ECE 526 – Network Processing Systems Design

1. ECE 526 – Network Processing Systems Design Network Address Translator II

2. Overview • Review • Network Address Translation Basic • Conceptually Implementation • Today • IXP Implementation • IXP Optimization • Header caching • Alignment • ARP • Checksum ECE 526

3. NAT Basic • What is NAT: • System allowing multiple computers share single global IP address • Changing packet header: address, port number, IDs and etc • Located between a set of computers at a site and rest of the Internet • Mechanism: header modification • Source/destination IP address, source/destination port, checksum ECE 526

4. Conceptual Implementation • Simplifying assumptions • No fragmentation (why) • No packet with IP option (why) • Flows initiated from local computer only (why) • NAT table • Creation: automatic • Updating policy: • LRU and priority heuristic • Optimization • Bucket hashing • Independent tables ECE 526

5. NAT Complexity • Fragmentation causes header of encapsulated protocol are only present in first fragment • E.g., no port number available for later fragments • IP datagram with options • Memory requirement for NAT table ECE 526

6. NAT Table Management • Creation • Automatically • Add one entry when it is first packet for new flow • Table entry lifetime • Based on packet header • E.g., Fin or reset for TCP • Aging • Counter down timer • Reset to maximum value once used • Decrease one every cycle • Table entry update • Least recent used • Priority heuristic • TCP > UDP > ARP >others ECE 526

7. NAT IXP Implementation ECE 526

8. NAT Overall Structure • What are implementation ideas to improve throughput? ECE 526

9. NAT Microblock Organization ECE 526

10. IXP Optimization • Header caching • Byte alignment • ARP lookup: • Checksum computation: incremental modification ECE 526

11. Header caching • NAT need access multiple header fields • External DRAM access is very slow • Read packet header from buffer in DRAM to local memory • Modify headers using local memory copy • Write back from local memory to DRAM after processing ECE 526

12. Byte alignment • Local memory is organized 4-byte words • Ethernet frame header is 14 byte • Simply copy: IP source/destination spanned into two entries • Use byte_align_be or byte_align_le to read and write local memory ECE 526

13. ARP Lookup • ( IP address, MAC address) • 16 bytes each entry: IP address in one entry of LM; easily convert index to address by shifting • Checksum computation: incremental modification ECE 526

14. Reminder • Read Chapter 10 ECE 526