FreeIPMI Design

by 

Albert Chu
chu11@llnl.gov
Last Updated: August 27, 2013

These are some notes on various design decisions made in FreeIPMI.

1) Fiid vs. other Marshalling/Unmarshalling Styles
--------------------------------------------------

Several programmers have asked us why we have chosen a relatively
unpopular/different method to marshall/unmarshall IPMI packets and
build network packets.

First, lets discuss several classic methods for
marshalling/unmarshalling data when using structs to represent a
packet.

Method A: Marshall/Unmarshall "manually":
-----------------------------------------

struct packet
{
  uint8_t field_1; /* 1 bit */
  uint8_t field_2; /* 3 bits */
  uint8_t field_3; /* 4 bits */
  int16_t field_4; /* 16 bits */
};

my_marshall_function(struct packet *pkt, char *buf, unsigned int buflen)
{
   buf[0] |= pkt->field_1 & 0x1;
   buf[0] |= (pkt->field_2 << 1) & 0x0E;
   buf[0] |= (pkt->field_3 << 4) & 0xF0;
   /* assuming network byte order here */
   buf[1] |= (pkt->field_4 & 0xFF00) >> 8;
   buf[2] |= pkt->field_4 & 0x00FF;
}

my_unmarshall_function(struct packet *pkt, char *buf, unsigned int buflen)
{
   pkt->field_1 = buf[0] & 0x01;
   pkt->field_2 = buf[0] & 0x0E >> 1;
   pkt->field_3 = buf[0] & 0xF0 >> 4;
#if LITTLE_ENDIAN_HOST
   pkt->field_4 = buf[2] | buf[1] << 8;;
#else
   pkt->field_4 = buf[1] | buf[2] << 8;;
#endif
}

general_usage_example()
{
   struct packet pkt;
   char buf[1024];
   int len;
 
   pkt.field_1 = 1;
   pkt.field_2 = 2;
   pkt.field_3 = 3;
   pkt.field_4 = 5;

   my_marshall_function(&pkt, buf, 1024);
   my_send_data_function(buf);

   len = my_receive_data_function(buf);
   my_unmarshall_function(&pkt, buf, len);

   printf("field_1 is: %d\n", pkt.field_1);
}

Pros:

A) No need to deal with struct packing issues in the compiler.
B) The struct definition describes packets closely and is relatively
   easy to use and understand.
C) Relatively efficient.
D) General usage code size is relatively small.
E) General usage need not determine field type (e.g. is it an unsigned
   or signed integer).

Cons:

A) Have to deal with endian problems.
B) Lots of marshalling and unmarshalling code are required for each
   packet type.
C) Relatively difficult to deal with optional fields. (You'll need
   flags in the struct to indicate if a field was set/unset, or validate
   the fields via protocol definition knowledge.)
D) Relatively difficult to deal with variable length fields. (You'll
   need a length parameter in the struct to indicate the length of a
   field.)
E) Packet dumps/debugging is relatively poor (you only get hex) or you
   have to create debug functions to handle each packet type.
F) Struct changes (e.g. due to IPMI errata changes) may break ABI if
   the structs are part of a public interface.

Method B: Cast a buffer to a packed struct:
-------------------------------------------

For Example:

struct packet
{
  uint8_t field_1 : 1;
  uint8_t field_2 : 3;
  uint8_t field_3 : 4;
  int16_t field_4; 
};

my_marshall_function(struct packet *pkt, char *buf, unsigned int buflen)
{
   memcpy(buf, pkt, sizeof(struct packet));
#if LITTLE_ENDIAN_HOST
    swap(&buf[1], &buf[2]);
#endif
}

my_unmarshall_function(struct packet *pkt, char *buf, unsigned int buflen)
{
   *pkt = *((struct packet *)buf);
#if LITTLE_ENDIAN_HOST
    pkt->field_4 = ntohs(pkt->field_4);
#endif
}

general_usage_example()
{
   struct packet pkt;
   char buf[1024];
   int len;
 
   pkt.field_1 = 1;
   pkt.field_2 = 2;
   pkt.field_3 = 3;
   pkt.field_4 = 5;

   my_marshall_function(&pkt, buf, 1024);
   my_send_data_function(buf);

   len = my_receive_data_function(buf);
   my_unmarshall_function(&pkt, buf, len);

   printf("field_1 is: %d\n", pkt.field_1);
}

Pros:

A) Not too much marshalling and unmarshalling code is required.
B) General usage code size is relatively small.
C) The struct definition describes packets exactly and is relatively
   easy to use and understand.
D) Very efficient (little actual marshalling/unmarshalling needs to be done.)
E) General usage need not determine field type (e.g. is it an unsigned
   or signed integer).

