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Network Working Group P. Kuehn
Request for Comments: 1329 May 1992
Thoughts on Address Resolution for Dual MAC FDDI Networks
Status of this Memo
This memo provides information for the Internet community. It does
not specify an Internet standard. Distribution of this memo is
unlimited.
1. Abstract
In this document an idea is submitted how IP and ARP can be used on
inhomogeneous FDDI networks (FDDI networks with single MAC and dual
MAC stations) by introducing a new protocol layer in the protocol
suite of the dual MAC stations. Thus two dual MAC stations are able
to do a load splitting across the two rings and use the double
bandwidth of 200 Mbits/s as single MAC stations. The new layer is an
extension of layer 3. For the user, the higher layer protocols, IP
and ARP the property "dual MAC" is transparent. No modification is
required in the protocol suite of single MAC stations and transparent
bridges.
2. Acknowledgements
This paper is a result of a diploma thesis prepared at the Technical
University of Munich, Lehrstuhl fuer Kommunikationsnetze, in co-
operation with the Siemens Nixdorf AG. The author would like to
thank Jrg Eberspher and Bernhard Edmaier from the university, Andreas
Thimmel and Jens Horstmeier from the SNI AG at Augsburg for the
helpful comments and discussions.
3. Conventions
Primary MAC, P-MAC MAC, placed on the primary ring
Secondary MAC, S-MAC MAC, placed on the secondary ring
Inhomogeneous ring configuration of a dual FDDI ring with
single MAC and dual MAC stations
DMARP Dual MAC Address Resolution Protocol
4. Assumptions
When a dual FDDI ring wraps, both MACs in a dual MAC station are
assumed to remain connected to the ring. ANSI is just investigating
whether the Configuration Management in the Station Management of a
Kuehn [Page 1]
RFC 1329 Address Resolution for Dual MAC FDDI Networks May 1992
FDDI station can be modified to allow this. According to the FDDI
SMT standard [1], different addresses are required for all MACs on
the primary and the secondary ring.
In this paper, the MAC in a single MAC station is assumed to reside
on the primary ring. The application of single MAC stations which
have their MAC attached to the secondary ring is not precluded, but
therefor additional connectivity between the two rings is required.
These configurations are beyond the scope of this document.
5. The Application of Transparent Bridges
Transparent bridges can provide links to other 802 LANs or further
inhomogeneous FDDI rings. The connection between two inhomogeneous
FDDI rings can be realized by one or two transparent bridges. When
two transparent bridges are used, one transparent bridge links the
primary rings, the other the secondary rings. If two secondary rings
are connected by a transparent bridge, a path of transparent bridges
must exist between the two primary rings. No transparent bridges are
allowed between the primary and the secondary ring.
6. Protocol Layers in Single MAC Stations
The new protocol layer, named load sharing layer, is drafted to be
introduced only in dual MAC stations. In single MAC stations, IP and
ARP are working on top of the Subnetwork Access Protocol (SNAP) 04]
and the Logical Link Control protocol (802.2 LLC) [3]. LLC type 1 is
used because connectionless services are investigated only.
Kuehn [Page 2]
RFC 1329 Address Resolution for Dual MAC FDDI Networks May 1992
+--------------------------+
| IP |
+--------------------------+
+--------------------------+
| ARP |
+--------------------------+
| |
| ARP frames | IP frames
| |
+--------------------------+
| SNAP |
+--------------------------+
+--------------------------+
| LLC |
+--------------------------+
+--------------------------++-------+
| FDDI-MAC || F |
+--------------------------+| D S |
+--------------------------+| D M |
| FDDI PHY and PMD || I T |
+--------------------------++-------+
For the ARP layer, the following model is assumed:
+-------------------------------------------------------X-----------+
| - ARP entity - | |
| | IP frames |
| +----------------+ +----------------+ read | |
| | Cache | | | entries +-------------+ |
| | Administration |->-| Address Cache |------>--| Address | |
| +----------------+ | | | Conversion | |
| | +----------------+ | Unit | |
| | ARP frames +-------------+ |
| | / | |
| | ___________ <- ARP requests _________________/ | IP frames |
| |/ | |
+-----X-------------------------------------------------X-----------+
The Address Conversion Unit handles the actual conversion of IP
addresses to hardware addresses. For this purpose, it uses the
information in the ARP cache. The cache administration communicates
with other ARP entities by ARP and creates, deletes and renews the
entries in the cache.
