Network Working Group J. Renwick
Request for Comments: 2067 NetStar, Inc.
Category: Standards Track January 1997
Obsoletes: 1374
IP over HIPPI
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.
Abstract
ANSI Standard X3.218-1993 (HIPPI-LE[3]) defines the encapsulation of
IEEE 802.2 LLC PDUs and, by implication, IP on HIPPI. ANSI X3.222-
1993 (HIPPI-SC[4]) describes the operation of HIPPI physical
switches. The ANSI committee responsible for these standards chose
to leave HIPPI networking issues largely outside the scope of their
standards; this document describes the use of HIPPI switches as IP
local area networks.
This memo is a revision of RFC1374, "IP and ARP on HIPPI", and is
intended to replace it in the Standards Track. RFC1374 has been a
Proposed Standard since November, 1992, with at least 10
implementations of IP encapsulation and HIPPI switch discipline. No
major changes to it are required. However, the ARP part of RFC1374
has not had sufficient implementation eXPerience to be advanced to
Draft Standard. The present document contains all of RFC1374 except
for the description ARP, which has been moved into a separate
document.
TABLE OF CONTENTS
1 IntrodUCtion............................................. 2
2 Scope.................................................... 3
2.1 Changes from RFC1374.............................. 3
2.2 Terminology........................................ 4
3 Definitions.............................................. 4
4 Equipment................................................ 5
5 Protocol ................................................ 7
5.1 Packet Format...................................... 7
5.2 48 bit Universal LAN MAC addresses................. 11
5.3 I-Field Format..................................... 12
5.4 Rules For Connections.............................. 13
5.5 MTU................................................ 15
6 Camp-on ................................................. 16
7 Path MTU Discovery....................................... 17
8 Channel Data Rate Discovery.............................. 17
9 Performance.............................................. 18
10 Sharing the Switch....................................... 20
11 References............................................... 21
12 Security Considerations.................................. 21
13 Author"s Address......................................... 21
14 Appendix A -- HIPPI Basics............................... 22
15 Appendix B -- How to Build a Practical HIPPI LAN......... 27
1 Introduction
The ANSI High-Performance Parallel Interface (HIPPI) is a simplex
data channel. Configured in pairs, HIPPI can send and receive data
simultaneously at nearly 800 megabits per second. (HIPPI has an
equally applicable 1600 megabit/second option.) Between 1987 and
1991, the ANSI X3T9.3 HIPPI working group drafted four documents that
bear on the use of HIPPI as a network interface. They cover the
physical and electrical specification (HIPPI-PH [1]), the framing of
a stream of bytes (HIPPI-FP [2]), encapsulation of IEEE 802.2 LLC
(HIPPI-LE [3]), and the behavior of a standard physical layer switch
(HIPPI-SC [4]). HIPPI-LE also implies the encapsulation of Internet
Protocol[5]. The reader should be familiar with the ANSI HIPPI
documents, copies of which are archived at the site "FTP.network.com"
in the Directory "hippi", and may be oBTained via anonymous FTP.
HIPPI switches can be used to connect a variety of computers and
peripheral equipment for many purposes, but the working group stopped
short of describing their use as Local Area Networks. This memo
takes up where the working group left off, using the guiding
principle that except for length and hardware header, Internet
datagrams sent on HIPPI should be identical to the same datagrams
sent on a conventional network, and that any datagram sent on a
conventional 802 network[6] should be valid on HIPPI.
2 Scope
This memo describes the HIPPI interface between a host and a
crosspoint switch that complies with the HIPPI-SC draft standard.
Issues that have no impact on host implementations are outside the
scope of this memo. Host implementations that comply with this memo
are believed to be interoperable on a network composed of a single
HIPPI-SC switch. They are also interoperable on a simple point-to-
point, two-way HIPPI connection with no switch between them. They
may be interoperable on more complex networks as well, depending on
the internals of the switches and how they are interconnected;
however, these details are implementation dependent and outside the
scope of this memo.
Within the scope of this memo are:
1. Packet format and header contents, including HIPPI-FP, HIPPI-
LE, IEEE 802.2 LLC[7] and SNAP.
2. I-Field contents
3. Rules for the use of connections.
Outside of the scope are
1. Address Resolution (ARP)
2. Network configuration and management
3. Host internal optimizations
4. The interface between a host and an outboard protocol
processor.
2.1 Changes from RFC1374
RFC1374 described the use of ARP on HIPPI, but because of
insufficient implementation experience, the description of ARP has
been separated from IP encapsulation and moved to an Informational
memo. It may be returned to the standards track in the future if
interest and implementations warrant it.
RFC1374"s specification of IP over HIPPI has been changed in this
document. Certain packet format options, permitted in RFC1374, are
no longer allowed:
1. Optional short burst first;
2. D1 fill bytes;
3. Nonzero D2 offset.
That is, the header format is no longer variable and is required to
be that which is recommended by RFC1374.
With these changes, it is possible to send packets which conform to
the ANSI standards but not to this memo. Because there are no RFC
1374 implementations in use that used these options, we believe that
all existing RFC1374 implementations are compliant with the
requirements of this memo, and there should be no interoperability
problems associated with these changes.
2.2 Terminology
In this document the use of the Word SHALL in capital letters
indicates mandatory points of compliance.
3 Definitions
Conventional
Used with respect to networks, this refers to Ethernet, FDDI and
802 LAN types, as distinct from HIPPI-SC LANs.
Destination
The HIPPI implementation that receives data from a HIPPI Source.
Node
An entity consisting of one HIPPI Source/Destination pair that is
connected by parallel or serial HIPPI to a HIPPI-SC switch and
that transmits and receives IP datagrams. A node may be an
Internet host, bridge, router or gateway. This memo uses the term
node in place of the usual "host" to indicate that a host might be
connected to the HIPPI LAN not directly, but through an external
adaptor that does some of the protocol processing for the host.
