ipop part A session 1

IPv4 address

(IPv4) A 32 bit address that identifies a host's network interface.

IPv6 address

(IPv6) A 128 bit address. It is written as 8 groups of 16 bits.

Prefix length /n

(IPv4) The number of bits at the start of the address that make up the network part. The rest is the host part.

NetID

(IPv4) The network part of an IP address. It says which network the host is in.

HostID

(IPv4) The host part of an IP address. It says which host it is inside the network.

Network address

(IPv4) The first address of a block, where all host bits are 0. It names the network and cannot be given to a host.

Directed broadcast address

(IPv4) The last address of a block, where all host bits are 1. It reaches every host in the network and cannot be given to a host.

Addresses in an IPv4 /n block

(IPv4) 2^(32 − n) addresses.

Usable hosts in an IPv4 /n block

(IPv4) 2^(32 − n) − 2. You subtract the network address and the broadcast address.

/24 mask and block

(IPv4) Mask 255.255.255.0. A block of 256 addresses.

/25 mask and block

(IPv4) Mask 255.255.255.128. A block of 128 addresses.

/26 mask and block

(IPv4) Mask 255.255.255.192. A block of 64 addresses.

/27 mask and block

(IPv4) Mask 255.255.255.224. A block of 32 addresses.

/28 mask and block

(IPv4) Mask 255.255.255.240. A block of 16 addresses.

/29 mask and block

(IPv4) Mask 255.255.255.248. A block of 8 addresses.

/30 mask and block

(IPv4) Mask 255.255.255.252. A block of 4 addresses.

/23 mask and block

(IPv4) Mask 255.255.254.0. A block of 512 addresses, which is 2 steps in the third number.

/22 mask and block

(IPv4) Mask 255.255.252.0. A block of 1024 addresses, which is 4 steps in the third number.

How to find the network address

(IPv4) Find the block size. Then take the biggest multiple of the block size that is not larger than your number.

How to find the broadcast address

(IPv4) Take the network address and add the block size minus 1.

Private IPv4 ranges

(IPv4) 10.0.0.0/8, 172.16.0.0/12 and 192.168.0.0/16. They are not routed on the Internet.

Routing

(Routing) Working out the best paths through the network. Routers do this in the background with a routing protocol.

Forwarding

(Forwarding) Sending each arriving packet out on the right interface, using the forwarding table.

Aggregation

(Forwarding) Merging several routes into one route with a shorter prefix. This makes the forwarding table smaller.

When can two networks be merged?

(Forwarding) When they have the same prefix length n and their first n − 1 bits are equal. Then together they fill one /(n − 1) block exactly.

Blackholing

(Forwarding) Announcing addresses that you do not actually have. Packets sent to those addresses are dropped. It can happen if you merge routes into a block that is too big.

Longest prefix match

(Forwarding) If several rows in the forwarding table match the destination, the row with the longest prefix wins, because it is the most specific.

Default route

(Forwarding) 0.0.0.0/0. It matches every address, so it is only used when no other row matches.

Flag U

(Forwarding) Up. The route works and can be used.

Flag G

(Forwarding) Gateway. The destination is further away, so the packet is handed to the router in the next hop column.

Flag H

(Forwarding) Host. The row points to one single host, not a whole network.

No G flag

(Forwarding) The network is directly connected to this router. The router delivers the packet itself.

MAC address

(Link layer) The hardware address of a network card. On a local network, frames are delivered by MAC address.

Subnet

(IP) One local network. All its hosts have the same network part in their IP addresses.

ARP

(ARP) Address Resolution Protocol. It finds the MAC address that belongs to an IP address, but only inside the same subnet.

ARP request is sent as

(ARP) A broadcast to everyone on the local network, because the sender does not know the MAC address yet.

ARP reply is sent as

(ARP) A unicast, only back to the host that asked.

ARP cache

(ARP) A table of IP to MAC answers the host has already learned. The entries time out after a while.

ARP snooping

(ARP) Hosts that hear an ARP request save the sender's IP and MAC address, even though the request was not for them.

Gratuitous ARP

(ARP) A host sends an ARP request for its own IP address. It announces itself and checks that nobody else uses the address.

Proxy ARP

(ARP) A router answers ARP requests on behalf of a host that sits behind it.

To reach another subnet, a host ARPs for

(ARP) Its router (the default gateway). ARP cannot reach beyond the subnet, so the packet goes to the router first.

Router

(Router) Connects different subnets. It forwards packets by looking at the IP address.

Bridge (switch)

(Bridge) Connects computers inside one subnet. It forwards frames by looking at the MAC address.

How a bridge learns where a MAC is

(Bridge) It reads the source MAC address of each incoming frame and notes which port it came from.

Does a bridge change addresses?

(Bridge) No. It passes frames on without changing the MAC or IP addresses.

What changes when a packet passes a router?

(Router) The MAC addresses. The router writes a new Ethernet header for the next link. The IP addresses stay the same.

ICMP

(ICMP) A protocol that carries error and test messages for IP.

Ping uses

(ICMP) ICMPv4 Echo Request (type 8) and Echo Reply (type 0).

TTL

(IP) Time To Live. A counter in the IP header that each router lowers by 1. When it runs out, the packet is dropped, so packets cannot loop forever.

MTU

(Fragmentation) The largest packet a link can carry.

