One document matched: draft-ietf-pwe3-fc-flow-00.txt
PWE3 Moran Roth (Ed.)
Internet-Draft Ronen Solomon
Intended status: Standards Track Corrigent Systems
Expires: July 15, 2009 Munefumi Tsurusawa
KDDI
January 15, 2009
Reliable Fibre Channel Transport Over MPLS Networks
draft-ietf-pwe3-fc-flow-00.txt
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Abstract
A Fibre Channel pseudowire (PW) is used to carry Fibre Channel frames
over an MPLS network. This enables service providers to offer
"emulated" Fibre Channel services over existing MPLS networks. This
document specifies the mechanisms controlling the reliable transport
of Fibre Channel PW over MPLS networks. The encapsulation of Fibre
Channel PDUs within a pseudowire and the procedures for using a PW to
provide a Fibre Channel service are specified in [FC-encap].
Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [1].
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Table of Contents
1. Introduction...................................................4
2. Congestion Control.............................................4
2.1. Rate Control..............................................5
2.1.1. Protocol Mechanism......................................5
2.1.2. Data Sender Protocol....................................6
2.1.3. Data Receiver Protocol..................................8
2.2. Selective Retransmission overview.........................8
2.2.1. FC Encapsulation Header................................10
2.2.2. Encapsulation Header field parameters..................12
2.2.3. Selective reject (SR-SREJ) frame.......................13
2.2.4. Exception condition reporting and recovery.............14
2.3. Selective Retransmission procedures......................16
2.3.1. SR mode of operation...................................16
2.3.2. SR procedure for addressing............................16
2.3.3. SR procedure for the use of the Poll/Final bit.........16
2.3.4. Procedures for information transfer....................17
2.3.5. List of SR system parameters...........................24
3. Timing Consideration..........................................26
4. Applicability Statement.......................................28
5. Normative References..........................................28
6. Informative references........................................29
7. Author's Addresses............................................29
8. Contributing Author Information...............................30
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1. Introduction
As metro transport networks migrate towards a packet-oriented network
infrastructure, the PSN is being extended in order to allow all
services to be transported over a common network infrastructure. This
has been accomplished for services such as Ethernet [RFC4448], Frame
Relay [RFC4619], ATM [RFC4717] and SONET/SDH [RFC4842] services.
Another such service, which has yet to be addressed, is the transport
of Fibre Channel (FC) frames over the PSN. This will allow network
service providers to transparently carry FC services over the packet-
oriented network, along with the aforementioned data and TDM
services.
FC frames encapsulation within PW PDUs and the procedures for using a
PW to provide a Fibre Channel service are specified in a companion
document [FC-encap]. However, complementary mechanisms are required
to provide reliable FC transport between FC entities.
This document specifies the mechanisms to provide reliable transport
for FC traffic over MPLS networks, specifically, two mechanisms are
specified:
(1) Congestion avoidance: this mechanism is intended to alleviate
congestion conditions. In order to provide TCP-friendly
operation the transmission rate is controlled by the
throughput equation specified in [RFC3448].
(2) Selective Retransmission: this mechanism enable lossless
transmission of FC frames by retransmission of PW PDUs that
were discarded in transit in order to provide lossless
transmission for the FC service. A selective retransmission
mechanism is specified.
2. Congestion Control
FC PW traffic can be transmitted over networks that may experience
congestion due to statistical multiplexing. When congestion
conditions are experienced frames may be discarded within the MPLS
PSN. Congestion control mechanism is required to prevent congestion
collapse and provide fairness among the different connections.
Fairness is usually defined with respect to TCP flow control
[RFC2914]. The FC PW relies on a congestion control mechanism that
provides TCP-friendly behavior by controlling the transmission rate
into the PSN by a rate shaper, whose output rate is a function of
network congestion.
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Frame loss within the MPLS PSN also requires a reliable transmission
mechanism in the PE to support faithful emulation of FC service,
providing in-order, no-loss transport of FC traffic between CE1 and
CE2. Reliable transmission is provided by a sliding-window selective
retransmission (SR) mechanism to allow efficient retransmission of
lost frames. This was standardized for FC transport in [FC-BB]. The
SR mechanism also provides congestion indication (i.e. Frame loss
events) to the rate control mechanism.
2.1. Rate Control
The rate control mechanism provides adaptive shaper control in
response to network congestion indications. The rate shaper is
configured with BW attributes, such as CIR and EIR, assigned to the
FC PW service. The rate control operation is based on [RFC3448]. In
the following sections the applicability of [RFC3448] to FC PW is
analyzed, and rate control operation is detailed.
[RFC3448] is a receiver-based congestion control mechanism, where the
congestion control information (i.e., the loss event rate) is
calculated by the receiver. In FC PW, on the other hand, the
congestion control information is calculated by the sender. This
approach is more appropriate for the point-to-point nature of FC PW.
This sender-based approach is also mentioned in [RFC3448] as a
possible variant of the protocol.
2.1.1. Protocol Mechanism
In accordance with [RFC3448] the actual allowed sending rate is
directly computed by a throughput equation, as a function of lost
frames and round trip time. In general, the congestion control
mechanism works as follows:
o The receiver detects lost frames and feeds this information
back to the sender as part of the SR mechanism.
o The sender calculates the frame loss probability and measures
the round-trip time (RTT) as defined in [FC-BB].
o The lost frame probability and RTT are then fed into the
throughput equation, calculating the acceptable transmission
rate.
o The sender then adjusts its transmission rate to match the
calculated rate in accordance with the service BW attributes
(CIR, EIR).
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As the CIR is guaranteed, the throughput equation controls only the
excess transmission rate. The parameters of the throughput equation
are set as follows:
o The retransmission timeout (t_RTO) is replaced by the T1 timer
of the SR mechanism as defined in section 6.3.
o The number of frames acknowledged by a single SR acknowledgment
frame (b) is set to b = 1, as recommended in [RFC3448].
Different implementation MAY use delayed acknowledgement by
increasing the value of b.
Frame loss probability (p) and RTT (R) are calculated as specified in
Section 6.1.2.
