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[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] TCP Framing Support in iSCSI - Proposal
The following rough proposal for iSCSI TCP Framing
Support has been generated by the Framing Team as a result
of the Face-to-Face Design Teams meeting and subsequent
discussions. It consists of changes to the "ULP Framing
for TCP" and "iSCSI" internet-drafts as summarized below.
An I-D detailing these, and other related, changes is
forthcoming.
Regards,
Jim Wendt
jim_wendt@hp.com
-------- iSCSI TCP Framing Support Proposal - Summary ------
A) Proposed changes to the "ULP Framing for TCP" I-D are:
1) Modify I-D to include two framing modes:
- "Marker mode" for unmodified TCP stacks
- "PDU-alignment mode" for modified TCP stacks
Note: An updated version of the "ULP Framing for TCP I-D"
reflecting these changes has been posted (7/9/01) to TSVWG
for comments (draft-ietf-tsvwg-tcp-ulp-frame-00)
2) ULP is responsible for negotiating use of a framing mode
and enabling framing behavior on the TCP connection in an
unambiguous manner.
3) There are "Senders" and "Receivers" on each unidirectional
data flow of a framing TCP connection. The use of framing,
the framing mode, and framing operational parameters values
are negotiated separately for each direction of a framing
TCP connection. This means that framing behavior may be in
use on one direction of a TCP connection but not the
reverse direction, or that different framing modes may
be in use.
4) Creation of the following operational parameters and
semantics:
- Marker period (in Marker mode)
- Receiver maximum PDU size (in Marker mode)
- Framing keys (in PDU-alignment mode)
- ULP packing behavior (in PDU-alignment mode)
5) ULP is responsible for negotiating use of a specific
framing mode over the TCP connection, and for negotiating
values for the framing operation parameters
6) Change the marker fields to be 16-bits rather than 32-bits
(and refer to as "offsets" rather than "pointers")
B) Proposed changes to the iSCSI spec are:
1) Remove Markers appendix from iSCSI spec (Appendix D. Synch
and Steering with Fixed Interval Markers)
2) iSCSI spec adds wording to the effect of:
Framing Protocol:
* iSCSI initiator and target framing behavior over a TCP
connection is defined in draft-ietf-tsvwg-tcp-ulp-frame-00
(or eventual RFC#)
* an iSCSI initiator or target is both a sender and receiver
with respect to framing behavior over a TCP connection.
iSCSI Sender:
* an iSCSI framing sender SHOULD implement PDU-alignment mode,
as defined in <Framing I-D>
* if an iSCSI framing sender does not implement PDU-alignment
mode, it MUST implement Marker mode, as defined in
<Framing I-D>
iSCSI Receiver:
* an iSCSI receiver may choose to not implement any
framing mode
* an iSCSI framing receiver MAY implement PDU-alignment
mode, or Marker mode, or both as defined in <Framing I-D>
* an iSCSI framing receiver on a TCP connection dictates
use of the highest framing mode desired from the sender
by progressing through the following sequence:
- if the receiver requests PDU-alignment mode from the
sender, and the sender supports PDU-alignment mode,
then the sender MUST enable use of PDU-alignment mode
on the TCP connection
- else if the sender does not support PDU-alignment mode,
then the receiver MAY request Marker mode from the
sender. If the sender also supports Marker mode, then
the sender MUST enable use of Marker mode
- otherwise, the receiver has not requested use of
a framing mode and the sender MUST NOT enable use of
any framing mode on the TCP connection
- [Note: the above rules may be moved to the Framing I-D]
Interoperation:
* the framing mode or modes that a receiver implementation
chooses to support will determine which senders it can
perform direct data placement for (since senders can choose
to implemented either of the framing modes). It is
anticipated that there will be receiver implementations
which support the following combinations of framing modes:
1. PDU-alignment + Markers
2. PDU-alignment only
3. no framing
Chunking:
* Use of PDU-alignment mode requires that a dynamic
chunking layer be implemented above the framing TCP
layer.
3) > Still need to determine iSCSI mechanism for turning on
Framing protocol Marker mode operation
4) > Still need to determine iSCSI mechanism for negotiating
values for framing operational parameters
5) Perhaps there is some description of probable framing
scenarios capturing the most likely combinations of
the following attributes:
- initiator or target
- software implementation or hardware implementation
- unmodified or modified TCP stack
- sender AND receiver framing behaviors (no framing,
or Marker mode, or PDU-Alignment mode)
- values for framing operational parameters
-------------------------------------------------------
The reasoning for these proposed changes is as follows:
1) iSCSI use of direct data placement and framing
a) Direct data placement allows a HW-accelerated
interface card to place each incoming ULP PDU (and
ideally each TCP segment payload) directly into its
final application buffer in end-system memory, even
when the underlying TCP segments arrive out of order.
