1) The sender periodically transmits NORM_CMD(CC) messages as needed
to initialize and collect roundtrip timing and congestion control
feedback from the receiver set.
2) The sender transmits an ordinal set of NormObjects segmented in
the form of NORM_DATA messages labeled with NormTransportIds and
logically identified with FEC encoding block numbers and symbol
identifiers. NORM_INFO messages may optionally precede the
transmission of data content for NORM transport objects.
3) As receivers detect missing content from the sender, they initiate
repair requests with NORM_NACK messages. Note the receivers track
the sender's most recent objectId::fecPayloadId transmit position
and NACK _only_ for content ordinally prior to that transmit
position. The receivers schedule random backoff timeouts before
generating NORM_NACK messages and wait an appropriate amount of
time before repeating the NORM_NACK if their repair request is not
4) The sender aggregates repair requests from the receivers and
logically "rewinds" its transmit position to send appropriate
repair messages. The sender sends repairs for the earliest
ordinal transmit position first and maintains this ordinal repair
transmission sequence. Previously untransmitted FEC parity
content for the applicable FEC coding block is used for repair
transmissions to the greatest extent possible. If the sender
exhausts its available FEC parity content on multiple repair
cycles for the same coding block, it resorts to an explicit repair
strategy (possibly using parity content) to complete repairs.
(The use of explicit repair is expected to be an exception in
general protocol operation, but the possibility does exist for
extreme conditions). The sender immediately assumes transmission
of new content once it has sent pending repairs.
5) The sender transmits NORM_CMD(FLUSH) messages when it reaches the
end of enqueued transmit content and pending repairs. Receivers
respond to the NORM_CMD(FLUSH) messages with NORM_NACK
transmissions (following the same suppression backoff timeout
strategy as for data) if they require further repair.
6) The sender transmissions are subject to rate control limits
determined by congestion control mechanisms. In the baseline
NORM-CC operation, each sender in a NormSession maintains its own
independent congestion control state. Receivers provide
congestion control feedback in NORM_NACK and NORM_ACK messages.
NORM_ACK feedback for congestion control purposes is governed
using a suppression mechanism similar to that for NORM_NACK
While this overall concept is relatively simple, there are details to
each of these aspects that need to be addressed for successful,
efficient, robust, and scalable NORM protocol operation.
5.1. Sender Initialization and Transmission
Upon startup, the NORM sender immediately begins sending NORM_CMD(CC)
messages to collect round trip timing and other information from the
potential group. If NORM-CC congestion control operation is enabled,
the NORM-CC Rate header extension MUST be included in these messages.
Congestion control operation SHALL be observed at all times when
operating in the general Internet. Even if congestion control
operation is disabled at the sender, it may be desirable to use the
NORM_CMD(CC) messaging to collect feedback from the group using the
baseline NORM-CC feedback mechanisms. This proactive feedback
collection can be used to establish a GRTT estimate prior to data
transmission and potential NACK operation.
In some cases, applications may wish for the sender to also proceed
with data transmission immediately. In other cases, the sender may
wish to defer data transmission until it has received some feedback
or request from the receiver set indicating that receivers are indeed
present. Note, in some applications (e.g., web push), this
indication may come out-of-band with respect to the multicast session
via other means. As noted, the periodic transmission of NORM_CMD(CC)
messages may precede actual data transmission in order to have an
initial GRTT estimate.
With inclusion of the OPTIONAL NORM FEC Object Transmission
Information Header Extension, the NORM protocol sender message
headers can contain all information necessary to prepare receivers
for subsequent reliable reception. This includes FEC coding
parameters, the sender NormSegmentSize, and other information. If
this header extension is not used, it is presumed that receivers have
received the FEC Object Transmission Information via other means.
Additionally, applications may leverage the use of NORM_INFO messages
associated with the session data objects in the session to provide
application-specific context information for the session and data
being transmitted. These mechanisms allow for operation with minimal
pre-coordination among the senders and receivers.
The NORM sender begins segmenting application-enqueued data into
NORM_DATA segments and transmitting it to the group. The
segmentation algorithm is described in Section 5.1.1. The rate of
transmission is controlled via congestion control mechanisms or is a
fixed rate if desired for closed network operations. The receivers
participating in the multicast group provide feedback to the sender
as needed. When the sender reaches the end of data it has enqueued
for transmission or any pending repairs, it transmits a series of
NORM_CMD(FLUSH) messages at a rate of one per 2*GRTT. Receivers may
respond to these NORM_CMD(FLUSH) messages with additional repair
requests. A protocol parameter "NORM_ROBUST_FACTOR" determines the
number of flush messages sent. If receivers request repair, the
repair is provided and flushing occurs again at the end of repair
transmission. The sender may attach an OPTIONAL "acking_node_list"
to NORM_CMD(FLUSH) containing the NormNodeIds for receivers from
which it expects explicit positive acknowledgment of reception. The
NORM_CMD(FLUSH) message may be also used for this optional function
any time prior to the end of data enqueued for transmission with the
NORM_CMD(FLUSH) messages multiplexed with ongoing data transmissions.
The OPTIONAL NORM positive acknowledgment procedure is described in
5.1.1. Object Segmentation Algorithm
NORM senders and receivers must use a common algorithm for logically
segmenting transport data into FEC encoding blocks and symbols so
that appropriate NACKs can be constructed to request repair of
missing data. NORM FEC coding blocks are comprised of multi-byte
symbols which are transmitted in the payload of NORM_DATA messages.
Each NORM_DATA message contains one source or encoding symbol and the
NormSegmentSize sender parameter defines the maximum symbol size in
bytes. The FEC encoding type and associated parameters govern the
source block size (number of source symbols per coding block). NORM
senders and receivers use these FEC parameters, along with the
NormSegmentSize and transport object size to compute the source block
structure for transport objects. These parameters are provided in
the FEC Transmission Information for each object. The algorithm
given below is used to compute a source block structure such that all
source blocks are as close to being equal length as possible. This
helps avoid the performance disadvantages of "short" FEC blocks.
Note this algorithm applies only to the statically-sized
NORM_OBJECT_DATA and NORM_OBJECT_FILE transport object types where
the object size is fixed and predetermined. For NORM_OBJECT_STREAM
objects, the object is segmented according to the maximum source
block length given in the FEC Transmission Information, unless the
FEC Payload ID indicates an alternative size for a given block.
The NORM block segmentation algorithm is defined as follows. For a
transport object of a given length (L_obj) in bytes, a first number
of FEC source blocks (N_large) is delineated of a larger block size
(B_large), and a second number of source blocks (N_small) is
delineated of a smaller block size (B_small). Given the maximum FEC
source block size (B_max) and the sender's NormSegmentSize, the block
segmentation for a given NORM transport object is determined as
B_max = Maximum source block length (i.e., maximum number of source
symbols per source block)
L_sym = Encoding symbol length in bytes (i.e., NormSegmentSize)
L_obj = Object length in bytes
N_total = The total number of source blocks into which the transport
object is partitioned.
