One document matched: draft-williams-ipvlx-ipbridging-00.txt
Network Working Group A. Williams
Internet-Draft NICTA
Expires: January 11, 2005 July 12, 2004
Bridging IP at Layer-3
draft-williams-ipvlx-ipbridging-00
Status of this Memo
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Copyright Notice
Copyright (C) The Internet Society (2004). All Rights Reserved.
Abstract
Joining incompatible links together as an IP subnet by bridging IP
packets at Layer-3 is an attractive goal. Several challenges that
need to be addressed before IPbridging becomes a reality are listed
in this document.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Complications with IPbridging . . . . . . . . . . . . . . . . 4
3. IPbridging and Rbridge . . . . . . . . . . . . . . . . . . . . 5
4. Other Notes . . . . . . . . . . . . . . . . . . . . . . . . . 6
4.1 IPv4 DHCP . . . . . . . . . . . . . . . . . . . . . . . . 6
4.2 Multicast forwarding . . . . . . . . . . . . . . . . . . . 6
4.3 IPv6 ND-proxy . . . . . . . . . . . . . . . . . . . . . . 6
5. Security Considerations . . . . . . . . . . . . . . . . . . . 7
6. References . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Author's Address . . . . . . . . . . . . . . . . . . . . . . . 7
Intellectual Property and Copyright Statements . . . . . . . . 8
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1. Introduction
IP protocols are widely used to build networks from multiple links of
differing types by assigning IP address ranges to each link and
connecting links with an IP router. Assignment of address ranges to
links is usually a manual process unsuited to plug-and-play
networking. Automatic assignment of address ranges to links is
possible with inefficient use of address space, possible link
renumbering with changes in network topology and host renumbering
when changing attachment point.
802-style LAN bridging provides a good example of plug and play
networking, however it too has some drawbacks. Layer-2 bridging
cannot be used between links with different L2 address types,
problems arise bridging links with the same L2 address type but
differing MTUs and multicast addressing semantics can differ between
links with the same L2 address type. Furthermore, L2 spanning trees
result in inefficient paths between end nodes and concentrates
traffic on a subset of the available links. See [Deering-email] and
[Rbridge] for more details.
A device connecting several links in an IP subnet together by
bridging IP packets at Layer 3 (an IPbridge) could combine the best
of both worlds and achieve plug and play operation over links that
cannot be bridged at Layer-2. IPbridges would behave as IP routers
with a host route for each IP device in the subnet. IPbridges could
use a more sophisticated routing protocol than spanning tree
resulting in more efficient paths and spreading of traffic over the
available links.
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2. Complications with IPbridging
Although IPbridging appears superficially similar to LAN bridging
there are a number of complicating differences:
IP address bootstrapping:
IP addresses are not pre-configured into devices like EUI-48
MAC addresses and therefore IPbridges must support address
autoconfiguration protocols (e.g. DHCP and IPv6 Stateless
Autoconfiguration) in a network without IP addresses.
Autoconfiguration protocols are hard to handle with vanilla
IPbridging because the device does not yet have a "bridgeable"
IP address and link specific information is often used.
IPbridges carrying out the normal functions of an IP router
(e.g. fragmenting packets, decrementing the IP TTL, issuing
redirects) will also need an IP address to use as a source
address in ICMP messages.
IP address to link address mapping:
Final delivery of an IP packet to an end device often requires
mapping the destination IP address to a link layer address.
This mapping is link specific and is not required in LAN
bridging because the link layer address is assumed to be the
same.
Discovery and use of exit routers:
IP networks are usually connected to other IP networks and to
the Internet via routers. IPbridges forwarding packets to
destinations outside the local IP subnet need to decide which
exit router should be used. Protocols like DHCP pass default
router information to the client -- should IPbridges honour
that or should they discover all available exit routers and
choose? Should clients using router discovery see IPbridges or
routers? Should redirects from exit routers affect routing
between IPbridges or only the clients?
