💾 Archived View for gemini.bortzmeyer.org › rfc-mirror › rfc6326.txt captured on 2022-03-01 at 17:33:40.
⬅️ Previous capture (2021-11-30)
-=-=-=-=-=-=-
Internet Engineering Task Force (IETF) D. Eastlake
Request for Comments: 6326 Huawei
Category: Standards Track A. Banerjee
ISSN: 2070-1721 D. Dutt
Cisco
R. Perlman
Intel
A. Ghanwani
Brocade
July 2011
Transparent Interconnection of Lots of Links (TRILL) Use of IS-IS
Abstract
The IETF has standardized the Transparent Interconnection of Lots of
Links (TRILL) protocol, which provides transparent Layer 2 forwarding
using encapsulation with a hop count and IS-IS link state routing.
This document specifies the data formats and code points for the
IS-IS extensions to support TRILL.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc6326.
Copyright Notice
Copyright (c) 2011 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
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
Eastlake, et al. Standards Track [Page 1]
RFC 6326 TRILL Use of IS-IS July 2011
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction ....................................................3
1.1. Conventions Used in This Document ..........................3
2. TLV and Sub-TLV Extensions to IS-IS for TRILL ...................3
2.1. The Group Address TLV ......................................3
2.1.1. The Group MAC Address Sub-TLV .......................4
2.2. Multi-Topology-Aware Port Capability Sub-TLVs ..............5
2.2.1. The Special VLANs and Flags Sub-TLV .................6
2.2.2. Enabled-VLANs Sub-TLV ...............................7
2.2.3. Appointed Forwarders Sub-TLV ........................8
2.3. Sub-TLVs for the Router Capability TLV .....................9
2.3.1. The TRILL Version Sub-TLV ...........................9
2.3.2. The Nickname Sub-TLV ...............................10
2.3.3. The Trees Sub-TLV ..................................11
2.3.4. The Tree Identifiers Sub-TLV .......................11
2.3.5. The Trees Used Identifiers Sub-TLV .................12
2.3.6. Interested VLANs and Spanning Tree Roots Sub-TLV ...12
2.3.7. The VLAN Group Sub-TLV .............................15
2.4. MTU Sub-TLV of the Extended Reachability TLV ..............15
2.5. TRILL Neighbor TLV ........................................16
3. The MTU PDUs ...................................................18
4. Use of Existing PDUs and TLVs ..................................19
4.1. TRILL IIH PDUs ............................................19
4.2. Area Address ..............................................19
4.3. Protocols Supported .......................................19
5. IANA Considerations ............................................20
5.1. Allocations from Existing Registries ......................20
5.2. New Sub-Registries Created ................................21
6. Security Considerations ........................................22
7. References .....................................................22
7.1. Normative References ......................................22
7.2. Informative References ....................................23
8. Acknowledgements ...............................................23
Appendix A. Initial IS-IS PDU Registry ............................24
Eastlake, et al. Standards Track [Page 2]
RFC 6326 TRILL Use of IS-IS July 2011
1. Introduction
The IETF has standardized the TRILL protocol [RFC6325], which
provides transparent Layer 2 forwarding using encapsulation with a
hop count and link state routing. TRILL provides optimal pair-wise
forwarding without configuration, safe forwarding even during periods
of temporary loops, and support for multipathing of both unicast and
multicast traffic as well as supporting VLANs. Intermediate Systems
(ISs) implementing TRILL can incrementally replace IEEE [802.1Q-2005]
bridges.
This document, in conjunction with [RFC6165], specifies the data
formats and code points for the IS-IS [ISO-10589] [RFC1195]
extensions to support TRILL.
1.1. Conventions Used in This Document
The terminology and acronyms defined in [RFC6325] are used herein
with the same meaning.
Additional acronyms used in this document are:
IIH - IS-IS Hello
IS - Intermediate System (for this document, all relevant
intermediate systems are RBridges)
NLPID - Network Layer Protocol Identifier
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. TLV and Sub-TLV Extensions to IS-IS for TRILL
This section, in conjunction with [RFC6165], specifies the data
formats and code points for the TLVs and sub-TLVs added to IS-IS to
support the TRILL standard. Information as to the number of
occurrences allowed, such as for a TLV in a PDU or set of PDUs or for
a sub-TLV in a TLV, is provided in Section 5.
2.1. The Group Address TLV
The Group Address (GADDR) TLV, IS-IS TLV type 142, is carried only in
an LSP PDU and carries sub-TLVs that in turn advertise multicast
group listeners. Section 2.1.1 below specifies a sub-TLV that
advertises listeners by MAC address. It is anticipated that
Eastlake, et al. Standards Track [Page 3]
RFC 6326 TRILL Use of IS-IS July 2011
additional sub-TLVs for additional address types such as IP addresses
will be specified in other documents. The sub-TLVs under GADDR
constitute a new series of sub-TLV types (see Section 5.2).
GADDR has the following format:
+-+-+-+-+-+-+-+-+
|Type=GADDR-TLV | (1 byte)
+-+-+-+-+-+-+-+-+
| Length | (1 byte)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| sub-TLVs...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
o Type: TLV Type, set to GADDR-TLV 142.
o Length: variable depending on the sub-TLVs carried.
o sub-TLVs: The Group Address TLV value consists of sub-TLVs
formatted as described in [RFC5305].