N_large = Number of larger source blocks (first set of blocks)
B_large = Size (in encoding symbols) of the larger source blocks
N_small = Number of smaller source blocks (second set of blocks)
B_small = Size (in encoding symbols) of the smaller source blocks
L_final = Length (in bytes) of the last source symbol of the last
source block (All other symbols are of length L_sym).
1) The total number of source symbols in the transport object is
computed as: S_total = L_obj/L_sym [rounded up to the nearest
2) The transport object is partitioned into N_total source blocks,
where: N_total = S_total/B_max [rounded up to the nearest
3) The average length of a source block is computed as: B_ave =
S_total/N_total (this may be non-integer)
4) The size of the first set of larger blocks is computed as:
B_large = B_ave [rounded up to the nearest integer] (Note it will
always be the case that B_large <= B_max)
5) The size of the second set of smaller blocks is computed as:
B_small = B_ave [rounded down to the nearest integer] (Note if
B_ave is an integer B_small = B_large; otherwise B_small = B_large
6) The fractional part of B_ave is computed as: B_fraction = B_ave -
7) The number of larger source blocks is computed as: N_large =
B_fraction * N_total (Note N_large is an integer in the range 0
through N_total - 1)
8) The number of smaller source blocks is computed as: N_small =
N_total - N_large
9) Each of the first N_large source blocks consists of B_large source
symbols. Each of the remaining N_small source blocks consists of
B_small source symbols. All symbols are L_sym bytes in length
except for the final source symbol of the final source block which
is of length (in bytes):
L_final = L_obj - (N_large*B_large + N_small*B_small - 1) * L_sym
5.2. Receiver Initialization and Reception
The NORM protocol is designed such that receivers may join and leave
the group at will. However, some applications may be constrained
such that receivers need to be members of the group prior to start of
data transmission. NORM applications may use different policies to
constrain the impact of new receivers joining the group in the middle
of a session. For example, a useful implementation policy is for new
receivers joining the group to limit or avoid repair requests for
transport objects already in progress. The NORM sender
implementation may wish to impose additional constraints to limit the
ability of receivers to disrupt reliable multicast performance by
joining, leaving, and rejoining the group often. Different receiver
"join policies" may be appropriate for different applications and/or
scenarios. For general purpose operation, default policy where
receivers are allowed to request repair only for coding blocks with a
NormTransportId and FEC coding block number greater than or equal to
the first non-repair NORM_DATA or NORM_INFO message received upon
joining the group is RECOMMENDED. For objects of type
NORM_OBJECT_STREAM it is RECOMMENDED that the join policy constrain
receivers to start reliable reception at the current FEC coding block
for which non-repair content is received.
5.3. Receiver NACK Procedure
When the receiver detects it is missing data from a sender's NORM
transmissions, it initiates its NACKing procedure. The NACKing
procedure SHALL be initiated _only_ at FEC coding block boundaries,
NormObject boundaries, and upon receipt of a NORM_CMD(FLUSH) message.
The NACKing procedure begins with a random backoff timeout. The
duration of the backoff timeout is chosen using the "RandomBackoff"
algorithm described in the NORM Building Block document  using
(Ksender*GRTTsender) for the "maxTime" parameter and the sender
advertised group size (GSIZEsender) as the "groupSize" parameter.
NORM senders provide values for GRTTsender, Ksender and GSIZEsender
via the "grtt", "backoff", and "gsize" fields of transmitted
messages. The GRTTsender value is determined by the sender based on
feedback it has received from the group while the Ksender and
GSIZEsender values may determined by application requirements and
expectations or ancillary information. The backoff factor "Ksender"
MUST be greater than one to provide for effective feedback
suppression. A value of K = 4 is RECOMMENDED for the Any Source
Multicast (ASM) model while a value of K = 6 is RECOMMENDED for
Single Source Multicast (SSM) operation.
T_backoff = RandomBackoff(Ksender*GRTTsender, GSIZEsender)
To avoid the possibility of NACK implosion in the case of sender or
network failure during SSM operation, the receiver SHALL
automatically suppress its NACK and immediately enter the "holdoff"
period described below when T_backoff is greater than (Ksender-
1)*GRTTsender. Otherwise, the backoff period is entered and the
receiver MUST accumulate external pending repair state from NORM_NACK
messages and NORM_CMD(REPAIR_ADV) messages received. At the end of
the backoff time, the receiver SHALL generate a NORM_NACK message
only if the following conditions are met:
1) The sender's current transmit position (in terms of
objectId::fecPayloadId) exceeds the earliest repair position of
2) The repair state accumulated from NORM_NACK and
NORM_CMD(REPAIR_ADV) messages do not equal or supersede the
receiver's repair needs up to the sender transmission position at
the time the NACK procedure (backoff timeout) was initiated.
If these conditions are met, the receiver immediately generates a
NORM_NACK message when the backoff timeout expires. Otherwise, the
receiver's NACK is considered to be "suppressed" and the message is
not sent. At this time, the receiver begins a "holdoff" period
during which it constrains itself to not reinitiate the NACKing
process. The purpose of this timeout is to allow the sender worst-
case time to respond to the repair needs before the receiver requests
repair again. The value of this "holdoff" timeout (T_rcvrHoldoff)
as described in  is:
The NORM_NACK message contains repair request content beginning with
lowest ordinal repair position of the receiver up through the coding
block prior to the most recently heard ordinal transmission position
for the sender. If the size of the NORM_NACK content exceeds the
sender's NormSegmentSize, the NACK content is truncated so that the
receiver only generates a single NORM_NACK message per NACK cycle for
a given sender. In summary, a single NACK message is generated
containing the receiver's lowest ordinal repair needs.
For each partially-received FEC coding block requiring repair, the
receiver SHALL, on its _first_ repair attempt for the block, request
the parity portion of the FEC coding block beginning with the lowest
ordinal _parity_ "encoding_symbol_id" (i.e., "encoding_symbol_id" =
"source_block_len") and request the number of FEC symbols
corresponding to its data segment erasure count for the block. On
_subsequent_ repair cycles for the same coding block, the receiver
SHALL request only those repair symbols from the first set it has not
yet received up to the remaining erasure count for that applicable
coding block. Note that the sender may have provided other
different, additional parity segments for other receivers that could
also be used to satisfy the local receiver's erasure-filling needs.
