TRILL WG J. Touch
Internet Draft USC/ISI
Intended status: Informational R. Perlman
Expires: August 2008 Sun
February 24, 2008
Transparent Interconnection of Lots of Links (TRILL):
Problem and Applicability Statement
draft-ietf-trill-prob-02.txt
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Abstract
Current Ethernet (802.1) link layers use custom routing protocols
that have a number of challenges. The routing protocols need to
strictly avoid loops, even temporary loops during route propagation,
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because of the lack of header loop detection support. Routing tends
not to take full advantage of alternate paths, or even non-
overlapping pairwise paths (in the case of spanning trees). The
convergence of these routing protocols and stability under link
changes and failures is also of concern. This document addresses
these concerns and suggests that they are related to the need to be
able to apply modern network layer routing protocols at the link
layer. This document assumes that solutions would not address issues
of scalability beyond that of existing bridged (802.1D) links, but
that a solution would be backward compatible with 802.1D, including
hubs, bridges, and their existing plug-and-play capabilities.
This document is a work in progress; we invite you to participate on
the mailing list at http://www.postel.org/rbridge
Table of Contents
1. Introduction...................................................3
2. The TRILL Problem..............................................3
2.1. Inefficient Paths.........................................4
2.2. Convergence Under Reconfiguration.........................5
2.3. Robustness to Link Interruption...........................6
2.4. Other Ethernet Extensions.................................6
2.5. Problems Not Addressed....................................7
3. Desired Properties of Solutions to TRILL.......................8
3.1. No Change to Link Capabilities............................8
3.2. Zero Configuration and Zero Assumption....................8
3.3. Forwarding Loop Mitigation................................9
3.4. Spanning Tree Management..................................9
3.5. Multiple Attachments.....................................10
3.6. VLAN Issues..............................................10
3.7. Equivalence..............................................10
3.8. Optimizations............................................11
3.9. Internet Architecture Issues.............................11
4. Applicability.................................................12
5. Security Considerations.......................................12
6. IANA Considerations...........................................13
7. Acknowledgments...............................................13
8. References....................................................13
8.1. Normative References.....................................13
8.2. Informative References...................................13
9. Author's Addresses............................................15
Intellectual Property Statement..................................15
Disclaimer of Validity...........................................16
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1. Introduction
Conventional Ethernet networks - known in the Internet as Ethernet
link subnets - have a number of attractive features, allowing hosts
and routers to relocate within the subnet without requiring
renumbering and are automatically configuring. Unfortunately, the
basis of the simplicity of these subnets is the spanning tree, which
although simple and elegant, can have substantial limitations. In
subnets where bridges are also frequently relocated, convergence of
the spanning tree protocol can be slow. Because all traffic flows
over a single tree, all traffic is concentrated on a subset of links,
increasing susceptibility to the effects of link failures and
limiting the bandwidth across the subnet.
The alternative to an Ethernet link subnet is often a network subnet.
Network subnets can use link-state routing protocols that allow
traffic to traverse least-cost paths rather than being aggregated on
a spanning tree backbone, providing higher aggregate capacity and
more resistance to link failures. Unfortunately, IP - the dominant
network layer technology - requires that hosts be renumbered when
relocated in different network subnets, interrupting network (e.g.,
tunnels, IPsec) and transport (e.g., TCP, UDP) associations that are
in progress during the transition.
It is thus useful to consider a new approach that combines the
features of these two existing solutions, hopefully retaining the
desirable properties of each. Such an approach would develop a new
kind of bridge system that was capable of using network-style
routing, while still providing Ethernet service. It allows reuse of
well-understood network routing protocols to benefit the link layer.
This document describes the challenge of such a combined approach in
detail. This problem is known as "Transparent Interconnection of Lots
of Links" or "TRILL". The remainder of this document makes as few
assumptions about a solution to TRILL as possible.
2. The TRILL Problem
Ethernet subnets have evolved from 'thicknet' to 'thinnet' to twisted
pair with hubs to twisted pair with switches, becoming increasingly
simple to wire and manage. Each level has corresponding topology
restrictions; thicknet is inherently linear, whereas thinnet and hub-
connected twisted pair have to be wired as a tree. Switches, added in
802.1D, allow network managers to avoid thinking in trees, where the
spanning tree protocol finds a valid tree automatically;
unfortunately, this additional simplicity comes with a number of
associated penalties [12].
