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Table of Contents
1. Introduction ....................................................31.1. Conventions Used in This Document ..........................32. TRILL Header Extensions .........................................32.1. RBridge Extended Flag Handling Requirements ................52.2. No Critical Surprises ......................................52.3. Extended Header Flags ......................................62.3.1. Critical Summary Bits ...............................72.4. Conflict of Extensions .....................................83. Specific Extended Header Flags ..................................93.1. RBridge Channel Alert Extended Flags .......................94. Additions to IS-IS ..............................................95. IANA Considerations ............................................106. Security Considerations ........................................107. Acknowledgements ...............................................118. References .....................................................118.1. Normative References ......................................118.2. Informative References ....................................11
The base IETF Transparent Interconnection of Lots of Links (TRILL)
protocol [RFC6325] provides a TRILL Header extension feature and
describes minimal hooks to safely support header extensions. (This
feature is called "options" in Section 3.8 of [RFC6325].) But,
except for the first two bits, the TRILL base protocol document does
not specify the structure of extensions to the TRILL Header nor the
details of any particular extension.
This document is consistent with [RFC6325] and provides further
details. It specifies an initial extension word providing additional
flag bits and specifies some of those bits. Additional extensions,
including TLV-encoded options, may be specified in later documents,
for example, [Options] and [Options2].
Section 2 below describes some general principles of TRILL Header
extensions and an initial extension. Section 3 specifies a pair of
flags in this initial extension.
1.1. Conventions Used in This Document
The terminology and acronyms defined in [RFC6325] are used herein
with the same meaning. Devices implementing the TRILL protocol are
referred to as RBridges (Routing Bridges) or TRILL Switches.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
2. TRILL Header Extensions
The base TRILL protocol includes a feature for extension of the TRILL
Header (see [RFC6325], Sections 3.5 and 3.8). The 5-bit Op-Length
header field gives the length of the extensions to the TRILL Header
in units of 4 octets, which allows up to 124 octets of header
extension. If Op-Length is zero, there are no header extensions
present; else, the extension area follows immediately after the
Ingress RBridge Nickname field of the TRILL Header. The first 32-bit
word of the optional extensions area consists of an extended flags
area and critical summary bits as specified in this document.
As described below, provision is made for
o hop-by-hop flags, which might affect any RBridge that receives a
TRILL Data frame with such a flag set,
o ingress-to-egress flags, which would only necessarily affect the
RBridge(s) where a TRILL frame is decapsulated,
o flags affecting an as-yet-unspecified class of RBridges, for
example, border RBridges in a TRILL campus extended to support
multi-level IS-IS (Intermediate System to Intermediate System)
o both "critical" and "non-critical" flags.
Any RBridge receiving a frame with a critical hop-by-hop extension
that it does not implement MUST discard the frame because it is
unsafe to process the frame without understanding such a critical
Any egress RBridge receiving a frame with a critical ingress-to-
egress extension it does not implement MUST drop the frame if it is a
unicast frame (TRILL Header M bit = 0); if it is a multi-destination
TRILL Data frame (M=1), then it MUST NOT be egressed at that RBridge,
but the egress RBridge still forwards such a frame on the
Non-critical extensions can be safely ignored.
Any extended flag indicating a significant change in the structure or
interpretation of later parts of the frame that, if the extended flag
were ignored, could cause a failure of service or violation of
security policy MUST be a critical extension. If such an extended
flag affects any fields that transit RBridges will examine, it MUST
be a hop-by-hop critical extended flag.
Note: Most RBridge implementations are expected to be optimized
for simple and common cases of frame forwarding and processing.
