Internet DRAFT - draft-ietf-ipng-trans-tokenring
draft-ietf-ipng-trans-tokenring
IPng Working Group Stephen Thomas
Internet Draft TransNexus
June 15, 1997
Transmission of IPv6 Packets over Token Ring Networks
<draft-ietf-ipngwg-trans-tokenring-00.txt>
Status of this Memo
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This Internet Draft expires December 15, 1997.
1. Introduction
This memo specifies the MTU and frame format for transmission
of IPv6 packets on Token Ring networks. It also specifies the
method of forming IPv6 link-local addresses on Token Ring
networks and the content of the Source/Target Link-layer
Address option used the Router Solicitation, Router
Advertisement, Neighbor Solicitation and Neighbor
Advertisement messages when those messages are transmitted on
a Token Ring network.
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2. Maximum Transmission Unit
IEEE 802.5 networks have a maximum frame size based on the
maximum time a node may hold the token. This time depends on
many factors including the data signaling rate and the number
of nodes on the ring. Because the maximum frame size varies,
implementations must rely on manual configuration or router
advertisements [DISC] to determine actual MTU sizes. Common
default values include approximately 2000, 4000, and 8000
octets.
In the absence of any other information, an implementation
should use a default MTU of 1500 octets. This size offers
compatibility with all common 802.5 defaults, as well as with
Ethernet LANs in an environment using transparent bridging.
In an environment using source route bridging, the process of
discovering the MAC-level path to a neighbor can yield the
MTU for the path to that neighbor. The information is
contained in the largest frame (LF) subfield of the routing
information field. This field limits the size of the
information field of frames to that destination, and that
information field includes both the LLC [LLC] header and the
IPv6 datagram. Since, for IPv6, the LLC header is always 8
octets in length, the IPv6 MTU can be found by subtracting 8
from the maximum frame size defined by the LF subfield. If an
implementation uses this information to determine MTU sizes,
it must maintain separate MTU values for each neighbor.
A detailed list of the LF values and the resulting maximum
frame size can be found in [BRIDGE]. To illustrate the
calculation of IPv6 MTU, the following table lists several
common values. Note that some of the 802.1D LF values would
result in an IP MTU less than 576 bytes. This size is less
than the IPv6 minimum, and communication across paths with
those MTUs is generally not possible using IPv6.
LF (base) LF (extension) MAC MTU IP MTU
001 000 1470 1462
010 000 2052 2044
011 000 4399 4391
100 000 8130 8122
101 000 11407 11399
110 000 17749 17741
111 000 41600 41592
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When presented with conflicting MTU values from several
sources, an implementation should choose from those sources
according to the following priorities:
1. Largest Frame values from source route bridging
(only for specific, unicast destinations)
2. Router advertisements
3. Manual configuration (including DHCP)
4. Default of 1500
3. Frame Format
IPv6 packets are transmitted in LLC/SNAP frames. The data
field contains the IPv6 header and payload. The following
figure shows a complete 802.5 frame containing an IPv6
datagram.
+-------+-------+-------+-------+
| SD | AC | FC | |
+-----------------------+ |
| Destination Address |
| +-----------------------+
| | Source |
+-------+ Address +-------+
| | DSAP |
+-------+-------+-------+-------+
| SSAP | CTL | OUI |
+-------+-------+-------+-------+
| OUI | EtherType | |
+-------+---------------+ |
| |
~ IPv6 header and payload... ~
| |
+-------------------------------+
| FCS |
+-------+-------+---------------+
| ED | FS |
+-------+-------+
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Token Ring Header Fields
SD: Starting Delimiter
AC: Access Control
FC: Frame Control
Destination Address: 48-bit IEEE address of destination
station
Source Address: 48-bit IEEE address of source station
DSAP: Destination Service Access Point (for LLC/SNAP
format, shall always contain the value 0xAA)
SSAP: Source Service Access Point (for LLC/SNAP format,
shall always contain the value 0xAA)
CTL: Control Field (for Unnumbered Information, shall
always contain the value 0x03)
OUI: Organizationally Unique Identifier (for EtherType
encoding, shall always contain the value 0x000000)
EtherType: Protocol type of encapsulated payload (for
IPv6, shall always contain the value 0x86DD)
FCS: Frame Check Sequence
ED: Ending Delimiter
FS: Frame Status
In the presence of source route bridges, a routing
information field (RIF) may appear immediately after the
source address. A RIF is present in frames when the most
significant bit of the source address is set to one. (This is
the bit whose position corresponds to that of the
Individual/Group bit in the Destination Address.)