The ULP PDU carries the buffer location information
for doing the direct placement. This means that the
TCP doesn't need to hold application data in an
internal receive queue while sequence gaps are filled,
nor subsequently copy that data into final
application buffers. This allows the interface
card to minimize or eliminate the very fast and
large receive queue memory that would normally be
required when running over networks with large
bandwidth-delay products (10-100 Gbps,
200msec RTT).
b) In order to perform direct data placement, the
placement function must be able to locate ULP
headers in the TCP segment stream, and extract
placement information, even when TCP segments
arrive out of order. A framing mechanism provides
the underlying wire protocol and behaviors to
enable this.
c) The I-D "The Case for RDMA"
(draft-csapuntz-caserdma-00.txt) discusses
the benefits of direct data placement in the
context of a generalized RDMA facility.
2) Merging Marker mode into "ULP Framing for TCP" I-D
a) The TCP-related framing work already has mindshare
in TSVWG and this work is embodied in the current
framing I-D. Rather than dilute the framing effort
with additional I-Ds, all framing related work
should be collected into a modified version of the
existing framing I-D.
b) Other ULPs may also find Marker mode useful in
software-only unmodified-TCP client scenarios
c) The framing I-D appears to be a reasonable literary
vehicle for documenting the collection of framing
schemes. The I-D could be extended in the future to
include a byte or word stuffing frame marker method
such as COBS.
d) A single framing I-D may help to encourage a single
consistent interface with the ULP regardless of which
framing mode is employed.
e) The iSCSI spec can simply reference the one framing I-D.
3) Specifying iSCSI sender and receiver framing support
a) Receivers can choose to not implement framing.
Software implementations of receivers may incur extra
data movements in processing framing and generally get
no benefit from using framing. It is anticipated that
these receiver software implementations will not
support framing.
b) Hardware-accelerated receivers that want to perform
direct data placement and eliminate or minimize the
amount of TCP reassembly memory (for links with large
bandwidth-delay products) will require senders to
support framing behavior. These receiver
implementations are only viable if they can rely
on the fact that all senders are capable of
supporting some framing mode.
c) Framing is done for the receiver's benefit, and is
mostly a minor inconvenience for the sender. However,
senders may have limitations regarding which framing
mode(s) they can support. So, a sender is allowed to
implement the framing mode(s) most suited to it, and
the receiver is allowed to select from these supported
framing modes, or choose not to utilize framing on
the connection.
d) There isn't one framing mode that is best for all
senders:
- Marker mode is best suited to software
implementations that run over unmodified TCP stacks
- PDU-alignment mode is best suited to hardware
implementations that want to minimize or eliminate
buffer memory and reduce per-packet processing
complexity
e) Marker mode is the best choice for a framing mechanism
that can operate completely above a client TCP stack
and not require any modification to that stack.
- Other interval-based approaches (periodic PDU
alignment, fixed length PDU) require padding
and waste bandwidth
- bit-stuffing and byte-stuffing schemes (COBS,
etc) have a much higher processing overhead
f) Marker mode supports one potentially compelling
application for iSCSI involving software-only
implementations on mainstream desktops and laptops
operating over unmodified TCP stacks to access
centralized storage arrays. These software
implementations are likely to exist far into the
future. Individual software-only clients may not operate
at 10Gbps, but may be combined together with other
clients that could aggregate to 10Gbps, thus making
direct data placement compelling for a 10Gbps receiver
even if the senders are only operating at 1Gbps.
g) Marker mode doesn't completely eliminate the need for
buffer memory on the receiver. The receiver still needs
to use "eddy buffers" that temporarily hold incoming data
after a dropped segment containing a ULP header up until
the next ULP header is located in the packet stream, and
which exist for as long as the original ULP header segment
is outstanding. But Marker mode does greatly reduce the
amount of memory needed as compared to a traditional TCP
receiver's reassembly memory requirements (often equal to
number-of-connections X round-trip-pipe-size). The Marker
mode small memory requirements are dependent upon the
period of the marker, and the size of the ULP PDUs being
restricted to a reasonably small value. The larger that
either one is, the larger the eddy buffer memory
requirements. Also, an eddy buffer is required each time
a ULP header is dropped, so that multiple ULP header drops
in close proximity may cause multiple eddy buffers to be
temporarily pending on a connection.