Although the hard limit on the header extensions area length, the
32-bit alignment of the extension area, and the presence of
critical extension summary bits, as described below, are intended
to assist in the efficient hardware processing of frames with a
TRILL Header extensions area, nevertheless the inclusion of
extensions may cause frame processing using a "slow path" with
inferior performance to "fast path" processing. Limited slow path
throughput of such frames could cause some of them to be
2.1. RBridge Extended Flag Handling Requirements
All RBridges MUST check whether there are any critical flags set that
are necessarily applicable to their processing of the frame. To
assist in this task, critical summary bits are provided that cover
not only the extended flags specified herein but will cover any
further extensions that may be specified in future documents, for
example, [Options] and [Options2]. If an RBridge does not implement
all critical flags in a TRILL Data frame, it MUST treat the frame as
having an unimplemented critical extension as described in Section 2.
A transit or egress RBridge may assume that the critical summary bits
In addition, a transit RBridge:
o MAY set or clear hop-by-hop flags as specified for such flags;
o MUST adjust the length of the extensions area, including changing
Op-Length in the TRILL Header, as appropriate if it adds or
removes the extended header flags word;
o MUST, if it adds the word of extended header flags or changes any
critical flags, correctly set the critical summary bits in the
extended header flags word;
o MUST NOT remove the extended header flags word unless it is all
zero (either on arrival or after permitted modifications); and
o MUST NOT set or clear ingress-to-egress or reserved extended
header flags except as specifically permitted in the specification
of such flags.
2.2. No Critical Surprises
RBridges advertise the extended header flags they support in IS-IS
PDUs (Protocol Data Units) [RFC7176]. Unless an RBridge advertises
support for a critical extended header flag, it will not normally
receive frames with that flag set. An RBridge is not required to
support any extensions.
An RBridge SHOULD NOT set a critical extended flag in a frame unless,
o for a critical hop-by-hop extended header flag, it has determined
that the next hop RBridge or RBridges that will accept the frame
support that flag,
o for a critical ingress-to-egress extended header flag, it has
determined that the RBridge or RBridges that will egress the frame
support that flag, or
o for a critical reserved extended header flag, it may set such a
flag only if it understands which RBridges it is applicable to and
has determined that those RBridges that will accept the frame
support that flag.
"SHOULD NOT" is specified above since there may be cases where it is
acceptable for those frames, particularly for the multi-destination
case, to be discarded or not egressed by any RBridges that do not
implement the extended flag.
2.3. Extended Header Flags
If any extensions are present in a TRILL Header, as indicated by a
non-zero Op-Length field, the first 32 bits of the extensions area
consist of extended header flags, as described below. The remainder
of the extensions area, if any, after the initial 32 bits may be
specified in later documents, for example, [Options] and [Options2].
Any RBridge adding an extensions area to a TRILL Header must set the
first 32 bits to zero except when permitted or required to set one or
more of those bits as specified. For TRILL Data frames with
extensions present, any transit RBridge that does not discard the
frame MUST transparently copy the extended flags word, except for
modifications permitted by an extension implemented by that RBridge.
The extended header flags word is illustrated below and the meanings
of these bits is further described in the list following the figure.
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
|Crit.| CHbH | NCHbH |CRSV | NCRSV | CItE | NCItE |
| ... additional optional 32-bit aligned words of extension |
| possibly including TLV extensions ...
(The first two critical summary bits are as specified in [RFC6325].
In this document, an "S", for Summary, has been added at the end of
their acronyms. A third critical summary bit is also specified
herein and its acronym also ends with an "S" for consistency.)
0-2 Crit.: Critical summary bits.
0 CHbHS: Critical Hop-by-Hop extension(s) are present.
1 CItES: Critical Ingress-to-Egress extension(s) are present.
2 CRSVS: Critical Reserved extension(s) are present.
3-7 CHbH: Critical Hop-by-Hop extended flag bits.
8-13 NCHbH: Non-critical Hop-by-Hop extended flag bits.
14-16 CRSV: Critical Reserved extended flag bits.
17-20 NCRSV: Non-critical Reserved extended flag bits.
21-26 CItE: Critical Ingress-to-Egress extended flag bits.
27-31 NCItE: Non-critical Ingress-to-Egress extended flag bits.