Cons:

A) Have to deal with endian problems.
B) Have to deal with portability of struct packing techniques between
   different compilers (there are differences in compilers, but nowadays,
   this may be easier/more portable than I originally believed it to be).
C) Difficult to deal with optional fields (no flags can be put
   in the struct to indicate if a field was set/unset, can only
   validate the fields via protocol definition knowledge.)
D) No mechanism to deal with variable length fields (no length
   field can be put in the struct to indicate the field length.)
E) Packet dumps/debugging is relatively poor (you only get hex) or you
   have to create debug functions to handle each packet type.
F) Struct changes (e.g. due to IPMI errata changes) may break ABI if
   the structs are part of a public interface.

Our Method C: string_name -> bitmask mapping
--------------------------------------------

The "FreeIPMI Interface Definition" or 'fiid' API in libfreeipmi uses
a string_name/bit_count template and an API to get and set values in a
packet to handle marshalling/unmarshalling.

The following are a few of the API functions used for FIID to give you
an idea for the fiid API:

fiid_obj_t fiid_obj_create (fiid_template_t tmpl);
int32_t fiid_obj_errnum(fiid_obj_t obj);
int8_t fiid_obj_clear (fiid_obj_t obj);
int8_t fiid_obj_set (fiid_obj_t obj, char *field, uint64_t val);
int8_t fiid_obj_get (fiid_obj_t obj, char *field, uint64_t *val);
int32_t fiid_obj_get_all (fiid_obj_t obj, uint8_t *data, uint32_t data_len);
int32_t fiid_obj_set_all (fiid_obj_t obj, uint8_t *data, uint32_t data_len);

The following is the fiid equivalent in the previous examples:

fiid_template_t tmpl_example =
  {
    {1, "field_1", FIID_FIELD_REQUIRED | FIID_FIELD_LENGTH_FIXED},
    {3, "field_2", FIID_FIELD_REQUIRED | FIID_FIELD_LENGTH_FIXED},
    {4, "field_3", FIID_FIELD_REQUIRED | FIID_FIELD_LENGTH_FIXED},
    {16, "field_4", FIID_FIELD_REQUIRED | FIID_FIELD_LENGTH_FIXED},
    {0, "", 0}
  };

general_usage_example()
{
   fiid_obj_t obj;
   char buf[1024];
   int len;
   uint64_t val;

   obj = fiid_obj_create(tmpl_example);
   fiid_obj_set(obj, "field_1", 1);
   fiid_obj_set(obj, "field_2", 2);
   fiid_obj_set(obj, "field_3", 3);
   fiid_obj_set(obj, "field_4", 5);

   /* "marshall" the packet */
   fiid_obj_get_all(obj, buf, 1024);
   my_send_data_function(buf);

   fiid_obj_clear(obj);

   len = my_receive_data_function(buf);

   /* "unmarshall" the packet */
   fiid_obj_set_all(obj, buf, len);
  
   fiid_obj_get(obj, "field_1", &val);   
   printf("field_1 is: %d\n", (int16_t)val);
}

The pros and cons of the fiid method are:

Pros:

A) No need to deal with endian problems (handled internally in the API).
B) No need to deal with struct packing issues (bit shifts are handled
   internally in the API).
C) Easier to deal with optional fields (For marshalling, don't set a
   field.  For unmarshalling, the api can identify if a field is set or not).
D) Easier to deal with variable length fields (For marshalling, set
   whatever length you want.  For unmarshalling, the api can identify the
   length of the field read).
E) Templates describe the packets exactly.
F) Easy to do large packet dumps and debug (fields and values easily
   output and identified).
G) Significantly reduce the amount of marshalling, unmarshalling, and
   debug code needed (the API handles it all already).
F) Template changes (e.g. due to IPMI errata changes) shouldn't break
   ABI.  (You can publish the template strings, need not publish the
   template itself.)
 
