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Unformatted text preview: framing overhead in a SLIP frame (Figure 2.2), we see that PPP adds three
additional bytes: I byte for the protocol field, and 2 bytes for the CRC. Additionally, using the IP
network control protocol, most implementations then negotiate to use Van Jacobson header
compression (identical to CSLIP compression) to reduce the size of the IP and TCP headers.
In summary, PPP provides the following advantages over SLIP: (1) support for multiple protocols
on a single serial line, not just IP datagrams, (2) a cyclic redundancy check on every frame, (3)
dynamic negotiation of the IP address for each end (using the IP network control protocol), (4)
TCP and IP header compression similar to CSLIP, and (5) a link control protocol for negotiating
many data-link options. The price we pay for all these features is 3 bytes of additional overhead
per frame, a few frames of negotiation when the link is established, and a more complex
Despite all the added benefits of PPP over SLIP, today there are more SLIP users than PPP users. As
implementations become more widely available, and as vendors start to support PPP, it should (eventually) replace
SLIP. 2.7 Loopback Interface
Most implementations support a loopback interface that allows a client and server on the same
host to communicate with each other using TCP/IP. The class A network ID 127 is reserved for
the loopback interface. By convention, most systems assign the IP address of 127.0.0.1 to this
interface and assign it the name localhost. An IP datagram sent to the loopback interface must
not appear on any network.
Although we could imagine the transport layer detecting that the other end is the loopback
address, and short circuiting some of the transport layer logic and all of the network layer logic,
most implementations perform complete processing of the data in the transport layer and network
layer, and only loop the IP datagram back to itself when the datagram leaves the bottom of the
Figure 2.4 shows a simplified diagram of how the loopback interface process...
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This test prep was uploaded on 04/04/2014 for the course ECE EL5373 taught by Professor Guoyang during the Spring '12 term at NYU Poly.
- Spring '12