NOTE: NCP header compression is not used for NCP packets using the Packet BurstTM protocol. Because IPX headers are the standard, IPX header compression is used.
Header compression increases the throughput of IPX and NCP packets over low-speed serial lines (except for NCP packets using the Packet Burst protocol). An IPX packet header is 30 bytes and is typically followed by an upper-layer protocol header, such as an SPX header. Header compression reduces the size of this combined packet header to just a few bytes.
Header compression is negotiated by the IPXWAN protocol when a call is established over any WAN connection type. Header compression is not used on the connection if IPXWAN detects that one of the routers does not support it. The routers at each end of the connection must have header compression enabled and must allocate the same number of compression slots.
For more information about header compression, refer to
When you enable header compression, you can also specify the number of compression slots. A compression slot is a location in router memory that stores packet header information. The compression algorithm uses this information to compress outgoing---and decompress incoming---packet headers.
IMPORTANT: You must allocate the same number of compression slots on each router. If the values are different, the IPXWAN protocol chooses the lesser of the two.
If too few compression slots are allocated for the number of different-style packets crossing the connection, the values in the following IPXCON counters become large:
The compression algorithm is running efficiently if the number of compressed packets sent is significantly higher than the values in these counters.
A router sends an uncompressed packet when it is considered beneficial not to reuse a compression slot.
Allocating too many compression slots has its own consequences. More memory is required to store all the headers, and the compression algorithm must scan through more stored headers to find a match for each transmitted packet. This results in a higher processing load and slower performance.
Five packet types are used to exchange compression-state information about packets sent over a connection on which header compression is enabled. Three of these packet types---slot initialization, reject, and acknowledgment packets---manage the flow of compressed and uncompressed packets over the connection; these are the compression protocol packets. The packet type, along with other information, is indicated in the first byte of a compressed packet. Compression packet types are defined as follows:
Uncompressed packet ---A standard, uncompressed IPX or NCP packet. A router sends an uncompressed packet when the packet cannot be compressed or a decision was made not to compress it. When the remote router receives an uncompressed packet, it simply removes the 1-byte compression header and passes the packet to IPX.
IPXCON tracks uncompressed packets exchanged on a connection in the Uncompressed Packets Sent and Uncompressed Packets Received counters.
Compressed packet ---A compressed IPX or NCP packet. Compressed packets do not contain the standard packet header. Instead, they contain the number of a compression slot on the receiving router. This slot contains the information necessary for the compression algorithm to decompress the packet header before passing the packet to IPX. IPXCON tracks compressed packets in the Packets Sent and Packets Received counters.
Slot initialization packet ---A compression protocol packet that a router sends to prepare a compression slot on the receiving router for use. Initialization packets can prepare new slots or previously used slots for reuse. The routing software uses two different initialization packets: one for IPX packets and one for NCP packets. IPXCON tracks initialization packets in the Initialization Packets Sent and Initialization Packets Received counters.
Reject packet --- A compression protocol packet that a router sends when it receives another compression protocol packet that it does not understand. IPXCON tracks reject packets in the Reject Packets Sent and Reject Packets Received counters.
Acknowledgment packet --- A compression protocol packet that acknowledges receipt of an IPX slot initialization packet. The transmitting node continues to send slot initialization packets until it receives an acknowledgment packet. After receiving the acknowledgment packet, the transmitting node begins to send compressed packets. IPXCON does not track acknowledgment packets.