The RIF is a variable-length field that (when present)
contains a two-octet Routing Control (RC) header, followed by
zero or more two-octet Route Designator fields:
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0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Routing Control: |Bcast| Length |D| LF |rsvd |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Route Designator 1: | Segment 1 |Bridge1|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ ... ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Route Designator N: | Segment N |BridgeN|
(0 <= N <= 7) +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Route Designator Fields:
Bcast: Broadcast Indicator, Defined values:
10x: All Routes Explorer
11x: Spanning Tree Explorer
0xx: Specifically Routed Frame
Length: Total length of RIF field in octets
D: Direction of source route. A value of 0 means that
the left-to-right sequence of Route Designators
provides the path from the sender to recipient. A
value of 0 indicates the sequence goes from
recipient to sender.
LF: Largest Frame
rsvd: Reserved
On transmission, the Route Designator fields give the
sequence of (bridge, LAN segment) numbers the packet is to
traverse. It is the responsibility of the sender to provide
this sequence for Specifically Routed Frames, i.e., unicast
IP datagrams.
4. Stateless Autoconfiguration
The interface token [CONF] for a Token Ring interface is the
EUI-64 identifier [EUI64] derived from the interface's built-
in 48-bit IEEE 802 address. The OUI of the Token Ring address
(the first three octets) becomes the company_id of the EUI-64
(the first three octets). The fourth and fifth octets of the
EUI are set to the fixed value FFFE hexadecimal. The last
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three octets of the Token Ring address become the last three
octets of the EUI-64.
For example, the interface token for a Token Ring interface
whose built-in address is, in hexadecimal and in canonical
bit order,
34-56-78-9A-BC-DE
would be
34-56-78-FF-FE-9A-BC-DE.
A different MAC address set manually or by software should
not be used to derive the interface token.
An IPv6 address prefix used for stateless autoconfiguration
of a Token Ring interface must have a length of 64 bits.
5. Link Local Address
The IPv6 link-local address [AARCH] for a Token Ring
interface is formed by appending the interface token, as
defined above, to the prefix FE80::/64.
10 bits 54 bits 64 bits
+----------+-----------------------+----------------------------+
|1111111010| (zeros) | Interface Token |
+----------+-----------------------+----------------------------+
6. Address Mapping -- Unicast
The procedure for mapping IPv6 addresses into Token Ring
link-layer addresses is described in [DISC]. The
Source/Target Link-layer Address option has the following
form when the link layer is Token Ring.
0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+- Token Ring -+
| |
+- Address -+
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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Option fields:
Type: 1 for Source Link-layer address.
2 for Target Link-layer address.
Length: 1 (in units of 8 octets).
Token Ring Address: The 48 bit Token Ring IEEE 802
address, in canonical bit order. This is the
address the interface currently responds to, and
may be different from the built-in address used as
the address token.
When source routing bridges are used, the source route for
the path to a destination can be extracted from the RIF field
of received Neighbor Advertisement messages. Note that the
RIF field of received packets can be reversed into a source
route suitable for transmitting return traffic by toggling
the value of the 'D' bit and insuring that the Bcast field is
set to indicate a Specifically Routed Frame.
7. Address Mapping -- Multicast
All IPv6 packets with multicast destination addresses are
transmitted to Token Ring functional addresses. The following
table shows the specific mapping between the IPv6 addresses
and Token Ring functional addresses (in canonical form). Note
that protocols other than IPv6 may use these same functional
addresses, so all Token Ring frames destined to these
functional addresses are not guaranteed to be IPv6 datagrams.
