Chapter 5 Basic Packet Processing ： Algorithms And Data Structures

Chapter 5Basic Packet Processing：Algorithms And Data Structures

Outline • State information and Resource Exhaustion • Packet Buffer Allocation • Packet Buffer Size And Copying • Protocol Layering And Copying • Heterogeneity And Network Byte Order • Bridge Algorithm • Table Lookup And Hashing • IP Datagram Fragmentation And Reassembly • IP Datagram Forwarding • IP Fragment Algorithm • High-Speed IP Forwarding • TCP Connection Recognition Algorithm • TCP Splicing Algorithm

State information and Resource Exhaustion • Information stored indefinitely in memory is known state information • The amount of state information depends on traffic • To allow it to run arbitrarily long, a network system must be designed with limits on all resources, and the limits must be fixed independent of arriving traffic; designs that violate this principle will not be considered

Packet Buffer Allocation • Memory is perhaps the most obvious resource：Asymmetric traffic is common • Buffer management is closely related to Qos • Network system usually allocate fixed-size buffer：Variable-size buffer allocation does not work well → memory fragmentation • How large buffer be：Choose a size appropriate for the network technology

Packet Buffer Size And Copying • Using a single set of buffers helps avoid an inefficiency：packet copying • Because copying an entire packet from one locate to another requires time proportional to the entire packet and because memory speed is often a bottleneck, network systems avoid copying whenever possible

Protocol Layering And Copying • Higher layer protocols also affect the buffer allocation decision • Layering implies encapsulation

Two buffer allocation schemes • Large buffer • Input：Place the incoming frame in a buffer, pass the buffer address among layers, and arrange for each layer to locate the appropriate header in the buffer • Output：Leave sufficient space in the buffer before the payload, pass the address of the buffer among layers, and allow each layer to prepend another header • Linked List • Output：Allocate a separate buffer for each header and for the data; construct a packed by forming a linked list where each node on the list points to one of the header buffers or the data buffer • Input：Create an initial list of one node that points to the frame buffer; as the packet passed up the stack, append a node to the list that points to the next header

Heterogeneity And Network Byte Order • To ensure interoperability among heterogeneous computers, many protocol specify representation and transmission details • Two integer representations Increasing memory adresses Little endian Increasing memory adresses Big endian

Integer Conversion • Needed when heterogeneous computers communicate • Protocols define network byte order • Computers convert to network byte order • Typical library functions

Examples Of Algorithms ImplementedWith Software-Based Systems • Layer 2 • Ethernet bridge • Layer 3 • IP forwarding • IP fragmentation and reassembly • Layer 4 • TCP connection recognition and splicing • Other • Hash table lookup

Ethernet Bridge • Commercial Ethernet bridges are often implemented in software on conventional hardware：the code run from ROM on an embedded system that consist of a microprocessor, memory, and two network interfaces • The bridge need receives a copy of any fame：the interface operate in promiscuous mode • A physical interface on a bridge must not change the source address in outgoing frames because the bridge must replicate an exact copy the frame

Bridge Filtering • Uses source address in frames to identify computers on each network • Uses destination address to decide whether to forward frame

Bridge Algorithm Assume: two network interfaces each operating in promiscuous mode. Create an empty list, L, that will contain pairs of values; Do forever { Acquire the next frame to arrive; Set l to the interface over which the frame arrived; Extract the source address, S; Extract the destination address, D; Add the pair (S, I) to list L if not already present. If the pair (D, l) appears in list L { Drop the frame; } Else { Forward the frame over the other interface; } }

Table Lookup And Hashing • Many network systems use a table to store state information or auxiliary data needed for processing, and lookup must be table for each packet • Software-based systems typically use hashingfor table lookup • Double hashing works well if the hash table contains many empty slots

Table Lookup Alogrithm Given: a key, a table in memory, and the table size N. Produce: a slot in the table that corresponds to the key or an empty table slot if the key is not in the table. Method: double hashing with open addressing Choose P1 and P2 to be prime numbers; Fold the key to produce an integer, K; Compute table pointer Q equal to (P1 * K) modulo N; Compute increment R equal to (P2 * K) modulo N; While (table slot Q not equal to K and nonempty) { Q  (Q + R) modulo N; } At this point, Q either points to an empty table slot or to the slot containing the key

IP Datagram Fragmentation And Reassembly • Maximum Transmission Unit (MTU)：the largest payload a given network can accommodate • Fragmentation and Reassembly：a router can divide a large datagram into a set of smaller datagram called fragments • Each fragment travels independently • Ultimate destination reassembles fragments to reproduce the original datagram • Implemented in software：The task is straightforward

Summarizes • Each fragment is a datagram that begins with a datagram header • Header fields in a fragment are derived from the original datagram • In a fragment, the following fields differ from the original datagram • TOTALLENGTH • FLAGS • FRAGMENT OFFSET • HEADER CHECKSUM • The size of a fragment is determined by the MTU of the outgoing network • The FLAGS and FRAGMENT OFFSET fields together identify a datagram as a fragment; if both contain zero the datagram is not a fragment

Uses FLAGS Bits in Datagram Header FLAGS bits 0 = last fragment; 1 = more fragments 0 = many fragment; 1 = do not fragment Reserved (must be 0)