2.1.1. The Group MAC Address Sub-TLV
The Group MAC Address (GMAC-ADDR) sub-TLV is sub-TLV type number 1
within the GADDR TLV. In TRILL, it is used to advertise multicast
listeners as specified in Section 4.5.5 of [RFC6325]. It has the
following format:
+-+-+-+-+-+-+-+-+
|Type=GMAC-ADDR | (1 byte)
+-+-+-+-+-+-+-+-+
| Length | (1 byte)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| RESV | Topology-ID | (2 bytes)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| RESV | VLAN ID | (2 bytes)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Num Group Recs | (1 byte)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| GROUP RECORDS (1) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ................. |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| GROUP RECORDS (N) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Eastlake, et al. Standards Track [Page 4]
RFC 6326 TRILL Use of IS-IS July 2011
where each group record is of the form:
+-+-+-+-+-+-+-+-+
| Num of Sources| (1 byte)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Group Address (6 bytes) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Source 1 Address (6 bytes) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Source 2 Address (6 bytes) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ..... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Source M Address (6 bytes) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
o Type: GADDR sub-TLV type, set to 1 (GMAC-ADDR).
o Length: Variable, minimum 5.
o RESV: Reserved. 4-bit fields that MUST be sent as zero and
ignored on receipt.
o Topology-ID: This field is not used in TRILL, where it is sent as
zero and ignored on receipt, but is included for use by other
technologies.
o VLAN ID: This carries the 12-bit VLAN identifier for all
subsequent MAC addresses in this sub-TLV, or the value zero if no
VLAN is specified.
o Number of Group Records: A 1-byte integer that is the number of
group records in this sub-TLV.
o Group Record: Each group record carries the number of sources. It
then has a 48-bit multicast address followed by 48-bit source MAC
addresses. If the sources do not fit in a single sub-TLV, the
same group address may be repeated with different source addresses
in another sub-TLV of another instance of the Group Address TLV.
2.2. Multi-Topology-Aware Port Capability Sub-TLVs
TRILL makes use of the Multi-Topology-Aware Port Capability
(MT-PORT-CAP) TLV as specified in [RFC6165]. The remainder of this
section specifies the sub-TLVs that TRILL uses the MT-PORT-CAP TLV to
transport.
Eastlake, et al. Standards Track [Page 5]
RFC 6326 TRILL Use of IS-IS July 2011
2.2.1. The Special VLANs and Flags Sub-TLV
In TRILL, a Special VLANs and Flags (VLAN-Flags) sub-TLV is carried
in every IIH PDU. It has the following format:
+-+-+-+-+-+-+-+-+
| Type | (1 byte)
+-+-+-+-+-+-+-+-+
| Length | (1 byte)
+---------------+---------------+
| Port ID | (2 bytes)
+-------------------------------+
| Sender Nickname | (2 bytes)
+--+--+--+--+-------------------+
|AF|AC|VM|BY| Outer.VLAN | (2 bytes)
+--+--+--+--+-------------------+
|TR|R |R |R | Desig.VLAN | (2 bytes)
+--+--+--+--+-------------------+
o Type: sub-TLV type, set to MT-PORT-CAP VLAN-FLAGs sub-TLV 1.
o Length: 8.
o Port ID: An ID for the port on which the enclosing TRILL IIH PDU
is being sent as specified in [RFC6325], Section 4.4.2.
o Sender Nickname: If the sending IS is holding any nicknames as
discussed in [RFC6325], Section 3.7, one MUST be included here.
Otherwise, the field is set to zero. This field is to support
intelligent end stations that determine the egress IS (RBridge)
for unicast data through a directory service or the like and that
need a nickname for their first hop to insert as the ingress
nickname to correctly format a TRILL encapsulated data frame. See
[RFC6325], Section 4.6.2, point 8.
o Outer.VLAN: A copy of the 12-bit outer VLAN ID of the TRILL IIH
frame containing this sub-TLV when that frame was sent, as
specified in [RFC6325], Section 4.4.5.
o Desig.VLAN: The 12-bit ID of the designated VLAN for the link, as
specified in [RFC6325], Section 4.2.4.2.
o AF, AC, VM, BY, and TR: These flag bits have the following
meanings when set to one, as specified in the listed section of
[RFC6325]:
Eastlake, et al. Standards Track [Page 6]
RFC 6326 TRILL Use of IS-IS July 2011
RFC 6325
Bit Section Meaning if bit is one
--------------------------------------
AF 4.4.2 Originating IS believes it is appointed forwarder
for the VLAN and port on which the containing IIH
PDU was sent.
AC 4.9.1 Originating port configured as an access port
(TRILL traffic disabled).
VM 4.4.5 VLAN mapping detected on this link.
BY 4.4.2 Bypass pseudonode.
TR 4.9.1 Originating port configured as a trunk port (end-
station service disabled).
o R: Reserved bit. MUST be sent as zero and ignored on receipt.
2.2.2. Enabled-VLANs Sub-TLV
The optional Enabled-VLANs sub-TLV specifies the VLANs enabled for
end station service at the port of the originating IS on which the
Hello was sent, as specified in [RFC6325], Section 4.4.2. It has the
following format:
+-+-+-+-+-+-+-+-+
| Type | (1 byte)
+-+-+-+-+-+-+-+-+
| Length | (1 byte)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| RESV | Start VLAN ID | (2 bytes)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| VLAN bit-map....
