Commit db5f9c29 authored by Mike Hibler's avatar Mike Hibler
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In contemplating a rewrite of the delay-agent and in dealing with PELAB

requirements I started to make some notes on how the delay agent works
and how delays are setup.  These are those partial notes...
parent 06de6fec
[ This file explains how traffic shaping is implemented with the emphasis
on how the delay-agent works. ]
We can shape network links or LANs. Links can have their characteristics
set either symmetrically (duplex) or asymmetrically (simplex). LANs can
have characteristics set either uniformly for the entire LAN or individually
per node on the LAN. Note that shaped LANs are mostly used to emulate
"clouds" where you are modeling the last hop of nodes connected to some
opaque network. We can shape bandwidth, delay and packet loss rate.
Shaping is usually done using a dedicated "delay node" which is interposed
between nodes on a link or LAN. A single shaping node can shape one link
per two interfaces. So in Emulab, where nodes typically have four experimental
network interfaces, we can shape two links per shaping node. For a LAN in
Emulab, one shaping node can handle two nodes connected to the LAN.
More details are given below.
A lower-fidelity method is to shape the links at the end points ("end node
shaping"). Larger networks can be emulated in this way.
Our shaping nodes currently use dummynet, configured via IPFW, running on
FreeBSD. Much of the terminology below (e.g., "pipe") comes from this
heritage though hopefully the parameters are general enough for other
implementations. The particular implementation sets up a layer 2 bridge
between the two interfaces composing a link. An IPFW rule is setup for
each interface, and a dummynet pipe associated with each rule. Shaping
characteristics are then applied to those pipes.
Complicating factors: LANs, PELAB, endnode shaping...
1. Specifying shaping.
Shaping can be specified statically in the NS file using (largely) standard
NS commands and syntax. Some commands were added by us, in particular to
handle LANs. Commands:
* create a link between nodes with specified parameters:
$ns duplex-link <node1> <node2> <bw> <delay> <q-behavior>
* set loss rate on a link:
tb-set-link-loss <src-node> <dst-node> <plr>
tb-set-link-loss <linkname> <plr>
* set simplex (individual direction) parameters on a link:
tb-set-link-simplex-params <linkname> <src-node> <delay> <bw> <plr>
And for LANs:
* create a LAN with N nodes:
$ns make-lan "<node0> <node1> ... <nodeN>" <bw> <delay>
* to set loss rate for an entire LAN or per-node:
tb-set-lan-loss <lan> <loss>
tb-set-node-lan-loss <node> <lan> <loss>
* to set per-node delay and bandwidth:
tb-set-node-lan-delay <node> <lan> <delay>
tb-set-node-lan-bandwidth <node> <lan> <bw>
Shaping can also be modified dynamically using the web page or tevc.
End node shaping can be set globally or per-link/LAN:
tb-use-endnodeshaping <enable?>
tb-set-endnodeshaping <link-or-lan> <enable?>
Semantic differences between links and LANs...
LANs of two nodes...
2. Shaping info in the database.
Shaping information is stored in the DB in a variety of tables...
3. Shaping info on the client.
The DB state is returned in the tmcd "delay" command and looks like:
DELAY INT0=<mac0> INT1=<mac1> \
PIPE0=<pipe0> DELAY0=<delay0> BW0=<bw0> PLR0=<plr0>
PIPE1=<pipe1> DELAY1=<delay1> BW1=<bw1> PLR1=<plr1>
<queue0 params> <queue1 params>
VNODE0=<node0> VNODE1=<node1> NOSHAPING=<0|1>
<mac0> and <mac1> are used to identify the physical interfaces which are
the endpoints of the link. The client runs a program called findif to map
a MAC address into an interface to configure. Identification is done in
this manner since different OSes have different names for interfaces
(e.g., "em0", "eth0") and even different versions of an OS might label
interfaces in different orders.
<pipe0> and <pipe1> identify the two directions of a link, with <delayN>,
<bwN>, <plrN> and <queueN params> being the associated characteristics.
How these are used is explained below.
<linkname> is the name of the link as given in the NS file and is used to
identify the link in the delay-agent.
<vnode0> and <vnode1> are the names of the nodes at the end points of the
link as given in the NS file.
The NOSHAPING parameter is not used by the delay agent. It is used for
link monitoring to indicate that a bridge with no pipes should be setup.
For LANs of three or more nodes it looks like...
This information is used at boottime to create two files.