Serial HIPPI
An implementation of HIPPI in serial fashion on coaxial cable or
optical fiber, informally standardized by implementor"s agreement
in the Spring of 1991.
Switch Address
A value used as the address of a node on a HIPPI-SC network. It
is transmitted in the I-field. HIPPI-SC switches may map Switch
Addresses to physical port numbers.
Source
The HIPPI implementation that generates data to send to a HIPPI
Destination.
Universal LAN Address (ULA)
A 48 bit globally unique address, administered by the IEEE,
assigned to each node on an Ethernet, FDDI, 802 network or HIPPI-
SC LAN.
4 Equipment
A HIPPI network can be composed of nodes with HIPPI interfaces, HIPPI
cables or serial links, HIPPI-SC switches, gateways to other
networks.
Each HIPPI interconnection between a node and a switch SHALL consist
of a pair of HIPPI links, one in each direction.
If a link between a node and the switch is capable of the 1600
Megabit/second data rate option (i.e. Cable B installed for 64 bit
wide operation) in either direction, the node"s HIPPI-PH
implementation SHALL also be capable of 32 bit operation (Cable B
data suppressed) and SHALL be able to select or deselect the 1600Mb/s
data rate option at the establishment of each new connection.
The following figure shows a sample HIPPI switch configuration.
+-----+
H 4
+--+--+
+----+ +----+ +----+
H1 H2 H3 +-++
+--+ +-++-+ +-++-+ +-++-+ PP
+---+H5 ++++
+--+
+---++--------++--------++------++----+

+----+ HIPPI-SC
+---+ G1 +--------+
+--------+ Switch
+----+
+---++--------++--------++------++----+
+--+
+---+H6 ++++
+--+ +-++-+ PP
+-++
G2
+--+--+
+--+-+ H 7
+-----+

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


+--+--+ +--+--+ +--+--+ +--+--+
H 8 H 9 H10 H11
+-----+ +-----+ +-----+ +-----+
Legend: ---+---+---+-- = 802 network, Ethernet or FDDI
= Paired HIPPI link
H = Host computer
PP = Outboard Protocol Processor
G = Gateway
A possible HIPPI configuration
A single HIPPI-SC switch has a "non-blocking" characteristic, which
means there is always a path available from any Source to any
Destination. If the network consists of more than one switch, the
path from a Source to a Destination may include a HIPPI link between
switches. If this link is used by more than one Source/Destination
pair, a "blocking" network is created: one Source may be blocked from
Access to a Destination because another Source is using the link it
shares. Strategies for establishing connections may be more
complicated on blocking networks than on non-blocking ones.
This memo does not take blocking issues into account, assuming that
the HIPPI LAN consists of one HIPPI-SC switch or, if the network is
more complex than that, it presents no additional problems that a
node must be aware of.
5 Protocol
5.1 Packet Format
The HIPPI packet format for Internet datagrams SHALL conform to the
HIPPI-FP and HIPPI-LE draft standards, with further restrictions as
imposed by this memo. Because this memo is more restrictive than the
ANSI standards, it is possible to send encapsulated IP datagrams that
conform to the ANSI standards, but are illegal according to this
memo. Destinations may either accept or ignore such datagrams.
To summarize the additional restrictions on ANSI standards found
here:
Any short burst must be the last burst of the packet.
Leading short bursts are not permitted.
Nonzero values for the HIPPI-FP D2_Offset field are not
permitted.
The D1_AreaSize SHALL be 3 (64-bit words). No D1 Fill is
permitted.
Note: Although this document is for IP over HIPPI, the encapsulation
described below accommodates ARP as well.
The HIPPI-FP D1_Area SHALL contain the HIPPI-LE header. The HIPPI-FP
D2_Area, when present, SHALL contain one IEEE 802.2 Type 1 LLC
Unnumbered Information (UI) PDU. Support of IEEE 802.2 XID, TEST and
Type 2 PDUs is not required on HIPPI, and Destinations that receive
these PDUs may either ignore them or respond correctly according to
IEEE 802.2 requirements.
The length of a HIPPI packet, including trailing fill, SHALL be a
multiple of eight bytes as required by HIPPI-LE.
+----------+-----------+---------------------+----------- ------+
0 - 7
HIPPI-FP HIPPI-LE IEEE 802.2 LLC/SNAP IP . . . bytes
(8 bytes) (24 bytes) (8 bytes) fill
+----------+-----------+---------------------+----------- ------+
HIPPI Packet Structure
ULP-id (8 bits) SHALL contain 4.
D1_Data_Set_Present (1 bit) SHALL be set.
Start_D2_on_Burst_Boundary (1 bit) SHALL be zero.
Reserved (11 bits) SHALL contain zero.
D1_Area_Size (8 bits) SHALL be sent as 3.
D2_Offset (3 bits) SHALL be zero.
D2_Size (32 bits) Shall contain the number of bytes in the
IEEE 802.2 LLC Type 1 PDU, or zero if no PDU is present. It
SHALL NOT exceed 65,288. This value includes the IEEE 802.2
LLC/SNAP header and the IP datagram. It does not include
trailing fill bytes. (See "MTU", below.)
HIPPI-LE Header
FC (3 bits) SHALL contain zero unless otherwise defined by local
administration.
Double_Wide (1 bit) SHALL contain one if the Destination associated
with the sending Source supports 64 bit HIPPI operation. Otherwise
it SHALL contain zero.
Message_Type (4 bits) contains a code identifying the type of HIPPI-
LE PDU. Defined values are:
0 Data PDU
1 Address Resolution Request PDU (AR_Request)
2 Address Resolution Response PDU (AR_Response)
3 Self Address Resolution Request PDU (AR_S_Request)
4 Self Address Resolution Response PDU (AR_S_Response)
Destination_Switch_Address is a 24-bit field containing the
Switch Address of the Destination if known, otherwise zero.
If the address comprises less than 24 bits, it SHALL be right
justified (occupying the least significant bits) in the
field.