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
o Type: sub-TLV type, set to MT-PORT-CAP Enabled-VLANs sub-TLV 2.
o Length: Variable, minimum 3.
o RESV: 4 reserved bits that MUST be sent as zero and ignored on
receipt.
o Start VLAN ID: The 12-bit VLAN ID that is represented by the high
order bit of the first byte of the VLAN bit-map.
Eastlake, et al. Standards Track [Page 7]
RFC 6326 TRILL Use of IS-IS July 2011
o VLAN bit-map: The highest order bit indicates the VLAN equal to
the start VLAN ID, the next highest bit indicates the VLAN equal
to start VLAN ID + 1, continuing to the end of the VLAN bit-map
field.
If this sub-TLV occurs more than once in a Hello, the set of enabled
VLANs is the union of the sets of VLANs indicated by each of the
Enabled-VLAN sub-TLVs in the Hello.
2.2.3. Appointed Forwarders Sub-TLV
The DRB on a link uses the Appointed Forwarders sub-TLV to inform
other ISs on the link that they are the designated VLAN-x forwarder
for one or more ranges of VLAN IDs as specified in Section 4.2.4 of
[RFC6325]. It has the following format:
+-+-+-+-+-+-+-+-+
| Type | (1 byte)
+-+-+-+-+-+-+-+-+
| Length | (1 byte)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Appointment Information (1) | (6 bytes)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ................. |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Appointment Information (N) | (6 bytes)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
where each appointment is of the form:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Appointee Nickname | (2 bytes)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| RESV | Start.VLAN | (2 bytes)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| RESV | End.VLAN | (2 bytes)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
o Type: sub-TLV type, set to MT-PORT-CAP AppointedFwrdrs sub-TLV 3.
o Length: 6*n bytes, where there are n appointments.
o Appointee Nickname: The nickname of the IS being appointed a
forwarder.
o RESV: 4 bits that MUST be sent as zero and ignored on receipt.
Eastlake, et al. Standards Track [Page 8]
RFC 6326 TRILL Use of IS-IS July 2011
o Start.VLAN, End.VLAN: These fields are the VLAN IDs of the
appointment range, inclusive. To specify a single VLAN, the
VLAN's ID appears as both the start and end VLAN. As specified in
Section 4.4 of [RFC6325], appointing an IS forwarder on a port for
a VLAN not enabled on that port has no effect.
An IS's nickname may occur as appointed forwarder for multiple VLAN
ranges by occurrences of this sub-TLV within the same or different MT
Port Capability TLVs within an IIH PDU.
2.3. Sub-TLVs for the Router Capability TLV
The Router Capability TLV is specified in [RFC4971]. All of the sub-
sections of this Section 2.3 below specify sub-TLVs that can be
carried in the Router Capability TLV for TRILL.
2.3.1. The TRILL Version Sub-TLV
The TRILL Version (TRILL-VER) sub-TLV indicates the maximum version
of the TRILL standard supported. By implication, lower versions are
also supported. If this sub-TLV is missing, the originating IS only
supports the base version of the protocol [RFC6325].
+-+-+-+-+-+-+-+-+
| Type | (1 byte)
+-+-+-+-+-+-+-+-+
| Length | (1 byte)
+-+-+-+-+-+-+-+-+
| Max-version | (1 byte)
+-+-+-+-+-+-+-+-+
o Type: Router Capability sub-TLV type, set to 13 (TRILL-VER).
o Length: 1.
o Max-version: Set to maximum version supported.
Eastlake, et al. Standards Track [Page 9]
RFC 6326 TRILL Use of IS-IS July 2011
2.3.2. The Nickname Sub-TLV
The Nickname (NICKNAME) Router Capability sub-TLV carries information
about the nicknames of the originating IS, along with information
about its priority to hold those nicknames as specified in [RFC6325],
Section 3.7.3. Multiple instances of this sub-TLV may be carried.
+-+-+-+-+-+-+-+-+
|Type = NICKNAME| (1 byte)
+-+-+-+-+-+-+-+-+
| Length | (1 byte)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| NICKNAME RECORDS (1) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ................. |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| NICKNAME RECORDS (N) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
where each nickname record is of the form:
+-+-+-+-+-+-+-+-+
| Nickname.Pri | (1 byte)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Tree Root Priority | (2 byte)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Nickname | (2 bytes)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
o Type: Router Capability sub-TLV type, set to 6 (NICKNAME).
o Length: 5*N, where N is the number of nickname records present.
o Nickname.Pri: An 8-bit unsigned integer priority to hold a
nickname as specified in Section 3.7.3 of [RFC6325].
o Tree Root Priority: This is an unsigned 16-bit integer priority to
be a tree root as specified in Section 4.5 of [RFC6325].
o Nickname: This is an unsigned 16-bit integer as specified in
Section 3.7 of [RFC6325].