In the case where the erasure count for a partially-received FEC
coding block exceeds the maximum number of parity symbols available
from the sender for the block (as indicated by the NORM_DATA
"fec_num_parity" field), the receiver SHALL request all available
parity segments plus the ordinally highest missing data segments
required to satisfy its total erasure needs for the block. The goal
of this strategy is for the overall receiver set to request a lowest
common denominator set of repair symbols for a given FEC coding
block. This allows the sender to construct the most efficient repair
transmission segment set and enables effective NACK suppression among
the receivers even with uncorrelated packet loss. This approach also
requires no synchronization among the receiver set in their repair
requests for the sender.
For FEC coding blocks or NormObjects missed in their entirety, the
NORM receiver constructs repair requests with NORM_NACK_BLOCK or
NORM_NACK_OBJECT flags set as appropriate. The request for
retransmission of NORM_INFO is accomplished by setting the
NORM_NACK_INFO flag in a corresponding repair request.
5.4. Sender NACK Processing and Response
The principle goal of the sender is to make forward progress in the
transmission of data its application has enqueued. However, the
sender must occasionally "rewind" its logical transmission point to
satisfy the repair needs of receivers who have NACKed. Aggregation
of multiple NACKs is used to determine an optimal repair strategy
when a NACK event occurs. Since receivers initiate the NACK process
on coding block or object boundaries, there is some loose degree of
synchronization of the repair process even when receivers experience
uncorrelated data loss.
5.4.1. Sender Repair State Aggregation
When a sender is in its normal state of transmitting new data and
receives a NACK, it begins a procedure to accumulate NACK repair
state from NORM_NACK messages before beginning repair transmissions.
Note that this period of aggregating repair state does _not_
interfere with its ongoing transmission of new data.
As described in , the period of time during which the sender
aggregates NORM_NACK messages is equal to:
T_sndrAggregate = (Ksender+1)*GRTT
where "Ksender" is the same backoff scaling value used by the
receivers, and "GRTT" is the sender's current estimate of the group's
greatest round-trip time.
When this period ends, the sender "rewinds" by incorporating the
accumulated repair state into its pending transmission state and
begins transmitting repair messages. After pending repair
transmissions are completed, the sender continues with new
transmissions of any enqueued data. Also, at this point in time, the
sender begins a "holdoff" timeout during which time the sender
constrains itself from initiating a new repair aggregation cycle,
even if NORM_NACK messages arrive. As described in , the value of
this sender "holdoff" period is:
T_sndrHoldoff = (1*GRTT)
If additional NORM_NACK messages are received during this sender
"holdoff" period, the sender will immediately incorporate these "late
messages" into its pending transmission state ONLY if the NACK
content is ordinally greater than the sender's current transmission
position. This "holdoff" time allows worst case time for the sender
to propagate its current transmission sequence position to the group,
thus avoiding redundant repair transmissions. After the holdoff
timeout expires, a new NACK accumulation period can be begun (upon
arrival of a NACK) in concert with the pending repair and new data
transmission. Recall that receivers are not to initiate the NACK
repair process until the sender's logical transmission position
exceeds the lowest ordinal position of their repair needs. With the
new NACK aggregation period, the sender repeats the same process of
incorporating accumulated repair state into its transmission plan and
subsequently "rewinding" to transmit the lowest ordinal repair data
when the aggregation period expires. Again, this is conducted in
concert with ongoing new data and/or pending repair transmissions.
5.4.2. Sender FEC Repair Transmission Strategy
The NORM sender should leverage transmission of FEC parity content
for repair to the greatest extent possible. Recall that the
receivers use a strategy to request a lowest common denominator of
explicit repair (including parity content) in the formation of their
NORM_NACK messages. Before falling back to explicitly satisfying
different receivers' repair needs, the sender can make use of the
general erasure-filling capability of FEC-generated parity segments.
The sender can determine the maximum erasure filling needs for
individual FEC coding blocks from the NORM_NACK messages received
during the repair aggregation period. Then, if the sender has a
sufficient number (less than or equal to the maximum erasure count)
of previously unsent parity segments available for the applicable
coding blocks, the sender can transmit these in lieu of the specific
packets the receiver set has requested. Only after exhausting its
supply of "fresh" (unsent) parity segments for a given coding block
should the sender resort to explicit transmission of the receiver
set's repair needs. In general, if a sufficiently powerful FEC code
is used, the need for explicit repair will be an exception, and the
fulfillment of reliable multicast can be accomplished quite
efficiently. However, the ability to resort to explicit repair
allows the protocol to be reliable under even very extreme
NORM_DATA messages sent as repair transmissions SHALL be flagged with
the NORM_FLAG_REPAIR flag. This allows receivers to obey any
policies that limit new receivers from joining the reliable
transmission when only repair transmissions have been received.
Additionally, the sender SHOULD additionally flag NORM_DATA
transmissions sent as explicit repair with the NORM_FLAG_EXPLICIT
Although NORM end system receivers do not make use of the
NORM_FLAG_EXPLICIT flag, this message transmission status could be
leveraged by intermediate systems wishing to "assist" NORM protocol
performance. If such systems are properly positioned with respect to
reciprocal reverse-path multicast routing, they need to sub-cast only
a sufficient count of non-explicit parity repairs to satisfy a
multicast routing sub-tree's erasure filling needs for a given FEC
coding block. When the sender has resorted to explicit repair, then
the intermediate systems should sub-cast all of the explicit repair
packets to those portions of the routing tree still requiring repair
for a given coding block. Note the intermediate systems will be
required to conduct repair state accumulation for sub-routes in a
manner similar to the sender's repair state accumulation in order to
have sufficient information to perform the sub-casting.
Additionally, the intermediate systems could perform additional
NORM_NACK suppression/aggregation as it conducts this repair state
accumulation for NORM repair cycles. The detail of this type of
operation are beyond the scope of this document, but this information
is provided for possible future consideration.
5.4.3. Sender NORM_CMD(SQUELCH) Generation
If the sender receives a NORM_NACK message for repair of data it is
no longer supporting, the sender generates a NORM_CMD(SQUELCH)
message to advertise its repair window and squelch any receivers from
additional NACKing of invalid data. The transmission rate of
NORM_CMD(SQUELCH) messages is limited to once per 2*GRTT. The
"invalid_object_list" (if applicable) of the NORM_CMD(SQUELCH)
message SHALL begin with the lowest "object_transport_id" from the
invalid NORM_NACK messages received since the last NORM_CMD(SQUELCH)
transmission. Lower ordinal invalid "object_transport_ids" should be
included only while the NORM_CMD(SQUELCH) payload is less than the
sender's NormSegmentSize parameter.
