Frequently asked questions 

1. How does the watchdog algorithm work in SPXS.NLM? 

Watchdog packets are sent only if there is an extended period with no traffic on the session.
The default WATCHDOG_TIMEOUT (the period between the last packet traffic on the session and 
the first watchdog packet) is 60 seconds. This is configurable by the SPX application 
(through the TLI interface). Any packet that arrives for a session will reset the watchdog 
timer for that session. This includes system packets as well as user data packets. A Watchdog 
request packet consists of an SPX header with SYS and ACK bits set. The receiver will respond 
with the SYS bit set. A data ACK could be piggy-backed on a watchdog packet. Before the 
watchdog algorithm can terminate a connection it must perform one half (1/2) the standard 
number of retries with appropriate time between attempts as is done for the data packets. If 
the connection partner has still not responded, the watchdog algorithm must also attempt to 
locate a new route. If a new route is found then the retries are continued. If the watchdog 
algorithm has completed all these steps and still has nor received a response from the 
connection partner, the algorithm will perform an Unilateral Abort to terminate the connection.

2. What parameters could be set in SPXS.NLM using SPXCONFG.NLM? 

The retry count and the number of configured SPX sessions could be set in SPXS using 
SPXCONFG.NLM. Rest of the parameters do not affect SPXS. 

3. How is the Round Trip Time calculated in SPXS? 

The calculation of Round Trip Time is based on the Van Jacobson method of using the average 
as well as the deviation of the measured round trip time. The time-out value used to determine 
that a packet has not arrived is round trip time + 2 mean deviations of round trip time. The 
initial value for the round trip time will be two times the transport time returned by the get 
local target call. An upper limit is also enforced on the value that the round trip time could 
reach. 

4. How can we disable watchdog mechanism in SPXS? 

SPXWDOG.NLM is a patch that is used to disable SPX watchdog mechanism . This is a work around 
to solve problems with spoofing SPX Watchdogs in remote access products like Netware Connect. 
SPX Watchdogs are not labeled and hence it becomes difficult to spoof them. 
Loading SPXWDOG.NLM will add the following file server set parameter. 

"set spx watchdogs=ON/OFF" The default is ON.

This program runs on 3x and 4x servers. To fully disable watchdogs, the remote client/server 
should also disable the watchdogs. IPXODI v3.02 and IPX.NLM (32 bit IPX for the client) support 
a NET.CFG parameter to disable SPX watchdogs. Patchman is not required for this patch since 
this program runs on all versions of the NW OS. 

5. What are the advantages of SPXII over SPX? 

Windowing: 

The SPX packet header has fields for managing sequencing, acknowledgments and window size. In 
the earliest implementations of SPX these fields were not used to the fullest extent of the 
definition. The number of receive buffers, posted to the IPX sockets, dictated the window size. 
Having more than one SPX session multiplexed over an IPX socket lead to the problem of 
determining which buffers belonged to which session. Since this could not be accurately 
determined, the reported window size could not be trusted. The fields of the SPX header are 
used by SPX-II to fully support windowing and SPX II is flexible enough to support different 
window management algorithms. 

Large Packets: 

The original XNS specification defined a network packet size as being 576 bytes long. This 
allowed 512 bytes of data and 64 bytes for the header. SPX II supports a mechanism for 
determining the largest packet size supported between endpoints and provides a message level 
service rather than a packet level service. SPX-II partitions large messages into appropriately 
sized media packets. The maximum packet size is determined during connection establishment and 
re-negotiated on route failure. SPX-II also deals with the possibility of different send and 
receive packet sizes, which results from the non-deterministic nature of IPX RIP routers with 
parallel routes between two points. 

TLI Interface: 

SPX does not provide all the features supported through TLI. SPX-II has improved TLI support. 

6. What is the difference between SPXS.NLM and SPX in the Server OS? 

SPX in the OS supports the conventional ECB interface used by the various applications. This is 
not supported by SPXS.NLM. SPXS.NLM is implemented using the STREAMS mechanism and it supports 
the TLI interface. 

7. Can you configure the maximum packet size in SPXS?

SPXS gets the maximum packet size supported by the network board from IPX. This is not a 
configurable option at an SPX level. This value is used by SPX to negotiate the packet size 
with its peer during connection establishment phase.