Eastlake, et al. Standards Track [Page 10]
RFC 6326 TRILL Use of IS-IS July 2011
2.3.3. The Trees Sub-TLV
Each IS providing TRILL service uses the TREES sub-TLV to announce
three numbers related to the computation of distribution trees as
specified in Section 4.5 of [RFC6325]. Its format is as follows:
+-+-+-+-+-+-+-+-+
|Type = TREES | (1 byte)
+-+-+-+-+-+-+-+-+
| Length | (1 byte)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Number of trees to compute | (2 byte)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Maximum trees able to compute | (2 byte)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Number of trees to use | (2 byte)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
o Type: Router Capability sub-TLV type, set to 7 (TREES).
o Length: 6.
o Number of trees to compute: An unsigned 16-bit integer as
specified in Section 4.5 of [RFC6325].
o Maximum trees able to compute: An unsigned 16-bit integer as
specified in Section 4.5 of [RFC6325].
o Number of trees to use: An unsigned 16-bit integer as specified in
Section 4.5 of [RFC6325].
2.3.4. The Tree Identifiers Sub-TLV
The tree identifiers (TREE-RT-IDs) sub-TLV is an ordered list of
nicknames. When originated by the IS that has the highest priority
tree root, it lists the distribution trees that the other ISs are
required to compute as specified in Section 4.5 of [RFC6325]. If
this information is spread across multiple sub-TLVs, the starting
tree number is used to allow the ordered lists to be correctly
concatenated. The sub-TLV format is as follows:
Eastlake, et al. Standards Track [Page 11]
RFC 6326 TRILL Use of IS-IS July 2011
+-+-+-+-+-+-+-+-+
|Type=TREE-RT-IDs| (1 byte)
+-+-+-+-+-+-+-+-+
| Length | (1 byte)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Starting Tree Number | (2 bytes)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Nickname (K-th root) | (2 bytes)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Nickname (K+1 - th root) | (2 bytes)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Nickname (...) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
o Type: Router Capability sub-TLV type, set to 8 (TREE-RT-IDs).
o Length: 2 + 2*n, where n is the number of nicknames listed.
o Starting Tree Number: This identifies the starting tree number of
the nicknames that are trees for the domain. This is set to 1 for
the sub-TLV containing the first list. Other Tree-Identifiers
sub-TLVs will have the number of the starting list they contain.
In the event a tree identifier can be computed from two such sub-
TLVs and they are different, then it is assumed that this is a
transient condition that will get cleared. During this transient
time, such a tree SHOULD NOT be computed unless such computation
is indicated by all relevant sub-TLVs present.
o Nickname: The nickname at which a distribution tree is rooted.
2.3.5. The Trees Used Identifiers Sub-TLV
This Router Capability sub-TLV has the same structure as the Tree
Identifiers sub-TLV specified in Section 2.3.4. The only difference
is that its sub-TLV type is set to 9 (TREE-USE-IDs), and the trees
listed are those that the originating IS wishes to use as specified
in [RFC6325], Section 4.5.
2.3.6. Interested VLANs and Spanning Tree Roots Sub-TLV
The value of this Router Capability sub-TLV consists of a VLAN range
and information in common to all of the VLANs in the range for the
originating IS. This information consists of flags, a variable
length list of spanning tree root bridge IDs, and an appointed
forwarder status lost counter, all as specified in the sections of
[RFC6325] listed with the respective information items below.
Eastlake, et al. Standards Track [Page 12]
RFC 6326 TRILL Use of IS-IS July 2011
In the set of LSPs originated by an IS, the union of the VLAN ranges
in all occurrences of this sub-TLV MUST be precisely the set of VLANs
for which the originating IS is appointed forwarder on at least one
port, and the VLAN ranges in multiple VLANs sub-TLVs for an IS MUST
NOT overlap unless the information provided about a VLAN is the same
in every instance. However, as a transient state these conditions
may be violated. If a VLAN is not listed in any INT-VLAN sub-TLV for
an IS, that IS is assumed to be uninterested in receiving traffic for
that VLAN. If a VLAN appears in more than one INT-VLAN sub-TLV for
an IS with different information in the different instances, the
following apply:
- If those sub-TLVs provide different nicknames, it is
unspecified which nickname takes precedence.
- The largest appointed forwarder status lost counter is used.
- The originating IS is assumed to be attached to a multicast
IPv4 router for that VLAN if any of the INT-VLAN sub-TLVs
assert that it is so connected and similarly for IPv6 multicast
router attachment.
- The root bridge lists from all of the instances of the VLAN for
the originating IS are merged.
To minimize such occurrences, wherever possible, an implementation
SHOULD advertise the update to an interested VLAN and Spanning Tree
Roots sub-TLV in the same LSP fragment as the advertisement that it
replaces. Where this is not possible, the two affected LSP fragments
should be flooded as an atomic action. An IS that receives an update
to an existing interested VLAN and Spanning Tree Roots sub-TLV can
minimize the potential disruption associated with the update by
employing a hold-down timer prior to processing the update so as to
allow for the receipt of multiple LSP fragments associated with the
same update prior to beginning processing.