2.1.2. Data Sender Protocol
The data sender sends a stream of data frames to the data receiver at
a controlled rate. When a feedback frame is received from the data
receiver, the data sender calculates the frame loss probability and
changes its sending rate accordingly. If the sender does not receive
a feedback frame during a timeout period, it reduces its sending
rate. This is achieved by the SR T1 timer.
We specify the sender-side protocol in the following steps:
o Sender initialization.
o The sender behavior when a feedback frame is received.
o The sender calculation of the frame loss probability.
o The sender behavior when a feedback frame is not received for
a timeout period.
The sender rate shaper is initialized to transmit at the CIR. The SR
mechanism is also initialized by resetting the sequence numbers.
The sender calculates RTT (R), based on delay measurement frames
transmitted by the NSP (as defined in [FC-BB]). These frames MUST be
sent at least every 100 milliseconds, and are used to measure round
trip samples that are averaged to obtain RTT (refer to [RFC3448]
section 4.3 for details). If an RTT measurement is missed (either due
to a loss of a delay measurement frame, or to an RTT larger than the
measurement period), PE1 SHOULD shut the PW down, as specified in
[FC-BB].
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The sender calculates the frame loss probability based on feedback
frames generated by the receiver. A feedback frame with accordance to
the SR mechanism defined in [FC-BB] is one of the following:
o Receiver Ready (RR) - a frame that includes the N(R) counter to
acknowledge the sender frames up to frame N(R).
o Receiver Not Ready (RNR) - a frame that includes the N(R)
counter to acknowledge the sender frames up to frame N(R), and
pause the sender from sending additional frames.
o Selective Reject (SREJ) - a frame that includes lost frames
indication (sequence numbers).
When the sender receives a feedback frame it re-calculates the frame
loss probability. RR and RNR will effectively decrease the frame loss
probability due to no frame loss. On the other hand, reception of a
SREJ frame tends to increase the frame loss probability. An
implementation MAY consider sending feedback frames, in a controlled
network environment, with expedite forwarding (EF) CoS to assure
delivery.
After the frame loss probability is updated, the sender calculates a
new transmission rate for the rate shaper. The transmission rate is
calculated as: Rate = MIN( PIR, MAX(CIR,X) ), where CIR is the
Committed Information Rate, PIR is the Peak Information Rate (PIR =
CIR+EIR), X is the outcome of the throughput equation as specified in
[RFC3448], and MIN/MAX are functions returning the smallest/largest
value among their operands, respectively. Note that the transmission
rate as controlled by the above function, is bounded in the range
[CIR,PIR].
No feedback in accordance with [RFC3448] is defined by the timer T1.
When the sender does not receive a feedback for such an interval it
halves its transmission rate as defined in [RFC3448]. The
transmission rate equation as specified above MUST still be applied
to guarantee that the CIR is the lower limit for the throughput. The
procedure controlling timer T1 (refer to Section 6.3) guarantees that
transmission rate is not halved during idle periods, as the timer is
not activated during these periods.
The maximum burst size allowed MUST be limited to a round-trip time's
worth of packets, to achieve efficient transmission while conforming
to [RFC3448].
In case the transmission rate is equal to CIR for a period greater
than RTT, and transmitted frames are still lost in transit, as
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indicated to the sender by receiving SREJ frames, the sender MUST
signal PW status of "Unable to maintain minimum transmission rate"
(refer to Section 9 - "IANA Considerations" for details) as specified
in [RFC4447], and MUST stop transmission over the PW for a duration
of 10 seconds (this period allows a transient network problem to
resolve itself, and guarantees that no more than one HELLO message
[FC-SW] is lost, and the link between the two FC devices is not
affected). The sender and receiver MUST discard all frames residing
in the buffers associated with the congested PW. The sender MUST also
discard all frames received from the attached FC device. If within 10
seconds after transmission was restarted severe congestion conditions
are encountered, as described above (i.e., CIR cannot be maintained),
the sender MUST shut the PW down, as described in Section 6.5 of
[RFC3985]. If the PW has been set up using the PWE3 control protocol
[RFC4447], the regular PW teardown procedures SHOULD be used. The PW
MUST NOT be automatically restarted, and administrative intervention
is required. Upon PW shutdown the sender and receiver MUST discard
all frames associated with the PW. Note that congestion may be
avoided by employing connection admission control (CAC) mechanism,
which assures that congestion conditions will not be reached when a
PW is transmitting at its configured CIR.
2.1.3. Data Receiver Protocol
The data receiver receives a stream of data frames from the data
sender, generates SR feedback frames (SR-RR, SR-RNR and SR-SREJ), and
sends them to the data sender. The details of feedback frames
generation and transmission are specified in section 6.3.
2.2. Selective Retransmission overview
The selective retransmission mechanism provides efficient
retransmission of lost frames to enable faithful emulation of FC
service, with no frame loss experienced by the CE. The proposed
selective retransmission mechanism was standardized for FC transport
in [FC-BB], and is specified in details in this standard.
The SR protocol is an efficient sliding window full-duplex protocol
that supports both the flow control and error recovery functions. SR
has been adopted from ITU's Link Access Protocol B (LAPB) that was
derived from ISO/IEC's High-level Data Link Control (HDLC) balanced
classes. Use of LAPB in SR is limited to a subset of the synchronous
modulo 32768 super sequence numbering service option.
SR works between two PE devices (see figure 7). SR flow control works
by streaming multiple messages within an allowed window, bounded by
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the system parameter k, and awaits acknowledgements before sending
more messages. Acknowledgements indicate which messages were
correctly received and there is a provision for requesting
retransmission of selected messages in the current window. Fibre
Channel Sequences and Exchanges are not visible to the SR flow
control protocol which sees the PW packets constructed from the FC
frames.
Some benefits of the SR protocol are summarized below:
a) it is used for reliable transport of all Class 2, 3, 4, and F
frames between two PE devices;
b) it optimizes buffer management at the PE devices;
c) it acts as a congestion avoidance technique to match the capacity
of the sender to the capacity of the network that carries the
payload;
d) it ensures correct delivery of messages (i.e., an error control
and recovery function); and
e) it provides a continuous stream of traffic across the MPLS
PSN thus leading to a higher throughput (i.e., optimizes bandwidth
utilization at each BBW device).
