Internet DRAFT - draft-ietf-trill-rbridge-multilevel
draft-ietf-trill-rbridge-multilevel
TRILL Working Group Radia Perlman
INTERNET-DRAFT EMC
Intended status: Informational Donald Eastlake
Mingui Zhang
Huawei
Anoop Ghanwani
Dell
Hongjun Zhai
JIT
Expires: January 3, 2018 July 3, 2017
Alternatives for Multilevel TRILL
(Transparent Interconnection of Lots of Links)
<draft-ietf-trill-rbridge-multilevel-07.txt>
Abstract
Although TRILL is based on IS-IS, which supports multilevel unicast
routing, extending TRILL to multiple levels has challenges that are
not addressed by the already-existing capabilities of IS-IS. One
issue is with the handling of multi-destination packet distribution
trees. Other issues are with TRILL switch nicknames. How are such
nicknames allocated across a multilevel TRILL network? Do nicknames
need to be unique across an entire multilevel TRILL network or can
they merely be unique within each multilevel area?
This informational document enumerates and examines alternatives
based on a number of factors including backward compatibility,
simplicity, and scalability and makes recommendations in some cases.
Status of This Memo
This Internet-Draft is submitted to IETF in full conformance with the
provisions of BCP 78 and BCP 79. Distribution of this document is
unlimited. Comments should be sent to the TRILL working group
mailing list <trill@ietf.org>.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet-
Drafts.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
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The list of current Internet-Drafts can be accessed at
http://www.ietf.org/1id-abstracts.html. The list of Internet-Draft
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Table of Contents
1. Introduction............................................4
1.1 The Motivation for Multilevel..........................4
1.2 Improvements Due to Multilevel.........................5
1.2.1. The Routing Computation Load........................5
1.2.2. LSDB Volatility Creating Too Much Control Traffic...5
1.2.3. LSDB Volatility Causing To Much Time Unconverged....6
1.2.4. The Size Of The LSDB................................6
1.2.5 Nickname Limit.......................................6
1.2.6 Multi-Destination Traffic............................7
1.3 Unique and Aggregated Nicknames........................7
1.4 More on Areas..........................................8
1.5 Terminology and Acronyms...............................8
2. Multilevel TRILL Issues................................10
2.1 Non-zero Area Addresses...............................11
2.2 Aggregated versus Unique Nicknames....................11
2.2.1 More Details on Unique Nicknames....................12
2.2.2 More Details on Aggregated Nicknames................13
2.2.2.1 Border Learning Aggregated Nicknames..............14
2.2.2.2 Swap Nickname Field Aggregated Nicknames..........16
2.2.2.3 Comparison........................................17
2.3 Building Multi-Area Trees.............................17
2.4 The RPF Check for Trees...............................18
2.5 Area Nickname Acquisition.............................18
2.6 Link State Representation of Areas....................19
3. Area Partition.........................................20
4. Multi-Destination Scope................................21
4.1 Unicast to Multi-destination Conversions..............21
4.1.1 New Tree Encoding...................................22
4.2 Selective Broadcast Domain Reduction..................22
5. Co-Existence with Old TRILL switches...................24
6. Multi-Access Links with End Stations...................25
7. Summary................................................27
8. Security Considerations................................28
9. IANA Considerations....................................28
Normative References......................................29
Informative References....................................29
Acknowledgements..........................................31
Authors' Addresses........................................32
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1. Introduction
The IETF TRILL (Transparent Interconnection of Lot of Links) protocol
[RFC6325] [RFC7177] [RFC7780] provides optimal pair-wise data routing
without configuration, safe forwarding even during periods of
temporary loops, and support for multipathing of both unicast and
multicast traffic in networks with arbitrary topology and link
technology, including multi-access links. TRILL accomplishes this by
using IS-IS (Intermediate System to Intermediate System [IS-IS]
[RFC7176]) link state routing in conjunction with a header that
includes a hop count. The design supports data labels (VLANs and Fine
Grained Labels [RFC7172]) and optimization of the distribution of
multi-destination data based on data label and multicast group.
Devices that implement TRILL are called TRILL Switches or RBridges.
Familiarity with [IS-IS], [RFC6325], and [RFC7780] is assumed in this
document.
1.1 The Motivation for Multilevel
The primary motivation for multilevel TRILL is to improve
scalability. The following issues might limit the scalability of a
TRILL-based network:
1. The routing computation load
2. The volatility of the link state database (LSDB) creating too much
control traffic
3. The volatility of the LSDB causing the TRILL network to be in an
unconverged state too much of the time
4. The size of the LSDB
5. The limit of the number of TRILL switches, due to the 16-bit
nickname space (for further information on why this might be a
problem, see Section 1.2.5)
6. The traffic due to upper layer protocols use of broadcast and
multicast
7. The size of the end node learning table (the table that remembers
(egress TRILL switch, label/MAC) pairs)
As discussed below, extending TRILL IS-IS to be multilevel
(hierarchical) can help with all of these issues except issue 7.
IS-IS was designed to be multilevel [IS-IS]. A network can be
partitioned into "areas". Routing within an area is known as "Level
1 routing". Routing between areas is known as "Level 2 routing".
The Level 2 IS-IS network consists of Level 2 routers and links
between the Level 2 routers. Level 2 routers may participate in one
or more Level 1 areas, in addition to their role as Level 2 routers.
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Each area is connected to Level 2 through one or more "border
routers", which participate both as a router inside the area, and as
a router inside the Level 2 "area". Care must be taken that it is
clear, when transitioning multi-destination packets between Level 2
and a Level 1 area in either direction, that exactly one border TRILL
switch will transition a particular data packet between the levels or
else duplication or loss of traffic can occur.
1.2 Improvements Due to Multilevel
Partitioning the network into areas directly solves the first four
scalability issues listed above as described in Sections 1.2.1
through 1.2.4. Multilevel also contributes to solving issues 5 and 6
as discussed in Section 1.2.5 and 1.2.6 respectively.
In the subsections below, N indicates the number of TRILL switches in
a TRILL campus. As a simplifying assumption, it is assumed that each
TRILL switch has k links to other TRILL switches. An "optimized"
multilevel campus is assumed to have Level 1 areas containing sqrt(N)
switches.