The sub-TLV layout is as follows:
+-+-+-+-+-+-+-+-+
|Type = INT-VLAN| (1 byte)
+-+-+-+-+-+-+-+-+
| Length | (1 byte)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Nickname | (2 bytes)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+...+-+-+-+-+
| Interested VLANS | (4 bytes)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+...+-+-+-+-+
| Appointed Forwarder Status Lost Counter | (4 bytes)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+...+-+-+
| Root Bridges | (6*n bytes)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+...+-+-+
Eastlake, et al. Standards Track [Page 13]
RFC 6326 TRILL Use of IS-IS July 2011
o Type: Router Capability sub-TLV type, set to 10 (INT-VLAN).
o Length: 10 + 6*n, where n is the number of root bridge IDs.
o Nickname: As specified in [RFC6325], Section 4.2.4.4, this field
may be used to associate a nickname held by the originating IS
with the VLAN range indicated. When not used in this way, it is
set to zero.
o Interested VLANS: The Interested VLANs field is formatted as shown
below.
0 1 2 3 4 - 15 16 - 19 20 - 31
+----+----+----+----+------------+----------+------------+
| M4 | M6 | R | R | VLAN.start | RESV | VLAN.end |
+----+----+----+----+------------+----------+------------+
- M4, M6: These bits indicate, respectively, that there is an
IPv4 or IPv6 multicast router on a link for which the
originating IS is appointed forwarder for every VLAN in the
indicated range as specified in [RFC6325], Section 4.2.4.4,
item 5.1.
- R, RESV: These reserved bits MUST be sent as zero and are
ignored on receipt.
- VLAN.start and VLAN.end: This VLAN ID range is inclusive. A
range of one VLAN ID is indicated by setting them both to that
VLAN ID value.
o Appointed Forwarder Status Lost Counter: This is a count of how
many times a port that was appointed forwarder for the VLANs in
the range given has lost the status of being an appointed
forwarder as discussed in Section 4.8.3 of [RFC6325]. It is
initialized to zero at an IS when the zeroth LSP sequence number
is initialized. No special action need be taken at rollover; the
counter just wraps around.
o Root Bridges: The list of zero or more spanning tree root bridge
IDs is the set of root bridge IDs seen for all ports for which the
IS is appointed forwarder for the VLANs in the specified range as
discussed in [RFC6325], Section 4.9.3.2. While, of course, only
one spanning tree root could be seen on any particular port, there
may be multiple ports in the same VLAN connected to different
bridged LANs with different spanning tree roots.
Eastlake, et al. Standards Track [Page 14]
RFC 6326 TRILL Use of IS-IS July 2011
An INT-VLAN sub-TLV asserts that the information provided (multicast
router attachment, appointed forwarder status lost counter, and root
bridges) is the same for all VLANs in the range specified. If this
is not the case, the range MUST be split into subranges meeting this
criteria. It is always safe to use sub-TLVs with a "range" of one
VLAN ID, but this may be too verbose.
2.3.7. The VLAN Group Sub-TLV
The VLAN Group Router Capability sub-TLV consists of two or more VLAN
IDs as specified in [RFC6325], Section 4.8.4. This sub-TLV indicates
that shared VLAN learning is occurring at the announcing IS between
the listed VLANs. It is structured as follows:
+-+-+-+-+-+-+-+-+
|Type=VLAN-GROUP| (1 byte)
+-+-+-+-+-+-+-+-+
| Length | (1 byte)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| RESV | Primary VLAN ID | (2 bytes)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| RESV | Secondary VLAN ID | (2 bytes)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| more Secondary VLAN IDs ... (2 bytes each)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
o Type: Router Capability sub-TLV type, set to 14 (VLAN-GROUP).
o Length: 4 + 2*n, where n is the number of secondary VLAN ID
fields, which may be zero.
o RESV: a 4-bit field that MUST be sent as zero and ignored on
receipt.
o Primary VLAN ID: This identifies the primary VLAN ID.
o Secondary VLAN ID: This identifies a secondary VLAN in the VLAN
Group.
o more Secondary VLAN IDs: zero or more byte pairs, each with the
top 4 bits as a RESV field and the low 12 bits as a VLAN ID.
2.4. MTU Sub-TLV of the Extended Reachability TLV
The MTU sub-TLV is used to optionally announce the MTU of a link as
specified in [RFC6325], Section 4.2.4.4. It occurs within the
Extended Reachability TLV (type 22).
Eastlake, et al. Standards Track [Page 15]
RFC 6326 TRILL Use of IS-IS July 2011
+-+-+-+-+-+-+-+-+
| Type = MTU | (1 byte)
+-+-+-+-+-+-+-+-+
| Length | (1 byte)
+-+-+-+-+-+-+-+-+
|F| Reserved | (1 byte)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MTU | (2 bytes)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
o Type: Extended Reachability sub-TLV type, set to MTU sub-TLV 28.
o Length: 3.
o F: Failed. This bit is a one if MTU testing failed on this link
at the required campus-wide MTU.
o Reserved: 7 bits that MUST be sent as zero and ignored on receipt.
o MTU: This field is set to the largest successfully tested MTU size
for this link, or zero if it has not been tested, as specified in
Section 4.3.2 of [RFC6325].
