iSER and DA Frequently Asked
Q1. What is iSER?
iSER stands for "iSCSI Extensions for RDMA". As the name indicates, it is
an extension of the data transfer model of iSCSI, a storage networking standard
for TCP/IP. iSER enables the iSCSI protocol to take advantage of the direct data
placement technology of the RDMA (Remote Direct Memory Access) protocol suite (RDMAP/DDP/MPA)
publicly released by the RDMA Consortium in October 2002.
Q2. What is the purpose of iSER?
The iSER protocol seeks to increase the connectivity of the iSCSI end nodes so
that implementations can take advantage of the RDMA over TCP/IP protocol suite.
Q3. What are the advantages of iSER?
The iSER data transfer protocol allows iSCSI implementations to have data
transfers which achieve true zero copy behavior - eliminating TCP/IP processing
overhead as network speeds approach 10Gb/s - on generic RDMA network interface
controllers (RNIC), while preserving the compatibility with iSCSI
infrastructure. Also, in an iSCSI/iSER implementation, certain protocol aspects
of iSCSI, such as data integrity management and some error recovery features are
Q3a Why is true zero copy
As networking speeds approach 10Gb/s, much of the overhead of processing
networking traffic is related to memory-to-memory copying, particularly at the
receiver. True zero copy eliminates this overhead.
Q3b Is an RNIC the only
way to achieve true zero copy?
No, but other methods of achieving true zero copy in the NIC require the NIC to
become aware of the actual upper layer protocol being utilized (iSCSI, NFS, DAFS
etc.). An RNIC is the only way to achieve true zero copy without upper layer
protocol specific extensions in the NIC itself.
Q4. Is iSCSI being replaced by iSER?
No, in fact, an iSCSI/iSER implementation requires iSCSI components - such as
login negotiation, discovery, boot, security, and authentication including the
PDU formats – defined by the iSCSI specification. As already noted, iSER
is an extension of the data transfer model of the iSCSI protocol, but does not
change the other areas of the iSCSI protocol. One should view iSER only as a
Datamover for the iSCSI protocol.
Q5. Is iSER a new "iSCSI 2.0"?
No, an iSCSI/iSER implementation must be compliant to the same iSCSI protocol
defined by the IETF iSCSI specification. The iSER protocol utilizes an
existing, compatible iSCSI mechanism (login key negotiation) to determine
whether to use iSER or standard iSCSI data transfer models. Other aspects of the
iSCSI protocol (discovery, boot, security, authentication etc.) are left
unchanged. One should view iSER only as a Datamover for the iSCSI protocol.
Q6. What is DA (or Datamover Architecture)?
The Datamover Architecture for iSCSI (DA) specification defines an abstract
model in which the movement of data between iSCSI end nodes is logically
separated from the rest of the iSCSI protocol in order to allow iSCSI to adapt
to innovations available in new IP transports such as RDMA over TCP/IP.
Q7. How is DA related to iSER? [ or vice
The iSER protocol is a “Datamover protocol” as defined in the Datamover
Architecture for iSCSI specification. The iSER protocol thus applies the
Datamover Architecture for iSCSI in extending the data-movement capabilities of
iSCSI to include RDMA. The Datamover Architecture for iSCSI itself is agnostic
about the specifics of iSER or other supporting protocols.
Q8. Why did you have to define two specs -
DA and iSER?
The architectural intent behind each specification is different. The DA
specification defines a logical separation of data movement from the rest of the
iSCSI protocol in a way that is useful for increasing the connectivity of iSCSI
in the future (such as running iSCSI on other RDMA protocols or even running on
SCTP). The iSER protocol specification applies the Datamover Architecture in
defining a specific mapping of iSCSI’s data movement features to the RDMA
protocol suite released by the RDMA Consortium in October 2002.
Q9. Doesn't the publication of DA and iSER
specs confuse the nascent iSCSI industry?
No. This work extends the connectivity of iSCSI and is a clear demonstration of
the commitment of RDMA Consortium members to the iSCSI technology and its
advancement. The involvement of several individuals involved in the IETF
iSCSI specification effort also reinforces the complementary nature of iSCSI, DA
and iSER specifications.
