network
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... performance of a transport
protocol is important, it is not the only consideration when
constructing a network containing satellite links. For example, data
link protocol, application protocol ...
... satellite mitigations proposed and studied by the research community.
While these mitigations may prove useful and safe for shared networks
in the future, this document only considers TCP mechanisms which are
...
...
This document is divided up as follows: Section 2 provides a brief
outline of the characteristics of satellite networks. Section 3
outlines two non-TCP mechanisms that enable TCP ...
... TCP
mechanisms defined by the IETF that may benefit satellite networks.
Finally, Section 5 provides a summary of what modern TCP
...
... satellite propagation time. The RTT will be
increased by other factors in the network, such as the transmission
time and propagation time of other links in the network ...
... network, such as the transmission
time and propagation time of other links in the network path and
queueing delay in gateways. Furthermore, the satellite ...
... bandwidth is the capacity of the bottleneck link
in the network path. Because the delay in some satellite
environments is large, TCP ...
... bit-error rate (BER) than
typical terrestrial networks. TCP uses all packet drops as
signals of network ...
... networks. TCP uses all packet drops as
signals of network congestion and reduces its window size in an
attempt to alleviate the congestion ...
... congestion or corruption), TCP
must assume the drop was due to network congestion to avoid
congestion ...
... window, even though these packet drops do not signal congestion in
the network.
Asymmetric use
...
... Due to the expense of the equipment used to send data to
satellites, asymmetric satellite networks are often constructed.
For example, a host connected to a satellite network ...
... satellite networks are often constructed.
For example, a host connected to a satellite network will send all
outgoing traffic over a slow terrestrial link ...
... satellite channels only exhibit a subset of the above
characteristics. Furthermore, satellite networks are not the only
environments where the above characteristics are found. However,
satellite networks ...
... satellite networks are not the only
environments where the above characteristics are found. However,
satellite networks do tend to exhibit more of the above problems or
the above problems are aggravated in the satellite environment. The
...
... the above problems are aggravated in the satellite environment. The
mechanisms outlined in this document should benefit most networks,
especially those with one or more of the above characteristics (e.g.,
gigabit ...
... especially those with one or more of the above characteristics (e.g.,
gigabit networks have large delay*bandwidth products).
...
... satellite channels in their
networks should use the following two non-TCP mechanisms which can
increase TCP ...
... performance of higher layer protocols. While
beyond the scope of this document, those constructing satellite
networks should tune these protocols in an appropriate manner to
ensure that the data link protocol does not limit TCP ...
... MD90] is used to determine the maximum packet
size a connection can use on a given network path without being
subjected to IP fragmentation ...
... fragmentation. The sender transmits a packet that is
the appropriate size for the local network to which it is connected
(e.g., 1500 bytes on an Ethernet) and sets the IP ...
... bit. If the packet is too large to be forwarded without being
fragmented to a given channel along the network path, the gateway
that would normally fragment ...
... DF bit set. The packet will be forwarded by G1,
however this does not ensure all subsequent gateways in the network
path will be able to forward the segment. If a second gateway ...
... path
MTU discovery can spend a large amount of time determining the
maximum allowable packet size on the network path between the sender
and receiver. Satellite delays can aggravate this problem (consider
...
... segments should
provide better performance, as with any network [MSMO97]. While the
exact method ...
... acknowledgments (see section 4), TCP spends a long time recovering
from loss when operating in satellite networks. When packet loss is
due to corruption, rather than congestion ...
... TCP to operate efficiently, the channel
characteristics should be such that nearly all loss is due to network
congestion. The use of forward error correction ...
... outages can also cause problems in satellite systems that do not
occur as frequently in terrestrial networks. Finally, FEC is not
without cost. FEC ...
... packet loss must be assumed to indicate
congestion to preserve network stability. Incorrectly interpreting
loss as caused by corruption and not reducing the transmission rate
...
... satellite or hybrid satellite/terrestrial networks to better utilize
the available capacity of the link. These mechanisms may also be
...
... these mechanisms do not fundamentally hurt performance in a shared
terrestrial network. Each of the following sections outlines one
mechanism and why that mechanism may be needed.