5.4.4. Sender NORM_CMD(REPAIR_ADV) Generation
When a NORM sender receives NORM_NACK messages from receivers via
unicast transmission, it uses NORM_CMD(REPAIR_ADV) messages to
advertise its accumulated repair state to the receiver set since the
receiver set is not directly sharing their repair needs via multicast
communication. The NORM_CMD(REPAIR_ADV) message is multicast to the
receiver set by the sender. The payload portion of this message has
content in the same format as the NORM_NACK receiver message payload.
Receivers are then able to perform feedback suppression in the same
manner as with NORM_NACK messages directly received from other
receivers. Note the sender does not merely retransmit NACK content
it receives, but instead transmits a representation of its aggregated
repair state. The transmission of NORM_CMD(REPAIR_ADV) messages are
subject to the sender transmit rate limit and NormSegmentSize
limitation. When the NORM_CMD(REPAIR_ADV) message is of maximum
size, receivers SHALL consider the maximum ordinal transmission
position value embedded in the message as the senders "current"
transmission position and implicitly suppress requests for ordinally
higher repair. For congestion control operation, the sender may also
need to provide information so that dynamic congestion control
feedback can be suppressed as needed among receivers. This document
specifies the NORM-CC Feedback Header Extension that is applied for
baseline NORM-CC operation. If other congestion control mechanisms
are used within a NORM implementation, other header extensions may be
defined. Whatever content format is used for this purpose should
ensure that maximum possible suppression state is conveyed to the
5.5. Additional Protocol Mechanisms
In addition to the principal function of data content transmission
and repair, there are some other protocol mechanisms that help NORM
to adapt to network conditions and play fairly with other coexistent
5.5.1. Greatest Round-trip Time Collection
For NORM receivers to appropriately scale backoff timeouts and the
senders to use proper corresponding timeouts, the participants must
agree on a common timeout basis. Each NORM sender monitors the
round-trip time of active receivers and determines the group greatest
round-trip time (GRTT). The sender advertises this GRTT estimate in
every message it transmits so that receivers have this value
available for scaling their timers. To measure the current GRTT, the
sender periodically sends NORM_CMD(CC) messages that contain a
locally generated timestamp. Receivers are expected to record this
timestamp along with the time the NORM_CMD(CC) message is received.
Then, when the receivers generate feedback messages to the sender, an
adjusted version of the sender timestamp is embedded in the feedback
message (NORM_NACK or NORM_ACK). The adjustment adds the amount of
time the receiver held the timestamp before generating its response.
Upon receipt of this adjusted timestamp, the sender is able to
calculate the round-trip time to that receiver.
The round-trip time for each receiver is fed into an algorithm that
weights and smoothes the values for a conservative estimate of the
GRTT. The algorithm and methodology are described in the NORM
Building Block document  in the section entitled "One-to-Many
Sender GRTT Measurement". A conservative estimate helps feedback
suppression at a small cost in overall protocol repair delay. The
sender's current estimate of GRTT is advertised in the "grtt" field
found in all NORM sender messages. The advertised GRTT is also
limited to a minimum of the nominal inter-packet transmission time
given the sender's current transmission rate and system clock
granularity. The reason for this additional limit is to keep the
receiver somewhat "event driven" by making sure the sender has had
adequate time to generate any response to repair requests from
receivers given transmit rate limitations due to congestion control
When the NORM-CC Rate header extension is present in NORM_CMD(CC)
messages, the receivers respond to NORM_CMD(CC) messages as described
in Section 5.5.2, "NORM Congestion Control Operation". The
NORM_CMD(CC) messages are periodically generated by the sender as
described for congestion control operation. This provides for
proactive, but controlled, feedback from the group in the form of
NORM_ACK messages. This provides for GRTT feedback even if no
NORM_NACK messages are being sent. If operating without congestion
control in a closed network, the NORM_CMD(CC) messages may be sent
periodically without the NORM-CC Rate header extension. In this
case, receivers will only provide GRTT measurement feedback when
NORM_NACK messages are generated since no NORM_ACK messages are
generated. In this case, the NORM_CMD(CC) messages may be sent less
frequently, perhaps as little as once per minute, to conserve network
capacity. Note that the NORM-CC Rate header extension may also be
used proactively solicit RTT feedback from the receiver group per
congestion control operation even though the sender may not be
conducting congestion control rate adjustment. NORM operation
without congestion control should be considered only in closed
5.5.2. NORM Congestion Control Operation
This section describes baseline congestion control operation for the
NORM protocol (NORM-CC). The supporting NORM message formats and
approach described here are an adaptation of the equation-based TCP-
Friendly Multicast Congestion Control (TFMCC) approach described in
. This congestion control scheme is REQUIRED for operation
within the general Internet unless the NORM implementation is adapted
to use another IETF-sanctioned reliable multicast congestion control
mechanism (e.g., PGMCC ). With this TFMCC-based approach, the
transmissions of NORM senders are controlled in a rate-based manner
as opposed to window-based congestion control algorithms as in TCP.
However, it is possible that the NORM protocol message set may
alternatively be used to support a window-based multicast congestion
control scheme such as PGMCC. The details of that alternative may be
described separately or in a future revision of this document. In
either case (rate-based TFMCC or window-based PGMCC), successful
control of sender transmission depends upon collection of sender-to-
receiver packet loss estimates and RTTs to identify the congestion
control bottleneck path(s) within the multicast topology and adjust
the sender rate accordingly. The receiver with loss and RTT
estimates that correspond to the lowest result transmission rate is
identified as the "current limiting receiver" (CLR).
As described in , a steady-state sender transmission rate, to be
"friendly" with competing TCP flows can be calculated as:
Rsender = --------------------------------------------------------------
tRTT * (sqrt((2/3)*p) + 12 * sqrt((3/8)*p) * p *
(1 + 32*(p^2)))
S = Nominal transmitted packet size. (In NORM, the "nominal"
packet size can be determined by the sender as an
exponentially weighted moving average (EWMA) of transmitted
packet sizes to account for variable message sizes).
tRTT = The RTT estimate of the current "current limiting receiver"
p = The loss event fraction of the CLR.
To support congestion control feedback collection and operation, the
NORM sender periodically transmits NORM_CMD(CC) command messages.
NORM_CMD(CC) messages are multiplexed with NORM data and repair
transmissions and serve several purposes:
1) Stimulate explicit feedback from the general receiver set to
collect congestion control information.
2) Communicate state to the receiver set on the sender's current
congestion control status including details of the CLR.
3) Initiate rapid (immediate) feedback from the CLR in order to
closely track the dynamics of congestion control for that current
"worst path" in the group multicast topology.