2.5. TRILL Neighbor TLV
The TRILL Neighbor TLV is used in TRILL IIH PDUs (see Section 4.1
below) in place of the IS Neighbor TLV, as specified in Section
4.4.2.1 of [RFC6325] and in [RFC6327]. The structure of the TRILL
Neighbor TLV is as follows:
+-+-+-+-+-+-+-+-+
| Type | (1 byte)
+-+-+-+-+-+-+-+-+
| Length | (1 byte)
+-+-+-+-+-+-+-+-+
|S|L| RESV | (1 byte)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Neighbor RECORDS (1) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ................. |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Neighbor RECORDS (N) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Eastlake, et al. Standards Track [Page 16]
RFC 6326 TRILL Use of IS-IS July 2011
The information present for each neighbor is as follows:
+-+-+-+-+-+-+-+-+
|F| RESV | (1 bytes)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MTU | (2 bytes)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+...+-+-+-+-+-+-+
| MAC Address | (6 bytes)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+...+-+-+-+-+-+-+
o Type: TLV Type, set to TRILL Neighbor TLV 145.
o Length: 1 + 9*n, where n is the number of neighbor records which
may be zero.
o S: Smallest flag. If this bit is a one, then the list of
neighbors includes the neighbor with the smallest MAC address
considered as an unsigned integer.
o L: Largest flag. If this bit is a one, then the list of neighbors
includes the neighbor with the largest MAC address considered as
an unsigned integer.
o RESV: These 7 bits are reserved use and MUST be sent as zero and
ignored on receipt.
o F: failed. This bit is a one if MTU testing to this neighbor
failed at the required campus-wide MTU (see [RFC6325], Section
4.3.1).
o MTU: This field is set to the largest successfully tested MTU size
for this neighbor or to zero if it has not been tested.
o MAC Address: The MAC address of the neighbor as in the IS Neighbor
TLV (6).
As specified in [RFC6327] and Section 4.4.2.1 of [RFC6325], all MAC
addresses may fit into one TLV, in which case both the S and L flags
would be set to one in that TLV. If the MAC addresses don't fit into
one TLV, the highest MAC address in a TRILL Neighbor TLV with the L
flag zero MUST also appear as a MAC address in some other TRILL
Neighbor TLV (possibly in a different TRILL IIH PDU). Also, the
lowest MAC address in a TRILL Neighbor TLV with the S flag zero MUST
also appear in some other TRILL Neighbor TLV (possibly in a different
TRILL IIH PDU). If an RBridge believes it has no neighbors, it MUST
send a TRILL Neighbor TLV with an empty list of neighbor RECORDS,
which will have both the S and L bits on.
Eastlake, et al. Standards Track [Page 17]
RFC 6326 TRILL Use of IS-IS July 2011
3. The MTU PDUs
Two PDUs are added to IS-IS, the MTU-probe and MTU-ack PDUs. They
are used to optionally determine the MTU on a link between ISs as
specified in [RFC6325], Section 4.3.2.
The MTU PDUs have the IS-IS PDU common header (up through the Maximum
Area Addresses byte) with two new PDU Type numbers, one each, as
listed in Section 6. They also have a 20-byte common fixed MTU PDU
header as shown below.
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| PDU Length | (2 bytes)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+.....+-+-+
| Probe ID (6 bytes) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+.....+-+-+
| Probe Source ID (6 bytes) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+.....+-+-+
| Ack Source ID (6 bytes) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+.....+-+-+
As with other IS-IS PDUs, the PDU length gives the length of the
entire IS-IS packet starting with and including the IS-IS common
header.
The Probe ID field is an arbitrary 48-bit quantity set by the IS
issuing an MTU-probe and copied by the responding IS into the
corresponding MTU-ack. For example, an IS creating an MTU-probe
could compose this quantity from a port identifier and probe sequence
number relative to that port.
The Probe Source ID is set by an IS issuing an MTU-probe to its
System ID and copied by the responding IS into the corresponding
MTU-ack.
The Ack Source ID is set to zero in MTU-probe PDUs. An IS issuing an
MTU-ack sets this field to its System ID.
The TLV area follows the MTU PDU header area. This area MAY contain
an Authentication TLV and MUST be padded to the exact size being
tested with the Padding TLV. Since the minimum size of the Padding
TLV is 2 bytes, it would be impossible to pad to exact size if the
total length of the required information bearing fixed fields and
TLVs added up to 1 byte less than the desired length. However, the
length of the fixed fields and substantive TLVs for MTU PDUs will be
quite small compared with their minimum length (minimum 1470-byte MTU
on an 802.3 link, for example), so this will not be a problem.
Eastlake, et al. Standards Track [Page 18]
RFC 6326 TRILL Use of IS-IS July 2011
4. Use of Existing PDUs and TLVs
The sub-sections below provide details of TRILL use of existing PDUs
and TLVs.
4.1. TRILL IIH PDUs
The TRILL IIH PDU is the variation of the LAN IIH PDU used by the
TRILL protocol. Section 4.4 of the TRILL standard [RFC6325]
specifies the contents of the TRILL IIH and how its use in TRILL
differs from Layer 3 LAN IIH PDU use. The adjacency state machinery
for TRILL neighbors is specified in Section 4.4 of [RFC6325] and in
[RFC6327].
In a TRILL IIH PDU, the IS-IS common header and the fixed PDU Header
are the same as a Level 1 LAN IIH PDU. The Maximum Area Addresses
octet in the common header MUST be set to 0x01.
The IS-IS Neighbor TLV (6) is not used in a TRILL IIH and is ignored
if it appears there. Instead, TRILL IIH PDUs use the TRILL Neighbor
TLV (see Section 2.5).