Protocol semantics:
Various IP protocols (e.g. IPv6 Neighbor Discovery, DHCP,
IGMP/MLD) assume that certain types of broadcast and multicast
messages are not forwarded by IP routers and that they are
delivered to all instances of a particular network service
within a subnet. Many of these protocols set and check the IP
TTL field to ensure that forwarding does not occur or detect
when it has. For these protocols, an IPbridge would need to
proxy services (e.g. act as a DHCP relay or default router) or
proxy the required information (e.g. IGMP/MLD).
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3. IPbridging and Rbridge
Rbridge [Rbridge] is an improved system for bridging 802 addressed
networks. L2 frames are encapsulated for transport between rbridges
and decapsulated for transmission onto the destination link. The
encapsulation header contains a hop count which is decremented each
time the packet is forwarded by an Rbridge. Packets are discarded
when the hop count reaches zero preventing catastrophic packet
looping. Rbridges run a link state routing protocol to exchange end
node reachability information, to compute optimal network paths and
to support fast fast recovery after link failure.
Since a great deal of traffic on 802 networks is IP traffic and since
IP packets already have a TTL, forwarding IP packets without rbridge
encapsulation is an attractive optimisation for Rbridges. In this
case the Rbridge would behave as an IPbridge and decrement the IP TTL
when forwarding a packet.
An Rbridge forwarding bare IP packets is not restricted to pure
IPbridge operation -- for example, it can make use of the L2
forwarding information for an destination IP address it does not know
how to reach. When an Rbridge is bridging between two incompatible
links using bare IP packets it is operating as a pure IPbridge. In
general, it may make sense for Rbridges using optimised IP forwarding
to treat some IP traffic as L2 traffic in order to avoid the issues
described above.
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4. Other Notes
4.1 IPv4 DHCP
DHCP, as typically deployed on a bridged 802 network, will not
operate in an IPbridged network. There are several issues. Firstly,
DHCPDISCOVERs and other messages are sent to a local IPv4 broadcast
address 255.255.255.255 that should not be forwarded by an IPbridge.
Secondly, DHCPOFFERs can be unicast to the requesting client using
the MAC address supplied in the DHCPDISCOVER message. There has been
no information before the DHCPOFFER that would allow an IPbridge to
correctly forward to the requesting client without using L2
information.
An alternative is for IPbridges to act as a DHCP relays. This would
work however configuration is required. Configuration information
for the DHCP relay could be distributed in the routing protocol.
4.2 Multicast forwarding
Currently, Rbridge and 802.1 Spanning Tree variants distribute
multicast over a single spanning tree. Multicast forwarding in
Rbridged/IPbridged networks could be made more efficient by snooping
and distributing group membership and multicast source information
via the link state routing protocol. This approach was used for
MOSPF [RFC1584][RFC1585].. Rbridges may still need to generate IGMP
packets to build efficient paths through the L2 networks between
Rbridges.
4.3 IPv6 ND-proxy
[IPv6-multilink] describes a method for connecting multiple links
sharing a common IPv6 prefix with routers that perform Neighbor
Discovery proxying for hosts not connected to the local link.
Topologies with loops are not supported.
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5. Security Considerations
None.
6 References
[Deering-email]
Deering, S., "Four problems with link layer bridging", Oct
2002.
http://internet.motlabs.com/pipermail/zerouter/
2002-October/000029.html
[IPv6-multilink]
Thaler, D. and C. Huitema, "Multi-link Subnet Support in
IPv6", draft-ietf-ipv6-multilink-subnets-00 (work in
progress), July 2002.
[RFC1584] Moy, J., "Multicast Extensions to OSPF", RFC 1584, March
1994.
[RFC1585] Moy, J., "MOSPF: Analysis and Experience", RFC 1585, March
1994.
[Rbridge] Perlman, R., "RBridges: Transparent Routing",
draft-perlman-rbridge-00 (work in progress), May 2004.
Author's Address
Aidan Williams
National ICT Australia (NICTA)
Bay 15, Locomotive Workshop
Australian Technology Park
Eveleigh, NSW 1430
Australia
Phone: +61 2 8374 5558
EMail: aidan@nicta.com.au
URI: http://nicta.com.au/
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