Removing a Restriction on the use of MPLS Explicit NULL
MPLS Explicit NULL labels may now sit anywhere in the label stack, not just at the bottom.
Rosen, E., "Removing a Restriction on the use of MPLS Explicit NULL", RFC 4182, DOI 10.17487/RFC4182, September 2005, <https://www.rfc-editor.org/info/rfc4182>.
@misc{rfc4182,
series = {Request for Comments},
number = 4182,
howpublished = {RFC 4182},
publisher = {RFC Editor},
doi = {10.17487/RFC4182},
url = {https://www.rfc-editor.org/info/rfc4182},
author = {E. Rosen},
title = {{Removing a Restriction on the use of MPLS Explicit NULL}},
pagetotal = 7,
year = 2005,
month = sep,
} RFC text
Contents
Network Working Group E. Rosen
Request for Comments: 4182 Cisco Systems, Inc.
Updates: 3032 September 2005
Category: Standards Track
Removing a Restriction on the use of MPLS Explicit NULL
Status of This Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2005).
Abstract
The label stack encoding for Multi-protocol Label Switching (MPLS)
defines a reserved label value known as "IPv4 Explicit NULL" and a
reserved label value known as "IPv6 Explicit NULL". Previously,
these labels were only legal when they occurred at the bottom of the
MPLS label stack. This restriction is now removed, so that these
label values may legally occur anywhere in the stack.
This document updates RFC 3032.
Table of Contents
1. Introduction ....................................................2
2. Detail of Change ................................................2
3. Reasons for Change ..............................................3
4. Deployment Considerations .......................................5
5. Security Considerations .........................................5
6. Acknowledgments .................................................5
7. Normative References ............................................5
8. Informative References ..........................................5
1. Introduction
RFC 3032 defines a reserved label value known as "IPv4 Explicit NULL"
and a reserved label value known as "IPv6 Explicit NULL" [RFC3032].
It states that these label values are only legal at the bottom of the
MPLS label stack. However, no reason is given for this restriction.
It has turned out that in practice there are some situations in which
it is useful to send MPLS packets that have Explicit NULL occur
somewhere other than at that bottom of the label stack. While the
intended semantics are obvious enough, the fact that such packets are
gratuitously declared by RFC 3032 to be illegal has made it difficult
to handle these situations in an interoperable manner.
This document updates RFC 3032 by removing the unnecessary
restriction, so that the two aforementioned label values are legal
anywhere in the label stack.
2. Detail of Change
RFC 3032 states on page 4:
There are several reserved label values:
i. A value of 0 represents the "IPv4 Explicit NULL Label". This
label value is only legal at the bottom of the label stack.
It indicates that the label stack must be popped, and the
forwarding of the packet must then be based on the IPv4
header.
iii. A value of 2 represents the "IPv6 Explicit NULL Label". This
label value is only legal at the bottom of the label stack.
It indicates that the label stack must be popped, and the
forwarding of the packet must then be based on the IPv6
header.
Paragraph i is hereby changed to read:
i. A value of 0 represents the "IPv4 Explicit NULL Label".
An IPv4 Explicit NULL at the top of the label stack means that
the stack must be popped.
If the NULL was not the only label on the stack, this will
cause the label beneath it to rise to the top of the stack.
The disposition of the packet is based on the label that has
now risen to the top.
If, on the other hand, the NULL was the only label on the
stack, then the stack is now empty. The resulting packet is
treated as an IPv4 packet, and its disposition is based on the
IPv4 header.
Paragraph iii is hereby changed to read:
iii. A value of 2 represents the "IPv6 Explicit NULL Label".
An IPv6 Explicit NULL at the top of the label stack means that
the stack must be popped.
If the NULL was not the only label on the stack, this will
cause the label beneath it to rise to the top of the stack.
The disposition of the packet is based on the label that has
now risen to the top.
If, on the other hand, the NULL was the only label on the
stack, then the stack is now empty. The resulting packet is
treated as an IPv6 packet, and its disposition is based on the
IPv6 header.
3. Reasons for Change
Restricting Explicit NULL to the bottom of the stack has caused some
problems in practice.
With this restriction in place, one should not distribute, to a
particular label distribution peer, a binding of Explicit NULL to a
particular Forwarding Equivalence Class (FEC), unless the following
condition (call it "Condition L") holds: all MPLS packets received by
that peer with an incoming label corresponding to that FEC contain
only a single label stack entry. If Explicit NULL is bound to the
FEC, but Condition L doesn't hold, the peer is being requested to
create illegal packets. None of the MPLS specifications say what the
peer is actually supposed to do in this case. This situation is made
more troublesome by the facts that, in practice, Condition L rarely
holds, and it is not possible, in general, to determine whether it
holds or not.