Destination_Address_Type (4 bits) and Source_Address_Type (4
bits) contain codes identifying the type of addresses in the
Destination_Switch_Address and Source_Switch_Address fields
respectively. Defined values (binary) are:
0 Unspecified
1 HIPPI-SC Source Route (24 bits)
2 HIPPI-SC Address (12 bits)
Source_Switch_Address is a 24-bit field containing the Switch
Address of the Source. If the address comprises less than 24
bits, it SHALL be right justified (occupying the least
significant bits) in the field.
Reserved (16 bits) SHALL contain zero.
Destination_IEEE_Address (48 bits) SHALL contain the 48 bit
Universal LAN MAC Address of the Destination if known,
otherwise zero.
LE_Locally_Administered (16 bits) SHALL contain zero UNLESS
otherwise defined by local administration.
Source_IEEE_Address (48 bits) SHALL contain the 48 bit
Universal LAN MAC Address of the Source if known, otherwise
zero.
IEEE 802.2 LLC
The IEEE 802.2 LLC Header SHALL begin in the first byte of the
HIPPI-FP D2_Area.
SSAP (8 bits) SHALL contain 170 ("AA"h).
DSAP (8 bits) SHALL contain 170 ("AA"h).
CTL (8 bits) SHALL contain 3 (Unnumbered Information).
SNAP
Organization Code (24 bits) SHALL be zero.
EtherType (16 bits) SHALL be set as defined in Assigned Numbers [8]:
IP = 2048 ("0800"h), ARP = 2054 ("0806"h), RARP = 32,821 ("8035"h).
31 28 23 21 15 10 7 2 0
+-----+---------+-+-+-----------+---------+-----+---------+-----+
0 04 10 Reserved 03 0
+---------------+-+-+---------------------+---------------+-----+
1 (n+8)
+-----+-+-------+-----------------------------------------------+
2 [LA] WM_Type Destination_Switch_Address
+-----+-+-------+-----------------------------------------------+
3 D_A_T S_A_T Source_Switch_Address
+-------+-------+---------------+-------------------------------+
4 Reserved [Destination_IEEE_Address]
+-------------------------------+
5
+-------------------------------+-------------------------------+
6 [LA] [Source_IEEE_Address]
+-------------------------------+
7
+---------------+---------------+---------------+---------------+
8 AA AA 03 00
+---------------+---------------+---------------+---------------+
9 00 00 [EtherType]
+---------------+---------------+---------------+---------------+
10 Message byte 0 Message byte 1 Message byte 2 . . .
+---------------+---------------+---------------+---
. . .

-------+---------------+---------------+---------------+
. . . byte (n-2) byte (n-1) FILL
+---------------+---------------+---------------+---------------+
N-1 FILL FILL FILL FILL
+---------------+---------------+---------------+---------------+
HIPPI Packet Format
Words 0-1: HIPPI-FP Header
Words 2-7: D1 Area (HIPPI-LE Header)
Words 8-9: D2 Area (IEEE 802.2 LLC/SNAP)
Words 10-(N-1): D2 Area (IP message)
(n) is the number of bytes in the IP message.
[LA] fields are zero unless used otherwise locally.
Abbreviations: "W" = Double_Wide field;
"M_Type" = Message_Type field;
"D_A_T" = Destination_Address_Type;
"S_A_T" = Source_Address_Type;
[FILL] bytes complete the HIPPI packet to an even
number of 32 bit words. The number of fill bytes
is not counted in the data length.
IEEE 802.2 Data
The IEEE 802.2 Data SHALL begin in the byte following the EtherType
field. Fill bytes SHALL be used following the Data as necessary to
make the number of bytes in the packet a multiple of 8. In
accordance with HIPPI-FP, the amount of this fill is not included in
the D2_Size value in the HIPPI- FP Header.
The order of the bytes in the data stream is from higher numbered to
lower numbered data signal (left to right) within the HIPPI word, as
specified in HIPPI-FP Clause 7, "Word and byte formats." With the
1600 megabit/second data rate option (64 bit) bits 32 through 63 are
on Cable B, so that the four bytes on Cable B come logically before
those on Cable A. Within each byte, the most significant bit is the
highest numbered signal.
5.2 48 bit Universal LAN MAC Addresses
IEEE Standard 802.1A specifies the Universal LAN MAC Address. The
globally unique part of the 48 bit space is administered by the IEEE.
Each node on a HIPPI-SC LAN should be assigned a ULA. Multiple ULAs
may be used if a node contains more than one IEEE 802.2 LLC protocol
entity.
The format of the address within its 48 bit HIPPI-LE fields follows
IEEE 802.1A canonical bit order and HIPPI-FP bit and byte order:
31 23 15 7 0
+-------------------------------+---------------+---------------+
(not used for ULA) ULA byte 0 LG ULA byte 1
+---------------+---------------+---------------+---------------+
ULA byte 2 ULA byte 3 ULA byte 4 ULA byte 5
+---------------+---------------+---------------+---------------+
Universal LAN MAC Address Format
L (U/L bit) = 1 for Locally administered addresses, 0 for
Universal.
G (I/G bit) = 1 for Group addresses, 0 for Individual.
The use of ULAs is optional, but encouraged. Although ULAs are not
used by HIPPI-SC switches, they may be helpful for HIPPI Switch
Address resolution, and for distinguishing between multiple logical
entities that may exist within one node. They may also be used by
gateway devices that replace HIPPI hardware headers with the MAC
headers of other LANs. Carrying the ULAs in the HIPPI header may
simplify these devices, and it may also help if HIPPI is used as an
interface to some future HIPPI based LAN that uses ULAs for
addressing.
5.3 I-Field format
fi The I-field bits, as defined in HIPPI-SC, SHALL be set as follows:
Locally Administered (bit 31) SHALL be zero.
Reserved (bits 30, 29) should be zero. Destinations SHALL
accept any value for these bits.
Double wide (bit 28) SHALL be set when Source Cable B is
connected and the Source wants a 64 bit connection. It SHALL
be zero otherwise.