7. Protocol Layers in Dual MAC Stations
The load sharing layer provides the same interface to ARP as SNAP
does. To exchange information about addresses and reachability, the
load sharing entities in dual MAC stations communicate with the Dual
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RFC 1329 Address Resolution for Dual MAC FDDI Networks May 1992
MAC Address Resolution Protocol (DMARP). For the transmission of
DMARP frames the SNAP SAP of LLC is used, as for IP and ARP, too.
The Organizationally Unique Identifier (OUI) in the SNAP header is
set to zero (24 bit), the EtherType field (16 bit) contains a new
number indicating DMARP, which is not defined yet.
+---------------------------------------------------------+
| IP |
+---------------------------------------------------------+
+---------------------------------------------------------+
| ARP |
+---------------------------------------------------------+
| ARP frames | IP frames
+---------------------------------------------------------+
| Load Sharing Layer |
+---------------------------------------------------------+
| | | | | |
| ARP | DMARP | IP | ARP | DMARP | IP
| frames | frames | frames | frames | frames | frames
| | | | | |
+-------------------------+ +----------------------------+
| SNAP 1 | | SNAP 2 |
+-------------------------+ +----------------------------+
+-------------------------+ +----------------------------+
| LLC 1 | | LLC 2 |
+-------------------------+ +----------------------------+
+-------------------------+ +----------------------------++-------+
| Primary MAC | | Secondary MAC || F |
+-------------------------+ +----------------------------+| D S |
+---------------------------------------------------------+| D M |
| FDDI PHY and PMD || I T |
+---------------------------------------------------------++-------+
8. Running Inhomogeneous FDDI Rings
8.1. Exchange of Primary MAC Addresses between Stations
IP and higher layer protocols only use the network independent IP
addresses. The ARP entity takes upon the conversion of an IP address
to the appropriate hardware address. To make the property dual MAC"
transparent, ARP may only know the addresses of MACs on the primary
ring. Therefore, the load sharing entity always delivers ARP frames
to SNAP 1 for transmission. By this way, communication with ARP is
done over the primary ring in normal state. A secondary MAC can
receive an ARP frame when the dual ring is wrapped and the
destination hardware address is a multicast or broadcast address.
These frames will be discarded because they were received twice.
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RFC 1329 Address Resolution for Dual MAC FDDI Networks May 1992
By this way, the associations of IP addresses to primary MAC
addresses for the single MAC and dual MAC stations are stored in the
ARP cache. The ARP cache contains no secondary MAC addresses.
8.2. Exchange of Secondary MAC Addresses between Dual MAC Stations
The load sharing layer needs to know the secondary MAC addresses of
the other dual MAC stations. The DMARP is used to get these
addresses. Whenever the load sharing entity delivers an ARP frame to
SNAP 1, a DMARP reply frame will be sent on the secondary ring,
containing the stations primary and secondary MAC address. The
destination hardware address in this DMARP frame is the broadcast MAC
address, the EtherType field in the SNAP header identifies DMARP.
The IP destination address is copied from the ARP frame. If the ARP
frame that was transmitted parallel to the DMARP reply was a request,
an ARP reply frame will be sent back to the sending station by the
ARP entity in the receiving station. When the load sharing layer in
the receiving station delivers this ARP reply frame to SNAP 1, it
sends a DMARP reply frame on the secondary ring.
By this way, DMARP exchanges the additionally required secondary MAC
addresses between the dual MAC stations. This is done parallel to
the exchange of the ARP frames.
8.3. Communication of Dual MAC Stations on Different Dual FDDI Rings
If two inhomogeneous dual FDDI rings are connected by one transparent
bridge, dual MAC stations placed on different dual FDDI rings cannot
perform a load sharing. If both dual FDDI rings remain in normal
state, no DMARP reply frames get from one secondary ring to the other
secondary ring. A dual MAC station realizes another dual MAC station
placed on the other dual ring as a single MAC station, because it
only receives ARP frames from it. If one of the dual rings is
wrapped, a DMARP reply frame can get on the primary ring of the other
dual ring. A target station on the unwrapped ring receives this
DMARP frame by the primary MAC and the load sharing entity stores the
contained addresses in an entry in the address cache. This entry is
marked with a control bit, named the OR-bit Other ring bit"). No
load sharing will be done with a station related to an entry with the
OR-bit set.
If both dual FDDI rings are wrapped, the MACs of all stations reside
on one ring. Now, dual MAC stations placed on different dual rings
can communicate with DMARP. If a DMARP reply frame is received by
the primary MAC and no entry exists for the sending station, a new
entry with OR-Bit set will be created. Otherwise, the OR-bit will be
set in the existing entry. If a DMARP reply frame is received by the
secondary MAC and an entry with OR-bit set already exists for the
Kuehn [Page 5]
RFC 1329 Address Resolution for Dual MAC FDDI Networks May 1992
sending station, the bit will not be reset.
