MPLS WG K. Kompella
Internet-Draft V. Beeram
Intended status: Standards Track T. Saad
Expires: January 13, 2022 Juniper Networks
I. Meilik
Broadcom
July 12, 2021
Multi-purpose Special Purpose Label for Forwarding Actions
draft-kompella-mpls-mspl4fa-01
Abstract
A Slice Selector is packet metadata that dictates the packet's
forwarding handling in order to conform to its slice requirements.
There are multiple proposals for carrying slice selectors in MPLS
networks. One of the more practical proposals is the "Global
Identifier for Slice Selector" (GISS). Global uniqueness requires
the GISS label be identified as such, via a special purpose label
(ideally a base special purpose label (bSPL)). However, bSPLs are a
precious commodity, and there are many requests for them. This
document serves two purposes: to define a bSPL for carrying a GISS,
and to show how this bSPL can consolidate many current requests for
special purpose labels while carrying associated data compactly and
efficiently.
Status of This Memo
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
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Internet-Drafts are draft documents valid for a maximum of six months
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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 January 13, 2022.
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Copyright Notice
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
1.2. Revision History . . . . . . . . . . . . . . . . . . . . 3
1.2.1. Changes from -00 to -01 . . . . . . . . . . . . . . . 3
1.3. Slice Selector . . . . . . . . . . . . . . . . . . . . . 4
2. Multi-purpose bSPL: the Forwarding Actions Indicator . . . . 4
2.1. The FAI bSPL . . . . . . . . . . . . . . . . . . . . . . 4
2.1.1. LS FAD vs PL FAD . . . . . . . . . . . . . . . . . . 5
2.2. Format of the FAI bSPL . . . . . . . . . . . . . . . . . 5
2.2.1. Definitions of the FAI Flag Bits . . . . . . . . . . 6
2.2.2. Processing the FAI Flags and the LS FAD . . . . . . . 7
2.2.3. Example of the FAI . . . . . . . . . . . . . . . . . 8
3. Issues to be Resolved . . . . . . . . . . . . . . . . . . . . 9
3.1. Preventing FAI From Reaching Top of Stack . . . . . . . . 9
3.2. Repeating the FAI at "Readable Stack Depth" . . . . . . . 10
3.3. PL FAD . . . . . . . . . . . . . . . . . . . . . . . . . 10
4. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 10
5. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 10
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11
7. Security Considerations . . . . . . . . . . . . . . . . . . . 11
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 11
8.1. Normative References . . . . . . . . . . . . . . . . . . 11
8.2. Informative References . . . . . . . . . . . . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 12
1. Introduction
Network slicing is an important ongoing effort both for network
design, as well as for standardization, in particular at the IETF
[I-D.nsdt-teas-ns-framework]. A key issue is identifying which slice
a packet belongs to, by means of a "slice selector" carried in the
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packet header. [I-D.bestbar-teas-ns-packet] describes several such
methods for MPLS networks, of which the Global Identifier for Slice
Selector (GISS) is one of the more practical solutions. This
document shows how to realize the GISS using a base special purpose
label (bSPL).
Base Special Purpose Labels are a precious commodity; there are only
16 such values, of which 8 have already been allocated. There are
currently five requests for bSPLs that the authors are aware of; this
document proposes another use case for a bSPL, in all consuming
nearly all the remaining values. Therefore, this document also
suggests a method whereby a single bSPL can be used for all the
purposes currently documented. This leads to perhaps the more
valuable long-term contribution of this document: an approach to the
definition and use of bSPLs (and SPLs in general) whereby a single
value can be used for multiple purposes, and provide a flexible and
efficient means of carrying associated data.
1.1. Terminology
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.
1.2. Revision History
This section (to be removed before publication) highlights the
draft's revision history.
1.2.1. Changes from -00 to -01
1. This section added.
2. Added a section discussing when data should be put in the LS FAD
vs in the PL FAD.
3. Tweaked the bits in the FAI. Added a field "edist".
4. Elaborated on the use of the H bit and the FAH data.
5. Updated the processing of the LS FAD.
6. Added processing of edist.
7. Updated the FAI example.
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8. Updated the Issues section.