Direction (bit 27) should be sent as zero, however
Destinations SHALL accept either zero or one and interpret
the Routing Control field accordingly, per HIPPI-SC.
Path Selection (bits 26, 25) SHALL be 00, 01, or 11 (binary)
at the Source"s option. 00 (source route mode) indicates
that the I-field bits 23-00 contain a 24 bit source route; 01
or 11 (logical address mode) indicate that bits 23-00 contain
12 bit Source and Destination Addresses. The value 11 is
meaningful when more than one route exists from a Source to a
Destination; it allows the switch to choose the route. Use
of 01 forces the switch always to use the same route for the
same Source/Destination pair.
Camp-on (bit 24) may be 1 or 0; however, a Source SHALL NOT
make consecutive requests without Camp-on to the same
Destination while the requests are being rejected. The
purpose of this restriction is to prevent a node from
circumventing the fair share arbitration mechanism of the
switch by repeating requests at a very high rate.
If logical address mode is used:
Source Address (bits 23-12) is not used.
Destination Address (bits 11-0) SHALL contain the Switch
Address of the Destination.
If source route mode is used:
Routing control (bits 23-00) SHALL contain the route to
the Destination.
5.4 Rules For Connections
The following rules for connection management by Source and
Destination are intended to insure frequent, fair share access to
Destinations for which multiple Sources are contending. If possible,
nodes should transfer data at full HIPPI speeds and hold connections
no longer than necessary.
A source may hold a connection for as long as it takes to send 68
HIPPI bursts at what ever speed the two connected nodes can achieve
together. The number of packets sent in one connection is not
limited, except that the number of bursts over all the packets should
not exceed 68. This is not a recommendation to send as many packets
as possible per connection; one packet per connection is acceptable.
The purpose of this limit is to give each Source an fair share of a
common Destination"s bandwidth. Without a limit, if there is a
Destination that is constantly in demand by multiple Sources, the
Source that sends the most data per connection wins the greatest
share of bandwidth.
The limit of 68 bursts is not absolute. An implementation may check
the burst count after transmission of a packet and end the connection
if it is greater than or equal to some threshold. If this is done,
the threshold should be less than 68 depending on the typical packet
size, to ensure that the 68 burst limit is not normally exceeded.
For instance, a Source sending 64K packets would send two per
connection (130 bursts) if it checked for 68 at the end of each
packet. In this situation the Source is required to check for a
value small enough that it will not send a second packet in the same
connection.
Destinations SHALL accept all packets that arrive during a
connection, and may discard those that exceed its buffering capacity.
A Destination SHALL NOT abort a connection (deassert CONNECT) simply
because too many bursts were received; however a Destination may
abort a connection whose duration has exceeded a time period of the
Destination"s choosing, as long as the Source is allowed ample time
to transmit its quota of bursts.
The rules admonish the node to do certain things as fast as it can,
however there is no absolute measure of compliance. Nodes that
cannot transfer data at full HIPPI speeds can still interoperate but
the faster the implementation, the better the performance of the
network will be.
Assuming that bursts flow at the maximum rate, the most important
factor in network throughput is the connection switching time,
measured from the deassertion of REQUEST by the Source at the end of
one connection to its first assertion of BURST after the
establishment of the new connection.
Implementations should keep this time as short as possible. For a
guideline, assuming parallel HIPPI and a single HIPPI-SC switch, ten
microseconds permits nearly full HIPPI throughput with full-sized
packets, and at 60 microseconds the available throughput is reduced
by about 10%. (See "Performance", below.)
All HIPPI electrical signaling SHALL comply with HIPPI-PH. In every
case, the following rules go beyond what HIPPI-PH requires.
Rules for the Source
1. Do not assert REQUEST until a packet is ready to send.
2. Transmit bursts as quickly as READYs permit. Except for
the required HIPPI Source Wait states, there should be no
delay in the assertion of BURST whenever the Source"s READY
counter is nonzero.
3. Make a best effort to ensure that connection durations do
not exceed 68 bursts.
4. Deassert REQUEST immediately when no packet is available
for immediate transmission or the last packet of the
connection has been sent.
Rules for the Destination
1. Reject all connections if unable to receive packets.
This frees the requesting Source to connect to other
Destinations with a minimum of delay. Inability to receive
packets is not a transient condition, but is the state of the
Destination when its network interface is not initialized.
2. A HIPPI node should be prepared to efficiently accept
connections and process incoming data packets. While this
may be best achieved by not asserting connect unless 68
bursts worth of buffers is available, it may be possible to
meet this requirement with fewer buffers. This may be due to
a priori agreement between nodes on packet sizes, the speed
of the interface to move buffers, or other implementation
dependent considerations.
3. Accept a connection immediately when buffers are
available. The Destination should never delay the acceptance
of a connection unnecessarily.
4. Once initialized, a Destination may reject connection
requests only for one of the following reasons:
1. The I-field was received with incorrect parity.
2. The I-field contents are invalid, e.g. the "W" bit set when the
Destination does not support the 1600 megabit data rate option,
the "Locally Administered" bit is set, the Source is not
permitted to send to this Destination, etc.
Transient conditions within the Destination, such as temporary
buffer shortages, must never cause rejected connections.
5. Ignore aborted connection sequences. Sources may time
out and abandon attempts to connect; therefore aborted
connection sequences are normal events.
5.5 MTU
Maximum Transmission Unit (MTU) is defined as the length of the IP
packet, including IP header, but not including any overhead below IP.
Conventional LANs have MTU sizes determined by physical layer
specification. MTUs may be required simply because the chosen medium
won"t work with larger packets, or they may serve to limit the amount
of time a node must wait for an opportunity to send a packet.
HIPPI has no inherent limit on packet size. The HIPPI-FP header
contains a 32 bit D2_Size field that, while it may limit packets to
about 4 gigabytes, imposes no practical limit for networking
purposes. Even so, a HIPPI-SC switch used as a LAN needs an MTU so
that Destination buffer sizes can be determined.