Further, if one is supporting the Pipe Model of RFC 3270 [RFC3270],
there are good reasons to create label stacks in which Explicit NULL
is at the top of the label stack, but a non-null label is at the
bottom.
RFC 3270 specifies the procedures for MPLS support of Differentiated
Services. In particular, it defines a "Pipe Model" in which (quoting
from RFC 3270, Section 2.6.2):
"tunneled packets must convey two meaningful pieces of Diff-Serv
information:
- the Diff-Serv information which is meaningful to intermediate
nodes along the LSP span including the LSP Egress (which we refer
to as the 'LSP Diff-Serv Information'). This LSP Diff-Serv
Information is not meaningful beyond the LSP Egress: Whether
Traffic Conditioning at intermediate nodes on the LSP span
affects the LSP Diff-Serv information or not, this updated Diff-
Serv information is not considered meaningful beyond the LSP
Egress and is ignored.
- the Diff-Serv information which is meaningful beyond the LSP
Egress (which we refer to as the 'Tunneled Diff-Serv
Information'). This information is to be conveyed by the LSP
Ingress to the LSP Egress. This Diff-Serv information is not
meaningful to the intermediate nodes on the LSP span."
When the Pipe Model is in use, it is common practice for the LSP
Egress to bind Explicit Null to the tunnel's FEC. The intention is
that the LSP Diff-Serv information will be carried in the EXP bits of
the Explicit Null label stack entry, and the tunneled Diff-Serv
information will be carried in whatever is "below" the Explicit Null
label stack entry, i.e., in the IP header DS bits or in the EXP bits
of the next entry on the MPLS label stack.
Naturally, this practice causes a problem if the Pipe Model LSP is
being used to tunnel MPLS packets (i.e., if Condition L does not
hold). With strict adherence to RFCs 3031 and 3036, this practice
results in an MPLS packet where Explicit NULL is at the top of the
label stack, even though it is not the only entry in the label stack.
However, RFC 3032 makes this packet illegal.
Some implementations simply transmit the illegal packet. Others try
to convert it to a legal packet by stripping off the Explicit NULL
before transmitting it. However, that breaks the Pipe Model by
discarding the LSP Diff-Serv information. It is conceivable that
there may be an implementation that drops the illegal packet
entirely; this would also break the Pipe Model, as it would lose not
only the LSP Diff-Serv information, but the entire packet.
Of course the LSP egress is not compelled to bind Explicit NULL to
the tunnel's FEC; an ordinary label could be used instead. However,
using Explicit NULL enables the egress to determine immediately
(i.e., without need for lookup in the Label Information Base) that
the further forwarding of the packet is to be determined by whatever
is below the label. Avoiding this lookup can have favorable
implications on forwarding performance.
Removing the restriction that Explicit Null only occur at the bottom
of the stack is the simplest way to facilitate the proper operation
of the Pipe Model.
4. Deployment Considerations
Implementations that adhere to this specification will interoperate
correctly, and will correctly support the Pipe Model.
Implementations that do not adhere to this specification may not
interoperate. In particular, if a router advertises a binding of
Explicit NULL, and if that router has an upstream LDP peer that will
not transmit a packet that has multiple label stack entries with
Explicit Null at top of the stack, then it will not be possible to
use Explicit NULL to support the Pipe Model until the upstream LDP
peer is brought into compliance with this specification.
It is possible that there may be a router implementation, preceding
this specification, which will discard any received packet with
multiple label stack entries and a top label value of Explicit Null.
It is advisable to configure any such routers so that they do not
advertise any bindings to Explicit Null.
5. Security Considerations
This document updates RFC 3032 by allowing Explicit NULL to occur at
any position in the label stack. This modification does not impose
any new security considerations beyond those discussed in RFC 3032.
6. Acknowledgments
Thanks to Rahul Aggarwal, Francois LeFaucheur, Yakov Rekhter, and Dan
Tappan for their helpful comments.
7. Normative References
[RFC3032] Rosen, E., Tappan, D., Fedorkow, G., Rekhter, Y.,
Farinacci, D., Li, T., and A. Conta, "MPLS Label Stack
Encoding", RFC 3032, January 2001.
8. Informative References
[RFC3270] Le Faucheur, F., Wu, L., Davie, B., Davari, S., Vaananen,
P., Krishnan, R., Cheval, P., and J. Heinanen, "Multi-
Protocol Label Switching (MPLS) Support of Differentiated
Services", RFC 3270, May 2002.
Author's Address
Eric C. Rosen
Cisco Systems, Inc.
1414 Massachusetts Avenue
Boxborough, MA 01719
EMail: erosen@cisco.com
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Copyright (C) The Internet Society (2005).
