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Serial Line Internet Protocol (SLIP)
(Page 3 of 3)
Problems and Limitations of SLIP
SLIP sounds great, rightnice
and simple? Unfortunately, the cost of this simplicity is that SLIP
just doesn't provide many of the features and capabilities we really
need on modern serial links. Some of the most significant problems with
SLIP include the fact that it is deficient in all of the following areas:
- Standardized Datagram Size Specification:
SLIPs maximum datagram size supported is not standardized and
depends on the implementation. The usual default is 1006 bytes, which
becomes the maximum
transmission unit (MTU) for the link.
If a different size is used this must be programmed into the IP layer.
- Error Detection/Correction Mechanism:
SLIP doesn't provide any way of detecting or correcting errors in transmissions.
While such protection is provided at higher layers through IP
header checksums and other mechanisms,
it is a job traditionally also done at layer two. The reason
is that relying on those higher layers means that errors are only detected
after an entire datagram has been sent and passed back up the stack
at the recipient. Error correction can only come in the form of re-sending
any datagrams that were corrupted. This is inefficient, especially considering
that serial links are generally much slower than normal LAN links.
- Control Messaging: SLIP provides no way
for the two devices to communicate control information between them
to manage the link.
- Type Identification: Since SLIP includes
no headers of its own, it is not possible to identify the protocol it
is sending. While developed for IP, you can see that there is no reason
other layer three protocols could not be sent using SLIP. However, without
type identification there is no way to mix datagrams from two or more
layer three protocols on the same link.
- Address Discovery Method: Addressing isn't
needed at layer two due to the point-to-point nature of the connectionthere
are only two devices so the intended recipient of each message is obvious.
However, devices do need some way of learning each other's IP addresses
for routing at layer three. SLIP provides no method for this.
- Support For Compression: Compression would
improve performance over serial lines that are, again, slow compared
to other technologies. SLIP provides no compression features. Note,
however, that modems usually do support compression at layer one, for
serial connections that use them. There was also a variant on SLIP called
Compressed SLIP or CSLIP that was created in the late
1980s, but it was not as widely deployed as regular SLIP.
- Security Features: SLIP provides no methods
for authentication of connections or encryption of data, which means
even the basics of security are not provided.
Sounds pretty bad, doesn't it? The
many shortcomings of SLIP have led most implementations to move from
SLIP over to the newer Point-to-Point Protocol (PPP), which is a much
richer data link protocol for direct connections that resolves the problems
listed above. SLIP is now outdated, and some even consider it a historical
protocol. Despite that, SLIP is still used in many places. Simplicity
is attractive, and computer people are famous for their inertia: if
something is implemented and is working well, many will refuse to change
unless they are forced to do so.
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The TCP/IP Guide (http://www.TCPIPGuide.com)
Version 3.0 - Version Date: September 20, 2005
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