Outline

• IPv6 - Packets, Autoconfiguration, Source Directed Routing
• Quiz
• NAT
• Multicast
• MPLS

Introduction

• Last week, we were looking at internetworks.
• We looked at how switching is done and forwarding tables are computed within areas and autonomous systems (using OSPF).
• We then started talking about how switches can be implemented in software and hardware.
• We talked about disaggregation of network control and data plaes and how this leads to software defined networks.
• We looked at how forwarding and routing is done between autonomous systems using BGP.
• Finally, we started talk about addressing in IPv6. We said addresses are 128 bits, written as eight, 4 digit hex numbers separated by colons.
• We went over common abbreviations for 0's in addresses.
• We went over some reserved IPv6 prefixes loopback, multicast, link local, and site local situations.
• Finally, we said how IPv4 addresses can be written as IPv6 addresses.
• We start today with a little bit more on addresses.

More on IPv6 Unicast addressing

• Global unicast addresses, addresses to a single host, are by far the most important addressing IPv6 provides.
• Address allocation determines how unicast addresses are assigned to service providers, autonomous systems, networks, hosts, and routers.
• Define a nontransit or a multi-homed AS as a subscriber and transit AS as a provider. Call a provider direct if it directly connects to a subscriber, otherwise, call it indirect (usually, belong to the backbone).
• In CIDR for IPv4, we has some_address1/m was a subnetwork of some_address2/n for `m > n` if some_address1 shared its first n bits with some_address2/n, so there was a notion of hierarchy.
• This allows for the aggregation of networks when we do advertisements in things like BGP.
• We want to be able to deal with situations where the concept of this hierarchy may become ambiguous such as where one subscriber directly connects to the backbone, or when one subscriber connects to two or more providers.
• The scheme above shows one ways to use prefixes to allow for aggregations at different levels of scale: Regional (say country-level), Provider Level, Subscribers within a provider and so on.
• One technique to handle subscribers with two providers X and Y is to use one providerID for X and one for Y and then to use an additional provider ID for X and Y.

Packet Format Details

• Version contains 6 as this is IPv6
• Traffic Class and FlowLabel are related to QoS and will be discussed later in the semester.
• PayloadLen is the length of the packet excluding the header.
• NextHeader replaces the IP options and Protocol of IPv4. If options are required they are carried in one or more special headers called extension headers following the IP header; otherwise, NextHeader identifies the higher-level protocol running over IP. (e.g., TCP or UDP).
• HopLimit corresponds to IPv4 TTL.

More on Extensions Headers

• The NextHeader field indicates the type of extension header that will follow.
• There is a particular order in which header fields must occur if present, and by knowing the type of header the router also knows its length, although if it not required that extension headers be the same length. This ordering to extensions makes it easier to determine what processing needs to be done on the packet quickly rather than the IPv4 use of unordered triples.
• The above diagram shows an example of what a fragmentation header might look like. NextHeader in this case would be 44.
• As we said on the previous slide, the last extension header, is followed by a transport layer header, which indicates the transport layer protocol being used such as UDP or TCP.

Autoconfiguration

• Autoconfiguration, obtaining an IP address when a new host is added to an IPv4 network, requires using something like DHCP and having a DHCP server.
• In IPv6 this can be done without having to have a separate server in a two step process:
  1. Obtain an interface ID that is unique on the link to which the host is attached.
  2. Obtain the correct prefix for the subnet.
• The first step can be done by using the MAC address (which will be unique) and embedding it into the IPv6 address. i.e., 1111 1110 10, followed by additional 0's, followed by the EUI-64 converted MAC address (invert seventh bit of first character of MAC address, insert ff:fe between the 3 and 4 character of MAC address).
• Recall this would specify an IPv6 link local address (unique to the subnet).
• Just this address would be sufficient for things like printers.
• To get a global address if needed, the host assumes a router on the same link will periodically broadcast the network prefix, and then the host pads with 0's and adds its MAC address.
• It should be noted using autoconfiguration might allow tracking of users because one can just look at the Ethernet address portion of the IP address which is unique. To avoid this Windows since Vista, and Macos since Sierra use a hash-based chosen 64 bit identifier.