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AI ELI5
On this page
§1Introduction
RFC 3032 defines two reserved label values, the IPv4 Explicit NULL and the IPv6 Explicit NULL. It also says they are legal only at the bottom of the MPLS label stack, but gives no reason for that restriction.
In practice there are situations where it is useful to send MPLS packets with Explicit NULL somewhere other than the bottom. The meaning of such a packet is obvious enough, yet RFC 3032 declares it illegal, which made these situations hard to handle in an interoperable way. RFC 4182 removes the restriction, so both labels are legal anywhere in the stack.
An old rule said a special sticker could only go on the last page of a book, and nobody knew why. This memo tears up the rule because people kept needing the sticker on other pages.
§2What the Change Says
RFC 3032 listed the reserved labels. Paragraph i defined value 0 as the IPv4 Explicit NULL and paragraph iii defined value 2 as the IPv6 Explicit NULL. Each said the label was only legal at the bottom of the stack and that the stack must be popped and the packet forwarded on its IPv4 or IPv6 header. RFC 4182 replaces both paragraphs.
Under the new wording, an Explicit NULL at the top of the stack means the stack must be popped. If it was not the only label, the label beneath it rises to the top, and the packet's disposition is based on that label. If it was the only label, the stack is now empty. The packet is then treated as an IPv4 packet (for label 0) or an IPv6 packet (for label 2), and its disposition is based on that IP header.
When the special plate is on top, you lift it off. If another plate is underneath, you now look at that one. If there was only the special plate, you look at what was sitting on it, the actual food.
| Document | Where Explicit NULL is legal | Label values |
|---|---|---|
| RFC 3032 | Only at the bottom of the label stack | 0 (IPv4), 2 (IPv6) |
| RFC 4182 THIS RFC | Anywhere in the label stack | 0 (IPv4), 2 (IPv6) |
§3The Problem with the Old Restriction
With the restriction in place, a router should not give a label distribution peer a binding of Explicit NULL to a Forwarding Equivalence Class (FEC) unless Condition L holds: every MPLS packet the peer receives for that FEC carries only a single label stack entry. If Explicit NULL is bound anyway and Condition L fails, the peer is being asked to create illegal packets, and none of the MPLS specifications say what it should do then.
The RFC adds that Condition L rarely holds in practice and that in general it is not possible to tell whether it holds. Removing the restriction removes the question.
It was like being told to hand out a special ticket only if every guest had exactly one coat. Nobody could check that, and nobody knew what to do when someone had two coats.
§4The Pipe Model and Explicit NULL
The strongest reason for the change is the Pipe Model of RFC 3270, which describes MPLS support of Differentiated Services. In that model a tunneled packet carries two pieces of Diff-Serv information: the LSP Diff-Serv information, meaningful to nodes along the LSP up to and including the egress, and the tunneled Diff-Serv information, meaningful beyond the egress.
When the Pipe Model is used, it is common for the LSP egress to bind Explicit NULL to the tunnel's FEC. The LSP Diff-Serv information then travels in the EXP bits of the Explicit NULL entry, and the tunneled information travels below it, in the IP header DS bits or the EXP bits of the next label stack entry. If the LSP is tunneling MPLS packets, this puts Explicit NULL at the top of a stack that has other entries, which RFC 3032 made illegal.
Implementations handled the illegal packet differently. Some simply transmitted it. Others stripped the Explicit NULL, which discards the LSP Diff-Serv information and breaks the Pipe Model. The RFC notes an implementation might even drop the whole packet. An egress could use an ordinary label instead, but Explicit NULL lets it decide right away, without a Label Information Base lookup, that forwarding depends on whatever is below the label, which can help forwarding performance.
Think of a parcel with a note for the delivery drivers on the outside label and a different note for the receiver inside. The special plate carries the drivers' note. Throwing it away to follow the old rule loses the note, so the rule had to go.
§5Deployment Considerations
Implementations that follow this specification interoperate correctly and correctly support the Pipe Model. Implementations that do not may fail to interoperate. In particular, if a router advertises a binding of Explicit NULL and its upstream LDP peer will not send packets with multiple label stack entries and Explicit NULL on top, then the Pipe Model cannot use Explicit NULL until that peer is brought into compliance.
A router built before this specification might discard any received packet that has multiple label stack entries and Explicit NULL as its top label. The RFC advises configuring any such router so that it does not advertise bindings to Explicit NULL.
If one friend still follows the old rule, the new trick will not work with that friend until they learn it. Until then, do not offer them the special plate.
§6Security Considerations
The document allows Explicit NULL to occur at any position in the label stack. The RFC states that this modification does not impose any new security considerations beyond those discussed in RFC 3032.
Letting the special plate go anywhere in the stack does not open any new doors for trouble beyond the ones that were already there.