The format of the NORM_CMD(CC) message is describe in Section 4.2.3
of this document. The NORM_CMD(CC) message contains information to
allow measurement of RTTs, to inform the group of the congestion
control CLR, and to provide feedback of individual RTT measurements
to the receivers in the group. The NORM_CMD(CC) also provides for
exciting feedback from OPTIONAL "potential limiting receiver" (PLR)
nodes that may be determined administratively or possibly
algorithmically based on congestion control feedback. PLR nodes are
receivers that have been identified to have potential for (perhaps
soon) becoming the CLR and thus immediate, up-to-date feedback is
beneficial for congestion control performance. The details of PLR
selection are not discussed in this document.
22.214.171.124. NORM_CMD(CC) Transmission
The NORM_CMD(CC) message is transmitted periodically by the sender
along with its normal data transmission. Note that the repeated
transmission of NORM_CMD(CC) messages may be initiated some time
before transmission of user data content at session startup. This
may be done to collect some estimation of the current state of the
multicast topology with respect to group and individual RTT and
congestion control state.
A NORM_CMD(CC) message is immediately transmitted at sender startup.
The interval of subsequent NORM_CMD(CC) message transmission is
determined as follows:
1) By default, the interval is set according to the current sender
GRTT estimate. A startup GRTT of 0.5 seconds is recommended when
no feedback has yet been received from the group.
2) If a CLR has been identified (based on previous receiver
feedback), the interval is the RTT between the sender and CLR.
3) Additionally, if the interval of nominal data message transmission
is greater than the GRTT or RTT_clr interval, the NORM_CMD(CC)
interval is set to this greater value. This ensures that the
transmission of this control message is not done to the exclusion
of user data transmission.
The NORM_CMD(CC) "cc_sequence" field is incremented with each
transmission of a NORM_CMD(CC) command. The greatest "cc_sequence"
recently received by receivers is included in their feedback to the
sender. This allows the sender to determine the "age" of feedback to
assist in congestion avoidance.
The NORM-CC Rate Header Extension is applied to the NORM_CMD(CC)
message and the sender advertises its current transmission rate in
the "send_rate" field. The rate information is used by receivers to
initialize loss estimation during congestion control startup or
The "cc_node_list" contains a list of entries identifying receivers
and their current congestion control state (status "flags", "rtt" and
"loss" estimates). The list may be empty if the sender has not yet
received any feedback from the group. If the sender has received
feedback, the list will minimally contain an entry identifying the
CLR. A NORM_FLAG_CC_CLR flag value is provided for the "cc_flags"
field to identify the CLR entry. It is RECOMMENDED that the CLR
entry be the first in the list for implementation efficiency.
Additional entries in the list are used to provide sender-measured
individual RTT estimates to receivers in the group. The number of
additional entries in this list is dependent upon the percentage of
control traffic the sender application is willing to send with
respect to user data message transmissions. More entries in the list
may allow the sender to be more responsive to congestion control
dynamics. The length of the list may be dynamically determined
according to the current transmission rate and scheduling of
NORM_CMD(CC) messages. The maximum length of the list corresponds to
the sender's NormSegmentSize parameter for the session. The
inclusion of additional entries in the list based on receiver
feedback are prioritized with following rules:
1) Receivers that have not yet been provided RTT feedback get first
priority. Of these, those with the greatest loss fraction receive
precedence for list inclusion.
2) Secondly, receivers that have previously been provided RTT are
included with receivers yielding the lowest calculated congestion
rate getting precedence.
There are "cc_flag" values in addition to NORM_FLAG_CC_CLR that are
used for other congestion control functions. The NORM_FLAG_CC_PLR
flag value is used to mark additional receivers from that the sender
would like to have immediate, non-suppressed feedback. These may be
receivers that the sender algorithmically identified as potential
future CLRs or that have been pre-configured as potential congestion
control points in the network. The NORM_FLAG_CC_RTT indicates the
validity of the "cc_rtt" field for the associated receiver node.
Normally, this flag will be set since the receivers in the list will
typically be receivers from which the sender has received feedback.
However, in the case that the NORM sender has been pre-configured
with a set of PLR nodes, feedback from those receivers may not yet
have been collected and thus the "cc_rtt" and "cc_rate" fields do not
contain valid values when this flag is not set.
126.96.36.199. NORM_CMD(CC) Feedback Response
Receivers explicitly respond to NORM_CMD(CC) messages in the form of
a NORM_ACK(RTT) message. The goal of the congestion control feedback
is to determine the receivers with the lowest congestion control
rates. Receivers that are marked as CLR or PLR nodes in the
NORM_CMD(CC) "cc_node_list" immediately provide feedback in the form
of a NORM_ACK to this message. When a NORM_CMD(CC) is received,
non-CLR or non-PLR nodes initiate random feedback backoff timeouts
similar to that used when the receiver initiates a repair cycle (see
Section 5.3) in response to detection of data loss. The backoff
timeout for the congestion control response is generated as follows:
T_backoff = RandomBackoff(K*GRTTsender, GSIZEsender)
The "RandomBackoff()" algorithm provides a truncated exponentially
distributed random number and is described in the NORM Building Block
document . The same backoff factor K = Ksender MAY be used as
with NORM_NACK suppression. However, in cases where the application
purposefully specifies a very small Ksender backoff factor to
minimize the NACK repair process latency (trading off group size
scalability), it may still be desirable to maintain a larger backoff
factor for congestion control feedback, since there may often be a
larger volume of congestion control feedback than NACKs in many cases
and congestion control feedback latency may be tolerable where
reliable delivery latency is not. As previously noted, a backoff
factor value of K = 4 is generally recommended for ASM operation and
K = 6 for SSM operation. A receiver SHALL cancel the backoff timeout
and thus its pending transmission of a NORM_ACK(RTT) message under
the following conditions:
1) The receiver generates another feedback message (NORM_NACK or
other NORM_ACK) before the congestion control feedback timeout
2) A NORM_CMD(CC) or other receiver feedback with an ordinally
greater "cc_sequence" field value is received before the
congestion control feedback timeout expires (this is similar to
the TFMCC feedback round number),
3) When the T_backoff is greater than 1*GRTT. This prevents NACK
implosion in the event of sender or network failure,
4) "Suppressing" congestion control feedback is heard from another
receiver (in a NORM_ACK or NORM_NACK) or via a
NORM_CMD(REPAIR_ADV) message from the sender. The local
receiver's feedback is "suppressed" if the rate of the competing
feedback (Rfb) is sufficiently close to or less than the local
receiver's calculated rate (Rcalc). The local receiver's feedback
is canceled when:
Rcalc > (0.9 * Rfb)
Also note receivers that have not yet received an RTT measurement
from the sender are suppressed only by other receivers that have
not yet measured RTT. Additionally, receivers whose RTT estimate
has "aged" considerably (i.e., they haven't been included in the
NORM_CMD(CC) "cc_node_list" in a long time) may wish to compete as
a receiver with no prior RTT measurement after some expiration
When the backoff timer expires, the receiver SHALL generate a
NORM_ACK(RTT) message to provide feedback to the sender and group.