Fragmentation

(Fragmentation) Cutting an IP datagram into smaller pieces when it is bigger than the MTU of the next link.

IPv4 header size

(IPv4) 20 bytes without options. With options it can be up to 60 bytes.

UDP header size

(UDP) 8 bytes.

TCP header size

(TCP) 20 bytes without options.

Data in a fragment (not the last)

(Fragmentation) Must be a multiple of 8 bytes. Only the last fragment may have any size.

Max data per fragment

(Fragmentation) MTU minus 20 (the IP header), rounded down to a multiple of 8.

Fragment offset is counted in

(Fragmentation) Units of 8 bytes. Offset = the position of the data divided by 8.

MF flag

(Fragmentation) More Fragments. It is 1 on every fragment except the last one.

DF flag

(Fragmentation) Don't Fragment. If it is set and the packet is too big, the router drops it and sends an error back.

Who reassembles fragments?

(Fragmentation) Only the destination host. Routers never put fragments back together.

One fragment is lost

(Fragmentation) The whole datagram is thrown away.

Who fragments in IPv6?

(Fragmentation) Only the sending host. Routers in IPv6 do not fragment.

Transport header (UDP/TCP) sits in

(Fragmentation) Only the first fragment. Add it to the data before you split.

Distance vector

(Routing) Each router sends its list of distances to its neighbours. No router knows the whole map.

Link state

(Routing) Each router floods information about its own links to all routers. So every router knows the whole map and computes the paths itself.

Path vector

(Routing) Like distance vector, but each route also carries the full path to the destination.

IGP

(Routing) Interior Gateway Protocol. Routing inside one network, for example RIP and OSPF.

EGP

(Routing) Exterior Gateway Protocol. Routing between networks, for example BGP.

RIP

(RIP) A routing protocol used inside one network. Every 30 seconds, each router sends its list of distances to its neighbours. The distance is counted in hops, meaning the number of routers on the way.

RIP infinity

(RIP) 16 means "this network cannot be reached", so a route can be at most 15 hops long. When a route stops working, the router sets it to 16 and tells its neighbours straight away, without waiting the usual 30 seconds. This is called a triggered update.

Triggered update

(RIP) Sending a change to the neighbours right away, instead of waiting for the next 30 second update.

Distance vector update

(RIP) New cost = link cost to the neighbour + the neighbour's own cost. The router keeps the new route if it is cheaper.

Count to infinity

(RIP) After a link breaks, two routers keep telling each other old routes, so the cost keeps going up: 3, 4, 5 … It stops when it reaches 16.

Split horizon

(RIP) A router never advertises a route back to the neighbour it learned it from. This stops two routers from fooling each other.

Poison reverse

(RIP) The router does advertise the route back to the neighbour it learned it from, but with cost 16, meaning "do not use me for this".

OSPF

(OSPF) A link state routing protocol used inside one network. Each router computes the paths with Dijkstra's algorithm. The metric is often based on link capacity.

LSA

(OSPF) Link state advertisement. The message in which an OSPF router describes its links. It is flooded to all routers.

OSPF areas

(OSPF) A big network (AS) is split into smaller areas, which are joined through area 0.

BGP

(BGP) The routing protocol between autonomous systems, the separate networks that make up the Internet. It is a path vector protocol and runs over TCP port 179.

Autonomous system (AS)

(BGP) A network with one routing policy and its own AS number.

Why BGP can spot loops

(BGP) Each route lists the AS numbers on the path. If a router sees its own AS in the list, the route is a loop.

Port number

(UDP/TCP) A 16 bit number that says which program on the host the data is for.

UDP

(UDP) Sends single messages without a connection. There are no guarantees: messages can be lost or arrive out of order.

TCP

(TCP) Sets up a connection and gives a reliable byte stream. The data arrives complete and in order.

Pseudoheader

(UDP) Information from the IP header, such as the IP addresses. It is used when computing the checksum but is never sent.

Stop and Wait

(Reliable transfer) The sender sends one packet and waits for its ACK before sending the next.

Go Back N max window

(Reliable transfer) 2^m − 1, where m is the number of bits in the sequence number. The receiver does not keep packets that arrive out of order.

Selective Repeat max window

(Reliable transfer) 2^(m−1). The receiver keeps packets that arrive out of order, so only the lost ones are resent.

Utilisation U

(Reliable transfer) The share of time the link is busy sending. U = N × (L/B) ÷ (RTT + L/B), at most 1. N is the window, L the packet length and B the link speed. For Stop and Wait, N = 1.

How to improve utilisation

(Reliable transfer) Use a bigger window N, so more packets are sent before waiting for an ACK.

TCP sequence numbers count

(TCP) Bytes, not segments.

ACK number

(TCP) The number of the next byte the receiver expects.

SYN

(TCP) The flag used to open a connection.

FIN

(TCP) The flag used to close a connection.

Three way handshake

(TCP) The client sends SYN, the server answers SYN+ACK, and the client answers ACK. Then the connection is open.

SYN and FIN each use

(TCP) One sequence number, even though they carry no data.

MSS

(TCP) Maximum Segment Size. The most data one TCP segment can carry. For IPv4 without options it is MTU − 20 − 20.

RTT

(TCP) Round trip time. The time from sending a segment until its ACK comes back.

RTO

(TCP) Retransmission timeout. How long TCP waits for an ACK before it resends the segment.