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The spanning tree often results in inefficient use of the link
topology; traffic is concentrated on the spanning tree path, and all
traffic follows that path even when other more direct paths may be
available. The spanning tree configuration is affected by even small
topology changes, and small changes can have large effects. Each of
these inefficiencies can cause problems for current link layer
deployments.
2.1. Inefficient Paths
The Spanning Tree Protocol (STP) helps break cycles in a set of
interconnected bridges, but it also can limit the bandwidth among
that set and cause traffic to take circuitous paths.
Consider the network shown in Figure 1, which shows a number of
bridges and their interconnecting links. End hosts and routers are
not shown; they would connect to the bridges that are shown, labeled
A-H. Note that the network shown has cycles which would cause packet
storms if hubs (repeaters) were used instead of STP-capable bridges.
One possible spanning tree is shown by double lines.
A
// \ C
// \ / \\ D
// \ / \\ //
B=======H===== E
\ // ||
\ // ||
\ // ||
G----------F
Figure 1 Bridged subnet with spanning tree shown
The spanning tree limits the capacity of the resulting subnet. Assume
that the links are 100 Mbps. Figure 2 shows how traffic from hosts on
A to hosts on C goes via the spanning tree path A-B-H-E-C (links
replaced with '1' in the figure); traffic from hosts on G to F go via
the spanning three path G-H-E-F (links replaced by '2' in the
figure). The link H-E is shared by both paths (alternating '1's and
'2's), resulting in an aggregate capacity for both A..C and G..F
paths of a total of 100 Mbps.
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A
1 C
1 1
1 1
B1111111H121212E
2 2
2 2
2 2
G F
Figure 2 Traffic from A..C (1) and G..F (2) share a link
If traffic from G to F were to go directly using full routing, e.g.,
from G-F, both paths could have 100 Mbps each, and the total
aggregate capacity could be 200 Mbps (Figure 3). In this case, the H-
F link carries only A-C traffic ('1's) and the G-F traffic ('2's) is
more direct.
A
1 C
1 1
1 1
B1111111H111111E
G2222222222F
Figure 3 Traffic from A..C (1) and G..F (2) with full routing
There are a number of features of modern layer 3 routing protocols
which would be beneficial if available at layer 2, but which cannot
be integrated into the spanning tree system. Multipath routing can
distribute load simultaneously among two different paths; alternate
path routing supports rapid failover to backup paths. Layer 3 routing
typically optimizes paths between pairs of endpoints, conventionally
based on hopcount but also including bandwidth, latency, or other
policy metrics.
2.2. Convergence Under Reconfiguration
The spanning tree is dependent on the way a set of bridges are
interconnected, i.e., the link layer topology. Small changes in this
topology can cause large changes in the spanning tree. Changes in the
spanning tree can take time to propagate and converge.
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One possible case occurs when one of the branches connected to the
root bridge fails, causing a large number of ports to block and
unblock before the network reconverges [4][9]. Consider a ring as
shown in Figure 4.
A----B----C----D----E
| |
+-----F-----G-------+
Figure 4 Ring with poor convergence under reconfiguration
If A is the root bridge, then the paths A->B->C->D and A->F->G->E are
the two open paths, while the D->E link is blocked in both
directions. If the A->B link fails, then E must unblock its port to D
for traffic to flow again, but it may require recomputation of the
entire tree through BPDUs.
The spanning tree protocol is inherently global to an entire layer 2
subnet; there is no current way to contain, partition, or otherwise
factor the protocol into a number of smaller, more stable subsets
that interact as groups. Contrast this with Internet routing, which
includes both intradomain and interdomain variants, split to provide
exactly that containment and scalability within a domain while
allowing domains to interact freely independent of what happens
within a domain.
2.3. Robustness to Link Interruption
Persistent changes to the link topology, as described in Section 2.2,
are not the only effects on subnet stability. Transient link
interruptions have similar effects, with similar scalability issues.
