Internet DRAFT - draft-wakeley-iscsi-msgbndry


Internet Draft                                               M. Wakeley
<draft-wakeley-iscsi-msgbndry-00.txt>              Agilent Technologies
Expires March 2001                                       September 2000

               iSCSI Message Boundary Detection Proposal

Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

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Copyright Notice

   Copyright (C) Agilent Technologies (2000). All Rights Reserved.

1. Abstract

   The iSCSI working group is currently considering the
   framing/delimiting of iSCSI messages in a TCP stream. This I-D
   presents a proposal to discover iSCSI message boundaries using the
   TCP urgent pointer.

Wakeley                                                       [Page 1]
Internet-Draft          iSCSI Message Boundary          September 2000

2. Introduction

   The iSCSI I-D presents an encapsulation of the SCSI protocol onto
   TCP/IP.  This encapsulation is accomplished by sending iSCSI
   "messages" that are of varying length. Unfortunately, TCP does not
   have a built-in mechanism for signaling message boundaries at the
   TCP layer.  iSCSI overcomes this obstacle by placing the message
   length at the beginning of each message (in the iSCSI message
   "header"). This serves to delineate the end of the current message
   as well as the beginning of the next message.

   In situations where IP packets are delivered in-order from the
   network, framing issues are not an issue (messages are processed one
   after the other). In the presence of IP packet reordering (mostly
   due to frames being dropped), it is best to minimize the
   dependencies between TCP segments, so as to be able to do as much
   processing of the received out-of-order segments as possible. Such
   processing ensures that data can be copied to correct buffers the
   first time, decreasing the need for dedicated reassembly buffers as
   well as the latency and bandwidth related to extra copies.

   Another area where it would be extremely helpful to be able to
   delineate iSCSI messages is when using a protocol analyzer to
   monitor or debug an iSCSI session.  Typically, an analyzer will not
   be snooping continuously from the time the session is established,
   and thus it will not be "aligned" with the iSCSI messages.  An iSCSI
   message delimiter would enable the analyzer to discover and decode
   the iSCSI messages.

   Unfortunately, when relying solely on the "message length in the
   iSCSI message" scheme to delineate iSCSI messages, a missing TCP
   segment that contains an iSCSI message header (with the message
   length) makes it impossible to find message boundaries in subsequent
   TCP segments. The missing TCP segment must be received before later
   segments can be parsed and processed.

   There are several ways to get around this.

   * Make each message fixed size

   * Put synchronization information in the stream at fixed intervals

   * Use a marker byte to mark message boundaries

   However, each of these techniques has its drawbacks. Fixed sized
   messages introduce deadspace for small messages and extra control
   overhead for large messages. Marker bytes may work well for hardware
   but the escape sequences make it difficult for software. Putting
   information at fixed intervals in the stream introduces extra

   This document proposes usage of the urgent field in the TCP header
   to delineate iSCSI messages.

Wakeley                                                       [Page 2]
Internet-Draft          iSCSI Message Boundary          September 2000

3. iSCSI Message Boundary Delineation

3.1. Use the TCP Urgent Pointer

   The proposal is pretty simple.  The first byte of every iSCSI
   message is marked "urgent" - the TCP urgent pointer will point to
   the first byte of the iSCSI message in the TCP segment.

3.2. Restrict iSCSI PDU Size

   When a large iSCSI message is sent, the first TCP segment will
   contain the iSCSI header, but the remaining TCP segments do not
   contain any iSCSI framing information.  To minimize the amount of
   buffering required when an iSCSI header is lost, it is recommended
   that the iSCSI PDU size be restricted to a small value (perhaps a
   few TCP segments in length).

3.3. Differing Interpretations of Urgent Pointer

   Unfortunately, there are differing interpretations of whether the
   Urgent pointer points to the last (only) byte of urgent data (as
   defined by RFC1122), or the byte after the urgent data (typically
   BSD implementations) (see section 20.8 in [Stevens]).

   In order to allow an iSCSI implementation to resolve which
   interpretation is being used on the data received, the following
   change is proposed for the iSCSI draft.

   Define a bit in the first byte of the iSCSI message (the opcode
   field) that shall always be zero.  Likewise, define the same bit in
   the following byte (typically a "flags" field) that shall always be
   one.  When an iSCSI implementation receives an out of order TCP
   segment with the Urgent pointer defined, it shall look at byte
   pointed to by the Urgent pointer.  If the bit is clear, the sender
   is RFC1122 compliant.  If the bit is set, the sender has implemented
   the BSD interpretation, and must "back up" one byte to find the
   beginning of the iSCSI message.

   The proposed bit to be used is bit 7.  Bit 7 is currently used in
   the opcode field as the "retry" bit, and used in the "SCSI Command"
   as a field describing immediate data.  The "retry" bit could be
   moved to bit 5, and the immediate data field moved elsewhere in the
   flags field.

4. Security considerations

   A malicious interceptor could modify the urgent pointer information,
   falsely indicating message boundaries and corrupting the stream.
   However, such an interceptor could probably manipulate the stream to
   corrupt it in other ways. In addition, the urgent pointers should
   only be used to attempt iSCSI message synchronization when a TCP
   segment is missing, and not during in order packet reception.

Wakeley                                                       [Page 3]
Internet-Draft          iSCSI Message Boundary          September 2000

5. Authors' Addresses

   Matt Wakeley
   1101 Creekside Ridge Drive Suite 100
   Roseville, CA 95661
   Phone: 916-788-5670


   [Stevens] W Richard Stevens, "TCP/IP Illustrated, Volume 1," ISBN

   [TCP]  Postel, J., "Transmission Control Protocol - DARPA Internet
   Program Protocol Specification", RFC 793, September 1981

   [Requirements]  Braden, R., "Requirements for Internet Hosts --
   Communication Layers", RFC 1122, October 1989

   Expires February 2001

Wakeley                                                       [Page 4]