MAC Addr (canonical) IPv6 Multicast Addresses
03-00-80-00-00-00 all nodes (FF01::1 and FF02::1) and
solicited node (FF02:0:0:0:0:1:FFXX:XXXX)
addresses
03-00-40-00-00-00 all routers addresses (FF0X::2)
03-00-00-80-00-00 any other multicast address with three
least significant bits = 000
03-00-00-40-00-00 any other multicast address with three
least significant bits = 001
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03-00-00-20-00-00 any other multicast address with three
least significant bits = 010
03-00-00-10-00-00 any other multicast address with three
least significant bits = 011
03-00-00-08-00-00 any other multicast address with three
least significant bits = 100
03-00-00-04-00-00 any other multicast address with three
least significant bits = 101
03-00-00-02-00-00 any other multicast address with three
least significant bits = 110
03-00-00-01-00-00 any other multicast address with three
least significant bits = 111
In a bridged token ring network, all multicast packets SHOULD
be sent with a RIF header specifying the use of the Spanning
Tree Explorer.
Note: it is believed that some (very) old bridge
implementations do not properly support the Spanning Tree
Explorer mechanism. In such environments, multicast traffic
sent through bridges must use a RIF with the All Routes
Explorer. Consequently, an implementation MAY wish to allow
the sending of IP multicast traffic using an All Routes
Explorer. However, such an ability must be configurable by a
system administrator and the default setting of the switch
MUST be to use the Spanning Tree Explorer.
8. Security Considerations
Token Ring, like most broadcast LAN technologies, has
inherent security vulnerabilities. For example, any sender
can claim the identity of another and forge traffic. It is
the responsibility of higher layers to take appropriate steps
in those environments where such vulnerabilities are
unacceptable.
9. Acknowledgments
Several members of the IEEE 802.5 Working Group contributed
their knowledge and experience to the drafting of this
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specification, including Jim, Andrew Draper, George Lin, John
Messenger, Kirk Preiss, and Trevor Warwick. The author would
also like to thank many members of the IPng working group for
their advice and suggestions, including Ran Atkinson, Scott
Bradner, Matt Crawford, Steve Deering, Francis Dupont, Robert
Elz, Thomas Narten, and Matt Thomas. A special thanks is due
Steve Wise, who gave the most relevant advice of all by
actually trying to implement this specification while it was
in progress.
10. References
[802.5] 8802-5 : 1995 (ISO/IEC) [ANSI/IEEE 802.5, 1995
Edition] Information technology--Telecommunications
and information exchange between systems--Local and
metropolitan area networks--Specific requirements--
Part 5: Token ring access method and physical layer
specification.
[AARCH] R. Hinden, S. Deering, "IP Version 6 Addressing
Architecture", draft-ietf-ipngwg-addr-arch-v2-01.txt.
[BRIDGE] 10038: 1993 (ISO/IEC) [ANSI/IEEE Std 802.1D, 1993
Edition] Information technology--Telecommunications
and information exchange between systems--Local
area networks--Media access control (MAC) bridges.
[CONF] S. Thomson, T. Narten, "IPv6 Stateless Address
Autoconfiguration", RFC 1971.
[DISC] T. Narten, E. Nordmark, W. A. Simpson, "Neighbor
Discovery for IP Version 6 (IPv6)", RFC 1970.
[EUI64] "64-Bit Global Identifier Format Tutorial", http:
//standards.ieee.org/db/oui/tutorials/EUI64.html.
[IPV6] S. Deering, R. Hinden, "Internet Protocol, Version
6 (IPv6) Specification", RFC 1883.
[LLC] 8802-2 : 1994 (ISO/IEC) [ANSI/IEEE 802.2, 1994
Edition] Information technology--Telecommunications
and information exchange between systems--Local and
Metropolitan area networks--Specific requirements--
Part 2: Logical link control.
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11. Author's Address
Stephen Thomas
TransNexus
590 Thornton Road Suite C
Lithia Springs, GA 30057
US
Email: sthomas@transnexus.com
Phone: +1 770.924.6119
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