Cons:

A) Need to learn/use a reasonably large API and learn/use all the
   templates.
B) Pretty inefficient (lots of string comparisons).
C) General usage code size is increased.
D) General usage must determine and cast field to appropriate type
   (e.g. is it an unsigned or signed integer).

(Side Comments: 

Some other networking APIs have a similar API, but use
macros/enums for the field names rather than strings.  Many of the
above benefits are identical, except the debug dump output
capabilities are weaker in exchange for better performance.

Some other networking APIs may return a type of a field (e.g. signed
vs unsigned, 16bit vs 32bit, etc.).  That would remove need to
determine casting in general usage in exchange for larger general
usage code size.)

The big reasons why this was developed and chosen over traditional
methods.

A) The IPMI specification is very large, so reducing code size weighed
in as an important factor for the FreeIPMI authors.  This allowed
there to be fewer marshalling/unmarshalling/debug functions.  By one
FreeIPMI author's counting in the specification, there are 304
different base payloads in the IPMI specification.  This does not
include permutations of payloads due to different versions, optional
fields, headers, trailers, encryption, oem extensions, record formats
data is stored in, etc.

B) There are a relatively large number of optional fields and variable
length fields in the IPMI specification.  As stated above, the
traditional struct based marshalling/unmarshalling have issues
with handling these.

C) The lack of IPMI compliance from vendors is a well known problem in
the open-source community.  The templates have saved developers
countless hours of debugging time due to the easy method by which
packets can be dumped with their fields and values quickly identified.
It is very easy to find vendor IPMI compliance problems very quickly.

Here's an example of a dump:

pwopr2: : RMCP Header:
pwopr2: : ------------
pwopr2: [               6h] = version[ 8b]
pwopr2: [               0h] = reserved[ 8b]
pwopr2: [              FFh] = sequence_number[ 8b]
pwopr2: [               7h] = message_class.class[ 5b]
pwopr2: [               0h] = message_class.reserved[ 2b]
pwopr2: [               0h] = message_class.ack[ 1b]
pwopr2: : IPMI Session Header:
pwopr2: : --------------------
pwopr2: [               0h] = authentication_type[ 8b]
pwopr2: [               0h] = session_sequence_number[32b]
pwopr2: [               0h] = session_id[32b]
pwopr2: [               9h] = ipmi_msg_len[ 8b]
pwopr2: : IPMI Message Header:
pwopr2: : --------------------
pwopr2: [              20h] = rs_addr[ 8b]
pwopr2: [               0h] = rs_lun[ 2b]
pwopr2: [               6h] = net_fn[ 6b]
pwopr2: [              C8h] = checksum1[ 8b]
pwopr2: [              81h] = rq_addr[ 8b]
pwopr2: [               0h] = rq_lun[ 2b]
pwopr2: [              26h] = rq_seq[ 6b]
pwopr2: : IPMI Command Data:
pwopr2: : ------------------
pwopr2: [              38h] = cmd[ 8b]
pwopr2: [               Eh] = channel_number[ 4b]
pwopr2: [               0h] = reserved1[ 3b]
pwopr2: [               1h] = get_ipmi_v2.0_extended_data[ 1b]
pwopr2: [               2h] = maximum_privilege_level[ 4b]
pwopr2: [               0h] = reserved2[ 4b]
pwopr2: : IPMI Trailer:
pwopr2: : --------------
pwopr2: [              1Fh] = checksum2[ 8b]

2) Non-generic error messages
-----------------------------

Under some circumstances, it may be preferred to return generic error
messages to the user, so that a malicious user cannot infer remote
login information from different error messages returned.  For
example, returning a generic error message of "Permission Denied"
would not give a malicious user information on whether the username or
password was input incorrectly.

Although implemented earlier on, the FreeIPMI authors have elected to
not implement this now.  There are many vendor implementations of IPMI
and many configuration options (authentication mechanism, cipher suite
id, username, password, K_g, privilege level) needed for proper IPMI
session establishment.  The number of error messages that could be
mapped into a generic "Permission Denied" would make it too difficult
for users to determine why they failed to connect properly.  The
overall worth of implementing a generic "Permission Denied" error
message just doesn't seem worth it now.