Note that the synchronization of the Sender PE and the Receiver PE at
the PW message level, which is required for correct SR operation is
performed through PW signaling.
The four different SR messages described in section 6.2.1 have a
correspondence to the LAPB frame types. Note that only the
information transfer SR-I message is flow-controlled while all other
messages are control messages of the protocol.
The SR protocol specifies the maximum number (k) of outstanding
messages at any given time. k is a system parameter that is not
negotiated and is fixed in a given implementation. The value of this
system parameter depends on the MPLS PSN delay characteristics and
the number of buffers available. Typically, the value of k is
expected to be far below the maximum number of 32767.
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+--------------------+ +--------------------+
| PE | | PE |
| | | |
| +--------------+ | | +--------------+ |
| | Flow Control |<---------------->| Flow Control | |
| | Protocol | | | | Protocol | |
| +--------------+ | | +--------------+ |
| | | | | |
| | | | | |
| +--------------+ | | +--------------+ |
| | PW Interface | | | | PW Interface | |
| +--------------+ | | +--------------+ |
| | | |
+--------------------+ +--------------------+
| - -- |
| -- / \ / \ |
| / \/ -- \ |
| \ / |
| \ MPLS \ |
+---------/ PSN /-----------+
/ /
-------------
Figure 7 - SR flow control protocol between two PEs
2.2.1. FC Encapsulation Header
The FC Encapsulation Header defines two types of field formats that
are used to perform information transfer (i.e., I-format frames), and
supervisory functions (i.e., S-format frames). SR makes use of four
different types of messages:
a) I-format (1): SR-I frame.
This frame is used to perform an information transfer. The
Encapsulation Header of an I-format frame is shown in figure 8.
The I-frame Encapsulation Header contains the following fields:
(1) N(S): Transmitter send sequence number.
(2) N(R): Transmitter receive sequence number.
(3) P: Poll bit (1 = Poll).
A detailed description of the different fields and explanation of
The functionality involved is provided in section 6.2.2.
An SR-I frame is a command message (i.e., the A-bit in the Control
Word is set to 1), and carries an encapsulated FC frame.
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b) S-format (3): SR-RR, SR-RNR, SR-SREJ frames.
These frames are used to perform supervisory control functions of
the Selective Retransmission mechanism, such as acknowledge SR-I
messages, request retransmission of SR-I messages, and to request
a temporary suspension of transmission of SR-I messages. The
Encapsulation Header of an S-format frame is shown in figure 9.
The S-frame Encapsulation Header contains the following fields:
(1) N(R): Transmitter receive sequence number.
(2) S: Supervisory function bits to define the frame type.
S = 00: SR-RR.
S = 01: Reserved.
S = 10: SR-RNR.
S = 11: SR-SREJ.
(3) P: Poll/Final bit (refer to section 6.2.2 for detailed
description).
(4) Reserved: MUST be set to 0 by the ingress PE, and MUST be
ignored by the egress PE.
A detailed description of the different fields and explanation of
The functionality involved is provided in section 6.2.2.
An SR-RR frame carries no payload, and may be either a command or
response message (the A-bit in the Control Word is set to 1 for a
Command, and to 0 for a Response). It indicates Ready to Receive
SR-I messages (negates busy condition) and acknowledges previous
SR-I messages.
An SR-RNR frame carries no payload, and may be either a command or
response message. It indicates Receiver not Ready to accept more
SR-I messages (busy condition) and acknowledges previous SR-I
messages.
An SR-SREJ frame may be either a command or response message, and
carries a payload that indicate SR-I frames in need of selective
retransmission.
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1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-----------------------------+-+-----------------------------+
|0| N(S) |P| N(R) |
+-+-----------------------------+-+-----------------------------+
Figure 8 - FC Encapsulation Header format for I-frame
1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+---+-----------------------+-+-----------------------------+
|1|0| S | Reserved |P| N(R) |
+-+-+---+-----------------------+-+-----------------------------+
Figure 9 - FC Encapsulation Header format for S-frame
2.2.2. Encapsulation Header field parameters
The following describes the different fields of the Encapsulation
Header and details how these fields are handled.
a) Modulus of SR - Each SR-I message is sequentially numbered and may
have the value 0 through modulus minus 1, where "modulus" is equal
to 32768 (i.e., the maximum value of the sequence numbers). The
sequence numbers cycle through the entire range.
b) Send state variable V(S) - The send state variable V(S) denotes
the sequence number of the next-in-sequence SR-I message to be
transmitted. V(S) may take on the values 0 through modulus minus
1. The value of V(S) is incremented by 1 with each successive SR-I
message transmission, but cannot exceed the N(R) of the last
received SR-I or supervisory message by more than the maximum
number of outstanding SR-I messages k. The value of k is defined
in section 6.3.5.
c) Send sequence number N(S) - Only SR-I messages contain N(S), the
send sequence number of the transmitted SR-I message. At the time
that an in-sequence SR_I message is designated for transmission,
the value of N(S) is set to the value of the send state variable
V(S).
d) Receive state variable V(R) - The receive state variable V(R)
denotes the sequence number of the next-in-sequence SR-I message
expected to be received. V(R) may take on the values 0 through
modulus minus 1. The value of V(R) is incremented by 1 by the
receipt of an error-free, in-sequence SR-I message whose send
sequence number N(S) equals the receive state variable V(R).
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e) Rceive sequence number N(R) - All SR-I messages and supervisory
messages, except SR-SREJ messages with the F bit set to 0, SHALL
contain N(R), the expected send sequence number of the next
received SR-I message. At the time that a message of the above
types is designated for transmission, the value of N(R) is set to
the current value of the receive state variable V(R). N(R)
indicates that the PE transmitting the N(R) has correctly received
all SR_I messages numbered up to and including N(R)-1.
f) Functions of the Poll/Final bit (P-bit) - All messages contain P-
bit, the Poll/Final bit. In command messages, the P-bit is
referred to as the Poll bit. In response messages it is referred
to as the Final bit.
The Poll bit set to 1 is used by the PE to solicit (i.e., poll) a
response from the remote PE.
The Final bit set to 1 is used by the PE to indicate the response
message transmitted by the remote PE, as a result of the
soliciting (i.e., poll) command.
The use of the P/F bit is further described in section 6.3.3.