/var/emulab/boot/rc.delay is a file containing shell commands that is
run to configure the bridge and pipes. This script sets up the static,
boot time shaping parameters.
/var/emulab/boot/delay_mapping contains information about which interfaces
are associated with which pipes. It is used by the delay-agent which
handles dynamic changes to shaping.
4. Dynamic shaping with the delay-agent.
4a. Physical configuration of delay nodes.
The delay-agent uses a mapping file created at boot time to determine
what names are associated with what interfaces and delay pipes.
/var/emulab/logs/delay_mapping contains a link describing each link
which is to be shaped by this node. Lines looks like:
<linkname> <linktype> <node0> <node1> <if0> <if1> <pipe0> <pipe1>
<linkname> is what is specified in the ns file as the link/lan name.
It is used as the ID for operations (events) on the link/lan.
<linktype> is duplex or simplex.
<node0> and <node1> are the ns names of nodes which are the endpoints
of a link. For a lan, then will be the same name.
<if0> and <if1> are the interfaces *on the delay node* for the two sides
of the link. <if0> is associated with <node0> and <if1> with <node1>.
For a lan, <if0> is associated with <node0> and <if1> with "the lan"
(see below for more info).
For a link (or a LAN of 2 nodes) this translates into:
+-------+ +-------+ +-------+
| | +-----+ +-----+ | |
| node0 |--- pipe0 -->| if0 | delay | if1 |<-- pipe1 ---| node1 |
| | +-----+ +-----+ | |
+-------+ +-------+ +-------+
In terms of physical connectivity, node0's interface and delay's
interface <if0> are in a switch VLAN together while node1's interface and
delay's <if1> are in another VLAN. The delay node bridges the two
interfaces together, applying dummynet shaping via IPFW at each interface.
The IPFW rules on delay (setup via /var/emulab/boot/rc.delay) are:
pipe <pipe0> ip from any to any in recv <if0>
pipe <pipe1> ip from any to any in recv <if1>
For a duplex link, the specified shaping values (e.g., 4ms delay) are applied
in both directions (e.g., 8ms round trip). For simplex links, each pipe
is set to the appropriate characteristics for the direction implied.
For a lan of two nodes, with each node having independent characteristics ...
A LAN of 3 or more nodes is considerably different. Each node will have
two pipes again, one between the node and the delay node and one between
the delay node and "the lan". The delay_mapping file now looks like:
<linkname> <linktype> <node0> <node0> <if0> <if1> <pipe0> <pipe1>
<linkname> <linktype> <node1> <node1> <if2> <if3> <pipe2> <pipe3>
<linkname> <linktype> <node2> <node2> <if4> <if5> <pipe4> <pipe5>
[ Of course, our delay nodes can only delay two links since they have only
4 interfaces, so there will actually be two delay nodes in an experiment
of three nodes. But for this explanation, we pretend that one delay node
has six interfaces and would have the above lines... ]
This translates into:
+-------+ +-------+ +-------+
| | +-----+ +-----+ | |
| node0 |--- pipe0 -->| if0 | | if1 |<-- pipe1 ---| |
| | +-----+ +-----+ | |
+-------+ | | | |
| | | |
+-------+ | | | |
| | +-----+ +-----+ | |
| node1 |--- pipe2 -->| if2 | delay | if3 |<-- pipe3 ---| "lan" |
| | +-----+ +-----+ | |
+-------+ | | + +
| | | |
+-------+ | | | |
| | +-----+ +-----+ | |
| node2 |--- pipe4 -->| if4 | | if5 |<-- pipe5 ---| |
| | +-----+ +-----+ | |
+-------+ +-------+ +-------+
and the IPFW rules:
pipe <pipe0> ip from any to any in recv <if0>
pipe <pipe1> ip from any to any in recv <if1>
pipe <pipe2> ip from any to any in recv <if2>
pipe <pipe3> ip from any to any in recv <if3>
pipe <pipe4> ip from any to any in recv <if4>
pipe <pipe5> ip from any to any in recv <if5>
4b. Dynamic configuration via events.
Emulab events are used to communicate and effect changes on links.
delay-agent specific events have the following arguments.
OBJNAME: the link being controlled.
The name is of the form <linkname>-<nodename>.
Duplex links have two such names, one each for src/dst nodename.
Duplex lans also have two, one each toward/from the LAN "node".