This mechanism provides that no load sharing will be done between
Dual MAC stations on different dual rings if the dual rings are
linked with one transparent bridge. An additional DMARP error frame
is used to provide against errors when a DMARP reply frame gets lost
on the ring.
8.4. Timeout of Entries Marked with OR-Bit Set
If a FDDI ring is wrapped, the DMARP reply frames are received by the
primary and secondary MACs of the target dual MAC stations. In that
case, the entries for dual MAC stations on the same dual ring are
also marked with the OR-bit, although the load sharing is possible
between these stations.
When an OR-bit in an entry is set for the first time, a timer entity
is started. If the timer entity runs out, a DMARP request frame is
sent over SNAP 2 to the secondary MAC of the associated target)
station. Then the entry will be discarded.
If the request cannot be received by the target station because the
network configuration has changed, there is no entry in the address
cache for this station any more and no load sharing is computed. If
the target station receives the DMARP request frame, it sends back a
DMARP reply frame.
8.5. Problems with the Application of Large FDDI Networks
With an increasing number of dual FDDI rings, each one linked
together by two transparent bridges, the probability increases, that
one of these inhomogeneous dual FDDI rings is wrapped in the moment
when two dual MAC stations exchange ARP frames and DMARP replies.
If two dual MAC stations are communicating for the first time, the
probability decreases that a load sharing is really computed after
the exchange of DMARP replies, although this would be possible
according to the network configuration. It relies upon the fact,
that DMARP replies get to the primary ring over the wrapped dual ring
and only entries marked with the OR-bit set are created. To solve
this problem further expedients are invented:
At first, entries in the address cache can be marked read-only by the
setting of the R-bit. In dual MAC stations, entries can be written
manually for other dual MAC stations that are frequently talked to or
that have a special importance. The control bits of these entries
cannot be changed by DMARP.
Kuehn [Page 6]
RFC 1329 Address Resolution for Dual MAC FDDI Networks May 1992
Next, additional control bits are introduced. One of these bits is
the Hold-bit (H-bit). When two dual MAC stations exchange ARP frames
and DMARP replies to create entries in their address caches, one
station starts sending a DMARP reply, first. According to the
network state, it sends an additional DMARP error frame, a moment
later. Within a maximum period of time (see "Configuring the Timer
Parameters"), all frames arrive at the neighbour station and are
received by the primary and/or secondary MAC. If the OR-bit was not
set for an entry within this period of time, it is clear, that no
further DMARP frames will be received, which result in setting the
OR-bit. For such an entry the H-bit is set. As the reception of
reply and error frames is not sufficient for setting the OR-bit when
the H-bit is set, the load sharing is assumed to be sure. The
correctness of the H-bit will be verified in relatively long time
periods by queries (query and hold frames) at the station associated.
For two communicating stations there exists a possibility to get
information from a third station. Always, when the OR-bit is set for
an entry in a dual MAC station, a search frame is transmitted by the
secondary MAC, containing the own primary MAC address and the primary
MAC address of the counter station. If a third station can compute a
sure load sharing with both stations (the H-bit is set for the
associated entries), the stations can perform a load sharing between
them, too. The third station informs these stations by sending found
frames to them.
8.6. Multicast and Broadcast Addresses in IP Frames
If the destination hardware address of an IP frame is a multicast or
broadcast hardware address, the frame is always delivered to SNAP 1
and sent on the primary ring, because one of the addressed stations
could be a single MAC station. IP frames which are delivered to the
load sharing entity by SNAP 2 are discarded by the load sharing
entity. Thus, the duplication of these frames can be prevented.
9. Internal Structure
One load Sharing entity exists in the load sharing layer. This load
sharing entity consists of the address cache, the cache
administration and the multiplexer.
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RFC 1329 Address Resolution for Dual MAC FDDI Networks May 1992
to ARP to ARP
+----X----------------------------------------------------X--------+
| | | IP |
| | ARP frames read | frames |
| | entries | |
| +----------------------------+ +---------+ +----------+ |
| | Cache Administration |->-| Address |---->--| Multi- | |
| +----------------------------|->-| Cache | | plexer | |
| | | | | +---------+ | | |
| | | | | +----------+ |
| | ARP | DMARP | ARP | DMARP | | |
| | frames | frames | frames | frames IP | IP | |
| | | | | frames | frames | |
| | | | | | | |
+--X--------X--------X--------X-----------------------X--------X---+
to SNAP 1 to SNAP 2 to SNAP 1 to SNAP 2
9.1. The Address Cache
In the address cache, the associations of primary MAC addresses to
secondary MAC addresses are stored for other dual MAC stations on the
network. There are no entries for single MAC stations.