2.2.3. Selective reject (SR-SREJ) frame
The SR-SREJ supervisory message is used by a PE to request
retransmission of one or more, not necessarily contiguous, SR-I
messages. The N(R) field SHALL contain the sequence number of the
earliest SR-I message to be retransmitted and the information field
(see figure 9) SHALL contain, in ascending order (i.e., 32767 is
higher than 32766 and 0 is higher than 32767 for modulo 32768), the
sequence numbers of additional SR-I message(s), if any, that needs to
be retransmitted.
The payload field SHALL be encoded such that there is a 2-byte field
for each standalone SR-I message in need of retransmission, and a 4-
byte span list for each sequence of two or more contiguously numbered
SR-I messages in need of retransmission, as depicted in figure 9.
Standalone SR-I messages are identified in the payload field by the
appropriate N(R) value preceded by a 0 bit in the 2-byte field used.
Span lists are identified in the payload field by the N(R) value of
the first SR-I message in the span list preceded by a 1 bit in the 2-
byte field used, followed by the N(R) value of the last message in
the span list preceded by a 1 bit in the 2-byte field used.
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The maximum payload size of a SR-SREJ message is 2148 bytes
corresponding to a maximum possible encoding of 1074 standalone SR-I
messages or a maximum possible encoding of 537 span list sets.
If the P-bit in an SR-SREJ message is set to 1, then SR-I messages
numbered up to N(R)-1 inclusive, N(R) being the value in the
Encapsulation Header field, shall be considered as acknowledged. If
the P-bit in an SR-SREJ message is set to 0, then the N(R) in the
Encapsulation Header field of the SR-SREJ message does not indicate
acknowledgement of SR-I messages.
1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+---------------------------------------------------------------+
| FC PW Control Word |
+---------------------------------------------------------------+
| FC Encapsulation Header |
+-+-----------------------------+-+-----------------------------+
|0| N(R) of standalone SR-I |1| N(R) of first SR-I in span |
+-+-----------------------------+-+-----------------------------+
|1| N(R) of last SR-I in span |0| N(R) of standalone SR-I |
+-+-----------------------------+-+-----------------------------+
|1| N(R) of first SR-I in span |1| N(R) of last SR-I in span |
+-+-----------------------------+-+-----------------------------+
| |
. .
. .
+---------------------------------------------------------------+
Figure 10 - SR-SREJ frame format example
2.2.4. Exception condition reporting and recovery
The error recovery procedures that are available to effect recovery
following the detection/occurrence of an exception condition are
described in this section. Exception conditions described are those
situations that may occur as the result of transmission errors, PE
device malfunction, or operational situations.
a) Busy condition - The busy condition results when the PE is
temporarily unable to continue to receive SR-I messages due to
internal constraints (e.g., receive buffering limitations). Upon
entering the busy condition, a PE transmits an SR-RNR message.
SR-I messages pending transmission may be transmitted from the
busy PE prior to or following the SR-RNR message.
An indication that the busy condition has cleared is communicated
by the transmission of SR-RR or SR-SREJ.
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b) N(S) sequence error condition - The information field of all
received SR-I messages whose N(S) is not in the range V(R) and
V(R)+k-1 inclusive, SHALL be discarded. The information field of
all SR-I messages received by the PE whose N(S) is in the range
V(R) and V(R) + k -1 inclusive, SHALL be saved in the receive
buffer.
An N(S) sequence error exception condition occurs in the receiver
when a received SR-I message contains an N(S) that is not equal to
the receive state variable V(R) at the receiver. The receiver
SHALL not acknowledge (i.e., increment its receive state variable)
the SR-I message causing the sequence error, or any SR-I message
that may follow, until an SR-I message with the correct N(S) is
received.
A PE that receives one or more valid SR-I messages having sequence
errors or subsequent supervisory messages (i.e., SR-RR, SR-RNR, or
SR-SREJ) shall accept and handle the N(R) field and the P-bit.
The means specified below shall be available for initiating the
retransmission of lost or errored SR-I messages following the
occurrence of an N(S) sequence error condition.
(1) SR-SREJ recovery - The SR-SREJ message shall be used to
initiate more efficient error recovery by selectively
requesting the retransmission of one or more, not necessarily
contiguous, lost or errored SR-I message(s) following the
detection of sequence errors, rather than requesting the
retransmission of all SR-I messages.
When a PE receives an out-of-sequence message, the SR-I
message shall be saved in a receive buffer. The SR-I message
shall be delivered to the upper layer only when all SR-I
messages numbered below N(S) are correctly received. If
message number N(S)-1 has not been received previously, then
an SR-SREJ response message with the P-bit set to 0 shall be
transmitted containing the sequence numbers of the block of
consecutive missing SR-I messages ending at N(S)-1. On
receiving such an SR-SREJ message the PE shall retransmit all
requested SR-I messages. After retransmitting these SR-I
messages, the BBW may transmit new SR_I messages, if they
become available.
When a PE receives a command message with the P-bit set to 1,
if there are out-of-sequence SR-I messages saved in the
receive buffer, it shall transmit an SR-SREJ message, with the
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F bit set to 1, containing a complete list of missing sequence
numbers. The PE that receives the SR-SREJ message shall
retransmit all requested SR-I messages, except those that were
transmitted subsequent to the last command message with the P
bit set to 1.
(2) Time-out recovery - If a PE, due to a transmission error, does
not receive, or receives and discards, a single SR-I message
or the last SR-I message in a sequence of SR-I messages, it
shall not detect a N(S) sequence error condition and,
therefore, shall not transmit an SR-SREJ message.
The PE that transmitted the unacknowledged SR-I message(s)
shall, following the completion of a system specified time-out
period (see section 6.3.4 items b) and j) below), send a
supervisory command message (i.e., SR-RR or SR-RNR) with the
P-bit set to 1. SR-I messages shall be retransmitted on the
receipt of an SR-RR response message with the F bit set to 1
or an SR-SREJ message.
c) Invalid message condition - Any message that is invalid shall be
discarded, and no action is taken as the result of that message.
An invalid message is defined as one that contains:
(1) the Control Word with an invalid encoding; or
(2) the Encapsulation Header with an invalid encoding.