Interpretation Of The Fragment Offset Field • The FRAGMENT OFFSET fields the relationship between data in the fragment and data in the original datagram • The FRAGMENT OFFSET specifies how far into the original payload the data in the fragment belongs • A receiver uses the offset during reassembly to put data from each fragment in its original position • The amount of data in a frame must be a multiple of eight octets • The limit is imposed even if the network MTU allows extra octets

IP Fragment Algorithm Given: IP datagram, D , and a network MTU. Produce: a set of fragments for D. If the DO THE FRAGMENT bit is set { Stop and report an error ; } Compute the size of the datagram header, H; Chooses N to be the largest multiple of 8 such that H + K ≤ MTU ; Initialize an offset counter , O , to zero ; Repeat until datagram empty { Create a new fragment that has a copy of D’s header; Extract up to the next N octets of data from D and Place the data in the fragment ; Set the MORE FRAGMENTS bit in fragment header; Set TOTAL LENGTH field in fragment header to be H + K; Set FRAGMENT OFFSET field in fragment header to be O ; Computer and set the CHECKSUM field in fragment header; Increment O by N/8; }

Fragmenting A Fragment • A datagram may pass over networks with various size MTUs • All offset refer to the original datagram：each fragment is handled independently • The assignment of the MORE FRAGMENTS bit in fragments • The same rules apply to fragment or a datagram

IP Reassembly • When all fragments arrive, the system reassembles them to produce the original datagram • Out-Of-Order Delivery：routes can change at any time • Duplication：overlap • Loss：Not retransmit • Concurrent Reception：A receiving system must be prepared to accept incoming fragment from multiple datagrams concurrently

Grouping Fragment Together • The identification field (ID) in a datagram header is unique to a given source • A receiving system uses both the ID and IP source address fields to determine whether two fragment came from the same datagram

Fragment Position • The receiver must be able to accommodate a fragment that carries an arbitrary amount of data with an arbitrary offset • Reassembly maintains two pieces of information • The data octets themselves • Information about which data has been received • Uses a reassembly buffer Reassembly buffer Fragment in reassenbly buffer

IP Reassembly Algorithm Give: a fragment, F ,add to a partial reassembly Method: maintain a set of fragment for each datagram Extract the IP source address, S , and ID fields from F; Combine S and ID to produce a lookup key, K; Find the fragment set with key K or create a new set; Insert F into the set; If the set combine all the data for the datagram { From a completely reassembled datagram and process it; }

IP Datagram Forwarding • IP forwarder uses the address in each incoming packet to determine how to forward the packet • IP forwarding operates in layer 3, forwards IP datagrams, and use layer 3 (i.e., IP) address • Use routing table for each forwarding decision • Conceptual mapping • (next hop, interface)  f(datagram, routing table)

IP Routing Table • One entry per destination • Entry contains • 32-bit IP address of destination • 32-bit address mask • 32-bit next-hop address • N-bit interface number

Example IP Routing Table • Values stored in binary • Interface number is for internal use only • Zero mask produces default route

IP Forwarding Algorithm Given: destination address A and routing table R. Find: a next hop and interface used to route datagrams to A. For each entry in table R { Set MASK to the Address Mask in the entry; Set DEST to the Destination Address in the entry; If (A & MASK) == DEST { Stop; use the next hop and interface in the entry; } } If this point is reached, declare error: no route exists;

High-Speed IP Forwarding • Binary trie is the most popular • Uses individual bits of the lookup key to navigate through the trie — the search starts at the root and proceeds toward the leaves • At each node, a bit of the lookup key specifies whether to follow the left subtree or the right subtree • Each terminal node must contain a complete IP Address

Binary Trie 0 1 A 1 0 1 0 0 1 D H 1 0 0 E 0 1 1 0 B C F G

TCP Connection • Keep information about the state of each connection • Involves a pair of endpoints • Started with SYN segment • Terminated with FIN or RESET segment • Identified by 4-tuple • ( src addr, dest addr, src port, dest port )

TCP Connection Recognition Algorithm Given: a copy of traffic passing across a network. Produce: a record of TCP connections present in the traffic. Initialize a connection table, C, to empty; For each IP datagram that carries a TCP segment { Extract the IP source, S, and destination, D, addresses; Extract the source, P1, and destination, P2, port numbers; Use (S,D,P1,P2) as a lookup key for table C and create a new entry, if needed; If the segment has the RESET bit set, delete the entry; Else if the segment has the FIN bit set, mark the connection closed in one direction, removing the entry from C if the connection was previously closed in the other; Else if the segment has the SYN bit set, mark the connection as being established in one direction, making it completely established if it was previously marked as being established in the other; }

TCP Splicing • Join two TCP connections • Allow data to pass between them • To avoid termination overhead translate segment header fields • Acknowledgement number • Sequence number

Illustration Of TCP Splicing TCP connection #1 TCP connection #2 Host A splicer Host B Sequence 200 Sequence 50 Sequence 800 Sequence 1200

TCP Splicing Algorithm Given: two TCP connections. Produce: sequence translations for splicing the connection. Compute D1, the difference between the starting sequences on incoming connection 1 and outgoing connection 2; Compute D2, the difference between the starting sequences on incoming connection 2 and outgoing connection 1; For each segment { If segment arrived on connection 1 { Add D1 to sequence number; Subtract D2 from acknowledgement number; } else if segment arrived on connection 2 { Add D2 to sequence number; Subtract D1 from acknowledgement number; } }

QUESTION？

Chapter 5 Basic Packet Processing ： Algorithms And Data Structures