1.
1.3. Slice Selector
In MPLS networks, a GISS is a data plane construct identifying
packets belonging to a slice aggregate (the set of packets that
belong to the slice). The GISS dictates forwarding actions for the
slice aggregate: QoS behavior and next hop selection. The purpose of
the GISS is detailed in [I-D.bestbar-teas-ns-packet]. To embed a
GISS in a label stack, one must preface it with a bSPL identifying it
as such. For reasons that will become apparent, this bSPL is called
the Forwarding Actions Indicator (FAI).
2. Multi-purpose bSPL: the Forwarding Actions Indicator
This document proposes the use of a single bSPL to tell routers one
or more forwarding actions they should take on a packet, e.g.:
o to treat a packet according to its slice, given its GISS;
o to load balance a packet, given its entropy;
o whether or not to perform fast reroute on a failure
[I-D.kompella-mpls-nffrr];
o whether or not a packet has a Flow ID;
o whether or not a packet has metadata relevant to intermediate hops
along the path;
o a faster way of finding the End of Stack;
o and perhaps other functions in the future.
This bSPL is called the "Forwarding Actions Indicator" (FAI). The
FAI uses the label's TC bits and TTL field to inform the forwarding
plane of the required actions. Each of these actions may have
associated data, the Forwarding Actions Data (FAD). The FAD may be
carried in the Label Stack (LS FAD) or in the payload (PL FAD).
2.1. The FAI bSPL
The design of the bSPL hinges on a key insight: forwarding engines do
not interpret the TC bits or the TTL field for labels that are not at
the top of the label stack (ToS). For non-ToS labels, the important
bit fields are the label value field (to compute entropy and identify
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SPLs) and the End of Stack (S) bit (to know when the label stack
ends). [If you know of a forwarding engine that looks at other bit
fields of labels below the ToS, please contact the authors.] This
means that for a bSPL that will never appear at the ToS, the TC bits
and the TTL bits can be used to carry additional information.
Furthermore, for the LS FAD, the entire 4-octet label word, the S bit
excepted, can be used to carry data. We use this technique to make
the FAI bSPL multipurpose, and to make the LS FAD words compact and
efficient.
2.1.1. LS FAD vs PL FAD
A pertinent question is whether one should put non-label data in the
label stack. The alternative is to put all such data in the PL FAD.
However, this would mean that accessing such information would
require finding the End of Stack, and parsing the PL FAD. For
certain types of data, this would be a severe burden on the packet
forwarding engine. Examples of such data are the Entropy label
(needed for efficient load balancing), the GISS (needed for accurate
packet forwarding) and the Flow ID (needed for telemetry). Having
any of this data in the PL FAD would hurt forwarding performance.
This memo will document criteria for when data should be in the LS
FAD versus in the PL FAD.
2.2. Format of the FAI bSPL
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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
TC S TTL
----- ---------------
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| (previous forwarding label | TC |0| TTL |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Forwarding Actions Indicator |H|A|N|S|E G|F|h| edist |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS Forwarding Actions Header |S| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS Forwarding Actions Data |S| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| More LS FAD and/or other labels |S| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| End of Stack label |1| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|b b b b| Payload (potentially, PL FAD) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Payload |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: Format for FAI, LS FAD and PL FAD
The FAI's label value MUST be the IANA allocated value. The S bit
MUST be reflect whether the label stack ends at this label or not.
2.2.1. Definitions of the FAI Flag Bits
The TC and TTL bits are used as flags, defined as follows:
H: if set, the FAI is followed by a Forwarding Actions Header (FAH).
A: Associated data (LS FAD) is present (1) or not (0).
N: If set, do not do fast reroute (NFFRR).
S: MUST be set if the FAI is the end of stack, and clear otherwise.
EG: this is a 2-bit flag indicating whether the LS FAD carries
Entropy and/or GISS information.