...
...
To avoid generating an inappropriate amount of network traffic for
the current network conditions, during a connection ...
... To avoid generating an inappropriate amount of network traffic for
the current network conditions, during a connection TCP employs four
...
... fast recovery. These algorithms are used to adjust the amount of
unacknowledged data that can be injected into the network and to
retransmit segments dropped by the network ...
... congestion window (cwnd). This is an upper
bound on the amount of data the sender can inject into the network
before receiving an acknowledgment (ACK ...
... is increased or decreased during the transfer based on the inferred
amount of congestion present in the network. The second variable is
the slow start threshold ...
... host has no
knowledge of the current state of the network between itself and the
data receiver. In order to avoid transmitting ...
... Congestion avoidance is used to slowly probe the network for
additional capacity. During congestion avoidance, cwnd is increased
...
... channel bandwidth when using long-delay
satellite networks [All97]. For example, transmission begins with
the transmission of one segment ...
... segment as an indication
of congestion in the network. In response to the congestion, the
value of ssthresh is set to half of the cwnd and the value of cwnd is
...
... segment that just arrived. For example, assume segment 11 has been
dropped somewhere in the network and segment 12 arrives at the
receiver ...
... ACK represents 1 segment that has left
the network. When the cwnd permits, TCP is able to transmit new
data. This allows TCP ...
... TCP is able to transmit new
data. This allows TCP to keep data flowing through the network at
half the rate it was when loss was detected. When an ACK for the
...
... ACKs, TCP knows that packets are still flowing
through the network and can therefore infer that the congestion is
not that bad. However, when resending a packet due to the expiration
...
... TCP cannot infer anything about the
state of the network and therefore must proceed conservatively by
sending new data using the slow start algorithm.
...
... connections are able to
inject enough new segments into the network during recovery to
trigger the multiple fast retransmits. Reducing cwnd multiple times
...
... algorithms slowly
probe the network for additional capacity, which in turn wastes
bandwidth. This is especially true over long-delay satellite ...
... algorithms are necessary to prevent congestive collapse in a shared
network [Jac88]. Therefore, the negative impact on a given
connection ...
... Jac88]. Therefore, the negative impact on a given
connection is more than offset by the benefit to the entire network.
...
... that are able to adjust the buffer capacity as dictated by the
current network conditions is currently underway [SMM98]. This will
allow stock TCP ...
... allow stock TCP implementations and applications to better utilize
the capacity provided by the underlying network.
...
... ACKs. And, second, increasing
the number of ACKs by a factor of two in a shared network may have
consequences that are not yet understood. Therefore, disabling
delayed ACKs ...
... retransmitted next. While waiting for a timeout, the data segments
and their acknowledgments drain from the network. In the absence of
incoming ACKs to clock new segments ...
... incoming ACKs to clock new segments into the network, the sender must
use the slow start algorithm ...
... IETF standards
track mechanisms that are recommended by the authors for use in
networks containing satellite channels. Those mechanisms marked
...
... channels
is ongoing and will be summarized in a planned memo along with other
considerations, such as satellite network architectures.
...
... links to transfer user data
and/or network control traffic, one should be aware of the intrinsic
security implications ...
... traffic control
information that would jeopardize the proper functioning of the
network. These attacks could reduce the ability of the network to
...
... network. These attacks could reduce the ability of the network to
provide data transmission services efficiently. Eavesdroppers could
...
... end-to-end
security mechanisms are not in use. While passive monitoring can
occur on any network, the wireless broadcast nature of satellite
links ...
... satellite
links allows reception of signals without physical connection to the
network which enables monitoring to be conducted without detection.
However, it should be noted that the resources needed to monitor a
satellite link ...
... channels. However, this still leaves
traffic vulnerable to eavesdropping on networks before and after
traversing the satellite link. Therefore, end-to-end security ...
... recommendations as to which security mechanisms should be employed.
However, those operating and using satellite networks should survey
the currently available network security mechanisms and choose those
...
... However, those operating and using satellite networks should survey
the currently available network security mechanisms and choose those
that meet their security requirements.
...