| Multicast group (G) |
A class D address (224.0.0.0/4). Packets sent to it are copied to every receiver that has joined |
| Source (S) |
The host sending the multicast. For market data, the exchange’s feed publisher |
| IGMP |
How hosts tell the local router or switch which groups they want. v2 names only the group; v3 can also name the source |
| IGMP Report / Query |
A Report says “I am in this group”; a Query is the querier asking every so often “who is still here?” |
| Querier |
The device that sends Queries on a segment: usually a PIM router, or a switch running the snooping querier |
| IGMP snooping |
A layer-2 switch listens to IGMP and sends multicast only to ports that have receivers, instead of flooding |
| mrouter port |
The port on a snooping switch that leads to a multicast router |
| PIM-SM / ASM |
Sparse mode: receivers first get traffic through the RP, from any source (Any-Source Multicast) |
| RP |
Rendezvous Point: the router where sources and receivers meet in PIM-SM |
| RPT / shared tree (*,G) |
The distribution tree rooted at the RP |
| SPT (S,G) |
The shortest-path tree rooted at the source |
| SSM |
Source-Specific Multicast: receivers ask for (S,G) directly, with no RP. Default range 232/8 |
| RPF |
Reverse Path Forwarding check: a multicast packet must arrive on the interface the router would use to reach the source by unicast, or it is dropped |
| A/B feeds, Line 1/2 |
The exchange sends the same data over two independent paths |
| A/B arbitration |
The feed handler merges both feeds by sequence number: the first copy wins, and a gap on one side is filled from the other |
| Gap |
A break in the sequence numbers, meaning lost packets. UDP does not retransmit; recovery goes through the exchange’s retransmission or snapshot service |
| Microburst |
A very short burst that overruns egress bandwidth or the buffer and causes queuing or drops; see 02 |
| Fan-in / fan-out |
Many-to-one / one-to-many. Multicast itself is fan-out; several feeds converging on one egress port is fan-in |
| Feed handler |
The program that receives market data, decodes it, arbitrates A/B and handles gaps |
| Line handler |
The part of a feed handler that reads one line (A or B) and extracts sequence numbers; see 05 |
| MLD |
IPv6’s version of IGMP (MLDv1 ≈ IGMPv2, MLDv2 ≈ IGMPv3), carried in ICMPv6 |
| MoldUDP64 |
Nasdaq’s framing for sequenced messages over UDP: session, 64-bit sequence number, message count (0 = heartbeat, 0xFFFF = end of session) |
| ITCH |
Nasdaq’s order-by-order market data message format; many venues use variants (SGX ITCH, Millennium’s MITCH) |
| SBE |
Simple Binary Encoding: fixed-layout binary messages that can be read in place; used by CME MDP 3.0 and iLink 3 |
| Snapshot / market recovery |
A feed or service that sends the current book state so a receiver can rebuild after a large gap or a late start |
| Re-request / replay |
Asking the exchange to resend a range of sequence numbers (unicast UDP or TCP); for small gaps |
| Kernel bypass |
Receiving packets in user space without the kernel network stack (Onload, ef_vi, DPDK, AF_XDP) |
| PTP |
Precision Time Protocol (IEEE 1588): clock sync to sub-microsecond accuracy using hardware timestamps |