2.3.1. Critical Summary Bits
The top three bits of the extended header flags area, bits 0, 1, and
2 above, are called the critical summary bits. They summarize the
presence of critical extensions as follows:
CHbHS: If the CHbHS (Critical Hop-by-Hop Summary) bit is one, one or
more critical hop-by-hop extensions are present. These might be
critical hop-by-hop extended header flags or critical hop-by-hop
extensions after the first word in the extensions area. Transit
RBridges that do not support all of the critical hop-by-hop
extensions present, for example, an RBridge that supported no
critical hop-by-hop extensions, MUST drop the frame. If the CHbHS
bit is zero, the frame is safe, from the point of view of
extensions processing, for a transit RBridge to forward,
regardless of what extensions that RBridge does or does not
CItES: If the CItES (Critical Ingress-to-Egress Summary) bit is a
one, one or more critical ingress-to-egress extensions are
present. These might be critical ingress-to-egress extended
header flags or critical ingress-to-egress extensions after the
first word in the extensions area. If the CItES bit is zero, no
such extensions are present. If either CHbHS or CItES is non-
zero, egress RBridges that do not support all critical extensions
present, for example, an RBridge that supports no critical
extensions, MUST drop the frame. If both CHbHS and CItES are
zero, the frame is safe, from the point of view of extensions, for
an egress RBridge to process, regardless of what extensions that
RBridge does or does not support.
CRSVS: If the CRSVS (Critical Reserved Summary) bit is a one, one or
more critical extensions are present that are reserved to apply to
a class of RBridges to be specified in the future, for example,
border RBridges in a TRILL campus extended to support multi-level
IS-IS. This class will be a subset of transit RBridges. RBridges
in this class MUST drop frames with the CRSVS bit set unless they
implement all critical hop-by-hop and all critical reserved
extensions present in the frame.
The critical summary bits enable simple and efficient processing of
TRILL Data frames by egress RBridges that support no critical
extensions, by transit RBridges that support no critical hop-by-hop
extensions, and by RBridges in the reserved class that support no
critical hop-by-hop or reserved extensions. Such RBridges need only
check whether Op-Length is non-zero and, if it is, check the top one,
two, or three bits just after the fixed portion of the TRILL Header.
Based on those three bits, such RBridges can decide whether to
discard or forward/process the frame.
2.4. Conflict of Extensions
Defining TRILL extensions including extended header flags that
conflict with each other would be undesirable. Should conflicting
extensions appear in the same packet, the results would be
unpredictable if different implementations processed them in
different orders. While rules could be defined to specify how to
predictably process conflicting extensions, such rules would also
limit implementation flexibility and could impose substantial
Conflicting extensions SHOULD NOT be defined, but if they are,
careful thought should be given as to whether and how to specify the
handling of conflicting extensions.
3. Specific Extended Header Flags
The table below shows the state of TRILL Header extended flag
assignments. See Section 5 for IANA Considerations.
Bits Purpose Section
0-2 Critical Summary Bits 2.3.1
3-6 available critical hop-by-hop flags
7 Critical Channel Alert flag 3.1
8 Non-critical Channel Alert flag 3.1
9-13 available non-critical hop-by-hop flags
14-16 available critical reserved flags
17-20 available non-critical reserved flags
21-26 available critical ingress-to-egress flags
27-31 available non-critical ingress-to-egress flags
Table 1: Extended Header Flags Area3.1. RBridge Channel Alert Extended Flags
The RBridge Channel Alert extended header flags indicate that the
frame is an RBridge Channel frame [RFC7178] that requests processing
at each hop.
If the Critical Channel Alert flag (bit 7) is a one and the RBridge
does not implement the RBridge Channel feature or the particular
RBridge Channel protocol involved [RFC7178] or the frame does not
actually appear to be an RBridge Channel message, then the frame is
discarded. This permits implementation, for example, of a channel
message requiring strict source routing or the like, with assurance
that it will be discarded rather than deviate from the directed path.