The MTU for HIPPI-SC LANs is 65280 bytes.
This value was selected because it allows the IP packet to fit in one
64K byte buffer with up to 256 bytes of overhead. The overhead is 40
bytes at the present time; there are 216 bytes of room for expansion.
HIPPI-FP Header 8 bytes
HIPPI-LE Header 24 bytes
IEEE 802.2 LLC/SNAP Headers 8 bytes
Maximum IP packet size (MTU) 65280 bytes
------------
Total 65320 bytes (64K - 216)
6 Camp-on
When several Sources contend for a single Destination, the Camp-on
feature allows the HIPPI-SC switch to arbitrate and ensure that all
Sources have fair access. (HIPPI-SC does not specify the method of
arbitration.) Without Camp-on, the contending Sources would simply
have to retry the connection repeatedly until it was accepted, and
the fastest Source would usually win. To guarantee fair share
arbitration, Sources are prohibited from making repeated requests to
the same Destination without Camp-on in such a way as to defeat the
arbitration.
There is another important reason to use Camp-on: when a connection
without Camp-on is rejected, the Source cannot determine whether the
rejection came from the requested Destination or from the switch.
The Source also cannot tell the reason for the rejection, which could
be either that the Destination was off line or not cabled, or the I-
field was erroneous or had incorrect parity. Sources should not
treat a rejection of a request without Camp-on as an error. Camp-on
prevents rejection due to the temporary busy case; with one
exception, rejection of a Camp-on request indicates an error
condition, and an error event can be recorded. The exception occurs
when a 64 bit connection is attempted to a Destination that does not
have Cable B connected, resulting in a reject. This case is covered
in "Channel Data Rate Discovery", below.
7 Path MTU Discovery
RFC1191 [9] describes the method of determining MTU restrictions on
an arbitrary network path between two hosts. HIPPI nodes may use
this method without modification to discover restrictions on paths
between HIPPI-SC LANs and other networks. Gateways between HIPPI-SC
LANs and other types of networks should implement RFC1191.
8 Channel Data Rate Discovery
HIPPI exists in two data rate options (800 megabit/second and 1600
megabit/second). The higher data rate is achieved by making the
HIPPI 64 bits parallel instead of 32, using an extra cable containing
32 additional data bits and four parity bits. HIPPI-SC switches can
be designed to attach to both. Source and Destination HIPPI
implementations can be designed to operate at either rate, selectable
at the time a connection is established. The "W" bit (bit 28) of the
I-field controls the width of the connection through the switch.
Sources with both cables A and B attached to the switch may set the
"W" bit to request a 1600 megabit/second connection. If the
requested destination also has both cables attached, the switch can
connect Source to Destination on both cables. If the requested
Destination has only Cable A, the switch rejects the request.
Sixty-four bit Sources can connect to 32 bit Destinations by
requesting with the "W" bit clear and not using Cable B. Sixty-four
bit Destinations must examine the "W" bit in the received I-field and
use or ignore Cable B accordingly. Note that both INTERCONNECT
signals stay active while a 64 bit HIPPI is used in 32 bit mode.
The following table summarizes the possible combinations, the
switch"s action for each, and the width of the resulting connection.
Destination
+-------------------+-------------------+
32 64
+----+-----+-------------------+-------------------+
W=0 Accept 32 Accept 32
32 +-----+-------------------+-------------------+
W=1 N/A N/A
Source +----+-----+-------------------+-------------------+
W=0 Accept 32 Accept 32
64 +-----+-------------------+-------------------+
W=1 Reject Accept 64
+----+-----+-------------------+-------------------+
HIPPI Connection Combinations
If the path between a 64 bit Source and a 64 bit Destination includes
more than one switch, and the route between switches uses a link that
is only 32 bits wide, the switch rejects 64 bit connection requests
as if the Destination did not have 64 bit capability.
In a mixed LAN of 32 bit and 64 bit HIPPIs, a 64 bit Source needs to
know the data rates available at each Destination and on the path to
it. This can be known a priori by manual configuration, or it can be
discovered dynamically. The only reliable method of discovery is
simply to attempt a 64 bit connection with Camp-on. As long as 64
bit connections succeed, the Source knows the Destination and path
are double width. If a 64 bit connection is rejected, the Source
tries to connect for 32 bits. If the 32 bit connection succeeds, the
Source assumes that the Destination or path is not capable of double
width operation, and uses only 32 bit requests after that. If the 32
bit request is rejected, the Source assumes that the Destination or
path is down and makes no determination of its capability.
The Double_Wide bit in the HIPPI-LE header, if nonzero, gives the
node that receives it a hint that the 64 bit connection attempt may
be worthwhile when sending on the return path.
Note that Camp-on must be used at least in the 64 bit attempt,
because it removes some ambiguity from the meaning of rejects. If
the request is made with the "W" bit and no Camp-on, a reject could
mean either that the Destination has no Cable B or that it is simply
busy, and no conclusion can be drawn as to its status for 64 bit
connections.
9 Performance
The HIPPI connection rules are designed to permit best utilization of
the available HIPPI throughput under the constraint that each
Destination must be made available frequently to receive packets from
different Sources. This discipline asks both Sources and
Destinations to minimize connection setup overhead to deliver high
performance. Low connection setup times are easily achieved by
hardware implementations, but overhead may be too high if software is
required to execute between the initial request of a connection and
the beginning of data transfer. Hardware implementations in which
connection setup and data transfer proceed from a single software
action are very desirable.
HIPPI connections are controlled by HIPPI Sources; a Destination,
being unable to initiate a disconnect without the possibility of data
loss, is a slave to the Source once it has accepted a connection.
Optimizations of connection strategy are therefore the province of
the HIPPI Source, and several optimizations are permitted.