Because the OR- and the LS-bit (see table) always have inverted
values, one of the bits is redundant. Afterwards the examination of
an entry state gets easier by the introduction of both bits, they are
defined together. The ARP is able to support other protocol address
formats than the IP format. To support this ARP property by DMARP,
the protocol type number as used in the ARP frames is stored in every
entry of the address cache. So, a dual MAC station is able to
communicate with another station with DMARP, even if the other
station does not use IP. The numbers used in DMARP frames and the
address cache for the protocol type and the address length are taken
over from ARP.
name length comment
--------------------------------------------------------------------
P-MAC address 48 bit Address of the primary MAC
in an other dual MAC station
S-MAC address 48 bit Address of the secondary MAC
in that station
LS-bit 1 bit A load sharing can be performed
with that station
("Load sharing bit")
Kuehn [Page 8]
RFC 1329 Address Resolution for Dual MAC FDDI Networks May 1992
OR-bit 1 bit No load sharing may be done
with that station
("Other ring bit")
H-bit 1 bit The load sharing with that
station is trusty.
("Hold bit")
Q-bit 1 bit A query frame was sent to that
station, no hold frame was
received yet ("Query bit")
R-bit 1 bit This entry cannot be changed by
DMARP ("Read-only bit")
V-bit 1 bit The entry is valid
("Valid bit")
subscript 32 bit Unique number, identifying this
entry
protocol type 16 bit Number of the protocol type
that was last used in that
station
9.2. The Multiplexer
The multiplexer deals with multiplexing the IP frames upon the two
FDDI rings. Broadcast and multicast frames are always sent on the
primary ring. Otherwise, the contents of the address cache and a load
sharing criteria are used to decide on which of the rings an IP frame
has to be transmitted. If there is no entry for the primary MAC
address of the destination station in the cache, the IP frame is
transmitted on the primary ring. If there is an entry for the
destination station and the LS-bit is set, a load sharing can be done
with this station. Later on a load sharing criteria, which is beyond
the scope of this document, decides, which one of the rings is used
for transmission. An example for a load sharing criteria is the
length of the transmit queues in the MACs. The multiplexer requires an
abstract function only, which returns the appropriate ring for the
transmission of an actual IP frame.
Additionally, the multiplexer filters the received IP frames:
multicast or broadcast frames received from the secondary MAC are
discarded.
Kuehn [Page 9]
RFC 1329 Address Resolution for Dual MAC FDDI Networks May 1992
9.3. The Cache Administration
The cache administration creates and deletes the entries in the
address cache. For this purpose, it communicates with other load
sharing entities in other dual MAC stations with the DMARP. The
cache administration handles the delivery of ARP frames to the ARP
and the SNAP entity in the station, respectively.
The cache administration needs three timers for the communication with
the DMARP, which have to be supported by the system environment. Each
of these timers must support a timer entity for each entry in the
address cache, whereby a single one is running at a time.
Supported timer services:
TIMER_request(time, name, subscript)
TIMER_response(name, subscript)
TIMER_cancel(name, subscript):
A timer entity is started by the service TIMER_request and cancelled
by the TIMER_cancel service request. The TIMER_response service
indicates that a timer entity has run out. The parameter name is the
name of a timer: OR-Entry-Timer, Hold-Timer, or Query-Timer. Each
entry in the address cache is uniquely identified by a number
subscript). This number is also the number of an associated timer
entity. How to dispose these numbers is a question of
implementation. The parameter time determines the time period when
the timer runs out. This parameter has the value OR-set-timeout for
the OR-Entry-Timer, Hold-time for the Hold-Timer and Query-time for
the Query-Timer.
9.4. Configuring the Timer Parameters
The OR-set-timeout parameter for the OR-Entry-Timer
The period of time, determined by this parameter, should be
essentially longer than the maximum time for a frame to travel
around the entire network. The expression entire network means
the network which is constituted by the subnetworks linked
together with transparent bridges. When entries with OR-bit set
are created continuously for a dual MAC station by the timeout
mechanism, this parameter determines the periods of time between
the consecutive requests that are sent to this station. If the
state of the dual FDDI ring changes and an entry with LS-bit set
could be created, this parameter additionally determines the
maximum time until the new entry is created. (If an entry could
not be created by transmission of search frames.) Therefore, the
OR-set-timeout parameter should be set to some 10 seconds.
