64Chapter 2: EIU messaging protocols

medium (1024 bytes)

large (1600 bytes)

When the IP layer comes into service, the number of buffers allocated by default is 10 for the small buffer size and 5 each for the medium and large buffer sizes. These values are load and application dependent.

When the TCP layer comes into service, it allocates its own pool of transmit buffers and adds buffers to the common pool of receive buffers. The common pool of buffers is used for receiving IP and TCP packets.

The transmit pool owned by TCP is used for transmission of TCP segments. The TCP transmission mechanism does not consume buffers from the common pool. The allocation of TCP transmit and receive buffers depends on the datafill for the number of TCP connections on the node. For each TCP connection, TCP allocates 3 small, 2 medium, and 1 large buffer for the transmit side. It adds as many buffers to the common pool of buffers owned by ICBM for receiving TCP segments.

For example, assume that the number of TCP connections for the node is 10. Therefore, the total number of transmit buffers allocated is 30, 20, and 10 for small, medium, and large sizes, respectively. The same number of buffers for all three sizes are added to the existing common pool owned by ICBM. If the TCP layer is brought out of service, all the transmit buffers owned by TCP, as well as receive buffers that are added by TCP to the common pool (owned by ICBM), are deallocated. Initial allocation of the buffers in the common pool is a static allocation regardless of the service state of IP layer. For every two endpoints reserved, TCP allocates a 4-kbyte buffer for compaction.

Buffer allocation is summarized in table 7.

Table 7 Buffer allocation per end point

Protocol

Buffer type

Number

Size (in

 

 

of buffers

bytes)

 

 

 

 

TCP

Receive

1

1518

 

 

 

 

 

ICBM common pool

2

1024

 

 

 

 

 

 

3

128

 

 

 

 

 

Transmit

1

1518

 

 

 

 

 

TCP own

2

1024

 

 

 

 

 

 

3

128

 

 

 

 

UDP

Receive

1

1518

 

 

 

 

(Sheet 1 of 2)

 

 

 

 

 

 

297-8991-910 Standard 03.01 August 1999

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Cabletron Systems DMS-100 manual Receive 1518 Icbm common pool 1024 128 Transmit TCP own