It would be more useful for subnet configuration to be tolerant of
such transients, e.g., supporting alternate, backup paths.
Contrast this to network layer intradomain and interdomain routing,
both of which include provisions for backup paths. These backups
allow routing to be more stable in the presence of transients, as
well as to recover more rapidly when the transient disappears.
2.4. Other Ethernet Extensions
There have been a variety of 802.1 protocols beyond the initial
shared-media variant, including:
o 802.1D - added bridges (i.e., switches) and a spanning tree
protocol (STP) (incorporates 802.1w, below) [6]
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o 802.1w - extension for rapid reconvergence of the spanning tree
protocol (RTSP) [6]
o 802.1Q - added VLAN support, where each link address maps to one
VLAN (incorporates 802.1v and 802.1s, below) [7]
o 802.1v - added VLANs where segments map to VLANs based on link
address together with network protocol and transport port [7]
o 802.1s - added support for multiple spanning trees, one per VLAN
(MSTP) [7]
These variants are further complicated by different versions updated
periodically.
It is useful to note that these extensions do not address the issue
of independent, localized routing in a single spanning tree - which
is the focus of TRILL. This document presumes the above variants are
supported on the Ethernet subnet, i.e., that a TRILL solution would
support all of the above.
2.5. Problems Not Addressed
There are other challenges to deploying Ethernet subnets that are not
addressed in this document. These include:
o increased Ethernet link subnet scale
o increased node relocation
o Ethernet link subnet management protocol security
o flooding attacks on a Ethernet link subnet
Solutions to TRILL are not intended to support deployment of
increasingly larger scales of Ethernet link subnets than current
broadcast domains can support (e.g., around 1,000 end-hosts in a
single bridged LAN of 100 bridges, or 100,000 end-hosts inside 1,000
VLANs served by 10,000 bridges).
Similarly, solutions to TRILL are not intended to address link layer
node migration, which can complicate the caches in learning bridges.
Similar challenges exist in the ARP protocol, where link layer
forwarding is not updated appropriately when nodes move to ports on
other bridges. Again, the compartmentalization available in network
routing, like that of network layer ASes, can help hide the effect of
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migration. That is a side effect, however, and not a primary focus of
this work.
Current link control plane protocols, including Ethernet link subnet
management (STP) and link/network integration (ARP), are vulnerable
to a variety of attacks. Solutions to TRILL are not intended to
directly address these vulnerabilities. Similar attacks exist in the
data plane, e.g., source address spoofing, single address traffic
attacks, traffic snooping, and broadcast flooding. TRILL solutions do
not address any of these issues, although it is critical that they do
not introduce new vulnerabilities in the process (see Section 5).
3. Desired Properties of Solutions to TRILL
This section describes some of the desirable or required properties
of any system that would solve the TRILL problems, independent of the
details of such an architecture. Most of these are based on retaining
useful properties of bridges, or maintaining those properties while
solving the problems listed in Section 2.
3.1. No Change to Link Capabilities
There must be no change to the service that Ethernet subnets already
provide as a result of deploying a TRILL solution. Ethernet supports
unicast, broadcast, and multicast natively. Although network
protocols, notably IP, can tolerate link layers that do not provide
all three, it would be useful to retain the support already in place
[8]. Zeroconf, as well as existing bridge autoconfiguration, are
dependent on broadcast as well.
Current Ethernet ensures in-order delivery and no duplicated packets
under normal operation (excepting transients during reconfiguration).
These criteria apply in varying degrees to the different variants of
Ethernet, e.g., basic Ethernet up through basic VLAN (802.1Q) ensures
that all packets between two link addresses have both properties, but
protocol/port VLAN (802.1v) ensures this only for packets with the
same protocol and port. There are subtle implications to such a
requirement. Bridge autolearning already is susceptible to moving
nodes between ports, because previously learned associations between
port and link address change. A TRILL solution could be similarly
susceptible to such changes.
3.2. Zero Configuration and Zero Assumption
Both bridges and hubs are zero configuration devices; hubs having no
configuration at all, and bridges being automatically self-
configured. Bridges are further zero-assumption devices, unlike hubs.
