HP 200 Series Services And Applications page 287

Congestion notifications are ignored for a couple of reasons. First, routers
do not have good mechanisms for notifying end systems on attached LANs
to slow their transmissions to a particular destination. The TCP/IP and OSI
transport layers, however, have implicit congestion avoidance mechanisms.
When these protocols begin to detect network delays, they reduce their
transmission windows on the affected sockets or SAPs and thereby help
relieve congestion conditions.
Second, since routers are not required to respond to congestion notifica-
tions, there is an issue of fairness. If more than 1 router (different vendors)
is attached to a node in a Frame Relay network and one behaves responsibly
while the other does not, the one behaving responsibly is penalized by realiz-
ing lower throughput rates on the affected PVCs.
Optional Pads
Pad characters may optionally be included in the Frame Relay header. Pads
allow the frame to be extended to an even or odd number of bytes or words
in length to suit the needs of the frame's transmitter.
NLPID
The Network Layer Protocol Identifier (NLPID) identifies the type of
network on which the packet was generated, for example, TCP/IP, Novell
IPX, AppleTalk, etc. NLPID values are defined in ISO 9577. Of the routable
protocols supported by HP routers, only Internet Protocol (the IP in TCP/IP)
has a defined NLPID field. Therefore, a couple of additional fields may be
required to encode the network type. Additional fields may also be added if
the packet is to be bridged. Table 3 shows the encoding of the NLPID field.
Table 3. NLPID Field Encoding
00 (hex)
80 (hex)
81 (hex)
82 (hex)
83 (hex)
CC (hex)
Internet Protocol (IP)
CE (hex)
Linking Up with Frame Relay
Frame Relay Data-Link Interface
Not Used
SNAP
ISO CLNP
ISO ES to IS
ISO IS to IS
EtherType
3-93