1.2.1. The Routing Computation Load
The Dijkstra algorithm uses computational effort on the order of the
number of links in a network (N*k) times the log of the number of
nodes to calculate least cost routes at a router (Section 12.3.3
[InterCon]). Thus, in a single level TRILL campus, it is on the order
of N*k*log(N). In an optimized multilevel campus, it is on the order
of sqrt(N)*k*log(N). So, for example, assuming N is 3,000, the level
of computational effort would be reduced by about a factor of 50.
1.2.2. LSDB Volatility Creating Too Much Control Traffic
The rate of LSDB changes is assumed to be approximately proportional
to the number of routers and links in the TRILL campus or N*(1+k) for
a single level campus. With an optimized multilevel campus, each area
would have about sqrt(N) routers and proportionately fewer links
reducing the rate of LSDB changes by about a factor of sqrt(N).
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1.2.3. LSDB Volatility Causing To Much Time Unconverged
With the simplifying assumption that routing converges after each
topology change before the next such change, the fraction of time
that routing is unconverged is proportional to the product of the
rate of change occurrence and the convergence time. The rate of
topology changes per some arbitrary unit of time will be roughly
proportional to the number of router and links (Section 1.2.2). The
convergence time is approximately proportional to the computation
involved at each router (Section 1.2.1). Thus, based on these
simplifying assumptions, the time spent unconverged in a single level
network is proportional to (N*(1+k))*(N*k*log(N)) while that time for
an optimized multilevel network would be proportional to
(sqrt(N)*(1+k))*(sqrt(N)*k*log(N)). Thus, in changing to multilevel,
the time spent unconverged, using these simplifying assumptions, is
improved by about a factor of N.
1.2.4. The Size Of The LSDB
The size of the LSDB, which consists primarily of information about
routers (TRILL switches) and links, is also approximately
proportional to the number of routers and links. So, as with item 2
in Section 1.2.2 above, it should improve by about a factor of
sqrt(N) in going from single to multilevel.
1.2.5 Nickname Limit
For many TRILL protocol purposes, RBridges are designated by 16-bit
nicknames. While some values are reserved, this appears to provide
enough nicknames to designated over 65,000 RBridges. However, this
number is effectively reduced by the following two factors:
- Nicknames are consumed when pseudo-nicknames are used for the
active-active connection of end stations. Using the techniques in
[RFC7781], for example, could double the nickname consumption if
there are extensive active-active edge groups connected to
different sets of edge TRILL switch ports.
- There might be problems in multilevel campus wide contention for
single nickname allocation of nicknames were allocated
individually from a single pool for the entire campus. Thus it
seems likely that a hierarchical method would be chosen where
blocks of nicknames are allocated at Level 2 to Level 1 areas and
contention for a nickname by an RBridge in such a Level 1 area
would be only within that area. Such hierarchical allocation leads
to further effective loss of nicknames similar to the situation
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with IP addresses discussed in [RFC3194].
Even without the above effective reductions in nickname space, a very
large multilevel TRILL campus, say one with 200 areas each containing
500 TRILL switches, could require 100,000 or more nicknames if all
nicknames in the campus must be unique, which is clearly impossible
with 16-bit nicknames.
This scaling limit, namely, 16-bit nickname space, will only be
addressed with the aggregated nickname approach. Since the aggregated
nickname approach requires some complexity in the border TRILL
switches (for rewriting the nicknames in the TRILL header), the
suggested design in this document allows a campus with a mixture of
unique-nickname areas, and aggregated-nickname areas. Thus a TRILL
network could start using multilevel with the simpler unique nickname
method and later add aggregated areas as a later stage of network
growth.
With this design, nicknames must be unique across all Level 2 and
unique-nickname area TRILL switches taken together, whereas nicknames
inside an aggregated-nickname area are visible only inside that area.
Nicknames inside an aggregated-nickname area must still not conflict
with nicknames visible in Level 2 (which includes all nicknames
inside unique nickname areas), but the nicknames inside an
aggregated-nickname area may be the same as nicknames used within one
or more other aggregated-nickname areas.
With the design suggested in this document, TRILL switches within an
area need not be aware of whether they are in an aggregated nickname
area or a unique nickname area. The border TRILL switches in area A1
will indicate, in their LSP inside area A1, which nicknames (or
nickname ranges) are available, or alternatively which nicknames are
not available, for choosing as nicknames by area A1 TRILL switches.
1.2.6 Multi-Destination Traffic
Scaling limits due to protocol use of broadcast and multicast, can be
addressed in many cases in a multilevel campus by introducing
locally-scoped multi-destination delivery, limited to an area or a
single link. See further discussion of this issue in Section 4.2.
1.3 Unique and Aggregated Nicknames
We describe two alternatives for hierarchical or multilevel TRILL.
One we call the "unique nickname" alternative. The other we call the
"aggregated nickname" alternative. In the aggregated nickname
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alternative, border TRILL switches replace either the ingress or
egress nickname field in the TRILL header of unicast packets with an
aggregated nickname representing an entire area.
The unique nickname alternative has the advantage that border TRILL
switches are simpler and do not need to do TRILL Header nickname
modification. It also simplifies testing and maintenance operations
that originate in one area and terminate in a different area.
The aggregated nickname alternative has the following advantages:
o it solves scaling problem #5 above, the 16-bit nickname limit,
in a simple way,
o it lessens the amount of inter-area routing information that
must be passed in IS-IS, and
o it logically reduces the RPF (Reverse Path Forwarding) Check
information (since only the area nickname needs to appear,
rather than all the ingress TRILL switches in that area).
In both cases, it is possible and advantageous to compute multi-
destination data packet distribution trees such that the portion
computed within a given area is rooted within that area.
For further discussion of the unique and aggregated nickname
alternatives, see Section 2.2.
1.4 More on Areas
Each area is configured with an "area address", which is advertised
in IS-IS messages, so as to avoid accidentally interconnecting areas.
For TRILL the only purpose of the area address would be to avoid
accidentally interconnecting areas although the area address had
other purposes in CLNP (Connectionless Network Layer Protocol), IS-IS
was originally designed for CLNP/DECnet.
