rfc1190.txt
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parameters other than the number of hops that the packets will
take, such as delay, local network bandwidth consumption, or
total internet bandwidth consumption.
The result of the routing function is a set of next-hop ST
agents and the parameters of the intervening network(s). The
latter permit the ST agent to determine whether the selected
network has the resources necessary to support the level of
service requested in the FlowSpec.
3.1.3. Reserving Resources
The intent of ST is to provide a guaranteed level of service by
reserving internet resources for a stream during a setup phase
rather than on a per packet basis. The relevant resources are
not only the forwarding information maintained by the ST
agents, but also packet switch processor bandwidth and buffer
space, and network bandwidth and multicast group identifiers.
Reservation of these resources can help to increase the
reliability and decrease the delay and delay variance with
which data packets are delivered. The FlowSpec contains all
the information needed by the ST agent to allocate the
necessary resources. When and how these resources are
allocated depends on the details of the networks involved, and
is not specified here.
If an ST agent must send data across a network to a single
next-hop ST agent, then only the point-to-point bandwidth needs
to be reserved. If the agent must send data to multiple next-
hop agents across one network and network layer multicasting is
not available, then bandwidth must be reserved for all of them.
This will allow the ST agent to
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RFC 1190 Internet Stream Protocol October 1990
use replication to send a copy of the data packets to each
next-hop agent.
If multicast is supported, its use will decrease the effort
that the ST agent must expend when forwarding packets and also
reduces the bandwidth required since one copy can be received
by all next-hop agents. However, the setup phase is more
complicated. A network multicast address must be allocated
that contains all those next-hop agents, the sender must have
access to that address, the next-hop agents must be informed of
the address so they can join the multicast group identified by
it (see Section 4.2.2.7 (page 86)), and a common HID must be
negotiated.
The network should consider the bandwidth and multicast
requirements to determine the amount of packet switch
processing bandwidth and buffer space to reserve for the
stream. In addition, the membership of a stream in a Group may
affect the resources that have to be allocated; see Section
3.7.3 (page 56).
Few networks in the Internet currently offer resource
reservation, and none that we know of offer reservation of all
the resources specified here. Only the Terrestrial Wideband
Network (TWBNet) [7] and the Atlantic Satellite Network
(SATNET) [9] offer(ed) bandwidth reservation. Multicasting is
more widely supported. No network provides for the reservation
of packet switch processing bandwidth or buffer space. We hope
that future networks will be designed to better support
protocols like ST.
Effects similar to reservation of the necessary resources may
be obtained even when the network cannot provide direct support
for the reservation. Certainly if total reservations are a
small fraction of the overall resources, such as packet switch
processing bandwidth, buffer space, or network bandwidth, then
the desired performance can be honored if the degree of
confidence is consistent with the requirements as stated in the
FlowSpec. Other solutions can be designed for specific
networks.
3.1.4. Sending CONNECT Messages
A VLId and a proposed HID must be selected for each next-hop
agent. The control packets for the next-hop must carry the
VLId in the SVLId field. The data packets transmitted in the
stream to the next-hop must carry the HID in the ST Header.
The ST agent sends a CONNECT message to each of the ST agents
identified by the routing function. Each CONNECT message
contains the VLId, the proposed HID (the HID Field option bit
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RFC 1190 Internet Stream Protocol October 1990
must be set, see Section 3.6.1 (page 44)), an updated FlowSpec,
and a TargetList. In general, the HID, FlowSpec, and
TargetList will depend on both the next-hop and the intervening
network. Each TargetList is a subset of the received (or
original) TargetList, identifying the targets that are to be
reached through the next-hop to which the CONNECT message is
being sent. Note that a CONNECT message to a single next-hop
might have to be fragmented into multiple CONNECTs if the
single CONNECT is too large for the intervening network's MTU;
fragmentation is performed by further dividing the TargetList.
If multiple next-hops are to be reached through a network that
supports network level multicast, a different CONNECT message
must nevertheless be sent to each next-hop since each will have
a different TargetList; see Section 4.2.3.5 (page 105).
However, since an identical copy of each ensuing data packet
will reach each member of the multicast group, all the CONNECT
messages must propose the same HID. See Section 3.7.4 (page
58) for a detailed discussion on HID selection.
In the example of Figure 2, the routing function might return
that B is reachable via Agent 1 and C and D are reachable via
Agent 2. Thus A would create two CONNECT messages, one each
for Agents 1 and 2, as illustrated in Figure 5. Assuming that
the proposed HIDs are available in the receiving agents, they
would each send a responding HID-APPROVE back to Agent A.
