SLIP and PPP in Software Add Code 3/9 in Software SLIP and PPP

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4.4 SLIP and PPP use software code-39 generator toreceive code-39 in software Elipse BIRT Project To use IP Code 3 of 9 for None over ISDN or acoustic modems we need to use a link layer protocol. There are two common link layer standards for encapsulation of IP (and other) packets over telephone lines: SLIP and PPP. SLIP was common until PPP was developed and took over as the protocol of choice.

. RFC 1618 SLIP RFC 1055 Serial Lin barcode 39 for None e IP, or SLIP, is a simple encapsulation for IP over serial lines. It is a point-topoint protocol, meaning it is designed to connect just two machines together. A frame is very simple (Figure 4.

6): the data is terminated by a byte containing the hex value c0, called END. Often, implementations will start the frame with c0, too, just to be sure..

4.4 / SLIP AND PPP Data Figure 4.6 SLIP frame. This means barcode code39 for None we can t have bytes with the value c0 inside the frame as they would be interpreted as the end of a frame. To get around this, SLIP uses byte stuf ng. If we need to transmit a c0 it is replaced by two bytes db dc.

In this context the character db is called ESC. Now to prevent the misinterpretation of a db that happens to be followed by a dc in the original data, we must stuff db, too. To send a value of db, we replace this by two bytes db dd.

This causes a minor expansion of data, but not much. Thus the byte stream 00 c0 db 01 gets transmitted in a frame as c0 00 db dc db dd 01 c0 When reading the frame at the destination it is a simple matter to replace any occurrence of db dc by c0 and db dd by db to recover the original data. (This is a non-trivial example of encapsulation, Section 2.

4.) There is no de ned limit on the frame size, but it is suggested that you should be able to cope with frames at least 1006 bytes long, though many implementations use 296 bytes (which accommodates 40 bytes of TCP/IP header plus 256 bytes of data). This is because 1006 is too large a frame: at 9600b/s it would take about 1 second to transmit a full frame.

Suppose we are copying a large le across the network which results in a continuous stream of full frames. If we want to have an interactive login session at the same time we will have to wait about 0.5 s on average for the current frame to nish before the packet containing our keystrokes can get through; similarly for the response from the other end.

Empirically, an interactive response time of more than 100 200 ms is felt by users to be too long. Reducing the frame to 296 bytes is a good compromise between interactive response and bulk data throughput. For modern machines with faster modems, 576 bytes or more is the size of choice.

For example, on a 56K modem you may want to increase the frame to 1500 bytes. More importantly, the larger packet size reduces the amount of IP fragmentation. Most DNS packets can t inside 576 bytes.

SLIP works, but it has problems. 1. The ends must have pre-agreed IP addresses: there is no mechanism for one end to tell the other its address.

2. No type eld. SLIP only supports IP.

3. No checksum. Essential on noisy telephone lines to spot data corruption.

4. No authentication. Needed on dial-up lines to check on who is trying to connect.

. CHAPTER 4 / THE PHYSICAL AND LINK LAYERS 2 Telephone Software Code 39 Full ASCII lines are slow (4 kHz or 56Kb/s maximum), so any opportunities to improve throughput are welcome. TCP/IP has a large overhead of 40 bytes or more of header per packet (see Section 9.4) and typically you send many packets with nearly identical headers.

A variant of SLIP, compressed SLIP (CSLIP), takes advantage of this repetition. Instead of sending a full header, Van Jacobson devised a scheme that only sends the small differences and which reduces the average overhead to about 3 bytes. This greatly improves throughput, particularly in interactive traf c when there are many small packets.

Van Jacobson also investigated compressing UDP/IP and IP separately but found the gains not to be worth the effort. Note that this is in complete disregard of layering and ties CSLIP to TCP/IP, but the bene ts are too large to ignore..

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