2.3. Selective Retransmission procedures
2.3.1. SR mode of operation
The SR protocol shall be limited to a subset of the synchronous
modulo 32768 super sequence numbering service option operation of the
LAPB protocol. The SR protocol is initialized upon PW set-up
following a successful signaling session.
2.3.2. SR procedure for addressing
The Address Bit field in the Control Word (see figure 3) identifies a
message as either a command or a response. This field is used in
conjunction with the P-bit (Poll/Final).
2.3.3. SR procedure for the use of the Poll/Final bit
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The PE receiving a supervisory command (i.e., SR-RR, SR-RNR, SR-
SREJ), or SR-I message with the P bit set to 1 shall set the F bit to
1 in the next response message it transmits.
The response message returned by the PE to an SR-I message with the P
bit set to 1, shall be an SR-RR, SR-SREJ, or SR-RNR response with the
F bit set to 1.
The response message returned by the PE to a supervisory command with
the P bit set to 1, shall be an SR-RR, SR-RNR, or SR-SREJ response
with the F bit set to 1.
The P bit may be used by the PE in conjunction with the timer
recovery condition (see section 6.3.4. item j) below).
2.3.4. Procedures for information transfer
a) Procedures for SR-I messages
The procedures that apply to the transmission of SR-I messages in
each direction using multi-selective reject are described below.
b) Sending new SR-I messages
When the PE has a new SR-I message to transmit (i.e., an SR-I message
not already transmitted), it shall transmit it with a N(S) equal to
its current send state variable V(S), and a N(R) equal to its current
receive state variable V(R). At the end of the transmission of the
SR-I message, it shall increment its send state variable V(S) by 1.
If the SR timer T1 is not running at the time of transmission of the
SR-I message, it shall be started.
If the SR send state variable V(S) is equal to the last value N(R)
received plus k, where k is the maximum number of outstanding SR-I
frames (see section 6.3.5), the PE shall not transmit any new SR-I
frames.
If the remote PE is busy, the PE shall not transmit any new SR-I
messages.
When the PE is in the busy condition, it may still transmit SR-I
messages, provided that the remote PE is not busy.
c) Receiving an in-sequence SR-I message
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When the PE is not in a busy condition and receives a valid SR-I
message whose send sequence number N(S) is equal to its receive state
variable V(R), the PE shall accept the information field of this
message and increment by one the receive state variable V(R). If the
SR-I message, whose N(S) is equal to the incremented value of V(R),
is present in the receive buffer, then the PE shall remove it from
the receive buffer, deliver it to the upper layer and increment V(R)
by one. The PE shall repeat this procedure until V(R) reaches a value
such that the SR-I message whose N(S) is equal to V(R) is not present
in the receive buffer. The PE shall then take one of the following
actions:
(1) if the PE is now in the busy condition, it shall transmit an
SR-RNR message with N(R) equal to the value of the SR receive
variable V(R) (see item i) below); or
(2) if the PE is still not in a busy condition:
- if the P bit is set to 1, then the PE shall transmit a
response message with the F bit set to 1, as specified in
item l) below;
- if an SR-I message is available for transmission the PE
shall act as described in item b) above, sending new SR-I
messages and acknowledging the received SR-I message by
setting N(R) in the Encapsulation Header field of the next
transmitted SR-I message to the value of the SR receive
state variable V(R), or the PE shall acknowledge the
received SR-I message by transmitting an SR-RR message
with the N(R) equal to the value of the SR receive state
variable V(R); or
- the PE shall transmit an SR-RR message with N(R) equal to
the value of the SR receive state variable V(R).
When the PE is in a busy condition, it may ignore the information
field contained in any received SR-I message.
d) Reception of invalid messages
When the PE receives an invalid message (see 6.2.4. item c), it shall
discard the message.
e) Reception of out-of-sequence SR-I messages
When the PE is not in a busy condition and it receives a valid SR-I
message whose send sequence number N(S) is out-of-sequence, (i.e.,
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not equal to the receive state variable V(R)), then it shall perform
one of the following actions:
1) if N(S) is less than V(R) or greater than or equal to V(R) + k,
then it shall discard the information field of the SR-I
message. If the P bit of the SR-I message is set to 1, then the
PE shall transmit a response message with the F bit set to 1,
as specified in item l) below; or
2) if N(S) is greater than V(R) and less than V(R) + k, then it
shall save the SR-I message in the receive buffer. It shall
then perform one of the following actions:
- if the P bit of the SR-I message is set to 1, then the PE
shall transmit a response message with the F bit set to 1,
as specified in item l) below;
- if the PE is now in a busy condition, it shall transmit an
SR-RNR message with N(R) equal to the value of the receive
variable V(R), as specified in item i) below; or
- if the SR-I message numbered N(S)-1 has not yet been
received, then the PE shall transmit an SR-SREJ response
message with the F bit set to 0. The PE shall create a list
of contiguous sequence numbers N(X), N(X)+1, N(X)+2,...,
N(S)-1, where N(X) is greater than or equal to V(R) and
none of the SR-I messages N(X) to N(S)-1 have been
received. The N(R) field of the SR-SREJ message shall be
set to N(X) and the information field set to the list
N(X)+1,...,N(S)-1. If the list of sequence numbers is too
large to fit into the information field of the SR-SREJ
message, then the list shall be truncated to fit in one
SR-SREJ message, by including only the earliest sequence
numbers.
When the PE is in the busy condition, it may ignore the information
field contained in any received SR-I message.
f) Receiving acknowledgement
When correctly receiving an SR-I message or a supervisory message
(i.e., SR-RR, SR-RNR, or SR-SREJ with the F bit set to 1), even in
the busy condition, the PE shall consider the N(R) contained in this
message as an acknowledgement for all the SR-I messages it has
transmitted with a N(S) up to and including the received N(R)-1. The
PE shall stop the timer T1 if the received supervisory message has
the F bit set to 1 or if there is no outstanding poll condition and
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the N(R) is higher than the last received N(R), actually
acknowledging some SR-I messages.