Currently, the TRILL specification says that the area address must be
zero. If we change the specification so that the area address value
of zero is just a default, then most of IS-IS multilevel machinery
works as originally designed. However, there are TRILL-specific
issues, which we address below in Section 2.1.
1.5 Terminology and Acronyms
This document generally uses the acronyms defined in [RFC6325] plus
the additional acronym DBRB. However, for ease of reference, most
acronyms used are listed here:
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CLNP - ConnectionLess Network Protocol
DECnet - a proprietary routing protocol that was used by Digital
Equipment Corporation. "DECnet Phase 5" was the origin of IS-IS.
Data Label - VLAN or Fine Grained Label [RFC7172]
DBRB - Designated Border RBridge
ESADI - End Station Address Distribution Information
IS-IS - Intermediate System to Intermediate System [IS-IS]
LSDB - Link State Data Base
LSP - Link State PDU
PDU - Protocol Data Unit
RBridge - Routing Bridge, an alternative name for a TRILL switch
RPF - Reverse Path Forwarding
TLV - Type Length Value
TRILL - Transparent Interconnection of Lots of Links or Tunneled
Routing in the Link Layer [RFC6325] [RFC7780]
TRILL switch - a device that implements the TRILL protocol
[RFC6325] [RFC7780], sometimes called an RBridge
VLAN - Virtual Local Area Network
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2. Multilevel TRILL Issues
The TRILL-specific issues introduced by multilevel include the
following:
a. Configuration of non-zero area addresses, encoding them in IS-IS
PDUs, and possibly interworking with old TRILL switches that do
not understand non-zero area addresses.
See Section 2.1.
b. Nickname management.
See Sections 2.5 and 2.2.
c. Advertisement of pruning information (Data Label reachability, IP
multicast addresses) across areas.
Distribution tree pruning information is only an optimization,
as long as multi-destination packets are not prematurely
pruned. For instance, border TRILL switches could advertise
they can reach all possible Data Labels, and have an IP
multicast router attached. This would cause all multi-
destination traffic to be transmitted to border TRILL switches,
and possibly pruned there, when the traffic could have been
pruned earlier based on Data Label or multicast group if border
TRILL switches advertised more detailed Data Label and/or
multicast listener and multicast router attachment information.
d. Computation of distribution trees across areas for multi-
destination data.
See Section 2.3.
e. Computation of RPF information for those distribution trees.
See Section 2.4.
f. Computation of pruning information across areas.
See Sections 2.3 and 2.6.
g. Compatibility, as much as practical, with existing, unmodified
TRILL switches.
The most important form of compatibility is with existing TRILL
fast path hardware. Changes that require upgrade to the slow
path firmware/software are more tolerable. Compatibility for
the relatively small number of border TRILL switches is less
important than compatibility for non-border TRILL switches.
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See Section 5.
2.1 Non-zero Area Addresses
The current TRILL base protocol specification [RFC6325] [RFC7177]
[RFC7780] says that the area address in IS-IS must be zero. The
purpose of the area address is to ensure that different areas are not
accidentally merged. Furthermore, zero is an invalid area address
for layer 3 IS-IS, so it was chosen as an additional safety mechanism
to ensure that layer 3 IS-IS packets would not be confused with TRILL
IS-IS packets. However, TRILL uses other techniques to avoid
confusion on a link, such as different multicast addresses and
Ethertypes on Ethernet [RFC6325], different PPP (Point-to-Point
Protocol) code points on PPP [RFC6361], and the like. Thus, using an
area address in TRILL that might be used in layer 3 IS-IS is not a
problem.
Since current TRILL switches will reject any IS-IS messages with non-
zero area addresses, the choices are as follows:
a.1 upgrade all TRILL switches that are to interoperate in a
potentially multilevel environment to understand non-zero area
addresses,
a.2 neighbors of old TRILL switches must remove the area address from
IS-IS messages when talking to an old TRILL switch (which might
break IS-IS security and/or cause inadvertent merging of areas),
a.3 ignore the problem of accidentally merging areas entirely, or
a.4 keep the fixed "area address" field as 0 in TRILL, and add a new,
optional TLV for "area name" to Hellos that, if present, could be
compared, by new TRILL switches, to prevent accidental area
merging.
In principal, different solutions could be used in different areas
but it would be much simpler to adopt one of these choices uniformly.
A simple solution would be a.1 above with each TRILL switch using a
dominant area nickname as its area address. For the unique nickname
alternative, the dominant nickname could be the lowest value nickname
held by any border RBridge of the area. For the aggregated nickname
alternative, it could be the lowest nickname held by a border RBridge
of the area or a nickname representing the area.
2.2 Aggregated versus Unique Nicknames
In the unique nickname alternative, all nicknames across the campus
must be unique. In the aggregated nickname alternative, TRILL switch
nicknames within an aggregated area are only of local significance,
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and the only nickname externally (outside that area) visible is the
"area nickname" (or nicknames), which aggregates all the internal
nicknames.
The unique nickname approach simplifies border TRILL switches.
The aggregated nickname approach eliminates the potential problem of
nickname exhaustion, minimizes the amount of nickname information
that would need to be forwarded between areas, minimizes the size of
the forwarding table, and simplifies RPF calculation and RPF
information.
2.2.1 More Details on Unique Nicknames
With unique cross-area nicknames, it would be intractable to have a
flat nickname space with TRILL switches in different areas contending
for the same nicknames. Instead, each area would need to be
configured with or allocate one or more block of nicknames. Either
some TRILL switches would need to announce that all the nicknames
other than that in blocks available to the area are taken (to prevent
the TRILL switches inside the area from choosing nicknames outside
the area's nickname block), or a new TLV would be needed to announce
the allowable or the prohibited nicknames, and all TRILL switches in
the area would need to understand that new TLV.
Currently the encoding of nickname information in TLVs is by listing
of individual nicknames; this would make it painful for a border
TRILL switch to announce into an area that it is holding all other
nicknames to limit the nicknames available within that area. Painful
means tens of thousands of individual nickname entries in the Level 1
LSDB. The information could be encoded as ranges of nicknames to make
this manageable by specifying a new TLV similar to the Nickname Flags
APPsub-TLV specified in [RFC7780] but providing flags for blocks of
nicknames rather than single nicknames. Although this would require
updating software, such a new TLV is the preferred method.