3) Get Channel Authentication Capabilities Command
--------------------------------------------------

The Get Channel Authentication Capabilities Command is typically the
first packet sent in the IPMI session.  It returns information 
on the remote machine's support of:

A) IPMI 1.5 authentication mechanisms (e.g. md2, md5, etc.)
B) IPMI 1.5 and/or IPMI 2.0
C) per msg authentication
D) K_g status
E) null username/non-null username/anonymous logins

Currently in FreeIPMI, we check each of these values during the
session setup to determine if a person can connect to the remote
machine later in the protocol:

A) If the user input an unsupported authentication mechanism, we
return an error.

B) If the user requested IPMI 2.0, but the remote machine doesn't
support IPMI 2.0, we return an error.

C) We determine if per msg authentication should be considered later
in the protocol session.

D) If the user was required/not-required to input a K_g value, we
return an error appropriately.

E) If the user input an unsupported username/password combination, we
return an error appropriately.

There is a question as to what values above, if any, need to be
checked and appropriate errors returned to the user.  The Get Channel
Authentication Capabilities command is often implemented incorrectly
by a number of vendors, so that overall benefit of checks has been put
in question.  The FreeIPMI authors have elected to keep all the checks
for the following reasons.

* 'A' and 'B' should be checked to avoid potential timeouts:

  - Later in the protocol, the password could be sent/hashed
    incorrectly, leading to a timeout because packets are not accepted by
    the remote machine.

  - If the remote machine does not support IPMI 2.0, later packets
    could timeout because the remote machine does not recognize the packet
    format.

* 'C''s checks could be skipped as long as per msg authentication was not
  supported.

* 'D''s checks could be skipped, because an improper null vs non-null K_g
will be caught later during IPMI 2.0 authentication.

* 'E''s checks are the most complicated.  An improper null vs non-null
username will be caught later during IPMI 1.5 and IPMI 2.0
authentication.  An improper null vs non-null password can be caught
later during IPMI 2.0 authentication, but may result in a timeout
during IPMI 1.5 authentication.

An argument could also be made that the speed at which an invalid
username/password error is returned to a user could also give a
malicious user information on the username/password of the remote BMC.

In the end, the authors have felt the overall positive benefits
provided by the checking of these values provides more than the
negative implications.  Changes in the overall industry implementation
could change this viewpoint later.

4) Configuration tool callback design
-------------------------------------

Ipmi-config is coded with a archicture that reads/writes each
configurable field in the BMC separately.

As an example, suppose we have the following BMC configuration file
we'd like to commit.

FieldA       Value1
FieldB.1     Value2
FieldB.2     Value3
FieldB.3     Value4
FieldB.4     Value5

Suppose FieldA is read/written using a single IPMI packet and fields
FieldB.1-FieldB.4 can be read/written using a single IPMI packet.

In the architecture that ipmi-config is currently based on, the above
would require 5 read requests to read all 5 values.  It would require
1 read request for FieldA, 4 read requests for FieldB.1-FieldB.4, and
5 write requests to write the values.

Obviously, this sounds like (and is!) very inefficient. 

The authors acknowledge that the code is very inefficient b/c it will
cause an excess number of request/response packets to be generated.  With
a large number of inputs the Configtools can be slow.

Here are some of the major reasons why this was done and is still
kept.

A) Due to widely varying IPMI versions and implementations, this
handles the write configuration case best.  Suppose FieldB.2 is only
configurable on IPMI 2.0 systems but not IPMI 1.5 systems.  Suppose
(perhaps b/c it is optional in the IPMI specification) FieldB.3 is
supported by some vendors but not other vendors.  Suppose FieldB.4 is
simply not implemented correctly by the vendor.

This architecture allows the majority of the configuration to succeed
on a specific platform, and allows the end user to know exactly what
fields may or may not be configurable.  If all 4 fields of
FieldB.1-FieldB.4 were written at the same time, there is currently no
method in the IPMI protocol to know what field was configured
incorrectly and why (only a generic error of "invalid input" is
returned, but you won't know which field it is).

In the future, functionality could be added to retry each field
separately if there was such a failure, however that would add another
piece of complexity into the code we currently don't have time to add.
In addition, with so many IPMI firmware implementations, it may
difficult to add such functionality because of the wide array of error
cases that might occur.

B) There are several (and possibly more future) vendor compliance
problems that can be (or will need to be) worked around.  By using
this architecture, each specific field can be worked around
independently depending on the vendor.  These workarounds need to be
handled on both the read and write conditions.

One of the major fallouts from this design is that if an
invalid/illegal configuration exists on the motherboard by default,
some configuration values may not be configurable.  For example,
suppose we want to write the following config to the BMC.