If timer T1 has been stopped by the receipt of an SR-I message, an
SR-RR command message, an SR-RR response message with the F bit set
to 0, or an SR-RNR message, and if there are outstanding SR-I
messages still unacknowledged, the PE shall restart timer T1. If
timer T1 has been stopped by the receipt of an SR-SREJ message with
the F bit set to 1, the PE shall follow the retransmission procedure
specified in item g.2) below. If timer T1 has been stopped by the
receipt of an SR-RR message with the F bit set to 1, the PE shall
follow the retransmission procedure specified in item k) below.
g) Receiving an SR-SREJ response message
1) Receiving an SR-SREJ response message with the F bit set to 0
When receiving an SR-SREJ response message with the F bit set
to 0, the PE shall retransmit all SR-I messages, whose sequence
numbers are indicated in the N(R) field and the information
field of the SR-SREJ message, in the order specified in the
SR-SREJ message. Retransmission shall conform to the following:
- if the PE is transmitting a supervisory or SR-I message
when it receives the SR-SREJ message, it shall complete
that transmission before commencing transmission of the
requested SR-I messages; or
- if the PE is not transmitting any message when it receives
the SR-SREJ message, it shall commence transmission of the
requested SR-I messages immediately.
If there is no outstanding poll condition, then a poll shall be
sent, either by transmitting an SR-RR command, or SR-RNR
command if the PE is in the busy condition, with the P bit set
to 1 or by setting the P bit in the last retransmitted SR-I
message and timer T1 shall be restarted.
If there is an outstanding poll condition, then timer T1 shall
not be restarted.
2) Receiving an SR-SREJ response message with the F bit set to 1
When receiving an SR-SREJ response message with the F bit set
to 1, the PE shall retransmit all SR-I messages, whose sequence
numbers are indicated in the N(R) field and the information
field of the SR-SREJ message, in the order specified in the
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SR-SREJ message, except those messages that were sent after the
message with the P bit set to 1 was sent. Retransmission shall
conform to the following:
- if the PE is transmitting a supervisory message or SR-I
message when it receives the SR-SREJ message, it shall
complete that transmission before commencing transmission
of the requested SR-I messages; or
- if the PE is not transmitting any message when it receives
the SR-SREJ message, it shall commence transmission of the
requested SR-I messages immediately.
If any messages are retransmitted, then a poll shall be sent,
either by transmitting an SR-RR command, or SR-RNR command if
the PE is in the busy condition, with the P bit set to 1 or by
setting the P bit in the last retransmitted SR-I message.
Timer T1 shall be restarted.
h) Receiving an SR-RNR message
After receiving an SR-RNR message, the PE shall stop transmission of
SR-I messages until an SR-RR or SR-SREJ message is received.
The PE shall start timer T1, if necessary, as specified in section
6.3.5.
When timer T1 runs out before receipt of a busy clearance indication,
the PE shall transmit a supervisory message (i.e., SR-RR, SR-RNR),
with the P bit set to 1 and shall restart timer T1, in order to
determine if there is any change in the receive status of the remote
PE. The remote PE shall respond to the P bit set to 1 with a
supervisory response message (i.e., SR-RR, SR-RNR, SR-SREJ) with the
F bit set to 1 indicating continuation of the busy condition (i.e.,
SR-RNR message) or clearance of the busy condition (i.e., SR-RR, SR-
SREJ). Upon receipt of the remote PE response, timer T1 shall be
stopped. The PE shall process the supervisory response message as
follows:
1) if the response is an SR-RR message, the busy condition shall
be assumed to be cleared and the PE may retransmit messages as
specified in item k) below. New SR-I messages may be
transmitted as specified in item b) above;
2) if the response is an SR-SREJ message, the busy condition shall
be assumed to be cleared and the PE may retransmit messages as
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specified in item g.2) above. New SR-I messages may be
transmitted as specified in item b) above; or
3) if the response is an SR-RNR message, the busy condition shall
be assumed to still exist and the PE, after a period of time
(e.g., the duration of timer T1), shall repeat the enquiry of
the remote PE receive status.
If timer T1 runs out before a status response is received, the
enquiry process above shall be repeated. If N2 attempts to get a
status response fail, the PE MAY declare the PW as down.
If, at any time during the enquiry process, an unsolicited SR-RR or
SR-SREJ message is received from the remote PE, it shall be
considered to be an indication of clearance of the busy condition.
Should the unsolicited SR-RR message be a command message with the P
bit set to 1, the appropriate response message with the F bit set to
1 shall be transmitted (see item l) below) before the PE may resume
transmission of SR-I messages. The PE shall not clear the outstanding
poll condition. The PE shall not stop timer T1. If an unsolicited SR-
SREJ message is received, then the PE shall perform retransmissions
as specified in item g.1) above.
i) BBW busy condition
When the PE enters a busy condition, it shall transmit an SR-RNR
message at the earliest opportunity. The SR-RNR message shall be a
command frame with the P bit set to 1 if an acknowledged transfer of
the busy condition indication is required, otherwise the SR-RNR
message may be a command or response message. While in the busy
condition, the PE shall accept and process supervisory messages,
accept and process the N(R) field of SR-I, SR-RR, and SR-SREJ
messages with the F bit set to 1, and return an SR-RNR response with
the F bit set to 1 if it receives a supervisory command or SR-I
command message with the P bit set to 1. Received SR-I messages may
be discarded or saved as specified in items c) and e) above, however,
SR-RR or SR-SREJ messages shall not be transmitted. To clear the busy
condition, the PE shall transmit an SR-RR message, with the N(R)
field set to the current receive state variable V(R). The SR-RR
message shall be a command message with the P bit set to 1 if an
acknowledged transfer of the busy-to-non-busy transition is required,
otherwise the SR-RR message may be either a command or response
message.
j) Awaiting acknowledgement
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If the timer T1 runs out while waiting for the acknowledgement of an
SR-I message from the remote PE, the PE shall restart timer T1 and
transmit an appropriate supervisory command message (i.e., SR-RR, SR-
RNR) with the P bit set to 1. The PE may transmit new SR-I messages
after sending this enquiry message.
If the PE receives an SR-SREJ response message with the F bit set to
1, the PE shall restart timer T1 and retransmit SR-I messages as
specified in item g.2) above.
If the PE receives an SR-SREJ response message with the F bit set to
0, the PE shall retransmit SR-I messages as specified in item g.2)
above.