Source-Directed Routing

• One possible extension header that can be used with IPv6 is a routing header.
• In the absence of a routing header, routing in IPv6 is much like that in IPv4.
• The routing header contains a list of nodes or other topological areas that must be visited en route to the destination.
• Thus, a host might say that it wants some packet to go through a provider that is cheap or through a provider that is known to be reliable, etc.

Quiz

Which of the following is true?

1. One solution to the count to infinity problem in distance vector routing is to use a split horizon.
2. All modern switches are software switches running on Raspberry Pi's.
3. The Border Gateway Protocol, which uses path-vector routing, was replaced with the Exterior Gateway Protocol in the mid-nineties.

NAT

• One technology which is being used to try to get around the problem of 4 byte IP addresses without having to make as big a change as IPv6 is NAT (Network Address Translation).
• The idea is that each company has a single IP address for internet traffic.
• Within the company each machine gets a unique IP address, which can be used to route traffic within the companies network.
• When a packet exits the company to go to an ISP, an address translation takes place.
• Three ranges of IP address are declared as private and no packets with these addresses are allowed on the internet:
  • 10.0.0.0 -- 10.255.255.255/8 (16millionish hosts)
  • 172.16.0.0 -- 172.31.255.255/12 (1millionish hosts)
  • 192.168.0.0 -- 192.168.255.255/16 (65thousandish hosts)
• Recall (or know for the first time) in IP a 0 octet refers to the current network, and -1 (255) is used to broadcast to all hosts, so not all of these addresses are usable.

More on NAT

• When a packet is sent through a NAT box the internal source IP address is converted to the companies IP address.
• The source port address of the packet is replaced by a new source port. The NAT box has a table consisting of entries (internal IP, internal source port, external source port).
• When a packet comes into the NAT Box from outside the company the reverse conversion is done.
• NAT has many problems: (1) each IP no longer corresponds to a unique machine, (2) makes internet less connectionless, (3) network and transport layer no longer independent, (4) some apps insert the IP address into the data and these will fail if NAT is used (for example, FTP).

Multicast

• Sometimes we would like to send data to many but not all hosts on the internet (live concert, video conferences, etc)
• In which case we would like to use multicast.
• IP multicast is a many-to-many model based on the concept of multicast groups.
• It is sometimes called Any Source Multicast (ASM).
• Each such group has its own multicast address.
• Any host can send to such an address. i.e., they don't have to be members of that group.
• The IP packet is just labeled with destination as the multicast address.
• Different implementation of IP multicast have been done.
• The original implementation of IP multicast was supplemented by a form of one-to-many multicast called Source Specific Multicast (SSM).
• In this, a receiving host would specify both a multicast group and a specific sending host.
• A host signals its desire to join or leave multicast groups by communicating with its local router using the Internet Group Management Protocol (IGMP) in IPv4 or Multicast Listener Discovery in IPv6.

Multicast Addresses

• In both IPv4 (first octet begins with 224) and IPv6 (top byte 255) a portion of the address space is reserved for multicast addresses.
• There are 28 bits of possible multicast addresses in IPv4
• Ethernet multicast addresses have 23 bits when the shared prefix is ignored.
• When mapping to Ethernet the high 5 bits are ignored. So there are 32 addresses which will map to the same Ethernet multicast address.
• When a host on an Ethernet joins an IP multicast group, it configures its Ethernet interface to receive any packets with the corresponding Ethernet multicast address.
• The IP at the receiving layer then determines if the packet was truely destined to the multicast group to which the host belonged.
• PONs (passive optical networks), which are often used for IPTV, use a similar idea.

Multicast Forwarding and Routing

• A router's unicast forwarding table indicates for any packet destined to an IP address which link to use to forward the packet.
• In the multicast setting one might have multiple entries in the forwarding table for a given IP address and the router needs to send the packet to each of them.
• Thus, if we look at the set of nodes that a packet might follow in each setting, then in the unicast setting they will form a path in the network graph and in the multicast setting they will form a forest of trees.
• This forest is called the multicast distribution trees.
• We next look at some different routing algorithms for building multicast forwarding tables.