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Bridges can be interconnected in arbitrary topologies, without regard
for cycles or even self-attachment. STP removes the impact of cycles
automatically, and port autolearning reduces unnecessary broadcast of
unicast traffic.
A TRILL solution should strive to have similar zero configuration,
zero assumption operation. This includes having TRILL solution
components automatically discover other TRILL solution components and
organize themselves, as well as to configure that organization for
proper operation (plug-and-play). It also includes zero configuration
backward compatibility with existing bridges and hubs, which may
include interacting with some of the bridge protocols, such as STP.
VLANs add a caveat to zero configuration; a TRILL solution should
support automatic use of a default VLAN (like non-VLAN bridges), but
should require explicit configuration where the VLANS require them as
well.
Autoconfiguration extends to optional services, such as multicast
support via IGMP snooping, broadcast support via serial copy, and
supporting multiple VLANs.
3.3. Forwarding Loop Mitigation
Spanning tree avoids forwarding loops by construction, although
transient loops can occur, e.g., via the appearance of a new link.
Solutions to TRILL are intended to use adapted network layer routing
protocols which may introduce transient loops during routing
convergence. TRILL solutions thus need support for mitigating the
effect of such routing loops.
In the Internet, loop mitigation is provided by a decrementing
hopcounts (TTL); in other networks, packets include a trace
(sometimes referred to as 'serialized' or 'unioned') of visited nodes
[2]. These mechanisms (respectively) limit the impact of loops or
detect them explicitly. A mechanism with similar effect should be
included in TRILL solutions.
3.4. Spanning Tree Management
In order to address convergence under reconfiguration and robustness
to link interruption (Sections 2.2 and 2.3), participation in the STP
must be carefully managed. The goal is to provide the desired
stability of the TRILL solution and of the entire Ethernet link
subnet while not interfering with the operation of STP of the
Ethernet on which the TRILL resides. This may involve TRILL solutions
participating in the STP, where the protocol is used for TRILL might
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dampen interactions with STP, or it may involve severing the STP into
separate STPs on 'stub' external Ethernet link subnet segments.
A requirement is that a TRILL solution must not require modifications
or exceptions to the existing spanning tree protocols (STP, MSTP).
3.5. Multiple Attachments
In STP, a single NIC with multiple attachments to a single spanning
tree will always only get traffic over one of the two attachment
points, TRILL allows load sharing between the attachment points.
Further, TRILL must manage multicast and broadcast traffic so as not
to create feedback loops on Ethernet segments which are attached at
multiple TRILL access points.
3.6. VLAN Issues
A TRILL solution should support multiple VLANs (802.1Q, 802.1V, and
802.1S). This may involve ignorance, just as many bridge devices do
not participate in the VLAN protocols. It may alternately support
direct VLAN support, e.g., by the use of separate TRILL routing
protocol instances to separate traffic for each VLAN traversing a
TRILL solution.
3.7. Equivalence
As with any extension to an existing architecture, it would be useful
- though not strictly necessary - to be able to describe or consider
a TRILL solution as a model of an existing link layer component. Such
equivalence provides a validation model for the architecture and a
way for users to predict the effect of the use of a TRILL solution on
a deployed Ethernet. In this case, 'user' refers to users of the
Ethernet protocol, whether at the host (data segments), bridge (ST
control segments), or VLAN (VLAN control).
This provides a sanity check, i.e., "we got it right if we can
replace a TRILL solution with an X" (where "X" might be a single
bridge, a hub, or some other link layer abstraction). It does not
matter whether "X" can be implemented on the same scale as the
corresponding TRILL solution. It also does not matter if it can -
there may be utility to deploying the TRILL solution components
incrementally, in ways that a single "X" could not be installed.
For example, if TRILL solution were equivalent to a single 802.1D
bridge, it would mean that the TRILL solution would - as a whole -
participate in the STP. This need not require that TRILL solution
would propagate STP, any more than a bridge need do so in its on-
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board control. It would mean that the solution would interact with
BPDUs at the edge, where the solution would - again, as a whole -
participate as if a single node in the spanning tree. Note that this
equivalence is not required; a solution may act as if an 802.1 hub,
or may not have a corresponding equivalent link layer component at
all.
