8
Traditional Demultiplexing
Chapter Demultiplexing
UDP protocol ID IP (type) Ethernet TCP
OSI?
Early Demultiplexing
Prioritizing packets Service differentiation Specializing paths Specialized code to process each packet Fast dispatching Avoids control overhead
Goals
Safety Speed Composability
CSPF
OR 4 compares 4 Booleans AND
All IP and ARP packets from X
AND
ARP.SRC =X
ETHER.TYPE = ARP
IP.SRC =X
ETHER.TYPE = IP
Expression tree Described in a stack based language
BPF
ETHER.TYPE = IP
F
ETHER.TYPE = ARP
3 compares
T
IP.SRC =X
T
ARP.SRC =X
Control ow graph
Performance
Tcpdump rarpd USER KERNEL TCP/IP Protocol stack buffer BPF filter filter buffer
link level driver
link level driver KERNEL NETWORK
Pathnder
Ethernet Cells ARP IP cells OSI
TCP cells Add branches after best matching prefix when adding new filter.
Trie-like data structure
Port Port 2 5
Port 17
Cells in Pathnder
Packet AND L HdrLength + 0ffset offset length (L) Mask Value EQUAL
fires
CELL
Generalized trie nodes 200ns to process 40 byte TCP message 1.5 Gbps at 100MHz
Other Features
Loadable cells to handle fragmentation Loaded with packet ID after the rst
fragment arrives
OR links
Hardware vs. Software
Hardware Software
Performance
High
Low
Flexibility
Low
High
Dynamic Packet Filter
Dynamic code generation Recompile the classier into machine Optimize code for each cell
code each time a lter is added/deleted
Interpretation Overhead
LOAD R1, C(Offset); LOAD R2, C(length); LOAD R3, R2); P(R1, LOAD R1, C(mask); AND R3, R1; LOAD R2, C(value); BNE R2, R3 /* /* /* /* /* /* /* load offset specified in cell into R1 */ load length specified in cell into R2 */ load packet field specified by offset into R3 */ load mask specified in cell into R1 */ mask packet field as specified in cell */ load value specified in cell into R2 */ branch if masked field is not equal to value */
Safety-Checking Overhead
Checks to ensure that it stays within the
current packet being demultiplexed memory alignment
Checks to see that references have right
Exterminating Interpretation Overhead
LOAD R1, C(Offset); LOAD R2, C(length); LOAD R3, P(R1, R2); LOAD R1, C(mask); AND R3, R1; LOAD R2, C(value); BNE R2, R3 /* /* /* /* /* /* /* load offset specified in cell into R1 */ load length specified in cell into R2 */ load packet field specified by offset into R3 */ load mask specified in cell into R1 */ mask p...