4.2. Area Address
TRILL uses a fixed zero Area Address as specified in [RFC6325],
Section 4.2.3. This is encoded in a 4-byte Area Address TLV (1) as
follows:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0x01, Area Address Type | (1 byte)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0x02, Length of Value | (1 byte)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0x01, Length of Address | (1 byte)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0x00, zero Area Address | (1 byte)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
4.3. Protocols Supported
NLPID 0xC0 has been assigned to TRILL [RFC6328]. A Protocols
Supported TLV (129, [RFC1195]) including that value MUST appear in
TRILL IIH PDUs and LSP number zero PDUs.
Eastlake, et al. Standards Track [Page 19]
RFC 6326 TRILL Use of IS-IS July 2011
5. IANA Considerations
IANA has allocated the existing registry code points listed in
Section 5.1 and created two new registries with the initial contents
as described in Section 5.2.
5.1. Allocations from Existing Registries
This document specifies two new IS-IS TLV types -- namely, the Group
Address TLV (GADDR-TLV, type 142) and the TRILL Neighbor TLV (type
145). The PDUs in which these TLVs are permitted for TRILL are shown
in the table below along with the section of this document where they
are discussed. The final "NUMBER" column indicates the permitted
number of occurrences of the TLV in their PDU, or set of PDUs in the
case of LSP, which in these two cases is "*" indicating that the TLV
MAY occur 0, 1, or more times.
IANA registered these two code points in the IANA IS-IS TLV registry
(ignoring the "Section" and "NUMBER" columns, which are irrelevant to
that registry).
Section TLV# IIH LSP SNP NUMBER
GADDR-TLV 2.1 142 - X - *
TRILL Neighbor TLV 2.5 145 X - - *
This document specifies eleven new sub-TLVs from existing sub-TLV
sequences -- namely, VLAN-FLAGS, Enabled-VLANs, AppointedFwrdrs,
TRILL Version (TRILL-VER), NICKNAME, TREES, TREE-RT-IDs,
TREE-USE-IDs, INT-VLAN, VLAN-GROUP, and MTU. The TLVs in which these
sub-TLVs occur are shown in the table below along with the section of
this document where they are discussed.
Those sub-TLVs with an "X" in the column labeled "MT Port Capabil."
are sub-TLVs of TLV 143 [RFC6165], the MT-PORT-CAP-TLV. Those sub-
TLVs with an "X" in the column labeled "Router Capabil." are sub-TLVs
of TLV 242, the IS-IS Router CAPABILITY TLV. Those sub-TLVs with an
"X" in the column labeled "Extended IS Reach" are sub-TLVs of TLV 22,
the Extended IS reachability TLV.
The final "NUM" column indicates the permitted number of occurrences
of the sub-TLV cumulatively within all occurrences of their TLV in
that TLV's carrying PDU (or set of PDUs in the case of LSP), as
follows:
Eastlake, et al. Standards Track [Page 20]
RFC 6326 TRILL Use of IS-IS July 2011
0-1 = MAY occur zero or one times. If it occurs more than once,
results are unspecified.
1 = MUST occur exactly once. If absent, the PDU is ignored. If
it occurs more than once, results are unspecified.
* = MAY occur 0, 1, or more times.
The values in the "Section" and "NUM" columns are irrelevant to the
IANA sub-registries.
Section sub- MT Port Router Extended NUM
TLV# Capabil. Capabil. IS Reach
VLAN-FLAGS 2.2.1 1 X - - 1
Enabled-VLANs 2.2.2 2 X - - *
AppointedFwrdrs 2.2.3 3 X - - *
NICKNAME 2.3.2 6 - X - *
TREES 2.3.3 7 - X - 0-1
TREE-RT-IDs 2.3.4 8 - X - *
TREE-USE-IDs 2.3.5 9 - X - *
INT-VLAN 2.3.6 10 - X - *
TRILL-VER 2.3.1 13 - X - 0-1
VLAN-GROUP 2.3.7 14 - X - *
MTU 2.4 28 - - X 0-1
5.2. New Sub-Registries Created
This document creates two new IS-IS PDUs -- namely, the MTU-PROBE-PDU
and MTU-ACK-PDU, as described in Section 3. IANA assigned new PDU
types to these PDUs and reflect them in a newly created PDU registry
(see Appendix A).
MTU-PROBE-PDU PDU Number: 23
MTU-ACK-PDU PDU Number: 28
IANA created a new sub-TLV IS-IS sub-registry for sub-TLVs within the
Group Address (GADDR) TLV and specified an initial sub-TLV within
that registry -- namely, the Group MAC Address (GMAC-ADDR) sub-TLV
(1). The GMAC-ADDR sub-TLV may occur 0, 1, or more times in a GADDR
TLV.
The initial sub-registry is shown below.
Registry Name: IS-IS Group Address Type Codes for TLV 10
Reference: This document
Registration Procedures: Expert Review [RFC5226]
Eastlake, et al. Standards Track [Page 21]
RFC 6326 TRILL Use of IS-IS July 2011
Registry:
Value Group Address Type Code Reference
------- ----------------------------- ---------
0 Reserved This document
1 GMAC-ADDR This document
2-254 Unassigned This document
255 Reserved This document
6. Security Considerations
For general TRILL protocol security considerations, see the TRILL
base protocol standard [RFC6325].