h) PDU-alignment framing mode allows each ULP PDU to be
aligned with, and sent in, a single TCP segment under
normal conditions (with the added requirement that a
chunking layer needs to be implemented between iSCSI and
the framing TCP stack). This behavior allows each TCP
segment to be fully self-describing with respect to
direct placment. Thus, each incoming TCP segment payload
can be processed and direct placed as it arrives with
no residual state information nor eddy buffer memory
required.
i) PDU-alignment framing mode does require the use of
a small number of "eddy buffers" when dynamic changes
in the network path MTU occur and packets arrive out
of order.
j) The PDU-alignment framing mode is preferred. However, it
may be several years before all of the different software
TCP/IP implementations will be able to support framing
behavior. To do this, software interfaces will need to
change, and something needs to drag them there. This
will take some time, so we have markers to help out in the
meantime. If all receivers that can use framing can do
either one, and senders that can do PDU-alignment should
do so, we will have a larger set of PDU-alignment
implementations that may help pull the rest of the
software interfaces along with the
k) It is anticipated that a receiver will not
implement only Markers. The receiver implementations will
probably be:
1. PDU-alignment + Markers
2. PDU-alignment only
3. no framing
-----------------------------------------------------------
Open Issues:
1) Interoperability between sender and receiver
- Given that both senders and receivers have a choice in
which framing mode(s) they implement, there is the
potential for the sender to implement one framing mode
and the receiver to implement a different framing mode
(e.g. the sender implements only Marker mode, and the
receiver implements only PDU-alignment mode).
- In this situation, the receiver and sender would not
enable framing on the TCP connection, and the receiver
would not be able to perform direct data placement.
Throughput from sender to receiver would likely be
greatly reduced should any TCP segment drops occur.
2) None of the current framing schemes take TCP data integrity
into account. It either needs to be decided:
a) how to detect when a data integrity problem occurs
within a framing header, and what to do about it
(even if it just kills the TCP connection),
b) or that a sufficient level of data integrity needs
to be provided for all protocols running over TCP
via a more holistic approach.
3) Acceptability of the PDU-Alignment framing mode's reliance
on "key+length" matching across resegmenting middleboxes
- In PDU-Alignment mode each TCP segment payload contains
one complete framing PDU (consisting of an 8 byte
framing header followed by one or more complete ULP
PDUs). Thus, every TCP segment has the TCP header
followed immediately by the framing header.
- In certain cases a single framing PDU must be broken
across multiple TCP segments (such as dynamic Path MTU
reductions), resulting in TCP segments where a framing
header doesn't immediately follow the TCP header.
- The framing I-D defines sender behaviors that allow
PDU-alignment mode to function deterministically and
correctly in all cases where the TCP segmentation
flowing from sender to receiver is not altered.
- If the TCP segmentation from sender to receiver is
altered by an intermediary (resegmenting middlebox),
and a framing-header-containing segment drop or
reordering has occurred such that the receiver is
attempting to locate the next framing header in the
segment stream, then the receiver must examine the
first 8 bytes of each incoming TCP segment payload for
a valid framing header containing valid Key(6B) and
Length(2B) fields.
- A false-positive occurs if, upon resegmentation by a
middlebox, the receiver gets a TCP segment in which
the first 8 bytes of the payload indicate a valid
framing header (the first 6 bytes match the
previously exchanged random key value, and the next
2 bytes contain a valid length), yet the TCP segment
payload isn't actually a framing header.
- While it is felt that the probability of a
false-positive in these resegmenting-middlebox scenarios
will be sufficiently low, further analysis work may be
may be required in this area.
- Note that this mechanism is NOT a scanning technique
for locating start-of-frame across an arbitrary byte
stream. It only provides an indication of PDU
alignment or not. The first 8 bytes of the TCP segment
payload are examined to determine if the segment
contains the start of a ULP PDU.
4) Do Markers work at 10Gbps?
- The feasibility of markers at 10Gbps has been questioned.
It would be beneficial to hear specifics regarding why
Markers won't work at 10Gbps. Markers don't allow for a
no-memory direct data placement NIC since eddy-buffers
are required. So, support for clients with unmodified
TCP stacks comes at a cost, which is the cost of
supporting eddy buffers on the NIC.
- One question is whether the eddy buffers can be contained
entirely in the ASIC or need to be in off-chip memory.
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