Application Agent A Agent 1 Agent 2
1.1. (open B,C,D)
V
1.2. +-> (routing to B,C,D)
V
1.3. +->(reserve resources from A to Agent 1)
| V
1.4. | +-> CONNECT B --------->>
| <RVLId=0><SVLId=4>
| <Ref=10><HID=1200>
V
1.5. +->(reserve resources from A to Agent 2)
V
1.6. +-> CONNECT C,D ------------------>>
<RVLId=0><SVLId=5>
<Ref=15><HID=2400>
Figure 5. Origin Sending CONNECT Message
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RFC 1190 Internet Stream Protocol October 1990
3.1.5. CONNECT Processing by an Intermediate Agent
An ST agent receiving a CONNECT message should, assuming no
errors, quickly select a VLId and respond to the previous-hop
with either an ACK, a HID-REJECT, or a HID-APPROVE message, as
is appropriate. This message must identify the CONNECT to
which it corresponds by including the CONNECT's Reference
number in its Reference field. Note that the VLId that this
agent selects is placed in the SVLId of the response, and the
previous-hop's VLId (which is contained in the SVLId of the
CONNECT) is copied into the RVLId of the response. If the
agent is not a target, it must then invoke the routing
function, reserve resources, and send a CONNECT message(s) to
its next-hop(s), as described in Sections 3.1.2-4 (pages 19-
20).
Agent A Agent 1 Agent B
[1.4] >>-> CONNECT B -------->+--+
<RVLId=0><SVLId=4> | V
2.1. <Ref=10><HID=1200> | (routing to B)
| V
2.2. V +->(reserve resources from 1 to B)
2.3. +<- HID-APPROVE <------+ V
2.4. <RVLId=4><SVLId=14> +-> CONNECT B ---------->>
<Ref=10><HID=1200> <RVLId=0><SVLId=15>
<Ref=110><HID=3600>
Agent A Agent 2 Agent C
[1.6] >>-> CONNECT C,D ------>+-+
<RVLId=0><SVLId=5> | V
2.5. <Ref=15><HID=2400> | (routing to C,D)
| V
2.6. V +-->(reserve resources from 2 to C)
2.7. +<- HID-APPROVE <------+ | V
2.8. <RVLId=5><SVLId=23> | +-> CONNECT C ---------->>
<Ref=15><HID=2400> | <RVLId=0><SVLId=25>
| <Ref=210><HID=4800>
|
| Agent D
V
2.9. +->(reserve resources from 2 to D)
V
2.10. +-> CONNECT D ---------->>
<RVLId=0><SVLId=26>
<Ref=215><HID=4800>
Figure 6. CONNECT Processing by an Intermediate Agent
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RFC 1190 Internet Stream Protocol October 1990
The resources listed as Desired in a received FlowSpec may not
correspond to those actually reserved in either the ST agent
itself or in the network(s) used to reach the next-hop
agent(s). As long as the reserved resources are sufficient to
meet the specified Limits, the copy of the FlowSpec sent to a
next-hop must have the Desired resources updated to reflect the
resources that were actually obtained. For example, the
Desired bandwidth might be reduced because the network to the
next-hop could not provide all of the desired bandwidth. Also,
the delay and delay variance are appropriately increased, and
the link MTU may require that the DesPDUBytes field be reduced.
(The minimum requirements that the origin had entered into the
FlowSpec Limits fields cannot be altered by the intermediate or
target agents.)
3.1.6. Setup at the Targets
An ST agent that is the target of a CONNECT, whether from an
intermediate ST agent, or directly from the origin host ST
agent, must respond first (assuming no errors) with either a
HID-REJECT or HID-APPROVE. After inquiring from the specified
application process whether or not it is willing to accept the
connection, the agent must also respond with either an ACCEPT
or a REFUSE.
In particular, the application must be presented with
parameters from the CONNECT, such as the Name, FlowSpec,
Options, and Group, to be used as a basis for its decision.
The application is identified by a combination of the NextPcol
field and the SAP field in the (usually) single remaining
Target of the TargetList. The contents of the SAP field may
specify the "port" or other local identifier for use by the
protocol layer above the host ST layer. Subsequently received
data packets will carry a short hand identifier (the HID) that
can be mapped into this information and be used for their
delivery.
The responses to the CONNECT message are sent to the previous-
hop from which the CONNECT was received. An ACCEPT contains
the Name of the stream and the updated FlowSpec. Note that the
application might have reduced the desired level of service in
the received FlowSpec before accepting it. The target must not
send the ACCEPT until HID negotiation has been successfully
completed.
Since the ACCEPT or REFUSE message must be acknowledged by the
previous-hop, it is assigned a new Reference number that will
be returned in the ACK. The CONNECT to which the ACCEPT or
REFUSE is a reply is identified by placing the CONNECT's
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