F: If set, the LS FAD has a Flow ID.
h: If set, the PL FAD contains hop-by-hop information. Every node in
the path SHOULD attempt to process the hop-by-hop information, but
not at the expense of exceeding the processing time budget, which
could cause this (or other) packets to be dropped.
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edist: ("distance to End of Stack") a 4-bit field that indicates how
many 4-octets labels to skip to reach End of Stack.
The FAH consists of a single 4-octet word, and is used if more FAD
flags are needed. As these bits are defined, processing of the
associated data MUST also be defined. The format of the FAH is TBD.
The EG field is used as follows:
00: No Entropy or GISS present
01: LS FAD 0 contains 16 bits of Entropy in the high order 16 bits
and 15 bits of GISS in the low order 16 bits (S bit excepted).
10: LS FAD 0 contains 20 bits of Entropy in the high order 20 bits
and 11 bits of GISS in the low order 12 bits (S bit excepted).
11: LS FAD 0 contains the 31-bit Entropy; LS FAD 1 contains the
31-bit GISS. In LS FAD 0, the S bit MUST be 0; the packet
forwarding engine may choose to use this as part of the Entropy,
as it doesn't affect the outcome. In LS FAD 1, the S bit may be 0
or 1.
2.2.2. Processing the FAI Flags and the LS FAD
Here's how the LS FAD is parsed. One must keep track of the S bit to
know when the stack ends. The LS FAD data appears in the order of
the corresponding flags.
It is an error if the label stack ends while there are more LS FAD
words to process. In particular, it is an error if the FAI's S bit
is set, but the H flag is set, or the A flag is set and any of EG or
F or edist is non-zero.
It is not an error if H, A, N, EG, F and h are all zero; however, in
that case, it's not clear what purpose the FAI serves.
1. Set CL ("current label") to the FAI label. LL is the last label
(End of Stack); PL ("payload") is the first 4-octet word of the
payload.
2. Process H:
1. If set, increment CL; process the FAH.
2. Otherwise, CL is unchanged.
3. If A is 0, done: there is no associated LS FAD.
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4. Process N. CL is unchanged.
5. Process EG:
1. If EG is 00, CL is unchanged.
2. If EG is 01 or 10, increment CL. CL now contains both GISS
and Entropy.
3. If EG is 11, CL+1 contains Entropy; CL+2 contains GISS.
Increment CL by 2.
6. Process F:
1. If F is set; increment CL. CL contains the Flow ID.
7. Process edist:
1. LL = CL + edist
2. while LL's S bit == 0, LL++
3. PL = LL+1
The edist field is used to expedite reaching the PL FAD (e.g., to
process hop-by-hop information). A forwarding engine can skip
forward edist 4-octet words, i.e., CL += edist. This word MUST be a
label, which may or may not have S = 1. If not, keep parsing until a
label with S = 1 is hit; this is the End of Stack. PL FAD follows
this label.
Details for parsing the PL FAD are outside the scope of this memo,
and will be addressed in the document describing its format.
2.2.3. Example of the FAI
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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
TC S TTL
----- ---------------
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Tunnel Label-1 | TC |0| TTL |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Tunnel Label-2 | TC |0| TTL |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Forwarding Actions Indicator |0|1|1|0|0 1|0|1|0 0 0 1|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Entropy | GISS ... |0| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| VPN Label |TC |1| TTL |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|b b b b| PL FAD |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| real payload starts ...
H = 0; no FAH.
A = 1: there is LS FAD.
N = 1: NFFRR is set.
EG = 01: LS FAD 0 contains Entropy + GISS.
F = 0: No Flow ID.
h = 1: There is hop-by-hop PL FAD.
edist = 1: one more label to End of Stack.
Figure 2: Example of FAI + LS FAD + hop-by-hop PL FAD
The real payload starts after the PL FAD.
3. Issues to be Resolved
This section captures issues to be resolved, in this memo and others.
As the issues are fixed, they should be removed from here; ideally,
this section should be empty before publication.
