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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 it’s 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, but
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