The TCP/IP Guide  9  TCP/IP Lower-Layer (Interface, Internet and Transport) Protocols (OSI Layers 2, 3 and 4)       9  TCP/IP Network Interface Layer (OSI Data Link Layer) Protocols            9  TCP/IP Serial Line Internet Protocol (SLIP) and Point-to-Point Protocol (PPP)

SLIP and PPP Overview and Role In TCP/IP 1 2 3 Point-to-Point Protocol (PPP)

Problems and Limitations of SLIP

SLIP sounds great, right—nice 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: SLIP’s 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 connection—there 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.

SLIP and PPP Overview and Role In TCP/IP 1 2 3 Point-to-Point Protocol (PPP)