3.1. Preventing FAI From Reaching Top of Stack
As was said earlier, the FAI MUST NOT be at the top of stack, since
its TC and TTL bits have been repurposed. There are two ways to
prevent this. If an LSR X pops a label and encounters an FAI, X can
pop the FAI and all LS FAD words. To do that, it must be able to
parse the FAI to determine how many LS FAD words there exist. This
can be used in conjunction with Section 3.2. However, there are
cases when it is desired to preserve the FAI+FAD until the egress.
In this case, X should push an explicit NULL (label value 0 or 2)
onto the stack above the FAI, with the correct TC and TTL values.
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Other options will be pursued.
3.2. Repeating the FAI at "Readable Stack Depth"
For LSRs which cannot parse the entire label stack, or would prefer
not to unless needed, it is possible to repeat the FAI at "readable
stack depth", say every 4 labels. In the above case, the FAI+LS FAD
can be repeated every 4 labels; or a truncated version, just the FAI
with A set to 0 can be used. Only the last FAI would contain the
full information, reducing the burden on the label stack. However,
in this case, LSRs that don't process the whole stack may not load
balance less effectively, and potentially not adhere to the slice
service level objectives.
Other options will be described in future versions of this document.
3.3. PL FAD
The format of the PL FAD, whether or not a Control Word is present,
and handling of the first nibble, is outside the scope of this
document. The FAI will not contain details about the contents of the
PL FAD, besides the single flag on whether or not the PL FAD contains
information relevant to (most) intermediate hops. It is assumed that
another memo will document the format of the PL FAD, and that this
memo will provide a means of parsing the PL FAD (e.g., a TLV
structure) and thus determining its contents.
The PL FAD memo should also comment on the impact of processing the
PL FAD on forwarding performance, especially in the case of hop-by-
hop info.
4. Contributors
Many thanks to Colby Barth, Chandra Ramachandran and Srihari Sangli
for their contributions to this draft.
5. Acknowledgments
We'd like to acknowledge the helpful discussions with Swamy SRK and
folks from the Broadcom team on the impacts to existing and future
forwarding engines.
The edist field was added thanks to Haoyu Song, who suggested the
optimization to find End of Stack.
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6. IANA Considerations
If this draft is deemed useful and adopted as a WG document, the
authors request the allocation of a bSPL for the FAI. We suggest the
early allocation of label 8 for this.
7. Security Considerations
A malicious or compromised LSR can insert the FAI and associated data
into a label stack, preventing (for example) FRR from occurring. If
so, protection will not kick in for failures that could have been
protected, and there will be unnecessary packet loss. Similarly,
inserting or removing a Fragmentation Header means that a packet's
contents cannot be accurately reconstructed. Inserting or changing a
GISS means that the packet will be misclassified, perhaps leaving or
entering a high-value slice and causing damage.
8. References
8.1. Normative References
[I-D.bestbar-teas-ns-packet]
Saad, T., Beeram, V. P., Wen, B., Ceccarelli, D., Halpern,
J., Peng, S., Chen, R., Liu, X., and L. M. Contreras,
"Realizing Network Slices in IP/MPLS Networks", draft-
bestbar-teas-ns-packet-02 (work in progress), February
2021.
[I-D.kompella-mpls-nffrr]
Kompella, K. and W. Lin, "No Further Fast Reroute", draft-
kompella-mpls-nffrr-01 (work in progress), November 2020.
[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>.
[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>.
8.2. Informative References
[I-D.nsdt-teas-ns-framework]
Gray, E. and J. Drake, "Framework for IETF Network
Slices", draft-nsdt-teas-ns-framework-05 (work in
progress), February 2021.
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Authors' Addresses
Kireeti Kompella
Juniper Networks
1133 Innovation Way
Sunnyvale, CA 94089
United States
Email: kireeti.ietf@gmail.com
Vishnu Pavan Beeram
Juniper Networks
1133 Innovation Way
Sunnyvale, CA 94089
United States
Email: vbeeram@juniper.net
Tarek Saad
Juniper Networks
1133 Innovation Way
Sunnyvale, CA 94089
United States
Email: tsaad@juniper.net
Israel Meilik
Broadcom
Email: israel.meilik@broadcom.com
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