Kuehn [Page 10]
RFC 1329 Address Resolution for Dual MAC FDDI Networks May 1992
The Hold-time parameter for the Hold-Timer
The period of time, determined by this parameter, should as well
be essentially longer than the maximum time for a frame to travel
around the entire network. When two stations communicate for the
first time, they exchange ARP frames and DMARP replies. The
Hold-time parameter determines the period of time until the load
sharing is assumed to be accomplished after the setting of the
LS-bit. In this period of time, the frames mentioned above must
have reached its destination. If an entry would be marked with
the H-bit incorrectly, the time until it gets corrected will be
relatively long (Query time). Proposed dimension: several
minutes.
The Query-time parameter for the Query-Timer
When an entry is marked with LS- and H-bit it is assumed, that
load sharing can be performed with the associated station. To
allow the correction of a wrong value of the H-bit, the
correctness of the H-bit is tested in periods of time, determined
by the parameter Query-time. It is tested whether a frame is
received, which was sent by the secondary MAC to the secondary MAC
address of the target station. (The target station acknowledges
the reception of the query frame by a hold frame.) To limit the
traffic caused by the query and hold frames, the parameter Query-
time should be set to several minutes.
9.5. Format of DMARP Frames
fieldname length comment
--------------------------------------------------------------------
hardware type 16 bit 1 = "ethernet"
protocol type 16 bit 2048D = "Internet
Protocol"
length of hardware 8 bit Value in octets,
addresses 6 for 48 bit MAC addresses
length of protocol 8 bit Value in octets,
addresses 4 for Internet addresses
operation 16 bit 1: "reply"
2: "request"
3: "error"
4: "search"
5: "found"
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RFC 1329 Address Resolution for Dual MAC FDDI Networks May 1992
6: "query"
7: "hold"
1. hardware address ... octets
2. hardware address ... octets
protocol address ... octets
sender
protocol address ... octets
receiver
--------------------------------------------------------------------
The value for the field "protocol type" is the same as in ARP frames.
9.6. Contents of DMARP Frames
In the following tables of DMARP frames, the fields containing the
length and type of protocol and hardware addresses are omitted.
Format:
+-------------------------------------------------------------+
| Operation | 1. hardware | 2. hardware | protocol | protocol |
| | address | address | address | address |
| | | | sender | receiver |
+-------------------------------------------------------------+
Operation = 1 (reply), 2 (request), 3 (error):
+-----------------------------------------------------------------+
| Operation | P-MAC address | S-MAC address | protocol | protocol |
| | sender | sender | address | address |
| | | | sender | receiver |
+-----------------------------------------------------------------+
+-------------------------------------------------------------------+
| Operation=4 | P-MAC | P-MAC address | protocol | broadcast |
| (search) | address | counter- | address | protocol |
| | sender | station | sender | address |
+-------------------------------------------------------------------+
+-------------------------------------------------------------------+
| Operation=5 | P-MAC | S-MAC address | protocol | broadcast |
| (found) | address | counter- | address | protocol |
| | sender | station | sender | address |
+-------------------------------------------------------------------+
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RFC 1329 Address Resolution for Dual MAC FDDI Networks May 1992
+-------------------------------------------------------------------+
| Operation=6 | S-MAC | P-MAC address | protocol | broadcast |
| (query) | address | counter- | address | protocol |
| | sender | station | sender | address |
+-------------------------------------------------------------------+
+-------------------------------------------------------------------+
| Operation=7 | P-MAC address | S-MAC address | protocol | protocol |
| (hold) | sender | sender | address | address |
| | | | sender | receiver |
+-------------------------------------------------------------------+
Apart from the error frames all frames are sent on the secondary
ring. The reply, error and search frames are addressed to the
broadcast hardware address. The request, found, query and hold
frames are addressed to an individual secondary MAC address.
10. Formal Description
The following description is written in ESTELLE.
10.1. Global Constants, Variables and Types
default individual queue;
timescale ...;
type
PDU_type = ... ; (* format of a Protocol Data Unit:
String of variable length *)
HW_addr_type = ... ; (* format of a 48 bit MAC address *)
PR_addr_type = ... ; (* General: format of a protocol address
in an ARP or DMARP frame *)
IP_addr_type = ... ; (* General: format of an IP address *)
QoS_type = ... ; (* General: format of a Quality-of-
-Service statement *)
timer_name_type = ... ; (* Type for the name of a system timer *)
flag = (reset,set);
var
(*
The values of these variables are set in the initialization part or
by external management functions.