This document raises no new security issues for IS-IS. IS-IS
security may be used to secure the IS-IS messages discussed here.
See [RFC5304] and [RFC5310]. Even when IS-IS authentication is used,
replays of Hello packets can create denial-of-service conditions; see
[RFC6039] for details. These issues are similar in scope to those
discussed in Section 6.2 of [RFC6325], and the same mitigations may
apply.
7. References
7.1. Normative References
[ISO-10589] ISO/IEC 10589:2002, Second Edition, "Intermediate
System to Intermediate System Intra-Domain Routing
Exchange Protocol for use in Conjunction with the
Protocol for Providing the Connectionless-mode Network
Service (ISO 8473)", 2002.
[RFC1195] Callon, R., "Use of OSI IS-IS for Routing in TCP/IP and
Dual Environments", 1990.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC4971] Vasseur, JP. and N. Shen, "Intermediate System to
Intermediate System (IS-IS) Extensions for Advertising
Router Information", 2007.
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing
an IANA Considerations Section in RFCs", BCP 26, RFC
5226, May 2008.
[RFC5305] Li, T. and H. Smit, "IS-IS Extensions for Traffic
Engineering", 2008.
Eastlake, et al. Standards Track [Page 22]
RFC 6326 TRILL Use of IS-IS July 2011
[RFC6165] Banerjee, A. and D. Ward, "Extensions to IS-IS for
Layer-2 Systems", RFC 6165, April 2011.
[RFC6325] Perlman, R., Eastlake, D., Dutt, D., Gai, S., and A.
Ghanwani, "RBridges: Base Protocol Specification", RFC
6325, July 2011.
[RFC6327] Eastlake, D., Perlman, R., Ghanwani, A., Dutt, D., and
V. Manral, "RBridges: Adjacency", RFC 6327, July 2011.
[RFC6328] Eastlake, D., "IANA Considerations for Network Layer
Protocol Identifiers", RFC 6328, July 2011.
7.2. Informative References
[802.1Q-2005] "IEEE Standard for Local and metropolitan area networks
/ Virtual Bridged Local Area Networks", 802.1Q-2005, 19
May 2006.
[RFC5304] Li, T. and R. Atkinson, "IS-IS Cryptographic
Authentication", RFC 5304, October 2008.
[RFC5310] Bhatia, M., Manral, V., Li, T., Atkinson, R., White,
R., and M. Fanto, "IS-IS Generic Cryptographic
Authentication", RFC 5310, February 2009.
[RFC6039] Manral, V., Bhatia, M., Jaeggli, J., and R. White,
"Issues with Existing Cryptographic Protection Methods
for Routing Protocols", RFC 6039, October 2010.
8. Acknowledgements
The authors gratefully acknowledge the contributions and review by
the following: Mike Shand, Stewart Bryant, Dino Farinacci, Les
Ginsberg, Sam Hartman, Dan Romascanu, Dave Ward, and Russ White. In
particular, thanks to Mike Shand for the detailed and helpful
comments.
Eastlake, et al. Standards Track [Page 23]
RFC 6326 TRILL Use of IS-IS July 2011
Appendix A. Initial IS-IS PDU Registry
The following is the suggested initial IS-IS PDU registry before
MTU-PROBE-PDU and MTU-ACK-PDU, which should be added with this
document as REFERENCE:
Registry Name: IS-IS PDUs
Reference: This document
Registration Procedures: IETF Review [RFC5226]
MNEMONIC PDU# REFERENCE
Unassigned 0-14
L1-LAN-HELLO-PDU 15 [ISO-10589]
L2-LAN-HELLO-PDU 16 [ISO-10589]
P2P-HELLO-PDU 17 [ISO-10589]
L1-LSP-PDU 18 [ISO-10589]
Unassigned 19
L2-LSP-PDU 20 [ISO-10589]
Unassigned 21-23
L1-CSNP-PDU 24 [ISO-10589]
L2-CSNP-PDU 25 [ISO-10589]
L1-PSNP-PDU 26 [ISO-10589]
L2-PSNP-PDU 27 [ISO-10589]
Unassigned 28-31
Eastlake, et al. Standards Track [Page 24]
RFC 6326 TRILL Use of IS-IS July 2011
Authors' Addresses
Donald Eastlake
Huawei
155 Beaver Street
Milford, MA 01757 USA
Phone: +1-508-333-2270
EMail: d3e3e3@gmail.com
Ayan Banerjee
Cisco Systems
170 West Tasman Drive
San Jose, CA 95134 USA
EMail: ayabaner@cisco.com
Dinesh Dutt
Cisco Systems
170 West Tasman Drive
San Jose, CA 95134-1706 USA
Phone: +1-408-527-0955
EMail: ddutt@cisco.com
Radia Perlman
Intel Labs
2200 Mission College Blvd.
Santa Clara, CA 95054-1549 USA
Phone: +1-408-765-8080
EMail: Radia@alum.mit.edu
Anoop Ghanwani
Brocade
130 Holger Way
San Jose, CA 95134 USA
Phone: +1-408-333-7149
EMail: anoop@alumni.duke.edu
Eastlake, et al. Standards Track [Page 25]