This message may be multicast to the group for most effective
suppression in ASM topologies or unicast to the sender depending upon
how the NORM protocol is deployed and configured.
Whenever any feedback is generated (including this NORM_ACK(RTT)
message), receivers include an adjusted version of the sender
timestamp from the most recently received NORM_CMD(CC) message and
the "cc_sequence" value from that command in the applicable NORM_ACK
or NORM_NACK message fields. For NORM-CC operation, any generated
feedback message SHALL also contain the NORM-CC Feedback header
extension. The receiver provides its current "cc_rate" estimate,
"cc_loss" estimate, "cc_rtt" if known, and any applicable "cc_flags"
via this header extension.
During slow start (when the receiver has not yet detected loss from
the sender), the receiver uses a value equal to two times its
measured rate from the sender in the "cc_rate" field. For steady-
state congestion control operation, the receiver "cc_rate" value is
from the equation-based value using its current loss event estimate
and sender<->receiver RTT information. (The GRTT is used when the
receiver has not yet measured its individual RTT).
The "cc_loss" field value reflects the receiver's current loss event
estimate with respect to the sender in question.
When the receiver has a valid individual RTT measurement, it SHALL
include this value in the "cc_rtt" field. The NORM_FLAG_CC_RTT MUST
be set when the "cc_rtt" field is valid.
After a congestion control feedback message is generated or when the
feedback is suppressed, a non-CLR receiver begins a "holdoff" timeout
period during which it will restrain itself from providing congestion
control feedback, even if NORM_CMD(CC) messages are received from the
sender (unless the receive becomes marked as a CLR or PLR node). The
value of this holdoff timeout (T_ccHoldoff) period is:
T_ccHoldoff = (K*GRTT)
Thus, non-CLR receivers are constrained to providing explicit
congestion control feedback once per K*GRTT intervals. Note,
however, that as the session progresses, different receivers will be
responding to different NORM_CMD(CC) messages and there will be
relatively continuous feedback of congestion control information
while the sender is active.
188.8.131.52. Congestion Control Rate Adjustment
During steady-state operation, the sender will directly adjust its
transmission rate to the rate indicated by the feedback from its
currently selected CLR. As noted in , the estimation of
parameters (loss and RTT) for the CLR will generally constrain the
rate changes possible within acceptable bounds. For rate increases,
the sender SHALL observe a maximum rate of increase of one packet per
RTT at all times during steady-state operation.
The sender processes congestion control feedback from the receivers
and selects the CLR based on the lowest rate receiver. Receiver
rates are either determined directly from the slow start "cc_rate"
provided by the receiver in the NORM-CC Feedback header extension or
by performing the equation-based calculation using individual RTT and
loss estimates ("cc_loss") as feedback is received.
The sender can calculate a current RTT for a receiver (RTT_rcvrNew)
using the "grtt_response" timestamp included in feedback messages.
When the "cc_rtt" value in a response is not valid, the sender simply
uses this RTT_rcvrNew value as the receiver's current RTT (RTT_rcvr).
For non-CLR and non-PLR receivers, the sender can use the "cc_rtt"
value provided in the NORM-CC Feedback header extension as the
receiver's previous RTT measurement (RTT_rcvrPrev) to smooth
RTT_rcvr = 0.5 * RTT_rcvrPrev + 0.5 * RTT_rcvrNew
For CLR receivers where feedback is received more regularly, the
sender SHOULD maintain a more smoothed RTT estimate upon new feedback
from the CLR where:
RTT_clr = 0.9 * RTT_clr + 0.1 * RTT_clrNew
"RTT_clrNew" is the new RTT calculated from the timestamp in the
feedback message received from the CLR. The RTT_clr is initialized
to RTT_clrNew on the first feedback message received. Note that the
same procedure is observed by the sender for PLR receivers and that
if a PLR is "promoted" to CLR status, the smoothed estimate can be
There are some additional periods besides steady-state operation that
need to be considered in NORM-CC operation. These periods are:
1) during session startup,
2) when no feedback is received from the CLR, and
3) when the sender has a break in data transmission.
During session startup, the congestion control operation SHALL
observe a "slow start" procedure to quickly approach its fair
bandwidth share. An initial sender startup rate is assumed where:
Rinitial = MIN(NormSegmentSize / GRTT, NormSegmentSize) bytes/second.
The rate is increased only when feedback is received from the
receiver set. The "slow start" phase proceeds until any receiver
provides feedback indicating that loss has occurred. Rate increase
during slow start is applied as:
Rnew = Rrecv_min
where "Rrecv_min" is the minimum reported receiver rate in the
"cc_rate" field of congestion control feedback messages received from
the group. Note that during "slow start", receivers use two times
their measured rate from the sender in the "cc_rate" field of their
feedback. Rate increase adjustment is limited to once per GRTT
during slow start.
If the CLR or any receiver intends to leave the group, it will set
the NORM_FLAG_CC_LEAVE in its congestion control feedback message as
an indication that the sender should not select it as the CLR. When
the CLR changes to a lower rate receiver, the sender should
immediately adjust to the new lower rate. The sender is limited to
increasing its rate at one additional packet per RTT towards any new,
higher CLR rate.
The sender should also track the "age" of the feedback it has
received from the CLR by comparing its current "cc_sequence" value
(Seq_sender) to the last "cc_sequence" value received from the CLR
(Seq_clr). As the "age" of the CLR feedback increases with no new
feedback, the sender SHALL begin reducing its rate once per RTT_clr
as a congestion avoidance measure.
The following algorithm is used to determine the decrease in sender
rate (Rsender bytes/sec) as the CLR feedback, unexpectedly,
Age = Seq_sender - Seq_clr;
if (Age > 4) Rsender = Rsender * 0.5;
This rate reduction is limited to the lower bound on NORM
transmission rate. After NORM_ROBUST_FACTOR consecutive NORM_CMD(CC)
rounds without any feedback from the CLR, the sender SHOULD assume
the CLR has left the group and pick the receiver with the next lowest
rate as the new CLR. Note this assumes that the sender does not have
explicit knowledge that the CLR intentionally left the group. If no
receiver feedback is received, the sender MAY wish to withhold
further transmissions of NORM_DATA segments and maintain NORM_CMD(CC)
transmissions only until feedback is detected. After such a CLR
timeout, the sender will be transmitting with a minimal rate and
should return to slow start as described here for a break in data
When the sender has a break in its data transmission, it can continue
to probe the group with NORM_CMD(CC) messages to maintain RTT
collection from the group. This will enable the sender to quickly
determine an appropriate CLR upon data transmission restart.