There is also an issue with the unique nicknames approach in building
distribution trees, as follows:
With unique nicknames in the TRILL campus and TRILL header
nicknames not rewritten by the border TRILL switches, there would
have to be globally known nicknames for the trees. Suppose there
are k trees. For all of the trees with nicknames located outside
an area, the local trees would be rooted at a border TRILL switch
or switches. Therefore, there would be either no splitting of
multi-destination traffic within the area or restricted splitting
of multi-destination traffic between trees rooted at a highly
restricted set of TRILL switches.
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As an alternative, just the "egress nickname" field of multi-
destination TRILL Data packets could be mapped at the border,
leaving known unicast packets un-mapped. However, this surrenders
much of the unique nickname advantage of simpler border TRILL
switches.
Scaling to a very large campus with unique nicknames might exhaust
the 16-bit TRILL nicknames space particularly if (1) additional
nicknames are consumed to support active-active end station groups at
the TRILL edge using the techniques standardized in [RFC7781] and (2)
use of the nickname space is less efficient due to the allocation of,
for example, power-of-two size blocks of nicknames to areas in the
same way that use of the IP address space is made less efficient by
hierarchical allocation (see [RFC3194]). One method to avoid nickname
exhaustion might be to expand nicknames to 24 bits; however, that
technique would require TRILL message format and fast path processing
changes and that all TRILL switches in the campus understand larger
nicknames.
2.2.2 More Details on Aggregated Nicknames
The aggregated nickname approach enables passing far less nickname
information. It works as follows, assuming both the source and
destination areas are using aggregated nicknames:
There are at least two ways areas could be identified.
One method would be to assign each area a 16-bit nickname. This
would not be the nickname of any actual TRILL switch. Instead, it
would be the nickname of the area itself. Border TRILL switches
would know the area nickname for their own area(s). For an
example of a more specific multilevel proposal using unique
nicknames, see [DraftUnique].
Alternatively, areas could be identified by the set of nicknames
that identify the border routers for that area. (See [SingleName]
for a multilevel proposal using such a set of nicknames.)
The TRILL Header nickname fields in TRILL Data packets being
transported through a multilevel TRILL campus with aggregated
nicknames are as follows:
- When both the ingress and egress TRILL switches are in the same
area, there need be no change from the existing base TRILL
protocol standard in the TRILL Header nickname fields.
- When being transported between different Level 1 areas in Level
2, the ingress nickname is a nickname of the ingress TRILL
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switch's area while the egress nickname is either a nickname of
the egress TRILL switch's area or a tree nickname.
- When being transported from Level 1 to Level 2, the ingress
nickname is the nickname of the ingress TRILL switch itself
while the egress nickname is either a nickname for the area of
the egress TRILL switch or a tree nickname.
- When being transported from Level 2 to Level 1, the ingress
nickname is a nickname for the ingress TRILL switch's area while
the egress nickname is either the nickname of the egress TRILL
switch itself or a tree nickname.
There are two variations of the aggregated nickname approach. The
first is the Border Learning approach, which is described in Section
2.2.2.1. The second is the Swap Nickname Field approach, which is
described in Section 2.2.2.2. Section 2.2.2.3 compares the advantages
and disadvantages of these two variations of the aggregated nickname
approach.
2.2.2.1 Border Learning Aggregated Nicknames
This section provides an illustrative example and description of the
border learning variation of aggregated nicknames where a single
nickname is used to identify an area.
In the following picture, RB2 and RB3 are area border TRILL switches
(RBridges). A source S is attached to RB1. The two areas have
nicknames 15961 and 15918, respectively. RB1 has a nickname, say 27,
and RB4 has a nickname, say 44 (and in fact, they could even have the
same nickname, since the TRILL switch nickname will not be visible
outside these aggregated areas).
Area 15961 level 2 Area 15918
+-------------------+ +-----------------+ +--------------+
| | | | | |
| S--RB1---Rx--Rz----RB2---Rb---Rc--Rd---Re--RB3---Rk--RB4---D |
| 27 | | | | 44 |
| | | | | |
+-------------------+ +-----------------+ +--------------+
Let's say that S transmits a frame to destination D, which is
connected to RB4, and let's say that D's location has already been
learned by the relevant TRILL switches. These relevant switches have
learned the following:
1) RB1 has learned that D is connected to nickname 15918
2) RB3 has learned that D is attached to nickname 44.
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The following sequence of events will occur:
- S transmits an Ethernet frame with source MAC = S and destination
MAC = D.
- RB1 encapsulates with a TRILL header with ingress RBridge = 27,
and egress = 15918 producing a TRILL Data packet.
- RB2 has announced in the Level 1 IS-IS instance in area 15961,
that it is attached to all the area nicknames, including 15918.
Therefore, IS-IS routes the packet to RB2. Alternatively, if a
distinguished range of nicknames is used for Level 2, Level 1
TRILL switches seeing such an egress nickname will know to route
to the nearest border router, which can be indicated by the IS-IS
attached bit.
- RB2, when transitioning the packet from Level 1 to Level 2,
replaces the ingress TRILL switch nickname with the area nickname,
so replaces 27 with 15961. Within Level 2, the ingress RBridge
field in the TRILL header will therefore be 15961, and the egress
RBridge field will be 15918. Also RB2 learns that S is attached to
nickname 27 in area 15961 to accommodate return traffic.
- The packet is forwarded through Level 2, to RB3, which has
advertised, in Level 2, reachability to the nickname 15918.
- RB3, when forwarding into area 15918, replaces the egress nickname
in the TRILL header with RB4's nickname (44). So, within the
destination area, the ingress nickname will be 15961 and the
egress nickname will be 44.
- RB4, when decapsulating, learns that S is attached to nickname
15961, which is the area nickname of the ingress.
Now suppose that D's location has not been learned by RB1 and/or RB3.