If the PE receives an SR-RR response message with the F bit set to 1,
the PE shall restart timer T1 and retransmit SR-I messages as
specified in item k) below.
If the PE receives an SR-RR response message with the F bit set to 0,
or an SR-RR command message or SR-I message with the P bit set to 0
or 1, the PE shall not restart timer T1, but shall use the received
N(R) as an indication of acknowledgement of transmitted SR-I messages
up to and including SR-I message numbered N(R)-1.
If timer T1 runs out before a supervisory response message with the F
bit set to 1 is received, the PE shall retransmit an appropriate
supervisory command message (i.e., SR-RR, SR-RNR) with the P bit set
to 1. After N2 such attempts, the PE MAY declare the PW as down.
k) Receiving an SR-RR response messages with the F bit set to 1
When receiving an SR-RR response message with the F bit set to 1, the
PE shall process the N(R) field as specified in item f) above. If
there are outstanding SR-I messages that are unacknowledged and no
new SR-I messages have been transmitted subsequent to the last
message with the P bit set to 1, then the PE shall retransmit all
outstanding SR-I messages except those that were sent after the
message with the P bit set to 1 was sent. Retransmission shall
conform to the following:
1) if the PE is transmitting a supervisory or SR-I message when it
receives the SR-RR message, it shall complete that transmission
before commencing transmission of the requested SR-I messages;
2) if the PE is not transmitting any message when it receives the
SR-RR message, it shall commence transmission of the requested
SR-I messages immediately.
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If any messages are retransmitted, then a poll shall be sent, either
by transmitting an SR-RR command, or SR-RNR command if the PE is in
the busy condition, with the P bit set to 1 or by setting the P bit
in the last retransmitted SR-I message.
The timer T1 shall be stopped. If any SR-I messages are outstanding,
then timer T1 shall be started.
l) Responding to command messages with the P bit set to 1
When receiving an SR-RR, SR-RNR, or-SR_I command message with the P
bit set to 1, the PE shall generate an appropriate response message
as follows:
1) if the PE is in the busy condition, it shall transmit an SR-RNR
response message with the F bit set to 1;
2) if there are some out-of-sequence messages in the receive
buffer, then it shall transmit an SR-SREJ message with the F
bit set to 1; N(R) shall be set to the receive state variable
V(R) and the information field set to the sequence numbers of
all missing SR-I messages, except V(R). If the list of sequence
numbers is too large to fit in the information field of the
SR-SREJ message, then the list shall be truncated by including
only the earliest sequence numbers; or
3) if there are no out-of-sequence messages in the receive buffer,
then an SR-RR response message with the F bit set to 1 shall be
sent.
2.3.5. List of SR system parameters
a) SR Poll Timeout (Timer T1)
The same value of the timer T1 shall be made known and agreed to by
the two PEs.
The period of timer T1, at the end of which retransmission of a
message may be initiated (see 6.3.4), shall take into account whether
T1 is started at the beginning or the end of the transmission of a
message.
The proper operation of the procedure requires that the transmitter's
timer T1 be greater than the maximum time between transmission of a
message (i.e., SR_I, or supervisory command) and the reception of the
corresponding message returned as an answer to that message (i.e.,
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acknowledging message). Therefore, the receiver should not delay the
response or acknowledging message returned to one of the above
messages by more than a value T2, where T2 is a system parameter (see
item b) below).
The PE shall not delay the response or acknowledging message returned
to one of the above remote PE messages by more than a period T2.
b) SR Response Timeout (Timer T2)
The same value of the parameter T2 shall be made known and agreed to
by the two PEs. The period of parameter T2 shall indicate the amount
of time available at the PE before the acknowledging message shall be
initiated in order to ensure its receipt by the remote PE, prior to
timer T1 running out at the PEs (parameter T2 < timer T1).
The period of parameter T2 shall take into account the following
timing factors:
1) the transmission time of the acknowledging message;
2) the propagation time over the access link;
3) the stated processing times at the PEs; and
4) the time to complete the transmission of the message(s) in the
PE transmit queue that are neither displaceable nor modifiable
in an orderly manner.
Given a value for timer T1 for the PEs, the value of parameter T2
shall be no larger than T1 minus 2 times the propagation time over
the access data link, minus the message processing time at the PE,
minus the message processing time at the remote PE, and minus the
transmission time of the acknowledging message by the PE.
c) SR Poll Retries (N2)
The same value of the N2 system parameter shall be made known and
agreed to by the two PEs.
The value of N2 shall indicate the maximum number of attempts made by
the PE to complete the successful transmission of a message to the
remote PE.
d) SR Window Size (k)
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The same value of the k system parameter shall be made known and
agreed to by the two PEs.
The value of k shall indicate the maximum number of sequentially
numbered SR-I messages that the PEs may have outstanding (i.e.,
unacknowledged) at any given time. The value of k shall never exceed
32767 for modulo 32768 operation.
3. Timing Consideration
Correct Fibre Channel information exchange requires that the inherent
latency between CE1 and CE2 (refer to Figure 1) be:
a) no more than one-half of the R_T_TOV (Receiver Transmitter Timeout
Value, default value: 100 milliseconds, defined in [FC-FS]) of the
attached devices for Primitive Sequences;
b) no more than one-half of the E_D_TOV (Error Detect Timeout Value,
default value: 2 seconds, defined in [FC-FS]) of the attached
devices for frames; and
c) within the R_A_TOV (Resource Allocation Timeout Value, default
value: 10 seconds, defined in [FC-FS]) of the attached fabric(s),
if any.
Requirement a) above, controlling the latency associated with FC
Primitive Sequence transport is addressed by [FC-BB], stating that in
case Primitive Sequences are received from the CE or the remote PE,
while the device is unable to forward these Primitive Sequence due to
backpressure indication, it shall flush its respective buffer (PSN-
facing if the Primitive Sequences were received from the CE, CE-
facing if they were received from the remote PE), and shall forward
the Primitive Sequences.
Another case is when there is no specific backpressure indication,
rather Primitive Sequences are being delayed due to retransmission of
dropped frames. In this case also, the PE shall flush its PSN-facing
buffer and shall forward the Primitive Sequences.