However, the sender should exponentially reduce its target rate to be
used for transmission restart as time since the break elapses. The
target rate SHOULD be recalculated once per RTT_clr as:
Rsender = Rsender * 0.5;
If the minimum NORM rate is reached, the sender should set the
NORM_FLAG_START flag in its NORM_CMD(CC) messages upon restart and
the group should observer "slow start" congestion control procedures
until any receiver experiences a new loss event.
5.5.3. NORM Positive Acknowledgment Procedure
NORM provides options for the source application to request positive
acknowledgment (ACK) of NORM_CMD(FLUSH) and NORM_CMD(ACK_REQ)
messages from members of the group. There are some specific
acknowledgment requests defined for the NORM protocol and a range of
acknowledgment request types that are left to be defined by the
application. One predefined acknowledgment type is the
NORM_ACK_FLUSH type. This acknowledgment is used to determine if
receivers have achieved completion of reliable reception up through a
specific logical transmission point with respect to the sender's
sequence of transmission. The NORM_ACK_FLUSH acknowledgment may be
used to assist in application flow control when the sender has
information on a portion of the receiver set. Another predefined
acknowledgment type is NORM_ACK(CC), which is used to explicitly
provide congestion control feedback in response to NORM_CMD(CC)
messages transmitted by the sender for NORM-CC operation. Note the
NORM_ACK(CC) response does NOT follow the positive acknowledgment
procedure described here. The NORM_CMD(ACK_REQ) and NORM_ACK
messages contain an "ack_type" field to identify the type of
acknowledgment requested and provided. A range of "ack_type" values
is provided for application-defined use. While the application is
responsible for initiating the acknowledgment request and interprets
application-defined "ack_type" values, the acknowledgment procedure
SHOULD be conducted within the protocol implementation to take
advantage of timing and transmission scheduling information available
to the NORM transport.
The NORM positive acknowledgment procedure uses polling by the sender
to query the receiver group for response. Note this polling
procedure is not intended to scale to very large receiver groups, but
could be used in large group setting to query a critical subset of
the group. Either the NORM_CMD(ACK_REQ), or when applicable, the
NORM_CMD(FLUSH) message is used for polling and contains a list of
NormNodeIds for receivers that should respond to the command. The
list of receivers providing acknowledgment is determined by the
source application with "a priori" knowledge of participating nodes
or via some other application-level mechanism.
The ACK process is initiated by the sender that generates
NORM_CMD(FLUSH) or NORM_CMD(ACK_REQ) messages in periodic "rounds".
For NORM_ACK_FLUSH requests, the NORM_CMD(FLUSH) contain a
"object_transport_id" and "fec_payload_id" denoting the watermark
transmission point for which acknowledgment is requested. This
watermark transmission point is "echoed" in the corresponding fields
of the NORM_ACK(FLUSH) message sent by the receiver in response.
NORM_CMD(ACK_REQ) messages contain an "ack_id" field which is
similarly "echoed" in response so that the sender may match the
response to the appropriate request.
In response to the NORM_CMD(ACK_REQ), the listed receivers randomly
spread NORM_ACK messages uniformly in time over a window of (1*GRTT).
These NORM_ACK messages are typically unicast to the sender. (Note
that NORM_ACK(CC) messages SHALL be multicast or unicast in the same
manner as NORM_NACK messages).
The ACK process is self-limiting and avoids ACK implosion in that:
1) Only a single NORM_CMD(ACK_REQ) message is generated once per
2) The size of the "acking_node_list" of NormNodeIds from which
acknowledgment is requested is limited to a maximum of the sender
NormSegmentSize setting per round of the positive acknowledgment
Because the size of the included list is limited to the sender's
NormSegmentSize setting, multiple NORM_CMD(ACK_REQ) rounds may be
required to achieve responses from all receivers specified. The
content of the attached NormNodeId list will be dynamically updated
as this process progresses and NORM_ACK responses are received from
the specified receiver set. As the sender receives valid responses
(i.e., matching watermark point or "ack_id") from receivers, it SHALL
eliminate those receivers from the subsequent NORM_CMD(ACK_REQ)
message "acking_node_list" and add in any pending receiver
NormNodeIds while keeping within the NormSegmentSize limitation of
the list size. Each receiver is queried a maximum number of times
(NORM_ROBUST_FACTOR, by default). Receivers not responding within
this number of repeated requests are removed from the payload list to
make room for other potential receivers pending acknowledgment. The
transmission of the NORM_CMD(ACK_REQ) is repeated until no further
responses are required or until the repeat threshold is exceeded for
all pending receivers. The transmission of NORM_CMD(ACK_REQ) or
NORM_CMD(FLUSH) messages to conduct the positive acknowledgment
process is multiplexed with ongoing sender data transmissions.
However, the NORM_CMD(FLUSH) positive acknowledgment process may be
interrupted in response to negative acknowledgment repair requests
(NACKs) received from receivers during the acknowledgment period.
The NORM_CMD(FLUSH) positive acknowledgment process is restarted for
receivers pending acknowledgment once any the repairs have been
In the case of NORM_CMD(FLUSH) commands with an attached
"acking_node_list", receivers will not ACK until they have received
complete transmission of all data up to and including the given
watermark transmission point. All receivers SHALL interpret the
watermark point provided in the request NACK for repairs if needed as
for NORM_CMD(FLUSH) commands with no attached "acking_node_list".
5.5.4. Group Size Estimate
NORM sender messages contain a "gsize" field that is a representation
of the group size and is used in scaling random backoff timer ranges.
The use of the group size estimate within the NORM protocol does not
require a precise estimation and works reasonably well if the
estimate is within an order of magnitude of the actual group size.
By default, the NORM sender group size estimate may be
administratively configured. Also, given the expected scalability of
the NORM protocol for general use, a default value of 10,000 is
recommended for use as the group size estimate.
It is possible that group size may be algorithmically approximated
from the volume of congestion control feedback messages which follow
the exponentially weighted random backoff. However, the
specification of such an algorithm is currently beyond the scope of
6. Security Considerations
The same security considerations that apply to the NORM, and FEC
Building Blocks also apply to the NORM protocol. In addition to
vulnerabilities that any IP and IP multicast protocol implementation
may be generally subject to, the NACK-based feedback of NORM may be
exploited by replay attacks which force the NORM sender to
unnecessarily transmit repair information. This MAY be addressed by
network layer IP security implementations that guard against this
potential security exploitation. It is RECOMMENDED that such IP
security mechanisms be used when available. Another possible
approach is for NORM senders to use the "sequence" field from the
NORM Common Message Header to detect replay attacks. This can be
accomplished if the NORM packets are cryptographically protected and
the sender is willing to maintain state on receivers which are
NACKing. A cache of receiver state may provide some protection
against replay attacks. Note that the "sequence" field of NORM
messages should be incremented with independent values for different
destinations (e.g., group-addressed versus unicast-addressed messages
versus "receiver" messages). Thus, the congestion control loss
estimation function of the "sequence" field can be preserved for
sender messages when receiver messages are unicast to the sender.