If the frame is not discarded as described above, then the presence
of either the Critical or Non-critical Channel Alert flag alerts
transit RBridges to the presence of an RBridge Channel message
[RFC7178] that may require special handling. The non-critical alert
flag supports, for example, an RBridge Channel protocol message
including a "record route" function where not recording transit
RBridges that do not support this function is acceptable.
4. Additions to IS-IS
RBridges use IS-IS Link State PDUs (LSPs) to inform other RBridges
which extended header flags they support. The IS-IS PDU(s), TLV(s),
or sub-TLV(s) used to encode and advertise this information are
specified in a separate document [RFC7176].
5. IANA Considerations
IANA has created a "TRILL Extended Header Flags" subregistry within
the TRILL Parameters registry. The "TRILL Extended Header Flags"
subregistry is initially populated as specified in Table 1 in Section
3. References in that table to sections of this document have been
replaced in the IANA subregistry by references to this document as an
New TRILL extended header flags are allocated by IETF Review
To indicate support of extended header flags, IANA has assigned the
following bits in the TRILL-VER and PORT-TRILL-VER Sub-TLV Capability
Flag registries created by [RFC7176]:
o Bits 3-13 of the PORT-TRILL-VER Sub-TLV Capability Flags have been
assigned to indicate support of TRILL hop-by-hop extended header
o Bits 14-31 of the TRILL-VER Sub-TLV Capability Flags have been
assigned to indicate support of TRILL extended header flags 14-31.
6. Security Considerations
For general TRILL protocol security considerations, see [RFC6325].
For security considerations related to extended header flags, see the
document where the flag is specified.
It is important that the critical summary bits in the extended header
flags word be set properly. If set when critical extensions of the
appropriate category are not present, frames may be unnecessarily
discarded. If not set when critical extensions are present, frames
may be mishandled or corrupted, and intended security policies may be
The RBridge Channel Alert extended header flags have the following
security considerations. Implementations should keep in mind that
they might be erroneously set in a frame. If either RBridge Channel
Alert flag is found set in a frame that is not an RBridge Channel
message [RFC7178], the flag MAY be cleared and should have no effect
except, possibly, delaying processing of the frame. If either
RBridge Channel Alert flag is erroneously omitted from a frame,
desired per-hop processing for the frame may not occur.
The following, listed in alphabetic order, are thanked for their
valuable contributions: Ben Campbell, Adrian Farrel, Barry Leiba,
and Thomas Narten.
8.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing
an IANA Considerations Section in RFCs", BCP 26, RFC
5226, May 2008.
[RFC6325] Perlman, R., Eastlake 3rd, D., Dutt, D., Gai, S., and
A. Ghanwani, "Routing Bridges (RBridges): Base Protocol
Specification", RFC 6325, July 2011.
[RFC7176] Eastlake 3rd, D., Senevirathne, T., Ghanwani, A., Dutt,
D., and A. Banerjee, "Transparent Interconnection of
Lots of Links (TRILL) Use of IS-IS", RFC 7176, May
[RFC7178] Eastlake 3rd, D., Manral, V., Li, Y., Aldrin, S., and
D. Ward, "Transparent Interconnection of Lots of Links
(TRILL): RBridge Channel Support", RFC 7178, May 2014.
8.2. Informative References
[MultiLevel] Perlman, R., Eastlake 3rd, D., Ghanwani, A., and H.
Zhai, "Flexible Multilevel TRILL (Transparent
Interconnection of Lots of Links)", Work in Progress,
[Options] Eastlake 3rd, D., Ghanwani, A., Manral, V., and C.
Bestler, "RBridges: Further TRILL Header Extensions",
Work in Progress, June 2012.
[Options2] Eastlake 3rd, D., "RBridges: More Proposed TRILL Header
Options", Work in Progress, October 2011.
Donald Eastlake 3rd
Huawei R&D USA
155 Beaver Street
Milford, MA 01757
5450 Great America Parkway
Santa Clara, CA 95054
4100 Moorpark Ave.
San Jose, CA 95117
101 Software Avenue,
455 El Camino Real
Santa Clara, CA 95050