IXP1200 Network Processor Family ATM OC-3/12/Ethernet IP Router Example Design
Application Note 9
Modified on: 3/20/02,
1.3.2 Frame and PDU Length vs. IP Packet Length
Figure 2 shows the relationship between IP Packet Length (X axis), Ethernet Frame Length, and
AAL5 PDU length (Y axis). Packet lengths 20 - 128 bytes are shown to illustrate 1-, 2-, and 3-cell
PDUs. The same pattern continues through the maximum Ethernet MTU size - the 1500 byte
packet, which requires 32 cells. There are a few important items to notice on this graph:
1.The smallest possible Ethernet frame is 64-bytes, which includes the IP packet in addition to
a 14-byte Ethernet header and 4-byte FCS. Adding an 8-byte preamble and 12-byte interframe
gap (960ns) to this frame increases its wire-occupancy time to 84 bytes. After IP packet length
exceeds 46 bytes, Ethernet frame length is a linear function of IP packet length.
AAL5 PDU length is a step-wise function of IP packet length, due to rounding up to ATM cell
boundaries. At 53 bytes per cell, the 4-byte ATM header and 1 byte HEC are included here, b ut
the physical layer SONET overhead is not shown.
The smallest possible IP packet, 20 bytes, corresponds to an IP header that does not contain an
IP payload. This packet fits into a single cell PDU, as do packets up to size 32 bytes (20 byte
IP header plus 12 payload bytes).
Minimized TCP/IP packets are 40 bytes - 20 byte IP header, 20 byte TCP header, and 0 TCP
payload bytes. These "40 byte packets" require 2 cell PDUs - they do not fit into single cell
Figure 1. IP over ATM Encapsulation Format
A8921-01
(LLC/SNAP)
Encapsulation LLC
3 bytes OUT
3 bytes PID
2 bytes IP Packet
AAL5
CS Padding
0-47 bytes UU
1 byte Length
2 bytes CRC
4 bytes
CPI
1 byte
CS-SDU Info Field
Ethernet
Data
Ethernet
to ATM
ATM to
Ethernet
IP
Data
Enet Header IP Packet
IP Header Payload
SAR
Sub-layer
Payload
48 bytes Payload
48 bytes
ATM Cell
Payload
48 bytes
GFC
4 bits VPI
8 bits VCI
16 bits PTI
3 bits CLP
1 bit HEC
8 bits ATM Header
(5 bytes)
Cells from other VCs
can be interleaved with
cells from this VC