What will happen, as it would in TRILL today, is that RB1 will
forward the packet as multi-destination, choosing a tree. As the
multi-destination packet transitions into Level 2, RB2 replaces the
ingress nickname with the area nickname. If RB1 does not know the
location of D, the packet must be flooded, subject to possible
pruning, in Level 2 and, subject to possible pruning, from Level 2
into every Level 1 area that it reaches on the Level 2 distribution
tree.
Now suppose that RB1 has learned the location of D (attached to
nickname 15918), but RB3 does not know where D is. In that case, RB3
must turn the packet into a multi-destination packet within area
15918. In this case, care must be taken so that in the case in which
RB3 is not the Designated transitioner between Level 2 and its area
for that multi-destination packet, but was on the unicast path, that
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border TRILL switch in that area does not forward the now multi-
destination packet back into Level 2. Therefore, it would be
desirable to have a marking, somehow, that indicates the scope of
this packet's distribution to be "only this area" (see also Section
4).
In cases where there are multiple transitioners for unicast packets,
the border learning mode of operation requires that the address
learning between them be shared by some protocol such as running
ESADI [RFC7357] for all Data Labels of interest to avoid excessive
unknown unicast flooding.
The potential issue described at the end of Section 2.2.1 with trees
in the unique nickname alternative is eliminated with aggregated
nicknames. With aggregated nicknames, each border TRILL switch that
will transition multi-destination packets can have a mapping between
Level 2 tree nicknames and Level 1 tree nicknames. There need not
even be agreement about the total number of trees; just that the
border TRILL switch have some mapping, and replace the egress TRILL
switch nickname (the tree name) when transitioning levels.
2.2.2.2 Swap Nickname Field Aggregated Nicknames
There is a variant possibility where two additional fields could
exist in TRILL Data packets that could be called the "ingress swap
nickname field" and the "egress swap nickname field". This variant is
described below for completeness but would require fast path hardware
changes from the existing TRILL protocol. The changes in the example
above would be as follows:
- RB1 will have learned the area nickname of D and the TRILL switch
nickname of RB4 to which D is attached. In encapsulating a frame
to D, it puts an area nickname of D (15918) in the egress nickname
field of the TRILL Header and puts a nickname of RB3 (44) in a
egress swap nickname field.
- RB2 moves the ingress nickname to the ingress swap nickname field
and inserts 15961, an area nickname for S, into the ingress
nickname field.
- RB3 swaps the egress nickname and the egress swap nickname fields,
which sets the egress nickname to 44.
- RB4 learns the correspondence between the source MAC/VLAN of S and
the { ingress nickname, ingress swap nickname field } pair as it
decapsulates and egresses the frame.
See [DraftAggregated] for a multilevel proposal using aggregated swap
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nicknames with a single nickname representing an area.
2.2.2.3 Comparison
The Border Learning variant described in Section 2.2.2.1 above
minimizes the change in non-border TRILL switches but imposes the
burden on border TRILL switches of learning and doing lookups in all
the end station MAC addresses within their area(s) that are used for
communication outside the area. This burden could be reduced by
decreasing the area size and increasing the number of areas.
The Swap Nickname Field variant described in Section 2.2.2.2
eliminates the extra address learning burden on border TRILL switches
but requires changes to the TRILL data packet header and more
extensive changes to non-border TRILL switches. In particular, with
this alternative, non-border TRILL switches must learn to associate
both a TRILL switch nickname and an area nickname with end station
MAC/label pairs (except for addresses that are local to their area).
The Swap Nickname Field alternative is more scalable but less
backward compatible for non-border TRILL switches. It would be
possible for border and other level 2 TRILL switches to support both
Border Learning, for support of legacy Level 1 TRILL switches, and
Swap Nickname, to support Level 1 TRILL switches that understood the
Swap Nickname method based on variations in the TRILL header but this
would be even more complex.
The requirement to change the TRILL header and fast path processing
to support the Swap Nickname Field variant make it impractical for
the foreseeable future.
2.3 Building Multi-Area Trees
It is easy to build a multi-area tree by building a tree in each area
separately, (including the Level 2 "area"), and then having only a
single border TRILL switch, say RBx, in each area, attach to the
Level 2 area. RBx would forward all multi-destination packets
between that area and Level 2.
People might find this unacceptable, however, because of the desire
to path split (not always sending all multi-destination traffic
through the same border TRILL switch).
This is the same issue as with multiple ingress TRILL switches
injecting traffic from a pseudonode, and can be solved with the
mechanism that was adopted for that purpose: the affinity TLV
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[RFC7783]. For each tree in the area, at most one border RB
announces itself in an affinity TLV with that tree name.
2.4 The RPF Check for Trees
For multi-destination data originating locally in RBx's area,
computation of the RPF check is done as today. For multi-destination
packets originating outside RBx's area, computation of the RPF check
must be done based on which one of the border TRILL switches (say
RB1, RB2, or RB3) injected the packet into the area.
A TRILL switch, say RB4, located inside an area, must be able to know
which of RB1, RB2, or RB3 transitioned the packet into the area from
Level 2 (or into Level 2 from an area).
This could be done based on having the DBRB announce the transitioner
assignments to all the TRILL switches in the area, or the Affinity
TLV mechanism given in [RFC7783], or a New Tree Encoding mechanism
discussed in Section 4.1.1.
2.5 Area Nickname Acquisition
In the aggregated nickname alternative, each area must acquire a
unique area nickname or can be identified by the set of border TRILL
switches. It is probably simpler to allocate a block of nicknames
(say, the top 4000) to either (1) represent areas and not specific
TRILL switches or (2) used by border TRILL switches if the set of
such border TRILL switches represent the area.
The nicknames used for area identification need to be advertised and
acquired through Level 2.
Within an area, all the border TRILL switches can discover each other
through the Level 1 link state database, by using the IS-IS attach
bit or by explicitly advertising in their LSP "I am a border
RBridge".
Of the border TRILL switches, one will have highest priority (say
RB7). RB7 can dynamically participate, in Level 2, to acquire a
nickname for identifying the area. Alternatively, RB7 could give the
area a pseudonode IS-IS ID, such as RB7.5, within Level 2. So an
area would appear, in Level 2, as a pseudonode and the pseudonode
could participate, in Level 2, to acquire a nickname for the area.