If the rate of available message traffic is less than the available
HIPPI throughput and Destinations are seldom busy when a connection
is requested, connection optimizations do not pay off and the
simplest strategy of waiting indefinitely for each connection to be
made and sending messages strictly in the order queued cannot be
improved upon. However if some nodes are slow, or network
applications can send or receive messages at a higher aggregate rate
than the available HIPPI bandwidth, Sources may frequently encounter
a busy Destination. In these cases, certain host output queuing
strategies may enhance channel utilization. Sources may maintain
separate output queues for different HIPPI Destinations, and abandon
one Destination in favor of another if a connection attempt without
Camp-on is rejected or a connection request with Camp-on is not
accepted within a predetermined interval. Such a strategy results in
aborted connection sequences (defined in HIPPI-PH: REQUEST is
deasserted before any data is sent). Destinations must treat these
as normal events, perhaps counting them but otherwise ignoring them.
Two components of connection setup time are out of the control of
both Source and Destination. One is the time required for the switch
to connect Source to Destination, currently less than four
microseconds in the largest commercially available (32 port) switch.
The second component is the round trip propagation time of the
REQUEST and CONNECT signals, negligible on a standard 25 meter copper
HIPPI cable, but contributing a total of about 10 microseconds per
kilometer on fiber optic links. HIPPI-SC LANs spanning more than a
few kilometers will have reduced throughput. Limited span networks
with buffered gateways or bridges between them may perform better
than long serial HIPPI links.
A Source is required to drop its connection after the transmission of
68 HIPPI bursts. This number was chosen to allow the transmission of
one maximum sized packet or a reasonable number of smaller sized
packets. The following table lists some possibilities, with
calculated maximum burst and throughput rates in millions (10**6) of
bytes per second:
Maximum HIPPI Throughput Rates
Number Number Hold Burst ------Max throughput MB/sec-------
User of of Time Rate Connection Setup Overhead (usec)
Data Packets Bursts (usec) MB/sec 10 30 60 90 120 150
---- ------- ------ ------ ------ ---- ---- ---- ---- ---- ----
63K 1 64 654 98.7 97.2 94.4 90.4 86.8 83.4 80.3
32K 2 66 665 98.6 97.1 94.3 90.4 86.8 83.5 80.4
16K 4 68 667 98.3 96.8 94.1 90.2 86.6 83.3 80.2
8K 7 63 587 97.8 96.1 93.0 88.7 84.8 81.2 77.8
4K 13 65 551 96.7 95.0 91.7 87.2 83.1 79.4 76.0
2K 22 66 476 94.6 92.7 89.0 84.0 79.6 75.6 72.0
1K 34 68 384 90.8 88.5 84.2 78.5 73.5 75.8 65.3
These calculations are based 259 40 ns clock periods to transmit a
full burst and 23 clock periods for a short burst. (HIPPI-PH
specifies three clock periods of overhead per burst.) A packet of "n"
kilobytes of user data consists of "n" full bursts and one short
burst equal in length to the number of bytes in the HIPPI, LLC, IP
and TCP headers. "Hold Time" is the minimum connection duration
needed to send the packets. "Burst Rate" is the effective transfer
rate for the duration of the connection, not counting connection
switching time. Throughput rates are in megabytes/second, accounting
for connection switching times of 10, 30, 60, 90, 120 and 150
microseconds. These calculations ignore any limit on the rate at
which a Source or Destination can process small packets; such limits
may further reduce the available throughput if small packets are
used.
10 Sharing the Switch
Network interconnection is only one potential application of HIPPI
and HIPPI-SC switches. While network applications need very frequent
transient connections, other applications may favor longer term or
even permanent connections between Source and Destination. Since the
switch can serve each Source or Destination with hardware paths
totally separate from every other, it is quite feasible to use the
same switch to support LAN interconnects and computer/peripheral
applications simultaneously.
Switch sharing is no problem when unlike applications do not share a
HIPPI cable on any path. However if a host must use a single input
or output cable for network as well as other kinds of traffic, or if
a link between switches must be shared, care must be taken to ensure
that all applications are compatible with the connection discipline
described in this memo. Applications that hold connections too long
on links shared with network traffic may cause loss of network
packets or serious degradation of network service.
11 References
[1] ANSI X3.183-1991, High-Performance Parallel Interface -
Mechanical, Electrical and Signalling Protocol Specification
(HIPPI-PH).
[2] ANSI X3.210-1992, High-Performance Parallel Interface - Framing
Protocol (HIPPI-FP).
[3] ANSI X3.218-1993, High-Performance Parallel Interface -
Encapsulation of IEEE 802.2 (IEEE Std 802.2) Logical Link
Control Protocol Data Units (802.2 Link Encapsulation) (HIPPI-
LE).
[4] ANSI X3.222-1993, High-Performance Parallel Interface - Physical
Switch Control (HIPPI-SC).
[5] Postel, J., "Internet Protocol", STD 5, RFC791, USC/Information
Sciences Institute, September 1981.
[6] IEEE, "IEEE Standards for Local Area Networks: Logical Link
Control", IEEE, New York, New York, 1985.
[7] IEEE, "IEEE Standards for Local Area Networks: Logical Link
Control", IEEE, New York, New York, 1985.
[8] Reynolds, J.K., and Postel, J., "Assigned Numbers", STD 2, RFC
1340, USC/Information Sciences Institute, July 1992.
[9] Mogul, J.C., and Deering, S.E., "Path MTU discovery", RFC1191,
Stanford University, November, 1990.
12 Security Considerations
Security issues are not discussed in this memo.
13 Author"s Address
John K. Renwick
NetStar, Inc.
10250 Valley View Road
Minneapolis, MN USA 55344
Phone: (612) 996-6847
EMail: jkr@NetStar.com
Mailing List: hippi-ext@think.com
14 Appendix A -- HIPPI Basics
This section is included as an aid to readers who are not completely
familiar with the HIPPI standards.
HIPPI-PH describes a parallel copper data channel between a Source
and a Destination. HIPPI transmits data in one direction only, so
that two sets are required for bidirectional flow. The following
figure shows a simple point-to-point link between two computer
systems:
+----------+ +----------+

+--------+ +--------+
HIPPI Cable HIPPI
+--------------------->
Source Dest.
