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RFC2019 - Transmission of IPv6 Packets Over FDDI

王朝other·作者佚名  2008-05-31
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Network Working Group M. Crawford

Request for Comments: 2019 Fermilab

Category: Standards Track October 1996

A Method for the Transmission of IPv6 Packets over FDDI Networks

Status of this Memo

This document specifies an Internet standards track protocol for the

Internet community, and requests discussion and suggestions for

improvements. Please refer to the current edition of the "Internet

Official Protocol Standards" (STD 1) for the standardization state

and status of this protocol. Distribution of this memo is unlimited.

IntrodUCtion

This memo specifies the MTU and frame format for transmission of IPv6

[IPV6] packets on FDDI networks, including a method for MTU

determination in the presence of 802.1d bridges to other media. It

also specifies the method of forming IPv6 link-local addresses on

FDDI networks and the content of the Source/Target Link-layer Address

option used the the Router Solicitation, Router Advertisement,

Neighbor Solicitation, and Neighbor Advertisement messages described

in [DISC], when those messages are transmitted on an FDDI network.

Maximum Transmission Unit

FDDI permits a frame length of 4500 octets (9000 symbols), including

at least 22 octets (44 symbols) of Data Link encapsulation when

long-format addresses are used. SuBTracting 8 octets of LLC/SNAP

header, this would, in principle, allow the IPv6 packet in the

Information field to be up to 4470 octets. However, it is desirable

to allow for the variable sizes and possible future extensions to the

MAC header and frame status fields. The default MTU size for IPv6

packets on an FDDI network is therefore 4352 octets. This size may

be reduced by a Router Advertisement [DISC] containing an MTU option

which specifies a smaller MTU, or by manual configuration of a

smaller value on each node. If a Router Advertisement is received

with an MTU option specifying an MTU larger than the default or the

manually configured value, that MTU option may be logged to system

management but must be otherwise ignored.

For purposes of this document, information received from DHCP is

considered "manually configured".

Frame Format

FDDI provides both synchronous and asynchronous transmission, with

the latter class further subdivided by the use of restricted and

unrestricted tokens. Only asynchronous transmission with

unrestricted tokens is required for FDDI interoperability.

Accordingly, IPv6 packets shall be sent in asynchronous frames using

unrestricted tokens. The robustness principle dictates that nodes

should be able to receive synchronous frames and asynchronous frames

sent using restricted tokens.

IPv6 packets are transmitted in LLC/SNAP frames, using long-format

(48 bit) addresses. The data field contains the IPv6 header and

payload and is followed by the FDDI Frame Check Sequence, Ending

Delimiter, and Frame Status symbols.

+-------+ ^

FC

+-------+-------+-------+-------+-------+-------+

Destination FDDI address

+-------+-------+-------+-------+-------+-------+ FDDI

Source FDDI address header

+-------+-------+-------+-------+-------+-------+

DSAP SSAP CTL OUI

+-------+-------+-------+-------+-------+-------+

Ethertype v

+-------+-------+-------+-------+-------+------+------+

IPv6 header and payload ... /

+-------+-------+-------+-------+-------+------+------+

FDDI Header Fields:

FC The Frame Code must be in the range 50 to 57 hexadecimal,

inclusive, with the three low order bits indicating the

frame priority. The Frame Code should be in the range 51 to

57 hexadecimal, inclusive, for reasons given in the next

section.

DSAP, SSAP Both the DSAP and SSAP fields shall contain the value AA

hexadecimal, indictating SNAP encapsulation.

CTL The Control field shall be set to 03 hexadecimal, indicating

Unnumbered Information.

OUI The Organizationally Unique Identifier shall be set to

000000 hexadecimal.

Ethertype The ethernet protocol type ("ethertype") shall be set to the

value 86DD hexadecimal.

Interaction with Bridges

802.1d MAC bridges which connect different media, for example

Ethernet and FDDI, have become very widespread. Some of them do IPv4

packet fragmentation and/or support IPv4 Path MTU discovery [PMTU],

many others do not, or do so incorrectly. Use of IPv6 in a bridged

mixed-media environment should not depend on support from MAC

bridges.

For correct operation when mixed media are bridged together, the

smallest MTU of all the media must be advertised by routers in an MTU

option. If there are no routers present, this MTU must be manually

configured in each node which is connected to a medium with larger

default MTU. Multicast packets on such a bridged network must not be

larger than the smallest MTU of any of the bridged media. Often, the

subnetwork topology will support larger unicast packets to be

exchanged between certain pairs of nodes. To take advantage of

high-MTU paths when possible, nodes transmitting IPv6 on FDDI should

implement the following simple mechanism for "FDDI adjacency

detection".