Within Level 2, all the border TRILL switches for an area can
advertise reachability to the area, which would mean connectivity to
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a nickname identifying the area.
2.6 Link State Representation of Areas
Within an area, say area A1, there is an election for the DBRB,
(Designated Border RBridge), say RB1. This can be done through LSPs
within area A1. The border TRILL switches announce themselves,
together with their DBRB priority. (Note that the election of the
DBRB cannot be done based on Hello messages, because the border TRILL
switches are not necessarily physical neighbors of each other. They
can, however, reach each other through connectivity within the area,
which is why it will work to find each other through Level 1 LSPs.)
RB1 can acquire an area nickname (in the aggregated nickname
approach) and may give the area a pseudonode IS-IS ID (just like the
DRB would give a pseudonode IS-IS ID to a link) depending on how the
area nickname is handled. RB1 advertises, in area A1, an area
nickname that RB1 has acquired (and what the pseudonode IS-IS ID for
the area is if needed).
Level 1 LSPs (possibly pseudonode) initiated by RB1 for the area
include any information external to area A1 that should be input into
area A1 (such as nicknames of external areas, or perhaps (in the
unique nickname variant) all the nicknames of external TRILL switches
in the TRILL campus and pruning information such as multicast
listeners and labels). All the other border TRILL switches for the
area announce (in their LSP) attachment to that area.
Within Level 2, RB1 generates a Level 2 LSP on behalf of the area.
The same pseudonode ID could be used within Level 1 and Level 2, for
the area. (There does not seem any reason why it would be useful for
it to be different, but there's also no reason why it would need to
be the same). Likewise, all the area A1 border TRILL switches would
announce, in their Level 2 LSPs, connection to the area.
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3. Area Partition
It is possible for an area to become partitioned, so that there is
still a path from one section of the area to the other, but that path
is via the Level 2 area.
With multilevel TRILL, an area will naturally break into two areas in
this case.
Area addresses might be configured to ensure two areas are not
inadvertently connected. Area addresses appear in Hellos and LSPs
within the area. If two chunks, connected only via Level 2, were
configured with the same area address, this would not cause any
problems. (They would just operate as separate Level 1 areas.)
A more serious problem occurs if the Level 2 area is partitioned in
such a way that it could be healed by using a path through a Level 1
area. TRILL will not attempt to solve this problem. Within the Level
1 area, a single border RBridge will be the DBRB, and will be in
charge of deciding which (single) RBridge will transition any
particular multi-destination packets between that area and Level 2.
If the Level 2 area is partitioned, this will result in multi-
destination data only reaching the portion of the TRILL campus
reachable through the partition attached to the TRILL switch that
transitions that packet. It will not cause a loop.
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4. Multi-Destination Scope
There are at least two reasons it would be desirable to be able to
mark a multi-destination packet with a scope that indicates the
packet should not exit the area, as follows:
1. To address an issue in the border learning variant of the
aggregated nickname alternative, when a unicast packet turns into
a multi-destination packet when transitioning from Level 2 to
Level 1, as discussed in Section 4.1.
2. To constrain the broadcast domain for certain discovery,
directory, or service protocols as discussed in Section 4.2.
Multi-destination packet distribution scope restriction could be done
in a number of ways. For example, there could be a flag in the packet
that means "for this area only". However, the technique that might
require the least change to TRILL switch fast path logic would be to
indicate this in the egress nickname that designates the distribution
tree being used. There could be two general tree nicknames for each
tree, one being for distribution restricted to the area and the other
being for multi-area trees. Or there would be a set of N (perhaps 16)
special currently reserved nicknames used to specify the N highest
priority trees but with the variation that if the special nickname is
used for the tree, the packet is not transitioned between areas. Or
one or more special trees could be built that were restricted to the
local area.
4.1 Unicast to Multi-destination Conversions
In the border learning variant of the aggregated nickname
alternative, the following situation may occur:
- a unicast packet might be known at the Level 1 to Level 2
transition and be forwarded as a unicast packet to the least cost
border TRILL switch advertising connectivity to the destination
area, but
- upon arriving at the border TRILL switch, it turns out to have an
unknown destination { MAC, Data Label } pair.
In this case, the packet must be converted into a multi-destination
packet and flooded in the destination area. However, if the border
TRILL switch doing the conversion is not the border TRILL switch
designated to transition the resulting multi-destination packet,
there is the danger that the designated transitioner may pick up the
packet and flood it back into Level 2 from which it may be flooded
into multiple areas. This danger can be avoided by restricting any
multi-destination packet that results from such a conversion to the
destination area as described above.
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Alternatively, a multi-destination packet intended only for the area
could be tunneled (within the area) to the RBridge RBx, that is the
appointed transitioner for that form of packet (say, based on VLAN or
FGL), with instructions that RBx only transmit the packet within the
area, and RBx could initiate the multi-destination packet within the
area. Since RBx introduced the packet, and is the only one allowed
to transition that packet to Level 2, this would accomplish scoping
of the packet to within the area. Since this case only occurs in the
unusual case when unicast packets need to be turned into multi-
destination as described above, the suboptimality of tunneling
between the border TRILL switch that receives the unicast packet and
the appointed level transitioner for that packet, might not be an
issue.
4.1.1 New Tree Encoding
The current encoding, in a TRILL header, of a tree, is of the
nickname of the tree root. This requires all 16 bits of the egress
nickname field. TRILL could instead, for example, use the bottom 6
bits to encode the tree number (allowing 64 trees), leaving 10 bits
to encode information such as:
o scope: a flag indicating whether it should be single area only, or
entire campus
o border injector: an indicator of which of the k border TRILL
switches injected this packet
If TRILL were to adopt this new encoding, any of the TRILL switches
in an edge group could inject a multi-destination packet. This would
require all TRILL switches to be changed to understand the new
encoding for a tree, and it would require a TLV in the LSP to
indicate which number each of the TRILL switches in an edge group
would be.
While there are a number of advantages to this technique, it requires
fast path logic changes and thus its deployment is not practical at
this time. It is included here for completeness.