3.8. Optimizations
There are a number of optimizations that may be applied to TRILL
solutions. These must be applied in a way that does not affect
functionality as a tradeoff for increased performance. Such
optimizations address broadcast and multicast frame distribution,
VLAN support, and snooping of ARP and IPv6 neighbor discovery.
3.9. Internet Architecture Issues
TRILL solutions are intended to have no impact on the Internet
network layer architecture. In particular, the Internet and higher
layer headers should remain intact when traversing a TRILL solution,
just as they do when traversing any other link subnet technologies.
This means that the IP TTL field cannot be co-opted for forwarding
loop mitigation, as it would interfere with the Internet layer
assuming that the link subnet was reachable with no changes in TTL
(Internet TTLs are changed only at routers, as per RFC 1812, and even
if IP TTL were considered, TRILL is expected to support non-IP
payloads, and so requires a separate solution anyway) [2].
TRILL solutions should also have no impact on Internet routing or
signaling, which also means that broadcast and multicast, both of
which can pervade an entire Ethernet link subnet, must be able to
transparently pervade a TRILL solution. Changing how either of these
capabilities behaves would have significant effects on a variety of
protocols, including RIP (broadcast), RIPv2 (multicast), ARP
(broadcast), IPv6 neighbor discovery (multicast), etc.
Note that snooping of network layer packets may be useful, especially
for certain optimizations. These include snooping multicast control
plane packets (IGMP) to tune link multicast to match the network
multicast topology, as is already done in existing smart switches
[3][5]. This also includes snooping IPv6 neighbor discovery messages
to assist with governing TRILL solution edge configuration, as is the
case in some smart learning bridges [10]. Other layers may similarly
be snooped, notably ARP packets, for similar reasons for IPv4 [14].
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4. Applicability
As might be expected, TRILL solutions are intended to be used to
solve the problems described in Section 2. However, not all such
installations are appropriate environments for such solutions. This
section outlines the issues in the appropriate use of these
solutions.
TRILL solutions are intended to address problems of path efficiency
and stability within a single Ethernet link subnet. Like bridges,
individual TRILL solution components may find other TRILL solution
components within a single Ethernet link subnet and aggregate into a
single TRILL solution.
TRILL solutions are not intended to span separate Ethernet link
subnets where interconnected by network layer (e.g., router) devices,
except via link layer tunnels that are in place prior to their
deployment, where such tunnels render the distinct subnet
undetectably equivalent from a single Ethernet link subnet.
A currently open question is whether a single Ethernet link subnet
should contain only one TRILL solution instance, either of necessity
of architecture or utility. Multiple TRILL solutions, like Internet
ASes, may allow TRILL routing protocols to be partitioned in ways
that help their stability, but this may come at the price of needing
the TRILL solutions to participate more fully as nodes (each modeling
a bridge) in the Ethernet link subnet STP. Each architecture solution
should decide whether multiple TRILL solutions are supported within a
single Ethernet link subnet and mechanisms should be included to
enforce whatever decision is made.
TRILL solutions are not intended to address scalability limitations
in bridged subnets. Although there may be scale benefits of other
aspects of solving TRILL problems, e.g., of using network layer
routing to provide stability under link changes or intermittent
outages, this is not a focus of this work.
As also noted earlier, TRILL solutions are not intended to address
security vulnerabilities in either the data plane or control plane of
the link layer. This means that TRILL solutions should not limit
broadcast frames, ARP requests, or spanning tree protocol messages
(if such are interpreted by the TRILL solution or solution edge).
5. Security Considerations
TRILL solutions should not introduce new vulnerabilities compared to
traditional bridged subnets.
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TRILL solutions are not intended to be a solution to Ethernet link
subnet vulnerabilities, including spoofing, flooding, snooping, and
attacks on the link control plane (STP, flooding the learning cache)
and link-network control plane (ARP). Although TRILL solutions are
intended to provide more stable routing than STP, this stability is
limited to performance, and the subsequent robustness is intended to
address non-malicious events.
There may be some side-effects to the use of TRILL solutions that can
provide more robust operation under certain attacks, such as those
interrupting or adding link service, but TRILL solutions should not
be relied upon for such capabilities.