A node which implements FDDI adjacency detection and has it enabled

on an FDDI interface must set a non-zero LLC priority in all Neighbor

Advertisement, Neighbor Solicitation and, if applicable, Router

Advertisement frames transmitted on that interface. (In IEEE 802

language, the user_priority parameter of the M_UNITDATA.request

primitive must not be zero.) If FDDI adjacency detection has been

disabled on an FDDI interface, the priority field of those frames

must be zero.

Note that an IPv6 frame which originated on an Ethernet, or traversed

an Ethernet, before being translated by an 802.1d bridge and

delivered to a node's FDDI interface will have zero in the priority

field, as required by [BRIDGE]. (There's a fine point here: a

conforming bridge may provide a management-settable Outbound User

Priority parameter for each port. However, the author is unaware of

any product that provides this optional capability and, in any case,

the default value for the parameter is zero.)

If a node N1 receives, in an FDDI frame with a non-zero LLC priority,

a valid Router Advertisement, Neighbor Advertisement, or Neighbor

Solicitation from a node N2, then N1 may send unicast IPv6 packets to

N2 with sizes up to the default IPv6 FDDI MTU (4352 octets),

regardless of any smaller MTU configured manually or received in a

Router Advertisement MTU option. N2 may be the IPv6 destination or

the next hop router to the destination.

Nodes implementing FDDI adjacency detection must provide a

configuration option to disable the mechanism. This option may be

used when a smaller MTU is desired for reasons other than mixed-media

bridging. By default, FDDI adjacency detection should be enabled.

The only contemplated use of the LLC priority field of the FC octet

is to aid in per-destination MTU determination. It would be

sufficient for that purpose to require only that Router

Advertisements, Neighbor Advertisements, and Neighbor Solicitations

sent on FDDI always have non-zero priority. However, it may be

simpler or more useful to transmit all IPv6 packets on FDDI with

non-zero priority.

Stateless Autoconfiguration and Link-Local Addresses

The address token [CONF] for an FDDI interface is the interface's

built-in 48-bit IEEE 802 address, in canonical bit order and with the

octet in the same order in which they would appear in the header of

an ethernet frame. (The individual/group bit is in the first octet

and the OUI is in the first three octets.) A different MAC address

set manually or by software should not be used as the address token.

An IPv6 address prefix used for stateless autoconfiguration of an

FDDI interface must be 80 bits in length.

The IPv6 Link-local address [AARCH] for an FDDI interface is formed

by appending the interface's IEEE 802 address to the 80-bit prefix

FE80::.

+-------+-------+-------+-------+-------+-------+------+------+

FE 80 00 00 00 00 00 00

+-------+-------+-------+-------+-------+-------+------+------+

00 00 FDDI Address

+-------+-------+-------+-------+-------+-------+------+------+

Address Mapping -- Unicast

The procedure for mapping IPv6 addresses into FDDI link-layer

addresses is described in [DISC]. The Source/Target Link-layer

Address option has the following form when the link layer is FDDI.

+-------+-------+-------+-------+-------+-------+------+------+

Type Length FDDI Address

+-------+-------+-------+-------+-------+-------+------+------+

Option fields:

Type 1 for Source Link-layer address.

2 for Target Link-layer address.

Length 1 (in units of 8 octets).

FDDI Address

The 48 bit FDDI 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.

Address Mapping -- Multicast

An IPv6 packet with a multicast destination address DST is

transmitted to the FDDI multicast address whose first two octets are

the value 3333 hexadecimal and whose last four octets are the last

four octets of DST, ordered from more to least significant.

+-------+-------+-------+-------+-------+-------+

33 33 DST13 DST14 DST15 DST16

+-------+-------+-------+-------+-------+-------+

Security Considerations

Security considerations are not addressed in this memo.

Acknowledgments

Erik Nordmark contributed to the method for interaction with bridges.

References

[AARCH] Hinden, and S. Deering, "IP Version 6 Addressing

Architecture", RFC1884, December 1995.

[BRIDGE]ISO/IEC 10038 : 1993 [ANSI/IEEE Std 802.1D] Media Access

control (MAC) bridges.

[CONF] Thomson, S., and T. Narten, "IPv6 Stateless Address

Autoconfiguration", RFC1971, August 1996.

[DISC] Narten, T., Nordmark, E., and W. Simpson, "Neighbor Discovery

for IP Version 6 (IPv6), RFC1970, August 1996.

[IPV6] Deering, S., and R. Hinden, "Internet Protocol, Version 6

(IPv6) Specification", RFC1883, August 1996.

[PMTU] Mogul, J., and S. Deering, "Path MTU Discovery", RFC1191,

November 1990.

Author's Address

Matt Crawford

Fermilab MS 368

PO Box 500

Batavia, IL 60510

USA

Phone: +1 708 840-3461

EMail: crawdad@fnal.gov

 
 
 
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