System +--------+ +--------+ System
X +--------+ +--------+ Y
HIPPI Cable HIPPI
<---------------------+
Dest. Source
+--------+ +--------+

+----------+ +----------+
A Simple HIPPI Duplex Link
Parallel copper cables may be up to 25 meters in length.
In this document, all HIPPI connections are assumed to be paired
HIPPI channels.
HIPPI-PH has a single optional feature: it can use a single cable in
each direction for a 32 bit parallel channel with a maximum data rate
of 800 megabit/second, or two cables for 64 bits and 1600
megabit/second. Cable A carries bits 0-31 and is used in both modes;
Cable B carries bits 32-63 and is use only with the 1600
megabit/second data rate option.
HIPPI Signal Hierarchy
HIPPI has the following hardware signals:
Source to Destination
INTERCONNECT A
INTERCONNECT B (64 bit only)
CLOCK (25 MHz)
REQUEST
PACKET
BURST
DATA (32 or 64 signals)
PARITY (4 or 8 signals)
Destination to Source
INTERCONNECT A
INTERCONNECT B (64 bit only)
CONNECT
READY
The INTERCONNECT lines carry DC voltages that indicate that the cable
is connected and that the remote interface has power. INTERCONNECT
is not used for signaling.
The CLOCK signal is a continuous 25 MHz (40 ns period) square wave.
All Source-to-Destination signals are synchronized to the clock.
The REQUEST and CONNECT lines are used to establish logical
connections. A connection is always initiated by a Source as it
asserts REQUEST. At the same time it puts 32 bits of data on DATA
lines 0-31, called the I-field. The Destination samples the DATA
lines and can complete a connection by asserting CONNECT. Packets
can be transmitted only while both REQUEST and CONNECT are asserted.
A Destination can also reject a connection by asserting CONNECT for
only a short interval between 4 and 16 HIPPI clock periods (160-640
nanoseconds). The Source knows a connection has been accepted when
CONNECT is asserted for more than 16 clocks or it receives a READY
pulse.
Either Source or Destination can terminate a connection by
deasserting REQUEST or CONNECT, respectively. Normally connections
are terminated by the Source after its last Packet has been sent. A
Destination cannot terminate a connection without potential loss of
data.
+------+-------------------------+------+
Idle Connected Idle . . .
+------+-------------------------+------+
/
/
/
/
/
+-------+ +-------+ +-------+ +-------+
I-field Packet Packet Packet
+-------+ +-------+ +-------+ +-------+
/
/
/
/
/
/
/
+-----+ +-----+ +-----+
Burst Burst...Burst
+-----+ +-----+ +-----+
HIPPI Logical Framing Hierarchy
The Source asserts PACKET for the duration of a Packet transmission,
deasserting it to indicate the end of a Packet. A sequence of Bursts
comprise a Packet. To send a burst, a Source asserts the BURST
signal for 256 clock periods, during which it places 256 words of
data on the DATA lines. The first or last Burst of a Packet may be
less than 256 clock periods, allowing the transmission of any
integral number of 32 or 64 bit words in a Packet.
The READY signal is a pulse four or more clock periods long. Each
pulse signals the Source that the Destination can receive one Burst.
The Destination need not wait for a burst before sending another
READY if it has burst buffers available; up to 63 unanswered READYs
may be sent, allowing HIPPI to operate at full speed over distances
of many kilometers. If a Source must wait for flow control, it
inserts idle periods between Bursts.
+------------------------------------------------+
REQUEST---+ +----
+--------------------------------------------+
CONNECT---------+ +--
+---------------------------------------+
PACKET-------------+ +----
+-+ +-+ +-+ +-+ +-+ +-+ +-+ +-+
READY------------+ +---+ +---+ +---+ +---+ +---+ +---+ +---+ +--
+-------+ +-------+ +-------+ +-----+
BURST--------------+ +-+ +-+ +-+ +--------
DATA------I-field----DATA------DATA------DATA-----DATA----------
HIPPI Signal Timing Diagram
Serial HIPPI
There is no ANSI standard for HIPPI other than the parallel copper
cable version. However an implementors" agreement exists, specifying
a serial protocol to extend HIPPI signals on optical fiber or coaxial
copper cable. Serial links may be used interchangeably with parallel
links to overcome HIPPI distance limitations; they are transparent to
the Source and Destination, except for the possibility of longer
propagation delays.
I-Field and Switch Control
The REQUEST, CONNECT and I-field features of HIPPI-PH were designed
for the control of switches as described in HIPPI-SC. A switch is a
hub with a number of input and output HIPPI ports. HIPPI Sources are
cabled to switch input ports, and switch output ports are cabled to
HIPPI Destinations. When a HIPPI Source requests a connection, the
switch interprets the I-field to select an output port and
electrically connects the HIPPI Source to the HIPPI Destination on
that port. Once connected, the switch does not interact with the
HIPPIs in any way until REQUEST or CONNECT is deasserted, at which
time it breaks the physical connection and deasserts its output
signals to both sides. Some existing switch implementations can
switch connections in less than one microsecond. There is the
potential for as many simultaneous connections, each transferring
data at HIPPI speeds, as there are input or output ports on the
switch. A switch offers much greater total throughput capacity than
broadcast or ring media.
31 28 26 23 11 0
+-+---+-+-+---+-+-----------------------+-----------------------+
L WDPS C Source Address Destination Address
+-+---+-+-+---+-+-----------------------+-----------------------+
HIPPI-SC I-field Format (Logical Address Mode)
L = Locally defined (1 => entire I-field is locally defined)
W = Width (1 => 64 bit connection)
D = Direction (1 => swap Source and Destination Address)
PS = Path Selection (01 => Logical Address Mode)
C = Camp-on (1 => wait until Destination is free)
HIPPI-SC defines I-field formats for two different addressing modes.