RESET forces a complete re-running of "delaysetup" which tears down
all existing dummynet and bridging, and sets it up again. Currently
only used as part of the Flexlab infrastructure below.
UP, DOWN will take the indicated link up or down. Taking a link down
is done by setting the packet loss rate to 1. Up returns the plr to
its previous value.
MODIFY is used for all other changes.
BANDWIDTH (in kilobits/sec), DELAY (in millis), PLR (0 to 1),
LIMIT (q-limit in packets, unless...), QUEUE-IN-BYTES (q limit is in bytes),
PIPE (to apply changes to)
5. Flexlab configuration.
5a. Hybrid mode setup:
The current so-called "hybrid mode" setup for the current so-called "simple
model" allows for per-node pairs in an internet "cloud" (i.e., a LAN) to have
individual (per pair) delay and plr, but potentially a shared bandwidth.
To setup unique characteristics per pair, the event should specify a DEST
tevc -e pid/eid now link-node DEST= DELAY=10 PLR=0
would say that the link "link-node" from us to should have the
indicated characteristics. To setup a shared bandwidth, omit the DEST:
tevc -e pid/eid now link-node BANDWIDTH=1000
which says that all traffic to all hosts reachable on link-node should share
a 1000Kb *outgoing* bandwidth. To allow some hosts to have per-pair
bandwidth while all others share, then use a command with DEST and BANDWIDTH:
tevc -e pid/eid now link-node DEST= BANDWIDTH=5000
tevc -e pid/eid now link-node BANDWIDTH=1000
which says that traffic between us and has an outgoing "private"
BW of 5000Kb while traffic from us to all other nodes in the cloud shares
a 1000Kb outgoing bandwidth.
This whole thing is implemented using the two shaping pipes that connect
every node to a LAN. The delay and PLR are set on the incoming (lan-to-node)
pipe, while the BW is applied to the outgoing (node-to-lan) pipe. Note that
this is completely different than the normal shaping done on a LAN node.
Normally, the delay/plr are divided up between the incoming and outgoing pipes.
So it looks like:
+-------+ +-------+ +-------+
| | +-----+ +-----+ | |
| node0 | -- pipe0 -->| if0 | | if1 |<-- pipe1 -- | |
| | +-----+ +-----+ | |
+-------+ | | | |
| | | |
+-------+ | | | |
| | +-----+ +-----+ | |
| node1 | -- pipe2 -->| if2 | delay | if3 |<-- pipe3 -- | "lan" |
| | +-----+ +-----+ | |
+-------+ | | + +
| | | |
+-------+ | | | |
| | +-----+ +-----+ | |
| node2 | -- pipe4 -->| if4 | | if5 |<-- pipe5 -- | |
| | +-----+ +-----+ | |
+-------+ +-------+ +-------+
There are additional event parameters for hybrid pipes.
# "flow" pipe events
CREATE: create "flow" pipes. Each link has two pipes associated with each
possible destination (destinations determined from /etc/hosts file).
The first pipe is used for most situations and contains BW/delay
values. The second pipe is used when operating in PELAB hybrid mode.
In that case the first pipe is used for delay, the second for BW.
CLEAR: destroy all "flow" pipes
# "flow" pipe specification params
DEST: destination IP address
SRCPORT: source port number
DSTPORT: destination port number
Additional MODIFY arguments:
5b. Hybrid model mods
We want to be able to specify, at a destination, a source delay from a
specific node. For example with nodes H1-H5 we might issue commands:
to H1: "10ms from H2 to me, 20ms from H3 to me"
tevc ... elabc-h1 SRC= DELAY=10ms
tevc ... elabc-h1 SRC= DELAY=20ms
delay from H4 to H1 and H5 to H1 will be the "default" (zero?)
We want to be able to specify, at a source, that some set of destinations
will share outgoing BW. Currently we support a single, implied set of
destinations in the sense that you can specify individual host-host links
with specific outgoing bandwidth, and then all remaining destinations can
share the "default" BW. We want to be able to support multiple, explicit
sets. For example, with hosts H1-H5 we might issue:
to H3: "1Mbs to {H1,H2}, 2Mbs to H4"
tevc ... elabc-h3 DEST=, BANDWIDTH=1000
tevc ... elabc-h3 DEST= BANDWIDTH=2000
The "default" in this case will be whatever was setup with an earlier
tevc ... elabc-h3 BANDWIDTH=2000
or unlimited if there was no such command.
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