attempt to describe TCP hol with some images

en/why-tcphol.md

@@ -2,18 +2,26 @@
 
 HTTP/2 is done over TCP and with much fewer TCP connections than when using
 earlier HTTP versions. TCP is a protocol for reliable transfers and you can
-basically think of it as an imaginary string between two machines. What is
+basically think of it as an imaginary chain between two machines. What is
 being put out on the network in one end will end up in the other end, in the
 same order - eventually. (Or the connection breaks.)
 
+![a TCP chain between two computers](../images/tcp-chain.png)
+
 With HTTP/2, typical browsers do tens or hundreds of parallel transfers over
 that single TCP connection.
 
 If a single packet is dropped, lost, in the network somewhere between two
 endpoints that speak HTTP/2, it means the entire TCP connection is brought to
-a halt while the lost packet needs to be re-transmitted and find its way to the
-destination. Since TCP is this "string", it means everything that would come
-after the lost packet needs to wait.
+a halt while the lost packet needs to be re-transmitted and find its way to
+the destination. Since TCP is this "chain", it means that if one link is
+suddenly missing, everything that would come after the lost link needs to
+wait.
+
+An illustration using the chain metaphor when sending two streams over this
+connection. A red stream and a green stream:
+
+![the chain showing links in different colors](../images/tcp-chain-streams.png)
 
 It becomes a TCP-based head of line block!
 
@@ -24,3 +32,20 @@ connections up to distribute the lost packet over so for each lost packet the
 other connections without loss can still continue.
 
 Fixing this issue is not easy, if at all possible, to do with TCP.
+
+## Independent streams avoids the block
+
+With QUIC there's still a connection setup between the two end-points that
+makes the connection secure and the data delivery reliable.
+
+![a QUIC chain between two computers](../images/tcp-chain.png)
+
+But when setting up two different streams over this connection, they are
+treated independently so that if any link goes missing for one of the streams,
+only that stream, that particular chain, has to pause and wait for the missing
+link to get retransmitted.
+
+Illustrated here with one yellow and one blue stream sent between two
+end-points.
+
+![two QUIC streams between two computers](../images/quic-chain-streams.png)