FieldA.1     Value1
FieldA.2     Value2
FieldA.3     Value3
FieldA.4     Value4

The architecture of the config tools will read FieldA.1-FieldA.4 from
the BMC, change only FieldA.1, then try to write all the fields back
to the BMC.  Then it would be repeated for FieldA.2, etc.

However, suppose the default setting on the motherboard for FieldA.4
is illegal.  Then each time we attempt to write FieldA.1, FieldA.2,
and FieldA.3, an invalid input error will be returned b/c FieldA.4 is
illegal.  Things cannot change until FieldA.4 is modified.

In a worse scenario, suppose the default setting on the motherboard is
illegal for both FieldA.3 and FieldA.4.  That means we will receive an
invalid input error for the config of FieldA.1 through FieldA.4.

Currently, this has been seen a very small minority of systems and
work arounds have been added for those systems.

Another similar fallout from this design is that the vendor must allow
"piecemeal" configuration.  In other words, the vendor must allow a
subset of the fields to perhaps be configured "incorrectly" while the
other subset may be configured "correctly".  Some vendors require that
fields be written "simultaneously", and do not support the ability to
alter configuration one by one.

Again, this has been seen a very small minority of systems and work
arounds have been added for those systems.

5) Dealing with workarounds
---------------------------

There is an admitted conflict in determining whether vendor compliance
issues should be handled automatically vs. a specified workaround
(e.g. on the commandline or via a flag in a library).

On one hand, we would like for the tools to operate as simply for the
users as possible without the need to specify strange workarounds or
options on the command line.  For example, we could detect vendor
product-IDs early in the protocol, and if necessary for a particular
vendor, turn on the workarounds.

On the other hand, some workarounds cannot be detected properly all of
the time.  For example, the workaround may exist on one firmware
release vs. another firmware release.  It may exist between one
product of a vendor vs. another product from the vendor.  Another
example, is that while we can make a pretty decent guess what the
vendor intended, ultimately, there's no real way to know if the guess
is correct.

A number of these workarounds are due to vendor compliance problems
that are sometimes so intrusive (e.g. using a different hashing
algorithm for keys) they must require a workaround on the command line
b/c there is really no other way to handle it.  However, some could be
handled seemlessly, but would require altered behavior to handle the
"common case" or "lowest common denominator" of all IPMI protocols.

The general rule that the FreeIPMI authors have come to is that if the
workaround changes some "normal" or "good" behavior, it must require a
specified workaround.  Although it may/will be annoying to a number of
users, I feel it is better for the long term.  It can hopefully also
pressure vendors into fixing their implementations.

As an example, on some motherboards, we found that System Event Log
(SEL) records reported an invalid sensor generator ID.  We found that
the reported generator ID was shifted off by one.  Thus, as a
workround, if a SDR entry cannot be found for a respective system
event, we will also search for a SDR entry using the generator ID
shifted by one.  If the resulting SDR entry is found, we assume the
original generator ID was just off by one and we use the located SDR
record.  This workaround is seemless and doesn't involve an option on
the command line.

In contrast, we found on some other motherboards that some SEL records
report an invalid event record type.  Unlike the above situation,
there is no additional information from this record that can tell us
how to parse the record.  For the particular motheboard, these illegal
SEL records were normal system event records with improperly coded
record types.  Therefore, we implemented a workaround called
"assumesystemevent", which the user can specify to assume a valid
system event record no matter what.

Admittedly, the area is grey, and at some point, it's a judgement call
:-)

6) Dealing with OEM extensions
------------------------------

Similar to the "Dealing with workarounds" question above, there is a
similar question of how to deal with OEM extensions.  Should code
automatically detect the manufacturer and product to determine if OEM
extensions can be handled or should be output?

We would like the tools to operate as simply for the users without
specifying options on the command line.  However, can we trust that a
vendor will implement their extensions consistently across
motherboards, products, or even firmware revisions?

The general decision is that there will be an option for the user to
specify if they would like OEM interpreted output if available.  Many
FreeIPMI tools come with a --interpret-oem-data option for this
situation.  If a motherboard is specifically supported by FreeIPMI,
the user is free to use and trust the OEM support.  However, if OEM
extensions happen to work for a unlisted motherboard, the user must
take the output with some grain of salt.