Requirements b) and c) above apply to the latency associated with
transporting FC frames, and the system MUST comply with the lower of
the two timeouts. The mechanism controlling this latency is described
below for the PE1 --> PE2 direction. A duplicate mechanism MUST be
used to control the PE2 --> PE1 direction.
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PE2 (the receiver) maintain a timer Td, set to the minimum timeout
set by requirements b) and c), with a safety margin to allow a
deviation of the estimated RTT from the actual RTT. PE2 SHOULD set
Td = 0.8 x (0.5 x MIN(E_D_TOV,R_A_TOV) - 2 x RTT), where RTT is an
average value calculated as specified in Section 6.1.2, and the
factor 0.8 provides safety margins for RTT fluctuations. In case the
calculated Td < 2 x RTT for two consecutive calculation periods, the
FC PW MUST be shut down (this avoids getting into conditions where Td
expiration is too frequent).
To guarantee correct operation of this mechanism FC PW SHOULD NOT be
used in environments where the RTT may have high variability, i.e.,
environments where the estimated RTT may not be off by more than 5%
of MIN(E_D_TOV,R_A_TOV). If the FC PW is used in an environment where
this limit is exceeded, the safety margins MUST be increased to
encompass twice the expected maximum variability in the RTT.
Upon Td expiration PE2 declares WAN Down as defined in [FC-BB], send
the Not Operational (NOS) Primitive Sequence to CE2, and flush its
buffers.
The timer Td is started when either of the following two conditions
occurs:
a) SR-RNR is sent by PE2 toward PE1, i.e., PE2 indicates
backpressure, and stop PE1 from transmitting;
b) SR-SREJ is sent by PE2 toward PE1, requiring retransmission.
PE2 notes the sequence number (Nx) of the first frame to be received
following the timer initiation (for SR_RNR this will be the next
frame to be transmitted by PE1, i.e., the sequence number of the last
frame received by PE2 before starting the timer plus 1. For SR_SREJ
this frame will be the first sequence number in the retransmission
list).
The timer continues counting and is initialized in one of two cases:
a) PE2 receives the frame with sequence number (Nx + k), where k is
the Selective Retransmission window size;
b) PE2 identifies idle period (all sent frames were acknowledged, and
the CE-facing buffer is empty).
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4. Applicability Statement
The methods specified in this document MUST be applied for the
transport of FC PW over uncontrolled (i.e., providing excess
information rate for the service) networks, in order to provide
reliable Fibre Channel transport and TCP-friendly behavior under
network congestion.
5. Normative References
[FC-encap] Roth, M., et al, "Encapsulation Methods for Transport of
Fibre Channel frames Over MPLS Networks", RFC TBD, to
appear.
RFC Editor: Please contact authors to obtain the correct
RFC number and date for the "to appear" in the above
reference prior to publication.
[RFC3985] Bryant, S., et al, "Pseudo Wire Emulation Edge-to-Edge
(PWE3) Architecture", RFC 3985, March 2005.
[RFC3916] Xiao, X., et al, "Requirements for Pseudo Wire Emulation
Edge-to-Edge (PWE3)", RFC 3916, September 2004.
[RFC3448] Floyd, S., et al, "TCP Friendly Rate Control (TFRC):
Protocol Specification", draft-ietf-dccp-rfc3448bis-06,
April 2008.
[RFC4446] Martini, L., "IANA Allocations for Pseudowire Edge to
Edge Emulation (PWE3)", RFC 4447, April 2006.
[RFC4447] Martini, L., et al, "Pseudowire Setup and Maintenance
using the Label Distribution Protocol (LDP)", RFC 4447,
April 2006.
[RFC4385] Bryant, S., et al, "Pseudowire Emulation Edge-to-Edge
(PWE3) Control Word for use over an MPLS PSN", RFC 4385,
February 2006.
[RFC4623] Malis, A., Townsley, M., "PWE3 Fragmentation and
Reassembly", RFC 4623, August 2006.
[FC-BB] "Fibre Channel Backbone-4" (FC-BB-4), ANSI INCITS
419:2008, to appear.
RFC Editor: Please contact authors to obtain the correct
date for the "to appear" in the above reference prior to
publication.
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[FC-SW] "Fibre Channel - Switch Fabric - 4" (FC-SW-4), ANSI
INCITS 418:2006, April 2006.
[FC-FS] "Fibre Channel - Framing and Signaling - 2" (FC-FS-2),
ANSI INCITS 424:2007, February 2007.
[BCP14] Bradner, S., "Key words for use in RFCs to Indicate
requirement Levels", BCP 14, RFC 2119, March 1997.
6. Informative references
[RFC3668] Bradner, S., "Intellectual Property Rights in IETF
Technology", RFC 3668, February 2004.
[RFC3821] M. Rajogopal, E. Rodriguez, "Fibre Channel over TCP/IP
(FCIP)", RFC 3821, July 2004.
[RFC2914] Floyd, S., "Congestion Control Principles", RFC 2914,
September 2000.
[RFC2581] Allman, M., et al, "TCP Congestion Control", RFC 2581,
April 1999.
7. Author's Addresses
Moran Roth
Corrigent Systems
101, Metro Drive
San Jose, CA 95110
Phone: +1-408-392-9292
Email: moranr@corrigent.com
Ronen Solomon
Corrigent Systems
126, Yigal Alon st.
Tel Aviv, ISRAEL
Phone: +972-3-6945316
Email: ronens@corrigent.com
Munefumi Tsurusawa
KDDI R&D Laboratories Inc.
Ohara 2-1-15, Fujimino-shi,
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Saitama, Japan
Phone: +81-49-278-7828
8. Contributing Author Information
David Zelig
Corrigent Systems
126, Yigal Alon st.
Tel Aviv, ISRAEL
Phone: +972-3-6945273
Email: davidz@corrigent.com
Leon Bruckman
Corrigent Systems
126, Yigal Alon st.
Tel Aviv, ISRAEL
Phone: +972-3-6945694
Email: leonb@corrigent.com
Luis Aguirre-Torres
Corrigent Systems
101 Metro Drive
San Jose, CA 95110
Phone: +1-408-392-9292
Email: Luis@corrigent.com
Roth, et al. Expires - July 15, 2009 [Page 30]
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