Base Header, OAM | RFC 8300 | +=======================================================================+ | Code points allocated for Pseudowires (PW) | +=======================================================================+ | Value | PSH Type | Reference | +-------+---------------------------------+-----------------------------+ | 0x0 | PW Control Word | RFC 4385 | +-------+---------------------------------+-----------------------------+ | 0x1 | PW Associated Channel | RFC 4385 | +=======================================================================+ | Code points allocated for the MPLS Generic Associated Channel | +=======================================================================+ | Value | PSH Type | Reference | +-------+---------------------------------+-----------------------------+ | 0x1 | MPLS G-ACh | RFC 5586 | +=======================================================================+ | Reserved Code Points, not to be allocated | +=======================================================================+ | Value | Usage | Reference | +-------+---------------------------------+-----------------------------+ | 0x4 | IPv4 Protocol Number | RFC 791 | +-------+---------------------------------+-----------------------------+ | 0x6 | IPv6 Protocol Number | RFC 8200 | +=======================================================================+ | Unassigned Code Points | +=======================================================================+ | Value | PSH Type | Reference | +--------+---------------------------------+----------------------------+ | 0x3 | - | - | +--------+---------------------------------+----------------------------+ | 0x7-0xF| - | - | +--------+---------------------------------+----------------------------+
All new values registered here MUST use the Standards Action policy [RFC8126].¶
This document proposes a new IANA registry and does not raise any security concerns or issues in addition to ones common to networking and those specific to MPLS networks.¶