Q10. Is the RDMA Consortium advising iSCSI users to move to
No. The iSER protocol definition is aimed at making iSCSI more pervasive
and is not meant to advise the customers on specific solutions. We expect
each customer to choose a solution that best fits their particular needs.
Q11. What is the roadmap for DA and iSER specs?
The iSER and DA specifications have been released by the RDMA Consortium and are
suitable for industry implementation today. The RDMA Consortium members will
maintain the specifications for any errata discovered during product
implementation. The specifications have also been forwarded to the IETF for
their consideration. Further development of DA and iSER specifications and their
eventual roadmap will be decided by the IETF.
Q12. Is iSER ready for product development?
Yes, the member companies of the RDMA Consortium regard the iSER specification
as suitable for iSCSI/iSER product implementation at the time of the iSER
specification release from the RDMA Consortium.
Q13. What about the interoperability problems between iSCSI
end nodes and the new iSCSI/iSER end nodes?
We do not expect any interoperability problems between iSCSI end nodes and iSCSI/iSER
end nodes. The iSER functionality extends the iSCSI protocol via standard
iSCSI architectural elements (login key negotiation) defined by the iSCSI
specification so that each end node is clearly aware of the mode of operation of
the other end node. Furthermore, the design of the iSER protocol requires
a node which supports iSCSI/iSER to be capable of providing iSCSI protocol login
service compliant with the current iSCSI specification, and to move into the
iSER mode as may be determined necessary during the login negotiation.
Therefore, new iSCSI/iSER implementations should be easily integrated with
current iSCSI networks.
Q14. Does iSER need a special RDMA-capable NIC (RNIC) design?
No. The iSER protocol is designed with an explicit intent to allow iSER to
run on any generic off-the-shelf RNIC. An equally important design goal
also was to allow innovations from vendors who seek to more tightly integrate
iSCSI/iSER into the RNIC hardware.
Q15. Why would such integration be important to vendors?
Many vendors want to continue to use their same APIs to the NICs whether or not
they are operating in traditional iSCSI or iSER mode. By integrating the
iSCSI/iSER functions within the RNIC hardware, the operating mode can be
transparent to the End-Node.
Q16. Does iSER work only on RDMA hardware?
The iSER protocol has no such constraints. The iSER protocol as defined,
should work transparently with either a hardware or software RDMA protocol
stack. Note however that an RNIC-based solution is likely to yield better
performance, assuming typical software processing power.
Q17. Does iSER require support for the recently released RDMA
No. The iSER protocol specification itself is completely independent of
the Verbs specification. However, an iWARP implementation compliant to the
RDMA Verbs specification will naturally satisfy all the expectations of the iSER
protocol in the most efficient manner.
Q18. Does iSCSI/iSER code running on an RNIC perform better
than an offloaded iSCSI NIC?
The relative performance depends on the specific iSCSI/iSER/RNIC design versus
the iSCSI NIC design. The iSER specification allows iSCSI end nodes to
take advantage of generic RDMA and data placement technologies, but equally
well-performing iSCSI NIC-based designs are feasible.
Q19. How does iSCSI/iSER with an RNIC position itself against
an iSCSI NIC?
The iSER Datamover is envisioned to be implemented in an iSCSI software
environment which exploits an RNIC, and can thereby offer performance
improvement and reduced overhead to the iSCSI software implementation.
iSCSI implementations that have been included in hardware, which are known as
iSCSI NICs, do not require iSER type of functions since they already do direct
memory placement, have the TCP/IP processing offloaded, have often been tailored
to the needs of iSCSI and tuned to produce the best possible performance.
The iSER specification was intended to permit both types of implementations to
co-exist, and thereby bring the compelling features of iSCSI to all platforms
with as low overhead as possible.
Q20. Where can I get additional information on the RDMA
protocol and iSER?
Additional information is available at the RDMA Consortium website which is
located at www.rdmaconsortium.org.