Networking (Cont’d)

Congestion Control l Is achieved by informing nodes along a route that congestion has occurred and asking them to reduce their packet transmission rate. l Congestion information can be supplied by explicit transmission of special messages (called choked packets). implementation of a specific transmission control protocol (e.g., TCP takes packet loss as an indication of congestion and will hence reduce its congestion window – the number of packets allowed to be transmitted before receipt of a positive acknowledgement).

Mobile IP Sender Home Mobile host MH Foreign agent FA Internet agent First IP packet addressed to MH Address of FA returned to sender First IP packet tunnelled to FA Subsequent IP packets tunnelled to FA

Transport Layer l Function 1 (Message decomposition and reassembly): Breaks messages into packets at the transmitting end and reassembles packets into messages at the receiving end. l Function 2 (Multiplexing and demultiplexing): Multiplexes several lower-rate sessions, all from the same source and all going to the same destination, into one session at the network layer. l Function 3 (Reliable communication): Performs message retransmission if the underlying network layer is not reliable. l Function 4 (End-to-end congestion/flow control): Avoids sending data faster than the destination can absorb it and cooperates with network layer entities (e.g., routers) for congestion control.

TCP Header

TCP Window Mechanism

TCP Congestion Control Slow start Congestion avoidance

TCP Congestion Control -- AIMD l Slow start phase During slow start, a TCP increments cwnd by at most one TCP segment for each ACK received. Slow start ends when cwnd exceed ssthresh, or when congestion is observed. l Congestion avoidance phase During congestion avoidance, cwnd is incremented by 1 segment per round-trip time. In practice, TCP does not wait for an entire window's worth of ACKs to add one segment to the congestion window, but instead increments cwnd by a little for each ACK that arrives, i.e., cwnd  cwnd + 1/ cwnd. l Upon arrival of 3 duplicate ACKs (i.e., 4 identical ACKs), cwnd  cwnd/2. l Upon timeout cwnd  1. l Why does TCP not perform well in wireless environments? Uses packet losses as indication of congestion.

TCP Error Control – Packet Retransmission l A TCP receiver sends an immediate duplicate ACK when an l out-of-order packet arrives, informing the sender of the SN expected. l Upon arrival of 3 duplicate ACKs (i.e., 4 identical ACKs) or timeout, The sender performs a retransmission of the missing packet.

TCP Flow Control l From time to time the receiver may include a receiver advertisement window RcvWin telling the sender its maximum allowable buffer for this connection. l The sender makes sure LastByteSent – LastByteAcked <= min(CongWin, RcvWin)

Session/Presentation/Application Layers l The session layer deals with access rights in setting up sessions and other interactions between the two end points in setting up sessions. l The presentation layer deals with data encryption, data compression, and code conversion. l The application layer actually does the work required by the users, e.g., FTP, telnet, WWW. l The application/presentation/session layers are not clearly distinguished in the Internet protocol stack.

DNS: Domain Name System People: many identifiers: SSN, name, Passport # Internet hosts, routers: IP address (32 bit) - used for addressing datagrams “name”, e.g., sal.cs.uiuc.edu - used by humans Q: map between IP addresses and name ? Domain Name System: l distributed database implemented in the hierarchy of many name servers l application-layer protocol that is responsible for resolving names (address/name translation)

DNS Name Servers l no server has all name-to-IP address mappings local name servers: each ISP, company has local (default) name server host DNS query first goes to local name server authoritative name server: for a host: stores that host’s IP address, name can perform name/address translation for that host’s name Why not centralize DNS? l single point of failure l traffic volume l distant centralized database l maintenance doesn’t scale!

DNS: Root Name Servers l contacted by local name server that can not resolve name l root name server: contacts authoritative name server if name mapping not known gets mapping returns mapping to local name server l ~ dozen root name servers worldwide

Simple DNS Example host surf.eurecom.fr wants IP address of dragon.cs.uiuc.ed u 1. Contacts its local DNS server, dns.eurecom.fr 2. dns.eurecom.fr contacts root name server, if necessary 3. root name server contacts authoritative name server, dns.cs.uiuc.edu, if necessary requesting host surf.eurecom.fr dragon.cs.uiuc.edu root name server authorititive name server dns.cs.uiuc.edu local name server dns.eurecom.fr

DNS Example Root name server: l may not know the authoritiative name server l may know intermediate name server: who to contact to find authoritative name server requesting host surf.eurecom.fr dragon.cs.uiuc.edu root name server local name server dns.eurecom.fr authoritative name server dns.cs.uiuc.edu intermediate name server dns.uiuc.edu 7 8

DNS: Iterated Queries recursive query: l puts burden of name resolution on contacted name server l heavy load? iterated query: l contacted server replies with name of server to contact l “I don’t know this name, but ask this server” requesting host surf.eurecom.fr dragon.cs.uiuc.edu root name server local name server dns.eurecom.fr authoritative name server dns.cs.uiuc.edu intermediate name server dns.uiuc.edu 7 8 iterated query

DNS: Caching and Updating Records l once (any) name server learns mapping, it caches mapping cache entries timeout (disappear) after some time l update/notify mechanisms under design by IETF RFC

Firewalls  A set of processes that monitor & control all comm. Into and out of an interanet, for:  Service control  Behavior control  User control  Firewall filtering can be done at diff. Levels  IP packet filtering  TCP gateway filtering  Application gateway filtering

Firewall Configurations