4.2 Selective Broadcast Domain Reduction
There are a number of service, discovery, and directory protocols
that, for convenience, are accessed via multicast or broadcast
frames. Examples are DHCP, (Dynamic Host Configuration Protocol) the
NetBIOS Service Location Protocol, and multicast DNS (Domain Name
Service).
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Some such protocols provide means to restrict distribution to an IP
subnet or equivalent to reduce size of the broadcast domain they are
using and then provide a proxy that can be placed in that subnet to
use unicast to access a service elsewhere. In cases where a proxy
mechanism is not currently defined, it may be possible to create one
that references a central server or cache. With multilevel TRILL, it
is possible to construct very large IP subnets that could become
saturated with multi-destination traffic of this type unless packets
can be further restricted in their distribution. Such restricted
distribution can be accomplished for some protocols, say protocol P,
in a variety of ways including the following:
- Either (1) at all ingress TRILL switches in an area place all
protocol P multi-destination packets on a distribution tree in
such a way that the packets are restricted to the area or (2) at
all border TRILL switches between that area and Level 2, detect
protocol P multi-destination packets and do not transition them.
- Then place one, or a few for redundancy, protocol P proxies inside
each area where protocol P may be in use. These proxies unicast
protocol P requests or other messages to the actual campus
server(s) for P. They also receive unicast responses or other
messages from those servers and deliver them within the area via
unicast, multicast, or broadcast as appropriate. (Such proxies
would not be needed if it was acceptable for all protocol P
traffic to be restricted to an area.)
While it might seem logical to connect the campus servers to TRILL
switches in Level 2, they could be placed within one or more areas so
that, in some cases, those areas might not require a local proxy
server.
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5. Co-Existence with Old TRILL switches
TRILL switches that are not multilevel aware may have a problem with
calculating RPF Check and filtering information, since they would not
be aware of the assignment of border TRILL switch transitioning.
A possible solution, as long as any old TRILL switches exist within
an area, is to have the border TRILL switches elect a single DBRB
(Designated Border RBridge), and have all inter-area traffic go
through the DBRB (unicast as well as multi-destination). If that
DBRB goes down, a new one will be elected, but at any one time, all
inter-area traffic (unicast as well as multi-destination) would go
through that one DRBR. However this eliminates load splitting at
level transition.
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6. Multi-Access Links with End Stations
Care must be taken in the case where there are multiple TRILL
switches on a link with one or more end stations, keeping in mind
that end stations are TRILL ignorant. In particular, it is essential
that only one TRILL switch ingress/egress any given data packet
from/to an end station so that connectivity is provided to that end
station without duplicating end station data and that loops are not
formed due to one TRILL switch egressing data in native form (i.e.,
with no TRILL header) and having that data re-ingressed by another
TRILL switch on the link.
With existing, single level TRILL, this is done by electing a single
Designated RBridge per link, which appoints a single Appointed
Forwarder per VLAN [RFC7177] [RFC8139]. This mechanism depends on the
RBridges establishing adjacency. But suppose there are two (or more)
TRILL switches on a link in different areas, say RB1 in area A1 and
RB2 in area A2, as shown below, and that the link also has one or
more end stations attached. If RB1 and RB2 ignore each other's
Hellos because they are in different areas, as they are required to
do under normal IS-IS PDU processing rules, then they will not form
an adjacency. If they are not adjacent, they will ignore each other
for the Appointed Forwarder mechanism and will both ingress/egress
end station traffic on the link causing loops and duplication.
The problem is not avoiding adjacency or avoiding TRILL Data packet
transfer between RB1 and RB2. The area address mechanism of IS-IS or
possibly the use of topology constraints or the like does that quite
well. The problem stems from end stations being TRILL ignorant so
care must be taken that multiple RBridges on a link do not ingress
the same frame originated by an end station and so that an RBridge
does not ingress a native frame egressed by a different RBridge
because the RBridge mistakes the frame for a frame originated by an
end station.
+--------------------------------------------+
| Level 2 |
+----------+---------------------+-----------+
| Area A1 | | Area A2 |
| +---+ | | +---+ |
| |RB1| | | |RB2| |
| +-+-+ | | +-+-+ |
| | | | | |
+-----|----+ +-----|-----+
| |
--+---------+-------------+--------+-- Link
| |
+------+------+ +--+----------+
| End Station | | End Station |
+-------------+ +-------------+
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A simple rule, which is preferred, is to use the TRILL switch or
switches having the lowest numbered area, comparing area numbers as
unsigned integers, to handle all native traffic to/from end stations
on the link. This would automatically give multilevel-ignorant legacy
TRILL switches, that would be using area number zero, highest
priority for handling end station traffic, which they would try to do
anyway.
Other methods are possible. For example doing the selection of
Appointed Forwarders and of the TRILL switch in charge of that
selection across all TRILL switches on the link regardless of area.
However, a special case would then have to be made for legacy TRILL
switches using area number zero.
These techniques require multilevel aware TRILL switches to take
actions based on Hellos from RBridges in other areas even though they
will not form an adjacency with such RBridges. However, the action is
quite simple in the preferred case: if a TRILL switch sees Hellos
from lower numbered areas, then they would not act as an Appointed
Forwarder on the link until the Hello timer for such Hellos had
expired.
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7. Summary
This draft describes potential scaling issues in TRILL and discusses
possible approaches to multilevel TRILL as a solution or element of a
solution to most of them.
The alternative using aggregated areas in multilevel TRILL has
significant advantages in terms of scalability over using campus wide
unique nicknames, not just in avoiding nickname exhaustion, but by
allowing RPF Checks to be aggregated based on an entire area.
However, the alternative of using unique nicknames is simpler and
avoids the changes in border TRILL switches required to support
aggregated nicknames. It is possible to support both. For example, a
TRILL campus could use simpler unique nicknames until scaling begins
to cause problems and then start to introduce areas with aggregated
nicknames.
Some multilevel TRILL issues are not difficult, such as dealing with
partitioned areas. Other issues are more difficult, especially
dealing with old TRILL switches that are multilevel ignorant.
R. Perlman, et al [Page 27]
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8. Security Considerations
This informational document explores alternatives for the design of
multilevel IS-IS in TRILL and generally does not consider security
issues.