Finally, TRILL solutions should not interfere with other protocols
intended to address these vulnerabilities, such as those under
development to secure IPv6 neighbor discovery [1].
6. IANA Considerations
This document has no IANA considerations.
This section should be removed by the RFC Editor prior to final
publication.
7. Acknowledgments
Portions of this document are based on documents that describe a
preliminary solution, and on a related network layer solution
[11][13][15].
This document was prepared using 2-Word-v2.0.template.dot.
8. References
8.1. Normative References
None.
8.2. Informative References
[1] Arkko, J., J. Kempf, B. Sommerfield, B. Zill, P. Nikander,
"Secure Neighbor Discovery (SeND)", RFC 3971 (Proposed
Standard), Mar. 2005.
[2] Baker, F., "Requirements for IP Version 4 Routers", RFC 1812
(Proposed Standard), Jun. 1995.
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[3] Cain, B., S. Deering, I. Kouvelas, B. Fenner, A. Thyagarajan,
"Internet Group Management Protocol, Version 3", RFC 3376
(Proposed Standard), Oct. 2002.
[4] Elmeleegy, K., A.L. Cox, T.E. Ng, "On Count-to-Infinity Induced
Forwarding Loops in Ethernet Networks", Proc. Infocom 2006,
Apr. 2006.
[5] Haberman, B., J. Martin, "Multicast Router Discovery", RFC 4286
(Proposed Standard), Dec. 2005.
[6] IEEE 802.1D bridging standard, "IEEE Standard for Local and
metropolitan area networks: Media Access Control (MAC)
Bridges", (incorporates 802.1w), Jun. 2004.
[7] IEEE 802.1Q VLAN standard, "IEEE Standards for Local and
metropolitan area networks: Virtual Bridged Local Area
Networks", (incorporates 802.1v and 802.1s), May 2003.
[8] Karn, P., (ed.), C. Bormann, G. Fairhurst, D. Grossman, R.
Ludwig, J. Mahdavi, G. Montenegro, J. Touch, L. Wood, "Advice
for Internet Subnetwork Designers", RFC-3819 / BCP 89 (Best
Current Practice), Jul. 2004.
[9] Myers, A., T.E. Ng, H. Zhang, "Rethinking the Service Model:
Scaling Ethernet to a Million Nodes", Proc. ACM Third Workshop
on Hot Topics in Nnetworks (HotNets-III), Mar. 2004.
[10] Narten, T., E. Nordmark, W. Simpson, H. Soliman, "Neighbor
Discovery for IP version 6 (IPv6)", RFC 4861 (Draft Standard),
Sep. 2007.
[11] Perlman, R., "RBridges: Transparent Routing", Proc. Infocom
2005, Mar. 2004.
[12] Perlman, R., "Interconnection: Bridges, Routers, Switches, and
Internetworking Protocols", Addison Wesley, Chapter 3, 1999.
[13] Perlman, R., J. Touch, A. Yegin, "RBridges: Transparent
Routing," (expired work in progress), Apr. 2004 - May 2005.
[14] Plummer, D., "Ethernet Address Resolution Protocol: Or
converting network protocol addresses to 48.bit Ethernet
address for transmission on Ethernet hardware", RFC 826 / STD
37 (Standard), Nov. 1982.
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[15] Touch, J., Y. Wang, L. Eggert, G. Finn, "A Virtual Internet
Architecture", ISI Technical Report ISI-TR-570, Presented at
the Workshop on Future Directions in Network Architecture
(FDNA) 2003 at Sigcomm 2003, March 2003.
9. Author's Addresses
Joe Touch
USC/ISI
4676 Admiralty Way
Marina del Rey, CA 90292-6695
U.S.A.
Phone: +1 (310) 448-9151
Email: touch@isi.edu
URL: http://www.isi.edu/touch
Radia Perlman
Sun Microsystems
Email: Radia.Perlman@sun.com
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Copyright (C) The IETF Trust (2008).
This document is subject to the rights, licenses and restrictions
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Acknowledgment
Funding for the RFC Editor function is currently provided by the
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Touch & Perlman Expires August 24, 2008 [Page 16]