DVMRP (Distance Vector Multicast Routing Protocol)

• This is a so-called flood and prune protocol.
• When a multicast packet comes into a router with source S, the router checks if it arrived on the port that its forwarding tables says is the shortest path to S. If and only if the answer is yes, the packet is flooded out on each of the other ports.
• If two or more routers are connected to the same network, each keeps a parent bit for every forwarding entry. For a given entry, if the parent bit is not set then it doesn't do the forwarding in the above.
• Initially this bit is set. When table information is exchanged between routers, the router determines if anyone either has a shorter route or the same length route but smaller address. If so, the bit is turned off. This mechanism is sometimes called (Reverse Path Broadcast or Reverse Path Forwarding).
• This completes the description of the flood part of the protocol.
• The above mechanism essentially floods the packet to the whole internetwork. Next a pruning mechanism needs to be used to reduce this to just the next networks that have members of the multi-cast group.
• If the parent router on a network is the only router on that network then it is a leaf.
• Periodically, each host in the group is supposed to send a packet announcing that it is in the group.
• When a parent router of a leaf, exchanges (Destination, Cost) pairs with it neighbors, it augments this with the set of groups for which it has hosts.
• This information is then propagated from router to router, so that a given router knows for what groups it should forward multicast packets.

PIM-SM (Protocol-independent multicast)

• PIM mean the multicast protocol is independent of the unicast routing of the network in question.
• The original PIM-DM (dense mode) protocol used a flood and prune algorithm much like DVMRP.
• PIM-SM (sparse mode) has become the dominant multicast routing protocol.
• Unlike DVRMP, routers in PIM-SM must explicity join the multicast distribution tree using Join messages.
• PIM-SM assigns each group a special router called a rendezvous point (RP).
• If a router is configured to be an RP it will periodically announce this candidacy to a bootstrap router.
• Groups are assigned to RP's by applying a known hash function to the Group IP address to get an RP number.
• The multicast forwarding tree is built as a result of sending Join messages using unicast to the RP.
• In order to get to an RP, a host looks up the RP IP address in the BSR that correspond to the RP number it got in applying the hash function to the Group IP.
• PIM-SM can operate in two modes shared (anyone can send to group) and source specific.

Sending Multicast Data PIM-SM

• Once the tree is built, to send to the multicast group a sender sends on its local network to a router known as the designated router (DR), this then opens a tunnel to the RP using a PIM Register message and the packet is sent there, finally the RP then sends the packet to the group.
• To make the scheme more efficient the RP sends join messages back along the path to the DR, so the DR can send as native multicast rather than tunneled multicast
• The need to make it easier to create so-called source specific multicast led to a variation on PIM-SM called PIM-SSM (source specific multicast), which uses a notion of a channel. A channel is a combination of the source and group address.
• To send multi-cast across domains, an extension PIM-SM called Multicast Source Discovery Protocol was devised, that makes use of MSDP peers in each domain.

MPLS

• To finish off our discussion of networking we are going to look at one more important protocol: Multiprotocol Label Switching (MPLS).
• This protocol combines some of the properties of virtual circuits with the flexibility and robustness of datagrams.
• MPLS is mainly used today for:
  • To enable IP capabilities on devices that do not have the capability to forward datagrams in the usual manner.
  • To forward IP packets along "explicit routes" -- explicit routes that don't necessarily match those that normal IP would select.
  • To support certain types of virtual private networks.

More MPLS

• The idea of MPLS is to add a label to an IP packet.
• When MPLS is enabled on a router, the router allocates a label for each prefix in its routing table.
• Basically, it just uses the index of that entry in the table. So if 18.1.1/24 was the 3rd row in the table, the label might be 3.
• It advertises this label and network prefix to its neighbors according to the Label Distribution Protocol.
• When a packet arrives at the first router in an MPLS network, a label edge router, a complete IP lookup on the packet is done, and the label of the router that the packet should be sent to is applied to that packet.
• The packet is then sent to the next router that understand MPLS. This router only needs to look at the label (not the IP address) to know which port to go out on and which label to replace the current label with.
• One advantage to doing this is that ATM networks can now be used to forward IP packets using MPLS rather than need to do special tricks to try to carry the IP packets across the network.
• Labels are attached between the layer 2 and layer 3 headers.