The first, called Source Routing, encodes a string of port numbers in
the lower 24 bits. This string specifies a route over a number of
switches. A Destination"s address may differ from one Source to
another if multiple switches are used.
The second format, called Logical Address Mode, defines two 12 bit
fields, Source Address and Destination Address. A Destination"s 12
bit Switch Address is the same for all Sources. Switches commonly
have address lookup tables to map 12 bit logical addresses to
physical ports. This mode is used for networking.
Control fields in the I-field are:
L The "Locally Defined" bit, when set, indicates that the I-field
is not in the standard format. The meaning of bits 30-0 are
locally defined.
W The Width bit, when set, requests a 64 bit connection through
the switch. It is meaningless if Cable B is not installed at
the Source. If W is set and either the Source or the requested
Destination has no Cable B to the switch, the switch rejects
the connection. Otherwise the switch connects both Cable A and
Cable B if W is set, or Cable A only if W is clear. This
feature is useful if both Source and Destination
implementations can selectively disable or enable Cable B on
each new connection.
D The Direction bit, when set, reverses the sense of the Source
Address and Destination Address fields. In other words, D=1
means that the Source Address is in bits 0-11 and the
Destination Address is in bits 12-23. This bit was defined to
give devices a simple way to route return messages. It is not
useful for LAN operations.
PS The Path Selection field determines whether the I-field
contains Source Route or Address information, and in Logical
Address mode, whether the switch may select from multiple
possible routes to the destination. The value "01" selects
Logical Address mode and fixed routes.
C The Camp-on bit requests the switch not to reject the
connection if the selected Destination is busy (connected to
another Source) but wait and make the connection when the
Destination is free.
15 Appendix B -- How to Build a Practical HIPPI LAN
"IP on HIPPI" describes the network host"s view of a HIPPI local area
network without providing much information on the architecture of the
network itself. Here we describe a network constructed from
available HIPPI components, having the following characteristics:
1. A tree structure with a central HIPPI-SC compliant hub and
optional satellite switches
2. Each satellite is connected to the hub by just one bidirectional
HIPPI link.
3. Serial HIPPI or transparent fiber optic HIPPI extender devices
may be used in any link.
4. Some satellites may be a particular switch product which is not
HIPPI-SC compliant.
5. Host systems are attached either directly to the hub or to
satellites, by single bidirectional links in which both HIPPI cables
go to the same numbered switch port.
Switch Address Management
Switch addresses use a flat address space. The 12-bit address is
subdivided into 6 bits of switch number and 6 bits of port number.
11 5 0
+-----------------------+-----------------------+
Switch Number Port Number
+-----------------------+-----------------------+
Logical Address Construction
Switches may be numbered arbitrarily. A given host"s address
consists of the number of the switch it is directly attached to and
the physical port number on that switch to which its input channel is
attached.
In the singly-connected tree structure, there is exactly one path
between any pair of hosts. Since each satellite must be connected
directly to the hub, the maximum length of this path is three hops,
and the minimum length is one. Each HIPPI-SC compliant switch is
programmed to map each of the host switch addresses to the
appropriate output port: either the port to which the host is
directly attached or a port that is linked to another switch in the
path to it.
Special Treatment of Nonstandard Switches
There is one commercially available switch that was designed
before the drafting of HIPPI-SC and is not fully compliant. It is
in common use, so it is worth making some special provisions to
allow its use in a HIPPI LAN. This switch supports only the
Source Route mode of addressing with a four bit right shift that
can be disabled by a hardware switch on each input port.
Addresses cannot be mapped. The switch does not support the "W",
"D", or "PS" fields of the I-field; it ignores their contents.
Use of this switch as a satellite will require a slight deviation
from normal I-field usage by the hosts that are directly attached
to it. Hosts attached to standard switches are not affected.
For a destination connected to a non compliant satellite, the
satellite uses only the least significant four bits of the I-field
as the address. Since the address contains the destination"s
physical port number in the least significant bits, its port will
be selected. Nonstandard switches should be set to disable I-
field shifting at the input from the hub, so that the destination
host will see its correct switch address in the I-field when
performing self-address discovery. I-field shifting must be
enabled on the satellite for each input port to which a host is
attached.
Hosts attached to nonstandard satellites must deviate from the
normal I-field usage when connecting to hosts on another switch.
It is suggested that all host implementations have this capability
as long as the nonstandard switches remain in use. The host must
know, by some manual configuration method, that it is connected to
a nonstandard switch, and it must have its "link port" number;
that is, the number of the port on the satellite that is connected
to the hub.
The normal I-field format for a 32-bit connection, per the
document, is this:
31 26 23 11 0
+---------+---+-+-----------------------+-----------------------+
0 0 0 0 0x 1C Unused Destination Address
+---------+---+-+-----------------------+-----------------------+
The special I-field format is:
31 26 24 15 4 3 0
+---------+---+-+---------------+-----------------------+-------+
0 0 0 0 0x 1C Unused Destination Address Link
+---------+---+-+---------------+-----------------------+-------+
This I-field is altered by shifting the lower 24 bits left by four
and adding the link port number. Camp-on is optional, and the PS
field is set to 01 or 11 (the host"s option) as if the switch
supported logical address mode. All other I-field bits are set to
zero. When the host requests a connection with this I-field, the
switch selects a connection through the link port to the hub, and
shifts the lower 24 bits of the I-field right by four bits. The link
port number is discarded and the I-field passed through to the hub is
a proper HIPPI-SC I-field selecting logical address mode.
A host on a nonstandard satellite may use the special I-field format
for all connection requests. If connecting to another host on the
same satellite, this will cause the connection to take an
unnecessarily long path through the hub and back. If an optimization
is desired, the host can be given additional information to allow it
to use the standard I-field format when connecting to another host on
the same switch. This information could consist of a list of the
other hosts on the same switch, or the details of address formation,
along with the switch number of the local satellite, which would
allow the host to analyze the switch address to determine whether or
not the destination is on the local switch. This optimization is
fairly complicated and may not always be worthwhile.