The NORM protocol is compatible with the use of the IP security
(IPsec) architecture described in . It is important to note that
while NORM does leverage FEC-based repair for scalability, this does
not alone guarantee integrity of received data. Application-level
integrity-checking of data content is highly RECOMMENDED.
7. IANA Considerations
No information in this specification is currently subject to IANA
registration. However, several Header Extensions are defined within
this document. If/when additional Header Extensions are developed,
the first RFC MUST establish an IANA registry for them, with a
"Specification Required" policy  and all Header Extensions,
including those in the present document, MUST be registered
thereafter. Additionally, building blocks components used by NORM
may introduce additional IANA considerations. In particular, the FEC
Building Block used by NORM does require IANA registration of the FEC
codecs used. The registration instructions for FEC codecs are
provided in .
8. Suggested Use
The present NORM protocol is seen as useful tool for the reliable
data transfer over generic IP multicast services. It is not the
intention of the authors to suggest it is suitable for supporting
all envisioned multicast reliability requirements. NORM provides a
simple and flexible framework for multicast applications with a
degree of concern for network traffic implosion and protocol overhead
efficiency. NORM-like protocols have been successfully demonstrated
within the MBone for bulk data dissemination applications, including
weather satellite compressed imagery updates servicing a large group
of receivers and a generic web content reliable "push" application.
In addition, this framework approach has some design features making
it attractive for bulk transfer in asymmetric and wireless
internetwork applications. NORM is capable of successfully operating
independent of network structure and in environments with high packet
loss, delay, and misordering. Hybrid proactive/reactive FEC-based
repairing improve protocol performance in some multicast scenarios.
A sender-only repair approach often makes additional engineering
sense in asymmetric networks. NORM's unicast feedback capability may
be suitable for use in asymmetric networks or in networks where only
unidirectional multicast routing/delivery service exists. Asymmetric
architectures supporting multicast delivery are likely to make up an
important portion of the future Internet structure (e.g.,
DBS/cable/PSTN hybrids) and efficient, reliable bulk data transfer
will be an important capability for servicing large groups of
9. Acknowledgments (and these are not Negative)
The authors would like to thank Rick Jones, Vincent Roca, Rod Walsh,
Toni Paila, Michael Luby, and Joerg Widmer for their valuable input
and comments on this document. The authors would also like to thank
the RMT working group chairs, Roger Kermode and Lorenzo Vicisano, for
their support in development of this specification, and Sally Floyd
for her early input into this document.
10.1. Normative References
 Kermode, R. and L. Vicisano, "Author Guidelines for Reliable
Multicast Transport (RMT) Building Blocks and Protocol
Instantiation documents", RFC 3269, April 2002.
 Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
 Deering, S., "Host Extensions for IP Multicasting", STD 5, RFC
1112, August 1989.
 Adamson, B., Bormann, C., Handley, M., and J. Macker,
"Negative-Acknowledgment (NACK)-Oriented Reliable Multicast
(NORM) Building Blocks", RFC 3941, November 2004.
 Luby, M., Vicisano, L., Gemmell, J., Rizzo, L., Handley, M., and
J. Crowcroft, "Forward Error Correction (FEC) Building Block",
RFC 3452, December 2002.
 Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA
Considerations Section in RFCs", BCP 26, RFC 2434, October 1998.
10.2. Informative References
 Handley, M. and V. Jacobson, "SDP: Session Description
Protocol", RFC 2327, April 1998.
 Handley, M., Perkins, C., and E. Whelan, "Session Announcement
Protocol", RFC 2974, October 2000.
 S. Pingali, D. Towsley, J. Kurose, "A Comparison of Sender-
Initiated and Receiver-Initiated Reliable Multicast Protocols",
In Proc. INFOCOM, San Francisco CA, October 1993.
 Luby, M., Vicisano, L., Gemmell, J., Rizzo, L., Handley, M., and
J. Crowcroft, "The Use of Forward Error Correction (FEC) in
Reliable Multicast", RFC 3453, December 2002.
 Macker, J. and B. Adamson, "The Multicast Dissemination Protocol
(MDP) Toolkit", Proc. IEEE MILCOM 99, October 1999.
 Nonnenmacher, J. and E. Biersack, "Optimal Multicast Feedback",
Proc. IEEE INFOCOMM, p. 964, March/April 1998.
 J. Macker, B. Adamson, "Quantitative Prediction of Nack Oriented
Reliable Multicast (NORM) Feedback", Proc. IEEE MILCOM 2002,
 H.W. Holbrook, "A Channel Model for Multicast", Ph.D.
Dissertation, Stanford University, Department of Computer
Science, Stanford, California, August 2001.
 D. Gossink, J. Macker, "Reliable Multicast and Integrated Parity
Retransmission with Channel Estimation", IEEE GLOBECOMM 98',
 Whetten, B., Vicisano, L., Kermode, R., Handley, M., Floyd, S.,
and M. Luby, "Reliable Multicast Transport Building Blocks for
One-to-Many Bulk-Data Transfer", RFC 3048, January 2001.
 Mankin, A., Romanow, A., Bradner, S., and V. Paxson, "IETF
Criteria for Evaluating Reliable Multicast Transport and
Application Protocols", RFC 2357, June 1998.
 Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson,
"RTP: A Transport Protocol for Real-Time Applications", STD 64,
RFC 3550, July 2003.
 J. Widmer and M. Handley, "Extending Equation-Based Congestion
Control to Multicast Applications", Proc ACM SIGCOMM 2001, San
Diego, August 2001.
 L. Rizzo, "pgmcc: A TCP-Friendly Single-Rate Multicast
Congestion Control Scheme", Proc ACM SIGCOMM 2000, Stockholm,
 J. Padhye, V. Firoiu, D. Towsley, and J. Kurose, "Modeling TCP
Throughput: A Simple Model and its Empirical Validation", Proc
ACM SIGCOMM 1998.
 Kent, S. and R. Atkinson, "Security Architecture for the
Internet Protocol", RFC 2401, November 1998.
11. Authors' Addresses
Naval Research Laboratory
Washington, DC, USA, 20375
Universitaet Bremen TZI
D-28334 Bremen, Germany
Department of Computer Science
University College London
Naval Research Laboratory
Washington, DC, USA, 20375
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