If aggregated nicknames are used in two areas that have the same area
address and those areas merge, there is a possibility of a transient
nickname collision that would not occur with unique nicknames. Such a
collision could cause a data packet to be delivered to the wrong
egress TRILL switch but it would still not be delivered to any end
station in the wrong Data Label; thus such delivery would still
conform to security policies.
For general TRILL Security Considerations, see [RFC6325].
9. IANA Considerations
This document requires no IANA actions. RFC Editor: Please remove
this section before publication.
R. Perlman, et al [Page 28]
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Normative References
[IS-IS] - 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.
[RFC6325] - Perlman, R., Eastlake 3rd, D., Dutt, D., Gai, S., and A.
Ghanwani, "Routing Bridges (RBridges): Base Protocol
Specification", RFC 6325, July 2011.
[RFC7177] - Eastlake 3rd, D., Perlman, R., Ghanwani, A., Yang, H.,
and V. Manral, "Transparent Interconnection of Lots of Links
(TRILL): Adjacency", RFC 7177, May 2014, <http://www.rfc-
editor.org/info/rfc7177>.
[RFC7780] - Eastlake 3rd, D., Zhang, M., Perlman, R., Banerjee, A.,
Ghanwani, A., and S. Gupta, "Transparent Interconnection of
Lots of Links (TRILL): Clarifications, Corrections, and
Updates", RFC 7780, DOI 10.17487/RFC7780, February 2016,
<http://www.rfc-editor.org/info/rfc7780>.
[RFC8139] - Eastlake, D., Li, Y., Umair, M., Banerjee, A., and F. Hu,
"Transparent Interconnection of Lots of Links (TRILL):
Appointed Forwarders", RFC 8139, DOI 10.17487/RFC8139, June
2017, <http://www.rfc-editor.org/info/rfc8139>.
Informative References
[InterCon] - Perlman, R., "Interconnections, Second Edition; Bridges,
Routers, Switches, and Internetworking Protocols", Addison
Wesley, ISBN 0-201-63448-1, September 1999.
[RFC3194] - Durand, A. and C. Huitema, "The H-Density Ratio for
Address Assignment Efficiency An Update on the H ratio", RFC
3194, DOI 10.17487/RFC3194, November 2001, <http://www.rfc-
editor.org/info/rfc3194>.
[RFC6361] - Carlson, J. and D. Eastlake 3rd, "PPP Transparent
Interconnection of Lots of Links (TRILL) Protocol Control
Protocol", RFC 6361, August 2011.
[RFC7172] - Eastlake 3rd, D., Zhang, M., Agarwal, P., Perlman, R.,
and D. Dutt, "Transparent Interconnection of Lots of Links
(TRILL): Fine-Grained Labeling", RFC 7172, May 2014
[RFC7176] - Eastlake 3rd, D., Senevirathne, T., Ghanwani, A., Dutt,
R. Perlman, et al [Page 29]
INTERNET-DRAFT Multilevel TRILL
D., and A. Banerjee, "Transparent Interconnection of Lots of
Links (TRILL) Use of IS-IS", RFC 7176, May 2014.
[RFC7357] - Zhai, H., Hu, F., Perlman, R., Eastlake 3rd, D., and O.
Stokes, "Transparent Interconnection of Lots of Links (TRILL):
End Station Address Distribution Information (ESADI) Protocol",
RFC 7357, September 2014, <http://www.rfc-
editor.org/info/rfc7357>.
[RFC7781] - Zhai, H., Senevirathne, T., Perlman, R., Zhang, M., and
Y. Li, "Transparent Interconnection of Lots of Links (TRILL):
Pseudo-Nickname for Active-Active Access", RFC 7781, DOI
10.17487/RFC7781, February 2016, <http://www.rfc-
editor.org/info/rfc7781>.
[RFC7783] - Senevirathne, T., Pathangi, J., and J. Hudson,
"Coordinated Multicast Trees (CMT) for Transparent
Interconnection of Lots of Links (TRILL)", RFC 7783, DOI
10.17487/RFC7783, February 2016, <http://www.rfc-
editor.org/info/rfc7783>.
[DraftAggregated] - Bhargav Bhikkaji, Balaji Venkat Venkataswami,
Narayana Perumal Swamy, "Connecting Disparate Data
Center/PBB/Campus TRILL sites using BGP", draft-balaji-trill-
over-ip-multi-level, Work In Progress.
[DraftUnique] - M. Zhang, D. Eastlake, R. Perlman, M. Cullen, H.
Zhai, D. Liu, "TRILL Multilevel Using Unique Nicknames", draft-
ietf-trill-multilevel-unique-nickname, Work In Progress.
[SingleName] - Mingui Zhang, et. al, "Single Area Border RBridge
Nickname for TRILL Multilevel", draft-ietf-trill-multilevel-
single-nickname, Work in Progress.
R. Perlman, et al [Page 30]
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Acknowledgements
The helpful comments and contributions of the following are hereby
acknowledged:
Alia Atlas, David Michael Bond, Dino Farinacci, Sue Hares, Gayle
Noble, Alexander Vainshtein, and Stig Venaas.
The document was prepared in raw nroff. All macros used were defined
within the source file.
R. Perlman, et al [Page 31]
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Authors' Addresses
Radia Perlman
EMC
2010 256th Avenue NE, #200
Bellevue, WA 98007 USA
EMail: radia@alum.mit.edu
Donald Eastlake
Huawei Technologies
155 Beaver Street
Milford, MA 01757 USA
Phone: +1-508-333-2270
Email: d3e3e3@gmail.com
Mingui Zhang
Huawei Technologies
No.156 Beiqing Rd. Haidian District,
Beijing 100095 P.R. China
EMail: zhangmingui@huawei.com
Anoop Ghanwani
Dell
5450 Great America Parkway
Santa Clara, CA 95054 USA
EMail: anoop@alumni.duke.edu
Hongjun Zhai
Jinling Institute of Technology
99 Hongjing Avenue, Jiangning District
Nanjing, Jiangsu 211169 China
EMail: honjun.zhai@tom.com
R. Perlman, et al [Page 32]
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R. Perlman, et al [Page 33]
